Electrochemical promotion of catalyst surfaces deposited on ionic and mixed conductors

The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion (EP) was investigated on Pt films deposited on Y₂O₃-stabilized-ZrO₂ (YSZ), an O²⁻ conductor, TiO₂, a mixed conductor, and Nafion 117 solid polymer electrolyte (SPE), a H⁺ conductor and also on Pd films deposited on YSZ and β″-Al₂O₃ a Na⁺ conductor. Four catalytic systems were investigated, i.e. C₂H₆ oxidation on Pt/YSZ, C₂H₄ oxidation on Pd/YSZ and Pd/β″-Al₂O₃, C₂H₄ oxidation on Pt/TiO₂ and H₂ oxidation on Pt/Nafion 117 in contact with 0.1 M aqueous KOH solution. In all cases pronounced and reversible non-Faradaic electrochemical modification of catalytic rates was observed with catalytic rate enhancement up to 2000% and Faradaic efficiency values up to 5000. All reactions investigated exhibit a pronounced electrophobic behaviour which is due to the weakening of chemisorptive oxygen bond at high catalyst potentials. Ethane oxidation, however, also exhibits electrophilic behaviour at low potentials due to weakened binding of carbonaceous species on the surface. The general features of the phenomenon are similar for all four cases presented here showing that the NEMCA effect is a general, electrochemically induced, promoting catalytic phenomenon not depending on the reaction and the type of supporting electrolyte. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

High resolution Auger electron spectroscopy and microscopy of a supported metal catalyst

High spatial resolution Auger electron spectra and scanning Auger microscope (SAM) images of supported metal catalysts have been obtained in a UHV scanning transmission electron microscope. Ag/α-Al₂O₃ was used as a model catalyst system, where silver was evaporated, in situ, onto polycrystalline alumina carriers. Silver particles, as small as 2 nm in diameter, were clearly revealed in SAM images with high contrast. On large islands, an edge resolution < 3 nm was achieved. Information about surface and bulk properties of supported catalysts can be extracted from images formed with different signals generated from the same area which are obtained simultaneously.     

Influence of the spacing between metal particles on the kinetics of reaction with spillover on the supported metal catalyst

The influence of the spacing between active metal particles placed on the supported metal catalyst on the kinetics of the catalytic reaction with spillover was investigated. The 2A+B₂→2AB reaction, modelling the CO oxidation on Pd/Al₂O₃ catalyst, was studied using Dynamic Monte Carlo simulations. It was shown that there exists an optimal spacing, that provides the maximum reaction rate. It was postulated that this optimum is a consequence of both competition and cooperative effects occurring between metal particles.     

Electrochemical promotion of environmental catalysis

Electrochemical catalysts were used for environmental applications, such as the clean production of energy from propane and propene combustion, the elimination of VOC's like propene and the NOx abatement All the selected electrochemical catalysts were composed of a Pt film deposited on YSZ, NASICON or CGO. It was found that all these chemical reactions can be electropromoted. Moreover, the reaction rates can be in-situ tuned by applying a polarisation. Furthermore, the selectivity of Pt-based electrochemical catalysts can be modified in order to avoid the formation of pollutant. Finally, EPOC can improve the lifetime of a catalyst by inhibiting its poisoning by carbonaceous species. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14–18, 2004.     

Electrochemical promotion of CH4 oxidation on Pd

The effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) or Electrochemical Promotion (EP) was used to promote the methane oxidation reaction to CO₂ and H₂O over Pd polycrystalline films interfaced with yttria-stabilized zirconia in galvanic cells of the type: CH₄, O₂, CO₂, Pd/YSZ/Au, CH₄, O₂, CO₂ It was found that by applying positive potentials or currents and thus, supplying O²⁻ onto the catalyst surface, up to 90-fold increases in CH₄ oxidation catalytic rate can be obtained. The induced changes in catalytic rate were two orders of magnitude higher than the corresponding rate of ion transfer to the catalyst-electrode surface, i.e. faradaic efficiency Λ values above 100 can be attained. The reaction exhibits electrophobic behavior under the experimental conditions of the investigation. The results can be rationalized on the basis of the theoretical considerations invoked to explain NEMCA behavior, i.e. the effect of changing work function on chemisorptive bond strengths of catalytically active electron donor or acceptor adsorbates. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Dielectric supported ring-shaped metal disks on a metal film for ultrasensitive refractive index sensing

We experimentally report a magnetic plasmonic metamaterial, which is constructed with a metal ring-shaped disk array supported by a dielectric layer on a metal film for high sensitive refractive index sensing. An ultrasensitive refractive index sensitivity of about 1842?nm per refractive index unit is achieved through the reflection spectrum measurement. We attribute the high sensitivity to greatly enhanced electric field intensity and its large spatial overlapping with the surrounding medium to sense. The present plasmonic structure provides an effective way for high sensitive chip-based biochemical sensors and integrated devices.     

Electrochemical promotion in emission control catalysis

Electrochemical promotion for the catalytic reduction of NO by CO and of NO by ethylene over a Pt catalyst are reported for the first time. Both reactions are of importance in the catalytic control of automotive emissions and both exhibit strong rate enhancement when Na is pumped to the Pt catalyst electrode from a β″ solid electrolyte. Complementary data obtained with a Pt(111)/Na model system indicate that electrochemically-pumped Na acts by inducing dissociation of chemisorbed NO, which is the reaction initiating step. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994.     

Electrochemical promotion of environmentally important catalytic reactions

The performance of conventional heterogeneous metal catalysts may be enhanced by the addition of so-called promoter species that are used to modify the intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical methods provide an alternative, radically different and uniquely efficacious method of catalyst promotion. Substantial and reversible changes in catalyst perfomance can be induced by back-spillover ions pumped from a solid electrolyte to the surface of a catalytically active electrode: one hasin situ control of the working catalyst. Studies of the electrochemical promotion of NO reduction over Pt films supported on β″-alumina (a sodium ion conductor) demonstrate that major enhancements in activity are possible when Na is pumped to the catalyst surface. We have examined the NO+CO reaction and the reaction of NO with propene. Both reactions are relevant to control of automotive and other emissions, and both exhibit strong electrochemical promotion. By simulating lean-burn engine conditions, we have also demonstrated that EP of a Pt catalyst very substantially enhances the ability of NO to oxidise propene in an oxygen-rich atmosphere. Reaction kinetic data obtained as a function of catalyst potential, temperature and gas composition indicate that Na increases the strength of NO chemisorption relative to CO or propene, a process that is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is the critical reaction-initiating step. XP spectroscopy under the appropriate conditions of temperature and catalyst potential confirms that the mode of operation of the elctrochemically promoted Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Electrochemical promotion of ammonia decomposition over Fe catalyst films interfaced with K+- & H+- conductors

The electrochemical promotion of the ammonia decomposition reaction over Fe catalyst films interfaced with K₂YZr(PO₄)₃, a K⁺-conductor, and CaZr_0.9In_0.1O_3−a, a H⁺-conductor, was investigated at temperatures 500–600 °C. At the higher temperatures, the catalytic rate was found to decrease significantly and reversibly upon decreasing the catalyst potential, V_WR, i.e., upon pumping K⁺ or H⁺ to the Fe surface. The effect of potassium was more pronounced than that of protons leading to almost complete poisoning of the reaction. At the lower temperatures, it was found that electrochemical supply of moderate amounts of potassium causes an enhancement in the catalytic rate while higher amounts poison the reaction. The study reports, for the first time, electrochemical promotion over a Fe catalyst and shows that K⁺-conductors can also be used to induce the effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA). The effect of backspillover potassium ions and protons on the catalytic activity is discussed in terms of the theory of electrochemical promotion by considering the effect of varying catalyst work function on the coverages and chemisorptive bond strengths of NH₃, N and H. The results show the possibility of using electrically promoted catalyst pellets to alter the performance of industrial reactions. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997.     

Electrochemical promotion of propane deep oxidation on doped lanthanum manganites

Electrochemical catalysts based on LSM deposited on YSZ were used for propane deep oxidation. Electrochemical characterizations such as impedance spectroscopy and current–overpotential measurements were performed. Different thicknesses of LSM films were catalytically characterized and EPOC experiments were carried out on those electrochemical catalysts. It was found that the reaction rates can be in situ tuned by applying an anodic polarization but with weak promotional effects. The faradaic efficiency decreased as the applied potential, the temperature, and the thickness of the catalyst increased. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Electrochemical promotion of NO reduction by propene on Pt/YSZ

The reduction of NO by C₃H₆ in the presence of oxygen, is of great environmental importance. Platinum-based catalysts are very active but not selective towards N₂ production and mainly convert NO into N₂O, which participates to the greenhouse effect. Moreover, their operating temperature window is quite narrow. Electrochemical promotion was used to improve platinum catalytic behaviour. Platinum was deposited on YSZ (Y₂O₃ — stabilised ZrO₂), an O²-conductor. It was found that a negative current increased the rate of NO reduction and CO₂ formation. This rate enhancement was non-Faradaic with an apparent Faradaic efficiency (Λ) close to 180 indicating the manifestation of a NEMCA effect. However, the current application had no effect on the N₂ selectivity Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Electrochemical promotion of Pt during the oxidation of ethanol

The reaction of ethanol oxidation has been examined over a Pt polycrystalline film deposited on an yttria stabilized zirconia (YSZ) electrochemical reactor. Experiments were carried out at 325 °C at atmospheric total pressure and under open and closed circuit conditions. It was found that the electrochemically-induced change in the catalytic rate of ethanol consumption is four orders of magnitude higher than the rate of ion transport through the solid electrolyte. The galvanostatic transient behavior of the consumption rates of ethanol and oxygen and of the formation rates of carbon dioxide and acetaldehyde have been also recorded. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electrochemical promotion of electronically isolated and dispersed Pt catalysts

In this paper we discuss the first attempts to induce the effect of Electrochemical Promotion or Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) on highly dispersed catalyst-electrodes systems which can compete in terms of dispersion and surface area with industrial catalysts. Three systems are discussed:

1. Electrochemical promotion of C₂H₄ oxidation on electronically isolated Pt catalysts on Y₂O₃-stabilized-Zirconia (YSZ) where NEMCA is induced via potential application between two terminal Au electrodes also supported on the solid electrolyte (bipolar design).
2. Electrochemical promotion of C₂H₄ oxidation on a finely dispersed Pt catalyst deposited on a Au electrode which is supported on YSZ.
3. Induction of NEMCA during CH₃OH oxidation on Pt without external voltage application by utilizing the potential difference developed between the catalyst and a catalytically inert counter electrode.

In all cases significant non faradaic behavior has been obtained. The underlying catalytic/ electro-catalytic phenomena are discussed together with some of the engineering challenges for potential practical applications.     

扫描隧道显微镜针尖调制的薄膜表面的原子扩散

采用原子尺度的模拟方法,探讨了在零偏压下扫描隧道显微镜(STM)针尖调制的金属表面岛上原子运动及岛边的层间质量输运. 研究结果显示STM的移动对岛上及岛边的原子扩散有重要的影响. 针尖与吸附原子的交互作用及岛和基体中强的形状变化影响了岛上吸附原子的跳跃扩散及岛边的跳下扩散和交换扩散过程. 研究发现,通过调节针尖与基体的垂直距离及针尖与吸附原子的水平距离,可以降低岛上吸附原子的跳跃扩散能垒及岛边的跳下扩散和交换扩散能垒,从而实现薄膜由三维生长模式向二维生长模式的转变. 关键词： 扫描隧道显微镜 原子运动 质量输运     

Electrochemical sensors based on transition metal oxide bronzes

Tungsten and molybdenum oxide bronzes of the general formulas A_xMO₃ and A_xM₆O₁₇ (A=K, Li, Na; M=W, Mo) are well known as electrode materials with good response to pH changes. However, there is also a high cross sensitivity to other ions, e.g. Na⁺ or Li⁺. Currently, the synthesis of tailored materials for special applications is the priority task. Preparation routes such as fused salt electrolysis, solid state reactions in vacuum or hydrothermal synthesis were studied. New compounds of the general formula Li_xMo_1−yW_yO₃ and also A_xWO₃ (A = bi- or trivalent) have a high technological potential. Preliminary experiments have shown that some of these new materials possess properties of reference electrodes within a restricted pH range. To prepare electrodes, usually single crystals are required. This is adverse because of high operating expense of single crystal synthesis. In some cases it is also possible to use polycrystalline materials for the preparation. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Electrochemical activities of Pt-Ru-Co and Pt-Ru-Ni electrocatalysts supported by a carbon fiber web

Dynamic cyclic voltammetry is performed as functions of cycle number and potential window for the electrodeposition of Pt-Ru-Co and Pt-Ru-Ni catalysts on a carbon fiber (CF) web. Surface morphologies of the electrodeposited catalysts are observed by a scanning electron microscope. The electrochemical activity in methanol oxidation is examined by the cyclic voltammetry using a mixed aqueous solution of methanol as an electrolyte component. As the cycle number increases, both the current density and the amount of the electrodeposited metallic particles on the CF web are increased. The Pt-Ru-Ni/CF catalyst prepared in the potential range of −0.6 to 0.2 V and from the 1:2:2 mixed solution of H₂PtCl₆, RuCl₃·3H₂O, and Ni(NO₃)₂·6H₂O, respectively, shows the higher current density, corresponding to the higher catalytic activity.     

Peculiarities of carrier scattering at the boundary of a normal metal and a quasi-one-dimensional conductor with a charge-density wave

Nonlinear effects at the interface between a normal metal and the quasi-one-dimensional conductor with a charge-density wave K_0.3MoO₃ are investigated experimentally. It is found that the scattering of the normal carriers with energy E less than the Peierls gap Δ_p gives rise to an excess resistance of the N-CDW boundary. The character of the scattering is similar to Andreev reflection. In contrast to superconductors, however, the sign of the charge of the reflected particle does not change. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 259–264 (25 August 1996)     

Guided subwavelength plasmonic mode supported by a slot in a thin metal film

We demonstrate the existence of a bound optical mode supported by a slot in a thin metallic film deposited on a substrate, with slot dimensions much smaller than the wavelength. The modal size is almost completely dominated by the near field of the slot. Consequently, the size is very small compared with the wavelength, even when the dispersion relation of the mode approaches the light line of the surrounding media. In addition, the group velocity of this mode is close to the speed of light in the substrate, and its propagation length is tens of micrometers at the optical communication wavelength.