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Al2O3 catalysts
B EWIRCNMENTAL
ELSEVIER
Applied Catalysis B: Environmental
6 ( 1995) 105-I 16
Enhancement effect of gold and silver on nitric oxide decomposition over Pd/A1203 catalysts R.J. Wu, T.Y. Chou, C.T. Yeh * Department
of Chemistry,
National
Tsing Hua University,
Received 22 April 1994; revised 12 December
Hsinchu,
30043,
Taiwan,
ROC
1994; accepted 10 January 1995
Abstract The effect of alloying palladium supported on alumina with Group 1B metals was studied with the catalytic decomposition of nitric oxide into nitrogen and oxygen. Pd/Al,OX was inactive for the decomposition at low temperatures, but the activity increased rapidly when the system temperature was raised to 1023 K. TPO studies revealed that supported PdO became unstable at elevated temperatures and started to decompose to metallic palladium by desorbing oxygen also around 1023 K. Therefore, the desorption of oxygen is regarded as the rate determining step for the catalytic decomposition at T< 1023 K. An addition of gold or silver to Pd/Al,O, increased its catalytic activity towards the decomposition. TPR and hydrogen chemisorption studies confirmed that added gold and silver alloyed with palladium on the surface of A&O,. TPO studies also demonstrated that the enhanced activity on alloying palladium with gold and silver can be correlated with a decrease in the temperature required for the desorption of oxygen. Keywords: Palladium; Alloy; Decomposition programmed oxidation
of nitric oxide; Temperature-programmed
reduction; Temperature-
1. Introduction NO, is one of the major pollutants in the atmosphere. The main source of this noxious pollutant from the civilized world is exhausts of high temperature combustion including motor vehicles and industrial plants. Many techniques have already been developed and attempted to reduce the emission level of this reactive and acidic molecule. Presently, 70% of the NO, emission from the exhaust of the vehicles can be removed by redox reaction with the help of decent control of air/ fuel ratio and using three-way catalysts. Under fuel lean conditions, the efficiency * Corresponding 0926.3373/95/$09.50 .SSDIO926-3373(
author. Fax. ( + 886-35) 711082. 0 1995 Elsevier Science B.V. All rights reserved 95)00005-4
106
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of the three-way catalyst toward NO, removal has been seriously reduced. NO, in the exhaust of a power plant is often reduced to nitrogen through the SCR process using ammonia as the reducing reagent and V205/Ti02 as the catalyst. Nevertheless, the SCR process still suffers disadvantages of high costs associated with the facility and maintenance as well as from the leakage of annoying ammonia. Thermodynamically, NO, is unstable and tends to decompose through reactions such as
NO = iN2+ +O,
AGj=
-86
kJ mall’
(1)
The equilibrium concentration of nitric oxide in the air is high at high temperatures (for example, 2. lo4 ppm at 2273 K) but decreases with the temperature (less than 200 ppm, a generally accepted maximum emission, at T< 1273 K). Therefore, a direct decomposition reaction at moderate temperature should be an attractive method for NO, abatement. However, the obstacle in using reaction ( 1) for the NO, abatement involves finding a proper catalyst for decreasing its huge energy barrier(E,=364kJmol-‘) [I]. Many catalysts, i.e., Cu/zeolite, perovskites, oxides and supported metals have been reported in previous literature to exhibit catalytic activity towards the reaction ( 1) .Among the tested catalysts, Cu/ZSM-5 is the most active catalyst at low temperatures with a maximum activity (85% N2 conversion) around 773 K [ 21. The activity of this catalyst, however, is seriously inhibited by the presence of oxygen and SO2 in the reaction stream [ 31. Oxides, supported metals, and perovskites are generally active at higher temperatures. Pt/Al,O,, for example, exhibited good initial decomposition activity (80% N2 conversion) at 973 K. However, its activity decayed quickly with time (25% N2 conversion after 10 h) [ 41. Finding a good catalyst for the reaction ( 1) is a worthwhile task to pursue. Palladium generally displays similar catalytic activity as platinum but has better thermal stability. It is therefore the interest of our laboratory to develop a long-life supported palladium catalyst for nitric oxide decomposition. We have found that Pd/A1,03 catalysts are quite active and being immune from the inhibition of oxygen and SO2 as well as have a long life time at T> 1100 K.
2. Experimental
2.1. Sample preparation Palladium catalysts of various composition were prepared by impregnating an aqueous solution of Pd(N03)2, H,AuCl, and AgN03 into A1203 (Merck, 108 m2/ g). Prepared samples were dried overnight in an oven at 393 K, calcined in air at 773 K for 2 h and then saved for reaction and characterization use.
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-116
107
2.2. Adsorption measurements Hydrogen chemisorption was performed in a vacuum system utilizing a precision pressure gauge (Texas Instruments Model 145) at 300 K for measuring the dispersion of palladium. Samples for chemisorption measurement were pretreated with 1 h reduction in flowing hydrogen at 523 K and I h evacuation at 523 K. 2.3. TPRLTPO experiments These two measurements were performed on the same flow setup. However, 10% H2 in Ar (30 ml/min) and 5% O2 in He (mUmin) was employed as the flowing gas in the TPR and TPO experiments, respectively. Accepted flowing gas passed sequentially through the reference side of a thermal conductivity detector (TCD), the sample (in a quartz tubing of 4 mm id.), a cell containing silica gel (to remove water formed during reduction) and the sample side of the TCD. The temperature of the quartz tubing was raised with a heating rate of 7 K/min to 1173 K (in the heating phase of the temperature cycle) by an oven and controlled by a temperature programmer. After reaching 1173 K, the temperature was naturally cooled down to RT (in cooling phase of temperature cycle) by turning off the power of the oven. Each sample was pretreated with a calcination at 773 K in air for 2 h prior to the TPR and to TPO measurements. 2.4. Measurement
of catalytic activity
The catalytic activity of nitric oxide decomposition was carried out in a conventional flow reactor at W/F of 1.6 g s ml- ‘. The flow-rate of feed gas (comprising 4% or 2000 ppm v/v of NO in He) was regulated at 30 ml/min with a mass flow controller (Tylan, Fc-280). The reactor was constructed of 8 mm i.d. quartz tubing in which a catalyst sample of 0.8 g was mounted. The composition of outflow gas was analyzed by gas chromatography using porapak Q ( N20) and molecular sieve 5A ( 02, N2 and NO) columns. The conversion of nitric oxide was estimated from the concentration of nitric oxide at the inlet ( [NO] i) and the concentration of nitrogen at the outlet ( [NJ J of the catalyst bed according to: Conversion
to N2 = 2 [NJ “/ [NO] i
3. Results and discussion 3.1. Catalytic activity An addition of gold to 0.5% Pd/A1203 alters the catalytic activity. Fig. 1 displays a volcano shape of the nitrogen conversion at 1173 K versus the Au-Pd composi-
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Catalysis B: Environmental 6 (1995) 105-116
20 -
10 -
Fig. 1. N2 conversion for NO decomposition over Pd/A1203 with various amounts of Au. 0.5% Pd-Au/A1,03, NO/He=4%, W/F=1.6gscm~3,T=1173K,X=Au/(Pd+Au).
tion. A maximum activity was found at Au/(Pd+ Au) weight ratio of l/20. A similar volcano shape of nitrogen conversion is also shown in Fig. 2 for Pd-Ag/ A1203 alloy catalysts. The maximum conversion was at Ag/ (Pd + Ag) ratio of 1/ 10 when the loading of palladium on A1203 was 0.5%. Fig. 3 shows the dependency of catalytic activity of Pd-Ag/Alz03, Pd-Au/ A120, and Pd/A1203, all with 0.5% Pd loading on the reaction temperature. An addition of silver or gold to 0.5% Pd/Al*O, apparently enhanced the catalytic activity of Pd/Al*O, over the entire temperature range examined. A screen test on the activity of a y-alumina supported metallic catalyst has been made in our laboratory. Platinum and palladium displayed the highest nitrogen conversion among all the tested metals. Pt/Al,O, samples showed impressive initial conversion at low temperatures ( 80% at 873 K) . Nevertheless, we do not suggest using Pt/Al,O, catalysts to promote nitric oxide decomposition because its activity decayed rapidly with time (the nitrogen conversion decreased to 15% after a 4 h test at 873 K). In contrary, catalytic activity of Pd/A1203 is quite stable. Fig. 4 illustrates some results of a lifetime test. All the tested samples in the figure display satisfactory lifetime. In fact, we have not found any significant activity decay from Pd/A1203 or modified Pd/A1203 samples in tests run at T< 1173 K. Table 1 displays that Pd/A1203 catalysts also persisted with decent activity in the presence of oxygen and SOz. Conceivably, adsorbed oxygen and SO* are
R.J. Wu et al. /Applied
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Catalysis B: Environmental
6 (1995) 105-116
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Fig. 2. N2 conversion for NO decomposition over Pd/A1203 with various amounts of Ag. 0.5% Pd-Ag/Al,O,, NO/He=4%,W/F=1,6gscm-‘,T=1173K,X=Ag(Pd+Ag).
unstable on the surface of palladium at 1173 K. Observed thermal durability and immunity from the poison by SO2 pursued us that decomposition of nitric oxide may possibly be developed into a practical process at the presence of the Pd/Al,O, catalyst. 3.2. Hydrogen chemisorption Isotherms of hydrogen chemisorption of Pd-Au/Alz03 catalysts are shown in Fig. 5. Two distinct uptake-stages were observed for each isotherm curve. These two stages reflected the capability of palladium towards adsorbing hydrogen on the palladium surface and absorbing hydrogen into the bulk of palladium crystallites [5], i.e., Pd,+&H,
+ Pd,-H
Pd,+$H,
+ Pdi,-H,
( 10 Tort-)
(2) (3)
where Pd, and Pdi, denote the palladium atom at the surface and in the bulk of palladium crystallites, respectively, and x is the absorption stoichiometry, which has a value of 0.6 for pure palladium crystallites at 300 K [ 61. The first stage of hydrogen uptake, as observed in the isotherm of Fig. 5, occurred due to the chem-
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I
I
I
1
70
800
900
1000
Temperature
1100
/ K
Fig. 3. Nz conversion for NO decomposition over Pd/A1202, Pd-Au/Al,O, and Pd-Ag/A120, catalysts. NO/ He=4%, W/F=1.6gscm-3, “: 0.5% Pd-O.O3% Au/A1203, v : 0.5% Pd-O.O6% Ag/A1,03, 0: 0.5% Pd/Al,O,.
100
OL 1
I
I
I
I
10
100
Time
On Stream
I
I 1000
10000
( min )
Fig. 4. Lifetime activity test of NO decomposition over Pd catalysts. v: 9% Pd/Al,O, at 1173 K, v : 0.056% Ag0.5% Pd/A1,03 at 1073 K, 0: 0.056% Au-O.5% Pd/AI,O, at 1073 K, 0: 0.13% Ag-O.5% Pd/AI,O, at 1173 K.
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-116
Table 1 Effect of O2 and SO, on the conversion
of NO
Sample
Conversion
into NZ
02 (%)
0.5% Pd-O.O6% Ag/AI,O”, Cu/ZSM-Sb Ag-Co,O: Pt/Al,O;
111
SO, @pm)
0
5
10
0
480
540
62.6 48 47 62
63.2
61.8 2.1
62.6 25
62.2 0
65.4
20 0.7
a 1173K. [NO] =2OOOppm, W/F=1.52gscmm’. ‘Cu/ZSM-5 (773 K), Pt/A&O, (973 K), [NO] = lOOOppm, W/F=O.I ‘873K,[NO]=2%,WIF=2.0gscm~7(see[14]).
g s cmm3 (see [2]).
isorption and the second stage was due to the absorption. The dispersion of palladium could be estimated from the hydrogen uptake at PHZ= 10 Torr in adsorption isotherm performed at 300 K. The dispersion (D) of 0.50% Pd/A&O, sample found in Fig. 5 was 0.23. Figs. 5 and 6 reveal that both D and x values for Pd/ A&O3 catalysts decreased on alloying the supported palladium with gold and silver 1
1.0,
I
I
I
I
I
I
0.0 -
0.8 0.7
0-b)
-
O,y
-
(b)
/ 0.6
-
0.0 0
CY
I 10
I
I
1
1
I
1
20
30
40
SO
60
70
H, Pressure Fig. 5. Adsorption isotherms for chemisorption Au/A120,, (c) 0.50% Pd-o.13% Au/A120s.
80
/ TQ~-
of hydrogen at 298 K. (a) 0.50% Pd/A1,03,
(b) 0.50% Pd-O.O6%
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Catalysis B: Environmental 6 (1995) 105-116
0.0 0.8 0.7 0.8 0.5 0.4 0.3 0.2 0.1 0.0
0
10
20
H, Pressure Fig. 6. Adsorption isotherms for chemisorption Ag/AlZ03, (c) 0.50% Pd-O.5% Ag/A1,03.
30
40
SO
/ Tom
of hydrogen at 298 K. (a) 0.50% Pd/A1203, (b) 0.50% Pd-O.O6%
metals. The decrease in D on adding a small amount of gold and silver can be regarded as a surface enrichment of gold and silver over palladium [ 71. This surface enrichment comes from the tendency of alloy crystallites to minimize the surface energy [ 81. A decrease in the hydrogen absorption into palladium with the addition of silver has also been observed by G. Alefeld and J. Volkl [ 91 and was attributed to filling the valence s electron from gold and silver in the alloy into the d-band holes of palladium [ lo]. 3.3. TPR measurements The TPR profiles for Pd-Au/A1,03 catalysts are shown in Fig. 7. A positive peak was observed around 340 K for Pd/Al,O,. This peak around 340 K was attributed to hydrogen consumption during the reduction of PdO to palladium : 1.5H2 + Pd,O + Pd,H + H,O
(4)
(1 +x/2)H,+Pd,O
(5)
+ Pd,H,+H,O
A negative peak around 380 K in the profile 7a was assigned to the desorption of absorbed hydrogen from PdbHx. Profiles 7b and 7c displayed that an additional
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-116
113
1 (4
1 200
1
300
I
1
400
I
so0
Temperature / K Fig. 7. TPR profiles for Pd-Au/A&O, catalysts. (a) 0.50% Pd/Al_,O,, 0.50% Pd-OSO% Au/Al,O,, (d) I .OO% Au/Al,03.
(b) 0.50% Pd-O.13%
Au/A120,,
(c)
peak of hydrogen consumption occurred around 270 K and the relative intensity of the peak around 340 K was decreased as the Pd/A1203 sample was modified with gold. Since Au/Al,O, did not show any noticeable TPR peak, the peak around 270 K in profiles 8b and 8c was assigned to the reduction of Pd-Au mixed oxide on the Pd-Au/A1203 catalysts. The temperature of this new peak shifted to a lower temperature and its intensity relative to that of the peak around 340 K was increased as the amount of added gold was increased. Probably, gold on the samples of AuPd ahoy could weaken the bond strength of Pd-0 and enhance the reduction of PdO into palladium. 3.4. TPO measurements Fig. 8 shows the TPO profiles for catalysts of palladium and palladium with gold. Profile 8a (of 0.50% Pd/Al,O,) exhibits a negative peak with desorption around 1100 K during the heating phase and a positive peak with consumption around 840 K during the cooling phase. These two peaks account for with the following reversible reaction: PdO + Pd + $02
added oxygen oxygen can be (6)
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Catalysis B: Environmental 6 (1995) 105-116
Fig. 8. TPO profiles of Pd and Au added Pd catalysts. (a) 0.50% Pd/Al,O,, (c) 0.50% Pd-O.O6% Au/A120,, (d) the second TPO of sample (c).
(b) the second TPO of sample (a),
The supported PdO became unstable at T> 1037 K and was thermally decomposed into metallic palladium in the heating phase according to reaction (6), and the metallic palladium was then reoxidized during the cooling phase by oxygen molecules in the TPO system. In order to confirm the reversibility of reaction (6), a same heating-cooling cycle was again performed on this sample in the TPO system. A negligible difference between the first TPO (Profile 8a) and the second TPO (Profile 8b) was observed from this catalyst. Addition of a small amount of gold to Pd/A1203, however, changed the TPO profile significantly. The first TPO (Profile 8c) for 0.50% Pd-O.OB%Au/Al,O, revealed that the oxygen desorption, besides a major peak at 1100 K, had a shoulder at a lower temperature ( 1040 K) and reoxidation in the cooling phase was shifted to a lower temperature. Apparently, the bond strength of Pd-0 was weakened and the equilibrium of reaction (6) was shifted towards the left by the presence of nearby gold. The 1040 K shoulder became the major desorption peak in the second TPO (Profile 8d) of this gold alloyed palladium sample. The extent of alloying between palladium and gold was most likely increased during high temperature treatment in the first TPO process. Fig. 9 shows the TPO profiles of palladium and Pd-Ag catalysts. Similar to the results of Fig. 8, a shoulder of oxygen desorption at 1023 K was also observed by the addition of silver, and this shoulder became enhanced in the second TPO profile,
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-l I6
_L
I
60
80
1
loo
I
I
120
140
I
loo
1
180
115
J 200
Time/MiIl Fig. 9. TPO profiles of Pd and Ag added Pd catalysts. (a) 0.50% Pd/Al,O,, (b) the second TPO of sample (a), (c) 0.50% Pd-O.13% Ag/AI,O,, (d) the second TPO of sample (c), (e) 0.50% Pd-O.50% Ag/AI,O,.
and increased with silver loading. Evidently, addition of silver to the Pd/A1,03 catalyst could also weaken the bond strength of Pd-0. Modification the electronic property of supported Group VIII metals by alloying them with a Group IB metal has been suggested and established [ 11,121 for quite a long time. A decrease in the energy of hydrogen adsorption on nickel by alloying nickel with copper has been demonstrated in our laboratory through TPD of hydrogen [ 131. In this report, we have additionally confirmed that the bonding strength of PdO on supported palladium samples may be weakened by alloying palladium with the group IB metals. The catalytic activity of all of the catalysts studied was negligible at low temperature. The abrupt increases in catalytic activity around 1023 K (Fig. 3) were regarded as having resulted from an acceleration in the rate of PdO decomposition into metallic palladium over that temperature (Fig. 8). The higher the system temperature is, the faster becomes the rate of oxygen desorption and, consequently, the higher is the catalytic activity of Pd/A1203 catalysts toward nitric oxide decomposition. Therefore, the oxygen desorption from PdO is considered as the rate determining step of the nitric oxide decomposition over Pd/A1203 at T< 1023 K. Above this temperature, desorption of oxygen becomes a fast step and adsorption of nitric oxide and decomposition of adsorbed nitric oxide on the palladium surface into adsorbed 0 and N atoms may become the rate determining step. An addition
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of silver or gold to Pd/A1203 plausibly decreased the decomposition temperature of PdO crystallites supported and thereby enhancing their catalytic activity. In conclusion, formation of Pd-Au or Pd-Ag alloy on the surface of alumina was verified to have occurred via the hydrogen chemisorption or the TPR measurement. TPO results indicated that the oxides of this bimetal alloy desorbed their oxygen at a lower temperature than PdO. The catalytic activity of Pd/A1203 catalyst towards nitric oxide decomposition was also enhanced by the gold or silver added.
Acknowledgements The authors appreciate the financial support of this study by the National Science Council of ROC.
References [ 11 H.S. Glick, J.J. Klien and W. Squire, .I. Chem. Phys., 27 (1957) 850. [2] M. Iwamoto and H. Hamada, Catal. Today, 10 ( 1991) 57. 131 M. Iwamoto, H. Yahiro, S. Shundo, Y. Yuu and N. Mizuno, Appl. Catal., 69 (1991) Ll5. [4] Japanese Pat. 01-104345. [5] T.C. Chang, J.J. Chen and C.T. Yeh, J. Catal., 89 (1984) 498. [6] Y. Soma-noto and W.H. Sachtler, J. Catal., 32 (1974) 315. [7] Frank L. Williams and D. Nason, Surf. Sci., 45 ( 1974) 377. [ 81 G.N. Burland and P.J. Dobson, Thin Solid Films, 75 (1981) 83. [9] G. Alefeld and J. Volkl, Hydrogen in Metals 2, Springer Verlag, Berlin, 1987. [ 101 D.D. Eley and P.B. Moore, Surf. Sci., 111 (1981) 325 and references therein. [ 111 W.M.H. Sachtler and R.A. Van Santen, Adv. Catal., 26 (1977) 69, 1121 J.J. Burton and R.L. Garten, Advanced Materials in Catalysts, Academic Press, New York, 1977, Chapters land2. [ 131 MI. Chen, C.T. Cheng and CT. Yeh, J. Catal., 95 (1985) 346. [ 141 H. Hamada, Y. Kintaichi, M. Sasacki and I. Ito, Chem. Lett., (1990) 1069.
ELSEVIER
Applied Catalysis B: Environmental
6 ( 1995) 105-I 16
Enhancement effect of gold and silver on nitric oxide decomposition over Pd/A1203 catalysts R.J. Wu, T.Y. Chou, C.T. Yeh * Department
of Chemistry,
National
Tsing Hua University,
Received 22 April 1994; revised 12 December
Hsinchu,
30043,
Taiwan,
ROC
1994; accepted 10 January 1995
Abstract The effect of alloying palladium supported on alumina with Group 1B metals was studied with the catalytic decomposition of nitric oxide into nitrogen and oxygen. Pd/Al,OX was inactive for the decomposition at low temperatures, but the activity increased rapidly when the system temperature was raised to 1023 K. TPO studies revealed that supported PdO became unstable at elevated temperatures and started to decompose to metallic palladium by desorbing oxygen also around 1023 K. Therefore, the desorption of oxygen is regarded as the rate determining step for the catalytic decomposition at T< 1023 K. An addition of gold or silver to Pd/Al,O, increased its catalytic activity towards the decomposition. TPR and hydrogen chemisorption studies confirmed that added gold and silver alloyed with palladium on the surface of A&O,. TPO studies also demonstrated that the enhanced activity on alloying palladium with gold and silver can be correlated with a decrease in the temperature required for the desorption of oxygen. Keywords: Palladium; Alloy; Decomposition programmed oxidation
of nitric oxide; Temperature-programmed
reduction; Temperature-
1. Introduction NO, is one of the major pollutants in the atmosphere. The main source of this noxious pollutant from the civilized world is exhausts of high temperature combustion including motor vehicles and industrial plants. Many techniques have already been developed and attempted to reduce the emission level of this reactive and acidic molecule. Presently, 70% of the NO, emission from the exhaust of the vehicles can be removed by redox reaction with the help of decent control of air/ fuel ratio and using three-way catalysts. Under fuel lean conditions, the efficiency * Corresponding 0926.3373/95/$09.50 .SSDIO926-3373(
author. Fax. ( + 886-35) 711082. 0 1995 Elsevier Science B.V. All rights reserved 95)00005-4
106
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-116
of the three-way catalyst toward NO, removal has been seriously reduced. NO, in the exhaust of a power plant is often reduced to nitrogen through the SCR process using ammonia as the reducing reagent and V205/Ti02 as the catalyst. Nevertheless, the SCR process still suffers disadvantages of high costs associated with the facility and maintenance as well as from the leakage of annoying ammonia. Thermodynamically, NO, is unstable and tends to decompose through reactions such as
NO = iN2+ +O,
AGj=
-86
kJ mall’
(1)
The equilibrium concentration of nitric oxide in the air is high at high temperatures (for example, 2. lo4 ppm at 2273 K) but decreases with the temperature (less than 200 ppm, a generally accepted maximum emission, at T< 1273 K). Therefore, a direct decomposition reaction at moderate temperature should be an attractive method for NO, abatement. However, the obstacle in using reaction ( 1) for the NO, abatement involves finding a proper catalyst for decreasing its huge energy barrier(E,=364kJmol-‘) [I]. Many catalysts, i.e., Cu/zeolite, perovskites, oxides and supported metals have been reported in previous literature to exhibit catalytic activity towards the reaction ( 1) .Among the tested catalysts, Cu/ZSM-5 is the most active catalyst at low temperatures with a maximum activity (85% N2 conversion) around 773 K [ 21. The activity of this catalyst, however, is seriously inhibited by the presence of oxygen and SO2 in the reaction stream [ 31. Oxides, supported metals, and perovskites are generally active at higher temperatures. Pt/Al,O,, for example, exhibited good initial decomposition activity (80% N2 conversion) at 973 K. However, its activity decayed quickly with time (25% N2 conversion after 10 h) [ 41. Finding a good catalyst for the reaction ( 1) is a worthwhile task to pursue. Palladium generally displays similar catalytic activity as platinum but has better thermal stability. It is therefore the interest of our laboratory to develop a long-life supported palladium catalyst for nitric oxide decomposition. We have found that Pd/A1,03 catalysts are quite active and being immune from the inhibition of oxygen and SO2 as well as have a long life time at T> 1100 K.
2. Experimental
2.1. Sample preparation Palladium catalysts of various composition were prepared by impregnating an aqueous solution of Pd(N03)2, H,AuCl, and AgN03 into A1203 (Merck, 108 m2/ g). Prepared samples were dried overnight in an oven at 393 K, calcined in air at 773 K for 2 h and then saved for reaction and characterization use.
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-116
107
2.2. Adsorption measurements Hydrogen chemisorption was performed in a vacuum system utilizing a precision pressure gauge (Texas Instruments Model 145) at 300 K for measuring the dispersion of palladium. Samples for chemisorption measurement were pretreated with 1 h reduction in flowing hydrogen at 523 K and I h evacuation at 523 K. 2.3. TPRLTPO experiments These two measurements were performed on the same flow setup. However, 10% H2 in Ar (30 ml/min) and 5% O2 in He (mUmin) was employed as the flowing gas in the TPR and TPO experiments, respectively. Accepted flowing gas passed sequentially through the reference side of a thermal conductivity detector (TCD), the sample (in a quartz tubing of 4 mm id.), a cell containing silica gel (to remove water formed during reduction) and the sample side of the TCD. The temperature of the quartz tubing was raised with a heating rate of 7 K/min to 1173 K (in the heating phase of the temperature cycle) by an oven and controlled by a temperature programmer. After reaching 1173 K, the temperature was naturally cooled down to RT (in cooling phase of temperature cycle) by turning off the power of the oven. Each sample was pretreated with a calcination at 773 K in air for 2 h prior to the TPR and to TPO measurements. 2.4. Measurement
of catalytic activity
The catalytic activity of nitric oxide decomposition was carried out in a conventional flow reactor at W/F of 1.6 g s ml- ‘. The flow-rate of feed gas (comprising 4% or 2000 ppm v/v of NO in He) was regulated at 30 ml/min with a mass flow controller (Tylan, Fc-280). The reactor was constructed of 8 mm i.d. quartz tubing in which a catalyst sample of 0.8 g was mounted. The composition of outflow gas was analyzed by gas chromatography using porapak Q ( N20) and molecular sieve 5A ( 02, N2 and NO) columns. The conversion of nitric oxide was estimated from the concentration of nitric oxide at the inlet ( [NO] i) and the concentration of nitrogen at the outlet ( [NJ J of the catalyst bed according to: Conversion
to N2 = 2 [NJ “/ [NO] i
3. Results and discussion 3.1. Catalytic activity An addition of gold to 0.5% Pd/A1203 alters the catalytic activity. Fig. 1 displays a volcano shape of the nitrogen conversion at 1173 K versus the Au-Pd composi-
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Catalysis B: Environmental 6 (1995) 105-116
20 -
10 -
Fig. 1. N2 conversion for NO decomposition over Pd/A1203 with various amounts of Au. 0.5% Pd-Au/A1,03, NO/He=4%, W/F=1.6gscm~3,T=1173K,X=Au/(Pd+Au).
tion. A maximum activity was found at Au/(Pd+ Au) weight ratio of l/20. A similar volcano shape of nitrogen conversion is also shown in Fig. 2 for Pd-Ag/ A1203 alloy catalysts. The maximum conversion was at Ag/ (Pd + Ag) ratio of 1/ 10 when the loading of palladium on A1203 was 0.5%. Fig. 3 shows the dependency of catalytic activity of Pd-Ag/Alz03, Pd-Au/ A120, and Pd/A1203, all with 0.5% Pd loading on the reaction temperature. An addition of silver or gold to 0.5% Pd/Al*O, apparently enhanced the catalytic activity of Pd/Al*O, over the entire temperature range examined. A screen test on the activity of a y-alumina supported metallic catalyst has been made in our laboratory. Platinum and palladium displayed the highest nitrogen conversion among all the tested metals. Pt/Al,O, samples showed impressive initial conversion at low temperatures ( 80% at 873 K) . Nevertheless, we do not suggest using Pt/Al,O, catalysts to promote nitric oxide decomposition because its activity decayed rapidly with time (the nitrogen conversion decreased to 15% after a 4 h test at 873 K). In contrary, catalytic activity of Pd/A1203 is quite stable. Fig. 4 illustrates some results of a lifetime test. All the tested samples in the figure display satisfactory lifetime. In fact, we have not found any significant activity decay from Pd/A1203 or modified Pd/A1203 samples in tests run at T< 1173 K. Table 1 displays that Pd/A1203 catalysts also persisted with decent activity in the presence of oxygen and SOz. Conceivably, adsorbed oxygen and SO* are
R.J. Wu et al. /Applied
do
Catalysis B: Environmental
6 (1995) 105-116
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(XI
100 k
Fig. 2. N2 conversion for NO decomposition over Pd/A1203 with various amounts of Ag. 0.5% Pd-Ag/Al,O,, NO/He=4%,W/F=1,6gscm-‘,T=1173K,X=Ag(Pd+Ag).
unstable on the surface of palladium at 1173 K. Observed thermal durability and immunity from the poison by SO2 pursued us that decomposition of nitric oxide may possibly be developed into a practical process at the presence of the Pd/Al,O, catalyst. 3.2. Hydrogen chemisorption Isotherms of hydrogen chemisorption of Pd-Au/Alz03 catalysts are shown in Fig. 5. Two distinct uptake-stages were observed for each isotherm curve. These two stages reflected the capability of palladium towards adsorbing hydrogen on the palladium surface and absorbing hydrogen into the bulk of palladium crystallites [5], i.e., Pd,+&H,
+ Pd,-H
Pd,+$H,
+ Pdi,-H,
(
(2) (3)
where Pd, and Pdi, denote the palladium atom at the surface and in the bulk of palladium crystallites, respectively, and x is the absorption stoichiometry, which has a value of 0.6 for pure palladium crystallites at 300 K [ 61. The first stage of hydrogen uptake, as observed in the isotherm of Fig. 5, occurred due to the chem-
110
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80
Catalysis B: Environmental 6 (1995) 105-116
I
I
I
1
70
800
900
1000
Temperature
1100
/ K
Fig. 3. Nz conversion for NO decomposition over Pd/A1202, Pd-Au/Al,O, and Pd-Ag/A120, catalysts. NO/ He=4%, W/F=1.6gscm-3, “: 0.5% Pd-O.O3% Au/A1203, v : 0.5% Pd-O.O6% Ag/A1,03, 0: 0.5% Pd/Al,O,.
100
OL 1
I
I
I
I
10
100
Time
On Stream
I
I 1000
10000
( min )
Fig. 4. Lifetime activity test of NO decomposition over Pd catalysts. v: 9% Pd/Al,O, at 1173 K, v : 0.056% Ag0.5% Pd/A1,03 at 1073 K, 0: 0.056% Au-O.5% Pd/AI,O, at 1073 K, 0: 0.13% Ag-O.5% Pd/AI,O, at 1173 K.
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-116
Table 1 Effect of O2 and SO, on the conversion
of NO
Sample
Conversion
into NZ
02 (%)
0.5% Pd-O.O6% Ag/AI,O”, Cu/ZSM-Sb Ag-Co,O: Pt/Al,O;
111
SO, @pm)
0
5
10
0
480
540
62.6 48 47 62
63.2
61.8 2.1
62.6 25
62.2 0
65.4
20 0.7
a 1173K. [NO] =2OOOppm, W/F=1.52gscmm’. ‘Cu/ZSM-5 (773 K), Pt/A&O, (973 K), [NO] = lOOOppm, W/F=O.I ‘873K,[NO]=2%,WIF=2.0gscm~7(see[14]).
g s cmm3 (see [2]).
isorption and the second stage was due to the absorption. The dispersion of palladium could be estimated from the hydrogen uptake at PHZ= 10 Torr in adsorption isotherm performed at 300 K. The dispersion (D) of 0.50% Pd/A&O, sample found in Fig. 5 was 0.23. Figs. 5 and 6 reveal that both D and x values for Pd/ A&O3 catalysts decreased on alloying the supported palladium with gold and silver 1
1.0,
I
I
I
I
I
I
0.0 -
0.8 0.7
0-b)
-
O,y
-
(b)
/ 0.6
-
0.0 0
CY
I 10
I
I
1
1
I
1
20
30
40
SO
60
70
H, Pressure Fig. 5. Adsorption isotherms for chemisorption Au/A120,, (c) 0.50% Pd-o.13% Au/A120s.
80
/ TQ~-
of hydrogen at 298 K. (a) 0.50% Pd/A1,03,
(b) 0.50% Pd-O.O6%
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Catalysis B: Environmental 6 (1995) 105-116
0.0 0.8 0.7 0.8 0.5 0.4 0.3 0.2 0.1 0.0
0
10
20
H, Pressure Fig. 6. Adsorption isotherms for chemisorption Ag/AlZ03, (c) 0.50% Pd-O.5% Ag/A1,03.
30
40
SO
/ Tom
of hydrogen at 298 K. (a) 0.50% Pd/A1203, (b) 0.50% Pd-O.O6%
metals. The decrease in D on adding a small amount of gold and silver can be regarded as a surface enrichment of gold and silver over palladium [ 71. This surface enrichment comes from the tendency of alloy crystallites to minimize the surface energy [ 81. A decrease in the hydrogen absorption into palladium with the addition of silver has also been observed by G. Alefeld and J. Volkl [ 91 and was attributed to filling the valence s electron from gold and silver in the alloy into the d-band holes of palladium [ lo]. 3.3. TPR measurements The TPR profiles for Pd-Au/A1,03 catalysts are shown in Fig. 7. A positive peak was observed around 340 K for Pd/Al,O,. This peak around 340 K was attributed to hydrogen consumption during the reduction of PdO to palladium : 1.5H2 + Pd,O + Pd,H + H,O
(4)
(1 +x/2)H,+Pd,O
(5)
+ Pd,H,+H,O
A negative peak around 380 K in the profile 7a was assigned to the desorption of absorbed hydrogen from PdbHx. Profiles 7b and 7c displayed that an additional
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-116
113
1 (4
1 200
1
300
I
1
400
I
so0
Temperature / K Fig. 7. TPR profiles for Pd-Au/A&O, catalysts. (a) 0.50% Pd/Al_,O,, 0.50% Pd-OSO% Au/Al,O,, (d) I .OO% Au/Al,03.
(b) 0.50% Pd-O.13%
Au/A120,,
(c)
peak of hydrogen consumption occurred around 270 K and the relative intensity of the peak around 340 K was decreased as the Pd/A1203 sample was modified with gold. Since Au/Al,O, did not show any noticeable TPR peak, the peak around 270 K in profiles 8b and 8c was assigned to the reduction of Pd-Au mixed oxide on the Pd-Au/A1203 catalysts. The temperature of this new peak shifted to a lower temperature and its intensity relative to that of the peak around 340 K was increased as the amount of added gold was increased. Probably, gold on the samples of AuPd ahoy could weaken the bond strength of Pd-0 and enhance the reduction of PdO into palladium. 3.4. TPO measurements Fig. 8 shows the TPO profiles for catalysts of palladium and palladium with gold. Profile 8a (of 0.50% Pd/Al,O,) exhibits a negative peak with desorption around 1100 K during the heating phase and a positive peak with consumption around 840 K during the cooling phase. These two peaks account for with the following reversible reaction: PdO + Pd + $02
added oxygen oxygen can be (6)
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Catalysis B: Environmental 6 (1995) 105-116
Fig. 8. TPO profiles of Pd and Au added Pd catalysts. (a) 0.50% Pd/Al,O,, (c) 0.50% Pd-O.O6% Au/A120,, (d) the second TPO of sample (c).
(b) the second TPO of sample (a),
The supported PdO became unstable at T> 1037 K and was thermally decomposed into metallic palladium in the heating phase according to reaction (6), and the metallic palladium was then reoxidized during the cooling phase by oxygen molecules in the TPO system. In order to confirm the reversibility of reaction (6), a same heating-cooling cycle was again performed on this sample in the TPO system. A negligible difference between the first TPO (Profile 8a) and the second TPO (Profile 8b) was observed from this catalyst. Addition of a small amount of gold to Pd/A1203, however, changed the TPO profile significantly. The first TPO (Profile 8c) for 0.50% Pd-O.OB%Au/Al,O, revealed that the oxygen desorption, besides a major peak at 1100 K, had a shoulder at a lower temperature ( 1040 K) and reoxidation in the cooling phase was shifted to a lower temperature. Apparently, the bond strength of Pd-0 was weakened and the equilibrium of reaction (6) was shifted towards the left by the presence of nearby gold. The 1040 K shoulder became the major desorption peak in the second TPO (Profile 8d) of this gold alloyed palladium sample. The extent of alloying between palladium and gold was most likely increased during high temperature treatment in the first TPO process. Fig. 9 shows the TPO profiles of palladium and Pd-Ag catalysts. Similar to the results of Fig. 8, a shoulder of oxygen desorption at 1023 K was also observed by the addition of silver, and this shoulder became enhanced in the second TPO profile,
R.J. Wu et al. /Applied Catalysis B: Environmental 6 (1995) 105-l I6
_L
I
60
80
1
loo
I
I
120
140
I
loo
1
180
115
J 200
Time/MiIl Fig. 9. TPO profiles of Pd and Ag added Pd catalysts. (a) 0.50% Pd/Al,O,, (b) the second TPO of sample (a), (c) 0.50% Pd-O.13% Ag/AI,O,, (d) the second TPO of sample (c), (e) 0.50% Pd-O.50% Ag/AI,O,.
and increased with silver loading. Evidently, addition of silver to the Pd/A1,03 catalyst could also weaken the bond strength of Pd-0. Modification the electronic property of supported Group VIII metals by alloying them with a Group IB metal has been suggested and established [ 11,121 for quite a long time. A decrease in the energy of hydrogen adsorption on nickel by alloying nickel with copper has been demonstrated in our laboratory through TPD of hydrogen [ 131. In this report, we have additionally confirmed that the bonding strength of PdO on supported palladium samples may be weakened by alloying palladium with the group IB metals. The catalytic activity of all of the catalysts studied was negligible at low temperature. The abrupt increases in catalytic activity around 1023 K (Fig. 3) were regarded as having resulted from an acceleration in the rate of PdO decomposition into metallic palladium over that temperature (Fig. 8). The higher the system temperature is, the faster becomes the rate of oxygen desorption and, consequently, the higher is the catalytic activity of Pd/A1203 catalysts toward nitric oxide decomposition. Therefore, the oxygen desorption from PdO is considered as the rate determining step of the nitric oxide decomposition over Pd/A1203 at T< 1023 K. Above this temperature, desorption of oxygen becomes a fast step and adsorption of nitric oxide and decomposition of adsorbed nitric oxide on the palladium surface into adsorbed 0 and N atoms may become the rate determining step. An addition
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Catalysis B: Environmental 6 (1995) 105-116
of silver or gold to Pd/A1203 plausibly decreased the decomposition temperature of PdO crystallites supported and thereby enhancing their catalytic activity. In conclusion, formation of Pd-Au or Pd-Ag alloy on the surface of alumina was verified to have occurred via the hydrogen chemisorption or the TPR measurement. TPO results indicated that the oxides of this bimetal alloy desorbed their oxygen at a lower temperature than PdO. The catalytic activity of Pd/A1203 catalyst towards nitric oxide decomposition was also enhanced by the gold or silver added.
Acknowledgements The authors appreciate the financial support of this study by the National Science Council of ROC.
References [ 11 H.S. Glick, J.J. Klien and W. Squire, .I. Chem. Phys., 27 (1957) 850. [2] M. Iwamoto and H. Hamada, Catal. Today, 10 ( 1991) 57. 131 M. Iwamoto, H. Yahiro, S. Shundo, Y. Yuu and N. Mizuno, Appl. Catal., 69 (1991) Ll5. [4] Japanese Pat. 01-104345. [5] T.C. Chang, J.J. Chen and C.T. Yeh, J. Catal., 89 (1984) 498. [6] Y. Soma-noto and W.H. Sachtler, J. Catal., 32 (1974) 315. [7] Frank L. Williams and D. Nason, Surf. Sci., 45 ( 1974) 377. [ 81 G.N. Burland and P.J. Dobson, Thin Solid Films, 75 (1981) 83. [9] G. Alefeld and J. Volkl, Hydrogen in Metals 2, Springer Verlag, Berlin, 1987. [ 101 D.D. Eley and P.B. Moore, Surf. Sci., 111 (1981) 325 and references therein. [ 111 W.M.H. Sachtler and R.A. Van Santen, Adv. Catal., 26 (1977) 69, 1121 J.J. Burton and R.L. Garten, Advanced Materials in Catalysts, Academic Press, New York, 1977, Chapters land2. [ 131 MI. Chen, C.T. Cheng and CT. Yeh, J. Catal., 95 (1985) 346. [ 141 H. Hamada, Y. Kintaichi, M. Sasacki and I. Ito, Chem. Lett., (1990) 1069.