Ind. Eng. Chem. Res. 1994,33, 825-831
825
A New Approach to Decomposition of Nitric Oxide Using Sorbent/ Catalyst without Reducing Gas: Use of Heteropoly Compounds Ralph T. Yang' and Ning Chen Department of Chemical Engineering, State University of New York at Buffalo,Buffalo,New York 14260
Effective decomposition of NO to NZin flue gas can be accomplished by a two-step approach using sorbent/catalyst. No reducing gas is needed. A heteropoly compound, H ~ P W ~ Z O ~ * ~isHused ZO, as the sorbent/catalyst in this study. At a space velocity of 5000 h-l, 70% of NO in a simulated flue gas is absorbed in the fixed bed a t 150 "C. Upon heating the NO-saturated bed to 450 "C (at 150 "C/min), 68.3% of the absorbed NO is decomposed into Nz. Based on the results of fixed-bed NO absorption and TGA and IR analyses, the absorption of NO is a bulk reaction where the six HzO linkages in the secondary structure are substituted by three NO linkages, and the Keggin structure (primary structure) is preserved. X-ray diffraction data show that the spatial arrangement in the secondary structure is also preserved with a 5% reduction in the lattice constant. 02 and HzO are needed for NO absorption. SO2 and COz have no effects on either absorption or decomposition. The basic rationale for the two-step approach is to concentrate NO from flue gas concentrations (hundreds of ppm) into a bulk solid phase (where the NO partial pressure is of the order of 1 atm) thereby taking advantage of the kinetic law of catalyzed NO decomposition (which is either first or second order with respect to NO partial pressure).
Introduction Although NO is thermodynamically unstable, it exhibits a high resilience to decomposition. The search for catalysts for NO decomposition dates back to the turn of the century. The most active catalysts are noble metals and oxides of transition metals (Hightower and van Leirsburg, 1975). On these catalysts, the reaction is mostly first order and in some cases second order with respect to NO partial pressure. Major reasons prohibiting the application of these catalysts to post-combustionstationary-source flue gas NO decomposition are that 02 has a strong inhibiting effect and SO2 is a strong catalyst poison, in addition to the fact that these catalysts are not active enough at the temperatures conveniently accessible in the combustion systems (Hightower and van Leirsburg, 1975;Bosch and Janssen, 1988). In the utility boiler system, there exist two convenient locations for inserting a NO catalytic reactor: one after the economizer and one after the air preheater (and before the stack inlet). The respective temperature ranges are, approximately,350-450"C and 100-150 "C. The selective catalytic reduction (SCR) technology has been widely accepted for the post-economizer operation (350-450"C). In SCR, NH3 is used as the reducing agent on Vz05/TiOz catalyst (Bosch and Janssen, 1988)or WOa + VzOa/TiOz catalyst (Chen and Yang, 1992). The low-temperature window (100-150 "C) provides an economically more Some catalysts attractive alternative for SCR (with "3). have been found for the low-temperature SCR, such as supported Pt (Otto et al., 1970),iron and nickel sulfates (Chen et al., 1990),amorphous chromia (Curry-Hyde et al., 1990), NbzOb + Vz05/Ti02 (Tanabe, 19911, and manganese oxides (Singoredjo et al., 1992), but more research is needed before a practical catalyst is developed. The noble metals, Rh and Pt, are being used in automobile catalytic converters where the temperature is around 900 K and NO is reduced by Hz, CO, and hydrocarbons (Taylor, 1984). For the above reasons, despite the large number of catalysts that have been found for NO decomposition, hopes for direct decomposition of NO-without using reducing gases-have been essentially abandoned. The obstacle for effective NO decomposition has been the low
concentration of NO in the flue gas, which results in low rates. This obstacle is overcome by a two-step approach described in this work.
Basic Idea for Enhanced NO Decomposition The kinetics of NO decomposition (to N2 and 02)on a large number of catalysts have been found to be either first-order or second-order with respect to NO partial pressure (Hightower and van Leirsburg, 1975). The concentration of NO in flue gas is typically of the order of hundreds of ppm. This low concentration makes it impractical to decompose NO directly without using a reducing gas at mild temperatures (e.g., below 600 "C). Catalyst poisoning by SO2 and kinetic inhibition by 02 also contribute to the impracticality of NO decomposition. The approach in this work is to use a sorbent/catalyst. NO, is first absorbed, or chemically fixed, in the bulk of the sorbent/catalyst particles at a low temperature, such as the flue gas temperature of 150 "C. The product of the absorption reaction may be nitrate, nitrite, or chemically bonded NO,. After NO, saturation, the product is rapidly heated to a higher temperature for decomposition, e.g., 450"C. The NO, fixation reaction is reverseduponheating causing NO, to penetrate through an outer product catalyst layer of the particle. During decomposition, the partial pressure of NO, within the sorbent/catalyst particle is of the order of 1 atm. The high partial pressure within the particle results in increases in the NO, decomposition Eates of 3-4 orders of magnitude (for first-order kinetics) or 6-8 orders of magnitude (for second-order kinetics) over that by directly contacting the flue gas on the same catalyst. The intimate contact of NO, with the catalyst within the particle may also contribute to enhanced decomposition. The basic idea of approach is illustrated in Figure 1. There exist many possibilities for the sorbent/catalyst. Metal oxides can form nitrates and nitrites in flue gas. For example, nitration of CaO in simulated flue gas is rapid at low temperatures (Ramanathan and Yang, 1982) (but no Nz was detected upon rapid heating of calcium nitrate because CaO is not active enough). The oxides of transition metals are promising candidates. Another class of promising candidates are composites made by co-impregnation
Q888-5885/94/2633-Q825$04.5Q/Q0 1994 American Chemical Society
826 Ind. Eng. Chem. Rea., Vol. 33, No. 4.1994
a. Primary structure (PW,,O.O.
6N2+02
W
‘Keggin’ structure)
P
0 Absorption
i
Figure I. A SorbenWcatalyst is uaed firat to absorb NO from flue gas. Upon decomposition by rapid heating, the NO, within the particle penetrates through a product catalyst layer where the NO. partial preasure is near 1 atm, thus achieving a high rate of NO decompositionthat isapproximately 3-4ordem ofmagnitude higher than that by direct contacting with the flue gas. b. Secondary SIrucIure (H,PW,20a.6 H,O)
of an NO decomposition catalyst and an alkali or alkaline earth oxide in a porous support. The class of sorbent/ catalyst to be described here is heteropoly acids.
Heteropoly Acids Oxides of the early transition elements form oligomeric anions (e.g., MorOd-) in aqueous solution at low pH. Heteropoly anions are formed when two or more 0x0metallate ions are present, e.g.: 12W04z + HPO,”
+ 23H’
-
(PW,,O,)”
+ 12H20
The product, H~PWI~OK,, belongs to a large class of heteropoly acids and salts which have been the subject of long-standing investigation (Pope, 1983). The crystal structure of the PW120, anion belongs to the Keggin structure of XM12Oa, shown in Figure 2. In thisstructure, 12 M06 octahedra surround a central X04 tetrahedron, where M is usually W or Mo and X can be P, As, Si, Ge, B, etc. Although the structure of the heteropoly anion (e.g., the Keggin structure) is well-defined and stable, the structure by which the Keggin structures are linked together is less understood. However, a distinct X-ray diffraction pattern is obtained for H3PW120~6H20.As shown in Figure 2, the Keggin polyanions are linked in a three-dimensionalnetwork through H+(H20)2bridges,and these linkages can be easily (i.e., at rmm temperature) replaced by polar molecules such as alcohols and amines (Misono, 1987). Generally,2-6alcoholor aminemolecules per Keggin anion irreversibly replace the water linkages (of 6H20). The Keggin structure is called the primary structure and the linked structure is called the secondary structure. Due to the flexible form of the secondary structure, Misono referred to the solid structure as a ‘pseudoliquid phase.” The importance of the “pseudoliquid phase” in many reactions catalyzed by the heteropoly compounds has been demonstrated (Misono, 1988; Ono, 1992). Experimental Section Reactor System and Analyses. A schematic diagram of the reactor is shown in Figure 3. The same reactor was used for both NO, absorption and the subsequent decomposition step. The reactor was a quartz tube with a fritted support. The reactor was equipped with a thermocouple well (inserted in the catalyst bed) and a gas preheating section, in addition to heating coils around the reactor. The preheating section was situated below the fritted support and consisted of a nichrome heating coil
Figure 2. (a) Primary structure: Keggin structure. (h) Secondary structure: each Keggin anion is linked by 3H+(HzO)z linkages to form H&’WIZO&HZO.
I t
.-
-- t
~~~
Figure 3. Schematic diagram of reactar. (A) Chemiluminescent NOINO, analyzer during NO absorptionor massspectrometer during decomposition. (B)Temperatun-pmgrammedcontroUer. (C)Water vaporgenerator(heated). (D)Reactorwithimbedded thermocouple well and a gas preheating section below.
imbedded in quartz chips. The preheating section was used only during the decomposition step for rapid heating. During the NO, absorption step, the feed gas contained lo00 ppm NO which was made by blending premixed NO/ He (1.0 % by vol) with a He carrier, and other gases such as 02, COa, and SO2 were added also by blending. The blending was accomplished by using an FM 4575 (Linde Division)mass flowcontrol blending system. Water vapor was added by using a heated gas wash bottle. The same synthetic flue gases were used for the decomposition step. Two different detectors. however, were used for the two steps. During NO, absorption, a chemiluminescent NO/
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 827 1200,
'
'
'
"
7
'
1
'
'
'
'
'
'
'
I
450
&loo0
l----Im
a
Gc
8
800
8400
600
3 ~
.-
350
200 0 " " " " ' " " " "
0
E75000
U
O
BM
400
1
2
A
4 6 8 101214 Time, hr
B
4
1
550
2508
0 1 2 3 4 5 6 7 8 9 Time, min Figure 4. The NO absorption-decomposition cycle with HaPW120~.6HzOas the sorbent-catalyst. (A) Effluent concentration from a fixed-bed reactor during absorption: 6.0 g of powder (3.2 mL), space velocity = 5000 h-l, feed NO = lo00 ppm, 02 = 5%, HzO = 5%, balance = He, T = 150 "C. (B)Nz production during decomposition: heating rate = 150OC/min, He containing lo00 ppm NO, 500 ppm S02,2% each of 02 and H20, space velocity = 7100 h-* with a flow of 386 cm* STP/min. 68.3% of the NO absorbed in A is converted to Nz in B.
NO, analyzer (Thermoelectron Corporation, Model 10) was used to record the effluent NO, concentration. For the decomposition step, a quadrupole mass spectrometer (UTI Model 100 C) was used to measure the N2 concentration in the product stream. By fixing the mass number at 28, the area of the N2 peak was integrated with a SpectroPhysics Model SP4600 Integrator. The integrated area was calibrated by injecting a known amount of N2 (e.g., 20000 ppm NOin He at 500 cm3/min for 4 min) and comparing the total integrated areas. The thermogravimetric analysis (TGA) was performed by using a Cahn 1000 electrobalance. The powder XRD patterns were obtained with a Siemens transmission powder diffractometer with CuKa source. The infrared spectra were measured with a FTIR Nicolet Impact 400 model, using KBr pellets. Materials. The heteropoly acid was reagent grade powder 12-tungstophosphate,H~PW~~O.WXHZO, supplied by Alfa Products, Ward Hill, MA. A sample size fraction 6O-80 mesh was used. The premixed NO in He was 1.0 % , from Scott Specialty Gases. All other gases were from the Linde division: He (High Purity grade), C02 (Precision Aquarator grade), 0 2 (Extra Dry grade), and SO2 (Commercial grade).
Results and Discussion The NO Absorption-Decomposition Cycle. Results of a complete absorption-decomposition cycle are shown in Figure 4. At a gas hourly space velocity of 5000 h-' and 150 "C, 70% of the NO in the feed gas was absorbed by the fixed bed of heteropoly acid. As will be shown shortly, C02 and SO2do not affect the NO absorption rate. During decomposition at a heating rate of 150 OC/min, the total amount of NZproduced was 38.9 cm3STP. Since each N2 molecule formed from two NO molecules, 68.3% of the NO that was absorbed initially was converted to N2 during
300
h
-1 3 p
1
,
,
,
,
,
,
,
,
,
,/I,, , I: ,
,
,
,
,
2
25 50 0 1 2 3 4 - 5 Time, min hr Figure 5. Weight change of heteropoly acid (H$WIZO~~.~HZO, 382 mg) upon exposure to NO and 02 at 150 O C .
decomposition. An overall nitrogen mass balance showed that nearly 50% of the NO originally contained in the "flue gas" was converted into NZby this two-step cycle. In the experiment, 6.0 g of heteropoly acid was used, which contained substantially more than 6 H2O for each Keggin unit. Based on our TGA TPD data, the net amount of H3PW1204~6H20was actually 5.5 g. The choice for the temperature of 150 "C for NO absorption was based on the practical reason that this temperature is conveniently availablein utility combustion systems. The temperature for decomposition was limited by the thermal stability of the heteropoly acid. Hodnett and Moffat (1984) showed that deprotonation of the acid with concurrent nonreductive loss of lattice oxygen became vigorous at 500 OC (but with the retention of the primary Keggin structure). NO Absorption Product. The fixed-bed absorption results shown in Figure 4 indicated that 114 cm3 STP of NO was absorbed in 5.5 g of H3PW120~6H20. This amount of NO corresponded to 2.8 NO molecules per Keggin unit (H3PW12040). From the results shown in Figure 4, one could not determine if 0 2 was also absorbed into the secondary structure. The TGA technique could provide accurate data on weight changes. A number of TGA experiments were performed, and Figure 5 shows the typical data. At 150 "C, the weight was stabilized a t 382 mg, as H3PW12040'6H20. Exposure to NO and 02resulted in a net weight loss of 2.6 mg. This weight loss corresponded to the substitution of 6 H2O molecules by 2.9 molecules of NO for each Keggin unit. The TGA result agreed, within experimental error, with the fixed-bed NO absorption results. Moreover, the TGAresult ruled out the possibility of residual HzO molecules in the secondary structure during NO absorption, because even one of the six H2O remaining would have resulted in a significantly smaller weight loss in the TGA experiments. Infrared (IR) absorption spectra of the heteropoly acid before and after NO absorption are shown in Figure 6. The spectrum of the acid sample is the same as that reported in the literature (Pope, 1983; Misono, 1987).The four bands between 1100 and 600 cm-l are, respectively, stretching frequencies for P-0, W-0, and W-0-W (the two lowest frequencies). The bands at 3170 and 3350 cm-l are stretching frequencies for water, and that a t 1600cm-' is the OH bending frequency. The band at 1710 cm-1 has been assigned to the bending mode of the hydronium ion, H30+ (Misono, 1987). Also given in Figure 6 is the spectrum of the sample saturated by NO. The four bands due to the Keggin structure, 1100-600 cm-1, remained unchanged upon NO absorption. The water bands have essentially disappeared, except small signals which were likely due to moisture residue trapped in the KBr pellet.
828 Ind. Eng. Chem. Res., Vol. 33, No. 4,1994
3600
rxx)
2500
2ooo
1500
1WO
WAVENUMBER (anm1)
Figure 6. Infrared spectra of heteropoly acid (HsPW120~~6Hz0) (A) before and (B)after NO/O2 exposure, showing that H2O linkages are substituted by NO linkages.
The hydronium ion band has completelyvanished. Three new bands appeared upon NO absorption: 1295,1390and 2270 cm-1. The assignments for the 1295- and 1390-cm-l bands are for NO linkages whereas for the 2270-cm-l band is for NO+ ion. Explanations for the assignments are as follows. IRspectra of metal complexes of nitro (NO2) and nitrito ( O - N 4 ) compounds have been studied (Nakamoto, 1970),particularly for NO linkages between Ni or Co atoms. A pair of IR bands in the region 1200-1400 cm-l are associated with both nitro and nitrito linkages. For example, the asymmetric and symmetric stretching frequencies for Ni-O-N-O-Ni linkages are, respectively, 1346 and 1319 cm-l. Based on the band assignments in the literature, the bands at 1390 and 1295 cm-1 in Figure 6 are assigned to the NO linkages in the secondary structure. The nitrosonium ion (NO+)has a characteristic vibration frequencyin the 2200-2300-cm-l region (Nakamoto,1970). The band for the hydronium (H30+)ion disappeared upon NO substitution of the water linkages. The nitrosonium ion was apparently formed upon charge transfer to the NO bond from the proton in the hydronium ion. The NO absorption results, along with the TGA and IR data, show that at 150 O C (and other temperatures higher than room temperature), approximately three NO molecules can substitute six H2O molecules for each Keggin anion unit. The NO-substituted compoundwas not stable a t room temperature when exposed to the ambient atmosphere; the NO was slowly resubstituted by H2O molecules. To further understand the secondary structures, i.e., the water and NO linkages, X-ray diffraction patterns were measured, as shown in Figure 7. As noted
in the literature (Pope, 1983; Hodnett and Moffat, 1984; Misono, 19871, the XRD patterns vary widely depending on the number of water molecules in the secondary structure. However, the XRD pattern became well-defined after the sample was purged in dry N2 at room temperature as shown in Figure 7. The XRD pattern is identical to that shown in the literature. The XRD pattern was caused by the secondarystructure, reflectingthe relative positions of the Keggin units linked by the six H20 molecules per Keggin unit (Pope, 1983; Misono, 1987). The pattern shown in Figure 7 was due to the structure of H3PW120~6H20.After being heat treated at 300 "C,minor changes occurred as discussed in the literature (Hodnett and Moffat, 1984). The XRD pattern for the sample after complete substitution of 6H20 by 3N0, also shown in Figure 7, exhibited the same lines with the same relative intensities as that of the fresh, purged sample, with the exception that all lines were shifted to higher diffraction angles. The amount of shift followed Bragg's law, i.e., lattice spacing is inversely proportional to sin 6 where 6 is the diffraction angle. The results indicated that the secondary structure was preserved upon NO substitution as linkages for the Keggin anions, but the lattice constant for the secondarystructure was reduced by approximately 5%. The present conclusion of 3N0 rather than 2N02 (which could also account for the TGA data) replacing the 6H20 linkages in the secondary structure is based on both structural consideration and literature information on linkage substitutions by other molecules. In the secondary structure of H ~ P W ~ ~ O M -a~'unit H ~ Ocell* , consists of a cube with a Keggin sphere at each corner position, while
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 829
-
1 500 C E s t e p 4 c y c l e s
300 C p u r g e d
-
I
r o o m temp. p u r g e d 0
" ' 1 " " 1 " " 1 " ~ ' 1 " " " " I " ' ' I " ' ' I ' ' '
10.0
15.0
20.0
25.0 30.0 Two-Theta
35.0
40.0
45.0
Figure 7. X-ray diffraction patterns of HflW12Ow6H20. From bottom to top: the secondary structure (with 6Hz0) after purge with dry
He at room temperature; after heating at 300 "C; after NO absorption at 150 O C ; after four complete cycles of absorption (150 "C) and decomposition (600 "C).
2H20 molecules form a planar linkage on each face of the cube (Misono, 1987),resulting in 6H20per Keggin sphere. To maintain this secondary structure (as the XRD data shows that it is indeed maintained), one NO is the minimum number required to replace 2H20 on each face. For the same reason, the number of alcohol (such as CzHsOH) molecules in the secondary structure of H3PW120~ is 3, 6, or 9, to replace 6H2O (Lee et al., 1989). Further structural studies as well as gas product analysis (upon decomposition) are in progress in our laboratory. The above is our tentative conclusion. NO Absorption Kinetics and Resistance to SO2 and C02. The kinetics of NO absorption were investigated mainly using the fixed-bed reactor, and a TGA reactor was employed to determine the form of the rate equation. The most important question to be addressed was possible effects from SO2 and C02. This was studied with the fixed-bed reactor under the following conditions: T = 150and 200 "C, NO = 1000ppm, 0 2 = 2 % ,space velocity = 10 000 and 15 000 h-l. The effects of SO2 were examined by adding 500 and 1000 ppm SO2 in the feed stream, and those of C02 by adding 10% and 20% C02 in the feed stream while keeping the concentrations of the other components fixed. No changes in the effluent NO concentrations were observed by these additions. It is therefore concluded that SO2 and C02 had no effects on NO absorption into the heteropoly acid. Since NO absorption is a bulk reaction, the above results indicate that SO2 and C02 do not substitute the HzO linkages in the secondary structure as does NO. The results also indicate that SO2 and C02 do not influence the diffusion rates of NO in the bulk substitution. In contrast, a strong effect was observed with "3. Injection of 1000ppm NH3 at 150 "C rapidly decreased the NO absorption rate to zero. This was consistent with the literature that 3.2 molecules of NH3 was absorbed (to substitute the H2O linkages) per PW12040anion (Ono, 1992). Since pyridine and alcohols are also known to substitute the H2O linkages (Misono, 1988; Ono, 19921, it is expected that they may also have a strong influence on the NO absorption rate.
Table 1. NO Absorption by a Fixed Bed of HflW12Ou.GH:O. Conditione (Unless Specified): 8V = 10 000 h-1, NO = 1000 ppm, 0 2 = HsO = 2W,He = balance, T = 200 "C 170 190 210 230 300 130 150 T ("0 110 16 5 10 22 23 22 22 0 % abs 1.6 0 1.0 2.0 3.0 5.0 0 2 (%) 14 0 7 40 22 32 % abs 11 2 5 H2O (%I 0 0 24 24 24 % abs SV (h-1) 20000 15000 10 OOO 7500 5Ooo 24 27 51 70 % absa 48 0
0 2
= 5% in this series.
However, neither NH3 nor these molecules are found in any significant concentrations in the flue gas, so no effects are expected in practical application. Effects of temperature, space velocity (SV),and 0 2 and HzO concentrations on the absorption of NO by the fixed bed are summarized in Table 1. The optimum temperature range is 150-200 "C. 02has a strong positive effect on NO absorption. Increasing 02 concentration beyond 5 % appeared to further increase, although slightly, the NO absorption, but detailed data are not included here. Although NO, rather than NOz, substitutes HzO as the linkages in the secondary structure, 0 2 clearly plays an important role in the bulk substitution reaction. The role of 0 2 and the mechanism of substitution are under further investigation. Water vapor is also necessary in NO absorption. When HzOwas cut off in the feed stream, the NO absorption rate declined slowly to zero (in about 1/2 h). This result indicates that the H2O linkages were removed slowly in a dry atmosphere a t the reaction temperature and that the H2O linkages were necessary for the NO substitution. Moreover, only a minimum amount of water vapor was needed to retain the water linkages and to sustain NO absorption. The NO absorption kinetics were further investigated using a TGA reactor. Nearly a monolayer of particles was spread on the sample holder pan in order to eliminate the
830 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 Table 2. N2 Production by Rapid Heating from 150 to 450 O C Expressed as Percent NO Initially Absorbed by H8Wl2040
heating rate (OC/min) 50 100 150
Nz production (%) 35.4 59.6 68.3
interparticle diffusion effect. A high gas flow rate was used to minimize or eliminate the film diffusion resistance. In the low-concentration range (NO below 1000 ppm and 0 2 below 5 % ) ,the kinetics followed the general form of the rate equation: rate = k [NO]'/2[0z1 Again, the reason for the strong 0 2 effect is not understood. The one-half order for NO is also not understood, although it may imply dissociation of NO in the substitution reaction. Nz Formation upon Decomposition. The results given in Figure 4 showed that nearly 70% of the NO that was initially absorbed in the absorption step was decomposed into NZmolecules during the rapid heating step. It was also found that the amount of NZproduced was not affected by NO, SOZ,COz, 02,and HzO that were present in the simulated flue gas, i.e., pure He and simulated flue gas (in He) yielded the same results. As shown in Figure 4, the decomposition step was very rapid, generally completed while the final temperature (ca. 450 "C) was being reached. In order to utilize the catalytic activity of the outer layer in the particle, fast heating rates are necessary, and in fact, the heating rate is the single most important factor in determining the amount of Nz produced. The effect of heating rate on the amount of Nz produced is shown in Table 2. The sample particle size was in the fraction of 60-80 US mesh, or 0.1770.25 mm. Clearly, the particle size should have a strong effect on Na production; larger size should improve NZ production because a longer diffusion path through the catalyst (outer) layer would be available. However, large size would decrease the NO absorption rate during the first step. The effects of particle size are under further investigation. The strong effect of heating rate on NZproduction is clearly shown in Table 2. The highest heating rate achievable in our apparatus was 150 OC/min. The heating in our apparatus was not efficient; faster heating in largescale reactors by direct flow through a hot flue gas (at near 500 "C) should be possible. By further increasing the heating rate, more NZproduction should be possible. Cyclic experiments were also performed. The X-ray diffraction pattern of the heteropoly acid after four complete absorption-decomposition cycles is included in Figure 7. The pattern indicated that the secondary structure was preserved. It should be noted that for samples subjected to decomposition in dry He, subsequent cooling and exposure to the ambient air restored the water linkages and the secondary structure.
Conclusion (1)Nitric oxide at low concentrations can be decomposed (to Nz) effectively at mild temperatures by a two-step approach using a sorbent/catalyst. A heteropoly compound, H3PW1~04~6Hz0, is used as the sorbent/catalyst in this study. NO is first absorbed into the bulk of the heteropoly compound particles at about 150 "C. Upon rapid heating to about 450 "C for decomposition, N2 is
formed. Both temperatures are conveniently available in utility boiler systems. A simulated flue gas containing 1000 ppm NO is contacted with a fixed bed of the particles at 150 "C. At a space velocity of 5000 h-l, the effluent contains 300 ppm NO. After NO saturation, the bed is heated to 450 "C at 150 "C/min in a flow of simulated flue gas. Nearly 70% of the NO absorbed in the particles is converted into Nz. SO2 and COZin the simulated flue gas have no effects on NO absorption and decomposition. (2) The NO absorption product is identified by results from fixed-bed absorption and TGA and IR analysis. The combined results are represented by the following reactions: Absorption: NOlOa
Decomposition: rapid A
H3PWlz040*3N0
_*
Ha0
H3PW1~040*6Hz0 + Nz
X-ray diffraction results show that the six HzO linkages in the secondary structure of the heteropoly compound are substituted by three NO linkages, and the spatial arrangement of the secondary structure ispreserved, with a reduction of approximately 5 7% in the lattice constant. (3) Although 02 does not appear to be a reactant, the NO absorption kinetics depend strongly on 02.The rate of absorption is 1/2-order with respect to the NO partial pressure and first-order in 0 2 . The role of 0 2 in the reaction mechanism is not understood. (4) Nz production depends strongly on the heating rate of the NO-saturated heteropoly compound. Higher heating rates favor Nz production.
Acknowledgment This work was partially supported by the Electric Power Research Institute. Literature Cited Bosch, H.; Janssen, F. Catalytic Reduction of Nitrogen Oxides. Catal. Today 1988,2(4),369-531. Chen, J. P.; Yang, R. T. Role of WO3 in Mixed VzOa-WOa/TiOz Catalysts for SCR of NO with "3. Appl. Catal. 1992,80,135. Chen, J. P.; Yang, R. T.; Buzanowski, M. A.; Cichanowicz,J. E. Cold Selective Catalytic Reduction of NO for Flue Gas Applications. Znd. Eng. Chem. Res. 1990,29,1431. Curry-Hyde, H. E.; Musch, H.; Baiker, A. Selective Catalytic Reduction of NO over Amorphousand Crystalline Chromia. Appl. Catal. 1990,65,211. Hightower, J. W.; van Leirsburg,D. A. Current Status of the Catalytic Decomposition of NO. In The Catalytic Chemistry of Nitrogen Oxides: Klimisch. R. L.. Larson. J. G.., Eds.:, Plenum: New York. 1975;pp 63-94. Hodnett. B. K.: Moffat. J. B. ADDliCatiOn of TemDerature-Programmed Desorption 'to the Siidy of Heteropoly Compounds: Desorption of Water and Pyridine. J . Catal. 1984,88,253. Lee, K. Y.; Mizuno, N.; Okuhara, T.; Misono, M. Catalysis by Heteropoly Compounds. XIII. IR Study of CzHsOH and (C&&)zO in PseudoliquidPhase of H3PW120~.Bull. Chem.SOC.Jpn. 1989, 62,1731. Misono, M. Heterogeneous Cataysis by Heteropoly Compounds of Molybdenum and Tungsten. Catal.Rev.-Sci. Eng. 1987,29,269. Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, 2nd ed.; Wiley: New York, 1970; Section 111-2. Ono, Y. Heteropoly Acid Catalysts-A Unique Blend of Acid-Base and Redox Properties. In Perspectives in Catalysis; Thomas, J. M.; Zamaraev, K. I., Eds.; Blackwell Scientific Publ.: London, 1992;pp 431-464. '
Ind. Eng. Chem. Res., Vol. 33, No. 4, Otto, K.; Shelef, M.; Kummer, J. T. Studies of Surface Reactions of NO by Isotope Labeling. 2. Deuterium Kinetic Isotope Effect in the "3-NO Reaction on Supported Pt. J. Phys. Chem. 1971,75,
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Tanabe, K. Niobium, Catalyst Repair Kit. Chemtech 1991,20,628. Taylor, K. C. Automobile Catalytic Converter; Springer-Verlag: New York, 1984, pp 14,36.
875. Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: New York, 1983. Ramanathan, K.; Yang, R. T. Kinetics of NO. Absorption in Calcined Linestone and Dolomite, Chem. Eng. Commun. 1982,18, 107. Singoredjo, L.; Korver, R.; Kapteijn, F.; Moulijn, J. Alumina Supported Manganese Oxides for the Low Temperature Selective Catalytic Reduction of NO with "3. Appl. Catal. B 1992, I, 297.
Received for review July 23, 1993 Revised manuscript received December 13, 1993 Accepted December 28, 1993' Abstract published in Advance ACS Abstracts, March 1, 1994.
