DRIFTS studies of carbon monoxide coverage on highly dispersed bimetallic Pt-Cu and Pt-Au catalysts

Silica supported platinum-gold and platinum-copper catalysts prepared from organometallic cluster precursors were characterized with diffuse reflectance Fourier-transform spectroscopy (DRIFTS) of adsorbed CO. Because of the effects that dipole coupling of adsorbed carbon monoxide molecules can have on the infrared spectrum, coverage studies were employed to evaluate the interactions between CO and the catalyst surfaces. DRIFTS spectra of the catalysts prepared from bimetallic molecular cluster precursors showed significantly lower CO stretching frequencies relative to a traditionally prepared platinum catalyst. The lowering of the CO stretching frequency may be the result of the formation of unique alloys and particle morphologies. In the case of the catalyst prepared from the platinum-copper cluster, two peaks were identified in the PtCO stretching region, one of which is postulated to be a bridging or semi-bridging mode between Pt and Cu. Infrared spectroscopy results are also discussed in relation to kinetic data from the n-hexane conversion reaction. Differences in catalyst activities and selectivities are concluded to be primarily due to geometric (i.e. structural or morphological) effects.