Poisoning of the catalyst surface

The temperature dependence of the conversion and product formation in the ammonia oxidation over a platinum sponge catalyst is investigated. Positron emission profiling experiments demonstrate that below 413K, the catalyst deactivates due to the poisoning of the catalyst surface, mainly by... 

The rate of carbon formation is far less on noble metals than on nickel and this behaviour appears to be related to the difficulty found by noble metals of dissolving carbon in bulk. ° The carbon formed on the surface of noble metals was found to be almost indistinguishable from the catalyst particles. High-resolution TEM images taken from a ruthenium catalyst employed in the SMR reaction revealed a structure in which a few carbon layers were deposited on the surface of the Ru particles. The mechanism by which whiskers grow on the surface of the nickel particles becomes blocked by sulphur poisoning of the catalyst surface. In this specific case, several octopus carbon filaments or whiskers are formed on a given nickel particle. A similar carbon structure has been reported to develop on Ni-Cu alloy catalysts with low nickel contents (20 wt%).ii... 

For instance, DMFC uses frequently PtRu alloys as the anode catalyst to prevent fast poisoning of the catalyst surface by CO molecules produced during methanol oxidation. 

Poisoning of the catalyst surface by irreversible adsorption and/or reaction of a chemical species, thus makingthe active centers required for the catalyzed reaction inactive. Example is CO adsorption on iron catalysts used for the ammonia... 

The adsorption rate, coverage, and stability of H2S species are strongly affected by temperature [18]. It is known that the adsorbed H2S and SH are highly unstable on Pt, while the adsorbed S and H are the most stable intermediates on Pt [19]. The formation of the sulfide film on the platinum surface not only causes poisoning of the catalyst surface but also makes it impossible for the fuel cell to recover from contamination [17, 20, 21]. 

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

In many of the other processes that use base metal catalysts, irreversible poisoning of the catalyst occurs as a result of deposition of metal contaminants from the process feedstock onto the catalyst surface. These catalysts are not considered to be regenerable by ordinary techniques. 

This equation indicates that a small amount of poisoned surface can lead to a sharp decline in apparent activity. For example, if only 10% of the catalyst surface has been deactivated in the case where the Thiele modulus for the unpoisoned reaction is 40, 3F = 0.200 so that the... 

Poisoning of the catalyst by presence of a catalytic poison may be either due to chemical reaction between catalyst and poison (e.g. Fe + H2S Fe + H2) or poison may render surface of the catalyst unavailable for adsorption of reactants. 

Finally, consider side-by-side deactivation. Whatever the concentration of reactants and products may be, the rate at which the poison from the feed reacts with the surface determines where it deposits. For a small poison rate constant the poison penetrates the pellet uniformly and deactivates all elements of the catalyst surface in the same way. For a large rate constant poisoning occurs at the pellet exterior, as soon as the poison reaches the surface. 

A definite theoretical explanation of this behavior is not available. It is important to realize that the preference of a metal for 3C as opposed to 2C complexes or for 5C as opposed to 3C complexes may be either intrinsic or induced by adsorption of less reactive carbonaceous fragments and carbon (for simplicity, we shall refer to both of these as carbon ) on the metal (alloy) surface. Also, the choice of the reaction conditions (apparent contact time, poisoning or self-poisoning of the catalyst, etc.) influences the temperature range in which the catalysts can be tested, and since the selectivity in various complex formations is also temperature dependent, one must always analyze which aspects of the product distributions are intrinsic properties of a metal and which are induced by often unavoidable side reactions. 

A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. 

Selectivity of a catalyst process is improved by also doping of the catalyst in other cases usually it has been ascribed to partial poisoning (i.e., to the poisoning of the catalyst by the dope with respect to undesirable reactions without poisoning it, or poisoning to a smaller extent, with respect to the reaction of formation of the desirable product, implying that different reactions are catalyzed by different surface sites). Our studies showed that such an explanation cannot be applied to the oxidation of ethylene. 

In addition, organometallic compounds produce metallic products that influence the reactivity of the catalyst by deposition on its surface. The transfer of metal from the feedstock to the catalyst constitutes an irreversible poisoning of the catalyst. After combustion to remove the carbonaceous deposits, the catalysts are treated to redisperse active metals. 

For example, the cracking and desulfurization activity of constituents containing the polynuclear aromatic nucleus may be sluggish and are more refractory to desulfurization (Gates et al., 1979 Nash, 1989 Ma et al., 1994). For example, alkyldibenzothiophenes (in particular, 4,6-dimethyldibenzothiophene) are very resistant to desulfurization. This unreactivity, or refractory behavior, may inhibit the reactivity of paraffinic and naphthenic constituents. In addition, organometal-lic compounds produce metallic products that influence the reactivity of the catalyst by deposition on its surface. The transfer of metal from the feedstock to the catalyst constitutes an irreversible poisoning of the catalyst. 

Another important issue is the permanence of the catalyst inhibition or poisoning. The permanence of catalyst inhibition is dependent on the mechanism of the chemical interaction of the poison with the catalyst. Catalyst inhibition and the resulting reduction in reaction rate could result from competition between the poison and the preferred reactant at the catalytic site, either because of a high affinity of the poison for the catalyst site or because of its slow reaction once on the catalyst site. If the affinity is too high, as when the poison actually reacts with the catalyst to form a new compound, the catalyst is permanently poisoned. If the inhibition is only related to a slow rate of reaction, it may be possible to remove the poison from the catalyst surface and restore catalyst activity. 

The Fe203 superacid was found to be quite effective for oxidation of hydrocarbons to CO and C02 when the reaction was performed at temperatures above 100°C. The catalyst gave a 29% conversion for the reaction of butane at 300°C to form CO and C02 in the ratio 4 6 under the conditions in which none of the reactions occurred at 300°C over Fe203, without the sulfate treatment (177). The decrease in oxygen of the catalyst surface was observed together with the complete recovery of activity by supply of 02. The catalyst was entirely poisoned by the addition of pyridine, the oxidation being related to the surface acidity. The activity enhancement of oxidation by the sulfate addition was also observed with the Sn02 superacid (135, 145). Iron and tin oxides are known to be oxidation catalysts thus those superacids would be the oxidation catalysts with superacidity. 

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