Transition metal surface composition

The catalytic activity of samples as prepared appeared to be higher than the activity of samples prepared by ceramic method [121]. The specific catalytic activity in CO and butane was found to be a function of the calcination temperature with its maximum at 700°C. Probably, the reason consists in the differences of chemical composition and the structure of the surface. SIMS data have demonstrated that middle- and high-temperature samples differ by the transition metal surface concentration. In some cases, high level of activity of middle-temperature defective samples can be assigned to segregation of Co ions on the surface of their particles. 

Theoretical methods are required to derive structural information from spectroscopic data, which usually concern measurements of electronic features. Because of the availability of large and efficient computer power and the current state of the art of theoretical chemistry, electronic structure calculations on model systems of relevance to experimental studies can be made. In addition, the catalytic chemist needs insight into the factors that determine the transition-state potential energy surface of reacting molecules. Also methods are needed to predict the geometry of the adsorption site as a function of metal surface composition or charge distribution in the zeolite. These methods will be extensively discussed in the next chapters. 

The final section in Chapter 2 deals with the molecular asp>ects of transition-metal catalysis. It serves as an introduction to Chapter 3. A characteristic feature of the transition-metal surfaces under catalytic conditions is their potential to restructure. Adsorbate overlayer adsorption can induce the surface to reconstruct with rapid diffusion of the metal as well as the overlayer atoms. The state of the surface may start to resemble that of a solid state compound. The state of the surface is not only strongly influenced by the composition of the reactant gas, but can also be strongly affected by the addition of promoters or other modifiers, that can result in alloy formation or new complex surface phases. 

Surface reconstruction is inherent to surface oxidation and sulfidation chemistry. In involves essentially surface corrosion and surface compound formation phenomena. The state of a surface can change from a metallic state to that of a solid oxide, sulfide, carbide or nitride depending upon the reaction environment. The surface of the epoxidation catalyst, discussed earlier, in the absence of Cl or Cs, for example, has a composition similar to AgO in the oxidizing reaction environment of the epoxidation system. The oxidation of CO over Ru can readily lead to the formation of surface RUO2 (see Chapter 5). In desulfurization reactions the transition-metal surface is converted to a sulfide form. The reactivity of the surface in these systems begins to look chemically more similar to that of coordination complexes. This we will illustrate in Chapter 5 for the C0S/M0S2 system. 

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

It is necessary to note the limitation of the approach to the study of the polymerization mechanism, based on a formal comparison of the catalytic activity with the average oxidation degree of transition metal ions in the catalyst. The change of the activity induced by some factor (the catalyst composition, the method of catalyst treatment, etc.) was often assumed to be determined only by the change of the number of active centers. Meanwhile, the activity (A) of the heterogeneous polymerization catalyst depends not only on the surface concentration of the propagation centers (N), but also on the specific activity of one center (propagation rate constant, Kp) and on the effective catalyst surface (Sen) as well ... 

It is evident [see Eq. (5), Section II[] that for catalysts of the same or similar composition the number of active centers determined must be consistent with the catalytic activity it can be expected that only in the case of highly active supported catalysts a considerable part of the surface transition metal ions will act as propagation centers. However, the results published by different authors for chromium oxide catalysts are hardly comparable, as the polymerization parameters as a rule were very different, and the absolute polymerization rate was not reported. 

Stamenkovic V, Mun BS, Blizanac BB, Mayrhofer KJJ, Ross PN Jr, Markovic NM. 2006a. The effect of surface composition on electronic structure, stability and electrocehmical properties of Pt-transition metal alloys Pt-skin vs. Pt-skeleton surfaces. J Am Chem Soc 137 1. 

The second and third method allow the measurement of surface complexation constants at various transition metal loadings and consequently yield apparent composition dependent constants. In the first method on the contrary a truly thermodynamic constant is obtained under standard state conditions. 

Figure 18. A comparison of (a) Fe 2p, and (b) Cu 2p and Zn 2p core level features on fresh and after two different stages of reaction on Cu0.5Zn0.5Fe204 catalyst. Note a drastic change in the transition metal composition accompanied with some reduction after different stages of reaction on the surface. Reprinted from Journal of Catalysis, 241, Vijayaraj M., et al, 2006, 83-95 with permission from Elsevier.

By applying this technique, it is not only possible to prepare relatively well-defined catalysts that may be alloys of a given composition but also catalysts in which adatoms of main group elements may be located on the surface of transition metal particles or organometallic fragments that are likely adsorbed (coordinated) at some particular crystallographic positions of the metallic particles. Each of these three different types of materials exhibits interesting and unusual selec-tivities in many catalytic reactions [33, 34]. 

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