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.     

掺杂二氧化锡的二氧化钛的光学性质及其光催化特性

二氧化钛多相催化是一种极具前途的环境污染深度净化技术。 本文以钛酸四丁酯和四氯化锡为原料,无水乙醇为溶剂,采用溶胶-凝胶法制备了掺杂二氧化锡的二氧化钛薄膜和复合氧化物粉体。通过测量薄膜的吸收光谱推算光学能隙,结果发现掺杂样品的光学能隙比纯二氧化钛样品有所变小。随着热处理温度的提高,掺杂和纯二氧化钛样品的光学能隙都略微降低。X-射线衍射分析表明,复合氧化物粉体的热处理温度对样品的晶体结构和光催化性能有重要影响。以掺杂二氧化锡5 % 摩尔比的样品与纯二氧化钛对照,500 ℃以下热处理样品以锐钛矿结构为主,600 ℃热处理样品为锐钛矿与金红石相共存,并显示了较好的光催化性能。透射电子显微镜观察显示,同样600 ℃热处理,掺杂样品要比纯二氧化钛具有更小的颗粒尺寸。在700 ℃热处理的样品中,掺杂样品只存在金红石相而纯二氧化钛样品中仍存有锐钛矿相。用阿伦尼乌斯经验关系式推测的晶粒生长的活化能,纯二氧化钛47.486 kJ.mol, 掺杂5 % 摩尔比的复合氧化物样品33.103 kJ.mol。以亚甲基蓝为降解物质,考察了掺杂量和热处理温度对样品的光催化性能。     

Probing Surface Sites of TiO2: Reactions with [HRe(CO)5] and [CH3Re(CO)5]

Two carbonyl complexes of rhenium, [HRe(CO)₅] and [CH₃Re(CO)₅], were used to probe surface sites of TiO₂ (anatase). These complexes were adsorbed from the gas phase onto anatase powder that had been treated in flowing O₂ or under vacuum to vary the density of surface OH sites. Infrared (IR) spectra demonstrate the variation in the number of sites, including Ti⁺³? OH and Ti⁺⁴? OH. IR and extended X‐ray absorption fine structure (EXAFS) spectra show that chemisorption of the rhenium complexes led to their decarbonylation, with formation of surface‐bound rhenium tricarbonyls, when [HRe(CO)₅] was adsorbed, or rhenium tetracarbonyls, when [CH₃Re(CO)₅] was adsorbed. These reactions were accompanied by the formation of water and surface carbonates and removal of terminal hydroxyl groups associated with Ti⁺³ and Ti⁺⁴ ions on the anatase. Data characterizing the samples after adsorption of [HRe(CO)₅] or [CH₃Re(CO)₅] determined a ranking of the reactivity of the surface OH sites, with the Ti⁺³? OH groups being the more reactive towards the rhenium complexes but the less likely to be dehydroxylated. The two rhenium pentacarbonyl probes provided complementary information, suggesting that the carbonate species originate from carbonyl ligands initially bonded to the rhenium and from hydroxyl groups of the titania surface, with the reaction leading to the formation of water and bridging hydroxyl groups on the titania. The results illustrate the value of using a family of organometallic complexes as probes of oxide surface sites.     

Surface Modification of (001) Facets Dominated TiO2 with Ozone for Adsorption and Photocatalytic Degradation of Gaseous Toluene

This study investigated the positive effect of surface modification with ozone on the photocatalytic performance of anatase TiO₂ with dominated (001) facets for toluene degradation. The performance of photocatalyst was tested on a home-made volatile organic compounds degradation system. The ozone modi cation, toluene adsorption and degradation mechanism were established by a combination of various characterization methods, in situ diffuse reflectance infrared fourier transform spectroscopy, and density functional theory calculation. The surface modi cation with ozone can significantly enhance the photocatalytic degradation performance for toluene. The abundant unsaturated coordinated 5c-Ti sites on (001) facets act as the adsorption sites for ozone. The formed Ti-O bonds reacted with H₂O to generate a large amount of isolated Ti_5c-OH which act as the adsorption sites for toluene, and thus significantly increase the adsorption capacity for toluene. The outstanding photocatalytic performance of ozone-modified TiO₂ is due to its high adsorption ability for toluene and the abundant surface hydroxyl groups, which produce very reactive OH radicals under irradiation. Furthermore, the O₂ generated via ozone dissociation could combine with the photogenerated electrons to form superoxide radicals which are also conductive to the toluene degradation.     

The influence of cobalt doping on photocatalytic nano-titania: Crystal chemistry and amorphicity

Photocatalysts of nominal composition (Ti_1−xCo_x)O_2−δ with 0.001?x?0.05 were prepared via a sol-gel technique followed by air firing (200-1000 °C). The incorporation of cobalt inhibited crystal growth and slightly raised the anatase to rutile transformation temperature (∼700 °C). An amorphous component was invariably significant with the maximum content (41-53 wt%) appearing simultaneously with the removal of anatase, suggesting that rutile crystallizes via an aperiodic structure. While the introduction of cobalt shifted the apparent band gap to visible light energies this did not enhance performance as there was limited miscibility of cobalt in titania, non-catalytic secondary phases were present, and active Ti³⁺ sites were displaced by cobalt.     

Room temperature synthesis of Ti-SBA-15 from silatrane and titanium-glycolate and its catalytic performance towards styrene epoxidation

A novel room temperature sol–gel synthesis of Ti-SBA-15 is described using moisture stable silatrane and titanium glycolate precursors, and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer (EO₂₀PO₇₀EO₂₀) as the structure directing agent. Catalyst performance was optimized by systematically investigating the influence of acidity, reaction time and temperature, and titanium loading. Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) showed well-ordered 2D mesoporous hexagonal structures, while N₂ adsorption/desorption measurements yielded high surface areas (up to 670 m²/g), with large pore diameters (5.79 nm) and volumes (0.83 cm³/g). Diffuse reflectance UV–visible spectroscopy (DRUV) was found that tetravalent titanium as Ti⁴⁺O₄ tetrahedra were incorporated in the framework through displacement of Si⁴⁺O₄ after calcination (550°C/6 h) to loadings of 7 mol% Ti without perturbation of the ordered mesoporous structure, or decoration by extra-framework anatase containing Ti⁴⁺O₆ octahedra. The catalytic activity and selectivity of styrene epoxidation using hydrogen peroxide (H₂O₂) showed that the conversion of styrene increases significantly at higher titanium contents. The only products of this reaction were styrene oxide and benzaldehyde, with selectivity of 34.2 and 65.8%, respectively, at a styrene conversion of 25.8% over the 7 mol% Ti-SBA-15 catalyst. Beyond this titanium loading, anatase is deposited on the framework and catalytic activity degrades. The performance of the new catalyst is also shown to be superior to conventional materials produced by incipient wetness impregnation where Ti resides on the surface of SBA-15, giving a styrene conversion of 11.9% under identical reaction conditions.     

Rate coefficient for the reaction of CCl3 radicals with ozone

The rate coefficient for the reaction of CCl₃ radicals with ozone has been measured at 303 ± 2 K. The CCl₃ radicals were generated by the pulsed laser photolysis of carbon tetrachloride at 193 nm. The time profile of CCl₃ concentration was monitored with a photoionization mass spectrometer. Addition of the O₃–O₂ mixture to this system caused a decay of the CCl₃ concentration because of the reactions of CCl₃ + O₃ → products (5) and CCl₃ + O₂ → products (4). The decay of signals from the CCl₃ radical was measured in the presence and absence of ozone. In the absence of ozone, the O₃–O₂ mixture was passed through a heated quartz tube to convert the ozone to molecular oxygen. Since the rate coefficient for the reaction of CCl₃ + O₂ could be determined separately, the absolute rate coefficient for reaction ( 5 ) was obtained from the competition among these reactions. The rate coefficient determined for reaction ( 5 ) was (8.6 ± 0.5) × 10^?13 cm³ molecule^?1 s^?1 and was also found to be independent of the total pressure (253–880 Pa of N₂). This result shows that the reaction of CCl₃ with O₃ cannot compete with its reaction with O₂ in the ozone layer. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 310–316, 2003     

Arrangement of acid sites on the surfaces of anatase titanium dioxide nanoparticles according to cluster models

Computer-implemented cluster models have been devised for ～2-nm anatase nanoparticles with exposed (001) and (100) faces. The Lewis acid sites occurring in these faces have been characterized by calculating the enthalpy of CO adsorption. In the Ti₁₁₄O₂₂₈ and Ti₁₈₇O₃₇₆H₄ clusters, the corner oxygen atoms compensating the electric charge are bound to titanium atoms by double bonds with a length of approximately 1.7 Å, which is in agreement with experimental data. The average enthalpy of CO adsorption on the (001) and (100) faces at a zero coverage is ?87.62 and ?135.31 kJ/mol, respectively. The deviation from the average value is 20.2 and 8.8%, respectively. The average enthalpy of CO adsorption for the Ti₁₁₄O₂₂₈ cluster is ?129.40 kJ/mol, and that for the Ti₁₈₇O₃₇₆H₄ cluster is ?119.79 kJ/mol.     

Kinetics of Active Oxygen Species with Implications for Atmospheric Ozone Chemistry

Photochemical processes involving singlet oxygen (O₂(a¹Δ)), oxygen atoms_, and ozone are critical in determining atmospheric ozone concentrations. Here we report on kinetic measurements and modeling that examine the importance of the reactions of vibrationally excited ozone. Oxygen atoms and O₂(a¹Δ) were produced by UV laser photolysis of ozone. Time‐resolved absorption spectroscopy was used for O₃ concentration measurements. It was found that vibrationally excited ozone formed by O + O₂ + M → O₃(ν) + M recombination reacts effectively with O₂(a¹Δ) and O atoms. The reaction O₃(υ) + O₂(a¹Δ) → O + 2O₂ results in a reduction of the ozone recovery rate due to O atom regeneration, whereas the reaction O₃(υ) + O → 2O₂ removes two odd oxygen species, resulting in incomplete ozone recovery. The possible impact of these reactions on the atmospheric O₂(a¹Δ) and O₃ budgets at altitudes in the range of 80–100 km is considered.     

Methanol Conversion into Dimethyl Ether on the Anatase TiO2(001) Surface

Exploring reactions of methanol on TiO₂ surfaces is of great importance in both C1 chemistry and photocatalysis. Reported herein is a combined experimental and theoretical calculation study of methanol adsorption and reaction on a mineral anatase TiO₂(001)‐(1×4) surface. The methanol‐to‐dimethyl ether (DME) reaction was unambiguously identified to occur by the dehydration coupling of methoxy species at the fourfold‐coordinated Ti⁴⁺ sites (Ti_4c), and for the first time confirms the predicted higher reactivity of this facet compared to other reported TiO₂ facets. Surface chemistry of methanol on the anatase TiO₂(001)‐(1×4) surface is seldom affected by co‐chemisorbed water. These results not only greatly deepen the fundamental understanding of elementary surface reactions of methanol on TiO₂ surfaces but also show that TiO₂ with a high density of Ti_4c sites is a potentially active and selective catalyst for the important methanol‐to‐DME reaction.     

On the formation and stability of O− and O−2 radicals in type a zeolites and demonstration of cation interactions

Electron spin resonance studies show tha O^? is formed as the major paramagnetic oxygen species in γ-irradiated Ca₆-A zeolite followed by oxygen adsorption. This is a new method to generate this highly reactive catalytic intermediate. O^?₂ is formed in addition to O^? if oxygen is adsorbed prior to irradiation. In Na₁₂-A zeolite O^? is also seen but it transforms to O^?₂ in several hours. Thus O^? appears to be more stable in divalent exchanged zeolites. By electron spin echo modulation spectrometry interactions fo O^?₂ with Li⁺ have been detected which suggests that oxygen species locations in zeolites can be delineated.     

Heterostructured composite of NiFe-LDH nanosheets with Ti4O7 for oxygen evolution reaction

Developing oxygen evolution reaction (OER) electrocatalyst based on earth-abundant materials holds great promise for ascertaining water-splitting to surmount its deprived kinetics. In this regard, NiFe-LDH (layered double hydroxide) receives considerable attention owing to their layered structure. However, they still suffer from poor electronic conductivity and structural stability. We combined NiFe-LDH nanosheets with Magnéli phase Ti₄O₇ into a heterostructured composite. A series of analyses reveal that decorating Ti₄O₇ facilitates charge transfer to enhance the conductivity of NiFe-LDH-Ti₄O₇. During electrochemical measurement, Ni²⁺ is transformed to metastable Ni³⁺ (Ni (OH)→ NiOOH) before the OER onset potential. Thus, the presence of Ni³⁺ as the main active sites could improve the chemisorption of OH^? to facilitate OER. As a result, the NiFe-LDH-Ti₄O₇ catalyst delivers as low as onset potential (1.43 V). Combining the holey structure (NiFe-LDH and Ti₄O₇) and the defect engineering generated on NiFe-LDH-Ti₄O₇ as a synergistic effect improves the OER performance. The inclusion of Ti₄O₇ in the composite leads to more vacancy sites, as evidenced by the extended X-ray absorption fine structure (EXAFS) analysis. The obtained defective structure with a low coordination environment would improve the electronic conductivity and facilitate the adsorption process of H₂O onto metal cations, thereby increasing the intrinsic catalytic activity of NiOOH. The strong coupling of NiFe-LDH and Ti₄O₇ also increases the stability, and the heterostructured composite helps maintain the structural robustness of the LDH.     

Ni-Ti-O混合氧化物的制备、表征及其富氧选择性催化还原NO

采用均匀共沉淀法制备了不同Ni/Ti摩尔比的Ni-Ti-O混合氧化物,考察了它们在富氧条件下丙烯选择性催化还原NO反应中的催化性能,并运用X射线衍射,N₂吸附-脱附、吡啶吸附、程序升温脱附和原位红外光谱对催化剂进行了表征.结果表明,Ni/Ti摩尔比为1的催化剂表现出最佳催化活性,430℃时NO_x转化率达68%.该催化剂具有锐钛矿结构,比表面积较高(149m²/g),有利于提高催化活性;其表面Lewis酸性位有利于硝酸盐物种的吸附,而硝酸盐物种是该反应的重要中间体.     

Dissociative double electron capture,dynamics of fragmentation of the water and hydrogen sulfide negative ions

The technique of ion kinetic energy spectrometry has been used to observe the unimolecular decompositions of H₂O^?? and H₂S^?? generated by charge exchange of the corresponding high velocity positive ions. The method involves dissociative double electron capture by a high velocity ion and allows the study of unstable negative ions that may be directly observable by conventional electron capture techniques. Information on the energetics of the reaction is obtained from the kinetic energy of the product ion. The reactions under consideration are shown in (1) and (2) where X = O or S.

The kinetic energy releases accompanying the reactions given in (1) and (2) have been measured and compared to those for the collision-induced reactions which produce the corresponding positive ions. The results have been used to deduce that the sequence of steps in the formation of the fragment negative ions is that given in (1) and (2). The cross section of OH^? formation is observed to be somewhat greater than for O^? production. This result is in contrast with dissociative electron capture cross sections from the neutral species and is interpreted on the basis of the energetic requirements for the reactions under consideration. H₂O^? reacts from different electronic states in yielding OH^? on the one hand and O^? on the other. The energy partitioning associated with reaction (2) suggests that the neutral productions 2H' rather than H₂. The kinetic energy losses accompanying excitation and kinetic energy releases upon fragmentation were similar for the corresponding reactions of the sulfur and oxygen-containing ions indicating related mechanisms in the two sets of reactions.     

Oxygen reduction on electrode surfaces modified by foreign metal ad-atoms: Lead ad-atoms on gold

Oxygen reduction on gold is considerably catalysed by foreign metal ad-atoms. The catalytic effects of lead have been studied in more detail as most illustrative. The two-electron reduction of O₂ to HO₂^? on Au changes into a four-electron process on Au modified by lead. In the potential region where AuOH constitutes the surface, the interaction of Pb ions with AuOH causes catalytic effects. At more negative potentials, on bare Au surface, the underpotential deposition of Pb ad-atoms gives rise to the catalytic effects.At AuOH surface modified by Pb ions the O₂ reduction involves a “series” mechanism, with only minute quantities of HO₂^? leaving the electrode surface. The reduction of HO₂^? is considerably catalysed. The mechanism of this reaction is changed from the rate-determining chemical step into the charge-transfer rate-determining step. The rate-determining step for O₂ reduction involves the first charge transfer: O₂+e→O₂^?(ads)The mechanism of HO₂^? formation is uncertain, while its reduction most probably involves a direct process. There are indications that on Au surface with Pb ad-atoms a “parallel” mechanism may be operative.The catalytic effect originates in the interaction of Pb²⁺ with AuOH surface, which considerably reduces a partial negative charge on OH. Such a surfaces, as well as that of Au covered by Pb ad-atoms, are more suitable for adsorption of O₂, O₂^? and HO₂^? which considerably alters the free energy of adsorption of these species.     

GO/Fe3O4/有机胺复合材料的制备及对结晶紫染料的吸附性能

用改进的Hummers法制备了氧化石墨烯,用乙二胺、乙二胺与丁二胺/己二胺混溶来改性氧化石墨烯。用水热法制备了Fe3O4,并用物理混合法制备了GO/Fe3O4/有机胺的三元复合体系。用透射电镜、扫描电镜、红外光谱、热重分析、X射线衍射、VSM和XPS等对所制得的样品进行了结构表征和性能测试,研究了三元复合粒子对结晶紫染料的吸附性能及影响结晶紫染料吸附效果的因素。结果表明:所制备的Fe3O4的平均粒径约为200 nm,粒径分布均匀;复合物中GO为典型的片状结构,GO及有机胺的掺杂没有影响Fe3O4的尖晶石结构;复合物为超顺磁性,Ms为53.0 emu·g~(-1)。吸附结果表明:石墨烯/Fe3O4/有机胺的三元复合材料对结晶紫染料的最大吸附量随浓度增大而增大,而吸附结晶紫染料的移除率却随结晶紫染料浓度增大而减小,并趋向一定值;乙二胺和己二胺混溶比例为5∶1的GO/Fe3O4复合材料吸附性能最佳:结晶紫浓度为400 mg·L~(-1),最大吸附量为164.3 mg·L~(-1)。     

Model of the catalytic reaction 2H<Subscript>2</Subscript> + O<Subscript>2</Subscript> → H<Subscript>2</Subscript>O with the participation of molecules in a precursor state

The absence of experimental evidence for the occurrence of the catalytic reaction 2H₂ + O₂ → 2H₂O on platinum in accordance with the Langmuir-Hinshelwood mechanism was established. It was found that the heterogeneous process can be described more adequately and its nature can be better understood with consideration for chemical transformations involving molecules in a precursor state in a model of the above reaction. The inverse kinetic problem was solved. It was found that the model in which an unambiguously specified set of rate constants for the elementary steps of the reaction was used provided an opportunity to describe experimental data obtained by various authors concerning the oxidation of hydrogen on platinum over the detonating gas pressure range 10^?3-10⁵ Pa. The signs of the occurrence of heterogeneous reactions by an adsorption mechanism were found.     

Highly Dispersed Ruthenium Hydroxide Supported on Titanium Oxide Effective for Liquid‐Phase Hydrogen‐Transfer Reactions

Supported ruthenium hydroxide catalysts (Ru(OH)_x/support) were prepared with three different TiO₂ supports (anatase TiO₂ (TiO₂(A), BET surface area: 316 m² g^?1), anatase TiO₂ (TiO₂(B), 73 m² g^?1), and rutile TiO₂ (TiO₂(C), 3.2 m² g^?1)), as well as an Al₂O₃ support (160 m² g^?1). Characterizations with X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), and X‐ray absorption fine structure (XAFS) showed the presence of monomeric ruthenium(III) hydroxide and polymeric ruthenium(III) hydroxide species. Judging from the coordination numbers of the nearest‐neighbor Ru atoms and the intensities of the ESR signals, the amount of monomeric hydroxide species increased in the order of Ru(OH)_x<Ru(OH)_x/TiO₂(C)<Ru(OH)_x/Al₂O₃<Ru(OH)_x/TiO₂(B)<Ru(OH)_x/TiO₂(A). These supported ruthenium hydroxide catalysts, especially Ru(OH)_x/TiO₂(A), showed high catalytic activities and selectivities for liquid‐phase hydrogen‐transfer reactions, such as racemization of chiral secondary alcohols and the reduction of carbonyl compounds and allylic alcohols. The catalytic activities of Ru(OH)_x/TiO₂(A) for these hydrogen‐transfer reactions were at least one order of magnitude higher than those of previously reported heterogeneous catalysts, such as Ru(OH)_x/Al₂O₃. These catalyses were truly heterogeneous, and the catalysts recovered after the reactions could be reused several times without loss of catalytic performance. The reaction rates monotonically increased with an increase in the amount of monomeric ruthenium hydroxide species, which suggests that the monomeric species are effective for these hydrogen‐transfer reactions.     

Synthesis, structure and electrical properties of Cu3.21Ti1.16Nb2.63O12 and the CuOx-TiO2-Nb2O5 pseudoternary phase diagram

Subsolidus phase relations in the CuO_x-TiO₂-Nb₂O₅ system were determined at 935 °C. The phase diagram contains one new phase, Cu_3.21Ti_1.16Nb_2.63O₁₂ (CTNO) and one rutile-structured solid solution series, Ti_1−3xCu_xNb_2xO₂: 0<x<0.2335 (35). The crystal structure of CTNO is similar to that of CaCu₃Ti₄O₁₂ (CCTO) with square planar Cu²⁺ but with A site vacancies and a disordered mixture of Cu⁺, Ti⁴⁺ and Nb⁵⁺ on the octahedral sites. It is a modest semiconductor with relative permittivity ∼63 and displays non-Arrhenius conductivity behavior that is essentially temperature-independent at the lowest temperatures.