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

Temperature-programmed desorption (TPD) spectroscopy was used to characteri ze the desorption of H-2 from gamma-Al2O3-supported samples prepared from { Pt[W(CO)(3)(C5H5)](2)(PhCN)(2)} and {Pt2W2(CO)(6)(C5H5)(2)(PPh3)(2)} as wel l 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 na rrow distributions of desorption energies (E-d). The slope of the near-line ar 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. T hese interactions are strong in the samples prepared from the bimetallic pr ecursors but not in those prepared from the monometallic precursor(s). The narrow and nearly coverage-independent desorption energy distribution curve s characterizing the former samples are consistent with the presence of hig hly dispersed, nearly uniform (4-6 atom) and energetically homogeneous plat inum clusters in these samples (as indicated by extended X-ray absorption f ine structure spectroscopy), in contrast to those prepared from the combina tion of monometallic precursors, which incorporated larger, nonuniform part icles of platinum largely segregated from tungsten. The data indicate that desorption energy distribution functions and compensation effect plots dete rmined from TPD profiles are helpful in characterizing the surface heteroge neity of highly dispersed supported metals.