Xinbin Ma

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/SiO(2) catalysts. The remarkable stability and efficiency of the catalysts are ascribed to the unique lamellar structure and the cooperative effect between surface Cu(0) and Cu(+) obtained by an ammonia evaporation hydrothermal method. Characterization results indicated that the Cu(0) 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 Cu(0) and Cu(+) site densities suggested that Cu(0) 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.

Recent advances in capture of carbon dioxide using alkali-metal-based oxides

Carbon dioxide (CO2) is a major greenhouse gas and makes a significant contribution to global warming and climate change. Thus CO2 capture and storage (CCS) have attracted worldwide interest from both fundamental and practical research communities. Alkali-metal-based oxides such as alkali-metal oxides, binary oxides, and hydrotalcite-like compounds are promising adsorbents for CO2 capture because of their relatively high adsorption capacity, low cost, and wide availability. They can also be applied to the adsorption-enhanced reactions involving CO2. The microstructures (e.g., surface area, porosity, particle size, and dispersion) of these oxides determine the CO2 adsorption capacity and multicycle stability. This perspective critically assesses and gives an overview of recent developments in the synthesized method, adsorption mechanism, operational conditions, stability, and regenerability of a variety of oxides. Both pros and cons of these oxides are also discussed. Insights are provided into several effective procedures regarding microstructural control of alkali-metal-based oxides, including preparation optimization, modification, stream hydration, etc.

A copper-phyllosilicate core-sheath nanoreactor for carbon–oxygen hydrogenolysis reactions

Hydrogenolysis of carbon-oxygen bonds is a versatile synthetic tool in organic synthesis. Copper-based catalysts have been intensively explored as the copper sites account for the highly selective hydrogenation of carbon-oxygen bonds. However, the inherent drawback of conventional copper-based catalysts is the deactivation by metal-particle growth and unstable surface Cu(0) and Cu(+) active species in the strongly reducing hydrogen and oxidizing carbon-oxygen atmosphere. Here we report the superior reactivity of a core (copper)-sheath (copper phyllosilicate) nanoreactor for carbon-oxygen hydrogenolysis of dimethyl oxalate with high efficiency (an ethanol yield of 91%) and steady performance (>300 h at 553 K). This nanoreactor, which possesses balanced and stable Cu(0) and Cu(+) active species, confinement effects, an intrinsically high surface area of Cu(0) and Cu(+) and a unique tunable tubular morphology, has potential applications in high-temperature hydrogenation reactions.

Branched TiO2 nanoarrays sensitized with CdS quantum dots for highly efficient photoelectrochemical water splitting

This paper describes the design, characterization, and utilization of branched TiO2 nanoarrays sensitized with CdS quantum dots as anodes for photoelectrochemical water splitting. The remarkable photocurrent density (∼4 mA cm(-2) at a potential of 0 V versus Ag/AgCl) and high solar to hydrogen efficiency of the materials obtained were ascribed to the novel branched nanostructure and efficient electron transfer from CdS to TiO2.

Sorption enhanced steam reforming of ethanol on Ni–CaO–Al2O3 multifunctional catalysts derived from hydrotalcite-like compounds

This paper describes the sorption of carbon dioxide for enhanced steam reforming of ethanol to produce hydrogen via Ni–CaO–Al2O3 multifunctional catalysts derived from hydrotalcite-like compounds (HTlcs). The catalysts were characterized by N2 adsorption–desorption, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR) and thermogravimetric analysis (TGA) and tested in sorption enhanced steam reforming of ethanol (SESRE), in which products were monitored by an online mass spectrometer (MS). The Ni–CaO–Al2O3 catalysts possess uniform distribution of Ni, Ca and Al, contributing significantly to the excellent CO2 adsorbent capacity and reforming activity in SESRE. We have also examined the effect of Ca/Al ratios on Ni dispersion, CaO particle size, and catalytic reactivity; a Ca/Al of 3.0 was optimized. The Ni–CaO–Al2O3 catalysts outperform the conventional mixture of CaO adsorbents and Ni/Al2O3 catalysts for SESRE.

Morphology control of ceria nanocrystals for catalytic conversion of CO2 with methanol

This paper describes the synthesis of ceria catalysts with octahedron, nanorod, nanocube and spindle-like morphologies via a template-free hydrothermal method. The surface morphologies, crystal plane and physical-chemical structures were investigated via field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and temperature-programmed desorption of ammonia and carbon dioxide (NH3-TPD and CO2-TPD). The catalytic performance over these ceria catalysts with different exposed planes were tested for dimethyl carbonate (DMC) synthesis from CO2 and methanol. The results showed that the spindle-like CeO2 showed the highest DMC yields, followed by nano-rods, nano-cubes and nano-octahedrons. A synergism among the exposed (111) plane, defect sites, and acid-basic sites was proposed to be crucial to obtaining the high reactivity of DMC formation.

An Alternative Synthetic Approach for Efficient Catalytic Conversion of Syngas to Ethanol

Ethanol is an attractive end product and a versatile feedstock because a widespread market exists for its commercial use as a fuel additive or a potential substitute for gasoline. Currently, ethanol is produced primarily by fermentation of biomass-derived sugars, particularly those containing six carbons, but coproducts 5-carbon sugars and lignin remain unusable. Another major process for commercial production of ethanol is hydration of ethylene over solid acidic catalysts, yet not sustainable considering the depletion of fossil fuels. Catalytic conversion of synthetic gas (CO + H2) could produce ethanol in large quantities. However, the direct catalytic conversion of synthetic gas to ethanol remains challenging, and no commercial process exists as of today although the research has been ongoing for the past 90 years, since such the process suffers from low yield and poor selectivity due to slow kinetics of the initial C-C bond formation and fast chain growth of the C2 intermediates. This Account describes recent developments in an alternative approach for the synthesis of ethanol via synthetic gas. This process is an integrated technology consisting of the coupling of CO with methanol to form dimethyl oxalate and the subsequent hydrogenation to yield ethanol. The byproduct of the second step (methanol) can be separated and used in circulation as the feedstock for the coupling step. The coupling reaction of carbon monoxide for producing dimethyl oxalate takes place under moderate reaction conditions with high selectivity (∼95%), which ideally leads to a self-closing, nonwaste, catalytic cycling process. This Account also summarizes the progress on the development of copper-based catalysts for the hydrogenation reaction with remarkable efficiencies and stability. The unique lamellar structure and the cooperative effect between surface Cu(0) and Cu(+) species are responsible for the activity of the catalyst with high yield of ethanol (∼91%). The understanding of nature of valence states of Cu could also guide the rational design of Cu-based catalysts for other similar reactions, particularly for hydrogenation catalytic systems. In addition, by regulating the reaction condition and the surface structure of the catalysts, the products in the hydrogenation steps, such as ethanol, methyl glycolate, and ethylene glycol, could be tuned efficiently. This synthetic approach enables a more sustainable ethanol, methyl glycolate, and ethylene glycol synthesis in industry and greatly reduces the dependence on petroleum resources and the emission of the greenhouse gas.

Understanding electronic and optical properties of anatase TiO2 photocatalysts co-doped with nitrogen and transition metals

This paper describes an investigation into the general trend in electronic properties of anatase TiO2 photocatalysts co-doped with transition metals and nitrogen employing first-principles density functional theory. Fourteen different transition metals (M), including Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, and Cd, have been considered. The characteristic band structures of the co-doping systems involving the transition metal series are presented. Our results indicate that the absorption edges of TiO2 are shifted to the visible-light region upon introduction of dopants, due to the reduced conduction band minimum (CBM) and the formation of impurity energy levels (IELs) in the band gap. These IELs are primarily formed from (a) the anti-bonding orbitals of the M-O (M indicates the doped transition metal) bonds, (b) the unsaturated nonbonding d orbitals of the doped transition metal (mainly d(xy), d(yz), and d(xz)), and (c) the Ti-O bonding/Ti-N anti-bonding orbitals of the bond next to the doped transition metal. When the valence d electrons of the doped metal are between 3 and 7, all three types of IELs appear in the band gap of the (M, N) co-doped systems. For systems doped with a metal of more than 7 valence electrons, only types (a) and (c) of IELs as well as the unoccupied pz state of N are observed. Based on our analysis, we propose that the co-doping systems such as (V, N), (Cr, N), and (Mn, N), which have the IELs with a significant bandwidth, are of great potential as candidates for photovoltaic applications in the visible light range.

Sintering-resistant Ni-based reforming catalysts obtained via the nanoconfinement effect

This communication describes the design and synthesis of anti-sintering and -coke nickel phyllosilicate (PS) nanotubes (Ni/PSn) for hydrogen production via reforming reactions. The introduction of nickel particles in PS nanotubes could effectively maintain the Ni size and increase the resistance of metal particles for carbon deposition.

Selective oxidation of methanol to dimethoxymethane over bifunctional VOx/TS-1 catalysts

This communication describes the design of bifunctional VO(x)/TS-1 catalysts with enhanced redox and acidic character via doping SO(4)(2-) and PO(4)(3-) for selective oxidation of methanol to dimethoxymethane. Redox sites enable the production of formaldehyde, while acidic sites favor the condensation of formaldehyde to DMM.