Density Funcional Studies On The Reaction Mechanisms Of The Reduction Of N₂O With CO Catalyzed By Os⁺、Al₂O₃⁺ And Ga-ZSM-5

Atmospheric environmental pollution is harmful to the health of human beings, results in a serious threat to the sustainable development of the social economy, even causes many other environmental problems, and leads to a series of unpredictable consequences. Catalytic conversion of harmful gases is of utmost importance both environmentally and economically. CO and N2O both are pollution gases. Although the reaction between these two gases is strongly exothermic, the process is kinetically unfavorable due to a substantial barrier, and does not proceed at all at ambient temperature and normal atmospheric pressure without a suitable catalyst. In this article, based on the results of the relevant experiment, systematical density functional studies have been performed for exploring the mechanisms of the reduction of N2O with CO catalyzed by three different types of catalysts:single metal ion Os+. metal oxide ion Al2O3+ and Ga-ion exchange zeolites Ga-ZSM-5 to mimic the the process for catalytic purifying air pollutants CO and N2O. The detailed potential energy surfaces, mechanisms and energetics for the reaction pathways in all catalytic cycles have been obtained. Specific contents are as follows:Firstly, density functional calculations with UB3I.YP, modified LanL2dz and 6-311G* for Os and other elements, respectively, have been performed to investigate the mechanism of the reaction between N2O and CO catalyzed by gas-phase Os". The sextet, quartet, and doublet potential energy surfaces of the reaction have been explored. The results show that this reaction occurs adiabaticaly on two potential energy surfaces with different spin states, namely through two state reactivity (TSR) mechanism. Totally, only sextet and quartet spin states are found to be involved in the minimum energy reaction path. The minimum energy pathway can be described as 6Os++N2O�'6IM1�'4TS1/2�'4IM2�'4OsO++N2;4OsO+ +CO�'4IM3�'4TS3/4�'6IM4�'6Os++CO2 Specifically, the overall reaction is calculated to be exothermic by 357.2 kJ/mol, and its rate-determining step is the formation of complex Os+-CO2 with a barrier of 162.6 kJ/mol. which is much lower than that of 202.5 kJ/mol for the reaction occuring directly.Secondly, the reaction mechanism of the reduction of N2O with CO catalyzed by Al2O3+ has been investigated using the density functional theory (DFT) method with UB3LYP and 6-311+G* bassis set. Five possible pathways are considered. From each of the potential energy surfaces we can see all steps of the reactions are exothermic, the total exothermicity is 362.3 kJ/mol, and the activation energy barriers for rate-determining steps in five pathways (the highest barrier is only 90.5 kJ/mol) are much lower than the barrier (197.2 kJ/mol) in the direct reaction.Thirdly, the reaction mechanism of the reduction of N2O with CO catalyzed by gallium ion-exchanged zeolites has been investigated using 3T cluster model and B3LYP/6-31+G(d) method. From the potential energy surfaces calculated, each branch reaction is exothermic, the total exothermicity is 347.1 kJ/molwith a barrier of 74.3 kJ/mol for its rate-determining step, which is much lower than that of 196.7 kJ/mol for the direct reaction.In a summary, the calculated results show that the three catalysts. Os+. Al2O3+/and Ga-ZSM-5 exhibit effective catalyzation to the reaction of CO and N2O, in which they lower the barrier for the reaction effectively. And the detailed catalytic reaction mechanisms are obtained. These results will not only help to deepen the understanding to the reaction mechanisms, but also help to develop the efficient catalyst for catalytic purifying air pollutants such as carbon oxides and nitrogen oxides.