Characterization of platinum-molybdenum catalysts supported on alumina and titania.

摘要

The acidity of molybdenum on titania was dependent on Mo loading, pretreatment, and support. The difference between the titania and alumina supported catalysts is ascribed to the different extent of support interaction.; CO adsorption on Pt-Mo catalysts shows that there are several adsorption states depending on the loading and Pt/Mo ratio. The same result was observed for NO adsorption. At low Pt loading, CO oxidation reaction destroyed Pt{dollar}sp{lcub}2+{rcub}{dollar} ion site, especially on titania. A new band was observed for both CO and NO adsorption on Pt-Mo catalysts. This band is assigned to CO and NO adsorbed on Pt-Mo ensemble sites since the new band was not observed for Pt or Mo catalyst. The band position of adsorbed CO on the ensemble site was different depending on the support, presumably due to the different degree of interactions of Pt-Mo/TiO{dollar}sb2{dollar} from Pt-Mo/Al{dollar}sb2{dollar}O{dollar}sb3{dollar}. Based on the FTIR spectra of adsorbed CO, a model for Pt-Mo sites was suggested to explain the different adsorption characteristics. The NO adsorption is consistent with this model. The NO bidentate structure is the dominant species on oxidized catalyst. When NO was adsorbed on CO saturated Pt surface, a strong isocyanate band is observed.; The thermal stability and the surface structure of Mo were studied by TGA and Raman spectroscopy. The Raman results show that the molybdenum oxides have less interaction with alumina than with titania. However, the thermal stability of titania was lower due to its non-porous structure. The appearance of a new Raman band, which cannot ascribed to either TiO{dollar}sb2{dollar}, MoO{dollar}sb3{dollar} or PtO, supports the presence of Pt-Mo ensemble.; The activity and selectivity of the NO reduction by CO were studied as a function of Pt-Mo loading and the type of support material. The synergism effect was observed when Mo was added to Pt. The lower activities of the titania supported catalysts are attributed to the strong metal support interaction and are in consistent with the CO and NO adsorption. The N{dollar}sb2{dollar}O concentration passed through a maximum with the temperature increase, indicating the change of reaction mechanism. Despite the lower activity at high temperature, the titania supported catalysts have higher N{dollar}sb2{dollar} selectivity.