A chemical probe for catalyst nonuniformity

Abstract

IntroductionThis note reports the results of an experimental study of diffusion and reaction in a catalyst particle that has a radial profile of active ingredient (Pt) described by the function rn (1 -x)". The deuterium-neopentane exchange (DNE) reaction was employed in the study (Dwyer et al., 1968;Ernst and Daugherty, 1978;Ernst and Wei, 1975). The theory is based upon the work of Wei and Prater (1962) and Wei (1 962a, b). Ernst and Daugherty (1978) showed that the highly coupled system of rate equations which describe the behavior of Dexchanged hydride species such as neopentane can be uncoupled if the following transformation is made. Wang and Varma (1980) solved the intraparticle diffusionreaction problem for a catalyst pellet that has an activity profile described by the equation k ( x ) = m z ( 1 -x )~. Their solution,can be used to calculate the effectiveness factor for each bi species if the Thiele modulus is defined as @i = R The selectivity for the DNE reaction defined as the average number of D atoms in the initial hydride product (Kemball, 1954) was shown to be related to the effectiveness factor ratio (Dwyer et al., 1968)where n is the number of exchangeable hydrogen sites on the reactant hydride. (For DNE reaction studies, the t-butyl ion is observed rather than the neopentane molecule; therefore n = 9.) ExperimentalThe method of preparation of nonuniformly active Pt catalysts was described in a previous article (Shyr and Ernst, 1980). The method involved impregnation of y-alumina particles of spherical geometry with chloroplatinic acid solution containing the coingredients citric acid and NaBr, which modified the Pt profile. The catalyst support was gamma alumina (Rhone Poulenc Inc. Type GOD-30). Properties of the support include: particle dia. 0.36 cm; surface area, 150 m2/g; total pore volume 1.7 mL/g; and average pore dia., 450A (45 nm).A sample of catalyst from the preparation batch was examined by electron probe microanalysis (EPM) and found to have a radial profile of Pt that could be represented by the function m(1 -x)" where cr = 1.1The batch reaction system was a slightly modified version of an earlier system (Ernst and Wei, 1975). It consisted of a 500 mL spherical Pyrex reactor containing a porous pedestal for 0.04 (Hiltzik, 1983).