I.V. Yentekakis

Methane to Ethylene with 85 Percent Yield in a Gas Recycle Electrocatalytic Reactor-Separator

Methane was oxidatively coupled to ethylene with an ethylene yield up to 85 percent and a total C2 hydrocarbon yield up to 88 percent in a gas recycle high-temperature (800�C) electrocatalytic or catalytic reactor where the recycled gas passes continuously through a molecular sieve trap in the recycle loop. Oxygen is supplied either electrocatalytically by means of the solid electrolyte support of the silver-based catalyst or in the gas phase. The C2 products are obtained by subsequent heating of the molecular sieve trap. The selectivity to ethylene is up to 88 percent for methane conversion up to 97 percent.

The effect of electrochemical oxygen pumping on the steady-state and oscillatory behavior of CO oxidation on polycrystalline Pt

The effect of electrochemically pumping 02- to or from a porous polycrystalline Pt catalyst film used for CO oxidation at atmospheric pressure and temperatures 250-600°C was studied. The Pt film served both as a catalyst and as an electrode of the solid electrolyte cell CO, 02, PtiZrOz (8 mole% Y203)/Pt,02. Under open-circuit conditions the Pt catalyst film operates as a regular CO oxidation catalyst. It was found that electrochemical O*- pumping has a dramatic non-Faradaic effect on the steady-state and oscillatory behavior of CO oxidation on Pt. The steady-state reaction rate typically increases or decreases by a factor of 2 but a 500% increase in reaction rate is observed under severely reducing conditions. The induced changes in reaction rate are typically two orders of magnitude higher than the rate of 02- transfer to or from the catalyst and are always accompanied by the appearance of activation overpotential at the catalyst electrode. Reaction rate oscillations can be induced or stopped at will by adjusting the rate of O*- transfer and consequently the potential of the catalyst-electrode. The frequency of electrochemically induced oscillations is linearly related to the applied O*m current. The observed phenomena are completely reversible and are due to electrochemically induced changes in the oxidation state and catalytic properties of the platinum surface. These changes appear to result from changes in the work function of the metal due to the interaction of 0 anions with the Pt surface. The very pronounced reaction rate increase upon 02- removal under reducing conditions appears to be caused by CO decomposition followed by fast carbon combustion by gaseous 02.

Chemical Cogeneration in Solid Electrolyte Cells

The anodic oxidation of H2S was investigated in the solid electrolyte fuel cell H2S, Sx, SO2, Pt/ZrO2(8% Y~O3)/Pt, air operating at atmospheric pressure and temperatures 650 ~ to 800~ It was found that the fuel cell product selectivity crucially depends on the ratio M of the fluxes of oxygen anions 02_ and H2S reaching the porous Pt anode. When M < 0.33, elemental sulfur is the major product, and the anode is severely polarized. For higher M values, the product selectivity to SO2 exceeds 99% at H2S conversions as high as 99%. The cell appears to be a promising candidate for the cogeneration of electric energy and sulfur dioxide.

Solid electrolyte aided study of the mechanism of CO oxidation on polycrystalline platinum

The mechanism of CO oxidation on polycrystalline Pt at atmospheric pressure has been investigated by combining kinetic and simultaneous potentiometric studies in a gradientless reactor containing one or two polycrystalline Pt films supported on stabilized zirconia. The initial oxidation state of the catalyst was found to have an important effect both on the steady-state behavior and on the waveform of rate and emf oscillations. A simple kinetic model where both oxygen adsorption and surface reaction are rate limiting is found to describe semiquantitatively the steady-state kinetic and potentiometric results both on preoxidized and on prereduced surfaces. The oscillatory behavior of the system was studied in detail by simultaneous mass spectroscopic monitoring of the concentrations of O/sub 2/ and CO/sub 2/. The kinetic and potentiometric results suggest strongly that the oscillations are caused by periodic formation and consumption of surface PtO/sub 2/. The formation of PtO/sub 2/ is verified by a series of surface CO-O/sub 2/ titration experiments. The experiments with two polycrystalline films show that oscillation synchronization occurs via the gas phase as the two films exposed to the same gaseous environment exhibit synchronous oscillations in the surface oxygen activity.

Support-induced promotional effects on the activity of automotive exhaust catalysts: 1. The case of oxidation of light hydrocarbons (C2H4)

Abstract The kinetics of oxidation of a light hydrocarbon (C 2 H 4 ) were studied on catalysts comprising of combinations of one of three metals, Pt, Pd or Rh supported on five different supports, that is, SiO 2 , γ-Al 2 O 3 , ZrO 2 (8% Y 2 O 3 ), TiO 2 or TiO 2 (W 6+ ). Significant variation of turnover frequency with the carrier was observed, which cannot be explained by structure sensitivity considerations and is attributed to interactions between the metal crystallites and the carrier. The catalytic activity of these metal-support combinations was investigated over a wide range of partial pressures of ethylene and oxygen. In a separate set of experiments, the kinetics of C 2 H 4 oxidation were also investigated on polycrystalline Rh films interfaced with ZrO 2 (8 mol% Y 2 O 3 ) solid electrolyte in a galvanic cell of the type: C 2 H 4 , O 2 , Rh/YSZ/Pt, air, during regular open-circuit conditions as well as under Non-Faradic Electrochemical Modification of Catalytic Activity (NEMCA), that is, closed-circuit conditions. Up to 100-fold increase in catalytic activity was observed by supplying O 2− ions to the catalyst surface via positive potential application to the catalyst. The observed kinetic behavior upon increasing catalyst potential parallels qualitatively the observed alteration of turnover frequency with variation of the support of the Rh crystallites.

Non-Faradaic Electrochemical Modification of Catalytic Activity in Solid Electrolyte Cells

The catalytic activity and selectivity of metal catalysts used as electrodes in high temperature solid electrolyte cells can be altered dramatically and in a reversible manner. This is accomplished by electrochemically supplying oxygen anions onto catalytic surfaces via polarized metal-solid electrolyte interfaces. Oxygen anions, forced electrochemically to adsorb on the metal catalyst surface, alter the catalyst work function in a predictable way and lead to reaction rate increases as high as 4000%. Changes in catalytic rates typically exceed the rate of O2− transport to or from the catalyst surface by 102-3 · 105. Significant changes in product selectivity have been also observed. The case of several catalytic reactions in which this new phenomenon has been observed is presented and the origin of the phenomenon is discussed.

Catalysis, Electrocatalysis and Electrochemical Promotion of the Steam Reforming of Methane over Ni Film and Ni-YSZ Cermet Anodes

AbstractThe kinetics of the steam reforming reaction of CH4 were investigated at temperatures 750 to 950°C under both open-circuit and closed-circuit conditions on Ni-YSZ (Yttria Stabilized Zirconia) solid oxide fuel cell (SOFC) anodes and polycrystalline Ni film SOFC anodes of measured Ni surface area. It was found that the rate of methane reforming on the Ni surface exhibits a Langmuir-Hinshelwood type dependence on

The effect of sodium on the Pd-catalyzed reduction of NO by methane

P_{CH_4 }