Structure sensitivity of ammonia synthesis

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

Table 1 Surface structure sensitivity of ammonia synthesis...

Fig. 6. The surface structure sensitivity of ammonia synthesis catalyzed by single-crystal iron under high-pressure conditions.

The reader who has followed the arguments up to this point may ask the following questions what is the average typical structure of the activated catalyst and what is the difference between metallic iron and the activated catalyst If the reader is practically minded, he/she may further ask what is the use of this information or, less provocatively, given the well-established structure sensitivity of ammonia synthesis over single crystals, is there a similar relationship between the structure of the technical material and its catalytic usefulness An artist s impression of the main structure features of the technical activated catalyst has been developed, to respond to these questions. 

Figure 4.2. The rate of ammonia synthesis over the five iron surfaces studied exhibiting the structure sensitivity of ammonia synthesis/...

Figure 4.4. The structure sensitivity of ammonia synthesis over rhenium single-crystal surfaces/ Schematics of the atomic structure of each surface is given above each bar.

Table 3.6. Structure sensitivity of ammonia synthesis on Re single crystals...

Table 2.10 Change of a real rate with crystal-lograpic orientation for a structure-sensitive reaction (ammonia synthesis on iron single crystal) ...

A large amount of Mossbauer work has been devoted to in situ surface characterization of iron catalyst. These experiments were carried out, however, using small particles of iron oxide supported on inert carriers. These materials represent a different class of catalysts to the industrial ammonia synthesis catalyst. The Mossbauer studies which has helped to evaluate the concept of structure sensitivity in ammonia synthesis will therefore not be discussed here. 

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... 

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... 

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). 

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

Fig. 39. The remarkable surface structure sensitivity of the iron-catalyzed, ammonia synthesis...

The platinum group metals demonstrate clearly the impact of the key parameters that influence the efficiency of ammonia synthesis catalysts structure sensitivity, the heat of adsorption of reactants and products, and the roles of promoters and supports. The key requirement to minimize the activation energy for nitrogen dissociation limits the active metals to alkali-promoted ruthenium and osmium. Similar activities are then obtained, irrespective of the genesis of the alkali metal promoter (salt or metal), indicating that both convert to a similar... 

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