Structural and kinetic effects on a simple catalytic reaction: oxygen reduction on Ni(110)

Oxygen hydrogenation at 100 K by gas phase atomic hydrogen on Ni(110) has been studied under ultrahigh vacuum conditions by temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). Formation of adsorbed water and hydroxyl species was observed and characterized. The coverage of the reaction products was monitored as a function of both temperature and initial oxygen precoverage. On the contrary, when high coverage oxygen overlayers were exposed to gas phase molecular hydrogen, no hydrogenation reaction took place. The results are compared to the inverse process, exposing the hydrogen covered surface to molecular oxygen. In this case, at 100 K, simple Langmuir-Hinshelwood modeling yields an initial sticking coefficient for oxygen adsorption equal to 0.26, considerably lower than for the clean surface. Moreover, formation of hydroxyl groups is found to be twice as fast as the final hydrogenation of OH groups to water. Assuming a preexponential factor of 10(13) s(-1), an activation barrier of 6.7 kcal/mol is obtained for OH formation, thus confirming the high hydrogenating activity of nickel with respect to other transition metals, for which higher activation energies are reported. However, oxygen is hardly removed by hydrogen on nickel: this is explained on the basis of the strong Ni-O chemical bond. The hydrogen residual coverage is well described including a contribution from the adsorption-induced H desorption process which takes place during the oxygen uptake and which is clearly visible from the TPD data.     

Chemistry of O‐ and H‐Containing Species on the (001) Surface of Anatase TiO2: A DFT Study

The chemistry of oxygen, hydrogen, water, and other species containing both oxygen and hydrogen atoms on the anatase TiO₂ (001) surface is investigated by DFT. The adsorption energy of atoms and radicals depends appreciably on the position and mode of adsorption, and on the coverage. Molecular hydrogen and oxygen interact weakly with the clean surface. However, H₂O dissociates spontaneously to give two nonidentical hydroxyl groups, and this provides a model for hydroxylation of TiO₂ surfaces by water. The mobility of the hydroxyl groups created by water splitting is initially impeded by a diffusion barrier close to 1 eV. The O₂ adsorption energy increases significantly in the presence of H atoms. Hydroperoxy (OOH) formation is feasible if at least two H atoms are present in the direct vicinity of O₂. In the adsorbed OOH, the O? O bond is considerably lengthened and thus weakened.     

A Density Functional Theory Study on the Effect of Zero‐Point Energy Corrections on the Methanation Profile on Fe(100)

The thermodynamics and kinetics of the surface hydrogenation of adsorbed atomic carbon to methane, following the reaction sequence C+4 H?CH+3 H?CH₂+2 H?CH₃+H?CH₄, are studied on Fe(100) by means of density functional theory. An assessment is made on whether the adsorption energies and overall energy profile are affected when zero‐point energy (ZPE) corrections are included. The C, CH and CH₂ species are most stable at the fourfold hollow site, while CH₃ prefers the twofold bridge site. Atomic hydrogen is adsorbed at both the twofold bridge and fourfold hollow sites. Methane is physisorbed on the surface and shows neither orientation nor site preference. It is easily desorbed to the gas phase once formed. The incorporation of ZPE corrections has a very slight, if any, effect on the adsorption energies and does not alter the trends with regards to the most stable adsorption sites. The successive addition of hydrogen to atomic carbon is endothermic up to the addition of the third hydrogen atom resulting in the methyl species, but exothermic in the final hydrogenation step, which leads to methane. The overall methanation reaction is endothermic when starting from atomic carbon and hydrogen on the surface. Zero‐point energy corrections are rarely provided in the literature. Since they are derived from C? H bonds with characteristic vibrations on the order of 2500–3000 cm^?1, the equivalent ZPE of 1/2 hν is on the order of 0.2–0.3 eV and its effect on adsorption energy can in principle be significant. Particularly in reactions between CH_x and H, the ZPE correction is expected to be significant, as additional C? H bonds are formed. In this instance, the methanation reaction energy of +0.77 eV increased to +1.45 eV with the inclusion of ZPE corrections, that is, less favourable. Therefore, it is crucial to include ZPE corrections when reporting reactions involving hydrogen‐containing species.     

HCN Adsorption on the Fe(100), Fe(111) and Fe(110) Surfaces: a Density Functional Theory Study

Twenty kinds of adsorptions of HCN on the Fe(100), Fe(111) and Fe(110) surfaces at the 1/4 monolayer coverage are found using the density functional theory. For Fe(100), the adsorption energy of the most stable configuration where the HCN locates at the fourfold site with the C-N bonded to four Fe atoms is 1.928 eV. The most favored adsorption structure for HCN on Fe(111) is f-η3(N)-h-η3(C), in which the C-N bond is almost parallel to the surface, and the adsorption energy is 1.347 eV. On Fe(110), the adsorption energy in the most stable configuration in which HCN locates at the two long-bridge sites is 1.777 eV. The adsorption energy of the parallel orientation for HCN is larger than that of the perpendicular configuration. The binding mechanism of HCN on the Fe(100), Fe(111) and Fe(110) surfaces is also analyzed by Mulliken charge population and the density of states in HCN. The result indicates that the configurations in which the adsorbed HCN becomes the non-linear are beneficial to the formation of the addition reaction for hydrogen. The nature that the introduction of Fe into the catalyst could increase the catalytic activity of the bimetallic catalyst in the addition reaction of hydrogen for nitriles is revealed.     

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.     

Initial Subsurface Incorporation of Oxygen into Ru(0001): A Density Functional Theory Study

The adsorption and diffusion of oxygen on Ru(0001) surfaces as a function of coverage are systematically investigated by using density functional theory. A high incorporation barrier of low‐coverage adsorbed oxygen into the subsurface is discovered. Calculations show that the adsorption of additional on‐surface oxygen can lower the penetration barrier dramatically. The minimum penetration barrier obtained is 1.81 eV for a path starting with oxygen in mixed on‐surface hcp and fcc sites at an oxygen coverage of 0.75 ML, which should be regarded as close to 1 ML. Energy diagrams show that oxygen‐diffusion barriers on the surface and in the subsurface are much lower than the penetration barrier. Oxygen diffusion on the surface is an indispensable step for its initial incorporation into the subsurface.     

H2-induced CO adsorption and dissociation over Co/Al2O3 catalyst

The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissocia-tion. The H2 was adsorbed first, and activated to form H* over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH*, which reacted with adsorbed CO subsequently via CO*+OH* → CO2*+H*; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO onthe surface via CO*+2H* → CH*+OH*. The formation of the surface species (CH) from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.     

Sensitivity of methyl thiolate desulfurization selectivity to reaction temperature and hydroxyl coverage

The adsorption and reaction of methanethiol (CH3SH) and dimethyl disulfide (CH3SSCH3) on Mo(110)-(1 x 6)-O have been studied using temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy over the temperature range of 110-550 K. The S-H bond is broken upon adsorption to form adsorbed OH, water, and methyl thiolate (CH3S-) at low temperature. Water is evolved at 210 and 310 K via molecular desorption and disproportionation of OH, respectively. Some hydroxyl remains on the surface up to 350 K. Methyl thiolate is also formed from CH3SSCH3 on Mo(110)-(1 x 6)-O. Methyl thiolate undergoes C-S cleavage above 300 K, yielding methane and methyl radicals. There is also a minor amount of nonselective decomposition leading to the formation of carbon and hydrogen. Methane production is promoted by adsorbed hydroxyl. As the hydroxyl coverage increases, the yield of methyl radicals relative to methane diminishes. Accordingly, there is more methane produced from methanethiol reaction than from dimethyl disulfide, since S-H dissociation in CH3SH produces OH. The maximum coverage of the thiolate is approximately 0.5 monolayers, based on the amount of sulfur remaining after reaction measured by Auger electron spectroscopy. In contrast to cyclopropylmethanethiol (c-C3H5CH2SH), for which alkyl transfer from sulfur to oxygen is observed, there is no evidence for transfer of the methyl group of methyl thiolate to oxygen on the surface. Specifically, there is no evidence for methoxy (CH3O-) in infrared spectroscopy or temperature-programmed reaction experiments.     

Pd掺杂Fe催化剂上愈创木酚加氢脱氧理论研究

通过密度泛函理论(DFT)计算研究了愈创木酚在Fe(211)表面上的吸附活化行为和加氢脱氧(HDO)反应性能.讨论了Pd的掺杂和H_2O~*的参与对Fe催化剂活性和选择性的影响.计算结果表明,通过苯环水平吸附在催化剂表面的愈创木酚的稳定性高于仅通过羟基的垂直吸附构型,这有利于苯环, C_(Ar)-OCH_3键和O-CH_3键的活化.在Fe(211)表面上,愈创木酚通过脱甲基再加氢生成邻苯二酚在动力学上比通过脱甲氧基生成苯酚和通过脱羟基生成苯甲醚更有利. Pd掺杂对愈创木酚的吸附稳定性影响较小(0.05 eV),但增加了其加氢脱氧反应的活化能垒,抑制了C_(Ar)-OCH_3, O-CH_3和C_(Ar)-OH键的断裂以及随后加氢生成苯酚,邻苯二酚和苯甲醚的反应过程.在Fe(211)表面上, H_2O~*通过与-CH_3形成氢键作用(H-bonding机理)对反应产生影响,从而降低了愈创木酚脱甲基和脱甲氧基反应的活化能垒.在Fe(211)-1Pd表面上, H_2O~*通过H转移参与反应(H-shuttling机理),促进了愈创木酚向邻苯二酚和苯酚产物的转化,并提高了加氢脱氧反应对苯酚的选择性.     

Room temperature water splitting at the surface of magnetite

An array of surface science measurements has revealed novel water adsorption behavior at the Fe(3)O(4)(001) surface. Following room temperature exposure to water, a low coverage of hydrogen atoms is observed, with no associated water hydroxyl group. Mild annealing of the hydrogenated surface leads to desorption of water via abstraction of surface oxygen atoms, leading to a reduction of the surface. These results point to an irreversible splitting of the water molecule. The observed phenomena are discussed in the context of recent DFT calculations (Mulakaluri, N.; Pentcheva, R.; Scheffler, M. J. Phys. Chem. C 2010, 114, 11148), which show that the Jahn-Teller distorted surface isolates adsorbed H in a geometry that could kinetically hinder recombinative desorption. In contrast, the adsorption geometry facilitates interaction between water hydroxyl species, which are concluded to leave the surface following a reactive desorption process, possibly via the creation of O(2).     

Surface structure and energetics of hydrogen adsorption on the Fe(111) surface

Spin-polarized density functional theory calculations have been performed to characterize the hydrogen adsorption and diffusion on the Fe(111) surface at 2/3-, 1-, and 2-monolayer (ML) coverages. It is found that the most favored adsorption site for atomic hydrogen (H) is the top-shallow bridge site (tsb), followed by the quasi 4-fold site (qff) with the energy difference of about 0.1 eV, while the top site (t) is not competitive. Furthermore, the adsorbed atomic hydrogen (H) has a high mobility, as indicated by the small diffusion barriers. The local density of state (LDOS) analysis reveals that the Fe-H (tsb or qff) bond involves mainly the Fe 4s and 4p and H 1s orbitals with less contribution of the Fe 3d orbital, while the Fe 4s, 4p, and 3d orbitals all participate in the Fe-H (top) bond. In addition, the coverage effects on the adsorption configurations and adsorption energies are addressed.     

氧化铜表面臭氧分解路径及表面氧物种生成机理研究

基于密度泛函理论（DFT）计算研究了O3在完整和具有氧空位的CuO(111)表面吸附的吸附位、吸附结构、吸附能和电子转移情况,比较了O3在完整表面和具有氧空位的表面分解的路径和能垒,分析了氧空位和表面吸附氧的生成机理。结果表明,在完整CuO表面,O3分子通过化学吸附或物理吸附表面结合,吸附能最高为-1.22eV（构型bri(2)）。O3在具有氧空位的CuO表面均为化学吸附,吸附能最高为-2.95eV（构型ovbri(3)）,显著高于完整表面的吸附能。O3吸附后,Cu吸附位的电荷密度减小,O3中的O原子附近的电荷密度显著增强,电荷从CuO表面转移到O3,并形成Cu-O离子键。O3分解后形成了超氧物种,提高了表面的氧化活性。在完整表面,以构型bri(2)为起始构型的路径反应能垒最低,为0.52eV;O2*在完整表面的脱附所需要的最低能量为0.42eV,形成氧空位的O2*脱附能为2.06eV。在具有氧空位的表面,O3分解的反应能垒为0.30eV（构型ovbri(1)）和0.12eV（构型ovbri(3)）,均低于完整表面的反应能垒;分解形成的O2*的最低脱附能也低于完整表面,为0.27eV。可见,氧空位的形成提高了吸附能,降低了反应能垒,使O3分子更容易吸附在CuO表面,并加快了O3的催化分解。     

Mechanisms of and effect of coadsorption on water dissociation on an oxygen vacancy of the MgO(100) surface

The dissociation mechanism of a water molecule at an oxygen vacancy on the MgO(100) surface was studied by using the embedded cluster method at the DFT/B3 LYP level, while the energetic information was refined by using the IMOMO method at the CCSD level. We found that a water molecule initially adsorbs on one of the magnesium ions surrounding the vacancy site with a binding energy of 15.98 kcal mol(-1). It then can dissociate on the MgO(100) surface along two possible dissociation pathways. One pathway produces a hydroxyl group bonded to the original magnesium with a proton filling the vacancy via a transition state with a barrier of 4.67 kcal mol(-1) relative to the adsorbed water configuration. The other pathway yields two hydroxy groups; the hydroxy group originally belonging to the water molecule fills the vacancy, while the hydrogen atom binds with the surface oxygen to form the other hydroxy group. Hydrogen atoms of these hydroxy groups can recombine to form a hydrogen molecule and the surface is healed. Although the barrier (14.09 kcal mol(-1)) of the rate-controlling step of the latter pathway is higher than that of the former one, the energies of all of its stationary points are lower than that of the separated reactants (H(2)O+cluster). The effects of water coadsorption are modeled by placing an additional water molecule near the active center, which suggests that the more coadsorbed water molecules further stabilize the hydroxy species and prevent the hydrogen molecule formation through the latter pathway. The results support the photoemission spectral evidence of water dissociation on the defective MgO(100) surface at low water coverage.     

Au(111)表面上乙醇选择性氧化反应机理的密度泛函理论研究(英文)

采用密度泛函理论研究了吸附有O原子的Au(111)表面上乙醇选择性氧化的反应机理.反应结果表明,除O原子和中间产物二齿醋酸根(CH3CHOO)外,其他中间产物在Au(111)表面扩散能垒均较低,不会对反应速控步骤的确定造成影响.乙醇羟基氧化脱氢为反应的第一步骤,当氧化剂为吸附态的O原子或者为OH基时,反应活化能分别为0.20和0.17eV.氧化产物乙氧基(CH3CH2O)进一步氧化脱氢生成乙醛则需要表面吸附的O原子或另一表面吸附的OH基的参与,所需活化能为0.29或0.27eV.同时,乙醛易与表面吸附的乙氧基反应生成乙氧基半缩醛(CH3CHOOC2H5),其可进一步与O原子作用,脱氢形成乙酸乙酯.此外,在乙醛深度氧化成酸的过程中需要克服较高的反应能垒,因而在表面反应温度较低时无法进行,这与实验结果相符.     

Prediction of tetraoxygen formation on rutile TiO2(110)

In this paper, we propose a new adsorption model for molecular oxygen on reduced TiO2(110), based on extensive first principles density functional calculations. For the first time, our calculations predict formation of tetraoxygen (O4) anchored at the vacancy site, which in turn allows adsorption of three O2 molecules per vacancy in saturation coverage. We present the structure, bonding, and energetics of adsorbed oxygen species by changing the number of adsorbed oxygen molecules per vacancy. We also find that thermally activated O2 desorption may take place via two channels that require overcoming barriers of 0.41 and 1.25 eV, respectively. In addition, our study provides strong theoretical evidence for the change in O2 reactivity with O2 coverage. Our findings associated with tetraoxygen complexes are consistent with existing experimental results.     

第一性原理研究C在Fe(111)表面和次表面的积炭机理

使用密度泛函方法对C原子在Fe(111)表面吸附团聚和次表层的吸附扩散进行了研究。在炭覆盖度θ_C <1 ML时,C主要以孤立的原子态存在并导致表面重构;1 ML≤θ_C ≤2 ML,"mC₂+nC"为主要的吸附形式;θ_C≥2 ML时,复杂的吸附形态比如碳链和岛状碳团簇开始生成。这些复杂岛状碳团簇是Fe(111)表面石墨沉积或碳纳米管生长的成核中心。在次表层,C原子在八面体位稳定存在。C在表面的迁移能垒为0.45 eV,由表面迁移到次表面的的能垒为0.73 eV。虽然C₂团簇的生成是热力学有利的,但是C向次表层的迁移动力学上占优。     

FT-IR、XPS和DFT研究水杨酸钠在针铁矿或赤铁矿上的吸附机理

采用傅里叶变换红外(FT-IR)光谱、X射线光电子能谱(XPS)以及基于周期平面波的密度泛函理论(DFT)分别研究了水杨酸钠在针铁矿或赤铁矿表面上的吸附结构,并将计算得到的光电子能谱移动(CLS)和电荷转移与实验得到的XPS结果进行对比。FT-IR结果表明,水杨酸钠可能以双齿双核(V)和双齿单核(IV)的形式分别吸附于针铁矿或赤铁矿表面。由DFT计算结果可知,水杨酸钠在针铁矿(101)晶面上形成双齿双核化合物(V)的吸附能为-5.46 eV。而水杨酸钠在针铁矿(101)晶面上形成双齿单核化合物(IV)的吸附能为3.80 eV,因此水杨酸钠在针铁矿上基本不以双齿单核化合物(IV)构型存在。水杨酸钠在赤铁矿(001)晶面上形成双齿单核化合物(IV)时吸附能为-4.07 eV,说明水杨酸钠在赤铁矿(001)晶面上形成了双齿单核化合物(IV)。另外,理论计算的针铁矿(101)晶面上吸附位点铁原子的Fe 2p的CLS值(-0.68 eV)与实验观察到的Fe 2p的CLS值(-0.5 eV)吻合。理论计算的赤铁矿(001)晶面上吸附位点铁原子的Fe 2p的CLS值(-0.80 eV)与实验观察到的Fe 2p的CLS值(-0.8 eV)吻合。因此,水杨酸钠吸附在针铁矿表面时能够通过羧酸基团上一个氧原子和酚羟基上的氧原子与针铁矿(101)表面上的两个铁原子形成双齿双核(V)结构,而在赤铁矿(001)表面上,水杨酸钠中羧酸基团上一个氧原子和酚羟基上的氧原子与赤铁矿(001)表面上的一个铁原子形成了双齿单核(IV)结构。     

Adsorption and protonation of CO2 on partially hydroxylated gamma-Al2O3 surfaces: a density functional theory study

Adsorption and protonation of CO2 on the (110) and (100) surfaces of gamma-Al2O3 have been studied using density functional theory slab calculations. On the dry (110) and (100) surfaces, the O-Al bridge sites were found to be energetically favorable for CO2 adsorption. The adsorbed CO2 was bound in a bidentate configuration across the O-Al bridge sites, forming a carbonate species. The strongest binding with an adsorption energy of 0.80 eV occurs at the O3c-Al5c bridge site of the (100) surface. Dissociation of water across the O-Al bridge sites resulted in partially hydroxylated surfaces, and the dissociation is energetically favorable on both surfaces. Water dissociation on the (110) surface has a barrier of 0.42 eV, but the same process on the (100) surface has no barrier with respect to the isolated water molecule. On the partially hydroxylated gamma-Al2O3 surfaces, a bicarbonate species was formed by protonating the carbonate species with the protons from neighboring hydroxyl groups. The energy difference between the bicarbonate species and the coadsorbed bidentate carbonate species and hydroxyls is only 0.04 eV on the (110) surface, but the difference reaches 0.97 eV on the (100) surface. The activation barrier for forming the bicarbonate species on the (100) surface, 0.42 eV, is also lower than that on the (110) surface (0.53 eV).     

Adsorption of hydrogen on palladium nanoparticle surfaces

The behaviors of hydrogen (H) adsorbed on the palladium (Pd) nanoparticles (NPs) are examined with the modified analytic embedded‐atom method potentials and MORSE potentials. We study the effects of particle size and H coverage, and compare their adsorption properties of nanoparticle's facets with that of flat surfaces. We simulate the Pd truncated octahedron NPs with atoms from 38 to 2406 and the coverage of adsorbed H up to 1.0 monolayer (ML). Site preferences, adsorption geometries, adsorption energies, and bond lengths of H? Pd are calculated. We have also calculated the potential energy surface (PES). It is clear that the H atom binding to particle facets is quite stronger than that of flat surfaces when the particle size is smaller than 3.2 nm. We have found a significant variation that adsorption energies ascend gradually with increasing the particle size or surface coverage of H, and the adsorption energy varies about 0.6 eV for (111) facet and 0.3 eV for (100) facet as the coverage up to 1.0 ML. Our results are in reasonable agreement with the experimental values and other calculations. Copyright © 2009 John Wiley & Sons, Ltd.     

Electrocatalytic Hydrogenation and Oxidation in Aqueous Conditions†

Water molecule contains one oxygen and two hydrogen atoms, making it a potential oxygen and hydrogen source. Electrocatalytic organic reduction and oxidation using water as oxygen and/or hydrogen donors provide an environmentally friendly and sustainable strategy to replace traditional chemical‐driven stoichiometric reactions that use sacrificial reagents. Furthermore, the development of electrochemical synthesis provides a potential application for low tension photoelectricity, which is not cost‐effective during boosted voltage and application. In the last decade, electrocatalytic redox reactions of organic molecules in aqueous media had shown progress owing to the development of electrode materials and water‐splitting technology. This paper highlights several electrocatalytic systems and corresponding mechanisms for both hydrogenation and oxidative transformation of representative compounds. The activation process of protons and water on the working electrode surface has received special focus. Furthermore, paired electrolysis using water as the oxygen and hydrogen source has been demonstrated. This paired system combines hydrogenation and oxidation half‐reactions in one cell using water as the hydrogen and oxygen source, resulting in high atomic and electron utilization rates.