DEVELOPMENT OF GAS SENSORS AND DECOMPOSITION CATALYST SYSTEM FOR NITROUS OXIDE USED IN OPERATING ROOMS

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Nitrous oxide (N_2O), called "laughing gas", is an anesthetic gas used extensively for surgical operations where waste N_2O is released into the atmosphere without treatment. This release may be hazardous to the atmospheric environment, since N_2O is a green-house-effect gas, the global warming coefficient of which is about 300 times higher than that of CO_2, beside being one of the gases leading to ozone layer disruption. It is thus desirable to catalytically decompose N_2O into N_2 and O_2 before release. It has recently been reported that contamination of indoor atmospheres with N_2O may cause health hazards for staff routinely working under such conditions. This project aims to develop a catalytic N_2O decomposition system and N_2O sensor.1).N_2O sensorsIn order to develop a semiconductor type gas sensor applicable to the monitoring of N_2O in air, a search for the semiconducting oxides sensitive to N_2O was carried out. Among the 23 kinds of single oxides tested, SnO_2 turned out … More to give the highest sensitivity to N_2O, although the sensitivity was not high enough. The N_2O sensitivity was found to be promoted effectively when SnO_2 was loaded with a small amount of a basic oxide such as SrO, CaO, BaO, Bi_2O_3 and Sm_2O_3. The promotion was particularly conspicuous with SrO loading. For example, 0.5wt%SrO-loaded SnO_2, exhibited the N_2O sensitivity about 3 times as high as that of pure SnO_2, and could detect N_2O in air fairly well in the concentration range of 10-300 ppm.A stabilized-zirconia based electrochemical device attached with an oxide electrode was developed for monitoring N_2O in air at elevated temperatures. Among the 11 kinds of monoxides tested for tubular zirconia device, SnO_2 was found to exhibit the best sensing electrode properties, giving a relatively high sensitivity in air at 475 ℃. The addition of a foreign oxide (e.g., Bi_2O_3 and Sm_2O_3 to SnO_2 was effective to improve the N_20 sensitivity. Especially the element using Sm_2O_3 (0.5wt%)-SnO_2 sensing electrode exhibited the N_2O sensitivity about 1.5 times as high as that using the pure SnO_2 electrode, allowing to detect about 35 ppm N_2O in air. On the basis of the measurements of anodic and cathodic polarization curves, the sensing mechanism was confirmed to involve a mixed potential at the sensing electrode.2).N_2O decomposition catalystsMore than 40 catalysts were subjected to the screening test ; they were transition or precious metal catalysts loaded onto oxide supports such as zeolites, Al_2O_3, TiO_2, or SiO_2. The direct decomposition of N_2O proceeded with all the catalysts tested, but the activity was strongly dependent on both the active ingredient and the support. Among these catalysts, metal-loaded Al_2O_3 exhibited the most potent decomposition activity, the actual level depending on the active ingredient (Pd>Ru>Pt). With the most active catalyst (5wt% Pd/Al_2O_3), the direct decomposition of N_20 from the dry model gas started around 250℃ and reached completion at 300℃. Although the decomposition activity was inhibited by co-existing water, 5wt%Pd/Al_2O_3 retained its high activity even in the presence of 2.3% water, N_2O decomposition starting and being completed at 300℃ and 400℃, respectively.3).Field monitoring of N_2O at a operating roomIt was found that the ambient N_2O concentration in an operating room reached 100-150ppm when a scavenging system did not work well. Especially, the N_2O concentration near a patient and/or equipment for anesthetic gas supply was more than 500 ppm. Less