Ethylene epoxide

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... 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

Despite the poisoning action of Cl for oxygen dissociative adsorption on Ag, it is used as moderator in the ethylene epoxidation reaction in order to attain high selectivity to ethylene oxide. The presence of Cl adatoms in this... 

In the following we will concentrate on three important cases, i.e. CO oxidation on alkali doped Pt, ethylene epoxidation on promoted Ag and synthesis gas conversion on transition metals. We will attempt to rationalize the observed kinetic behaviour on the basis of the above simple rules. 

In summary one can view the ethylene epoxidation system as one where selectivity maximization requires the coexistence of the following two adsorption reactant states ... 

The commonly held view of the uniqueness of Ag for ethylene epoxidation may soon change in view both of the propene epoxidation work of Haruta and coworkers on Au/Ti02 catalysts upon cofeeding H2 123 and also in view of the recent demonstration by Lambert and coworkers124 126 that Cu(lll) and Cu(110) surfaces are both extremely efficient in the epoxidation of styrene and butadiene to the corresponding epoxides. In fact Cu was found to be more selective than Ag under UHV conditions with selectivities approaching 100%.124-126 The epoxidation mechanism appears to be rather similar with that on Ag as both systems involve O-assisted alkene adsorption and it remains to be seen if appropriately promoted Cu124 126 can maintain its spectacular selectivity under process conditions. 

C.T. Campbell, and M.T. Paffett, The role of chlorine promoters in catalytic ethylene epoxidation over the Ag(110) surface, Applications of Surface Science 19, 28-42 (1984). 

C. Karavasilis, S. Bebelis, and C.G. Vayenas, Non-Faradaic Electrochemical Modification of Catalytic Activity 10. Ethylene epoxidation on Ag deposited on stabilized Zr02 in presence of chlorine moderators, J. Catal. 160, 190-204 (1996). 

Figure 4.42. Ethylene epoxidation on Ag/p"-Al203.101 Steady-state effect of catalyst potential on the selectivity to ethylene oxide at various levels of gas-phase dichloroethane (a) and 3-dimensional representation of the effect of dichloroethane concentration, catalyst potential and corresponding Na coverage on the selectivity to ethylene oxide (b).101 Reprinted with permission from Academic Press.

One of the most striking results is that of C2H4 oxidation on Pt5 where (xads,o ctact = -1, i.e. the decreases in reaction activation energy and in the chemisorptive bond strength of oxygen induced by increasing work function ethylene epoxidation and deep oxidation on Ag.5... 

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). 

S. Boghosian, S. Bebelis, C.G. Vayenas, and G.N. Papatheodorou, In Situ High Temperature SERS on Ag Catalysts and Electrodes during Ethylene Epoxidation, J. Catal. 117,561-565 (1989). 

Figure 8.38. Steady state effect of current on the increase in the rates of ethylene epoxidation (rj) and deep oxidation to CO2 (r2) of C2H4 on Ag and comparison with the rate Go2=I/4F of electrochemical oxygen supply42 pC2H4=l-6 kPa, pO2=10 kPa, T=400°C intrinsic (1=0) selectivity 0.5, Reprinted with permission from Academic Press.

Figure 8.41 Effect of Ag/YSZ catalyst overpotential on the activation energy E and preexponential factor k° of ethylene epoxidation (open symbols) and oxidation to C02 (closed symbols) pC2H4=2.48 kPa, p02=3.15 kPa.45 Reprinted with permission from Academic Press.

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.

M. Stoukides The name of M. Stoukides is associated with the first electrochemical promotion studies and publications in 1981 (Chapter 1) when he as a graduate student of C. Vayenas at MIT was investigating ethylene epoxidation on Ag/YSZ. In recent years the group of Professor M. Stoukides in Thessaloniki has made interesting electrochemical promotion studies ofH2S decomposition and C2H4 and NH3 synthesis at elevated temperatures near the border of electrochemical promotion and electrocatalysis. 

Electrochemical promotion, or non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) came as a rather unexpected discovery in 1980 when with my student Mike Stoukides at MIT we were trying to influence in situ the rate and selectivity of ethylene epoxidation by fixing the oxygen activity on a Ag catalyst film deposited on a ceramic O2 conductor via electrical potential application between the catalyst and a counter electrode. 

A good example is provided by the selective oxidation of ethylene to ethylene epoxide, an important intermediate towards ethylene glycol (antifreeze) and various polyethers and polyurethanes (Fig. 1.6). 

Figure 1.6. Ethylene epoxide, an important intermediate in the chemical industry.

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