Ammonia synthesis structure sensitivity

Surface Science of Ammonia Synthesis Structure Sensitivity of Ammonia Synthesis Kinetics of Dissociative Nitrogen Adsorption Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis Characterization of the Restructured Surfaces Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV... 

Structure Sensitivity over Pe. Table II presents the rates of ammonia synthesis over each of the low Miller index planes of Pe. 

Structure Sensitivity over Re. As in the case of the Fe catalysts the rate of ammonia synthesis varies greatly over Re single crystal surfaces of different orientations. This phenomenon has been studied over the (0001), (loTo), (1120) and 0121) planes in a 3 1 Hp/N mixture at a total pressure of 20 atm. and a temperature or 870 K. Under these conditions these surfaces catalyze the reaction with relative rates of 1 94 920 2820 respectively, showing a range of activities even greater than that observed on Fe. 

The studies of ammonia synthesis over Fe and Re and the hydrodesulfurization of thiophene over Mo, described above, illustrate the importance and success of our approach of studying catalysis over single crystal samples at high pressures. The use of surfaces having a variety of orientations allows the study of reactions that are surface structure sensitive 6Uid provides insight into the nature of the catalytic site. Here we have shown that the ammonia synthesis... 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

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... 

Reactions Exhibiting Strong Structure Sensitivity. Reactions for which there is at least an order of magnitude difference in activity as a function either of particle size or of exposed crystal planes include the ammonia synthesis reaction and the hydrogenolysis of hydrocarbons. 

The structure sensitivity of catalytic reactions is often striking. As an example, Spencer et al. [5] demonstrated a factor of over 400 difference between the activity of the Fe (111) surface and that of the Fe (110) surface (the former being the more active one) for ammonia synthesis. This type of investigation, while of course not predictive in nature, has the promise... 

The application of the fusion process can lead to a control over structure-sensitive reactions for unsupported catalysts. The prototype example for such a catalyst is the multiply-promoted iron oxide precursor used for ammonia synthesis. In Section B.2.1.1 a detailed description is given of the necessity for oxide fusion and the consequences of the metastable oxide mixture for the catalytic action of the final metal catalyst. 

Of the group of metals forming stable nitrides, only molybdenum is of some interest. Under synthesis conditions it is present as a nitride with some ammonia formation activity and structural sensitivity [393], Molybdenum also seems to exhibit activity in biological nitrogen fixation [394] and is synthetically active at ambient conditions in the air-sensitive Glemser compounds [394]. 

The synthesis of ammonia over both Fe and Re single-crystal surfaces has been studied by the Somoijai group and found to be extremely structural sensitive (83-85). The activity of iron increases in the order (110) < ... 

Fig. 7. Structure sensitivity in the ammonia synthesis over rhenium single-crystal faces. The turnover frequencies (TF) are given as NH3 molecules/cm2/s (PIotai = 20 atm, H2 N2 = 3 1, Tcryslai = 870 K). Schematics of the atomic structure of each surface are given above each bar. From Ref. 85.

The reaction of H2 and N2 to form ammonia is a reaction for which an enormous literature exists, dating to the beginning of the century. Recent reviews are those of Boudart (309) and Ertl (310). Industrial fused iron catalysts present iron particles of 25 nm or higher, and it has not been possible to prepare well-reduced small iron particles (1-10 nm) on conventional silica and alumina supports. Dumesic et al. (311a-c) were able to prepare iron crystallites of diameters from 1.5 to 30 nm by using MgO as the support. The state of the iron was monitored by Mossbauer-effect spectroscopy. The synthesis rates were measured from 300 to 405°C at atmospheric pressure the conversion was small enough so that the reverse reaction could be neglected. It was found that the reaction showed antipathetic structure sensitivity. 

We have seen how studies on the various faces of Fe and Ni have contributed to the understanding of the structure sensitivity of the ammonia synthesis and of the hydrogenolysis of alkanes. Clearly, the effect of crystal faces, steps, and kinks on other reactions should be pursued. In some cases, kinetics can be measured at vacuum conditions, permitting the use of transient methods. The example of CO oxidation is striking (315). 

Ruthenium has long been known to be an effective catalyst for ammonia synthesis. However, compared to the traditional iron-based catalysts, studies on ruthenium-based catalysts are limited. The rate determining step of ammonia synthesis, the dissociative adsorption of dinitrogen, has been shown to extremely structure sensitive on both iron and mthenium catalysts. To study this structure sensitivity on ruthenium, density functional theory calculations were performed on Ru(OOl) and Ru(llO) clusters. End-on, side-on, and dissociated adsorption states were investigated on both surfaces. While the Ru(llO) cluster could stabilize aJl three adsorption modes, a minimum energy structure for the side-on adsorption on Ru(OOl) could not be found. It is likely that this side-on mode can provide a low energy pathway to the dissociated state, thereby resulting in faster dissociative adsorption on Ru(llO). 

Ammonia Synthesis. - Ammonia (NH3) synthesis is another well-known structure-sensitive reaction. The two most widely accepted mechanisms involve adsorption of nitrogen as the rate limiting step. A main question has been whether nitrogen adsorption is dissociative [37,38] or associative [38]. For NH3... 

Structure Sensitivity of Ammonia Synthesis An ultrahigh vacuum chamber equipped with a high-pressure cell was developed to study the ammonia synthe-... 

Figure 2.6. Top figure Surface structure sensitivity of iron catalysed ammonia synthesis.

It is found in many reactions that a particular surface is favored. For example the (111) surface is particularly active in fee and hep metals. A strong dependence is foimd for ammonia synthesis on iron catalysts (Table 5-13) [T35]. Ammonia synthesis is one of the most structurally sensitive reactions. The opposite order was foimd for the decomposition of ammonia on copper, i.e., (111)>(100). In the decomposition of formic acid, the (111) surface is three times more active than (110) or (100). 

Single crystal studies based on pure Fe show that the ammonia synthesis is a highly structure sensitive reaction (36). Different crystal planes display pronounced variations in reaction rate, and by studies of the most densely packed surfaces the difference in reactivity was found to decrease by more than two orders of magnitude in the sequence (111) > (100) > (110) (37). 

The carbides and nitrides were also found to be active in ammonia decomposition. The activities of vanadium carbides were 1-2 orders of magnitude lesser than that of Mo nitride catalyst, but 1-2 orders of magnitude higher than that of a Pt/C catalyst (138). Vanadium carbides were 1 order of magnitude more active than vanadium nitrides. Both the synthesis and decomposition of NH3 were found to be structure-sensitive reactions over carbide and nitride catalysts with activities depending strongly on the particle sizes of the catalysts. 

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