Electrochemical promotion of Pd, Fe and distributed Pt catalyst-electrodes (original) (raw)
Related papers
Electrochimica Acta, 1994
The catalytic activity and selectivity of the gasexposed electrode surface of metal electrodes in solid electrolyte cells is altered dramatically and reversibly upon polarizing the metal-solid electrolyte interface. The induced steady-state change in catalytic rate can be up to 9OW% higher than the normal (open-circuit) catalytic rate and up to 3 x 10' higher than the steady-state rate of ion supply. This new effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) has been already demonstrated for more than 30 catalytic reactions on Pt, Pd, Rh, Ag, Au and Ni surface. by using 03-, F-, Na+ and H+ conducting solid electrolytes. There is also a recent demonstration for an aqueous electrolyte system. In this paper the common features of previous NEMCA studies are summarized and the origin of the effect is discussed in light of recent in situ work function and XPS measurements which showed that: (1) solid electrolyte cells with metal electrodes are work function probes and work function controllers, via potential application, for their gas-exposed electrode surfaces; and (2) NEMCA is due to an electrochemically driven and controlled spillover of ions from the solid electrolyte onto the gasexposed electrode surface. These spillover ions establish an effective electrochemical double layer and act as promoters for catalytic reactions. This interfacing of electrochemistry and catalysis offers several exciting theoretical and technological possibilities.
Electrochemical Promotion of a Dispersed Platinum Catalyst☆
Journal of Catalysis, 1998
The effect of electrochemical promotion or nonfaradaic electrochemical modification of catalytic activity (NEMCA effect) was investigated for the first time on highly dispersed Pt catalyst (dispersion 0.2-1) supported on polycrystalline Au films which had been deposited on Y 2 O 3 -stabilized ZrO 2 (YSZ), an O 2− conductor. The oxidation of ethylene to CO 2 was chosen as a model reaction. It was found that the catalytic rate of ethylene oxidation could be reversibly enhanced by up to a factor of 5 via electrical current or potential application between the supported Pt/Au catalyst and a counter Au electrode also deposited on the YSZ ionic conductor. The increase in catalytic rate is typically a factor of 10 3 higher than the rate, I/2F, of electrochemical supply or removal of O 2− to (from) the dispersed Pt/Au catalyst from (to) the solid electrolyte. Analysis of the time constant of the catalytic rate upon current application or interruption suggests that, as in previous electrochemical promotion studies with continuous metal film catalysts, the observed promotional phenomena are due to electrochemically controlled migration (spillover and backspillover) of anionic oxygen between the solid electrolyte and the metal catalyst surface and to the effect of this anionic oxygen on the chemisorptive bond strength of coadsorbed oxygen and ethylene.
Non-faradaic electrochemical modification of catalytic activity: A status report
Catalysis Today, 1992
The catalytic activity of Pt for the oxidation of ethylene to CO 2 can be markedly and reversibly affected by interfacing polycrystalline Pt films with TiO 2 and applying currents or potentials between the catalyst film and a Au counter electrode at temperatures near 500 • C. The increase in the rate of C 2 H 4 oxidation is up to 20 times higher that the open-circuit (unpromoted) catalytic rate and at least a factor of 5000 higher than the rate of O 2− supply through the mixed conducting TiO 2 support. The latter is and remains catalytically inert during electrical bias. This electrochemically induced Schwab effect of the second kind has all the same qualitative features with the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) when using pure O 2− conductors. Work function measurements and X-ray photoelectron spectroscopic (XPS) investigation of the Pt catalyst surface under UHV conditions has also provided evidence consistent with the electrochemically controlled promoting oxide ion backspillover mechanism which is operative with NEMCA when using pure O 2− conductors. Under reaction conditions in atmospheric pressure or oxidizing environments in UHV the TiO 2 support exhibits mixed electronic (n-type)-ionic conductivity and thus the catalyst work function and catalytic activity can be controlled by the applied potential. In reducing environments the electronic conductivity of TiO 2 dominates and the catalyst work function remains constant upon application of potential.
Non-Faradaic Electrochemical Modification of Catalytic Activity
Journal of Catalysis - J CATAL, 1995
The catalytic activity of polycrystalline Rh films deposited on 8 mol% Y2O3-stabilized ZrO2, (YSZ), an O2-− conductor, can be altered dramatically and reversibly by varying the potential of the Rh catalyst film. The complete oxidation of ethylene was investigated as a model reaction in the temperature range 300 to 400°C and at atmospheric total pressure. The rate of C2H4 oxidation can be reversibly enhanced by up to 100 times by supplying O2− to the catalyst via positive potential application (up to 1.5 V). This is the highest rate enhancement observed so far with in situ electrochemical promotion studies. The steady-state rate increase is typically 104 times larger than the steady-state rate of O2− supply to the catalyst. It was also found that varying the catalyst potential causes the appearance of the well-known compensation effect with an isokinetic point at 372°C. As in previous studies of the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) the...
Electrochemical promotion of catalyst surfaces deposited on ionic and mixed conductors
Ionics, 1995
The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion (EP) was investigated on Pt films deposited on Y203-stabilized-~ (YSZ), an 02-conductor, TiO2, a mixed conductor, and Nation 117 solid polymer electrolyte (SPE), a H + conductor and also on Pd films deposited on YSZ and ~'-Al203 a Na + conductor. Four catalytic systems were investigated, i.e. C2I-I6 oxidation on Pt/YSZ, C2H4 oxidation on Pd/YSZ and Pd~"-AI203, C2H4 oxidation on Pt/TiO2 and H 2 oxidation on Pt/Nafion 117 in contact with 0.1 M aqueous KOH solution.
The Journal of Physical Chemistry
It was found that the catalytic activity and selectivity of Pt for the oxidation of methanol to formaldehyde and COz can be altered significantly and reversibly by depositing a Pt catalyst film on an yttria-stabilized zirconia (YSZ) disc and by applying current or potential between the catalyst film and a Ag film deposited on the other side of the 02-conducting YSZ disc. Both the catalyst film and the Ag counter and reference electrodes are exposed to the reacting CHsOH-02 mixture. The observed increase in the rate of HzCO production was typically a factor of 100 higher than the rate of 02supply to the catalyst with a concomitant 2-fold increase in selectivity. This demonstration of the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) to reversibly modify catalyst activity and selectivity in a single-pellet flow reactor is a new result. It has considerable practical importance as it shows that the NEMCA effect can be utilized in conventional flow-type catalytic reactors. The present study has also shown the spontaneous generation of significant reactiondriven potential differences between the catalyst and the counter electrode. This observation is significant both for catalytic and also for sensor applications.
The catalytic activity of Pt for the oxidation of ethylene to CO 2 can be markedly and reversibly affected by interfacing polycrystalline Pt films with TiO 2 and applying currents or potentials between the catalyst film and a Au counter electrode at temperatures near 500 • C. The increase in the rate of C 2 H 4 oxidation is up to 20 times higher that the open-circuit (unpromoted) catalytic rate and at least a factor of 5000 higher than the rate of O 2− supply through the mixed conducting TiO 2 support. The latter is and remains catalytically inert during electrical bias. This electrochemically induced Schwab effect of the second kind has all the same qualitative features with the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) when using pure O 2− conductors. Work function measurements and X-ray photoelectron spectroscopic (XPS) investigation of the Pt catalyst surface under UHV conditions has also provided evidence consistent with the electrochemically controlled promoting oxide ion backspillover mechanism which is operative with NEMCA when using pure O 2− conductors. Under reaction conditions in atmospheric pressure or oxidizing environments in UHV the TiO 2 support exhibits mixed electronic (n-type)-ionic conductivity and thus the catalyst work function and catalytic activity can be controlled by the applied potential. In reducing environments the electronic conductivity of TiO 2 dominates and the catalyst work function remains constant upon application of potential.
Materials Science Forum, 1991
It was found that the catalytic activity and selectivity of polycrystalline Pt for the oxidation of methanol to formaldehyde and CO2 can be dramatically and reversibly affected when oxygen anions 02-are electrochemically pumped to or from the Pt catalyst surface. The experiments were conducted using a stabilized zirconia solid electrolyte at temperatures 600 to 900 K. The steady state increases in the catalytic rates of H2CO and CO2 formation, are typically 103-104 higher than the rate of 0 2-transport to or from the catalyst surface. Over a wide range of experimental conditions the catalytic rates depend exponentially on the catalyst-solid electrolyte overpotential, which is proportional to the induced change in catalyst work function. The product selectivity to H2CO can be varied between 35 and 60% by controlling the catalyst potential. The phenomena are reversible and show that catalyst work function and catalytic activity and selectivity can be varied at will by adjusting the catalyst potential. As in previous studies of non-Faradaic electrochemical modification of catalytic activity one can interpret the observed behaviour by taking the change in catalyst work function with changing catalyst potential and the concomitant changes in the strength of chemisorptive bonds into account. ~:~ 1991 Academic Pres~, Inc.
Solid State Ionics, 2000
Electrochemical promotion (EP), discovered and developed by Vayenas and co-workers provides a novel in situ reversible and highly controllable means of catalyst promotion. We found that Pt-group metal catalysts exhibit strong EP by sodium during reactions related to emission control catalysis, such as NO reduction by hydrocarbons. Close similarities are found between the performance of Pt-film catalyst promoted electrochemically with Pt highly dispersed on large surface area carriers (e.g. g-Al O ) promoted by conventional means (impregnation). These similarities include (i) the overall kinetic 2 3 behaviour and (ii) the dependence of the activity and selectivity on Na loading. Using both methods of Na-promotion, the catalytic reduction of NO by propene over Pt exhibited rate enhancements as high as two orders of magnitude accompanied by very pronounced increases of the system selectivity towards N . The results serve to validate further the interpretation 2 offered for the EP (or NEMCA) phenomenon. More importantly, they demonstrate that the insight obtained from EP studies can be used to design conventional type effective catalyst formulations that were previously untried, thus opening up new areas for investigation in the frontiers between catalysis and electrochemistry.