Redox properties of vanadium ions in SBA-15-supported vanadium oxide: an FTIR spectroscopic study

Synergistic effects of VO(x)-Pt probed by the oxidation of propane on VO(x)/Pt(111)

VO(x)/Pt(111), which was grown layer-by-layer and exhibited a well-defined structure, was used as a model catalytic surface to study the intrinsic catalytic activity of Pt, as well as the effect of VO(x) additive, for the oxidation of propane. A special sample system was designed to ensure a reliable analysis of the trace amount of model catalytic reaction products. The results show that the catalytic activities for the oxidation of C3H8 on the Pt(111) surface as adding VO(x) are suppressed apparently at temperatures below 400 K, but enhanced significantly at temperatures above 400 K. Maximum reaction rates are achieved at a VO(x) coverage of about 0.3 ML at the test temperatures of 423 and 473 K. The infrared reflection-absorption spectroscopy (IRAS) results show that the redox property of the VO(x)-Pt is much better than that of the bulklike VO(x). This is confirmed by CO poisoning tests, in that the oxidation of VO(x)/Pt(111) is significantly suppressed by the coadsorbed CO. The kinetic data demonstrate that there are at least two catalytically active sites, metallic Pt and VO(x)-Pt interface, for the activation and oxidation of C3H8. The promotion effects of VO(x) on Pt for the oxidation of C3H8 can be attributed to the synergy between VO(x) and Pt.

Structural characterization of vanadium oxide catalysts supported on nanostructured silica SBA-15 using X-ray absorption spectroscopy

The local structure of vanadium oxide supported on nanostructured SiO2 (VxOy/SBA-15) was investigated by in situ X-ray absorption spectroscopy (XAS). Because the number of potential parameters in XAS data analysis often exceeds the number of "independent" parameters, evaluating the reliability and significance of a particular fitting procedure is mandatory. The number of independent parameters (Nyquist) may not be sufficient. Hence, in addition to the number of independent parameters, a novel approach to evaluate the significance of structural fitting parameters in XAS data analysis is introduced. Three samples with different V loadings (i.e. 2.7 wt %, 5.4 wt %, and 10.8 wt %) were employed. Thermal treatment in air at 623 K resulted in characteristic structural changes of the V oxide species. Independent of the V loading, the local structure around V centers in dehydrated VxOy/SBA-15 corresponded to an ordered arrangement of adjacent V2O7 units. Moreover, the V2O7 units were found to persist under selective oxidation reaction conditions.