金属铂表面复杂脱氢氧化反应机理的理论研究
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摘要
自从密度泛函理论(Density Functional Theory, DFT)建立以来,它就被广泛应用于金属表面各种简单催化反应的机理研究当中,例如氢气在过渡金属表面的解离反应,CO和NO的氧化与还原反应等。然而,应用周期性密度泛函方法开展对金属表面复杂脱氢氧化反应机理的理论研究是近年来刚刚兴起的领域,目前的可见报道的研究还很少。作为一个典型的例子,有机小分子如甲醇、乙醇以及甲酸等在过渡金属表面的脱氢氧化反应是目前实验研究中广泛涉及到的热点非均相催化过程。这主要是因为甲醇、乙醇以及甲酸作为理想的氢的载体,在醇类重整制氢体系以及直接燃料电池反应体系中有着广阔的应用,对于缓解当前的能源危机和环境问题具有重要的意义。目前,关于这两个反应体系的研究均处于机理研究和催化剂探索的初步研究阶段。因此,开展醇类以及甲酸在过渡金属表面的催化氧化机理的理论研究不仅可以进一步拓展密度泛函方法的应用领域,开发DFT方法在处理金属表面复杂催化反应体系的巨大潜力,而且可以加深人们对反应过程的微观机理认识,为实验探索新型有效的催化剂提供重要的理论线索。
由于反应机理的复杂性加上实验手段的局限性,目前人们对于醇类重整制氢反应以及直接燃料电池反应过程的机理认识还不够深入,尤其是对于反应物的C-C键断裂模式以及决定反应选择性的关键因素了解很少。因此,本论文的主要工作是以乙醇和甲酸(在有水存在条件下)的脱氢氧化反应作为模型反应体系,以电化学实验为背景,使用第一性原理的密度泛函计算方法,研究过渡金属铂表面的复杂脱氢氧化反应的完全氧化反应机理。在此基础上,重点围绕复杂脱氢反应体系的反应选择性问题研究了以下几个方面的内容：
由特殊反应途径决定的反应选择性：从原子水平上来看,反应选择性是不同反应通道之间的速率比较结果,是微观反应动力学的宏观表现。因此,反应选择性与反应发生的微观过程密切相关。据文献报道,在乙醇电化学氧化过程中,不完全脱氢产物乙醛的选择性很高,而且不随反应条件(如电压)的变化而变化。这是制约直接乙醇燃料电池实际能量效率的关键因素。传统的逐步脱氢反应机理并不能合理的解释乙醛的高选择性问题,尤其是乙醛的选择性始终高于它的下游氧化产物乙酸的问题。因此,在本论文中我们首先对乙醇生成乙醛和乙酸的过程进行了研究,全面考察了乙醇生成乙醛的多种反应通道,通过对不同反应路径的反应速率的比较,并考虑了表面其它共吸附物种对反应活性影响,我们发现乙醇氧化生成乙醛的过程是通过一步脱两个氢的协同反应机理实现的。这种特殊的协同脱氢反应通道与反应物分子的构型有关,在金属表面催化过程中并不多见。它的存在合理的解释了乙醇电氧化反应中乙醛的高选择性问题。
反应选择性之表面结构敏感性特点：如何提高目标产物的选择性是化学研究的首要问题。因此,对于反应选择性的研究是机理研究中的重中之重。大量实验研究表明,催化剂表面结构是影响反应活性和反应选择性的一个重要因素。电化学实验结果显示,乙醇在传统Pt电极上主要发生部分氧化反应生成乙醛和乙酸,而完全氧化产物二氧化碳很少；当加入Sn之后,可以进一步促进C2产物(含有两个C原子)的生成,但CO2的选择性仍然很低；在单晶Pt电极如Pt(557),Pt(335)上,随着电极表面阶梯位的增多,乙酸的选择性明显降低；当电极表面有高密度的(100)面时,CO2的选择性明显增强。这些结果表明乙醇的反应选择性与电极表面的微观结构密切相关。
本论文即以乙醇的脱氢氧化反应为例,系统的考察了乙醇在铂的不同晶面(如Pt(111),Pt(211)及Pt(100)等)上的完全脱氢氧化机理。通过密度泛函理论计算,我们发现了决定乙醇反应选择性的两个关键步骤,即乙醇的第一步脱氢反应和乙酰基(CH3CO)的氧化反应。前者主要决定了乙醇生成乙醛的选择性问题,而后者主要决定了乙酸和CO2的选择性问题。这两个反应步骤在不同配位数的Pt表面的反应行为不同。在Pt(111)表面,乙醇主要发生部分氧化生成乙醛和乙酸,而在低配位的Pt(211)和Pt(100)表面则主要发生连续的碳氢断键反应,并最终发生C-C键断裂生成C1物种。通过电子结构分析和对关键反应能垒的解析,我们找出了导致乙醇氧化反应选择性随表面结构的不同而变化的物理原因：ⅰ)Δd-PDOS分析结果显示,α-脱氢反应过渡态与表面有较强的成键相互作用,从而导致反应能垒随表面结构的改变有明显的变化；ii)在低配位的Pt(211)和Pt(100)表面,OH由于在桥位吸附太稳定,导致对CH3CO的氧化能力降低,从而抑制了乙酸的生成。在此研究基础上,我们还指出所涉及到的反应之间存在Bronsted-Evans-Polanyi (BEP)线性关系,而且这种关系只与参与反应的化学键的极性有关(如C-C键,C-H键等),与Pt的晶面类型无关。应用这种BEP线性关系,可以很好的预测碳碳解离反应的断键模式,包括反应的活性位点和活性前驱体等。
反应选择性与反应物吸附构型和溶剂化作用的关系：溶剂化作用对反应活性和反应选择性有着重要影响。关于非均相催化反应的溶剂化效应的研究,尤其是水／金属界面反应的理论研究,一直是理论研究界的研究难点。作为一个典型的例子,甲酸的脱氢氧化反应在超高真空实验条件下和电化学环境下的反应行为截然不同,这说明水溶液对甲酸的脱氢氧化反应有着重要影响。但是目前,实验研究者对于甲酸的电催化氧化反应机理还没有统一的认识,对于反应过程中甲酸根中间体所起的作用,究竟是反应的活性中间体还是电极表面的中毒物种,还存在着很大的分歧。采用密度泛函计算方法并结合一种离散-连续组合溶剂化模型,我们对甲酸在干净的Pt(111)/H2O界面以及甲酸根(HCOO)覆盖的Pt(111)/H2O界面的吸附和反应行为进行了研究,发现当甲酸从水相转移到Pt/H2O界面时,损失了大量的溶剂化能△Esol,而△Esol正是影响甲酸吸附构型的关键因素。我们指出ⅰ)甲酸在金属/水的界面主要经过直接反应通道生成CO2,它的反应活性前驱体是以CH键垂直指向表面(CH-down构型)吸附的溶剂化甲酸分子；水在甲酸的直接氧化反应通道中起了重要的作用;ⅱ)甲酸根的存在促进了活性前驱体CH-down构型在Pt表面的吸附,从而有利于甲酸的直接氧化。主要原因是甲酸根改变了水在金属界面的分布方式,从而降低了CH-down构型吸附过程中所要消耗的溶剂化能。
反应选择性与表面修饰之间的关系：在电极表面修饰上其它物种或者金属原子是改善电极催化活性的一种有效方法。大量的电化学实验证实,当Pt电极表面有Sb修饰时,能够有效的抑制甲酸的解离吸附(生成CO),提高甲酸直接氧化生成CO2的反应选择性。有研究者认为,Sb的作用主要是通过几何位阻效应阻止了中毒物种CO在Pt表面的生成。然而,单纯的几何位阻效应并不能完全解释甲酸在Sb/Pt电极上的高反应活性。我们研究得知,甲酸生成CO2的反应选择性与甲酸在金属表面的吸附构型密切相关。因此本节中我们应用密度泛函理论方法对比了甲酸(有水存在下)在洁净的Pt(111)及不同Sb覆盖度的Sb/Pt (111)表面上的吸附行为。我们发现,当Sb覆盖度增加到一定程度(如0.375 ML),并在表面形成二维岛状结构(2D-island)时,甲酸在铂表面主要以CH-down构型吸附。Bader电荷分析结果显示,Sb的修饰作用主要是转移了部分电子给Pt表面,与表面Pt原子之间形成垂直于表面的静电偶极矩Sbδ+-Ptδ-。正是这个静电偶极矩的存在促使了甲酸的活性前驱体CH-down构型在Pt表面的吸附,从而有利于提高甲酸的电化学活性。我们的计算结果得到了实验的大力支持。
另外,鉴于传统过渡态搜索方法对于研究复杂脱氢氧化反应体系所存在的局限性,本论文在原有方法的基础上,发展了一种新的过渡态的搜索方法。新方法继承了Quasi-Newton Broyden几何优化算法的优点,加快了电子结构的收敛速度,使得短时间内建立复杂反应的完全反应势能面成为可能。此外,我们还建立了一种周期性连续介质溶剂化模型来研究金属/水界面的溶剂化效应,采用离散-连续组合方法(combined discrete-continnum method)模拟体系的溶剂化层,即选取一定数目(四个或六个)的溶剂分子与反应物分子形成超分子,来模拟反应物分子的最近溶剂化层,而对剩余的其它溶剂化层用文献报道的介电模型函数(dielectric model function)来描述。
With the advent of Density Functional Theory (DFT), it has shown a powerful functionality in clarifying mechanisms of simple surface reactions, such as H2 dissociation, NO or CO reduction/oxidation on transition metal surfaces and so on. However, exploring mechanisms of complex dehydrogenation reaction networks at transition metals with DFT is an emerging area in the recent years, and very limited research work has been reported. As a typical example, oxidation of small organic molecules like methanol, ethanol and formic acid on transition metals is widely involved in many important heterogeneous catalytic processes, because they are of great interest in alcohol reforming processes and fuel cell applications as ideal hydrogen carrier, which is important in relieving the current energy crisis and environmental pressures. At present, experimental studies on the alcohol reforming processes and direct alcohol/acid fuel cells are at the early stages in clarifying reaction mechanisms and screening proper catalysts. It is therefore of both theoretical and practical interest in investigating mechanisms of alcohol and formic acid dehydrogenation on transition metal surfaces applying with DFT calculations, which not only can broaden the DFT functionality in dealing with more complex reaction network on metal surfaces, but also can help to understand the atomic-level reaction mechanisms and provide important clues for choosing new efficient metal catalysts.
Due to the complexity of reaction networks and limitation of experimental methods, there is inadequate knowledge about mechanisms of alcohol reforming for hydrogen production and fuel cell reactions, especially for the pattern of C-C bond breaking and the key factors in dictating reaction selectivity. Therefore, in the present work, we take the dehydrogenation of ethanol and formic acid (in the presence of water) as model systems and comprehensively investigate the complex dehydrogenation reaction networks on platinum surfaces within DFT frameworks mainly in the context of electro-oxidation experiments. On this basis, we put emphasis on the following contents which are related to reaction selectivity.
Selectivity determined by unique reaction pathways:From an atomic-level view, reaction selectivity is a comparison result of the rates between different reaction channels. It is macro-performance of reaction microkinetics. Apparently, reaction selectivity is closely related to the micro-reaction processes. It has been reported that in ethanol electro-oxidation, ethanol is mainly partially oxidized into acetaldehyde and acetic acid instead of being fully oxidized and the highest selectivity to CH3CHO is observed to be slightly affected by electric potential. This undesired selectivity severely limits the practical energy efficiency of direct ethanol fuel cells (DEFCs). The traditionally proposed stepwise dehydrogenation mechanism can not rationalize the high selectivity of CH3CHO, and no answer for the issue that selectivity to CH3CHO is always higher than that of its oxidation product CH3COOH. By extensive density functional theory calculations on the distinct reaction channels from ethanol to acetaldehyde and acetic acid on Pt(111), the rate constants of the different reaction pathways were compared and effect of the coadsorbates were considered. We demonstrated that ethanol is partially oxidized into CH3CHO via a unique concerted dehydrogenation path which is rarely occurred in surface reactions. Such a concerted path is shown to be largely affected by the reactants'structures and its existence can well explain the observed high selectivity to acetaldehyde.
Structure sensitivity of reaction selectivity:How to selectively activate chemical bond towards making desired product must rank the top concern in chemistry. Research on reaction selectivity is the key point in the mechanistic studies. Amounts of experiments have shown that reaction selectivity and activity is largely affected by the surface structure of metal catalysts. For example, in ethanol electro-oxidation with traditional Pt catalysts, the observed main products are CH3CHO and CH3COOH and the complete oxidation product CO2 is very low; addition of Sn to Pt can accelerate the production of C2 products (species containing 2-C atoms) while the selectivity to CO2 is little affected; on the stepped surfaces like Pt(557) and Pt(335), it was found that the selectivity to acetic acid decreased with the increasing surface steps; when the electrode catalysts composed of high density of (100) terraces, the selectivity to CO2 was greatly enhanced. All these experimental facts indicate that selectivity of ethanol oxidation on platinum is surface structure sensitive.
Therefore, in the present work, we performed an extensive investigations on the whole reaction network of the complete oxidation of ethanol at different Pt surfaces, including Pt(111), Pt(211) and Pt(100). Two critical steps in dictating the selectivity of ethanol oxidation were clarified, namely the initial dehydrogenation of ethanol and the oxidation of the acetyl (CH3CO) intermediate. The former mainly determines the selectivity to CH3CHO and the latter determines the selectivity to CH3COOH and CO2. These two selectivity-determining steps have distinct behaviors on differently coordinated Pt surfaces. On Pt(111) surface, ethanol is mainly oxidized into CH3CHO and CH3COOH, while on Pt(211) and Pt(100), ethanol mainly proceeds consecutive C-H bond breaking and finally via C-C bond splitting into C1 species. By detailed electronic structure analysis and barrier decomposition, we clarified the physical origin of the surface structure-sensitivity of the reaction selectivity:i)Δd-PDOS results show that transition state of the a-dehydrogenation has strong bonding interaction with surface Pt atom which can account for the surface structure-sensitivity of the a-dehydrogenation barrier; ii) on low coordinated Pt(211) and Pt(100), hydroxyl (OH) bonds at bridge sites too strongly to oxidize other species like acetyl, which suppresses the formation of acetic acid. Besides, we found that a linear Bronsted-Evans-Polanyi (BEP) relation holds for different bond-breaking reactions across different surfaces, which is only related to the type of bond (say, C-C, C-H etc.) but not related to the surface structures. Applied with this BEP relationship, it can qualitatively predict the general pattern in C-C bond dissociation reactions, such as reaction sites and the reaction precursor.
Relationships of selectivity with reactant adsorption configurations and solvation effect:water solvation has important influences on reaction activity and selectivity. It has been a long-standing challenge for theorists to investigate the solvation effect on the heterogeneous catalytic processes within DFT framework, especially simulations at water/metal interfaces. As a typical example, formic acid shows distinct degradation behaviors under the ultra-high vacuum conditions and electro-oxidation environment. This indicates that the aqueous environment plays an important role in formic acid oxidation. However, there is still no consensus about the reaction mechanism of formic acid electro-oxidation under water solution at the present, and the role of the intermediate formate (HCOO), whether an active intermediate or a spectator species, is still open to discussion. Combined DFT calculations with a discrete-continuum solvation model, we extensively studied the adsorption and reaction behaviors of formic acid on both clean Pt(111)/H2O interface and formate covered Pt(111)/H2O interface. We found that during the adsorption processes in which formic acid transfers from bulk phase to Pt/H2O interfaces, it cost lots of solvation energiesΔEsol, which have important influences on the adsorption configurations of formic acid at the metal/water interfaces. It was concluded that i) formic acid is directly oxidized into CO2 with the reactive precursor of CH-down configured formic acid, and water plays a key role in the direct oxidation pathway; ii) the presence of formate promotes the adsorption of reactive CH-down configuration on Pt sites, which is beneficial to the direct oxidation of formic acid. The physical reason is that the adsorbed formate results in rearrangement of the H-bonding network of water at the water/metal interface, which then leads to reduction of the solvation energy cost during the adsorption of CH-down configuration.
Relationships between selectivity and surface modification:Modification of Pt with other species and secondary metal atoms is considered to be an efficient way to improve the catalytic activity of electrodes in electro-chemistry. It was generally reported that modification of Sb on Pt (Sb/Pt) hindered the dissociative adsorption of formic acid (leading to CO formation) and promoted the direct oxidation of formic acid into CO2. While some one proposed that the function of Sb on Pt can be attributed to the steric effect which blocks the surface active sites for CO formation, it is still elusive for the high reactivity of formic acid on Sb/Pt electrodes. As we reported, the selectivity of formic acid direct oxidation to CO2 is greatly related to the adsorption configurations. Therefore, we then compared the adsorption behaviors of formic acid (in the presence of water) on clean Pt(111) and various Sb-covered Pt(111). It was found that as the Sb coverage achieves 0.375 ML and forms a 2D island structure on Pt(111), the CH-down configuration becomes the dominant species on the surface, which will benefit the direct oxidation of formic acid into CO2. According to Bader charge analysis, Sb adlayer donates some electron to Pt surface and leads to the formation of a Sbδ+-Ptδ- dipole normal to the surface. It is such an electrostatic dipole that enhances the adsorption energy of CH-down configuration on Pt surfaces and thus the reactivity of formic acid direct oxidation. Our mechanisms get well supported by experimental facts.
Besides, considering the limitation of the traditional transition state (TS) searching method in dealing with complex dehydrogenation reaction networks, we developed a new TS searching method. This new method inherits the advantages of Quasi-Newton Broyden optimization in the searching processes with a significant speed-up in convergence. It allowed us to study explicitly the whole reaction network of complex dehydrogenation systems in reduced computational time. Besides, we built a periodic continuum solvation model to investigate the solvation effect on reactions at metal/water interfaces. We applied a discrete-continuum method to simulate the solvation shells of reaction systems, namely, we surrounded the reaction centre with up to six discrete water molecules as the core solvation shells and simulated the rest of solvation shells with a reported dielectric model function in a continuum solvation model.
由于反应机理的复杂性加上实验手段的局限性,目前人们对于醇类重整制氢反应以及直接燃料电池反应过程的机理认识还不够深入,尤其是对于反应物的C-C键断裂模式以及决定反应选择性的关键因素了解很少。因此,本论文的主要工作是以乙醇和甲酸(在有水存在条件下)的脱氢氧化反应作为模型反应体系,以电化学实验为背景,使用第一性原理的密度泛函计算方法,研究过渡金属铂表面的复杂脱氢氧化反应的完全氧化反应机理。在此基础上,重点围绕复杂脱氢反应体系的反应选择性问题研究了以下几个方面的内容：
由特殊反应途径决定的反应选择性：从原子水平上来看,反应选择性是不同反应通道之间的速率比较结果,是微观反应动力学的宏观表现。因此,反应选择性与反应发生的微观过程密切相关。据文献报道,在乙醇电化学氧化过程中,不完全脱氢产物乙醛的选择性很高,而且不随反应条件(如电压)的变化而变化。这是制约直接乙醇燃料电池实际能量效率的关键因素。传统的逐步脱氢反应机理并不能合理的解释乙醛的高选择性问题,尤其是乙醛的选择性始终高于它的下游氧化产物乙酸的问题。因此,在本论文中我们首先对乙醇生成乙醛和乙酸的过程进行了研究,全面考察了乙醇生成乙醛的多种反应通道,通过对不同反应路径的反应速率的比较,并考虑了表面其它共吸附物种对反应活性影响,我们发现乙醇氧化生成乙醛的过程是通过一步脱两个氢的协同反应机理实现的。这种特殊的协同脱氢反应通道与反应物分子的构型有关,在金属表面催化过程中并不多见。它的存在合理的解释了乙醇电氧化反应中乙醛的高选择性问题。
反应选择性之表面结构敏感性特点：如何提高目标产物的选择性是化学研究的首要问题。因此,对于反应选择性的研究是机理研究中的重中之重。大量实验研究表明,催化剂表面结构是影响反应活性和反应选择性的一个重要因素。电化学实验结果显示,乙醇在传统Pt电极上主要发生部分氧化反应生成乙醛和乙酸,而完全氧化产物二氧化碳很少；当加入Sn之后,可以进一步促进C2产物(含有两个C原子)的生成,但CO2的选择性仍然很低；在单晶Pt电极如Pt(557),Pt(335)上,随着电极表面阶梯位的增多,乙酸的选择性明显降低；当电极表面有高密度的(100)面时,CO2的选择性明显增强。这些结果表明乙醇的反应选择性与电极表面的微观结构密切相关。
本论文即以乙醇的脱氢氧化反应为例,系统的考察了乙醇在铂的不同晶面(如Pt(111),Pt(211)及Pt(100)等)上的完全脱氢氧化机理。通过密度泛函理论计算,我们发现了决定乙醇反应选择性的两个关键步骤,即乙醇的第一步脱氢反应和乙酰基(CH3CO)的氧化反应。前者主要决定了乙醇生成乙醛的选择性问题,而后者主要决定了乙酸和CO2的选择性问题。这两个反应步骤在不同配位数的Pt表面的反应行为不同。在Pt(111)表面,乙醇主要发生部分氧化生成乙醛和乙酸,而在低配位的Pt(211)和Pt(100)表面则主要发生连续的碳氢断键反应,并最终发生C-C键断裂生成C1物种。通过电子结构分析和对关键反应能垒的解析,我们找出了导致乙醇氧化反应选择性随表面结构的不同而变化的物理原因：ⅰ)Δd-PDOS分析结果显示,α-脱氢反应过渡态与表面有较强的成键相互作用,从而导致反应能垒随表面结构的改变有明显的变化；ii)在低配位的Pt(211)和Pt(100)表面,OH由于在桥位吸附太稳定,导致对CH3CO的氧化能力降低,从而抑制了乙酸的生成。在此研究基础上,我们还指出所涉及到的反应之间存在Bronsted-Evans-Polanyi (BEP)线性关系,而且这种关系只与参与反应的化学键的极性有关(如C-C键,C-H键等),与Pt的晶面类型无关。应用这种BEP线性关系,可以很好的预测碳碳解离反应的断键模式,包括反应的活性位点和活性前驱体等。
反应选择性与反应物吸附构型和溶剂化作用的关系：溶剂化作用对反应活性和反应选择性有着重要影响。关于非均相催化反应的溶剂化效应的研究,尤其是水／金属界面反应的理论研究,一直是理论研究界的研究难点。作为一个典型的例子,甲酸的脱氢氧化反应在超高真空实验条件下和电化学环境下的反应行为截然不同,这说明水溶液对甲酸的脱氢氧化反应有着重要影响。但是目前,实验研究者对于甲酸的电催化氧化反应机理还没有统一的认识,对于反应过程中甲酸根中间体所起的作用,究竟是反应的活性中间体还是电极表面的中毒物种,还存在着很大的分歧。采用密度泛函计算方法并结合一种离散-连续组合溶剂化模型,我们对甲酸在干净的Pt(111)/H2O界面以及甲酸根(HCOO)覆盖的Pt(111)/H2O界面的吸附和反应行为进行了研究,发现当甲酸从水相转移到Pt/H2O界面时,损失了大量的溶剂化能△Esol,而△Esol正是影响甲酸吸附构型的关键因素。我们指出ⅰ)甲酸在金属/水的界面主要经过直接反应通道生成CO2,它的反应活性前驱体是以CH键垂直指向表面(CH-down构型)吸附的溶剂化甲酸分子；水在甲酸的直接氧化反应通道中起了重要的作用;ⅱ)甲酸根的存在促进了活性前驱体CH-down构型在Pt表面的吸附,从而有利于甲酸的直接氧化。主要原因是甲酸根改变了水在金属界面的分布方式,从而降低了CH-down构型吸附过程中所要消耗的溶剂化能。
反应选择性与表面修饰之间的关系：在电极表面修饰上其它物种或者金属原子是改善电极催化活性的一种有效方法。大量的电化学实验证实,当Pt电极表面有Sb修饰时,能够有效的抑制甲酸的解离吸附(生成CO),提高甲酸直接氧化生成CO2的反应选择性。有研究者认为,Sb的作用主要是通过几何位阻效应阻止了中毒物种CO在Pt表面的生成。然而,单纯的几何位阻效应并不能完全解释甲酸在Sb/Pt电极上的高反应活性。我们研究得知,甲酸生成CO2的反应选择性与甲酸在金属表面的吸附构型密切相关。因此本节中我们应用密度泛函理论方法对比了甲酸(有水存在下)在洁净的Pt(111)及不同Sb覆盖度的Sb/Pt (111)表面上的吸附行为。我们发现,当Sb覆盖度增加到一定程度(如0.375 ML),并在表面形成二维岛状结构(2D-island)时,甲酸在铂表面主要以CH-down构型吸附。Bader电荷分析结果显示,Sb的修饰作用主要是转移了部分电子给Pt表面,与表面Pt原子之间形成垂直于表面的静电偶极矩Sbδ+-Ptδ-。正是这个静电偶极矩的存在促使了甲酸的活性前驱体CH-down构型在Pt表面的吸附,从而有利于提高甲酸的电化学活性。我们的计算结果得到了实验的大力支持。
另外,鉴于传统过渡态搜索方法对于研究复杂脱氢氧化反应体系所存在的局限性,本论文在原有方法的基础上,发展了一种新的过渡态的搜索方法。新方法继承了Quasi-Newton Broyden几何优化算法的优点,加快了电子结构的收敛速度,使得短时间内建立复杂反应的完全反应势能面成为可能。此外,我们还建立了一种周期性连续介质溶剂化模型来研究金属/水界面的溶剂化效应,采用离散-连续组合方法(combined discrete-continnum method)模拟体系的溶剂化层,即选取一定数目(四个或六个)的溶剂分子与反应物分子形成超分子,来模拟反应物分子的最近溶剂化层,而对剩余的其它溶剂化层用文献报道的介电模型函数(dielectric model function)来描述。
With the advent of Density Functional Theory (DFT), it has shown a powerful functionality in clarifying mechanisms of simple surface reactions, such as H2 dissociation, NO or CO reduction/oxidation on transition metal surfaces and so on. However, exploring mechanisms of complex dehydrogenation reaction networks at transition metals with DFT is an emerging area in the recent years, and very limited research work has been reported. As a typical example, oxidation of small organic molecules like methanol, ethanol and formic acid on transition metals is widely involved in many important heterogeneous catalytic processes, because they are of great interest in alcohol reforming processes and fuel cell applications as ideal hydrogen carrier, which is important in relieving the current energy crisis and environmental pressures. At present, experimental studies on the alcohol reforming processes and direct alcohol/acid fuel cells are at the early stages in clarifying reaction mechanisms and screening proper catalysts. It is therefore of both theoretical and practical interest in investigating mechanisms of alcohol and formic acid dehydrogenation on transition metal surfaces applying with DFT calculations, which not only can broaden the DFT functionality in dealing with more complex reaction network on metal surfaces, but also can help to understand the atomic-level reaction mechanisms and provide important clues for choosing new efficient metal catalysts.
Due to the complexity of reaction networks and limitation of experimental methods, there is inadequate knowledge about mechanisms of alcohol reforming for hydrogen production and fuel cell reactions, especially for the pattern of C-C bond breaking and the key factors in dictating reaction selectivity. Therefore, in the present work, we take the dehydrogenation of ethanol and formic acid (in the presence of water) as model systems and comprehensively investigate the complex dehydrogenation reaction networks on platinum surfaces within DFT frameworks mainly in the context of electro-oxidation experiments. On this basis, we put emphasis on the following contents which are related to reaction selectivity.
Selectivity determined by unique reaction pathways:From an atomic-level view, reaction selectivity is a comparison result of the rates between different reaction channels. It is macro-performance of reaction microkinetics. Apparently, reaction selectivity is closely related to the micro-reaction processes. It has been reported that in ethanol electro-oxidation, ethanol is mainly partially oxidized into acetaldehyde and acetic acid instead of being fully oxidized and the highest selectivity to CH3CHO is observed to be slightly affected by electric potential. This undesired selectivity severely limits the practical energy efficiency of direct ethanol fuel cells (DEFCs). The traditionally proposed stepwise dehydrogenation mechanism can not rationalize the high selectivity of CH3CHO, and no answer for the issue that selectivity to CH3CHO is always higher than that of its oxidation product CH3COOH. By extensive density functional theory calculations on the distinct reaction channels from ethanol to acetaldehyde and acetic acid on Pt(111), the rate constants of the different reaction pathways were compared and effect of the coadsorbates were considered. We demonstrated that ethanol is partially oxidized into CH3CHO via a unique concerted dehydrogenation path which is rarely occurred in surface reactions. Such a concerted path is shown to be largely affected by the reactants'structures and its existence can well explain the observed high selectivity to acetaldehyde.
Structure sensitivity of reaction selectivity:How to selectively activate chemical bond towards making desired product must rank the top concern in chemistry. Research on reaction selectivity is the key point in the mechanistic studies. Amounts of experiments have shown that reaction selectivity and activity is largely affected by the surface structure of metal catalysts. For example, in ethanol electro-oxidation with traditional Pt catalysts, the observed main products are CH3CHO and CH3COOH and the complete oxidation product CO2 is very low; addition of Sn to Pt can accelerate the production of C2 products (species containing 2-C atoms) while the selectivity to CO2 is little affected; on the stepped surfaces like Pt(557) and Pt(335), it was found that the selectivity to acetic acid decreased with the increasing surface steps; when the electrode catalysts composed of high density of (100) terraces, the selectivity to CO2 was greatly enhanced. All these experimental facts indicate that selectivity of ethanol oxidation on platinum is surface structure sensitive.
Therefore, in the present work, we performed an extensive investigations on the whole reaction network of the complete oxidation of ethanol at different Pt surfaces, including Pt(111), Pt(211) and Pt(100). Two critical steps in dictating the selectivity of ethanol oxidation were clarified, namely the initial dehydrogenation of ethanol and the oxidation of the acetyl (CH3CO) intermediate. The former mainly determines the selectivity to CH3CHO and the latter determines the selectivity to CH3COOH and CO2. These two selectivity-determining steps have distinct behaviors on differently coordinated Pt surfaces. On Pt(111) surface, ethanol is mainly oxidized into CH3CHO and CH3COOH, while on Pt(211) and Pt(100), ethanol mainly proceeds consecutive C-H bond breaking and finally via C-C bond splitting into C1 species. By detailed electronic structure analysis and barrier decomposition, we clarified the physical origin of the surface structure-sensitivity of the reaction selectivity:i)Δd-PDOS results show that transition state of the a-dehydrogenation has strong bonding interaction with surface Pt atom which can account for the surface structure-sensitivity of the a-dehydrogenation barrier; ii) on low coordinated Pt(211) and Pt(100), hydroxyl (OH) bonds at bridge sites too strongly to oxidize other species like acetyl, which suppresses the formation of acetic acid. Besides, we found that a linear Bronsted-Evans-Polanyi (BEP) relation holds for different bond-breaking reactions across different surfaces, which is only related to the type of bond (say, C-C, C-H etc.) but not related to the surface structures. Applied with this BEP relationship, it can qualitatively predict the general pattern in C-C bond dissociation reactions, such as reaction sites and the reaction precursor.
Relationships of selectivity with reactant adsorption configurations and solvation effect:water solvation has important influences on reaction activity and selectivity. It has been a long-standing challenge for theorists to investigate the solvation effect on the heterogeneous catalytic processes within DFT framework, especially simulations at water/metal interfaces. As a typical example, formic acid shows distinct degradation behaviors under the ultra-high vacuum conditions and electro-oxidation environment. This indicates that the aqueous environment plays an important role in formic acid oxidation. However, there is still no consensus about the reaction mechanism of formic acid electro-oxidation under water solution at the present, and the role of the intermediate formate (HCOO), whether an active intermediate or a spectator species, is still open to discussion. Combined DFT calculations with a discrete-continuum solvation model, we extensively studied the adsorption and reaction behaviors of formic acid on both clean Pt(111)/H2O interface and formate covered Pt(111)/H2O interface. We found that during the adsorption processes in which formic acid transfers from bulk phase to Pt/H2O interfaces, it cost lots of solvation energiesΔEsol, which have important influences on the adsorption configurations of formic acid at the metal/water interfaces. It was concluded that i) formic acid is directly oxidized into CO2 with the reactive precursor of CH-down configured formic acid, and water plays a key role in the direct oxidation pathway; ii) the presence of formate promotes the adsorption of reactive CH-down configuration on Pt sites, which is beneficial to the direct oxidation of formic acid. The physical reason is that the adsorbed formate results in rearrangement of the H-bonding network of water at the water/metal interface, which then leads to reduction of the solvation energy cost during the adsorption of CH-down configuration.
Relationships between selectivity and surface modification:Modification of Pt with other species and secondary metal atoms is considered to be an efficient way to improve the catalytic activity of electrodes in electro-chemistry. It was generally reported that modification of Sb on Pt (Sb/Pt) hindered the dissociative adsorption of formic acid (leading to CO formation) and promoted the direct oxidation of formic acid into CO2. While some one proposed that the function of Sb on Pt can be attributed to the steric effect which blocks the surface active sites for CO formation, it is still elusive for the high reactivity of formic acid on Sb/Pt electrodes. As we reported, the selectivity of formic acid direct oxidation to CO2 is greatly related to the adsorption configurations. Therefore, we then compared the adsorption behaviors of formic acid (in the presence of water) on clean Pt(111) and various Sb-covered Pt(111). It was found that as the Sb coverage achieves 0.375 ML and forms a 2D island structure on Pt(111), the CH-down configuration becomes the dominant species on the surface, which will benefit the direct oxidation of formic acid into CO2. According to Bader charge analysis, Sb adlayer donates some electron to Pt surface and leads to the formation of a Sbδ+-Ptδ- dipole normal to the surface. It is such an electrostatic dipole that enhances the adsorption energy of CH-down configuration on Pt surfaces and thus the reactivity of formic acid direct oxidation. Our mechanisms get well supported by experimental facts.
Besides, considering the limitation of the traditional transition state (TS) searching method in dealing with complex dehydrogenation reaction networks, we developed a new TS searching method. This new method inherits the advantages of Quasi-Newton Broyden optimization in the searching processes with a significant speed-up in convergence. It allowed us to study explicitly the whole reaction network of complex dehydrogenation systems in reduced computational time. Besides, we built a periodic continuum solvation model to investigate the solvation effect on reactions at metal/water interfaces. We applied a discrete-continuum method to simulate the solvation shells of reaction systems, namely, we surrounded the reaction centre with up to six discrete water molecules as the core solvation shells and simulated the rest of solvation shells with a reported dielectric model function in a continuum solvation model.
引文
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SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms)适用于分子和固体的电子结构计算和分子动力学模拟[80]。SIESTA使用标准的Kohn-Sham自洽密度泛函方法,结合局域密度近似(LDA-LSD)或广义梯度近似(GGA)。计算使用完全非局域形式(Keinman-Bylander)的标准模守恒赝势。基组是数值原子轨道的线性组合(LCAO)。它允许任意个角动量,多个zeta,极化和截断轨道。计算中把电子波函数和密度投影到实空间网格中,以计算Hartree和XC势,以及矩阵元素。除了标准的Rayleigh-Ritz本征态方法外,程序还允许使用占据轨道(价键或类Wannier函数)的局域线性组合,使得计算时间和内存与原子大小成线性标度,因而可以在一般的工作站上模拟几百个原子的体系,计算效率较高。
除了上面提到的几个程序外,还有多个用于周期性体系计算的软件,如PWSCF、 CPMD[81]、 DACAPO、Dmol3、 CP2K[82]、 CRYSTAL、 ONETEP[83-85]、 OPENMX[86]、 SeqQuest和ConQuest[87]等。详细的介绍可以参考各个程序的使用手册或在线介绍。
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从结构上看,甲醇和乙醇之间只差了一个甲基基团。然而,从脱氢氧化反应选择性上看,两者差别很大。所以我们可以认为,甲基是决定乙醇反应选择性的一个关键的结构因素。Mulliken电荷分析结果显示,在乙醇的协同脱氢反应过渡态中,[CH3HC=OH]基团带有部分正电荷([CH3HC-OH]δ+)。这可能是由于过渡态中形成了部分分子内双键,导致羟基氢原子部分质子化所造成的。显然,对比[CH3HC=OH]δ+和[H2C=OH]δ+可知,甲基供电子作用有利于稳定带正电的过渡态[CH3HC=OH]δ+,因此使得乙醇的协同反应途径能垒降低。而在甲醇的协同反应过程中却不存在这种作用。
本章采用周期性密度泛函方法研究了乙醇在Pt(111)表面的脱氢氧化反应过程,得到了乙醇氧化生成乙醛进而生成乙酸的反应势能面。重点研究了乙醇脱氢氧化生成乙醛的三种不同反应通道,定量计算了它们的反应速率常数,并考察了其它共吸附物种如H和H2O的存在对反应活性和选择性的影响。结果显示,乙醇氧化生成乙醛的过程是通过一步脱两个氢的协同反应机理实现的,而且这种协同反应途径不受电极表面其它吸附物种的影响。
在金属表面催化领域,这是首次通过理论计算的方法确定了协同脱氢反应过渡态。它的存在合理的解释了乙醇电氧化反应中产物乙醛的高选择性问题。我们的反应机理解释了很多实验现象,计算结果得到众多实验事实的支持。通过Mulliken电荷分析以及与甲醇在Pt(111)表面的脱氢氧化反应进行对比结果显示,乙醇中甲基基团的存在是稳定乙醇协同脱氢反应过渡态的重要因素。这说明在实验设计中,可以通过改变反应物的配体构型或者改变金属催化剂的组成(将催化剂表面视为反应过渡态的一个配体)来改善醇类的氧化反应选择性,从而达到提高醇类燃料电池氧化性能的目的。[1] Lynd LR. Overview and evaluation of fuel ethanol from cellulosic biomass: Technology, economics, the environment, and policy [J]. Annu. Rev. Energ. Environ.,1996,21:403-465. [2] Vigier F, Coutanceau C, Perrard A, Belgsir EM, Lamy C. Development of
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SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms)适用于分子和固体的电子结构计算和分子动力学模拟[80]。SIESTA使用标准的Kohn-Sham自洽密度泛函方法,结合局域密度近似(LDA-LSD)或广义梯度近似(GGA)。计算使用完全非局域形式(Keinman-Bylander)的标准模守恒赝势。基组是数值原子轨道的线性组合(LCAO)。它允许任意个角动量,多个zeta,极化和截断轨道。计算中把电子波函数和密度投影到实空间网格中,以计算Hartree和XC势,以及矩阵元素。除了标准的Rayleigh-Ritz本征态方法外,程序还允许使用占据轨道(价键或类Wannier函数)的局域线性组合,使得计算时间和内存与原子大小成线性标度,因而可以在一般的工作站上模拟几百个原子的体系,计算效率较高。
除了上面提到的几个程序外,还有多个用于周期性体系计算的软件,如PWSCF、 CPMD[81]、 DACAPO、Dmol3、 CP2K[82]、 CRYSTAL、 ONETEP[83-85]、 OPENMX[86]、 SeqQuest和ConQuest[87]等。详细的介绍可以参考各个程序的使用手册或在线介绍。
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从结构上看,甲醇和乙醇之间只差了一个甲基基团。然而,从脱氢氧化反应选择性上看,两者差别很大。所以我们可以认为,甲基是决定乙醇反应选择性的一个关键的结构因素。Mulliken电荷分析结果显示,在乙醇的协同脱氢反应过渡态中,[CH3HC=OH]基团带有部分正电荷([CH3HC-OH]δ+)。这可能是由于过渡态中形成了部分分子内双键,导致羟基氢原子部分质子化所造成的。显然,对比[CH3HC=OH]δ+和[H2C=OH]δ+可知,甲基供电子作用有利于稳定带正电的过渡态[CH3HC=OH]δ+,因此使得乙醇的协同反应途径能垒降低。而在甲醇的协同反应过程中却不存在这种作用。
本章采用周期性密度泛函方法研究了乙醇在Pt(111)表面的脱氢氧化反应过程,得到了乙醇氧化生成乙醛进而生成乙酸的反应势能面。重点研究了乙醇脱氢氧化生成乙醛的三种不同反应通道,定量计算了它们的反应速率常数,并考察了其它共吸附物种如H和H2O的存在对反应活性和选择性的影响。结果显示,乙醇氧化生成乙醛的过程是通过一步脱两个氢的协同反应机理实现的,而且这种协同反应途径不受电极表面其它吸附物种的影响。
在金属表面催化领域,这是首次通过理论计算的方法确定了协同脱氢反应过渡态。它的存在合理的解释了乙醇电氧化反应中产物乙醛的高选择性问题。我们的反应机理解释了很多实验现象,计算结果得到众多实验事实的支持。通过Mulliken电荷分析以及与甲醇在Pt(111)表面的脱氢氧化反应进行对比结果显示,乙醇中甲基基团的存在是稳定乙醇协同脱氢反应过渡态的重要因素。这说明在实验设计中,可以通过改变反应物的配体构型或者改变金属催化剂的组成(将催化剂表面视为反应过渡态的一个配体)来改善醇类的氧化反应选择性,从而达到提高醇类燃料电池氧化性能的目的。[1] Lynd LR. Overview and evaluation of fuel ethanol from cellulosic biomass: Technology, economics, the environment, and policy [J]. Annu. Rev. Energ. Environ.,1996,21:403-465. [2] Vigier F, Coutanceau C, Perrard A, Belgsir EM, Lamy C. Development of
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