Jean-Paul Hindermann

Application of chemical trapping to the determination of surface species and to the study of their evolution under reaction conditions in heterogeneous catalysis

Abstract The detection of surface species by chemical trapping is described and discussed. The results obtained by scavenging intermediates in CO-H 2 and CO 2 -H 2 reactions show the relevance of this method to the study of reaction mechanisms in heterogeneous catalysis. Quantitative detection of some species (carboxylate, formyi, methoxy) is achieved and a correlation has been found between activity and surface species on a Pd/MgO-SiO 2 catalyst. A reaction scheme is proposed for the CO-H 2 and CO 2 -H 2 reactions.

Ethanol promotion by the addition of cerium to rhodium–silica catalysts

The selectivity of rhodium-based catalysts is very dependent on the choice of support or the promotors. Cerium oxide is very effective for ethanol promotion when added to Rh/SiO2 catalysts or used as support (70–80% ethanol selectivity). The role of the cerium oxide has been studied by various techniques, namely chemical trapping, Fourier-transform infrared spectroscopy and the use of probe molecules. The results show that when cerium oxide is added to rhodium catalysts, a new absorption band appears at 1725 cm–1 in the infrared spectra which may be attributed to a CO molecule bonded through both the carbon and oxygen atoms. Such an interaction could be responsible for the formation and stabilization of the formyl species which, from chemical trapping experiments, is proposed as a key intermediate. These results and those obtained with probe molecules would be in accordance with a reactional pathway where the formyl species is a building block of the C2-oxygenates and the acetaldehyde a primary product of the synthesis. In addition to its role in the activation of carbon monoxide towards hydrogenation, it is also shown that cerium oxide is responsible for the reduction of acetaldehyde to ethanol.

Characterization of dioxymethylene species over cu-zn catalysts

Abstract Dioxymethylenic species have been characterized upon adsorption of formaldehyde on zinc aluminate and copper supported on zinc aluminate, by both FTIR spectroscopy and chemical trapping. Dioxymethylene could be an intermediate in the synthesis of methanol on these catalysts in CO + H 2 and CO 2 + H 2 reactions, as shown by chemical trapping experiments.

Oxidative dehydrogenation of propane on rare earth vanadates influence of the presence of CO2 in the feed

Abstract The oxidative dehydrogenation of propane on rare earth vanadates is always in competition with the total oxidation of the alkane which leads to the formation of CO 2 and CO. Addition of CO 2 into the feed results in an increased C 3 H 6 selectivity together with a decreased CO formation through the total oxidation of the alkane or the formed alkene. For the tested rare earth vanadates as well as for the niobium promoted catalysts an enhancement of the C 3 H 6 selectivity at isoconversion and thus of the C 3 H 6 yields is observed upon CO 2 addition into the feed.

Chapter 4: Heterogeneous Carbon Monoxide Hydrogenation

Laboratoirede Chimie Organique Appliquee,Unite AAssocie Aeau CNRS 469,Ecole Europe Aenne des Hautes Etudes des Industries Chimiques de Strasbourg (EHICS), 1, rue Blaise Pascal, 67008 Strasbourg Cedex (France).

Temperature-programmed desorption studies on Pd/CeO2 after methanol and formic acid adsorption and carbon monoxide—hydrogen reaction

Abstract Temperature-programmed desorption (TPD) after methanol and formic acid adsorption and COzH2 reaction were studied on palladium catalysts. No difference in the nature of the products after decomposition of methanol on 3% Pd/SiO2 and 3% Pd/CeO2 was found. Both catalysts decompose methanol into CO+H2. After adsorption of formic acid on CeO2 alone, methanol was desorbed with a maximum at 540–550 K. This result can be interpreted by possible hydrogenation of formate species into methanol. On the other hand, no methanol was observed after adsorption of formic acid on 3% Pd/CeO2 (methane was desorbed with a maximum at 545 K). Two peaks of methanol desorption were found after COzH2 reaction on CeO2 at 383 and 535 K. The first could be formed after methoxy hydrogenation and the second may be due to hydrogenation of formate species. TPD after COzH2 reaction on 3% Pd/CeO2 gave results similar to those found after methanol adsorption. These results suggest that formyl species are involved in methanol synthesis on Pd/CeO2 catalyst, whereas on CeO2 formate species could be the main intermediate.

CO + H2 Reactions on Rhodium Catalysts: Study of the Reactivity and Mechanism

The reactivity of Rh-SiO 2 catalysts, doped with cerium is studied in CO/H 2 synthesis. At 200°C and atmospheric pressure, a Rh/siO 2 catalyst gives essentially hydrocarbons. The addition of cerium, in a concentration varying from 0,5 to 5 weight %, changes its selectivity and it reaches 70% in oxygenated products. Most of it being ethanol. Using the chemical trapping, we could detect formyl species on both Rh/SiO 2 and Rh-Ce/SiO 2 catalysts. Their possible intermediacy in the Formation of oxygenated compounds is discussed.

EFFECT OF ADDITION OF PD, CO AND PD-CO ON CEO2: SYNGAS CONVERSION AND ACETALDEHYDE REACTION

Abstract Ethanol was observed from the reaction of CO/H 2 + CH 2 Cl 2 on Pd/CeO 2 as well as from the reaction of CO/H 2 on Pd-Co/CeO 2 . This result clearly indicates that Co sites provide CH x species responsible for ethanol formation. Intimate contact between Pd and Co was most likely formed since TPR indicated that both metals were reduced at the same temperature. Moreover, acetaldehyde TPD showed that ethanol produced by acetaldehyde hydrogenation desorbed at only one peak temperature which might be indicative of a single type of active site.