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Born-Oppenheimer
The Leiden theoretical chemists Ernst Pijper, Roar
Olsen and Geert-Jan Kroes, together with colleagues from Amsterdam and
from Spain, demonstrated that the Born-Oppenheimer approach can be used
in predicting the reaction of hydrogen molecules with a metal surface.
This is an approach which considerably simplifies chemical calculations
by splitting them in two. A heated debate is currently raging on the applicability
of the Born-Oppenheimer approach to reactions of molecules with a metal
surface - a class of reactions which is very important in this application.
’Hot results’
The researchers published their findings last Thursday
on the site of Science Express. This site publishes on the web a number
of what they call hot results, sometimes weeks before they appear in the
journal Science. The Born-Oppenheimer approach can be used, according
to the researchers in their article, because a hydrogen molecule which
is fired at a platinum surface, first splits into two atoms or is scattered
at the surface before any strong interaction with the platinum takes place.
Complicated
Chemical reactions are generally so complicated that
it is difficult to make predictions using quantum mechanical calculations,
the specialism of theoretical chemistry. The Born-Oppenheimer approach
is a useful tool for making these chemical calculations.
Born and Oppenheimer in 1927 postulated the idea that
you can split a chemical calculation in two. First you solve the movement
of the electrons and then the movement of the atomic nuclei which are
so much heavier and slower than electrons that you can imagine they are
standing still in comparison.
This makes the calculation much easier, and the Born-Oppenheimer
approach is then also a welcome tool for predicting the progress of chemical
reactions of complex systems. It works particularly well in calculating
the behaviour of many gas phase reactions.
Out of step
The Born-Oppenheimer approach can only be applied if
the reaction processes are adiabatic, which means that the electrons follow
the movements of the nuclei and do not get out of step. And a lot of reactions
of molecules with a metal surface are not adiabatic.
It has been shown for a number of molecules investigated
to date which have been fired at a metal surface, that they already form
a strong bond with the surface before they dissociate or are scattered.
The consequence is that the molecules affect the metal surface, by forming
so-called electron-hole pairs. This process is non-adiabatic; the electrons
no longer follow the movement of the nuclei, and the Born-Oppernheimer
approach cannot therefore be used.
Firing
Other molecules, like nitrogen, which when stretched
like to take up electrons, can, if they are fired in a highly excited
state at a metal surface that weakly binds electrons, shoot electrons
out of a metal surface. This case also constitutes a breakdown of the
Born-Oppenheimer approach.
But, based on calculations and experiments, the three
Leiden researchers and their colleagues have been able to show that hydrogen
molecules behave very differently if they are fired at a metal surface.
Back-scattering
At first, hydrogen molecules split into two atoms,
or are back-scattered to the platinum surface before they form the strong
bond with the metal whereby electron-hole pairs can be formed.
Secondly, a hydrogen molecule has a low electron affinity,
which means that it does not readily take up an electron. Therefore, it
does not tend to ’take’ an electron of the metal far from the surface.
So, here too there is no risk of forming electron-hole pairs.
The reaction of molecular hydrogen with a platinum
surface, and the scattering of the molecule on the surface is then an
adiabatic process, and the Born-Oppenheimer approach can readily be applied
to this class of reactions.
Hydrogen economy
‘It is not the case that hydrogen economy will be a
fact next year as a result of this discovery,’ says Prof. dr. Geert-Jan
Kroes, who is conducting fundamental research into hydrogen as a source
of clean energy, and who transformed his faculty into a hydrogen plaza
on the Science Day in 2005. ’But it is encouraging, because some hydrogen
storage systems are based on the disassociation of hydrogen, that is the
breaking of the hydrogen-hydrogen bond. Sometimes a metal is added, such
as palladium. And disassociation can be described with the Born-Oppenheimer
approach. Platinum is not a candidate for promoting the storage of hydrogen.
We do know now that a whole class of reactions, particularly dissociation
of hydrogen on metal surfaces, can be solved simply, so that we can conduct
large scale testing of theories which describe the interaction of molecules
with metal surfaces.‘
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