Jaan Leis

Carbon nanostructures produced by chlorinating aluminium carbide

A number of carbon samples with different nanostructures such as: amorphous, nanoparticles and turbostratic, were synthesised through the reaction between aluminium carbide and gaseous chlorine at fixed temperatures between 300 and 900°C. The synthesised carbon samples were characterised using high-resolution transmission electron microscopy and X-ray powder diffraction techniques, as well as low temperature nitrogen sorption measurements. The carbon produced at T=300°C was amorphous with a surface area of ∼1400 m2 g−1. At 700°C, a large amount of carbon nanoparticles and with a lower surface area ∼710 m2 g−1 was obtained. At 900°C, mainly a turbostratic carbon with a surface area of ∼680 m2 g−1 was produced.

Catalytic effects of metals of the iron subgroup on the chlorination of titanium carbide to form nanostructural carbon

Abstract The effect of the reaction temperature and the metals of an iron subgroup on the thermo–chemical treatment of titanium carbide with a chlorine gas and their influence on the carbon structure obtained thereby was studied. Different analytical methods such as porosity measurements, X-ray diffraction spectrometry and a high-resolution electron microscopy revealed the catalytic behaviour of the above-mentioned metals, which appeared to support the formation of graphitised carbon at much lower temperatures compared to those needed for the ordinary thermo–chemical chlorination of titanium carbide.

Reverse Monte Carlo studies of nanoporous carbon from TiC

The structures of nanoporous carbon prepared by chlorination of TiC at five different temperatures (700–1100 °C) have been studied by means of reverse Monte Carlo modelling of neutron diffraction data S(q), 0.3<q<10.5 A−1, using an atomic configuration (8000 atoms) with a density corresponding to 0.62 of graphite. Four different starting models were tested: (i) random atom configuration, (ii) separated graphite sheets and (iii) two defect models created by removing atoms in a graphite structure to obtain the wanted density. To increase the feasibility of the resulting atom configurations, a number of hard and soft constraints were introduced into the software. The hard constraints were (i) a minimum C–C distance of 1.0 A, (ii) a co-ordination constraint for nearest-neighbour distances of up to 1.6 A to avoid zero- or single- co-ordinated atoms and (iii) no atoms between 1.7 and 2.1 A to avoid small unphysical peaks in the radial distribution function. A soft constraint was centred C–C–C angles around 120° with a variance of 6°. The best fit between observed and calculated S(q) was obtained for the defect models. An evaluation of the porosity and surface area corresponding to the atomic configuration showed a significant difference between the 700 and 1000 °C samples and the one prepared at 1100 °C in agreement with HREM and sorption studies.

Barrel-like carbon nanoparticles from carbide by catalyst assisted chlorination

Abstract The nanostructure of carbon materials synthesised via chlorination of various metal and metalloid carbides and their mixtures has been investigated by low-temperature nitrogen sorption, X-ray powder diffraction and high-resolution electron microscopy techniques. The carbon nanostructure and its crystallinity are strongly affected by transition metal catalysts and synthesis temperature. A clear relationship between carbon nanostructure formation and catalysts concentration revealed that only very low concentration (approximately 1 mg per gram of carbide) of Cobalt (Ni, Fe) in reaction medium supports the conversation of Al4C3 to nanobarrel-like carbon nanoparticles.

Nanoporous Carbide-Derived Carbon Material-Based Linear Actuators

Devices using electroactive polymer-supported carbon material can be exploited as alternatives to conventional electromechanical actuators in applications where electromechanical actuators have some serious deficiencies. One of the numerous examples is precise microactuators. In this paper, we show for first time the dilatometric effect in nanocomposite material actuators containing carbide-derived carbon (CDC) and polytetrafluoroetylene polymer (PTFE). Transducers based on high surface area carbide-derived carbon electrode materials are suitable for short range displacement applications, because of the proportional actuation response to the charge inserted, and high Coulombic efficiency due to the EDL capacitance. The material is capable of developing stresses in the range of tens of N cm-2. The area of an actuator can be dozens of cm2, which means that forces above 100 N are achievable. The actuation mechanism is based on the interactions between the high-surface carbon and the ions of the electrolyte. Electrochemical evaluations of the four different actuators with linear (longitudinal) action response are described. The actuator electrodes were made from two types of nanoporous TiC-derived carbons with surface area (SA) of 1150 m2 g-1 and 1470 m2 g-1, respectively. Two kinds of electrolytes were used in actuators: 1.0 M tetraethylammonium tetrafluoroborate (TEABF4) solution in propylene carbonate and pure ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITf). It was found that CDC based actuators exhibit a linear movement of about 1% in the voltage range of 0.8 V to 3.0 V at DC. The actuators with EMITf electrolyte had about 70% larger movement compared to the specimen with TEABF4 electrolyte.

Solvent polarity effects on the EZ conformational equilibrium of N-1-naphthylamides

Abstract The ratio of the E and Z conformers of N-alkyl-N-1-naphthylformamides has been measured in a series of solvents using NMR spectroscopy. A linear relationship was found between the free energy of the conformational equilibrium, ΔG0, and the relative dielectric permittivity of the solvent. The comparison of NMR data with quantum-chemically calculated SCRF heats of equilibria reveals that the solvent effect is a combination of both the electrostatic and specific solute-solvent interactions, the latter being directly connected to the solvent-induced sterical deformations of the solute molecule.

Dynamic processes in N-acylated 1,2-dihydro-2,2,4-trimethylbenzo(h)quinoline: A comparative study by NMR spectroscopy and quantum chemistry

Abstract The results of conformational studies on several N-acyl derivatives of 1,2-dihydro-2,2,4-trimethylbenzo(h)quinoline are reported. A comparative study by NMR spectroscopy and semiempirical quantum chemical modelling using the AM1 SCF method revealed that the nitrogen atom is pyramidal with a substantial out-of-plane torsion of the acyl group and that the molecules adopt the E conformation in the ground state. Also, the 1H NMR signals revealed the interconversion of a pair of enantiomers for all compounds studied, with ΔG≠ in the range 56.1–74.1 kJ mol−1. A good correlation exists between the experimental ΔG≠ values and the energy barriers, EA, predicted by the semiempirical AM1 SCF model.

Synthesis of highly-active Fe–N–C catalysts for PEMFC with carbide-derived carbons

Proton exchange membrane fuel cells (PEMFC) offer a viable alternative to internal combustion engines, but highly performing stacks still require large amounts of platinum-based catalysts. Fe–N–C catalysts have recently emerged as potential substitutes. Carbide-derived carbon (CDC) can be designed to have various pore size distributions (PSD), in the microporous and/or mesoporous domains, which can be used for defining the number and/or accessibility of active sites in Fe–N–C catalysts based on the CDC. In this work, we compare two sets of Fe–N–C catalysts derived from two different CDCs, one with most frequent pore size of 8.5 A, (CDC-2) and another one with most frequent pore sizes at 7.8 and 30 A (CDC-1). The CDC-based Fe–N–C catalysts show excellent half-wave potential for oxygen reduction reaction (ORR) of 0.81 V vs. RHE in 0.5 M H2SO4. This work presents the first study of CDC-based catalysts in a PEMFC, where the performance of the CDC-2 based catalyst rivaled that of the best Fe–N–C materials in the literature. The catalyst derived from CDC-2 showed ca. 5 times higher activity at 0.8 V vs. RHE than the one derived from CDC-1. We show that the residual presence of boron in CDC-1 is the main reason for the lower activity of CDC-1 derived catalysts, leading to the formation of iron boride instead of ORR-active FeNxCy moieties. Higher Fe contents were investigated for CDC-2, but lead to unmodified activity, which is explained from Mossbauer spectroscopy measurements by the increasing formation of ORR-inactive Fe species at high Fe content. In summary, we demonstrate the excellent potential for CDC materials to be used in catalyst design and also identify some key issues that may arise from the possible residual presence of secondary atoms from the starting carbide.

Low voltage linear actuators based on carbide-derived carbon powder

Novel linear electromechanical actuators based on nanoporous TiC-derived carbons were prepared and studied. Traditionally, thin membranes containing mobile ions are used for bending actuators. We describe a linear actuator which consists of carbon material thin film and an ionic liquid. The thin film is made from nanoporous TiC-derived carbon powder and polytetrafluoroethylene (PTFE) as a binder agent. The working mechanism of the actuators is based on the interactions between the high-surface-area carbide-derived carbon (CDC) and the ions of the electrolyte. These actuators are able to generate linear actuation of about 1% from their thickness under voltages less than 3 V. The motion starts already at 0.8V and the magnitude of actuation depends on the electrical charge stored by the device. Two different types of electrolyte were used: 1) Ionic liquid (EMITf) and 2) Tetra-alcyl-ammonium salt in propylene carbonate (PC) solution. The actuators with ionic liquid have 60% higher movement. The electromechanical parameters of the actuators were studied by using cyclic voltammetry and electrochemical impedance spectroscopy methods.

A QSPR model for the prediction of the gas-phase free energies of activation of rotation around the NC(O) bond

A novel approach to predict the gas-phase rotational activation energies of amides is presented. The quantitative structure property relationship (QSPR) treatment, using the statistical package, comprehensive descriptors for structural and statistical analysis (CODESSA), resulted in a three-parameter equation with R2 = 0.982 for the deltaG(double dagger)gas, of a set of 24 N,N-dialkylamides.