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Al2O3 catalyst
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APPLIED CATALYSS I A:GENERAL
Applied Catalysis A: General 151 (1997) 179-191
ELSEVIER
Oxidation of volatile organic compounds on a
Ag/A1203 catalyst Eric M. Cordi, John L. Falconer * Department of Chemical Engineering, Universi~. of Colorado, Boulder, CO 80309-0424, USA
Received 24 September 1995; revised 5 April 1996; accepted 12 July 1996
Abstract
Temperature-programmed desorption (TPD) and oxidation (TPO) were used to study decomposition and oxidation of methanol, ethanol, acetaldehyde, formic acid, and acetic acid on an Ag/AlzO 3 catalyst. The volatile organic compounds (VOCs) were adsorbed on the alumina support. Silver was inactive for decomposition of adsorbed VOCs; all dehydration and dehydrogenation products evolved from reaction on the A120 3 support during TPD. Silver was active for oxidation of VOCs during TPO, however, and a portion of the VOCs reacted on A120 3 sites in parallel. Adsorbed VOCs reacted at silver sites with adsorbed oxygen; oxygen was supplied to silver sites from the gas phase, and surface diffusion on A1203 transported adsorbed VOCs to the silver sites. Adsorbed oxygen also activated neighboring silver sites for alcohol dehydrogenation in parallel with oxidation. Dehydrogenation was not observed for the aldehyde and acids. Keywords: Temperature-programmed desorption; Silver/alumina; Temperature-programmed oxidation; Volatile organic compounds
1. Introduction
Fundamental knowledge of catalytic oxidation mechanisms can be valuable for developing effective methods for using catalysts to control air pollution. Catalytic incineration of volatile organic compounds (VOCs) is preferred over thermal incineration for several reasons. A thermal incinerator operates at higher temperatures (973-1473 K) than a catalytic unit (673-773 K), and thermal incineration must often be supplemented with fuel to maintain a flame in a dilute VOC waste stream (less than 1000 ppm). The objective of this study is to *
Correspondingauthor, e-mail [email protected]
0926-860X/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PH S0926-860X(96)00264-5
180
E.M. Cordi, J.L. Falconer~Applied Catalysis A: General 151 (1997) 179-191
compare decomposition and oxidation of VOCs on Ag/A1203. Though silver is not the best oxidation catalyst, this study provides a useful comparison to catalysts that are more active. Five VOCs were used in this study: methanol, ethanol, acetaldehyde, formic acid, and acetic acid. Methanol and ethanol are commonly emitted from industrial solvent processes and as unburned fuel from mobile sources. Acetaldehyde is a partial oxidation product that is often emitted from ethanol-fueled engines. Formic acid and acetic acid are partial oxidation products of methanol and ethanol, respectively. Temperature-programmed desorption and oxidation (TPD, TPO) were used for this study of VOC oxidation because they have advantages for studying supported metal catalysts. Reaction steps are separated in time by their relative rates, and reactants are pre-adsorbed on the catalyst, so adsorption is separated from the reaction process. All the VOCs used in this study adsorb on AI203 at room temperature. Thus, TPD and TPO were also carried out on A1203 [1] to determine the contribution of A1203 to the VOC oxidation mechanisms on Ag/A1203. Ethylene oxidation has been the reaction most often studied on silver, which is the only metal that selectively catalyzes the epoxidation to ethylene oxide [2]. As a result, extensive work has been performed to establish the interaction of oxygen with silver surfaces [2-13]. The presence of adsorbed diatomic oxygen at reaction conditions is in dispute, but it is well known that 02 dissociates on silver above room temperature and is adsorbed as an O - species [4,5]. The O species is reported to be the most active of adsorbed oxygen forms. Furthermore, the reactivity of oxygen results from its relatively weak bond with silver compared to other metals [5]. The stoichiometry of adsorbed O - on silver is one, and above oxygen monolayer coverage (obtained at 443 K) [6], small amounts of oxygen are incorporated into the silver lattice. Oxygen is one of few gases that adsorb on clean silver [8]. An intriguing aspect of a silver surface is its ability to strongly adsorb other gases once the surface contains adsorbed oxygen. Anderson et al. [9] measured the heats of adsorption of 02, C2H 4, and C4H 6 (1,3-butadiene) on Ag/A1203 and found that the alkenes adsorbed stronger on oxygen-covered silver crystallites than on clean silver at 300 and 443 K. According to Khasin [4] and Goncharova et al. [10], adsorbed oxygen activates neighboring silver sites for C2H 4 adsorption. Therefore, C2H 4 oxidation at steady state is a Langmuir-Hinshelwood process that is dependent upon 02 adsorption as the first step.
2. Experimental Temperature-programmed desorption (TPD) and oxidation (TPO) were performed on a 2.1% Ag/A1203 catalyst. The organic compounds under consideration were adsorbed on the catalyst at room temperature in He flow. After
E.M. Cordi, J,L. Falconer/Applied Catalysis A: General 151 (1997)179-191
181
adsorption of an organic compound, the catalyst temperature was ramped at 1 K / s . During TPD, pure He was used as a flow gas for desorbed products, and (3% O2)/(1% Ar)/(96% He) was used during TPO. The flow system has been described previously [ 14]. The catalytic reactor consisted of a 1-cm-OD quartz tube with a 0.5-mm-OD chromel-alumel thermocouple placed in the center of a bed of 60-80 mesh catalyst particles (25 mg). The thermocouple measured temperature and provided feedback to the temperature programmer, which regulated heating of the electric furnace. A Balzers quadrupole mass spectrometer detected products immediately downstream of the reactor as they were desorbed from the catalyst, and a computer allowed multiple signals and the thermocouple output to be recorded simultaneously. The VOCs were adsorbed on A1203 and A g / A I 2 0 3 by injecting 0.5 txL samples of the liquid into He flow. The liquid evaporated from the side of the quartz tube, so only vapor contacted the catalyst bed. Carbon monoxide was adsorbed on silver by injecting 0.15 cm 3 (STP) pulses of a 10% C O / H e mixture twice each minute into He flow for a 30-min period to saturate the catalyst. Each reactant and product was calibrated by injecting a measured amount of each into the flow gas stream between the reactor and mass spectrometer. Extensive corrections were made for cracking in the mass spectrometer to obtain the final plots of product signals versus temperature. Water that formed during TPD and TPO is not shown in the figures. Since H20 readsorbs on the A1203 support, the appearance of H 2 0 in the gas phase is limited by desorption. Therefore, the H 2 0 spectra provide little information about the reaction mechanism, except for the fact that H 2 0 is a reaction product. By not including H zO, the other products are easier to observe in the figures. The 2.1% A g / A I 2 0 3 was prepared using the method of Plummer et al. [15]. An incipient wetness technique was used to contact a solution of AgC1 dissolved in NH4OH with the A1203 support. The catalyst was vacuum dried at 373 K for 8 h, and then reduced in flowing 3% H 2 for 4 h at 773 K. Before each use, the catalyst was pretreated in flowing 3% 02 at 773 K for 30 min to remove water and contaminants. Finally, the catalyst was reduced in flowing H 2 for 15 min at 573 K, before switching to He and raising the temperature to 773 K to remove any adsorbed H 2 and H20. The weight loading of silver in the catalyst was determined by atomic absorption spectroscopy.
3. Results and discussion
3.1. Temperature-programmed desorption (TPD) The same TPD spectra were observed for Ag/A1203 as for A1203 [1,16] for all the VOCs; i.e., the addition of silver to A1203 has no effect on the
E.M. Cordi, J.L. Falconer~Applied Catalysis A: General 151 (1997) 179-191
182
H2 2.5
2
CO
1.5 6 E
(CH3)20
1
rr 0.5
0
-
300
,
,
400
i
500
i
600
f
700
i
800
900
Temperature (K)
Fig. l. TPD of methanol on Ag/AI203.
decomposition activity, and all reaction was on the A1203 surface. The VOCs were adsorbed almost exclusively on the A1203, whose surface area is much larger than the silver surface area, and thus decomposition on the silver might be difficult to detect. For Pd or Ni supported on the same alumina, however, decomposition on the metal was readily detected during TPD [ 1,16]. Alumina is both a dehydration and dehydrogenation catalyst for the VOCs [16]. As shown in Fig. 1, methanol decomposed by both dehydration and dehydrogenation on Ag/A1203. Less than 20% of the CH3OH desorbed intact from the catalyst; most dehydrogenated to CO and H 2 between 600 K and 900 K. Dimethyl ether and H 2 0 desorbed at much lower temperature, and a small amount of CO 2 formed above 700 K. In contrast, dehydration was the dominant decomposition pathway for ethanol. As shown in Fig. 2, more than 60% of adsorbed CzH5OH dehydrated to C2H 4 and H20. The C2H 4 peak was centered at 630 K. Simultaneously, dehydrogenation formed acetaldehyde and H 2, but the amount of H 2 detected was three times that expected from C2H40 formation. The additional H 2 was from dehydrogenation to form surface carbon, and approximately 1 / 3 of the original carbon in ethanol remained on the surface at 873 K. Adsorbed acetaldehyde decomposed the slowest of the five VOCs on Ag/A1203. The only gas-phase products were CO, CO z, and H 2, and each was detected over a wide temperature range, as shown in Fig. 3. More than two-thirds of the carbon in C z H 4 0 remained on the surface at the end of TPD to 873 K.
183
E.M. Cordi, J.L. Falconer/Applied Catalysis A: General 151 (1997)179-191 2
H4
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e3 (3
1
"-6 E t~ rr 0.5
0
,
300
400
,
500
,
600
i
A--
700
800
900
Temperature (K) Fig. 2. TPD of ethanol on Ag/AI203.
Formic acid dehydrated to CO and H20 on Ag/A1203 (Fig. 4) as it did on A1203. The CO peak had a maximum at 610 K, whereas H20 desorbed slowly until heating was stopped at 873 K. A small fraction of the HCOOH dehydrogenated to CO 2 and H E, which formed simultaneously with CO. Acetic acid
0.5
0.4CO
>, to
0.3
0.2
002
0.1
300
400
500
600
700
Temperature (K)
Fig. 3. TPD of acetaldehyde on Ag/AI203.
800
900
184
E.M. Cordi, J.L. Falconer/Applied Catalysis A: General 151 (1997) 179-191 5
(~ 3
2
rr
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-'
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i
r
i
400
r
,
500
600
,
700
i
900
800
Temperature (K)
Fig. 4. TPD of formic acid on Ag/A1203.
decomposed much slower than HCOOH, and reaction was not complete by 873 K, when heating was stopped. As shown in Fig. 5, products were only detected above 600 K. The carbon-carbon bond was cleaved to form CO and CO 2 from the a-carbon, and some of the resulting C H 3 groups were hydrogenated to C H 4.
1.2
1]
~ C O
0.8 ~
0.6
~
0.4
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300
[
i
400
500
J
r
i
600
700
Temperature (K) Fig. 5. TPD of acetic acid on Ag/AI203.
800
900
E.M. Co rdi, J.L. Falconer~Applied Catalysis A: General 151 (1997)179-191
185
The C H 4 amount was much smaller than the CO and C02, however, because the rest of the [3-carbon remained on the surface. The increasing rate of formation of H 2 up to 873 K most likely corresponds to continued dehydrogenation of CH 3 to surface carbon and hydrogen. The addition of palladium [1] or nickel [16] to A1203 dramatically increases the rate of VOC decomposition, but silver does not. Silver is expected to be inactive for decomposition, however, since Barteau and Madix [8] reported that very few gases or vapors adsorb on clean silver. On P d / A I 2 0 3 and N i / A I 2 0 3 , VOCs adsorbed on A1203 and diffused along the surface to the active Pd and Ni sites. On Ag/A1203, VOCs also adsorbed on A1203, and the rate of surface diffusion is expected to be the same as on N i / A I 2 0 3 or Pd/A1203, but the VOCs did not react with the silver crystallites. As a result, all the VOCs decomposed on A1203. Thus, diffusion on the alumina surface does not appear to be limiting the rate of decomposition on silver.
3.2. Temperature-programmed oxidation (TPO) The TPO spectra were quite different from the TPD spectra, and they show that in the presence of oxygen, Ag/A1203 is both a decomposition and an oxidation catalyst for adsorbed VOCs. Both decomposition and oxidation rates are higher on A g / A I 2 0 3 than on A1203 [1]. As shown in Fig. 6, the dehydration rate of methanol was almost unchanged in the presence of O 2 , and dimethyl ether formed at the same temperature and in the same amount as during TPD. The dehydrogenation rate increased dramati-
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113
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1
0.5
0
i
3O0
i
400
-
500
i
600
Temperature (K) Fig. 6. TPO of methanol on Ag/AI203.
700
800
186
E.M. Cordi, J.L. Falconer/Applied Catalysis A: General 151 (1997) 179-191
cally in the presence of O 2, however, and H 2 formed in a peak at 610 K, which is 170 K lower than during TPD. The amount of H z is much smaller because it oxidizes to form water, which desorbed over a wide temperature range. The amount of CO from dehydrogenation was also much smaller than during TPD because of CO oxidation to CO 2. The CO also formed at a lower temperature than during TPD. The CO was at a higher temperature than H 2 during TPO, whereas during TPD on Ag/A1203, CO and H 2 formed at the same temperature. Apparently H 2 is more weakly held by the catalyst, causing it to desorb at lower temperatures than CO. Khasin [4] noted that silver is inert toward H z adsorption, and thus H 2 probably desorbs as soon as it forms. Because CO is more strongly bound and remains on the surface to higher temperature, more of it is oxidized to CO 2. Since dehydrogenation was faster in the presence of O 2, and since dehydrogenation was faster on A g / A I 2 0 3 than on A1203, dehydrogenation was catalyzed by the silver crystallites. Since methanol adsorbs on the alumina surface, the methanol diffuses across the AlzO 3 to the silver, as has been observed for methanol on Ni/AI203 and Pd/A1203 [1,16]. Adsorbed oxygen on silver apparently creates activated silver sites that adsorb VOCs, as suggested by Khasin [4] for the oxidation of ethylene on silver. These sites than catalyze CH3OH dehydrogenation. Oxidation of methanol or methanol decomposition products only took place on silver at relatively high temperatures, and therefore, dehydration at A1203 sites occurred in parallel with oxidation of adsorbed CH3OH. Similar changes took place for ethanol on A g / A I 2 0 3 during TPO. The TPO spectra for adsorbed ethanol, which are shown in Fig. 7, are quite different from the ethanol TPD spectra. Hydrogen started to form at lower temperature, showing that in the presence of oxygen, silver was more active for dehydrogenation. More acetaldehyde formed during TPO than during TPD, but the amount of H a was unchanged. The additional acetaldehyde could result from a higher dehydrogenation rate or partial oxidation of ethanol. The amount of ethylene from C2HsOH dehydration on AlzO 3 was significantly decreased during TPO, but was detected in the same temperature range as during TPD. No other carbon-containing product formed at lower temperature, but CO and CO 2 formed at higher temperature than ethylene. Either ethylene was partially oxidized before it could desorb or ethanol was partially oxidized. The resulting species was then oxidized to CO and CO 2 at elevated temperatures, as shown in Fig. 7. During TPD, formation of gas phase species was complete by 750 K, but the formation of m o r e H 2 than expected from the dehydrogenation reaction to form acetaldehyde indicated that carbon was deposited on the surface. During TPO, both CO and CO 2 continued to form up to 873 K, where heating was stopped. Apparently the carboneous species that formed as a result of dehydrogenation of ethanol was subsequently oxidized at high temperature to CO and CO 2. It is
E.M. Cordi, J.L. Falconer~Applied Catalysis A: General 151 (1997)179-191
187
4
J 03
-6 E v
0 300
400
500
600
700
800
900
Temperature (K) Fig. 7. TPO of ethanol on Ag/A1203.
possible, however, that a partial oxidation product of C2HsOH formed during TPO, and it was then oxidized at highest temperature. As shown below, possible partial oxidation products like acetaldehyde and acetic acid were oxidized to CO 2 and CO slowly during TPO. Since CO 2 and H20 formed at lower temperatures on Ag/A1203 than on AI203, oxygen adsorbed on silver was active for oxidation. Oxygen is one of the few gases that strongly adsorb on clean silver [8], and many studies have sought to establish its adsorbed form [3-13]. According to Khasin [4] and Barteau and Madix [8], adsorbed oxygen allows other gases, such as C2H 4, to adsorb on activated silver sites. Ethylene adsorption on oxygen-covered silver is often studied because silver is the only metal known to catalyze the direct epoxidation of ethylene to ethylene oxide [2]. Thus, ethylene may have formed by dehydration on A1203 during TPO, but then adsorbed on the silver and been oxidized or partially oxidized before it could desorb. In contrast, during TPD the ethylene did not adsorb on silver in the absence of oxygen, and thus ethylene appeared in the gas phase. As seen in Fig. 8, acetaldehyde was oxidized to CO 2 and CO, but reaction was not complete by 773 K, when heating was stopped, and the rates of CO 2 and CO formation were still high at 773 K. Water also formed, but is not shown in the figure. A comparison to the TPD spectra in Fig. 3 shows that acetaldehyde was more reactive during TPO than during TPD. Note the difference in scales in the two figures; CO 2 formed at rates 10 times greater during TPO than TPD. Slightly less CO formed in TPO, and a small amount of unreacted C 2H 4°
188
E.M. CordL J.L. Falconer/Applied Catalysis A: General 151 (1997) 179-191 4
3
g t~ t~ o)
2
"5 E
#1
02H40
f
0 300
400
500 600 Temperature (K)
700
800
Fig. 8. TPO of acetaldehyde on A g / A I ~ O 3.
was detected, but no H2 was seen during TPO. Apparently any H e was oxidized at the high temperature where it formed. Since the rate of acetaldehyde decomposition was so low during TPD, silver either increased the rate of decomposition in the presence of oxygen, and the reaction products were oxidized, or the oxidation rate was increased directly. Acetaldehyde adsorbed strongly on A1203 so that little decomposed during TPD on either AlzO 3 or Ag/A1203. Oxygen increased the rate of product evolution on both catalysts, but to a greater extent on A g / A I e O 3. The rate of product formation during TPO of formic acid (Fig. 9) did not change significantly from that during TPD. The peak temperature for CO 2 formation was essentially unchanged. The main difference was that much more CO 2 formed and less CO formed. About 35% of the carbon in HCOOH was converted to CO e. Water was produced as a result of dehydration, and it desorbed slowly. Since the H 2 formation rate did not increase, the Ag/A1203 catalyst was relatively inactive for formic acid dehydrogenation, even in the presence of oxygen. Instead it appears that during TPO, formic acid dehydrated and the CO product were then oxidized to CO 2. Even thought acetic acid decomposed slowly during TPD on Ag/A1203, the presence of 02 did not significantly decrease the temperature where products formed. As shown in Fig. 10, the main product during TPO was CO e, much smaller amounts of CO and CH 4 formed, and no H 2 was detected. In addition, water was observed but is not shown in the figure. Apparently carbon-carbon bond cleavage was not increased by O 2, and instead, products of decomposition
E.M. Cordi, J.L. Falconer~Applied Catalysis A: General 151 (1997)179-191
2.5
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189
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2
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0
.
300
.
.
.
.
.
.
.
.
.
.
.
400
-
-
-
- -
500
600
700
800
Temperature (K) Fig. 9. TPO of formic
acid on
Ag/AlzO 3.
were subsequently oxidized during TPO. In addition t o H 2 and C H 4, the [3-carbon that remained on the surface during TPD was also oxidized during TPO. Carbon dioxide formed at the same temperature on A1203 and A g / A I 2 0 3 , so it is likely that reaction took place on both silver and alumina sites in parallel.
th
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~_ 2 t~
rr
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400
500
600
Temperature (K) Fig. 10. TPO of acetic
acid on
Ag/AI203.
700
800
E,M. CordL J.L. Falconer/Applied Catalysis A: General 151 (1997) 179-191
190
0.25
0.2
0.15 ¢D
E
0.1
0.05
I
200
[
300
]
400
I
500
I
600
I
700
800
Temperature (K) Fig. l I. TPO of carbon monoxide on Ag/A1203.
Since CO does not adsorb on alumina, CO that adsorbs on Ag/A1203 must be on the silver surface. During TPO of adsorbed CO, all the CO was oxidized to CO 2, as shown in Fig. 11. Though oxidation began at room temperature, the rate of oxidation was slow and CO 2 continued to form until heating was stopped. The rate of CO desorption was slower than oxidation since no unreacted CO was detected. Monolayer coverage cannot be obtained with CO, and this is a reason that oxygen chemisorption is a preferred method of measuring silver surface area [11]. Adsorption of oxygen is an activated process up to monolayer coverage at 443 K [6], and this may be why the oxidation rate is slow relative to CO oxidation on Pd/A1203 for example. Meima et al. [7] studied CO oxidation on Ag/A1203 and observed a drop in oxidation activity above 473 K as multicrystalline silver converted to Ag(111) facets. The main purpose of studying CO oxidation is to show that CO oxidizes faster than any of the adsorbed VOCs, and thus the CO 2 formation rates during TPO are not limited by CO oxidation. Thus, when CO formed at much higher temperature on A g / A I 2 0 3 , in most cases it was CO from AI203, since CO on Ag would be expected to oxidize at these temperatures.
4. Conclusions Silver added to alumina does not increase the rate of VOC decomposition. None of the VOCs decompose on silver during TPD, but they all react on the
E.M. Cordi, J.L, Falconer/Applied Catalysis A: General 151 (1997)179-191
191
AI203 support by dehydration and dehydrogenation. The Ag/A1203 catalyst is more active than A1203 for oxidation to CO 2 and H20, however. Adsorbed VOCs diffuse along the A1203 surface and react at the silver sites, where oxygen adsorbs. Because oxidation is at high temperatures on silver, VOCs react in parallel on silver and alumina sites. Based upon the largest CO 2 peak observed during TPO, the relative oxidation reactivities are HCOOH > CH3OH > CH3COOH > C2HsOH > C2H40 Oxygen adsorption on silver activates neighboring silver sites for alcohol dehydrogenation. Methanol and ethanol dehydrogenate in parallel with oxidation on the silver sites, but the aldehyde and acids do not.
Acknowledgements Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. We also gratefully acknowledge partial support by the National Science Foundation, Grant CTS-90-21194.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
E.M. Cordi and J.L. Falconer, J. Catal., 162 (1996) 104. Y. Yong and N.W. Cant, J. Catal., 122 (1990) 22. D.I. Kondarides and X.E. Verykios, J. Catal., 143 (1993) 481. A.V. Khasin, Kin. and Catal., 34 (1993) 42. S. Imamura, M. Ikebata, T. Ito and T. Ogita, Ind. Eng. Chem. Res., 30 (1991) 217. G.R. Meima, R.J. Vis, M.G.J. van Leur, A.J. van Dillen, J.W. Geus and F.R. van Buren, J. Chem. Soc., Faraday Trans. I, 85 (1989) 279. G.R. Meima, R.J. Vis, M.G.J. van Leur, A.J. van Dillen, J.W. Gues and F.R. van Buren, J. Chem. Soc., Faraday Trans. I, 85 (1989) 293. M.A. Barteau and R.J. Madix, The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Fundamental Studies of Heterogeneous Catalysis, Elsevier, Amsterdam, 1981. K.L. Anderson, J.K. Plischke and M.A. Vannice, J. Catal., 128 (1991) 148. S.N. Goncharova, A.V. Khasin, S.N. Filimonova and D.A. Bulushev, Kin. and Catal., 32 (1991) 768. S.R. Seyedmonir, D.E. Strohmayer, G.J. Guskey, G.L. Geoffroy and M~A. Vannice, J. Catal., 93 (1985) 288. G.R. Meima, M. Hasselaar, A.J. van Dillen, F.R. van Buren and J.W. Geus, J. Chem. Soc., Faraday Trans. I, 5 (1989) 1267. S.R. Seyedmonir, J.K. Plischke, M.A. Vannice and H.W. Young, J. Catal., 123 (1990) 534. J.L. Falconer and J.A. Schwarz, Catal. Rev.-Sci. Eng., 25 (1983) 141. H.K. Plummer, Jr., W.L.H. Watkins and H.S. Gandhi, App. Catal., 29 (1987) 261. B. Chert and J.L. Falconer, J. Catal., 144 (1993) 214.
APPLIED CATALYSS I A:GENERAL
Applied Catalysis A: General 151 (1997) 179-191
ELSEVIER
Oxidation of volatile organic compounds on a
Ag/A1203 catalyst Eric M. Cordi, John L. Falconer * Department of Chemical Engineering, Universi~. of Colorado, Boulder, CO 80309-0424, USA
Received 24 September 1995; revised 5 April 1996; accepted 12 July 1996
Abstract
Temperature-programmed desorption (TPD) and oxidation (TPO) were used to study decomposition and oxidation of methanol, ethanol, acetaldehyde, formic acid, and acetic acid on an Ag/AlzO 3 catalyst. The volatile organic compounds (VOCs) were adsorbed on the alumina support. Silver was inactive for decomposition of adsorbed VOCs; all dehydration and dehydrogenation products evolved from reaction on the A120 3 support during TPD. Silver was active for oxidation of VOCs during TPO, however, and a portion of the VOCs reacted on A120 3 sites in parallel. Adsorbed VOCs reacted at silver sites with adsorbed oxygen; oxygen was supplied to silver sites from the gas phase, and surface diffusion on A1203 transported adsorbed VOCs to the silver sites. Adsorbed oxygen also activated neighboring silver sites for alcohol dehydrogenation in parallel with oxidation. Dehydrogenation was not observed for the aldehyde and acids. Keywords: Temperature-programmed desorption; Silver/alumina; Temperature-programmed oxidation; Volatile organic compounds
1. Introduction
Fundamental knowledge of catalytic oxidation mechanisms can be valuable for developing effective methods for using catalysts to control air pollution. Catalytic incineration of volatile organic compounds (VOCs) is preferred over thermal incineration for several reasons. A thermal incinerator operates at higher temperatures (973-1473 K) than a catalytic unit (673-773 K), and thermal incineration must often be supplemented with fuel to maintain a flame in a dilute VOC waste stream (less than 1000 ppm). The objective of this study is to *
Correspondingauthor, e-mail [email protected]
0926-860X/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PH S0926-860X(96)00264-5
180
E.M. Cordi, J.L. Falconer~Applied Catalysis A: General 151 (1997) 179-191
compare decomposition and oxidation of VOCs on Ag/A1203. Though silver is not the best oxidation catalyst, this study provides a useful comparison to catalysts that are more active. Five VOCs were used in this study: methanol, ethanol, acetaldehyde, formic acid, and acetic acid. Methanol and ethanol are commonly emitted from industrial solvent processes and as unburned fuel from mobile sources. Acetaldehyde is a partial oxidation product that is often emitted from ethanol-fueled engines. Formic acid and acetic acid are partial oxidation products of methanol and ethanol, respectively. Temperature-programmed desorption and oxidation (TPD, TPO) were used for this study of VOC oxidation because they have advantages for studying supported metal catalysts. Reaction steps are separated in time by their relative rates, and reactants are pre-adsorbed on the catalyst, so adsorption is separated from the reaction process. All the VOCs used in this study adsorb on AI203 at room temperature. Thus, TPD and TPO were also carried out on A1203 [1] to determine the contribution of A1203 to the VOC oxidation mechanisms on Ag/A1203. Ethylene oxidation has been the reaction most often studied on silver, which is the only metal that selectively catalyzes the epoxidation to ethylene oxide [2]. As a result, extensive work has been performed to establish the interaction of oxygen with silver surfaces [2-13]. The presence of adsorbed diatomic oxygen at reaction conditions is in dispute, but it is well known that 02 dissociates on silver above room temperature and is adsorbed as an O - species [4,5]. The O species is reported to be the most active of adsorbed oxygen forms. Furthermore, the reactivity of oxygen results from its relatively weak bond with silver compared to other metals [5]. The stoichiometry of adsorbed O - on silver is one, and above oxygen monolayer coverage (obtained at 443 K) [6], small amounts of oxygen are incorporated into the silver lattice. Oxygen is one of few gases that adsorb on clean silver [8]. An intriguing aspect of a silver surface is its ability to strongly adsorb other gases once the surface contains adsorbed oxygen. Anderson et al. [9] measured the heats of adsorption of 02, C2H 4, and C4H 6 (1,3-butadiene) on Ag/A1203 and found that the alkenes adsorbed stronger on oxygen-covered silver crystallites than on clean silver at 300 and 443 K. According to Khasin [4] and Goncharova et al. [10], adsorbed oxygen activates neighboring silver sites for C2H 4 adsorption. Therefore, C2H 4 oxidation at steady state is a Langmuir-Hinshelwood process that is dependent upon 02 adsorption as the first step.
2. Experimental Temperature-programmed desorption (TPD) and oxidation (TPO) were performed on a 2.1% Ag/A1203 catalyst. The organic compounds under consideration were adsorbed on the catalyst at room temperature in He flow. After
E.M. Cordi, J,L. Falconer/Applied Catalysis A: General 151 (1997)179-191
181
adsorption of an organic compound, the catalyst temperature was ramped at 1 K / s . During TPD, pure He was used as a flow gas for desorbed products, and (3% O2)/(1% Ar)/(96% He) was used during TPO. The flow system has been described previously [ 14]. The catalytic reactor consisted of a 1-cm-OD quartz tube with a 0.5-mm-OD chromel-alumel thermocouple placed in the center of a bed of 60-80 mesh catalyst particles (25 mg). The thermocouple measured temperature and provided feedback to the temperature programmer, which regulated heating of the electric furnace. A Balzers quadrupole mass spectrometer detected products immediately downstream of the reactor as they were desorbed from the catalyst, and a computer allowed multiple signals and the thermocouple output to be recorded simultaneously. The VOCs were adsorbed on A1203 and A g / A I 2 0 3 by injecting 0.5 txL samples of the liquid into He flow. The liquid evaporated from the side of the quartz tube, so only vapor contacted the catalyst bed. Carbon monoxide was adsorbed on silver by injecting 0.15 cm 3 (STP) pulses of a 10% C O / H e mixture twice each minute into He flow for a 30-min period to saturate the catalyst. Each reactant and product was calibrated by injecting a measured amount of each into the flow gas stream between the reactor and mass spectrometer. Extensive corrections were made for cracking in the mass spectrometer to obtain the final plots of product signals versus temperature. Water that formed during TPD and TPO is not shown in the figures. Since H20 readsorbs on the A1203 support, the appearance of H 2 0 in the gas phase is limited by desorption. Therefore, the H 2 0 spectra provide little information about the reaction mechanism, except for the fact that H 2 0 is a reaction product. By not including H zO, the other products are easier to observe in the figures. The 2.1% A g / A I 2 0 3 was prepared using the method of Plummer et al. [15]. An incipient wetness technique was used to contact a solution of AgC1 dissolved in NH4OH with the A1203 support. The catalyst was vacuum dried at 373 K for 8 h, and then reduced in flowing 3% H 2 for 4 h at 773 K. Before each use, the catalyst was pretreated in flowing 3% 02 at 773 K for 30 min to remove water and contaminants. Finally, the catalyst was reduced in flowing H 2 for 15 min at 573 K, before switching to He and raising the temperature to 773 K to remove any adsorbed H 2 and H20. The weight loading of silver in the catalyst was determined by atomic absorption spectroscopy.
3. Results and discussion
3.1. Temperature-programmed desorption (TPD) The same TPD spectra were observed for Ag/A1203 as for A1203 [1,16] for all the VOCs; i.e., the addition of silver to A1203 has no effect on the
E.M. Cordi, J.L. Falconer~Applied Catalysis A: General 151 (1997) 179-191
182
H2 2.5
2
CO
1.5 6 E
(CH3)20
1
rr 0.5
0
-
300
,
,
400
i
500
i
600
f
700
i
800
900
Temperature (K)
Fig. l. TPD of methanol on Ag/AI203.
decomposition activity, and all reaction was on the A1203 surface. The VOCs were adsorbed almost exclusively on the A1203, whose surface area is much larger than the silver surface area, and thus decomposition on the silver might be difficult to detect. For Pd or Ni supported on the same alumina, however, decomposition on the metal was readily detected during TPD [ 1,16]. Alumina is both a dehydration and dehydrogenation catalyst for the VOCs [16]. As shown in Fig. 1, methanol decomposed by both dehydration and dehydrogenation on Ag/A1203. Less than 20% of the CH3OH desorbed intact from the catalyst; most dehydrogenated to CO and H 2 between 600 K and 900 K. Dimethyl ether and H 2 0 desorbed at much lower temperature, and a small amount of CO 2 formed above 700 K. In contrast, dehydration was the dominant decomposition pathway for ethanol. As shown in Fig. 2, more than 60% of adsorbed CzH5OH dehydrated to C2H 4 and H20. The C2H 4 peak was centered at 630 K. Simultaneously, dehydrogenation formed acetaldehyde and H 2, but the amount of H 2 detected was three times that expected from C2H40 formation. The additional H 2 was from dehydrogenation to form surface carbon, and approximately 1 / 3 of the original carbon in ethanol remained on the surface at 873 K. Adsorbed acetaldehyde decomposed the slowest of the five VOCs on Ag/A1203. The only gas-phase products were CO, CO z, and H 2, and each was detected over a wide temperature range, as shown in Fig. 3. More than two-thirds of the carbon in C z H 4 0 remained on the surface at the end of TPD to 873 K.
183
E.M. Cordi, J.L. Falconer/Applied Catalysis A: General 151 (1997)179-191 2
H4
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e3 (3
1
"-6 E t~ rr 0.5
0
,
300
400
,
500
,
600
i
A--
700
800
900
Temperature (K) Fig. 2. TPD of ethanol on Ag/AI203.
Formic acid dehydrated to CO and H20 on Ag/A1203 (Fig. 4) as it did on A1203. The CO peak had a maximum at 610 K, whereas H20 desorbed slowly until heating was stopped at 873 K. A small fraction of the HCOOH dehydrogenated to CO 2 and H E, which formed simultaneously with CO. Acetic acid
0.5
0.4CO
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0.3
0.2
002
0.1
300
400
500
600
700
Temperature (K)
Fig. 3. TPD of acetaldehyde on Ag/AI203.
800
900
184
E.M. Cordi, J.L. Falconer/Applied Catalysis A: General 151 (1997) 179-191 5
(~ 3
2
rr
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-'
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i
r
i
400
r
,
500
600
,
700
i
900
800
Temperature (K)
Fig. 4. TPD of formic acid on Ag/A1203.
decomposed much slower than HCOOH, and reaction was not complete by 873 K, when heating was stopped. As shown in Fig. 5, products were only detected above 600 K. The carbon-carbon bond was cleaved to form CO and CO 2 from the a-carbon, and some of the resulting C H 3 groups were hydrogenated to C H 4.
1.2
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~ C O
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0.6
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300
[
i
400
500
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i
600
700
Temperature (K) Fig. 5. TPD of acetic acid on Ag/AI203.
800
900
E.M. Co rdi, J.L. Falconer~Applied Catalysis A: General 151 (1997)179-191
185
The C H 4 amount was much smaller than the CO and C02, however, because the rest of the [3-carbon remained on the surface. The increasing rate of formation of H 2 up to 873 K most likely corresponds to continued dehydrogenation of CH 3 to surface carbon and hydrogen. The addition of palladium [1] or nickel [16] to A1203 dramatically increases the rate of VOC decomposition, but silver does not. Silver is expected to be inactive for decomposition, however, since Barteau and Madix [8] reported that very few gases or vapors adsorb on clean silver. On P d / A I 2 0 3 and N i / A I 2 0 3 , VOCs adsorbed on A1203 and diffused along the surface to the active Pd and Ni sites. On Ag/A1203, VOCs also adsorbed on A1203, and the rate of surface diffusion is expected to be the same as on N i / A I 2 0 3 or Pd/A1203, but the VOCs did not react with the silver crystallites. As a result, all the VOCs decomposed on A1203. Thus, diffusion on the alumina surface does not appear to be limiting the rate of decomposition on silver.
3.2. Temperature-programmed oxidation (TPO) The TPO spectra were quite different from the TPD spectra, and they show that in the presence of oxygen, Ag/A1203 is both a decomposition and an oxidation catalyst for adsorbed VOCs. Both decomposition and oxidation rates are higher on A g / A I 2 0 3 than on A1203 [1]. As shown in Fig. 6, the dehydration rate of methanol was almost unchanged in the presence of O 2 , and dimethyl ether formed at the same temperature and in the same amount as during TPD. The dehydrogenation rate increased dramati-
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1
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0
i
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i
400
-
500
i
600
Temperature (K) Fig. 6. TPO of methanol on Ag/AI203.
700
800
186
E.M. Cordi, J.L. Falconer/Applied Catalysis A: General 151 (1997) 179-191
cally in the presence of O 2, however, and H 2 formed in a peak at 610 K, which is 170 K lower than during TPD. The amount of H z is much smaller because it oxidizes to form water, which desorbed over a wide temperature range. The amount of CO from dehydrogenation was also much smaller than during TPD because of CO oxidation to CO 2. The CO also formed at a lower temperature than during TPD. The CO was at a higher temperature than H 2 during TPO, whereas during TPD on Ag/A1203, CO and H 2 formed at the same temperature. Apparently H 2 is more weakly held by the catalyst, causing it to desorb at lower temperatures than CO. Khasin [4] noted that silver is inert toward H z adsorption, and thus H 2 probably desorbs as soon as it forms. Because CO is more strongly bound and remains on the surface to higher temperature, more of it is oxidized to CO 2. Since dehydrogenation was faster in the presence of O 2, and since dehydrogenation was faster on A g / A I 2 0 3 than on A1203, dehydrogenation was catalyzed by the silver crystallites. Since methanol adsorbs on the alumina surface, the methanol diffuses across the AlzO 3 to the silver, as has been observed for methanol on Ni/AI203 and Pd/A1203 [1,16]. Adsorbed oxygen on silver apparently creates activated silver sites that adsorb VOCs, as suggested by Khasin [4] for the oxidation of ethylene on silver. These sites than catalyze CH3OH dehydrogenation. Oxidation of methanol or methanol decomposition products only took place on silver at relatively high temperatures, and therefore, dehydration at A1203 sites occurred in parallel with oxidation of adsorbed CH3OH. Similar changes took place for ethanol on A g / A I 2 0 3 during TPO. The TPO spectra for adsorbed ethanol, which are shown in Fig. 7, are quite different from the ethanol TPD spectra. Hydrogen started to form at lower temperature, showing that in the presence of oxygen, silver was more active for dehydrogenation. More acetaldehyde formed during TPO than during TPD, but the amount of H a was unchanged. The additional acetaldehyde could result from a higher dehydrogenation rate or partial oxidation of ethanol. The amount of ethylene from C2HsOH dehydration on AlzO 3 was significantly decreased during TPO, but was detected in the same temperature range as during TPD. No other carbon-containing product formed at lower temperature, but CO and CO 2 formed at higher temperature than ethylene. Either ethylene was partially oxidized before it could desorb or ethanol was partially oxidized. The resulting species was then oxidized to CO and CO 2 at elevated temperatures, as shown in Fig. 7. During TPD, formation of gas phase species was complete by 750 K, but the formation of m o r e H 2 than expected from the dehydrogenation reaction to form acetaldehyde indicated that carbon was deposited on the surface. During TPO, both CO and CO 2 continued to form up to 873 K, where heating was stopped. Apparently the carboneous species that formed as a result of dehydrogenation of ethanol was subsequently oxidized at high temperature to CO and CO 2. It is
E.M. Cordi, J.L. Falconer~Applied Catalysis A: General 151 (1997)179-191
187
4
J 03
-6 E v
0 300
400
500
600
700
800
900
Temperature (K) Fig. 7. TPO of ethanol on Ag/A1203.
possible, however, that a partial oxidation product of C2HsOH formed during TPO, and it was then oxidized at highest temperature. As shown below, possible partial oxidation products like acetaldehyde and acetic acid were oxidized to CO 2 and CO slowly during TPO. Since CO 2 and H20 formed at lower temperatures on Ag/A1203 than on AI203, oxygen adsorbed on silver was active for oxidation. Oxygen is one of the few gases that strongly adsorb on clean silver [8], and many studies have sought to establish its adsorbed form [3-13]. According to Khasin [4] and Barteau and Madix [8], adsorbed oxygen allows other gases, such as C2H 4, to adsorb on activated silver sites. Ethylene adsorption on oxygen-covered silver is often studied because silver is the only metal known to catalyze the direct epoxidation of ethylene to ethylene oxide [2]. Thus, ethylene may have formed by dehydration on A1203 during TPO, but then adsorbed on the silver and been oxidized or partially oxidized before it could desorb. In contrast, during TPD the ethylene did not adsorb on silver in the absence of oxygen, and thus ethylene appeared in the gas phase. As seen in Fig. 8, acetaldehyde was oxidized to CO 2 and CO, but reaction was not complete by 773 K, when heating was stopped, and the rates of CO 2 and CO formation were still high at 773 K. Water also formed, but is not shown in the figure. A comparison to the TPD spectra in Fig. 3 shows that acetaldehyde was more reactive during TPO than during TPD. Note the difference in scales in the two figures; CO 2 formed at rates 10 times greater during TPO than TPD. Slightly less CO formed in TPO, and a small amount of unreacted C 2H 4°
188
E.M. CordL J.L. Falconer/Applied Catalysis A: General 151 (1997) 179-191 4
3
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f
0 300
400
500 600 Temperature (K)
700
800
Fig. 8. TPO of acetaldehyde on A g / A I ~ O 3.
was detected, but no H2 was seen during TPO. Apparently any H e was oxidized at the high temperature where it formed. Since the rate of acetaldehyde decomposition was so low during TPD, silver either increased the rate of decomposition in the presence of oxygen, and the reaction products were oxidized, or the oxidation rate was increased directly. Acetaldehyde adsorbed strongly on A1203 so that little decomposed during TPD on either AlzO 3 or Ag/A1203. Oxygen increased the rate of product evolution on both catalysts, but to a greater extent on A g / A I e O 3. The rate of product formation during TPO of formic acid (Fig. 9) did not change significantly from that during TPD. The peak temperature for CO 2 formation was essentially unchanged. The main difference was that much more CO 2 formed and less CO formed. About 35% of the carbon in HCOOH was converted to CO e. Water was produced as a result of dehydration, and it desorbed slowly. Since the H 2 formation rate did not increase, the Ag/A1203 catalyst was relatively inactive for formic acid dehydrogenation, even in the presence of oxygen. Instead it appears that during TPO, formic acid dehydrated and the CO product were then oxidized to CO 2. Even thought acetic acid decomposed slowly during TPD on Ag/A1203, the presence of 02 did not significantly decrease the temperature where products formed. As shown in Fig. 10, the main product during TPO was CO e, much smaller amounts of CO and CH 4 formed, and no H 2 was detected. In addition, water was observed but is not shown in the figure. Apparently carbon-carbon bond cleavage was not increased by O 2, and instead, products of decomposition
E.M. Cordi, J.L. Falconer~Applied Catalysis A: General 151 (1997)179-191
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.
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.
.
.
.
.
.
.
.
.
.
.
400
-
-
-
- -
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Temperature (K) Fig. 9. TPO of formic
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Ag/AlzO 3.
were subsequently oxidized during TPO. In addition t o H 2 and C H 4, the [3-carbon that remained on the surface during TPD was also oxidized during TPO. Carbon dioxide formed at the same temperature on A1203 and A g / A I 2 0 3 , so it is likely that reaction took place on both silver and alumina sites in parallel.
th
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400
500
600
Temperature (K) Fig. 10. TPO of acetic
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Ag/AI203.
700
800
E,M. CordL J.L. Falconer/Applied Catalysis A: General 151 (1997) 179-191
190
0.25
0.2
0.15 ¢D
E
0.1
0.05
I
200
[
300
]
400
I
500
I
600
I
700
800
Temperature (K) Fig. l I. TPO of carbon monoxide on Ag/A1203.
Since CO does not adsorb on alumina, CO that adsorbs on Ag/A1203 must be on the silver surface. During TPO of adsorbed CO, all the CO was oxidized to CO 2, as shown in Fig. 11. Though oxidation began at room temperature, the rate of oxidation was slow and CO 2 continued to form until heating was stopped. The rate of CO desorption was slower than oxidation since no unreacted CO was detected. Monolayer coverage cannot be obtained with CO, and this is a reason that oxygen chemisorption is a preferred method of measuring silver surface area [11]. Adsorption of oxygen is an activated process up to monolayer coverage at 443 K [6], and this may be why the oxidation rate is slow relative to CO oxidation on Pd/A1203 for example. Meima et al. [7] studied CO oxidation on Ag/A1203 and observed a drop in oxidation activity above 473 K as multicrystalline silver converted to Ag(111) facets. The main purpose of studying CO oxidation is to show that CO oxidizes faster than any of the adsorbed VOCs, and thus the CO 2 formation rates during TPO are not limited by CO oxidation. Thus, when CO formed at much higher temperature on A g / A I 2 0 3 , in most cases it was CO from AI203, since CO on Ag would be expected to oxidize at these temperatures.
4. Conclusions Silver added to alumina does not increase the rate of VOC decomposition. None of the VOCs decompose on silver during TPD, but they all react on the
E.M. Cordi, J.L, Falconer/Applied Catalysis A: General 151 (1997)179-191
191
AI203 support by dehydration and dehydrogenation. The Ag/A1203 catalyst is more active than A1203 for oxidation to CO 2 and H20, however. Adsorbed VOCs diffuse along the A1203 surface and react at the silver sites, where oxygen adsorbs. Because oxidation is at high temperatures on silver, VOCs react in parallel on silver and alumina sites. Based upon the largest CO 2 peak observed during TPO, the relative oxidation reactivities are HCOOH > CH3OH > CH3COOH > C2HsOH > C2H40 Oxygen adsorption on silver activates neighboring silver sites for alcohol dehydrogenation. Methanol and ethanol dehydrogenate in parallel with oxidation on the silver sites, but the aldehyde and acids do not.
Acknowledgements Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. We also gratefully acknowledge partial support by the National Science Foundation, Grant CTS-90-21194.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
E.M. Cordi and J.L. Falconer, J. Catal., 162 (1996) 104. Y. Yong and N.W. Cant, J. Catal., 122 (1990) 22. D.I. Kondarides and X.E. Verykios, J. Catal., 143 (1993) 481. A.V. Khasin, Kin. and Catal., 34 (1993) 42. S. Imamura, M. Ikebata, T. Ito and T. Ogita, Ind. Eng. Chem. Res., 30 (1991) 217. G.R. Meima, R.J. Vis, M.G.J. van Leur, A.J. van Dillen, J.W. Geus and F.R. van Buren, J. Chem. Soc., Faraday Trans. I, 85 (1989) 279. G.R. Meima, R.J. Vis, M.G.J. van Leur, A.J. van Dillen, J.W. Gues and F.R. van Buren, J. Chem. Soc., Faraday Trans. I, 85 (1989) 293. M.A. Barteau and R.J. Madix, The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Fundamental Studies of Heterogeneous Catalysis, Elsevier, Amsterdam, 1981. K.L. Anderson, J.K. Plischke and M.A. Vannice, J. Catal., 128 (1991) 148. S.N. Goncharova, A.V. Khasin, S.N. Filimonova and D.A. Bulushev, Kin. and Catal., 32 (1991) 768. S.R. Seyedmonir, D.E. Strohmayer, G.J. Guskey, G.L. Geoffroy and M~A. Vannice, J. Catal., 93 (1985) 288. G.R. Meima, M. Hasselaar, A.J. van Dillen, F.R. van Buren and J.W. Geus, J. Chem. Soc., Faraday Trans. I, 5 (1989) 1267. S.R. Seyedmonir, J.K. Plischke, M.A. Vannice and H.W. Young, J. Catal., 123 (1990) 534. J.L. Falconer and J.A. Schwarz, Catal. Rev.-Sci. Eng., 25 (1983) 141. H.K. Plummer, Jr., W.L.H. Watkins and H.S. Gandhi, App. Catal., 29 (1987) 261. B. Chert and J.L. Falconer, J. Catal., 144 (1993) 214.