Application of AXAFS Spectroscopy to Transition‐Metal Oxides: Influence of the Nearest and Next Nearest Neighbour Shells in Vanadium Oxides

Abstract

IntroductionMany parameters affect the catalytic properties of transition-metal oxide catalysts, for example, metal oxide loading, pre-treatment conditions, molecular structure, electronic structure, and the nature of the supporting oxide. [1][2][3][4][5][6][7] Various researchers have investigated the activities and selectivities of metal oxide catalysts. However, little is known about the parameters that are truly responsible for the catalytic action of transition-metal oxides.To study the effect of the support on the catalytic performance, one has to find a tool that is capable of probing the influence of the local environment (e.g., the support or promoters) on the properties of the catalytically active species, that is, a metal particle or metal oxide cluster. Metal-A C H T U N G T R E N N U N G (oxide)-support interactions can be ascribed to a structural (particle/cluster size and shape) effect, but can also have an electronic origin. For supported metal particles, the effect of the support on the catalytic properties has been determined by several techniques, for example, XPS, FT-IR analysis of adsorbed CO, and newly developed delta XANES and atomic XAFS (AXAFS) techniques. [8][9][10][11][12][13][14][15] In particular, the application of AXAFS has, in our opinion, led to a better understanding of metal-support interaction effects. [8,[15][16][17] AXAFS is sensitive to changes in the potential around the X-ray absorbing atom. The potential field around an atom is mainly determined by the first and second coordination spheres. O Grady et al. have shown for Pt/Ru alloys that the AXAFS intensity changes systematically with the composition of the alloy. [18] Thus, when the number of Ru atoms around a Pt atom increases, the intensity of the FT AXAFS peak increases and the peak centroid shifts to lower values of R. Comparison of these effects with the effects observed for a Pt electrode as a function of the applied voltage suggests that the first coordination shell of the Pt in the Pt/Ru alloy influences the electronic properties (inter-A C H T U N G T R E N N U N G atomic potential) of the Pt atom. [18,19] The effect of changes in the coordination around the absorber atom has been further illustrated for Pt by van Dorssen et al., who compared the AXAFS of a Pt foil with that of bulk Na 2 Pt(OH) 6 . The AXAFS intensity was found to be significantly higher in the Abstract: The influence of changes in coordination number, interatomic distances, and oxidation state on the intensity and centroid position of the Fourier transform (FT) of the atomic X-ray absorption fine structure (AXAFS) peak of vanadium oxide bulk model compounds and aluminasupported vanadium oxide clusters has been investigated. Using Na 3 VO 4 and V 2 O 5 as model compounds, it has been shown that the nearest neighbour shells have a pronounced influence on the AXAFS intensity; specifically, a 40 % decrease in intensity was observed between these two compounds. Secondly, the influence of partial reduction of the vanadium oxide species has been dete...

A combinedin situXAS/Raman and WAXS study on nanoparticulate V2O5in zeolites ZSM-5 and Y

“…Fitting one average oxygen shell for this sample gave an unsatisfactory result from the refinements ( Table 1). The bond distances from the EXAFS refinements show a consistency with previous studies on bulk V 2 O 5 as adapted from Keller et al [16]. In comparison, zeolite Y shows a major broad peak in the Fourier Transform (Figure 5), suggesting a more symmetric oxygen environment around vanadium.…”

Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework

“…7G and H), indicating that these features are the result of artifacts in the background. [138][139][140][141] The spectra are most effectively modeled using bulk pathways alone. The lack of such a contribution suggests that the ThO 2 and UO 2 NPs were not strongly bound to the COF-5 host, and that any interactions must have been weak (e.g., van der Waals) and/or with low coverage.…”

Pentane, Hexane, Cyclopentane, Cyclohexane, 1-Hexene, 1-Pentene,cis-2-Pentene, Cyclohexene, and Cyclopentene at Vapor/α-Alumina and Liquid/α-Alumina Interfaces Studied by Broadband Sum Frequency Generation

“…One of the major recent advances in the field is that it is now understood that the molecular orientations of the adsorbed organic species are critical for controlling interfacial reactivity and kinetics in heterogeneous catalysis. Although much work has been carried out on understanding reactivity and kinetics on metal substrates relevant to catalysis, − vibrational studies focusing on the interaction of catalytically relevant oxides − with hydrocarbons are more sparse − and tend not to focus on determining the molecular orientations of the hydrocarbon species at the catalytically relevant surface unless they are computational. , Bridging this knowledge gap could improve many research fields, including catalyst performance and design, − chromatographic separations, , environmental chemistry, − and tribology. − …”