High Throughput Study of Pt-Pd Catalysts for DOC Applications: XRD and STEM Characterization

Lean-burn gasoline and diesel engines provide significant increase in fuel economy over conventional gasoline engines for transportation applications, but the catalytic after-treatment of exhaust, notably NOx and soot, presents unique challenges. Oxidation of NO plays a prominent role in the catalytic after-treatment of exhaust gas from lean-combustion vehicles. Although alumina-supported Pt is one of the best NO oxidation catalysts, cost and durability must also be considered in selecting a practical catalyst.

At Ford, the search for novel catalysts for DOC applications has been conducted by high-throughput synthesis and testing of a variety of materials, including alumina-supported Pt with partial substitution of Pd.  Alloying of Pt with other metals such as Pd has been shown to inhibit particle growth, particularly that of anomalously large particles, under high-temperature lean conditions without excessively degrading NO oxidation activity, but the mechanisms underlying this thermal stabilization remain unknown.

This presentation focuses on high-temperature x-ray diffraction (XRD) of alumina-supported Pt and Pt-Pd bimetallic catalysts.   For example, under lean aging conditions, XRD shows that average particle size grows as a power-law in time.  The fact that this growth rate follows Arrhenius behavior allows us to combine time and temperature into a single “exposure” variable.  These and other findings are considered along with scanning transmission electron microscopy (STEM) results to provide a somewhat more complete picture of the particle coarsening processes at work in these catalysts.