Sergio R. de Miguel

State of metallic phase in PtSn/Al2O3 catalysts prepared by different deposition techniques

Abstract The state of the metallic phase after thermal treatments in Pt Sn/Al 2 O 3 catalysts, prepared by different deposition techniques (coimpregnation and successive impregnations), was investigated by temperature programmed reduction, X-ray photoelectron spectroscopy and cyclohexane dehydrogenation and cyclopentane hydrogenolysis reactions. In all catalysts the presence of zerovalent Sn was detected due to a catalytic effect of platinum on tin reduction. Besides, there seems to exist a relationship between the amount of [Pt Cl 2 (Sn Cl 3 )2] 2- complex deposited on the alumina after the impregnation step, tin reducibility and Pt Sn alloy formation.

FTIR and XPS study of supported PtSn catalysts used for light paraffins dehydrogenation

A comparison between the characteristics of the metallic phase (studied by FTIR and XPS) of Pt and PtSn catalysts supported on Al2O3, K-doped Al2O3 and MgO (used for light paraffins dehydrogenation reactions) is reported in this paper. The beneficial effects produced by tin addition to platinum, both in the increase of the selectivity to propene and the low coke formation, would be related with the possible electronic modifications of Pt by Sn, with probable formation of alloys, mainly for Al2O3 and MgO supported bimetallic catalysts. On the other hand, the modification of the electronic state of Pt by Sn addition appears to be of a minor importance in bimetallic catalysts supported on K-doped Al2O3.

Hydrogenation of carvone on Pt–Sn/Al2O3 catalysts

Abstract The effect of Sn concentration in Pt–Sn/Al 2 O 3 catalysts prepared by different procedures on the catalytic behavior in the carvone hydrogenation in liquid phase was studied. Results indicated that the increase of the Sn amount added to Pt modified the catalytic behavior, favoring the formation of unsaturated ketones and the simultaneous production of small quantities of unsaturated alcohols as reaction intermediaries. On the other hand, Pt/Al 2 O 3 catalyst only produced carvomenthone as the main reaction intermediary.

PtSn/Al2O3 catalysts: Studies of the impregnation step

Abstract The impregnating solution (H2PtCl6 + SnCl2 + 1 M HCl; Sn(II)/Pt(IV) molar ratio = 1.6) used to prepare Pt Sn/Al2O3 catalysts was studied by uv-visible spectroscopy. Three different species were found in the above-mentioned solution: Pt(II), Sn(IV) and a Pt Sn complex with the structure (PtCl2(SnCl3)2)2−. Moreover, solutions with Sn(II)/Pt(IV) molar ratio of 1 and 3 were studied. The results indicate that the complex formation takes place only for solutions with Sn(II)/Pt(IV) molar ratio higher than 1. The Pt(0.3 wt%)-Sn(0.3 wt%)/Al2O3 catalyst precursors prepared by using three different deposition techniques: a) coimpregnation, b) two-step impregnation (Pt deposition, drying, Sn deposition) and c) two-step impregnation as in b) but in an inverse sequence of metal deposition, were characterized by diffuse reflectance spectroscopy and temperature programmed reduction. When the catalyst is prepared using the deposition techniques named a or b, the (PtCl2(SnCl3)2)2− complex is adsorbed on the alumina surface maintaining its structure. But when the catalyst is prepared using the technique named c, the complex formation does not take place, and only Pt(IV) and Sn(IV) species are detected on the alumina. Hence, the use of different impregnation techniques leads to different adsorbed species in the Pt Sn/Al2O3 catalyst precursors.

Nature of the metallic phase in platinum-germanium/alumina catalysts

Abstract Bimetallic Pt Ge(0.3-0.3 wt.-%)/Al2O3 catalysts prepared by different deposition techniques (coimpregnation and successive impregnations) were characterized by temperature-programmed reduction (TRP) and test reactions (cyclohexane dehydrogenation and cyclopentane hydrogenolysis). From TPR and test reactions results, a catalytic effect of Pt on Ge reduction and the presence of Pt Ge alloy particles (with very low dehydrogenation and hydrogenolysis activities) were observed. It was also found that the use of different impregnation techniques led to catalysts with similar metallic phase characteristics while addition of chlorine produced a lower quantity of Pt Ge alloy.

Highly selective and stable multimetallic catalysts for propane dehydrogenation

Different mono (Pt), bi (Pt–Sn, Pt–Pb, Pt–Ga) and trimetallic (Pt–Sn–Ga) catalysts based on Pt and supported on different materials (Al2O3, Al2O3–K and ZnAl2O4) were tested under severe process conditions in the propane dehydrogenation reaction (both in continuous and in pulse reactors). Results show that the Pt–Sn–Ga/ZnAl2O4 catalyst has a better and more stable performance in propane dehydrogenation (high yield to propene and low coke deposition), than the other bi- and trimetallic systems and a commercial catalyst. Thus, the use of an adequate support (ZnAl2O4) in combination with the addition of Ga to the Pt–Sn bimetallic system enhances the catalytic performance. © 2000 Society of Chemical Industry

Stability and regeneration of supported PtSn catalysts for propane dehydrogenation

This paper reports the catalytic performance for propane dehydrogenation of Pt and PtSn catalysts on inert supports, studied by means of pulse and flow reaction techniques, in order to determine effects of support on catalytic behavior. K addition to Pt/Al 2 O 3 and PtSn/Al 2 O 3 significantly decreases catalyst deactivation, by lowering the amounts of coke deposited on the support and metal, while producing a coke with a lowe,degree of polymerization. Sn addition to Pt/Al 2 O 3 improves the activity and selectivity to propylene and decreases deactivation. The addition of Sn to Pt/ZnAl 2 O 4 not only enhances activity but also improves catalyst stability. The existence of strong interactions between Pt and Sn, with probable alloy formation, is suggested both on an acidic support like Al 2 O 3 , and on a pH-neutral support like ZnAl 2 O 4 . Bimetallic PtSn/ZnAl 2 O 4 catalysts appear to have the highest stability and reversibility after several reaction steps including the corresponding regeneration steps between them.

Effect of lithium addition upon γ-Al2O3 for isopropanol dehydration

The effect of lithium addition to γ-Al 2 O 3 on the acidity and on the catalytic behaviour in the isopropanol dehydration reaction is studied. Results show that there is an important blocking or poisoning effect of lithium on the Lewis Al 3+ sites of the alumina surface ; these sites play an important role in the isopropanol dehydration reaction to di-isopropylether and propylene. The di-isopropylether formation is drastically reduced by lithium addition, while the propylene formation can be maintained, even at a high lithium content, by increasing the reaction temperature. Results are interpreted by considering that the ether would be produced by a concerted mechanism (E 2 ), and the olefin formation could be carried out by both E 2 or E 1 mechanisms.

Effect of the addition of alkali metals on the metallic phase of Pt/Al2O3 catalysts

The effect of the addition of alkali metals to Pt/Al2O3 catalysts for dehydrogenation of paraffins was studied. Results can be interpreted in terms of different effects. Thus, the addition of alkali metals produces a poisoning of the acid sites of the alumina support and a modification of the characteristics of the metallic phase. The latter effect involves not only an increase of the Pt particles size, but also an electronic modification of the metallic phase. These two effects are more marked with alkali metals with higher ionic radius, such as K.

Characterization of γ-alumina doped with Li and K by infrared studies of CO adsorption and27Al-NMR

The influence of alkali metals on the Lewis acid sites of gamma-alumina has been studied with NMR and IR of adsorbed CO. The physical properties of the alumina are slightly changed by the addition of low alkali metal concentrations. Li and K are adsorbed selectively on tetrahedral Lewis Al3+ sites of the alumina surface. These effects on the tetrahedral cations are more marked with increasing molar concentration and ionic radius of the alkali metal added to alumina. Results demonstrate that the influence of the alkali metals is not only due to steric factors but also to possible electronic modifications.