Removal of NOx through sorption-desorption cycles over metal oxides and zeolites

ELSEVIER

catalysis today Catalysis Today 22 (1994) 97-109

Removal of NO, through sorption-desorption cycles over metal oxides and zeolites H. Araia,*, M. Machidab “Department of Materials Sciences and Technology, Graduate School of Engineering Sciences, Kyushu University, 61 Kasugakoen. Kasuga, Fukuoka 816, Japan bDepartment of Materials Sciences, Faculty of Engineering, Mlyazaki University l-l Gakuen-Kibanadai-Nishi, Miyazaki 889-21, Japan

Abstract Recent progress in NO, removal through sorption/desorption over metal oxides and zeolites is reviewed. The NO, removal method of this category employs adsorption, absorption and/or solidgas reactions as a separation or concentration process of diluted NO,. These sorption/desorption processes contain various novel and interesting phenomena from physicochemical aspects of solidgas interactions.

1. Introduction Much interest in environmental catalysis is now focused on the development of novel catalytic reactions for depolluting NO, exhausts in lean-bum conditions. These catalytic de-NO, processes, which are most useful for the continuous treatment of exhaust gases, are indispensable for a wide variety of practical applications, such as diesel engines, on-site combustors, and/or boilers. However, it is necessary to take into account NO, concentrations of far less than 1 vol.-% in almost all these applications. This is one of the reasons why catalytic de-NO, reactions are seriously inhibited by various coexisting gases. Also, from the point of view of efficiency and running costs, catalytic processes are not always the most appropriate for the removal of diluted NO,. The development of de-NO, processes strongly requires an efficient technique for the enrichment of NO,. In this regard, sorption/desorption processes are considered to be useful for the separation of NO, diluted in air [ 11. Moreover, by combining it with a temperature or pressure swing cycle system, *Corresponding author. 0920-5861/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0920-5861(94)00069-E

98

Tabfe 1 Sorption/desorption Adsorption

Absorption

Ei. Arai, M. Machida / Catalysis Today 2.2 (1994) 97IO9

processes for NO removal by inorganic

solids

Physical adsorption (Micropore filling) Chemical adsorption Solid-gas reaction Direct incorporation f Intercalation)

so~tion/deso~tion can be extremely important not only as a supplement for catalytic processes but also as a primary de-NO, process, especially for the removal of low concentration NO,. Table 1 represents a brief summary of recent attempts at de-NO, by sorption/ desorption processes. Adsorption is one of the conventional methods used for the selective separation of gases, being divided into physical and chemical adsorption. Physical adsorption equilibrium is very rapid and reversible. Physical adsorption occurs as a result of nonspecific intermolecular forces, such as the condensation of vapor to liquid, and is usually effective for vapors (gases below their critical temperature, T,). This means that physical adsorption is less selective to specific gas species. The amount of physical adsorption greatly exceeds monolayer capacity, being comparable to the volume of micropores when micropore filling occurs. As far as NO, is concerned, however, conventional porous materials (activated carbons, silica gels, and zeolites) adsorbing NOz by physical adsorption, are not available for supercritical NO (T,= I80K). On the other hand, chemisorption usually occurs above or below the Tcof the adsorbate because of the specific interaction of the adsorbate molecule with the adsorption site, which leads to marked selectivity to the specific gas species. However, the amount of chemisorption is limited to less than the monolayer capacity. First, an extensive study of the chemisorption of NO on metal oxides was conducted by researcher workers at the Ford Motor Co. who reported that Co304 and NiO exhibited the greatest ~hemiso~tion activity [ 231. Thereafter, many oxide systems were submitted to NO adso~tion as shown in Table 2 [ 41. Of these studies [ 2,3,513 ] , the high NO chemisorption activity onto F+O, and Fe,O, should be noted. However, the amount of NO chemisorption was not sufficient for practical use even though a wide variety of support materials were examined to enhance the surface area. Thus, NO, removal by adsorption is now directed at developing various microporous adsorbents because of their considerable adsorption capacity. Many absorption processes have been extensively developed for the removal of NO, to date. While almost all these attempts have been concerned with wet processes, e.g., oxidation to NO2 followed by absorption into alkali solutions, the application of dry processes have been confined to the use of molten salts [ 143. Solid absorbents are not only useful for various purposes including the environment, they can also be combined into convenient and compact equipment. Recent progress in cataIysis concerned with NO, has stimulated the elucidation of various novel

H. Arai, M. Machida /Catalysis Today 22 (1994) 97-109

99

Table 2 Amounts of NO adsorbed on various adsorbents near room temperature [4] Adsorbent

Amount of NO adsorbed* (mg/g)

Ref.

SnO, CeO, NiO Co@* CuOly-A&O, NiO/ y-A120s Co,O,/ y-AlzOs Fe,O,/ y-A&O3 Fez03/SiOz Fe-Y zeolite Fe304 FGh a-FezO:, Jaosites wFeOOH p-, y-FeOOH GFeOOH

5 5 1 6 36 36 7 45 5 18 22 24 9 3-10 10-20 4-6 13

151 [61 [31 [3] [31 [31 131 131 171 181 191 I21 [21 [lOI [Ill [I21 1131

“At 13 kPa and room temperature.

interactions between solids and NO,, such as complex formation, solid-gas reactions, direct inco~oration into a solid (inte~alation) , and so on. These interactions needs to be investigated extensively from the practical view point of depolluting applications. This review is an attempt to provide updated information on the development of NO, removal techniques through the sorption/desorption cycle on solid materials, including metal oxides, zeolites, and other microporous substrates. We have to note that these NO, sorption/desorption processes are based on novel interactions between NO, and solids, which are strongly promoted by the assistance of catalysis. 2. Adsorption and desorption on zeolites 2.I. NO, sepurutiun by u~u~tiun Selective adsorption is one of the most suitable NO, removal processes in combination with a pressure or thermal swing technique [ 15,161. Fig. 1 schematically shows the principle of pressure or thermal swing adsorption (PSA and TSA, respectively). In the PSA process, adsorption at high pressure and desorption at low pressure are cycled alternately, while the TSA process utilizes differences in adsorption capacity at different temperatures. In particular, PSA, which has been widely used in various industrial processes, such as the separation of oxygen or carbon dioxide from air, is expected to be an excellent method for removing NO, diluted in air. This process requires NO, adsorbents with a high capacity for reversible

100

H. Arai. M. Machida / Catalysis Today 22 (1994) 97-109

Pressure

P2

Fig. 1. Principles of pressure. or temperature

swing adsorption

adsorption in order to attain sufficient removal efficiency. However, for conventional adsorbents, such as activated carbons, silica gels, and zeolites, the NO, adsorption capacity is not sufficient for practical applications. Separate measurement of reversible and irreversible adsorption is also needed to evaluate the NO, removal performance. Recently, Iwamoto and coworkers [ 17-191 studied the use of various ionexchanged zeolites for practical applications to the PSA process. They noted that zeolites have a large surface area because of microporosity and their physicochemical properties can be widely controlled by a selection of zeolite structure, silica/ alumina ratio, and/or metal ion loaded. 2.2. Adsorption-desorption of NO on metal ion-exchanged zeolites [17-191 Iwamoto and coworkers [ 17-191 examined the adsorption properties of nitrogen monoxide on various metal ion-exchanged zeolites in a fixed-bed flow adsorption apparatus. The amount of reversible (qre,) and irreversible NO adsorption (qI1+) measured at 273 K on various cation-exchanged MFI zeolites are summarized in Table 3. The qrevand ql= are strongly dependent on the type of metal ion and the resultant adsorption properties can be classified into two groups, i.e., ion-exchanged samples with alkali, alkaline-earth, and rare-earth metals and those with transition metals. In the case of transition metal ion-exchanged zeolites, the values Of qirr were larger than those of qrevexcept for Zn-MFI and Ag-MFI. In contrast, qrevwas greater than qIrrfor the other group of zeolites. The order of qrev was transition metal ion N alkaline earth metal ion > rare earth metal ion N alkaline metal ion N proton. This result indicates that reversibly adsorbed NO is associated with transition metal ions, such as Cu and Co. Take the Cu-MFI sample, for instance, since both the qrev and qi= were proportional to the exchange level of the copper ion, the ratios of qrev/ Cu and qi,/CU are constant. From an IR study on the Cu-MFI sample, the dominant species of reversible adsorption was attributed for the most part to NO+ while those of irreversible adsorption were attributed to NO+, NO:, NO; and NO;. The amount of NO adsorption was also influenced by the zeolite structure. As shown in Table 4, the amount of reversible as well as irreversible adsorption of NO per copper ion greatly varied with the zeolite structure and decreased in the follow-

H. Arai, M. Machida /Catalysis Today 22 (1994) 97-109 Table 3 NO adsorption

properties of various cation-exchanged

Adsorbent

Na-MFI(23.3)-100b Ca-MFI(23.3)-54 Sr-MF1(23.3)-105 Ba-MFI(23.3)-80 Mg-MFI(23.3)-46 Cu-MFI(23.3)-157 Ag-MFI(23.3)-90 Co-MFI(23.3)-90 Mn-MFI(23.3)-127 Ni-MFI(23.3)-68 Zn-MFI(23.3)-96 Fe-MFI(23.3)-62 Cr-MFI(23.3)-41 Ce-MFI(23.3)-8 La-MFI(23.3)-7 H-MFI(23.3)-100

MFI zeolite$

Content of cation (wt.-%)

2.81 1.32 5.45 6.44 0.69 5.90 10.85 3.06 4.20 2.41 3.79 2.12 0.87 0.43 0.40 0.13

101

[ 181

Amount of NO adsorbed ( cm3 g - ’ ) Reversible

Irreversible

0.16(0.006)= 1.81(0.246) 2.71(0.195) 1.50(0.143) 0.69(0.109) 4.28(0.206) 3.38(0.150) 1.52(0.131) 1.19(0.069) 1.03(0.112) l.Ol(O.078) 0.52(0.061) 0.38(0.101) 0.34(0.496) 0.25(0.388) 0.12(0.004)

O.OO(0.000)’ 1.56(0.212) 0.20(0.014) 1.44(0.137) 0.22(0.035) 14.90(0.716) 0.54( 0.024) 19.69( 1.693) 5.81(0.339) 6.64(0.727) 0.50( 0.039) 3.08(0.362) 1.16(0.308) 0.34(0.496) 0.24(0.372) 0.32(0.011)

aAdsorption time, 45 min; desorption time, 60 min; concentration of NO, 997 ppm; adsorption temperature, 273 K; adsorbent weight, 0.5 g; flow-rate, 100 cm min- ‘. The sample was abbreviated as ‘cation-zeolite structure ( SiOJA1?03 ratio)-degree of exchange’. ME1 means ZSM-5 zeolites. bConcentration of NO, 1910 ppm. ‘Unit/NO molecules (cation) -‘.

ing order, MFI > OFF/ERI > MOR > LTL > FER > FAU. This order is consistent with that of the increment of the Al content in the zeolites. The effect of the structure is reflected by the change of electronic state, which depends on the Al content of the zeolites. Table 4 Effect of zeolite structure on NO adsorbability Adsorbent”

Cu-MFI(23.3)-68 Cu-OFF/ERI(7.7)-81 Cu-MOR( 10.5)-76 Cu-LTL(6.0)-34 Cu-FER( 12.3)-66 Cu-FAU( 2.6) -60 Cu-FAU(5.6)-83 aAdsorption time, 45 K; adsorbent weight, bMFI, ZSM-5; MOR, ‘Unit/NO molecules

of copper ion-exchanged

Content of cation (wt.-%)

2.63 5.45 5.26 3.22 3.89 9.27 7.99

zeolites”

[ 18 I

Amount of adsorption

of NO ( cm3 g- ’ )

Reversible

Irreversible

2.29(0.247)’ 2.81(0.146) 2.11(0.114) 1.23(0.108) 1.42(0.104) 1.15(0.035) 0.86(0.031)

7.46(0.805)’ 5.55(0.270) 6.69(0.361) 2.38(0.210) 4.82(0.353) 0.62(0.019) 1.52(0.055)

min; desorption time, 60 min; concentration of NO, 1910 ppm; adsorption temperature, 273 0.5 g; flow-rate, 100 cm3 min-‘. mordenite; FER, ferrierite; OEE/ERI, offretite/erionite; FAU, Y- or X-type; LTL, L-type. (cation) -‘.

H. Arai, M.

102 Table 5 Effect of preadsorbed Preadsorbed

gash

N0,(4680 ppm)/He 0,(99.5%) CO,(20%) /He S02(2170 ppm)He C0(1890ppm)/He Hz0(3%)/He None

Mach&

gases on adsorption

/Catalysis

Today 22 (1994) 97-109

property of Cu-MFI(23.3)-147’

[ 181

Amount of adsorption of NO (cm3 g-‘) Reversible

Irreversible

7.14 4.26 4.25 3.92 1.39 0.22 4.35

2.21 14.38 12.19 7.86 4.15 0.45 17.83

“Adsorption time, 60 min; desorption time, 120 min; concentration of NO, 1000 ppm; adsorption temperature, 273 K; adsorbent weight, 0.5 g; flow-rate, 100 cm3 min-‘. bathe adsorbent was heated at 773 K for 5 h under a helium stream (50 cm3 min-‘) before the preadsorption treatment. After the preadsorption the sample was purged with helium at room temperature.

The influence of coexisting gases plays an important role in the practical application of NO adsorption. Their effect on NO adsorption on the Cu-MFI sample is shown in Table 5. The preadsorption of NOz results in the enhancement of qrev, because the adsorbed NO2 provides a new adsorption site for NO molecules to produce N,03. While preadsorption of 02, CO*, or SO2 onto the Cu-MFI sample hardly deteriorated the reversible adsorption of NO, poisoning due to CO or HZ0 was serious. On the other hand, qim is always decreased by the preadsorption of other gases though the degree of the decrease is dependent on the preadsorbed gas. A study of the poisoning mechanism is necessary to elucidate and overcome this problem.

3. NO removal by micropore filling A large quantity of adsorption is required to remove NO from the atmospheric environment through adsorption. As mentioned in the introductory section, the amount of adsorption strongly depends on the adsorption mechanism as well as on the physical properties of adsorbents and adsorbates. The most adsorption can be attained when adsorbate molecules are packed into a porous material through micropore filling [ 20,211. This type of enhanced physical adsorption is a dominant process for vapor; the microporous solid has a great adsorption rate for vapors by micropore filling. However, micropore filling is not effective for supercritical gases whose critical temperatures are less than the adsorption temperature, even in microporous solids like zeolites and activated carbons. This means that micropore filling at approximately room temperatures cannot be adopted for nitrogen oxide with a critical temperature of 180 K. However, the systematic study by Kaneko et al. [ 4,22-241 revealed the micropore filling of NO into activated carbon fibers ( ACFs) modified by iron oxide.

H. Arai, M. Machida /Catalysis Today 22 (1994) 97-109

9 ii ;

103

100

.s ! 4

0 O

200 NO Pressure

400

600

/ mmHg

Fig. 2. Adsorption isotherms of NO [ 221. Top curve, FezOx-dispersed ACF; second from top curve, FeOOHdispersed ACF; third from top curve, ACF; bottom curve, activated carbon.

Fig. 3. Schematic

model of the NO dimer in the slit-shaped ACF micropores.

Fig. 2 shows NO adsorption isotherms over FeOOH- and Fe,O,-dispersed ACFs at various temperatures [ 221. Desorption of FeOOH provides the micropore filling of NO and the decomposition into FezO, markedly enhances the micropore filling of NO into ACF up to 320 mg/g at 30°C. This corresponds to 85% filling of the micropore volume. All isotherms consistent with the Langmuir equation exhibit remarkable hysteresis; the adsorbed NO cannot be removed by evacuation at room temperature. The incorporated NO can be reversibly recovered as NO by heating above 200°C. The initial adsorption rate of NO over Fe,O,-dispersed ACF was approximately 70 times higher than that over the neat ACF sample. Micropore filling of NO was confirmed by the measurement of pore volume using Nz adsorption on ACF after exposure to NO. The whole mechanism of micropore filling of supercritical NO is not clear at the present stage. However, the presence of highly-dispersed Fe species is obviously essential for this phenomenon to take place. From the temperature dependence of the isotherm, this type of NO adsorption seems to possess both chemisorption and physical adsorption characteristics. Actually, gaseous NO molecules produce strong chemisorption onto Fe203 highly-dispersed around the entrance of the slit-shaped micropores of ACF. After the adsorption step, almost all the absorbed NO molecules

migrate to fill the micropore. Almost all the NO molecules in the micropores are dimerized (NO), even above room temperature (Fig. 3) [ 25,261. This intermolecular interaction plays a key role in the micropore filling. A more interesting feature of micropore filling is selectivity; micropore filling of NO can remove 93% of NO from 300 ppm NO in the presence of 02, SOZ,COZ, and HZ0 [ 25-281. The performance of adsorption/desorption in the flow system or in the TSA system also needs to be examined. A further study is now directed at examining practical evaluations for NO, removal.

4. NO absorption into metal oxides Nitrogen oxide shows various interactions with metal oxides. Recently, new types of interactions between nitrogen oxide and metal oxides have been noticed from the point of view of depollutional applications. Misono and coworkers have been the first to report the rapid uptake of NO as well as CO into superconducting YBa$Zu,O? [ 291. Table 6 summarizes the uptake of NO and CO by YBa&&O,. After pre-evacuation at 3OO”C,the sample absorbed ca. 2 mob’mol oxide of NO at the same tem~ra~re. The NO molecules thus absorbed were almost completely recovered as NO by heating above 4OO’C.The abso~t~on ability is strongly dependent on the pre-evacuation conditions; the NO uptake at 300°C increased to 2.3 mall mol oxide, and the recovered gas was a mixture of NO and NZ when the sample was pre-evacuated at 30°C. A large quantity of gaseous CO was also incorporated into Y13a2Cu30,.,but the absorbed CO molecules cannot be recovered below 500°C probably because of the stronger interaction. More striking is that, on exposure to NO/CO mixtures at 300°C NO was taken up initially, then CO absorption and N2 formation took place, and finally the stationary catalytic reaction, 2N0 + 2CO -+ N2 + C02, proceeded over YBa,Cu,O, [ 301. This absorptionTable 6 Uptake of NO and CO by YBazCu,Osa 1291 Pretreatment

Uptake of NO (mm01 g-‘)

NO desorbedb (mm01 g-l)

Temperature

573

573

Evacuation at 573 K for 1 h Evacuation at 298 K for I h

2.9

0.55 (19) 2.7 (93) 2.9 (100) 7.0

3.4

0 (0)

673

uptake of co’ (mm01 gg’) 773

0.5 (IS) 2.2 (65)

573

9.0

CO desorbedd (mm01 gg ‘) < 713 0 0

“Uptake was measured in separate experiment using NO alone and CO alone. “Total amounts desorbed below the temperature indicated; percentage recovery as NO in parentheses. difference between amount of CO decrease and COz formed. “Nz (0.48 mmollg) was formed.

H. Arai, M. Machida / Catalysis Today 22 (1994) 97-109

105

assisted catalytic reaction showed a high-turnover frequency comparable to that over Rh/A1203 catalysts, being expected as one of excellent de-NO, catalysis. In the layer structure of this material, oxide ion sites in the Cu-0 plane between two Ba-0 sheets are only half-occupied and ordered to form a Cu-0 chain. The resultant nonstoichiometric feature strongly influenced the superconductive characteristics. Since such a large NO/CO uptake cannot be explained only by adsorption to the surface, Mizono and coworkers proposed the absorption of NO into the solid through intercalation or the solid-gas reaction [ 301. Their subsequent study revealed that NO absorption is easily influenced by the surface conditions of the sample. In particular, NO absorption is significantly accelerated only after exposure to saturated water vapor at room temperature. NO absorption into YBa$&O,, was also examined by Arakawa and Adachi [ 311, who proposed the following reaction mechanism: 2YBa,Cu307

+ 2N0 + 3Cu0+Y,BaCuOS

+Ba(N02)*

BaCuO, + 2N0 + CuO + BaNz O3

+ 2BaCu02

(1) (2)

Reaction ( 1) can be confirmed by X-ray diffraction after the termination of NO absorption (ca. 2.5 mol/mol oxide) at 3OO”C, which showed that YBa$&O, disappeared with simultaneous appearance of YzBaCu05 and BaCuO,. Actually, however, since NO absorption into a single BaCuOz phase is negligible, reaction (2) should be expressed as a solid-gas reaction with a Ba-Cu-0 system to produce barium nitrate or nitrite, which is accelerated in the presence of CuO. A detailed description of NO absorption into the Ba-Cu-0 system appears in the following section. In addition to Ba-containing oxides, various metal oxides have been studied for the removal of NO by absorption. Yamashita and coworkers [ 321 reported that the NO absorptivity of Y-containing oxides decrease in the following order; YSr,Co,O, > YzBa,Co,O, > YSr,Mn,O, > YSr,V,O,. From TPD and JR results, absorbed NO molecules were oxidized by lattice oxygen to form N03-. The absorbed NO was desorbed as a mixture of NO/O*. Layertitanate, NazTi307, shows NO absorption in the presence of transition metals or noble metals. The sample impregnated with Pd, for instance, absorbed ca. 0.8 mol/mol oxide of NO. The absorbed NO produces NaNOx, which is decomposed at 400°C to liberate NOx. [331.

5. NO removal based on solid-gas reactions with Ba-Cu-0 As mentioned in the preceding section, mixed oxides containing Ba species as well as other alkaline-earth, rare earth, or alkali elements possess a characteristic affinity to gaseous NO. The authors investigated NO absorption through solid-gas reactions from the point of view of depollutional applications [ 34-361. Since the

106

H. Arai, M. Machida /Catalysis

Today 22 (1994) 97-109

solid-gas reaction generally accompanies a change in free energy larger than adsorption, it is expected to attain almost complete removal of diluted gaseous NO even at less than 10 ppm in air. Most of the solid-gas reactions between NO, and metal oxides are reversible; NO absorption and desorption corresponds to the formation of nitrate (low temperature) and decomposition of nitrate (high temperature), respectively. Thus, we believe that the solid-gas reaction is the most suitable for the de-NO, process in combination with the TSA process. 5.1.

Solid-gas

reaction between NO and Ba-Cu-0

Various Ba-M-O systems were examined for the removal of NO and NO2 (Table 7). These mixed oxides showed NO absorption activity to some extent at 2OO“C, which was accelerated in the presence of oxygen. A much higher removal rate was observed when a NO/NO2 mixture was fed to these oxides. The most outstanding activity for NO/NO2 absorption was observed for the Ba-Cu-0 system, which completely removed 0.09-0.1% of NO, after 20 min of the reaction. In the cases of NO/NO,? removal in the presence of 02, a large amount of NO, was liberated from samples above ca. 500°C. The amount of liberated NO,, which was several times larger than that estimated from adsorption onto the surface, was equal to the cumulative amount of NO removed. The NO removal results from absorption into the oxide. The Ba-Cu-0 system, which showed the highest NO absorption rate, contained two different types of nonstoichiometric compounds, BaCuOz,, and BaCuO,.S. Insitu X-ray diffraction analysis in a NO/air flow revealed that these mixed oxide phases disappeared with simultaneous formation of Ba(N03)JCu0. This means that the NO absorption results from the solid-gas reaction between NO and these oxides to produce Ba( N03) ,/CuO. The single phase samples of BaCu02.5 and BaCuOz,r were prepared from calcination of a mixture of nitrates at 650 and 750°C respectively. The formation of Table 7 Removal of NO and NO* by Ba-M-O 20 min of the reaction is shown M

Cr Mn Fe co Ni CU

systems

[ 361. All samples were calcined at 750°C. NO/NO* removal after

NO removal (%)

NO2 removal (%)

02 O%a

02 10%b

02 10%’

8.0 2.8 6.9 18.6 1.4 100.0

18.6 7.9 10.7 33.3 2.5 100.0

70.0 30.2 15.1 34.5 21.9 100.0

“NO 0.1%. N2 balance, SV = 6000 hh ‘. bNO 0.09%. O2 lo%, N, balance, SV = 6000 hh ‘. ‘NO, 0.07%. O2 IO%, Nz balance, S.V, =6000 hh’

Crystal phase

BaCrO, BaMnO, + Ba3Mn20s BaFeO,_x BaCo03 _ ~ NiO + BaNiO, + Ba3Ni,0s BaCuOz , + BaCuOz 5 + CuO

H. Arai, M. Machida /Catalysis

n

cue+

Today 22 (1994) 97-109

107

2NO + 1.250,

Ba(NO&

\

J

BaCuO, 5

Fig. 4. Schematic model of NO absorption

into BaCuO, 5 [ 361,

BaCuOz,S phase is peculiar to the nitrate precursor, i.e., this phase could not be produced from a mixture of acetates or carbonates. Copper ions in the high oxidation state (3 + ) bring about a high initial NO absorption rate ( 1.33 mol/mol Ba min at 250°C) due to the promotion of NO oxidation and the formation of Ba(N03)2 as shown in Fig. 4. However, the high chemical activity of Cu3+ also resulted in the reaction with CO, to produce BaC03. This led to not only a significant decrease in NO removal but also to the imperfect reproduction of the active phase in a heating course. The other phase, BaCuOz.I, possesses less activity for NO absorption, e.g., when the BaCuO,., sample was exposed to a NO/air mixture at 25O”C, no solid-gas reaction was observed in in-situ X-ray diffraction. This result corresponds to the small content of the high oxidation state copper ions in the BaCu02., phase. However, substantial NO absorption was observed for the BaCuO,,, sample physically mixed with CuO. The CuO/BaCuO,,, mixture attained complete NO removal from the gas mixture of 0.09% NO, 10% 02, and N2 balance at 250°C (SV = 6000 h- ‘) and achieved a cumulative NO absorption of ca. 2 mol/mol Ba when the oxide phase was completely converted to a Ba(NO,),/CuO mixture. 5.2. Reaction mechanism of NO absorption into BaCu02,1 To elucidate the role of CuO in the acceleration of the solid-gas reaction between NO and BaCuO,.,, the catalytic reaction of NO over CuO was examined. When a

b)

50 Temperature

200

250

300

350

/ “C

Fig. 5. (a) NO oxidation over CuO (0) and BaCuO? , (0). Equilibrium conversion (-) (0.09% NO, 10% Oz. N, balance, SV=6000 h-l). (b) NOremovalby BaCuO,, (greysquares);andCuO/BaCuO~, (blanksquares); and NO2 removal by BaCuO,, (black squares): (0.09% NO or 0.08% NOZ, 10% Oz. Nz balance, SV=6000 h- ’ ) Removal is after 20 min reaction [ 361.

108

H. Arai, M. Mach& /Catalysis Today 22 (1994) 97-109

cue+

0.4502

Bm2

Fig. 6. Schematic model of NO absorption

into BaCuO,

, [ 361.

100

e 80 a g60 a c is 40 20

co2

concentration I%

Fig. 7. Effect of CO2 concentration on NO removal by MnOJBaCuO, 0.09% NO, 10% 02, N, balance, SV = 6000 h-l.

1 (0) and CuO/BaCuO,

, (0)

[36].

NO/O2 mixture was fed to CuO, NO was converted to NO* with a rise in temperature up to 250°C (Fig. 5). But CuO had no contribution to the absorption of NO or NO*. The BaCu02,, sample, which showed much less catalytic activity for NO oxidation, completely removed diluted NO* (0.08%) up to 400°C (Fig. 5). These results indicate that the NO absorption into BaCu02,, follows the catalytic NO oxidation by CuO as shown in Fig. 6. From a practical view point, the influence of coexisting gas on NO removal should be subsequently noted. The most favorable feature of NO removal by BaCuOz,l is that the adsorption rate is enhanced in the presence of oxygen. On the other hand, the NO absorption into BaCuO,., was significantly inhibited by CO*, which reacts with Ba species to produce an inactive surface layer of BaCO,. As shown in Fig. 7, NO removal by CuO/BaCuO *.I completely disappeared in the presence of CO2 (8 vol.-%). However, this problem could be overcome by employing MnOz as a preoxidation catalyst. The higher catalytic activity of MnO, for NO oxidation seems effective in reducing the COz inhibition. Furthermore, we confirmed that the BaCuO,., combined with MnO, could be successfully reproduced by heating above 800°C after NO absorption in the presence of Oz. [ 361

6. Concluding remarks The present survey shows that dry NO, sorption/desorption processes have become an important approach for the development of an efficient de-NO, process. Studies in this field are now directed toward installations using sorption/desorption cycles by means of TSA and PSA. The problem of inhibition due to coexisting gas

H. Arai, M. Machida /Catalysis Today 22 (1994) 97-109

109

has not, as yet, been reliably solved for real exhausts. Unlike automotive three-way catalysis, future de-NO, processes must offer a high performance in the presence of considerable quantities of oxygen. An excess amount of water vapor and/or carbon dioxide is also considered to deteriorate NO, removal activity. Despite much interest in this matter, the inhibition mechanism in each case is not clearly understood. The authors believe that the development of the de-NO, process will be ensured by developments in the chemistry of solid-gas interactions.

References [ 1 I J.A. Ritter and R.T. Yang, Ind. Eng. Chem. Res., 29 (1990) 1023. [2] K. Otto and M. Shelef, J. Catal., 18 (1970) 184. [3] H.C. Yao and M. Shelef, in R.L. Klimisch and J.G. Larson (Editors), The Catalytic Chemistry of Nitrogen Oxides, Plenum Press, New York, 1978, p. 45. [4] K. Kaneko and K. Inouye, Adsorp. Sci. Tech., 5 (1988) 239. [5] F. Solimosi and J. Kiss, J. Catal., 41 ( 1976) 202. [6] M. Niwa, Y. Furukawa and Y. Murakami, J. Colloid Interface Sci., 86 ( 1982) 260. [7] K. Segawa, Y. Chen, J.E. Kubsh, W.W. Delgass, J.A. Dumesic and W.K. Hall, J. Catnl., 76 (1982) 112. 181 S. Yuen, Y. Chen, J.E. Kubsh, J.A. Dumesic, N. Topsee and H. Topsee, J. Phys. Chem., 86 ( 1982) 3022. [9] C.R.F. Lund, J.J. Schorfheide and J.A. Dumesic, J. Catal., 57 (1979) 105. [ 101 K. Inouye, I. Nagumo, K. Kaneko and T. Ishiiawa, 2. Phys. Chem., 131 (1982) 199. [ 111 K. Kaneko and K. Inoeye, J. Chem. Tech. Biotech., 37 ( 1987) 11. [ 121 T. Hattori, K. Kaneko, T. Ishiiawa and K. Inouye, Nippon Kagaku Kaishi, ( 1979) 423. [ 131 K. Kaneko and K. Inoeye, Adsorption Sci. Tech., 3 ( 1986) 11. [ 141 M. Iwamoto and N. Mizuno, Shokubai, 32 ( 1990) 462. [ 151 H. Lee and D.E. Stahl, AI&E Symp. Ser., 69 (1973) 1. [ 161 M. Takeuchi, R. Tanibana and S. Nishida, Nenryo Kyokai-shi, 62 (1983) 989. [ 171 W. Zhang, H. Yahiro, N. Mizuno, J. Izumi and M. Iwamoto, Chem. Lett., (1992) 851. [ 181 W. Zhang, H. Yahiro, N. Mizuno, J. Izumi and M. Iwamoto, Langmuir, 9 (1993) 2337. [ 191 W. Zhang, H. Yahiro, N. Mizuno, J. Izumi and M. Iwamoto, J. Mater. Sci. Len., 12 (1993) 1197. [20] M.M. Dubinin, Chem. Rev., 60 (1980) 235. [21] P.J.M. Carrot and K.S.W. Sing, in K.K. Unger et al. (Editors [22] K. Kaneko, Langmuir, 3 (1987) 357. [23] K. Kaneko, in K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Krol (Editors), Characterization of Porous Solids, Elsevier, Amsterdam, 1988, p. 183. [24] K. Kaneko, Colloid Surf., 37 (1989) 879. [25] K. Kaneko, N. Fukuzaki and S. Ozeki, J. Chem. Phys., 87 (1987) 776. [26] K. Kaneko, A. Kobayashi, T. Suzuki, S. Ozeki, K. Kekei, S. Kosugi and H. Kuroda, J. Chem. Sot., Faraday Trans. I, 84 (1988) 1975. [27] K. Kaneko, S. Ozeki, and K. Inouye, Atmosph. Environ., 21 (1987) 2053. [28] K. Kaneko, T. Funamoto and S. Ozeki, Ind. J. Environ. Protect., 8 (1988) 681. [29] K. Tabata, H. Fukuda, S. Kohiki and M. Misono, Chem. Lett., (1988) 799. [30] N. Mizuno, M. Yamato and M. Misono, J. Chem. Sot., Chem. Commun., (1988) 887. [31] T. Arakawa and G. Adachi, Mater. Res. Bull., 24 (1989) 529. [ 321 M. Kishida, T. Tachi, A. Kato, H. Yamashita and H. Miyadera, Shokubai, 33 (1991) 163. [33] M. Machida, S. Ogata, K. Eguchi and H. Arai, unpublished results. [34] M. Machida, K. Yasuoka, K. Eguchi and H. Arai, J. Chem. Sot., Chem. Commun., (1990) 1165. [35] M. Machida, K. Yasuoka, K. Eguchi and H. Arai, J. Solid State Chem., 91 (1991) 176. [ 361 M. Machida, S. Ogata, K. Yasuoka, K. Eguchi and H. Arai, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), Proc. of the 10th Int. Congress on Catalysis, Akademiai Kiad6, Budapest, 1992, p. 2645.