Pressure and Temperature Effects on the Formation of a Pd/C Surface Carbide: Insights into the Role of Pd/C as a Selective Catalytic State for the Partial Hydrogenation of Acetylene

The stability of the near-surface region of palladium(111) under a pressure of acetylene and hydrogen is studied via a combination of density functional theory calculations and atomistic ab initio thermodynamics. Gibbs free energy maps give insight into the deposition of C in the near surface of Pd(111) due to acetylene fragmentation. Our results indicate that under working conditions Pd is able to incorporate large amounts of C that will be distributed equally among the top metal interlayers. We further explored factors such as the partial hydrogen pressure, temperature, and presence of strain and found that whereas high temperatures and large hydrogen pressures inhibit C accumulation, the presence of tensile strain enhances the dissolution of C. Additional calculations indicated that the accumulation and distribution of C in the near surface of Pd critically weaken the adsorption energy of ethylene, hence favoring its desorption and yielding a high selectivity for partial hydrogenation. We claim that the study of Gibbs free energy maps can be particularly useful in gaining atomic understanding of solute distribution in materials under realistic environmental situations.