Jean Massardier

Crotonaldehyde and methylcrotonaldehyde hydrogenation over Pt(111) and Pt80Fe20(111) single crystals

Abstract Hydrogenation reactions of crotonaldehyde and methylcrotonaldehyde have been carried out on Pt(111) and Pt 80 Fe 20 (111) single crystals. The reactions have been performed in the gaseous phase with a large excess of hydrogen. A noticeable enhancement of activity has been observed by alloying the platinum with iron. The additional methyl group present in the methylcrotonaldehyde greatly increases the selectivity toward formation of the unsaturated alcohol, while only a slight increase of such selectivity is induced by alloying. The results have been compared with the previous experimental results on crotonaldehyde hydrogenation on Pt/SiO 2 catalysts and cinnamaldehyde hydrogenation on Pt and Pt 80 Fe 20 particles supported on carbon.

PtMo bimetallic catalysts supported on Y-zeolite: I. Characterization of the catalysts

Bimetallic PtMoY catalysts have been prepared by adsorption and decomposition of Mo(CO)6 in a Y-type zeolite containing 10-A Pt particles trapped in the zeolite cages. Part of the Mo atoms are deposited onto the Pt particles which produces a weak increase of the particle size measured by CTEM and STEM and a progressive decrease of the amount of H2 or CO chemisorbed on platinum. At high Mo contents these chemisorptive properties are completely masked. It was checked by XPS, X-ray absorption edge spectroscopy, and ir spectroscopy with CO and NO as probe molecules that the Pt particles are initially electron deficient but the electronic structure becomes similar to that of bulk platinum once the Mo atoms were added. This is probably because the Pt atoms are then shielded from the electron acceptor sites of the support. The Mo atoms bonded to Pt are probably in a low valence state, but there is a fraction of Mo not bonded to Pt, especially in the zeolite outer layers of Mo-rich samples, which is oxidized to MoIV-MoVI by oxido-reduction reactions with the zeolite protons.

Synergistic alloying behaviour of Pd50Cu50 single crystals upon adsorption and co-adsorption of CO and NO

Abstract We have studied the surface reactivity of the Pd 50 Cu 50 (111) and (110) single crystals in the adsorption and the co-adsorption of CO and NO at low temperature. The bond strengths of CO and No on Pd and Cu in the alloy are significantly different from those observed on pure metals. Carbon monoxide and nitric oxide are more tightly bound to Cu and less tightly bound to Pd than to the pure components. The calculated NO coverages on PdCu samples are approximately four times lower than for CO due to the dissociation of a fraction of NO on the surfaces. However, the CO or NO adsorption did not suggest an important difference of reactivity between the two surfaces studied. In the co-adsorption experiments, the total surface coverage of CO and NO on Pd 50 Cu 50 (111) and (110) appears quite dependent on the order in which the gases are adsorbed in the successive adsorptions. The pre-saturation with NO leads to molecular adsorbed states and irreversible dissociated species which are not removed upon an exposure to CO. The subsequent CO co-adsorption induced only a slight replacement of molecular NO by CO and the amount of adsorbed CO is quite low ( θ CO =0.06 ML) as compared to the coverage measured with CO alone ( θ CO =0.40 ML). The pre-saturation with CO gives a different behaviour, the subsequent co-adsorption of NO leads to the replacement of CO by NO up to the saturation coverage found for NO alone ( θ NO =0.10 ML). Finally, when NO is adsorbed on a partially CO covered surface, NO displaces CO up to around 0.15 ML but the total amount of surface species (0.30 ML) is lower than the amount (0.55 ML) of pre-adsorbed CO, indicating a strong displacement of CO by NO. The competitive adsorption of CO and NO on PdCu surfaces is discussed in terms of dissociation of NO at low temperature which partially inhibits the further adsorption of CO on the PdCu alloy surfaces.

New supported palladium-chromium catalysts: characterization and catalytic properties

New silica-supported Pd-Cr catalysts have been prepared either by using mixed inorganic salts of the two metals as precursors or through the deposition of Cr(CO)6 on a monometallic palladium catalyst. Analytic electron microscopy shows that palladium-enriched particles are formed. The EXAFS experiments show that when the reduction temperature reaches 870 K, part of the chromium is reduced and interacts with palladium. In the partial hydrogenation of 1,3-butadiene, these catalysts are less active but 100% selective for the formation of butenes. The explanation, proposed on the basis of hydrogen chemisorption and confirmed by XPS data, is a modification of the electronic structure of palladium, leading to a variation of the adsorption coefficients of the diene and alkenes, compared to the values measured on monometallic palladium catalysts.

Pt-Mo bimetallic catalysts supported on Y-zeolite: II. Activity and selectivity in n-butane conversion

Pt-Mo bimetallic catalysts prepared by adsorption and decomposition of Mo(CO)6 on 1-nm Pt aggregates supported on Y-zeolite exhibit enhanced hydrogenolysis activity with respect to PtY zeolite in n-butane conversion. The activity curve vs PtMo composition is volcano-shaped with a maximum near equiatomic composition. The rate at maximum is 7 and 34 times larger than on PtNaHY and PtSiO2, respectively. The reaction orders with respect to n-C4 and H2 are positive in a large range of pressures. The kinetic parameters are interpreted in terms of the participation of both Pt and Mo atoms in the reaction mechanism. Mo atoms act primarily as strong adsorption sites for the hydrocarbon whereas hydrogen atoms dissociated on Pt are required to hydrogenate the hydrocarbon fragments.

ESR study of deep-bed calcined NH4Y and aluminum deficient zeolites

Abstract Electron spin resonance and irradiation by γ-rays at 77 °K or at room temperature were used to study the removal of alumina from the Y type zeolite framework by heat treatment in a deep bed geometry. X and Q band ESR spectra were recorded at 77 °K and showed respectively 12 and 16 lines corresponding to an isotropic hyperfine interaction with two aluminum atoms (I = 5 2 ) (c ∥∼- c ⊥ ∼- a iso Al = 10.0 Oe ) , the eleven hyperfine lines being split by a uniaxial symmetry of the g tensor ( g ∥ = 2.0125, g ⊥ = 2.0030, and g iso = 2.0062). The paramagnetic center was identified with a V center, namely a hole of an electron trapped on an oxygen atom bonding two nearby aluminum atoms. This center is localized on alumina which has been removed from the zeolite framework. The removal of alumina occurred only when the samples were heated above 400 °C in the presence of both water vapor and ammonia. It is suggested that the mechanism of migration involves the formation of aluminic acid in a dimer form and a reorganization of the lattice leaving no aluminum atom vacancies. A special emphasis is placed on the heat treatment conditions.

Evolution of adsorbed species during C2H2 adsorption on Ni(111) in relation to their vibrational spectra

Acetylene adsorption at room temperature on Ni (111) has been studied by low energy electron diffraction (l.e.e.d.), thermal desorption, work function changes and high resolution electron energy loss spectroscopy (h.r.e.l.s.). The existence of two adsorbed states has been demonstrated depending upon exposure.At low exposure, C2H2 is associatively adsorbed but a large rehybridization occurs, characterized by a carbon–carbon bond order close to 1.15. According to its hybridization state C2H2 should be adsorbed on a triangular site through two σ bonds and one π bond with nickel atoms.With increasing exposure, large modifications occur. In addition to self-hydrogenated species, benzene should be formed as shown by comparison of the vibrational spectra of adsorbed C2H2 and adsorbed C6H6. This conclusion is in agreement with the benzene formation as detected after contact between C2H2 and Ni/SiO2 catalysts with (111) exposed faces.

Y-type zeolites. Their heterogeneous nature as shown by electron spin resonance following γ-irradiation

The e.s.r. spectra of Y zeolites containing different exchangeable cations have been recorded following γ-irradiation at 77 K. The samples varied in their cation, proton and Al contents. The spectra were assigned to V centre type defects and were separated into two types: [graphic omitted] the unpaired electron spin being localized in a lone pair p orbital. The concentration in these centres follows an equilibrium of the type: [graphic omitted] the electron being trapped either by H+ ions, if they are present, or in the insulator material. The stability of the V centre is strongly dependent on the presence of electron scavengers such as H+.Discussion of the e.s.r. spectra leads to the conclusion that there is a random distribution of g3 and to a lesser extent of g2 components of the g tensor showing that some fluctuations occur in T—O—Si (T = Si or Al) bond angles and T—O bond lengths. This heterogeneity of the material closely corresponds to its heterogeneity in acid strength and catalytic activity.It is postulated and evidence is given that the paramagnetic defects correspond to T—O—Si groups which constitute the zeolitic framework itself, i.e., that the method proposed allows the solid itself to be studied. This conclusion has been applied to the study of aluminium deficient zeolites. It is concluded that silica migrates towards aluminium vacancies during aluminium removal, rebuilding the framework, and that heterogeneity of the material decreases during removal of aluminium.

Sulphur resistant palladium-platinum catalysts prepared from mixed acetylacetonates

Well-defined mixed acetylacetonates precursors were synthesised on a silica support and identified by their X-ray differaction spectra. After a calcination treatment to decompose the ligands and a reduction in hydrogen, bimetallic particles of 3.5 to 5 nm have been obtained. Analytical microscopy shows that these catalysts have a perfectly homogeneous composition. Low energy ion scattering experiments show that the surface of the particles is palladium enriched, as expected from Monte Carlo simulations. However, in the simple hydrogenation of cyclooctadiene to cyclooctene, where Pd is two orders of magnitude more active than Pt, it is observed that the rate is not proportional to the Pd surface concentration. This is interpreted as an ensemble effect on the chemisorption of the reactant. A study of the hydrogenation of tetralin in the presence of H 2 S shows a maximum of sulphur resistance for the Pd 20 Pt 80 atomic composition.

Propylene Selective Oxidation as Studied by Oxygen-18 Labelling on Well-Denned MoO3 Catalysts

The propylene oxidation reaction has been studied using 18O2 on well defined M0O3 crystallites. Results show that the formation of products -acrolein and carbon dioxide-involves the lattice oxygen. The different 18O-labelling, according to selective or total oxidation products, agrees with the structure sensitivity of this reaction. Post reaction analysis (by LRS, SIMS, LEIS) support a redox equilibrium which determines the 18O concentration in the surface layers.