Patrice Marecot

Propane and propene oxidation over platinum and palladium on alumina: Effects of chloride and water

Abstract The oxidation of propane and propene was investigated on palladium and platinum supported catalysts. Catalyst intrinsic activities, evaluated by light-off temperatures in slightly oxidizing reactant mixture (5% excess oxygen), show an optimum particle size which maximizes the catalytic activity for a given metal loading. On catalysts prepared from chloride containing precursor salts, chloride poisons the metallic activity whatever the particle size. Moreover, reaction isotherms under varying oxygen levels point out that the effect of chloride is more detrimental under oxidizing conditions. After successive oxidation cycles, this poisoning effect disappears as a consequence of the removal of chloride from the catalyst surface by water produced during propane and propene combustion. On the other hand, addition of relatively large quantities of water (equivalent to the content of the exhaust gas) inhibits the oxidation of hydrocarbon. Poisoning effects of chloride and water are explained by a decreasing active surface for the reactions under consideration.

In situ Raman and in situ XRD analysis of PdO reduction and Pd° oxidation supported on γ-Al2O3 catalyst under different atmospheres

Reduction of Pd° and decomposition of palladium oxide supported on γ-alumina were studied at atmospheric pressure under different atmospheres (H(2), CH(4), He) over a 4 wt% Pd/Al(2)O(3) catalyst (mean palladium particle size: 5 nm with 50% of small particles of size below 5 nm). During temperature programmed tests (reduction, decomposition and oxidation) the crystal domain behaviour of the PdO/Pd° phase was evaluated by in situ Raman spectroscopy and in situ XRD analysis. Under H(2)/N(2), the reduction of small PdO particles (<5 nm) occurs at room temperature, whereas reduction of larger particles (>5 nm) starts at 100 °C and is achieved at 150 °C. Subsequent oxidation in O(2)/N(2) leads to reoxidation of small crystal domain at ambient temperature while oxidation of large particles starts at 300 °C. Under CH(4)/N(2), the small particle reduction occurs between 240 and 250 °C while large particle reduction is fast and occurs between 280 and 290 °C. Subsequent reoxidation of the catalyst reduced in CH(4)/N(2) shows that small and large particle oxidation of Pd° starts also at 300 °C. Under He, no small particle decomposition is observed probably due to strong interactions between particles and support whereas large particle reduction occurs between 700 and 750 °C. After thermal decomposition under He, the oxidation starts at 300 °C. Thus, the reduction phenomenon (small and large crystal domain) depends on the nature of the reducing agent (H(2), CH(4), He). However, whatever the reduction or decomposition treatment or the crystal domain, Pd° oxidation starts at 300 °C and is completed only at temperatures higher than 550 °C. Under lean conditions, with or without water, the palladium consists of reduced sites of palladium (Pd°, Pd(δ+) with δ < 2 or PdO(x) with x < 1) randomly distributed on palladium particles.

Deactivation of steam-reforming model catalysts by coke formation: I. Kinetics of the Formation of Filamentous Carbon in the Hydrogenolysis of cyclopentane on Ni/Al2O3 Catalysts

Abstract Coke formation in the hydrogenolysis of cyclopentane on Ni/Al 2 O 3 catalysts in the temperature range 300–500°C was studied. A similar change in apparent activation energy for coking obtained with catalysts presenting different surface states coincides with the appearance of filamentous carbon on the catalyst surface. A definite carbon-to-surface nickel ratio seems to be necessary for the nucleation of the filaments. Coked catalysts reveal different activities for hydrogenolysis and hydrogenation reactions depending on the nature of the carbon deposit. The free-metal surface area decreases by two orders of magnitude during coke deposition accompanying cyclopentane hydrogenolysis. Nevertheless, as soon as the filamentous carbon appears on the catalyst, the nickel recovers a significant part of its initial activity, which shows that the accessible fraction of metal increases at the initial stage of the whisker formation.

Resistance to sulfur poisoning of metal catalysts: dehydrogenation of cyclohexane on Pt/Al2O3 catalysts

Abstract Model Pt catalysts, containing ~0.6 wt% Pt, were prepared by deposition of dinitrodiamino-platinum salt on a nonmicroporous alumina. The carrier has been modified by addition of K or Cl. The dispersion of the metallic phase was varied through different decomposition-reduction processes, but the final activation conditions were identical in all cases. The various catalysts have been tested in the dehydrogenation of cyclohexane which was confirmed to be a structure-insensitive reaction. The thiotolerance level of the various catalysts was measured with a feed contaminated with 0.6 ppm of thiophene. The thiotolerance level is more affected by the acidity of the support (presence of K or Cl) than by variations of Pt particle size. This has been confirmed by making sudden injections of CH2Cl2 or (C2H5)3N into the feed. On removing thiophene from the reactant mixture partial decontamination occurred and the activity recovered to ~50% of the initial value.

Preparation of alumina supported palladium–platinum catalysts by surface redox reactions. Activity for complete hydrocarbon oxidation

Abstract The complete oxidation of hydrocarbons (methane, propane and propene) and the 18 O / 16 O isotopic exchange were investigated on supported palladium–platinum catalysts. The bimetallic catalysts were prepared by controlled surface reactions (the “refilling” RC and the “direct redox reaction” RD) and by coimpregnation CI. The preparation methods lead to different kinds of platinum deposit, which involve different behaviours in hydrocarbon oxidation and in isotopic exchange. Even if the structure of bimetallic catalysts is modified under oxygen, the results show that the bimetallic catalysts keep a memory effect of their preparation method under the reaction conditions used in this work.

Location and structure of coke deposits on alumina-supported platinum catalysts by EELS associated with electron microscopy

Abstract The location and structure of carbonaceous deposits formed upon coking Pt Al 2 O 3 catalysts with cyclopentane have been studied by conventional TEM and by electron energy loss spectroscopy (EELS) associated with high resolution scanning transmission electron microscopy. The TEM investigation shows patches of amorphous coke covering the support surrounding each metal particle. This is substantiated by EELS showing that the coke coverage can extend as far as 20 nm from a given particle. From the absence of fine structure on the EELS peak of carbon and from the comparison with reference compounds (graphite, coronene, and pentacene), it is concluded that the local structure of coke is not graphitic or pregraphitic but may consist in a disordered arrangement of polyaromatic molecules. After partial combustion at 563 K, the coke covering the metal particles is removed but the coke on the support is still present. This confirms previous temperature-programmed Oxidation results.

Influence of the redox properties of ceria on the preparation of three-way automotive platinum—rhodium/ alumina—ceria catalysts

Abstract The redox properties of ceria were investigated by temperature-programmed reduction performed under hydrogen and the catalysts were evaluated by determining the temperatures at which 50% conversion for propene and propane oxidation were reached. The results obtained show that ceria plays a decisive role within the preparation of bimetallic PtRh/Al2O3CeO2 catalysts by successive impregnations (SI) when reduced capping cerium atoms (Ce3+) are present on the parent Pt/Al2O3CeO2 catalyst before introduction of the rhodium salt. Thus, rhodium deposition would occur by a redox reaction between Ce3+ and Rh3+. The preparation procedure of PtRh/Al2O3CeO2 catalysts by the SI method increases the thermal stability of the metallic species under oxidizing conditions. This improvement would be the consequence of a higher resistance to the formation of PtRh alloys with surface enrichment in rhodium.

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.

Preparation of PtReAl2O3 catalysts by surface redox reactions I. Influence of operating variables on Re deposit in the presence of hydrochloric acid

Abstract The preparation of bimetallic PtRe Al 2 O 3 catalysts was investigated by surface redox reaction between hydrogen adsorbed on a ‘parent’ platinum/alumina catalyst and rhenium ions. Rhenium can be deposited under an inert atmosphere (‘recharge’ method), but the rhenium deposit is limited by the amount of hydrogen irreversibly adsorbed on surface platinum. When the reaction is carried out under hydrogen atmosphere (‘catalytic reduction’), this technique allows to deposit large amounts of rhenium. The partial pressure of hydrogen, the temperature of the reaction, the concentration and the nature of the rhenium salt and the dispersion of the ‘parent’ monometallic platinum catalyst strongly influence the kinetic and the extent of rhenium deposition by catalytic reduction.

Influence of chlorine content on PtRe interaction and coke deposition

Abstract The influence of the chlorine content on the PtRe interaction and the modification of the catalytic properties of reforming catalysts prepared by co-impregnation were studied. Chlorine inhibits PtRe interaction and produces a decrease in the deposition of coke over the metallic function. In monometallic Pt catalysts, chlorine produces lower coke deposits, but its toxicity in cyclohexane dehydrogenation is the same and the catalytic activity is independent of chlorine content. For bimetallic catalysts, the greater the PtRe interaction, the lower the deactivation by coking of the metallic function.