Synthesis of multi-walled carbon nanotubes on ‘red mud’ catalysts

Red mud, a toxic waste product from bauxite processing, was used as a catalyst for the synthesis of multi-walled carbon nanotubes (MWCNTs) by fluidised bed chemical vapour deposition. The products were analysed using thermogravimetric analysis, Raman spectroscopy, and transmission electron microscopy. Using ethylene at 650 °C a MWCNT yield of 375% (with respect to Fe loading) was obtained. Carbon products were approximately 75% MWCNTs with an I_G/I_D ratio from Raman spectroscopy of 1.43. The production technique and reaction conditions used are conducive to large-scale CNT production, offering a potential value-added commercial use for red mud.     

Influence of the nanostructure of silica supports on the growth and morphology of MWCNTs synthesized by CCVD method

Due to the influence of its superficial physical parameters on the interaction with catalyst, mesoporous silica is commonly referred as suitable support to grow carbon nanotubes (CNTs) by catalytic chemical vapor deposition (CCVD) method. Faced with the various possibilities of applying nanostructured SiO₂/CNTs composites, this work aims to evaluate and clarify the influence of the silica mesoporosity and morphology on the quality and amount of CNTs produced by CCVD process. Five different nanostructured silicas (n-SiO₂) were produced by sol-gel method. Basically, four silica samples were synthesized with the addition of an acidic catalyst and one with a basic catalyst. Thermogravimetric analysis, Raman spectroscopy, transmission electron microscopy and scanning electron microscopy were used to characterize the silica supports and the as-grown CNTs produced in this work. The obtained results show differences in the morphology of the synthesized CNTs according to the physical properties of each n-SiO₂. The mesoporous silica structure, due to different pore size distribution and volume, affected the interaction between the support and the catalyst, and, consequently, the quality and amount of the synthesized multi-walled carbon nanotubes (MWCNTs). Silica supports with either high mesopore volume, or high mesopore size, provide the highest quantities of as-grown CNTs materials. However, in terms of quality of as-grown CNTs, the supports with lower mesopores volume were more adequate to the MWCNTs synthesis. Nevertheless, the presence of pores with compatible size may have allowed an improved anchorage of catalyst particles inside these pores favoring the growth of CNTs with good quality.     

Synthesis of Supported Catalysts by Impregnation and Calcination of Low-Temperature Polymerizable Metal-Complexes

The low-temperature polymerizable metal complex solution has been used as an active component precursor to prepare supported (structured) catalysts by a straight-forward sequence of impregnation, drying and calcination. Two typical samples, 5 wt% Zr _x Ce_1?x O₂/Al₂O₃ nanocomposite and structured carbon nanofiber supported Cu?CCeO₂ catalyst, are prepared to explore the potential of this method in the controlled synthesis of catalysts and catalyst supports. The interaction between the active component precursor and the surface of the solid matrices during impregnation and drying is investigated by infrared spectroscopy (IR) and transmission electron microscopy (TEM), demonstrating that the in situ polymerization process is crucial for the deposition of the active component on the surface of solid matrices. The evolution of the phase transformation and the structure of the produced materials during the calcination step is studied by coupling thermal gravimetric analysis?Cdifferential thermal analysis?Cmass spectroscopy (TGA?CDTA?CMS), TEM and X-ray diffraction (XRD) measurements, indicating that higher thermal stable particles or smaller particles can be obtained with the presence of the Al₂O₃ or structured carbon nanofibers (SCNF), respectively, after calcination. This method combines the advantages of sol?Cgel and impregnation, representing a promising route for preparing supported catalysts and catalyst supports. The limitations of the method are also discussed.     

Thermal and mechanical stabilities of composite films from thiadiazol bearing poly(amide-thioester-imide) and multiwall carbon nanotubes by solution compounding

An amino acid containing poly(amide-thioester-imide) (PATEI) possessing a conjugated thiadiazol ring was shown to be effective for dispersing multiwall carbon nanotubes (MWCNTs) in N,N′-dimethylacetamide. Through casting of these dispersions, MWCNT/PATEI composite films were successfully fabricated on substrates and showed no signs of macroscopic aggregation. To increase the compatibility between PATEI matrix and MWCNTs, carboxyl-functionalized MWCNTs (f-MWCNTs) were used in this study. The f-MWCNTs were dispersed homogeneously in the PATEI matrix while the structure of the polymer and the MWCNTs structure were stable in the preparation process as revealed by transmission electron microscopy. Tensile tests and thermal analysis were carried out on free-standing composite films for different MWCNT loading levels. Results showed that overall mechanical and thermal properties of the composites were greatly improved as compared with the neat PATEI film. Fourier transform infrared spectroscopy, powder X-ray diffraction, and field emission electron microscopy were also used to evaluate the MWCNT/PATEI composite system.     

Preparation of multi-walled carbon nanotube supported TiO2 and its photocatalytic activity in the reduction of CO2 with H2O

Multi-walled carbon nanotube (MWCNT) supported TiO₂ composite catalysts were prepared by sol-gel and hydrothermal methods. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy and N₂-adsorption analysis were carried out to characterize the composite catalysts. In using the sol-gel method, the MWCNTs were coated with anatase TiO₂ nanoparticles, and by the hydrothermal method, rutile TiO₂ nanorods were uniformly deposited on the MWCNTs. The photocatalytic activities of the composite catalysts were evaluated by the reduction of CO₂ with H₂O. The results indicate that the addition of an appropriate amount of MWCNTs as supports for TiO₂ could remarkably improve the efficiency of the photocatalytic reaction. The composite catalysts prepared by the sol-gel method lead to the main formation of C₂H₅OH, while HCOOH is found to be the major product on the sample prepared by the hydrothermal method.     

Effect of metal-support interaction on the structural and enhanced photocatalytic performance of mesoporous M–TiO<Subscript>2</Subscript>/SBA-16 (M = Ag and Fe)

In this paper, a series of metal supported TiO₂/SBA-16 photocatalysts were successfully synthesized by wet impregnation method. The synthesized samples were characterized by X-ray diffraction (XRD), Raman spectra, N₂ adsorption–desorption, transmission electron microscopy (TEM), UV–visible absorption spectra (UV-vis) and X-ray photoelectron spectroscopy (XPS). It was found that SBA-16 retained the ordered mesostructure after modification. The results shown that loading different metal elements was found to have significant influences on the crystallographic structure and physical properties. Moreover, the prepared materials were evaluated on photodegradation of Rhodamine B (RhB). Experiment results showed that the degradation of RhB by these catalysts follows the pseudo-first-order kinetic model. The effect of metal element on photocatalytic activity can be attributed that the appropriate amounts of metal concentration can enhance the photocurrent due to the reduced electron–hole recombination, which can improve the efficiency of photocatalytic reactions.     

Preparation of alumina/carbon nanotubes composites by chemical precipitation

Continuous alumina coating on multi-walled carbon nanotubes (MWCNTs) was successfully prepared by a new method of chemical precipitation using aluminum nitrate and ammonia as starting materials. Structure and morphology of the alumina/multi-walled carbon nanotubes (Al₂O₃/MWCNTs) composites were characterized by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), infrared spectra (IR), thermo gravimetric analysis (TG), differential thermal analysis (DTA) and N₂ adsorption–desorption. The results show that polyvinyl alcohol (PVA) modification on the surface of MWCNTs contributes to form continuous alumina coating, γ-Al₂O₃ layers with thickness of 1–3 nm cover the surface of MWCNTs and the original structure of MWCNTs is retained during the coating process.     

Effect of the Synthesis Method on Alumina Supported Silver Based Catalyst for NO x Selective Reduction by Ethanol

Alumina supported indium oxide, silver or bimetallic Ag–In catalysts with metal loading of ca. 2.5 wt% were prepared by excess solvent impregnation or incipient wetness impregnation. The influence of the impregnation was studied by testing the catalysts in DeNO _x activity with ethanol in Lean Burn Conditions.     

Synthesis of well-dispersed PtRuSnOx by ultrasonic-assisted chemical reduction and its property for methanol electrooxidation

PtRuSnO_x supported on multi-wall carbon nanotubes (MWCNTs) was prepared by ultrasonic-assisted chemical reduction method. The as-prepared catalyst was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns indicate that Pt exists as the face-centered cubic structure, Ru is alloyed with platinum, while non-noble metal oxide SnO_x exists as an amorphous state. From TEM observation, PtRuSnO_x is well dispersed on the surface of MWCNTs with the particle size of several nanometers. The electrochemical properties of the as-prepared catalyst for methanol electrooxidation were studied by cyclic voltammetry (CV) and chronoamperometry (CA). The onset potential of methanol oxidation on PtRuSnO_x and PtRu catalysts is much more negative than that on Pt catalyst, shifting negatively by about 0.20 V, while the peak current density of methanol oxidation on PtRuSnO_x is higher than that on PtRu. Electrochemical impedance spectroscopy (EIS) studies also show that the reaction kinetics of methanol oxidation is improved with the presence of SnO_x. The addition of non-noble metal oxide SnO_x to PtRu promotes the catalytic activity for methanol electrooxidation and the possible reaction mechanism is proposed.     

Characterization and reactivity of RHA–Al2O3 composite oxides supported nickel catalysts

RHA–Al₂O₃ composite oxide supports were prepared by impregnation of rice husk ash (RHA) with an aluminum sulfate solution, and were then used to prepare supported nickel catalysts by the ion exchange method. The supports and catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption and temperature-programmed desorption (TPD) of hydrogen. The reactivities of nickel catalysts were tested by CO₂ hydrogenation with H₂/CO₂ ratios of 4/1 for temperatures between 573 and 873 K. These results show that nickel compound with a layer structure was formed after the drying step. Furthermore, the nickel crystallites were finely distributed on the support even at high loading. The BET surface area of the unreduced catalysts decreased with the nickel loading up to 2.44 wt% and then increased with further deposits. The hydrogenation reactivity increases with an increase in nickel loading. Furthermore, the hydrogenation reactivity increases with an increasing reaction temperature up to 773 K and then remained constant.     

Influence of support porosity and Pt content of Pt/carbon aerogel catalysts on metal dispersion and formation of self-assembled Pt-carbon hybrid nanostructures

Three carbon aerogels with different meso-macropore networks were used as supports for Pt catalysts using [Pt(NH₃)₄]Cl₂ as precursor salt. Results obtained showed mesopore volume and mean mesopore size to be important parameters that control Pt particle size and dispersion in catalysts containing 2 wt.% Pt. Once the most appropriate porosity to obtain the highest dispersion was determined, the metal content was increased to 20 wt.% Pt. However, the mean Pt particle size only increased from 1 to 2 nm, indicating the importance of an appropriate mesoporosity for obtaining a high dispersion at high metal loading. Mean Pt particle size was always slightly smaller by transmission electron microscopy than by H₂ chemisorption, because some Pt particles were not reduced during pre-treatment, as confirmed by X-ray photoelectron spectroscopy. Finally, transmission electron microscopy observations of catalysts with metal loading of 8-20 wt.% before pre-treatment showed the formation of self-assembled Pt-carbon hybrid nanorods and nanowires. To the best of our knowledge, this is the first observation of this phenomenon in Pt/C catalysts.     

Water Denitration over a Pd–Sn/Al2O3 Catalyst

Bimetallic Pd–Sn catalysts were synthesized by incipient-wetness impregnation of the metals on alumina and employed for the reduction of nitrates from aqueous solutions. The catalysts were characterized by FTIR spectroscopy of adsorbed CO, X-ray diffraction (XRD), transmission electron microscopy (TEM), and H₂ chemisorption. The influence of the metal ratio was evaluated in reaction measurements. The bimetallic Pd–Sn catalysts exhibited high selectivity for nitrate removal forming less NO₂⁻ and NH₄⁺ than the Pd–Cu catalysts.     

Preparation of macrospherical magnesia-rich magnesium aluminate spinel catalysts for methanolysis of soybean oil

In order to fully exploit the green characteristics of solid base catalysts they should be fabricated into macrostructured rather than powder form. Magnesia-rich magnesium aluminate spinel (MgO·MgAl₂O₄) framework catalysts with tunable basicity have been prepared by using γ‐Al₂O₃ macrospheres (0.5-1.0 mm in diameter) as a hard template. The process involves in situ growth of magnesium-aluminum layered double hydroxides (MgAl-LDHs) in the channels of the γ‐Al₂O₃ macrospheres by the urea hydrolysis method, followed by calcination, tuning of the basicity through etching of excess aluminum with aqueous alkali and a final calcination step. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), elemental analysis and low temperature N₂ adsorption-desorption studies demonstrate that the composite MgO·MgAl₂O₄ materials are composed of nanosized rod-like particles aggregated into a spherical framework. Catalytic reactivity was investigated by using methanolysis of soybean oil as probe reaction. The MgO·MgAl₂O₄ composite shows a higher biodiesel yield compared to an MgO/MgAl₂O₄/γ‐Al₂O₃ material with the same loading of magnesium prepared by a conventional impregnation method. The enhanced catalytic activity of the former material can be ascribed to its higher basicity, specific surface area, pore volume and pore size.     

Transformation of methanol to gasoline range hydrocarbons using HZSM-5 catalysts impregnated with copper oxide

Various CuO/HZSM-5 catalysts were studied in a fixed bed reactor for the conversion of methanol to gasoline range hydrocarbons at 673 K and at one atmospheric pressure. The catalysts were prepared by wet impregnation technique. Copper oxide loading over HZSM-5 (Si/Al=45) catalyst was studied in the range of 0 to 9 wt%. XRD, BET surface area, metal oxide content, scanning electron microscopy (SEM) and thermogravimetric (TGA) techniques were used to characterize the catalysts. Higher yield of gasoline range hydrocarbons (C₅-C₁₂) was obtained with increased weight % of CuO over HZSM. Effect of run time on the hydrocarbon yields and methanol conversion was also investigated. The activity of the catalyst decreased progressively with time on-stream. Hydrocarbon products’ yield also decreased with the increase in wt% of CuO. Relatively lower coke deposition over HZSM-5 catalysts was observed compared to CuO impregnated HZSM-5 catalyst.     

Characterization and photodegradation characteristics of organic dye for Pt–titania combined multi-walled carbon nanotube composite catalysts

Multi-walled carbon nanotubes (MWCNTs), titanium(IV) isopropoxide (TIP) and potassium hexachloroplatinate(IV) (K₂PtCl₆) were used for the preparation of Pt/MWCNT/TiO₂ composites. The composites were comprehensively characterized by Brauer–Emett–Teller surface area, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray and UV–vis absorption spectroscopy. The photoactivity of the prepared materials under UV irradiation was tested using the conversion of methylene blue (MB) in aqueous solution. According to the results of MB removal experiment, it can be considered that the MB removal effect of the Pt/MWCNT/TiO₂ composites is affected by two kinds of effects: adsorption effect by MWCNTs and photocatalytic effect by TiO₂. Finally, the photocatalytic effect increases due to photo-induced-electron absorption effect by MWCNTs and electron trap effect by Pt metal.     

Catalytic activity of Ni-Co supported metals in carbon dioxides methanation

This work deals with the catalytic performance of nickel-cobalt supported on ceria-doped gadolinia (GDC) catalyst in the single and in the simultaneous methanation of carbon monoxide and carbon dioxide. The catalysts have been prepared by impregnation method, starting from metal salts precursors. Samples have been characterized by x-ray diffraction (XRD), thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), hydrogen temperature programmed reduction (TPR-H₂), transmission electronic microscopy (TEM), and scanning electron microscopy (SEM/EDX) technique. The temperature examined for methanation tests ranged from 200°C-600°C. The results show that the prepared and optimized catalysts possess the main characteristics of materials suitable for SOECs (solid oxide electrolyzer cells) applications: high metal content (50% wt/wt with respect to the support), high activity, and high stability. The catalytic performance of bimetallic catalysts highlights that the cobalt does not improve the activity of the nickel catalysts.     

Platelet-like catalyst design for high yield production of multi-walled carbon nanotubes by catalytic chemical vapor deposition

We investigated the effect of catalyst design on the synthesis of multi-walled carbon nanotubes (MWCNTs) by chemical vapor deposition (CVD). A set of highly active supported sol–gel Co–Mo/MgO and Ni–Mo/MgO catalysts was prepared systematically modifying the calcination temperature. First, the evolution of catalysts’ crystallographic phases and their morphology were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning electron (SEM) and transmission electron (TEM) microscopy. Second, the catalysts were used for the CVD growth of MWCNTs. The resulting materials were analysed by SEM and TEM, Raman and XRD to establish a relation between catalyst design and MWCNT yield. We show that our catalyst synthesis route leads to the formation of laminar non-porous catalyst systems, which at a calcination temperature of 800 °C stabilize in a crystallographic phase of Me_xMg_1−xMoO₄ (Me = Co or Ni). We give evidence that increased MWCNT yields of more than 3000 wt.% with respect to the catalysts are directly related to the aforementioned crystallographic phase. Finally, we propose a growth model based on the continuous exfoliation of platelet-like catalyst systems. This consistently explains the high catalytic activity towards MWCNT production using a non-porous catalyst. Our findings provide important insights for catalyst design strategies towards large-scale MWCNT production.     

Large-scale production of high-quality multi-walled carbon nanotubes: Role of precursor gas and of Fe-catalyst support

The crucial role of precursor gas (PG) and of catalyst support (CS) in the growth of multi-walled C nanotubes (MWCNTs) by iron-catalysed chemical vapour deposition (CVD) is evidenced. This is accomplished by comparing structural and morphological properties of MWCNTs synthesised by the use of different PGs (ethane and isobutane) and CSs (silica and alumina). The results of analyses, carried out on catalysts and C deposits by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy (RS), thermo-gravimetry (TG) and X-ray diffraction (XRD), demonstrate that Al₂O₃-supported catalysts are more efficient than SiO₂-supported ones in decomposing hydrocarbons. The use of i-C₄H₁₀ as PG allows reducing Fe-encapsulation and improving yield (Y_C) and selectivity, so as the large-scale production (Y_C > 900 wt.%) of high-quality nanotubes can be operated even at moderate reaction temperature (600 °C) after proper calibration of Fe-load (29 wt.%) and catalyst reduction temperature (500 °C).     

Synthesis of PtRu/carbon nanotube composites in supercritical fluid and their application as an electrocatalyst for direct methanol fuel cells

PtRu nanoparticles were decorated on multi-walled carbon nanotubes (MWCNTs) using H₂PtCl₆ and RuCl₃ as precursors with the aid of supercritical CO₂, and the resulting composites were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy. TEM observation showed that nanoparticles of size about 5 nm were distributed evenly on the MWCNTs, and XRD analysis showed that the particles had a face-centered cubic crystal structure. The loading content of the nanoparticles on the MWCNTs could be adjusted by manipulating the relative ratio of the precursor to MWCNTs. The as-prepared PtRu/MWCNT composites exhibited high activity for methanol electro-oxidation.     

Co‐Metal Catalysts on SiO2‐TiO2 for Methanol Production from CO2 – Effect of Preparation Methods

The effect of quantity, composition, and different impregnation sequences on the catalytic properties of Cu‐Zn‐Al/SiO₂‐TiO₂ in the CO₂ hydrogenation for methanol production was investigated. The Cu‐Zn‐Al catalysts supported on SiO₂ and TiO₂ were prepared by incipient wetness impregnation. Then, their performances in CO₂ hydrogenation were tested under defined conditions. The composition variation of Cu and Zn catalysts resulted in a high methanol production for Cu catalysts with a higher content of Cu, which was the active site for CO₂ activation. Regarding the metal quantity of catalysts, a relatively low loading of co‐metal (Cu‐Zn‐Al) led to the maximum methanol yield when compared with higher loadings as a result of the largest surface area.