A first-principles study of molecular oxygen dissociation at an electrode surface: a comparison of potential variation and coadsorption effects

Influences of coadsorbed sodium and water, aqueous solvent, and electrode potential on the kinetics of O(2) dissociation over Pt(111) are systematically investigated using density functional theory models of vacuum and electrochemical interfaces. Na coadsorption alters the electronic states of Pt to stabilize the reactant (O(2)*), transition, and product (2O*) states by facilitating electron donation to oxygen, causing a more exothermic reaction energy (-0.84 eV for Na and O(2), -0.81 eV for isolated O(2)) and a decrease in dissociation barrier (0.39 eV for Na and O(2), 0.57 eV for isolated O(2)). Solvation decreases the reaction energy (-0.67 eV) due to enhanced hydrogen bond stabilization of O(2)* compared to 2O*. The influence of Na is less pronounced at the solvated interface (barrier decreases by only 0.11 eV) because H(2)O screens Na charge-donation. In the electrochemical model system, the dissociation energy becomes more exothermic and the barrier decreases toward more positive potentials. Potential-dependent behavior results from changes in interfacial dipole moment and polarizability between O(2)*, the dissociation transition state, and 2O*; each are influenced by changes in adsorption and hydrogen bonding. Coadsorption of Na in the solvated system dampens the dipole moment change between O(2)* and 2O* and significantly increases the polarizability at the dissociation transition state and for 2O*; the combination causes little change in the reaction energy but reduces the activation barrier by 0.08 eV at 0 V versus NHE. The potential-dependent behavior contrasts that determined at a constant surface charge or from an applied electric field, illustrating the importance of considering the electrochemical potential at the fully-solvated interface in determining reaction energetics, even for non-redox reactions.     

The reaction pathways for HSCH3 adsorption on Au(111): a density functional theory study

Density functional theory was used to investigate the reaction pathways for HSCH(3) adsorption on Au(111) at low coverage. A molecular adsorbed state was found with the S atom bond on Top sites (E approximately -0.38 eV) and molecular adsorption is nonactivated. The H-SCH(3) dissociation process is energetically less favorable and becomes slightly exothermic only when surface relaxation is considered (DeltaE approximately -0.2 eV). All the reaction pathways present a sizable activation energy barrier, with the lowest being approximately 0.52 eV (0.41 eV taking into account slab relaxation). In the corresponding saddle point of the potential energy surface, the S atom of the methylthiolate molecule is placed on Top sites and the H near a Bridge site. The high barrier obtained explains the complete absence of reactive methanethiol dissociation found in recent experiments.     

Theoretical study of the geometries and dissociation energies of molecular water on neutral aluminum clusters Al(n) (n = 2-25)

Geometries and dissociation energies of water molecules on Al(n) (n = 2-25) clusters were investigated using density functional theory with all electron relativistic spin-polarized calculations under the generalized gradient approximation. An extensive structure search was performed to identify the low-energy conformations of Al(n)H(2)O complexes for each size. Optimal adsorption sites were assigned for low-energy isomers of the clusters. Size and site specific dependences were studied for the Al(n)H(2)O complexes in stabilities, geometries, adsorption energies, dissociation energies, Al-O bond lengths, and other characteristic quantities. The stabilities and geometries revealed that H atom in H(2)O is not inclined to bond with Al atoms. The most stable Al(n)H(2)O configurations for each size tend to correspond to the most stable bare Al(n) cluster except of Al(6) and Al(24) clusters. The HO bond lengths increase generally 0.01 ? with respect to the isolated H(2)O in all of the adsorption complexes. The dissociation energy of an isolated H(2)O into HO and H was 5.39 eV, which decreased about two-thirds to the energy range of 0.83-2.12 eV with the help of Al(n) clusters. In spite of the fluctuations, the dissociation energies of Al(n)H(2)O complexes rise with the size increasing as a whole. In addition, we also found that the bare Al(n) clusters with high vertical ionization potentials usually have high dissociation energies of H(2)O in the corresponding adsorption models. The energetically preferred spin-multiplicity of all the odd-n Al(n)H(2)O complexes is doublet, and it is singlet for all the even-n complexes with exception of Al(2)H(2)O which is triplet.     

甲硫醇在Cu(111)表面的解离吸附: 密度泛函理论研究

采用基于密度泛函理论的第一性原理方法和平板模型研究了CH₃SH分子在Cu(111)表面的吸附反应.系统地计算了S原子在不同位置以不同方式吸附的一系列构型, 第一次得到未解离的CH₃SH分子在Cu(111)表面顶位上的稳定吸附构型,该构型吸附属于弱的化学吸附, 吸附能为0.39 eV. 计算同时发现在热力学上解离结构比未解离结构更加稳定. 解离的CH₃S吸附在桥位和中空位之间, 吸附能为0.75-0.77 eV. 计算分析了未解离吸附到解离吸附的两条反应路径, 最小能量路径的能垒为0.57 eV. 计算结果还表明S―H键断裂后的H原子并不是以H₂分子的形式从表面解吸附而是以与表面成键的形式存在. 通过比较S原子在独立的CH₃SH分子和吸附状态下的局域态密度, 发现S―H键断裂后S原子和表面的键合强于未断裂时S原子和表面的键合.     

A DFT study of H2O dissociation on metal‐precovered Fe (100) surface

The H₂O adsorption and dissociation on the Fe (100) surface with different precovered metals are studied by density functional theory. On both kinds of metal‐precovered surface, H₂O molecules prefer adsorb on hollow sites than bridge and top sites. The impurity energy difference is proportional to the adsorption energy, but the adsorbates are not sensitive to the adsorption orientation and height relative to the surface. The Hirshfeld charge analysis shows that water molecules act as an electron donor while the surface Fe atoms act as an electron acceptor. The rotation and dissociation of H₂O molecule occur on the Co‐ and Mn‐precovered surfaces. Some H₂O molecules are dissociated into OH and H groups. The energy barriers are about 0.5 to 1.0 eV, whose are consistence with the experimental data. H₂O molecules can be dissociated more easily at the top site on Co‐precovered surface 1 than that at bridge site on Mn‐precovered surface 2 because of the lower reaction barrier. The dispersion correction effects on the energies and adsorption configurations on Co‐precovered surface 1 were calculated by OBS + PW91. The dispersion contributions can improve a bit of the bond energy of adsorbates and weaken the hydrogen bond effect between adsorption molecules a little.     

Ru掺杂对Fe催化剂上酚类化合物加氢脱氧影响的密度泛函理论研究北大核心CSCD

采用密度泛函理论(DFT)计算研究了苯酚、邻甲酚、愈创木酚在不同结构Ru-Fe(211)表面上吸附活化性能和加氢脱氧反应路径.结果表明,Ru掺杂能促进H2分子在Fe(211)表面上解离,提高加氢脱氧反应速率.酚类在1Ru_(ads)-Fe(211)表面上吸附比在1Ru_(sub)-Fe(211)表面上更稳定,苯酚和邻甲酚脱羟基步骤能垒分别降低0.13和0.28 eV,有利于生成芳烃.愈创木酚在1Ru_(sub)-Fe(211)表面上加氢脱氧优势路径是先脱甲氧基生成苯酚,苯酚再加氢脱氧生成产物苯(速控步骤能垒1.16 eV);而在1Ru_(ads)-Fe(211)表面上愈创木酚先脱羟基再脱甲基生成苯酚的路径更具有动力学优势(速控步骤能垒1.21 eV).计算结果表明Ru掺杂方式影响Fe催化剂对酚反应分子的吸附稳定性以及加氢脱氧反应路径和性能.与1Ru掺杂Fe(211)催化剂相比,增加Ru原子数形成4Ru_(ads)-Fe(211),能够进一步提高酚类反应物的吸附强度,但导致加氢脱氧反应能垒升高.因此,在Fe催化剂上以表面吸附的形式掺杂少量贵金属Ru更利于酚类加氢脱氧生成芳烃.     

Effect of Hydrogen on O2 Adsorption and Dissociation on a TiO2 Anatase (001) Surface

The effect of hydrogen on the adsorption and dissociation of the oxygen molecule on a TiO₂ anatase (001) surface is studied by first‐principles calculations coupled with the nudged elastic band (NEB) method. Hydrogen adatoms on the surface can increase the absolute value of the adsorption energy of the oxygen molecule. A single H adatom on an anatase (001) surface can lower dramatically the dissociation barrier of the oxygen molecule. The adsorption energy of an O₂ molecule is high enough to break the O?O bond. The system energy is lowered after dissociation. If two H adatoms are together on the surface, an oxygen molecule can be also strongly adsorbed, and the adsorption energy is high enough to break the O?O bond. However, the system energy increases after dissociation. Because dissociation of the oxygen molecule on a hydrogenated anatase (001) surface is more efficient, and the oxygen adatoms on the anatase surface can be used to oxidize other adsorbed toxic small gas molecules, hydrogenated anatase is a promising catalyst candidate.     

Agostic interactions and dissociation in the first layer of water on Pt(111)

Recent quantum mechanical (QM) calculations for a monolayer of H(2)O on Ru(0001) suggested a novel stable structure with half the waters dissociated. However, different studies on Pt(111) suggested an undissociated bilayer structure in which the outer half of the water has the OH bonds toward the surface rather than the O lone pair. Since water layers on Pt are important in many catalytic processes (e.g., the fuel cell cathode), we calculated the energetics and structure of the first monolayer of water on the Pt(111) surface using QM [periodic slab using density functional calculations (DFT) with the PBE-flavor of exchange-correlation functional]. We find that the fully saturated surface ((2)/(3) ML) has half the water almost parallel to the surface (forming a Pt-O Lewis acid-base bond), whereas the other half are perpendicular to the surface, but with the H down toward the surface (forming a Pt-HO agostic bond). This leads to a net bond energy of 0.60 eV/water = 13.8 kcal/mol (the standard ice model with the H up configuration of the water molecules perpendicular to the surface is less stable by 0.092 eV/water = 2.1 kcal/mol). We examined whether the partial dissociation of water proposed for Ru(0001) could occur on Pt(111). For the saturated water layer ((2)/(3) ML) we find a stable structure with half the H(2)O dissociated (forming Pt-OH and Pt-H covalent bonds), which is less favorable by only 0.066 eV/water = 1.51 kcal/mol. These results confirm the interpretation of combined experimental (XAS, XES, XPS) and theoretical (DFT cluster and periodic including spectrum calculations) studies, which find only the H down undissociated case. We find that the undissociated structure leads to a vertical displacement between the two layers of oxygens of approximately 0.42 A (for both H down and H up). In contrast, the partially dissociated system leads to a flat structure with a separation of the oxygen layers of 0.08 A. Among the partially dissociated systems, we find that all subsurface positions for the dissociated hydrogen are less favorable than adsorbing on top of the free Pt surface atom. Our results suggest that for less than (1)/(3) ML, clustering would be observed rather than ordered monolayer structures.     

Coverage dependence and hydroperoxyl-mediated pathway of catalytic water formation on Pt (111) surface

Hydrogen oxidation on Pt (111) surface is modeled by density functional theory (DFT). Previous DFT calculations showed too large O2 dissociation barriers, but we find them highly coverage dependent: when the coverage is low, dissociation barriers close to experimental values (approximately 0.3 eV) are obtained. For the whole reaction, a new pathway involving hydroperoxyl (OOH) intermediate is found, with the highest reaction barrier of only approximately 0.4 eV. This may explain the experimental observation of catalytic water formation on Pt (111) surface above the H2O desorption temperature of 170 K, despite that the direct reaction between chemisorbed O and H atoms is a highly activated process with barrier approximately 1 eV as previous calculations showed.     

Pd掺杂对ZnO(1120)面上水解离的影响

采用密度泛函理论计算研究了清洁的以及Pd掺杂的ZnO(1120)面上水分子的吸附和解离.结果表明,在清洁ZnO(1120)上,水分子倾向于分子吸附,解离吸附较为困难.在Pd掺杂的ZnO上,水分子仍倾向吸附在Zn原子上,且吸附能与其在清洁ZnO表面的相当.然而,Pd的掺杂可增强水解离产物OH和H的吸附,从而显著提高了水的解离活性,相应的水解离能垒为0.36eV,放热0.21eV.     

Density functional theory study of water dissociation in a double water bilayer with or without coadsorption of CO on Pt(111)

Using density functional theory, we investigate the structure of the double water bilayer with or without coadsorption of CO on Pt(111). The double water bilayer consists of two bilayers. Each bilayer is buckled with every second water molecule being closer to the surface than every other water molecule. CO is found to adsorb most strongly when substituting in the first bilayer, the water molecule closest to the surface. Dissociation of H2O in the water bilayer (with or without CO) is further considered. A great number of pathways for the dissociation are studied. These include homolytic pathways where both dissociation products end up adsorbed on the Pt surface and heterolytic pathways where only the OH is adsorbed, while a proton is transferred to the water adlayers. We find that the heterolytic dissociation pathways are energetically more favorable than the homolytic ones, yet they are all rather endothermic. The most favorable pathways found have reaction energies of 0.60 and 0.52 eV without and with CO present. The corresponding activation energies are 0.99 and 0.53 eV, respectively.     

Comparisons of multilayer H2O adsorption onto the (110) surfaces of alpha-TiO2 and SnO2 as calculated with density functional theory

Mono- and bilayer adsorption of H2O molecules on TiO2 and SnO 2 (110) surfaces has been investigated using static planewave density functional theory (PW DFT) simulations. Potential energies and structures were calculated for the associative, mixed, and dissociative adsorption states. The DOS of the bare and hydrated surfaces has been used for the analysis of the difference between the H2O interaction with TiO2 and SnO 2 surfaces. The important role of the bridging oxygen in the H2O dissociation process is discussed. The influence of the second layer of H2O molecules on relaxation of the surface atoms was estimated.     

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。     

H2分子在LaFeO3表面吸附的第一性原理研究

基于密度泛函理论的第一性原理方法,通过计算表面能确定LaFeO₃(010)表面为最稳定的吸附表面,研究了H₂分子在LaFeO₃(010)表面的吸附性质。LaFeO₃(010)表面存在LaO和FeO₂两种终止表面,但吸附主要发生在FeO₂终止表面,由于LaFeO₃(010)表面弛豫的影响,使得凹凸不平的表面层增加了表面原子与H原子的接触面积,表面晶胞的纵向体积增加约2.5%,有利于H原子向晶体内扩散。研究发现,H₂分子在LaFeO₃(010)表面主要存在3种化学吸附方式:第一种吸附发生在O-O桥位,2个H原子分别吸附在2个O原子上,形成2个-OH基,这是最佳吸附位置,此时H原子与表面O原子的作用主要是H1s与O2p轨道杂化作用的结果,H-O之间为典型的共价键。H₂分子的解离能垒为1.542 eV,说明表面需要一定的热条件,H₂分子才会发生解离吸附;第二种吸附发生在Fe-O桥位,1个H原子吸附在O原子上形成1个-OH基,另一个H原子吸附在Fe原子上形成金属键;第三种吸附发生在O顶位,2个H原子吸附在同一个O原子上,形成H₂O分子,此时H₂O分子与表面形成物理吸附,H₂O分子逃离表面后容易形成氧空位。此外,H₂分子在LaFeO₃(010)表面还可以发生物理吸附。     

Metal-Oxo Electronic Tuning via In Situ CO Decoration for Promoting Methane Conversion to Oxygenates over Single-Atom Catalysts

Direct methane conversion (DMC) to oxygenates at low temperature is of great value but remains challenging due to the high energy barrier for C−H bond activation. Here, we report that in situ decoration of Pd₁-ZSM-5 single atom catalyst (SAC) by CO molecules significantly promoted the DMC reaction, giving the highest turnover frequency of 207 h⁻¹ ever reported at room temperature and ~100 % oxygenates selectivity with H₂O₂ as oxidant. Combined characterizations and DFT calculations illustrate that the C-atom of CO prefers to coordinate with Pd₁, which donates electrons to the Pd₁−O active center (L−Pd₁−O, L=CO) generated by H₂O₂ oxidation. The correspondingly improved electron density over Pd−O pair renders a favorable heterolytic dissociation of C−H bond with low energy barrier of 0.48 eV. Applying CO decoration strategy to M₁-ZSM-5 (M=Pd, Rh, Ru, Fe) enables improvement of oxygenates productivity by 3.2–11.3 times, highlighting the generalizability of this method in tuning metal-oxo electronic structure of SACs for efficient DMC process.     

Density-functional study for the NOx (x = 1, 2) dissociation mechanism on the Cu(1 1 1) surface

Spin-polarized density functional theory calculation is employed to study the adsorption and dissociation of NO₂ molecule on Cu(1 1 1) surface. It is shown that the most favorable adsorption structure is the NO₂ (T,T-O-,O′-nitrito) configuration which has an adsorption energy of −1.49 eV. The barriers for step-wise NO₂ dissociation reaction, NO_2(g) → N_(a) + 2O_(a), are 1.05 (for O–N–O bond activation), and 2.08 eV (for N–O bond activation), respectively, and the entire process is 0.6 eV exothermic. The energetics of single N–O dissociation with and without the presence of N atom or O atom on the surface are also calculated. The results indicate that in the presence of O atom on Cu(1 1 1) surface would raise the N–O dissociation barrier, whereas in the presence of N atom decrease it. The interaction nature between adsorbates and substrate is analyzed by the local density of states (LDOS) calculation.     

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.     

Slab model studies of water adsorption and decomposition on clean and X- (X = C, N and O) contaminated Pd(111) surfaces

To explore the effect of surface contaminants on water chemistry at metallic surfaces, adsorption and decomposition of water monomers on clean and X/Pd(111)(X = C, N and O) surfaces are investigated based on density functional theory calculations. It is revealed that H(2)O binds to Pd(111) surface primarily through the mixing of its 1b(1) with the Pd 4d(z(2)) state. A charge accumulation between the oxygen atom of water and the bound Pd atom is calculated, which is found to be relevant to the H(2)O-Pd interaction. Water adsorption results in a reduction of surface work function and the polarization of the X 2p states. The O-H bond scission of H(2)O on the clean Pd(111) is an energy unfavorable process. In the case of X-assisted O-H bond breaking on X/Pd(111) surfaces, however, the reaction barrier tends to be lower than that on the clean surface and decreases from C/Pd(111) to O/Pd(111). In particular, water decomposition is found to become feasible on O/Pd(111), in agreement with the experimental observations. The calculated barrier is demonstrated to be correlated linearly with the density of X 2p states at the Fermi level. A thorough energy analysis demonstrates that the following geometrical and electronic factors favor the barrier reduction on X/Pd(111) with respect to water decomposition on clean Pd(111): (i) the less deformed structure of water in TS; (ii) the decreased bonding competition between the fragments OH and H. The remarkable decrease of the barrier on O/Pd(111) is revealed to be due to the largest stabilization of the split H atom and the least deformation of water in the TS.     

Electron scattering on OH-(H2O)n clusters (n = 0-4)

The cross sections for electron scattering on OH-(H2O)n for n = 0-4 were measured from threshold to approximately 50 eV. All detachment cross sections were found to follow the classical prediction given earlier [Phys. Rev. Lett. 74, 892 (1995)] with a threshold energy for electron-impact detachment that increased upon sequential hydration, yielding values in the range from 4.5 eV +/- 0.2 eV for OH- to 12.10 eV +/- 0.5 eV for OH-(H2O)4. For n > or = 1, we found that approximately 80% of the total reaction events lead to electron detachment plus total dissociation of the clusters into the constituent molecules of OH and H2O. Finally, we observed resonances in the cross sections for OH-(H2O)3 and for OH-(H2O)4. The resonances were located at approximately 15 eV and were ascribed to the formation of dianions in excited states.     

CO诱导的FeO(111)/Ru(0001)负载Au原子吸附位和电荷的改变

采用密度泛函理论研究了CO气氛对FeO(111)/Ru(0001)负载Au原子吸附位、电荷及其稳定性的影响. 首先考察了FeO(111)单层薄膜在Ru(0001)表面上的界面结构. 研究发现,表面莫尔超晶胞内的HCP区域有最小的Fe-O层间距(rumpling),且Fe和O原子均与衬底Ru形成化学键. Au原子在FeO/Ru(0001)上最稳定的吸附在HCP区域的Fe-bridge位. 其中,Au原子诱导两个Fe原子从O原子层的下面翻转到其上面,形成两个Au-Fe键,且Au带负电. 当把体系暴露在CO气氛下后,CO能诱导Au原子从原来最稳定的Fe-bridge位转移到其邻近的O-top位,伴随着Au的电荷从负变到正,形成非常稳定的Au⁺-CO羰基物. 结果表明,反应气氛对负载金属催化剂的化学状态及其稳定性的影响很大; 同时也强调了反应条件下催化剂原位表征的重要性.