Why certain catalyst materials work more efficiently when they are surrounded by water instead of a gas phase is unclear. RUB chemists have now gleamed some initial answers from computer simulations. They showed that water stabilises specific charge states on the catalyst surface.
Snapshot of the charge transfer: Water-induced charge transfer at the interface between water and catalyst. The red and blue areas in the left and right image quantify the decrease or increase of the electron density due to the charge transfer at a given time. The blue and red mesh in the lower image section represents the oxide, the grey and yellow balls at the oxide surface the metal. The small blue and red molecules in the upper image section are water molecules.
Image: M. Farnesi Camellone, D. Marx
“The catalyst and the water sort of speak with each other” says Professor Dominik Marx, depicting the underlying complex charge transfer processes. His research group from the Centre for Theoretical Chemistry also calculated how to increase the efficiency of catalytic systems without water by varying pressure and temperature. The researchers describe the results in the journals “Physical Review Letters” and “Journal of Physical Chemistry Letters.”
Heterogeneous catalysis: water or gas as the second phase
In heterogeneous catalysis, researchers combine substances with two different phases - usually solid and gas. Chemical reactions work faster at the resulting interfaces than without a catalyst. Industry uses heterogeneous catalysis for many processes, for example to transform alcohols into certain aldehydes. Titanium dioxide with gold particles bonded to the surface, for example, is suitable as the solid phase. Water - instead of a gas - as the second phase has several advantages: environmentally harmful substances which are required in traditional procedures for the oxidation of alcohols can easily be replaced by atmospheric oxygen. Also, the whole reaction in water is very efficient, even at moderate temperatures.
Charge transfer between water and catalyst
The theoretical chemists have studied what happens in the catalysis at the molecular level by means of so-called ab initio molecular dynamics simulations. The result: a charge transfer takes place between the water and the catalyst. Electrons, or more specifically portions of electron densities, are moved between the solid and the liquid phase. The researchers speculate that in this way the liquid phase stabilises charge states on the gold surface. The sites where this occurs could be the active centres of the catalyst, where the chemical reactions work efficiently. Unlike water, a gas phase is not able to “talk” to the catalyst in this way, because no charge transfer is possible with the gas phase.
Increasing the efficiency through thermodynamics
In a further study, the team led by Dominik Marx examined a related metal/oxide catalyst of copper and zinc oxide, which is used for the large-scale industrial synthesis of methanol. As the computer simulations showed, especially the interplay between the solid phase and the gas phase is important here for the efficiency. Depending on the pressure and temperature conditions, hydrogen binds to the catalyst surface and thus indirectly stabilises catalytically active centres that occur in this case due to an electron transfer between the metal and the oxide. “Without the hydrogen, put bluntly the centres would not exist”, says Marx. In this way, the thermodynamic conditions in the gas phase put the surface into a certain state which is particularly favourable for the work of the catalyst.
Added value through combination
The two studies thus show that the catalytic efficiency can be controlled both by a solvent and by thermodynamics – namely through the pressure and temperature of the gas phase. However, completely different mechanisms are responsible for this, which the researchers were nevertheless able to elucidate using the same simulation methods. This makes the results directly comparable. In this way, the theorists aim to study in future whether they can improve the copper/zinc oxide system even further by replacing the gas phase with a suitable solvent.
The chemists at the RUB explore the active role of the solvent in catalytic reactions in the Excellence Cluster “Ruhr Explores Solvation” RESOLV (EXC 1069), which was approved by the German Research Foundation in June 2012.
M. Farnesi Camellone, D.Marx (2013). On the Impact of Solvation on a Au/TiO2 Nanocatalyst in Contact with Water, The Journal of Physical Chemistry Letters, doi: 10.1021/jz301891v
L. Martínez-Suárez, J. Frenzel, D. Marx, B. Meyer (2013): Tuning the Reactivity of a Cu/ZnO Nanocatalyst via Gas Phase Pressure, Physical Review Letters, doi: 10.1103/PhysRevLett.110.086108
Prof. Dr. Dominik Marx, Centre for Theoretical Chemistry, Department of Chemistry and Biochemistry at the Ruhr-Universität, 44780 Bochum, Germany, Tel.+49/234/32-28083, E-mail: email@example.com
Click for moreTheoretical Chemistry at the RUB
Dr. Josef König | idw
Researchers uncover protein-based “cancer signature”
05.12.2016 | Universität Basel
The Nagoya Protocol Creates Disadvantages for Many Countries when Applied to Microorganisms
05.12.2016 | Leibniz-Institut DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH
Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.
The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...
Broadband rotational spectroscopy unravels structural reshaping of isolated molecules in the gas phase to accommodate water
In two recent publications in the Journal of Chemical Physics and in the Journal of Physical Chemistry Letters, researchers around Melanie Schnell from the Max...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
05.12.2016 | Power and Electrical Engineering
05.12.2016 | Information Technology
05.12.2016 | Earth Sciences