Michael Stoukides

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.

Solid-Electrolyte Membrane Reactors: Current Experience and Future Outlook

This review article covers the research work that has been conducted in solid-electrolyte membrane reactors (SEMR). An overview of the types of solid electrolytes and their applications in heterogeneous catalysis is first presented, followed by a survey of SEMR studies published until 1998. Earlier and recent developments are discussed. The technoeconomic aspects and the requirements to be met for further development into industrial practice are presented and discussed.

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.

Methane activation on a La0.6Sr0.4Co0.8Fe0.2O3 perovskite catalytic and electrocatalytic results

The catalytic and electrocatalytic behavior of the La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) perovskite deposited on yttria-stabilized zirconia (YSZ) was studied during the reaction of methane oxidation. Experiments were carried out in a well-mixed (CSTR) reactor at atmospheric pressure, in the 600–900°C range. When, instead of co-feeding with methane in the gas phase, oxygen was electrochemically supplied as O2−, considerable changes in the methane conversion and product selectivity were observed. Catalytic and electrocatalytic results were compared to those obtained when the LSCF served as a dense mixed-conducting membrane supplying oxygen to the methane feed stream because of the oxygen partial-pressure gradient across the membrane.

Electrocatalytic nonoxidative dimerization of methane over Ag electrodes

Abstract The electrocatalytic nonoxidative dimerization of CH 4 to C 2 H 4 and C 2 H 6 was studied in a continuous stirred tank reactor at 600–750°C and atmospheric total pressure. The electrochemical reactors were one- and two-chamber cells configured as Ad|SCY|Ag where the ionic conductivity of the ytterbia-doped strontia-ceria (SCY) electrolyte was studied. The presence of H 2 O or O 2 was found to increase the protonic conductivity of the electrolyte. The oxygen-ion transport number of the SCY was small (0.02-0.03) while that of protons dominated (0.60-0.98). By applying current to the cell, the reaction rate of CH 4 dehydrogenation to C 2 H 4 and C 2 H 6 was enhanced to as much as 8 times the open-circuit rate.

Electrochemical Synthesis of Ammonia in Solid Electrolyte Cells

Developed in the early 1900s, the “Haber-Bosch” synthesis is the dominant NH3 synthesis process. Parallel to catalyst optimization, current research efforts are also focused on the investigation of new methods for ammonia synthesis, including the electrochemical synthesis with the use of solid electrolyte cells. Since the first report on Solid State Ammonia Synthesis (SSAS), more than 30 solid electrolyte materials were tested and at least 15 catalysts were used as working electrodes. Thus far, the highest rate of ammonia formation reported is 1.13×10−8 mol s−1 cm−2, obtained at 80°C with a Nafion solid electrolyte and a mixed oxide, SmFe0.7Cu0.1Ni0.2O3, cathode. At high temperatures (>500oC) the maximum rate was 9.5*10-9 mol s−1 cm−2 using Ce0.8Y0.2O2-δ -[Ca3(PO4)2 -K3PO4] as electrolyte and Ag-Pd as cathode. In this paper, the advantages and the disadvantages of SSAS vs the conventional process and the requirements that must be met in order to promote the electrochemical process into an industrial level, are discussed.

Catalytic oxidation of methane on polycrystalline palladium supported on stabilized zirconia

The oxidation of methane on porous polycrystalline palladium supported on yttria-stabilized zirconia was studied at atmospheric total pressure in a continuous-stirred tank reactor (CSTR) at temperatures between 450 and 600 °C. The technique of solid-electrolyte potentiometry (SEP) was used to measure in situ the thermodynamic activity of oxygen adsorbed on the catalyst surface. The reaction kinetics are consistent with an Eley-Rideal model according to which adsorbed atomic oxygen reacts with gaseous methane. The potentiometric measurements suggest that only one oxygen atom is involved in the rate-limiting step. A kinetic model that satisfies both kinetic and SEP results is discussed.

Synthesis Gas Production from Methane over an Iron Electrode in a Solid Electrolyte Cell

The production of synthesis gas, CO + H[sub 2], from methane oxidation was studied in an yttria-stabilized zirconia reactor at 950C and atmospheric pressure. The anodic electrode was Fe and the cathode that was exposed to air was Pt. Reduced iron was more active than oxidized iron for syngas formation. The effect of two oxygen sources, gaseous oxygen and ionically transported oxygen, on methane conversion and selectivity to syngas was studied. O[sup 2[minus]] transported through the electrolyte promoted CO formation more favorably than gaseous oxygen. The maximum CO selectivity and yield using O[sup 2[minus]] was nearly 100 and 73%, respectively. Ionically transported oxygen and gaseous oxygen, however, did not differ significantly in H[sub 2] formation. Carbon formation occurred with both oxidants; however, less with O[sup 2[minus]].

Modelling of equilibrium limited hydrogenation reactions carried out in H+ conducting solid oxide membrane reactors

A model is prepared for the hydrogenation reactions of ammonia and methanol synthesis that are carried out in solid electrolyte cell-reactors. Hydrogen is supplied in the form of H+ through the proton conducting wall and the thermodynamic activity of hydrogen on the catalyst surface is controlled electrochemically. Under certain conditions, the yield of the cell-reactor is orders of magnitude higher than that of the corresponding conventional catalytic reactor. The key parameters that effect the product yield of either reaction are identified and their role is discussed.

Electrochemical studies of methane activation

The direct conversion of methane into useful and more versatile chemicals is a subject that has attracted the interest of numerous researchers. Methane is a refractory molecule and therefore very difficult to convert to upgraded products. In the last thirty years, the electrochemical studies of methane activation have contributed significantly by adding various alternative solutions to this very challenging research problem. In the present communication, the most important findings of low, moderate and high temperature electrochemical studies are reviewed. Since methane activation is easier at elevated temperatures, solid electrolyte cells have been used more extensively. Most of these high-temperature works focused on the production of either synthesis gas or of C2 compounds. A third vital alternative is the development of the internally reformed methane fuel cell. Results are discussed and compared with those of conventional catalytic processes.