Rate of ethylene oxidations

Figure 4.30. Volcano-type behaviour Effect of catalyst potential on the rate of ethylene oxidation on a Pt film deposited on NASICON (Na3Zr2Si2PO 2), a Na+ conductor T=430°C, P02 =7.2 kPa, Pc2H4= kPa.102 Reproduced by permission of The Electrochemical Society.

Figure 6.4. Examples for the four types of global classical promotion behaviour. Work function increases with the x-axis. (a) Steady-state (low conversion) rates of ethylene oxide (EtO) and C02 production from a mixture of 20 torr of ethylene and 150 torr of 02 for various Cs predosed coverages on Ag(lll) at 563 K19 (b) Rate of water-gas shift reaction over Cu(l 11) as a function of sulphur coverage at 612 K, 26 Torr CO and 10 Torr H202° (c) Effect of sodium loading on NO reduction to N2 by C3H6 on Pd supported on YSZ21 at T=380°C (d) Effect of sodium loading on the rate of NO reduction by CO on Na-promoted 0.5 wt% Rh supported on Ti02(4% W03).22...

Figure 9.10. Ethylene epoxidation on Ag/p"-Al203 Transient effect of a negative applied current (Na supply to the catalyst) on the rates of ethylene oxide and C02 formation and on catalyst potential (work function) and Na coverage22 T=260°C, P=5 atm, p02=17,5 kPa, Pc2H4=49 kPa, 0.6 ppm C2H4CI2. Reprinted with permission from Academic Press.

Figure 12.7. Transient effect of an applied current on the rate of ethylene oxidation (expressed in mol O/s) for three different electrode configurations of Fig. 12.6 config. 1, I=+20 pA (solid curve) config. 5, I=-20 pA (dotted curve) and configs. 6, I=+20 and -20 pA (dashed curves). Conditions T=353°C, Pc2h4=065 kPa, p02=17.5 kPa, flowrate=200 ml/min.8 Reprinted with permission from Academic Press.

Figure 12,9, Transient effect of an applied potential, UAP, between the two terminal gold electrodes (30 V) on the catalytic rate of ethylene oxidation (expressed in molO/s) for a multi-dotted platinum configuration.10 Reprinted with permission from Elsevier Science.

Structural Sensitivity. Figure 1 shows the steady-state rates of ethylene oxide (EtO) and CO2 production as a function of temperature, in Arrhenius form, at an ethylene pressure (P-.) of 20 torr and P. 

Alternatively, it may be possible to demonstrate for the pure metals that the catalytic activity is independent of film weight in a certain weight range. For example, rates of ethylene oxidation were constant over pure palladium films, deposited and annealed at 400°C and weighing between 4 and 40 mg (73). Then, if electron micrographs show that the crystallite size is relatively independent of composition, a satisfactory comparison of catalytic activity can be made at the various alloy compositions. Finally, surface area measurements are less urgently needed when activity varies by orders of magnitude, or where the main interest lies outside the determination of absolute reaction rates. 

Figure 11. Transient galvanostatic response of the rates of ethylene oxidation r, (O) and deep oxidation rt (0) and of the cell overvoltage AV for reactor RC 9. Electrolyte breakdown occurred at 40 min. Conditions RC 9, 420°C, i — 100 pA, P0l = 0.095 bar, PET = 0.012 bar.

The selectivity of ethylene oxidation was found to be independent of feed composition at zero conversion. This was interpreted to mean that each of the two parallel processes is initiated by a similar type of transformation. Selectivity at zero conversion appeared to approach, a value considerably different from 100%. Therefore the Initial rate of carbon dioxide formation does not approach zero, as it should if it has to arise exclusively from ethylene oxide. The initial rate of ethylene oxide oxidation was found to depend on the partial pressure of both ethylene oxide and oxygen. Orzechoweki and Mac-Cormack concluded from this, in conflict with Twigg s earlier proposal,1771 that isomerization of ethylene oxide to acetaldehyde ie not a significant step in its further oxidation, Ethylene oxide could undergo oxidation either on the catalyst surface or in the gas phase by ooffision with an adsorbed oxygen atom.127 ... 

The kinetics of the reaction between ethylene oxide and various alcohols has been a subject of some interest as an application of the theory of consecutive reactions. 8 1,M 12l7>1SW With ethanol, for example, a sequence of steps may be written in the manner depicted in Rq. (650), and a complex kinetic expression derived for the rate of ethylene oxide disappearance. 

If one measures the rate of ethylene oxide disappearance at constant hydroxyl concentration for a series of polyglycols in the molecular weight range 100—1500, one finds that the rate reaches a maximum at a molecular weight of about 400 and then falls off (Fig. 1). The decrease... 

A complex system is one in which more than one reaction occurs. This can lead to ijiultiple products, some of which are more desirable than others from a practical standpoint. For example, in the air oxidation of ethylene the desired product is ethylene oxide, but complete oxidation to carbon -dioxide and—water—alse-0eeu-FS--The-im-perta-nt-per-for-ma-nee-factor-is-i he production rate of ethylene oxide and its purity in the reaction products, rather than the total amount of ethylene reacted. To characterize this performance two parameters are used yield and selectivity. The yield of a specific product is defined as the fraction of reactant converted to that product. The point selectivity is the ratio of the rate of production of one product to the rate for another product. With multiple products there is a separate selectivity based on each pair of products. The overall, or integrated, selectivity is the ratio of the amount of one product produced to the amount of another. Selectivity and yield are related to each other through the total conversion, i.e., the total fraction of reactant converted to all products. 

A gaseous stream consisting of 60% C2H4, 30% O2, and 10% N2 (by mole) is fed at a rate of 40 mol/min into a flow reactor operating at steady state. If the mole firaction of oxygen in the reactor effluent stream is 0.08, calculate the production rate of ethylene oxide. 

Electrochemical promotion can be used to modify significantly the product selectivity, of catalytic oxidation reactions. An example is presented in Fig. 5 which shows the effect of catalyst potential and corresponding work function change on the selectivity to ethylene oxide (Fig. 5a) and acetaldehyde (Fig. 5b) of ethylene oxidation on AgA SZ at various levels of gas phase chlorinated hydrocarbon moderators [31] (The third, unde.sirable, product is CO2). As shown in the Figure a 500 mV decrease in catalyst potential causes the Ag surface to change from selective (up to 70%) ethylene oxide production to selective (up to 55%) acetaldehyde production. The same study [31] has shown that the total rate of ethylene oxidation varies by a factor of 200 upon varying the catalyst potential. (Fig. 6)... 

In all papers published the rate of ethylene oxidation is but slightly... 

Temkin et al. (159) studied the kinetics of ethylene oxidation over a stationary silver surface. It was shown by means of the flow-circulating method that the rate of ethylene oxide and carbon dioxide formation was proportional to ethylene concentration in the gas phase, and that there was inhibition with reaction products. 

Reyerson and Swearingen (9) found that the rate of ethylene oxidation over platinum was directly proportional to oxygen concentration and inversely proportional to ethylene concentration. 

The electron work function is related to oxygen pressure A = ylogC0t Substituting this value into the equation for the rate of ethylene oxide formation, we obtain... 

When y = 1 the reaction rate will be independent of oxygen concentration. It is also necessary to take into account the effect of reaction products on kinetics of the reaction. The rate of ethylene oxide formation is known to be inhibited both by ethylene oxide and C02. The measured shifts of with adsorption of these substances on silver have shown... 

It was shown above that the activation energy is related to . There is almost no information available on as a function of activation energies and rates of catalytic reactions over silver. According to Hayes (130) the activation energy for NaO decomposition on alloys of silver with various -decreasing metals will be low. Sosnovsky (175) has investigated the catalytic activities (E and K0) for different planes of silver crystals, with respect to the decomposition of formic acid. E and Ko were found to increase with plane indexes. The relation between and the rate of ethylene oxidation to ethylene oxide was not established. 

After removal of the load from the sterilizer, further dissipation of residues is a function of time, temperature, and ventilation. Generally, the rate of ethylene oxide dissipation doubles for every 10 C rise in temperature (the Q]q is... 

Poisoning can affect the selectivity as well as the rate of conversion, and mild poisoning may be beneficial. The oxidation of ethylene is carried out using silver catalysts that are deliberately poisoned with chlorine compounds, and the selectivity is improved, because the total oxidation reaction is suppressed more than the rate of ethylene oxide formation [14]. The presence of sulfur compounds changes the selectivity for competitive hydrogenation, such as the hydrogenation of acetylenes or diolefins in the olefins [15]. 

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