Two recently developed electrochemical techniques are presented as tools for the characterization of solid surfaces with possible catalytic or electrocatalytic capacities. The techniques are the voltammetry of immobilized micro particles (VIMP) and the electrochemical scanning microscopy (SECM). The VIMP is a simple and economical technique that only requires basic electrochemical instrumentation. Higher sophistication is possible with relatively low investments for non-specialized laboratories. The SECM requires more investment and technical advice because it requires a somewhat sophisticated instrumentation not usually found in an electrochemical laboratory. However, associated costs are relatively low compared to other solid surface exploration techniques. The historical development, improvements over time, and some applications for surface characterization are presented for each technique. Finally, some cases of coupling with other techniques allowing the expansion of the capacities for the topographical and reactivity characterization of solid surfaces are briefly presented.
TopIntroduction
The progress of chemistry since the beginning of the last century is largely due to the use of physical techniques to evaluate a wide variety of materials, chemical systems and the processes they undergo. Such techniques have enabled the obtaining of information at atomic or molecular levels through small-scale, often non-destructive experiments. Of particular importance has been the study of catalytic processes, since these are a primordial part of the chemical phenomenon of transformation of matter. This involves physical and chemical processes that take place in solid-solid, solid-liquid and/or solid-gas interfaces, which may be present in the successive steps of catalyst life, from its preparation to its final use in the catalytic reaction. Many such techniques are now routinely used, essentially due to the progress of technology and the availability of more powerful and easy-to-use computers. The main physical techniques used to characterise solid materials and investigate their surface reactivity are of the spectrometric type. On the other hand, there are the techniques that make use of sources of stimulus different to the electromagnetic radiation; for example, techniques based on the direct application of magnetic or electric fields (Che & Védrine, 2012, pp. XXXI-XLIV). This chapter will deal with one of the last: the techniques of electrochemical analysis or electroanalytical techniques. These techniques are traditionally associated with studies of dissolved species that undergo redox processes in solution, with the help of electrodes.
In this sense, it is worth to mention a few definitions. Electrochemistry is defined as “the science of structures and processes associated with the phenomena of transfer and transport of charge occurring at the interface and through the interphase between an electronic conductor (electrode) and an ionic conductor (electrolyte) or between two ionic conductors (Doménech-Carbó, Labuda, & Scholz, 2013). However, since 1960, a new branch of electrochemistry began to be developed, dedicated to the study of the phenomena that occur to the solid phases during an electrochemical process; this is the Solid State Electrochemistry (Scholz, Schröder, Gulaboski, & Doménech-Carbó, 2015, p. v). Paraphrasing Fritz Scholz and Birgit Meyer (Scholz & Meyer, 1998, p. 2), “Solid State Electrochemistry” is the science that investigates the role and fate of solid phases in the course of electrochemical reactions. Within this context emerges the Solid State Electroanalytical Chemistry (SSEAC), dedicated to obtaining information of solid materials by electrochemical methods and according to the IUPAC (Doménech-Carbó, Labuda, & Scholz, 2013), SSEAC deals with studies of processes, materials and methods specifically aimed at obtaining analytical information (elemental quantitative composition, phase composition, structure and reactivity) of solid materials by electrochemical means. Thus, SSEAC methods allow the characterisation of minerals having the same chemical composition; for example, the identification species such as hematite, α-Fe2O3 and maghemite, γ-Fe2O3.
Much of the electrochemical methods are based on the use of energy changes that initiate reactions; that is to say, inducing the transformation of one compound into another. The thermodynamics of these reactions depends on Gibbs’ free energies of reagents and products, associated with differences in electrical potential; while their reaction rates (kinetics), associated with the electric current, depend on the reaction pathways and the activation barriers between the reactants and the products. Therefore, electrochemical measurements provide information on the reactions of the compounds and since the reactivity of a compound always depends on its composition and structure, the respective information can be deduced from electrochemical measurements.