Study On The Structures And Performance Of Carbonate-based Lean-burn Nox Storage And Reduction Catalysts

Lean NOx storage and reduction (NSR) technique proposed by Toyota inmid-1990is now still regarded as one of the most promising techniques for lean NOxremoval. NSR catalyst should have good storage capacity to prevent NOx gasesescape in rich period. So, exploring novel NSR catalysts with large NOx storagecapacity is of great significance. In present work, a series of potassiumcarbonates-based NSR catalysts Pt-K2CO3/ZrO2, Pt-K2CO3/CeO2, Pt-K2CO3/Al2O3with different K2CO3loading were prepared by impregnation. The effects of K2CO3loading and calcination temperature of the support on the kinds and distribution of Kspecies were carefully investigated. Meanwhile, the function of K2CO3during NOxstorage is discussed.The results of NOx storage capacity indicate that the samples Pt-XK2CO3/ZrO2orPt-K2CO3/CeO2with different K2CO3loadings show extremely good performance forlean NOx storage and reduction. The catalysts containing15wt.%K2CO3exhibitlarge NOx storage capacities (2.16and2.21mmol/g for Pt-K2CO3/ZrO2,Pt-K2CO3/CeO2, respectively) and very high NOx reduction percentages (99%).Multiple characterizations including XRD, HR-TEM, TPD, FT-IR and in-situDRIFTS reveal that at room temperature the K species exist as amorphous K2CO3;while at NOx storage temperature (350oC) three kinds of K species including–OKgroups, K2O and K2CO3are simultaneously present in the catalysts. The amount ofaccessible surface carbonate species reaches the maximum on Pt-K2CO3/ZrO2andPt-K2CO3/CeO2with15wt.%K2CO3. Higher K2CO3loading leads to the formationof more bulk or bulk-like K2CO3species, which are unfavorable to NOx storage. AsK2CO3loading is10wt.%or less the NOx is mainly stored as nitrates species such asmonodentate nitrates, ionic nitrates and bridging bidentate nitrates, while at higherK2CO3loading the NOx is only stored as bidentate nitrite species. The presence ofexcess amount of K2CO3can decrease the ability of the catalysts for NO adsorptionand oxidation, making the NOx oxidized only to nitrite species.The NOx storage capacities of Pt-5%K2CO3/CeO2-X with the support calcined atdifferent temperature show that as the calcination temperature increases the NSCexhibit a volcano-type tendency, with the maximum appearing at700oC. Thecharacterization results show that the support calcined at700oC possesses less surface hydroxyl groups but more accessible surface carbonate species; when the calcinationtemperature of the support is increased to850oC, the specific surface areas of thesupport decline evidently, which is unfavorable to the dispersion of K species, leadingto the formation of more bulk or bulk-like K2CO3species.The catalyst Pt-25%K2CO3/Al2O3exhibits a very large NOx storage capacity(2.53mmol/g) and a very high NOx reduction percentage (99%). When K2CO3loading is very low (less than10%), most of the K2CO3reacts with the surfacehydroxyl groups on Al2O3, forming surface–OK groups, as a result, little surfaceK2CO3is detected. On the contrary, as the K2CO3loading is higher than10%, thesurface K2CO3species are obviously formed. The largest amount of surface K2CO3isachieved on Pt-25%K2CO3/Al2O3. Further increase of K2CO3loading leads to theformation of more bulk or bulk-like K2CO3species, decreasing the NOx storage andreduction performance of the catalysts.