Instituto de Investigaiones en Catálisdis y Petroquímica -INCAPE- (UNL-CONICET),

Sgo. del Estero 2654, (3000) Santa Fe, Argentina. Email: capesteg@fiqus.unl.edu.ar

Platinum-based catalysts are highly active for oxidative removal of small amounts of aromatics from effluent streams. In previous work it has been observed that the catalysts are activated on stream, ab-initio of the hydrocarbon combustion reaction. The causes of initial activation periods remain unclear. Several authors have specifically investigated the effect of varying the metallic particle size on the catalytic combustion of different hydrocarbons. Nevertheless, the results obtained are conflicting, probably due to the fact that the correlation between catalytic activity and metallic dispersion depends on the type of hydrocarbon to be abated. In this work, the sensitivity of cyclopentane and methane oxidation turnover rates to Pt crystallite size was studied over Pt/Al₂O₃ catalysts. Our goal was obtain further information on the catalyst activation phenomenon and on the structure sensitivity of the hydrocarbon combustion reaction.

A Pt(0.3%)/Al₂O₃ catalyst (catalyst A) was prepared by impregnation of g -Al₂O₃ with an aqueous solution of H₂PtCl₆.6H₂O and HCl. A set of three catalysts with D_Pt (Pt dispersion) of 38%, 24%, and 15%, respectively, was prepared by sintering catalyst A (D_Pt = 65%) in a 2% O₂/N₂ atmosphere at different temperatures. Accessible metal fractions were measured by H₂ chemisorption. The metallic phase was examined by transmission electron microscopy (TEM) in a JEOL 100 CX microscope. The hydrocarbon oxidation was carried out at 1 atm in a fixed-bed tubular reactor. Benzene (0.65%) and methane (2%) were fed in a 10% O₂/N₂ mixture. Two experimental procedures were used for catalyst testing. The complete oxidation of hydrocarbons was studied by obtaining curves of hydrocarbon conversion (X) as a function of temperature (light-off curves). The temperature was raised by steps of about 25 K, from 298 to 673 K. More fundamental differential reactor experiments (less than 10% conversion) were performed at constant temperature.

Fig. 1 shows the X vs T curves obtained on catalyst A (D_Pt = 65%) in two consecutive catalytic tests. The CH₄ combustion on Pt occurs at temperatures significantly higher as compared with cyclopentane combustion. In fact, the temperature at X = 50% was = 793 K for methane and = 493 K for cyclopentane. In cyclopentane combustion, the X vs T curve of the second run is clearly shifted to lower temperatures as compared to that obtained in the first run. On the contrary, the consecutive light-off curves in CH₄ combustion are similar and the activation phenomenon is not verified. TEM examination showed that the metallic fraction appears severely sintered after the two consecutive catalytic tests; the D_Pt value of fresh catalyst A as measured by H₂ chemisorption (65%) diminished to 18% (cyclopentane) and 15% (methane).

In order to establish the effect of the Pt crystallite size on catalyst activity, additional

Fig. 1: Consecutive light-off curves Fig.2: Turnover rates as a function of D_Pt

kinetically-controlled catalytic test were performed. In cyclopentane combustion, the activity of catalyst A slowly increased with time along the 20 hour runs; in contrast the CH₄ conversion was constant. As shown in Fig. 2, methane combustion turnover rates (TOF, s^-1) do not significantly change with D_Pt. On the contrary, TOF values on cyclopentane combustion drastically increase with increasing Pt particle size, thereby showing that the deep oxidation of cyclopentane is a sensitive-structure reaction. The initial activation of well-dispersed Pt catalysts in cyclopentane combustion is caused by the sintering of the metallic phase, which occurs in reaction conditions even if the cyclopentane combustion reaction is performed at low-temperature and low-conversion regimes. The reaction is highly exhotermic and the Pt crystallite temperature is significantly increased in reaction conditions. Hot-spots on the metallic particles together with the presence of gaseous water cause the metal phase sintering at mild reaction conditions and the formation of larger, more active, Pt particles.

Kinetic data were interpreted by considering a power-law rate equation: , where r₀ is the initial reaction rate. Over all the samples, the reaction orders in the cyclopentane combustion were a @ 0 and b @ 1 while values of a @ 1 and b @ 0 were determined for CH₄ combustion. The apparent activation energies (E_a) were 11 ± 1 kcal/mol (cyclopentane) and 17 ± 1 kcal/mol (CH₄), irrespective of the mean Pt crystallite size of the sample. This suggests that the structure sensitivity exhibited by the cyclopentane combustion is not caused by a change in the reaction mechanism when the metallic dispersion varies.

The kinetic results show that the two molecules are oxidized by different mechanisms. It is proposed that the cyclopentane combustion occurs through a Mars-Van Krevelen type redox mechanism, in which the dissociative adsorption of oxygen on Pt is 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 combustion is interpreted by considering a Langmuir-Hinshelwood mechanism, where the rate-determining step is the abstraction of the first hydrogen on the adsorbed methane molecule and the OH^- groups are the predominant surface species.

Opportunities for collaboration (C.R. Apesteguia)

Main research lines of our group on environmental catalysis are:

We are using Pt-supported catalysts for deep oxidation of hydrocarbons, particularly aromatic hydrocarbon mixtures. Fundamental studies are focused on the reaction mechanism pathways and on the sensitivity of hydrocarbon oxidation turnover rates to Pt crystallite size

We are also studying the catalytic combustion of chlorinated volatile organic compounds (CVOC´s) over transition metal ion-exchanged zeolites. Our target is to relate the physicochemical, structural and acid/base properties of Co, Cr, Mn and Fe exchanged zeolites with the catalytic behavior in the combustion of chloride compounds, such as carbon tetrachloride (TCC) and trichloroethylene (TCE).

In this project we explore the use of solid bases to replace environmentally problematic and corrosive liquid bases currently employed in important industrial processes. Magnesium oxide promoted with group IA or IIA metals and ex-hydrotalcite Mg_yAlO_x mixed oxides are used in valuable organic reactions requiring carbanion intermediates, such as aldol condensation of aldehydes and ketones and linear alcohol coupling to branched higher alcohols. Basic site density and strength are characterized and related with site requirements of the rate-determining steps.

We also study the use of alkali-exchanged Y and X zeolites for the side-chain alkylation of aromatic rings. Specifically, we investigate the side-chain alkylation of toluene with methanol for producing a mixture of styrene and ethylbenzene. This reaction offers potential economical advantages as compared with the conventional homogeneously catalyzed Friedel-Crafts process, which use ethylene and benzene as reactants.

We explore the use of HPA and its salts for acid-catalyzed reactions in liquid phase reactors. These materials are potential replacements for harmful liquid acid catalysts. Homogeneous (HPA) and heterogeneous (supported HPA, nonsoluble HPA salts) catalysts are employed in esterification and acylation reactions. The catalyst performance is evaluated and compare to current conventional catalysts (sulfuric acid, resins).