(362b) CRE Division Practice Award Lecture: Water Gas Shift Over Industrial Cu Catalysts: A Mechanistic and Microkinetic Investigation

Low temperature water gas shift (LTS) is a commercially important reaction that takes place over Cu-ZnO-Al2O3 catalyst. A very large number of fundamental studies have been carried out for this reaction including investigations of the reaction mechanism as typified by Refs. [1-4]. In short, discussions have centered around (a) the redox mechanism in which adsorbed H2O is dissociated to O* and OH* and the O* is removed via CO* to form CO2 ? where * indicates an active site, and (b) formate as a reactive intermediate. Recently, Gokhale et al. [5] using a DFT investigation of the LTS reaction on Cu(111) proposed a new mechanism that involves a reactive surface carboxyl. Our study is aimed at resolving some of the argument over the elementary steps that best describe the catalytic cycle for the LTS reaction. To achieve this, we used the microkinetic modeling methodology pioneered by Dumesic [6], and analyzed our reactivity data using all elementary steps including those that described the redox mechanism, the formate mechanism, and the carboxyl mechanism. Importantly, by using all reactions together, we ensured that there was no bias towards any particular reactions to fit our data. We found the catalytic cycle for LTS on Cu to include the formation of COOH*, and its reaction with OH* to form CO2* and H2O*. We further found the reaction of CO2* and H* to form formate to be equilibrated but an important reaction, because even though its net rate was significantly lower than the rates of the reactions in the LTS catalytic cycle, the step ensured that there was significant coverage of bidentate formate on the Cu surface. Bidentate formate is a spectator species whose coverage increases with increasing pressure and decreases with increasing temperature. In summary, our investigation confirms that the formate mediated and redox mechanisms are unimportant, and shows that the LTS catalytic cycle involves the formation and reaction of a surface carboxyl intermediate.

References

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6. Dumesic, J. A., et al. ?The Microkinetics of Heterogeneous Catalysis?, American Chemical Society, Washington, D. C., 1993.