Heterogeneous Nano-particle Catalysis and Novel Photo-Catalyst Structures

Nanoscale heterogeneous catalysts can provide good catalytic properties, excellent selectivity and good recoverability and recyclability. Thus, recent research has investigated nanomaterials either on their own or as substrates for homogeneous catalysts, to produce a broad range of innovative heterogeneous catalysts [peb9, peb10]. The performance of these systems is heavily dependent on size, morphology and composition of the system components, opening them up to rational design. In Fig. peb6, we show a Pd nanoparticle-based catalytic system, embedded in organic-functionalized nano-porous silica for hydrogen production formic acid in this example. We use a high temperature reduction of Pd(II) precursors in the presence of hydrogen to generate ultra-small-nanoparticles, linked to an amine-grafted silica surface. The superior reactivity/selectivity of the catalysts can be attributed to the ultra-small size of the nanoparticles as well as to the basic amine sites, which help the deprotonation process of formic acid. 

The hunt for alternatives to noble-metal-based catalysts has led to mesoporous carbon, derived from easily available sources, and a promising candidate as an electro-catalyst for various renewable energy applications (e.g., fuel cells and hydrogen production from water—a sustainable hydrogen source).  Metal-free catalysts built from nitrogen containing carbon nanotubes have been found to electro-catalyze the production of hydrogen from water (hydrogen evolution reaction, HER) under a wide range of conditions, with activities comparable to that of noble metals such as Pt  [peb10].

Plasmonic photo-catalysis, in which visible light is harvested by surface plasmon resonance (SPR) active metal nanoparticles to drive chemical reactions via selective bond activation (Fig. peb7), is an emerging sub-discipline in catalysis with many proven potential applications [peb12]. Furthermore, as the proposed catalytic reactions involve photo-chemical activation, including that of sunlight, they are expected to be less energy intensive than current catalytic methods. This work incorporates emerging areas of nanoscience and nanotechnology with self-assembly to create new classes of core-shell plasmonic photocatalysts containing: 1) metallic NPs such as Au and Ag NPs, 2) metal oxides such as TiO2, ZrO2, and ZnO, and 3) nanostructured carbons such as graphene, graphene oxide, and carbon nanoneedles.

These methods have contributed to a new synthesis method to make highly porous TiO2 microsphere material with high surface area (400-500 m2/g) (Fig. peb8). Furthermore, these materials can be doped with Ag NPs using a simple one-step redox or galvanic reaction. More importantly, the resulting materials were shown to strongly absorb visible light (Fig. peb8C) thanks to the Ag NPs, since pure TiO2 cannot absorb visible light.