Selective Catalytic Reduction Of NO By CH₄ Over Mn/HFBZ In The Presence Of Excess Oxygen

NO_X (NO+NO₂, NO accounts for 95%) is a main pollutant in the atmosphere.Its emission causes serious impacts on human health and the ecologicalenvironment. Therefore, reducing the emission of NO_X has become an extremelyemergent task of the environmental protection. Nowadays, selective catalyticreduction of NO by methane(CH4-SCR) is one of highly efficient way in theremoval of nitrogen oxides. Preparing catalysts with high activity, selectivity andSO₂ and H₂O tolerance should be solved before practical application of theCH₄-SCR technology. The investigating tasks are put forward under thisresearch background in this paper.A type of new zeolite composite FBZ with FAU and BEA topology structurewas synthesized. Selective catalytic reduction of NO_x in the presence of excessoxygen was investigated systemically over Mn- ion-exchange H-FBZ catalysts.XRD, FT-IR, FE-SEM, NH₃-TPD, H₂-TPR techniques were applied tocharacterize the catalysts. The H₂O and SO₂ tolerance of the catalysts were alsostudied. It is found that the topology structure of the carrier strongly affects thecatalytic properties of the catalysts. The SO₂ and H₂O tolerance of the catalystsis also affected by the carrier. The main research results obtained in this paperwill be shown as follows:1. Zeolite composite with FAU and BEA topology structure can besynthesized succefully with two step hydrothermal crystalyzing method. Onlycharacteristic peaks of FAU and BEA topology structure are observed over theXRD patterns of FBZ. The relative content of the two kinds of topology structure can be obtained by the XRD results. Characteristic peaks of FAU andBEA topology structure are observed in the FTIR spectra of the catalysts. Nooctahedral morphology is observed in the SEM images of the zeolite composite,whereas it cannot be excluded completely. Homogenous ellipse morphology isobserved in the SEM image of zeolite composite. The morphology is similar tothat of the Beta zeolite because the zeolite composite FBZ is synthesized byovergrowing or epitaxially growing a layer of zeolite Beta on the pseudo-crystalof FAU zeolite.2. NH₃-TPD results show that a new kind of strong acidic sites is formed onH-FBZ. These acidic sites can be ion-exchanged by the metal ions. The averageacidity of this type of acidic sites increases after ion-exchange of Mn cations.Temperature programmed oxidizing (TPO) and temperature programmedreducing (TPR) results show that the ion-exchange Mn cations in the zeolite areresist to the reducing and oxidizing process.3. Activity test results show that the catalytic activity is significantlyaffected by the topology structure of the catalyst. Compared to the physicalmixture catalysts of MnH-Y and MnH-Beta, with comparable FAU and BEAtopology structure, MnH-FBZ catalysts exhibit much higher catalytic activity,particularly the catalysts with the mass content of BEA topology structurebetween 60 % and 80%. MnH-FBZ catalysts also exhibit higher CH₄ selectivitythan the physical mixture catalysts of MnH-Y and MnH-Beta. The catalyticactivity increases with the Mn content in the catalysts. Higher gas hourly spacevelocity (GHSV) results in lower NO to N₂ conversion. The acidity of the carriercan promote the CH₄-SCR. O₂ is another essential factor in the CH₄-SCR. In thereaction system absence of oxygen at 773 K, i.e. 2050×10^-6 CH₄/2180×10^-6 NO,the catalytic activity is very low in the whole test temperature range. Increasingthe oxygen concentration, the NO to N₂ conversion increases significantly andreaches a maximum while the oxygen concentration is up to 2.00%. Furtherincreasing the oxygen concentration, the catalytic activity decreases. 4. In the presence of H₂O or SO₂, the catalytic activity of the catalystdecreases considerably. Co-existence of H₂O and SO₂, NO to N₂ conversiondecreases further, whereas, the composite catalyst MnH-FBZ exhibits betterH₂O and SO₂ than the catalysts MnH-Beta with single topology structure.