Elementary surface reaction steps structure sensitivity

Finally, the kinetics approximations cannot be forgotten. Usually, in order to put in evidence structure-activity relationships, a simple parameter, the TOF, is used. The TOF, which reflects the rate per accessible site, contains the combination of all the adsorption and surface reaction elementary steps. Each of these steps is dependent on adsorption and/or rate constants. For that reason, the significance of TOF dependence as a function of structural parameters, e.g., the particle size, is not obvious since the rate equation can be particle-size-dependent [17]. Moreover, the adsorption and surface reaction steps may exhibit very different sensitivities to electronic and geometrical features. 

The undoubtedly structure-sensitive reaction NO -r CO has a rate that varies with rhodium surface structure. A temperature-programmed analysis (Fig. 10.8) gives a good impression of the individual reaction steps CO and NO adsorbed in relatively similar amounts on Rh(lll) and Rh(lOO) give rise to the evolution of CO, CO2, and N2, whereas desorption of NO is not observed at these coverages. Hence, the TPRS experiment of Fig. 10.8 suggests the following elementary steps ... 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

In our discussion of the influence of structure on the turnover rate our understanding is frequently hampered by lack of information on the ratedetermining step and the most abundant surface intermediate. It would be logical to consider the structure sensitivity of the rate of an elementary step, such as the desorption of a chemisorbed gas. Results on temperature programmed desorption as a function of particle size might be simpler to interpret than those of global reactions consisting of a sequence of steps. However, few such data are available. 

NO reduction using CO was also analysed in this context. In this reaction as well as for stoichiometric CO + NO -1- Og mix-tures " the main question arises as to which is the rate limiting step, which in some cases is believed to be the NO dissocia-tioni93 i98 in other (possibly more frequent) cases seems to be related to the N-coupling steps. Both elementary steps need a cluster of noble metal atoms and thus NO reduction processes are generally structure sensitive and size dependent. " The rate of these two steps and the performance of the catalyst under reaction conditions seems closely connected with the availability of noble metal reduced (zero-valent) sites, the formation of which are strongly inhibited at the surface of the metal by the presence of... 

A widespread interest for the electrochemical oxygen reduction reaction (ORR) has two aspects. The reaction attracts considerable attention from fundamental point of view, as well as it is the most important reaction for application in electrochemical energy conversion devices. It has been in the focus of theoretical considerations as four-electron reaction, very sensitive to the electrode surface structural and electronic properties. It may include a number of elementary reactions, involving electron transfer steps and chemical steps that can form various parallel-consecutive pathways [1-3]. 

---