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Dry reforming (DRM) is as an efficient way for CH 4 and CO 2 valorisation because the produced syn -gas has an H 2 /CO ratio equal to that suitable for the synthesis of oxygenated hydrocarbons and synthetic fuels. The development of Ni (10 wt%) based structured and unstructured catalysts promoted by a small amount of Ru (0.5 wt%) for DRM has been investigated. Unstructured catalysts were prepared by wet impregnation method and a combination of wash coating-wet impregnation methods was used for cordierite monoliths. Samples were characterized by XRD, BET, H 2 -TPR, TEM, FE-SEM, XPS techniques and the catalytic activity for DRM was evaluated in the temperature range 600–800 °C. The catalyst stability was followed at 800 °C during time on stream. Ru promoted catalyst (Ni-Ru) was remarkable active and stable whereas Ni catalyst deactivated due to the formation of Ni 2 + -containing inactive phases. Ni-Ru monolith was initially much more active than monometallic Ni stating the positive effect of Ru on maintaining Ni reduced. Reaching steady state condition, Ni rapidly deactivated due to carbon formation, whereas Ni-Ru monolith remained stable confirming that Ru behaves as an efficient and cheap promoter of Ni for DRM.

a b s t r a c t The effects of electrical contact configuration for La2CuO4 sensing electrodes for a planar-based poten- tiometric gas sensor were studied in order to further quantify the effects of processing on sensor performance. Five configurations of La2CuO4 were used for the sensing electrode. The La2CuO4 sens- ing electrode was screen printed opposite a Pt counter electrode on a tape cast YSZ electrolyte. Three sensors were prepared for each configuration to test repeatability. Each sensor was tested at tempera- tures from 400 to 700 ◦C at various concentrations of NO2, NO, and CO in an environment of 3% O2 with a balance of N2. Results show that these sensors exhibit suitable sensitivity to all the gases tested. More importantly, it was shown that the sensitivity, selectivity/cross-sensitivity, and repeatability of these sensors are dependent on the sensing electrode configuration.

Cermet anode substrates Ni–BaCe 0.9 Y 0.1 O 3− δ (Ni–BCY10) with varied ratios between Ni and BCY10 were prepared. BCY10 powders were prepared by the citrate–nitrate auto-combustion method and anode substrates were prepared by the method of evaporation and decomposition of solutions and suspensions. Sintered anode substrates were reduced and their properties were examined before and after reduction as a function of the ratio between Ni and BCY10. Microstructural properties of the pellets were investigated using X-ray diffraction analysis and field emission scanning electron microscopy. The influence of the Ni:BCY10 ratio on the microstructure of conducting paths through ceramic and metal parts was discussed. Impedance spectroscopy measurements were used for evaluation of electrical properties of the anode pellets. The high conductivity values of reduced anodes confirmed percolation through Ni particles even for an anode with a reduced amount of nickel. Fuel cell tests were carried out in order to examine the influence of the Ni:BCY10 ratio on fuel cell performance and to compare characteristics of cermet anodes with platinum electrodes. The ratio between Ni and BCY10 did have a slight influence on the power output of fuel cells. Fuel cells with a cermet anode demonstrated a higher power output compared to fuel cells with a platinum electrode.

In this work, high temperature planar sensors based on tape-cast YSZ layers with Nb2O5 n-type semiconducting oxide as sensing electrode were studied. The investigated gases were CO and different saturated and unsaturated hydrocarbons. A study on the presence of the metallic electrode under the metal oxide and on the nature of that metallic (Pt or Au) electrode was performed. Thus, different types of sensors were prepared and investigated. The sensors were tested in potentiometric devices at various concentrations of gases in the 500–700 °C temperature range. The most promising sensor in terms of selectivity, sensitivity and stability was Pt/YSZ/Nb2O5, without any metallic electrode under the Nb2O5 powder. In fact, this sensor was found totally selective to propylene with very large EMF response even at temperature as high as 700 °C (−115 mV/decade).

The discussion about the sensing mechanism of non-Nernstian electrochemical gas sensors with a semiconducting sensing electrode is still open. This work reports a study of the influence of the metallic electrode under the semiconducting metal oxide, Nb 2 O 5 , on the sensor response. Planar sensors based on tape-cast YSZ layers were fabricated. Two Pt or Au finger electrodes were deposited on one side of the layers. One of the electrodes was covered with a Nb 2 O 5 thick film. The response to CO was studied performing electromotive force (EMF) measurements in the 450–650 °C temperature range. In order to better understand the sensing mechanism, the electrical measurements were correlated to catalytic measurements performed on both electrodes. With no catalytic electrodes, the adsorption-Fermi level related mechanism seems to prevail, while in the case of sensors with Nb 2 O 5 |Pt|YSZ electrode, the mixed potential theory is more adapted to explain the results.

Planar electrochemical sensors based on tape-cast yttria stabilized zirconia (YSZ, 8 wt.% of Y2O3) layers with two Pt parallel electrodes, one coated with WO3 powders were prepared. The WO3 powders were synthesized in the laboratory using different chemical routes. The NO2 and CO potentiometric response of the sensors was measured at high temperature and compared to the response of the sensors fabricated using commercial WO3 powders. Promising results in terms of selectivity, sensitivity, stability, reproducibility and response time were obtained for synthesized WO3-based sensors in comparison to the response of commercial WO3-based sensors.

Electrochemical planar sensors based on yttria-stabilized zirconia (YSZ) with semiconducting oxides (WO 3 and LaFeO 3 ) and mixed conductors (La 0.8 Sr 0.2 FeO 3 ) as sensing electrodes were investigated. The electromotive force (EMF) of the sensors was measured at fixed temperature (450–700 °C) and different concentrations of NO 2 and CO in air in the range 20–1000 ppm. The sensors were wholly exposed to the same gas atmosphere, alternatively air and different gases in air, without using reference air. Fast and stable response was measured for all the different sensors. The EMF of p- and n-type semiconducting-based sensors was measured in opposite directions upon the same gas exposure. These results obtained for the response of planar sensors with different semiconductor sensing electrodes showed that the sensing mechanism cannot be explained by mixed potential theory. The response of these sensors is better described by a differential electrode equilibria sensing mechanism.

Solid-state NO 2 sensors based on yttria stabilized zirconia (YSZ), an O 2- conductor, combined either with an n- (WO 3 ), or a p-type semiconducting oxide (LaFeO 3 ), or a mixed electronic and ionic conductor (La 0.8 Sr 0.2 FeO 3 ) were investigated. Platinum parallel finger electrodes were applied on the surface of tape-casted YSZ layers and attached with gold wires for current collection. Nanocrystalline perovskite powders were prepared using different chemical methods: LaFeO 3 by the thermal decomposition of the LaFe-hexacyanide complex, and La 0.8 Sr 0.2 FeO 3 by a sol-gel route. A sub-micrometric commercial WO 3 powder was used. The oxide powders were mixed with a screen-printing oil and deposited on one Pt finger electrode. The presence of the oxide powder makes one of the electrodes different from the other in terms of catalytic activity, specific surface area, gas adsorption and reaction kinetics. Both electrodes were wholly exposed to the same gas atmosphere, without using reference air. The sensors were investigated at fixed temperature (450-700 °C) by measuring the electromotive force (EMF) at different concentrations of NO 2 and CO in air in the range 20-1000 ppm. A fast and stable response was measured for all the tested sensors. An EMF of opposite sign was measured for p- and n-type semiconducting based sensors upon exposure to the same gas. After increasing the grain size of the nano-structured La 0.8 Sr 0.2 FeO 3 powder by a heat treatment at 900 °C for 4 h, the response to NO x became small, slow and unstable.

The heat treatment of commercial zinc oxide based varistor ceramics at temperatures of 200–300 °C in air with 1000 ppm of NO 2 causes strong shift of voltage-current characteristic (VCC) to low voltages. Subsequent exposure to pure air leads to partial restoration of VCC. Scanning electron microscopy analysis showed microstructural changes of samples after heat treatment in air with 1000 ppm of NO 2 . Chemical reactions between NO 2 molecules and oxygen chemisorbed or Bi-rich oxide phases at grain boundaries followed by the lowering of grain boundary potential barriers can be the reason of observed degradation.

Solid-state ceramic NO x sensors based on interfacing an ionic conductor (NASICON) with semiconducting oxides (rare earth perovskite-type oxides) were investigated. NASICON powders were pressed into thimbles 12 mm long with 3 mm inner diameter and 4 mm outer diameter, then sintered at 1270°C in air. A Pt wire was attached to the outer surface of the tubes using a platinum paste. A uniform Au/Pd (60 wt.%) coating, permeable to oxygen but not to NO x , was sputtered for 40 min on the sensor external surface to allow the exposure of both electrodes to the gas atmosphere without using reference air. Windowless energy-dispersive spectroscopy (EDS) was used to evaluate the chemical composition of the Au–Pd layer before and after the performance of sensing tests. Sodalite powder as an auxiliary phase was tightly packed into the NASICON thimbles with a Pt lead for the electrical contact. To get an in-situ NO conversion to NO 2 , a Pt-loaded alumina powder was used as a catalyst and incorporated with the sensor on the top of the auxiliary phase. Nano-sized and chemically-pure rare earth perovskite-type oxide (LaFeO 3 , SmFeO 3 , NdFeO 3 and LaCoO 3 ) powders, prepared by the thermal decomposition of the corresponding hexacyanocomplexes, were also used in the electrochemical cells. Each of the tested oxides was packed into the thimbles replacing the sodalite and the Pt-loaded alumina catalyst. Tests were performed also using only the perovskitic oxides. The microstructure of the materials tested was evaluated using scanning electron microscopy (SEM). The NO 2 sensing properties of the prototype sensors were investigated at controlled temperature (in the range 300–600°C) by measuring the electromotive force (EMF) at different NO 2 concentrations (in the range 2–2000 ppm in air). Some measurements were performed at various NO concentrations diluted with Ar. The results obtained showed a promising NO 2 sensing performance when ferrites were used. SmFeO 3 has a lower catalytic effect on NO oxidation than the Pt-loaded alumina catalyst, and has a similar effect to sodalite when used as auxiliary phase. The perovskite-type oxides are more preferable as auxiliary phase than sodalite because they improve the stability of the electrochemical sensor performances.

NO2 sensing properties of SnO2-based varistor-type sensors have been investigated in the temperature range of 400-650 °C and in the NO2 concentration range of 15-30 ppm. Pure SnO2 exhibited a weak nonlinear I-V characteristic in air, but clear nonlinearity in NO2 at 450 °C. The breakdown voltage of SnO2 shifted to a high electric field upon exposure to NO2 and the magnitude of the shift was well correlated with NO2 concentration. Thus, SnO2 exhibited some sensitivity to NO2 as a varistor-type sensor. When SnO2 particles coated with a SiO2 thin film were used as a raw material for fabricating a varistor, the breakdown voltage in air was approximately the double that of pure SnO2 and the sensitivity to 15 ppm NO2 was enhanced slightly. However, the sensitivity to 30 ppm NO2 decreased. The Cr2O3-loading on SnO2 also led to an increase in the breakdown voltage in air, but the Cr2O3 addition was not effective for promoting the NO2 sensitivity under the present experimental conditions.

The humidity dependence of the voltage-current characteristics of nonsymmetrical and symmetrical heterocontacts ZnO/BaPb0·8Bi0·2O3, ZnO/BaPb0·8Bi0·2O3/ZnO and ZnO/ZnO have been studied at fixed temperature in air with different relative humidity values. Nonlinear characteristics have been observed for all the contacts at any humidity values. The increase in the nonlinearity coefficient for the ZnO/BaPb0·8Bi0·2O3/ZnO contact with respect to the ZnO/ZnO contact, and the voltage-current relationship at forward bias for ZnO/BaPb0·8Bi0·2O3 heterocontacts are attributed to surface states. The increase in relative humidity causes the decrease in the Schottky potential barrier height, thereby resulting in a redistribution of the voltage drop at the heterocontact interface and in the current growth at lower voltages.