Mechanistic insights into the emission and trapping of mobile species during sintering of Pt based diesel oxidation catalysts

Abhaya  Datye, University of New Mexico

To gain insight into factors that control the durability and reactivity of Pt-Pd diesel oxidation catalysts (DOCs), we have studied the emission of mobile species and their trapping on catalyst supports and the nature of the Pt-Pd working catalyst during alkane oxidation.  Recent in-situ TEM as well as STM studies suggest that particle migration plays only a secondary role during catalyst sintering [1], the dominant mechanism appears to be Ostwald ripening [2].  Our work sheds light on how Pd helps improve the durability of Pt-Pd catalysts:
1. When heated in air, Pt forms volatile PtO2 which is transported to the vapor phase and can also diffuse on the surface. Addition of Pd helps to slow the rates of emission of adatoms [3].
2. PdO can trap mobile Pt species to form Pt-Pd metallic particles (see schematic below) [4].  This provides a self-regenerating mechanism to slow the rates of Ostwald ripening.
3. Smaller PdO particles are better at trapping Pt [5].
4. At low temperatures, metallic Pd is more effective than PdO for methane oxidation.  The Pt helps keep Pd in metallic form, improving the reactivity of these catalysts [3, 5].
These results help explain how Pd modifies the performance of Pt based catalysts, improving the durability and performance.
Financial Support provided by NSF CBET-1067803 CBET- 1438765, OISE-0730277 & GM Global R&D.
1. A.T. DeLaRiva, T.W. Hansen, S.R. Challa, and A.K. Datye, In situ Transmission Electron Microscopy of catalyst sintering. Journal of Catalysis, 2013. 308: p. 291-305.
2. T.W. Hansen, A.T. Delariva, S.R. Challa, and A.K. Datye, Sintering of Catalytic Nanoparticles: Particle Migration or Ostwald Ripening? Accounts of Chemical Research, 2013. 46(8): p. 1720-1730.
3. T.R. Johns, R.S. Goeke, V. Ashbacher, P.C. Thune, J.W. Niemantsverdriet, B. Kiefer, C.H. Kim, M.P. Balogh, and A.K. Datye, Relating adatom emission to improved durability of Pt-Pd diesel oxidation catalysts. Journal of Catalysis, 2015. 328: p. 151-164.
4. C. Carrillo, T.R. Johns, H. Xiong, A. DeLaRiva, S.R. Challa, R.S. Goeke, K. Artyushkova, W. Li, C.H. Kim, and A.K. Datye, Trapping of Mobile Pt Species by PdO Nanoparticles under Oxidizing Conditions. Journal of Physical Chemistry Letters, 2014. 5(12): p. 2089-2093.
5. H. Xiong, E. Petersen, G. Qi, A. K. Datye, Trapping mobile Pt species by PdO in Diesel Oxidation Catalysts: Smaller is Better, Catalysis Today, 2016, in press.

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