Structure and reactivity of nanoparticles: CO dissociation on Ruthenium, CO induced surface reconstruction of Platinum, Platinum and Platinum-Yttrium alloys for electrochemical Oxygen Reduction Reaction

The correlation between the structure and reactivity of mass-selected nanoparticles have been investigated by the surface science approach for two model systems: 1) The dissociation of CO on ruthenium and 2) platinum and platinum-yttrium alloy tested for the electrochemical Oxygen Reduction Reaction (ORR). Furthermore, surface reconstructions of platinum nanoparticles induced by CO have been studied for various particle sizes.

All the model catalysts consisted of mass-selected nanoparticles supported on
planar oxide or carbon substrates. The nanoparticles were produced in a ultra
high vacuum (UHV) setup by the magnetron sputter gas-aggregation technique and mass-selected before deposition onto the support. This approach provides well-defined model catalysts with mono-dispersed nanoparticles where the particle size and particle coverage can be varied independently, ideal for studying particle size effects. The model catalysts are furthermore compatible with UHV surface science techniques which enables detailed characterisation of the particle structure, elemental composition and reactivity.

In the study of CO dissociation on ruthenium, nanoparticles in the size range from 3 to 15 nm were deposited onto Highly Ordered Pyrolytic Graphite (HOPG) and the active sites for CO dissociation were probed by temperature programmed desorption spectroscopy using isotopically labelled CO. Combined with transmission electron microscopy we gain insight on how the size and morphology of the nanoparticles affect the CO dissociation activity. Surprisingly, it was found that larger particles exposed a higher fraction of active sites. It is suggested that this is due to larger particles exposing a more rough surface, that contain a high fraction of undercoordinated sites, than the smaller particles. The variation in surface roughness with particle size is a proposed to be a consequence of the growth processes in the gas-aggregation chamber. Furthermore, we provided a link in the CO desorption behaviour from Ru single crystal model catalysts to nanoparticulate model catalyst, in order to narrow the materials gap.

The activity of platinum and platinum-yttrium nanoparticles supported on glassy carbon for the electrochemical ORR was studied in the size range from 2 to 11 nm, and a size dependence for both pure platinum and platinum-yttrium nanoparticles was established. For pure platinum, the specic activity was found to increase with increasing particle size, and the specic activity correlates to the fraction of terrace sites. It was therefore concluded that the active sites for ORR on platinum are located on the terraces, in good agreement with earlier theoretical predictions. A maximum in the mass activity was found for ∼3 nm diameter particles. For the Pt_xY nanoparticles, a clear enhancement of the ORR activity compared to pure platinum nanoparticles was observed, and to the best of our knowledge, the 9 nm Pt₅Y nanoparticles display the highest ORR activity ever measured on supported nanoparticles. The origin of the enhanced activity is speculated to be caused by a compressed platinum overlayer covering a Pt_xY alloy core.
The topic of adsorbate induced surface reconstructions was elucidated for a model catalyst of platinum nanoparticles on SiO₂ support and a Pt(111) single crystal, which was exposed to an elevated pressure of CO. Exposing the roughened Pt(111) crystal to mbar range pressures of CO at elevated temperatures gave rise to a promotion of the annealing process whereas the same treatment of the 3 nm and 6 nm platinum particles induced an apparent surface roughening. The 11 nm particles showed only a minor increase in surface roughening. We hypothesize that the dierences in CO induced surface reconstruction with particle size is related to the facets size and the distribution of surface atoms with low coordination number.
The results highlight the structure sensitivity of catalytic reactions and the influence of adsorbates and gaseous environment on the structure of nanoparticles. The results furthermore demonstrated the use of temperature programmed desorption experiments as a sensitive probe of the conguration of surface sites and the ability to distinguish between low index facets and under-coordinated atoms on both single crystal and nanoparticle model catalysts.