A. Borgna

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 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.

Relationship between the kinetic parameters of different catalyst deactivation models

Abstract This paper presents a mathematical relationship between the parameters of Levenspiel’s Deactivation Kinetic Model (LDKM) and those of the Deactivation Models with Residual Activity (DMRA) and their evolution over time. This correlation provides an explanation for the erroneous variation obtained in the kinetic parameters (deactivation order and deactivation function) over time when LDKM is used to fit deactivation data having a certain level of residual activity, which frequently leads to systematic errors in estimating intrinsic parameters, such as activation energies. The variations of the LDKM parameters cannot in fact be related to a physical phenomenon, but are only a consequence of a mathematical artifact. The methodology developed in this work provides a valuable tool for the comparison and discrimination between different models used in kinetic studies. The equations here presented are applied to analyze the deactivation by fouling of Pt/Al2O3 reforming catalysts during methyl cyclohexane dehydrogenation.

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.

Activity, selectivity and coking of bimetallic Ni-Co-spinel catalysts in selective hydrogenation reactions

The influence of the calcination and reduction temperatures of a Ni-Co-ZnAl 2 O 4 catalyst was correlated with its catalytic activity in the hydrogenation of acetylene. A well interdispersed Ni-Co, catalyst supported in a ZnAl 2 O 4 spinel-like structure was obtained by using a coprecipitation method. Cobalt apparently effects a dilution of Ni surface ensembles, increasing the selectivity to ethylene. The influence of the operating temperature on activity, coking rate and selectivity was analyzed using a deactivation kinetic model.

Effect of alloying on the sulfur resistance of bimetallic Pt-based catalysts

The effect of alloying on the thiotolerance of bimetallic Pt-Ge/Al2O3 and Pt-Sn/Al2O3 catalysts was studied. Unalloyed and alloyed samples were obtained by reducing both catalysts at 573 and 773 K, respectively. Gas chemisorption, catalytic testing, temperature programmed reduction (TPR) and temperature programmed desorption (TPD) experiments showed that after H2 reduction at 773 K a part of tin (germanium) cations reduces and forms a metallic Pt-Sn(Ge) alloy. The relative sensitivity to sulfur poisoning of the catalysts was established using a cyclohexane dehydrogenation as test reaction and thiophene as the poisoning molecule. It was found that alloying does not increase the thiotolerance of Pt-Sn catalysts compared to monometallic Pt catalyst. On the contrary, alloyed Pt-Ge/Al2O3 was the most thiotolerant catalyst because Ge decreases the density of charge of alloyed Pt thereby weakening the bond strength with electrophilic sulfur.

High-thiotolerant Pt-Ge/Al2O3 naphtha reforming catalysts by in-situ alloying

The effect of alloying on the generation of high-thiotolerant Pt-Ge/Al2O3 naphtha reforming catalysts was studied. The state of the metallic fraction after successively reducing the bimetallic catalyst with H2 at 300°C and 500°C was characterized by a variety of physical, spectroscopic, and catalytic techniques. It was found that the hightemperature treatment reduces a part of germanium cations to metallic Ge and forms PtGe alloy clusters. In such alloyed bimetallic particles, Ge acts as an electron-acceptor element, decreasing the electronic density of platinum. Alloying diminishes dramatically both the H2 uptake and the activity for dehydrogenation reactions but, on the other hand, clearly enhances the resistance to sulfur poisoning. Catalyst thiotolerance is increased because alloying weakens the strength of the S-Pt bond and thereby reduces the amount of irreversibly held sulfur on platinum. It was found that the alloy formation is reversible when the catalyst is submitted to oxidation-reduction cycles. Although treatment with an oxidizing atmosphere at 450°C destroys the alloyed bimetallic particles, subsequent reduction with H2 at 500°C restores the Pt-Ge alloying and enhances the catalyst thiotolerance.

Effect of the addition of acidic and basic compounds on the catalytic behavior of exchanged zeolites in the alkylation of toluene with methanol

The alkylation of toluene with methanol over alkali and alkaline-earth exchanged Y zeolites was studied. While Mg(Ca)-exchanged zeolites produced the selective alkylation of the benzene ring, alkali exchanged zeolites were highly selective for side-chain alkylation. “in situ” poisoning experiments were performed by doping the reactant mixture with an acid or a base. Alkaline-earth exchanged zeolites were only deactivated by adding base compounds. By the contranst, alkali exchanged zeolites were strongly deactivated by the addition of either acid or base compounds, demonstrating that a cooperative action of acid/base pairs is required for side chain alkylation.

Modeling of Simultaneous Deactivation Processes: Thioresistance and Coking on Pt Based Catalysts

Summary This paper deals with the modeling of simultaneous deactivation processes. Deactivation models were developed to measure the thioresistance of naphtha reforming catalysts in presence of simultaneous coke deactivation. The hydrogenation of benzene on Pt/Al2O3, Pt-Re/Al2O3 and Pt-Ir/Al2O3 catalysts was studied in a differential reactor; thiophene was the poisoning molecule. The results were first interpreted in terms of empirical parameters like initial deactivation, steady-state activity, and power-law equations, by accepting that the overall deactivation rate was a simple sum of the each individual deactivation rate (hypothesis of independence). In a second part, the thioresistance was evaluated from the overall deactivation function, ψt, by using different mechanisms in a LHHW kinetics. The sulfur resistance was in the order Pt-Ir=Pt> Pt-Re. The sulfur poisoning mechanism was the same in all the cases, the hydrogenolysis of adsorbed thiophene being the rate-determining step.