Predicting the heats of adsorption for gas physisorption from isotherm measurements

This thesis consists of an introduction which includes a brief survey of literature relevant to the topic of predicting the heats of gas physical adsorption, three chapters that have been written as a series of papers on the topic and a conclusion which discusses the significance of each of the chapters to the field of adsorption. In Chapter I a new method of predicting the heats of adsorption is presented based on forming the Tóth potential function from isotherm models. This new method requires the measurement of only a single isotherm, in contrast to methods based on Clapeyron or Clausius-Clapeyron which require multiple isotherms. The method is compared to calorimetrically obtained isosteric heats for a variety of adsorbate/adsorbent systems showing type I and type II isotherm behavior; the method of predicting the heats of adsorption is shown to agree with the measured values to within ±10-15%. In Chapter II the previously introduced method of predicting the heats of adsorption is compared to predictive methods predicated on the Clausius-Clapeyron equation, assuming ideal gas behavior and negligible adsorbed phase volume and a modified Clapeyron approach that incorporates real gas behavior but still neglects the adsorbate specific volume. Three adsorbate/adsorbent systems are considered for which multiple isotherms and corresponding integral heats of adsorption have been measured. Based on comparison with the data, the new method is found to be superior to both Clausius-Clapeyron and the modified Clapeyron approach. In Chapter III the new method for predicting the heats of adsorption is adapted to model the adsorbate specific heat capacity and is compared to a model derived from Clausius-Clapeyron. Models for residual, integrated-isosteric heat are fit to data for an argon on rutile phase titanium dioxide system to obtain parameters for specific heat capacity modeling. The specific heat capacity model derived from the Tóth potential function is better at predicting the trend found in the data of decreasing heat capacity with increasing uptake than is the Clausius-Clapeyron inspired model and predicts values for the heat capacity closer to the data than would be found from assumptions commonly found in the literature such as the adsorbed phase heat capacity being equal to that of a saturated liquid or an ideal gas.