Speaker
Description
Ceria ($\mathrm{CeO}_2$) was recently identified as a promising catalyst in the selective hydrogenation of alkynes to alkenes[1]. This reaction occurs primarily on highly dispersed metal catalysts, such as Pd, but rarely on oxide surfaces. The origin of the outstanding activity and selectivity observed on $\mathrm{CeO}_2$ remains unclear. In this work, we show that one key aspect of the hydrogenation reaction – the interaction of $\mathrm{H}_2$ with the oxide – depends strongly on the presence of O vacancies within $\mathrm{CeO}_2$.[2] By infrared reflection absorption spectroscopy (IRAS) on well-ordered $\mathrm{CeO}_2$(111) thin films and density functional theory (DFT) calculations, we show that the preferred heterolytic dissociation of molecular hydrogen on $\mathrm{CeO}_2$(111) requires $\mathrm{H}_2$ pressures in the mbar regime. Hydrogen depth profiling with nuclear reaction analysis (NRA) indicates that H species stay on the surface of stoichiometric $\mathrm{CeO}_2$(111) films, whereas H incorporates as a volatile species into the volume of partially reduced $\mathrm{CeO}_{2-x}$(111) thin films (x$\approx$1.8-1.9). Complementary DFT calculations suggest that oxygen vacancies facilitate H incorporation below the surface and that they are the key to the stabilization of hydridic H species in the volume of reduced ceria. Finally, we currently assess the role of O vacancies in the selective semi-hydrogenation of alkynes by gas chromatography (GC) reactivity studies.
References
[1] Vilé, G., Bridier, B., Wichert, J. Pérez-Ramirez, J.,Angew. Chem. Int. Ed., 2012, 51(34): 8620-8623.
[2] K. Werner, X. Weng, F. Calaza, M. Sterrer, T. Kropp, J. Paier, J. Sauer, M. Wilde, K. Fukutani, S. Shaikhutdinov, H.-J. Freund, J. Am. Chem. Soc., 2017, 139 (48), pp 17608–17616.
