Modeling surface reactivity

Gorb LG, Rivail JL, Thery V, Rinaldi D (1996) Modification of the local self-consistent field method for modeling surface reactivity of covalent solids, Int J Quant Chem 60 313—324... 

By combining the results of the Newns-Andersons model and the considerations from the tight binding model it is now possible to explain a number of trends in surface reactivity. This has been done extensively by Norskov and coworkers and for a thorough review of this work we refer to B. Hammer and J.K. Norskov, Adv. Catal. 45 (2000) 71. We will discuss the adsorption of atoms and molecules in separate sections. 

In addition to performing experiments under pressures similar to those encountered in real processes to bridge the pressure gap , surface scientists have also been increasing the level of complexity of the model surfaces they use to better mimic real supported catalysts, thus bridging the materials gap . A few groups, including those of Professors Freund and Henry, have extended this approach to address the catalytic reduction of NO. The former has published a fairly comprehensive review on the subject [23], Here we will just highlight the information obtained on the reactivity of NO + CO mixtures on these model supported catalysts. 

Over the past decade or so, our understanding of oxide surfaces has markedly improved, especially in the case of the model oxide surface-Ti02(l 10). In particular, the influence of defects such as oxygen vacancies on the surface reactivity has been demonstrated in exquisite detail and it is clear that they play a crucial role in many surface reactions. As we have shown in this chapter, scanning probes have been instrumental in evaluating the surface reactivity as they allow individual defects, reactants, and products to be imaged on relatively short timescales (seconds or minutes). 

A theoretical model for the adsorption of metals on to clay particles (<0.5 pm) of sodium montmorillonite, has been proposed, and experimental data on the adsorption of nickel and zinc have been discussed in terms of fitting the model and comparison with the Gouy-Chapman theory [10]. In clays, two processes occur. The first is a pH-independent process involving cation exchange in the interlayers and electrostatic interactions. The second is a pH-dependent process involving the formation of surface complexes. The data generally fitted the clay model and were seen as an extension to the Gouy-Chapman model from the surface reactivity to the interior of the hydrated clay particle. 

The importance of these radicals in catalytic processes may be evaluated by studying their behavior in stoichiometric reactions and by extrapolating this information to catalytic conditions. In following the stoichiometric reactions, magnesium oxide has been an excellent model surface since the three types of oxygen ions may be selectively formed and are stable at temperatures where most hydrocarbons of interest will react. Magnesium oxide, on the other hand, is basic and reactive itself therefore intermediates may react differently on this surface than on silica, for example. 

Computational efforts using DPT calculations as well as kinetic modeling of reactivities based on Monte Carlo simulations or mean field mefh-ods have been employed to study elementary processes on Pt surfaces. 2 228 Unraveling systematic trends in structure versus reactivity relations remains a formidable challenge due to fhe complex nafure of sfrucfural effects in electrocatalysis. 

The significance of the development of photoelectron spectroscopy over the last decade for a better understanding of solid surfaces, adsorption, surface reactivity, and heterogeneous catalysis has been discussed. The review is illustrative rather than exhaustive, but nevertheless it is clear that during this period XPS and UPS have matured into well-accepted experimental methods capable of providing chemical information at the molecular level down to 10% or less of a monolayer. The information in its most rudimentary state provides a qualitative model of the surface at a more sophisticated level quantitative estimates are possible of the concentration of surface species by making use of escape depth and photoionization cross-section data obtained either empirically or by calculation. 

For this complex, molecular chemistry does not adequately model the surface reactivity and the latter is strongly influenced by the presence of surface hydroxyl groups [22]. The organometallic fragments immobilized on silica have been reacted with trimethylphosphine to afford different silica-supported phosphine complexes of rhodium. The course of the reaction depends strongly on the hydroxyl content of the silica surface [23] (Scheme 7.2). 

Given that interfacial solvation affects chemical transport/ surface reactivity and electron transfer/ and macromolecular self-assembly/ predictive models of solvent-solute interactions near surfaces will afford researchers deeper insights into a host of phenomena in biology, physics, and engineering. Research in this area should aid efforts to develop a general, experimentally tested, and quantitative understanding of solution-phase surface chemistry. 

In this paper, we will report the electronic and catalytic reactivities of the model VC/V(110) surface, and our attempt to extend them to VC powder catalysts. By using high-resolution electron energy loss spectroscopy (HREELS) and NEXAFS techniques, we observed that the surface properties of V(110) could be significantly modified by the formation of vanadium carbide some of the experimental results on these model surfaces were published previously.3-5 We will discuss the selective activation of the C-H bond of isobutane and the C=C bond of isobutene on V(110) and on VC/V(110) model systems. These results will be compared to the catalytic performances of vanadium and vanadium carbide powder materials in the dehydrogenation of isobutane. 

These electronic properties in turn give rise to some unique catalytic properties for vanadium carbide. Compared to metallic vanadium, vanadium carbide shows an enhancement in the activation of the C-H bond of alkanes and a reduction in the interaction with the C=C bond of alkenes. The surface reactivity of VC/V(110) can be generally described as similar to those of Pt group metals, although the VC/V(110) surface might have an even higher activity towards the activation of C-H bonds. The dehydrogenation of isobutane on VC powder catalysts will be compared to the reactivities of the VC/V(110) model surfaces. 

The intentional design of model systems can be envisioned, as for instance binary or multiple assemblies (clusters) of active components and poisons, for the examination of their activity in chemisorption, or specific reactions. The results can then be compared with respective clusters containing the active species only. Perhaps, such model systems will be amenable to computational methods capable of predicting their chemisorptive behavior and their surface reactivity. Such approaches are now employed for the design of improved multicomponent catalysts and can, obviously, be used to study the reverse effect, i.e., the mutual deactivation of the cluster components. 

Sakariassen KS, Hanson SR, Cadroy Y. Methods and models to evaluate shear-dependent and surface reactivity-dependent antithrombotic efficacy. Thromb Res 2001 104 149-174. Review. 

Quantum-Chemical Approximations for the Modeling of Surface Reactivity... 

Chlorosilanes are discussed, considering both those with short-chain and long-chain organic groups. Short-chain chlorosilanes are important as model compounds in the general study of silica surface reactivity in vapour phase modification. Furthermore,... 

As far as phenomenological modeling is concerned, an excellent review of earlier thermodynamic approaches to chemisorption and surface reactivity was given by Benziger (156), who also developed some general thermodynamic criteria for dissociative versus nondissociative adsorption of diatomic and polyatomic molecules on transition metal surfaces (137, 156). In particular, for quantitative estimates of QA, A = C, N, or O, Benziger (156) used the heats of formation of bulk metal carbides, nitrides, and oxides. The BOC-MP approach is different, however, not only analytically but also in making direct use of experimental values of QA. 

The theorists are incurable optimists in their belief that there exists a comprehensible order of things. The only problem is to find a framework within which the intricacies of real phenomena are coherently interrelated. For chemisorption phenomena on transition metal surfaces, including surface reactivity, the BOC-MP model appears to provide such a framework. 

---