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A calorimetric technique is described for measuring the reaction enthalpies of reduction of PtOx/Al2O3 with hydrogen and oxidation of Pt/Al2O3 with oxygen. The enthalpies of reduction–oxidation were determined at Tred=670 and Tox=770 K for PtOx/Al2O3 and Pt/Al2O3, respectively. The measurements revealed that the heat of reduction (ΔHred) is relatively constant and has a value of − 103 kJ ±3 kJ (mol H2)−1 for various platinum oxides. The particle size of platinum metals, however, had a profound effect on the bulk oxide formed and heat of oxide formation (ΔHf). The platinum oxide formed at 770 K was PtO2 for d 4.0 nm. The ΔHf also varied with platinum oxide species from −190 kJ (mol O2)−1 for PtO2 to −168 kJ (mol O2)−1 for Pt2O. Based on a thermodynamic cycle, it was demonstrated that the consistency in ΔHred for various platinum oxides was attributed to the variations of heat of adsorption of water (ΔHad,H2O) on alumina-supported platinum. |
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