**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 [1–3]. In dry reforming of biogas or natural gas containing CH4 + CO2 these greenhouse gases are transformed into syngas with H2/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 [1–12]. 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.

1. Instead of traditional supports (SiO2, alumina, zeolites, etc.) use mixed oxides of rare earth and transition metals with variable oxidation states of cations/oxygen stoichiometry. As the result, such oxides with fluorite [13–29], perovskite [30–67] and spinel [68–76] structures as well as their nanocomposites [35,72,73] have sufficient amount of reactive surface/bulk oxygen species characterized also by a high mobility providing their fast migration to metal particles, where they react with activated

**Citation:** Sadykov, V.; Simonov, M.; Eremeev, N.; Mezentseva, N. Modern Trends in Design of Catalysts for Transformation of Biofuels into Syngas and Hydrogen: From Fundamental Bases to Performance in Real Feeds. *Energies* **2021**, *14*, 6334. https://doi.org/10.3390/en14196334

Academic Editor: Byong-Hun Jeon

Received: 1 September 2021 Accepted: 1 October 2021 Published: 4 October 2021

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fuel molecules, thus preventing coking [71–73]. These oxides are the most promising supports for catalysts of hydrocarbons or oxygenates reforming to syngas without coking [11–77]. Their oxygen mobility can be tuned by changing their composition as well as synthesis procedures [13,32,35].


For any practical application catalysts for transformation of biofuels into syngas are to be supported as thin layers on heat-conducting monolithic substrates, which allows to minimize or even avoid heat and mass transfer limitations typical for granulated catalysts beds [77–86].

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 18O2 or C18O2 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].

#### **2. 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

ultrasonic spray pyrolysis were used for synthesis of oxides [30,33,35,36,87]. Among advanced methods, synthesis in flow regimes (including that in supercritical conditions) characterized by continuous generation of nanoparticles appears to be very promising [20–26,87]. Note that single-phase complex oxides (such as ceria-zirconia mixed oxides, etc.) including cations differing by charge and size and, hence, inherent acidity, could not be prepared by traditional precipitation with alkaline solutions added to a mixed metal salts solution [87].

#### *2.1. Pechini Method*

Ester polymeric precursors (Pechini) method [88] is based upon using citric acid and ethylene diamine as chelating agents. With ethylene glycol solution single-phase nanocrystalline doped cerium–zirconium oxides [13,18,89], perovskites [31,32,34,35] and spinel oxides [69] were obtained possessing a high spatial uniformity of cations distribution. In the case of aqueous solutions such spatial uniformity was not obtained [62–65]. For preparation of perovskite-fluorite nanocomposites Pechini method was further modified. It was made by adding fluorite oxide nanopowder into the polymeric precursor solution containing cations of perovskite followed by ultrasonic treatment and evaporation. After polymeric matrix decomposition and calcinations under air this provides nanocomposites with a high specific surface area and developed interphases between perovskite and fluorite domains [31].

#### *2.2. Synthesis in Supercritical Alcohols*

Complex Ce1−xZrxO2−<sup>δ</sup> oxides were synthesized in supercritical ethanol and isopropanol using Zr(OBu)4, ZrOCl2 and Ce(NO3)3 · 6H2O solutions in isopropanol at 400–480 ◦C and pressure 120–140 atm [22–26]. Single-phase samples with uniform spatial distribution of cations were obtained only with solutions containing acetylacetone (AA) with AA/Zr molar ratio 2. This method allowed also to obtain single-phase samples of Ce1−xZrxO2−<sup>δ</sup> doped with Ti and Nb cations [21,22] as well as to promote them with Ni cations in so-called one-pot route of synthesis [21–24]. In reducing conditions Ni cations are exsolved from the fluorite lattice providing small Ni clusters strongly interacting with support, which helps to suppress coking and sintering.

#### *2.3. Mesoporous Nanocomposites*

Specific surface area of perovskites prepared via Pechini method is in the range of 10–15 m2/g, which is too small for their good performance as active components of structured catalysts. To deal with this problem perovskites were loaded on Mg-doped γ-Al2O3 [90] or mesoporous MgAl2O4 prepared by self-assembly method induced by evaporation (EISA) with copolymer Pluronic P123 [78].

Even though for (Ru + Ni)-promoted doped MnCr2O4 spinels specific surface areas were reasonably high (~100 m2/g), to improve their sintering resistance these active components were loaded on Mg-doped γ-Al2O3 as well [90].

#### **3. Characterization of Nanocomposite Materials**

#### *3.1. Structural Features*

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 [73,77,89–98]. For Ce0.5Zr0.5O2−<sup>y</sup> composition having the highest oxygen mobility, doping with La, Gd, Pr, Sm cations (thus producing Lnx(Ce0.5Zr0.5)1−xO2−<sup>y</sup> 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 Ce0.5Zr0.5O2−<sup>y</sup> 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 Ce3+ 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).

Perovskites of LnFe0.7−xRuxNi0.3O3−<sup>δ</sup> (Ln = La, Pr, Sm; x = 0–0.1) composition prepared by Pechini method are single-phase rhombohedral samples. Their reduction produces nanocomposites comprised of Ni–Fe–(Ru) nanoalloys and LnOx situated in the surface layers of remaining Ln–Fe–O particles [32,34,35].

Freshly prepared Ru/(La0.8Pr0.2Mn0.2Cr0.8O3 + 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 MnCr2O4 prepared by Pechini method [69,90] have 2–40% of admixture phase with corundum structure due to segregation of (Mn,Cr)2O3 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 MgAl2O4 or 10 wt.% Mg-doped γ-Al2O3 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].

#### *3.2. 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 [13,94–97]. 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 MnCr2O4 spinel is enriched by Mn as judged by XPS data, which is explained by segregation of Mn2+ 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 O2 platinum was found to be present in three oxidation states: 0, 2+ and 4+ (XPS binding energies BE equal to 71, 72 and 75 eV, respectively), the content of Pt cations being the highest in the case of Pr-doped samples [13,95,97]. FTIRS of adsorbed CO also revealed several states of Pt on the surface reflected in bands of linear carbonyls Pt0-CO (*ν*CO 2046–2084 cm–1), Pt+-CO (*ν*CO 2125–2140 cm–1) and Pt2+-CO (*ν*CO 2170–2180 cm–1) [13,94–98]. FTIRS spectra did not contain bands which could be assigned to Pt4+-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 Pt2+ 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 Ptn+ cations on the surface.

For Ni/Ce–Zr–O samples, mainly FTIRS bands of terminal carbonyls Ni-CO at *ν*CO~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].
