J. Phys. Chem. 1994,98, 7153-7161
7153
Isotopic Labeling Studies of the Effects of Temperature, Water, and Vanadia Loading on the Selective Catalytic Reduction of NO with NH3 over Vanadia-Titania Catalysts Bronwyn L. Duffy and H. Edward Curry-Hyde' School of Chemical Engineering and Industrial Chemistry, P.O. Box I , Kensington, New South Wales 2033, Australia
Noel W. Cant School of Chemistry, Macquarie University, New South Wales 2109, Australia
Peter F. Nelson' CSIRO Division of Coal and Energy Technology, P.O. Box 136, North Ryde, New South Wales 2113, Australia Received: February 8, 1994; In Final Form: May 3, 1994'
Isotopic labeling studies of the reaction between l5NOand 14NH3have been performed over a range of vanadiatitania-based SCR catalysts (pure VtO5 and catalysts containing 1.4-23.2 wt % V205) for the extended temperature range of 200-500 OC. For temperatures less than 350 O C , I4Nl5N is always the major product. At higher temperatures, however, product distributions are very sensitive to vanadia content; ammonia oxidation to 14NO is particularly dominant for pure VZOSand, at 500 OC,accounts for more than 70% of the nitrogen-containing products. Pure VZOSalso produces significantly more 14N'SN0,and at much lower temperatures, than that observed for a 1.4 wt % Vz05/Ti02 catalyst. On the basis of these results it is clear that ammonia oxidation to I4NO is the major reason for the observed decrease in the N O conversion over vanadia-based catalysts at temperatures greater than 400 OC. Ammonia oxidation to nitrogen and nitrous oxide is less significant; I4NzO and 14N2each comprise less than 10%of the total products for both the pure and supported vanadia catalysts. Addition of 1.6% water decreases the amount of nitrous oxide (largely I4N15NO)produced over the supported catalyst at 450 OC by over 90%. A simultaneous increase in the amount of l4NISN is also observed. The presence of water also suppresses the 14NH3oxidation to 14N2,I4N2O, and 14N0,even at 500 OC. By contrast, for pure VzO5 at 500 OC, water has a relatively minor effect on the product distribution, and the major product remains 14N0. In general, high temperatures, dry feed gas conditions, and high vanadia contents favor both the production of l4N'5NO relative to l4NIsN and the ammonia oxidation reaction producing 14N0. Results from this and previous studies suggest that there is a relationship between NzO formation and NH3 oxidation capability.
Introduction Titania-supported vanadia is a highly active catalyst for the selective catalytic reduction (SCR) of NO with NH3 in the presence of O2 and is the most commonly used catalyst in the commercial process for reducing nitrogen oxide emissions from power stations.' Thevanadia loading isoptimized for a particular application and will depend on, inter alia, the degree of control required and the SO2 concentrationin the flue gas. High loadings lead to enhanced rates of conversion of SO2 to SO3 and are only used for low sulfur fuels, since fouling problems due to the formation of ammonium compounds,such as bisulfate, have been encountered in the practical application of this technology.' Vanadia loading has also been shown to affect the activity of the catalysts for NO conversion over a variety of supports. A number of studies" have demonstrated that maximum activity coincides with a loading of 10-20 wt % VZOS. Industrial applicationsof these catalysts thus require a compromisebetween maximizing NO conversion and minimizing SO2 oxidation. Rather less attention has been given to the effects of vanadia loading on the relative contributions of, on the one hand, the selective reaction of NO with NH3 and, on the other, the competitive oxidation of NH3. It is clear that at temperatures less than about 350 OC the selective reaction to produce Nz dominates.' At higher temperatures, however, N O conversion Abstract published in Advance ACS Absrracrs, June 15, 1994.
decreases and ammonia oxidation becomes increasingly important.' This featureof the behavior of V205/Ti02 catalysts, when taken together with their ability to oxidize S02, effectively determines the maximum catalyst operating temperature. Isotopic labeling experiments are the most definitive way to determine the relative rates of the selective and nonselective reactions and the products of ammonia oxidation, particularly under competitive reaction conditions. Some previous labeling studies of vanadia-based systems>' have been performed, one of which used reactant concentrations very different from those found in flue gases. Additionally, none of these studies has examined the effects of vanadia loading or of temperatures greater than 400 OC. Similarly, inadequate attention has been given to the effects of water, which is always present in'flue gases at high concentrations, on the isotopic product distributions. In this study reactant conversions and product distributions have been determined for pure VZOSand a series of V205/Ti02 catalysts with vanadia loadings of 1.4-23.2 wt %. The experiments were conducted at temperatures in the range 185-500 OC to encompass the full range of possible operatingconditions. Specific aims of this study were threefold: for the range of vanadia loadings, (1) to evaluate the relative contributions of the selective reaction and the ammonia oxidation reaction to product formation; (2) to determine, more accuratelythan previously possible using mass spectrometry alone, the products of the ammonia oxidation reactions; and (3) to determine the effects of added water on the reactions and product distributions.
0022-3654/94/2098-7 153%04.50/0 0 1994 American Chemical Society
Duffy et al.
7154 The Journal of Physical Chemistry, Vol. 98, No. 29, 1994 Experimental Section The preparation of the catalysts and the analytical techniques used in this study have been reported previously.* Some pertinent additional details are given here. Supported catalysts were prepared by incipient wetness impregnation of Ti02 support (Degussa, P25) with an oxalic acid solution of ammonium metavanadate. After impregnation, the materials were dried in air at 60 OC and calcined in air at 500 OC for 3 h. Unsupported V205 was prepared by decomposition of pure ammonia metavanadate in air at 500 OC for 3 h. After calcination, the catalysts were crushed and the 300-500 Nm fraction was separated for use. BET surface areas of the catalysts were determined by N2 adsorption at 77 K using a Quantachrome surface area analyzer (QS- 17). X-ray diffraction (XRD) patterns were measured with a Philips PW 1700 automated powder X-ray diffractometer equipped with a graphite monochrometer. Diffraction patterns were recorded with Cu Ka radiation, and the sample was analyzed as a step-scan using a step interval of 0.02' (28)with a count time of 0.8 s per step over an angular range 2-70° (28). Vanadium contents were determined by ICP-AES after digestion in sulfuric acid. Catalytic testing was carried out in a continuous flow system at atmospheric pressure. The feed stream was made by blending mixtures of NO/He, NH3/He, and 02/He with additional He to provide a resultant mixture of composition 900 ppm NO, 900 ppm NH3, and 18 000 ppm 0 2 . When desired, water could be introduced into the reactant mixture by diverting the combined 0 2 / H eand He stream through a standard laboratory glass water bubbler equipped with a sintered frit and immersed in a constant temperature water bath. A range of water vapor pressures was obtained by varying the bath temperature. Background levels of water in the dry feed gas were about 0.05% and arose, for the most part, from the NH3/He mixture. The reactants were passed over 25-100 mg of catalyst contained in a quartz tube of internal diameter 5 mm and heated to temperatures between 185 and 500 OC. The exit gas was monitored continuously by a mass spectrometer with periodic sampling for analysis by gas chromatography (Hewlett-Packard 5890 with Chromosorb 102 column operated cryogenically) and off-line Fourier transform infrared (FTIR) spectroscopy (Digilab FTS 15/80). Thesteadystate activity and selectivity of the catalysts at each condition were first determined using unlabeled mixtures (I4N0/I4NH3) before I4NO was substituted with 15NO(Isotec Inc., 99.4% l5N), and after the switch to ISNO was made, the reaction was continued until the mass spectrometer signals were stable. The quadrupole mass spectrometer was operated in multiple ion monitoring mode and component intensities calculated using a least squares fitting procedure, assuming fragmentation was unaffected by isotopic substitution. The least accurate analyses are for I4N2(due to a substantial background signal at m / e 28) and I5N2 (due to interference by 14NO produced by oxidation of ammonia on the filament). With a low filament current (0.2 mA) ammonia oxidation was negligible with feeds free of oxygen but reached the equivalent of a90 ppm for mixtures containing 900 ppm NH3 and 18 000 ppm oxygen. Signals a t m / e = 30 were adjusted for such contributions using a second-order correction function established by regression of data on the amount of 14N0formed in the mass spectrometer with mixtures containing various concentrations of l5NO and 14NH3. FTIR was used to assess the amounts of I4NO formed by the reaction itself, and of 14N15N0versus 15N14NO. Direct 15NO/ 14NH3 isotopic exchange was discounted as 1SNH3 and '4NO were never observed in the infrared spectra concurrently. Spectra were recorded a t a resolution of 0.25 cm-1 on samples collected for off-line analysis by passing the product stream through a multiple reflection cell of total path length 3.2 m. Signal accumulation was restricted to 16 scans when NO and NH3 were analyzed since their concentrations declined steadily with time
TABLE 1: Vanadia Content and Surface Areas of the Series of V205-BasedCatalysts vanadia content surface catalyst (wt % ' vios) area (mZ/g) Ti02 0.0 43 V205/Ti02 1.4 42 VzOs/Ti02 3.O 46 VzOs/Ti02 6.6 33 V205/Ti02 23.2 28 v205
100.0
5
due to a combination of oxidation of the former to NO2 and adsorption or deposition on cell surfaces. Back-extrapolation of infrared intensities to the time of collection showed that NO2 concentrations were then less than 5% of the inlet N O concentration. Nitrous oxide concentrations were stable, which allowed the use of 256 scans to obtain better signal-to-noise ratios. Conversions of 15N0and NH3 were determined from the mass spectral analysis of m / e = 31 and m / e = 17, respectively. In experiments with added water, the latter are less certain due to a contribution from fragment ions of H2O ( m / e = 18). However, conversionscould also be obtained from the FTIR measurements using the sR (J = 10) line of 14NH3a t 1 1 76.6cm-1 and the '$NO line a t 185 1.2cm-1. In most cases there was agreement to within f5% with similarly good agreement between the mass spectral and gas chromatographic analyses for total nitrogens and nitrous oxides.
Results and Discussion Effects of Vanadia Content and Temperature. The vanadia contents and surface areas of the series of VzOs-based catalysts (ranging from 1.4wt % V205/Ti02 to pure V205) are given in Table 1, together with that of the Ti02 material used both as support material and, for some experiments, as a catalyst bed diluent. Figure 1 shows the corresponding XRD patterns for the region from 10 to 35' 20. The sample of Ti02 shows only those features expected for an anatase/rutile mixture as expected for this typeof fumed titania (Degussa, P25). The supportedcatalysts with 1.4 and 3.0wt % V205 also have patterns similar to that of the support. At 6.6wt % V205 and above, however, additional features due to crystalline vanadia become evident, as confirmed by a comparison with the XRD pattern of the 100% V205 sample. Previous s t ~ d i e susing ~ - ~ laser ~ Raman spectroscopy (LRS) have shown that crystallites of V205 form on TiO2-supported catalysts if the vanadia loading increases above the dispersive capacity of the support. At lower loadings the vanadia is present in the form of monomeric and polymeric vanadyls with the monomeric vanadyls predominating at low loadings;"2 in the LRS studies, crystalline vanadia was first detected at about 10 wt % V205, in good agreement with the present study. The catalyst samples were further characterized by infrared spectroscopy,and Figure 2shows absorbance spectra in the region 1075-725 cm-1 for samples of pure V205,23.2wt % V205, and 6.6wt % V2O5 in the form of KBr pellets. Absorption due to the pure Ti02 support has been subtracted where applicable. Previous workersl2-15 have assigned the feature a t 1020cm-1 to the V=O stretching vibration, and that at about 820 cm-I to V-0-V linkages. Another broad feature at about 980 cm-l is observed for 6.6wt % V205/Ti02. This feature has been attributed9J2J4 to the V=O stretching vibration of distorted surface vanadate species present on Ti02 in a monolayer, the strained structure bringing about a shift in the vibrational frequency from 1020 to 980 cm-1. The spectra show that the ratio of the V=O feature to that of the V-0-V feature is highest in the pure vanadia sample and decreases with decreasing amounts of V2O5. The relationship of this characteristic of the catalysts to their activity for ammonia oxidation is discussed below. The performance of this series of catalysts for the reaction of 14N0with 14NH3was determined as a function of temperature
The Journal of Physical Chemistry, Vol. 98, No. 29, 1994 7155
Selective Catalytic Reduction of NO with NH3
15
20
25
:
30
2 0 (degrees)
Figure 1. XRD patterns for V20s and V20s/Ti02 catalysts and Ti02 support.
00
r
1000
1000
900
800
900
800
,
Wavenumbera (cm")
Figure 2. FTIR spectra of VzOs and VzOs/Ti02 catalysts (prepared as KBr pellets) in the region 1075-725 cm-'. Features due to the Ti02 support were subtracted from the 23.2 and 6.6 wt B VZOs/Ti02 spectra; the vertical line is drawn at the position assigned to the V = O stretching vibration.
in the range 200-500 O C in the presence of 1.8%0 2 . The upper part of Figure 3 shows data for the two catalysts with lowest
vanadium contents, whereas the lower part shows results for the three catalysts which contain crystalline V205. Note that when unlabeled reactants are used, the NO present in the feed gas cannot be distinguished from the NO produced by catalytic oxidation of ammonia. Consequently, selectivities to N 2 0 can only be calculated as percentages of the nitrogen and nitrous oxide produced and not as a percentage of the total reaction products. Negative conversions of NO, observed for pure V205 and 23.2 wt 9% V205/Ti02, are a consequence of higher amounts of NO in the product gases from the reactor than in the reactant gases (Le. NH3 oxidation to N O in excess of NO reduction by the SCR reaction). Discussion of the effects of vanadia loading is probably best done by comparison of the results for the 1.4 wt % V205 sample and pure V205. At temperatures up to 300 OC both catalysts convert NO and NH3 in a 1:l ratio; pure vanadia exhibits a selectivity to N20 of approximately 10% at 300 OC,whereas the supported catalyst has low selectivity to N2O even at 400 OC.At higher temperatures much greater differencesemerge. For pure V205 significantlyless NO is converted than NH3; at 500 OC the apparent conversion of NO is negative. Selectivity to N20, as a proportion of the total NZ and N20 produced, increases to approximately 70% (Figure 3), whereas that for the supported material is approximately 40% at 500 OC. The only totally satisfactory way to determine the source of the various productsunder these conditions is by means of isotopic labeling experiments. Most of the experiments were performed with the supported catalyst with the lowest V205 loading (1.4 wt %) and pure vanadia, since these materials showed the greatest differences in NO conversions and selectivities. Table 2 shows the range of conditions used with ISNO with 14NH3 mixtures and the corresponding product distributionsobtained. The activity of the supported vanadia catalysts was so high at temperatures greater than 300 OC that experiments conducted above this temperature were confined to the 1.4%V205/Ti02 catalyst which was diluted 1:2 with a Ti02 support of the same particle size. As can be seen from Table 2, Ti02 itself exhibited significant activity in blank tests at 450 OC and gave NO and NH3 conversions to 14N15N approximately 20% of those observed with the diluted V205/Ti02 sample. Table 2 also shows the isotopic distributions derived from the MS and FTIR measurements. The mass spectral analyses are derived from signals with m / e of 28,29, and 30 (for the nitrogens) and m / e 44,45, and 46 (for the nitrous oxides). There are two potential ambiguities. First, the signal at m / e of 30 could arise from 14N0or from I5N2, FTIR spectra were used to determine the concentrations of I4NO produced, and Figure 4 gives an example of the determinationof 14N0in the presence of unreacted 15N0. This technique provides a better method for determining the amounts of '4NO and 15NO in this system than does mass spectrometry. When significant amounts of 14N0 (>200 ppm) were produced from ammonia oxidation,concentrationsof l4NO determined from FTIR spectra showed reasonable agreement with the values calculated from mass spectrometry data, indicating that negligible amounts of lSN2 are produced under these conditions. Past studies using 15NH3as the labeled species also suggest that NO decomposition to N2 is negligible in the presence of excess oxygen.' Second, the signal with m / e 45 could be due toeither 14N15NO or 15N14NO. FTIR spectra showed bands characteristic of I4N15NO(vI = 1280.4 cm-', v2 = 575.4 cm-l, and v3 = 2177.7 cm-l) but no trace of absorption at the positions expected for 15N14N0(vI = 1269.9 cm-1, v2 = 583.3 cm-l, v3 = 2201.6 cm-I). Thus, all the nitrous oxide made from a combination of 15NO and 14NH3 retains the starting nitrogen-xygen bond. Figure 5 shows the relative contribution of the various species to the total nitrogen-containing products as a function of temperature for the experiments without added water (i.e. with
7156 The Journal of Physical Chemistry, Vol. 98, No. 29, 1994
Duffy et al.
c
2
E" Z
U Q
8 200 300 400 500
200 300 400 500
Temperature ("C)
Temperature ("C)
50
25 E"
z
......
'1
I
................I?......
U
5
8
......
i
1
23.2 wt% V,O, I TIO,
v,p,
,
,
1
\?
200 300 400 500
200 300 400 500
200 300 400 500
Temperature ("C)
Temperature ("C)
Temperature ("C)
-25
Figure 3. N O ( 0 )and NH3 (0)conversions and selectivity to NzO (V) as a function of temperature for V205 and V205/Ti02 catalysts. Negative conversions of N O imply higher concentrations of N O in product gas than in reactant gas. Conditions: (a-c), 20 mg of catalyst diluted with 40 mg Ti02 and a flow rate of 80 mL/min; (d) 50 mg of catalyst and a flow rate of 80 mL/min; (e) 50 mg of catalyst and a flow rate of 40 mL/min.
TABLE 2 Product Distributions and Experimental Conditions for the Reaction of 15N0 and 14NH3 in 1.8%0 2 over V205/Ti02 Catalysts at Temperatures from 185 to 500 OC
catalyst
now mass rate H2O selectivity* "N I NO/ temp of cat (mL/ conc conversion' (%) to N 2 0 product distributione (ppm) ('4N15N ("C) (mg) min) (a) "NO "NH, (96) 14N2 I4Nl5N I5N2 '"20 14N15N0 15N20 14N@ "NO l4Nl5NO)(96)
V205/Ti02 185 (3.0wt % V205) V205/Ti02 185 (1.4wt% V2O5) 350 450 500 450 450 500 500 200 350 450 500 500 500 Ti02 450
+
100
40 0.05
89
85
1
8
774
-
0
0
0
0
84
0
100 2od 204 2od 3W
40 0.05 80 0.05 160 0.05 80 0.05 120 0.05 120 1.60 80 0.05 80 1.50 40 0.05 40 0.05 40 0.05 40 0.05 40 0.05 40 1.70 160 0.05
51 89 82 60 80 82 66 75 21 68 37 11 16 13 19
40 85 89 96 87 73 98 85 20 72 92 91 98 97 20
1 1
4 0
21 37 23 2 43 9 4 28 41 18 19 12 7
-4
345 827 655 292 510 601 314 517 152 384 95 39 40 42 189
-
2 3 5 44 7 -10 43 3 7 5 69 74 59 42 1
0 8 167 223 151 13 280 60 0 146 219 79 85 61 15
0 0 0 2 0 0 2 0 0 1 2 1 2 1 0
0 0 0 100 0 0 80 69 0 0 277 513 515 629 0
356 103 190 395 140 133 306 247 648 274 601 827 751 729 730
0 0 20 43 23 2 47
3W 204 204 50 50 50 50 50 50 50
63 21 6 44 34 0 -5 52 53 79 58 18
-
10
0 28 70 67 68 59 7
+
By Fourier transform infrared spectroscopy. Defined as (N20/(N2 NzO + 14NO)). By mass spectrometry. 20 mg of 1.4 wt % V205/Ti02 diluted with 40 mg of Ti02 catalyst on stream at 185 "C for 24 h. 30 mg of 1.4wt % VzOs/Ti02 diluted with 20 mg of Ti02 catalyst on stream a t 450 "C for 240 h.
only about 500 ppm H20, which is the amount present in the reactant gases). One principal difference between the 1.4 wt % V205/Ti02 catalyst and the pure vanadia is their propensity for ammonia oxidation to NO. This reaction is particularly dominant for the pure vanadia catalyst and, at 500 O C , accounts for more than 70% of the nitrogen-containing products. As may be calculated from the data of Table 2, approximately 40% of all the N O leaving the reactor is 14N0, derived from ammonia. Another significant difference is the amount of 14NlSNOproduced. Figure 5 and the final column of Table 2 also show the selectivity to I4Nl5NO as a percentage of the products formed by a combination reaction of I5NO and 14NH3, Le. 14N15N and 14NlSNO. For pure vanadia, I4N15NO production becomes significant at much lower temperatures (250 "C compared to 400 O C for supported vanadia). The other ammonia oxidation
products, 14N2and 14N20,each comprise less than 10%of the total products for both 1.4 wt % VzOs/Ti02 and pure V205. Effects of Water. As noted in the introduction, water is always present in flue gas at high concentrations. In this study the effects of water addition on conversions, selectivitiesto NzO, and isotopic product distributions were studied, particularly at high temperatures. Figure 6 shows NO conversions and selectivities to N20 as a function of time on stream for the 1.4 wt % V205/Ti02 catalyst a t 190 and 450 O C . At the lower temperature, addition of water has an immediateand significant effect on NO conversion; at this temperature selectivities to N20 are insignificant. At higher temperatures, addition of water has only small effects on N O conversion but substantially reduces N20 selectivity. Figure 6 also demonstrates that, on this time scale, the performance of the catalyst is not permanently altered by exposure to water. A
Selective Catalytic Reduction of N O with NH3
The Journal of Physical Chemistry, Vol. 98, No. 29, 1994 7157
0.6
0.3 0.0
0.6
d.
i
14N0
0.3 0.0
1960
1 940
1920
1900
1880
1860
1840
1820
Wavenumbers (cm-l)
Figure 4. FTIR spectra in the 196&1820 cm-I region of (a) ISNOand 14NH3reactants; (b) lSNOand 14NH3products; (c) difference spectrum of b - 0.89(a); (c) reference spectrum of 14N0. V20s catalyst at 500 ' C .
- E c
00-
o
. zN 60 -
NOconv.
H'i :;: 0-
L-
0.44%
V
7- 1.65% HZ0
......................................
(b) 1.4 wt% V,Ofl10,
at 450'C
0.56%
1.66%
40
Temperature (OC)
Temperature ("C)
Figure 5. Distribution of nitrogen-containingproducts and lrNkSNO/ (I4N15N + l4Nl5NO) ratio as a function of tempcrature for (a) 1.4 wt W V20s/Ti02 and (b) V205. Nitrogen-containing products: selective reaction, 14NISN(a),14N1sNO(r);ammonia oxidation products, I4N2
(o),1
(01.
4 ~ (VI, ~ 01 4 ~ 0
series of isotopic experiments were performed to help explain these observed effects. Table 2 includes results for conversions and product distributions for matched pairs of experiments performed in the presence and absence of added water. Conversions and product distributions were first obtained without added water (0.05% HzO) and then, under identical conditions of space velocity and temperature and without removing thecatalyst samples from the reactor, with about 1.6% added water. In the presence of water at 450 O C , the amount of nitrous oxide produced over the supported catalyst decreases from 151 to 13 ppm, a reduction of over 90%. A
.............. L
.......
0
0
............. .......
!
40 Tlme on stream (hrs)
20
60
1
Figure 6. NO conversions(solid line) and N20 selectivities(dashed line) as a function of time on stream for 1.4 wt % V205/Ti02 catalyst at (a) 190 ' C and (b) 450 ' C . The experimentwas performed in the absence of added water and with the addition of amounts noted. simultaneous increase in the amount of I4N15N is also observed. It is unclear whether this increase in the amount of nitrogen produced is due to the greater availability of reactant 15NO and 14NH3or whether water promotes the reaction leading to the formation of 14N15N. A decrease is also observed in the amount of 14Nz,which arises from the oxidation of ammonia. Similar effects of water are observed for this catalyst at 500 O C , and in addition, the amount of 14NO produced also decreases. Hence
7158 The Journal of Physical Chemistry, Vol. 98, No. 29, 1994
Duffy et al.
TABLE 3 Comparison of the Results of This Study with Previous Isotopic Labeling Investigations
temp reaction catalyst ("C) system V20s/Ti02 400 plugflow plug flow et al. V205 400 Vogtetal. VzOs/TiOz 400 plugflow Miyamoto V205/Ti02 200 pulsed etal. V205 0-300 pulsed v2os 400 pulsed thisstudy V205/Ti02 185 plug flow V205/Ti02 185 plugflow V205/Ti02 350 plugflow V205/Ti02 450 plug flow V20s/Ti02 450 plugflow VZOs/TiOz 500 plug flow Vz05/Ti02 500 plugflow v205 200 plugflow vzos 350 plugflow v205 450 plugflow 500 plug flow vzos 500 plugflow v205 ref Janssen
wt W
V2Os 3.4 100 3.6
20.2 100 100 3.0 1.4
1.4 1.4 1.4 1.4
product distribution (mol W of total products) source of nitrogens source of nitrous oxides H20 selectivity NO partia'pressure(lOdatm) mnc toN2O NH3 NO+ NO NH3 NO+ NO NH3 0 2 NO NH3 (W) (7%) alone NH3 alone alone NH3 alone alone 495 500 473 25 - 7 3 2 - 2 3 2 495 500 473 66 28 6 6 1 5 20000 500 500 9 10 81 9 nil 39000 39 000 0 100 nil 39000 39000 0 100 nil 39000 39000 14 8 17 1 1 3 1 18 000 900 900 0.05 1 1 99 18000 900 900 0.05 1 1 99 18000 900 900 0.05 1 - 99 1 18000 900 900 0.05 21 80 20 18000 900 900 1.6 2 1 97 2 18 000 900 900 0.05 31 9 40 6 31 14
1.4 100
18000 18000
100 100
18000 18000 18 000
100 100
18000
900 900 900 900 900 900
the overall effect of water is to significantly improve the selectivity to N2 and to suppress the oxidation of NH3. Pure V2O5 behaves in a similar fashion, but with one important difference. Addition of water significantly increases the concentration of 14N0in the product gases. Again it is difficult to decide whether water promotes the formation of this product or somehow changes the overall product formation pathways. Mechanistic Implications. Results of the previous isotopic labeling studies over VzOs and V20~/TiO2catalysts (all in the absence of water) are compared with the results of the present study under the same conditions in Table 3. At comparable temperatures the results are generally in accord; the most abundant product is almost always I4N15N,except for pure vanadia at high temperatures (>400 "C) for which 14N15NO (or lsNI4NO when ISN-labeled NH3 rather than ISNO was used) is the most prevalent product. The present study extends the conditions to significantly higher temperatures and provides a comprehensive framework for evaluating mechanisms proposed for the SCR reaction. The mechanism of the SCR reaction has been extensively studied,' particularly over vanadia-based catalysts, and continues to receive considerable attention.I6l8 However, a generally accepted mechanism has failed to emerge from these studies. Hence, for vanadia-containing catalysts alone, both LangmuirHinshelwoodlg and Eley-Rideal5 mechanisms have had their supporters. In spite of these mechanistic uncertainties, the major product of this reaction over VzO5/TiO2 catalysts, N2, was, until recently, agreed to arise from a combination reaction of N O and NH3. The evidence for this is based on the above isotopic labeling studies (Table 3) in which one of N O or NH3 is labeled with lSN,and I4Nl5N is the major molecular nitrogen (N2) product observed. An alternate mechanism has, however, recently been proposed. Odriozola and co-workers studied the adsorption of N O and NH3 on unsupported V2O5 and pure Ti02 surfaces using Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS),Zo and on V205/Ti02 catalysts using electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS),21 which led them to propose that nitrogen was produced by two routes. One was via N20,
V20,-x
+ xN20
-
and the other directly from NO,
V205+ xN,
900 1.6 900 0.05 900 0.05
900 0.05 900 0.05 900 1.6
9 4 28
41 18 12
5 16 - 9 6 12 7 13 5 5 7 5
-
-
-
9 -
4 1 10 7
-
-
27 31
11
5
I
-
-
-
10
-
39 72 76
(3) with further catalyst oxidation according to (4) Reaction 4 accounts for the increased rate of the SCR reaction in the presence of 0 2 . A suitable combination of eq 1-4 then results in the overall reaction ( 5 ) : 4N0
+ 4NH3 + 0,
-
4N2
+ 6H20
(5)
which is in agreement with the stoichiometry observed experimentally. However, if the reduction of nitric oxide with ammonia in the presence of 0 2 over V2O5 and supported V205/Ti02 catalysts occurred via the combination of steps 1-4 required to give reaction 5,then the reaction of 15NO and 14NH3would produce nitrogen which was 50% I5N2 and 50% 14N2. This product distribution is clearly inconsistent with all previous isotopic labeling studies which show that I4Nl5Nis the major form of nitrogen produced, andalsowith the resultsofthe present study. Theresultspresented in Table 2 and Figure 5 demonstrate that for pure VzOs, 14N15N is the major molecular nitrogen species produced, even at temperatures as high as 450 OC; for the supported material I4NI5N is always the major nitrogen-containing product. Odriozola et d . 2 ' also suggested that ammonia oxidation is the major reason for the increase in N2O formationQ2.23 observed over V2O~/Ti02catalysts at temperatures greater than 350 "C. They postulate that NH3 interacts with reduced vanadia sites at high temperatures to form strongly adsorbed NH2 species (equations 6 and 7 are not reproduced as they appear in the original,20 as we suspect that the equations given there contain a typesetting error):
which subsequently react with 0 2 at temperatures higher than 350 "C to produce N2O
(2)
In this case, the reaction of ISNO and 14NH3should produce 14N20as the predominant form of nitrous oxide. Again this
The Journal of Physical Chemistry, Vol. 98, No. 29, 1994 7159
Selective Catalytic Reduction of NO with NH3 proposition is not supported by the isotopic labeling results: 14NI5NO is always the most common nitrous oxide, for both catalysts, at all temperatures and in the presence and absence of added water. Thus, the results of the isotopic labeling experiments must be accommodated by a fundamentally different mechanism than that proposed by Odriozola et ~ 1 on . the~ basis ~ of TPD measurements. There are three main points of contention regarding the mechanism of the SCR reaction: the form of adsorbed ammonia, whether V - 0 or V - O H is the active site, and whether the reaction of N O and NH3 proceeds via adsorbed or gas phase NO. There is abundant spectroscopicevidenceIgs4-28 that NH3 adsorbs on vanadia-based catalysts on Bronsted acid sites as NH4+groups and on Lewis acid sites as NH, type species ( i = 0-3). The reactive NHI species proposed by several authors varies from NH26JJ7 to NH4+.5J6J9,27Similarly, there is considerablespeculation about whether V-OH,16 V=0,18.29 or some combination of b0th2~928are the active surface sites involved in the catalytic reaction. The more common view is that the crucial step in the catalytic reduction of NO is the activation of ammonia by reaction with V=O groups.I8.29 By contrast to the situation with NH3, no convincing spectroscopic evidence has been reported for adsorbed NO on SCR catalysts under reaction conditions where NH3 is present. However, the observation6J0that N O is converted to N'80 by exchange with 1 8 0 2 does provide evidence for adsorbed NO. Recently Bell and co-workersllJ8 have proposed a mechanism, without specifying the adsorption sites for NH3 and NO explicitly, in which N2 formation arises from the reaction of an NH2 species and adsorbed NO. In the case of labeled species, reaction 8 describes this process:
+
+
14NH2(a) 15NO(a)- I4Nl5N H,O
(8)
which is analogous to the gas phase reaction thought responsible for N2 formation in the thermal DeNO, process.31 Thus, one possible model for nitrogen formation, consistent with the overall stoichiometry of reaction 5 , is V=O+NH3(a)+V-OH NH2(a) 2V-OH
+ NO(a)
-
2vo
-
V=O
+ H,O + VO + H 2 0
(11)
2v=0
(12)
-
+ 0,
(9)
+NH2(a) N,
(10)
where V u represents a reduced surface site. If the slow step was eq 11 (or eq 9 with eq 11 an equilibrium well to the left), then thevacancyconcentrationwould be low. This model would clearly account for the effect of 0 2 , with small, nearly stoichiometric quantities having a large effect on both the rate and product distributions.' The mechanism by which 14N15NOis formed over VzOs-based catalysts has received less attention, since under most industrial conditions where supported catalysts and temperatures of 350400 O C are employed, only small amounts of this product are formed. Went et al.18 propose a reaction similar to that for nitrogen formation, which again involves an NH2 species and can be represented by the following:
+
14NH2(a) "NO(a)
+ 2V=O
-
+
14Ni5N0 2V-OH (13)
Two recent papers investigating the effect of water on the
SCR reaction propose alternate mechanisms for the formation of I4N15Nand 14NISNO. Topspre et al.,32 who also observed a large decrease in N20 production in experiments with added
water, suggest that nitrous oxide formation involves the same intermediate (denoted as NH,NO) as that yielding I4N15N but that the reaction is dehydrogenationnear Lewis sites rather than dehydrationnear Bronsted sites. An alternative, althoughrelated, ~ u g g e s t i o nis~that ~ nitrogen formation (which predominates at low temperatures) is characteristic of hydroxylated surfaces whereas nitrous oxide (favored at high temperatures) arises on more dehydroxylated surfaces. In this scheme the first step in the reaction leading to 14N15Nis the adsorption of NH3 on Bronsted acid (V-OH) groups to form an NH4 species which subsequently reacts with gas phase NO to form nitrogen: 2V-ONH4
-
+ V-0-V + 2 N 0 2N2 + 3 H 2 0 + 2V-OH + VOV (1 4)
An oxygen vacancy with one vanadyl group in close proximity is suggested as the active site for production of 14NlSNO. Adsorption of ammonia on the vacancy is thought to lead to a dissociation yielding an NH2 group, which reacts further to form nitrous oxide: 4V=O
+ 2V0V + 4NH3 * 4V-NH2 + 4 V - O H 4V-NH, + 4 N 0 4 N 2 0 + 4 H 2 0 +
(1 5 ) (16)
In order to regenerate the active site for N20 formation, Odenbrand et al." suggest a dehydration step similar to reaction 11. The forward reaction is favored by high temperature and retarded by the presence of water. This mechanism would indeed account for the inhibition of 14N15N0formation with water addition. Several W O ~ ~ ~ ~ S have ~ J suggested ~ J ~ , ~NH4 ~ , (or ~ NH4+) ~ as the active species for nitrogen formation, and the response of the NH4+species to reaction conditions has been measured19 in siru using FTIR spectroscopy. It seems unlikely, given the complicated nature of the rearrangement which would be necessary, that a direct reaction between NH4+ and NO could give rise to N2. However, it is not possible to discount some role for NH4+. Formationof I4NI5NOis not only favored by high temperatures anddry conditions but also by catalysts with higher V2O5 contents (and thus higher relative amounts of V - 0 groups). Another possible, although again perhaps related, explanation for the formation of ''N15N0 is that both higher temperatures and catalysts having a greater number of V - 0 groups favor the further dissociation of NH2 groups to NH, in a process similar to that of eq 9. Reaction between N H and NO may then lead to nitrous oxide: I4NH(a)
+ "NO(a) + V = O
-
I4Ni5NO+ V - O H (17)
This mechanism would also account for the large decrease in 14N15N0formation in the presence of added water (see Table 2) by shifting the equilibrium of eq 11 to the left, thereby decreasing the number of V - 0 groups and vacancies on the surface and retarding NH3 dissociation. It is clear that the formation of 14N15NO is a complex function of temperature, vanadia content, and water concentration. A number of suggestions have been made to account for the decrease in NO conversion over vanadia-based catalysts at temperatures above 400 0C.5.33The reaction of NH3 with 0 2 to produce N2 was invoked by Miyamoto et aL5as the major reason for decreases in the N O conversion. However, the results of the present study show that the amount of 14N2arising from ammonia oxidation is always less than 10%of the products, even at 500 OC. Odenbrand et aL33investigated the performanceof vanadia-based catalysts under conditions similar to those of the present study, but without the benefit of isotopic labeling. They ~uggested,~3 in addition to NH3 oxidation reactions, oxidation of N2O to NO
Duffy et al.
7160 The Journal of Physical Chemistry, Vol. 98, No. 29, 19'94
4N20
+ 20,
-
8NO
(18)
as one reason for the apparent decline in NO conversion at these temperatures. As part of this work, a specific test was carried out for the possible Occurrence of this reaction over V2Os using conditions appropriate to the SCR process. Formation of N O was not detectable at any temperature in the range 250-500 OC, and hence reaction 18 can be excluded. It may be noted that the same tests showed no decomposition of NzO to N2 either, as long as oxygen was present. The present study, as noted above, provides isotopic labeling results for much higher temperatures than previously investigated. Under these conditions ammonia oxidation becomes significant, particularly for the pure V2O5 catalyst. Using a V205/Si02/ Ti02 catalyst, Odenbrand et al.33 showed that the crystalline phases of vanadia seem to be responsible for ammonia oxidation to nitric oxide. More recently, Ozkan et ~ 1 . prepared '~ V205 samples which preferentiallyexposed different crystal planes and showed that the sample with a higher V-0 content had a much higher overall activity for ammonia oxidation, and that the rate of NO formation from this material showed a very sharp increase at temperatures above 350 OC. It was concluded34that the V=O sites promote direct oxidation of NH3 to NO. It is also clear from the results of this study that supported catalysts with V2O5 contents greater than 10 wt % (see Figure 3), and above all pure V2O5, possess a much higher propensity for ammonia oxidation. Previous work on the SCR reaction has suggested that there may be a correlation between the oxidation of NH3 to NO and the formation of nitrous oxide over oxide catalysts. Odenbrand et 01-33showed that the crystalline phases of vanadia seem to be responsible for both the formation of nitrous oxide and ammonia oxidation to nitric oxide. Similarly, Ozkan et al.34claimed that the V=O sites promote direct oxidation of NH3 to NO and that, in addition, these sites also promote the formation of N20 from the reaction of NO and NH3. A correlation between Nz0 production and activity for NH3 oxidation has also been observed for carbon-supported Cu catalyst^.'^ The present results provide additional support for the existence of a connection betweenN20 production and ammonia oxidation. The FTIR spectra of Figure 2 confirm the higher relative proportion of V=O groups in pure VzO5, and this material clearly has the greatest activity both for N20 production at intermediate temperatures and for NO formation from ammonia oxidation at high temperatures. In addition, our recent labeling studies36 of the SCR reaction over chromia catalysts also suggest a relationship between N20 production and ammonia oxidation. These studies36 show that crystalline (a-Cr2O3) chromia produces NzO with high selectivity (-50%), even at temperatures as low as 150 OC, and has a much higher activity for ammonia oxidation than amorphous chromia. Selectivities for N20 production over amorphous chromia, by contrast, remain below 30% at temperatures less than 200 OC. However, by contrast to the present results for vanadia-based catalysts, the major ammonia oxidation product over crystalline chromia catalyst was 14Nz,with significant I4NO formation only observed at temperatures above ~ 2 8 0 OC. It is possible that the formation of NH, (where i = 0 or 1) groups may also account for the observation of the ammonia oxidation product I4NO:
+ 20, V-I4N + 20,
V-I4NH
-
+
+ I4NO + V=O
I4NO V-OH
(hydrogen abstracting) power. This model would also account for the apparent relationship between high selectivities to N20 and high activities for ammonia oxidation. The addition of water moderates the activity of the catalysts for ammonia oxidation somewhat, but at the highest temperature studied, formation of 14N0from ammonia oxidation is not much affected for 1.4wt % VzO~/Ti02.It is likely that the equilibrium shown in eq 11 lies well to the right at these temperatures. Over pure V2O5 at 500 OC, the amount of 14N0 formed actually increasesslightly with water addition from 72 to 76%, calculated asa molar percentageof the total products (Table 3). Theincrease in the 14N0formed is roughly equivalent to the combined decrease in 14NH3consumed due to a reduction in the amount of I4N15NO, I4N2,and I4N2Oproduced. It is possible that the oxidation of I4NHpto 14N0is so facile at this temperature that any reduction in I4NH3 consumption is channeled into the oxidation reacting forming 14N0. It is interesting to note that Odenbrand et al.33 also observed a decrease in the NO conversion, consistent with increasedammonia oxidation,at higher temperatures (>500 "C) in the presence of water.
Conclusions The reaction of 15N0and 14NH3over pure V2O5 and 1.4 wt % V205/Ti02 has been performed at temperatures in the range
200-500 OC and for water concentrationsof 0.05 and 1.6%.The results show the following: (1) Product distributions vary significantly with vanadia content. Ammonia oxidation to I4NO is particularly dominant for pure VZOSand, at 500 OC, accounts for more than 70% of the nitrogen-containingspecies. Under dry feed conditions (
