Phillip F. Britt

Pyrolysis mechanisms of lignin: surface-immobilized model compound investigation of acid-catalyzed and free-radical reaction pathways

Abstract The pyrolysis of surface-immobilized β-alkyl aryl ethers, m ≈ PhOCH 2 CH 2 Ph , ≈ PhCH 2 CH 2 OPh , and ≈ PhCH 2 CH 2 OPh - o - OCH 3 , has been studied with a dispersed acid catalyst at 300–450 °C in order to gain an insight into the relative importance of ionic reactions vs. free-radical reactions in the thermal depolymerization of lignin. In the absence of acid catalyst, surface-immobilization does not hinder the free-radical reaction pathway to produce phenols, alkenes, alkanes, and aldehydes as the major products. Pyrolysis in the presence of small particle sized (15 nm) SiO 2 -1% Al 2 O 3 at 300 °C, where free-radical reactions are suppressed, produces an acceleration in the rate of decomposition and a substantial alteration in the product distribution as a consequence of the solid-solid interactions between the catalyst and substrate. Products from the silica-alumina catalyzed reactions are characteristic of acid-catalyzed cracking reactions which involve carbocation intermediates to produce products from ether cleavage and aromatic alkylation and dealkylation reactions resulting most notably in the near absence of alkenes. The surface-immobilized primary products continue to undergo acid-catalyzed reactions to form larger aromatics and coke. At temperatures in which the free-radical and acid-catalyzed reactions can compete (> 375 °C), products from the acid-catalyzed reaction dominate. A comparison of the products from the pyrolysis of lignin, biomass, and the surface-immobilized model compounds provides new evidence that the thermal degradation of lignin occurs principally by a free-radical reaction pathway.

Individually addressable vertically aligned carbon nanofiber-based electrochemical probes

In this paper we present the fabrication and initial testing results of high aspect ratio vertically aligned carbon nanofiber (VACNF)-based electrochemical probes. Electron beam lithography was used to define the catalytic growth sites of the VACNFs. Following catalyst deposition, VACNF were grown using a plasma enhanced chemical vapor deposition process. Photolithography was performed to realize interconnect structures. These probes were passivated with a thin layer of SiO2, which was then removed from the tips of the VACNF, rendering them electrochemically active. We have investigated the functionality of completed devices using cyclic voltammetry (CV) of ruthenium hexammine trichloride, a highly reversible, outer sphere redox system. The faradaic current obtained during CV potential sweeps shows clear oxidation and reduction peaks at magnitudes that correspond well with the geometry of these nanoscale electrochemical probes. Due to the size and the site-specific directed synthesis of the VACNFs, these ...

A novel self-assembled monolayer (SAM) coated microcantilever for low level caesium detection

We report a new sensor concept based on an ion-selective SAM modified microcantilever which can detect caesium ion concentrations in situ in the range 10−11–10−7 M and shows potential for use in developing a new family of real time in situ metal ion sensors with high sensitivity/selectivity and low cost, for chemical and biological applications.

Synthesis and characterization of single-wall carbon nanotube-amorphous diamond thin-film composites

Thin-film single-wall carbon nanotube (SWNT) composites synthesized by pulsed laser deposition (PLD) are reported. Ultrahard, transparent, pure-carbon, electrically insulating, amorphous diamond thin films were deposited by PLD as scratch-resistant, encapsulating matrices for disperse, electrically conductive mats of SWNT bundles. In situ resistance measurements of the mats during PLD, as well as ex situ Raman spectroscopy, current–voltage measurements, spectroscopic ellipsometry, and field-emission scanning electron microscopy, are used to understand the interaction between the SWNT and the highly energetic (∼100 eV) carbon species responsible for the formation of the amorphous diamond thin film. The results indicate that a large fraction of SWNT within the bundles survive the energetic bombardment from the PLD plume, preserving the metallic behavior of the interconnected nanotube mat, although with higher resistance. Amorphous diamond film thicknesses of only 50 nm protect the SWNT against wear, providi...

A study of the reductive pyrolysis behaviour of sulphur model compounds

Abstract The difficulties inherent in the direct determination of sulphur functionalities in complex solid matrices by various techniques often make the need for reference compounds indispensable. One of the pyrolysis techniques used for sulphur determination is atmospheric pressure–temperature programmed reduction (AP-TPR). Experiments on sulphur model compounds have served successfully as a reference for both the temperature region in which the reduction or hydrogenation occurs and the efficiency of the reduction reaction. In this study, the pyrolysis behaviour of several organic and inorganic sulphur model compounds is investigated by AP-TPR using a mass spectrometer detector interfaced with the pyrolysis reactor (AP-TPR-MS). This technique permits a more complete description of the competitive and successive reactions that are occurring during the pyrolysis of the model compounds, providing new information regarding the reduction efficiency of oxidised and non-oxidised sulphur compounds.

The electrodeposition of metal at metal/carbon nanotube junctions

Abstract We deposited a semiconducting single-walled carbon nanotube on Pd electrodes, and the initial charge transport measurements showed the usual large contact resistance between the electrodes and the nanotube. We electroplated Au over the electrodes with no obvious deposition of Au along the sidewalls of the nanotube between the electrodes. Post deposition charge transport measurements indicated more than a factor of six decrease in the electrode/nanotube contact resistance, yet the semiconducting behavior of the nanotube was maintained. A significant difference in the post deposition I – V characteristics may be explained by an electronic or mechanical modification of the nanotube/electrode junction.

Confinement effects on product selectivity in the pyrolysis of phenethyl phenyl ether in mesoporous silica

Pyrolysis of phenethyl phenyl ether confined in mesoporous silicas by covalent grafting results in significantly increased product selectivity compared with fluid phases.

Development of a technique for the analysis of inorganic mercury salts in soils by gas chromatography/mass spectrometry

Abstract A technique has been developed to analyze environmentally relevant samples for organic and inorganic mercury compounds. A solid phase microextraction (SPME) fiber was used as a sampling medium in both water and water/soil slurries. Quantification of inorganic mercury was accomplished through a chemical alkylation reaction designed to convert an inorganic mercury salt to an organomercury compound prior to GC/MS analysis; this was found to be the rate limiting step in the analysis. Two alkylating reagents were investigated: methylpentacyanocobaltate (III) (K3[Co(CN)5CH3]) and methylbis(dimethylglyoximato)pyridinecobalt (III) (CH3Co(dmgH)2Py). Methylbis(dimethylglyoximato)pyridinecobalt (III) was found to be superior for this application because it produced a single reaction product, methylmercury iodide, with an efficiency of ∼95%. Detection limits were ∼7 ppb in water and ∼2 ppm in soil. The poorer results in soil were due to an increase in background signal (∼10 times compared to water) and a reduction in analyte signal (as much as 100 times). This reduction in signal intensity is believed to be caused by complex soil chemistry. Manipulation of the solution chemistry [e.g. oxidation of mercury (0) → mercury (II)], before or during the alkylation step, may improve the detection limits and increase the number of elements amenable to analysis. Keywords: Gas chromatography/mass spectrometry; Inorganic analysis; Elemental; Mercury compounds; Chemical alkylation

Elemental and Organometallic Analyses of Soil Using Glow Discharge Mass Spectrometry and Gas Chromatography/Mass Spectrometry

Glow discharge mass spectrometry (GDMS) and gas chromatography/mass spectrometry (GC/MS) have been evaluated as techniques for total elemental assay in soil. GDMS analysis demonstrated accurate elemental quantification for lead and tin (approximately 20% error at the 10 ppm level). Limitations were encountered, however, when the element of interest was volatile, as in the case of mercury, or when the element was not an inorganic salt but a volatile organometallic compound. GC/MS was investigated as an alternative means of providing both organometallic compound analysis and elemental quantification. A solid-phase microextraction fiber was demonstrated to be an effective sampling medium for several organometallic compounds in both water and a water/soil slurry. Quantification of inorganic mercury species was facilitated by using an alkylating reagent (methylpentacyanocobaltate (III)) to produce an organomercurial that could be analyzed by GC/MS. Although the reaction proceeded as anticipated, several additional observations were made that may be exploited in future studies.

In situ electric-field-induced contrast imaging of electronic transport pathways in nanotube-polymer composites

An electric-field-induced contrast mechanism for scanning electron microscopy is reported which permits the visualization of embedded nanomaterials inside various matrices with high contrast and high definition. The high contrast is proposed to result from localized enhancement of secondary electron emission from the nanomaterials due to electric-field-induced changes in their work functions. By utilizing a stage that allows in situ current-voltage measurements inside a scanning electron microscope, single-walled carbon nanotubes embedded within polymethyl methacrylate films were visualized directly. In addition to the rapid assessment of nanotube dispersion within polymers, electric-field-induced contrast imaging enables the determination of percolation pathways. From the contrast in the images, the relative voltage at all points in the electron micrograph can be determined, providing a new mechanism to understand electronic percolation through nanoscale networks.