Structure of Monomeric Chromium(VI) Oxide Species Supported on Silica: Periodic and Cluster DFT Studies

Abstract

International audienceSilica-supported chromium oxide systems are efficient catalysts for many important chemical processes. Despite many years of investigations, the structure of the surface Cr species is not unambiguously determined. In this work, comprehensive DFT investigations of the monomeric Cr(VI) oxide species on silica under dehydrated conditions are performed. A large number of advanced periodic and cluster models of the SiO2 surface, based on the beta-cristobalite structure and different amorphous structures, have been applied. The calculated relative energies of the rnonooxo and dioxo Cr(VI) species depend on their location on the surface and on the structure of the model. It is concluded that the dioxo Cr(VI) species are thermodynamically preferred, but the presence of the monoxo Cr(VI) species, being in minority, cannot be excluded. According to the vibrational frequency analysis, the asymmetric O=Cr=O stretching mode for the dioxo species and the Cr=O stretching mode for the monooxo species can overlap

The Nature of Surface CrOx Sites on SiO2 in Different Environments

“…Deconvolution of the H 2 -TPR spectrum of the supported 3 % CrO x /SiO 2 catalyst found that the surface chromia phase consists of *2/3 surface dioxo sites and *1/3 surface mono-oxo sites (exact dioxo:mono-oxo ratio *1.7). These findings are in agreement with the DFT calculations by Handzlik et al that predicted that dioxo surface (O=) 2 CrO 2 sites should be much more stable than monooxo surface O=CrO 4 sites on the SiO 2 support [19]. The H 2 reduction kinetics for the surface dioxo and mono-oxo chromia sites on silica are also provided from the peak temperatures (Tp) during the H 2 -TPR experiments by application of the Redhead equation for first-order reactions [34,35].…”

“…The corresponding Raman spectrum, with 442 nm laser excitation, exhibited bands at 987 (s) and 1014 (m) cm -1 , and the selective reduction of the 987 cm -1 band by H 2 showed that both bands originate from two distinct surface CrO x sites on silica [14]. Corresponding isotopic 18 [19]. This suggests that the surface dioxo CrO 4 species should have a higher concentration than the surface mono-oxo CrO 5 species on the silica support (as will be experimentally shown below with H 2 -TPSR).…”

Microporous titania–silica nanocomposite catalyst-adsorbent for ultra-deep oxidative desulfurization

“…It is believed that anatase titania in the equilibrium morphology displays primarily the (101) surface [71]. Amorphous silica has often been modeled as a surface of crystalline β-cristobalite [72,73]. Here we used (001) surface of β-cristobalite, which has shown to resemble amorphous silica very closely [73].…”

“…Amorphous silica has often been modeled as a surface of crystalline β-cristobalite [72,73]. Here we used (001) surface of β-cristobalite, which has shown to resemble amorphous silica very closely [73]. The surfaces were considered to be dehydrated corresponding to the high calcination temperature of the samples.…”

Theoretical identification of structural heterogeneities of divalent nickel active sites in NiMCM-41 nanoporous catalysts

“…1). For the comparisons made here, it will be assumed [78] that the dispersion of the Ni 2? species at the defect sites of MCM-41 is controlled by their thermodynamic stability and not by any kinetic phenomenon in the preparation method.…”