Mo Coverage Research Articles

Photoreducibility of [formula omitted] catalysts with CO

The photoreduction of supported Mo SiO 2 catalysts by CO has been investigated at room temperature as a function of Mo coverage and wavelength of the incident UV irradiation. The CO adsorbed on photoreduced Mo ions has been investigated by IR spectroscopy. In the literature, it has been reported that CO photoreduction of Mo SiO 2 catalysts leads to the formation of Mo 4+ and Mo 5+ ions. Results of this study indicate that CO photoreduction also leads to the formation of Mo ions in valence states lower than Mo 4+. In particular, the pairs of IR bands observed at 2100, 2135 cm −1 and 2075, 2125 cm −1 were attributed to pairs of CO molecules adsorbed on Mo 4+ or Mo 3+, and Mo 2+ sites, respectively. These sites are assumed to be formed from the CO photoreduction of dimeric surface molybdates. Another IR band observed at 2035 cm −1 may be attributed to the stretching vibrations of a CO molecule adsorbed in a bridged configuration between two adjoining Mo 2+ or to a single CO adsorbed linearly on Mo 2+ sites. Thus, our results indicate that the low-temperature CO photoreduction is a complex process and that Mo ions in several valence and coordination states may be formed simultaneously during CO photoreduction in Mo SiO 2 catalysts.

The interaction of water with SiMo interface. A core and valence photoemission investigation

Core level photoemission with MgKα radiation ( hv = 1253.6 eV) and valence photoemission ( hv = 21.2 eV) are presented for Si(1 1 1)-Mo interfaces at various Mo coverages (up to 3.2·10 15 at cm −2) exposed to 6000 L of H 2O. The metal deposition strongly enhances the oxidation of Si with the complete H 2O dissociation and the formation of a SiO 2-like compound. The results are consistent with an interface growth with the formation of silicide islands.

Stacking of elementary non-porous particles as an XPS intensity model for supported heterogeneous catalysts: experimental assessment of validity

Mo 3 d/Al 2 p XPS intensity ratios ( R Mo 3 d ) have been determined for a series of Al 2O 3-supported molybdenum oxides, characterized by a roughly constant Mo coverage and surface areas ( S BET) ranging from 10 to 300 m 2 g −1. These experimental R Mo 3 d values are compared with theoretical values calculated according to three XPS intensity models. Of these three models, the stacking model we have proposed earlier is shown to take the effect of surface area the best into account and to predict correctly the experimental values for the entire S BET range. In contrast, the two other models are shown experimentally to be valid only at low or high surface area. Grain size turns out to be an important factor in XPS, in relation to diffusion-controlled impregnation which leads to concentration profiles