Oxidation Catalysis by Gold Nano-Particles supported on h-BN Nanomesh
Final Report Abstract
The original objective of the project was directed to gain both: in-depth understanding of the gold nano-particles in oxidation catalysis and the importance of the oxidic (especially O defects) character of the oxide support for Au-related catalysis. For this reason we prepared and characterized Au particles on h-BN nanomesh as an oxygen-free support which was uniformly coated on Rh(111) or Ru(0001). From such model catalytic systems we expected thermal stability and stability against oxygen exposure. Unfortunately, h-BN has shown to be not chemically stable against oxygen exposure at elevated temperature above 500 K so that we were forced to alter the scientific direction of the project. Rather than studying Au on h-BN/Ru(0001) we studied the growth and oxidation of Au on O-precovered Ru(0001) as well as its redox behavior. Already the growth of Au on Ru(0001) as function of the O-precoverage reveal interesting properties. Au islands have shown not to grow on O-precovered but rather need bare Ru(00 1) regions. Therefore Au adsorption is accompanied by a continued compression of the O-overlayer. In this way the island size as well as the island height can be controlled by the O-precoverage on Ru(0001). The oxidation of Au islands needs the exposure of atomic oxygen. Oxidation of Au islands already proceeds at 300 K and has found to be dependent on the island thickness. 3 ML thick Au islands can readily be oxidized leading to a fragmentation of the Au islands into small cluster whose arrangement reflects the shape of the former Au islands. The initial oxidation of 3 ML thick Au islands starts from the top layer at the rim of the islands. From Au 4f core level spectroscopy these particles consist of a Au oxide shell and a metallic core. Quite in contrast, thicker Au islands are less prone to be oxidized. The oxidation of 4-5 ML thick Au islands starts also from the top layer at the rim of the islands. However, no fragmentation of the Au islands into smaller particles is observed. Rather the entire Au island is transformed into one big particle that also consists of a Au-oxide shell covering a metallic Au core. The oxidized Au islands can be reduced by thermal annealing to 670 K or by chemical reduction via CO exposure at 670 K. Exposing the oxidized Au particles to a mixture of CO and O2 with various feed compositions indicate that the Au-oxide particles are chemically reduced and accordingly not be stable. Therefore, steady state oxidation of CO is not sustained with our oxidized Au particles supported on Ru(0001).
