Transition Metal-Promoted LDH-Derived CoCeMgAlO Mixed Oxides as Active Catalysts for Methane Total Oxidation

Abstract

A series of M(x)CoCeMgAlO mixed oxides with different transition metals (M = Cu, Fe, Mn, and Ni) with an M content x = 3 at. %, and another series of Fe(x)CoCeMgAlO mixed oxides with Fe contents x ranging from 1 to 9 at. % with respect to cations, while keeping constant in both cases 40 at. % Co, 10 at. % Ce and Mg/Al atomic ratio of 3 were prepared via thermal decomposition at 750 °C in air of their corresponding layered double hydroxide (LDH) precursors obtained by coprecipitation. They were tested in a fixed bed reactor for complete methane oxidation with a gas feed of 1 vol.% methane in air to evaluate their catalytic performance. The physico-structural properties of the mixed oxide samples were investigated with several techniques, such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), elemental mappings, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction under hydrogen (H₂-TPR) and nitrogen adsorption–desorption at −196 °C. XRD analysis revealed in all the samples the presence of Co₃O₄ crystallites together with periclase-like and CeO₂ phases, with no separate M-based oxide phase. All the cations were distributed homogeneously, as suggested by EDX measurements and elemental mappings of the samples. The metal contents, determined by EDX and ICP-OES, were in accordance with the theoretical values set for the catalysts’ preparation. The redox properties studied by H₂-TPR, along with the surface composition determined by XPS, provided information to elucidate the catalytic combustion properties of the studied mixed oxide materials. The methane combustion tests showed that all the M-promoted CoCeMgAlO mixed oxides were more active than the M-free counterpart, the highest promoting effect being observed for Fe as the doping transition metal. The Fe(x)CoCeMgAlO mixed oxide sample, with x = 3 at. % Fe displayed the highest catalytic activity for methane combustion with a temperature corresponding to 50% methane conversion, T₅₀, of 489 °C, which is ca. 40 °C lower than that of the unpromoted catalyst. This was attributed to its superior redox properties and lowest activation energy among the studied catalysts, likely due to a Fe–Co–Ce synergistic interaction. In addition, long-term tests of Fe(3)CoCeMgAlO mixed oxide were performed, showing good stability over 60 h on-stream. On the other hand, the addition of water vapors in the feed led to textural and structural changes in the Fe(3)CoCeMgAlO system, affecting its catalytic performance in methane complete oxidation. At the same time, the catalyst showed relatively good recovery of its catalytic activity as soon as the water vapors were removed from the feed.

1. Introduction

Volatile organic compounds arising from different activities or industrial sectors, such as transportation, chemical and petrochemical industries, and textile manufacturers, are known as major air pollutants, considering that their continuous release into the environment brings serious concerns, owing to their toxic nature [1]. These compounds are associated with multiple hazardous impacts on the atmosphere, such as the depletion of the stratospheric ozone, the formation of harmful ground-level ozone, and the increase in photochemical smog, which can cause harmful effects on the biosphere, including human beings, due to their potential carcinogenic inducing properties [1,2,3,4]. The main organic compounds that belong in the VOCs category are alcohols, aldehydes, aromatics, alkanes, ketones, olefins, ethers, esters, halogenated hydrocarbons, and sulfur-containing compounds [5].

VOCs found in low concentrations in residual flue gases resulting from different industrial activities must be eliminated as a viable option to control their emissions in the environment; their recovery from the flue gas mixtures is disadvantageous from an economic point of view [1,6]. The methods used for VOC abatement fall into two main categories: (1) recovery methods, such as condensation, absorption, adsorption, and membrane separation, with good results in highly VOC-concentrated flue gases, and (2) destructive methods, which include biofiltration, flame combustion, and catalytic combustion, being, in general, more effective in VOC elimination [1]. Catalytic combustion is considered the most effective method for the destruction of low concentrations of VOC, presenting more advantages than traditional thermal combustion [5,7]. It involves lower temperatures for the removal process, which, as a result, reduces energy consumption and inhibits, at the same time, the formation of NO_x gases and partial oxidation products [8,9].

Most of the catalysts used in the combustion of VOC are based on noble metals (Pd and Pt), which are highly active in oxidation reactions [10,11,12]. However, they have some drawbacks, as follows: easy deactivation at high reaction temperatures, owing to their volatility and sintering properties, poisoning when in the presence of sulfur compounds and water vapor, and their high cost [2,13,14]. On the other hand, to find viable alternatives for the noble metal catalysts, research directed towards transition metal mixed oxides has increased in the last decade, as they show several advantages, such as facile preparation, cheaper raw materials, higher thermal stability, and better resilience to poisoning [15,16]. Multiple works have been focused on the topic of active mixed oxide-based catalytic materials, including different oxides, such as Co₃O₄ [17,18,19,20], Fe₂O₃ [21,22], CuO [23,24], CeO₂ [25,26], ordered porous transition metal oxides [27], and mixed oxides derived from layered double hydroxides (LDH) [28], aiming to achieve highly active systems for both VOC abatement [1,5,29] and methane total oxidation [12,17], and to replace the currently used noble metal catalysts.

Mixed oxide catalysts obtained from transition metal-based LDH precursors via thermal decomposition have shown encouraging results as active materials for combustion processes [28]. The LDHs are known as anionic clays or hydrotalcite-like compounds with layered structures, which can behave as anionic exchangers and whose general chemical formula is [M^II_{1 − x}M^III_x(OH)₂]^x+(Aⁿ⁻)_x/n·mH₂O, where M^II and M^III represent divalent and trivalent metal cations, Aⁿ⁻ stands for interlayered anions (inorganic or organic), m is the number of water molecules, and 2 ≤ (1 − x)/x ≤ 4 [30]. A homogeneous distribution of the metal ions with different oxidation states in the brucite-like layers can be achieved for the LDH materials with various cationic compositions, also including transition metals. Their controlled thermal calcination leads to mixed oxide-based catalysts with high activity in VOC abatement, possessing high surface areas, excellent thermal stability, and adjustable redox and acid-base properties [31,32].

Among VOCs, methane (CH₄) has an important contribution to many facilities available to the current standard of living in society, mostly involving energy production, as it is the main component of natural gas. However, the use of methane also comes with some environmental issues, having severe consequences on the global climate, being considered an air pollutant with a global warming potential (GWP) 25 times higher than that of CO₂ over a 100-year time horizon due to its greenhouse gas (GHG) ability to trap energy and its durability in the atmosphere [33]. Therefore, among the anthropogenic GHGs released in the atmosphere on a global scale, methane is the second largest pollutant responsible for climate change, behind carbon dioxide [34]. At the same time, methane is a molecule containing C–H bonds with high dissociation energy (438 kJ mol⁻¹) and, thus, possesses high chemical inertness [35]. Hence, CH₄ is often used as a test molecule in studies of catalytic combustion to develop active catalytic systems for efficient VOC abatement applications. In the last decades, methane combustion has been studied extensively on different catalytic systems, most of them being based on metal mixed oxides, achieving highly active materials for the total oxidation reaction of various hydrocarbons and other VOCs to outline the potential of mixed oxide catalysts in pollution abatement by CH₄ oxidation tests at different temperatures [1,5,36,37].

Regarding the activity of LDH-derived mixed oxides, it has been shown that Cu- and Co-containing systems presented excellent catalytic performances in methane combustion processes [38,39]. Also, among the ex-LDH LnMgAlO (Ln = Ce, Sm, Dy, and Yb) mixed oxides investigated for the methane total oxidation, the highest activity was noticed for the Ce-based catalyst with an optimum of 10 at. % Ce [40]. Recently, the combination of cerium and cobalt provided a series of mixed oxides derived from Ce- and Co-based LDH precursors with high activity for the total oxidation of methane, the optimum composition corresponding to a Mg/Al atomic ratio of 3, 40% Co and 10% Ce (at. % with respect to cations) [41]. In this series, it was highlighted that there was a clear dependence of the activity in methane combustion on the Co content in the mixed oxide, while the catalyst composition takes advantage of the high activity of cobalt spinel oxide, Co₃O₄, in lean methane total oxidation, due to different oxidation states of cobalt (Co²⁺/Co³⁺) and lower Co–O bonds energy [42,43,44]. Previous results indicated, besides the synergy of Co–Ce species, a strong synergy effect between Co and other transition metal cations M, such as Ni, Cu, Mn, or Fe [45,46,47,48]. At the same time, iron oxide-based catalysts showcased the high activity provided by the iron cations with variable oxidation states [49,50].

Therefore, this work aims to consider the benefits of mixed oxide catalysts obtained from LDH precursors containing cobalt oxide in methane total oxidation, presenting a Co-Ce synergistic effect [51] and also an expected promoting effect arising from the doping with low content of different transition metal ions, to enhance the catalytic activity of the LDH-derived CoCeMgAlO system. Thus, this work includes the study of two series of M-promoted CoCeMgAlO mixed oxides (M = Cu, Fe, Mn, Ni) prepared from LDH precursors and tested for methane combustion: the first series comprised of M(x)CoCeMgAlO mixed oxides with fixed M content x = 3 at. %, and the second series of Fe(x)CoCeMgAlO mixed oxides with Fe contents x varying from 1 to 9 at. % with respect to cations, while keeping constant in both cases a Mg/Al atomic ratio of 3, 40 at. % Co and 10 at. % Ce.

2. Results and Discussion

2.1. Catalysts Characterization

The as-prepared M(3)CoCeMgAl and Fe(x)CoCeMgAl LDH precursors showed, in the XRD patterns, similar diffraction lines corresponding to poorly crystallized LDH (ICDD No. 37-0630) and boehmite AlOOH (ICDD No. 83-2384) phases (Figure 1). No other diffraction lines that can be attributed to cerium- or transition metal-based additional phases could be evidenced (Figure 1a). This suggests that these cations were well dispersed in the LDH phase of the precursor samples, which shows good compatibility with the LDH lattice at low amounts of all doping transition metal ions (3 at. % with respect to the cations). Even at increasing Fe contents, in the series of Fe(x)CoCeMgAl LDH precursors, no signs of another phase besides the LDH and boehmite were noticed in the XRD patterns, indicating excellent insertion of the Fe ions into the LDH lattice (Figure 1b).

The mean crystallite size of the LDH phase in the precursors was calculated using the Scherrer equation (Table S1). All the precursor samples consist of nanometric LDH crystallites with sizes in the range from 9 to ca. 15 nm. Among the M(3)CoCeMgAl hydrotalcite precursors, only the Fe-containing sample presents a higher crystallite size (15.3 nm) than the M-free CoCeMgAl LDH sample (12.7 nm), whereas the other M-containing samples were characterized by lower crystallite size (9.0, 11.9 and 12.0 nm for Ni, Mn, and Cu, respectively). The crystallite size of the Fe(3)CoCeMgAl LDH sample is still the highest in the Fe(x)CoCeMgAl LDH precursors series. However, the mean sizes of the Fe(x)CoCeMgAl LDH samples are close together in the range of 14.1–15.3 nm. Additionally, the interplanar distance d₀₀₃ for the LDH phase was determined using Bragg’s law applied to the (003) reflection and was included in Table S1 together with the corresponding 2-theta position and the unit cell parameter c calculated considering a hexagonal symmetry for the LDH structure with the following equation: c = 3 × d₀₀₃. It can be observed that all the M-doped LDH precursors present an interplanar d₀₀₃ distance and, hence, cell parameter c values lower than the undoped counterpart, obviously due to the transition metal doping. Also, the insertion of more Fe ions into the LDH layers favors smaller interplanar d₀₀₃ distances and cell parameter c values in the Fe(x)CoCeMgAl LDH series, which continuously decrease with increasing the Fe content. Thus, an increase in the average charge in the brucite-like layers is evidenced by the increase in Fe(III) content in the LDH precursor.

The XRD patterns of the as-prepared M(3)CoCeMgAlO and Fe(x)CoCeMgAlO mixed oxides after the calcination process are illustrated in Figure 2. In all the mixed oxide samples, three phases were evidenced: Mg(Al)O periclase-like mixed oxide (ICDD No. 04-0829), CeO₂ fluorite (ICDD No. 75-0076), and Co₃O₄ spinel (ICDD No. 42-1467). Thus, the diffraction lines at 2θ = 42.92 and 62.15° correspond to the (200) and (220) crystalline planes in the periclase-like structure. The diffraction lines at 2θ = 28.6, 33.2, 47.5, and 56.4° are ascribed to the (111), (200), (220), and (311) reflection planes, respectively, in the ceria structure, while those at 2θ = 19.1, 31.4, 36.9, 44.7, 59.2, and 65.2°, correspond to the (111), (220), (311), (400), (511), and (440) crystalline planes, respectively, in the cobalt spinel structure. Notably, for both mixed oxide series, irrespective of the type of the transition metal promoter or its content, no other oxide phase was observed besides periclase, ceria, and cobalt spinel, already present in the unpromoted counterpart (Figure 2).

These findings indicate that the transition metal ions used for promoting the CoCeMgAlO mixed oxide are accommodated into the lattices of the three oxide phases identified. The doping of the ceria and periclase phases with Co species was already assessed in the ex-LDH mixed oxides [41]. Furthermore, the lattice parameters and the crystallite sizes of the different phases found in the mixed oxide samples are displayed in

Table 1. Textural and crystallographic properties for M(3)CoCeMgAlO and Fe(x)CoCeMgAlO mixed oxide catalysts.

Catalyst	Surface Area (m2 g−1)	Pore Volume (cm3 g−1)	Pore Size a (nm)	CeO2 Lattice Parameter (nm)	CeO2 Crystallite Size (nm)	Periclase Lattice Parameter (nm)	Periclase Crystallite Size (nm)	Co3O4 Lattice Parameter (nm)	Co3O4 Crystallite Size (nm)
CoCeMgAlO	92.7	0.22	22.1	0.5416	8.0	0.4247	14.0	0.8078	13.9
Cu(3)CoCeMgAlO	40.1	0.21	13.8	0.5408	9.0	0.4220	9.4	0.8077	17.9
Mn(3)CoCeMgAlO	71.4	0.41	22.7	0.5403	8.6	0.4196	9.0	0.8099	9.1
Fe(3)CoCeMgAlO	62.8 (58.8) b	0.28 (0.28) b	17.6 (23.5) b	0.5408	8.6 (9.8) b	0.4203	8.9 (10.0) b	0.8107	15.9 (18.3) b
Ni(3)CoCeMgAlO	67.6	0.31	14.3	0.5409	8.0	0.4216	10.3	0.8087	13.1
Fe(1)CoCeMgAlO	74.9	0.36	17.6 and 11 c	0.5420	7.8	0.4205	8.1	0.8097	12.4
Fe(6)CoCeMgAlO	77.3	0.39	17.5	0.5415	8.0	0.4213	8.0	0.8123	13.0
Fe(9)CoCeMgAlO	72.3	0.32	17.8	0.5411	8.7	0.4251	8.7	0.8138	15.2

^a Maxima of pore size distribution. ^b Obtained after catalytic tests in dry/humid conditions cycles. ^c Shoulder.

. The ceria lattice parameter in the M(3)CoCeMgAlO series is lower compared to the undoped CoCeMgAlO sample. The cerium ions (Ce⁴⁺/Ce³⁺ with a radius of 0.97 Å/1.14 Å) are large cations, whereas the transition metal ions exhibit smaller ionic radii (0.7–0.8 Å) [52]; thus, upon insertion of transition metal ions into the ceria phase, a lattice contraction is observed. Similarly, as the ionic radii of the transition metal ions are slightly lower than that of Mg, the periclase-like phase in the M(3)CoCeMgAlO series shows a lower lattice parameter compared to the undoped CoCeMgAlO sample. Nevertheless, in the Fe(x)CoCeMgAlO series, the ceria and periclase lattice parameters vary irrespective of the Fe content.

The mean crystallite size of the CeO₂ phase varied within a small range, i.e., 7.8–9 nm (Table 1), with no significant influence of the doping transition metal ions on the size. At the same time, the mean crystallite size for the periclase-like phase was estimated in the 8.0–10.3 nm range for all the promoted mixed oxides, while the unpromoted CoCeMgAlO sample presented the highest value of 14.0 nm. The range for the Co₃O₄ crystallite size is more extended, ranging from 9.1 to 17.9 nm, irrespective of the nature of the transition metal ion intended for doping or its content (Table 1).

In addition, an increase in the cobalt spinel lattice parameter of the promoted oxides compared to the unpromoted counterpart was observed (Table 1). Moreover, the expansion effect of the cobalt oxide lattice was more pronounced in the Fe(x)CoCeMgAlO series; the lattice parameter increased with the Fe content in the mixed oxides from 8.097 Å for Fe(1)CoCeMgAlO to 8.138 Å for Fe(9)CoCeMgAlO, in line with the slightly larger size of iron cations than cobalt cations from the Co₃O₄ phase.

The high compatibility of the iron species with the cobalt oxide spinel structure, owing to similar ionic radii of the iron and cobalt ions as transition metals, allows the insertion of the iron cations into the Co₃O₄ phase, even at higher concentrations of Fe (6 and 9 at. %), with no segregation of separate phases of iron oxides in the ex-LDH mixed oxide samples. This proves the advantage provided by the homogeneous distribution of the cations in the LDH precursors, as no segregated phases containing transition metal ions were evidenced in their XRD patterns. Furthermore, in the promoted mixed oxide samples, the homogeneous distribution found in the LDH precursors was confirmed and maintained after the controlled calcination, as transition metal-doped Co₃O₄ crystallites coexisted with doped ceria and periclase particles, with no transition metal promoter-based segregated phases evidenced.

It is noteworthy that the XRD patterns of the mixed oxide samples after the catalytic tests displayed profiles similar to those of the fresh mixed oxides, as illustrated in Figure S1 for the Fe(3)CeMgAlO sample.

The textural properties, such as the specific surface area, the pore volume, and the pore size of the M(3)CoCeMgAlO and Fe(x)CoCeMgAlO samples, are included in Table 1, while their adsorption–desorption isotherms and pore size distributions are depicted in Figures S2 and S3, respectively. The values of the specific surface area of all the transition metal-promoted mixed oxides are lower in comparison to the unpromoted counterpart, as observed previously for ex-LDH M-CuCeMgAlO systems [45]. For the M(3)CoCeMgAlO samples, the surface area varied in the range of 40–71 m² g⁻¹ depending on the type of the transition metal M promoter, while in the case of Fe(x)CoCeMgAlO samples, it varied in the range of 63–77 m² g⁻¹ irrespective of the Fe content. No clear correlation between the specific surface areas of the mixed oxides and the crystallite sizes of the different phases constituting them could be observed. The samples exhibit type IV isotherms with H2b-type hysteresis loops, specific to the mesoporous materials with complex pore structures, according to the IUPAC classification (Figure S2) [53,54]. From the desorption branch of the isotherms, the pore size distributions of the solids were obtained (Figure S3), indicating unimodal pore size structures with well-defined maxima within the range of 13.8–22.7 nm for all the promoted samples, except for the Fe(1)CoCeMgAlO sample which displayed a shoulder at 11 nm besides the maxima of 17.6 nm. The values in Table 1 show a shrinkage of the pore size in most of the samples after the addition of transition metal ions to the CoCeMgAlO system. At the same time, except for Cu(3)CoCeMgAlO, the pore volume of the transition metal-promoted mixed oxides is larger compared to the M-free CoCeMgAlO sample (Table 1).

The metal composition of the transition metal-promoted catalysts has been evaluated by EDX spectroscopy and presented in

Table 2. Chemical composition of the calcined M(x)CoCeMgAlO mixed oxides determined by EDX spectroscopy.

Mixed Oxide	Atomic Content (%) a					Atomic Ratio b
	M	Co	Ce	Mg	Al	Mg/Al	Co/Ce	Co/M
CoCeMgAlO	-	39.8	9.2	37.6	13.4	2.8	4.3	-
Cu(3)CoCeMgAlO	3.0	41.8	9.9	33.2	12.1	2.7	4.2	13.9
Mn(3)CoCeMgAlO	3.0	40.0	9.4	35.0	12.6	2.8	4.2	13.3
Fe(3)CoCeMgAlO	2.6	40.5	9.3	34.6	13.0	2.7	4.3	15.6
Ni(3)CoCeMgAlO	3.6	41.1	9.7	34.1	11.5	3.0	4.2	11.4
Fe(1)CoCeMgAlO	0.6	39.5	9.1	37.7	13.1	2.9	4.3	65.8
Fe(6)CoCeMgAlO	5.7	42.6	9.6	31.2	10.9	2.9	4.4	7.5
Fe(9)CoCeMgAlO	8.2	39.6	9.3	32.0	10.9	2.9	4.2	4.8

^a With respect to cations. ^b Within the Co₃O₄-CeO₂-MgO-Al₂O₃ mixed oxide.

. The values of Co and Ce contents are in line with the theoretical values expected for all mixed oxide samples. In addition, the transition metal M content was around 3 at. % for the M(3)CoCeMgAlO catalysts, whereas the Fe content was not far but slightly lower than intended for the Fe(x)CoCeMgAlO series. The Mg/Al atomic ratio was also found to be close to the fixed value of 3, while the Co/Ce atomic ratio was slightly higher compared to the intended value for all the catalysts. Notably, the EDX results for each mixed oxide are almost identical in different areas, which emphasizes that the preparation of their LDH precursors affords a homogeneous distribution of the metal ions within the samples. The EDX mapping images for several M(3)CoCeMgAlO (M = Mn, Cu, and Fe) and Fe(1)CoCeMgAlO samples, presented in Figure S4, have also confirmed the homogeneous distribution of the elements.

The M(3)CoCeMgAlO and Fe(x)CoCeMgAlO samples presented a good crystallinity in the SEM images (Figures S5 and S6), which featured structures comprised of micrometric clusters containing crystallites of nanometric sizes, as observed from the Scherrer crystallite size in both mixed oxide series. In the samples with high Fe content, an increase in surface roughness and larger aggregates are noticed (Figure S6).

The ICP-OES measurements of the calcined M(x)CoCeMgAlO mixed oxides revealed the overall chemical composition of the catalysts, as shown in

Table 3. Cationic composition of the calcined M(x)CoCeMgAlO mixed oxides determined by ICP-OES technique.

Mixed Oxide	Atomic Content (%) a					Atomic Ratio
	M	Co	Ce	Mg	Al	Mg/Al	Co/Ce	Co/M
CoCeMgAlO	-
Cu(3)CoCeMgAlO	3.1	41.4	10.0	33.4	12.1	2.8	4.1	13.3
Mn(3)CoCeMgAlO	3.1	41.5	10.3	33.1	12.0	2.8	4.0	13.4
Fe(3)CoCeMgAlO	3.2	42.0	10.8	31.9	12.1	2.6	3.9	13.1
Ni(3)CoCeMgAlO	3.0	40.2	10.1	34.9	11.8	3.0	4.0	13.4
Fe(1)CoCeMgAlO	1.0	41.1	10.1	35.2	12.6	2.8	4.1	41.1
Fe(6)CoCeMgAlO	6.2	41.1	10.1	31.4	11.2	2.8	4.1	6.6
Fe(9)CoCeMgAlO	9.3	40.7	10.2	29.3	10.5	2.8	4.0	4.4

^a With respect to cations.

. The values of Co and Ce contents were very close to the intended values in all the mixed oxides, and the Mg/Al ratio reached values close to 3. At the same time, the doping transition metal content was found to be as intended, at around 3 at. % in the M(3)CoCeMgAlO series and within the 1–9 at. % range for the Fe(x)CoCeMgAlO series. The results obtained from the ICP-OES technique are in good agreement with those from EDX spectroscopy, confirming the successful synthesis of the ex-LDH mixed oxides and the good dispersion of the cations in the materials.

X-ray photoelectron spectroscopy (XPS) measurements have been performed to study the surface composition of the M(3)CoCeMgAlO and Fe(x)CoCeMgAlO catalysts and to identify the oxidation states of the different elements. All the expected metal ions, besides oxygen and carbon, were present on the surface of the samples, and their surface atomic content was included in

Table 4. Chemical composition of the mixed oxide samples from X-ray photoelectron spectroscopy results. (Reference binding energy: C 1s = 284.8 eV).

Sample	Surface Atomic Content (at.%)							Surface Atomic Content with Respect to Cations (at.%)
	Ce	Mg	Al	Co	M	O	C	Co	M	Ce
CoCeMgAlO	3.2	15.9	5.7	9.2	-	54.6	11.5	27.2	-	9.3
Cu(3)CoCeMgAlO	2.7	15.7	4.7	8.0	1.5	52.9	14.4	24.6	4.7	8.2
Mn(3)CoCeMgAlO	2.8	15.1	7.6	8.5	0.6	52.1	13.2	24.6	1.8	8.1
Fe(3)CoCeMgAlO	3.8	10.5	2.5	8.8	0.8	55.0	18.6	33.3	3.0	14.4
Ni(3)CoCeMgAlO	3.0	14.9	5.6	9.9	1.1	50.9	14.5	28.7	3.3	8.7
Fe(1)CoCeMgAlO	2.5	9.6	3.0	6.7	0.2	42.4	35.6	30.4	0.9	11.4
Fe(6)CoCeMgAlO	3.7	10.6	2.6	9.7	1.4	52.4	19.6	34.6	5.0	13.2
Fe(9)CoCeMgAlO	4.1	11.0	3.2	10.4	1.9	52.0	17.4	34.0	6.2	13.4
Fe(3)CoCeMgAlO-used	4.7	10.8	3.1	6.1	0.6	52.9	21.8	24.1	2.4	18.6

.

In the XPS spectra of O 1s core level for all the samples (Figure 3a and Figure S7), it was evidenced a peak that was deconvoluted in two main components, which correspond to the lattice oxygen in oxide (ca. 530 eV) and oxygen in the lateral structure (ca. 532 eV), respectively [55]. The oxygen in the lateral structure can arise from the presence of hydroxyl or carbonate species on the surface [56] or subsurface oxygen ions with specific coordination and electron density lower than that of the lattice oxygen [55]. An important contribution to the component at 532 eV is likely to arise from the hydroxylation and carbonatation of the oxide surface, owing to the origins and chemical composition of these oxides. At the same time, the subsurface oxygen ions can appear at the interface of the crystalline Co₃O₄ and CeO₂ phases, bringing an additional contribution to the XPS peak component at 532 eV. The XPS spectra of C 1s core level (Figure 3b and Figure S8) displayed two main peaks, attributed to the presence of adventitious hydrocarbon species (284.8 eV) and carbon from carbonate groups (ca. 289.2 eV) [56]. These results validated the necessity of prior thermal treatment of the LDH-derived mixed oxides in the reactor under air at 600 °C for 30 min before each combustion test to clean their surface of miscellaneous deposition [40].

Figure 4a illustrates the Co 2p core level XPS spectra for Fe(x)CoCeMgAlO mixed oxides (Figure S9 for the M(3)CoCeMgAlO series), showing two major peaks for all the samples. The two major peaks correspond to the presence of Co 2p_3/2 (ca. 780 eV) and Co 2p_1/2 (ca. 796 eV) with a spin-orbit splitting of ca. 16 eV, indicating the presence of both Co²⁺ and Co³⁺ ions [43,57]. Moreover, the satellite peak at around 787 eV is pointing out the presence of Co²⁺ ions at the surface [58,59]. The cobalt surface content has been determined after the deconvolution of the Co 2p_3/2 peak into four components: Co³⁺ in octahedral coordination (780.0 eV), Co²⁺ in tetrahedral coordination (781.5 eV), and two satellite peaks (783.7 eV; 787.5 eV) [43]. The Co(III)/Co(II) surface atomic ratios included in

Table 5. Surface atomic ratios from XPS results of the mixed oxide samples ^a.

Sample	Surface Atomic Ratios
	Co/Ce	Mg/Al	Co/M	Ce(IV)/Ce	Co(III)/Co(II)	M(n + 1) /M(n) b
CoCeMgAlO	2.9	2.8	-	0.94	0.3	-
Cu(3)CoCeMgAlO	3.0	3.3	5.2	0.85	2.5	0.48
Mn(3)CoCeMgAlO	3.0	2.0	13.7	0.80	1.9	0.94
Fe(3)CoCeMgAlO	2.3	4.2	11.0	0.88	1.5	0.81
Ni(3)CoCeMgAlO	3.3	2.7	8.7	0.77	1.8	-
Fe(1)CoCeMgAlO	2.7	3.2	33.5	0.87	1.7	n.d. c
Fe(6)CoCeMgAlO	2.6	4.1	6.9	0.86	1.3	0.74
Fe(9)CoCeMgAlO	2.5	3.4	5.5	0.84	1.0	0.80
Fe(3)CoCeMgAlO-used	1.3	3.5	10.2	0.86	3.1	0.75

^a Reference binding energy: C 1s = 284.8 eV. ^b Cu²⁺/Cu⁺, Mn⁴⁺/Mn³⁺ and Fe³⁺/Fe²⁺ for M = Cu, Mn and Fe, respectively. ^c Not determined.

were obtained from the component areas corresponding to the Co²⁺ and Co³⁺ species resulting from the Co 2p_3/2 peak, according to the following equation:

$${\displaystyle \frac{Co\left(III\right)}{Co \left(II\right)}}={\displaystyle \frac{B}{A}}$$

where A and B represent the peak component areas in percentage attributed to the Co²⁺ and Co³⁺ species, respectively.

It can be observed that the inclusion of a transition metal promoter in the CoCeMgAlO oxide leads to a strong increase in the surface Co(III)/Co(II) atomic ratio, indicating a surface enrichment in Co(III) species (Table 4 and Table 5), which emphasize the higher potential of the promoted mixed oxides in the methane combustion process compared to their unpromoted counterpart [40]. In the M(3)CoCeMgAlO series, the surface Co(III)/Co(II) atomic ratio varies in the range from 1.5 for Fe(3)CoCeMgAlO to 2.5 for Cu(3)CoCeMgAlO, being five to ca. eight times higher than that of the unpromoted sample (0.3). In the Fe(x)CoCeMgAlO mixed oxide series, the Co(III)/Co(II) atomic ratio decreases from 1.7 to 1 with the increasing Fe content from x = 1 to x = 9, indicating a surface enrichment in Co(II) species (Table 4 and Table 5). The increase in iron cations content favors the low oxidation state of the cobalt species upon insertion into the cobalt oxide spinel phase. However, the Fe-promoted samples show the highest surface Co concentration among the mixed oxides studied (Table 4 and Table 5). Notably, for all the mixed oxides, a surface Co content lower than its corresponding bulk content (Table 3 and, for CoCeMgAlO, Table 4) was observed, the latter being quite similar for all samples, i.e., ca. 40 at. %. In the M(3)CoCeMgAlO series (M = Cu, Mn, Fe and Ni), for Cu(3)- and Mn(3)-CoCeMgAlO samples, the surface-to-bulk Co ratio (0.59 for both of them) was lower compared to the unpromoted CoCeMgAlO sample (0.68), while for Fe(3)- and Ni(3)-CoCeMgAlO samples, it was higher (0.79 and 0.71, respectively). In the Fe(x)-CoCeMgAlO series, the surface-to-bulk Co ratio was higher compared to the unpromoted CoCeMgAlO sample (0.68) and increased with increasing the Fe content, from 0.74 for Fe(1)-CoCeMgAlO to 0.84 for Fe(6)- and Fe(9)-CoCeMgAlO.

The X-ray photoelectron spectra of the Ce 3d core level for the Fe(x)CoCeMgAlO samples are illustrated in Figure 4b, while for the M(3)CoCeMgAlO series, in Figure S10. In general, pure CeO₂ displayed six peaks in the XPS profile (three pairs of spin-orbit doublets), which are marked in order of decreasing energy as U’’’, U’’, and U (Ce 3d_3/2 level) and V’’’, V’’, and V (Ce 3d_5/2 level) [60]. At the same time, the XPS profile of Ce₂O₃ showed four peaks (two pairs of spin-orbit doublets), which are marked as U’, U⁰ (Ce 3d_3/2 level) and V’, V⁰ (Ce 3d_5/2 level) [60].

The XPS profiles of all the transition metal-promoted mixed oxides display a combination of all these ten features owing to the presence of both types of Ce species on the catalyst surface, as evidenced in Figure 4b. The specific binding energies of all these peaks found in the high-resolution XPS spectra of the Ce 3d core level were confirmed from Ref. [61]. The U’’’ peak was absent in the XPS spectrum of Ce 3d core level in the case of pure Ce₂O₃, which was entirely ascribed to Ce(IV) species and, hence, it can be used in the quantitative determination of the Ce⁴⁺ content [62]. Present in the case of pure CeO₂, the U’’’ peak constitutes ca. 14% of total integral intensity [62], and the surface Ce(IV) content in percentage can be obtained using the following equation:

$$\%Ce\left(IV\right)=\frac{{\%U}^{\prime \prime \prime }}{14}\times 100$$

where %U’’’ is the U”’ peak area expressed in percentage with respect to the total area of the Ce 3d peak. The surface Ce(IV)/Ce atomic ratio of the unpromoted CoCeMgAlO was high, i.e., 0.94, indicating the presence of low content of surface Ce(III) in the respective sample. The incorporation of a transition metal promoter resulted in lower Ce(IV)/Ce surface atomic ratios, corresponding to higher amounts of surface Ce(III) species as previously observed for transition metal-promoted CuCeMgAO mixed oxides [45]. Thus, in the M(3)CoCeMgAlO series, the Ce(IV)/Ce surface atomic ratio varied between 0.77 for Ni(3)CoCeMgAlO and 0.88 for Fe(3)CoCeMgAlO, while in the Fe(x)CoCeMgAlO series it varied in a narrower range, from 0.84 for Fe(9)CoCeMgAlO to 0.88 for Fe(3)CoCeMgAlO, irrespective of the Fe content (Table 5). Notably, Ce was quite homogeneously distributed in the CoCeMgAlO system, which showed a surface-to-bulk Ce ratio of 1.01. With a quite similar bulk Ce content (10.4 ± 0.4 at. %) in all M-modified mixed oxides, the Ce content found at the surface was lower than that found in the bulk for the M(3)-CoCeMgAlO series, except for Fe(3)-CoCeMgAlO. All the Fe-promoted samples show an excess of surface Ce with respect to its corresponding bulk content, a maximum surface-to-bulk Ce ratio of 1.33 being noticed for Fe(3)-CoCeMgAlO mixed oxide. While the bulk Co/Ce mol ratio is close to 4 for all mixed oxides, the surface Co/Ce ratio was lower and varies in the M(3)-CoCeMgAlO series between 2.3 for Fe(3)-CoCeMgAlO and 3.3 for Ni(3)-CoCeMgAlO, and, in the Fe(x)-CoCeMgAlO series, between 2.3 for Fe(3)-CoCeMgAlO and 2.7 for Fe(1)-CoCeMgAlO, irrespective of the Fe content. Notably, in all the Fe-containing samples, the surface Co/Ce ratio was lower compared to the unpromoted CoCeMgAlO (2.9), the lowest value being noticed for Fe(3)-CoCeMgAlO.

The X-ray photoelectron profiles of the Al 2p and Mg 2p in the M(3)- and Fe(x)-CoCeMgAlO catalyst series are represented in Figures S11 and S12, respectively. The relative peak positions were similar for both elements along the catalyst series and were attributed to Al³⁺ and Mg²⁺ found in their corresponding oxides. The surface Mg/Al ratio was equal to the corresponding bulk ratio (2.8) in the unpromoted CoCeMgAlO; it was lower compared to the bulk Mg/Al ratio in Mn(3)- and Ni(3)-CoCeMgAlO samples and higher in all other samples. In the Fe(x)-CoCeMgAlO series, the surface Mg/Al ratio varied between 3.2 for Fe(1)-CoCeMgAlO and 4.2 for Fe(3)-CoCeMgAlO, irrespective of the Fe content.

The XPS spectra of Cu 2p, Mn 2p, and Ni 2p of the Cu(3)-, Mn(3)-, and Ni(3)CoCeMgAlO samples, are displayed in Figure S13, whereas their surface ion species contents are tabulated in Table 4. The X-ray photoelectron profile of the Cu 2p region consists of multiple peaks: Cu 2p_3/2, Cu 2p_1/2, and their shake-ups. The Cu 2p_3/2 emission was deconvoluted into two components centered at 932.2 and 934.2 eV, ascribed to the existence of both Cu⁺ and Cu²⁺ surface species, respectively, as illustrated in Figure S13a, which allowed obtaining the Cu²⁺/Cu⁺ surface ratio of 0.48. The shake-ups of the Cu 2p_3/2 were also identified at 941.2 and 943.7 eV, owing to the existence of Cu²⁺ species at the surface [63,64]. In Figure S13b, the XPS spectrum of Mn 2p showed two emission lines as well (2p_3/2 and 2p_1/2), which were deconvoluted into three peaks attributed to Mn³⁺, Mn⁴⁺, and shake-up [65]. The Mn 2p_3/2 components positioned at 641.7 and 643.3 eV were ascribed to Mn³⁺ and Mn⁴⁺, respectively, which were further used for determining the Mn⁴⁺/Mn³⁺ surface ratio of 0.94. In the case of the Ni 2p photoelectron spectrum (Figure S13c), the presence of Ni 2p_3/2 peak and its shake-up located at 856 and 861.9 eV, respectively, was confirmed and ascribed to high spin Ni²⁺ species found in the mixed oxide sample [57,66]. Notably, in the M(3)CoCeMgAlO series (M = Cu, Mn, Fe, and Ni), the surface metal M content strongly varied with respect to its corresponding bulk content, which was close to 3 for all samples (Table 3). Thus, for Fe(3)- and Ni(3)-CoCeMgAlO systems, the promoter surface content was only slightly different compared to its bulk content (surface-to-bulk ratio close to 1), which corresponds to a nearly homogeneous distribution of the promoting Fe and Ni cations in the mixed oxide. Contrarily, for Cu(3)-and Mn(3)-CoCeMgAlO samples, the surface Cu content is significantly higher (surface-to-bulk Cu ratio = 1.52), while the Mn surface content is significantly lower compared to their respective bulk content (surface-to-bulk Mn ratio = 0.58). This shows that Cu was rather concentrated in the mixed oxide at the surface level, while Mn prefers the bulk.

The XPS profiles of Fe 2p core level for the Fe(x)CoCeMgAlO mixed oxides are illustrated in Figure 5, and the surface contents of iron species for the corresponding samples are included in Table 4. Two emission lines were identified in XPS spectra at 712 and 725.4 eV, ascribed to the Fe 2p_3/2 and Fe 2p_1/2, respectively. Two components at the binding energies of 711.2 and 713.4 eV were obtained from splitting the Fe 2p_3/2 peak, which was ascribed to Fe²⁺ and Fe³⁺ species, respectively [67]. Using the area of the components, the Fe³⁺/Fe²⁺ surface atomic ratio was calculated for all the Fe-doped catalysts, except for the Fe(1)CoCeMgAlO sample whose low counts for iron species in high-resolution XPS spectra allowed no deconvolution to determine this ratio. It can be observed in Table 5 that the surface Fe³⁺/Fe²⁺ ratio varied in a narrow range and irrespective of the Fe content, from 0.74 for Fe(6)CoCeMgAlO to 0.81 for Fe(3)CoCeMgAlO. The presence of increased Fe³⁺ species concentrations at the surface confers high activity to the catalysts in combustion reactions, favoring electron withdrawal from organic molecules that are to be oxidized [48]. Notably, in the Fe(x)-CoCeMgAlO series, the surface Fe concentration increased with increasing bulk content. However, for Fe(1)- and Fe(3)-CoCeMgAlO samples, the surface Fe content was only slightly lower than its corresponding bulk content (surface-to-bulk Fe ratio of 0.90 and 0.94, respectively), while for Fe(6)- and Fe(9)-CoCeMgAlO samples, the surface Fe content became significantly lower (surface-to-bulk Fe ratio of 0.81 and 0.67, respectively). Considering these results, at low Fe content, this cation is nearly homogeneously distributed in the mixed oxide (surface-to-bulk Fe ratio close to 1), while at high Fe content, it is rather concentrated in the bulk; the difference between surface and bulk Fe concentrations becoming greater at higher Fe content in the mixed oxide. A maximum surface-to-bulk Fe ratio of 0.94 was noticed for Fe(3)-CoCeMgAlO mixed oxide.

H₂-TPR measurements were performed to establish the reducibility behavior in the series of M(3)CoCeMgAlO and Fe(x)CoCeMgAlO samples. The reducibility profiles are illustrated in Figure 6, and the calculated H₂ consumptions are included in

Table 6. Hydrogen consumption in the H₂-TPR experiments.

Catalyst	Experimental H2 Consumption (mmol g−1)							Calculated H2 Consumption (mmol g−1)
	At Low Temperatures (Peak Maximum, °C)			Total Below 500 °C	At High Temperatures (Peak Maximum, °C)	Total Below 750 °C	Total Below 850 °C	Necessary for (M + Co) Reduction a	Necessary for (M + Co + Ce) Reduction a
CoCeMgAlO	0.05 (280)	0.91 (434)	-	0.96	4.75 (845)	2.42	5.71	7.62	8.28
Cu(3)CoCeMgAlO	0.29 (198)	0.39 (239)	2.43 (432)	3.11	3.74 (790)	3.94	6.85	7.90	8.59
Mn(3)CoCeMgAlO	0.03 (298)	1.1 (404)	-	1.13	5.84 (825)	3.03	6.97	7.88	8.59
Fe(3)CoCeMgAlO	0.01 (157)	0.03 (239)	0.56 (322)	0.60	6.12 (769)	3.48	6.73	8.13	8.86
Ni(3)CoCeMgAlO	0.04 (254)	1.04 (420)	-	1.08	5.74 (805)	2.89	6.82	7.87	8.58
Fe(1)CoCeMgAlO	0.07 (265)	0.8 (430)	-	0.87	4.03 (835)	1.74	4.90	7.82	8.52
Fe(6)CoCeMgAlO	0.01 (207)	0.11 (332)	0.17 (440)	0.29	6.07 (784)	2.10	6.36	8.48	9.17
Fe(9)CoCeMgAlO	0.01 (210)	0.13 (353)	0.13 (448)	0.27	7.04 (771)	2.78	7.31	8.81	9.49

^a Calculated according to the oxide composition and the oxidation states found, assuming the total reduction of all cobalt and other transition metal ion species (Fe, Ni, Cu), the reduction of manganese ions Mn(IV/III) to Mn(II) and of Ce(IV) to Ce(III).

. Figure 6 evidenced the effect of the doping transition metal on the reducibility of the CoCeMgAlO system, which presents three reduction peaks, among them two situated below and another one above 500 °C. The two peaks identified in the low-temperature region (below 500 °C) correspond to the reduction of highly dispersed surface cobalt species found in the Co₃O₄ spinel oxide or other Co-doped phases, i.e., periclase and ceria, with maxima at 280 and 434 °C, ascribed to the successive reduction of Co³⁺ to Co²⁺ and Co²⁺ to Co⁰, respectively [43,44]. Moreover, the surface Ce⁴⁺ species from highly reducible small ceria particles can contribute to the hydrogen consumption in this temperature range since the hydrogen spillover on the metallic Co particles can produce atomic hydrogen by dissociation of the hydrogen molecule, which can favor the ceria phase reduction at lower temperature [24]. In the high-temperature region (above 500 °C), it is observed an extended H₂ consumption starting from around 600 °C and reaching a maximum at 845 °C, owing to the reduction of Co ions from the cobalt spinel bulk and other Co-doped phases, as well as the reduction of Co²⁺ species to metallic Co from less reducible compounds such as CoAl₂O₄ or Co₂AlO₄ [68,69]. At the same time, larger and less reducible ceria particles dispersed in the periclase-like phase, which are characterized by strong interactions with the support, also contribute to hydrogen consumption in the high-temperature region. The reduction of small and large ceria particles can be described in the same model as that of the surface and bulk ceria, explaining the reduction profiles of Ce-based materials [70]. Indeed, the small particles present mostly available surface in reaction with hydrogen, whereas the large particles provide mostly bulk phase.

The introduction of a doping transition metal in the CoCeMgAlO system has brought significant changes to the reducibility of the M(3)CoCeMgAlO samples due to the different nature of the transition metal cations used for promoting, as evidenced in Figure 6. The TPR profiles indicated similar reduction peaks, however, being shifted to lower temperatures with 50–100 °C compared to the M-free CoCeMgAlO mixed oxide, which indicates enhanced reducibility for the doped samples in terms of ease of reduction. The reduction peaks showed by the transition metal-promoted mixed oxides in the low-temperature region are assigned to the same reduction processes as in the case of the undoped CoCeMgAlO system, together with the reduction of the additional promoting transition metal ions. Even the high-temperature peak is shifted toward lower temperature upon promotion with transition metal cations M, where the less reducible forms of Co and Ce species present a more favorable reduction than in the M-free CoCeMgAlO system. Comparing the temperature of the first reduction peak in the low-temperature region, the reducibility of the M(3)CoCeMgAlO mixed oxides varies as follows, in descending order: Fe(3)CoCeMgAlO (157 °C) > Cu(3)CoCeMgAlO (198 °C) > Ni(3)CoCeMgAlO (254 °C) > Mn(3)CoCeMgAlO (298 °C). A similar descending reducibility is observed when comparing the maximum high-temperature peak: Fe(3)CoCeMgAlO (769 °C) > Cu(3)CoCeMgAlO (790 °C) > Ni(3)CoCeMgAlO (805 °C) > Mn(3)CoCeMgAlO (825 °C) > CoCeMgAlO (840 °C), emphasizing the increase in redox ability due to the transition metal doping.

The transition metal-promoted mixed oxides presented an increased H₂ consumption compared to the undoped catalyst, as expected for the addition of redox-active cations (Table 6). However, the total experimental H₂ uptake from room temperature to 850 °C is significantly lower than the theoretical H₂ amount calculated according to the oxide composition and the oxidation states found, assuming the total reduction of all cobalt and other transition metal ion species, the reduction of manganese ions to Mn(II) and that of Ce(IV) to Ce(III). This shows that a significant amount of reducible cations in the mixed oxides cannot be reduced under H₂ below 850 °C. At the same time, the H₂ uptakes obtained below 750 °C, the maximum temperature allowed for the catalytic reaction, are markedly lower compared to those obtained below 850 °C, suggesting that the most reducible species found in the catalysts at the surface and sub-surface level are mostly involved in the reduction processes. Less reducible bulk Co species from cobalt spinel or other Co-doped phases can be considered, as well as the presence of CoAl₂O₄ or Co₂AlO₄, which may explain the lower experimental hydrogen consumption to some extent, in comparison to the theoretical consumption, as previously indicated in Refs. [41,71,72]. Although the ceria phase present in the sample has been reported to hinder the apparition of CoAl₂O₄ [44,73,74], by functioning as a physical barrier between alumina and Co₃O₄ phase to prevent its formation, the respective aluminate phase in the samples presents poor reducibility, resulting in lower experimental H₂ consumption [71,73,75]. Notably, M(3)CoCeMgAlO mixed oxides show reducibility in terms of hydrogen consumption below 750 °C much higher than that of their unpromoted counterpart and varies as follows: Cu(3)CoCeMgAlO > Fe(3)CoCeMgAlO > Mn(3)CoCeMgAlO > Ni(3)CoCeMgAlO > CoCeMgAlO. The hydrogen consumption below 500 °C follows the same order, except for the Fe(3)CoCeMgAlO system, which shows the lowest value. As the hydrogen spillover on the first reduced Co and M (M = Co, Cu, and Fe) metallic particles is expected to be involved in the reduction of the oxide at lower temperatures, this change in the reducibility of the Fe(3)CoCeMgAlO sample may be due to the inability of metallic iron to facilitate fast hydrogen spillover [76]. The Mn-containing oxide, for which the final reduction product of higher valency manganese oxide species is MnO [77,78], keeps a greater hydrogen consumption, likely because MnO strongly favors hydrogen diffusion [79].

In the Fe(x)CoCeMgAlO series, the reduction peaks in the low-temperature region appear at different temperatures as a function of the Fe content (Figure 7). Considering the temperatures found for the first reduction peaks, the Fe(x)CoCeMgAlO catalysts present reducibilities varying in the following decreasing order in terms of ease of reduction: Fe(3)CoCeMgAlO (157 °C) > Fe(6)CoCeMgAlO (207 °C) > Fe(9)CoCeMgAlO (210 °C) > Fe(1)CoCeMgAlO (265 °C). Even at the lowest Fe concentration (1 at. %), its addition to the CoCeMgAlO mixed oxide improves the redox properties of the system, as the reduction temperatures for the Fe(1)CoCeMgAlO mixed oxide (265, 430, and 835 °C) are lower compared to the M-free CoCeMgAlO sample (280, 434 and 845 °C). These results show the effect of the iron cations on the catalysts’ reducibility upon insertion in several crystalline phases (cobalt oxide and ceria) without segregation of other compounds as inferred by XRD analysis, resulting in enhanced redox properties.

The H₂ consumption for the Fe(x)CoCeMgAlO series, also included in Table 6, shows that the theoretical hydrogen consumption increases proportionally with the Fe content in the catalyst series. For the temperature range below 850 °C, the experimental H₂ consumption is lower than the theoretical one and varies in direct relation to the Fe content, in the following order: Fe(9)CoCeMgAlO > Fe(3)CoCeMgAlO > Fe(6)CoCeMgAlO > Fe(1)CoCeMgAlO, except for Fe(3)CoCeMgAlO that shows a higher than expected hydrogen consumption. Mainly at higher Fe loadings of the mixed oxides, some diffusion limitations appear as a consequence of the formation of iron aluminates [80,81], which act as barriers, inhibiting the H₂ access for the reduction in the samples. Notably, when the temperature range below 750 °C is considered, the experimental H₂ consumption reaches a maximum for the Fe(3)CoCeMgAlO sample among the Fe(x)CoCeMgAlO series and decreases following the order: Fe(3)CoCeMgAlO > Fe(9)CoCeMgAlO > Fe(6)CoCeMgAlO > Fe(1)CoCeMgAlO. This suggests a synergistic effect between the reducible cations corresponding to an optimum for the concentration of promoting iron cations, which provides the most benefits in terms of reducibility, with an expected great impact on the catalytic activity in methane complete oxidation. However, in the low-temperature range (<500 °C), with the increase in Fe content, the hydrogen consumption decreases in the mixed oxide series. Taking into consideration that the first reduced metallic particles favor the hydrogen spillover, which is involved in the oxide reduction at lower temperatures, this could be explained by the inability of metallic iron to facilitate fast hydrogen spillover [76].

2.2. Catalytic Properties

The evaluation of the catalytic properties of the transition metal-promoted mixed oxides was performed through combustion reactions in a temperature domain lower than the calcination temperature of the oxides, using methane as a model molecule. The light-off curves for the methane combustion over the M(3)CoCeMgAlO and Fe(x)CoCeMgAlO catalysts are depicted in Figure 8. All the tested catalysts showed complete selectivity towards CO₂, regardless of the nature of the transition metal cation or its loading. These results support their high potential for complete oxidation reactions of VOCs. The T₁₀, T_50, and T₉₀ temperatures, which correspond to the methane conversion of 10, 50 and 90%, respectively, and the intrinsic and specific activities (reaction rates per unit of surface area and per unit of catalyst mass, respectively) at 400 and 450 °C are included in

Table 7. Catalytic performances in the methane total oxidation for the transition metal-promoted CoCeMgAlO catalysts ^a.

Catalyst	T10 (°C)	T50 (°C)	T90 (°C)	Reaction Rate at 400 °C		Reaction Rate at 450 °C		Ea (kJ mol−1)
				Specific (107 mol g−1 s−1)	Intrinsic (109 mol m−2 s−1)	Specific (107 mol g−1 s−1)	Intrinsic (109 mol m−2 s−1)
CoCeMgAlO	438	528	619	0.83	0.90	3.72	4.02	84.6
Cu(3)CoCeMgAlO	426	514	620	1.19	2.97	3.66	9.14	91.8
Mn(3)CoCeMgAlO	433	521	631	0.94	1.32	3.90	5.46	118.3
Fe(3)CoCeMgAlO	391	489	595	2.46	3.92	5.90	9.40	54.9
Ni(3)CoCeMgAlO	431	507	613	1.09	1.62	3.39	5.02	93.8
Fe(1)CoCeMgAlO	405	495	603	2.15	2.87	5.11	6.82	74.7
Fe(6)CoCeMgAlO	437	523	634	0.94	1.22	3.76	4.86	103.3
Fe(9)CoCeMgAlO	429	521	607	0.88	1.21	3.42	4.73	106.7

^a Reaction conditions: 1 vol.% methane in air, GHSV of 16,000 h⁻¹, 1 cm³ of catalyst.

.

As indicated in Figure 8a, all the promoted CoCeMgAlO systems displayed methane total conversion, irrespective of the nature of the transition metal cation. The Fe-doped catalyst showed the highest activity, with the lowest temperature for complete combustion at 675 °C among the catalyst series. The catalytic activity for the promoted mixed oxide series with regard to T₁₀ follows the order: Fe(3)CoCeMgAlO > Cu(3)CoCeMgAlO > Ni(3)CoCeMgAlO > Mn(3)CoCeMgAlO > CoCeMgAlO, which corresponds with the order found from the H₂-TPR measurements in terms of the ease of reduction. Indeed, the addition of the transition metal M enhances the catalytic performance of the CoCeMgAlO mixed oxide system in the methane total oxidation, likely owing to a synergistic interaction between M, Co, and Ce species, with improvement in the redox properties of the system. This synergistic interaction consists of a process of electron transfer involving all the reducible cations, i.e., M, Co and Ce, present in the solid: Co(III) + Ce(III) → Co(II) + Ce(IV); Co(III) + M(n) → Co(II) + M(n + 1); M(n + 1) + Ce(III) → M(n) + Ce(IV). Notably, the synergy between the metal ions has a more important contribution in determining the catalytic performance of the M(3)CoCeMgAlO materials than their crystallinity or surface area [45]. However, no correlation can be found between the order of activity in terms of T₁₀ and the oxide reducibility in terms of H₂ consumption at low temperatures in the H₂-TPR experiments, pointing out that not all the reduced species under hydrogen are involved in catalysis as well. This is obviously due to the hydrogen spillover on the metallic Co and M (M = Ni, Cu, and Fe) particles, which favors the reduction under the H₂-TPR conditions.

In addition, the catalytic activity of the M-doped catalysts varies in terms of T₅₀, as follows: Fe(3)CoCeMgAlO > Ni(3)CoCeMgAlO > Cu(3)CoCeMgAlO > Mn(3)CoCeMgAlO > CoCeMgAlO. This order of activity does not entirely correspond with the order for the ease of reduction in the H₂-TPR measurements, as the activity of the Cu(3)CoCeMgAlO became lower. This reveals that the most reducible species during H₂-TPR measurements do not correspond to the most active species in the methane combustion conditions for this catalytic system. On the other hand, no correlation could be established between the H₂ consumption below 750 °C during the H₂-TPR experiments and the catalytic activity of the M-promoted CoCeMgAlO catalysts in terms of T₅₀.

The activity of the Fe(3)CoCeMgAlO catalyst, in terms of T₉₀, remains the highest, but that of Cu- and Mn-promoted systems becomes as low as or lower than that of the unpromoted CoCeMgAlO likely due to increased mass transfer limitations at high reaction temperatures.

The activity of the transition metal-promoted catalysts expressed by the intrinsic and specific reaction rates in the low-temperature range (at 400 and 450 °C), where the conversion rates remained low, showed dependence on the nature of the promoter and had higher values compared to the M-free CoCeMgAlO system, reaching a maximum for the Fe(3)CoCeMgAlO mixed oxide (Table 7). Taking this into consideration, the Fe content was varied into the Fe-promoted CoCeMgAlO system to study its effect on the catalytic performance in methane combustion. A complex influence of the iron content was noticed on the catalytic properties of the Fe(x)CoCeMgAlO mixed oxide series (Figure 8b). When comparing the T₅₀ values and both intrinsic and specific reaction rates, their activity followed the order: Fe(3)CoCeMgAlO > Fe(1)CoCeMgAlO > Fe(6)CoCeMgAlO > Fe(9)CoCeMgAlO > CoCeMgAlO. Thus, the catalytic activity of all Fe-promoted catalysts is higher than that of their unpromoted counterpart and achieves a maximum, which corresponds to the Fe(3)CoCeMgAlO system, when the Fe content varies from 1 to 9 at. %. These results point to a Co–Ce–Fe synergistic interaction.

The T₅₀ of the Fe(3)CoCeMgAlO showed a value of around 40 °C lower than that of the unpromoted CoCeMgAlO mixed oxide, outlining the increased catalytic activity of the promoted system and the successful approach of transition metal doping of the CoCeMgAlO to obtain more active catalysts in the methane total oxidation. Table S2 compares the catalytic performance of the Fe(3)CoCeMgAlO mixed oxide, the most active catalyst from this study, with other oxide-based catalysts found in the literature, specifically other Co-containing and ex-LDH systems, for the methane total oxidation. These data demonstrate that its catalytic activity, in terms of T₅₀, aligns closely with other findings reported in the literature.

In the Fe(x)CoCeMgAlO mixed oxide series, the intrinsic rates of methane conversion at 400 and 450 °C showed similar variations as the total H₂ consumption below 750 °C, which is the calcination temperature of the LDH precursors and the limit temperature for the combustion tests, when represented against the Fe content, with maxima for the Fe(3)CoCeMgAlO system (Figure 9). This dependence indicates again the superiority of Fe(3)CoCeMgAlO mixed oxide in the methane total oxidation and highlights the pivotal contribution of the redox active species in assessing the catalytic performance of the Fe(x)CoCeMgAlO mixed oxides. Clearly, a synergistic interplay among the Fe, Ce, and Co species can be pointed out, reaching an optimal state for a specific catalyst composition, exemplified by the Fe(3)CoCeMgAlO sample.

Interestingly, the changes in the Ce/Co surface atomic ratio and the intrinsic activities mostly respect the same course when increasing the Fe content (Figure 10a), with a maximum for Fe(3)CoCeMgAlO catalyst. This points to the surface Ce species, which play an important role in determining the catalytic activity of the mixed oxides. Moreover, a good linear relation was observed between the intrinsic methane conversion rate over the Fe-promoted mixed oxides at 400 °C and the Ce⁴⁺/Ce surface atomic ratio (Figure 10b), with the highest reaction rate for the greatest Ce⁴⁺/Ce ratio in the Fe(3)CoCeMgAlO sample, showing the importance of Ce species distribution on the catalyst surface. Specifically, the surface Ce⁴⁺ species seem to play an essential role in the Fe–Co–Ce synergistic interaction.

At the same time, the specific reaction rates expressed per unit mass of total transition metal (mol (g_Co+M)⁻¹ s⁻¹), cobalt, and doping cations, in relation to the total hydrogen consumption below 500 °C (Figure S14), present a similar dependence with that found for the specific reaction rate expressed per unit mass of catalyst, with Fe(3) sample showing the highest activity. This indicates that the active components are mainly the phases containing Co species, e.g., Co₃O₄, together with the Fe species, which increased the redox capacities of the mixed oxides, resulting in higher performances for the total oxidation of methane.

As diffusion influences can arise at high reaction temperatures, only conversion data obtained at low temperatures have been used to calculate the apparent activation energies (E_a) across the transition metal-promoted mixed oxide samples. These values were derived from the slope of the linear dependence of lnr_i on 10³/T which were depicted in Figure S15 and were included in Table 7. Interestingly, the activation energy in the M(3)CoCeMgAlO series increased with the addition of the promoting transition metal compared to the unpromoted counterpart, except for the case of iron doping, which decreased considerably the activation energy, as evidenced by the highest activity of the Fe(3)CoCeMgAlO mixed oxide. These shifts in the activation energies suggest changes in the characteristics of the catalytic sites needed for the combustion reaction, emphasizing a Co–Ce–Fe synergistic interaction for the Fe-promoted system. The higher activity of the systems promoted with M = Ni, Cu, and Mn compared to their unpromoted counterpart is most likely attributed to a higher density of the active sites in the former. In the Fe(x)CoCeMgAlO series, the activation energy decreases, reaching a minimum for Fe(3)CoCeMgAlO, and then, it increases with increasing the Fe content. This evidences changes in the characteristics of the catalytic active sites, implying the formation of less active forms at higher Fe contents. Only at lower Fe contents, i.e., x = 1 and 3, the activation energy was improved, showing lower values compared to the Fe-free CoCeMgAlO system, proving the promotion effect of the iron cations. At higher Fe contents, i.e., x = 6 and 9, the activation energy was greater, but the density of the active sites was obviously larger to explain their better activity compared to the unpromoted catalyst. Remarkably, Fe(1) and Fe(3)CoCeMgAlO systems exhibit activation energy values (55–75 kJ mol⁻¹), which are comparable to other results found for catalysts with similar composition employed in methane combustion [24,44,45] and quite close or even lower than that of a bulk Co₃O₄ catalyst (78 kJ mol⁻¹) [82]. Taking into account these results, the Co₃O₄ spinel oxide can be considered the main active phase in the Fe(x)CoCeMgAlO catalyst. Indeed, the Fe(x)CoCeMgAlO catalysts represent intricate mixed oxide systems comprising iron-doped Co₃O₄ particles intertwined with dispersed CeO₂ particles across a periclase-like matrix. However, in accordance with the observed decrease in the surface Co/Fe atomic ratio (Table 5), at high Fe contents, Fe₂O₃ and different ferrite phases, i.e., CoFe₂O₄, MgFe₂O₄, may be formed, also acting as surface active phases. This is supported by the activation energies of the methane reaction in the 84–125 kJ mol⁻¹ range measured in the case of the latter [83,84], comparable to those observed for Fe(6)- and Fe(9)-CoCeMgAlO.

Generally, the methane catalytic oxidation reaction on the active phase of Co₃O₄ spinel oxide mainly occurs according to the Mars–van Krevelen mechanism for the tested temperature range above 450 °C, as illustrated in Scheme 1 [85]. The catalytic reaction involves the cleavage of the C–H bond of methane on the transition metal cations—oxide anions pairs and the use of lattice oxygen and interfacial oxygen in the dehydroxylation and decarboxylation of the catalyst surface with the formation of H₂O and CO₂. The lattice oxygen ions involved in the methane oxidation lead to the reduction of the catalyst surface and the formation of oxygen vacancies, which are refilled by oxygen ion migration through the lattice and reoxidation of the catalyst with gaseous oxygen at other sites than the methane activation site. At the same time, the suprafacial Langmuir–Hinshelwood mechanism can operate in parallel, being prevalent at temperatures lower than 450 °C [85,86].

Furthermore, the impact of gas hourly space velocity (GHSV) on the catalytic activity was examined for the Fe(3)CoCeMgAlO catalyst with the highest activity in the series. The increase in GHSV at a constant concentration of methane in the feed gas of 1 vol.% produced shifts toward higher temperatures in the methane combustion curves (Figure 11). The effect is more accentuated at higher GHSV, as the T₅₀ value increased from 489 to 505 and 530 °C, measured at GHSV of 16,000, 20,000, and 30,000 h⁻¹, respectively.

Typically, LDH-derived catalysts are known for good stability in the methane total oxidation reactions [24,39,40]. Thus, the Fe(3)CoCeMgAlO mixed oxide, with the highest catalytic activity from both series, underwent methane combustion tests for a period of approximately 55 h at 600 °C to study its stability. No significant changes in methane conversion during the combustion tests were observed, accounting for its good stability after the long-term tests, as illustrated in Figure 12, for the specified reaction conditions and the studied time interval. Mg(Al)O periclase-like support present in the complex oxide composition offers more thermal stability in high-temperature combustion conditions.

The impact of water vapors on the catalytic performance of the Fe(3)CoCeMgAlO mixed oxide was assessed by combustion tests at 600 °C in cycles of dry/humid conditions. A peristaltic pump was used to add to the dry mixture a flow of 0.14 mL min⁻¹ of deionized liquid water, corresponding to a water vapor content of around 40 vol. %. During the first run in humid conditions, a significant decrease in methane conversion was observed (Figure 13). These results can be attributed to the presence of the water vapor coupled with an increase in the GHSV and the subsequent decrease in the methane concentration in the reaction mixture. Further, the catalytic activity had partially recovered with an increase in methane conversion when the mixed oxide was exposed to a run under dry conditions for the methane combustion reaction. It is noticed that the water vapors have a detrimental impact on the performance of the Fe(3)CoCeMgAlO sample, which appears to suffer irreversible effects. Nevertheless, the Fe(3)CoCeMgAlO catalyst showed a relatively stable methane conversion after multiple runs of combustion tests in dry and humid conditions. Notably, the promotion of the CoCeMgAlO system with iron species has enhanced its catalytic performance in humid conditions by increasing the methane conversion by ca. 10%, compared to the unpromoted CoCeMgAlO [41].

Several analyses by XRD, XPS, and nitrogen adsorption–desorption were performed on the Fe(3)CoCeMgAlO catalyst after the multiple runs of methane combustion tests in dry/humid conditions to investigate the effect of the water vapors on the catalyst sample, in order to explain its activity loss in methane total oxidation. The XRD patterns of the used and fresh Fe(3)CoCeMgAlO catalyst samples were compared in Figure S16, which showed the presence of strong diffraction lines specific for the periclase, ceria, and cobalt spinel phases in both samples, even in the profile of the tested sample, suggesting its stability from crystallographic point of view during the test in humid conditions. However, all the phases have their crystallite sizes modified as a result to the dry/humid tests, as follows: the crystallite size of Co₃O₄ phase increased from 15.9 to 18.3 nm, evidencing an observable sintering effect on the respective phase with impact over the recovery of the catalytic activity; also, the periclase and ceria phases showed increased crystallite sizes, from 8.9 to 10 nm, and from 8.6 to 9.8 nm, respectively. Also, the XPS measurements indicated a decrease in the Ce⁴⁺/Ce (from 0.88 to 0.86) and Fe³⁺/Fe²⁺ (from 0.81 to 0.75) surface atomic ratios, which are redox species in the higher oxidation state that impact the catalytic activity. While the fresh and used samples present similar profiles of the nitrogen adsorption–desorption isotherms (Figure S17a), the latter suffers a small decrease in surface area from 62.8 to 58.8 m² g⁻¹ with its pore volume kept the same (Table 1). At the same time, significant changes in the pore width profile were observed, as the distribution in size is much broader for the used sample with a maximum at 23.5 nm, showing an increase in pore size compared to the fresh sample with a narrower distribution and a maximum at 17.6 nm (Figure S17b). The decrease in the catalytic activity caused by water vapors in the reaction mixture can be attributed to these structural and textural changes.

3. Materials and Methods

3.1. Catalysts Preparation

All chemicals used in the catalysts preparation, such as Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, Ce(NO₃)₃·6H₂O, Co(NO₃)₂·6H₂O, Cu(NO₃)₂·3H₂O, Fe(NO₃)₃·9H₂O, Mn(NO₃)₂·4H₂O, Ni(NO₃)₂·6H₂O, NaOH, were purchased from Sigma-Aldrich (St. Louis, MO, USA). The purity of the chemicals was of analytical grade, and they were used directly without any further purification.

Two series of multicationic M-CoCeMgAl-LDH precursors were prepared by coprecipitation under ambient atmosphere, followed by their controlled calcination into the corresponding mixed oxides. The first series of mixed oxide samples considers the use of different transition metal ions as promoters and consists of four M(x)CoCeMgAlO mixed oxides with the metal M content set to x = 3 at. % (M = Cu, Fe, Mn, Ni). The second series includes the variation of Fe contents and comprises four Fe(x)CoCeMgAlO mixed oxides containing x = 1, 3, 6, and 9 at. % with respect to cations. In both series, Mg/Al atomic ratio was fixed to 3, and Co and Ce contents were kept constant at 40 and 10 at. %, respectively.

Thus, a solution (400 mL) containing adequate amounts of metal nitrates (Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, Ce(NO₃)₃·6H₂O, Co(NO₃)₂·6H₂O and the doping metal nitrate) and a 2 M NaOH solution were simultaneously added dropwise at room temperature into a beaker containing 200 mL deionized water with controlled rate to maintain the solution pH close to 10. The metal nitrates used as a source for the promoting metal ions were, as follows: Cu(NO₃)₂·3H₂O, Fe(NO₃)₃·9H₂O, Mn(NO₃)₂·4H₂O and Ni(NO₃)₂·6H₂O. After precipitation, the obtained slurry was aged at 80 °C overnight under continuous stirring. Then, the mixture was cooled down at room temperature in order to separate the precipitate through centrifugation. After separation, it was thoroughly washed with deionized water and finally dried overnight at 80 °C to afford the LDH precursors. The M-CoCeMgAlO mixed oxide catalysts, with different doping metal ions and contents, were obtained by calcination of the LDH precursors in air in a muffle furnace at 750 °C for 8 h, with a heating rate of 2 °C min⁻¹. The unpromoted CoCeMgAlO mixed oxide sample was obtained following the same protocol above.

3.2. Catalysts Characterization

Powder X-ray diffraction (XRD) patterns of both LDH precursors and mixed oxides were acquired with a 9 kW Rigaku SmartLab diffraction system (Tokyo, Japan) with a rotating anode equipped with a Cu-K_α1 radiation source with the wavelength, λ = 1.5406 Å, operated at 40 kV and 75 mA. They were recorded over the 5–70° 2θ angular range at a scanning rate of 3° min⁻¹ and with a 0.01° step width. Crystalline phases were identified using standard ICDD (International Centre for Diffraction Data) powder diffraction patterns. The mean crystallite size, D, of the oxide materials, was calculated by using the Scherrer Equation (1):

$$D={\displaystyle \frac{K\lambda }{\beta cos\theta }}$$

(1)

where K is the shape factor, taken as 0.93, λ is the Cu K_α1 X-ray wavelength, θ is the Bragg diffraction angle, and β is the full width at half maximum (FWHM) of the diffraction line, in radians. The interplanar distance for the LDH phase was calculated using the Bragg’s law: $n\lambda =2d·\mathrm{s}\mathrm{i}\mathrm{n}\theta$, where n is the diffraction order, λ is the X-ray wavelength, d is the interplanar distance, and θ is the Bragg diffraction angle.

Scanning electron microscopy (SEM), together with energy dispersive X-ray analysis (EDX), were used to examine the morphology and chemical composition of the mixed oxides. Elemental mappings were carried out to evidence the uniform distribution of the different elements within the mixed oxide. SEM/EDX analysis was performed using a field emission scanning electron microscope (FE-SEM, MERLIN^® VP Compact, Zeiss, Oberkochen, Germany) equipped with an EDX detector (XFlash 6/30, Bruker, Berlin, Germany). The samples were mounted on heavy metal-free Al-SEM-carrier (PLANO, Wetzlar, Germany) with adhesive conductive carbon tape (Spectro Tabs, TED PELLA INC, Redding, USA) and coated with carbon (5.0 nm thickness) under vacuum (CCU 010 HV-Coating Unit, Safematic GmbH, Zizers, Switzerland). Representative areas of the samples were analyzed and mapped for elemental distribution based on the EDX-spectra data by QUANTAX ESPRIT Microanalysis software (version 2.0). SEM images were taken from the selected regions (conditions, including applied detector, magnification, accelerating voltage, and working distance are shown on the micrographs). For each sample, three different areas were analyzed, whereas the reported composition was the average of these measurements.

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was employed to determine the elemental composition of the mixed oxide samples using a Varian 715-ES ICP emission spectrometer (Agilent, Santa Clara, CA, USA). Approximately 6 mg of a sample was mixed with 8 mL of aqua regia. The digestion was carried out in a microwave-assisted sample preparation system, “Multiwave PRO”, from Anton Paar (Graz, Austria) at 200 °C and 50 bar pressure. The digested solution was made up to 100 mL and measured using ICP-OES. The measurement data was evaluated using the Varian 715-ES software “ICP Expert II”, version 2.0.5.283.

X-ray photoelectron spectroscopy (XPS) was used to identify the chemical state of the elements on the catalyst surface. A SPECS spectrometer equipped with a PHOIBOS 150 analyzer (SPECS, Berlin, Germany) using a monochromatic Al Kα radiation source (1486.7 eV) was used for the M-free CoCeMgAlO and Fe(x)CoCeMgAlO samples. The acquisition was operated at a pass energy of 20 eV for the high-resolution spectra and 50 eV for the extended spectra. The analysis of the spectra has been performed with the Spectral Data Processor v2.3 software using Voigt functions and usual sensitivity factors. An ESCALAB 220iXL (Thermo Fisher Scientific, Waltham, MA, USA) with monochromatic Al Kα radiation (E = 1486.7 eV) was used for the M(3)CoCeMgAlO samples with M = Mn, Ni and Cu. Samples were prepared on a stainless-steel holder with conductive double-sided adhesive carbon tape. The measurements were performed with charge compensation using a flood electron system combining low energy electrons and Ar+ ions (Ar pressure of 1 × 10⁻⁷ mbar). For quantitative analysis, the peaks were deconvoluted with Gaussian–Lorentzian curves using the software Unifit 2023. The peak areas were normalized by the transmission function of the spectrometer and the element-specific sensitivity factor of Scofield. The electron binding energies are referenced to the C 1s core level of carbon at 284.8 eV (C–C and C–H bonds) for all samples.

The textural characterization was performed using the conventional low-temperature (−196 °C) nitrogen adsorption–desorption method with a Micromeritics ASAP 2020 apparatus (Norcross, GA, USA). Prior to nitrogen adsorption, the oxide samples were degassed under vacuum at 300 °C for 16 h. The surface areas were calculated using the BET method in the relative pressure range, P/P₀ = 0.065–0.2, while the desorption branch of the isotherms was used to determine the average pore sizes with the Barrett–Joyner–Halenda (BJH) method. The total pore volume was calculated from the amount adsorbed at the relative pressure of 0.99.

The redox properties of the mixed oxides were evaluated by temperature-programmed reduction under hydrogen (H₂-TPR). The measurements were performed using a 3-Flex instrument (Micromeritics, Norcross, GA, USA) equipped with a quartz tube reactor and a thermal conductivity detector (TCD). The TCD was used for the quantification of the amount of H₂ consumed in µmol/g after its calibration. Around 70–100 mg of calcined mixed oxide was loaded in the reactor and pretreated for 30 min at 400 °C (20 °C min⁻¹) under a flow of Ar to remove any adsorbed species on the surface of the sample. Then, the sample was cooled down to room temperature under the Ar stream, and an H₂/Ar flow (50 mL min⁻¹) was passed over the sample, which was heated from room temperature to 850 °C with a heating rate of 10 °C·min⁻¹. The temperature was held at 850 °C for 60 min. The hydrogen consumption peaks were recorded with temperature, and quantitative analysis of the TPR data, based on the peak areas, was done after calibration.

3.3. Catalytic Test

The catalytic tests for the methane combustion over the promoted M-CoCeMgAlO mixed oxide catalysts were performed in a fixed bed quartz tube down-flow reactor at atmospheric pressure. If not otherwise specified, a mixture of CH₄ and air containing 1 vol. % methane was passed through 1 cm³ (ca. 0.8 g) catalyst bed with a total flow rate of 267 mL min⁻¹ corresponding to a gas hourly space velocity (GHSV) of 16,000 h⁻¹. Before the combustion tests, the catalyst was pre-treated for 30 min in a stream of air at 600 °C to clean its surface. Activity measurements were performed by increasing the reaction temperature from 400 to 730 °C at regular intervals. Combustion tests in humid conditions were performed by introducing, with a peristaltic pump, deionized water (0.14 mL min⁻¹) in the reaction mixture containing 1 vol. % methane in air fed into the reactor with a flow rate of 267 mL min⁻¹, corresponding to a water vapor content of ca. 40 vol. %. The reactants and product gases were analyzed online by a Clarus 500 Gas-Chromatograph equipped with a thermal conductivity detector, using two packed columns in series (6 ft Hayesep and 10 ft molecular sieve 5 Å). The activity of the catalysts was evaluated by comparison of T₁₀, T₅₀, and T₉₀ values, which represent the temperatures of methane conversions of 10, 50, and 90%, respectively. The conversion was calculated as the amount of methane transformed in the reaction divided by the amount that was fed to the reactor by using the following Formula (2):

$$Conversion \left(\%\right)={\displaystyle \frac{C_{C{H}_4,in}-C_{C{H}_4,out}}{C_{C{H}_4,in}}}\times 100$$

(2)

where C_CH4,in and C_CH4,out represent the methane concentration (v/v) in the feed and effluent gases, respectively.

Complete selectivity to CO₂ and H₂O was always observed. The carbon balance was calculated based on the following Equation (3):

$$C_{C{H}_4,in}=C_{C{H}_4,out}+C_{C{O}_2,out}$$

(3)

where C_CO2,out is the concentration of carbon dioxide (v/v) in the effluent gas. It was satisfactory in all runs to within ±2%.

4. Conclusions

Two series of M(3)CoCeMgAlO and Fe(x)CoCeMgAlO mixed oxides with fixed Ce and Co content of 10 at. % and 40 at. %, with respect to cations, respectively, and a Mg/Al mol ratio of 3, but with different transition metals M = Cu, Mn, Fe, Ni, and iron loadings x in the 1–9 at. % range were prepared by controlled calcination in air at 750 °C of their corresponding LDH precursors. The as-prepared mixed oxides contained periclase-like Mg(Al)O, CeO₂ fluorite, and Co₃O₄ phases in all the cases. The most important phase among them is represented by the cobalt oxide, which interacts with ceria, with both phases being doped with low concentrations of promoting transition metal. The type and content of the doping transition metal significantly impact the morpho-structural and chemical features of the mixed oxides, as well as their redox properties, thus controlling their catalytic activity. As a result, all the promoted mixed oxides presented higher catalytic activities than the M-free CoCeMgAlO system. The catalytic activity was found to be influenced by both the redox properties and the different surface atomic ratios. Having the highest reducibility properties and the lowest activation energy, the Fe-promoted CoCeMgAlO mixed oxide with 3 at. % Fe displayed the highest performance for methane combustion among the promoted catalysts, with a T₅₀ value of 489 °C, owing to the synergistic interaction between Co, Cu, and Fe species. The Fe(3)CoCeMgAlO catalyst demonstrated good stability while operating at high temperatures for a prolonged time on stream due to the presence of the Mg(Al)O periclase matrix. Although the water vapors in the feed have caused a detrimental effect on the catalytic performance of the promoted mixed oxide, its catalytic activity was partially recovered during the run in dry conditions. Changes in the textural, structural, and redox properties of the iron-doped oxide-based catalyst took place in humid conditions.