Methane catalytic decomposition over ordered mesoporous carbons: A promising route for hydrogen production

Methane decomposition offers an interesting route for the CO₂-free hydrogen production. The use of carbon catalysts, in addition to lowering the reaction temperature, presents a number of advantages, such as low cost, possibility of operating under autocatalytic conditions and feasibility of using the produced carbons in non-energy applications. In this work, a novel class of carbonaceous materials, having an ordered mesoporous structure (CMK-3 and CMK-5), has been checked as catalysts for methane decomposition, the results obtained being compared to those corresponding to a carbon black sample (CB-bp) and two activated carbons, presenting micro- (AC-mic) and mesoporosity (AC-mes), respectively. Ordered mesoporous carbons, and especially CMK-5, possess a remarkable activity and stability for the hydrogen production through that reaction. Under both temperature programmed and isothermal experiments, CMK-5 has shown to be a superior catalyst for methane decomposition than the AC-mic and CB-bp materials. Likewise, the catalytic activity of CMK-5 is superior to that of AC-mes in spite of the presence of mesoporosity and a high surface area in the latter. The remarkable stability of the CMK-5 catalyst is demonstrated by the high amount of carbon deposits that can be formed on this sample. This result has been assigned to the growth of the carbon deposits from methane decomposition towards the outer part of the catalyst particles, avoiding the blockage of the uniform mesopores present in CMK-5. Thus, up to 25 g of carbon deposits have been formed per gram of CMK-5, while the latter still retains a significant catalytic activity.     

Hydrogen production via the aqueous phase reforming of ethylene glycol over platinum-supported ordered mesoporous carbon catalysts: Effect of structure and framework-configuration

Structurally and framework-configurationally different kinds of ordered mesoporous carbon (OMC) supported platinum catalyst were applied to aqueous phase reforming (APR) of ethylene glycol (EG) for hydrogen production. Wide-angle XRD patterns, CO chemisorption results, and TEM analyses clearly provide evidence that Pt nanoparticles on 3-dimensional (3-D) OMC support (CMK-8, CMK-9) are less sintered than Pt nanoparticles on 2-dimensional (2-D) OMC support (CMK-3, CMK-5). In addition, due to the large surface area caused by the carbon microporosity and the additional mesopores of hollow-type OMC support, OMC with a hollow-type framework-configuration (CMK-5, CMK-9) provides Pt metal nanoparticles smaller than those of the rod-type OMC (CMK-3, CMK-8). Therefore, the Pt/CMK-9 catalyst with a 3-D OMC and hollow-type framework configuration exhibits the best hydrogen production in the APR with EG due to both the synergetic effect of having a low amount of metal sintering during the reaction process and the more favorable transport and diffusion of the reactants and products.     

Pt-Ru electrocatalysts supported on ordered mesoporous carbon for direct methanol fuel cell

Pt-Ru electrocatalysts supported on ordered mesoporous carbon (CMK-3) were prepared by the formic acid method. Catalysts were characterized applying energy dispersive X-ray analyses (EDX) and X-ray diffraction (XRD). Methanol and carbon monoxide oxidation was studied electrochemically by cyclic voltammetry, and current-time curves were recorded in a methanol solution in order to establish the activity towards this reaction under potentiostatic conditions. The physicochemical and electrochemical properties of the Pt-Ru catalysts supported on CMK-3 carbon were compared with those of electrocatalysts supported on Vulcan XC-72 and commercial catalyst from E-TEK. Additionally, in order to complete this study, Pt electrocatalysts supported on CMK-3 and Vulcan XC-72 were prepared by the same method and were used as reference. Results showed that the Pt-Ru/CMK-3 catalyst presented the best electrocatalytic activity towards the CO oxidation and, therefore, good perspectives to its application in DMFC anodes. On the other hand, the activity of the Pt-Ru/CMK-3 catalyst towards methanol oxidation was higher than that of the commercial Pt-Ru/C (E-TEK) catalyst on all examined potentials, confirming the potential of the bimetallic catalysts supported on mesoporous carbons.     

Aqueous phase reforming of polyols for hydrogen production using supported PtFe bimetallic catalysts

3-D cubic ordered mesoporous carbon (CMK-9) supported PtFe bimetallic catalysts with a range of PtFe compositions were applied to the aqueous phase reforming (APR) of polyols for hydrogen production. The catalytic performance with respect to the polyol and support used was also studied. The catalysts and supports were characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM), N₂ sorption, temperature programmed reduction (TPR), and CO chemisorption techniques. The polyols investigated include ethylene glycol (EG), glycerol, xylitol, and sorbitol. It was found that the addition of Fe to the Pt/CMK-9 catalyst significantly improved catalytic performance, with the optimum Pt:Fe ratio for APR activity being 1:3. It was also observed that, in the PtFe (1:3) system, the CMK-9 support demonstrated better catalytic performance than commercially available activated carbon or alumina. In addition, the catalytic activity of the PtFe/CMK-9 catalyst was successfully increased by both the effect of the water-gas shift reaction, promoted by Fe addition to Pt, and by the structural properties and nature of the CMK-9 support. Moreover, the PtFe (1:3)/CMK-9 catalyst showed efficient catalytic activity for different biomass derivatives (EG, glycerol, xylitol, and sorbitol), with the activity decreasing with increase in the number of carbon atoms.     

Study on the electrochemical properties of cubic ordered mesoporous carbon for supercapacitors

Highly ordered, three-dimensional (3D) cubic mesoporous carbon CMK-8 is prepared by a facile nanocasting approach using cubic mesoporous silica KIT-6 as starting template. Afterwards, in order to increase the active sites of surface electrochemical reactions and promote the wettability in aqueous electrolyte, a chemical surface modification is carried out on the CMK-8 by nitric acid treatment. Two electrodes are prepared from the CMK-8 and the acid-modified CMK-8 (H-CMK-8) and used as the active materials for supercapacitors. The unique 3D mesoporous network combined with high specific surface area makes the nano-channel surfaces of the CMK-8 carbon favorable for charging the electric double-layer, resulting in that the CMK-8 and the H-CMK-8 electrodes both show well supercapacitive properties. Furthermore, the specific capacitance of the CMK-8 can be further improved by acid treatment, so that the H-CMK-8 exhibits the largest specific capacitance of 246 F g⁻¹ at a current density of 0.625 A g⁻¹ in 2 M KOH electrolyte. Also, the two carbon electrodes both exhibit good cycling stability and lifetime. Therefore, based on the above investigations, such CMK-8 carbon, especially H-CMK-8 carbon can be a potential candidate for supercapacitors.     

Highly ordered mesoporous carbons as the support for Pt catalysts towards alcohol electrooxidation: The combined effect of pore size and electrical conductivity

Pt nanoparticles were successfully deposited on ordered mesoporous carbons (CMK-3) using a pulse microwave-assisted polyol method. CMK-3 with three different pore sizes, obtained by using boric acid as the pore-expanding agent, were adopted. The pore size was controlled to be 4.4, 6.1, and 6.7 nm while the highly ordered structure was still maintained. With these different CMK-3 samples as the support, the particle size of Pt was identical, about 2 nm. It was found that the electrochemical surface area was almost the same in these three cases and the alcohol electrooxidation activity was not increased but decreased a little along with the pore size increment. The increased pore size of CMK-3 had no obviously positive effect on easier mass transfer and then a better electrocatalytic activity. This could be attributed to the very good 3-D interconnection of the nanospacings of CMK-3 per se. Moreover, combined with the theoretical calculation and confirmed by alternative current impedance results, the electrical conductivity of CMK-3 was dramatically decreased along with the pore size increasing, which counteracted the beneficial effect from easier mass transfer in the case of bigger pore size. Based on the present and reported results, it hints that when the carbon skeleton of carbon materials is different, the modification skills are changed for improving their structure in order to make them suitable for electrocatalysts supports.     

Synthesis and performance of platinum supported on ordered mesoporous carbons as catalyst for PEM fuel cells: Effect of the surface chemistry of the support

Platinum electrocatalysts supported on ordered mesoporous carbon (CMK-3) have been prepared as alternative catalysts for PEM fuel cells. Their performance has been compared with that of a commercial Pt-carbon black on carbon cloth electrode (E-TEK) for the hydrogen oxidation in a PEM single cell. Ordered mesoporous carbon was synthesized using nanocasting method and then platinum was deposited by incipient wetness impregnation. Before the platinum deposition, carbon support was functionalized using HNO₃ as oxidizing agent to modify its surface chemistry. The characterization study of the electrocatalysts demonstrated that the surface chemistry of the support has an important effect on both the physicochemical and electrochemical properties of electrocatalysts. For this catalyst synthesis method, functionalization did not improve the preparation of the catalyst, since the presence of surface oxygen groups facilitated the aggregation of metal particles. However, the Pt/CMK-3 based electrodes showed a better performance than the commercial one, which could be attributed to the porous structure of the support.     

Hydrogen production through the aqueous phase reforming of ethylene glycol over supported Pt-based bimetallic catalysts

The catalytic activities of supported Pt-based bimetallic catalysts (Pt-M) were studied for hydrogen production via aqueous phase reforming (APR) using a 10 wt% ethylene glycol solution. The catalysts and supports used were characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption-desorption, CO chemisorption, and temperature programmed reduction (TPR) techniques. It was found that the Pt–Mn (Pt:Mn = 1:1, molar ratio) bimetallic catalyst significantly enhanced the catalytic performances such as the hydrogen yield and hydrogen production when compared with monometallic catalysts and other bimetallic catalysts that were examined. The XRD and TPR studies confirmed the interaction between the Pt and Mn species, leading to the Pt–Mn alloys supported on CMK-3. Related to the effect of the type of support, the CMK-3 support demonstrated better performance than the commercial activated carbon and alumina. Accordingly, it can be understood that the better catalytic performance of the APR reaction over Pt–Mn/CMK-3 catalyst is dependent on the alloy effect as well as the structural properties and nature of support given by the addition of the second metal.     

Ordered CoO/CMK-3 nanocomposites as the anode materials for lithium-ion batteries

A novel ordered mesoporous carbon hybrid composite, CoO/CMK-3, is prepared by an infusing method using Co(NO₃)₂·6H₂O as the cobalt source. The products are characterized by X-ray diffraction, transmission electron microscopy and N₂ adsorption-desorption analysis techniques. It is observed that the CoO nanoparticles are loaded in the channels of mesoporous carbon. The mesopore structure of CMK-3 is destroyed gradually with increasing of the CoO content. The electrochemical properties of samples as the anode materials for lithium-ion batteries are studied by galvanostatic method. The results show that the CoO/CMK-3 composites have higher reversible capacities (more than 700 mAh g⁻¹) and better cycle performance in comparison with the pure mesoporous carbon (CMK-3). Based on the above results, a mechanism is proposed to explain the reason of such a substantial improvement of electrochemical performance in the CoO/CMK-3 composites.     

Synergistic effects played by CMK-3 and NbF5 co-additives on de/re-hydrogenation performances of NaAlH4

In the present work, a strategy for simultaneously reducing the thermal stability of NaAlH₄ and enhancing its dehydrogenation kinetics was suggested by means of synergistic effects from co-additives of mesoporous carbon material CMK-3 and NbF₅. The ball milled NaAlH₄ + 10 wt% (NbF₅ + CMK-3) (NbF₅: CMK-3 = 1:1 in weight ratio) composite can liberate hydrogen at an onset temperature of 358 K, which was drastically decreased by 93 K from that of pristine NaAlH₄. By means of Kissinger's method, the activation energy of NaAlH₄ + 10 wt% (NbF₅ + CMK-3) can be identified as 99.2 kJ mol^?1, which was greatly reduced from that of pristine NaAlH₄ (121 kJ mol^?1). Investigations on the dehydrogenation process revealed that CMK-3 was beneficial to reducing the particle size of NaAlH₄ during ball milling, while NbF₅ was actively involved in the decomposition of NaAlH₄ and yielded some Nb-relevant intermediate phases NbH_0.89 during the heating process. The modified dehydrogenation pathway of NaAlH₄ also results in the destabilization of dehydrogenation by 2.13 kJ mol^?1 H₂ from that of pristine NaAlH₄. During the hydrogenation process, the NbH_0.89 and the mesoporous carbon material CMK-3 played synergistic roles in improving the dehydrogenation performance of NaAlH₄.     

Evaluation of activated carbons based on olive stones as catalysts during hydrogen production by thermocatalytic decomposition of methane

Catalytic decomposition of methane over carbon materials has been intensively studied as an environmental approach for CO₂-free hydrogen production without further by-products except hydrogen and valuable carbon. In this work, we will investigate the catalytic activity of activated carbons based on olive stones prepared by two different processes. Additionally, the effect of three major operational parameters: temperature, weight of catalyst and flow rate of methane, was determined. Therefore, a series of experiments were conducted in a horizontal-flow fixed bed reactor. The outflow gases were analysed using a mass spectrometer. The textural, structural and surface chemistry properties of both fresh and used activated carbons were determined respectively by N₂ gas adsorption, X-Ray Diffraction and Raman and Temperature Programmed Desorption. The results reveal that methane decomposition rate increases with temperature and methane flow however it decreases with catalyst weight. The two carbon samples exhibit a high initial activity followed by a rapid decay. Textural characterization of the deactivated carbon presents a dramatic drop of surface area, pore and micropore volumes against an increase of average pore diameter confirming that methane decomposition occurs mainly in micropores. XRD characterization shows a turbostratic structure of fresh samples with more graphitization in deposed carbon explaining the lowest activity at the end of reaction. Raman spectra reveal the domination of the two bands G and D which varying intensities affirm that the different carbons tend to organise in aromatic rings. Finally the surface chemistry qualitatively changes greatly after methane dissociation for CAGOC unlike CAGOP but quantitatively a small difference is observed which indicates that these functionalities may have a role in this heterogeneous reaction but cannot be totally responsible. Among the two catalysts tested, CAGOC has the highest initial methane decomposition rate but CAGOP is the most stable one.     

Mesoporous carbon-encapsulated NiO nanocomposite negative electrode materials for high-rate Li-ion battery

In this study, a novel mesoporous carbon-encapsulated NiO nanocomposite is proposed and demonstrated for Li-ion battery negative electrode. The nanostructure of the electrode composes of an ordered mesoporous CMK-3 as a 3D nanostructured current collector with micorporous channels for Li⁺ transportation. In addition, exclusive formation of NiO nanoparticles in the confined space of the ordered mesoporous carbon is achieved using the hydrophobic encapsulation route. The half-cell assembled with the synthesized NiO/CMK-3 nanocomposite is able to deliver a high charge capacity of 812 mAh g⁻¹ at the first cycle at a C-rate of 1000 mA g⁻¹ and retained throughout the test with only 0.236% decay per cycle. Even the C-rate as high as 3200 mA g⁻¹, a charge capacity of 808 mAh g⁻¹ contributed by the NiO nanoparticles in CMK-Ni is obtained, which shows excellent rate capability for NiO with utilization close to 100%. The result suggests fast kinetics of conversion reaction for NiO with Li⁺. It also indicates the blockage of the pore channels by NiO nanoparticles does not take place in the synthesized NiO/CMK-3.     

New three-dimensional electrode structure for the lithium battery: Nano-sized γ-Fe2O3 in a mesoporous carbon matrix

A new electrode structure based on a three-dimensional mesoporous matrix was developed. Nanoparticles of γ-iron oxide (Fe₂O₃) were introduced into the mesopores of a carbon matrix (mesoporous carbon, CMK-3) by oxidizing metallic iron, which was electroplated in the matrix. The resulting structure was found to have a high charge-discharge capacity when used as the positive electrode of a lithium battery. The iron oxide nanoparticles bonded tightly to the electrically conductive electrode framework, and showed a high activity for the electrochemical reaction: Fe₂O₃ + 6Li → 3Li₂O + 2Fe.     

Sulphur–carbon composites for Li/S batteries

Abstract

Structural and electrochemical properties of various types of sulphur–carbon composites were reviewed to propose approaching ways for the development of lithium/sulphur battery with high energy density and good cycle performance. To improve the electrochemical properties of a sulphur cathode, carbon and polymer materials are applied to sulphur composites: multiwalled nanotube (MWNT), graphene, CMK-3 and activated carbon; and polyaniline (PANi), polyacrylonitrile and polythiophene (PTh). These can serve conducting paths and a polysulphide reservoir to enhance the electrical conductivity of the sulphur cathode and effectively prevent dissolution of polysulphides. And the composites are categorised in two parts such as mixed type sulphur composites and embedded type sulphur composites. Among the sulphur–carbon composites, the hollow carbon capsule/S composite prepared by a geometric control and an infusion method of sulphur in sulphur carbon composite electrode, demonstrated the best electrochemical properties.     

Growth of polyaniline nanowhiskers on mesoporous carbon for supercapacitor application

Vertically aligned polyaniline nanowhiskers (PANI-NWs) doped with (1R)-(−)-10-Camphorsulfonic acid (L-CSA) have been successfully synthesized on the external surface of ordered mesoporous carbon (CMK-3) by chemical oxidative polymerization. The specific surface area of the PANI-NWs/CMK-3 nanocomposite remains as high as 497 m² g⁻¹ by removing mesoporous silica template after the polymerization of aniline. Structural and morphological characterizations of the nanocomposite were further investigated by XRD, FTIR and FE-SEM measurements. The result shows that the nanocomposite with 40 wt% PANI applying in supercapacitor devices possesses a large specific capacitance of 470 F g⁻¹ and good capacitance retention of 90.4% is achieved after 1000 cycles at a current density of 1.0 A g⁻¹. The synergistic effect of small PANI nanowhisker arrays and well-ordered mesoporous carbon endows the composite with high electrochemical capacitance and good cycling stability.     

The modification of the mesoporous carbon CMK-3 with the aqua regia and its adsorption desulfurization mechanism

Ordered mesoporous carbon CMK-3 was synthesized and modified with aqua regia. Dibenzolthiophene (DBT) was employed to evaluate the adsorptive desulfurization performance of the adsorbents. It was found that such modification considerably enhanced the saturated sulfur capacity and breakthrough sulfur capacity toward DBT by 2 times, respectively. The modified CMK-3 possessed good reused performance. In order to make a deep study of main reasons for such increase, some characterizations including XRD, BET, TEM, FTIR, XPS, and Boehm titration were carried out. Based on the characterization results, the increased adsorption performance of modified CMK-3 was mainly attributed to the changes of the surficial acidic oxygen-containing functional groups. On such surface, the phenolic hydroxyl and the carboxyl groups have the chemical interactions with the S and the aromatic rings of DBT. Such interaction concerning the carboxyl groups devoted itself to the desulfurization more than the phenolic hydroxyl, and the cracking of the C-S bond of DBT also happen.     

Characterization and performance evaluation of Pt–Ru electrocatalysts supported on different carbon materials for direct methanol fuel cells

The paper addresses the effect of the carbon support on the microstructure and performance of Pt–Ru-based anodes for direct methanol fuel cells (DMFC), based on the study of four electrodes with a carbon black functionalized with HNO₃, a mesoporous carbon (CMK-3), a physical mixture of TiO₂ and carbon black and a reference carbon thermally treated in helium atmosphere (HeTT). It is shown that CMK-3 hinders the growth of the electrocatalyst nanoparticles (2.7 nm) and improves their distribution on the support surface, whereas the oxidized surfaces of HNO₃ carbon and TiO₂+carbon lead to larger (4–4.5 nm), agglomerated particles, and the lowest electrochemical active areas (54 and 26 m² g⁻¹, in contrast with 90 m² g⁻¹ for CMK-3), as determined from CO stripping experiments. However, HNO₃ and TiO₂ are characterized by the lowest CO oxidation potential (0.4 V vs. RHE), thus suggesting higher CO tolerance for the se electrodes. Tests in DMFC configuration show that the three modified electrodes have clearly better performance than the reference HeTT. The highest power density attained with electrodes supported on carbon treated with HNO₃ (65 mW cm⁻²/300 mA cm⁻² at 90 °C) and the equally interesting performance of the TiO₂-based electrodes (53 mW cm⁻²/300 mA cm⁻²), is a strong indication of the positive effect of the presence of oxygenated groups on the methanol oxidation reaction. The results are interpreted in order to identify separate microstructural (electrocatalyst particle size, porosity) and compositional (oxygenated surface groups, presence of oxide phase) effects on the electrode performance.     

Facile fabrication of titania-ordered cubic mesoporous carbon composite: Effect of Ni doping on photocatalytic hydrogen generation

Hitherto TiO₂ is the most popular catalyst for photocatalytic H₂ generation and reduction of organic pollutants due to its chemical inertness, high activity, and abundance. In this work, ordered mesoporous carbon CMK-8 supported TiO₂ nanoparticles loaded with non-noble metal Ni, denoted as NiTiO₂@CMK-8 are fabricated for application in photocatalytic H₂ generation. Our results demonstrate that the developed composite exhibit exceptional photocatalytic activity for H₂ generation (3556 μmol g⁻¹) under 300 W Xe lamp irradiation (with an external quantum efficiency of 37.9%), which is around 10 times higher than pure TiO₂ and 4 times that of TiO₂@CMK-8. The enhanced photocatalytic performance is attributed to the well-dispersed TiO₂ in the ordered mesoporous carbon structure of CMK-8. The performance is boosted by the black body-like light absorbance of CMK-8 and plasmonic effect of Ni. Enhanced UV–visible light absorbance, large surface area and the loaded Ni metal synergistically improve the charge-carrier kinetics to attain highly efficient photocatalytic H₂ generation. The NiTiO₂@CMK-8 composite shows enhanced activity for the reduction of 4-nitrophenol as well.     

Fabrication of a mesoporous Pt-carbon catalyst by the direct templating of mesoporous Pt-alumina for the methanol electro-oxidation

Mesoporous Pt-carbon catalysts were directly fabricated using mesoporous Pt-alumina as a template with a metal source and using poly(divinylbenzene) as a carbon precursor. Two types of mesoporous Pt-alumina templates were prepared by employing different calcination conditions (PtAl-A and PtAl-N were produced by the calcination in a stream of air and nitrogen, respectively). Both the mesoporous Pt-aluminas served as efficient templates for the fabrication of replicated Pt-carbon catalysts (PtC-A and PtC-N). The PtC catalysts showed high surface area with a narrow pore size distribution centered at ca. 4.0 nm. Together with pore-confined metal growth, the characteristic feature of the template, such as a strong interaction of metal species with the support was beneficial for the formation of highly dispersed Pt particles on the replicated mesoporous carbon catalysts. The mesoporous Pt-carbon (PtC) catalysts exhibited a higher metal dispersion than Pt catalyst impregnated on CMK-3 (Pt/CMK-3). Futhermore, the PtC-N catalyst exhibited a higher metal dispersion than the PtC-A catalyst. Methanol electro-oxidation experiments revealed the catalytic performance was closely related to the metal dispersion in the supported catalysts. The PtC-N catalyst with the highest metal dispersion exhibited the best catalytic performance in methanol electro-oxidation.     

Nanoconfinement degradation in NaAlH4/CMK-1

An ordered mesoporous carbon scaffold (CMK-1) has been synthesized and infiltrated with NaAlH₄ nanoparticles by solvent- and melt-infiltration techniques. Small angle X-ray scattering (SAXS), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission Electron Microscopy (TEM) and energy dispersive spectroscopy (EDS) are used to characterize the structure, composition and morphology before and after thermal treatment. This study illuminates some of the problems that can be associated with nanoconfinement of hydrogen storage materials including scaffold contamination, residual solvent contamination, sample morphology changes after heating, and other factors that can be detrimental to the application of these systems. Of particular interest is the expulsion of NaAlH₄ decomposition products from the scaffold after heating beyond its melting point under vacuum. This results in the surface of mesoporous carbon particles having arrays of multi-micron-long Al filaments that are >100 nm in diameter.