S.R. de Miguel

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.

Effect of acid treatment on the properties of an activated carbon

The enhancement of the activity of a carbon treated with HF is due to the drastic diminution of its inorganic impurities, which produces an increase of the surface area of the carbon, a stronger Pt-C interaction and a higher Pt dispersity.AbstractУвеличение активности углерода, обработанного HF, является результатом сильного уменьшения количества его неорганических примесей, что приводит к увеличению поверхностной площади углерода, более сильному взаимодействию Pt-C и более высокой дисперсности Pt.

Competitive Adsorption of H2PtCl6, and HCl on Al2O3 in the Preparation of Naphtha Reforming Catalysts

The HCl competition in the H 2 PtCl 6 deposition on Al 2 O 3 pellets to obtain 0.38% Pt was studied. The radial profiles of platinum and chlorine surface concentration for different HCl concentrations in the impregnating solution were measured. For low HCl concentrations the Pt penetrations into the pellet are incomplete and the Pt profiles sharply decrease towards the interior whereas the Cl profiles increase. Both profiles are uniform for high HCl concentration. The results were interpreted through a mathematical model based on a diffusion adsorption process with very fast and irreversible H 2 PtCl 6 , and HCl adsorptions.

Effect of Sn addition on the catalytic properties of the metallic phase in Pt−Sn/Al2O3 catalysts

Platinum-tin/alumina supported catalysts have been studied. The TPR results show an assisted reduction of tin by platinum. The effect of alloying in cyclohexane dehydrogenation, o-xylene hydrogenation and cyclopentane hydrogenolysis is discussed.AbstractИсследовали платино-оловянные катализаторы, нанесенные на оксид алюминия. Результаты ТПВ свидетельствуют о восстановлении олова платиной. Овсуждают эффект сплавления в реакциях дегидрогенизации циклогексана, гидрировании о-ксилола и гидрогенолизе циклопентана.

Performance of Pt−Re/Al2O3 catalysts with different radial distribution profiles

Pt−Re/Al2O3 prepared under different impregnating conditions were characterized by TPR and tested in naphtha and n-heptane reforming. Catalysts showed different Pt and Re radial distributions and reduction profiles. When both metals interact, higher dehydrocyclization and conversion are retained after a deactivation step, meanwhile, when Re is segregated from Pt the catalytic performance is negatively affected.AbstractКатализатор Pt−Re/Al2O3, приготовленный при различных условиях пропитки, характеризовали с помощью ТПВ и тестировали в реформинге лигроина и н-гептана. Наблюдались различные распределения радикалов Pt и Re и профилей восстановления на катализаторах. Когда оба металла взаимодействуют, после ступени дезактивации наблюдались повыщенные дегидроциклизация и конверсия, в то время как разделение Re от Pt не оказывает отрицательного эффекта на каталитическую активность.

n-DECANE DEHYDROGENATION ON BIMETALLIC PtSn AND PtGe CATALYSTS PREPARED BY DIP-COATING

The catalytic performance of Pt, PtSn and PtGe supported on γ-Al2O3 (γ-A) deposited by dipcoating of spheres of α-Al2O3 (α-A) was studied in the n-decane dehydrogenation. The effect of Sn and Ge addition to Pt on the activity and selectivity was analyzed. The catalytic characterization was carried out using cyclohexane dehydrogenation (CHD), cyclopentane hydrogenolysis (CPH), temperature-programmed reduction (TPR), hydrogen chemisorption, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and scanning electronic microscopy (SEM). Pt(0.5)Sn/γ-A/α-A catalyst had the best catalytic performance and showed a low electronic interaction between the metals, with a surface segregation of Sn and the presence of oxidized Sn stabilized on the support. PtGe catalysts presented strong interactions with probable alloy formation. The catalytic performance of these catalysts is comparable to that reported in the patents.

EFFECT OF THE CARBON BURN-OFF ON THE PERFORMANCE OF ACTIVATED CARBON AS A SUPPORT FOR HYDROGENATION CATALYSTS

The hydrogenation activity of Pt supported on activated carbon would depend on both the Pt dispersity (which can be modified by a partial carbon burn-off in air) and the impurities concentration.

Performance of Pt−Sn/Al2O3−Cl catalysts in isopropanol decomposition

Pt−Sn/Al2O3, Pt/Al2O3 and Sn/Al2O3 catalysts were tested for isopropanol decomposition. Dehydration activity was related to catalysts acidity. Pt/Al2O3 and bimetallic catalysts show a similar dehydrogenation activity, however, monometallic catalysts have a deactivation rate non-observable in Pt−Sn/Al2O3.AbstractКатализаторы Pt-Sn/Al2O3, Pt/Al2O3 и Sn/Al2O3 тестировали в реакции разложения изопропанола. Дегидратирующую активность связывали с кислотностью катализаторов. Для Pt/Al2O3 и биметаллических катализаторов обнаружена подобная дегидрирующая активность, однако, для монометаллических катализаторов скорость дезактивации была измерена, а для Pt-Sn/Al2O3 не обнаружена.