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 (∼ 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 (∼ 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 (⩽2 μmol).