Jacques Barbier

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°).

Coke formation on platinum-alumina catalyst of wide varying dispersion

The effect of metallic dispersion on coke formation from cyclopentane reactions was studied on a series of Pt/Al2O3 catalysts of widely varying accessibility. While small metal crystallites presented a strong resistance to coke deposition, the big particles showed a higher sensitivity to this poison. The results can be explained by means of a higher stabilization of the adsorption of the cyclopentadiene produced during the reaction, by the big platinum crystallites than by the small ones which present an electron eficiency due to metal-support interactions.

Thioresistance of supported metal catalysts: structure sensitivity of hydrogen sulphide adsorption on Pt/A1203, Ir/A1203 and Pt-Ir/A1203 catalysts

Abstract Different series of Pt/A1 2 0 3 , Ir/A1 2 03 catalysts, of widely varying metallic dispersions, have been supported on two aluminas, one acidic and the other weakly acidic. It has been observed that various parameters (nature of the metallic phase, acidity of the support, metallic dispersion) can modify the adsorption capacity of hydrogen sulphide on the metal at high temperature (773 K). All results can be explained by taking into account that sulphur adsorption is essentially sensitive to the electronic properties of the metal.

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).

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.

Coke formation on bimetallic platinum/rhenium and platinum/iridium catalysts

Abstract The coke deposition during the reaction of cyclopentane has been investigated on Pt-Re/Al 2 O 3 and Pt-Ir/Al 2 O 3 catalysts. It appears that the amount of coke deposited on the catalyst is lowered by the addition of Ir or Re to Pt. On the other hand, the amount of coke oxidizable at low temperature (deposited on the metal) decreases in the order Pt-Re > Pt > Re > Pt-Ir > Ir. Nevertheless, toxicities of those carbonaceous deposits for catalytic hydrogenation reaction are the lowest on Pt-Re and the highest on Pt-Ir. These results can be explained by a ligand effect which can stabilize the adsorption of coke precursor or by a change in the nature of the coke.

Coking of PtIr/Al2O3 and PtRe/Al2O3 catalysts in different pressures of cyclopentane

Abstract The coking reaction with cyclopentane was investigated at various pressures on bimetallic PtRe/Al 2 0 3 and PtIr/Al 2 O 3 catalysts. At atmospheric pressure addition of iridium or rhenium to platinum acts in the same way on the amount of deposited coke as an increasing working pressure on pure Pt/Al 2 O 3 catalysts. Such modification induces a lowering of coke content and yields more dehydrogenated carbonaceous deposits. Nevertheless, the toxicity of coke defined by its fouling effect on the pure metallic reaction of benzene hydrogenation depends on the nature of the metallic phase and the overall pressure. These toxicities are linked to the presence of light polyaromatic compounds which are precursors of graphitic coke.

A “refilling” technique for catalyst particle modification

Abstract Chloroplatinic ions are reduced by hydrogen preadsorbed on the metallic particles of an alumina-supported platinum catalyst. This reduction induces a uniform growth of the particles. The transformation in the catalyst was followed by H 2 -0 2 titrations and T.E.M.

Deactivation by Coking of Platinum/Alumina Catalysts: Effects of Operating Temperature and Pressure

Abstract Coke deposition is a complex reaction that results from the production of coke precursors and from their destruction. At low temperatures, when the destruction reaction (high apparent activation energy) is negligible, the coking rate is increased owing to increasing metal accessibility. In contrast, at high temperatures the coking reaction is hindered by an increasing metal loading. A change in the coking temperature does not alter the chemical nature of the carbonaceous deposits and their distribution between the metallic and acidic phases of a bifunctional catalyst. On the other hand, an increase in coking pressure (with a constant hydrogen:hydrocarbon molar ratio) promotes the graphitization of coke on the support, protecting the catalytic activity of the metallic phase.

Coking of Pt-Ir/Al sub 2 O sub 3 and Pt-Re/Al sub 2 O sub 3 catalysts in different pressures of cyclopentane

Abstract The coking reaction with cyclopentane was investigated at various pressures on bimetallic PtRe/Al 2 0 3 and PtIr/Al 2 O 3 catalysts. At atmospheric pressure addition of iridium or rhenium to platinum acts in the same way on the amount of deposited coke as an increasing working pressure on pure Pt/Al 2 O 3 catalysts. Such modification induces a lowering of coke content and yields more dehydrogenated carbonaceous deposits. Nevertheless, the toxicity of coke defined by its fouling effect on the pure metallic reaction of benzene hydrogenation depends on the nature of the metallic phase and the overall pressure. These toxicities are linked to the presence of light polyaromatic compounds which are precursors of graphitic coke.