Active Sites and Turnover Number

They have an exceedingly high specific activity per active site the turnover number y is as high as 10 to 10 s in certain enzyme reactions, while at ordinary electrocatalysts having a number of reaction sites on the order of 10 cm , yhas a value of about 1 s at a current density of lOmA/cm. Thus, the specific catalytic activity of tfie active sites of enzymes is many orders of magnitude fiigher tfian tfiat of all other known catalysts for electrochemical (and also chemical) processes. 

However, catalytic chemists should not despair, because there is a technique which is capable of measuring both the turnover frequency of the active sites and the number of these sites simultaneously under real reaction conditions. This technique is Isotope Transient Kinetics (ITK) and uses a simple experimental approach to obtain very powerful data. This paper will introduce the ITK technique and outline its potential by demonstrating how it has been used to identify and address problems pertaining to autocatalysis. 

When it is possible to fill a reactor with catalyst up to the bulk density, these surface areas per unit volume can be achieved. We now will calculate the rate of production per unit volume of catalyst that can be obtained provided transport limitations do not interfere. We assume a product having a molecular weight of 80. Usually the number of catalytically active atoms at the surface of a solid catalyst is smaller than the total number of surface atoms, which in metal surfaces is of the order of lO m . Here we will assume that 2 X lO atoms m are catalytically active. The turnover number indicates the number of molecules reacting s per active site. The turnover number usually ranges between 10 and 10 s [1]. For this calculation we will take a turnover number of 1 s. Finally we use a working day of 8 h-the fine-chemical industry does not usually work continuously. 

Number of Surface Active Sites and Turnover Frequency Why are... 

Briand, L.E., Earneth, W.E., and Wachs, I.E. Quantitative determination of the number of active sites and turnover frequencies for methanol oxidation over metal oxide catalysts. I. Fundamentals of the methanol chemisorption technique and application to monolayer supported molybdenum oxide catalysts. Catal Today 2000, 62, 219-229. 

Before deriving the rate equations, we first need to think about the dimensions of the rates. As heterogeneous catalysis involves reactants and products in the three-dimensional space of gases or liquids, but with intermediates on a two-dimensional surface we cannot simply use concentrations as in the case of uncatalyzed reactions. Our choice throughout this book will be to express the macroscopic rate of a catalytic reaction in moles per unit of time. In addition, we will use the microscopic concept of turnover frequency, defined as the number of molecules converted per active site and per unit of time. The macroscopic rate can be seen as a characteristic activity per weight or per volume unit of catalyst in all its complexity with regard to shape, composition, etc., whereas the turnover frequency is a measure of the intrinsic activity of a catalytic site. 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

The activity of a catalyst can be written as a product of two factors, the number of active sites and the turnover frequency. The turnover frequency is the subject of this section, we will return to the number of active sites later. 

In this system, the catalyst G3-I9 showed a similar reaction rate and turnover number as observed with the parent unsupported NCN-pincer nickel complex under the same conditions. This result is in contrast to the earlier observations for periphery-functionalized Ni-containing carbosilane dendrimers (Fig. 4), which suffer from a negative dendritic effect during catalysis due to the proximity of the peripheral catalytic sites. In G3-I9, the catalytic active center is ensconced in the core of the dendrimer, thus preventing catalyst deactivation by the previous described radical homocoupling formation (Scheme 4). 

Table 1 shows Hj uptake, the number of surface nickel atoms and turnover number. The addition of samarium increases the turnover number as well as the number of surface nickel atoms, i.e., the number of active sites. The increase in the turnover number seems to be mostly due to the predominant formation of tetragonal zirconia. 

L34. Lockridge, O., and La Du, B. N., Gomparison of atypical and usual human serum cholinesterase. Purification, number of active sites, substrate affinity, and turnover number. /. Biol. Chem. 253, 361-366 (1978). 

The overall catalytic activity of a catalyst is the product of two independent properties the total number of active sites on the catalyst and the intrinsic activity per site (turnover frequency) of each of these sites. The only technique capable of measuring both the number of active sites and their turnover frequency simultaneously under actual operating conditions is the Isotope Transient Kinetics (ITK) technique. This makes it one of the most powerful techniques available to the catalytic chemist today. 

The data within Table 3 show that ageing reduces the performance of the Pd/A catalyst. This decrease in activity is brought about by reductions in both the turnover frequency of the active sites, and in the number of active sites. The extent of the reduction in performance is lower over this catalyst than it was over the Pd-only material, although the overall performance of the Pd-only catalyst still exceeds that of the promoted Pd/A sample when the rates are expressed in terms of pmol/s/g. Note, however, that one gram of the Pd-only sample contains more Pd than one gram of the promoted Pd/A sample due to the effect of the promoter. When the aged activities of these two samples are compared at the same Pd content, the activities are fairly similar, with the exception of the RHSA data, where the promoted catalyst is significantly more active. 

Catalysis is a kinetic phenomenon we would like to carry out the same reaction with an optimum rate over and over again using the same catalyst surface. Therefore, in the sequence of elementary reactions leading to the formation of the product molecule, the rate of each step must be of steady state. Let us define the catalytic reaction turnover frequency, as the number of product molecules formed per second. Its inverse, 1 /, yields the turnover time, the time necessary to form a product molecule. By dividing the turnover frequency by the catalyst surface area, (i, we obtain the specific turnover rate, (R (molecules/cm /sec) = /Ct ((R often called the turnover frequency also, in the literature). This type of analysis assumes that every surface site is active. Although the number of catalytically active sites could be much smaller (usually uncertain) than the total number of available surface sites, the specific rate defined this way gives a conservative lower limit of the catalytic turnover rate. If we multiply (R by the total reaction time, bt, we obtain the turnover number, the number of product molecules formed per surface site. A turnover number of one corresponds to a stoichiometric reaction. Because of the experimental uncertainties, the turnover number must be on the order of 10 or larger for the reaction to qualify as catalytic. 

Sometimes it is given as reaction rate per unit weight or specific surface area of the catalyst, but more instructive is its definition in terms of the turnover frequency (TOP). This is defined as the number of reaction events per active site and unit time and relies on the possibility to evaluate the correct density of active sites. 

This work demonstrates the principle of using a supramolecular complex containing both a photosensitizer and a catalytically active site, and is conceptually related to the antenna effect used in energy-storage reactions. The advantage of the supramolecular complexes was demonstrated by comparing the turnover numbers for the... 

Enzyme activity can be measured in terms of the so-called turnover number, which represents the net number of substrate molecules reacted per catalytic activity site and per unit time. The turnover number, therefore. 

Krilov OV, KisUev VF (1981) Adsorption and catalysis on the transition metals and their oxides. Chemistry, Moscow Kroger FA, Vink HJ (1956) Relations between concentrations of imperfections in crystaUine solids. In Seitz F, TumbuU D (eds) Solid state physics, vol 3. Academic, New York, pp 307-435 Kulkami D, Wachs IE (2002) Isopropanol oxidation by pure metal oxide catalysts number of active surface sites and turnover frequencies. Appl Catal A 237 121-137 Kulwicki BM (1991) Humidity sensors. J Am Ceram Soc 74 697-708... 

Briand, L., Hirt, A. and Wachs, I. (2001). Quantitative Determination of the Number of Surface Active Sites and the Turnover Frequencies for Methanol Oxidation over Metal Oxide Catalysts Application to Bulk Metal Molybdates and Pure Metal Oxide Catalysts, J. Catal., 202, pp. 268-278. 

Kulkami, D. and Wachs, I.E. Isopropanol oxidation hy pure metal oxide catalysts number of active surface sites and turnover frequencies, Catal 2002, 237, 121-137. 

It has also been proposed that hydrogen transfer rates are proportional to the time the reacting species stay on the active site, and so may be inversely related to acid strength (42). An increase in acid strength results in higher turnover numbers, less time on the site and therefore less hydrogen transfer. It is likely that both site densities and the time on site affect hydrogen transfer rates. 

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