4.2.2. 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 [32,63–67,108–110,115,116]. The same factors exsolution 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/La2O3 catalysts obtained from LaNiO3 have a higher stability to coking than catalysts of the same composition prepared by traditional impregnation.

In our work [32], LnFe0.7−xMxNi0.3O3 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% H2O+N2 at contact time 70 ms they were either reduced by H2 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<sup>−</sup>1m<sup>−</sup>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.

#### 4.2.3. 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 [31,32,35,72,73,77,87,117–120]. 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.% LnFeNi0.3 + x wt.% Ce0.9Gd0.1O2−<sup>δ</sup> (GDC) (x = 5–50 wt.%) nanocomposites were prepared from polymeric precursors containing all cations. Their testing in MDR (feed 10% CH4 +10% CO2 in He, contact time 15 ms) revealed that at 800 ◦C specific first-order rate constant *k* (s<sup>−</sup>1m<sup>−</sup>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.

Nanocomposite (10 wt.% Ni + 2 wt.% Ru)/(La0.8Pr0.2Mn0.2Cr0.8O3 + 10 wt.% YSZ) [72,73,77] showed a high and stable performance in MDR. Its specific activity is close to that of (Ru + Ni)/SmPrCeZrO catalyst (Figure 5a), and it provides a high methane conversion into syngas at high temperatures even at short contact times (Figure 5b).

**Figure 5.** (**a**) Temperature dependence of specific rate constants for methane dry reforming on fractions of (Ru+Ni)/SmPrCeZrO and (Ru+Ni)/LaPrMnCr/YSZ catalysts. Feed 7% CH4 + 7% CO2, contact time 15 ms; (**b**) Temperature dependence of CH4 conversion in CH4 dry reforming on fraction of (Ru+Ni)/(LaPrMnCr+YSZ) catalyst (1, 2; feed 7% CH4 + 7% CO2 in He, contact time 15 ms) and on the stack of microchannel plates with this active component (3, 4; feed 20% CH4 + 20% CO2 in Ar, contact time 0.4 s). 1,3-experimental data, 2,4-fitting.

#### *4.3. Catalysts Based on Spinels*

Co1.8Mn1.2O4, Ni0.33Co1.33Mn1.33O4 and Ni0.6Co1.2Mn1.2O4 catalysts were prepared by thermal decomposition of nitrates and studied in ethanol steam reforming reaction. The highest activity was found for Ni0.6Co1.2Mn1.2O4 catalyst, which is explained by a high content of mixed Ni-Co clusters segregated at the surface in reaction conditions along with the highest oxygen mobility (Table 1) preventing coking [68].

Mixed manganese-chromium oxides MnxCr3−xO4 (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. The highest conversion of ethanol and yield of syngas were obtained for stoichiometric MnCr2O4 composition. Turnover frequencies (TOF, s<sup>−</sup>1) at 500 ◦C vary in the row (Ru+Ni)/Mn0.3Cr2.7O4 (0.88) < (Ru+Ni)/Mn2CrO4 (1.13) < (Ru+Ni)/Mn2.7Cr0.3O4 (1.36) < (Ru+Ni)/MnCr2O4 (3.28). In this series of catalysts, the highest oxygen diffusion coefficient was found for (Ru+Ni)/Mn2.7Cr0.3O4 sample with the excess of manganese. This implies that namely metal-support interaction provides the highest TOF for (Ru+Ni) MnCr2O4 catalyst, while sufficient oxygen mobility (Table 1) ensures coking stability. Even in concentrated feed 10% C2H5OH + 40% H2O in N2 at short contact time 70 ms concentration of byproduct CH4 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)/MnCr2O4 catalyst in the autothermal reforming of glycerol (feed 10.9% C3H8O3 + 9.5% O2 + 44.5% H2O + 35.1% N2, 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 CH4 and C2H4 less than 2% and syngas yield approaching equilibrium.

#### *4.4. Catalysts Based on High Surface Area Supports*

Since specific surface area of perovskites, fluorites and spinel is lower than that of traditional supports such as γ-alumina, silica, zeolites, etc., a lot of research was devoted to design of catalysts for fuels reforming where active components—metals or their combination with reactive oxides are loaded on high surface area supports [12,28,39,72,75,77–80,90,122–133]. Since acidity of supports is well known to be responsible for coking, the best results for activity and

stability of Ni-loaded catalysts in fuels reforming, as expected, were obtained in the case of Mg-doped alumina [72,90,129,131,133] or MgAl2O4 [12,28,70,75,78–80,123,124,126,128,129].

In our studies [72,73,77,90,104,129], both Mg–γ-Al2O3 supports prepared by supporting Mg on γ-Al2O3 as well as mesoporous Mg–γ-Al2O3 and MgAl2O4 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 MgAl2O4 with supported PrNi0.9Ru0.1O3, MnCr2O4 or Ce0.35Zr0.35Pr0.3O3 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/CeZrO2/MgAl2O4 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/MgAl2O4 washcoated FeCrAlloy honeycomb monolith was observed and explained by a high active metal dispersion as well as absence of heat and mass transfer limitations.
