Iron-ammonia catalysts composition

Another important application of iron is as an industrial catalyst. It is used in catalyst compositions in the Haber process for synthesis of ammonia, and in Fischer-Tropsch process for producing synthetic gasoline. 

FIGURE 5 Characterization of iron ammonia synthesis catalyst. High-resolution laboratory diffraction indicated a reversible modification of the iron (111) line profile. Under catalytic reaction conditions, a sub-nitride with x = 15-18 is present in addition to the bulk iron matrix. The fitting and assignment of the data were substantiated by observations of the line profile during step changes in the composition of the gas atmosphere. Details and references are given in the text. 

Table 16 gives a composition survey of commercial ammonia catalysts in the years 1964-1966. The principal component of oxidic catalysts is more or less stoichiometric magnetite, Fe304, which transforms after reduction into the catalytically active form of a-iron. 

In addition to the BET equation, Paul Emmett made enduring contributions to the experimental determination of gas-solid equilibria and the understanding of ammonia synthesis over iron-based catalysts. He also pioneered the development of selective chemisorption methods to estimate the surface composition of multicomponent catalysts and the use of tracer methods to explore the mechanism of Fischer-Tropsch synthesis and catalytic cracking. 

Since the realization of ammonia synthesis on the industrial scale in 1916 there have been no fundamental changes in the composition of the iron synthesis catalyst. Essentially, potassium carbonate, aluminum oxide, and small amounts of other promoters are fused with magnetite, followed by reduction in situ as described fully in Chapter 2,... 

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 ... 

Table 5.1 Mole percentage compositions from XPS study of the promoted iron catalyst used in ammonia synthesis171,172...

At present, the main industrial catalyst of ammonia oxidation is platinum and its alloys with aluminium and rhodium. Taking into account the deficit and high cost of platinum metals, the dcCTcasing of the consumption and losses of platinum metals is an urgent problem. Therefore, several compositions of complex oxide catalysts have been developed with iron (111), cobalt and chromium oxides as an active component. Complex oxides with perovskite structure are used as new catalysts they provide selective oxidation of ammonia with an yield not less than 90 %. The authors of [33] proposed to use perovskite powders LaMeOj, where Me=Fe, Co, Ni, Cr, Mn, and La,.,Sr,Me03, where Me=Co, Mn and x=0.25-0.75. To prepare these compounds, they used the precipitation by tetraethyl ammonia from diluted nitrate solutions taken at necessary ratios. The powders as prepared are poorly molded as in the form of honeycomb stractures as well as in the form of simple granules. 

From the early days of ammonia production to the present, the only catalysts that have been used have been iron catalysts promoted with nonreducible oxides. Recently, a ruthenium-based catalyst promoted with rubidium has found industrial application. The basic composition of iron catalysts is still very similar to that of the first catalyst developed by BASF. 

The use of selective chemisorption to determine the surface area of a metal component of a composite catalyst was first demonstrated by Emmett and Brunauer in their classic studies of promoted iron catalysts for the synthesis of ammonia (17). Other applications of selective chemisorption to determine the surface area of supported metals have been collected by Gregg and Sing (18). 

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

The yields of ammonia generally ranged from 0 to 606 x 10 8 g NH, m 2 of powder surface. Absolute yields (which we calculated from the surface area and amount of putative catalyst used in each experiment) ranged from 0 to 52.3/rmol NH3. The highest yield was with a sample of coprecipitated Ti02 (0.2 mol % Fe) which was irradiated for 28 h. The most effective catalysts appeared to be Ti02 coprecipitated with 0.2 and 0.5 mol % Fe. For some samples there appeared to be a correspondence between ammonia yield and reaction temperature, but in others no consistent trend was reported. No correlation was observed between the surface area of titania samples and their apparent catalytic activity, but the surface area was never varied without also varying the composition and manner of preparation of the powder. A net decline in apparent rate of ammonia production was observed after a few hours of irradiation the decline did not depend on the reactor temperature, the composition of the putative catalyst, or the apparent yield of ammonia. The maximum yield was reported to correspond to a turnover of 6 electrons per iron atom before the powder was deactivated. 

Surface-science studies succeeded to identify many of the molecular ingredients of surface catalyzed reactions. Each catalyst system that is responsible for carrying out important chemical reactions with high turnover rate (activity) and selectivity has unique structural features and composition. In order to demonstrate how these systems operate, we shall review what is known about (a) ammonia synthesis catalyzed by iron, (b) the selective hydrogenation of carbon monoxide to various hydrocarbons, and (c) platinum-catalyzed conversion of hydrocarbons to various selected products. 

In 1986, Liu et at found that the iron catalyst with wiistite as the precursor has extremely high ammonia synthesis activity and rapid reduction rate, which led to the invention of wiistite (Fei xO) based catalyst for ammonia synthesis. The relationship between the activity and the iron oxides (Fe304, FeO and Fe203) and their mixtures in the precursor were studied systematically, and a hump type curve was found between the activity and the ratio (Fe +/Fe +). It was speculated that the monophase of iron oxide phase in the precursor is an essential condition for high activity of the catalyst and a uniform distribution of iron oxide phase and promoters is a key to make a better performance of catalyst. The hump type curve was interpreted by the ratio of phase compositions in the precursor, that is, the activity change of the fused iron catalyst depends essentially on the molecule ratio of different iron oxides but not on the atomic ratio of Fe + and Fe +, or Fe +/Fe +, in the precursor under certain promoters. Thus we found that Fei xO based catalyst with wiistite phase structure (Fei xO, 0.04 < x < 0.10) for ammonia synthesis has the highest activity among all the fused iron catalysts for ammonia synthesis. 

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