G. Del Angel

Effect of lanthanum on the catalytic properties of PtSn/γ-Al2O3 bimetallic catalysts prepared by successive impregnation and controlled surface reaction

Abstract Bimetallic PtSn/ γ -Al 2 O 3 –La 2 O 3 catalysts were prepared by two comparative methods, successive impregnation (SI) and controlled surface reaction (CSR). FTIR-pyridine adsorption revealed a decrease in support acidity as a function of lanthanum content. Oxidized Pt 2+ was identified by CO-FTIR chemisorption and by XPS spectra on catalysts prepared by SI. When the CSR preparation method was used, reduced Pt 0 was observed and oxidized Pt 2+ was not detected. As the lanthanum content increased a diminution in cyclohexane dehydrogenation and n -heptane conversion was observed. Low hydrogenolysis and high selectivity to C 7 -olefins were obtained in both preparations. It is proposed that lanthanum in SI catalysts stabilizes the platinum in an oxidation state, Pt 2+ , and diminishes the support acidity. In CSR preparations the lanthanum effect is limited to inhibit the support acidity and hence to the bifunctional reactions occurring during the n -heptane conversion.

Effect of HCl acid on the hydrodechlorination of chlorobenzene over palladium supported catalysts

Ammonia and hydrochloric acid effects have been studied in the chlorobenzene hydrodechlorination in liquid phase on Pd/C catalysts. The addition of NH4OH to the reactant medium does not modify the reaction rate while in hydrochloric acid medium the activity was increased. The role of HCl acid in the activity is explained by a partial oxidation of the Pd particles.

Selective hydrogenation of carvone and o-xylene on Pd–Cu catalysts prepared by surface redox reaction

A series of Pd and Pd–Cu bimetallic catalysts were prepared by surface redox reaction and coimpregnation methods. A comparison between the preparation methods in the activity and selectivity for o-xylene and carvone hydrogenation was done. FTIR studies showed that the Cu is deposited mainly in the low coordination Pd sites when the catalyst was prepared by redox, whereas a random distribution of Cu on the Pd surface was obtained for the catalyst prepared by coimpregnation. The activity and selectivity of bimetallic catalysts, were found to depending on the method of preparation used. For o-xylene hydrogenation in redox catalyst, the selectivity to trans-1,2-dimethylcyclohexane was lower than that obtained in the Pd/SiO2 reference catalyst, whereas in the coimpregnated catalyst, the trans isomer selectivity was higher. In the redox preparation, the selective hydrogenation of carvone favors the formation of carvotanacetone, whereas the coimpregnated catalyst favors the selectivity to carvomenthone.

Ammonia and sulfur poisoning effects on hydrogenation of phenylacetylene over Pd supported catalysts

Abstract The effect of palladium dispersion, on the selectivity for phenylacetylene hydrogenation was studied. Low dispersed palladium catalysts showed to be more selective to partial hydrogenation than high dispersed ones. The initial selectivity to styrene is increased with time on stream. Carbonaceous deposits were assumed to be responsible for this phenomenon. Selectivity was also modified by ammonia and sulfur addition: ammonia increased the styrene selectivity while sulfur had a negative effect. The poisoning effects were explained through an electronic effect. Ammonia is an electron donor modifier whereas sulfur is an electron acceptor modifier.

Selective hydrogenation of phenylacetylene on Pd/Al2O3: Effect of the addition of Pt and particle size

Phenylacetylene hydrogenation on Pd, Pt and Pd−Pt/Al2O3 catalysts has been studied. In all catalysts activity was found not to depend on particle size. However, selectivity to styrene was found to depend on Pd/Al2O3 catalysts. Carbon deposition in both metal and support explains such a behavior. Nevertheless, in small Pd particles a longer residence time of styrene may control the selectivity.

Platinum-palladium catalysts in hydrogenation of methylbenzenes

Hydrogenation of benzene, toluene, o,m and p-xylene over Pt, Pd and Pt−Pd/Al2O3 catalysts has been studied. Experimental data show that when Pd is added to Pt catalysts, a decrease in activity and changes in selectivity occur. This is explained by the formation of electron-deficient and electrorich metallic species.AbstractБыло исследовано гидрирование бензола, толуола, о-, м- и п-ксилолов на катализаторах Pt, Pd и Pt−Pd/Al2O3. На основе экспериментальных данных было установлено, что при добавлении Pd к катализатору Pt происходит уменьшение активности и изменение селективности. Это объясняется образованием электронообедненных и электрониообогащенных металлических частиц.

Catalytic behavior and active sites structure of PtAu/Al2O3 bimetallic catalysts prepared by surface redox reactions

Abstract Alumina supported Pt–Au bimetallic catalysts, in which Au was deposited onto Pt by reduction of AuCl4− with adsorbed hydrogen, were studied using hydrogen TPD and X-ray diffraction. Their catalytic behavior was tested in methylcyclopentane reaction. The gold inhibits the hydrogen adsorption indicating that gold atoms are deposited on platinum sites. In methylcyclopentane reaction, the catalysts containing the largest amounts of Au, promoted the formation of dehydrogenation products although in conversions very low. However, the environmental conditions or oxidation treatments irreversibly affect the catalytic behavior in these systems, suggesting a rearrangement of the metallic sites created by the redox reaction. This rearrangement effect was investigated by X-ray diffraction and by high resolution electron microscopy (HREM) studies.

o-Xylene hydrogenation on Ru, Pt, Rh and Pd supported catalysts: Dispersity effect on selectivity

Selectivity to 1,2-cis- and 1,2-trans-dimethylcyclohexane products was studied in o-xylene hydrogenation over Pt, Pd, Rh and Ru supported catalysts. The results show that selectivity is sensitive to particle size.AbstractИсследовали селективность к образованию 1,2-цис- и 1,2-транс-диметилциклогексанов в гидрировании о-ксилола на нанесенных катализаторах Pt, Pd, Rh и Ru. Результаты свидетельствуют о чувствительности реакции к размерам частиц.

Chemical analysis of small supported Pt-Ru bimetallic clusters by EDS

Abstract Energy dispersive X-ray spectroscopy was used in conjunction with transmission electron microscopy to determine the extent of bimetallic clustering for silica supported PtRu catalysts. The effects of various artifacts which interfere with the X-ray analysis were examined. Two different techniques, zone analysis and particle analysis, were utilized. The elemental analysis performed by zones is in good agreement with a single particle analysis performed by Analytical Electron Microscopy. Single particle analysis performed using a 20 nm beam confirms the formation of small PtRu bimetallic clusters while larger particles (> 500 A) consist nearly exclusively of Ru. No correlation between particle size and metallic composition was observed.

Selective Hydrogenation of Carvone on Pt and Pt-Au Catalysts

Abstract The initial selectivity of platinum catalysts in carvone hydrogenation depends on the platinum dispersion. For example carvotanacetone (2-methyl-5-isopropylcyclo 2-ene-1-one) is the major product on catalysts with large platinum particles, whereas carvomenthone (2-methyl-5-isopropylcyclohexanone) is the major product on highly dispersed platinum catalysts. The previous platinum catalysts were modified by gold addition using a surface Redox reaction : Au3+ ions were reduced by hydrogen preadsorbed on the Pt surface area. The addition of gold to platinum on the highly dispersed catalyst modifies the initial selectivity toward that obtained with large platinum particles. In agreement with comparable electronic affinities of Pt and Au and comparable selectivities in O-xylene hydrogenation on Pt and Pt-Au catalysts, such change in selectivity in carvone hydrogenation cannot be explained by a ligand effect. On the other hand, results can be explained by assuming that gold adds selectively on the corners or the edges of the platinum particles. X-ray emission analysis of platinum-gold catalysts bears out that gold decorates the platinum particles rims pointing out that low coordination platinum atoms which are selectively deactivated by gold deposition, are responsible for the direct transformation of carvone into carvomenthone.