Influence of the thickness of sputter-deposited platinum films on the electrochemical promotion of propane combustion

8 to 120 nm-thick Pt coatings were sputter-deposited on Yttria-Stabilised Zirconia (YSZ) membranes, 17 mm in diameter, by magnetron sputtering of a Pt target at low pressure (0.3 Pa). The catalytic activity of propane combustion under open-circuit conditions is first measured near to the stoichiometry (0.2%C₃H₈/1%O₂) and shows that the coatings present a rather high catalytic activity. Close-circuit measurements were finally performed at 337 °C and 400 °C. They show that rather high faradaic efficiencies in the range 10⁶ can be reached as soon as the Pt film is thick enough to allow a bias of its whole area. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Electrochemical characterisation of the Pt/YSZ interface exposed to a reactive gas phase

The Pt/yttria-stabilized cubic zirconia (YSZ) interface exposed to a reactive gas was characterised by solid electrolyte potentiometry and cyclic voltammetry. The catalytic reactions included total combustion of C₃H₈ and C₃H₆ to CO₂ and H₂O as well as NO reduction by C₃H₆ in the presence of O₂ under oxygen-rich and stoichiometric conditions. The solid electrolyte potentiometry as a function of the temperature in C₃H_x/O₂ (with x=6 or 8) reflected the catalytic properties of Pt for C₃H_x oxidation. In C₃H₆/NO/O₂, the reduction of NO was evidenced below 300 °C. The cyclic voltammetry evidenced the formation of an oxygen chemisorbed layer on the Pt surface under anodic potential. Propane had no effect on this chemisorbed layer, whereas propene weakened significantly the strength of this Pt–O bond. Addition of NO to C₃H₆/O₂ led to the disappearing of this chemisorbed layer. The use of solid electrolyte potentiometry in conjunction with cyclic voltammetry allowed us to determine the surface oxidation state of Pt during the catalytic reactions.     

Kinetics of the O2, Pt/YSZ interface at moderate temperature in the presence of C3H8 in the gas phase

The catalytic activity of polycrystalline Pt deposited on Yttria Stabilized Zirconia (YSZ) for the oxidation of propane to CO₂ can be affected using the effect of Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA). It was found that by applying positive overpotentials and thus, supplying O^2- onto catalyst surface, up to 3.2-fold increase in the catalytic rate of C₃H₈ oxidation could be obtained at 365 °C. At 305 °C, no effect was evidenced. Using cyclic voltammetry and impedance spectroscopy, we have shown the modifications induced by the addition of C₃H₈ on the kinetics of the 0₂, Pt/YSZ interface in the temperature range 300–400 °C. A decrease of the coverage of adsorbed oxygen species produced electrochemically was evidenced as well as a decrease of the oxygen electrode reaction rate under anodic potential. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Electrochemical promotion of NO reduction by C3H6 and CO on Rh/YSZ catalyst — Electrodes

The selective catalytic reduction of NO by propylene or CO in the presence of excess oxygen is a system of great technological importance. The effect of Electrochemical Promotion (or Non-faradaic Electrochemical Modification of Catalytic Activity — NEMCA) was used to promote this reaction (C₃H₆ or CO/NO/O₂) on Rh/YSZ catalyst-electrodes. It was found that both the catalytic activity and the selectivity of the Rh catalyst-electrode is affected dramatically upon varying its potential with respect to a Au pseudoreference electrode. Catalytic rate enhancements up to 15000% and 6000% were observed in the case of NO reduction by propylene, while the product selectivity to N₂ production is affected significantly (up to 200%) upon positive potential application. Remarkable promotion of the catalytic activity was also observed in the case of NO reduction by CO, since up to 20-fold increases both in catalytic rates and in NO conversion were obtained under NEMCA conditions. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Catalytic and electrocatalytic activation of methane over manganese oxides deposited on YSZ

The catalytic and the electrocatalytic behavior of MnO_x oxides deposited on Yttria Stabilized Zirconia (YSZ) in the form of thin porous films, was studied during the reaction of methane activation at high methane to oxygen ratios. Experiments were carried out in a continuous flow well-mixed reactor (CSTR), at atmospheric total pressure and in a temperature range between 500–850 °C. It was found that the electrochemical pumping of oxygen anions (O²⁻) through the solid electrolyte (YSZ) affect drastically the rates of CO₂, C₂H₄ and C₂H₆ formation and consequently the C₂ selectivity. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

New advanced in situ carbon monoxide sensor for the process application

Mixed potential solid electrolyte CO sensors with sensing electrodes based on composite with various semiconducting oxides were extensively studied in the temperature range 500–650 °C for sensitivity, stability and cross-sensitivity besides CO to other combustion components like CO₂, H₂O, O₂, and SO₂. The highest CO sensitivity was found for the CO sensor with composite electrode based on Au/Ga₂O₃ showing also good reproducibility and stability in hazardous combustion environment. CO sensor response behavior in non equilibrated oxygen containing gas mixtures is mainly determined by the catalytic activity of the measuring electrode and depends strongly on preparation and measuring conditions. Mixed oxides based on doped chromites show only a little sensitivity to CO. CO sensor based on Au/Ga₂O₃ composite electrodes was showing good CO selectivity in the presence of other combustion gas species and finally was tested in combustion environment at power plant. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Electrochemical solid electrolyte gas sensors — hydrocarbon and NOx analysis in exhaust gases

For the in situ measurement of the free oxygen concentration and the equilibrium oxygen partial pressure oxygen sensors based on zirconia solid electrolytes are widely used in order to monitor and control technical high temperature processes. Similarly combustibles (HC, CO) and NO_x can be determined in non equilibrated oxygen containing gas mixtures of exhausts by mixed potential sensors and amperometric solid electrolyte sensors. It is expected that their long-term stability is similar to that of oxygen sensors. In both cases the electrode material with the desired electrochemical and catalytic properties is the key component. Different electrode materials made of perovskites (La_1-xSr_xCr_1-yGa_yO_3-δ) and composites (Au/Metal oxide) were investigated in different combustibles including CO, C₃H_6/8, C₇H₈ and CH₄. The response behaviour of mixed potential sensors is determined by the catalytic activity of the measuring electrode, which is closely connected with the defect structure and depends on the measuring conditions. Furthermore the electrode response can be understood by electrokinetic data. Gas symmetrical mixed potential sensors with electrodes made of Au/Nb₂O₅ composites show maximum sensitivity. By using Ptreference electrodes without equilibrium behaviour the sensors are applicable in lean and rich mixtures as well. In the amperometric sensor mode the consecutive determination of oxygen and NO_x or combustibles at two working electrodes is possible. The catalytic activity of the oxygen pumping electrode should be low in order to avoid the decomposition of NO and HC respectively. Alternatively, the electrochemical reduction of NO can be performed at a single working electrode, made of materials with improved NO selectivity, without the previous reduction of oxygen. Paper presented at th 8th EuroConference on Ionics, Ixia, Rhodos, Greece, Sept. 15–21, 2002.     

Possibilities of NOx and CHx determination using galvanic cells with perovskite-electrodes on YSZ

Perovskite-type mixed oxides with the formula La_1−xA_xMe_1−yB_yO_3±δ (x=0.01...0.2; y=0.01...0.5; A=Ca, Sr; Me=Cr, Mn, Fe, Co; B=Mg, Ga) were investigated with the aim to use the oxides as electrode materials for galvanic cells with Y₂O₃-stabilized ZrO₂ solid electrolytes (YSZ). The catalytic activity of the oxides for the oxidation, reduction and decomposition of gas components was varied by changing the transition metal and by partial substitution of lanthanum and the metal. The behaviour of different substituted lanthanum chromite/YSZ and manganite/YSZ electrodes in the presence of nitric oxides (NO_x) and combustible components (CH_x) in gases containing oxygen was investigated. Various electrode combinations were tested in gas-symmetrical cells. For the NO determination cells with a catalytically active Pt electrode and a La_0.8Sr_0.2MnO₃ electrode with a low catalytic activity for the decomposition of NO were used. The results show the possible quantitative measurement at low and constant p(O₂) on the basis of a calibration. For the determination of C₃H₆ a CH_x-sensitive Au and an O₂-sensitive La_0.99Sr_0.01CrO₃ electrode were combined in a potentiometric thick film sensor. In gases containing oxygen with small amounts of C₃H₆ the sensor provided a utilizable voltage in dependence of the C₃H₆ concentration. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

The research on the synthesis and structure of some new ferrocene derivatives

The reaction of C₅H₄RLi with FeCl₂ gave nine new compounds of Fe(C₅H₄R)₂ [R=C(CH₃)₂C₆H₄CH₃-p(-m,-o), C₆H₁₀C₆H₅, C(Me)₂C₆C₄OCH_3-o, C₆H₁₀C₆H₄CH₃-p(,-m,-o), C₆H₁₀C₆H₄OCH₃-p]. The compositions of compounds were determined through elementary analysis. The structural determination was made by IR and H²NMR. Mossbauer spectia were taken at room temperature. The IS and QS values are 0.41–0.45mm/s and 2.3–2.5mm/s., respectively. The solid state structure of the complex has been determined by a single crystal x-ray diffraction study, crystal data for Fe[C₅H₄C(CH₃)₂C₆H₅]₂: a=17.988(2)A, b=17. 411(2)A, c=7.496(1)A, α=β=90°, r=112.23°, Z=4, monoclinic form, space group C2/c. Our conclusions are: in π-acceptor ligand, the nucleophilic substituents decrease and the electrophilic substituts increase the metal to ligand electron cloud shift, which results in a decrease or an increase in the strength of the coordinate bonds and in the stabilization of the complexes by their steric effect.     

Gold-composite electrodes for hydrocarbon sensors based on YSZ solid electrolyte

In potentiometric zirconia based sensors gold electrodes show a high sensitivity for hydrocarbons (HC's) when the measurements are carried out in non equilibrated oxygen containing gas mixtures at temperatures <700 °C. This behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To modify the electrode behaviour composites consisting of gold and gallium oxide were investigated. Gold pastes with different amount of Ga₂O₃ were prepared and screen printed on YSZ pellets. After sintering at defined temperatures between 900 and 950 °C the cells were tested regarding the electrode behaviour in a C₃H₆, O₂ gas mixture using a platinum air reference electrode. These composite electrodes show as compared with pure gold an enhanced sensitivity at low propylene concentrations and a time-independent characteristic at high concentrations of C₃H₆. The optimal composition is found to be at 20 mass-% Ga₂O₃. This electrode can be treated in reducing gases at temperatures 850 °C without changing its characteristics. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electrochemical reduction of NO and O2 on oxide based electrodes

Cyclic voltammetry has been used to investigate the electrocatalytic properties of oxide based electrodes for the reduction of NO and oxygen. Perovskites of compositions La_xSr_1-xMnO_3+δ, (LSMx), with 0.05 ≤ x ≤ 0.5 and La_0.6Sr_0.4Fe_0.8Mn_0.2O_3-δ, (LSFM), the spinels Co₃O₄ and CuCr₂O₄ and the mixed oxide V₂O₅ have been selected and investigated in the temperature range 300 – 500 °C. The materials show good catalytic properties against both NO oxidation and NO reduction. The catalytic activity as well as the selectivity is dependent both on the redox behavior of the compound and on the oxygen defect concentration. Paper presented at 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electrode reactions of La0.8Sr0.2MnO3±δ-electrodes on stabilized zirconia with oxygen and the nitrogen oxides NO and NO2

The kinetics for the electrode reactions with oxygen and with NO and NO₂ in the presence of oxygen has been studied for La_0.8Sr_0.2MnO_3±δ-electrodes on stabilized zirconia (8 mol% Y₂O₃=YSZ) in the temperature range between 500°C and 900°C for oxygen partial pressures between 1 kPa and 20 kPa by means of electrochemical methods (impedance, I-U characteristics) and temperature programmed desorption (TPD). For oxygen reduction below 900°C a mechanism is proposed which describes the formation of peroxidic ions at the electrode surface and a subsequent rate-determining electron transfer at the three-phase-boundary. At temperatures below 650°C the electrode reaction between NO and NO₂ is much faster than the oxygen reduction. The results for the NO₂-reduction to NO can be explained by a two-step mechanism consisting of a fast one-electron transfer to adsorbed NO₂ at the electrode surface and a subsequent rate-determining transfer of the second electron to NO₂ at the three-phase-boundary. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Direct methanol fuel cells: The influence of methanol on the reduction of oxygen over Pt/C catalysts with different particle sizes

Two Pt/C catalysts with different particle sizes (Pt/C: 2.5 nm, Pt/C-700Ar: 5.1 nm) were investigated by applying a half-cell configuration —rotating disk electrode (RDE) technique in H₂SO₄ aqueous solutions in the absence of or in the presence of methanol with different concentrations. Pt/C catalyst exhibited higher mass activity in H₂SO₄ aqueous solution without methanol and slightly lower mass activity in H₂SO₄ plus 0.1 mol/L CH₃OH in comparison with that of Pt/C-700Ar catalyst. On the contrary,single direct methanol fuel cell (DMFC) tests showed that Pt/C exhibited higheroxygen reduction reaction (ORR) activity and better cell performance, mainly due to the different kinds of electrolyte properties. Furthermore, it suggested that a better single DMFC performance could be obtained with a smaller particle size Pt-based cathode catalyst. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 — 18, 2004.     

An automated setup for impedance and electrochemical measurements of NOX sensors

An automated setup for impedance measurements of NO_X sensors and sensor elements has been assembled. The NO_X atmosphere is generated using NO and NO₂ calibration mixtures and N₂. The mixture is passed to the measurement chamber, where impedance measurements are performed. The sample can be subjected to temperature between 50 and 800 °C. The gas mixture is passed to the chemiluminescence analyser, where NO_X and NO amount is determined. The automation is based on National Instruments LabView programming language. Gas flow control, impedance measurement and NO_X level analysis can be simultaneously performed using a single PC computer. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Hydrogen production over a commercial Pd/Al2O3 catalyst for fuel cell utilization

In order to produce a hydrogen stream for fuel cell utilization, the study of the ethanol steam reforming process over an alumina supported palladium commercial catalyst was carried out. The effect of the reaction temperature, the H₂O/C₂H₆O molar ratios and the contact time on catalytic activity and stability was studied. It was found that even at very low temperature values complete ethanol conversion was possible. Hydrogen selectivities up to 98% were obtained at temperature values close to 625 °C. It was also observed that for different reactant molar ratios carbon monoxide concentration exhibits a minimum at a temperature value close to 450 °C. Furthermore, carbon formation was found to be negligible even for H₂O/C₂H₆O molar ratios equal to the stoichiometric one. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002     

Adsorption of carbon monoxide,oxygen, hydrogen,nitrogen, ethylene and benzene on an iridium (110) surface; Correlation with other iridium crystal faces

The interaction of CO, O₂, H₂, N₂, C₂H₄ and C₆H₆ with an Ir(110) surface has been studied using LEED, Auger electron spectroscopy and flash desorption mass spectroscopy. Adsorption of oxygen at 30°C produces a (1× 2) structure, while a c(2 × 2) structure is formed at 400°C. Two peaks have been detected in the thermal desorption spectrum of oxygen following adsorption at 30°C. The heat of adsorption of hydrogen is slightly higher on Ir(110) than on Ir(111). Adsorption of carbon monoxide at 30°C produces a (2 × 1) surface structure. The main CO desorption peak is found around 230, while two other desorption peaks are observed around 340 and 160°C. At exposures between 250 and 500°C carbon monoxide adsorption yields a c(2 × 2) structure and a desorption peak around 600°C. Carbon monoxide is adsorbed on an Ir(110) surface partly covered with oxygen or carbon in a new binding state with a significantly higher desorption temperature than on the clean surface. Adsorption of nitrogen could not be detected on either clean or on carbon covered Ir(110) surfaces. The hydrocarbon molecules do not form ordered surface structures on Ir(110). The thermal desorption spectra obtained after adsorption of C₆H₆ or C₂H₄ are similar to those reported previously for Ir(111) consisting mostly of hydrogen. Heating the (110) surface above 700°C in the presence of C₆H₆ or C₂H₄ results in the formation of an ordered carbonaceous overlayer with (1 × 1) structure. The results are compared with those obtained previously on the Ir(111) and Ir(755) or stepped [6(111) × (100)] surfaces. The CO adsorption results are discussed in relation to data on similar surfaces of other Group VIII metals.     

Promotion of surface SOx on the selective catalytic reduction of NO by hydrocarbons over Ag/Al2O3

The promotion of sulfur oxides on the selective catalytic reduction (SCR) of NO by hydrocarbons in the presence of a low concentration of sulfur oxides over Ag/Al₂O₃ has been investigated by a flow reaction test and in situ infrared spectroscopy. When the C₃H₆ (or C₁₀H₂₂) + NO + O₂ feed-flow reaction was tested, maximum NO reduction was below 30% over fresh Ag/Al₂O₃. After the addition of SO₂ to the feed flow, conversion increased slightly. Conversion increased further after SO₂ was cut-off from the feed flow. This demonstrated that the increase in NO reduction activity of the catalyst was related to SO_x adsorbed on the catalyst. SO_x adsorbed on the catalytic surface (1375 cm⁻¹) was detected by IR spectroscopy and was stable within the temperature range. NCO species, as an intermediate in NO reduction, on SO_x-adsorbed Ag/Al₂O₃ in a C₃H₆ + NO + O₂ feed flow was observed in in situ IR spectra during the elevation of the reaction temperature from 473 to 673 K, while it was only observed at 673 K on fresh Ag/Al₂O₃ under the same experimental conditions. We suggest that SO_x in low concentrations depressed the combustion of reductants by contaminating hydrocarbon combustion active sites on the catalyst, resulting in an increase in NO reduction efficiency of the reductants.     

The oxidation of dodecane on a vanadium-molybdenum catalyst

The catalytic oxidation of dodecane by air oxygen on a mixed vanadium-molybdenum oxide (V₂O₅ · MoO₃, 40 mol % MoO₃) was studied over the temperature range 300–350°C. The reaction at 300–330°C occurred on oxygen vacancies with the rupture of C-H bonds and formation of α-acid. Oxidation above 350°C occurred with the splitting of the C-C bond and formation of two and more acids. Singlet oxygen ¹O₂ generated in the oxidation of oxide catalyst lattice oxygen participated in the reaction. A possible mechanism of the process was considered.     

Selectivity maximization of ethylene epoxidation via NEMCA with zirconia and β″-Al2O3 solid electrolytes

The silver-catalyzed epoxidation of ethylene is a reaction of great technological importance and also represents one of the most challenging and thoroughly studied catalytic systems. It was found that the catalytic activity and selectivity of polycrystalline Ag for the epoxidation and complete oxidation of ethylene can be affected in a pronounced and reversible manner by electrochemically supplying or removing oxygen ions O^2- or Na⁺ to or from the silver catalyst surface in ZrO₂ (8 mol%Y₂O₃) or β″-Al₂O₃ solid electrolyte cell reactors and in the presence or absence of traces of chlorinated hydrocarbons in the gas phase. The steady-state changes in catalytic rates of formation of C₂H₄O and CO₂ are typically 10 to 100 times larger than the corresponding rate of ion transport to or from the catalyst surface, i.e., the reaction exhibits the effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) or “Electrochemical Promotion”. The selectivity to C₂H₄O can be very significantly altered, relative to open circuit conditions. Under fuel rich conditions, temperatures near 250°C and in the presence of traces of 1,2-C₂H₄Cl₂ in the gas phase selectivity values as high as 88% can be obtained, well above the ones reported in the open literature. The observed phenomena are discussed and interpreted within the framework of previous NEMCA studies and the currently prevailing ideas regarding the mechanism of ethylene epoxidation. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Ultrafine layered LiCoO2 obtained from citrate precursors

A new method for the preparation of ultrafine LiCoO₂ with a layered crystal structure was developed, which consists in thermal pyrolysis of homogeneous lithium-cobalt-citrate precursors. Atomic scale mixing of Li and Co is achieved by citric acid acting as a chelating agent. Electron spectroscopy of concentrated Li-Co-citrate solutions with Li:Co:Cit=1:1:1 and Li:Co:Cit=1:1:2 reveals that the predominant species at pH=7 are [Co(C₆H₅O₇)]⁻ and [Co(C₆H₅O₇)₂]⁴⁻ complexes. Freeze-drying of the two types of solutions leads to the formation of LiCo(C₆H₅O₇).nH₂O and (NH₄)₃LiCo(C₆H₅O₇)₂.nH₂O precursors, where Co²⁺ ions are complexed by one and two triionized citrate ions, respectively, and Li⁺ ions serve as counter ions. Between 400–600 °C, the thermal decomposition of these metal-citrate precursors yields LiCoO₂ with layered and pseudo-spinel structure, the proportion between them being depending on: (i) the Co/citrate ratio; (ii) the concentration of the freeze-dried solution; (iii) the heating rate. At 400 °C, the most defectless layered LiCoO₂, consisting of hexagonal individual particles with dimensions of 120–170 nm, is a product of the bis-citrate decomposition with a slow heating rate. For this sample, heating up to 600 °C does not affect the crystal size dimensions. For ultrafine layered LiCoO₂ and LiCoO₂ obtained by solid state reaction at high-temperatures (850 °C), the deintercalation and intercalation reactions proceed in the 3.95 – 3.99 and 3.86 – 3.88 voltage intervals, respectively. For defect trigonal LiCoO₂, additional oxidation and reduction peaks at 3.7 – 3.8 and 3.4 – 3.5 V were observed. We did not succeed in preparing monophase LiCoO₂ with pseudo-spinel structure. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Sept. 14–21, 1996