Good news for heterogeneous catalysis and the hydrogen economy: computers
can now be used to make accurate predictions of the reactions of (hydrogen)
molecules with surfaces. An international team of researchers, headed by Leiden
theoretical chemist Geert-Jan Kroes, published on this subject this week in
the journal Science.
Hydrogen on copper
The team developed a new method of modelling what happens when hydrogen molecules
separate on a copper surface. The way is now open for calculating the interaction
between more complex molecules and surfaces.
Chemical processes on surfaces
Kroes: 'It's amazing how little we actually know about chemical processes on
surfaces. Processes that take place openly and under our very noses.' Even the
interaction between hydrogen – with its two atoms, the simplest of all
molecules – and metal surfaces is so complex that it has so far never
been possible to describe what happens with quantitative precision.
Catalytic converters
Yet at the same time the reaction of molecules with surfaces is highly important
for society. Their interaction plays a crucial role in heterogeneous catalysis
whereby surfaces function as an intermediary allowing two other substances to
react with one another. This is what happens with catalysers in cars, for example,
and also in the production of the majority of synthetic compounds.
Storage of hydrogen
In the specific case of hydrogen, the interaction between a metal surface and
hydrogen molecules also has an important function in the storage of hydrogen,
in its turn one of the mainstays in the use of hydrogen as a clean fuel. Separation
of hydrogen molecules into two atoms is generally the first step in chemical
hydrogen storage methods.
Forcefield
Research into chemical reactions is no longer restricted to the lab; complex
computer calculations have become an essential element of this research. Since
the sixties, theoreticians have tried to find methods of calculating the forcefield
between the atoms of the molecules that are involved in the reactions of molecules
with surfaces. The forces between the atoms and therefore the barriers for the
activation of energy exert an exponential influence on reaction speed.
Different subsystems
The more precise the calculations of the inter-atomic forcefield, the more
precise the prediction of the reactions that take place between molecules and
surface. However, it is very difficult to calculate the force field, because
it calls for an accurate description of two totally different subsystems: that
of individual molecules and that of complete metal surfaces.
Chemical precision
Kroes and his team members have now developed a method of making computer models
of an important class of molecule-surface reactions, namely the dissociation
of hydrogen on a metal surface, with so-called chemical precision.
Margin of error: 1 kilocalorie per mol
Kroes: ' "Chemical precision" means that we can calculate the energy
of the interaction between molecules and surface with a margin of error not
greater than 1 kilocalorie per mol. The kilocalorie is the unit of energy familiar
to us from diet lists, and a mol is the unit that expresses the number of molecules.
There are about 6 x 1023 molecules in one mol. To give you an idea: 1 mol of
water weights approximately 18 grams.'
The method
To achieve "chemical precision" an advance first had to be made
in so-called density function theory (DFT). In this theory, the Hohenberg-Kohn
theorems state that the energy of the system (and therefore also the reaction
barrier) is determined by the density of the electrons in the system. However,
the theory does not explain how exactly the energy is determined from the electron
density. The trick that the Leiden researchers applied is to take a so-called
functional with a parameter that could be fitted to one experiment on the reaction
of 'heavy hydrogen' (D2) with copper. The functional gives the energy as a function
of the electron density. Subsequent calculations showed that other experiments
on the reaction of H2 on that copper surface could be reproduced accurately
using the same functional.
Posted November 6th, 2009
