Surface Structure of Catalysts

While understanding the bulk structure of a catalyst is of obvious importance it is often desirable to acquire information selectively about the nature of the catalyst surface. Such informahon may include the nature of any acid sites present, or differences in the coordinahon of atoms at the surface as compared to those in the bulk. NMR is ideally suited to such studies owing to its inherent sensitivity to the local environment of the nuclei under observahon. 

---

Somorjai, G. A., Introduction to Surf ace Chemistry and Catalysis, Wiley, New York, 1994. (Undergraduate level. This in-depth treatment of surface chemistry and catalysis brings the experience and perspectives of a pioneer in the field to the general audience. The book is meant to be an introductory-level description of modern developments in the area for students at the junior level. However, it is also an excellent source of the current literature and contains numerous, extensive tables of data on kinetic parameters, surface structure of catalysts, and so on. Chapter 3, Thermodynamics of Surfaces, and Chapter 7, Catalysis by Surfaces, cover information relevant to the present chapter. Chapter 8 discusses applications in tribology and lubrication (not discussed in this chapter).)... 

Although addition of alkali metal reduced the catalytic activity of Fe UFP for FT synthesis, catalyst deactivation was suppressed by alkali promotion. Figure 2 shows the average STY s of hydrocarbons, oxygenates, and CO2 over the Fe UFP catalysts promoted by various kinds of alkali metals in a comparison with the precipitated catalyst. These data were taken for the products in the initial 6 hr of run. The activities of UFP catalysts were higher than that of the ordinary K-promoted Fe precipitation catalyst, in spite of comparable surface areas. This is interpreted as due to an effect of surface structure of catalyst. In the case of the precipitated catalyst having a rather porous structure compared with UFP, the reactant diffuses into the pores and reacts on the catalyst surface. If the reaction is faster than diffusion processes, the concentration of reactant falls along with the distance from the pore mouth. Thus, a limited portion of the surface of the precipitated catalyst can be used for reaction (ref. 7). 

The decision of K.H. Meyer to engage in basic research was probably influenced by the example of the pioneering work of Irving Langmuir at General Electric, especially studies on the surface structure of catalysts. ... 

Electrical current is generated at highly dispersed catalyst nanoparticles. Atomistic surface structures of catalyst particles determine their specific activities. Detailed models in nanoparticle electrocatalysis have to distinguish the contributions of distinct surface sites to the relevant electrocatalytic... 

When relating the surface nature and surface structure of catalyst according to the adsorption capacity of solid surface, it is indispensable to understand the relationship between the probe-molecule and the metal elements on surface, including adsorption species and stoichiometry of the probe-molecule, both of which are closely related. 

Many information of surface structure of catalyst can be obtained by surface EXAFS method. It can be seen from the following examples that this information of the structure is often related to a more sophisticated molecular or atomic scale features. 

Ambient pressure XPS permits in situ investigation of the surface structure of catalysts. Catalytic oxidation, reduction, and CO oxidation were carried out on Ru nanoparticle arrays and the surface oxidation states were measured and monitored using AP-XPS to understand the relationship between the oxidation states and catalytic activity under realistic conditions [48]. AP-XPS showed that the smaller Ru nanoparticles form bulk RuOa on the surface, which is responsible for the lower catalytic activity. 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

STM has particularly great potential for in situ chemical studies. While our present knowledge of the atomic structure of catalyst surfaces is largely limited to those structures which are stable in ultra-high vacuum before and after reaction, STM may provide an insight into both adsorbate and catalyst surface structure in situ during the reaction. The following issues to be characterized by STM may be most relevant to characterization of catalysts and catalysis ... 

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

The development of experimental methods over the last 50 years has been at the forefront of new strategies that emerged, driven by the need to obtain molecular information relevant to the structure of catalyst surfaces and the dynamics of surface reactions. The ultimate aim was in sight with the atomic resolution that became available from STM, particularly when this was coupled with chemical information from surface-sensitive spectroscopies. 

In HRTEM, very thin samples can be treated as weak-phase objects (WPOs) whereby the image intensity can be correlated with the projected electrostatic potential of crystals, leading to atomic structural information. Furthermore, the detection of electron-stimulated XRE in the electron microscope (energy dispersive X-ray spectroscopy, or EDX, discussed in the following sections) permits simultaneous determination of chemical compositions of catalysts to the sub-nanometer level. Both the surface and bulk structures of catalysts can be investigated. 

Promoters are generally divided in two classes. Structural promoters help to stabilize certain surface structures of the catalyst, or to prevent sintering. Structural promoters are not involved in the catalytic reaction itself and have no interaction with the reacting species. Chemical promoters, on the other hand, directly influence the reacting species on the surface of the catalyst. Obviously, alkali promoters fall into the latter category. 

Au(OH) c. Additionally, silica does not stabilize goldNPs against agglomeration. It appears that, on silica, one of the primary roles of Pt is to help stabilize small particles. Pt may also add additional functionality to the catalyst by binding O2 and locating bound or activated O2 near active Au sites. Alternately, the presence of Pt may affect the surface structure of Au, helping it to adopt a more active geometry. 

Duplication of the chemical constitution of a good catalyst is no guarantee that the solid produced will have any catalytic activity. This observation suggests that it is the physical or crystalline structure which somehow imparts catalytic activity to a material. This view is strengthened by the fact that heating a catalyst above a certain critical temperature may cause it to lose its activity, often permanently. Thus present research on catalysts is strongly centered on the surface structure of solids. 

---