Methanol oxidation channels

GP 4] [R 11] For methanol oxidation over sputtered silver catalyst, conversion is higher when using micro channels of smaller diameter (8.5 vol.-% methanol balance oxygen 510 °C 4—27 ms sHghfly > 1 atm) [72]. For two channels of the same width, but different depths (70 pm, 130 pm), concentration differences of nearly 10% at the same residence time were detected, all other parameters being equal. 

The large amount of gaseous CO2 produced in the anode can also reduce the cell performance. Methanol oxidation gives one CO2 per each methanol molecule. Already at medium current densities (100 mA.cm ) the amount of dissolved CO2 becomes negligible and the molar flux of gaseous CO2 in the anode channel is comparable to the flux of liquid methanol. Thus, most of the CO2 formed during methanol oxidation is in the gas phase and has to be removed via the feed channel through the gas diffusion layer. There, CO2 bubbles form at the outlet of the pores. 

Carbon dioxide bubbles produced in the methanol oxidation reaction enter the anode channel and disturb the flow of the methanol-water solution. The direct effect of gaseous bubbles is a dramatic acceleration of the flow experiments show that in typical situations the outlet flow velocity exceeds the inlet velocity by an order of magnitude (Yang et al., 2005). This is easy to understand gas density is much lower than the density of liquid and simple mass conservation prescribes that due to an increasing concentration of gaseous bubbles, the two-phase flow must accelerate. 

The reactor consists of two parallel channels that include rows of steam reforming (SR) channels and methanol oxidation (OX) channels. Modeling was used to... 

A similar retardation effect of cosolvent was reported previously for benzene oxidation [90, 91]. The solvent may compete with reactant for diffusion in the channels and adsorption at the active sites of TS-1 catalyst. The activity Ti-beta for 1-hexene and cydohexanol oxidations is highest in acetonitrile, which is a polar, nonprotic solvent [92]. This is in contrast with the observed enhancement of the activity of TS-1 by methanol and protic solvents [68]. These differences have been... 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

Fig. 11.10 Parallel chronoamperometric screening of a 64-element, thin film electrocatalyst library for the oxidation of methanol. The library contained a diverse set of binary, ternary and quaternary electrocatalyst compositions consisting of Pt in combination with W, Ni, Co and Ru. The graph plots current vs. time and channel number. Conditions 1 M methanol, 0.5 M H2S04, room temperature, = + 450 mV/RHE, test time = 5 min. For clarity, channel numbers 2-4,10,12,19, 20, 23, 26-29, 42,45 and 57 are omitted. (Reproduced from [18]).

Bravo et al. [29] dealt with the coating of a commercial CuO/ZnO catalyst on quartz and fused-silica capillaries for future application in micro channels. The catalyst was mixed with boehmite as binder and water at a mass ratio of44 11 100. The boehmite was treated with hydrochloric or nitric acid before. The capillaries were pretreated with a hot sulfuric acid/solid oxidation step before coating. The capillaries were filled with the catalyst/binder suspension and then cleared with air. In this way, catalyst coatings up to 25 pm thick were obtained. The coatings were applied to methanol steam reforming (see Section 2.4.1). 

Selectivity on metal oxide catalysts is ultimately determined by complex intermolecular and surface-adsorbate interactions. Competing reaction channels are facilitated or hindered by the coordination geometry around metal cations, the ease of reduction of the surface, and the resulting stabilization of surface intermediates. The decomposition of relatively simple organic molecules like methanol and formic acid can be surprisingly complex, but attention to a few concepts may help to understand the reaction processes ... 

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