Temperature Dependence of Adsorption-Desorption Heats

In adsorption microcalorimetry, surface equilibration depends not only on the chosen probe molecule but also on the adsorption temperature. It is worth mentioning that the literature contains some controversial articles on this subject [29]. 

Heats of adsorption of probe molecules have frequently been measured at room temperature [8] however, the results obtained from such measurements can sometimes be of questionable accuracy as a result of non-equilibrium conditions and non-specific adsorption. 

In fact, the adsorption temperature should not be too low, in order to allow the detection of differences among the sites otherwise under certain circumstances the measured evolved heat can be just an average value. Another important issue is that one must ensure that chemisorption predominates over physisorption. 

Cardona-Martinez and Dumesic [30] have analyzed the problem of surface mobility of the adsorbates for the particular case of adsorption of basic probe molecules on acid sites of oxides. Without equilibration of adsorbate with surface sites, the measured differential heat would only be an average value of the sites that the molecules adsorb on, and differences among sites would not be detected. Thus, ideally, measurements should be made at as a high a temperature as feasible without desorbing or decomposing the adsorbate. 

In addihon to isothermal measurements, temperature programmed calorimetry experiments can also be performed using DSC. For such measurements, the sample is equilibrated with the adsorbate at low temperatures. The temperature is then increased in a controlled manner, and the heat absorbed (or evolved) by the sample over a certain temperature region is related to the heat of the event occurring at that point. 

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