Selective Catalytic Reduction Of NO_x By CH₄ Over Co, Mn Loaded Zeolite Catalysts In The Presence Of Excess Oxygen

The combustion of coal, petroleum and natural-gas etc. meets the mankind's needs of energy. However, the emission of flue gas has caused serious environmental pollutions. SO₂, NO_x (NO accounts for 90%), CO and CO₂ results in great threat to environment, particularly NO_x can not only lead to formation of acid rain and photochemical smog, but also to "green-house" effect. Many diseases are more or less related to NO_x. How to abate NO_x in the flue gas is a tough task in the worldwide. Although selective catalytic reduction of NO_x with NH₃ has been put into commercial application in the disposal of stationary source of NO_x such as emission gas in the electric power plant, application of this technology in the moving source of NO_x such as the traffic flue gas is impossible because of the well-known reasons such as storage and leakage of NH₃, costly equipment, strict operation conditions and formation of sulfate leading to pipe jam. Three-Way-Catalysts (TWC) is an effective way to abate CO, HC, and NO_x in the traffic flue gas simultaneously. However, this type of catalysts is only effective in the narrow range of air to fuel ratio adjacent to 14.6. Combustion of fuel in the presence of excess oxygen is not only an effective way to improve the fuel utilizing efficiency, but also can decrease the emission of pollutants. However, TWC catalysts exhibit low activity for NO_x reduction in these conditions. Selective catalytic reduction of NO_x with hydrocarbon (HC-SCR) is considered to be the potential technology substituting for NH₃-SCR, particularly selective catalytic reduction of NO_x by methane (CH₄-SCR). Preparing catalysts with high activity, selectivity and SO₂ and H₂O tolerance should be solved before practical application of the CH4-SCR technology. The investigating tasks are put forward under this research background in this paper.A type of new zeolite composite FBZ with FAU and BEA topology structure was synthesized. Selective catalytic reduction of NO_x in the presence of excess oxygen was investigated systemically over the Co- or Mn- ion-exchange H-FBZ catalysts. XRD, FT-IR, DRS-UV-Vis, FE-SEM, NH₃-TPD, H₂-TPR techniques were applied to characterize the catalysts. Combined NO, NO+O₂,NO₂ adsorption and temperature programmed desorption (TPD) were applied to investigate the adsorption species contained N and O over the catalysts. The H₂O and SO₂ tolerance of the catalysts were also studied. SO₂ temperature programmed surface reaction (TPSR) was applied to investigate the sulfur species formed during the poisonous tests. CH₄-SCR was also investigated over the traditional catalyst CoH-ZSM-5. Combined NO, NO+O₂, NO₂ and NO+NO₂ adsorption and TPD were carried out over CoH-ZSM-5 to investigate the interaction of the N and O contained species with the catalyst surface. This paper would focus on acquiring novel catalyst, and valuable information for the reaction mechanism by which the CH₄-SCR occurs over Co-zeolite catalyst. It is found that the topology structure of the carrier strongly affects the catalytic properties of the catalysts. The SO₂ and H₂O tolerance of the catalysts is also affected by the carrier. The main research results obtained in this paper will be shown as follows:I) Research progresses of CH₄-SCR in the Co and Mn contained zeolite composite CoH-FBZ and MnH-FBZ1 Zeolite composite with FAU and BEA topology structure can be synthesized succefully with two step hydrothermal crystalyzing method. Only characteristic peaks of FAU and BEA topology structure are observed over the XRD patterns of FBZ. The relative content of the two kinds of topology structure can be obtained by the XRD results. Characteristic peaks of FAU and BEA topology structure are observed in the FTIR spectra of the catalysts. No octahedral morphology is observed in the SEM images of the zeolite composite, whereas it cannot be excluded completely. Homogenous ellipse morphology is observed in the SEM image of zeolite composite. The morphology is similar to that of the Beta zeolite because the zeolite composite FBZ is synthesized by overgrowing or epitaxially growing a layer of zeolite Beta on the pseudo-crystal of FAU zeolite.2 NH₃-TPD results show that a new kind of strong acidic sites is formed on H-FBZ. These acidic sites can be ion-exchanged by the metal ions. The average acidity of this type of acidic sites increases after ion-exchange of Co or Mn cations. Temperature programmed oxidizing (TPO) and temperature programmed reducing (TPR) results show that the ion-exchange Co and Mn cations in the zeolite are resist to the reducing and oxidizing process.3 Different DRS-UV-Vis spectra are obtained over CoH-FBZ, CoH-Y, CoH-Beta and the physical mixture of both. Combining the NH₃-TPD results and the properties of the zeolite composite, new ion-exchange Co sites could be formed in CoH-FBZ. The combined NO, NO+O₂ and NO₂ adsorption and TPD as shown in the follows support this viewpoint.4 It is found that the composite catalysts CoH-FBZ and the physical mixture catalysts of CoH-Y and CoH-Beta exhibit significant different properties. The topology structure of the carrier, acidity of the zeolite carrier, the metal cations and their ligand and the reaction conditions significantly affects the catalytic properties of the catalysts, which are discussed in detail as follows:a. Activity test results show that the catalytic activity is significantly affected by the topology structure of the catalyst. Compared to the physical mixture catalysts of CoH-Y and CoH-Beta, with comparable FAU and BEA topology structure, CoH-FBZ catalysts exhibit much higher catalytic activity, particularly the catalysts with the mass content of BEA topology structure between 60 % and 80%. CoH-FBZ catalysts also exhibit higher CH₄ selectivity than the physical mixture catalysts of CoH-Y and CoH-Beta. The catalytic activity increases with the cobalt content in the catalysts. Higher gas hourly space velocity (GHSV) results in lower NO to N₂ conversion. The acidity of the carrier can promote the CH₄-SCR. O₂ is another essential factor in the CH₄-SCR. In the reaction 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. Increasing the oxygen concentration, the NO to N₂ conversion increases significantly and reaches a maximum while the oxygen concentration is up to 2.00%. Further increasing the oxygen concentration, the catalytic activity decreases. O₂ can react with NO to form adsorbed -NO_y, species over the catalyst. Too higher oxygen concentration leads to the rate of direct CH₄ combustion increase quickly, and the CH₄-SCR is inhibited.In the presence of H₂O or SO₂, the catalytic activity of the catalyst decreases considerably. Co-existence of H₂O and SO₂, NO to N₂ conversion decreases further, whereas, the composite catalyst CoH-FBZ exhibits better H₂O and SO₂ than the catalysts CoH-Beta with single topology structure.b. Similar catalytic properties are obtained over MnH-FBZ and CoH-FBZ catalysts. MnH-FBZ catalysts exhibit much higher catalytic activity than physical mixture catalysts with comparable FAU and BEA topology, particularly over the catalysts with BEA topology structure mass content between 60-80%. The catalytic activity increases with the Mn content in the catalysts. As that in CoH-FBZ catalysts, the acidity of the catalyst promotes the CH₄-SCR activity. The composite catalyst MnH-FBZ exhibits better H₂O and SO₂ than the catalysts MnH-Beta with single topology structure.c. The CH₄-SCR activity over the Co ion-exchange catalysts with single topology structure is higher than that over the Mn ion-exchange catalysts with comparable metal content. However, the activity of the two types of composite catalysts is almost comparable on both kind catalysts. In some reaction conditions, the catalytic activity of MnH-FBZ is even higher than that of CoH-FBZ. These results illustrate that the zeolite composite exhibit different properties to the physical mixtures, and the CH₄-SCR activity is influenced by the topology structure of the zeolite singnificantly. Mn catalysts with either single topoloigy structure or composite topology structure better H₂O tolerance than Co catalysts, whereas addition of SO₂, the difference decreases.5 Combined NO, NO₂, NO+O₂ adsorption and TPD results show that the N and O contained species formed over the catalyst are significantly affected by the topology structure of the carrier. NO-TPD results show that the NO species formed over the catalysts is unstable and desorbed at temperature lower than 573 K. The NO species is adsorbed more strongly on CoH-FBZ catalysts. Compared with the CoH-Beta and CoH-Y with single topology structure, adsorbed -NO_y species formed by NO+O₂ co-adsorption (or NO₂ adsorption) is adsorbed more strongly on CoH-FBZ than on CoH-Y and CoH-Beta. Furthermore, at least two NO₂ desorption centers are observed over TPD profiles of CoH-FBZ. N and O contained species are only adsorbed on Co sites. This supports that new Co sites are formed over CoH-FBZ.6 SO₂-TPSR results show that stable sulfur compounds are formed over the SO₂ poisoning catalysts. The sulfur compounds occupy part of the active sites, lead to the amount of the active species -NO_y decrease, and the catalytic activity of the catalysts is inhibited. In contrast to that over the catalysts CoH-Beta with single topology structure, the sulfur compounds is less stable over the catalysts CoH-FBZ with composite topology structure. Increasing the reaction temperature, part of the sulfur compounds is desorbed and the active sites are released. As a result, the composite catalyst CoH-FBZ exhibit better SO₂ tolerance than CoH-Beta due to the less stable adsorption of sulfur species over CoH-FBZ than over CoH-Beta, as revealed by SO₂-TPSR results.II) Research progresses of CH₄-SCR in CoH-ZSM-5Combined NO₂, NO (O₂) adsorption and temperature programmed desorption (TPD) have been studied systematically to probe into the selective catalytic reduction of NO by methane (CH₄-SCR) over CoH-ZSM-5 (SiO₂/Al₂O₃=25). Selective catalytic reduction of NO or NO₂ by CH₄ over CoH-ZSM-5 are also investigated.1 Catalytic activity results show that in the absence of oxygen, low NO to N₂ conversion is obtained. Addition of oxygen, the catalytic activity increases significantly. In the reaction system of NO/CH₄/O₂ and NO₂/CH₄/O₂, same NO or NO₂ to N₂ conversion is obtained, the CH₄ conversion is also comparable. In the absence of oxygen, same NO₂ to N₂ conversion is obtained in the NO₂/CH₄ reaction as that in the presence of oxygen at temperature lower than 673 K. Further increasing the reaction temperature, catalytic activity decreases.2 Adsorption conditions significantly affect the adsorption of NO, NO₂ and NO+O₂. Adsorbed NO species are unstable and desorbed below the reactive temperature 523 K. Increasing adsorption temperature results in the decrease of the adsorbed NO species amount.3 The amount of-NO_y species formed from NO₂ adsorption increases with the increase of NO₂ concentration in the adsorption process, while decreases significantly with the increase of adsorption temperature. Though NO species are adsorbed weakly on CoH-ZSM-5, competitive adsorption between NO and -NO_y species decreases the amount of adsorbed -NO_y species. Similar desorption profiles of NO₂ was obtained over CoH-ZSM-5 while it was contacted with NO₂ or NO+O₂ followed by TPD. If NO₂ was essential to form adsorbed -NO_y species, the amount of adsorbed -NO_y species for NO+O₂ adsorption should be the least among the adsorption of NO₂, NO+O₂ and NO+NO₂ because of the lowest NO₂ concentration and highest NO concentration. In fact, the amount of adsorbed -NO_y species is between the other two adsorption processes. These indicate that formation of adsorbed -NO_y species may not originate from NO₂.4 O₂ promotes the CH₄-SCR activity by reacting with adsorbed NO or NO₂ to form sufficient amount of active -NO_y species.