Important Trends in Surface Reactivity

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. 

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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. 

Experiments also have the capacity to reveal new trends in reactivity and to discover new phenonena concerning reaction dynamics. In addition, experiments provide the ultimate testing ground of any theoretical prediction. What is more, new experimental discoveries can demonstrate to theorists what important elements their theories should contain, and what kind of simplifications are appropriate. Experiments should not be done at the level of overall rates, because very differentmicroscopic dynamics can fortuitously lead to the same overall macroscopic kinetics. Rather, experiments should investigate the surface reaction dynamics directly at the molecular level. In this way, theory and experiment can complement each other ideally, and benefit from the mutual feedback. 

Studies on reactions between different oxidants and FejO, Fe2Co04 and Fe2Ni04 reveal similar trends in reactivity for the oxidants. An important exception is observed for H2O2 where the reactivity is markedly lower than expected from the trend for the other oxidants.The rationale for this is probably that H2O2 is consumed by catalytic decomposition (as will be discussed below) on the surface while the other (stronger) oxidants are capable of oxidizing the metal oxides used in this study. 

We first discuss the overall chemical process predicted, followed by a discussion of reaction mechanisms. Under the simulation conditions, the HMX was in a highly reactive dense fluid phase. There are important differences between the dense fluid (supercritical) phase and the solid phase, which is stable at standard conditions. Namely, the dense fluid phase cannot accommodate long-lived voids, bubbles, or other static defects, since it has no surface tension. Instead numerous fluctuations in the local environment occur within a timescale of 10s of femtoseconds. The fast reactivity of the dense fluid phase and the short spatial coherence length make it well suited for molecular dynamics study with a finite system for a limited period of time. Under the simulation conditions chemical reactions occurred within 50 fs. Stable molecular species were formed in less than a picosecond. We report the results of the simulation for up to 55 picoseconds. Figs. 11 (a-d) display the product formation of H2O, N2, CO2 and CO, respectively. The concentration, C(t), is represented by the actual number of product molecules formed at the corresponding time (. Each point on the graphs (open circles) represents a 250 fs averaged interval. The number of the molecules in the simulation was sufficient to capture clear trends in the chemical composition of the species studied. These concentrations were in turn fit to an expression of the form C(/) = C(l- e ), where C is the equilibrium concentration and b is the effective rate constant. From this fit to the data, we estimate effective reaction rates for the formation of H2O, N2, CO2, and CO to be 0.48, 0.08,0.05, and 0.11 ps, respectively. 

A representative example of the important role of surface composition for the interaction of ceria materials with NO was a study about the effect of the amount of cerium and zirconium on the adsorption of NO on Ce Zri x02 mixed oxides. This study was carried out with a set of Ce,Zrj.,02 samples with x = 1, 0.76, 0.56, 0.36, 0.16, and 0, and it concluded that their catalytic activity for NO oxidation to NOg mainly depends on the cerium molar fraction. A linear relation between NOg production capacity and cerium content was found for these ceria-zirconia mixed oxides. However, bare CeOg did not follow the same trend as the mixed oxides, and its catalytic activity for NO oxidation was lower than expected considering its cerium content. This was mainly attributed to the availability of reactive oxygen needed for NO oxidation, which is enhanced in doped cerias. 

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