(614b) Catalytic Activity of Platinum Nanoparticles for H2 Formation
Theoretical calculations based on density functional theory (DFT) and transition state theory have led to a deeper understanding of catalytic activity and thus helped develop improved heterogeneous catalysts, a task that is vital for the design of processes with higher energy and atom efficiency in the chemical industry. By providing information about the factors controlling the reactivity, such as the energetics of molecule-surface interaction and the kinetics of elementary processes at solid surfaces, trends in the catalytic activity as the chemical composition of the catalyst is varied have been established and predictions made for new and improved catalysts.
The interaction of hydrogen with the surface of platinum metal is of fundamental importance to a wide range of technologies including electrolysis of water and hydrogen fuel cells. The metal is typically dispersed as small particles embedded in a matrix. The active sites of such particles can be under-coordinated atoms at edges where crystal facets meet, or at corners where edges meet. A 3 nm particle, which is at the lower end of the size range used in fuel cells today, contains on the order of 1000 atoms. DFT calculations of the catalytic activity of a whole nanoparticle of this size generally require a prohibitively large computational effort. An alternative approach is to construct a periodic array of the sites of interest, such as edges between micro-facets. The missing-row reconstructed Pt(110)-(1x2) surface can be considered a very small model of edges between (111) facets and has been studied both experimentally and theoretically [1,2].
In the current work , the associative desorption of hydrogen at edges and facets on Pt nanoparticles was studied using DFT. The goal was to identify catalytically active sites on Pt nanoparticles for the hydrogen evolution reaction as well as the hydrogen oxidation reaction. The adsorption sites were modeled by periodic, face centered cubic slabs representing an array of edges between two (111) micro-facets or edges between (111) and (100) micro-facets. The width of the facets in the periodic representations was systematically increased to reach converged results for binding and activation energy. Under typical electrochemical conditions (for small applied voltage vs. SHE), edges between (111) micro-facets were found to be several orders of magnitude more active than edges between (100) and (111) micro-facets or at terraces.
 S. Gudmundsdóttir, E. Skúlason, and H. Jónsson, Phys. Rev. Lett, 108, 156101 (2012).
 S. Gudmundsdóttir, E. Skúlason, K.-J. Weststrate, L. Juurlink, and H. Jónsson, Phys. Chem. Chem. Phys. 15, 6323 (2013).
 E. Skúlason, A. A. Faraj, L. Kristinsdóttir, J. Hussain, A. L. Garden and H. Jónsson, Top. Catal. (submitted).