New Insights in the Gas-Phase Ion Chemistry of Group IVB Transition Metal Oxides: Cluster Models for Catalysis
Open Access
- Author:
- Harmon, Christopher L
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 10, 2016
- Committee Members:
- Albert Welford Castleman Jr., Dissertation Advisor/Co-Advisor
Albert Welford Castleman Jr., Committee Chair/Co-Chair
Nicholas Winograd, Committee Member
Lasse Jensen, Committee Member
Robert Martin Rioux Jr., Outside Member - Keywords:
- clusters
ion chemistry
catalysis - Abstract:
- The study of gas-phase cluster models has emerged as a useful tool to model catalytic active sites on surfaces. By observing how the reactivity of TMO clusters changes with cluster size, active site, and metal composition, we draw insights about the molecular factors governing reactivity and selectivity. The aim is to provide a conceptual framework to drive the development of improved catalysts with higher activity and selectivity. As group IVB transition metal oxides (TMOs) are widely used as catalysts and supports, we have undertaken mass spectrometric reactivity studies of titanium, zirconium, and hafnium oxide clusters with small organic molecules of interest in catalysis. We particularly note the differences in reactivity observed as the identity of the metal in the cluster changes. We report the reactivity of MxO2x+ (M = Ti, Zr) clusters that contain radical atomic oxygen, O•−, with methane, ethane, propane, ethylene, and propylene. We observe that in many cases the titanium clusters undergo a type of “etching” reaction, where the cluster loses a TiO unit and associates the reactant gas. Zirconium oxides strongly prefer the hydrogen atom abstraction pathway, and no evidence for an etching type of reaction is found for zirconium clusters. Next, we examine the reactivity of TixO2x+1+ clusters that are theoretically predicted to contain a superoxide active site, O2•−, and find that the reactivity is quite different from ZrxO2x+1+. We also present one of the first studies of the reactivity of hafnium oxide clusters, and show that HfxO2x+1+ acts very similarly to ZrxO2x+1+ clusters, attributed to a superoxide active site. However, Hf2O4+, which contains a radical oxygen atom, shows drastically different reactivity from Zr2O4+. Finally, we discuss these studies in the context of isoelectronic superatoms.