Investigating Charge Transfer Interactions in AuCe2On Clusters Using Photoionization Efficiency Spectroscopy and Density Functional Theory.

The properties of small cerium oxide and gold-cerium oxide clusters were explored as analogues for gold deposition at defect sites on a cerium oxide surface. Ce2On (n = 0-2) and AuCe2On (n = 0-2) clusters were prepared in the gas phase and investigated using photoionization efficiency spectroscopy complemented by spectral simulations based on DFT calculations; purely theoretical investigations were conducted on the Ce2O3, Ce2O4, AuCe2O3, and AuCe2O4 clusters due to these species not being detected. The optimized AuCe2On (n = 0-3) cluster geometries are consistent with Au adsorption to oxygen vacancy sites while the AuCe2O4 cluster correlates with Au adsorption to a CeO2 vacancy site. The electronic properties of the adsorbed Au atom depend strongly on the nature of the ceria adsorption site: O vacancy-adsorbed Au is negatively charged with a Ce → Au charge transfer occurring at the adsorption interface, whereas Au adsorbed to a CeO2 vacancy is positively charged with an Au → Ce charge transfer. The adsorbed Au atom is proposed to enhance the catalytic properties of the AuCe2On cluster by (i) stabilizing the negatively charged Au atom on reduced AuCe2On clusters to enhance nucleophilicity; (ii) increasing the electron accepting capability of the AuCe2O4 species; (iii) destabilizing the HOMO of the AuCe2O4 cluster; and (iv) facilitating the abstraction of additional surface oxygen atoms by reactants.