Laurence Pirault-Roy

Hydrogen Peroxide Decomposition on Various Supported Catalysts Effect of Stabilizers

The decomposition of hydrogen peroxide (H 2 O 2 ) has been studied on various catalysts (platinum supported on silica; silver, iridium, platinum-tin or manganese oxides supported on alumina). The experiments were performed using two reactors: 1) a conventional constant pressure reactor for the determination of the volume increase vs time using diluted H 2 O 2 solutions; 2) a constant volume reactor to measure the pressure increase using more concentrated solutions. The first reactor leads to the determination of the kinetic order of the reaction, to the comparison of the activities of the different samples, and to the characterization of the influence of some stabilizers of H 2 O 2 solutions on the catalytic activity. Two kinetic orders were found, depending on the catalyst: a zero order and a first order. The shape of the catalysts samples is an important parameter, with powders always being more reactive than grains and pellets. The catalyst activities are sorted as follows: Pt-Sn/Al 2 O 3 < Ir/Al 2 O 3 < Pt/SiO 2 < MnO x /Al 2 O 3 < Ag/Al 2 O 3 . The presence of pyrophosphate stabilizer leads to a loss of activity mainly as a result of passivation in the case of MnO x -supported samples, whereas the presence of stannate increases slightly the activity of silver and displays no influence on manganese samples.

Methylcyclopentane reactions on Rh Ge/Al2O3 catalysts prepared by controlled surface reaction

Abstract The reaction mechanism of methylcyclopentane (MCP) ring-opening on Rh catalysts (the participation of each intermediate in further hydrogenolysis versus their desorption) was studied on a special series of Rh(Ge)/Al 2 O 3 catalysts. These were obtained by wet impregnation and adding different amounts of Ge by anchoring of Ge( n -C 4 H 9 ) 4 on the surface of Rh with preadsorbed hydrogen. As shown earlier [Appl. Catal. A: Gen. 245 (2003) 15], low amounts of Ge were deposited selectively on low-Miller-index microfacets, whereas excess use of Ge( n -C 4 H 9 ) 4 caused statistical deposition on Rh. This difference induced also alterations in the classical ring-opening pattern: the sample with randomly located Ge behaved like the Parent catalyst with dispersion of 80%. Each ring-opening intermediate hydrogenolyzed further nearly to the same extent; the ring-opening product distribution (ROPD) showing thus no variation as a function of reaction conditions. The catalyst with selective Ge deposition followed, however, the pattern of a sintered sample: the surface intermediate of 2-methylpentane underwent preferential hydrogenolysis to smaller fragments. Thus, changing the position of Ge deposits (without modifying the particle size) induced changes in the prevailing reaction route.

A new approach of selective Ge deposition for RhGe/Al2O3 catalysts: Characterization and testing in 2,2,3-trimethylbutane hydrogenolysis

Abstract A series of Rh monometallic and RhGe/Al 2 O 3 catalysts was prepared in a dynamic reactor by anchoring Ge( n -C 4 H 9 ) 4 on Rh surface, containing preadsorbed hydrogen and by subsequent hydrogenolysis of the surface organometallic complexes. Different amounts of organometallic Ge compound were introduced, corresponding nominally to 1/20, 1/2, 2 Ge monolayers. The samples were characterized by FTIR of CO, transmission electron microscopy (TEM), hydrogen chemisorption and hydrogenolysis of 2,2,3-trimethylbutane (223TMB) as a test reaction. Hydrogen chemisorption and TEM results are consistent with (i) the effect of preparation procedure on the morphology of the monometallic sample, (ii) the surface deposition of Ge on Rh particles. Concerning the effect of preparation, the grafting procedure led for blank experiments (Ge-free catalysts) to a slight sintering which can be avoided by Ge anchoring. On the other hand, we used the relative normalized intensities of linear and bridged CO species of FTIR measurements to identify the location of Ge deposits. The amount of the Ge compound introduced had a pronounced effect on the final landing position of Ge. Excess Ge( n -C 4 H 9 ) 4 resulted in a statistical deposition on the surface of Rh or a slight diffusion in Rh lattice, while low-Miller-index microfacets were preferentially occupied when the amount of tetra- n -butylgermanium was markedly lower than that for monolayer coverage. The presence of quaternary carbon atom in the reactant molecule (223TMB) induced changes in the chemisorption geometry deduced from the shift of maximum activity towards lower p (H 2 ). In the wide negative hydrogen order region breaking of CC bonds can be the slowest step. Low-Miller-index microfacets of Rh particles may be active in multiple CC bond cleavage during one residence time of the reactant molecule.

Modelling of the metallic phases of different Pt-Rh/Al2O3-CeO2 catalysts : influence of the rhodium loading and nature of the metallic precursors

Abstract The activities for propane–propene mixture oxidation under lean conditions were found to be strongly different in the case of three-way Pt–Rh/Al2O3–CeO2 catalysts prepared either by coimpregnation of the metallic salts (CI catalysts) or by an original method of successive impregnations with an intermediary reducing treatment (SI catalysts). Based upon the analysis of results of catalytic tests and physical characterisations including EXAFS at the Pt edge, it was shown previously that such a different behaviour for propane oxidation could easily be explained. The formation of clusters of inactive Pt–Rh alloys of a rather homogeneous composition was suggested for CI catalysts while active bimetallic catalysts obtained by the SI preparation procedure were modelled by Pt and Rh clusters more or less mixed together. To achieve the complete description of the metallic species (especially for SI catalysts), the influence of rhodium loading and the nature of the metallic precursors were examined by studying the different catalysts by EXAFS mainly at the Pt edge but also at the Rh edge.

Oxygen Storage Capacity of Pt–CeO2 and Pt–Ce0.5Zr0.5O2 Catalysts

Cerium dioxide containing materials have involved much interest in recent years owing to their broad range of applications in various fields. The most important property that makes ceria as an excellent catalytic material is its oxygen storage and release capacity via the ability of ceria to rapidly switch from Ce3+ to Ce4+ under reducing and oxidizing conditions, respectively. Zirconia is incorporated into the ceria matrix as it greatly improves thermal stability and oxygen diffusion through the bulk of the crystallites.

Study of effect of chromium on titanium dioxide phase transformation

MTiX samples with different atomic chromium percentages were synthesized by sol–gel method and calcined at 400 °C under air. The effects of Cr and temperature on titanium dioxide phase transition were studied. In situ measurement showed the presence of anatase phase for all samples at temperature < 500 °C. Without Cr content, the anatase–rutile transition takes place at 600 °C and the rutile fraction increases with increase of temperature. In the presence of Cr content, rutile phase appeared at 700 °C. Cr2O3 phase was shown only in the case of CrTi20 content at 800 °C which indicates that the segregation remains modest. We have also studied the anatase–rutile transition kinetics by using in situ X-ray measurements. It was found that the anatase phase stability increases as the chromium content increases. Results confirm that the transformation of anatase–rutile is of first order.

The Influence of the Nature of the Support on the Copper–Palladium Catalysed Suzuki–Miyaura-Coupling

Copper–palladium bimetallic catalysts supported on mixed oxides (MgO)0.75(Al2O3)0.25 (MgAlO) were prepared with two step impregnation (TSI) and co-impregnation (CI). These materials were studied to determine the effect of the support on the activity and stability in the Suzuki–Miyaura reaction and on the properties of a catalyst. The catalyst prepared by CI was active and selective during 6 catalytic cycles in Suzuki coupling, whereas the activity of the catalyst prepared with TSI dropped at the sixth use. The kinetics of the reaction was examined along with its scope. The relationship between the basic properties of the support and the catalytic performances were examined too. The importance of the nature of the support was also studied by catalyst characterization methods.Graphical Abstract

Palladium, Iridium, and Rhodium Supported Catalysts: Predictive H2 Chemisorption by Statistical Cuboctahedron Clusters Model

Chemisorption of hydrogen on metallic particles is often used to estimate the metal dispersion (D), the metal particle size (d), and the metallic specific surface area (SM), currently assuming a stoichiometry of one hydrogen atom H adsorbed per surface metal atom M. This assumption leads to a large error when estimating D, d, and SM, and a rigorous method is needed to tackle this problem. A model describing the statistics of the metal surface atom and site distribution on perfect cuboctahedron clusters, already developed for Pt, is applied to Pd, Ir, and Rh, using the density functional theory (DFT) calculation of the literature to determine the most favorable adsorption sites for each metal. The model predicts the H/M values for each metal, in the range 0–1.08 for Pd, 0–2.77 for Ir, and 0–2.31 for Rh, depending on the particle size, clearly showing that the hypothesis of H/M = 1 is not always confirmed. A set of equations is then given for precisely calculating D, d, and SM for each metal directly from the H chemisorption results determined experimentally, without any assumption about the H/M stoichiometry. This methodology provides a powerful tool for accurate determination of metal dispersion, metal particle size, and metallic specific surface area from chemisorption experiments.

Influence of rhodium content on the behavior of Rh/SiO2–Al2O3 catalysts for selective ring opening of decalin

The influence of the Rh content of catalysts supported on SiO2–Al2O3 (SIRAL 70 and 80, with 70 and 80 wt% of SiO2) on the activity and selectivity for selective ring opening of decalin was studied at 325 and 350 °C. An optimum metal content was found at 1.5 wt% of rhodium. Both supports present moderate and strong acid sites. For the selective opening of decalin, the best metal/support combination was found with Rh(1.5)/SIRAL 70 balancing the support acidity together with the hydrogenolytic activity of the metal. A linear correlation was established between the amount of coke deposited and the formation of dehydrogenated compounds.

Study of the Effect of Transition Metals on Titanium Dioxide Phase Transformation

MTiX samples with different atomic metal percentage were synthesised by sol-gel method and calcined at 400 °C under air. The anatase-rutile transformation in TiO2 in the presence of transition metals (Cr, V and Mn) was investigated. The kinetics of anatase-rutile transformations were determined by XRD over the temperature range 500–800 °C. It was found that the presence of V and Mn accelerate the transformation anatase-to-rutile. However, the anatase phase stability increases as the chromium content increases.