Computer predicts reactions between molecules and surfaces, with 'chemical precision'

Nov 06, 2009

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.

The team developed a new method of modelling what happens when hydrogen molecules separate on a surface. The way is now open for calculating the interaction between more complex molecules and 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.

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.

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 methods.

Research into 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.

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.

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 on a , with so-called chemical precision.

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.'

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.

More information: Chemically accurate simulation of a prototypical surface reaction: H2 dissociation on Cu(111).
C. Díaz, E. Pijper, R.A. Olsen, H.F. Busnengo, D.J. Auerbach, G.J. Kroes. Science 6 November 2009.

Source: Leiden University

Explore further: Dolphin 'breathalyzer' could help diagnose animal and ocean health

add to favorites email to friend print save as pdf

Related Stories

Scattering of hydrogen makes calculation easier

Feb 23, 2006

The chemical reaction of hydrogen molecules (H2) with a platinum surface can be calculated much more straightforwardly than many researchers to date had thought. This is encouraging for research into hydro ...

Nanotechnology could promote hydrogen economy

Mar 29, 2005

Say "nanotechnology" and people are likely to think of micro machines or zippy computer chips. But in a new twist, Rutgers scientists are using nanotechnology in chemical reactions that could provide hydrogen for tomorrow's ...

Researchers Observe Hydrogen-Bond Exchange

May 08, 2008

Hydrogen bonds are quite small, on the level of a few angstroms. They can also be passed between two different molecules very quickly, at speeds of tens of times per second. But in spite of these properties, ...

Recommended for you

Triplet threat from the sun

13 hours ago

The most obvious effects of too much sun exposure are cosmetic, like wrinkled and rough skin. Some damage, however, goes deeper—ultraviolet light can damage DNA and cause proteins in the body to break down ...

Towards controlled dislocations

Oct 20, 2014

Crystallographic defects or irregularities (known as dislocations) are often found within crystalline materials. Two main types of dislocation exist: edge and screw type. However, dislocations found in real ...

User comments : 0