Plasma-driven CO2 hydrogenation to CH3OH over Fe2O3/γ-Al2O3 catalyst

We report a plasma-assisted CO₂ hydrogenation to CH₃OH over Fe₂O₃/γ-Al₂O₃ catalysts, achieving 12% CO₂ conversion and 58% CH₃OH selectivity at a temperature of nearly 80°C atm pressure. We investigated the effect of various supports and loadings of the Fe-based catalysts, as well as optimized reaction conditions. We characterized catalysts by X-ray powder diffraction (XRD), hydrogen temperature programmed reduction (H₂-TPR), CO₂ and CO temperature programmed desorption (CO₂/CO-TPD), high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), x-ray photoelectron spectroscopy (XPS), Mössbauer, and Fourier transform infrared ( FTIR). The XPS results show that the enhanced CO₂ conversion and CH₃OH selectivity are attributed to the chemisorbed oxygen species on Fe₂O₃/γ-Al₂O₃. Furthermore, the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) and TPD results illustrate that the catalysts with stronger CO₂ adsorption capacity exhibit a higher reaction performance. In situ DRIFTS gain insight into the specific reaction pathways in the CO₂/H₂ plasma. This study reveals the role of chemisorbed oxygen species as a key intermediate, and inspires to design highly efficient catalysts and expand the catalytic systems for CO₂ hydrogenation to CH₃OH.     

Catalytic conversion of glucose to 5-hydroxymethylfurfural over nano-sized mesoporous Al2O3–B2O3 solid acids

A series of nano-sized mesoporous Al₂O₃–B₂O₃ catalysts with different molar ratios of Al/B were prepared from aluminum isopropoxide and boric acid through an evaporation-induced self-assembly (EISA) process, and were characterized by ICP-AES, FTIR (pyridine adsorption), XRD, NH₃-TPD, SEM, TEM, and N₂ adsorption–desorption. These catalysts were further used as solid acids in the catalytic conversion of glucose to 5-hydroxymethylfurfural (HMF). An optimized HMF yield of 41.4% was obtained within 120 min at 140 °C over Al₂O₃–B₂O₃ (Al/B = 5:5). It was demonstrated that catalysts with the presence of Lewis acid sites were more favorable for the formation of HMF.     

CH4 reforming with CO2 for syngas production over nickel catalysts supported on mesoporous nanostructured γ-Al2O3

Nanostructured γ-Al₂O₃ with high surface area and mesoporous structure was synthesized by sol-gel method and employed as catalyst support for nickel catalysts in methane reforming with carbon dioxide. The prepared samples were characterized by XRD, BET, TPR, TPH, SEM and TPO techniques. The BET analysis showed a high surface area of 204m²g^?1 and a narrow pore-size distribution centered at a diameter of 5.5 nm for catalyst support. The results revealed that an increase in nickel loading from 5 to 15 wt% decreased the surface area of catalyst from 182 to 160 m²g^?1. In addition, the catalytic results showed an increase in methane conversion with increase in nickel content. TPO analysis revealed that the coke deposition increased with increasing in nickel loading, and the catalyst with 15 wt% of nickel showed the highest degree of carbon formation. SEM and TPH analyses confirmed the formation of whisker type carbon over the spent catalysts. Increasing CO₂/CH₄ ratio increased the methane conversion. The BET analysis of spent catalysts indicated that the mesoporous structure of catalysts still remained after reaction.     

Direct Synthesis of Diethyl Carbonate from CO2 and CH3CH2OH Over Cu–Ni/AC Catalyst

Cu?CNi/AC (Active carbon) catalysts were synthesized and characterized by temperature programmed reduction, X-ray diffraction, Scanning electron microscopy, chemical analysis, and N₂ adsorption. Their activities (in terms of TOF) in the direct synthesis of diethyl carbonate from CO₂ and CH₃CH₂OH were also evaluated. The presence of a Cu?CNi alloy phase may explain the significant increase in activity of bimetallic catalysts compared with monometallic samples.     

Enhanced the photocatalytic activity of B–C–N–TiO2 under visible light:Synergistic effect of element doping and Z-scheme interface heterojunction constructed with Ag nanoparticles

In order to enhance the photocatalytic activity of TiO₂ under visible light, Ag nanoparticles were introduced into tridoped B–C–N–TiO₂ (TT) catalyst by photoreduction deposition. Ag/B–C–N–TiO₂ (ATT) catalysts with the functions of reducing band gap and carrier recombination were prepared. At the same time, the effect of the amount of Ag on the photocatalytic performance of ATT catalyst was investigated. Through XRD, XPS, PL and other characterization methods, the (211)/(101)/Ag interface heterojunction mechanism similar to the traditional Z-scheme heterojunction was proposed. The intervention of Ag nanoparticles changed the P–N interface heterojunction between (211)/(101) to the (211)/(101)/Ag Z-scheme interface heterojunction. The results show that ATT catalyst exhibits the highest photocatalytic activity when the molar amount of Ag is 0.005% with the MB degradation rate of the ATT catalyst (0.01707 min^?1), which is 14.59 times of TiO₂ (0.00117 min^?1) and 2.02 times of TT (0.00847 min^?1). In addition, the four cycles efficiencies of ATT for MB degradation were all above 94.00%.This study reveals the possibility of construction of Z-scheme heterojunctions between precious metal nanoparticles and different interfaces of TiO₂, and provides a reference for the construction of Z-scheme interface heterojunctions.     

The catalytic conversion of CO2 to hydrocarbons over Fe–K supported on Al2O3–MgO mixed oxides

Al₂O₃–MgO mixed oxides prepared by a co-precipitation method have been used as supports for potassium-promoted iron catalysts for CO₂ hydrogenation to hydrocarbons. The catalysts have been characterized by XRD, BET surface area, CO₂ chemisorption, TPR and TPDC techniques. The CO₂ conversion, the total hydrocarbon selectivity, the selectivities of C₂–C₄ olefins and C₅₊ hydrocarbons are found to increase with increase in MgO content upto 20 wt% in Fe–K/Al₂O₃–MgO catalysts and to decrease above this MgO content. The TPR profiles of the catalysts containing pure Al₂O₃ and higher (above 20 wt%) MgO content are observed to contain only two peaks, corresponding to the reduction of Fe₂O₃ to Fe⁰ through Fe₃O₄. However, the TPR profile of 20 wt% MgO catalyst exhibits three peaks, which indicate the formation of iron phase through FeO phase. The TPDC profiles show the formation of three types of carbide species on the catalysts during the reaction. These profiles are shifted towards high temperatures with increasing MgO content in the catalyst. The activities of the catalysts are correlated with physico-chemical characteristics of the catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation of cobalt coated TiO2 and WO3–TiO2 nanotube films via photo-assisted deposition with enhanced photocatalytic activity under visible light illumination

Highly ordered TiO₂ and WO₃–TiO₂ nanotubes were prepared by one-step electrochemical anodizing method and cobalt has been successfully deposited on these nanotubes by photo-assisted deposition process. The morphology, crystal structure, elemental composition and light absorption capability of samples were characterized by field emission scanning electron microscope, X-ray diffraction, energy dispersive X-ray spectrometer and ultraviolet–visible spectroscopy methods. All cobalt loaded samples show an appearance of red shift relative to the unloaded samples. The degradation of methylene blue was used as a model reaction to evaluate the photocatalytic activity of these novel visible-light-responsive photocatalysts. Results showed that the photocatalytic activity of bare WO₃–TiO₂ samples is higher than that with undoped TiO₂ sample. Compared with unmodified TiO₂ and WO₃–TiO₂, the Co/TiO₂ and Co/WO₃–TiO₂ samples exhibited enhanced photocatalytic activity in the degradation of methylene blue. Kinetic research showed that the reaction rate constant of Co/WO₃–TiO₂ is approximately 2.26 times higher than the apparent reaction rate constant of bare WO₃–TiO₂. This work provides an insight into designing and synthesizing new TiO₂–WO₃ nanotubes-based hybrid materials for effective visible light-activated photocatalysis. The catalysts prepared in this study exhibit industrially relevant interests due to the low cost and high photocatalytic activity.     

Graphene–WO3 nanobelt composite: Elevated conduction band toward photocatalytic reduction of CO2 into hydrocarbon fuels

Graphene oxide (GO), tungsten trioxide (WO₃) and graphene–WO₃ nanobelt composites (GW) were synthesized and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), UV–vis diffuse reflection spectra (DRS) and X-ray photoelectron spectroscopy (XPS) valence band spectra. We demonstrated that the graphene can elevate the conduction band of WO₃ toward photocatalytic reduction of CO₂ into hydrocarbon fuels under visible-light irradiation. And the photocatalytic activity of GW is higher than that of GO, WO₃ and P25 TiO₂.     

Low-temperature synthesis of DME from CO2/H2 over Pd-modified CuO–ZnO–Al2O3–ZrO2/HZSM-5 catalysts

Three series of Pd-modified CuO–ZnO–Al₂O₃–ZrO₂/HZSM-5 catalysts were prepared and characterized by BET, XRD and TPR analysis. The catalytic system was evaluated in the development of direct synthesis of dimethyl ether (DME) from carbon dioxide hydrogenation at low temperature (T=200 °C, P=3.0 MPa). The results indicated that the addition of palladium markedly enhanced the DME synthesis and retarded the CO formation. An explanation of this promoting effect of Pd on the DME synthesis could be attributed to the spillover of hydrogen from Pd⁰ to the neighboring phase.     

Reduction of NO by CO under oxidizing conditions over Pt and Rh supported on Al2O3–ZrO2 binary oxides

The platinum and rhodium particles supported on Al₂O₃–ZrO₂ binary oxides were prepared by adding the metal precursors to the binary gel. High specific surface areas (220–250 m²/g) and small metallic particle size (∼20 Å) were obtained. In the reduction of NO by CO without oxygen in the reactant flow high levels of N₂O were achieved, whereas in the presence of oxygen the formation of N₂O notably increases. This selectivity behavior was not observed in catalysts prepared by impregnation with the metallic precursors of Al₂O₃–ZrO₂ mixed oxide stabilized after calcination at 500 °C, since in these catalysts the selectivity to N₂O is the higher with or without oxygen. Thus, it is proposed that the metallic impregnation of gels strongly modifies the mechanism by which the NO reduction by CO occurs.     

Enhanced ultraviolet–visible–near infrared driven photocatalytic activity of 1D/0D BiVO4:Er/Yb@Ag/Ag3PO4 Z-scheme heterostructure via a synergetic strategy of plasmonic effect and upconversion luminescence

Expanding the spectral response range to near-infrared (NIR) region and improving carrier separation are the two critical strategies to obtain highly efficient photocatalysts for water pollution control. Herein, a new type of photocatalyst one-dimensional/zero-dimensional (1D/0D) BiVO₄:Er/Yb@Ag/Ag₃PO₄ Z-scheme heterostructure with full spectral response is constructed by an electrospinning coupled with an ethylene glycol assisted hydrothermal method. The optimized BiVO₄:Er/Yb@Ag/Ag₃PO₄ heterostructured nanofibers degrade 69.5% (9 h) and 91.3% (30 min) of tetracycline hydrochloride (TC), 76.7% (9 h) and 99.4% (24 min) of methylene blue (MB) and 38.2% (9 h) and 94.5% (60 min) of bisphenol A (BPA) under NIR light and simulated sunlight excitation, respectively. Under NIR light and simulated sunlight excitation, the MB removal efficiencies of the optimal composite are 9.833 and 1.094 times of Ag₃PO₄ and 20.184 and 11.558 times of BiVO₄, respectively. The superior photocatalytic activity can be attributed to the synergistic effects of the unique 1D/0D contact interface, the porous outer wall of BiVO₄:Er/Yb nanofibers, Ag bridged Z-scheme heterostructure, upconversion (UC) luminescence and the surface plasmon resonance (SPR) effect. This makes BiVO₄:Er/Yb@Ag/Ag₃PO₄ heterostructured nanofibers have the advantages of broadened absorption spectrum, abundant active sites, increased specific surface area, fast electron transfer channels, improved carrier separation efficiency, enhanced photocorrosion resistance and stability. This work provides a new insight in designing and constructing high-performance Ag bridged Z-scheme heterostructure photocatalysts with full spectral response for water pollution control.     

Pulse‐MS studies on CH4/CD4 isotope effect in the partial oxidation of methane to syngas over Pt/α‐Al2 3

By replacing CH₄/+O₂ with CD₄+O₂, the deuterium isotope effect in the partial oxidation of methane over Pt/α‐Al₂O₃ was studied in the temperature range of 550–650 °C using the pulse‐MS method. The effect of space velocity of carrier gas and CO₂ reforming of CH₄ to syngas were also investigated. No deuterium isotope effect was observed for CH₄ conversion whereas CO formation showed a normal deuterium isotope effect. The surface reaction between adsorbed hydrocarbon species and adsorbed oxygen species to CO formation may be a relatively slow step. The results support the parallel mechanism, namely CO and CO₂ are simultaneously formed in parallel from the direct oxidation of methane. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rapid visible and solar photocatalytic Cr(VI) reduction and electrochemical sensing of dopamine using solution combustion synthesized ZnO–Fe2O3 nano heterojunctions: Mechanism Elucidation

In recent times cost-effective advanced materials with dual applications in photocatalytic water treatment and electrochemical sensing have been explored and developed. Herein we report facile solution combustion synthesis of ZnO/Fe₂O₃ (ZF) type-II heterojunction for electrochemical sensing of dopamine and visible assisted photocatalytic reduction of carcinogenic Cr(VI) into Cr(III). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to evaluate the compatibility of the electrodes for DA sensing. Furthermore, DPV curves revealed that ZF2 (ZnO: Fe₂O₃- 3:0.5) modified GCEs possessed 0.27 μM and 0.7 μM of LOD and LOQ with good linear range from 3 μM to 30 μM. Overall, reproducibility and interference studies confirm the efficient use of material for DA sensing. The junction ZF2 shows best performance with 88% Cr(VI) photo-reduction under visible light in 90 min and 100% reduction with tartaric acid as sacrificial agent (utilizing holes) in just 45 min. The effect of sacrificial agents and scavengers suggest the photogenerated electrons were major active species followed by ^●O₂⁻. The CB edges have enough potential for rapid reduction of hexavalent chromium under visible light and solar light. The photoluminescence and electrochemical impedance suggest lowered recombination, high charge separation and charge transfer capacity in the heterojunction. The transfer of electrons from conduction band of Fe₂O₃ to that of ZnO having high enough negative potential to reduce Cr(VI), thus utilizing the CB potential of wide band gap ZnO for rapid visible photocatalysis. In addition shortcomings as low conductivity Fe₂O₃ and high band gap of ZnO are both minimized in the junction. This study confirmed that popular semiconductors as ZnO and Fe₂O₃ have enough potential in dual role as electrochemical sensors and visible photocatalysts with best performance by optimizing the conditions and ratios.     

Preparation and characterization of spindle-shaped nanoporous anatase TiO<Subscript>2</Subscript>–Ag<Subscript>3</Subscript>PO<Subscript>4</Subscript> heterostructure with enhanced visible light driven photocatalytic performance

A novel spindle-shaped nanoporous anatase TiO₂–Ag₃PO₄ heterostructure with high photocatalytic activity was successfully prepared by a simple method. The Ag₃PO₄ nanoparticles with a diameter of 20–50 nm were deposited on the surface of the spindle-shaped TiO₂. The effect of Ag₃PO₄ nanoparticles amounts on the photocatalytic activity was investigated. The results showed that the TiO₂–Ag₃PO₄ composite with the mass ratio of TiO₂:Ag₃PO₄ = 1:2 displayed the highest photodegradation efficiency of methylene blue (MB) and Bisphenol A (BPA), which was more highly than that of Ag₃PO₄ nanoparticles and also indicated a high stability of photocatalytic degradation. The improved activity of the TiO₂–Ag₃PO₄ composite could be attributed to the efficient separation of the photogenerated electron–hole pairs.     

Reaction Pathways in the Reduction of NO x Species by CO over Pt–Ba/Al2O3: Lean NO x Trap Catalytic Systems

The reduction by CO of NO _x species stored over Pt–Ba/Al₂O₃ Lean NO _x Trap systems is analysed in this work. The reaction mechanisms and pathways leading to N₂ formation both under dry and wet conditions are investigated by complementary transient dynamic experiments and FTIR analyses.