Heterogeneous catalysis theoretical considerations

How important the breakdown of the Born-Oppenheimer approximation is in limiting our ability to carry out ab initio simulations of chemical reactivity at metal surfaces is the central topic of this review. Stated more provocatively, do we have the correct theoretical picture of heterogeneous catalysis. This review will restrict itself to a consideration of experiments that have begun to shed light on this important question. The reader is directed to other recent review articles, where aspects of this field of research not mentioned in this article are more fully addressed.10-16... 

Considerable interest has been shown in the new processes of stereospecific polymerization, not only so far as they concern the production of new classes of polymers, having unusual characteristics and improved properties, but also because they are representative of a peculiar new type of heterogeneous catalysis, of great interest from the practical and the theoretical points of view 1-5). 

Any real system is known to suffer constantly from the perturbing effects of its environment. One can hardly build a model accounting for all the perturbations. Besides, as a rule, models account for the internal properties of the system only approximately. It is these two factors that are responsible for the discrepancy between real systems and theoretical models. This discrepancy is different for various objects of modem science. For example, for the objects of planetary mechanics this discrepancy can be very small. On the other hand, in chemical kinetics (particularly in heterogeneous catalysis) it cannot be negligible. Strange as it is, taking into consideration such unpredictable discrepancies between theoretical models and real systems can simplify the situation. Perturbations "smooth out some fine details of dynamics. 

In a first part, the application of combinatorial chemistry to heterogeneous catalysis is analysed in terms of current strategies and perspectives on the industrial and academic levels. Potential methodologies for academic research laboratories are proposed with emphasis on both theoretical and practical considerations. 

Researchers in the area of heterogeneous catalysis have recently focussed considerable attention to the relationships among catalytic activity, product selectivity and the size and shape of metal particles for reactions catalyzed by metals (15). Reactions that are influenced by the size and shape of metal particles or electronic interactions of the metal particles with the support are known as structure sensitive reactions. Theoretical calculations of various crystallographic structures (16) have shown that the number of specific type of surface atoms (face, corner, edge) change as a function of particle size. For example, for a face centered cubic system, the number of face atoms decreases as particle size decreases. If, therefore, a reaction is catalyzed on a face and there are a substantial number of face atoms necessary for catalysis to occur, then as particle size decreases catalytic activity will decrease. This idea often runs counter to principles discussed in general science texts (17). 

The computational studies on surface chemistry of Co catalysts have offered significant supports to the investigation of FTS on Co catalysts. However, the work is far from decent. As the experimental studies indicated, surface reconstruction and phase transition were certain to take place under practical FTS conditions. The theoretical studies about surface reconstruction and phase transition of Co catalysts, however, are fairly rare. In addition, cluster models were less studied in the previous theoretical work compared to slab models. However, practical catalytic reactions do not always happen as proposed in ideal plane models, nor the active sites distribute homogeneously on the surface. The investigation on cluster models is acting a crucial role in the study of heterogeneous catalysis. Accordingly, more considerations on surface reconstruction, phase transition, and cluster models should be taken into account in future work. 

Metallic oxides having different shapes and sizes have received considerable attention because of their theoretical, technological applications in various organic reactions. Catalysts, for example, are mostly nanoscale particles, and catalysis is a nanoscale phenomenon. In the case of various reactions, separation of the catalyst from the reaction mixture is the main problem and loss of product occurs. Nano-particles prepared on the resin can easily be applied as a heterogeneous catalyst for efficient recovery and recycling of the photocatalyst from liquid-phase reactions. 

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