Costas G. Vayenas

The effect of electrochemical oxygen pumping on the rate and selectivity of ethylene oxidation on polycrystalline silver

Abstract The selectivity and ethylene oxide yield of ethylene oxidation on polycrystalline silver can be affected significantly by electrochemical oxygen pumping. The reaction was studied in the solid electrolyte cell C 2 H 4 , C 2 H 4 O, CO 2 , O 2 , Ag¦ZrO 2 (Y 2 O 3 )¦Ag, air at temperatures near 400 °C and atmospheric pressure. Under open-circuit conditions the porous silver electrode exposed to the ethylene-O 2 mixture acts as a normal catalyst for C 2 H 4 , oxidation to C 2 H 4 O and CO 2 . It was found that when external voltages are applied to the cell and O 2− is “pumped” to the catalyst, the ethylene oxide selectivity and yield increase considerably. The opposite effect is observed upon inversion of the voltage polarity. The increase in the rate of C 2 H 4 O production can exceed the rate of O 2− pumping by a factor of 400, indicating a dramatic change in the properties of the silver catalyst. The phenomenon is reversible and typical relaxation times are of the order of a few minutes. A simple model is proposed in order to explain this new phenomenon.

Kinetics, limit cycles, and mechanism of the ethylene oxidation on platinum

The oxidation of ethylene on polycrystalline Pt films was studied in a CSTR at atmospheric pressure and temperatures between 200 and 400 °C. The new technique of solid electrolyte potentiometry (SEP) was used to monitor the activity of oxygen on the metal catalyst. To this end the platinum film catalyst also served as one of the electrodes of a solid-electrolyte oxygen concentration cell and the open-circuit emf of the cell was monitored during reaction. It was found that the steady-state surface oxygen activity a0 satisfies the equation a0 = KsPO2PET, where Ks depends on temperature only. The reaction rate is first order in ethylene and adsorbed oxygen. Over a certain range of temperature and gas-phase composition both the surface oxygen activity and the reaction rate exhibit self-sustained oscillations. Limit cycles appear only over a well-defined range of surface oxygen activity a0 values. The oscillations can be explained in terms of the stability of a surface platinum oxide. The reaction mechanism is discussed in light of these observations.

The role of PtOx in the isothermal rate oscillations of ethylene oxidation on platinum

Abstract A kinetic model has been developed to explain the oscillatory phenomena observed during the oxidation of ethylene on polycrystalline Pt films in a CSTR. Direct measurement of the oxygen activity on the Pt catalyst indicates that rate and oxygen activity oscillations are caused by the periodic formation and decomposition of surface platinum oxide. The model explains semiquantitatively all the experimental observations, i.e., that (1) oscillations occur on the fuel rich side only; (2) increasing rates correspond to decreasing surface oxygen activity; (3) there exist an upper and a lower temperature limit for oscillations; (4) the frequency of oscillations is a linear function of both the ethylene/O2 ratio and the residence time in the CSTR. The thermodynamic properties of PtOx are estimated from the oxygen activity measurements. The model may be applicable to other Ptcatalyzed oscillatory reactions as well.

Solid electrolyte aided study of the ethylene oxide oxidation on silver

The oxidation of ethylene oxide on polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at atmospheric pressure and temperatures between 250 and 400 °C. The new technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the metal catalyst. To this end the silver film catalyst also served as one of the electrodes of a solid electrolyte oxygen concentration cell and the open-circuit emf of the cell was monitored during reaction. It was found that the steady-state surface oxygen activity αo is given by a0 = PO212/(1 + KETOXPETOX2), with KETOX = 3.3 · 10−5 exp (10,600T). This equation as well as the kinetics can be explained in terms of a simple reaction mechanism.

Kinetics of vapor-phase electrochemical oxidative dehydrogenation of ethylbenzene

Abstract The vapor-phase electrochemical oxidative dehydrogenation of ethylbenzene to styrene was studied at 575 °C and atmospheric pressure on a polycrystalline platinum electrocatalyst in a stabilized zirconia electrochemical reactor. Electrochemical supply of oxygen to the electrocatalytic surface increases both the dehydrogenation rate and the deep oxidation rate. Carbon dioxide formation is oxygen-limited and its rate is linear in current density. The dehydrogenation rate is enhanced as much as 600% by moderate current densities; this electrocatalytic enhancement reaches an ethylbenzene-concentration-dependent asymptote at larger current densities. Addition of gas-phase hydrogen suppresses both the deep oxidation rate and the current-induced increase of the dehydrogenation rate. A two-site Langmuir-Hinshelwood type reaction mechanism is proposed which quantitatively describes these results. In this model, the electrocatalytic enhancement of the dehydrogenation rate results from a surface oxidative dehydrogenation step in which adsorbed ethylbenzene reacts with oxidized surface sites to form styrene and water.

Steady-state analysis of high temperature fuel cells

Abstract A mathematical model is presented to describe the steady state behavior of high temperature solid electrolyte fuel cells. The resulting equations are solved for the case of anodic oxidation of hydrogen and carbon monoxide. Similar to chemical reactors, fuel cells are found to exhibit steady-state multiplicity over a wide range of parameters. The paper discusses the relative importance of the pertinent design and operating parameters in order to maintain ignited steady states corresponding to high current densities and nearly complete fuel conversion.

Kinetics and rate oscillations of the oxidation of propylene oxide on polycrystalline silver

Abstract The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 250 and 400 °C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the silver catalyst. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillatory behavior. Oxidation of propylene, ethylene, and ethylene oxide on the same surface under similar conditions does not produce oscillations. A possible cause for this phenomenon will be discussed.

Solid electrolyte-aided study of propylene oxidation on polycrystalline silver

Abstract The oxidation of propylene on porous polycrystalline Ag films supported on yttria-stabilized zirconia was studied in a CSTR at temperatures between 290 and 400 °C and atmospheric total pressure. Steady state kinetic measurements were combined with simultaneous in situ monitoring of the chemical potential of oxygen adsorbed on the silver catalyst, using the technique of solid electrolyte potentiometry (SEP). The kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood reaction model with two types of chemisorbed oxygen. The reaction network kinetics are studied thoroughly and compared with those obtained during oxidation of propylene oxide, ethylene, and ethylene oxide on the same catalytic surface.

Ammonia oxidation to nitric oxide in a solid electrolyte fuel cell

Abstract The oxidation of ammonia to nitric oxide was investigated in the fuel cell NH 3 ,NO,N 2 ,PtƒZrO 2 (8%Y 2 O 3 )ƒPt, air at temperatures between 1000 and 1200 K and at atmospheric pressure. The product selectivity to NO can exceed 95% with simultaneous electrical energy generation. A new design of the cell has resulted in an 800-fold increase in power density over the original design of Farr and Vayenas. In the new design the decreased electrode surface area suppresses the undesirable catalytic side reaction of NH3 and NO to form N2 while the thinner electrolyte (≈200 μm) increases power density.

The effect of homogeneous gas phase oxidations in char particle gasification

Abstract Two steady state models have been developed for the simultaneous oxidation and gasification of a single coal char particle. Because of the uncertainty in the relative importance of solid phase and gas phase reaction kinetics in a gasifier environment, the models assume that the velocities for the gas phase oxidations of hydrogen and carbon monoxide are either zero or infinitely fast. The global rates and steady state multiplicities are discussed as functions of particle size, film thickness, gas composition and ambient temperature. The restriction of infinite hydrogen diffusivity assumed by previous investigators is removed. The two models lead to considerably different predictions about the maximum temperature of char particles in coal gasifiers.