TRANSIENT KINETIC STUDIES OF CARBON MONOXIDE HYDROGENATION OVER SILICA-SUPPORTED RHODIUM AND RHODIUM-IRON CATALYSTS (METHANATION, ISOTOPE, TRACING)
Abstract
The mechanism of CO hydrogenation over Rh/SiO(,2) and RhFe/SiO(,2) catalysts was studied with transient kinetic experiments at 1 atmosphere and near 250(DEGREES)C. The catalysts were perturbed by sudden changes in reactant concentrations or isotope content while a computer-controlled mass spectrometer monitored the reactor effluent. CH(,4) responses from step changes from H(,2) to 3:2:1 H(,2),He,CO to H(,2) indicated that CO dominates the active surface of the catalysts during reaction and suggested that the accumulation of inactive carbon-containing intermediates on the catalyst surface leads to catalyst deactivation. ('13)CO tracing experiments, in which H(,2),He,('12)CO was replaced by H(,2),Ar,('13)O revealed the following: (1) the catalyst surface contains nearly a monolayer of CO which exchanges readily with gas phase CO. (2) the coverage of CO on the catalyst surface does not change appreciably for reaction times up to 4 hours. (3) the coverage of a lumped pool of active intermediates on the catalyst surface remains small ((TURN)0.03 monolayers or less) and decreases with reaction time in parallel with the decrease in the methane TON. (4) the CO dissociation step must be relatively slow since the observed time lags between ('13)CO(,s) forcing functions and ('13)CH(,4) responses were small. The appearance of ('13)C('18)O and ('12)C('16)O when ('13)C('16)O and ('12)C('18)O were passed over the catalysts at reaction temperature indicated reversible CO dissociation. Consequently, CO dissociation is a plausible step for CO hydrogenation over these catalysts. A comparison of simulated and experimental isotopic CO responses led to estimates for the forward (50 to 100 sec('-1) site('-1)) and reverse (0.3 to 1.0 sec('-1) site('-1)) CO dissociation rate constants. Transient experiments with D(,2) indicated that during reaction: H(,2) adsorbs reversibly and dissociatively, adsorbed H is nearly in equilibrium with gas phase H(,2), the production of methane is nearly in equilibrium with gas phase hydrogen, and the total amount of H on the surface is larger than expected. The apparent equilibrium between methane and gas phase hydrogen indicates small coverages of active intermediates and/or reversible hydrogenation steps.
Degree
Ph.D.
Subject Area
Chemical engineering
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