Phosphorus metal catalyst poisoning

Other substances decrease or annihilate, even in traces, the catalytic properties of iron. Such catalyst poisons had already been known as a nuisance in the catalytic oxidation of sulfur dioxide. With the ammonia catalysis several elements, particularly sulfur proved to be harmful, even in amounts of 4oo of one per cent. Chlorine, phosphorus and arsenic showed a similar behavior (30), just as certain metals, such as lead, tin and zinc. 

B. Angele, and K. Kirchner (1980) The poisoning of noble metal catalysts by phosphorus compounds. I. Chemical processes, mechanisms, and changes in the catalyst, Chem. Eng. Sci 35 2089-2091... 

The main causes of the deactivation of diesel catalysts are poisoning by lubrication oil additives (phosphorus), and by SOx, and the hydrothermal instability. The SCR by HC is less sensitive to SOx than the NO decomposition. The Cu-based catalysts are slightly inhibited by water vapor and SOx, and suffer deactivation at elevated temperature. Noble metal catalysts such as Pt-MFI undergo low deactivation under practical conditions, are active at temperatures below 573 K but the major and undesired reduction product is N20 (56). 

Electronic promoters, for example, the alkali oxides, enhance the specific activity ofiron-alnmina catalysts. However, they rednce the inner snrface or lower the thermal stability and the resistance to oxygen-containing catalyst poisons. Promoter oxides that are rednced to the metal during the activation process, and form an alloy with the iron, are a special group in which cobalt is an example that is in industrial use. Oxygen-containing compounds such as H2O, CO, CO2, and O2 only temporarily poison the iron catalysts in low concentrations. Sulfur, phosphorus, arsenic, and chlorine compounds poison the catalyst permanently. 

The papers included in this symposium cover the full gamut of problems that had to be addressed. The physical and chemical stability of the catalysts had to be significantly improved over known catalysts in order to meet the 50,000 mile life requirement prescribed by the regulations. The effects of catalyst poisons such as lead, sulfur, phosphorus, etc. were also critical in relation to the limits of deposition that could be tolerated while maintaining catalyst effectiveness. The nature of the catalyst support or substrate became significant in relation to its interaction with the metallic components of the catalyst—adherence, distribution, and reactivity at high temperature. 

Introduction. - The effect of phosphorus compounds on poisoning of noble metal catalysts has been widely studied. This section deals with deactivation of... 

Catalytic deactivation may occur for a number of reasons, both chemical and physical in nature. Several authors have reported that chemical poisoning of the noble metal catalysts is the primary mechanism for phosphorus compounds. Nevertheless, inhibition also takes place. The difference between phosphorus inhibitors and poisons is that inhibitors absorb weakly on the surface and the process is often reversible. On the other hand poisoning is the irreversible loss of activity due to the strong chemisorption of the impurities in the feed on the catalytic active sites. 

The Influence of Phosphorus Poisoning. - Catalytic oxidation using noble metal catalysts has been used to reduce the concentration of unburned hydrocarbons, carbon monoxide pollutants released from internal combustion engines, and similar applications. It is well known that contaminants arising from lubricants, (P, Ca, and Zn) deactivate these catalysts. Phosphorus compounds in printing processes are the source of decay of noble metal catalysts used to control these emissions. 

In a recent modeling study, Angele and Krishner have developed a model for poisoning of supported noble metal catalysts by phosphorus compounds in an isothermal fixed bed reactor. They have solved the model for a single pellet. Their model is based on the following assumptions ... 

Carbon-phosphorus bonds may also be formed. Chemists at Merck developed a synthesis of either enantiomer of the valuable ligand BINAP 133 from the more easily resolved BINOL 2.612, using a triflate-phosphine coupling reaction (Scheme 2.183). They reasoned that nickel catalysis would be more effective as this metal is harder than palladium and, therefore, less susceptible to catalyst poisoning by the product. BINAP 133 could be obtained with no loss of chirality. They also reported a resolution procedure for BINOL 2.612.224... 

Poisoning of metal catalysts The soft metals like palladium, platinum act as catalyst in different chemical reactions. These catalysts are easily poisoned by carbon mono-oxide, phosphorus or arsenic (all soft bases). These bases are adsorbed on the surface of these metals and block the active sites. 

Information on phosphorus retention (Table V) is less abundant than that on lead. The presence of lead phosphates in used catalysts has been noted (26, 35). The retention and possibly its ability to poison a catalyst, as well, of phosphorus originating from fuel, will depend on the presence of lead. The work of McArthur (26) shows very low P retention from the fuel as compared with that from oil. The ad hoc explanation offered is that whereas P205 is the most likely form for the transport of fuel phosphorus, other forms may prevail for the oil phosphorus. This, indeed, may be so if one realizes that the oil contains species such as Zn and alkaline earth metals which form very stable phosphates. The harmful effects and the distribution of phosphorus might well be influenced by such differences, as will be discussed subsequently. 

Metal location is but one of a number of applications for scanning electron microscope studies in catalysis. Other applications are the study of the morphology of platinum-rhodium gauzes used in the oxidation of ammonia and the poisoning of catalysts, in which the scanning electron microscope results show the location of poisons such as compounds containing sulfur, phosphorus, heavy metals, or coke relative to the location of the catalytic components. 

Supported metal catalysis are employed in a variety of commercially important hydrocarbon conversion processes. Such catalysts consist, in general, of small metal crystallites (0.S to 5 nm diameter) dispersed on non-metallic oxide supports. One of the major ways in which a catalyst becomes deactivated is due to accumulation of carbonaceous deposits on its surface. Catalyst regeneration, or decoking, is normally achieved by gasification of the deposit in air at about 500°C. However, during this process a further problem is frequently encountered, which contributes to catalyst deactivation, namely particle sintering. Other factors which can contribute to catalyst deactivation include the influence of poisons such as sulfur, phosphorus, arsenic and... 

The catalytic activity of fine nickel metal is very much reduced and modified when prepared by SHOP or the cation-exchange method, as shown in the previous sections. The partial poisoning of the nickel catalyst with phosphorus compounds brings about almost the same effect. These facts suggest the possible formation of specific and mild active sites of nickel, if nickel metal makes an alloy with phosphorus. 

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