FUNDAMENTAL STUDY OF NOBLE METAL AND TRANSITION METAL OXIDE CLUSTERS IN THE PRESENCE OF CO: INSIGHT INTO MECHANISMS OF HETEROGENEOUS CATALYSIS
Open Access
- Author:
- Reilly, Nelly Moore
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 01, 2007
- Committee Members:
- Albert Welford Castleman Jr., Committee Chair/Co-Chair
James Bernhard Anderson, Committee Member
Nicholas Winograd, Committee Member
Robert John Santoro, Committee Member - Keywords:
- CO oxidation
catalysis
gold oxide
gas-phase - Abstract:
- The conversion of carbon monoxide (CO) to carbon dioxide (CO2) is one of the most important catalytic reactions. Fundamental studies aimed at providing information to aid in the design of more efficient and selective catalysts for the abatement of this harmful environmental pollutant were conducted. Gas phase metal oxide cluster studies are a valuable complementary technique to surface methods for elucidating the mechanistic details and active sites of catalytic systems. To this end, gas phase studies were utilized to uncover possible species responsible for the increased activity and selectivity of noble metal and transition metal nanoparticle catalysts. A guided ion beam mass spectrometer was employed to study the dissociation patterns and interactions with CO of mass selected clusters. This provides information into specific reaction pathways based on cluster size, ionic charge and oxidation state, stoichiometry and coordinative saturation. There has been recent interest in the activity of gold nanoparticles toward the oxidation of CO and studies herein have focused on the effects of charge state in elucidating the mechanism. Most reactions of gold oxide cations underwent oxygen replacement by CO, however evidence of other reaction pathways including CO oxidation for both Au4O2+ and Au4O3+ species were also observed. Compared to anionic gold oxide clusters of the same stoichiometry, CO oxidation was previously found to be the dominant reaction channel. Anionic clusters were governed by a structure-reactivity relationship in which a peripheral oxygen atom was the most efficient reaction center for effecting CO oxidation. Density functional calculations found molecularly bound oxygen in the ground state structure of cationic clusters in which oxygen atom replacement by CO was observed in experiments. High barriers to O-O bond dissociation suggest the role of ionic charge may be an important factor governing the reaction. A mechanism based on a triplet-singlet spin state transition was proposed that may provide a pathway with lower barriers for O2 dissociation in order to explain the experimentally observed reaction products. Next, studies involving mixed metal oxide clusters were undertaken to probe to effects of catalyst-dopant interactions and gain a deeper understanding of the principles governing the oxidation of CO. Comparison of the reactions of pure metal oxides with CO to experiments of bimetallic oxides can aid in determining the influence that each individual metal exhibits and the changes that occur in reactivity when another metal is present. Studies involving silver-gold dimer oxide clusters were conducted and provide insight into the role of charge transfer between different metals. AgAuO1,2+ species showed similar reaction products as pure gold oxide cationic clusters, while AgAuO3,4+ reacted similar to pure silver oxide clusters with the same stoichiometry. Only AgAuO2- exhibited oxygen atom transfer which was different from reactions of either pure metal oxide. This provides experimental evidence of density functional calculations that previously predicted the reaction of AgAu- with O2 and CO promotes CO oxidation. Studies concerning 3d transition metals were conducted to investigate their activity as common catalytic support materials and their ability toward direct oxidation of CO. Structural elucidation was conducted through CID experiments and density functional calculations were performed for iron oxide ionic clusters. Reactions of iron oxide cations with CO were compared to anionic clusters. Both ionic charge states were shown promote CO oxidation; however, attachment of CO onto the metal cluster was only observed for cationic clusters. Calculated energy profiles of the reaction demonstrated that the oxidation efficiency was governed by the strength of oxygen binding to the iron atom. The results provide important insight into the nature of the active site for condensed phase catalysis, shedding light on the role of charge state for CO oxidation in the presence of iron oxide clusters. The behavior of several other 3d transition metal oxide clusters, cobalt, nickel and copper, were undertaken to examine possible periodic trends. The dissociation patterns and reactivity in the presence of CO for these metal oxides were similar to iron oxides for each respective charge state. The most active stoichiometry for CO oxidation was MxOy+ where y=x for cationic clusters and MxOy- where y=x+1 for anionic clusters with the exception of Cu3O3-. The same number of Cu atoms and oxygen atoms was a particularly active stoichiometry. This may be a consequence of the electron density for copper, which possesses a full orbital shell. Comparison of cations and anions showed that cations were more efficient in their particular reaction channels requiring less CO reactant gas to observe reaction products.