Search for a better water gas shift catalyst

Luis Bollmann, Joshua L. Ratts, W. Damion Williams, Jorge Pazmino, Jeffrey T. Miller1, W. Nicholas Delgass, Fabio H. Ribeiro

School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West
Lafayette, IN 47907-2100, 1BP Research Center, E-1F, 150 W.Warrenville Rd.,
Naperville, IL 60563

Abstract

We are attempting to synthesize a catalyst for the WGS reaction that has the high turnover rate (TOR) of the Cu-based system and the capability typical of noble metal systems to recover from operational upsets. The kinetic measurements were carried out at 300 °C, 1 bar, 6.8% CO, 22% H₂O, 8.5% CO₂, 37.3% H₂, and balance Argon. As a reference, the TOR for a Cu based catalyst at these conditions was about 2 s^-1. We studied catalysts based on Pd and Pt. For Pt on alumina, particle size (2-15 nm) caused no change in the TOR, which was about 1 s^-1 for Pt on CeO₂, TiO₂, and ZrO₂ supports, implying that these supports do not influence the reactivity. However, catalysts supported on silica and alumina showed a TOR about 10 times lower than those on these three supports. The addition of, for example, Mo, Fe, and Zn to Pd and Pt catalysts on alumina significantly increased the rate (up to a factor of 100), but only up to the rate on the noninteracting supports. The rates of Pd and Pt catalysts supported on the non-interacting supports CeO₂, TiO₂, and ZrO₂ could not be further increased by any of these additives. For a series of Pd-Zn catalysts ranging from 2 to 19 wt% Zn on Al₂O₃, it was observed by EXAFS and DRIFTS of CO that Pd alloyed with Zn, and that the alloy exhibited an increased TOR of up to 20 times as compared to Pd on Al₂O₃. The presence of zinc aluminate was also observed. The addition of 2 wt% Zn to Pd catalysts supported on TiO₂, CeO₂ and ZrO₂ did not enhance the WGS rate, although alloying was verified by EXAFS and DRIFTS of CO. The TORs of Pd on these three supports were as high as the TOR on the best PdZn on alumina. One explanation of the results is that Zn forms an aluminate and prevents a deleterious interaction between Pd and alumina. Zinc is thus not a true promoter for WGS, although it decreased the undesirable methanation reaction to below detection limits. While we have found ways to mitigate the deleterious effects of alumina and silica on the WGS rate, we have not yet found a promoter that will increase the TOR significantly for Pd or Pt on non-interacting supports.

Speaker’s Bio - Dr. Ribeiro is Professor at the School of Chemical Engineering, Purdue University. His research interest is in kinetics of heterogeneous catalytic processes. Before starting on academia, Dr. Ribeiro has worked in industry and in research using model catalysts and surface science techniques.