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Atomically precise cerium oxo clusters offer a platform to investigate structure–property relationships that are much more complex in the ill-defined bulk material cerium dioxide. We investigated the activity of the MCe₇₀ torus family (M = Cd, Ce, Co, Cu, Fe, Ni, and Zn), a family of discrete oxysulfate-based Ce₇₀ rings linked by monomeric cation units, for CO oxidation. CuCe₇₀ emerged as the best performing MCe₇₀ catalyst among those tested, prompting our exploration of the role of the interfacial unit on catalytic activity. Temperature-programmed reduction (TPR) studies of the catalysts indicated a lower temperature reduction in CuCe₇₀ as compared to CeCe₇₀.more » In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that CuCe₇₀ exhibited a faster formation of Ce³⁺ and contained CO bridging sites absent in CeCe₇₀. Isothermal CO adsorption measurements demonstrated a greater uptake of CO by CuCe₇₀ as compared to CeCe₇₀. The calculated energies for the formation of a single oxygen defect in the structure significantly decreased with the presence of Cu at the linkage site as opposed to Ce. Furthermore, this study revealed that atomic-level changes in the interfacial unit can change the reducibility, CO binding/uptake, and oxygen vacancy defect formation energetics in the MCe₇₀ family to thus tune their catalytic activity.« less

Oxidative dehydrogenation of propane (ODHP) to propylene could have a significant impact on the production of this critical chemical intermediate, if appropriate catalysts can be discovered. Recently, heterogeneous catalysts based on boron (oxides and nitrides) have been demonstrated to be promising for ODHP, but their active sites have not been conclusively identified. Here, we report that the deposition of differently sized boronic acids into the micropores of silica supports results in different distributions of surface borate species after calcination. Furthermore, these materials, in turn, display a wide range of rates in ODHP but similar selectivity, suggesting that they differ onlymore » in the numbers of active sites. Features identified by in situ Raman, IR, and magic-angle-spinning ¹¹B solid-state NMR spectroscopies are compared to catalyst activity. This correlation identifies the S2 borate species, a hydroxylated nonring boron, as the likely active site and provides a target for directed syntheses of future catalysts.« less

Selective oxidation of cyclohexene to 2-cyclohexen-1-one over titania supported vanadia (VO_x/TiO₂) has been studied using temperature dependent in-situ FTIR spectroscopy in both the presence and absence of oxygen. The VO_x/TiO₂ samples were prepared using one atomic layer deposition (ALD) cycle and characterized by Raman spectroscopy. In-situ FTIR data for the oxidation of cyclohexene and perdeuterocyclohexene allow for the formulation of a molecular level reaction mechanism, which is initiated by the transfer of an allyl hydrogen. Oxidation of perdeuterocyclohexene provides a direct probe of the formation of OD and HDO moieties that support the involvement of specific steps in the proposedmore » mechanism. The presence of gas phase oxygen does not lead to a change in the products versus anaerobic conditions. However, gas phase oxygen is significantly incorporated in the CO₂ over-oxidation product above ~250 °C. Data were also obtained with cyclohexene epoxide as the reactant in an effort to determine whether there is a parallel reaction pathway, which is initiated by C=C activation in cyclohexene, that involves cyclohexene epoxide as an intermediate. Furthermore, though a minor pathway involving a cyclohexene epoxide intermediate cannot be ruled out, these data demonstrate that, under experimental conditions, the dominant pathway from cyclohexene to cyclohexene-1-one is initiated by an allyl-H activation step and does not involve an epoxide intermediate.« less