Temperature-programmed desorption of hydrogen from γ-Al2O3-supported platinum catalysts with and without tungsten

Abstract

Temperature-programmed desorption (TPD) spectroscopy was used to characterize the desorption of H₂ from γ-Al₂O₃-supported samples prepared from {Pt[W(CO)₃(C₅H₅)]₂(PhCN)₂} and {Pt₂W₂(CO)₆(C₅H₅)₂(PPh₃)₂} as well as those prepared from [PtCl₂(PhCN)₂] and from [PtCl₂(PhCN)₂] and [W(CO)₆]. The latter two are characterized by broad and the former two by narrow distributions of desorption energies (E_d). The slope of the near-linear plot of the logarithm of the preexponential factor for desorption vs. E_d (indicating a compensation effect) is correlated with the strength of the interactions between platinum atoms and tungsten cations in the samples. These interactions are strong in the samples prepared from the bimetallic precursors but not in those prepared from the monometallic precursor(s). The narrow and nearly coverage-independent desorption energy distribution curves characterizing the former samples are consistent with the presence of highly dispersed, nearly uniform (4–6 atom) and energetically homogeneous platinum clusters in these samples (as indicated by extended X-ray absorption fine structure spectroscopy), in contrast to those prepared from the combination of monometallic precursors, which incorporated larger, nonuniform particles of platinum largely segregated from tungsten. The data indicate that desorption energy distribution functions and compensation effect plots determined from TPD profiles are helpful in characterizing the surface heterogeneity of highly dispersed supported metals.