Clusters: Addressing Material and Environmental Issues

Open Access
Author:
Jones, Charles Elwood
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 11, 2008
Committee Members:
  • Albert Welford Castleman Jr., Committee Chair
  • Nicholas Winograd, Committee Member
  • John V Badding, Committee Member
  • Renee Denise Diehl, Committee Member
Keywords:
  • clusters
  • materials
  • aluminum
  • niobium
  • carbides
  • formic acid
Abstract:
The present thesis work is part of a continuing effort in the Castleman group to study the physical and chemical properties of clusters as they relate to materials and the environment. With regards to materials, metal-based clusters are synthesized in the gas phase in order to identify uniquely stable species. Just as atoms or molecules are the building blocks of matter in nature, our group envisions the synthesis of future cluster-assembled materials (CAMs) built with clusters. By varying the size and composition of a cluster by even one atom, the electronic properties can be dramatically changed. A general introduction of the cluster chemistry and physics pertinent to this thesis work is presented in Chapter 1. Anion photoelectron spectroscopy is the experimental technique utilized in this work to study metal and metal-based clusters. A home-built instrument was employed, and descriptions of its components are presented in Chapter 2. This instrument was used to study a series of AlnBi- (n = 1-5) clusters. As explained in Chapter 3, experimental data was combined with theoretical work from our collaborators in order to show that the introduction of a bismuth atom to aluminum clusters creates uniquely stable clusters. One of these, Al3Bi, is demonstrated be an all-metal aromatic cluster. Another cluster, Al5Bi, is shown to be stabilized by the jellium model. The findings are a practical example of how small changes in size and/or composition of a cluster give rise to exciting properties. Based on the results, a relationship is drawn between the aromatic and jellium models herein, and potential for use in CAMs is briefly discussed. In Chapter 4, niobium carbide clusters of the form Nb3Cn- (n = 5-10) are investigated with similar methods in order to elucidate the formation of larger metal-carbon clusters, most notably the niobium Met-Car (Nb8C12). When the experimental data is merged with theory from our collaborators, isomers with various structural motifs are identified. Structures of tri-niobium carbide clusters are proposed to follow two general trends. One structural trend of Nb3Cn- clusters features a triangular niobium unit at the cluster core. The second structural motif entails a single carbon atom that links to all three metal atoms, and C2 units that bridge the individual niobium atoms. Nice agreement between the experimental and computational results is shown, lending credence to our findings. Comparisons are drawn between the results presented here and previous theoretical and experimental studies of small niobium carbide clusters. The last study presented in this work does not deal with materials, but rather environmental chemistry. In Chapter 5, reactions of formic acid (HCOOH) and protonated water clusters of the form H+(H2O)n (n = 3-27) are examined in a fast-flow reactor under well-defined thermal conditions. The product distributions of H+(H2O)n(HCOOH)m show that nucleation occurs with these clusters on a molecular scale more easily than both pure water and methanol-water clusters do under similar conditions. Calculations are performed using the Thompson liquid drop model to interpret the experimental results. The findings are also in good agreement with recent theoretical work regarding a “cooperative bonding” effect with formic acid molecules and water. The atmospheric implications of this study are briefly discussed. A summary of all the results from this thesis work is presented in Chapter 6.