Adatoms electrooxidation

The use of adatoms of foreign metals obtained by imderpotential deposition on the platinum surface is another convenient method for investigating the effect of a promoter on the electrocatalytic properties of platinum. However, the effect of adatoms in this case has been shown to be not as effective for electrooxidation of methanol as for the oxidation of other organic molecules such as formic acid adatoms of tin, however, showed a positive effect on the rate of methanol oxidation. ... 

However, for some electrocatalytic reactions, such as the electrooxidation of alcohols, aldehydes or acides, and also the electro reduction of oxygen, lead adatoms can exhibit a promoting effect (3-7). Moreover, lead can change the selectivity in the case of electrocatalytic reductions of nitrocompounds (8), whereas it inhibits the adsorption of hydrogen on platinum (9,10),... 

The main aim of these studies is how to enhance the oxidation of glucose therefore, upd studies are also involved. As in the previous cases, the role of the crystalline surface structure of platinum electrodes in the electrooxidation is also studied. A series of studies was carried out by Wilde and Zhang using EQCM technique, in the presence and absence of adatoms in both acid and alkaline media. Unfortunately, owing to the very nature of the technique used, only phenomenological conclusions can be drawn from these latter studies and no information is furnished about the nature of the adsorbed species. The same refers to some other studies. ... 

Since around 1960, the formic acid electrooxidation mechanism has been investigated, resulting in several review articles [15-18]. Formic acid electrooxidation studies have been carried out on pure metal electrodes, such as platinum (Pt) [19], palladium (Pd) [20], gold [21-23], rhodium [24, 25], and iridium [26]. Studies have also been performed on alloys, intermetallics, and adatoms. The conversion efficiency is determined by the rate of a series of steps (a) reactant adsorption, (b) electrooxidation, and (c) product desorption. The electrooxidation... 

Demirci investigated the degree of segregation and shifting of d-band centers by metal alloy combinations to improve the direct liquid fuel cell catalyst activity through electronic promotion of the dehydrogenation pathway [57]. He focused on Pt- and Pd-based catalyst for formic acid electrooxidation and looked at the potential impact of surface adatom adsorption of other 3d, 4d, and 5d transition metals. The criteria he imposed for improved catalytic activity on Pt and Pd... 

Catalyst activity towards formic acid electrooxidation is strongly influenced by preparation method and nanoparticle size. As discussed in the previous chapter, the optimal sizes for Pt/C and Pd/C are 4 nm and 5.2-6.5 imi, as determined by Park et al. [14] and Zhou et al. [15], respectively. This chapter is segregated into two sections bimetallic catalysts and catalyst supports. The section on bimetallic catalysts is subdivided into adatoms, alloys, and intermetallics. 

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... 

To illustrate the primary effects of adatom addition, single-crystal electrodes are discussed here. Feliu and Herrero have extensively studied formic acid electrooxidation on Pt single-crystal substrates modified with an array of various adatoms. They have established a connection between the electronegativity of the adatoms in relation to Pt and the type of active enhancement mechanism incurred as a function of adatom coverage [42]. Their results support inhibition of the indirect pathway on Pt(lll) terraces and they have demonstrated that COads formation occurs at step and defect sites. For Pt(l 11) substrates decorated with electropositive adatoms, such as Bi, Pb, Sb, and Te, the electronic enhancement is extended to the second or third Pt atom shell from the adatom. While for electronegative adatoms, in respect to Pt, the third-body effect dominates with increased coverages, such as S and Se. 

Table 4.2 highlights the extensive work done in Feliu s group on adatom-decorated Pt(lll) substrates—specifically Se, As, Te, Pd, and Bi. Cyclic voltammogram results are compared for the first forward sweep in 0.25 M formic acid and 0.5 M H2SO4 at 50 mV s vs. RHE. Two distinct phenomena can be differentiated based on their results (a) formic add electrooxidation activity at low overpotentials and (b) the... 

Fig. 4.2 Plot of direct formic acid fuel cell performance at 0.6 V for Pt/C anodes as a function of Pb and Sb adatom coverages. The experimental data is compared to the two formic acid electrooxidation models proposed by Leiva (solid line) electronic enhancement and (dashed line) third-body effect [29]...

Wieckowski s group has studied formic acid electrooxidation on Pt nanoparticles decorated with controlled amounts of Pd and Pd-l-Ru adatoms [41]. They reported two orders of magnitude increase in the reactivity of the Pd-decorated catalyst compared to pure Pt towards formic acid oxidation. Also, it was concluded that the impact of COads on the Pt/Pd catalyst through the dual pathway mechanism is much lower even though the potential required to remove COads from the surface was the highest. 

Bismuth has attracted significant interest as a Pt/C modifier for formic acid electrooxidation [21, 24, 26, 27]. A wide range of stable and well-characterized electrode surfaces modified by irreversible Bi adatom adsorption on Pt have been reported in the literature for a range of Bi coverages 6). Chen et al. have explored Bi adatom decoration on 81 nm tetrahexahedral Pt nanoparticles that while composed of (100) and (110) facets that are the least active for formic acid electrooxidation, they are boimd by 730 and vicinal high-index facets that are extremely active [18]. They have measured current densities of 10 mA cm for Bi coverages up to 0.9 at 0.4 V in 0.25 M formic acid and 0.5 M H2SO4 solution see Fig. 4.4. They also showed steady-state activity at 0.3 V of 2.8 mA cm after 1 min vs. 0.0003 mA cm for the non-modified Pt baseline. 

To improve the electrocatalytic activity of platinum and palladium, the ethanol oxidation on different metal adatom-modified, alloyed, and oxide-promoted Pt- and Pd-based electrocatalysts has been investigated in alkaline media. Firstly, El-Shafei et al. [76] studied the electrocatalytic effect of some metal adatoms (Pb, Tl, Cd) on ethanol oxidation at a Pt electrode in alkaline medium. All three metal adatoms, particularly Pb and Tl, improved the EOR activity of ft. More recently, Pt-Ni nanoparticles, deposited on carbon nanofiber (CNE) network by an electrochemical deposition method at various cycle numbers such as 40, 60, and 80, have been tested as catalysts for ethanol oxidadmi in alkaline medium [77]. The Pt-Ni alloying nature and Ni to ft atomic ratio increased with increasing of cycle number. The performance of PtNi80/CNF for the ethanol electrooxidation was better than that of the pure Pt40/CNF, PtNi40/CNF, and PtNi60/CNF. 

Kadirgan F, BouhiercharbonnierE, Lamy C, Leger JM, Beden B (1990) Mechanistic study of the electrooxidation of ethylene-glycol on gold and adatom-modified gold electrodes in alkaline-medium. J Electroanal Chem 286(1-2) 41-61... 

Based on early work by Watanabe and Motoo [89] (originally on the enhanced oxidation of CO and methanol at Pt electrodes with various oxygen-adsorbing adatoms [34, 39, 81-83, 90, 93, 94]), a bifunctional mechanism involving water activation by Ru (Equation 16.11) and subsequent CO electrooxidation on a neighboring Pt atom (Equation 16.12) has been postulated by a number of groups [20, 102,103, 110-112]. 

Kokkinidis and Xonoglou have examined the electrooxidation of glucose and of other monosaccharides like galactose and fructose on Pt surfaces modified by submonolayers of heavy metals (Tl, Bi and Pb) deposited at underpotentials. The catalytic action due to the adatom submonolayers has been interpreted in terms of the decrease of the electrode poisoning, resulting from a stable glucono-lactone type adsorbate, according to the model of the "third body effect . ... 

More recently, the influence of various factors like temperature, concentration, anions, adatoms (Bi, Cd, Tl, Pb, Re) was systematically reexamined in acidic and alkaline media . The complexity of the electrooxidation of glucose on platinum, mainly due to the hemiacetal group, was again evidenced. 

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