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Here, a series of supported MoO_x catalysts on different oxide supports (Al2O3, TiO2, ZrO2, SiO2) were synthesized and investigated for propylene metathesis, characterized with in situ spectroscopies (DRIFTS, Raman, UV-vis) and chemically probed with propylene-TPSR-MS, propylene-TPSR-IR, and ethylene/2-butene titration. Under dehydrated conditions at monolayer coverage or maximum surface dispersion, the surface MoO_x sites are present as a mixture of isolated di-oxo (O=)₂Mo(–O–Al)₂ and oligomeric mono-oxo O=Mo(–O–Al)_4/5 sites on Al₂O₃, primarily oligomeric mono-oxo O=Mo(–O–Ti)_4/5 on TiO₂, isolated di-oxo (O=)₂Mo(–O–Zr)₂ and oligomeric mono-oxo O=Mo(–O–Zr)_4/5 on ZrO₂, and isolated di-oxo (O=)₂Mo(–O–Si)₂ on SiO₂. The bridged (S₂-OH) and tri-coordinated (S₃-OH) anchoring surface hydroxyls ofmore » the oxide supports with strong support cation electronegativity control the activation and number of active surface MoO_x sites at low temperatures (<100 °C). The isolated anchoring surface hydroxyls (S-OH) of the oxide supports with strong support cation electronegativity control the activation and number of active surface MoO_x sites at high temperatures (>350 °C). Olefin metathesis by the more redox active supported MoO_x/TiO₂ and MoO_x/ZrO₂ catalysts is retarded by the formation of stable surface acetone and acetate species that block olefin adsorption. The oxide supports are potent ligands that tune the activation and surface chemistry of the surface MoOx sites for olefin metathesis. This is the first time that the influence of oxide supports on the activation and surface chemistry of supported MoO_x sites has been systematically examined.« less

A series of TiO₂-supported MO_x catalysts (M=V, W, Ce, Cu and S) were investigated for their SCR activity. In situ Raman spectroscopy indicated that the supported MO_x phases were completely dispersed as surface sites on the TiO₂ support. In situ IR revealed that surface VO_x, WO_x and SO_x sites anchored at both CeO_x/CuO_x and TiO₂ sites. The number of surface Lewis acid sites decreased with the addition of basic (CeO_x/CuO_x) and acidic (VO_x/WO_x) sites in all catalysts, and acidic SO_x in the unpromoted and Ce-promoted catalysts. The surface VO_x, WO_x and SO_x sites introduced surface Brønsted acid sites. The redoxmore » promoters increased the NO conversion, but SO_x impregnation inhibited their effect due to acid (SO_x)-base (CeO_x/CuO_x) interactions. Finally, the SCR reaction was shown to efficiently proceed via either surface NH₃* or NH₄^+* species, resolving the long-standing dispute on the involvement of these species in the SCR reaction.« less

The SCR activity of MO_x (M=Al, P, Si, Zr) promoted V₂O₅-WO₃/TiO₂ was investigated before and after sulfation. In situ IR spectroscopy indicated that the VO_x active sites preferentially anchor on promoter generated surface hydroxyl. In situ Raman spectroscopy confirmed that all oxides are completely dispersed on the TiO₂ surface. In situ NH₃-IR spectroscopy showed that the oxides can increase the Lewis (AlO_x and ZrO_x) and Brønsted (PO_x, SiO_x, VO_x, WO_x and SO_x) acid site concentrations. The SiO_x and ZrO_x promoters had little effect on NO conversion, while the AlO_x and PO_x promoters and surface sulfation generally inhibited it. The SiO_xmore » promoted catalyst was highly SCR active despite lacking Lewis acid sites, indicating that they are not vital for SCR. Finally, the N₂O formation activity of the catalyst was inhibited by surface sulfation and the promoters, correlating with the promoter and SO_x induced increase in the Brønsted acid sites’ strength.« less

The influence of basic oxide promoters on copper-chromium-iron oxide catalysts was investigated to determine the nature of surface oxygen species and structure-activity relationship for the reverse water-gas shift reaction. The catalysts were characterized with in situ XRD, in situ Raman, in situ XPS, in situ HS-LEIS and H₂-TPR. Two surface oxygen sites with different reduction characteristics were found to be present. The overall CO₂ activation rate was found to correlate with both the number and reducibility of the more active oxygen species that were likely associated with the Cu-FeO_x interfacial regions for enhanced hydrogen spillover. While addition of K₂O somewhatmore » preserved the interfacial regions and facilitated the reduction kinetics of surface oxygen, both Na₂O and CaO significantly suppressed the availability of metallic Cu as well as the Cu-FeO_x interfaces, leading to decreased reactivity. These findings provide a direction to promote the copper-iron catalysts by creating more metal-metal oxide interfacial sites.« less

We report that selective catalytic reduction (SCR) of NO_x with NH₃ by supported vanadium oxide catalysts is an important technology for reducing acidic NO_x emissions from stationary sources and mobile diesel vehicles. However, rational design of improved catalysts is still hampered by a lack of consensus about reaction pathways and kinetics of this critical technology. The SCR fundamentals were resolved by applying multiple time-resolved in situ spectroscopies (ultraviolet–visible light (UV-vis), Raman and temperature-programmed surface reaction (TPSR)) and isotopically labeled molecules (¹⁸O₂, H₂¹⁸O, ¹⁵N¹⁸O, ND₃). Finally, this series of experiments directly revealed that the SCR reaction occurs at surface V⁵⁺O₄ sitesmore » that are maintained in the oxidized state by O₂ and the rate-determining step involves the reduction of V⁵⁺O₄ sites by NO and NH₃, specifically the breaking of N–H bonds during the course of formation or decomposition of the NO–NH₃ intermediate.« less

We compared the molecular structures, surface acidity and catalytic activity for NO/NH₃/O₂ SCR of V₂O₅-WO₃/TiO₂ catalysts for two different synthesis methods: co-precipitation of aqueous vanadium and tungsten oxide precursors with TiO(OH)₂ and by incipient wetness impregnation of the aqueous precursors on a reference crystalline TiO₂ support (P25; primarily anatase phase). Bulk analysis by XRD showed that co-precipitation results in small and/or poorly ordered TiO₂(anatase) particles and that VO_x and WO_x do not form solid solutions with the bulk titania lattice. Surface analysis of the co-precipitated catalyst by High Sensitivity-Low Energy Ion Scattering (HS-LEIS) confirms that the VO_x and WO_x aremore » surface segregated for the co-precipitated catalysts. In situ Raman and IR spectroscopy revealed that the vanadium and tungsten oxide components are present as surface mono-oxo O = VO₃ and O = WO₄ sites on the TiO₂ supports. Co-precipitation was shown for the first time to also form new mono-oxo surface VO₄ and WO₄ sites that appear to be anchored at surface defects of the TiO₂ support. IR analysis of chemisorbed ammonia showed the presence of both surface NH₃^* on Lewis acid sites and surface NH₄^+* on Brønsted acid sites. TPSR spectroscopy demonstrated that the specific SCR kinetics was controlled by the redox surface VO₄ species and that the surface kinetics was independent of TiO₂ synthesis method or presence of surface WO₅ sites. SCR reaction studies revealed that the surface WO5 sites possess minimal activity below ~325 °C and their primary function is to increase the adsorption capacity of ammonia. A relationship between the SCR activity and surface acidity was not found. The SCR reaction is controlled by the surface VO₄ sites that initiate the reaction at ~200 °C. The co-precipitated catalysts were always more active than the corresponding impregnated catalysts. Finally, we ascribe the higher activity of the co-precipitated catalysts to the presence of the new surface WO_x sites associated surface defects on the TiO₂ support that increase the ammonia adsorption capacity.« less

A series of supported WO₃/TiO₂ catalysts was prepared by a new synthesis procedure involving co-precipitation of an aqueous TiO(OH)₂ and (NH₄)₁₀W₁₂O₄₁*5H₂O slurry under controlled pH conditions. The morphological properties, molecular structures, surface acidity and surface chemistry of the supported WO₃/TiO₂ catalysts were determined with BET, in situ Raman, in situ IR and temperature-programmed surface reaction (TPSR) spectroscopy, respectively. Isotopic ¹⁸O-¹⁶O exchange demonstrated that tungsten oxide was exclusively present as surface WO_x species on the TiO₂ support with mono-oxo W=O coordination. In contrast to previous studies employing impregnation synthesis that found only surface one mono-oxo O=WO₄ site on TiO₂, the co-precipitationmore » procedure resulted in the formation of two distinct surface WO_x species: mono-oxo O=WO₄ (~1010-1017 cm^-1) on low defect density patches of TiO₂ and a second mono-oxo O=WO₄ (~983-986 cm^-1) on high defect density patches of TiO₂. The concentration of the second WO_x surface species increases as a function of solution pH. Both surface WOx sites, however, exhibited the same NO/NH₃ SCR reactivity. The co-precipitated WO₃-TiO₂ catalysts synthesized in alkaline solutions exhibited enhanced performance for the NO/NH₃ SCR reaction that is ascribed to the greater number of surface defects on the resulting TiO2 support. For the co-precipitated catalyst prepared at pH10, surface NH₄⁺ species on Br nsted acid sites were found to be more reactive than surface NH₃* species on Lewis acid sites for SCR of NO with NH₃.« less