Gold Catalysed Syngas Conversion: Pathways and Size Effects
Günter Klatt , Eric Van Steen,
Despite the accepted d10s1 electronic configuration for elemental gold, supported nanoparticles of this metal have been shown to be active catalysts for many reactions of industrial interest . For most of the reported gold catalysed reactions, e.g CO oxidation and methanol synthesis, the reaction rate responds to changes in the gold crystallite size in a manner than cannot be explained by dispersion effects alone.
Insights from Computational Studies
Consequences of size effect on syngas conversion potential of clean gold surfaces were investigated by comparing chemisorptive properties of methanol synthesis intermediates on gold surfaces using the cluster approach (as models for small particles) and plane-wave slab model (to model an extended gold surface). It is shown that the chemisorptive properties of methanol synthesis intermediates are different on the cluster and the slab models, interpreted as a size effect.
The electronic structure of small gold clusters of different sizes (up to Au309) was explored using the semiempirical Extended Hückel Theory, with trends in the Fermi energy being shown to be similar to DFT predictions for Au2 – Au13 clusters. The results reveal that the separation between the d-band and the Fermi level increases as the cluster size increases (see Figure 1). A small d-band – Fermi gap will result in lowering of the Pauli repulsion between the occupied orbitals of an approaching molecule and the d-band, making small gold clusters intrinsically less noble, from a catalytic point of view, than bulk gold. Figure 2 shows the minimum structures of a formaldehyde species asorbed onto an Au13 cluster and an extended Au(111) surface.
The results further suggest that one of the feed molecules, CO2, does not adsorb at all onto a gold surface (cluster or extended), unless the surface is modified by oxygen. This points to the need to have both oxidised and metallic gold particles during methanol synthesis. The roles of the CO2/CO ratio and the metal-support interactions in generating oxidised gold particles will be discussed.
1. Haruta, M., Catalysis Today 36 (1997) 153-166.
Cluster Size, Mechanisms, Pauli Repulsion, Methanol Synthesis