Surface Termination and CO₂ Adsorption onto Bismuth Pyrochlore Oxides

Abstract

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 BiO x -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.

CO2 Activation on Heterostructures of Bi2O3-Nanocluster Modified TiO2: Promoting the Critical First Step in CO2 Conversion

“…The atomic structure of the Bi2O3 nanoclusters can further facilitate CO2 adsorption due the flexibility in these supported nanoclusters upon relaxation. We can also relate these findings to the work from Walker et al 41 on the Bi-containing pyrochlore. In this system the surface is terminated by a Bi-O layer, similar to our Bi2O3-nanocluster modified TiO2, and the Bi 5s/5p electronic states are mixed with O 2p states at the top of the valence band.…”

“…Upon examining recent literature on this topic, the recent experimental work from Rosenthal and co-workers [37][38][39][40] and Walker et al 41 strongly suggests that p-block metal containing systems, in particular Bismuth, are able to activate CO2 or convert it to other molecules with good efficiency. The catalysts in refs [37][38][39][40] are Bi-containing materials supported on glassy carbon and Bi nanoparticles, which have large amounts of Bi 3+ .…”

“…The bismuth pyrochlore oxide Bi2Ti2O7 was studied in ref. 41 and has high CO2 chemisorption capacity. In this system, low energy ion scattering (LEIS) shows clearly that Bi 3+ species are present in the surface layer.…”

“…Bi 3+ is an interesting species as it has a sterochemically active lone pair which results in the presence of Bi-derived electronic states at the top of the valence band. This gives rise to the distorted structure of bulk Bi2O3 43 and the Bi-rich surface region in the pyrochlore Bi2Ti2O741 . Other metal oxides with +3 cations do not show these structural distortions.In the PEDOS of the Bi2O3-TiO2 heterostructures, the Bi electronic states are present at the valence band edge of all composite systems studied.…”

CO2 and water activation on ceria nanocluster modified TiO2 rutile (110)

“…70 Bismuth pyrochlore oxides have been shown to have a high CO2 chemisorption capacity as identified through FTIR and attributed to the Bi2O3 motif with a Bi 3+ lone pair. 71 Cu2O has emerged as a potential candidate for CO2RR due to its favourable band gap position and width 72 and studies have focussed on the interaction of CO2 molecules with various cuprous oxide surfaces and terminations. [72][73][74][75][76][77] Wu and colleagues studied the adsorption of CO2 at the Cu2O (111): O terminated surface with oxygen vacancies, 75 finding that oxygen vacancies have a negative impact on the interaction energy; the most stable adsorption configuration at the perfect surface was more favourable by 0.15 eV than adsorption at the O vacancy surface.…”