Methane Decomposition and Carbon Growth on Y₂O₃, Yttria-Stabilized Zirconia, and ZrO₂

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. By comparison of the three oxides we could moreover 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

Reduction of Different GeO₂ Polymorphs

A combination of volumetric adsorption, thermal desorption and structure-determining methods was used to study and compare the hydrogen reduction behavior of three different GeO2 polymorphs: tetragonal, water-free hexagonal, and water-containing (hydroxylated) commercial hexagonal GeO2. Marked differences in the onset and extent of reduction between the two water-free polymorphs have been observed. Tetragonal GeO2 adsorbs more hydrogen at temperatures T ≤ 673 K and tends to be more easily reducible at low temperatures, but extended Ge metal formation at elevated temperatures is rather suppressed compared to both hexagonal GeO2 phases. Temperature-programmed hydrogen desorption spectra indicate the reduction-induced formation of weakly bonded hydrogen adsorption sites both on hydroxylated and pure hexagonal GeO2. The existence of the most weakly bonded hydrogen is linked to the transformation of initially present hydroxylated species into a tetragonal structure fraction upon annealing at 500 K. Thus, analogous forms of hydrogen were not observed on any of the pure (dehydroxylated) structures

Water−Gas Shift and Formaldehyde Reforming Activity Determined by Defect Chemistry of Polycrystalline In₂O₃

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

Quantum mechanical calculations of the vibrational spectra of quartz and rutile type GeO2

Heat treatment and stepwise cooling of as delivered, water containing quartz type GeO2 powder resulted in transformation into a water free form. A rutiletype modification could be prepared by impregnation of the quartz type phase with RbOH solutions, drying and annealing. Raman and FTIR absorption spectra of quartz and rutile type GeO2 were measured and compared to quantum mechanical ab initio calculations based on a hybrid functional using the Perdew Burke Ernzerhof correlation functional with 16.7 Hartree Fock exchange density functional. Maximum and mean deviations between measured spectral bands and assigned vibrational modes are 14 and 8 cm 1 for the quartz type and 30 and 13 cm 1 for the rutile type polymorphic form. Water is incorporated into GeO4 entities of quartz type GeO2; a water free and structurally stable form can be prepared by a heating up to 1425 K, tempering at 1323 K and stepwise cooling. Spectral bands not explained by the calculations suggest defects and distortions in both quartz and rutile type structures, in case of the quartz type one by incomplete transformation into an ideal structure after removing the water, whereas the rutile type modification most probably incorporates Rb during its synthesi