Tolerance Toward CO

Platinum alloys offer better tolerance towards CO. Especially, PtRu and PtMo alloys show superior tolerance towards CO [62], Nonetheless, the performance of presently known catalysts is far from satisfactory. 

Catalyst development has lead to formulations more effective than PtRu, especially at higher CO concentrations. As shown in Fig. 14.17, which gives the drop in performance for different anode formulations when increasing amounts of CO are added to hydrogen, both PtPd [64] as well as PtRuMo [65] lead to a strong improvement in tolerance towards CO. 

ATR) process. The type and number of affordable downstream units also determine the reformer characteristics. For example, if the fuel is methanol, then a high-temperature water-gas shift reactor is not required, given that CO levels in the reformate do not significantly exceed 1%. High-temperature FCs (new generation of HT PEMFC, MOFC or MCFC) are more tolerant towards CO and the number of clean-up units in the reformer can be reduced. 

Better stabilities were achieved investigating heat-treated catalysts (section Molecular Centers in Carbonized Materials ). Nevertheless, in most of the cases, even after the heat treatment, considerable decreases of performance were already observed after a few hours of operation time [69, 159, 168, 184, 188, 213-216]. For catalysts prepared by a heat treatment of either iron porphyrin or iron phthalocyanine, it was shown that they are tolerant toward CO [217, 218]. Therefore, we assume that in general, CO-poisraiing can be mled out as degradation mechanism for carbonized materials. Possible degradation mechanisms could be addressed to (1) a corrosion of carbon as known from platinum catalysts, (2) an inactivation by leaching of active sites, or (3) a deactivation of active species (e.g., by blocking of the centers by intermediates or the final products) [71, 98, 99, 102, 190, 191, 210, 211, 219, 220],... 

Platinum supported on high surface area WO3 seems to have received considerable attention in recent years, mainly because of its improved tolerance toward CO poisoning during MOR and its enhanced electrocatalytic activity. However, WO3 is chemically unstable in add medium. The latter is shown to improve by H substitution in the WC framework [57]. 

Chiappe and co-workers reported chloroperoxide (CPO)-catalyzed oxidation in hydrophilic ILs as co-solvents (Fig. 21). The authors investigated the hydrophilic ILs on the activity of CPO and found that CPO showed a higher tolerance toward IL than organic solvent good activity was obtained when the reaction was carried out in a mixed solvent of [mmim][Me2P04] and buffer (pH 5.0) (1 1) rather than buffer solution. 

The strategy towards CO tolerance has therefore been changed, towards the development of proton conducting polymers suitable for high-temperature operation of the PEMFC, i.e., 120 °C and higher. It is already demonstrated [68] that at this temperature, 1000 ppm CO leads to only minor loss of performance. The high temperature operation will be further addressed in the final section of this chapter. 

Grubbs and co-workers have developed a new class of neutral Ni complexes incorporating bulky phenoxy-imine ligands F13-16 that show considerable tolerance toward functional groups and even remain active in the presence of water. These complexes can co-polymerize ethylene with functionalized NBs such as 5-norbornen-2-ol to provide cyclic olefin co-polymers with hydroxy or ester functionality. It has been reported that these Ni complexes are ineffective for the polymerization of MA due to /3-H transfer from the enolate-Ni complex to the Ni metal. 

In the beginning, the major focus was on the early transition metals such as Ti, Zr and Hf, but today the potential of the late transition metals complexes of Ni, Pd, Co and Fe is well recognised. As the late transition metals are characteristically less oxophilic than the early metals, they are more tolerant towards polar groups. Therefore, it was assumed that with late transition metals catalysts one could produce a wide range of different polymers. 

It is known that addition of mthenium or tin to platinum leads to greatly increase its tolerance towards the presence of CO. Wa-... 

Table 1 Rank of the Evaluation of Alloying with Pt Toward CO-Tolerant H2 Oxidation... 

Similarly to the early-metal-based systems, the late-metal-complexes can yield high molecular-weight polymers. Moreover, they can lead to poly-olefins with different microstructures, depending on the catalysts and the reaction conditions (temperature, olefin pressure) [16, 21]. Unlike the heterogeneous catalysts and the early-metal-complexes, the late-metal-based systems exhibit substantial tolerance toward polar groups, and as such they may be useful for the co-polymerization of a-olefins with polar monomers [lb, 16,17,22]. 

DMFCs are PEMFCs fed with methanol as fuel. The technologies required by DMFCs are similar to those of PEMFCs. What differ between DMFCs and PEMFCs are in the following two aspects The proton exchange membrane used for DMFCs must possess low methanol permeability or crossover, and the anode catalyst must possess high activity toward the oxidation of methanol and high tolerance to CO and other intermediates from methanol oxidation. In this section, the applications of NMR techniques for the development of DMCFs as well as essential materials are going to be briefly reviewed. 

Subsequently, the first generation fuel processor prototype described above was linked to a meso-scale high temperature PEM fuel cell developed at Case Western University by Holladay et al. [600], which was astonishingly tolerant towards carbon monoxide up to 10 vol.%. Thus, no CO clean-up was necessary to run the fuel processor together with the fuel cell. A power output of 23 mW was demonstrated by Holladay et al. [599]. This value was lower than the calculated value of 100 mW, which was due to a lower hydrogen supply of the reformate (more than 99% conversion of 0.03 mL h methanol could be achieved at a 420 °C reformer temperature), lower cell voltage of the fuel cell by the 2 vol.% carbon monoxide present in the reformate and a dilution effect by carbon dioxide at the gas diffusion layer material of the fuel cell. 

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