Phosphorus compounds, catalyst poisoning

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

In catalytic incineration, there are limitations concerning the effluent streams to be treated. Waste gases with organic compound contents higher than 20% of LET (lower explosion limit) are not suitable, as the heat content released in the oxidation process increases the catalyst bed temperature above 650 °C. This is normally the maximum permissible temperature to which a catalyst bed can be continuously exposed. The problem is solved by dilution-, this method increases the furnace volume and hence the investment and operation costs. Concentrations between 2% and 20% of LET are optimal, The catalytic incinerator is not recommended without prefiltration for waste gases containing particulate matter or liquids which cannot be vaporized. The waste gas must not contain catalyst poisons, such as phosphorus, arsenic, antimony, lead, zinc, mercury, tin, sulfur, or iron oxide.(see Table 1.3.111... 

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... 

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. 

These catalysts are extremely sensitive to catalyst poisons, which reduce chemisorption of hydrogen and nitrogen on the active surfaces of the catalyst and thereby reduce its activity. Gaseous oxygen-, sulfur-, phosphorus-and chlorine compounds, such as water, carbon monoxide, carbon dioxide, the latter being reduced to water under ammonia synthesis conditions, are particularly troublesome in this regard. Catalyst poisoned with oxide compounds can be reactivated by reduction with pure synthesis gas. 

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. 

In continuous operation consecutive phosphorus-containing products are formed which also influence the activity of the rhodium center and thus contribute to the catalyst deactivation. One of the main degradation products from TPPTS is the sodium salt of m-formylbenzenesulfonic acid, which indicates the insertion of the rhodium atom into the P—C bond. The arylrhodium species in which rhodium has replaced one phosphorus atom presumably exists as a phosphido-bridged dimer which is inactive. This compound may subsequently be converted to a series of consecutive products, e.g., alkyl-diarylphosphines, which act as catalyst poisons. 

Platinum Catalysts for Exhaust Emission Control The Mechanism of Catalyst Poisoning by Lead and Phosphorus Compounds... 

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. 

In general, organophosphorus compounds show relatively low thermal stability. They decompose relatively quickly under the process conditions, either being converted to phosphorus pentoxide, or in the presence of water vapour to higher, condensed phosphoric acids. It is phosphorus compounds such as these which reach the surface of the catalyst. Most researchers have concluded that phosphorus poisoning is non-selective and can therefore additionally serve as a model for other non-selective poisons such as lead and zinc. Most researchers have... 

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. 

The slope of the saturation concentration versus surface area line was about 2.2 X 10 phosphorus atoms per cm of support surface, quite close to the number of atoms per cm of solid surfaces. Therefore, this indicates a monolayer-equivalent coverage of the alumina surface by phosphorus. The fact that there was very little change in the effective dilfusivity upon poisoning by phosphorus compounds suggests that the poisons tend to deposit in a monolayer-like concentration over the surfaces of the poisoned shell. The simple pore-mouth-poisoning mechanism was adapted for catalyst deactivation by phosphorus compounds. ... 

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 ... 

Angele and Kirchner carried out the same mathematical modeling for poisoning of a honeycomb catalyst by phosphorus compounds. They ignored axial diffusion compared to convection and simplified the transport equation as ... 

Both the MPT and DTS use CATOX units to destroy VOCs in the gaseous effluent streams. The CATOX units use a Pt/Pd oxidation catalyst. AEA uses a scrubber and filter upstream of the CATOX units to remove phosphorus, fluorine, and chlorine compounds that could poison the catalyst. 

Example 22 removal of allyl group attached to a phosphorus centre with Pd, Pt and Rh complexes is a well established procedure [51] but is inconvenient for synthesis of therapeutic compounds on a large scale. During the deprotection step the palladium catalyst is susceptible to poisoning especially with P-S compounds resulting in loss of catalytic efficiency. Furthermore traces of organometallic compounds remain in the product after deprotection. In the paper of Manoharan et al. other methods of deprotection of allyl... 

Most lubricating oils for engine use contain additives designed to improve such properties as lubricity, detergency, oxidation resistance, and viscosity. The additives contain elements that could be potentially harmful to catalysts. Table I lists these elements and their typical concentration in lubrication oils of 1973. The first three elements are combined usually in one compound, zinc dialkyldithiophosphate. Thus, before combustion, sulfur and phosphorus in oil are in a different chemical state than the same elements are in fuel. Little is known whether combustion nullifies these differences partially or fully. Some data, to be discussed subsequently, are available on the separate poisoning effects of these elements as derived either from the fuel or from the oil. 

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. 

Sulfur, Phosphorus, and Arsenic Compounds. Sulfur, occasionally present in synthesis gases from coal or heavy fuel oil, is more tightly bound on iron catalysts than oxygen. For example, catalysts partially poisoned with hydrogen sulfide cannot be regenerated under the conditions of industrial ammonia synthesis. Compounds of phosphorus and arsenic are poisons but are not generally present in industrial synthesis gas. There are... 

The main cause of deactivation are elements or compounds which chemically attack the catalytically active material or its support. Also, structural changes and pore blocking are important issues of deactivation. A variety of poison compounds containing elements such as halogens, alkah metals, alkaline earth metals, arsenic, lead, phosphorus, and sulfur are mentioned in the hterature. AS2O3 is the most severe poison in coal-fired power plant operation in Germany. In power plants equipped with wet-bottom boilers alkah metal oxides mostly remain in the molten ash, whereas AS2O3 tends to escape into the flue gas and deposits on the catalyst. 

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