Potassium oxide-iron-alumina catalyst

From a study of the mechanism of the poisoning action of water vapors mill oxygen on iron ammonia catalysts 21 and by making certain assumptions, Almquistsu has been able to calculate that in pure iron catalysts about one atom in two thousand is active toward ammonia synthesis, whereas in iron catalysts promoted by alumina about one atom in two hundred is active. This shows the remarkable added activity obtainable by the use of promoters. That the effect is complicated beyond any simple explanation is evidenced further by some of the results of Almquist and Black, These workers have shown that whereas an iron-alumina catalyst shows greater activity toward ammonia synthesis at atmospheric pressure than an iron catalyst containing both alumina and potassium oxide, the hitter catalyst is 50 per cent more active when the pressure is raised to 1(X) atmospheres. 

Promoter. In some cases, a relatively small quantity of one or more substances, the promoter or promoters, when added to a catalyst improves the activity, the selectivity, or the useful lifetime of the catalyst. In general, a promoter may either augment a desired reaction or suppress an undesired one. There is no formal system of nomenclature for designating promoted catalysts. One may, however, for example, employ the phase iron promoted with alumina and potassium oxide. ... 

Ammonia synthesis catalysts have traditionally been based on iron and have been made by the reduction of magnetite (Fe304). The difference between different commercially available products lies in optimized levels of metal oxide promoters that are included within the magnetite structure. These metal oxides promote activity and improve the thermal stability of the catalyst. Typical promoters are alumina (AI2O3X potassium oxide (K2O), and calcium oxide (CaO). The interactions between the many components in the catalyst can radically affect 1) the initial reducibility, 2) the level of catalyst activity that is achieved, 3) the long-term catalyst performance and 4) the long-term catalyst stability204. 

Successful ammonia conversion required discovery of a catalyst, which would promote a sufficiently rapid reaction at 100-300 atm and 400-500°C to utilize the moderately favorable equilibrium obtained under these conditions. Without this, higher temperatures would be required to obtain sufficiently rapid rates, and the less favorable equilibrium at higher temperatures would necessitate higher pressures as well, in order to obtain an economic conversion to ammonia. The original synthesis experiments were conducted with an osmium catalyst. Haber later discovered that reduced magnetic iron oxide (Fe304) was much more effective, and that its activity could be further enhanced by the presence of the promoters alumina (AI2O3 3%) and potassium oxide (K2O 1%), probably from the introduction of iron lattice defects. Iron with various proprietary variations still forms the basis of all ammonia catalyst systems today. 

Where the Fischer-Tropsch process has been used on an industrial scale, iron or cobalt are the essential catalyst components. Technical catalysts also contain oxidic promoters, such as alumina and potassium oxide. Ruthenium and nickel are most attractive for academic research since they produce the simplest product packages. Nickel is used for methanation (production of substitute natural gas and removal of carbon monoxide impurities from hydrogen). 

The reaction uses a catalyst, typically an iron catalyst. Magnetite ( 6304) is used with other oxides such as alumina, calcium oxide, potassium oxide, magnesium oxide and silica. The magnetite is reduced in the process to the active iron catalyst [23]. Alumina acts as a structural stabilizer and gives improved surface area in the catalyst. Particle size must be balanced. Smaller particles are more effective catalysts but very small particles can cause pressure drops through the reactor. Certain substances may interfere with the catalyst. These are called poisons. In general, a catalyst that is resistant to poisons is desirable, but there must still be efforts to exclude poisons from the reactor feed. 

On the other hand, the effect of the electronic promoter, potassium, has been shown to be related to the kinetics of ammonia formation. Addition of 0.5% potassium oxide to the Fe-alumina catalyst resulted in a decrease in the free iron surface from 40% to ca 35% and an increase in the ammonia yield at 613 K from 0.2mol% to 0.34 mol%. The surface fractional coverage was determined by chemisorption methods to be ca 25%. This leads to the conclusion that potassium... 

Both Schlogl and Somorjai report on the interaction between oxidized iron and the promoters, mainly alumina and potassium oxide. In particular, the interaction with the structural promoter, alumina, can alter significantly the crystallography of the metallic iron surfaces formed during reduction. Since supported iron catalysts can contain much more alumina than normal for the industrial ammonia synthesis catalyst, any interaction between the iron oxide being reduced and the alumina support is much more easily observed. Some results, indicating unambiguously the effect of the interaction of iron with the alumina, will therefore be reviewed. 

In the case of Haber-type catalysts the potassium is present in the unreduced catalyst, predominantly in a compound form associated with both the iron oxide and alumina phases, while in supported platinum group metal catalysts the alkali may be added in salt form (nitrate, carbonate, hydroxide, etc.) or as the metal. In single-crystal studies the alkali may be fired at the crystal surface from molecular sieve ion sources or evaporated onto the surface as the metals. 

The industrial catalysts for ammonia synthesis consist of far more than the catalyticaHy active iron (74). There are textural promoters, alumina and calcium oxide, that minimise sintering of the iron and a chemical promoter, potassium (about 1 wt % of the catalyst), and possibly present as K2O the potassium is beheved to be present on the iron surface and to donate electrons to the iron, increasing its activity for the dissociative adsorption of N2. The primary iron particles are about 30 nm in size, and the surface area is about 15 m /g. These catalysts last for years. 

CO oxidation, 28 108 iron catalyst, 30 168 kinetics, 28 250-257 complicated, 28 257-263 latest developments in, 5 1 over amorphous metal alloys, 36 372-374 over iron, 36 24-25 on alumina support, 36 47 antipathetic behavior, 36 150, 152 particle size and, 36 131-132 promotion by potassium, 36 36-37 over rhenium. 36 24-25 promotion by potassium, 36 37 photocatalysis over perovskites, 36 304 Anunoxidation, 30 136-137 allyl alcohol, 30 157-158... 

After heating, the EB is mixed with superheated steam and fed to the first stage reactor. Both the first and second stage reactors are packed with a catalyst of metal oxide deposited on an activated charcoal or alumina pellets. Iron oxide, sometimes combined with chromium oxide or potassium carbonate, is commonly used. 

Styrene (phenyl ethylene, vinyl benzene freezing point -30.6°C, boiling point 145°C, density 0.9059, flash point 31.4°C) is made from ethylbenzene by dehydrogenation at high temperature (630°C) with various metal oxides as catalysts, including zinc, chromium, iron, or magnesium oxides coated on activated carbon, alumina, or bauxite (Fig. 1). Iron oxide on potassium carbonate is also used. 

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