Turn-over frequencies of catalytic reactions on nanocatalysts measured by pulsed molecular beams and quantitative mass spectrometry

We present an experimental scheme for measuring turn-over frequencies (TOFs) and obtaining mechanistic details of catalytic processes on small clusters on surfaces with high sensitivity. The scheme uses a newly designed pulsed, piezo-electric driven valve with extremely high pulse-to-pulse stability in combination with quantitative mass spectrometry. The experimental scheme is applied for studying the oxidation of CO on mass-distributed palladium clusters deposited from a molecular beam onto well-characterized oxide surfaces. The obtained results are compared to the ones of palladium single crystals and supported palladium particles on oxide surfaces. The measured TOF is 47 +/- 8 CO2 molecules per deposited atom per second at 350 K and the observed mechanistic details can be explained by different active centers on the clusters and by the competitive adsorption of the two reactant molecules. The interpretation is in agreement with earlier studies on the oxidation of CO on palladium model catalysts. (C) 2003 Elsevier B.V. All rights reserved.