Understanding the influence of support alkalinity on the hydrogen and oxygen chemisorption properties of Pt particles Comparison of X-ray absorption near edge data from gas phase and electrochemical systems

Abstract

The influence of the alkalinity of the support on the chemisorption of oxygen on Pt particles has been studied using Pt L₃ and L₂ X-ray absorption near edge data from Pt/Al₂O₃ doped with F or Rb. The O binding site is determined by using a procedure similar to that previously utilised for H chemisorption. The Pt L₃ X-ray absorption edge data of the sample in vacuum is subtracted from the data obtained after chemisorption of oxygen. The signature of this difference spectrum is compared with that resulting from a similar procedure applied to theoretical data, the latter obtained with the help of full multiple scattering ab initio calculations on small model Pt₄ clusters using the FEFF8 code. After O₂ exposure of the F doped alumina, oxygen is found in hcp/sub-surface binding sites, whereas in the Rb doped case oxygen is present in the bridged/fcc sites. These results imply that the properties of the support have a strong effect on the oxygen chemisorption properties of small Pt particles. Since O is known to bond in a bridged position near step edges or corners at low coverage, followed by 3-fold fcc sites on flat faces or terraces at higher coverage, and then finally at hcp sites and/or in the subsurface region, these data indicate that the Pt–O bond strength and hence coverage increases with support acidity. This is in contrast to that found previously for the Pt–H bond, which decreases in strength with support acidity. Consistent with these results, it is concluded from density functional theory (DFT) calculations on model Pt₄ clusters that the isolated Pt–O bond strength is indeed significantly altered by the support and in the opposite direction to that reported previously for Pt–H bonding. The calculations also show that the Pt–O bond ionic character is dependent on the properties of the support. The increased Pt–O ionic character in a basic support gives rise to a stronger O–O lateral repulsion. This lateral interaction, along with the decreased Pt–O bond strength, decreases the O coverage for basic supports. A survey of the recent literature reporting DFT and experimental results on H and O adsorption on Pt(111) and Pt clusters is given.