Ethylene electrocatalytic

Statistical Estimates of Kinetic Parameters for Ethylene Electrocatalytic Hydrogenation"... 

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

Y. Jiang, I.V. Yentekakis, and C.G. Vayenas, Methane to Ethylene with 85% Yield in a Gas-Recycle Electrocatalytic Reactor-separator, Science 264, 1583-1586 (1994). 

M. Stoukides, and C.G. Vayenas, Transient and steady-state vapor phase electrocatalytic ethylene epoxidation, ACS Symposium Series 178 ("Catalysis under transient conditions") A.T. Bell and L.L. Hegedus, Eds., pp. 181-202 (1982). 

The molecular sieve adsorbent traps ethylene quantitatively, thus practically freezing step 4. Ethane trapping is only partial, thus the desired step 3 is not decelerated significantly. Steps 1,3 and 4 are predominantly catalytic or electrocatalytic, depending on the mode of oxygen addition. 

Methane can be oxidatively coupled to ethylene with very high yield using the novel gas recycle electrocatalytic or catalytic reactor separator. The ethylene yield is up to 85% for batch operation and up to 50% for continuous flow operation. These promising results, which stem from the novel reactor design and from the adsorptive properties of the molecular sieve material, can be rationalized in terms of a simple macroscopic kinetic model. Such simplified models may be useful for scale up purposes. For practical applications it would be desirable to reduce the recycle ratio p to lower values (e.g. 5-8). This requires a single-pass C2 yield of the order of 15-20%. The Sr-doped La203... 

Transient and Steady-State Vapor Phase Electrocatalytic Ethylene Epoxidation... 

Fig. M8.1. Voltammograms of R in H2S04 0.5 M+EQ 0.1 M at 25° C. Effect of the variation in the sweep rates. (Reprinted from F. Kadirgan, B. Beden, and C. Larry, Electrocatalytic Oxidation of Ethylene-Glycol, J. Electroansi. Chem. 136, Fig.

Electrode — Electrocatalytic electrode — Figure. Plots of the exchange current density of the hydrogen electrode reaction (left) as a function of the enthalpy of the metal-hydrogen bond and the current density of the electrochemical oxidation of ethylene as a function of the enthalpy of sublimation of various metals and alloys (right). These displays are also called Volcano plots... 

The presence of electrolyte, its possible adsorption on the electrocatalyst, and the electrode-electrolyte potential can alter the strength of reactant adsorption, the surface coverage, and the reaction rate (5,7,8). Thus, electro-generative hydrogenation of ethylene on platinum and palladium electrodes in acidic electrolytes proceeds more slowly than the corresponding gas phase catalytic reactions (33). However, electrocatalytic reduction of cyclopropane is faster than the catalytic one, probably due to a decrease in hydrogen and reactant competitive chemisorption. Some electrolyte ions and impurities can also poison the electrocatalysts (34). 

Equation (1) can serve as the basis for determining the highest reduction potential or the lowest oxidation potential of an electrocatalytic reaction. Figure 3 shows that spontaneous electroreductions result in a positive E° vs. NHE, often approached with the aid of a catalytic electrode (25, 26, 31, 33, 36). For the example of ethylene shown in Fig. 3, reduction can occur below its reversible potential away from equilibrium. If the ethylene electrode is combined with a hydrogen anode, an electrogenerative cell is formed (16,17,25,26,31,33,36), similar to a fuel cell, that can generate low-voltage. 

Similarly, oxidation of ethylene can occur above its E° (3-13,28,29). With an oxygen counterelectrode, conventional electrolytic oxidation starts when the anode potential becomes more positive than the cathode. Halogenation of hydrocarbons is also possible electrogeneratively, above E°, on appropriate electrocatalytic anodes (47, 50, 51). 

Observed and Estimated A ctivation Energies for Electrocatalytic Reduction of Ethylene on Platinum Black (25 -70 C... 

Similarly, the difficulty for electrocatalytic, electrogenerative hydrogenation of alkenes on platinum parallels the strength of gas phase adsorption of the substrate (55) acetylene > ethylene > propylene > cyclopropane. Palladium is a more active electrocatalyst for ethylene reduction than platinum (55), in agreement with adsorption strength on each metal. Selectivity and reduction rate of substituted alkenes also depends on adsorption... 

Apart from poisoning by adsorbing impurities, the working electrode potential can also contribute to suppress electrocatalytic activity. Platinum metals, for instance, passivate or form surface oxygen and oxide layers above 1 V (Section IV,D), which inhibit Oj reduction (779,257,252) and oxidation of carbonaceous reactants (7, 78, 253, 254) however, decomposition of hydrogen peroxide on platinum is accelerated by oxygen layers (255). Some electrocatalysts may corrode or dissolve, especially in acidic electrolytes, while reactants may contribute to dissolution. Thus, ethylene oxidation on palladium to acetaldehyde proceeds via a Pd-ethylene complex, which releases colloidal palladium in solution (28, 29). Equivalent to this is the surface roughening and the loss of Pt in gas phase ammonia oxidation (256, 257). 

Reduction of acetylenic and ethylenic compounds on catalysts other than platinum, such as Pd, Ru, Au, Ag, C, and W, are few 33,117,118,356a). The electrocatalytic mechanisms and the action of these electrodes are currently far from being understood. 

Reaction or exchange with stable isotopic tracers and quantitative identification of all products by mass spectrometry provides indications for molecular interactions on the surface. Reactions can be studied at steady state or by following the transient distribution of isotopic products. Langer and co-workers (25,26) presented the first steady-state mechanistic analysis for the electrocatalytic hydrogenation of ethylene on Pt in deuterated electrolytes. Proton abstraction in electroorganic synthesis has also been verified using deuterated solvents (374, 375). On-line mass spectrometry permitted indirect identification of adsorbed radicals in benzene and propylene fuel cell reactions (755,795,194). Isotopic radiotracers provided some notion on adsorption isotherms (376, 377) and surface species on electrocatalysts (208, 378, 379). 

Evaluation of the electrocatalytic isotopic reaction of alkenes was facilitated by using Kemball s statistical model to determine the origin of each species and the probability of each step (380,381). The model assumes olefin adsorption and reaction with deuterium or hydrogen to form ethyl radicals. These can revert to ethylene or they can add H or D to give ethane (i = 0, 1,...,4) ... 

The investigation of the electrocatalytic activity of codeposited Pt-Pd electrodes in the oxidation of ethylene glycol constitutes a continuation of former studies with alloys. ... 

Unsaturated C2 Hydrocarbons. - The adsorption and the electrocatalytic transformations of ethylene and acetylene (reduction and oxidation) on Pt and Au electrodes have been the subject of several studies (see, for instance. References 309-311 and literature cited therein). 

The results of Pt single crystal experiments show that Pt(110) has a high activity compared with Pt(lll) and Pt(l(X)) in the electrocatalytic reduction of acetylene. The main product on Pt(l 10) was ethylene where only 10 percent ethane was found. 

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