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Selective Catalytic Reduction of Nitric Oxide by Ammonia over V2O5/TiO2, V2O5/TiO2/SiO2, and V2O5-WO3/TiO2 Catalysts: Effect of Vanadia Content on the Activation Energy Michael D. Amiridis* Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208
Jeffrey P. Solar† Research Division, W. R. Grace & CompanysConn., Columbia, Maryland 21044
A systematic variation of the V2O5 content of the title catalysts reveals a dependence of the activation energy of the selective catalytic reduction of nitric oxide by ammonia on the surface concentration of vanadia. A significant decrease in activation energy (from approximately 80 to 40 kJ/mol) is observed, as the vanadia surface concentration is increased (from 0.3 to 8 µmol/ m2). The same results were obtained from tests conducted both in the presence and absence of H2O and SO2. A possible mechanistic explanation is discussed. Introduction Selective catalytic reduction (SCR) of nitric oxide by ammonia has been used for over 10 years for the removal of nitrogen oxides from power plant and industrial boiler flue gases. The most common catalysts in commercial practice are those based on vanadia, or mixtures of vanadium and tungsten oxides, supported on titania (Bosch and Jannsen, 1988). Recent work has focused both on improvements of these catalysts and on the development of an understanding of the mechanism of the SCR reaction. The reaction between NH3 and NO, in the presence of oxygen, has been found to satisfy the following stoichiometry:
4NO + 4NH3 + O2 f 4N2 + 6H2O Various mechanistic schemes for this reaction have been proposed. Since the ammonia is known to be strongly adsorbed on the catalyst surface, both Langmuir-Hinshelwood (Lintz and Turek, 1992) and EleyRideal (Topsøe et al., 1995) kinetics have been used to describe the system. An extensive body of literature exists on the characterization of V2O5/TiO2 surfaces. The presence of multiple structures of vanadium oxide on the titania surface has been demonstrated using both vibrational (laser Raman and infrared) (Wachs, 1990; Went et al., 1992a; Busca, 1988) and NMR spectroscopies (Eckert and Wachs, 1989). Monomeric and polymeric vanadyl species have been observed on titania (and other oxides) at coverages below a monolayer. The relative distribution of these different types of vanadia structures has been shown to vary with the surface vanadia concentration. Similar surface structures are also present on V2O5/TiO2/SiO2 catalysts (Solar et al., 1992). Attempts have been made to correlate the structure of the vanadia sites to their catalytic activity. Nobe and coauthors, for example, noted a decrease in the SCR activity of V2O5-based catalysts at high loadings (Bauerle et al., 1978). More recently, Went et al. (1992b) studied a series of catalysts with differing loadings of * Author to whom correspondence is addressed. E-mail:
[email protected]. Fax: 803-777-8265. † Current address: BDM Federal, Gaithersburg, MD 20878.
0888-5885/96/2635-0978$12.00/0
vanadium oxide on titania. They proposed that the specific activity of the polymeric vanadates is substantially higher than that of the monomeric species. Baiker et al. (1992) have also correlated changes in the turnover frequency for SCR with the distinct vanadia structures which are present at varying V2O5 loadings. None of these reports, however, addressed differences in the temperature dependency of the catalytic performance. Other studies of the SCR reaction over V2O5/TiO2 catalysts have reported activation energies ranging from 94 to 40 kJ/mol (Tronconi et al., 1992; Wong and Nobe, 1984, 1986; Miyamoto et al., 1982). In this paper, we examine changes in the activation energy of the SCR reaction with the vanadia content. By systematically varying the vanadia loading in a variety of catalysts, we show that the activation energy decreases significantly as the vanadia coverage of the catalyst increases. This is true both in the presence and in the absence of H2O and SO2 in the reacting gas mixture, for V2O5/TiO2, V2O5/TiO2/SiO2, and V2O5-WO3/ TiO2 catalysts. Furthermore, our studies suggest an explanation for the range of activation energies reported by other workers. Experimental Methods The V2O5/TiO2 catalysts were prepared by incipient wetness impregnation of vanadium citrate solutions onto Kemira Grade 905 TiO2 which had been calcined at 773 K prior to impregnation. The citrate was decomposed by calcination at 723 K for 6 h and 793 K for 3 h. The preparation of the V2O5/TiO2/SiO2 catalysts also followed the procedure outlined above, utilizing TiO2/SiO2 supports prepared according to the procedure described in U.S. Patent 4,929,586 (Hegedus et al., 1990). Finally, the V2O5-WO3/TiO2 catalysts were prepared by incipient wetness impregnation of a vanadium oxalate and ammonium metatungstate solution onto Kemira Grade 907 TiO2. To facilitate studies of catalysts with varied surface-specific V2O5 loading, the TiO2 was calcined using differing profiles. This pretreatment resulted in different surface areas prior to impregnation. Following the impregnation step, the catalysts were further calcined for 2 h at 623 K. The activity tests were carried out in a stainless steel one-pass flow reactor. The reacting gases were mixed © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 979 Table 1. Simulated Flue Gas Composition component
“wet” conditions
“dry” conditions
NO NH3 SO2 H2O O2 N2
400 ppm 400 ppm 800 ppm 8% 4% bal.
900 ppm 900 ppm 4% bal.
and preheated prior to the reactor entrance. During some experiments, H2O and SO2 were added to the reacting gas mixture to simulate realistic conditions for commercial SCR operation. The two different gas compositions utilized in this work are shown in Table 1. Analyzed certified mixtures of 12% NO in N2, 5% NH3 in N2, and 2% SO2 in N2 were used as the sources of the flue gas components. Air was mixed in as the source of O2 and N2 as the carrier gas. Water was introduced to the system through a high-performance pump (Shimadzu). A thermocouple projecting into the center of the catalyst bed was utilized for measurements of the catalyst temperature. The NO concentration at both the inlet and outlet of the reactor was analyzed by the use of a chemiluminescent analyzer (Thermo Electron). Each run utilized approximately 0.1 g of catalyst in the form of 40/60 mesh particles. The total flowrate over the catalyst was controlled at approximately 3300 standard cm3/min (200 standard L/h). Results and Discussion Rate constants were estimated by treating the reactor as a plug flow or integral reactor, according to the following equation:
k ) -FNO/(mcatCNO) ln(1 - x)
Figure 1. Arrhenius plot for the SCR activity of V2O5/TiO2 catalysts: (O) 0.3 µmol of V2O5/m2; (0) 3.7 µmol of V2O5/m2. Table 2. Comparison of Activation Energies in the Presence and Absence of H2O and SO2
catalyst
V2O5 content
“wet” activation energy (kJ/mol)
V2O5/TiO2 V2O5/TiO2 V2O5/TiO2/SiO2
0.3 µmol/m2 3.7 µmol/m2 3.5%
85 45 49
“dry” activation energy (kJ/mol) 96 38 49
Table 3. Activation Energies of V2O5/TiO2/SiO2 Catalysts under Dry, SO2-Free Conditions V2O5 content (%)
activation energy (kJ/mol)
V2O5 content (%)
activation energy (kJ/mol)
1.0 2.0 3.5 3.9
65 43 49 46
6.6 8.5 11.1 15.9
40 41 39 37
(1)
This equation assumes a first-order dependence on NO and a zero-order dependence on NH3, in agreement with numerous published results over V2O5-based SCR catalysts (Marangozis, 1992). Under our experimental conditions, internal diffusion limitations were important for some of the catalysts tested. Therefore, intrinsic rates were calculated according to standard correction methods (Beeckman and Hegedus, 1991). In particular, the model of Wakao and Smith (1962) was used to calculate the effective diffusion coefficients. For similar catalysts, coefficients thus obtained were found to be within 10% of the experimentally measured values (Beeckman, 1991). The experiments with the 5% V2O5/TiO2 catalyst were repeated using three different particle sizes (7/10, 40/ 60, and 120/200 mesh). In all cases the intrinsic rates and activation energies were reproduced within 10%, validating the correction methods. The observed conversions in this study varied between 5 and 50%, while the calculated effectiveness factors varied between 0.4 (for the 5% V2O5/TiO2 catalyst) and 0.9 (for the V2O5/ TiO2/SiO2 catalysts). The intrinsic SCR activities for a pair of V2O5/TiO2 catalysts containing 0.3 and 3.7 µmol/m2 V2O5 (0.5 and 5.1%, respectively) were measured in the presence of H2O and SO2 at temperatures between 548 and 648 K. Within this temperature range, the data give a good fit to straight lines in an Arrhenius plot (Figure 1). Activation energies of 85 and 45 kJ/mol were calculated from the fit to the data for the low and high vanadium content catalysts, respectively. Similar results were also obtained under dry, SO2-free conditions (Table 2),
suggesting that the presence of these components in the flue gas has little effect on the activation energy. This appears to be the case regardless of the level of vanadia loading or the type of support used. The same dependence of activation energy on V2O5 content was also observed with catalysts prepared using a mixed TiO2/SiO2 support. In this case, V2O5 contents are expressed on a mass percentage basis, since the determination of the V2O5 surface concentration on the TiO2 phase requires knowledge of both the amount of V2O5 associated with TiO2 and the surface area of TiO2, which cannot be directly obtained from analytical characterization. These studies, carried out under dry, SO2-free conditions, reveal a sharp initial drop in the activation energy at low loadings and a slower decline as the V2O5 level is further increased (Table 3). Finally, we have determined the activation energy of V2O5-WO3/TiO2 catalysts containing 0.3-0.5 µmol/m2 (0.7-0.9%) WO3 and 0.6-7.8 µmol/m2 (1-12%) V2O5. While WO3/TiO2 is not an effective SCR catalyst, WO3 is frequently used as a promoter in commercial V2O5/ TiO2 catalysts. The results of these studies are shown in Figure 2. Once again, the activation energy decreases significantly as the V2O5 surface concentration is increased up to 2 µmol/m2, while a slower decrease is observed at higher vanadia levels up to monolayer coverage (6-7 µmol of V2O5/m2). Reported literature values for the SCR activation energy vary between 94 kJ/mol reported for a “commercial SCR catalyst” (Tronconi et al., 1992) and 4055 kJ/mol reported for V2O5/TiO2 catalysts loaded with 5-10% V2O5 (Wong and Nobe, 1984, 1986; Miyamoto et al., 1982) and bulk V2O5 (Baiker et al., 1987). This
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Conclusions We have examined the effect of the vanadia surface coverage on the activation energy of the SCR reaction by systematically varying the vanadia loading on a series of V2O5/TiO2, V2O5/TiO2/SiO2, and V2O5-WO3/ TiO2 catalysts. For all three catalyst series, our results show a significant decrease in activation energy as the vanadia surface concentrations increase up to 2 µmol/ m2 and a slower decrease at higher vanadia levels. The same trend is observed both in the presence and in the absence of H2O and SO2. Acknowledgment Figure 2. Effect of V2O5 surface concentration on the activation energy of V2O5/TiO2 and V2O5-WO3/TiO2 catalysts: (O) V2O5/TiO2; (0) V2O5-WO3/TiO2 (V2O5 surface concentration varied by varying the V2O5 weight percent); (4) V2O5-WO3/TiO2 (V2O5 surface concentration varied by varying the TiO2 surface area).
apparent discrepancy is explained by our results, since commercial SCR catalysts usually contain low amounts of V2O5 (on the order of 1%). When the V2O5 content is considered, these previously reported values are in good agreement with the activation energies observed in this study. The changes observed in the activation energy with the V2O5 content (30-40 kJ/mol) would result in changes in the overall rate of the reaction at 623 K of more than 3 orders of magnitude. Although we have observed an effect of the V2O5 content on the rate of the SCR reaction, this effect is not so dramatic. At 623 K, for example, the rate of V2O5/TiO2 catalysts increases with the V2O5 surface concentration in the 0.5-3.5 µmol/m2 range. The total increase observed, however, is less than 1 order of magnitude (see Figure 1). This is because the overall preexponential factors also change with the V2O5 content (from 6.4 × 109 to 8.4 × 106 cm3/ g‚s) and partially compensate for the change in the activation energy. Topsøe et al. (1995) have recently proposed a catalytic cycle for the SCR reaction over V2O5/TiO2 catalysts which is in agreement with a variety of available kinetic and spectroscopic data. This mechanism suggests that, at the O2 partial pressures employed in our work, both the activation of NH3 toward a partially dehydrogenated NHx species and the subsequent reaction of NHx with NO are kinetically significant steps. The observed changes in the activation energy with the vanadia content may be related to changes in both of these steps. We expect, for example, that an increased surface density of VOx groups facilitates the NH3 activation process, by providing an additional adsorption site for the dissociation products adjacent to the adsorbed NH3. The subsequent reaction of NHx with NO may also be effected by changes in the vanadia content in the following way. It has been reported that the dominant sites in an ammonia TPD experiment change with V2O5 content (Srnak et al., 1992). At a low loading (0.6% V2O5/TiO2), sites with enthalpies of 92 and 109 kJ/mol are prominent, while the sites which adsorb ammonia less strongly (approximately 75 kJ/mol) are important for samples with higher loadings (6%). It is therefore possible that, in the higher vanadia content catalysts, the weaker bonding between NHx and the catalytic site facilitates the reaction with NO and, hence, the reduction in the activation energy.
The authors acknowledge valuable discussions with C. J. Pereira, the experimental support of P. Myers, C. Paris, and R. Perlish, and W. R. Grace & Co.sConn. for permission to publish this work. Symbols k ) reaction rate constant (cm3/g‚s) FNO ) molar feed rate of NO to the reactor (mol/s) mcat ) mass of the catalyst in the reactor (g) CNO ) concentration of the NO at the inlet of the reactor (mol/cm3) x ) fractional conversion of NO across the reactor
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Received for review July 20, 1995 Revised manuscript received December 6, 1995 Accepted December 29, 1995X IE950452Y X Abstract published in Advance ACS Abstracts, February 1, 1996.
