In situ FT-IR spectroscopy was exploited to study the adsorption of CO2 and CO on commercially available yttria-stabilized ZrO2 (8 mol % Y, YSZ-8), Y2O3, and ZrO2. All three oxides were pretreated at high temperatures (1173 K) in air, which leads to effective dehydroxylation of pure ZrO2. Both Y2O3 and YSZ-8 show a much higher reactivity toward CO and CO2 adsorption than ZrO2 because of more facile rehydroxylation of Y-containing phases. Several different carbonate species have been observed following CO2 adsorption on Y2O3 and YSZ-8, which are much more strongly bound on the former, due to formation of higher-coordinated polydentate carbonate species upon annealing. As the crucial factor governing the formation of carbonates, the presence of reactive (basic) surface hydroxyl groups on Y-centers was identified. Therefore, chemisorption of CO2 most likely includes insertion of the CO2 molecule into a reactive surface hydroxyl group and the subsequent formation of a bicarbonate species. Formate formation following CO adsorption has been observed on all three oxides but is less pronounced on ZrO2 due to effective dehydroxylation of the surface during high-temperature treatment. The latter generally causes suppression of the surface reactivity of ZrO2 samples regarding reactions involving CO or CO2 as reaction intermediates.

The adsorption, bonding, defect formation, and reactivity of hydrogen on different In 2 O 3 powder samples were studied by a combination of volumetric adsorption, thermal desorption, diffraction, and spectroscopic techniques. Surface reduction was observed in dry hydrogen up to 400 K, followed by reduction of surfacenear regions. Above 500 K bulk reduction, along with the formation of metallic In, sets in. Raman spectra indicate a considerable reordering of the In 2 O 3 structure in this temperature regime. Despite their TPD proven presence, the related adsorbed H-containing species were not detectable by Fourier transform infrared spectroscopy and/or Raman spectroscopy, in strong contrast to related experiments on -Ga 2 O 3 . Hydrogeninduced oxygen vacancies were found to be easily replenished by traces of water in the gas feed.

Carbon deposition following thermal methane decomposition under dry and steam reforming conditions has been studied on yttria-stabilized zirconia (YSZ), Y2O3, and ZrO2 by a range of different chemical, structural, and spectroscopic characterization techniques, including aberration-corrected electron microscopy, Raman spectroscopy, electric impedance spectroscopy, and volumetric adsorption techniques. Concordantly, all experimental techniques reveal the formation of a conducting layer of disordered nanocrystalline graphite covering the individual grains of the respective pure oxides after treatment in dry methane at temperatures T ≥ 1000 K. In addition, treatment under moist methane conditions causes additional formation of carbon-nanotube-like architectures by partial detachment of the graphite layers. All experiments show that during carbon growth, no substantial reduction of any of the oxides takes place. Our results, therefore, indicate that these pure oxides can act as efficient nonmetallic substrates for methane-induced growth of different carbon species with potentially important implications regarding their use in solid oxide fuel cells. Moreover, by comparing the three oxides, we could elucidate differences in the methane reactivities of the respective SOFC-relevant purely oxidic surfaces under typical SOFC operation conditions without the presence of metallic constituents.

The interaction of In2O3 with methanol steam reforming reactants (H2O), intermediates (formaldehyde), and products (CO, CO2) as well as (inverse) water−gas shift reaction mixtures is studied by volumetric adsorption, temperature-programmed reaction, electric impedance measurements, and Fourier-transform infrared spectroscopy to clarify the high CO2 selectivity of pure In2O3 in methanol steam reforming. Reduction in dry CO occurs already slightly above 300 K, yielding CO2 by reaction with reactive lattice oxygen. Replenishment of any lattice oxygen species by defect quenching with CO2 is strongly suppressed. Adsorption of dry CO or CO2 leads to formation of weakly (monodentate HCO3) or more strongly bound carbonate species (bidentate or bridged CO3), for CO at least partly via reaction with lattice oxygen to CO2 (gas) and readsorption of CO2 (gas) on the In2O3 surface. Whereas CO2 evolution via reaction of a CO + H2O mixture on In2O3 starts at 430 K and accelerates above 550 K, only trace amounts of CO are formed upon reaction in a CO2 + H2 mixture. Formaldehyde is converted with 95% selectivity to CO2 under typical steam reforming conditions and temperatures of ∼550 K, i.e., at rates and selectivities comparable to methanol.

This Minireview summarizes the fundamental results of a comparative inverse-model versus real-model catalyst approach toward methanol steam reforming (MSR) on the highly CO₂-selective H₂-reduced states of supported Pd/ZnO, Pd/Ga₂O3, and Pd/In₂O3 catalysts. Our model approach was extended to the related Pd/GeO₂ and Pd/SnO₂ systems, which showed previously unknown MSR performance. This approach allowed us to determine salient CO₂-selectivity-guiding structural and electronic effects on the molecular level, to establish a knowledge-based approach for the optimization of CO₂ selectivity. Regarding the inverse-model catalysts, in situ X-ray photoelectron spectroscopy (in situ XPS) studies on near-surface intermetallic PdZn, PdGa, and PdIn phases (NSIP), as well as bulk Pd₂Ga, under realistic MSR conditions were performed alongside catalytic testing. To highlight the importance of a specifically prepared bulk intermetallic[BOND]oxide interface, unsupported bulk intermetallic compounds of Pd_xGa_y were chosen as additional MSR model compounds, which allowed us to clearly deduce, for example, the water-activating role of the special Pd₂Ga-β-Ga₂O₃ intermetallic[BOND]oxide interaction. The inverse-model studies were complemented by their related “real-model” experiments. Structure–activity and structure–selectivity correlations were performed on epitaxially ordered PdZn, Pd₅Ga₂, PdIn, Pd₃Snv, and Pd₂Ge nanoparticles that were embedded in thin crystalline films of their respective oxides. The reductively activated “thin-film model catalysts” that were prepared by sequential Pd and oxide deposition onto NaCl(001) exhibited the required large bimetal[BOND]oxide interface and the highly epitaxial ordering that was required for (HR)TEM studies and for identification of the structural and catalytic (bi)metal[BOND]support interactions. To fully understand the bimetal[BOND]support interactions in the supported systems, our studies were extended to the MeOH- and formaldehyde-reforming properties of the clean supporting oxides. From a direct comparison of the “isolated” MSR performance of the purely bimetallic surfaces to that of the “isolated” oxide surfaces and of the “bimetal[BOND]oxide contact” systems, a pronounced “bimetal[BOND]oxide synergy” toward optimum CO2 activity/selectivity was most evident. Moreover, the system-specific mechanisms that led to undesired CO formation and to spoiling of the CO2 selectivity could be extracted

The surface reactivity of Y2O3, YSZ, and ZrO2 polycrystalline powder samples toward H2 has been comparatively studied by a pool of complementary experimental techniques, comprising volumetric methods (temperature-programmed volumetric adsorption/oxidation and thermal desorption spectrometry), spectroscopic techniques (in situ electric impedance and in situ Fourier-transform infrared spectroscopy), and eventually structural characterization methods (X-ray diffraction and scanning electron microscopy). Reduction has been observed on all three oxides to most likely follow a surface or near-surface-limited mechanism involving removal of surface OH-groups and associated formation of water without formation of a significant number of anionic oxygen vacancies. Partly reversible adsorption of H2 was proven on the basis of molecular H2 desorption. Dictated by the specific hydrophilicity of the oxide, readsorption of water eventually takes place. The inference of this surface-restricted mechanism is further corroborated by the fact that no bulk structural and/or morphological changes were observed upon reduction even at the highest reduction temperatures (1173 K). We anticipate relevant implications for the use of especially YSZ in fuel cell research, since in particular the chemical state and structure of the surface under typical reducing high-temperature conditions affects the operation of the entire cell.

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