Effect of Crystal Planes of Pd and the Structure of Interfacial Water on the Electrocatalytic Hydrogenation of Alkynes to Alkenes

Efficient electrochemical semi-hydrogenation (ECSH) of alkynes to alkenes is a promising alternative to traditional thermal semi-hydrogenation. Mechanistic understanding of crystal plane-dependent ECSH performance is extremely important for the subsequent catalyst design. Here, we investigate the role of low-index Pd(hkl) surfaces, including Pd (100), (111), and (110) planes, in ECSH of alkynes. As a result, alkyne conversion rate and alkene selectivity of Pd(hkl) surfaces follow the order of Pd (100) > Pd (111) > Pd (110). In situ Raman spectroscopic evidence indicates that the Pd (100) plane effectively enhances the formation of K+ ion hydrated water and two-coordinated hydrogen-bonded water to accelerate semi-hydrogenation rate and improve alkene selectivity, respectively. The order of the ability to regulate the interfacial water structure is ideally consistent with that of ECSH performance on Pd(hkl) surfaces. Pd nanocubes enclosed by the (100) surface exhibit excellent ECSH activity, alkene selectivity, and cycling stability. These findings demonstrate that the interfacial water structure is crystal plane-dependent, which is the determining factor for ECSH.