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Ind. Eng. Chem. Res. 2003, 42, 2756-2766
APPLIED CHEMISTRY Integration of in Situ FTIR Studies and Catalyst Activity Measurements in Reaction Kinetic Analysis Juha Ahola,* Mika Huuhtanen, and Riitta L. Keiski Department of Process and Environmental Engineering, University of Oulu, P.O. Box 4300, FIN-90014 Oulu, Finland
This paper describes a method to utilize DRIFT measurements with catalyst activity data in reaction kinetic analysis. The method is applied to construction and discrimination of reaction rate models for NO reduction with CO in the presence of oxygen over a Pd-containing threeway catalyst. DRIFT measurements have a special advantage to discriminate reaction rate models by comparing semiquantitative measurement of surface complexes to simulated surface coverages. Reaction kinetic models based on three different mechanisms (direct decomposition, bimolecular reaction, and N2O formation via isocyanate complex formation) were compared. Only minor differences between the models exist in the fitting of the models to measured gas-phase concentrations, whereas predicted surface coverages differ significantly from each other. The direct decomposition reaction model can most correctly predict the semiquantitative DRIFT measurements. Moreover, the explanation of the slow transient effects, detected on light-off experiments, was found by DRIFT measurements. Introduction One of the most relevant features needed in reactor analysis and design is the correct kinetic information on reactions occurring in the operation conditions. Especially in environmental catalysis, variation of operation conditions is wide. Thus, the reaction kinetic model should be valid in a broad temperature and concentration range. It has been believed1 that the kinetic models based on the mechanism have a higher ability to predict reaction rates than empirical models especially in environmental catalysis. The same steady-state kinetic model can be derived from several hypotheses. Several mechanism hypotheses with different simplifications and assumptions result in huge amounts of possible kinetic models that should be evaluated. Moreover, more than one model can be fitted equally even in a large number of experimental data. A special feature of a mechanism-based model is that it predicts also the surface complexes that are not detectable in traditional kinetic measurements. However, there are some methods by which it is possible to detect surface complexes and to evaluate them semiquantitatively. It is possible to use this information in the construction of mechanism hypotheses and kinetic equation derivations as well as in model validation. In our research, the integration of DRIFT measurements and gas-phase analysis at all stages of kinetic analysis is applied. Moreover, methodology for kinetic model construction is discussed. The reaction system studied in this research was the reaction between CO and NO in the presence of oxygen at a supported palladium catalyst. Many research groups have investigated NO + CO reactions by using the DRIFT technique. Also numerous catalyst supports and precious metals (e.g., Pt, Rh, Pd,
Ru) have been studied.2-8 Still there are a lot of uncertainties in the reaction mechanisms and routes. Almusaiteer et al.9 measured simultaneously DRIFT spectra on the Pd/Al2O3 surface and gas-phase concentrations on the reactor outlet in pulse experiments. They detected correlations between conversions and surface complexes. However, surface science measurements and traditional activity measurements have seldom been combined in the kinetic model construction, and procedures to do that have not yet matured. Reaction Mechanism and Kinetics The first step in the construction of a reaction kinetic model is the building of a mechanism hypothesis. An overall reaction describes the conversion of raw materials to products. In the same way a mechanistic step describes the conversion of reactants to products. It is not possible to acquire phenomenon information directly but via material which it affects. Thus, the first goal is to find components taking part in the reaction. This can be detected from the concentration changes of the reacting component, when reaction conditions are changed, particularly as a function of the extent of a reaction. When the components are surface complexes, it is not self-evident which materials are taking part in the reaction and which ones are only adsorbed at the surface. In that case it may be reasonable to apply the described deduction. Moreover, materials, the concentrations of which change in the same manner, may take part in the same mechanistic step. This provides an additional guidance to construct the frame of the reaction mechanism, which should be completed to an overall catalytic cycle in a more intuitive way. Thus, a composite reaction has been disaggregated to its topological nodes, from which some are eminently credible
10.1021/ie020113m CCC: $25.00 © 2003 American Chemical Society Published on Web 05/15/2003
Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003 2757
and the others more speculative. Microscopic information helps to reduce speculative nodes. In the next stage, the reaction rate relation of the composite reaction is derived. Typically, some steps will be assumed fast, and in steady-state kinetics, this leads to quasi-equilibrium. Additional simplifications neglect the effect of concentration and other terms, the effect of which has been evaluated to be insignificant. In some cases, these asymptotical assumptions lead to curious behavior of the model10 and more often they weaken or even destroy the mechanism bases of the model. When direct information from sources other than the total behavior of a reactor is available, it is possible to reduce the number of assumptions. Especially, microscopic information can also help in this aggregation stage. NO Reduction by CO. Reaction between NO and CO is widely studied especially on a Rh catalyst, but there is still confusion on the reaction mechanism and the dominant reaction steps may differ by reaction conditions. Two main types of hypothetical mechanisms are bimolecular11,12 and regenerative reaction mechanisms.13,14 In the bimolecular mechanism, nitrogen- and carbon-containing materials react directly with each other. The regenerative mechanism refers to reaction types in which CO reacts with atomic oxygen that is adsorbed on the surface during NO decomposition. Dissociation of NO has been proposed in many hypothetical mechanisms. Almusaiteer et al.9 also made this proposition on a Pd catalyst, but later they made more guarded deductions on a Rh catalyst.15 They discussed NO dissociation, but in the formal mechanism, only the decomposition of NO is proposed in which gaseous nitrogen and an oxidized surface are formed. Another questionable block is the role of N2O and isocyanate surface complexes during the reaction. Recently, Kondarides et al.16 suggested that isocyanates are responsible for the production of N2O and CO on a Rh catalyst. The formation of nitrate species on alumina has been observed by DRIFT to be significantly greater when the reaction gas mixture contains oxygen rather than when it does not.3 Possibly NO dissociates to adsorbed N and O species, which can react to nitrate species. CO has been reported to adsorb on the palladium surface linearly and in bridged and triply bonded forms as well as on the oxidized site. An overview of adsorption bands and CO surface complexes related to them is given, for example, by Keiski et al.17 The potential materials at gas phase on NO reduction by CO are NO, CO, O2, CO2, N2, N2O, and NO2, from which the formation of NO2 was not detected. Thus, the composite reactions are
2NO + 2CO f 2CO2 + N2
(1)
2NO + CO f CO2 + N2O
(2)
2CO + O2 f 2CO2
(3)
The potential materials at the surface are NO*, CO*, O*, N*, N2O*, NCO*, and (NO*)2. Moreover, CO and NO can adsorb on different active sites and via different bond types, with the materials being composites of these complexes. When material, e.g., adsorbed CO, is seen as a composite, it can be disaggregated further to materials “triply bonded CO”, “bridge-bonded CO”, and “linearly adsorbed CO”. In most cases in chemical reaction engineering applications, composite material “adsorbed CO” is convenient without further detailing.
Thus, linear, bridge-bonded, and triply bonded complexes compose one CO* complex in typical reaction kinetic studies in chemical engineering. In the same way, nitric surface intermediates result in composites such as NO(ad), N-N-O(ad), or N(ad). Surface isocyanate NCO(ad) can be taken into account as well. Evidence of existing materials can be acquired by properties of comparable molecules, which are a result of the state and structure of the molecule. A great part of measurement methods is based on the ability of a molecule to transmit or absorb electromagnetic radiation. In the case of heterogeneous catalysis, the vibrations of the molecules can be detected by DRIFT spectroscopic analysis, and in cases where IR spectroscopy is not applicable, other spectroscopic methods, such as electron spectroscopy for chemical analysis (ESCA), could be applied. In NO reductions, all species at the surface except (atomic) oxygen and nitrogen can be, in principle, detected by DRIFT and oxygen and nitrogen can be detected by ESCA18 at least during a limited part of operation conditions. Elementary reactions, which are self-evident, are
NO + * h NO* CO + * h CO* O2 + 2* h 2O*
(4) (5) (6)
CO* + O* f CO2 + 2*
(7)
If material N* exists, the following reactions are possible:
NO* + * f N* + O* NO* + CO* f CO2 + N* + *
(8) (9)
NO* + N* f N2O + 2*
(10)
2N* f N2 + 2*
(11)
If isocyanate NCO* exists, reaction steps
NO* + CO* f NCO* + O* NCO* + NO* f N2 + CO2 + 2*
(12) (13)
NCO* + NO* f N2O + CO* +*
(14)
are possible, whereas the existence of N2O* results in steps like
2NO* f N2O* + O*
(15)
N2O* h N2O + *
(16)
N2O* f N2 + O
(17)
Finally, coupled nitric complex (NO*)2 leads to
2NO* f (NO*)2
(18)
(NO*)2 f N2O + O* + *
(19)
(NO*)2 f N2 + 2O*
(20)
More hypothetical reaction steps can be imagined if more than one speculative material exists and reacts at the surface. If just NO* and O* exist at the surface in addition to CO*, surface reactions
2NO* f N2O + O* + *
(21)
2NO* f N2 + 2O*
(22)
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Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003
Figure 1. DRIFT spectra on a rich NO + CO + O2 mixture at 290 °C with deconvolution.
are possible. Derivation of a kinetic equation for the steady state and complexity of the equation has to be taken into account so that aggregation of state is not gratuitously complicated and time-consuming compared to the purpose of the model. Experimental Section The catalyst used in the experiments was a supported Pd catalyst on a thin (50 µm) metal foil. The support (40 g/m2) contained Al2O3, La2O3, and CexZr1-xO2. The noncommercial catalyst was prepared by Kemira Metalkat Oy. Experiments were carried out in reducing and oxidizing CO- and NO-containing atmospheres with or without oxygen. The λ value (redox ratio) was 0.9 or 1.1 in all mixtures. The inlet gas flow contained 4000 ppm CO and 3600 ppm NO in reducing atmospheres and 4400 ppm NO in oxidizing atmospheres. In some experiments, a part of NO was replaced with oxygen in such a way that the inlet gas stream contained 2000 ppm NO and 800 ppm O2 in a reducing atmosphere and 1200 ppm O2 in an oxidizing atmosphere. In this study the DRIFT measurements were carried out in a continuous-flow reaction chamber (environmental chamber, Specac 19930). The DRIFT chamber operated as a continuous stirred tank reactor, in which a small flat metallic foil coated with a catalyst material was placed. The vibrations of surface components were detected in the range from 4000 to 600 cm-1. The resolution of the measurements was 4 cm-1, and each spectrum contained 20 scans. The measurements were done at several temperatures from room temperature to 380 °C. The catalyst samples were pretreated with oxygen for 20 min at 400 °C and cooled to room temperature under a small oxygen flow to prevent selfreduction. The catalyst activity was measured in a tube reactor constructed from a metallic monolith with a volume of 1.4 cm3. The kinetic experiments were carried out with a constant heating rate of 20 °C/min from ambient temperature to 400 °C. The gas flow rate was 1000 cm3/ min at NTP, giving the time constant of the gas phase of the reactor as 60 ms. The inlet concentrations were
selected to be the same in the DRIFT measurements and in the activity measurements. The gas flow rate per catalyst weight was approximately the same in both experimental methods. Peak overlapping in DRIFT spectra was deconvoluted into Gaussian bands by a quasi-Newton algorithm. Baseline correction was made by proposing a piecewise linear baseline which follows through absorbance at a wavenumber of 2464 cm-1 and a minimum absorbance value between 2264 and 1964 cm-1 as well as 1674 and 1189 cm-1. At least the first point is at the real baseline, the second one is near the real baseline but the band at 1674 cm-1 might be slightly disturbed by a nitric band at 1750 cm-1, and the last band is the most speculative. The treatment was evaluated to be a reasonable way to take into account the effect of temperature on the baseline. However, a higher-order polynomial or exponential function could be applied as a baseline, but this is not reasonable based on the temperature dependence of the spectrum measured on the adsorbate-free surface. Examples of deconvoluted spectra are shown in Figure 1. Results and Discussion Qualitative Analysis. The light-off temperature of the catalyst, referred as 50% conversion, was approximately 200 °C. Nitrous oxide formation was observed at around the light-off temperature. A fractional yield of N2O was, in the worst case, as high as 50%. In lean conditions, fractional NO conversion achieved unity at 300 °C, but at higher temperatures, conversion decreased at the same time as when selectivity to nitrogen increased. On the other hand, in rich conditions, maximum conversion was achieved and just excess CO stayed in the outlet gas. At rich conditions, when oxygen was present, the CO outlet concentration went through a minimum in the light-off measurement, as shown at Figure 2c. This behavior was observed in repeated trials. In the experiments at constant temperature, the CO outlet concentration decreased monotonically to the final value.
Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003 2759
Figure 2. Measured CO (/), NO (×), and N2O (3) concentrations in light-off experiments with regression (solid line) on a direct decomposition model for a (a) rich CO + NO mixture, (b) lean CO + NO mixture, (c) rich CO + NO + O2 mixture and (d) lean CO + NO + O2 mixture. In addition, CO concentrations in stationary experiments are marked with O in part c.
From the experiments done with a NO + CO mixture, the formation of isocyanate adsorbed on alumina at 2250-2230 cm-1 was observed.2,3,5 The formation of isocyanate species began at temperatures above 260 °C. In lean conditions with oxygen, the isocyanate band had a maximum at around 320 °C. At temperatures above 320 °C, bands due to isocyanate species started to attenuate and also the adsorption band around 1550 cm-1 caused by adsorbed carbon (CO32-) and nitrogen (NOx-) species on an alumina support decreased.5 The spectra of the DRIFT experiment are shown in Figure 3. In other experiments, the height of the isocyanate band remained high at the measured temperature range. The broad band at 1300-1250 cm-1 can be identified as nitrates and nitrites on an alumina support.3,6 The band almost vanished above 320 °C. The band at 2152 cm-1 at temperatures from 25 to 260 °C is due to adsorbed CO on palladium.6,7 At higher temperatures, the band at 2190 cm-1 can possibly be specified as a vibration caused by adsorbed CN species on an alumina support.19,20 The CtN vibrations can also be detected5 at 1620 cm-1. In the literature,8,19 the bands at 1553 and 1460 cm-1 are assigned as Al-NO3-, and we can agree with this. The broad band at around 1750 cm-1 can be assigned to NO on palladium.5,6,8,17,21,22 The impact of oxygen (O2) was significant. When oxygen was present, the formation of isocyanate was very strong and started at 260 °C. In conditions when oxygen was absent, the formation did not start before the temperature had reached 290 °C and the band was not so strong either. The strongest isocyanate formation can be detected with the rich reaction gas mixture NO + CO + O2. With a lean NO + CO + O2 reaction gas mixture, the isocyanate formation reached a maximum
at around 320 °C. When the temperature was raised above 320 °C, the isocyanate band at 2250 cm-1 diminished very quickly. The same effect can also be detected in the band at 2190 cm-1. The formation of nitrates and nitrites was much greater when oxygen gas was present. Evidently,3 oxygen reacts more effectively with NO and nitrates formed in a NO + CO + O2 gas mixture than in the reaction between NO and CO. A rapid decreasing effect can also be detected as a strong triplet band (13101260 cm-1) due to adsorbed nitrates and nitrites on alumina at temperatures above 290 °C. Kinetic Models. The DRIFT measurements gave information on the possible intermediates for reaction steps, but they did not give complete structural guidance. Thus, three different mechanisms were compared: direct decomposition, bimolecular reaction, and N2O formation via isocyanate complex formation. The first one is an example of a regenerative mechanism, in which CO reacts with oxygen adsorbed on the surface during NO decomposition, the second one is an example of a coupled mechanism, in which NO decomposition is assisted with CO, and the last one is a combination of the two previous types. The mechanisms are representatives of the typically proposed mechanism types over platinum group metal catalysts. However, several other variants of the hypotheses are possible to imagine. The proposed mechanism on direct decomposition includes reaction steps (4)-(7), (21), and (22). Reaction steps (4) and (5) were assumed to be in quasi-equilibrium, and (6) was assumed to be irreversible. The frequently used23-25 assumption was argued on the low desorption rate of oxygen at temperatures below 400 °C on the catalyst. The steady-state kinetic model was
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Figure 3. Adsorption of a lean NO + CO + O2 reaction gas mixture over Pd catalyst as a function of temperature.
derived as follows:
r6 ) k6cO2/D2 r21 ) k21KNO2cNO2/D2 ) ka21cNO/D2 r22 ) k22KNO2cNO2/D2 ) ka22cNO2/D2 D ) 1 + KNOcNO + KCOcCO +
(23)
2k6 (c /c ) + (k21 + k7KCO O2 CO
2k22)(KNO2/k7KCO)(cNO2/cCO)
Thus, the formation rates of gas-phase components are ∂cCO/∂t ) -r7 - r9, ∂cNO/∂t ) -r9 - r10, ∂cO2/∂t ) -(1/2)r7, ∂cN2O/∂t ) r10, and ∂cCO2/∂t ) r7 + r9. The proposed mechanism of N2O formation via isocyanate includes reaction steps (4)-(7), (12), (14), and (22). When (4) and (5) were assumed to be in quasiequilibrium and (6) was assumed to be irreversible, the steady-state kinetic model was derived as follows:
r6 ) k6cO2/D2
) 1 + KNOcNO +
KCOcCO + Ka1(cO2/cCO) + Ka2(cNO2/cCO) Thus, the formation rates of gas-phase components are ∂cCO/∂t ) -2r5 - r21 - 2r22, ∂cNO/∂t ) -2r21 - 2r22, ∂cO2/ ∂t ) -r5, ∂cN2O/∂t ) r21, and ∂cCO2/∂t ) 2r5 + r21 + 2r22. The proposed mechanism on the bimolecular reaction includes reaction steps (4)-(7) and (9)-(11). When (4)(6) were assumed to be in quasi-equilibrium, the steadystate kinetic model was derived as follows: 2
r9 ) k9KNOKCOcNOcCO/D ) kb9cNOcCO/D
2
r10 ) (k102KNO2cNO2/4k11)[-1 +
x1 + (8k11k9KCO/k102KNO)(cCO/cNO)]/D2 ) kb10[-1 +
x1 + Kb2(cCO/cNO)]/D2
r7 ) k7KO21/2KCOcCOcO21/2/D2 ) kb7cCOcO21/2/D2
(24)
D ) 1 + KNOcNO + KCOcCO + KO21/2cO21/2 + (k10KNOcNO/4k11)[-1 +
x1 + (8k11k9KCO/k102KNO)(cCO/cNO)] ) 1 + KNOcNO + KCOcCO + KO21/2cO21/2 + Kb1cNO[-1 +
x1 + Kb2(cCO/cNO)]
r12 ) r14 ) k12KNOKCOcNOcCO/D2 ) kc12cNOcCO/D (25) r22 ) k22KNO2cNO2/D2 ) kc22cNO2/D2 If cNO > 0,
D ) 1 + KNOcNO + KCOcCO +
k12KCO 2k6 cCO + k14 k7KCO
k12KNO 2k22KNO2 (cO2/cCO) + cNO + (c 2/c ) ) 1 + k7 k7KCO NO CO Kc1cCO + Kc2(cO2/cCO) + Kc3cNO + Kc4(cNO2/cCO) (26) and if cNO ) 0,
D ) 1 + KNOcNO + KCOcCO +
2k6 (c /c ) + k7KCO O2 CO
k12KNO cNO + (2k22KNO2/k7KCO)(cNO2/cCO) ) 1 + k7 KCOcCO + Kc2(cO2/cCO) (27) Thus, the formation rates of gas-phase components are ∂cCO/∂t ) -2r6 - r14 - 2r22, ∂cNO/∂t ) -2r14 - 2r22, ∂cO2/ ∂t ) -r6, ∂cN2O/∂t ) r14, and ∂cCO2/∂t ) 2r6 + r14 + 2r22.
Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003 2761 Table 1. Parameters for the Kinetic Model of Direct Decomposition parametera
value
standard error (%)
k6 Ea6 k21 k22 Ea22 Ka2 Ha2 KNO HNO KCO
2.30 m3/kg‚s 22.7 kJ/mol 14.1 m6/mol‚kg‚s 13.0 m6/mol‚kg‚s 46.9 kJ/mol 5.98 m3/mol -37.5 kJ/mol 45.0 m3/mol -9.31 kJ/mol 7.0 m3/mol
12.6 16.3 13.3 13.6 4.4 50.0 15.7 15.8 12.8 30.2
a
Reaction rate and equilibrium constants at 520 K.
Table 2. Parameters for the Kinetic Model of Bimolecular Reaction parametera
value
standard error (%)
kb9 Eab9 kb10 Eab10 kb7 Eab7 KNO KCO Kb1 Kb2 Hb2
380 m3/kg‚s 46.2 kJ/mol 0.260 mol/kg‚s 20.8 kJ/mol 254 m41/2/mol1/2‚kg‚s 31.2 kJ/mol 0.24 m3/mol 205 m3/mol 115 m3/mol 0.043 11.7 kJ/mol
10.3 2.8 18.4 10.0 10.4 5.5 19.7 5.6 22.8 32.0 17.5
a
Reaction rate and equilibrium constants at 520 K.
Table 3. Parameters for the Kinetic Model of Reaction with Isocyanate parametera
value
standard error (%)
k6 Ea6 kc12 Eac12 kc22 Eac22 Kc1 Hc1 Kc2 Kc3 Hc3
0.79 m3/kg‚s 21.0 kJ/mol 3.15 m6/mol‚kg‚s 1.75 kJ/mol 4.69 m6/mol‚kg‚s 49.7 kJ/mol 1.5 m3/mol -17.9 kJ/mol 1.0 × 10-4 22.0 m3/mol -16.6 kJ/mol
0.3
