Ammonia synthesis surface science

D.A. Rudd, L.A. Apuvicio, J.E. Bekoske and A.A. Trevino, The Microkinetics of Heterogeneous Catalysis (1993), American Chemical Society, Washington DC]. Ideally, as many parameters as can be determined by surface science studies of adsorption and of elementary steps, as well as results from computational studies, are used as the input in a kinetic model, so that fitting of parameters, as employed in Section 7.2, can be avoided. We shall use the synthesis of ammonia as a worked example [P. Stoltze and J.K. Norskov, Phys. Rev. Lett. 55 (1985) 2502 J. Catal. 110 (1988) Ij. 

In comparison to most other methods in surface science, STM offers two important advantages STM gives local information on the atomic scale and it can do so in situ [51]. As STM works best on flat surfaces, applications of the technique in catalysis concern models for catalysts, with the emphasis on metal single crystals. A review by Besenbacher gives an excellent overview of the possibilities [52], Nevertheless, a few investigations on real catalysts have been reported also, for example on the iron ammonia synthesis catalyst, on which... 

N2 and CO, respectively [31,32], Empirical knowledge about the promoting effect of many elements has been available since the development of the iron ammonia synthesis catalyst, for which some 8000 different catalyst formulations were tested. Recent research in surface science and theoretical chemistry has led to a fairly complete understanding of how a promoter works [33,34],... 

For surface science studies of ammonia synthesis, see also G. Ertl, J. Vac. 

Ertl G. Elementary steps in ammonia synthesis the surface science approach. In Jennings JR, editor. Catalytic ammonia synthesis fundamentals and practice, fundamental and applied catalysis. New York Plenum Press 1991. p. 

The addition of potassium to industrial Fe catalysts leads to an increase in activity for ammonia synthesis (N2 -I- 3H2 - 3NH3) (136). This promotion effect has been the subject of considerable attention from the surface science community, particularly with regard to the coadsorption of K or K + O and N2 (136-139). Ertl and co-workers have shown that potassium addition to single-crystal Fe surfaces can lead to a 10- to 100-fold enhancement in the rate of dissociative N2 adsorption, which is thought to be the rate-determining step in NH3 synthesis (136-139). However, Bare et al. (140) were unable to promote the activity of Fe(l 11), (100), or (110) surfaces for this reaction at 20-atm pressure with either K, K + O, or K + AlO, addition. They interpreted this result to indicate that the promotional role of K in industrial catalysis may be cooperation with other promoters, such as the support material, to cause structural rather than electronic promotion. These results were for very low conversions, however, so that the product (NH3) partial pressure was low. Strongin and... 

Surface science studies of thin films may be very helpful for the understanding of the mechanisms of heterogeneous catalysis on intermetallics. This was true in particular for the AES study of the Ru(0001)-Ce-H2 interface performed by Walker and Lambert (1992) in the context of ammonia synthesis or the growth of cerium films on polycrystalline rhodium (Warren et al. 1993) on top of which carbon monoxide oxidation was performed. 

In the kinetic modelling of catalytic reactions, one typically takes into account the presence of many different surface species and many reaction steps. Their relative importance will depend on reaction conditions (conversion, temperature, pressure, etc.) and as a result, it is generally desirable to introduce complete kinetic fundamental descriptions using, for example, the microkinetic treatment [1]. In many cases, such models can be based on detailed molecular information about the elementary steps obtained from, for example, surface science or in situ studies. Such kinetic models may be used as an important tool in catalyst and process development. In recent years, this field has attracted much attention and, for example, we have in our laboratories found the microkinetic treatment very useful for modelling such reactions as ammonia synthesis [2-4], water gas shift and methanol synthesis [5,6,7,8], methane decomposition [9], CO methanation [10,11], and SCR deNO [12,13]. 

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... 

Surface-science studies succeeded to identify many of the molecular ingredients of surface catalyzed reactions. Each catalyst system that is responsible for carrying out important chemical reactions with high turnover rate (activity) and selectivity has unique structural features and composition. In order to demonstrate how these systems operate, we shall review what is known about (a) ammonia synthesis catalyzed by iron, (b) the selective hydrogenation of carbon monoxide to various hydrocarbons, and (c) platinum-catalyzed conversion of hydrocarbons to various selected products. 

G. Ertl. Elementary Steps in Ammonia Synthesis The Surface Science Approach. In J.R. Jennings, editor. Catalytic Ammonia Synthesis Fundamentals and Practice, Fundamental and Applied Catalysis. Plenum Press, New York, 1991. 

J G. Ertl. Surface Science and Catalysis—Studies on the Mechanism of Ammonia Synthesis The P.H. Emmett Award Address. Catal. Rev. Sci. Eng. 21 201 (1980). 

P. Stolze. Surface Science as the Basis for the Understanding of the Catalytic Synthesis of Ammonia. Phys. Scr. 36 824 (1987). 

P. Stolze and J.K. Norskov. A Description of the High-Pressure Ammonia Synthesis Reaction Based on Surface Science. J. Vacuum Sci. Technol. A 5 581 (1987). 

E. A. Quadrelli. Dinitrogen Dissociation on an Isolated Surface Tantalum Atom, Science 317 (2007) 1,056-1,060. This research article describes an alternative route to ammonia synthesis that possibly is more efficient and less expensive than the traditional Haber-Bosch process. 

In 1993, almost 20 years later, another symposium was held in the honor of H. Topsoe and A. Nielsen, two of the leaders in industrial ammonia synthesis, and there, a general agreement was reached that the essential aspects of the kinetics and mechanism of this process are now more or less understood (20). This progress has essentially to be attributed to the large number of experimental studies applying surface physical techniques to well-defined, single-crystal surfaces (the surface science approach) that in combination with quantum-chemical calculations and theoretical treatment of the kinetics of the various elementary reactions (microkinetics) provided a rather closed picture. 

We will provide the reader with an introduction to fundamental concepts in catalytic reactivity and catalyst synthesis derived from the results of computational analysis along with physical and chemical experimental studies. The tremendous advances in nanoscale materials characterization, in itu spectroscopy to provide atomic and molecular level resolution of surfaces and adsorbed intermediates under reaction conditions, predictive ab initio quantum mechanical methods and molecular simulations that have occurred over the past two decades have helped to make catalysis much more of a predictive science. This has significantly enhanced the technology of catalysis well beyond the historical ammonia synthesis and petrochemical processes. 

S. Dahl, J. Sehested, C. Jacobsen, E. Tornqvist, and I. Chorkendorff, Surface science based microkinetic analysis of ammonia synthesis over ruthenium catalysts, Journal of Catalysis, vol. 192, pp. 391-399,2000. 

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