Abstract
The demands of stricter diesel engine emission regulations have created challenges for current exhaust systems. With advances in low-temperature internal combustion engines and their operations, advances must also be made in vehicle exhaust catalysts. Most current diesel oxidation catalysts use heavy amounts of precious group metals (PGMs) for hydrocarbon (HC), CO, and NO oxidation. These catalysts are expensive and are most often synthesized with poor bimetallic interaction and dispersion. The goal of this work was to study the effect of aging on diesel emission abatement of Pt-Pd bimetallic nanoparticles precisely prepared with different morphologies: well dispersed core-shell vs. well dispersed homogeneously alloyed vs. poorly dispersed, poorly alloyed particles. Alumina and silica supports were studied. Particle morphology and dispersion were analyzed before and after hydrothermal treatments by XRD, EDX, and STEM. Reactivity as a function of aging was measured in simulated diesel engine exhaust. While carefully controlled bimetallic catalyst nanoparticle structure has a profound influence on initial or low temperature catalytic activity, the differences in behavior disappear with higher temperature aging as thermodynamic equilibrium is achieved. The metallic character of Pt-rich alumina-supported catalysts is such that behavior rather closely follows the Pt-Pd metal phase diagram. Nanoparticles disparately composed as well-dispersed core-shell (via seq-SEA), well-dispersed homogeneously alloyed (via co-SEA), and poorly dispersed, poorly alloyed (via co-DI) end up as well alloyed, large particles of almost the same size and activity. With Pd-rich systems, the oxidation of Pd also figures into the equilibrium, such that Pd-rich oxide phases appear in the high temperature forms along with alloyed metal cores. The small differences in activity after high temperature aging can be attributed to the synthesis methods, sequential SEA and co-DI which give rise, after aging, to a bimetallic surface enriched in Pd.
Original language | English |
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Pages (from-to) | 145-156 |
Number of pages | 12 |
Journal | Catalysis Today |
Volume | 267 |
DOIs | |
State | Published - Jun 1 2016 |
Funding
The support of the National Science Foundation, grant CBET-1160036, is gratefully acknowledged. A portion of this research was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program, the Center of Catalysis for Renewable Fuels (CReF) at the University of South Carolina, and the Nation Science Foundation. The authors wish to express their gratitude to program managers Ken Howden and Gurpreet Singh for their support. This manuscript has been co-authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The authors would also like to thank Dr. Alan Nicholls at UIC for acquisition of STEM images and elemental maps.
Keywords
- Bimetallic
- Diesel oxidation catalyst
- NO
- Pd
- Pt
- Pt-Pd
- Strong electrostatic adsorption