First-principles evaluation of carbon diffusion in Pd and Pd-based alloys

The absorption and diffusion of carbon in Pd and Pd-based alloys can be important in applications of these materials as catalysts or membranes, but little is known about these processes. We used first principles calculations to characterize the absorption of C in pure Pd and PdM alloys with M=Ag, Au, and Cu. Our calculations show that the preferred configuration of C in Pd is as an interstitial atom; effects from Pd vacancies are minimal and substitutional sites are much less favorable. When the effects of thermal lattice expansion are included, our calculations predict C diffusivities in good agreement with experimental data, which is only available at elevated temperatures. Characterizing the binding energy and hopping energies of interstitial C in terms of lattice expansion or contraction is also a useful way to understand the effect of small amounts of M=Ag, Au, or Cu in PdM alloys. We also examined the properties of interstitial C in Pd77.7Ag22.3. Our calculations predict that the diffusivity of C in this alloy is two to three orders of magnitude lower than in pure Pd.