γ-Al2O3-supported Pt–W catalysts prepared from molecular organometallic precursors: Characterization by infrared spectroscopy

Abstract

γ-Al₂O₃-supported catalysts were prepared from molecular bimetallic precursors, {Pt[W(CO)₃(C₅H₅)]₂(PhCN)₂} and {Pt₂W₂(CO)₆(C₅H₅)₂(PPh₃)₂}; samples for comparison were prepared from mononuclear precursors, [PtCl₂(PhCN)₂] or a mixture of [PtCl₂(PhCN)₂] and [W(CO)₆]. The samples were treated in H₂ at 400°C to remove the organic ligands and characterized by infrared spectroscopy before and after the treatment and after exposure to CO or NO. The infrared spectra show that the precursors were adsorbed intact on γ-Al₂O₃, interacting weakly by hydrogen bonding of their organic ligands with surface hydroxy groups of γ-Al₂O₃; consequently, the supported clusters were completely recovered by extraction with CH₂Cl₂. The capacities for chemisorption of CO or NO of the samples prepared from the bimetallic clusters (after treatment in H₂ at 400°C) were found to be less than those of samples made from mononuclear precursors. Electronic interactions between platinum atoms and tungsten cations in the former samples are inferred to lead to increased electron density on platinum, which is reflected in shifts of the terminal CO and NO bands to lower frequencies. The close proximity of platinum clusters and tungsten cations in the samples facilitates the formation of adsorbed CO with the carbon atom bonded to a platinum atom and the oxygen atom to an oxophilic tungsten cation. The formation of such species is an activated process, suggested to take place via terminal and bridging forms of CO adsorbed on platinum. NO on these samples gave adsorbed NO₂ species characterized by infrared bands at 1319 and 1225 cm^-1. The formation of NO₂ species is explained by the reaction of NO adsorbed on platinum with oxygen atoms formed as a result of dissociative NO adsorption on the platinum–tungsten bimetallic sites or partially reduced tungsten cations located in close proximity to platinum atoms.