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.