TITLE: EVALUATION OF THE EFFECT OF WATER VAPOR ON

THE PERFORMANCE OF NASA'S NMRO CATALYSTS

FOR CARBON MONOXIDE OXIDATION

GRANT NUMBER: NAG-I-2223

PRINCIPAL INVESTIGATORS: Ares Akyurtlu

Jale F. Akyurtlu

INSTITUTION: Hampton University

Department of Chemical Engineering

Hampton, Virginia 23668

SUPPORTED BY: NASA Langley Research Center

Office of Education

Hampton, VA 23665

TECHNICAL MONITOR: David Schryer, Jeff Jordan

STUDENT RESEARCHERS: Vaughnery Ammons, Taikelia Battle, Amy Gay, Kyle Bray, Boe

Washington

FINAL REPORT

Summary of Research

August 1, 1999 - March 31, 2002

https://ntrs.nasa.gov/search.jsp?R=20020061271 2018-09-27T17:18:19+00:00Z

TABLE OF CONTENTS

INTRODUCTION ................................................................................................................. 1

Background Inforcnation .......................................................................... 1

EXPERIMENTAL .................................................................................................................... 2

RESULTS ............................................................................................................................. 3

Effect of Water or the Reduction-Oxidation Properties of the 15%Pt/SnO2 Catalyst .... 3

TPR of Other Catalysts Prepared by NASA/LaRC ............................................ 5

TPD of Other Catalysts Prepared by NASA/LaRC ............................................. 5

Temperature Programmed Reaction Studies with GC-MS Analyzer ......................... 6

CONCLUSIONS ....................................................................................................................... 7

FIGURES ....................................................................................................................................... 9

REFERENCES ............................................................................................... 50

±

INTRODUCTION

The Noble Metal Reducible Oxide (NMRO) catalysts for the low temperature oxidation of carbon

monoxide were developed by NASA for the reoxidation of carbon monoxide which forms by the

dissociation of carbon dioxide during the operation of sealed carbon dioxide lasers. The NMRO

catalyst, which consi:;ts of a noble metal in conjunction with a reducible metal oxide, was evaluated

under conditions that will be encountered in a CO2 laser operation, namely temperatures in the range

298 to 373 K and no 6gnificant reaction gas components other than CO, C02, and 02. The NMRO

catalysts may have significant potential for spin-off applications such as the prevention of carbon

monoxide build-up iv closed spaces as in space vehicle cabins or submarines, and the elimination of

the cold start-up problem of automobile exhaust catalysts. The most significant difference in the

conditions of these possible future applications is the high moisture content of the gases to be

processed. Lack of understanding of the effects of water vapor and high temperature on catalyst

activity and operatiol_ for extended periods are currently the main stumbling blocks for the transfer

of this NASA technology to be used for commercial purposes.

In the original propo _al the following objectives were stated: To obtain experimental data on the

adsorption, desorptio n and reaction characteristics of CO and 02 on the catalysts under high moisture

conditions; to collect evidence on the presence of carbonate and hydroxyl surface species and their

involvement in the CO oxidation mechanism; and to model the reaction system using a Monte-Carlo

simulation to gain insight on the various steps involved. After the work has commenced the NASA

technical monitor Mr David Schryer informed us that there was increased interest in the possible use

of the NMRO catal3sts as automobile exhaust catalysts and therefore NASA wanted to know

whether the catalysts can operate at high temperatures as well as with high moisture gases. At that

meeting it was decided that investigation of the high temperature performance of the NMRO

catalysts should be gi yen priority and replace the Monte-Carlo simulation objective. As a result, the

modified objectives of the investigation were taken as the investigation of the high-temperature

activity of the NMRC. catalysts, and the effect of water vapor on the performance of these catalysts.

Background Information

The noble metal recucible oxide (NMRO) catalysts have very important applications mostly

involving oxidation or reduction reactions where transfer of oxygen atoms takes place. These

catalysts exhibit a strong synergy between the two metal components producing significantly higher

catalytic activities compared to the single metal catalysts. This synergy may arise from three

different types of interactions w. The presence of one component may alter the properties of the other,

the two components nay independently catalyze different catalytic reaction steps, and/or the two

components may combine to form a different surface species that may create new catalytic sites.

The NMRO catalyst,'; that are of interest for low temperature CO oxidation are based on either

platinum or gold as noble metal. Platinum is generally associated with SnO2 as the reducible oxide

for operation below 373 K as in CO reoxidation catalyst in sealed CO2 lasers, and with ceria or

titania for higher temperature service such as automobile exhaust applications. With Au, the highest

activities were obtained when titanium and manganese oxides were used as the reducible oxides.

Pt/SnO2-based catal)sts were extensively studied for COz laser applications by NASA/LaRC

researchers and investigators in associated universities _2"3"4"5"6"7'8_.These studies have led to the

optimization of the P_:/SnO2-based catalysts with 15-20 % Pt and 5% Pd pretreated at 398 K under

3_

reducingconditions.TI lere is evidence that surface OH groups participate in the oxidation of CO

chemisorbed on Pt sit_:s. In fact, humidification of the catalyst surface after pretreatment or

humidification of the reaction gas mixture, was reported to increase catalyst activity. The Pt/SnO2

catalyst exhibited both long-term and short-term deactivation. Outgassing the CO2 could restore

catalyst activity after short-term decay, and the long-term deactivation could be reversed by the

reduction of the cataly:;t. Isotopic studies have revealed the formation of some carbonate and

bicarbonate species on the surface probably contributing to the short-term activity decay. About 15

% of the CO2 was formcd by the reaction of adsorbed CO with a lattice oxygen from the tin oxide

situated adjacent to the noble metal and the rest resulted from the reaction of CO adsorbed on Pt

reacting with the dissociatively adsorbed oxygen at the Pt-SnO2 interface. Due to the strength of the

O2 double bond, direct reaction of adsorbed CO and oxygen on the catalyst surface is not probable.

Instead, Schryer and Upchurch 99_propose two possible mechanisms involving surface hydroxyl

groups. One mechanism involves oxidation of chemisorbed CO by OH to form CO2 and hydrogen.

The CO 2 desorbs to forn_ gas phase carbon dioxide while the OH is regenerated by the reaction of

hydrogen with the chemisorbed oxygen. The second mechanism is supported by the DRIFTS data

and involves the chemisc.rption of CO on Pt sites while oxygen is chemisorbed on hydroxylated Sn

sites through six hydroxyl groups. This type of chemisorption weakens the 02 double bond thus

allowing for the reaction of oxygen with the adsorbed CO on a neighboring Pt site.

Haruta, et. al. °-_ reported high catalytic activity for CO oxidation on gold supported on TiO2, ot-

Fe203, Co3Oa, NiO, Be(OH)2, and Mg(OH)2 even at temperatures below 273 K. Their results were

taken as an indication of the involvement of adsorbed oxygen in the chemisorption of CO. Their

proposed reaction mechai_ism involved the formation ofbidentate carbonate species by the reaction

of CO adsorbed on gold _nd activated oxygen formed on the support at the gold-support interface.

Gardner, et. al _ synthesi _,¢d and tested several alternative low temperature CO oxidation catalysts

mostly based on manganese oxide as the reducible oxide. They have observed that Au/MnOx catalyst

exhibited significantly higher activity than that of the optimized Pt/SnO2 catalyst and almost no

deactivation. The Att/MnO_ catalyst was cheaper and required no pretreatment. However, Au/MnO_

is deactivated by high concentrations of CO2 and therefore, Au/MnOx is less useful than Pt/SnO2 in

closed CO2 lasers and the treatment of combustion gases.

In contrast to the involvement of surface hydroxyl groups in CO oxidation on Pt/SnO_ catalysts,

measurements on Pt/CeO2 catalysts do not indicate the involvement of hydroxyl groups. Jin, et.al. _

concluded that the lattice oxygen at the Pt-CeO 2interface was involved in the formation of CO2 from

CO and the formation of CO from CO2 occurred through the donation of oxygen from COz to a

lattice vacancy. Pre-dosin_, of water before the CO-TPD indicated adsorption of water but did not

change the CO and CO_ ¢!esorption profiles indicating that surface water did not take part in the

formation of CO and CO_. The CO-TPR results ofMartinez-Arias, et. al. _ also do not indicate any

involvement of surface wa_er or hydroxyl groups below 550 K on Pt/CeO2/A1203. They attributed the

enhanced activity at low temperatures to the enhancement of the reducibility of both the Pt and the

ceria in close vicinity of the Pt-Ceria interface.

EXPERIMENTAL

For the temperature programmed reduction (TPR), Temperature Programmed Oxidation (TPO), and

Temperature Programmed Reaction (TPR.n) experiments Micromeritics Pulse Chemisorb 2705 apparatus

with TPD/TPR 027 option was used in conjunction with a Varian 3800 Gas Chromatograph (GC)-

2

Satum2000MassSpectrometer(MS) fortheanalysisofthegaseousproducts.Gasmixturesusedforthesepurposeswere 6.69'70H2in Ar, 3.6%02 in He, and 10.1% CO in He. Watervaporwasintroducedbybubblingthefeedgasstreamthroughwateratroomtemperature(20-25°C).Toallowfor betterestimationof the:extentof reductiona 3 °C/minheatingratewasusedfor theTPRrunswhilea 5°C/minheatingn_te was used in TPO experiments to reduce the run time.

TPD experiments were performed by injecting 0.993-ml doses of 10.1% CO in He into the test cell

until the observed peak areas remained constant. Helium was used as the cartier gas. Subsequently,

the temperature of the test cell was increased at a rate of 5°C/min to obtain the desorption peaks.

A cold trap containing propanol slush, prepared by mixing propanol with liquid nitrogen, was used

during TPR runs with hydrogen and for all runs with wet gases.

RESULTS

The investigation consisted of two main sections. Since the activity of the tin oxide-based oxidation

catalysts will depend on their reduction-oxidation behavior under the conditions of the proposed

commercial applications, the first part of the investigation focused on the TPR of the fresh and re-

oxidized baseline 15% Pt/SnO2 catalyst up to 1000 °C and under wet or dry reduction/oxidation

conditions, and the TPR of some modified tin oxide based catalysts. The second part involved a

series of TPD and TPRn sta_dies aimed at obtaining information on the mechanism of CO oxidation.

The results that were obtaii_ed can be classified into four groups: TPR results that show the effect of

water vapor on the reducti(,n-oxidation ability of the 15% Pt/SnO2 catalyst, TPR results for various

different catalysts prepared by NASA Langley Research Center (NASA/LaRC), TPD results to show

the effect of various pre-t_ catments, and results of the TPRn studies combined with gas analysis

using a gas chromatograph-mass spectrometer system.

Effect of Water on the Reduction-Oxidation Properties of the 15%Pt/SnO2 Catalyst

Three sets ofTPR-TPO rm_s were made to find out if the reduction and re-oxidation of the catalyst

was affected by the presen,:e of water.

Figures 1-3 show the results for dry reduction, dry re-oxidation, and dry reduction of the re-oxidized

catalyst. Two distinct redt, ction peaks, one at around 130 °C and the other at around 425 °C are

observed for the fresh catalyst (Figure 1). The small jump in the TCD signal just above 500 °C is

probably due to the flow disturbance introduced by the addition of liquid nitrogen to the cold trap to

re-freeze the propanol. The addition of liquid nitrogen to the trap always produced severe

fluctuations in the TCD signal, which in most cases returned to normal. Occasionally, the condensed

moisture froze and partially or totally blocked the flow path, which resulted in permanent flow

disturbance until the cold i:rap is heated to restore the flow. To illustrate this, the points of liquid

nitrogen additions and total flow path blockage are shown on some of the TPR curves. Dry re-

oxidation of the reduced catalyst is seen to be slow, with a rate proportional to the temperature

indicating mass transfer control of this process (Figure 2). The TPR of the re-oxidized catalyst

exhibits two distinct reduction regions, a wide peak around 350 °C, and an incomplete high

temperature peak, which :;tarts at around 750 °C. The shape of the low-temperature peak also

suggests mass transfer limitation at temperatures below 300 °C. Figures 4 and 5 show the amount of

hydrogen consumption by _:he _esh and re-oxidized catalysts. From these graphs it can be observed

that the oxidation-reductioTl capacity of the catalyst has decreased significantly when the catalyst was

subjected to dry TPR up to about 1000 o C. This may be due to the loss of OH groups that were

attachedto tin oxide, but since Figures 2 and 3 indicate the presence of mass transfer limitations, the

deterioration of the low temperature reduction-oxidation activity may be, at least partially, due to the

loss of active sites due to sintering at the high temperatures involved.

Figures 6-8 show the results for dry reduction, wet re-oxidation, and dry reduction of the re-oxidized

catalyst. Figure 6 shows the results of a TPR under the same conditions of those of Figure 1 and

gives an indication of the reproducibility of the results. Comparison of Figure 7 with Figure 2 shows

a higher oxidation rale during wet oxidation, which starts after 500 °C with a peak at around 725 °C.

The higher oxidation rate is also confirmed by the larger reduction peaks that were obtained during

subsequent dry reduction (Figure 8) compared to those shown in Figure 3. A more quantitative

indication of the reproducibility of the results can be obtained by comparing the hydrogen

consumption figures :_hown in Figures 4 and 9. Reproducibility of the total hydrogen consumption is

quite good (8.18 ml STP H2 versus 8.25 ml). The slight discrepancy (1.87 ml STP versus 1.29 ml) in

the amounts of hydrogen consumption corresponding to the low temperature reduction peak is

mainly due to the placement of the baseline, which is complicated by the disturbances introduced

during the addition o f liquid nitrogen to the cold trap as discussed above. In order to obtain a more

reasonable quantitative result, the flow disturbance seen in Figure 8 is omitted in Figure 10 and the

results are reported as estimates. Figure 10 shows that the low temperature reduction peak after wet

re-oxidation is about 2.5 times larger than the low temperature reduction peak obtained after dry re-

oxidation. It is also ebserved that a more significmat increase in the high temperature reduction is

obtained following _ et re-oxidation.

Figures 11-13 preseI_t the results for the wet reduction, dry re-oxidation, and wet reduction of the re-

oxidized catalyst and Figures 14 and 15 provides quantitative results for the wet reduction of the

fresh and dry re-oxidized catalyst. Figures 16-20 present the corresponding results for the case of wet

reduction, wet re-oxidation, and wet reduction. Comparison of Figures 11 and 16 with Figures 1 and

6 show that wet redu,ztion does not change the position of the low temperature reduction peak and

moves the high temperature reduction peak to slightly higher temperature (about 30 °C). The

oxidation peak observed at 760 °C in Figure 12 during dry re-oxidation of wet reduced catalyst is

similar to the oxidation peak observed in Figure 7 for the wet re-oxidation of dry reduced catalyst,

but occurs at a slight ly higher temperature. The quantitative results presented in Figures 9 and 14

indicate that the amount of wet reduction obtained both at low temperature (around 150 °C) and high

temperature (around 450 °C) is somewhat higher than those for the dry reduction. On the other hand,

the wet TPR of dry re-oxidized catalyst shown in Figure 13 is significantly different compared to the

dry TPR of the wet re-oxidized catalyst given in Figure 8. The low temperature peak observed at 150

°C during the wet or dry TPR of the fresh catalyst re-appears in the wet reduction of the dry re-

oxidized catalyst, and the high temperature reduction peak that was seen starting around 750 °C in

Figures 3 and 8, sta..-ts at around 500 °C in Figure 13 and peaks at around 850 °C. When the

quantitative results presented in Figures 10 and 15 are compared, it is seen that the total amounts of

reduction of the re-oxidized catalyst in both cases are comparable.The results shown in Figures 16-

20 for the wet reduction, wet re-oxidation case are almost identical to the results of wet reduction,

dry re-oxidation case presented in Figures 11-15.

In summary, the pre_ence of water vapor during the reduction and/or re-oxidation of the catalyst

plays a significant role in retaining the reduction-oxidation activity of the catalyst. While about the

same amount of reduction is obtained for dry reduction-wet re-oxidation and wet reduction-dry or

wet re-oxidation cases, the reduction temperatures are significantly lower for the wet reduction cases.

4

From these observations it may be tentatively concluded that the effect of water vapor during

reduction is to reduce sintering and retain reduction and oxidation activity at lower temperatures

while the effect of water vapor during re-oxidation may be attributed to the enhancement of the

oxidation rate by the presence of OH groups.

TPR of Other Catalysts Prepared by NASA/LaRC

Figure 21 shows the dry TI'R of the baseline 15% Pt/SnO2 catalyst with hydrogen and Figures 22-26show the TPR curves for w_rious combinations of tin oxide with the oxides of cerium, zirconium

and aluminum.

Table 1. Summary ot TPR Results for Various Tin Oxide-Based Mixed Oxide Catalysts

Catalyst

15% Pt/SnOz ..

Sn (011019/1)

SrgCe (011019/2)

SrgCe/Zr

(011019/3)SrgCe/Zr/A1

(011019/4)

Sn/Zr (011019/5)

First ?eak Second Peak

Temp. Range

(pk temp), °C

280-544 (456)

440--690 (613)500-760 (642)

First Peak Size 2ndPeak Size Total reduction

ml STP HJ100

mg cat.2.68

ml STP H2/

100 mg ca t.7.93

ml STP HJ

100 mg cat.10.6

1.36 12.4 13.8

2.05 15.6

13.3

Temp. Range

(pk tO), °C

, 44-222 (148)

150-350(250)175-43 7 (248,

33o)135-330 (238) 390-706 (540) 2.24

13.5

11.1

152-440 (304) 447-840 (600) 2.43 4.65 7.08

150-470 (290) 490-670 (600) 5.78 12.3 18.1

Since the amount of tin o _ide in each mixture is different, relative amounts of reduction cannot be

observed directly from these figures. Therefore, the quantitative results are calculated and presented

in Table 1 along with the information about the reduction peak temperatures. The lowest reduction

peak temperatures are obtained with the Pt-containing baseline catalyst. The other catalysts have

significantly higher peak temperatures, low around 300 °C and high around 600 °C, with the

exception of Srt/ Ce/Zr catalyst, which has reduction peaks around 240 °C and 540 °C. An

interesting observation from the table is the increase in the amounts of both low and high

temperature reductions for Sn/Zr catalyst and the increase in the amount of high temperature

reduction for the Sn/Ce This can be attributed to the ability to create oxygen vacancies and high

oxygen mobility of the cerium and zirconium oxides.

These observations make the investigation of the enhancement of the activity and high temperature stabilityof tin oxide-based catalysts by the introduction of appropriate metal oxides an attractive topic for future

investigations.

TPD of Other Catalysts Prepared by NASA/LaRC

For these experiments the catalyst sample was subjected to various pretreatments. Pretreatments with

dry gases consisted of 3owing a gas stream (He, He+10.1% CO, He+3.6% 02) over the catalyst

sample for two hours at 50 °C. Pretreatment with wet He involved flowing dry He over the catalyst

for 2 hours followed b- a 10-minute flow of He bubbled through de- ionized water at 25 °C. All

pretreatments were followed by He flow over the catalyst at 50 °C for two hours to desorb the

physically adsorbed sp_.cies. This was followed by the injection of 1-ml doses of the CO-containing

gas into the carrier gas str.-am at 30 °C. This dosing was continued until the peak areas stayed

relatively constant. Only CO2 was observed in the exit gas, using a gas chromatograph equipped with

a thermal conductivity detector, indicating that the catalyst was very effective in oxidizing CO. The

CO2 peaks eventually react_ed a relatively constant area, which was smaller than the CO peak of the

feed.

Figure 27 shows the temperature programmed desorption results for catalyst samples subjected to

various pretreatments. The TPD results were different alter the pretreatment of the catalyst in He,

CO, 02, and wet He. Two desorption peaks were obtained with the catalysts pretreated with dry

gases, while only the first desorption peak was obtained with the catalyst pretreated with wet He.

The low temperature peak observed around 75 °C for dry He and CO pre-treatment and at 100 °C for

02 pre-treatment was tentatively attributed to the desorption of physically adsorbed species. The

absence of this low temperature peak for the TPD of the catalyst sample pretreated with wet He can

be explained by the filling of active sites with adsorbed water. Due to the presence of the cold trap

the desorption of water is not observed. The high temperature peak observed around 225-250 °C was

attributed to the formation and subsequent desorption of CO2. Two possible mechanisms may be

possible for the formation of CO2. One may be through the reaction of a CO molecule adsorbed on a

platinum site with the lattice oxygen of an adjacent tin oxide site. Another route to CO2 formation

may be the formation of surface carbonates at the metal oxide sites and the subsequent

decomposition of these surface carbonates. The slightly elevated temperature of the second peak for

oxygen pretreatment will be consistent with this second mechanism. These arguments are based on

the gas chromatographic ar:alysis ofdesorption products, which indicated the presence of only CO2;

but the analysis of the desorption products were not detailed enough to arrive at a more definite

conclusion. Figure 28 shows the effect ofpretreatment temperature on the TPD results after CO pre-

treatment. The low tempelature desorption peak is absent for pretreatment at 125 °C because the

pretreatment temperature is higher than the temperature for the desorption of physically adsorbed

species. The difference in tiae shapes of the desorption curves for the 50 °C and 125 °C pre-treatment

with CO may be due to the decrease of the importance of CO2 formation through the first mechanism

as the result of the decrea:;e in the amount of CO adsorbing at Pt sites at the high pre-treatment

temperature. After the TPD runs up to 600 °C the catalysts were re-oxidized with oxygen at 600 °C

and the re-oxidized catalysts produced TPD curves with no peaks indicating very slow, almost

constant rate desorption (Figure 29).

Temperature Programmed Reaction Studies with GC-MS Analyzer

Since the results of the gas chromatographic analyses were not sufficiently sensitive to provide an

accurate gas compositions, it was decided to replace the gas chromatograph with a new GC-mass

spectrometer system for more detailed product gas analysis and to repeat the CO chemisorption-TPD

runs under He-O2 mixture to obtain more insight into the processes occurring on the catalyst.

Figure 30 shows the relati','e amounts of CO and C O2 obtained when the fresh 15 % Pt/SnO2 catalyst

was subjected to repeated 0.993 ml doses of 10.1% CO in He under flowing He at 50 °C. The

corresponding thermal conductivity detector peak areas indicating the total gas composition change

are also shown. It is seen tltat negligible amounts of CO are present in the product gases and almost

all the injected CO is oxidized to CO2 possibly through the formation and subsequent decomposition

of tin carbonates. Figure 3 l shows the TPD curve obtained after the CO injections of Figure 30,

while the relative amounts of the evolved species are presented in Figure 32. These figures show that

the first TPD peak below 100 °C is due to the desorption of adsorbed air and moisture and the peak

around 220 °C is due to the CO2 evolution as the result of the dissociation of surface carbonate

6

species.Nosignificantquaz_titiesof COweredetected.FollowingtheTPDof Figure31,TPOof thecatalystwasperformedupto 530°C.Figure33showstherelativeamountsof COandCO2obtainedwhenthere-oxidizedcatalbstwassubjectedto repeated0.993ml dosesof 10.1% CO in Heunderflowing Heat 50°C.Deactivationof thecatalystis clearlyvisible. A significantamountof CO isobservedalongwith theC()2(theratioof COto CO2is about2:3).Thefollowing TPD producedaflat curveandtheanalysisof thegasesdid not showanypresenceof CO or CO2 indicating the

absence of carbonate species on the surface. This observation combined with Figure 33 implies that

not all CO z was formed b2y the dissociation of the carbonates, but the reduced catalyst has some

activity for direct CO oxidation as well, possibly involving Pt surface sites.

Figure 34 shows the TPD c_Jrve obtained after CO chemisorption under a carrier gas containing 3.6

% O2 in He. This is very similar to Figure 32 except that the observed peak temperature is slightly

higher. The species evolved during TPD is presented in Figure 35. The most important observation

from this figure is the small CO peak occurring at the same temperature as the CO 2 peak. Figures 36

and 37 show the species preduced with fresh and used catalyst during the injection of 0.993 ml doses

of CO under an oxygen-containing gas stream at 50 °C, respectively. With the fresh catalyst no CO is

observed and the CO2 evohation decreases slightly to a steady state value atter about 6 injections. On

the other hand, with the used catalyst the COz evolution increases from zero to about the same steady

state value after about 14 injections, while the observed CO, which is initially about one third of the

steady state CO2 value, decreases continuously with each CO dose. Apparently, presence of oxygen

in the feed inhibits the deactivation of the catalyst, which maintains significant activity even when

heated up to 530 °C. This may be attributed either to the involvement of gaseous oxygen in the

oxidation of CO on the Pt sites, or to the enhancement of the CO oxidation by the oxygen in the gas

phase, which fills the oxygt;n vacancies created in the tin oxide lattice when a CO molecule on the Pt

surface reacts with the lattice oxygen of an adjacent metal oxide site.

Figure 38 shows the species evolved during 0.993-ml injections of 10.1% CO in He at 50 °C into a

carrier gas stream of moist He containing 2,8 % water vapor. As with the other carrier gas streams

almost all CO is oxidized to CO2. The amount of CO observed is slightly more than those when dry

and oxygen-containing helium is used as the carder gas stream. Figure 39 shows the TPD curve

obtained after CO chemisc, rption under a carrier gas containing 2.8% H20 in He. The desorption

peak occurs at the same temperature as with dry He, but the size of the peak is significantly smaller.

The species evolved during desorption is very similar to those observed after CO injections under

oxygen-containing carrier gas, but the amounts are much smaller (Figure 40). From these

observations it can be coacluded that presence of water during CO injections inhibit surface

carbonate formation, thus trte much smaller desorption peak, but enhance direct CO oxidation on the

Pt sites with water supplyirig the oxygen by some sort of water gas shift mechanism, as evidenced by

the high conversion to CO, shown in Figure 38.

CONCLUSIONS

The main conclusion from the TPR experiments is that the presence of water during reduction, re-

oxidation, or both helps tt:e 15 % Pt/SnO2 catalyst to retain its oxidation-reduction activity even

when heated up to about 1000 °C. This is attributed to the presence of hydroxyl groups attached to

tin oxide, which may facil tate the re-oxidation of the reduced oxide.

The TPD and TPRn result,,; indicate the existence of two different mechanisms for the oxidation of

CO on the

15 % Pt/SnO2 catalyst. One mainly involves the platinum sites and leads to the oxidation of adsorbed

CO either by the lattice oxygen supplied by a neighboring tin oxide site or an adsorbed oxygen atom,

the second going through the formation of surface carbonates and their subsequent desorption. The

extremely high conversion rate for the flesh catalyst during CO injections and the relatively lower conversion

rate on the re-oxidized cataly,,;t indicate the importance of the first mechanism. The second mechanism is the

primary source of CO2 that is observed during temperature programmed desorption.

Both the presence of oxygen or water facilitates the first mechanism by providing external oxygen. High

temperature reduction appea,s to kill the catalyst's activity for the carbonate formation but the catalystmaintains its ability to promc.te CO oxidation via the first mechanism.

Presence of water vapor during CO oxidation diminishes carbonate formation significantly. This is rather

disappointing because water vapor was also found to facilitate the re-oxidation of the catalyst and help retainits reduction-oxidation activity. This indicates the need for finding an appropriate additive to retain the

catalyst activity in high temperature applications. Oxides like ceria, which possess a large oxygen capacity,

high oxygen mobility, and file ability to form oxygen vacancies, are prime candidates for this purpose.Preliminary TPR studies presented in this report for the effect of some additives indicate that results with

ceria and zirconia are very promising. Finding an optimum catalyst composition will be the focus of futureactivities.

8

FIGURES

Figure 1. Dry TPR of fresh 1: % Pt/SnO2 catalyst. Temperature plot.

Figure 2. Dry re-oxidation of dry reduced 15 % PffSnO2 catalyst. Temperature plot.

Figure 3. Dry TPR of dry re-c, xidized 15 % Pt/SnO2catalyst. Tempera_re plot.Figure 4. Dry TPR of fresh 15 % Pt/SnO2 catalyst.. Time plot

Figure 5. Dry TPR of dry re-_,xidized 15 % Pt/SnO2 catalyst. Time plotFigure

FigureFigure

FigureFigure

FigureFigure

FigureFigure

FigureFigure

FigureFigure

FigureFigure

FigureFigure

FigureFigureFigure

FigureFigure

Figure

FigureFigure

FigureFigure

Figure

FigureFigure

Figure

Figure

Figure

Figure

Figure

6. Dry TPR of fresh 15; % Pt/SnO2 catalyst. Temperature plot.7. Wet TPO of dry reduced 15 % Pt/SnO2 catalyst. Temperature plot.

8. Dry TPR of wet re-oxidized 15 % Pt/SnO2 catalyst. Temperature plot.9. Dry TPR of fresh 15 % Pt/SnO2 catalyst. Time plot

10. Dry TPR of wet re-oxidized 15 % Pt/SnO2 catalyst. Time plot11. Wet TPR of fresh 15 % Pt/SnO2 catalyst. Temperature plot.

12. Dry TPO of wet reduced 15 % PffSnO2 catalyst. Temperature plot.13. Wet TPR of wet reduced, dry re-oxidized 15 % Pt/SnO2 catalyst. Temperature plot.

14. Wet TPR of fresh 15 % Pt/SnO2 catalyst. Time plot15. Wet TPR of wet reduced, dry re-oxidized 15 % Pt/SnO2 catalyst. Time plot

16. Wet TPR of fresh 15 % Pt/SnO2 catalyst. Temperature plot.17. Wet TPO of wet reduced 15 % Pt/SnO2 catalyst. Temperature plot.18. Wet TPR of wet re duced, wet re-oxidized 15 % Pt/SnO2 catalyst. Temperature plot.

19. Wet TPR of fresh 15 % Pt/SnO2 catalyst. Time plot20. Wet TPR of wet reduced, wet re-oxidized 15 % Pt/SnO2 catalyst. Time plot

21. TPR of 15 % Pt/SnO2 catalyst with 6.69% H2 in At.22. TPR of NASA catalyst SnEH only (011019/1) with 6.69% H2 in At.

23. TPR of NASA catalyst Sn/Ce (011019/2) with 6.69% H2 in Ar.24. TPR of NASA catalyst Sn/Ce/Zr (011019/3) with 6.69% H2 in At.25. TPR of NASA catalyst SrgCe/Zr/A1 (011019/4) w_th 6.69% H2 in Ar.

26. TPR of NASA catalyst SrdZr (011019/5) with 6.69% H2 in Ar.27. TPD of 15 % Pt/SlaO2 catalyst. Effect of pre-treatment.28. TPD of 15 % Pt/S_IO2 catalyst after pre-treatment with 10.1% CO in He. Effect ofpre-

treatment temperature.29. TPD of re-oxidized 15 % Pt/SnO2 catalyst.

30. Species evolved a_ld TCD peak areas on fresh 15 % Pt/SnO2 catalyst during injection of0.993-ml doses of 10.1% CO in He at 50 °C, with He as carrier.

31. TPD of fresh 15 _J Pt/SnO2 catalyst after CO injections at 50 °C, with He as carrier.

32. Species evolved daring TPD of 15 % Pt/SnO2 catalyst after CO injections under dry He.

33. Species evolved a_ad TCD peak areas on 15 % Pt/SnO2 catalyst re-oxidized after CO sorptionand TPD to 530 "C, during injection of 0.993-ml doses of 10.1% CO in He with He asearner.

34. TPD of fresh 15 o; Pt/SnO2 catalyst after CO injections at 50 °C, with He+ 3.6% 0 2 as carrier.35. Species evolved during TPD of 15 % Pt/SnO2 catalyst after CO injections under He+3.6% 02

as carrier.

36. Species evolved and TCD peak areas on fresh 15 % Pt/SnO2 catalyst during injection of0.993-ml doses t,f 10.1% CO in He at 50 °C, with He+3.6% O5 as carrier.

37. Species evolved and TCD peak areas during re-dosing with CO at 50 °C after TPD to 530 °Cwith He+3.6% Ch as carrier.

38. Species evolved and TCD peak areas on fresh 15 % Pt/SnO2 catalyst during injection of0.993-ml doses of 10.1% CO in He at 50 °C, with He+2.8% H20 as carrier.

39. TPD of fresh 15 o; PtJSnO2 catalyst after CO injections at 50 °C, with He+ 2.8% H20 ascarrier.

40. Species evolved during TPD of 15 % Pt/SnO2 catalyst after CO injections at 50 °C, withHe+3.6% 02 as carrier.

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REFERENCES

. Herz, R. K., in "Low Temperature CO-Oxidation Catalysts for Long-Life CO2 Lasers", NASA

Conf. Publ. 3076, D. R Schryer and G. B. Hoflund, Eds., pp. 21-40, National Information

Service, Springfield, VA, 1990.

. Sehryer. D. R., Upchurch, B. T., Hess, R. V., Wood, G. M., Sydney, B. D., Miller, I. M.,

Brown, K. G., Van Norman, J. D., Schryer, J., Brown, D. R., Hoflund, G. B., and Herz, R.

K., in "Low Temperature CO-Oxidation Catalysts for Long-Life CO2 Lasers", NASA Conf.

Publ. 3076, D. R. Schrycr and G. B. Hoflund, Eds., pp. 41-53, National Information Service,

Springfield, VA, 1990.

. Upehureh, B. T., Kielin, E. J., and Miller, I. M., in "Low Temperature CO-Oxidation

Catalysts for Long-Life CO2 Lasers", NASA Conf. Publ. 3076, D. R. Schryer and G. B.

Hofltmd, Eds., pp. 69-90, National Information Service, Springfield, VA, 1990.

. Van Norman, J. D., Brown, K. G., Schryer, J., Sehryer. D. R., Upehurch, B. T., Sydney, B.

D., in "Low Temperature CO-Oxidation Catalysts for Long-Life CO2 Lasers", NASA Conf.

Publ. 3076, D. R. Schryt._r and G. B. Hoflund, Eds., pp. 181-191, National Information Service,

Springfield, VA, 1990.

. Upchureh, B. T., Wood, G. M., Schryer. D. R., Hess, R. V., Miller, I. M., Kielin, E. J., in

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