【作者】 王永钊；

【导师】 刘滇生； 赵永祥；

【作者基本信息】 山西大学 ， 无机化学， 2007， 博士

【摘要】 CO低（常）温催化氧化由于在许多方面具有重要的实用价值而颇受关注，如用于地下矿井的过滤式自救器和消防自救呼吸器、CO气体传感器、封闭式CO₂激光器以及密闭系统内CO的消除等方面。同时，由于反应体系简单，在理论研究方面CO氧化还常常用作研究氧化催化剂的探针反应，以揭示催化剂的结构与性能之间的关系，探讨反应机理。Co₃O₄作为一种非贵金属催化剂用于CO的低温催化氧化近年来已成为研究的热点之一。目前来看制备高活性Co₃O₄催化剂的制备方法存在制备过程复杂、后处理繁琐等缺点；根据研究发现预处理条件对Co₃O₄催化剂的CO低温氧化活性有较大的影响；对Co₃O₄催化剂上CO低温氧化反应的机理和失活原因也有研究，取得了一些共识，但仍有许多问题需要解决，同时对Co₃O₄催化剂的表征及其CO低温催化性能缺乏系统研究。本文利用简单的液相沉淀方法制备了高活性的Co₃O₄催化剂，首先考察了各种制备条件对其CO低温氧化性能的影响，然后着重考察了后处理温度以及工艺条件等对其CO低温催化氧化性能的影响，并通过TG-DTG、XRD、N₂-物理吸附、TEM、IR、XPS等表征手段对催化剂的结构进行了系统研究，同时还利用CO滴定、O₂-TPD以及In-situ DRIFT等实验手段，对Co₃O₄催化剂上CO和O₂的吸附性能以及催化CO低温氧化反应的机理和可能的失活原因进行了探索，得到的主要结果和结论如下：1．制备方法和制备参数对Co₃O₄催化剂CO低温催化氧化性能的影响研究（1）通过比较直接焙烧法、均匀沉淀法和液相沉淀法制备的Co₃O₄催化剂的CO催化氧化性能得出，均匀沉淀法和液相沉淀法制备的催化剂在25℃均能将CO完全转化，并且具有相似的稳定性，直接焙烧法制备的催化剂虽然也能将CO完全转化，但稳定性相对较差。（2）采用液相沉淀法，以NH₄HCO₃为沉淀剂，通过比较以三种不同钴盐为前驱物制备的Co₃O₄的CO催化氧化性能得出，以Co（NO₃）₂·6H₂O、COSO₄·7H₂O为前驱物制备的催化剂在25℃均能将CO完全转化，并且具有相似的稳定性，以COCl₂·6H₂O为前驱物制备的催化剂初始CO转化率只有80％，并且稳定性较差。（3）采用液相沉淀法，以Co（NO₃）₂·6H₂O为前驱物，通过比较三种不同沉淀剂制备的Co₃O₄的CO催化氧化性能得出，以NH₄HCO₃和NH₃·H₂O为沉淀剂制备的催化剂在25℃均能将CO完全转化，前者稳定性优于后者，以KHCO₃为沉淀剂制备的催化剂初始CO转化率只有80％，并且稳定性较差。（4）通过液相沉淀法以Co（NO₃）₂·6H₂O为前驱物，NH₄HCO₃为沉淀剂，通过比较分别采用普通干燥和超临界干燥制备的Co₃O₄催化剂的CO催化氧化性能得出，两种干燥方式对Co₃O₄催化剂的催化性能影响不大。（5）以液相沉淀法制备的Co₃O₄负载Pd制备Pd／Co₃O₄催化剂，在25℃具有与Co₃O₄相同的催化活性，但催化稳定性低于Co₃O₄。（6）以液相沉淀法制备的Co₃O₄与膨润土机械混合制备不同Co₃O₄含量的Co₃O₄／膨润土催化剂，在25℃具有与Co₃O₄相同的催化活性，稳定性随Co₃O₄含量的降低而下降。2．焙烧温度对Co₃O₄催化剂织构、结构、表面性质和CO低温催化氧化性能的影响研究（1）在所考察的温度范围内（250～600℃），焙烧温度对Co₃O₄催化剂的体相结构没有影响，均以尖晶石结构存在，但对Co₃O₄催化剂的粒径形貌具有较大影响，经300℃焙烧的催化剂具有小的粒径和相对好的分散状态，随焙烧温度的升高，催化剂颗粒不断聚集，粒径明显长大。（2）焙烧温度对Co₃O₄催化剂的比表面积和比孔体积有较大影响，300℃焙烧的催化剂比表面积和比孔体积分别为54.3m²／g和0.20cm³／g，当焙烧温度达到600℃时，分别仅有9m²／g和0.02cm³／g，催化剂基本上变为无孔材料。（3）在所考察的温度范围内，焙烧温度对Co₃O₄催化剂表面Co的价态没有明显影响。（4）Co₃O₄催化剂表面存在三种形态的氧，随着焙烧温度的升高，吸附氧略有减少，羟基氧明显减少，晶格氧占总氧的百分数显著增加。（5）CO滴定实验表明，经氧化预处理的Co₃O₄催化剂表面具有活性氧物种，焙烧温度对催化剂表面的活性氧物种的数量有较大影响，300℃焙烧的催化剂具有最多的活性氧物种，约为143.74μmol／g。（6）CO催化氧化实验结果表明，在考察的焙烧温度范围内所有的催化剂都具有较高的CO氧化初始活性，经300℃焙烧的催化剂具有最好的稳定性。高于或低于300℃的焙烧均引起催化剂稳定性的下降。（7）通过对不同温度焙烧的Co₃O₄催化剂的结构、织构、表面性质以及CO催化氧化性能的研究，发现通过液相沉淀法经300℃焙烧制备的Co₃O₄催化剂以尖晶石结构存在，粒径细小，具有相对好的分散状态，从而具有最多的活性氧物种和最好的催化性能，显然小的粒径、好的分散状态和多的活性氧物种是Co₃O₄催化剂具有高的低温催化活性和稳定性的重要因素。3．Co₃O₄催化剂上CO低温催化氧化工艺条件的优化选择（1）在反应温度为25℃时，经过200℃氧化预处理的Co₃O₄催化剂具有高的CO催化氧化活性和稳定性，经惰性气氛预处理的催化剂也具有较高的催化活性，稳定性相对变差，不经过预处理的催化剂几乎没有催化活性。（2）在反应温度为25℃时，空速在2500～20000h^-1的范围内，随空速的提高，催化稳定性下降；原料气中CO含量在0.5～2％的范围内，随CO含量的提高，催化稳定性下降。（3）在反应温度为-78～80℃的范围内，Co₃O₄催化剂均具有较高的CO催化氧化活性，在-78～25℃的范围内随温度的升高，稳定性提高，在25～80℃的范围内随温度的升高，稳定性降低。（4）通过工艺条件的优化选择，液相沉淀法制备的Co₃O₄催化剂的最佳反应条件为：反应温度25℃，空速2500h^-1，原料气中CO含量0.5％，在此条件下，可以将CO完全催化氧化维持时间达630min，优于文献报道的结果。（5）原料气中水蒸气的引入对Co₃O₄催化剂的稳定性有极大的负作用，在Co₃O₄催化剂中添加微量的Pd明显可以提高催化剂的抗水性能。（6）失活的Co₃O₄催化剂在高于200℃的温度下，经流动的干燥空气处理30min，完全可以再生。（7）该催化剂不但具有较高的连续催化CO低温氧化的活性和稳定性，而且可以间断使用。4．Co₃O₄催化剂催化CO低温氧化的反应机理和失活原因的探索（1）失活前后催化剂的O₂-TPD分析表明：Co₃O₄催化剂表面有多种氧物种的吸附，其中α峰对应的低温脱附的氧物种O₂^-和β、γ峰对应脱附的O₂^2-和O^-具有CO低温氧化活性，δ峰对应的高温脱附的晶格氧对CO低温氧化没有作用。（2）失活前后催化剂的XPS结果表明，催化剂表面Co的价态没有明显变化，表明催化剂失活不是由于Co的价态变化引起的。在失活的催化剂表面上明显有吸附水存在，同时催化剂失活后与失活前相比吸附氧与晶格氧的比值下降。（3）In situ DRIFT分析结果表明：在未经预处理的Co₃O₄催化剂表面，存在一定量的吸附水，与CO竞争吸附同一活性位，导致CO的吸附减少，且与催化剂表面吸附水的-OH形成甲酸盐物种，导致吸附的CO不能与氧反应生成CO₂。在经过氧化预处理的Co₃O₄催化剂上，CO发生明显的线性吸附，并且能够与催化剂表面的氧物种反应生成CO₂，其中碳酸盐物种是可能的中间产物。（4）推测Co₃O₄催化剂上CO低温氧化的反应机理可能为：在催化剂表面存在CO吸附和氧的吸附，吸附的活性氧与吸附的CO反应生成CO₂，碳酸盐物种是可能的中间产物；在反应过程中很可能由于原料气中微量的水吸附在催化剂表面并逐渐积累，导致CO吸附减少，并且吸附的CO与-OH生成可能的甲酸盐物种，从而导致催化剂失活。更多还原

【Abstract】 The process of low-temperature CO catalytic oxidation has become animportant research topic due to its many potential areas of applications.Particularly, these applications include air-purification devices for respiratoryprotection, carbon monoxide gas sensors, closed-cycle carbon dioxide lasers,and removing trace quantities of CO from the ambient air in sealed cabins. Inaddition, in the aspect of academic study the CO oxidation has been regardedas a probe reaction to show the relationship between the structure and theperformance, and also to investigate the reaction mechanism.The research about the base metal oxide Co₃O₄ catalyst forlow-temperature CO catalytic oxidation has become a hotspot over the years.At present, the methods for preparing the Co₃O₄ catalyst with high activityexist many disadvantages such as the complicated process of operation andthe post treatment. According to the literatures, the pretreatment conditionshave an great effect on the catalytic activity of Co₃O₄ for low-temperatureCO oxidation. There are some researches about the reaction mechanism andthe original factors of deactivation of the Co₃O₄ catalyst, and a littleconsensus has been obtained, however, there are many problems awaitingfurther investigation, and there is a lack of perfect study about thecharacterization and catalytic performance of the Co₃O₄ catalyst. Without useof any surfactant or oxidant, Co₃O₄ catalyst with high activity has beenprepared by a simple liquid-precipitation method in the dissertation. Firstly,the effects of preparation conditions on the catalytic performance forlow-temperature CO oxidation have been studied; secondly, the effects of calcination temperatures and evaluation conditions on the catalyticperformance for low-temperature CO oxidation have been studied. Thestructures of the catalysts have been investigated by TG-DTG、XRD、TEM、N₂-adsorption、IR and XPS. The adsorption performance of CO and O₂ andthe reaction mechanism and the reason of deactivation of low-temperatureCO oxidation have been investigated via CO titration, O₂-TPD and in situDRIFT. The main results and conclusions are as follows:1. The study on the effect of preparation methods and preparationparameters on the catalytic performance for low-temperature CO oxidationover the Co₃O₄ catalysts.（1） The Co₃O₄ catalyst with high catalytic activity can be obtained by theliquid-precipitation and the well-distributed precipitation method, and canconvert CO completely at 25℃with the same stability. However, the catalystprepared by the direct calcination has a poor catalytic performance.（2） The Co₃O₄ catalysts obtained by the liquid-precipitation can convertCO completely at 25℃with the same stability using Co（N_O3）₂·6H₂O orCoSO₄·7H₂O as the cobalt salt and NH₄HCO₃ as the precipitator. However,the conversion of CO over the catalyst is only 80% obtained usingCoCl₂·6H₂O as the cobalt salt and the stability is poor.（3） The Co₃O₄ catalysts obtained by the liquid-precipitation can convertCO completely at 25℃with the same stability using NH₄HCO₃ or NH₃·6H₂Oas the precipitator and Co（NO₃）₂·6H₂O as the cobalt salt. However, theconversion of CO over the catalyst is only 80% obtained using KHCO₃ as theprecipitator and the stability is poor.（4） The drying methods including the common drying and the super criticaldrying have no obvious effects on the catalytic performance.（5） The Pd/Co₃O₄ catalyst does not show superiority compared to theCo₃O₄ catalyst for CO oxidation at 25℃.（6） The Co₃O₄/Bent catalysts have the same catalytic activity as the Co₃O₄catalyst at 25℃, and the stability decreases with the content of Co₃O₄ increasing.2. The study on the effect of calcination temperature on the structure,surface properties and the low-temperature CO oxidation performance overthe Co₃O₄ catalysts.（1） The calcination temperatures have no obvious effect on the structure ofthe Co₃O₄ catalyst, and all the catalysts exist as a pure Co₃O₄ phase with thespinel structure within the calcination temperature 250～600℃. However,the calcinations temperatures have a great effect on the crystal size and thegeometry of the catalysts. The catalyst calcined at 300℃has the highestdispersion level, and the particles are apt to aggregate together randomly withthe increase of calcination temperature.（2） The calcination temperatures also have an obvious effect on the BETspecific surface area and the specific pore volume. The BET specific surfacearea and the specific pore volume of the catalyst calcined at 300℃are 54.3m²/g and 0.20 cm³/g respectively, and they are 9 m²/g and 0.02 cm³/g whenthe calcination temperature is at 600℃, so the catalyst has transformed into amaterial of none pore.（3） Within the temperature 250～600℃, the calcination temperatures haveon obvious effect on the valence state of Co on the Co₃O₄ surface.（4） There are three kinds of oxygen species on the Co₃O₄ surface. Theamount of the adsorption oxygen species decrease a little while the hydroxideradical oxygen species decrease a lot, and the percent of the lattice oxygen atthe whole oxygen species increase.（5） The results of CO titration indicate the Co₃O₄ catalysts possess theactive oxygen species after pretreatment. The calcinations temperature havean obvious effect on the amount of active oxygen species, and the catalystcalcined at 300℃possesses the most amounts of active oxygen species,about 143.74μmol/g.（6） All the catalysts have good catalytic activity for CO oxidation in thetemperature range used. The catalyst calcined at 300℃shows the beststability. The calcination temperatures that higher or lower than 300℃lead to a decrease of the catalytic stability.（7） The results of investigation about the structure、surface properties andthe low-temperature CO oxidation performance indicate that the catalystcalcined at 300℃has the small particle size and the highest dispersion levelexisting as the spinel structure, and so possesses the most amounts of activeoxygen species and the best catalytic performance. Obviously, the smallparticle size、high dispersion level and many active oxygen species are theimportant factors for the high catalytic activity and stability oflow-temperature CO oxidation over the Co₃O₄ catalysts.3. The optimization of the technological condition of low-temperature COoxidation over the Co₃O₄ catalyst（1） The pre-oxidized Co₃O₄ catalyst shows very high CO oxidation activityand stability at 25℃. Treatment of catalyst with inert gas causes the catalyticstability lower, while the catalyst has bad catalytic activity without anypretreatment.（2） The stability decreases with the space velocity increasing within2500～20000 h^-1, and the stability decreases with the content of COincreasing within 0.5～2.0% at 25℃.（3） The pre-oxidized Co₃O₄ catalyst shows very high CO oxidation activitywithin -78～80℃. The stability increases with the reaction temperatureincreasing within -78～25℃, and the stability decreases with the reactiontemperature increasing within 25～80℃（4） The results of optimization of the technological condition indicate that:the pre-oxidized Co₃O₄ catalyst can be able to maintain its activity for COcomplete oxidation more than 630 min under the reaction conditions: CO0.5%、space velocity 2500 h^-1 and 25℃. The result is better than theliteratures.（5） The low-temperature activity of Co₃O₄ is inhibited by the presence ofmoisture in the feed gas, and the resistance to water is promoted greatly bythe addition of traces of Pd. （6） The regeneration of the deactivated catalyst could be achieved via theheat treatment at 200℃under the following drying air.（7） The catalyst not only has the high catalytic performance forlow-temperature CO oxidation in continuous flowing conditions, but also canbe used inconsecutively.4. The investigation of the reaction mechanism and the reason ofdeactivation for the low-temperature CO catalytic oxidation over the Co₃O₄catalysts.（1） The O₂-TPD analysis of the fresh and deactivated Co₃O₄ catalystsindicate the Co₃O₄ catalysts have several kinds of oxygen species, and theO₂^-、O₂^2- and O^- species responsible for the possible active oxygen species.The lattice oxygen has no effect on the catalytic performance of Co₃O₄ inlow-temperature CO oxidation.（2） The valence state of Co has no obvious difference between the freshand the deactivated Co₃O₄ catalysts, and it indicates that the deactivationdoes not originate in the change of the valence state of Co. There is water onthe surface of the deactivated, and the ratio of the adsorption oxygen to thelattic oxygen decreases after the deactivation.（3） The results of in situ DRIFT indicate that: there is a little wateradsorbing on the catalyst without pretreatment, and the water and CO arecompeting for the same site which leads to the weak adsorption of CO, andthe CO can react with the—OH to form the formate species. There is linearadsorbed CO on the surface of the pre-oxidized Co₃O₄ catalyst, and the COcan react with the active oxygen species to form CO₂, and the carbonates arethe possible intermediate product.（4） The presumable reaction mechanism of low temperature CO oxidationover the Co₃O₄ catalyst is that: there is adsorption of CO and O₂ on thesurface of the catalyst, and the CO can react with the active oxygen species toform CO₂, and the carbonates are the possible intermediate product. In thereaction process, the deactivation of the catalyst probably due to traces of moisture in the feed gas adsorption and accumulating on the surface leadingto the decrease of CO adsorption and the formation of the formate species.更多还原