Structure and Surface Chemistry of Gold-Based Model Catalysts

Structure and Surface Chemistry of Gold-Based Model Catalyst

Enhanced Lattice Oxygen Reactivity over Ni-Modified WO₃‑Based Redox Catalysts for Chemical Looping Partial Oxidation of Methane

Partially oxidizing methane into syngas via a two-step chemical looping scheme is a promising option for methane transformation. Providing the optimum lattice oxygen to selectively produce syngas represents the major challenge for the development of oxygen carrier materials in chemical looping processes. This paper describes the design of WO₃-based oxygen carriers as the primary source of lattice oxygen with high melting points and attractive syngas selectivity. To further enhance the lattice oxygen availability and methane conversion capacity, NiO nanoclusters are introduced, considering the doping effect on chemical bonding disruption in both bulk and surface regions. For Ni_0.5WO_<i>x</i>/Al₂O₃, the nickel cations incorporated into the bulk of WO₃ can strongly weaken the tungsten–oxygen bond strength and increase the availability of lattice oxygen. The surface-grafted nickel species can effectively activate methane molecules and catalyze the partial oxidation reaction. Total methane conversion and syngas yield can be substantially increased by about 2.7-fold in comparison with unmodified WO₃/Al₂O₃. This work demonstrates that the bulk and surface modifications are feasible to tailor the active lattice oxygen of oxygen-carrying materials in chemical looping processes

Nature of the Active Sites of VO_<i>x</i>/Al₂O₃ Catalysts for Propane Dehydrogenation

Supported VO_<i>x</i> catalysts are promising for use in propane dehydrogenation (PDH) because of the relatively superior activity and stable performance upon regeneration. However, the nature of the active sites and reaction mechanism during PDH over VO_<i>x</i>-based catalysts remains elusive. We examined active species by attaining various fractions of V⁵⁺, V⁴⁺, and V³⁺ ions by adjusting the surface vanadium density on an alumina support. The results reveal a close relationship between TOF and the fraction of V³⁺ ion, indicating that V³⁺ was more active for PDH. <i>In situ</i> diffuse reflectance infrared Fourier transform spectroscopy showed the same strong adsorbed species during both propane dehydrogenation and propylene hydrogenation. The results indicated that such an intermediate may correspond to V species containing a CC bond, i.e., V–C₃H₅, and a reaction mechanism was proposed accordingly

Coverage Effect on the Activity of the Acetylene Semihydrogenation over Pd–Sn Catalysts: A Density Functional Theory Study

The existence of acetylene impurities in ethylene feedstock is harmful to downstream polymerization reactions. The removal of acetylene can be achieved via semihydrogenation reaction, which is normally catalyzed by Pd-based catalysts. This paper describes the coverage effect on the activity of acetylene hydrogenation reactions over Pd and PdSn alloy surfaces. High-coverage models are presented to construct coverage-dependent adsorption energies of C₂H₂, C₂H₄, and H₂ on Pd(111) and Pd₃Sn­(111) surfaces. It has been validated that the downshift of d-band center caused by preadsorbed molecules makes the adsorption weaker along with the increase of coverage, and the geometric effect can be neglected. An iterative method has been applied to predict surface coverages of reaction intermediates. Previous calculations with low-coverage models indicate that alloying Pd with late or post-transition metals, in general, enhances ethylene selectivity, accompanied with lower hydrogenation activity. However, by applying a high-coverage model, we show that the predicted hydrogenation barriers are comparable over Pd(111) and Pd₃Sn­(111) surfaces

Platinum-Modified ZnO/Al₂O₃ for Propane Dehydrogenation: Minimized Platinum Usage and Improved Catalytic Stability

Compared to metallic platinum and chromium oxide, zinc oxide (ZnO) is an inexpensive and low-toxic alternative for the direct dehydrogenation of propane (PDH). However, besides the limited activity, conventional zinc-based catalysts suffer from serious deactivation, because of ZnO reduction and/or carbon deposition. Considering the high cost of platinum, reducing the amount of platinum in the catalyst is always desirable. This paper describes a catalyst comprising ZnO modified by trace platinum supported on Al₂O₃, where the Zn²⁺ species serve as active sites and platinum acts as a promoter. This catalyst contains less platinum than traditional platinum-based catalysts and is much more stable than conventional ZnO catalyst or commercial chromium-based systems during PDH. It is proposed that ZnO was promoted to a stronger Lewis acid by platinum; thus, easier C–H activation and accelerated H₂ desorption were achieved

Propane Dehydrogenation over Pt/TiO₂–Al₂O₃ Catalysts

This paper describes an investigation on understanding catalytic consequences of Pt nanoparticles supported on a TiO₂–Al₂O₃ binary oxide for propane dehydrogenation (PDH). The TiO₂–Al₂O₃ supports were synthesized by a sol–gel method, and the Pt/TiO₂–Al₂O₃ catalysts were prepared by an incipient wetness impregnation method. Both as-prepared and post-experiment catalysts were characterized employing N₂ adsorption–desorption, X-ray diffraction, Raman spectra, H₂–O₂ titration, temperature-programmed desorption, thermogravimetric analysis, temperature-programmed oxidation, transmission electron microscopy, and Fourier-transform infrared spectra of chemisorbed CO. We have shown that TiO₂ is highly dispersed on Al₂O₃, and the addition of appropriate amount of TiO₂ improves propylene selectivity and catalytic stability, which is ascribed to the electron transfer from partially reduced TiO_<i>x</i> (<i>x</i> < 2) to Pt atoms. The increased electron density of Pt could reduce the adsorption of propylene and facilitate the migration of coke precursors from the metal surface to the support. The addition of TiO₂, however, also increases the amount of strong acid centers on the supports and the excessive TiO₂ addition might lead to a significant amount of coke formation. The electron transfer effect and the acid sites effect of TiO₂ addition exert an opposite influence on catalytic performance. The trade-off between the electron transfer effect and the acid sites effect is studied by varying the amount of TiO₂ loading. An optimal loading content of TiO₂ is 10 wt %, which results in a higher propylene selectivity and a better stability

Hydrogen Production via Glycerol Steam Reforming over Ni/Al₂O₃: Influence of Nickel Precursors

This paper describes an investigation regarding the influence of Ni precursors on catalytic performances of Ni/Al₂O₃ catalysts in glycerol steam reforming. A series of Ni/Al₂O₃ is synthesized using four different precursors, nickel nitrate, nickel chloride, nickel acetate, and nickel acetylacetonate. Characterization results based on N₂ adsorption–desorption, X-ray diffraction, H₂ temperature-programmed reduction, H₂ chemisorption, transmission electron microscopy, and thermogravimetric analysis show that reduction degrees of nickel, nickel dispersion, and particle sizes of Ni/Al₂O₃ catalysts are closely dependent on the anion size and nature of the nickel precursors. Ni/Al₂O₃ prepared by nickel acetate possesses the moderate Ni reduction degree, high Ni dispersion, and small nickel particle size, which possesses the highest H₂ yield. Reaction parameters are also examined, and 550 °C and a steam-to-carbon ratio of 3 are optimized. Moreover, coke deposition, mainly graphite species, leads to the deactivation of Ni/Al₂O₃ catalysts in glycerol steam reforming. Nickel chloride-derived Ni/Al₂O₃ catalysts suffer from severe coke deposition and low reaction activity due to large Ni particle size, low Ni dispersion, and residual chloride

Hydrogen Production via Steam Reforming of Ethanol on Phyllosilicate-Derived Ni/SiO₂: Enhanced Metal–Support Interaction and Catalytic Stability

This paper describes the design of Ni/SiO₂ catalysts obtained from a phyllosilicate precursor that possess high activity and stability for bioethanol steam reforming to sustainably produce hydrogen. Sintering of metal particles and carbon deposition are two major issues of nickel-based catalysts for reforming processes, particularly at high temperatures; strong metal–support interaction could be a possible solution. We have successfully synthesized Ni-containing phyllosilicates by an ammonia evaporation method. Temperature programmed reduction results indicate that the metal–support interaction of Ni/SiO₂ catalyst prepared by ammonia evaporation method (Ni/SiO_2P) is stronger due to the unique layered structure compared to that prepared by conventional impregnation (Ni/SiO_2I). With the phyllosilicate precursor nickel particles highly disperse on the surface, remaining OH groups in the unreduced phyllosilicates promote nickel dispersion and carbon elimination. We also show that high dispersion of Ni and strong metal–support interaction of Ni/SiO_2P significantly promote ethanol conversion and H₂ production in ethanol steam reforming. Ni/SiO_2P produces less carbon deposition compared to Ni/SiO_2I; for the latter, a surface layer of Ni₃C formed during the deactivation

Reduced Graphene Oxide (rGO)/BiVO₄ Composites with Maximized Interfacial Coupling for Visible Lght Photocatalysis

This paper describes the construction of reduced graphene oxide (rGO)/BiVO₄ composites with maximized interfacial coupling and their application as visible light photocatalysts. Thin rGO sheets (<5 nm) could completely cover BiVO₄ polyhedrons with highly active (040) facets exposed through an evaporation-induced self-assembly process. In addition to the increased surface adsorption effect of rGO, a considerable enhancement of the photoactivity of BiVO₄ has been demonstrated through the degradation of methylene blue upon the covering of rGO. The improved photocatalytic activity is attributed to the formation of well-defined rGO/BiVO₄ interfaces, which greatly enhances the charge separation efficiency

Synthesis of Ethanol via Syngas on Cu/SiO₂ Catalysts with Balanced Cu⁰–Cu⁺ Sites

This paper describes an emerging synthetic route for the production of ethanol (with a yield of ∼83%) via syngas using Cu/SiO₂ catalysts. The remarkable stability and efficiency of the catalysts are ascribed to the unique lamellar structure and the cooperative effect between surface Cu⁰ and Cu⁺ obtained by an ammonia evaporation hydrothermal method. Characterization results indicated that the Cu⁰ and Cu⁺ were formed during the reduction process, originating from well-dispersed CuO and copper phyllosilicate, respectively. A correlation between the catalytic activity and the Cu⁰ and Cu⁺ site densities suggested that Cu⁰ could be the sole active site and primarily responsible for the activity of the catalyst. Moreover, we have shown that the selectivity for ethanol or ethylene glycol can be tuned simply by regulating the reaction temperature