CH3OH在ZnO(0001)表面的光催化解离

本文利用266 nm波长的激光及程序升温脱附的方法研究了甲醇在ZnO(0001)表面的光催化反应. TPD结果显示部分的CH₃OH以分子的形式吸附在ZnO(0001)表面,而另外一部分在表面发生了解离. 实验过程中探测到H₂,CH₃^·,H₂O,CO,CH₂O,CO₂和CH₃OH这些热反应产物. 紫外激光照射实验结果表明光照可以促进CH₃OH/CH₃O^·解离形成CH₂O,在程序升温或光照的过程中它又可以转变为HCOO^-. CH₂OH_Zn与OH_ad反应在Zn位点上形成H₂O分子. 升温或光照都能促进CH₃O^·转变为CH₃^·. 该研究对CH₃OH在ZnO(0001)表面的光催化反应机理提供了一个新的见解.     

表面缺陷对一氧化碳和甲基基团在金红石TiO2(110)表面吸附的影响

本文利用程序升温脱附技术研究了氧空位浓度对甲基基团和CO在R-TiO₂(110)表面吸附的影响. 结果表明,随着氧空位浓度的变化,吸附在桥氧位的甲基基团和吸附在五配位Ti⁴⁺位点上的CO分子的脱附温度呈现了不同的趋势,揭示了表面缺陷可能对R-TiO₂(110)不同位点上的物质吸附具有重要影响.     

用光电子成像技术研究Cu-的飞秒双光子光脱附过程

用光电子成像技术研究了Cu^-的飞秒双光子光脱附过程．光脱附产生的光电子能量分布和角度分布通过光电子成像直接获得．实验发现,在光强为6.0×10¹⁰ W/cm²下,双光子光脱附通道的光电子角度分布随激光波长出现剧烈的变化．通过低阶微扰理论和实验结果对比分析,Cu^-的双光子脱附产生的光电子角度分布特征可以归结于脱附过程中Cu^-的s和d轨道波函数之间的干涉效应．通过分析光电子角度分布特征,获得了不同波长下s和d轨道波函数对双光子脱附过程的贡献,该结果符合Wigner阈值理论的预期．     

甲醇在TiO2(110)表面光催化氧化的波长相关性

用程序升温脱附谱的方法研究了甲醇在TiO₂(110)表面光催化氧化的波长相关性．对于不同波长的激发光,甲醇光催化解离的产物相似,都是在光照的过程中通过O-H键和C-H键的断裂形成甲醛,解离出的氢原子转移到相邻的两配位的桥氧原子(O_b)上形成O_bH．但是反应速率与激发光波长强烈相关．在360 nm光照下,甲醇的反应速率是400 nm光照时的4.8倍．这与以前使用时间分辨的飞秒激光吸收光谱测量的结果是一致的,他们认为近带隙激发和过带隙激发产生的电荷相比,前者的复合速率要远快于后者．     

异丙醇在锐钛矿TiO2(101)表面的光化学

通过程序升温脱附谱的方法探究了266 nm激光照射下异丙醇在锐钛矿TiO₂（101）表面的光化学,并提出了详细的解离机理.在光照作用下,异丙醇发生分步解离：首先,O-H键断裂生成异丙氧基;然后,仲碳的C-H键在光照的作用下进一步发生解离,从而生成丙酮,两步生成的氢原子均转移到相邻的桥氧上.在升温过程中,这部分氢原子会与分子态的异丙醇发生置换反应,从而以低温水和高温异丙基的形式从表面脱附出来.     

TiO2表面氧空位对NO分子吸附的作用

采用程序升温热脱附(TPD)实验方法测定了NO在TiO₂表面吸附后的脱附谱，利用 分子轨道 理论研究了TiO₂吸附NO的原子簇模型及吸附前后的原子簇能级变化.结果表明， NO在TiO₂表面吸附后可在两个峰值温度450 和980 K脱附出N₂.TiO ₂表面经预覆氧处理后，N₂ 的脱附量降低.吸附时NO中的O能够占据TiO₂表面氧空位并与N脱离，而N原子则 相互结合成 为N₂脱附.分子轨道理论计算证明在TiO₂(110)表面能够存在氧空位 并具备吸附NO的结构条件.     

利用交流阻抗谱揭示Ir(111)电极在酸性溶液中的氢吸附动力学

本文利用阻抗谱研究Ir(111)电极在HClO₄和H₂SO₄中溶液中的氢吸附行为. 在HClO₄溶液中,随着施加电位从0.2 V降到0.1 V(vs RHE),Ir(111)电极上氢吸附速率从1.74×10^-8 mol·cm^-2·s^-1增大到 3.47×10^-7 mol·cm^-2·s^-1 . 与相同条件下Pt(111)电极上的氢吸附速率相比,Ir(111)上的氢吸附速率要小1∽2个数量级,这是由于Ir(111)电极与H₂O结合能力更强,因此位于水合氢键网络中的氢离子需要克服更高的能垒才能重新定向进而发生欠电位沉积. 在H₂SO₄溶液中,氢吸附电位负移了200 mV,吸附速率也下降了一个数量级,这是由于Ir(111)电极表面强吸附的硫酸根/硫酸氢根物种的阻碍作用. 结果表明,在电化学环境下,位于电极表面附近的水分子的取代和重新定向在很大程度上影响了氢吸附过程.     

用于团簇负离子的共线光电子速度成像仪

描述了包含共线的光电子速度成像仪,以及与双阀激光溅射团簇源联用的时间飞行质谱装置．为了避免预混气体,两个脉冲阀和一个反应通道被用来产生反应负离子．共线的光电子成像仪采用了改进的速度成像透镜系统,它的能量分辨率优于3%．此外还报道了Si₄^-团簇在532和355 nm,以及Si₃C^- 团簇在532 nm的光电子速度成像．从实验图像中可以获得光电子能谱和各向异性参数．从Si₄^-团簇能谱上获得了中性的Si₄团簇的基态和第一激发态的振动频率分别为330和312 cm^-1．初步的实验结果证明这套光电子成像装置对研究团簇负离子的电子结构以及光脱附动力学非常有效.     

小尺寸VmOn+团簇的激光光解和密度泛函计算

通过激光溅射法产生了V₂O_n⁺ (n=1, 2), V₃O_n⁺ (n=1, 2, 3)和V₄O₃⁺等缺氧的钒氧团簇,并采用532和266 nm波长的激光对它们进行了光解研究. 利用密度泛函理论计算与激光光解实验相结合确定了这些团簇的几何结构和可能的光解通道. 激光光解实验表明V相似文献   

锐钛矿型TiO2(001)-(1×4)表面还原型缺陷对乙醛偶联反应的作用

本文利用程序升温脱附技术(TPD)研究了乙醛吸附在锐钛矿型TiO₂(001)-(1×4)表面的化学性质. 实验结果表明完整晶格位点对乙醛反应表现极为惰性,而表面上的还原型缺陷位点在热驱动下可有效地使乙醛分子通过碳-碳偶联反应生成2-丁酮和丁烯. 提出了乙醛在锐钛矿型TiO₂(001)-(1×4)表面偶联反应主要是通过表面还原型缺陷位吸附成对的乙醛分子,因为表面已有的钛原子对还原型缺陷为乙醛分子提供了合适的吸附位点.     

甲醇分子在金红石TiO2(110)和锐钛矿TiO2(001)表面吸附和反应的活性位点

通过高分辨的扫描隧道显微术研究并比较了金红石型TiO₂(110)-(1×1)和锐钛矿型TiO₂(001)-(1×4)两种表面的活性位点． 在金红石型TiO₂(110)-(1×1)表面, 观察到氧空位缺陷是O₂和CO₂分子的活性吸附位点,而五配位的Ti原子是水分子和甲醇分子的光催化反应活性位点．在锐钛矿型TiO₂(001)-(1×4)表面,观察到完全氧化的表面,Ti原子更可能是六配位的,H₂O和O₂分子均不易在这些Ti原子上吸附．经还原后表面出现富Ti的缺陷位点, 这些缺陷位点对H₂O和O₂分子表现出明显的活性． 锐钛矿型TiO₂(001)-(1×4)表面的吸附和反应活性并不具有很高的活性,某种程度上其表现出的活性似乎低于金红石型TiO₂(110)-(1×1)表面．     

Acetaldehyde photochemistry on TiO2(1 1 0)

The ultraviolet (UV) photon induced decomposition of acetaldehyde adsorbed on the oxidized rutile TiO₂(1 1 0) surface was studied with photon stimulated desorption (PSD) and thermal programmed desorption (TPD). Acetaldehyde desorbs molecularly from TiO₂(1 1 0) with minor decomposition channels yielding butene on the reduced TiO₂ surface and acetate on the oxidized TiO₂ surface. Acetaldehyde adsorbed on oxidized TiO₂(1 1 0) undergoes a facile thermal reaction to form a photoactive acetaldehyde–oxygen complex. UV irradiation of the acetaldehyde–oxygen complex initiated photofragmentation of the complex resulting in the ejection of methyl radical into gas phase and conversion of the surface bound fragment to formate.     

甲醇在二氧化钛上的光化学性质与表面结构的相关性

通过双光子光电子的方法探测了TiO₂(011)-(2×1) 和TiO₂(110)-(1×1)表面的光催化氧化甲醇的性质. 在吸附了甲醇的二氧化钛(011)和(110)界面处探测到了一个费米能级以上2.5 eV的电子激发态,该电子激发态可作为测试二氧化钛界面还原性的探针使用. 利用此探针在甲醇/TiO₂(011)-(2×1)和甲醇/TiO₂(110)-(1×1)界面探测到了一个随光照时间的电子激发态信号变化,这一变化可以归于光催化生成的表面羟基对界面还原性的影响. 由此得出的光催化氧化甲醇的速率TiO₂(110)-(1×1)比TiO₂(011)-(2×1)快了大约11.4倍. 这可能由于表面原子结构排布的原因不同. 本工作不仅介绍了一个利用双光子光电子能谱探测到的甲醇/TiO₂界面电子结构的细节特征,还揭示了表面结构对二氧化钛光反应性质的重要影响.     

甲醛在金红石型TiO2(100)-(1×1)表面的光解离

We have investigated the photoinduced decomposition of formaldehyde (CH₂O) on a rutile TiO₂(100)-(1×1) surface at 355 nm using temperature-programmed desorption. Products, formate (HCOO^-), methyl radical (CH₃·), ethylene (C₂H₄), and methanol (CH₃OH) have been detected. The initial step in the decomposition of CH₂O on the rutile TiO₂(100)-(1×1) surface is the formation of a dioxymethylene intermediate in which the carbonyl O atom of CH₂O is bound to a Ti atom at the five-fold-coordinated Ti⁴⁺ (Ti_5c) site and its carbonyl C atom bound to a nearby bridge-bonded oxygen (O_b) atom, respectively. During 355 nm irradiation, the dioxymethylene intermediate can transfer an H atom to the O_b atom, thus forming HCOO^- directly, which is considered as the main reaction channel. In addition, the dioxymethylene intermediate can also transfer methylene to the O_b row and break the C-O bond, thus leaving the original carbonyl O atom at the Ti_5c site. After the transfer of methylene, several pathways to products are available. Thus, we have found that O_b atoms are intimately involved in the photoinduced decomposition of CH₂O on the rutile TiO₂(100)-(1×1) surface.     

Methylene bromide chemistry and photochemistry on rutile TiO2(110)

The chemistry and photochemistry of methylene bromide (CD₂Br₂) on the rutile TiO₂(110) surface was probed using temperature programmed desorption (TPD). CD₂Br₂ desorbed in three desorption states at 145, 160 and 250 K tentatively assigned to desorption from the multilayer, from an η¹-CD₂Br₂ species and a bridging η²-CD₂Br₂ species, respectively. The latter two TPD states presumably involve binding of CD₂Br₂ molecules to the surface through Br coordination at five-coordinate Ti⁴⁺ surface sites. The 160 and 250 K TPD states saturated at coverages of 1.0 and 0.33 ML, respectively, where 1 ML is equivalent to the surface Ti⁴⁺ site density (5.2 × 10¹⁴ cm^? 2). No thermal decomposition of CD₂Br₂ was observed on either the clean surface or with preadsorbed O₂. UV irradiation of CD₂Br₂ on TiO₂(110) resulted in predominately photodesorption, with trace amounts of photodecomposition evidenced in TPD. The rate of CD₂Br₂ photodesorption from TiO₂(110) occurred with a low cross section (~ 2 × 10^? 21 cm²) similar to that expected from direct optical excitation of CD₂Br₂. This observation suggests that charge carriers generated in TiO₂(110) were no more effective in activating adsorbed CD₂Br₂ molecules than would be expected through direct molecular excitation. These findings suggest that photocatalytic destruction of halocarbons such as CD₂Br₂ on TiO₂ may preferentially occur though indirect processes (such as OH radical attack) as opposed to direct electron transfer processes involving charge carriers generated in TiO₂ by bandgap excitation.     

Templating a face-centered cubic (110) termination of C60

Scanning tunneling microscopy has been used to study C₆₀ overlayers on TiO₂(100)-(1 × 3). Initial adsorption preferentially occurs on O vacancies (Ti³⁺), evidencing a site-specific interaction dominated by substrate-adsorbate charge transfer. At saturation coverage the molecules are incommensurate with Ti³⁺ sites along [001], suggesting a delicate balance between intermolecular and substrate interactions. The unit cell of the C₆₀ overlayer is 13.8Å× 10.0 ± 0.5Å, consistent with the first layer of fcc C₆₀(110); molecules also adopt sites in the troughs of the (1 × 3) structure to form the second layer.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

Phase transitions on clean Si(110) surfaces

The properties of the structure of clean Si(110) surfaces have been investigated by LEED. The phase transitions between surface structures Si(110)?(4 × 5), Si(110)?(2 × 1) and Si(110)(5 × 1) take place at about 600 and 750°C. The time of reconstruction from the high temperature phase to the low temperature phase may exceed the time of the sample cooling. That explains why the Si(110)?(2 × 1) and the Si(110)?(5 × 1) superstructures may be seen at room temperature. Surface defects favour the retaining of high temperature phases on the surface at room temperature. The transition from the Si(110)?(5 × 1) structure to the Si(110)?(2× 1) structure and conversely in the temperature range of 720–750°C apparently occurs through formation of the intermediate structures Si(110)?(7 × 1) and Si(110)?(9 × 1). The models are given of superstructures observed by LEED.