Anode catalyst Direct conversion

Ceria, particularly when doped with Gd203 or SmzOs," has received some attention for direct hydrocarbon conversion in SOFC. Dating back to Steele and co-workers,interesting properties have been demonstrated for ceria-based anodes in direct utilization of methane. Later work suggested that the performance of ceria-based anodes in hydrocarbons could be improved by the addition of precious-metal catalysts, at dopant levels,but the performance of these cells was still too low for practical considerations. The problem with doped ceria is likely that its electronic conductivity is not sufficient. In general, the electrode material should have a conductivity greater than 1 S/cm in order to be practical since a conductivity of 1 S/cm would lead to a cell resistance of 0.1 Q cm for an electrode thickness of 1 mm, even... 

While the membrane represents the heart of the fuel cell, determining the type of cell and feasible operating conditions, the two catalyst layers are its pacemakers. They fix the rates of electrochemical conversion of reactants. The anode catalyst layer (ACL) separates hydrogen or hydrocarbon fuels into protons and electrons and directs them onto distinct pathways. The cathode catalyst layer (CCL) rejoins them with oxygen to form liquid water. This spatial separation of reduction and oxidation reactions enables the electrons to do work in external electrical appliances, making the Gibbs free energy of the net reaction, —AG, available to them. 

In contrast to stationary applications, portable applications require frequent start and stop procedures. Therefore for SOFC, a robust cell design and adapted electrode-electrolyte assemblies are an important issue. Frequent thermal cycles between room temperature and an operation temperature of about 600-800 °C pose challenges to the layered system consisting of solid anode, ceranfic electrolyte and solid cathode with respect to thermal and mechanical stability. For several years, different approaches to developing tubular nficro SOFC have been undertaken but did not lead to a commercial product yet. As SOFC can be operated with pure hydrogen, reformate and hydrocarbons as fuel as well - the latter option means direct internal reforming at the anode catalyst — various investigations focused on reduced operation temperature and a parallel conversion of fuels [21]. 

In fuel cells hydrocarbons are dehydrogenated by active catalysts. The main purpose of these cells is the direct conversion of combustion energy into electrical energy. At the fuel cell cathode the hydrocarbon is in must cases oxidized to carbon dioxide (equation 26a), because the intermediates are more easily oxidized than the starting hydrocarbon. Only in few cases are substitution, dehydrogenation or coupling products of the hydrocarbon obtained. At the fuel cell anode oxygen is reduced to water (equation 26b). 

Until a few years ago, the TG/SC mode of SECM operation was the most common way to image an enzyme that catalyzes oxygen reduction. TG/SC mode is well suited for imaging activity of surfaces with morphological features because it is relatively insensitive to changes in the tip-substrate distance [14]. The main difference between this mode and classical FB mode is that the feedback diffusion process is not required for TG/SC mode, which enables a direct measurement of activity in acidic solutions. This mode is the converse of SG/TC mode used for the anode catalysts. TG/SC mode has been applied to the study of the kinetics of oxygen reduction reaction (ORR) [14], evaluation of catalytically active nonprecious metal alloy compositions [57,58], optimization of Cu(II) biomimetics [59], thermodynamics-based design of catalysts [60], and analysis of wired enzyme architectures [61]. 

Fuel cells can run on fuels other than hydrogen. In the direct methanol fuel cell (DMFC), a dilute methanol solution ( 3%) is fed directly into the anode, and a multistep process causes the liberation of protons and electrons together with conversion to water and carbon dioxide. Because no fuel processor is required, the system is conceptually vei"y attractive. However, the multistep process is understandably less rapid than the simpler hydrogen reaction, and this causes the direct methanol fuel cell stack to produce less power and to need more catalyst. 

Iodine is oxidised to iodine(l) at an anode and use has been made of this reagent for the conversion of styrenes to the phenylacetaldehyde dimethyl acetal [66]. Iodine functions as a catalyst in this process. However, a moderate concentration of iodine is required to suppress the direct oxidation of the styrene to give 1,2-dimethoxylated products. 

In Figure 5 the conversion of 1-phenylethanol and the open circuit potential of alumina-supported catalysts are plotted as a function of reaction time. There is a striking difference between the curves of unpromoted (a, a ) and bismuth-promoted (c, c ) catalysts. When air is introduced to the reactor, the potential of the platinum-on-alumina catalyst quickly increases to the anodic direction and after one minute the catalyst potential is above -300 mV. One may conclude that there is practically no hydrogen on the platinum surface and after a short period an increasing fraction of platinum is covered by OH. The influence of bismuth promotion is a higher reaction rate (final conversion) and lower catalyst potential during reaction. 

A convenient method to produce porous surfaces is the anodic oxidation of aluminum plates. Such microstructured aluminum platelets have been coated by wet impregnation with Pt-, V- and Zr-precursors [35], and tested under catalytic methane combustion conditions. The conversion rate of oxygen followed directly the platinum content in the catalysts. These data were well reproducible even after five different runs. 

A library of 35 different catalysts fixed on electrochemically oxidized aluminum either in oxalic acid (Lib 1) or sulfuric add (lib 2) was tested at 450 °C and 1.1 bar. The methane-to-oxygen ratio was set to 1 in order to establish the potential of the catalyst to form intermediates. Figure 3.20 shows experimental results for a residence time of 550 ms and a screening time of 60 s. The conversion rate followed directly the platinum content in the catalysts. The higher the platinum content, the higher is the degree of conversion. Catalyst carrier formed by anodization of... 

The Direct Methanol Fuel Cell, DMFC, (see Fig. 7-6 in section 7.2.2.4.) is another low temperature fuel cell enjoying a renaissance after significant improvements in current density. The DMFC runs on either liquid or, with better performance but higher system complexity, on gaseous methanol and is normally based on a solid polymer electrolyte (SPFC). R-Ru catalysts were found to produce best oxidation results at the anode, still the power density is relatively low [5, 29]. Conversion rates up to 34 % of the energy content into electricity were measured, an efficiency of 45 % is expected to be feasible in the future. SPFC in the power order of several kW to be used in automobile applications are currently in the development phase. 

This review will deal with the direct and indirect anodic oxidation of unactivated CH bonds in alkanes, and of remote CH bonds in substrates with various functional groups. The conversion of CH bonds activated by vinyl, aryl, amino or alkoxy groups will not be dealt with in this chapter. However, the catalytic oxidation of alkanes in fuel cells and the oxidation of alkanes with molecular oxygen and iron catalysts are included. 

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