Computer simulation of the effective double layer occurring on a catalyst surface under electrochemical promotion conditions

E.P.M. LEIVA; C. VÁZQUEZ; M. ROJAS; E.P.M. LEIVA

Thessaloniki, Grecia

Conferencia; First International Conference on the ORigin of Electrochemical Promotion of Catalysis; 2007

Computer simulation of the effective double layer occurring on a catalyst surface under electrochemical promotion conditions. Ezequiel P.M. Leiva , Cecila Vázquez, Mariana Rojas, Marcelo M. Mariscal Unidad de Matemática y Física, INFIQC, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba, Argentina Introduction The catalytic activity of metal films deposited on solid electrolytes may be drastically modified in a reversible way by means of the application of current or a potential difference between the metal catalyst and a counter electrode. This phenomenon, know as Non- Faradaic Electrochemical Modification of Catalytic Activity, NEMCA) [1], has been the subject of extensive experimental and theoretical research. When a current density is allowed to flow between the catalyst and the counter electrode, depending on the sign of the current, anions or cations migrate from(or to)the solid electrolyte towards(from) to the catalyst surface, in a process known as back spillover. Thus, an effective double layer is established, which modifies the work function of the metal exposed, and affects the binding strength of reacting adsorbed molecules. The change in the surface dipole of the catalyst, manifest in the change of the work function of the system, produces remarkable changes in the surface catalytic activity that are reversible and even predictable on the basis of models[2-5]. In the present work we perform simulations on the adsorption of ions on a charged surface, so simulate the formation of the above mentioned electrostatic double layer. The computer simulation is performed in the Grand Canonical ensemble, where the chemical potential of the adsorbed species, the volume of the system and the temperature are fixed, in order to emulate the experimental boundary conditions. The metal is represented as a perfect conductor and the ions interact with the surface through their image charge, as shown in the following scheme. A hard core represents the repulsion of the core electrons with the corresponding metal core ions. Different approximations were used to deal with the long-range interactions arising in the model: Shifted Potential (SP), Shifted Force (SF), and Damped Shifted Potential (DSP). Results and Discussion The structure of the system, the coverage degree and the surface dipole were analyzed as function of the chemical potential. The coverage degree presents a linear increase as a function of the chemical potential of the backspilled species, as predicted by previous analytic models. However, the structure of the effective layer appears to be considerably more compact than in models where a 1-D Poisson equation is solved. At very low coverage degrees the angular distribution function of the adsorbed species denotes a disordered structure, while at coverage degrees close to 0.01 it turns to show a solid-like structure. The simple computational model presented in the present work is able to predict an important number of qualitative features of the properties found in the experiments, opening new perspectives for the simulation of the effective double layer occurring in NEMCA and other related phenomena. C. G. Vayenas, S. Bebelis, I. V. Yentekakis, en: G. Erlt, H. Knötzinger, J. Weitcamp (Eds.), Handbook of Catalysis, VCH, Weinheim, 1997, 1310- 1338.