Adsorption configurations and energetics of BClx (x=0-3) on TiO2 anatase (101) and rutile (110) surfaces

This study investigates the adsorption and reactions of boron trichloride and its fragments (BClx) on the TiO2 anatase (101) and rutile (110) surfaces by first-principles calculations. The results show that the possible absorbates on the TiO2 anatase and rutile surfaces are very similar. The single- and double-site adsorption configurations are found for both anatase and rutile surfaces. The particular adsorbate feature on the anatase surface is its in-plane double-site adsorption by Ti and O from its sawtooth surface. The potential energy surface shows that BCl3 can be adsorbed on the O site for both the anantase and rutile surfaces and the most of the BClx reaction on both anatase and rutile surfaces are endothermic, except for the dissociative reaction on the rutile surface. The energy levels of the BClx reactions between the anatase and rutile surfaces show that the rutile surface has lower energy levels than those of anatase surface. This result reveals that the BClx dissociative adsorption more easily occurs on rutile surface than on anatase surface.     

The adsorption and reaction of a titanate coupling reagent on the surfaces of different nanoparticles in supercritical CO2

The adsorption and reaction in supercritical CO2 of the titanate coupling reagent NDZ-201 on the surfaces of seven metal oxide particles, SiO2, Al2O3, ZrO2, TiO2 (anatase), TiO2 (rutile), Fe2O3, and Fe3O4, was investigated. FTIR and TG analysis indicated that the adsorption and reaction were different on different particle surfaces. On SiO2 and Al2O3 particles, there was a chemical reaction of the titanate coupling reagent on the surfaces. On the surfaces of ZrO2 and TiO2 (anatase) particles, there were two kinds of adsorption, weak and strong adsorption. On the surfaces of TiO2 (rutile), Fe2O3, and Fe3O4 particles, there was only weak adsorption. The acidity or basicity of the OH groups on the particle surface was the key factor that determined if a surface reaction occurred. When the OH groups were acidic, the titanate coupling reagent reacted with these, but otherwise, there was no reaction. The surface density of OH groups on the original particles and the amount of titanate coupling reagent adsorbed and reacted were estimated from TG analysis. The reactivity of the surface OH groups of Al2O3 particles was higher than that of the SiO2 particles.     

Computational investigation of the adsorption and reactions of SiHx (x = 0–4) on TiO2 anatase (101) and rutile (110) surfaces

The adsorption and reactions of the SiH_x (x = 0–4) on Titanium dioxide (TiO₂) anatase (101) and rutile (110) surfaces have been studied by using periodic density functional theory in conjunction with the projected augmented wave approach. It is found that SiH_x (x = 0–4) can form the monodentate, bidentate, or tridentate adsorbates, depending on the value of x. H coadsorption is found to reduce the stability of SiH_x adsorption. Hydrogen migration on the TiO₂ surfaces is also discussed for elucidation of the SiH_x decomposition mechanism. Comparing adsorption energies, energy barriers, and potential energy profiles on the two TiO₂ surfaces, the SiH_x decomposition can occur more readily on the rutile (110) surface than on the anatase (101) surface. The results may be used for kinetic simulation of Si thin‐film deposition and quantum dot preparation on titania by chemical vapor deposition (CVD), plasma enhanced CVD, or catalytically enhanced CVD. © 2013 Wiley Periodicals, Inc.     

The adsorption and activation of formic acid on different anatase TiO_2 surfaces

Formic acid photodegradation is one of the most important reactions in organic pollution control, and helps to improve the hydrogen generation efficiency in titanium dioxide catalyzed water photodecomposition. Based on density functional theory and Reax FF molecular dynamics, the adsorption, diffusion and activation of formic acid on the different anatase TiO_2(101),(001),(010) surfaces are investigated.The result shows that the adsorption of COOH on anatase TiO_2 surface shrinks the energy gap between the dehydrogenation intermediate COOH and HCOO. On the anatase TiO_2(101) surface, the formic acid breaks the O–H bond at the first step with activation energy 0.24 eV, and the consequent break of α-H become much easier with activation energy 0.77 eV. The dissociation of α-H is the determination step of the HCOOH decomposition.     

Active Sites for Adsorption and Reaction of Molecules on Rutile TiO2(110) and Anatase TiO2(001) Surfaces (cited: 1)

The reactivity of specific sites on rutile TiO₂(110)-(1×1) surface and anatase TiO₂(001)-(1×4) surface has been comparably studied by means of high resolution scanning tunneling microscopy. At the rutile TiO₂(110)-(1×1) surface, we find the defects of oxygen vacancy provide distinct reactivity for O₂ and CO₂ adsorption, while the terminal fivefold-coordinated Ti sites dominate the photocatalytic reactivity for H₂O and CH₃OH dissociation. At the anatase TiO₂(001)-(1×4) surface, the sixfold-coordinated terminal Ti sites at the oxidized surface seem to be inert in both O₂ and H₂O reactions, but the Ti-rich defects which introduce the Ti³⁺ state into the reduced surface are found to provide high reactivity for the reactions of O₂ and H₂O. By comparing the reactions on both rutile and anatase surfaces under similar experimental conditions, we find the reactivity of anatase TiO₂(001) is actually lower than rutile TiO₂(110), which challenges the conventional knowledge that the anatase (001) is the most reactive TiO₂ surface. Our findings could provide atomic level insights into the mechanisms of TiO₂ based catalytic and photocatalytic chemical reactions.     

Adsorption of small organic molecules on anatase and rutile surfaces: a theoretical study

The adsorption of several small organic molecules on rutile (110) and (100) as well as on anatase (101) surfaces was investigated by Car-Parrinello molecular dynamics in aqueous solution and a new approach to the calculation of adsorption energies is proposed, taking into account the potential energy fluctuation of larger systems. Acetylene and ethylene insert into twin oxygen vacancies in the surface and form polarized covalent Ti-C bonds. In one case spontaneous coupling of two acetylene molecules to a C(4)H(3) molecule with a structure similar to trans-butadiene was observed. Neutral catechol and the singly charged anion were not reactive on any titanium dioxide surface, but the twofold-charged anion attained stable mono- and bidentated geometries on anatase. Methanol, ethanol, formaldehyde and acetaldehyde adsorbed with their functional groups. Very stable geometries provide a Ti-O bond and have adsorption energies of 60-200 kJ/mol. The adsorbates compete with water molecules for similar adsorption sites in point defects as well as on perfect surfaces.     

ATR-FTIR measurements and quantum chemical calculations concerning the adsorption and photoreaction of oxalic acid on TiO2

The adsorption and photoreaction of oxalic acid on the surface of anatase and rutile TiO2 nanoparticles have been studied using a combined experimental and theoretical approach. In the dark, the experimental adsorption reaches an equilibrium state that can be described as a mixture of adsorbed water and oxalic acid molecules, with the latter forming two different surface complexes on anatase and one on rutile particles. When the system is subsequently illuminated with UV(A) light, the surface becomes enriched with absorbed oxalic acid, which replaces photo-desorbed water molecules, and one of the adsorbed oxalic acid structures on anatase is favoured over the other.     

Quantum-chemical study of adsorption of Ag<Subscript>2</Subscript>, Ag<Subscript>4</Subscript>, and Ag<Subscript>8</Subscript> on different parts of the TiO<Subscript>2</Subscript> surface

Quantum-chemical study of the adsorption of two-, four- and eight-atomic silver clusters on stoichiometric and partially reduced rutile (110) surface, and of silver tetramer on the surface of anatase (101) was carried out in the framework of periodic DFT model. The most energetically favorable positions of clusters on the surface of TiO₂ and the mechanism of binding the clusters with the substrate were revealed. According to the calculations, the adsorption of silver clusters on the surface of stoichiometric rutile (110) is more preferable than on the partially reduced one. The mechanism of binding the clusters with the surface of anatase and rutile is shown to be similar.     

Unique adsorption behaviors of NO and O2 at hydrogenated anatase TiO2(101)

The extra electron on the hydrogenated anatase TiO₂(101) is localized at the nearest Ti_5c only, and the chargetransfer promoted NO and O₂ adsorptions are also site-selective. These results are totally different from those at hydrogenated rutile TiO₂(110).     

Computational study on the reactions of H2O2 on TiO2 anatase (101) and rutile (110) surfaces

This study investigates the adsorption and reactions of H(2)O(2) on TiO(2) anatase (101) and rutile (110) surfaces by first-principles calculations based on the density functional theory in conjunction with the projected augmented wave approach, using PW91, PBE, and revPBE functionals. Adsorption mechanisms of H(2)O(2) and its fragments on both surfaces are analyzed. It is found that H(2)O(2) , H(2)O, and HO preferentially adsorb at the Ti(5c) site, meanwhile HOO, O, and H preferentially adsorb at the (O(2c))(Ti(5c)), (Ti(5c))(2), and O(2c) sites, respectively. Potential energy profiles of the adsorption processes on both surfaces have been constructed using the nudged elastic band method. The two restructured surfaces, the 1/3 ML oxygen covered TiO(2) and the hydroxylated TiO(2), are produced with the H(2)O(2) dehydration and deoxidation, respectively. The formation of main products, H(2)O(g) and the 1/3 ML oxygen covered TiO(2) surface, is exothermic by 2.8 and 5.0 kcal/mol, requiring energy barriers of 0.8 and 1.1 kcal/mol on the rutile (110) and anatase (101) surface, respectively. The rate constants for the H(2)O(2) dehydration processes have been predicted to be 6.65 × 10(-27) T(4.38) exp(-0.14 kcal mol(-1)/RT) and 3.18 × 10(-23) T(5.60) exp(-2.92 kcal mol(-1)/RT) respectively, in units of cm(3) molecule(-1) s(-1).     

Reactions on alkali-modified low-index stepped copper surfaces

Interest in CO, CO₂ and H adsorption on alkali-promoted Cu surfaces stems from the promoting role of alkali metals in heterogeneous catalysis. Adsorption of an alkali metal on a Cu surface causes a substantial decrease in the work function. The change in the electronic structure of the surface has strong consequences for the adsorption and reactive properties of the Cu surfaces. Another important factor for the reaction yields is the corrugation of the surface. The influence of these two parameters, the alkali metal coverage and the corrugation, on the dissociation of CO and the reactions between low-molecular gases like H, CO and CO₂ has been investigated on a series of low-index and stepped Cu surfaces. Particularly the synthesis of formate HCOO⁻ from coadsorption of H and CO₂ has been studied. The experimental evidence for these findings is synchrotron radiation based measurements of valence band energy distributions, and work function measurements. It is demonstrated in this report that CO dissociates on the potassium-modified stepped surface, Cu(1 1 2), at 125 K. The dissociation process is conditioned by the presence of steps and the alkali metal coverage. Carbonate is formed via the process CO + CO →CO₂ + C and reaction with oxygen. Adsorption of atomic hydrogen in the presence of K gives rise to two H-1s-induced states in the valence band. The influence of temperature on the binding energies and the population of these states have been studied. Formate is synthesized when the alkali/Cu surface, precovered with hydrogen, is exposed to CO₂.     

First-principles study of CO adsorption and vibration on Au surfaces

A CO stretching frequency analysis is presented for the adsorption of CO on various Au(110) surfaces from density functional theory calculations. The structure sensitivity of the adsorption has been studied by considering the unreconstructed (1 x 1) surface, the missing-row reconstructed (1 x 2) surface, the vicinal stepped (20) surface, and the adsorption on adatoms deposited on the (110)-(1 x 2) surface. The calculated CO stretching frequencies are compared with infrared reflection-absorption spectroscopy (IRAS) measurements carried out at room temperature and pressure below 1 atm. The overall stability of the systems is discussed within the calculations of surface free energies at various coverages. At room temperature, the adsorption of CO on the ridge of the missing-row reconstructed surface competes in the high pressure regime with more complex adsorption structures where the molecule coadsorbs on the ridge and on adatoms located along the empty troughs of the reconstruction. This result is supported by the CO stretching frequency analysis.     

Preparation of controllable crystalline titania and study on the photocatalytic properties

Nanocrystalline TiO(2) catalysts with different anatase/rutile ratios and high surface area (113-169 m(2)/g) have been prepared at low temperature by the microemulsion-mediated hydrothermal method. The samples were characterized by x-ray diffraction (XRD), Fourier transform (FT)-IR spectra, UV-vis diffuse reflectance spectra, and nitrogen adsorption-desorption methods. The contents of anatase and rutile phases in the TiO(2) powders have been successfully controlled by simply changing the proportion of Cl(-) and SO(4)(2-) in the aqueous phase of the microemulsion. A proposed mechanism involving bidentately chelated sulfate is discussed to explain the variation of the crystalline phase in the TiO(2) powder. The photodegradation of methyl orange (MO) in water has been investigated over titanium dioxide consisting of different anatase/rutile ratios. The catalyst containing 74.2% anatase showed the highest photocatalytic activity, which is due to a synergistic effect between anatase and rutile. The synergism was also found for the photodegradation of MO with physically mixed anatase and rutile as catalysts.     

Dispersion behaviors of molybdena on titania (rutile and/or anatase)

Raman and FT-IR spectra were employed to investigate the dispersion of molybdena on mixed TiO2 (rutile and anatase, signed as R and A) with different BET surface ratios of rutile/TiO2(R + A). The results showed that (1) molybdena would preferentially disperse on the rutile surface in mixed TiO2; (2) for MoO3/rutile with low molybdena loading (e.g., 0.20 mmol/100 m2 rutile), a dispersed molybdena species existed on the rutile surface in an isolated tetrahedral coordination environment, while for MoO3/rutile with high molybdena loading (e.g. 0.82 mmol/100 m2 rutile), a polymeric molybdena species could be detected on the rutile surface; (3) for the MoO3/anatase sample, a dispersed molybdena species existed on the anatase surface in a polymeric coordination environment; and (4) the formation of the Bronsted acid site on the surface of rutile and anatase should be related to the polymeric molybdena species. All these results have been discussed via the interaction between OH groups of molybdena and OH groups of rutile and anatase, and it seems reasonable to suggest that, for the lower molybdena loading, the different states of the dispersed molybdena species should result from the different dehydration orders of OH groups of the molybdena and surface OH groups of rutile and anatase.     

CO吸附在过渡金属铂表面的微观动力学研究

建立了处理双原子分子-表面相互作用的推广的LEPS势.借助推广的LEPS势,系统研究了一氧化碳分子在铂低指数表面吸附的动力学特性,重现了低指数表面的分子吸附热、吸附几何及本征振动等实验数据；鉴定了某些不合理的文献信息,预测了实验尚未探测到的重要信息:预测到Pt(100)表面四重洞位的C－O伸缩振动频率为1 962.60 cm－1;预测到Pt(110)表面吸附态的C－O及C－Pt键长分别为115.1、147 pm.     

表面相结结构在促进光催化电荷分离中的应用

二十世纪八十年代以来,特别是近十年,光催化研究在利用可再生能源太阳能的道路上飞速发展。越来越多的研究表明,相结结构的构筑是有效提高半导体光催化剂性能的重要策略。其中, TiO2作为重要的模型光催化剂,其相关研究成果呈现出指数增长的趋势。本综述围绕TiO2模型光催化剂,主要介绍TiO2表面相结的研究成果,包括TiO2表面相的表征、锐钛矿：金红石TiO2相结用于光催化产氢研究、TiO2相结在光催化中作用的最新认识等。在表征方面,通过表面灵敏的紫外拉曼光谱研究了TiO2相变过程中表面相结构的变化,结合可见拉曼以及XRD表征揭示了TiO2独特的相变过程,即相变始于锐钛矿粒子的界面处,小粒子逐渐团聚为大粒子,致其相变从大粒子体相开始最终扩展到整个粒子。使用CO, CO2探针红外光谱,根据锐钛矿和金红石表面吸附物种的差异,进一步证实了锐钛矿：金红石表面相结结构,为紫外拉曼光谱的表面表征特性提供坚实证据。同时,利用发光光谱观察到锐钛矿晶相的可见发光带和金红石晶相的近红外发光带,并基于此给出了TiO2材料表面相结结构的荧光表征新方法。此外荧光光谱还提供了锐钛矿、金红石相中载流子动力学信息,揭示了束缚态在光催化中的作用。在光催化应用方面,观察到混相结构TiO2较单独锐钛矿及金红石相具有更高的光催化产氢活性,通过在较大金红石颗粒上担载纳米锐钛矿粒子,证明了相结结构在提高光催化活性中的核心作用,并首次提出了锐钛矿：金红石表面异相结结构概念,推断其对电荷分离的促进作用是最终提高反应活性的原因。之后将此概念应用到改善商品TiO2(P25)光催化活性中,通过可控热处理精细调控P25的表面相结构,在光催化重整生物质衍生物产氢实验中,成功将P25光催化产氢活性提高3?5倍。之后发展了新的TiO2表面控制方法,通过加入Na2SO4等相变控制剂,延缓了TiO2从锐钛矿向金红石的相变过程,在较高温度下实现TiO2相结结构的调控,最终可将P25光催化重整甲醇制氢的活性提高6倍,同时通过高分辨电镜清晰观察到锐钛矿：金红石相结的原子层生长接触。在相结作用机理方面,多种时间分辨光谱技术以及理论计算被用作探索锐钛矿：金红石相结处的电子转移机理。通过时间分辨红外光谱对TiO2表面相结结构作用的研究,特别是利用锐钛矿、金红石不同的瞬态吸收光谱特征,证明了锐钛矿：金红石相结处的载流子转移过程,存在锐钛矿向金红石的电子转移过程。模型光催化剂TiO2相结的研究成果,加深了对光催化机理的认识,促进新型高效光催化体系的设计合成。     

Studies of CO adsorption on Pt(100), Pt(410), and Pt(110) surfaces using density functional theory

Adsorption of CO on Pt(100), Pt(410), and Pt(110) surfaces has been investigated by density functional theory (DFT) method (periodic DMol(3)) with full geometry optimization and without symmetry restriction. Adsorption energies, structures, and vibrational frequencies of CO on these surfaces are studied by considering multiple possible adsorption sites and comparing them with the experimental data. The same site preference as inferred experiments is obtained for all the surfaces. For Pt(100), CO adsorbs at the bridge site at low coverage, but the atop site becomes most favorable for the c(2 x 2) structure at 1/2 monolayer. For Pt(410) (stepped surface with (100) terrace and (110) step), CO adsorbs preferentially at the atop site on the step edge at 1/4 monolayer, but CO populates also at other atop and bridge sites on the (100) terrace at 1/2 monolayer. The multiple possible adsorption sites probably correspond to the multiple states in the temperature-programmed desorption spectra for CO desorption. For Pt(110), CO adsorbs preferentially at the atop site on the edge for both the reconstructed (1 x 2) and the un-reconstructed (1 x 1) surfaces. When adjacent sites along the edge row begin to be occupied, the CO molecules tilt alternately by ca. 20 degrees from the surface normal in opposite directions for both the (1 x 2) and (1 x 1) surfaces.     

Physicochemical characterization of an anatase TiO2 surface and the adsorption of a nonionic surfactant: an atomic force microscopy study

Atomic force microscopy was used to characterize an anatase TiO2 surface, prepared by the helical vapor preparation method. The forces between two bare TiO2 surfaces were measured in the presence of water at various pH values. This TiO2 isoelectric point (iep) was characterized by the presence of only a van der Waals attraction and was measured at pH 5.8; this value is similar to that for a rutile TiO2 surface. The adsorption mechanism of a nonionic surfactant molecule to this anatase TiO2 surface was investigated by measuring the forces between two such TiO2 surfaces at their iep pH in the presence of linear dodecanol tetraethoxylate (C12E4), a poly(ethoxylene oxide) n-alkyl ether. C12E4 was seen by the presence of steric forces to adsorb to the uncharged TiO2 surface. For low surfactant concentrations, C12E4 adsorbed with its hydrophobic tail facing the TiO2 substrate, to reduce its entropically unfavorable contacts with water. Additional surfactant adsorption occurred at higher surfactant concentrations by the hydrophobic and hydrophilic interactions between the surfactant tails and heads, respectively, and gave sub-bilayers. A two-step adsorption isotherm was subsequently proposed with four regions: (1) submonolayer, (2) complete monolayer, (3) sub-bilayer, and (4) bilayer. The absence of a long-range repulsive force between the two TiO2 surfaces in the presence of the C12E4 surface aggregates indicated that a C12E4 nonionic surfactant aggregate did not possess charge.     

锐钛矿TiO2（101）表面电子能带结构的理论研究

二氧化钛作为一种理想的光催化和光电转换半导体材料,受到了广泛的关注和研究,其表面的电子能带结构作为其本征的化学性质之一,决定着表面上氧化还原反应发生的可能性. 对二氧化钛表面电子能带结构进行深入研究对于我们从微观上认识并改良二氧化钛这一光电催化材料,以及进一步开发利用更好的光催化材料都具有非常好的指导意义. 本论文采用密度泛函理论,计算研究了锐钛矿TiO₂（101）表面的电子能带结构,并通过与金红石TiO₂（110）晶面的对比,系统分析了两个表面电子能带结构的不同以及水分子的溶剂化作用对电子能带结构的影响.     

First principles study of CO oxidation on TiO2(110): the role of surface oxygen vacancies

The reactivities of the stoichiometric and partially reduced rutile TiO2(110) surfaces towards oxygen adsorption and carbon monoxide oxidation have been studied by means of periodic density functional theory calculations within the Car-Parrinello approach. O2 adsorption as well as CO oxidation are found to take place only in the presence of surface oxygen vacancies (partially reduced surface). The oxidation of CO by molecularly adsorbed O2 at the O-vacancy site is found to have an activation energy of about 0.4 eV. When the adsorbed O2 is dissociated, the resulting adatoms can oxidize incoming gas-phase CO molecules with no barrier. In all studied cases, once CO is oxidized to form CO2, the resulting surface is defect-free and no catalytic cycle can be established.