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

一氧化二氮(N2O)，又称笑气，是一种广泛用于外科手术的麻醉性气体，在外科手术中，废N2O未经处理就被释放到大气中。这种排放可能对大气环境造成危害，因为N_2O是温室效应气体，其全球变暖系数是CO_2的300倍左右，同时也是导致臭氧层破坏的气体之一。因此，在释放之前将N_2O催化分解为N_2和O_2是可取的。最近有报道称，室内空气中N_2O的污染可能会对经常在这种条件下工作的工作人员的健康造成危害。本项目旨在开发一种催化N_2O分解体系和N_2O传感器。N_2O传感器为了研制一种适用于空气中N_2O监测的半导体型气体传感器，对对N_2O敏感的半导体氧化物进行了研究。在测试的23种单一氧化物中，SnO_2被证明是…对N_2O的敏感度最高，但敏感度不够高。在SnO_2中加入少量的碱性氧化物，如SRO、CaO、BaO、Bi2O_3和Sm_2O_3，可以有效地提高其对N_2O的敏感性，这种促进作用在SRO的负载量下尤为明显。例如，0.5wt%的氧化锶负载SnO_2的N_2O灵敏度约为纯SnO_2的3倍，在10-300ppm的浓度范围内可以很好地检测空气中的N_2O。在11种用于管状氧化锆器件的氧化物中，SnO2具有最好的传感电极性能，在475℃的空气中具有相对较高的灵敏度。在SnO_2中添加外加氧化物(如Bi2O_3和Sm_2O_3)可以有效地提高N_20的灵敏度。尤其是使用Sm_2O_3(0.5wt%)-SnO_2敏感电极的元件对N_2O的灵敏度是纯SnO_2电极的1.5倍，可以检测到空气中约35ppm的N_2O。通过对阳极和阴极极化曲线的测量，确定了N_2O的敏感机理为敏感电极上的混合电势。N_2O分解催化剂有40多种进行了筛选，它们是负载在沸石、Al_2O_3、TiO_2或SiO_2等氧化物载体上的过渡金属或贵金属催化剂。N_2O的直接分解是在所有被测试的催化剂上进行的，但其活性强烈依赖于活性成分和载体。在这些催化剂中，负载金属的Al_2O_3表现出最强的分解活性，其实际水平取决于活性成分(Pd&Gt；Ru&Gt；Pt)。在活性最高的催化剂(5wt%Pd/Al_2O_3)作用下，干模型气直接分解N_20的反应在250℃左右开始，300℃时完成。尽管共存水抑制了分解活性，但5wt%Pd/Al_2O_3在水含量为2.3%的情况下仍保持高活性，N_2O分解开始和完成的时间分别为300ppm和400ppm。3)手术室对N_2O的现场监测发现，当扫气系统不能正常工作时，手术室内的环境N_2O浓度可达100-150ppm。尤其是病人和/或麻醉供气设备附近的N_2O浓度超过500ppm。较少

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