This diagram shows how catalysts of two gold atoms can help convert methane into ethylene at room temperature (shown in red) and into formaldehyde at lower temperatures (shown in blue). Credit: Uzi Landman |
Scientists have discovered a method to control the
gas-phase selective catalytic combustion of methane, so finely that if done at
room temperature the reaction produces ethylene, while at lower temperatures it
yields formaldehyde. The process involves using gold dimer cations as
catalysts—that is, positively charged diatomic gold clusters. Being able to
catalyze these reactions, at or below room temperature, may lead to significant
cost savings in the synthesis of plastics, synthetic fuels and other materials.
The research was conducted by scientists at the Georgia Institute of Technology
and the Univ. of Ulm. It appears in The Journal of
Physical Chemistry C.
“The beauty of this process is that it allows us to selectively control the
products of this catalytic system, so that if one wishes to create
formaldehyde, and potentially methyl alcohol, one burns methane by tuning its
reaction with oxygen to run at lower temperatures, but if it’s ethylene one is
after, the reaction can be tuned to run at room temperature,” said Uzi Landman,
Regents’ and Institute Professor of Physics and director of the Center for
Computational Materials Science at Georgia Tech.
Reporting last year in the journal Angewandte Chemie International
Edition, a team that included theorists Landman and Robert Barnett from
Georgia Tech and experimentalists Thorsten Bernhardt and Sandra Lang from the Univ. of Ulm, found that by using gold dimer
cations as catalysts, they can convert methane into ethylene at room
temperature.
This time around, the team has discovered that, by using the same gas-phase
gold dimer cation catalyst, methane partially combusts to produce formaldehyde
at temperatures below 250 Kelvin or -9 degrees Fahrenheit. What’s more, in both
the room temperature reaction-producing ethylene, and the formaldehyde
generation colder reaction, the gold dimer catalyst is freed at the end of the
reaction, thus enabling the catalytic cycle to repeat again and again.
The temperature-tuned catalyzed methane partial combustion process involves
activating the methane carbon-to-hydrogen bond to react with molecular oxygen.
In the first step of the reaction process, methane and oxygen molecules
coadsorb on the gold dimer cation at low temperature. Subsequently, water is
released and the remaining oxygen atom binds with the methane molecule to form
formaldehyde. If done at higher temperatures, the oxygen molecule comes off the
gold catalyst, and the adsorbed methane molecules combine to form ethylene
through the elimination of hydrogen molecules.
In both the current work, as well as in the earlier one, Bernhardt’s team at
Ulm conducted
experiments using a radio-frequency trap, which allows temperature-controlled
measurement of the reaction products under conditions that simulate realistic
catalytic reactor environment. Landman’s team at Georgia Tech performed
first-principles quantum mechanical simulations, which predicted the mechanisms
of the catalyzed reactions and allowed a consistent interpretation of the
experimental observations.
In future work, the two research groups plan to explore the use of
multi-functional alloy cluster catalysts in low temperature-controlled
catalytic generation of synthetic fuels and selective partial combustion
reactions.
