The Role of Radical Oxygen in Metal Oxide Based Heterogeneous Oxidation Catalysis

Open Access
Author:
Johnson, Grant Edward
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 28, 2009
Committee Members:
  • Albert Welford Castleman Jr., Dissertation Advisor
  • Albert Welford Castleman Jr., Committee Chair
  • Nicholas Winograd, Committee Member
  • Ayusman Sen, Committee Member
  • Jorge Osvaldo Sofo, Committee Member
Keywords:
  • cluster
  • radical
  • oxygen
  • zirconium
  • tungsten
  • aluminum
Abstract:
Employing guided-ion-beam mass spectrometry, a series of gas-phase cationic stoichiometric zirconium oxide clusters (ZrO2)x+ (x = 1-4) have been identified that exhibit enhanced activity and selectivity for three oxidation reactions of widespread chemical importance. Density functional theory calculations reveal that these clusters all possess the same active site consisting of a radical oxygen (Zr-O•) with an elongated zirconium-oxygen bond (elongation ≈ 0.24 ± 0.02 Å) and a spin-unpaired electron located on the terminal oxygen atom. Calculated energy profiles demonstrate that each oxidation reaction is favorable energetically and involves easily surmountable barriers. Furthermore, it is found that the active series of stoichiometric clusters (ZrO2)x+ (x = 1-4) may be regenerated by reacting a series of oxygen-deficient clusters (ZrxO2x-1)+ (x = 1-4) with a strong oxidizer. This indicates that the series of clusters may promote multiple cycles of oxidation reactions and, therefore, exhibit true catalytic behavior. The series of cationic stoichiometric clusters, having structures that resemble specific sites in bulk zirconia, are promising candidates for potential incorporation into a cluster assembled catalyst material. Guided-ion-beam mass spectrometry experiments and density functional theory calculations have also been conducted to determine the influence of ionic charge state on the catalytic activity of gas-phase zirconium oxide clusters containing radical oxygen centers. Experiment and theory indicate that by adding one oxygen atom with a full octet of valence electrons (O2-) to a series of stoichiometric cationic zirconium oxide clusters (ZrO2)x+ (x = 1-4), a series of anionic clusters (ZrxO2x+1)- (x = 1-4) are formed which contain radical oxygen with elongated (elongation ≈ 0.24 ± 0.02 Å) metal-oxygen bonds and a spin unpaired electron located on the terminal oxygen atom. The series of anionic clusters oxidize carbon monoxide, strongly associate acetylene, and weakly associate ethylene, in contrast to the series of cationic clusters that was found to be highly active toward the oxidation of all three molecules. Theoretical investigations indicate that a critical hydrogen transfer step necessary for the oxidation of ethylene and acetylene at metal oxide clusters containing radical oxygen centers is energetically favorable for cationic clusters but unfavorable for the corresponding anionic species. The calculated electrostatic potential of the cluster reveals that in the case of cations, a favorable interaction with nucleophilic molecules takes place over the whole surface of the (ZrO2)x+ (x = 1-4) series of clusters, compared to a restricted interaction of ethylene and acetylene with the less coordinated zirconium atom in the case of the anionic (ZrxO2x+1)- (x = 1-4) series. Therefore, in spite of the common presence of radical oxygen in specific anionic and cationic stoichiometries, the extent to which various classes of reactions are promoted is directly determined by ionic charge state. Moreover, the (ZrxO2x+1)- (x = 1-4) series of anionic clusters may be regenerated by reacting a series of oxygen deficient anionic clusters (ZrO2)x- (x = 1-4) with a strong oxidizer. This indicates that not only cationic species, but also anionic clusters may promote multiple cycles of carbon monoxide oxidation. The guided-ion-beam technique also has been applied to investigate the reactivity of gas-phase cationic tungsten oxide clusters with carbon monoxide and propylene. A series of cationic clusters having the same stoichiometry as bulk tungsten oxide (WO3) exhibit enhanced activity and selectivity for the transfer of a single oxygen atom to propylene, suggesting the formation of propylene oxide, an important monomer used, for example, in the industrial production of plastics. Furthermore, the series of cationic stoichiometric clusters have been demonstrated to be active for the oxidation of carbon monoxide to carbon dioxide, a reaction of significance to environmental pollution abatement. The findings suggest that the enhanced oxidation reactivity of the stoichiometric cationic clusters also may be due to the presence of radical oxygen (W-O•) with elongated metal-oxygen bonds. In other studies, small cationic and anionic aluminum oxide clusters have been investigated to determine their reactivity toward carbon monoxide and nitrous oxide employing guided-ion-beam mass spectrometry. Clusters with the same stoichiometry as bulk alumina, Al2O3, exhibited atomic oxygen transfer products when reacted with carbon monoxide, suggesting the formation of carbon dioxide. Anionic clusters were less reactive than cationic species but showed higher selectivity towards the transfer of only a single oxygen atom. Cationic clusters, in contrast, exhibited additional products corresponding to the sequential transfer of two oxygen atoms and the loss of an aluminum atom. To determine if these stoichiometric clusters may be generated from oxygen-deficient species, clusters having a stoichiometry with one less oxygen atom than bulk alumina, Al2O2, were reacted with nitrous oxide. Cationic clusters were found to be selectively oxidized to Al2O3+, while anionic clusters added both one and two oxygen atoms forming Al2O3- and Al2O4-. The oxygen-rich Al2O4- cluster exhibited comparable reactivity to Al2O3- when reacted with carbon monoxide.