Conventional supported metal catalysts lose high performance over time. A key reason for catalyst deactivation is the sintering of small particles that occurs at high temperatures. While relatively easy to make, metallic nanoparticles do not maintain stability under harsh conditions, such as those employed in biomass processing (high temperature and aqueous conditions).
As researchers identify promising reaction mechanisms and catalytic surfaces, they can synthesize potential catalysts for testing. To do that, researchers need atomically precise materials that are stable under biomass processing conditions, specifically, high temperature liquid water. Nature provides the perfect tools for this — enzymes — which yield unrivaled selectivity through atomically precise structure and composition. The challenge for IACT scientists is to achieve enzyme-like specificity in the laboratory using robust materials; this requires a close coupling of synthesis, characterization, theory, and evaluation.
IACT researchers are focusing on nanobowls as ideal media for demonstrating the effectiveness of potential catalysts. Nanobowls offer isolated catalytic sites, inhibited catalyst sintering, size and/or shape selectivity towards reactants, precise spacing between catalyst and co-catalyst, and selective access to substrate surfaces, all of which are crucial for evaluating performance in the proverbial vacuum.
Using atomic layer deposition (ALD), researchers can create highly specific nanobowls, controlling composition and relative position of multiple catalytic species; depth, width, and shape of the bowl cavity; and access to support and substrate. ALD allows researchers to apply many materials in atomic layer-by-layer coatings to all exposed surfaces of a substrate, permitting complete control over the substrates, support layers, and metals they are evaluating.
Fructose molecules adsorbed on a zirconium oxide surface (right) and in a zirconium oxide nanobowl (left). The nanobowl structure was calculated using density functional theory.
IACT's overarching goals for the Materials Synthesis effort are to:
- Synthesize "nanobowl" metal/metal oxide catalysts using templates or blocking agents with atomic layer deposition (ALD);
- Develop stable catalytic support materials by hydrothermal synthesis;
- Create nano-bowls with multiple catalytic sites to promote bifunctional catalysis.
To date, the IACT Materials Synthesis team has:
- Synthesized nanobowls of Al2O3/TiO2, Pd/Al2O3 and Ir/Nb2O5 with exceptional thermal stability;
- Developed well-defined nanosized catalyst supports of t-ZrO2 and SrTiO3 with superior hydrothermal stability and high water-gas shift reactivity.
Going forward, the IACT Materials Synthesis team will investigate the effects of blocking agent deposition and removal on catalytic activity, conduct in-situ probes to gain an understanding of surface chemistry of nanobowl synthesis, and establish stable supports for aqueous environments. These steps will pave the way for synthesizing nanocavity structures with three or more components in order to form proximate acid/metal catalytic functions, and for synthesizing "super nanobowls" that can support multiple pores, multiple supports, multiple catalytic functions, and multiple reactions.