(464e) The Importance of Vapor Phase Transport during Accelerated Aging of Emission Control Catalysts

Emission control catalysts, such as those used to treat exhaust from diesel engines or cars, involve platinum group metals (PGMs) in the form of nanoparticles on a support. Decreasing size of the nanoparticles leads to better utilization of the precious metals and higher reactivity. When these catalysts are subjected to high temperatures, surface atoms become mobile causing inter particle transport and growth of particle size. Our research is focused on developing methods to control the growth of particle size by studying the mechanisms of catalyst sintering [1]. Specifically, we focus on the accelerated aging protocols which have been developed by industry to ensure the catalysts are durable and will continue to perform over the lifetime of the vehicle. The low temperature after treatment (LTAT) protocol [2] for diesel engines involves heating the catalyst in flowing air at 800 °C for 50 hr. The protocol for 3-way catalysts involves even higher temperatures, ranging from 900 °C – 1100 °C. At these temperatures, the vapor pressure of the PGMs is quite high and can cause transport of mobile species through the vapor phase. For example, the vapor pressure of PtO₂ derived from Pt particles in air is 1.6x10^-8 atm at 800 °C. The vapor pressure of Pd at 1050 °C is 6x10^-9 atm. Under these conditions, the Pt or Pd can be emitted to the vapor phase, but the loss of the PGM may not be readily apparent. To enable study of these phenomena we have used model catalysts where the powder is dispersed as a thin film on a Si wafer, or on an electron transparent Si₃N₄ membrane. By using SEM – EDS as well as TEM – EDS, we can accurately quantify the loss of the platinum group metals and derive mechanistic understanding of the particle growth processes during accelerated aging. These studies help explain the role of Pd in improving the durability of Pt catalysts for diesel oxidation [3, 4]. This presentation will describe our recent work on the role of pore structure on the durability of catalysts subjected to accelerated aging protocols.

References

1. 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 46(8) (2013) 1720-1730.
2. USDRIVE Low Temperature Oxidation Catalyst Test Protocol. 2015; Available from: http://www.cleers.org/acec-lowt/includes/2015CLEERS_Low_T_protocol_poster.pdf.
3. 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 5(12) (2014) 2089-2093.
4. C. Carrillo, A. DeLaRiva, H.F. Xiong, E.J. Peterson, M.N. Spilde, D. Kunwar, R.S. Goeke, M. Wiebenga, S.H. Oh, G.S. Qi, S.R. Challa, and A.K. Datye, Regenerative trapping: How Pd improves the durability of Pt diesel oxidation catalysts Applied Catalysis B-Environmental 218(2017) 581-590.