- Email: [email protected]
Production of synthesis gas from natural gas using ZrO2-supported platinum
Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 9 Elsevier B.V. All rights reserved.
133
Production of synthesis gas from natural gas using ZrO2-supported platinum Mariana M.V.M.
Souza
a
and M a r t i n S c h m a l *a' b
aNUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, 21945-970, Rio de Janeiro, Brazil. E-mail: [email protected] bEscola de Quimica, Universidade Federal do Rio de Janeiro, C.P. 68542, 21940-900, Rio de Janeiro, Brazil. ABSTRACT Reforming of methane with steam, partial oxidation of methane and a combination of both reactions were camed out using Pt/AI203, Pt/ZrO2 and Pt/10%ZrOz/A1203 catalysts, in temperature range between 350-900~ For steam reforming, the H2/CO product ratio is strongly influenced by the watergas shift reaction, with the Ptl0Zr sample showing the highest activity and stability. When oxygen is present in the feed, the total combustion is the main reaction at low temperatures. The catalyst with 10% of ZrO2 also presented the best performance for partial oxidation and oxy-steam reforming. 1. INTRODUCTION Renewed attention in both academic and industrial research has recently been focused on alternative routes for conversion of natural gas to synthesis gas, a mixture of H2 e CO, which can be used to produce chemical products with high added value. The steam reforming of methane (SRM) is the most important industrial process for the production of synthesis gas. SRM is a very energyintensive process because of the highly endothermic property of the reaction; the high H2/CO ratio (3:1) is useful for processes requiring a Hz-rich feed such as ammonia synthesis and petroleum refining process [1, 2]. Methane partial oxidation (POM) is an advantageous route for synthesis gas production for both economical and technical reasons: it makes the process less energy and capital cost intensive because of its exothermic nature and the lower H2/CO ratio (about 2) is more favorable with respect to downstream processes such as methanol synthesis and Fischer-Tropsch synthesis of higher hydrocarbons [3-6].
134
On the other hand, oxy-steam reforming, which integrates SRM and POM simultaneously, has many advantages such as: low-energy requirements due to the opposite contribution of the exothermic methane oxidation and endothermic steam reforming; low specific consumption; variable H2/CO ratio regulated by varying the feed composition [7, 8]. We have previously reported work on CO2 reforming of methane showing that Pt/ZrO2 and Pt/ZrOz/AI203 are effective formulations for this reaction [9]. This paper gives results on the testing of these novel zirconia-supported catalysts for the steam reforming and partial oxidation of methane, as well as a combination of this both reactions.
2. EXPERIMENTAL 2.1. Catalyst Preparation A1203 and ZrO2 supports were prepared by calcination of 7-alumina (Engelhard) and zirconium hydroxide (MEL Chemicals) at 550~ for 2h under flowing air. 10%ZrO2/A1203 was prepared by impregnation over alumina powder with a nitric acid solution of zirconium hydroxide; the mixture was stirred for 2h at 90~ dried and calcined at the same conditions as the bare supports [9]. The catalysts were prepared by incipient wetness technique, using an aqueous solution of H2PtC16 (Aldrich), followed by drying at 120~ for 16 h and calcination in air at 550~ for 2 h. The platinum content was around l wt%, which was measured by atomic absorption spectrometry. Prepared catalysts will be referred to as PtA1 for Pt/A1203, PtZr for Pt/ZrO2 and Ptl0Zr for Pt/10%ZrO2/A1203.
2.2. Catalyst testing Catalyst testing was performed in a flow tubular quartz reactor loaded with 20mg of catalyst, under atmospheric pressure. The total feed flow rate was held constant at 200 cm3/min (WHSV- 600,000 cm3/h.gcat), with flowing He. The activity tests were performed at different temperatures, ranging from 350 to 900~ in steps of 50~ that were kept for 30 rain at each temperature. The loss in catalyst activity at 800~ was monitored up to 70 h on stream. The reaction products were analyzed by on-line gas chromatograph (CHROMPACK CP9001), equipped with a Hayesep D column and a thermal conductivity detector. 3. RESULTS AND DISCUSSION
3.1. Steam Reforming The comparison of catalyst activities for steam reforming under stoichiometric feed (CH4:H20-I:I) was carried out as a function of temperature
135
(400-800~ and the results are displayed in Figure 1A and B, in terms of CH4 conversion and H2/CO product ratio. The experimental CH4 conversion increased almost linearly with increasing reaction temperature from 400 to 650~ for PtA1 and Ptl 0Zr catalysts, with PtA1 showing the highest conversions at high temperatures. The PtZr catalyst presented low conversions up to 600~ simultaneously this catalyst exhibited the highest H2/CO ratio in this temperature range. The observed high H2/CO ratio (>8.0) suggests that watergas shift (WGS) reaction occurs to a great extent with SRM, as already observed in the literature [7, 10]. At the same time, the decrease in H2/CO ratio with increasing temperature is consistent with the fact that the WGS reaction is thermodynamically unfavorable at higher temperatures. The stability of the catalysts for steam reforming of methane at 800~ is displayed in Figure 2. The PtA1 showed an oscillatory behavior around 72% of conversion, while the PtZr deactivated at a rate of 0.25%/h, probably due to deposition of inactive carbon. The Ptl0Zr sample presented the highest activity and stability, giving a constant conversion of about 81% throughout the duration of the experiment (40h). These results are comparable to the reported values for Pt/ZrO2 of Hegarty et al. [ 10]. lOO
H20/CH4 = 1
16
90 ..-..
80
g
60
>
50
9
B
H20/CH4= 1
12 10
\ 9 % \ ~ ~x,,~
w 4o f o
9
14
20
_
lO o
_
4
9
4;0
5;0
6;0 T e m p e r a t u r e
~.
6-
um
ao
equilibrium --m-- PtAI --t--PEr _A_ Ptl0Zr
Ptl0Zr
~;o
2
5~o
~;o
6~o
40
8~o
Temperature (~
(~
Fig. 1. CH4 conversion (A) and H2/CO product ratio (B) of Pt catalysts for steam reforming of methane as a function of temperature. 84
A
82-
i 8
H20/CH4=I
80. 78-
'~
9 9 9
9 9
9
PtAI PtZr Ptl0Zr
74.
0 7270,
,
0
,
,
5
.
,
10
.
,
15
.
,
.
20
. . . . . . .
25
30
35
40
time (h)
Fig. 2. CH4 conversion as a function of time on stream at 800~ methane.
for steam reforming of
136
3.2. Partial Oxidation The effect of temperature on the CH4 conversion and H2/CO product ratio for partial oxidation of methane, with OJCH4 feed ratio of 0.5, is displayed in Figure 3A and B. The activities of supported Pt catalysts were influenced by the nature of the support, with a better activation of PtZr at low temperatures. At temperatures higher than 550~ PtA1 and Ptl0Zr exhibited activities greater than PtZr. As temperature rises from 500 to 800~ the H2/CO ratio decreases from 2.5-3 to 1.5. At low temperatures the high HJCO ratio is associated with the reforming of methane with steam produced by total combustion, which suggests that the steam reforming is faster than reforming with CO2. At higher temperatures, the HJCO ratio is mainly related to the partial oxidation of methane, with simultaneous RWGS reaction. The catalyst stabilities for partial oxidation of methane at 800~ are shown in Figure 4. The catalysts exhibited similar initial activities and the deactivation rates of PtA1 and PtZr were very close: 0.21%/h and 0.18%/h, respectively. The deactivation is related to the deposition of inactive carbon over the active surface and the amount of coke on these catalysts, as quantified by thermogravimetric analysis carried out in an oxygen-containing atmosphere, was about 0.6-0.8 mg coke/h.gcat. On the other hand, the Ptl0Zr remained stable up to 54h of reaction, with a deactivation rate of only 0.03%/h.
100t A
3,2-'
H
80
2,8~ 2,6-
60 O O
40
T
200
' ,
-=- ptAI
I/
,
,
,
,
equilibrium
-:--~'~
2,4-
2,21,8
+ PtZr ---.A-- I~10Zr
II ,
~
\\
.
w e q u i l i b r i u m
~(t ,
~"k~
B
2,0
/ //
I
O2/CH =1/2
|
,
,
,
|
,
|
,
,
,
|
1,6 ,
,
1,4
,
300 350 400 450 500 550 600 650 700 750 800 850 Temperature
i
500
.
|
550
(~
9
|
9
600
|
650
Temperature
9
,
700
.
|
750
9
,
800
(~
Fig. 3. CH4 conversion (A) and H2/CO product ratio (B) of Pt catalysts for partial oxidation of methane as a function of temperature.
i
O,CH40 5
0
5
10 15 20 25 30 35 40 45 50 55 t i m e (h)
Fig. 4. CH4 conversion as a function of time on stream at 800~ methane.
for partial oxidation of
137
3.3. Oxy-steam reforming The comparison of catalyst activities in terms of CH4 conversion is presented in Figure 5 for oxy-steam reforming of methane. The PtZr showed the highest activity at 400~ with 20% of CH4 conversion, maintaining this conversion up to 550~ The Ptl0Zr was the most active catalyst over the midtemperature range (450-600~ while at higher temperatures PtZr exhibited slightly better activity than PtA1 and Pt 10Zr. The composition profiles for PtZr catalyst are shown in Figure 6. The 02 conversion is 100% over the whole temperature range. The production of steam and CO2 decreases rapidly with the temperature increase due to the exothermic nature of total combustion. The lower production of H2 and CO up to 600~ is also related to the occurrence of total combustion. At 800~ the H2/CO product ratio is 1.4, showing the influence of the water-gas shift reaction that occurs simultaneously with partial oxidation. These results suggest that the reaction sequence is probably initiated by methane combustion, followed by reforming of the remaining methane with CO2 and H20, which is in agreement with reported studies of oxy-steam reforming over supported noble metal catalysts in the literature [ 11, 12]. The effect of time on stream at 800~ for oxy-steam reforming is displayed in Figure 7. PtZr showed the highest initial activity, but deactivated at a rate of 0.86%/h during 28 h on stream. Also in this case the Ptl0Zr exhibited the best performance, with CH4 conversion decreasing from 82 to 68% in the first 10h, maintaining this conversion for over 60h.
7O
80.
O2/CH4=0.25 H
60-
.._.,
> w O
- - " H2 - - O - 02 A - CO
- - a - CH 4 --O-- CO 2 - v - H20
5060.
z
OJCH4=0.25 H20/CH4= 0.5
40-
40. 20.
~o ~ "/// ]'
- - n - - PtAI - - t - - PtZr --v-- Pt 10Zr
o.
20-
100-
400
500
600
700
800
900
Temperature (~
Fig. 5. C H 4 conversion as a function of time on stream at 800~ for oxy-steam reforming of methane.
400
500
600
700
8(~0
900
Temperature (~
Fig. 6. Composition profiles for oxy-steam reforming of methane as a function of temperature over the PtZr catalyst.
138
909
OJCH4:0.25 H20/CH,=0.5
9 9
85o~"
80-
~
75-
~
7o,
~
65.
9
PtAI PtZr Ptl0Zr
60-
;'
,'o'
;'
;
2o'
~'o ' 6'o' ~'o
Time (h)
Fig. 7. CH4 conversion as a function of time on stream at 800~ for oxy-steam reforming of methane. 4. C O N C L U S I O N S The Pt/10%ZrO2/A1203 material proved to be an effective catalyst for both steam reforming and partial oxidation of methane, besides CO2 reforming. The higher stability of this catalyst is closely related to its coking resistance, which has been attributed to Pt-Zr n+ interactions at metal-support interface [9, 13]. The interfacial sites on Pt-ZrOx are active for CO adsorption and CO2 dissociation, providing active species of oxygen that may react with carbon formed by CH4 decomposition on the metal particle, suppressing carbon accumulation. Moreover, zirconia is a well-know oxygen supplier, and its oxygen mobility is fast, which helps to keep the metal surface free of carbon.
REFERENCES [1] S.C. Tsang, J.B. Claridge and M.L.H. Green, Catal. Today 23 (1995) 3. [2] M.A. Pefia, J.P. G6mez and J.L.G Fierro, Appl. Catal. A 144 (1996) 7. [3] D.A. Hickman and L.D. Schmidt, Science 259 (jan.1993) 343. [4] D.A. Hickman, E.A. Haupfear and L.D. Schmidt, Catal. Lett. 17 (1993) 223. [5] D.A. Hickman and L.D. Schmidt, J. Catal. 138 (1992) 267. [6] S. Liu, G. Xiong, H. Dong and W. Yang, Appl. Catal. A 202 (2000) 141. [7] Z.-W. Liu, K.-W. Jun, H.-S. Roh and S.-E. Park, J. Power Sources 111 (2002) 283. [8] S. Ayabe, H. Omoto, T. Utaka, R. Kikuchi, K. Sasaki, Y. Yeraoka and K. Eguchi, Appl. Catal. A 241 (2003) 261. [9] M.M.V.M. Souza, D.A.G. Aranda and M. Schmal, J. Catal. 204 (2001) 498. [10] M.E.S. Hegarty, A.M. O'Connor and J.R.H. Ross, Catal. Today 42 (1998) 225. [11] D. Dissanayaki, M.P. Rosynek and J.H. Lunsford, J. Phys. Chem. 97 (1993) 3644. [12] E.P.J. Mallens, J.H.B. Hoebink and G.B. Matin, Catal. Lett. 33 (1995) 291. [13] M.M.V.M. Souza, D.A.G. Aranda and M. Schmal, Ind. Eng. Chem. Res. 41 (2002) 4681.
133
Production of synthesis gas from natural gas using ZrO2-supported platinum Mariana M.V.M.
Souza
a
and M a r t i n S c h m a l *a' b
aNUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, 21945-970, Rio de Janeiro, Brazil. E-mail: [email protected] bEscola de Quimica, Universidade Federal do Rio de Janeiro, C.P. 68542, 21940-900, Rio de Janeiro, Brazil. ABSTRACT Reforming of methane with steam, partial oxidation of methane and a combination of both reactions were camed out using Pt/AI203, Pt/ZrO2 and Pt/10%ZrOz/A1203 catalysts, in temperature range between 350-900~ For steam reforming, the H2/CO product ratio is strongly influenced by the watergas shift reaction, with the Ptl0Zr sample showing the highest activity and stability. When oxygen is present in the feed, the total combustion is the main reaction at low temperatures. The catalyst with 10% of ZrO2 also presented the best performance for partial oxidation and oxy-steam reforming. 1. INTRODUCTION Renewed attention in both academic and industrial research has recently been focused on alternative routes for conversion of natural gas to synthesis gas, a mixture of H2 e CO, which can be used to produce chemical products with high added value. The steam reforming of methane (SRM) is the most important industrial process for the production of synthesis gas. SRM is a very energyintensive process because of the highly endothermic property of the reaction; the high H2/CO ratio (3:1) is useful for processes requiring a Hz-rich feed such as ammonia synthesis and petroleum refining process [1, 2]. Methane partial oxidation (POM) is an advantageous route for synthesis gas production for both economical and technical reasons: it makes the process less energy and capital cost intensive because of its exothermic nature and the lower H2/CO ratio (about 2) is more favorable with respect to downstream processes such as methanol synthesis and Fischer-Tropsch synthesis of higher hydrocarbons [3-6].
134
On the other hand, oxy-steam reforming, which integrates SRM and POM simultaneously, has many advantages such as: low-energy requirements due to the opposite contribution of the exothermic methane oxidation and endothermic steam reforming; low specific consumption; variable H2/CO ratio regulated by varying the feed composition [7, 8]. We have previously reported work on CO2 reforming of methane showing that Pt/ZrO2 and Pt/ZrOz/AI203 are effective formulations for this reaction [9]. This paper gives results on the testing of these novel zirconia-supported catalysts for the steam reforming and partial oxidation of methane, as well as a combination of this both reactions.
2. EXPERIMENTAL 2.1. Catalyst Preparation A1203 and ZrO2 supports were prepared by calcination of 7-alumina (Engelhard) and zirconium hydroxide (MEL Chemicals) at 550~ for 2h under flowing air. 10%ZrO2/A1203 was prepared by impregnation over alumina powder with a nitric acid solution of zirconium hydroxide; the mixture was stirred for 2h at 90~ dried and calcined at the same conditions as the bare supports [9]. The catalysts were prepared by incipient wetness technique, using an aqueous solution of H2PtC16 (Aldrich), followed by drying at 120~ for 16 h and calcination in air at 550~ for 2 h. The platinum content was around l wt%, which was measured by atomic absorption spectrometry. Prepared catalysts will be referred to as PtA1 for Pt/A1203, PtZr for Pt/ZrO2 and Ptl0Zr for Pt/10%ZrO2/A1203.
2.2. Catalyst testing Catalyst testing was performed in a flow tubular quartz reactor loaded with 20mg of catalyst, under atmospheric pressure. The total feed flow rate was held constant at 200 cm3/min (WHSV- 600,000 cm3/h.gcat), with flowing He. The activity tests were performed at different temperatures, ranging from 350 to 900~ in steps of 50~ that were kept for 30 rain at each temperature. The loss in catalyst activity at 800~ was monitored up to 70 h on stream. The reaction products were analyzed by on-line gas chromatograph (CHROMPACK CP9001), equipped with a Hayesep D column and a thermal conductivity detector. 3. RESULTS AND DISCUSSION
3.1. Steam Reforming The comparison of catalyst activities for steam reforming under stoichiometric feed (CH4:H20-I:I) was carried out as a function of temperature
135
(400-800~ and the results are displayed in Figure 1A and B, in terms of CH4 conversion and H2/CO product ratio. The experimental CH4 conversion increased almost linearly with increasing reaction temperature from 400 to 650~ for PtA1 and Ptl 0Zr catalysts, with PtA1 showing the highest conversions at high temperatures. The PtZr catalyst presented low conversions up to 600~ simultaneously this catalyst exhibited the highest H2/CO ratio in this temperature range. The observed high H2/CO ratio (>8.0) suggests that watergas shift (WGS) reaction occurs to a great extent with SRM, as already observed in the literature [7, 10]. At the same time, the decrease in H2/CO ratio with increasing temperature is consistent with the fact that the WGS reaction is thermodynamically unfavorable at higher temperatures. The stability of the catalysts for steam reforming of methane at 800~ is displayed in Figure 2. The PtA1 showed an oscillatory behavior around 72% of conversion, while the PtZr deactivated at a rate of 0.25%/h, probably due to deposition of inactive carbon. The Ptl0Zr sample presented the highest activity and stability, giving a constant conversion of about 81% throughout the duration of the experiment (40h). These results are comparable to the reported values for Pt/ZrO2 of Hegarty et al. [ 10]. lOO
H20/CH4 = 1
16
90 ..-..
80
g
60
>
50
9
B
H20/CH4= 1
12 10
\ 9 % \ ~ ~x,,~
w 4o f o
9
14
20
_
lO o
_
4
9
4;0
5;0
6;0 T e m p e r a t u r e
~.
6-
um
ao
equilibrium --m-- PtAI --t--PEr _A_ Ptl0Zr
Ptl0Zr
~;o
2
5~o
~;o
6~o
40
8~o
Temperature (~
(~
Fig. 1. CH4 conversion (A) and H2/CO product ratio (B) of Pt catalysts for steam reforming of methane as a function of temperature. 84
A
82-
i 8
H20/CH4=I
80. 78-
'~
9 9 9
9 9
9
PtAI PtZr Ptl0Zr
74.
0 7270,
,
0
,
,
5
.
,
10
.
,
15
.
,
.
20
. . . . . . .
25
30
35
40
time (h)
Fig. 2. CH4 conversion as a function of time on stream at 800~ methane.
for steam reforming of
136
3.2. Partial Oxidation The effect of temperature on the CH4 conversion and H2/CO product ratio for partial oxidation of methane, with OJCH4 feed ratio of 0.5, is displayed in Figure 3A and B. The activities of supported Pt catalysts were influenced by the nature of the support, with a better activation of PtZr at low temperatures. At temperatures higher than 550~ PtA1 and Ptl0Zr exhibited activities greater than PtZr. As temperature rises from 500 to 800~ the H2/CO ratio decreases from 2.5-3 to 1.5. At low temperatures the high HJCO ratio is associated with the reforming of methane with steam produced by total combustion, which suggests that the steam reforming is faster than reforming with CO2. At higher temperatures, the HJCO ratio is mainly related to the partial oxidation of methane, with simultaneous RWGS reaction. The catalyst stabilities for partial oxidation of methane at 800~ are shown in Figure 4. The catalysts exhibited similar initial activities and the deactivation rates of PtA1 and PtZr were very close: 0.21%/h and 0.18%/h, respectively. The deactivation is related to the deposition of inactive carbon over the active surface and the amount of coke on these catalysts, as quantified by thermogravimetric analysis carried out in an oxygen-containing atmosphere, was about 0.6-0.8 mg coke/h.gcat. On the other hand, the Ptl0Zr remained stable up to 54h of reaction, with a deactivation rate of only 0.03%/h.
100t A
3,2-'
H
80
2,8~ 2,6-
60 O O
40
T
200
' ,
-=- ptAI
I/
,
,
,
,
equilibrium
-:--~'~
2,4-
2,21,8
+ PtZr ---.A-- I~10Zr
II ,
~
\\
.
w e q u i l i b r i u m
~(t ,
~"k~
B
2,0
/ //
I
O2/CH =1/2
|
,
,
,
|
,
|
,
,
,
|
1,6 ,
,
1,4
,
300 350 400 450 500 550 600 650 700 750 800 850 Temperature
i
500
.
|
550
(~
9
|
9
600
|
650
Temperature
9
,
700
.
|
750
9
,
800
(~
Fig. 3. CH4 conversion (A) and H2/CO product ratio (B) of Pt catalysts for partial oxidation of methane as a function of temperature.
i
O,CH40 5
0
5
10 15 20 25 30 35 40 45 50 55 t i m e (h)
Fig. 4. CH4 conversion as a function of time on stream at 800~ methane.
for partial oxidation of
137
3.3. Oxy-steam reforming The comparison of catalyst activities in terms of CH4 conversion is presented in Figure 5 for oxy-steam reforming of methane. The PtZr showed the highest activity at 400~ with 20% of CH4 conversion, maintaining this conversion up to 550~ The Ptl0Zr was the most active catalyst over the midtemperature range (450-600~ while at higher temperatures PtZr exhibited slightly better activity than PtA1 and Pt 10Zr. The composition profiles for PtZr catalyst are shown in Figure 6. The 02 conversion is 100% over the whole temperature range. The production of steam and CO2 decreases rapidly with the temperature increase due to the exothermic nature of total combustion. The lower production of H2 and CO up to 600~ is also related to the occurrence of total combustion. At 800~ the H2/CO product ratio is 1.4, showing the influence of the water-gas shift reaction that occurs simultaneously with partial oxidation. These results suggest that the reaction sequence is probably initiated by methane combustion, followed by reforming of the remaining methane with CO2 and H20, which is in agreement with reported studies of oxy-steam reforming over supported noble metal catalysts in the literature [ 11, 12]. The effect of time on stream at 800~ for oxy-steam reforming is displayed in Figure 7. PtZr showed the highest initial activity, but deactivated at a rate of 0.86%/h during 28 h on stream. Also in this case the Ptl0Zr exhibited the best performance, with CH4 conversion decreasing from 82 to 68% in the first 10h, maintaining this conversion for over 60h.
7O
80.
O2/CH4=0.25 H
60-
.._.,
> w O
- - " H2 - - O - 02 A - CO
- - a - CH 4 --O-- CO 2 - v - H20
5060.
z
OJCH4=0.25 H20/CH4= 0.5
40-
40. 20.
~o ~ "/// ]'
- - n - - PtAI - - t - - PtZr --v-- Pt 10Zr
o.
20-
100-
400
500
600
700
800
900
Temperature (~
Fig. 5. C H 4 conversion as a function of time on stream at 800~ for oxy-steam reforming of methane.
400
500
600
700
8(~0
900
Temperature (~
Fig. 6. Composition profiles for oxy-steam reforming of methane as a function of temperature over the PtZr catalyst.
138
909
OJCH4:0.25 H20/CH,=0.5
9 9
85o~"
80-
~
75-
~
7o,
~
65.
9
PtAI PtZr Ptl0Zr
60-
;'
,'o'
;'
;
2o'
~'o ' 6'o' ~'o
Time (h)
Fig. 7. CH4 conversion as a function of time on stream at 800~ for oxy-steam reforming of methane. 4. C O N C L U S I O N S The Pt/10%ZrO2/A1203 material proved to be an effective catalyst for both steam reforming and partial oxidation of methane, besides CO2 reforming. The higher stability of this catalyst is closely related to its coking resistance, which has been attributed to Pt-Zr n+ interactions at metal-support interface [9, 13]. The interfacial sites on Pt-ZrOx are active for CO adsorption and CO2 dissociation, providing active species of oxygen that may react with carbon formed by CH4 decomposition on the metal particle, suppressing carbon accumulation. Moreover, zirconia is a well-know oxygen supplier, and its oxygen mobility is fast, which helps to keep the metal surface free of carbon.
REFERENCES [1] S.C. Tsang, J.B. Claridge and M.L.H. Green, Catal. Today 23 (1995) 3. [2] M.A. Pefia, J.P. G6mez and J.L.G Fierro, Appl. Catal. A 144 (1996) 7. [3] D.A. Hickman and L.D. Schmidt, Science 259 (jan.1993) 343. [4] D.A. Hickman, E.A. Haupfear and L.D. Schmidt, Catal. Lett. 17 (1993) 223. [5] D.A. Hickman and L.D. Schmidt, J. Catal. 138 (1992) 267. [6] S. Liu, G. Xiong, H. Dong and W. Yang, Appl. Catal. A 202 (2000) 141. [7] Z.-W. Liu, K.-W. Jun, H.-S. Roh and S.-E. Park, J. Power Sources 111 (2002) 283. [8] S. Ayabe, H. Omoto, T. Utaka, R. Kikuchi, K. Sasaki, Y. Yeraoka and K. Eguchi, Appl. Catal. A 241 (2003) 261. [9] M.M.V.M. Souza, D.A.G. Aranda and M. Schmal, J. Catal. 204 (2001) 498. [10] M.E.S. Hegarty, A.M. O'Connor and J.R.H. Ross, Catal. Today 42 (1998) 225. [11] D. Dissanayaki, M.P. Rosynek and J.H. Lunsford, J. Phys. Chem. 97 (1993) 3644. [12] E.P.J. Mallens, J.H.B. Hoebink and G.B. Matin, Catal. Lett. 33 (1995) 291. [13] M.M.V.M. Souza, D.A.G. Aranda and M. Schmal, Ind. Eng. Chem. Res. 41 (2002) 4681.