Temperature-Programmed Desorption of Hydrogen from Platinum Particles on γ-Al2O3: Evidence of Platinum-Catalyzed Dehydroxylation of γ-Al2O3

Samples of Pt on γ-Al2O3 were prepared by reduction in H2 at 400°C of [PtCl2(PhCN)2], adsorbed intact from n-pentane solution onto γ-Al2O3 powder that had been partially dehydroxylated under vacuum at 400°C. The Pt dispersion was determined to be about 0.58 by standard methods, namely, chemisorption of hydrogen, of oxygen, and of CO, titration of chemisorbed oxygen with hydrogen, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The supported Pt particles were resistant to sintering in the presence of O2 at temperatures up to 400°C, as shown by chemisorption and EXAFS data, but the latter indicated that a combination of O2 and H2 treatments at 200–400°C led to changes in particle morphology. The samples were investigated by temperature-programmed desorption (TPD) of preadsorbed hydrogen; a TPD peak centered at 120°C represents hydrogen adsorbed on Pt and confirms the value of 0.58 for the dispersion. TPD of preadsorbed H2 gave evidence of a second desorption peak, at 580°C, which was always accompanied by desorption of water at the same temperature; its presence was found to depend chiefly on the sample reduction temperature and evacuation time (it was absent when there was no Pt on the γ-Al2O3). The desorption of H2 or D2 that had been adsorbed on Pt/γ-Al2O3 reduced with D2 or H2, respectively, shows that hydrogen species desorbed at about 580°C originate from water (hydroxyl) species as a result of Pt-catalyzed decomposition.