Mass-selected supported cluster catalysts: Size effects on CO oxidation activity, electronic structure, and thermal stability of Pdn/alumina (n ≤ 30) model catalysts

• CO oxidation studied over size-selected Pdn clusters supported on thin alumina films.
• Highly efficient and stable Pdn clusters (∼50% CO oxidation).
• Size dependent reactivity controlled by O2 activation and CO desorption kinetics.

An intense, mass-selected cluster source for preparation of model supported clusters is described, and results are presented for cluster size effects on CO oxidation activity for Pdn clusters supported on alumina films grown on Re(0001) or Ta(110) single crystals. The electronic structure of the samples was probed by X-ray and UV photoelectron spectroscopy, and low energy ion scattering was used to probe binding morphology. The Pdn activity was monitored via both steady state and temperature-programmed reaction (TPR) methods. In both cases, the samples appear to be stable in repeated reaction cycles for temperatures up to 600 K. In the TPR experiments, CO oxidation activity per Pd atom varied by ∼40% between the most and least reactive clusters, while under steady state conditions, the activity varied by ∼55%, but with a different pattern of activity vs. size. The difference in the effects of cluster size is attributed to the fact that TPR experiments were done under conditions where the rate-limiting step is oxygen activation, while the steady-state reactivity conditions were most sensitive to the strength of CO binding. Two distinct types of CO binding were observed and characterized by a combination of temperature-programmed desorption (TPD), and temperature-dependent ISS (TD-ISS).

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