Hengyong Xu

CO oxidation catalyzed by oxide-supported Au25(SR)18 nanoclusters and identification of perimeter sites as active centers.

In this work, we explore the catalytic application of atomically monodisperse, thiolate-protected Au(25)(SR)(18) (where R = CH(2)CH(2)Ph) nanoclusters supported on oxides for CO oxidation. The solution phase nanoclusters were directly deposited onto various oxide supports (including TiO(2), CeO(2), and Fe(2)O(3)), and the as-prepared catalysts were evaluated for the CO oxidation reaction in a fixed bed reactor. The supports exhibited a strong effect, and the Au(25)(SR)(18)/CeO(2) catalyst was found to be much more active than the others. Interestingly, O(2) pretreatment of the catalyst at 150 °C for 1.5 h significantly enhanced the catalytic activity. Since this pretreatment temperature is well below the thiolate desorption temperature (~200 °C), the thiolate ligands should remain on the Au(25) cluster surface, indicating that the CO oxidation reaction is catalyzed by intact Au(25)(SR)(18)/CeO(2). We further found that increasing the O(2) pretreatment temperature to 250 °C (above the thiolate desorption temperature) did not lead to any further increase in activity at all reaction temperatures from room temperature to 100 °C. These results are in striking contrast with the common thought that surface thiolates must be removed-as is often done in the literature work-before the catalyst can exert high catalytic activity. The 150 °C O(2)-pretreated Au(25)(SR)(18)/CeO(2) catalyst offers ~94% CO conversion at 80 °C and ~100% conversion at 100 °C. The effect of water vapor on the catalytic performance is also investigated. Our results imply that the perimeter sites of the interface of Au(25)(SR)(18)/CeO(2) should be the active centers. The intact structure of the Au(25)(SR)(18) catalyst in the CO oxidation process allows one to gain mechanistic insight into the catalytic reaction.

Directly converting CO 2 into a gasoline fuel

The direct production of liquid fuels from CO2 hydrogenation has attracted enormous interest for its significant roles in mitigating CO2 emissions and reducing dependence on petrochemicals. Here we report a highly efficient, stable and multifunctional Na–Fe3O4/HZSM-5 catalyst, which can directly convert CO2 to gasoline-range (C5–C11) hydrocarbons with selectivity up to 78% of all hydrocarbons while only 4% methane at a CO2 conversion of 22% under industrial relevant conditions. It is achieved by a multifunctional catalyst providing three types of active sites (Fe3O4, Fe5C2 and acid sites), which cooperatively catalyse a tandem reaction. More significantly, the appropriate proximity of three types of active sites plays a crucial role in the successive and synergetic catalytic conversion of CO2 to gasoline. The multifunctional catalyst, exhibiting a remarkable stability for 1,000 h on stream, definitely has the potential to be a promising industrial catalyst for CO2 utilization to liquid fuels.

CeO2-supported Au38(SR)24 nanocluster catalysts for CO oxidation: a comparison of ligand-on and -off catalysts

The catalytic properties of atomically precise, thiolate-protected Au38(SR)24 (R = CH2CH2Ph) nanoclusters supported on CeO2 were investigated for CO oxidation in a fixed bed quartz reactor. Oxygen (O2) thermal pretreatment of Au38(SR)24/CeO2 at a temperature between 100 and 175 °C largely enhanced the catalytic activity, while pretreatment at higher temperatures (>200 °C) for removing thiolate instead gave rise to a somewhat lower activity than that for 175 °C pretreatment, and the ligand-off clusters were also found to be less stable. The CO conversion in the case of wet feed-gas (i.e. the presence of H2O vapor) was appreciably higher than the case of dry feed-gas when the reaction temperature was kept relatively low (between 60 and 80 °C), and interestingly the ligand-on and ligand-off catalysts exhibited opposite response to water vapor. Finally, we discussed some insights into the catalytic reaction involving the well-defined gold nanocluster catalyst.

Recent progress in Pd-based composite membranes

Pd-based composite membranes have received great attention from both academia and industry as they exhibit significant potential in several important fields, such as hydrogen purification and small-scale hydrogen generation as well as CO2 capture. Thin and defect-free Pd-based membranes must have high hydrogen flux, strong thermal stability and resistance to poisoning species. This work critically reviews both the experimental and modeling studies of Pd-based composite membranes over the last few decades, particularly focusing on the critical issues of thermal and chemical stability, which are the main obstacles blocking their commercial applications.

Permeation hysteresis in PdCu membranes

H 2 permeation hysteresis has been observed during cycling of a 3 mum thick supported PdCu membrane with approximately 50 atom % Pd through the fcc/bcc (face-centered cubic/body-centered cubic) miscibility gap between 723 and 873 K. Structural investigations after annealing of membrane fragments under H 2 at 823 K reveal retardation of the fcc(H) --> bcc(H) transition, which is attributed to the occurrence of metastable hydrogenated fcc PdCu(H) phases. The H(2) flux at 0.1 MPa H(2) pressure difference in the well-annealed bcc single phase regime below 723 K can be described by J(H2) = (1.3 +/- 0.2) mol.m (-2).s (-1) exp[(-11.1 +/- 0.6) kJ.mol (-1)/( RT)] and that in the fcc single phase regime above 873 K by J(H2) = (7 +/- 2) mol.m (-2).s (-1) exp[(-30.3 +/- 2.5) kJ.mol (-1)/( RT)].

CO2 decomposition over Pd membrane surfaces.

The interaction of pure CO 2 with a 3 microm thin, supported Pd membrane has been investigated between 473 and 773 K. Diagnostic H 2 permeation measurements indicate a reduction of the H 2 flux after CO 2 exposure at the lower and upper ends of this temperature range. Temperature-programmed oxidation and desorption in combination with scanning electron microscopy analyses reveal the dissociation of CO 2 above 523 K, yielding molecularly and/or dissociatively adsorbed CO below 623 K and nanoscopic carbon deposits above 723 K. CO 2 is obviously not inert over Pd surfaces at practical Pd membrane operation temperatures but could be kinetically stabilized in a narrow temperature window around 673 K.

Structural and Permeation Kinetic Correlations in PdCuAg Membranes

Addition of Ag is a promising way to enhance the H2 permeability of sulfur-tolerant PdCu membranes for cleanup of coal-derived hydrogen. We investigated a series of PdCuAg membranes with at least 70 atom % Pd to elucidate the interdependence between alloy structure and H2 permeability. Membranes were prepared via sequential electroless plating of Pd, Ag, and Cu onto ceramic microfiltration membranes and subsequent alloying at elevated temperatures. Alloy formation was complicated by a wide miscibility gap in the PdCuAg phase diagram at the practically feasible operation temperatures. X-ray diffraction showed that the lattice constants of the fully alloyed ternary alloys obey Vegards law closely. In general, H2 permeation rates increased with increasing Ag and decreasing Cu content of the membranes in the investigated temperature range. Detailed examination of the permeation kinetics revealed compensation between activation energy and pre-exponential factor of the corresponding H2 permeation laws. The origin of this effect is discussed. Further analysis showed that the activation energy for H2 permeation decreases overall with increasing lattice constant of the ternary alloy. The combination of these correlations results in a structure-function relationship that will facilitate rational design of PdCuAg membranes.

Highly Efficient Alcohol Oxidation on Nanoporous VSB‐5 Nickel Phosphate Catalyst Functionalized by NaOH Treatment

Selective catalytic oxidation of alcohols to corresponding aldehydes or ketones under mild conditions with molecular oxygen has attracted much attention in recent decades, because the aforementioned reaction is the foundation of many important fine chemical processes, which are currently being subjected to intense economical and environmental scrutiny. From the viewpoint of industry, the heterogeneous catalytic oxidation of alcohols over solid catalysts in liquid phase under mild conditions is in high demand because of several limitations of homogeneous catalysis, such as corrosion of the reactor wall, product purification, catalyst activity loss, extra engineering on recovery, and recycling of catalyst. In general, solid catalysts prepared by conventional loading methods often suffer the leaching of active species 2] due to the corrosion of liquid-phase reactants, products, or solvents, which are frequently excellent chelating agents, resulting in the irreversible deactivation of the catalyst. Therefore, complicated preparation methods are often necessary to protect or immobilize the active phase. Much effort has been devoted to alcohol oxidation, commonly using noble metal catalysts such as Ru, Pd, Pt, or Au, and rarely using transition metal catalysts such as Cu, Co, Ti, Ni, Mn. Although the noble metal-based catalysts, especially Ru, show promising activity, cheaper transition metal catalysts are always desired. The aerobic oxidation of various alcohols was reported over a nickel-containing hydrotalcite-like anionic clay catalyst, and either Ni + in association with aluminum oxide or octahedrally coordinated Ni + was thought to be responsible for the activation of molecular oxygen. 13] Furthermore, high valence Ni + species could act as a catalyst for the oxidation of benzyl alcohol, while simultaneously acting as a stoichiometric oxidant to convert the alcohol into the acid. Nanoporous VSB-5, a nickel phosphate based on octahedral NiO6 units linked by tetrahedral PO4 to build up one-dimensional 24-member ring channel structures, received much attention because of its novel properties, such as a high BET surface area and ion-exchange ability. It was applied for several purposes, such as shape-selective catalysis, hydrogen storage, and hydrogenation reactions. 18] Utilization of this easily-prepared nickel-rich nanoporous material is attractive because the catalytically active nickel centers dispersed in porous channels allow for catalyst shape selectivity and high catalytic selectivity, which are of importance in the development of green chemistry technology. Herein, we report findings about the property and catalytic behavior of VSB-5 molecular sieve that has been functionalized by simple NaOH treatment. It is to our knowledge, the first clear demonstration that an active phase of Ni(OH)2 efficiently catalyzes the oxidation of various alcohols with molecular oxygen to the corresponding aldehydes or ketones under mild reaction conditions. This active phase was prepared in situ in the channels of VSB-5 by NaOH treatment. Despite the fact that coordinatively unsaturated Ni + and octahedral NiO6 are present and accessible in the pore walls of VSB-5, 17] the calcined VSB-5 catalyst sample was inactive in the aerobic oxidation of benzyl alcohol at 353 K (Table 1,

Synergetic effect between NiO and Ni3V2O8 in propane oxidative dehydrogenation

The oxidative dehydrogenation (ODH) of propane was investigated on Ni-V-O catalysts in a wide range of vanadium contents (5-40%). The addition of a small amount of vanadium significantly increased the catalytic activity of NiO for oxidative dehydrogenation of propane to propene. The formation of propene has a good correlation with the coexistence of NiO and Ni3V2O8. This result strongly suggests that a synergetic effect exists between them in NiXV1-XOY (X = 0.95 to 0.6). The best results were obtained with a high Ni/V ratio (e.g. X = 0.95 to 0.85). The active sites and selective oxygen species are discussed. The influence of the catalyst preparation technique and the redox properties of the catalyst were also examined.

A new explanation for the carbon deposition and elimination over supported Ni, Ni-Ce and Ni-Co catalysts for CO2-reforming of methane

The influence of the addition of CeO2 or Co3O4 to Ni-based catalysts on carbon deposition by CH4 and carbon elimination by CO2 was studied with a pulse reaction as well as BET, TGA, XPS and hydrogen chemisorption techniques. It was found that addition of the CeO2 decreased the carbon deposition activity of CH4 and increased the carbon elimination ability of CO2, which could be explained by the interaction between the metal (Ni) and n-type semiconductor (CeO2). However, the addition of the Co3O4, which was transformed to Co0 during the reaction, accelerated carbon deposition by CH4 and resisted carbon elimination by CO2. A novel model of carbon deposition/carbon elimination over Ni-based catalysts has been proposed.