Zero coverage procedure

Figure 2.14 Simulated TDS spectra and the results of a number of different analysis procedures for determining the activation energy of desorption. The solid line represents the input for the simulations. Note that only the complete analysis [16] and the leading edge procedure of Habenschaden and Kiippers [29] give reliable results. The Chan-Aris-Weinberg curves [28] extrapolate to the correct activation energies at zero coverage (from de Jong and Niemantsverdriet [31]).

Several methods are used to calculate the amount of adsorbate corresponding to monolayer coverage. Extrapolation of the nearly linear high pressure portion of the adsorption isotherm back to zero pressure, and calculation of the amount of gas adsorbed at zero pressure is the most usual procedure. One reason for chosing this method is that it minimizes errors due to weak adsorption on the support since at low pressures this is directly proportional to the adsorption pressure. An alternative means of determining the monolayer coverage is to measure the quantity of gas adsorbed at a fixed reference pressure (e.g., about 25 kPa). 

In this experience, the colloid suspension was introduced into a reactor of volume equal to 50 mL and stirred to ensure homogeneity. At time zero, the polymer solution was injected at constant rate and periodically the corresponding volume of the suspension was collected for a certain time to determine the aggregate mass characteristics and to start fragmentation experiments by immersion of these aggregate samples in the polymer solution of concentration C /30. This procedure allows the surface coverage T(f) to be a linear function of the injection time t due to fast total adsorption of the injected polymer. The variations with time of the average masses N(t) and... 

For many adsorbate/support systems Henry s Law is obeyed and, as a consequence, this procedure is simplified for the experimenter because Line 3 in Figure 3.3 can simply be extrapolated to its zero-pressure intercept to obtain b, which is equal to a, thus eliminating the need to obtain a second isotherm. This latter procedure does not represent the gas uptake at zero pressure, as sometimes mis-stated, rather it represents subtraction of adsorption on the support at saturation coverage on the metal. 

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