Water molecules are gold for nanocatalysis
Some catalysts show remarkable high activity - thanks to water

Date:
July 27, 2020
Source:
Ruhr-University Bochum
Summary:
Nanocatalysts made of gold nanoparticles dispersed on metal oxides
are very promising for the industrial, selective oxidation of
compounds, including alcohols, into valuable chemicals. They show
high catalytic activity, particularly in aqueous solution. A team
of researchers has been able to explain why: Water molecules play
an active role in facilitating the oxygen dissociation needed for
the oxidation reaction.



FULL STORY ========================================================================== Nanocatalysts made of gold nanoparticles dispersed on metal oxides are
very promising for the industrial, selective oxidation of compounds,
including alcohols, into valuable chemicals. They show high catalytic
activity, particularly in aqueous solution. A team of researchers from Ruhr-Universita"t Bochum (RUB) has been able to explain why: Water
molecules play an active role in facilitating the oxygen dissociation
needed for the oxidation reaction. The team of Professor Dominik Marx,
Chair of Theoretical Chemistry, reports in the high-impact journal ACS Catalysis on 14 July 2020.


========================================================================== Rushing for gold Most industrial oxidation processes involve the use
of agents, such as chlorine or organic peroxides, that produce toxic or
useless by-products. Instead, using molecular oxygen, O2, and splitting
it to obtain the oxygen atoms needed to produce specific products would
be a greener and more attractive solution. A promising medium for this
approach is the gold/metal oxide (Au/TiO2) system, where the metal oxide titania (TiO2) supports nanoparticles of gold. These nanocatalysts
can catalyse the selective oxidation of molecular hydrogen, carbon
monoxide and especially alcohols, among others. A crucial step behind
all reactions is the dissociation of O2, which comprises a usually high
energy barrier. And a crucial unknown in the process is the role of water, since the reactions take place in aqueous solutions.

In a 2018 study, the RUB group of Dominik Marx, Chair of Theoretical
Chemistry and Research Area coordinator in the Cluster of Excellence
Ruhr Explores Solvation (Resolv), already hinted that water molecules
actively participate in the oxidative reaction: They enable a stepwise charge-transfer process that leads to oxygen dissociation in the
aqueous phase. Now, the same team reveals that solvation facilitates the activation of molecular oxygen (O2) at the gold/ metal oxide (Au/TiO2) nanocatalyst: In fact, water molecules help to decrease the energy barrier
for the O2 dissociation. The researchers quantified that the solvent curbs
the energy costs by 25 per cent compared to the gas phase. "For the first
time, it has been possible to gain insights into the quantitative impact
of water on the critical O2 activation reaction for this nanocatalyst -
- and we also understood why," says Dominik Marx.

Mind the water molecules The RUB researchers applied computer simulations,
the so-called ab initio molecular dynamics simulations, which explicitly included not only the catalyst but also as many as 80 surrounding water molecules. This was key to gain deep insights into the liquid-phase
scenario, which contains water, in direct comparison to the gas phase conditions, where water is absent. "Previous computational work employed significant simplifications or approximations that didn't account for
the true complexity of such a difficult solvent, water," adds Dr. Niklas
Siemer who recently earned his PhD at RUB based on this research.

Scientists simulated the experimental conditions with high temperature
and pressure to obtain the free energy profile of O2 in both liquid and
gas phase.

Finally, they could trace back the mechanistic reason for the solvation
effect: Water molecules induce an increase of local electron charge
towards oxygen that is anchored at the nanocatalyst perimeter; this in
turn leads to the less energetic costs for the dissociation. In the end,
say the researchers, it's all about the unique properties of water: "We
found that the polarizability of water and its ability to donate hydrogen
bonds are behind oxygen activation," says Dr. Munoz-Santiburcio. According
to the authors, the new computational strategy will help to understand
and improve direct oxidation catalysis in water and alcohols.


========================================================================== Story Source: Materials provided by Ruhr-University_Bochum. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Niklas Siemer, Daniel Mun~oz-Santiburcio, Dominik Marx. Solvation-
Enhanced Oxygen Activation at Gold/Titania Nanocatalysts. ACS
Catalysis, 2020; 8530 DOI: 10.1021/acscatal.0c01326 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200727145426.htm

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