Oxygen reduction reaction methanol concentrations

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Other Oxygenated Hydrocarbons Reductants. Other oxygenated hydrocarbons— 2-propanol, ethanol, methanol, iso-butanol, ethyl ether—were also tested. The inlet carbon atom concentration of these hydrocarbons was calculated to be equal to that of acetone in the tests reported above, e.g., equivalent to 1,300 ppm acetone. The NO conversion at steady state for each reductant is shown in Table II. All reductants showed 100% selectivity to N2. Since the carbon concentration of all reductants was the same, the activity of the reductants can be compared by comparing the NO conversion. The only reductant with activity close to acetone is 2-propanol with 31% conversion. Others showed much less activity than acetone. Specifically, methanol showed negligible NO reduction activity. It is speculated that the NO selective reduction activity is closely related to the ability of hydrocarbons to form oxygenated surface intermediates at these reaction conditions. This is being investigated further. 

The paper [106] reports data on the effect of dilution with helium on the selectivity of methanol formation and complete oxygen conversion temperature for the DMTM reaction in a small d = l mm) fused silica capillary at P = 100 atm and = 10 s. At a constant total pressure and reaction time, the increase of the helium fraction (decrease in the concentrations of the reactants) in the mixture was accompanied by the monotonic growth of the complete oxygen conversion temperature and the reduction of the selectivity of methanol formation to zero at [He] = 90% (Fig. 4.10). 

It should be emphasized that the positive effect of promotion with NOx manifests itself only at low pressure, becoming less pronounced at higher pressures [158]. Even at a pressure of 10 atm, the reduction in the reaction onset temperature (at 4% methane conversion) does not exceed 25 °C. The selectivity of formation of oxygenates is almost independent of the NOx concentration, with the selectivity of methanol formation in the absence of NOx constituting the same 30% (Fig. 9.7). 

As the initial concentration of oxygen in the mixture was increased from 11% to 15.5%, the selectivity of formation of most (except formaldehyde) oxygenates, especially ethanol and acetic acid, dropped markedly. However, the maximum temperature of heating of the mixture also increased from 283.5 to 316.8 °C, which could give a significant contribution to this reduction. Adding sequential portions of oxygen to the reaction mixture had little effect on the concentrations ethanol, aldehydes, acids, and methane — only the concentrations of methanol and carbon oxides increased monotonically [22]. 

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