Study On Microwave Catalytic Conversion Of Nitric Oxide And Microwave Effects

Nitrogen oxides（NOx） are considered to be one of the most harmful air pollutants for their devastating effect on the ecosystem and human health. The increasingly serious air pollution and hazy weather has been attracted significant attention, and it has become the most pressing environmental problems that how to remove NO depth efficiently to eliminate air pollution and haze. The selective catalytic reduction with NH3（NH3-SCR） is currently a popular method widely used in industry, in spite of several drawbacks, such as storage and leakage inevitably and un-reacted reducing agents. Hence, the reduction of NO by carbon provides an interesting alternative since the problems of NH3 can be avoided. However, NOx reduction by AC occurs at very high temperature, and carbon would be mainly consumed by O2. It remains a challenge that the reduction of NO by AC by catalysis is of high activity and selectivity to N2 at a low temperature under excess oxygen. The direct catalytic decomposition of NO is the most desirable method with respect to both environmental and economic considerations, since it does not involve any addition of toxic reducing agents, and the technology is simple, and it does not produce secondary pollution. Currently, although the direct catalytic decomposition of NO has been widely reported and some progress has been made, there are still significantly low in denitrification rate and strong oxygen inhibition. Therefore, one of the great challenges in catalysis is to develop novel and more efficient strategies for the direct decomposition of NO.Recently, microwave-enhanced catalytic reaction has been aroused tremendous attention with drastic acceleration on catalytic reaction rate and selectivity under mild condition. This significant microwave features may provide a new route for addressing the problems in the NO reduction by AC and the direct catalytic decomposition of NO. However, rare investigations have been paid on the microwave direct catalytic decomposition of NO. For example, direct decomposition of NO over Fe/ZSM-5 catalyst in the microwave heating mode could reach 70% NO conversion, while the Fe/ZSM-5 catalyst is almost inactive to decompose NO in the conventional heating mode. Moreover, up to now, none of microwave catalyst with high activity and selectivity has been investigated for the microwave direct catalytic decomposition of NO. A huge amount of experiments and researches on the microwave- heating or microwave-assisted chemical reactions or catalytic reactions has observed and confirmed that microwave can speed up the reaction rate. Although it was proposed the “non-thermal microwave effects” or “special microwave effect” and microwave “hot-spots” hypothesis to try to explain the phenomena and results of accelerating reaction rates by microwave, how to accelerate the chemical reaction by microwave and its mechanism of action is still unclear understanding. Recently, although the use of microwave irradiation to enhance chemical reactions is growing at a rapid rate, the intrinsic nature of the effect of microwave irradiation on chemical reactions remains unclear. On account of these above problems and the challenge of high-efficient catalytic conversion of NO to remove NOx, in this paper, It was conduct research on the microwave catalytic conversion of NO and microwave effects, and the specific content includes the following sections:（1） It was investigated microwave catalytic reduction of NO into N2 by AC supported Mn₂O₃ at low temperature（below 300?C） under excess oxygen. It was found microwave-assisted catalytic reduction of NO into N2 by AC supported Mn₂O₃ is of high efficiency with 98.7% NO conversion and 99.8% NO selectivity to N2 at temperature as low as 300?C under O2 excess. Microwave irradiation exhibits microwave selective effect. The activity of catalysts in the microwave catalytic reaction mode（MCRM） is much higher than that in the conventional reaction mode（CRM）. The inlet gas doesn’t require preheating and the outlet gas temperature is maintained at room temperature hence MCRM reduces the heat loss during reaction and lowers the energy consumption greatly. Mn₂O₃/AC is more active than AC for NO reduction in the MCRM.（2） It was investigated the direct decomposition of NO into N2 and O2 by microwave catalysis over MeOx-Cu-ZSM-5（Me=Mn,N i） at excessive oxygen. It was found microwave direct catalytic decomposition of NO at excessive oxygen is high efficiency with the NO conversions are 94.3% of MnO 2-Cu-ZSM-5 at 300 °C and 92.3% of N i2O3-Cu-ZSM-5 at 350 °C. Meanwhile, the N2 selectivity remains more than 98%. Moreover, the apparent activation energies of C u-ZSM-5 is 75.6 kJ/mol in the CRM, while the apparent activation energies of MnO2-Cu-ZSM-5 and Ni2O3-Cu-ZSM-5 are respectively as low as 15.5 kJ/mol and 25.7 kJ/mol in the MCRM, which indicate that MnO2-Cu-ZSM-5 and N i2O3-Cu-ZSM-5 catalysts exhibit microwave catalytic effect in the MCRM. MnO2-Cu-ZSM-5 and Ni2O3-Cu-ZSM-5 are good microwave catalysts while the single C u-ZSM-5 is not. Microwave irradiation exhibits microwave selective effect in the MC RM. The oxygen concentration has surprisingly no effect on the activity of direct catalytic decomposition of NO over MeOx-Cu-ZSM-5 catalysts in the MCRM.（3） It was investigated the direct decomposition of NO into N2 and O2 by microwave catalysis over MeOx/Al2O3（Me=C u,Mn,Ce） at low temperature and excess oxygen. It was found the direct decomposition of NO by microwave catalysis over CeCuMnOx/Al2O3 is very impressive with NO conversion and N2 selectivity up to 94.8% and 98.7%, respectively, even at a low temperature of 250 °C and coexistence oxygen of 10%. Microwave irradiation enhances the activity of MeOx/Al2O3 catalysts for NO decomposition remarkably. Although MeOx/Al2O3（Me=Cu,Mn,Ce） catalysts display almost no activity for the decomposition of NO in the CRM below 300?C, they show excel ent catalytic performance at low temperature in the MCRM.（4） It was investigated the direct decomposition of NO into N2 and O2 by microwave catalysis over BaMn1-x Mgx O3 mixed oxides at low temperature and excess oxygen. It was found the microwave selective effect: a new approach to oxygen inhibition removal for a difficult（strong oxygen inhibition） and important catalytic decomposition reaction for the direct decomposition of NO under excess oxygen. Importantly, the oxygen concentration has surprisingly no effect on the catalytic performance of decomposition of NO over BaMn_xMg_1-xO₃ catalysts in the MCRM. A complete NO decomposition with NO conversion and N2 selectivity respectively up to 99.8% and 99.9% was achieved over the BaMn0.9Mg0.1O3 catalyst at the environment of excess oxygen and temperature as low as 250 °C in the MCRM.（5） It was investigated the influence of B-sites over BaBO3（B=Mn,Co,Fe） catalysts. It was found the direct NO decomposition through microwave catalysis is impressively high efficient in NO conversion and N2 selectivity with the value separately up to 99.8% and 99.9% for the BaCoO3 even at coexistence of 10% oxygen and low temperature of 250 °C. Moreover, BaCoO3 shows superior endurance to water vapor. Comparatively, the best NO conversion is 93.7% for BaMnO3 and only 64.1% for BaFeO3. Importantly, apparent activation energies of BaMnO3, BaCoO3, and BaFeO3 separately decrease to as low as 33.4, 13.7, and 46.7 kJ/mol, suggesting a significant microwave catalytic effect. The whole catalytic activity in the MCRM is much higher than that in the CRM. Furthermore, microwave irradiation shows microwave selective effect for these Ba BO3（B=Mn, Co, Fe） catalysts, and the catalytic activity of NO decomposition under microwave irradiation almost has no influence on oxygen concentration.（6） It was investigated the effect of A-site substitution over Ba0.8A0.2MnO3（A=Ca, K, La） catalysts. It was found the best NO conversion is highly up to 93.7% for BaMnO3, 92.3% for Ba0.8Ca0.2MnO3, and 99.8% for Ba0.8K0.2MnO3, and 95.5% for Ba0.8La0.2MnO3 in the MCRM. Comparatively, under identical conditions in the CRM, these catalysts show very low activity. Moreover, the effect of Ca, K and La substitution is different in the MCRM and CRM. That is, K and La substitution can improve the catalytic activity of BaMnO3 in the MC RM and C RM, whereas the Ca substitution in the MC RM results in lowering of the catalytic performance even though the Ca substitution in the C RM can increase the catalytic activity. The synergistic effect between excellent the microwave absorbing property, the best reducibility, and the better oxygen desorption capability of Ba0.8K0.2MnO3 accounts for its best catalytic performance among these four BaMnO3 and Ba0.8A0.2MnO3（A=Ca, K, La） catalysts for NO decomposition in the MCRM. Importantly, the Ea’ values for BaMnO3 and Ba0.8A0.2MnO3（A=Ca, K, La） catalysts in the C RM and MCRM is remarkable agreement with the ranking of the catalytic performance of these catalysts.（7） It has found and elucidated that microwave catalytic effect is a new exact reason for microwave-accelerated chemical reaction. Based on the direct catalytic decomposition of NO under microwave irradiation can result in NO conversion exceeding these realized through conventional heating at any specific temperature, it has been demonstrated the significant evidence that the lowering of apparent activation energy rather than solely “hot-spots” hypothesis is a new exact reason for the microwave-accelerated heterogeneous gas-phase catalytic reactions. O ur findings challenge the classic view of “hot-spots” hypothesis as a dominant role in mechanisms responsible for microwave-accelerated heterogeneous gas-phase catalytic reaction.（8） It has found MW irradiation is a new type of power energy for speeding up chemical reactions, and revealed that MW irradiation can increase the rate of chemical reactions by direct reduction of the Ea’ that is required to activate reactant molecules. Based on the reaction temperature was decreased by several hundred degrees centigrade and the apparent activation energy（Ea’） decreased substantially under MW irradiation, it was found MW irradiation is partially transformed to reduce the Ea’, and then it has found that MW irradiation is a new type of power energy for speeding up chemical reactions. It was proposed a model of the interactions between microwave electromagnetic waves and molecules to elucidate the intrinsic reason for the reduction in the Ea’ under MW irradiation, and determined a formula for the quantitative estimation of the decrease in the Ea’. The nature and mechanism of the MW irradiation on the chemical reaction was elucidated. An explanation was provided for the experimental phenomena and chemical reaction results from MW irradiation of chemical reactions.