Modifications required on a power-compensated differential scanning calorimeter to obtain heat of adsorption measurements

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Abstract
A commercial, power‐compensated, differential scanning calorimeter (DSC) has been modified and incorporated into a gas‐handling system to improve its performance and to allow its use to accurately measure heats of adsorption on supported metal catalysts. The use of gas flow controllers, needle valves, and appropriate purge gas mixtures minimized base‐line perturbations due to uneven flow rates to the sample and references cavities and to changes in thermal conductivity upon introduction of the adsorbate into the purge gas. However, errors in the energy trace were still observed when large differences existed between the thermal conductivities of the adsorbate and the purge gas. These errors can be especially pronounced with processes involving the adsorption or reaction of hydrogen (if Ar is the purge gas, for example), and the severity of the problem has not previously been recognized. However, the use of a purge gas mixture with a thermal conductivity near that of the adsorbate minimized this problem, and employing a DSC block temperature near that of the adsorption temperature further decreased this error. Utilizing He as the purge gas allowed accurate values to be obtained for the enthalpy of Pd hydride formation and for heats of adsorption of H2 on Pt surfaces. The difficulties with CO adsorption were much less severe because its thermal conductivity is much more similar to that of Ar, and Qad values for CO on Pt were more easily obtained. Finally, the error in the energy trace for H2 when Ar was used was proportional to the amount of H2 adsorbed; therefore a correction factor could be obtained which alternatively allowed Ar to be used as a purge gas and provided values in good agreement with those obtained under optimum conditions using He.