Closed-cycle pulsed CO2 laser operation requires the use of an efficient CO-O2 recombination catalyst for these dissociation products which otherwise would degrade the laser operation. The catalyst must not only operate at low temperatures but also must operate efficiently for long periods. In the case of the Laser Atmospheric Wind Sounder (LAWS) laser, an operational lifetime of 3 years is required. Additionally, in order to minimize atmospheric absorption and enhance aerosol scatter of laser radiation, the LAWS system will operate at 9.1 micrometers with an oxygen-18 isotope CO2 lasing medium. Consequently, the catalyst must not only operate at low temperatures but must also preserve the isotopic integrity of the rare-isotope composition in the recombination mode. Several years ago an investigation of commercially available and newly synthesized recombination catalysts for use in closed-cycle pulsed common and rare-isotope CO2 lasers was implemented at the NASA Langley Research Center. Since that time, mechanistic efforts utilizing both common and rare oxygen isotopes have been implemented and continue. Rare-isotope studies utilizing commercially available platinum-tin oxide catalyst have demonstrated that the catalyst contributes oxygen-16 to the product carbon dioxide thus rendering it unusable for rare-isotope applications. A technique has been developed for modification of the surface of the common-isotope catalyst to render it usable. Results of kinetic and isotope label studies using plug flow, recycle plug flow, and closed internal recycle plug flow reactor configuration modes are discussed

Hess, Robert V.

Kielin, Erik J.

Miller, Irvin M.

Schryer, David R.

Upchurch, Billy T.

Wood, George M.

English

NASA Technical Reports Server

N90-24598
A A
RARE-ISOTOPE AND KINETIC STUDIES OF Pt/SnO 2 CATALYSTS
Billy T. Upchurch, George M. Wood, David R. Schryer, Robert V. Hess
NASA Langley Research Center
Hampton, VA 23665
Irvin M. Miller
Science and Technology Corporation
Hampton, VA 23666
Erik J. Kielin
Old Dominion University
Norfolk, VA 23529
ABSTRACT
Closed-cycle pulsed CO2 laser operation requires the use of an
efficient CO-O 2 recombination catalyst for these dissociation products
which otherwise would degrade the laser operation. The catalyst must not
only operate at low temperatures but also must operate efficiently for
long periods. In the case of the Laser Atmospheric Wind Sounder (LAWS)
laser, an operational lifetime of 3 years is required. Additionally, in
order to minimize atmospheric absorption and enhance aerosol scatter of
laser radiation, the LAWS system will operate at 9.1 micrometers with an
oxygen-18 isotope CO2 lasing medium. Consequently, the catalyst must not
only operate at low temperatures but must also preserve the isotopic
integrity of the rare-isotope composition in the recombination mode.
Several years ago an investigation of commercially available and
newly synthesized recombination catalysts for use in closed-cycle pulsed
common and rare-isotope CO 2 lasers was implemented at the NASA Langley
Research Center. Since that time, mechanistic efforts utilizing both
common and rare oxygen isotopes have been implemented and continue.
Rare-isotope studies utilizing commercially available platinum-tin oxide
catalyst have demonstrated that the catalyst contributes oxygen-16 to the
product carbon dioxide thus rendering it unusable for rare-isotope
applications. A technique has been developed for modification of the
surface of the common-isotope catalyst to render it usable. In this paper
we report on results of kinetic and isotope label studies using plug flow,
recycle plug flow, and closed internal recycle plug flow reactor
configuration modes.
INTRODUCTION
A problem affecting the use of pulsed CO2 lasers for applications
involving atmospheric transmission is the attenuation of the intensity of
the laser output due to absorption by atmospheric CO2 which is present at
about 330 ppm concentration. As this is predominantly the common isotope,
12C1602, the use of a rare-isotope of CO2 would enable the laser to emit
at frequencies which would not be readily absorbed by the atmosphere.
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Among the available rare-isotope CO 2 compounds, 12C1802 is the least
expensive. Furthermore, in addition to providing enhanced atmospheric
transmission, it has the added advantage of enhanced aerosol scattering
due to its comparatively short wavelength of 9.1 micrometers which may be
further enhanced by surface interactions with some aerosol species (ref.
3). Frequency-envelopes or "windows" which are favorable for atmospheric
transmission are presented for various CO2 isotopes in references 1 and 2.
A second problem associated with the operation of pulsed CO2 lasers
is that the electrical discharge normally used to excite such lasers
inevitably decomposes some of the CO2 to CO and 02 . This decomposition is
harmful to long-life laser operation both because of the loss of CO2 and
because of the buildup of 02 . The loss of CO2 results in a corresponding
gradual loss of laser power; the buildup of even small concentrations
(about 0.1 percent to 1.0 percent) of 02 can cause rapid power loss and
even complete laser failure. To obviate these problems with no wastage of
costly rare-isotope gases the product CO and 02 must be recombined with a
suitable catalyst.
Rare-isotope 12C1802 laser operation has been carried out at 300°C
with a common-isotope Pt/A]203 catalyst at Los Alamos National Laboratory
(ref. 4). In these studies, while the alumina was considered an inert
substrate and not a participant in the catalytic recombination of 12C180
and 1802 to form C02, special attention and care were necessary to reduce
the exchange or scrambling of the normal-isotope oxygen in A121603 with
the 1802 and the 12C180 dissociation products of the laser medium. A
platinum-tin oxide catalyst (Pt/Sn02) has been shown to operate with a
respectable recombinative efficiency at considerably lower temperatures
than achieved in the Los Alamos study of platinum on alumina (ref. 5), but
again the possibility of isotope exchange between a common-isotope
catalyst and rare-isotope gases exists.
The primary goals of the present study were to measure isotopic
exchange between the rare-isotope laser gases 12C1802, 12C180, and 1802
and common-isotope Pt/SnO 2 catalyst material and to develop techniques to
maximize isotopic integrity of the rare-isotope laser gases to maintain
laser power at the desired frequency. A further goal was to utilize
common and rare-isotope gases to determine mechanistic details of CO
oxidation on the Pt/SnO 2 catalyst.
Experiments described in this paper demonstrate that, while some
oxygen exchange between the gaseous oxygen and carbon monoxide species and
the Pt/SnO 2 substrate does indeed occur even at low reaction temperatures,
isotopic scrambling may be eliminated by an inexpensive isotope-exchange
surface-labeling technique developed at Langley Research Center (ref. 6).
Also, evidence is presented for the formation of a carbonate species on
the catalyst when the conversion of CO and CO2 occurs.
194
EXPERIMENTAL
The experimental apparatus for isotope measurements consisted of a
test gas cylinder connected through a gas-drying chamber, flow controller,
and a temperature-controlled catalyst reactor-chamber to a mass spectro-
meter gas-sampling inlet. The drying chamber was charged with anhydrous
magnesium perchlorate. A Hastings mass-flow controller was placed
upstream from the catalyst chamber. Excess gas flow not passing into the
mass spectrometer was diverted through a Hastings mass-flow meter to the
outside atmosphere. The mass spectrometer was a DuPont CEC Model 21-104
magnetic-sector unit. The catalyst reactor-chamber was built from
components and maintained temperature control within 0.5°C. The catalyst
of 1 percent Pt/SnO 2 was obtained from Englehard Industries. All catalyst
charges were placed in the chamber enclosed within a 6.35 mm internal
diameter by an approximately 40 cm long quartz tube. The catalyst was
held in place by quartz wool plugs on each end of the charge. All rare-
isotope gas compositions were obtained from Cambridge Isotope Laboratories
with stated isotopic purities of better than 98 atomic-percent and were
analyzed mass spectrometrically in our laboratory prior to use. All other
gases were from Scott Specialty Gases. Chemisorption measurements were
carried out using a Shimadzu thermal-conductivity-detector gas-chromato-
graph. All test-gas flow rates were 5 standard cubic centimeters per
minute. All CO concentrations were 2 percent by volume in neon. All 02
concentrations were 1 percent by volume in neon. All stoichiometric
mixtures of CO and 02 were 2 percent and 1 percent, respectively, with a 2
percent neon spike (as an internal standard) and the balance helium.
Hydrogen was 7.5 percent by volume in helium.
For kinetic studies a recycle plug-flow reactor was operated in a
recirculatins volume of approximately 200 ml with a recirculating flow
rate of 200 ml/min. Samples were contained in a temperature-controlled
quartz reactor tube as in the isotope measurement experiment. The recycle
plug-flow reactor was evacuated and filled with test gas and operated in a
closed recycle configuration for all kinetic studies. Internal recycle
flow rates were nominally 200 ml/min for all kinetic experiments.
Catalyst test specimens were 0.050 g in order to lower conversion rates
enough to allow sufficient gas chromatographic analyses to follow the
reaction through several half-lives. During tests to determine the
kinetic order for oxygen in the catalytic reaction the CO concentration
was nominally held constant at 50 percent. Initial oxygen concentrations
were nominally 2 percent. Repetitive tests were carried over a
temperature range of 0-125°C using a NASA prepared platinum oxide/silica
gel catalyst (refs. 7, 8, 9).
RESULTS AND DISCUSSION
During carbon monixide chemisorptlon studies, it was found that some
CO chemisorbed onto the Pt/SnO 2 catalyst at room temperature while other
CO was oxidized and immediately evolved as CO2 with the consequent
reduction of the catalyst surface by removal of some oxide. A room-
temperature CO chemisorption titration of a 1.0 gram sample of 1 percent
Pt/SnO 2 catalyst was found to bind 42 microliters of CO in some form.
Thermal desorption yielded 42 microliters of CO 2 as measured mass
195
spectrometrically. To determine whether or not the CO was converted to
CO2 by the heat involved in the thermal desorption another sample of
catalyst with chemisorbed CO was treated with gaseous HCI. It evolved CO 2
at room temperature, which indicated that oxidation of the CO had already
occurred, possibly as a carbonate-like species. Subsequently, chemisorp-
tion of 12C180 upon a normal isotope Pt/SnO 2 catalyst substrate followed
by thermal desorption of CO 2 yielded an approximately 4:4:1 ratio of
12C160180:12C1802:12C1602 as would be expected from a carbonate species,
thus lending further support to the existence of such an intermediate
species in the recombinative mechanism.
Since the primary goal of this research was to develop a catalytic
CO-O 2 recombination system for the operation of a rare-isotope closed-
cycle pulsed CO 2 laser system, isotope-exchange studies were subsequently
carried out to thoroughly investigate the Pt/SnO 2 catalyst system with
regard to isotopic interactions with all laser gas species as shown in
table I.
As is noted in table I, 12C180 was found to readily extract some
oxygen-16 from common-isotope Pt/SnO 2 at temperatures from room tempera-
ture upwards. Thus, the common-isotope catalyst cannot be used, in
unmodified form, for regeneration of the dissociation products which would
be encountered in a closed-cycle rare-isotope CO2 laser. Rows 2, 3, and 4
in table I show that there was no exchange or scrambling reaction observed
between 1802 (alone or in combination with 1602) and the Pt/Sn1602
catalyst substrate until temperatures substantially greater than the
expected operating temperatures of the laser catalyst bed were attained.
When a stoichiometric mixture of the oxygen-18 labeled carbon
monoxide and oxygen was passed over the common-isotope Pt/SnO 2 catalyst at
100°C, 85 percent 12C1802 and 15 percent 12C160180 were initially formed
(row 5, table I). These yields gradually changed to 90 percent and 10
percent, respectively, after 8 hours or more of operation. Evidently,
there are two reactive mechanisms occurring. The dominant reaction
appears to involve CO and 02 adsorbed on the catalyst surface, while the
lesser reaction involves oxidation of CO by the SnO 2 surface. In
addition, the SnO 2 surface then slowly become isotopically labeled with
oxygen-18 via the interactive exchange mechanism. Complete surface
labeling by this technique would, however, require an inordinately long
time even if diffusion from the bulk were inoperative.
With the foregoing data in hand, it was decided to deliberately label
the surface of the Pt/Sn1602 catalyst with oxygen-18 by first removing all
readily reactive oxygen-16 by hydrogen reduction followed by reoxidation
of the reduced surface to Sn1802 . If a11 of the active normal-isotope
oxygen at the surface could be exchanged in this way, then the catalyst
would be suitable for use in a rare-isotope closed-cycle CO2 laser if
diffusion of the bulk matrix oxygen-16 to the surface is negligable.
The chemical reduction of the Pt/SnO 2 catalyst was accomplished by
exposing it to a flowing stream of 7.5 percent H2 in helium at 225°C. The
active surface-oxygen removal was judged complete after the mass
spectrometrically monitored H2160 concentration in the stream had dropped
to the instrument background level. The reduced substrate surface was
196
then reoxidized at 225°C with a gas stream containing I percent 1802 until
the 1802 concentration exiting the catalyst chamber had attained and
remained at the 1 percent concentration level for at least 1 hour as
measured on the mass spectrometer. The temperature was then reduced to
ambient under neon flow. The preceding isotope-exchange labeling of the
metal-oxide catalyst was accomplished in about 5 hours and is covered in a
patent (ref. 6).
The 1 percent Pt/lsn1802 surface-labeled catalyst was then evaluated
under conditions listed in row 6 of table I and was found to maintain the
isotopic integrity of the rare-isotope gas composition in a test of 17
days duration. The subsurface normal oxygen-16 isotope in the bulk of the
catalyst material obviously does not diffuse to the surface at or below
the test temperature of I00°C during the test duration.
Kinetic studies were undertaken for the Pt/tin oxide/silica gel
catalyst using the recycle reactor in the internal recycle mode. The
graphical results of these studies were consistent with a rate law showing
first order oxygen dependency over the temperature range of 0-125°C. In
earlier studies, we and others reported that the overall reaction took
place with an overall first order rate law (refs. 10, 11). Activation
energies over the above temperature interval are tabulated in table II for
the CO oxidation reaction. The activation energy for the reaction was
found to be 36 kJ/mol over the 0-75°C interval with an abrupt decrease to
17 kJ/mol over the 75-125°C interval. The former value is somewhat lower
than that reported by Stark and Harris for unsupported platinum on tin
oxide catalyst (ref. 10). The decrease in the activation energy at 75°C
has mechanistic implications which have not thus far been explained.
Furthermore, if the overall reaction is first order then the obvious
conclusion is that the reaction must be zero order in CO. Preliminary
studies indicate that such is indeed the case under excess oxygen
conditions. However, under excess CO conditions the reaction seems to be
inhibited, thus indicating otherwise. As is noted in table II, the
activation energy for the stoichiometric condition does not show a change
at 75°C as was observed in the excess CO experimental condition. Kinetic
studies are continuing.
CONCLUDING REMARKS
Rare-isotope studies have demonstrated that when CO and 02 are
reacted on a Pt/SnO 2 catalyst the dominant reaction mechanism involves
these adsorbed species and only a minor amount of oxidation of the CO by
oxygen from the SnO 2 occurs. Furthermore, we have developed a simple,
economical surface-labelling technique, validated in a 17-day test, to
prevent isotopic scrambling from this minor reaction between rare-isotope
CO and common-isotope Pt/SnO 2. Additionally, we have obtained supportive
evidence for the formation of some carbonate-like intermediate species
when CO is oxidized on Pt/SnO 2.
Results of kinetic studies on NASA prepared Pt/SnO2/silica gel
cataysts have demonstrated a first order dependence of 02 for the CO
oxidation reaction over the temperature range of 0-125°C with an abrupt
decrease in activation energy at 75°C. Investigations are continuing with
mechanistic and other studies for improving the performance of CO
oxidation catalysts and to better predict a successful outcome for their
application in long-life closed-cycle CO2 lasers.
197
REFERENCES
i.
1
.
.
.
.
.
.
go
10.
Freed, C.; Ross, C.; and O'Donnell, R. C., J. Mol. Spec., 4_99,
439-453, (1974).
Freed, L. E.; Freed, C.; and O'Donnell, R. G., IEEE J. quant. Elect.,
QE-18, 1229-1239, (1982).
Hess, R. V.; Brockman, P.; Schryer, D. R.; Miller, I. M.; Bair, C.
H.; Sidney, B. D.; Wood, G. M.; Upchurch, B. T.; and Brown, K. G.,
NASA TM-86415, (1985).
Sorem, M. S. and Faulkner, A., Rev. Sci. Instrum., 5__22,1193-1196,
(1981).
Brown, K. G.; Sidney, B. D.; Schryer, D. R.; Upchurch, B. T.; Miller,
I. M.; Wood, G. M.; Hess, R. V.; Burney, L. G.; Paulin, P. A.; Hoyt,
R. F.; and Schryer, J., SPIE Proceedings, 663, 136-144, (1986).
Hess, R. V.; Upchurch, B. T.; Brown, K. G.; Miller, I, M.; Schryer,
D. R.; Sidney, B. D.; Wood, G. M.; and Hoyt, R. F., Isotope Exchange
in Oxide Containing Catalyst. U.S. Patent No. 4,839,330, June 13,
1989.
Upchurch, B. T.; Schryer, D. R.; Wood, G. M.; and Hess, R. V., SPIE
Proceedings, 106____22,(1989).
Upchurch, B. T.; Miller, I. M.; Brown, D. R.; Davis, P. P.; Schryer,
D. R.; Brown, K. G.; and Van Norman, J. D., Catalyst for Carbon
Monoxide Oxidation. U.S. Patent No. 4,912,082, March 27, 1990.
Upchurch, B. T.; Miller, I. M.; and Davis, P. P., Process for Making
a Noble Metal on Tin Oxide Catalyst. U.S. Patent No. 4,855,274,
August 8, 1989.
Stark, D. S., and Harris, M. R., J. Phys. E.: Sci. Instrum., 1__66,
492-496, (1983).
11. Miller, I. M.; Batten, C. E.; and Wood, G. M., NASA CP 2456, (1987).
198
TABLE I - OXYGEN ISOTOPE LABEL STUDIES
Reactants
C180
1802
1802 + 1602
1802 + 1602
C180 + 1/21802
Catalyst T, °C
Pt/Snl602 24-150
Pt/Snl602 25-225
Pt/Snl602 25-225
Pt/Sn1602 > 350
Pt/Snl602 i00
Pt/Snl802 i00C180 + 1/21802
TABLE II. 02 ACTIVATION ENERGY VALUES FOR
Pt/SnO2/SiO2/H20 CATALYST (CO EXCESS)
184 36.1 O- 35
185 36.8 0- 35
I74 36.9 33- 57
175 35.7 35- 57
176 '28.6" 35- 52
177 34.5 41- 57
178 44.6* 35- 62
179 35.2 35- 73
t81 16.5 75- 100
182 15.7 75- 125
183 17.8 75 -126
This Work E = 35.9 kJ/mol 0 - 73
This Work E = 16.7 kJ/mo[ 75 - 125
Stark and Hams E = 41.4 kJ/mol 21 - 60
This Work E = 35.0 kJ/mol 35 - 100t
*Excluded Value
t Stoichiometric Gas Mix
Product Yields
C160180
No Reaction
No Reaction
160180
85-90% C1802
15-10% C160180
C1802
199



Rare-isotope and kinetic studies of Pt/SnO2 catalysts

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