仿生光电催化固氮

固氮是将游离的N₂转变为生物可用形式的过程,主要包括生物固氮和工业固氮。前者通过固氮酶进行,利用ATP水解提供的能量,可以在常温常压下将N₂还原成NH₃,同时有H₂形成。工业固氮主要指Haber-Bosch过程,在铁催化剂和促进剂的共同作用下,可以高效地将N₂催化成NH₃。这个100多年前发明的过程需要400~500 ℃高温和高于100 atm的反应条件,会消耗大量的能量。合成H₂的甲醇水蒸气重整过程也会消耗大量能量。如果能进一步认识固氮酶的固氮机制,利用太阳能驱动实现常温常压下的固氮反应将会非常有前景。本文概述了近年来固氮酶启发的光催化固氮领域的进展,并结合了相关的电化学领域的固氮研究,对本领域作了展望。目前还没有催化剂能取代传统Haber-Bosch过程所采用的催化体系,但是通过总结过去的研究进展和经验,可为未来设计高效催化剂提供非常有益的启示。     

太阳能光(电)催化固氮研究进展（英文）

氨(NH₃)是一种现代社会必需的化学物质。目前,工业上合成NH₃仍然采用的是Haber-Bosch过程,即以H₂和N₂为反应物在铁基催化剂的作用下于高温(400-600℃)高压(20-40Mpa)下将N₂转化为NH₃。然而,其效率只有10%-15%,同时造成大量的能源消耗,而且CO₂排放不可避免。开发构建可持续发展的清洁友好的新能源体系是解决能源危机和环境污染问题、实现碳达峰和碳中和的关键战略。半导体光(电)催化固氮可以利用绿色无污染的太阳能制取重要的基础化工原料氨,有望代替传统的化工制氨工艺,解决其能源消耗严重和环境污染的问题。本文概述了光(电)催化固氮反应及其影响因素、光催化、电催化和光电催化固氮反应实验装置与基本特征、光(电)催化固氮反应催化剂研究进展、光电催化固氮反应机理,着重论述了半导体光催化剂、光(电)催化固氮体系以及光催化固氮机理的最新进展,并对太阳能光催化固氮技术加以评述和展望。     

钼铁硫簇合物催化乙炔还原反应的研究

钼铁辅基的分离及其EXAFS(外延X射线精细结构)的测定结果表明,固氮酶活性中心是一种Mo-Fe-S原子簇结构.为模拟固氮酶活性中心结构及研究其性能,人们开展了钼铁硫簇合物的合成、结构和性质的研究.迄今已经合成了上百种钼铁硫簇合物,从而形成了钼铁硫簇合物化学. 固氮酶是由钼铁蛋白和铁蛋白组成的,活性中心在钼铁蛋白中,铁蛋白的作用是传递外部还原剂提供的电子.固氮酶的基本功能是具有固氮活性.生物学家在研究固氮酶的     

基于Li-N2电池体系的“连续式”氮气还原合成氨

电化学合成氨近年来受到较多关注, 直接的电化学固氮法(NRR)存在产氨来源不明的问题, 而间接的锂式合成氨(LiNR)被认为是一种可行的固氮方案. LiNR的研究多为电沉积锂, 本工作以Li-N₂电池体系为基础, 利用电池的放电反应固定N₂, 质子源H₂O同时参与反应, 理论上提高了Li-N₂电池的放电电压. 结合充电反应锂盐分解, 构成了清晰的锂循环方案. 研究发现, 当N₂和H₂O共同通入电池, 可以实现连续式的NH₃生产, 且放电电位与理论值接近. 充放电循环显示, 每个循环均可以产生NH₃, 产氨量随循环次数而增加. 该方案可循环利用锂, 对于开发新型的固氮方式有较大的研究与利用价值.     

滑动弧等离子体固氮研究进展

传统工业固氮采用哈伯-博施(Haber-Bosch)工艺,但是需要高温高压,能耗高,污染严重。滑动弧等离子体(Gliding arc plasma, GAP)兼具热等离子体和冷等离子体的优点,能够高效地产生活性物种,显著提高能量效率,使其在固氮领域具有很大的潜在应用价值,近年来受到人们的广泛关注。然而,目前GAP固氮相关研究还比较零散,有必要对具体内容进行总结归纳。本文主要综述了近10年来国内外GAP固氮研究进展,主要包括GAP放电机制、反应器设计、工艺参数研究以及固氮反应机理研究。GAP放电存在击穿伴随滑动放电的B-G模式和持续稳定放电A-G模式,A-G模式放电有助于提高固氮效率。随着滑动弧放电技术的不断发展,GAP反应器中电极结构从传统的2D刀片结构演变到了多种3D柱形结构。通过工艺优化,GAP有助于N₂分子的振动激发,从而促进N₂分子的分裂转化。最后,对GAP固氮研究进行了展望。     

固氮酶催化活性中心及其化学模拟

固氮酶是固氮微生物在常温常压下固氮成氨的催化剂,其催化机理和化学模拟一直是国际上长期致力研究的对象.钼铁蛋白高分辨1.0单晶X射线衍射分析表明,固氮酶催化活性中心铁钼辅基的结构为Mo Fe7S9C(R-homocit),其中,Mo原子和3个u2-硫配体、1个组氨酸和1个高柠檬酸配位,形成八面体构型.高柠檬酸以α-烷氧基氧和α-羧基氧与钼螯合形成双齿配位,氨基酸残基上的组氨酸咪唑氮和半胱氨酸巯基与钼和铁单齿配位.在固氮酶铁钼辅基的生物合成过程中,高柠檬酸和咪唑侧基是在最后的合成步骤插入铁硫碳簇前驱体中,其中高柠檬酸和咪唑侧基有可能对质子传递以及稳定Mo Fe7S9C簇起到重要作用.本文从固氮酶铁钼辅基结构出发,结合最近本课题组从化学模拟出发,将固氮酶催化活性中心铁钼辅基结构修订为加氢新结构Mo Fe7S9C(R-Hhomocit)的研究,着重介绍了近年来国内外固氮酶活性中心、生物合成和催化作用机理的研究进展,并展望了固氮酶的研究前景.     

人工光合作用

光合作用将太阳能储存在化学反应中,是绿色高效的能量转换途径。模拟自然光合作用系统活性中心的结构和功能,实现小分子物质(H₂O、CO₂、N₂等)中惰性化学键的活化转化,对于解决能源和环境等问题具有重要意义。本文综述了人工光合作用在水分解、二氧化碳及氮气还原领域取得的重要进展,分析了相关光化学转换体系的设计思路和工作原理,并对人工光合作用面临的挑战和未来发展方向进行讨论。     

五嗪含氮取代基衍生物结构和性质的理论研究

采用密度泛函理论(DFT)方法在B3LYP/aug-cc-pvDZ理论水平上研究了CN, NO₂, NH₂, N₃, N₂H, NHNH₂, N₄H和N₄H₃等含氮取代基取代五嗪环上的氢原子生成的衍生物, 预测了它们的分子构型、分解能及含能性质. 对衍生物分解能的研究结果表明, CN和NH₂取代的衍生物的分解能比未取代时更高, 而其余基团的取代使分解能降低; 取代基化合物的生成热越大, 取代五嗪中的氢原子后生成衍生物的生成热也越大. N₄H₃, NO₂, H, N₂H, N₄H, N₃和CN取代的五嗪衍生物的单位原子生成热为72.6～108.9 kJ, 比文献报道的三叠氮基-均三嗪的(70.2 kJ)更高. 对于CN, N₂H, N₄H₃, N₃和N₄H取代的衍生物, 其生成热为871.4～1159.3 kJ•mol^－1, 但N₄H和N₄H₃的分解能较小, 稳定性较差. 因此, N₃, N₂H和CN取代的衍生物可能成为高能量、低感度的含能材料.     

梯型In2O3/ZnIn2S4杂化体系的构建及其光催化固氮增效

工业化固氮合成氨主要采用Haber-Bosch法.然而,该工艺条件苛刻,需要氮气与氢气在高温高压和使用催化剂的条件下反应,耗费大量能源,同时产生温室气体.与Haber-Bosch法不同,光催化固氮不需要使用氢气,而是利用清洁的太阳能和水直接提供固氮反应所需的还原电子和质子,反应耗能低且绿色无污染,是一种理想的固氮方法.然而,目前光催化固氮合成氨受限于光催化剂载流子分离效率低、氮气吸附和活化难,总体固氮效率仍然很低.大量研究证明,构建梯型异质结是一种改善光催化活性的有效手段,这是因为梯型异质结体系不仅有效分离光生载流子,而且保留了光生空穴和电子的强氧化还原能力.另外,表面缺陷不仅可以充当吸附位点,有效调控表面N2分子的吸附特性,还可以起到活化N2分子的作用.本文设计了富含空位的In₂O₃/ZnIn₂S₄梯型异质结,系统考察了复合体系中组分配比对晶型结构、微结构和光学吸收等的影响,并通过XPS谱研究了In₂O₃和ZnIn₂S₄之间存在强的相互作用,这为光生载流子的高效分离奠定了基础.同时,结合XPS、Raman和EPR测试揭示了材料中表面空位的成功构筑.在此基础上,深入研究了In₂O₃/ZnIn₂S₄梯型异质结在室温常压下光催化固氮合成氨的活性.研究结果表明,所有的梯型异质结均展现出明显的光催化固氮活性,其中50 wt%In₂O₃/ZnIn₂S₄梯型体系具有最高的光催化固氮活性,自然光照射2 h产生的氨气浓度达到18.1±0.77 mg·L-1,分别是In₂O₃和ZnIn₂S₄的21.0和2.72倍.并且该复合体系具有较高的光催化稳定性,在连续循环使用6次时,产生氨气浓度仍然可达到16.3±0.86 mg·L^-1.荧光光谱测试、光电化学测试和表面光电压测试证明了电荷的有效分离和转移.综上,构建In₂O₃/ZnIn₂S₄梯型体系后,所制备的In₂O₃/ZnIn₂S₄活性得到增强,这主要归因于空位对氮气的吸附和活化作用以及梯型异质结中载流子的高效分离机制.另外,研究表明·CO₂-物种是光催化固氮合成氨的主要活性物种.     

不同组分过渡金属氧化物催化剂对介质阻挡放电固氮的影响机制

为了促进介质阻挡放电(DBD)协同催化固氮效果, 制备了不同组分Mn/Co/W元素的单一型、 二元和三元复合型负载催化剂, 并将催化剂置入DBD气隙中进行等离子体协同催化固氮反应. 通过X射线衍射(XRD)、 扫描电子显微镜(SEM)和X射线能谱(EDS)表征了催化剂的性质, 并采用紫外分光光度法测定了液相中的总氮浓度. 结果表明, 填充催化剂实验组比未填充催化剂组的总氮浓度明显提升. 采用傅里叶变换红外光谱仪对有/无催化剂填充两种情况下DBD的气相产物进行了检测, 结果表明, 填充催化剂能促进空间内NO₂和N₂O₅的生成. 通过DBD气相链式反应和催化原理揭示了总氮浓度得以提升的原因, 是由于催化剂在等离子协同过程中提供了大量的氧空位, 使得NO _x 充分氧化. 多元复合型催化剂能在单一型的基础上, 通过金属元素价态的变换和能量的传递进一步促进固氮效果, 三元复合型催化剂Mn₃WCo/γ-Al₂O₃在电压为22 kV时的固氮最高总氮浓度为119.13 mg/L, 较未填充催化剂组的最大值提升了71.61%, 能耗降低了21.70%.     

Studies on the hydrogenation steps of the nitrogen molecule at the Azotobacter vinelandii nitrogenase site

We follow the initial activation of the nitrogen molecule at the FeMo cofactor of nitrogenase and subsequently model the hydrogenation of N₂ up to the fourth protonation step using the intermediate neglect of differential overlap quantum-chemical model. The results obtained favor a reaction mechanism going through hydrazido intermediates on the 4-Fe surfaces, externally to the FeMo cofactor. Calculations using density functional theory on smaller model systems also support the suggested mechanism over other possible schemes that involve early release of the first molecule of ammonia as a product of the enzymatic reaction. We also demonstrate that dielectric stabilization due to the protein around the cofactor could lower markedly the barrier for the product release as an ammonium ion. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1159–1168, 1998     

Enzymatic and catalytic reduction of dinitrogen to ammonia: Density functional theory characterization of alternative molybdenum active sites

We used density functional calculations to model dinitrogen reduction by a FeMo cofactor containing a central nitrogen atom and by a Mo‐based catalyst. Plausible intermediates, reaction pathways, and relative energetics in the enzymatic and catalytic reduction of N₂ to ammonia at a single Mo center are explored. Calculations indicate that the binding of N₂ to the Mo atom and the subsequent multiple proton–electron transfer to dinitrogen and its protonated species involved in the conversion of N₂ are feasible energetically. In the reduction of N₂ the Mo atom experiences a cycled oxidation state from Mo(IV) to Mo(VI) by nitrogenase and from Mo(III) to Mo(VI) by the molybdenum catalyst, respectively, tuning the gradual reduction of N₂. Such a wide range of oxidation states exhibited by the Mo center is crucial for the gradual reduction process via successive proton–electron transfer. Present results suggest that the Mo atom in the N‐centered FeMo cofactor is a likely alternative active site for dinitrogen binding and reduction under mild conditions once there is an empty site available at the Mo site. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Ammonia production at the FeMo cofactor of nitrogenase: results from density functional theory

Biological nitrogen fixation has been investigated beginning with the monoprotonated dinitrogen bound to the FeMo cofactor of nitrogenase up to the formation of the two ammonia molecules. The energy differences of the relevant intermediates, the reaction barriers, and potentially relevant side branches are presented. During the catalytic conversion, nitrogen bridges two Fe atoms of the central cage, replacing a sulfur bridge present before dinitrogen binds to the cofactor. A transformation from cis- to trans-diazene has been found. The strongly exothermic cleavage of the dinitrogen bond takes place, while the Fe atoms are bridged by a single nitrogen atom. The dissociation of the second ammonia from the cofactor is facilitated by the closing of the sulfur bridge following an intramolecular proton transfer. This closes the catalytic cycle.     

Modeling a central ligand in the nitrogenase FeMo cofactor

In very recent work by Einsle et al. (Science 2002, 297, 1696), a new X-ray crystallographic structure of the FeMo cofactor of nitrogenase with a central ligand was presented. The central ligand is a light atom (N, O, or C), and Einsle et al. suggest that it is nitrogen. We present density functional calculations on the FeMo cofactor, and we investigate N, O, and C as central ligands. We show that both N and O lead to energetically stable FeMo cofactor structures, whereas C is energetically unfavorable. By comparison of bond geometries with the crystallographically determined values, we show that the central ligand is most likely nitrogen.     

Elucidating the Coordination Chemistry and Mechanism of Biological Nitrogen Fixation

How does the enzyme nitrogenase reduce the inert molecule N₂ to NH₃ under ambient conditions that are so different from the energy‐expensive conditions of the best industrial practices? This review focuses on recent theoretical investigations of the catalytic site, the iron–molybdenum cofactor FeMo‐co, and the way in which it is hydrogenated by protons and electrons and then binds N₂. Density functional calculations provide reaction profiles and activation energies for possible mechanistic steps. This establishes a conceptual framework and the principles for the coordination chemistry of FeMo‐co that are essential to the chemical mechanism of catalysis. The model advanced herein explains relevant experimental data.     

Electron transfer coupled with ATP hydrolysis in nitrogenase

Nitrogenase, which is not a membrane protein in vivo, performs energy coupling: the transfer of an electron coupled with ATP hydrolysis from one protein component of nitrogenase, Fe protein (Av2), to another its protein component, MoFe protein (Av1), to form the so-called super-reduced state of the active site responsible for the reduction of the substrates, FeMo cofactor (FeMoco) containing Fe, Mo, S, and homocitrate. The review discusses recent publications on studying the electron transfer coupled with ATP hydrolysis in nitrogenase and evaluates a possible value of the redox potential of the super-reduced FeMoco.     

Chemical activity of the nitrogenase FeMo cofactor with a central nitrogen ligand: density functional study

We investigate the chemical consequences of a central ligand in the nitrogenase FeMo cofactor using density functional calculations. Several studies have shown that the central ligand most probably is a nitrogen atom, but the consequences for the chemical reactivity of the cofactor are unknown. We investigate several possible routes for insertion of the central nitrogen ligand and conclude that all routes involve barriers and intermediate states, which are inaccessible at ambient conditions. On this basis we suggest that the central nitrogen ligand is present at all times during the reaction. Furthermore, we investigate how the FeMoco with the central ligand can interact with N(2) and reduce it.     

The Interstitial Carbon of the Nitrogenase FeMo Cofactor is Far Better Stabilized than Previously Assumed

The first quantum-mechanical calculations of all relevant potential constants in both the iron-molybdenum cofactor and the iron-vanadium cofactor of nitrogenase suggest that the carbide is bound to the center of the enzyme much more strongly than hitherto assumed. Previous studies seemed to indicate a dummy function of the interstitial carbon, with a weak force constant (ca. 0.32 N cm⁻¹). Our new investigations confirm a different picture: the central carbon atom binds the iron-sulfur cluster through six covalent C−Fe bonds. With a potential constant of more than 1.3 N cm⁻¹, the interstitial carbon also appears to be dynamically persistent. According to our investigations, the values for the elasticity within the iron-sulfur cluster have to be corrected too. These new details on the mechano-chemical properties of the FeMo cofactor will be important for elucidating the catalytic cycle of nitrogen fixation. By implementing our new algorithm in the freely available COMPLIANCE program, the dependence on the coordinates during the calculation of Hesse matrices is eliminated completely.     

Nonenzymatic Simulation of Nitrogenase Reactions and the Mechanism of Biological Nitrogen Fixation

The development of biologically relevant model systems of nitrogenase permitted the simulation of virtually all known reactions of the nitrogen reducing enzymes under nonenzymatic conditions. On the basis of these experiments, a mechanism of biological nitrogen fixation is formulated which is in accord with the available enzymological evidence. The key reactions of the substrates of nitrogenase occur at a molybdenum active site. The non-heme iron, which is bound to sulfur and protein-S^? groups, mediates the transport of electrons to the molybdenum active site but does not participate directly in the reduction of the substrates. ATP is required for the acceleration of the reduction and activation of the molybdenum site and is hydrolyzed to ADP and inorganic phosphate. Diimine and hydrazine were detected as intermediates in the reduction of molecular nitrogen under nonenzymatic conditions.     

Unique peculiarities of the structure of the Fe-Mo cofactor of nitrogenase favorable for multicenter coordination of a nitrogen molecule

The probable coordination mode of a nitrogen molecule on the Fe-Mo cofactor of nitrogenase has been theoretically considered, taking into account both the well-known data on the structure of the Fe-Mo cofactor and the substrate selectivity of nitrogenase and the results of semiempirical calculations of the electronic structures of the cofactor and its complexes with molecular nitrogen. The distances between the Fe atoms in the cofactor are favorable for different multicenter coordination modes of a nitrogen molecule: above the Fe₄ face along its diagonal, through this face, and inside the Fe₆ prism perpendicularly to its axes. It is important that the nitrogen atoms are open for protonation in all coordination modes. The first mode is disadvantageous due to steric hindrances. Of the other variants, the latter is the most favorable both energetically and from the viewpoint of weakening of the N-N bond.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1928–1933, August, 1996.