Elizabeth A Dawson

A new approach to the statistical optimisation of catalyst preparation

Abstract The statistical design of experiments is not widely used due to the apparent complexity of the procedures involved. A simplified approach, based on the methods of Taguchi, is described and is illustrated by applying it to the optimisation of the preparation of a metal-carbon catalyst. The surface area was increased from 8 to 250 m 2 g −1 . It is shown that the method gives five significant advantages. It enables the amount of experimentation to be significantly reduced; it provides a means for assessing the significance of synergistic effects (interactions) between variables (factors) and it can yield evidence for the existence of previously unsuspected interactions. The method has the advantage of being applicable to existing production processes by assessing the consequences of variations in operating parameters found within normal run conditions. Finally, the experimental design process is simplified and the analysis of results is facilitated by the ready availability of the required statistical techniques in commercial software for microcomputers.

An investigation of the porosity of carbons prepared by constant rate activation in air

Nutshell carbon was activated in air/N2 mixtures using controlled rate (CR) methods and the porosity characteristics compared with carbons activated conventionally in CO2 at 800 °C to the same degree of burn off. The advantages of CR activation in air include the use of lower temperatures and the avoidance of thermal runaway. It was also possible to prepare activated carbons with significant microporosity, showing that excessive external burn off was prevented. In the CR experiments, the rate of evolution of CO2 was controlled and constrained at a set level, either by altering the furnace temperature or the concentration of air in the activating gas. Although the highest micropore volumes (0.4 cm3 g−1) were obtained at 40% burn off with the conventional method, at 20% burn off, the CR method using air concentration to control CO2 evolution yielded carbons with similar micropore volumes (0.2 cm3 g−1) to those activated conventionally.

High temperature ceramics for use in membrane reactors: the development of microporosity during the pyrolysis of polycarbosilanes

The pyrolysis of polycarbosilane (PCS), a ceramic precursor polymer, at temperatures up to 700 °C under an inert atmosphere results in the development of amorphous microporous materials which have a number of potential applications, such as gas separation membranes. This paper investigates the development of microporosity during pyrolysis under nitrogen, at temperatures ranging from 300 to 700 °C, of both the cross-linked and non-cross-linked starting materials. The products are characterised by nitrogen adsorption, to determine surface areas and pore volumes, solid-state NMR, electron microscopy and FTIR, and their formation is studied using thermal analysis and evolved gas analysis with on-line mass spectrometry. The cross-linked and non-cross-linked PCSs have a maximum micropore volume of 0.2 cm3 g−1 at pyrolysis temperatures of between 550 and 600 °C. The microporosity is stable in air at room temperature, but is lost in oxidising atmospheres at elevated temperatures.

Preparation, Characterisation and Application of Metal-doped Carbons for Hydrogen Cyanide Removal

A novel method for the production of metal-doped activated carbons was developed. Evaluation of the materials produced showed that the metals were well dispersed throughout the carbon pore structure and had a high degree of accessibility. The filtration performance of these materials was assessed against HCN using inverse gas chromatography. The performance was affected both by the selection of metals and the conditions used in the production of the carbon. In-depth studies varying the synthesis parameters were performed with the aim of optimising the manufacturing process. Significant HCN adsorption capacity was developed which exceeded that of the current best available ASC-type carbons.

A Study of the Activation of Carbon Using Sample Controlled Thermal Analysis

A constant rate method involving the control of the concentration of evolved CO2 at a constant level was used to study the air activation of pure and copper-doped carbon prepared from sodium carboxymethylcellulose. Whereas under a linear heating regime, both types of carbon reacted suddenly and quickly with O2, under constant rate conditions this violent reaction was avoided and oxidation proceeded steadily at a lower temperature until complete burn off of the carbon was achieved. The catalytic effect of the copper on carbon gasification was noted with lower reaction temperatures for both linear heating (380°C compared to 500°C) and for the constant rate experiments (320°C compared to 400°C).

Activation of novel copper–carbon catalysts using temperature-programmed mass spectrometry

An entirely new method of preparing metal–carbon catalysts is reported in which the metal is present as small crystallites distributed in a three-dimensional array throughout the bulk of the carbon matrix. The metal particles are linked by a network of pores which exposes virtually all of the metal surface to the gas phase. By virtue of their structure, these new materials exhibit a remarkable resistance to sintering, even near the melting point of the metal. While many catalytically important metals can be incorporated into our materials, this paper concentrates on the structure of one of the copper–carbon systems. These are made by the reduction of a copper(II) cellulose precursor to produce a copper(0) cellulose material, followed by thermal degradation of the cellulose and activation of the resulting carbon to produce the pore network. New techniques, using a temperature-programmed gas-flow micro-reactor linked to an on-line mass spectrometer, are used to investigate the nature of the critical activation process. The resulting pore structure is studied and a mechanism is proposed to explain the catalytic effect of the copper particles on the formation of the pore network. Poisoning experiments demonstrate that access to the active sites is determined by the pore size, thus giving a measure of size selectivity. Using temperature-programmed oxidation with on-line MS it was demonstrated that the pores can be widened in a controlled manner, so reducing any diffusion-induced limitation on reaction rates. This offers the possibility of designing a catalyst with a specified balance between the conflicting requirements for size selectivity and high reaction rates.

Comparison of new thermal and reactant gas blending methods for the controlled oxidation of carbon

Abstract Two approaches to sample controlled thermal analysis (SCTA) used preparatively were contrasted using the air activation of a nutshell derived carbon. The rate of reaction, corresponding to the level of evolved carbon dioxide, was monitored using a quadrupole mass spectrometer and controlled via software using a feedback loop. In the first approach, control of the rate was achieved via temperature under a constant concentration of oxygen in nitrogen, while in the second the reactant gas concentration (oxygen/nitrogen ratio) was changed while the furnace was held isothermally. Due to the exothermic nature of the carbon–oxygen reaction, temperature control of the activation process at high reaction rates was difficult and produced oscillatory behaviour, while good control of the process could be achieved using the gas concentration method at higher reaction rates. By using oxygen rather than the more usual CO 2 or H 2 O activation at 800–1000°C, the reaction takes place at 200–300°C with a consequent significant saving in energy costs.

Development and applications of a preparative scale sample controlled thermogravimetric system

The successful development of a high sensitivity SCTG system [1] resulted in the construction of a larger scale sample controlled thermobalance, which enables samples of 500 mg and above to be studied. One objective of the system was to allow the preparation, under precisely controlled thermal conditions, of sufficiently large amounts of coal chars that could then be characterised using a range of analytical techniques.

Synthesis of Vegetable-Based Activated Carbons with Mixed Micro- and Mesoporosity for Use in Cigarette Filters

Activated carbons with micropores for adsorption and filtration of the volatile constituents of mainstream cigarette smoke, together with mesopores for enhanced mass transport were prepared by a novel route. Treatment of coconut shell or other lignocellulosic precursors with aqueous NaOH, followed by thorough washing, charring and steam activation produced carbons with enhanced adsorption characteristics in smoking trials, compared with their microporous analogues. The mechanism of formation of these carbons is explained in terms of initial partial dissolution of the precursor in an aqueous alkali solution, followed by catalytic gasification of carbon in steam involving residual sodium.

Preparation and dynamic adsorption properties of activated carbons with tailored micro-and mesoporosity

Carbons containing controlled levels of micro- and mesoporosity are prepared using a combination of templating and activation. Analysis by nitrogen adsorption shows that the templated carbons are predominantly mesoporous. Subsequent activation of the template carbons increases the micro:mesopore ratio. Adsorption of pentane from a dry flowing airstream reveals that the dynamic uptakes are consistent with the uptake from adsorption isotherms. Comparison with the isotherm data shows that only a limited amount of the microporosity is utilized. Increase in the micropore volume results in an increase in breakthrough time and uptake. When in competition with water vapor, the breakthrough time and capacity for pentane are both significantly reduced. Introduction of high levels of microporosity can reduce performance, as a result of the high quantities of water present. The effect of water vapor is less pronounced for octane than for pentane, as a result of its lower volatility and hence higher partial pressure. This enables octane to more readily displace water from the micropore volume, thus limiting the reduction in breakthrough time and capacity. In this case, additional microporosity does provide a beneficial effect.