The Research On V₂O₅-WO₃-TiO₂ Sensing Materials For Potentiometric Ammonia Sensor

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.