Size-dependent electronic structure controls activity for ethanol electro-oxidation at Ptn/indium tin oxide (n = 1 to 14).

Understanding the factors that control electrochemical catalysis is essential to improving performance. We report a study of electrocatalytic ethanol oxidation - a process important for direct ethanol fuel cells - over size-selected Pt centers ranging from single atoms to Pt14. Model electrodes were prepared by soft-landing of mass-selected Ptn(+) on indium tin oxide (ITO) supports in ultrahigh vacuum, and transferred to an in situ electrochemical cell without exposure to air. Each electrode had identical Pt coverage, and differed only in the size of Pt clusters deposited. The small Ptn have activities that vary strongly, and non-monotonically with deposited size. Activity per gram Pt ranges up to ten times higher than that of 5 to 10 nm Pt particles dispersed on ITO. Activity is anti-correlated with the Pt 4d core orbital binding energy, indicating that electron rich clusters are essential for high activity.

Oxygen activation and CO oxidation over size-selected Pt(n)/alumina/Re(0001) model catalysts: correlations with valence electronic structure, physical structure, and binding sites.

Oxidation of CO over size-selected Ptn clusters (n = 1, 2, 4, 7, 10, 14, 18) supported on alumina thin films grown on Re(0001) was studied using temperature-programmed reaction/desorption (TPR/TPD), X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and low energy ion scattering spectroscopy (ISS). The activity of the model catalysts was found to vary by a factor of five with deposited Ptn size during the first reaction cycle (TPR) and by a factor of two during subsequent cycles, with Pt2 being the least active and Pt14 the most active. The limiting step in the reaction appears to be the binding of oxygen; however, this does not appear to be an activated process as reaction is equally efficient for 300 K and 180 K oxidation temperatures. Size-dependent shifts in the valence band onset energy correlate strongly with CO oxidation activity, and there is also an apparent correlation with the availability of a particular binding site, as probed by CO TPD. The morphology of the clusters also becomes more three dimensional over the same size range, but with a distinctly different size-dependence. The results suggest that both electronic structure and the availability of particular binding sites control activity.

Alumina support and Pdn cluster size effects on activity of Pdn for catalytic oxidation of CO.

A series of model catalysts were prepared by depositing different size Pd(n) clusters on alumina films grown to variable thickness on a Ta(110) support. Samples were characterized by a combination of X-ray photoelectron spectroscopy, low energy He+ scattering, and temperature-programmed reaction and desorption (TPR/ TPD). For the activity studies, the samples were first exposed to 18O2 at Tox, and then to 13CO at 180 K, where CO sticks to Pd, but not to the alumina support. CO oxidation activity increased with increasing thickness of the alumina support up to approximately 4.5 nm, but was constant for greater thicknesses. Activity increased, with Tox up to 400 K, but then declined for Tox = 500 K. Activity was also found to be non-monotonically dependent on deposited cluster size, with Pd(n) (n < or = 6) being generally more reactive than the larger clusters studied. Activity was only weakly correlated with exposed Pd binding sites, which decreased with increasing cluster size, however, there does appear to be a correlation between activity and electronic structure, as probed via the Pd 3d binding energy. Unlike previous systems we have studied, the activity of small Pd(n) on these alumina films was quite stable, with essentially no changes observed in up to eight successive TPR experiments.