60th Annual Report on Research 2015

The ability to control structure and composition of materials with nanoscale precision has introduced many opportunities to create catalysts by design. Access to nanomaterials with precisely controlled size, shape and surfaces with parallel advances in characterization tools have provided key insights into fundamental aspects of heterogeneous catalysis. The inherently large specific surface area of nanoparticles (NPs) can be very advantageous for catalytic activity, but at the same time presents a challenge due to the tendency for the structures to sinter and deactivate during extended exposure to generally harsh environments encountered in the catalytic reactor. Maintaining catalyst structures with high exposure of catalytically active surfaces remains a key challenge in heterogeneous catalysis. Recent discoveries in the directed assembly of colloidal nanomaterials into ordered superstructures present great promise in addressing this challenge. The initial objective of this project was to establish the scientific foundation for the design, processing, and catalytic performance of binary NP catalysts. In context of NP catalysis our goal was to combine the directed assembly and tunable processing conditions to create binary NP catalysts with secondary structures optimized for the high density of active sites in a stable framework with optimized synergistic interactions. Fig. 1 provides an overview of the integration of specific research activities in three main tasks: (1) processing binary NP catalyst assemblies via non-equilibrium methods, (2) characterizing the NP structures using synchrotron-based X-ray scattering, X-ray absorption spectroscopy, and high-resolution electron microscopy, and (3) probing the catalytic performance of NP assemblies to understand basic processing-structure-performance relationships. Forming high-quality nanoparticle superlattices is a critical prerequisite towards establishing a robust relationship between the structure and their catalytic performance; towards that goal, we have refined the self-assembly of binary nanoparticles at fluid interfaces (see Fig.2). Our first goal towards creating catalytic materials from the binary nanoparticle assemblies was to explore non-equilibrium processing methods to decouple the dynamics of NP melting and recrystallization at the sub-nanometer scale with NP coalescence. The hypothesis driving this work is graphically summarized in Fig.3. Specifically, we sought to identify processing conditions that will allows us to selectively fuse a specific sublattice based on the size and composition dependent melting points. As a first model system, we formed assemblies of Au and Fe2O3 NPs. We found that even relatively mild thermal annealing conditions (400C) resulted in significant agglomeration of the Au catalyst NPs. We found that the Au NP agglomeration is articularly pronounced in regions of grain boundaries of the superlattice (Fig. 4). High-resolution transmission electron micrographs revealed that the agglomerated structures are crystalline gold. (Fig.5). These results support the interpretation that the secondry structure of the binary NP superlattice plays a critical role in stabilizing the catalytic particles. Despite initial success in forming high-quality binary NP superlattices (Fig. 2), our annealing experiments revealed significant sample-to-sample variation which precluded us from establish a robust relationship between the non-equilibrium processing conditions (laser pulse duration and fluence) and the evolution of the primary and secondary structure of the binary NP superlattice catalyst. In response to this challenge, we revised the main hypothesis of the project and turned to inspirations from nature for creating hierarchically structured catalyst. The biomimetic approach to create nanostructured catalysts for photochemical reduction of CO2 using perovskites metal oxide with metal co-catalyst and our initial results in this new direction are described below. Inspired by the complex, yet beautiful, hierarchical structure of natural photosynthetic systems we sought to create biomimetic catalytic structures for photochemical CO2 reduction based on materials that nature has not yet had the opportunity to work with. A recent report by Zhou et al. ("Leaf-Architectured 3D Hierarchical Artificial Photosynthetic System of Perovskite Titanates Towards CO2 Photoreduction Into Hydrocarbon Fuels" Scientific Reports 3, (2013): doi:10.1038/srep01667) illustrated the approach to use natural leaf as a template for photocatalytic systems with a hierarchical structure optimized for the coupled sub-processes of light-absorption, gas transport, liquid transport, and catalytic reaction. Notably, the structure of a natural leaf has evolved in response to a complex interplay of mechanical and chemical constraints. It is not clear whether the complex hierarchical vasculature present in natural leaves is necessary for optimized photochemical performance. To resolve this question, our first step was to reproduce the work by Zhou et al. and employ a biotemplated approach to form strontium titanate catalyst support structures. Initial results in this new direction were successful and we were able to create strontium titante structures from a variety of leaves as biotemplates (Fig. 6). We used bamboo leaf as a template, and decomposed by calcination at 600oC. Figure 6 shows the morphology of the artificial leaf with porous and channels network of the natural leaf maintained even after calcination. XRD analysis confirmed the composition of the artificial leaf. The photochemical reduction of CO2 with Au NP co-catalysts on the artificial leaf was carried out in the system built for measuring catalytic activity. The gas product was sampled and analyzed in a GC-MS. Detailed analysis of the catalytic performance is ongoing and will be discussed in a future report. Although the biomimetic approach is elegant, we believe that the complex hierarchical vasculature may not be necessary for optimized light-coupling and photochemical performance. To probe this conjecture we have started to create 3D printed leaf structures from strontium titanate inks. Comparison of the catalytic performance of the bio-templated and 3D printed structures will be discussed in a future report. In summary, with the support of the ACS new directions grant, our group has been able to explore exiting new opportunities at the confluence of nanoscience and additive nanofabrication to create bio-inspired novel catalytic structures.