Catalysts: The platinum riddle

Date:
April 4, 2022
Source:
Vienna University of Technology
Summary:
Platinum is an important catalyst. But up until now, nobody know
how exactly single platinum atoms behave during catalysis.



FULL STORY ========================================================================== Platinum is an important catalyst. But up until now, nobody know how
exactly single platinum atoms behave during catalysis.


==========================================================================
What happens when a cat climbs onto a sunflower? The sunflower is
unstable, will quickly bend, and the cat will fall to the ground. However,
if the cat only needs a quick boost to catch a bird from there, then the sunflower can act as a "metastable intermediate step." This is essentially
the mechanism by which individual atoms of a catalyst capture molecules
in order to chemically transform them.

Several years ago, the Vienna University of Technology surface physics
group discovered that platinum "single-atom" catalysts could oxidize
carbon monoxide at temperatures which, according to their theoretical
models, should not have been possible. Now, with the help of atomic-scale microscope images and complex computer simulations, they have been able
to show that both the catalyst itself and the material on which it is
anchored assume energetically unfavorable "metastable" states for a short
time to allow the reaction to happen in a special way. The results have
been published in the journal Science Advances.

Single atoms as catalysts The research group of Prof. Gareth Parkinson
at the Institute of Applied Physics at the TU Wien is investigating the smallest catalysts possible: Individual platinum atoms are placed on
an iron oxide surface. They then come into contact with carbon monoxide
gas and convert into carbon dioxide, like happens in a modern car exhaust.

"This process is technically very important, but exactly what happens
when the catalyst is reduced in size to the single atom limit has not
been clear until now," says Gareth Parkinson. "In our research group,
we study such processes in a number of ways: on the one hand, we use
a scanning tunneling microscope to produce extremely high-resolution
images on which you can study the movement of individual atoms. And
on the other hand, we analyze the reaction process with spectroscopy
and computer simulations." Whether the platinum atoms are active as a
catalyst depends on the temperature.

In the experiment, the catalyst is heated slowly and evenly until the
critical temperature is reached, and carbon monoxide is converted to
carbon dioxide.

That threshold is about 550 Kelvin. "However, this did not fit our
original computer simulations," says Matthias Meier, first author of the current publication. "According to density functional theory, which is
normally used for such calculations, the process could only take place
at 800 Kelvin. So we knew: Something important had been overlooked
here until now." A metastable state: short-lived, but important For
several years, the team gathered extensive experience with the same
materials in other reactions, and as a result, a new picture emerged
step by step. "With density functional theory, you normally calculate
that state of the system that has the lowest energy," says Matthias
Meier. "That makes sense, because that is the state that the system most
often assumes. But in our case, there is a second state that plays a
central role: A so-called metastable state." Both the platinum atoms
and the iron oxide surface can switch back and forth between different
quantum physical states. The ground state, with the lowest energy, is
stable. When the system changes to the metastable state, it inevitably
returns to the ground state after a short time -- like the cat trying
to get to the top on an unstable climbing pole. But in the catalytic
conversion of carbon monoxide, it is enough for the system to be in the metastable state for a very short time: Just as a brief moment in a wobbly climbing state might be enough for the cat to catch a bird with its paw,
the catalyst can convert carbon monoxide in the metastable state.

When the carbon monoxide is first introduced, two platinum atoms attach together to make a dimer. When the temperature is high enough, the dimer
can move to a less favourable position where the surface oxygen atoms
are less weakly bound. In the metastable state, the iron oxide changes
its atomic structure precisely at this point, releasing the oxygen atom
that the carbon oxide needs to form carbon dioxide, which instantly
flies away -- completing the catalysis process. "If we include these
previously unaccounted for short- term states in our computer simulation,
we get exactly the result that was also measured in the experiment,"
says Matthias Meier.

"Our research result shows that in surface physics you often need a
lot of experience," says Gareth Parkinson. "If we hadn't studied very
different chemical processes over the years, we probably never would
have solved this puzzle." Recently, artificial intelligence has also
been used with great success to analyze quantum chemical processes --
but in this case, Parkinson is convinced, it probably would not have
been successful. To come up with creative solutions outside of what was previously thought possible, you probably need humans after all.


========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Iron_oxide_surface_with_two_platinum_atoms ========================================================================== Journal Reference:
1. Matthias Meier, Jan Hulva, Zdenek Jakub, Florian Kraushofer, Mislav
Bobić, Roland Bliem, Martin Setvin, Michael Schmid, Ulrike
Diebold, Cesare Franchini, Gareth S. Parkinson. CO oxidation
by Pt 2 /Fe 3 O 4 Metastable dimer and support configurations
facilitate lattice oxygen extraction. Science Advances, 2022; 8
(13) DOI: 10.1126/sciadv.abn4580 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220404105717.htm

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