Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces

磷化物 催化作用 过渡金属 吸附 无机化学 化学 化学计量学 密度泛函理论 水溶液 活动站点 金属 钝化 材料科学 物理化学 计算化学 有机化学 图层(电子)
作者
Robert B. Wexler,John Mark P. Martirez,Andrew M. Rappe
出处
期刊:ACS Catalysis 卷期号:7 (11): 7718-7725 被引量:113
标识
DOI:10.1021/acscatal.7b02761
摘要

Optimizing catalysts for the hydrogen evolution reaction (HER) is a critical step toward the efficient production of H2(g) fuel from water. It has been demonstrated experimentally that transition-metal phosphides, specifically nickel phosphides Ni2P and Ni5P4, efficiently catalyze the HER at a small fraction of the cost of archetypal Pt-based electrocatalysts. However, the HER mechanism on nickel phosphides remains unclear. We explore, through density functional theory with thermodynamics, the aqueous reconstructions of Ni2P(0001) and Ni5P4(0001)/(0001̅), and we find that the surface P content on Ni2P(0001) depends on the applied potential, which has not been considered previously. At −0.21 V ≥ U ≥ −0.36 V versus the standard hydrogen electrode and pH = 0, a PHx-enriched Ni3P2 termination of Ni2P(0001) is found to be most stable, consistent with its P-rich ultrahigh-vacuum reconstructions. Above and below this potential range, the stoichiometric Ni3P2 surface is instead passivated by H at the Ni3-hollow sites. On the other hand, Ni5P4(0001̅) does not favor additional P. Instead, the Ni4P3 bulk termination of Ni5P4(0001̅) is passivated by H at both the Ni3 and P3-hollow sites. We also found that the most HER-active surfaces are Ni3P2+P+(7/3)H of Ni2P(0001) and Ni4P3+4H of Ni5P4(0001̅) due to weak H adsorption at P catalytic sites, in contrast with other computational investigations that propose Ni as or part of the active site. By looking at viable catalytic cycles for HER on the stable reconstructed surfaces, and calculating the reaction free energies of the associated elementary steps, we calculate that the overpotential on the Ni4P3+4H surface of Ni5P4(0001̅) (−0.16 V) is lower than that of the Ni3P2+P+(7/3)H surface of Ni2P(0001) (−0.21 V). This is due to the abundance of P3-hollow sites on Ni5P4 and the limited surface stability of the P-enriched Ni2P(0001) surface phase. The trend in the calculated catalytic overpotentials, and the potential-dependent bulk and surface stabilities explain why the nickel phosphides studied here perform almost as well as Pt, and why Ni5P4 is more active than Ni2P toward HER, as is found in the experimental literature. This study emphasizes the importance of considering aqueous surface stability in predicting the HER-active sites, mechanism, and overpotential, and highlights the primary role of P in HER catalysis on transition-metal phosphides.
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