Single-molecule nanocatalysis reveals heterogeneous reaction pathways and catalytic dynamics

Nanoparticles are important catalysts for many chemical transformations. However, owing to their structural dispersions, heterogeneous distribution of surface sites and surface restructuring dynamics, nanoparticles are intrinsically heterogeneous and challenging to characterize in ensemble measurements. Using a single-nanoparticle single-turnover approach, we study the redox catalysis of individual colloidal Au nanoparticles in solution, using single-molecule detection of fluorogenic reactions. We find that for product generation, all Au nanoparticles follow a Langmuir-Hinshelwood mechanism but with heterogeneous reactivity; and for product dissociation, three nanoparticle subpopulations are present that show heterogeneous reactivity between multiple dissociation pathways with distinct kinetics. Correlation analyses of single-turnover waiting times further reveal activity fluctuations of individual Au nanoparticles, attributable to both catalysis-induced and spontaneous dynamic surface restructuring that occurs at different timescales at the surface catalytic and product docking sites. The results exemplify the power of the single-molecule approach in revealing the interplay of catalysis, heterogeneous reactivity and surface structural dynamics in nanocatalysis.     

Synthesis and characterization of heterogeneous ZnO/CuO hierarchical nanostructures for photocatalytic degradation of organic pollutant

In the current study, ZnO, CuO and ZnO/CuO mixed metal oxide nano-composites with different molar ratio of Zn/Cu (10:1, 8:1, 6:1, 4:1, 2:1) were prepared via low temperature hydrothermal synthesis technique. The consequences of different synthesis conditions such as molar ratio, pH and processing temperature on physicochemical properties of ZnO/CuO nano-composites were also studied. The surface morphology, elemental composition, crystal structure, chemical states and optical characteristics of the prepared nano-composite materials were determined using various techniques such as field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX), X-ray diffraction analysis (XRD), X-ray photo-electron spectroscopy (XPS) and UV–visible diffused reflectance spectra (UVDRS). A morphological change in bitter gourd structured ZnO as well as improved optical response of ZnO after incorporation of CuO was observed. The decreased recombination rate of charge carriers, effectual generation of photoinduced charge carriers, formation of hetero-junction system and unique morphology are the responsible factors for improved photodegradation characteristics of prepared ZnO/CuO nano-composites. Role of active species and pH of dye solution in degradation process was also studied.     

ZnO:Cd纳米棒的制备及其光催化降解性能研究

采用水热法制备了ZnO和不同Cd掺杂浓度的ZnO:Cd纳米棒。通过x射线衍射仪、扫描电子显微镜、紫外-可见-近红外分光光度计和拉曼光谱对ZnO:Cd纳米棒的结构和光学特性进行了系统研究。结果显示,样品为一维纳米棒结构,Cd的掺杂可以减小ZnO纳米棒的晶粒尺寸和光学带隙。利用分光光度计检测ZnO:Cd纳米棒对偶氮结构染料(甲基橙溶液)的光催化降解效率,结果表明Cd掺杂可以改善ZnO的光催化性能,掺杂浓度为16%时ZnO:Cd纳米棒对甲基橙溶液的光催化降解效率最高。     

室温固相法制备纳米氧化锌及光催化降解活性黑5动力学

以Na2C2O4和ZnSO4.7H2O为原料,利用室温固相化学反应法合成纳米氧化锌,并研究该氧化锌光催化降解活性黑5。结果表明,平均粒径约为30nm左右,UV/ZnO催化过程符合拟一级动力学,催化剂表面吸附平衡常数k0和吸附平衡常数KZnO分别是0.0163min-1和9.42L/g,最佳ZnO投加量为0.3g/L。UV/ZnO/H2O2和UV/ZnO/Fenton催化氧化过程不符合经典的拟一级动力学,建立一种双曲线动力学模型能够较好地描述该过程。     

Photocatalytic degradation of para-hydroxybenzoic acid: Relationship between substrate adsorption and photocatalytic degradation

 Dark adsorption and photocatalytic degradation of para-hydroxybenzoic acid (PHBA) was studied in the presence of Degussa P-25 titanium dioxide photocatalyst under concentrated solar radiation. This work includes the study of the effect of pH, and of the presence of different anions and cations on dark adsorption and photocatalytic degradation (PCD) of PHBA on titanium dioxide. The results obtained clearly indicate that there is a link between the extent of adsorption of PHBA and its PCD. Conditions that favor greater adsorption of PHBA also favor a greater PCD. This points to the possibility that the mechanism of PCD involves a surface reaction between adsorbed PHBA and OH ° radical. Anions were generally found to have a detrimental effect on the photocatalytic degradation of PHBA. Cl^– caused the greatest decrease in the PCD followed by NO₃ ^–, HCO₃ ^– and SO₄ ^2–. Cations Cu²⁺ and Fe²⁺ were not found to assist the degradation of PHBA, possibly due to the anions associated with them. Received: 15 May 2000 / Accepted: 15 July 2000     

Removal and degradation pathway study of sulfasalazine with Fenton-like reaction

The Fenton-like degradation of sulfasalazine solution is studied in this work. The effects of reaction parameters such as Fe(3+) concentration, initial H(2)O(2) dosage and the reaction temperature are evaluated. For sulfasalazine of 100mg/L, the removal of sulfasalazine, chemical oxygen demand (COD) and total organic carbon (TOC) reached 99.5%, 84.2% and 41% in 60 min with 0.20mM Fe(3+) and 16 mM H(2)O(2) at 35°C, respectively. The complexed Fe(3+) presents a reaction constant of 0.062 min(-1)mM(-1) while that of free Fe(3+) is 2.526 min(-1)mM(-1) for sulfasalazine degradation. LC-MS technology was used to analysis the possible degradation intermediates. The degradation of sulfasalazine principally begins with the attack of hydroxyl radical on the azo-group as well as the sulfanilamido group. Both intramolecular rearrangement and bimolecular reaction occur simultaneously after the hydroxyl radical attack. Further attack of the active oxidative species results in the cleavage of the aromatic rings and the production of CO(2). The degradation of industrial sulfasalazine wastewater with a COD of 3425 mg/L has also been achieved by Fenton reaction with different dosage of H(2)O(2). Relatively better removal efficiency is observed at moderate Fe/H(2)O(2) molar ratio from 1/5 to 2/5 for industrial sulfasalazine wastewater treatment.     

Cd/CdS光催化降解甲基橙的研究

采用胶体化学法制备表面富镉的Cd／CdS纳米粒子为催化剂对水溶液中甲基橙的光催化降解进行了研究。探讨了光催化反应机理,讨论了光催化剂用量,双氧水的用量,试液的pH值,光照时间与甲基橙脱色率的关系。实验结果表明,当甲基橙起始浓度为20mg／L,Cd／CdS用量为0．500g,双氧水用量为5．88mmol／L,pH值为7．0时,光照6h,甲基橙的脱色率可达到95．5％。     

Strong enhancement on dye photocatalytic degradation by ball-milled TiO_2: A study of cationic and anionic dyes

TiO_2 particles with desirable properties were produced by undergoing specific durations of ball milling.Characterizations of the TiO_2 particles before and after ball milling were investigated via X-ray diffraction(XRD), Brunauer-Emmett-Teller(BET), particle size analysis, zeta potential, and scanning electron microscope(SEM). The equilibrium adsorption data were well fitted to Langmuir, Freundlich, and Dubinin-Radushkevich(D-R) isotherms. Compared to the as-received TiO_2(mean particle size d150= 0.78μm, specific surface area = 88.17 m~2g~(-1), pore volume = 0.41 cm~3g~(-1)), the 60 min ballmilled TiO_2(d50= 0.55 μm, specific surface area = 99.48 m2g~(-1), pore volume = 0.48 cm3g~(-1)) enhanced the adsorption quantity of congo red and methylene blue from 10.4 mg g~(-1)to 13.6 mg g~(-1), and from17.0 mg g~(-1)to 22.2 mg g~(-1), respectively; and also improved the kinetic rates from k = 0.1325 to 0.2193, and k = 0.0944 to 0.1553, respectively. Dye adsorption and degradation efficiency of congo red was enhanced in acidic p H range(2–5.14), and methylene blue was enhanced in alkaline p H range(7.58–12).     

Heterogeneous photocatalytic degradation of monochlorobenzene in water

This investigation evaluated the photocatalytic degradation of monochlorobenzene (MCB) in an aqueous TiO(2) suspension. In accordance with the experimental results, the degradation of MCB was a function of the initial substrate concentration, incident light intensity, and TiO(2) dosage. However, the solution pH had insignificant effect on the degradation efficiency. The heterogeneous photocatalytic degradation of MCB followed the Langmuir-Hinshelwood kinetics. The adsorption coefficient of MCB (K) and the observed degradation rate constant (k) were calculated as 13.4 mM(-1) and 0.0054 mM min(-1), respectively. In addition, a 0.255 dependency of the initial degradation rate on the light intensity revealed the considerable adverse effect of e(-)-h(+) pair recombination. Both mineralization and dechlorination occurred during the photocatalytic degradation of MCB. Under the operating condition of initial MCB concentration of 0.1mM, light intensity of 5.68 microEinsteins(-1), TiO(2) dosage of 1.0 g L(-1), and solution pH of 7, about 93.7% of MCB was mineralized after 240 min of irradiation. Nevertheless, 64.3% of the stoichiometric amount of Cl(-) ions was released into the bulk solution. The simulation results derived from the X-ray photoelectron spectroscopy (XPS) analysis was suggested that the interaction between Cl(-) ions and TiO(2) surface tended to lower the released amount of Cl(-) ions.     

Photocatalytic degradation of triazophos in aqueous titanium dioxide suspension: identification of intermediates and degradation pathways

The photocatalytic degradation of triazophos in aqueous TiO2 suspension has been studied in a photoreactor operating with simulated solar radiation. The decrease in triazophos concentration followed first-order kinetics with a half-life of 4.76+/-0.42 h at a TiO2 suspension concentration of 10 mg/L. Seventeen degradation products were identified using HPLC-UV, HPLC/MS/MS, GC/MS/MS and IC, and by comparing retention times and spectra with commercially available authentic standards. On the basis of the observed transformation products, two routes were proposed, one based on the initial oxidative cleavage of PS bond to PO bond, and the other on initial cleavage of the ester P-O bonds. Photocatalysis holds promise for the solar treatment of pesticide-contaminated waters.     

纳米TiO2光催化剂的制备及在降解有机物方面的研究进展

纳米二氧化钛及其复合催化剂是催化科学中的热点之一。本文综述了近期用溶胶－凝胶法制备纳米二氧化钛光催化剂及其在光催化降解有机污染物方面的研究进展，并对一些有待于进一步研究的问题作了展望。     

Solar assisted photocatalytic and photochemical degradation of Reactive Black 5

The photocatalytic oxidative degradation of Reactive Black 5 (RB 5) has been investigated using TiO(2)-P25 as photocatalyst and sunlight as irradiation source in slurry form. The degradation was carried out at different experimental conditions to optimize the parameters such as amount of catalyst, concentration of dye and pH. A complete degradation of 3.85 x 10(-4) M dye solution under solar irradiation was observed in 3.5 h. The photochemical degradation using hydrogen peroxide results in the partial removal of the dye.     

光催化降解甲醛功能泡沫炭的制备与表征

以聚氨酯为骨架,在泡沫成型过程中加入颗粒活性炭,经炭化得到孔结构发达的聚氨酯泡沫炭,以其为基底采用浸渍法制备出TiO_2负载的具有光催化性能的复合材料。采用SEM观测不同活性炭量泡沫炭的表面形貌;采用BET吸附-脱附等温曲线考察制备不同的泡沫炭及复合材料的孔径结构;XRD分析不同产物的晶型结构;TG分析碳骨架热分解过程。以气相甲醛为模型物,评价泡沫炭/TiO_2复合材料在紫外灯光下对甲醛气体的光催化降解性能。结果表明,聚氨酯泡沫炭/TiO_2复合材料具有良好的催化降解甲醛功能,是吸附与降解协同作用的结果;当活性炭含量为35%,TiO_2的负载量为2%时,对甲醛吸附降解能力最好,达到85.3%。     

Ag/TiO2纳米粒子的制备及其光催化降解苯酚的性能研究

采用溶胶-凝胶法制备了TiO₂和Ag/TiO₂纳米粒子,采用涂覆法制备了TiO₂和Ag/TiO₂纳米粒子光催化剂基板样品。使用XRD、SEM和拉曼光谱等手段,对TiO₂和Ag/TiO₂纳米粒子进行了晶格结构和表面形貌研究;通过UV-Vis,研究了TiO₂和Ag/TiO₂纳米粒子光催化剂基板样品在光催化反应器中对苯酚的光催化降解性能。结果表明,制备的TiO₂和Ag/TiO₂纳米粒子均为纯净的金红石相,二者表面形貌并没有明显区别,Ag单质粒子成功负载在TiO₂纳米材料上;Ag单质粒子的负载,明显增强了TiO₂纳米粒子对可见光的吸收,且Ag/TiO₂纳米粒子薄膜对苯酚的光催化降解性能明显优于TiO₂纳米粒子薄膜;在光催化降解1 h后,TiO₂纳米粒子薄膜仅催化降解了溶液中30%(质量分数)的苯酚,且光催化降解出现了饱和趋势,而Ag/TiO₂纳米粒子薄膜可催化降解溶液中50%(质量分数)的苯酚,且在光催化降解3 h后,仍未出现饱和趋势。     

Effectiveness and pathways of electrochemical degradation of pretilachlor herbicides

Pretilachlor used as one kind of acetanilide herbicides is potentially dangerous and biorefractory. In this work, electrochemical degradation of lab-synthetic pretilachlor wastewater was carried out with Sb doped Ti/SnO(2) electrode as anode and stainless steel as cathode. The effect of current density on pretilachlor degradation was investigated, and the degradation pathway of pretilachlor was inferred by analyzing its main degradation intermediates. The results showed that the removal of pretilachlor and TOC in treatment time of 60 min were 98.8% and 43.1% under the conditions of current density of 20 mA cm(-2), initial concentration of pretilachlor of 60 mg L(-1), Na(2)SO(4) dosage of 0.1 mol L(-1), pH of 7.2, respectively, while the energy consumption was 15.8 kWhm(-3). The main reactions for electrochemical degradation of pretilachlor included hydroxylation, oxidation, dechlorination, C-O bond and C-N bond cleavage, resulting in the formation of nine main intermediates.     

Study on photocatalytic degradation of several volatile organic compounds

The gas-phase photolytic and photocatalytic reactions of several aromatics and chlorohydrocarbons were investigated. The experimental results revealed that chlorohydrocarbons like trichloroethylene, dichloromethane and chloroform could be degraded through either photolysis or photocatalysis under irradiation of germicidal lamp, and the elimination rate of chlorohydrocarbons through photolysis was quicker than that through photocatalysis. UV light from a germicidal lamp could directly lead to degradation of toluene but could hardly act on benzene. The photodegradation rate for these volatile organic compounds (VOCs) through photolysis followed an order: trichloroethylene>chloroform>dichloromethane>toluene>benzene>carbon tetrachloride, and through photocatalysis followed: trichloroethylene>chloroform>toluene>dichloromethane>benzene>carbon tetrachloride. Besides, a series of modified TiO2 photocatalysts were prepared by depositing noble metal, doping with transition metal ion, recombining with metal oxides and modifying with super strong acid. Activity of these catalysts was examined upon photocatalytic degradation of benzene as a typical compound that was hard to be degraded. It indicated that these modification methods could promote the activity of TiO2 catalyst to different extent. The apparent zero-order reaction rate constant for degrading benzene over SnO2/TiO2 catalyst had the highest value, which was nearly three times as that over P25 TiO2. But it simultaneously had the lowest rate for mineralizing the objective compound. In spite that Fe3+/TiO2 catalyst behaved slightly less active than SnO2/TiO2 for degradation of benzene, the mineralization rate over Fe3+/TiO2 was the highest one among the prepared catalysts.     

Non-thermal plasma-induced photocatalytic degradation of 4-chlorophenol in water

TiO(2) photocatalyst (P-25) (50mgL(-1)) was tentatively introduced into pulsed high-voltage discharge process for non-thermal plasma-induced photocatalytic degradation of the representative mode organic pollutant parachlorophenol (4-CP), including other compounds phenol and methyl red in water. The experimental results showed that rate constant of 4-CP degradation, energy efficiency for 4-CP removal and TOC removal with TiO(2) were obviously increased. Pulsed high-voltage discharge process with TiO(2) had a promoted effect for the degradation of these pollutants under a broad range of liquid conductivity. Furthermore, the apparent formation rates of chemically active species (e.g., ozone and hydrogen peroxide) were increased, the hydrogen peroxide formation rate from 1.10x10(-6) to 1.50x10(-6)Ms(-1), the ozone formation rate from 1.99x10(-8) to 2.35x10(-8)Ms(-1), respectively. In addition, this process had no influence on the photocatalytic properties of TiO(2). The introduction of TiO(2) photocatalyst into pulsed discharge plasma process in the utilizing of ultraviolet radiation and electric field in pulsed discharge plasma process enhanced the yields of chemically active species, which were available for highly efficient removal and mineralization of organic pollutants.