Invited Paper SS1-MoM5
The Fabrication and Reactivity of Anchored Metal Nanoparticles on TiO2(110)
Monday, October 15, 2007, 9:20 am, Room 608
We have investigated the interaction of Pd particles with the TiO2(110) surface in some detail using high temperature STM, XPS and a molecular beam reactor. Upon formation of nanoparticles at low anneal temperatures (<600K), a high adsorption probability of CO is found, much higher than expected from the fraction of the surface covered by Pd. This is due to the influence of a precursor state; this is a weakly-held form of CO on the support, which has a short lifetime at 300K (~ 1 micros), but which can nevertheless find nearby Pd nanoparticles (at which it gets trapped) by surface diffusion, which is extremely efficient. Nonetheless the CO is actually held more weakly on the nanoparticles than on bulk Pd, due, we believe, to modifications of the Pd by the presence of Ti in the surface of the particle. Upon heating to only 700K, the CO is further destabilised on the Pd, such that the sticking probability and uptake become very low; this is NOT due to sintering which is minimal at this temperature. Investigation by XPS shows that Ti has migrated onto the surface of the Pd as the surface was heated. From the chemical shifts this Ti is probably present as surface TiO, with Ti in the 2+ oxidation state. STM shows that, at least for surfaces annealed above 700K, beautiful ordered structures of the surface layer are formed, with large unit cells. The two main structures are a pinwheel and a zig-zag. In this talk I will try to assign the detailed structures of these layers which is proposed to be intermetallic-like, with the two structures being very specific intermetallics with Pd:Ti ratios of 1:1 and 2:1 respectively. This material causes the loss of activity seen for CO adsorption and for catalysis.