Langmuir 1995,11, 143-148
143
Reaction between Methylamine and Oxygen on Polycrystalline Platinum? Y. S. Gong, Chiapyng Lee,* and Y. Ku Department of Chemical Engineering, National Taiwan Institute of Technology, Taipei, Taiwan 10672, Republic of China Received January 17, 1994. I n Final Form: October 17, 1994@ Heterogeneous ignition and steady-state kinetics of reaction between methylamine and excess oxygen on polycrystalline platinum have been studied in a mixed reactor. The oxidation of hydrogen initiates the ignition, and the released heat which increases the temperature of platinum wire accelerates the decomposition of methylamine, and then more hydrogen is released until a final steady-state of the system is reached. CH3NHz HCN 2H2,2Hz 0 2 2H20, and HCN + ' 1 4 0 2 NO COz + VzHzO, are the reactions which react when a mixture of methylamine and oxygen contacts a high-temperature Pt wire. The flux limited kinetics of these three reactions can be described by Langmuir-Hinshelwood (LH) rate expressions with empirical expressions for the sticking coefficients. It is found that these LangmuirHinshelwood rate expressions correlate all rate data within f15% at all temperatures and pressures. The results suggest that the reactive species are strongly inhibited by molecular oxygen. The selectivities of products can be quantitatively determined.
-
+
+
-
1. Introduction The catalytically stabilized thermal (CST) combustor surmounts the problem that the volumetric heat release rates of conventional catalytic oxidation reactors are far too low to be competitive with the flame combustor.l I n the CST combustor, a heterogeneous catalyst is used to promote gas-phase combustion at temperatures well below those possible in flame combustors. Crucial to the operation of the CST combustor, catalytic oxidation reactions represent an important class of catalytic processes because of altered selectivity, ease of ignition, and stability of the ignited reaction compared to their homogeneous counterparts. The selectivity of catalytic oxidation reactions makes the choice of catalyst to be important for the reduction of air pollution, and the ease and stability of ignition make catalysts excellent flame ignitors and stabi1izers.l Steady-state rates of decomposition and oxidation of methylamine on polycrystalline platinum between 600 and 1500 K have been studied in a mixed reactor by Hwang and Schmidt2 with methylamine and oxygen partial pressures from 0.2 to 6 Torr. HCN and H2 are the dominant products in both excess methylamine and oxygen. Reaction rates rise rapidly up to -950 K, and most reactions appear to become flux limited by -1100 K. These results suggest that CHsNH2 HCN 2Hz is the dominant reaction channel even in excess oxygen and that oxygen mostly reacts with atomic hydrogen but does not significantly affect the C-N bond in CH3NH2. Cordonier e t aL3have examined the effects of pressure, electrical current, oxygen content, and wire dimensions on steady state and oscillatory temperature profiles, oscillation wave shape, amplitude, frequency, and bifurcation behavior in methylamine decomposition on Pt, Rh, and Ir witldwithout the presence of 0 2 . The methylamine pressure has a strong effect on the existence of oscillations
-
+
* Author to whom correspondence should be addressed. + This work was supported by NSC under Grant NSC 81-0421EOll-16Z. Abstract published in Advance ACS Abstracts, December 1, @
1994.
(1) Pfefferle, L. D.; Pfefferle, W. C. Cutal. Rev. Sci. Eng. 1987,29, 219. (2) Hwang, S. Y.;Schmidt, L. D. J. Catal. 1988, 114, 230. (3) Cordonier, G. A.; Schuth, F.; Schmidt, L. D. J.Chem.Phys. 1989, 91, 5374.
0743-746319512411-0143$09.00/0
-
+
and their characteristics. The influence of oxygen concentration on the oscillatory dynamics has been investigated by varying P02/PcH3m2 ratio from 0.025 to 1 for fmed PCH~N ofH2 ~Torr. 0 2 has little influence on oscillation amplitude at low concentrations. As the oxygen concentration is increased beyond about 5%)the amplitude and NH 1,~ range of oscillation both decrease. At P O J P C H ~= oscillations disappear, coinciding with the appearance of hysteresis and multiple study-state temperatures. However, research on the flux-limited kinetics under which the CST combustor is operated is very rare. In this work, we use a low-pressure reactor equipped with a mass spectrometer to study the chemistry of the reaction between methylamine and oxygen over platinum wires, the heterogeneous ignition, and also the influence of Po4 P C H ~ ratio N H ~on the selectivities of products. At the same time, we examine and model the flux-limited kinetics over a wide range of temperature and feed compositions.
2. Experimental Section A schematicdiagram of the apparatus used for the experiments is shown in Figure 1. Reactions were carried out in a 400-cm3, six-way cross stainless steel reactor. Reactant gases that effised from two cylinders, were independently controlled by the two designated needle valves and then premixed before introducing into the reactor. Gases were pumped out ofthe reaction chamber by a 35 m3/h mechanical pump. A bellow sealed right angle valve, connected at the bottom of the reactor, was used to vary the volumetric flow rate. The residence time in the reactor could be varied from 0.3 t o 10 s, as desired by the reaction conditions. All data were collected with a residence time of 5 s in this study. Total pressures between 10-2 and 10 Torr were measured by a capacitance manometer. Gas composition was determined by leaking a small fraction of the gas in the reactor through a leak valve into the ultrahigh vacuum (UHV)system equipped with a quadrupole mass spectrometer. Partial pressures of CH3NH2 and 0 2 were calibrated against mass spectrometer readings by passing known mixtures of CH3NHz and 0 2 through the reactor. Sensitivitiesof H2 and COz were calibrated against CH3NH2
by introducing a certain pressure of CH3NHz into the reactor and recording the mass spectrometer signal with a fixed orifice. After CH3NH2 was evacuated, Hz or COz was introduced with the same pressure and the signal was recorded and the sensitivitiesofHz and COz were obtained. "he mass spectrometer sensitivities of HzO, HCN, and NO were determined with the mass balance to be closed. CrackingofCH3NH2 on the filament ofthe mass spectrometer produced background signals as those of the reaction products.
0 1995 American Chemical Society
Gong et al.
144 Langmuir, Vol. 11, No. 1, 1995
Oxygen Cylinder
Ibrbo-Runping Station
Figure 1. Schematic diagram of the apparatus. Since major signals of CHsNH2 appeared at 2, 27, and 30, correctionswere essential when measuring the rates of H2, HCN, and NO. The correctionswere achieved by recording the signals of these mass units at different CH3NH2partial pressures before reaction and the appropriate background signals corresponding to the partial pressure of CH3NH2 during reaction being subtracted from the full readings of these mass units. The polycrystallineplatinum wire of 0.03 cm in diameter and 8 cm in length was spot-welded on two 0.9-mm nickel leads, which were screwed to the feedthrough that was sealed on the six-way cross. The sample was resistively heated and its temperature was monitored by a R-13% Rh thermocouple, which was spot-welded to the center of the sample. The pressure of CH3NH2 was particularly troublesome to be measured accurately, because it adsorbed strongly on the wall of the reactor. The temperature of the chamber wall was kept at about 55 “C when experiments were proceeding to maintain pump down time and prevent CH3NH2from freezing in the valve orifice. At this temperature, the wall was inert to reactant gases. This could be proven fromthe mass spectrometer and capacitance manometer readings. Most kinetic data were obtained at total pressures between 1 and 10 Torr. Under these conditions, it was believed that the boundary layer effect and homogeneous reactions were negligible and mixing times were short compared to the residence times. The steady-state reaction rates were determined using the mixed reactor mass balance
400
-- 700
1000
1300 1600 1900
Temperature (K) Figure 2. Changes of mass spectrometer signals of all species during the proceeding of ignition.
where Ri is the specific rate of species“i” in molecules per square centimeter per second, Mi is the change in partial pressure of species “i” during reaction, S is the volumetric flow rate, NOis Avogadro’s constant, R is the gas constant, Tg is the gas temperature, and A is the catalyst area. The fresh platinum wire was pretreated by heating to 1800 K in 0.2 Torr oxygen for 4 h and was cleaned with the same conditions for approximately 1 h prior to each day’s experiment. After such treatment the surface of the platinum wire was suggested to be free of contamination by Hwang and Schmidt.2 Auger electronspectroscopy analysis of the wire after treatment showedthe surfacewas free of any contamination except carbon which presumably formedduring the transfer for surfaceanalysis. The reactant gases used in this study were methylamine (98%, Matheson) and oxygen (99.999%)without further purification.
3. Results and Discussion 3.1. Heterogeneous Ignition. Our experimental results show that no appreciable reaction occurs until the
wire is heated to some critical temperatures when the reaction is ignited. Similar observations were reported by Hwang et aL2 However, before the ignition there is a drastic increase in the mass spectrometer signal of H1. The typical mass spectrometer signalsof all species plotted as a function of temperature are shown in Figure 2 when the partial pressures of CH3NH2 and 0 2 are 0.5 and 4 Torr,respectively. The scale on the y-axis of Figure 2 is a linear one. The data plotted in Figure 2 were the raw intensities of species measured by the mass spectrometer. Since the sensitivity factors of these species are different, the ratios of intensities would not coincide with the stoichiometry without further calculation. When ignition takes place, the signals of hydrogen and oxygen decrease abruptly and that of water increases dramatically. No steady-state kinetics and temperature could be obtained
Reaction between Methylamine and Oxygen I
10
:
I
0
e 0
coz
HZO
+
I 1200
o HzO
1400
r 1600
*"
1800
lo
po,/p
c
CHa"
Figure 4. Rates of consumption of CH3NH2 and 0 2 and the rates of formation of Hz, HCN, HzO, NO, and COz as functions of P O J P C H ~ratio N H ~at 0.3 Torr of CH3NHz and 1523 K.
clear that all rates are parallel to each other. No CH4 and NH3 are observed. On the other hand, Figure 2 also shows the decrease of HCN signal accompanies the increase of COZand NO signals during the ignition. Figure 4 is the reaction rates of all species related to the P O J P C H ~ratio N H ~obtained a t 1523 K with the fixed methylamine partial pressure to be 0.3 Torr. When the ratio of P O J P C H ~isN less H ~ than loll, the HZ and HCN formation rates are following the rate of methylamine decomposition. This implies that Hz and HCN are coming from the methylamine decomposition, C H ~ N H Z HCN 2Hz, which is consistent with that reported by Hwang and Schmidt.2 Since HzO is the major product and the rate of HZformation is always less than t h a t calculated from the rate of methylamine decomposition, we expect that, 2Hz 0 2 2Hz0, is another reaction occurring simultaneously. This reaction consumes hydrogen and is the cause of heterogeneous ignition. As the ratio is above 16/1, decompositionof methylamine proceeds continuously, but the formation rate of HCN decreases more rapidly and those of HzO, NO, and COz increase simultaneously. It also can be observed t h a t the rates of formation of NO and COz are identical to each other within the experimental error in Figures 3a, 3b, and 4. This also implies t h a t oxidation of HCN is another reaction which reacts simultaneously. Therefore we conclude that
-
+
+
1200
40
10
1
Temperature( K ) 10
Hz
0 2
t 10
CH3NHz
HCN NO
A
.
P C H ~ =N H 0.3~ Torr
CHjNHZ H2
A t
Langmuir, Vol. 11,No.1, 1995 145
1400
1600
1800
Temperature(K) Figure 3. (a)Rates of consumption of CH3NH2 and 0 2 and the rates of formation of H2, HCN, H20, NO, and COz as functions of temperature with 0 . 3 Torr of methylamine and 0.6 Torr of oxygen. (b) Rates of consumption of CH3NH2 and 0 2 and the rates of formation of H2, HCN, HzO, NO, and C02 as functions of temperature with 0.2 Torr of methylamine and 3.0 Torr of oxygen.
during the ignition, but after t h a t the reaction system reaches a steady state. Aforementioned results tell t h a t the oxidation of hydrogen initiates the ignition, and the released heat which increases the temperature of platinum wire accelerates the decomposition of methylamine, and then more hydrogen is released until a final steady state of the system is reached. 3.2. Reaction Chemistry. Parts a and b of Figure 3 show the rates of consumption of CH3NHz and 0 2 and the rates of formation of Hz, HCN, HzO, NO, and COZ as functions of temperature with 0.3 and 0.2 Torr of methylamine and 0.6 and 3 Torr of oxygen, respectively. It is
-
CH,NH, 2H, HCN
+7402
-
HCN
+ 2H,
+ 0, - 2Hz0 - NO + co, + V,H,O
(2) (3)
(4)
are the reactions which take place when a mixture of methylamine and oxygen contacts a high-temperature Pt wire. The rate limiting step is obviously to be the decomposition of CHSNHz, which is consistent with t h a t reported by Hwang and Schmidt.2 3.3. Flux-LimitedKinetics. Parts a and b of Figure 5 indicate the reaction rates of consumption of methyl-
Gong et al.
146 Langmuir, Vol. 12, No. 1, 1995
also been reported on the NO decomposition on polycrys.~ the talline Pt with the presence of o ~ y g e n . ~Therefore surface coverage ofoxygen, e, can still be assumed to follow the Langmuir type isotherm as
Po, i n Torr
where K is the adsorption equilibrium constant of oxygen and PO,is the partial pressure of oxygen. Since the decomposition of CH~NHZ should occur on the surface which is not covered with oxygen and is fluxlimited, a t steady state the rate ofreaction can be described as
and
T e m p e r a t u r e (K ) 10
where FCHW,is the flux of CH3NH2 onto the metal surface, P C H ~ is NH the~ partial pressure of CH3NH2, M C H ~ N isH the ~ is the sticking molecular weight of CH3NH2, and coefficient of CH3NHz on the bare surface.
'OL
POI= 3 Torr
PCHm, in Torr
According to our model, under flux-limited conditions rates increasing slightly with temperature in Figure 5 are due to the decrease of oxygen coverage. If we fit the reaction rate data of parts a and b of Figure 5 with eq 6, the sticking coefficient @I which should not be a function of temperature can be fitted very well with the following expression:
N 3 . 0 W0.5
&*
0.3'7:
0.2
@, = 0 . 0 5 7 8 / 7
+ 0.0465
(8)
CH3NH2
J
0
T e m p e r a t u r e (K ) Figure 6. (a) Rates of CH3NH2 consumption as a function of temperature at 0.3 Torr of methylamine and indicated oxygen partial pressures. (b) Rates of CH3NH2 consumption as a function of temperature at 3.0 Torr of oxygen and indicated methylamine partial pressures. amine as a function of the Pt wire temperature with methylamine and oxygen pressures being kept at 0.3 and 3 Torr, respectively. Since the rates are not a strong function of temperature as shown in Figures 3 and 5 , the reactions are under the flux-limited kinetics at high temperatures. At the same time, the surface temperature is well above the desorption temperatures of methylamine and HZ4so the only absorbed species on the surface would be oxygen. It was reported t h a t oxygen adsorption on Pt(ll1) probably occurs either through a precursor state or a n undissociated transition state, and the first order kinetics (molecular adsorption) was used to describe the surface coverage of ~ x y g e n .Similar ~ observations have (4) Hwang, S. Y . ;Seebauer, E. G.; Schmidt, L. D. Su$. Sci. 1987, 188,219.
and the result is shown in Figure 6. Since the elementary surface reaction steps for catalytic combustion are usually complicated and not well understood, the use of the empirical expression (eq 8)to describe the variation of sticking coefficient is an extension of the idea that the sticking coefficient may not be a constant.8 The use of a n empirical expression to describe the change of sticking Coefficient due to the interaction of coadsorbed species was also used by Marteney and Kesteng for the oxidation of CO on platinum with the presence of propane. The oxygen adsorption equilibrium constant K is also obtained by fitting the experimental data and can be expressed as
K = 1.421 exp(11927lRT)
(9)
where R is equal to 1.987 c d m o l K. Amirnazmi and BoudartlO determined heats of adsorption of O2between 6.6 and 14 kcallmol which are in reasonable agreement with our value of 12 kcallmol. Therefore, the rate of methylamine decomposition is (5) Monroe, D. R.; Merrill, R. P. J.Catal. 1980,65, 461. (6) Mummey, M. J.; Schmidt, L. D. Surf. Sci. 1981,109,29. (7) Pancharatnam, S.;Lim, K. J.; Mason, D. M. Chem. Eng. Sci. 1975,30,781. (8) Ertl, G.In Catalysis: Science and Technology; Anderson, J. R., Boudard, M., Eds.; Springer: Berlin, 1983; Vol. 4, p 209. (9) Marteney, P. J.; Kesten, A. S. Eighteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; p 1899. (lO)Amimazmi, A.; Boudart, M. J.Catal. 1975,39,383.
Reaction between Methylamine and Oxygen
OV4
Langmuir, Vol. 11, No. I , 1995 147 0.02
c7
/
/
/.
/
@H2
-nn
0.00
V."
0.0
4.0
2.0
0'01
6.0
J POI/PCH3NH2 Figure 6. Stickingcoefficient ofmethylamine,@I, as a function of ( P O ~ P C H ~ N H Z ) ~ ' .
5
0
15
10
Figure 7. Sticking coefficient of H2,@ Has ~ ,a function of 1/PH,.
-rc'cy,NH, =
Following the similar idea and procedure, the consumption rate of 0 2 , r3,02,through the reaction between Hz and 0 2 (eq 3) can be represented as
FH,
= (3.51 x 1 0 2 2 ) P , J J m
(12)
+ 0.00022
(13)
@H2 = O.O0153/P~,
where F H is~ the flux of Hz onto the metal surface, is the partial pressure of Hz, is the molecular weight of Hz, and C P H ~is the sticking coefficient ofHz. The oxidation rate of HCN (eq 41, r4,HCN, also can be described as
(14) (15) @HCN
- 0.00015 PHCN
= 0.00401-
(16)
where FHCNis the flux of HCN onto the metal surface, PHCN is the partial pressure of HCN, MHCN is the molecular weight of HCN, and QHCN is the sticking coefficient of HCN. Both Q H ~and CPHCN can be described very well with eqs 13 and 16. The results are shown in Figures 7 and 8, respectively. The solid lines in Figures 3 through 5 are the predictions for the mixed reactor using eqs 5 through 16. The agreement of data with the simple LH rate expressions is within f 1 5 % over a wide range of temperatures and pressures.
li 0.00
1f
0
P
I
10
I
0
Figure 8. Sticking coefficient of HCN, @HCN, as a function of PO~PHCN.
It is clear that kinetics, no matter how extensive, do not reveal a detailed picture of the mechanism of the reaction of the nature of adsorbed intermediates especially for such a complicated system. However, it is surprising that simple modified LH rate expressions can be used to model complicated catalytic oxidation reactions. This study may provide a new way to analyze the CST combustors. 3.4. Selectivity. Figure 9 indicates that the selectivities of HCN and NO on the various P O J P C H ~ratios, NH~ when the pressure of methylamine and Pt wire temperature are 0.3 Torr and 1523K, respectively. The solid lines are theoretical values calculated for the mixed reactor using eqs 5 through 16. I t could be observed t h a t as the ratio is less than 16/1, the main product is HCN. This indicates that methylamine decomposition is the dominant
Gong et al.
148 Langmuir, Vol. 11,No.1, 1995 loo
accompanied with the increase of NO selectivity which is the product of HCN oxidation.
1 PCHINHI = 0.3 Torr
tY
t W
NO / 0
4. Conclusions
By examining the reaction between methylamine and oxygen on polycrystalline platinum wires using a low pressure reactor equipped with a mass spectrometer, we conclude the following: (1)The oxidation of hydrogen initiates the ignition, and the released heat which increases the temperature of platinum wire accelerates the decomposition of methylamine, and then more hydrogen is released until a final steady-state of the system is reached. (2)
~ C H ~ N H ~
r H C N /rCHsNHz
601
h
4
.A
3
3
40
i
CH,NH,
2ov 0
4
8
12
16
HCN
+ 2H,
+ 0, - 2H,O HCN + 7/402 - NO + CO, + l/,H2O 2H2
t
0
-
20
24
28
p 02/PCH3NHz Figure 9. Selectivities of HCN and NO of methylamine oxidation as a function of P ~ J P c H at ~ N1523 H ~ K. reaction. As the ratio is greater than 1611, NO is the dominant product. The decrease of HCN selectivity is
are the reactions which react when a mixture of methylamine and oxygen contacts a high temperature Pt wire. (3) The flux limited kinetics of these three reactions can be formulated by Langmuir-Hinshelwood type rate expressions with the sticking coefficients to be expressed empirically. (4) The selectivities of products can be quantitatively determined. LA940054K
