Characterization of active sites

One key aspect of SOMC is the determination of the structure of surface complexes at a molecular level one of the reasons being that our goal is to assess structure-activity relationships in heterogeneous catalysis, which requires a firm characterization of active sites or more exactly active site precursors. While elemental analysis is an essential first step to understand how the organometallic complex reacts with the support, it is necessary to gather spectroscopic data in order to understand what are the ligands and... 

Tanaka N, Hasan Z, Wever R (2003) Kinetic Characterization of Active Site Mutants Ser402Ala and Phe397His of Vanadium Chloroperoxidase from the Fungus Curvularia inaequalis. Inorg Chim Acta 356 288... 

S. Salmaso, S. Bersani, and A. Semenzato. 2005. Preparation and characterization of active site protected polyethylene glycol) avidin bioconjugates. Biochim. Biophys. Acta 1726 57-66. 

The adsorption of ammonia leads to a variety of chemically distinct species in most cases. Different types of sites are responsible for the formation of these surface species. Any correlation between rates of catalytic reactions and quantities of adsorbed ammonia may, therefore, be misleading and the characterization of active sites becomes ambiguous. Furthermore, ammonia is a very strong... 

The Isolation and Characterization of Active Site Peptides in Lysyl Oxidase... 

J. Keranen, P. Carniti, A. Gervasini, E. Iiskola, A. Auroux, L. Niinisto, Preparation by atomic layer deposition and characterization of active sites in nanodispersed vanadia/ titania/silica catalysts, Catal. Today 91-92 (2004) 67. 

The peculiar TS-1 catalytic activity, never observed before with other Ti-contalning materials, open new and very interesting perspectives for industrial applications of shape selective catalysis. In the recent years, several papers dealing with the synthesis, the characterization and the catalytic activity of the TS-1 have been published, nevertheless many questions about its preparation, activation and characterization of active sites remain still open. 

Aebischer, A., Hostettler, M., Hauser, J., Kramer, K., Weber, T., Gudel, H.U., et al., 2006. Structural and spectroscopic characterization of active sites in a family of light-emitting sodium lanthanide tetrafluorides. Angew. Chem. Int. Ed. 45, 2802—2806. 

Figueiredo JL, Pereira MFR, Freitas MMA, Orfao JJM (2006) Characterization of active sites on carbon catalysts. Ind Eng Chem Res 46 4110-4115... 

McLean KJ, Carroll P, Lewis DG, DunfordAJ, Seward HE, Neeli R, Cheesman MR, Marsollier L, Douglas P, Smith WE, Rosenkrands I, Cole ST, Leys D, Parish T, Munro AW (2008) Characterization of active site structure in CYP121. A cytochrome P450 essential for viability of Mycobacterium tuberculosis H37Rv. J Biol Chem 283 33406-33416... 

Manoli JM, Costa PD, Brun M, Vrinat M, Mauge F, Potvin C (2004) Hydrodesulfurization of 4, 6-dimethyldibenzothiophene over promoted (Ni, P) alumina-supported molybdenum carbide catalysts activity and characterization of active sites. J Catal 221 365-377... 

Characterization of active sites. In more fundamental studies, the reaction rateshould not refer to the mass of catalyst or metal, but to the active site. The detection of active sites, including the density (A) and the intensity of active site is a central subject of heterogeneous catalysis. The turnover number or turnover frequency (TOF) on per unit active site per unit time under the given conditions can be calculated according to active site determined. At present, TOF is a basis to obtain the intrinsic activity of metals as well as determine the reaction mechanisms. Due to the dimension of TOF is the reciprocal of second, the comparisons of the... 

As the total number of the active sites is unknown, it is common practice, to substitute it with the total number of surface metal atoms. The reaction rate expressed as the number of reacting molecules transformed per surface metal atom per second is often called turnover frequency (TOF) and is expressed as sec TOF is currently used to draw fundamental conclusions about the intrinsic activity of metals and the mechanism of the reaction. If defining the amount of iron atoms exposed on the per unit surface as the characterization of active site density (A), then ... 

French SA et al (2003) Identification and characterization of active sites and their catalytic processes - the Cu/ZnO methanol catalyst. Top Catal 24 161-172... 

The details that explain more closely how the data obtained from TPD experiment can be used to get the information concerning the characterization of active sites, kinetic and thermodynamic parameters, are given in the following sections. 

The characterization of active sites of solid catalysts includes the determination of active sites nature, the estimation of their density (or population, i.e. the number of active sites per unit of mass or per unit of surface area), their strength and strength distribution. Active sites can be acidic, basic and, in certain cases, amphoteric. All mentioned characteristics are very important for catalysts functionality therefore, many experimental techniques are invented and adapted for their investigation. Among others, mainly spectroscopic methods (like NMR, IR, XPS, XRF...), temperature-programmed desorption is particularly important because it can be useful in the characterization of all mentioned features. 

This chapter has presented the fundamentals, the experimental setups and the applications of temperature-programmed desorption (TPD). TPD is widely utilized for characterization of active sites present on the surface of solid materials and determination of kinetic and thermodynamic parameters of desorption processes. It is a powerful technique, even if it does not provide direct information about molecular nature of adsorbed species. Given its relative simplicity and low cost, this technique will continue to find more applications in the future. 

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