Investigating Charge Transfer Interactions in AuCe₂O_n Clusters Using Photoionization Efficiency Spectroscopy and Density Functional Theory

Abstract

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.

Investigating Pt Atom Addition to Gas Phase Ce2On Clusters Using Photoionization Efficiency Spectroscopy and Density Functional Theory

“…Charge transfer from Pt → Ce then begins from n = 3, coinciding with the sharp reduction in negative charge on the Pt atom. The decrease in NBO charge on oxygen observed for Ce 2 O 4 contrasts with the Hirshfeld charges calculated previously, which remains relatively constant along the series. This decrease in NBO charge suggests that charge is being transferred away from both O and Pt toward Ce, though the qualitative difference between calculated NBO and Hirshfeld charges reduces confidence in this conclusion.…”

“…The low-energy structures for PtCe 2 O n ( n = 0–5) exhibit a similar base structural moiety, with the platinum atom bridging the two cerium atoms (Figure and Figure S2). The bare metal and the singly oxygenated clusters show similar structures to that of the analogous AuCe 2 and AuCe 2 O clusters as well as the pure ceria clusters, Ce 2 O and Ce 2 O 2 , which correspond to the replacement of the transition metal (Au or Pt) with an oxygen atom . The LES for PtCe 2 O is a planar C 2 v structure, while the doubly oxygenated PtCe 2 O 2 remains C 2 v though no longer planar, with an oxygen atom above and below the plane of the base metal trimer.…”

“…The inability to detect these more oxidized clusters is likely due to high ionization energy and will be discussed later. Adding a Pt atom to Ce 2 O n appears to reduce the ionization onset for n = 0 and 1, with the platinum containing species’ IEs lowered by ∼0.3 eV each . Ce 2 O 2 is ionized at ∼4.6 eV, whereas PtCe 2 O 2 is not observed in the experiment, signaling a sharp increase in IE ( vide infra ).…”

“…Calculated relevant neutral structures and NBO atomic charges for PtCe 2 O n ( n = 0 – 5) clusters. Contrasting AuCe 2 O n and Ce 3 O 5 low-energy structures (from refs and ) are inserted above their corresponding PtCe 2 O n counterparts. Structures presented for PtCe 2 O 5 represent the LES ( VIA ) and a structure with associatively bound O 2 ( VIB ).…”

“…In this work, we aim to extend and complement our previous work on AuCe 2 O n , identifying the trends in geometric and electronic structure of neutral and cationic PtCe 2 O n ( n = 0–5) clusters and their relevance to atom-on-surface catalytic systems. Generally, our PtCe 2 O n ( n = 3–5) cluster structures were consistent with Pt bonding to cerium vacancies and tended toward charge being transferred away from the transition metal into the cerium oxide, resulting in positively charged Pt, consistent with the charge state observed with Pt on ceria surfaces.…”