Promoters nitrogen chemisorption, effect

There is evidence of a promoting action of chromium on nickel catalysts for the reaction of hydrogenation of valeronitrile in our conditions. Introduction of chromium increased the initial specific activity and the selectivity. The promoting effect of chromium on activity could be correlated to the increase of the metallic surface. Another explanation could be that the Cr+ segregated at the surface of the catalyst may play the role of a Lewis acid center and may be responsible for a better chemisorption of valeronitrile on the catalysts, through nitrogen lone pair electrons or the n orbital of the CN bond. However, further examination of the results obtained (see Fig. 3)... 

Metals or metal alloys are suitable as ammonia catalysts - especially those metals in the transition-metal group. Metals or metal compounds whose chemisorption energy of nitrogen is neither too high nor too low show the greatest effectiveness. Most catalysts are complex and contain other metal oxides that are hard to reduce. This promotes the activity of metallic iron74. 

Extensive information concerning distribution of the promoters, penetration below the promoters of adsorbed atoms, and chemical behavior of the promoters was obtained by Brunauer and Emmett (25,26). They used chemisorption of carbon monoxide, carbon dioxide, nitrogen, hydrogen, and oxygen, individually and successively measuring the influence of one type of chemisorption upon another type. It was concluded that CO and C02 were chemisorbed as molecules, H2 and N2 as atoms, and 02 probably as ions. C02 is chemisorbed on the alkali molecules located at the surface, whereas H2, N 2, CO, and 02 are chemisorbed on the iron atoms. From the effect of presorbed CO upon the chemisorption of C02 and vice versa it was concluded that the promoters are concentrated on the surface and are distributed so effectively that most surface iron atoms are near to a promoter atom. Strong indication... 

Ozaki et al. (33) compared the rate of ammonia synthesis on a doubly promoted iron catalyst with that of deuteroammonia, and found that deuterium reacts markedly faster than hydrogen imder the same reaction condition. From the kinetic data, as well as the isotope effect, they reached the conclusion that the rate-determining step of the overall reaction is the chemisorption of nitrogen on a surface mainly covered with NH radicals, and that the isotope effect is due to the fact that NH is adsorbed more strongly than ND. 

Alkali promotion is vital for Fe catalysts. The basicity of the surface determines its activity and, in particular, the selectivity toward longer chain hydrocarbons (see the section Control of Selectivity ). Thus, the effect of K2O is much more pronounced than that of the weaker base Li20. The presence of silica, which reacts with the alkali to form less basic silicates, lowers the basicity of alkali-promoted catalysts (20). Thus, the presence of silica in alkali-promoted Fe catalysts lowers the heavy hydrocarbon selectivity. CO2 chemisorption can be used as a measure of the surface basicity. It has been found that chemisorbed nitrogen lowers the amount of CO2 that can subsequently be chemisorbed, which indicates that surface nitrides lower the basicity. This is in agreement with the observation that nitriding iron catalysts results in a lower heavy hydrocarbon selectivity (14). CO2 chemisorption data nevertheless need to be interpreted with care. For instance, promotion with CaO increases the CO2 chemisorption but it has little effect on the FT selectivity. 

One of the earliest studies about the smface compositions of ammonia sjmthesis catalysts is the classical works conducted by means of chemisorption method by Emmett et al. The results are as follows Under the reaction atmosphere, molecular state carbon dioxide is chemisorbed particularly on those potassimn atoms or ions on surfaces, while the atomic state hydrogen and nitrogen, molecular state CO and ionic state oxygen can be used to probe the iron atoms on sm-faces. According to the interactions between the chemisorption of CO and CO2, it can be inferred that the promoters emich on catalyst surfaces and disperse effectively, so as to make the most of iron atoms being closer to promoter atoms. It was found... 

The role of boron in the dissociative chemisorption process (H) may be either to promote adsorption or dissociation (or both). Trigonally coordinated boron is a Lewis acid it can accept the lone pair electrons of nitrogen, promoting NHj adsorption (left-side of scheme H). Low and Ramasubramanian [102] have pointed out that B-O-Si bonds are more easily ruptured than Si-O-Si bonds, thus promoting dissociation (right side of scheme H). The combined effects result in increased nitridation compared to pure silica gels. 

As will be described in the section on kinetics, the rate of ammonia synthesis is a function of the rate of N2 chemisorption, the amount of adsorbed nitrogen (retardation), and the amount of adsorbed hydrogen (retardation). These factors in turn depend on the reaction conditions (temperature, pressure and flow rate) and the nature of the element. Thus, we might change the reaction rate or kinetics on a new active center which is composed of two elements (ensemble effect). A support and a promoter also influence such characteristics and will be described in the next section. However, if an alloy is used as a starting material, and the two elements are separated and turned into an active metal and an inert oxide, then the resulting activity should be classified as a support effect. 

The heat of chemisorption of nitrogen on Ru-K is estimated from the adsorption constant in the above equation to be 167 kJ/mol. It is to be noted, however, that both isotopic equilibration and N2 chemisorption are enhanced by hydrogen on potash-promoted iron catalysts [26]. No enhancement by hydrogen was found on pure iron [181, 182]. Recently, N2 isotopic equilibration was found to be enhanced by hydrogen on rare earth-promoted Ru catalysts [122]. The hydrogen effect on Ru seems to depend on the kind of support and promoter. The guiding principle of this phenomena has not been solved, yet, the study is quite important to construct an effective Ru catalyst under high pressure. 

FIGURE 7.8 Turnover frequency (TOP) of ammonia synthesis as a function of the dissociative chemisorption energy of nitrogen. Top panel Experimental data from Aika et al. (1973). Middle panel Result of the microkinetic model for stepped metal surfaces (blue Une). Reaction conditions are 673 K, 100 bar, Hj N2 ratio of 3 1, and y = 0.1. The effect of potassium promotion has been included (red Une). Effects of promotion will be discussed in Chapter 12. Lower panel Microkinetic model using a two-site model for the adsorption of intermediates. Adapted from Vojvodic et al. (2014). 

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