Reduction of nitrate on electrochemically pre-reduced tin-modified palladium electrodes

The electrochemical pretreatment of Sn-modified Pd electrodes at the negative potentials of ⩽−0.2 V vs. Ag/AgCl (sat. KCl) enhanced their electrocatalytic properties for the reduction of nitrate. Namely, the nitrate reduction current increased by several times depending on the potentials for the nitrate reduction and the pretreatment, and the product selectivity for the formation of N2 increased from 29% for the as-prepared (untreated) electrodes to 51%. The STM, XPS and cyclic voltammetric measurements revealed that the following structural changes were induced by the pretreatment. (1) Spontaneously adsorbed divalent tin was reduced to zero-valent tin. (2) The hydrogen adsorption and absorption ability of Pd was almost lost while the surface Sn fraction decreased. (3) Primary particles of ca. 30 nm observed on a Pd and an as-prepared Sn/Pd electrode were reconstructed to larger particles of 45 nm. Based on these results on the structural change and the literature on d-band filling in binary metal systems, it was concluded that the surface Sn/Pd alloy was formed, and thereby the surface electronic structure was changed. This accelerated N–O bond cleavage and decreased the hydrogenation ability.