Catalysts, general active sites

Catalytic activity is generally measured in terms of turnovers. Thus the Turnover Number, TON, is the number of times the catalytic reaction occurs per active site, while the Turnover Frequency (TOF) is the TON per unit of time. The TON is easily understood for defined reactions where the number of active sites is known homogeneous catalyses that occur at clearly understood metal centres, enzymatic catalysts or some heterogeneous catalysts whose active sites are easily determined. If the number of active sites on a heterogeneous catalyst is not easily determined, then that number is often replaced by the total surface area of the catalyst. In industry TOF is sometimes measured as grams of product produced per gram catalyst per hour, as this gives a useful measure of the catalyst cost. 

This principle appears amenable to generalization active sites and catalyst promoters can be positioned in the same cage in order to systematically study catalyst promoter effects due to direct interaction of metal particles and metal ions. Quantum chemical calculations by van Santen et al. have resulted in detailed predictions, e.g., of the effects of Mg ions, that are in direct contact with zeolite-encaged Ir4 tetrahedra, on the adsorption of H2 (i72) or CO 373) on these clusters. These theoretical results should be verified experimentally, as they could form a basis for general predictions on the action of ionic promoters on chemisorbing transition metals. 

The reaction of triphasc catalysis is carried out in a three-phase liquid (organic) - solid (catalyst) - liquid (aqueous) condition. In general, the reaction mechanism of the triphasc catalysis is (i) mass transfer of reactants form the bulk solution to the surface of the catalyst pellet, (ii) diffusion of reactants to the interior of the catalyst pellet (active sites) through pores, and (iii) surface or intrinsic reaction of reactants with active sites. For step (iii). the substitution reaction in the organic phase and ion exchange reaction in the aqueous phase occurred. 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Enzymes are efficient catalysts for cathodic and anodic reactions relevant to fuel cell electrocatalysis in terms of overpotential, active site activity, and substrate/reaction specificity. This means that design constraints (e.g., fuel containment and anode-cathode separation) are relaxed, and very simple devices that may take up ambient fuel or oxidant from their environment are possible. While operation is generally confined to conditions close to ambient temperature, pressure, and pH, and power densities over about 10 mW cm are rarely achieved, enzyme fuel cells may be particularly useM in niche environments, for example scavenging trace H2 released into air, or sugar and O2 from blood. Thus, trace or unusual fuels become viable for energy production. 

In general, if condensation polymers are prepared with methylated aryl repeat units, free radical halogenatlon can be used to introduce halomethyl active sites and the limitations of electrophilic aromatic substitution can be avoided. The halogenatlon technique recently described by Ford11, involving the use of a mixture of hypohalite and phase transfer catalyst to chlorinate poly(vinyl toluene) can be applied to suitably substituted condensation polymers. 

In general, several spectroscopic techniques have been applied to the study of NO, removal. X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are currently used to determine the surface composition of the catalysts, with the aim to identify the cationic active sites, as well as their coordinative environment. 

Main group organometallic polymerization catalysts, particularly of groups 1 and 2, generally operate via anionic mechanisms, but the similarities with truly coordinative initiators justify their inclusion here. Both anionic and coordinative polymerization mechanisms are believed to involve enolate active sites, (Scheme 6), with the propagation step akin to a 1,4-Michael addition reaction. 

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