【作者】 王超；

【导师】 肖建中； 夏风；

【作者基本信息】 华中科技大学 ， 材料学， 2018， 博士

【摘要】 汽车尾气中NO_x的排放对环境造成了日趋严重的危害,为此全球各国相继制定了越来越苛刻的尾气排放标准,同时开发了许多减排技术。基于优异的脱硝效率,选择性催化还原（SCR）技术被广泛用于NO_x的还原。为了提升SCR系统的NO_x转化效率,并防止还原剂NH₃过量造成泄露,尾气用高性能NH₃传感器的需求日益迫切。在各种电化学型气体传感器中,基于氧化钇稳定氧化锆（YSZ）固体电解质的电势型传感器成为近年来的研究热点,主要是由于其具有高敏感性、高稳定性、结构简单,成本较低的优点。敏感电极作为气体传感器的重要组成部分,发挥着气体吸附和电化学催化的关键作用。而电极材料成分、物相组成、微观形貌等参数又影响着电极的物理化学性能,进而影响气体传感器的敏感性、选择性和响应恢复速率等特性。作为一种商用SCR催化材料,V₂O₅-WO₃-TiO₂（VWT）混合金属氧化物因其较高的NH₃吸附性、催化活性和抗毒化能力,满足NH₃传感器对敏感电极材料的基本要求,表现出较大的潜能。本文使用VWT催化剂作为敏感电极材料制备了基于YSZ固体电解质的混合电势型NH₃传感器,测试了敏感性、选择性及响应和恢复时间,并结合电化学阻抗谱和极化曲线等电化学方法研究了VWT成分、电极显微结构、Au和YSZ修饰及电极厚度对传感器响应特性的影响,并分析了各因素的作用机理。采用浸渍法制备了钒含量分别为0、0.5、1.0、2.0、3.0和5.0 wt.%的VWT粉体。通过XRD和SEM表征,发现制得的粉体由锐钛矿TiO₂颗粒及覆盖在其表面的WO₃和高分散分布的V₂O₅构成。当钒含量不超过3.0 wt.%时,不同成分的VWT粉体形貌相似,颗粒直径为100 nm左右。当钒升高到5 wt.%后,粉体中出现条状和棒状晶体,经TEM能谱分析,该条状物相为氧化钒晶体。这主要是由于钒含量超过阈值后,V₂O₅不能再以高分散状态分布于载体TiO₂表面,而是聚集形成晶态钒氧化物。将不同钒含量的VWT作为敏感电极材料烧结制备电势型NH₃传感器,并从物相组成、电极微观结构、敏感性、选择性、响应/恢复时间及电化学催化活性等方面探讨了钒含量对传感器性能的影响机理。随着钒含量的增加,传感器的NH₃敏感性和响应恢复速率均呈先增后减的趋势,钒含量为2.0 wt.%的样品敏感性最大。钒含量的变化对NH₃选择性的影响较小。敏感性的变化主要是由于钒含量改变所引起的NH₃吸附能力、电极反应催化活性的变化导致的。钒的少量增加促进了VWT催化活性的同时,也提高了对NH₃的吸附,从而增强了NH₃敏感性;过量的钒形成的条状棒状晶体造成了VWT比表面积的降低,弱化了NH₃的吸附及参与电极反应的NH₃浓度,从而降低了传感器敏感性和响应/恢复速度。在550°C的工作温度和40 h的持续时间内,基于VWT敏感电极的NH₃传感器耐久性较好,其对低浓度NH₃的响应值保持稳定。分析了由不同烧结温度产生的电极显微结构对NH₃敏感性能及电化学阻抗谱的影响。当烧结温度低于750°C时,VWT电极微观形貌变化不明显。当温度达到800°C后,VWT颗粒长大到200 nm以上,比表面积显著降低,并发生TiO₂载体由锐钛矿向金红石的相变。在较低温度范围内,烧结温度的提高促进了NH₃敏感性的提升,这是由于电极与电解质结合更为充分,增加了三相界面数量。表现在阻抗谱方面,是电极反应强度的增加,带来三相界面电阻的减小。烧结温度超过800°C后,过烧引起的金红石相变、氧化钒的减少,使VWT电极催化活性降低,进而弱化了NH₃敏感性,同时增加了NO_x的交叉敏感性。VWT晶粒长大引起的NH₃吸附性能减退及三相界面的减少,也是造成敏感性下降的重要原因。分析了基于VWT敏感电极的NH₃传感器的抗氧干扰能力。发现当O₂含量高于1 vol.%时,O₂浓度的变化对NH₃敏感性的影响较小;当O₂含量处于1 vol.%时,高NH₃浓度的响应值显著高于其他较高O₂浓度下的响应。这是由于高温下VWT对NH₃具有气相催化作用,较低的O₂浓度减少了NH₃的消耗,从而提升了NH₃敏感性。研究了Au及YSZ的修饰对VWT物相、微观组织、NH₃敏感性和响应/恢复时间的影响,并利用电极极化曲线分析其机理。5 wt.%以下的Au可以提高VWT的微区导电性和电极电化学反应活性,加强NH₃参与的电极反应强度,同时也加快了响应和恢复速率。低于10 wt.%的YSZ可以促进传感器的NH₃敏感性,主要是由于适量的YSZ增加了三相界面数量,提升了NH₃催化氧化反应速度,从而提高NH₃敏感性的同时也加快了响应和恢复速率。但过量的YSZ颗粒覆盖到VWT表面,阻碍了NH₃的吸附和扩散,使参与电极电化学反应的NH₃浓度降低,从而导致NH₃敏感度的下降及响应和恢复时间的延长。作为重要的电极结构参数,电极厚度对NH₃传感器性能的影响也进行了分析,并利用电化学阻抗谱和极化曲线探讨了影响机理。通过不同丝网印刷次数制得厚度分别为9.8μm、15.2μm和24.5μm的VWT电极。随着电极厚度的增加,传感器的NH₃敏感性呈递减趋势,由71.9 mV/decade锐减到17.0 mV/decade,响应和恢复速度也迅速放缓,且增加了NO_x的交叉敏感性。结合电极电化学分析结果,认为随着VWT厚度的增加,由于气相催化反应而消耗更多的NH₃,使参与三相界面电极反应的NH₃浓度降低,从而弱化了阳极反应的催化活性,降低VWT的NH₃敏感性。增加的电极厚度引起NH₃分子扩散的减缓,最终增加了传感器响应和恢复时间。更多还原

【Abstract】 The emission of NO_x from automobile exhaust has caused increasing pollution of the environment.Based on this fact,stricter emission regulations and mitigation technologies have been introduced in recent years by many countries.Selective catalytic reduction（SCR）technology is widely used in the reduction of NO_x due to its excellent denitrification efficiency.In order to improve the NO_x conversion efficiency of SCR system and prevent the leakage of reducing agent NH₃,the demand of high performance NH₃ sensor for exhaust gas is becoming more urgent.Among various kinds of electrochemical gas sensors,yttria-stabilized zirconia（YSZ）-based potentiometric sensor becomes a research focus mainly due to its high sensitivity,durability,simple structure and lower cost.As an important part of gas sensor,sensing electrode plays a key role in gas adsorption and electrochemical catalysis,and these properties could be influenced by the material composition,phase composition and electrode morphology.As a commercial SCR catalyst,V₂O₅-WO₃-TiO₂（VWT）mixed metal oxides meet the basic requirements of the sensing electrode materials for NH₃ sensors because of the excellent abilities of NH₃ adsorption,catalytic activity and anti-poisoning,and display a great research potential.In this thesis,VWT was prepared as sensing electrode material for the YSZ-based mixed-potential NH₃ sensors,and the sensitivity,selectivity,response and recovery time were tested.The influence mechanisms of VWT composition,electrode microstructure,Au/YSZ modification and electrode thickness on NH₃ sensing performances were evaluated combined with the electrochemical impedance spectroscopy（EIS）and polarization curves.Six VWT powders with different V-content of 0,0.5,1.0,2.0,3.0 and 5.0 wt%were prepared by co-impregnation method.Through the characterizations of XRD and SEM,it was found that the powder particles were composed of anatase TiO₂ particles,WO₃ and highly dispersed V₂O₅ on the TiO₂ surface.When the content of vanadium was lower than3 wt.%,the VWT powder particle was similar,and the diameter was about 100 nm.When the vanadium increased to 5 wt.%,the strip and rod crystals appeared.They were considered as vanadium oxide crystal after been analyzed by TEM and EDS.The formation of crystalline vanadium oxide is mainly due to the excess vanadium can not be distributed on the limited surface of TiO₂ in a high dispersion state.The NH₃ sensors with different V-content VWT sensing materials were prepared,and the phase composition,microstructure,sensitivity,selectivity,response/recover times and electrochemical catalytic activity were analyzed to investigate the influence mechanisms of sensing properties from vanadium content.With the increase of V-content,the sensitivity and response/recovery rates of the sensors increased first and then decreased,and the sample with 2.0 wt.%V₂O₅ obtained optimum NH₃ sensitivity.The content of vanadium has less influence on selectivity.The change of sensitivity is mainly due to the changes of NH₃ adsorption capacity,electrochemical catalytic activity caused by the increased V-content.The addition of a moderate amount of vanadium promotes the catalytic activity of VWT,and also increases the NH₃ adsorptive capacity,thus enhances the NH₃ sensitivity.The rod-shaped and block-shaped crystalline formed by excessive vanadium reduces the specific surface area of VWT,this weakens the NH₃ adsorption and the concentration of the NH₃ participated in the electrode reactions,and thus reduces the sensitivity and response/recovery speed.Under the working temperature of 550°C and the holding time of 40 h,NH₃ sensors based on VWT sensing electrodes have enough durability,response values for low concentrations of NH₃ remain stable.The influences of sintering temperature on electrode microstructures,NH₃ sensing properties and electrochemical impedance spectroscopy were analyzed.When the temperature reaches 800°C,VWT particle grows up to more than 200 nm,and the specific surface area decreases significantly.Phase transformation from anatase to rutile takes place on the TiO₂.In the lower temperature range of 650-750°C,the increase of sintering temperature leads to enhanced NH₃ sensitivity because of the more sufficient connection of VWT electrode and YSZ electrolyte and the increased number of three phase boundary（TPB）.The increase of the TPB resulted in a more intense electrode reaction,which decreases the interface resistance and improves the NH₃ sensitivity.When sintering temperature is over 800°C,the phase transformation and reduction of vanadium species caused by oversintering lower the electrode catalytic activity and increase NO_x cross sensitivity at the same time.The decrease of the NH₃ adsorption and TPB caused by VWT grain growth is also an important reason for the degradation of sensitivity.The anti-oxygen interference ability of the NH₃ sensor based on VWT sensing electrode was analyzed.It was found that when the O₂ was higher than 1 vol.%,the changes of O₂ concentration had little effect on the NH₃ sensitivity.When the O₂concentration was 1 vol.%,the response values for the high NH₃ concentrations were significantly higher than that of other higher O₂ concentrations.This is due to the gas phase catalysis of VWT at high temperatures.The lower O₂ concentration reduces the NH₃consumption,thus enhances the sensitivity of NH₃.The effects of Au and YSZ modification on VWT phases,microstructures,NH₃sensitivity and response/recovery time were also studied,and the mechanism was analyzed by electrode polarization curves.Au modification of less than 5 wt.%could improve the microzone conductivity and the electrochemical activity to electrode reactions,enhance the exchange current density,and eventually improve the NH₃ sensitivity and response/recovery rates.The YSZ modification of below 10 wt.%promotes NH₃sensitivity which is due to the increased TPB caused by nano-YSZ.Too much YSZ particles cover on the VWT surface,hinders the adsorption and diffusion of NH₃ and reduces the concentration of NH₃ involved in the electrode reactions,which leads to a decline of NH₃ sensitivity and response/recovery rates of the sensors.As an important electrode structure parameter,the influence of electrode thickness on sensor performances was also investigated with EIS and polarization curves.The VWT electrodes with the thickness of 9.8,15.2 and 24.5μm were made by different screen printing times.As the thickness increased,the sensitivity of the NH₃ sensor decreased from 71.9 to 17.0 mV/decade,and the response/recovery rate also decreased rapidly,but the cross-sensitivity of NO_x increased.Considering electrochemical analysis results,we believe that with the increase of electrode thickness,more NH₃ is consumed by gas phase catalytic reactions in VWT catalyst,which reduces the concentration of NH₃ participated in the electrode reactions on TPB.The weakened anode reaction then reduces the NH₃sensitivity of sensor.The increased VWT thickness retards the diffusion of NH₃ molecular,and finally slows down the response/recovery rates.更多还原