what is poison reaction mechanism
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Poisoning refers specifically to chemical deactivation, rather than other mechanism of catalyst degradation such as thermal decomposition or physical damage. ...Poisoned sites can no longer accelerate thereaction with which the catalyst was supposed to catalyze.
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Poisoning, the strong chemisorption of reactants, products or impurities on active catalytic sites, is an important, difficult deactivation problem in many catalytic processes and particularly in processes involving nickel catalysts. For example, only a few ppm of sulfur impurities can reduce the activity of nickel methanation catalysts by several orders of magnitude within just a few days or weeks; moreover the sulfur thus adsorbed is essentially irreversibly held and cannot be conveniently removed by treatments with air, oxygen, steam, or hydrogen.
Catalyst poisons for nickel can be classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites. For example, sulfur in the form of H2S and organic sulfides is a highly selective, strongly adsorbing poison for nickel hydrogenation, methanation, and steam reforming catalysts. That surface sulfur-nickel bonds are substantially more stable than the corresponding bulk nickel-sulfur bonds is emphasized by the fact that hydrogen sulfide adsorbs dissociatively with a heat of 250 kJ/mole compared to a heat of bulk formation of 75 kJ/mole. Adsorbed sulfur blocks the adsorption of most other species and generally causes a substantial or complete loss of nickel catalyst activity. Whether this loss of activity is a geometric blocking or a longer range electronic effect is somewhat controversial and probably depends on coverage. The evidence is in favor of a combination of geometric and electronic effects at low sulfur coverages and geometric blocking effects at high coverages.
The toxicity of poisons for nickel depends on the stability of the nickel-additive bond as well as the electronegativity of the adsorbing species. Species with a high degree of shielding such as SO3 are less toxic than poorly shielded species such as elemental sulfur and H2S. The toxicity of some poisons, e. g. carbonaceous species, depends not only on their strength of adsorption but also on their reactivity with gasifying agents such as hydrogen or water vapor.
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Poisoning, the strong chemisorption of reactants, products or impurities on active catalytic sites, is an important, difficult deactivation problem in many catalytic processes and particularly in processes involving nickel catalysts. For example, only a few ppm of sulfur impurities can reduce the activity of nickel methanation catalysts by several orders of magnitude within just a few days or weeks; moreover the sulfur thus adsorbed is essentially irreversibly held and cannot be conveniently removed by treatments with air, oxygen, steam, or hydrogen.
Catalyst poisons for nickel can be classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites. For example, sulfur in the form of H2S and organic sulfides is a highly selective, strongly adsorbing poison for nickel hydrogenation, methanation, and steam reforming catalysts. That surface sulfur-nickel bonds are substantially more stable than the corresponding bulk nickel-sulfur bonds is emphasized by the fact that hydrogen sulfide adsorbs dissociatively with a heat of 250 kJ/mole compared to a heat of bulk formation of 75 kJ/mole. Adsorbed sulfur blocks the adsorption of most other species and generally causes a substantial or complete loss of nickel catalyst activity. Whether this loss of activity is a geometric blocking or a longer range electronic effect is somewhat controversial and probably depends on coverage. The evidence is in favor of a combination of geometric and electronic effects at low sulfur coverages and geometric blocking effects at high coverages.
The toxicity of poisons for nickel depends on the stability of the nickel-additive bond as well as the electronegativity of the adsorbing species. Species with a high degree of shielding such as SO3 are less toxic than poorly shielded species such as elemental sulfur and H2S. The toxicity of some poisons, e. g. carbonaceous species, depends not only on their strength of adsorption but also on their reactivity with gasifying agents such as hydrogen or water vapor.
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