Amperometric Ammonia Sensor

NH₃ is widely used in the industry.However,NH₃ is a toxic gas.Even small amount of NH₃ is very harmful to the human body.Therefore,monitoring NH₃ for the living and working environments is very important.The amperometric NH₃ sensor possesses some advantages,such as their miniaturization,low cost,high sensitivity and good selectivity. Therefore,the development of the amperometric NH3 sensor has been became a hot research area.This thesis mainly investigated the catalysts with the high electrocatalytic performance for the NH₃ oxidation in neutral solution and the best composite of the electrolyte.The amperometric NH₃ sensor with the possible application was studied and assembled.The main results obtained are as follows:.1.Comparison of electrochemical properties of NH₃ and NH₄OHIt was found that the electrochemical properties of NH3 and NH4OH are similar in the neutral solution.Thus,instead of NH3,NI-I4OH can be used to investigate the electrocatalytic performances of the catalysts for the NH₃ oxidation.2.Comparison of electrocatalytic performances of Ir and Pt catalysts for the NH₃ oxidationThe Pt and Ir catalysts were prepared with the liquid phase reduction method.Their electrocatalytic performances and selectivity for the NH₃ oxidation in the neutral solution was compared.It was found that in the NaClO₄ solution,the peak current density of the NH₃ oxidation at the Ir catalyst is 2.5 times larger than that at the Pt catalyst.Furthermore, CO does not disturb the measurement of NH3 at the Ir catalyst,but obviously disturbs the measurement of NH₃ at the Pt catalyst.Therefore,the Ir catalyst is suitable to be used as the catalyst in the amperometric NH₃ sensor.3.Study on effect of structure of Ir catalyst on its electrocatalytic performancesThe Ir catalysts were prepared with the electrodeposition method,the NH₄F complex reduction method and the THF complex reduction method.The effect of the structure of the Ir catalysts on the electrocatalytic performances for the NH3 oxidation was investigated.(1)After the Ir catalyst is prepared with the electrodeposition method and undergoes the cyclic voltametric treatment,the average sizes of the Ir particles become small and the active surface area is increased.Thus,the electrocatalytic performance of the Ir catalyst for the NH₃ oxidation is increased.It was found that the potential of the oxidation peak of NH₃ at the Ir catalyst electrode in the NaClO₄ solution is at about 0.85V and the current density of the oxidation peak of NH₃ is linearly proportional with the concentration of NH₃,indicating that.the electrocatalytic oxidation of NH₃ is diffusion-controlled.(2)It was found that when the Ir catalyst is prepared with the NH₄F complex reduction method the average size of the Ir particles in the Ir-1 catalyst is as small as 2.8 nm and the crystallinity is low because NH₄F can complex with IrCl₃.The average size of the Ir particles in the Ir-2 catalyst prepared with the general liquid phase reduction method is as large as 7.5 nm and the crystallinity is high.Therefore,the electrocatalytic activity and stability of the Ir-1 catalyst for the NH₃ oxidation are better than that of the Ir-2 catalyst. Thus,it is hopeful to have apractical application in the amperometric NH₃ sensor.(3)A series of the Ir catalysts with the different average sizes of the Ir particles were prepared in the THF complex reduction method.It was found that the average size and the crystallinity of the Ir particles in the Ir catalysts prepared are decreased with increasing the volume ratio of THF and H₂O.It is due to that the complex between THF and IrCl₃ can be formed,leading the decrease in the congregation of the Ir particles.However,the electrocatalytic performance of the Ir catalyst is not always increased with the decease in the average size and the crystallinity of the Ir particles.When the volume ratio of THF and H₂O is 5:5 and the average size of the Ir particles in the Ir catalyst is 3.8 nm the electrocatalytic performance of the Ir catalyst for the NH3 oxidation is highest.4.Comparison of structure and performances of Ir-Pt catalysts with different weight ration of Ir and PtA series of Ir-Pt catalysts with the different weight ratios of Ir and Pt were prepared with the THF complex reduction method.It was found that Ir and Pt have formed the alloy in the Ir-Pt catalysts.In addition,the average size of the Ir-Pt particles in the Ir-Pt catalysts. is increased with decreasing the weight ratio of Ir and Pt.The electrocatalytic activity and the stability of the Ir-Pt catalysts for the NH₄OH oxidation are better than that of the Ir catalyst.Especially,the electrocatalytic performance of the Ir-Pt catalysts with the weight ratio of 2 is best.Although the anti-CO disturbance performance of Pt is poor,the CO disturbance for the Ir-Pt catalysts would decrease with increasing the weight ratio of Ir and Pt.When the weight ratio of Ir and Pt reaches 2 CO does not disturb the measurement of NH₄OH.Thus,the performance of the Ir-Pt catalyst with the weight ratio of 2 is best. 5.Effect of catalyst and electrolyte on performance of amperometric NH₃ sensor(1)The anti-dryness performances of the sensors with the LiCl aqueous solution,the aqueous solution of ion liquid and ion liquid as the electrolyte were investigated.It was found that the LiCl aqueous solution and ion liquid possess the good anti-dryness performance.(2)The effects of the LiCl aqueous solution and ion liquid as the electrolyte on the responsive signals of the sensor to NH₃ were compared.It was found that the stability of the responsive signal of the sensor with ion liquid as the electrolyte is slightly poor. However,the time of returning to zero is short.Thus,the total performance of the sensor with ion liquid as the electrolyte is good.(3)The performances of the NH₃ sensors with the 20%Ir/C,30%IrPt/C and 45%IrPt/C catalysts were compared.It was found that the electrocatalytic stability of the NH₃ sensors with 20%Ir/C catalyst is the best.(4)The disturbance tests of CO and H₂S indicated that CO does not disturb the measurement of NH₃,but H₂S obviously disturbs the measurement of NH₃.However, the disturbance of H₂S can removed through using the filter or the sorbent of H₂S.