Electrocatalytic Oxidation of CO

The electrochemical reduction of NO discussed in the previous section was found to be independent of surface steps. By way of comparison, we discuss in this final section the electrochemical oxidation of CO which, in contrast, is strongly affected by the presence of surface steps. 

The potential dependence of the apparent rate constant is found to be structure insensitive and gives a Tafel slope of (80 mV), which is consistent with step (6.50) being rate limiting (see Addendum to this chapter). 

The reaction preferably takes place at the steps, whereas the terraces supply CO through fast surface diffusion. Infrared spectra acquired during he oxidation of the CO 

The apparent reaction rates fall in the range lO -lO s. The oxidative activation of water to form OH groups appears to occur at the step sites. The OH intermediates can subsequently go on to oxidize CO. The intrinsic catalytic reactivity of the steps is shown to be independent of the step density. In the oxidation of a saturated CO adlayer, two processes are distinguished reaction initiation, which is a zero-order process in which CO desorbs to generate reaction centers, and an oxidation process that is of the Langmuir-Hinshelwood form with competitive adsorption of CO and OH. CO diffusion is thought to be fast dco 10 cm /s) on the terraces towards the step where OH is generated. 

Step (6.49) preferably occurs at the step edges and is fast. This implies that OHads builds up concentration at the step edges. The CO binds to strong on step edges to react. The more weakly bonded, mobile CO adsorbed to terrace, therefore, reacts first with adsorbed OH. 

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Figure 12. Electrocatalytic oxidation of CO (from a CO-saturated 0.1 M HCIO4 solution) on different Pt-based electrodes (sweep rate 5 mV/s, 25 "C) ( ) smooth Pt, (0) 0.1 mg cm Pt dispersed in a polyaniline film,(A) 0.1 mg cm" R-Sn dispersed in a polyaniline film.

Takasu Y, Zhang XG, Minoura S, Murakami Y. 1997. Size effects of ultrafine palladium particles on the electrocatalytic oxidation of CO. Appl Surf Sci 121 596-600. 

Figure 10.9 Electrocatalytic oxidation of CO in a CO saturated 0.1 M HCIO4 solution at dilTerent modified polyaniline...

The first example of successful electrocatalytic oxidation of CO was reported using networks of thiolate-capped Au particles by Maye et al. [153]. This type of self-assembled electrode is a popular motif for nanosensor architectures. Authors reported that particles with average sizes of both 2 and 5 nm result in active catalyst systems (i.e., the onset ofelectrocatalytic activity at the metal-to-nonmetal transition size threshold of about 2-3 nm was not observed) and that thiolate ligands allow access of CO to the metal cores. 

Later, Hayden et al. [154] demonstrated a sharp volcano-like size effect plot and enhanced activity of Au nanoparticles supported on Ti02 in electrocatalytic oxidation of CO using a high-throughput approach an electrode array system with series of Au-Ti02 catalysts (Figure 9.13). 

Venkataramab et al. reported that MP, MPc, and cyclam complexes, employed as co-catalysts with Pt electrodes, acted as redox mediators which generated a species that catalyzes CO oxidation. Shi and Anson showed that adsorbed Co OEP catalyzed the oxidation of CO to CO2 in aqueous media. The first step was the oxidation of Co OEP to Co OEP, followed by coordination of CO. Van Baar et al. reported that electrocatalytic oxidation of CO occurs in the presence of Rh and Ir porphyrins in aqueous acid solutions. In strongly alkaline media Co and Ee porphyrin counterparts showed excellent catalysis for CO oxidation. The proposed catalytic mechanism was as follows (i) CO adsorption to metal center, (ii) nucleophilic attack by H2O (acid media) or OH (basic media) on the adsorbed CO, and (iii) decarboxylation. The differences in the behavior of the metalloporphyrin complexes was explained in terms of CO affinity for the central metal in the different oxidation states. ... 

In the case of continuous CO oxidation CO gets re-adsorbed on the smface of the electrode after the electrode surface is cleaned upon deep anodic excursion, which leads to a steady state due to continuous CO supply [79]. Recorded RDE polarization emv es show dependence on the potential sweep rate and the electrode rotation rate (not only in the region of diffusion control). Onset for CO oxidation is always higher in the experiments of continuous CO oxidation than in CO stripping voltammetry measurements due to self-poisoning effect. For complete overview of theory of electrocatalytic oxidation of CO the reader is referred to couple excellent existing reviews [77, 79],... 

In this section, we focus on the Pt-induced modifications of the electrocatalytic properties of Ru(OOOl), using the electro-oxidation of CO (CO bulk oxidation) as example. 

Platinum carbonylate anion clusters like [Pt3(CO)6] can be obtained by alkaline reduction of [PtCh] in a CO atmosphere. From [Pt3(CO)s] other higher nuclear-ity anions can be obtained. In this context, several examples have been reported in which this type of anionic cluster is used in the preparation of catalysts by impregnation or exchange methods. Salts of [Pt3 (CO)6 ] (n = 3, 5) have been used to prepare, by impregnation, dispersed platinum on ZnO and MgO [49] and, by ion exchange methods, to prepare Pt3 /C electrodes for the electrocatalytic oxidation of methanol [50]. A salt of [Pti2(CO)24] has recently been used to prepare... 

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

In acidic medium, the electrocatalytic oxidation of glyoxal on platinum in the potential range 1 to 1.5 V/RHE leads mainly to formic acid (60%) and CO (40%). With lead adatoms, it becomes possible to oxidize glyoxal between 0.4 and 1.0 V/RHE leading mainly to CO2 formation (46%), while the selectivity towards, glyoxylic acid is sensibly increased (28%). At pH=7 and 1,9 V/RHE, the main oxidation product is formic acid (99%). Otherwise, in acidic medium the oxidation is more selective towards glyoxylic acid (70%), when the applied potential is in the range of 1.80 to 2.13 V/RHE. 

Until recently there has been surprisingly little interest in high oxidation state complexes of terpy. Meyer and co-workers have demonstrated that the ruthenium(IV) complex [Ru(terpyXbipy)0] is an effective active catalyst for the electrocatalytic oxidation of alcohols, aromatic hydrocarbons, or olefins (335,443,445,446). The redox chemistry of the [M(terpy)(bipy)0] (M = Ru or Os) systems has been studied in some detail, and related to the electrocatalytic activity (437,445,446). The complexes are prepared by oxidation of [M(terpy)(bipyXOH2)] . The related osmium(VI) complex [Os(terpyXO)2(OH)] exhibits a three-electron reduction to [Os(terpyXOH2)3] (365,366). The complex [Ru(terpy)(bipyXH2NCHMe2)] undergoes two sequential two-electron... 

The electrocatalytic oxidation of ethanol has been investigated for many years on different platinum-based electrodes, including Pt/X alloys (with X = Ru, Sn, Mo, etc ), and dispersed nanocatalysts. Pme platinum smooth electrodes are rapidly poisoned by some strongly adsorbed intermediates, such as carbon monoxide, resulting from the dissociative chemisorption of the molecule, as shown by the first experiments in infrared reflectance spectroscopy (EMIRS). Both kinds of adsorbed CO, either linearly-bonded or bridge-bonded to the platinum surface, are observed. Besides, oth-... 

In situ FTIR studies of the electrocatalytic oxidation of ethanol at iridium and rhodium electrodes were carried out by de Tacconi et al. According to this study the ethanol electrosorption leads to the formation of linearly bonded and bridge-bonded CO on Rh surfaces, but only linearly bonded CO is formed on Ir. 

The interaction of CO with alloy or bimetallic surfaces is of special interest because of the importance of bimetallic catalysts in both the electrochemical and gas-phase oxidation of CO. Platinum-ruthenium alloys have long been known to be superior catalysts for the electrochemical CO oxidation, but the details of their catalytic action are still disputed. We will have more to say about the mechanism of the electrocatalytic CO oxidation on both metals and alloys in section III.8, in particular about the relevant ab initio quantum-chemical studies. Here, we will simply discuss how the chemisorption properties of CO on PtRu depend on the stmcture and composition of the bimetallic surface. 

The formation of surface-bonded OH is of great interest for another important electrocatalytic process, namely the oxidation of CO. It is usually assumed by experimentalists that the surface-bonded OH reacts with CO to form CO2 in the reaction scheme ... 

In this section we will discuss the role of surface modification to enhance electrocatalytic oxidation of methanol, one of the interesting components for fuel cell technology. Perhaps the most successful promoter of methanol electrooxidation is ruthenium. Pt/Ru catalysts appear to exhibit classical bifunctional behavior, whereas the Pt atoms dissociate methanol and the ruthenium atoms adsorb oxygen-containing species. Both platinmn and ruthenimn atoms are necessary for eomplete oxidation to occur at a significant rate. The bifunctional mechanism can account for a decrease in poisoning from methanol, as observed for Pt/Ru alloys. Indeed, CO oxidation has been attributed to a bifimctional mechanism that reduces the overpotential of this reaction by 0.1 V on the Pt/Ru surface. 

Electrocatalytic oxidation of oxalic acid by a polymer from 51a (M = Co) [168]. 

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