Thermodynamic Stability of Buta-1,3-diene and But-1-ene on Pd(111) and (100) Surfaces under H₂ Pressure: A DFT Study

Experimental studies suggest that hydrogenation reactions on metal catalysts are sensitive to the structure and reaction conditions. In the present work, the interactions of buta-1,3-diene and but-1-ene molecules with Pd(111) and Pd(100) surfaces in the presence of hydrogen were investigated as a function of the temperature and of the pressures of the three molecules. For that purpose, we used periodic density functional theory calculations coupled to a thermodynamic model, to establish thermodynamic diagrams for the three molecules on the two surfaces. Under typical reaction conditions, both Pd(111) and Pd(100) surfaces are covered by at least one hydrogen monolayer, and surface hydride for Pd(100). On both surfaces, the impact of hydrogen on the adsorption modes and stability of the hydrocarbons was quantified. In particular, the monolayer of hydrogen destabilizes butadiene on Pd(111) but not on Pd(100). Moreover, the quantification of adsorption competitions between hydrogen and butadiene or butene shows that the affinity for both molecules is higher on Pd(100) than on Pd(111). The adsorption of hydrocarbons on hydrogenated surfaces infers a free energy cost, which is found to be larger on Pd(111) than on Pd(100). The results are discussed in comparison with experimental data.