Poisoning of iron catalysts

The catalyst may combine chemically with the impurity The poisoning of iron catalyst by H2S comes in this class. 

Hydrogen sulfide causes a permanent poisoning of iron catalysts. Methane does not poison ammonia catalysts under normal synthesis conditions. Equilibrium data (Browning, De Witt, and Emmett, 77 Browning and Emmett, 78) should be mentioned in this connection. 

Poisoning of iron catalysts during ammonia synthesis by sulfur compounds has received relatively little attention (154, 240-244). Nevertheless, the previous work provides information on the poisoning mechanism and interesting examples of how oxide promoters may influence the sulfur poisoning behavior of a catalytic metal. 

Almquist and Black (38) studies the poisoning of iron catalysts by water por. In experiments conducted at 1 atmosphere pressure, they showed that quantities of water vapor 0.3% as large as those required to convert the iron to Fe O would nonetheless form oxygen-Fe bonds on the surface. The amount of surface oxygen increased approximately with the square root of the pressure of water vapor. If the poison was added as O2 it all was converted to H O and emerged from the catalyst in that form and not as 0. The catalyst recovered its initial activity if the poison was shut off and the catalyst was treated for an hour or two with pure dry 3H2 N2 gas. Oxygen is, therefore, a reversible poison. 

Remaining trace quantities of CO (which would poison the iron catalyst during ammonia synthesis) are converted back to CH4 by passing the damp gas from the scmbbers over a Ni methanation catalyst at 325° CO -t- 3H2, CRt -t- H2O. This reaction is the reverse of that occurring in the primary steam reformer. The synthesis gas now emerging has the approximate composition H2 74.3%, N2 24.7%, CH4 0.8%, Ar 0.3%, CO 1 -2ppm. It is compressed in three stages from 25 atm to 200 atm and then passed over a promoted iron catalyst at 380-450°C ... 

How can this enigma be answered Put away a sample of pure harmaline, with its spectral identification, onto the shelf for 50 or 100 years, and then re-analyze it Who knows, but what might be needed for this conversion is heat, or a bit of iron catalyst, or some unknown species of South American mold. Acid is certainly known to promote this oxidation. It would be very much worth while to answer this question because some, perhaps much, of the results of human pharmacological studies that involve harmaline as a metabolic poison, may be influenced by the independent action of harmine as a harmaline contaminant. 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

One strong point of SIMS is its ability to detect elements that are present in trace amounts, and as such the technique is highly suited to the detection of poisons on a catalyst caused by contaminants in the reactor feed. Chlorine, for example, poisons the iron catalyst used in ammonia synthesis because it suppresses the dissociation of nitrogen molecules. Plog et al. [18] used SIMS to show that chlorine impurities may coordinate to potassium promoters, as evidenced by a KCI2- signal, or to iron, visible by an FeCh- peak. The SIMS intensity ratio... 

Deactivation by sulfur has been explained by the withdrawing of electrons from the catalyst surface. It has also been shown that sulfur inhibits the dissociation of CO on iron surfaces l]. The deliberate partial poisoning of iron/manganese cataly.sts with sulfur has been used to shift the product selectivity towards short-chain hydrocarbons. At higher sulfur concentrations (0.7 mg S/g catalyst) the activity is significantly decreased and the olefin selectivity increased [82]. Sulfur poisoning of nickel catalysis has recently been shown to inhibit the chemisorption of hydrogen 83.84). 

Addition of promoters to neutralize poisons. Sulfur poisoning of nickel is reduced in the presence of copper chromite, since copper and chromium ions preferentially form sulfides. Another example is heavy metals poisoning of cracking catalyst, in which iron, nickel, and vanadium are alloyed with antimony added to the feed and deposited on the catalyst. 

All heavy crude oil residues have heavy metals such as Ni, V or Fe in their structure. These metals are bonded as organometalic compounds. At high temperatures and for hydrogenation reactions, these compounds are cracked and heavy metals are deposited on the catalyst surface. These metals can also react with hydrogen sulfur from the gas phase to form metal sulfides. The deposition of sulfides of iron, vanadium or nickel leads to irreversible poisoning of the catalyst. This is the difference between catalyst deactivation by metals and deactivation by coke the former leads to an irreversible loss of the catalyst activity. 

Electronic promoters, for example, the alkali oxides, enhance the specific activity of iron-alumina catalysts. However, they reduce the inner surface or lower the thermal stability and the resistance to oxygen-containing catalyst poisons. Promoter oxides that are reduced 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 permanendy. 

It is sometimes possible to reactivate a catalyst that has lost its activity, especially when the loss in activity is due to some absorbed materials. Passing hydrogen alone over the catalyst for a period of time may help or, in some cases, it is possible to reactivate the catalyst by passing a small amount of oxygen over it. In these cases, the adsorbed materials are probably burned off. Oxygen was found to reactivate catalysts used in the synthesis of methanol from carbon monoxide and hydrogen but quite effectively to poison an iron catalyst used in the synthesis of ammonia. ... 

Pure iron(iii) oxide performs rather poorly as a WGS catalyst, due to rapid catalyst deactivation by sintering. Traditional iron catalysts typically consist of iron(iii) oxide (80-90% by mass), chromium(iii) oxide (8-10% by mass) and small amounts of other stabilisers and promoters such as copper(ii) oxide, aluminium oxide, alkali metals, zinc oxide and magnesium oxide. The small fraction of chromium(iii) oxide acts to prevent catalyst sintering, and also promotes the catalytic activity of iron. Catalyst deactivation is typically caused by poisons in the feedstock gases and by deposition of solids on the catalyst surface. 

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... 

Ammonia production requires a gas phase with hydrogen and nitrogen present in a ratio of 3 1 (Section 6.1), and the ammonia syngas must be free of CO and CO2 because these compounds poison the iron catalyst used in the ammonia synthesis reactor. 

The reduction rate of iron catalyst is related with the type and content of promoters. Many researchers studied the inhibitory effect of promoter content on reductions, indicating that structural promoters, such as AI2O3 etc. have strong inhibitory effect on the reduction of Fc304. MgO, Ti02, Si02 etc. also inhibit the reduction rate. Electronic promoters such as K2O etc. can help to speed up the reduction rate of the catalyst. In addition, addition of CuO, NiO promotes the reduction of iron oxide, but these oxides will reduce ammonia sjmthesis activity. Therefore, it is commonly accepted that the promoters which can reduce the reduction rate can increase the activity of ammonia sjmthesis, and it also can improve the thermal stability and anti-poisoning ability. 

It can be seen that, with both organic and inorganic sulfur, the irreversible poisoning of the catalyst occurs after the sulfide is reduced to H2S and adsorbed on the surface of the fused iron catalyst. 

Wlien a strong electron-donor ligand such as pyridine is added to tlie reaction mixture, it can bond so strongly to tlie Rli tliat it essentially drains off all tlie Rli and shuts down tlie cycle it is called a catalyst poison. A poison for many catalysts is CO it works as a physiological poison in essentially the same way as it works as a catalyst poison it bonds to tlie iron sites of haemoglobin in competition witli O. ... 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

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... 

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