Ammonia synthesis activation methods

Bridger, G.W. Snowden, C.B. (1970) Ammonia synthesis catalysts. In Catalyst Handbook. Wolfe Scientific Books, 126-147 Brindley, G.W. Bish, D.L. (1976) Green rust a pyroaurite type structure. Nature 263 353 Bromfield, S.M. Williams, E.G. (1963) An examination of the biological reduction method for estimating active iron in soils. J. Soil Sd. 14 346-359... 

Reactions which may occur on sites consisting of one or two atoms only on the surface of the catalyst are generally known as facile reactions. Reactions involving hydrogenation on metals are an example. Eor such reactions, the state of dispersion or preparation methods do not greatly affect the specific activity of a catalyst. In contrast, reactions in which some crystal faces are much more active than others are called structure sensitive. An example is ammonia synthesis (discovered by Fritz Haber in 1909 (Moeller 1952)) over Fe catalysts where (111) Fe surface is found to be more active than others (Boudart 1981). Structure-sensitive reactions thus require sites with special crystal structure features, which... 

An exhaustive study has been carried out recently on the synthesis of BN nanotubes and nanowires by various CVD techniques.17 The methods examined include heating boric acid with activated carbon, multi-walled carbon nanotubes, catalytic iron particles or a mixture of activated carbon and iron particles, in the presence of ammonia. With activated carbon, BN nanowires are obtained as the primary product. However, with multi-walled carbon tubes, high yields of pure BN nanotubes are obtained as the major product. BN nanotubes with different structures were obtained on heating boric acid and iron particles in the presence of NH3. Aligned BN nanotubes are obtained when aligned multi-walled nanotubes are used as the templates (Fig. 40). Prior to this report, alignment of BN nanotubes was achieved by the synthesis of the BN nanotubule composites in the pores of the anodic alumina oxide, by the decomposition of 2,4,6-trichloroborazine at 750 °C.116 Attempts had been made earlier to align BN nanotubes by... 

The rate used here is what has been called the nominal turnover rate (4), moles reacted per second per mole of sites, taken as surface atoms for metal catalysts. These sites are measured, for example, by hydrogen chemisorption, electron microscopy, X-ray diffraction, and magnetic methods. Of course, only a fraction of these sites may be active, but this fraction has to be learned by kinetic experiments and is subject to change as new kinetic results become available (6). This fraction seems to be known for ammonia synthesis over iron and can be measured by nitrogen chemisorption. 

The ammonia synthesis reaction is one of the most studied and best understood reactions in heterogeneous catalysis, but it has been the subject of few papers involving transient methods. SSITKA experiments have been performed at 350-500°C and 204-513 kPa using a commercial Haldor-Tops0e KMIR catalyst, with iron triply promoted by AI2O3, K2O, and CaO (262). Similar studies using K-promoted Ru/Si02 have also been reported 263). The promoted Ru catalyst is much more active than Ru alone, and new, very active sites are detected on the promoted catalyst. It seems that the analysis of this type of experiment would benefit from the elementary-step approach, as exemphfied by Kao et al. 107) two kinds of sites can be included in such a model. 

Exhaustive studies have been carried out on the synthesis of BN nanotubes and nanowires by various CVD techniques [225]. The methods examined include heating boric acid with activated carbon, multi-walled carbon nanotubes, catalytic iron particles or a mixture of activated carbon and iron particles, in the presence of ammonia. With activated carbon, BN nanowires are obtained as the primary prod-... 

The preparation of Ru supported catalysts by sol-gel method, indeed, was extended to obtain new formulations by changing the type of support. Alkali-promoted Ru/MgO systems were prepared starting from magnesium ethoxide, Ru3(CO)i2 and a cesium compound [9]. The gels were subjected to an activation/reduction procedure to substantially obtain Ru-CsOH/MgO and then tested as catalysts in the ammonia synthesis at atmospheric pressure. It was evidenced that the sol-gel prepared Cs-promoted Ru/MgO catalysts are much more active, under similar reaction comlitions, than the analogous catalysts prepared by the impregnation procedures reported in literature [10]. 

The reverse of reaction (2.1) is methanation. Used to remove residual CO traces from ammonia synthesis feedstocks, it was also developed as an important source of substitute natural gas (SNG) in the synthetic fuels industry. Since this reaction is exothermic, equilibrium yields are better at low temperatures (300-500 C). Thus, high activity is critical. Nickel must be highly dispersed. Preparational methods are required to produce small nickel crystallites. This high metal area must be maintained in the presence of extreme exothermicity, so that sintering must be avoided. This is partially accomplished through proper catalyst design, but process reactor type must also be considered. Recycle, fluidized, and slurry reactors are appropriate. 

Their results showed that the catalytic activity for Fe (111) crystal face is 430 times higher than that of Fe (110) crystal face and 13 times higher than that of Fe (100) crystal face for ammonia synthesis. The exposed crystal face for industrial iron catalysts for ammonia synthesis mainly is Fe (111). Their results also showed that the catalytic activity is much higher over the flat hexagonal crystal face than over tetragonal crystal face of platinum for the reaction of normal heptane dehydrogen and cyclization to form toluene. These results confirm that the concept of structure suitability and sensitivity are valid. It is believed that once the dynamic study methods are widely used, it is not far for the explanation of catal Tic reaction mechanism based on the results obtained from dynamic techniques. 

In summary, the precipitation-washing processes can be considered as the new method of preparing the efficient Ru based ammonia synthesis catalyst. It not only significantly increases the catal3dic activity, but also simplifies operation, decreases the complexity of the catalyst preparation technology, and reduces the... 

It is hard to find out the most appropriate equipment/method and performance index to measure and assess the quality of catalysts, which is difficult in unified definition and standardization of methods. So far, some methods for evaluating the activities of a few catalysts such as those for catalytic ammonia synthesis and catalytic cracking have been standardized. The evaluation methods are different due to the varieties of catalysts and the experiences of researchers. Sometimes, the technical details of evaluation methods are a part of patent, such as formula and preparation process is confidential. Even then, some basic conceptions and methods for the evaluation and testing of catalysts are still worthy being introduced, which is the main focus of this chapter. 

Activity determination and calculation. The activity of ammonia synthesis catalyst is commonly expressed by the ammonia concentration of the outlet of reactor, and measured by neutralization method of sulfuric acid solution. The experiment is carried out in the fixed-bed reactor as shown in Fig. 7.4, which can measure four samples in one experiment. The inner diameter of reactors is 12 mm, with size of catalysts being 1.0-1.4 mm and volume being 2 ml. After reduction in the syngas, the outlet concentration of ammonia (NH3 %) is measured at given temperature, pressure and space velocity. System of determining the outlet concentration of ammonia of reactor and the space velocity is shown in Fig. 7.5. 

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