| Present work firstly investigated the catalytic activities of a series of transition metal ions (TMI)(Fe, Co, Cu) modified zeolite catalysts with different structures (BEA, FER, MOR, MCM-49) during N2O direct decomposition, in which the influences of SO2(100ppm)/NO (1vol%)/CO (1vol%) were also evaluated. It was found that the TMI(Fe, Co, Cu)-BEA exhibited the superior activity for N2O dissociation, with respect to other zeolite samples. Therefore, the N2O direct decomposition over TMI(Fe, Co, Cu)-BEA zeolites were thereafter systematically investigated by employing both experimental and theoretical approaches (density functional theory, DFT), which includes the intrinsic kinetic analysis, reaction mechanism investigation, microkinetic analysis, charge transfer (CT) analysis, and electric filed effect (EFE) of TMI(Fe, Co, Cu)-BEA. Finally, based on the powder Fe-, Co-BEA with excellent N2O decomposition activity, a detailed monolithic zeolite catalyst preparation method was proposed using honeycomb cordierite as the support, which contributes to the development of industrial catalyst for N2O abatement. The main contents and conclusions of present work were detailedly described as follows.1. A series of Fe, Co, Cu ions modified zeolite catalysts (BEA, FER, MOR, MCM-49)(SiO2/Al2O3=30) was prepared by the wet ion exchange (WIE) method with the theoretical metal loading of1wt%. The activity evaluation revealed that TMI(Fe, Co, Cu)-BEA exhibited the better activity than other zeolite samples. The N2adsorption/desorption suggested that the better activities of TMI(Fe, Co, Cu)-BEA for N2O dissociation were correlated with its highest specific surface area and pore volume.2. Based on above study, the N2O direct decomposition over Fe-BEA was systematically investigated by means of both experimental and theoretical (DFT) approaches. Firstly, the characterizations of XRD and H2-TPR revealed that as Fe%<1%the atomic Fe mainly existed as the Fe+ions on the Fe-BEA zeolites, which was also believed to be the active center for N2O dissociation. However, as Fe%>1%the FeOx could be formed, which exhibited low N2O decomposition activity. The activity performance evaluations were thereafter conducted, suggesting that as Fe%>1%no promotion effect was found during N2O direct decomposition. It was probably attributed to that as Fe%>1%the FeOx could be formed having low N2O dissociation activity and that the reaction was probably controlled by the internal mass transfer rather than the kinetic as Fe%>1%. 3. The experiments of N2O-DRIFTS, N2O/NO2-TPD-MS, and N2O-TPSR were further performed to investigate the N2O direct decomposition mechanism over Fe-BEA-1%zeolite. On the basis of that, two kinds of reaction mechanisms were proposed named O2formation mechanism (the main reaction mechanism) and NOx formation mechanism, respectively. Moreover, the quantum chemistry based on the DFT was employed to simulate above two kinds of reaction mechanisms using5T-Fe-BEA model and aiming at giving deeper insight. The energy barriers ΔE of the main reaction steps were thereafter calculated, revealing that the O2desorption step with the energy barrier of63.20kcal mol-1was the rate determining step (RDS) for O2formation mechanism, and the reaction step B2with the ΔE of26.92kcal mol-1was the RDS for NOx formation mechanism.4. Present work also investigated the CT of each reaction step of O2formation mechanism (Fe-BEA-1%) based on Mulliken population analysis and with purpose of better understanding the reaction mechanism. It was found that the zeolite framework played important role in decomposition of the first N2O (Part1). However, during the second N2O dissociation (Part2), the active center Fe and the formed aO played major roles. The CT behavior of the aO in Part2can well explain the high activity of αO in the electric point of view. Additionally, the frontier molecular orbital (FMO) analysis and N2O-DRIFTS were also applied to investigate the CT during N2O direct decomposition over Fe-BEA-1%, with the results being in good agreement with those revealed by Mulliken CT analyses.5. Based on above studies, present work further systematically investigated the active differences of TMI(Fe, Co, Cu)-BEA (with the TMI loading of1wt%) during N2O direct decomposition, wherein both the experimental and theoretical approaches were employed. Firstly, the characterizations of XRD, H2-TPR, XPS, and UV-vis were conducted to investigate the chemical states of Fe, Co, and Cu on TMI(Fe, Co, Cu)-BEA. It is revealed that the Fe, Co, and Cu were mainly in the form of metal ions on the zeolite, which also formed the active center for N2O dissociation. The experiments of activity evaluation, turn over frequency (TOF), and intrinsic kinetic analysis suggested that the N2O active sequence was in the order of Co-BEA> Fe-BEA> Cu-BEA. Thereafter, the theoretical approach (DFT) was employed to investigate the reaction mechanism of Co-and Cu-BEA during N2O direct decomposition, in which the energy barriers of main reaction steps were calculated. Through the DFT energy calculation, it was found that the O2desorption step (Part3) was the RDS for both Co-and Cu-BEA. Thus, comparing the energy barriers of RDS among TMI(Fe, Co, Cu)-BEA it can be clearly found that Co-BEA having the lowest energy barrier exists the best N2O activity. Additionally, an intermediate (IM) was found in Part1during the reaction mechanism study for Co-BEA. The generation of IM can reduce the N2O decomposition rate resulting in the curve shape being different from those of Fe-and Cu-BEA during the activity evaluation and N2O-TPD experimental studies. The microkinetic analysis based on DFT was further conducted with the result well confirming above finding.6. The modified zeolite catalysts can affect the reactant molecule through their electric field effect (EFE). Therefore, present work investigated the EFE of TMI(Fe, Co, Cu)-BEA during N2O direct decomposition, with aim to give deeper insight into the activity differences of TMI(Fe, Co, Cu)-BEA for N2O dissociation based on the EFE point of view. The approaches of Mulliken CT analysis, FMO analysis, and N2O-DRIFTS were employed. It can be concluded that the EFE of TMI(Fe, Co, Cu)-BEA influences the N2O direct decomposition in two ways:firstly the EFE of TMI(Fe, Co, Cu)-BEA facilitates the adsorption of N2O through accepting large amounts of chares from N2O; then in the following step the EFE of TMI(Fe, Co, Cu)-BEA favors the bond polarization of O-N2by donating the chares to the N2O. In Part2it was interestingly found that Cu-BEA and its αO exhibited no EFE on O-N2bond polarization, which is correlated with the lowest N2O dissociation activity of Cu-BEA in Part2. For the purpose of quantitatively calculating the CT abilities of TMI(Fe, Co, Cu)-BEA during N2O direct decomposition, the FMO analysis was conducted. It was found that the LUMO-HOMO orbital gaps between TMI(Fe, Co, Cu)-BEA and N2O molecule decided the related CT abilities, which further decided the activity of TMI(Fe, Co, Cu)-BEA for N2O dissociation. The N2O-DRIFTS was used to experimentally evaluate the CT ability of TMI(Fe, Co, Cu)-BEA during N2O dissociation by comparing the maximum v(N-N) band shift values. It was found that the N2O-DRIFTS results agreed well with that of FMO quantitative analysis, verifying that the applied FMO analysis in present work was correct.7. On the basis of above studies, present work investigated the preparation methods of Fe-and Co-BEA monolithic zeolite catalysts, using silica or alumina sols as the binders and honeycomb cordierite as the support. The activity evaluations and characterizations of SEM and ultrasonic test revealed that the5%alumina sol prepared Fe-BEA and5%silica sol prepared Co-BEA exhibited the best catalytic activities. Additionally, the computational fluid dynamics (CFD) was also employed to simulate the N2O dissociation process over Fe-, Co-BEA monolith catalyst. The simulated N2O conversion results were in good agreement with those of experimental activity evaluations. |
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