Paul R. Norman

Characterisation of surface oxides on carbon and their influence on dynamic adsorption

Abstract The aim of this work is to gain a better fundamental understanding of the nature of surface oxide sites present on carbon surfaces, and their role in the adsorption process. A number of model carbon substrates with different degrees of surface oxidation and similar textural properties were prepared using a wide range of solution and gaseous phase oxidation techniques. Some of the carbons were characterised using established techniques including flow microcalorimetry, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The results showed that both carboxylic acid (–CO2H) and ether/hydroxyl (C–O) surface oxygen complexes were introduced to all of these carbons as a result of the oxidation processes. The number, strength and thermal stability of the surface groups formed were dependent on the nature of the base material and the oxidation conditions employed. The dynamic adsorption performance of the carbons against hexane, under humid conditions, was found to be mainly determined by the quantity of acidic surface functional groups. However, the location, strength of interaction and availability of the surface oxygen complexes to the adsorbate molecules, are also thought to affect the breakthrough characteristics of the carbons used in this work.

Phosphorus-31 NMR studies of adsorption onto activated carbon

Abstract Magic-angle spinning 31 P NMR spectra have been obtained for trimethylphosphate, triethylphosphate and dimethylmethyl phosphonate on three activated carbons for a variety of loading conditions up to 60 w/w%. At low loadings a broad resonance band is seen at ca. 8 ppm to low frequency of the signal for the neat liquid adsorbate. This is ascribed to overlap of bands from adsorbate molecules in different micropores, with proximity of the adsorbate to aromatic carbon rings. At high loadings a second band appears, close to the resonance for neat liquid adsorbate. It may be assigned to molecules weakly adsorbed at surfaces and particle interfaces relatively remote from aromatic molecular fragments. Variations with the nature of the adsorbate, of the carbon type and of loading are discussed. Quantitative data are presented. A significant process of slow adsorbate transport was found, indicating some subtle re-arrangement of the adsorbate–adsorbent system with time.

Enhanced hydrolysis of a phosphonate ester by mono-aquo metal cation complexes

Abstract The hydrolysis of the ester 2,4-dinitrophenyl- ethyl methylphosphonate has been examined by both stop-flow spectrophotometric and pH-stat techniques. These reactions have been carried out in the presence of several nucleophiles including simple non-labile (w.r.t. substitution) mono-aquo metal ion complexes. Comparison of reaction rates of the metal complexes with sterically hindered organic nucleophiles has led to the conclusion that the metal ions function predominantly as general base catalysts in dilute aqueous solution. Reaction rates for the various nucleophiles studied are tabulated together with solvolysis constants for hydroxide ion and water at 35 °C and I =0.1 mol dm −3 (KNO 3 ). These later two values are respectively 32.7 mol −1 dm 3 s −1 and 1.37 x 10 −4 s −1 . A Bronsted β value of 0.52 for the phosphonate ester studied has also been derived.

A magic-angle spinning NMR study into the adsorption of deuterated water by activated carbon

Magic-angle spinning deuterium NMR spectra have been used to evaluate the effects of activation to different extents of nutshell and coal-based carbon substrates on the adsorption of water (D2O). In general, separate signals are seen for water adsorbed in the micropores and in larger channels or at particle surfaces. The unactivated carbon shows negligible micropore adsorption except when it is pretreated in a vacuum. This fact is attributed to a blocking effect of oxygen, nitrogen and/or water molecules in the pores.

Adsorption competition onto activated carbon, studied by magic-angle spinning NMR

The competition between water and two phosphorus-containing compounds for adsorption sites on two activated carbon materials has been studied by 2H and 31P magic-angle spinning (MAS) NMR as a function of time after addition of the second adsorbate. Both 2H and 31P spectra show two bands, one close to the resonance of the relevant liquid compound and the other (assigned to molecules in micro- and meso-pores) shifted by 6–9 ppm to low frequency. Addition of the phosphorus-containing compounds to carbons with pre-adsorbed water rapidly causes displacement of most of the water from the strong adsorption sites. Other changes occur over a period of up to 24 h. Addition of water to carbons with pre-adsorbed phosphate causes some transfer of phosphate from the macropore/surface sites to the micro/mesopores although these were already heavily occupied by phosphate.

HIGH-RESOLUTION 2H SOLID-STATE NMR OF 2H2O ADSORBED ONTO ACTIVATED CARBON

Adsorption of deuteriated water onto activated carbon has been studied using magic-angle spinning (MAS)2H NMR. At 2H2O vapour concentrations below 80% relative humidity a single line of moderate width is observed. The peak is chemically shifted by 6–8 ppm to low frequency of the signal for bulk 2H2O. At high relative humidities ( > 80%) a further, sharper, 2H NMR signal was observed close to the chemical shift of bulk 2H2O. The observed chemical shifts and line widths are discussed in terms of the adsorption of the 2H2O into the porous carbon substrate.Signal intensities were measured relative to a standard sample and an adsorption isotherm plotted. The results are consistent with the standard gravimetric isotherm method for the sample. However, NMR offers the advantage of the direct observation of the chemical species present in various locations within the sample.

Oxygen-17 and deuterium NMR investigation into the adsorption of water on activated carbon

Magic‐angle spinning (MAS) 2H and 17O NMR spectra were obtained for water, following addition to an activated carbon. Shortly after addition two peaks are seen: a ‘free liquid’ resonance and a second broad resonance shifted to low frequency. The latter is attributed to water in the pores of the activated carbon. Over time, some intensity from the free water peak shifts into the lower frequency peak and a further decrease in the average chemical shift is seen. This is ascribed to gradual diffusion of water into the smaller pores with time. Comparisons between the 2H and 17O spectra are drawn with regard to the likely orientation of water molecules on the carbon surface in relation to the effects on chemical shifts by ring currents due to the graphene sheets. Spectra are presented for sealed samples spun at different MAS rates, and it is demonstrated that rates above 3 kHz induce loss of water from the pores, increasing the intensity of the free water peak. The implications of this centrifugation effect are discussed. Copyright

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).

Estimation of activated carbon adsorption efficiency for organic vapours: I. A strategy for selecting test compounds

Abstract Strategies for developing quantitative structure–affinity relationships (QSAfR) for the prediction of break-through performance of 31 chlorinated hydrocarbons on activated carbon have been studied. Two different approaches for the selection of a limited set of compounds for modelling were evaluated through the predictive power of the resulting QSAfR models. When the model was based on a training-set selected without a rational strategy, the developed QSAfR model showed poor predictive performance. Accordingly, such models have a limited capability to produce information concerning the important adsorbate related parameters influencing adsorption. By using a strategy where multivariate data analytical techniques are used in conjunction with statistical experimental design to select a balanced set of compounds for break-through performance evaluation, it was possible to develop QSAfR models with high predictive capability.