Investigating Charge Transfer Interactions in AuCe₂O Clusters Using Photoionization Efficiency Spectroscopy and...

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. Ce₂O (n = 0-2) and AuCe₂O (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 AuCe₂O (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 AuCe₂O cluster by (i) stabilizing the negatively charged Au atom on reduced AuCe₂O 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.