基于DFT模拟不同拓扑结构分子筛的氨氧化机理

Simulation of ammonia oxidation mechanism over zeolites with different topologies based on DFT

采用铜离子交换后的分子筛对NH_3具有较好的N_2选择性和催化氧化脱除效果,但对于不同拓扑结构分子筛上氨氧化机理间的对比研究鲜有报道。研究了三种不同拓扑结构(MFI、BEA、MOR)分子筛上NH_3的催化氧化性能,并借助密度泛函理论(DFT)对反应机理进行详细阐释。三种分子筛的氨氧化机理对比研究结果表明,离子交换率相同的条件下,MFI构型的Cu-ZSM-5分子筛在300℃时,NH_3转化率高达90%,是三种分子筛中NH_3催化净化效果最好的催化剂。DFT计算结果表明,反应机理包括以下三部分:(1)吸附气态NH_3分子,随后与活性氧原子作用生成OHNH_2;(2)通过E-R机理,由生成的OHNH_2和气相的NH_3分子反应生成中间物种N_2H_4;(3)N_2H_4物种和活性氧原子继续作用,逐步脱氢,最后生成N_2和H_2O。通过反应路径活化能计算明确OHNH_2物种的生成所需能量最高,是反应的速控步骤。

After ion-exchange with Cu ion,zeolites have good efficiency on selective catalytic oxidation of NH 3 ,but there are few reports on the mechanism of NH 3 -SCO over zeolites with different topologies.Therefore,the catalytic oxidation of ammonia over three topological zeolites (MFI, BEA, MOR) were studied and the mechanisms were investigated by density functional theory (DFT).The results show that MFI (Cu-ZSM-5) exhibits the highest NH 3 conversion of 90% at 300 ℃ under the same copper exchange rate.The DFT simulation suggests that the mechanism consists of three major parts:(1)adsorption of gaseous ammonia molecules,followed by interaction with reactive oxygen atoms to form OHNH 2;(2)gas phase NH 3 molecule reacts with OHNH 2 to produce stable N 2 H 4 species through E-R mechanism;(3)N 2 H 4 species and active oxygen atoms continue to act,and finally generate nitrogen and water.The formation of OHNH 2 species is the rate-determining step of the reaction and energy required for it is highest.