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Al2O3 catalysts
Reaction Kinetics and the Development of Catalytic Processes G.F. Froment and K.C. Waugh (Editors) 9 1999 Elsevier Science Ltd. All rights reserved.
341
A comparative kinetic study of CH 4 oxidation by NiO/A1203, Pt/A1203 and NiO-Pt/A1203 catalysts T.-N. Angelidis and V. Tzitzios Aristotle University, Faculty of Chemistry (Box 114), 54006 Thessaloniki, Greece
Abstract
A NiO-Pt/fi-A1203 oxidation-reduction catalyst (3%Ni and 0.1%Pt, C53-1, Catalyst and Chemicals Europe S.A.) is applied in industry (EKO refinery, Thessaloniki, Greece) for nitrogen gas production, through LPG combustion by atmospheric air. The aim of the present research work is to compare the kinetics of total oxidation of methane by the separate constituents of the above catalyst, as well as by the mixed catalyst. The ~-alumina based NiO (3%Ni), Pt (0.1%Pt) and NiO-Pt (3%Ni, 0.1%Pt) catalysts were prepared by the impregnation method and characterized by BET, XRD and SEM-EDX. The activity tests show that activity follows the order, NiO-Pt>Pt>NiO. The reaction kinetics were measured at low partial pressures of oxygen and methane near the stoichiometric point. The kinetic results were interpreted with the Langmuir-Hinshelwood (L-H) and Elye-Riedel (E-R) mechanisms. The reaction follows an E-R mechanism on the NiO catalyst. On the Pt catalyst, reaction follows a mixed L-H/E-R mechanism, but at low methane partial pressures and high oxygen partial pressures an E-R mechanism prevails. The reaction rate kinetic and adsorption constants and activation energies were calculated according to the above mechanisms. The validation of the proposed mechanisms was checked statistically and through thermodynamic consideration. The reaction rate over the Pt catalyst is considerably higher (ten times) than over the NiO catalyst at the stoichiometric point. The rate of the reaction over the mixed catalyst is the sum of the rates over the separate catalysts and a synergistic effect is not seems possible.
1. INTRODUCTION An oxidation/reduction catalyst (3% Ni and 0.1% Pt on a-alumina, C53-1, Catalyst and Chemicals Europe S.A.) is applied in industry for Nz(g ) and CO2(g) production. The catalyst is described in detail elsewhere [1 ]. The flowsheet of the production unit is shown in Figure 1. A desulfurization ZnO catalyst (XC-117) is applied before the main catalyst to prevent poisoning by sulfur. The above catalyst may be of interest in environmental applications for removal of NOx, CO and hydrocarbons from combustion exhaust gases, due to his oxidation/reduction properties. The presence of nickel will limit his application only to stationery sources, where preheating is easier, to prevent nickel carbonyl formation (nickel carbonyls are stable up to 200~ [2]). The kinetics of the deNOx action of this catalyst has been studied in the earlier stages of his development [3].
342
HEATER ~ CATALYTIC ELECTRICAL
A: CATALYSTXC-117 B: CATALYSTC-53-1 GASES
- MEA
;;%
NITROGEN
315"Cl ~lR LPG~:~ REBOILER~ I I
AIR
ilI
Ico, I
I
ABSORBER . . . . . J'~
[ 'I
MEA
]
g_ _ ~ Figure 1. The industrial unit flowsheet (EKO refinery and petrochemical plant, Thessaloniki, Greece). The main effort of the present research was to examine the kinetics of the oxidation action of the separate constituents of the above catalyst and of the mixed one during methane oxidation. Methane was selected, since is the most refractory hydrocarbon and thus, is often used as model hydrocarbon compound for activity tests. In addition, methane itself is a potent greenhouse gas and the emission control of unburned methane from exhaust gases (specially from natural gas combustion) is essential. High load nickel based catalysts are applied in SYNGAS reforming by partial oxidation of methane and extensive research work is published concerning the improvement of the process conditions [4-5]. The application of nickel based catalysts in CO2 reforming by methane over nickel based catalyst is also the subject of recent publications [6-9]. Pt based catalysts are extensively applied in oxidation of CO and hydrocarbons from combustion exhaust gases [ 10]. The generally proposed mechanism for methane oxidation over noble metals (Rh, Pd, Pt) as well as transition metal oxides [ 1l-13] can be summarized as follows: CH4(g )
HCHO(g)
CO(g)
;T CH4(a ) - tt ~
C t t 3 . (a) + O
~
ttCItO(a)
H2(g )
T; ~
CO(a) + 2H(a) + O
----*CO2(g) + H 2 0 ( g )
In both cases the formation of adsorbed methyl radicals is the rate controlling step. The main differences between noble metals and transition metal oxides are: 9 methane is adsorbed on the noble metals surface at reasonable partial pressures, while is not adsorbed at all on metal oxides in the same partial pressures region [ 13] 9 oxygen is readily adsorbed and dissociates on noble metals catalysts, while is not easily adsorbed on metal oxides. In the last case oxidation may be under mixed control (oxygen adsorption and adsorbed methyl radicals formation) [13] 9 in the case of nickel oxide oxygen is readily adsorbed and dissociates due to the p-type semiconductor' s characteristics of the oxide [ 12-13]. According to the Langmuir-Hinshelwood (L-H) and the Eley-Riedel (E-R) approach, the acceptance of the above statements leads to the conclusion that an E-R mechanism prevails (methane from the gas phase reacts with adsorbed oxygen at substantial partial pressures of
343 oxygen). This is what most researchers observed during the kinetic study of methane oxidation over noble metals and active metal oxides catalysts [ 14-15].
2. E X P E R I M E N T A L 2.1. Catalyst Preparation and Characterization The supported catalysts' samples with composition similar to that of the industrial catalyst (3% Ni in NiO/A1203, 0.1% Pt in Pt/A1203 and 3%Ni-0.1%Pt in NiO-Pt/A1203) were prepared by conventional wet impregnation. Hydrogen hexachloroplatinate (IV) hydrate (Riedel-deHaen) and nickel nitrate hydrate (Riedel-de Haen) were applied to provide the active metals and fi-Al203 (HARSHAW A1-3971P) was used as the support material. The required quantities of metals and support were mixed with water. Water was removed by heating under agitation. The resulting solids were dried at 105~ for 24h, crushed and sieved, the 75-212im fraction being retained. They were then heated to 600~ in a current of hydrogen (2h) and in a current of helium (lh). The catalyst characterization was performed by AAS (PERKIN ELMER 2380) for bulk chemical analysis, by nitrogen adsorption/desorption for BET surface and pore volume measurements, by XRD (SIEMENS D-500 CuKfi) for crystalline identification and by SEM-EDX (JEOL 120CX, LINK AN 10S, ZAF4) for the texture and composition dispersion of the catalyst particles. Bulk analyses show that the prepared catalysts were of the expected chemical composition. XRD studies show that alumina exists mainly in gamma crystalline form and nickel mainly as NiA104. SEM-EDX studies show that the NiO-Pt/AI203 particles have a more clear crystalline form that confirms their low porosity compared with the other catalysts and that there is no observable difference of chemical composition between particles. 2.2. Activity and kinetics measurements The reactor was a 1cm i.d. x 35 cm long quartz tube heated by a temperature controlled tubular furnace. The reaction temperature was monitored by a thermocouple placed near the packed catalyst bed. Oxygen and methane certified calibration gas mixtures balanced by helium were used as reacting gases and pure helium was used as diluent (all from AIR LIQUID). The gas streams were measured with mass flow controllers and mixed before the reactor inlet. The resulting gas mixture flowed through the packed bed. For kinetic measurements, the reactor was operated in a differential mode with the conversion not exceeding 10%, so that the temperature was nearly uniform in the packed catalyst bed. Separate experimental tests showed that bulk mass transfer and intraparticle mass transfer resistance could be eliminated by using a gas space velocity greater than 26500 h ~ and catalyst particles less than 212im in size. The reaction conversion was controlled by the catalyst loading. The partial pressure of the reacting gas species was varied over the range of 0.01-0.08 bar for the Ni catalyst and 0.01-0.05 bar for the Pt and mixed catalysts and the temperature was varied over the range 525-575~ and 475-550~ respectively. Before any kinetic measurement, the catalysts were always treated with a gas flow of hydrogen for 0.5h and helium for 0.5h at 600~ The chemical analysis of reactants and the products gas mixtures was performed by gas chromatography (SHIMADZU GC-14B) equipped with Poropac Q and Molecular Sieve 5A columns and a TCD (Thermal Conductivity Detector). The mass balance of the reaction was always checked by analysis of all the feed and the product gases (O2, CH4, CO2 and CO). Carbon monoxide production was only observed during the activity experiments at temperatures over 800~ So the production of CO: was used to calculate the reaction rate:
344 (1)
Rate(cH4) = Rate(c02) = N t X c o 2 / S
where N t is the total molar gas flowrate in mole sec-1, Xco2 is the molar fraction of C O 2 in the product gas stream and S is the surface of the considered catalyst (calculated from the specific surface and the weight of the applied catalyst sample).
3. RESULTS AND DISCUSSION 3.1 Activity tests Figure 1 shows the light-off curves of C H 4 oxidation over the three prepared catalysts and the industrial one. The experiments were carried out at the stoichiometry of the methane total oxidation reaction (PcHn/Po2 = 89 The calculated light-off temperatures were 780, 610, 560 and
CONVERSION
CH 4 , %
CONVERSION
CH 4, %
100
I
N iO/A 80
I
i2 0 3
m
m TOTA L T O CO TO
60
_ __
-t
40
CO 2
_
NiO/AI2 03
-~ / ..~ / NiO-Pt/AIz 031 P t / A l : O~
20
INDUSTRIAL] .
_
O
600 TEMPERATURE,
800 oC
1000
Figure 2. Light-off of C H 4 oxidation over the tested catalysts (PCH4 = 0.01 bar and Po2 = 0.02bar)
400
u=
--
1
.
.
.
.
6~) . . . . . TEMPERATURE,
.
myd~ljm oC
1000
Figure 3. Light-off of C H 4 oxidation over the NiO/A1203 catalyst with CO formation (PCH4 = 0.01 bar and Po2 = 0.02 bar).
570~ for NiO, Pt, Pt-NiO and industrial catalyst respectively. The oxidation activity of the mixed and Pt catalysts is considerably higher than the respective activity of the NiO catalyst. So, the oxidation activity of the mixed catalyst is mainly attributed to the presence of Pt. The prepared in laboratory mixed catalyst and the industrial one seems to have comparable activities, and the results obtained by the first one are representative for the later. In Figure 2 the partial oxidation of methane to CO at temperatures over 800~ is shown. CO production at high temperatures disappears in the presence of Pt in the mixed and industrial catalyst. 3.2. Kinetic results
Figures 4 and 5 show the variation of C H 4 oxidation rate with the partial pressure of 02 and respectively. Various rate equations derived from different reaction mechanisms as well as
CH 4
345 rate, moleCH4/sec
m2
rate, moleCC4/sec
NiO/AI2 O3 TEMPERATURE 8E- l0
~
5o0" C
ll
52s,.c
O
540" C
9
550" C
[
m 2
3E-9 NiO/AI2 03 TEMPERATURE
2E-9
_
~
5oo, c
1
525" c
O
540" C
9
sso,, c
9
-
4E-10 1E-9
0 0.00
I 0.08
0.04
0.12
pO 2 , bar
Figure 4. Variation of the C H 4 oxidation rate over the N i O / A 1 2 0 3 catalyst with Po2 (PCH4 = 0.01 bar).
_
0 0.00
I 0.08
0.04
0.12
p U l l 4 , bar
Figure 5. Variation of the C H 4 oxidation rate over the NiO/A1203 catalyst with PCH4(1902 = = 0.02 bar).
the empirical power order equation were evaluated to regress the experimental data. It was found that the experimental data were best represented by the equation: RCH 4 --
k PfH4KoPo/(1+KoPo)
(2)
where R C H 4 is the methane oxidation rate, k and Ko can be considered as the surface reaction rate constant and the adsorption equilibrium constant of oxygen respectively. Eq.1 gives a linear relation between the methane oxidation rate and the partial pressure of methane for constant partial pressure of oxygen. The results of the statistical check of the experimental data were applied for the calculation of the constants k and Ko. The curves in Figures 4 and 5 represent the fitting of Eq. 1. Eq. 1 is representative of an E-R mechanism. According to the discussion of the possible mechanisms made in the introduction the formation of adsorbed methyl radicals is the rate controlling step and methane reacts from the gas phase with adsorbed oxygen. The activation energy of the surface reaction was found to be 36.5 kcal mole l from the Arrhenius equation. The heat of adsorption calculated from the Ko values at various temperatures was found to be 11.4 kcal mole -I and the respective entropy of adsorption (AS) -6.2cal mole-lK 1. The entropy value seems to be in agreement with the fundamental thermodynamic rules: A~S < 0 and [AS] < Sg where Sg is the entropy of oxygen gas (56,5-57.2 cal mole l K -1 at the experiments temperature range [16]). The entropy value does not agree with the Boudart rules: [A,S[ > 10 and [AS[ < 12.2-(0.0014Q) where Q is the heat of adsorption in cal mole 1. This means that the calculated heat of adsorption is low and the dependence of the respective adsorption constants on temperature is less than thermodynamically expected. This fact may be explained by crystalline changes of nickel compounds from NiA104 to NiO as temperature increases. The later is a more active oxygen adsorbent [4] and changes the adsorption behavior of the catalyst. Figures 6 and 7 show the variation of the methane oxidation as a function of the partial pressure of methane and the partial pressure of oxygen respectively. A mixed E-R/L-H mechanism described by Eq.3 was the basis for the discussion of the kinetic behavior:
346
RCH 4 --
(3)
klKoPoPcH4/(1 + KoPo) + k2KoPoKcH4PcH4/(1 + KoPo)2 Eley- Riedel Langmuir-Hinshelwood
Where k~ and k 2 are reaction rate constants and Ko and KCH 4 a r e the respective adsorption constants of oxygen and methane. For constant partial pressure of oxygen Eq.3 becomes a linear function of the partial pressure of methane, while at low partial pressures of methane only oxygen is adsorbed and the E-R mechanism prevails. This consideration is confirmed by the experimental data of Figure 6, since they can be analyzed by two separate linear functions with different slopes. The same happens with the experimental results of Figure 7, where at low partial pressures of oxygen, methane is partially adsorbed and the mixed mechanism prevails and at higher partial pressure of oxygen the E-R mechanism prevails and the rate becomes constant. At the stoichiometric point both figures show that the reaction follows an E-R mechanism that is described by the linear relation: RCH 4 --
(4)
klPcH 4
since KoPo2 >> 1. The values of the reaction rate constant were calculated at various temperatures from the first linear part of Figure 6 and the activation energy was found to be 33 kcal mole -1 (this value is in accordance with the activation energies found by other researchers [17-19] for methane oxidation over Pt catalysts, which were between 24 and 44.7 kcal mole1). The lack of experimental data, specially in the experimentally difficult range of low partial pressure of oxygen, does not permit a complete kinetic study. Figures 8 and 9 show the kinetic behavior of the two separate catalysts compared with the mixed one. The oxidation rate on the mixed catalyst seems to be the sum of the rates of the separate catalysts. A definite conclusion is difficult to be obtained since the oxidation rate of methane on the nickel catalyst is about ten times lower than the oxidation rate on the platinum catalyst. rate, m o l e C H 4 / s e c
rate, m o l e C H 4 / s e c
m 2
1.6E-8
6E
I
t Pt/AI 20j i TEMPERA TURE
~/
1.2E-8
ll
475; C
O
500" C
9
525"C
m 2
Pt/AI2 03 TEMPERATURE 475 ~ C
----l--4E
500" C 525"C
9
550" C
8E-9
2E 4E-9
0 0.00
I 0.04
0.02 pCH 4 ,
,, 0.06
bar
Figure 6. Variation of the C H 4 oxidation rate over the Pt/A1203 catalyst with PCH4 (Po2 = 0.02 bar).
0 0.0 0
"
I 0.0 2
I 0.0 4 pO 2 ,
0.0 6
bar
Figure 7. Variation of the C H 4 oxidation rate over the Pt/A1203 catalyst with Po2 (PCH4 = = 0.01 bar).
347
moleCH
rate,
4 / m 2 sec '
I
moleCH
rate,
1E-8 '
'
--
6E-9
4 / m 2 sec I
I 550" C
_
8E-9
Pt-NiO/AI203 4E-9
6E-9
_
Pt/AI2 03
n
NiO/AI 2 03
_ 550" c
4E-9_
A~--
Pt-NiO/AI203
-~-
Pt/AI2 03
-1---
NiO/AIz 03
2E-9
2E-9
_
_
0E+0
I
9
9
0.00
m
I---IJ
I-
T
9
n
I-~
I
0.02
0.04
0E+0 0.06
p C H 4 , bar
Figure 8. Variation of the C H 4 oxidation rate over the laboratory catalysts with PCH4(Po2 = 0.02 bar). 4. CONCLUSION
0.00
I 0.08
0.04
B
0.12
p O 2 , bar
Figure 9. Variation of the C H 4 oxidation rate over the laboratory catalysts with Po2 (PCH4 = 0.01 bar).
NiO/A1203, Pt/AI203 and NiO-Pt/A1203 catalysts were examined in laboratory scale for total methane oxidation concerning their activity and kinetics. At the applied conditions (low partial pressures of methane and oxygen near the stoichiometric point and temperatures in the range of 475-575~ the oxidation reaction seems to follow an E-R mechanism for the three catalyst (methane from the bulk of the gas phase reacts with adsorbed oxygen). The oxidation activity is attributed mainly to Pt. The crystalline structure of nickel seems to influence the oxidation rate. No evidence of synergistic effect of nickel and platinum in the mixed catalyst was observed.
Ackwoledgments
The authors want to thanks the administration of the HELPE (ex-EKO) refinery (Thessaloniki, Greece) for the provision of catalyst samples.
5. REFERENCES 1. T. N. Angelidis, V. G. Papadakis, E. Pavlidou, Applied Catalysis B: Environmental No.4 (1994) 301. 2. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th ed.J. Wiley&Sons, 1988. 3. R. R. Sakaida, R. G. Rinker, Y. L. Wang, and W. H. Corcoran, A.I.Ch.E. Journal No. 4 (1961)658. 4. D. Dissanayake, M.P. Rosynek, K.C.C. Kharas and J.H. Lunsford, Journal of Catalysis No. 132 (1991) 117. 5. F. van Looij and J.W. Geus, Journal of Catalysis No. 168 (1997) 154. 6. S.B. Wang and G,Q.M. Lu, Applied Catalysis B: Environmental No. 16(3) (1998) 269.
348 7. Z. Zhaolong and X. Verykios, J. Chem. Soc. Chem. Commun. (1995) 71. 8. J. R. Rostrup-Nielsen and J-H. Bak Hansen, Joumal of Catalysis No. 144 (1993) 38. 9. E. Ruckenstein and Y.H. Hu, Joumal of Catalysis No. 162 (1996) 230. 10.F.R. Hartley (ed.), Chemistry of the Platinum Group Metals-Recent developments, Studies in Inorganic Chemistry No. 11, Elsevier, 1991. 11 .S.H. Oh, P.J. Mitchell and R.M. Siewert, Journal of Catalysis No. 132 (1991) 287. 12.H. Borchert and M. Baems, Journal of Catalysis No. 168 (1997) 315. 13.J.J. Spivey, Ind.Eng.Chem.Res. No. 26 (1987) 2165. 14.W.L. Liu and M. Flytzani-Stephanopoulos, Journal of Catalysis No. 153 (1995) 317. 15.J.J. Carberry, Chemical Engineering Progress No.2 (1988) 51. 16.A. Roin, HSC CHEMISTRY ver. 2.03 Outokumpu Research Oy, Pori, Finland 17.R.F. Hicks, H. Qi, M.L. Young and R.G. Lee, Journal of Catalysis No. 122 (1990) 280. 18.C.F. Cullis and B.M. Willatt, Journal of Catalysis, No. 83 (1983)267. 19.Y.F.Y. Yao, Ind. Eng. Prod. Res. Dev. No. 19 (1980) 293.
341
A comparative kinetic study of CH 4 oxidation by NiO/A1203, Pt/A1203 and NiO-Pt/A1203 catalysts T.-N. Angelidis and V. Tzitzios Aristotle University, Faculty of Chemistry (Box 114), 54006 Thessaloniki, Greece
Abstract
A NiO-Pt/fi-A1203 oxidation-reduction catalyst (3%Ni and 0.1%Pt, C53-1, Catalyst and Chemicals Europe S.A.) is applied in industry (EKO refinery, Thessaloniki, Greece) for nitrogen gas production, through LPG combustion by atmospheric air. The aim of the present research work is to compare the kinetics of total oxidation of methane by the separate constituents of the above catalyst, as well as by the mixed catalyst. The ~-alumina based NiO (3%Ni), Pt (0.1%Pt) and NiO-Pt (3%Ni, 0.1%Pt) catalysts were prepared by the impregnation method and characterized by BET, XRD and SEM-EDX. The activity tests show that activity follows the order, NiO-Pt>Pt>NiO. The reaction kinetics were measured at low partial pressures of oxygen and methane near the stoichiometric point. The kinetic results were interpreted with the Langmuir-Hinshelwood (L-H) and Elye-Riedel (E-R) mechanisms. The reaction follows an E-R mechanism on the NiO catalyst. On the Pt catalyst, reaction follows a mixed L-H/E-R mechanism, but at low methane partial pressures and high oxygen partial pressures an E-R mechanism prevails. The reaction rate kinetic and adsorption constants and activation energies were calculated according to the above mechanisms. The validation of the proposed mechanisms was checked statistically and through thermodynamic consideration. The reaction rate over the Pt catalyst is considerably higher (ten times) than over the NiO catalyst at the stoichiometric point. The rate of the reaction over the mixed catalyst is the sum of the rates over the separate catalysts and a synergistic effect is not seems possible.
1. INTRODUCTION An oxidation/reduction catalyst (3% Ni and 0.1% Pt on a-alumina, C53-1, Catalyst and Chemicals Europe S.A.) is applied in industry for Nz(g ) and CO2(g) production. The catalyst is described in detail elsewhere [1 ]. The flowsheet of the production unit is shown in Figure 1. A desulfurization ZnO catalyst (XC-117) is applied before the main catalyst to prevent poisoning by sulfur. The above catalyst may be of interest in environmental applications for removal of NOx, CO and hydrocarbons from combustion exhaust gases, due to his oxidation/reduction properties. The presence of nickel will limit his application only to stationery sources, where preheating is easier, to prevent nickel carbonyl formation (nickel carbonyls are stable up to 200~ [2]). The kinetics of the deNOx action of this catalyst has been studied in the earlier stages of his development [3].
342
HEATER ~ CATALYTIC ELECTRICAL
A: CATALYSTXC-117 B: CATALYSTC-53-1 GASES
- MEA
;;%
NITROGEN
315"Cl ~lR LPG~:~ REBOILER~ I I
AIR
ilI
Ico, I
I
ABSORBER . . . . . J'~
[ 'I
MEA
]
g_ _ ~ Figure 1. The industrial unit flowsheet (EKO refinery and petrochemical plant, Thessaloniki, Greece). The main effort of the present research was to examine the kinetics of the oxidation action of the separate constituents of the above catalyst and of the mixed one during methane oxidation. Methane was selected, since is the most refractory hydrocarbon and thus, is often used as model hydrocarbon compound for activity tests. In addition, methane itself is a potent greenhouse gas and the emission control of unburned methane from exhaust gases (specially from natural gas combustion) is essential. High load nickel based catalysts are applied in SYNGAS reforming by partial oxidation of methane and extensive research work is published concerning the improvement of the process conditions [4-5]. The application of nickel based catalysts in CO2 reforming by methane over nickel based catalyst is also the subject of recent publications [6-9]. Pt based catalysts are extensively applied in oxidation of CO and hydrocarbons from combustion exhaust gases [ 10]. The generally proposed mechanism for methane oxidation over noble metals (Rh, Pd, Pt) as well as transition metal oxides [ 1l-13] can be summarized as follows: CH4(g )
HCHO(g)
CO(g)
;T CH4(a ) - tt ~
C t t 3 . (a) + O
~
ttCItO(a)
H2(g )
T; ~
CO(a) + 2H(a) + O
----*CO2(g) + H 2 0 ( g )
In both cases the formation of adsorbed methyl radicals is the rate controlling step. The main differences between noble metals and transition metal oxides are: 9 methane is adsorbed on the noble metals surface at reasonable partial pressures, while is not adsorbed at all on metal oxides in the same partial pressures region [ 13] 9 oxygen is readily adsorbed and dissociates on noble metals catalysts, while is not easily adsorbed on metal oxides. In the last case oxidation may be under mixed control (oxygen adsorption and adsorbed methyl radicals formation) [13] 9 in the case of nickel oxide oxygen is readily adsorbed and dissociates due to the p-type semiconductor' s characteristics of the oxide [ 12-13]. According to the Langmuir-Hinshelwood (L-H) and the Eley-Riedel (E-R) approach, the acceptance of the above statements leads to the conclusion that an E-R mechanism prevails (methane from the gas phase reacts with adsorbed oxygen at substantial partial pressures of
343 oxygen). This is what most researchers observed during the kinetic study of methane oxidation over noble metals and active metal oxides catalysts [ 14-15].
2. E X P E R I M E N T A L 2.1. Catalyst Preparation and Characterization The supported catalysts' samples with composition similar to that of the industrial catalyst (3% Ni in NiO/A1203, 0.1% Pt in Pt/A1203 and 3%Ni-0.1%Pt in NiO-Pt/A1203) were prepared by conventional wet impregnation. Hydrogen hexachloroplatinate (IV) hydrate (Riedel-deHaen) and nickel nitrate hydrate (Riedel-de Haen) were applied to provide the active metals and fi-Al203 (HARSHAW A1-3971P) was used as the support material. The required quantities of metals and support were mixed with water. Water was removed by heating under agitation. The resulting solids were dried at 105~ for 24h, crushed and sieved, the 75-212im fraction being retained. They were then heated to 600~ in a current of hydrogen (2h) and in a current of helium (lh). The catalyst characterization was performed by AAS (PERKIN ELMER 2380) for bulk chemical analysis, by nitrogen adsorption/desorption for BET surface and pore volume measurements, by XRD (SIEMENS D-500 CuKfi) for crystalline identification and by SEM-EDX (JEOL 120CX, LINK AN 10S, ZAF4) for the texture and composition dispersion of the catalyst particles. Bulk analyses show that the prepared catalysts were of the expected chemical composition. XRD studies show that alumina exists mainly in gamma crystalline form and nickel mainly as NiA104. SEM-EDX studies show that the NiO-Pt/AI203 particles have a more clear crystalline form that confirms their low porosity compared with the other catalysts and that there is no observable difference of chemical composition between particles. 2.2. Activity and kinetics measurements The reactor was a 1cm i.d. x 35 cm long quartz tube heated by a temperature controlled tubular furnace. The reaction temperature was monitored by a thermocouple placed near the packed catalyst bed. Oxygen and methane certified calibration gas mixtures balanced by helium were used as reacting gases and pure helium was used as diluent (all from AIR LIQUID). The gas streams were measured with mass flow controllers and mixed before the reactor inlet. The resulting gas mixture flowed through the packed bed. For kinetic measurements, the reactor was operated in a differential mode with the conversion not exceeding 10%, so that the temperature was nearly uniform in the packed catalyst bed. Separate experimental tests showed that bulk mass transfer and intraparticle mass transfer resistance could be eliminated by using a gas space velocity greater than 26500 h ~ and catalyst particles less than 212im in size. The reaction conversion was controlled by the catalyst loading. The partial pressure of the reacting gas species was varied over the range of 0.01-0.08 bar for the Ni catalyst and 0.01-0.05 bar for the Pt and mixed catalysts and the temperature was varied over the range 525-575~ and 475-550~ respectively. Before any kinetic measurement, the catalysts were always treated with a gas flow of hydrogen for 0.5h and helium for 0.5h at 600~ The chemical analysis of reactants and the products gas mixtures was performed by gas chromatography (SHIMADZU GC-14B) equipped with Poropac Q and Molecular Sieve 5A columns and a TCD (Thermal Conductivity Detector). The mass balance of the reaction was always checked by analysis of all the feed and the product gases (O2, CH4, CO2 and CO). Carbon monoxide production was only observed during the activity experiments at temperatures over 800~ So the production of CO: was used to calculate the reaction rate:
344 (1)
Rate(cH4) = Rate(c02) = N t X c o 2 / S
where N t is the total molar gas flowrate in mole sec-1, Xco2 is the molar fraction of C O 2 in the product gas stream and S is the surface of the considered catalyst (calculated from the specific surface and the weight of the applied catalyst sample).
3. RESULTS AND DISCUSSION 3.1 Activity tests Figure 1 shows the light-off curves of C H 4 oxidation over the three prepared catalysts and the industrial one. The experiments were carried out at the stoichiometry of the methane total oxidation reaction (PcHn/Po2 = 89 The calculated light-off temperatures were 780, 610, 560 and
CONVERSION
CH 4 , %
CONVERSION
CH 4, %
100
I
N iO/A 80
I
i2 0 3
m
m TOTA L T O CO TO
60
_ __
-t
40
CO 2
_
NiO/AI2 03
-~ / ..~ / NiO-Pt/AIz 031 P t / A l : O~
20
INDUSTRIAL] .
_
O
600 TEMPERATURE,
800 oC
1000
Figure 2. Light-off of C H 4 oxidation over the tested catalysts (PCH4 = 0.01 bar and Po2 = 0.02bar)
400
u=
--
1
.
.
.
.
6~) . . . . . TEMPERATURE,
.
myd~ljm oC
1000
Figure 3. Light-off of C H 4 oxidation over the NiO/A1203 catalyst with CO formation (PCH4 = 0.01 bar and Po2 = 0.02 bar).
570~ for NiO, Pt, Pt-NiO and industrial catalyst respectively. The oxidation activity of the mixed and Pt catalysts is considerably higher than the respective activity of the NiO catalyst. So, the oxidation activity of the mixed catalyst is mainly attributed to the presence of Pt. The prepared in laboratory mixed catalyst and the industrial one seems to have comparable activities, and the results obtained by the first one are representative for the later. In Figure 2 the partial oxidation of methane to CO at temperatures over 800~ is shown. CO production at high temperatures disappears in the presence of Pt in the mixed and industrial catalyst. 3.2. Kinetic results
Figures 4 and 5 show the variation of C H 4 oxidation rate with the partial pressure of 02 and respectively. Various rate equations derived from different reaction mechanisms as well as
CH 4
345 rate, moleCH4/sec
m2
rate, moleCC4/sec
NiO/AI2 O3 TEMPERATURE 8E- l0
~
5o0" C
ll
52s,.c
O
540" C
9
550" C
[
m 2
3E-9 NiO/AI2 03 TEMPERATURE
2E-9
_
~
5oo, c
1
525" c
O
540" C
9
sso,, c
9
-
4E-10 1E-9
0 0.00
I 0.08
0.04
0.12
pO 2 , bar
Figure 4. Variation of the C H 4 oxidation rate over the N i O / A 1 2 0 3 catalyst with Po2 (PCH4 = 0.01 bar).
_
0 0.00
I 0.08
0.04
0.12
p U l l 4 , bar
Figure 5. Variation of the C H 4 oxidation rate over the NiO/A1203 catalyst with PCH4(1902 = = 0.02 bar).
the empirical power order equation were evaluated to regress the experimental data. It was found that the experimental data were best represented by the equation: RCH 4 --
k PfH4KoPo/(1+KoPo)
(2)
where R C H 4 is the methane oxidation rate, k and Ko can be considered as the surface reaction rate constant and the adsorption equilibrium constant of oxygen respectively. Eq.1 gives a linear relation between the methane oxidation rate and the partial pressure of methane for constant partial pressure of oxygen. The results of the statistical check of the experimental data were applied for the calculation of the constants k and Ko. The curves in Figures 4 and 5 represent the fitting of Eq. 1. Eq. 1 is representative of an E-R mechanism. According to the discussion of the possible mechanisms made in the introduction the formation of adsorbed methyl radicals is the rate controlling step and methane reacts from the gas phase with adsorbed oxygen. The activation energy of the surface reaction was found to be 36.5 kcal mole l from the Arrhenius equation. The heat of adsorption calculated from the Ko values at various temperatures was found to be 11.4 kcal mole -I and the respective entropy of adsorption (AS) -6.2cal mole-lK 1. The entropy value seems to be in agreement with the fundamental thermodynamic rules: A~S < 0 and [AS] < Sg where Sg is the entropy of oxygen gas (56,5-57.2 cal mole l K -1 at the experiments temperature range [16]). The entropy value does not agree with the Boudart rules: [A,S[ > 10 and [AS[ < 12.2-(0.0014Q) where Q is the heat of adsorption in cal mole 1. This means that the calculated heat of adsorption is low and the dependence of the respective adsorption constants on temperature is less than thermodynamically expected. This fact may be explained by crystalline changes of nickel compounds from NiA104 to NiO as temperature increases. The later is a more active oxygen adsorbent [4] and changes the adsorption behavior of the catalyst. Figures 6 and 7 show the variation of the methane oxidation as a function of the partial pressure of methane and the partial pressure of oxygen respectively. A mixed E-R/L-H mechanism described by Eq.3 was the basis for the discussion of the kinetic behavior:
346
RCH 4 --
(3)
klKoPoPcH4/(1 + KoPo) + k2KoPoKcH4PcH4/(1 + KoPo)2 Eley- Riedel Langmuir-Hinshelwood
Where k~ and k 2 are reaction rate constants and Ko and KCH 4 a r e the respective adsorption constants of oxygen and methane. For constant partial pressure of oxygen Eq.3 becomes a linear function of the partial pressure of methane, while at low partial pressures of methane only oxygen is adsorbed and the E-R mechanism prevails. This consideration is confirmed by the experimental data of Figure 6, since they can be analyzed by two separate linear functions with different slopes. The same happens with the experimental results of Figure 7, where at low partial pressures of oxygen, methane is partially adsorbed and the mixed mechanism prevails and at higher partial pressure of oxygen the E-R mechanism prevails and the rate becomes constant. At the stoichiometric point both figures show that the reaction follows an E-R mechanism that is described by the linear relation: RCH 4 --
(4)
klPcH 4
since KoPo2 >> 1. The values of the reaction rate constant were calculated at various temperatures from the first linear part of Figure 6 and the activation energy was found to be 33 kcal mole -1 (this value is in accordance with the activation energies found by other researchers [17-19] for methane oxidation over Pt catalysts, which were between 24 and 44.7 kcal mole1). The lack of experimental data, specially in the experimentally difficult range of low partial pressure of oxygen, does not permit a complete kinetic study. Figures 8 and 9 show the kinetic behavior of the two separate catalysts compared with the mixed one. The oxidation rate on the mixed catalyst seems to be the sum of the rates of the separate catalysts. A definite conclusion is difficult to be obtained since the oxidation rate of methane on the nickel catalyst is about ten times lower than the oxidation rate on the platinum catalyst. rate, m o l e C H 4 / s e c
rate, m o l e C H 4 / s e c
m 2
1.6E-8
6E
I
t Pt/AI 20j i TEMPERA TURE
~/
1.2E-8
ll
475; C
O
500" C
9
525"C
m 2
Pt/AI2 03 TEMPERATURE 475 ~ C
----l--4E
500" C 525"C
9
550" C
8E-9
2E 4E-9
0 0.00
I 0.04
0.02 pCH 4 ,
,, 0.06
bar
Figure 6. Variation of the C H 4 oxidation rate over the Pt/A1203 catalyst with PCH4 (Po2 = 0.02 bar).
0 0.0 0
"
I 0.0 2
I 0.0 4 pO 2 ,
0.0 6
bar
Figure 7. Variation of the C H 4 oxidation rate over the Pt/A1203 catalyst with Po2 (PCH4 = = 0.01 bar).
347
moleCH
rate,
4 / m 2 sec '
I
moleCH
rate,
1E-8 '
'
--
6E-9
4 / m 2 sec I
I 550" C
_
8E-9
Pt-NiO/AI203 4E-9
6E-9
_
Pt/AI2 03
n
NiO/AI 2 03
_ 550" c
4E-9_
A~--
Pt-NiO/AI203
-~-
Pt/AI2 03
-1---
NiO/AIz 03
2E-9
2E-9
_
_
0E+0
I
9
9
0.00
m
I---IJ
I-
T
9
n
I-~
I
0.02
0.04
0E+0 0.06
p C H 4 , bar
Figure 8. Variation of the C H 4 oxidation rate over the laboratory catalysts with PCH4(Po2 = 0.02 bar). 4. CONCLUSION
0.00
I 0.08
0.04
B
0.12
p O 2 , bar
Figure 9. Variation of the C H 4 oxidation rate over the laboratory catalysts with Po2 (PCH4 = 0.01 bar).
NiO/A1203, Pt/AI203 and NiO-Pt/A1203 catalysts were examined in laboratory scale for total methane oxidation concerning their activity and kinetics. At the applied conditions (low partial pressures of methane and oxygen near the stoichiometric point and temperatures in the range of 475-575~ the oxidation reaction seems to follow an E-R mechanism for the three catalyst (methane from the bulk of the gas phase reacts with adsorbed oxygen). The oxidation activity is attributed mainly to Pt. The crystalline structure of nickel seems to influence the oxidation rate. No evidence of synergistic effect of nickel and platinum in the mixed catalyst was observed.
Ackwoledgments
The authors want to thanks the administration of the HELPE (ex-EKO) refinery (Thessaloniki, Greece) for the provision of catalyst samples.
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348 7. Z. Zhaolong and X. Verykios, J. Chem. Soc. Chem. Commun. (1995) 71. 8. J. R. Rostrup-Nielsen and J-H. Bak Hansen, Joumal of Catalysis No. 144 (1993) 38. 9. E. Ruckenstein and Y.H. Hu, Joumal of Catalysis No. 162 (1996) 230. 10.F.R. Hartley (ed.), Chemistry of the Platinum Group Metals-Recent developments, Studies in Inorganic Chemistry No. 11, Elsevier, 1991. 11 .S.H. Oh, P.J. Mitchell and R.M. Siewert, Journal of Catalysis No. 132 (1991) 287. 12.H. Borchert and M. Baems, Journal of Catalysis No. 168 (1997) 315. 13.J.J. Spivey, Ind.Eng.Chem.Res. No. 26 (1987) 2165. 14.W.L. Liu and M. Flytzani-Stephanopoulos, Journal of Catalysis No. 153 (1995) 317. 15.J.J. Carberry, Chemical Engineering Progress No.2 (1988) 51. 16.A. Roin, HSC CHEMISTRY ver. 2.03 Outokumpu Research Oy, Pori, Finland 17.R.F. Hicks, H. Qi, M.L. Young and R.G. Lee, Journal of Catalysis No. 122 (1990) 280. 18.C.F. Cullis and B.M. Willatt, Journal of Catalysis, No. 83 (1983)267. 19.Y.F.Y. Yao, Ind. Eng. Prod. Res. Dev. No. 19 (1980) 293.