【作者】 马军；

【导师】 熊宇杰； 龙冉；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2021， 博士

【摘要】 随着天然气储量的不断探明,天然气作为重要的化石能源和碳原料受到越来越多的关注。然而由于甲烷极其稳定的分子结构,目前的甲烷转化技术通常需要比较苛刻的条件(700℃以上的高温),反应过程不仅需要大量的能量输入,还会带来比较严重的积炭和催化剂烧结等问题。太阳能作为广泛可得的清洁能源,可以代替热能来驱动化学反应。因此,温和条件下太阳能驱动的甲烷转化被认为是实现天然气有效利用的重要途径,一方面太阳能可以提供活化甲烷的能量而减少化石能源的消耗,另一方面在温和条件下反应能够极大地提高目标产物的选择性和催化剂稳定性。但如何抑制光驱动甲烷转化的过度氧化尤其是晶格氧参与的过度氧化,提高目标产物的选择性仍然具有较大挑战。本论文旨在通过对半导体纳米催化剂的缺陷、晶面等参数进行设计和调控,在半导体材料表面构筑高效催化活性位点以稳定甲烷转化过程中的中间体,如甲氧基等,从而抑制甲烷的过度氧化,提高目标产物选择性。同时我们开发了气-固、气-液等光驱动甲烷转化体系,极大地扩展了光驱动甲烷转化的应用场景。此外,借助原位表征技术对催化剂结构演变以及催化反应过程中间体、活性氧物种等进行监测,从表面化学的角度揭示了催化剂结构与催化性能间的构效关系以及光驱动甲烷分子转化的机理。本论文为设计高效的光驱动甲烷转化催化剂以及理解表面活性位点对甲烷活化过程的影响奠定了研究基础。本论文所取得的主要研究成果如下:1.我们首先利用溶剂热合成法制备了 Cu和Si共掺杂的超薄TiO2纳米片光催化剂。Cu和Si掺杂不仅能提高TiO2纳米片的载流子浓度还能促进TiO2纳米片的光生电荷的分离和迁移。此外,原位自由基捕获实验证实Cu和Si掺杂可以调控TiO2光催化时的活性氧物种,进而抑制甲烷的过度氧化。基于同步辐射的原位红外表征发现Cu和Si掺杂能够在表面构建合适的甲烷吸附活化位点,促进甲烷分子在表面解离成中间体甲氧基。气液相光催化甲烷转化结果发现合适的Cu和Si掺杂比例表现出优异的温和条件下光催化转化甲烷的性能。2.我们通过溶胶凝胶法制备了 Cu掺杂多孔TiO2催化剂,并且我们利用非常廉价易得的NaCl作为氯源进行光催化甲烷氯化合成一氯甲烷。同时我们利用原位红外、近常压光电子能谱(NAP-XPS)等同步辐射谱学技术证实掺杂的Cu元素能够在TiO2表面构筑活性位点以稳定关键中间体甲氧基,同时调控甲烷分子吸附构型从而抑制甲烷的过度氧化提高氯甲烷的选择性。Cu元素的变价离子Cu2+还能促进多孔TiO2的光生电荷分离和迁移。Cu掺杂的多孔TiO2能够实现高选择性光催化甲烷氯化合成氯甲烷,并且体系中催化剂和氯源都能大批量且低成本制备(获得),具有大规模应用的潜力。光催化甲烷氯化反应的研究不仅拓展了甲烷转化的研究体系还为烷烃C-H的活化提供了新的途径。3.我们结合密度泛函理论计算和自由基捕获实验结果,揭示了单斜相W03不同晶面上羟基自由基·OH反应活性的差异,发现WO3{010}面上的OH具有较强的反应活性,同时{010}晶面上成对出现的W原子能够为C-C偶联提供足够的位点促进C2+产物形成。之后我们制备了不同{010}面比例的WO3纳米阵列。为提高WO3催化稳定性,我们选择光电催化甲烷转化体系,结果发现其中拥有最高{010}晶面比例的光阳极表现出最优的光电化学甲烷转化性能,实现了在较低外加偏压下甲烷到乙二醇的转化。我们通过原位红外表征发现反应中间体甲醇被高活性的·OH进攻后形成的羟甲基自由基能够在成对出现的W原子上有效偶联生成乙二醇。更多还原

【Abstract】 With the continuous exploration of natural gas reserves,natural gas,as an alternative and promising clean energy and feedstocks,has attracted more and more attention.However,methane molecule is very stable due to its symmetrical tetrahedral molecule configuration and huge thermal energy is required to activate inert C-H bonds of methane,during which severe sintering and coke would be formed.Solar energy,the most abundant and clean energy,has been utilized as a stimulus to drive chemical reactions that is difficult to happen under mild conditions.Therefore,increasing research has been paid on solar energy-driven methane conversion.Solar energy as an alternative to thermal energy can reduce consumption of fossil fuel in methane conversion.Besides,the photoenergy could help to activate C-H bonds of methane at lower temperatures,which has significant benefit to the selectivity of target products and stability of catalysts.However,it is still a great challenge to depress the over oxidation of solar-driven methane conversion to improve the selectivity of target products.In this dissertation,we mainly aimed to design and construct active sites on semiconductors nanomaterials to stabilize the intermediates of solar energy-mediated methane conversion with the aim of repressing the overoxidation of methane.Meanwhile,we developed various reaction system e.g.gas-solid,gas-liquid,for methane conversion.Moreover,we intended to explore the working mechanism of solar energy-mediated methane conversion by detecting of intermediates and reactive oxygen species using in situ characterizations.Our research findings offer new insight into direct conversion of methane into high value-added products and present significant guidance to future design of catalyst toward C-H bonds activation.In summary,the main results are summarized as follow:1.We prepared Cu and Si co-doped ultrathin TiO2 nanosheets with excellent photocatalytic performance for methane conversion by solvothermal method.Cu&Si doping can improve the carrier concentration and promote the separation and migration of photogenerated charge carriers of TiO2 nanosheets.Meanwhile,in situ radical capture experiments show that Cu Si doping can regulate the species of active oxygen in TiO2 photocatalysis,and then inhibit the excessive oxidation of methane.Finally,it benefiting from the construction of appropriate sites on the surface,Cu and Si co-doped ultrathin TiO2 nanosheets achieved excellent photocatalytic conversion of methane under mild conditions and greatly inhibited the over oxidation of methane.2.We presented a novel photocatalytic route for selective halogenation of methane to methyl chloride using chlorine salts,e.g.,NaCl,readily available at large scale and safe in storage and transport,as chlorinating agent under moderate condition(room temperature,ambient pressure)over a Cu-doped porous TiO2 photocatalyst.The doped low-valence Cu species plays a multi-role in CH4 dissociation and halogenation by porous TiO2:(1)promoting the separation and migration of photogenerated charge carriers in porous TiO2,which could enhance the efficiency of photoenergy utilization;(2)modulating the adsorption configuration of CH4 molecule on the surface of TiO2,which dramatically repressed the overoxidation of methane and improved selectivity towards desired products.As a result,the Cu-doped porous TiO2 with optimal Cu content achieves a methyl chloride production.Combining with in situ characterization,it is manifested that the methane molecule would be first dissociated into methoxy on the surface of photocatalyst and then the methoxy intermediate coupled with chloride to form methyl chloride under light irradiation.Furthermore,the selectivity of methyl chloride is positively relative to the Cu doped content.3.We began with an investigation on how facets of WO3 affect the reactivity of·OH.According to the density functional theory(DFT)materials simulation,the ·OH produced on {010} facets own much stronger reactivity than that on {100} and ｛001}facets.Such a result was then comprehended by experimental evidence via preparing the WO3 photoanode with different {010} to {100} and {001} facet ratios.The ·OH trapping test performed through the electron paramagnetic resonance(EPR)test affirmed the prediction of the DFT materials simulation.As a result,the WO3 photoanode with the highest {010} facet ratio among all the prepared samples achieved the greatest PEC CH4 conversion performance.Moreover,based on the in situ characterization,the methanol,which could be attacked by reactive.OH to form hydroxymethyl radicals,is confirmed to be the main intermediate for the production of ethylene glycol.更多还原