石墨炭负载单原子铁催化剂非均相还原NO的微观作用机理：DFT研究

基于密度泛函理论和经典过渡态理论,探究了石墨炭负载单原子Fe催化剂(Fe/G)异相还原NO的微观机理,并对催化剂失活原因进行分析。结果表明,基于E-R机理,NO还原反应依次经历了N₂O形成与释放、N₂形成与释放四个阶段。而基于L-H机理,NO还原反应主要经历了N₂形成与释放两个阶段。在E-R机理作用下,NO分子以N,O-down结构吸附在Fe原子上发生的NO还原反应的控速步骤能垒值仅为15.5 kJ/mol,小于其余路径控速步骤能垒值。由能垒角度分析,Fe原子上残留的活性氧被还原的能垒值高于NO还原生成N₂的能垒值。NO分解后残留在Fe原子表面的活性氧抑制了NO的吸附与还原,Fe原子活性位的缺失导致催化剂的失活,单原子Fe的存在促进了NO还原反应的进行。由动力学角度分析,随着反应温度的升高,NO还原速率较活性氧转移速率提升更为显著。     

功能离子液体[NH2p-bim]BF4吸收CO2的密度泛函研究

用密度泛函理论对功能化离子液体[NH2p-bim]BF4吸收CO2的作用机制进行了理论研究。在RB3LYP/6-311 G**的计算水平对离子液体[NH2p-bim]BF4的结构以及CO2与该离子液体反应可能生成的产物进行了全优化,获得了优化结构的振动频率和热力学数据。计算结果表明,离子液体[NH2p-bim]BF4吸收CO2主要是通过离子液体的阳离子[NH2p-bim] 自偶解离的[NHp-bim]与CO2分子结合生成[O2C-NHp-bim],其结合能为238-260 kJ/mol。     

铂催化2-烯炔苯甲醛水合环化反应的机理及化学选择性的理论研究

采用密度泛函理论(DFT)的计算方法, 研究了铂催化2-烯炔基苯甲醛水合环化反应的微观机理及化学选择性的根源. 计算结果表明, 首先炔基被催化活化而发生亲核环化生成吡喃铂中间体; 接着吡喃铂中间体与烯烃双键发生[3+2]环加成生成铂-碳卡宾复合物; 之后, 反应将沿2条路径进行, 得到产物3a或4a, 其中4a的生成需经两步水分子辅助的质子转移过程. 生成产物3a需要克服的活化能垒为146.5 kJ/mol; 对4a的生成, 烯醇式和酮式互变异构是决速步聚, 当一个水分子参与反应时, 对应的能垒为185.8 kJ/mol, 当2个和3个水分子参与反应时, 能垒分别降低到128.1和64.9 kJ/mol. 因此, 水分子参与催化得到产物4a的路径是有利的. 另外, 反应的选择性与在异构化过程中水的共催化作用有关. 以上结果很好地解释了实验现象, 并为铂催化水环化反应提供新的见解.     

HCN消除反应机理的理论研究

采用密度泛函理论方法从HCN氧化和水解两个方面研究了HCN消除反应机理,并考虑了HCN的直接消除反应(途径Ⅰ和途径Ⅱ)和CuO上的HCN消除反应(途径Ⅲ和途径Ⅳ)。途径Ⅰ为HCN与2个O2分子生成CO2、NO和H原子;途径Ⅱ为HCN与1个O2分子和1个H2O分子生成 CO2和NH3;途径Ⅲ为CuO上HNCO水解为CO2和NH3;途径Ⅳ为CuO上HCN水解为CO和NH3。研究发现,途径III速控步骤的活化自由能垒为157.32 kJ/mol,比途径Ⅱ中HNCO水解降低12.34 kJ/mol;比途径Ⅳ降低了63.8 kJ/mol。可见,HNCO是HCN净化过程中的重要中间体,CuO的加入降低了反应能垒,促进了HCN消除。     

铂催化2-烯炔苯甲醛水合环化反应的机理及化学选择性的理论研究（英文）

采用密度泛函理论(DFT)的计算方法,研究了铂催化2-烯炔基苯甲醛水合环化反应的微观机理及化学选择性的根源.计算结果表明,首先炔基被催化活化而发生亲核环化生成吡喃铂中间体;接着吡喃铂中间体与烯烃双键发生[3+2]环加成生成铂-碳卡宾复合物;之后,反应将沿2条路径进行,得到产物3a或4a,其中4a的生成需经两步水分子辅助的质子转移过程.生成产物3a需要克服的活化能垒为146.5 k J/mol;对4a的生成,烯醇式和酮式互变异构是决速步聚,当一个水分子参与反应时,对应的能垒为185.8 k J/mol,当2个和3个水分子参与反应时,能垒分别降低到128.1和64.9 k J/mol.因此,水分子参与催化得到产物4a的路径是有利的.另外,反应的选择性与在异构化过程中水的共催化作用有关.以上结果很好地解释了实验现象,并为铂催化水环化反应提供新的见解.     

气相中HRnCCH和X(X=H_2O,NH_3)反应机理的理论研究

在CCSD(T)//MP2/aug-cc-pVTZ-pp理论水平上,研究了HRnCCH与大气中H2O及NH3分子反应的机理,反应主要包括HRnCCH与HRnOH及HRnNH2之间的转化、H2O和NH3在HRnCCH中的碳碳三键上的加成反应以及HRnCCH与双分子水反应等.结果表明,HRnCCH与H2O反应生成HCCH和HRnOH及HRnCCH与NH3反应生成HCCH和HRnNH2的能垒分别为54.1和75.2 kJ/mol,而生成HRnCHC(OH)H,HRnC(OH)CH2,HRnCHC(NH2)H和HRnC(NH2)CH2的活化能分别为219.6,220.5,174.4和182.4kJ/mol,此结果表明HRnCCH反应性较弱且是稳态存在的.此外,在HRnCCH与H2O反应中加入单个水分子,仍然生成HRnCHC(OH)H,但反应活化能却降低了96.4 kJ/mol,说明水分子对该反应有明显的催化作用.     

锰催化苄基C—H键氟化反应机理的理论研究

采用密度泛函理论(Density Functional Theory,DFT),对锰催化剂Mn^III(salene)F作用下苄基C—H键氟化反应的机理进行了深入的理论研究。对该反应中涉及到的重要中间体和过渡态的能量、Mulliken电荷分布、前线分子轨道等进行了分析。计算结果表明采用[Mn^IV(OH)(salene)F]和[Mn^IV(salene)F₂]为催化剂催化苄基C—H键氟化反应所需的能垒分别为11.5 kcal·mol^-1和7.6 kcal·mol^-1。可见采用双氟催化剂[Mn^IV(salene)F₂],氟化反应的能垒较低,更有利于苄基C—H键的氟化。通过轨道分析,给出了反应过程中详细的电子转移情况,从本质上分析了苄基氟化反应的机理。研究表明中心金属上的氧原子可以获得和失去电子,在C—H活化过程中起到传递电子的作用。在电子转移过程中,Mn原子是最终的电子接受体。以上结果很好地解释了实验现象,为进一步研究金属催化C—H键氟化反应提供理论支持。     

CH3O·2+NO气相反应的密度泛函理论研究

用密度泛函方法分别研究了单态和三态 CH3 O·2 NO CH3 O· NO2 气相反应 .结果表明 ,反应中 NO进攻 CH3 O·2 经过了一个顺反异构化的过程 ,摘取 CH3 O·2 的端基氧 .整个反应是吸热反应 ,理论计算吸热值为 5 0 .93k J/ mol,单态为多通道多步骤反应 ,决定速度步骤的能垒为 1 90 .6 1 k J/ mol.而三态为单通道反应 ,其决定速度步骤的能垒为 1 6 3.31 k J/ mol.三态反应为最佳反应通道 .该反应的研究将为保护臭氧层及大气环境提供重要的理论依据 .     

Mo／ZSM-5催化剂上NO催化还原反应机理与动力学研究

采用浸渍法制备了Mo/ZSM-5催化剂样品,并以氨为还原剂对其NO选择性催化还原活性、以及NO转化的反应速率进行测定.结果表明,在Mo/ZSM-5催化剂上不会发生NO氧化成NO2的反应,也没有N2O生成,然而有少量的NO分解反应发生.在氧气存在条件下,Mo/ZSM-5催化剂上NO-NH3-O2的SCR反应遵循LH机理.NO、O2和NH3首先吸附在Mo/ZSM-5表面,吸附态NO物种与吸附NH3物种直接反应生成氮气,气相氧的作用是加强NO吸附、补充催化剂表面吸附氧物种.并由此推导出NO转化的速率方程式,分别计算和模拟了在不同O2浓度、NO浓度和反应温度条件下NO的反应速率rNO值及其变化关系.结果表明,理论模拟值能够与实验值很好地吻合,所推测的机理能够很好地描述Mo-ZSM-5催化剂上NO选择性催化还原行为.     

N2O在焦炭表面异相生成和分解机理的密度泛函理论研究

选用合理简化的焦炭模型,对煤焦燃烧过程中N₂O的异相生成和分解机理进行了分子水平上的研究。采用UB3LYP/6-31G(d)密度泛函理论方法优化得到了反应路径上反应物、产物、中间体和过渡态的几何构型和各中间反应的活化能和反应焓变。NO与其预先吸附在焦炭表面解离生成的表面氮组分反应生成N₂O的路径有两个,需要克服的势垒分别为69.3kJ/mol和200.0kJ/mol;NO亦可直接与焦炭中的吡啶氮结合释放出N₂O,该反应路径所需克服的最大势垒为418.0kJ/mol。N₂O可在焦炭表面分解释放出N₂,异相分解反应为一步反应,计算所得活化能为100.8kJ/mol。N₂O的异相生成和异相分解反应均为放热反应。采用经典过渡态理论计算得到了各路径中速率控制步骤的反应速率常数。低温条件下,N₂O的异相分解反应速率略低于其异相生成速率,随着温度的升高,两者逐渐接近,说明高温条件有利于N₂O的异相分解。     

New insight into selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites: a theoretical study

Density functional theory calculations were carried out to investigate the reaction mechanism of selective catalytic reduction of nitrogen oxides by ammonia in the presence of oxygen at the Br?nsted acid sites of H-form zeolites. The Br?nsted acid site of H-form zeolites was modeled by an aluminosilicate cluster containing five tetrahedral (Al, Si) atoms. A low-activation-energy pathway for the catalytic reduction of NO was proposed. It consists of two successive stages: first NH(2)NO is formed in gas phase, and then is decomposed into N(2) and H(2)O over H-form zeolites. In the first stage, the formation of NH(2)NO may occur via two routes: (1) NO is directly oxidized by O(2) to NO(2), and then NO(2) combines with NO to form N(2)O(3), which reacts with NH(3) to produce NH(2)NO; (2) when NO(2) exceeds NO in the content, NO(2) associates with itself to form N(2)O(4), and then N(2)O(4) reacts with NH(3) to produce NH(2)NO. The second stage was suggested to proceed with low activation energy via a series of synergic proton transfer steps catalyzed by H-form zeolites. The rate-determining step for the whole reduction of NO(x) is identified as the oxidation of NO to NO(2) with an activation barrier of 15.6 kcal mol(-1). This mechanism was found to account for many known experimental facts related to selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites.     

Computational study of the release of H2 from ammonia borane dimer (BH3NH3)2 and its ion pair isomers

High-level electronic structure calculations have been used to map out the relevant portions of the potential energy surfaces for the release of H2 from dimers of ammonia borane, BH3NH3 (AB). Using the correlation-consistent aug-cc-pVTZ basis set at the second-order perturbation MP2 level, geometries of stationary points were optimized. Relative energies were computed at these points using coupled-cluster CCSD(T) theory with the correlation-consistent basis sets at least up to the aug-cc-pVTZ level and in some cases extrapolated to the complete basis set limit. The results show that there are a number of possible dimers involving different types of hydrogen-bonded interactions. The most stable gaseous phase (AB)2 dimer results from a head-to-tail cyclic conformation and is stabilized by 14.0 kcal/mol with respect to two AB monomers. (AB)2 can generate one or two H2 molecules via several direct pathways with energy barriers ranging from 44 to 50 kcal/mol. The diammoniate of diborane ion pair isomer, [BH4-][NH3BH2NH3+] (DADB), is 10.6 kcal/mol less stable than (AB)2 and can be formed from two AB monomers by overcoming an energy barrier of approximately 26 kcal/mol. DADB can also be generated from successive additions of two NH3 molecules to B2H6 and from condensation of AB with separated BH3 and NH3 molecules. The pathway for H2 elimination from DADB is characterized by a smaller energy barrier of 20.1 kcal/mol. The alternative ion pair [NH4+][BH3NH2BH3-] is calculated to be 16.4 kcal/mol above (AB)2 and undergoes H2 release with an energy barrier of 17.7 kcal/mol. H2 elimination from both ion pair isomers yields the chain BH3NH2BH2NH3 as product. Our results suggest that the neutral dimer will play a minor role in the release of H2 from ammonia borane, with a dominant role from the ion pairs as observed experimentally in ionic liquids and the solid state.     

Cycloaddition of benzene on Si(100) and its surface conversions

A comprehensive ab initio study of the adsorption of benzene on the silicon(100) surface is presented. Five potential candidates ([2+2] adduct, [4+2] adduct, two tetra-sigma-bonded structures, and one radical-like structure) for the reaction product are examined to determine the lowest energy adsorption configuration. A [4+2] butterfly structure is determined to be the global minimum (-29.0 kcal/mol), although one of the two tetra-sigma-bonded structures (-26.7 kcal/mol) is similar in energy to it. Multireference perturbation theory suggests that the [4+2] addition mechanism of benzene on Si(100) is very similar to the usual Diels-Alder reaction (i.e., small or zero activation barrier), even though benzene adsorption entails the loss of benzene aromaticity during the reaction. On the other hand, the [2+2] cycloaddition mechanism is shown to require a relatively high activation barrier (17.8 kcal/mol), in which the initial step is to form a (relatively strongly bound) van der Waals complex (-8.9 kcal/mol). However, the net activation barrier relative to reactants is only 8.9 kcal/mol. Careful examination of the interconversion reactions among the reaction products indicates that the two tetra-sigma-bonded structures (that are energetically comparable to the [4+2] product) can be derived from the [2+2] adduct with activation barriers of 15.5 and 21.4 kcal/mol. However, unlike the previous theoretical predictions, it is found that the conversion of the [4+2] product to the tetra-sigma-bonded structures entails huge barriers (>37.0 kcal/mol) and is unlikely to occur. This suggests that the [4+2] product is not only thermodynamically the most stable configuration (lowest energy product) but also kinetically very stable (large barriers with respect to the isomerization to other products).     

Nitric oxide adsorption and reduction reaction mechanism on the Rh7(+) cluster: a density functional theory study

The transition metal rhodium has been proved the effective catalyst to convert from NO(x) to N(2.) In the present work, we are mainly focused on the NO adsorption and decomposition reaction mechanism on the surface of the Rh(7)(+) cluster, and the calculated results suggest that the reaction can proceed via three steps. First, the NO can adsorb on the surface of the Rh(7)(+) cluster; second, the NO decomposes to N and O atoms; finally, the N atom reacts with the second adsorbed NO and reduces to a N(2) molecule. The N-O bond breaks to yield N and O atoms in the second step, which is the rate-limiting step of the whole catalytic cycle. This step goes over a relatively high barrier (TS(12)) of 39.6 kcal/mol and is strongly driven by a large exothermicity of 55.1 kcal/mol during the formation of stable compound 3, accompanied by the N and O atoms dispersed on the different Rh atoms of the Rh(7)(+) cluster. In addition, the last step is very complex due to the different possibilities of reaction mechanism. On the basis of the calculations, in contrast to the reaction path II that generates N(2) from two nitrogen atoms coupling, the reaction path I for the formation of intermediate N(2)O is found to be energetically more favorable. Present work would provide some valuable fundamental insights into the behavior of the nitric oxide adsorption and reduction reaction mechanism on the Rh(7)(+) cluster.     

Investigation of ethylene dimerization over faujasite zeolite by the ONIOM method.

Ethylene dimerization was investigated by using an 84T cluster of faujasite zeolite modeled by the ONIOM3(MP2/6-311++G(d,p):HF/6-31G(d):UFF) method. Concerted and stepwise mechanisms were evaluated. In the stepwise mechanism, the reaction proceeds by protonation of ethylene to form the surface ethoxide and then C--C bond formation between the ethoxide and the second ethylene molecule to give the butoxide product. The first step is rate-determining and has an activation barrier of 30.06 kcal mol(-1). The ethoxide intermediate is rather reactive and readily reacts with another ethylene molecule with a smaller activation energy of 28.87 kcal mol(-1). In the concerted mechanism, the reaction occurs in one step of simultaneous protonation and C--C bond formation. The activation barrier is calculated to be 38.08 kcal mol(-1). Therefore, the stepwise mechanism should dominate in ethylene dimerization.     

Model of uptake of OH radicals on nonreactive solids

A model of adsorption and recombination of OH radicals was developed for nonreactive solid surfaces of atmospheric interest. A parametrization of this heterogeneous mechanism was carried out to determine the role of the catalytic properties of these solid surfaces, taking into account the adsorption energy, defects, surface diffusion, and chemical reactions in the gas-solid interface. The uptake process was simulated for diffusion-controlled chemical reactions on the surface on the basis of Langmuir-Hinshelwood and Eley-Rideal mechanisms. Using an analytical approach and the Monte Carlo technique, we show the dependencies of the uptake probability of the heterogeneous reactions on the OH concentration and adsorption energy. The model is employed in the analysis of the empirically derived uptake coefficient for water ice, Al(2)O(3), NaCl, NH(4)NO(3), NH(4)HSO(4), and (NH(4))(2)SO(4). We found the following values for the free energy of adsorption of OH radicals: E(ice) = 7.3-7.6 kcal/mol, E(Al)(2)(O)(3) = 11-11.7 kcal/mol, E(NH)(4)(NO)(3) = 10.2 kcal/mol, E(NaCl) = 10.2 kcal/mol, E(NH)(4)(HSO)(4) = 9.8 kcal/mol, and E((NH)(4))(2)(SO)(4) = 9.8 kcal/mol. The atmospheric implications of the catalytic reactions of OH with adsorbed reactive molecules are discussed. The results of the modeling of the uptake process showed that the heterogeneous decay rate can exceed the corresponding gas-phase reaction rate under atmospheric conditions.     

Chemo- and periselectivity in the addition of [OsO2(CH2)2] to ethylene: a theoretical study

Quantum chemical calculations by using density functional theory at the B3LYP level have been carried out to elucidate the reaction course for the addition of ethylene to [OsO2(CH2)2] (1). The calculations predict that the kinetically most favorable reaction proceeds with an activation barrier of 8.1 kcal mol(-1) via [3+2] addition across the O=Os=CH2 moiety. This reaction is -42.4 kcal mol(-1) exothermic. Alternatively, the [3+2] addition to the H2C=Os=CH2 fragment of 1 leads to the most stable addition product 4 (-72.7 kcal mol(-1)), yet this process has a higher activation barrier (13.0 kcal mol(-1)). The [3+2] addition to the O=Os=O fragment yielding 2 is kinetically (27.5 kcal mol(-1)) and thermodynamically (-7.0 kcal mol(-1)) the least favorable [3+2] reaction. The formal [2+2] addition to the Os=O and Os=CH2 double bonds proceeds by initial rearrangement of 1 to the metallaoxirane 1 a. The rearrangement 1-->1 a and the following [2+2] additions have significantly higher activation barriers (>30 kcal mol(-1)) than the [3+2] reactions. Another isomer of 1 is the dioxoosmacyclopropane 1 b, which is 56.2 kcal mol(-1) lower in energy than 1. The activation barrier for the 1-->1 b isomerization is 15.7 kcal mol(-1). The calculations predict that there are no energetically favorable addition reactions of ethylene with 1 b. The isomeric form 1 c containing a peroxo group is too high in energy to be relevant for the reaction course. The accuracy of the B3LYP results is corroborated by high level post-HF CCSD(T) calculations for a subset of species.     

Mechanisms of [1,3]-Sigmatropic Migrations of the Nitroso Group in the ON-X-CH=X Systems (X = O,S, Se,NH, CH<Subscript>2</Subscript>)

[1,3]-Sigmatropic migrations of the nitroso group in the systems ON-X-CH=X (X = O, S, Se, NH, CH₂) were studied by MP2(fc)/6-311+G** and B3LYP/6-311+G** quantum-chemical calculations. The energy barrier in the process was estimated at 2.4 (2.5), 20.0 (25.0), and 22.3 (23.4) kcal/mol for X = O, NH, and CH₂, respectively. The energy minima for X = S and X = Se correspond to cyclic structures with two-coordinate NO group, which are more stable than acyclic structures by 9.3 (4.3) (X = S) or 13.1 (5.7) kcal/mol (X = Se).     

Mechanistic Study on the Monofunctional Binding of Hydrolysis Products of Non-planar Heterocyclic Complexes Trans-[PtCl_2NH_3（piperidine）] and Trans-[PtCl_2（piperidine）_2] to DNA Purine Bases

The monofunctional substitution reactions between trans-[PtCl(H2O)(NH3)(pip)]+,trans-[Pt(H2O)2(NH3)(pip)]2+,trans-[PtCl(H2O)(pip)2]+,trans-[Pt(H2O)2(pip)2]2+ (pip = piperidine) and adenine/guanine nucleotides are explored by using B3LYP hybrid functional and IEF-PCM salvation models. For the trans-[Pt(H2O)2(NH3)(pip)]2+ and trans-[PtCl(H2O)(NH3)(pip)]+ complexes,the computed barrier heights in aqueous solution are 13.5/13.5 and 11.6/11.6 kcal/mol from trans-Pt-chloroaqua complex to trans/cis-monoadduct for adenine and guanine,and the corresponding values are 20.7/20.7 and 18.8/18.8 kcal/mol from trans-Pt-diaqua complex to trans/cis-monoadduct for adenine and guanine,respectively. For trans-[PtCl(H2O)(pip)2]+ and trans-[Pt(H2O)2(pip)2]2+,the corresponding values are 21.5/21.3 and 19.4/19.4 kcal/mol,and 26.0/26.0 and 20.7/20.8 kal/mol for adenine and guanine,respectively. Our calculations demonstrate that the barrier heights of chloroaqua are lower than the corresponding values of diaqua for adenine and guanine. In addition,the free energies of activation for guanine in aqueous solution are all smaller than that for adenine,which predicts a preference of 1.9 kcal/mol when trans-[PtCl(H2O)(NH3)(pip)]+ and trans-[Pt(H2O)2(NH3)(pip)]2+ are the active agents and ～1.9 and ～ 5.3 kcal/mol when trans-[PtCl(H2O)(pip)2]+ and trans-[Pt(H2O)2(pip)2]2+ are the active agents,respectively. For the reaction of trans-Pt-chloroaqua (or diaqua) to cis-monoadduct,we obtain the same transition-state structure as from the reaction of trans-Pt-chloroaqua (or diaqua) to trans-monoadduct,which seems that the trans-Pt-chloroaqua (or diaqua) complex can generate trans-or cis-monoadduct via the same transition-state.