(148b) Interaction Between Highly Dispersed Ionic Pd2+ and Mn-Ce Solid Solution Support for Low Temperature CO Oxidation

Chao Wang, Cun Wen, Erdem Sasmaz, Jochen Lauterbach*

Department of Chemical Engineering, University of South Carolina, Columbia, SC, 29208, U.S.A.

*lauteraj@cec.sc.edu

Supported Pd catalysts are widely used in automobile three-way catalysts (TWC) and diesel oxidation catalysts. While Pd catalysts perform well for CO and hydrocarbon emission control during continuous engine operation, 80â??90% of CO and hydrocarbons are emitted into the atmosphere during the cold start phase of an automobile when the catalyst temperature is low [1]. Hence, promotion of catalytic activity at low temperatures is desired to achieve better removal of pollutants. Pd-CeO₂ catalyst is a widely studied category of supported Pd catalysts for low temperature CO oxidation. By doping Mn or both Mn and Sn into the CeO₂ lattice and forming a solid solution, we found that Pd-Ce based catalysts show excellent CO oxidation activity at even ambient temperature [2]. Metal-support interaction has been proposed as one of the most important factors that determine the Pd-CeO₂ catalyst activity at low temperature, while doping a secondary metal into the CeO₂ support may improve and change such interaction between Pd and Ce based supports. So far, the dynamic Pd structure change and interaction with the Ce based solid solution supports before and during reaction are not yet fully understood.

In this study, Pd supported on Mn doped CeO₂ solid solution (Pd-MC) and pure CeO₂ are synthesized and compared to evaluate metal-support interaction. CO oxidation tests show that Pd-MC achieves almost 100% CO conversion at 50^oC, which is much higher than Pd-CeO₂ (40%). The high activity of Pd-MC can be attributed to the formation of more highly dispersed ionic Pd²⁺ on MC and strong oxygen transfer interaction between Pd species and MC support. Detailed dynamic changes of Pd sites on MC and CeO₂ before and during reaction were characterized and compared by a series of ex-situ and in-situ spectroscopic experiments, including X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs). H₂ temperature-programmed reduction (H₂-TPR) and CO-O₂ pulse were used to investigate the effect of metal-support interaction on the redox properties of the catalysts. In addition, for practical implementation of low temperature CO oxidation catalysts, effects of hydrothermal treatment, steam, hydrocarbon, and space velocity on the catalytic activity were evaluated on Pd-MC.

References

1. M. Shelef, R. W. McCabe, Catal. Today 62, (2000) 35-50.

2. C. Wang, E. Sasmaz, C.Wen, J. Lauterbach, Catal. Today 258 (2015) 481â??486.