Reactivity of surfaces

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Volume 48 Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Volume 49 Zeolites Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14,1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen... 

Volume 48 Structure and Reactivity of Surfaces. Proceedings of a European Conference,... 

Step 4 Chemical Reactivity of Surface Complexes Reactivity with Alcohol and H2O... 

Since heterogeneous catalysis is a phenomenon which is exclusively based on the reactivity of surface atoms, a high fraction of the latter, exposed towards reactants, is desired. This demand can be equated with a high degree of dispersion of the metal or a very small particle size, that is, in the lower nanometer range of approximately 1-5 nm. 

M. W. Roberts, The nature and reactivity of chemisorbed oxygen and oxide overlayers at metal surfaces as revealed by photoelectron spectroscopy, in Structure and Reactivity of Surfaces, ed. C. Morterra, A. Zechina and G. Costa, Elsevier, Amsterdam, 1989, p. 787. 

Satsuma, A., Enjoji, T., Shimizu, K.I. et al. (1998) Reactivity of surface nitrate species in the selective reduction of NO with propene over Na-H-mordenite as investigated by dynamic FTIR spectroscopy,./. Chem. Soc., Faraday Trans., 94, 301. 

Kondarides, D.I., Chafik, T. and Verykios, X.E. (2000) Catalytic reduction of NO by CO over rhodium catalysts. 2. effect of oxygen on the nature, population, and reactivity of surface species formed under reaction conditions, J. Catal., 191, 147. 

Koerts, T. 1992. The reactivity of surface carbonaceous intermediates. PhD thesis, Eindhoven University of Technology, The Netherlands. 

Adsorption influences the reactivity of surfaces. It has been shown that the rates of processes such as precipitation (heterogeneous nucleation and surface precipitation), dissolution of minerals (of importance in the weathering of rocks, in the formation of soils and sediments, and in the corrosion of structures and metals), and in the catalysis and photocatalysis of redox processes, are critically dependent on the properties of the surfaces (surface species and their strucutral identity). 

This short analysis of the molecular reactivity of [Mo(CO)6] shows that one or more CO ligands can be displaced quite easily, and that various oxidation states can be reached. Though such chemical behavior carmot be directly applied to the reactivity of surface functionalities with [Mo(CO)6], it might provide useful suggestions about possible similarities. 

Regazzoni, A.E. Blesa, M.A. (1991) Reactivity of surface iron(III)-thiocyanate complexes characterized by the dissolution of hematite in acidic thiocyanate solutions. Langmuir 7 473-478... 

Mineral-liquid or mineral-gas interfaces under reactive conditions cannot be studied easily using standard UHV surface science methods. To overcome the pressure gap between ex situ UHV measurements and the in situ reactivity of surfaces under atmospheric pressure or in contact with a liquid, new approaches are required, some of which have only been introduced in the last 20 years, including scanning tunneling microscopy [28,29], atomic force microscopy [30,31], non-linear optical methods [32,33], synchrotron-based surface scattering [34—38], synchrotron-based X-ray absorption fine structure spectroscopy [39,40], X-ray standing wave... 

To unambiguously elucidate the reactivity of surface methoxy species, the preparation of pure methoxy species on the catalyst surface is an important prerequisite. This preparation can be achieved by a SF protocol, which starts with a flow of C-enriched methanol into acidic zeolites at room temperature, followed by a purging of the catalyst with dry nitrogen at room temperature and subsequently at higher temperatures (74,262. The latter step progressively removes the surplus of methanol and DME, together with water produced by the conversion of methanol. 

The reactivity of surface methoxy species was further investigated with various probe molecules that were thought to possibly be involved in the MTO process, including water, toluene (representing aromatics), and cyclohexane (representing saturated hydrocarbons) (263). It was found that surface methoxy species react at room temperature with water to form methanol, which indicates the occurrence of a chemical equilibrium between these species at low reaction temperatures (Scheme 15) (263). 

We conclude, therefore, that the identification of A and E with the concentration of the surface precursor to product formation and the energy barrier to a bond redistribution process in the dominant step of a surface reaction, respectively, is not always or necessarily justified and may not be a realistic representation of the kinetics of a surface change. More direct information concerning the concentrations and reactivities of surface intermediates is required to substantiate meaningfully the kinetic properties of reactions proceeding on surfaces. Such considerations also call into question the application of the transition state theory to systems for which the transition complex has not been characterized unambiguously. 

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