Ammonia Response Characteristics Of Spinel Ferrite

The environmental hazards caused by NO_x in vechile exhaust have gradually received attention,and many countries have formulated stringent exhaust emission standards.To meet these standards,some NO_x reduction technologies have been developed one after another.Among them,the technology of selective catalytic reduction of NO_x using urea hydrolysis to produce NH₃ is widely used to reduce diesel exhaust emission.In order to obtain high NO_x removal efficiency and prevent pollution caused by NH₃ leakage due to excessive urea injection,a sensor is urgently needed for NH₃monitoring of diesel exhaust.Among various electrochemical gas sensors,the potentiometric NH₃ sensor based on YSZ solid electrolyte has attracted wide attention,and the research and development of its sensing electrode material is the top priority.Spinel ferrite has excellent performance,simple preparation method and low cost,and has been extensively studied in many fields.Its melting point is high,and stable microstructure can be obtained after sintering at high temperature,so it has excellent high temperature stability,which has been favored by scholars in recent years.In addition,the adsorption and catalytic performance of spinel ferrite on ammonia is related to its structure and electronegativity.Based on this,ZnFe₂O₄,CuFe₂O₄ and NiFe₂O₄were selected as the research objects of this paper due to their different spinel structure or electronegativity.We tried to obtain the NH₃ sensing mechanism and influencing factors of spinel ferrite by comparing and analyzing their NH₃ sensing characteristics,so as to develop a sensing electrode material with high NH₃ sensitivity and selectivity.The thesis mainly includes the following contents:First,the normal-spinel ZnFe₂O₄ sensing material was prepared by the sol-gel method,and it was found to be insufficiently sensitive to NH₃.In order to improve the NH₃sensitivity of the ZnFe₂O₄ electrode,Au was attempted to modify the ZnFe₂O₄ electrode due to its high NH₃ sensitivity,and the response characteristics of the sensor to NH₃were evaluated between 650? and 750?.It was found that after gold sputtering,the catalytic activity of the anode electrode reaction was enhanced,which made the response signal of the sensor to 80 ppm NH₃ increased by 75%at 700?,while the response and recovery time were shortened by 50%.At 700?,the response value of the sensor to 80 ppm NH₃ is-42.3 m V,and the response and recovery times are 25 s and 49 s,respectively.The sensor has good anti-interference ability to water vapor,C₃H₈ and NO_x,and also has good long-term stability.Secondly,the inverse-spinel CuFe₂O₄ sensing material was prepared,and different electrode microstructures were obtained by sintering at 950?,1000? and 1050?,respectively.The results show that the response performance is strongly affected by the microstructure of the electrode.The sensor sintered at 1000? has the most three-phase boundary(TPB)sites,moderate electrode thickness and porosity,and thus has the best NH₃sensitivity.At 650?,its NH3 sensitivity is-68.5 m V/decade,and the response signal value to 320 ppm NH₃ is as high as-92.3 m V.However,its response and recovery rate are slow,and it is attempted to improve this problem by adding different amounts of 8YSZ to the CuFe₂O₄ electrode.It was found that the addition of 8YSZ had a significant effect on inhibiting the grain growth of CuFe₂O₄.When the addition amount increased to 2%and 5%,the porosity of the electrode gradually increased and the ammonia response signal also gradually increased.However,when the addition amount is 10%,the electrode porosity decreased due to excessive YSZ filling the pores,and the ammonia response signal and response rate decreased slightly.In addition,for the inverse-spinel NiFe₂O₄ sensing material,the sensors were prepared by sintering at 1100?,1150?,and 1200?,respectively,and the ammonia response performance was tested at 650?.It was found that the 1150? sintered sensor has the most TPB sites and moderate porosity,and thus has the largest response signal.Its response signal to 320 ppm NH₃ is-104.3 m V,and its sensitivity is as high as-77.56 m V/decade.The EIS and polarization tests prove that the sensor conforms to the mixed-potential sensing mechanism.The sensor has good anti-interference ability to water vapor,oxygen and NO_x,and also has good long-term stability,indicating that it has good prospects in the practical application of the potentiometric NH₃ sensor.Then,by comparing the sensing characteristics of ZnFe₂O₄,CuFe₂O₄ and NiFe₂O₄,it was found that the inverse-spinel NiFe₂O₄ with the highest electronegativity has the best ammonia response performance,the inverse-spinel CuFe₂O₄ with slightly lower electronegativity has the slightly worse performance,and the normal-spinel ZnFe₂O₄ with the lowest electronegativity has the worst performance.It shows that the inverse-type has better ammonia response than the normal-type of spinel ferrite.For different inverse-spinel ferrites,the ferrite with higher electronegativity and higher melting point has better ammonia response.Finally,NiFe₂O₄ was used to prepare the split type and the thick film stacked type NH₃sensor devices.The two electrodes of the split type are in the same test atmosphere,while the reference electrode of the thick film stacked type is in the atmosphere,and the sensing electrode is in the test atmosphere.The split type has better NH₃ sensitivity,NH₃ selectivity,and resistance to oxygen/NO_x interference than the thick-film stacked type sensor device.It shows that in order to obtain better NH₃ response performance in the design of the sensor,the reference electrode and the sensing electrode should be designed in the same test atmosphere.