Graduate Student, Missouri University of Science and Technology
Atomic/molecular layer deposition were employed to prepare size-selective catalysts and Ni catalysts
Atomic/molecular layer deposition (ALD/MLD) are gas-phase thin film coating technique. The thickness of the ALD/MLD deposited films can be well controlled down to subnanometer. ALD can also be employed to deposit small metal nanoparticles (<4nm) on different substrates and these metal nanoparticles showed good activity in catalyzing different reactions.
We deposited Ni nanoparticles on different substrates and employed the catalysts for different phases of reactions, such as liquid phase transfer hydrogenation of nitroarenes and gas phase dry reforming of methane. The product of hydrogenation of nitroarenes are aromatic amines which are important raw materials in dyes, pharmaceuticals, and agricultural products. In dry reforming of methane reaction, two main greenhouse gases (carbon dioxide and methane) can be converted to syngas (hydrogen and carbon monoxide). The syngas can be employed to synthesize more valuable chemicals. The synthesized nickel catalysts showed excellent performance in these reactions.
Supported metal nanoparticles catalysts are widely used due to many advantages, including the ease of separating the catalysts by filtration after reaction. For supported metal nanoparticles catalyst, it is typically hard to convert certain molecules in a reactants mixture to only desired products. Size-selective catalysis is an important concept to improve the selectivity of the catalysts. In most previous studies, unsupported and nonporous substrates supported metal nanoparticles were encapsulated in relatively thick porous structures to realize the size-selectivity of heterogeneous catalysts by the size discrimination effect of the porous shell. However, the catalytic activity of the catalysts greatly decreased due to mass transfer limitations brought by relatively thick shells and the contact areas between the porous shells and active sites. We deposited an ultra-thin porous oxide shell on porous substrate-supported Pt nanoparticle catalysts, using molecular layer deposition (MLD), followed by oxidation to remove the organic components in hybrid organic/inorganic MLD films. The encapsulated catalyst showed very high selectivity in catalyzing hydrogenation reactions of n-hexene over cis-cyclooctene. We also reduced the contact areas between the Pt nanoparticles and porous shells by introducing a sacrificial layer, which can be removed later, on Pt nanoparticles before the deposition of porous shells.
Abstract: We report the synthesis of a highly active, supported nanostructured metal nanoparticle catalyst with an ultra-thin 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.
Pub.: 13 Dec '16, Pinned: 27 Jun '17