富氧条件下金属催化CO还原NO的研究进展

CO广泛存在于燃煤烟气及汽车尾气中,利用未完全燃烧的CO催化还原NO可同时脱除NO和CO,过程中催化剂起着决定性作用。本文对近年来含氧条件下CO催化还原NO的研究成果进行了系统梳理,重点关注了Pd系、Ir系、Cu系、其他贵金属及金属氧化物催化剂的研究进展,分析了催化剂制备方法、掺杂改性及反应条件对催化性能的影响,同时考察了O₂浓度、H₂O以及SO₂对催化反应的影响,总结并对比了不同体系催化剂的活性位点及其催化机理,指明了O₂在催化还原过程中的抑制机理,得出了几种体系催化剂催化CO还原NO的活性顺序。最后,针对富氧条件下CO催化还原NO所存在的问题和难点,提出深入研究O₂抑制机理、降低贵金属用量、添加活性助剂是今后的研究方向。     

Nitric oxide reduction and carbon monoxide oxidation over carbon-supported copper-chromium catalysts

Carbon supported copper-chromium catalysts are shown to be very active for both the reduction of nitric oxide with carbon monoxide and the oxidation of carbon monoxide with oxygen. Mixed copper-chromium oxide active phases have good activity in the simultaneous removal of nitric oxide and carbon monoxide from exhaust gases. The influence of several catalyst variables has been investigated. The activity per volume of catalyst increases with increasing loading, while the intrinsic activity shows a maximum around C/M=100−50. An optimum catalyst for nitric oxide reduction and carbon monoxide oxidation has a copper/chromium ratio of 2/1. The apparent activation energy for the carbon monoxide oxidation over carbon supported copper-chromium catalysts is 77 kJ/mol, suggesting that the Cu---O bond rupture is the rate-limiting process. The reduction of nitric oxide takes place at higher temperatures. Since all catalysts have a low selectivity for molecular nitrogen formation at lower temperatures, the dissociation of nitric oxide is probably rate determining, resulting in a slightly reduced catalyst system. In an excess of carbon monoxide the reaction is first-order in nitric oxide and zero-order in carbon monoxide. Moisture inhibits the reaction by reversible competitive adsorption, whereas carbon dioxide does not. Oxygen completely inhibits the reduction of nitric oxide due to the more rapid reoxidation of the catalytic sites compared to nitric oxide. Therefore, the reduction of nitric oxide takes place only when all oxygen has been converted and, hence, is shifted to higher temperatures. As a possible consequence, the production of nitrous oxide is reduced. Nitric oxide and molecular oxygen react preferentially with carbon monoxide, so, in an excess of oxidizing component, gasification of the carbon support occurs at higher temperatures after carbon monoxide has been completely consumed.     

Catalytic Properties of Ceria and CeO2-Containing Materials

Over the past several years, cerium oxide and CeO₂-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO₂-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO_x reduction catalysts is described. A survey of the use of CeO₂-containing materials as oxidation and reduction catalysts is also presented.     

CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM-BASED CATALYSTS

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.     

CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM-BASED CATALYSTS

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.     

Catalytic Properties of Ceria and CeO2-Containing Materials

Over the past several years, cerium oxide and CeO₂-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO₂-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO _x reduction catalysts is described. A survey of the use of CeO₂-containing materials as oxidation and reduction catalysts is also presented.     

Influence of NH3 and NO oxidation on the SCR reaction mechanism on copper/nickel and vanadium oxide catalysts supported on alumina and titania

The influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina have been investigated, paying special attention to N₂O formation. In the SCR reaction, the VTi catalyst had a higher activity than VAl at low temperatures, while the CuNiAl catalyst had a higher activity than CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H₂-TPR showed that the formation rate of NH species in copper/nickel catalysts would be higher than in vanadia catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that copper/nickel catalysts presented ammonia coordinated on Lewis acid sites, whereas ammonium ion adsorbed on Brønsted acid sites dominated on vanadia catalysts. The NO oxidation experiments revealed that copper/nickel catalysts had an increase of the NO₂ and N₂O concentrations with the temperature. NO could be adsorbed on copper/nickel catalysts and the NO₂ intermediate species could play an important role in the reaction mechanism. It was suggested that the presence of adsorbed NO₂ species could be related to the N₂O formation.     

Diesel Oxidation Catalysts

The diesel oxidation catalyst (DOC) has been part of diesel exhaust systems since regulations were introduced to limit the amount of harmful emissions released to the environment from diesel engines. The DOC primary functions are oxidation of CO, unburned hydrocarbons, and NO, while active hydrocarbon oxidation can also be used to generate exotherms required for downstream components. This review will describe work that has attempted to understand the reactions, both desired and undesired, that occur over the catalyst. First, the history, configuration, and components of the DOC will be discussed, followed by in-depth coverage of the fundamental reactions that occur over a DOC, including reaction mechanisms, reaction inhibition, and other reactivity effects. Finally, DOC deactivation mechanisms and their effects on the DOC are described. While there is a lot of research literature regarding Pt- and Pd-based catalysts for many different reaction schemes, this review tries to highlight work most relevant to DOC applications.     

CuSO_4-CeO_2/TS催化氧化NO及其抗H_2O和SO_2毒化性能

采用共沉淀法制备了载体TiO2-SiO2(TS),用浸渍法制备了催化剂CuSO4-CeO2/TS,考察了组分配比、焙烧温度等制备条件和反应温度、NO进口浓度、O2含量、空速等操作条件对其催化氧化NO活性的影响及其抗H2O和SO2毒化的能力,对载体和催化剂分别进行了分析.结果表明,在最佳条件下制备的催化剂,在反应温度350℃、进口NO浓度500×10-6(φ)及O2浓度8%(φ)、空速10000h-1条件下,NO转化率可达67.5%,能满足NOx高效吸收的要求.但因NO催化氧化过程的反应温度和O2含量均较高,其抗SO2和H2O毒化能力低于NO催化还原过程.     

金属氧化物催化剂上H_2S选择性催化氧化研究进展

选择性催化氧化法作为一种新型的脱除H_2S尾气技术,重点是开发具有高活性和多种性能特点的催化剂形成系列产品。总结了金属氧化物催化剂上H_2S选择性催化氧化反应的研究进展。重点介绍了不同载体、活性组分和助剂对催化剂活性的影响;阐述了H_2S选择性催化氧化的反应机理和失活机理,并对金属氧化物催化剂的未来发展方向上进行了展望。     

Selective Catalytic Oxidation (SCO) of Ammonia to Nitrogen over Hydrotalcite Originated Mg–Cu–Fe Mixed Metal Oxides

Abstract  

Mg–Cu–Fe oxide systems, obtained from hydrotalcite-like precursors, were tested as catalysts for the selective catalytic oxidation (SCO) of ammonia. Copper containing catalysts were active in low-temperature SCO processes; however, their selectivity to nitrogen significantly decreased at higher temperatures. The optimum composition of the catalyst to guarantee high activity and selectivity to N₂ was proposed. Temperature-programmed experiments, SCO catalytic tests performed with various contact times and additional tests on the samples in the selective catalytic reduction of NO with ammonia showed that the SCO process over the studied calcined hydrotalcites proceeds according to the internal SCR mechanism and oxidation of ammonia to NO is a rate-determining step in the low-temperature range.     

金属氧化物催化CO还原NO的研究进展

一氧化碳（CO）广泛存在于烧结/球团/焦化烟气或汽车尾气中,应用CO-选择性催化还原（SCR）技术同时脱除烟气中CO和NO是烟气治理的理想方案之一。目前,在NO-CO反应研究中较多的是贵金属催化剂,但由于其价格昂贵、高温失活、易中毒等问题难以在工业中实现应用。本文将近几年来金属氧化物催化CO还原NO的研究成果进行了系统的梳理与总结,重点介绍Fe基、Ce基、Co基、Cu基这4种金属氧化物催化剂的研究进展,分析催化剂的制备方法、掺杂助剂种类和比例、NO-CO反应条件等因素与催化活性之间的关系,总结催化剂抗水抗硫性能及可能的CO-SCR反应机理,并探讨O₂存在的条件下对催化剂活性的影响,为提高金属氧化物催化剂抗氧性研究提供理论参考。     

THREE-WAY CATALYTIC CONVERTER MODELING AND APPLICATIONS

The advent of stricter U.S. and European exhaust emissions regulations has increased the need for reliable 3-way catalytic converter models supporting the design of demanding exhaust systems for low-emitting vehicles. Although a number of tunable models have been presented in the literature, their efficient performance in actual 3-way applicaions requires further development and validation. The major difficulties posed in such modeling efforts arise from the complexities in the reaction schemes and the respective rate expressions for the multitude of currently used catalyst formulations. This paper addresses the details of tuning and real world application of a two-dimensional catalytic converter model, which accounts for the HC (hydrocarbons) and CO oxidation, as well as NO reduction reactions. The model features a number of innovations regarding the catalyst transient behaviour modelling and the reaction kinetics scheme. The advanced oxygen storage submodel presented is capable of accounting for the redox and temperature dependence of the oxygen availability under transient operation. The redox sensitivity of the reaction scheme allows to get clearer insight in the “lambda-window” behavior of 3-way catalysts. It is concluded, that mathematical modelling may successfully describe important aspects of real world three-way catalytic converter operation under dynamic conditions, and thus, is a valid tool in exhaust aftertreatment systems optimization.     

Co3O4 based catalysts for NO oxidation and NOx reduction in fast SCR process

Reaction activities of several developed catalysts for NO oxidation and NO_x (NO + NO₂) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co₃O₄ based catalysts are the most active ones for both NO oxidation and NO_x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co₃O₄ catalyst, the effects of calcination temperatures, SO₂ concentration, optimum SV for 50% conversion of NO to NO₂ were determined. Also, Co₃O₄ based catalysts (Co₃O₄-WO₃) exhibit significantly higher conversion than all the developed DeNO_x catalysts (supported/unsupported) having maximum conversion of NO_x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N₂O formation over Co₃O₄-WO₃ catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N₂O over all the catalysts. The effect of SO₂ concentration on NO_x reduction is found to be almost negligible may be due to the presence of WO₃ that resists SO₂ oxidation.     

CO-NO/O2 redox reactions over Cu substituted cobalt oxide spinels

Catalytic conversion of NO and CO over Cu substituted cobalt oxide spinels show excellent activity for CO-O₂ and NO-CO reactions. Lower concentration of Cu in cobalt oxide spinel is having an enhancing effect on the catalytic conversion. Best activity among the tested catalyst was found for Co_2.9Cu_0.1O₄ and complete conversion (100%) is observed at 93 °C for CO oxidation by O₂ and 209 °C for NO reduction by CO. Prepared catalysts show promising activity compared to few of the precious metal based catalysts reported in the literature. The influence of moisture and oxygen on catalytic conversion has been studied.     

Pd/Al2O3催化剂上CH4选择性还原NO的研究

考察了Pd/Al₂O₃、In/Al₂O₃和Co/Al₂O₃对甲烷选择性还原NO的催化活性。结果表明，采用浸渍法制备的Pd/Al₂O₃、In/Al₂O₃和Co/Al₂O₃三种催化剂，在有氧气氛下，用CH₄作还原剂催化还原NO时，Pd/Al₂O₃催化剂的活性最佳，热稳定性好，在550 ℃，用CH₄选择还原NO，Pd/Al₂O₃催化剂表现出较强的催化能力，NO的转化率达到100%。在高空速实验中，该催化剂亦表现出较高的活性，其活性顺序为Pd/Al₂O₃>In/Al₂O₃>Co/Al₂O₃。实验研究了助催化剂、氧含量以及空速对Pd/Al₂O₃催化剂活性的影响。     

Detailed surface reaction mechanism for reduction of NO by CO

In order to meet the stringent regulatory norms of NO_x and CO emitted by automobiles, reduction of these pollutants has become an intense field of research. Various catalysts like Pt, Rh, Ir, Cu, and Fe have been found to possess high activity for the reduction of NO. However, the available detailed surface reaction mechanisms are not satisfactory in clarifying all the aspects of the simultaneous reduction of NO and oxidation of CO. Here we have developed a quantitative surface reaction mechanism based on elementary steps, in order to comprehend the phenomena of catalytic reduction of NO by CO. Eleven elementary steps are proposed for the NO–CO and NO–CO–O₂ systems on Pt group catalysts. The elementary reaction mechanism is coupled with the continuously stirred tank reactor/packed bed reactor models and the simulation results are validated against literature experiments for the NO–CO reaction on Pt, and the NO–CO–O₂ reaction on Ir catalyst. Despite the simplicity, the CSTR model is able to capture the observed phenomena well on Pt and Ir catalysts. The effect of O₂ on the activity of CO for NO reduction is also analysed in detail through the simulations.     

Low-temperature selective catalytic reduction of NO over MnOx/CNTs catalysts: Effect of thermal treatment condition

Carbon nanotubes (CNTs) supported manganese oxide catalysts were prepared through different thermal treatment routes and used for low-temperature selective catalytic reduction of NO with NH₃. The MnO_x/CNTs catalyst prepared by calcined the precursor in air at 300 °C showed lower NO conversions than that treated at 250 °C, while it showed higher NO conversions than the one calcined in nitrogen. BET, TGA, XRD and H₂-TPR results indicated that CNTs may impose effects on the oxidation state and redox ability of the manganese oxide and hence on the catalytic activity during the calcination process at given temperatures.     

Investigation of Co-Based DeNOx Catalysts for the Treatment of Natural Gas Lean-Burn Engine Exhaust

The catalytic reduction of nitric oxide with methane in lean conditions has been investigated on cobalt-based powder and monolith catalysts such as Co-exchanged ferrierite, Co-exchanged beta zeolite and Co–ZrO₂. From catalyst laboratory testing the following activity scale was obtained: Co-ferrierite > Co-beta >>> Co–ZrO₂, suggesting a significant role of the support catalytic matrix. In the tests at the engine exhaust, the best results were obtained with Co-ferrierite, after water separation from the exhaust. However, in the presence of the high concentration of water, typical of the engine exhaust, the performances were dramatically reduced.     

同时消除柴油机尾气排放炭颗粒和NOx催化剂的研究进展

介绍了简单氧化物、复合氧化物(尖晶石和钙钛矿)催化剂都具有同时消除柴油机尾气中的炭颗粒和NO的活性．但是炭颗粒的起燃温度较高,生成N2的选择性差。在消除炭颗粒和氮氧化物时,有N2O生成,造成二次污染,炭颗粒与催化剂的接触形式直接影响炭颗粒的燃烧温度。