Modern Trends in Design of Catalysts for Transformation of Biofuels into Syngas and Hydrogen: From Fundamental Bases to Performance in Real Feeds

: This review considers problems related to design of efﬁcient structured catalysts for natural gas and biofuels transformation into syngas. Their active components are comprised of ﬂuorite, perovskite and spinel oxides or their nanocomposites (both bulk and supported on high surface area Mg-doped alumina or MgAl 2 O 4 ) promoted by platinum group metals, nickel and their alloys. A complex of modern structural, spectroscopic and kinetic methods was applied to elucidate atomic-scale factors controlling their performance and stability to coking, such as dispersion of metals/alloys, strong metal-support interaction and oxygen mobility/reactivity as dependent upon their composition and synthesis procedures. Monolithic catalysts comprised of optimized active components loaded on structured substrates with a high thermal conductivity demonstrated high activity and stability to coking in processes of natural gas and biofuels reforming into syngas. A pilot-scale axial reactor equipped with the internal heat exchanger and such catalysts allowed to efﬁciently convert into syngas the mixture of natural gas, air and liquid biofuels in the autothermal reforming mode at low (~50–100 ◦ C) inlet temperatures and GHSV up to 40,000 h − 1 .


Introduction
Production of syngas by methods alternative to steam reforming of methane now attracts a lot of attention due to both environmental and commercial reasons [1,2]. Conversion of oxygenates obtained from biomass and dry reforming of natural gas appear to be most promising [1][2][3]. In dry reforming of biogas or natural gas containing CH 4 + CO 2 these greenhouse gases are transformed into syngas with H 2 /CO ratio~1, which is a suitable feed for Fischer-Tropsch and oxygenates synthesis. Oxygenates obtained from biomass and glycerol-byproduct of biodiesel production are considered as attractive alternatives to fossil fuels for syngas production [1,4,5].
Efficient catalysts of such processes are usually based upon supported noble (Pt, Rh, Ru) or transition (mainly Ni) metals [1][2][3][4][5][6][7][8][9][10][11][12]. The main problem of these processes is coking of catalysts leading to their deactivation. Even though noble metals are much more stable to coking, their high price makes their broad-scale application impossible. Hence, great efforts were devoted to design of Ni-based catalysts stable to coking. Next approaches were found to be successful.
This review is devoted to analysis of these trends in design of catalysts for transformation of biofuels into syngas based on results of our research in frames of international collaboration in last 10 years compared with those published in literature. The most important new aspect of this review is detailed description of oxygen mobility in catalysts of fuels transformation into syngas comprised of mixed oxides with fluorite, perovskite and spinel structures promoted with Ni and Pt-group metals. It is based on application of unique techniques of oxygen isotope heteroexchange of these catalysts with 18 O 2 or C 18 O 2 in the gas phase in flow installations including experiments in the temperature-programmed mode as well as in the steady-state of catalytic reactions (Steady-State Isotope Transients Kinetic Analysis, SSITKA) [18,35,68,69,71,72,87]. Even though it is well known that a high oxygen mobility and reactivity in these catalysts allows to prevent coking in the reactions of biofuels transformation into syngas by a fast transfer of oxygen species to the metal-support interface, where they interact with activated fuels fragments transforming them into syngas, only in our works such strict characteristics of oxygen mobility as oxygen self-diffusion coefficients were systematically estimated using sophisticated software for isotope exchange kinetics data analysis. Atomic-scale features controlling oxygen mobility in these catalysts were elucidated using modern structural and spectroscopic methods, while their surface oxygen bonding strength was estimated by pulse microcalorimetry, which provided foundations for optimization of their compositions and synthesis procedures [78,82,87].

Synthesis of Active Components
The method of synthesis should provide a high dispersion of complex oxides along with spatial uniformity of elements distribution in their particles. A lot of methods including co-precipitation, solvothermal method, sol-gel method, Pechini polymeric precursor method, microemulsions, sonochemical method, microwave-assisted self-combustion and For doped ceria and Ce-Zr-O oxides, a complex of modern techniques was applied for studies of their structure. This includes high resolution transmission electron microscopy with elemental mapping, diffraction studies using X-ray synchrotron radiation and neutron diffraction, wide-angle X-ray scattering (WAXS), infrared and Raman spectroscopies. This allowed to elucidate effects of samples chemical composition and preparation procedures on their phase homogeneity, spatial cations distribution in particles, types and concentrations of defects and features of local coordination environment of Ce and Zr cations [73,77,[89][90][91][92][93][94][95][96][97][98]. For Ce 0.5 Zr 0.5 O 2−y composition having the highest oxygen mobility, doping with La, Gd, Pr, Sm cations (thus producing Ln x (Ce 0.5 Zr 0.5 ) 1−x O 2−y oxides with x = 0.1 ÷ 0.3) stabilizes the pseudo-cubic structure in humid environment and reduces domain sizes. For these samples the effect of domain boundaries on the oxygen mobility is significant [18,94,95,98].
Prepared in optimized (with addition of AA complexing agent) supercritical conditions Ce 0.5 Zr 0.5 O 2−y samples have a cubic structure with the crystallite size of~5.5 nm. Doping by Ti and Nb cations increases oxygen deficiency due to generation of Ce 3+ cations [21,22]. According to TEM data, nickel oxide particles supported by impregnation (5 wt.%) have sizes from 20 to 40 nm, while for one-pot route they are smaller (~10 nm).
Freshly prepared Ru/(La 0.8 Pr 0.2 Mn 0.2 Cr 0.8 O 3 + 10 wt.% NiO + 10 wt.% YSZ) nanocomposite mainly consists of the perovskite phase, with Ni and Ru cations being mainly dissolved in its surface layers. YSZ disorders perovskite structure and hampers sintering due to interfaces between its nanoparticles and perovskite domains [32].
Perovskite + fluorite (P+F) nanocomposites prepared by optimized procedures [31,35] are characterized by the developed interphase, a higher specific area as compared to the mechanical mixture of P+F phases. Cations redistribution between perovskite and fluorites nanodomains helps to improve oxygen mobility.
Oxides with a spinel structure based on MnCr 2 O 4 prepared by Pechini method [69,90] have 2-40% of admixture phase with corundum structure due to segregation of (Mn,Cr) 2 O 3 oxide during annealing in air. Doping with Fe and Zn cations as well as supporting Ru + Ni decrease the content of this admixture due to spinel structure stabilization.
After supporting up to 10 wt.% of spinel, fluorite or perovskite oxides on MgAl 2 O 4 or 10 wt.% Mg-doped γ-Al 2 O 3 followed by supporting Ru + Ni by impregnation when required, epitaxial layers of these oxides are formed along with incorporation of rare-earth and transition metal cations into the surface layers of these supports [78,90]. In reducing conditions Ni-Ru alloy nanoparticles are formed strongly interacting with layers of rareearth or transition metal oxides on the surface of these high surface area supports [78,90].

Surface Properties
The most detailed characteristics of the surface of catalysts were obtained with the help of X-ray Photoelectron Spectroscopy (XPS) and Fourier-transformed Infra-red Spectroscopy of adsorbed CO (FTIRS of adsorbed CO). While the first method gives information about the charge state of ions on the surface (as judged by their binding energies (BE) in XPS spectra) as well as their surface concentrations, the second one allows to estimate the number of coordinatively unsaturated sites (metal atoms, cations) as well as their charges reflected in intensities as well as frequencies of carbonyl absorption bands in FTIRS spectra [13,[94][95][96][97]. Secondary Ions Mass Spectrometry (SIMS) allows to estimate variation of the content of cations along the depth of the surface layer sputtered by the beam of argon ions [13].
For doped Ce-Zr oxides the surface was found to be enriched by large Pr, Ce and La cations as revealed by XPS and SIMS [94,95]. This implies domain boundaries enrichment by the same cations, which could affect their transport properties.
The surface layer of MnCr 2 O 4 spinel is enriched by Mn as judged by XPS data, which is explained by segregation of Mn 2+ cations on the surface of spinel obtained by Pechini method, where decomposition of polymeric precursor under contact with air initially occurs in rather reducing conditions [69]. Apparently even after complete oxidation of all organic residues and transformation of charge state of Mn surface cations mainly to 3+ state with an admixture of 4+ state, they remain on the surface as revealed by its enrichment by Mn. This helps to provide a high mobility and reactivity of the surface oxygen having a lower bonding strength with Mn cations than with Cr cations, thus ensuring a high coking resistance of these catalysts in fuels reforming [68,69,71].
For Pt/Ln-Ce-Zr-O catalysts (Ln = La, Pr, Gd) pretreated in O 2 platinum was found to be present in three oxidation states: 0, 2+ and 4+ (XPS binding energies BE equal to  [13,95,97]. FTIRS of adsorbed CO also revealed several states of Pt on the surface reflected in bands of linear carbonyls Pt 0 -CO (νCO 2046-2084 cm -1 ), Pt + -CO (νCO 2125-2140 cm -1 ) and Pt 2+ -CO (νCO 2170-2180 cm -1 ) [13,[94][95][96][97][98]. FTIRS spectra did not contain bands which could be assigned to Pt 4+ -CO carbonyls, since these cations are able to oxidize CO even at liquid nitrogen temperature, being reduced to 2+ and 1+ states. For Pt/La-Ce-Zr-O sample the highest concentration of coordinatively unsaturated Pt 2+ cations was revealed by these methods, which can be explained by their stabilization with strongly basic La cations. Hence, strong metal-support interaction for these catalysts results in stabilization of Pt n+ cations on the surface.
For Ni/Ce-Zr-O samples, mainly FTIRS bands of terminal carbonyls Ni-CO at νCO~2105 cm -1 are observed [89]. This is explained by decoration of Ni nanoparticles surface by Ce-Zr-O fragments due to strong metal-support interaction, thus hampering appearance of neighboring Ni atoms able to stabilize bridging carbonyls. In a similar way, FTIRS bands corresponding to terminal Ni/Ru carbonyls were mainly observed for Ni + Ru loaded oxides, where alloys are formed after reduction [78,90,91]. Hence, these effects of dilution and decoration are vital to prevent coking, since carbon nucleation on Ni particles requires ensembles of the same surface atoms >6 or stepped faces having coordinatively unsaturated atoms [8].
For catalysts based on fluorite, perovskite and spinel oxides containing transition and rare-earth cations with variable charges, the bonding strength of surface oxygen species depends on their stoichiometry. In the initial oxidized state, it is~150-200 kJ/mol O 2 , corresponding to M-O forms of oxygen [35,99,100]. After removing more than one monolayer of oxygen its binding strength increases to~400 kJ/mol for spinels (MnCrO x ),~500 kJ/mol for perovskites (PrFeOx, LaPrMnCrOx) and~650 kJ/mol for LnCeZrO fluorites, respectively, corresponding to M 2 O bridging forms of the surface oxygen [99][100][101][102][103]. In the stationary state of these catalysts in the reactions of fuels reforming, only bridging forms of oxygen are present on the surface being regenerated by CO 2 or H 2 O pulses forming also CO and H 2 as products [101][102][103]. For PrCeZrO fluorite and MnCrO x spinel layers supported on mesoporous MgAl 2 O 4 the coverage by reactive oxygen species decreases while their bonding strength increases due to interaction with support [104].
Diffusion coefficients of oxygen for these catalysts are high enough (up to 10 -12 cm 2 /s at 700 • C, Table 1, Figure 1) to provide fast oxygen migration to the metal-support interface, which is required to transform activated fuel fragments into syngas.
Even though for stoichiometric spinel MnCr 2 O 4 the oxygen diffusion in the bulk is not too fast, it is at least close to that of La-doped ceria-zirconia, which allows to suggest usage of much less expensive spinel for the catalysts design. Oxygen diffusion coefficients along domain boundaries in Pt-supported fluorites exceed by 1-3 order of magnitude those for the bulk diffusion ( Table 1). The amount of oxygen involved into this fast diffusion channel decreases when using oxygen exchange with C 18 O 2 or SSITKA instead of heteroexchange with 18 O 2 , apparently reflecting also its lower amount (oxygen storage capacity) in real reaction conditions of biofuels reforming. spinel layers supported on mesoporous MgAl2O4 the coverage by reactive oxygen species decreases while their bonding strength increases due to interaction with support [104].
Diffusion coefficients of oxygen for these catalysts are high enough (up to 10 -12 cm 2 /s at 700 °C, Table 1, Figure 1) to provide fast oxygen migration to the metal-support interface, which is required to transform activated fuel fragments into syngas.

Catalysts Based on Cerium-Zirconium Mixed Oxides
In methane dry reforming (MDR) Ni-supported biphasic ceria-zirconia sample prepared in supercritical ethanol without adding AA complexion was very fast completely deactivated in MDR due to coking [20]. In contrary, a high and stable performance of catalysts with Ni supported by impregnation on single-phase Ce 0.5 Zr 0.5 O 2 oxide prepared either by modified Pechini route or in supercritical alcohols with addition of AA (vide supra) Energies 2021, 14, 6334 7 of 25 was maintained even at 600 • C [20][21][22][23][24]89]. At 700 • C, the effective first-order rate constants for these catalysts were in the range of 1-4 s -1 cm -2 , being close to values for Pt or Ru-supported Pr (Pr+Sm)-doped ceria-zirconia ( Figure 2) and exceeding by an order of magnitude k values for Ni supported on these fluorites (~0.2 s -1 cm -2 at 700 • C) [87]. This stresses importance of the spatial homogeneity of ceria-zirconia mixed oxide for ensuring a high oxygen mobility and, hence, stability to coking, which is usually not taken into account.  Oxygen diffusion coefficients (Do) estimated by the isotope exchange method, of the order of 10 −15 -10 -14 cm 2 /s ( Table 2), are quite close to those for Ni/ Ce0.5Zr0.5O2 with single-phase oxide support prepared via modified Pechini route ( Figure 1). They are high enough for efficient oxygen transport to the metal-support interface and contribute to a high value of catalytic activity and stability against catalyst coking.
For catalysts based on SmPrCeZrO2 oxide support, in MDR the rate constant was higher for supported Ru (~7 s -1 m -2 at 850 °C) than for Pt ( Figure 2) [87]. Ni-supported catalyst is much less active (k ~ 0.2 s -1 cm -2 at 700 °C), which is explained by coking of the catalyst. For supported LaNiO3 activity is higher by an order of magnitude (k ~ 1.8 s -1 cm -2 at 700 °C). This is explained by decoration of Ni nanoparticles by LaOxCO3 species preventing coking. Ru+Ni-supported catalyst demonstrates the highest activity ( Figure 2) since NiRu clusters are not coked [87]. For Ru co-supported with LaNiO3, specific activity is significantly lower being identical with that for Ru alone supported on this fluorite. According to CO chemisorption data the metal surface area for this catalyst (~0.05 m 2 /g) is much lower than that for Ru + Ni-loaded sample (0.5 m 2 /g). Hence, the rate constant related to the metal surface is even higher for Ru+LaNiO3-loaded catalyst due to stronger metal-support interaction/ interface.
For Pt/Lnx(Ce0.5Zr0.5)1-xO2-y catalysts specific rate of hydrogen production in methane partial oxidation into syngas (POM) in diluted feed ( Figure 3) is the highest for La-doped catalysts due to a higher content of Pt n+ cations transformed into clustered metal species in reaction media [13,18,72,94,95].  (Table 2) were prepared in supercritical isopropanol with addition of AA. 5 wt.% Ni were supported either by impregnation (I) or via one-pot synthesis from mixed solutions in supercritical conditions (O) [24]. For samples with Ni added in one-pot route its surface content estimated as Ni/Ce+Zr atomic ratio from X-ray photoelectron spectroscopy data was twice as low in comparison with impregnated samples. This indicates Ni incorporation into the bulk of fluorite particles for one-pot samples reflected in the increase of oxygen diffusion coefficients ( Table 2) due to generation of additional oxygen vacancies. Such disordering also leads to stronger sintering of samples during calcinations step, so specific surface area for one-pot samples was twice as low [24]. However, both reagents' conversions and reaction rate related to the surface concentration of nickel atoms estimated by hydrogen chemisorption (TOF, turn-over frequency) of one-pot samples were quite close to those of impregnated samples of the same chemical composition (Table 2), which indicates on a higher dispersion of Ni on the surface of one-pot samples. TOF values strongly depend on the support composition, and the catalyst with titanium and niobium co-doped ceriazirconia support prepared by impregnation is three times more active than that with the unmodified ceria-zirconia support, apparently due to optimized interaction of Ni with more disordered doped ceria-zirconia. Table 2. Kinetic parameters of MDR and oxygen mobility for catalysts prepared in supercritical conditions [24]. Oxygen diffusion coefficients (D o ) estimated by the isotope exchange method, of the order of 10 −15 -10 -14 cm 2 /s ( Table 2), are quite close to those for Ni/Ce 0.5 Zr 0.5 O 2 with single-phase oxide support prepared via modified Pechini route ( Figure 1). They are high enough for efficient oxygen transport to the metal-support interface and contribute to a high value of catalytic activity and stability against catalyst coking.
For catalysts based on SmPrCeZrO 2 oxide support, in MDR the rate constant was higher for supported Ru (~7 s -1 m -2 at 850 • C) than for Pt ( Figure 2) [87]. Ni-supported catalyst is much less active (k~0.2 s -1 cm -2 at 700 • C), which is explained by coking of the catalyst. For supported LaNiO 3 activity is higher by an order of magnitude (k~1.8 s -1 cm -2 at 700 • C). This is explained by decoration of Ni nanoparticles by LaO x CO 3 species preventing coking. Ru+Ni-supported catalyst demonstrates the highest activity ( Figure 2) since NiRu clusters are not coked [87]. For Ru co-supported with LaNiO 3 , specific activity is significantly lower being identical with that for Ru alone supported on this fluorite. According to CO chemisorption data the metal surface area for this catalyst (~0.05 m 2 /g) is much lower than that for Ru + Ni-loaded sample (0.5 m 2 /g). Hence, the rate constant related to the metal surface is even higher for Ru+LaNiO 3 -loaded catalyst due to stronger metal-support interaction/interface.
For Pt/Ln x (Ce 0.5 Zr 0.5 ) 1−x O 2−y catalysts specific rate of hydrogen production in methane partial oxidation into syngas (POM) in diluted feed ( Figure 3) is the highest for La-doped catalysts due to a higher content of Pt n+ cations transformed into clustered metal species in reaction media [13,18,72,94,95].  In the autothermal reforming of acetone ( Figure 4) [95], specific catalytic activity of Pt-supported doped ceria-zirconia catalysts correlates with the oxygen mobility required to prevent coking, being the highest for Pt/Pr0.3Ce0.35Zr0.35O2 catalyst [18].
While testing catalysts in real concentrated feeds in POM, methane steam reforming and MDR, the temperature gradient along the length of reactor equipped with the catalyst fraction or granules caused by exothermicity or endothermicity of these reactions emerges, which complicates data analysis. To avoid this problem, catalytic layers were supported onto the inner walls of single channels cut from corundum honeycomb monoliths, which allowed to avoid any temperature gradients and estimate rate constants (Table 3) [109][110][111][112]. Among Pt/Lnx(Ce0.5Zr0.5)1-xO2-ycatalysts the highest activity was revealed for Pr-doped catalyst possessing the highest oxygen mobility, which stresses importance of this characteristic. In the autothermal reforming of acetone ( Figure 4) [95], specific catalytic activity of Pt-supported doped ceria-zirconia catalysts correlates with the oxygen mobility required to prevent coking, being the highest for Pt/Pr 0.3 Ce 0.35 Zr 0.35 O 2 catalyst [18].
While testing catalysts in real concentrated feeds in POM, methane steam reforming and MDR, the temperature gradient along the length of reactor equipped with the catalyst fraction or granules caused by exothermicity or endothermicity of these reactions emerges, which complicates data analysis. To avoid this problem, catalytic layers were supported onto the inner walls of single channels cut from corundum honeycomb monoliths, which allowed to avoid any temperature gradients and estimate rate constants (Table 3) [109][110][111][112]. Among Pt/Ln x (Ce 0.5 Zr 0.5 ) 1−x O 2−y catalysts the highest activity was revealed for Pr-doped catalyst possessing the highest oxygen mobility, which stresses importance of this characteristic. lyst fraction or granules caused by exothermicity or endothermicity of these reactions emerges, which complicates data analysis. To avoid this problem, catalytic layers were supported onto the inner walls of single channels cut from corundum honeycomb monoliths, which allowed to avoid any temperature gradients and estimate rate constants (Table 3) [109][110][111][112]. Among Pt/Lnx(Ce0.5Zr0.5)1-xO2-ycatalysts the highest activity was revealed for Pr-doped catalyst possessing the highest oxygen mobility, which stresses importance of this characteristic.   To accelerate preparation of ceria-zirconia based catalysts, an automated workstation was used allowing wet impregnation of La-doped γ-Al 2 O 3 by mixed solutions, so series of samples with supported CeZrO layers doped by Pr or Sm and promoted by Pt, Ru, Cu, Cu + Ni were made [113]. The ruthenium-promoted Ru/Ce 0.4 Zr 0.4 Sm 0.2 O 2−y /La-γ-Al 2 O 3 catalyst demonstrated the best activity and coking stability in ethanol steam reforming, which was explained by a high mobility and reactivity of oxygen in this sample.
A high and stable performance in MDR without any coking was demonstrated also for LaMnO 3 -based perovskite with in-situ exsolved Ni nanoparticles strongly interacting with perovskite surface layers [114].
Detailed review on methane dry reforming over perovskite derived catalysts is presented in [39], where basically the same trends in ensuring high and stable performance related to oxygen mobility and strong interaction of metal nanoparticles with remaining oxide matrix are analyzed.

Ethanol Reforming on Bulk Perovskites
In reactions of oxygenates reforming a high activity and stability of catalysts based on perovskites precursors was shown as well [32,[63][64][65][66][67][108][109][110]115,116]. The same factorsexsolution of Ni-containing nanoalloys from perovskite lattice in reducing conditions, their stabilization by strong interaction with remaining matrix and oxygen transfer to metal-support interface determine their resistance to coking and activity. It has been shown that even Ni/La 2 O 3 catalysts obtained from LaNiO 3 have a higher stability to coking than catalysts of the same composition prepared by traditional impregnation.
In our work [32], LnFe 0.7−x M x Ni 0.3 O 3 perovskites (Ln = La, Pr, M = Mn, Ru, x = 0-0.3) as precursors of robust catalysts were prepared by modified Pechini route. Before testing in EtOH steam reforming in concentrated feed 10% EtOH+40% H 2 O + N 2 at contact time 70 ms they were either reduced by H 2 at 850 • C or pretreated in the reaction feed. The highest activity was shown by Pr and Ru-containing samples, with specific rate constants at 800 • C varying from 4 to 8 s −1 m −2 , and their performance was stable due to presence of only a trace admixture of ethylene in products, which is usually responsible for coking in this reaction.

Perovskite-Fluorite Nanocomposites
Since perovskite + fluorite nanocomposites possess a higher oxygen mobility compared to separate phases [106], a lot of research was devoted to their design not only as cathodes of solid oxide fuel cells (SOFC) and materials for oxygen separation membranes [35,87], but also as active components of structured catalysts for fuels reforming and SOFC anodes operating in the internal reforming mode [31,32,35,72,73,77,87,[117][118][119][120]. As fluorites doped ceria or zirconia were employed. Different methods of synthesis were used for preparation, the most efficient was modified Pechini method when prepared nanocrystalline fluorite was dispersed in a polymeric precursor of perovskite followed by its decomposition and calcinations [31], while (100 − x) wt.% LnFeNi 0.3 + x wt.% Ce 0.9 Gd 0.1 O 2−δ (GDC) (x = 5-50 wt.%) nanocomposites were prepared from polymeric precursors containing all cations. Their testing in MDR (feed 10% CH 4 +10% CO 2 in He, contact time 15 ms) revealed that at 800 • C specific first-order rate constant k (s −1 m −2 ) goes through the maximum at x = 10 wt.% (k = 5.5), its values being nearly the same at x = 0 (k = 2.5) and 20% (k = 2.0). Such trend is apparently explained by the positive effect of perovskite structure disordering at a low content of fluorite dopant followed by subsequent surface blocking by Ce and Gd cations due to their segregation as a result of their bigger sizes.
ovskite structure disordering at a low content of fluorite dopant followed by subsequent surface blocking by Ce and Gd cations due to their segregation as a result of their bigger sizes.
Mixed manganese-chromium oxides Mn x Cr 3−x O 4 (x = 0.3-2.7) prepared by modified Pechini route and promoted by 2 wt.% Ni + 2 wt.% Ru were studied in steam reforming of ethanol [69]. The density of surface metal sites was estimated by CO pulse chemisorption.  (3.28). In this series of catalysts, the highest oxygen diffusion coefficient was found for (Ru+Ni)/Mn 2.7 Cr 0.3 O 4 sample with the excess of manganese. This implies that namely metal-support interaction provides the highest TOF for (Ru+Ni) MnCr 2 O 4 catalyst, while sufficient oxygen mobility (Table 1) ensures coking stability. Even in concentrated feed 10% C 2 H 5 OH + 40% H 2 O in N 2 at short contact time 70 ms concentration of byproduct CH 4 was only 2% at 700 • C, while only trace admixtures of usual byproducts such as ethylene and acetaldehyde were observed [69]. This means that such known intermediates as ethoxy complexes and acetaldehyde are rapidly transformed into syngas due to a high concentration of reactive oxygen forms and metal sites [104,121].
Testing (Ru + Ni)/MnCr 2 O 4 catalyst in the autothermal reforming of glycerol (feed 10.9% C 3 H 8 O 3 + 9.5% O 2 + 44.5% H 2 O + 35.1% N 2 , contact time~40 ms) also revealed its high efficiency and stable performance [73]. Thus, already at 750 • C complete conversion of glycerol was achieved with the content of main byproducts CH 4 and C 2 H 4 less than 2% and syngas yield approaching equilibrium.
In our studies [72,73,77,90,104,129], both Mg-γ-Al 2 O 3 supports prepared by supporting Mg on γ-Al 2 O 3 as well as mesoporous Mg-γ-Al 2 O 3 and MgAl 2 O 4 supports prepared by EISA method with Pluronic P123 were used. Ni was supported either by impregnation or during one-pot synthesis. Nanocomposite active components comprised of mixed oxides with perovskite, fluorite and spinel structures described above were supported by impregnation and then promoted by Ni+Ru. Catalysts were tested in dry reforming of methane and steam/autothermal reforming of ethanol as fractions as well as layers supported on small plates of heat-conducting substrates. Even for catalysts containing only Ni prepared by one-pot synthesis a high and stable performance in ethanol steam reforming was demonstrated due to a high dispersion of metal and acidity suppression [129]. The most promising active components comprised of mesoporous MgAl 2 O 4 with supported PrNi 0.9 Ru 0.1 O 3 , MnCr 2 O 4 or Ce 0.35 Zr 0.35 Pr 0.3 O 3 promoted with Ni+Ru demonstrated a high efficiency and resistance to coking in dry reforming of natural gas and autothermal reforming of such fuels as ethanol and ethyl acetate [78]. In a similar way, Ni/CeZrO 2 /MgAl 2 O 4 catalyst revealed a high activity and coking stability in tri-reforming of methane due to a small size of Ni nanoparticles and moderate basicity of support [28]. In steam reforming of methane [80] a high activity of Rh-Ni/MgAl 2 O 4 washcoated FeCrAlloy honeycomb monolith was observed and explained by a high active metal dispersion as well as absence of heat and mass transfer limitations.

Partial Oxidation and Dry Reforming of Methane
Mechanism of these reactions was studied by using such methods as SSITKA, kinetic transients and pulse techniques (pulse microcalorimetry, pulse studies in flow conditions including those carried out in vacuum systems called TAP) [19,72,82,87,101,102,108,109,111,112,[134][135][136][137]. For majority of efficient catalysts based on mixed oxides with a high mobility and storage capacity of reactive oxygen species with supported Pt group metals and/or Ni mechanism can be described by so called bifunctional type, where molecules of oxidants are activated on support vacancies producing oxygen species (and CO in the case of CO 2 ), while methane is activated on metal sites by C-H bond rupture (rate-limiting stage). These steps are conjugated by rapid transfer of surface oxygen species to the metal sites where they interact with CH x fragment transforming them into CO and H 2 . A typical feature of such redox scheme in the case of MDR is the same degree of methane conversion and syngas selectivity in pulses containing only CH 4 or CH 4 + CO 2 , as well as identical CO 2 conversion into CO in mixed and CO 2 -containing pulses. Simplified scheme of methane dry reforming and sequence of elementary steps successfully applied for modeling of transient over Ni-Ru-Sm 0. 15  surface oxygen species to the metal sites where they interact with CHx fragment tran forming them into CO and H2. A typical feature of such redox scheme in the case of MD is the same degree of methane conversion and syngas selectivity in pulses containin only CH4 or CH4 + CO2, as well as identical CO2 conversion into CO in mixed and CO -containing pulses. C-H bond rupture (rate-limiting stage). These steps are conjugated by rapid transfe surface oxygen species to the metal sites where they interact with CHx fragment tra forming them into CO and H2.
A typical feature of such redox scheme in the case of M is the same degree of methane conversion and syngas selectivity in pulses contain only CH4 or CH4 + CO2, as well as identical CO2 conversion into CO in mixed and -containing pulses.  [28]. In steam reforming of methane [80] a high activity of Rh-Ni/MgAl2O washcoated FeCrAlloy honeycomb monolith was observed and explained by a high a tive metal dispersion as well as absence of heat and mass transfer limitations.

Partial Oxidation and Dry Reforming of Methane
Mechanism of these reactions was studied by using such methods as SSITKA, k netic transients and pulse techniques (pulse microcalorimetry, pulse studies in flo conditions including those carried out in vacuum systems called TA [19,72,82,87,101,102,108,109,111,112,[134][135][136][137]. For majority of efficient catalysts based o mixed oxides with a high mobility and storage capacity of reactive oxygen species wi supported Pt group metals and/or Ni mechanism can be described by so called bifun tional type, where molecules of oxidants are activated on support vacancies producin oxygen species (and CO in the case of CO2), while methane is activated on metal sites b C-H bond rupture (rate-limiting stage). These steps are conjugated by rapid transfer surface oxygen species to the metal sites where they interact with CHx fragment tran forming them into CO and H2.
A typical feature of such redox scheme in the case of MD is the same degree of methane conversion and syngas selectivity in pulses containin only CH4 or CH4 + CO2, as well as identical CO2 conversion into CO in mixed and CO -containing pulses.  [28]. In steam reforming of methane [80] a high activity of Rh-Ni/MgA washcoated FeCrAlloy honeycomb monolith was observed and explained by a high tive metal dispersion as well as absence of heat and mass transfer limitations.

Partial Oxidation and Dry Reforming of Methane
Mechanism of these reactions was studied by using such methods as SSITKA netic transients and pulse techniques (pulse microcalorimetry, pulse studies in f conditions including those carried out in vacuum systems called T [19,72,82,87,101,102,108,109,111,112,[134][135][136][137]. For majority of efficient catalysts based mixed oxides with a high mobility and storage capacity of reactive oxygen species w supported Pt group metals and/or Ni mechanism can be described by so called bifu tional type, where molecules of oxidants are activated on support vacancies produc oxygen species (and CO in the case of CO2), while methane is activated on metal site C-H bond rupture (rate-limiting stage). These steps are conjugated by rapid transfe surface oxygen species to the metal sites where they interact with CHx fragment tra forming them into CO and H2. A typical feature of such redox scheme in the case of M is the same degree of methane conversion and syngas selectivity in pulses contain only CH4 or CH4 + CO2, as well as identical CO2 conversion into CO in mixed and -containing pulses. Here →-denotes irreversible steps; ⇔-reversible steps; s-surface sites; O ls -lattice oxygen atoms; V Os -lattice oxygen vacancy, sm-surface metal sites For catalysts comprised of Pt/doped CeZrO oxides direct route of CH 4 partial oxidation into syngas, which generates CO and H 2 even in the presence of oxygen, was reliably demonstrated in our studies. It is explained by stabilization of Pt cations due to metal-support interaction and fast migration of oxygen species activated on support to Pt. Moreover, Pt cations are less efficient in oxidation of CO and H 2 , while being more efficient in C-H bond activation in CH 4 .
Basic scheme of methane partial oxidation [109] is presented here:

+ Pt
Clearly this mechanism is impossible in the case of such supported metals as Ru, Pd, Ni, Co, etc., which can only combust methane, CO and H 2 in the oxidized state. For these catalysts indirect scheme of methane partial oxidation is realized, in which all oxygen is consumed for methane combustion in the inlet part of the catalytic layer, while syngas is generated via steam and dry reforming of methane in the main part of catalytic layer where O 2 is absent in the gas phase and supported metals are in the reduced state [82].
These unique features of Pt cations were also reflected in specificity of MDR relaxation after contact of oxidized Pt/PrCeZrO catalyst with reaction feed, where both CH 4 and CO 2 conversions decline with time-on-stream apparently caused by the catalyst progressing reduction [19,136,137]. Mathematical modeling using scheme of methane dry reforming mechanism given below allowed to describe such transients taking into account a high efficiency of Pt cations in CH 4 activation, while CO 2 transformation occurs via carbonates adsorbed on Pt n+ -Pr 4+ -O oxidized sites: Note that for catalysts with all other supported transition and precious metals in oxidized state catalytic activity in MDR is negligible and begins to increase to the steady-state value in the process of catalysts reduction by reaction mixture at sufficiently high temperatures.

Reactions of Ethanol Transformation into Syngas
For steady-state of catalysts based on oxides with high oxygen mobility and reactivity in ethanol reforming reactions pulse studies revealed that ethanol conversion and products selectivities in pulses containing reaction mixture or only ethanol are practically the same [103,104,121]. This proves realization of step-wise redox mechanism of these reactions. FTIRS studies combined with SSITKA identified main intermediates of ethanol conversion into syngas, such as ethoxy species and acetaldehyde and estimated rate constants of their transformation, while acetates were shown to be spectators. The rate-limiting step is the cleavage of C-C bond in ethoxy species fixed on rare-earth or transition metal cations due to incorporation of terminal oxygen species located on neighboring Me sites as supported by DFT calculations for the (001) face of MnCr 2 O 4 spinel doped by Ru [104]. Kinetic scheme of the reaction of ethanol partial oxidation used in analysis of SSITKA data is as follows:  [104].

Development of Structured Catalysts
For design of such catalysts substrates made of metals, alloys and cermets were used [72,73,77,82,84,86,87,90,91,98,117]. Detonation spraying [138] was applied to cover thin foils, microchannel plates or gauzes by protective nonporous layer of alumina or zirconia. Known procedures of stacking and winding were applied to produce monolithic substrates from these constituents. Foam substrates comprised of NiAl alloys, ceramics, etc. were prepared via polyurethane foam duplication followed by required mechanical and thermal treatment [73,86]. These substrates were covered by catalytic layers using suspensions of active components in isopropanol. Structured catalysts were tested in pilot reactors in processes of natural gas, liquid fuels and biofuels reforming/autothermal reforming under realistic conditions. For best catalysts on substrates with a high thermal conductivity a high syngas yield was provided even at short contact times due to more uniform temperature profile along the catalysts' length [73,82,84]. In the autothermal reforming mode it helps to transfer heat generated in the inlet part of monolith due to exothermal combustion of a part of fuel into the following parts where O 2 in the gas phase is absent, so endothermic reactions of steam and dry reforming occur. For strongly endothermic reaction of MDR this positive effect of a high thermal conductivity of substrate is demonstrated by the identical temperature dependence of methane conversion in the layer of catalyst fraction for diluted feed and for the stack of microchannel plates with the same active component even in more concentrated feed (Figure 5b). In dry reforming of real natural gas (NG) containing up to 6% of C 2 -C 4 alkanes also high and stable performance of structured catalyst was demonstrated for concentrated feed ( Figure 6). where [ZO] and [Z] correspond to oxidized and reduced site of catalysts surface, respectively, without any differentiation of the nature of active sites related to different transition metal cations (Mn, Cr) and metal atoms (Ni, Ru) at this level of analysis similar to that for catalysts based on bulk MnCr2O4 [104].

Development of Structured Catalysts
For design of such catalysts substrates made of metals, alloys and cermets were used [72,73,77,82,84,86,87,90,91,98,117]. Detonation spraying [138] was applied to cover thin foils, microchannel plates or gauzes by protective nonporous layer of alumina or zirconia. Known procedures of stacking and winding were applied to produce monolithic substrates from these constituents. Foam substrates comprised of NiAl alloys, ceramics, etc. were prepared via polyurethane foam duplication followed by required mechanical and thermal treatment [73,86]. These substrates were covered by catalytic layers using suspensions of active components in isopropanol. Structured catalysts were tested in pilot reactors in processes of natural gas, liquid fuels and biofuels reforming/autothermal reforming under realistic conditions. For best catalysts on substrates with a high thermal conductivity a high syngas yield was provided even at short contact times due to more uniform temperature profile along the catalysts' length [73,82,84]. In the autothermal reforming mode it helps to transfer heat generated in the inlet part of monolith due to exothermal combustion of a part of fuel into the following parts where O2 in the gas phase is absent, so endothermic reactions of steam and dry reforming occur. For strongly endothermic reaction of MDR this positive effect of a high thermal conductivity of substrate is demonstrated by the identical temperature dependence of methane conversion in the layer of catalyst fraction for diluted feed and for the stack of microchannel plates with the same active component even in more concentrated feed (Figure 5b). In dry reforming of real natural gas (NG) containing up to 6% of C2-C4 alkanes also high and stable performance of structured catalyst was demonstrated for concentrated feed ( Figure 6). This advantage of structured catalysts on heat-conducting metal/cermet substrates allowed to carry out efficient transformation of a lot of fuels including gasoline and diesel into syngas via partial oxidation and steam/autothermal reforming [82][83][84][85][86][87]117,[139][140][141][142].
Reforming of biofuels such as acetone, ethyl acetate and glycerol is known to be accompanied by gas-phase reactions yielding ethylene, which is easily transformed into coke on catalysts. However, structured catalysts with nanocomposite active components demonstrated high and stable performance in the autothermal reforming of such biofuels as ethanol, acetone, ethyl acetate ( Figure 7) and glycerol ( Figure 8) with fuels content up to 25% and O 2 content in the mixture with steam up to 20% [73,82,86,87,117]. Note that Figures 7 and 8 show a high yield of syngas achieved at very short contact times, which is provided by a high efficiency of active components supported on mesoporous MgAl 2 O 4 or Mg-doped -γ-Al 2 O 3 . Even anisole (content in the feed up to 10%), sunflower oil (content up to 0.7%) [82,117] and turpentine oil (commercial bio-oil mainly containing C 10 H 16 and C 10 H 18 O components, such as α-pinen, α-terpineol, etc., content in the feed up to 6% [90]) ( Figure 9) were successfully converted into syngas in the autothermal reforming on de- veloped structured catalysts. Another important problem in design of efficient syngas generators, especially for the small-scale application, is related to the heat management, since endothermic reactions of fuels steam and dry reforming require extensive preheat of inlet streams. This can be dealt with by using the exit stream for preheating the inlet feed in specially designed heat exchangers conjugated with catalytic reactors. Another aspect of energy efficiency problem solution is bound with possibility to conjugate exothermal partial oxidation of methane into syngas with endothermal processes of biofuels steam/dry reforming. These problems were solved in design of a radial-type reactor equipped with the heat exchanger described in detail in our previous work [82,117]. Here a cylindrical stack of catalytic microchannel washers was wrapped by gauze sheets with supported active components as well as with microspherical catalysts loaded between gauzes. The feed enters the central part of the stack of washers and flows in the radial direction. The reformed gas is collected into a plenum around the catalyst arrangement and exited from a single pipe, while inlet feed is heated by passing through the heat exchanger situated around the reactor as an outer shell. Such design made it possible to efficiently carry out partial oxidation of a mixture of natural gas and liquid fuel (ethanol, ethyl acetate, turpentine oil) to syngas at high flow rates (up to 40,000 h −1 ) with preheating the mixture up to 50-100 • C at the reactor inlet (Figures 10-12).
ethanol, acetone, ethyl acetate ( Figure 7) and glycerol (Figure 8) with fuels content up to 25% and O2 content in the mixture with steam up to 20% [73,82,86,87,117]. Note that Figures 7 and 8 show a high yield of syngas achieved at very short contact times, which is provided by a high efficiency of active components supported on mesoporous MgAl2O4 or Mg-doped -γ-Al2O3. Even anisole (content in the feed up to 10%), sunflower oil (content up to 0.7%) [82,117] and turpentine oil (commercial bio-oil mainly containing C10H16 and C10H18O components, such as α-pinen, α-terpineol, etc., content in the feed up to 6% [90]) ( Figure 9) were successfully converted into syngas in the autothermal reforming on developed structured catalysts. Another important problem in design of efficient syngas generators, especially for the small-scale application, is related to the heat management, since endothermic reactions of fuels steam and dry reforming require extensive preheat of inlet streams. This can be dealt with by using the exit stream for preheating the inlet feed in specially designed heat exchangers conjugated with catalytic reactors. Another aspect of energy efficiency problem solution is bound with possibility to conjugate exothermal partial oxidation of methane into syngas with endothermal processes of biofuels steam/dry reforming. These problems were solved in design of a radial-type reactor equipped with the heat exchanger described in detail in our previous work [82,117]. Here a cylindrical stack of catalytic microchannel washers was wrapped by gauze sheets with supported active components as well as with microspherical catalysts loaded between gauzes. The feed enters the central part of the stack of washers and flows in the radial direction. The reformed gas is collected into a plenum around the catalyst arrangement and exited from a single pipe, while inlet feed is heated by passing through the heat exchanger situated around the reactor as an outer shell. Such design made it possible to efficiently carry out partial oxidation of a mixture of natural gas and liquid fuel (ethanol, ethyl acetate, turpentine oil) to syngas at high flow rates (up to 40,000 h −1 ) with preheating the mixture up to 50-100°C at the reactor inlet (Figures 10-12).          Figure 11. Dependence of products concentration on ethyl acetate (a) or O2 (b) content in the autothermal reforming of the mixture of natural gas and ethyl acetate in radial reactor with the heat exchanger and LaNi(Pt)O3/La0.1Ce0.45Zr0.45O2 active component. Feed 20% NG +18 % O2 +N2 +EtAc (a) or 10% NG + O2 + N2 + 15% EtAc (b), flow rate 2.5 m 3 /h.

Conclusions
Pechini method and synthesis in supercritical alcohols allowed to provide uniformity of the spatial distribution of elements in nanodomains of fluorite, perovskite and spinel oxides required for controlling their oxygen mobility and reactivity. Promoting these oxides and their nanocomposites of optimized composition by platinum group metals and nickel allowed to create effective and coking-resistant catalysts for biogas and biofuels transformation into syngas. Due to developed metal-support interface and strong metalsupport interactions bifunctional scheme of reaction mechanism is realized with activation of oxidants (O 2 , CO 2 , H 2 O) on the oxide support sites, while fuel molecules are activated on metal centers. Fast diffusion of surface oxygen species to the metal-oxide interface provides conjugation of these steps resulting in efficient syngas generation and coking suppression. Preparation of mesoporous supports comprised of MgAl 2 O 4 or Mg-doped alumina with enhanced basicity allowed to design core-shell systems with a high working area of supported catalytic nanocomposite layers, increase their activity, thermal and coking stability and decrease content of rare-earth elements. Structured heat-conducting substrates loaded with optimized active components provide efficient heat-and masstransfer in reactors for biofuels transformation into syngas. Pilot reactors with internal heat exchangers permit efficient operation in the autothermal mode on the mixture of natural gas, air and real biofuels such as ethanol, glycerol and turpentine oil with the inlet temperature 50-100 • C and GHSV up to 40,000 h −1 .