Reformate Tolerant Catalyst Development

In recent years, the search for more CO- (and reformate-) tolerant catalysts has resumed. Given that CO formation and removal are central to the development of efficient methanol oxidation catalysts, much overlap between DMFC and reformate-tolerant catalyst research has occurred. It is no coincidence that the current anode catalyst of choice for both fuel cell systems is PtRu.

A large number of additives and promoters to Pt have been investigated for improved CO tolerance properties (Wilkinson and Thompsett, 1997). The most promising group of catalyst materials harks back to the early GE work, where recently carbon-supported PtMo, PtW, PtCoMo, and PtCoW catalysts have all been shown to give enhanced CO tolerance when compared to PtRu formulations (Cooper et al., 1997; Mukerjee et al., 1999; Gunner et al., 1999). One recent report showed that a carbon-supported PtMo catalyst with a Pt:Mo ratio of 4:1 gave only a ca. 50 mV loss in performance on 100 ppm CO in H2, compared to pure H2. In contrast, a carbon-supported PtRu catalyst (Pt:Ru = 1:1) showed a ca. 160-mV loss (Grgur et al., 1999).

These additives have been found to operate in two ways. First, the modifying component promotes the electro-oxidation of adsorbed CO on neighboring sites at low potential by co-adsorbing H2O to act as the oxidizing agent. The oxidation of CO then exposes additional sites to support H2 oxidation. Second, the modifying component alters the catalyst's H2 and CO adsorption properties, which results in the reduction in CO coverage with respect to H2 oxidation sites.

Indeed, it not completely clear how PtRu achieves CO tolerance. Studies on bulk PtRu alloys have shown that at higher potentials (ca. 0.4 at 62°C), CO tolerance is achieved by the electro-oxidation of CO from the catalyst surface (Iwase and Kawatsu, 1995). However, at lower potentials (<0.2 V at ca. 80°C), the mechanism is not as equivocal. It is clear that PtRu catalysts (and bulk surfaces) achieve greater CO tolerances than Pt at potentials below that at which bulk CO electro-oxidation occurs (0.25 V at 80°C). It has been calculated that to remove sufficient CO to give effective CO tolerance, only CO oxidation currents of nanoamps are required (Springer et al., 1997). However, it is also reasonable to argue that the second mechanism can also apply. EXAFS3 and modeling studies have shown that the introduction of Ru into Pt particles does modify the electronic properties of Pt by removing electron density from Pt (Russell et al., 2001; Mitchell et al., 1997). This would have the effect of weakening the Pt-CO bond and hence would modify the equilibrium coverages of CO and H2, favoring an increased population of H2 electro-oxidation sites.

3EXAFS: Extended x-ray absorption fine structure.

100 time/min

FIGURE 6.6 Comparison of cell performances of PtRu and PtMo anode catalysts on 40 ppm CO/25% CO2 in H2, 0.5 Acm-2, 80°C.

100 time/min

FIGURE 6.6 Comparison of cell performances of PtRu and PtMo anode catalysts on 40 ppm CO/25% CO2 in H2, 0.5 Acm-2, 80°C.

However, more recent work has shown that although PtMo shows good CO tolerances, it suffers from significant CO2 intolerance (Ball et al., 2002) (see Fig. 6.5). When compared to PtRu on full reformate mixes, PtMo catalysts are generally inferior to PtRu in anode performance (see Fig. 6.6).

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