Oxidation Catalysis by Gold Nano-Particles supported on h-BN Nanomesh

Supported Au nano-particles are considered as promising oxidation catalysts with superior low temperature activity and selectivity in a variety of chemical reactions. However, supported nano Au-catalysts are facing a major problem with sintering under typical reaction conditions. For the case of Au/TiO2(110) oxygen vacancies or oxidized (alkaline) TiO2(110) may provide nucleation sites for immobilizing the Au clusters up to 500K. However, the microscopic processes responsible for the low temperature activity of Au particles on TiO2(110) are still controversially discussed in the literature. The recently discovered nanomesh of hexagonal BN (h-BN) on Rh(111) and on Ru(0001) offers a unique sturdy oxygen-free template for supporting Au nanoparticles. The h-BN/Ru(0001) nanomesh consists of a periodic hexagonal array of 2 nm wide pores with a lattice constant of 3.25 nm. The h-BN nanomesh can be viewed as a highly regular network of trapping sites in which deposited Au atoms preferentially condense into Au nano particles. This allows for the preparation of well-ordered model catalysts. With the Au/h- BN/Ru(0001) model catalyst we shall perform in-situ experiments and ab-initio calculations to elucidate both the oxidation behavior of Au particles and its catalytic behavior in oxidation reactions, including the simple CO oxidation and the "dream reaction" of propylene epoxidation. The oxygen-free h-BN nanomesh support facilitates significantly the identification of the catalytically active oxygen species on the Au particles with spectroscopic methods. The major goal of the present project is to advance the molecular understanding on the low-temperature activity of supported Au clusters which are not affected by defect sites of the supporting oxide surface. Electronic properties of the Au and oxidized Au clusters will be studied by photoemission spectroscopy and compared with first principles electronic structure calculations. The catalyzed oxidation reaction will be studied by in-situ infrared experiments/on-line mass spectrometry and modeled in detail by ab-initio calculations.

Gold has attracted much scientific attention over the past two decades spurred by the pioneering work of Haruta and coworkers, indicating a tremendous difference in the catalytic activity between bulk Au and (supported) Au nano-particles. Au nano-particles are typically grown on metal-oxide supports and considered as promising oxidation catalysts with superior low temperature activity and selectivity in a variety of chemical reactions. However, supported Au-catalysts are facing a major problem with sintering of the Au nanoparticles under typical reaction conditions. The recently discovered nanomesh of hexagonal BN (h-BN) on Rh(111) or Ru(0001) offers a unique sturdy oxygen-free template for supporting Au nanoparticles. The h-BN/Ru(0001) nanomesh consists of a highly corrugated monolayer of h-BN on top of a metal substrate, which forms a periodic hexagonal array of 2 nm wide pores with a lattice constant of 3.25 nm. h-BN resembles quite some similarities to the now very famous and intensively studied graphene layers since it is isoelectronic and isostructural with graphene. However, it also has distinct differences, namely some ionic bonding contribution between B and N as well as a large band gap. We have found that the h-BN nanomesh can be viewed as a highly regular network of trapping sites in which deposited Au atoms preferentially condense into Au nano-particles. This allows for the preparation of well-ordered model catalysts. The Au atoms are strongly bound to the B atoms in the pores of the nanomesh and can easily form small clusters. On the other hand, there is a large energy barrier for Au atoms to migrate from one pore to the other and this will prevent the sintering of the individual Au nano-clusters into one big, catalytically inactive Au particle. The major result of the present project was to advance the fundamental understanding which leads to small supported Au clusters of high stability, which are not affected by defect sites of a supporting oxide surface. Electronic properties of the Au clusters have been studied by first principles electronic structure calculations and reveal that Au is negatively charged. This is commonly assumed to be a prerequisite for catalytic activity of Au.