CO2在金属表面活化的UBI-QEP方法研究

应用UBI-QEP方法估算了金属表面上形成的活化吸附态CO₂^-在Cu(111),Pd(111),Fe(111)和Ni(111)表面上的吸附热,计算了各种相关反应的活化能垒.结果表明,CO₂^-在4种过渡金属表面的相对稳定性的顺序为Fe>Ni>Cu>Pd;在Fe和Ni表面上CO₂^-较易生成,且容易进一步发生解离反应,在Fe表面会解离成C和O吸附原子,而在Ni表面上解离的最终产物为CO和O;在Cu表面上,CO₂^-虽较难形成,但其加氢反应的活化能比解离反应低,因此加氢反应是其进一步活化的有效模式;在Pd表面上,CO₂^-吸附态在能量上很不稳定,所以CO₂在Pd表面上不容易活化.     

CO和H2分子在Cu(111)面的吸附和溶剂化效应

采用广义梯度近似(GGA)密度泛函理论(DFT)的PW91方法结合周期性模型, 在DNP基组下, 利用Dmol3模块研究了CO和H2在真空和液体石蜡环境下在Cu(111)表面上不同位置的吸附. 计算结果表明, 溶剂化效应对H2和CO的吸附结构参数和吸附能的影响非常显著. 在液体石蜡环境下, H2平行吸附在Cu(111)表面是解离吸附, 而CO 和H2在两种环境下的垂直吸附都是非解离吸附. 相比真空环境吸附, 在液体石蜡环境中, Cu(111)吸附CO时, 溶剂化效应能够提高CO吸附的稳定性, 同时有利于CO的活化. 在真空中, H2只能以垂直方式或接近垂直方式吸附在Cu(111)表面. 当Cu(111)顶位垂直吸附H2, 相比真空环境吸附, 溶剂化效应能够提高H2吸附的稳定性, 但对H2的活化没有明显影响. Cu(111)表面的桥位或三重穴位(hcp和fcc)垂直吸附H2时, 溶剂化效应能明显提高H2的活化程度, 但降低H2的吸附稳定性; 在液体石蜡中, 当H2平行Cu(111)表面吸附时, 溶剂化效应使H—H键断裂, 一个H原子吸附在fcc位, 另一个吸附在hcp位.     

Pd掺杂的Ni催化剂表面CH4解离吸附的理论研究

使用密度泛函理论研究了Pd掺杂的Ni(111),Ni(100)和Ni(211)表面最稳定的结构,同时考察了干净的和Pd掺杂的Ni表面催化CH₄解离反应的活性.结果表明,由Pd原子取代最外层Ni原子而形成的表面Pd掺杂的Ni表面在热力学上最为稳定,亚表面Pd掺杂的Ni表面在热力学上都不稳定; 而对于表面Pd吸附的Ni表面,只有Pd/Ni(211)表面是稳定的.表面掺杂的Pd/Ni表面上CH₄解离中间体(CH₄,CH₃,CH,C,H)吸附能的计算结果表明,Pd的掺杂在不同程度上减弱了除CH₄之外各解离中间体的吸附能.另外,CH₄和CH均优先在Ni(211)和Pd/Ni(211)台阶面上解离,其次是在比较开阔的Ni(100)和Pd/Ni(100)表面上.Pd的掺杂不同程度上提高了CH₄和CH解离的能垒,对于活性最高的Ni(211)面,Pd的掺杂使得CH脱氢的能垒较CH₄脱氢的高,改变了其速率控制步骤,从而抑制了积碳的生成.     

Fe(100)表面Cu单层膜上CO的吸附,解离以及C-C偶合反应

采用密度泛函方法研究了Fe(100)表面Cu单层膜上CO的吸附,直接解离,氢助解离以及C-C偶合反应.相比洁净的Fe(100)表面,在Fe(100)的单层Cu膜上,CO的吸附和活化都减弱了.特别是,相比Fe(100)上CO的解离能垒1.08 eV,铜单层膜上CO解离能垒高达2.4 eV.在H原子共吸附的情况下,Fe(1...     

Cu单原子修饰对Fe(111)表面CO吸附性能及电子性质调变的第一性原理研究

采用密度泛函理论方法研究了Cu单原子修饰对Fe(111)表面CO吸附性能和电子性质的调变作用,其中,Cu单原子修饰研究了吸附和取代两种方式。结果表明,CO在Cu修饰的Fe(111)面吸附能力都会变弱,一是Cu原子自身提供的位点对CO的吸附较弱;二是Cu会使其附近的Fe对CO的吸附变弱。分析电子性质表明,Cu作用于Fe表面后,会导致Cu附近Fe原子部分电子向Cu原子转移,进而削弱了Fe与吸附分子间电子交互作用而改变Fe原子的吸附能力。故Cu原子改性Fe表面可以很好地调变CO的吸附、解离及后续反应催化活性,这为进一步探究Cu改性Fe表面的合成气催化反应机理提供了基础信息。     

不同粒径的Ni/SiO2催化剂上CH4和CO2吸附活化的漫反射傅里叶变换红外光谱研究

 采用原位漫反射傅里叶变换红外光谱研究了CH4和CO2在不同粒径的Ni/SiO2催化剂上的吸附及活化. 结果表明,在不同粒径的催化剂上,检测到有CH4解离生成的CHx(x=1～3)物种,以及催化剂表面吸附的CHx物种与表面-OH 作用生成的CHx-O物种. CH4的裂解强烈依赖于催化剂表面Ni颗粒的大小,在粒径8 nm左右的Ni颗粒上, CH4较易解离; CO2难以直接在Ni/SiO2催化剂表面发生解离吸附,但CH4解离生成的吸附H对CO2的解离吸附具有明显的促进作用; CH4与CO2共吸附时,较小粒径的Ni可以促进CO2与表面氧物种发生反应,生成单齿表面碳酸盐物种.     

DFT法研究HCOOH在Pd/WC(0001)上的分解机理（英文）

作为便携式电子设备的动力源,直接甲酸燃料电池(DFAFC)具有燃料跨界范围小、电动势大、甲酸无毒、低温下功率密度大等优点,因而引起了人们的极大兴趣.DFAFC商业化的主要挑战之一是阳极电催化剂材料的高成本和低CO耐受性.阳极通常需要高负载的贵金属电催化剂(Pt或Pd)氧化甲酸(HCOOH)以获得所需的电能.完全电氧化甲酸在Pt和Pd表面上会产生强吸附的CO,从而降低了Pt或Pd催化剂的活性.Pt和Pd储量少且价格昂贵,减少Pt和Pd含量且保持催化性能的燃料电池催化剂一直是研究者的奋斗目标.本文用周期性密度泛函理论(DFT)系统地研究了WC负载的单分子层Pd(Pd/WC(0001))催化剂对甲酸的分解机理,这可为所需的反应路径设计、筛选催化剂提供指导.Trans-HCOOH通过C-H,O-H,C-O键的活化发生分解.关于吸附,确定了可能反应中间体的最稳定吸附构型.trans-HCOOH,HCOO,mHCOO,cis-COOH,trans-COOH,CO,H2O,OH和H的吸附过程是化学吸附,而cis-HCOOH和CO2与Pd/WC(0001)表面的相互作用较弱,是物理吸附.此外,提出了trans-HCOOH分解的不同途径来探索分解机理.trans-HCOOH中O-H,C-H和C-O键的活化能垒分别为0.61,0.77和1.05 eV,O-H键断裂的能垒最小,则trans-HCOOH优先通过O-H键断裂生成HCOO.双齿HCOO是HCOOH分解的主要中间体,它可以转变为单齿HCOO,这条路线生成CO2的能垒比双齿HCOO的低0.04 eV.CO2是HCOO主要解离产物,这一步是总反应的决速步骤.对于cis-COOH和trans-COOH,CO是其主要解离产物.此外,trans-HCOOH也能直接生成CO,但克服的能垒较大.在Pd/WC(0001)表面上分解trans-HCOOH的最有利途径是HCOOH→HCOO→CO2,其中HCOO脱氢形成CO2的步骤是速率决定步骤.本文提供了HCOOH在Pd/WC(0001)表面上分解的活性中间体、能垒和机理的推测,CO形成主要是通过cis-COOH、trans-COOH及HCO的分解,CO2的形成主要是通过HCOO的分解,CO2占主导.该结论与Pd(111)面上甲酸分解结果一致,说明WC作为Pd载体没有改变Pd对甲酸的催化性能,但降低了Pd的使用量.综上,本文阐明了WC负载单分子层Pd催化剂上甲酸催化分解机理,得出甲酸分解的最佳反应路径,为直接甲酸燃料电池设计低贵金属含量、高活性的负载型Pd催化剂提供了理论指导;可用于预测不同载体负载Pd催化剂的性能,大大减少实验成本,以验证提出的实验假设.     

CO在CeO2(111)表面的吸附与氧化

采用密度泛函理论计算了CO在CeO2(111)表面的吸附与氧化反应行为. 结果表明, O2在洁净的CeO2(111)表面为弱物理吸附, 而在氧空位表面是强化学吸附, 且O2分子活化程度较大, O—O键长为0.143 nm. CO在CeO2(111)表面吸附行为的研究表明, CO在洁净表面及氧空位表面上为物理吸附, 吸附能均小于0.42 eV; 当表面氧空位吸附O2后, CO可吸附生成二齿碳酸盐中间体或直接生成CO2, 与原位红外光谱结果相一致. 表面碳酸盐物种脱附生成CO2的能垒仅为0.28 eV. 计算结果表明, 当CeO2表面存在氧空位时, Hubbard参数U对CO吸附能有一定的影响. CeO2载体在氧化反应中可能的催化作用为, 在氧气氛下, CeO2表面氧空位吸附O2分子, 形成活性氧物种, 参与CO催化氧化反应.     

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.     

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。     

A first principles study of oxygen reduction reaction on a Pt(111) surface modified by a subsurface transition metal M (M = Ni, Co, or Fe)

We have performed first-principle density functional theory calculations to investigate how a subsurface transition metal M (M = Ni, Co, or Fe) affects the energetics and mechanisms of oxygen reduction reaction (ORR) on the outermost Pt mono-surface layer of Pt/M(111) surfaces. In this work, we found that the subsurface Ni, Co, and Fe could down-shift the d-band center of the Pt surface layer and thus weaken the binding of chemical species to the Pt/M(111) surface. Moreover, the subsurface Ni, Co, and Fe could modify the heat of reaction and activation energy of various elementary reactions of ORR on these Pt/M(111) surfaces. Our DFT results revealed that, due to the influence of the subsurface Ni, Co, and Fe, ORR would adopt a hydrogen peroxide dissociation mechanism with an activation energy of 0.15 eV on Pt/Ni(111), 0.17 eV on Pt/Co(111), and 0.16 eV on Pt/Fe(111) surface, respectively, for their rate-determining O2 protonation reaction. In contrast, ORR would follow a peroxyl dissociation mechanism on a pure Pt(111) surface with an activation energy of 0.79 eV for its rate-determining O protonation reaction. Thus, our theoretical study explained why the subsurface Ni, Co, and Fe could lead to multi-fold enhancement in catalytic activity for ORR on the Pt mono-surface layer of Pt/M(111) surfaces.     

Prediction of Comparative Catalytic Activity in the Series of Single Crystalline Surfaces in a Water-Gas Shift Reaction

A method is proposed for predicting comparative catalytic activity. This method includes the following stages: (1) formulation of a general reaction mechanism that is applicable to the whole series of catalysts, (2) calculation of the Arrhenius activation energies and the preexponential factors, and (3) kinetic simulations for the preset conditions. The method is illustrated by the model water-gas shift reaction (WGSR) CO + H₂O = CO₂ + H₂ and a series of catalytic single crystalline surfaces Cu(111), Ag(111), Au(111), Ni(111), Pd(111), Pt(111), and Fe(110). The mechanism is formulated using the computer program MECHEM. The activation energies are calculated using the UBI-QEP method. The reaction kinetics is simulated for a plug-flow reactor. The following series of the catalytic activity is obtained: Cu(111) > Ni(111) > Fe(111) > Pt(111), Pd(111) > Ag(111) > Au(111).__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 543–549.Original Russian Text Copyright © 2005 by Zeigarnik, Callaghan, Datta, Fishtik, Shustorovich.     

Thermokinetic study of Fischer–Tropsch synthesis on Fe2Cu1 and FeCu surfaces with comparison to Fe(110) and Cu(111) catalysts by the UBI-QEP method

The purpose of this study is to predict the activation barriers and enthalpy for elementary steps in the process of Fischer–Tropsch (F-T) on the surfaces of Fe(110), Cu(111) and Fe/Cu alloys catalyst using “Unity Bond Index-Quadratic Exponential Potential” method aimed at predicting the activity and selectivity on the basis of energy criteria. The elementary steps, such as dissociation of CO, hydrogenation of carbidic carbon, C–C chain growth by insertion of CH₂ versus CO into the metal-alkyl bonds, and chain termination, which lead to hydrocarbons (alkanes versus α-olefins) or oxygenates are discussed in detail. The results show that metallic Fe(110) is necessary to produce the carbidic carbon from CO dissociation, but the synthesis of hydrocarbons and oxygenates can effectively proceed on Cu(111) surface. For optimum performance of F-T synthesis catalyst, these conflicting properties must be optimized. In this regard, we studied Fe/Cu alloy catalyst. On all the catalyst surfaces, the energetically preferred path to initiate the alkyl chain growth is via insertion of a CH_2,s group into the carbon–metal bond of a CH_3,s group. On FeCu catalyst surface, the activation barrier for termination of alkyl chain growth by β-elimination of hydrogen is found to be lower than that for α-addition of hydrogen and consequently for this catalyst, olefins are expected to form more readily than paraffins. The results of the model for a single metal surface are in agreement with the experimental data.     

First-principles study of C adsorption, O adsorption, and CO dissociation on flat and stepped Ni surfaces

The adsorption of atomic oxygen and carbon was studied with plane wave density functional theory on four Ni surfaces, Ni(110), Ni(111), Ni(210), and Ni(531). Various adsorption sites on these surfaces are examined in order to identify the most favorable adsorption site for each atomic species. The dependence of surface bonding on adsorbate coverage is also investigated. Adsorption energies and structural information are obtained and compared with existing experimental results for Ni(110) and Ni(111). In addition, activation barriers to CO dissociation have been determined on Ni(111) and Ni(531) by locating the transition states for these processes. Our results indicate that the binding energies of C are comparatively stronger on stepped surfaces than on flat surfaces, and the energy barriers associated with CO dissociation strongly favor reactions occurring near surface steps.     

Adsorption and Decarbonylation Reaction of Furfural on the Pd(111) and M/Pd(111)(M=Ni,Cu and Ru) Surfaces

By performing with density functional theory(DFT) method, the detailed adsorption process and the catalytic decarbonylation mechanisms of furfural over Pd(111) and M/Pd(111)(M = Ni, Cu, Ru) surfaces toward furan were clarified. The results of atomic size factor, formation energy and d-band center showed that Ru/Pd(111) surface was the most stable and active. The adsorption energies of furfural on the different surfaces followed the order Ru/Pd(111) Cu/Pd(111) Pd(111) Ni/Pd(111). After analyzing Mulliken atomic charge population and the deformation density, we can find that on Ru/Pd(111) surface, the number of charge transfer was the most and the interaction was the strongest. Therefore, its adsorption energy was the highest. Furthermore, the furfural decarbonylation pathway is more kinetically feasible on bimetallic surface, and the reaction is the most likely to occur on Ru/Pd(111).     

CO2 reforming of CH4 on Ni(111): a density functional theory calculation

CO(2) reforming of CH(4) on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH(4) sequential dissociation into surface CH (CH(4) --> CH(3) --> CH(2) --> CH) and hydrogen, and CO(2) dissociation into surface CO and O (CO(2) --> CO + O). The second step is CH oxygenation into CHO (CH + O --> CHO), which is more favored than dissociation into C and hydrogen (CH --> C + H). The third step is the dissociation of CHO into surface CO and H (CHO --> CO + H). This can explain the enhanced selectivity toward the formation of CO and H(2) on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C((ads)) + CO(2)) is more favored than by CH(4) sequential dehydrogenation. The major problem of CO(2) reforming of CH(4) is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH(4) activation into CH(3) and H as the rate-determining step of CO(2) reforming of CH(4) on Ni(111), in agreement with the isotopic experimental results.     

CO oxidation on inverse CeO(x)/Cu(111) catalysts: high catalytic activity and ceria-promoted dissociation of O2

A Cu(111) surface displays a low activity for the oxidation of carbon monoxide (2CO + O(2) → 2CO(2)). Depending on the temperature, background pressure of O(2), and the exposure time, one can get chemisorbed O on Cu(111) or a layer of Cu(2)O that may be deficient in oxygen. The addition of ceria nanoparticles (NPs) to Cu(111) substantially enhances interactions with the O(2) molecule and facilitates the oxidation of the copper substrate. In images of scanning tunneling microscopy, ceria NPs exhibit two overlapping honeycomb-type moire? structures, with the larger ones (H(1)) having a periodicity of 4.2 nm and the smaller ones (H(2)) having a periodicity of 1.20 nm. After annealing CeO(2)/Cu(111) in O(2) at elevated temperatures (600-700 K), a new phase of a Cu(2)O(1+x) surface oxide appears and propagates from the ceria NPs. The ceria is not only active for O(2) dissociation, but provides a much faster channel for oxidation than the step edges of Cu(111). Exposure to CO at 550-750 K led to a partial reduction of the ceria NPs and the removal of the copper oxide layer. The CeO(x)/Cu(111) systems have activities for the 2CO + O(2) → 2CO(2) reaction that are comparable or larger than those reported for surfaces of expensive noble metals such as Rh(111), Pd(110), and Pt(100). Density-functional calculations show that the supported ceria NPs are able to catalyze the oxidation of CO due to their special electronic and chemical properties. The configuration of the inverse oxide/metal catalyst opens new interesting routes for applications in catalysis.