Determination of adsorption and activation volumes and apparent transfer coefficients by pressure and potential modulation

Abstract

In this thesis the following investigations were carried out on Pt or Pt based electrodes: the determination of the apparent transfer coefficient (α′) for CO oxidation using an ac method, the determination of the molar adsorption volume of hydrogen and the effect of cations thereupon combining the methods of pressure modulation and ac voltammetry, and the determination of the activation volume for CO oxidation by pressure modulation.<br /> The ac method for the determination of apparent transfer coefficients or Tafel slopes has been developed in this thesis; it’s used here for CO oxidation on noble metals. This method involves a sinusoidal modulation of the potential and the simultaneous recording of the ac and the dc current. It allows to record α′ quasi continuously as a function of potential or time (i.e., in cyclic voltammetry or in potentiostatic experiments), with the reaction rate varying with time, much more accurately than the traditional method that can only measure the transfer coefficient or Tafel slope over a large range of potentials. Since the mechanism for CO oxidation on noble metals is in the focus of electrocatalytic research in recent years, I applied this ac method to the determination of the apparent transfer coefficient for the oxidation of pre-adsorbed CO at Pt electrodes in sulfuric acid. Electrodes of polycrystalline platinum and single crystalline Pt(111), Pt(665) and Pt(332) were investigated using either potential sweeps or potential steps while superimposing an ac voltage. The apparent transfer coefficients were measured and the transition of values from 1.5 to 0.5 with potential increase, which had been predicted in a simulation by Koper et al., was clearly observed experimentally for the first time. Some assumptions on the mechanism of CO oxidation on Pt by other authors could thus be rejected. The measurement of the apparent transfer coefficient by the ac method is also extended to CO oxidation on Ru and Sn decorated Pt and contributes to the understanding of those processes, as well. The ac method for the determination of the apparent transfer coefficient, which I used here, will be of great help also in many other cases, especially under steady state conditions, where the major limitations of the method are avoided.<br /> The volume of adsorbed hydrogen on Pt and the cation effects on it were investigated combining pressure modulation method and ac voltammetry. The pressure modulation method has been developed in our group by Loewe et al for the measurement of reaction and adsorption volumes and was further improved in this work. Instead of the traditional way of using high pressure in a complicated device, modulation by less than 1 bar is sufficient for the measurement of pressure dependences. The adsorption of hydrogen on noble metal surfaces is important for many other reactions, the partial molar volume of adsorbed hydrogen is of fundamental interest. In this thesis, the molar adsorption volume of hydrogen is measured to be 3.3±1 cm³·mol^-1 on polycrystalline Pt in sulphuric acid. Cation were observed to largely influence the molar volume of adsorbed hydrogen in Li⁺, K⁺ and Cs⁺ containing electrolytes. A preliminary explanation for these cation effects is proposed. This work will be of importance in understanding the surface structure of the double layer in the hydrogen adsorption region and the involvement of cations in the interface.<br /> The pressure modulation method has been extended in this work to the determination of the activation volume for CO oxidation. Activation volume is one of the few parameters that can be measured for the activated complex. Instead of the traditional way of using complicated high pressure method, in this thesis I introduced the pressure modulation method to the measurement of the activation volume of the oxidation of adsorbed CO on polycrystalline Pt. Reasonable values are obtained at low sweep rates and low step potentials with an average of -18.2 cm³·mol^-1. A preliminary structure for this activated complex is put forward. The applicability of pressure modulation method for measuring the activation volume of electrochemical processes is thus demonstrated.