Cluster size effects on CO oxidation activity, adsorbate affinity, and temporal behavior of model Aun/TiO2 catalysts.

Model catalysts were prepared by deposition of size-selected Au_n (n=1–7) on rutile TiO₂(110), and characterized by a combination of electron spectroscopy, ion scattering, temperature-programmed desorption, and pulse-dosing mass spectrometry. CO oxidation activity was found to vary strongly with deposited cluster size, with significant activity appearing at Au₃. Activity is not obviously correlated with affinity for CO, or with cluster morphology, but is strongly correlated with the clusters’ ability to bind oxygen (during O₂ exposure) on top of the gold. The temporal dependence of CO₂ evolution in reaction of O₂ pre-exposed samples with CO pulses shows an interesting cluster size dependence. For Au₅ and Au₆, the peak CO₂ production is coincident with the peak CO flux, but for Au₃, Au₄, and Au₇, there are significant induction periods for CO₂ evolution. In addition, it is observed that some of the most active cluster sizes have the slowest CO₂ evolution rates. Several mechanistic scenarios capable of accounting for the observations are laid out. [ABSTRACT FROM AUTHOR]