Paper SS1+NC-TuA8
Hydrogen Splitting on Ru and Pt Nanoparticles

Tuesday, October 21, 2008, 4:00 pm, Room 208

Here we investigate the hydrogen splitting rate as a function of particle size for Pt and Ru by measuring the rate of the H-D exchange reaction at 1 bar. The study is mainly motivated by fuel cell applications, where highly dispersed Pt is used as the anode catalyst in the Proton Exchange Membrane (PEM) fuel cell. It was found in an earlier study that at 1 bar, Ru gives a higher rate for the H-D exchange reaction than Pt.¹ The experiments are performed in an apparatus which combines an ultra-high vacuum chamber for sample preparation and surface analysis with a high-pressure cell.² Model catalysts in the form of spots with a diameter of 1 mm are prepared by electron-beam evaporation of metal onto a sputtered highly ordered pyrolytic graphite (HOPG) substrate. The amount of metal corresponds to an average thickness between 0.2 and 50 Å. The rate of the H-D exchange reaction for each catalyst spot is measured in the high-pressure cell with the help of a combined gas dispenser and gas sampling device. The sampled gas is analyzed with mass spectrometry. Measurements were carried out at 1 bar with a gas mixture containing 1 percent D₂ in H₂ and the temperature was varied in the range 40 to 200 °C. The model catalysts were characterized with Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM) before and after the high-pressure measurements. A simple model is used in order to extract the sticking probability for H₂ from data for the H-D exchange rate. Once the sticking probability has been obtained, the dissociative adsorption/desorption rate for H₂ can be calculated. The apparent desorption energy, E_des, for H₂ at the H coverage corresponding to 1 bar pressure is then extracted from data for the desorption rate as a function of temperature. It is found that at 0.2 Å average metal thickness, E_des is approximately 0.2 eV for Ru and 0.27 eV for Pt, and that the metals form particles with a mean diameter less than 10 Å. As the metal loading is increased, the particle size increases whereas E_des decreases. Above approximately 15 Å, an almost continuous metal film is formed and E_des becomes constant, 0.02 eV for Ru and 0.21 eV for Pt. Hence, the desorption energy for H₂ decreases with increasing particle size for both Pt and Ru, but the effect is more pronounced for Ru.

¹ M. Johansson, O. Lytken, I. Chorkendorff, J. Chem. Phys. 128 (2008) 034706.
² M. Johansson, J. Hoffmann Jørgensen, I. Chorkendorff, Rev. Sci. Instrum. 75 (2004) 2082.