Evolution of surface and bulk carbon species derived from propylene and their influence on the interaction of hydrogen with palladium

Abstract

The evolution of carbon species on the surface and in the bulk of palladium (Pd) foils during exposure to C3H6/H2/N2 gas mixtures is quantitatively analyzed in this work as a function of temperature (300–500 °C), C3H6 concentration (5–20% in 80% H2/balance N2), and C3H6 exposure time (0–16 h) by temperature-programmed oxidation (TPO) of the carbonaceous deposits. The influence of these carbonaceous deposits on the interaction of H2 with Pd is analyzed by measurements of H2 fluxes across the Pd foils during exposure to C3H6/H2/N2 gas mixtures in the 300–500 °C range with C3H6 concentrations in the 5–20% range (in 80% H2 with balance N2). TPO results indicate that there are at least four different carbon species that are deposited onto the Pd foil during C3H6 exposure, which we hypothesize are associated with (1) a surface-adsorbed CxHy species, (2) a sub-surface C species, (3) a bulk solid solution C species, and (4) a bulk carbidic C species. The bulk C species are not observed at low temperatures (300–400 °C) due to the large activation barrier for C diffusion in the bulk of Pd (~123 ± 7 kJ/mol). The carbonaceous deposits inhibit hydrogen permeation across the Pd foils to an extent that increases with increasing temperature and increasing C3H6 concentration. There is a particularly strong correlation between the TPO peak temperature associated with the surface-adsorbed CxHy species and the H2 flux across the Pd foil. As the TPO peak temperature associated with the surface-adsorbed carbonaceous species increases with increasing C3H6 exposure time, C3H6 concentration, and C3H6 exposure temperature, the H2 flux across the membrane decreases. This could indicate that surface-adsorbed CxHy species are primarily responsible for inhibiting H2 permeation, and their structure evolves to become a more potent poisoning species with increasing C3H6 exposure time, C3H6 concentration, and C3H6 exposure temperature. This is an important step towards a molecular-level understanding of how different carbon species control the distribution of H atoms on the surface and in the bulk of Pd, which is important for many Pd-based H2 technologies.