Tuning the electronic and chemical properties of metals: Bimetallics and transition metal carbides.

摘要

Improving the activity and selectivity of heterogeneous catalysts is a primary concern in the chemical industry. One method for changing the catalytic properties of a transition metal is to alloy it with another metal or with caxbon to form a transition metal carbide. However, it is well known that the surface structures and compositions of alloys may differ substantially from the bulk structure and composition. It can be non-trivial to characterize these surfaces, and the combinations of strain and complex interactions between the metals can make it difficult to relate surface structure and composition to catalytic properties. Consequently, it is still difficult to rationally design new catalytic materials. In this dissertation, a combined experimental and theoretical approach designed to develop an intuitive understanding of bimetallic and carbide surfaces and their chemical reactivities will be presented. This approach is a critical development towards the rational design of new bimetallic and carbide catalysts.;The adsorption of hydrogen, oxygen and caxbon monoxide was investigated on a wide range of model bimetallic and carbide surfaces and structures using combinations of surface science experiments and density functional theory (DFT). Using single crystals in ultrahigh vacuum as model surfaces, we were able to use surface science tools to fully characterize the structure and adsorption properties of a few bimetallic surfaces in atomic detail. By using these well-defined surfaces as models in DFT calculations, the experimental trends in adsorption energies for different bimetallic surface structures and compositions were explained, and the properties of new bimetallic surfaces were predicted. The use of DFT allowed us to understand the contributions of strain and metal-metal interactions to the modifications of the chemical properties of these bimetallic surfaces. Most importantly, a linear correlation between the average energy of the surface d-electron-band and the adsorption energy of sample probe molecules was identified for all of the surfaces examined. The implications of these findings for catalyst design are discussed.