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 BiOx-like structures. CO2 adsorption experiments in situ monitored by diffuse reflectance IR spectroscopy show a high CO2 chemisorption capacity for this series of cubic bismuth ternary oxides, indicating a high surface basicity. This can be associated with O 2p-Bi 6s-6p hybridized electronic states, which are more able to donate electronic density to adsorbed species than surface lattice oxygen ions, normally considered as the basic sites in metal oxides. The enhanced CO2 adsorption of these types of oxides is particularly relevant to the current growing interest in the development of technologies for CO2 reduction.