Selective Catalytic Reduction Of NO By CH₄ On Co-catalysts In The Presence Of Excess Oxygen

The combustion of coal, petroleum and natural-gas meet the needs of mankind energy. However, the emission of flue gas has caused serious environmental pollutions. NO can not only lead to formation of acid rain and photochemical smog, but also to"green-house"effect even threaten human lives. Therefore, how to abate NOx in the flue gas is a attractive task in the worldwide. Selective catalytic reduction of NO by methane (CH4-SCR) is considered to be the potential technology as methane is not only abundant and easy to get but also in the combustion of oil emission.A type of new zeolite composite Beta-mordenite zeolite was synthesized. Selective catalytic reduction of NO in the presence of excess oxygen was investigated systemically over the Co- ion-exchange H-BMZ catalysts. Morden techniques were applied to characterize the catalysts and investigate the probable reaction mechanism. The SO₂ tolerance of the Co-Beta and Mn-ZSM-5 catalysts were also studied. The effect of SO₂ for Co species on Co-Beta catalyst was characterized by H₂-TPR and DRS-UV-Vis techniques. NO-TPD and SO₂-TPSR was applied to investigate the sulfur species formed during the poisonous tests. The effect of alkali-treatment on the activity of Co-ZSM-5 over the traditional catalyst Co-ZSM-5 for NO reduction with CH4 was also investigated.Zeolite composite with BEA and MOR topology structure, observed over XRD, SEM, HRTEM, can be synthesized succefully with two step hydrothermal crystalyzing method. With increasing the mass content of MOR topology structure, Co-BMZ catalysts exhibit much lower catalytic activity. It is found that the composite catalyst Co-BMZ exhibit higher activity and better water tolerance than Co-MOR and Co-Beta catalyst. Pyridine-IR (Py-IR) results show that the numbers of acidic sites over CoH-BMZ are superior to that over Co-MOR and Co-Beta catalyst. With large pore catalyst and surface area, the composite zeolite loaded Co-BMZ catalyst mainly exists in the form Co²⁺ ion, hardly in Co oxides as shown by H₂-TPR results, thus inhibits the combustion of CH4. NO FT-IR results show that adsorbed NO primarily form NO+ and NOy- on the composite catalyst in low temperature. With the rise of reaction temperature and adsorption time, the species transform in between Co²⁺ and Co³⁺ ion, that indicates the Co²⁺/Co³⁺ redox cycle in composite catalyst.NO conversion on Co-Beta catalyst decreases by addition of SO₂, while the activity fully restores by removing it. The loss of catalytic activity is clearly revealed by XRD, H₂-TPR, DR UV-vis, NO-TPD. The reasons are that the zeolite surface and channels are reduced and partially blocked, respectively, reducing the diffusion of reactant to the active center on poisoned-catalyst. Besides, the number of Co²⁺ ions reduce, mainly theβ-type position, reducing the number of active sites, decreasing adsorption capacity of NO, resulting in catalystic activity decrease significantly. In the presence of SO₂, a substantial decrease in NO conversion was found at T≤550 oC but no effect was observed at T≥600 oC. Such a result can be attributed to the weaker adsorption of the species contained sulfur on active sites at higher temperatures.The alkali-treatment of ZSM-5 led to a formation of new mesopores, while the intrinsic micropores remained unchanged. The Co-loaded catalysts, derived from the alkali-treated ZSM-5 exhibited higher NO conversion and more tolerance to water and SO₂ poisoning than the conventional Co/ZSM-5 catalyst. Such a result can be attributed to the coexistence of mesopores and inherent micropores in the zeolite, which promoted the dispersion of Co species and improved the accessibility of active Co²⁺ ions for NO.