Hydrogen interaction with iron catalyst

The hypothesis of formation of oxygenated compounds as intermediate products was rejected by Eidus on the basis of experiments on the conversion over cobalt of methyl and ethyl alcohols and formic acid which were found to form carbon monoxide and hydrogen in an intermediate step of the hydrocarbon synthesis (76). Methylene radicals are thought to be formed on nickel and cobalt catalysts (76) by hydrogenation of the unstable group CHOH formed by interaction of adsorbed carbon monoxide and hydrogen, while on iron catalysts methylene radicals are probably formed by hydrogenation of the carbide (78,81). Carbon dioxide was found to interact with the alkaline promoters on the surface of iron catalysts as little as 1 % potassium carbonate was found to occupy 30 to 40% of the active surface area. The alkali also promotes carbide formation and the synthesis reaction on iron (78). 

Elemental and surface analysis measurements had shown that the cokes contained significant amounts of well-dispersed iron chlorides and other contaminants. XPS measurements had shown the importance of the chemical nature of the cokes and their interactions with the catalyst components (and not simply the degree of carbon coverage of the catalyst). IINS focused on the hydrogen-containing part of the coke provided a quite different view of finely divided, highly contaminated, bulk samples of cokes from commercial processes. 

The present paper focuses on the interactions between iron and titania for samples prepared via the thermal decomposition of iron pentacarbonyl. (The results of ammonia synthesis studies over these samples have been reported elsewhere (4).) Since it has been reported that standard impregnation techniques cannot be used to prepare highly dispersed iron on titania (4), the use of iron carbonyl decomposition provides a potentially important catalyst preparation route. Studies of the decomposition process as a function of temperature are pertinent to the genesis of such Fe/Ti02 catalysts. For example, these studies are necessary to determine the state and dispersion of iron after the various activation or pretreatment steps. Moreover, such studies are required to understand the catalytic and adsorptive properties of these materials after partial decomposition, complete decarbonylation or hydrogen reduction. In short, Mossbauer spectroscopy was used in this study to monitor the state of iron in catalysts prepared by the decomposition of iron carbonyl. Complementary information about the amount of carbon monoxide associated with iron was provided by volumetric measurements. 

Cyclic photophosphorylation is also a highly energetic reaction. The bipyridyliums, paraquat and diquat (Figure 2.2), divert the electron flow of cyclic photophosphorylation (photosystem I). The capture of an electron from the chlorophyll reduces the herbicide and the reduced herbicide reacts with oxygen to form superoxide. Superoxide produces hydrogen peroxide within the chloroplast and these two compounds interact to form hydroxyl radicals in the presence of an iron catalyst. Hydroxyl radicals are very damaging and lead to the destruction of the cellular components leading to rapid plant death. 

Warburg and Christian showed that the color of this old yellow enzyme came from a flavin and proposed that its cyclic reduction and reoxidation played a role in cellular oxidation. When NADP+ was isolated the proposal was extended to encompass a respiratory chain. The two hydrogen carriers NADP+ and flavin would work in sequence to link dehydrogenation of glucose to the iron-containing catalyst that interacted with oxygen. While we still do not know the physiological function of the old yellow enzyme,b the concept of respiratory chain was correct. 

As mentioned above, the addition of promoters, and even the formation of bimetallic particles, can provide carbon-supported iron catalysts with better performances in CO hydrogenation. The method of preparation of these systems is going to determine the final effect, always taking advantage of the relative inertness of the carbon surface. The interaction between the different components of the active phase can be maximized by using mixed-metal carbonyl complexes. Furthermore, use of these precursors allows for the preparation of catalysts with... 

Methanol is prepared by the interaction of carbon monoxide and hydrogen, usually at 300-400°C and 275-360 atmospheres. A catalyst of zinc oxide promoted with chromic oxide is generally employed and conversions of about 15% per pass are usual. The methanol is condensed out and unreacted gases, with fresh make-up gas, recycled to the converters. In the second stage, methanol is oxidized to formaldehyde. In one process a mixture of methanol vapour and air is passed over a catalyst of molybdenum oxide promoted with iron at 350—450°C. The exit gases are scrubbed with water and the formaldehyde is isolated as an aqueous solution. 

Finally, the chapters by Schlogl and Somorjai deal with the effects of water on the action of the promoters, in particular alumina, with an oxidized iron surface. The structure and catalytic properties obtained by the iron surface after reduction appear to be affected significantly by previous exposure to water vapor (and hydrogen) at high temperatures. The small amount of alumina present in industrial synthesis catalysts renders assessment of the interaction of the excess oxidized iron with the alumina difficult. We therefore will review some results obtained on iron catalysts supported on alumina at a much lower iron content. 

---