Catalysts Electrocatalytic oxidation

Techniques for attaching such ruthenium electrocatalysts to the electrode surface, and thereby realizing some of the advantages of the modified electrode devices, have been developed.512-521 The electrocatalytic activity of these films have been evaluated and some preparative scale experiments performed. The modified electrodes are active and selective catalysts for oxidation of alcohols.5 6-521 However, the kinetics of the catalysis is markedly slower with films compared to bulk solution. This is a consequence of the slowness of the access to highest oxidation states of the complex and of the chemical reactions coupled with the electron transfer in films. In compensation, the stability of catalysts is dramatically improved in films, especially with complexes sensitive to bpy ligand loss like [Ru(bpy)2(0)2]2 + 51, 519 521... 

J. Luo, M. M. Maye, N. N. Kariuki, L. Wang, P. Njoki, Y. Lin, M. Schadt, H. R. Naslund, and C. J. Zhong, Electrocatalytic oxidation of methanol Carbon-supported gold-platinum nanoparticle catalysts prepared by two-phase protocol, Catal. Today 99, 291-297 (2005). 

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... 

The electrocatalytic oxidation of several secondary and primary alcohols has been also described, in keeping with the original work by Masui and coworkers it resorts to HPI or to X-substituted HPIs as electron carriers . The tetrafluoroaryl-substituted HPI was the most efficient among these catalysts. Secondary alcohols gave carbonyl compounds primary alcohols gave the corresponding aldehyde exclusively under anaerobic conditions, whereas a mixture of aldehyde plus carboxylic acid was formed in the presence of 2 . 

Scheme 12 Electrocatalytic oxidation of benzylic alcohol to benzylic aldehyde using [Cr" (0H2)PWii039] as redox catalyst taken from Ref 8).

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... 

Jones, Anne K. Sillery, Emma Albracht, Simon P. J. Armstrong, Fraser A. Direct comparison of the electrocatalytic oxidation of hydrogen by an enzyme and a platinum catalyst. Chemical Communications (Cambridge, UK) 2002 (8) 866-867. 

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... 

An electrocatalytic oxidation of guanine in oligonucleotides and DNA using the [Ru(bpy)3] +/ + redox couple has been observed and its mechanism investigated [261]. Metal-polypyridine complexes with 0x0 ligands act as electrochemical catalysts of water, Cl, or hydrocarbon oxidations [128, 166, 167, 262]. 

In another example, a mixed monolayer composed of a photoisomerizabie component and an electrochemical catalyst was applied to switch the electrocatalytic properties of a modified eleetrode between ON and OFF states. A Au-electrode surface functionalized with a nitrospiropyran mono-layer and PQQ moieties incorporated into the monolayer was applied to control the electrocatalytic oxidation of 1,4-dihydri-P-nicotinamide adenine dinucleotide (NADH) by light. The positively charged nitromerocyanine-state interface resulted in the repulsion of Ca cations, which are promoters for the NADH oxidation by the PQQ, thus resulting in the inhibition of the electrocatalytic process. In the nitrospiropyran state, the monolayer does not prevent the association of the PQQ catalyst and promoter thus it provides efficient electrocatalytic oxidation of NADH. Similar outcomes have been achieved using a combination of the photo- and thermal effects resulting... 

The electrocatalytic oxidation of many small organic molecules was carried out at Pt-based catalysts dispersed in an ECP, particularly that of Cl molecules (formic acid, formaldehyde, and methanol). 

Hable and Wrighton were the first to study the electrocatalytic oxidation of ethanol on Pt-Ru and Pt-Sn catalyst particles in PAni [46]. They found that dispersion of Pt, Pt-Ru, and Pt-Sn in PAni greatly enhanced the oxidation current of ethanol, the Pt-Sn electrocatalyst being far superior to the two others, with oxidation current... 

Intensive research is currently being carried out to obtain efficient catalysts for this reaction. Most of the research is devoted to different metal (Pt, in particular)-based materials, but several approaches include porous materials such as Ni-impregnated zeolites, obtained from soaking of zeolites in, for instance, NiSO4 solutions (Abdel Rahim et al., 2006). The performance of gold-zeolite-modified electrodes toward electrocatalytic oxidation of ethanol, an alternative to methanol fuel, has been recently reported by Ouf et al. (2008). 

Electrocatalytic oxidation of methanol on platinum based catalysts... 

In order to improve the electrocatalytic properties of methanol electrodes, and to reduce the poisoning phenomenon usually observed with bulk platinum, different platinum based alloys were considered such as Pt-Ru, and Pt-Sn, etc. [153]. Therefore such alloys were also dispersed into electron conducting polymers. Hable et al. [53] were apparently the first authors to disperse Pt-Sn catalyst particles in a polyaniline matrix, in order to activate the oxidation of methanol. They evaluated the Pt/Sn ratio by X-ray Photoelectron Spectroscopy and found that small amounts of Sn (e.g. Pt/Sn ratios of 10/1) were sufficient to enhance the electrocatalytic oxidation of methanol. Pt was found to be in the Pt(0) state whereas Sn was in an oxidized form. Similar observations concerning the enhanced electrocatalytic activity of Pt-Sn particles incorporated in PAni films were made by Laborde et al. [154]. Such Pt-Sn alloys are also very active for the electrocatalytic oxidation of ethanol [68,154]. 

Since bacterial cells behave like bags of enzymes in catalyzing redox reactions of substrates, using artificial redox compounds as electron acceptors or donors, they may work as catalysts to produce catalytic currents for the electrocatalytic oxidation or reduction of substrates in the presence of appropriate electron-transfer mediators. In fact, whole bacterial cells both in a suspension and in an immobilized state produce similar bioelectro-catalytic currents to those obtained with enzymes. 

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. 

Pd nanoparticles supported on PANI-NFs are efficient semi-heterogeneous catalysts for Suzuki coupling between aryl chlorides and phenylboronic acid, the homocoupling of deactivated aryl chlorides, and for phenol formation from aryl halides and potassium hydroxide in water and air [493], PANl-NF-supported FeCl3 as an efficient and reusable heterogeneous catalyst for the acylation of alcohols and amines with acetic acid has been presented [494]. Vanadate-doped PANI-NFs and PANI-NTs have proven to be excellent catalysts for selective oxidation of arylalkylsulfides to sulfoxides under nuld conditions [412]. Heterogeneous Mo catalysts for the efficient epoxidation of olefins with ferf-butylhydroperoxide were successfully synthesized using sea urchin-Uke PANI hollow microspheres, constructed with oriented PANI-NF arrays, as support [495]. Pt- and Ru-based electrocatalyst PANI-NFs—PSSA—Ru—Pt, synthesized by the electrodeposition of Pt and Ru particles into the nanofibrous network of PANI-PSSA, exhibited an excellent electrocatalytic performance for methanol oxidation [496]. A Pt electrode modified by PANI-NFs made the electrocatalytic oxidation reaction of methanol more complete [497]. Synthesis of a nanoelectrocatalyst based on PANI-NF-supported... 

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). 

Thiols are impurities distributed among petroleum products. They cause foul odor and deterioration of additives in finished products. Some thiol compounds such as 6-mercaptopurine and 6-thioguanine are used in medicine (e.g., treatment of leukemia) while others form essentials components of biological systems (e.g., amino acids). Some thiols (e.g., cysteine) are not readily detected on bare electrodes, hence the need for chemical modification of electrodes with electroactive catalysts. The use of CME also enhances sensitivity for the detection of thiols even for those that can be determined directly on bare electrodes. Phthalo-cyanines and porphyrins have been studied extensively as electrocatalysts for the detection of thiols. Electrocatalytic oxidations of cysteine and 2-mercaptoethanol have received considerable attention over many years. The aim is to lower the... 

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. ... 

Chen G, Li Y, Wang D, Zheng L, You G, Zhong C-J, Yang L, Cai F, Cai J, Chen BH (2011) Carbon-supported PtAu alloy nanoparticle catalysts for enhanced electrocatalytic oxidation of formic acid. J Power Sources 196 8323-8330... 

Philips ME, Gopalan Al, Lee K-P (2011) Enhanced electrocatalytic performance of cyano groups containing conducting polymer supported catalyst for oxidation of formic acid. Catal Commun 12 1084-1087... 

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