Effect of K loadings on nitrate formation/decomposition and on NOx storage performance of K-based NOx storage-reduction catalysts

Abstract

We have investigated the effect of K loadings on the formation and the decomposition of KNO3 over K2O/Al2O3, and measured NOx storage performance of Pt–K2O/Al2O3 catalysts with various potassium loadings. After NO2 adsorption on K2O/Al2O3 at room temperature, ionic and bidentate nitrates were observed by Fourier transform infra-red (FTIR) spectroscopy. The ratio of the former to the latter species increased with increasing potassium loading up to 10wt%, and then stayed almost constant with additional K, demonstrating a clear dependence of loading on potassium nitrate formed. Although both K2O(10)/Al2O3 and K2O(20)/Al2O3 samples formed similar nitrate species identified by FTIR obtained after NO2 adsorption, the latter has more thermally stable nitrate species as evidenced by FTIR and NO2 temperature programmed desorption (TPD) results. With regard to NOx storage-reduction performance of Pt–K2O/Al2O3 samples, the temperature of maximum NOx uptake (Tmax) is 573K up to a potassium loading of 10wt%. As the potassium loading increases from 10wt% to 20wt%, Tmax shifted from 573K to 723K. Moreover, the amount of NO uptake (38cm3 NOx/gram of catalyst) at Tmax increased more than three times, indicating that efficiency of K in storing NOx is enhanced significantly at higher temperature, in good agreement with the NO2 TPD and FTIR results. Thus, a combination of characterization and NOx storage performance results demonstrates an unexpected effect of potassium loading on nitrate formation and decomposition processes; results important for developing Pt–K2O/Al2O3 as potential catalysts as high temperature NOx storage-reduction applications.