J. Phys. Chem. B 2001, 105, 6895-6906
6895
A Kinetic Study of NO Oxidation and NOx Storage on Pt/Al2O3 and Pt/BaO/Al2O3 Louise Olsson,†,‡,§ Hans Persson,‡,§ Erik Fridell,‡,§ Magnus Skoglundh,‡,| and Bengt Andersson*,†,‡ Department of Chemical Reaction Engineering, Competence Centre for Catalysis, Department of Applied Physics, and Department of Applied Surface Chemistry, Chalmers UniVersity of Technology, SE-412 96 Go¨ teborg, Sweden ReceiVed: January 25, 2001; In Final Form: May 1, 2001
Modeling and flow reactor experiments were used to study the kinetics of NOx storage/release on a Pt/BaO/ Al2O3 model catalyst. The mechanism for this concept can be divided into four steps: (i) NO to NO2 oxidation on Pt, (ii) NO2 storage on BaO, (iii) NOx release, and (iv) NOx reduction to N2. In this paper, we have focused on the first three steps. From the NO oxidation study on Pt/Al2O3 compared to Pt/BaO/Al2O3, we observed that the presence of BaO decreases the formation of NO2. To test the importance of this step for effective storage, experiments were performed with a Pt/Al2O3 catalyst placed before the Pt/BaO/Al2O3 catalyst. This resulted in increased NOx storage for the combined system compared to the Pt/BaO/Al2O3 case. To resolve the second and third steps, an experimental investigation of NOx storage/release on BaO/Al2O3 was performed using only NO2 and N2 in the gas feed. We propose a kinetic model, which first includes adsorption of NO2, which oxidizes the surface, followed by nitrate formation. Finally, NO3-BaO-NO2, i.e., Ba(NO3)2, is formed. By using the kinetic parameters from the NO oxidation on Pt/BaO/Al2O3 and the NOx storage on BaO/Al2O3, a kinetic model was constructed to describe NOx storage/release experiments on Pt/BaO/Al2O3. However, the rate for NOx release was increased when Pt was present, and the kinetic model could not accurately describe this phenomenon. Therefore, the mechanism was modified by including a reversible surface spillover step of NO2 between Pt sites and BaO sites. Further, experiments with NO2 exposure followed by a temperature ramp with NO/N2 showed that the desorption behaviors from the BaO/Al2O3 and Pt/BaO/Al2O3 were significantly different, which further supports the spillover mechanism. Finally, the models describing NOx storage on BaO/Al2O3 and on Pt/BaO/Al2O3 were successfully validated with independent experiments.
1. Introduction The demand for a lower fuel consumption for gasoline engines, driven by the need to decrease the emissions of greenhouse gases, has led to the introduction of lean-burn engines where the engine is operated at oxygen excess. This concept yields a more complete combustion and reduces the fuel consumption compared to stoichiometric combustion and thus reduces the amounts of CO2, carbon monoxide (CO), and hydrocarbons (HC) emitted during driving. However, with a lean exhaust the conventional three-way catalyst is not able to efficiently reduce nitrogen oxides. One solution to this problem is the so-called NOx storage concept, where a storage function in the catalytic converter is combined with mixed lean operation of the engine. During lean periods NOx is trapped by a storage material, e.g., BaO, and during short rich intervals the catalyst is regenerated, where NOx is released and reduced with HC and CO to form N2, H2O, and CO2. Despite the fact that this concept has been known for some years, there is a lack of detailed knowledge about how the system works mechanistically. For improving temperature stability, NOx storage ability, regenera* To whom correspondence should be addressed. E-mail: bengt@ cre.chalmers.se. † Department of Chemical Reaction Engineering. Fax: +46(0)31-772 3035. ‡ Competence Centre for Catalysis. Fax: +46(0)31-772 2967. § Department of Applied Physics. Fax: +46(0)31-772 3134. | Department of Applied Surface Chemistry. Fax: +46(0)31-16 00 62.
tion times, and hindering of sulfur deactivation, detailed knowledge about the storage and release mechanisms is important. To summarize earlier work on this subject, there are several papers presenting experimental results for NOx storage,1-11 and a few papers have focused on the storage mechanism.5,6,8-10 Following the results from these studies, the mechanism can be divided into four steps: (i) NO oxidation to NO2 on Pt, (ii) NOx storage on BaO, (iii) NOx release, and (iv) NOx reduction to N2. Specifically, Mahzoul et al.8 propose a mechanism where two types of Pt sites, close to barium and far from barium, are responsible for nitrate formation and NO oxidation, respectively. Moreover, Fridell et al.6 suggest two possible surface mechanisms for storage; the first is that NO2 oxidizes BaO to BaO2. The second reaction pathway includes an initial formation of nitrite followed by an oxidation of the nitrite to nitrate with NO2. Other studies by Balcon et al.9 and Kobayashi et al.,10 which both use more complex gas mixtures including hydrocarbons and/or CO2, suggest that the competition between carbonate and nitrate formation is crucial for the storage/release mechanism. This paper, which is a continuation of refs 6 and 12, consists of a detailed mechanistic study of the NOx storage/release process, where we try to resolve and identify the different key steps by using flow reactor experiments and kinetic mean-field modeling. In particular, we have improved our NO oxidation model for a Pt/Al2O3 catalyst12 and investigated the oxidation capacity when adding barium oxide to the catalyst. Moreover,
10.1021/jp010324p CCC: $20.00 © 2001 American Chemical Society Published on Web 06/29/2001
6896 J. Phys. Chem. B, Vol. 105, No. 29, 2001 a kinetic model for NOx storage on BaO/Al2O3 is presented and validated. Finally, a NOx storage/release model for Pt/BaO/Al2O3 has been constructed and tested with separate experiments, not included in the fitting procedure. 2. Experimental Section 2.1. Catalyst Preparation. In this study four different catalysts were used: 7.7 wt % Pt/γ-Al2O3, 2.3 wt % Pt/γ-Al2O3, 2.0 wt % Pt/20 wt % BaO/γ-Al2O3, and 20 wt % BaO/γ-Al2O3. We denote them C1, C2, C3, and C4. The catalysts are monoliths with 69 channels (∼1 × 1 mm) and a length of 15 mm. The catalyst preparation procedure is described in detail elsewhere.4,12,13 The total washcoat weights are 294, 273, 204, and 206 mg for C1, C2, C3, and C4, respectively. The Pt dispersions are 2.5% for C1 and 3.3% for C2. The dispersions were obtained from CO temperature-programmed desorption (TPD) experiments, assuming a maximum coverage of 0.7 CO molecule per Pt site.14 For catalyst C3 the dispersion is not measured by chemisorption since it is difficult to separate CO on Pt from CO on BaO. Therefore, the dispersions of the corresponding powder catalysts were measured in a FTIR study of CO adsorbed on Pt, resulting in a Pt dispersion decrease of about 5 times when BaO was present. 2.2. Reactor Experiments. Two different flow reactors were used in this study. The first setup was used for the O2 and CO TPD experiments, and the other reactor was used for all other experiments. In both setups the pressure is 1 atm and the gases are controlled by mass flow controllers. The catalyst is placed in a quartz tube with one thermocouple placed inside the catalyst and one about 10 mm in front of the catalyst. In the first reactor, described by Lo¨o¨f et al.15 and Lundgren et al.,16 the gases are detected with a mass spectrometer (Balzer QMA 120). In the second setup, described in ref 13, the NO and NO2 concentrations are measured with a chemiluminescense detector (Tecan CLD 700 EL ht) and the N2O concentration with an IR instrument (Maihak, UNOR 610). The inert balance for the two setups was Ar and N2, respectively. The experiments with NOx were conducted with a total flow of 2600 mL/min (space velocity (SV) ) 87000 h-1). O2 and CO TPD. The oxygen TPD experiments were performed by exposing the catalyst (C1, 7.7 wt % Pt/γ-Al2O3) to 2 vol % O2 for about 5 min at different temperatures, 40, 200, 300, 350, 400, 450, 500, and 550 °C. The catalyst was then exposed to Ar for 1 min and cooled to room temperature in an Ar flow. Finally, the catalyst was heated from room temperature to ∼700 °C at a rate of 40 °C/min in an Ar flow of 50 mL/min (SV ) 1680 h-1 ). In a similar experiment the catalyst was cooled to room temperature under a continuous exposure of oxygen, followed by a temperature ramp in an inert gas flow. The CO TPD experiments were performed by oxidizing the catalysts in 2% O2 followed by reduction in 4% H2 in Ar at 600 °C to clean and reduce the catalyst surface. The catalyst was then cooled to 5 °C in 4% H2 and flushed with Ar prior to the CO exposure. The sample was exposed to CO for 10 min, and after 15 min in Ar, a temperature ramp from 5 to 600 °C (40 °C/min) was conducted. NO Oxidation on Pt/Al2O3 (C2). A temperature ramp was made with a heating rate of 5 °C/min from 25 to 500 °C in 600 ppm NO and 8% oxygen. Further, steady-state experiments were performed at different temperatures, 300, 350, 400, and 450 °C with 680 ppm NO2 in the flow. NO Oxidation on Pt/BaO/Al2O3 (C3). A temperature ramp, such as the one described above, was conducted with 600 ppm
Olsson et al. NO and 8% O2. The temperature was raised from room temperature to 500 °C with a rate of 5 °C/min. Also steadystate experiments were performed at 250, 275, 300, 325, 350, 400, 425, and 450 °C with 600 ppm NO and 8% O2 and at 350, 400, and 450 °C with 680 ppm NO2. NOx Storage on BaO/Al2O3 (C4) and on Pt/BaO/Al2O3 (C3). The catalyst was initially heated to 600 °C in N2 and then exposed to NO2 for 10 min at 300, 350, or 400 °C, followed by 5 min of flushing with N2. Finally, the temperature was increased to 600 °C, with a rate of 20 °C/min, with an inert flow over the catalyst. These experiments were performed for both catalysts C3 and C4. A similar experiment was also performed, with an exposure of 640 ppm N2O 15 min prior to the NO2 exposure and during the NO2 adsorption. This experiment was conducted at 400 °C for the BaO/Al2O3 sample. In another type of experiment conducted for both catalysts C3 and C4, the sample was exposed to NO2 for 10 min at 300, 350, or 400 °C, followed by 1 min with N2. Then the catalyst was exposed to 600 ppm NO/N2 for 4 min, and finally the sample was heated to 600 °C (with a rate of 20 °C/min) in 600 ppm NO in N2. Transient experiments were performed for the Pt/BaO/Al2O3 sample at 300, 350, and 400 °C, with 600 ppm NO and a sequence with 8%, 0%, 4%, 0%, 1%, and 0% oxygen. Each interval lasted 5 min, and the whole cycle was repeated three times. This experiment was also conducted at 300, 350, and 400 °C with the Pt/Al2O3 catalyst (C2) placed about 15 mm in front of the Pt/BaO/Al2O3 catalyst. 3. Kinetic Model The kinetic model is a mean-field model, and the monolith is described as a series of continuously stirred tank reactors. For the oxygen TPD experiments the flow was low (50 mL/ min), and one tank was used in the modeling of these results. However, for the other experiments the flow was relatively high (2600 mL/min), and the monolith could be viewed as a plug flow reactor, which was described using 15 tanks. The rate constant, ki, for reaction i is described with the Arrhenius expression
ki ) Aie-EA,i /RT
(1a)
where Ai is the preexponential factor, EA,i is the activation energy, R is the molar gas constant (8.314 J/(mol K)), and T is the temperature. For the reactions, where both the preexponential factor and the activation energy were fitted, a centered expression was used for describing the rate constant:
ki ) Aie(-EA,i /R)[(1/T)-(1/Tref)]
(1b)
where the reference temperature, Tref, was set to 600 K. This was done to decrease the correlation between the parameters. The preexponential factors for adsorption, Aads, are determined from kinetic gas theory:17
Aads )
NART
ANcatS(0) [m3/(s (kg of catalyst))] (2) (2πMRT)1/2
where NA is Avogadro’s number, M is the molar mass (kg/ mol), A is the area of a Pt site (8 × 10-20 m2/site18), Ncat is the number of active sites (mol/(kg of catalyst)), and S(0) is the sticking coefficient for zero coverage. The temperature dependence for the preexponential factors for adsorption was ne-
NO Oxidation and NOx Storage on Pt Catalysts
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6897
glected, and a reference temperature of 600 K was used in eq 2. The mass transport is described with the film model,19 and the Sherwood number is calculated from a correlation given by Tronconi et al.:20
Sh ) Sh∞ + 6.874(1000z*)
-0.488
exp(-57.2z*)
(3)
where Sh∞ is 3 for a square channel and z* is a dimensionless axial distance. The transition-state theory17,21 is used to investigate whether the entropy changes due to adsorption are reasonable. The partition functions for translation in two (qtr,2D) and three dimensions (qtr,3D) and the partition function for rotation (qrot) for linear and nonlinear molecules are given by
(2πmkBT)3/2 qtr,3D ) V h3 qtr,2D )
(2πmkBT) A h2
2
qrot )
qrot )
(8π IxkBT)
which together with eq 10 gives
rtotal ) ri+(1 - e∆G/RT)
(12)
If ∆G is a large negative value, then ri+/rtotal f 1 and the step is rate-determining. Accordingly, both methods will give the same results.
4.1. Oxygen Adsorption/Desorption on Pt/Al2O3. The following two reactions are used to describe the dissociative adsorption and the associative desorption of oxygen on Pt
(5) r1
} 2Pt-O O2(g) + 2Pt {\ r
(linear)
(nonlinear)
(6)
(7)
where m is the mass of one molecule (kg), T is the temperature (K), kB is the Boltzmann constant (1.381 × 10-23 J/K), h is the Planck constant (6.626 × 10-34 J s), V is the molecule volume (m3), A is the area of a site (m2), σr is the symmetry number, and finally Ii represents the rotational moment of inertia. The resulting entropy for a specific partition function is given by
S ) R ln(q) + RT
(11)
(13)
2
8π2(8π3IxIyIzx)1/2(kBT)3/2 σrh2
rtotal ) ri+ - ri-
4. Results and Discussion
(4)
3/2
σrh3
for a specific reaction contributes to the total rate, i.e., comparing ri+ with rtotal, and if ri+/rtotal is equal to 1, then the step is ratedetermining, according to Campbell.23 The reaction rate r can be expressed as
d(ln(q)) dT
(8)
and the total entropy is the sum of the entropy contributions from translation, rotation, and vibration. The adsorbed species may be described by two extreme cases: as a two-dimensional gas or as a completely localized species. The entropy change between the gas phase and a two-dimensional gas can be described by the following equation:
∆S2D ) Str,3D + Srot,2D - (Str,2D + Srot,2D)
(9)
where it, in the calculations, is assumed that Srot,3D ) Srot,2D for NO and NO2 and Srot,2D ) 0 for O2, because it adsorbs dissociatively. The value for the localized entropy change is calculated from the sum of Str,3D and Srot,3D. A comparison between our parameters and ∆S2D and also ∆Sloc can be found below. The rate-limiting step is assumed to be the step/steps that “consumes the most free energy”.22 The Gibbs free energy for process i is obtained from
∆Gi ) -RT ln(ri+/ri-)
(10)
where ri+ and rI- are the reaction rates for the forward and backward reactions, respectively. The rate-limiting step could also be determined by comparing how much the reaction rate
where Pt denotes a Pt site. The kinetic model for NO oxidation over Pt used here is a development of a model described earlier.12 In this work, we have used more experiments in the fitting procedure and we have made a more thorough analysis of the dispersion. We have also increased the preexponential factor for oxygen desorption. By also taking into account translational and rotational degrees of freedom for the activated complex, we have increased the value for the preexponential factor from 1013 to 1015 s-1.24 This value will be compared to calculated values using transition-state theory in the next section. The activation energy for oxygen desorption is dependent on the oxygen coverage, due to repulsive interactions between the oxygen atoms on the surface. In the present model, this phenomenon is taken into account by using a coverage-dependent activation energy for desorption:
E2(θ) ) E2(0)(1 - R2θPt-O)
(14)
where R2 is a constant and θPt-O the coverage of oxygen on Pt. Three experiments, oxygen exposure at 350, 400, or 450 °C followed by TPD from room temperature, with one of the Pt/ Al2O3 catalysts (C1) were used to obtain the kinetic parameters. The initial coverage of oxygen on the surface prior to the oxygen TPD was calculated from the total amount of desorbing oxygen, together with the number of surface sites resulting from a CO TPD. The sticking coefficient of oxygen, S0(O2), on Pt(111) is 0.023 at 327 °C and 0.054 at 27 °C according to Elg et al.,25 0.025 at 327 °C and 0.06 at 27 °C according to Campbell et al.,26 and 0.064 at 27 °C according to Yeo et al.27 As 327 °C is the reference temperature in our study, we use 0.023 for S0(O2) in the model. It was not possible to determine the activation energies for both adsorption and desorption of oxygen, with these TPD experiments, due to a large correlation between these parameters. Therefore, only the heat of adsorption for oxygen was determined, by assuming the activation energy for oxygen adsorption to be 21 kJ/mol, which it is for Pt supported on CeO2.28 However, for describing the NO oxidation/NO2 dissociation experiments, the actual values for both these activation energies (EO2,ads and EO2,des) were crucial, and therefore, the activation energy for oxygen adsorption was fitted to these
6898 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Olsson et al. TABLE 1: Kinetic Parameters for Oxygen Adsorption/ Desorption on Pt/Al2O3a rate expression
Ai
units
r1 ) k1CO2(g)θ2v,Pt
1.7 × 103, fixed
2 r2 ) k2θPt-O
9.8 × 1012, fixed
m3/(s (kg of catalyst)) mol/(s (kg of catalyst))
Ei(0) (kJ/mol) 21, fixed 200.0 ( 0.9
a Ai, Ei(0), ki, and ri denote the preexponential factor, activation energy at zero coverage, rate constant, and rate for a specific process, respectively. Ci and θi denote the gas-phase concentration and surface coverage for a certain species. θv,Pt is equal to 1 - ∑θi,Pt.
Figure 1. Results from a TPD experiment on a Pt/Al2O3 catalyst (C1), where oxygen is adsorbed at 400 °C. The top graph shows the measured and calculated outlet concentrations of oxygen, and the bottom graph displays the calculated oxygen coverage on the Pt surface.
experiments, keeping the heat of adsorption constant at the value obtained from the TPD experiments. Figure 1 shows the result of a heating ramp after oxygen adsorption at 400 °C. The top graph shows measured and calculated outlet concentrations of oxygen, and the bottom graph shows the calculated oxygen atom coverage on the Pt surface. The parameter values and their 95% confidence intervals (approximate, linearized) are shown in Table 1. The activation energy for oxygen desorption (200 kJ/mol) agrees well with the value 213 kJ/mol that Campbell et al.26 obtained for Pt(111). The number of sites is fixed at 9.8 × 10-3 mol/(kg of catalyst), from the CO TPD measurements, and the result for the coverage dependence, R2 (given in eq 14), is 0.115 ( 0.008. 4.2. NO Oxidation on Pt/Al2O3. The previous kinetic model for NO oxidation12 is improved by using more experiments in the fitting procedure and using the new parameters for oxygen adsorption/desorption, obtained in the previous section. The results from five different experiments are used for modeling: a temperature ramp under 8% O2 and 600 ppm NO and four steady-state experiments with 680 ppm NO2 in the inlet gas flow at 300, 350, 400, and 450 °C. Three different kinetic models are examined: a Langmuir-Hinshelwood model (LH), an Eley-Rideal model (ER), and finally a combined LangmuirHinshelwood and Eley-Rideal (LH-ER) model. In all models the following six reactions are used: r1′
O2(g) + 2Pt {\ } 2Pt-O r
(15)
2′
r3
NO(g) + Pt {\ } Pt-O r
(16)
4
r5
} Pt-NO2 NO2(g) + Pt {\ r
(17)
6
where ri denotes the reaction rate for step i and Pt is a Pt site. Then the following different reactions are added for the different models: r7
} Pt-NO2 + Pt LH: Pt-NO + Pt-O {\ r
(18)
8
r7′
ER: NO2(g) + Pt-O {\ } Pt-NO2 r
(19)
8′
and for the LH-ER model both reactions in eqs 18 and 19 are used. The number of free parameters have been reduced to decrease the correlation, and only 3 parameters out of 17 in the
LH model are obtained by fitting to experimental data. The parameters used are described as follows. (1) The preexponential factors and the coverage-dependent heat of adsorption for oxygen adsorption and desorption are fixed to the values obtained from the oxygen TPD experiments. (2) The adsorption of NO2 is reported to be almost nonactivated, 0.97) and independent of temperature below 27 °C according to Bartram et al.29 We therefore used S0(NO2) ) 1 in our model. (4) The preexponential factor for NO desorption from Pt(100), Pt(110), or Pt(111) is reported to be 1016 s-1,32 for the major tightly bound states, and the corresponding activation energies are reported to be 151, 140, and 105 kJ/mol, respectively. In the model a preexponential factor of 1016 s-1 is used, and the activation energy for NO desorption is fitted. (5) Bartram et al.29 performed a study of NO2 on a clean Pt(111) surface. They were not able to determine the preexponential factor for desorption of NO2. However, they assumed it to be 1013 s-1 and determined an activation energy for NO2 desorption of 79.5 kJ/mol. In our case, with a sticking coefficient of 1, these values result in an entropy change that is slightly lower than the entropy change if the adsorbed NO2 is regarded as a two-dimensional gas. In our model, we prefer to use the same preexponential factor as for NO desorption (1016 s-1), and the activation energy has been calculated to be 98 kJ/mol. This is done by setting the rate constants (with preexponential factor 1013 or 1016 s-1) equal for NO2 desorption at 47 °C (the desorption peak temperature in the work by Bartram et al.29). The activation energy for desorption of NO2 is decreased if oxygen is adsorbed before the adsorption of NO2.33 The reason for this may be that NO2 changes binding state, from a bridge bonded to a state with only one bond to the surface. In our model the activation energy for desorption of NO2 decreases linearly with the oxygen coverage, according to
E6(θ) ) E6(0)(1 - R6θPt-O)
(20)
where R6 is a constant and θPt-O the coverage of oxygen on Pt. (6) The preexponential factor for the surface reaction PtNO + Pt-O w Pt-NO2 + Pt is set to 1013 s-1. (7) Parameters k8, E8, and R6 for the LH model, k8′, E8′, and R6 for the ER model, and k8, E8, k8′, E8′, and R6 for the LHER model are calculated from the thermodynamic restriction for the reaction NO(g) + 1/2O2(g) S NO2(g). At 600 K, which is used as a reference temperature, ∆H ) -58.3 kJ/mol and ∆S ) -76.1 J/(mol K).34
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J. Phys. Chem. B, Vol. 105, No. 29, 2001 6899
Figure 2. NO oxidation model (LH) and experiments on Pt/Al2O3 (C2). The lower panels display the calculated coverages on Pt. The experimental sequence is as follows: a temperature ramp with 600 ppm NO/N2 and 8% O2 (a) and 680 ppm NO2/N2 at 300 °C (b), 350 °C (c), and 400 °C (d).
(8) The parameters fitted are, for the LH model, E1, E4, and E7, for the ER model, k7′, E1, E4, and E7′, and, for the LH-ER model, k7′, E1, E4, E7, and E7′. The three models have about the same standard deviation (LH model, 2.78 × 10-5; ER model, 2.80 × 10-5; LH-ER model, 2.80 × 10-5), and therefore it cannot be determined which mechanism is the most probable for NO oxidation. According to Bartram et al.,33 who studied the decomposition of NO2 with high-resolution electron energy loss spectroscopy (HREELS), NO2 should be bridge-bonded to decompose to NO and O on the surface. These results imply that NO2 needs two sites to decompose, and thus the Langmuir-Hinshelwood mechanism is the most probable. We therefore choose to continue with the Langmuir-Hinshelwood mechanism. It is crucial to include experiments with NO2 in the fitting, since the decomposition of NO2 to NO and oxygen is slower than the oxidation of NO to form NO2 at low temperatures. To explain this low decomposition capacity of Pt, two different mechanisms are proposed. (i) When NO2 dissociates to NO, it leaves oxygen atoms on the surface which, at low temperatures, will not desorb and thus lead to oxygen poisoning. (ii) NO2 and oxygen atoms block the surface. The second mechanism results in both a low preexponential factor and a low activation energy for NO2 desorption. The low preexponential factor is needed to get a high NO2 coverage, and the low activation energy is crucial for the production of NO2 from NO at low temperatures. However, the change in entropy for the adsorption/desorption in this model is unrealistic, and therefore we reject mechanism ii. Mechanism i, where only oxygen atoms block the surface, results in a reasonable entropy loss and is therefore chosen. Figure 2 shows measured and calculated (using the LH model) outlet concentrations of NO and NO2 (upper panel) for NO oxidation on Pt/Al2O3. The lower panels show the calculated mean coverages on the surface. The experimental sequence is
TABLE 2: Kinetic Parameters for NO Oxidation, with a Langmuir-Hinshelwood Mechanism on Pt/Al2O3a rate expression
Ai
units
r1 ) k1CO2(g)θ2v,Pt
6.6 × 102, fixedb
2 r2 ) k2θPt-O
3.8 × 1012, fixedb
r3 ) k3CNO(g)θv,Pt
2.5 × 104, fixed
r4 ) k4θPt-NO
3.8 × 1013, fixed
r5 ) k5CNO2(g)θv,Pt
2.4 × 104, fixed
r6 ) k6θPt-NO2
3.8 × 1013, fixed
r7 ) k7θPt-NOθPt-O
3.8 × 1010, fixed
r8 ) k8θPt-NO2θv,Pt
2.2 × 1010, fixedc
m3/(s (kg of catalyst)) mol/(s (kg of catalyst)) m3/(s (kg of catalyst)) mol/(s (kg of catalyst)) m3/(s (kg of catalyst)) mol/(s (kg of catalyst)) mol/(s (kg of catalyst)) mol/(s (kg of catalyst))
Ei(0) (kJ/mol) 30.4 ( 0.2 209.4, fixedb 0, fixed 115.5 ( 3.0 0, fixed 97.9, fixed 101.3 ( 6.1 52.5, fixedc
a The notations are the same as in Table 1. b The preexponential factors and heat of adsorption are fixed from the O2 TPD experiments. c Calculated from the thermodynamic restriction.
the following: a temperature ramp with 600 ppm NO and 8% O2 (Figure 2a) followed by steady-state experiments with 680 ppm NO2 in N2 at 300 °C (Figure 2b), 350 °C (Figure 2c), and 400 °C (Figure 2d). For all experiments the model predicts a high oxygen coverage. For low temperatures the model predicts a very slow NO2 decomposition because of oxygen poisoning of the Pt surface. The parameter values and their 95% confidence intervals (approximate, linearized) are shown in Table 2. The number of Pt sites in the model is 3.8 × 10-3 mol/(kg of catalyst), which is obtained from CO TPD measurements. The value for the constant R2, described in eq 14, is 0.11 (fixed from the O2 TPD experiments), and that for the constant R6, described in eq 20, is 0.12 (calculated from the thermodynamic restriction). The entropy changes due to adsorption calculated from the transition-state theory are shown in Table 3. The
6900 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Olsson et al.
TABLE 3: A Comparison among the Entropy Changes for the NO Oxidation Model, a 2D Gas, and a Localized Adsorbatea ∆Smodel ∆S2D ∆Sloc Sgas (J/(mol K)) (J/(mol K)) (J/(mol K)) (J/(mol K)) O2 adsorption NO adsorption NO2 adsorption
-162 -150 -152
-103 -98 -101
-212 -216 -254
-226 -232 -269
a Also shown is the entropy for the gas phase. The reference temperature is 600 K.
Figure 3. O2 concentration during TPD measurements following oxygen adsorption at 40, 200, and 300 °C on Pt/Al2O3 (C1). Also shown is the O2 signal during a TPD, following O2 exposure of the sample while being cooled from 600 to 40 °C.
entropy change for all species is between the values for a twodimensional gas and a completely localized adsorbed species, which seems reasonable. The activation energy for NO desorption (115 kJ/mol) is within the range 105-151 kJ/mol reported by Gorte et al.32 for different Pt surfaces. The activation energy for oxygen adsorption is relatively high, above 30 kJ/mol. Adsorption of oxygen is often regarded as nonactivated. However, in our experiments at atmospheric pressure, we know from the oxygen TPD measurements that the coverage is high, and we infer that the adsorption of oxygen is activated. Figure 3 shows O2 TPD spectra after oxygen adsorption at different temperatures. There is no significant difference whether oxygen is adsorbed at 40 °C or at 200 °C. However, if oxygen is adsorbed at 300 °C, the resulting coverage of oxygen on Pt becomes higher. This indicates that oxygen adsorption is activated. An even larger difference is seen if the catalyst is exposed to oxygen while being cooled from 600 to 40 °C prior to the TPD. The rate-limiting step according to the Langmuir-Hinshelwood mechanism for NO2 reduction to NO on Pt/Al2O3 is the oxygen desorption. This is shown in Figure 4, where the Gibbs free energy versus the distance in the monolith is plotted for NO2 decomposition at two different temperatures (300 and 450 °C). At low temperatures the NO2 reduction is hindered by the oxygen poisoning, whereas at higher temperatures the thermodynamic equilibrium levels are reached. The results, including the determination of the rate-limiting steps, are sensitive to the value of the dispersion of catalyst C1, obtained from CO TPD experiments, and also the chosen literature values that were used for decreasing the number of free parameters. 4.3. NO Oxidation on Pt/BaO/Al2O3. The capacity of Pt to oxidize NO to NO2 and also to reduce NO2 to NO is decreased when BaO is present in the catalyst in addition to γ-Al2O3. This is illustrated in Figure 5, where the results from a temperature ramp with 600 ppm NO and 8% O2 for two different catalysts (C2 (2.3 wt % Pt/γ-Al2O3) and C3 (2.0 wt % Pt/20 wt % BaO/
γ-Al2O3)) are shown. It should be mentioned that the Pt/BaO/ Al2O3 catalyst contains less Pt than the Pt/Al2O3 catalyst. However, this difference alone cannot explain the large difference seen in Figure 5. If the NO2 production rate [(mol/s)/(mg of Pt)] is compared at 300 °C, the rate is 11 × 10-8 mol/(s (mg of Pt)) for Pt/Al2O3 and 5.6 × 10-8 mol/(s (mg of Pt)) for Pt/ BaO/Al2O3. There are different possible explanations for the different behavior of Pt when BaO is present. One possiblity is that the Pt dispersion is decreased, possibly because the BaO covers some of the Pt particles. In an FTIR study with CO, as a probe molecule, with the corresponding powder catalysts the dispersion of Pt was observed to decrease markedly (by a factor of 5) with BaO present. In our kinetic model we decreased the number of Pt sites by fitting the dispersion, which resulted in a factor of about 15 less active Pt sites, for the BaO-containing catalyst. There could be different reasons for this discrepancy. One possible explanation may be that since the volume of Ba(NO3)2 is a factor of 3 larger than for BaO, the nitrates may sterically hinder the oxidation of some Pt particles. Of course, it would also have been possible to explain the experiments by refitting the parameters, which would have been the case if the platium was affected by BaO, but we chose to only reduce the number of sites. Figure 5 also shows steady-state values at different temperatures, which are quite close to the temperature ramp results. Thus, the temperature ramp results may be used for modeling NO oxidation over the Pt/BaO/Al2O3 catalyst. Figure 6 shows measured and calculated (using the LH model) outlet concentrations of NO and NO2 (upper panel) for NO oxidation on Pt/BaO/Al2O3. The four experiments shown are as follows: a temperature ramp with 600 ppm NO and 8% O2 (a) and 680 ppm NO2 at 350 °C (b), 400 °C (b), and 450 °C (c). The lower panels in Figure 6 show the corresponding calculated coverages on the Pt surface. In the NO2 reduction experiments the capacity of the Pt/BaO/Al2O3 catalyst to reduce NO2 to NO is observed to slowly decrease with time. This phenomenon is most obvious at 400 °C and is seen during the whole 80 min long experiment. This behavior is not included in the model, and only the last 5 min in the experiments is used for the fitting. To model this phenomenon, a more thorough analysis of this behavior needs to be performed. There may be several possible explanations; for example, Pt particles may be slowly oxidized by the strong oxidizing agent NO2 into an inactive platinum oxide. Another explanation may be that formed barium nitrates gradually cover some of the Pt particles or alternatively block some pores in the washcoat. 4.4. NOx Storage on BaO/Al2O3. Figure 7 shows the NO and NO2 outlet concentrations when the BaO/Al2O3 catalyst is exposed to 680 ppm NO2 at 400 °C. The adsorption is followed by an inert period of 5 min, and finally a temperature ramp is conducted. During the initial storing period NO is formed, due to an oxidation of the surface or due to reactions between species adsorbed on it. Also shown in Figure 7 are the results of a similar experiment, but with an exposure to about 640 ppm N2O prior to and during the NO2 adsorption. The N2O does not influence the results, which indicates that the surface is not oxidized by N2O. This result also indicates that the surface is not likely to be oxidized by oxygen, since N2O is a stronger oxidizing agent than O2. The kinetic model used contains five different reversible reactions, i.e., ten reactions (r9-r18), and these will be described below. The NO2 adsorption experiment, shown in Figure 7, indicates that about two NO2 molecules are stored for every NO molecule produced. Therefore, it is not possible to describe the experiments with only a production of S-NO3, where S
NO Oxidation and NOx Storage on Pt Catalysts
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6901
Figure 4. Gibbs free energy (∆G) versus the position in the Pt/Al2O3 monolith (C2) for experiments with 680 ppm NO2 in the inflow at (a) 300 °C and (b) 450 °C.
the stoichiometry between NO formed and NO2 stored in the experiment is preserved: r15
NO2(g) + S-(NO3) {\ } Ba(NO3)2 r
(24)
16
Figure 7 shows that the NOx desorption peak is broad. To be able to model this behavior, we assume that the activation energy for reaction 16 decreases as the coverage of Ba(NO3)2 increases:
E16(θ) ) E16(0)(1 - R16θBa(NO3)2)
Figure 5. Measured NO and NO2 outlet concentrations as a function of the catalyst temperature, when Pt/Al2O3 (C2) and Pt/BaO/Al2O3 (C3) are exposed to 600 ppm NO and 8 vol % O2 in N2. Also shown are steady-state values at some temperatures for the Pt/BaO/Al2O3 catalyst.
denotes a BaO site, since this will give the stoichiometry 1:1 between stored NO2 and produced NO. We have therefore assumed that different species can adsorb on either the barium or the oxide part of BaO and in this way form (NO3)-BaO(NO2), which gives this 2:1 relation. Ba(NO3)2 will be used for (NO3)-BaO-(NO2) in the rest of the paper. In the model NO2 first adsorbs loosely on the BaO site, S: r9
NO2(g) + S {\ } S-(NO2) r
(21)
10
To explain the NO desorption in the experiments (Figure 7), we include the following step, where the surface is oxidized: r11
} S-O + NO(g) S-(NO2) {\ r
(22)
12
The formation of BaO2 has previously been observed by in situ Raman spectroscopy.35 In the next step of the storage process NO2(g) is adsorbed and a nitrate is formed: r13
} S-(NO3) NO2(g) + S-O {\ r
(23)
14
This nitrate is localized on the barium part of BaO, since FTIR shows that monodentate nitrate is the dominating surface species on BaO.36 Another NO2(g) molecule is then adsorbed on the oxide part of the site, and Ba(NO3)2 is formed. By this reaction
(25)
where R16 is a constant and θBa(NO3)2 the coverage of Ba(NO3)2. This behavior may be explained by repulsive interactions between NO3 groups on the barium, similar to the case for oxygen atoms on Pt. The final reaction step added is the formation of O2(g), because there is a continuous reduction of NO2 to NO on BaO/Al2O3 at high temperatures (about 500 °C), according to experiment. In our model we describe this by the following reaction: r17
} 2S + O2(g) 2S-O {\ r
(26)
18
This continuous production of O2(g) is another reason for introducing S-O. The activation energy and the preexponential factor for reaction 18, eq 26, are calculated from the enthalpy and entropy changes for the oxidation of NO with oxygen to form NO2. The preexponential factors A10, A11, A14, A16, and A17 are kept constant at a value of 1013 s-1 to decrease the correlation and to prevent unrealistic values of these parameters in the fitting. Three experiments were used in the fitting procedure: first the catalyst was exposed to 680 ppm NO2 at 300, 350, or 400 °C, followed by 5 min of inert atmosphere, and finally the temperature was increased to 600 °C. The fitting resulted in reactions 12 and 13 being almost nonactivated (
