Poisoning of platinum catalysts

Figure 61. Rudolf Bottger (1806-1887). (Courtesy E. Berl.) Professor at Frankfort-on-the-Main. Discovered guncotton independently of Schon-bein but somewhat later, in the same year, 1846. He also invented matches, and made important studies on the poisoning of platinum catalysts. Reproduced from original in Kekule s portrait album.

Lead is generally known as a poison of platinum catalysts (1,2), whose toxicity, for different catalytic reactions, depends on the way it is deposited. 

Somorjai, G. A. On the mechanism of sulfur poisoning of platinum catalysts. Journal of Catalysis 11, 453-456 (1972). 

G.A. Somorjai - On the Mechanism of Sulfur Poisoning of Platinum Catalysts,... 

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]. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

The poisoning of a catalyst may be shown by adding some hydrogen sulfide solution to the hydrogen peroxide before the colloidal platinum is introduced. No decomposition of the peroxide is observed in this case, since the platinum has been poisoned by the presence of the hydrogen sulfide. This method is applicable to most of the metals below hydrogen in the electrochemical series. 

The poisoning of metallic catalysts by sulfur has been extensively studied, essentially on nickel, palladium, and platinum, for numerous reactions and consequently under very different experimental conditions, particularly for temperatures ranging from 300 to 1300 K. Experiments carried out at high temperatures (1-4) have shown that poisoning can be balanced by the following reaction ... 

Benzene and aikyibenzenes are quantitatively converted to cyclohexanes by catalytic hydrogenation. Modem procedures employ liquid-phase hydrogenation over nickel catalysts at 100-200° or over platinum catalysts at room temperature. Nickel catalysts are poisoned by traces of thiophene and water. Small quantities of hydrogen halide increase the effectiveness of platinum catalysts. Isomerization occurs during the reduction of benzene over nickel at 170° the cyclohexane formed is probably contaminated with methylcyclopentane, Partial reduction of benzene to 1,4-dihydrobenzene is accomplished by sodium in liquid ammonia at —45°. ... 

Poisoning of metal catalysts may provide a tool for improving selec> tivity by affecting the concentrations of ensembles required by different reaction paths. This is illustrated by steam reforming on sulfur passivated nickel catalysts and the results are compared with observations for sulfided platinum-rhenium catalysts for catalytic reforming and for a chlorine poisoned palladium catalyst for partial oxidation of methane. 

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