甲烷在清洁 Pd(111) 及氧改性的 Pd(111) 表面解离的密度泛函理论研究

 采用广义梯度近似 (GGA) 的密度泛函理论 (DFT) 并结合平板模型, 研究了 CH4 在清洁 Pd(111) 及 O 改性的 Pd(111) 表面发生 C朒 键断裂的反应历程. 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能. 结果表明, CH4 采用一个 H 原子指向表面的构型在 Pd(111) 表面的顶位吸附, CH3 的最稳定的吸附位置为顶位, OH, O 和 H 的最稳定吸附位置均为面心立方. CH4 在清洁 Pd(111) 表面裂解的活化能为 0.97 eV, 低于它在 O 原子改性 (O 没有参与反应) 的 Pd(111) 表面的活化能 1.42 eV, 说明表面氧原子抑制了 CH4 中 C朒 键的断裂. 当亚表面 O 原子和表面 O 原子 (O 参与反应) 共同存在时, C朒 键断裂的活化能为 0.72 eV, 低于只有表层氧存在时的活化能 (1.43 eV), 说明亚表面的 O 原子对 CH4 分子的活化具有促进作用. CH4 在 O 原子改性的 Pd(111) 表面裂解生成 CH3 和 H, 以及生成 CH3 和 OH 的反应活化能分别为 1.42 和 1.43 eV, 说明 CH4 在 O 原子改性的 Pd(111) 表面发生这两种反应的难易程度相当.     

乙烷在Ni(111)表面的吸附和分解

研究了乙烷在Ni(111)表面解离的可能反应机理, 使用完全线性同步和二次同步变换(complete LST/QST)方法确定解离反应的过渡态. 采用基于第一性原理的密度泛函理论与周期平板模型相结合的方法, 优化了C2H6裂解反应过程中各物种在Ni(111)表面的top, fcc, hcp和bridge位的吸附模型, 计算了能量, 并对布居电荷进行分析, 得到了各物种的有利吸附位. 结果表明, 乙烷在Ni(111)表面C—C解离的速控步骤活化能为257.9 kJ·mol-1, 而C—H解离速控步骤活化能为159.8 kJ·mol-1, 故C—H键解离过程占优势, 主要产物是C2H4和H2.     

甲胺在清洁及磷改性Mo(100)表面的解离

采用广义梯度近似(GGA)的密度泛函理论(DFT)(DFT-GGA)并结合平板模型, 研究了甲胺在清洁及磷(P)改性的Mo(100)表面(P-Mo(100))发生C—N键断裂的反应历程(CH3NH2→CH3+NH2). 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能数据. 计算结果表明, 在清洁和磷改性的Mo(100)表面, 甲胺均稳定吸附在顶位, 甲基和氨基最稳定的吸附位置均为桥位. 甲胺的C—N键在P-Mo(100)表面裂解的活化能为2.39 eV, 高于其在清洁表面的活化能(1.99 eV). 这表明Mo(100)表面被预吸附的P原子钝化了. 电子结构分析表明, 改性P原子使得金属Mo的供电子能力减弱, 导致它的d带中心下移, 从而降低了该表面的反应活性, 提高了甲胺的C—N键裂解的活化能. 活化能的分解表明, C—N键在P-Mo(100)与Mo(100)表面裂解的活化能的差异主要体现在初态到过渡态时甲胺的结构变化引起的能量变化(△EdefCH3NH2)、过渡态仅有甲基存在时的吸附能(ETSCH3)和过渡态甲基和氨基的相互作用(EintCH3…NH2). △EdefCH3NH2和ETSCH3使活化能升高幅度大于EintCH3…NH2使活化能降低幅度, 最终导致甲胺的C—N键在P-Mo(100)表面裂解的活化能要高于在Mo(100)表面裂解的活化能.     

Pd催化木质素醚类二聚体分子内氢转移断裂C—O键研究

结合氢转移方法,研究了木质素模型物2-(2'-甲氧基苯氧基)-1-苯乙醇(1a)分子在无外加氢源的条件下利用金属钯催化剂催化发生C—O键断裂反应.合成并表征了一系列Pd负载型催化剂,通过优化发现反应体系在环己烷溶剂和弱碱添加剂Na2HPO4条件下显示出较好的催化效率.结合反应特点将催化剂进行改进,使用MgO作为载体的催化剂Pd/MgO高效完成了木质素模型物的分子自供氢降解.反应过程可能分为两步进行:首先,模型物在钯表面先进行脱氢过程,含羟基的木质素模型物二聚体1a脱去氢后生成酮式中间体2-(2'-甲氧基苯氧基)-1-苯乙酮(1b),被脱去的氢原子吸附于钯表面.随后,脱氢中间体1b在Pd催化下与其表面吸附的H作用,发生催化C—O键断键过程.     

甲醇在FeS2(100)完整表面的吸附和分解

采用基于第一性原理的密度泛函理论结合周期平板模型方法, 研究了甲醇分子在FeS₂(100)完整表面的吸附与解离. 通过比较不同吸附位置的吸附能和构型参数发现: 表面Fe位为有利吸附位, 甲醇分子通过氧原子吸附在表面Fe位, 吸附后甲醇分子中的C―O键和O―H键都有伸长, 振动频率发生红移; 甲醇分子易于解离成甲氧基CH3O和H, 表面Fe位仍然是二者有利吸附位. 通过计算得出甲醇在FeS₂(100)表面解离吸附的可能机理: 甲醇分子首先发生O―H键的断裂, 生成甲氧基中间体, 继而甲氧基C―H键断裂, 得到最后产物HCHO和H₂.     

水滑石负载钯催化剂上的醇无氧脱氢反应

 研究了多种载体负载 Pd 催化剂上苯甲醇无氧脱氢反应. 结果发现, 以兼具较强酸性和碱性的水滑石 (HT) 为载体时, Pd 催化剂具有优异的苯甲醇转化活性和苯甲醛选择性, 当 Pd 含量为 0.32%~0.55% 时催化性能最佳. Pd/HT 催化剂可重复使用, 且对于含推电子取代基的芳香醇、2-噻吩甲醇、α,β-不饱和醇与环状脂肪醇等的直接脱氢反应均具有较好催化性能. HT 表面的 Pd(II) 物种反应后转变为平均粒径为 2.0~2.5 nm 的 Pd 纳米粒子或纳米簇. 具有较高分散度的 Pd(II) 物种易转变为较小的 Pd 纳米粒子, 从而具有较佳的催化性能. 本文推测, 催化剂表面的碱性位可促进苯甲醇 O–H 键的活化, 形成 Pd-苯甲氧基中间体, 该中间体进一步脱氢生成苯甲醛和 Pd-H 物种; 而催化剂表面的质子酸位可与 Pd-H 作用, 促进 H2 的脱除.     

CO2在Cu表面还原成碳氢化合物的DFT计算研究

应用密度泛函理论（DFT）反应能计算及最小能量路径分析研究了CO2在气相和电化学环境中于Cu(111)单晶表面的还原过程。气相中,CO2还原为碳氢化合物的反应路径可能为：CO2(g) + H* → COOH* → (CO +OH)* → CHO*;CHO + H* → CH2O* → (CH2 + O)*;CH2* + 2H* → CH4或2CH2* → C2H4。整个反应由CO2(g) + H* → COOH* → (CO +OH)*,(CO + H)* → CHO*和CH2O* → (CH2 + O)*等几个步骤联合控制。在-0.50V (vs RHE) 以正的电势下,CO2在Cu(111)表面电化学还原主要形成HCOO-和CO吸附物;随着电势逐渐负移,CO2加氢解离形成CO的反应越来越容易,CO成为主要产物;随电势进一步变负,形成碳氢化合物的趋势逐渐变强。与CO2的气相化学还原不同的是,电化学环境下CO质子化形成的CHO中间体倾向于解离形成CH,而在气相中CHO中间体则倾向于进一步质子化形成CH2O中间体。     

1-乙基-3-甲基咪唑阳离子与天冬酰胺阴离子的相互作用

利用密度泛函理论B3LYP方法, 在6-311+G(d,p)水平上, 对1-乙基-3-甲基咪唑阳离子[Emim]+与天冬酰胺阴离子[Asn]-形成的氨基酸离子液体气态阴阳离子对([Emim][Asn])进行理论研究. 通过几何结构优化和频率分析得到势能面上的五个稳定构型. [Emim]+和[Asn]-之间能够形成较强的氢键相互作用, 零点能校正后的能量在-373.96至-326.28 kJ·mol-1之间. 其稳定化能主要来源于[Asn]-中羰基O的孤对电子lp(O)与[Emim]+中C—H反键轨道σ*(C—H)之间的相互作用: lp(O)→σ*(C—H). 红外光谱特征和自然布居分析(NPA)计算表明咪唑阳离子中参与形成氢键的C—H键振动的红移值、阴阳离子间的电荷转移与氢键相互作用能成正比关系. 分子中的原子(AIM)理论分析得到[Emim]+和[Asn]-之间的氢键相互作用以静电作用为主. 通过计算结果初步探讨影响氨基酸离子液体玻璃化温度Tg的结构因素.     

反常蓝移单电子锂键Y…Li-CH3[Y=CH3, CH2CH3, CH(CH3)2, C(CH3)3]体系的结构与性质

在密度泛函理论B3LYP/6-311++G(d,p)及MP2/6-311++G(d,p)水平上研究了单电子锂键复合物Y…Li—CH3[Y=CH3, CH2CH3, CH(CH3)2, C(CH3)3]的结构与性质. 结果表明, 三种单电子锂键复合物H3CH2C…Li—CH3(II), (H3C)2HC…Li—CH3(III)和(H3C)3C…Li—CH3(IV)单电子锂键强度依II(-26.7 kJ·mol-1)相似文献   

CH3SS与OH自由基反应机理的理论研究

应用密度泛函理论(DFT)对CH3SS与OH自由基单重态反应机理进行了研究.在B3PW91/6-311+G(d,p)水平上优化了反应通道上各驻点(反应物、中间体、过渡态和产物)的几何构型,用内禀反应坐标(IRC)计算和频率分析方法对过渡态进行了验证.在QCISD(T)/6-311++G(d,p)水平上计算了各物种的单点能,并对总能量进行了零点能校正.研究结果表明,CH3SS与OH反应为多通道反应,有5条可能的反应通道.反应物首先通过不同的S—O键相互作用形成具有竞争反应机理的中间体IM1和IM2.再经过氢迁移、脱氢和裂解等机理得到主要产物P1(CH2SS+H2O),次要产物P2(CH2S+HSOH),P3(CH3SH+1SO)和P4(CH2SSO+H2),其中最低反应通道的势垒为174.6kJ.mol-1.     

Kinetic mechanism of methanol decomposition on Ni(111) surface: a theoretical study

The decomposition of methanol on the Ni(111) surface has been studied with the pseudopotential method of density functional theory-generalized gradient approximation (DFT-GGA) and with the repeated slab models. The adsorption energies of possible species and the activation energy barriers of the possible elementary reactions involved are obtained in the present work. The major reaction path on Ni surfaces involves the O-H bond breaking in CH(3)OH and the further decomposition of the resulting methoxy species to CO and H via stepwise hydrogen abstractions from CH(3)O. The abstraction of hydrogen from methoxy itself is the rate-limiting step. We also confirm that the C-O and C-H bond-breaking paths, which lead to the formation of surface methyl and hydroxyl and hydroxymethyl and atom hydrogen, respectively, have higher energy barriers. Therefore, the final products are the adsorbed CO and H atom.     

A first-principles study of methanol decomposition on Pt(111)

A periodic, self-consistent, Density Functional Theory study of methanol decomposition on Pt(111) is presented. The thermochemistry and activation energy barriers for all the elementary steps, starting with O[bond]H scission and proceeding via sequential hydrogen abstraction from the resulting methoxy intermediate, are presented here. The minimum energy path is represented by a one-dimensional potential energy surface connecting methanol with its final decomposition products, CO and hydrogen gas. It is found that the rate-limiting step for this decomposition pathway is the abstraction of hydroxyl hydrogen from methanol. CO is clearly identified as a strong thermodynamic sink in the reaction pathway while the methoxy, formaldehyde, and formyl intermediates are found to have low barriers to decomposition, leading to very short lifetimes for these intermediates. Stable intermediates and transition states are found to obey gas-phase coordination and bond order rules on the Pt(111) surface.     

盐对甲醇微观结构的影响

利用拉曼光谱研究盐对甲醇微观结构的影响.比较了不同盐/甲醇体系的O—H伸缩谱段和C—O伸缩谱段的超额拉曼光谱,对比给出了阴、阳离子与甲醇的相互作用.O—H伸缩谱段的超额拉曼光谱明显地显示了阴离子与甲醇形成弱氢键,氢键强度排序为CH3OH-CH3OHCl--CH3OHNO3--CH3OHClO4--CH3OH,在这个波段内,基本观察不到阳离子与甲醇的相互作用.在C—O伸缩谱段内,阴阳离子均有显著的体现,且与它们作用的甲醇C—O伸缩振动频率有如下的关系:CH3—OH(阴离子)CH3—OH(体相)CH3—OH(阳离子).根据C—O伸缩谱段的超额拉曼光谱,拟合了该谱段的拉曼光谱,由分解的谱峰强度得到阴、阳离子第一溶剂化层中甲醇分子的数目,结果显示在该浓度(～0.005)下离子对第一溶剂化层以外的甲醇氢键网络结构没有明显影响.     

Imaging adsorbate O-H bond cleavage: methanol on TiO2(110)

We investigated methanol adsorption and dissociation on bridge-bonded oxygen vacancies of the TiO2(110)-(1x1) surface using in situ scanning tunneling microscopy. We provide the first direct evidence that methanol dissociates on oxygen vacancies via O-H bond scission rather than C-O scission. For CH3OH coverages lower than the oxygen vacancy concentration, stationary methoxy-hydroxyl pairs form. At CH3OH coverages close to the oxygen vacancy concentration undissociated mobile CH3OH interacts with methoxy-hydroxyl pairs and facilitates the movement of hydroxyl away from the methoxy group.     

Theoretical Study on the Adsorption and Decomposition of Methanol over the Pt-Mo（111）/C Surface

The density functional theory(DFT) and self-consistent periodic calculation were used to investigate the methanol adsorption on the Pt-Mo(111)/C surface.The adsorption energies,equilibrium geometries and vibration frequencies of CH3OH on nine types of sites on the Pt-Mo(111)/C surface were predicted and the favorite adsorption site for methanol is the top-Pt site.Both sites of valence and conduction bands of doped system have been broadened,which are favorable for electrons to transfer to the cavity.The possible decomposition pathway was investigated with transition state searching and the calculation results indicate that the O-H bond is first broken,and then the methanol decomposes into methoxy.The activation barrier of O-H bond breaking with Pt-Mo catalyst is only 104.8 kJ mol-1,showing that carbon supported Pt-Mo alloys have promoted the decomposition of methanol.Comparing with the adsorption energies of CH3OH on the Pt(111)/C surface and that of CO,the adsorption energies of CO are higher,and Pt(111)/C is liable to be oxidized and loses the activity,which suggests that the catalyst Pt-Mo(111)/C is in favor of decomposing methanol and has better anti-poisoning ability than Pt(111)/C.     

How the C-O bond breaks during methanol decomposition on nanocrystallites of palladium catalysts

Experimental findings imply that edge sites (and other defects) on Pd nanocrystallites exposing mainly (111) facets in supported model catalysts are crucial for catalyst modification via deposition of CH(x) (x = 0-3) byproducts of methanol decomposition. To explore this problem computationally, we applied our recently developed approach to model realistically metal catalyst particles as moderately large three-dimensional crystallites. We present here the first results of this advanced approach where we comprehensively quantify the reactivity of a metal catalyst in an important chemical process. In particular, to unravel the mechanism of how CH(x) species are formed, we carried out density functional calculations of C-O bond scission in methanol and various dehydrogenated intermediates (CH3O, CH2OH, CH2O, CHO, CO), deposited on the cuboctahedron model particle Pd79. We calculated the lowest activation barriers, approximately 130 kJ mol(-1), of C-O bond breaking and the most favorable thermodynamics for the adsorbed species CH3O and CH2OH which feature a C-O single bond. In contrast, dissociation of adsorbed CO was characterized as negligibly slow. From the computational result that the decomposition products CH3 and CH2 preferentially adsorb at edge sites of nanoparticles, we rationalize experimental data on catalyst poisoning.     

Kinetics of the reaction of methyl radical with hydroxyl radical and methanol decomposition

The CH3 + OH bimolecular reaction and the dissociation of methanol are studied theoretically at conditions relevant to combustion chemistry. Kinetics for the CH3 + OH barrierless association reaction and for the H + CH2OH and H + CH3O product channels are determined in the high-pressure limit using variable reaction coordinate transition state theory and multireference electronic structure calculations to evaluate the fragment interaction energies. The CH3 + OH --> 3CH2 + H2O abstraction reaction and the H2 + HCOH and H2 + H2CO product channels feature localized dynamical bottlenecks and are treated using variational transition state theory and QCISD(T) energies extrapolated to the complete basis set limit. The 1CH2 + H2O product channel has two dynamical regimes, featuring both an inner saddle point and an outer barrierless region, and it is shown that a microcanonical two-state model is necessary to properly describe the association rate for this reaction over a broad temperature range. Experimental channel energies for the methanol system are reevaluated using the Active Thermochemical Tables (ATcT) approach. Pressure dependent, phenomenological rate coefficients for the CH3 + OH bimolecular reaction and for methanol decomposition are determined via master equation simulations. The predicted results agree well with experimental results, including those from a companion high-temperature shock tube determination for the decomposition of methanol.     

Effect of steps on the decomposition of CH3O at PdZn alloy surfaces

The decomposition of methoxide (CH(3)O) on a PdZn alloy is considered to be the rate-limiting step of steam re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir 2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-free flat (111) and (100) PdZn surfaces to promote C-H or C-O bond breaking of CH(3)O. Thus, we applied the same DF periodic slab-model approach to investigate these two routes of CH(3)O decomposition on PdZn(221) surfaces that expose Pd, (221)(Pd), and Zn, (221)(Zn), steps. C-H bond cleavage of CH(3)O is greatly facilitated on (221)(Pd): the calculated activation energy is dramatically reduced, to approximately 50 kJ mol(-1) from approximately 90 kJ mol(-1) on flat PdZn surfaces, increasing the rate constant by a factor of 10(8). The lower barrier is mainly due to a weaker interaction of the reactant CH(3)O and an enhanced interaction of the product CH(2)O with the substrate. The activation energy for C-O bond scission did not decrease on the (221)(Pd) step. On the (221)(Zn) step, the calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the stronger adsorption of CH(3)O. Steps (and other defects) appear to be crucial for methanol steam re-forming on Pd/ZnO catalyst; the stepped surface PdZn(221)(Pd) is a realistic model for studying the reactivity of this catalyst.