Surface crystallography of three catalytically important structures in the Au{111}-Pd system

Automated Tensor LEED has been used to determine the detailed crystallography of structures formed during Pd deposition on Au{111}. At 200 K, deposition of 0.2 monolayer Pd results in a complete lifting of the clean surface Au{111} reconstruction, with excess Au atoms being squeezed into a disordered adlayer with the added Pd adatoms. At monolayer coverage at this temperature a disordered structure is formed, which orders on heating to 300 K to a (1 × 1) structure with a predominantly Pd top layer. Further deposition of Pd produces a disordered overlayer. On annealing to ≥ 500 K, this converts to an ordered alloy structure with ( 3× 3)R30° periodicity. This structure consists of at least 2 layers of Pd₂Au alloy. It is unique in showing a constant splitting of all Bragg beams, attributed to facetting into domains oriented only 0.75° from the planar Au{111} surface. This rumpled structure possesses domains which are larger than 100 Å in width. The results are discussed in relation to the known catalytic activity for the various structures in the cyclisation of ethyne to benzene.