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Analysis of N2O decomposition over fixed bed mixed metal oxide catalysts made from hydrotalcite-type precursors
Applied Catalysis A: General 273 (2004) 223–231 www.elsevier.com/locate/apcata
Analysis of N2O decomposition over fixed bed mixed metal oxide catalysts made from hydrotalcite-type precursors Kil Sang Chang*, Haak Song, Yong-Sung Park, Je-Wan Woo Department of Industrial Chemistry, Sangmyung University, Seoul, Korea Received in revised form 16 June 2004; accepted 21 June 2004 Available online 31 July 2004
Abstract The kinetic modeling and analysis for the catalytic decomposition of nitrous oxide into nitrogen and oxygen have been performed based on some experiments over a fixed bed of mixed metal oxide catalysts made from calcined hydrotalcite-type precursors. The hydrotalcite-type compounds were obtained by co-precipitation of various metal nitrates with the addition of the precipitants and under controlled pH. The reactions were carried out under atmospheric pressure in the temperature range 250–500 8C and space velocities were set between 15,000 and 60,000 h1. A reaction rate model derived from the mechanisms of adsorption and sequential decomposition of nitrous oxide has been applied to the decomposition operation of a fixed bed reactor. The mechanistic aspects of all the relevant reactions could be represented exactly for the operation range of temperature, concentration and parameters of reactor concern. # 2004 Elsevier B.V. All rights reserved. Keywords: Nitrous oxide; Catalytic decomposition; Reaction kinetics; Fixed bed; Hydrotalcite
1. Introduction Nitrous oxide (N2O), one of the major greenhouse gases, is a colorless gas and an important component of the earth’s atmosphere with about 150 years of life time in atmospheric condition. It also exhibits a global warming potential 310 times that of carbon dioxide (CO2) on a per molecule basis [1–3]. The global concentration of N2O is now increasing faster than at anytime in the past, and this has been attributed to anthropogenic contributions. The partial trapping of the thermal radiation by so-called greenhouse gases and radiation-absorbing particles is causing the surface and tropospheric temperatures of the earth to increase by several degrees [4]. Nitrous oxide is also the main source of stratospheric NOx which is expected to play an important role as an ozone sink when atmospheric levels of CFCs decrease. Hence, it is environmentally important to reduce the emission of nitrous oxide into atmosphere, and catalytic decomposi-
* Corresponding author. Tel.: +82 222 875 296; fax: +82 239 495 85. E-mail address: [email protected] (K.S. Chang). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.06.036
tion of nitrous oxide is an energy-efficient and promising process to attain this objective. Nitrous oxide can be decomposed into nitrogen and oxygen at higher temperatures rather easily. However, high reaction temperature is a great burden for economical and operational efficiencies. The reduction in operation temperature and high economical efficiencies would be the main objective of a new catalyst development. Variety of catalysts for the decomposition of nitrous oxide such as metal oxides, metal exchanged zeolites and mixed metal oxides has been studied by many authors, whose works are precisely reviewed by Kapteijn et al. [5,6]. Among those catalysts, Cu-ZSM-5 and Rh-ZSM-5 by Li and Armor [7], and Rh supported on ZnO by Oi et al. [8] showed excellent activities among the catalysts screened so far. Fe ion-exchanged zeolites are also noticeable for their selective catalytic reduction efficiencies with hydrocarbon reducing agents in the presence of oxygen [9,10]. Meanwhile, mixed metal oxide catalysts obtained from calcined hydrotalcites or hydrotalcite-type compounds have been discovered and found to be efficient for the decomposition of nitrous oxide [11–14].
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Nomenclature List of C C0 E k ka kdb kd ke k0 M r R ra rd re t ts T uz x x N2 O x0 x O2 z Q0 Qi QO
symbols concentration input concentration activation energy reaction rate constant reaction rate constant of adsorption reaction rate constant of backward desorption reaction rate constant of desorption reaction rate constant for Eley–Rideal reaction Arrhenius constant catalyst reaction rate gas law constant reaction rate of adsorption reaction rate of desorption reaction rate of Eley–Rideal reaction time space time temperature interstitial velocity in z direction mole fraction mole fraction of N2O input mole fraction mole fraction of O2 catalyst bed depth portion of free active sites portion of active sites occupied by species i portion of active sites occupied by oxygen
Hydrotalcite-type compounds are intrinsically anionic clay materials which have the form of layered double hydroxides, with the general formula [Mg1x Alx(OH)2] [Anx/nzH2O]. The ratio of Mg:Al varies from 4:1 to 2:1 (0.20 x 0.33). The host layers are charged positively by replacement of divalent metal cations with trivalent ones. The positive charges are compensated by the interlayer anions. Hydrotalcite, whose interlayer anions are carbonates (CO3 hydrotalcite), decomposes to oxide at 500 8C, releasing CO2 and H2O. Although the application of hydrotalcites was mainly as precursors of oxide catalysts, various intercalation methods, which can exhibit new functions by inserting various anions into interlayers, are also actively being studied [15,16]. Various compositions of mixed metal oxide catalysts obtained by calcination of hydrotalcite-type compound precursors synthesized by the co-precipitation method have been studied with respect to the reactivity, kinetics and impurities other than nitrous oxide. Dandl and Emig [17], through the kinetic modelling and simulation in a recycle reactor, reported that a new class of catalyst, calcined hydrotalcite, has a good activity even in the presence of high humidity. For the kinetic study of N2O decomposition, reaction activities were tested for alumina-supported metal
catalysts by Doi et al. [18]. Also, it has been reported that the N2O decomposition over mixed metal oxide catalysts followed the first order reaction law in obtaining the kinetic data [12]. The catalytic reaction mechanism of N2O decomposition seems straightforward, with a series of mass transfer, adsorption, decomposition and desorption processes. However, an analytical attempt to describe all these processes becomes messy and it seems hard to define the real characters. In this article, a reaction kinetic model for the calcined hydrotalcite-type catalyst is suggested with respect to the reaction rates of adsorption, instant cleavage of nitrous oxide on adsorption, desorption of oxygen and other related reactions. The model has been created through the analytical and experimental investigations of the catalytic fixed bed reactor under various conditions.
2. Experimental The catalyst precursor, hydrotalcite-type compounds, were prepared by the co-precipitation method following Armor et al. [19]; here the NaOH/Na2CO3 solution was added to aluminum and various transition metal nitrate solutions drop-wise for about 4 h while controlling pH under stirring for the precipitation reaction. The final pH was adjusted to 10. The precipitate slurry was stirred for about 30 min more and heated to 65–67 8C for 16 h. The resulting products were filtered out and washed with plenty of distilled water several times to remove excess sodium and nitrate, and then dried in an oven at 110 8C overnight. The compound thus prepared is single phase with hydrotalcite-type structure. The compounds were then calcined to activate as a mixed metal oxide catalysts under flowing dry air and slowly increasing temperature condition up to 500 8C, at which value the temperature was held constant for 4 h. The catalytic reaction tests were carried out in an allquartz glass fixed bed reactor under ambient pressure employing 0.1–0.5 g of mixed metal oxide catalysts made from hydrotalcite-type precursors. The catalytic decomposition was performed at an initial concentration of 5000– 10,000 ppm N2O in the temperature range from 250 to 500 8C. Before each decomposition run, the catalyst was conditioned with nitrogen flowing at the activation temperature of 500 8C for 3 h, which caused the regeneration of active surface and confirmed the reproducibility of the catalysts. The reactor was heated by a temperature controlled heating tape, and owing to the exothermic character of the decomposition reaction of N2O, the input and output temperatures were measured, though no appreciable thermal change was detected due to the low concentration of reacting component. The flow rate of the feed gas was modulated to produce GHSV from 15,000 to 60,000. The concentration analysis was performed using an on-line gas chromatograph
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225
Fig. 1. Schematic diagram of fixed bed catalytic experimental system for N2O decomposition.
with a pulse-discharged helium ionization detecter. The schematic diagram of reactor system is shown in Fig. 1.
3. Mechanistic kinetics modeling A catalytic decomposition takes place due to the contact of reactants with catalyst through an adsorption process. The reaction mechanisms can be described as three steps as mentioned by the authors [5,17]. ka
N2 O þ M ! N2 þ OM kd
2OM @ O2 þ 2M kdb
ke
N2 O þ OM ! N2 þ O2 þ M
(1) (2) (3)
where M indicates an active site of the catalyst and OM implies an adsorbed oxygen. Reaction (1) is a condensed expression of two steps of adsorption and decomposition reactions of nitrous oxide into nitrogen and adsorbed state of oxygen, which implies that the decomposition progresses as soon as the adsorption of nitrous oxide takes place; it can be ascribed to the weak adsorbability of nitrous oxide in comparison with oxygen radical. Reactions (2) and (3) may occur competitively unless the reverse reaction rate by high oxygen concentration is significant or one of them may occur predominantly over the other. The reaction rate for reactions (1) to (3) can be shown as: ra ¼ ka xN2 O Q0
(4)
rd ¼ kd ðQO Þ2 kdb xO2 ðQ0 Þ2 re ¼ ke xN2 O QO
(5) (6)
where Q0 and QO are the portions of the free active sites of the catalyst and the sites covered by oxygen respectively. If we let Qi be the portion of sites occupied by component i, the
sum of all portions should be 1. That is, X Qi ¼ 1 Q0 þ
(7)
Also we can obtain Eq. (8) from the stoichiometry of reaction rates based on the elemental oxygen adsorbed on the catalyst at steady state, from which the portion of free or adsorbed active sites can be derived as a function of x N2 O . ra ¼ 2rd þ re
(8)
The overall decomposition rate of nitrous oxide then can be expressed as the sum of two rates of reactions (4) and (6) as: r ¼ ra þ re ¼ ½ka Q0 þ ke ð1 Q0 Þ xN2 O
(9)
A kinetic study based on the reaction mechanism needs to consider all the internal and product behavior of the reactor along with the reactor type, feed concentration and other operational conditions. One may recollect here how the fixed bed reactor behaves and the presence of oxygen as shown in reaction (2) which may have a definite role in the overall reaction with the activity of reverse reaction against desirable reactive direction. For further study of the reaction mechanism in detail, we may consider two possible operational conditions separately: these are the cases when oxygen-free nitrogen and nitrogen with oxygen in excess are used as feed gases. 3.1. Oxygen effect Some studies on the reaction mechanism with respect to the adsorption and desorption of oxygen on the catalyst surface are of interest. Tanaka et al. [20] reported by observing the adsorption and recombinative desorption behavior of oxygen on rhodium impregnated zeolite that the Langmuir–Hinshelwood mechanism of reaction (2) was
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prevailing at lower temperature. On the other hand, the Eley–Rideal mechanism of reaction (3) was found to be prevailing on Fe-zeolite catalyst at higher temperature [21]. These results are quite contrasting to the suggestion of Dandl and Emig [17] for the mixed metal oxide catalyst, where the Eley–Rideal mechanism prevails at lower temperature and both Eley–Rideal and Langmuir–Hinshelwood mechanisms prevail at higher temperature. Tanaka et al. [20] also suggested that oxygen adsorption to rhodium is irreversible to some extent for the temperatures under 600 8C. In that case, it may take some time to reach to the full activity operation of the catalyst at steady state. However, with respect to the adsorption of oxygen on the mixed metal oxide catalyst which has the lattice oxygen at least one to twice the number of catalyst metals, the reaction progresses instantly on a reversible basis. Our experimental scope is within the reversible adsorption range of oxygen depending on the ambient concentration as can be shown in reaction (2), which can be ascertained through the preparation steps of the catalyst. When no oxygen is in the feed gas, the oxygen is produced only by the result of N2O decomposition. When it is produced as adsorbed oxygen, however, for the operational character of the fixed bed reactor, the oxygen will be desorbed by reaction (3) or recombined by reaction (2), and will be swept away by the effluent gas instantly, except for leaving a very small amount of adsorbed oxygen maintaining dynamic equilibrium with effluent oxygen concentration at steady state. Thus the oxygen can not affect the reverse reaction of reaction (2) at least in the once-through type fixed bed reactor, and the reverse reaction can be neglected. For the primitive access of adsorption and reaction of the process, the adsorption rate of nitrous oxide was assumed to be the same as in reaction (1); if we assume that the desorption reaction of OM to O2 takes place between neighboring sites, we may suggest the reaction mechanisms by replacing reaction (2) as follows: ka
N2 O þ M ! N2 þ OM kd
2OM ! O2 þ 2M ke
N2 O þ OM ! N2 þ O2 þ M
(1) (10) (3)
The reaction rate of each reaction can be expressed as follows: ra ¼ ka xN2 O Q0
(4)
rd ¼ kd ðQO Þ2
(11)
re ¼ ke xN2 O QO
(6)
Though the rates of these three reactions are different, the rates become balanced at steady state in the sense of Eq. (8). If we use x instead of xN2 O for convenience, the solution of
these two equations at steady state can be derived as: ½fðka þ ke Þ2 x2 þ 8ka kd xg1=2 ðka þ ke Þx
ð4kd Þ Q0 ¼ 1 QO
QO ¼
¼1
½fðka þ ke Þ2 x2 þ 8ka kd xg1=2 ðka þ ke Þx
ð4kd Þ
(12)
(13)
Then from Eq. (9), the overall reaction rate can be expressed as a complex function of xin the power series as: " # fððka þ ke Þ2 x2 þ 8ka kd xÞ1=2 ðka þ ke Þxg r ¼ ka x 1 ð4kd Þ þ ke x
½fka þ ke Þ2 x2 þ 8ka ke Þx
ð4kd Þ
(14)
Since the reaction rate of Eq. (14) is very hard to define, a modification should be necessary for the analysis of a reaction. We can go further to evaluate the relative magnitude of QO, the portion of sites covered by oxygen. Using the triangle inequality rule for Eq. (12), where the values of parameters and variables are all positive, one can express the magnitude of QO as: 1=2 ka x fðka þ ke Þ2 x2 þ 8ka kd xg1=2 QO (15) 2kd 4kd Then, for ka, kd and ke assumed to have almost the same order of magnitude and for a very small range of nitrous oxide concentration, QO may have a value far less than 1, so that it may not affect the overall reaction rate of nitrous oxide decomposition. This implies that the adsorption rate is mainly dependent on the first power of nitrous oxide concentration x. Then, when QO = 0 and Q0 = 1 are assumed approximately, the overall reaction rate constant for Eq. (9) can be represented as: k ¼ ka Q0 þ ke ð1 Q0 Þ
(16)
The involvement of reaction (3) in the overall decomposition of nitrous oxide is of interest. This reaction is typically called Eley–Rideal surface reaction, in which nitrous oxide decomposes on collision with adsorbed oxygen without any adsorption procedure. It was not possible to ascertain whether the mechanism is actually involved in this metal oxide catalytic reaction from other reports [5,17]. From a practical point of view, for a low concentration of nitrous oxide and the value of Q0 near 1, much involvement of this reaction is not expected from Eq. (16). However, actual involvement of this reaction in the mechanism may be estimated with respect to the effect of ke and the factor (1 Q0), which will be discussed below after the analysis of the case when oxygen is present in the feed gas.When oxygen is present constantly in excess compared with the concentration of nitrous oxide, the product oxygen on the catalyst from reaction (1) will be far below the equilibrium under excess oxygen environment, and for the supplement of
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equilibrium deficit the reverse reaction of reaction (2) will be dominant rather than the forward reaction. Then reaction (3) may contribute for the decomposition of nitrous oxide. Recall that the concentration of nitrous oxide is generally very low in comparison with oxygen in the air or in the industrial flue gases. The major reactions would then be reactions (1) and (3) as: ka
N2 O þ M ! N2 þ OM ke
N2 O þ OM ! N2 þ O2 þ M
(1) (3)
Then, introducing Q0 = 1 QO into the overall reaction rate model of Eq. (9), the reaction rate constant can be expressed as Eq. (17) with respect to the portion of oxygen-occupied sites. k ¼ ka þ ðke ka ÞQO
(17)
When the oxygen concentration is very high, most of the free active sites will be covered by oxygen and the portion of the oxygen-adsorbed sites, QO will have the value near 1. Hence the overall reaction rate will follow the first order rate law. Especially, when ke is greater than ka, then the overall decomposition reaction will proceed faster than the oxygen-free feed condition. Meanwhile, when ke is smaller than ka, the overall decomposition rate can be diminished. Moreover, if ke = 0, that is, there is no Eley–Riedal reaction involved, then the overall decomposition reaction will be seriously injured, wherein oxygen can be deemed as mere an impurity in excess.
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The output concentration of nitrous oxide through the fixed bed reactor with space time ts can be expressed as follows: C ¼ C0 expðkts Þ
(21)
This result coincides with the solution result of fixed bed with bed depth of z, which means that the product concentration and the reaction rate with fixed bed reactor are dependent only on the space time of the reaction and not on the bed depth. Therefore, the kinetic data obtained from the fixed bed catalytic reactor can be relied upon without considering the varying boundary conditions. Thus the fixed bed reactor can be said very useful in kinetic study of catalytic reaction. The decomposition rate constant can be obtained by measuring the product concentration and space time. The temperature dependence of the rate constant should be examined by the Arrhenius equation, which gives the information that the rate constants are consistent with the reaction rate model. E k ¼ k0 exp (22) RT The kinetic data of activation energy and the Arrhenius constants were obtained through the polynomial regression of the overall decomposition rate constants.
3.2. Fixed bed catalytic reaction
4. Results and discussion
Consider the fixed bed catalytic reactor with first order decomposition reaction where the eddy dispersion effect is negligible. Then, the concentration change and distribution of the reacting component is described as a partial differential equation,
The conversion efficiencies of N2O in a fixed bed reactor for different metal compositions of Co–Al hydrotalcite catalysts with the temperature change are compared in Fig. 2. The conversion efficiencies of Co–Al catalysts with the composition ratio are different but not peculiar in comparison with other catalysts illustrated in the next figures. The conversion results for other catalysts with
@C @C þ uz þ kC ¼ 0 @t @z
(18)
Here, C is the concentration of reacting component in fluid flowing at an interstitial velocity uz along axial direction, z. The transient solution of partial differential Eq. (18) for various types of nonlinear reactions is not easy to obtain. However, when the decomposition reaction is carried out in the fixed bed reactor, the reaction reaches steady state sooner or later, where the adsorption rate becomes equivalent to the decomposition rate and the product concentration does not change as time. The steady state equation can be obtained from Eq. (18) with the time dependent term deleted and the solution can be written as Eq. (19). kz C ¼ C0 exp (19) uz Here, the value z/uz can be considered as space time, and denoted by ts = z/uz, and the space velocity (SV) becomes, SV ¼
uz 1 ¼ ts z
(20)
Fig. 2. Comparison of experimental results with simulation for N2O decomposition of various Co-Al hydrotalcite catalysts.
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Fig. 3. Comparison of experimental results with simulation for N2O decomposition of various Co–La–Al and Co–Rh–Al hydrotalcite catalysts.
Fig. 4. Comparison of experimental results with simulation for N2O decomposition of Co–La–Al(4/1/1) hydrotalcite catalysts with the change of space velocity.
different metal compositions of Co–La–Al and Co–Rh–Al are compared in Fig. 3. Some catalysts containing Rh and La are showing high efficiencies even at temperatures below 350 8C. When the feed concentration of nitrous oxide was varied between 1500 and 5000 ppm, the conversion efficiency was not significantly influenced, as can be expected from the first order reaction model. The simulation results applied by the reaction rate model are also shown in the drawings; these simulation results show a good agreement with the experimental results. When the first order rate equation is applied, the period of transition state is very short and thus whole reaction may be considered to take place in a steady state. The overall reaction rate also can be considered to be mainly dependent on the adsorption rate for very low concentration range of nitrous oxide, which might have been caused by the low portion of the sites covered by adsorbed oxygen. When the space velocity was decreased, the conversion ratio rose. The change of space velocity for Co–La–Al catalyst is also shown in Fig. 4; more than 90% conversion with space velocity of 15,000 h1 can be achieved at 300 8C. The figure also shows that, if operated at 350 8C, more than 90% conversion can be achieved even at a space velocity of 30,000 h1. The simulation of the reaction rates shows very good approximation in agreement with the experimental results. With these results, we can deduce with the information on the small portion of the active sites covered by oxygen and the overall decomposition rate constant of Eq. (16) that the effect of Eley–Rideal reaction was not influencing much in nitrous oxide decomposition under nitrogen environment, for which the relatively small value of ke for the oxygen present case can be referred to for comparison. Thus, the desorption rate of oxygen cannot be a rate-determining step of the decomposition of N2O. From these circumstances the neglect of terms containing x because of their comparatively low magnitude can be justified from
these results. It would be more comprehensible to consider that the concentration of N2O is much smaller than the input concentration through most of the reactor. The effect of oxygen on the decomposition rate over the fixed bed catalysts was investigated with 2–21% oxygen content in the feed gas. Figs. 5 and 6 show the conversion results of the Co-Al hydrotalcite catalyst reactions in the presence of oxygen. The conversion rate drops with oxygen content. The portion of active sites covered by oxygen is thought to be almost full and can be thought to be constant throughout the bed for the feed of oxygen higher than 10%. Then the overall decomposition rate constant of Eq. (17) can be varied with the comparative magnitude of Eley–Rideal reaction rate as the difference of two rate constants, (ke ka). If Eley–Rideal reaction is fast enough, as ke > ka, then the overall decomposition proceeds faster. On the other hand, if Eley–Rideal reaction is not involved, as ke = 0,
Fig. 5. Comparison of experimental results with simulation for N2O decomposition of Co–Al(1/1) hydrotalcite catalysts with the concentration change of oxygen in the feed gas.
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229
Fig. 6. Comparison of experimental results with simulation for N2O decomposition of Co–Al(2/1) hydrotalcite catalysts with the concentration change of oxygen in the feed gas.
Fig. 8. Comparison of experimental results with simulation for N2O decomposition of Co–Rh–Al(1/0.2/1) hydrotalcite catalysts with the concentration change of oxygen in the feed gas.
oxygen will behave absolutely as an impurity that can diminish the overall reaction rate significantly. However, the Eley–Rideal reaction seemed not active enough to effectuate the gain in oxygen adsorbed sites, at least in this range of reaction temperatures, and consequently the overall decomposition rate was found to be reduced in the experiments. From this fact, it can also be deduced that the Eley– Rideal reaction prevails, but the rate constant, ke is somewhat less than the rate constant of reaction (1), ka, which means the Eley–Rideal reaction is rate-controlling. It is quite comprehensible that the relevant reaction mechanisms can be inferred from the rate constant of the overall reaction rate model with the support of some experimental results. Similar interpretations could have been possible for the experimental results of Co–La–Al and Co–Rh–Al hydrotalcite catalysts shown in Figs. 7 and 8. Their catalytic efficiencies for the whole range of temperature were excellent. If economic effect is put aside, the Co–Rh–Al catalyst
shows slightly higher efficiency. However, the sensitivity of the Co–La–Al hydrotalcite catalyst shows higher stability for oxygen content than Co–Rh–Al catalyst, from which Co–La–Al catalyst can be considered to have a merit if the oxygen content can be kept low. The estimation of rate constant and activation energy of the catalysts for the decomposition reaction can be obtained using an Arrhenius plot; such results are listed in Table 1. The activation energies of the catalysis reactions in the presence of oxygen in the feed gas show a tendency of slight increase with the oxygen content; this might have been caused by the adsorbability bias of oxygen with temperature change. As in most decomposition reactions, the higher reaction rate and the high conversion ratio at higher temperature can be interpreted as the complex results of Arrhenius effect and low adsorbability of oxygen at higher temperature. A question may be raised if the forward reaction of reaction (2) may occur at higher temperature range where oxygen adsorption equilibrium is below the product oxygen of reaction (1). In that case, the reaction should go with reaction (3) competitively, which means production of a positive effect on reaction (1) and a negative effect on reaction (3) with the decreased value of QO. Therefore, the decrease in adsorption equilibrium of reaction (2) and the increase in rate constants may cause the overall decomposition rate to increase further at higher temperature. The experimental kinetic data of Table 1 shows that the activation energy is slightly increasing with oxygen contents for the experimental temperature range, which implies that the rate constant is less sensitively dependent on the temperature change. Meanwhile, some of the Arrhenius constants are increasing with oxygen content, which means higher rate constants at higher temperatures. However, it is not yet possible to tell whether the adsorption equilibrium of product oxygen went over the oxygen feed condition in the experimental range.
Fig. 7. Comparison of experimental results with simulation for N2O decomposition of Co–La–Al(4/1/1) hydrotalcite catalysts with the concentration change of oxygen in the feed gas.
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Table 1 Parameters and kinetic data obtained from the experimental results and simulation of nitrous oxide decomposition over the fixed bed catalysts under various conditions Catalysts
Oxygen contents (%)
E (KJ/mole)
k0 (104/s)
Co–Al(1/1) Htlc
0 10 21 0 10 21 0 0 2 10 21 0 10 21 0
43.00 45.00 45.30 69.00 79.00 82.00 61.20 48.99 51.67 59.00 59.10 66.34 67.70 71.00 69.00
3.00 3.53 3.16 152.00 410.00 560.00 25.38 42.14 37.75 79.92 71.59 983.36 863.48 750.67 151.55
Co–Al(2/1) Htlc
Co–Al(3/1) Htlc Co–Rh–Al(1/0.2/1) Htlc
Co–La–Al(4/1/1) Htlc
Co–La–Al(6/1/1) Htlc
Dandl and Emig [17], using a simulation model, reported the calculation results of the free active sites with the effect of water and oxygen; here high levels of N2O and water concentration in the feed gas were used. The concentration level of 15% N2O which can occur in adipic or nitric acid manufacturing industries is beyond our scope, wherein the portion of free active sites were greatly reduced with the impurity level and increased with the reaction temperature. Thus, the desorption rate of oxygen is sufficiently faster than the adsorption rate and the adsorption rate can be a ratedetermining step for the whole decomposition. Therefore the reduction capability of the catalytic reactor depends on how large the portion of active sites is that can be kept free so that the adsorption may progress faster.
free for further adsorption of N2O. Thus, the overall reaction rate is determined by the product of the portion of free active sites and the concentration of N2O, which leads approximately to the rate equation of the first order of the concentration of N2O. The low coverage of active sites by surface oxygen did not allow the situation of the Eley–Rideal reaction either. The presence of high concentration of oxygen in the feed gas provides the dominant coverage of active sites, which diminishes the portion of free active sites, and the overall reaction becomes reduced. Meanwhile the Eley– Rideal reaction, though it prevails in the desorption of oxygen, was proven to be not active enough to promote the overall decomposition reaction in the temperature range between 250 and 500 8C. Thus, the presence of oxygen did not help to the decomposition reaction, which is different from the cases of selective catalytic reduction processes where oxygen may help the cleavage of organic reducing agents to produce nitrogen molecules from nitrogen oxides [22]. In the range of concentration under 10,000 ppm N2O, the first order reaction rate model can be taken for a catalytic decomposition of N2O without any problems for industrial and engineering purposes. If any impurities are present in the feed gas, they can be treated as chemicals adsorbing to the active sites of the catalyst. They may share the free active sites of the catalyst and gradually diminish the overall decomposition rate. Most of all, the mixed metal oxide catalysts made from various hydrotalcite-type precursors have been proved to be very efficient catalysts for effective removal of nitrous oxide, which is known as one of the major greenhouse gases. The experimental data and the reaction rate model suggested in this paper will be very useful when a scale-up for industrial engineering is needed.
5. Conclusions The decomposition experiments of nitrous oxide and the mechanistic analysis were performed over fixed bed catalyzer made from hydrotalcite-type precursors. For the concentration range between 5000 and 10,000 ppm of N2O with and without the presence of oxygen in the feed gas, we could draw the following results. The fixed bed reaction system is very useful in the study of catalytic reduction mechanism and reaction rate of nitrous oxide decomposition. The results are very uniform throughout the operation and are not affected by the concentration change of reactants with time and position, which often can be met as a problem in batch processes. The overall reaction rate is mainly dependent on the adsorption rate of N2O which is instantly cleaved into nitrogen and oxygen on contact with the catalyst, while the oxygen remains adsorbed on the catalyst. The rates of molecular oxygen formation and desorption of the adsorbed oxygen are fast enough to leave the active sites
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Analysis of N2O decomposition over fixed bed mixed metal oxide catalysts made from hydrotalcite-type precursors Kil Sang Chang*, Haak Song, Yong-Sung Park, Je-Wan Woo Department of Industrial Chemistry, Sangmyung University, Seoul, Korea Received in revised form 16 June 2004; accepted 21 June 2004 Available online 31 July 2004
Abstract The kinetic modeling and analysis for the catalytic decomposition of nitrous oxide into nitrogen and oxygen have been performed based on some experiments over a fixed bed of mixed metal oxide catalysts made from calcined hydrotalcite-type precursors. The hydrotalcite-type compounds were obtained by co-precipitation of various metal nitrates with the addition of the precipitants and under controlled pH. The reactions were carried out under atmospheric pressure in the temperature range 250–500 8C and space velocities were set between 15,000 and 60,000 h1. A reaction rate model derived from the mechanisms of adsorption and sequential decomposition of nitrous oxide has been applied to the decomposition operation of a fixed bed reactor. The mechanistic aspects of all the relevant reactions could be represented exactly for the operation range of temperature, concentration and parameters of reactor concern. # 2004 Elsevier B.V. All rights reserved. Keywords: Nitrous oxide; Catalytic decomposition; Reaction kinetics; Fixed bed; Hydrotalcite
1. Introduction Nitrous oxide (N2O), one of the major greenhouse gases, is a colorless gas and an important component of the earth’s atmosphere with about 150 years of life time in atmospheric condition. It also exhibits a global warming potential 310 times that of carbon dioxide (CO2) on a per molecule basis [1–3]. The global concentration of N2O is now increasing faster than at anytime in the past, and this has been attributed to anthropogenic contributions. The partial trapping of the thermal radiation by so-called greenhouse gases and radiation-absorbing particles is causing the surface and tropospheric temperatures of the earth to increase by several degrees [4]. Nitrous oxide is also the main source of stratospheric NOx which is expected to play an important role as an ozone sink when atmospheric levels of CFCs decrease. Hence, it is environmentally important to reduce the emission of nitrous oxide into atmosphere, and catalytic decomposi-
* Corresponding author. Tel.: +82 222 875 296; fax: +82 239 495 85. E-mail address: [email protected] (K.S. Chang). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.06.036
tion of nitrous oxide is an energy-efficient and promising process to attain this objective. Nitrous oxide can be decomposed into nitrogen and oxygen at higher temperatures rather easily. However, high reaction temperature is a great burden for economical and operational efficiencies. The reduction in operation temperature and high economical efficiencies would be the main objective of a new catalyst development. Variety of catalysts for the decomposition of nitrous oxide such as metal oxides, metal exchanged zeolites and mixed metal oxides has been studied by many authors, whose works are precisely reviewed by Kapteijn et al. [5,6]. Among those catalysts, Cu-ZSM-5 and Rh-ZSM-5 by Li and Armor [7], and Rh supported on ZnO by Oi et al. [8] showed excellent activities among the catalysts screened so far. Fe ion-exchanged zeolites are also noticeable for their selective catalytic reduction efficiencies with hydrocarbon reducing agents in the presence of oxygen [9,10]. Meanwhile, mixed metal oxide catalysts obtained from calcined hydrotalcites or hydrotalcite-type compounds have been discovered and found to be efficient for the decomposition of nitrous oxide [11–14].
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Nomenclature List of C C0 E k ka kdb kd ke k0 M r R ra rd re t ts T uz x x N2 O x0 x O2 z Q0 Qi QO
symbols concentration input concentration activation energy reaction rate constant reaction rate constant of adsorption reaction rate constant of backward desorption reaction rate constant of desorption reaction rate constant for Eley–Rideal reaction Arrhenius constant catalyst reaction rate gas law constant reaction rate of adsorption reaction rate of desorption reaction rate of Eley–Rideal reaction time space time temperature interstitial velocity in z direction mole fraction mole fraction of N2O input mole fraction mole fraction of O2 catalyst bed depth portion of free active sites portion of active sites occupied by species i portion of active sites occupied by oxygen
Hydrotalcite-type compounds are intrinsically anionic clay materials which have the form of layered double hydroxides, with the general formula [Mg1x Alx(OH)2] [Anx/nzH2O]. The ratio of Mg:Al varies from 4:1 to 2:1 (0.20 x 0.33). The host layers are charged positively by replacement of divalent metal cations with trivalent ones. The positive charges are compensated by the interlayer anions. Hydrotalcite, whose interlayer anions are carbonates (CO3 hydrotalcite), decomposes to oxide at 500 8C, releasing CO2 and H2O. Although the application of hydrotalcites was mainly as precursors of oxide catalysts, various intercalation methods, which can exhibit new functions by inserting various anions into interlayers, are also actively being studied [15,16]. Various compositions of mixed metal oxide catalysts obtained by calcination of hydrotalcite-type compound precursors synthesized by the co-precipitation method have been studied with respect to the reactivity, kinetics and impurities other than nitrous oxide. Dandl and Emig [17], through the kinetic modelling and simulation in a recycle reactor, reported that a new class of catalyst, calcined hydrotalcite, has a good activity even in the presence of high humidity. For the kinetic study of N2O decomposition, reaction activities were tested for alumina-supported metal
catalysts by Doi et al. [18]. Also, it has been reported that the N2O decomposition over mixed metal oxide catalysts followed the first order reaction law in obtaining the kinetic data [12]. The catalytic reaction mechanism of N2O decomposition seems straightforward, with a series of mass transfer, adsorption, decomposition and desorption processes. However, an analytical attempt to describe all these processes becomes messy and it seems hard to define the real characters. In this article, a reaction kinetic model for the calcined hydrotalcite-type catalyst is suggested with respect to the reaction rates of adsorption, instant cleavage of nitrous oxide on adsorption, desorption of oxygen and other related reactions. The model has been created through the analytical and experimental investigations of the catalytic fixed bed reactor under various conditions.
2. Experimental The catalyst precursor, hydrotalcite-type compounds, were prepared by the co-precipitation method following Armor et al. [19]; here the NaOH/Na2CO3 solution was added to aluminum and various transition metal nitrate solutions drop-wise for about 4 h while controlling pH under stirring for the precipitation reaction. The final pH was adjusted to 10. The precipitate slurry was stirred for about 30 min more and heated to 65–67 8C for 16 h. The resulting products were filtered out and washed with plenty of distilled water several times to remove excess sodium and nitrate, and then dried in an oven at 110 8C overnight. The compound thus prepared is single phase with hydrotalcite-type structure. The compounds were then calcined to activate as a mixed metal oxide catalysts under flowing dry air and slowly increasing temperature condition up to 500 8C, at which value the temperature was held constant for 4 h. The catalytic reaction tests were carried out in an allquartz glass fixed bed reactor under ambient pressure employing 0.1–0.5 g of mixed metal oxide catalysts made from hydrotalcite-type precursors. The catalytic decomposition was performed at an initial concentration of 5000– 10,000 ppm N2O in the temperature range from 250 to 500 8C. Before each decomposition run, the catalyst was conditioned with nitrogen flowing at the activation temperature of 500 8C for 3 h, which caused the regeneration of active surface and confirmed the reproducibility of the catalysts. The reactor was heated by a temperature controlled heating tape, and owing to the exothermic character of the decomposition reaction of N2O, the input and output temperatures were measured, though no appreciable thermal change was detected due to the low concentration of reacting component. The flow rate of the feed gas was modulated to produce GHSV from 15,000 to 60,000. The concentration analysis was performed using an on-line gas chromatograph
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225
Fig. 1. Schematic diagram of fixed bed catalytic experimental system for N2O decomposition.
with a pulse-discharged helium ionization detecter. The schematic diagram of reactor system is shown in Fig. 1.
3. Mechanistic kinetics modeling A catalytic decomposition takes place due to the contact of reactants with catalyst through an adsorption process. The reaction mechanisms can be described as three steps as mentioned by the authors [5,17]. ka
N2 O þ M ! N2 þ OM kd
2OM @ O2 þ 2M kdb
ke
N2 O þ OM ! N2 þ O2 þ M
(1) (2) (3)
where M indicates an active site of the catalyst and OM implies an adsorbed oxygen. Reaction (1) is a condensed expression of two steps of adsorption and decomposition reactions of nitrous oxide into nitrogen and adsorbed state of oxygen, which implies that the decomposition progresses as soon as the adsorption of nitrous oxide takes place; it can be ascribed to the weak adsorbability of nitrous oxide in comparison with oxygen radical. Reactions (2) and (3) may occur competitively unless the reverse reaction rate by high oxygen concentration is significant or one of them may occur predominantly over the other. The reaction rate for reactions (1) to (3) can be shown as: ra ¼ ka xN2 O Q0
(4)
rd ¼ kd ðQO Þ2 kdb xO2 ðQ0 Þ2 re ¼ ke xN2 O QO
(5) (6)
where Q0 and QO are the portions of the free active sites of the catalyst and the sites covered by oxygen respectively. If we let Qi be the portion of sites occupied by component i, the
sum of all portions should be 1. That is, X Qi ¼ 1 Q0 þ
(7)
Also we can obtain Eq. (8) from the stoichiometry of reaction rates based on the elemental oxygen adsorbed on the catalyst at steady state, from which the portion of free or adsorbed active sites can be derived as a function of x N2 O . ra ¼ 2rd þ re
(8)
The overall decomposition rate of nitrous oxide then can be expressed as the sum of two rates of reactions (4) and (6) as: r ¼ ra þ re ¼ ½ka Q0 þ ke ð1 Q0 Þ xN2 O
(9)
A kinetic study based on the reaction mechanism needs to consider all the internal and product behavior of the reactor along with the reactor type, feed concentration and other operational conditions. One may recollect here how the fixed bed reactor behaves and the presence of oxygen as shown in reaction (2) which may have a definite role in the overall reaction with the activity of reverse reaction against desirable reactive direction. For further study of the reaction mechanism in detail, we may consider two possible operational conditions separately: these are the cases when oxygen-free nitrogen and nitrogen with oxygen in excess are used as feed gases. 3.1. Oxygen effect Some studies on the reaction mechanism with respect to the adsorption and desorption of oxygen on the catalyst surface are of interest. Tanaka et al. [20] reported by observing the adsorption and recombinative desorption behavior of oxygen on rhodium impregnated zeolite that the Langmuir–Hinshelwood mechanism of reaction (2) was
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prevailing at lower temperature. On the other hand, the Eley–Rideal mechanism of reaction (3) was found to be prevailing on Fe-zeolite catalyst at higher temperature [21]. These results are quite contrasting to the suggestion of Dandl and Emig [17] for the mixed metal oxide catalyst, where the Eley–Rideal mechanism prevails at lower temperature and both Eley–Rideal and Langmuir–Hinshelwood mechanisms prevail at higher temperature. Tanaka et al. [20] also suggested that oxygen adsorption to rhodium is irreversible to some extent for the temperatures under 600 8C. In that case, it may take some time to reach to the full activity operation of the catalyst at steady state. However, with respect to the adsorption of oxygen on the mixed metal oxide catalyst which has the lattice oxygen at least one to twice the number of catalyst metals, the reaction progresses instantly on a reversible basis. Our experimental scope is within the reversible adsorption range of oxygen depending on the ambient concentration as can be shown in reaction (2), which can be ascertained through the preparation steps of the catalyst. When no oxygen is in the feed gas, the oxygen is produced only by the result of N2O decomposition. When it is produced as adsorbed oxygen, however, for the operational character of the fixed bed reactor, the oxygen will be desorbed by reaction (3) or recombined by reaction (2), and will be swept away by the effluent gas instantly, except for leaving a very small amount of adsorbed oxygen maintaining dynamic equilibrium with effluent oxygen concentration at steady state. Thus the oxygen can not affect the reverse reaction of reaction (2) at least in the once-through type fixed bed reactor, and the reverse reaction can be neglected. For the primitive access of adsorption and reaction of the process, the adsorption rate of nitrous oxide was assumed to be the same as in reaction (1); if we assume that the desorption reaction of OM to O2 takes place between neighboring sites, we may suggest the reaction mechanisms by replacing reaction (2) as follows: ka
N2 O þ M ! N2 þ OM kd
2OM ! O2 þ 2M ke
N2 O þ OM ! N2 þ O2 þ M
(1) (10) (3)
The reaction rate of each reaction can be expressed as follows: ra ¼ ka xN2 O Q0
(4)
rd ¼ kd ðQO Þ2
(11)
re ¼ ke xN2 O QO
(6)
Though the rates of these three reactions are different, the rates become balanced at steady state in the sense of Eq. (8). If we use x instead of xN2 O for convenience, the solution of
these two equations at steady state can be derived as: ½fðka þ ke Þ2 x2 þ 8ka kd xg1=2 ðka þ ke Þx
ð4kd Þ Q0 ¼ 1 QO
QO ¼
¼1
½fðka þ ke Þ2 x2 þ 8ka kd xg1=2 ðka þ ke Þx
ð4kd Þ
(12)
(13)
Then from Eq. (9), the overall reaction rate can be expressed as a complex function of xin the power series as: " # fððka þ ke Þ2 x2 þ 8ka kd xÞ1=2 ðka þ ke Þxg r ¼ ka x 1 ð4kd Þ þ ke x
½fka þ ke Þ2 x2 þ 8ka ke Þx
ð4kd Þ
(14)
Since the reaction rate of Eq. (14) is very hard to define, a modification should be necessary for the analysis of a reaction. We can go further to evaluate the relative magnitude of QO, the portion of sites covered by oxygen. Using the triangle inequality rule for Eq. (12), where the values of parameters and variables are all positive, one can express the magnitude of QO as: 1=2 ka x fðka þ ke Þ2 x2 þ 8ka kd xg1=2 QO (15) 2kd 4kd Then, for ka, kd and ke assumed to have almost the same order of magnitude and for a very small range of nitrous oxide concentration, QO may have a value far less than 1, so that it may not affect the overall reaction rate of nitrous oxide decomposition. This implies that the adsorption rate is mainly dependent on the first power of nitrous oxide concentration x. Then, when QO = 0 and Q0 = 1 are assumed approximately, the overall reaction rate constant for Eq. (9) can be represented as: k ¼ ka Q0 þ ke ð1 Q0 Þ
(16)
The involvement of reaction (3) in the overall decomposition of nitrous oxide is of interest. This reaction is typically called Eley–Rideal surface reaction, in which nitrous oxide decomposes on collision with adsorbed oxygen without any adsorption procedure. It was not possible to ascertain whether the mechanism is actually involved in this metal oxide catalytic reaction from other reports [5,17]. From a practical point of view, for a low concentration of nitrous oxide and the value of Q0 near 1, much involvement of this reaction is not expected from Eq. (16). However, actual involvement of this reaction in the mechanism may be estimated with respect to the effect of ke and the factor (1 Q0), which will be discussed below after the analysis of the case when oxygen is present in the feed gas.When oxygen is present constantly in excess compared with the concentration of nitrous oxide, the product oxygen on the catalyst from reaction (1) will be far below the equilibrium under excess oxygen environment, and for the supplement of
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equilibrium deficit the reverse reaction of reaction (2) will be dominant rather than the forward reaction. Then reaction (3) may contribute for the decomposition of nitrous oxide. Recall that the concentration of nitrous oxide is generally very low in comparison with oxygen in the air or in the industrial flue gases. The major reactions would then be reactions (1) and (3) as: ka
N2 O þ M ! N2 þ OM ke
N2 O þ OM ! N2 þ O2 þ M
(1) (3)
Then, introducing Q0 = 1 QO into the overall reaction rate model of Eq. (9), the reaction rate constant can be expressed as Eq. (17) with respect to the portion of oxygen-occupied sites. k ¼ ka þ ðke ka ÞQO
(17)
When the oxygen concentration is very high, most of the free active sites will be covered by oxygen and the portion of the oxygen-adsorbed sites, QO will have the value near 1. Hence the overall reaction rate will follow the first order rate law. Especially, when ke is greater than ka, then the overall decomposition reaction will proceed faster than the oxygen-free feed condition. Meanwhile, when ke is smaller than ka, the overall decomposition rate can be diminished. Moreover, if ke = 0, that is, there is no Eley–Riedal reaction involved, then the overall decomposition reaction will be seriously injured, wherein oxygen can be deemed as mere an impurity in excess.
227
The output concentration of nitrous oxide through the fixed bed reactor with space time ts can be expressed as follows: C ¼ C0 expðkts Þ
(21)
This result coincides with the solution result of fixed bed with bed depth of z, which means that the product concentration and the reaction rate with fixed bed reactor are dependent only on the space time of the reaction and not on the bed depth. Therefore, the kinetic data obtained from the fixed bed catalytic reactor can be relied upon without considering the varying boundary conditions. Thus the fixed bed reactor can be said very useful in kinetic study of catalytic reaction. The decomposition rate constant can be obtained by measuring the product concentration and space time. The temperature dependence of the rate constant should be examined by the Arrhenius equation, which gives the information that the rate constants are consistent with the reaction rate model. E k ¼ k0 exp (22) RT The kinetic data of activation energy and the Arrhenius constants were obtained through the polynomial regression of the overall decomposition rate constants.
3.2. Fixed bed catalytic reaction
4. Results and discussion
Consider the fixed bed catalytic reactor with first order decomposition reaction where the eddy dispersion effect is negligible. Then, the concentration change and distribution of the reacting component is described as a partial differential equation,
The conversion efficiencies of N2O in a fixed bed reactor for different metal compositions of Co–Al hydrotalcite catalysts with the temperature change are compared in Fig. 2. The conversion efficiencies of Co–Al catalysts with the composition ratio are different but not peculiar in comparison with other catalysts illustrated in the next figures. The conversion results for other catalysts with
@C @C þ uz þ kC ¼ 0 @t @z
(18)
Here, C is the concentration of reacting component in fluid flowing at an interstitial velocity uz along axial direction, z. The transient solution of partial differential Eq. (18) for various types of nonlinear reactions is not easy to obtain. However, when the decomposition reaction is carried out in the fixed bed reactor, the reaction reaches steady state sooner or later, where the adsorption rate becomes equivalent to the decomposition rate and the product concentration does not change as time. The steady state equation can be obtained from Eq. (18) with the time dependent term deleted and the solution can be written as Eq. (19). kz C ¼ C0 exp (19) uz Here, the value z/uz can be considered as space time, and denoted by ts = z/uz, and the space velocity (SV) becomes, SV ¼
uz 1 ¼ ts z
(20)
Fig. 2. Comparison of experimental results with simulation for N2O decomposition of various Co-Al hydrotalcite catalysts.
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Fig. 3. Comparison of experimental results with simulation for N2O decomposition of various Co–La–Al and Co–Rh–Al hydrotalcite catalysts.
Fig. 4. Comparison of experimental results with simulation for N2O decomposition of Co–La–Al(4/1/1) hydrotalcite catalysts with the change of space velocity.
different metal compositions of Co–La–Al and Co–Rh–Al are compared in Fig. 3. Some catalysts containing Rh and La are showing high efficiencies even at temperatures below 350 8C. When the feed concentration of nitrous oxide was varied between 1500 and 5000 ppm, the conversion efficiency was not significantly influenced, as can be expected from the first order reaction model. The simulation results applied by the reaction rate model are also shown in the drawings; these simulation results show a good agreement with the experimental results. When the first order rate equation is applied, the period of transition state is very short and thus whole reaction may be considered to take place in a steady state. The overall reaction rate also can be considered to be mainly dependent on the adsorption rate for very low concentration range of nitrous oxide, which might have been caused by the low portion of the sites covered by adsorbed oxygen. When the space velocity was decreased, the conversion ratio rose. The change of space velocity for Co–La–Al catalyst is also shown in Fig. 4; more than 90% conversion with space velocity of 15,000 h1 can be achieved at 300 8C. The figure also shows that, if operated at 350 8C, more than 90% conversion can be achieved even at a space velocity of 30,000 h1. The simulation of the reaction rates shows very good approximation in agreement with the experimental results. With these results, we can deduce with the information on the small portion of the active sites covered by oxygen and the overall decomposition rate constant of Eq. (16) that the effect of Eley–Rideal reaction was not influencing much in nitrous oxide decomposition under nitrogen environment, for which the relatively small value of ke for the oxygen present case can be referred to for comparison. Thus, the desorption rate of oxygen cannot be a rate-determining step of the decomposition of N2O. From these circumstances the neglect of terms containing x because of their comparatively low magnitude can be justified from
these results. It would be more comprehensible to consider that the concentration of N2O is much smaller than the input concentration through most of the reactor. The effect of oxygen on the decomposition rate over the fixed bed catalysts was investigated with 2–21% oxygen content in the feed gas. Figs. 5 and 6 show the conversion results of the Co-Al hydrotalcite catalyst reactions in the presence of oxygen. The conversion rate drops with oxygen content. The portion of active sites covered by oxygen is thought to be almost full and can be thought to be constant throughout the bed for the feed of oxygen higher than 10%. Then the overall decomposition rate constant of Eq. (17) can be varied with the comparative magnitude of Eley–Rideal reaction rate as the difference of two rate constants, (ke ka). If Eley–Rideal reaction is fast enough, as ke > ka, then the overall decomposition proceeds faster. On the other hand, if Eley–Rideal reaction is not involved, as ke = 0,
Fig. 5. Comparison of experimental results with simulation for N2O decomposition of Co–Al(1/1) hydrotalcite catalysts with the concentration change of oxygen in the feed gas.
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229
Fig. 6. Comparison of experimental results with simulation for N2O decomposition of Co–Al(2/1) hydrotalcite catalysts with the concentration change of oxygen in the feed gas.
Fig. 8. Comparison of experimental results with simulation for N2O decomposition of Co–Rh–Al(1/0.2/1) hydrotalcite catalysts with the concentration change of oxygen in the feed gas.
oxygen will behave absolutely as an impurity that can diminish the overall reaction rate significantly. However, the Eley–Rideal reaction seemed not active enough to effectuate the gain in oxygen adsorbed sites, at least in this range of reaction temperatures, and consequently the overall decomposition rate was found to be reduced in the experiments. From this fact, it can also be deduced that the Eley– Rideal reaction prevails, but the rate constant, ke is somewhat less than the rate constant of reaction (1), ka, which means the Eley–Rideal reaction is rate-controlling. It is quite comprehensible that the relevant reaction mechanisms can be inferred from the rate constant of the overall reaction rate model with the support of some experimental results. Similar interpretations could have been possible for the experimental results of Co–La–Al and Co–Rh–Al hydrotalcite catalysts shown in Figs. 7 and 8. Their catalytic efficiencies for the whole range of temperature were excellent. If economic effect is put aside, the Co–Rh–Al catalyst
shows slightly higher efficiency. However, the sensitivity of the Co–La–Al hydrotalcite catalyst shows higher stability for oxygen content than Co–Rh–Al catalyst, from which Co–La–Al catalyst can be considered to have a merit if the oxygen content can be kept low. The estimation of rate constant and activation energy of the catalysts for the decomposition reaction can be obtained using an Arrhenius plot; such results are listed in Table 1. The activation energies of the catalysis reactions in the presence of oxygen in the feed gas show a tendency of slight increase with the oxygen content; this might have been caused by the adsorbability bias of oxygen with temperature change. As in most decomposition reactions, the higher reaction rate and the high conversion ratio at higher temperature can be interpreted as the complex results of Arrhenius effect and low adsorbability of oxygen at higher temperature. A question may be raised if the forward reaction of reaction (2) may occur at higher temperature range where oxygen adsorption equilibrium is below the product oxygen of reaction (1). In that case, the reaction should go with reaction (3) competitively, which means production of a positive effect on reaction (1) and a negative effect on reaction (3) with the decreased value of QO. Therefore, the decrease in adsorption equilibrium of reaction (2) and the increase in rate constants may cause the overall decomposition rate to increase further at higher temperature. The experimental kinetic data of Table 1 shows that the activation energy is slightly increasing with oxygen contents for the experimental temperature range, which implies that the rate constant is less sensitively dependent on the temperature change. Meanwhile, some of the Arrhenius constants are increasing with oxygen content, which means higher rate constants at higher temperatures. However, it is not yet possible to tell whether the adsorption equilibrium of product oxygen went over the oxygen feed condition in the experimental range.
Fig. 7. Comparison of experimental results with simulation for N2O decomposition of Co–La–Al(4/1/1) hydrotalcite catalysts with the concentration change of oxygen in the feed gas.
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Table 1 Parameters and kinetic data obtained from the experimental results and simulation of nitrous oxide decomposition over the fixed bed catalysts under various conditions Catalysts
Oxygen contents (%)
E (KJ/mole)
k0 (104/s)
Co–Al(1/1) Htlc
0 10 21 0 10 21 0 0 2 10 21 0 10 21 0
43.00 45.00 45.30 69.00 79.00 82.00 61.20 48.99 51.67 59.00 59.10 66.34 67.70 71.00 69.00
3.00 3.53 3.16 152.00 410.00 560.00 25.38 42.14 37.75 79.92 71.59 983.36 863.48 750.67 151.55
Co–Al(2/1) Htlc
Co–Al(3/1) Htlc Co–Rh–Al(1/0.2/1) Htlc
Co–La–Al(4/1/1) Htlc
Co–La–Al(6/1/1) Htlc
Dandl and Emig [17], using a simulation model, reported the calculation results of the free active sites with the effect of water and oxygen; here high levels of N2O and water concentration in the feed gas were used. The concentration level of 15% N2O which can occur in adipic or nitric acid manufacturing industries is beyond our scope, wherein the portion of free active sites were greatly reduced with the impurity level and increased with the reaction temperature. Thus, the desorption rate of oxygen is sufficiently faster than the adsorption rate and the adsorption rate can be a ratedetermining step for the whole decomposition. Therefore the reduction capability of the catalytic reactor depends on how large the portion of active sites is that can be kept free so that the adsorption may progress faster.
free for further adsorption of N2O. Thus, the overall reaction rate is determined by the product of the portion of free active sites and the concentration of N2O, which leads approximately to the rate equation of the first order of the concentration of N2O. The low coverage of active sites by surface oxygen did not allow the situation of the Eley–Rideal reaction either. The presence of high concentration of oxygen in the feed gas provides the dominant coverage of active sites, which diminishes the portion of free active sites, and the overall reaction becomes reduced. Meanwhile the Eley– Rideal reaction, though it prevails in the desorption of oxygen, was proven to be not active enough to promote the overall decomposition reaction in the temperature range between 250 and 500 8C. Thus, the presence of oxygen did not help to the decomposition reaction, which is different from the cases of selective catalytic reduction processes where oxygen may help the cleavage of organic reducing agents to produce nitrogen molecules from nitrogen oxides [22]. In the range of concentration under 10,000 ppm N2O, the first order reaction rate model can be taken for a catalytic decomposition of N2O without any problems for industrial and engineering purposes. If any impurities are present in the feed gas, they can be treated as chemicals adsorbing to the active sites of the catalyst. They may share the free active sites of the catalyst and gradually diminish the overall decomposition rate. Most of all, the mixed metal oxide catalysts made from various hydrotalcite-type precursors have been proved to be very efficient catalysts for effective removal of nitrous oxide, which is known as one of the major greenhouse gases. The experimental data and the reaction rate model suggested in this paper will be very useful when a scale-up for industrial engineering is needed.
5. Conclusions The decomposition experiments of nitrous oxide and the mechanistic analysis were performed over fixed bed catalyzer made from hydrotalcite-type precursors. For the concentration range between 5000 and 10,000 ppm of N2O with and without the presence of oxygen in the feed gas, we could draw the following results. The fixed bed reaction system is very useful in the study of catalytic reduction mechanism and reaction rate of nitrous oxide decomposition. The results are very uniform throughout the operation and are not affected by the concentration change of reactants with time and position, which often can be met as a problem in batch processes. The overall reaction rate is mainly dependent on the adsorption rate of N2O which is instantly cleaved into nitrogen and oxygen on contact with the catalyst, while the oxygen remains adsorbed on the catalyst. The rates of molecular oxygen formation and desorption of the adsorbed oxygen are fast enough to leave the active sites
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