6 Search Results

The reaction mechanism of CO₂ with H₂ is studied on platinum nanoparticles supported on fumed silica. It is found that platinum nanoparticle size, reaction temperature, and metal oxide promoters play important roles in determining the reaction rate and the mechanism of forming surface carbonyl species. Metal oxide promoters consist of sub-monolayer titanium oxide or aluminum oxide overcoated onto the catalysts by atomic layer deposition (ALD). Furthermore, these alter the CO formation rate, influence the adsorption and desorption behavior, and switch the surface reaction channel from Eley-Rideal to Langmuir-Hinshelwood mechanism due to an enhancement of CO₂ affinity to the metal-metal oxidemore » interface. At the temperatures relevant for catalytic turnover, ALD overcoating significantly increases catalytic activity in CO₂ hydrogenation to CH₄ and CO, while the identity of the oxide overcoat helps control product selectivity.« 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

Tandem catalysis couples multiple reactions and promises to improve chemical processing, but precise spatiotemporal control over reactive intermediates remains elusive. We used atomic layer deposition to grow In ₂ O ₃ over Pt/Al ₂ O ₃ , and this nanostructure kinetically couples the domains through surface hydrogen atom transfer, resulting in propane dehydrogenation (PDH) to propylene by platinum, then selective hydrogen combustion by In ₂ O ₃ , without excessive hydrocarbon combustion. Other nanostructures, including platinum on In ₂ O ₃ or platinum mixed with In ₂ O ₃ , favor propane combustion because they cannot organize the reactions sequentially.more » The net effect is rapid and stable oxidative dehydrogenation of propane at high per-pass yields exceeding the PDH equilibrium. Tandem catalysis using this nanoscale overcoating geometry is validated as an opportunity for highly selective catalytic performance in a grand challenge reaction.« less

Propylene, a precursor for commodity chemicals and plastics, is produced by propane dehydrogenation (PDH). An increase in PDH yield via added catalyst activity, lifetime, or selectivity represents significant energy and economic savings. Using Pt dispersed on Al2O3 extrudate supports as a commercially relevant model system, we demonstrate that atomic layer deposition (ALD) metal oxide overcoats, used to tailor metal-active sites, can increase PDH yield and selectivity. We investigate the interplay of Pt loading, ALD overcoat thickness, and Al₂O₃ support surface area on PDH activity, selectivity, and catalyst stability to show that applying a 6-8 A thick layer of Al₂O₃ onmore » low-surface area Al₂O₃ supports of similar to 90 m²/g surface area yields the optimal combination of stability and activity, while increasing propylene selectivity from 91 to 96%. Increased stability upon steaming deactivation occurs because the Al₂O₃ overcoat prevents the Pt nanoparticles from sintering. We speculate that the ALD overcoat selectively binds to the undercoordinated sites on the Pt nanoparticles, while leaving the more selective terrace sites available for dehydrogenation.« 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

AlO_X thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Furthermore, understanding the factors that affect ALD thin film porosity would enable future design of layered catalytic structures with tunable nanoscale features on industrially relevant high surface area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2 - 7 nm AlO_X by ALD deposited at temperatures of 100 °C, 200 °C and 300 °C. Hydroxylmore » loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlO_x deposition temperature and presence of Pd. Crystallization of the overcoat to γ-Al₂O₃ is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlO_X ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlO_X ALD coatings.« less