T.F. Garetto

Oxidative catalytic removal of hydrocarbons over Pt/Al2O3 catalysts

The reaction kinetics, structure sensitivity, and in situ activation of cyclopentane and methane combustions were studied on Pt/Al2O3 catalysts of different platinum and chlorine loadings. The catalyst activities were evaluated through both conversion vs. temperature (light-off curves) and conversion vs. time catalytic tests. Cyclopentane oxidation turnover rates (TOF) increased dramatically with increasing Pt crystallite size while TOF values for methane oxidation increased only three times by diminishing the Pt dispersion from 65 to 15%. The reaction orders in oxygen were one (cyclopentane) and zero (CH4). For both reactions, the orders and activation energies did not change by changing the Pt dispersion. Results are interpreted in basis of two different reaction mechanisms over the metallic Pt active sites. Cyclopentane oxidation proceeds via a surface redox mechanism, being the dissociative adsorption of oxygen the rate-determining step. The observed turnover rate increase with increasing Pt particle size reflects an increase in the density of reactive Pt–O species resulting from higher Pt oxidation rates. The methane oxidation mechanism is interpreted in terms of Mars–van Kravelen reduction–oxidation pathways which include the abstraction of the first hydrogen on the adsorbed methane molecule as the rate-determining step. Low-conversion catalytic tests performed at constant temperature showed that on well-dispersed Pt/Al2O3 catalysts the cyclopentane conversion increases with time on stream, while the methane activity decreases. Activating induction periods during the oxidation of cyclopentane are related to the sintering of the metallic phase in reaction conditions. Hot-spots on the metallic particles together with the presence of gaseous water cause the formation of larger, more reactive, Pt crystallites, even at mild reaction conditions. The activation phenomenon ab initio of the reaction is not verified for methane oxidation on Pt/Al2O3 catalysts. The different structure sensitivity of the slowest steps in the reaction–oxidation mechanisms explains the existence of induction periods on well-dispersed Pt catalysts only for cyclopentane oxidation.

Structure sensitivity and in situ activation of benzene combustion on Pt/Al2O3 catalysts

Abstract The structure sensitivity and in situ activation of benzene combustion on Pt/Al 2 O 3 catalysts of different platinum and chlorine loadings were studied. The catalyst activities were evaluated through both conversion versus temperature (light-off curves) and conversion versus time catalytic tests. The light-off curves shifted to lower temperature with increasing Pt particle size, thereby suggesting that benzene combustion is a structure sensitive reaction. Kinetically-controlled catalytic tests confirmed that benzene oxidation turnover rates are preferentially promoted by larger Pt crystallites. Kinetic studies showed that the reaction orders and the apparent activation energy are not changed by changing the metallic dispersion. Results are explained by considering that benzene oxidation proceeds via a Langmuir–Hinshelwood mechanism which involves the rapid and strong adsorption of benzene on metallic platinum and assumes that the rate constant of oxygen adsorption is very low compared to the rate constant of the surface reaction. The number of PtO bonds of lower binding energy, i.e. the site density of more reactive surface oxygen, increases on larger Pt particles. Low-conversion catalytic tests performed at constant temperature showed that on well-dispersed Pt/Al 2 O 3 catalysts the benzene conversion increases with time, irrespective of the chlorine content on the sample. TEM examination and hydrogen chemisorption measurements suggested that the activity increase parallels a concomitant increase in the platinum particle size. In contrast, sintered samples (platinum dispersions lower than 10%) did not exhibit initial activation periods. It is proposed that the initial in situ activation of well-dispersed Pt catalysts is caused by the sintering of the metallic phase. Hot-spots on the metallic particles together with the presence of gaseous water cause the formation of larger Pt crystallites, even at mild reaction conditions. As a result, the benzene conversion increases with time until the formation of larger steady state Pt particles is completed.

On the sulfur-aided metal-support interaction in PtAl2O3Cl catalysts

The SMSI behavior displayed by sulfated PtAl2O3Cl catalysts with relatively low sulfur content has been explained by the alternative reactions or Redox reaction (2) would be sulfur catalyzed, giving the so-called sulfur-aided SMSI. In this paper the SMSI behavior displayed by sulfided samples with only irreversibly held sulfur (SPt = 0.20–0.35) was studied by the changes in H2 and O2 chemisorption, infrared spectroscopy, temperature-programmed reduction and H2S desorption at increasing temperatures. The changes in the 1380-cm−1 band (SO2−4 on the carrier) and in the 2060-cm−1 band (CO adsorption on Pt in linear form) were used to investigate the effects of the high-temperature reduction treatment. The TPR profiles of sulfated aluminas showed only one reduction peak at 650–680 °C; the presence of Pt gave rise to a low-temperature peak which appeared at 450–500 °C. Such a peak shifted to lower temperatures when the Pt content was increased. The data showed that the SMSI behavior of sulfated PtAl2O3Cl samples is better explained in terms of redox reaction (1). During the HTR treatment Pt cata-lyzes the reduction of sulfate ions. The H2S formed is adsorbed on the metal so that the H2 chemisorption is suppressed by the S blockage of the surface. Subsequent oxidation treatment causes oxidized S species in the metal surface to migrate back to the support. The metallic fraction is regenerated and, as a consequence, the H2 chemisorption capacity is restored.

Sulfurization of PtAl2O3Cl catalysts: VI. Sulfur-platinum interaction studied by infrared spectroscopy

Sulfur-platinum interaction was studied by ir spectroscopy of the coadsorbed CO, competitive hydrogenation reactions, and chemisorption of O2, CO, and H2, PtAl2O3 catalysts of different metal loadings and mean particle size were used. Infrared spectra for variable coverages of sulfur and CO were obtained. The predeposited S on Pt caused an upward shift of v(CO) from 2068 to 2083 cm−1; a superimposed band that shifted to the lower frequencies for decreasing CO coverages was detected. The kTB values (kTB, ratio of the adsorption equilibrium constant) for the competitive hydrogenation of benzene and toluene were determined. It was found that the kTB ratio increases from 4.2 for unsulfided samples to 6.0 for sulfided samples. On the other hand, the irreversibly held sulfur on the metal increased the ratio of the reversibly adsorbed H2 to the total adsorbed H2. These results are discussed considering electronic, geometric, and surface heterogeneity effects. A “localized” modification of the metal by the electron-acceptor properties of sulfur is advanced as a possible explanation.

Sintering of Pt/Al2O3 reforming catalysts: EXAFS study of the behavior of metal particles under oxidizing atmosphere

The effect of an oxidative atmosphere (300 °C) is studied on fresh and sintered unchlorinated naphtha reforming catalysts containing 0.6–1% Pt. The TPR profiles show that only one species is formed using our experimental conditions, regardless of the mean crystallite size of the metal particles. The structural information supplied by EXAFS compared with cuboctahedral particle modeling, implies that such species is a surface platinum oxide, the structure of which is close to that of PtO2, but largely distorted. This is true whether the catalyst is sintered or not.

Sintering and redispersion of Pt/γ-Al2O3 catalysts: a kinetic model

Abstract The sintering and redispersion kinetics of a Pt/Al2O3 naphtha reforming catalyst have been studied. The effect of the operating conditions, temperature, and oxygen and HCl concentration on the sintering and redispersion rates have been investigated. It was found that the rate of sintering depends on temperature and oxygen partial pressure. The rate of redispersion depends both on oxygen and HCl concentrations. A new kinetic model to study the sintering and redispersion phenomena has been developed. The model considers the evolution of the metallic dispersion as a reversible process and includes the effect of the operating conditions on the dispersion variation rate.

Simultaneous deactivation by coke and sulfur of bimetallic Pt–Re(Ge, Sn)/Al2O3 catalysts for n-hexane reforming

The simultaneous deactivation by coke and sulfur of monometallic Pt/Al2O3 and bimetallic Pt–Re(Ge, Sn)/Al2O3 catalysts was studied using n-hexane reforming as bifunctional test reaction and thiophene as poisoning molecule. The residual activities in the activity decay curves were used for measuring the catalyst sensitivity to coke formation and sulfur poisoning. Sulfur and carbonaceous deposits accumulated essentially on the metallic fraction and affected the catalyst activity for both monofunctional metallic and bifunctional metal–acid catalyzed reactions. The overall deactivation rate for n-hexane conversion increased in the order Pt–Ge<Pt⪡Pt–Sn≤Pt–Re. This deactivation trend resulted from the combination of the catalyst resistance to each individual deactivation process. Pt–Ge/Al2O3 was the most stable catalyst essentially because of its high thiotolerance for n-hexane transformation reactions and also because it showed low activity for dehydrogenation reactions leading to the formation of coke precursors. Sulfur poisoning on Pt/Al2O3 decreased monofunctional metal-catalyzed reactions but concomitantly increased the activity for acid-controlled skeletal rearrangement reactions; as a result, n-hexane conversion was only slightly diminished by the addition of sulfur. Pt–Sn/Al2O3 showed high resistance to coke deactivation but was severely poisoned by the addition of sulfur. The Pt–Re/Al2O3 activity was significantly decreased by both deactivation processes. Changes in catalyst selectivity are interpreted in terms of selective deactivation by coke and sulfur of individual reaction pathways involved in the n-hexane reforming mechanism.

Modelling of sulfur deactivation of naphtha-reforming catalysts Structure sensitivity in cyclopentane hydrogenolysis

The effect of particle size and the addition of Ir on the relative sulfur sensitivity of Pt-based catalysts has been studied. Cyclopentane hydrogenolysis, a structure-sensitive reaction, was employed as a test reaction and thiophene as the poisoning molecule. Fresh and sintered monometallic Pt/Al 2 O 3 and bimetallic Pt–Ir/Al 2 O 3 catalysts were used. Sulfur poisoning in the presence of simultaneous coke deactivation was characterised by two deactivation kinetic models. Model I assumes a single deactivation order for both deactivation causes, whereas in model II different deactivation orders were assumed (d c =1, d s =0.5). Thioresistance, calculated from the above models as the number of sulfur atoms initially needed to deactivate one atom of exposed Pt, was in the order: Pt-2Pt-1>Pt-1APt–IrPt-2A. According to the deactivation models, thioresistance mainly depends on k s , the specific rate constant of hydrogenolysis of adsorbed thiophene. The higher the hydrogenolytic constant, the lower the thioresistance. Moreover, both cyclopentane hydrogenolysis and sulfur poisoning depend on the mean particle size. When the particle size was increased, a higher hydrogenolytic activity and a lower thioresistance were observed. Thus sulfur deactivation is also a structure-sensitive reaction.

Modelling of sintering kinetics of naphtha-reforming Pt/Al2O3-Cl catalysts

The sintering kinetics of a conventional naphtha-reforming supported metal catalyst have been studied. Kinetic data were analysed using a general power-law equation. A quantitative relationship between the operational variables (T, po2) and the kinetic parameters was established. Sintering can be satisfactorily described as a first-order process. The observed activation energy was 132.6 kJ mol–1, within the range of activation energies for processes occurring at an atomic level. As po2 was increased, the sintering deactivation decreased, while the initial sintering temperature values increased.

Kinetic study of trichloroethylene combustion on exchanged zeolites catalysts.

In this paper is presented a kinetic study of the catalytic combustion of trichloroethylene (TCE) over Y-zeolites exchanged with several cations. The catalysts, based on zeolite, were prepared by ion exchange and characterized by means of physico-chemical techniques and then tested under kinetic conditions. The kinetic results obtained were interpreted using kinetic models of power-law type and Eley-Rideal. The results obtained indicate that catalyst Y-Cr is more active than Y-Co catalyst. The greater activity of catalyst exchanged with Cr can be attributed to the higher acidity that presented these catalysts.