Electron driven chemistry in microreactors

This thesis describes the development of novel process windows by the combination of atmospheric pressure plasmas with microreaction technology. In the first chapter, recent literature on microreactor technology and non-equilibirum microplasma chemistry is discussed. The focus is on microplasmas in confined microchannels for the purpose of chemical synthesis and environmental applications. Study of oxidative conversion of propane using dielectric barrier discharge in a microreactor is described. This generates a cold microplasma at atmospheric pressure and ambient temperature. Surprisingly, large amounts of products with molecular weight higher than propane, such as, C4 and C4+ were mainly observed due to C-C bond formation, in contrast to what is usually observed for this reaction when it is carried out under thermal activation, which leads to cracking products. Development of a plasma catalytic reactor based on a dielectric barrier discharge for oxidative cracking of hexane with Li/MgO based catalysts is reported. The effect of temperature, oxygen concentration, helium flow, and MgO support, and the role of Li/MgO catalysts on the conversion of hexane, and on the selectivity and yield of olefin formation are described. Direct synthesis of liquid oxygenates from partial oxidation of methane is demonstrated in a multi-phase flow, non-equilibrium plasma microreactor near 0oC at atmospheric pressure. A method for liquid-water injection into the microreactor was introduced in order to remove incomplete oxidation products such as methanol, which prevents further oxidation with excited species originating from the microplasma. In the sixth chapter, development of an in-situ CVD method for the growth of CNFs on Ni/alumina and nickel thin film catalyst coated inside a closed channel fused silica microreactor is described. Synthesis, characterization and atmospheric pressure field emission operation of tungsten oxide W18O49 nanorods is discussed. Atmospheric pressure field emission measurements in air showed a turn-on field of 3.3 V/μm, and a stable and reproducible emission current density of 28 mA/cm2. Carbon nanofibers (CNFs) and tungsten oxide nanorods have been incorporated in a continuous flow microplasma reactor to increase the reactivity and efficiency of the barrier discharge at atmospheric pressure

Characterization of LaMnAl11O19 by FT-IR spectroscopy of adsorbed NO and NO/O-2

Cataloged from PDF version of article.The nature of the NOx species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on LaMnAl11O19 sample with magnetoplumbite structure obtained by a sol–gel process has been investigated by means of in situ FT-IR spectroscopy. The adsorption of NO leads to formation of anionic nitrosyls and/or cis-hyponitrite ions and reveals the presence of coordinatively unsaturated Mn3+ ions. Upon NO/O2 adsorption at room temperature various nitro–nitrato structures are observed. The nitro–nitrato species produced with the participation of electrophilic oxygen species decompose at 350 8C directly to N2 and O2. No NO decomposition is observed in absence of molecular oxygen. The adsorbed nitro–nitrato species are inert towards the interaction with methane and block the active sites (Mn3+ ions) for its oxidation. Noticeable oxidation of the methane on the NOx -precovered sample is observed at temperatures higher than 350 8C due to the liberation of the active sites as a result of decomposition of the surface nitro–nitrato species. Mechanism explaining the promoting effect of the molecular oxygen in the NO decomposition is proposed. (c)2005 Elsevier B.V. All rights reserved

Oxidative Conversion of Hexane to Olefins-Influence of Plasma and Catalyst on Reaction Pathways

An integrated plasma-Li/MgO system is efficient for the oxidative conversion of hexane. In comparison to the Li/MgO catalytic system, it brings considerable improvements in the yields of light olefins (C 2 = –C 5 = ) at relatively low temperatures indicating synergy from combination of plasma and catalyst. The study on the influence of temperature on the performance of the integrated plasma-Li/MgO system shows dominancy of plasma chemistry at the lower temperature (500°C), while contribution from the catalyst both in hexane activation and in enhancing olefin formation becomes significant at the higher temperature (600°C). At 500°C significant amount of acetylene formation is observed. This is minimized at 600°C at oxygen depleting condition

Charge injection from carbon nanofibers into hexane under ambient conditions

The observation of charge injection from carbon nanofibers (CNFs) into liquid hexane under ambient conditions is reported. A CNF-coated electrode and a counter electrode are brought into micrometer proximity in a quasi-parallel geometry using a strain-gauge-based proximity sensor. Controlled charge injection is obtained at interelectrode distances of 4, 6, 9, and 15 μm. The resulting emission current shows an onset of about 3 V/μm , and it follows the Fowler-Nordheim behavior. The work reported here opens new applications for free electron chemistry in liquids and novel liquid field emitter devices

On-chip microplasma reactors using carbon nanofibres and tungsten oxide nanowires as electrodes

Carbon nanofibres (CNFs) and tungsten oxide (W18O49) nanowires have been incorporated into a continuous flow type microplasma reactor to increase the reactivity and efficiency of the barrier discharge at atmospheric pressure. CNFs and tungsten oxide nanowires were characterized by high-resolution scanning electron microscopy, transmission electron microscopy and nanodiffraction methods. Field emission of electrons from those nanostructures supplies free electrons and ions during microplasma production. Reduction in breakdown voltage, higher number of microdischarges and higher energy deposition were observed at the same applied voltage when compared with plane electrodes at atmospheric pressure in air. Rate coefficients of electron impact reaction channels to decompose CO2 were calculated and it was shown that CO2 consumption increased using CNFs compared with plane electrode in the microplasma reactor

Particle suspension reactors and materials for solar-driven water splitting

Reactors based on particle suspensions for the capture, conversion, storage, and use of solar energy as H_2 are projected to be cost-competitive with fossil fuels. In light of this, this review paper summarizes state-of-the-art particle light absorbers and cocatalysts as suspensions (photocatalysts) that demonstrate visible-light-driven water splitting on the laboratory scale. Also presented are reactor descriptions, theoretical considerations particular to particle suspension reactors, and efficiency and performance characterization metrics. Opportunities for targeted research, analysis, and development of reactor designs are highlighted

High Performance Field Emitters.

The field electron emission performance of bulk, 1D, and 2D nanomaterials is here empirically compared in the largest metal-analysis of its type. No clear trends are noted between the turn-on electric field and maximum current density as a function of emitter work function, while a more pronounced correlation with the emitters dimensionality is noted. The turn-on field is found to be twice as large for bulk materials compared to 1D and 2D materials, empirically confirming the wider communities view that high aspect ratios, and highly perturbed surface morphologies allow for enhanced field electron emitters.M.T.C. thanks the Oppenheimer Trust, Cambridge University, for generous financial support. This work was supported by an EPSRC Impact Acceleration grant and an Innovate UK Advanced Materials Feasibility Study award. CC thanks the EPSRC Centre for Doctoral Training in Ultra Precision.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/advs.20150031

Synthesis and Atmospheric Pressure Field Emission Operation of W18O49 Nanorods

Tungsten oxide W18O49 nanorods with diameters of 15−20 nm were grown on tungsten thin films exposed to ethene and nitrogen at 700 °C at atmospheric pressure. It was found that tungsten carbide formation enhances nucleation and growth of nanorods. Atmospheric pressure field emission measurements in air showed a turn-on field of 3.3 V/μm and stable and reproducible emission current density (28 mA/cm2). At low current densities (<20 mA/cm2), the field emission current follows the Fowler−Nordheim model; saturation effects were observed above 20 mA/cm2