| With the continuous development of industrial civilization, environmental pollution is increasing and has gradually posed a threat to human health life. Air pollution as the main part of the environmental pollution has seriously affected the global climate and human health. Nitrogen oxides are the main component of greenhouse gases, and also an important reason for the formation of photochemical smog and acid rain. The combustion of fossil fuel, such as oil, coal, biomass, etc., is the mainly man-made emission of nitrogen oxides. In China,24,043,000 tons of nitrogen oxides emissions in 2011, which means imminently effective governance needed. The NOx pollution control techniques can be divided into the combustion control and post-combustion purification. It is the select catalytic reduction of NO in the excess of oxygen by hydrocarbons (HC-SCR) studied widely. In addition, CH4-SCR has a good prospect used to effectively removed methane and nitrogen oxides in the exhaust gas.After years of research, the selective catalytic reduction of NO by methane has made great progress, but some limitations of single molecular sieve catalysts still existed, such as effect of structure on active sites and acidic sites. Composites zeolites can improve active and acidic sites through topology structure’s complexed. Therefore, this paper further studied the structure-activity relationship of composites CH4-SCR catalyst and catalytic reaction mechanism.In this thesis, we used the composite BZZ, BMZ, BFZ as the carrier for methane selective catalytic reduction of NO. The role of the active site and catalytic reaction mechanism were studied systematically on the Co-based catalysts. All the three composite zeolite Na-BZZ, Na-BMZ and of Na-BFZ are synthesized by a two-step crystallization. After then, the physical mixture CoH-M, which were mixed by BZZ, BMZ and the BFZ with different proportions, were studied and thought to be the good catalysts for NO-SCR by CH4. The Na-type samples were exchanged with 0.5M NH4NO3 solution and 0.01M Co(AC)2, respectively, to form CoH-zeolites. Afterwards, they were calcined at 550℃ and crushed and sieved to 40-60 mesh granules. The structure, acidic, active sites and surface adsorption state of Co-based catalysts was investigated by various characterizations, such as X-ray diffraction (XRD), nitrogen adsorption-desorption characterization of diffuse reflectance UV visible spectroscopy (DRS-UV-Vis), NH3 program temperature desorption(NH3-TPD), H2 temperature-programmed reduction (H2-TPR), NO temperature-programmed desorption (NO-TPD), pyridine adsorption IR spectra (Py-IR), the probe molecule adsorption in situ infrared Characterization (in-situIR) and X-ray photoelectron spectroscopy (XPS).Exchanged metal Co ion species charge balanced at the cationic sites can occur as three forms. (ⅰ) "bare" cations coordinated exclusively to the framework oxygen atoms and thus exhibiting open coordination sphere, (ⅱ) Co-oxo species coordinated to the framework but simultaneously bearing extra-framework oxygen atom(s)-they might be represented by Co-oxo or dinuclear Co-oxo-Co species, and (ⅲ) Co oxide-like species supported in the zeolite inner volume or mostly at the outer surface of the crystals. Three typical α-, β-and γ-type Co2+ ions located in the corresponding α, β and γ cationic framework sites were suggested in mordenite, ZSM-5 and Beta zeolites. The Co ions in pentasil ring zeolites, and transition divalent cations generally, are coordinated to framework six-membered rings (α- and β-types) and γ-type cations are in boat-shape site of mordenite and ZSM-5.In the opinion, there is an optimal ratio range for good reactivity between β-type Co2+ and α-type Co2+, which was valued from 1.5 to 2.5. When Cop/a> 2.5, the NO adsorption on the catalyst is relatively limited and the adsorption and combustion of methane are more conducive; when Cop/a<1.5, more adsorption of NO but less activation of methane occurred which lead to poor activity, however, the β-type Co2+ located rich on the catalysts.There are important effects of ion-exchange degree on the activity of catalysts. With the increase of ion-exchange degree, the α-type Co2+ and Co-oxo ion clusters were first saturated, and then it was β-type Co2+. But Co oxide phase is constantly enriched in this process. When the α-, β-type Co2+ andCo-oxo ion clusters are saturated, the increase of the Co oxide would gradually plug part of the active site of Co ions, resulting in a lower catalyst activity.Composite based catalysts preserve more exchanged bare Co+ cations, which lead to stronger Lewis acid production. Furthermore, more complex acid sites are located on them. The weak acids weaken a little, while the strong acids get stronger. However, the amounts of strong acids decreased a little. Particularly, some small amounts of strong acid depicted above 800℃ appeared on CoH-BMZ and CoH-BZZ. Moreover, β-and a-type bare Co2+ ions were considered to be the two different NO adsorption site on CoH-BMZ and CoH-BZZ. Finally, the porosity of composite zeolite was richer than the one of single zeolite, and the diffusion of the reactant molecules was improved on CoH-BZZ and CoH-BMZ. Where more reaction active sites exist, CoH-BZZ and CoH-BMZ have a better response molecular accessibility than CoH-Beta, which promote the reactivity well. The ratio of Copβ/α on CoH-BMZ, CoH-BZZ was 1.9 and 2.3, respectively, which was with in the optimal range. However, it was 3.3 on CoH-Beta.In the absence of vapor, CoH-BMZ expressed the best NO conversion for 71% at 500℃, CoH-BZZ showed the best NO conversion for 60% at 550℃, however, it was 40% on CoH-Beta at 600℃. After reaction for 140h, CoH-BZZ expressed the best stability, but CoH-BMZ played less stability. It was because that the relatively high Si/Al ratio showed on BZZ, and more effectively polarized such that the bond between the silicon and oxygen was made than it was with just (structural) ZSM5 and Beta present. In another word, the polarization boosted the generation of the electron hole produces substantial geometry relaxation.In the presence of vapor, the NO conversion on CoH-Beta decreseased 17%, about which was 22% on CoH-BZZ and 25% on CoH-BMZUnder wet condition, Co oxide species would share part of the water adsorption in the presence of vapor, which reduce the competitive adsorption of water and the reactive feeds on bare Co2+active sites.After physical mixed of BMZ, BZZ and BFZ with different proportion, the mixture sample catalyst imaged the most powerful profiles for all the three Co species. More Co oxide species and Co-oxo ions generated on the interface of the different crystal, and all the bare Co ions sites were maintained in the sample CoH-M. As expected, the CoH-M1, which possessed plenty of all the three Co species, took one higher activity, smaller deactivation amount in the presence of water, and also one short response time for activity recovery.Additives of Zn improved effectively the selectivity of methane towards NOx reduction. Cobalt on both CoZnBZZ and CoZnBMZ catalysts presented as bare Co species, and the bind energy decreased by strong metal-support interaction (SMSI) and the interaction between cobalt and the additive metals zinc. Reactivity of both CoZnBZZ and CoZnBMZ is much higher than that found on Co based catalysts at temperature range between 500 and 650℃.The bare Co2+ cations are the main active site for selective catalytic reduction of NO with methane. The Co-oxo-phase promoted the reaction by the adsorption of NO and the formation of NO3-1 adsorbed intermediates. The metal oxide phases on outer surface play a negative effect on the catalytic reaction for methane combustion.The adsorption and activation of CH4 and the formation of C=N intermediates on β-type Co2+ are the control step in the whole reaction. The a-type Co ions play the role as the NO adsorption and activation. Co-oxo ion clusters promoted the formation of NO3- mainly. Then, NO3-1 and C≡N generated to the final reaction products N2 and CO2 on the Lewis acid site. |
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