Metal–Perovskite Interfacial Engineering to Boost Activity in Heterogeneous Catalysis

: In this review, we have assessed the possibility of metal–perovskite interfacial engineering to enhance the catalytic activity and selectivity in a range of heterogeneous catalytic reactions. We embarked on a literature screening of different perovskite material classes and reactions to show the versatility of the perovskite structures to induce the formation of such hetero-interfaces and the widespread nature of the phenomenon in catalytic research. There is almost no limitation on the chemical composition of the used perovskites and the nature of the catalyzed reaction, be it under reduction or oxidation conditions. We attempted to classify the perovskite materials, discuss the different strategies leading to the hetero-interfaces, and detail the synergistic action of the components of the respective interfaces. We also provide a critical assessment of the large body of data that is available in terms of a knowledge-based approach to the comparison of differently prepared interfaces with varying interfacial extent to gain a deeper understanding of the bi-functional operation of the interfaces and the urgent necessity to study and characterize such interfaces under realistic operation conditions.


Introduction: Simple Exsolution vs. Activity of the Metal-Perovskite Interface in Heterogeneous Catalysis
Perovskites are one of the fastest evolving materials classes in heterogeneous catalysis with promising catalytic properties in a wide field of research, including, but not limited to nitrogen, carbon, and oxygen (electro-) chemistry [1].As such, perovskite materials are especially used for environmental and energy applications, such as hydrogen production, advanced oxidation processes, oxidative removal of air pollutants, wastewater treatment, degradation of volatile organic compounds, and photocatalytic applications, e.g., carbon dioxide reduction [2].Comprehensive review articles, encompassing not only selected applications but also synthesis procedures and the influence of perovskite size and morphology, already exist in literature [1][2][3][4][5][6][7][8][9][10].The main interest in the use of perovskite materials lies in the eventual replacement of noble metal-containing catalysts by various perovskite classes, thereby exploiting their unique feature of steering their properties by adjusting the chemical composition, redox properties, defect structure, oxygen vacancy concentration, or ion conductivity, likewise [1].Perovskites have the general formula ABX 3 , where X is usually oxygen, a halid, sulfur, or nitrogen.A and B are (transition) bi-or trivalent metal cations, but depending on the doping level, higher oxidation states can also be accommodated to tune the physico-chemical and catalytic properties [1].
A unique feature inherent to perovskite materials, setting them apart from other classes of heterogeneous catalysts, is the potential gas-atmosphere-dependent instability of the perovskite structure [1][2][3][4][5][6][7][8][9][10].Often viewed as a nuisance, it has been shown, especially for perovskites, that it can be used to tailor metal-oxide interfacial systems if the decomposition is carried out in a controlled way [11][12][13].In this respect, the perovskite structure is used strategies.The review will close in Section 4 with a balance of advantages and drawbacks of the preparation strategies and will present an outlook to pending questions.

Hydrocarbon Dry Reforming
Methane dry reforming is by far the most studied reaction with respect to perovskites and the potential formation and activity of the evolving metal-perovskite or metal-oxide interface upon decomposition.A very large body of all kinds of perovskite structures has been studied with respect to doping and cation exchange at the B or B' site, but Ni-containing perovskites on nickelate basis are still in the vast majority [11][12][13].

Nickelates
Nickelates with La at the A-site are the archetypical materials, where the Ni exsolution and the formation of the active Ni/La 2 O 3 interface has been observed at first.Almost all aspects of the mechanism and its structural and morphological prerequisites are known [11][12][13].The structural decomposition pathway of LaNiO 3 is known from in situ measurements and involves a sequence of structural transformations, the formation of oxygen-deficient perovskite structures, and the formation of a Ruddlesden-Popper structure La 2 NiO 4 en route to full decomposition.The reversible formation of two polymorphs of La 2 O 2 CO 3 oxy-carbonates has also been observed (Figure 1A) [11][12][13]16,17].The Ni-La 2 NiO 4 and Ni-La 2 O 3 interfaces resulting from decomposition of La 2 NiO 4 have been studied by Gallego et al., who revealed strong differences in the coking propensity [17].Ni particles obtained from La 2 NiO 4 decomposition, in contact with La 2 O 3 , are much less prone to carbon decomposition, owing to the efficient carbon reactivity at the Ni-La 2 O 3 interface (Figure 1B) [17].The effect of Cu-, Fe-and Ru-doping is also known to reduce the formation of coke, alongside steering the Ni crystallite size, the chemical composition of the exsolved Ni by Cu and Fe co-alloying and the reactivity of the oxy-carbonate structures [12,[18][19][20]23,25,27,29,31,34,35].Das et al. reported differences in the mechanistic pathway, switching between a La-oxy-carbonate-dominated pathway over Ni-La 2 O 3 interfaces to an oxygen-vacancy-dominated pathway over a NiFe-La 2 O 3 interface-both obtained via hydrogen reduction of La 0.9 Sr 0.1 NiO 3 and La 0.9 Sr 0.1 Ni 0.5 Fe 0.5 O 3 , respectively (Figure 1C) [18].An adverse effect of Ni-Fe alloying has also been reported on Fe-doped LaNiO 3 perovskites, where formation of LaFeO 3 -encapsulated Ni particles with diminished reactivity was observed [23].The role of reactivity of surface graphite has been found to be crucial for DRM activity over earth-alkaline-doped perovskite catalysts.The effect of Zn-and Ce-doping on LaNi 1−x Zn x O 3 and La 1−x Ce x NiO 3 has also been studied: Zn stabilizes the LaNiO 3 structure relative to pure LaNiO 3 and, thus, allows steering the extent of Ni exsolution [31].Instead, Ce provides activation of C-H bonds and increases the H 2 selectivity [30].Ce substitution in LaNiO 3 has also been reported to act beneficial due to the formation of active Ce 3+ cations in the Ni-CeO 2 -(LaCe)(NiFe)O 3 interfacial mixture of the decomposed perovskite [32,33].C-H bond activation by mobile lattice oxygen species was also reported over A-site doped Ni-M (M=Bi, Co, Cr, Cu, Fe) [26].Addition of Ru to LaNi 1-x Ru x O 3 perovskites enhances the stability of the evolving Ni-perovskite interface [27].A similar observation has been made for a trimetallic LaNi 0.34 Co 0.33 Mn 0.33 O 3 catalyst upon reduction [25].

Ferrites
Bimetallic Ni-Co catalysts supported on LaFeO 3 -La 2 O 3 were prepared by steered decomposition of La(Co x Ni 1−x ) 0.5 Fe 0.5 O 3 precursors and revealed promotion of catalytic DRM activity and carbon removal by Co addition [37].Oxygen vacancies in the LaFe 0.9 Ni 0.1 O 3 perovskite were found to facilitate the activation/dissociation of CO 2 in catalytic ethane dry reforming [38].Effects of Fe partial substitution of La 2 NiO 4 and LaNiO 3 catalyst precursors have been studied by Song et al. and revealed a suppressed formation tendency of the Ni-La 2 O 3 interface for the Fe-doped samples [24,35].Provendier et al. studied LaNi x Fe 1−x O 3 structures in DRM and found that Ni-Fe alloys exsolved from the perovskite structures can prevent surface poisoning by deposited carbon [29].

Niobates
Perovskite-related mixed oxides of LaNiO 3 and LaNbO 4 were reported to form a Ni-La 2 O 3 -NbO x interface with increased metallic dispersion favoring the activity and stability of the catalysts [39].

Titanates
Ni doping into Ca 0.8 Sr 0.2 TiO 3 and BaTiO 3 yielded highly active DRM catalysts due to in situ formation of a Ni/perovskite interface and easy reverse spillover from the perovskite to the Ni particles, where it helps in oxidative removal of deposited carbon [21].Joo et al. combined the exsolution process with atomic layer deposition (ALD) on La 0.6 Sr 0.2 Ti 0.85 Ni 0.15 O 3−δ .This increased the number of exsolved nanoparticles, increased the catalytic activity, and prolonged the stability compared to the pristine catalyst in DRM [40].

Cuprates
A Ni-LaCuO 3 interface has been compared in DRM to a Ni-LaCuO 3 interface developing from a LaCu 0.53 Ni 0.47 O 3 perovskite and found to be more active, essentially due to a smaller Ni particle size as well as a strong interaction between NiO and the LaCuO 3 support.(Figure 1D) [41].

Cobaltites
Osuzawa showed the excellent carbo-removal properties of an in situ-developed Co-Sm 2 O 3 interface developing from a SmCoO 3 perovskite upon DRM treatment [42].

Aluminates
Deactivation studies of impregnating Ni in different amounts on Ca-doped La-containing aluminate perovskites La 0.9 Ca 0.1 AlO 2.95 yielded different deactivation mechanisms as a function of Ni particle size.Small Ni particles tended to deactivate via metal particle sintering, but for larger ones, deactivation by carbon deposition becomes more important [43].A strongly reduced sintering of exsolved metallic Ni particles has been observed for LaNi x A l−x O 3 perovskites due to strong interaction of Ni with the perovskite [44].Ni supported on LaAlO 3 has been reported by Urasaki et al. to exhibit excellent carbon suppression properties due to oxygen migration from the perovskite to metallic Ni, outperforming LaFeO 3 , SrTiO 3 , BaTiO 3 , or a doped La-Ba ferrite perovskite [22].Ma et al. reported the favorable anti-sintering and anti-carbon properties of a deliberately prepared Ni-LaAlO 3 interface [45].

Zirconates
An array of MZr 0.8 Ni 0.2 O 3 (M=Sr, Ba and Ca) perovskites has been investigated by Dama et al. to study the effect of basicity of the perovskite oxide, metal-support interaction, Ni crystallite size, surface hydroxyl groups, and oxygen defects, and revealed that, for CaZr 0.8 Zr 0.2 O 3 , the physicochemical properties are balanced in the most beneficial way to yield an excellent DRM catalyst [46].

Manganites
A comprehensive comparison of differently prepared Ni-LaMnO 3 interfaces has been provided by Wei et al., revealing the smaller size distribution of exsolved Ni particles and a better attachment to the perovskite surface compared to Ni impregnated on the perovskite surface [47].An atypical stability of exsolved Ni-Fe alloy particles anchored on an oxygen-excess PrBaMn 1.6 Ni 0.3 Fe 0.1 O 5+δ perovskite under severe coking conditions was reported by Yao et al. [48].

Chromates
The importance of the Ni-LaCrO 3 interface in providing strong exsolved Ni particlesupport interaction, a high basicity, and a high oxygen vacancy concentration for improved methane dry reforming has been demonstrated on Sr-and Ni-co-doped chromate-based perovskites [49].
and a better attachment to the perovskite surface compared to Ni impregnated on the perovskite surface [47].An atypical stability of exsolved Ni-Fe alloy particles anchored on an oxygen-excess PrBaMn1.6Ni0.3Fe0.1O5+δperovskite under severe coking conditions was reported by Yao et al. [48].

Chromates
The importance of the Ni-LaCrO3 interface in providing strong exsolved Ni particlesupport interaction, a high basicity, and a high oxygen vacancy concentration for improved methane dry reforming has been demonstrated on Sr-and Ni-co-doped chromatebased perovskites [49].La2NiO4 resulting in a Ni-La2O3 interface.Decomposed La2NiO4 facilitates CO2 adsorption sites via lanthanum oxy-carbonates, which can react on Ni-adsorbed carbon to form CO, while unreduced La2NiO4 leads to coking [17].(B) Sequence of crystalline phases observed during in situ decomposition of LaNiO3 in a DRM mixture [11].(C) Different pathways of methane dry reforming over Srand Fe-doped LaNiO3.Switching from a La-oxycarbonate-mediated pathway over Ni-La2O3 interfaces to a parallel Mars-van Krevelen mechanism over a NiFe-La2O3 interface resulting from hydrogen reduction which was observed by isotopic experiments and DRIFTS analysis [18].(D) Catalytic methane and carbon dioxide conversions over LaCuO3, Cu-doped LaNiO3, and NiO-decorated LaCuO3 [41].Reprinted with permission from Elsevier (A,D) [17,41] and American Chemical Society (B,C) [11,18].

Nickelates
Pure and Sr/Ca/Co-doped LaNiO3 perovskites have been shown by Choudhary et al. to be active in the oxidative conversion of methane to syngas [50].Doping significantly decreased both activity and selectivity, but pure LaNiO3 shows a very good performance in the simultaneous oxidative conversion and steam and CO2 reforming reactions due to the formation of a Ni-La2O3 interface in the initial stage of the reactions (Figure 2A).In a similar line of argumentation, Provendier et al. showed the capabilities of mixed LaNixFe1-xO3 materials, permitting to control the reversible migration of Ni from the bulk to the surface and the stabilization of Ni by Fe substitution [51].Interestingly, this is a system where only partial decomposition of the perovskite structure has been observed after the catalytic tests, i.e., a structurally complex interface of still intact perovskite, La2O3 and Ni  [18].(D) Catalytic methane and carbon dioxide conversions over LaCuO 3 , Cu-doped LaNiO 3 , and NiO-decorated LaCuO 3 [41].Reprinted with permission from Elsevier (A,D) [17,41] and American Chemical Society (B,C) [11,18].

Nickelates
Pure and Sr/Ca/Co-doped LaNiO 3 perovskites have been shown by Choudhary et al. to be active in the oxidative conversion of methane to syngas [50].Doping significantly decreased both activity and selectivity, but pure LaNiO 3 shows a very good performance in the simultaneous oxidative conversion and steam and CO 2 reforming reactions due to the formation of a Ni-La 2 O 3 interface in the initial stage of the reactions (Figure 2A).In a similar line of argumentation, Provendier et al. showed the capabilities of mixed LaNi x Fe 1-x O 3 materials, permitting to control the reversible migration of Ni from the bulk to the surface and the stabilization of Ni by Fe substitution [51].Interestingly, this is a system where only partial decomposition of the perovskite structure has been observed after the catalytic tests, i.e., a structurally complex interface of still intact perovskite, La 2 O 3 and Ni species, is formed.Clean LaNiO 3 was studied by Nguyen et al. in the partial oxidation of methane, with formation of syngas prevailing over decomposed LaNiO 3 (i.e., Ni and La 2 O 3 ) [52].2B) [53].Noble metal (Pd, Pt and Rh) incorporated LaFeO 3 perovskites were tested in the oxidative cracking of n-propane.Formation of a Rh/LaFeO 3 and Pt/LaFeO 3 interface was observed, which increased the redox properties of the systems and the number of oxygen moieties [54].species, is formed.Clean LaNiO3 was studied by Nguyen et al. in the partial oxidation of methane, with formation of syngas prevailing over decomposed LaNiO3 (i.e., Ni and La2O3) [52].

Ferrites
A Pd-LaFeO3 interface was prepared by atomic layer deposition on a MgAl2O4 support by Onn et al. and exhibited a stable catalytic performance in methane oxidation after a high-temperature annealing treatment at 900 °C (Figure 2B) [53].Noble metal (Pd, Pt and Rh) incorporated LaFeO3 perovskites were tested in the oxidative cracking of n-propane.Formation of a Rh/LaFeO3 and Pt/LaFeO3 interface was observed, which increased the redox properties of the systems and the number of oxygen moieties [54].Representative SEM images for LCNT and LCCNT before testing, after testing, and after carbon oxidation experiment.(g) Ni/Al2O3 sample after testing displaying extensive carbon fiber growth all over the surface of the sample.[55] Reprinted with permission from Elsevier (A) [50], American Chemical Society (B) [53] and Royal Society of Chemistry (C) [55].[55] Reprinted with permission from Elsevier (A) [50], American Chemical Society (B) [53] and Royal Society of Chemistry (C) [55].

Ferrites
Ca-doped Nd-based ferrate perovskite structures have been studied by Lindenthal et al. to correlate the exsolution of metal particles with reverse water-gas shift activity.They identified a Mars-van Krevelen-type reaction mechanism with hydrogen activation at the exsolved metal, spillover of hydrogen to the interface for oxygen vacancy generation, and subsequent replenishment of the vacancies by CO 2 activation (Figure 3A) [56,57].
A Ni/La 0.9 Fe 0.1 O 3 perovskite was demonstrated by Huang et al. to exhibit a higher activity in the water-gas shift reaction than the corresponding perovskite-free materials.A combination of experiment and theory revealed the importance of water as the oxygen source for the water-gas shift reaction and highlighted the cooperative role of the perovskite as oxygen transmitter following water dissociation and the role of Ni in CO activation [58].

Nickelates
K-doped LaNiO 3 has been studied as a promising catalyst for the high-temperature water-shift reaction.In that work, it was shown that the CO 2 methanation could be effectively suppressed by K doping of the structure.It is inferred that this is due to the formation of hydroxylated K-OH species at the interface of the decomposed Ni-La 2 O 3 interface.K-OH also promotes the formation of formate species, as a key intermediate for the water-gas shift reaction over reduced LaNiO 3 [59].Maluf et al. observed that the replacement of Co by Ni favored the reduction of the perovskite at lower temperatures (Figure 3B) [60] and accordingly boosted the water-gas shift activity over La 1−y Sr y Ni x Co 1−x O 3 perovskites (Figure 3C) [61].

Titanates
A combined experimental and theoretical study of the water-gas shift reaction over Cumodified SrTiO 3 revealed an optimum Cu content of x Cu = 0.2 in SrTi 1−x Cu x O 3 materials with favorable interfacial dimensions to suppress Cu sintering and boosting the water-gas shift activity (Figure 3D,E) [61].

Cobaltites
Pt and Pd supported/incorporated LaCoO 3 catalysts have been assessed in the watergas shift reaction and it was found that they outperformed Pd and Pt particles supported on SiO 2 , indicating the importance of the metal-perovskite interface.The dominant reaction mechanism is of the Mars-van Krevelen type, including CO and steam (Figure 3F) [62].In an additional study, Sekine revealed that the water-gas shift reactivity of those Pd-and Pt-LaCoO 3 interfaces was not derived from Pt/Pd or the perovskite alone but, in fact, the interaction between the two entities promotes the water-gas shift reaction [63].Carbon dioxide conversion by the reverse water-gas shift chemical looping over La 1−x Sr x CoO 3−δ revealed the importance of metallic Co in contact with perovskite and La 2 O 3 after hydrogen reduction to facilitate the role of the interface as an oxygen carrier in the reaction [64].

Yttrates
A Cu-Y 2 O 3 interface derived by decomposition of a Cu 2 Y 2 O 5 perovskite precursor was found to be active in the water-gas shift reaction due to facile water activation on differently exposed Cu facets [65].

Molybdates
Molybdates have essentially been used as double perovskite precursors in the reverse water-gas shift reaction, where exsolution of Ni-Fe alloys and the associated formation of a Ruddlesden-Popper structure in a core-shell structure has been found to be favorable for performance and stability [66].[61].(F) Catalytic water-gas shift activities over Pt-(diamonds) and Pd-doped (squares) LaCoO3 perovskites [62].Reprinted with permission from Elsevier (A) [56], Springer (B,C,F) [60,62], and American Chemical Society (D,E) [61].

Ferrites
Ploner et al. have demonstrated the use of Co3O4-and Ag-impregnated SrTi0.7Fe0.3O3−δ(STF) as catalysts for methanol steam reforming.Ag and Co3O4 both promote the complex methanol chemistry with respect to pure STF.For Ag-STF, a clear effect of the Ag-STF heterointerface on the total oxidation of methanol to CO2 has been observed, ascribed to the fast and efficient transfer of lattice oxygen from STF to Ag via the extended metalperovskite interface (Figure 4A) [67].The effect of Ni doping on LaFeO3 perovskites has been studied by Provendier et al.Highly efficient catalysts for syngas production via steam reforming of methane were prepared, with the effect of Ni exsolution and the formation of a Ni/La2O3 by decomposition of LaFeO3 most pronounced for x > 0.4, if LaNixFe1−xO3 [68].Chemical-looping steam reforming of methane over Ce-doped perovskites was shown by Zhang et al. to yield in situ-developed Fe 0 /La0.5Ce0.5)2O3and Fe 0 /La0.5Ce0.5)2O2−xphases with improved water-splitting capabilities and superior reforming activity due to the amount of Ce 3+ [69].Ou et al. studied an array of Ni/ABO3 interfaces (A = Ca, Ba, Ce, Zn, Sr, Mg; B=Al, Fe, Mn), as well as different A-site doped materials, and highlighted the importance of the higher dispersion of Ni at the interface, in combination with a higher amount of surface oxygen for surface carbon removal and a stronger interaction between Ni and support.The Mg-doped aluminate perovskite-Ni interface showed the best performance [70].Natile et al. provided a direct comparison of the methanol and ethanol steam-reforming performance of La0.6Sr0.4CoyFe1−yO3catalysts and found that the (F) Catalytic water-gas shift activities over Pt-(diamonds) and Pd-doped (squares) LaCoO 3 perovskites [62].Reprinted with permission from Elsevier (A) [56], Springer (B,C,F) [60,62], and American Chemical Society (D,E) [61].(STF) as catalysts for methanol steam reforming.Ag and Co 3 O 4 both promote the complex methanol chemistry with respect to pure STF.For Ag-STF, a clear effect of the Ag-STF heterointerface on the total oxidation of methanol to CO 2 has been observed, ascribed to the fast and efficient transfer of lattice oxygen from STF to Ag via the extended metalperovskite interface (Figure 4A) [67].The effect of Ni doping on LaFeO 3 perovskites has been studied by Provendier et al.Highly efficient catalysts for syngas production via steam reforming of methane were prepared, with the effect of Ni exsolution and the formation of a Ni/La 2 O 3 by decomposition of LaFeO 3 most pronounced for x > 0.4, if LaNi x Fe 1−x O 3 [68].Chemical-looping steam reforming of methane over Ce-doped perovskites was shown by Zhang et al. to yield in situ-developed Fe 0 /La 0.5 Ce 0.5 ) 2 O 3 and Fe 0 /La 0.5 Ce 0.5 ) 2 O 2−x phases with improved water-splitting capabilities and superior reforming activity due to the amount of Ce 3+ [69].Ou et al. studied an array of Ni/ABO 3 interfaces (A = Ca, Ba, Ce, Zn, Sr, Mg; B=Al, Fe, Mn), as well as different A-site doped materials, and highlighted the importance of the higher dispersion of Ni at the interface, in combination with a higher amount of surface oxygen for surface carbon removal and a stronger interaction between Ni and support.The Mg-doped aluminate perovskite-Ni interface showed the best performance [70].Natile et al. provided a direct comparison of the methanol and ethanol steam-reforming performance of La 0.6 Sr 0.4 Co y Fe 1−y O 3 catalysts and found that the differences in the activity, especially toward ethanol steam reforming, are attributable to a different stabilization of Co 0 nanoparticles after the high-temperature reduction at 873 K, with particularly pronounced stability and high activity for La 0.6 Sr 0.4 Co 0.5 Fe 0.5 O 3 (Figure 4B) [71].

Titanates
A series of Sr n+1 Ti n−x Ni x O 3n+1 Ruddlesden-Popper perovskite structures have been evaluated for methane, CO 2 , and bi-reforming of methane.For SrTi 0.8 Ni 0.2 O 3−δ , the effect of the interface manifested itself in the spillover from labile oxygen vacancies from the bulk to the surface, assisting in the removal of surface-bound carbon [72].

Nickelates
Ce-doped La 1−x Ce x NiO 3 nickelate structures have been identified as promising catalysts for steam CO 2 reforming of methane.For La 0.9 Ce 0.1 NiO 3 , a high degree of active Ni species (consequently forming a Ni/La 2 O 3 /CeO 2 /La 2 O 2 CO 3 interface in the spent catalyst state) with promising activity and a high resistance to coke deposition, has been observed [73].
The ethanol steam-reforming reaction is a particularly well studied reaction in nickelatetype perovskites.Aguero et al. [74,75], Lima et al. [76], Lin et al. [77], and Zhao et al. [78] all studied different perovskite formulations on a lanthanum-nickelate basis; there is common agreement that a Ni-perovskite or Ni-La 2 O 3 interface is present after reduction and that it exhibits beneficial properties with respect to stabilization of the Ni particles at the interface or the highest amount of oxygen vacancies, in correlation with high oxygen mobility and improved carbon clean-off performance.For example, Lima et al. identified a La 0.9 Ce 0.1 NiO 3 composition exhibiting a very high ethanol conversion and particularly low amount of acetaldehyde.Structurally, this composition exhibits a high amount of exsolved Ni, alongside La 2 O 3 and CeO 2 , after a hydrogen pre-treatment.The oxygen vacancy concentration is also highest for this composition, indicating a high carbon clean-off effectivity due to the high oxygen mobility (Figure 4C,D).
A Cu-decorated LaNi 0.9 Cu 0.1 O 3 structure was also found to be active for low-temperature hydrogen production by steam reforming of glycerol.In the hydrogen-reduced state, a mix of Ni and Cu in contact with La 2 O 3 was obtained, with consequently better resistance to coking [79].Wu et al. showed the beneficial properties of smaller Ni particle sizes and a stronger metal-support interaction for reduced carbon deposition and higher stability in glycerol steam reforming [80].A comparison of LaNiO 3 and LaCoO 3 by Aman et al. revealed the better glycerol steam-reforming performance of the latter due to the lower tendency of carbon deposition [81].
The promotional effect of Fe in LaNi 1−x Fe x O 3 catalysts for toluene steam reforming was highlighted by Oemar et al. and ascribed to the formation of Ni-Fe alloy particles at the bimetal-perovskite interface, also yielding a higher sintering stability [82].

Aluminates
Ni/LaAlO 3 interfaces were studied by Sekine et al. also in the steam reforming of toluene.It was found that a Sr-doped La 0.7 Sr 0.3 AlO 3−δ catalyst (prepared by a citrate complexing method) in contact with Ni exhibited the best bifunctional synergistic action due to the active oxygen species derived from the perovskite for coke suppression on the metallic Ni particles [83].Urasaki et al. studied the production of hydrogen by ethanol steam reforming over Co and Ni particles supported on LaAlO 3 in comparison to SrTiO 3 and BaTiO 3 .LaAlO 3 supported catalysts showed high catalytic activity and long-term stability with suppressed coke formation.It was concluded that the lattice oxygen plays a major role in suppressing coke formation and that the Co size has a great impact on the participation of lattice oxygen [84].

Ferrites
Investigation of metal-support interaction effects on Ni-and Rh-decorated La0.6Sr0.4FeO3−δperovskites have been investigated by Thalinger et al. [86,87].These results highlighted the extreme variety in structure, extent, and electronic properties of the metalperovskite interface induced upon hydrogen reduction.A clear correlation of deactivation with perovskite reducibility, as well as metal encapsulation effects and CO2 methanation, has been established (Figure 5A,B).Ni-LaFeO3 and Ni-Fe-LaFeO3 interfaces have also been identified to exhibit a superior resistance to coking in CO methanation.Comparison of Ni and Ni-Fe also allowed for deciphering the difference between metal-perovskite prior to reaction, that indicate a decreasing amount of oxygen vacancies with increasing ceria content [76].Reprinted with permission from American Chemical Society (A,B) [67,71] and Elsevier (C,D) [76].

Methanation and Hydrogenation Reactions 2.5.1. Ferrites
Investigation of metal-support interaction effects on Ni-and Rh-decorated La 0.6 Sr 0.4 FeO 3−δ perovskites have been investigated by Thalinger et al. [86,87].These results highlighted the extreme variety in structure, extent, and electronic properties of the metal-perovskite interface induced upon hydrogen reduction.A clear correlation of deactivation with perovskite reducibility, as well as metal encapsulation effects and CO 2 methanation, has been established (Figure 5A,B).Ni-LaFeO 3 and Ni-Fe-LaFeO 3 interfaces have also been identified to exhibit a superior resistance to coking in CO methanation.Comparison of Ni and Ni-Fe also allowed for deciphering the difference between metal-perovskite and alloy-perovskite interfaces, with the latter showing higher stability and better coking resistance.LaFeO 3 perovskite supported Ni and Ni-Fe catalysts were studied by Wang et al.They revealed that NiFe@Ni/LaFeO 3 -La 2 O 2 CO 3 and Ni/LaFeO 3 formed after hydrogen reduction and syngas methanation.The former shows a core-shell structure with high stability and very good resistance to coking, while the latter is more active [88].

Titanates
Ni-and Rh-SrTi 0.7 Fe 0.3 O 3−δ metal-perovskite interfaces prepared by impregnation have been assessed in CO and CO 2 methanation in reference to a clean Ni/Al 2 O 3 catalyst.Due to the small extent of reduction of SrTi 0.7 Fe 0.3 O 3−δ , only a minor degree of Ni-Fe and Rh-Fe alloying has been observed and the deviation of the catalytic properties from the perovskite-free catalyst were equally less pronounced [86,87].Cu-doped LaTi 1−x Cu x O 3 perovskites have been found to be active for CO hydrogenation to methane, methanol, and CO 2 .XP spectra after pre-reduction indicated the presence of Cu 0 species on the surface, which yielded the conclusion that the reaction proceeds at the interface via H x CO intermediates, followed by decomposition to H x C fragments and, subsequently, hydrocarbons [89].Yan et al. provided evidence of a cooperative effect of a Rh-SrTiO 3 interface in CO 2 hydrogenation.The superior activity is assigned to a bi-functional cooperation between Rh (for efficient hydrogen dissociation) and oxygen vacancies on SrTiO 3 for preferential adsorption and activation of carbon dioxide [90].

Aluminates
A three-dimensional porous LaAlO 3 perovskite structure doped with Ni and Rh showed unique CO 2 methanation capabilities, owing to the exsolution of Rh-Ni nanoalloys and the associated formation of a Ni-Rh/LaAlO 3 interface.This interface shows that the low-temperature reducibility, rich-surface-adsorbed oxygen species, and associated basic sites have a distinct beneficial effect on CO 2 methanation (Figure 5C) [91].

Rhodates and Platinates
CO hydrogenation has also been tested over BaMO 3 structures (M=Rh, Ru, Pt, and Ir) and, in contrast to BaIrO 3 and BaRuO 3 , crystalline BaRhO 3 and BaPtO 3 are both unstable in the synthesis gas mixture and an Rh (Pt)/BaCO 3 interface was obtained in the spent catalyst state.Both perovskites have been found to exhibit a high oxygenate selectivity to methanol [92,93].In a similar line of argumentation, LaRhO 3 has been tested in both CO methanation and the hydroformylation of ethene after different hydrogen pre-treatments.After pre-reduction in hydrogen at 300 • C, the high selectivity to propionaldehyde and the observed chemical shift of the Rh 3d 5/2 peak led to the conclusion of Rh 0 species on the surface; thus, a Rh/LaRhO 3 interface was most likely present [94].

Car Exhaust Catalysis 2.6.1. Ferrites
The archetypical perovskite system, where the exsolution of metal from the perovskite lattice and the reversible formation has been demonstrated in a systematic way, is the Pd-doped ferrite system LaFe 0.57 Co 0.38 Pd 0.05 O 3 [95][96][97][98].Oxidative and reductive cycling allows for reversible insertion and exsolution of Pd into and out from the lattice with improved long-term NO and CO conversion rates (Figure 6A).[86].Methanation capability of Rh-Ni alloys on a three dimensionally ordered microporous lanthanum aluminate perovskite catalyst prepared via exsolution (C) [91].Reprinted with permission from Elsevier (A,B) [86] and American Chemical Society (C) [91].

Ferrites
The archetypical perovskite system, where the exsolution of metal from the perovskite lattice and the reversible formation has been demonstrated in a systematic way, is the Pd-doped ferrite system LaFe0.57Co0.38Pd0.05O3[95][96][97][98].Oxidative and reductive cycling allows for reversible insertion and exsolution of Pd into and out from the lattice with improved long-term NO and CO conversion rates (Figure 6A).[86].Methanation capability of Rh-Ni alloys on a three dimensionally ordered microporous lanthanum aluminate perovskite catalyst prepared via exsolution (C) [91].Reprinted with permission from Elsevier (A,B) [86] and American Chemical Society (C) [91].

Titanates and Zirconates
In line with similar studies on the Pd-ferrite system discussed in the preceding section, also the respective earth-alkaline A-site perovskites in the titanate and zirconate systems have been evaluated, specifically CaTi .095Rh 0.05 O 3 , SrTi .095Rh 0.05 O 3 , BaTi .095Rh 0.05 O 3 , CaTi .095Pt 0.05 O 3 , and CaZr .095Rh 0.05 O 3 .Although the solid solution properties in the oxidized state are comparable and the formation of a metal-perovskite interface occurs for all systems, their oxidative regeneration properties are vastly different.The high stability of small Pt particles (1-3 nm) on the perovskite surface for CaTi .095Pt 0.05 O 3 is particularly pronounced.Titanates in the composition La 0.4 Ca 0.3925 Ba 0.0075 Pt 0.005 Ti 0.995 O 3 and La 0.4 Sr 0.3925 Ba 0.0075 Pt 0.005 Ti 0.995 O 3 were also tested by Kothari et al. in various exhaustrelated reactions, including diesel oxidation catalysis, CO and NO oxidation, and CO oxidation.Excellent integration of the Pt particles by a sophisticated "trojan horse" solidstate synthesis into the perovskite lattice enabled the controlled exsolution of well-anchored Pt particles on the surface during hydrogen reduction, in turn reaching high activity in the oxidation reactions (Figure 6B) [99].[95].(B) Reaction mechanism of Pt+LCT (top) and conversion rates of La0.4Ca0.3925Ba0.0075Pt0.005Ti0.995O3under car exhaust NO+CO reduction conditions for different emergent Pt/La0.4Ca0.3925Ba0.0075Pt0.005Ti0.995O3catalysts (bottom) [99].Reprinted with permission from Wiley (A) [95] and Nature (B) [99].Rights in the material are owned by a third party.

Titanates and Zirconates
In line with similar studies on the Pd-ferrite system discussed in the preceding section, also the respective earth-alkaline A-site perovskites in the titanate and zirconate systems have been evaluated, specifically CaTi.095Rh0.05O3,SrTi.095Rh0.05O3,BaTi.095Rh0.05O3,CaTi.095Pt0.05O3, and CaZr.095Rh0.05O3.Although the solid solution properties in the oxidized state are comparable and the formation of a metal-perovskite interface occurs for all systems, their oxidative regeneration properties are vastly different.The high stability of small Pt particles (1-3 nm) on the perovskite surface for CaTi.095Pt0.05O3 is particularly pronounced.Titanates in the composition La0.4Ca0.3925Ba0.0075Pt0.005Ti0.995O3 and La0.4Sr0.3925Ba0.0075Pt0.005Ti0.995O3were also tested by Kothari et al. in various exhaust-related reactions, including diesel oxidation catalysis, CO and NO oxidation, and CO oxidation.Excellent integration of the Pt particles by a sophisticated "trojan horse" solid-state synthesis into the perovskite lattice enabled the controlled exsolution of well-anchored Pt particles on the surface during hydrogen reduction, in turn reaching high activity in the oxidation reactions (Figure 6B) [99].

Cobaltites
Glisenti et al. presented a Cu-doped LaCo1−xCux perovskite, which yielded surfacesegregated Cu clusters and increased oxygen mobility due to the formation of vacancies after hydrogen reduction.Of particular interest is the LaCo0.5Cu0.5O3composition, which  [99].Reprinted with permission from Wiley (A) [95] and Nature (B) [99].Rights in the material are owned by a third party.

Cobaltites
Glisenti et al. presented a Cu-doped LaCo 1−x Cu x perovskite, which yielded surfacesegregated Cu clusters and increased oxygen mobility due to the formation of vacancies after hydrogen reduction.Of particular interest is the LaCo 0.5 Cu 0.5 O 3 composition, which yielded promising results in simple NO+CO, CO oxidation and complex exhaust-modelled gas mixtures under stoichiometric and lean oxygen conditions.Thus, a strong dependence on the amount of Cu and the associated extent of the Cu-perovskite interface has been reported [100].

CO Oxidation 2.7.1. Titanates
In addition to the cobaltite and titanate structures discussed in the previous section, also a Ir-doped SrIr 0.005 Ti 0.995 O 3 perovskite showed Ir exsolution under hydrogen reduction and an associated high CO oxidation capability (Figure 7).The distinct socketing of the Ir particles with the perovskite support yields a high iridium stability.It has been concluded that metallic iridium is primarily responsible for the high CO oxidation activity, as the unreduced doped perovskite catalyst does not exhibit any activity [101].

Manganites
Differences in the CO oxidation behavior for Pt and Pd particles supported on thinfilm LaMnO 3 have been addressed at the high dispersion of Pd particles on LaMnO 3 after several CO oxidation cycles associated with a low activation energy and a positive reaction order in CO [102].

deNOx NO+CO 2.8.1. Manganites
The role of Cu in model NO+CO, as well as realistic exhaust gas mixtures, has been assessed on Cu-and Pd-doped lanthanum manganite perovskites LaCu x Mn 1−x O 3 to economize the use of noble metals by Thurner et al. [103,104].It was shown that the Cu/reduced perovskite interface can indeed provide efficient NO activation sites in contact with in situ exsolved surface-bound monometallic Cu and bimetallic CuPd nanoparticles (Figure 8A,B).A similar result was obtained on Fe-and Pd-doped LaFe 0.7 Mn 0.3 O 3 perovskites [105].

Cobaltites
An adverse effect of NO+CO reactivity has been observed on a Co 3 O 4 -LaCoO 3 interface: intermediate N 2 O decomposition has been seemingly blocked upon the presence of the Co 3 O 4 -LaCoO 3 interface because of the decrease of active oxygen vacancy numbers.Part of this drawback could be overcome by doping the catalyst with potassium, rendering the catalyst a promising NO storage material at low temperatures [106].

Aluminates
A promotive effect of water on low-temperature NO reduction by CO has been observed over Pd/La 0.9 Ba 0.9 AlO 3−δ .Steam addition promotes the desorption of surfacebound carbonates to accelerate the reaction of nitrate with CO.It has been stated that the mechanism specifically proceeds on the Pd-perovskite interface via participation of lattice oxygen and oxygen from La 0.9 Ba 0.9 AlO 3−δ [107].

Cobaltites
An adverse effect of NO+CO reactivity has been observed on a Co3O4-LaCoO3 interface: intermediate N2O decomposition has been seemingly blocked upon the presence of the Co3O4-LaCoO3 interface because of the decrease of active oxygen vacancy numbers.Part of this drawback could be overcome by doping the catalyst with potassium, rendering the catalyst a promising NO storage material at low temperatures [106].

Aluminates
A promotive effect of water on low-temperature NO reduction by CO has been observed over Pd/La0.9Ba0.9AlO3−δ.Steam addition promotes the desorption of surface-bound carbonates to accelerate the reaction of nitrate with CO.It has been stated that the mechanism specifically proceeds on the Pd-perovskite interface via participation of lattice oxygen and oxygen from La0.9Ba0.9AlO3−δ[107].

Titanates
A LaTi0.5Mg0.5O3catalyst with Pd added showed promising activity in the reduction of NO with CO and it was concluded that the distribution of Pd at the interface is responsible for the steering of the reaction between the reduction of NO by CO and direct NO decomposition [108].
Figure 9 intermediary summarizes the present chapter and, at the same time, provides the link to the following section on the strategies of preparing the metal-perovskite and metal-oxide interfaces derived from controlled synthesis or decomposition.Panel A

Titanates
A LaTi 0.5 Mg 0.5 O 3 catalyst with Pd added showed promising activity in the reduction of NO with CO and it was concluded that the distribution of Pd at the interface is responsible for the steering of the reaction between the reduction of NO by CO and direct NO decomposition [108].
Figure 9 intermediary summarizes the present chapter and, at the same time, provides the link to the following section on the strategies of preparing the metal-perovskite and metal-oxide interfaces derived from controlled synthesis or decomposition.Panel A reveals that nickelate perovskite systems represent by far the largest group of all perovskite compositions, owing to their widespread use in methane (dry) reforming reactions.Equally important are ferrite, cobaltite, titanate, and manganite materials.They are used in a wide variety of reactions, where oxygen vacancies and an associated redox couple, e.g., Mn 3+ /Mn 4+ , Fe 2+ /Fe 3+ are important for charge stabilization.Aluminate perovskites also represent a large group, and are used in a variety of mostly steam-reforming reactions ranging from methane over toluene and glycerol to ethanol [109].In terms of reactions, the overwhelming majority of reactions where the discussed hetero-interfaces play a dominant role is due to methane dry reforming.This is essentially due to the fact that the invoked bi-functional synergism is very pronounced, with job-sharing duties between the components for methane activation (mostly nickel), carbon dioxide activation (e.g., La 2 O 3 ), and enhancing carbon reactivity (oxygen-deficient perovskite with oxygen spillover to the metal entity).The same is true for steam reforming reactions, where water activation usually takes place on the oxidic part, and NO reduction by CO, with shared duties for NO and CO activation.As a link to the subsequent chapter, Panel C highlights the overall synthesis strategies that are used to prepare the hetero-interfaces.The exsolution approach is by far the most used one, making up for more than 75% of all screened cases.Within this group, exsolution by hydrogen pre-reduction is strongly dominant, and only a small number of studies provide results on exsolution directly by decomposition in the reaction mixture.Characterization is strongly dominated by ex situ methods; if in situ characterization is provided, it is usually connected with monitoring the hydrogen pre-reduction.
and CO activation.As a link to the subsequent chapter, Panel C high thesis strategies that are used to prepare the hetero-interfaces.The e by far the most used one, making up for more than 75% of all scree group, exsolution by hydrogen pre-reduction is strongly dominant, ber of studies provide results on exsolution directly by decompositi ture.Characterization is strongly dominated by ex situ methods; if i is provided, it is usually connected with monitoring the hydrogen A summary of the materials and the reactions is provided in T A summary of the materials and the reactions is provided in Table 1.The formation of a metal-perovskite interface by exsolution following a reduction treatment was first observed by Nishihata et al. upon reducing a Co-doped lanthanum ferrite structure modified with Pd, LaFe 0.57 Co 0.38 Pd 0.05 O 3 , in a hydrogen/nitrogen mixture.Following this treatment, the reversible partial decomposition of the parent perovskite lattice and the formation of La 2 O 3 , in addition to exsolution of Pd, has been observed.The active state under NO reduction by CO conditions relevant for automotive exhausts, hence, is a structurally extremely complex Pd/La 2 O 3 /reduced LaFe 0.57 Co 0.38 Pd 0.05 O 3 heterointerface.The presence of the interface has been essentially inferred from XANES and EXAFS data [95].Interestingly, direct observation of the Pd-perovskite interface has been only obtained by atomic-resolution electron microscopy on model Pd-LaFeO 3 compounds, mimicking the reduction conditions by Nishihata et al.Pd exsolution was indeed observed with the according formation of a Pd/reduced LaFeO 3 interface, but the process was deemed to be very limited and localized, with a limited impact on catalysis [98].(Figure 10) .

Formation by Metal Exsolution from Structure-Pure (Doped) Perovskites
The formation of a metal-perovskite interface by exsolution following a reduction treatment was first observed by Nishihata et al. upon reducing a Co-doped lanthanum ferrite structure modified with Pd, LaFe0.57Co0.38Pd0.05O3, in a hydrogen/nitrogen mixture.Following this treatment, the reversible partial decomposition of the parent perovskite lattice and the formation of La2O3, in addition to exsolution of Pd, has been observed.The active state under NO reduction by CO conditions relevant for automotive exhausts, hence, is a structurally extremely complex Pd/La2O3/reduced LaFe0.57Co0.38Pd0.05O3heterointerface.The presence of the interface has been essentially inferred from XANES and EX-AFS data [95].Interestingly, direct observation of the Pd-perovskite interface has been only obtained by atomic-resolution electron microscopy on model Pd--LaFeO3 compounds, mimicking the reduction conditions by Nishihata et al.Pd exsolution was indeed observed with the according formation of a Pd/reduced LaFeO3 interface, but the process was deemed to be very limited and localized, with a limited impact on catalysis [98].(Figure 10)  This introductory example already underlines one of the core problems that occurs upon using exsolution as a preparation approach to metal-perovskite or metal-oxide interfaces: while the reduction process in hydrogen (or directly in the reaction mixture) can be steered by the experimental conditions, the resulting hetero-interface is, without exception, extremely complex and heterogeneous.In most cases, it is not a total decomposition but only a partial decomposition of the perovskite that is observed.In other words, in most cases, it is not a "metal-oxide" interface that is present but, rather, a "metal-reduced perovskite" or, even worse, a "metal-oxide-reduced perovskite" hetero-interface.This renders the establishment of the active site or a bi-functional synergism extremely complicated.

Methane Dry Reforming
Kühl et al. provided electron microscopy evidence of the formation of a Ni-La 2 O 3 interface following hydrogen reduction of a Ru (2.5%)-doped LaNiO 3 catalyst at 700 • C. The large, structurally homogeneous LaNiO 3 particles were found to be transformed into aggregates of smaller La 2 O 3 and Ni domains in random orientation.The authors also provided in situ reduction data by X-ray diffraction and were, thus, able to follow the structural decomposition of the perovskite through two distinct reduction steps associated with the formation of an oxygen-deficient LaNiO 2.5 structure and then full decomposition at 475 • C [35].In a similar line of argumentation, Bonmassar et al. studied the decomposition of LaNiO 3 in hydrogen and within a dry reforming mixture in situ.Clear differences in the appearance and stability of transient structures have been observed, most importantly, a La 2 NiO 4 Ruddlesden-Popper structure, which appears only in a carbon dioxide-methane mixture (Figure 11A,B) [11].Similarly, the influence of A-and B-site doping of La 2 NiO 4 has been studied by Bekheet et al.It was revealed that the differences in catalytic activity essentially arise due the formation of Ni-Cu alloy phases, the resulting Ni particle size and the inherently different reactivity of the present (oxy)carbonate phases.Nezhad et al. in turn focused on the formation, structure, and other aspects of the undoped La 2 NiO 4 as a precursor material in contrast to the LaNiO 3 structure.A much stronger coking and encapsulation propensity of Ni in contrast to LaNiO 3 was observed, despite full decomposition of La 2 NiO 4 [12].Sr-doped samarium nickelate structures were also studied by Nezhad et al. in dry reforming of methane to reveal the role of earth-alkaline doping.It was found, that despite the often-reported beneficial role of the earth-alkaline metal in DRM catalysis, the detrimental role manifests itself in the sole formation of a SrCO 3 phase and a generally destabilized Ni/Sm 2 O 3 interface with pronounced Ni particle sintering [34].The intermediate formation of La 2 NiO 4 during DRM has also been observed by Batiot-Dupeyrat et al. at 700 • C. At higher reaction temperatures of 800 • C, La 2 NiO 4 vanished in favor of full decomposition.The material switched from a Ni-La 2 NiO 4 hetero-interface to Ni-La 2 O 3 at higher temperatures [24].
Gao et.al. [36], Alvarez et al. [39], Su et al. [30], Wang et al. [33], Gallego et al. [17], Valderrama et al. [16], Rabelo-Neto et al. [32], and Moradi et al. [31] all reported full decomposition of La 2 NiO 4 /LaNiO 3 , Nb-, Cu-, or Ce-doped LaNiO 3 into Ni and LaNiO 3 upon hydrogen pre-reduction or by ex situ XRD characterization after the catalytic test.M-doped (M=Bi, Co, Cr, Cu, and Fe) La 0.8 Sr 0.2 Ni 0.8 M 0.2 O 3 catalysts were found to yield very different metal-oxide interfaces after a dry reforming mixture: La 2 NiO 4 was found in all structural mixtures after the catalytic test (alongside La oxy-carbonate structures and SrCO 3 ) except for the Co-doped samples.Nevertheless, the analysis is severely hampered by the structurally impure starting materials containing at least SrCO 3 , NiO, SrO, La 2 O 3 , and Bi 2 O 3 as parasitic structures [26].Kim et al. also revealed full decomposition of Mnand Co-doped LaNiO 3 , with clear differences in the formation propensity of the La-oxycarbonate phases for the Co-and Mn-co-doped sample.While the manganese-free sample exhibited extensive coke formation due to the insertion of CO 2 into the La 2 O 3 support and thereby forming lanthanum oxycarbonate (La 2 O 2 CO 3 ), this is not the case for the manganese-doped sample.MnO thereby strengthens the interaction of metal and support, leading to a smaller particle size (due to slower reduction) of the metallic particles initially and ascertains a constant particle size of Ni against their crystal growth during DRM.Additionally, MnO doping also provides more CO 2 adsorption sites to supply oxygen species during the reaction to Ni, where the methane cracking takes place [25].[16], Rabelo-Neto et al. [32], and Moradi et al. [31] all reported full decomposition of La2NiO4/LaNiO3, Nb-, Cu-, or Ce-doped LaNiO3 into Ni and LaNiO3 upon hydrogen pre-reduction or by ex situ XRD characterization after the catalytic test.Mdoped (M=Bi, Co, Cr, Cu, and Fe) La0.8Sr0.2Ni0.8M0.2O3catalysts were found to yield very different metal-oxide interfaces after a dry reforming mixture: La2NiO4 was found in all structural mixtures after the catalytic test (alongside La oxy-carbonate structures and SrCO3) except for the Co-doped samples.Nevertheless, the analysis is severely hampered by the structurally impure starting materials containing at least SrCO3, NiO, SrO, La2O3, and Bi2O3 as parasitic structures [26].Kim et al. also revealed full decomposition of Mnand Co-doped LaNiO3, with clear differences in the formation propensity of the La-oxycarbonate phases for the Co-and Mn-co-doped sample.While the manganese-free sample exhibited extensive coke formation due to the insertion of CO2 into the La2O3 support and thereby forming lanthanum oxycarbonate (La2O2CO3), this is not the case for the manganese-doped sample.MnO thereby strengthens the interaction of metal and support, leading to a smaller particle size (due to slower reduction) of the metallic particles initially and ascertains a constant particle size of Ni against their crystal growth during DRM.Additionally, MnO doping also provides more CO2 adsorption sites to supply oxygen species during the reaction to Ni, where the methane cracking takes place.[25] Stabilization of the perovskite-La2O2CO3-metallic Ni interface by Fe doping of La2NiO4 and LaNiO3 was reported by Song et al. [24].In turn, the authors suggest that the interaction between Ni and the perovskite effectively suppresses the sintering of Ni particles alongside suppressing the coking propensity.The samples with equal nominal amounts of Ni and Fe are reported to be particularly promising.
In contrast, de Lima et al. studied Fe-doped LaNiO3 catalysts at varying stoichiometry and revealed the presence of a small amount of La2O3 in contact with LaFeO3 and metallic Ni particles.It was suggested that the presence of the oxidic phases gives rise to an oxidizing character, which destabilizes the carbonaceous species on the active Ni sites [28].Tsoukalou also reported stabilization of Fe-doped LaNiO3 samples by in situ XRD measurements in a 1:1 = CO2:CH4 reaction mixture.While a Ni-LaFeO3-La2O2CO3 interface was present for this sample, the respective clean Ni and Co-doped samples shows full decomposition into Ni, La2O3, and La2O2CO3 [23].Das et al. also pointed out that partial substitution of Ni by Fe in LaxSr1−xNiO3 perovskites increased the stability of the perovskite and the perovskite structure is obviously retained even under harsh reduction conditions (hydrogen, 700 °C).It was also reported that exposure of the hydrogen-reduced sample to Reprinted with permission from American Chemical Society, Ref. [11].
Stabilization of the perovskite-La 2 O 2 CO 3 -metallic Ni interface by Fe doping of La 2 NiO 4 and LaNiO 3 was reported by Song et al. [24].In turn, the authors suggest that the interaction between Ni and the perovskite effectively suppresses the sintering of Ni particles alongside suppressing the coking propensity.The samples with equal nominal amounts of Ni and Fe are reported to be particularly promising.
In contrast, de Lima et al. studied Fe-doped LaNiO 3 catalysts at varying stoichiometry and revealed the presence of a small amount of La 2 O 3 in contact with LaFeO 3 and metallic Ni particles.It was suggested that the presence of the oxidic phases gives rise to an oxidizing character, which destabilizes the carbonaceous species on the active Ni sites [28].Tsoukalou also reported stabilization of Fe-doped LaNiO 3 samples by in situ XRD measurements in a 1:1 = CO 2 :CH 4 reaction mixture.While a Ni-LaFeO 3 -La 2 O 2 CO 3 interface was present for this sample, the respective clean Ni and Co-doped samples shows full decomposition into Ni, La 2 O 3 , and La 2 O 2 CO 3 [23].Das et al. also pointed out that partial substitution of Ni by Fe in La x Sr 1−x NiO 3 perovskites increased the stability of the perovskite and the perovskite structure is obviously retained even under harsh reduction conditions (hydrogen, 700 • C).It was also reported that exposure of the hydrogen-reduced sample to DRM conditions at least partially regenerates the perovskite structure.This Mars-van Krevelen-type mechanism is attributed to this particular regeneration cycle and the associated high oxygen mobility and storage capability of the perovskite [18].Different Fe-doped LaNiO It is suggested that the stable perovskite at the Ni-La 2 O 3 interface affects the active oxygen species in the perovskite, which is highly beneficial for removal of deposited coke, boosting the catalytic DRM activity [37].
An illuminating example of the importance of the hetero-interface has been provided by Touahra et al. using surface-dispersed Ni particles in contact with LaCuO 3 prepared by exsolution and by impregnation.Reduction in hydrogen, in all cases, resulted in the total decomposition into Ni and La 2 O 3 .After a subsequent DRM reaction, a La 2 O 2 CO 3 structure in contact with metallic Ni and Cu was observed in both cases, and, interestingly, in the case of the impregnated sample, part of the initial LaCuO 3 structure is restored during the DRM reaction [41].
Dama et al. focused on the structure and coking propensity of different Ni-doped zirconate structures and revealed that the particular stability of CaZr 0.8 Ni 0.2 O 3 in comparison to the Sr-and Ba-doped variants is due to the suppressed sintering of the Ni particles resulting from strong interaction between Ni and the support.In addition, for the Ca-doped sample, only amorphous carbon was observed at the interface between Ni and perovskite, but no carbon filaments, which would cause deactivation of the catalyst (Figure 12A) [46].

Methane Oxidation
Ni(Co) particles in contact with Ce-and Co-doped La0.8Ce0.1Ni0.4TiO6−δwere shown by Kousi et al. to be much more strongly coking-resistant due to the strong interaction of the exsolved Ni(Co) particles with the perovskite, in contrast to a Ni/Al2O3 reference catalyst.The peculiarity of this system is that it contains so-called exo and endo Ni(Co) particles, i.e., Ni(Co) particles on the surface and within the bulk of a porous perovskite material.The system therefore contains two interfaces, which synergistically act to lower the methane oxidation temperature to about 450 °C (Figure 13A) [55].The peculiarity of this system is that it contains so-called exo and endo Ni(Co) particles, i.e., Ni(Co) particles on the surface and within the bulk of a porous perovskite material.The system therefore contains two interfaces, which synergistically act to lower the methane oxidation temperature to about 450 • C (Figure 13A) [55].

Water-Gas Shift Reaction
Formation of Ni, as well as Co-perovskite interfaces in the Ca-doped neodym-ferrite system has been studied by Lindenthal et al. in the reverse water-gas shift reaction [56,57].It has been concluded that the combination of an easily reducible element (such as Co or Ni) in combination with a less reducible element (Fe, in this case) allows for the simple formation of a metal-perovskite interface, as the less reducible element acts as a stability factor for retaining the parent perovskite structure preventing full decomposition.Similar observations have been made by Huang et al. using La0.9Fe0.95Ni0.05O3materials in the water-gas shift reaction.Ni particles were observed by reduction in hydrogen, but the L-FeO3 backbone was also retained.
Maneerung et al. provided evidence for the formation of a structurally complex Ni-K2O-La2O3 interface following hydrogen reduction of a 5 wt.-%K-doped LaNiO3 sample.Electron microscopy analysis evidenced the overall smaller Ni particle size compared to a conventionally impregnated Ni-La2O3 system and the formation of the Ni-K2O-La2O3 triple structure boundary, i.e., K2O formed after reduction is found in direct contact with both metallic Ni and La2O3.This subsequently gives rise to easier water activation at the interface by formation of K-OH species (Figure 13B) [59].
Daza et al. provided evidence for the dynamics of the metal-perovskite interface during several cycles of reduction and reverse water-gas shift reaction for Sr-doped La1−xSrx-CoO3−d phases.Starting with a cubic perovskite phase, the initial perovskite was reduced to La2O3, Co, and SrCO3 and minor amounts of the parent perovskite.After the first water-

Water-Gas Shift Reaction
Formation of Ni, as well as Co-perovskite interfaces in the Ca-doped neodym-ferrite system has been studied by Lindenthal et al. in the reverse water-gas shift reaction [56,57].It has been concluded that the combination of an easily reducible element (such as Co or Ni) in combination with a less reducible element (Fe, in this case) allows for the simple formation of a metal-perovskite interface, as the less reducible element acts as a stability factor for retaining the parent perovskite structure preventing full decomposition.Similar observations have been made by Huang et al. using La 0.9 Fe 0.95 Ni 0.05 O 3 materials in the water-gas shift reaction.Ni particles were observed by reduction in hydrogen, but the L-FeO 3 backbone was also retained.
Maneerung et al. provided evidence for the formation of a structurally complex Ni-K 2 O-La 2 O 3 interface following hydrogen reduction of a 5 wt.-%K-doped LaNiO 3 sample.Electron microscopy analysis evidenced the overall smaller Ni particle size compared to a conventionally impregnated Ni-La 2 O 3 system and the formation of the Ni-K 2 O-La 2 O 3 triple structure boundary, i.e., K 2 O formed after reduction is found in direct contact with both metallic Ni and La 2 3 .This subsequently gives rise to easier water activation at the interface by formation of K-OH species (Figure 13B) [59].
Daza et al. provided evidence for the dynamics of the metal-perovskite interface during several cycles of reduction and reverse water-gas shift reaction for Sr-doped La 1−x Sr x CoO 3−d phases.Starting with a cubic perovskite phase, the initial perovskite was reduced to La 2 O 3 , Co, and SrCO 3 and minor amounts of the parent perovskite.After the first water-gas shift cycle, Co and La 2 O 3 were oxidized to CoO and a Sr-doped La 2 CoO 4 structure.After yet another two reductions, CoO was partially reduced to Co and re-oxidized to CoO during the next two catalytic cycles [64].
A structural dependence of Cu particles exsolved from a Cu 2 Y 2 O 5 perovskite, yielding differently facetted Cu particles attached to Y 2 O 3 as a result of total perovskite decomposition in a pre-reduction step in hydrogen, was provided by Wang et al.Most importantly, the faceting could be directly steered by the reduction temperature.Reduction at 320 • C produced more Cu(111) facets and higher reduction temperatures (500 • C) and yielded an increasing number of Cu(110) facets.It was shown that the so-resulting interface had different abilities to dissociate and activate water (Figure 13C) [65].
As a glimpse into the complexity of using double perovskite structures as precursor materials to form a metal-perovskite interface, Orsini et al. studied a Sr 2 FeMo 0.6 Ni 0.4 O 6−d double perovskite in the reverse water-gas shift reaction and revealed that, depending on temperature and number of reduction cycles, at least four different structures were present: Ni-Fe alloy particles, Sr 3 MoO 6 , Sr 3 FeMoO 7−d , and Sr 2 FeMoO 6 .Despite the structural complexity, the authors inferred a bi-functional synergism, especially between the Ni-Fe alloy (activation of reactants H 2 and CO 2 ) and the Sr 3 FeMoO 7−δ structure (facilitating fast oxygen exchange) [66].14) [68].Similarly, Yang et.Studied the effect of Ce-doping on interface formation in the La x Ce 1−x NiO 3 system in the steam CO 2 reforming of methane.Higher amounts of Ce also in the calcined initial samples cause formation of NiO-perovskite-CeO 2 interfaces, which is, in due course, also reflected in the spent catalysts: Samples without Ce show the typical La-oxy-carbonate phases in contact with Ni in the spent catalyst state, but the Ce samples show a transition to a Ni-CeO 2 interface with minor oxy-carbonate formation with increased Ce content [73].A very similar observation has been made by Sarkari et al. in the case of oxidative steam reforming of ethanol [76].
Ou et al. focused on the formation of a Ni-LaAlO 3 interface also during methane dry reforming.Ni deposited on clean LaAlO 3 was compared to Ni on La 0.7 Mg 0.3 AlO 3−δ .While the interface for both samples appeared stable even after 35 h on stream, the coking propensity was very much suppressed for the Mg-doped sample due to the higher dispersion of Ni and the increased amount of surface oxygen [70].
Lin et al. reported that the behavior of the archetypical LaNiO 3 in ethanol steam reforming, in fact, yields a very similar interfacial structure after reduction and reaction, with metallic Ni, La 2 O 2 CO 3 , and LaNiO 3 still present [77].A different approach to a metalperovskite interface was taken by Zhao et al.LaNixFe1−xO3 structure.For values x ≤ 0.4, formation of small NiO particles on LaFeO3 have been observed, while higher x values lead to deconstruction of the perovskite lattice and the formation of a Ni-La2O3 interface.The system, therefore, switches from pure LaNixFe1−xO3 (a) over NiO-LaFeO3 (b) to Ni-La2O3 (c) as a function of Fe-doping (Figure 14) [68].Similarly, Yang et.Studied the effect of Ce-doping on interface formation in the LaxCe1−xNiO3 system in the steam CO2 reforming of methane.Higher amounts of Ce also in the calcined initial samples cause formation of NiO-perovskite-CeO2 interfaces, which is, in due course, also reflected in the spent catalysts: Samples without Ce show the typical La-oxy-carbonate phases in contact with Ni in the spent catalyst state, but the Ce samples show a transition to a Ni-CeO2 interface with minor oxy-carbonate formation with increased Ce content [73].A very similar observation has been made by Sarkari et al. in the case of oxidative steam reforming of ethanol [76].The general behavior of Ca-, Co, and Co-doped LaNiO 3 catalysts and the respective formation of a metal-oxide interface in glycerol steam reforming is the decomposition of the parent perovskite lattice into a mix of Ni, La 2 O 2 CO 3 , and CaCO 3 [79][80][81].
For CO 2 reduction, Yan et al. compared Rh-SrTiO 3 interfaces developed either from the exsolution of Rh or from Rh-doped SrTiO 3 with Rh particles supported on SrTiO 3 .The extent of the metal-perovskite interface was much larger for the exsolved sample after reduction, giving rise to enhanced adsorption and activation of CO 2 (Figure 15A) [90].SrTiO 3 was also used by Cali et al. as a host for iridium (in a composition of SrIr 0.005 Ti 0.995 O 3 ) and, in turn, to exsolve it during reduction under forming gas at and above 900 • C to form a Ir/SrTiO 3 interface as a catalyst in CO oxidation.Ir particle sizes were found to be in the range of 2-4 nm (Figure 15B) [101].In NO reduction by CO, titanates in composition of La 0.4 Ca 0.3925 Ba 0.00775 Pt 0.05 Ti 0.995 O 3 and La 0.4 Sr 0.3925 Ba 0.00775 Pt 0.05 Ti 0.995 O 3 have also been used by Kothari et al. to form an interface composed of Pt nanoparticles in contact with the perovskite backbone after reduction in forming gas at 700 • C (Figure 15C) [99].

Impregnation Strategies
Impregnation of perovskites with metal and bimetals or alloys is the second most strategy to prepare metal (or bimetal)-perovskite hetero-interfaces.In some cases, the structure and the catalytic properties are compared to an interface obtained by an exsolution approach.Mohammadi et al. have studied a range of Pd-impregnated and Pd-incorporated lanthanum iron manganite compounds in the NO+CO reaction and revealed a strong dependence of CO adsorption and deNO x reactivity.Strong CO adsorption without a pronounced Pd-perovskite interface acts as an inhibitor for adsorption and desorption of NO, while samples with an extended interface show reduced CO poisoning.At the interface, the optimized use of lattice oxygen for CO oxidation and its replenishment from NO dissociation allows for the formation of sites for NO activation without CO poisoning.This directly translates into an increase in N 2 selectivity [105].
SrTiO3 was also used by Cali et al. as a host for iridium (in a composition of SrIr0.005Ti0.995O3)and, in turn, to exsolve it during reduction under forming gas at and above 900 °C to form a Ir/SrTiO3 interface as a catalyst in CO oxidation.Ir particle sizes were found to be in the range of 2-4 nm (Figure 15B) [101].In NO reduction by CO, titanates in composition of La0.4Ca0.3925Ba0.00775Pt0.05Ti0.995O3and La0.4Sr0.3925Ba0.00775Pt0.05Ti0.995O3have also been used by Kothari et al. to form an interface composed of Pt nanoparticles in contact with the perovskite backbone after reduction in forming gas at 700 °C (Figure 15C) [99].Urasaki et al. studied an array of Ni-impregnated aluminate, ferrite, and titanate samples, including LaAlO 3 , LaFeO 3 , SrTiO 3 , and BaTiO 3 , in the methane steam-reforming reaction.Utilization of lattice oxygen at the Ni-perovskite interface appeared as the key parameter for promotion of adsorbed hydrocarbon fragments adsorbed on metallic Ni.A Mars-van Krevelen-type reaction mechanism was inferred, with the Ni/LaAlO 3 and Ni/SrTiO 3 interface performing best due to the easiest accessibility of lattice oxygen at the interface [84].In a similar line of argumentation, Hayakawa inferred participation of lattice oxygen in the dry reforming methane reaction over Ni/BaTiO 3 interfaces [21].Similarly, Sekine et al. [63], Qi et al. [43] and Ma et al. [45] revealed that Ni-La 0.9 Ca 0.1 AlO 2.95 and Ni/LaAlO 3 hetero-interfaces performed outstandingly in methane dry reforming, due to the stabilization of small Ni particles at the interface and an enhanced metalsupport interaction.
Onn et al. revealed the smart behavior of Pd particles supported on LaFeO 3 for the beneficial redox cycling for CO oxidation and methane oxidation.The LaFeO 3 film with a thickness of less than 1.5 nm was shown to be stable to at least 900 • C [53].
Sekine et al. and Watanabe et al. studied Pt-and Pd-LaCoO 3 interfaces in the watergas shift reaction.A Pt-LaCoO 3 interface is present in the fresh catalyst, but after reduction (causing deactivation), the structure of the interface changes to Pt-Co-La 2 O 3 .Interestingly, similar reduction of a Pd-LaCoO 3 causes the corresponding changes in the structure of the interface, but no deactivation [62].Pt and Pd particles were also deposited on a LaMnO 3 perovskite by Cao et al. as catalysts for CO oxidation.The authors report that, while the Pd-LaMnO 3 interface is stable during a reductive pre-treatment, the Pt-LaMnO 3 is not stable and decomposes into a Pt-Mn alloy, Mn 2 AlO 4 , and remains of LaMnO 3 [102].
The importance of surface and bulk characterization in judgement of the structural properties of metal-perovskite interfaces were revealed by Ploner et al. on Ag-and Co 3 O 4 impregnated SrTi 0.7 Fe 0.3 O 3−d in methanol steam reforming.During catalytic pre-treatment, it was observed that Ag remained metallic in the bulk state during pre-oxidation, but XP spectra indicated a Ag 2 O shell around the Ag particles.After pre-reduction, the Ag 2 O shell turns metallic again.The interface of Co 3 O 4 to SrTi 0.7 Fe 0.3 O 3−d appears even more interesting, as it does not show up crystalline during any of the pre-treatments, but the surface-as monitored by XPS-followed the expected trends in Co chemistry during pre-oxidation and pre-reduction.Most importantly, highlighting the need for characterization in the reaction mixture, it was revealed, for both samples, that hydrogen formed during the methanol steam reforming reaction reduces Ag 2 O and Co 3 O 4 to Ag and CoO, respectively.This underlines the sensitivity of the structure of the interface to the gas-phase environment [67].
Urasaki et al. revealed the dependence of the Co particle size in contact with BaTiO 3 ans SrTiO 3 .The smaller Co particle size on the latter caused more frequent participation of lattice oxygen in the oxidation of intermediate species, causing a higher catalytic activity and stability [22].
Wang et al. studied Ni-LaFeO 3 interfaces in syngas methanation following different preparation pathways.A strong dependence of the extent, stability, and chemical composition of the Ni-LaFeO 3 interface has been observed.A NiO/LaFeO 3 interface prepared by wet impregnation was revealed to be very stable during reaction, whereas preparation via a citrate complexing routine caused partial collapse of the Ni-LaFeO 3 structure into Ni-Fe particles in contact with intact LaFeO 3 and La 2 O 2 CO 3 [88].
Ni and Rh interfaces with SrTi 0.7 Fe 0.3 O 3-d and La 0.6 Fe 0.4 FeO 3 were studied by Thalinger et al. in methane steam reforming and carbon dioxide methanation.Pre-reduction in hydrogen or catalytic gas mixtures induced a variety of different effects, including Fe exsolution, the formation of Ni-Fe (or Fe-Rh) alloy particles (Figure 16A).As La 0.6 Fe 0.4 FeO 3 is more reducible than SrTi 0.7 Fe 0.3 O 3−d , these effects are reported to be more pronounced on the former [86,87].Ni and Rh interfaces with SrTi0.7Fe0.3O3-dand La0.6Fe0.4FeO3were studied by Thalinger et al. in methane steam reforming and carbon dioxide methanation.Pre-reduction in hydrogen or catalytic gas mixtures induced a variety of different effects, including Fe exsolution, the formation of Ni-Fe (or Fe-Rh) alloy particles (Figure 16A).As La0.6Fe0.4FeO3 is more reducible than SrTi0.7Fe0.3O3−d,these effects are reported to be more pronounced on the former [86,87].
A stable Pd/LaAlO3 and Pd/La0.9Ba0.1AlO3interface for low-temperature NO reduction by CO was prepared by Higo et al. by a citrate complexing method.As observed for other metal-perovskite interfaces, the key factor for activity is the promotion of surface carbonate species by perovskite lattice oxygen at the interface [107].

Deliberate Over-doping
Over-doping of perovskite structures, thereby creating a structural mix even in initial catalyst entities, is somewhat common in perovskite research.In essence, this means that, even before any catalytic reaction, a normally ill-defined hetero-interface is present.While this should be generally avoided, it can indeed be helpful, if it is deliberately done to create a bi-functionally synergistically acting interface.Thurner et al. provided evidence, how copper can indeed substitute noble metals in the reduction of NO by CO, if a Cuperovskite interface is initially prepared.Specifically, the Cu doping level of a LaCuxMn1-xO3 perovskite structure was deliberately exceeded at a composition of LaCu0.5Mn0.5O3 to yield a Cu/LaCu0.5Cu0.5MnO3interface with increased Cu(0) surface loading, improved exsolution kinetics and a higher number of oxygen vacancies.It has been proven that this catalyst is similarly active and selective as a structure-pure 0.86 wt.-% Pd-doped LaCu0.3Mn0.7O3catalyst [103].A stable Pd/LaAlO 3 and Pd/La 0.9 Ba 0.1 AlO 3 interface for low-temperature NO reduction by CO was prepared by Higo et al. by a citrate complexing method.As observed for other metal-perovskite interfaces, the key factor for activity is the promotion of surface carbonate species by perovskite lattice oxygen at the interface [107].

Deliberate Over-Doping
Over-doping of perovskite structures, thereby creating a structural mix even in initial catalyst entities, is somewhat common in perovskite research.In essence, this means that, even before any catalytic reaction, a normally ill-defined hetero-interface is present.While this should be generally avoided, it can indeed be helpful, if it is deliberately done to create a bi-functionally synergistically acting interface.Thurner et al. provided evidence, how copper can indeed substitute noble metals in the reduction of NO by CO, if a Cu-perovskite interface is initially prepared.Specifically, the Cu doping level of a LaCu x Mn 1-x O 3 perovskite structure was deliberately exceeded at a composition of LaCu 0.5 Mn 0.5 O 3 to yield a Cu/LaCu 0.5 Cu 0.5 MnO 3 interface with increased Cu(0) surface loading, improved exsolution kinetics and a higher number of oxygen vacancies.It has been proven that this catalyst is similarly active and selective as a structure-pure 0.86 wt.-% Pd-doped LaCu 0.3 Mn 0.7 O 3 catalyst [103].

Combination of Exsolution and Impregnation
Combining two standard preparation pathways to metal-perovskite interfaces, Thurner et al. highlighted how Pd impregnation of a LaCu 0.5 Mn 0.5 O 3 catalyst in combination with enhanced Cu exsolution during the NO reduction by CO, can overcome the oxidative deactivation issues of Cu under realistic water-containing model exhaust gas mixtures.Minute impregnation with Pd (0.86 wt.-%) leads to formation of a Cu-Pd alloy, which essentially suppresses the oxidation of Cu to CuO under humid conditions (equivalent to 5 vol.% water in the NO+CO feed).This in turn enhances the N 2 O decomposition kinetics and improves the other main reaction branches of the deNO x network, including water-gas shift reaction and CO oxidation (Figure 16B) [104].

Further Development and Current Understanding of the Metal-Perovskite Interface in Heterogeneous Catalysis
In summary, we have shown that the formation of metal-perovskite or metal-oxide hetero-interfaces developing from decomposition of a perovskite precursor is a quite common phenomenon occurring for a vast amount of different perovskite classes and in many catalytic reactions.We have attempted to classify the perovskite materials, discuss the different strategies leading to the hetero-interfaces, and detail the synergistic action of the components of the interface.Despite the large body of data that is, in principle, available, we highlight four points that provide a combination of summary and outlook for further development and understanding.
• Knowledge-based steering of the metal-perovskite interface: Striking is the fact that, upon screening the literature, it is evident that, in the vast majority of cases, the formation of the interface is "by accident", rather than the result of a knowledge-based approach.This is mostly the case if perovskite precursors are used.In this respect, one should be careful to discriminate between the understanding of the exsolution process itself and the interpretation of the synergistic action of the formed interface.While the former has been scrutinized in almost every aspect already, a deeper understanding of the latter is still mostly missing.To develop this understanding, a two-pronged approach is necessary: correlation of differently prepared interfaces, allowing for varying the extent of the interface, and in situ formation and analysis of the interfaces.

• Different approaches allow altering the extent of the metal-perovskite interface:
There is a high need for detailed studies of hetero-interfaces with varying metalperovskite or metal-oxide (if started from perovskite precursors) extent prepared by different approaches.This can for example achieved by varying the composition in the parent doped perovskite and steering the decomposition process, or by entirely different preparation approaches.The combination of impregnation, exsolution, and/or a combination of those appears particularly promising.A comparison of interfaces prepared by impregnation in comparison with exsolved ones is the absolute minority in the discussed cases studies, but it is still imperative for a deeper understanding.

•
Mind the difference between a metal-perovskite and metal-oxide interface: Speaking about the knowledge-based approach to such interfaces, a clear discrimination must be made between the interfaces gained from a deliberate support of the metal particles on the perovskite surface, the metal-perovskite interface arising from a partial decomposition of a perovskite precursor and a metal-oxide interface developing from a total decomposition of such a perovskite precursor.While it is evident that, usually, only a comparison of such interfaces reveals the true mechanistic understanding, the literature-reported cases fail to a large extent in this respect.In connection with this deficiency, it has to be stated that, unfortunately, the bad habit of starting from structurally impure initial perovskite structures has wormed itself into research.While the appearance of parasitic stray phases might not have a strong effect on the decomposition process, it cannot be ruled out at will.
• Only in situ and operando analysis gives insight into the structure-activity correlation of metal-perovskite and metal-oxide interfaces derived from (partial) perovskite decomposition: A thorough understanding of the interface developing under operational reaction conditions is equally imperative.This is, sadly, still under-represented in most of the studies, despite the fact that such in situ and operando characterization exists for many methods nowadays.In addition, in the vast majority of the discussed cases, the interface is triggered by a pre-reduction step in hydrogen and not by the reaction mixture itself.This is a particular pity, as it has been shown, e.g., for methane dry reforming or the reduction of NO by CO, that the nature and reactivity in terms of, e.g., the size of exsolved metal particles (hence, extent of the interface), the structural pre-steps to the formation of the interface, and the reactivity of the interface itself, is strongly depending on the gas-phase chemical potential.It is the rule, rather than the exception, that the physico-chemical and catalytic properties of interfaces formed by reduction in hydrogen or in the reaction mixture are different.The archetypical material in methane dry reforming, LaNiO 3 , serves as the prime example: upon decomposition in the reaction mixture, the reactivity of the interface is governed by the intermediate appearance of the Ruddlesden-Popper structure La 2 NiO 4 .However, upon hydrogen reduction, this phase does not appear during perovskite decomposition.In consequence, the Ni particle size and the reactivity of the interface are different.

Figure 1 .
Figure 1.Mechanistic details of methane dry reforming over metal-perovskite interfaces.(A) Difference in CO2 reforming between unreduced La2NiO4 with Ni metal decoration and decomposed La2NiO4 resulting in a Ni-La2O3 interface.Decomposed La2NiO4 facilitates CO2 adsorption sites via lanthanum oxy-carbonates, which can react on Ni-adsorbed carbon to form CO, while unreduced La2NiO4 leads to coking [17].(B) Sequence of crystalline phases observed during in situ decomposition of LaNiO3 in a DRM mixture[11].(C) Different pathways of methane dry reforming over Srand Fe-doped LaNiO3.Switching from a La-oxycarbonate-mediated pathway over Ni-La2O3 interfaces to a parallel Mars-van Krevelen mechanism over a NiFe-La2O3 interface resulting from hydrogen reduction which was observed by isotopic experiments and DRIFTS analysis[18].(D) Catalytic methane and carbon dioxide conversions over LaCuO3, Cu-doped LaNiO3, and NiO-decorated LaCuO3[41].Reprinted with permission from Elsevier (A,D)[17,41] and American Chemical Society (B,C)[11,18].

Figure 1 .
Figure 1.Mechanistic details of methane dry reforming over metal-perovskite interfaces.(A) Difference in CO 2 reforming between unreduced La 2 NiO 4 with Ni metal decoration and decomposed La 2 NiO 4 resulting in a Ni-La 2 O 3 interface.Decomposed La 2 NiO 4 facilitates CO 2 adsorption sites via lanthanum oxy-carbonates, which can react on Ni-adsorbed carbon to form CO, while unreduced La 2 NiO 4 leads to coking [17].(B) Sequence of crystalline phases observed during in situ decomposition of LaNiO 3 in a DRM mixture [11].(C) Different pathways of methane dry reforming over Sr-and Fe-doped LaNiO 3 .Switching from a La-oxycarbonate-mediated pathway over Ni-La 2 O 3 interfaces to a parallel Mars-van Krevelen mechanism over a NiFe-La 2 O 3 interface resulting from hydrogen reduction which was observed by isotopic experiments and DRIFTS analysis[18].(D) Catalytic methane and carbon dioxide conversions over LaCuO 3 , Cu-doped LaNiO 3 , and NiO-decorated LaCuO 3[41].Reprinted with permission from Elsevier (A,D)[17,41] and American Chemical Society (B,C)[11,18].

2. 2
.3.Titanates Chemical looping partial oxidation of methane was studied by Kousi et al. on a complex and especially tailored La 0.7 Ce 0.1 Co 0.3 Ni 0.1 Ti 0.6 O 3−δ perovskite, where Co-doping of perovskite-supported exo/endo Ni particles at the interface allow to decrease the activation temperature of methane to as low as 450 • C (Figure 2C) [55].

2. 2
.3.Titanates Chemical looping partial oxidation of methane was studied by Kousi et al. on a complex and especially tailored La0.7Ce0.1Co0.3Ni0.1Ti0.6O3−δperovskite, where Co-doping of perovskite-supported exo/endo Ni particles at the interface allow to decrease the activation temperature of methane to as low as 450 °C (Figure 2C) [55].

Figure 2 .
Figure 2. Methane oxidation behavior of selected metal-perovskite materials.(A) Effect of Co-doping of LaNiO3 on the methane conversion and syngas selectivity [50].(B) Improvement of the thermal stability and redox cycling ability by introducing a Pd-LaFeO3 interface to a MgAl2O4 spinel structure [53].(C) Co-doping of La0.8Ce0.1Ni0.4Ti0.6O3−δleading to an improvement of chemical looping partial oxidation of methane.In picture: (a) Conversion of methane to CO, CO2 and H2 for LCNT and LCCNT as a function of temperature.(b) X-ray diffraction patterns for the two samples (c).Calculated oxygen capacity for the catalysts in (a).(d) Temperature-programmed oxidation experiment after activity testing.(e) Carbon deposition calculated by integration of the curves in (d).(f)Representative SEM images for LCNT and LCCNT before testing, after testing, and after carbon oxidation experiment.(g) Ni/Al2O3 sample after testing displaying extensive carbon fiber growth all over the surface of the sample.[55]Reprinted with permission from Elsevier (A)[50], American Chemical Society (B)[53] and Royal Society of Chemistry (C)[55].

2. 3
.1.Ferrites Ca-doped Nd-based ferrate perovskite structures have been studied by Lindenthal et al. to correlate the exsolution of metal particles with reverse water-gas shift activity.They

Figure 2 .
Figure 2. Methane oxidation behavior of selected metal-perovskite materials.(A) Effect of Codoping of LaNiO 3 on the methane conversion and syngas selectivity [50].(B) Improvement of the thermal stability and redox cycling ability by introducing a Pd-LaFeO 3 interface to a MgAl 2 O 4 spinel structure [53].(C) Co-doping of La 0.8 Ce 0.1 Ni 0.4 Ti 0.6 O 3−δ leading to an improvement of chemical looping partial oxidation of methane.In picture: (a) Conversion of methane to CO, CO 2 and H 2 for LCNT and LCCNT as a function of temperature.(b) X-ray diffraction patterns for the two samples (c).Calculated oxygen capacity for the catalysts in (a).(d) Temperature-programmed oxidation experiment after activity testing.(e) Carbon deposition calculated by integration of the curves in (d).(f) Representative SEM images for LCNT and LCCNT before testing, after testing, and after carbon oxidation experiment.(g) Ni/Al 2 O 3 sample after testing displaying extensive carbon fiber growth all over the surface of the sample.[55]Reprinted with permission from Elsevier (A)[50], American Chemical Society (B)[53] and Royal Society of Chemistry (C)[55].

2. 4
.5.Cobaltites Morales et al. studied a La 0.6 Sr 0.4 CoO 3−δ precursor in the steam reforming of ethanol and revealed decomposition by hydrogen reduction into stabilized Co nanoparticles within a matrix of metal carbonates and metal oxides, leading to promising total ethanol conversion and high hydrogen selectivity [85].2.4.6.Manganates A Ni-doped La 0.8 Ce 0.2 Mn 0.6 Ni 0.4 O 3 catalyst was studied with and without Cu addition for ethanol steam reforming by Wang et al.Hydrogen pre-reduction yielded Ni exsolution and accordingly, partial formation of a Ni/CeO 2 /perovskite interface, with complete ethanol conversion and a high syngas yield [86]. Surfaces 2024, 7, FOR PEER REVIEW 10 2.4.5.Cobaltites Morales et al. studied a La0.6Sr0.4CoO3−δprecursor in the steam reforming of ethanol and revealed decomposition by hydrogen reduction into stabilized Co nanoparticles within a matrix of metal carbonates and metal oxides, leading to promising total ethanol conversion and high hydrogen selectivity [85].2.4.6.Manganates A Ni-doped La0.8Ce0.2Mn0.6Ni0.4O3catalyst was studied with and without Cu addition for ethanol steam reforming by Wang et al.Hydrogen pre-reduction yielded Ni exsolution and accordingly, partial formation of a Ni/CeO2/perovskite interface, with complete ethanol conversion and a high syngas yield [86].

Figure 4 .
Figure 4. Representative overview of the performance of different metal-perovskite interfaces in alcohol and hydrocarbon steam reforming reactions.(A) Display of methanol steam reforming over Ag-and Co 3 O 4 -decorated SrTi 0.7 Fe 0.3 O 3−δ interfaces [67].(B) Catalytic profiles of La 0.6 Sr 0.4 Co 1−y Fe y O 3−δ perovskites in methanol steam reforming, with Co = (A) 0.8, (B) 0.5 and (C) 0.2 [71].(C,D) Oxidative ethanol steam reforming performance of La 0.9 Ce 0.1 NiO 3 (C) in correlation with the structural evolution of Ni particles and La 2 O 3 during hydrogen reduction of La 1−x Ce x NiO 3prior to reaction, that indicate a decreasing amount of oxygen vacancies with increasing ceria content[76].Reprinted with permission from American Chemical Society (A,B)[67,71] and Elsevier (C,D)[76].

Figure 5 .
Figure 5. Overview of the CO2 methanation capability of metal-perovskite interfaces derived via deliberate impregnation of metal particles on perovskite surfaces (A).Normalized methane steam reforming (A,C) as well as CO methanation reaction rates (B,D) for Ni-LSF and Ni-STF (B)[86].Methanation capability of Rh-Ni alloys on a three dimensionally ordered microporous lanthanum aluminate perovskite catalyst prepared via exsolution (C)[91].Reprinted with permission from Elsevier (A,B)[86] and American Chemical Society (C)[91].

Figure 5 .
Figure 5. Overview of the CO 2 methanation capability of metal-perovskite interfaces derived via deliberate impregnation of metal particles on perovskite surfaces (A).Normalized methane steam reforming (A,C) as well as CO methanation reaction rates (B,D) for Ni-LSF and Ni-STF (B)[86].Methanation capability of Rh-Ni alloys on a three dimensionally ordered microporous lanthanum aluminate perovskite catalyst prepared via exsolution (C)[91].Reprinted with permission from Elsevier (A,B)[86] and American Chemical Society (C)[91].

Figure 6 .
Figure 6.Behavior of in situ-formed metal-perovskite interfaces in car exhaust catalysis.(A) Catalyst activity of LaFe0.57Co0.38Pd0.05O3 in NO reduction by CO in comparison to a Pd/Al2O3 reference catalyst (top) ascribed to reversible exsolution and incorporation of Pd out of and into the perovskite structure.XRD patterns are shown on the bottom (a), the energy dependency for Pd(100) (b) and Pd (110) (c) near the Pd K-edge indicates the occupation of the B-site in the perovskite structure in the oxidized state by Pd)[95].(B) Reaction mechanism of Pt+LCT (top) and conversion rates of La0.4Ca0.3925Ba0.0075Pt0.005Ti0.995O3under car exhaust NO+CO reduction conditions for different emergent Pt/La0.4Ca0.3925Ba0.0075Pt0.005Ti0.995O3catalysts (bottom)[99].Reprinted with permission from Wiley (A)[95] and Nature (B)[99].Rights in the material are owned by a third party.

Figure 6 .
Figure 6.Behavior of in situ-formed metal-perovskite interfaces in car exhaust catalysis.(A) Catalyst activity of LaFe 0.57 Co 0.38 Pd 0.05 O 3 in NO reduction by CO in comparison to a Pd/Al 2 O 3 reference catalyst (top) ascribed to reversible exsolution and incorporation of Pd out of and into the perovskite structure.XRD patterns are shown on the bottom (a), the energy dependency for Pd(100) (b) and Pd (110) (c) near the Pd K-edge indicates the occupation of the B-site in the perovskite structure in the oxidized state by Pd) [95].(B) Reaction mechanism of Pt+LCT (top) and conversion rates of La 0.4 Ca 0.3925 Ba 0.0075 Pt 0.005 Ti 0.995 O 3 under car exhaust NO+CO reduction conditions for different emergent Pt/La 0.4 Ca 0.3925 Ba 0.0075 Pt 0.005 Ti 0.995 O 3 catalysts (bottom)[99].Reprinted with permission from Wiley (A)[95] and Nature (B)[99].Rights in the material are owned by a third party.

Surfaces 2024, 7 309 2 . 7
.2. Manganites Differences in the CO oxidation behavior for Pt and Pd particles supported on thinfilm LaMnO3 have been addressed at the high dispersion of Pd particles on LaMnO3 after several CO oxidation cycles associated with a low activation energy and a positive reaction order in CO[102].

Figure 7 .
Figure 7. CO oxidation profile of an in situ developed iridium-SrTiO3 interface via exsolution.Catalysts prepared at different exsolution temperatures gradually close the gap to a commercially available 1% Ir/Al2O3.Reprinted with permission from American Chemical Society [101].
2.8.deNOx NO+CO2.8.1.ManganitesThe role of Cu in model NO+CO, as well as realistic exhaust gas mixtures, has been assessed on Cu-and Pd-doped lanthanum manganite perovskites LaCuxMn1−xO3 to economize the use of noble metals by Thurner et al.[103,104]It was shown that the Cu/reduced perovskite interface can indeed provide efficient NO activation sites in contact with in situ exsolved surface-bound monometallic Cu and bimetallic CuPd nanoparticles (Figure8A,B).A similar result was obtained on Fe-and Pd-doped LaFe0.7Mn0.3O3perovskites[105].

Figure 7 .
Figure 7. CO oxidation profile of an in situ developed iridium-SrTiO 3 interface via exsolution.Catalysts prepared at different exsolution temperatures gradually close the gap to a commercially available 1% Ir/Al 2 O 3 .Reprinted with permission from American Chemical Society [101].

Figure 8 .
Figure 8. NO reduction by CO over pure and Pd-doped LaCuxMn1−xO3 perovskite catalysts with in situ-developed Cu/perovskite and alloyed CuxPdy/perovskite interfaces.(A) Effect of perovskite chemical composition on the consumption temperature.(B) Correlation of in situ X-ray photoelectron spectra collected under catalytic NO+CO reaction conditions at different temperatures (left) with the La 3d5/2-normalized integral intensity of Cu(0/I) compared to the formation of N2O (right).Reprinted with permission from Elsevier [104].

Figure 8 .
Figure 8. NO reduction by CO over pure and Pd-doped LaCu x Mn 1−x O 3 perovskite catalysts with in situ-developed Cu/perovskite and alloyed Cu x Pd y /perovskite interfaces.(A) Effect of perovskite chemical composition on the consumption temperature.(B) Correlation of in situ X-ray photoelectron spectra collected under catalytic NO+CO reaction conditions at different temperatures (left) with the La 3d 5/2 -normalized integral intensity of Cu(0/I) compared to the formation of N 2 O (right).Reprinted with permission from Elsevier [104].

Figure 9 .
Figure 9. Statistical analysis of key features of the studies of the formation of metal-perovskite and metal-oxide (from perovskite decomposition) interfaces.(A) Chemical composition across all studied reactions, (B) Appearance of hetero-interfaces among different reactions, and (C) Formation and characterization of the hetero-interfaces across all reactions.

Figure 10 .
Figure 10.Exsolution of Pd particles from a LaFe0.95Pd0.05O3−δPLD thin film after various thermal treatments.XP spectra shown in (a), the high-angle annular dark-field image of a Pd particle found on the LaFe0.95Pd0.05O3−δsurface in (b).Reprinted with permission from Ref [98].American Chemical Society.

Figure 10 .
Figure 10.Exsolution of Pd particles from a LaFe 0.95 Pd 0.05 O 3−δ PLD thin film after various thermal treatments.XP spectra shown in (a), the high-angle annular dark-field image of a Pd particle found on the LaFe 0.95 Pd 0.05 O 3−δ surface in (b).Reprinted with permission from Ref [98].American Chemical Society.

Surfaces 2024, 7 ,Figure 11 .
Figure 11.Comparison of the development of the Ni-La2O3 interface during in situ heating of LaNiO3 in a CO2:CH4 (1:1) mixture (A) and in hydrogen (B) from room temperature to 800 °C.Reprinted with permission from American Chemical Society, Ref. [11].Gao et.al.[36], Alvarez et al.[39], Su et al.[30], Wang et al.[33], Gallego et al.[17], Valderrama et al.[16], Rabelo-Neto et al.[32], and Moradi et al.[31] all reported full decomposition of La2NiO4/LaNiO3, Nb-, Cu-, or Ce-doped LaNiO3 into Ni and LaNiO3 upon hydrogen pre-reduction or by ex situ XRD characterization after the catalytic test.Mdoped (M=Bi, Co, Cr, Cu, and Fe) La0.8Sr0.2Ni0.8M0.2O3catalysts were found to yield very different metal-oxide interfaces after a dry reforming mixture: La2NiO4 was found in all structural mixtures after the catalytic test (alongside La oxy-carbonate structures and SrCO3) except for the Co-doped samples.Nevertheless, the analysis is severely hampered by the structurally impure starting materials containing at least SrCO3, NiO, SrO, La2O3, and Bi2O3 as parasitic structures[26].Kim et al. also revealed full decomposition of Mnand Co-doped LaNiO3, with clear differences in the formation propensity of the La-oxycarbonate phases for the Co-and Mn-co-doped sample.While the manganese-free sample exhibited extensive coke formation due to the insertion of CO2 into the La2O3 support and thereby forming lanthanum oxycarbonate (La2O2CO3), this is not the case for the manganese-doped sample.MnO thereby strengthens the interaction of metal and support, leading to a smaller particle size (due to slower reduction) of the metallic particles initially and ascertains a constant particle size of Ni against their crystal growth during DRM.Additionally, MnO doping also provides more CO2 adsorption sites to supply oxygen species during the reaction to Ni, where the methane cracking takes place.[25]Stabilization of the perovskite-La2O2CO3-metallic Ni interface by Fe doping of La2NiO4 and LaNiO3 was reported by Song et al.[24].In turn, the authors suggest that the interaction between Ni and the perovskite effectively suppresses the sintering of Ni particles alongside suppressing the coking propensity.The samples with equal nominal amounts of Ni and Fe are reported to be particularly promising.In contrast, de Lima et al. studied Fe-doped LaNiO3 catalysts at varying stoichiometry and revealed the presence of a small amount of La2O3 in contact with LaFeO3 and metallic Ni particles.It was suggested that the presence of the oxidic phases gives rise to an oxidizing character, which destabilizes the carbonaceous species on the active Ni sites[28].Tsoukalou also reported stabilization of Fe-doped LaNiO3 samples by in situ XRD measurements in a 1:1 = CO2:CH4 reaction mixture.While a Ni-LaFeO3-La2O2CO3 interface was present for this sample, the respective clean Ni and Co-doped samples shows full decomposition into Ni, La2O3, and La2O2CO3[23].Das et al. also pointed out that partial substitution of Ni by Fe in LaxSr1−xNiO3 perovskites increased the stability of the perovskite and the perovskite structure is obviously retained even under harsh reduction conditions (hydrogen, 700 °C).It was also reported that exposure of the hydrogen-reduced sample to

Figure 11 .
Figure 11.Comparison of the development of the Ni-La 2 O 3 interface during in situ heating of LaNiO 3 in a CO 2 :CH 4 (1:1) mixture (A) and in hydrogen (B) from room temperature to 800 • C.Reprinted with permission from American Chemical Society, Ref.[11].
3 materials have been characterized by Provendier et al.A clear dependence of the decomposition propensity as observed: higher Fe contents (0.2 ≤ x ≤ 0.7, x = Fe) lead to undecomposed LaFeO 3 , including the formation of exsolved Ni-Fe alloy plus La 2 O 3 .Hence, a complex Ni-Fe alloy, LaFeO 3 -La 2 O 3 , is present under DRM conditions [29].Similar observations were made by Wang et al. on bimetallic Ni-Co catalysts supported on La 2 O 3 -LaFeO 3 formed by Co-Fe-doped LaNiO 3 .

Figure 12 .
Figure 12.Transmission electron microscopy images of CaZr 0.8 Ni 0.2 O 3-δ (a) SrZr 0.8 Ni 0.2 O 3-d (b), and BaZr 0.8 Ni 0.2 O 3-δ (c) after methane dry reforming at 900 • C (A) and Ni particle size, as well as coking propensity of La 0.9 Mn 0.8 Ni 0.2 O 3 (a) in comparison with Ni impregnated on LaMnO 3 in the spent catalyst state after methane dry reforming at 800 • C (b) (B).Reprinted with permission from Elsevier, adapted from Refs [46,47].A structurally extremely stable Ni-LaMnO 3 perovskite has been studied by Wei et al.Reduction and/or treatment in the DRM reaction up to 800 • C caused Ni exsolution, but the perovskite remained stable.If, compared to an impregnated Ni-LaMnO 3 system, the coking propensity of the interface developed from exsolution is very much suppressed.It was concluded that the weak bonding of the Ni particles to the LaMnO 3 surface for the impregnated catalyst results in accelerated sintering and a higher carbon filament formation rate (Figure 12B) [47].Similar observations have been made by Zhao et al. in the dry reforming of ethane (DRE) with respect to LaFeO 3 stability during DRE.In particular, the authors provided a direct structural comparison of the Ni-NiFe-La 2 O 3 interface resulting from exsolution and Ni and NiFe deliberately supported on La 2 O 3 .The spent NiFe-La 2 O 3 catalyst showed clear indications of partial formation of LaFeO 3 , alongside La 2 O 2 CO 3 species.Interestingly, this hetero-interface turned out to be negatively affected by the interaction of LaFeO 3 and NiFe particles.The interface developed during DRM by exsolution and perovskite decomposition was found to be more stable and active compared to the interfaces prepared via impregnation, due to the smaller Ni particle size and the higher number of oxygen vacancies (for CO 2 dissociation) at the interface[38].

3. 1 . 4 .
Steam Reforming of Hydrocarbons and Alcohols Provendier et al. provided evidence for a composition-dependent formation and stability of a Ni-LaFeO 3 interface during steam reforming of methane, arising from an initial LaNi x Fe 1−x O 3 structure.For values x ≤ 0.4, formation of small NiO particles on LaFeO 3 have been observed, while higher x values lead to deconstruction of the perovskite lattice and the formation of a Ni-La 2 O 3 interface.The system, therefore, switches from pure LaNi x Fe 1−x O 3 (a) over NiO-LaFeO 3 (b) to Ni-La 2 O 3 (c) as a function of Fe-doping (Figure by impregnating ZrO 2 by a LaNi 1−x Co x O 3 perovskite, which decomposes into Ni-Co alloy particles highly dispersed on ZrO 2 and La 2 O 3 .The anti-sintering and anti-coking propensity of this hetero-interface is pronounced.Reversible formation and deconstruction of an interface developing from La 0.6 Sr 0.4 CoO 3 after reduction and ethanol steam reforming has been studied by Morales et al.After reaction at 600 • C, highly dispersed cobalt nanoparticles within a matrix of metal carbonates and metal oxides, including La 2 O 3 La 2 O 2 CO 3 , SrO, and SrCO 3 , were present.Regeneration in air at 900 • C, in turn, caused re-formation of the initial perovskite [78].

Figure 15 .
Figure 15.Comparison of the Rh-SrTiO3 interface derived from exsolution of Rh-doped SrTiO3 samples and Rh impregnated on SrTiO3 with the respective Rh particle-size distributions during CO2 hydrogenation (A).Emerging Ir-SrTiO3 interface developing during a reduction process prior to CO oxidation as a function of reduction temperature (B).Pt particles exsolved from La0.4Ca0.3925Ba0.00775Pt0.05Ti0.995O3during reduction in forming gas at 700 °C (C).Reprinted and adapted with permission from Elsevier (A), American Chemical Society (B), and Nature (C) from Refs.[90,99,101].

Figure 15 .
Figure 15.Comparison of the Rh-SrTiO 3 interface derived from exsolution of Rh-doped SrTiO 3 samples and Rh impregnated on SrTiO 3 with the respective Rh particle-size distributions during CO 2 hydrogenation (A).Emerging Ir-SrTiO 3 interface developing during a reduction process prior to CO oxidation as a function of reduction temperature (B).Pt particles exsolved from La 0.4 Ca 0.3925 Ba 0.00775 Pt 0.05 Ti 0.995 O 3 during reduction in forming gas at 700 • C (C). Reprinted and adapted with permission from Elsevier (A), American Chemical Society (B), and Nature (C) from Refs.[90,99,101].

Surfaces 2024, 7 ,
FOR PEER REVIEW 42 Wang et al. studied Ni-LaFeO3 interfaces in syngas methanation following different preparation pathways.A strong dependence of the extent, stability, and chemical composition of the Ni-LaFeO3 interface has been observed.A NiO/LaFeO3 interface prepared by wet impregnation was revealed to be very stable during reaction, whereas preparation via a citrate complexing routine caused partial collapse of the Ni-LaFeO3 structure into Ni-Fe particles in contact with intact LaFeO3 and La2O2CO3 [88].

Figure 16 .
Figure 16.EDX mapping of Ni-STF and Ni-LSF hetero-interfaces prepared by impregnation of the perovskites with Ni particles in the as prepared state and after CO2 methanation.Bullet points of the key parameters are also shown (A).Cu particles exsolved during NO reduction by CO from a LaCu0.3Mn0.7O3catalyst alongside reaction mechanism observed at the interface (B).Reprinted with permission from Elsevier (A) and American chemical Society (B) from Refs.[86,103].

Figure 16 .
Figure 16.EDX mapping of Ni-STF and Ni-LSF hetero-interfaces prepared by impregnation of the perovskites with Ni particles in the as prepared state and after CO 2 methanation.Bullet points of the key parameters are also shown (A).Cu particles exsolved during NO reduction by CO from a LaCu 0.3 Mn 0.7 O 3 catalyst alongside reaction mechanism observed at the interface (B).Reprinted with permission from Elsevier (A) and American chemical Society (B) from Refs.[86,103].
LaFeO 3 interface was prepared by atomic layer deposition on a MgAl 2 O 4 support by Onn et al. and exhibited a stable catalytic performance in methane oxidation after a high-temperature annealing treatment at 900 • C (Figure

Table 1 .
Overview of the systems, reaction conditions, and observed structures after reduction and/or reaction and pathway of hetero-interface formation.* e/i indicates if the interface is obtained by exsolution or impregnation, respectively.

Strategies for the In Situ Steering of the Extent of Metal-Perovskite Interface and Correlation to Activity in Heterogeneous Catalysis
3.1.Formation by Metal Exsolution from Structure-Pure (Doped) Perovskites