Turnover-frequency

Despite the limited solubility of 1-octene in the ionic catalyst phase, a remarkable activity of the platinum catalyst was achieved [turnover frequency (TOP) = 126 h ]. However, the system has to be carefully optimized to avoid significant formation of hydrogenated by-product. Detailed studies to identify the best reaction conditions revealed that, in the chlorostannate ionic liquid [BMIM]Cl/SnCl2 [X(SnCl2) = 0.55],... 

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 5.3-4 Turnover frequency of Rh-catalyzed hydroformylation as a function of 1-hexene...

Figure 1.3. Rate and catalyst potential response to step changes in applied current during C2H4 oxidation on Pt deposited on YSZ, an O2 conductor. T = 370°C, p02=4.6 kPa, Pc2H4=0.36 kPa. The catalytic rate increase, Ar, is 25 times larger than the rate before current application, r0, and 74000 times larger than the rate I/2F,16 of 02 supply to the catalyst. N0 is the Pt catalyst surface area, in mol Pt, and TOF is the catalytic turnover frequency (mol O reacting per surface Pt mol per s). Reprinted with permission from Academic Press.

Figure 2.3. Catalysis (0), classical promotion ( ), electrochemical promotion ( , ) and electrochemical promotion of a classically promoted (sodium doped) ( , ) Rh catalyst deposited on YSZ during NO reduction by CO in presence of gaseous 02.14 The Figure shows the temperature dependence of the catalytic rates and turnover frequencies of C02 (a) and N2 (b) formation under open-circuit (o.c.) conditions and upon application (via a potentiostat) of catalyst potential values, UWr, of+1 and -IV. Reprinted with permission from Elsevier Science.

Figure 8.20. Effect of p02 (a) and pC3H6 (b) on the rate and turnover frequency of propylene oxidation on Pt/YSZ.28 Reprinted with permission from Academic Press.

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.

Figure 9.35. Transient effect of applied negative current on the rate and turnover frequency of C2H4 oxidation on Pt/CaZro,9Ino ]03.a (solid curve) and on catalyst potential (dashed curve).45 Reprinted with permission from the Institute of Ionics.

Figure 10.7. Turnover frequency of S02 catalytic oxidation [mol S02 (conv.)/mol V2Os/s] vs working electrode polarization for the molten 10 mol % V20s - 90 mol% K2S207 catalyst at (1) 440°C and (2) 460°C.12 Reproduced by permission of the Electrochemical Society.

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. 

Table 11.2 and assume A=100, which is rather conservative value, to compute J via Eq. (11.32) and O via Eq. (11.22). The results show t p 0.91 which implies that the O2 backspillover mechanism is fully operative under oxidation reaction conditions on nanoparticle metal crystallites supported on ionic or mixed ionic-electronic supports, such as YSZ, Ti02 and Ce02. This is quite reasonable in view of the fact that, as already mentioned an adsorbed O atom can migrate 1 pm per s on Pt at 400°C. So unless the oxidation reaction turnover frequency is higher than 103 s 1, which is practically never the case, the O8 backspillover double layer is present on the supported nanocrystalline catalyst particles. 

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

The turnover frequency (TOP) based on surface-exposed atoms significantly increases with a decrease in the diameter of the gold particle from 5 nm [66]. This feature is unique to gold, because other noble metals usually show TOFs that decrease or remain the same with a decrease in the diameter [7]. The decrease in particle size gives rise to an increase in corner or edge and perimeter of NPs and change in electronic structure however, the origin of size effects on catalytic activity for CO oxidation is not clear. 

Turnover frequency - the number of moles of product produced per mole of catalyst per second (low turnover frequencies will mean large amounts of catalyst are required, resulting in higher cost and potentially more waste). 

The propane aromatization was conducted under the differential condition by using Ga203/Ga-MOR catalysts thus characterized. The contributions of L, HI, and H2 sites to the propane conversion and the aromatics formation were estimated by assuming that the observed reaction rates are the sum of the reaction rate on each site which is equal to the product of the turnover frequency (TFij) and the amount of active sites per weight of catalyst (Aj) ... 

Table 1 Turnover frequencies of propane conversion and aromatics formation over L, HI and H2 sites of Ga203/Ga-M0R catalysts.

Reaction conditions 0.014 mmol Ru, H2 lOOpsig, temp. 50"C, 30 ml EtOH, olar ratio of substrate to catalyst. Turnover frequency. 

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. 

For each step there is a corresponding rate (for convenience we drop the total number of sites from the expressions, i.e. r becomes a rate per site, or a turnover frequency) ... 

Minimize the effects of transport phenomena If we are interested in the intrinsic kinetic performance of the catalyst it is important to eliminate transport limitations, as these will lead to erroneous data. We will discuss later in this chapter how diffusion limitations in the pores of the catalyst influence the overall activation energy. Determining the turnover frequency for different gas flow velocities and several catalyst particle sizes is a way to establish whether transport limitations are present. A good starting point for testing catalysts is therefore ... 

Obtain meaningful data on the catalyst Usually for kinetic purposes is it the turnover frequency per active site (TOP) that of interest. But other parameters such as selectivity and yield are also of great importance for judging the potential of the catalyst. Instead of expressing the activity as a turnover frequency, it can also be given in terms of ... 

Figure 8.14. Turnover frequency for methanol synthesis from H2 and CO2 at 18 bar and 523 K as a function ofZn coverage on polycrystalline copper. [From j. Nakamura, I. Nakamura,...

Figure 8.25. Predicted volcano plots for ammonia synthesis, showing the turnover frequency versus the relative bonding strength of N atoms to the surface for ammonia concentrations of 5%, 20%, and 90%. The left-hand panel corresponds to conditions of...

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

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