Role of Au(+) in supporting and activating Au(7) on TiO(2)(110)

The adhesion properties and catalytic activity of rutile TiO(2)(110)-supported Au(7) nanoclusters in different oxidation states are investigated by means of density functional theory. The calculations cover both surface science conditions of reduced TiO(2) and real catalyst conditions of oxidized (alkaline) TiO(2) supports. Large adhesion energies of Au(7) are found only when modeling real catalysts where the cluster becomes cationic with Au(+) ions in Au-O or Au-OH bonds. The full catalytic cycle for oxidation of CO by O(2) over Au(7) on alkaline TiO(2)(110) is calculated and found to involve only small activation barriers. In the presence of the CO reductant, the Au(+) sites are capable of cycling between bonding of atomic and molecular oxygen. We confirm our findings by comparison of calculated and experimental infrared stretch frequency data for adsorbed CO.     

Selective oxidation of CO in the presence of air over gold-based catalysts Au/TiO2/C (sonochemistry) and Au/TiO2/C (microwave)

Two model catalysts, Au/TiO2/C (S) (sonochemically derived) and Au/TiO2/C (M) (microwave derived), were produced by employing ultrasound irradiation and microwave irradiation, respectively. The deposition of gold colloids onto the support powders, TiO2/C, was accomplished by using a solvated metal atom impregnation (SMAI) method. The SMAI technique provides highly-dispersed gold particles on the TiO2/C support. The catalytic performance of Au based catalysts 1 wt% Au-TiO2/C (S) and 1 wt%Au-TiO2(M)/C (M) have been tested for the oxidation of CO in the temperature range of 0-300 degrees C and compared to that of 1 wt% Au-TiO2 (Degussa-P25). A boost in the conversion of CO was observed for the sonochemically-derived catalyst, Au/TiO2/C (S), at low temperature. Hence, the reactivity order found for CO oxidation is (Au/TiO2/C (S)>Au/TiO2 (P25)>Au/TiO2/C (M)).     

Influence of Au and TiO2 structures on hydrogen dissociation over TiO2/Au(100)

We performed H₂–D₂ exchange reactions over TiO_x/Au(100) and compared the observed reaction kinetics with those reported for TiO_x/Au(111) in order to clarify the influence of the Au and TiO₂ structures on dissociation of H₂ molecules. Low energy electron diffraction observations showed that the TiO₂ produced on Au(100) was disordered, in contrast to the comparatively ordered TiO₂ structure formed on Au(111). The activation energies and the turnover frequencies for HD formation over TiO₂/Au(100) agreed well with those for TiO₂/Au(111), clearly indicating that the hydrogen dissociation sites created over TiO₂/Au(100) were the perimeter interface between stoichiometric TiO₂ and Au, as was previously concluded for TiO₂/Au(111). We concluded that the creation of active sites for hydrogen dissociation was independent of the Au and TiO₂ structures consisting perimeter interface, and that local bonds that formed between Au and O atoms of stoichiometric TiO₂ were essential for the creation of active sites.     

Titanium overlayers on TiO2(110)

Non-stoichiometric, Ti-rich TiO₂(110) surfaces were prepared by evaporation of Ti on stoichiometric TiO₂(110). Changes in the spectra of core levels, valence bands, Auger emissions, electron energy losses, conductivities and work functions were investigated during overlayer formation in the monolayer range and during subsequent annealing of non-stoichiometric surfaces at high temperatures and high oxygen partial pressures. These annealing procedures make it possible to restore the ideal stoichiometry of TiO₂(110). The results are discussed quantitatively by calculating concentrations of (sub)surface intrinsic defects in a space-charge layer model.     

TiO2纳米层(110)表面对DNA强吸附的第一性原理研究

纳米二氧化钛(TiO₂)由于具有卓越的生物相容性和优异的物理性能,因此有望在生物医学领域中发挥重要的作用,且应用前景广阔.利用第一性原理计算,深入地研究了金红石型TiO₂纳米层(110)表面与脱氧核糖核酸(DNA)不同碱基在界面之间的吸附性能及相互作用的原子机制.通过分析结合能和功函数的计算结果发现,TiO₂纳米层(110)表面对DNA碱基的吸附强度显著增强,比典型二维纳米材料的吸附强度大两倍以上.进而,通过研究电子能带结构和态密度计算结果,阐明了二者在界面之间的吸附机制,其起源于吸附体系显著降低的能级和C、N和/或O的2p轨道与费米能级附近Ti原子的3d轨道的强烈杂化.纳米TiO₂为DNA传感器和测序仪的设计提供了一种极具潜力的候选材料.     

甲醇/TiO2(110)界面激发态的表征

用光电子能谱的方法研究了甲醇/TiO₂(110)界面的电子结构．在激发波长为400 nm的双光子光电子能谱(2PPE)中,探测到了一个末态能量在费米能级以上5.5 eV的共振信号．之前的研究[Chem. Sci. 1, 575 (2010)]表明,这个共振信号与甲醇在5配位的钛离子(Ti_5c)上的光催化解离相关．双光子光电子能谱同时携带初态和中间态的信息．为此设计了一个调谐激发光波长的2PPE实验以及一个单光子光电子能谱(1PPE)和2PPE对比的实验,结果一致表明这个共振信号来自于未占据的中间态,也就是激发态．能带色散关系测量表明这个激发态是局域的．时间分辨2PPE测得这个激发态的寿命是24 fs．     

Influence of temperature on the interaction between Pd clusters and the TiO2 (110) surface

The behavior of a Pd nanocluster on the rutile TiO2 (110) surface has been analyzed by extensive first principles molecular dynamics simulations between 100 K and 1073 K. Calculations predict a steep change in the morphological and electronic cluster structure around 800 K in excellent agreement with previous experimental evidence. At low temperature, the cluster geometry is mainly controlled by the substrate structure; however, upon the transition temperature, the cluster-substrate interaction decreases appreciably, and the cluster adopts a geometry more stable in vacuum and becomes metallic. These results illustrate at an atomistic level the influence of temperature on the geometrical and electronic properties of oxide-supported clusters.     

Role of nanostructured dual-oxide supports in enhanced catalytic activity: theory of CO oxidation over Au/IrO2/TiO2

The synergetic effect in multicomponent catalysts is a topic of profound industrial importance and intense academic interest. On a newly identified multicomponent catalyst, Au/IrO(2)/TiO(2), first-principles density-functional theory is analyzed to clarify the outstanding catalytic activity of the system for oxidative reactions at high temperatures. By comparing CO oxidation on interfaces and single-component surfaces, it is revealed that a high dispersion of a more active oxide (IrO2), on a more inert oxide (TiO2) is the key. It preserves the sintering resistance of Au supported on less active oxides, while at the same time promoting oxidative reactions that occur at the Au/active-oxide interface.     

Enhanced light absorption of TiO(2) in the near-ultraviolet band by Au nanoparticles

We propose a scheme to enhance near-UV band absorption of a rutile TiO(2) nanoparticle by placing Au nanoparticles in its neighborhood. The discrete-dipole approximation method was employed to calculate the absorption spectrum of pure rutile TiO(2) and that of TiO(2) mixed with Au nanoparticles. The results indicate that pure rutile TiO(2) has its maximum absorption located in the deep-UV band. With the existence of Au nanoparticles, a significant light harvesting effect occurs, and this maximum shifts to the near-UV band, where usual excitation wavelength falls.     

Direct evidence for ethanol dissociation on rutile TiO2(110)

We have studied the interaction of ethanol with reduced TiO(2)(110)-(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory calculations. The STM data revealed direct evidence for the coexistence of molecularly and dissociatively adsorbed ethanol species on surface Ti sites. In addition, we found evidence for dissociation of ethanol at bridge-bonded O vacancies. The density functional theory calculations support these findings and rationalize the distinct diffusion behaviors of molecularly and dissociatively adsorbed ethanol species, as revealed in time-lapsed STM images.     

Adsorption of L-cysteine on rutile TiO2(110)

We have used X-ray photoelectron spectroscopy to study the adsorption of L-cysteine on a rutile TiO₂(110) surface at room temperature and ? 65 °C. For the molecules in direct contact with the surface our results suggest that the molecules bind dissociatively to the fivefold-coordinated Ti atoms of the surface through their deprotonated carboxylic groups. A second, dissociative interaction occurs between the molecular thiol groups and the surface. It is attributed to a dissociative bond to the bridging oxygen vacancies. Most likely, the thiol groups are deprotonated and a bond is formed between the thiolates and defects. In an alternative scenario, the C–S bond is cleaved and atomic sulfur binds to the defects. With regard to the molecular amino groups, they remain neutral at the lowest investigated coverages (0.3–0.5 ML), but already starting from around 0.7 ML nominal coverage protons are being transferred to them. The fraction of protonated amino groups increases with coverage and becomes dominating in multilayers prepared at room temperature and ? 65 °C. In these multilayers the carboxylic groups are deprotonated.     

DFT calculations for Au adsorption onto a reduced TiO2 (110) surface with the coexistence of Cl

Residual chlorines, which originate from HAuCl₄, enhance the aggregation of gold (Au) nanoparticles and clusters, preventing the generation of highly active supported Au catalysts. However, the detailed mechanism of residual-chlorine-promoted aggregation of Au is unknown. Herein to investigate this mechanism, density functional theory (DFT) calculations of Au and Cl adsorption onto a reduced rutile TiO₂ (110) surface were performed using a generalised gradient approximation Perdew, Burke, and Ernzerhof formula (GGA–PBE) functional and plane-wave basis. Although both Au and Cl atoms prefer to mono-absorb onto oxygen defect sites, Cl atoms have a stronger absorption onto a reduced TiO₂ (110) surface, abbreviated as rTiO₂ (110) in the following, than Au atoms. Additionally, co-adsorption of a Cl atom and a Au atom or Au nanorod onto a rTiO₂ surface was investigated; Cl adsorption onto an oxygen defect site weakens the interaction between a Au atom or Au nanorod and rTiO₂ (110) surface. The calculation results suggest that the depletion of interaction between Au and rTiO₂ surface is due to strong interaction between Cl atoms at oxygen defect sites and neighbouring bridging oxygen (O^B) atoms.     

Imaging cluster surfaces with atomic resolution: the strong metal-support interaction state of Pt supported on TiO2(110)

Nanosized platinum clusters were grown on a TiO2(110) surface and annealed in ultrahigh vacuum at high temperatures. This leads to the so-called strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the clusters with a reduced titanium oxide layer. We present atomically resolved scanning tunneling microscopy measurements of the cluster surfaces and an atomic model of the SMSI state. The ability to resolve the cluster surface geometry with atomistic detail may help to identify the active sites responsible for the SMSI.     

Comparison of the CO and D2 oxidation reactions on Pd supported on MgO(100), MgO(110) and MgO(111)

Oxidation of D₂ and CO on oxygen pre-exposed 200 nm thick Pd films, epitaxially grown on MgO(100), MgO(110) and MgO(111), has been investigated in the temperature range 100–300°C. Oxygen initial sticking coefficients have been determined to be close to 1 for the 100 and 110 films, and around 0.8 for the 111 film. The sticking coefficient and reactive sticking coefficient for CO oxidation on Pd/MgO(100) is also close to 1, and the maximum reactive sticking coefficient for hydrogen oxidation is determined to be around 0.9 at temperatures above 200°C. It is shown that the reactivities for the different surfaces vary strongly with surface and oxygen coverage, and the consequence of this for supported particle catalysts is pointed out.     

The adsorption of carbon monoxide on TiO2(110) supported palladium

Palladium overlayers deposited on TiO₂(110) by metal vapour deposition have been investigated using LEED, XPS and FT-RAIRS of adsorbed CO. Low coverages of palladium (<3 ML) deposited at 300 K adsorb CO exclusively in a bridged configuration with a band (B₁ at 1990 cm⁻¹) characteristic of CO adsorption on Pd(110) and Pd(100) surfaces. When annealed to 500 K, XPS and LEED indicate the nucleation of Pd particles on which CO adsorbs predominantly as a strongly bound linear species which we associate with edge sites on the Pd particles (L* band at 2085 cm⁻¹). Both bridged and linear CO bands are exhibited as increases in reflectivity at the resonant frequency, indicating the retention of small particle size during the annealing process. Palladium overlayers of intermediate coverages (10–20 ML) deposited at 300 K undergo some nucleation during growth, and adsorbed CO exhibits both absorption and transmission bands in the B₁ (1990 cm⁻¹) and B₂ (1940 cm⁻¹) regions. The latter is associated with the formation of Pd(111) facets. Highly dispersed Pd particles are produced on annealing at 500 K. This is evidenced by the dominance of transmission bands for adsorbed CO and a significant concentration of edge sites, which accommodate the strongly bound linear species at 300 K. Adsorption of CO at low temperature also allows the identification of the constituent faces of Pd and the conversion of Pd(110)/(100) facets to Pd(111) facets during the annealing process. High coverages of palladium (100 ML) produce only absorption bands in FT-RAIRS of adsorbed CO associated with the Pd facets, but annealing these surfaces also shows a conversion to Pd(111) facets. LEED indicates that at coverages above 10 ML, the palladium particles exhibit (111) facets parallel to the substrate and aligned with the TiO₂(110) unit cell, and that this ordering in the particles is enhanced by annealing.     

Low temperature and low pressure CO oxidation on gold clusters supported on MgO(100)

CO oxidation has been investigated on Au/MgO(100) model catalysts at RT and low pressure. The gold particles prepared by UHV evaporation on clean MgO surfaces are characterized by HRTEM. The gold particles are FCC single crystals or multiple twins with five-fold symmetry. Infrared spectroscopy indicates that two types of adsorption sites are present which correspond to loosely and strongly bound CO. The equilibrium CO coverage for the strongly bound CO is smaller than 0.1 ML. CO titration experiments show that oxygen does not dissociate on the gold nanoparticles. The CO oxidation reaction is studied at RT by molecular beam methods. A steady state CO reaction probability up to 0.50 is measured, for the first time at low pressure, for gold particles with a mean size of 1.5 nm. A reaction mechanism is proposed in which CO adsorbed on low coordinated gold atoms reacts with oxygen adsorbed molecularly, possibly at the Au/MgO interface.     

CO adsorption on small Aun（n=1-7） clusters supported on a reduced rutile TiO2（110） surface:a first-principles study

CO adsorption on small Au n(n = 1-7) clusters which are supported by a partially reduced rutile TiO 2(110) surface has been investigated by the first-principles method.The low coordinated sites of Au clusters are favorable for CO adsorption.CO-Au n-TiO 2 system displays surface magnetism.There is a strong interaction between the adsorbed CO molecule and the supported Au clusters.