Theoretical study of NH₃ decomposition on Pd-Cu (1 1 1) and Cu-Pd (1 1 1) surfaces: A comparison with clean Pd (1 1 1) and Cu (1 1 1)

Highlights: • Dehydrogenation mechanisms of NH₃ on Pd-Cu (1 1 1) and Cu-Pd (1 1 1) were investigated using periodic DFT calculations. • The optimized structures and adsorption energies of intermediates were obtained. • The minimum energy path for the dissociation of NH₃ into adsorbed N and H was identified on different surfaces. • The effect of surface structure composition of the catalysts and the role of the doped-metal were analyzed. - Abstract: The adsorption and successive dehydrogenation mechanisms of NH₃ on Pd-Cu (1 1 1) and Cu-Pd (1 1 1) surfaces (the Pd atoms substitution of the first and second layers of Cu (1 1 1) surfaces) have been systematically investigated by density functional theory (DFT) method with a periodic slab model. All possible adsorption configurations of relevant intermediates on Pd-Cu (1 1 1) and Cu-Pd (1 1 1) surfaces are identified. It is revealed that the adsorption configurations and corresponding adsorption energies of adsorbates are slightly changed on Pd-Cu (1 1 1) and Cu-Pd (1 1 1) surfaces. The adsorption energies of NH_x(x = 0–3) species exhibit the following trend: NH₃ < NH₂ < NH < N. Then, the minimum energy path for the complete dehydrogenation of NH₃ into adsorbed N and H is identified to explore the dehydrogenation mechanisms on different surfaces. The highest energy barrier and reaction energy on Pd-Cu (1 1 1) surface are greatly reduced to 1.56 and 0.99 eV, implying that the complete dehydrogenation of NH₃ on Pd-Cu (1 1 1) surface is favorable both kinetically and thermodynamically, namely, the doped-Pd atoms in the first layer are the reaction active center. Compared to that on clean Pd (1 1 1) and Cu (1 1 1) surfaces, it is found that the synergistic effect exits in different layers of catalyst surfaces. The calculated results show that the layer-substituted Pd atoms on the surface of Cu catalysts exhibit a better catalytic activity for NH₃ dehydrogenation compared to the clean Cu (1 1 1) surface.