Due to the uniform and stable pore structure, mesoporous silica has attracted increasing research attention as a catalyst support material. As a large family of mesoporous silica-supported materials, noble-metal nanoparticles supported on mesoporous silica catalysts have demonstrated desirable properties across a broad platform of reactions. In this review article, we first introduce systems of metal nanoparticles dispersed on mesoporous silica, and then, we focus on next generation systems, in which the noble metal is not supported on the mesoporous silica but rather entrapped/intercalated within the silica matrix, thus enhancing particle stability and in some cases, enhanced activity. Herein, research and future directions on both synthesizing hybrid noble-metal nanoparticles/mesoporous silica composite catalysts and their resultant properties will be discussed.
Catalytic fast pyrolysis (CFP) is a conversion process that integrates rapid thermochemical depolymerization of solid feedstocks with catalytic transformation to yield small molecules for fuel and chemical products. This process is well-suited for the conversion of nonfossil feedstocks such as biomass and waste plastics, and thereby holds great potential for the production of renewable commodities. In spite of many technological developments in various aspects of CFP achieved over decades of research, this technology has yet to attain commercial success for the production of fuels and chemicals from renewable feedstocks. Effective CFP processes require careful coordination of chemical and physical phenomena that span very large length and time scales. A broad spectrum of scientific progress in both pyrolysis and catalytic upgrading has provided the foundation for successful deployment of CFP, although additional progress in process-scale integration is yet required for commercial realization. Modeling and simulation tools provide an important framework wherein the CFP technologies by be better understood and evaluated from a holistic perspective. Here we provide a detailed description of the multiscale phenomena underlying CFP, describe challenges and associated technical progress, and suggest strategies for an integrated approach to advance this technology toward commercialization.
Nanostructured noble-metal catalysts
traditionally suffer from
sintering under high operating temperatures, leading to durability
issues and process limitations. The encapsulation of nanostructured
catalysts to prevent loss of activity through thermal sintering, while
maintaining accessibility of active sites, remains a great challenge
in the catalysis community. Here, we report a robust and regenerable
palladium-based catalyst, wherein palladium particles are intercalated
into the three-dimensional framework of SBA-15-type mesoporous silica.
The encapsulated Pd active sites remain catalytically active as demonstrated
in high-temperature/pressure phenol hydrodeoxygenation reactions.
The confinement of Pd particles in the walls of SBA-15 prevents particle
sintering at high temperatures. Moreover, a partially deactivated
catalyst containing intercalated particles is regenerated almost completely
even after several reaction cycles. In contrast, Pd particles, which
are not encapsulated within the SBA-15 framework, sinter and do not
recover prior activity after a regeneration procedure.
The design and synthesis of shape-directed nanoscale noble metal particles have attracted much attention due to their enhanced catalytic properties and the opportunities to study fundamental aspects of nanoscale systems. As such, numerous methods have been developed to synthesize crystals with tunable shapes, sizes, and facets by adding foreign species that promote or restrict growth on specific sites. Many hypotheses regarding how and why certain species direct growth have been put forward, however there has been no consensus on a unifying mechanism of nanocrystal growth. Herein, we develop and demonstrate the capabilities of a mathematical growth model for predicting metal nanoparticle shapes by studying a well known procedure that employs AgNO3 to produce {111} faceted Pt nanocrystals. The insight gained about the role of auxiliary species is then utilized to predict the shape of Pd nanocrystals and to corroborate other shape-directing syntheses reported in literature. The fundamental understanding obtained herein by combining modeling with experimentation is a step toward computationally guided syntheses and, in principle, applicable to predictive design of the growth of crystalline solids at all length scales (nano to bulk).
Colloidal iron pyrite nanocrystals (or FeS 2 NC inks) are desirable as active materials in lithium ion batteries and photovoltaics and are particularly suitable for large-scale, roll-to-roll deposition or inkjet printing. However, to date, FeS 2 NC inks have only been synthesized using the hot-injection technique, which requires air-free conditions and may not be desirable at an industrial scale. Here, we report the synthesis of monodisperse, colloidal, spherical, and phase-pure FeS 2 NCs of 5.5 ± 0.3 nm in diameter via a scalable solvothermal method using iron diethyldithiocarbamate as the precursor, combined with a postdigestive ripening process. The phase purity and crystallinity are determined using X-ray diffraction, transmission electron microscopy, farinfrared spectroscopy, and Raman spectroscopy techniques. Through this study, a hypothesis has been verified that solvothermal syntheses can also produce FeS 2 NC inks by incorporating three experimental conditions: high solubility of the precursor, efficient mass transport, and sufficient stabilizing ligands. The addition of ligands and stirring decrease the NC size and led to a narrow size distribution. Moreover, using density functional theory calculations, we have identified an acid-mediated decomposition of the precursor as the initial and critical step in the synthesis of FeS 2 from iron diethyldithiocarbamate.
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