University of Utah chemists
demonstrated the first conclusive link between the size of catalyst
particles on a solid surface, their electronic properties and their
ability to speed chemical reactions. The study is a step toward the
goal of designing cheaper, more efficient catalysts to increase
energy production, reduce Earth-warming gases and manufacture a
wide variety of goods from medicines to gasoline.
|
| University of Utah chemistry Professor Scott
Anderson and doctoral student Bill Kaden work on the elaborate
apparatus they use to produce and study catalysts, which are
substances that speed chemical reactions without being consumed.
The world economy depends on catalysts, and the Utah research is
aimed at making cheaper, more efficient catalysts, which could
improve energy production and reduce emissions of Earth-warming
gases. Credit: William Kunkel, University of Utah. |
Catalysts are substances that speed chemical reactions without
being consumed by the reaction. They are used to manufacture most
chemicals and many industrial products. The world's economy depends
on them.
"One of the big uncertainties in catalysis is that no one really
understands what size particles of the catalyst actually make a
chemical reaction happen," says Scott Anderson, a University of
Utah chemistry professor and senior author of the study in the
Friday, Nov. 6 issue of the journal Science. "If we could
understand what factors control activity in catalysts, then we
could make better and less expensive catalysts."
"Most catalysts are expensive noble metals like gold or
palladium or platinum," he adds. "Say in a gold catalyst, most of
the metal is in the form of large particles, but those large
particles are inactive and only nanoparticles with about 10 atoms
are active. That means more than 90 percent of gold in the catalyst
isn't doing anything. If you could make a catalyst with only the
right size particles, you could save 90 percent of the cost or
more."
In addition, "there's a huge amount of interest in learning how
to make catalysts out of much less expensive base metals like
copper, nickel and zinc," Anderson says. "And the way you are going
to do that is by 'tuning' their chemical properties, which means
tuning the electronic properties because the electrons control the
chemistry."
The idea is to "take a metal that is not catalytically active
and, when you reduce it to the appropriate size [particles], it can
become catalytic," Anderson says. "That's the focus of our work -
to try to identify and understand what sizes of metal particles are
active as catalysts and why they are active as catalysts."
In the new study, Anderson and his students took a step toward
"tuning" catalysts to have desired properties by demonstrating, for
the first time, that the size of metal catalyst "nanoparticles"
deposited on a surface affects not only the catalyst's level of
activity, but the particles' electronic properties.
Anderson conducted the study with chemistry doctoral students
Bill Kaden and William Kunkel, and with former doctoral student
Tianpin Wu. Kaden was first author.
The Economy Depends on Catalysts
"Catalysts are a huge part of the economy," Anderson says.
"Catalysts are used for practically every industrial process, from
making gasoline and polymers to pollution remediation and rocket
thrusters."
Catalysts are used in 90 percent of U.S. chemical manufacturing
processes and to make more than 20 percent of all industrial
products, and those processes consume large amounts of energy,
according to the U.S. Department of Energy (DOE).
In addition, industry produces 21 percent of U.S. Earth-warming
carbon dioxide emissions - including 3 percent by the chemical
industry, DOE says.
Thus, improving the efficiency of catalysts is "the key to both
energy savings and carbon dioxide emissions reductions," the agency
says.
Catalysts also are used in drug manufacturing; food processing;
fuel cells; fertilizer production; conversion of natural gas, coal
or biomass into liquid fuels; and systems to reduce pollutants and
improve the efficiency of combustion in energy production.
The North American Catalysis Society says catalysts contribute
35 percent or more of global Gross Domestic Product. "The biggest
part of this contribution comes from generation of high energy
fuels (gasoline, diesel, hydrogen), which depend critically on the
use of small amounts of catalysts in … petroleum
refineries," the group says.
"The development of inexpensive catalysts … is pivotal to
energy capture, conversion and storage," says Henry White,
professor and chair of chemistry at the University of Utah. "This
research is vital to the energy security of the nation."
Catalyst Research: What Previous Studies and the New Study
Showed
Many important catalysts - such as those in catalytic converters
that reduce motor vehicle emissions - are made of metal particles
that range in size from microns (millionths of a meter) down to
nanometers (billionths of a meter).
As the size of a catalyst metal particle is reduced into the
nanoscale, its properties initially remain the same as a larger
particle, Anderson says. But when the size is smaller than about 10
nanometers - containing about 10,000 atoms of catalyst - the
movements of electrons in the metal are confined, so their inherent
energies are increased.
When there are fewer than about 100 atoms in catalyst particles,
the size variations also result in fluctuations in the electronic
structure of the catalyst atoms. Those fluctuations strongly affect
the particles' ability to act as a catalyst, Anderson says.
Previous experiments documented that electronic and chemical
properties of a catalyst are affected by the size of catalyst
particles floating in a gas. But those isolated catalyst particles
are quite different than catalysts that are mounted on a metal
oxide surface - the way the catalyst metal is supported in real
industrial catalysts.
Past experiments with catalysts mounted on a surface often
included a wide variety of particle sizes. So those experiments
failed to detect how the catalyst's chemical activity and
electronic properties vary depending with the size of individual
particles.
Anderson was the first American chemist to sort metal catalyst
particles by size and demonstrate how their reactivity changes with
size. In previous work, he studied gold catalyst particles
deposited on titanium dioxide.
The new study used palladium particles of specific sizes that
were deposited on titanium dioxide and used to convert carbon
monoxide into carbon dioxide.
The study not only showed how catalytic activity varies with
catalyst particle size, "but we have been able to correlate that
size dependence with observed electronic differences in the
catalyst particles," Kaden says. "People had speculated this should
be happening, but no one has ever seen it."
Anderson says it is the first demonstration of a strong
correlation between the size and activity of a catalyst on a metal
surface and electronic properties of the catalyst.
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