Through an interaction with hydrogen atoms (green), a water molecule (magenta and blue) moves rapidly across a metal oxide surface. This atomic-scale speed leads to more efficient chemical reactions.

An
international team of researchers has discovered how adding trace
amounts of water can tremendously speed up chemical reactions-such as
hydrogenation and hydrogenolysis-in which hydrogen is one of the
reactants, or starting materials.

Led
by Manos Mavrikakis, the Paul A. Elfers professor of chemical and
biological engineering at the University of Wisconsin-Madison, and
Flemming Besenbacher, a professor of physics and astronomy at the
University of Aarhus, Denmark, the team published its findings in the
May 18 issue of the journal Science.

Hydrogenation
and hydrogenolysis reactions have huge applications in many key
industrial sectors, including the petrochemical, pharmaceutical, food
and agricultural industries. “In the petrochemical industry, for
example, upgrading of oil to gasoline, and in making various
biomass-derived products, you need to hydrogenate molecules-to add
hydrogen-and all this happens through catalytic transformations,” says
Mavrikakis, who is among the top 100 chemists of the 2000-10 decade,
according to Thomson Reuters.

A
chemical reaction transforms a set of molecules (the reactants) into
another set of molecules (the products), and a catalyst is a substance
that accelerates that chemical reaction, while not itself being consumed
in the process.

In
industrial applications, the speed of catalytic transformations is
important, says Mavrikakis. “The rate at which the hydrogen atoms
diffuse on the surfaces of the catalyst determines, to a large extent,
the rate of the chemical reaction-the rate at which we produce the
products we want to produce,” he says.

While
many researchers have observed that water can accelerate chemical
reactions in which hydrogen is a reactant or a product, until now, they
lacked a fundamental grasp of how that effect was taking place, says
Mavrikakis. “Nobody had appreciated the importance of water, even at the
parts per million level,” he says.

In
their research, Mavrikakis and Besenbacher drew on their respective
theoretical and experimental expertise to study metal oxides, a class of
materials often used as catalysts or catalyst supports. They found that
the presence of even the most minute amounts of water-on the order of
those in an outer-space vacuum-can accelerate the diffusion of hydrogen
atoms on iron oxide by 16 orders of magnitude at room temperature.

In
other words, water makes hydrogen diffuse 10,000 trillion times faster
on metal oxides than it would have diffused in the absence of water.
Without water, heat is needed to speed up that motion.

Besenbacher
and his colleagues have one of the world’s fastest scanning tunneling
microscopes, which has atomic-scale resolution. With it, they could see
how quickly hydrogen atoms diffused across iron oxide in the presence of
water.

To
explain the fundamental mechanisms of how that happened, Mavrikakis and
his team used quantum mechanics, a branch of physics that explains the
behavior of matter on the atomic scale; and massively parallel
computing.

Essentially,
when water is present, hydrogen diffuses via a proton transfer, or
proton “hopping,” mechanism, in which hydrogen atoms from the oxide
surface jump onto nearby water molecules and make hydronium ions, which
then deliver their extra proton to the oxide surface and liberate a
water molecule. That repeated process leads to rapid hydrogen atom
diffusion on the oxide surface.

It’s a process that doesn’t happen willy-nilly, either.

The
researchers also showed that when they roll out the proverbial red
carpet—a nanoscale “path” templated with hydrogen atoms-on iron oxide,
the water will find that path, stay on it, and keep moving. The
discovery could be relevant in nanoscale precision applications mediated
by water, such as nanofluidics, nanotube sensors, and transfer across
biological membranes, among others.

The
U.S. Department of Energy Office of Basic Energy Sciences funded the
UW-Madison research. Other UW-Madison authors on the Science paper
include chemical and biological engineering research scientist Guowen
Peng, PhD student Carrie Farberow, and Ph.D. alumnus Lars Grabow (now an
assistant professor at the University of Houston). Other authors
include Lindsay Merte, Ralf Bechstein, Felix Rieboldt, Wilhelmine
Kudernatsch, Stefan Wendt and Erik Laegsgaard of Aarhus University.

           

Source: University of Wisconsin-Madison