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Selective catalytic reduction of NOx with methane over metal exchange zeolites
Apphed Catiysls B Enocronmental, 2 (1993) 239-256 Elsevler Science Pubhshers B V , Amsterdam
239
APCAT BOO44
Selective catalytic reduction of NO, with methane over metal exchanged zeolites Yuejm Li and John N. Armor ALLProducts and Chemrcals, Inc , 7201 Ham&on Boulevard, Allentown, PA 18195 (USA) (Fkcewed 21 January 1993, revised manuscnpt received 8 March 1993)
Abstract Catalytic reduction of NO, with methane m an oxldlzmg atmosphere was studled over metal exchanged zeohtes We found that the combmatlons of Co’+, Mn’+ and Nl*+ with certain types of zeohtes, e g , ZSM-5 and mordemte, are active catalysts for this reactlon For a Co-ZSM-5 catalyst, the mtnc oxide conversion displays a volcano-shape curve as temperature mcreases, which 1s reversible upon decreasing temperature The mtnc oxide reduction actlvlty 1s proportional to the level of Co2+ exchanged into ZSM-5, but excess amounts of cobalt do not contnhute to the activity Mn-ZSM-5 1s very slmdar to Co-ZSM-5 m the mtnc oxide reduction actlvlty, and NI-ZSM-5 has slightly lower activity compared to Co-ZSM-5 Under an oxldlzmg condltlons, Cu-ZSM-5, however, 1s meffectlve for the rutnc oxide reduction Co-Y, which has much more Co2+, 1smuch less active compared to Co-ZSM-5, or Comordemte The amount of mtnc oxide adsorbed, measured by TPD, on Co-Y 18extremely small (NO/ Co=0 06) compared to Co-ZSM-5 (NO/Co> 1 1) and Co-mordemte (NO/Co=0 8) Keywords CH,, Cobalt, emlsslon control, metal exchange, methane, nitrogen oxldes, NO,, reduction, zeohte, ZSM-5
INTRODUCTION
Recently, there has been a strong mterest worldwrde m pursumg alternative approaches to abate NO, ermssions from both stationary and mobile sources The reduction of NO, with hydrocarbons, instead of using ammonia, m an oxldrzmg atmosphere is a subject of intense research By usmg hydrocarbons, the problems, e g , ammoma shp, transportation of ammonia through resldenteal areas and equipment corrosion, associated with the selective catalytic reduction (SCR) process can be avoided Currently, propane, propene and ethylene are the most mtensely mvestigated hydrocarbons for NO, reduction [l31 In all these early reports, the presence of O2 is essential for tHe NO, reduction, demonstratmg that hydrocarbons can be effective for the selective reducCorrespondence to Dr J N Armor, Air Products and Chemicals, Inc ,720l Ham&on Allentown, PA 18195, USA Tel ( + l-215)4815792, fax ( + l-215)4812989
0926-3373/93/$06
00 0 1993 Elsevler Science Publishers
B V All nghts reserved
Boulevard,
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tlon of NO, However, the use of methane as a selective reducmg agent for NO, was never reported. In fact, methane 1s often regarded as a non-selective reducing agent for NO, [4,5 ] Methane 1s &fficult to activate because of the strong C-H bond (101 kcal/mol) which often necessitates high reaction temperatures It 1s well known that methane reacts much faster with oxygen than with mtrlc oxide on most catalysts, and to use methane selectively for NO, reduction 1s indeed a great challenge On the other hand, with the abundance of natural gas and the wide spread use of natural gas as a fuel by many utlhtles, it certamly would be desirable to use methane as a reductant Recently, we Ascovered [6] that NO, could be selectively reduced with methane m an ox&zmg atmosphere over a class of metal exchanged zeohtes We observed (7) complete nitric oxide conversion at 400’ C with methane over a Co-ZSM-5 catalyst Contrary to previous reports on this reaction [8], we found that on Co-ZSM-5 the catalytic actlvlty 1sgreatly enhanced by the presence of oxygen, and the actlvlty 1s proportional to the level of methane m the feed We also found that Co2+ exchanged zeohte Y, which has much higher cobalt loadmg compared to Co-ZSM-5, IS much less active for nltrlc oxide reduction Supported cobalt oxide was completely inactive Our earlier results suggest that certain types of zeohtes are required to posltlon cobalt cations provldmg a suitable electronic environment to carry out the selective reduction of nitric oxide with methane In this paper, we wish to elaborate upon our earlier report by describing the role of zeohtes, demonstrating the importance of the metal center, and reporting our temperature-programmed desorptlon stu&es EXPERIMENTAL
Catalyst preparatwn All our zeohte samples were prepared by exchangmg an appropriate cation into a zeohte m an aqueous solution Na-ZSM-5, a starting material for cation exchange, was synthesized m house via a template-free method [6,9] Zeohte Y (LZ-Y-52) and mordenlte (LZ-M-5) were obtamed from Union Carbide m the Na+ form Acetate metal salts were used for the exchange The metal exchange was typically carried out at 80’ C for 24 h with a &h&d (0 01-O 02 M) metal cation solution After exchange the sample was exhaustwely washed mth deionized water, filtered and dried at 110°C overnight The Co-H-ZSM-5 was made by exchangmg Co2+ into an NH,-ZSM-5, where NH,-ZSM-5 was obtamed by three successive exchanges of Na-ZSM-5 with NH4N03 solution (1 M) at room temperature H-ZSM-5 was obtamed by heating a fully exchanged NH,-ZSM-5 at 500°C m hehum for 1 h The metal exchanged zeohte samples were analyzed for Sl, Al, Na and the exchanged metal using mductlvely coupled plasma-atomic emlsslon spectroscopy CoO/Al,O, and other supported cobalt
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oxide samples were prepared by the standard mclplent wetness technique usmg a cobalt rutrate solution; the preparation was drred at llO”C, and later calclned 1n a 10% O,/He mixture at 500°C for 1 h in situ. Co/slhca-alumma was obtamed by exchangrng Co2+ mto an amorphous silica-alumma support (Davrson S111ca-Alumina Catalyst Support, Grade 135, W R Grace & Co.) m a cobalt acetate solution 1n a manner similar to the Co-zeollte preparation. CoT102 was made by exchangmg Co2+ mto a Na+ containing hydrous T102 prepared by hydrolyzing the product of the reaction between tltanlum 1sopropox1de and NaOH 1n CH30H [lo] Reactma studces The catalytic actlvltles were measured using a micro-catalytic reactor 1n a steady-state plug flow mode The reactor was a U-shaped glass tube wrth l/4 m 0 D at the inlet and 3/8 m 0 D at the outlet. To reduce pressure drop, the catalyst was pelletlzed, crushed and then sieved to 60-80 mesh before use A 0 10 g to 0.4 g of sample was used for actlvlty measurements and partial pressure dependence &u&es However, kmetlc measurements, e.g , reaction order determmatlons, were made using a 0 05 g sample, and the conversions were controlled below 30%. At temperature programmer (Yokogawa, Model UP 40 ) with a J-type of thermocouple m contact with the catalyst bed was used to control the temperature The typical temperature ramp rate was 5”C/mm and the flow-rate of the feed was 100 cm3/mm (GHSV= 30 000 or 7506 based on the apparent density of the zeohte catalysts, ca 0 5 g/cm3) with the flows independently controlled by a 4-channel mass flow meter/controllers (Brooks 5850) The reaction mixture typically consisted of 1640 ppm NO, 1025 ppm CH4, and 2.5% 0, (balance as He) Zeol1te catalysts were pretreated m situ 1n flowing helium at 500’C for 1 h, and supported metals were oxuhzed m a 10% O,/He mixture at 500°C for 1 h before reaction A Var1an 6000 gas chromatograph (GC) with a TCD detector was used to monitor catalytic actlvlty A molecular sieve 5A column (l/8 1n x 10 ft ) was used to separate 02, N, and CH4 at 25°C The n1tnc oxide conversion was calculated based on the N2 formation, and the methane conversion based on the CH, consumption Selectlv1ty of methane IS defined as the molar rat10 of the CHI reacted with NO to the CH, totally consumed, and it was calculated based on the followmg equation Selectivity =
(~c~,/~~o)xC~ox~tx
[NO],
G-t, xFt x [(-Xl,
where, oCHl and ONoare the stolchlometrrc numbers of methane and mtrrc oxide, respectively based on reaction 1 (see Results and Discussion ), Ft 1stotal volumetric flow-rate of the feed, [NO], and [ CHIlo are the inlet concentrat1ons (ppm) of nitric oxide and methane, respectively, CNo and CCHlare the conversions of n1trlc oxide and methane, respectively Occasionally, a Porapak
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Q column (column temperature = 50 o C ) was used to analyze for carbon &oxide formation and check the material balance Near 100% carbon balance was achieved (Methane was oxidized to carbon dioxide) An on-hne mass spectrometer (MS) (UT1 106C) equipped wrth an on-hne, atmospheric sampling device, was also used to monitor the gas effluent Both the GC and MS could be operated simultaneously Temperature-programmed
desorptux
studtes
For a typical TPD measurement, a 0 1 g sample was used and pretreated with a helium flow at 500 oC for 1 h (The catalyst bed was about 0 5 cm deep ) The mtrrc oxide adsorption was carried out at 25 oC by flowing a NO/He mixture (1620 ppm, 100 cm3/mm) through the sample, and the effluent from the exit of the reactor was continuously monitored by a mass spectrometer, a levehng-off in mtrrc oxide mtensrty &mated the saturation of the sample with nitric oxide A color change of the catalyst (from light blue to light brown) was observed during the mtrlc oxide adsorption on a dry Co-zeohte sample, and this color change progresses down the bed as the adsorption proceeds Typlcahy, 15 mm is sufficient to achieve a saturation for nitric oxide adsorption After the mtric oxide adsorption, the sample was then flushed wrth a stream of hehum at 25 oC to eliminate gaseous nitric oxide and weakly adsorbed nitric oxide As the gaseous mtrlc oxide level, monitored by a mass spectrometer, returned to near the background level of the mass spectrometer, the sample was heated up to 506 oC with a ramp rate of 8 oC/mm in flowing of hehum ( 100 cm3/mm), and the desorbed species were monitored continuously by the mass spectrometer as a function of time/temperature The mass spectrometer was calibrated for NP, Oz, NO, N,O and other relevant species, and quantitative analysis was possible RESULTS AND DISCUSSION
Co-ZSM-5 Recently Co-ZSM-5 was described as one of the more active catalysts for the reduction of mtrlc oxide with methane, and the presence of oxygen greatly enhanced the catalytic activity [7] Because oxygen is essential to the nitric oxide reduction, the stolchlometry of this reaction can be expressed by reaction 1 In addition, methane combustion also occurs (reaction 2 ) These two reactions compete for use of methane The temperature dependencies of nitric oxide and methane conversions were studied on a Co-ZSM-5 catalyst 2NO+CH4+02+N2+C02+2H20
(1)
CH4+202+COz+2H20
(2)
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243
Fig 1 shows the mtric oxide conversson to rutrogen (reaction 1) over a CoZSM-5 catalyst as a function of temperature at two space velocltles At a GHSV=30 000 the conversion increases with reaction temperature, but the conversion curve bends down at T> ca 460 oC The optimum temperature is dependent on the space velocity, at a GHSV = 7500, the maximum mtrlc oxide conversion was obtamed at ca 420”C In either case, the bendmg over of conversion is entirely reversible upon decreasing temperature, suggesting that this effect is a kinetic phenomenon, rather than due to any structural damage of catalyst at high temperatures At first sight, the bending over of conversion on Co-ZSM-5 catalyst appears similar to that observed on Cu-ZSM-5 for the direct decomposltlon of mtrlc oxide [ 111 A key difference between the two catalysts is that methane combustion is a side reaction (reaction 2) for mtrlc oxide reduction on Co-ZSM-5 At high temperatures the methane combustion rate on Co-ZSM-5 becomes ngmticant. As illustrated in Fig 2, with GHSV = 30 000 the methane conversion 1s nearly complete at 500”C at which point the nitric oxide conversion begms to fall off At GHSV = 7500, the methane combustion rate IS even higher at a given temperature, reaching 100% at 450 ’C Apparently, at either space velocity a maximum nitric oxide conversion is obtained at the temperature at which ca 80% of methane was consumed It is known that the mtrlc oxide reduction rate IS proportional to the level of methane m the feed [ 71 The bending over of conversion at high temperature, to a large extent, may be the result of slgmficantly lowered partial pressure of
300
350
400
Temperature
450
500
550
(“C)
Fig 1 Nltnc oxide converslon as a function of temperature and space velocity on a Co-ZSM-5 catalyst [NO] = 1640 ppm, [CH,] = 1025 ppm, [OJ =2 5% (0) GHSV 7500, (0 ) GHSV 30
000
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Catal B 2 (1993) 239-256
60
40
20
0
I 300
350
400
Temperature
450
500
550
(“C)
Fig 2 Methane converslon as a function of temperature and space velocity on a Co-ZSM-5 catalyst [NO] = 1640 ppm, [ CH,] = 1025 ppm, [ 0, ] =2 5% Symbols as in Fig 1
methane m the reactor Running the reaction at high space velocltles, therefore with low conversions, certamly raises the temperature at which catalyst performance for reactlon 1 falls off With GHSV=30 000, the selectlvltles are 96, 51, 38 and 23% at 350, 400, 450 and 500’ C, respectively Note that the level of oxygen ISmuch higher than those of other reactants ( [O,] / [NO] = 15 6 and [O,]/ [CH,] = 25 at the mlet), and the selectlvltles for methane to reaction 1 are especially high at low temperatures The high selectlvlty of Co-ZSM-5 IS m clear contrast to metals, metal oxides and some other metal exchanged zeohtes, e g , Cu-ZSM-5, as catalysts A reactlon was carried out on a Co-ZSM-5 catalyst at 400 “C to test the short term stablhty of the catalyst As shown m Fig 3, a stable conversion (45% ) was obtained lmmedately at the begmnmg of the run, and the conversion &d not decrease even after 20 h of contmuous reaction The effect of water vapor on the mtrlc oxide conversion was also tested A moderate decrease m activity was observed For example, on a Co-ZSM-5 the mtrlc oxide conversion decreases from 53 to 28% and from 40 to 35% at 450 and 5OO”C, respectively upon addmg 2% H,O vapor to the feed (GHSV= 30 000, [NO] =800 ppm, [ CH4] = 1000 ppm, IO,] =2 5% ) However, the effect was entirely reversible after ehmmatmg water from the feed Moreover, the nltrlc oxtde conversion m the presence of 2% H,O IS stable overnrght at these high temperatures with Co-ZSM-5 Thus, water inhibits, but not poisons, the mtrlc oxide reduction on
245
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0
5
10
Time
15
20
25
(h)
Fig 3 Nltnc ox& convemlon aa a fun&on of time at 400”C on a Co-ZSM-5 catalyst [NO ] = 1640 ppm, [CH,] =1025ppm, [O,] =2 5%, GHSV=7500
Co-ZSM-5 The nature of the water effect and its influence on kinetics will be &scussed m dem elsewhere [ 121 The catalytic activities of Co-ZSM-5 for reaction 1 were measured on a series of Co-ZSM-5 catalysts with different cobalt exchange levels but with the same &/Al ratio, 14 The conversslons of mtrlc oxide over these catalysts at 400,450 and 500°C are shown m Fig 4. At 400 and 45O”C, the conversion increases mth the level of cobalt exchanged and levels off when Co/Al > ca 0 5, the theoretical cation exchange capacity At 5OO”C, the conversion increases wrth the cobalt loadmg when Co/Al 0 5. On the over exchanged Co-ZSM-5 (Co/Al > 0.5)) conversions at 500” C are lower than those at 450°C However, this bendmg over was not observed for the under exchanged samples (Co/Al < 0 5) up to 500’ C Thrs 18probably due to the lower extent of the competltlve reaction (methane combustion) observed on these low cobalt loading samples The catalytic actlvltles are also expressed as turnover frequencies (TOF) the number of nitrrc oxrde molecules converted per cobalt cation per second Table 1 shows the TOF’s as a function of cobalt-exchange level at 400,450 and 500’C For low cobalt loadings, the TOF’s are essentially independent of the level of cobalt exchanged, e g , ca. 10 10e3 at 450” C for Co/Al between 0.22 to 0 33. The TOF is much lower for over exchanged Co-ZSM-5 The invariance of the TOF for low loadmg catalysts su gesta that the cobalt catmns are equally active below a certain exchange level ! f Co2+, and the activity 1sduectly proportlonal to the exchanged cobalt sites. On the over exchanged catalysts, per-
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25 20 15 10 5 0 0
01
02
03
04
05
06
07
06
Co/Al
Fig 4 Nltnc oxide conversion as a function of cobalt loading and temperature on Co-ZSM-5 catalysts [NO]=1640ppm, [CH,]=1025ppm, [0,]=25%,GHSV=30000 (0) 400°C (0) 45O"C,(A) 500°C TABLE 1 Turnover frequencies of Co-ZSM-5 for the nltnc oxide reduction as a function of cobalt loading Co/AI”
0 22 0 26 0 33 0 70 081
Turnover frequencyb X lo3 (8-l) 400°C
450°C
500°C
0 62 0 71 061 0 34 0 35
098 100 100 0 55 0 54
120 1 12 1 12 0 47 0 48
“Sl/AI = 14 qhe turnover frequencies were measured at [NO I = 1640 ppm, and total pressure = 1 atm
[CI-LI= 10% PPm,1% I= 2 5%
haps two kmds of s&es exrst This findmg 1sm contrast to the situation for CuZSM-5 for mtrrc oxide decomposition [ 71, where the TOF mcreases with the copper exchange level and the over exchanged Cu-ZSM-5 is more active. In the case of Cu-ZSM-5 copper pairs (or dimmers) were suggested [ 131 as the catalytic sites for the nitric oxide decompositron m the Cu-ZSM-5 system. It seems that on Co-ZSM-5 a single Co2+ cation may be a sufficient catalyst site for the mtric oxide reduction with methane Table 2 shows the mtric oxide conversron as a function of &/Al ratio of
247
Y LL and J N Arnaor/Appl Catal B 2 (1993) 239-256 TABLE 2 Nltnc oxide convermon as a function of Sl/Al ratlo Sl/Al
Co/Al
Conversion”
14 23 38
0 70 102 0 67
34 19 9
“rhe conversions GHSV = 30 000
(% )
were measured at 45O”C, [NOI = 1640 ppm,
[CKI =lO% ppm, 1% =2 5%,
TABLE 3 Nltnc oxide and methane conversions” sample
Co-ZSM-5 Mn-ZSM-5 NI-ZSM-5 Cu-ZSM-5 Co-H-ZSM-5 H-ZSM-5
metal/Al
0 70 0 53 0 70 060 038 -
on metal cation-exchanged
ZSM-5 zeohtes 450°C
metal loadmg (wt%)
400°C NO
CH,
NO
CH,
NO
CH,
40 31 43 37 23
23 17 16 8 18 4
26 20 12 60 16 5
34 30 26 8 34 6
70 58 40 96 42 10
30 32 20 na 42 10
100 92 73 na 78 13
“All samples were tested at GHSV=30 ppmand [O,] =2 5% - Conversion was not detected na Data not available
000 (0 1 g, 100 cm3/mm),
500°C
[NOI =X40,
[C%I =I025
ZSM-5 All these ZSM-5 samples had excess amounts of Co2+ Given the fact that excess amounts of Co2+ do not contribute to the mtrlc oxide reduction, we assume the mtrlc oxide conversrons obtained on those catalysts reflect the actlvlty of the fully exchanged ZSM-5 catalysts We found the mtnc oxide conversions increase rather linearly with decreasing &/Al ratio, 1 e with mcreasing the cation exchange capacity Thus the overall actlvlty of a catalyst 1sproportional to the number of the exchanged Co2+ m the zeohte Therefore, the TOFs of these catalysts are comparable Other catwn exchanged ZSM-5 Table 3 shows the mtrlc oxide conversrons on a varrety of catron-ZSM-5 catalysts as a functron of temperature Mn-ZSM-5, which has slmllar metal loading as Co-ZSM-5, has comparable actlvltles for nitric oxide reduction, but, unlike Co-ZSM-5, the conversion on Mn-ZSM-5 does not bend over up to
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500°C Ni-ZSM-5 has slightly lower activltles than Co-ZSM-5, and, hke CoZSM-5, the conversion bends over when Z’z=450’C The mtrlc oxide conversion on Cu-ZSM-5 is quite low Interestingly, H-ZSM-5 has some activity for reaction 1 Mn-ZSM-5 and Co-ZSM-5 are very similar not only in catalytic activity but also in their partial pressure dependencies Fig 5 compares the actlvltles of Mn-ZSM-5 and Co-ZSM-5 as a function of methane partial pressure The nltrlc oxide conversions on both catalysts monotonically increase with methane partial pressure; but Co-ZSM-5 has a higher conversion at any given methane concentration, and this difference increases with mcreasing methane partial pressure The reaction orders with respect to methane are 0 59 and 0 56, on Mn-ZSM-5 and Co-ZSM-5, respectively (Note the reaction orders were determined separately based on experiments with a sample size of 0.05 g, where conversions were maintained below 30% ) The dependencies of nitric oxide over these two catalysts on the nltrlc oxide partial pressure also show amazing slmllarity (Fig 6). Nitric oxide conversion actually increases with decreasing level of nitric oxide The reaction order with respect to nltrlc oxide are 0 52 and 0 44 over Mn-ZSM-5 and Co-ZSM-5, respectively Although the shapes of the convesion-temperature curves for the two catalysts are essentially identlCal,the optimum temperature for Mn-ZSM-5 is shifted to a high temperature The difference is also consistent with their methane conversions, Co-ZSM-5 has slightly higher methane conversions compared to Mn-ZSM-5 at any temperature below 450°C (Fig 7)
Ia
0
500
1
I
I
1000
I
I,
c
1
““I””
1500
2000
2500
lCH,l(ppm) Fig 5 Nhc ox& conversion as a fimctlon of methane mlet concentration on (H) Co-ZSM-5 ad(o)Mn-ZSM-5at4OO”C [NO]=1640ppm, [CH,I=1025ppm, [0,1=25%GHSV=7500
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-
40
-
Catal B 2 (1993) 239-256
249
. --2
20
-
OL”“““““““‘,” 0
1000
500
WI
1500
2000
ivm
Fg 6 Nltnc oxide conversIon as a function of nltnc oxide mlet concentration an ( n) Co-ZSMppm, [OS]=2 5% 5 and (0) Mn-ZSM-5 at 400°C [NO]=1640 ppm, [CH,]=1025 GHSV= 7500
20
1 -
10
o
)
.=
1
/.
/ It' I
20
i!
l.~,,,.,..,....l,...,...,10 300
350
400
450
Temperature Fq 7 NI~IIC ox& (&a ) and methane ZSM-5 (W,Ct) and Mn-ZSM-5 (0,O) GHSV = 7500
500
550
(“C)
( QO ) conversIons as a fun&on of texpperature on Co[NO]=1640 ppm, [CH,]=1025 pF/m, [0,1=25X,
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Interestmgly, the remarkable slmllarlty between Co-ZSM-5 and Mn-ZSM5 a8 catalyst for the mtrlc oxide reduction with methane doe8 not translate to other reactions A8 we reported earlier [ 141, Co-ZSM-5 was an effective cata-
lyst for the decomposition of nitrous oxide to nitrogen and oxygen, but MnZSM-5 was practically inactive obviously, more fundamental work on the interaction between nitrogen oxides (nitric oxide and mtrous oxide) and metal exchanged zeohtes (e g , Mn-ZSM-5 and Co-ZSM-5) 1s necessary The meffectlveness of Cu-ZSM-5 for the mtnc oxide reduction with methane 1s apparently due to Its strong ablhty to activate oxygen, on Cu-ZSM-5, methane 1smainly combusted when excess oxygen ISpresent m the feed (Table 3) In the absence of oxygen, however, Cu-ZSM-5 1s very active m reducmg mtrlc oxide with CHI In fact, a rutrlc oxide conversion of 53% was obtained at 400°C at GHSV = 30 000 m the absence of oxygen, the conversion decreased to 8% when 2 5% of oxygen was added In contrast, on Co-ZSM-5, oxygen enhance8 the rutrlc oxide reduction [ 71 Here, the nature of the transltlon metal center 1s very important m determmmg the actlvlty and selectlvlty for nltrlc oxide reduction We also tested some noble metal exchanged ZSM-5 samples, 1 e , Rh, Pd, Pt, and Ru None of the noble metal exchanged ZSM-5 samples showed any detectable nitric oxide reduction actlvltles under an oxl&zmg atmosphere, except Rh-ZSM-5 However, Rh-ZSM-5 was not stable under the expenmental con&tlons, deactivating with time The nitric oxide adsorption capacity on the used Rh-ZSM-5 was decreased dramatically compared to the fresh one We suspect that rhodmm oxide was formed durmg the reactlon, and rho&urn oxide (now no longer ion exchanged) 1s inactive for mtnc oxide reduction m an oxihzmg atmosphere H-ZSM-5 ha8 Borne actlvlty for the nitric oxide reduction, and the methane conversion 1s very low Therefore H-ZSM-5 1s very selective for the reduction of nltrlc oxide m the presence of oxygen Co-H-ZSM-5 has higher nltrlc oxide conver8lon at temperature8 higher than 450°C compared to Co-ZSM-5 and has lower methane conversion Note, on this catalyst the bendmg over of nltrlc oxide conversion was not observed up to 5OO”C, which 1s consistent with Its low methane conver8lon It 1s not clear at this pomt that this actlvlty mcrea8e 1s caused by the mteractlon between H+ and Co2+ or because NH,+ influences the Co2+ siting m ZSM-5 dunng the preparation
Cobalt on other supports To study the role of zeohte a8 a support, we exchanged Co2+ mto several type8 of zeohte and supported cobalt onto a number of oxides via either lmpregnatlon or ion-exchange These samples were tested for the nitric oxide reduction with methane, and their actlvltles are compared m Table 4 Co-morden& 1s active for the reaction but ha8 slightly lower activity compared to Co-
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TABLE 4 Conversions” over cobalt exchanged zeohtes and cobalt oxides Sample
&/Al
Me/AI
Metal loading (wt-%)
400°C
450°C
500°C
Co-ZSM-5 Co-M CO-Y Co-KL CoO/Al,O, CoO/T102 Co/TIOzb
140 53 25 29
0 0 0 0
40 56 118 35 11 26 10 16 30
23 17
34 27 5 9
30 24 6 11
coo/slllcallte
CO/SIO~-A&O,
70 47 67 18
7
na 6
“All zeohte samples were tested at GHSV=30 000 (0 1 g, 100 cm3/mm), [CH,] =O 10% and [0,]=2 5% “Co exchanged hydrous TIOz, and it was prepared according to ref 10 - Conversion was not detected na data not avadable
5
na [NO] =O 16%,
ZSM-5 Although the cobalt loadmg m Co-Y is three times of that m Co-ZSM5, Co-Y has very low activity for the mtnc oxide reduction even at 500°C These results demonstrate that not only the nature of the cation but also the type of zeohte is important in catalyzing this reaction It has also been observed that for other mtrlc oxide removal reactlons, e g , mtric oxide decomposition [ 111 and mtric oxide reduction with other hydrocarbons [ 151, metal exchanged zeohte Y is much less active than metal exchanged ZSM-5 or mordenite The zeohte topology and/or the crystal field must play a very important role m activating metal cation for selective nitric oxide reduction The importance of zeohte structure is further illustrated by the fact that none of the oxide supported cobalt samples have any appreciable activity for the n&c oxide reduction Some of the metal oxide supported samples were preparedvia the ion exchange method and have similar chemical composltrons, e.g , Co/slhca-alumma (117% A120, and 3 0% unreduced Co by weight), and the cobalt should be atomically dispersed Sihcahte on the other hand has the ZSM-5 topology but does not have the ion exchange capacity, and therefore the cobalt m slhcahte is m oxide form Thus clearly the zeohte exerts an important electromc effect as a support for the Co2+ It does not appear that the traditional features of zeohte, 1 e , acidity and shape selectivity, are important for reaction 1 Temperature-programmed
desorptum of nrtru: omde
Some Co-zeohte samples were characterized by TPD (temperature-programmed desorptlon of mtrlc oxide) Fig 8 shows the TPD profiles of nitric
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0
100
200
Temperature
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400
Catal B 2 (1993) 239-256
500
(“C)
Fig 8 TPD of mtnc oxide on Co-zeohtes The samples were pretreated m flowmg hehum at 500 ’C for 1 h followed by mtrlc oxide adsorptlon at 25°C (m) Co-Y, (0 ) Co-mordemte, (A ) Co-ZSM5 TABLE 5 Nltrlc oxide TPD results The amounts of the mtnc oxide adsorbed were obtained by integration of TPD profiles (desorption rate vs tune) Sample”
Co/Al
Na/Al
Co content (mmol/g)
NO adsorbed (mmol/g)
NO/Co
CO-Y
0 67
Co-MOR Co-ZSM-5 Co-ZSM-5 Co-ZSM-5
0 47 0 70 049 0 23
0 27 0 41 006 0 20 0 46
20 0 95 0 68 0 45 0 22
0 0 0 0 0
0 06 0 75 109 158 132
12 71 74 71 29
“All samples were pretreated with flowing helium at 500°C for 1 h before nitric oxide adsorption at 25°C
oxide on Co-ZSM-5 (S1/Al= 13 6, Co/Al = 0.70)) Co-morden1t.e (S1/Al= 5.3, Co/Al=0 47) and Co-Y (S1/Al= 2 4, Co/Al=0 67) Co-ZSM-5 and Co-morden1te had very similar TPD profiles for both desorptlon peak-temperature and intensity. Five desorptlon peaks were observed at about 90,150,210,250 and 38O”C, and the peak mtenslty decreased with increasing peak-temperature QuanWatlvely, the amount of nitric oxide adsorbed/desorbed (Table 5 ) was 0 74 and 0 71 mmol/g for Co-ZSM-5 and Co-mordenlte, respectively Be-
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cause of their different cobalt loadings, the ratios of NO/Co are 11 and 0.8 for Co-ZSM-5 and Co-mordenite, respectively. Na-ZSM-5 does not adsorb mtnc oxide under the same conditions The TPD profile on Co-Y, however, is substantmlly different from those on Co-ZSM-5 and Co-mordemte. Nitric oxide desorptlon peaks were detected at 90,150,210 and 36O”C, the peak at 360°C 1svery broad, from 300 to 480°C Although Y zeohte contams the largest amount of cobalt cations, its mtnc oxide adsorption capacity is the smallest (0 12 mmol/g ), and the NO/Co ratio is therefore even smaller, 0.06 The very broad mtnc oxide desorption peak of Co-Y at ca 360°C is hkely the result of a diffusion hmlted process from the sodahte cage into the gas phase, and those sites may not be readily accessible by the reactants Apparently, the mtnc oxide conversion of a catalyst is proportional to its mtnc oxide adsorption capacity at room temperature (Table 5), and the very low catalytic activity of Co-Y may be related to the mabihty of mtnc oxide to adsorb on this material. The dramatic difference between the adsorption capacities (NO/Co) of Co-ZSM-5 and Co-Y suggests the very &fferent degrees of coordinative saturation of the cobalt ion. A cobalt cation m ZSM-5 can accommodate more than one nitric oxide molecule, but a cobalt cation m Y can only hold 0 06 nitric oxide molecule If the formation of dmitrosyl species on a transition metal cation is a necessary condition for nitric oxide conversion to mtrogen, Co-Y obviously does not meet this requirement The TPD of nitnc oxide was also measured on Co-ZSM-5 as a function of cobalt exchange level, where 3 cobalt loadings were compared. As shown m Fig 9, the TPD profiles are very similar, but with different mtric oxide adsorption capacities. The NO/Co ratios for all three samples are larger than 1, with the highest being 16 (Table 5). This indicates that at least on some cobalt cations mtnc oxide adsorbed as a dimtrosyl (2 NO molecules per Co2+ ion) The adsorption stoichiometry may vary with the adsorption temperature and partial pressure of mtnc oxide used It is mterestmg to compare the TPD profiles between Co-ZSM-5 and CuZSM-5 [lo] Cu-ZSM-5 1sa umque catalyst for the direct mtnc oxide decomposition to nitrogen and oxygen [ 71, but a poor catalyst for mtnc oxide reduction with methane in the presence of oxygen. Conversely, Co-ZSM-5 is not active for mtnc oxide decomposition but an effective catalyst for mtnc oxide reduction Further, these two materials demonstrate completely opposite 0, effects on the mtnc oxide conversion for NO+CH4, addition of oxygen suppresses the activity on Cu-ZSM-5, but enhances the activity greatly on CoZSM-5. A drastic difference was seen between the mtnc oxide adsorption at room temperature on the two samples. On Cu-ZSM-5 a stoichiometnc reaction was observed when nitnc oxide was adsorbed at 25°C [ 151,whildon Co-ZSM5,no appreciable amount of other species was observed except mtnc oxide On Cu-ZSM-5, nitrous oxide was also desorbed at low temperatures Oxygen formed
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0
100
200
Temperature
300
400
Catal B 2 (1993) 239-256
500
(“C)
Fig 9 TPD profiles of Co-ZSM-5 as a function of cobalt loading The samples were pretreated m flowing hehum at 500’ C for 1 h followed by nltrlc oxide adsorption at 25” C ( 0 ) Co/Al = 0 70, (0) Co/Al=0 49, (A) Co/Al=0 23
during the room-temperature reaction was desorbed at ca 36O”C, comcldent wrth the highest nitric oxrde desorptron peak On Cu-ZSM-5 the extra lattice oxygen 1s activated, the oxygen held by copper ions can be readily lost and gamed during a redox cycle (e g , CO/O2 cycle ) and the copper ions cycle between Cu2+ and Cu+ [ 111 Carbon monoxtde can react wrth an extra-lattice oxygen atom forming a carbon droxrde molecule, as a consequence, the Cu2+ 1s reduced to Cu+ The loss or gam of oxygen can be easrly monitored by observing the weight change with a mrcrobalance, and the change of copper valence state can be morutored with electron spin resonance (ESR) spectroscopy To test the redox ability of cobalt m Co-ZSM-5, we carried out a temperature-programmed reductron study on the Co-ZSM-5 wrth a l%CO/He mrxture (The sample was pre-oxrdrzed at 500°C m a lO%O,/He mixture for 1 h, cooled to room temperature m the flowing O,/He mixture, then the catalyst was flushed with a stream of He ) Carbon monoxrde consumptron and carbon droxlde formatron were not observed up to 500” C This suggests that the extra lattice oxygen atoms m Co-ZSM-5 are not active enough to oxrdrze carbon monoxrde wrth a change of the cobalt valence state Under our experimental condrtrons, an oxrdrzmg atmosphere, It 1s unlikely that a redox process for cobalt catrons 1s mvolved durmg the reaction Perhaps the stronger bondmg of the extra lattrce oxygen m Co2+ m Co-ZSM-5 reflects the selectlvlty of this catalyst toward rutrlc oxide reduction The requirement of the presence of gaseous oxygen for the NO, reduction
Y LI and J N Armor/Appl Catal B 2 (1993) 239-256
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suggests gaseous oxygen may be mvolved m the formation of reaction mtermediates Unhke nitric oxide, mtrogen dioxide can be effectively reduced by methane in the absence of oxygen Nitrogen &oxide is known be a stronger oxldmmg agent than mtnc oxide Tlus leads us to beheve that a mtrogen &oxide species may be formed on catalyst when mtric oxide and oxygen coexist, and perhaps this bound nitrogen &oxide species is an important catalytic mtermedate This, however, is not to exclude other posslbihtles, e g., forming oxygenated carbon species On the other hand, methane as a reducmg agent must be activated Although detectable amount of methane adsorption was not observed on Co-ZSM-5 at room temperature, activated adsorption of methane is known to occur on Ni-ZSM-5 [ 171 and Ni0/A1203 [ 181, either dlssociatlvely ( formmg C and Hz) or associatively Quahtatively, we also detected apparent carbon deposition on Co-ZSM-5 with a steam of CH,/He mixture flowing through the catalyst at elevated temperatures (the temperature was ramped from 25 to 500”C ) as evidenced by the formation of carbon &oxide when the catalyst was titrated with a mixture of O,/He at 500°C At this point we do not have direct observation of any mtermediate species Further we do not understand what makes Co-ZSM-5 (or Co-mordemte) so &fferent from Co-Y and what are the structural requirements for an effective catalyst All these however, remam a challenge for future research CONCLUSION
We found that Co2+, Mn2+ and Ni2+ exchanged ZSM-5 and mordemte are active for the reduction of mtnc oxide with methane m the presence of excess oxygen The nitric oxide conversion bends over with mcreasmg temperature, and this behavior is reversible upon decreasing temperature Apparently, the bending over of nitric oxide conversion is related to methane combustion reaction and therefore to the level of methane remammg m the feed The nitric oxide conversion on a Co-ZSM-5 catalyst is proportional to the level of exchanged Co2+ , but excess amounts of Co2+ do not contribute to the mtnc oxide reduction Below a certam exchange level, the turnover frequency for mtrrc oxide reduction IS independent from Co2+ exchange level We beheve that a single Co2+ cation is an active site The type of metal ion is very important m determining the selectivity and activity for the nitric oxide reduction, Co2+ is selective but Cu2+ is not The type of zeohte as a support IS also crucial, CoZSM-5 and Co-mordemte are active but Co-Y not Our TPD results show that the amount of nltnc oxide adsorbed on Co-Y ISvery small (0 06 NO/Co) compared to Co-ZSM-5 (NO/Co > 11) or Co-mordemte (NO/Co = 0 8) It seems that the traditional properties associated with zeobtes, e g , acidity and shape selectivity, are not important for this reaction, the electromc influence of the cations by the zeohtes maybe more important
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ACKNOWLEDGEMENTS
Thanks are due to Paula Battavlo for the actlvlty measurements and Tom Braymer for the catalyst preparations We thank Air Products and Chemicals, Inc for the permission to publish this work
REFERENCES
5 6 7 8 9 10 11 12 13 14 15 16 17
18
H Hamada, Y Kmtalchl, M Sasakl and T Ito, M Tabata, Appl Catal ,64 (1990) Ll H Hamada, Y Kmtalchl, M Sasakl and T Ito and M Tabata, Appl Catal ,70 (1991) L15 S S Sato, H Hlrabay, H Yahlro, N Mlzuno and M Iwamoto, Catal Lett ,12 (1992) 193 B Hmson, M , Wyatt and K G Gough, Catalysis, Royal Society of Chemistry, London, 1982, Vol 5, p 127-171 M IwamotoandN Mxzuno, J Auto Eng, m press Y Ll and J N Armor, US Patent 5 149 512 (1992) Y Ll and J N Armor, Appl Catal B, 1 (1992) L31 0 J Adlhart, S G Hmdm and R E Kenson, Chem Eng Prog ,76 (1971) 73 V P Shlrabar and A Clearfield, Zeohtes, 9 (1989) 363 H P Stephens and R G Dosch, m B Delmon, P Grange, P A Jacobs and G Poncelet (Editors), Preparation of Catalysts IV, Elsevler, Amsterdam, 1987, p 271 Y Ll and W K Hall, J Catal, 129 (1991) 202 Y LlandJ N Armor, J Catal,mpress M Iwamoto, H Yashlro, H Ooe, K Y Banno, F Okamoto, Shokubal, 32 (1990) 91 Y Ll and J N Armor, Appl Catal B, 1 (1992) L2 1 M Sasakl, H Hamada, Y Kmtalchl and T Ito, Catal Lett ,15 (1992) 297 Y Ll and J N Armor, Appl Catal ,76 (1991) Ll J Hoffman, R Bauermelster, B Hunger, K Hantel, G Ulhnann, K -H Stemberg and H Siegel, m G Ohhnann, H Pfelffer and R Frxke (Editors), Catalyst and Adsorption by Zeohtes (Studies m Surface Science and Catalysis, Volume 65), Elsevler, Amsterdam, 1991, p 367 C H , Barththolomew and H Y Hsleh, AIChE 1992 Annual Meetmg, Mlaml Beach, Florida, Nov l-6.1992, paper No 50b
239
APCAT BOO44
Selective catalytic reduction of NO, with methane over metal exchanged zeolites Yuejm Li and John N. Armor ALLProducts and Chemrcals, Inc , 7201 Ham&on Boulevard, Allentown, PA 18195 (USA) (Fkcewed 21 January 1993, revised manuscnpt received 8 March 1993)
Abstract Catalytic reduction of NO, with methane m an oxldlzmg atmosphere was studled over metal exchanged zeohtes We found that the combmatlons of Co’+, Mn’+ and Nl*+ with certain types of zeohtes, e g , ZSM-5 and mordemte, are active catalysts for this reactlon For a Co-ZSM-5 catalyst, the mtnc oxide conversion displays a volcano-shape curve as temperature mcreases, which 1s reversible upon decreasing temperature The mtnc oxide reduction actlvlty 1s proportional to the level of Co2+ exchanged into ZSM-5, but excess amounts of cobalt do not contnhute to the activity Mn-ZSM-5 1s very slmdar to Co-ZSM-5 m the mtnc oxide reduction actlvlty, and NI-ZSM-5 has slightly lower activity compared to Co-ZSM-5 Under an oxldlzmg condltlons, Cu-ZSM-5, however, 1s meffectlve for the rutnc oxide reduction Co-Y, which has much more Co2+, 1smuch less active compared to Co-ZSM-5, or Comordemte The amount of mtnc oxide adsorbed, measured by TPD, on Co-Y 18extremely small (NO/ Co=0 06) compared to Co-ZSM-5 (NO/Co> 1 1) and Co-mordemte (NO/Co=0 8) Keywords CH,, Cobalt, emlsslon control, metal exchange, methane, nitrogen oxldes, NO,, reduction, zeohte, ZSM-5
INTRODUCTION
Recently, there has been a strong mterest worldwrde m pursumg alternative approaches to abate NO, ermssions from both stationary and mobile sources The reduction of NO, with hydrocarbons, instead of using ammonia, m an oxldrzmg atmosphere is a subject of intense research By usmg hydrocarbons, the problems, e g , ammoma shp, transportation of ammonia through resldenteal areas and equipment corrosion, associated with the selective catalytic reduction (SCR) process can be avoided Currently, propane, propene and ethylene are the most mtensely mvestigated hydrocarbons for NO, reduction [l31 In all these early reports, the presence of O2 is essential for tHe NO, reduction, demonstratmg that hydrocarbons can be effective for the selective reducCorrespondence to Dr J N Armor, Air Products and Chemicals, Inc ,720l Ham&on Allentown, PA 18195, USA Tel ( + l-215)4815792, fax ( + l-215)4812989
0926-3373/93/$06
00 0 1993 Elsevler Science Publishers
B V All nghts reserved
Boulevard,
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tlon of NO, However, the use of methane as a selective reducmg agent for NO, was never reported. In fact, methane 1s often regarded as a non-selective reducing agent for NO, [4,5 ] Methane 1s &fficult to activate because of the strong C-H bond (101 kcal/mol) which often necessitates high reaction temperatures It 1s well known that methane reacts much faster with oxygen than with mtrlc oxide on most catalysts, and to use methane selectively for NO, reduction 1s indeed a great challenge On the other hand, with the abundance of natural gas and the wide spread use of natural gas as a fuel by many utlhtles, it certamly would be desirable to use methane as a reductant Recently, we Ascovered [6] that NO, could be selectively reduced with methane m an ox&zmg atmosphere over a class of metal exchanged zeohtes We observed (7) complete nitric oxide conversion at 400’ C with methane over a Co-ZSM-5 catalyst Contrary to previous reports on this reaction [8], we found that on Co-ZSM-5 the catalytic actlvlty 1sgreatly enhanced by the presence of oxygen, and the actlvlty 1s proportional to the level of methane m the feed We also found that Co2+ exchanged zeohte Y, which has much higher cobalt loadmg compared to Co-ZSM-5, IS much less active for nltrlc oxide reduction Supported cobalt oxide was completely inactive Our earlier results suggest that certain types of zeohtes are required to posltlon cobalt cations provldmg a suitable electronic environment to carry out the selective reduction of nitric oxide with methane In this paper, we wish to elaborate upon our earlier report by describing the role of zeohtes, demonstrating the importance of the metal center, and reporting our temperature-programmed desorptlon stu&es EXPERIMENTAL
Catalyst preparatwn All our zeohte samples were prepared by exchangmg an appropriate cation into a zeohte m an aqueous solution Na-ZSM-5, a starting material for cation exchange, was synthesized m house via a template-free method [6,9] Zeohte Y (LZ-Y-52) and mordenlte (LZ-M-5) were obtamed from Union Carbide m the Na+ form Acetate metal salts were used for the exchange The metal exchange was typically carried out at 80’ C for 24 h with a &h&d (0 01-O 02 M) metal cation solution After exchange the sample was exhaustwely washed mth deionized water, filtered and dried at 110°C overnight The Co-H-ZSM-5 was made by exchangmg Co2+ into an NH,-ZSM-5, where NH,-ZSM-5 was obtamed by three successive exchanges of Na-ZSM-5 with NH4N03 solution (1 M) at room temperature H-ZSM-5 was obtamed by heating a fully exchanged NH,-ZSM-5 at 500°C m hehum for 1 h The metal exchanged zeohte samples were analyzed for Sl, Al, Na and the exchanged metal using mductlvely coupled plasma-atomic emlsslon spectroscopy CoO/Al,O, and other supported cobalt
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241
oxide samples were prepared by the standard mclplent wetness technique usmg a cobalt rutrate solution; the preparation was drred at llO”C, and later calclned 1n a 10% O,/He mixture at 500°C for 1 h in situ. Co/slhca-alumma was obtamed by exchangrng Co2+ mto an amorphous silica-alumma support (Davrson S111ca-Alumina Catalyst Support, Grade 135, W R Grace & Co.) m a cobalt acetate solution 1n a manner similar to the Co-zeollte preparation. CoT102 was made by exchangmg Co2+ mto a Na+ containing hydrous T102 prepared by hydrolyzing the product of the reaction between tltanlum 1sopropox1de and NaOH 1n CH30H [lo] Reactma studces The catalytic actlvltles were measured using a micro-catalytic reactor 1n a steady-state plug flow mode The reactor was a U-shaped glass tube wrth l/4 m 0 D at the inlet and 3/8 m 0 D at the outlet. To reduce pressure drop, the catalyst was pelletlzed, crushed and then sieved to 60-80 mesh before use A 0 10 g to 0.4 g of sample was used for actlvlty measurements and partial pressure dependence &u&es However, kmetlc measurements, e.g , reaction order determmatlons, were made using a 0 05 g sample, and the conversions were controlled below 30%. At temperature programmer (Yokogawa, Model UP 40 ) with a J-type of thermocouple m contact with the catalyst bed was used to control the temperature The typical temperature ramp rate was 5”C/mm and the flow-rate of the feed was 100 cm3/mm (GHSV= 30 000 or 7506 based on the apparent density of the zeohte catalysts, ca 0 5 g/cm3) with the flows independently controlled by a 4-channel mass flow meter/controllers (Brooks 5850) The reaction mixture typically consisted of 1640 ppm NO, 1025 ppm CH4, and 2.5% 0, (balance as He) Zeol1te catalysts were pretreated m situ 1n flowing helium at 500’C for 1 h, and supported metals were oxuhzed m a 10% O,/He mixture at 500°C for 1 h before reaction A Var1an 6000 gas chromatograph (GC) with a TCD detector was used to monitor catalytic actlvlty A molecular sieve 5A column (l/8 1n x 10 ft ) was used to separate 02, N, and CH4 at 25°C The n1tnc oxide conversion was calculated based on the N2 formation, and the methane conversion based on the CH, consumption Selectlv1ty of methane IS defined as the molar rat10 of the CHI reacted with NO to the CH, totally consumed, and it was calculated based on the followmg equation Selectivity =
(~c~,/~~o)xC~ox~tx
[NO],
G-t, xFt x [(-Xl,
where, oCHl and ONoare the stolchlometrrc numbers of methane and mtrrc oxide, respectively based on reaction 1 (see Results and Discussion ), Ft 1stotal volumetric flow-rate of the feed, [NO], and [ CHIlo are the inlet concentrat1ons (ppm) of nitric oxide and methane, respectively, CNo and CCHlare the conversions of n1trlc oxide and methane, respectively Occasionally, a Porapak
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Q column (column temperature = 50 o C ) was used to analyze for carbon &oxide formation and check the material balance Near 100% carbon balance was achieved (Methane was oxidized to carbon dioxide) An on-hne mass spectrometer (MS) (UT1 106C) equipped wrth an on-hne, atmospheric sampling device, was also used to monitor the gas effluent Both the GC and MS could be operated simultaneously Temperature-programmed
desorptux
studtes
For a typical TPD measurement, a 0 1 g sample was used and pretreated with a helium flow at 500 oC for 1 h (The catalyst bed was about 0 5 cm deep ) The mtrrc oxide adsorption was carried out at 25 oC by flowing a NO/He mixture (1620 ppm, 100 cm3/mm) through the sample, and the effluent from the exit of the reactor was continuously monitored by a mass spectrometer, a levehng-off in mtrrc oxide mtensrty &mated the saturation of the sample with nitric oxide A color change of the catalyst (from light blue to light brown) was observed during the mtrlc oxide adsorption on a dry Co-zeohte sample, and this color change progresses down the bed as the adsorption proceeds Typlcahy, 15 mm is sufficient to achieve a saturation for nitric oxide adsorption After the mtric oxide adsorption, the sample was then flushed wrth a stream of hehum at 25 oC to eliminate gaseous nitric oxide and weakly adsorbed nitric oxide As the gaseous mtrlc oxide level, monitored by a mass spectrometer, returned to near the background level of the mass spectrometer, the sample was heated up to 506 oC with a ramp rate of 8 oC/mm in flowing of hehum ( 100 cm3/mm), and the desorbed species were monitored continuously by the mass spectrometer as a function of time/temperature The mass spectrometer was calibrated for NP, Oz, NO, N,O and other relevant species, and quantitative analysis was possible RESULTS AND DISCUSSION
Co-ZSM-5 Recently Co-ZSM-5 was described as one of the more active catalysts for the reduction of mtrlc oxide with methane, and the presence of oxygen greatly enhanced the catalytic activity [7] Because oxygen is essential to the nitric oxide reduction, the stolchlometry of this reaction can be expressed by reaction 1 In addition, methane combustion also occurs (reaction 2 ) These two reactions compete for use of methane The temperature dependencies of nitric oxide and methane conversions were studied on a Co-ZSM-5 catalyst 2NO+CH4+02+N2+C02+2H20
(1)
CH4+202+COz+2H20
(2)
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243
Fig 1 shows the mtric oxide conversson to rutrogen (reaction 1) over a CoZSM-5 catalyst as a function of temperature at two space velocltles At a GHSV=30 000 the conversion increases with reaction temperature, but the conversion curve bends down at T> ca 460 oC The optimum temperature is dependent on the space velocity, at a GHSV = 7500, the maximum mtrlc oxide conversion was obtamed at ca 420”C In either case, the bendmg over of conversion is entirely reversible upon decreasing temperature, suggesting that this effect is a kinetic phenomenon, rather than due to any structural damage of catalyst at high temperatures At first sight, the bending over of conversion on Co-ZSM-5 catalyst appears similar to that observed on Cu-ZSM-5 for the direct decomposltlon of mtrlc oxide [ 111 A key difference between the two catalysts is that methane combustion is a side reaction (reaction 2) for mtrlc oxide reduction on Co-ZSM-5 At high temperatures the methane combustion rate on Co-ZSM-5 becomes ngmticant. As illustrated in Fig 2, with GHSV = 30 000 the methane conversion 1s nearly complete at 500”C at which point the nitric oxide conversion begms to fall off At GHSV = 7500, the methane combustion rate IS even higher at a given temperature, reaching 100% at 450 ’C Apparently, at either space velocity a maximum nitric oxide conversion is obtained at the temperature at which ca 80% of methane was consumed It is known that the mtrlc oxide reduction rate IS proportional to the level of methane m the feed [ 71 The bending over of conversion at high temperature, to a large extent, may be the result of slgmficantly lowered partial pressure of
300
350
400
Temperature
450
500
550
(“C)
Fig 1 Nltnc oxide converslon as a function of temperature and space velocity on a Co-ZSM-5 catalyst [NO] = 1640 ppm, [CH,] = 1025 ppm, [OJ =2 5% (0) GHSV 7500, (0 ) GHSV 30
000
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Catal B 2 (1993) 239-256
60
40
20
0
I 300
350
400
Temperature
450
500
550
(“C)
Fig 2 Methane converslon as a function of temperature and space velocity on a Co-ZSM-5 catalyst [NO] = 1640 ppm, [ CH,] = 1025 ppm, [ 0, ] =2 5% Symbols as in Fig 1
methane m the reactor Running the reaction at high space velocltles, therefore with low conversions, certamly raises the temperature at which catalyst performance for reactlon 1 falls off With GHSV=30 000, the selectlvltles are 96, 51, 38 and 23% at 350, 400, 450 and 500’ C, respectively Note that the level of oxygen ISmuch higher than those of other reactants ( [O,] / [NO] = 15 6 and [O,]/ [CH,] = 25 at the mlet), and the selectlvltles for methane to reaction 1 are especially high at low temperatures The high selectlvlty of Co-ZSM-5 IS m clear contrast to metals, metal oxides and some other metal exchanged zeohtes, e g , Cu-ZSM-5, as catalysts A reactlon was carried out on a Co-ZSM-5 catalyst at 400 “C to test the short term stablhty of the catalyst As shown m Fig 3, a stable conversion (45% ) was obtained lmmedately at the begmnmg of the run, and the conversion &d not decrease even after 20 h of contmuous reaction The effect of water vapor on the mtrlc oxide conversion was also tested A moderate decrease m activity was observed For example, on a Co-ZSM-5 the mtrlc oxide conversion decreases from 53 to 28% and from 40 to 35% at 450 and 5OO”C, respectively upon addmg 2% H,O vapor to the feed (GHSV= 30 000, [NO] =800 ppm, [ CH4] = 1000 ppm, IO,] =2 5% ) However, the effect was entirely reversible after ehmmatmg water from the feed Moreover, the nltrlc oxtde conversion m the presence of 2% H,O IS stable overnrght at these high temperatures with Co-ZSM-5 Thus, water inhibits, but not poisons, the mtrlc oxide reduction on
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0
5
10
Time
15
20
25
(h)
Fig 3 Nltnc ox& convemlon aa a fun&on of time at 400”C on a Co-ZSM-5 catalyst [NO ] = 1640 ppm, [CH,] =1025ppm, [O,] =2 5%, GHSV=7500
Co-ZSM-5 The nature of the water effect and its influence on kinetics will be &scussed m dem elsewhere [ 121 The catalytic activities of Co-ZSM-5 for reaction 1 were measured on a series of Co-ZSM-5 catalysts with different cobalt exchange levels but with the same &/Al ratio, 14 The conversslons of mtrlc oxide over these catalysts at 400,450 and 500°C are shown m Fig 4. At 400 and 45O”C, the conversion increases mth the level of cobalt exchanged and levels off when Co/Al > ca 0 5, the theoretical cation exchange capacity At 5OO”C, the conversion increases wrth the cobalt loadmg when Co/Al
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25 20 15 10 5 0 0
01
02
03
04
05
06
07
06
Co/Al
Fig 4 Nltnc oxide conversion as a function of cobalt loading and temperature on Co-ZSM-5 catalysts [NO]=1640ppm, [CH,]=1025ppm, [0,]=25%,GHSV=30000 (0) 400°C (0) 45O"C,(A) 500°C TABLE 1 Turnover frequencies of Co-ZSM-5 for the nltnc oxide reduction as a function of cobalt loading Co/AI”
0 22 0 26 0 33 0 70 081
Turnover frequencyb X lo3 (8-l) 400°C
450°C
500°C
0 62 0 71 061 0 34 0 35
098 100 100 0 55 0 54
120 1 12 1 12 0 47 0 48
“Sl/AI = 14 qhe turnover frequencies were measured at [NO I = 1640 ppm, and total pressure = 1 atm
[CI-LI= 10% PPm,1% I= 2 5%
haps two kmds of s&es exrst This findmg 1sm contrast to the situation for CuZSM-5 for mtrrc oxide decomposition [ 71, where the TOF mcreases with the copper exchange level and the over exchanged Cu-ZSM-5 is more active. In the case of Cu-ZSM-5 copper pairs (or dimmers) were suggested [ 131 as the catalytic sites for the nitric oxide decompositron m the Cu-ZSM-5 system. It seems that on Co-ZSM-5 a single Co2+ cation may be a sufficient catalyst site for the mtric oxide reduction with methane Table 2 shows the mtric oxide conversron as a function of &/Al ratio of
247
Y LL and J N Arnaor/Appl Catal B 2 (1993) 239-256 TABLE 2 Nltnc oxide convermon as a function of Sl/Al ratlo Sl/Al
Co/Al
Conversion”
14 23 38
0 70 102 0 67
34 19 9
“rhe conversions GHSV = 30 000
(% )
were measured at 45O”C, [NOI = 1640 ppm,
[CKI =lO% ppm, 1% =2 5%,
TABLE 3 Nltnc oxide and methane conversions” sample
Co-ZSM-5 Mn-ZSM-5 NI-ZSM-5 Cu-ZSM-5 Co-H-ZSM-5 H-ZSM-5
metal/Al
0 70 0 53 0 70 060 038 -
on metal cation-exchanged
ZSM-5 zeohtes 450°C
metal loadmg (wt%)
400°C NO
CH,
NO
CH,
NO
CH,
40 31 43 37 23
23 17 16 8 18 4
26 20 12 60 16 5
34 30 26 8 34 6
70 58 40 96 42 10
30 32 20 na 42 10
100 92 73 na 78 13
“All samples were tested at GHSV=30 ppmand [O,] =2 5% - Conversion was not detected na Data not available
000 (0 1 g, 100 cm3/mm),
500°C
[NOI =X40,
[C%I =I025
ZSM-5 All these ZSM-5 samples had excess amounts of Co2+ Given the fact that excess amounts of Co2+ do not contribute to the mtrlc oxide reduction, we assume the mtrlc oxide conversrons obtained on those catalysts reflect the actlvlty of the fully exchanged ZSM-5 catalysts We found the mtnc oxide conversions increase rather linearly with decreasing &/Al ratio, 1 e with mcreasing the cation exchange capacity Thus the overall actlvlty of a catalyst 1sproportional to the number of the exchanged Co2+ m the zeohte Therefore, the TOFs of these catalysts are comparable Other catwn exchanged ZSM-5 Table 3 shows the mtrlc oxide conversrons on a varrety of catron-ZSM-5 catalysts as a functron of temperature Mn-ZSM-5, which has slmllar metal loading as Co-ZSM-5, has comparable actlvltles for nitric oxide reduction, but, unlike Co-ZSM-5, the conversion on Mn-ZSM-5 does not bend over up to
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500°C Ni-ZSM-5 has slightly lower activltles than Co-ZSM-5, and, hke CoZSM-5, the conversion bends over when Z’z=450’C The mtrlc oxide conversion on Cu-ZSM-5 is quite low Interestingly, H-ZSM-5 has some activity for reaction 1 Mn-ZSM-5 and Co-ZSM-5 are very similar not only in catalytic activity but also in their partial pressure dependencies Fig 5 compares the actlvltles of Mn-ZSM-5 and Co-ZSM-5 as a function of methane partial pressure The nltrlc oxide conversions on both catalysts monotonically increase with methane partial pressure; but Co-ZSM-5 has a higher conversion at any given methane concentration, and this difference increases with mcreasing methane partial pressure The reaction orders with respect to methane are 0 59 and 0 56, on Mn-ZSM-5 and Co-ZSM-5, respectively (Note the reaction orders were determined separately based on experiments with a sample size of 0.05 g, where conversions were maintained below 30% ) The dependencies of nitric oxide over these two catalysts on the nltrlc oxide partial pressure also show amazing slmllarity (Fig 6). Nitric oxide conversion actually increases with decreasing level of nitric oxide The reaction order with respect to nltrlc oxide are 0 52 and 0 44 over Mn-ZSM-5 and Co-ZSM-5, respectively Although the shapes of the convesion-temperature curves for the two catalysts are essentially identlCal,the optimum temperature for Mn-ZSM-5 is shifted to a high temperature The difference is also consistent with their methane conversions, Co-ZSM-5 has slightly higher methane conversions compared to Mn-ZSM-5 at any temperature below 450°C (Fig 7)
Ia
0
500
1
I
I
1000
I
I,
c
1
““I””
1500
2000
2500
lCH,l(ppm) Fig 5 Nhc ox& conversion as a fimctlon of methane mlet concentration on (H) Co-ZSM-5 ad(o)Mn-ZSM-5at4OO”C [NO]=1640ppm, [CH,I=1025ppm, [0,1=25%GHSV=7500
Y Lz and J N ArmorjAppl
60
-
40
-
Catal B 2 (1993) 239-256
249
. --2
20
-
OL”“““““““‘,” 0
1000
500
WI
1500
2000
ivm
Fg 6 Nltnc oxide conversIon as a function of nltnc oxide mlet concentration an ( n) Co-ZSMppm, [OS]=2 5% 5 and (0) Mn-ZSM-5 at 400°C [NO]=1640 ppm, [CH,]=1025 GHSV= 7500
20
1 -
10
o
)
.=
1
/.
/ It' I
20
i!
l.~,,,.,..,....l,...,...,10 300
350
400
450
Temperature Fq 7 NI~IIC ox& (&a ) and methane ZSM-5 (W,Ct) and Mn-ZSM-5 (0,O) GHSV = 7500
500
550
(“C)
( QO ) conversIons as a fun&on of texpperature on Co[NO]=1640 ppm, [CH,]=1025 pF/m, [0,1=25X,
250
Y Li and J N ArmorjAppl
Catal B 2 (1993) 239-256
Interestmgly, the remarkable slmllarlty between Co-ZSM-5 and Mn-ZSM5 a8 catalyst for the mtrlc oxide reduction with methane doe8 not translate to other reactions A8 we reported earlier [ 141, Co-ZSM-5 was an effective cata-
lyst for the decomposition of nitrous oxide to nitrogen and oxygen, but MnZSM-5 was practically inactive obviously, more fundamental work on the interaction between nitrogen oxides (nitric oxide and mtrous oxide) and metal exchanged zeohtes (e g , Mn-ZSM-5 and Co-ZSM-5) 1s necessary The meffectlveness of Cu-ZSM-5 for the mtnc oxide reduction with methane 1s apparently due to Its strong ablhty to activate oxygen, on Cu-ZSM-5, methane 1smainly combusted when excess oxygen ISpresent m the feed (Table 3) In the absence of oxygen, however, Cu-ZSM-5 1s very active m reducmg mtrlc oxide with CHI In fact, a rutrlc oxide conversion of 53% was obtained at 400°C at GHSV = 30 000 m the absence of oxygen, the conversion decreased to 8% when 2 5% of oxygen was added In contrast, on Co-ZSM-5, oxygen enhance8 the rutrlc oxide reduction [ 71 Here, the nature of the transltlon metal center 1s very important m determmmg the actlvlty and selectlvlty for nltrlc oxide reduction We also tested some noble metal exchanged ZSM-5 samples, 1 e , Rh, Pd, Pt, and Ru None of the noble metal exchanged ZSM-5 samples showed any detectable nitric oxide reduction actlvltles under an oxl&zmg atmosphere, except Rh-ZSM-5 However, Rh-ZSM-5 was not stable under the expenmental con&tlons, deactivating with time The nitric oxide adsorption capacity on the used Rh-ZSM-5 was decreased dramatically compared to the fresh one We suspect that rhodmm oxide was formed durmg the reactlon, and rho&urn oxide (now no longer ion exchanged) 1s inactive for mtnc oxide reduction m an oxihzmg atmosphere H-ZSM-5 ha8 Borne actlvlty for the nitric oxide reduction, and the methane conversion 1s very low Therefore H-ZSM-5 1s very selective for the reduction of nltrlc oxide m the presence of oxygen Co-H-ZSM-5 has higher nltrlc oxide conver8lon at temperature8 higher than 450°C compared to Co-ZSM-5 and has lower methane conversion Note, on this catalyst the bendmg over of nltrlc oxide conversion was not observed up to 5OO”C, which 1s consistent with Its low methane conver8lon It 1s not clear at this pomt that this actlvlty mcrea8e 1s caused by the mteractlon between H+ and Co2+ or because NH,+ influences the Co2+ siting m ZSM-5 dunng the preparation
Cobalt on other supports To study the role of zeohte a8 a support, we exchanged Co2+ mto several type8 of zeohte and supported cobalt onto a number of oxides via either lmpregnatlon or ion-exchange These samples were tested for the nitric oxide reduction with methane, and their actlvltles are compared m Table 4 Co-morden& 1s active for the reaction but ha8 slightly lower activity compared to Co-
Y Lt and J N Armor/Appl
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Catal B 2 (1993) 239-256
TABLE 4 Conversions” over cobalt exchanged zeohtes and cobalt oxides Sample
&/Al
Me/AI
Metal loading (wt-%)
400°C
450°C
500°C
Co-ZSM-5 Co-M CO-Y Co-KL CoO/Al,O, CoO/T102 Co/TIOzb
140 53 25 29
0 0 0 0
40 56 118 35 11 26 10 16 30
23 17
34 27 5 9
30 24 6 11
coo/slllcallte
CO/SIO~-A&O,
70 47 67 18
7
na 6
“All zeohte samples were tested at GHSV=30 000 (0 1 g, 100 cm3/mm), [CH,] =O 10% and [0,]=2 5% “Co exchanged hydrous TIOz, and it was prepared according to ref 10 - Conversion was not detected na data not avadable
5
na [NO] =O 16%,
ZSM-5 Although the cobalt loadmg m Co-Y is three times of that m Co-ZSM5, Co-Y has very low activity for the mtnc oxide reduction even at 500°C These results demonstrate that not only the nature of the cation but also the type of zeohte is important in catalyzing this reaction It has also been observed that for other mtrlc oxide removal reactlons, e g , mtric oxide decomposition [ 111 and mtric oxide reduction with other hydrocarbons [ 151, metal exchanged zeohte Y is much less active than metal exchanged ZSM-5 or mordenite The zeohte topology and/or the crystal field must play a very important role m activating metal cation for selective nitric oxide reduction The importance of zeohte structure is further illustrated by the fact that none of the oxide supported cobalt samples have any appreciable activity for the n&c oxide reduction Some of the metal oxide supported samples were preparedvia the ion exchange method and have similar chemical composltrons, e.g , Co/slhca-alumma (117% A120, and 3 0% unreduced Co by weight), and the cobalt should be atomically dispersed Sihcahte on the other hand has the ZSM-5 topology but does not have the ion exchange capacity, and therefore the cobalt m slhcahte is m oxide form Thus clearly the zeohte exerts an important electromc effect as a support for the Co2+ It does not appear that the traditional features of zeohte, 1 e , acidity and shape selectivity, are important for reaction 1 Temperature-programmed
desorptum of nrtru: omde
Some Co-zeohte samples were characterized by TPD (temperature-programmed desorptlon of mtrlc oxide) Fig 8 shows the TPD profiles of nitric
252
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0
100
200
Temperature
300
400
Catal B 2 (1993) 239-256
500
(“C)
Fig 8 TPD of mtnc oxide on Co-zeohtes The samples were pretreated m flowmg hehum at 500 ’C for 1 h followed by mtrlc oxide adsorptlon at 25°C (m) Co-Y, (0 ) Co-mordemte, (A ) Co-ZSM5 TABLE 5 Nltrlc oxide TPD results The amounts of the mtnc oxide adsorbed were obtained by integration of TPD profiles (desorption rate vs tune) Sample”
Co/Al
Na/Al
Co content (mmol/g)
NO adsorbed (mmol/g)
NO/Co
CO-Y
0 67
Co-MOR Co-ZSM-5 Co-ZSM-5 Co-ZSM-5
0 47 0 70 049 0 23
0 27 0 41 006 0 20 0 46
20 0 95 0 68 0 45 0 22
0 0 0 0 0
0 06 0 75 109 158 132
12 71 74 71 29
“All samples were pretreated with flowing helium at 500°C for 1 h before nitric oxide adsorption at 25°C
oxide on Co-ZSM-5 (S1/Al= 13 6, Co/Al = 0.70)) Co-morden1t.e (S1/Al= 5.3, Co/Al=0 47) and Co-Y (S1/Al= 2 4, Co/Al=0 67) Co-ZSM-5 and Co-morden1te had very similar TPD profiles for both desorptlon peak-temperature and intensity. Five desorptlon peaks were observed at about 90,150,210,250 and 38O”C, and the peak mtenslty decreased with increasing peak-temperature QuanWatlvely, the amount of nitric oxide adsorbed/desorbed (Table 5 ) was 0 74 and 0 71 mmol/g for Co-ZSM-5 and Co-mordenlte, respectively Be-
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Catal B 2 (1993) 239-256
253
cause of their different cobalt loadings, the ratios of NO/Co are 11 and 0.8 for Co-ZSM-5 and Co-mordenite, respectively. Na-ZSM-5 does not adsorb mtnc oxide under the same conditions The TPD profile on Co-Y, however, is substantmlly different from those on Co-ZSM-5 and Co-mordemte. Nitric oxide desorptlon peaks were detected at 90,150,210 and 36O”C, the peak at 360°C 1svery broad, from 300 to 480°C Although Y zeohte contams the largest amount of cobalt cations, its mtnc oxide adsorption capacity is the smallest (0 12 mmol/g ), and the NO/Co ratio is therefore even smaller, 0.06 The very broad mtnc oxide desorption peak of Co-Y at ca 360°C is hkely the result of a diffusion hmlted process from the sodahte cage into the gas phase, and those sites may not be readily accessible by the reactants Apparently, the mtnc oxide conversion of a catalyst is proportional to its mtnc oxide adsorption capacity at room temperature (Table 5), and the very low catalytic activity of Co-Y may be related to the mabihty of mtnc oxide to adsorb on this material. The dramatic difference between the adsorption capacities (NO/Co) of Co-ZSM-5 and Co-Y suggests the very &fferent degrees of coordinative saturation of the cobalt ion. A cobalt cation m ZSM-5 can accommodate more than one nitric oxide molecule, but a cobalt cation m Y can only hold 0 06 nitric oxide molecule If the formation of dmitrosyl species on a transition metal cation is a necessary condition for nitric oxide conversion to mtrogen, Co-Y obviously does not meet this requirement The TPD of nitnc oxide was also measured on Co-ZSM-5 as a function of cobalt exchange level, where 3 cobalt loadings were compared. As shown m Fig 9, the TPD profiles are very similar, but with different mtric oxide adsorption capacities. The NO/Co ratios for all three samples are larger than 1, with the highest being 16 (Table 5). This indicates that at least on some cobalt cations mtnc oxide adsorbed as a dimtrosyl (2 NO molecules per Co2+ ion) The adsorption stoichiometry may vary with the adsorption temperature and partial pressure of mtnc oxide used It is mterestmg to compare the TPD profiles between Co-ZSM-5 and CuZSM-5 [lo] Cu-ZSM-5 1sa umque catalyst for the direct mtnc oxide decomposition to nitrogen and oxygen [ 71, but a poor catalyst for mtnc oxide reduction with methane in the presence of oxygen. Conversely, Co-ZSM-5 is not active for mtnc oxide decomposition but an effective catalyst for mtnc oxide reduction Further, these two materials demonstrate completely opposite 0, effects on the mtnc oxide conversion for NO+CH4, addition of oxygen suppresses the activity on Cu-ZSM-5, but enhances the activity greatly on CoZSM-5. A drastic difference was seen between the mtnc oxide adsorption at room temperature on the two samples. On Cu-ZSM-5 a stoichiometnc reaction was observed when nitnc oxide was adsorbed at 25°C [ 151,whildon Co-ZSM5,no appreciable amount of other species was observed except mtnc oxide On Cu-ZSM-5, nitrous oxide was also desorbed at low temperatures Oxygen formed
254
Y 0 and J N Armor/Appl
0
100
200
Temperature
300
400
Catal B 2 (1993) 239-256
500
(“C)
Fig 9 TPD profiles of Co-ZSM-5 as a function of cobalt loading The samples were pretreated m flowing hehum at 500’ C for 1 h followed by nltrlc oxide adsorption at 25” C ( 0 ) Co/Al = 0 70, (0) Co/Al=0 49, (A) Co/Al=0 23
during the room-temperature reaction was desorbed at ca 36O”C, comcldent wrth the highest nitric oxrde desorptron peak On Cu-ZSM-5 the extra lattice oxygen 1s activated, the oxygen held by copper ions can be readily lost and gamed during a redox cycle (e g , CO/O2 cycle ) and the copper ions cycle between Cu2+ and Cu+ [ 111 Carbon monoxtde can react wrth an extra-lattice oxygen atom forming a carbon droxrde molecule, as a consequence, the Cu2+ 1s reduced to Cu+ The loss or gam of oxygen can be easrly monitored by observing the weight change with a mrcrobalance, and the change of copper valence state can be morutored with electron spin resonance (ESR) spectroscopy To test the redox ability of cobalt m Co-ZSM-5, we carried out a temperature-programmed reductron study on the Co-ZSM-5 wrth a l%CO/He mrxture (The sample was pre-oxrdrzed at 500°C m a lO%O,/He mixture for 1 h, cooled to room temperature m the flowing O,/He mixture, then the catalyst was flushed with a stream of He ) Carbon monoxrde consumptron and carbon droxlde formatron were not observed up to 500” C This suggests that the extra lattice oxygen atoms m Co-ZSM-5 are not active enough to oxrdrze carbon monoxrde wrth a change of the cobalt valence state Under our experimental condrtrons, an oxrdrzmg atmosphere, It 1s unlikely that a redox process for cobalt catrons 1s mvolved durmg the reaction Perhaps the stronger bondmg of the extra lattrce oxygen m Co2+ m Co-ZSM-5 reflects the selectlvlty of this catalyst toward rutrlc oxide reduction The requirement of the presence of gaseous oxygen for the NO, reduction
Y LI and J N Armor/Appl Catal B 2 (1993) 239-256
255
suggests gaseous oxygen may be mvolved m the formation of reaction mtermediates Unhke nitric oxide, mtrogen dioxide can be effectively reduced by methane in the absence of oxygen Nitrogen &oxide is known be a stronger oxldmmg agent than mtnc oxide Tlus leads us to beheve that a mtrogen &oxide species may be formed on catalyst when mtric oxide and oxygen coexist, and perhaps this bound nitrogen &oxide species is an important catalytic mtermedate This, however, is not to exclude other posslbihtles, e g., forming oxygenated carbon species On the other hand, methane as a reducmg agent must be activated Although detectable amount of methane adsorption was not observed on Co-ZSM-5 at room temperature, activated adsorption of methane is known to occur on Ni-ZSM-5 [ 171 and Ni0/A1203 [ 181, either dlssociatlvely ( formmg C and Hz) or associatively Quahtatively, we also detected apparent carbon deposition on Co-ZSM-5 with a steam of CH,/He mixture flowing through the catalyst at elevated temperatures (the temperature was ramped from 25 to 500”C ) as evidenced by the formation of carbon &oxide when the catalyst was titrated with a mixture of O,/He at 500°C At this point we do not have direct observation of any mtermediate species Further we do not understand what makes Co-ZSM-5 (or Co-mordemte) so &fferent from Co-Y and what are the structural requirements for an effective catalyst All these however, remam a challenge for future research CONCLUSION
We found that Co2+, Mn2+ and Ni2+ exchanged ZSM-5 and mordemte are active for the reduction of mtnc oxide with methane m the presence of excess oxygen The nitric oxide conversion bends over with mcreasmg temperature, and this behavior is reversible upon decreasing temperature Apparently, the bending over of nitric oxide conversion is related to methane combustion reaction and therefore to the level of methane remammg m the feed The nitric oxide conversion on a Co-ZSM-5 catalyst is proportional to the level of exchanged Co2+ , but excess amounts of Co2+ do not contribute to the mtnc oxide reduction Below a certam exchange level, the turnover frequency for mtrrc oxide reduction IS independent from Co2+ exchange level We beheve that a single Co2+ cation is an active site The type of metal ion is very important m determining the selectivity and activity for the nitric oxide reduction, Co2+ is selective but Cu2+ is not The type of zeohte as a support IS also crucial, CoZSM-5 and Co-mordemte are active but Co-Y not Our TPD results show that the amount of nltnc oxide adsorbed on Co-Y ISvery small (0 06 NO/Co) compared to Co-ZSM-5 (NO/Co > 11) or Co-mordemte (NO/Co = 0 8) It seems that the traditional properties associated with zeobtes, e g , acidity and shape selectivity, are not important for this reaction, the electromc influence of the cations by the zeohtes maybe more important
256
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Catal B 2 (1993) 239-256
ACKNOWLEDGEMENTS
Thanks are due to Paula Battavlo for the actlvlty measurements and Tom Braymer for the catalyst preparations We thank Air Products and Chemicals, Inc for the permission to publish this work
REFERENCES
5 6 7 8 9 10 11 12 13 14 15 16 17
18
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