J. Barbier

Reactivity of steam in exhaust gas catalysis I. Steam and oxygen/steam conversions of carbon monoxide and of propane over PtRh catalysts

Pt (1 wt.-%), Rh (0.2 wt.-%) and Pt (1%) Rh (0.2%) catalysts supported on A2O3 (A) and on 12 wt.-% CeO2/Al2O3 (CeA) were prepared and characterized by the chemisorption and the titration of probe molecules. Their catalytic behavior in steam and oxygen/steam conversions of carbon monoxide and of propane was investigated. The reactions were carried out in oxygen-deficient medium, under isothermal or temperature-programmed reaction (TPR) conditions. Light-off temperatures as well as turnover frequency (Nt) deduced from the low-conversion branch of the TPR curves were used to evaluate the catalytic activities. In oxygen/steam conversion of carbon monoxide, the relative Nt values were: PtCeA, 2500 > PtA, 240 > RhCeA, 75 > RhA, 1 for carbon monoxide oxidation at 100°C and PtCeA, 43500 > RhCeA, 100 > PtA, 75 > RhA, 1 for the water-gas shift (WGS) at 200°C. This reaction, on Al2O3 and to a lesser extent on CeO2/Al2O3, was inhibited by oxygen, WGS activities being one to three orders of magnitude higher in the CO+H2O mixture. In oxygen/steam conversion of propane, the relative Nt values were: PtA, 76 > PtRhA, 5.4 > PtCeA, 3.6 > RhCeA, 1.8 > RhA, 1 > PtRhCeA, 0.8 for propane oxidation at 300°C while the activity order was: PtRhCeA > PtRhA > RhCeA > PtA > RhA for the propane steam reforming by 400°C. This reaction started when oxygen was consumed and was inhibited by carbon monoxide, which explains why there was a definite cooperation effect between RhCeA intrinsically active in steam reforming and platinum active in oxidation and in WGS.

Deactivation of supported copper based catalysts during polyol conversion in aqueous phase

Abstract Degradations and cyclodehydration of polyols in aqueous solution were studied on copper deposited on charcoal unmodified or modified by addition of chloride and a second metal (Pt or Au). These catalysts had properties very close to those of Raney copper based catalysts. Their deactivation was studied in a dynamic flow three-phase reactor. Sintering of copper particles, surface oxidation and copper(I) hydroxide dehydration were responsible for the deactivation of the unmodified Cu/C. In the cases of modified Cu/C, decrease of the cyclodehydration selectivity was attributed to the loss of chloride. Addition of chloride ions to the polyol solution to be converted increased the cyclodehydration selectivity but strongly decreased the activity.

Platinum modified by electrochemical deposition of adatoms

Abstract Platinized platinum or platinum/graphite catalysts can be modified ‘in situ’ by submonolayer deposition of different additives (sulphur, copper, lead, tin, iron). An electrochemical method is used based on the underpotential deposition of metals on another more noble metal substrate, by control of its potential at values higher than the Nernst potential of bulk deposition of this additive. Linear potential sweep cyclic voltammetry is used for ‘in situ’ characterization of modified platinum catalysts, allowing evaluation of the quantity and the degree of coverage of deposited adatoms. Moreover, their adsorption stoichiometry and their oxidation state can be determined. The study of the adsorption, on modified platinum, of olefinic acids (maleic acids), used as probe molecules, allowed evaluation of the ‘ligand effects’ induced by these additives. Sulphur, copper, lead and tin adatoms behave as ‘poisons’ of the hydrogenation properties of platinum. The deactivation curves obtained ( a = f(θ poison ) ) and the initial toxicities of these poisons can be explained by the occurrence of ‘ensemble effects’ (in the case of copper and sulphur), or by the high surface diffusion of adatoms having low melting points (lead, tin). A restructuring effect of iron, deposited on Pt/graphite has been remarked and can account for the obtained promotion of the hydrogenation catalytic activity of platinum.

Hydrogen formation in propane oxidation on Pt-Rh/CeO2/Al2O3 catalysts

Temperature-programmed oxidation (TPO) of propane (200–550°C) was carried out on PtRh/CeO2/ Al2O3 catalysts with oxygen-deficient C3H8/O2 mixtures. On these TPO profiles, two regions of reaction can be observed. ForT 350 ° C, there are two distinct zones in the catalyst bed: total oxidation catalyzed by platinum at the bed inlet and steam reforming catalyzed by rhodium in the outlet zone. Steam reforming is the slow step in the formation of hydrogen. Gas composition (H2, CO, CO2) is always very close to that predicted by the water-gas shift equilibrium, here this reaction being very rapid.

Preparation of platinum-gold bimetallic catalysts by redox reactions

Abstract Bimetallic Pt-Au/SiO2 catalysts were prepared by reduction of AuC1−4 using hydrogen preadsorbed on a parent monometallic Pt/SiO2 catalyst. In fact during that preparation a direct redox reaction can occur between the reduced platinum and AuCl−4. At low gold coverage the additive is deposited statistically on the whole surface of platinum particles (the reducer is H preadsorbed on Pt). At high gold coverage the additive is preferentially deposited on the high index planes of platinum particles (by direct redox reaction). During the formation of Pt-Au bimetallic entities sintering of platinum can occur as a result of simultaneous oxidation and reduction of platinum.

Benzene hydrogenation on platinum and iridium catalysts. Variation of the toxicity of sulfur with the nature of the support

Abstract Benzene hydrogenation was studied at 323 K on fresh and sulfurized catalysts. The activity of small platinum and iridium particles was sensitive to the electronic properties of the metal, which, in turn, depended on the acidity of the support. The toxicity of sulfur, which varies with both the nature of the metal and the support, can be explained by the combination of geometric (negative) and electronic (negative or positive) effects.

Influence of PtRe interaction on activity and selectivity of reforming catalysts

The influence of the preparation procedure on the Pt and Re interaction in PtRe/Al2O3 reforming catalysts has been studied. Three different preparation procedures have been used: the classical coimpregnation and successive impregnation techniques, and the recently reported catalytic reduction method. Catalyst activation was done either by direct reduction after metal deposition, or by calcination and reduction. The degree of interaction of the metals was indirectly measured by the cyclopentane hydrogenolysis reaction. It has been found that interaction between Pt and Re increases according to the sequence: coimpregnation (calcined and reduced), catalytic reduction (calcined and reduced), successive impregnations (reduced catalysts) and catalytic reduction (reduced catalysts). Calcination greatly diminishes the PtRe metal-metal interaction, as measured by cyclopentane hydrogenolysis. After sulfiding, the catalysts prepared by catalytic reduction with a calcination and reduction treatment display the highest n-heptane dehydrocyclization activity. Catalysts only reduced also have good activity for this reaction, but with poor stability.

Effect of PtRe interaction on sulfur adsorption and coke deposition

Abstract Bimetallic reforming Pt Re catalysts were prepared either by coimpregnation or by a surface redox reaction (catalytic reduction); this last technique allowing improvement of the Pt Re interaction. Bimetallic catalysts prepared by catalytic reduction and activated by direct reduction are less sensitive than coimpregnated ones to hydrogen sulfide adsorption at 500°C. The coking reaction was studied in the course of cyclopentane transformation at 450°C. For non-sulfided catalysts, the deposition of coke and its fouling effect for the metallic function decrease as the Pt Re interaction increases whatever the working pressure. After sulfidation, the deactivation of bimetallic catalysts by carbon deposition depends on both the Pt Re interaction and the working pressure. At normal pressure, the toxicity of coke increases with the degree of Pt Re interaction while the reverse effect is observed at high pressure (15 bar).

Isoprene hydrogenation on supported PdFe catalysts. Influence of the catalyst preparation procedure

Abstract Isoprene hydrogenation was investigated on Pd Fe/alumina catalysts in the liquid phase. Iron added by a controlled surface reaction (CSR) promotes the activity of small palladium particles while the opposite effect is generally observed on catalysts prepared by coimpregnation or successive impregnations of the metal precursor salts. Iron introduced by CSR would be deposited on the surface of palladium particles while the other preparation techniques would lead to alloy formation.

Reduction of nitrates by dihydrogen in CeO2 and Rh/CeO2 catalysts

Abstract Series of NO 3 − /CeO 2 and Rh/NO 3 − /CeO 2 catalyst with variable loadings of nitrate (0-1.5 wt.-%) and of rhodium (0-0.2 wt.-%) were prepared by impregnation of a cerium oxide (19 m 2 g −1 ) with aqueous solutions of sodium nitrate and of rhodium nitrate. The catalysts were dried at 120 ° C and characterized by temperature-programmed reduction in hydrogen. In NO 3 − /CeO 2 supports, the nitrate reduced into nitrogen at 460–480° C, while the ceria reduced in two peaks: a narrow peak at 530° C (surface of CeO 2 ) and a large peak above 600° C (bulk of CeO 2 ). The rhodium increased significantly the reducibility of all the surface species: the nitrates and the surface of CeO 2 were reduced at the same time as the metal in a single peak at 220 ° C. Conversely the nitrates retarded the reduction of Rh/CeO 2 which presented a single peak at 185 ° C if no sodium nitrate was added. At low rhodium content ( 2 O, N 2 , NH 3 ) could be observed.