CO adsorption on a Pd/TiO2(1 1 0) model catalyst

The adsorption of CO on model catalysts consisting of Pd nanoparticles on the (1×1) surface of TiO2(1 1 0) has been measured using a thermal molecular beam. The adsorption is dominated by weakly held `precursorʹ states, in this case mainly in the form of reverse spillover of CO from the support (where it only exists in a short lifetime state) to the Pd nanoparticles where it is trapped into a long lifetime state. The heat of adsorption of the weakly held state on the support is about 38 kJ mol−1. For a catalyst with 12% coverage of Pd, the whole of the support provides CO by diffusion to the metal particles up to a temperature of 330 K, at which point the diffusion range of the precursor on the support becomes shorter than the interparticle separation. Further, the CO appears to be significantly more weakly held on the nanoparticles than on a Pd single crystal surface (initial heat of adsorption of 110 kJ mol−1 versus 135 kJ mol−1), showing zero net sticking above 400 K. Annealing of the model catalyst above 573 K leads to a significant loss of CO uptake and sticking probability, probably due to a combination of sintering and an SMSI-like effect. The reduced reactivity of the nanoparticles is due to alloying of the Pd with Ti, the latter originating from Ti3+ interstitials in the bulk of the TiO2.