Atomic/molecular layer deposition prepared size-selective catalysts and supported nickel catalysts

Heterogeneous catalysts are widely used because of their many advantages. In this dissertation, the application of atomic/molecular layer deposition (ALD/MLD) in heterogeneous catalyst synthesis and modification was examined. A novel nanostructured size-selective catalyst was synthesized by depositing ultra-thin porous shells on the surface of catalysts. The ultra-thin porous shells were formed by oxidation of aluminum alkoxide films deposited by MLD. The catalytic activity of the size-selective catalyst was improved by introducing gaps between the porous shell and catalytic metal nanoparticles. The introduction of gaps greatly reduced the catalyst activity loss, which resulted from the contact areas between the active sites and porous shells. Ni nanoparticles supported by silica gel particles were prepared by ALD, and the catalyst showed both high activity and selectivity in catalyzing chemoselective transfer reduction of different nitroarenes to produce corresponding aromatic amines. A highly stable and active Ni/γ-Al₂O₃ catalyst was synthesized by depositing Ni nanoparticles on porous γ-Al₂O₃ particles by ALD. The catalyst showed exceptionally high catalytic activity and excellent stability for dry reforming of methane (DRM) reaction. A 4-channel α-Al₂O₃ hollow fiber was also studied as a support for Ni nanoparticles. The Ni/α-Al₂O₃ hollow fiber catalyst showed excellent performance in catalyzing the DRM reaction. The performance of the catalyst was further improved by alumina ALD overcoating on Ni nanoparticle surface to increase Ni-support interaction --Abstract, page iv

Theory of correlated insulating behaviour and spin-triplet superconductivity in twisted double bilayer graphene

Two monolayers of graphene twisted by a small `magic' angle exhibit nearly flat bands leading to correlated electronic states and superconductivity, whose precise nature including possible broken symmetries, remain under debate. Here we theoretically study a related but different system with reduced symmetry - twisted {\em double} bilayer graphene (TDBLG), consisting of {\em two} Bernal stacked bilayer graphene sheets, twisted with respect to one another. Unlike the monolayer case, we show that isolated flat bands only appear on application of a vertical displacement field $D$. We construct a phase diagram as a function of twist angle and $D$, incorporating interactions via a Hartree-Fock approximation. At half filling, ferromagnetic insulators are stabilized, typically with valley Chern number $C_v=2$. Ferromagnetic fluctuations in the metallic state are argued to lead to spin triplet superconductivity from pairing between electrons in opposite valleys. Response of these states to a magnetic field applied either perpendicular or parallel to the graphene sheets is obtained, and found to compare favorably with a recent experiment. We highlight a novel orbital effect arising from in-plane fields that can exceed the Zeeman effect and plays an important role in interpreting experiments.Comment: main 15 pages, appendix 11 page

“Core–Shell” Nanostructured Supported Size-Selective Catalysts with High Catalytic Activity

We report the synthesis of a highly active, supported nanostructured metal nanoparticle catalyst with an ultrathin porous shell and gaps between the metal nanoparticles and the shell for size-selective reactions. The size-selectivity of the catalysts could be realized through the porous shell. The gaps were able to reduce catalytic activity loss due to the contact areas between the shell and the catalytic sites. Evaluations of the activity and selectivity of the catalysts were made by catalytic hydrogenation of n-hexene versus cis-cyclooctene. Further verification of the high catalytic activity of the nanostructured catalysts was by oxidation of carbon monoxide

Highly Active and Stable Alumina Supported Nickel Nanoparticle Catalysts for Dry Reforming of Methane

A highly stable and extremely active nickel (Ni) nanoparticle catalyst, supported on porous γ-Al2O3 particles, was prepared by atomic layer deposition (ALD). The catalyst was employed to catalyze the reaction of dry reforming of methane (DRM). The catalyst initially gave a low conversion at 850°C, but the conversion increased with an increase in reaction time, and stabilized at 93% (1730 L h-1 g Ni-1 at 850°C). After regeneration, the catalyst showed a very high methane reforming rate (1840 h-1 g Ni-1 at 850°C). The activated catalyst showed exceptionally high catalytic activity and excellent stability of DRM reaction in over 300 h at temperatures that ranged from 700°C to 850°C. The excellent stability of the catalyst resulted from the formation of NiAl2O4 spinel. The high catalytic activity was due to the high dispersion of Ni nanoparticles deposited by ALD and the reduction of NiAl2O4 spinel to Ni during the DRM reaction at 850°C. It was verified that NiAl2O4 can be reduced to Ni in a reductive gas mixture (i.e., carbon monoxide and hydrogen) during the reaction at 850°C, but not by H2 alone

Nano‐engineered nickel catalysts supported on 4‐channel α‐Al 2

A nickel (Ni) nanoparticle catalyst, supported on 4-channel α-Al2O3 hollow fibers, was synthesized by atomic layer deposition (ALD). Highly dispersed Ni nanoparticles were successfully deposited on the outside surfaces and the inside porous structures of hollow fibers. The catalyst was employed to catalyze the dry reforming of methane (DRM) reaction and showed a methane reforming rate of 2040 Lh-1gNi-1 at 800°C. NiAl2O4 spinel was formed when Ni nanoparticles were deposited on alpha-alumina substrates by ALD, which enhanced the Ni-support interaction. Different cycles (two, five, and ten) of Al2O3 ALD films were applied on the Ni/hollow fiber catalysts to further improve the interaction between the Ni nanoparticles and the hollow fiber support. Both the catalyst activity and stability were improved with the deposition of Al2O3 ALD films. Among the Al2O3 ALD coated catalysts, the catalyst with five cycles of Al2O3 ALD showed the best performance

Effects of wind farm on surface soil in desert steppe, China

In recent years, the wind farm has developed rapidly in northern China. However, the construction and operation of wind farm directly or indirectly result in the change of soil structure, thus affecting the regional ecological environment. According to the characteristics of wind turbine engineering, the disturbance area of wind farm can be divided into the core disturbed zone, the secondary disturbed zone and the indirect disturbed zone. A field study was conducted to investigate the physical and chemical properties of surface soil in different disturbed zones. The results show that: (1) soil compaction, soil bulk density and soil salinity of the surface soil are sensitive to wind farm disturbance; compared with the undisturbed zone, there are still significant differences between the core disturbed zone and the secondary disturbed zone; (2) after nearly 10 years of natural recovery, apart from the phosphorus element, there are little differences of surface soil nutrients in different disturbed zones; (3) the impact of wind farm on surface soil will be gradually eliminated over time, but continuous and frequent disturbances caused by engineering maintenance during its operation should be avoided if possible

Nano-Engineered Nickel Catalysts Supported on 4-channel α-Al₂O₃ Hollow Fibers for Dry Reforming of Methane

A nickel (Ni) nanoparticle catalyst, supported on 4-channel α-Al2O3 hollow fibers, was synthesized by atomic layer deposition (ALD). Highly dispersed Ni nanoparticles were successfully deposited on the outside surfaces and the inside porous structures of hollow fibers. The catalyst was employed to catalyze the dry reforming of methane (DRM) reaction and showed a methane reforming rate of 2040 Lh-1gNi-1 at 800°C. NiAl2O4 spinel was formed when Ni nanoparticles were deposited on alpha-alumina substrates by ALD, which enhanced the Ni-support interaction. Different cycles (two, five, and ten) of Al2O3 ALD films were applied on the Ni/hollow fiber catalysts to further improve the interaction between the Ni nanoparticles and the hollow fiber support. Both the catalyst activity and stability were improved with the deposition of Al2O3 ALD films. Among the Al2O3 ALD coated catalysts, the catalyst with five cycles of Al2O3 ALD showed the best performance

Chemoselective Transfer Hydrogenation of Nitroarenes Catalyzed by Highly Dispersed, Supported Nickel Nanoparticles

A recyclable highly dispersed Ni/SiO₂ catalyst was prepared by atomic layer deposition. Chemoselective reduction of nitroarenes was studied using the prepared Ni/SiO₂ as the catalyst and hydrazine hydrate as a hydrogen donor. Different kinds of nitroarenes were converted to the corresponding anilines with high yields. The high activity of the catalysts could be a result of the highly dispersed Ni nanoparticles

Steam Reforming of N-Dodecane over Mesoporous Alumina Supported Nickel Catalysts: Effects of Metal-Support Interaction on Nickel Catalysts

Developing a highly active and stable Ni-based catalyst is still a challenge for the generation of on-site hydrogen through steam reforming of long-chained hydrocarbons, such as kerosene fuels. Ni nanoparticles (ca. 5 nm) on mesoporous alumina prepared by atomic layer deposition (ALD) were employed in steam reforming of n-dodecane, and exhibited a turnover frequency (TOF) of 477.6 h-1, whereas Ni nanoparticles on commercial alumina support prepared by impregnation method exhibited a TOF of 100 h-1. The high activity of ALD Ni catalysts was ascribed to high reduction degree, as confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and H2-chemisorption. A deactivation was also observed on the ALD prepared catalysts, which was ascribed to the weak metal-support interaction, as confirmed by H2 temperature-programmed reduction (TPR). The ALD Ni/Al2O3 catalysts were further modified with CeO2 and they showed enhanced stability with 8% deactivation degree in steam reforming of n-dodecane. Further characterizations of spent catalysts showed that the presence of CeO2 was favorable for stabilizing Ni nanoparticles by enhancing moderate metal-support interaction, and reducing the formation of coke on the interfaces of Ni-CeO2