6 Search Results

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

We report a unique reaction type for facile synthesis of ultrafine and well-dispersed Pt nanoparticles supported on chalcogel surfaces. The nanoparticles are obtained by in-situ Pt²⁺ reduction of a chalcogel network formed by the metathesis reaction between K₂PtCl₄ and Na₄SnS₄. The rapid catalytic ability of the chalcogelsupported Pt nanoparticles is demonstrated in a recyclable manner by using 4-nitrophenol reduction as a probe reaction.

Significance In nature, nitrogenase fixes nitrogen into biologically usable forms under ambient conditions. Today, half of the world’s nitrogen fixation is achieved through the industrial Haber–Bosch process, which operates at elevated temperature and pressure. Here, we present a synthetic nitrogenase mimic in the form of chalcogel composed of molybdenum and iron-containing biomimetic clusters that can accomplish photocatalytic N ₂ fixation and conversion to NH ₃ at ambient temperature and pressure. Surprisingly, the iron–sulfur chalcogels without molybdenum are observed to have a higher activity toward N ₂ reduction. The results reported here will greatly expand the scope of materials design andmore » engineering for the creation of highly active iron-based N ₂ reduction catalysts operating in mild conditions.« less

Collapsed titania nanotubes (cTiNT) were synthesized by the calcination of titania nanotubes (TiNT) at 650 °C, which leads to a collapse of their tubular morphology, a substantial reduction in surface area, and a partial transformation of anatase to the rutile phase. There are no significant changes in the position of the XPS responses for Ti and O on oxidation or reduction of the cTiNTs, but the responses are more symmetric than those observed for TiNTs, indicating fewer surface defects and no change in the oxidation state of titanium on oxidative and/or reductive pretreatment. The interaction of H₂O and CO₂ withmore » the cTiNT surface was studied. The region corresponding to OH stretching absorptions extends below 3000 cm-1, and thus is broader than is typically observed for absorptions of the OH stretches of water. The exchange of protons for deuterons on exposure to D₂O leads to a depletion of this extended absorption and the appearance of new absorptions, which are compatible with deuterium exchange. We discuss the source of this extended low frequency OH stretching region and conclude that it is likely due to the hydrogen-bonded OH stretches. Interaction of the reduced cTiNTs with CO₂ eads to a similar but smaller set of adsorbed carbonates and bicarbonates as reported for reduced TiNTs before collapse. Implications of these observations and the presence of proton sources leading to hydrogen bonding are discussed relative to potential chemical and photochemical activity of the TiNTs. These results point to the critical influence of defect structure on CO₂ photoconversion.« less