(Top frame) Temperature-programmed desorption (TPD) of 13CO from TiO2 and Pdn/TiO2 (n = 1, 2, 7, 20) following 5L 13CO exposure at 180 K. Points are raw m/q = 29 data, and curves are fits based on simulations of the experiments (see text). (Bottom frame) Edesorption distributions used to fit the TPD spectra in the top frame. The distributions are normalized so that the integrals give the number of CO molecules desorbing from the probe spot. (Inset) The low energy portions of the distributions.
Integrated intensities of the three CO desorption features: TiO2-bound (□), and 200 K (Δ) and 430 K (O) Pd-bound CO. The desorption scale gives the lower limit on the total number of CO molecules desorbing from the cluster spot.
Pd/(Ti+O) ISS intensity ratios for Pd2/TiO2 (bottom frame) and Pd20/TiO2 (top frame) as-deposited, after exposure to 5 L13CO at 180 K, and during a sequence where the samples are heated to the indicated temperatures. Calibrated rates for recovery of Pd signal from CO sputtering, and for loss of Pd signal by Pd sputtering are shown as sloping lines.
Pd 3d XPS for 0.1 ML Pd7/TiO2 as deposited (■), after exposure to 5 L 13CO at 180 K (•), and following 1 TPD cycle (▲). (Inset) Shifts in the Pd 3d binding energy upon exposure to 5 L 13CO at 180 K and following TPD for Pd7 and Pd20 on TiO2.
(a) m/z+ = 29 TPD after exposing 0.1 ML Pd2/TiO2 and Pd20/TiO2 to 5 L 13CO at 180 K (blue points and curve) and m/z+ = 28 desorption collected after sample preparation with no deliberate dose (red points and curve). (b) TD-ISS collected from 0.1 ML Pd2/TiO2 after preparation with no deliberate gas dose. (Inset) The extent of adventitious CO binding detected via TPD after sample preparation for 0.1 ML Pd2/TiO2 and Pd20/TiO2 is plotted (red bars) alongside the amount desorbing above room temperature following exposure to 5 L 13CO at 180 K (blue bars).
Pd 3d XPS peak positions are plotted for various Pd4/TiO2 samples as a function of the cluster deposition time required to create the surface.
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