Trends in Surface Reactivity

In the previous two sections we have described trends in the chemisorption energies of atoms and molecules on metallic surfaces. These express the final situation of the adsorption process. Here we consider what happens when a molecule approaches a surface. 

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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. 

Are there fields other than catalysis where trends in surface reactivity may be of value ... 

In the next section, we shall combine what we have learned from the Newns-Anderson and tight-binding models and apply that as a basis for understanding the choice of the descriptors that we use to explain trends in surface reactivity. 

In the preceding chapter it has been shown that the DFT methods currently available can be used to reproduce relative trends in both reactivities and transition-metal NMR chemical shifts. Thus, NMR/reactivity correlations can be modeled theoretically, at least when relative reactivities are reflected in relative energies on the potential energy surfaces (activation barriers, BDEs). It should in principle also be possible to predict new such correlations. This is done in the following, with the emphasis on olefin polymerization with vanadium-based catalysts. 

There should be increased study of bimetalUc surfaces to establish stmctural trends in surface behavior across the periodic table and the correlation between these trends and electrocatalytic reactivity. 

On a catalyst surface, it may sometimes be important to enhance or reduce the acid-base strength, for example when an irreversible adsorption of the reactants takes place. The technique of doping the catalyst with small cations or anions such as Li" ", Ca " ", Ni " ", SC ions, etc., or forming mixed oxides are then employed. When one calcines mixtures of coprecipitated hydroxides at high temperature, chemically mixed oxides are produced which involve an intimate mixing of both types of oxygen-cation bonds. But when mechanically mixed oxides are obtained by powder compression, the mixed bonds occur only at the grain boundaries. Clear and systematic trends in the reactivity and acidity of mixed oxides are not presently available. 

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

This is in principle all we need to understand chemical bonding on surfaces and trends in reactivity. For a more accurate description of molecular orbital theory we refer to P.W. Atkins, Molecular Quantum Mechanics (1983), Oxford University Press, Oxford. The main results from molecular orbital theory are summarized in Fig. 6.8 below. 

Nalewajski, R. F. and A. Michalak. 1998. Charge sensitivity/bond-order analysis of reactivity trends in allyl-[Mo03] chemisorption systems A comparison between (010)- and (100)-surfaces. J. Phys. Chem. A 102 636-640. 

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. 

General Trends in Metal Complex/Surface Reactivity, and Further Requirements for Metal-Supported Catalyst Preparation... 

An alternative method to investigate DNA strand breakage by OH radicals considers the surface accessibility of hydrogen atoms of the DNA backbone [102]. The solvent accessibility is 80% for the sugar-phosphates and —20% for the bases. This method allows a more direct determination of reaction of OH radicals with the individual deoxyribose hydrogens [103,104]. Recent studies show trends in reactivity of OH radicals closely follow the accessibility of the solvent to various deoxyribose hydrogens [105,106]. 

We need to develop methods to understand trends for complex reactions with many reaction steps. This should preferentially be done by developing models to understand trends, since it will be extremely difficult to perform experiments or DFT calculations for all systems of interest. Many catalysts are not metallic, and we need to develop the concepts that have allowed us to understand and develop models for trends in reactions on transition metal surfaces to other classes of surfaces oxides, carbides, nitrides, and sulfides. It would also be extremely interesting to develop the concepts that would allow us to understand the relationships between heterogeneous catalysis and homogeneous catalysis or enzyme catalysis. Finally, the theoretical methods need further development. The level of accuracy is now so that we can describe some trends in reactivity for transition metals, but a higher accuracy is needed to describe the finer details including possibly catalyst selectivity. The reliable description of some oxides and other insulators may also not be possible unless the theoretical methods to treat exchange and correlation effects are further improved. 

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