Mechanistic studies of the homogeneous catalysis of the water gas shift reaction by rhodium carbonyl iodide

TLDR: In this paper, the results of semiquantitative kinetic studies and of quenching experiments are reported, and a steady state kinetic analysis and assumptions about rate-determining steps show the mechanism to be consistent with the observed behavior of the system.

Abstract: The water gas shift reaction, H/sub 2/O + CO = H/sub 2/ + CO/sub 2/, is effectively catalyzed at low temperatures and pressures by a catalyst system based on rhodium carbonyl iodide. The results of semiquantitative kinetic studies and of quenching experiments are reported. In the temperature range 80 to 100/sup 0/C the catalytic reaction shows first-order dependence on the partial pressure of CO and inverse dependence on acid and iodide concentrations. The major species in solution are rhodium (III) carbonyls. However, below 65/sup 0/C the reaction is independent of CO pressure, and exhibits second-order dependence on iodide concentration and positive dependence on acid concentration. Rhodium (I) species now become predominant in solution. The temperature dependence of the rate shows corresponding behavior. Above 80/sup 0/C the Arrhenius plot is linear with a slope which yields an apparent activation energy of 9.3 kcal mol/sup -1/. However, below 70/sup 0/C it is concave downwards with a slope at 55/sup 0/C corresponding to 25.8 kcal mol/sup -1/. Limiting behavior is not reached on the low-temperature side. These results are explained in terms of a change in the rate-limiting step of the catalysis. At high temperature CO/sub 2/ production from reduction of Rh(III) speciesmore » by CO is viewed as rate determining, while at lower temperature hydrogen production from oxidation of Rh(I) species by protons is thought to be limiting.Possible intermediates are discussed and a mechanistic scheme is proposed. A steady-state kinetic analysis and assumptions about rate-determining steps show the mechanism to be consistent with the observed behavior of the system. 46 references, 3 figures, 3 tables.« less