Model catalysts metal single crystals

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

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... 

Model studies on single crystal surfaces are also helpful in developing an understanding of the effects of surface additives on catalyst performance. Electronegative, electroneutral (i.e. metals) and electropositive additives can all be studied. The influence of additives on the bond strengths and structure of... 

Wax et al. have studied the hydrogenolysis of ethane over W(100) (79). This surface is itself not active in this reaction until a monolayer of surface carbide is formed, after which an active and very stable model catalyst results. In general, the activity of cleaned metal single crystals for this reaction increased in the order W < Ni < Ru < Ir at the same conditions (573 K, 1 torr ethane, 100 torr H2) (72, 75, 79, 80). 

There are, of course, some limitations. An obvious one is that this method cannot be applied yet to the preparation of industrial catalysts, another one is its cost because ultra-high-vacuum (UHV) equipment is required. This drawback explains why this method is usually coupled to surface techniques such as XPS, UPS, RHEED, and AES, which also require UHV. The last disadvantage is that the best suited supports are those that are flat, i.e., oxide single-crystal faces, oxides produced by oxidation of a metal single crystal, or compressed powder oxides. There have been several examples where the preparation chamber also serves as sample chamber for surface techniques and is coupled to a catalytic reactor. Whereas there are a number of works using this approach for bulk metals (80), there are, by contrast, few studies dealing with metals supported on either single crystals (81) or polycrystalline supports (78, 79,82, 83). The latter type of system appears to be the model catalysts closest to the real catalyst. 

Introducing some of the real-life complexities of a working catalyst into more abstract modeis has been a distinguishing feature of work in the Department of Chemical Physics to date. In the above case study, it was the size of the nanoparticle and the presence of its oxide support that were key to developing an understanding of the elementary processes involved. A metal single crystal was too simple a model. 

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

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