Estudio de transferencias de carga acopladas a reacciones químicas en microinterfases y nanointerfases mediante técnicas electroquímicas

Resumo

In this Thesis, the theoretical and experimental study of heterogenous charge transfer processes across conventional, micrometric and nanometric electrode|solution interphases is tackled by electrochemical and spectroelectrochemical techniques. Specifically, in the systems considered such transfers are complicated by the heterogeneous kinetics, complex reaction stoichiometries and/or coupled homogeneous chemical reactions. Thus, the theory developed in this Thesis includes single-step reversible charge transfers with non-unity reaction stoichiometries (E-a:b), simple electron transfers (both kinetically limited and not) where the redox reagent is regenerated by a (pseudo)first order homogeneous chemical reaction that involves the redox product (catalytic mechanism), single-step Nernstian charge transfers where the redox reagent, product or both are involved in a homogeneous chemical reaction of (pseudo)first order (CE, EC or CEC mechanisms, respectively), two reversible electron transfers coupled to each other by (pseudo)first order chemical kinetics (ECEc and 4-member square mechanisms), as well as reversible multi-electron charge transfers coupled to chemical equilibria ('extended' square mechanism) have been studied theoretically. The electrochemical and/or spectroelectrochemical response is dependent on different physicochemical phenomena that determine the thermodynamics and kinetics of the charge transfer, as well as the availability of electroactive species around the interphase, mainly influenced by the mass transport and coupled chemical reactions. Thus, an adequate modelling of the electrochemical signal enables us to obtain information about these processes. In this context, obtaining analytical solutions is of great interest since they offer interesting advantages such as the a priori identification and analysis of the influence of the different key parameters of the system, the prediction of limiting cases and the establishment of simple procedures of characterization. According to the above, the main objective of this Thesis has been the deduction of analytical equations that describe adequately the electrochemical response of complicated heterogeneous charge transfers, considering the use of various electrochemical techniques in combination with microelectrodes, nanoelectrodes and arrays of them, given their well-known advantages for experimental quantitative studies. Based on the solutions obtained, the influence of the different variables over the electrochemical and/or spectroelectrochemical response has been evaluated. From this analysis, criteria to identify and determine parameters of interest have been developed. Regarding the methodology followed in order to achieve the objectives previously mentioned, first the mathematical problem corresponding to the electrochemical system considered has been solved, mainly by analytical methods, either rigorous or approximate (as the kinetic steady-state treatment). Numerical methods based on finite differences have also been employed for particularly complex situations. Then, the equations deduced have been implemented in own-designed calculation computer programs, mainly using the software Mathcad and/or programs in C++ language, in order to study the effect of the variables that affect the electrochemical response, to establish criteria for the system identification and characterization and to perform quantitative analyses of experimental data. Regarding the experimental studies, the use of multiple electrochemical techniques (such as cyclic voltammetry, square wave voltammetry and chronoamperometry), and of microelectrodes has been considered. In particular, the influence of ion pairing over the electroreduction of the Keggin type -phosphopolioxowolframate (PW12O403-) and thus, over its electrocatalytic performance, has been studied. The cation:anion stoichiometries and the corresponding ion association constants have been estimated from the analysis of the evolution of the voltammetric signal with the analytical equations deduced for the 'extended' square mechanism. Also, density functional theory calculations have been performed in order to complement the electrochemical results obtained, corroborating the conclusions deduced from the electrochemical study.