Oxygen poisoning, platinum catalysts

Figure 12. Transient HCN yield over platinum catalysts. A and B represent one experiment (Exp. 1) in which a sulfur-poisoned catalyst was regenerated on admission of 1% O2 into the inlet gas mixture between times t, and C shows the resistance of the catalyst to poisoning by SO when oxygen is simultaneously present in the inlet gas mixture. At t only oxygen is removed from the inlet gas mixture. (See Ref. 16 for details.)...

Some key adsorbates and reaction intermediates relevant to fuel-cell anodes are H2 as the fuel, CO and CO2 as poisons in hydrogen reformate feeds, and water as a co-adsorbate and potential oxidant. In the case of the cathode, oxygen is clearly the most important reactant. In the case of a number of these molecules, such as H2, O2, and H2O, not only is the molecular adsorption important on platinum (or promoted platinum catalysts), but the dissociative adsorption of the molecules is important as well. With this in mind, some details concerning the dynamics of adsorption of these molecules, the associated dissociation barriers, molecular degrees of freedom, and energy partition are important to the overall catalytic processes. In addition to the... 

The high theoretical efficiency of a fuel cell is substantially reduced by the finite rate of dynamic processes at various locations in the cell. Substantial efficiency losses at typical operating temperatures occur already in the anodic and cathodic catalyst layers due to the low intrinsic reaction rates of the oxygen reduction and, in the case of the DMFC, of the methanol oxidation reaction. (The catalytic oxidation of hydrogen with platinum catalysts is very fast and thus does not limit PEFC performance.) In addition, at low temperatures, turnover may be limited by noble metal catalyst poisoning due to sulfur... 

Supported PtRu alloys are so far considered the best anodic materials for DMFC [109, 110], It is well known that the oxidation of methanol on platinum catalysts generates CO as an intermediate, which is a poison that adsorbs on the active sites of the catalyst. Ru forms oxygenated species at lower potentials than Pt and its presence in the catalyst promotes the oxidation of CO to CO2, through the so-called bifunctional mechanism [111, 112],... 

Two methods have been studied that address the problem of catalyst poisoning by CO Mixing of a small amount of oxygen in the fuel [14, 15], and developing a catalyst that is resistant to catalyst poisoning [16, 17]. It has been demonstrated that it is possible to reduce the use of platinum catalyst to less than one-tenth of the traditional amount used by studying the reacting interface [18]. 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

The Pt-Rn catalysts have another important property. In contrast to pure platinum, they are almost insensitive to poisoning by carbon monoxide CO. They can be used, therefore, in the hydrogen electrodes of hydrogen-oxygen fuel cells operated with technical hydrogen containing marked amonnts of CO. 

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