Transient and isotopic tracing kinetic study on supported rhodium catalysts in the CO hydrogenation reaction

Date of Completion

January 1989

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

Methanation and hydrocarbon synthesis from CO and H$\sb2$ was studied using 5.2 wt% Rh/Al$\sb2$O$\sb3$ and 2.5 wt% Rh/MgO catalysts at 1 atm total pressure. Transient responses to the switches between various feed mixtures, including $\sp{13}$C labeled feeds, to a gradientless microreactor were used to obtain the in-situ surface composition of the catalysts.^ On Rh/Al$\sb2$O$\sb3$ in the temperature range of 180-260$\sp\circ$C the coverage of surface CO varies between 0.98 and 0.6 of a monolayer. This surface CO can be all exchanged off the surface by $\sp{13}$CO. The methanation reaction passes through a small reservoir of a very active carbon species CH$\sb{\rm x}$, between 0.03 and 0.08 of a monolayer. Large amounts of inactive carbon and formate species, between one and two monolayers were found. The formate and part of the inactive carbon species must be located on the alumina support. These two species do not participate in the mechanistic carbon pathway from CO to form methane, where the dissociation of CO must be considered as the rate determining step.^ On RH/MgO, it was found that the methane turnover frequency is an order of magnitude less than that when Rh/Al$\sb2$O$\sb3$ is used. Characterization of the catalyst before and during the kinetic study revealed drastic changes in the amount of H$\sb2$ and CO chemisorption and a large loss in its initial activity. The CO temperature programmed desorption study showed also a drastic change between the spectrum obtained from the catalyst before and after exposure to synthesis gas. The temperature programmed desorption and the CO temperature programmed reaction study for the two catalysts suggests that the binding energy of CO on Rh/MgO is higher than on Rh/Al$\sb2$O$\sb3$. Between 260$\sp\circ$C and 300$\sp\circ$C, the surface coverage of CO on Rh/MgO is about one monolayer, and that of active carbon species CH$\sb{\rm x}$ less than 0.02 of a monolayer. Therefore, as for the Rh/Al$\sb2$O$\sb3$, the rate of dissociation of surface CO largely controls the overall methanation rate. When the Rh/MgO catalyst reaches a reproducible activity, negligible amounts of inactive carbon are found. Formate may be present on the magnesia support, but is not reactive towards hydrogen and does not participate in the sequence of steps to produce methane. ^

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