MODELING OF THE HEAT-TRANSFER PROCESS IN A DIFFERENTIAL SCANNING CALORIMETER

The model developed predicts a priori potential errors associated with the energy trace recorded by an isoperibol differential power scanning calorimeter in the measurement of heat of adsorption of H-2 on Pt and Pd catalysts. The uptake of H-2 by the catalyst sample was approximated by a diffusion-limited quasi-steady-state moving boundary model. This approximation is valid only if the parameter [(adsorption capacity of cat. sample)/(inlet conc. of H-2)] is extremely large (approximately 24). The effect of flow rate, amount of H-2 adsorbed, sink temperature, and the thermal conductivity of the adsorbate mixture was examined. Model predictions indicate that the error in the energy trace recorded by the DSC is appreciable: if a large difference exists between the thermal conductivity of the inert carrier, Ar (k = 0.017 J/m.K.s), and the adsorbate, H-2 (k = 0.174 J/m.K.s); if the heat sink temperature is much lower (approximately 90 K) than the measurement temperature. However, these errors can be eliminated by matching the thermal conductivity of the inert carrier and adsorbate, such as He (k = 0.143 J/m.K.s) and H-2 (k = 0.174 J/m.K.s). The results agree well with the experimental observations of Vannice et al. (1987) on high-purity Pt and Pd powder and supported Pt catalysts, if the H-2 uptake by the catalyst sample in the calorimeter is small (less-than-or-equal-to 2-mu-mol).