单晶Au表面和Au/TiO2（110）模型催化剂表面CO氧化反应机理和活性位

在分子尺度上介绍了Au/TiO2(110)模型催化剂表面和单晶Au表面CO氧化反应机理和活性位、以及H2O的作用.在低温(<320 K), H2O起着促进CO氧化的作用, CO氧化的活性位位于金纳米颗粒与TiO2载体界面(Auδ+–Oδ––Ti)的周边. O2和H2O在金纳米颗粒与TiO2载体界面边缘处反应形成OOH,而形成的OOH使O–O键活化,随后OOH与CO反应生成CO2.300 K时CO2的形成速率受限于O2压力与该反应机理相印证.相反,在高温(>320 K)下,因暴露于CO中而导致催化剂表面重组,在表面形成低配位金原子.低配位的金原子吸附O2,随后O2解离,并在金属金表面氧化CO.     

The beneficial effect of hydrogen on CO oxidation over Au catalysts. A computational study

Density functional theory calculations have been carried out to explore the effect of hydrogen on the oxidation of CO in relation to the preferential oxidation of CO in the presence of excess hydrogen (PROX). A range of gold surfaces have been selected including the (100), stepped (310) surfaces and diatomic rows on the (100) surface. These diatomic rows on Au(100) are very efficient in H-H bond scission. O(2) hydrogenation strongly enhances the surface-oxygen interaction and assists in scission of the O-O bond. The activation energy required to make the reaction intermediate hydroperoxy (OOH) from O(2) and H is small. However, we postulate its presence on our Au models as the result of diffusion from oxide supports to the gold surfaces. The OOH on Au in turn opens many low energy cost channels to produce H(2)O and CO(2). CO is selectively oxidized in a H(2) atmosphere due to the more favorable reaction barriers while the formation of adsorbed hydroperoxy enhances the reaction rate.     

α-Fe2O3形貌对Au/α-Fe2O3催化一氧化碳氧化反应性能的影响

负载型纳米金催化剂由于其独特的化学性质在一系列氧化反应中受到广泛关注.其中,一氧化碳氧化不仅在实际应用领域(如汽车尾气处理)发挥重要作用,而且作为一种理想的模型反应用以深入研究和理解催化剂的构效关系.为了获得高效的纳米金催化剂,我们需要把金负载到载体上,载体不仅为金的分散提供必要的表面,而且还会和金产生相互作用,这种金属-载体相互作用对金的氧化态,金颗粒大小及其热稳定性均有重要影响.金属氧化物是负载金最常用的载体.为了提高纳米金催化剂的性能,需要调变金属氧化物的性质.常用的策略是调控金属氧化物的组成、晶相以及晶粒大小.此外,对金属氧化物的形貌进行精细调控也是一种重要的方法,因为具有不同形貌的氧化物可能会暴露出不同的晶面,而且可能具有不同的缺陷位点.α-Fe₂O₃是一种热稳定性强而且对环境友好的载体,可是有关其形貌对负载金催化剂在一氧化碳氧化反应中性能影响的研究尚不充分.因此,本文采用水热法合成了具有纳米球和纳米棒两种形貌的氧化铁,并采用沉积-沉淀的方法将金纳米颗粒负载于其表面.高分辨透射电镜照片显示,和氧化铁纳米球(α-Fe₂O₃(S))相比,氧化铁纳米棒(α-Fe₂O₃(R))的表面更为粗糙,具有更多的缺陷位点.Au和α-Fe₂O₃(R)之间有更强的金属载体相互作用,导致纳米棒氧化铁上的金纳米颗粒更小而且多呈半球形.相比之下,纳米球氧化铁上的金纳米颗粒较大,多呈球形,且分布不均匀.反应结果表明,Au/α-Fe₂O₃(R)具有更高的一氧化碳氧化活性.对反应后的催化剂进行表征发现,Au/α-Fe₂O₃(R)上金颗粒烧结程度较低,平均粒径从1.5增至2.4 nm,而Au/α-Fe₂O₃(S)上金颗粒烧结较为严重,平均粒径从2.0 nm增加到4.0 nm.氢气程序升温还原结果表明,Au/α-Fe₂O₃(R)具有更强的还原性,这也促进了其催化活性的提高.     

Kinetic study of a direct water synthesis over silica-supported gold nanoparticles

The reaction mechanism of water formation from H2 and O2 was studied over a series of silica-supported gold nanoparticles. The metal particle size distributions were estimated with TEM and XRD measurements. Hydrogen and oxygen adsorption calorimetry was used to probe the nature and properties of surface species formed by these molecules. DFT calculations with Au5, Au13, and Au55 clusters and with Au(111) and Au(211) periodic slabs were performed to estimate the thermodynamic stability and reactivity of surface species. Kinetic measurements were performed by varying the reactant partial pressures at 433 K and by varying the temperature from 383 to 483 K at 2.5 kPa of O2 and 5 kPa of H2. The measured apparent power law kinetic parameters were similar for all catalysts in this study: hydrogen order of 0.7-0.8, oxygen order of 0.1-0.2, and activation energy of 37-41 kJ/mol. Catalysts with Si-MFI (Silicalite-1) and Ti-MFI (TS-1 with 1 wt % Ti) exhibited similar activities. The activities of these catalysts with the MFI crystalline supports were 60-70 times higher than that of an analogous catalyst with an amorphous silica support. Water addition in the inlet stream at 3 vol % did not affect the reaction rates. The mechanism of water formation over gold is proposed to proceed through the formation of OOH and H2O2 intermediates. A rate expression derived based on this mechanism accurately describes the experimental kinetic data. The higher activity of the MFI-supported catalysts is attributed to a higher concentration of gold particles comparable in size to Au13, which can fit inside MFI pores. DFT results suggest that such intermediate-size gold particles are most reactive toward water formation. Smaller particles are proposed to be less reactive due to the instability of the OOH intermediate whereas larger particles are less reactive due to the instability of adsorbed oxygen.     

The effect of hydrogen adsorption on the electronic structure of gold nanoparticles

It has been demonstrated that hydrogen adsorption has an effect on the electronic structure of gold nanoparticles. The physicochemical properties of separate gold nanoparticles have been studied under an ultrahigh vacuum scanning tunneling microscope. The structure and electronic structure of gold–hydrogen clusters were modeled by the quantum-chemical density functional theory method. Hydrogen adsorption onto gold nanoparticles 4–5 nm is size at room temperature was experimentally revealed, and the lower limit of 1.7 eV for the Au–H bond energy was determined. The interaction of hydrogen with gold leads to a considerable rearrangement of the electronic subsystem of nanoparticles. The experimentally observed effects were supported by quantum-chemical calculations. The rearrangement mechanism is related to strong correlations in the electronic subsystem.     

金催化CO氧化湿度增强效应的理论研究

实验发现纳米金催化的CO氧化有良好的湿度增强效应,但有关机制仍不清楚.我们应用密度泛函理论研究了湿度增强效应的微观机制,以Au4团簇为例,研究了金催化CO氧化的微观机理,考察了H2O在反应中的角色和作用.计算结果表明,H2O与Au4团簇一样,在反应中扮演催化剂的角色,参与反应的进行、改变反应历程、降低反应能垒.催化循环包含4个基元步骤：O2＋H2O→OOH＋OH,CO＋OOH→CO2＋OH,CO＋OH→COOH,和COOH＋OH→CO2＋H2O,其中自由基OOH和OH的形成是催化循环的速控步骤,其能垒为100.31kJ/mol,明显低于非水参与反应的能垒（161.41kJ/mol）.目前的结果合理地解释了实验观测的CO催化氧化的湿度增强效应,给出了其微观反应机制.     

Comparison of the catalytic activity of Au3, Au4+, Au5, and Au5- in the gas-phase reaction of H2 and O2 to form hydrogen peroxide: a density functional theory investigation

We report a detailed density functional theory (B3LYP) analysis of the gas-phase H2O2 formation from H2 and O2 on Au3, Au4+, Au5, and Au5-. We find that H2, which interacts only weakly with the Au clusters, is dissociatively added across the Au-O bond, upon interaction with AunO2. One H atom is captured by the adsorbed O2 to form the hydroperoxy intermediate (OOH), while the other H atom is captured by the Au atom. Once formed, the hydroperoxy intermediate acts as a precursor for the closed-loop catalytic cycle. An important common feature of all the pathways is that the rate-determining step of the catalytic cycle is the second H2 addition to form H2O2. The H2O2 desorption is followed by O2 addition to AunH2 to form the hydroperoxy intermediate, thus leading to the closure of the cycle. On the basis of the Gibbs free energy of activation, out of these four clusters, Au4+ is most active for the formation of the H2O2. The 0 K electronic energy of activation and the DeltaGact at the standard conditions are 12.6 and 16.6 kcal/mol respectively. The natural bond orbital charge analysis suggests that the Au clusters remain positively charged (oxidic) in almost all the stages of the cycle. This is interesting in the context of the recent experimental evidence for the activity of cationic Au in CO oxidation and water-gas shift catalysts. We have also found preliminary evidence for a correlation between the activation barrier for the first H2 addition and the O2 binding energy on the Au cluster. It suggests that the minimum activation barrier for the first H2 addition is expected for the Au clusters with 7.0-9.0 kcal/mol O2 binding energy, i.e., in the midrange of the expected interaction energy. This represents a balance between more favorable H2 dissociation when the Aun-O2 interaction is weaker and high O2 coverage when the interaction is stronger. On the basis of this work, we predict that the hydroperoxy intermediate formation can be both thermodynamically and kinetically viable only in a narrow range of the O2 binding energy (10.0-12.0 kcal/mol)-a useful estimate for computationally screening Au-cluster-based catalysts. We also show that a competitive channel for the OOH desorption exists. Thus, in propylene epoxidation both OOH radicals and H2O2 can attack the active Ti in/on the Au/TS-1 and generate the Ti-OOH sites, which can convert propylene to propylene oxide.     

金催化一氧化碳氧化反应中锌锡复合氧化物的载体效应

氧化物负载的纳米金催化剂对CO氧化反应具有极高的活性,这不仅依赖于金的结构特性,也取决于氧化物载体的结构.近年来,除了氧化硅、氧化铝等惰性载体以及氧化钛、氧化铈、氧化铁等可还原性载体外,人们还致力于探索各类新型氧化物载体.另一方面,锡酸锌是具有反尖晶石结构的化合物,并且在透明导电氧化物、锂离子电池阳极材料、光电转换装置以及传感器等方面应用广泛.然而,迄今为止,锡酸锌仍未被用于负载纳米金催化剂,因此相关的构效关系作用研究也十分有限.基于此,本文采用氮气吸附-脱附实验、电感耦合等离子体原子发射光谱(ICP-AES)、X射线衍射(XRD)、X射线光电子能谱(XPS)、透射电子显微镜(TEM)和高分辨电镜(HRTEM)、高角环形暗场像-扫描透射电子显微镜(HAADF-STEM)、X射线吸收精细结构谱(XAFS)和氢气程序升温脱附(H2-TPD)等手段,系统研究了锡酸锌负载的纳米金催化剂在CO氧化反应中催化性能差异的原因.首先,利用水热法制备了锡酸锌(ZTO)载体,而其织构性质可由碱(N2H4·H2O)与金属离子(Zn2+)的比例在4/1(ZTO_1)、8/1(ZTO_2)和16/1(ZTO_3)之间进行调节.结果发现, ZTO_2具有最大的孔体积(0.223 cm3/g)和最窄的孔径分布.再采用沉积沉淀法将0.7 wt% Au负载于其上,得到金-锡酸锌(Au_ZTO)催化剂. ICP-AES测得样品中Au含量在0.57-0.59 wt%,与投料比接近. CO氧化反应结果显示, Au_ZTO_1和Au_ZTO_2的表观活化能相同,但后者的活性更高;而Au_ZTO_3在220°C以下没有活性,催化性能最差,与纯锡酸锌载体相当. XRD结果显示,反应过程中ZTO晶相、晶胞参数及晶粒尺寸变化不明显; TEM和HRTEM分析表明,载体ZTO在反应前后均为多面体形貌,平均颗粒尺寸在12-16 nm; XPS结果验证了Zn2+和Sn4+离子是新鲜和反应后样品中载体金属的存在形式; HAADF-STEM探测到所有样品中均含有1-2 nm的Au粒子; XAFS结果表明, Au以Au0形式存在,并且在Au_ZTO_3中Au平均粒径大于4 nm,而其它两样品约为2 nm. H2-TPR结果表明,金的引入对ZTO载体耗氢量影响不大,但还原峰温度向低温移动;金属-载体相互作用强弱与催化活性高低具有正相关性,即Au_ZTO_2> Au_ZTO_1>> Au_ZTO_3.这是由于不同织构性质的锡酸锌载体对于纳米金活性物种的稳定作用不同所致,具有最大孔体积和最窄孔径分布的ZTO_2负载的金纳米颗粒表现出最高活性.     

O与O2在Au团簇上吸附的密度泛函理论研究

采用基于密度泛函理论(DFT)的Dmol³程序系统研究了O原子与O₂在 Au₁₉与Au₂₀团簇上的吸附反应行为. 结果表明: O在Au₁₉团簇顶端洞位上的吸附较Au₂₀强; 在侧桥位吸附强度相近. O与O₂在带负电Au团簇上吸附较强, 在正电团簇吸附较弱. 从O―O键长看, 当金团簇带负电时, O―O键长较长, 中性团簇次之, 正电团簇中O―O键长较短, 因而O₂活化程度依次减弱. 电荷布居分析表明, Au团簇带负电时, O与O₂得电子数较中性团簇多, 而团簇带正电时, 得电子数较少. 差分电荷密度(CDD)表明, O₂与Au团簇作用时, 金团簇失电子, O₂的π^*轨道得电子, 使O―O键活化. O₂在Au₁₉^-团簇上解离反应活化能为1.33 eV, 较中性团簇低0.53 eV; 而在Au₁₉⁺上活化能为2.27 eV, 较中性团簇高0.41 eV, 这与O₂在不同电性Au₁₉团簇O―O键活化规律相一致.     

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.     

Direct deposition of gold nanoparticles onto polymer beads and glucose oxidation with H2O2

Gold nanoparticles (Au NPs) were deposited directly from aqueous solution of diethylenediaminegold(III) complex onto polymer beads commercially available, such as poly(methyl methacrylate) (PMMA), polystyrene (PS), and polyaniline (PANI) without surface modification. The dropwise addition of NaBH4 to reduce Au(III) was found to be very effective to obtain small Au0 NPs with a narrow size distribution except for PANI. The catalytic performance of Au NPs deposited on polymer beads for H2O2 decomposition and glucose oxidation with H2O2 were more significantly affected by the kinds of polymer supports than by the size of Au NPs. The equimolar oxidation of glucose with H2O2 could be operated by controlling the decomposition rate of H2O2 over Au/PMMA.     

Gold nanoparticles located at the interface of anatase/rutile TiO2 particles as active plasmonic photocatalysts for aerobic oxidation

Visible-light irradiation (λ > 450 nm) of gold nanoparticles loaded on a mixture of anatase/rutile TiO(2) particles (Degussa, P25) promotes efficient aerobic oxidation at room temperature. The photocatalytic activity critically depends on the catalyst architecture: Au particles with <5 nm diameter located at the interface of anatase/rutile TiO(2) particles behave as the active sites for reaction. This photocatalysis is promoted via plasmon activation of the Au particles by visible light followed by consecutive electron transfer in the Au/rutile/anatase contact site. The activated Au particles transfer their conduction electrons to rutile and then to adjacent anatase TiO(2). This catalyzes the oxidation of substrates by the positively charged Au particles along with reduction of O(2) by the conduction band electrons on the surface of anatase TiO(2). This plasmonic photocatalysis is successfully promoted by sunlight exposure and enables efficient and selective aerobic oxidation of alcohols at ambient temperature.     

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.     

Partial oxidation of propylene to propylene oxide over a neutral gold trimer in the gas phase: a density functional theory study

We report a B3LYP study of a novel mechanism for propylene epoxidation using H(2) and O(2) on a neutral Au(3) cluster, including full thermodynamics and pre-exponential factors. A side-on O(2) adsorption on Au(3) is followed by dissociative addition of H(2) across one of the Au-O bonds (DeltaE(act) = 2.2 kcal/mol), forming a hydroperoxy intermediate (OOH) and a lone H atom situated on the Au(3) cluster. The more electrophilic O atom (proximal to the Au) of the Au-OOH group attacks the C=C of an approaching propylene to form propylene oxide (PO) with an activation barrier of 19.6 kcal/mol. We predict the PO desorption energy from the Au(3) cluster with residual OH and H to be 11.5 kcal/mol. The catalytic cycle can be closed in two different ways. In the first subpathway, OH and H, hosted by the same terminal Au atom, combine to form water (DeltaE(act) = 26.5 kcal/mol). We attribute rather a high activation barrier of this step to the breaking of the partial bond between the H atom and the central Au atom in the transition state. Upon water desorption (DeltaE(des) = 9.9 kcal/mol), the Au(3) is regenerated (closure). In the second subpathway, H(2) is added across the Au-OH bond to form water and another Au-H bond (DeltaE(act) = 22.6 kcal/mol). Water spontaneously desorbs to form an obtuse angle Au(3) dihydride, with one H atom on the terminal Au atom and the other bridging the same terminal Au atom and the central Au atom. A slightly activated rearrangement to a symmetric triangular Au(3) intermediate with two equivalent Au-H bonds, addition of O(2) into the Au-H bond, and rotation reforms the hydroperoxy intermediate in the main cycle. On the basis of the DeltaG(act), which contains contribution from both pre-exponetial factor and activation energy, we identify the propylene epoxidation step as the actual rate-determining step (RDS) in both the pathways. The activation barrier of the RDS (epoxidation step: DeltaE(act) = 19.6 kcal/mol) is in the same range as that in the published computationally investigated olefin epoxidation mechanisms involving Ti sites (without Au involved) indicating that isolated Au clusters and possibly Au clusters on non-Ti supports can be active for gas-phase partial oxidation, even though cooperative mechanisms involving Au clusters/Ti-based-supports may be favored.     

纳米金催化一氧化碳氧化反应的理论研究

近年来,纳米金催化剂独特的催化性质,特别是其优异的低温催化氧化活性,引起了人们极大的研究热情.除低温选择氧化外,在精细化学品合成、大气污染物消除、氢能的转换和利用等领域也开发出了一系列有广泛应用前景的金催化反应.此外,体相金的化学惰性和纳米金的超高活性之间差异的“鸿沟”也引起了理论工作者浓厚兴趣,试图从原理上理解体相金和纳米金活性差异的根源. CO催化氧化是最具有代表性的研究金催化活性的化学反应,本文主要综述了近十多年来金催化 CO氧化反应理论计算方面的研究工作.一般认为, CO在纳米金表面的吸附是 CO氧化反应的初始步骤.密度泛函理论研究表明, CO在金表面的吸附强度主要与被吸附金原子的配位数有关：金配位数越低, CO的吸附能越强,部分研究结果表明两者之间存在近似的线性关系.我们研究发现, CO吸附强度也与被吸附金周围配位金原子的相对位置有关,其中位于正下方的配位金原子加强 CO吸附,而位于侧位的配位金原子则弱化 CO吸附,这显然削弱了 CO吸附与金配位数线性关系的可靠性.理论研究表明,在纯金表
　　面上 O2吸附强度一般很弱,只有在一些特殊结构的金团簇上才有较强的吸附,但在 Au/TiO2界面及 CeO2表面上 O2吸附较强.金表面原子氧的吸附和金的表面结构有关.我们发现,原子氧倾向于在金的表面形成一种线性的 O–Au–O结构以增加其稳定性.当金表面的氧覆盖度增大时,会形成一种金氧化物薄膜结构,其结构依赖于氧的化学势和金的表面结构.纳米金催化 CO氧化反应机理可能因体系、载体等的差异而不同.大部分理论计算结果表明,在纯金表面上 O2很难直接解离形成原子氧,因此反应机理可能是吸附的 CO先与 O2反应形成了一种 CO–O2中间体,然后解离形成 CO2.在 Au/TiO2和 Au/CeO2催化剂上 CO催化氧化机理争议很大,均有计算结果支持 LH机理和 M–vK机理.另外,根据实验上观察到了负载型纳米金能直接活化分子氧的结果,理论上也提出了分子氧先解离为原子氧再与 CO反应的氧解离机理.针对如何解离分子氧问题,人们分别提出了低配位金模型、正方形金结构模型、Ti5c模型及 Au/Ti5c模型等.我们也提出了一种独特的双直线 O–Au–O模型来理解 Au/TiO2或 Au/CeO2界面解离活化分子氧.理论计算结果表明,低配位的金,金和载体之间的电荷转移,以及金所表现出的强相对论效应对于纳米金的活性影响很大.需要特别指出的是,金的强相对论效应有助于理解金表面的 CO吸附与金配位的关系、金表面原子氧的吸附特性、金氧化物薄膜的结构和分子氧的活化等过程.我们认为,金的强相对论作用导致了体相金的化学惰性以及纳米金的活性,因此相对论效应的深入研究将有助于理解金催化 CO氧化反应机理,从而有助于深层次理解纳米金催化活性来源.     

Alkynylisocyanide gold mesogens as precursors of gold nanoparticles

Gold nanoparticles (Au NPs) have been synthesized using simple thermolysis, whether from the mesophase or from toluene solutions, of mesogenic alkynyl-isocyanide gold complexes [Au(C≡C-C(6)H(4)-C(m)H(2m+1))(C≡N-C(6)H(4)-O-C(n)H(2n+1))]. The thermal decomposition from the mesophase is much slower than from solution and produces a more heterogeneous size distribution of the nanoparticles. Working in toluene solution, the size of nanoparticles can be modulated from ~2 to ~20 nm by tuning the chain lengths of the ligands present in the precursor. Different experimental conditions have been analyzed to reveal the processes governing the formation of the gold nanoparticles. Experiments on the effect of adding ligands or bubbling oxygen support that the thermal decomposition is a bimolecular process that starts by decoordination of the isocyanide ligand, producing an oxidative coupling of the akynyl group to [R-C≡C-C≡C-R] and reduction of gold(I) to gold(0) as nanoparticles. The nanoparticles obtained behave as a catalyst in the oxidation of isocyanide (CNR) to isocyanate (OCNR), which in turn cooperates to catalyze the decomposition.     

Theoretical study of oxygen adsorption on pure Au(n+1)+ and doped MAu(n)+ cationic gold clusters for M = Ti, Fe and n = 3-7

A comparative study of the adsorption of an O2 molecule on pure Au(n+1)+ and doped MAu(n)+ cationic gold clusters for n = 3-7 and M = Ti, Fe is presented. The simultaneous adsorption of two oxygen atoms also was studied. This work was performed by means of first principles calculations based on norm-conserving pseudo-potentials and numerical basis sets. For pure Au4 +, Au6+, and Au7+ clusters, the O2 molecule is adsorbed preferably on top of low coordinated Au atoms, with an adsorption energy smaller than 0.5 eV. Instead, for Au5+ and Au8+, bridge adsorption sites are preferred with adsorption energies of 0.56 and 0.69 eV, respectively. The ground-state geometry of Au(n)+ is almost unperturbed after O2 adsorption. The electronic charge flows towards O2 when the molecule is adsorbed in bridge positions and towards the gold cluster when O2 is adsorbed on top of Au atoms, and both the adsorption energy and the O-O bond length of adsorbed oxygen increase when the amount of electronic charge on O2 increases. On the other hand, we studied the adsorption of an O2 molecule on doped MAu(n)+ clusters, leading to the formation of (MAu(n)O2+) ad complexes with different equilibrium configurations. The highest adsorption energy was obtained when both atoms of O2 bind on top of the M impurity, and it is larger for Ti doped clusters than for Fe doped clusters, showing an odd-even effect trend with size n, which is opposite for Ti as compared to Fe complexes. For those adsorption configurations of (MAu(n)O2+) ad involving only Au sites, the adsorption energy is similar to or smaller than that for similar configurations of Au(n)+1O2 + complexes. However, the highest adsorption energy of (MAu(n)O2+) ad is higher than that for (Au(n)+1O2+) ad by a factor of approximately 4.0 (1.2) for M = Ti (M = Fe). The trends with size n are rationalized in terms of O-O and O-M bond distances, as well as charge transfer between oxygen and cluster substrates. The spin multiplicity of those (MAu(n)O2+) ad complexes with the highest O2 adsorption energy is a maximum (minimum) for M = Fe (Ti), corresponding to parallel (anti-parallel) spin coupling of MAu(n)+ clusters and O2 molecules. Finally, we obtained the minimum energy equilibrium structure of complexes (Au(n)O2+) dis and (MAu(n)O2+) dis containing two separated O atoms bonded at different sites of Au(n)+ and MAu(n)+ clusters, respectively. For (MAu(n)O2 (+)) dis, the equilibrium configuration with the highest adsorption energy is stable against separation in MAu(n)+ and O2 fragments, respectively. Instead, for (Au(n)O2+) dis, only the complex n = 6 is stable against separation in Au(n)+ and O2 fragments. The maximum separation energy of (MAu(n)O2+) dis is higher than the O2 adsorption energy of (MAu(n)O2+) ad complexes by factors of approximately 1.6 (2.5), 1.6 (1.7), 1.5 (2.4), 1.5 (1.3), and 1.6 (1.8) for M = Ti (Fe) complexes in the range n = 3-7, respectively.     

New preparation method of gold nanoparticles on SiO2

It is shown that adsorption of the [Au(en)(2)](3+) cationic complex can be successfully employed for the deposition of gold nanoparticles (1.5 to 3 nm) onto SiO(2) with high metal loading, good dispersion, and small Au particle size. When the solution pH increases (from 3.8 to 10.5), the Au loading in the Au/SiO(2) samples increases proportionally (from 0.2 to 5.5 wt %), and the average gold particle size also increases (from 1.5 to 2.4 nm). These effects are explained by the increase in the amount of negatively charged sites present on the SiO(2) surface, namely, when the solution pH increases, a higher number of [Au(en)(2)](3+) species can be adsorbed. Extending the adsorption time from 2 to 16 h gives rise to an increase in the gold loading from 3.3 to 4.0 wt % and in the average particle size from 1.8 to 2.9 nm. Different morphologies of gold nanoparticles are present as a function of the particle size. Particles with a size of 3-5 nm show defective structure, some of them having a multiple twinning particle (MTP) structure. At the same time, nanoparticles with an average size of ca. 2 nm exhibit defect-free structure with well-distinguishable {111} family planes. TEM and HAADF observations revealed that Au particles do not agglomerate on the SiO(2) support: gold is present on the surface of SiO(2) only as small particles. Density functional theory calculations were employed to study the mechanisms of [Au(en)(2)](3+) adsorption, where neutral and negatively charged silica surfaces were simulated by neutral cluster Si(4)O(10)H(4) and negatively charged cluster Si(4)O(10)H(3), respectively. The calculation results are totally consistent with the suggestion that the deposition of gold takes place according to a cationic adsorption mechanism.     

Experimental and DFT studies of gold nanoparticles supported on MgO(111) nano-sheets and their catalytic activity

A wet chemical preparation of MgO with the (111) facet as the primary surface has recently been reported and with alternating layers of oxygen anions and magnesium cations, this material shows unique chemical and physical properties. The potential to utilize the MgO(111) surface for the immobilization of metal particles is intriguing because the surface itself offers a very different environment for the metal particle with an all oxygen interface, as opposed to the typical (100) facet that possesses alternating oxygen anion and magnesium cation sites on the surface. Gold nanoparticles have demonstrated a broad range of interesting catalytic properties, but are often susceptible to aggregation at high temperatures and are very sensitive to substrate effects. Here, we investigate gold-supported on MgO(111) nanosheets as a catalyst system for the aerobic oxidation of benzyl alcohol. Gold nanoparticles deposited on MgO(111) show an increased level of activity in the solvent-free benzyl alcohol aerobic oxidation as compared to gold nanoparticles deposited on a typical MgO aerogel. TEM studies reveal that the gold nanoparticles have a hemispherical shape while sitting on the main surface of MgO(111) nanosheets, with a large Au-MgO interface. Given that the gold nanoparticles deposited on the two types of MgO have similar size, and that the two types of unmodified MgO show almost the same activities in the blank reaction, we infer that the high activity of Au/MgO(111) is due to the properties of the (111) support and/or those of the gold-support interface. To understand the binding of Au on low-index MgO surfaces and the charge distribution at the surface of the support, we have performed density functional theory (DFT) calculations on all low-index MgO substrates (with and without gold), using a model Au(10) cluster. Due to similar lattice constants of Au(111) and MgO(111) planes, the Au cluster retains its structural integrity and binds strongly on MgO(111) with either oxygen or magnesium termination. Furthermore, we have found that for the (001) and (110) substrates the charges of the ions in the top surface layer have similar values as in bulk MgO, but that on (111) surfaces these charges are significantly different. This difference in surface charge determines the direction of the electronic transfer upon adsorption of gold, such transfer occurring so as to restore the bulk MgO charge values. Using the results from theoretical calculations, we provide an explanation of our observations of increased catalytic activity in the case of the Au/MgO(111) system.     

Surface chemical states of gold nanoparticles prepared using the solution-plasma method in a CsCl aqueous solution

In this study, gold nanoparticles (AuNPs) prepared in a 5 mM CsCl aqueous solution using the solution-plasma method are characterized via transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy with synchrotron radiation (SR-XPS). The particle diameter is measured over the process time via TEM. During the solution-plasma process, small particles of 2.1 to 2.2-nm diameter are generated in the CsCl aqueous solution; these particles then enlarge via Ostwald ripening over time until they reach an equilibrium size of ~13 nm after 36 days. In addition, the surface chemical states of the AuNPs are characterized at different depths via SR-XPS. The SR-XPS measurements obtained using incident X-ray energy (hν) of 945.0 eV revealed that Cs─Au, Cl─Au, and Cs─Cl─Au bonds are present 1.2 nm below the surface. The measurements obtained at an incident X-ray energy of 2515.0 eV showed that Cs─Cl─Au bonding is also present 2.5 nm below the surface, indicating that Cs and Cl strongly interact with Au. The TEM and SR-XPS measurements revealed that 2 processes occur cyclically during the growth process via Ostwald ripening: (i) the Cs and Cl in the aqueous solution adsorb on the AuNP surface and (ii) Au atoms subsequently bond to the AuNPs surface.