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)

Abstract

The adsorption and successive dehydrogenation mechanisms of NH3 on Pd-Cu (111) and Cu-Pd (111) surfaces (the Pd atoms substitution of the first and second layers of Cu (111) 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 (111) and Cu-Pd (111) surfaces are identified. It is revealed that the adsorption configurations and corresponding adsorption energies of adsorbates are slightly changed on Pd-Cu (111) and Cu-Pd (111) surfaces. The adsorption energies of NHx(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 (111) surface are greatly reduced to 1.56 and 0.99eV, implying that the complete dehydrogenation of NH3 on Pd-Cu (111) 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 (111) and Cu (111) 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 (111) surface.