Surface Termination and CO2 Adsorption onto Bismuth Pyrochlore Oxides

The catalytic activity and gas-sensing properties of a solid are dominated by the chemistry of the surface atomic layer. This study is concerned with the characterization of the outer atomic surfaces of a series of cubic ternary oxides containing Bi(III): Bi2M2O7 (M = Ti, Zr, Hf), using low-energy ion scattering spectroscopy. A preferential termination in Bi and O is observed in pyrochlore Bi2Ti2O7 and related cubic compounds Bi2Zr2O7 and Bi2Hf2O7, whereas all three components of the ternary oxide are present on the surface of a Bi-free pyrochlore oxide, Y2Ti2O7. This observation can be explained based on the revised lone-pair model for post-transition-metal oxides. We propose that the stereochemically active lone pair resulting from O 2p-assisted Bi 6s–6p hybridization is more energetically favored at the surface than within a distorted bulk site. This leads to reduction of the surface energy of the Bi2M2O7 compounds and, therefore, offers a thermodynamic driving force for the preferential termination in...