In-situ QXAFS study on CO and H2 adsorption on Pt in solid [PtAu8(PPh3)8]-H[PMo12O40]

Abstract

The adsorption behaviors of H2 and CO molecules in crown-motif [PtAu8(PPh3)8]-H[PMo12O40] (PtAu8-PMo12) solid were investigated by in-situ quick-scan X-ray absorption fine structure (QXAFS) measurements with time resolution of 0.1 s. The electronic state of Pt in the PtAu8-PMo12 was drastically changed by the adsorptions of H2 and CO molecules because of the formation of Pt-H2/Pt-CO interactions. The H2 adsorbed more rapidly (< 0.5 s) on Pt than CO did (~2.5 s) and showed reversible adsorption/desorption behavior on Pt atoms in the PtAu8-PMo12. The rapid adsorption of H2 is due to the fast diffusion of H2, which has smaller kinetic diameter than CO, in the narrow channels between the closed voids in PtAu8-PMo12. Meanwhile, CO irreversibly adsorbed on Pt, resulting in structural isomerization to the stable “chalice-motif” PtAu8, determined by XAFS analysis and density functional theory calculations. The structural isomerization is involved by pushing ligands aside to make space for CO adsorption since the void size near Pt in the crown-motif PtAu8-PMo12 is narrower than kinetic diameter of CO.