Catalyst poisoning sensitivity

The advent of a large international trade in methanol as a chemical feedstock has prompted additional purchase specifications, depending on the end user. Chlorides, which would be potential contaminants from seawater during ocean transport, are common downstream catalyst poisons likely to be excluded. Limitations on iron and sulfur can similarly be expected. Some users are sensitive to specific by-products for a variety of reasons. Eor example, alkaline compounds neutralize MTBE catalysts, and ethanol causes objectionable propionic acid formation in the carbonylation of methanol to acetic acid. Very high purity methanol is available from reagent vendors for small-scale electronic and pharmaceutical appHcations. 

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 all of the ethylene polymerization processes, the catalyst is sensitive to feed impurities and is poisoned by most polar compounds. Many of the properties of the polymer are determined by polymerization conditions, but catalyst composition and condition are critical determinants as well. 

Catalyst Poisons. Hausberger, Atwood, and Knight (33) reported that nickel catalysts are extremely sensitive to sulfides and chlorides. If all materials which adversely affect the performance of a catalyst were classified as poisons, then carbon laydown and, under extreme conditions, water vapor would be included as nickel methanation catalyst poisons. 

So far, certain biomimetic catalysts (1 and 2b in Fig. 18.17) have been shown to reduce O2 to H2O under a slow electron flux at physiologically relevant conditions (pH 7,0.2-0.05 V potential vs. NHE) and retain their catalytic activity for >10" turnovers. Probably, only the increased stability of the turning-over catalyst is of relevance to the development of practical ORR catalysts for fuel cells. In addition, biomimetic catalysts of series 1,2,3, and 5, and catalyst 4b are the only metalloporphyrins studied in ORR catalysis with well-defined proximal and distal environments. For series 2, which is by far the most thoroughly studied series of biomimetic ORR catalysts, these well-defined environments result in an effective catalysis that seems to be the least sensitive among all metalloporphyrins to the electrode material (whether the catalyst is adsorbed or in the film) and to chemicals present in the electrolyte or in the O2 stream, including typical catalyst poisons (CO and CN ). 

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. 

Feedstocks and products Next (or perhaps first) we need to consider the markets of products and the availabihty of feedstocks. The prices of these depend sensitively on purity levels that can be tolerated. For the reactants these are usually determined by the effects of impurities as catalyst poisons and on product distributions. For products different markets demand specific impurities. AH byproducts and unused reactants must be disposed of, either sold, recycled into the reactor, or incinerated. 

Sensitivity towards catalysts poisons low low high high no sensitivity... 

Raney-nickel catalysts are barely sensitive to catalyst poisoning (as are Pt-activated cathodes), e.g., by iron deposition, but they deteriorate due to loss of active inner surface because of slow recrystallization—which unavoidably leads to surface losses of 50% and more over a period of 2 years. A further loss mechanism is oxidation of the highly dispersed, reactive Raney nickel by reaction with water (Ni + 2H20 — Ni(OH)2 + 02) under depolarized condition, that is, during off times in contact with the hot electrolyte after complete release of the hydrogen stored in the pores by diffusion of the dissolved gas into the electrolyte. 

The nickel-based reforming catalysts which are commonly used in steam reforming are quite sensitive to sulphur, halogen and heavy metal poisons. Since these elements may all be found in natural gas, a feed gas purification section is normally required. Of the mentioned catalyst poisons, sulphur is by far the most important [6],... 

The main advantages of a batch reactor are as follows. It is simple and allows rapid measurements. Many experiments can be performed in a short period of time. It is convenient when using pure, expensive, corrosive, or high boiling temperature chemicals. Its use is recommended if the catalyst is sensitive to traces of poisons since there is no accumulation effect. In principle, by varying the stirring conditions it is possible to investigate the influence of heat and mass transfer processes. 

The minimum concentration required to eliminate the catalytic activity is one possible measure of the sensitivity of a catalyst to a poison. Sensitivity to poisoning is most properly defined by the amount, np, of the poison adsorbed on a unit amount of catalysts which causes a given fractional decrease of the catalytic activity (a = (a0 - ap)/a0), where aQ and ap are the activities in the absence or presence of poison, respectively) ot/rtp is a measure of the sensitivity to poisoning. One may also use the ratio of a to the concentration cp of the poison in the feed, namely a/cp, but this is less precise, as this depends on the adsorption coefficient of the poison. 

The catalyst is sensitive to sulfur and arsenic poisoning (the Utter being a permanent poison). Natural gas must, therefore be desulfurized. Carbon and coke deposits also damage the catalyst and must be removed by steam or by burning off with air. 

In the steam-reforming process, any sulfur compounds present in the hydrocarbon feedstock have to be removed because the nickel-containing catalysts are sensitive to poisons. This is either achieved by hydrodesulfurization (see Hydrodesulfurization Hydrodenitrogenation), generally with a combination of cobalt-molybdenum and zinc oxide... 

Acyl halides are sensitive to the same groups of reducing agents that are used with aldehydes and ketones. However, it is possible, by using a catalyst poison such as barium sulfate, to stop the reduction at the stage at which only the halogen atom has been replaced by hydrogen, as in the Rosenmund reduction (Scheme 3.65). 

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. 

Iron is also a fi equently analyzed contaminant. Corrosion or colloidal iron species (dust) may be the cause of this type of poisoning. Pd based catalysts are sensitive to this contaminant because... 

To carry out long-term lifetime tests. The catalyst encounters components of the feed for the first time. Although the designer may have anticipated the effect of substances such as poisons, sensitivity of the catalyst formulation can only be checked with experiment. For well-behaved processes, lifetime tests of several hundred hours are necessary before long-term... 

---