Sustained production of H2O2 on irradiated TiO2- fluoride systems

UV irradiation of fluorinated TiO(2) suspensions in water, in the presence of oxygen and a hole scavenger, leads to the production of H(2)O(2) with steady state concentration levels up to 1.3 millimolar; the H(2)O(2) formation rate follows the TiO(2) surface speciation, being maximum when the surface is completely covered by [triple bond]Ti-F groups; these results outline the importance of surface speciation on the photocatalytic process.     

负载型Sn2(OMe)2Cl2/SiO2催化剂的制备、表征与催化合成碳酸二甲酯

采用表面改性法制备了负载型Sn2(OMe)2Cl2/SiO2双核桥联配合物催化剂,用IR,TPD和微量反应技术研究了催化剂的表面结构、化学吸附性能和反应活性.结果表明,双核桥联配合物Sn2(OMe)2Cl2以O(Me)为桥,Cl为配体,并以Sn-O-Si形式键合到SiO2表面上;CO2与催化剂表面的金属离子Sn4+和桥基配体OMe的O2-形成桥式和甲氧碳酸酯基两种吸附态,CH3OH与催化剂表面的金属离子Sn4+仅形成一种分子吸附态;在413K以下,CO2和CH3OH在Sn2(OMe)2Cl2/SiO2催化剂表面上以近100%的选择性生成碳酸二甲酯;CO2在催化剂表面形成的甲氧碳酸酯基吸附态是生成DMC的关键物种,其与在同一活性中心的分子吸附态甲醇的反应决定了催化剂的活性和产物选择性.     

Surface dependence of CO2 adsorption on Zn2GeO4

An understanding of the interaction between Zn(2)GeO(4) and the CO(2) molecule is vital for developing its role in the photocatalytic reduction of CO(2). In this study, we present the structure and energetics of CO(2) adsorbed onto the stoichiometric perfectly and the oxygen vacancy defect of Zn(2)GeO(4) (010) and (001) surfaces using density functional theory slab calculations. The major finding is that the surface structure of the Zn(2)GeO(4) is important for CO(2) adsorption and activation, i.e., the interaction of CO(2) with Zn(2)GeO(4) surfaces is structure-dependent. The ability of CO(2) adsorption on (001) is higher than that of CO(2) adsorption on (010). For the (010) surface, the active sites O(2c)···Ge(3c) and Ge(3c)-O(3c) interact with the CO(2) molecule leading to a bidentate carbonate species. The presence of Ge(3c)-O(2c)···Ge(3c) bonds on the (001) surface strengthens the interaction of CO(2) with the (001) surface, and results in a bridged carbonate-like species. Furthermore, a comparison of the calculated adsorption energies of CO(2) adsorption on perfect and defective Zn(2)GeO(4) (010) and (001) surfaces shows that CO(2) has the strongest adsorption near a surface oxygen vacancy site, with an adsorption energy -1.05 to -2.17 eV, stronger than adsorption of CO(2) on perfect Zn(2)GeO(4) surfaces (E(ads) = -0.91 to -1.12 eV) or adsorption of CO(2) on a surface oxygen defect site (E(ads) = -0.24 to -0.95 eV). Additionally, for the defective Zn(2)GeO(4) surfaces, the oxygen vacancies are the active sites. CO(2) that adsorbs directly at the Vo site can be dissociated into CO and O and the Vo defect can be healed by the oxygen atom released during the dissociation process. On further analysis of the dissociative adsorption mechanism of CO(2) on the surface oxygen defect site, we concluded that dissociative adsorption of CO(2) favors the stepwise dissociation mechanism and the dissociation process can be described as CO(2) + Vo → CO(2)(δ-)/Vo → CO(adsorbed) + O(surface). This result has an important implication for understanding the photoreduction of CO(2) by using Zn(2)GeO(4) nanoribbons.     

Structures and energies of coadsorbed CO and H2 on Fe5C2(001), Fe5C2(110), and Fe5C2(100)

Density functional theory calculations have been carried out on the CO/H2 coadsorption on the (001), (110), and (100) surfaces of Fe5C2 for the understanding of the Fischer-Tropsch synthesis (FTS) mechanism. The stable surface species changes with the variation of the H2 and CO coverage. Along with dissociated hydrogen and adsorbed CO in 2-, 3-, and 4-fold configurations, methylidyne (C(s)H) (C(s), surface carbon), ketenylidene (C(s)CO), ketenyl (C(s)HCO), ketene (C(s)H2CO), and carbon suboxide (C(s)C2O2) are computed as thermodynamically stable surface species on Fe5C2(001) and Fe5C2(110) containing both surface iron and carbon atoms. These surface carbon species can be considered as the preliminary stages for FTS. On Fe5C2(100) with only iron atoms on the surface layer, the stable surface species is dissociated hydrogen and CO with top and 2-fold configurations. The bonding nature of these adsorbed carbon species has been analyzed.     

Coupling vs surface-etching reactions of alkyl halides on GaAs(100). 2. CH2I2 reactions

Surface reactions of CH2I2 on gallium-rich GaAs(100)-(4 x 1), studied by temperature programmed desorption and X-ray photoelectron spectroscopy (XPS), show CH2I2 adsorbs dissociatively at liquid nitrogen temperatures to form surface chemisorbed CH2(ads) and I(ads) species. Controlled hydrogenation of a fraction of the CH2(ads) species in the chemisorbed layer by the background hydrogen radicals results in a surface layer comprising both CH3(ads) and CH2(ads) species. This hydrogenation step initiates a plethora of further surface reactions involving these two species and I(ads). Thermal activation leads to three sequential methylene insertions (CH2(ads)) into the CH3-surface bond to form three higher alkyl (ethyl (C2), propyl (C3), and butyl (C4)) species, which undergo beta-hydride elimination to evolve the respective higher alkene (ethene, propene, and butene). In competition with beta-hydride elimination, reductive elimination of the ethyl and propyl species with I(ads) occurs to liberate the respective alkyl iodide. Beta-hydride elimination in the alkyls, in the temperature range 420-520 K, is the more dominant pathway, and it is also the rate-limiting step for further chain propagation. The evolution of the alkyl iodides represents the only pathway for the removal of surface iodines in this study and is different from previous investigations where gallium and arsenic iodide etch products (GaI(x), AsI(x) (x = 1-3)) formed instead. The desorption of methane and methyl iodide, formed from surface CH3(ads) species at high temperatures by the reaction between surface methylenes and hydrogens eliminated from the surface C2-C4 alkyls, terminates the chain propagation. We discuss the reaction mechanisms by which the observed reaction products form and postulate reasons for the reaction pathways adopted by the surface species.     

Surface characterization and properties of ordered arrays of CeO2 nanoparticles embedded in thin layers of SiO2

We demonstrated the surface composite character down to the nanometer scale of SiO(2)-CeO(2) composite high surface area materials, prepared using 5 nm colloidal CeO(2) nanoparticle building blocks. These materials are made of a homogeneous distribution of CeO(2) nanoparticles in thin layers of SiO(2), arranged in a hexagonal symmetry as shown by small-angle X-ray scattering and transmission electron microscopy. Since the preparation route of these composite materials was selected in order to produce SiO(2) wall thickness in the range of the CeO(2) nanoparticle diameter, these materials display surface nanorugosity as shown by inverse chromatography. Accessibility through the porous volume to the functional CeO(2) nanoparticle surfaces was evidenced through an organic acid chemisorption technique allowing quantitative determination of CeO(2) surface ratio. This surface composite nanostructure down to the nanometer scale does not affect the fundamental properties of the functional CeO(2) nanodomains, such as their oxygen storage capacity, but modifies the acid-base properties of the CeO(2) surface nanodomains as evidenced by Fourier transform IR technique. These arrays of accessible CeO(2) nanoparticles displaying high surface area and high thermal stability, along with the possibility of tuning their acid base properties, will exhibit potentialities for catalysis, sensors, etc.     

WO3/(TiO2-SiO2)催化剂的表面分散状态及催化性能的研究

用气相流动吸附法制备复合载体,用浸渍法制备WO3/(TiO2-S iO2)催化剂.应用LRS和TPR技术研究WO3在复合载体TIO2-S iO2表面的分散状态,发现TiO2在S iO2表面的分散可增强WO3与载体之间的相互作用,提高WO3在载体表面的分散阈值.另外TPR实验证明,TiO2的存在不仅大大改善WO3的分散状态,而且使WO3在TiO2-S iO2的还原温度升高,WO3与载体之间的作用增强.负载于经TiO2调变的S iO2上的催化剂其HDS、HYD、BHD活性高于负载于单纯S iO2上的催化剂的活性.     

Influence of long-term aqueous exposure on surface properties of plasma-sprayed oxides Cr2O3 and Cr2O3-25 wt% TiO2

The influence of water exposure on the surface properties of plasma-sprayed Cr(2)O(3) and Cr(2)O(3)-25 wt% TiO(2) was studied. It was shown that both plasma-sprayed materials contained Cr(VI) hydrous oxide phases, which dissolved rapidly at the beginning of water exposure. The dissolution continued slowly during the whole water exposure time. The Cr(VI) dissolution was accompanied by a rapid increase in surface IEP value. Both Cr(2)O(3) and Cr(2)O(3)-25 wt% TiO(2) showed similar dissolution, zeta potential, and surface oxidation states. Thus the addition of TiO(2) did not influence the surface properties of the plasma-sprayed Cr(2)O(3).     

吸附氢气后的钯团簇在氧化亚铜表面上的稳定性研究

为了探究吸附H2后的Pdn团簇在Cu2O（111）完整表面和铜缺陷表面上的稳定性,计算了负载在Cu2O（111）完整表面和铜缺陷表面上的Pdn（n=1-4）对H2分子的最稳定吸附结构;利用在给定H2压力和温度下Pdn / Cu2O表面吸附H2的相图揭示了Pdn团簇在Cu2O（111）两个表面的变化情况。结果表明,在吸附了H2分子以后,Pdn团簇更倾向于保持原有的结构,且随着Pd团簇的增大,吸附H2的数量也逐渐增长。     

A systematic computational study of the reactions of HO2 with RO2: the HO2 + CH2ClO2, CHCl2O2, and CCl3O2 reactions

Potential energy surfaces for the reactions of HO(2) with CH(2)ClO(2), CHCl(2)O(2), and CCl(3)O(2) have been calculated using coupled cluster theory and density functional theory (B3LYP). It is revealed that all the reactions take place on both singlet and triplet surfaces. Potential wells exist in the entrance channels for both surfaces. The reaction mechanism on the triplet surface is simple, including hydrogen abstraction and S(N)2-type displacement. The reaction mechanism on the singlet surface is more complicated. Interestingly, the corresponding transition states prefer to be 4-, 5-, or 7-member-ring structures. For the HO(2) + CH(2)ClO(2) reaction, there are two major product channels, viz., the formation of CH(2)ClOOH + O(2) via hydrogen abstraction on the triplet surface and the formation of CHClO + OH + HO(2) via a 5-member-ring transition state. Meanwhile, two O(3)-forming channels, namely, CH(2)O + HCl + O(3) and CH(2)ClOH + O(3) might be competitive at elevated temperatures. The HO(2) + CHCl(2)O(2) reaction has a mechanism similar to that of the HO(2) + CH(2)ClO(2) reaction. For the HO(2) + CCl(3)O(2) reaction, the formation of CCl(3)O(2)H + O(2) is the dominant channel. The Cl-substitution effect on the geometries, barriers, and heats of reaction is discussed. In addition, the unimolecular decomposition of the excited ROOH (e.g., CH(2)ClOOH, CHCl(2)OOH, and CCl(3)OOH) molecules has been investigated. The implication of the present mechanisms in atmospheric chemistry is discussed in comparison with the experimental measurements.     

凹凸棒黏土-SnO2-TiO2复合材料的制备及其光催化性能

以凹凸棒黏土为基体,通过原位溶胶-凝胶包覆工艺,将SnO2-TiO2复合氧化物负载于凹凸棒黏土表面制备凹凸棒黏土复合光催化剂(Att-SnO2-TiO2)。采用XRD、BET、TEM等分析测试技术对产物进行了表征。结果表明,SnO2-TiO2复合氧化物均匀地负载于凹凸棒黏土表面,其平均粒径约为10 nm;经过复合氧化物负载改性后,凹凸棒黏土的表面形貌得到明显改善,产物的包覆效果好、比表面大。以光催化降解甲基橙为探针,研究了凹凸棒黏土复合光催化剂的光催化性能。结果表明,凹凸棒黏土复合光催化剂显示出优异的光催化性能,其光催化活性顺序为Att-SnO2-TiO2>Att-SnO2>Att-TiO2。以性能最佳的Att-SnO2-TiO2为催化剂光降解甲基橙30min后,甲基橙的降解率达99%。实验表明,Att-SnO2-TiO2复合光催化剂可重复利用。     

Investigations of the structure of H2O clusters adsorbed on TiO2 surfaces by near-infrared absorption spectroscopy

The overtone and combination bands of the fundamental vibration modes (nu(1), symmetric stretching; nu(2), bending; and nu(3), asymmetric stretching) attributed to the H(2)O molecules adsorbed on a TiO(2) surface could be observed in the near-infrared (NIR) region. Especially, two absorption bands attributed to the combination (nu(2) + nu(3)) and (nu(1) + nu(3)) modes of the H(2)O molecules adsorbed on the TiO(2) surface were observed at around 1940 and 1450 nm, respectively. From detailed investigations on the (nu(2) + nu(3)) combination band, it was found that H(2)O molecules absorbed on a TiO(2) surface aggregate to form clusters due to the high surface tension of H(2)O arising from the intermolecular hydrogen bonds, and the hydrogen-bonded H(2)O in the bulk part of the cluster and the hydrogen-bond-free H(2)O in the outside spherical part of the cluster could be easily distinguished. Furthermore, it was quantitatively confirmed that the relaxation of the surface energy accompanying the adsorption of H(2)O on the TiO(2) surface stabilized the adsorption states of the hydrogen-bonded H(2)O molecules, while on the other hand, the hydrogen-bond-free H(2)O molecules became unstable as compared to the liquid-phase H(2)O molecules.     

Periodic density functional theory study of methane activation over La(2)O(3): activity of O(2-), O(-), O(2)(2-), oxygen point defect,and Sr(2+)-doped surface sites

Results of gradient-corrected periodic density functional theory calculations are reported for hydrogen abstraction from methane at O(s)(2-), O(s)(-), O(2)(s)(2-) point defect, and Sr(2+)-doped surface sites on La(2)O(3)(001). The results show that the anionic O(s)(-) species is the most active surface oxygen site. The overall reaction energy to activate methane at an O(s)(-) site to form a surface hydroxyl group and gas-phase (*)CH(3) radical is 8.2 kcal/mol, with an activation barrier of 10.1 kcal/mol. The binding energy of hydrogen at an site O(s)(-) is -102 kcal/mol. An oxygen site with similar activity can be generated by doping strontium into the oxide by a direct Sr(2+)/La(3+) exchange at the surface. The O(-)-like nature of the surface site is reflected in a calculated hydrogen binding energy of -109.7 kcal/mol. Calculations indicate that surface peroxide (O(2(s))(2-)) sites can be generated by adsorption of O(2) at surface oxygen vacancies, as well as by dissociative adsorption of O(2) across the closed-shell oxide surface of La(2)O(3)(001). The overall reaction energy and apparent activation barrier for the latter pathway are calculated to be only 12.1 and 33.0 kcal/mol, respectively. Irrespective of the route to peroxide formation, the O(2)(s)(2-) intermediate is characterized by a bent orientation with respect to the surface and an O-O bond length of 1.47 A; both attributes are consistent with structural features characteristic of classical peroxides. We found surface peroxide sites to be slightly less favorable for H-abstraction from methane than the O(s)(-) species, with DeltaE(rxn)(CH(4)) = 39.3 kcal/mol, E(act) = 47.3 kcal/mol, and DeltaE(ads)(H) = -71.5 kcal/mol. A possible mechanism for oxidative coupling of methane over La(2)O(3)(001) involving surface peroxides as the active oxygen source is suggested.     

Ni2(OCH3)2/SiO2催化剂上CO2和CH3 OH的吸附和反应性能

采用表面改性和离子交换相结合的方法制备了Ni2(OCH3)2/SiO2负载型双核金属甲氧基配合物催化剂,利用红外光谱(IR)、程序升温脱附(TPD)、程序升温表面反应(TPSR)和微反技术考察了催化剂的表面结构以及CO2和CH3OH的化学吸附和反应性能.结果表明:Ni2(OCH3)2/SiO2中Ni2+与载体SiO2表面O2-以双齿配位形式键合,甲氧基以桥基形式联结双金属离子形成双核物种Ni2(OCH3)2;CO2在催化剂表面存在甲氧碳酸酯基物种和桥式两种吸附态,CH3OH则只有一种分子吸附态;在100～200℃条件下,CO2和CH3OH在催化剂上的反应产物主要是DMC和H2O;根据反应结果,讨论了催化反应机理.     

超临界条件下CO2和丙烯直接合成甲基丙烯酸Ni2(OCH3)2/SiO2催化剂

超临界反应;负载型催化剂;双核桥联配合物;配合物;超临界条件下CO2和丙烯直接合成甲基丙烯酸Ni2(OCH3)2/SiO2催化剂     

Vibrational analysis of H2 and D2 adsorption on Pt/SiO2

Vibrational properties of surface species formed upon H2 and D2 exposure of silica supported platinum particles have been investigated with in situ diffuse reflection infrared Fourier transform spectroscopy. Experiments have been performed at 50-250 degrees C, using different platinum loading of the samples in the absence and presence of oxygen. In addition, electronic structure calculations and vibrational analysis have been performed within the density functional theory for H adsorption on a silica cluster, (HO)3SiOSi(OH)3. The spectroscopy experiments showed reversible formation of isolated OH and OD groups on the silica surface when the samples were exposed to H2 and D2, respectively. In addition to the absorption peak corresponding to isolated OH and OD groups, an intense broad band was observed around 3270 cm(-1) (2500 cm(-1)) during H2 (D2) exposure. Supported by the calculations, this band was assigned to perturbed OH groups on the silica surface. The surface coverage of new OH groups was found to correlate to the platinum loading in the samples, indicating that the new silanol groups were formed in the vicinity of the Pt particles. In the investigated temperature interval, the formation rate of OH groups was not found to be temperature dependent.     

Enhanced photocatalytic degradation of C.I. Basic Violet 2 using TiO2-SiO2 composite nanoparticles

In this report, TiO(2) -SiO(2) composite nanoparticles were prepared by the thermal hydrolysis method using titanium tetrachloride and tetraethylorthosilicate as TiO(2) and SiO(2) precursors, respectively. The prepared nanoparticles were characterized by X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), nitrogen adsorption/desorption and UV-Vis diffuse reflectance spectroscopy (DRS). The results indicated that, in comparison with pure TiO(2), TiO(2)-SiO(2) composite nanoparticles had a higher thermal stability, which prevents phase transformation from anatase to rutile. In addition, the TiO(2)-SiO(2) nanoparticles had a higher specific surface area, larger pore volume, greater band gap energy and smaller crystallite size. Thus, the surface area of TiO(2)-40% SiO(2) composite nanoparticles was about 17 times higher than that of pure TiO(2) nanoparticles. The photocatalytic activity of TiO(2)-SiO(2) composite nanoparticles in the photodegradation of C.I. Basic Violet 2 was investigated. The photodegradation rate of Basic Violet 2 using TiO(2)-40% SiO(2) nanoparticles calcined at 600°C was much faster than that using pure TiO(2) and Degussa P25 TiO(2) by 10.9 and 4.3 times, respectively. The higher photoactivity of the TiO(2)-SiO(2) composite nanoparticles was attributed to their higher surface area, larger pore volume, greater band-gap energy and smaller crystallite size compared with pure TiO(2).     

Reduction of the (001) surface of gamma-V2O5 compared to alpha-V2O5

The defect-free gamma-V(2)O(5)(001) surface and ordered structures of oxygen vacancies have been studied for a wide range of defect concentrations, Theta ((1)/(6) monolayer (ML) < or = Theta < or = 1 ML), combining density functional theory and statistical thermodynamics. The gamma polymorph of V(2)O(5) is characterized by two structurally different vanadium sites, V(A) and V(B). The V(A) sites having a weaker bond to an adjacent crystal layer are easier to reduce. Up to (1)/(2) ML, the V(A) defect structures with defects aligned along the [010] direction are increasingly more stable as in alpha-V(2)O(5)(001). At higher defect concentrations, the different coordination of the V(B) vanadium atoms at the gamma-V(2)O(5) surface causes an increase in the vacancy formation energy of approximately 0.8 eV/atom at Theta = 1.0 compared to Theta = (1)/(2). For alpha-V(2)O(5), this increase amounts to 0.2 eV/atom only. Under conditions (low oxygen partial pressures and high temperatures) at which the alpha-V(2)O(5)(001) surface would be fully reduced, the gamma-V(2)O(5)(001) surface is only partially reduced. The presence of surface vanadyl oxygen groups at V(B) sites may change the surface reactivity compared to that of alpha-V(2)O(5)(001).     

Competitive adsorption of NO, NO2, CO2, and H2O on BaO(100): a quantum chemical study

Density functional theory (DFT) quantum chemical calculations are used to determine adsorption energies and geometries of NO, NO(2), CO(2), and H(2)O on a barium oxide (100) surface. The study includes two adsorption geometries for NO(2). All species form thermodynamically stable adsorbates, and adsorption strength increases in the order NO(2) < H(2)O < NO 相似文献   

A systematic computational study of the reactions of HO2 with RO2: the HO2 + C2H5O2 reaction

The reaction of HO2 with C2H5O2 has been studied using the density functional theory (B3LYP) and the coupled-cluster theory [CCSD(T)]. The reaction proceeds on the triplet potential energy surface via hydrogen abstraction to form ethyl hydroperoxide and oxygen. On the singlet potential energy surface, the addition-elimination mechanism is revealed. Variational transition state theory is used to calculate the temperature-dependent rate constants in the range 200-1000 K. At low temperatures (e.g., below 300 K), the reaction takes place predominantly on the triplet surface. The calculated low-temperature rate constants are in good agreement with the experimental data. As the temperature increases, the singlet reaction mechanism plays more and more important role, with the formation of OH radical predominantly. The isotope effect of the reaction (DO2 + C2D5O2 vs HO2 + C2H5O2) is negligible. In addition, the triplet abstraction energetic routes for the reactions of HO2 with 11 alkylperoxy radicals (CnHmO2) are studied. It is shown that the room-temperature rate constants have good linear correlation with the activation energies for the hydrogen abstraction.