Frank Derbyshire

Continuous production of aligned carbon nanotubes: a step closer to commercial realization

High-purity aligned multi-walled carbon nanotubes MWNTs were synthesized through the catalytic decomposition of a ferrocene-xylene mixture at ; 6758C in a quartz tube reactor and over quartz substrates, with a conversion of ; 25% of the total hydrocarbon feedstock. Under the experimental conditions used, scanning electron microscope images reveal that the MWNT array grows perpendicular to the quartz substrates at an average growth rate of ; 25 mmrh. A process of this nature which does not require preformed substrates, and which operates at atmospheric pressure and moderate temperatures, could be scaled up for continuous or semi-continuous production of MWNTs. q 1999 Elsevier Science B.V. All rights reserved.

Model of carbon nanotube growth through chemical vapor deposition

Abstract This Letter outlines a model to account for the catalyzed growth of nanotubes by chemical vapor deposition. It proposes that their formation and growth is an extension of other known processes in which graphitic structures form over metal surfaces at moderate temperatures through the decomposition of organic precursors. Importantly, the model also states that the form of carbon produced depends on the physical dimensions of the catalyzed reactions. Experimental data are presented that correlate nanotube diameters to the size of the catalyst particles. Nanotube stability as a function of nanotube type, length and diameter are also investigated through theoretical calculations.

Activated carbons from yellow poplar and white oak by H3PO4 activation

Abstract Results are presented from continuing investigations of the phosphoric acid activation of hardwoods. Earlier work with white oak has been extended to include yellow poplar. It is found that the same general chemical and physical changes occur with both precursors. A discussion is presented on the possible mechanisms of phosphoric acid activation, drawing upon extensive research on the use of phosphorous compounds as fire retardants for wood and cellulose. Phosphoric acid appears to function both as an acid catalyst to promote bond cleavage reactions and the formation of crosslinks via processes such as cyclization, and condensation, and to combine with organic species to form phosphate and polyphosphate bridges that connect and crosslink biopolymer fragments. The addition or insertion of phosphate groups drives a process of dilation that, after removal of the acid, leaves the matrix in an expanded state with an accessible pore structure. It is considered that activation of the amorphous polymers produces mostly micropores, while activation of crystalline cellulose produces a mixture of pore sizes. The different response of crystalline cellulose is attributed to a much greater potential for structural expansion than is possible with the amorphous polymers due, among other factors, to its higher density and its chemical structure that allows for a more extensive degree of combination with phosphoric acid, and hence “bulking” of the cell walls. The pore size distribution obtained from crystalline cellulose can be altered by increasing the HTT and/or the ratio of acid to precursor such that, eventually, the structure is dominantly mesoporous. At temperatures above 450 °C, a secondary contraction of the structure occurs when the phosphate linkages become thermally unstable. The reduction in crosslink density allows the growth and alignment of polyaromatic clusters, producing a more densely packed and less porous structure.

NANOTUBE COMPOSITE CARBON FIBERS

Single walled carbon nanotubes (SWNTs) were dispersed in isotropic petroleum pitch matrices to form nanotube composite carbon fibers with enhanced mechanical and electrical properties. We find that the tensile strength, modulus, and electrical conductivity of a pitch composite fiber with 5 wt % loading of purified SWNTs are enhanced by ∼90%, ∼150%, and 340% respectively, as compared to the corresponding values in unmodified isotropic pitch fibers. These results serve to highlight the potential that exits for developing a spectrum of material properties through the selection of the matrix, nanotube dispersion, alignment, and interfacial bonding.

Evolution of carbon structure in chemically activated wood

Abstract 13C NMR and FTIR analyses have been employed to follow the evolution of chemical structure in relation to porosity development, as a function of heat treatment temperature (HTT), for activated carbons produced from white oak by phosphoric acid activation. The chemical changes effected by acid treatment at low HTT are: by 50 °C there is significant alteration of the lignin structure; by 100 °C a significant portion of the cellulose has reacted, with the formation of ketones and esters; the formation of phosphate esters becomes apparent around 150 °C; crosslinking reactions are initiated below 150 °C, consistent with the higher carbon yield obtained in chemical activation; and generally there is an increase in aromaticity and loss of aliphatic, carboxyl, and carbonyl groups. The low temperature phenomena precede, and relate to, the development of porosity and structural dilation that commences around 250 °C, and attains a maximum between 350 and 450 °C. Up to 450 °C, pore volume is found to correlate with crosslink density. Above 450 °C, there is a dimensional contraction and a reduction in porosity. Among the accompanying phenomena are: the elimination of cellulose phosphates and oxygen functionalities; and a dramatic increase in the estimated aromatic cluster size. The latter would require a reduction in crosslink density to facilitate cluster growth, and the resulting structural rearrangement and increased alignment of clusters would produce a more densely packed structure with reduced porosity.

Some considerations of the origins of porosity in carbons from chemically activated wood

Abstract Investigations have been made of the conversion of white oak to activated carbons by reaction with phosphoric acid at temperatures up to 650°C. At low temperatures, reaction with the acid promotes dehydration reactions, and the redistribution of biopolymers. It is considered that the subsequent formation of crosslinks leads to an increase in carbon yield above about 300°C. Detailed observations by optical microscopy, and measurements of dimensional changes, have established for the first time a link between the origins of porosity and reaction mechanisms. Following an initial contraction, the structure undergoes considerable expansion between 250 and 450°C, which corresponds to the development of high surface area. Above 450°C, there is secondary contraction and an accompanying loss of accessible porosity. It is concluded that porosity development is directly related to the retention and dilation of cellular material, which creates an extensive surface accessible to adsorbent molecules. The results may be relevant to understanding equivalent processes with other biomass and coal precursors that contain biopolymers or altered biopolymers.

Adsorbent carbon synthesis from coals by phosphoric acid activation

The chemical activation of a bituminous coal by reaction with phosphoric acid has been followed by studying chemical and structural changes to the activate and the development of porosity. There are clear differences in porosity development after thermal treatment in the range 350–650°C and after heat treatment in the same temperature range following reaction with phosphoric acid. It is proposed that chemical treatment causes rupture of weaker linkages in the coal structure and early development of a rigidly cross-linked product through the formation of new stronger linkages. This process is accompanied by development of a mainly microporous structure which reaches a maximum BET surface area of ~ 750 m2g−1 HTT ~ 550°C. Pore structure development can be tailored to some extent by varying both phosphoric acid strength and final heat-treatment temperature.

Molecular structure of coals: A debate

Abstract Recent evidence concerning the molecular phase present in coals is discussed. It is proposed that pyrolysis-field ionization mass spectrometry (py-f.i.m.s.) provides information on the chemical nature of such material, but caution is also suggested in these interpretations. N.m.r. experiments can give information concerning the spatial location of molecular species, and domains with different mobilities can be recognised. The importance of considering coal origins and petrology in discussions of coal structure is emphasized. An empirical perspective of the molecular phase is afforded by solvent-free catalytic hydrogenation. Coal depolymerization by solvated electrons yields products consistent with polymethylene as a major part of the macromolecular network. The relation of these observations to the molecular mass of extracted material is suggested as crucial.

Catalytic reduction of nitric oxide over activated carbons

Abstract The reduction of NO to N 2 using NH 3 as a reductant over activated carbons was investigated using an integral reactor system. The effects of varying the gas-phase and surface oxygen concentrations were studied in detail. Surface oxygen was introduced by treating a coconut shell activated carbon with sulphuric acid at 100–300 °C. The type and concentration of surface oxides were investigated by infrared spectroscopy, linear temperature-programmed desorption of surface oxides, and adsorption of reactant gases. Increasing the gas-phase oxygen concentration increased NO conversion significantly. The activity of activated carbon at reaction temperatures > 150 °C was enhanced by sulphuric acid treatment. Sulphuric acid acted as an oxidizing agent and created acidic surface functionalities including carboxyl and carbonyl groups.

Temperature-staged catalytic coal liquefaction

Abstract An investigation has been made of the liquefaction of a bituminous and a subbituminous coal under conditions where reaction is conducted in successive stages of increasing temperature and in the presence of a dispersed sulphided Mo catalyst. This sequence has been found to lead not only to high coal conversion but to greatly increase the selectivity of the liquefied products to oils at the expense of asphaltenes. These gains are made with marginal increases in the production of light hydrocarbon gases. Although no systematic attempt has yet been made to determine the specific influence of reaction parameters upon liquefaction behaviour, preliminary results show that there is substantial potential for further improvement through the suitable choice of solvent and reaction conditions in the two stages. The reasons for the effectiveness of temperature staged liquefaction are discussed in terms of the balance between hydrogenation and condensation reactions. Examination of the liquefaction residues by optical microscopy has provided strong supporting evidence to show that the staged reaction sequence favours hydrogenative processes. Moreover, the microscopic examination has proved to be a powerful diagnostic technique, showing, for example, that the first stage temperature should be lower for the subbituminous than the bituminous coal, and providing insight into the processes of catalysed liquefaction.