Electrocatalytic oxidation of hydrocarbons

The major problem with direct electrocatalytic oxidation of the hydrocarbon fuel at the anode is the marked tendency towards carbon deposition via hydrocarbon decomposition (Eqs. (1) and (2)). It is extremely difficult to avoid carbon deposition in the absence of a co-fed oxidant. However, some recent studies have reported anodes which show considerable promise for the direct electrocatalytic oxidation of hydrocarbons [29,68,69]. The conditions under which these anodes can be used may present problems for their widespread application, whilst their long-term durability with respect to carbon deposition must be established. Electrically conducting oxides have also been proposed in recent years as having potential for use as anodes for the direct electrocatalytic oxidation of hydrocarbons [67.70,74]. 

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]. 

Although some electrocatalytic reactions are first order in a key reactant [e.g., oxygen reduction (77)], several reactions of organic species exhibit other orders. Thus oxidation of hydrocarbons has a small fractional order in reactant and a negative order in H concentration (78). Alkene reduction is... 

The electrochemical immobilization of other metallic complexes like metalloporphyrins in PPy [147, 203, 204] led to materials exhibiting electrocatalytic properties with respect to the oxidation of hydrocarbons [147] and hindered phenols [204] by molecular oxygen. 

This chapter starts by discussing the range of possible fuels for SOFCs a brief discussion on the possibility of using renewable fuels in SOFCs is also included. The remainder of the chapter is devoted to approaches in fuel processing in SOFCs, and some of the issues and problems inherent in such fuel processing. The possibility of direct electrocatalytic oxidation of the hydrocarbon fuels at the anode is also discussed. The chapter concludes with a brief consideration of future prospects. 

Recent studies have also identified some alternative anodes, one being copper-based and incorporating significant quantities of ceria in addition to YSZ [68,114] and the other adding yttria-doped ceria to nickel and YSZ [69], both of which have been reported to show considerable promise for the direct electrocatalytic oxidation of the hydrocarbon fuels, without the need for any co-fed oxidant. However, the conditions under which such anodes can be used for direct hydrocarbon oxidation may be a problem for their widespread application, whilst their long-term performance in terms of deactivation resulting from carbon deposition remains to be investigated. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

One of the main objective of SOFCs in the future is the use of gaseous mixtures of C0-H2-H20 produced by coal gasification plants or by steam reforming a hydrocarbon fuel, especially methane. Very little data is available about the direct oxidation of methane in SOFCs [96, 97], Steele et al. [97] have recently confirmed the poor electrocatalytic activity of Pt electrodes for the anodic oxidation of methane at 800 °C. Although nickel fulfills major requirements for anode materials when H2 and CO are employed as fuels, its use for the direct oxidation of methane encourages carbon deposition. To overcome this problem, alternative anode materials must be... 

A single-chamber solid oxide fuel cell (SC-SOFC), which operates using a mixture of fuel and oxidant gases, provides several advantages over the conventional double-chamber SOFC, such as simplified cell structure with no sealing required and direct use of hydrocarbon fuel [1, 2], The oxygen activity at the electrodes of the SC-SOFC is not fixed and one electrode (anode) has a higher electrocatalytic activity for the oxidation of the fuel than the other (cathode). Oxidation reactions of a hydrocarbon fuel can... 

Similarly, oxidation of ethylene can occur above its E° (3-13,28,29). With an oxygen counterelectrode, conventional electrolytic oxidation starts when the anode potential becomes more positive than the cathode. Halogenation of hydrocarbons is also possible electrogeneratively, above E°, on appropriate electrocatalytic anodes (47, 50, 51). 

The relatively slow rate of hydrocarbon fuel cell oxidations prompted an intensive examination of the adsorption characteristics of organic reactants in the 1960s. Because of the low potential for the development of hydrocarbon fuel cells, such studies have largely subsided today and no modern surface analysis techniques have been applied to characterize intermediates. Conventional adsorption studies of carbonaceous species have been reviewed repeatedly (7, 9-12, 100 -, therefore, we summarize here only some essential adsorption features for fuel cell and selective electrocatalytic oxidations. 

From the above discussion it becomes apparent that some conflicting experimental evidence exists on hydrocarbon adsorption and on surface intermediates. This arises primarily from the use of electrocatalysts of varying histories and pretreatments. It should be stressed that many adsorption studies were performed on anodically pretreated platinum. The removal of surfaces oxides from such electrodes may have not been always accomplished when the surface was cathodically reduced in some experiments, as outlined in Section IV,D. Obviously, different surface species could exist on bare or on oxygen-covered electrocatalysts. Characterization of surface structure and activity and of adsorbed species using modern spectroscopic techniques would provide useful information for fuel cell and selective electrocatalytic oxidations and reductions. 

Unsaturated C2 Hydrocarbons. - The adsorption and the electrocatalytic transformations of ethylene and acetylene (reduction and oxidation) on Pt and Au electrodes have been the subject of several studies (see, for instance. References 309-311 and literature cited therein). 

Due to the intense efforts to develop fuel cell technology in the 1960 s, there has been continued interest in identifying efficient electrocatalytic processes for the electro-oxidation of organic molecules. The prospect of achieving the conversion of the chemical energy of hydrocarbons and their... 

The improved electro-oxidation behavior observed with the C-8 and C-12 acid coated electrodes might be attributed to (i) the higher concentration of sulfonic acid groups present in the electrocatalytic layer of the C-8 acid and C-12 acid coated electrode, (ii) enhanced diffusion of reactants and products, and/or (iii) the enhanced wettability" at the catalytic site of oxidation facilitating hydrocarbon adsorbtion. These results are in accordance with earlier findings where the addition of C-8 acid was observed to significantly enhance the ease of oxidation of various oxygenated molecules. 

Almost all studies regarding direct hydrocarbon SOFCs show comparatively poor performance (lower OCP and higher polarization resistance) with hydrocarbon fuels when compared to H2 fuel, Fig. 3.2. Since most of these tests are performed by switching fuel on the same cell, the drop in performance must be linked to the anode. It is possible that the increased polarization resistance may be due to lower diffusivity of the hydrocarbon fuels, but the electrodes are typically highly porous and the current density per unit area is relatively low. In addition, the oxidation of 1 mole of hydrocarbon fuel yields a significantly greater number of electrons than 1 mole of H2 fuel (H2, CH4, and C4H10 total oxidation yield 2, 8, and 26 moles of electrons, respectively). Furthermore, the cell OCP is an equilibrium, zero current, measurement and is therefore not directly influenced by gas diffusivity. Therefore, it is unlikely that gas diffusivity limits the performance for pure fuels at low conversion. The conclusion must then be that the anode electrocatalytic activity toward hydrocarbon oxidation is the primary factor in reduced SOFC performance. 

Bruce MK, van den Bossche M, McIntosh S (2008) The influence of current density on the electrocatalytic activity of oxide-based direct hydrocarbon SOFC anodes. J Electrochem Soc 155 B1202-B1209... 

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