Catalysts synthetic ammonia

Emmett P H and Brunauer S 1937 The use of low temperature van der Waals adsorption isotherms in determining the surface area of iron synthetic ammonia catalysts J. Am. Chem. See. 59 1553-64... 

From these results, it is concluded that, in a fully reduced catalyst, FeAl204 is not present furthermore, the aluminum inside the iron particle is present as a phase that does not contain iron (e.g., A1203), and this phase must be clustered as inclusions 3 nm in size. These inclusions may well account for the strain observed by Hosemann et al. From the Mossbauer effect investigation then, the process schematically shown in Fig. 17 was suggested for the reduction of a singly promoted iron synthetic ammonia catalyst. Finally, these inclusions and their associated strain fields provide another mechanism for textural promoting (131). 

If the material is of natural origin (natural molecular sieve, pumice) the desired entity size is obtained by crushing and/or grinding and subsequent size selection by sieving. Some catalysts are also obtained in this way from synthetically prepared bulky masses (e.g. ammonia catalyst). Grinding may take place in the absence (dry grinding) or presence (wet grinding) of a liquid, usually water. 

The theory of Kobozev is open to criticism of the character recently stated by Kummer and Emmett (184), who observed that there was a very rapid exchange between the isotopes of nitrogen in the presence of singly and doubly promoted iron synthetic ammonia catalyst. Kobozev (167) had concluded earlier that iron synthetic ammonia catalysts consisted of an ensemble of iron atoms to which were attached the promoter molecules. Each ensemble was capable of adsorbing only one nitrogen molecule further, in accordance with his theory, these ensembles, separated from one another by geometrical barriers, would make the... 

Figure 3. Adsorption of nitrogen on synthetic ammonia catalyst...

It is useful, however, to recall an example which does not conform to this condition, the decomposition of ammonia on doubly-promoted iron synthetic ammonia catalysts as studied by Love and EmmettT They found a kinetic equation,... 

P.H. Emmet and S. Brunauer, Accumulation of alkali promoters on surfaces of iron synthetic ammonia catalysts. /. Am. Chem. Soc., 59 (1937) 310. 

Brunauer and Emmett 120), in their extensive studies on synthetic ammonia catalysts have concluded, by a comparison of the CO uptakes and nitrogen adsorption surface area measurements, that on pure iron at temperatures between —78 and — 183°C CO chemisorbs up to one molecule per surface atom. Beebe and Stevens 121) from measurements of differential heats of adsorption confirmed that chemisorption rather than physical adsorption was occurring in this system. 

Magnetic methods are, like x-ray diffraction, a tool for gaining structural information. These methods have been used to measure the effective dispersion of a paramagnetic oxide such as chromia gel or chromia supported on alumina and to determine oxidation states and bonding types under conditions where other procedures are difficult or inapplicable. Magnetic methods are useful also in the identification and estimation of ferromagnetic components such as iron carbide in Pischer-Tropsch or synthetic ammonia catalysts. 

Many of the reactions catalysed involve the transfer of protons to and from the zeolite (Figure 2.8). As this is the characteristic function of an acid, the H-forms of the zeolite (those in which the adsorbed ions are H+) are sometimes referred to as solid acid catalysts. Synthetic zeolites are often prepared in concentrated solutions of sodium hydroxide, which leaves Na" as the counter ion in the zeolite pores, the Na-zeolite. The usual way to prepare H-zeolites is to exchange these metal ions for ammonium ions (NH4" ), and then heat the ammonium form of the zeolite to about 500 °C. Ammonia (NH3) is then driven off, leaving H+ ions to balance the negative charges on the silicate structure. This is the reverse of the reaction shown in Figure 2.8, when the base B is NH3. 

With doubly-promoted iron synthetic ammonia catalysts the same type of observations are repeated. Data on desorption and readsorption are shown in Fig. 7, while in Fig. 8 are the values obtained on raising and lowering the temperature. It will be noted that the latter curves do not show the double maxima at —78 and 110 C. earlier found by Brunauer and Emmett (10) and examined by them in detail as types A and B adsorption. 

Iron Synthetic Ammonia Catalysts Brunauer and Emmett (10) in their careful studies of iron catalysts had already observed (see especially Tables IV, X and XIII of the paper cited) the phenomenon of increasing adsorption with decreasing temperature that we have indicated as occurring in our studies, as recorded in Fig. 8, on other preparations of synthetic ammonia catalysts. 

Measuring Surface Promoter Distribution Attention has already been called to the use that has been made of surface area measurements in studying synthetic ammonia catalysts. It has been possible, for example, to show that certain promoters have a specific influence on the activity per unit surface area (4). Thus iron catalysts containing both K2O and AI2O3 as promoters have surfaces that are only about one-third as large as those containing only AhOs as promoter and yet are several fold more active under synthesis conditions. 

Fia. 6. Comparison of the isotherms for total carbon monoxide adsorption at —183° and for the physical adsorption on about 45 g. of pure iron synthetic ammonia catalyst (No. 973) and on a similar quantity of a doubly promoted iron catalyst (No. 931) (39). 

As is indicated in greater detail below, studies have shown that the intermetallics undergo extensive (perhaps total) decomposition in the course of the reactions that are being investigated, and it is highly probable that it is the decomposition products or the transformed intermetallics which are the active catalysts. Initially, the materials were examined as synthetic ammonia catalysts (14), More recently, attention has been directed toward their use as catalysts for the synthesis of hydrocarbons from CO and CO2. 

As has been alluded to above, the catalysts are extensively transformed when exposed to a mixture of CO and H2 at T > / 225°C. This transformation was first noted by Takeshita, Wallace, and Craig (14) in the use of these materials as synthetic ammonia catalysts. RCo. and RFca, intermetallics were converted into iron or cobalt rare earth nitride. This was established by conventional x-ray diffraction measurements. Coon (16) observed that the RNis compounds were transformed by the CO/H2 mixture into R2O3 and Ni, and a similar transformation also was observed by Elattar et al. (17) for ThNis, UNis, and ZrNis. SEM and EDAX results on ThNis show the formation of nickel nodules 0.5 /un in diameter situated on a Th02 substrate. 

Figure 1. Adsorption isotherms for a pure iron synthetic ammonia catalyst for various gases near their boiling points. Curve lA is for physical plus chemical adsorption of CO. Curve IB is for physical adsorption occurring at -1830C after the evacuation of the samples at -78 C for an hour. The solid symbols are for desorption. (Reproduced from Ref. 22. Copyright 1937, American Chemical Society.)...

Preparation of the H8gg carbide was started with a commercial synthetic ammonia catalyst containing 92.6% FesOi, 4.2% MgO, 0.7% SiOj, and 0.4% Cr203. Reduction and carburization were carried out in apparatus similar to that previously described (Hofer and Peebles, 49). A stream of purified hydrogen was passed over the sample held at 450° for 82 hours. The reduced sample was then treated at 240° for 539 hours with purified carbon monoxide, until the carbon/iron ratio became nearly constant, and closely corresponded to FeaC. The reaction vessel was always opened in a carbon dioxide atmosphere to prevent oxidation. 

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