Synthesis and Catalytic Features of Hybrid Metal Nanoparticles Supported on Cellulose Nanofibers
The structural and functional design of metal nanoparticles has recently allowed remarkable progress in the development of high-performance catalysts. Gold nanoparticles (AuNPs) are among the most innovative catalysts, despite bulk Au metal being regarded as stable and inactive. The hybridization of metal NPs has attracted major interest in the field of advanced nanocatalysts, due to electro-mediated ligand effects. In practical terms, metal NPs need to be supported on a suitable matrix to
... able matrix to avoid any undesirable aggregation; many researchers have reported the potential of polymer-supported AuNPs. However, the use of conventional polymer matrices make it difficult to take full advantage of the inherent properties of the metal NPs, since most of active NPs are imbedded inside the polymer support. This results in poor accessibility for the reactants. Herein, we report the topochemical synthesis of Au and palladium (Pd) bimetallic NPs over the surfaces of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs), and their exceptional catalytic performance. Highly-dispersed AuPdNPs were successfully synthesized in situ on the crystal surfaces of TOCNs with a very high density of carboxylate groups. The AuPdNPs @ TOCN nanocomposites exhibit excellent catalytic OPEN ACCESS Catalysts 2011, 1 84 efficiencies in the aqueous reduction of 4-nitrophenol to 4-aminophenol, depending on the molar ratios of Au and Pd. Metal nanoparticles have a variety of attractive properties, which enable them to play significant roles in a broad range of electronic, optical, biochemical and catalytic applications     . Gold nanoparticles (AuNPs) have become a central topic of academic and industrial interest as a promising candidate for next-generation nanocatalysts, even though bulk Au is stable and chemically inert    . Furthermore, in recent decades bimetallic NPs have also attracted attention for their specific characteristics being much different from those of the monometallic individuals     . The electronic interactions between two metal NPs are known to accelerate catalytic reactions. These so-called ligand effects result in a higher process efficiency compared to the monometallic equivalent. It has been reported that bimetallic NPs, comprising Au and palladium (Pd) have a higher catalytic activity towards the oxidation of alcohols in an aqueous solution than the individual monometallic NPs  . Similarly, Au-Ag and Au-Ni bimetallic NPs exhibit higher catalytic activities towards CO oxidation reactions  and the hydrolysis of ammonia borane  , respectively, in comparison to the monometallic equivalents. Therefore, the hybridization of two different metal NPs is of great importance in the structural and electrochemical design of catalysts. However, metal NPs are generally unstable due to their large active surface areas, so preventing their self-aggregation, which causes a huge drop in a catalytic activity, is critical for practical use. Surface coating with surfactants and/or various chemical modifications of the metal NPs have been carried out to stabilize them and maintain the original size dispersions    . Another effective approach to inhibit aggregation is to immobilize the metal NPs on various matrices, such as metal oxides or polymers    . In fact, polymer-NPs nanocomposites are attracting attention for practical applications; their mechanical and catalytic properties have been extensively investigated. Polymer-type matrices with thiol, pyridyl, amine and carboxyl groups as anchor sites for the metal NPs are highly tunable for further improvement. However, polymer matrices have the following disadvantages for catalyst immobilization: (1) poorly regulated anchor sites; (2) low thermal stability; and (3) the embedding of active metal NPs inside the polymer layers    . Therefore, to achieve high catalytic performances, the structural and functional design of catalyst supports for metal NPs is required for the regulated immobilization of exposed active metal NPs on the outer surfaces of thermally-stable matrices. Furthermore, many polymer matrices are made from non-renewable petrochemicals, so alternative, environmentally-friendly polymer supports are required. Cellulose is the most abundant natural polymer, originating from plants, tunicates and bacteria    . Native cellulose consists of nanometer-sized fibrils, 3-20 nm in width, and has an extremely high crystallinity up to 65-95%, depending on their origin       . Since cellulose nanofibrils strongly bind to each other through hydrogen bonding, it is very difficult to obtain individual cellulose nanofibers.