Surface chemical properties oxygen reduction

Ahmed et al. [116] carried ont a detailed stndy with the objective of identifying the properties of activated carbons that are important for the SCR of NO they concluded that chemical properties such as surface oxides and mineral matter play a more important role than their physical properties, such as surface area and pore structure. In effect, they found that the catalyst activity correlated directly with the oxygen content of the carbon samples and inversely with their pH. These results indicate that the NO conversion is favored on more acidic carbons. They also reported that NO reduction by ammonia was negligible in the absence of oxygen. Indeed, it has been shown [117] that oxygen enhances the C-NO reaction through the formation of surface oxygen complexes, which are essential for the C-NO reaction to proceed. 

As discussed above, the oxygen adsorption and reduction processes are often simulated either on small Pt clusters or flat surfaces. However, both experimental measurements [51, 52] and computational calculations [53, 54] indicate that nanosized electrocatalysts show a considerably different catalytic activity from extended flat surfaces. These investigations would suggest that effects observed with particle size reduction go well beyond the increase in surface area and involve fundamental physical and chemical changes in the reaction steps. Han and his coworkers [55] studied explicitly Pt nanoparticles with 1 and 2 nm sizes and compared their chemical adsorption properties to those of an extended flat Pt (111) surface. As atomic oxygen (O) and hydroxyl group (OH) are two species of considerable importance [56], they focused on effect of particle size and Pt coordination on the chemisorption energies of O and OH. 

Modification of the electrode started with academic studies on physical and chemical adsorption, i.e., with the appearance of fundamental researches on adsorption of different species on electrode surfaces, both under polarization and at open circuit potential [3]. The properties of similar chemically modified electrodes , in which the modifier consists of a monolayer of a variety of chemical species with different characteristics, possessing (or not) particular properties, were initially studied in a purely electrochemical context, aimed at the collection of fundamental physico-chemical data. A small group of electrochemists were among those involved in these basic studies, envisioning the perspectives opened by the novel systems. In the first, really fascinating, work with similar monomolecular layers, cobalt porphyrin and phthalocyanine, as well as deliberately synthesized dicobalt face-to-face porphyrins were adsorbed on Pt or C surfaces to catalyze molecular oxygen reduction [4]. However, similar systems were not always used or adequately tested in proper amperometric sensing by researchers more interested in electroanalysis dicobalt face-to-face porphirins still constitute a rare example of tailored materials for selective amperometric detection. 

Diamond s electrochemical properties are ideally suited for the detection of trace metal ions via anodic stripping voltammetry (ASV) [30,57,115] (1) a large overpotential for hydrogen evolution, (2) a large overpotential for oxygen reduction, (3) no metal complexation with the diamond surface, (4) excellent stability at extreme anodic and cathodic potentials, and (5) chemical inertness and environmental friendliness. These properties make it superior, in some respects, to Hg and other alternative electrode materials. 

A widespread interest for the electrochemical oxygen reduction reaction (ORR) has two aspects. The reaction attracts considerable attention from fundamental point of view, as well as it is the most important reaction for application in electrochemical energy conversion devices. It has been in the focus of theoretical considerations as four-electron reaction, very sensitive to the electrode surface structural and electronic properties. It may include a number of elementary reactions, involving electron transfer steps and chemical steps that can form various parallel-consecutive pathways [1-3]. 

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