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
Catalytic converters
Storage of hydrogen
Forcefield
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
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.
International team
Kroes worked together with former postdoc Cristina Díaz (now at the Universidad Autónoma Madrid), former PhD researcher Ernst Pijper (now working at SARA Reken- en Netwerkdiensten in Amsterdam), Roar Olsen from Oslo, who previously worked at the Vrije Universiteit and at the Theoretical Chemistry department in Leiden, Fabio Busnengo (Universidad Naçional de Rosario, Argentina) and Daniel Auerbach (GRT Inc., Santa Barbara USA). The calculations were made by the Huygens super-computer SARA Reken- en Netwerkdiensten in Amsterdam.
Geert-Jan Kroes has conducted research into molecule-surface reactions for many years, and into the chemistry behind the hydrogen economy.
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.
Science