ABSTRACT
The accurate determination of the gas sorption properties of a carbon nanomaterial is a prerequisite to the assessment of its suitability for use in practical gas sorption applications. Over the last decade or so, the hydrogen sorption properties of some nanostructured carbons, including nanotubes and nanofibres, have been the subject of controversy due to the large variation in the hydrogen storage capacities reported in the literature. Although the microstructural characterization of the materials in question was partly to blame, the technically demanding nature of hydrogen sorption measurements also played a significant role. Many of the error sources that can affect hydrogen sorption measurements can also, however, affect the determination of the gas sorption properties of other species of practical interest. In this chapter, we cover the gas sorption techniques typically used to characterize the adsorption properties of materials, and discuss experimental methodology and potential sources of error in sorption measurements performed on nanostructured and nanoporous carbons. We focus on elevated
pressure measurements, which are often required for practical applications, but are considerably more susceptible to error than those performed at sub-ambient pressure. 1.1 IntroductionThe quantitative determination of the gas sorption properties of materials is typically performed using the gravimetric and volumetric techniques. In addition, temperature-programmed methods have also been used recently to determine the hydrogen desorption properties of nanostructured and nanoporous carbons. The principal aim of a gas sorption measurement is often to determine a sorption isotherm, which is a plot of the quantity of gas adsorbed versus pressure at the measurement temperature. Gravimetric methods determine the amount of gas adsorbed or desorbed by measuring the weight change of a sample in response to its exposure to a step change in the gas pressure. Once equilibrium has been achieved, the weight reading is recorded and then the pressure changed. This process is repeated until a full isotherm has been determined. Volumetric techniques, on the other hand, determine the amount of gas adsorbed or desorbed using the real gas law, PV = nZRT (Eq. 1.1). The most common implementation of the technique measures the pressure change in a system of a fixed, known volume. An aliquot of gas is dosed from a calibrated volume into the sample cell. A decrease in pressure beyond that expected from the ratio of the dosing volume to the total system volume is then assumed to be due to adsorption. Further doses are delivered to the sample cell in order to construct a complete isotherm. In temperature-programmed hydrogen desorption techniques, a material is dosed with hydrogen and the temperature decreased below that required for desorption to occur. A temperature ramp is then applied and the desorbed quantity of hydrogen is monitored in one of a number of ways. Temperature-programmed desorption (TPD) from carbon nanomaterials typically requires a mass spectrometer. The total desorbed quantity can be determined from the integral of the mass spectrometer signal, providing it can be adequately calibrated. Otherwise, the temperature of the desorption peaks and the form of the spectrum provides information regarding the adsorbed state of the hydrogen.
