Three-Site Model for Hydrogen Adsorption on Supported Platinum Particles:  Influence of Support Ionicity and Particle Size on the Hydrogen Coverage

• Published in: Journal of the American Chemical Society 2005-02, Vol.127 (5), p.1530-1540
• Main Authors: Oudenhuijzen, Michiel K, van Bokhoven, Jeroen A, Miller, Jeffrey T, Ramaker, David E, Koningsberger, Diederik C
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja045286c

Pt L3 X-ray absorption edge data on small supported Pt particles (N < 6.5) reveals that at very low H2 pressure or high temperature the strongest bonded H is chemisorbed in an atop position. With decreasing temperature or at higher H2 pressure only n-fold (n = 2 or 3) sites are occupied. At high H2 pressure or low temperature, the weakest bonded H is positioned in an “ontop” site, with the chemisorbing Pt already having a stronger bond to a H atom in an n-fold site. DFT calculations show that the adsorption energy of hydrogen increases for Pt particles on ionic (basic) supports. The combination of the DFT calculations with hydrogen chemisorption data and the analysis of the Pt L3 X-ray absorption spectra implies that both the H coverage and/or the type of active Pt surface sites, which are present at high temperature catalytic reaction conditions, strongly depend on the ionicity of the support. The consequences for Pt catalyzed hydrogenolysis and hydrogenation reactions will be discussed.