Effects of particle size on the progressive oxidation of nanometer platinum by dioxygen

Platinum particles in nanometer size were dispersed on the gamma-Al2O3 support by the impregnation technique. Oxidation phenomena of the supported particles with dioxygen were pursued by a simultaneous TG-DSC technique. Observed extent of oxidation varies in four consecutive steps, i.e., adsorption of oxygen on the surface of supported platinum crystallites at ambient or lower temperatures, reconstruction of platinum surface for extended accommodation of oxygen above 300 K, formation of a stable surface layer of platinum oxides about 750 K, and desorption of oxygen and/or platinum dioxide over 800 K, while raising the temperature. Measured heat of oxidation (-Q(ox)) generally decreases with the extent of oxidation. The particle size (d) of dispersed platinum has a profound effect on the oxidation. The species of surface platinum oxide formed at 770 K was (PtO2)-O-s [-Delta H-f = 190 kJ (mol O-2)(-1)] as d < 1.3 nm but became (PtO)-O-s [-Delta H-f = 169 kJ (mol O-2)-1] On d > 2.0 nm. The heat of dioxygen adsorption (Delta H-ad) at T < 300 K could be also correlated with the average diameter (d) of supported platinum according to -Delta H-ad [kJ (mol O-2)(-1)] = 406 - 81 d/nm when the platinum diameter was less than 2.5 nm. (C) 1998 Academic Press.