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 properties of Fe-exchanged ferrierite were investigated by ab initio periodic DFT calculations. Stabilities of Al/Si substitutions were compared for all four irreducible tetrahedral (T) sites of the framework. For each T site, the most stable position of the extraframework Fe 2+ cation is located in the six-membered ring, in agreement with experimental data. Depending on the location of the framework Al/Si substitutions, differences in the total energies of the Fe-exchanged configurations can be as high as 200 kJ/mol. Simulated adsorption of NO shows that both ON-and NO-interactions with Fe 2+ are at least metastable. Adsorption through the N atom, however, is ∼2.5 times stronger. Two types of Fe-exchanged configurations were observed. Stable configurations with the cation located in a β site and exhibiting low adsorption energies of ∼180 kJ/mol were destabilized upon adsorption of NO. Less stable configurations, with the cation located in an R site and with higher adsorption energies of ∼240 kJ/mol, were stabilized upon adsorption. A strong interaction of NO with Fe 2+ can cause a migration of the extraframework cation to a new position in the zeolite framework. The interaction of NO with the Fe 2+ cation combines both σ and π bonding. σ bonding depletes the electron density in Fe d σ orbitals and leads to accumulation in the N p σ orbital. The π bonding causes an increase of the π-electron density on both the N and Fe atoms. Adsorption induces extensive changes in the electron density distribution within the NO molecule. An expansion of the N p σ density is accompanied by a depletion of the N p π density oriented toward the O atom. On the contrary, the O atom exhibits a depletion of the σ-electron density and an increase of the π-electron density. The complex polarization of the N-O bond leads to only a slight decrease of the bond length. Stretching frequencies calculated for configurations with different stabilities vary from 1866 to 1909 cm -1 . For several stable configurations, the calculated stretching frequency of 1876 cm -1 is in good agreement with the maximum of the IR band, and for most configurations, the frequency is within the width of the experimental band. Too-high frequencies calculated for two Al atoms in the small rigid ring indicate that no such configurations exist in ferrierite structures. The bonding of NO to the Fe 2+ cation is qualitatively different from the bonding on the surface of a transition metal characterized by the Blyholder scheme. The π back-donation on the surface leads to a weakening of the N-O bond and a downshift of the stretching frequency. In contrast, in the bonding of NO to a transition metal cation, the electron density accumulates within the newly formed N-Fe bond, proportionally depleting both the molecule and the cation. The withdrawing of antibonding electron density leads to a strengthening of the intramolecular bond and to higher N-O stretching frequencies.

Ab initio density functional calculations of the structural and electronic properties of V(2)O(5) bulk and its low-index surfaces are presented. For the bulk oxide and the (010) surface (the natural cleavage plane) a good agreement with experiment and with earlier ab initio calculations is found. For the first time, the investigations are extended to other low-index surfaces: (001) and (100). On both surfaces, termination conserving a bulk-like stoichiometry is preferred, but-in contrast to the (010) surface-a strong structural relaxation takes place. Relaxation reduces the surface energy from 1.16 to 0.48 J m(-2) for the (001) and from 0.61 to 0.55 J m(-2) for the (100) surface. Although the relaxed surface energies are still one order of magnitude higher than calculated for the (010) surface (0.047 J m(-2)), the Wulff construction demonstrates that (001) and (100) surfaces contribute about 15% of the total surface area of a V(2)O(5) crystallite, indicating a non-negligible role in the catalytic activity of V(2)O(5).

Cr/ZSM-5 catalysts with 0.5−2.6 wt % Cr loadings and zeolites with 15−140 Si/Al ratios were characterized with in situ UV−vis, IR, and Raman spectroscopies, including operando Raman measurements under reaction conditions in methane conversion at 773−1123 K with a simultaneous online analysis of reaction products. DFT calculations with cluster and periodic models were performed with DMol 3 , Gaussian 09, and VASP software packages. Isolated Cr(VI) dioxo and Cr(III) mono-oxo structures on framework Al anchoring sites were identified as the dominant species under most conditions. In the absence of gas-phase O 2 (under Ar flow) at 773 K, the Cr(VI) dioxo species on framework Al anchoring sites autoreduce, and the Cr(III) mono-oxo species remain the only observable surface Cr oxide structures. For ZSM-5 zeolites with a relatively low concentration of framework Al atoms (Si/Al ≥ 25), exposure to gas-phase O 2 at 773 K forces surface Cr oxide species to migrate from framework Al anchoring sites to Si sites on the external surface of the zeolite and form dioxo (Si− O−) 2 Cr(O) 2 and mono-oxo (Si−O−) 4 Cr(O) structures. The activity of Cr/ZSM-5 catalysts in methane conversion with the production of benzene and hydrogen as the main products is lower than that of Mo/ZSM-5 catalysts. The rate of benzene formation over Cr/ZSM-5 catalysts, however, is relatively stable with time on stream, in comparison to a rapidly declining rate over Mo/ZSM-5 catalysts. The zeolite-supported Cr species are highly mobile under the reaction conditions and can migrate between zeolite particles.