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Ind. Eng. Chem. Res. 2008, 47, 9240–9247
Selective Catalytic Reduction of NOx: Mechanistic Perspectives on the Role of Base Metal and Noble Metal Ion Substitution Sounak Roy,† A. Marimuthu,‡ Parag A. Deshpande,‡ M. S. Hegde,† and Giridhar Madras*,‡ Solid State and Structural Chemistry Unit and Chemical Engineering Department, Indian Institute of Science, Bangalore 560012, India
Selective catalytic reduction (SCR) of NO by NH3 in presence of excess oxygen is an industrially and environmentally important reaction. Determining a suitable catalyst substituted with noble metals or base metal ions with a suitable reaction mechanism is important. In this study, ionically substituted Mn and Pd in TiO2 catalysts were synthesized by the solution combustion technique and examined for SCR activity. A seven-step reaction mechanism was proposed to incorporate the role of NH3 oxidation in the SCR reaction that shows minima in the NO concentration profile. Both modeling and experimental results show that the reduction of NO in SCR condition follows the order Ti0.9Mn0.1O2-δ > Ti0.89Mn0.1Pd0.01O2-δ > Ti0.99Pd0.01O2-δ. However, the hydrogen uptake study showed that the noble metal ion (Pd2+) substituted TiO2 has better reducibility than the base metal ion (Mn3+) substituted TiO2. The rate of ammonia oxidation by different catalysts followed the reverse order such as that of NO reduction. This clearly indicates that catalysts with higher reducibility and that which exhibit higher rates of ammonia oxidation have poor SCR activity. Thus, the base metal ion substitution is better than noble metal ion substitution for SCR. Introduction NOx (NO and/or NO2) are normally produced during combustion, and they react with volatile organic compounds to form photochemical smog. The hazardous environmental impact and emission limits of NOx have made researchers investigate several methods to convert NOx into nitrogen. Selective catalytic reduction (SCR) of NOx by NH3 in the presence of excess oxygen is one of the potential processes for removal of NOx from the exhaust. SCR systems are typically used in large utility boilers, industrial boilers, large diesel engines, and, recently, in automobiles. The commercial catalyst is V2O5-WO3/TiO2,1-3 and the reaction occurs at around 300-400 °C. However, the first row transition metal doped on TiO2 exhibits better catalytic activity at lower temperatures.4-6 Noble metals such as Ag, Pd, and Pt doped on oxygen storage materials have also been used.7-10 For a catalyst to exhibit good SCR catalytic activity, the catalyst should exhibit low-temperature activity, have high N2 selectivity, and have a wide SCR window. The primary reaction in SCR-NH3 is NO reduction by NH3 in the presence of excess O2, 4NH3 + 4NO + O2 f 4N2 + 6H2O
(A)
However, other undesirable reactions such as NH3 oxidation by O2 also occur and produce NO and N2O. 4NH3 + 4NO + 3O2 f 4N2O + 6H2O
(B)
4NH3 + 5O2 f 4NO + 6H2O
(C)
The oxidation of ammonia, in addition to reaction C, can also occur by 4NH3 + 3O2 f 2N2 + 6H2O
(D)
2NH3 + 2O2 f N2O + 3H2O
(E)
* To whom correspondence should be addressed. Tel.: +91-80-22932321. Fax: +91-80-2360-8121. E-mail:
[email protected]. † Solid State and Structural Chemistry Unit. ‡ Chemical Engineering Department.
The primary aim of an SCR catalyst should be to enhance the rate of reaction A and reduce the rate of NH3 oxidation (reactions B and C). The kinetics of the reaction and understanding of the mechanism are important for process development. The important feature that needs to be accounted for during the development of a mechanism for the SCR reaction is the role of NH3 oxidation at higher temperatures. Several mechanisms for the SCR reaction have been proposed in the literature.11-15 These mechanistic studies for the NO + H2 + O2 reactions have not explained the role of NH3 oxidation in the overall SCR reaction. In most cases, the SCR and NH3 oxidation at particular reaction conditions were studied separately.13,14 Further, any comparisons are limited to a relatively narrow range of reaction conditions (400 °C).13,14 Tronconi et al.11 have proposed a model to predict the minima in the NO concentration and the corresponding maxima in the N2 concentration. However, the five-steps mechanism proposed in their study does not consider the role of NH3 oxidation.11 Boer et al.12 investigated the experimental work on ammonia oxidation and proposed two separate reaction paths for the SCR of NO and the NH3 oxidation. In most of these studies, no kinetic data that directly relates the role of NH3 oxidation in the SCR reaction have been established. The commercial catalyst that is used for SCR is V2O5-WO3/ TiO2. However, recent studies have investigated the use of first row transition metals for their low-temperature activity and the application of noble metals such as Ag, Pd, and Pt doped on oxygen storage materials. However, no comparative studies are available. In this study, the reactions have been conducted in the presence of the first row transition metal (Mn) doped TiO2, noble metal (Pd) doped TiO2, and their combination. By comparing the reaction mechanisms over these catalysts, one can obtain greater insights into the mechanism. Thus, the catalysts in the study were chosen with the special purpose of studying and comparing the effects of the noble metal and base metal substituted compound on the reaction kinetics. Since the process can be far more economical by the use of a base metal than a noble metal, it was intended to study the provision of
10.1021/ie8010879 CCC: $40.75 2008 American Chemical Society Published on Web 11/05/2008
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9241
base metal substitution in catalysis. This is vindicated by this study which shows that the base metal substituted compound shows higher catalytic activity. Thus, the objectives of this study are (a) to compare the SCR activity of substituted noble metal ion Pd2+ and base metal Mn3+ in TiO2 (Ti0.9Mn0.1O2-δ, Ti0.99Pd0.01O2-δ, and Ti0.89Mn0.1Pd0.01O2-δ) and (b) to develop an overall mechanism to incorporate the role of NH3 oxidation in SCR reaction based on the elementary steps that were proven in many independent studies. The overall mechanism consists of seven elementary steps and simulates the reaction rate observed experimentally over the entire temperature range.
Experimental Section Noble metal and base metal ion substituted TiO2 have been synthesized by solution combustion method as discussed in previous studies.4,5 The metal nitrates such as TiO(NO3)2 and Mn(NO3)2 · 4H2O and metal chloride such as PdCl2 were the precursors for synthesis. The catalysts were characterized by X-ray diffraction (Phillips X’Pert diffractometer using Cu KR radiation at a scan rate of 2θ ) 0.5° min-1), high-resolution transmission electron microscopy (TECNAI F30 electron microscope operated at 200 kV), X-ray photoelectron spectroscopy (ESCA-3 Mark II VG scientific spectrometer using Al KR radiation), and surface analysis (Quantachrome NOVA 1000). Hydrogen uptake experiments were carried out with 50 mg of catalyst of 40-80 mesh size from -100 to 600 °C. The catalyst samples were plugged with ceramic wool in a continuous flow of 5% H2 in Ar. The amount of H2 uptake was detected by a TCD detector that was calibrated against the uptake of H2 with a known amount of CuO. The catalytic studies were carried out in a temperature programmed reaction system equipped with a quadrupole mass spectrometer SX200 (VG Scientific Ltd.). A 150 mg amount of catalyst samples of 40-80 mesh were diluted with SiO2 and were packed between two glass wool plugs. The catalyst bed volume was 0.138 cm3, and the catalysts were placed in the center of a microreactor that consists of 4 mm i.d., 15 cm length quartz tube. The quartz tube was kept in a furnace whose temperature was measured by a chromel-alumel thermocouple dipped in the catalyst bed, and the temperature was controlled by a PID controller. The temperature was ramped at 10 °C/ min. The gaseous products were sampled through a fine leak valve to an ultrahigh-vacuum (UHV) system housing the quadrupole mass spectrometer. The product gas passing through the needle valve was leaked to a stainless steel chamber from 10-5 to 0.02 torr. The sampled gas from this chamber at 0.02 torr was leaked to an UHV chamber housing the mass spectrometer from 10-8 to 10-6 torr. Thus, the dead volume of the sampling gas is replaced 8-10 times in 1 s by the differential pumping method. The gases leaving the reaction zone were detected within 1 s in the mass spectrometer. The feed gas ratio was NO:NH3:O2 ) 1:1:5 (vol %); i.e., NO and NH3 were 10000 ppm and O2 was 50000 ppm at a flow rate of 100 cm3 min-1. For the rate experiments, the mass balance for the differential reactor was used. Thus, the rate of NO conversion was calculated as, -rNO ) FNOXNO/W, where W is the weight of the catalyst, FNO is the flow rate of NO, and XNO is the conversion. The flow rate was varied, and the rate was obtained from the linear
Figure 1. Hydrogen uptake profile over the three catalysts.
portion of the plot of XNO with W/FNO. Further details of the experimental procedure has been provided in our earlier work.4,16,20 Results and Discussion Structural Studies. X-ray diffraction studies showed that Ti0.9Mn0.1O2-δ and Ti0.99Pd0.01O2-δ crystallizes in pure anatase structure.4,5 The bright field image of the materials showed the particle size to be in the range of 8-10 nm, and HRTEM images showed lattice fringes of a distance of 3.1 Å, which corresponds to the (101) plane of anatase TiO2. Mn is in the 3+ state in Ti0.9Mn0.1O2-δ, whereas Pd is in the 2+ state in Ti0.99Pd0.01O2-δ as confirmed by X-ray photoelectron spectroscopy (XPS) studies. Ti (2p3/2,1/2) in Ti0.9Mn0.1O2-δ and Ti0.99Pd0.01O2-δ showed Ti in the 4+ oxidation state. H2 Uptake Studies. H2-uptake profiles for Ti0.9Mn0.1O2-δ, Ti0.99Pd0.01O2-δ, and Ti0.89Mn0.1Pd0.01O2-δ are shown in Figure 1. Ti0.99Pd0.01O2-δ showed the H2-uptake peak at the lowest temperature among the three catalysts, followed by Ti0.89Mn0.1Pd0.01O2-δ and Ti0.9Mn0.1O2-δ. For Ti0.99Pd0.01O2-δ and Ti0.89Mn0.1Pd0.01O2-δ, two peaks are observed. The first peak is attributed to the noble metal or base metal ion reduction, and the second peak is due to Ti4+/Ti3+ reduction.5 For the Mn3+ substituted TiO2, the peak is broad. The H2/M molar ratio taking only the first peak is H2/Pd ) 1.9 for Ti0.99Pd0.01O2-δ, H2/Pd ) 1.4 for Ti0.89Mn0.1Pd0.01O2-δ and H2/Mn ) 0.55 for Ti0.9Mn0.1O2-δ. When Pd2+ is substituted, the total H2/Pd is higher than unity. However, if only Pd2+ had been reduced, then H2/Pd will be unity. The higher value indicates that part of Mn3+ and Ti4+ get reduced at the same temperature as that of the Pd2+ ion. Thus, the effect of Pd2+ ion substitution is that Mn3+ and Ti4+ get reduced at a lower temperature. These results indicate that the reducibility of Ti0.99Pd0.01O2-δ is higher than Ti0.9Mn0.1O2-δ and follows the order of Ti0.99Pd0.01O2-δ > Ti0.89Mn0.1Pd0.01O2-δ > Ti0.9Mn0.1O2-δ. Catalytic Reaction and Mechanism. The concentration profiles of SCR with all reaction components over the three catalysts are given in Figure 2a,b. The profiles in terms of percent in the outlet are presented as Figure S1 (see the Supporting Information). The NO reduction over Ti0.9Mn0.1O2-δ starts at the lowest temperature, followed by Ti0.89Mn0.1Pd0.01O2-δ and Ti0.99Pd0.01O2-δ. Interestingly this order is just the opposite order of the reducibility of the catalysts. One more interesting observation is that the formation of NO takes place at higher temperature for Ti0.9Mn0.1O2-δ compared to Ti0.99Pd0.01O2-δ, making the SCR window the widest. We have explained these observations in the following section from a mechanistic point of view.
9242 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
In the development of an overall mechanism, it is necessary to incorporate all the overall reactions A-E (discussed in the Introduction) in terms of elementary reactions. We have confirmed the importance of oxide ion vacancy that exists in substituted Pd catalyst on the supports TiO2 and/or CeO2 in the earlier studies. The models15-20 were developed for CO + O2, NO + CO, N2O + CO, NO + H2, and N2O + H2 reactions over these catalysts by using a bifunctional (dual sites) mechanism. A similar kinetic modeling development was extended for SCR reaction. We have tested many mechanisms17 (based on both monofunctional and bifunctional mechanisms and their combinations) in which the adsorption and the reaction take place both in the metal site and on the ionic vacancy of the support. The following mechanism shows the best fit with the experimental data. NH3 + SMn S (NH3)Mn
(1a)
NH3 + STi S (NH3)Ti
(1b)
NH3 + SPd S (NH3)Pd
(1c)
O2 + 2 “ V ” S 2 “ O”
(2)
NO + “ V ” S N “ O” (3) (NH3)Mn + 3 “ O ” + “ O ” f NO + 3 “ O ” H + SMn + “ V” (4a) (NH3)Ti + 3 “ O ” + “ O ” f NO + 3 “ O ” H + STi + “ V” (4b) (NH3)Pd + 3 “ O ” + “ O ” f NO + 3 “ O ” H + SPd + “ V″ (4c) (NH3)Mn + 3 “ O ” + N “ O ” f N2O + 3 “ O ” H + SMn + “ V” (5a) (NH3)Ti + 3 “ O ” + N “ O ” f N2O + 3 “ O ” H + STi + “ V” (5b) (NH3)Pd + 3 “ O ” + N “ O ” f N2O + 3 “ O ” H + SPd + “ V” (5c) N2O + “ V ” f N2 + “ O”
(6)
“O ” H + “ O ” H f H2O + “ V ” + “ O”
(7)
Here “O” and “V” are the oxide ion and vacant site on the support, respectively. SMn, SPd, and STi are the active sites on the Mn, Pd, and Ti surfaces. On the surface of Mn, Pd, and Ti, the reactions are assumed to happen in parallel with the ionic site. All the possible elementary reaction steps before lumping are given in Appendix A1 (see the Supporting Information). The selection and formulation of the above-mentioned elementary reaction steps are explained below in detail. Several mechanisms have been proposed11,15 on the basis of the redox Mars and van Krevelen dual site mechanism for the SCR of NO by NH3.. Other studies have also used the monofunctional mechanism to model the SCR reaction.21,22 The Eley-Rideal mechanism12-14,23 is assumed where the adsorbed NHx species react with the gas-phase or weakly adsorbed NO molecule. Tronconi et al.11 confirm the presence of two different types of sites in V2O5-WO3/TiO2, one redox site for O2 and NO adsorption/activation and one acidic site for NH3 adsorption for the SCR reaction. Similar to our earlier studies,16-20 we have also assumed two different types of sites, one site for NHx species and one site for “O” and NOx species. TPD experiments24 over the TiO2 supported catalysts such as V2O5/TiO2 proved that the important step in the mechanism is the activation of the adsorbed NH3 to produce NHx. In the presence of excess oxygen, it is generally accepted that the activation of ammonia proceeds via the reaction between adsorbed ammonia species with oxide ion to produce NHx and OH species.25-27 The observation of these intermediates NH2, NH, and OH was confirmed in the literature.26 The mechanism discussed in this study thus considers that the reaction takes place by the interaction of the absorbed species, i.e., reaction by spillover of one of the adsorbed species. The other mechanisms that involve the reaction of a reactant in the gas phase with the adsorbed species did not fit the experimental data.17 The excess oxygen used in the feed gas is reversibly adsorbed on the oxide ion vacancy that exists in the catalyst and maintains the catalyst mostly in the oxidized state (step 2). On the basis of our earlier mechanism proposed for the NO reduction16-20 and the mechanism proposed in the literature (for the SCR reaction) over similar catalysts, the overall sequence of steps is
Figure 2. Concentration (a) of the reactants and (b) of the products at the reactor exit over the three catalysts. The open symbols are for the reactants, and the closed symbols are for the products.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9243
proposed for the mechanism, as given in eqs 1a-7. The sequence of the steps proposed in this mechanism is similar to that proposed by Boer et al.,12 “O”
Nads
“V”
NH3 98 NO(g) 98 N2O(g) 98 N2(g) + “ O”
(F)
This sequence describes the route through which the intermediates and the final products are formed. The mechanism is explained in detailed as follows. For the reactions over Ti0.9Mn0.1O2-δ, a mechanism based on three sites is proposed (steps 1a-c). This is in correspondence with the possibility of NH3 adsorption on the Ti surface..24 On the surfaces of Pd, Mn, and Ti, NH3 adsorption is assumed to be in equilibrium (steps 1a-c) and NO does not adsorb on an ammonia covered surface.24 The experimental data show that NO conversion increases until 200 °C and decreases at higher temperatures. The decreasing NO conversion above 200 °C is due to the influence of ammonia oxidation (reactions C, D and E) over its reduction with NO (reactions A and B). A similar trend is observed in many other studies.1,23 However, no attempt has been made in the literature to incorporate the influence of NH3 oxidation in the kinetic and mechanistic analysis. The role of catalytic oxidation of ammonia in the SCR mechanism is mainly incorporated in steps 3 and 4. For the NH3 + NO + O2 reaction system (SCR of NO), the NO that takes part in step 3 is both from the feed gas and the NO produced from step 4 due to the oxidation of ammonia. Step 5 is the reduction of NO by ammonia. Steps 4 and 5 in the mechanism are the lumped equations of the following three elementary steps.25 (NH3)ads + “ O ” f (NH2)ads + “ O ” H
(G)
(NH2)ads + “ O ” f NHads + “ O ” H
(H)
NH + “ O ” f Nads + “ O ” H
(I)
Because the surface nitrogen species (Nads) are more stable thermodynamically than surface imide (NH) and amide (NH2) species, the above lumping is reasonable. It is generally accepted that the formation of NO occurs by the reaction of adsorbed N species with lattice oxygen, and it is easily possible over the catalysts which are easily reducible.12 The possibility of NO formation from the adsorbed species (Nads and Oads) over the metal surface has been also confirmed in most of the earlier studies.25,27,28 The best possibility of N2O formation (step 5) is from the two different types of N species, i.e., one from NO and another from NH3.12-14,25 For the NH3 oxidation, this reaction path is called an internal SCR mechanism since NO is formed internally and it subsequently reacts with Nads to form N2O.12 For the SCR reaction, the vacancy is created by the reaction of NHx species with lattice oxygen “O” and it is refilled by O2 (step 2) and NO (step 3) from feed gas, N2O dissociation into N2 and “O” (step 6), and water formation (step 7) from two OH species.12,25,29,30 This applies for the other reactions also, i.e., NO reduction by CO and H2.16-20 The formation of nitrogen occurs through the dissociation of N2O12 (step 6). The formation of nitrogen from the recombination of two adsorbed N species (Nads) is negligible at this temperature range.12,16-20,30 This especially holds for reactions over reducible catalysts. The N2O dissociation into N2 and O is the faster step in several reactions:12,17-20,30 N2O + CO, NO + CO, N2O + H2, NO + H2, and NO + NH3. N2O dissociates to nitrogen and O to reoxidize the vacancy in the catalyst. Our earlier experimental work12 and kinetic analysis17-20 also confirm the formation of N2 from N2O. The derivation for the
rate of nitrogen formation is presented in Appendix A2 and the figure is shown as Figure S2 (see the Supporting Information). The water formation (step 7) occurs with the creation of oxide ion vacancy through the combination of two adjacent “OH” groups.12-14 Water is not considered to be an adsorbed species because water formation occurs through two adjacent OH species and thus it is unlikely.12,14 Further, in the presence of excess oxygen and N2O dissociation to refill the vacancy, it is unlikely for the water species to adsorb on the surface. Experimentally, all the water that is evolved in the reaction was observed at the outlet indicating no water adsorption. On the basis of the above arguments for establishing the reaction mechanism, the equations for the rates are derived for this mechanism. The adsorption of NH3 over the metal ions takes place reversibly given by eqs 1a-1c. For eq 1a, the rate of adsorption of NH3 on Mn is given by (r1)Mn ) (k1)MnCNH3θV - (k-1)Mnθ(NH3)Mn
(8)
Applying the quasi-equilibrium approximation and equating the above rate to zero yields θ(NH3)Mn ) (K1)MnCNH3θV
(9)
where (K1)Mn is the adsorption equilibrium constant and is given by (K1)Mn ) (k1)Mn/(k-)Mn. The site balance on Mn metal is θV + θ(NH3)Mn ) 1
(10)
Using eq 10 in eq 9 yields θ(NH3)Mn )
(K1)MnCNH3
(11a)
1 + (K1)MnCNH3
Studies show that the metal (Mn) and the Ti surfaces are predominantly occupied by (NH3)ads species.24 By assuming Langmuir adsorption isotherm for NH3 on Mn, Ti, and Pd surfaces, the fraction of sites occupied by the NH3 species are thus given by eq 11a. Similarly for eqs 1b and 1c, θ(NH3)Pd ) θ(NH3)Ti )
(K1)PdCNH3
(11b)
1 + (K1)PdCNH3 (K1)TiCNH3
(11c)
1 + (K1)TiCNH3
The equilibrium constants for all the three reactions are assumed to be equal as these steps are much faster as compared to the other steps in the reaction, and they have been found not to have any significant influence in the reaction kinetics. Thus, (K1)Mn ) (K1)Pd ) (K1)Ti ) K1
(12)
Therefore, eqs 11a-11c can be written as θ(NH3)Mn ) θ(NH3)Pd ) θ(NH3)Ti )
K1CNH3 1 + K1CNH3
) θ(NH3)M (13)
Similarly O2 and NO get dissociatively adsorbed in the oxide ion vacancy sites (steps 2 and 3). For step 2, the rate expression is r2 ) k2CO2θV22 - k-2θ“O”2
(14)
Equation 14 can be equated to zero and reduced to θ“O” ) √K2CO2θV2
(15)
9244 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 1. Rate Parameters for NO + NH3 + O2 Reaction over Ti0.9Mn0.1O2-δ, Ti0.89Mn0.1Pd0.01O2-δ, and Ti0.99Pd0.01O2-δ parameters 3
-1
Ti0.9Mn0.1O2-δ
Ti0.89Mn0.1Pd0.01O2-δ
Ti0.99Pd0.01O2-δ
5.8 × 10 T exp(3900/T) 600T exp(6110/T) 1.7 × 108T exp(1350/T) 2.0 × 108 exp(-10700/T) 4.5 × 1010 exp(-6600/T) 220 exp(-1800/T)
5.9 × 10 T exp(3730/T) 65T exp(5620/T) 1.1 × 108T exp(1000/T) 3.8 × 108 exp(-9700/T) 9.23 × 109 exp(-7050/T) 72 exp(-2500/T)
3.0 × 102T exp(3000/T) 60T exp(5200/T) 9.0 × 107T exp(840/T) 1.0 × 109 exp(-9000/T) 7.5 × 109 exp(-8100/T) 15 exp(-3700/T)
3
K1 (cm mol ) K2 (cm3 mol-1) K3 (cm3 mol-1) k4 (mol g-1 s-1) k5 (mol g-1 s-1) k7 (mol g-1 s-1)
2
For step 3, the rate expression is r3 ) k3CNOθV2 - k-3θN“O”
(16)
rate expression derived in this study is consistent with the rate determining step reported in the literature.25,31 The rate of formation of NO is given by
Equation 16 can be equated to zero and reduced to θN“O” ) K3CNOθV2
Next, the quasi-steady-state approximation is applied to the various intermediate species. For the species “O”H, 3r4 + 3r5 - 2r7 ) 0
Substituting eqs 15 and 17 in eq 19 yields the expression for θ“O”H, θ“O”H )
[ (
]
)
3 (k K C + k5√K2CO2K3CNO) θV2 2k7 1 + K1CNH3 4 2 O2 (20)
where k4 ) k4a + k4b + k4c
(20a)
k5 ) k5a + k5b + k5c
(20c)
The site balance on the support is θV2 + θ“O” + θN“O” + θ“O”H ) 1
(21)
rNO )
[
( ( k4
1 + K3CO2 + √K2CO2 +
σ )
1 + K3CO2 + √K2CO2 +
(
1 1 + K1CNH3
3K1CNH3
)(
2k7
)
(22)
For the reaction of NO reduction by ammonia the rates of oxide ion vacancy filling are expected to be faster (especially step 6).16-20,25,30 The rate determining step for the above mechanism is one of the steps where NHx species react with “O”, i.e., the oxygen consumption step (since its filling is much faster).25,31 In the mechanism presented in this study, step 4 includes this reaction as the rate limiting reaction. However, no rate limiting step is assumed in the derivation of our model. The rate expression for NO is rNO ) k4Θ(NH3)MnΘO2 - k3CNOΘV2 + k-3ΘN“O”
(23)
Because the NO adsorption step is assumed to be in equilibrium, k3CNOΘV2 ) k-3ΘN“O”. Thus, the rate expression for NO reduction reduces to eq 24. N2 is observed even when N2O is not observed, and thus it is justifiable to assume that N2O disappears as soon as it is formed and hence is a quasi-steadystate species. Therefore, the rate of step 5 is much higher as compared to the rate of step 4, as confirmed from the relative magnitudes of the rate constants given in Table 1. Thus, the
(K2CO2) k4K2CO2 + k5√K2CO2K3CNO 2k7
)]
2
(25)
∑ i)1
k4K2CO2 + k5√K2CO2K3CNO
) )(
1 + K1CNH3
N
2
3K1CNH3
K1CNH3
1 + K1CNH3
In the above rate expression for NO, the rate coefficients can be expressed in terms of the preexponential factor and activation energy16-20,32 ki ) ki0 exp(-(Ei/RT)), the equilibrium constant can be written as Ki ) ki/k- i ) (ki0T)/(k- i0 exp(-(Ei/RT)) ) Ki0T exp(Ei/RT), and thus the model can be used to fit the experimental data of the variation of rate with temperature. A nonlinear least-squares technique in Polymath 5.1 software was used to determine the parameters in the model. In this technique, the optimized values were obtained by minimizing the sum of squared differences of the experimentally observed rate, riexp, and the rate calculated by the model, rimodel. Thus,
Substituting eqs 15, 17, and 20 in eq 21 θV2 )
(24)
Substituting eq 22 in eq 15, the expression for θ“O” is obtained, which can be substituted in eq 23 to yield
(18)
3k4θ(NH3)Mθ“O”2 + 3k5θ(NH3)Mθ“O”θN“O” - 2k7θ“O”H2 ) 0 (19)
K1CNH3
rNO ) k4θ(NH3)Mθ“O”2
(17)
(riexp - rimodel)2 N-K
(26)
where N is the number of experimental data points and K is the number of parameters to be optimized. The experimental data were fitted to the above model for three catalysts. The number of data points (Figure 3) varies from 45 to 60, while the number of parameters (K) is 12. However, the range of activation energies and preexponential factors for the equilibrium constants, reactions 1a-3, are known in the literature. Further, the activation energies for reactions 4a, 5a, and 7 should be reasonable and be in the range published earlier. The initial guesses for the activation energies and preexponential factors for the rate coefficients were also taken from the literature.16-20,32 On the basis of these constraints, the equations were simulated and the optimized values were determined. A unique solution was found and is shown in Table 1. To determine whether other solutions could be obtained, the initial guesses were changed and the optimization routine yielded the same solution. Other solutions that are possible are not physically realistic, such as negative preexponential factors and positive activation energies. The optimized rate constant values in terms of temperature are given in Table 1 for the three catalysts. The calculated model fits well with the experimentally observed rate (Figure 3) for the three catalysts. It can be seen from Figures 2 and 3 that the rate of NO disappearance goes through a maximum and similarly the
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The activation energies for the reaction in the presence of these catalysts are in the ranges of 70-90 kJ/mol for step 4 and 55-70 kJ/mol for step 5 (Table 1). These activation energies are comparable to that reported in our previous study4 (58-75 kJ/mol) and that reported in the literature (40-90 kJ/mol) for zeolite based catalysts.33,34 From Figure 3, it is obvious that the rate of reduction for NO is in the order of Ti0.9Mn0.1O2-δ > Ti0.89Mn0.1Pd0.01O2-δ > Ti0.99Pd0.01O2-δ and the rate of ammonia oxidation at higher temperatures is in the opposite order. The rate of ammonia oxidation (step 4) thus plays an important role at higher temperatures. When the rate of ammonia oxidation is higher, then more NO is produced and the rate of NO reduction is reduced. Thus, the rate of ammonia oxidation for the different catalysts is in the reverse order to that of NO reduction.
Figure 3. Rate of NO reduction over three catalysts. The lines indicate model prediction.
concentration of NO goes through a minimum with the corresponding maximum concentration for N2 and N2O. The main reason for the maximum in the NO conversion is NH3 oxidation. The concentration profile of ammonia shows that the concentration decreases and then remains constant thereafter. One can also observe that the maxima in the N2 and N2O concentration profile correspond to the same temperature wherein the minima in the NO concentration profile is observed. This confirms the importance of ammonia oxidation in the reaction. If NO is merely reduced in the presence of NH3, then 1 mol of ammonia would produce 1 mol each of N2 and N2O (reactions A and B). However, if ammonia is oxidized, then 1 mol of ammonia would produce 1/2 mol each of N2 and N2O (reactions D and E). Thus, ammonia oxidation is the main reason for the maxima in the N2 and N2O concentration profiles. The NO in the reaction is both from the feed and from the oxidation of ammonia. In the reaction mechanism presented, the NO that reacts in step 3 is both from the feed gas and that produced from step 4. Step 4 represents the selective catalytic oxidation (SCO) of ammonia, while step 5 represents the reduction of NO by ammonia. The NO that is reduced in step 5 comes from the feed as well as NO produced in step 4. At lower temperatures, NO is hardly produced from step 4 and mainly comes from the feed. At high temperatures, a significant amount of NO is produced from step 4. To verify this, experiments were conducted for the oxidation of ammonia without NO in the feed.4 Ammonia can be oxidized to form NO, N2O, or N2.. It was found that at lower rates of oxidation of ammonia, the selectivity for nitrogen was the highest.4 The results obtained in the current study are in accordance with the above. At intermediate temperatures, the rate of formation of NO and N2O by NH3 oxidation is sufficient to support the reaction, and thus an overall decrease in the concentration of NO is observed. This causes the minima in the NO concentration profile. Since a significant amount of (NH3)ads is consumed for step 4 at higher temperatures, the (NH3)ads species available for the NO reduction in step 5 is comparatively less. This causes a maximum in the concentration profiles of N2O and N2. At higher temperatures, ammonia oxidation becomes less selective toward the N2 formation reaction and a decrease in the rate of disappearance of NO is observed.
This is also in accordance to the following rate parameter comparison across the catalysts. By comparing the rate constants for steps 4 and 5 over three catalysts from Table 1, it can be noted that the k5 values for the reduction step are on the order of Ti0.9Mn0.1O2-δ > Ti0.89Mn0.1Pd0.01O2-δ > Ti0.99Pd0.01O2-δ and k4 values for the NH3 oxidation step are in the opposite order of Ti0.99Pd0.01O2-δ > Ti0.89Mn0.1Pd0.01O2-δ > Ti0.9Mn0.1O2-δ. By comparing the rate constant values for the elementary steps, it can be concluded that NO formation (step 4) is the slowest step in the mechanism. This is in correspondence to the results observed in the literature.25,31 It is also observed that the rate constant value for the reduction step 5 is always higher than the oxidation step 4 both at lower temperatures and higher temperatures. Since the NO reduction (step 5) is much faster than its formation (step 4) in the mechanism, as soon as NO formed from step 4, it gets reduced in step 5. Thus, the maximum possible NO amount that can be accumulated in the NO + NH3 + O2 system will not be more than the concentration of NO in the feed gas mixture. This is in accordance with the concentration of the NO profile observed experimentally over three catalysts in the present study. By comparing the value of k5 to k4 across the catalysts, it can be found that the values are closer for Ti0.99Pd0.01O2-δ, which shows the NO concentration at the higher temperature region is equal to the NO concentration in the feed; i.e., only ammonia oxidation occurs at this region. However, for Ti0.9Mn0.1O2-δ, the k5 value is much higher than the k4 value and thus even at a higher temperature region the NO concentration is less than that in the feed, i.e., some amount of feed NO is reduced in addition to the NO produced from step 4 even at higher temperatures. The reaction mechanism indicates that as the reducibility of the catalyst increases, the SCR activity toward NO reduction decreases. This can be explained mathematically. The rates of reactions 4a, 5a, and 6 are proportional to ΘV22, ΘV22, and ΘV2, respectively. Thus, reaction 4a, which represents ammonia oxidation, is proportional to ΘV22, while the overall NO reduction is proportional to ΘV2. An increase in the reducibility of the catalyst leads to an increase in ΘV2, and thus, as the catalyst reducibility increases, the rate of ammonia increases much more than the rate of overall NO reduction. When the rate of ammonia oxidation is higher, then the rate of NO reduction reduces. Thus, the reducibility of the catalyst has a negative effect on SCR activity. Thus, a catalyst that shows higher activity for ammonia oxidation and higher reducibility in terms of hydrogen uptake exhibits lower activity for NO reduction with ammonia. This
9246 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
results in a base ion substituted catalyst exhibiting higher catalytic activity than a noble metal ion substituted catalyst. Conclusions (1) The role of NH3 oxidation in the SCR mechanism is welldescribed by the two main steps in the mechanism: NO reduction step and ammonia oxidation step. (2) The NO reduction step is faster at all temperatures than its formation step for the experimental data in this study. (3) The reducibility of the catalyst has a negative correlation with SCR activity. Thus, if the catalyst is more reducible, NH3 oxidation is better and SCR activity is poor. NO reduction follows the order Ti0.9Mn0.1O2-δ > Ti0.89Mn0.1Pd0.01O2-δ > Ti0.99Pd0.01O2-δ and the ammonia oxidation follows the reverse order, which is in accordance with the reducibility of the catalyst. (4) Substitution of a base metal and a noble metal in titania results in Ti0.89Mn0.1Pd0.01O2-δ but does not show any synergetic effect. (5) To achieve better SCR activity, the catalyst should have less reducibility. Thus, noble metal ion (Pd2+) substituted TiO2, which has better reducibility than the base metal ion (Mn3+) substituted TiO2, performs poorly for SCR. Future studies should investigate other base metal ion substituted titania that show even poorer reducibility, which may result in better SCR activity. Acknowledgment G.M. acknowledges the Department of Science and Technology, India, for financial support and the Swarnajayanthi Fellowship. Supporting Information Available: Figure S1, showing concentration profiles (%) at the outlet (a) of the reactants and (b) of the products over the three catalysts, Figure S2, showing rates of nitrogen formation over different catalysts, Appendix A1, giving the detailed reaction mechanism, and Appendix A2, discussing the derivation of the rate of nitrogen formation. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Tuenter, G.; Leeuwen, W. F.; Snepvangers, L. J. M. Kinetics and mechanism of the NOx reduction with NH3 on V2O5-WO3-TiO2 catalyst. Ind. Eng. Chem. Prod. Res. DeV. 1986, 25, 633. (2) Djerad, S.; Crocoll, M.; Kureti, S.; Tifouti, L.; Weisweiler, W. Effect of oxygen concentration on the NOx reduction with ammonia over V2O5sWO3/TiO2 catalyst. Catal. Today 2006, 113, 208. (3) Granger, P.; Dacquin, J. P.; Dhainaut, F.; Dujardin, C. The formation of N2O during sNOX conversion: Fundamental approach and practical developments. Stud. Surf. Sci. Catal. 2007, 171, 291. (4) Roy, S.; Viswanath, B.; Hegde, M. S.; Madras, G. Low-temperature selective catalytic reduction of NO with NH3 over Ti0.9M0.1O2-δ (M ) Cr, Mn, Fe, Co, Cu). J. Phys. Chem. C 2008, 112, 6002. (5) Roy, S.; Hegde, M. S.; Ravishankar, N.; Madras, G. Creation of redox adsorption sites by Pd2+ ion substitution in nano TiO2 for high photocatalytic activity of CO oxidation, NO reduction, and NO decomposition. J. Phys. Chem. C 2007, 111, 8153. (6) Pena, D. A.; Uphade, B. S.; Reddy, E. P.; Smirniotis, P. G. Identification of surface species on titania-supported manganese, chromium, and copper oxide low-temperature SCR catalysts. J. Phys.Chem. B 2004, 108, 9927. (7) Liotta, L. F.; Longo, A.; Macaluso, A.; Martorana, A.; Pantaleo, G.; Venezia, A. M.; Deganello, G. Influence of the SMSI effect on the catalytic activity of a Pt(1%)/Ce0.6Zr0.4O2 catalyst: SAXS, XRD, XPS and TPR investigations. Appl. Catal., B 2004, 48, 133. (8) Thomas, C.; Gorce, O.; Fontaine, C.; Krafft, J. M.; Villain, F.; Mariadassou, G. D. On the promotional effect of Pd on the propene-assisted decomposition of NO on chlorinated Ce0.68Zr0.32O2. Appl. Catal., B 2006, 63, 201.
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ReceiVed for reView July 16, 2008 ReVised manuscript receiVed September 19, 2008 Accepted September 19, 2008 IE8010879
