Dry reforming of methane (DRM) is a promising chemical approach to convert greenhouse gases CO2 and CH4 into valuable fuels. Previous experimental study has shown that the addition of alkaline earth can promote the activity and stability of the Ni-based catalyst. However, the physical structure of alkaline earth additives on supports and their interaction with Ni particles should have significant influence for the catalytic performance of catalysts. To clarify the synthesis–structure–activity relationship for further improving these catalysts, the underlying reaction mechanism for DRM over size-confined Ni–CaO catalysts on neutral supports and the structure/effect of CaO as promoter were investigated combining density functional theory (DFT) calculation and experimental studies. The favored active sites for all elementary reactions were identified, and the activation energies of the reactions were calculated for the determination of the primary reaction pathways. DFT results found a cooperation effect between Ni and CaO, where the interface dissociates CO2, Ni activates CH4 dehydrogenation, and CaO attracts CO2. The interface between Ni and CaO was found to provide another channel to activate CO2 and decrease the energy barrier of CHO formation, contributing to the high efficiency and long-term stability of the catalyst. On the basis of the DFT results, the optimum stacking order between Ni and CaO was proposed, in good agreement with the experimental studies that synthesized and compared four catalysts with different Ni–CaO structures. The proposed Ni–CaO composite catalyst should be a promising catalyst for potential application in industrial dry reforming processes.

Using solid acid catalysts to replace liquid acids in the liquid-phase Beckmann rearrangement of cyclohexanone oxime (CHO) into -caprolactam (CPL) is crucial for the environmentally friendly production of synthetic fibers, such as Nylon-6. In this work, we prepared aluminum-containing MCM-41 catalysts under ultrasonic irradiation with various Si/Al ratios for this purpose. Quantitative 1 H MAS NMR investigations show that ultrasonic irradiation significantly promotes the formation of active Brønsted acid sites (BAS) on the [Al]MCM-41 catalysts up to 8 times higher than those prepared at the same conditions without ultrasonic irradiation, and up to 12 times higher BAS density than those reported in the literatures. The catalytic performance of [Al]MCM-41 catalysts can be strongly improved with increasing the BAS density, particularly to the ratio of BAS/(weakly acidic SiOH groups). Moreover, [Al]MCM-41 catalysts dehydrated at 393 K obtained two time higher CHO conversion and CPL yield than that dehydrated at 473 K. Hydrogen-bonded water molecules retained at low dehydration temperature may block surface SiOH groups and suppress the side reaction for cyclohexanone. With higher BAS density resulting from ultrasonic irradiation, [Al]MCM-41 catalyst (Si/Al = 10) in this work obtained the highest CPL yield among all [Al]MCM-41 materials reported for liquid-phase Beckmann rearrangement up to now. Finally, the reusability of [Al]MCM-41 catalyst was tested and no significant activity loss can be observed after five reaction cycles.

N-doped mesoporous carbon nanospheres (N-MCN@M) impregnated with uniformly dispersed noble-metal (Au, Pt, Rh, Ru, Ag, Pd and Ir) nanoparticles are rationally designed and synthesized for hydrogenation reactions. This facile and generally applicable synthetic strategy ensured confinement of the noble-metal nanoparticles within different carbon morphologies, including mesoporous spheres, hollow particles and core-shell particles. High loading of the noble-metal nanoparticles from 8 to 44% was accomplished by tuning the initial concentration of metal salts. Even at very high loadings (440 wt%), a homogeneous dispersion of uniform metal nanoparticles throughout the carbon nanostructures was achieved. The proposed synthesis is also well suited for the fabrication of carbon spheres loaded with bimetallic nanoparticles (AuPt, AuRh and PtRh). Examination of these metal-loaded carbon particles as catalysts for the hydrogenation of benzaldehyde gave 100% selectivity toward carbonyl group at room and higher reaction temperatures. The outstanding performance of Au nanoparticles gave an unprecedented turn over frequency 2-4 times greater than those of Pt nanoparticles with the same size, loading and support.

Tuning the chemical composition during the synthesis is a widely used method to control the activity of catalysts. Here, we reported an alternative synthesis strategy to tune the catalytic properties of nanocatalysts without changing their precursors and compositions. We synthesized a series of Pd catalysts on the most popular SiO2-, Al2O3-, and silica-alumina supports using the double flame-spray pyrolysis (FSP) technique. It was observed that various flow rates used for the synthesis of catalysts with the same composition affected the formation of the catalyst particles and their structures to further tune the surface acidity due to the correlation between acidity and structure, but did not influence the electronic properties of Pd particles. It was observed that surface OH groups could associate Pd for the hydrogenation, but Lewis acid sites could not, as Pd/SA-30 and Pd/SiO2 showed much higher activity than Pd/Al2O3 for the same Pd size and surface properties. For Pd catalysts with Brønsted acid sites (silica-alumina) or weak/non-acidic SiOH groups (SiO2), their catalytic performance for the chemoselective hydrogenation of acetophenone was obviously enhanced by tuning the surface OH groups via changing the flow rates for the same precursor solution during this ultra-fast synthesis.

Hydrogen production from the thermochemical conversion of biomass was carried out with nano-sized NiZnAlOx catalysts using a two-stage fixed bed reactor system. The gases derived from the pyrolysis of wood sawdust in the first stage were catalytically steam reformed in the second stage. The NiZnAlOx catalysts were synthesized by a co-precipitation method with different Ni molar fractions (5, 10, 15, 25 and 35%) and a constant Zn:Al molar ratio of 1:4. The catalysts were characterized by a wide range of techniques, including N2 adsorption, SEM, XRD, TEM and temperature-programmed oxidation (TPO) and reduction (TPR). Fine metal particles of size around 10-11 nm were obtained and the catalysts had high stability characteristics, which improved the dispersion of active centers during the reaction and promoted the performance of the catalysts. The yield of gas was increased from 49.3 to 74.8 wt.%, and the volumetric concentration of hydrogen was increased from 34.7 to 48.1 vol.%, when the amount of Ni loading was increased from 5 to 35%. Meanwhile, the CH4 fraction decreased from 10.2 to 0.2 vol.% and the C2-C4 fraction was reduced from 2.4 vol.% to 0.0 vol.%. During the reaction, the crystal size of all catalysts was successfully maintained at around 10-11 nm with lowered catalyst coke formation, (particularly for the 35NiZn4Al catalyst where negligible coke was found) and additionally no obvious catalyst sintering was detected. The efficient production of hydrogen from the thermochemical conversion of renewable biomass indicates that it is a promising sustainable route to generate hydrogen from biomass using the NiZnAl metal oxide catalyst prepared in this work via a two-stage reaction system.

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ABSTRACT:The production of renewable hydrogen enriched gas from biomass waste is a promising technology for the development of a sustainable economy and society. Until now, there are still challenges of the technology in terms of the efficiency of hydrogen production. Catalyst is known and has been tested to enhance hydrogen production from biomass gasification. In particular using Ni-based catalysts, which have high reactivity for hydrogen production and are cost effective. However, developing a Ni-based catalyst with high thermal stability and resistance of coke deposition on the surface of the catalyst is still a challenging topic. In this work, Ni-Al catalysts doped with low-cost Fe metal were investigated for hydrogen enriched syngas production from gasification of biomass using a two-stage fixed bed reactor. NiOFe2O3-Al2O3 catalysts with various Ni:Fe molar ratios (9:1, 8:2, 6:4, 5:5, 4:6, 2:8 and 1:9) were studied aiming to understand the influence of Fe addition on the production of hydrogen and the catalyst stability in terms of coke deposition on surface. X-ray diffraction, temperature programme reduction and Transmission electron microscopy analysis of the fresh catalysts showed that nanoparticles (mainly NiAl2O4 spinel phase and Al2O3, ~5 nm) were identified in the catalysts. High dispersion of metal particles was obtained using a co-precipitation method of catalyst preparation. With the increase of Fe addition, hydrogen production was reduced from around 11 to 8 (mmol H2 g -1 biomass). However, the addition of Fe into the Ni-based catalyst significantly reduced the amount of coke deposited on the surface of the catalyst.H2/CO molar ratio was maximized to 1.28 when Ni:Fe molar ratio was 1:1. In addition, sintering of metal particles was not observed through the TEM analysis of the fresh and reacted catalysts.2