- Email: [email protected]
Ruthenium Pyrochlore Catalyst
K.J. Smith, E.C. Sanford (Editors), Progress in Catalysis 1992 Elsevier Science Publishers B.V.
359
Partial Oxidation of Methane over a Praseodymium/Ruthenium Pyrochlore Catalyst Michel G. Poirier, Gilles Jean and Martin P. Poirier Laboratoire de recherche en diversification CnergCtique, CANMET, EMR Canada, 2082 Boulevard Marie-Victorin, Suite 210, Varennes, Qukbec, J3X 1R3, CANADA Abstract The partial oxidation of methane to synthesis gas over a praseodymium ruthenium oxide catalyst with a pyrochlore structure (Pr2Ru207)was investigated by microreactor experiments and by thermal analysis (TG/DTA). The activity of the Pr,Ru,O, catalyst was compared to that observed with pure praseodymium oxide (Pr6OI1/PrzO3) and with 0.1 % (w/w) Ru/AI,03. TG/DTA analysis under a methane/oxygen mixture showed that PrZRu207has to be reduced to ruthenium metal and to praseodymium oxide to be active for methane conversion. Results obtained in microreactor experiments showed that ruthenium metal is the active site for the formation of synthesis gas, praseodymium oxide being active mainly for the combustion of methane to CO, and water.
1. INTRODUCTION The production of synthesis gas from natural gas is usually achieved through steam reforming of methane, which is a highly endothermic reaction:
An alternative reaction is the partial oxidation of methane, which is exothermic and leads to a H,/CO ratio of two instead of three: CH,
-
+ %02 CO + 2H, .
(2)
The latter reaction has recently been studied over various catalysts [l-41. In one of these studies, Ashcroft et al. [l] have reported excellent results obtained with several lanthanide ruthenium oxides having a pyrochlore structure (Ln,Ru207). At 777"C, atmospheric pressure and a 4/2/1 mixture of nitrogen/methane/oxygen with a total gas hourly space velocity of 4x104 h-I, methane conversion was over 89% and selectivity to carbon monoxide and hydrogen was higher than 94%. Their study with one particular catalyst, praseodymium ruthenium oxide (Pr2Ru20,), indicated a change in the catalyst structure upon reaction. That was determined by XRD and XPS analyses of a used catalyst sample, which showed the presence of ruthenium metal and graphitic-like carbon deposition on the surface. From these
360
informations, the authors suggested that the active catalyst was supported ruthenium metal and that a carbonaceous surface may have been involved. However, no indication was given on what the praseodymium structure may be during the reaction. The aim of the present work was to investigate the behaviour of a Pr2Ru,07 pyrochlore catalyst in the course of the methane partial oxidation reaction, and to get some information on the active species. Experiments in a fixed bed microreactor using Pr2Ru20,as well as pure praseodymium oxide and 0.1% (w/w) Ru/AI,O,, were performed in order to record the change in the reacted gas composition with temperature. Thermal analyses (TG/DTA) were performed with Pr2Ru20,to observe the change in the structure of the catalyst upon reaction. The results obtained are used to determine the active site for the partial oxidation of methane.
2. EXPERIMENTAL The praseodymium oxide (Pr6011)used was of 99.99% purity (Johnson Matthey AESAR Group, #11235). The praseodymium ruthenium oxide (Pr2Ru207)was prepared by mixing Ru02.xH,0 (Johnson Matthey - AESAR Group, #11803) with Pr,Oll and by heating the mixture at 1133 K in air for 100 hours. XRD analysis of the powder obtained confirmed that it was mainly Pr2Ru20,with a pyrochlore structure. Catalyst 0.1 % (w/w) Ru/AI,03 was prepared by mixing AI2O3(Aldrich, #19,997-4) with RuCI, .3H20, 99.9% (Johnson Matthey AESAR Group, #11043) dissolved in demineralized water. The suspension obtained was evaporated and dried overnight at 120°C. The powder was then heated in a hydrogen flow at 2"C/min from ambient temperature to 420"C, with a plateau of 2 hours at that temperature. The temperature was then lowered, hydrogen was replaced by oxygen and the catalyst was heated again at S"C/min from ambient temperature to 600"C, with a plateau of 20 minutes at 600°C. The catalysts were tested in a fixed-bed microreactor. The gases used were monitored by MKS mass flow controllers, mixed and directed to a quartz reactor (i.d. = 7 mm). The catalysts were held in the middle of the reactor by a small amount of quartz wool. The thermocouple used by the temperature controller was inserted in a quartz well and located into the catalytic bed. At the exit of the reactor, the gases were analyzed on line, in parallel, with a Hewlett-Packard gas chromatograph (5890A) modified by WASSON ECE, and with a VG Instruments mass spectrometer (model 8-80). The gas line between the microreactor and the gas chromatograph was heated above 120"C, and each gas, including water, was analyzed once every 18 minutes. A cold trap maintained below 0°C was used along the line between the microreactor and the mass spectrometer, thus allowing the rapid analysis of every gas except water. For each experiment, an amount of about 200 mg of catalyst was used. The feed gas mixture used was approximately 20% oxygen (Linde, extra dry grade), 76% methane (Linde, UHP grade) and 4% helium (Linde, UHP grade) added as an internal standard. The gas flow-rate was 54 mL NPT per minute. Temperature was first increased from ambient to 250°C in ten minutes, then from 250°C to 600°C in two hours. At this point, a plateau of twenty minutes was held at 6OO"C,then the temperature was further increased from 600°C to 900°C in ninety minutes and this final temperature was held for few minutes before cooling down the reactor. All the experiments were performed at a total pressure of 110 H a .
36 1
The thermal analysis studies were performed with a TG/DTA 220, model SSC 5200H, from Seiko Instruments. The apparatus was specially manufactured in order to have all of the internal heated parts made of ceramic. This allows the use of methane/oxygen mixtures at temperatures up to 1100°C.
3. RESULTS AND DISCUSSION Figure 1 shows the evolution of the gases after reaction over Pr2Ru207as a function of time. For clarity purposes, helium added as an internal standard is not shown on this Figure nor on the other Figures. The results in Figure 1 indicate that at temperatures lower than 420°C there is only a small production of C02. Then, at that temperature, a reaction occurs suddenly, leading to the complete conversion of oxygen. At this point, a thermocouple located into the catalytic bed indicates a large exotherm, the temperature having increased rapidly from 420°C to 500°C. After the first few minutes of reaction, oxygen is all consumed and the product gas is composed of unreacted methane, hydrogen, water, C02, and CO, the latter in smaller amount. With a further increase in temperature, the percentage of hydrogen and CO in the reacted gases increases while that of methane, water and CQ decreases. 80
69
v
900
70
aoo
60
100
50
600
40
30 20 10
0
f 0
5004
400G 300
200 100
0
0.8
1.6
2.4
3.2
4
Time ( h r )
Figure 1:
Change in gas composition when temperature is increased from ambient to 900°C over Pr2Ru20,catalyst.
These results indicate that Pr2Ru,07is a good catalyst for the reactions of methane with oxygen at low temperature, and is also active for the further reaction of methane with water and C 0 2 to give synthesis gas according to reactions 1 and 3:
362
CH,
+ CO, 2 2CO + 2 H 2 .
(3)
These reactions 1 and 3 are endothermic and thermodynamically favoured at higher temperature [ 5 ] . Prette et al. [6] obtained similar results some 45 years ago, when they studied the partial oxidation of methane. They observed a temperature increase in the first part of their catalytic bed, followed by a temperature decrease. This led them to suggest that the mechanism leading to synthesis gas from methane/oxygen mixtures involves two steps, an exothermic one followed by an endothermic one. They proposed that methane/oxygen mixtures are first converted according to the following three exothermic reactions:
CH, CH, CH,
+ 20, CO, + 2H,O + 1.50, CO + 2H20 + 0, CO, + 2H2 . +
+
+
These reactions consume all available oxygen. After that, CO, and water react further with the remaining methane to give synthesis gas according to reactions 1 and 3. The Pr,Ru,O, catalyst is thus very active in promoting the kinetics of reactions 1 and 3 and the product distribution observed from 500°C to 900°C are ensued from the equilibrium of those reactions. The almost complete conversion of water and CO, occurs at about 800" C. 150
16.2 16
100
15.8 15.6
2 15.4
50
0
v L
-g 1 5 . 2
.cn _
g
15 14. 8 14.6
-50
14.4 14.2
-
0
50
100
150
200
100
250
Time (min)
Figure 2:
Mass and DTA variations of Pr,Ru,07 catalyst in a mixture of 20% (v/v) oxygen in methane. Point A: 465"C, 15.97 mg; point B: 640"C, 14.36 mg; point C: 806"C, 14.27 mg.
The experimental conditions of the catalytic run were reproduced in a TG/DTA
363
apparatus. This allowed the thermal analysis of the catalyst during the methane partial oxidation reaction (Figure 2). The mass variation (TG) shows a large weight lost at 465°C (point A), and a further smaller lost at around 650°C.At the beginning of the weight lost, the DTA signal increases, reaches a maximum and then decreases and stays at a positive plateau. A positive DTA signal indicates an exothermic reaction. The DTA behaviour is thus in agreement with the increase in temperature inside the microreactor when oxygen starts to be consumed, and confirms that at this temperature (465°C) the overall reaction is exothermic. At higher temperatures, the DTA signal becomes negative, which indicates an endothermic reaction. The total weight lost observed between 465°C and 806°C (from point A to point C) is 10.64%, which is close to the value of 10.74% corresponding to the reduction of the pyrochlore catalyst according to: Pr,RuzO,
+ CH,
+
2Ru" + Pr203 + C02 + 2H20 .
(7)
The difference between 10.64% and 10.74% is probably due to a small amount of impurities in the Pr2Ruz0, sample. The small weight lost at around 650°C could result from a decrease in the oxidation state of praseodymium oxide, since it is kr m that this oxide can take several oxidation states according to the temperature and the oxygen partial pressure, even in methane/oxygen mixtures [7]. 30
,
,
90 80
25
70
-
60
20
- 15
50
be
v
40
10
3 0-
2
a
20
5
10
0
0 1.34
3-
1.39
1.44
1.49
Time ( h r )
Figure 3:
Expanded view of Figure 1 where oxygen starts to be fully consumed. Temperature is about 420°C. Water and helium are not shown.
It is thus suggested that at 465"C,the pyrochlore catalyst is reduced to ruthenium metal and praseodymium sesquioxide. Further proof is provided on Figure 3, which shows an
364
expanded view of Figure 1 where the catalyst starts to be fully active. This occurred at about 420°C on Figure 1 instead of 465°C as on Figure 2. The difference between these two reaction temperatures is due to the difference in catalyst amount and from the difference in contact between solid and gas. It can be seen on Figure 3 that oxygen becomes completely consumed in a two-minute interval. The CO, percentage starts from about 1%, reaches 27.5 % and then decreases and stabilizes at around 12%. Hydrogen and CO appear a few seconds after COz, increase less rapidly, reach their maximum values and then decrease slowly due to a small decrease in temperature following the large exothermic effect. CO, and water (not shown in Figure 3) are thus the primary products. The amount of C02produced from gaseous oxygen is 12%,and the higher percentage of CO, at the beginning comes from the reaction of methane with the oxygen of the catalyst during the reduction process. This partial reduction of the catalyst is largely responsible for the temperature exotherm recorded in the catalytic bed and is in agreement with the DTA signal observed in Figure 2. From the first three figures, one may conclude that praseodymium ruthenium oxide is not active and has to be reduced to ruthenium metal and to praseodymium sesquioxide to promote methaneloxygen reactions. Once reduced, the catalyst is very active towards methane combustion and synthesis gas formation, the change in gas composition observed being the one ensued from thermodynamic equilibrium. At 777"C, Ashcroft et al. [l] reported the presence of graphitic carbon on their used sample. Carbon deposition should be associated to a mass increase, but in Figure 2 there is no apparent mass gain at temperatures lower than 900°C
20
h
be W
r 80
75
f'" t Oxyge
70
65
2 3 60 2
l5
rn
55 .3 50 45 40
1
2
3
4
5
Time [ h r ) Figure 4:
Change in gas composition when temperature is increased from ambient to 900°C over praseodymium oxide.
806°C. There is, however, a slight mass increase at temperatures higher than 806"C, that
365
may be due to carbon formation. The role of praseodymium oxide and ruthenium metal have now to be determined. Praseodymium oxide (Pr6OlI)was tested in the microreactor and the results are shown
on Figure 4. This oxide allows the complete conversion of oxygen at 455°C. The products
obtained are mainly water and COz in a two to one ratio, with smaller amounts of ethane, ethylene, hydrogen, and CO, the last two increasing above 800°C. A TG/DTA experiment indicated that under a methane/oxygen mixture at a temperature higher than 455"C, Pr6OI1 is reduced, and that above 800"C, a sesquioxide structure (Pr203)is formed. These results indicate that praseodymium oxide is a good catalyst for methane combustion, but is not able to promote the reaction of methane with C 0 2 and water, and therefore to allow reactions 1 and 3 to reach equilibrium under our experimental conditions. Finally, a catalyst made of 0.1 weight percent of ruthenium supported on alumina (0.1 % (w/w) Ru/Al,O,) was tested in the fixed-bed microreactor (Figure 5). The activity of this catalyst for the partial oxidation of methane is quite similar to the one observed over Pr2Ru20,(Figure 1). This catalyst is thus active for the methane/oxygen reaction, as well as for the reactions of methane with COz and water to produce synthesis gas. 90
l-----l I
900
Methane
h
LO
800
700
600
50
500+
40
400-
30
300
20
200
10
100
0
0
0
0
1
2
3
4
Time ( h r )
Figure 5:
Change in gas composition when temperature is increased from ambient to 900°C over 0.1 % (w/w) Ru/Alz03.
From the results presented here, it appears that ruthenium metal is the active site promoting the formation of synthesis gas according to reactions 1 and 3. Praseodymium sesquioxide may play a role, but it is mainly for the first reactions of methane combustion.
366
Moreover, Figure 5 shows that under our reaction conditions, 0.1 % ruthenium is enough to allow reactions 1 and 3 to reach equilibrium.
4. CONCLUSIONS Praseodymium ruthenium oxide (Pr2Ru2O7)is not active by itself and has to be reduced to praseodymium sesquioxide and ruthenium metal to promote the partial oxidation of methane to synthesis gas. Praseodymium sesquioxide takes part in the methane/oxygen reactions leading to CO, and water, but does not promote the production of synthesis gas. On the other hand, ruthenium metal is very active in promoting the reaction of methane with water and C02 to produce synthesis gas. Under our experimental conditions, these reactions are brought to equilibrium with as little as 0.1 % (w/w) ruthenium on alumina.
5. REFERENCES 1-
A.T. Ashcroft, A.K. Cheetham, J.S. Foord, M.L.H. Green, C.P. Grey, A.J. Murrell and P.D.F. Vernon, NATURE, 344 (1990) 319. 2- P.D.F. Vernon, M.L.H. Green, A.K. Cheetham and A.T. Ashcroft, Catal. Lett., 6 (1990) 181. 3- R.H. Jones, A.T. Ashcroft, D. Waller, A.K. Cheetham and J.M. Thomas, Catal. Lett., 8 (1991) 169. 4- D. Dissanayake, M.P. Rosynek, K.C.C. Kharas and J.H. Lunsford, J. Catal., 132 (1991) 117. 5- G.A. Mills and F.W. Steffgen, Catal. Rev., 8 (1973) 159. 6- M. Prettre, Ch. Eichner and M. Pemn, Trans. Faraday SOC.,43 (1946) 335. 7- M.G. Poirier, R. Breault, S. Kaliaguine and A. Adnot, Appl. Catal., 71 (1991) 103.
359
Partial Oxidation of Methane over a Praseodymium/Ruthenium Pyrochlore Catalyst Michel G. Poirier, Gilles Jean and Martin P. Poirier Laboratoire de recherche en diversification CnergCtique, CANMET, EMR Canada, 2082 Boulevard Marie-Victorin, Suite 210, Varennes, Qukbec, J3X 1R3, CANADA Abstract The partial oxidation of methane to synthesis gas over a praseodymium ruthenium oxide catalyst with a pyrochlore structure (Pr2Ru207)was investigated by microreactor experiments and by thermal analysis (TG/DTA). The activity of the Pr,Ru,O, catalyst was compared to that observed with pure praseodymium oxide (Pr6OI1/PrzO3) and with 0.1 % (w/w) Ru/AI,03. TG/DTA analysis under a methane/oxygen mixture showed that PrZRu207has to be reduced to ruthenium metal and to praseodymium oxide to be active for methane conversion. Results obtained in microreactor experiments showed that ruthenium metal is the active site for the formation of synthesis gas, praseodymium oxide being active mainly for the combustion of methane to CO, and water.
1. INTRODUCTION The production of synthesis gas from natural gas is usually achieved through steam reforming of methane, which is a highly endothermic reaction:
An alternative reaction is the partial oxidation of methane, which is exothermic and leads to a H,/CO ratio of two instead of three: CH,
-
+ %02 CO + 2H, .
(2)
The latter reaction has recently been studied over various catalysts [l-41. In one of these studies, Ashcroft et al. [l] have reported excellent results obtained with several lanthanide ruthenium oxides having a pyrochlore structure (Ln,Ru207). At 777"C, atmospheric pressure and a 4/2/1 mixture of nitrogen/methane/oxygen with a total gas hourly space velocity of 4x104 h-I, methane conversion was over 89% and selectivity to carbon monoxide and hydrogen was higher than 94%. Their study with one particular catalyst, praseodymium ruthenium oxide (Pr2Ru20,), indicated a change in the catalyst structure upon reaction. That was determined by XRD and XPS analyses of a used catalyst sample, which showed the presence of ruthenium metal and graphitic-like carbon deposition on the surface. From these
360
informations, the authors suggested that the active catalyst was supported ruthenium metal and that a carbonaceous surface may have been involved. However, no indication was given on what the praseodymium structure may be during the reaction. The aim of the present work was to investigate the behaviour of a Pr2Ru,07 pyrochlore catalyst in the course of the methane partial oxidation reaction, and to get some information on the active species. Experiments in a fixed bed microreactor using Pr2Ru20,as well as pure praseodymium oxide and 0.1% (w/w) Ru/AI,O,, were performed in order to record the change in the reacted gas composition with temperature. Thermal analyses (TG/DTA) were performed with Pr2Ru20,to observe the change in the structure of the catalyst upon reaction. The results obtained are used to determine the active site for the partial oxidation of methane.
2. EXPERIMENTAL The praseodymium oxide (Pr6011)used was of 99.99% purity (Johnson Matthey AESAR Group, #11235). The praseodymium ruthenium oxide (Pr2Ru207)was prepared by mixing Ru02.xH,0 (Johnson Matthey - AESAR Group, #11803) with Pr,Oll and by heating the mixture at 1133 K in air for 100 hours. XRD analysis of the powder obtained confirmed that it was mainly Pr2Ru20,with a pyrochlore structure. Catalyst 0.1 % (w/w) Ru/AI,03 was prepared by mixing AI2O3(Aldrich, #19,997-4) with RuCI, .3H20, 99.9% (Johnson Matthey AESAR Group, #11043) dissolved in demineralized water. The suspension obtained was evaporated and dried overnight at 120°C. The powder was then heated in a hydrogen flow at 2"C/min from ambient temperature to 420"C, with a plateau of 2 hours at that temperature. The temperature was then lowered, hydrogen was replaced by oxygen and the catalyst was heated again at S"C/min from ambient temperature to 600"C, with a plateau of 20 minutes at 600°C. The catalysts were tested in a fixed-bed microreactor. The gases used were monitored by MKS mass flow controllers, mixed and directed to a quartz reactor (i.d. = 7 mm). The catalysts were held in the middle of the reactor by a small amount of quartz wool. The thermocouple used by the temperature controller was inserted in a quartz well and located into the catalytic bed. At the exit of the reactor, the gases were analyzed on line, in parallel, with a Hewlett-Packard gas chromatograph (5890A) modified by WASSON ECE, and with a VG Instruments mass spectrometer (model 8-80). The gas line between the microreactor and the gas chromatograph was heated above 120"C, and each gas, including water, was analyzed once every 18 minutes. A cold trap maintained below 0°C was used along the line between the microreactor and the mass spectrometer, thus allowing the rapid analysis of every gas except water. For each experiment, an amount of about 200 mg of catalyst was used. The feed gas mixture used was approximately 20% oxygen (Linde, extra dry grade), 76% methane (Linde, UHP grade) and 4% helium (Linde, UHP grade) added as an internal standard. The gas flow-rate was 54 mL NPT per minute. Temperature was first increased from ambient to 250°C in ten minutes, then from 250°C to 600°C in two hours. At this point, a plateau of twenty minutes was held at 6OO"C,then the temperature was further increased from 600°C to 900°C in ninety minutes and this final temperature was held for few minutes before cooling down the reactor. All the experiments were performed at a total pressure of 110 H a .
36 1
The thermal analysis studies were performed with a TG/DTA 220, model SSC 5200H, from Seiko Instruments. The apparatus was specially manufactured in order to have all of the internal heated parts made of ceramic. This allows the use of methane/oxygen mixtures at temperatures up to 1100°C.
3. RESULTS AND DISCUSSION Figure 1 shows the evolution of the gases after reaction over Pr2Ru207as a function of time. For clarity purposes, helium added as an internal standard is not shown on this Figure nor on the other Figures. The results in Figure 1 indicate that at temperatures lower than 420°C there is only a small production of C02. Then, at that temperature, a reaction occurs suddenly, leading to the complete conversion of oxygen. At this point, a thermocouple located into the catalytic bed indicates a large exotherm, the temperature having increased rapidly from 420°C to 500°C. After the first few minutes of reaction, oxygen is all consumed and the product gas is composed of unreacted methane, hydrogen, water, C02, and CO, the latter in smaller amount. With a further increase in temperature, the percentage of hydrogen and CO in the reacted gases increases while that of methane, water and CQ decreases. 80
69
v
900
70
aoo
60
100
50
600
40
30 20 10
0
f 0
5004
400G 300
200 100
0
0.8
1.6
2.4
3.2
4
Time ( h r )
Figure 1:
Change in gas composition when temperature is increased from ambient to 900°C over Pr2Ru20,catalyst.
These results indicate that Pr2Ru,07is a good catalyst for the reactions of methane with oxygen at low temperature, and is also active for the further reaction of methane with water and C 0 2 to give synthesis gas according to reactions 1 and 3:
362
CH,
+ CO, 2 2CO + 2 H 2 .
(3)
These reactions 1 and 3 are endothermic and thermodynamically favoured at higher temperature [ 5 ] . Prette et al. [6] obtained similar results some 45 years ago, when they studied the partial oxidation of methane. They observed a temperature increase in the first part of their catalytic bed, followed by a temperature decrease. This led them to suggest that the mechanism leading to synthesis gas from methane/oxygen mixtures involves two steps, an exothermic one followed by an endothermic one. They proposed that methane/oxygen mixtures are first converted according to the following three exothermic reactions:
CH, CH, CH,
+ 20, CO, + 2H,O + 1.50, CO + 2H20 + 0, CO, + 2H2 . +
+
+
These reactions consume all available oxygen. After that, CO, and water react further with the remaining methane to give synthesis gas according to reactions 1 and 3. The Pr,Ru,O, catalyst is thus very active in promoting the kinetics of reactions 1 and 3 and the product distribution observed from 500°C to 900°C are ensued from the equilibrium of those reactions. The almost complete conversion of water and CO, occurs at about 800" C. 150
16.2 16
100
15.8 15.6
2 15.4
50
0
v L
-g 1 5 . 2
.cn _
g
15 14. 8 14.6
-50
14.4 14.2
-
0
50
100
150
200
100
250
Time (min)
Figure 2:
Mass and DTA variations of Pr,Ru,07 catalyst in a mixture of 20% (v/v) oxygen in methane. Point A: 465"C, 15.97 mg; point B: 640"C, 14.36 mg; point C: 806"C, 14.27 mg.
The experimental conditions of the catalytic run were reproduced in a TG/DTA
363
apparatus. This allowed the thermal analysis of the catalyst during the methane partial oxidation reaction (Figure 2). The mass variation (TG) shows a large weight lost at 465°C (point A), and a further smaller lost at around 650°C.At the beginning of the weight lost, the DTA signal increases, reaches a maximum and then decreases and stays at a positive plateau. A positive DTA signal indicates an exothermic reaction. The DTA behaviour is thus in agreement with the increase in temperature inside the microreactor when oxygen starts to be consumed, and confirms that at this temperature (465°C) the overall reaction is exothermic. At higher temperatures, the DTA signal becomes negative, which indicates an endothermic reaction. The total weight lost observed between 465°C and 806°C (from point A to point C) is 10.64%, which is close to the value of 10.74% corresponding to the reduction of the pyrochlore catalyst according to: Pr,RuzO,
+ CH,
+
2Ru" + Pr203 + C02 + 2H20 .
(7)
The difference between 10.64% and 10.74% is probably due to a small amount of impurities in the Pr2Ruz0, sample. The small weight lost at around 650°C could result from a decrease in the oxidation state of praseodymium oxide, since it is kr m that this oxide can take several oxidation states according to the temperature and the oxygen partial pressure, even in methane/oxygen mixtures [7]. 30
,
,
90 80
25
70
-
60
20
- 15
50
be
v
40
10
3 0-
2
a
20
5
10
0
0 1.34
3-
1.39
1.44
1.49
Time ( h r )
Figure 3:
Expanded view of Figure 1 where oxygen starts to be fully consumed. Temperature is about 420°C. Water and helium are not shown.
It is thus suggested that at 465"C,the pyrochlore catalyst is reduced to ruthenium metal and praseodymium sesquioxide. Further proof is provided on Figure 3, which shows an
364
expanded view of Figure 1 where the catalyst starts to be fully active. This occurred at about 420°C on Figure 1 instead of 465°C as on Figure 2. The difference between these two reaction temperatures is due to the difference in catalyst amount and from the difference in contact between solid and gas. It can be seen on Figure 3 that oxygen becomes completely consumed in a two-minute interval. The CO, percentage starts from about 1%, reaches 27.5 % and then decreases and stabilizes at around 12%. Hydrogen and CO appear a few seconds after COz, increase less rapidly, reach their maximum values and then decrease slowly due to a small decrease in temperature following the large exothermic effect. CO, and water (not shown in Figure 3) are thus the primary products. The amount of C02produced from gaseous oxygen is 12%,and the higher percentage of CO, at the beginning comes from the reaction of methane with the oxygen of the catalyst during the reduction process. This partial reduction of the catalyst is largely responsible for the temperature exotherm recorded in the catalytic bed and is in agreement with the DTA signal observed in Figure 2. From the first three figures, one may conclude that praseodymium ruthenium oxide is not active and has to be reduced to ruthenium metal and to praseodymium sesquioxide to promote methaneloxygen reactions. Once reduced, the catalyst is very active towards methane combustion and synthesis gas formation, the change in gas composition observed being the one ensued from thermodynamic equilibrium. At 777"C, Ashcroft et al. [l] reported the presence of graphitic carbon on their used sample. Carbon deposition should be associated to a mass increase, but in Figure 2 there is no apparent mass gain at temperatures lower than 900°C
20
h
be W
r 80
75
f'" t Oxyge
70
65
2 3 60 2
l5
rn
55 .3 50 45 40
1
2
3
4
5
Time [ h r ) Figure 4:
Change in gas composition when temperature is increased from ambient to 900°C over praseodymium oxide.
806°C. There is, however, a slight mass increase at temperatures higher than 806"C, that
365
may be due to carbon formation. The role of praseodymium oxide and ruthenium metal have now to be determined. Praseodymium oxide (Pr6OlI)was tested in the microreactor and the results are shown
on Figure 4. This oxide allows the complete conversion of oxygen at 455°C. The products
obtained are mainly water and COz in a two to one ratio, with smaller amounts of ethane, ethylene, hydrogen, and CO, the last two increasing above 800°C. A TG/DTA experiment indicated that under a methane/oxygen mixture at a temperature higher than 455"C, Pr6OI1 is reduced, and that above 800"C, a sesquioxide structure (Pr203)is formed. These results indicate that praseodymium oxide is a good catalyst for methane combustion, but is not able to promote the reaction of methane with C 0 2 and water, and therefore to allow reactions 1 and 3 to reach equilibrium under our experimental conditions. Finally, a catalyst made of 0.1 weight percent of ruthenium supported on alumina (0.1 % (w/w) Ru/Al,O,) was tested in the fixed-bed microreactor (Figure 5). The activity of this catalyst for the partial oxidation of methane is quite similar to the one observed over Pr2Ru20,(Figure 1). This catalyst is thus active for the methane/oxygen reaction, as well as for the reactions of methane with COz and water to produce synthesis gas. 90
l-----l I
900
Methane
h
LO
800
700
600
50
500+
40
400-
30
300
20
200
10
100
0
0
0
0
1
2
3
4
Time ( h r )
Figure 5:
Change in gas composition when temperature is increased from ambient to 900°C over 0.1 % (w/w) Ru/Alz03.
From the results presented here, it appears that ruthenium metal is the active site promoting the formation of synthesis gas according to reactions 1 and 3. Praseodymium sesquioxide may play a role, but it is mainly for the first reactions of methane combustion.
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Moreover, Figure 5 shows that under our reaction conditions, 0.1 % ruthenium is enough to allow reactions 1 and 3 to reach equilibrium.
4. CONCLUSIONS Praseodymium ruthenium oxide (Pr2Ru2O7)is not active by itself and has to be reduced to praseodymium sesquioxide and ruthenium metal to promote the partial oxidation of methane to synthesis gas. Praseodymium sesquioxide takes part in the methane/oxygen reactions leading to CO, and water, but does not promote the production of synthesis gas. On the other hand, ruthenium metal is very active in promoting the reaction of methane with water and C02 to produce synthesis gas. Under our experimental conditions, these reactions are brought to equilibrium with as little as 0.1 % (w/w) ruthenium on alumina.
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