Surface Reactivity of YSZ, Y2O3, and ZrO2toward CO, CO2, and CH4: A Comparative Discussion

Abstract

The C1-surface chemistry of catalytically and technologically relevant oxides (YSZ, ZrO2, and Y2O3) toward CH4, CO, and CO2 was comparatively studied by electrochemical impedance (EIS) and spectroscopic (FT-IR) methods. Highly correlated in situ measurements yield a consistent picture with respect to qualitative and quantitative surface modifications as a function of temperature and gas phase composition. This includes not only a detailed study of carbon deposition in methane and adsorption of CO and CO2 but also proof of the strong influence of surface chemistry. On all studied oxides, carbon deposited during methane treatment grows dynamically forming interconnected islands and eventually a continuous conducting carbon layer at T ≥ 1073 K. Before methane dissociation via gas phase radical reactions/H-abstraction and carbon growth, a complex redox interplay of total oxidation as well as formate and carbonate formation leads to associated surface and grain conductivity changes. For CO adsorption, these measurements yield data on the time and temperature dependence of the adsorbate- and carburization-induced conductivity processes. In that respect, an equivalent circuit model in dry CO allows to disentangle the different contributions of grain interiors, grain boundaries, and electrode contributions. For YSZ, temperature regions with different charge carrier activation energies could be identified, perfectly corresponding to significant changes in surface chemistry. Hydroxyl groups, carbonates, or formates strongly influence the impedance properties, suggesting that the conductivity properties of YSZ, e.g., in a realistic reforming gas mixture, cannot be reduced to exclusive bulk ion conduction. Because of the different degree of hydroxylation and the different ability to chemisorb CO and CO2, the influence of the surface chemistry on the electrochemical properties is varying strongly: in contrast to ZrO2, the impact of the studied C1-gases on YSZ and Y2O3 is substantial. This also includes the reoxidation/reactivation behavior of the surfaces.