- Email: [email protected]
metal oxides for low temperature CO oxidation
J.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (Eds.) 11th International Congress on Catalysis -40th Anniversary Studies in Surface Science and Catalysis, Vol. 101 9 1996 Elsevier Science B.V. All rights reserved.
427
Au/Metal Oxides for Low T e m p e r a t u r e C O Oxidation Girish Srinivas, John Wright, C.-S. Bai, and Ron Cook TDA Research, Inc. 12345 W. 52 nd Avenue, Wheat Ridge, CO 80033, USA
Abstract Room temperature CO oxidation has been investigated on a series of Au/metal oxide catalysts at conditions typical of spacecraft atmospheres; CO = 50 ppm, CO 2 = 7,000 ppm, H20 -- 40% (RH) at 250C, balance = air, and gas hourly space velocities of 7,000- 60,000 hr1. The addition of Au increases the room temperature CO oxidation activity of the metal oxides dramatically. All the Au/metal oxides deactivate during the CO oxidation reaction, especially in the presence of CO 2 in the feed. The stability of the Au/metal oxide catalysts decreases in the following order: TiO2 > Fe203 > NiO > Co304. The stability appears to decrease with an increase in the basicity of the metal oxides. In situ FTIR of CO adsorption on Au/TiO 2 at 25~ indicates the formation of adsorbed CO, carboxylate, and carbonate species on the catalyst surface.
1. INTRODUCTION Low temperature oxidation is an attractive option for removing trace quantities of CO from ambient air in enclosed atmospheres such as submarines and spacecraft on long duration missions, and for industrial applications such as automotive cold start, ammonia synthesis, fuel cells, and CO 2 lasers. Traditionally, noble metals such as Pt and Pd supported on AI203 have been the catalysts of choice because of their high activity; however, they generally need to be operated at temperatures higher than 100~ Recent work [1-3] has shown that small particles of Au supported on various metal oxides such as C0304, Fe203, NiO, and TiO 2 are highly active for CO oxidation at low temperatures. The small size of Au particles and the method of preparation of the catalysts are both crucial to the high activity of the catalysts at low temperatures [1]. The objective of this study was to develop a low temperature CO oxidation catalyst that continually removes low concentrations of CO from the atmospheres of space stations. CO is a major contaminant in spacecraft environments. Since
428
power is scarce aboard spacecrafts, using a room temperature CO oxidation catalyst instead of conventional Pt or Pd supported on AI203 would result in substantial savings in energy. Thus, we studied catalysts composed of small particles of Au supported on various metal oxides as room temperature CO oxidation catalysts.
2. Experimental 2.1. Catalyst Preparation The activity of the Au/metal oxide catalysts is extremely sensitive to the method of preparation. The Au/metal oxide catalysts were prepared by the co-precipitating method [1]. During the course of this study, we have determined that the activity and the stability of the catalyst for room temperature CO oxidation were a function of Ph of the solution, temperature of precipitation, aging temperature and time, catalyst wash procedure, and calcination. In order to obtain reproducibility of catalyst synthesis, a semi-automated catalyst synthesis apparatus was used to prepare the catalyst. The catalysts were synthesized by co-precipitating a solution of hydrogen tetrachloroaurate (HAuCI4.XH20) and the metal nitrate using sodium carbonate (Na2CO3). The sodium carbonate was added dropwise to the solution of hydrogen tetrachloroaurate and metal nitrate at 60~ using an automated syringe pump. The precipitate was aged for 1 hour at 600C. The precipitate was then washed thoroughly with water three times to remove residual chlorine and sodium, dried in air overnight, and calcined at 350~ for 8 hours. These conditions were optimized to increase the low temperature performance of the catalysts. The Au/TiO 2 catalysts were made by precipitating Au particles on TiO 2 (p-25) similarly. 2.2. Reaction Studies The catalysts were tested for their CO oxidation activity in an automated microreactor apparatus. The catalysts were tested at space velocities of 7,000 60,000 hr 1. A small quantity of catalyst (typically 0.1 - 0.5 g.) was supported on a frit in a quartz microreactor. The composition of the gases to the inlet of the reactor was controlled by mass flow controllers and was: CO = 50 ppm, CO2 = 0, or 7,000 ppm, H20 = 40% relative humidity (at 25~ balance air. These conditions are typical of conditions found in spacecraft cabin atmospheres. The temperature of the catalyst bed was measured with a thermocouple placed half way into the catalyst bed, and controlled using a temperature controller. The inlet and outlet CO/CO2 concentrations were measured by non-dispersive infrared (NDIR) monitors. 2.3. In Situ FTIR Studies The in situ FTIR studies were conducted to monitor the formation and stability of surface species during CO chemisorption on the Au/metal oxide catalysts, using a Nicolet Magna IR 550 Fourier-Transform IR spectrometer equipped with a DTGS detector operating at a resolution of 4 cm 1. The IR reactor cell consisted of a central 314 in. hollow stainless steel tubing attached to circular stainless steel flanges. The reactor cell was sealed at the ends using CaF 2 windows with Viton O-
429
rings. The powdered catalyst was pressed into a wafer and placed in the reactor cell. Inlet and outlet lines were used to contact the pellet with the reactants. A thermocouple in contact with the pellet was used to monitor the temperature of the catalyst. The gaseous volume in the reactor was kept to a minimum by inserting two CaF 2 rods on either side of the pellet to minimize the intensity of the gas-phase bands in the IR spectra. The reactor could be heated with a heating mantle, capable of raising the temperature of the pellet to 500~ The IR spectra were obtained as a function of time and catalyst temperature. 3. RESULTS AND DISCUSSION CO oxidation tests on Au supported on various metal oxides were undertaken at low CO concentrations, where the adiabatic temperature rise in the bed is negligible. Since CO oxidation is highly exothermic, when high CO concentrations are present in the feed ( ~ 1%), and at high conversions, the adiabatic temperature rise in the catalyst bed due to the heat of reaction may be as high as 100~ Therefore, it is important to monitor the catalyst bed temperature when high CO concentrations are present in the feed.
100
c O Conversion
80 CO - 50 ppm COe-0 A
o<
H=O -
60
CO e selectivity
RH(2S~
25 ~ C balance air GHSV - 7,000 hr"1
40
20
0
2
4
6
8
Time (hr) Figure 1
CO oxidation on 5%Au/Co304 at a space velocity of 7,000 hr-1.
Figure 1 shows the CO oxidation on a 5%Au/Co304 catalyst at 25~ and a space velocity of 7,000 hr -1 in the absence of CO 2 in the feed. The CO conversion remains
430 at 100% throughout the 7 hour run. During the first hour, the CO 2 selectivity, {CO 2 formed/CO reacted} remained at 0. The CO 2 selectivity gradually increased and reached 100% only after 5 hours of reaction. This indicates that the catalyst adsorbed all the CO 2 that is formed from the CO during the first hour of reaction. This data also suggests that operating at 100% CO conversion could only provide transient data, and provide no information on the deactivation of the catalyst. As Figure 1 shows, the catalyst has a high enough activity at the low space velocity of 7,000 hr -~ to convert all the CO in the feed. If the catalyst is deactivating, it would be impossible to determine the extent of deactivation until the CO conversion decreased below 100%. In order to determine the extent of deactivation of the catalyst, if any, we performed the remaining tests at space velocities higher than 7,000 hr -1 and CO conversions less than 100%.
100 A
o~
s0 CO - 50 ppm CO==0
c:
0 m 60 0
a m
H=O - ~
s_..
> e~
0 o
0 0
RH(2S~
25 ~ C balance air GHSV - 60,000 hr"1
40
20
=
0
I
2
I
I
4
I
I
6
i
8
Time (hr) Figure 2 CO conversion on 5%Au/C0304 at a space velocity of 60,000 hr "1. Figure 2 shows the CO conversion as a function of time on stream in the absence of CO 2 in the feed on the 5%Au/Co304 at a space velocity of 60,000 hr 1. In contrast to the data shown in Figure 1, the catalyst showed an initial CO conversion of about 80% and showed considerable deactivation over eight hours. Figure 3 shows the CO conversion as a function of time at 25~ in the presence of CO 2 for C0304, TiO2, 5%Au/C0304, and 1%Au/TiO 2 catalysts at a space velocity of 60,000 hr -1. The data clearly shows that the addition of Au to the metal oxides
431
resulted in increasing the activity of the catalyst for CO oxidation at 25~ The data also showed that the initial activity and stability of the catalysts were different for different metal oxides.
100 i
Ii,l~r~am...
IL..
.
I
- - ~ ' . "~1l ~&..
'
, . J' . , ~ . ~ . 9 -- r,~,,m~
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-
= "~O 60 9> 0
..
', ~ 1 ~ - ~ , , ~ . . , .
L i, "---~.~_._
O c:
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l Aurno=
CO - 50 ppm CO2 - 7000 ppm H=O - 40% RH(25~ 25 ~ C
---
40
~
baJanoe Mr
-~_:..
GHSV - 7,000 hr"1 " :~-'~*~--__ o.:-
o
~
0
5
r-~e-li
n
n
i-i
r-!
I-I
10
i
r-'!
I
15
20
Time (hr) Figure 3
Effect of Au on the CO oxidation activity of metal oxides.
CO oxidation on 1%Au supported on various metal oxide catalysts was carried out to determine the effect of metal oxide on the activity and stability of the catalysts during room temperature CO oxidation. Figure 4 shows the CO conversion as a function of time on stream on 1%Au supported on various metal oxides such as Co304, Fe203, NiO, ZrO 2, and TiO 2. All the catalysts showed high initial CO conversions. The stability of the catalysts decreased in the following order: TiO 2 > ZrO 2 > NiO > Fe203 > C0304. The stability of the catalysts appears to decrease with increasing basicity of the metal. All of the Au/metal oxide catalysts deactivate quickly, under the conditions shown in Figure 4. In addition, the deactivation of the Au/metal oxide catalysts appears to be enhanced in the presence of CO 2. In support of the theory that increased basicity of the metal oxides leads to lower stability, we carried out CO 2 temperature programmed desorption experiments on the various catalysts. The CO 2 TPD data also confirmed that an increase in the basicity of the metal oxides leads to an increase in the amount of CO 2 adsorption on the catalysts. In situ FTIR studies of CO adsorption on a 1% Au/TiO 2 have identified various surface species on the catalysts. Figure 5 shows the in situ FTIR spectra of CO
432
100 L A
8o r 1%Au/TIO e
0 to L-0 c 0 o
\
1%Au/ZrO e
60
~"~'"~"~"~'~~--.----:---~W..
1%Au/NIO C O - 50 ppm
40
O0 e - 7000 ppm ~ 0 - 40% (eu4) T-25~
0 o
20
~
balance air
~--. 1%Au/FeeO.
GHSV -- eo,ooo hr"~
1%AulCosO4 0
,
0
Figure 4
I
5
,
,,,i .........
i
...... i
i
10
15 Time (hr)
20
I
I
25
,
80
CO oxidation on 1%Au on various metal oxides.
chemisorption on a 1%Au/TiO 2 catalyst. The spectrum prior to introduction of CO over the catalyst is shown for reference. Upon introduction of 1%CO in N2 over the catalyst, gas phase CO species appear at 2180 and 2130 cm 1. Flowing CO over the catalyst over time resulted in the development and increase in the intensity of bands in the 1700-1300 cm 1. The bands in the 1700-1300 cm 1 could be attributed to carboxylate and carbonate species. The uneven variation of the bands at 2180 and 2130 cm 1 could be due to the formation of adsorbed CO species on TiO 2 and on Au. These bands could be overlapping with the gas phase CO bands. Haruta et al. have attributed CO adsorbed on TP§ and on Au at 2183 and 2110 cm 1 [1]. Upon flushing the catalyst with N2 following introduction of CO (as shown in Figure 5), the gas phase and adsorbed CO bands disappeared. There was also a decrease in the intensity of the carboxylate and carbonate bands, suggesting that some of these species were weakly adsorbed at 25~ Further study is required to identify and determine the stability of all the adsorbed species. 4. CONCLUSION Au/metai oxides are active for low temperature CO oxidation. The activity of the catalysts is very sensitive to catalyst preparation. All the Au/metal oxides tested for room temperature CO oxidation deactivated substantially with time. The deactivation
433
0.04 -
g
o~o=""~176176,,,=~oo~
o~o=""~176176
t'~ I~ ~ i . .
0.08 o c .Q o 0.02.o <
T " 25~ flush In air
,
60 min
!
I,,,,
9
40 mln
~t
20 min
0.01 -
10 min
"I~O/N=
.
0.00
~ . . 2800
5 rain l . . . 2500 2200
t --0
. . 1900
Wavenumber Figure 5
. 1600
1800
'
{om"1}
In situ FTIR spectra of CO adsorption on 1%Au/mio 2.
increased in the presence of CO2 in the feed. The stability of the catalyst decreased with an increase in the basicity of the metal oxide. In situ FTIR studies suggest that adsorbed carboxylate or carbonate species on the catalyst surface could be inhibiting the room temperature CO oxidation reaction. A C KNOWLE DG EM ENTS The authors gratefully acknowledge support from NASA Marshall Space Flight Center under Contract No. NAS8-39344.
REFERENCES o
.
.
M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet and B. Delmon, J. Catal., 144 (1993) 175. S.D. Gardner, G.B. Hoflund, B.T. Upchurch, D.R. Schryer, E.J. Kielin, E. J. and J. Schryer, J. Catal., 129 (1991) 114. S.D. Lin, M. Bollinger, and M.A. Vannice, Catai. Lett. 17 (1983) 245.
427
Au/Metal Oxides for Low T e m p e r a t u r e C O Oxidation Girish Srinivas, John Wright, C.-S. Bai, and Ron Cook TDA Research, Inc. 12345 W. 52 nd Avenue, Wheat Ridge, CO 80033, USA
Abstract Room temperature CO oxidation has been investigated on a series of Au/metal oxide catalysts at conditions typical of spacecraft atmospheres; CO = 50 ppm, CO 2 = 7,000 ppm, H20 -- 40% (RH) at 250C, balance = air, and gas hourly space velocities of 7,000- 60,000 hr1. The addition of Au increases the room temperature CO oxidation activity of the metal oxides dramatically. All the Au/metal oxides deactivate during the CO oxidation reaction, especially in the presence of CO 2 in the feed. The stability of the Au/metal oxide catalysts decreases in the following order: TiO2 > Fe203 > NiO > Co304. The stability appears to decrease with an increase in the basicity of the metal oxides. In situ FTIR of CO adsorption on Au/TiO 2 at 25~ indicates the formation of adsorbed CO, carboxylate, and carbonate species on the catalyst surface.
1. INTRODUCTION Low temperature oxidation is an attractive option for removing trace quantities of CO from ambient air in enclosed atmospheres such as submarines and spacecraft on long duration missions, and for industrial applications such as automotive cold start, ammonia synthesis, fuel cells, and CO 2 lasers. Traditionally, noble metals such as Pt and Pd supported on AI203 have been the catalysts of choice because of their high activity; however, they generally need to be operated at temperatures higher than 100~ Recent work [1-3] has shown that small particles of Au supported on various metal oxides such as C0304, Fe203, NiO, and TiO 2 are highly active for CO oxidation at low temperatures. The small size of Au particles and the method of preparation of the catalysts are both crucial to the high activity of the catalysts at low temperatures [1]. The objective of this study was to develop a low temperature CO oxidation catalyst that continually removes low concentrations of CO from the atmospheres of space stations. CO is a major contaminant in spacecraft environments. Since
428
power is scarce aboard spacecrafts, using a room temperature CO oxidation catalyst instead of conventional Pt or Pd supported on AI203 would result in substantial savings in energy. Thus, we studied catalysts composed of small particles of Au supported on various metal oxides as room temperature CO oxidation catalysts.
2. Experimental 2.1. Catalyst Preparation The activity of the Au/metal oxide catalysts is extremely sensitive to the method of preparation. The Au/metal oxide catalysts were prepared by the co-precipitating method [1]. During the course of this study, we have determined that the activity and the stability of the catalyst for room temperature CO oxidation were a function of Ph of the solution, temperature of precipitation, aging temperature and time, catalyst wash procedure, and calcination. In order to obtain reproducibility of catalyst synthesis, a semi-automated catalyst synthesis apparatus was used to prepare the catalyst. The catalysts were synthesized by co-precipitating a solution of hydrogen tetrachloroaurate (HAuCI4.XH20) and the metal nitrate using sodium carbonate (Na2CO3). The sodium carbonate was added dropwise to the solution of hydrogen tetrachloroaurate and metal nitrate at 60~ using an automated syringe pump. The precipitate was aged for 1 hour at 600C. The precipitate was then washed thoroughly with water three times to remove residual chlorine and sodium, dried in air overnight, and calcined at 350~ for 8 hours. These conditions were optimized to increase the low temperature performance of the catalysts. The Au/TiO 2 catalysts were made by precipitating Au particles on TiO 2 (p-25) similarly. 2.2. Reaction Studies The catalysts were tested for their CO oxidation activity in an automated microreactor apparatus. The catalysts were tested at space velocities of 7,000 60,000 hr 1. A small quantity of catalyst (typically 0.1 - 0.5 g.) was supported on a frit in a quartz microreactor. The composition of the gases to the inlet of the reactor was controlled by mass flow controllers and was: CO = 50 ppm, CO2 = 0, or 7,000 ppm, H20 = 40% relative humidity (at 25~ balance air. These conditions are typical of conditions found in spacecraft cabin atmospheres. The temperature of the catalyst bed was measured with a thermocouple placed half way into the catalyst bed, and controlled using a temperature controller. The inlet and outlet CO/CO2 concentrations were measured by non-dispersive infrared (NDIR) monitors. 2.3. In Situ FTIR Studies The in situ FTIR studies were conducted to monitor the formation and stability of surface species during CO chemisorption on the Au/metal oxide catalysts, using a Nicolet Magna IR 550 Fourier-Transform IR spectrometer equipped with a DTGS detector operating at a resolution of 4 cm 1. The IR reactor cell consisted of a central 314 in. hollow stainless steel tubing attached to circular stainless steel flanges. The reactor cell was sealed at the ends using CaF 2 windows with Viton O-
429
rings. The powdered catalyst was pressed into a wafer and placed in the reactor cell. Inlet and outlet lines were used to contact the pellet with the reactants. A thermocouple in contact with the pellet was used to monitor the temperature of the catalyst. The gaseous volume in the reactor was kept to a minimum by inserting two CaF 2 rods on either side of the pellet to minimize the intensity of the gas-phase bands in the IR spectra. The reactor could be heated with a heating mantle, capable of raising the temperature of the pellet to 500~ The IR spectra were obtained as a function of time and catalyst temperature. 3. RESULTS AND DISCUSSION CO oxidation tests on Au supported on various metal oxides were undertaken at low CO concentrations, where the adiabatic temperature rise in the bed is negligible. Since CO oxidation is highly exothermic, when high CO concentrations are present in the feed ( ~ 1%), and at high conversions, the adiabatic temperature rise in the catalyst bed due to the heat of reaction may be as high as 100~ Therefore, it is important to monitor the catalyst bed temperature when high CO concentrations are present in the feed.
100
c O Conversion
80 CO - 50 ppm COe-0 A
o<
H=O -
60
CO e selectivity
RH(2S~
25 ~ C balance air GHSV - 7,000 hr"1
40
20
0
2
4
6
8
Time (hr) Figure 1
CO oxidation on 5%Au/Co304 at a space velocity of 7,000 hr-1.
Figure 1 shows the CO oxidation on a 5%Au/Co304 catalyst at 25~ and a space velocity of 7,000 hr -1 in the absence of CO 2 in the feed. The CO conversion remains
430 at 100% throughout the 7 hour run. During the first hour, the CO 2 selectivity, {CO 2 formed/CO reacted} remained at 0. The CO 2 selectivity gradually increased and reached 100% only after 5 hours of reaction. This indicates that the catalyst adsorbed all the CO 2 that is formed from the CO during the first hour of reaction. This data also suggests that operating at 100% CO conversion could only provide transient data, and provide no information on the deactivation of the catalyst. As Figure 1 shows, the catalyst has a high enough activity at the low space velocity of 7,000 hr -~ to convert all the CO in the feed. If the catalyst is deactivating, it would be impossible to determine the extent of deactivation until the CO conversion decreased below 100%. In order to determine the extent of deactivation of the catalyst, if any, we performed the remaining tests at space velocities higher than 7,000 hr -1 and CO conversions less than 100%.
100 A
o~
s0 CO - 50 ppm CO==0
c:
0 m 60 0
a m
H=O - ~
s_..
> e~
0 o
0 0
RH(2S~
25 ~ C balance air GHSV - 60,000 hr"1
40
20
=
0
I
2
I
I
4
I
I
6
i
8
Time (hr) Figure 2 CO conversion on 5%Au/C0304 at a space velocity of 60,000 hr "1. Figure 2 shows the CO conversion as a function of time on stream in the absence of CO 2 in the feed on the 5%Au/Co304 at a space velocity of 60,000 hr 1. In contrast to the data shown in Figure 1, the catalyst showed an initial CO conversion of about 80% and showed considerable deactivation over eight hours. Figure 3 shows the CO conversion as a function of time at 25~ in the presence of CO 2 for C0304, TiO2, 5%Au/C0304, and 1%Au/TiO 2 catalysts at a space velocity of 60,000 hr -1. The data clearly shows that the addition of Au to the metal oxides
431
resulted in increasing the activity of the catalyst for CO oxidation at 25~ The data also showed that the initial activity and stability of the catalysts were different for different metal oxides.
100 i
Ii,l~r~am...
IL..
.
I
- - ~ ' . "~1l ~&..
'
, . J' . , ~ . ~ . 9 -- r,~,,m~
o<
'"
ao
. . -.. ., ,., - - , , , . , . = ~ .
-
= "~O 60 9> 0
..
', ~ 1 ~ - ~ , , ~ . . , .
L i, "---~.~_._
O c:
i
~
l Aurno=
CO - 50 ppm CO2 - 7000 ppm H=O - 40% RH(25~ 25 ~ C
---
40
~
baJanoe Mr
-~_:..
GHSV - 7,000 hr"1 " :~-'~*~--__ o.:-
o
~
0
5
r-~e-li
n
n
i-i
r-!
I-I
10
i
r-'!
I
15
20
Time (hr) Figure 3
Effect of Au on the CO oxidation activity of metal oxides.
CO oxidation on 1%Au supported on various metal oxide catalysts was carried out to determine the effect of metal oxide on the activity and stability of the catalysts during room temperature CO oxidation. Figure 4 shows the CO conversion as a function of time on stream on 1%Au supported on various metal oxides such as Co304, Fe203, NiO, ZrO 2, and TiO 2. All the catalysts showed high initial CO conversions. The stability of the catalysts decreased in the following order: TiO 2 > ZrO 2 > NiO > Fe203 > C0304. The stability of the catalysts appears to decrease with increasing basicity of the metal. All of the Au/metal oxide catalysts deactivate quickly, under the conditions shown in Figure 4. In addition, the deactivation of the Au/metal oxide catalysts appears to be enhanced in the presence of CO 2. In support of the theory that increased basicity of the metal oxides leads to lower stability, we carried out CO 2 temperature programmed desorption experiments on the various catalysts. The CO 2 TPD data also confirmed that an increase in the basicity of the metal oxides leads to an increase in the amount of CO 2 adsorption on the catalysts. In situ FTIR studies of CO adsorption on a 1% Au/TiO 2 have identified various surface species on the catalysts. Figure 5 shows the in situ FTIR spectra of CO
432
100 L A
8o r 1%Au/TIO e
0 to L-0 c 0 o
\
1%Au/ZrO e
60
~"~'"~"~"~'~~--.----:---~W..
1%Au/NIO C O - 50 ppm
40
O0 e - 7000 ppm ~ 0 - 40% (eu4) T-25~
0 o
20
~
balance air
~--. 1%Au/FeeO.
GHSV -- eo,ooo hr"~
1%AulCosO4 0
,
0
Figure 4
I
5
,
,,,i .........
i
...... i
i
10
15 Time (hr)
20
I
I
25
,
80
CO oxidation on 1%Au on various metal oxides.
chemisorption on a 1%Au/TiO 2 catalyst. The spectrum prior to introduction of CO over the catalyst is shown for reference. Upon introduction of 1%CO in N2 over the catalyst, gas phase CO species appear at 2180 and 2130 cm 1. Flowing CO over the catalyst over time resulted in the development and increase in the intensity of bands in the 1700-1300 cm 1. The bands in the 1700-1300 cm 1 could be attributed to carboxylate and carbonate species. The uneven variation of the bands at 2180 and 2130 cm 1 could be due to the formation of adsorbed CO species on TiO 2 and on Au. These bands could be overlapping with the gas phase CO bands. Haruta et al. have attributed CO adsorbed on TP§ and on Au at 2183 and 2110 cm 1 [1]. Upon flushing the catalyst with N2 following introduction of CO (as shown in Figure 5), the gas phase and adsorbed CO bands disappeared. There was also a decrease in the intensity of the carboxylate and carbonate bands, suggesting that some of these species were weakly adsorbed at 25~ Further study is required to identify and determine the stability of all the adsorbed species. 4. CONCLUSION Au/metai oxides are active for low temperature CO oxidation. The activity of the catalysts is very sensitive to catalyst preparation. All the Au/metal oxides tested for room temperature CO oxidation deactivated substantially with time. The deactivation
433
0.04 -
g
o~o=""~176176,,,=~oo~
o~o=""~176176
t'~ I~ ~ i . .
0.08 o c .Q o 0.02.o <
T " 25~ flush In air
,
60 min
!
I,,,,
9
40 mln
~t
20 min
0.01 -
10 min
"I~O/N=
.
0.00
~ . . 2800
5 rain l . . . 2500 2200
t --0
. . 1900
Wavenumber Figure 5
. 1600
1800
'
{om"1}
In situ FTIR spectra of CO adsorption on 1%Au/mio 2.
increased in the presence of CO2 in the feed. The stability of the catalyst decreased with an increase in the basicity of the metal oxide. In situ FTIR studies suggest that adsorbed carboxylate or carbonate species on the catalyst surface could be inhibiting the room temperature CO oxidation reaction. A C KNOWLE DG EM ENTS The authors gratefully acknowledge support from NASA Marshall Space Flight Center under Contract No. NAS8-39344.
REFERENCES o
.
.
M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet and B. Delmon, J. Catal., 144 (1993) 175. S.D. Gardner, G.B. Hoflund, B.T. Upchurch, D.R. Schryer, E.J. Kielin, E. J. and J. Schryer, J. Catal., 129 (1991) 114. S.D. Lin, M. Bollinger, and M.A. Vannice, Catai. Lett. 17 (1983) 245.