Combined experimental infrared (IR) and theoretical approaches have been carried out in an attempt to specify the actual structure of the CO 2 species adsorbed on the magnesium oxide surface. The interaction of CO 2 with regular sites of the MgO(100), (111) and (110) surfaces as well as MgO(100) defect sites (steps, corners, kinks and di-vacancies) has been investigated by mean of Density Functional Theory study. Theoretical IR frequencies compared with IR experiments show distinguishable carbonate species, adsorbed on different planes and defects, vibrating in different IR-frequency ranges. In addition, by mean of thermodynamic model, the stability of carbonates as a function of temperature have been calculated and compared to the experiment. Analyzing the nature of basic sites, the results show that the most active site versus CO 2 , which is a Lewis acid, is not the same that the strongest site for the deprotonating adsorption of Brønsted acids. The present work revisits and improves the understanding of carbonate species that could exist on the magnesium oxide surface and gives a picture of the accessible planes of magnesium oxide as well as their surface Lewis basicity.

International audienceCombined Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were performed to study the distribution of Pd atoms in bimetallic AuPd nanoparticles in the presence of adsorbed CO. Compared to vacuum condition, the results showed evidence of Pd surface enrichment where both Pd monomers and Pd dimers could exist. The energetic stability calculated for several alloy configurations evidenced the preference of Pd to occupy undercoordinated edge sites in the presence of CO gas. Moreover, the calculation of the vibrational frequencies of adsorbed CO for the first time allowed the fine assignment of the complex experimental DRIFTS bands of CO interacting with the bimetallic nanoparticles and their evolution with time exposure. Electronic structure analysis shows preponderant p-back-donation from under-coordinated Pd to CO inducing strong bonding on edge sites

The recent developments in biomass-derivative fuelled electrochemical converters for electricity or hydrogen production together with chemical electrosynthesis have been reviewed.

The Cr/SiO2 system is investigated using periodic DFT. The model represents the amorphous character of the silica surface and allows the investigation of the effect of hydration on the Cr(VI) monomers. First, the geometry and energetics are discussed and compared with experimental data. The phase diagram plotted from an atomistic thermodynamics model confirms the higher stability of mono-oxo and dioxo chromium, in comparison with species containing Cr–OH groups. In addition, the effect of the siloxane ring size on the spectroscopic signature of chromium is analyzed. A preliminary study is presented on the surface doping effect by Ti on the structure and stability of chromium species. The results reveal that the charge transfer process between Ti and Cr can explain the observed change in the reactivity of chromium species.

The reaction mechanism for nitrous oxide (N 2 O) direct decomposition into molecular nitrogen and oxygen was studied on binuclear iron sites in Fe-ZSM-5 zeolite using the density functional theory (DFT + complex for the N 2 O decomposition. The first step of the catalytic reaction corresponds to a spontaneous adsorption of N 2 O over Fe II sites, followed by the surface atomic oxygen loading and the release of molecular nitrogen. The formation of molecular O 2 occurs through the migration of the atomic oxygen from one iron site to another one followed by the recombination of two oxygen atoms and the desorption of molecular oxygen. The computed reactivity over the binuclear iron core complex [Fe+ is consistent with experimental data reported in the literature. Although the dissociation steps of the N 2 O molecules, calculated with respect to adsorbed N 2 O intermediates, are highly energetic, the energy barrier associated with the atomic oxygen migration is the highest one. Up to 700 K, the oxygen migration step has the highest free energy barrier, suggesting that it is the rate-limiting step of the overall kinetics. This result explains the absence of O 2 formation in experimental study of N 2 O decomposition at temperatures below 623 K.

Periodic DFT calculations have been performed on molybdenum(VI) oxide species supported on the hydroxylated amorphous silica surface. The Mo grafting site has been investigated systematically for the type of silanol (geminate, vicinal, isolated or in a nest) accessible on the surface, as well as its effect on H-bond formation and stabilization, with the Mo-oxide species. Different grafting geometries, combined with different degrees of hydration of the Mo species are investigated using atomistic thermodynamics. The most stable Mo(VI) oxide species resulting from these calculations are confronted with experiment. Finally, calculated vibrational frequencies confirm the experimental evidence of the dominant presence of di grafted di-oxo Mo(VI) species on silica up to 700 K.

Despite intensive research eorts, the nature of the active sites for O 2 and H 2 adsorption/dissociation by supported gold nanoparticles (NPs) is still an unresolved issue 1 in heterogeneous catalysis. This stems from the absence of a clear picture of the evolution of the structural properties of Au NPs in the presence of these gases at near reaction conditions, i.e. at high pressures and high temperatures. We hereby report on the rst real-space observation of the equilibrium shapes of TiO 2 -supported model Au NPs under O 2 and H 2 at atmospheric pressure using window gas cell transmission electron microscopy (GCTEM). In situ GCTEM observations show instantaneous changes in the equilibrium shape of Au NPs under O 2 during cooling from 400°C to room temperature. In comparison, no instant change in equilibrium shape is observed under H 2 environment. To interpret these experimental observations, the equilibrium shape of Au NPs under O 2 , atomic oxygen and H 2 gas environments was predicted using a multiscale structure reconstruction model. Excellent agreement between GCTEM observations and theoretical modelling under O 2 provides strong evidence for the molecular adsorption of O 2 on the Au NPs below 120°C. Molecular adsorption takes place on specic Au facets which are identied in this work. In the case of H 2 , theoretical modelling predicts weak interactions with gold atoms which explain their high morphological stability under this gas. This work provides atomic structural information for the fundamental understanding of the O 2 and H 2 adsorption properties of Au NPs under real working conditions and also shows a new way to identify the active sites of heterogeneous nanocatalysts under reaction conditions by monitoring the structure reconstruction.

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