Enthalpies of reduction-oxidation of alumina-supported platinum

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 T-red = 670 and T-ox = 770 K for PtOx/Al2O3 and Pt/Al2O3, respectively, The measurements revealed that the heat of reduction (DeltaH(red)) is relatively constant and has a value of -103 kJ +/- 3 kJ (mol H-2)(-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 (DeltaH(f)). The platinum oxide formed at 770 K was PtO2 for d < 2.0 nm, but this became Pt2O for d > 4.0 nm. The DeltaH(f) also varied with platinum oxide species from -190 kJ (mol O-2)(-1) for PtO2 to -168 kJ (mol O-2)(-1) for Pt2O. Based on a thermodynamic cycle, it was demonstrated that the consistency in DeltaH(red) for various platinum oxides was attributed to the variations of heat of adsorption of water (DeltaH(ad,H2O)) on alumina-supported platinum. (C) 2002 Elsevier Science B.V. All rights reserved.