CO sensors

Barsan, N. and Weimar, U., Understanding the fundamental principles of metal oxide based gas sensors the example of CO sensors with SnOz sensors in the presence of humidity, Journal of Physics Condensed Matter, 15, R813-R839, 2003. 

Carbon Monoxide Evolution. Determination of the carbon monoxide evolved during combustion of polymer samples in NBS Chamber experiments was carried out using a Telegan CO Sensor (Type 3F). Quoted values are the numerical averages of three independant determinations. 

Fig. 5.40 A commercial Ga203-based CO-sensor in the flue gas of an independent vehicle heater (STEINEL Solutions AG SGAS 220)...

CO-sensors and sensor systems for indoor use should meet several specifications regarding their sensitivity, cross-sensitivity and stability. A reasonable approach for the development of CO indoor alarm or monitoring systems is to comply with BS... 

Suitable methods for CO indoor monitoring are based on using metal oxide sensors (chapter 5.3.2.1), electrochemical cells (chapters 5.3.2.3 and 5.3.2.4), pellis-tors (chapter 53.2.2) and optical methods (chapter 5.3.2.5). An overview of CO-sensors and systems is given in Tab. 5.9. 

With the introduction of modern electronics, inexpensive computers, and new materials there is a resurgence of voltammetric techniques in various branches of science as evident in hundreds of new publications. Now, voltammetry can be performed with a nano-electrode for the detection of single molecular events [1], similar electrodes can be used to monitor the activity of neurotransmitter in a single living cell in subnanoliter volume electrochemical cell [2], measurement of fast electron transfer kinetics, trace metal analysis, etc. Voltammetric sensors are now commonplace in gas sensors (home CO sensor), biomedical sensors (blood glucose meter), and detectors for liquid chromatography. Voltammetric sensors appear to be an ideal candidate for miniaturization and mass production. This is evident in the development of lab-on-chip... 

G. Faglia, E. Comini, A. Cristalli, G. Sberveglieri, and L. Dori. Very low power consumption micromachined CO sensors , Sensors and Actuators B55 (1999), 140-146. 

Figure 3. CO sensor cell voltage-current relationships (O) air, ( Z ) 40 ppm CO...

Effect of Feed Flow. The effect of feed flow on sensor response for the CO sensor cell is shown in Figure 9. Similar curves were observed for NO and NO2. Generally, there is a marked increase in sensor cell response with increasing flow (10 to 40 cc/min), followed by a slow rising slope as flow exceeds 50 cc/min. A flow of 60 cc/min was selected for practical use based on tradeoff studies of water management and flow dependence. 

Life Characteristics of the Sensor Cells. Typical life behavior of the SPE CO sensor cells is depicted in Figure 12 for sensor cells with responses in the range 1.8 to 2.k /ppm. The data reported are for cells that had been continuously potentiostated and operated on the test gas intermittingly for approximately 8 hours/ day. Similar stability has been observed for sensor cells during continuous operation with a test gas. 

During the first 15 to 20 days there is a decrease in signal amounting to 2 /day followed by a leveling in performance. In a practical instrument, calibration is only required daily to weekly to achieve 10 accuracy during initial life operation. Subsequently, weekly to monthly calibrations are required. Typical SPE CO sensor cells operated in commercial instruments have an output that is within of the original calibration signal after two to three years of operation. 

Figure 11. Temperature compensated output for CO sensor cells...

Figure 12. CO sensor cell response as a function of time potentiostatic voltage, 1.15 V gas flow, 60 cm3/min temperature, 25°C.

For domestic use, more useful humidity sensors, CO sensors and combustion monitoring sensors will be developed with their increased integration into safety systems for homes and offices. 

Fast Response CO Sensor. The sensor requirements for eddy covariance measurements are extreme. To be used within a few meters of a plant canopy, the sensor must have a frequency response in excess of 20 Hz. Additionally, because the large mean density of CO2 in the atmosphere (about 560 mg m-3) and the deviations around the mean associated with turbulent transfer are small (>10 mg m-3), the sensor must have a signal to noise ratio in excess of 3500 1. The sensor must maintain these specifications for long durations, while mounted on a tower above the canopy, where it is exposed to constant changes in temperature, solar irradiation, and background gas concentrations. The instrument must unobtrusively sense the natural turbulant fluctuations of the atmosphere. To effectively accomplish this it must be small and streamlined. 

CO sensor allows detection of CO in the presence of hydrocarbons and other adsorbable contaminants. The membrane Is usually chosen for Its ability to protect the sensing electrode. However, If It has low permeability to air, the sensor will have a slower response time. The electrolyte and counter electrode have also been reported as Influencing selectivity and device performance In the determination of hydrazines (5) and NO2 (9), respectively. Finally, materials of construction are typically Teflon and high-density plastics like polypropylene because such materials must be compatible with reactive gases and corrosive electrolytes. 

Fig. 8.2 Output signal changes of five parallel CO sensors with increasing catalytic reaction temperature. The catalysts whose effluents were analyzed by CO gas sensors were Rh/Sn02, Rh/W03, Rh/Si02, Rh/Ti02, Rh/Ce02 (reproduced by permission of Elsevier from [19]).

Products obtained by propane-selective oxidation have been analyzed by gas sensor systems [19, 26]. Usually, several or multiple kinds of compounds are produced during the selective oxidation of propane. The formation of CO, C02, aldehydes such as acrolein, and ketone were observed over iron-silica catalysts [28, 29]. During the initial stage of catalyst investigation, the conversion of propane and the selectivity toward useful oxygenate products as chemical resources are of interest. Semiconductor-type gas sensors selective toward the oxygenate were employed to estimate the yield of oxygenate products, with a combination of the potentiometric CO sensor and the ND-IR C02 sensor [30]. 

I 8 Cos Sensor Technology for High-Throughput Screening in Catalysis... 

Fig. 8.5 shows the change in output signals from oxygenate sensors 1 and 2, the potentiometric CO sensor and the ND-IR C02 sensor after the introduction of the reaction gas into the gas sensor system. A 90%-response requires 20, 90 and... 

Fig. 8.5 Time courses of output signals of semiconductor-type oxygenate sensors, potentiometric CO sensor and ND-IR C02 sensors. The delay of CO gas sensor was due to the resistance of active carbon filter which removes oxygenate compounds (reproduced by permission of Elsevier from [19]).

Temp. Control Temp. Control Temp. Control i i CO Sensor... 

Realization of two-terminal microsensors like that represented in Scheme I depends on the discovery of viable reference and indicator molecules that can be confined to electrode surfaces. Described herein are three different microelectrochemical sensing systems. The first is a three-terminal microelectrochemical sensing system for CO based upon an indicator molecule, ferrocenyl ferraazetine, that selectively reacts with CO (2). The second microelectrochemical system is based upon a disulfide functionalized ferrocenyl ferraazetine that can be adsorbed onto Au or Pt via monolayer self-assembly techniques. Efforts to make a two-terminal CO sensor based upon the... 

A CO Sensor Based Upon Self-assembled Ferrocenyl Ferraazetine. Having demonstrated the CO dependent solid-state electrochemistry of ferrocenyl ferraazetine, we synthesized a ferrocenyl ferraazetine molecule with disulfide functionality (Id). Scheme III. The specific aim was to design a CO sensitive molecule that could be confined to the working electrode of a two-terminal device via monolayer self-assembly techniques. Disulfides have been shown to irreversibly adsorb to Au and Pt surfaces (3-10). NMR and mass spectrometry are consistent with the proposed structure for compound Id. The FTIR spectrum of Id in THF exhibits metal carbonyl bands at 2067,2024,1989,1985 cm similar to the spectra for other ferraazetine derivatives la-c (2,5-6). Like derivatives la-c. Id reacts with CO (1 atm) at 298 K in CH2C12 to form a ferrapyrrolinone complex 2d, equation (3). 

Based on a newly developed sensor electrode reaction model, the gas electrode reactions for type III potentiometric CO, sensors... 

Polarized Electrochemical Vapor Deposition to Deposit Auxiliary Phases at the Working Electrode of Type III Potentiometric CO Sensors... 

The PEVD system for Na CO auxiliary phase formation at the working electrode of a type III potentiometric CO sensor is schematically shown in Eigure 10. The electrochemical cell for this PEVD process can be illustrated as ... 

The potential profiles in this PEVD system are illustrated in Figure 17. Although there is no driving force due to a difference in the chemical potential of sodium in the current PEVD system, the applied dc potential provides the thermodynamic driving force for the overall cell reaction (62). Consequently, electrical energy is transferred in this particular PEVD system to move Na COj from the anode to the cathode of the solid electrochemical cell by two half-cell electrochemical reactions. In short, this PEVD process can be used to deposit Na CO at the working electrode of a potentiometric CO sensor. 

Improvement of the geometric structure of the working electrode by a well-controlled PEVD process benefits the performance of a CO sensor in many ways. To optimize kinetic behavior, the response and recovery times of CO potentiometric sensors were studied at various auxiliary phase coverages. This was realized by a unique experimental arrangement to deposit the Na COj auxiliary phase in-situ at the working electrode of type III potentiometric CO sensors by PEVD in a step-wise fashion. Since the current and flux of solid-state transported material in a series of PEVD processes can be easily moiutoredto control the amount of deposit... 

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