Synthesis of Pd-Cu catalyst with modified composition and structure via galvanic displacement and its electrochemical applications

Abstract

Electronic structure and electrochemically active surface area of catalysts are the important factors to determine the catalytic activity of electrochemical reactions. The catalyst synthetic method using galvanic displacement reaction can provide highly active electro-catalysts by alloying and changing their geometric structure. For example, when immersing Cu substrate in a solution containing Pd2+ ions, Pd deposition occurs spontaneously by galvanic displacement reaction originated from the difference of standard reduction potentials between Pd and Cu. During Pd-Cu galvanic displacement reaction, Cu atoms in the substrate diffuse into Pd deposit forming Pd-Cu alloy by Kirkendall effect. The catalytic activity of Pd can be enhanced by alloying with Cu which can change the electronic structure of Pd by ligand effect. The structure of Pd-Cu catalyst can be controlled from planar to whisker shapes by manipulating the reaction kinetics of galvanic displacement. The addition of Cl- ions in the reaction bath accelerates the galvanic displacement reaction and develops a steep concentration gradient of Pd2+ ions near the surface of Cu substrate. This induced the deposition of Pd-Cu in the vertical direction to form a whisker instead of horizontal extension. To verify the advantage of whisker-structured catalyst, the catalytic activity of Pd-Cu whisker toward electrochemical ethanol oxidation reaction was investigated. Pd-Cu whisker showed 21 times higher electrocatalytic performance than planar Pd due to the large surface area of whisker structure which could provide more reactive sites and the modified electronic structure of catalyst by alloying Pd with Cu. The prepared Pd-Cu whisker catalyst was also applied for N2O reduction. To enhance the N2O approach to rough-surfaced catalyst and N2O dissolution, electrochemical system combined with Couette-Taylor flow (CTF) mixer was adopted. When Ta number exceeds the critical Ta number by increasing rotating speed of inner cylinder of CTF mixer, Taylor vortices evolve in the solution, resulting in the rapid N2O dissolution and enhancing N2O solubility. The effectiveness of enhanced N2O dissolution on the N2O reduction was investigated by applying electrical potential on the CTF mixer. Pd-Cu whisker catalyst was loaded on outer cylinder of CTF mixer which played a role of cathode for N2O reduction. N2O conversion of 99.99% was obtained with introduction of Taylor vortices in the solution using a CTF mixer. Therefore, it can be suggested that the synthetic method to control geometric and electronic structure of catalysts using galvanic displacement reaction and the CTF mixer/electrolysis reactor hybrid system for efficient reactant dissolution can significantly enhance the electrochemical performance. The electronic structure of Pd-Cu catalysts can be modulated by the addition of citric acid in the galvanic displacement bath. The atomic ratio of Pd to Cu in Pd-Cu catalysts, which determines the electronic structure, was controlled by varying citric acid concentration during the displacement. Citric acid was incorporated into the Pd-Cu deposit during galvanic displacement and restrained the diffusion of Cu from Cu substrate to deposit leading to decrease in Cu content in the Pd-Cu catalysts. The electrocatalytic activity for N2O reduction was strongly dependent on Pd/Cu composition in Pd-Cu catalysts. The optimum composition of Pd-Cu catalysts with the highest N2O reduction activity was Pd66Cu34. Moreover, the Pd66Cu34 catalyst showed remarkably enhanced mass activity for N2O reduction compared with a commercial Pd/C. The density functional theory (DFT) calculations revealed that the highest N2O reduction activity of Pd66Cu34 was attributed to the moderate bonding energy to reaction intermediates which resulted from interatomic charge transfer between Pd and Cu.