Thermal stability evolution of carbon nanotubes caused by liquid oxidation

Carbon nanotubes (CNTs) grown by chemical vapor decomposition of ethylene on alumina- and silica-supported Fe–Co bimetallic catalysts were examined before and after purification encompassing chemical oxidation treatment in 3 M NaOH and 3 M HNO₃ solutions, sequentially. Thermal properties were investigated and correlated with structural changes followed by TEM, X-ray diffraction and Raman spectroscopy characterization. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) were employed simultaneously in the regime of TPO and TPH, in diluted flow of either O₂ or H₂. TG revealed almost complete burning of both refined CNT samples in diluted O₂, indicating the efficiency of the purification method used to remove the catalyst remains. However, different trends and significant magnitudes of changes in the heat of combustion demonstrate changes in CNTs stability after purification as a function of the catalyst support type. This is the consequence of changes in carbon type, CNTs order degree and morphology, as well as the degree of functionalization, which have their own effects on the CNTs thermal stability.     

Effect of cobalt weight content on the structure and catalytic properties of Co/CNT catalysts in the fischer–tropsch synthesis

Cobalt-based Fischer–Tropsch synthesis (FTS) catalysts containing 1 to 40 wt % cobalt supported on multi-walled carbon nanotubes (CNTs) have been investigated. The CNTs have been characterized by low-temperature nitrogen adsorption, scanning electron microscopy, and X-ray photoelectron spectroscopy. All catalysts have been prepared by impregnating, with an ethanolic solution of cobalt nitrate, the CNTs preoxidized with concentrated nitric acid and have been tested in the FTS at 220°C and atmospheric pressure. Correlations have been established between the cobalt weight content of the catalyst and the Co particle size determined by transmission electron microscopy and X-ray diffraction. The Co content and particle size have an effect on the activity and selectivity of the catalyst and on the target fraction (C₅₊) yield in the FTS. The highest CO conversion is observed for the catalyst containing 20 wt % Co; the highest selectivity and activity, for the catalyst containing 5 wt % Co; the highest C₅₊ yield, for the catalyst containing 10 wt % Co.     

处理方法对多壁碳纳米管结构及负载Ru催化剂氨分解反应活性的影响

分别用H_2O_2、强碱(NaOH、 KOH)和强酸HNO_3处理CNTs.以处理后的CNTs为载体、通过浸渍RuCl_3水溶液结合高温H_2还原制备Ru/CNTs催化剂,并将其应用在氨分解催化反应中.利用XRD、 TPR、 TPD-MS表征手段研究了Ru在CNTs表面的分散、还原性能及CNTs表面化学基团,探究催化剂结构-性能间构效关系.结果表明,强碱及双氧水处理CNTs,为其表面引入了数量适宜的羧基、酸酐、酚等官能团,而传统硝酸处理则引入了大量的羧基、酸酐、酯、内酯、酚、醌和羰基等官能团,对CNTs本征结构性质影响很大.经强碱及双氧水处理CNTs上负载Ru后所得催化剂的效果明显优于传统硝酸处理CNTs上负载Ru催化剂.我们发展的CNTs的新型处理方法为研制高活性Ru/CNTs催化分解氨催化剂提供了新的思路.     

Surface site density and utilization of platinum group metal (PGM)-free Fe–NC and FeNi–NC electrocatalysts for the oxygen reduction reaction

Pyrolyzed iron-based platinum group metal (PGM)-free nitrogen-doped single site carbon catalysts (Fe–NC) are possible alternatives to platinum-based carbon catalysts for the oxygen reduction reaction (ORR). Bimetallic PGM-free M₁M₂–NC catalysts and their active sites, however, have been poorly studied to date. The present study explores the active accessible sites of mono- and bimetallic Fe–NC and FeNi–NC catalysts. Combining CO cryo chemisorption, X-ray absorption and ⁵⁷Fe Mössbauer spectroscopy, we evaluate the number and chemical state of metal sites at the surface of the catalysts along with an estimate of their dispersion and utilization. Fe L_3,2-edge X-ray adsorption spectra, Mössbauer spectra and CO desorption all suggested an essentially identical nature of Fe sites in both monometallic Fe–NC and bimetallic FeNi–NC; however, Ni blocks the formation of active sites during the pyrolysis and thus causes a sharp reduction in the accessible metal site density, while with only a minor direct participation as a catalytic site in the final catalyst. We also use the site density utilization factor, ϕ_{SDsurface/bulk}, as a measure of the metal site dispersion in PGM-free ORR catalysts. ϕ_{SDsurface/bulk} enables a quantitative evaluation and comparison of distinct catalyst synthesis routes in terms of their ratio of accessible metal sites. It gives guidance for further optimization of the accessible site density of M–NC catalysts.

The gravimetric surface density and ORR catalytic turnover frequency of Fe–NC and Fe/Ni–NC catalysts were investigated. Both catalysts feature chemically identical Fe sites, but the presence of Ni lowered the gravimetric surface density of Fe sites.     

Hydrodechlorination of 4-Chlorophenol on Pd-Fe Catalysts on Mesoporous ZrO2SiO2 Support

A mesoporous support based on silica and zirconia (ZS) was used to prepare monometallic 1 wt% Pd/ZS, 10 wt% Fe/ZS, and bimetallic FePd/ZS catalysts. The catalysts were characterized by TPR-H₂, XRD, SEM-EDS, TEM, AAS, and DRIFT spectroscopy of adsorbed CO after H₂ reduction in situ and tested in hydrodechlorination of environmental pollutant 4-chlorophelol in aqueous solution at 30 °C. The bimetallic catalyst demonstrated an excellent activity, selectivity to phenol and stability in 10 consecutive runs. FePd/ZS has exceptional reducibility due to the high dispersion of palladium and strong interaction between FeOx and palladium, confirmed by TPR-H₂, DRIFT spectroscopy, XRD, and TEM. Its reduction occurs during short-time treatment with hydrogen in an aqueous solution at RT. The Pd/ZS was more resistant to reduction but can be activated by aqueous phenol solution and H₂. The study by DRIFT spectroscopy of CO adsorbed on Pd/ZS reduced in harsh (H₂, 330 °C), medium (H₂, 200 °C) and mild conditions (H₂ + aqueous solution of phenol) helped to identify the reasons of the reducing action of phenol solution. It was found that phenol provided fast transformation of Pd⁺ to Pd⁰. Pd/ZS also can serve as an active and stable catalyst for 4-PhCl transformation to phenol after proper reduction.     

Selective hydrogenation of o-chloronitrobenzene (o-CNB) over supported Pt and Pd catalysts obtained by laser vaporization deposition of bulk metals

Carbon nanotubes (CNTs), γ-alumina (γ-Al₂O₃) and silica (SiO₂) supported Pt and Pd catalysts were produced by laser vaporization deposition of respective bulk metals. The catalysts were characterized by inductive coupled plasma emission spectrometer (ICP), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The catalytic properties of the catalysts were investigated in the liquid phase hydrogenation of o-chloronitrobenzene (o-CNB) to o-chloroaniline (o-CAN) under 333 K and 1.0 MPa hydrogen pressure. The results show that the catalytic properties are greatly affected by the supports. Pt/CNTs catalyst exhibits the best catalytic performance among the Pt-based catalysts, producing o-CAN with 99.6% selectivity at complete conversion. Pd/CNTs catalyst exhibits the best catalytic performance among the Pd-based catalysts, giving o-CAN with 95.2% selectivity at complete conversion. For Pt-based catalysts, geometric effect and the textures and properties of the supports play important roles on catalytic properties. On the other hand, geometric effect, electronic effect and the textures and properties of the supports simultaneously influence the catalytic properties of the Pd-based catalysts. In addition, hydrogenolysis of the C–Cl bond can be well inhibited over all catalysts prepared by laser vaporization deposition.     

A Reusable CNT-Supported Single-Atom Iron Catalyst for the Highly Efficient Synthesis of C−N Bonds

C−N bond formation is regarded as a very useful and fundamental reaction for the synthesis of nitrogen-containing molecules in both organic and pharmaceutical chemistry. Noble-metal and homogeneous catalysts have frequently been used for C−N bond formation, however, these catalysts have a number of disadvantages, such as high cost, toxicity, and low atom economy. In this work, a low-toxic and cheap iron complex (iron ethylene-1,2-diamine) has been loaded onto carbon nanotubes (CNTs) to prepare a heterogeneous single-atom catalyst (SAC) named Fe-N_x/CNTs. We employed this SAC in the synthesis of C−N bonds for the first time. It was found that Fe-N_x/CNTs is an efficient catalyst for the synthesis of C−N bonds starting from aromatic amines and ketones. Its catalytic performance was excellent, giving yields of up to 96 %, six-fold higher than the yields obtained with noble-metal catalysts, such as AuCl₃/CNTs and RhCl₃/CNTs. The catalyst showed efficacy in the reactions of thirteen aromatic amine substrates, without the need for additives, and seventeen enaminones were obtained. High-angle annular dark-field scanning transmission electron microscopy in combination with X-ray absorption spectroscopy revealed that the iron species were well dispersed in the Fe-N_x/CNTs catalyst as single atoms and that Fe-N_x might be the catalytic active species. This Fe-N_x/CNTs catalyst has potential industrial applications as it could be cycled seven times without any significant loss of activity.     

Enhancement of Field Emission Characteristics for Multi-Walled Carbon Nanotubes Treated with a Mixed Solution of Chromic Trioxide and Nitric Acid

A simple acid treatment method was applied to functionalize the surface and to modify the structures of multi-walled carbon nanotubes (CNTs) grown on silicon substrates using a mixed solution of chromic trioxide (CrO₃) and nitric acid (HNO₃). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and energy dispersive spectrometer (EDS) were employed to investigate the mechanism causing the modified field emission (FE) properties of the CNT films. After 20 min of CrO₃+HNO₃ treatment, the emitted currents were enhanced by more than one order of magnitude compared with those of the untreated CNTs. This large increase in emitted current can be attributed to the favorable surface morphologies, open-ended structures, and highly curved CNT surfaces in the CNT films. These factors altogether caused an increase in the field enhancement factors of CNTs. We also demonstrated that using a mixed solution of CrO₃+HNO₃ post-treatment exhibited a higher emission current and a lower turn-on electric field than in the CNTs treated with HNO₃. The method provides a simple, economical, and effective way to enhance the CNT field emission properties.     

Catalytic conversion of cellulose into polyols using carbon-nanotube-supported monometallic Pd and bimetallic Pd–Fe catalysts

A series of carbon nanotube (CNT)-supported monometallic Pd and bimetallic Pd–Fe catalysts were synthesized and employed for catalytic hydrogenolysis of cellulose into polyols, including hexitol, ethylene glycol (EG), 1,2-propanediol (1,2-PG), and glycerol (Gly). The physicochemical properties of the catalysts were characterized by nitrogen physical adsorption measurements, X-ray diffraction analysis, transmission electron microscopy, and X-ray photoelectron spectroscopy. The total yield of hexitol, EG, 1,2-PG, and Gly in hydrolytic hydrogenation of cellulose was 37, 55, and 53% for Pd/CNTs, Pd–Fe/CNTs (Pd:Fe = 1:1), and Pd–Fe/CNTs (Pd:Fe = 1:2), respectively. Addition of Fe to Pd significantly modified the physicochemical properties of the nanoparticles and their catalytic performance, especially regarding hexitol selectivity. The promoting effect of Fe, especially for hexitol selectivity, compared with the monometallic catalyst is due to the fact that incorporation of Fe may stabilize Pd⁰ nanoparticles and lead to downshift of the d-band center of Pd metal nanoparticles by charge transfer from Fe to Pd. Recycling experimental results showed that leaching of Fe resulted in a significant decrease in the hexitol yield obtained using the Pd–Fe/CNTs after the first recycle, further demonstrating that Fe element plays a promoting role for hexitol formation.     

Photocatalytic Activity of Fe and Ce Co‐doped Mesoporous TiO2 Catalyst under UV and Visible Light

The catalysts of un‐doped, single‐doped and co‐doped mesoporous titanium dioxide (MTiO₂) were prepared by a template method with tetrabutyltitanate (Ti(OC₄H₉)₄) as a Ti source material and Pluronic P123 as a template. The photo‐absorbance of the obtained catalysts was measured by UV‐vis absorption spectroscopy, and the photocatalytic activities of the prepared samples under UV and visible light were estimated by measuring the degradation rate of methyl orange (MO) (50 mg/L) in an aqueous solution. It was shown that the co‐doped MTiO₂ could be activated by visible light and could thus be used as an effective catalyst in photo‐oxidation reactions. The effect of Fe and Ce co‐dopants on the material properties was investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and N₂ adsorption‐desorption isotherm measurement. The characterizations indicated that the photocatalysts possessed a homogeneous pore diameter of ca. 10 nm with high surface area of ca. 150 m²/g. The photocatalytic activity of MTiO₂ co‐doped with Fe and Ce was markedly improved due to the synergistic actions of the two dopants.     

Prussian Blue/Multiwalled Carbon Nanotube Hybrids: Synthesis,Assembly and Electrochemical Behavior

Prussian blue/carbon nanotube (PB/CNT) hybrids with excellent dispersibility in aqueous solutions were synthesized by adding CNTs to an acidic solution of Fe³⁺, [Fe(CN)₆]^3? and KCl. Fourier transform infrared spectroscopy, UV‐vis absorption spectroscopy and scanning electron microscopy were employed to confirm the formation of PB/CNT hybrids. The PB nanoparticles formed on the CNT surfaces exhibit a narrow size distribution and an average size of 40 nm. The present results demonstrate that the selective reduction of Fe³⁺ to Fe²⁺ by CNTs is the key step for PB/CNT hybrid formation. The subsequent fabrication of the PB/CNT hybrid films was achieved by layer‐by‐layer technique. The thus‐prepared PB/CNT hybrid films exhibit electrocatalytic activity towards H₂O₂ reduction.     

Pt deposited TiO2 catalyst fabricated by thermal decomposition of titanium complex for solar hydrogen production

C, N codoped TiO₂ catalyst has been synthesized by thermal decomposition of a novel water-soluble titanium complex. The structure, morphology, and optical properties of the synthesized TiO₂ catalyst were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity of the Pt deposited TiO₂ catalysts synthesized at different temperatures was evaluated by means of hydrogen evolution reaction under both UV–vis and visible light irradiation. The investigation results reveal that the photocatalytic H₂ evolution rate strongly depended on the crystalline grain size as well as specific surface area of the synthesized catalyst. Our studies successfully demonstrate a simple method for the synthesis of visible-light responsive Pt deposited TiO₂ catalyst for solar hydrogen production.     

Cu2O/CNTs复合材料的制备及其光催化性能

以乙二醇为还原剂,采用溶剂热法在混酸(V_(H_2SO_4)/V_(HNO_3)=3∶1)超声处理的碳纳米管(CNTs)表面负载氧化亚铜(Cu_2O),通过改变CNTs的含量制备出球形Cu_2O/CNTs复合材料。采用X射线衍射(XRD)、傅里叶变换红外光谱(FTIR)、扫描电镜(SEM)、能谱分析(EDS)、N_2吸附-脱附、紫外-可见光漫反射(DRS)、有机总碳量(TOC)等对Cu_2O/CNTs进行表征;研究CNTs含量对Cu_2O/CNTs复合材料的结构、形貌、比表面积与孔径、光吸收特性的影响;结合光催化机理讨论CNTs对Cu_2O/CNTs光催化性能的影响。结果表明,当CNTs含量为0.2 g时,Cu_2O/CNTs的光催化性能最佳,在可见光照射60 min后,对甲基橙的降解率达到92.1%。     

Fe/Zn double metal cyanide catalyzed ring-opening polymerization of propylene oxide: 2. Characterization of active structure of double metal cyanide catalysts

Double metal cyanide (DMC) complexes based on Zn₃[Fe(CN)₆]₂ were synthesized using different molar ratios of ZnCl₂ to K₃[Fe(CN)₆] and special complexing agents. IR spectroscopy, electron spectroscopy for chemical analysis, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and other analytical techniques were employed to characterize these catalysts. The morphology and structure of these DMC catalysts were attributed to the different complexing agents as well as to the different molar ratios of ZnCl₂ to K₃[Fe(CN)₆]. In addition, the catalytic activity was strongly correlated with the morphology and noncrystalline content of DMC catalysts. High-activity catalysts could be prepared by controlling the structure of DMC catalysts by incorporating complexing agents. The active species of DMC catalysts for ring-opening polymerization are Zn²⁺, [Fe(CN)₆]^3–, Cl^–, and the compound of their ligands.     

Bifunctional Electrocatalysts for Overall Water Splitting from an Iron/Nickel‐Based Bimetallic Metal–Organic Framework/Dicyandiamide Composite

Pyrolysis of a bimetallic metal–organic framework (MIL‐88‐Fe/Ni)‐dicyandiamide composite yield a Fe and Ni containing carbonaceous material, which is an efficient bifunctional electrocatalyst for overall water splitting. FeNi₃ and NiFe₂O₄ are found as metallic and metal oxide compounds closely embedded in an N‐doped carbon–carbon nanotube matrix. This hybrid catalyst (Fe‐Ni@NC‐CNTs) significantly promotes the charge transfer efficiency and restrains the corrosion of the metallic catalysts, which is shown in a high OER and HER activity with an overpotential of 274 and 202 mV, respectively at 10 mA cm^?2 in alkaline solution. When this bifunctional catalyst was further used for H₂ and O₂ production in an electrochemical water‐splitting unit, it can operate in ambient conditions with a competitive gas production rate of 1.15 and 0.57 μL s^?1 for hydrogen and oxygen, respectively, showing its potential for practical applications.     

AAO模板法生长碳纳米管阵列及形成机理研究

采用阳极刻蚀法制备得到多孔氧化铝模板(AAO)，通过在二茂铁苯溶液中浸润而后热解的方法，得到内壁附着纳米铁颗粒的AAO模板。用化学气相沉积(CVD)法在AAO模板孔内生长出两端开口的碳纳米管(CNTs)阵列。仅用盐酸浸泡就可除去CNTs表面上的催化剂颗粒，得到高纯的CNTs阵列。高分辨透射电镜(HRTEM)和拉曼图谱(Raman)表明CNTs具有很低的石墨化结构。通过对CNTs形貌和形成过程的剖析，认为AAO模板孔道的导向作用以及模板孔内壁催化剂铁颗粒大小分布不均是形成低石墨化碳纳米管的主要原因。     

Controlled surface functionalization of carbon nanotubes by nitric acid vapors generated from sub‐azeotropic solution

Controlled surface modification of nanocarbons is crucial for their use in applications. The paper deals with the functionalization of carbon nanotubes (CNTs) with HNO₃ vapors. Sub‐azeotropic HNO₃ + H₂O + Mg(NO₃)₂ solution is used for the generation of nitric acid vapors. Because this approach allows tuning the HNO₃ concentration in the vapor phase, the effect of its variation on the surface chemistry and structural properties of the CNTs is investigated. A combination of analytical techniques is applied to evaluate oxidation extent of the CNT surface, selectivity towards the formation of carboxyl groups compared with other oxygenated functionalities, and CNT integrity. The comparison with liquid‐phase functionalization in H₂SO₄ + HNO₃ mixture (1 : 3–3 : 1 v/v), conventionally utilized for oxidizing CNTs, shows that vapor‐phase functionalization affords greater surface oxygen uptakes and higher selectivity towards the formation of carboxyl groups, with smaller tube damage; more importantly, it evidences that, regardless of the method and conditions chosen, the selectivity towards carboxyl groups increases linearly with the surface oxygen concentration. The presented results prove that the product of HNO₃ concentration in the vapor‐phase (25–93 wt%) and vapor‐phase functionalization duration (0.5–5 h) controls the surface oxygen concentration. A simple rate model is proposed to account for its increase. Copyright © 2015 John Wiley & Sons, Ltd.     

Transition metal loaded TiO2 for phenol photo-degradation

Photocatalytic degradation of phenol under both UV radiation and visible light, using TiO₂ (Degussa P-25) and TiO₂ loaded with some transition metal ions (Co, Cu, Fe and Mo) was examined. From the series of metal loaded catalysts, Mo/TiO₂ was the most efficient one. In the presence of Mo, neither TiO₂ anatase/rutile fraction nor its pore size diameter has been affected. However, Mo made its surface more acidic. The percentage of phenol degradation reached under visible light was significantly lower than that under UV radiation due to the lower degree of light absorption by the catalyst surface. From the series of studied catalysts, 2 wt% Mo/TiO₂ was the most efficient one. The synergetic effect between S_BET, mean pore size diameter, catalyst agglomerate size, band gap, ZPC and the type of Mo_xO_y species on TiO₂ surface, depending on Mo loading, created its photocatalytic performance.     

Fe–N/C nanofiber electrocatalysts with improved activity and stability for oxygen reduction in alkaline and acid solutions

Fe–N/C nanofiber (Fe–N/CNF) electrocatalysts were prepared by impregnating electrospun polyacrylonitrile nanofibers with iron nitrate (Fe(NO₃)₃) solution and subsequent heat treatment, exhibiting improved activity and stability during oxygen reduction reaction (ORR) both in 0.1 M KOH (pH?=?13) and 0.5 M H₂SO₄ (pH?=?0) electrolyte solutions. Higher treatment temperature and NH₃ atmosphere were preferred by the Fe–N/CNF catalysts, and especially the concentration of Fe(NO₃)₃ solution exerted great effects on the surface morphology, structure, and thus electrocatalytic performance of the catalysts. The Fe–N/CNFs prepared using 0.5 wt% Fe(NO₃)₃ solution showed relatively higher ORR activity in alkaline and acid solutions and better stability especially in 0.5 M H₂SO₄ solution than the catalyst without Fe, probably because Fe could promote the graphitization of the polymer-converted carbon species, enhancing the resistance to electrochemical oxidation and thus the stability of the Fe–N/CNF catalysts.     

Fe/Al-PILC催化C_3H_6选择性还原NO的实验研究

采用浸渍法制备了负载于铝柱撑黏土的铁基催化剂(Fe/Al-PILC),在固定床反应器上测试其催化C3H6选择性还原NO的性能。通过N2吸附-脱附、X射线衍射(XRD)、H2的程序升温还原(H2-TPR)、紫外可见光谱(Uv-vis)、吡啶吸附红外光谱(Py-FTIR)等手段对催化剂的物理化学性质进行表征。结果表明,9Fe/Al-PILC在400-550℃能够还原98%以上的NO,而且SO2和水蒸气对其催化性能的影响很小。XRD、N2吸附-脱附表征结果表明,Fe/Al-PILC催化剂中铁氧化物高度分散在载体表面,催化剂有较大的比表面积和孔容。H2-TPR结果表明,催化剂的活性主要由Fe_2O_3物相的还原性能决定。Uv-vis结果表明,催化剂的活性与铁氧低聚物种FexOy呈正相关性。Py-FTIR结果表明,催化剂表面同时存在Lewis酸和Brnsted酸,L酸性位是NO和C3H6反应的主要催化活性中心。