Poisoning phenomena

In add media, platinum appears to be the most efficient catal rst for the electrochemical oxidation of methanol. Indeed, other single component material showed very little catalytic activity. Most work, therefore, has been done to increase the catalytic activity and decrease the poisoning phenomena by modifying the surface nature of platinum. 

Clean Air Act redirected the interest toward the inherently more active noble metal catalysts and stimulated an interest in detailed examination of the poisoning phenomena. 

Because of the strength of the sulfur-metal bond, sulfur adsorption is frequently very nonuniform with respect to catalyst bed and particle, making fundamental interpretation of poisoning phenomena very difficult or impossible. 

Will the electrocatalytic activity be maintained in spite of (a) catalyst poisoning phenomena, particularly on the anode side, and (b) catalyst agglomeration phenomena, particularly on the cathode ... 

In the presence of poisons in the gas feed, the catalytic activity. A, decreases first as time t increases then tends toward a limit which is in some cases, equal to zero. The initial slope of the A(t curve can be used to define the resistance of the caitalyst to the poison. It is related to the initial toxicity of poison which is defined as the number of active sites which are poisoned by one moiecuie of poison. The limit observed after achieving a steady activity is called the tolerance and is often expressed in percentage of the initial activity. For a better understanding of poisoning phenomena, it can be useful to attempt to correlate these macroscopic parameters to microscopic data. 

Thus, in the diagnosis and analysts of poisoning deactivation, site heterogeneity, stoichiometry, and diffusion play important roles. Each situation must 1 considered on its own merits, with all these factors taken into consideration. Techniques for mathematical modeling of poisoning phenomena have advanced to the point where they are useful tools in this endeavor. ... 

Because of the poisoning phenomena occurring in long-term electrolysis and in order to maintain the electrode activity at a sufficient level, the oxidation reaction of carbohydrates was carried out using an optimized triple-pulse potential program (Figure 21.17). It consists in controlling the... 

Sudara, S., Tamiyavanich, S., and Wisessang, S., Red tide and paralytic shellfish poisoning phenomena in Thailand, in Toxic Red Tides and Shellfish Toxicity in Southeast Asia, White, A.W., Anraku, M., and Hooi, K., Eds., Southeast Asian Pisheries Development Center and the Ineternational Development Research Center, Singapore, 1984, 90. 

However, after 1980 the development of in situ infrared spectroscopic techniques have allowed the direct identification of adsorbed intermediates. In terms of methanol oxidation on R, electrochemically modulated infrared reflectance spectroscopy (EMIRS) led to the unambiguous identification of adsorbed CO as the poisoning species. Two adsorbed CO species were identified as being responsible for the poisoning phenomena (i) a linearly bonded species (IR absorption band around 2060 cm ) and (ii) a bridge-bonded species (a small band around 1850-1900 cm ). These results were... 

It has been observed that the rate of electrocatalytic oxidation of formic acid at R electrodes can be enhanced significantly by the incorporation of adatoms, such as Pb, Bi, Tl, and Cd. The most significant enhancement has been seen with Pb adatoms, which were shown to be accompanied by a dramatic decrease in the poisoning phenomena on R. ° A similar type of behavior has been observed with systems based on Pt/Cdg, Pt/TI and Pt/Biads, although to a lesser extent. One of the most plausible explanations given rationalizing the improved characteristics of electro-oxidation in the presence of adatoms was proposed by Shibata and Motoo ° who invoke a... 

Even though high-temperature fuel cells have good fuel flexibility with various kinds of fuels, some minor constituents (impurities) coming from, for example, low-purity fuels, raw materials, and system components can also react with electrode materials or can be adsorbed on the electrode reaction sites, hindering electrode reactions. Such poisoning phenomena can lead to fuel cell performance degradation with time. 

Carbon monoxide is the most understood poisoning phenomena due to extensive studies. The goal of this chapter (7) is to provide a full understanding of the CO poisoning phenomenon, sufficient to enable the reader to numerically simulate CO poisoning as well as its mitigation methods. Empirical models are usually developed and an example of fhis is described. Mathematical models, which include the fundamental physical and chemical properties of the system under study, are more useful for parametric studies, such as, for example, the effects of fhe catalyst layer structure. Several approaches to the mathematical models are briefly described. 

The poisoning of the anode of PEM fuel cells due to CO presence in the fuel stream can be said to be one of the most understood poisoning phenomena due to the abundant experimental, analytical, and mathematical studies in literature. The models found in literature are classified as either empirical or mathematical. Using curve-fitting schemes of experimental data, empirical models are usually developed. In PEM fuel cells, empirical models are used to understand the performance of the cell and are normally very specific. An empirical equation that correlates the cell voltage to the current density... 

The objective of the next section is to give the reader the mathematical formulation for estimating the concentration of gases at the flow channel/gas diffusion layer boimdary. The use of this estimation for understanding the poisoning phenomena is justified since no chemical reactions occur in the flow channels. The effects of the gas diffusion layer and the catalyst layer on the mass transport are of greater interest and impact. 

The aim of this chapter was to outline a comprehensive method to numerically simulate the many poisoning phenomena that occur in PEM fuel cells and their mitigation methods. A full mathematical formulation for CO, CO2, and H2S poisoning of the anode of PEM fuel cells was put forward. Further, the mathematical formulation for the mitigahon of CO effects using oxygen bleeding and CO-tolerant catalysts was also discussed. 

Many of the numerical models available, which look into the many poisoning phenomena in PEM fuel cells, are one-dimensional. Neglecting the dependency of the performance on dimensionalify limits the knowledge that could be acquired about these phenomena. In reality, the distribution of the current... 

S.3.2.4 Poisons and the poisoning phenomena for ammonia synthesis catalysts... 

The effects of Xenon poisoning phenomena exacerbated any A Reactor shutdown, and nearly guaranteed that every time the reactor was shut down the operators would lose at least one day s worth of Pu production. The Soviets expected to be able to produce 60-80 g of plutonium per day from the A Reactor. A critical mass of about 5 kg (Rhodes, 1986) was needed for a nuclear weapon, so about 3 months of Continuous full power operation should have been sufficient to generate the required plutonium. 

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