Osvaldo A. Scelza

Effect of the carbon pre-treatment on the properties and performance for nitrobenzene hydrogenation of Pt/C catalysts

Abstract This paper reports a study of the effect of the purification and functionalization treatments of a peach pit derived carbon on the properties and performance for nitrobenzene hydrogenation reaction of Pt/C catalysts. Results show that the elimination of inorganic impurities, mainly sulphur, enhances the nitrobenzene hydrogenation rate. Moreover, the functionalization treatments of purified carbon with ozone and hydrogen peroxide have a positive effect both on the Pt dispersion and on the hydrogenation capacity of the catalyst, while the HNO3-treatment has a lower effect. The effect of the different oxidants can be related to the nature of the functional groups developed on the carbon surface. Thus, HNO3-treated carbon displays a high density of both strong and weak acid sites, while H2O2- and O3-treated carbons show an important concentration of weak acid sites but a low concentration of strong acid sites, according to the TPD results. Moreover, the H2PtCl6 isotherms in liquid phase at 298 K show a stronger interaction of the metallic precursor with the carbons of low acidity (like those treated with H2O2 or O3) than with the most acidic carbon (treated with HNO3). Carbons functionalized with weak oxidants, which develop acidic sites with moderate strength and show strong interaction with H2PtCl6 during impregnation, would favour the Pt dispersion on the carbon surface and consequently the catalytic behaviour.

Preparation of Pt catalysts supported on activated carbon felts (ACF)

Activated carbon felts (ACFs) have been used as supports for Pt catalysts. The preparation was carried out by the impregnation method using chloroplatinic acid as metal precursor. The effect of impregnation time and surface chemistry of the support on the catalytic properties and the characteristics of the metallic phase have been investigated. Nitrobenzene (Nbz) hydrogenation in liquid phase at 25 °C and cyclohexane (CH) dehydrogenation in gas phase at 300 °C were used as catalytic tests. The state of platinum in reduced catalysts (at 100 and 350 °C) was studied by TPR and XPS. Oxygen surface groups only produce a slight effect on the catalytic properties. The use of low impregnation times (30 min) during the preparation of Pt/ACF leads to catalysts with Pt mainly deposited in the outer shell of the fibers, while at higher impregnation times, the metallic atoms seem to be deposited inside the pores. Pt(0) species appear in catalysts reduced at 100 °C by effect of the reducing properties of the carbon fiber exhibiting a considerable catalytic activity for Nbz hydrogenation.

States of Pt in Pt/C catalyst precursors after impregnation, drying and reduction steps

Abstract The characterization of Pt/C catalysts after impregnation and activation steps has been carried out by using different techniques: TPD; TPR; XPS; EXAFS and H 2 chemisorption. Furthermore, the catalytic activity of the samples has been tested in the cyclohexane dehydrogenation. The catalysts were prepared from a purified peach pit derived carbon and two H 2 O 2 -functionalized supports obtained from it. Results showed that the interaction of H 2 PtCl 6 with the carbon implies a redox process in which after impregnation and drying steps, the metal complex is stabilized as Pt 2+ on the carbon surface. The functionalization treatment seems to have no effect on the resulting state of platinum. During the activation step (thermal treatment in H 2 ), apart from the reduction of Pt species to Pt 0 , catalyzed and non-catalyzed reactions involving CO and CO 2 desorbed from the carbon surface, take place. It was also found that the Pt 2+ species are reduced to zerovalent Pt even by thermal treatment of the dried samples with He. The electronic state of reduced platinum is not modified by the differences in the support surface chemistry.

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.

Platinum—tin/alumina catalyst: Modification of the metallic phase after successive oxidation—reduction cycles

Abstract PtSn/Al2O3 catalysts, prepared by different deposition techniques and submitted to successive oxidation—reduction cycles at 500°C, were characterized by TPR and test reactions (cyclohexane dehydrogenation and cyclopentane hydrogenolysis), and compared with fresh samples. Interpretation of the results is based on the fact that in catalysts obtained by both coimpregnation and successive impregnation, a higher quantity of PtSn alloys was produced as a consequence of the oxidation—reduction cycles. In catalysts prepared by impregnating the support with the [PtCl2(SnCl3)2]2− complex, the cycles produced changes in the metallic phase probably due to either a surface enrichment of tin in alloy particles or a modification of the alloy phase composition.

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