Christophe Dujardin

Parallel between infrared characterisation and ab initio calculations of CO adsorption on sulphided Mo catalysts

Abstract Carbon monoxide adsorption on sulphided Mo catalysts has been investigated by means of IR spectroscopy and DFT ab initio calculations. IR experiments show that CO adsorption on the sulphide phase of Mo/Al2O3 catalysts gives rise to various ν(CO) bands, the intensities of which are strongly modified when post-treatment of the catalyst with H2 or H2S is performed before CO adsorption, therefore, revealing strong modifications in the nature and the number of sites present on the sulphide phase. Ab initio periodic DFT calculations allow to define two types of edges for MoS2, which sulphur coverage and structure depend on the H2/H2S ratio in the surrounding atmosphere. Adsorption energies and stretching wavenumber of CO adsorbed on the various sites of these surfaces were computed, providing the possibility to compare for the first time results from theoretical calculations and spectroscopic measurements on these systems. A novel attribution of the main IR features of CO adsorbed on MoS2 is proposed.

A simple and reproducible method for the synthesis of silica-supported rhodium nanoparticles and their investigation in the hydrogenation of aromatic compounds

Colloidal suspensions of rhodium nanoparticles have been easily prepared in aqueous solution by chemical reduction of the precursor RhCl3·3H2O in the presence of the surfactant N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium chloride (HEA16Cl) and further used to immobilize rhodium nanoparticles on silica by simple impregnation. The obtained silica-supported rhodium nanoparticles have been investigated by adapted characterization methods such as transmission electron microscopy and X-ray photoelectron spectroscopy. A particle size increase from 2.4 to 5 nm after the silica immobilization step and total elimination of the surfactant has been observed. This “heterogeneous” catalyst displayed good activities for the hydrogenation of mono-, di- alkylsubstituted and/or functionalized aromatic derivatives in water under atmospheric hydrogen pressure and at room temperature. In all cases, the catalyst could be recovered several times after a simple decantation or filtration and reused without any significant loss in catalytic activity. This supported catalyst has also been tested under higher hydrogen pressure giving rise to TOFs reaching 6430 h−1 at 30 bar and in terms of catalytic lifetime 30 000 TTO in 8.5 h for pure anisole hydrogenation at 40 bar.

A new experimental cell for in situ and operando X‐ray absorption measurements in heterogeneous catalysis

A new X-ray absorption cell dedicated to in situ and operando experiments in heterogeneous catalysis has been built and tested. The cell consists of several boron nitride and stainless steel plates linked together using graphite seals. It allows the measurement of XANES and EXAFS spectra of heterogeneous catalysts within a wide range of photon energies in transmission mode under the flow of various oxidative and reductive gas mixtures at elevated temperatures. The cell is compact and easy to build. Catalysts are loaded into the cell as powders. The use of boron nitride and a small beam pathlength in the cell result in a low absorption of the X-ray beam at lower energies. The cell was tested by in situ characterizing cobalt species during oxidative and reductive pre-treatments of a silica-supported Fischer-Tropsch catalyst. An operando study of methanol conversion over alumina-supported molybdenum catalysts was also carried out.

Catalytic Activity and Thermal Stability of LaFe1−xCuxO3 and La2CuO4 Perovskite Solids in Three-Way-Catalysis

LaFe1−xCuxO3 and La2CuO4 catalysts are investigated for three-way-catalysis application. LaFe1−xCuxO3 solids present interesting catalytic properties mainly with respect to hydrocarbon and CO oxidation and NO reduction in rich conditions with high N2 selectivity. The partial substitution of iron with copper leads to enhanced catalytic activity for oxidation reactions. This can be attributed to the higher reducibility of the solid as evidenced earlier by H2-temperature-programmed reduction. The orthorhombic structure associated with LaFe1−xCuxO3 solids (x up to 0.2) revealed good thermal stability during catalytic cycles in stoichiometric, lean and rich conditions whereas the Ruddlesden Popper phase associated with La2CuO4 completely decomposed in rich conditions.

Chapter 10 The formation of N2O during sNOX conversion: fundamental approach and practical developments

Abstract Presently, a growing interest is focused on unregulated emissions of nitrous oxide (N 2 O) from stationary and mobile sources in order to anticipate future restrictive legislations, since N 2 O exhibits a significant higher global warming power than that of CO 2 . The adoption of end-of-pipe technologies is appropriate. However, the simultaneous conversion of NO x and N 2 O over catalytic processes is still challenging both for industrial plants and automotive exhaust gases particularly at low temperature. Subsequent selectivity enhancements towards the formation of N 2 probably need better insights into the mechanisms involved in the formation and the subsequent conversion of N 2 O during the overall reduction of NO x , particularly in O 2 excess. Up to now practical solutions for mobile sources imply the use of noble metals. Conventional three-way catalysts (TWC) running under stoichiometric conditions are wide-spread through the world even if their efficiency is still restricted during the cold start engine with a substantial formation of undesired N-containing products such as N 2 O. Nowadays, the use of noble metals in TWC, particularly Rh, becomes more and more questionable with the continuous development of lean-burn engines because of their poor efficiency to convert NO x into nitrogen in those running conditions. In such a circumstance, there is a particular interest in developing non-noble metal-based catalysts and also the use of additives and alternative reducing agents. By way of illustration, hydrogen could be an interesting issue for both stationary and mobile sources for the reduction of NO x emissions at low temperature under lean conditions. This paper will discuss on such an opportunity and the correlative development of novel catalysts.

In situ characterization by Raman and IR vibrational spectroscopies on a single instrument: DeNOx reaction over a Pd/?-Al2O3 catalystPresented at the International Congress on Operando Spectroscopy, Lunteren, The Netherlands, March 2?6, 2003.

For the first time, in situ Raman and FTIR measurements have been carried out on a single bench-top instrument to characterize an alumina-supported palladium during the catalytic DeNOx reaction. While IR spectroscopy gives access to dedicated information on the NOx adsorbed species, Raman spectroscopy is highly sensitive to the molecular structure modifications of the active phase and allows us to characterize the adsorbed species as well as their bonds with the support. The ability to monitor the very same reaction via both techniques in a quasi-simultaneous approach provides full and complementary vibrational information giving a better insight into the catalytic reaction.