WEST
LAFAYETTE, Ind. - Chemical engineers at Purdue University
have made a discovery that may help to improve a
promising low-polluting energy technology that combusts
natural gas more cleanly than conventional methods.
The
finding revolves around the fact that catalysts
and other materials vital to industry have complex
crystalline structures with numerous sides, or facets.
Different facets sometimes provide higher performance
than others, so industry tries to prepare catalytic
materials that contain a large number of higher-performing
facets.
The
Purdue researchers have determined, however, that
the precious metal palladium, the catalyst used
in the clean energy technology - called catalytic
combustion - performs the same no matter which facet
is exposed.
"Palladium
is the best metal for the catalytic combustion of
methane, which is contained in natural gas,"
said Fabio Ribeiro, an associate professor of chemical
engineering at Purdue. "There is no other element
in the periodic table you can use that's better
than palladium for this reaction."
To
produce electricity, natural gas is burned in a
turbine similar to a jet engine, and the turbine
runs a generator. The conventional method, which
is widely used in commercial power generation, burns
natural gas with a flame. Researchers are trying
to eliminate the flame, replacing it with a catalyst
that combusts methane at lower temperature, emitting
less smog-producing nitrogen oxide pollution. The
catalytic combustion technology is promising as
a future energy source because it generates less
pollution without losing efficiency, but industry
is still trying to find higher-performance catalysts
to improve the process.
The
research findings are detailed in a paper presented
today (Tuesday, March 15) during a meeting of the
American Chemical Society in San Diego. The paper
was written by Ribeiro; Jinyi Han, a researcher
from Worcester Polytechnic Institute in Massachusetts;
Purdue postdoctoral student Guanghui Zhu; and Dmitri
Y. Zemlianov, a researcher from the University of
Limerick in Ireland.
Catalysts
are critical for numerous manufacturing processes
and everyday applications, such as a car's catalytic
converter. Industry prepares tiny catalyst clusters
only a few nanometers, or billionths of a meter,
in diameter that contain numerous facets. The clusters
are then coated on a spongelike, porous "support
material."
"Because
the support material is porous, it has a larger
overall surface area than a smooth material would
have, making it possible to increase the amount
of catalyst per unit of volume present and boosting
performance," Ribeiro said.
Another
way to increase performance is to find which facets
work best.
The composition of the catalyst is the same in each
facet, but the arrangement of atoms is different,
much like the way in which a pile of oranges can
be arranged in many different ways.
"Some
piles will have square shapes, others will be hexagonal,
others will have different kinds of troughs, or
spacing, between rows of oranges," Ribeiro
said. "What we want to know is whether the
arrangement on the surface will make a difference.
If industry researchers want to prepare catalysts,
they want to know whether it matters if they use
small particles, big particles, a particle that
has a certain shape on its surface, and so on. Suppose
I find out that, for a certain reaction, one of
the facets is much better than all the others -
perhaps as much as a thousand times better? And
there are cases where that is true. Then I can tell
the people who make that catalyst that only this
facet is important."
If
a particular facet is known to perform better than
others, industry prepares clusters that contain
a large number of those facets.
Finding
the best facet, however, is difficult because commercial
catalyst clusters contain a complex combination
of many different facets.
"Each
of these clusters can be very different, or non-uniform,
from one place to another, which makes it difficult
to pick out individual facets," Ribeiro said.
"It's like a million people screaming at the
same time. You can't distinguish one person's voice
from the next.
"It's
very difficult to learn exactly how a catalyst works
by studying these non-uniform particles. All you
can really do is get an average performance for
all of the different facets combined. But we want
to learn precisely how each separate facet performs."
The
engineers do that by using a large single crystal,
cutting it at the proper angle with special equipment,
and then polishing the surface to a mirror finish,
creating pieces about 1 centimeter in diameter that
expose only a particular configuration of atoms.
"I
can now simplify the problem by studying just one
facet at a time for this particular reaction."
Samples
have to be prepared under ultra-high vacuum - a
millionth of a millionth of Earth's normal atmospheric
pressure - so that they are not contaminated by
impurities. After being cleaned, the samples are
transferred to a chamber where the chemical reactions
can be studied at regular pressure.
Determining
how well a catalyst works requires engineers to
precisely duplicate the same conditions in which
the catalysts are used.
"You
must recreate the same temperature and the same
concentration that the real catalyst sees so that
you measure the rate at exactly the same conditions,"
Ribeiro said. "And then we can measure the
rates and say, 'That's the maximum you will ever
get from your catalyst.' So we tell industry that
this is the benchmark for a certain catalyst."
The
Purdue researchers used the method to study the
oxidation of methane on a palladium catalyst, a
reaction that is critical to the catalytic combustion
of natural gas.
"What
we are saying here is that, in this case, no matter
what surface you have exposed, no matter what size
the particles are or anything like that, the rate
is always the same. So don't spend your time trying
to make something that has a certain shape because
it doesn't matter.
"If
researchers want a rate beyond what is currently
possible with palladium, they need to find a totally
different catalyst."
The
research has been funded by the U.S. Department
of Energy, and Ribeiro's lab is associated with
the Birck Nanotechnology Center in Purdue's Discovery
Park, the university's hub for interdisciplinary
research. The research is ongoing and is supported
by Purdue's recently formed Center for Catalyst
Design.
Writer: Emil Venere, (765) 494-4709, venere@purdue.edu
Source:
Fabio Ribeiro, (765) 494-7799, Fabio@ecn.purdue.edu
Related
Web site:
Fabio
Ribeiro:
https://engineering.purdue.edu/ChE/Directory/Faculty/Ribeiro.html
ABSTRACT
Complete
oxidation of methane on Pd single crystal catalysts
Fabio
H. Ribeiro (1), Jinyi Han (2), Guanghui Zhu (1),
and Dmitri Y.
Zemlianov (3)
The
oxidation of methane was studied on the Pd single
crystal surfaces (111), (100) and (110) and a foil
and characterized by LEED, XPS, TPD and STM. The
reaction studies were performed in mixtures of CH4
and O2 at total pressures close to 1 bar and 600-900
K. As a first step, the metal oxidation kinetics
and morphological changes were studied. The first
phase of metal oxidation consisted of dissolution
of oxygen into the metal until a critical oxygen
concentration triggered transformation to bulk PdO.
The diffusion of oxygen through the metal and bulk
PdP followed the Mott-Cabrera parabolic law. The
oxide had an amorphous "cauliflower-like"
structure on all three crystals. The reaction rates
of methane oxidation per unit of active surface
area on all three single crystals were similar when
the metal or the oxide was the stable phase. The
rates are similar and slightly higher than the ones
obtained on supported catalysts. It is concluded
that the reaction is not sensitive to the structure
of the catalysts.
Purdue University News Service
400 Centennial Mall Drive Room 324
West Lafayette, IN 47906-2016
purduenews@purdue.edu
Office (765) 494-2096
Fax (765) 494-0401
