A new pathway for heterogenization of molecular catalysts by non-covalent interactions with carbon nanoreactors

A novel approach to heterogenisation of catalytic molecules is demonstrated using the nanoscale graphitic step-edges inside hollow graphitised carbon nanofibres (GNFs). The presence of the fullerene C60 moiety within a fullerene-salen CuII complex is essential for anchoring the catalyst within the GNF nanoreactor as demonstrated by comparison with the analogous catalyst complex without the fullerene group. The presence of the catalyst at the step-edges of the GNFs is confirmed by high resolution transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) with UV/Vis spectroscopy demonstrating only negligible (c.a. 3 %) desorption of the fullerene-salen CuII complex from the GNFs into solution under typical reaction conditions. The catalyst immobilised in GNFs shows good catalytic activity and selectivity towards styrene epoxidation, comparable to the analogous catalyst in solution. Moreover, the fullerene-salen CuII complex in GNFs demonstrates excellent stability and recyclability as it can be readily separated from the reaction mixture and employed in multiple reaction cycles with minimal loss of activity, which is highly advantageous compared to catalysts not stabilised by the graphitic step-edges that desorb rapidly from GNFs

Structural changes of thermal sprayed graphene nano platelets film into amorphous carbon under sliding wear

© 2020 The Authors Graphene has become a promising candidate to protect surfaces against friction due to its strength and lubricating ability. In this study, graphene nano platelets (GNP) thin films have been deposited onto stainless steel substrates by axially injecting GNP suspension through high velocity oxy fuel thermal spray gun. The tribological performance of the films under dry sliding wear was investigated through unlubricated ball on disc sliding wear test against a sintered alumina counter body ball under 5 N load. The understanding of the behaviour of the GNPs under sliding wear will be useful for improving the performance of graphene-based coatings which are in demand for wear resistant applications. A film was deposited showing significant improvements in friction with coefficient of friction value reduced by 7 times compared to uncoated stainless steel, even for a discontinuous film. A morphological analysis shows sliding wear led to change in particle shape from angular flakes into randomly oriented circles. Interatomic bonding and structural analysis performed reveals oxidation defect formations during wear test. Structural degradation and oxidation of GNPs during the process led to formation of amorphous carbon from graphene. Amorphous carbon formation reduces the lubricating ability and strength of the film, leading to failure

Movement of palladium nanoparticles in hollow graphitised nanofibres: the role of migration and coalescence in nanocatalyst sintering during the Suzuki–Miyaura reaction

The evolution of individual palladium nanoparticle (PdNP) catalysts, in graphitised nanofibres (GNF), in the liquid-phase Suzuki-Miyaura (SM) reaction has been appraised. The combination of identical location-transmission electron microscopy (IL-TEM) and a nano test tube approach allowed spatiotemporal continuity of observations at single nanopartcile level, revealing that migration and coalescence is the most significant pathway to coarsening of the nanocatalyst, rather than Ostwald ripening. IL-TEM gave unprecedented levels of detail regarding the movement of PdNP on carbon surfaces at the nanoscale, including size-dependent migration and directional movement, opening horizons for optimisation of future catalysts through surface morphology design

Molybdenum dioxide in carbon nanoreactors as a catalytic nanosponge for the efficient desulfurization of liquid fuels

The principle of a “catalytic nanosponge” that combines the catalysis of organosulfur oxidation and sequestration of the products from reaction mixtures is demonstrated. Group VI metal oxide nanoparticles (CrOx, MoOx, WOx) are embedded within hollow graphitized carbon nanofibers (GNFs), which act as nanoscale reaction vessels for oxidation reactions used in the decontamination of fuel. When immersed in a model liquid alkane fuel contaminated with organosulfur compounds (benzothiophene, dibenzothiophene, dimethyldibenzothiophene), it is found that MoO2@GNF nanoreactors, comprising 30 nm molybdenum dioxide nanoparticles grown within the channel of GNFs, show superior abilities toward oxidative desulfurization (ODS), affording over 98% sulfur removal at only 5.9 mol% catalyst loading. The role of the carbon nanoreactor in MoO2@GNF is to enhance the activity and stability of catalytic centers over at least 5 cycles. Surprisingly, the nanotube cavity can selectively absorb and remove the ODS products (sulfoxides and sulfones) from several model fuel systems. This effect is related to an adsorptive desulfurization (ADS) mechanism, which in combination with ODS within the same material, yields a “catalytic nanosponge” MoO2@GNF. This innovative ODS and ADS synergistic functionality negates the need for a solvent extraction step in fuel desulfurization and produces ultralow sulfur fuel

Formation of hollow carbon nanoshells from thiol stabilised silver nanoparticles via heat treatment

Uniform, less 10 nm sized, hollow carbon nano-shells (HCNS) have been prepared via a single-step, thermal treatment of alkanethiol stabilised Ag nanoparticles (TS-AgNP). Direct evidence for the formation of spherical HCNS from TS-AgNP is provided by in situ MEMS heating on Si3N4 supports within a TEM, and ex situ thermal processing of TS-AgNP on carbon nanotube supports. A mechanism is proposed for the thermally driven, templated formation of HCNS from the TS-AgNP stabilising layer, with Ag catalysing the graphitisation of carbon in advance of thermally induced AgNP template removal. This facile processing route provides for excellent size control of the HCNS product via appropriate AgNP template selection. However, a rapid rate of heating was found to be crucial for the formation of well-defined HCNS, whilst a slow heating rate gave a much more disrupted product, comprising predominantly lacy carbon with decreased levels of graphitic ordering, reflecting a competition between the thermal transformation of the TS-layer and the rate of removal of the AgNP template

Steric and electronic control of 1,3-dipolar cycloaddition reactions in carbon nanotube nanoreactors

The use of single-walled carbon nanotubes as effective nanoreactors for preparative bimolecular reactions has been demonstrated for the first time. We show that the extreme spatial confinement of guest reactant molecules inside host carbon nanotubes increases the regioselectivity for 1,4-triazole in thermally initiated azide–alkyne cycloaddition reactions. Through comparison of the internal dimensions of the nanotube and the steric bulk of the reactants, we demonstrate that the formation of the more linear 1,4-regioisomer can be enhanced by up to 55% depending on the extent of spatial restrictions imposed within the nanoreactors. Furthermore, through systematic variation of the substituents in the para-position of the alkyne reactants, we reveal the unexpected influence of the reactants’ electronic properties on the regioselectivity of reactions within nanoreactors, which act to either oppose or promote the preferential formation of the 1,4-regioisomer induced by steric effects, reflecting the unique ability of carbon nanotubes to stabilize the dipole moment of confined reactants. Thus, we show that the observed regioselectivity of azide–alkyne cycloaddition reactions confined within carbon nanotube nanoreactors reflects a subtle interplay between both steric and electronic factors

Synthesis of hydroxylated group IV metal oxides inside hollow graphitised carbon nanofibers: nano-sponges and nanoreactors for enhanced decontamination of organophosphates

The confinement and enhanced catalytic properties of hydroxylated group IV metal oxide nanostructures inside hollow graphitised carbon nanofibers (GNF) has been demonstrated. GNF – a structural analogue of carbon nanotubes – were effectively filled with suitable precursor molecules of metal chlorides from the gas and liquid phases. Subsequent basecatalysed hydrolysis afforded amorphous, nanostructured hydroxylated metal oxide (MOx(OH)y where M = Zr, Ti, and Hf) thin films, which coat the internal surfaces of GNF. This versatile and general strategy allows the chemical composition and morphology of the encapsulated material to be modified by varying the conditions used for hydrolysis and post-synthesis thermal treatment. The increased Lewis acidic properties and high surface area of the zirconium composite promote the catalysed hydrolysis of dimethyl nitrophenyl phosphate (DMNP) – a toxic organophosphorus chemical. A four-fold enhancement in the rate of DMNP hydrolysis relative to its separate constituent components was observed, highlighting the surprising synergistic abilities of this composite material to perform both as a ‘nano-sponge’, absorbing the harmful compounds inside the GNF, and a nanoreactor, enhancing the local concentration of organophosphate around the hydroxylated metal oxide species, leading to improved catalytic performance

Palladium nanoparticles in catalytic carbon nanoreactors: the effect of confinement on Suzuki-Miyaura reactions

We explore the construction and performance of a range of catalytic nanoreactors based on palladium nanoparticles encapsulated in hollow graphitised nanofibres. The optimum catalytic material, with small palladium nanoparticles located almost exclusively at the graphitic step-edges within nanoreactors, exhibits attractive catalytic properties in Suzuki-Miyaura cross-coupling reactions. Confinement of nanoparticles at the step-edges facilitates retention of catalytic centres and recycling of catalytic nanoreactors without any significant loss of activity or selectivity over multiple catalytic cycles. Furthermore, careful comparison of the catalytic properties of palladium nanoparticles either on or in nanoreactors reveals that nanoscale confinement of catalysts fundamentally affects the pathways of the Suzuki-Miyaura reaction, with the yield and selectivity for the cross-coupled product critically dependent on the steric properties of the aryl iodide reactant, whereas no effects of confinement are observed for aryl boronic acid reactants possessing substituents in different positions. These results indicate that the oxidative addition step of the Suzuki-Miyaura reaction occurs at the step-edge of nanofibres, where the mechanisms and kinetics of chemical reactions are known to be sensitive to nanoscale confinement, and thus the extent of confinement in carbon nanoreactors can be discretely controlled by careful selection of the aryl iodide reactant

Click chemistry in carbon nanoreactors

Copper-nanoparticle catalytic centres anchored at the 5 graphitic step-edges within hollow carbon nanoreactors exhibit superior activity and stability in cycloaddition reactions as compared to catalytic centres outside the nanoreactors. Nanoscale confinement enables efficient recycling of the catalyst in preparative-scale synthesis without 10 significant changes in activity

Tuning the interactions between electron spins in fullerene-based triad systems

A series of six fullerene-linker-fullerene triads have been prepared by the stepwise addition of the fullerene cages to bridging moieties thus allowing the systematic variation of fullerene cage (C60 or C70) and linker (oxalate or terephthalate) and enabling precise control over the inter-fullerene separation. The fullerene triads exhibit good solubility in common organic solvents, have linear geometries and are diastereomerically pure. Cyclic voltammetric measurements demonstrate the excellent electron accepting capacity of all triads, with up to 6 electrons taken up per molecule in the potential range between -2.3 and 0.2 V (vs. Fc+/Fc). No significant electronic interactions between fullerene cages are observed in the ground state indicating that the individual properties of each C60 or C70 cage are retained within the triads. The electron-electron interactions in the electrochemically generated dianions of these triads, with one electron per fullerene cage were studied by EPR spectroscopy. The nature of electron-electron coupling observed at 77 K can be described as an equilibrium between a doublet and triplet state biradical which depends on the interfullerene spacing. The shorter oxalate-bridged triads exhibit stronger spin-spin coupling with triplet character, while in the longer terephthalate-bridged triads the intramolecular spin-spin coupling is significantly reduced