Catalytic reaction, turnover frequency

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. 

H4Ru4(CO),2] were shown to transform propene into butyraldehyde under a partial CO pressure of 28 bar (PcjH. 12 bar) at 100°C for 10 hours in the presence of a large excess of aqueous trimethylamine. These catalytic systems are more active for the water gas shift reaction (turnover frequency 340 hour" ) than for the production of 0x0 compounds( 12 hour" ). However, no indications about the nature of the ruthenium species produced in basic media were given. 

All samples were reduced again at 500°C under hydrogen for two hours before the catalytic tests. Turnover frequencies (TOP) of the catalysts were calculated from initial rate data. A fixed bed tubular continuous axial flow reactor was used for all reactions. 

The major drawback of the nickel catalyst is poisoning of Ni surface by the interaction with CO to form nickel carbonyl at low temperature. Noble metal based catalysts are more active and stable catalyst in comparison with Ni based catalysts [100]. Among Ru nanoparticle dispersed on various supports (Al Oj, MgAljO, MgO, C, etc ), Ru/Al Oj showed highest catalytic activity (turnover frequency, TOF = 16.5 x 10 s" ) [101,102]. Yttrium addition to the Ru-based catalyst enhances the activity and stability for methanation reaction [103], Pd/Mg-SiO and platinum titanate nanotubes were also foimd... 

The cyclodimerization of 1,3-butadiene was carried out in [BMIM][BF4] and [BMIM][PF(3] with an in situ iron catalyst system. The catalyst was prepared by reduction of [Fe2(NO)4Cl2] with metallic zinc in the ionic liquid. At 50 °C, the reaction proceeded in [BMIM][BF4] to give full conversion of 1,3-butadiene, and 4-vinyl-cyclohexene was formed with 100 % selectivity. The observed catalytic activity corresponded to a turnover frequency of at least 1440 h (Scheme 5.2-24). 

Figure 8.65. Dependence of the catalytic rates and turnover frequencies of C02 on the reaction temperature and on the catalyst potential for the initially sodium free Rh/YSZ catalyst (labeled C2) during NO reduction by CO in presence of gaseous 02. Reprinted with permission from Elsevier Science.

In the early days of catalysis the porous high surface area support was usually thought to be inert. It soon became obvious, however, that the catalytic activity, or turnover frequency, of a catalytic reaction on a given active phase is quite often seriously affected both by the crystallite size and by the material of the support. 

Turnover frequency, TOF of the catalytic reaction, 4, 193 of the depletion of the promoting species, 193... 

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. 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

Finke has reported remarkable catalytic lifetimes for the polyoxoanion- and tetrabutylammonium-stabi-lized transition metal nanoclusters [288-292]. For example in the catalytic hydrogenation of cyclohexene, a common test for structure insensitive reactions, the lr(0) nanocluster [296] showed up to 18,000 total turnovers with turnover frequencies of 3200 h [293]. As many as 190,000 turnovers were reported in the case of the Rh(0) analogue reported recently. Obviously, the polyoxoanion component prevents the precious metal nanoparticles from aggregating so that the active metals exhibit a high surface area [297]. 

Application of small metal particles has attracted the attention of the scientists for a long time. As early as in the seventies Turkevich already prepared mono-dispersed gold particles [19], and later, using molecular transition metal carbonyl clusters [20], the importance of small nanoparticles increased considerably. One of the crucial points is whether turnover frequency measured for a given catalytic reaction increases or decreases as the particle size is diminished. 

Catalytic activity was measured as a function of turnover frequency [moles product/(mole catalyst) (hour)]. The standard run has a turnover frequency of 105 10. All the parameters investigated were perturbed about this standard and included the effects of catalyst, aldehyde, KOH and water concentration, initial CO pressure, and reaction time. In addition, a few selected runs were also conducted to examine the effects of hydrogen in the gas phase as well as the relative ease with which other aldehydes could be reduced. 

In general, the reaction rate is proportional to the amount of catalyst. This is true if the catalytic sites function independently. The number of turnovers per catalytic site per unit time is called the turnover frequency The reactivity of a catalyst is the product of the number of sites per unit mass or volume and the turnover frequency. 

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