Nanocrystal tandem catalysis a promising approach towards designing high-performance, multifunctional nanostructured catalysts; energy applications
12 April 2011
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| The tandem catalyst. (Image courtesy of Yang group) Click to enlarge. |
Researchers with the US Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have created bi-layered nanocrystals of metal-metal oxide that are the first to feature multiple catalytic sites on nanocrystal interfaces. A paper describing the research is published in the journal Nature Chemistry.
In the novel bi-layer nanocatalyst system, single layers of metal and metal oxide nanocubes are deposited to create two distinct metal–metal oxide interfaces—multiple catalytic sites that allow for multiple, sequential catalytic reactions to be carried out selectively and in tandem. The development holds intriguing possibilities for the future of industrial catalysis, as well as for promising energy technologies as artificial photosynthesis, according to the researchers.
High-performance metal-oxide nanocatalysts are central to the development of new-generation energy conversion and storage technologies. However, to significantly improve our capability of designing better catalysts, new concepts for the rational design and assembly of metal–metal oxide interfaces are needed.
The demonstration of rationally designed and assembled nanocrystal bi-layers with multiple built-in metal–metal oxide interfaces for tandem catalysis represents a powerful new approach towards designing high-performance, multifunctional nanostructured catalysts for multiple-step chemical reactions.
—Peidong Yang, corresponding author
Studies in recent years have shown that for nanocrystals, the size and shape—specifically surface faceting with well-defined atomic arrangements—can have an enormous impact on catalytic properties. This makes it easier to optimize nanocrystal catalysts for activity and selectivity than bulk-sized catalysts. Shape- and size-controlled metal oxide nanocrystal catalysts have shown particular promise.
It is well-known that catalysis can be modulated by using different metal oxide supports, or metal oxide supports with different crystal surfaces. Precise selection and control of metal-metal oxide interfaces in nanocrystals should therefore yield better activity and selectivity for a desired reaction.
—Peidong Yang
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| Transmission electron micrograph showing monolayer of a cerium oxide nanocube monolayer on a platinum monolayer in a new bi-layer nanocatalyst. (Image courtesy of Yang group) Click to enlarge. |
To determine whether the integration of two types of metal oxide interfaces on the surface of a single active metal nanocrystal could yield a novel tandem catalyst for multistep reactions, Yang and his coauthors used the Langmuir-Blodgett assembly technique to deposit nanocube monolayers of platinum and cerium oxide on a silica (silicon dioxide) substrate. The nanocube layers were each less than 10 nanometers thick and stacked one on top of the other to create two distinct metal–metal oxide interfaces: platinum-silica and cerium oxide-platinum.
The two distinct metal–metal oxide interfaces, CeO2–Pt and Pt–SiO2, can catalyze two distinct sequential reactions. First, the cerium oxide-platinum interface catalyzed methanol decomposition to produce carbon monoxide and hydrogen. These products then underwent ethylene hydroformylation catalyzed by the nearby platinum-silica interface. The final result of this tandem catalysis was propanal.
| “Integrating binary nanocrystals to form highly ordered superlattices is a new and highly effective way to form multiple interfaces with new functionalities.” |
| —Peidong Yang |
The cubic shape of the nanocrystal layers is ideal for assembling metal–metal oxide interfaces with large contact areas, according to Yang. Yang says that the concept of tandem catalysis through multiple interface design that he and his co-authors have developed should be especially valuable for applications in which multiple sequential reactions are required to produce chemicals in a highly active and selective manner.
A prime example is artificial photosynthesis, the effort to capture energy from the sun and transform it into electricity or chemical fuels. To this end, Yang leads the Berkeley component of the Joint Center for Artificial Photosynthesis, a new Energy Innovation Hub created by the US Department of Energy that partners Berkeley Lab with the California Institute of Technology (Caltech).
Artificial photosynthesis typically involves multiple chemical reactions in a sequential manner, including, for example, water reduction and oxidation, and carbon dioxide reduction. Our tandem catalysis approach should also be relevant to photoelectrochemical reactions, such as solar water splitting, again where sequential, multiple reaction steps are necessary. For this, however, we will need to explore new metal oxide or other semiconductor supports, such as titanium dioxide, in our catalyst design.
—Peidong Yang
This research was supported by the DOE Office of Science.
Resources
Y. Yamada, C. Tsung, W. Huang, Z. Huo, S. E. Habas, T. Soejima, C. E. Aliaga, G. A. Somorjai, P. Yang (2011) Nanocrystal Bilayer for Tandem Catalysis. Nature Chem. doi: doi:10.1038/nchem.1018
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