Low
pressure, low temperature plasmas
are commonly used in many of today’s modern technological industries,
and
continue to be the subject of intense basic and applied chemical
research. Within these plasmas there exists a separation between
electron temperature
( Te ) and plasma gas temperature (
Tg ) due to the
large differences in mass between electrons and nuclei (i.e., protons
and
neutrons), where Te is typically 10-1000 times
higher than Tg. From a synthetic point of view,
these plasmas provide a unique environment
consisting of chemically reactive species at relatively low
temperatures
that may aid in the production of metastable materials. Much of this
research on low pressure, low temperature plasmas however, focuses
mainly
on vapor deposition and surface modification, whereas bulk
decomposition
in plasmas is considered a mere consequence of the plasma etching
process. Using a homemade RF plasma reactor,
our group seeks
to explore the use of low pressure, low temperature plasmas toward the
production of new (metastable) materials through bulk plasma
decomposition
of various precursor materials.
Aluminum Trifluoride:
Plasma Synthesis
Metastable aluminum
trifluoride phases
such as b-AlF3
are commonly used in industry as halogen exchange catalysts, converting
mixed halogen-containing species such as the chlorofluorocarbons (CFCs)
into their corresponding higher and lower fluorinated analogues.1
Consequently, these solid fluorides played an important role in the
past
production of CFC refrigerants, and are now promising catalysts in the
synthesis of new CFC alternatives. Until recently, the synthesis of
these
metastable aluminum trifluoride phases required “soft chemistry”
techniques
involving the thermal decomposition of fluorometallate (e.g., (NH4)3AlF6))
or hydrate (e.g., a-AlF3•3H2O)
precursors. Recently, our group has shown that fluorine-containing
low-temperature
plasmas can also be used to synthesize metastable AlF3.
In our initial studies,2
we
have succeeded in synthesizing amorphous AlF3 in
an NF3-plasma
using zeolites as starting material. The impetus for the use of
zeolites
originated from the increase in spatial separation of the Al atoms,
thereby
allowing open structures to form more readily (1).
zeolite 
amorphous-AlF3 (s) + SiF4 + HF +
N2/NOx (1)
Preliminary
characterization of the zeolite-derived
plasma-synthesized AlF3 (plasma-AlF3)
has revealed
an unusually high BET (N2) surface area (190 m2/g)
compared with conventional "soft chemistry" techniques (18 m2/g),
as well as unexpected nanoscale morphologies. Moreover, since the
catalytic activity in b-AlF3
is believed to involve coordinatively unsaturated Al surface sites, we
might expect the highly strained, amorphous AlF3 structure
to
be active toward halogen exchange.
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 |
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TEM
images of AlF3
from zeolite / NF3
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Aluminum Trifluoride:
Characterization
of Reactivity
In the second phase of this
project, we
have probed the metastable nature of plasma-AlF3 and
exploring
its reactivity using a general-purpose gas-solid flow reactor (Figure
1),
designed and built to directly measure the catalytic activity of
plasma-AlF3
toward the dismutation (halogen exchange) reaction of CCl2F2
(Equation 2).
2 CCl2F2(g)  CCl3F(g) + CClF3(g) (2) |
 |
Figure 1. General Purpose
Gas-Solid Flow Reactor
Using the model dismutation
reaction (2),
we show that plasma-AlF3 is indeed active toward
halogen exchange,
while also being extremely sensitive toward both hydrolysis in the
presence
of H2O and coking in the presence of trace
hydrocarbons. Complementary studies on the temperature dependence of
dismutation ( TPR-CCl2F2 )
reveal unexpected features thought to be linked to structural changes,
while temperature-programmed desorption of ammonia ( TPD-NH3
)
experiments show an large amount of NH3 desorption
relative
to b-AlF3,
suggesting that a large fraction of the high BET (N2) surface
area consists of acid “sites.”
Continuing investigations
into the reactivity
of plasma-AlF3 include temperature-programmed
experiments of
bulk and surface structure and their possible correlations with the
temperature-dependent
reactivity data. Synthetic studies in the plasma decomposition of
modified
(e.g., ion-exchanged, isomorphously substituted) zeolites and
meso/microporous
(sol-gel) precursors for the production of new metastable, high surface
area materials are currently underway.
(1)
Kemnitz, E.; Menz,
D.-H. Prog. Solid State Chem. 1998, 26, 97-153.
(2) Delattre,
J. L.;
Chupas, P. J.; Grey, C. P.; Stacy, A. M. J. Am. Chem. Soc. 2001,
123,
5364-5365.
• People
Jamie Delattre - Plasma-puff
boy
Evan Hajime - Plasma monkey
• Collaborations
Characterization of new
materials is a
vital complement to virtually all synthetic studies, and may often
require
the use of advanced and/or complex characterization techniques far
outside
of one’s research focus. This is especially true when
conventional
techniques (e.g., Powder XRD) are unable to reveal the
structure/properties
of the material, as realized with our high surface area, x-ray
amorphous
AlF3 produced from the plasma decomposition of zeolite.
To meet these requirements
of advanced
structural characterization, we have formed a strong collaboration with
Professor
Clare P. Grey and Peter Chupas from the State University of New
York
at Stony Brook (SUNY-Stony Brook), where their expertise in solid-state
NMR have helped to reveal a highly distorted Al environment relative to
conventional phases of AlF3. Temperature-programmed
synchrotron
powder XRD studies with Dr. Jon Hansen at the National
Synchrotron Light Source (NSLS) have also provided useful
structural
transformation data to complement our studies in reactivity.
Synthetic aspects of the
project include
collaborations from Dr. Joseph Biscardi (Chevron
Central Research) with generous donations of zeolite precursors.
