Individual atoms of palladium, represented by the yellow peaks, in the surface of copper help break hydrogen molecules into two atoms, facilitating important chemical reactions. Image: Sykes Laboratory |
When you hear
the word hydrogenation, you might think of Crisco or margarine—plant oils made
thicker and more stable by adding hydrogen atoms. In fact, hydrogenation is a
key process in a large number of industries, such as oil refining, where it is
used to turn crude oil into gasoline.
Hydrogenation
happens thanks to the presence of a catalyst—usually a metal, such as nickel or
palladium, or an alloy—which allows the hydrogen atoms to bind with other
molecules. Typically, metal alloys are mixtures of cheap common metals, such as
nickel, and expensive precious metals, such as platinum or palladium. However,
it is hard to produce alloys that are selective hydrogenation catalysts, which
are able to attach the hydrogen atoms to specific sites on a molecule.
Now
scientists at Tufts
University have found a
way to create a selective hydrogenation catalyst by scattering single atoms of
palladium onto a copper base. This catalyst requires less of the expensive
metal, and the process is greener, too, offering potentially significant
economic and environmental benefits.
The team
reported its discovery in Science.
Led by
Charles Sykes, an associate professor of chemistry in the School of Arts
and Sciences, the group of researchers heated up very small amounts of palladium
to almost 1,000 C, or about 1,830 F. At that temperature, the metal evaporated
like a gas, so that single atoms were released. These atoms, less than half a
nanometer wide, embedded themselves into a copper metal surface about three
inches away.
Using a
scanning tunneling microscope, which can capture pictures of objects at the
atomic level, the researchers verified that single palladium atoms had indeed
embedded themselves at scattered intervals in the copper. In a conventional
metal catalyst, by contrast, palladium is used in clumps 5 to 10 nm wide. This
is significantly less economical, since it requires much greater quantities of
palladium, which costs more than $650 an ounce. It is less environmentally
friendly as well, because of the energy that must be used to extract the
additional necessary palladium from raw ore.
The new
catalyst also behaves differently, says Georgios Kyriakou, a research assistant
professor in chemistry and first author of the report. He helped determine that
the single atom alloy was more effective in catalyzing hydrogenation than
denser mixtures of palladium and copper.
“In the face
of precious metals scarcity and exorbitant prices, these systems are promising
in the search for sustainable global solutions,” says Maria
Flytzani-Stephanopoulos, the Robert and Marcy Haber Endowed Professor in Energy
Sustainability in the School
of Engineering, whose laboratory
is studying the effectiveness of the single-atom process. She and Sykes are
continuing to collaborate on advancing their research, funded by the National
Science Foundation, the United States Department of Energy, and Tufts
Collaborates, a grant program administered by the Office of the Provost.
Flytzani-Stephanopoulos and her
group in the School
of Engineering are now
looking into other approaches to achieve hydrogenation with different metal
pairs. She says that eventually single-atom alloy catalysts could be used as
low-cost alternatives for hydrogenation and dehydrogenation. That could be a boon
for the production of agricultural chemicals, foods, and pharmaceuticals.
