Poisoning by impurities

The goal of Haber s research was to find a catalyst to synthesize ammonia at a reasonable rate without going to very high temperatures. These days two different catalysts are used. One consists of a mixture of iron, potassium oxide. K20, and aluminum oxide. Al203. The other, which uses finely divided ruthenium, Ru. metal on a graphite surface, is less susceptible to poisoning by impurities. Reaction takes place at 450°C and a pressure of 200 to 600 atm. The ammonia... 

Chemical deactivation. In chemical deactivation the active surface area changes by strong chemisorption of impurities in the feed, by blocking of active sites by heavy products formed in parallel or sequential reactions, etc. The most important chemical causes of deactivation are poisoning by impurities in the feed and deposition of carbonaceous material, usually referred to as coke . 

Chemically induced decomposition, of which two further categories can be considered namely substrate induced decomposition and poisoning by impurities or products. 

We can readily understand these setbacks today if we consider the high sensitivity of iron as an ammonia catalyst toward numerous catalyst poisons. In those early years, this fact was unknown to us. Specifically, no one suspected the harm which is done to the catalyst by substances such as sulfur and sulfur compounds. Even Haber had never discussed a catalyst poisoning by impurities, because he had been able, apparently to avoid the presence of catalyst poisons in his small scale experiments. 

The more interesting situation occurs when the catalyst is partially and reversibly poisoned by impurities in the reactant gas. The degree of loss of catalyst activity then depends on the operating conditions. 

Other technical hurdles must be overcome to make fuel cells more appealing to automakers and consumers. Durability is a key issue and performance degradation is usually traceable to the proton exchange membrane component of the device. Depending on the application, 5,000 40,000 h of fuel cell lifetime is needed. Chemical attack of the membrane and electrocatalyst deactivation (due to gradual poisoning by impurities such as CO in the feed gases) are critical roadblocks that must be over come. 

A. Poisoning by impurities brought into the system with the synthesis gas, for example dust, tar, resin formers, hydrogen sulfide, organic sulfur compounds, and chlorine and other elements. 

The most pronounced problem of these materials is, nevertheless, the poisoning by impurities in the feedstream, and this often limits their applicability. Alloys of Pd-Ag are particularly prone to poisoning by sulfur. Other gases such as CO and H2O can also be detrimental to long-term membrane performance. The highest measured permeabilities of these materials are of the order of 10 to 10 mol H2 m i s i Pa-", where typically n= 0.5 [104-106]. 

Catalyst-poison-resistant promoters protect the active phase against poisoning by impurities, either present in the starting materials or formed in side reactions. 

The anhydrous ammonia and process air used must be free from catalyst poisons, dust, and oil. Platinum catalysts can be poisoned by such elements as As, Bi, P, Pb, S, Si, and Sn. Fortunately, synthetic ammonia is normally of high purity unless it is accidentally-contami -nated. However, since air can be contaminated by dust or many other pollutants, thorough air cleaning is necessary. Location of the air intake in an area relatively free from contaminants will help. If poisoning by impure ammonia or air ould arise, deep penetration may occur, leading to the formation of inactive compounds in the catalyst wires and, perhaps, to the extent of ruining the catalyst, fri other instances, contamination by traces of Cr, Fe, or Ni may temporarily reduce conversion efficiency, but this can often be restored by treatment with hydrochloric acid or certain sails. 

Chain growth (step b in Equation 22.5) occurs by a combination of olefin coordination and migratory insertion. A vacant site is required for the olefin to coordinate before alkyl insertion can take place. Because coordination is required, fast insertions occur with less-hindered olefins, such as ethylene, propylene, linear a-olefins, and vinylarenes. The relative rates for the polymerization of aUcenes typically follow the trend ethylene > propylene > a-olefin 1,2-disubstituted olefin = 1,1-disubstituted olefin, and enchainment of tri-and tetra-substituted olefins is rare or unknown. Many polymerization catalysts are sensitive to poisoning by impurities that bind the open coordination site. 

However, even though the theory was laid out, experimental work did not follow as soon as expected. This was because studies on electrode processes were made using solid electrodes. Solid, constant surface electrodes are extremely prone to poisoning by impurities if the solutions and containers are not extremely clean, the electrode surface will be covered with... 

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