In a unique new bilayer nanocatalyst system, single layers of metal and metal oxide nanocubes are deposited to create two distinct metal–metal oxide interfaces that allow for multiple, sequential catalytic reactions to be carried out selectively and in tandem. (Image courtesy of Yang group) |
In
a development that holds intriguing possibilities for the future of
industrial catalysis, as well as for such promising clean green energy
technologies as artificial photosynthesis, researchers with the U.S.
Department of Energy (DOE)’s Lawrence Berkeley National Laboratory
(Berkeley Lab) have created bilayered nanocrystals of a metal-metal
oxide that are the first to feature multiple catalytic sites on
nanocrystal interfaces. These multiple catalytic sites allow for
multiple, sequential catalytic reactions to be carried out selectively
and in tandem.
“The
demonstration of rationally designed and assembled nanocrystal bilayers
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,” says the leader of this research
Peidong Yang, a chemist who holds joint appointments with Berkeley Lab’s
Materials Sciences Division, and the University of California
Berkeley’s Chemistry Department and Department of Materials Science and
Engineering.
Yang is the corresponding author of a paper describing this research that appears in the journal Nature Chemistry.
The paper is titled “Nanocrystal bilayer for tandem catalysis.”
Co-authoring the paper were Yusuke Yamada, Chia-Kuang Tsung, Wenyu
Huang, Ziyang Huo, Susan Habas, Tetsuro Soejima, Cesar Aliaga and
leading authority on catalysis Gabor Somorjai.
Catalysts
– substances that speed up the rates of chemical reactions without
themselves being chemically changed – are used to initiate virtually
every industrial manufacturing process that involves chemistry. Metal
catalysts have been the traditional workhorses, but in recent years,
with the advent of nano-sized catalysts, metal,oxide and their interface
have surged in importance.
Peidong Yang (left), Wenyu Huang, and Gabor Somorjai were members of a Berkeley Lab team that developed the first bilayered metal-metal oxide nanocrystals to feature multiple catalytic sites on nanocrystal interfaces. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs) |
“High-performance
metal-oxide nanocatalysts are central to the development of
new-generation energy conversion and storage technologies,” Yang says.
“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.”
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,” Yang says. “Precise selection and control of metal-metal
oxide interfaces in nanocrystals should therefore yield better activity
and selectivity for a desired reaction.”
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. These two interfaces were then used to catalyze two
separate and sequential reactions. First, the cerium oxide-platinum
interface catalyzed methanol to produce carbon monoxide and hydrogen.
These products then underwent ethylene hydroformylation through a
reaction catalyzed by the platinum-silica interface. The final result of
this tandem catalysis was propanal.
Transmission electron micrograph showing monolayer of a cerium oxide nanocube monolayer on a platinum monolayer in a new bilyaer nanocatalyst. (Image courtesy of Yang group) |
“The
cubic shape of the nanocrystal layers is ideal for assembling
metal–metal oxide interfaces with large contact areas,” Yang says.
“Integrating binary nanocrystals to form highly ordered superlattices is
a new and highly effective way to form multiple interfaces with new
functionalities.”
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
U.S. 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,” says Yang. “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.”
For more about the research of Peidong Yang and his group, visit the Website at http://www.cchem.berkeley.edu/pdygrp/main.html
For more information about the Joint Center for Artificial Photosynthesis visit the Website at http://solarfuelshub.org/
For more information about the research of catalysis authority Gabor Somorjai, visit the Website at http://chem.berkeley.edu/faculty/somorjai/
