Ethylene Epoxidation on a Au Nanoparticle versus a Au(111) Surface: A DFT Study

The heterogeneously catalyzed epoxidation of alkenes is experimentally challenging, theoretically interesting, and technologically of vital importance. Recent experimental studies show that small gold nanoparticles supported on inert materials are efficient and robust catalysts for the selective oxidation of alkenes. The reasons for the outstanding catalyst of Au nanoparticles have been investigated and compared with the Au(111) surface by means of density functional theory. The nanoparticle is intrinsically much more selective than the surface in the epoxidation. The fundamental cause is the inversion in the ordering of activation barriers for the competing pathways to epoxide formation versus acetaldehyde formation. On the nanoparticle, epoxide formation is less activated than acetaldehyde formation, whereas the opposite is true on the (111) surface. This behavior is associated with a late transition state to epoxidation on the nanoparticle (i.e., product-like) compared to an early (reactant-like) transition state to epoxidation on the (111) surface.