Ethylene hydrogenation, rate

Table 4. Ag/Pt molar ratio and its influence on ethylene hydrogenation rates and apparent activation energy for nanoparticle encapsulated shape-controlled Pt nanoparticles [17].

Kinetics and activation energies for ethylene hydrogenation Rate = k Ph2 po1ua... 

Figure 7.42. Turnover rates for ethylene hydrogenation, the rehydrogenation of ethylidyne, and the deuteration of the methyl- group of ethylidyne on platinum and rhodium crystal surfaces [190]. Note that ethylene hydrogenation rates are orders of magnitude faster than the rate of removal of chemisorbed ethylidyne.

A particularly interesting phenomenon is the conformation of the polymer matrix that provides the favored configuration for the catalytic center. To illustrate this point let us describe the abnormal Arrhenius dependence of the ethylene hydrogenation rate in the gas phase by rhodium complexes immobilized on phosphynated polystyrene [41], A sharp increase of hydrogenation rate has been observed when the temperature increases across the glass transition point, Tg, of the polymer (341 K Fig. 12-4). 

Table 12-6. The influence of the P/Ir ratio on the ethylene hydrogenation rate.

Some generalizations that pertain are (1) Terminal olefins are more rapidly reduced than internal olefins (2) conjugated olefins are not reduced at 1 atmosphere (3) ethylene is not hydrogenated. Rates of reduction compare favorably with those obtained by heterogeneous catalysts such as Raney nickel or platinim oxide. In fact, the hydrogenation of some olefins may be so rapid that the temperature of the solution (benzene) is raised to the boiling point. 

Fig. 15. Kinetics of the ethylene hydrogenation on Ni and 0-Ni-hydride film catalysts m denotes mass of films, which as known is connected with the thickness and crystallite sizes of the films involved. Blank points—rate of reaction proceeding on Ni film catalysts black points—rate of reaction proceeding on nickel previously exposed to the atomic hydrogen action, i.e. transformed to some extent into /3-Ni-hydride.

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

Table 3. Ethylene hydrogenation reaction rates and kinetic parameters for both series of Pt/SBA-15 catalysts [13,16].

Reaction rate data were reported as a function of temperature and are shown in Figure 12P.4. Although the form of the intrinsic rate equation for ethylene hydrogenation for this specific catalyst is not known, one might anticipate an equation of the form... 

Doi and co-workers379 carried out kinetic and mechanistic studies for the hydrogenation of ethylene with [H4Ru(CO)i2] as the precatalyst. The hydrogenation rate is first-order with respect to cluster concentration, and increased to constant values with increasing ethylene and hydrogen pressure. An inverse dependence of the reaction rate on CO pressure was also observed. The mechanism proposed is in accordance with a cluster-catalyzed reaction (Scheme 74). 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

Fig. 33. Dependence of surface coverage of (a) ethyl radicals C3H5 , (b) hydrogen atoms, and (c) rate of ethylene hydrogenation at Pt cathodes.

Compared with the extensive studies of the kinetics of ethylene hydrogenation, the kinetics of the hydrogenation of propene have received little attention. Over platinum—pumice catalysts at 18° C, the kinetic rate law was observed to be... 

Examples of these favourable reaction conditions are seen in ammonia production (N2/H2) [3] and ethylene hydrogenation (C2H2/H2) [4], The reaction rates in these processes can be very high, 1-10 mmol/gcalmin [3,4] (i. e., about 2 000 - 20 000 kgprodycl/m3reactorh). 

Fig. 1. Arrhenius plots for hydrogenation rate of ethylene over nickel on doped zinc oxide.

The process of catalyst oxidation and reduction can be treated as a reversible phase transition [136]. It is to this process that the authors of recent investigations [37, 47-49, 85] ascribe critical effects. When studying kinetic self-oscillations in the oxidation of hydrogen over nickel [37] and measuring CPD, the authors established that the reaction performance oscillates between the states in which oxygen is adsorbed either on the reduced or on the oxidized nickel surface. Vayenas et al. [47-49], by using direct measurements of the electrochemical activity of 02 adsorbed on Pt, showed that the isothermal self-oscillations of the ethylene oxidation rate over Pt are due to the periodic formation and decomposition of subsurface Pt oxides. A mathemati-... 

Ethylene reacts somewhat more slowly than butylene. This is the opposite of the behavior observed with nickel (16) catalysts. The olefin hydrogenation rate with nickel catalyst decreases with increasing molecular weight of the olefin. The experiments were made with WS2 that was... 

---