Iron-alumina catalyst

To a certain extent the expression multicomponent catalysts is an arbitrary one. There is no doubt that the pure chemical elements and pure chemical compounds have to be called single component catalysts. It is, however, questionable whether a material such as steel should be classified as a single component system or as a multicomponent system. Some of the multicomponent catalysts, for instance, the iron-alumina catalyst consist of two separate solid phases but it would be misleading to accept the presence of more than one phase as the decisive criterion for multicomponent catalysts. The more than additive catalytic action of Cu-ions and Fe-ions in an homogeneous aqueous medium represents obviously a case of multicomponent catalysis, although it occurs in a single-phase system. As to solid multicomponent catalysts, they usually consist of more than one single phase, but there are exceptions to this rule, such as in cases in which mixed crystals or solid solutions are formed from the components. 

Further, a stabilization of the total surface of the main catalyst by added substances may explain some promoter effects, but this explanation holds only for a few multicomponent catalysts. For the iron-alumina catalyst, a beneficial stabilizing effect of the promoter alumina on the fine structure of the iron has to be accepted as a partial explanation. The fact that highly dispersed pure iron sinters at temperatures above 300°C. to a considerable extent, and that sintering practically does not occur with iron of the same high dispersion which contains 1 to 2% of alumina, is a strong qualitative support for this concept. In a quantitative way, the work of P. H. Emmett (47) and his associates has proved this point beyond any doubt it gives similarly valuable... 

It should be noted that the results for the formic acid decomposition donor reaction have no bearing for ammonia synthesis. On the contrary, if that synthesis is indeed governed by nitrogen chemisorption forming a nitride anion, it should behave like an acceptor reaction. Consistent with this view, the apparent activation energy is increased from 10 kcal/mole for the simply promoted catalyst (iron on alumina) to 13-15 kcal/mole by addition of K20. Despite the fact that it retards the reaction, potassium is added to stabilize industrial synthesis catalysts. It has been shown that potassium addition stabilizes the disorder equilibrium of alumina and thus retards its self-diffusion. This, in turn, increases the resistance of the iron/alumina catalyst system to sintering and loss of active surface during use. 

Rare earth oxides are useful for partial oxidation of natural gas to ethane and ethylene. Samarium oxide doped with alkali metal halides is the most effective catalyst for producing predominantly ethylene. In syngas chemistry, addition of rare earths has proven to be useful to catalyst activity and selectivity. Formerly thorium oxide was used in the Fisher-Tropsch process. Recently ruthenium supported on rare earth oxides was found selective for lower olefin production. Also praseodymium-iron/alumina catalysts produce hydrocarbons in the middle distillate range. Further unusual catalytic properties have been found for lanthanide intermetallics like CeCo2, CeNi2, ThNis- Rare earth compounds (Ce, La) are effective promoters in alcohol synthesis, steam reforming of hydrocarbons, alcohol carbonylation and selective oxidation of olefins. 

The H2/CO reaction has also been studied by various transient isotopic methods over the same iron/alumina catalyst. After 1.5 h of reaction of 10% CO/H2 at 285°C, the feed is switched to He and then to H2. The curve CH4.S of Fig. 29 represents the usual three peaks that arise from this titration, including a final temperature ramp to remove the most refractory surface carbon. Now, when the feed is changed to H2/ CO before the titration with H2, the produced after various times in CO/H2 is shown (190). Even after 30 min of exposure, only 86% of the first peak (CH) has been changed to the form, meaning that the rest of the surface and bulk carbonaceous species are spectator species. It is of course also interesting to measure the rate of formation of CH4 immediately after the switch... 

D. Bianchi and J.L. Gass, Hydrogenation of Carbonaceous Adsorbed Species on an Iron/Alumina Catalyst, J. Catal. 123 (1990) 310. 

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. 

Single-bed catalysts had been used to produce sulfur from dry sulfur dioxide gases. Ryason 1,2) used either Cu, Pd, Ag, Co, or Ni supported on alumina. Khalafalla and Haas (3) optimized the composition of iron-alumina catalysts to produce sulfur from dry gases containing sulfur dioxide and carbon monoxide. Querido and Short (4) demonstrated the feasibility of reducing sulfur dioxide by carbon monoxide on a copper-alumina catalyst at concentrations and temperatures typical of power plant stack gases. 

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. 

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