Electrocatalyst surface coverage

The use of cyclic voltammetry in conjunction with CO adsorption and CO stripping for estimating electrocatalyst surface area and CO coverage is shown in Fig. 22 for Pt and Rh electrocatalysts in acidic solution [45, 46]. 

The presence of electrolyte, its possible adsorption on the electrocatalyst, and the electrode-electrolyte potential can alter the strength of reactant adsorption, the surface coverage, and the reaction rate (5,7,8). Thus, electro-generative hydrogenation of ethylene on platinum and palladium electrodes in acidic electrolytes proceeds more slowly than the corresponding gas phase catalytic reactions (33). However, electrocatalytic reduction of cyclopropane is faster than the catalytic one, probably due to a decrease in hydrogen and reactant competitive chemisorption. Some electrolyte ions and impurities can also poison the electrocatalysts (34). 

Not only is the value of jQ important in electrocatalysis but also the experimental Tafel slope at the operating electrode potential. As expected in an electrocatalytic process, this complex heterogeneous reaction exhibits at least one intermediate (reactant or product) adsorbed species. Therefore, a single or simple Tafel slope for the entire process is not expected, but rather surface coverage and electrolyte composition potential dependent Tafel slopes within the whole potential domain are expected. Instead of calculating the most proper academic Tafel slope, the experimental current vs. potential curve is required for the selected electrocatalysts [4,6]. 

A common method for improving formic acid electrooxidation activity is through the incorporation of foreign adatoms in sub- or monolayer coverages onto metal electrocatalyst surfaces (substrates). Adatoms are usually deposited onto the metal surface either by under potential deposition (UPD) or by irreversible adsorption [17]. The two dominant reaction enhancement mechanisms for the direct dehydrogenation pathway, as described in Sect. 3.3 of the previous chapter for formic acid electrooxidation, are the third-body and electronic effects. The type of enhancement mechanism due to adatom addition is dependent on the substrate/adatom... 

In the equation 25 n is the number of electrons and F, k and C(02) are Faraday constant, rate constant and bulk O2 concentration, respectively. In addition, P and y are symmetry factors, while E is the electrode potential. The term 0ad relates to total surface coverage by OHads and adsorbed anions. The effect of surface oxides on the metal electrode surface was clearly demonstrated for series of Pt-based electrocatalysts [53, 54], leading to a general recipe for design of electrocatalysts with improved ORR activity. In specific, if oxide formation is hindered onset potential for ORR is shifted to higher anodic potentials. Underlying principles of this route have been set by combining electrochemical measurement... 

A way to circumvent the first problem is to ensure that all of the active material is present at the electrode surface. That is, employ a chemically modified electrode where a precursor to the active electrocatalyst is incorporated. The field of chemically modified electrodes Q) is approaching a more mature state and there are now numerous methodologies for the incorporation of materials that exhibit electrocatalytic activity. Furthermore, some of these synthetic procedures allow for the precise control of the coverage so that electrodes modified with a few monolayers of redox active material can be reproducibly prepared. Q)... 

XPS or AES is extensively used not only to indicate the cleanliness of the sample before transfer, but also to indicate the presence of adsorbates and their oxidation states following electrochemical experiments and transfer back into the UHV environment. In the case of model platinum-based electrocatalysts, the electron spectroscopies have been used to estimate the coverage of the adsorbate metal atoms or the alloy composition. In the case of alloys, or the nucleation and growth of metal adsorbate structures, the techniques give only the mean concentrations averaged over a depth determined by the inelastic mean free path of the emitted electrons. Adsorption and reaction at surfaces often depend on the... 

A prevailing view of carbon-supported metal nanoparticles is that they are in a form of cubo-octahedral or icosohedral structures.One such model for the active electrocatalyst with submonolayer Pt coverage, a cubo-octahedral Ru particle with Pt islands on its surface, is shown as an inset in Fig. 18b. Pt atoms are in 2D islands as expected from EXAFS measurements and the Ru nanoparticles are supported on a high surface area Vulcan XC-72 carbon. 

The PtML/Ru electrocatalysts were inaugurated for fuel cell anode reactions, and the catalysts were synthesized by two methods. The first method facilitated the formation of submonolayer-to-multilayer Pt deposits on Ru surfaces via the electroless (spontaneous) deposition of Pt on Ru [103-107]. The coverage and morphology of the Pt deposit on Ru depended on the concentration of platinum ions and the time of deposition. The activity and selectivity of the electrocatalyst was fine-tuned by changing the coverage (the cluster size) of the Pt deposit, and the optimized PtRu2o/C (with atomic ratio of Pt Ru as 1 20) electrocatalyst demonstrated superior CO tolerance and stability compared to conventional Pt-Ru/C alloy catalyst [104]. 

Recent work has been carried out by Conway and Novak " " on the role of the oxide film, plus coadsorbed CP ion at Pt electrodes in the chlorine evolution reaction optimum electrocatalysis appears to arise at a surface at which appreciable coverage of specifically adsorbed Cl" ion exists together with an incomplete film of OH and O species. Thick oxide films on Ir and Ru electrodes are also very effective electrocatalysts. 

In addition to the above mentioned catalyst, Schmidt et al. [60] reported the superior activity and CO tolerance of PdAu/C (Vulcan XC-72) (platinum-free electrocatalysts) at 60 °C compared to PtRu/C. This enhanced CO tolerance was explained by the significantly reduced CO adsorption energy on the PdAu surface, resulting in very low effective CO coverage and therefore in high HOR rates [60, 61],... 

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