Density functional theory study of hydrogen adsorption on Fe5C2(001), Fe5C2(110), and Fe5C2(100)

Density functional theory calculations have been carried out for hydrogen adsorption on the (001), (110), and (100) surfaces of Fe5C2. At 1/3 and 2/3 monolyer (ML) on (001), the most stable hydrocarbon species is CsH, while CsH and CsH3 can coexist at 1 ML. On (110), only dissociated hydrogen is found at 2/5 ML, while CsH is the most stable hydrogen carbon species at 4/5 ML, and CsH and CH3 coexist at 6/5 ML. On (001) and (110) surfaces, CsH2 is less stable and can dissociate into CsH or convert into CsH3, respectively. These results are in agreement with the experimental observations. On the metallic Fe5C2(100) surface which lacks surface carbon atoms on the surface monolayer, dissociated hydrogen is found at 1/2 ML, while both dissociated hydrogen and activated H2 are found at 1 ML.     

The Surface Chemistry of Water on Fe(100): A Density Functional Theory Study

The formation of water by hydrogenation of atomic oxygen is studied using density functional theory. Atomic oxygen preferentially adsorbs at the four‐fold hollow site, the hydroxyl group prefers the bridge site in a tilted configuration, and water is most stable when adsorbed at the top site with the two O? H bonds parallel to the Fe surface. Water formation by the hydrogenation of oxygen is a highly activated process on the Fe(100) surface, with similar activation energies, in the order of 1.1 eV, for the first and second hydrogen additions. A more favourable route for the addition of the second hydrogen atom involves the disproportionation of hydroxyl groups to form water and adsorbed oxygen. Dissociation of the OH is also likely since the activation energy is similar to that for disproportionation of 0.65 eV. Furthermore, the results show that the dissociation of water on Fe(100) is a non‐activated process: 0.16 eV for the zero‐coverage limit and 0.03 eV when surface oxygen is present. Herein, adsorption energies, structures and vibrational frequencies are presented for several adsorption states at 0.25 ML coverage, as well as the potential energy surface for water formation on Fe(100).     

DFT study of gas-phase adsorption of benzotriazole on Cu(111), Cu(100), Cu(110), and low coordinated defects thereon

The adsorption of benzotriazole--an outstanding corrosion inhibitor for copper--on Cu(111), Cu(100), Cu(110), and low coordinated defects thereon has been studied and characterized using density functional theory (DFT) calculations. We find that benzotriazole can either chemisorb in an upright geometry or physisorb with the molecular plane being nearly parallel to the surface. While the magnitude of chemisorption energy increases as passing from densely packed Cu(111) to more open surfaces and low coordinated defects, the physisorption energy is instead rather similar on all three low Miller index surfaces. It is pointed out that due to a large dipole moment of benzotriazole the dipole-dipole interactions are rather important. For perpendicular chemisorption modes the lateral repulsion is very long ranged, extending up to the nearest-neighbor distance of about 60 bohrs, whereas for parallel adsorption modes the lateral interactions are far less pronounced and the molecules experience a weak attraction at distances ?25 bohrs. The chemisorption energies were therefore extrapolated to zero coverage by a recently developed scheme and the resulting values are -0.60, -0.73, and -0.92 eV for Cu(111), Cu(100), and Cu(110), respectively, whereas the zero-coverage physisorption energy is about -0.7 eV irrespective of the surface plane. While the more densely packed surfaces are not reactive enough to interact with the molecular π-system, the reactivity of Cu(110) appears to be at the onset of such interaction, resulting in a very stable parallel adsorption structure with an adsorption energy of -1.3 eV that is ascribed as an apparent chemisorption+physisorption mode.     

氢原子在Pt及Pt系双金属催化表面吸附的密度泛函理论研究

采用密度泛函理论(dFT)考察了Pt(100)、(110)、(111)三种表面氢原子的吸附行为, 计算了覆盖度为0.25 ML时氢原子在Pt 三种表面和M-Pt(111)双金属(M=Al, Fe, Co, Ni, Cu, Pd)上的最稳定吸附位、表面能以及吸附前后金属表面原子层间弛豫情况. 分析了氢原子在不同双金属表面吸附前后的局域态密度变化以及双金属表面d 带中心偏离费米能级的程度并与氢吸附能进行了关联. 计算结果表明, 在Pt(100), Pt(110)和Pt(111)表面, 氢原子的稳定吸附位分别为桥位、短桥位和fcc 穴位. 三种表面中以Pt(111)的表面能最低, 结构最稳定. 氢原子在不同M-Pt(111)双金属表面上的最稳定吸附位均为fcc 穴位, 其中在Ni-Pt 双金属表面的吸附能最低, Co-Pt 次之. 表明氢原子在Ni-Pt 和Co-Pt 双金属表面的吸附最稳定. 通过对氢原子在M-Pt(111)双金属表面吸附前后的局域态密度变化的分析, 验证了氢原子吸附能计算结果的准确性. 掺杂金属Ni、Co、Fe 的3d-Pt(111)双金属表面在吸附氢原子后发生弛豫, 第一层和第二层金属原子均不同程度地向外膨胀. 此外, 3d金属的掺入使得其对应的M-Pt(111)双金属表面d带中心与Pt 相比更靠近费米能级, 吸附氢原子能力增强, 表明3d-Pt系双金属表面有可能比Pt具有更好的脱氢活性.     

氮原子在铁低指数面上的吸附位和吸附态

应用原子和表面簇合物相互作用的5参数Morse势方法(5-MP)对N-Fe低指数表面体系进行了系统的研究, 并获得了全部临界点特性, 如吸附位、吸附几何、结合能、正则振动频率等. 计算结果表明: 在Fe(100)面, N原子吸附在四重洞位; 在Fe(110)表面, 趋向于吸附在膺式三重位; 而在Fe(111)表面最稳定的吸附位是近似桥位.     

DFT characterization of adsorbed NH(x) species on Pt(100) and Pt(111) surfaces

Periodic density functional theory (DFT) calculations using plane waves have been performed to systematically investigate the adsorption and relative stability of ammonia and its dehydrogenated species on Pt(111) and Pt(100) surfaces. Different adsorption geometries and positions have been studied, and in each case, the equilibrium configuration has been determined by relaxation of the system. The vibrational spectra of the various ammonia fragments have been computed, and band assignments have been compared in detail with available experimental data. The adsorption of NH3 (on top) and NH2 (bridge) is more favorable on Pt(100) than on Pt(111), while similar adsorption energies were computed for NH (hollow) and N (hollow) on both surfaces. The remarkably lower adsorption energy of NH2 over Pt(111) as compared with Pt(100) (the difference being approximately 0.7 eV) can be related to different geometric and electronic factors associated with this particular intermediate. Accordingly, the type of platinum surface determines the most stable NH(x) fragment: Pt(100) has more affinity for NH2 species, whereas NH species are preferred over Pt(111).     

Adsorption and Vibration of O Atoms on Fe Low-index and Fe （211） High-index Surfaces

IntroductionThe interaction of oxygen with iron in lowcoverageregimes is considered to be an important step in the for-mation of oxides in corrosion science and in Fisher-Tro-psch process for the synthesis of ammonia over the het-erogeneous catalysts[1]. …     

Theoretical study of NH3 adsorption on Fe(110) and Fe(111) surfaces

With the aim of understanding the nature of the interactions between organic molecules and metal surfaces, the adsorption of NH3 onto model Fe(110) and Fe(111) surfaces has been studied with use of the molecular orbital and density functional theories. B3LYP calculations have revealed that the on-top site is most suitable for adsorption of NH3 both on Fe(110) and on Fe(111). Mulliken population analysis in terms of the MO's of the two fragment systems suggested that electron delocalization from NH3 to the Fe surface should play a key role in the adsorption. Then, our transformation scheme of fragment orbitals has demonstrated that the electron delocalization is represented well only by a pair of interaction orbitals. The NH3 molecule provides the occupied interaction orbital bearing a close resemblance to the highest occupied (HO) MO, whereas the Fe surface prepares the paired unoccupied orbital that is localized at the adsorption site and overlaps in-phase with the orbital of NH3. Not only the lowest unoccupied (LU) MO but also other unoccupied MO's have been shown to participate significantly in the interaction. The reason the on-top site is the most preferable position for NH3 attack has been elucidated by investigating the interaction orbitals.     

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.     

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.     

Ru修饰前后Pd(111)面的性质及对糠醛吸附的比较研究

采用密度泛函理论研究了Pd(111)面和Ru-Pd(111)面的性质及对糠醛的吸附.原子尺寸因素、相对键长、形成能及d带中心等计算结果表明,Ru-Pd(111)面比Pd(111)面稳定且活性强,Ru的修饰优化了Pd(111)面的几何构型.糠醛在Pd(111)面及Ru-Pd(111)面的初始吸附位分别为P(top-bridge)位及P(Pd-fcc-Ru-fcc)位时,吸附能最大,吸附构型最稳定.由电荷布局和差分电荷密度可得,糠醛在Ru-Pd(111)面上电荷转移数更多,相互作用更强烈,因此吸附能更大.分析态密度可知,产生吸附的主要原因是位于-7.34 eV处至费米能级处的p,d轨道杂化.吸附于Ru-Pd(111)面后糠醛分子的p轨道向低能级偏移程度更明显,使Ru改性后的Pd催化剂具有更好的催化活性.     

氢原子在Fe低指数面及高指数面(211)上的吸附和振动

摘要应用原子和表面簇合物相互作用的5参数Morse势方法(5-MP)对H-Fe低指数表面体系及高指数表面体系进行了系统研究, 并获得了吸附位、 吸附几何、 结合能和正则振动频率等全部临界点特性. 理论计算结果表明, 在Fe(100)面, H原子吸附在四重洞位, H-Fe的垂直振动频率为1 009 cm^-1; 在Fe(110)和Fe(211)表面, 趋向于吸附在赝式三重位, H-Fe的垂直振动频率分别为1 054和1 046 cm^-1; 而在Fe(111)表面最稳定的吸附位是近桥位, 频率为1 030 cm^-1.     

Adsorption of atoms on cu surfaces: a density functional theory study

The chemisorption of atoms (H, N, S, O, and C) on Cu surfaces has been systematically studied by the density functional theory generalized gradient approximation method with the slab model. Our calculated results indicate that the orders of the adsorption energy are H < N < S < O < C on Cu(111) and H < N < O < S < C on Cu(110) and Cu(100). Furthermore, the adsorption energies of the given atoms on Cu(100) are larger than those on Cu(111) and Cu(110). The preferred adsorption sites are a 3-fold hollow site on Cu(111) and a 4-fold hollow site on Cu(100), but the preferred adsorption sites on Cu(110) are different for different adatoms. The energy, as well as the geometry, is in good agreement with the experimental and other theoretical data. In addition, this study focuses on the electronic and geometric properties of the metal-atom (M-A) bond to explain the difference in adsorption energies among adatoms. A detailed investigation of the density of states curves explains the nature of the most stable site. Finally, we test the effect of the coverage and find that the surface coverage has no influence on the preferred adsorption sites of the given adatoms on Cu(110) with the exception of hydrogen and oxygen, but has much influence on the value of the adsorption energy.     

Behavior of molecular hydrogen on the platinum crystal surface: Quantum-chemical modeling

The interaction of molecular hydrogen with the (111), (110), and (100) surfaces of the platinum crystal has been modeled by the density functional theory method within the generalized gradient approximation (GGA). The (100) surface is the least energetically favorable one, while the (111) and (110) surfaces are close in energy. The hydrogen molecule is attached to all three types of surfaces without a barrier. The largest decrease in energy is realized for the (100) surface. The bidentate coordination of hydrogen atoms is typical of the (100) and (110) surfaces, and the tridentate coordination is characteristic of the (111) surface. The H atoms can migrate over the crystal surface, overcoming moderate potential barriers of ??0.1?C0.2 eV; however, over the (110) surface, migration is possible only along the ridges. The maximal number of attached atoms per surface atom is close to unity for the (111) or (110) surface and to 1.67 for the (100) surface.     

苯乙烯在Ag(111)和Ag(110)表面环氧化反应结构敏感性的理论研究

采用密度泛函理论(DFT)对苯乙烯在Ag(110)表面和Ag(111)表面的环氧化反应进行了计算研究. 经计算, 在Ag(110)表面预吸附氧原子更易吸附在3 重穴位(3h), 吸附能为-3.59 eV; 在Ag(111)表面预吸附氧原子的最稳定吸附位是fcc 位, 吸附能为-3.69 eV. 苯乙烯的环氧化反应过程首先经过一个金属中间体, 然后再进一步反应变为产物, 其中经过直链中间体较支链中间体更加有利. Ag(110)面的反应活化能一般大于Ag(111)面的, 并且微观动力学模拟结果表明, Ag(111)表面生成环氧苯乙烷的选择性要明显高于Ag(110)表面(0.38 与 0.003), 原因是Ag(111)面环氧化反应活化能小于苯乙醛及燃烧中间体的活化能, 而在Ag(110)上正相反.     

Prediction of a highly activated state of CO adsorbed on an Al/Fe(100) bimetallic surface

Using periodic slab density functional theory, we investigate CO adsorption, diffusion, and dissociation energetics on a monolayer of Al covering Fe(100) [Al/Fe(100)]. We predict a weakly chemisorbed state of CO to exist on Al/Fe(100), with CO adsorbing on the 4-fold hollow site in a very tilted fashion. This state is predicted to have an extremely low CO stretching frequency of only 883 cm(-1), indicating a dramatically weakened CO bond relative to gaseous CO, even though the molecule is predicted to bind to Al/Fe(100) quite weakly. We predict that dissociation of CO starting from this weakly adsorbed state has a barrier of only approximately 0.35 eV, which is approximately 0.70 eV lower than that on Fe(100). To understand how the underlying substrate changes the electronic properties of the supported Al monolayer, we compare CO adsorption on Al/Fe(100) to its adsorption on analogous pure Al(100) surfaces. This highly activated yet weakly bound state of CO on Al/Fe(100) suggests that Al/Fe(100) could be an effective low-temperature bimetallic catalyst in reducing environments.     

Density functional theory study of pyrrole adsorption on Mo(110)

The objective of the present study is to identify possible adsorption configurations of pyrrole on Mo(110) using density functional theory (DFT) calculations. Several adsorption configurations were studied including pyrrole and pyrrolyl adsorption as parallel, perpendicular, and tilted adsorption modes relative to the Mo(110) surface plane. Based on the DFT calculations, pyrrole is suggested to adsorb in a parallel mode with respect to the Mo(110) surface through its pi-orbital as mu3,eta(5)-Pyr-0 degrees with an adsorption energy of -28.7 to -31.5 kcal mol(-1). The possibility of a coexisting mode where pyrrole adsorbs on the surface with a slightly tilted molecular plane as mu3,eta(4)(N,C2,C3,C4)-Pyr-5 degrees is also likely to occur, particularly at higher pyrrole coverages. The slightly tilted configuration is suggested to arise from the lateral interactions of adsorbed pyrrole on Mo(110), and not the result of a phase transformation on the surface since this requires a relatively high activation energy as indicated by additional linear synchronous transit (LST)/quadratic synchronous transit (QST) calculations. Both adsorption geometries bond to three surface Mo atoms, and Mo(110) did not promote hydrogen abstraction. Pyrrolyl adsorption on Mo(110) is energetically possible, but unlikely to occur because gas-phase hydrogen has not been previously experimentally observed as a pyrrole decomposition product on Mo(110).     

吸附O的Cu(110)c(2×1)表面原子结构和电子态

采用第一性原理的密度泛函理论方法计算了清洁Cu(110)表面和吸附O原子的Cu(110) c(2×1)表面的原子结构, 结构弛豫和电子结构, 得到了各种表面结构参数. 分别计算了O原子在Cu(110)表面三个可能吸附位置吸附后的能量, 并给出了能量最低的吸附位置上各层原子的弛豫特性和态密度. 结果表明O吸附后的Cu(110)表面有附加列(added-row)再构的特性, O原子吸附在最表层铜原子上方, 与衬底Cu原子的垂直距离为0.016 nm, 以氧分子为能量基准的吸附能为－1.94 eV; 同时由于Cu 3d- O 2p态的杂化作用使得低于费米能级5.5~6.0 eV的范围内出现了局域的表面态. 计算得到清洁的和氧吸附的Cu(110)表面的功函数分别为4.51 eV和4.68 eV. 电子态密度的结果表明：在Cu(110) c(2×1) 表面O吸附的结构下, 吸附O原子和金属衬底之间的结合主要是由于最表层Cu原子3d态和O原子2p态的相互作用.     

氢原子在Ni(100), Ni(111)和Ni(110)面上吸附扩散势能面的结构

本文构造了氢-镍相互作用的5参数Morse势, 用经典的对势方法研究氢原子在Ni(100), Ni(111)和Ni(110)面上的吸附和扩散, 得到氢原子在三个表面上的吸附位、吸附几何、结合能及本征振动等数据, 和实验结果符合得很好。同时, 系统地研究了三个体系的吸附扩散势能面结构。     

Orientation dependent oxygen exchange kinetics on single crystal SrTiO(3) surfaces

The perovskite SrTiO(3) is arguably one of the most important oxide systems in condensed matter research. In this study, we report measurement of the orientation dependence of oxygen exchange on SrTiO(3) single crystal surfaces by dynamic conductivity measurements under electrochemical perturbations. Activation energy for electrical conduction in the 923-1223 K range at an oxygen partial pressure of ～10(-11) Pa of (100), (111), and (110) single crystals was found to be 2.6 eV, 2.7 eV, and 3.1 eV, respectively. The equilibration kinetics show profound dependence on the surface orientation and are modelled using a heterogeneous relaxation process. All surfaces show similar cationic sub-lattice limited rate behavior with (111), (100), and (110) having the fastest, intermediate, and slowest rates, respectively. We discuss the orientation dependence and its relation to local atomic structure in light of previous experimental and theoretical studies.