Catalytic membranes prepared by adsorption of polyelectrolyte/metal nanoparticle films in porous supports

Well-defined metal nanoparticles are attractive for catalytic applications because of their high specific surface area and unique electronic properties. However, the high surface energy of these particles often leads to their aggregation so the nanoparticles must be capped with a stabilizer or immobilized on a support. This dissertation demonstrates metal nanoparticle immobilization in porous membranes via layer-by-layer (LbL) adsorption of polyelectrolyte/metal nanoparticle films and the potential advantages of these membranes for catalytic applications including reduction of nitro compounds, hydrogenation, and wet air oxidation for wastewater treatment. The immobilization method utilizes electrostatic interactions to facilitate deposition of oppositely charged polyelectrolytes and nanoparticles, and controlled synthesis of the nanoparticles prior to their deposition allows tuning of the nanoparticle size and composition for specific catalytic processes. Nanoparticles can also be formed after film deposition by incorporating metal ions into the polyelectrolyte film and later reducing these metal ions to form nanoparticles with diameters of 2--4 nm.;LbL deposition of polyelectrolyte/Au nanoparticle films in porous alumina and polymer substrates yields catalytic membranes with a high density of well-separated Au nanoparticles in the membrane pores. These nanoparticles catalyze the reduction of nitroaromatic compounds by sodium borohydride with rate constants that are the same as those for nanoparticles in solution. Moreover, the membranes selectively catalyze the reduction of nitro groups in compounds containing other reducible functionalities such as cyano, chloro, and styrenyl moieties. These membranes are particularly attractive for controlling product distributions through variation of solution fluxes.;Membrane catalysts also facilitate contact between reactants in gas/liquid reactions by operating as flow-through or interfacial contactors. LbL deposition of polyelectrolyte/Pd nanoparticle films in flat membranes yields flow-through contactors that effectively hydrogenate allyl alcohol to 1-propanol when flowing H2-saturated solutions through these membranes. Hydrogen reacts completely after one pass of the solution through the modified membranes, and thus the reaction is limited by the solubility of H2 in the reactant solution. The interfacial contactor configuration, in which a tubular membrane separates the gas and liquid phases, provides enhanced contact between the immobilized catalytic nanoparticles and the gas/liquid interface. Control over the catalyst location in such membranes is crucial for efficient use of noble metals. LbL adsorption of polyelectrolyte/Pt nanoparticle films in tubular ceramic membranes allows deposition of the catalytic nanoparticles only near the interior of tube, where the gas/liquid interface is typically located. In wet air oxidation of formic acid, acetic acid, and phenol, tubular membranes modified by LbL deposition showed 2 to 10 times higher specific activities than similar membranes modified by conventional impregnation techniques, presumably due to controlled deposition of the Pt nanoparticles in the membrane.