Study Of The Reactions Between Metal Oxide Clusters And Small Molecules

Metal oxides （MOs） are widely used as both catalysts and catalytic-support materials innumerous industrial processes. Experimental studies has proved that mononuclearoxygen-centered radicals （O^-） are significant active species in catalytic oxidation reactionsover MOs surfaces. However, because the elementary reactions on the catalysts surfaces arecomplex, and the O^-species are usually with low concentrations and short life-times, theroles of the O^-in heterogeneous catalysis remain poorly understood in details. The oxideclusters, however, enable the investigation into surface reactions in the absence of complexfactors. Thus the study of O^--containing clusters is very helpful to understand the structuresand elementary steps of the O^-species over MO surfaces. The studies in this dissertation arefocused on bonding and reactivity of O^-over metal oxide clusters. The summary is asfollows:1. The inter-conversion between un-reactive peroxides O₂^2- and reactive atomic oxygenradicals O^-. A time-of-flight mass spectrometry （TOF-MS） and theoretical calculations areemployed to study the electronic structure and reactivity of oxygen-rich scandium oxidecluster anions ScO_3-5^-. The experimental results provide evidences that1) the ScO_3,4^-clusterscan react with n-butane to produce ScO₃H^-, ScO₃H₂^-and ScO₄H^-, while the moreoxygen-rich cluster ScO₅^-is inert;2) the un-reactive cluster isomers of ScO₃^-and ScO₄^-arealso present in the cluster source. Theoretical calculations are used to calculate the structuresand reaction mechanisms of the clusters. The theoretical results also indicate that theun-reactive and reactive cluster isomers of ScO_3,4^-contain O₂^2- and O^-species, respectively.With the best of our knowledge, this study is the first to investigate the inter-conversionbetween un-reactive peroxides O₂^2- and reactive atomic oxygen radicals O^-theoretically.2. The reactivity of Sc_xO_y^-toward C–H bond activation of n-butane. To identify largersize MO clusters that are able to activate C–H bonds, a reflectron time-of-flight massspectrometry （Re-TOF-MS） is employed to study the reactivity of large-sized oxide clusteranions of the first transition metal, scandium. Hydrogen atom abstraction （HAA） fromn-butane by （Sc₂O₃）_NO^-（N=1-19）,（Sc₂O₃）NO₅^-（N=3-19） and （Sc₂O₃）NScO₄^-（N=1-5）clusters are observed. The size of oxide clusters that are able to activate alkane C–H bonds isextended to nanosizes (-1.01nm for Sc₁₈O₂₈^-). Density function theoretical calculationssuggest that the experimentally observed C–H bond activation is facilitated by O^-over（Sc₂O₃）_NO^-. The size-dependent reactivity is attributed to the local charge environment aroundthe O^-. 3. The group metal and non-metal oxides （Al₂O₃, SiO₂） are extensively used as catalyticsupports but do not participate directly in the surface reactions. The experimental andtheoretical methods are collaborated to study the reactivity of aluminum oxide cluster anionstoward n-butane. The results indicate that aluminum oxide clusters Al₂O_4,6^-and Al₃O₇^-canabstract H atom from n-C₄H₁₀to produce Al₂O_4,6H^-and Al₃O₇H^-, respectively, while theAl₃O₆^-can abstract two H atoms from two n-C₄H₁₀molecules consecutively. The calculatedresults show that the O^-as the active sites are responsible for the observed HAA reactivity.Furthermore, the mechanism of the O^-generation in Al₃O₇^-from the adsorption of O₂molecule on un-reactive Al₃O₅^-is similar to that for the transition metal oxide clusters: O^2-+O₂→O^-+O₂^-. The present results indicate that the usually considered “inert” supportmaterials （Al₂O₃in this study） may play significant roles in surface reactions: they not onlyprovide a means of spreading active catalyst, but also have certain oxygen storage and O^-generation capability.