Oxygen reduction reaction poisoning effects

Methanol crossover is one of the major obstacles to prevent DMFC from commercialization. This effect, which is caused by diffusion of methanol through the membrane, reduces the cell efficiency. Competing with the oxygen reduction reaction on the cathode side, methanol adsorbs on the surface of Pt and is oxidized to CO2. The main poisoning species formed during the chemisorption and oxidation of methanol is carbon monoxide... 

Here we shall briefly summarize the effects of individual poisons on various catalytic reactions taking place on automotive catalysts. There are three main catalytic processes oxidation of carbon monoxide and hydrocarbons and reduction of nitric oxide. Among secondary reactions there are undesirable ones which may produce small amounts of unregulated emissions, such as NH3, S03 (6), HCN (76, 77), or H2S under certain operating conditions. Among other secondary processes which are important for overall performance, in particular of three-way catalysts, there are water-gas shift, hydrocarbon-steam reforming, and oxygen transfer reactions. Specific information on the effect of poisons on these secondary processes is scarce. 

The archetypical direct fuel cell is the DMFC. like the PEMFC, the DMFC uses a proton-conducting membrane to separate the anode and cathode, and protons liberated during electrocatalytic methanol oxidation [Eq. (15.6)] at the anode are involved in oxygen reduction at the cathode. However, whereas in the hydrogen fuel cell the anode reaction is straightforward, the methanol oxidation is comparably sluggish, which is mainly attributed to poisoning effects. 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

Reduction activity was measured by using the same reactant mix except that oxygen was varied from 0.05 to 0.60% with the temperature held constant at either 482° or 593 °C. Again space velocity was varied by using 1-3 monolithic units. Water-gas shift reaction was studied at 75,000/hr with the reactants being 1% CO, 10% H20, and N2 the balance. For sulfur dioxide poisoning studies, either 50 or 100 ppm sulfur dioxide was added stepwise to the sample at 593°C with the oxygen level maintained at 0.40% the effect on conversion was monitored continuously. 

NO addition in propane and oxygen flow increases up to 68 % propane oxidation whereas NO reduction reaches 45 %. This result shows a positive effect of NO in propane oxidation in agreement with recent work on H-ZSM-5 reporting that addition of small amounts of NOx drastically increases tlie propane conversion [17]. This suggests a direct reaction between propane and NOx. One of tlie O2 roles would be to eliminate strongly chemisorbed species evidenced under propane and NO flow, without O2 such species could poison active... 

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