The Surface Science Approach

This effect is a consequence of the fact that the energy released by this process is preferentially channeled into the separation y parallel to the surface (cf. Fig. 4.4). A simple estimate reveals that the time necessary to damp this excess energy of the hot adatoms into the heat bath of the solid is of the order of 10-12 s, 

Further inspection of Fig. 4.5 demonstrates that the O atoms are surrounded by dark zones. The STM technique probes not only the atomic topography but also the electronic structure, and the dark zones reflect the modification of the local electronic density in the vicinity of the adsorbates, this modification being responsible for the operation of indirect interactions (which may be either repulsive or attractive) between adsorbed particles mediated through the substrate. 

The Atomic Mechanism of a Catalytic Reaction Oxidation of Carbon Monoxide 

The oxidation of carbon monoxide, 2CO + 02 — C02 is one of the reactions (apart from reduction of nitric oxides and oxidation of unbumt hydrocarbons) taking place in the car exhaust catalyst. The latter consists of small noble metal particles (Pt, Rh or Pd) on a ceramic support. The reaction proceeds through the following steps [25] 

The situation is quite different for oxygen adsorption. The Oad atoms form the relatively open mesh of a 2 x 2 structure (Fig. 4.9b) where these are located in threefold coordinated sites separated from each other by 2a0 = 0.56 nm. At this stage further uptake of oxygen is pronouncedly slowed down, but the surface is still read- 

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How are fiindamental aspects of surface reactions studied The surface science approach uses a simplified system to model the more complicated real-world systems. At the heart of this simplified system is the use of well defined surfaces, typically in the fonn of oriented single crystals. A thorough description of these surfaces should include composition, electronic structure and geometric structure measurements, as well as an evaluation of reactivity towards different adsorbates. Furthemiore, the system should be constructed such that it can be made increasingly more complex to more closely mimic macroscopic systems. However, relating surface science results to the corresponding real-world problems often proves to be a stumbling block because of the sheer complexity of these real-world systems. 

This chapter will explore surface reactions at the atomic level. A brief discussion of corrosion reactions is followed by a more detailed look at growth and etchmg reactions. Finally, catalytic reactions will be considered, with a strong emphasis on the surface science approach to catalysis. 

Surface science studies of catalytic reactions certainly have shed light on the atomic-level view of catalysis. Despite this success, however, two past criticisms of the surface science approach to catalysis are that the... 

The surface science approach (Eq. 5.14) has the important advantage that both d>and are measurable quantities. This is not the case for the electrochemical approach (Eq. 5.15) since neither the chemical potential p nor the Galvani potential tp are measurable quantities. Only changes in tp are measurable. 

In this chapter, we have summarized (recent) progress in the mechanistic understanding of the oxidation of carbon monoxide, formic acid, methanol, and ethanol on transition metal (primarily Pt) electrodes. We have emphasized the surface science approach employing well-defined electrode surfaces, i.e., single crystals, in combination with surface-sensitive techniques (FTIR and online OEMS), kinetic modeling and first-principles DFT calculations. 

Nieuwenhuys, B.E. (2000) The surface science approach toward understanding automotive exhaust conversion catalysis at the atomic level, Adv. Catal. 44, 35. 

The strength of the surface science approach is that it can address the molecular details of catalytic issues by pooling information from a battery of specific analytical spectroscopies and techniques [174], As more complex model systems are developed, the wealth of characterization techniques available in vacuum environments can be used to better understand catalysis. 

In recent years the surface science approach has led to a dramatic increase in our knowledge of the surface chemistry and kinetics. Processes that have been studied using the surface science approach include the deposition and etching of semiconductors (e.g., Si, Ge,... 

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. 

At present we are beginning to understand the reaction mechanisms of many heterogeneous catalytic reactions at the molecular level. A major breakthrough came with the design of catalytic model systems, such as single crystal surfaces, enabling exhaustive structural characterization and model catalytic experiments. The surface science approach forms the basis of current developments of surface chemical reaction rate theory. 

The Surface Science Approach Toward Understanding Automotive Exhaust Conversion Catalysis at the Atomic Level... 

Surface science offers many opportunities in catalysis research because a variety of techniques are available to characterize in detail the composition and structure of the catalyst surface and to identify the adsorbed species. A frequent criticism of the surface science approach is that it is far removed from real catalysis since most of the surface science techniques can only be applied at low pressures and with model catalysts, often single-crystal surfaces. The so-called pressure gap has been bridged by combining, in the same apparatus, the techniques needed for surface analysis and characterization with the ability to measure reaction rates at elevated pressures. In addition, many techniques can also be apphed in situ at elevated pressures. 

With a few exceptions, most research applying the surface science approach to study ceria based systems has been performed in about the last five years. As evidence for this statement consider that in a 1995 book which presents a comprehesive review of surface science of oxide surfaces only a single reference to cerium oxide surfaces is cited or that in a 1997 review of structural, electronic and chemisorptive properties of metal films and particles on oxide surfaces there are six references to ceria surfaces or that in a 1998 review of surface studies of supported model catalysts there are only six references to ceria as a support. The... 

To date the surface science approach and techniques such as those described above have been used to study structure of ceria surfaces, the adsorption and desorption of several molecular species on ceria and model ceria supported catalysts, and the co-adsorption and reaction of certain of these molecular species. The results provide a basis for clarifying the elementary reaction steps underlying catalytic processes occurring on ceria based catalysts. In this Chapter it is attempted to review and summarize this research. 

The emergence in the 1960s of ultra-high-vacuum techniques, structural studies through LEED and surface sensitive spectroscopies became recognised as the surface science approach to catalysis, answering many of the issues raised in Taylor s paper. It was, however, the last paragraph of his paper that made a particular impression on me ... 

Returning to the mechanism of the CO + NO reaction, we can now list the kinetic parameters for several of the elementary steps, see Table 5.3. As the mechanism of any catalytic reaction is inevitably a sequence of several steps, the surface science approach for studying the kinetics of elementary steps is vitally important, because parameters such as those listed in Table 5.3 form the highly desirable input for the modeling of more complex reaction mechanisms. 

The Surface Science Approach In Situ Vapor Deposition Methods... 

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