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 vacancyadsorbed 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.

The characteristics of small cerium oxide and gold–cerium oxide clusters were investigated as models for gold attachment to various defect sites on a ceria surface. Photoionization efficiency (PIE) spectra of gas phase Ce3O n (n = 0–4) and AuCe3O n (n = 0–3) clusters were recorded and compared to spectral simulations based on DFT calculations. Calculated structures and PIE spectra for the Ce3O5,6 and AuCe3O4–6 clusters are also presented; however, these species were not detected during photoionization experiments. Addition of an Au atom to Ce3 was found to increase the energy of the ionization onset by ∼0.4 eV, whereas addition of one or more oxygen atoms decreases the onset by ∼0.25 eV. The optimized AuCe3O n (n = 0–4) cluster geometries correlate with Au atoms adsorbed to oxygen vacancy sites while the AuCe3O5 and AuCe3O6 clusters are consistent with Au adsorption to CeO3 and CeO2 vacancies, respectively. The interactions between the cerium oxide cluster surface and the adsorbed Au atom were found to strongly depend on the nature the of the adsorption site. Au adsorbed to O vacancies are negatively charged with a Ce → Au charge transfer, whereas Au adsorbed to CeO2 and CeO3 vacancies have a reversed Au → Ce charge transfer, resulting in a positively charged Au atom. Au adsorption to the Ce3O n clusters has the effect of (i) reducing the differences in the HOMO energies of the AuCe3O4, AuCe3O5, and AuCe3O6 clusters and (ii) lowering the binding energy of oxygen atoms for all AuCe3O n (n = 1–6) clusters. Au adsorption appears to have a minimal effect on CeO2 vacancy formation, although CeO2 vacancies were calculated to form more readily than O vacancies on both the Ce3O n and AuCe3O n clusters. The low energy fragmentation calculated for the Ce3O5,6 and AuCe3O4–6 clusters, via loss of either Au, O, or CeO2, could potentially make photoionization experiments unfeasible since these clusters may simply dissociate when exposed to high energy photons above the ionization threshold.

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Photoionization efficiency spectra for PtCe2O n (n = 0–1) clusters were recorded and compared with simulations based on density functional theory to determine adiabatic ionization energies (IE) and verify cluster structures. Calculated structures were also presented for PtCe2O2–5, though no comparison with experiment was made due to nondetection of these species in the mass spectrum. The IEs of PtCe2 and PtCe2O were determined to be 4.59 and 4.55 eV, respectively. The calculated IEs for PtCe2O2–5 sharply rise to >5.95 eV, coinciding with the HOMO of the clusters being lower energy Pt 6s-like, Ce 4f-like, and Pt 5d-like orbitals, from the higher energy Ce 6s-like HOMOs of PtCe2O0,1. The sharp increase in IE also explains the lack of PtCe2O2–5 ionization products observed in the experiment, though photodissociation is found to be a less likely possibility. As the level of oxidation in the clusters increased the charge on the Pt atom in the cluster became more positive, representing a transition from Ce → Pt to Pt → Ce charge transfer. This transition coincided with the formation of the first Pt–O–Ce structural motif and the sharp decrease in O bond dissociation energy in PtCe2O3. PtCe2O4 and PtCe2O5 each possess a second Pt–O–Ce site formed with the latter also having a sharp increase in calculated IE to 7.71 eV due to the low-energy Pt localized 5d-like HOMO.