Chemical looping partial oxidation over FeWOx/SiO2 catalysts

This paper describes the design of a FeWOx-based oxygen carrier for the chemical partial oxidation of methane(CLPOM).Thermodynamic screening and kinetic analyses both forecast the FeWOx-based oxygen carrier as a promising candidate for the production of syngas.The total methane conversion and syngas yield can be dramatically increased with this catalyst compared to the case with the unmodified WO3/SiO2,thereby enabling CLPOM with 62%methane conversion,93%CO gas-phase selectivity,94%H2 selectivity,and a 2.4 H2/CO ratio.The catalyst has the advantages of high availability of lattice oxygen to oxidize carbonaceous intermediates in time,together with the formation of an Fe-W alloy to promote the surface reaction.Consequently,it demonstrates excellent catalytic performance with no catalyst deactivation at 900°C and 1 atm.The excellent structural stability plays an essential role in CLPOM.As revealed via XPS and ICP,the phase segregation has not been observed due to the strong interaction between Fe and W,which resulted in the formation of the Fe-W alloy during the reduction processes and the match between the ion oxidation rates of the Fe and W ions in the oxidation stage.The results provide fundamental information on the reaction mechanism of FeWOx/SiO2,and present it as a promising candidate for CLPOM.

Water promoted structural evolution of Ag nanocatalysts supported on alumina

Water is often involved in many catalytic processes,which can strongly affect structural evolution of catalysts during pretreatments and catalytic reactions.In this work,we demonstrate a promotional effect of H_(2)O on both oxidative dispersion and spontaneous aggregation of Ag nanocatalysts supported on alumina.Ag nanoparticles supported onγ-Al_(2)O_(3) and Ag nanowires on Al_(2)O_(3)(0001)can be dispersed into nanoclusters via annealing in O_(2)above 300℃,which is accelerated by introduction of H_(2)O into the oxidative atmosphere.Furthermore,the formed highly dispersed Ag nanoclusters are subject to spontaneous aggregation in humid atmosphere at room temperature.Ex situ and in situ characterizations in both powder and model catalysts suggest that formation of abundant surface hydroxyls and/or water adlayer on the Al_(2)O_(3) surface in the H_(2)O-containing atmosphere facilitates the surface migration of Ag species,thus promoting both dispersion and aggregation processes.The aggregation of the supported Ag nanostructures induced by the humid oxidative atmosphere enhances CO oxidation but inhibits selective catalytic reduction of NO with C_(3)H_(6).This work illustrates the critical role of H_(2)O in structure and catalytic performance of metal nanocatalysts,which can be widely present in heterogeneous catalysis.

Effects of water adsorption on active site-dependent H_(2)activation over MgO nanoflakes

Understanding the effect of H_(2)O adsorption on reactant activation is of great importance in heterogeneous catalysis,which remains a grand challenge particularly in oxide catalyst systems with structural complexity.Herein,the effect of D_(2)O adsorption on D_(2)activation over MgO nanocatalysts at different temperatures has been investigated by transmission Fourier transform infrared(FT-IR)and temperature-programmed desorption(TPD).Two sets of hydride and hydroxyl species produced from D_(2)dissociation at more active and less active Mg-O pairs can be observed by FT-IR,which all desorb via the product of D_(2)as confirmed by TPD experiments.We find that the physically adsorbed D_(2)O overlayer does not affect the dissociation of D_(2)since D_(2)may pass through the molecular layer and access the surface-active sites.When D_(2)O is partially dissociated on the MgO surface,D_(2)can only dissociate at the remaining active sites until that dissociated-ODw groups from D_(2)O occupy all active sites.These findings provide a fundamental understanding of the effect of water adsorption on D_(2)activation on oxide catalysts.

Active sites in CO2 hydrogenation over confined VOx-Rh catalysts

Metal oxide-promoted Rh-based catalysts have been widely used for CO2hydrogenation,especially for the ethanol synthesis.However,this reaction usually suffers low CO2conversion and alcohols selectivity due to the formation of byproducts methane and CO.This paper describes an efficient vanadium oxide promoted Rh-based catalysts confined in mesopore MCM-41.The Rh-0.3VOx/MCM-41 catalyst shows superior conversion（～12%）and ethanol selectivity（～24%）for CO2hydrogenation.The promoting effect can be attributed to the synergism of high Rh dispersion by the confinement effect of MCM-41 and the formation of VOx-Rh interface sites.Experimental and theoretical results indicate the formation of til-CO at VOx-Rh interface sites is easily dissociated into*CHx,and then*CHxcan be inserted by CO to form CH3CO*,followed by CH3CO*hydrogenation to ethanol.

Defect-mediated reactivity of Pt/TiO2 catalysts:the different role of titanium and oxygen vacancies

Defects are ubiquitous in oxide supports and can often tune the catalytic property of supported metal catalysts.This work describes the distinct role of titanium and oxygen vacancies of TiO2 supports in the catalytic performance of supported Pt catalysts for CO oxidation.Pt was loaded on the TiO2 supports with oxygen vacancies(VO-TiO2)and titanium vacancies(VTiTiO2).It was found that different defects of TiO2 could distinctively modify the electron property of Pt and thereby CO adsorption strength.The strength of CO adsorption on Pt/VTi-TiO2 is enhanced,while that of Pt/VO-TiO2 becomes weaker.Additionally,the presence of defects would also promote the reducibility of catalysts.On the account of the superior redox ability,both Pt/VTi-TiO2 and Pt/VO-TiO2 exhibit a higher activity than Pt supported on normal TiO2 for CO oxidation.

Step-confined thin film growth via near-surface atom migration

Understanding of thin film growth mechanism is crucial for tailoring film growth behaviors,which in turn determine physicochemical properties of the resulting films.Here,vapor-growth of tungsten carbide overlayers on W(110)surface is investigated by real time low energy electron microscopy.The surface growth is strongly confined by surface steps,which is in contrast with overlayer growth crossing steps in a so-called carpet-like growth mode for example in graphene growth on metal surfaces.Density functional theory calculations indicate that the step-confined growth is caused by the strong interaction of the forming carbide overlayer with the substrate blocking cross-step growth of the film.Furthermore,the tungsten carbide growth within each terrace is facilitated by the supply of carbon atoms from near-surface regions at high temperatures.These findings suggest the critical role of near-surface atom diffusion and step confinement effects in the thin film growth,which may be active in many film growth systems.

A comparative study in structure and reactivity of “FeO_x-on-Pt” and “NiO_x-on-Pt” catalysts

Oxide nanostructures grown on noble metal surfaces are often highly active in many reactions,in which the oxide/metal interfaces play an important role.In the present work,we studied the surface structures of Fe Ox-on-Pt and Ni Ox-on-Pt catalysts and their activity to CO oxidation reactions using both model catalysts and supported nanocatalysts.Although the active Fe O1x structure is stabilized on the Pt surface in a reductive reaction atmosphere,it is prone to change to an Fe O2x structure in oxidative reaction gases and becomes deactivated.In contrast,a Ni O1x surface structure supported on Pt is stable in both reductive and oxidative CO oxidation atmospheres.Consequently,CO oxidation over the Ni O1x-on-Pt catalyst is further enhanced in the CO oxidation atmosphere with an excess of O2.The present results demonstrate that the stability of the active oxide surface phases depends on the stabilization effect of the substrate surface and is also related to whether the oxide exhibits a variable oxidation state.