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

gamma-A(2)O(3)-supported catalysts were prepared from molecular bimetallic precursors, {Pt[W(CO)(3)(C5H5)](2)(PhCN)(2)} and {Pt2W2(CO)(6)(C5H5)(2)(PPh 3)(2)}; samples for comparison were prepared from mononuclear precursors, [ PtCl2(PhCN)(2)] or a mixture of [PtCl2(PhCN)(2)] and [W(CO)(6)]. The sample s were treated in H-2 at 400 degrees C to remove the organic ligands and ch aracterized by infrared spectroscopy before and after the treatment and aft er exposure to CO or NO. The infrared spectra show that the precursors were adsorbed intact on gamma-Al2O3, interacting weakly by hydrogen bonding of their organic ligands with surface hydroxy groups of gamma-Al2O3; consequen tly, the supported clusters were completely recovered by extraction with CH 2Cl2. The capacities for chemisorption of CO or NO of the samples prepared from the bimetallic clusters (after treatment in H-2 at 400 degrees C) were found to be less than those of samples made from mononuclear precursors. E lectronic interactions between platinum atoms and tungsten cations in the f ormer samples are inferred to lead to increased electron density on platinu m, which is reflected in shifts of the terminal CO and NO bands to lower fr equencies. 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 cati on. The formation of such species is an activated process, suggested to tak e place via terminal and bridging forms of CO adsorbed on platinum. NO on t hese samples gave adsorbed NO2 species characterized by infrared bands at 1 319 and 1225 cm(-1). The formation of NO2 species is explained by the react ion of NO adsorbed on platinum with oxygen atoms formed as a result of diss ociative NO adsorption on the platinum-tungsten bimetallic sites or partial ly reduced tungsten cations located in close proximity to platinum atoms.