Palladium oxidation directly precedes methane oxidation in catalyzed reactions

Catalysts containing palladium and platinum have garnered widespread use in emissions control catalysis reactions that convert pollutants into less toxic materials. The role of oxidation in the active phase of the metals during these reactions remains unclear, made more complicated by the myriad nanoparticle sizes and compositions that vary between materials.

Goodman et al. have overcome these complications by elegantly controlling both size and composition, in a study of methane combustion in which the catalyst consists of platinum-palladium alloy nanoparticles. Using Quick Extended X-ray Absorption Fine Structure to study catalysts with palladium particle sizes from 3 to 16 nanometers, the group found a connection between increased catalytic activity to palladium oxidation.

“We believe that by understanding exactly how these precious metals perform each reaction most efficiently, we can minimize the quantity of precious metals in each catalytic converter, as well as extend the lifetime of automotive catalysts,” said author Emmett Goodman.

For all studied nanoparticle sizes, palladium oxidation directly preceded methane combustion to carbon dioxide, suggesting palladium oxidation is a required step in the process. Oxidation pre-treatment showed equal or better catalysis than a reduction pre-treatment.

Oxidative pre-treatments also enhanced low-temperature combustion in uniform alloyed platinum-palladium nanoparticles. Using microscopy facilities at the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory, they found that alloys rich in platinum initially kept palladium in a lower-activity reduced state, but eventually palladium oxide segregated under oxidizing combustion conditions.

Goodman hopes the work provides a clearer picture of the relationship between palladium oxidation state and activity for emissions-control catalysis. They next hope to use similar approaches for understanding catalyst stability under operating conditions.

Source: “Palladium oxidation leads to methane combustion activity: Effects of particle size and alloying with platinum,” by Emmett D. Goodman, Angela A. Ye, Aisulu Aitbekova, Oliver Mueller, Andrew R. Riscoe, Temy N. Taylor, Adam S. Hoffman, Alexey Boubnov, Karen C. Bustillo, Maarten Nachtegaal, Simon R. Bare, and Matteo Cargnello, Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5126219 .