Single surface reaction.measurement

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. 

The presence (or absence) of pore-diffusion resistance in catalyst particles can be readily determined by evaluation of the Thiele modulus and subsequently the effectiveness factor, if the intrinsic kinetics of the surface reaction are known. When the intrinsic rate law is not known completely, so that the Thiele modulus cannot be calculated, there are two methods available. One method is based upon measurement of the rate for differing particle sizes and does not require any knowledge of the kinetics. The other method requires only a single measurement of rate for a particle size of interest, but requires knowledge of the order of reaction. We describe these in turn. 

Weisz-Prater criterion. The relative significance of pore-diffusion resistance can be assessed by a criterion, known as the Weisz-Prater (1954) criterion, which requires only a single measurement of the rate, together with knowledge of De, Le, and the order of the surface reaction (but not of the rate constant). 

The Clean Single-Crystal-Surface Approach to Surface Reactions N. E. Farnsworth Adsorption Measurements during Surface Catalysis Kenzi Tamaru... 

The values of As and s are not necessarily identical with, or to be identified as those calculated from, measurements for the overall reaction, A and E, since the latter are composite terms that may include contributions from the temperature dependences of c, c2, and /4S, as described in Appendix I. The surface reaction is not completely represented by the consideration of this single step (the surface collision) and rate expressions should be more realistically regarded as the resultant of several contributory factors in the sequence of interdependent (55, 119,120) processes required to convert the reactants into products. In general, the overall surface reaction is composite kinetic behavior and thus more complicated than many of the homogeneous processes that have attracted greatest interest. In the heterogeneous reactions,... 

The rate of a structure sensitive reaction catalyzed by a metal single crystal is also a function of the exposed plane. As illustrated in Figure 5.1.3, the low-index planes of common crystal structures have different arratigctncnls of surface atoms. Thus, caution must be exercised when the rates of structure sensitive reactions measured on single crystals are compared to those reported for supported metal particles. 

Structural, spectroscopic, and dynamic measurements of adsorption at single-crystal surfaces form the basis of modern surface science, and more extensive reviews of these methodologies and the information that can be provided concerning adsorbates and surface reactions can be found elsewhere as well as within this volume. Most experiments on metal surfaces are carried out on single crystals of about 10-mm diameter which are cut and polished to within 0.5° of the desired crystal plane and cleaned and ordered in the UHV environment using ion bombardment, chemical reaction, and annealing. Single-crystal surfaces prepared... 

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