The Effect of Carbon Dioxide

CO2 poisoning on pure Pt catalysts is modest when compared to the effect of CO (see Fig. 6.3), especially when the large differences in relative concentrations in reformate are considered (typically 25% CO2 vs. 40 ppm CO). The poisoning effect comes from two possible mechanisms:

i/mAcm-2

FIGURE 6.3 Effect of CO2 on cell performance of Pt and PtRu anode catalysts.

i/mAcm-2

FIGURE 6.3 Effect of CO2 on cell performance of Pt and PtRu anode catalysts.

Both are forms of the "reverse water-gas shift" reaction with (1) being the familiar gas-phase reaction, and (2) the electrochemical equivalent. In both cases, the product is CO, which has the same effect as fuel stream CO. It has been calculated that under equilibrium conditions in a typical gas mixture of 75% H2/25% CO2, 100 to 200 ppm CO should be present, although this is highly dependent on the concentration of H2O present (Bellows et al., 1996).

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