Poisoning of the platinum catalyst

PAFCs are very efficient fuel cells, generating electricity at more than 50 % efficiency [13], About 85 % of the steam produced by the PAFC is used for cogeneration. This efficiency may be compared to about 35 % for the utility power grid in the United States. As with the PEMFC Pt or Pt alloys are used as catalysts at both electrodes [76]. The electrolyte is inorganic acid, concentrated phosphoric acid (100 %) which will conduct protons [77-79]. Operating temperatures are in the range of 150-220 °C. At lower temperatures, PAFC is a poor ionic conductor, and carbon monoxide (CO) poisoning of the platinum catalyst in the anode can become severe [76, 80,81]. 

Hydrosilylation. Historically, the synthesis of the silyl chloride intermediate from DAM and TCS requires between 100-200 ppm of platinum. Often, the hydrosilylation would terminate prior to completion, with yields as low as 40%. To determine whether DAM was responsible for poor reaction results, the diallyl fumarate (DAF) isomer was evaluated as an alternate substrate DAF is a weaker inhibitor of platinum The comparison revealed that when the DAF and DAM were distilled prior to use, there were no significant differences in reaction kinetics. It was determined that a sulfur contaminant found in commercial sources of DAM - assumed to be a residual of acid catalyst from the maleate esterification - results in poisoning of the platinum catalyst. Thus with distillation, DAM can now be hydrosilylated with TCS in the presence of only 10 ppm platinum catalyst. This observation was consistently reproduced when utilizing an active platinum catalyst complex. Consequently, the hydrosilylation offers > 99% yield with respect to depletion of either DAF or DAM, and severe coloration is completely eliminated from the final product by reduction of the amount of platinum catalyst required. 

R SiH and CH2= CHR interact with both PtL and PtL 1. Complexing or chelating ligands such as phosphines and sulfur complexes are exceUent inhibitors, but often form such stable complexes that they act as poisons and prevent cute even at elevated temperatures. Unsaturated organic compounds are preferred, such as acetylenic alcohols, acetylene dicarboxylates, maleates, fumarates, eneynes, and azo compounds (178—189). An alternative concept has been the encapsulation of the platinum catalysts with either cyclodextrin or in thermoplastics or siUcones (190—192). 

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

An example of an inhibitor is the well-known poisoning of the contact catalyst platinum by As and the inhibition of the explosion of T A(g) and O2 by argon or nitrogen. ... 

Catalytic reduction in the presence of platinum or palladium is a very mild method for splitting trityl ethers. The products are an alcohol and tritane. The latter may be separated from the alcohol by taking advantage of its solubility in petroleum ether. If the trityl ether contains sulfur, poisoning of the platinum or palladium catalyst may occur. ... 

The geometric and promoter effects on the selectivity to furfuryl alcohol were studied in the hydrogenation of furfural over various series of platinum catalysts. The results obtained with the PtCu catalysts indicate that the size of the active site does not affect the selectivity. Promotion of the platinum catalyst can lead to a considerable rise in the selectivity to furfuryl alcohol. The hydrogenation over the series of PtSn catalysts showed the influence of the reaction conditions. The experiments under isothermic conditions resulted in up to 80% selectivity for furfuryl alcohol, while the selectivity dropped to approximately 40% if non-isothermic conditions were applied. This change in selectivity is attributed to the self-poisoning of the catalyst at high temperatures. 

The selectivity to furfuryl alcohol is not influenced by the size of the active site. Promotion of the platinum catalyst by compoimds containing non-transition elements enhances the selective activation of the carbonyl group considerably. In the hydrogenation over the PtSn catalysts it is shown that the selectivity to furfuryl alcohol is dependent on the reaction temperature. At higher temperatures the catalyst is probably self-poisoned by carbonaceous deposits or polymers made from furfural or its fragments and the selectivity decreases. 

Fig. 3. Detoxication of cysteine by various metallic per acids. Relationship is shown between the amount of detoxicant added and the percentage suppression of the toxicity of the poison. The curve obtained with the reagent containing a perti-tanate is abnormal in form, and the subsequent fall in the activity of the platinum catalyst may be due to cloaking by an insoluble deposit.

The mildly endothermic steam reforming of methanol is one of the reasons why methanol is finding favour with vehicle manufacturers as a possible fuel for FCVs. Little heat needs to be supplied to sustain the reaction, which will readily occur at modest temperatures (e.g. 250°C) over catalysts of mild activity such as copper supported on zinc oxide. Notice also that carbon monoxide does not feature as a principal product of methanol reforming. This makes the methanol reformate particularly suited to PEM fuel cells, where carbon monoxide, even at the ppm level, can cause substantial losses in performance because of poisoning of the platinum anode electrochemical catalyst. However, it is important to note that although carbon monoxide does not feature in reaction 8.7, this does not mean that it is not produced at all. The water-gas shift reaction of reaction 8.5 is reversible, and carbon monoxide in small quantities is produced. The result is that the carbon monoxide removal methods described in Section 8.4.9 are still needed with PEM fuel cells, though the CO levels are low enough for PAFC. 

CO is one of the major poisons in low-temperature fuel cells [89]. In PEMFC and PAFC, CO poisoning occurs due to adsorption of the species to the active sites of the platinum catalysts so that no, or virtually no, sites are available for reaction with H2. To reactivate the surface, the CO can be oxidized to CO2. 

Catalytic reduction over a platinum catalyst fails because of poisoning of the catalyst (101). 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

The production of sulphuric acid by the contact process, introduced in about 1875, was the first process of industrial significance to utilize heterogeneous catalysts. In this process, SO2 was oxidized on a platinum catalyst to S03, which was subsequently absorbed in aqueous sulphuric acid. Later, the platinum catalyst was superseded by a catalyst containing vanadium oxide and alkali-metal sulphates on a silica carrier, which was cheaper and less prone to poisoning. Further development of the vanadium catalysts over the last decades has led to highly optimized modem sulphuric acid catalysts, which are all based on the vanadium-alkali sulphate system. 

Hydroprocessing and special absorption techniques are utilized to remove sulfur and nitrogen from the reformer. If not removed through hydroprocessing, feedstock sulfur will be converted to H2S in the reformer. The H2S will then serve as a poison to the platinum reformer catalyst and diminish the dehydrogenation and dehydrocyclization reactions. When present, H2S can neutralize the acid sites on the catalyst diminishing the ability of the catalyst to promote isomerization, dehydrocyclization, and hydrocracking reactions. 

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