Invited Paper SS2-MoA3
Well-defined, Highly Uniform Metallic Nano-Structures as Selective Heterogeneous Catalysts and Platforms for Chemical Characterization

Monday, November 9, 2009, 2:40 pm, Room N

I will present our recent work where we explored potential utilization of highly uniform metallic nano-structured materials as selective heterogeneous catalysts. The advantage of these materials compared to conventional catalytic materials is that their structure can be controlled with almost atomic precision, and that it is possible to synthesize highly homogeneous structures. We demonstrated some of these advantages recently when we showed that well-defined, tailored Ag nano-structures are much more selective in heterogeneous epoxidation of ethylene to form ethylene oxide (EO) (ethylene + ½ O₂ → EO) than conventional industrial catalysts.

We showed using quantum chemical Density Functional Theory (DFT) calculations, where we studied critical elementary chemical steps that govern the selectivity to EO in the process, that the Ag(100) surface should be inherently more selective than the Ag(111) surface. We note that catalytic particles, synthesized using conventional synthesis procedure and currently used in commercial ethylene epoxidation process, are dominated by the (111) surface. To synthesize Ag nano-structures which are dominated with the Ag(100) faces, we employed a synthesis procedure which uses organic stabilizer molecules to direct the growth of the nano-structure in a particular direction and to control the surface facets that terminate the nano-structure. This synthetics strategy allowed us to synthesize well-defined and highly uniform Ag nano-wires and nano-cubes which are dominated by the (100) facet. Subsequent experiments showed that Ag nano-wires and nano-cube catalysts can achieve selectivity to EO, which is, at differential conversion, by ~ 40 % higher than for conventional Ag catalysts.

We have also recently started exploring these metallic nano-structures as possible platforms for chemical characterization. The features of these nano-structures that are particularly appealing are: (i) the nanostructures are well defined on atomic level, and their surface to volume ratio is fairly high, which makes these structures inherently better suited for the studies of surface chemical processes compared to traditional single crystal model systems, which are while very well defined, characterized by low surface to volume ratio, (ii) we can synthesize the nanostructures with high degree of uniformity in size and shape, which rules out possible effects due to diversity in size and shape, i.e. these, (iii) the nanostructures are effective scatterers of electromagnetic radiation which make them suitable as platforms for a number of chemical characterization techniques including surface enhanced Raman (SERS) or IR spectroscopies. We will demonstrate the utility of the nano-structures for chemical characterization by a way of an example, where we monitored in-situ ethylene epoxidation.