Fundamental Studies of d-Band Transition Metal Alloys

This thesis will attempt to make some small contribution toward our understanding of how particle size effects, alloy effects and support effects impact the performance of heterogeneous catalysis. As a starting point, in chapter 2, we have examined the reverse spillover of hydrogen on Rh6 clusters using density functional theory over a variety of supports: TiO2, Al2O3, MgO, SiO2 and a LTA zeolite. In this work we attempt to elucidate how the identity of the support influences the presence of hydrogen on the metal cluster. While no universal descriptor of reverse spillover was identified, the support does have a strong influence on the thermodynamics of reverse hydrogen spillover. We have also examined alloying in heterogeneous catalysts. Through alloying one may manipulate the properties of a catalyst, thus creating highly selective materials that are of high interest to the industrial processes. Combining X-ray absorption spectroscopy and density functional theory calculations, Chapter 3 examines trends in the simulated X-ray Absorption Near Edge Structure (XANES) of bulk alloys of Pd with other late transition metals and relates the observed changes in the XANES upon alloying to features in the valence electronic states of these materials. We find that effects can be grouped into three categories: 1) changes in M-M bond distances 2) changes in bond overlap due to differing orbital extents and 3) charge transfer effects. When synthesizing alloy nanoparticles, not all systems form homogeneous structures. In fact, there are many prominent instances of segregation where the shell becomes enriched in one metal and the core is enriched in the other. These so-called core-shell alloys often display unique behavior which lies outside the range of behavior of the monometallic components. In chapter 4, we use a simple model to explain the changes in the electron density of states in these core-shell systems for a variety of combinations of late d-band transition metals. Chapter 5 introduces a method based upon changes in the XANES (combined with IR) for characterizing the surface composition of bimetallic nanoparticles. In chapter 5 we focus upon our contribution to that work which involved the calculation of CO adsorption on a variety of PdPt alloy surfaces which aided in understanding the observed spectra. Finally, using acrolein hydrogenation as a test reaction, chapter 6 examines how the presence of an alloying metal influences both the kinetics and thermodynamics of the complete mechanism. In addition, we have investigated the influence of nanoparticle structure on the observed reactivity and selectivity.