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NANO-CATALYSTS
The proposed research utilizes a bioengineering approach to develop nano-size photocatalytic materials. We, and others, have previously shown that protein-encapsulated transition metal oxide nanoparticles act as efficient photo-catalysts with visible and UV light (see Figure 10) 2-4. These materials are able to catalyze the reduction of environmental pollutants such as Cr(VI) to Cr(III) using visible light and simple electron donors such as organic acids (acetate, citrate, tartrate) 2. In addition, we have shown efficient photoreduction of Cu(II) to form Cu(0) nanoparticles, catalyzed by the Fe2O3 encapsulated nanoparticles 3. The catalytic nanoparticle thrust of the proposed MSU NSEC is aimed at investigating the synthesis, photo-reactivity, and marketability of protein cage-derived nanomaterial catalysts. Taking advantage of our protein cage constrained mineralization approach, we can control the size of the encapsulated inorganic nanoparticles to form homogeneous materials with selected particle sizes in the range of 2 to 25 nm in diameter. We are applying this approach to the synthesis of a variety of transition metal oxides such as Fe-, Mn-, Co-, and Ti-oxides, some of which are known to be active photocatalysts as bulk materials105-108.
Much of the excitement surrounding nanochemistry resides in the promise that materials fabricated at this size-scale may exhibit unique structural properties and chemical reactivity. “Nanocatalysts” have probably been important components of many heterogeneous catalyst systems but have only recently been identified as such 109. To date however, there are few clear examples where nanomaterials - for reasons other than increased surface-to-volume ratios - catalyze fundamentally different reactions than bulk materials: nanoclusters of gold have been shown to catalyze CO oxidation110-113 (which bulk gold does not); and nano-sized semiconductors such as TiO2 show an enhancement in their redox properties108.
The scientific merit of our effort utilizing the protein cage to encapsulate nanomaterials54 has a number of distinctive advantages over other nanoparticle catalysts. The desired material particles can be constrained in size by synthesis in any number of different cages from our library. This capability enables us to systematically probe the size-dependence of physical properties and chemical reactivity. The protein cage prevents corrosion and agglomeration of the nanoparticle catalysts. The cage provides controlled access to the nanoparticle from the bulk medium; pores can be opened, closed, and varied to control molecular access and contact with the nanoparticle. The protein cage can be easily derivatized either chemically55,56 or genetically6, allowing us to direct the reactivity through covalent modification. The cage-constrained nanoparticle can be coupled to other catalyst systems (enzymes, for example); this “handshake” allows the nanoparticle catalysts to drive the efficient and specific enzymatic catalysis.
NSEC Impact: Making use of the chemical selectivity offered by biological systems and the high quantum efficiency offered by high surface-to-volume particles, the NSEC will have demonstrated a fundamentally new pathway for catalytic research. The implications of this in areas of energy production and environmental remediation could have significant impacts on society at large. In particular, clean hydrogen production from renewable fuels produced in an efficient and low-cost manner is a substantial step forward in the hydrogen economy.
This multi-disciplinary research effort aims to develop a firm understanding of the photo-catalytic properties of metal oxide, metallic, and composite nanoparticles encapsulated within the library of protein cages at our disposal (Figure 3). We will additionally combine the protein-cage nanomaterials with electron transfer mediators (viologens and electroactive gels114) and highly selective biological catalysts (e.g. hydrogenase enzyme)115,116 to provide opportunities for the development of fundamentally new composite catalyst systems. Our attention will initially be focused towards applications in photocatalytic remediation of inorganic and organic pollutants and H2 generation.
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