CO and O overlayers on Pd nanocrystals supported on TiO2(110).

We have prepared a model catalytic system by depositing Pd onto a TiO2(110) surface held at approximately 720 K. Scanning tunneling microscopy (STM) reveals well-defined Pd nanocrystals consisting of (111) top facets with {111} and {100} side facets. The Pd nanocrystals go down to about 10 nm in width and 1.3 nm in height. Top facets can be imaged with atomic resolution, indicating the absence of TiOx encapsulation. The model catalyst was probed by exposure to CO and O2. By varying the CO exposure, different CO overlayers were formed on the (111) top facets, with coverages ranging from 0.33 to 0.75 of a monolayer. Near edge X-ray absorption fine structure (NEXAFS) measurements at 300 K reveal that at around 0.5 ML coverage, CO is oriented with the molecular axis more or less normal to TiO2(110). Dosing small amounts of 02 separately on a Pd/ TiO2(110) surface led to an overlayer of p(2 x 2)-O formed on the (111) top facet of the Pd nanocrystals at 190 K.