The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)-alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalyst functions: protonation of propene to surface Mo(VI)-isopropoxide species driven by surface Brønsted acid sites, subsequent oxidation of isopropoxide to acetone in the adsorbed state owing to the red-ox capability of molybdenum leaving naked Mo(IV) sites after desorption of acetone, and oxidative addition of another propene molecule yielding finally the active Mo(VI)-alkylidene species. This view is quite different from the one-step mechanism, which has been accepted in the community for three decades, however, fully consistent with the empirically recognized importance of acidity, reducibility, and strict dehydration of the catalyst. The knowledge acquired in the present work has been successfully implemented for catalyst improvement. Simple heat treatment after the initial propene adsorption doubled the catalytic activity by accelerating the oxidation and desorption-capturing steps, demonstrating the merit of knowledge-based strategies in heterogeneous catalysis. Molecular structure of active Mo(VI)-alkylidene sites derived from surface molybdena is discussed in the context of similarity to the highly active Schrock-type homogeneous catalysts.

The influence of water on the dispersion and structure of silica SBA-15-supported vanadia model catalysts has been studied using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, which were combined within one experimental setup, as well as UV-vis diffuse reflectance spectroscopy. By performing timedependent XPS experiments, the influence of UHV/X-ray radiation could be eliminated by extrapolation of the observed temporal changes to t ) 0. XPS characterization reveals that the V2p 3/2 emission consists of two contributions, which are assigned to vanadia with distinctly different cluster size. Dehydration by treatment in oxygen flow at elevated temperatures leads to a significant increase in total intensity and a substantial redistribution of spectral weight to higher binding energies as a result of an increase in the vanadia dispersion. The V/Si XPS intensity ratio of the dehydrated samples closely follows the corresponding bulk ratio over the whole range of vanadium loadings (0-22 wt % V) studied. This observation renders possible a correlation of XPS and Raman results allowing for quantification of Raman features such as the relative cross sections of the vanadyl surface species. It is shown that the observed changes in vanadia dispersion are directly associated with the changes in the molecular structure of the surface vanadia species.

The structure of molybdenum oxide supported by silica SBA-15 has been studied by visible Raman spectroscopy, diffuse reflectance UV-Vis spectroscopy and X-ray absorption spectroscopy in the dehydrated state obtained after thermal treatment at elevated temperatures (≥350°C). No dependence of the molybdenum oxide structure on preparation procedure or loading has been observed within the range of loadings studied in detail (0.2 to 0.8 Mo/nm 2 ). X-ray absorption spectroscopy (XAS) reveals that the dehydrated state consists of a mixture of monomeric and connected molybdenum oxide centres. While the presence of crystalline MoO3 can be excluded by Raman spectroscopy, tetrahedrally and octahedrally coordinated MoO4 and MoO6 units are identified by XAS. The MoO6 units possess connectivity similar to that of MoO3 building blocks, whereas the MoO4 units are isolated or connected to other MoxOy units. These results are supported by UV-Vis spectra showing intensity at wavelengths (>300 nm) typical for dimeric and/or oligomeric species.

The structures of different titania, vanadia, and vanadia–titania clusters located on the surface of mesoporous silica SBA-15 are optimized using density functional theory (DFT). The apparent absorption spectra arising from these clusters are calculated with the aid of the ORCA package. The silica support is shown to contribute to the absorption spectra at wavelengths much shorter than those observed for vanadia and titania clusters located on the SBA-15 surface. The comparison of calculated and experimental apparent absorption spectra of supported vanadia and titania catalysts reveals that titania species generally show a higher nuclearity compared to vanadia species at similar low loadings. SBA-15 based catalysts loaded with both vanadia and titania are supposed to contain two types of species: species in which the V ions are anchored to the titania ones and those in which V and Ti ions alternate and are mainly coupled to the support through M–O–Si (M = V, Ti) bridges. The latter provide the major contribution to the apparent absorption spectra at not very high Ti loadings

Dispersed vanadium oxide samples were prepared on the basis of two differently structured high surface area silica materials (Aerosil 300, SBA-15). For each support material incipient wetness impregnation and a grafting/ion exchange procedure were applied to prepare catalyst samples with comparable vanadium density. The influence of the silica support material and preparation method on the vanadium oxide structure and dispersion has been studied using diffuse reflectance UV-Vis spectroscopy, visible Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). By independent spectroscopic characterization a fully consistent picture regarding the relation of the structure (UV-Vis, Raman) and dispersion (XPS) is developed. Based on the present structural data and recent findings the dispersed vanadium oxide is proposed to consist of monomers and oligomers with a distorted tetrahedral (VO 4 ) structure containing one short VQO bond. A different degree of hydroxylation of vanadium gives rise to two VQO stretching bands at 1027 and 1040 cm À1. The structure and dispersion of vanadium oxide are more strongly influenced by the support material than by the synthesis method. In this regard the Aerosil 300 samples show a higher degree of oligomerization, i.e. less dispersion of the surface vanadium oxide species, than the SBA-15 samples. Likewise incipient wetness impregnation leads to more oligomerized species than grafting/ion-exchange.