Jean-Paul Viricelle

Physico-chemical contribution of gold metallic particles to the action of oxygen on tin dioxide sensors

The role of a metal( gold) on the electrical response on tin oxide sensor was investigated thanks to the development of a particular test bench : it allows to separate the atmosphere surrounding SnO 2 region in contact with gold electrode from the atmosphere in the region containing gold. This result associated with calorimetric tests indicates the creation of specific oxygen species at the metal-oxide interface. A qualitative physico-chemical model based on the electronic effect of these adsorbed species, resulting in the increase of space charge area is proposed.

Tin dioxide gas sensor as a tool for atmospheric pollution monitoring: Problems and possibilities for improvements

The monitoring of atmospheric pollution needs network of gas sensors in order to increase the number of measuring points. For this goal, tin dioxide sensors have been evaluated. Their potentials and limitations (selectivity and stability), are presented via on-site experiments of urban pollution control. The study shows that some global information indicating mainly the traffic pollution can be obtained. The general problem of long-term stability and selectivity of SnO2 sensors is discussed. Some specific solutions are proposed, for example, a chemical treatment in the case of instability due to SO2. The problem of the dual response to oxidising and reducing gases is discussed, especially in regard to CO/NO2 detection. To solve this problem, the use of active filters is proposed. For example, a thin film of rhodium deposited above the sensing material allows to filter NO2. The use of a platinum filter has also been applied to improve the selectivity for the reducing gases. The results point out the difficulty to control this type of device in thin film technology. On the contrary, with thick films obtained by screen-printing, the possibility to separate HC from VOC (CH4 from CO and C2H5OH) is demonstrated. Some preliminary results concerning the use of a MnO2 filter are also presented in order to control the ozone reaction. The possibilities offered by signal processing are finally discussed using a multi-variable approach with 90 days aged sensors. AC measurements are exploited to improve the selectivity for the oxidizing gases NO2 and O3. The results obtained with a model built with atmospheric air as carrier gas appear promising.

An all porous solid oxide fuel cell (SOFC): a bridging technology between dual and single chamber SOFCs

A novel concept that employs dual chamber SOFC technology with a porous electrolyte, which allows the controlled distribution of gaseous O2 at the anode side, was successfully designed using an all porous structure. The oxidative reforming of hydrocarbon streams can consequently operate in a similar fashion to single chamber SOFCs, but within a safer, better controlled process.

Development of a protected gas sensor for exhaust automotive applications

A /spl beta/-alumina-based gas sensor for automotive exhaust application (hydrocarbon, CO, NO/sub 2/ detection in 10-1000 ppm concentration range) has been developed by thick film technology (screen-printing) in the frame of a European project. The sensing device consists of a solid electrolyte (/spl beta/ alumina) and of two metallic electrodes having different catalytic properties, the whole system being in contact with the surrounding atmosphere to be analyzed. The detection principle is based on the chemisorption of oxygen which leads to a capacitance effect at the metal-electrolyte interface, resulting in a measurable difference of potential depending on nature and concentration of pollutants and on the sensor temperature. For application in exhaust pipe, a porous protective layer based on /spl alpha/-alumina for preserving the sensing material and the metal electrodes from contamination and deterioration was screen-printed on the sensing element. For limiting the possible interface interactions between the overlapped layers, a new concept of screen -printable ink was set up based on mixing the oxide powder and its gelly precursor without any inorganic binder addition. The performances of the sensor were tested both on laboratory and engine bench. The sensitivity is relevant for exhaust application, and the long-term stability is improved by the protective layer.

Transformation of cerium(III) hydroxycarbonate into ceria. Part 1.—Nucleation and growth rates of ceria

A kinetic study of the transformation of cerium(III) hydroxycarbonate into ceria has been achieved by means of isothermal thermogravimetry experiments under a controlled atmosphere. A geometrical modelling based on Mampels model allowed determination of the values and the variations of the specific rates of nucleation and growth of ceria for various partial pressures of oxygen and carbon dioxide.

Nucleation and growth of ceria from cerium III hydroxycarbonate

Abstract The kinetic study of the transformation of cerium III hydroxycarbonate into ceria has beenachieved by means of thermogravimetry experiments and using a methodology based on the concepts of nucleation and growth. Two methods were used to determine the specific rates of nucleation and growth: a geometrical modeling which allowed to determine their variations against the partial pressures of gases involved in the transformation; an experimental method which gave the variations of the specific rate of growth. The comparison of the results obtained by these two methods allowed to validate the geometrical modeling and, thus, the values of the nucleation specific rate.

Effect of a platinum membrane on the sensing properties of materials based on thin and thick tin dioxide films

Abstract A possible way to improve the selectivity of tin dioxide sensors is to deposit a membrane with specific properties above the sensing element. The purpose of this paper is to investigate the effect of a platinum membrane on the sensing properties of thin (chemical vapour deposition) and thick (screen-printing technology) SnO2 films under gases (air, diluted pollutant gases CO, CH4 or C2H5OH). The measurement of the catalytic activity of Pt has been investigated. At above 500 °C, this metal is very efficient for the total oxidation of CO and C2H5OH, but CH4 does not react. As a consequence, it should be possible to develop a selective sensor to CH4 in presence of C2H5OH and CO as interfering gases. Firstly, we tried to correlate this catalytic activity to the sensing detection properties of the thin SnO2 film+Pt structure. The membrane strongly decreases the sensor response under alcohol, and does not affect the CH4 one, which is in agreement with catalytic tests. In COs case, at high temperature (500 °C), although Pt is very efficient with regard to CO oxidation, it does not strongly decrease the sensor response. To explain this result, a mechanism based on the contact between Pt particles and thin SnO2 film is proposed. Hence, in order to decrease the COs response, Pt membrane has to be insulated from SnO2 film. A new structure, thick SnO2 film+thick insulator coating (SiO2)+Pt, has been tested. A good correlation between catalytic tests and the effects on the sensor response is obtained. However, contrary to thin film device, a good correlation is also obtained in the thick SnO2 film+Pt structure when a contact between Pt and SnO2 is materialised. In thick-film structures, no insulator coating seems to be necessary.

Preconcentration Modeling for the Optimization of a Micro Gas Preconcentrator Applied to Environmental Monitoring

This paper presents the optimization of a micro gas preconcentrator (μ-GP) system applied to atmospheric pollution monitoring, with the help of a complete modeling of the preconcentration cycle. Two different approaches based on kinetic equations are used to illustrate the behavior of the micro gas preconcentrator for given experimental conditions. The need for high adsorption flow and heating rate and for low desorption flow and detection volume is demonstrated in this paper. Preliminary to this optimization, the preconcentration factor is discussed and a definition is proposed.

Transformation of cerium(III) hydroxycarbonate into ceria. Part 2.—Experimental study of the growth rate

The variation of the specific rate of growth of ceria from cerium(III) hydroxycarbonate through a solid–gas reaction involving oxygen has been determined using an experimental method which consisted of modifying one of the chemical stresses (a partial pressure) during an experiment, whilst keeping the other partial pressures constant. The specific rate of growth and nucleation of ceria previously obtained from Mampels model were confirmed. Another application was to ensure that, under the experimental conditions of the study, nucleation and growth of ceria from the solid–gas reaction were the only processes to be taken into account in the mass balance of the transformation.

Nickel-Based Anodes for Single-Chamber Solid Oxide Fuel Cells: A Catalytic Study

Single-chamber solid oxide fuel cells (SOFCs) are an alternative concept to traditional SOFCs. Conventional anodes, which consist of a cermet of metallic Ni and an ionic conductor, have to work under a mixture of hydrocarbon and oxygen. This paper presents a catalytic study of C 3 H 8 oxidation by Ce 0.9 Gd 0.1 O 1.95 and nickel. The influences of temperature and oxygen to propane ratio on the nickel oxidation state are clarified, thanks to catalytic tests and electrical conductivity measurements. The results show that the temperature must be higher than a critical value to maintain the nickel particles in the metallic state. Furthermore, at high temperatures, the conversion of the initial mixture leads to the water-gas-shift equilibrium.