Final Report
Improved Sulfur Tolerance for Automotive
Emissions Control Catalysts
R.J. Gorte and J.M. Vohs
Department of Chemical Engineering
University of Pennsylvania
Philadelphia, PA
Project funded by the Coordinating Research Council
November 27, 2002
Executive Summary:
This report describes results from studies of sulfur poisoning of ceria-supported Pd catalysts performed at the University of Pennsylvania from 1997 to 2001, with support from the Coordinating Research Council. The aim of this work was to understand how sulfur poisons OSC and to develop TWC that are more sulfur tolerant. Seven publications resulted from this work and the following conclusions were reached:
1)Pd/ceria can be oxidized by both H2O and CO2 under conditions similar to that which exist in automotive exhausts. This property makes Pd/ceria an excellent WGS and steam-reforming catalyst and must play a role in OSC.
2)Under reducing conditions, a very stable carbonate is formed on ceria. This carbonate decomposes rapidly when ceria is exposed to O2 and it is unknown if it plays a major role in WGS or OSC.
3)Poisoning of Pd/alumina by exposure to 20 ppm of SO2 is quickly reversed in under CO oxidation conditions. Poisoning of Pd/ceria by exposure to 20 ppm of SO2 is not easily reversed under CO oxidation conditions. The enhancement that ceria provides is lost upon SO2 exposure and rates on poisoned Pd/ceria are identical to rates on Pd/alumina.
4)Exposure of Pd/ceria to SO2 at room temperature results in molecularly adsorbed SO2. A stable, surface-phase, cerium sulfate is formed by heating the adsorbed SO2 to 473 K. Heating Pd/ceria in SO2 and O2 results in the formation of bulk sulfates: a Ce+3 sulfate that decomposes to SO2 and O2 at 1073 K and a Ce+4 sulfate that decomposes to SO2 and O2 at 1023 K.
5)Under automotive conditions, the sulfates are easily reduced to Ce2O2S upon heating in CO or H2. In H2, a fraction of the sulfur is also reduced to H2S at about 723 K. The reduction of the sulfate and the re-oxidation of Ce2O2S is rapidly reversible, so that CO-O2 pulse measurements of the poisoned catalyst seem to suggest that OSC is actually improved by sulfur poisoning.
6)WGS and CO-H2O and CO-CO2 pulse studies show that, unlike CeO2-x, Ce2O2S is not easily re-oxidized by water or CO2. This leads to the conclusion, in agreement with the literature, that OSC is not a simple process but one that involves oxidation and reduction in the presence of water.
The detailed results that lead to the above conclusions are given in the following papers, which are attached at the end of this report.
Papers that Acknowledge CRC support
1) Evidence for Oxidation of Ceria by CO2, S. Sharma, S. Hilaire, J.M. Vohs, R.J. Gorte, and H.-W. Jen, J Catal., 190 (2000) 199.
2) Effect of SO2 on the Oxygen Storage Capacity of Ceria-Based Catalysts, S. Hilaire, S. Sharma, R.J. Gorte, J.M. Vohs, and H.-W. Jen, Cat. Lett., 70 (2000) 131.
3) SO2 Poisoning of Ceria-Supported Metal Catalysts, T. Luo and R.J. Gorte, a review article appearing in Catalysis by Ceria and Related Materials, A. Trovarelli, Ed., Imperial College Press, London, 2002.
4) A Comparative Study of Water-Gas-Shift Reaction Over Ceria-Supported Metallic Catalysts, S. Hilaire, X. Wang, T. Luo, R.J. Gorte, and J. Wagner, Appl. Catal. A 215 (2001) 271.
5) TPD and XPS Investigation of the Interaction of SO2 with Model Ceria Catalysts, R.M. Ferrizz, R.J. Gorte, J.M. Vohs, Cat. Lett., , 82 (2002) 123-129..
6) An Examination of Sulfur Poisoning on Pd/Ceria Catalysts, T. Luo, J.M. Vohs, and R.J. Gorte, J. Catal., 210 (2002) 397-404.
7) A Mechanistic Study of Sulfur Poisoning of the Water-Gas-Shift Reaction Over Pd/Ceria", T. Luo and R. J. Gorte, Catalysis Letters, accepted.
Students Trained
The following graduate and postdoctoral students have worked on and been partially supported by the CRC grant.
Post-docs:
Stephanie Hilaire
Graduate Students:
Rob Ferrizz
Tian Luo