Ethanol photo-oxidation on a rutile TiO2(110) single crystal surface

The reaction of ethanol has been studied on the surface of rutile TiO(2)(110) by Temperature Programmed Desorption (TPD), online mass spectrometry under UV excitation and photoelectron spectroscopy while the adsorption energies of the molecular and dissociative modes of ethanol were computed using the DFT/GGA method. The most stable configuration is the dissociative adsorption in line with experimental results at room temperature. At 0.5 ML coverage the adsorption energy was found equal to 80 kJ mol(-1) for the dissociative mode (ethoxide, CH(3)CH(2)O(a) + H(a)) followed by the molecular mode (67 kJ mol(-1)). The orientation of the ethoxides along the [001] or [110] direction had minor effect on the adsorption energy although affected differently the Ti and O surface atomic positions. TPD after ethanol adsorption at 300 K indicated two main reactions: dehydration to ethylene and dehydrogenation to acetaldehyde. Pre-dosing the surface with ethanol at 300 K followed by exposure to UV resulted in the formation of acetaldehyde and hydrogen. The amount of acetaldehyde could be directly linked to the presence of gas phase O(2) in the vacuum chamber. The order of this photo-catalytic reaction with respect to O(2) was found to be 0.5. Part of acetaldehyde further reacted with O(2) under UV excitation to give surface acetate species. Because the rate of photo-oxidation of acetates (acetic acid) was slower than that of ethoxides (ethanol), the surface ended up by being covered with large amounts of acetates. A reaction mechanism for acetaldehyde, hydrogen and acetate formation under UV excitation is proposed.     

Interaction of oxygen with TiN(001): N<-->O exchange and oxidation process

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450 K. At the lowest temperature, there was chemisorption of oxygen (O(2,gas)-->2O(ads)) without significant surface oxidation. In contrast, at 450 K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (DeltaE=2.84 eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2 -->TiOx + NO reaction is an important exit channel for nitrogen in the oxidation process.     

First principles analysis of H2O adsorption on the (110) surfaces of SnO2, TiO2 and their solid solutions

Both associative and dissociative H(2)O adsorption on SnO(2)(110), TiO(2)(110), and Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces have been investigated at low ((1)/(12) monolayer (ML)) and high coverage (1 ML) by density functional theory calculations using the Gaussian and plane waves formalism. The use of a large supercell allowed the simulation at low symmetry levels. On SnO(2)(110), dissociative adsorption was favored at all coverages and was accompanied by stable associative H(2)O configurations. Increasing the coverage from (1)/(12) to 1 ML stabilized the (associatively or dissociatively) adsorbed H(2)O on SnO(2)(110) because of the formation of intermolecular H bonds. In contrast, on TiO(2)(110), the adsorption of isolated H(2)O groups ((1)/(12) ML) was more stable than at high coverage, and the favored adsorption changed from dissociative to associative with increasing coverage. For dissociative H(2)O adsorption on Ti-enriched Sn(1-x)Ti(x)O(2)(110) surfaces with Ti atoms preferably located on 6-fold-coordinated surface sites, the analysis of the Wannier centers showed a polarization of electrons surrounding bridging O atoms that were bound simultaneously to 6-fold-coordinated Sn and Ti surface atoms. This polarization suggested the formation of an additional bond between the 6-fold-coordinated Ti(6c) and bridging O atoms that had to be broken upon H(2)O adsorption. As a result, the H(2)O adsorption energy initially decreased, with increasing surface Ti content reaching a minimum at 25% Ti for (1)/(12) ML. This behavior was even more accentuated at high H(2)O coverage (1 ML) with the adsorption energy decreasing rapidly from 145.2 to 101.6 kJ/mol with the surface Ti content increasing from 0 to 33%. A global minimum of binding energies at both low and high coverage was found between 25 and 33% surface Ti content, which may explain the minimal cross-sensitivity to humidity previously reported for Sn(1-x)Ti(x)O(2) gas sensors. Above 12.5% surface Ti content, the binding energy decreased with increasing coverage, suggesting that the partial desorption of H(2)O is facilitated at a high fractional coverage.     

Pressure-induced intermetallic valence transition in BiNiO3

The valence state change of BiNiO3 perovskite under pressure has been investigated by a powder neutron diffraction study and electronic-state calculations. At ambient pressure, BiNiO3 has the unusual charge distribution Bi(3+)(0.5)Bi(5+)(0.5)Ni(2+)O3 with ordering of Bi(3+) and Bi(5+)charges on the A sites of a highly distorted perovskite structure. High-pressure neutron diffraction measurements and bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high-pressure phase is metallic Bi(3+)Ni(3+)O3. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable.     

水在金红石型TiO2（110）表面〈001〉阶梯边缘吸附的第一性原理研究

利用第一性原理计算方法,研究了水在金红石型TiO2（110）表面及〈001〉阶梯边缘处的吸附．关于水在（11O）表面上的吸附,研究表明,对不同的吸附率,水都是以分子模式吸附在表面．关于水在〈001〉9梯边缘处的吸附,研究表明,其吸附模式和吸附率有密切的联系．当水的吸附率为一个单层（1ML）时,分子吸附和解离吸附对应的吸附能分别为0．92和0．60eV,分子吸附模式更稳定．当吸附率降为1／2ML时,分子吸附和解离吸附所对应的吸附能分别为0．86和0．84eV,两种吸附模式都可能存在．在表面上,不同吸附模式的吸附能随吸附率变化趋势是一致的．而在〈001〉阶梯边缘处,对于不同的吸附模式,吸附能随吸附率的变化呈现出不同的变化趋势．这是由在〈001〉阶梯边缘处低吸附率时解离模式的独特结构引起的．     

Adsorption and dissociation of the methanethiol (CH3SH) molecule on the Fe(100) surface

These contributions explore interaction modes between the methanethoil (CH₃SH) molecule and the Fe(100) surface via implementing accurate density functional theory (DFT) calculations with the inclusion of van der Waals corrections. We consider three adsorption sites over the Fe(100) surface, namely, top(T), bridge (B), and hollow (H) sites as potential catalytic active sites for the molecular and dissociative adsorption of the CH₃SH molecule. The molecular adsorption structures are found to occupy either B or T sites with former sites holding higher stability by 0.17 eV. The inclusion of van der Waals corrections refound to slightly alter adsorption energies. For instance, adsorption energies increased by ~ 0.18 and ~ 0.21 eV for B and T structure, respectively, in reference to values obtained by the plain generalized gradient approximation (GGA) functional. A stability ordering of the dissociation products was found to follow the sequence (CH₄, S) > (CH₃, S, H) > (─SCH_3, H) > (─CH₃, SH). The differential charge density distributions were examined to underpin prominent electronic contributing factors. Direct fission of C─S bond in the CH₃SH molecule attains exothermic values in the range 2.0 to 2.1 eV. The most energetically favorable sites for the surface-mediated fission of the thiol's S─H bond correspond to the structure where the ─SCH₃ and H are both situated on hollow sites with an adsorption energy of −2.43 eV. Overall, we found that inclusion of van der Waals functional to change the binding energies more noticeably in case of dissociative adsorption structures. The results presented herein should be instrumental in efforts that aim to design stand-alone Fe desulfurization catalysts.     

Insights into electrocatalysis

When standard reversible potentials for bulk solution reactions, U(0), are known, the reversible potentials when the reactant and product are adsorbed on an electrocatalyst surface, U(surf)(rev), are given in terms of these potentials and the adsorption Gibbs energy bond strengths: U(surf)(rev) = U0 + D(ads)G (Ox)/F-Δ(ads)G (R)/F (i). When the Δ(ads)G (Ox) and Δ(ads)G (Red) values are known at potential U, this equation is exact. When the overpotential for a multi-electron transfer reaction is minimal, each electron transfer takes place at the standard reversible potential for the overall reaction. In the case of O(2) reduction to water via the intermediate step OOH(aq) → O(aq) + OH(aq), or via O(2)(g) → 2O(aq), the respective endergonic O-O dissociation Gibbs energies are shown to be 2.52 eV and 4.76 eV. When the oxygen product and water reactant adsorb weakly, as on platinum, the adsorption Gibbs energies, Δ(ads)G, for O, OH, and OOH intermediates can be uniquely predicted using these data. All of the above depend exclusively on experimentally determined data. Reversible potentials have been calculated for oxygen reduction steps on the platinum electrocatalyst surface using Interface 1.0, a comprehensive computational code for the potential dependence of the electrochemical interface. Using these results as benchmarks, is found to be accurate to around 0.1 V when the Δ(ads)G are values calculated for the potentials of zero charge, instead of 1.229 V, which is a significant simplification. The variation in Δ(ads)G values between the calculated potentials of zero charge and 1.229 V are found to be 0.2 eV V(-1) or less. Prior work, using internal adsorption energies calculated at the potential of zero charge in place of Gibbs energies in was found to be accurate to within about 0.2 V. On platinum Δ(ads)G of the reaction OOH(ads) → O(ads) + OH(ads) is calculated at the potential of zero charge for the reactant and product to be about 1.2 eV exergonic under Langmuir conditions, and this Gibbs energy loss reduces the 1.229 V four-electron reversible potential on the platinum surface to an effective reversible potential of about 0.93 V for this mechanism on platinum. The effective reversible potential is a consequence of efficiency loss, not kinetics. Based on these values, the onset potential for four-electron oxygen reduction will be less than or equal to the effective reversible potential and on pure Pt(111) it appears to be equal to it.     

Reactivity of sub 1 nm supported clusters: (TiO2)n clusters supported on rutile TiO2 (110)

Metal oxide clusters of sub-nm dimensions dispersed on a metal oxide support are an important class of catalytic materials for a number of key chemical reactions, showing enhanced reactivity over the corresponding bulk oxide. In this paper we present the results of a density functional theory study of small sub-nm TiO(2) clusters, Ti(2)O(4), Ti(3)O(6) and Ti(4)O(8) supported on the rutile (110) surface. We find that all three clusters adsorb strongly with adsorption energies ranging from -3 eV to -4.5 eV. The more stable adsorption structures show a larger number of new Ti-O bonds formed between the cluster and the surface. These new bonds increase the coordination of cluster Ti and O as well as surface oxygen, so that each has more neighbours. The electronic structure shows that the top of the valence band is made up of cluster derived states, while the conduction band is made up of Ti 3d states from the surface, resulting in a reduction of the effective band gap and spatial separation of electrons and holes after photon absorption, which shows their potential utility in photocatalysis. To examine reactivity, we study the formation of oxygen vacancies in the cluster-support system. The most stable oxygen vacancy sites on the cluster show formation energies that are significantly lower than in bulk TiO(2), demonstrating the usefulness of this composite system for redox catalysis.     

Surface dependence of CO2 adsorption on Zn2GeO4

An understanding of the interaction between Zn(2)GeO(4) and the CO(2) molecule is vital for developing its role in the photocatalytic reduction of CO(2). In this study, we present the structure and energetics of CO(2) adsorbed onto the stoichiometric perfectly and the oxygen vacancy defect of Zn(2)GeO(4) (010) and (001) surfaces using density functional theory slab calculations. The major finding is that the surface structure of the Zn(2)GeO(4) is important for CO(2) adsorption and activation, i.e., the interaction of CO(2) with Zn(2)GeO(4) surfaces is structure-dependent. The ability of CO(2) adsorption on (001) is higher than that of CO(2) adsorption on (010). For the (010) surface, the active sites O(2c)···Ge(3c) and Ge(3c)-O(3c) interact with the CO(2) molecule leading to a bidentate carbonate species. The presence of Ge(3c)-O(2c)···Ge(3c) bonds on the (001) surface strengthens the interaction of CO(2) with the (001) surface, and results in a bridged carbonate-like species. Furthermore, a comparison of the calculated adsorption energies of CO(2) adsorption on perfect and defective Zn(2)GeO(4) (010) and (001) surfaces shows that CO(2) has the strongest adsorption near a surface oxygen vacancy site, with an adsorption energy -1.05 to -2.17 eV, stronger than adsorption of CO(2) on perfect Zn(2)GeO(4) surfaces (E(ads) = -0.91 to -1.12 eV) or adsorption of CO(2) on a surface oxygen defect site (E(ads) = -0.24 to -0.95 eV). Additionally, for the defective Zn(2)GeO(4) surfaces, the oxygen vacancies are the active sites. CO(2) that adsorbs directly at the Vo site can be dissociated into CO and O and the Vo defect can be healed by the oxygen atom released during the dissociation process. On further analysis of the dissociative adsorption mechanism of CO(2) on the surface oxygen defect site, we concluded that dissociative adsorption of CO(2) favors the stepwise dissociation mechanism and the dissociation process can be described as CO(2) + Vo → CO(2)(δ-)/Vo → CO(adsorbed) + O(surface). This result has an important implication for understanding the photoreduction of CO(2) by using Zn(2)GeO(4) nanoribbons.     

The role of surface and subsurface point defects for chemical model studies on TiO2: a first-principles theoretical study of formaldehyde bonding on rutile TiO2(110)

We report a systematic investigation of the effects of different surface and subsurface point defects on the adsorption of formaldehyde on rutile TiO(2)(110) surfaces using density functional theory (DFT). All point defects investigated--including surface bridging oxygen vacancies, titanium interstitials, and subsurface oxygen vacancies--stabilize the adsorption significantly by up to 56 kJ mol(-1) at a coverage of 0.1 monolayer (ML). The stabilization is due to a decrease of the coordination (covalent saturation) of the surface Ti adsorption sites adjacent to the defects, which leads to a stronger molecule-surface interaction. This change in the Ti is caused by the removal of a neighboring atom (oxygen vacancies) or substantial lattice relaxations induced by the subsurface defects. On the stoichiometric reference surface, the most stable adsorption geometry of formaldehyde is a tilted η(2)-dioxymethylene (with an adsorption energy E(ads)=-125 kJ mol(-1)), in which a bond forms to a nearby bridging O atom and the carbonyl-O atom in the formaldehyde binds to a Ti atom in the adjacent fivefold coordinated lattice site. The η(1)-top configuration on five-coordinate Ti(4+) is much less favorable (E(ads)=-69 kJ mol(-1)). The largest stabilization is exerted by subsurface Ti interstitials between the first and second layers. These defects stabilize the η(2)-dioxymethylene structure by nearly 40 kJ mol(-1) to an adsorption energy of -164 kJ mol(-1). Contrary to popular belief, adsorption in a bridging oxygen vacancy (E(ads)=-86 kJ mol(-1)) is much less favorable for formaldehyde compared to the η(2)-dioxymethylene structures. From these results we conclude that formaldehyde will bind in the η(2)-dioxymethylene structure on the stoichiometric surface as well as in the presence of Ti interstitials and bridging oxygen vacancies. In the light of these substantial effects, we conclude that it is essential to include all the types of point defects present in typical, reduced rutile samples used for model studies, at realistic concentrations to obtain correct adsorption sites, structures, energetic, and chemi-physical properties.     

Density functional study of the interfacial electron transfer pathway for monolayer-adsorbed InN on the TiO(2) anatase (101) surface

Density functional theory (DFT) in connection with ultrasoft pseudopotential (USP) and generalized gradient spin-polarized approximations (GGSA) is applied to calculate the adsorption energies and structures of monolayer-adsorbed InN on the TiO(2) anatase (101) surface and the corresponding electronic properties, that is, partial density of states (PDOS) for surface and bulk layers of the TiO(2) anatase (101) surface and monolayer-adsorbed InN, to shed light on the possible structural modes for initial photoexcitation within the UV/vis adsorption region followed by fast electron injection through the InN/TiO(2) interface for an InN/TiO(2)-based solar cell design. Our calculated adsorption energies found that the two most probable stable structural modes of monolayer-adsorbed InN on the TiO(2) anatase (101) surface are (1) an end-on structure with an adsorption energy of 2.52 eV through N binding to surface 2-fold coordinated O (O(cn2)), that is, InN-O(cn2), and (2) a side-on structure with an adsorption energy of 3.05 eV through both N binding to surface 5-fold coordinated Ti (Ti(cn5)) and In bridging two surface O(cn2), that is, (O(cn2))(2)-InN-Ti(cn5). Our calculated band gaps for both InN-O(cn2) and (O(cn2))2-InN-Ti(cn5) (including a 1.0-eV correction using a scissor operator) of monolayer-adsorbed InN on the TiO(2) anatase (101) surface are red-shifted to 1.7 eV (730 nm) and 2.3 eV (540 nm), respectively, which are within the UV/vis adsorption region similar to Gratzel's black dye solar cell. Our analyses of calculated PDOS for both surface and bulk layers of the TiO(2) anatase (101) surface and monolayer-adsorbed InN on the TiO(2) anatase (101) surface suggest that the (O(cn2))(2)-InN-Ti(n5) configuration of monolayer-adsorbed InN on the TiO(2) anatase (101) surface would provide a more feasible structural mode for the electron injection through the InN/TiO(2) interface. This is due to the presence of both occupied and unoccupied electronic states for monolayer-adsorbed InN within the band gap TiO(2) anatase (101) surface, which will allow the photoexcitation within the UV/vis adsorption region to take place effectively, and subsequently the photoexcited electronic states will overlap with the unoccupied electronic states around the lowest conduction band of the TiO(2) anatase (101) surface, which will ensure the electron injection through the InN/TiO(2) interface. Finally, another thing worth our attention is our preliminary study of double-layer-adsorbed InN on the TiO(2) anatase (101) surface, that is, (O(cn2))(2)-(InN)(2)-Ti(cn5), with a calculated band gap red-shifted to 2.6 eV (477 nm) and a different overlap of electronic states between double-layer-adsorbed InN and the TiO(2) anatase (101) surface qualitatively indicated that there is an effect of the thickness of adsorbed InN on the TiO(2) anatase (101) surface on both photoexcitation and electron injection processes involved in the photoinduced interfacial electron transfer through InN/TiO(2). A more thorough and comprehensive study of different layers of InN adsorbed in all possible different orientations on the TiO(2) anatase (101) surface is presently in progress.     

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.     

Probing the reaction pathways of DL-proline on TiO2 (001) single crystal surfaces

The reaction of DL-Proline on O2-annealed (stoichiometric) and O-defected (sub-stoichiometric) TiO2 (001) single-crystal surfaces has been investigated. This is of significance in trying to understand the concept of how biomolecules interact with the surfaces of biomedical implants (molecular recognition). On an O2-annealed TiO2 surface, proline is found to largely adsorb then desorb intact at approximately 350 K. DFT (B3LYP) calculations of proline bound to a Ti(OH)4 cluster suggest a binding through the carboxylate functional group rather than through the NH group of the ring. In contrast, proline reaction was considerably different on the O-defected surface. First, proline was further stabilized, evidenced by a shift of its desorption temperature (during temperature-programmed desorption) to approximately 530 K. Along with proline desorption, two distinctive sets of reaction processes occurred at 530 and 630 K, respectively. The first pathway (alpha) at 530 K shows desorption of large amounts of m/e 55 (attributed to 1-azetine) and m/e 42 (attributed to ketene). At still higher temperature, 630 K, a pathway (beta) dominated by the appearance of low masses, mainly m/e 28, 27, and 26, is seen. These masses are tentatively attributed to desorption of HCN, ethylene, and/or acetylene as they represent the logical further decomposition of the different fragments of proline.     

CO在α-U(001)表面的吸附

采用密度泛函理论中的广义梯度近似,计算了CO在α-U(001)表面的吸附、解离和扩散.结果表明:CO分子以CU3OU2构型化学吸附在α-U(001)表面,吸附能为1.78-1.99eV;吸附后表层U原子向上迁移,伴随着褶皱的产生;CO分子与表面U原子的相互作用主要是U原子的电子向CO分子最低空轨道2π*转移,以及CO2π*/5σ/1π-U6d轨道间杂化而生成新的化学键;CO解离吸附较分子吸附在能量上更为有利,h1(C)+h2(O)和h1(C)+h1(O)(h:空位)解离态吸附能分别为2.71和3.08eV;近邻三重穴位之间C、O原子的扩散能垒分别为0.57和0.14eV,预示O原子较C原子更易在U(001)表面扩散迁移.     

Adsorption of small Au(n) (n = 1-5) and Au-Pd clusters inside the TS-1 and S-1 pores

We used a hybrid quantum-mechanics/molecular-mechanics (QM/MM) approach to simulate the adsorption of Au(n)() (n = 1-5), AuPd, and Au(2)Pd(2) clusters inside the TS-1 and S-1 pores. We studied nondefect and metal-vacancy defect sites in TS-1 and S-1 for a total of four different environments around the T6 crystallographic site. We predict stronger binding of all clusters near Ti sites in Ti-substituted framework compared to adsorption near Si sites-consistent with the experimental finding of a direct correlation between the Ti-loading and the Au-loading on the Au/TS-1 catalysts with high Si/Ti ratio. The cluster binding is also stronger near lattice-metal vacancies compared to fully coordinated, nondefect sites. In all the cases, a trend of binding energy (BE) versus Au cluster size (n) shows a peak at around n = 3-4. Our results show that there is enough room for the attack of H(2)O(2) on the Ti-defect site even with Au(1-4) adsorbed-a result that supports the possibility of H(2)O(2) spillover from the Au clusters to the adjacent Ti-defect sites. Mulliken charge analysis indicates that in all the cases there is electron density transfer to adsorbed clusters from the zeolite lattice. In the case of both gas-phase and adsorbed Au-Pd clusters, all the Pd atoms were positively charged, and all the Au atoms were negatively charged due to the higher electron-affinity of Au. We also found a correlation between the BE and the charge transfer to the clusters (the higher the charge transfer to the clusters, the higher the BE), and a universal correlation was found for Au(2-5) when BE and charge transfer were plotted on a per atom basis. A relatively larger charge transfer to the adsorbed clusters was found for the Ti sites versus the Si sites, and for the defect sites versus the nondefect sites. The trends in the BE were corroborated using Gibbs free energy of adsorption (DeltaG(ads)), and the implications of DeltaG(ads) in sintering of Au clusters are also discussed. Our results confirm that electronic factors such as cluster-charging are potentially important support effects for the Au/TS-1 catalyst.     

NO2在Ag/Pt(110)双金属表面上的吸附和分解

利用俄歇电子能谱(AES)和程序升温脱附谱(TDS)研究了NO₂在Ag/Pt(110)双金属表面的吸附和分解.室温下NO₂ 在Ag/Pt(110)双金属表面发生解离吸附, 生成NO(ads)和O(ads)表面吸附物种. 在升温过程中NO(ads)物种发生脱附或者进一步分解. 500 K时NO₂在Ag/Pt(110)双金属表面发生解离吸附生成O(ads)表面吸附物种. Pt 向Ag传递电子, 从而削弱Pt－O键的强度, 降低O(ads)从Pt 表面的并合脱附温度. 发现能够形成具有稳定组成的Ag/Pt(110)合金结构, 其表现出与Pt(110)-(1×2)相似的解离吸附NO₂能力, 但与O(ads)的结合明显弱于Pt(110)-(1×2). 该AgPt(110)合金结构是可能的低温催化直接分解氮氧化物活性结构.     

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。     

Characterizing TiO2(110) surface states by their work function

The unreconstructed TiO(2)(110) surface is prepared in well-defined states having different characteristic stoichiometries, namely reduced (r-TiO(2), 6 to 9% surface vacancies), hydroxylated (h-TiO(2), vacancies filled with OH), oxygen covered (ox-TiO(2), oxygen adatoms on a stoichiometric surface) and quasi-stoichiometric (qs-TiO(2), a stoichiometric surface with very few defects). The electronic structure and work function of these surfaces and transition states between them are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). The character of the surface is associated with a specific value of the work function that varies from 4.9 eV for h-TiO(2), 5.2 eV for r-TiO(2), 5.35 eV for ox-TiO(2) to 5.5 eV for qs-TiO(2). We establish the method for an unambiguous characterization of TiO(2)(110) surface states solely based on the secondary electron emission characteristics. This is facilitated by analysing a weak electron emission below the nominal work function energy. The emission in the low energy cut-off region appears correlated with band gap emission found in UPS spectra and is attributed to localised electron emission through Ti(3+)(3d) states.     

Theoretical study of adsorption of O((3)P) and H(2)O on the rutile TiO(2)(110) surface

The adsorption of oxygen atoms O(3P) on both ideal and hydrated rutile TiO(2)(110) surfaces is investigated by periodic density functional theory (DFT) calculations within the revised Perdew-Burke-Ernzerhof (RPBE) generalized gradient approximation and a four Ti-layer slab, with (2 x 1) and (3 x 1) surface unit cells. It is shown that upon adsorption on the TiO(2) surface the spin of the O atom is completely lost, leading to stable surface peroxide species on both in-plane and bridging oxygen sites with O-binding energies of about 1.0-1.5 eV, rather than to the kinetically unstable terminal Ti-O and terminal O-O species with smaller binding energies of 0.1-0.7 eV. Changes in O-atom coverage ratios between 1/3 and 1 molecular layer (ML) and coadsorption of H(2)O have only minor effects on the O-binding energies of the stable peroxide configurations. High O-atom diffusion barriers of about 1 eV are found, suggesting a slow recombination rate of adsorbed O atoms on TiO(2)(110). Our results suggest that the TiOOTi peroxide intermediate experimentally observed in photoelectrolysis of water should be interpreted as a single spinless O adatom on TiO(2) surface rather than as two Ti-O* radicals coupled together.