J.M. Dominguez

The effect of shape and crystal structure of small particles on their catalytic activity

Abstract It is shown by the use of weak beam electron microscopy techniques, that the crystallographic structure of small particles on supported catalysts can be obtained. In the case of Rh, it is found that the particles are mainly icosahedral non-fcc or fcc cubo-octahedral depending on the strength of the interaction with the support. Pentane hydrogenolysis reaction using Rh particles of different structure is studied.

Preparation of platinum-gold bimetallic catalysts by redox reactions

Abstract Bimetallic Pt-Au/SiO2 catalysts were prepared by reduction of AuC1−4 using hydrogen preadsorbed on a parent monometallic Pt/SiO2 catalyst. In fact during that preparation a direct redox reaction can occur between the reduced platinum and AuCl−4. At low gold coverage the additive is deposited statistically on the whole surface of platinum particles (the reducer is H preadsorbed on Pt). At high gold coverage the additive is preferentially deposited on the high index planes of platinum particles (by direct redox reaction). During the formation of Pt-Au bimetallic entities sintering of platinum can occur as a result of simultaneous oxidation and reduction of platinum.

Preparation and characterization of copper based bimetallic catalysts

Abstract CuM (MPt, An, Ru) bimetallic catalysts were prepared by direct reduction of the corresponding metallic ions by Raney copper powders in aqueous media. The redox reaction was followed by pH and copper ions concentration measurements. The alkaline dealumination of a CuAl alloy powder (50/50 wt.%) produced non isotropic grains with various crystallographic planes (extended faces and corresponding rims). Depending on the relative redox potential of Mn+/M and Cun+/Cu the deposit on the grain exhibits a selectivity. The rim sites of copper (Cu(I)) are oxidized first and Ru3+ can only oxidize these sites. For Pt4+ and Au3+, in contrast, all Cu(I) and Cu(O) sites are involved. The location of the deposits is related to the selectivity for the polyol transformations on the bimetallic catalysts.

Modification of copper by redox reaction: location of Ru on Cu-Ru bimetallic catalysts

Raney copper catalysts were modified by addition of ruthenium by means of an oxido-re-duction reaction between copper surface and ruthenium chloride in aqueous solution. Energy dispersive spectroscopy fitted to a STEM unit allowed to conclude that the active sites for the selective conversion of glucitol into 1,4–3,6-dianhydroglucitol are composed of ruthenium and chlorine located on low coordinated copper atoms.

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.

Selective Modification of Pt Active Sites on Pt–Au/C Catalysts Prepared by Surface Redox Reactions

This study focused on the selective deposition of Au0 onto (111), (100) faces and (111)/(100) edges of cuboctahedral Pt particles present on the Pt/C(graphite) model system. The Pt–Au/C catalysts were prepared by novel surface redox methods involving the direct reduction (DR) of AuCl4− species onto the Pt particles or reducing these species on the Pt–H interface, i.e., the refilling (RE) method. The presence of Au on the Pt particles was verified by means of high-resolution energy dispersive spectroscopy (EDS), and, after treatment at 300 °C in H2, the formation of crystalline Au0 aggregates was verified by X-ray wide-angle diffraction; further treatments at 500 °C in H2 led to a true Pt–Au solid solution. The Monte Carlo simulation methods indicated the selective deposition of Au0 onto the (111)/(100) edges of the Pt cuboctahedral particles when the relative Au concentration varied from 10 to 50 wt% Au. The catalytic conversion of n-heptane on the Pt–Au/C (DR and RE solids) catalysts presented an oscillatory behavior with respect to Pt/C, indicating modification of the active Pt ensembles, driven by the energy released during the exothermic n-C7 dehydrogenation and cracking reactions, which should enhance the Au0 mobility at the Pt particle surface level.

Preparation of Pt-Rh/Al2O3-CeO2 catalysts by surface redox reactions

Abstract Three-way automotive Pt-Rh catalysts were prepared either by coimpregnation of the two noble metals (C.I. catalysts) or by an original method of successive impregnations with a reduction step after platinum impregnation (S.I. catalysts). It was shown that addition of rhodium can lead to opposite effects on the activity of platinum deposited on alumina-ceria for the reaction of propane oxidation, according to the preparation procedure: the coimpregnation of the two metals induces an inhibition of platinum activity when the addition of rhodium by successive impregnations leads to the reverse effect. The differences between the two preparations are greater on oxidized samples after aging at high temperature (900°C). These results are explained by the formation of alloy particles on coimpregnated catalysts while rhodium added by successive impregnations would be selectively deposited on surface cerium, avoiding the formation of Pt-Rh alloy. Energy dispersive spectroscopy fitted to a STEM unit allowed to bear out such hypothesis.

Hydrotreating of heavy vacuum gas oil on unsupported and supported Mo-, W-, Nb- nitrides

This work shows the results obtained in hydrotreating reaction from using a real fraction. The results showed a relationship with the structural and crystalographyc properties of the used materials.

Synthesis, Characterization and Catalytic Properties of Al 2 O 3 -TiO 2 Mixed Oxides Prepared by Sol-Gel

Some synthesis parameters like R-OH/OBu, H 20 /OBu and composition (Al/Ti) ratios influence the textural and catalytic properties of the mixed Al 2 O 3 -TiO 2 oxides. The Radial Distribution Function of the pure and mixed oxides shows that Ti 4+ incorporation favors the long-range-order in the mixed aggregates as well as the surface area loss at high temperatures, i.e. 900 °C. The catalytic dehydration of isopropanol towards propylene and the HDS of light gasoils indicate that catalytic properties of the sol-gel mixtures are comparable to the activity of reference catalysts, i.e. Al 2 O 3 and Ni/Al 2 O 3 .