The quest for an efficient process to convert methane efficiently to fuels and high value-added chemicals such as olefins and aromatics is motivated by their increasing demands and recently discovered large reserves and resources of methane. Direct conversion to these chemicals can be realized either oxidatively via oxidative coupling of methane (OCM) or nonoxidatively via methane dehydroaromatization (MDA), which have been under intensive investigation for decades. While industrial applications are still limited by their low yield (selectivity) and stability issues, innovations in new catalysts and concepts are needed. The newly emerging strategy using iron single sites to catalyze methane conversion to olefins, aromatics, and hydrogen (MTOAH) attracted much attention when it was reported. Because the challenge lies in controlled dehydrogenation of the highly stable CH4 and selective C–C coupling, we focus mainly on the fundamentals of C–H activation and analyze the reaction pathways toward selective routes of OCM, MDA, and MTOAH. With this, we intend to provide some insights into their reaction mechanisms and implications for future development of highly selective catalysts for direct conversion of methane to high value-added chemicals.

This study presents the discovery that porous boron nitride (p-BN) is active in acetylene hydrochlorination, although boron nitride (BN) is generally considered chemically inert. An acetylene conversion of 99% is achieved with a vinyl chloride selectivity over 99% at 280 °C at a gas hourly space velocity (GHSV) of 1.32 mL min–1 g–1. By contrast, the commercially available crystallized hexagonal BN (h-BN) exhibits no catalytic activity. Furthermore, this p-BN is rather durable as demonstrated by a 1000 h lifetime test. Catalytic tests, spectroscopic characterization, and theoretical calculations indicate that the activity likely originates from the defects and edge sites. Particularly, the armchair edges of BN can polarize and activate acetylene, which then reacts with gaseous HCl giving vinyl chloride as the product.Keywords: acetylene hydrochlorination; mercury-free catalyst; metal-free catalyst; porous boron nitride; stability;

The key of syngas (a mixture of CO and H2) chemistry lies in controlled dissociative activation of CO and C–C coupling. We demonstrate here that a bifunctional catalyst of partially reducible manganese oxide in combination with SAPO-34 catalyzes the selective formation of light olefins, which validates the generality of the OX-ZEO (oxide-zeolite) concept for syngas conversion. Results from in situ ambient-pressure X-ray photoelectron spectroscopy, infrared spectroscopy, and temperature-programmed surface reactions reveal the critical role of oxygen vacancies on the oxide surface, where CO dissociates and is converted into surface carbonate and carbon species. They are converted to CO2 and CHx in the presence of H2. The limited C–C coupling and hydrogenation activities of MnO enable the reaction selectivity to be controlled by the confined pores of SAPO-34. Thus, a selectivity of light olefins up to 80% is achieved, far beyond the limitation of Anderson–Shultz–Flory distribution. These findings open up possibilities to explore other active metal oxides for more efficient syngas conversion.Keywords: bifunctional catalysts; CO dissociation; heterogeneous catalysis; light olefins; manganese oxides; oxide-zeolite; syngas chemistry;

With increasing concern about the environmental impact of shale gas exploitation, nonaqueous fracturing with carbon dioxide has emerged as a promising alternative to increase gas production and, at the same time, to store large amounts of CO2. The key process of CH4 displacement by CO2 is worth a systematic investigation from aspects of both experiment and simulation. In this work, the CH4 and CO2 displacement was studied with in situ 13C NMR in the pores of silica (SBA-15), which were functionalized with organic groups such as phenyl and cyclohexyl, in order to model the organic matter in shale with different aromaticity. Due to the stronger adsorption strength and higher capacity of CO2 in SBA-15, CH4 can be easily stripped out of the pores by CO2, while the reverse process to displace CO2 with CH4 is not effective. Even though the displacement effect in the pores of SBA-15 with a higher aromaticity is relatively better at room temperature, the superiority is eliminated by high temperature. Furthermore, the results of pulse field gradient (PFG) NMR demonstrate that the self-diffusion coefficient of CO2 is an order of magnitude smaller than that of CH4, and the existence of CO2 slows down the diffusion of CH4 slightly. The gas diffusion in both scenarios follows the trend: SBA-15 > SBA-phenyl > SBA-cyclohexyl.

•A novel composite SiC@PDA is prepared.•The composite showed good activity as metal free catalyst in acetylene hydrochlorination.•The reason leading to the catalyst deactivation is found.•A regeneration method and the regeneration mechanism are proposed.Acetylene hydrochlorination is an important coal-based technology for production of vinyl chloride, the monomer of one of the world mostly used plastics. Despite of the great potentials demonstrated for carbon-based catalysts to replace the toxic mercury chloride, the stability and the deactivation mechanism are rarely discussed, which is essential for real applications. Herein, we present a detailed study on the deactivation mechanism of nitrogen doped carbon based catalyst in acetylene hydrochlorination. The results show that the deactivation was likely caused by the carbon-like deposition over the catalyst, which can be regenerated with high temperature NH3 treatment.Download high-res image (119KB)Download full-size image

An increasing number of studies have demonstrated that confinement within carbon nanotubes (CNTs) provides an effective approach for the modulation of catalysis. It was generally predicted that confinement became stronger with a decreasing diameter of CNTs. However, our present study here overturns the previous expectation: the influence on catalysis is not monotonic. Instead, it exhibits a volcano relationship with CNT diameter. Taking Pt catalyzing O2 conversion and Re catalyzing N2 conversion as probes using density functional theory, we show that only within tubes with an i.d. of ∼1 nm can the activity of metal clusters be enhanced to its maximum. Furthermore, confinement only enhances the catalytic activity of metals with strong intrinsic binding with reactants, whereas it is suppressed for those with weak binding. These findings shed further light on the fundamental effects of confinement on catalysis, and could guide more rational design of confined catalysts.

A composite catalyst combining the partially reducible ZnCrOx with zeolite ZSM-5 enables direct conversion of syngas to aromatics, with a selectivity to aromatics reaching 73.9% at a single pass CO conversion of 16.0%.

We demonstrate that the methanation activity of Ni catalysts is modulated by the crystal phase of the TiO2 support. The rutile TiO2 (r-TiO2)-supported Ni catalyst gives a turnover frequency almost two orders of magnitude higher than the anatase TiO2 (a-TiO2)-supported catalyst although the two catalysts exhibit a similar mean Ni particle size. Characterization by H2-TPR, H2-TPD, XPS and IR reveals a stronger interaction between Ni and r-TiO2, leading to a higher capability of CO activation and hydrogenation activity of Ni although Ni is partially covered by TiOx overlayers during reaction. Interestingly, the modulating effects of the titania crystal phases are much more pronounced on the activity of Ni in CO2 methanation.

To tailor the catalytic activities of metal catalysts at will to achieve efficient conversion in chemical processes remains a challenge, particularly for noble metals, such as Pt. We demonstrate herein that encapsulation within the carbon nanotube (CNT) channels with a diameter of 1.0–1.5 nm not only allows restriction of the size of Pt nanoclusters around 1.0 nm but also enables modulating of the Pt species at the active reduced states through host–guest interaction. The encapsulated Pt is protected from oxygen under reaction conditions in toluene oxidation up to 200 °C, as unveiled by in situ X-ray absorption spectroscopy and density functional theory calculations. As a result, the encapsulated Pt clusters deliver a remarkably higher activity and stability than the clusters located on the open surfaces of the CNT exterior walls and carbon black support, although the latter are much more accessible to reactants. This characteristic of the CNT channels can be explored to tune the properties of other metal catalysts for oxidation reactions.Keywords: carbon nanotube; cluster; confined catalysis; platinum; toluene oxidation

Rhenium (Re), a high-performance engineering material with a hexagonal close-packed (hcp) structure, remains stable even under pressures of up to 250 GPa and at temperatures up to its melting point (3453 K). We observed here that Re atoms self-assembled, within the confined space of carbon nanotubes (CNTs) with a diameter of <1.5 nm, into ultrathin nanowires stacking with an unusual face-centered cubic (fcc) structure along the CNTs. In contrast, only Re nanoparticles of hcp structure formed on an open surface of graphite and carbon black. Aberration-corrected electron microscopy unambiguously showed the atomic arrangements of the Re nanowires and their confinement within the CNTs, ∼80% exhibiting a four-atom and 15% a nine-atom configuration. Density functional theory calculations confirmed that the formation of unusual fcc-stacking Re nanowires is largely facilitated by the strong interaction between Re atoms and CNTs and the spatial restriction within the CNTs. The use of CNTs as nanoscale reactors to create novel structures not only is fundamentally interesting but also may find unique applications in catalysis, sensing, and nanoelectronics.

An increasing number of experimental studies have demonstrated that metal or metal oxide nanoparticles confined inside carbon nanotubes (CNTs) exhibit different catalytic activities with respect to the same metals deposited on the CNT exterior walls, with some reactions enhanced and others hindered. In this article, we describe the concept of confinement energy, which enables prediction of confinement effects on catalytic activities in different reactions. Combining density functional theory calculations and experiments by taking typical transition metals such as Fe, FeCo, RhMn, and Ru as models, we observed stronger strains and deformations within the CNT channels due to different electronic structures and spatial confinement. This leads to downshifted d-band states, and consequently the adsorption of molecules such as CO, N2, and O2 is weakened. Thus, the confined space of CNTs provides essentially a unique microenvironment due to the electronic effects, which shifts the volcano curve of the catalytic activities toward the metals with higher binding energies. The extent of the shift depends on the specific metals and the CNT diameters. This concept generalizes the diverse effects observed in experiments for different reactions, and it is anticipated to be applicable to an even broader range of reactions other than redox of metal species, CO hydrogenation, ammonia synthesis and decomposition discussed here.

We demonstrate here a concept that chemical reactions can be enhanced by utilizing the confined hydrophobic environment of carbon nanotube (CNT) channels to separate products from reactants during a reaction and hence shift the reaction equilibrium. Taking the hydroxylation of benzene to phenol as an example, we observed that benzene is enriched inside CNT channels while the product phenol was discriminatively expelled out of the channels, as shown by solid state NMR studies. Consequently, the reaction over a CNT-confined Re catalyst exhibited a 4 times higher activity than the same catalyst dispersed on the outer walls of the same CNTs. The effect of this selective enrichment of benzene on the reaction was further confirmed by varying the amount of benzene in the reaction over commercial activated carbon-supported catalyst. CNT channels discriminating hydrophobic from hydrophilic molecules are expected to be a general feature. It is of significance for many synthetic organic processes involving molecules with different hydrophobicity in the reactants and products.

An efficient anode with a superior high-rate capability and stability remains a challenge for development of high performance Li-ion batteries. We present a new concept by encapsulating 2 nm-sized SnO2 nanocrystals in the channels of carbon nanotubes (SnO2-in-CNTs). Characterization shows that the confined space does not only stabilize the small nanoparticles but also alleviates the stress caused by the large volume change of tin species during the charging–discharging process. In addition, well crystallized graphitic structure of CNTs with a positive curvature provides a good contact between SnO2 nanoparticles and graphene layers, and excellent electronic conductivity. As a result, SnO2-in-CNTs as an anode of lithium ion battery exhibit stable cyclability and superior high-rate capability relative to SnO2 nanoparticles dispersed on the outer walls of CNTs particularly under a high current density. The charge capacity remains about 560 mA h g−1 after 50 cycles at 50 mA g−1, and even around 400 mA h g−1 at 1000 mA g−1. Additionally, the facile preparation method we have developed makes such encapsulates appealing for further optimization and applications.

A C–SiC composite with a thin layer of carbon surrounding the SiC substrate has been produced by the reaction of SiC with CCl4. The pore structures, graphitization levels and the chemical compositions can be finely modulated by the synthesis temperature, and atmosphere. A higher synthesis temperature accelerates the chlorination rate, increases the thickness of carbon layers and enhances their graphitization. Mesopores can be generated in C–SiC composites in comparison to predominant micropores in commercial activated carbon (AC), particularly in the presence of reactive atmosphere such as CO2 and NH3. Furthermore, with cofeeding of NH3 with CCl4, N heteroatoms can be incorporated into the carbon layer and the N content varies in a range of 4.7–9.5 at.%, depending on the synthesis conditions. Both increased fraction of mesopores and their sizes, as well as N doping facilitate significantly hydrogenation of 4-carboxybenzaldehyde. The activity of Pd catalyst supported on N-doped C–SiC is five times that on commercially used AC under the same conditions.

We demonstrate that reactions confined within single-walled carbon nanotube (SWCNT) channels are modulated by the metallic and semiconducting character of the hosts. In situ Raman and X-ray absorption near-edge structure spectroscopies provide complementary information about the electronic state of carbon nanotubes and the encapsulated rhenium species, which reveal electronic interactions between encapsulated species and nanotubes. More electrons are transferred from metallic tubes (m-SWCNTs) to oxidic rhenium clusters, leading to a lower valence state rhenium oxide than that in semiconducting tubes (s-SWCNTs). Reduction in 3.5% (vol/vol) H2/Ar leads to weakened host–guest electronic interaction. The high valence state Re within s-SWCNTs is more readily reduced when raising the temperature, whereas only a sluggish change is observed for Re within m-SWCNTs. Only at 400 °C does Re reach a similar electronic state (mixture of Re0 and Re4+) in both types of tubes. Subsequent oxidation in 1% O2/Ar does not show changes for Re in s-SWCNTs up to 200 °C. In comparison, m-SWCNTs facilitate the oxidation of reduced rhenium (160 °C). This can be exploited for rational design of active catalysts with stable species as a desired valence state can be obtained by selecting specific-type SWCNTs and a controlled thermal treatment. These results also provide a chemical approach to modulate reversibly the electronic structure of SWCNTs without damaging the sidewalls of SWCNTs.

Carbon has been widely used as a catalyst support and adsorbent in industry. However, it suffers from poor stability due to its limited mechanical strength, particularly under high pressures and temperatures. We report here a carbide derived carbon (CDC) layer on a porous SiC surface, which has the properties of high mechanical strength and is easy to shape. The CDC exhibits an amorphous structure and contains mainly mesopores with a BET surface area of 125 m2 g−1. The CDC–SiC composite yields a comparable performance to coconut activated carbon (AC) as a catalyst support in the probe reaction hydrogenation of 4-carboxybenzaldehyde. The further introduction of TiO2 nanoparticles enhances the activity and stability significantly because of the improved dispersion of Pd particles on CDC–SiC. The activity is 4 times higher than the Pd/AC catalyst. Pd–TiO2/CDC–SiC shows great promise as an alternative to the current AC supported Pd catalyst for the crude terephthalic acid hydropurification industry.

Cubic FeN particles of a few nanometers in size were synthesized for the first time by encapsulation inside carbon nanotube (CNT) channels. Such an FeN catalyst exhibits a 5–7 times higher activity than a reduced Fe catalyst and a SiO2 supported iron nitride in CO hydrogenation. The confined FeN catalyst is also more active than iron nitride particles dispersed on the CNT exterior walls.

The unique tubular morphology of carbon nanotubes (CNTs) has trig-gered wide research interest. These structures can be used as nanoreactors and to create novel composites through the encapsulation of guest materials in their well-defined channels. The rigid nanotubes restrict the size of the encapsulated materials down to the nanometer and even the sub-nanometer scale. In addition, interactions may develop between the encapsulated molecules and nanomaterials and the CNT surfaces. The curvature of CNT walls causes the π electron density of the graphene layers to shift from the concave inner to the convex outer surface, which results in an electric potential difference. As a result, the molecules and nanomaterials on the exterior walls of CNTs likely display different properties and chemical reactivities from those confined within CNTs. Catalysis that utilizes the interior surface of CNTs was only explored recently. An increasing number of studies have demonstrated that confining metal or metal oxide nanoparticles inside CNTs often leads to a different catalytic activity with respect to the same metals deposited on the CNT exterior surface. Furthermore, this inside and outside activity difference varies based on the metals used and the reactions catalyzed.In this Account, we describe the efforts toward understanding the fundamental effects of confining metal nanoparticles inside the CNT channels. This research may provide a novel approach to modulate their catalytic performance and promote rational design of catalysts. To achieve this, we have developed strategies for homogeneous dispersion of nanoparticles inside nanotubes. Because researchers have previously demonstrated the insertion of nanoparticles within larger nanotubes, we focused specifically on multiwalled carbon nanotubes (MWCNTs) with an inner diameter (i.d.) smaller than 10 nm and double-walled carbon nanotubes (DWCNTs) with 1.0–1.5 nm i.d. The results show that CNTs with well-defined morphology and unique electronic structure of CNTs provide an intriguing confinement environment for catalysis.

Theoretical studies predicted that doping graphene with nitrogen can tailor its electronic properties and chemical reactivity. However, experimental investigations are still limited because of the lack of synthesis techniques that can deliver a reasonable quantity. We develop here a novel method for one-pot direct synthesis of N-doped graphene via the reaction of tetrachloromethane with lithium nitride under mild conditions, which renders fabrication in gram scale. The distinct electronic structure perturbation induced by the incorporation of nitrogen in the graphene network is observed for the first time by scanning tunnelling microscopy. The nitrogen content varies in the range of 4.5−16.4%, which allows further modulation of the properties. The enhanced catalytic activity is demonstrated in a fuel cell cathode oxygen reduction reaction with respect to pure graphene and commercial carbon black XC-72. The resulting N-doped materials are expected to broaden the already widely explored potential applications for graphene.Keywords (keywords): electrocatalysis; graphene; N-doped; oxygen reduction reaction; solvothermal;

Crystals of graphite nanosheets, achieved via a simple ball milling approach, show a significant size effect in electrocatalytic activation of oxygen.

The coating of nanoscaled carboxylated carbonaceous fragments on carbon nanotubes (CNTs) has been directly observed in chemical imaging with a concurrent identification of their electronic structure by scanning transmission X-ray microscopy. The coating also shields the detection of the CNT/nanoparticle interaction.

Homogeneous dispersion of metal oxide nanoparticles was achieved on carbon nanotubes (CNTs) even with a very small amount of surface oxygen functional groups (SOFGs) aided by using ethylene glycol (EG) and sodium hydroxide during the process. Similar particle size distributions were obtained for iron deposited on CNTs containing various amounts of SOFGs. We proposed that formation of hydrogen bonds between EG on the CNT surface and sodium hydroxide is likely responsible, which creates precipitating sites for iron ions on the CNT surface. This facile method is expected to find applications not only for catalysis but also in the fields such as sensors and magnetic materials in particular where a perfect sp2 hybridized carbon structure is preferred.Uniform dispersion of metal oxide nanoparticles was achieved on carbon nanotubes (CNTs) even with a very small amount of surface oxygen functional groups (SOFGs), mediated by ethylene glycol and hydroxide.Research Highlights► Metal nanoparticles have been uniformly dispersed on carbon nanotube (CNT) surfaces ► Particle size distribution does not vary with the amount of SOFGs groups on CNTs. ► A similar particle size distribution has been achieved for several transition metal oxides. ► It was mediated by the co-presence of ethylene glycol and NaOH on CNT surface.

With the in situ generated H2O2 tailored by the addition of p-tetrachlorobenzoquinone, the product can be effectively steered towards either HCOOH or the methanol derivative CF3COOCH3 during the direct oxidation of methane with molecular oxygen over palladium catalyst.

A theoretical study combining first-principles and Monte Carlo simulations has been carried out to investigate the interactions of H2 and CO molecules with carbon nanotube (CNT) surfaces. The results show that there are stronger interactions of both H2 and CO with the interior nanotube surface than with the exterior surface. In addition, CO interacts more strongly with CNT surfaces than H2. This can be explained by the nature of the molecules and the different electronic properties of the concave and convex surfaces of CNTs formed by graphene layers. As a result, syngas molecules are enriched inside CNTs and the enrichment generally becomes greater in smaller nanotubes. Furthermore, the ratio of CO/H2 inside CNTs increases with respect to the composition of syngas in the exterior gas phase. The enriched reactants and altered CO/H2 ratio inside nanotubes could be beneficial for the reaction rate and lead to modification of the product selectivity.

A novel method has been developed for homogeneous dispersion of metal nanoparticles inside short carbon nanotubes (CNTs) with an inner diameter smaller than 10 nm. The process involves controlled cutting of pristine long nanotubesviaoxidation catalyzed by Ag or Fe and introduction of metal nanoparticles inside the CNT channels using a wet chemistry method aided by ultrasonic treatment and extended stirring. The resulting metal particles are very uniform with sizes in the range of 2–4 nm. In addition, selective dispersion of such nanoparticles on the exterior surfaces of open CNTs has been achieved by temporary blocking of the channels with an organic solvent while decorating the CNT exterior surfaces with aqueous solution of the metal salt. The trick is the choice of this organic solvent, which is immiscible with, and has a higher boiling point than, water.

We review a new concept for modifying the redox properties of transition metals via confinement within the channels of carbon nanotubes (CNTs), and thus tuning their catalytic performance. Attention is also devoted to novel techniques for homogeneous dispersion of metal nanoparticles inside CNTs since these are essential for optimization of the catalytic activity.

Hydrogenation of benzaldehyde is a typical consecutive reaction, since the intermediate benzyl alcohol is apt to be further hydrogenated. Here we demonstrate that the selectivity of benzyl alcohol can be tuned via functionalization of carbon nanotubes (CNTs), which are used as the support of Pd. With the original CNTs, the selectivity of benzyl alcohol is 88% at a 100% conversion of benzaldehyde. With introduction of oxygen-containing groups onto CNTs, it drops to 27%. In contrast, doping CNTs with N atoms, the selectivity reaches 96% under the same reaction conditions. The kinetic study shows that hydrogenation of benzyl alcohol is significantly suppressed, which can be attributed to weakened adsorption of benzyl alcohol. This is most likely related to the modified electronic structure of Pd species via interaction with functionalized CNTs, as shown by XPS characterization.

Commercial production of vinyl chloride from acetylene relies on the use of HgCl2 as the catalyst, which has caused severe environmental problem and threats to human health because of its toxicity. Therefore, it is vital to explore alternative catalysts without mercury. We report here that N-doped carbon can catalyze directly transformation of acetylene to vinyl chloride. Particularly, N-doped high surface area mesoporous carbon exhibits a rather high activity with the acetylene conversion reaching 77% and vinyl chloride selectivity above 98% at a space velocity of 1.0 mL·min−1·g −1 and 200 °C. It delivers a stable performance within a test period of 100 h and no obvious deactivation is observed, demonstrating potentials to substitute the notoriously toxic mercuric chloride catalyst.Recent advances in the development of metal free catalyst using nitrogen doped ordered mesoporous carbon as catalyst in acetylene hydrochlorination to produce vinyl chloride was highlighted.Download full-size image

Ethanol is considered as a potential alternative synthetic fuel to be used in automobiles or as a potential source of hydrogen for fuel cells. In this paper we first undertake a brief overview of the catalyst development for syngas conversion to C2 oxygenates over Rh-based catalysts, mainly on the effects of various additives and supports on the activity and selectivity. Then we investigated the effects of carbon materials, which have been rarely studied as supports for Rh-based catalysts in this process. For example, rather well graphitized carbon black, very high surface area CMK-3 and activated carbon (AC) were compared to carbon nanotubes (CNTs), which exhibits a medium level surface area with well defined nanochannels. The CNT-supported catalyst shows a highest overall activity and yield of C2 oxygenates compared to the other carbon-supported catalysts. The catalysts are characterized by N2 adsorption–desorption, CO chemisorption, TEM, XRD and TPD. The graphitized structure combined with the tubular morphology of CNTs likely play an important role.

An increasing number of studies have demonstrated that confinement within carbon nanotubes (CNTs) provides an effective approach for the modulation of catalysis. It was generally predicted that confinement became stronger with a decreasing diameter of CNTs. However, our present study here overturns the previous expectation: the influence on catalysis is not monotonic. Instead, it exhibits a volcano relationship with CNT diameter. Taking Pt catalyzing O2 conversion and Re catalyzing N2 conversion as probes using density functional theory, we show that only within tubes with an i.d. of ∼1 nm can the activity of metal clusters be enhanced to its maximum. Furthermore, confinement only enhances the catalytic activity of metals with strong intrinsic binding with reactants, whereas it is suppressed for those with weak binding. These findings shed further light on the fundamental effects of confinement on catalysis, and could guide more rational design of confined catalysts.

We demonstrate that the methanation activity of Ni catalysts is modulated by the crystal phase of the TiO2 support. The rutile TiO2 (r-TiO2)-supported Ni catalyst gives a turnover frequency almost two orders of magnitude higher than the anatase TiO2 (a-TiO2)-supported catalyst although the two catalysts exhibit a similar mean Ni particle size. Characterization by H2-TPR, H2-TPD, XPS and IR reveals a stronger interaction between Ni and r-TiO2, leading to a higher capability of CO activation and hydrogenation activity of Ni although Ni is partially covered by TiOx overlayers during reaction. Interestingly, the modulating effects of the titania crystal phases are much more pronounced on the activity of Ni in CO2 methanation.

Crystals of graphite nanosheets, achieved via a simple ball milling approach, show a significant size effect in electrocatalytic activation of oxygen.

We review a new concept for modifying the redox properties of transition metals via confinement within the channels of carbon nanotubes (CNTs), and thus tuning their catalytic performance. Attention is also devoted to novel techniques for homogeneous dispersion of metal nanoparticles inside CNTs since these are essential for optimization of the catalytic activity.

A novel method has been developed for homogeneous dispersion of metal nanoparticles inside short carbon nanotubes (CNTs) with an inner diameter smaller than 10 nm. The process involves controlled cutting of pristine long nanotubesviaoxidation catalyzed by Ag or Fe and introduction of metal nanoparticles inside the CNT channels using a wet chemistry method aided by ultrasonic treatment and extended stirring. The resulting metal particles are very uniform with sizes in the range of 2–4 nm. In addition, selective dispersion of such nanoparticles on the exterior surfaces of open CNTs has been achieved by temporary blocking of the channels with an organic solvent while decorating the CNT exterior surfaces with aqueous solution of the metal salt. The trick is the choice of this organic solvent, which is immiscible with, and has a higher boiling point than, water.

Carbon has been widely used as a catalyst support and adsorbent in industry. However, it suffers from poor stability due to its limited mechanical strength, particularly under high pressures and temperatures. We report here a carbide derived carbon (CDC) layer on a porous SiC surface, which has the properties of high mechanical strength and is easy to shape. The CDC exhibits an amorphous structure and contains mainly mesopores with a BET surface area of 125 m2 g−1. The CDC–SiC composite yields a comparable performance to coconut activated carbon (AC) as a catalyst support in the probe reaction hydrogenation of 4-carboxybenzaldehyde. The further introduction of TiO2 nanoparticles enhances the activity and stability significantly because of the improved dispersion of Pd particles on CDC–SiC. The activity is 4 times higher than the Pd/AC catalyst. Pd–TiO2/CDC–SiC shows great promise as an alternative to the current AC supported Pd catalyst for the crude terephthalic acid hydropurification industry.

An efficient anode with a superior high-rate capability and stability remains a challenge for development of high performance Li-ion batteries. We present a new concept by encapsulating 2 nm-sized SnO2 nanocrystals in the channels of carbon nanotubes (SnO2-in-CNTs). Characterization shows that the confined space does not only stabilize the small nanoparticles but also alleviates the stress caused by the large volume change of tin species during the charging–discharging process. In addition, well crystallized graphitic structure of CNTs with a positive curvature provides a good contact between SnO2 nanoparticles and graphene layers, and excellent electronic conductivity. As a result, SnO2-in-CNTs as an anode of lithium ion battery exhibit stable cyclability and superior high-rate capability relative to SnO2 nanoparticles dispersed on the outer walls of CNTs particularly under a high current density. The charge capacity remains about 560 mA h g−1 after 50 cycles at 50 mA g−1, and even around 400 mA h g−1 at 1000 mA g−1. Additionally, the facile preparation method we have developed makes such encapsulates appealing for further optimization and applications.

We demonstrate here a concept that chemical reactions can be enhanced by utilizing the confined hydrophobic environment of carbon nanotube (CNT) channels to separate products from reactants during a reaction and hence shift the reaction equilibrium. Taking the hydroxylation of benzene to phenol as an example, we observed that benzene is enriched inside CNT channels while the product phenol was discriminatively expelled out of the channels, as shown by solid state NMR studies. Consequently, the reaction over a CNT-confined Re catalyst exhibited a 4 times higher activity than the same catalyst dispersed on the outer walls of the same CNTs. The effect of this selective enrichment of benzene on the reaction was further confirmed by varying the amount of benzene in the reaction over commercial activated carbon-supported catalyst. CNT channels discriminating hydrophobic from hydrophilic molecules are expected to be a general feature. It is of significance for many synthetic organic processes involving molecules with different hydrophobicity in the reactants and products.

With the in situ generated H2O2 tailored by the addition of p-tetrachlorobenzoquinone, the product can be effectively steered towards either HCOOH or the methanol derivative CF3COOCH3 during the direct oxidation of methane with molecular oxygen over palladium catalyst.

The coating of nanoscaled carboxylated carbonaceous fragments on carbon nanotubes (CNTs) has been directly observed in chemical imaging with a concurrent identification of their electronic structure by scanning transmission X-ray microscopy. The coating also shields the detection of the CNT/nanoparticle interaction.