(513ce) The Evolution of Propylene Derived Carbon Species on the Surface and in the Bulk of Palladium and Palladium Silver (Pd77/Ag23)

Palladium has long been exploited for its catalytic ability to dissociate and absorb hydrogen into its’ bulk, often being utilized as hydrogenation catalysts or H₂-selective membranes. However, unsaturated hydrocarbons that are common to these systems, tend to deposit carbonaceous species on the surface and in the bulk that can enhance, or degrade, Pd’s activity. Alloying Pd with other metals such as Ag is one strategy that is commonly used to control the deposition of carbonaceous species on Pd-based catalysts. However, the influence of alloying on the deposition and growth of carbonaceous species on the surface, and in the bulk, of Pd-based catalysts is not well-understood. In this work, the evolution of propylene-derived carbon species on the surface and in the bulk of Pd and Pd₇₇Ag₂₃ foils during exposure to C₃H₆/H₂/N₂ is quantitatively analyzed as a function of temperature (300-500°C) and C₃H₆ concentration (5-20% in 80% H₂/balance N₂) by temperature-programmed oxidation (TPO). TPO results for non-alloyed Pd indicate the existence of at least four carbon species deposited during exposure to C₃H₆, which we hypothesize are associated with a surface C_xH_y species, a subsurface C species, a bulk solid-solution C species, and a bulk carbidic C species. The results for Pd₇₇Ag₂₃ indicate four similar carbon species, however, TPO trends and carbon amounts vary largely from Pd. Bulk species are observable at lower temperatures of C₃H₆ exposure and TPO-peak positions shifted to lower TPO temperatures. Also, due to Pd₇₇Ag₂₃’s higher solubility towards interstitial carbon, the total C amount is near an order-of-magnitude higher than non-alloyed Pd. Interestingly, for both Pd and Pd₇₇Ag₂₃,the surface species appears to evolve, shifting to higher TPO temperatures with increasing C₃H₆ exposure temperature and concentration. These results further understanding of the alloy effect on carbon species interaction and may suggest Ag largely alters bulk C growth.