Study On The Application Of Calcined Hydrotalcite To Catalytic Reduction Of Nitrate In Water

The massive discharge of nitrogen-containing wastewater from industry and agriculture has led to the frequent occurrence of nitrate pollution in water,which was a serious threat to human health.Due to the high efficiency of the reaction and no secondary pollution,the catalytic reduction denitrification method using hydrogen as the reducing agent was widely concerned.Exploring the optimization of catalyst preparation and process conditions to achieve higher catalytic activity and nitrogen selectivity was the key to the study.As an excellent catalyst carrier,hydrotalcites materials could reduce the limitation of mass transfer during the reaction process and improve the reaction efficiency through a unique memory effect,however,there were also problems such as nitrite cumulation and poor nitrogen selectivity.Based on these issues,this article combined the reaction mechanism to study the catalytic performance from two aspects of the preparation method of the hydrotalcites catalyst and the reaction process conditions.Specific research contents were as follows:（1）A series of hydrotalcite carrier contained different metal species were made by coprecipitation method and characterized through XRD,SEM and BET,etc.The results showed that the Pd/CuMgAl hydrotalcite catalyst with relatively high specific surface area and regular morphology had the best catalytic effect.The high Cu content led to layer deformation due to the Jahn-Teller effect,which caused a rapid decrease in the catalytic activity.（2）A series of calcined Pd/CuMgAl hydrotalcites were prepared using different calcination methods and reduction processes.These catalysts were characterized by using XRD,FT-IR,and H₂-TPR to study the key factors affecting reaction conversion and selectivity.The results showed that the catalyst has the best catalytic effect when there was no calcination process before reduction（400℃,1 h）.It was found that calcination would reduce the reduction barrier of Cu,which would decrease the amount of Cu⁺after reduction,and be detrimental to the further reduction of nitrite.（3）The effect of reaction process conditions has been investigated.The results showed that the problem of nitrite accumulation could be reduced by decreasing the initial nitrate concentration in solution,increasing the amount of catalyst,increasing the hydrogen flow rate,and using organic acids as neutralizing agents,however,this would be accompanied by an increase in ammonium selectivity.（4）Hydrotalcites carriers were successfully prepared by the hydrothermal method and the urea hydrolysis method.The influence of the hydrothermal temperature and the amount of precipitant added on the catalytic effect has investigated by XRD and SEM.It was found that the number and size of the flaky crystals stacked layer by layer increased when the hydrothermal temperature and urea addition increased,and the adsorption effect of the“memory effect”was more significant.Under the optimal conditions,the NO₃^-conversion could reach more than 93%and N₂ selectivity was 88.6%.