Daniel Duprez

Investigation of the oxygen storage process on ceria- and ceria–zirconia-supported catalysts

Abstract A total of 10 noble metal (Rh, Pt, Pd, Ru and Ir) catalysts, either supported on CeO 2 or Ce 0.63 Zr 0.37 O 2 , were prepared. Catalysts were fully characterized using XRD, N 2 adsorption at −196xa0°C, TEM and H 2 chemisorption. Oxygen storage processes were carefully investigated. The influence of temperature was checked and a key role of oxygen diffusion was further demonstrated. A review of the reactions involved in the CO transient oxidation reaction is finally proposed.

Oxidation of carbon monoxide, propene, propane and methane over a Pd/Al2O3 catalyst. Effect of the chemical state of Pd

A 1 wt.-%Pd/γ-Al2O3 catalyst was prepared (38% dispersion) and sintered under a flow of 3 vol.% O2 in N2 at 900°C (9% dispersion). The temperature-programmed oxidation of CO, C3H6, C3H8 and CH4 in substoichiometry of O2 was carried out on the fresh and, in some cases, on the sintered catalyst. The chemical state of palladium in the sintered catalyst was determined by XRD after interruption of the reaction at different temperatures. The temperatures at which a 20% conversion was obtained on the fresh pre-oxidised catalyst were: CO, 210°C C3H6, 66 > C3H8, 15.3 > CH4, 7.9. Pre-reduction did not change the catalyst activity in CO and in C3H6 oxidation while it decreased the activity by a factor 2.4 for C3H8 and 3.2 for CH4. Except for a definite decrease of activity, the above conclusions remained valid for the sintered catalyst. XRD measurements showed that PdO initially present in the pre-oxidised sample was reduced before the oxidation of CO (< 180°C) and C3H6 (< 210°C) started, which explains why both pre-oxidised and pre-reduced samples had the same activity. In the case of C3H8, the reduction occurred during the reaction and led to a temporary decrease of activity. For CO and C3H8, the Pd° structure was clearly identified while a new structure PdOξ (0 < ξ ≪ 1) was formed with C3H6. This compound has the cfc structure of Pd° with a higher lattice parameter (3.990Ainstead of 3.889Ain Pd°).

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.

Selective steam reforming of aromatic compounds on metal catalysts

Abstract Steam reforming can be used to gasify aromatic molecules selectively. First, toluene steam dealkylation is reviewed. This reaction is catalyzed by Group VIII metal catalysts: rhodium is the most active metal and platinum, the most selective into benzene but both are extremely support-sensitive. We discuss the rate-determining step of OH group surface diffusion from support to metal as well as structural and chemical parameters which control selectivity. Steam reforming of other aromatics (alkylbenzenes, alkylnaphthalenes, oxygen-containing and nitrogen-containing aromatics) are reviewed in a second part. When possible, activity and selectivity results obtained in steam reforming are compared to those obtained in hydrogen conversions of the same compounds. The third part of this paper refers to the patent literature concerning the design of stable selective catalysts.

Effect of steam on the coking of platinum catalysts: I. Inhibiting effect of steam at low partial pressure for the dehydrogenation of cyclopentane and the coking reaction

Abstract The effect of steam on the coking of Pt/Al2O3 was studied at 400°C under atmospheric pressure in the presence of cyclopentane in dry or wet (0.67 kPa of water) nitrogen. Steam decreases the coking rate, owing to the inhibition of the dehydrogenation of cyclopentane to cyclopentadiene, the main coke precursor. Temperature-programmed oxidation profiles of coke show three peaks: peak I at 275–320°C, peak II at 380–440°C and peak III at 480–550°C. Peak I is ascribed to carbon deposited on the metal whereas peaks II and III correspond to two different forms of coke deposited on the support. Steam decreases the amount of carbon oxidized at high temperature (500°C).

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.

Reactivity of steam in exhaust gas catalysis III. Steam and oxygen/steam conversions of propane on a Pd/Al2O3 catalyst

Abstract A 1% Pd catalyst (38% dispersion) was prepared by impregnating a γ-alumina with palladium acetylacetonate dissolved in acetone. The behaviour of this catalyst in oxidation and steam reforming (SR) of propane was investigated. Temperature-programmed reactions of C 3 H 8 with O 2 or with O 2 + H 2 O were carried out with different stoichiometric ratios S ( S =[O 2 ]/5[C 3 H 8 ]). The conversion profiles of C 3 H 8 for the reaction carried out in substoichiometry of O 2 ( S 2 and carbon oxides by this reaction. Contrary to what was observed with Pt, an apparent deactivation between 310 and 385°C could be observed with Pd in oxidation. This is due to a reduction of PdO x into Pd 0 , which is much less active than the oxide in propane oxidation. Steam added to the reactants inhibits oxidation while it prevents the reduction of PdO x into Pd 0 . Compared to Pt and to Rh, Pd has a higher thermal resistance: no deactivation occurred after treatment up to 700°C and limited deactivation after treatment up to 900°C, provided that the catalyst is maintained in an oxygen-rich atmosphere during the cooling.

Intermetallic compounds as catalysts for reactions of heterogeneous catalysis

Abstract Intermetallic compounds such as RNi 5 exhibit an appreciable activity towards catalytic reactions, particularly in CO conversion and hydrocarbon hydrogenolysis. However, we found that the intermetallic compound decomposes into a metal-support system during the reaction. Therefore this transformation represents a new and interesting method for the preparation of supported metals which enables us to solve some of the problems associated with conventional methods of catalyst preparation.

The effects of support and of particle size on the redox properties of rhodium

Abstract Alumina, silica and zirconia supported rhodium catalysts were prepared by impregnation and sintered at different temperatures (700–900°C) to obtain metal dispersions ranging from 19–41%. Rhodium oxidation is very sensitive to particle size, the larger the particle size, the higher the temperature of the maximum rate of oxidation, but no significant effect of the support is noted. On the contrary, the reduction of rhodium oxide is less dependent on particle size than oxidation of metallic rhodium, since all the catalysts are reduced in the same temperature range 25–200°C. However, on alumina, the rate of Rh 2 O 3 reduction increases with the mean particle size while on zirconia a contrary effect can be observed, though it is less marked.

Intermetallic compounds as heterogeneous catalysts

Abstract Some intermetallic compounds such as RExNiy, where RE represents lanthanum or rare-earth mischmetal alloy exhibit an appreciable activity in catalytic reactions, particularly in carbon monoxide hydrogenation, toluene dealkylation and ethane hydrogenolysis. However, we found that, during the reaction, the intermetallic compound decomposed into a metal-support system, the support being an oxide or hydride according to the performed reaction.