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. Ce 2 O n ( n = 0–2) and AuCe 2 O n ( 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 Ce 2 O 3, Ce 2 O 4, AuCe 2 O 3, and AuCe 2 O 4 clusters due to these species not being detected. The optimized AuCe 2 O n ( n = 0–3) cluster geometries are consistent with Au adsorption to oxygen vacancy sites while the AuCe 2 O 4 cluster correlates with Au adsorption to a CeO 2 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 CeO 2 vacancy is positively charged with an Au → Ce charge transfer. The adsorbed Au atom is proposed to enhance the catalytic properties of the AuCe 2 O n cluster by (i) stabilizing the negatively charged Au atom on reduced AuCe 2 O n clusters to enhance nucleophilicity; (ii) increasing the electron accepting capability of the AuCe 2 O 4 species; (iii) destabilizing the HOMO of the AuCe 2 O 4 cluster; and (iv) facilitating the abstraction of additional surface oxygen atoms by reactants.