The activity of zeolite-supported
nanocatalysts is dependent on
both the dispersion, size, and location of metal nanoparticles around
the zeolite and the size and pore structure of the zeolite. In this
study, a synthetic approach was developed to encapsulate metal catalysts
within hollow interiors of single-crystal ZSM-5. Briefly, Stöber
silica spheres were synthesized and then transformed to single-crystal
nano-ZSM-5 (Si/Al = 60), followed by growth of embedded metal nanoparticles
and subsequently creation of a nanosized (30–50 nm shell thickness)
hollow hierarchical zeolite structure. Metal nanoparticles such as
Co, Cu, Cu–Zn, Fe, and Ni can be supported on the inner wall
of the hollow zeolite and the surrounding satellite mesopores, without
any particles present on the external zeolite surface. When evaluated
as a catalyst for the Fischer–Trøpsch reaction, the Fe@h-ZSM5
catalyst shows high activity, sintering and coking resistance (50%
longer stability than Fe@ZSM5), and secondary cracking reactions in
the acid sites in the ZSM-5 shell, which reduce C5+ hydrocarbon
selectivity and increase smaller-chain hydrocarbon selectivity. In
addition, when Pt was further deposited inside the hollow structure,
shape-selective alkene hydrogenation was demonstrated. These configured
nanoscale zeolite catalysts have potential applications for reactions
that involve supported metal nanoparticle catalysis, shape selectivity,
or secondary cracking reactions.
Molybdenum
disulfide (MoS2) is a two-dimensional transition-metal
dichalcogenide that can form layered nanosheets with catalytically
active sites present at edge or defect sites. The density of such
active sites can be further tuned by modifying the length, layer number,
strain, and surface defects of the sheets. Herein, a synthetic approach
has been developed to encapsulate nanoscale MoS2 nanosheets
inside a mesoporous silica shell. Small molybdenum(IV) oxide (MoO2) cores were synthesized and coated with a mesoporous silica
phase, followed by a conversion to MoS2@SiO2. The space constraint on the inner cores resulted in short, few-layered,
highly curved MoS2 nanosheets with circular or flowerlike
morphology. The MoS2@SiO2 was evaluated as a
catalyst for decomposition of hydrogen sulfide (H2S), which
shows high catalytic turnover frequency and superior thermal stability
in comparison to unconstrained MoS2 catalysts.
Reaction rates on photocatalytic surfaces would often benefit greatly if minority photocarriers could be driven more efficiently to the surface through the manipulation of electric fields within the semiconductor. Such field-induced manipulation of photocurrent is commonplace in conventional optoelectronics, but translation to photochemistry and photoelectrochemistry has lagged. The present work demonstrates quantitatively that manipulation of the spatial extent of band bending via background carrier concentration can increase photoreaction rates by a factor of five or more in the case of methylene blue photodegradation over thin-film polycrystalline anatase TiO2. A quantitative photocurrent model fits closely to experimental rate data with no adjustable parameters.
INTRODUCTIONThin-film semiconducting photocatalysts currently find applications in self-cleaning windows and other weather-exposed surfaces, 1, and in anti-microbial 3 and anti-fouling coatings. 4 Reaction rates on these surfaces would often benefit greatly if minority photocarriers could be driven more efficiently to the
The utility of thin-film TiO 2 for photocatalysis would be greatly improved if the spatial variation of the electronic band edges near the surface could be engineered a priori to control the current of photogenerated minority carriers. The present work demonstrates such a concept. In particular, remote oxygen plasma treatment of polycrystalline anatase TiO 2 with specified majority carrier concentration is employed in the test case of methylene blue photodegradation. The photoreaction rate varies by up to 35% in concert with a 0.4 eV change in built-in surface potential measured by photoelectron spectroscopy. The correlation between these changes agrees quantitatively with a photodiode−photocurrent model. The plasma treatment affects concentration of charged native defects within the first few atomic layers of the surface, most likely by lowering the concentration of oxygen vacancies within surface crystallites. In tandem, the position in the deep bulk is controlled via engineering the defect concentration at grain boundaries, thus illustrating the coordinated use of multiple defect engineering practices in polycrystalline material to accomplish quantitative manipulation of band bending and corresponding photocurrent.
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