Thermodynamic features of the Cu-ZSM-5 catalyzed NO decomposition reaction

Transcription

1 Koren J. Chem. Eng., 23(4), (2006) SHRT CMMUNICATIN Thermodynmic fetures of the Cu-ZSM-5 ctlyzed N decomposition rection Deuk Ki Lee Deprtment of Environmentl Engineering, Gwngju University, Gwngju , Kore (Received 30 August 2005 ccepted 23 Februry 2006) Abstrct ver Cu-ZSM-5 ctlyst with quntified mount of the ctive Cu 2+ -dimers (Cu Cu 2+ ), the kinetics of the ctlytic N decomposition to ws derived on the bsis of the proposed rection mechnism, nd such thermodynmic dt s dsorption enthlpies of N onto the Cu ion dimer sites were evluted. It ws reveled tht the enthlpy of the dsorption of N ( H= 34.1 kcl/mol) onto reduced Cu + -dimer, s the inititing step of N decomposition ctlysis, ws higher thn tht ( H= 27.8 kcl/mol) onto n oxidized Cu 2+ -dimer, or tht ( H= 27.4 kcl/mol) of the dissocitive dsorption of 2 onto the two reduced Cu + -dimers in neighbor. The strong inhibition effect of gs phse oxygen on the kinetic rte of N decomposition t o C could be explined by the thermodynmic predominnce of the oxidized Cu 2+ -dimers ginst the ctive reduced Cu + -dimers on the ctlyst even t high temperture nd under the low prtil pressure of oxygen. It ws lso found tht the mximum ctlytic ctivity t tempertures round 500 o C, which ws commonly observed in the Cu-ZSM-5 ctlyzed N decomposition rection, ws ttributed to the reltively lrge enthlpy of N dsorption onto the reduced Cu + -dimers s compred to tht of the rection ctivtion energy (=19.5 kcl/mol), resulting in less fvored N dsorption t the higher tempertures thn 500 o C. Key words: N Decomposition, Cu-ZSM-5, Cu-dimers, Mechnism, Kinetics, Enthlpy of Adsorption, Activtion Energy, Thermodynmics INTRDUCTIN To whom correspondence should be ddressed. E-mil: dklee@gwngju.c.kr Irrespective of the extensive studies for more thn couple of decdes, the direct decomposition of N to on Cu-ZSM- 5 ctlyst is still very interesting topic in such fundmentl respects s the identifiction of ctlytic ctive sites nd the elucidtion of rection mechnism leding to in reltion to the rection kinetics. As summrized by Kurod nd Iwmoto [2004] or by Groothert et l. [2003], it is generlly ccepted tht tomic oxygen-bridged Cu 2+ -dimer ions, Cu Cu 2+, re the ctlytic centers of N decomposition when reduced to the Cu + --Cu + by the removl of bridged oxygen. There re number of experimentl evidences [Anpo et l., 1997; Beutel et l., 1996; D Cost et l., 2002; Groothert et l., 2003; Kurod nd Iwmoto, 2004; Kurod et l., 1999; Lei et l., 1995; Lee, 2004; Modén et l., 2002; Skny et l., 1992; Yn et l., 1996] supporting tht oxygen-bridged Cu 2+ - dimer ions re vilble on the high Cu-exchnged zeolite ctlysts in the oxidized stte. In recent pper [Lee, 2004], it ws experimentlly demonstrted tht Cu 2+ -dimer ions over Cu-ZSM-5 ctlyst were involved in the decomposition of N to s ctlytic centers vi cyclic redox process. The loded copper on the ctlysts in oxidized stte ws found to be present s isolted Cu 2+, Cu Cu 2+ nd Cu, nd the molr composition of ech species on ctlyst ws quntittively determined. It ws lso reveled tht the mol frction of Cu d (copper involved in the Cu Cu 2+ species of ctlyst) ws higher for the ctlysts bsed on the ZSM-5 with the lower rtio of Si to Al f, nd incresed t the scrifice of the isolted Cu 2+ with incresing level of copper loding to ZSM-5, ensuring tht the Cu Cu 2+ species could be formed between the two isolted Cu 2+ in close proximity. Regrding the N decomposition mechnism bsed on the Cu 2+ - dimerspecies s ctlytic centers, it ws suggested tht the removl of the bridged oxygen from the Cu Cu 2+ led to the reduction of Cu 2+ to the Cu + --Cu + ion sites, on which N dsorbs inititing the decomposition ctlysis, nd tht two dsorbed N molecules on the Cu + --Cu + species rected to form s rection intermedite to product, resultntly leving the Cu Cu 2+ on surfce [Kurod nd Iwmoto, 2004; Modén et l., 2002]. In the kinetic expression like Lngmuir-Hinshelwood-Hougen-Wtson (LHHW) rte eqution, provided tht the rte-determining step is relevnt to, the kinetic term in the numertor would be expressed by the second order with respect to the N pressure, while the dsorption group of the denomintor would consist of such kinds of d-species on the ctive sites s tomic oxygen, moleculr oxygen, N, N 2,, nd/or N 3, s proposed by Modén et l. [2002]. For the Cu-ZSM-5 ctlyzed N decomposition, it ws commonly observed tht the rte of ws severely inhibited by the presence of 2 in gs phse [Modén et l., 2002; Li nd Hll, 1991], nd mximized t temperture round 500 o C [Modén et l., 2002; Li nd Hll, 1991; Yokomichi et l., 2000; Tomsic et l., 1998]; however, the resons for these re not well understood. To resolve such impliction involved in the Cu-ZSM-5 ctlyzed N decomposition, thermodynmic in on the ctivtion energy of nd the dsorption enthlpies of N or 2 to the reduced Cu + -dimers is indispensble. As prerequisite to this, kinetic expression bsed on the normlized rte of N decomposition to the ctive Cu ion dimer sites of Cu-ZSM-5 ctlyst should be obtined. In this pper, LHHW-type kinetics of N decomposition over Cu-ZSM-5 ws estblished using the Cu d -normlized rte of N 547

2 548 D. K. Lee Tble 1. Cu-ZSM-5 ctlysts prepred nd chrcterized [Lee, 2004] Cu(116)-Z(14.7) Cu(76)-Z(14.7) Cu(262)-Z(29) Cu(156)-Z(29) µmol Cu /g µmol Cu db /g Totl mount of the loded copper per grm ctlyst. b Amount of copper species in the oxidized dimeric form, (Cu--Cu) 2+, per grm ctlyst. decomposition to from the kinetic experiment over Cu-ZSM- 5 ctlyst of which the ctlytic mount of the ctive Cu 2+ -dimers hd been determined. And, thermodynmic dt of rection ctivtion energy nd enthlpies of dsorption/desorption of N from/to the ctive site were obtined nd discussed with respect to the observed impliction in the rection. EXPERIMENTAL 1. Ctlyst Preprtion Cu-ZSM-5 ctlysts with different Cu lodings, s listed in Tble 1, were prepred by ion exchnge of Cu 2+ ions into N-ZSM-5. Further detils of ctlyst preprtion nd chrcteriztion cn be found elsewhere [Lee, 2004]. For the nottion of ctlysts in Tble 1, the numbers in prentheses following Cu nd Z indicte the %Cu exchnge level (%Cu EL) nd the Si/Al f, respectively. The %Cu EL is defined s two hundred times the tomic rtio of totl copper to Al f of ZSM-5, i.e., 100% corresponds to one Cu 2+ exchnged per two Al f toms. 2. Mesurement of N Decomposition Rection Rte Two kinds of rectors were used for the determintion of the ctlytic rection rte. First, multitube rector designed for evluting the ctlysts up to eight smples simultneously t the sme rection condition ws used for the determintion of ctivities of ll the prepred ctlysts. The rector consisted of eight stinless steel tubes (inner dimeter 2.5 mm) in prllel, which were inserted within stinless steel block in order to ensure uniform tempertures. About 50 mg of fresh ctlyst smples ws loded to ech of the tube rectors. The ctlysts in the rectors were pretreted for the reduction with 30 cm 3 /min of He (UHP grde) for 2 h t 550 o C before their exposure to the rectnt gs. The rectnt gs ws the mixture of 1.0% N/9.98% Ar/89.02% He (1% N gs). The 1% N gs flow through ech rector ws controlled to 60 cm 3 /min (0.05 g/ (cm 3 /min)). Exit strems t stedy stte were routed to n online mss spectrometer (MS, MKS Instruments) one by one using n eight-port switching vlve (Vlco) for nlysis. Next, in order to obtin the kinetic dt of N decomposition over the ctlyst Cu(116)-Z(14.7), rection experiments were conducted using qurtz rector with porous qurtz frit, over which 50 mg of ctlyst smple ws so evenly supported to give differentil rector with shllow bed with height less thn 5 prticle dimeters. The ctlyst ws pre-reduced by the sme procedure s bove. Experiments were conducted t mbient pressure t tempertures, o C, nd the feed concentrtions of N were djusted to 0.2, 0.34, 0.6, nd 1% by mixing pure He nd the 1% N gs. The flow rte of totl feed gs t specified concentrtion of N ws chnged by using mss flow controller to give spce time, g-ct s/cm 3. Monitored species by the MS were s follows: He (4 mu), (16), H 2 (18), N (30), (28), 2 (32), Ar (40), (44) nd N 2 (36). In order to clibrte the MS, clibrtion gs mixture contining 0.05%, 0.05% 2, 0.05%, 10% Ar (used s internl stndrd) nd 89.85% He ws used. For the clibrtion of N 2 MS signl, mixed gs of 0.05% N 2 /9.94% Ar/90.01% He ws used. Tble 2. Mechnism of N decomposition over Cu-ZSM-5 Formtion of on reduced Cu-dimer 1. N+[Cu + --Cu + ] [Cu + (N)--Cu + ] 2. N+[Cu + (N)--Cu ] + [Cu Cu 2+ ] + Formtion of on the two oxidized Cu-dimers in vicinity 3. N+[Cu Cu 2+ ] [Cu 2+ (N)- 2 -Cu 2+ ] 4. [Cu 2+ (N)- 2 -Cu 2+ ] [Cu + (N 2 )--Cu + ] 5. N+[Cu + (N 2 )--Cu + ] [Cu + (N 2 )--Cu + (N)] 6. [Cu + (N 2 )--Cu + (N)]--[Cu + (N)--Cu + (N 2 )] 2Cu + (N 2 )+[Cu Cu 2+ ]+ Formtion of nd/or 2 on Cu-dimer 7. +[Cu + --Cu + ] [Cu Cu 2+ ]+ 8. +[Cu Cu 2+ ] [Cu + --Cu + ]+ + 2 Formtion of 2 on the two oxidized Cu-dimers in vicinity 9. 2[Cu Cu 2+ ] 2[Cu + --Cu + ]+ 2 Formtion of N 2 involving the reduction of Cu-dimer 10. [Cu + (N 2 )--Cu + ] [Cu + --Cu + ]+N 2 The rection in step 2 cn be divided into the two elementry steps s follows: 2. N+[Cu + (N)--Cu + ] [Cu + (N)--Cu + (N)] 2b. [Cu + (N)--Cu + (N)] [Cu Cu 2+ ]+ (rte determining step, RDS) (ctivted bove 50 o C) (swift even t RT) (swift even t RT) (ctivted bove 300 o C) (ctivted bove 250 o C) (qusiequilibrted, RDS) (swift even t RT) July, 2006

3 Thermodynmic fetures of the Cu-ZSM-5 ctlyzed N decomposition rection 549 DERIVATIN F KINETIC EXPRESSIN 1. Bckgrounds of the Proposed Rection Mechnism The rection mechnism proposed in this study is summrized in Tble 2. As briefly mentioned in the introduction, s fr s the Cu 2+ -dimers re regrded s ctlytic centers, there is little disgreement on tht the rection of two dsorbed N molecules on Cu + - dimer species led to the of s precursor of, leving the resultnt Cu Cu 2+ species on surfce (elementry rection steps: 1-2). The Cu + (N) d-species on the surfce ws evidenced by IR mesurement [Konduru nd Chung, 1999], nd the of prior to ws experimentlly observed [Modén et l., 2002]. ne N-dsorbed dimer species, Cu + (N)--Cu +, would be equilibrted with the pressure of N in gs phse, s represented in step 1. If the rection between the two N-dsorbed Cu + species in dimer leding to (step 2b) is fst enough to not permit the desorption of ny N, the rte of would be governed by tht of step 2, the second dsorption of N to the Cu + (N)--Cu +. Fig. 1 shows the swiftness of the rection forming even t room temperture. This result ws obtined t room temperture by switching the gs flow (100 cm 3 /min) over the reduced ctlyst smple of Cu(116)-Z(14.6) in qurtz rector from He to the gs of 0.464% N/Ar/He. For the reduction of the ctlyst, the smple stored fter drying ws therml-treted t 550 o C in He flow for 2 h nd cooled to room temperture. In Fig. 1, the dvent of Ar in MS indictes the beginning of N dsorption onto the ctlyst, nd tht of N indictes tht such N dsorption is ner n equilibrium stte. Just before reching the equilibrium stte of N dsorption, the of nd in more thn discernble mounts is observed, ensuring the swiftness of step 2b. This result suggests tht step 7 forming vi dsorption to reduced Cu-dimer is lso very swift process even t room temperture. Consequently, step 2 deserves to be regrded s the rte-determining step in the N decomposition rection to. As the other possible route of, there is need to consider elementry steps 3 through 6 since experimentl evidence for tht is vilble, s shown in Fig. 6(A) of the previous pper Fig. 1. Formtion of nd t room temperture during the flow of 0.464% N/Ar/He over the pre-reduced ctlyst of Cu(116)-Z(14.7). [Lee, 2004]. It shows the turnover frequency (TF) of the gses detected during the temperture-progrmmed surfce rection of N (N-TPSR) over the oxidized ctlyst smple of Cu(116)-Z(14.7). As previously described [Lee, 2004], for the pre-oxidtion, the ctlyst smple ws thermlly treted using gs mixture of 49% 2 / He ( 2 gs) t 500 o C for 2 h, nd fter tht, the rector ws cooled to room temperture. Then, pure He ws introduced insted of the 2 gs in order to remove physisorbed oxygen on the ctlyst. The fct tht most of Cu-dimers over the ctlysts fter such therml oxidtion existed s the Cu Cu 2+ species ws evidenced by the temperture-progrmmed desorption (TPD) of 2 in He flow [Lee, 2004]. For the N-TPSR, the flow of He ws switched to the flow (100 cm 3 /min) of 0.464% N/Ar/He over the ctlyst, nd fter the N-dsorption equilibrtion t room temperture, the rector temperture ws rised by 10 o C/min, vrious kinds of products ppered with incresing temperture. In this process, only ws observed to be formed in significnt mount t reltively low temperture region, followed by the N 2 t tempertures bove 250 o C. It should be noted tht ppering t tempertures lower thn 250 o C should hve resulted from different route thn the rection steps 1-2, becuse of the scrcity of the Cu + -- Cu + species on surfce due to the preoxidtion. In reference to the suggestion of Beutel et l. [1996] tht the dsorption of two N onto n oxidized Cu-dimer resulted in Cu + -N nd (Cu-N 2 ) + complex, steps 3-5 were estblished. Rection step 4 ws introduced becuse the dsorption of N onto n oxidized Cu-dimer ws not likely to directly led to the reduction of the Cu 2+ s the Cu + (N 2 )--Cu + t room temperture. During the period of N-dsorption equilibrtion t room temperture over the oxidized ctlyst, ny discernble mount of or ws not detected. As shown in Fig. 6(A) of the previous pper [Lee, 2004], the of strted t temperture bove 50 o C, indicting tht little extent of ctivtion energy ws required for the rection step 4. As described in step 6, if two of the resultnt Cu + (N)--Cu + (N 2 ) species re plced so close for the two Cu + (N) species to ssocite s dimer, this results in the of, resultntly leving new Cu Cu 2+ with two seprtely existing Cu + (N 2 ) species. This rection my be possible for Cu-ZSM-5 over which the copper species involved in the dimers re highly clustered, like the cse of the therml recombined of 2 from the two vicinl Cu Cu 2+ species, s previously reported [Lee, 2004, 2005]. Before the desorption of N 2, this process would be continued with concurrent of until quite lrge mount of the Cu Cu 2+ species trnsformed to Cu + (N 2 ) species. The resulting species of Cu + (N 2 ) on surfce observed to desorb t tempertures bove 250 o C. This desorption of N2 cn be resultntly thought s reduction process of the oxidized Cu 2+ -dimers, s described by step 10. In ctul N decomposition condition where 2 is lwys previling, the rection steps 3-6 would tke chrge of the of to some extent, but smll s compred to the steps 1-2 becuse of the low-efficiency of the process needing four ctive Cu ions. As routes of from, except for the elementry step 7 discussed prior in its swiftness, step 8 needs to be considered referring to Fig. 5(A) of the previous pper [Lee, 2004]. It shows the TF of the gses detected during the -TPSR over the oxidized ctlyst smple of Cu(116)-Z(14.7). Pre-oxidtion tretment ws the sme s for the N-TPSR, nd gs of 0.1% /He ws Koren J. Chem. Eng.(Vol. 23, No. 4)

4 550 D. K. Lee used for the -TPSR. At tempertures bove 300 o C, observed to decompose over the oxidized ctlyst into. Up to bout 360 o C, the TFs of evolved were prcticlly equivlent to ech other, indicting tht the rection step 8 ws working. According to the proposl of Kurod nd Iwmoto [2004] for vi the dsorption of onto n oxidized Cu-dimer site, Cu 2+ -( - )-Cu 2+ species remin on surfce; however, this figure suggests prompt desorption of 2 in prllel with t tempertures bove 300 o C. In respect of the removl of the Cu 2+ -bridged tomic oxygen, step 8 s well s step 10 cn be thought of s n efficient reduction process for oxidized dimers s compred to step 9, requiring two vicinl oxidized Cu 2+ -dimers. 2. Kinetic Rte Eqution To estblish n LHHW-type kinetic rte eqution, bsed on the mechnism in Tble 2, the kinds of previling d-species on the ctlytic sites re to be determined. According to the bove discussion on the rection mechnism, cndidte d-species would be [Cu + -- Cu + ], [Cu Cu 2+ ], [Cu + (N)--Cu + ], nd [Cu + (N 2 )--Cu + ], termed briefly s [*], [*], [N*], nd [N 2 *], respectively, to represent their site-occupying mol frctions. If the rection rte could be expressed on the bsis of one mol of the Cu d species involved in the Cu 2+ -dimer sites, the site blnce would be given by 1=[*]+[*]+[N*]+[N 2 *] (1) As discussed bove, step 2 is regrded s the rte-determining step for the. The Cu d -normlized rection rte of, r (mmol- /mol-cu d /s), is represented by r=k 2 P N [N*] (2) where k 2 (mmol- /mol-cu d /kp-n/s) is rte constnt in the rection step 2, nd P N is the N pressure in kp. The equilibrium reltionships in steps 1, 3 nd 9 led to Eqs. (3), (4) nd (5), respectively: [N*]=K 1 P N [*] (3) [N 2 *]=K 3 P N [*] (4) [*]=K 9 P 2 [*] (5) where K 1 (1/kP), K 3 (1/kP) nd K 9 (kp) re the dsorption equilibrium constnts in the steps, 1, 3 nd 9. By substituting Eqs. (3)- (5) into Eq. (1), nd replcing [N*] of Eq. (2) with Eq. (3), nd fter some rrngements, finl eqution for the rte of is obtined: Tble 3. Prmeters for the kinetics of N decomposition over Cu(116)-Z(14.7) Temperture, o C r = 2 k pp P N D k pp K 1b, 1/kP where k pp (mmol- /mol-cu d /kp 2 /s), n pprent rte constnt of from the N decomposition rection, nd the denomintor D, re given by, respectively k pp =k 2 K 1 (7) D=1+K 9 P 2 +K 1 P N +K 3 K 9 P N P 2 K 3b, 1/kP K 9 1, 1/kP RESULTS AND DISCUSSIN 1. Determintion of Kinetic Prmeters Tble 3 lists the vlues of the prmeters in Eq. (6) nd k 2 clculted by using Eq. (7), which were obtined through multi-nonliner regression for the rte dt expressed s mmol- per mol-cu d per second t the rection tempertures o C. For the ctlyst, Cu(116)-Z(14.7), the mount of Cu d ws determined to 198 µmol per grm of the ctlyst [Lee, 2004], s shown in Tble 1. At ech temperture level, rte dt set consisting of 20 points were obtined by vrying the rector spce-time of feed gs nd the feed pressure of N. The pressure of N in the ctlyst bed ws clculted from the feed N conversion to the MSmesured, nd N 2, nd tht of 2 ws bsed on the MSmesured intensity of 2. The rte dt t 400 nd 450 o C were fitted very well to Eq. (6), k 2 c SD d (0.65) (0.12) (0.12) (0.04) (0.03) (0.02) mmol-n2 /mol-cu d /s/kp 2 b Numeric vlues in prentheses were tken to get stright lines for ll dt in the Arrhenius plots shown in Fig. 3. c k 2 (mmol- /mol-cu d /s/kp)=k pp /K 1 d Stndrd devition of the clculted rte dt from the experimentl. (6) (8) Fig. 2. Prity plots of the clculted rte of ginst the experimentl t tempertures 400, 500, nd 600 o C. July, 2006

5 Thermodynmic fetures of the Cu-ZSM-5 ctlyzed N decomposition rection 551 Tble 4. Rection ctivtion energy nd enthlpy of dsorption E A_2 (ctivtion energy of the rection in step 2), kcl/mol 19.5 H 1 (het of dsorption of N to * in step 3), kcl/mol 34.1 H 3 (het of dsorption of N to * in step 3), kcl/mol 27.8 H 9 (het of dissocitive dsorption of 2 in step 9), kcl/mol 27.4 E A_pp (pprent ctivtion energy=e A_2 + H 1 ), kcl/mol 14.6 Fig. 3. Arrhenius plot for the kinetic prmeters in Tble 3. giving ech of the prmeter vlues listed in Tble 3. However, for the dt set t 500 o C nd the bove tempertures, it ws not possible to obtin the vlues of prmeters K 1 nd K 3 with dequte physicl significnce from the regression due to their so much smll vlues s compred to K So, K1 nd K 3 vlues in prentheses in Tble 3 were so properly tken tht ll the dt of K 1, k 2 nd K 3 could be represented s linerly s possible on the Arrhenius plot. With these pre-set vlues of K 1 nd K 3, the vlues of K pp nd K 9 were obtined from the regression of the rte dt using Eq. (6). Fig. 2 shows the prity plot of the clculted rte of using Eq. (6) with the prmeters in Tble 3 ginst the experimentl t typiclly 400, 500 or 600 o C. Fig. 3 shows the Arrhenius plots for ech of the prmeters determined. In view of good correspondence between the rtes of clculted nd experimentl, nd the liner Arrhenius reltionships of the prmeters, the kinetic eqution given by Eq. (6) with the prmeters determined could be thought to resonbly depict the rection kinetics of N decomposition to over the Cu-dimers s ctlytic centers. 2. Enthlpies of the Adsorption of N onto the Cu- Dimers The rection ctivtion energy nd the enthlpies of dsorption clculted from the Arrhenius reltionship shown in Fig. 3 re summrized in Tble 4. The dsorption of N onto Cu + in the reduced Cu + -dimer ccompnies the exothermic het of dsorption of 34.1 kcl/mol. This vlue is closely correspondent to the clculted result, 34.4 kcl/mol, by Yokomichi et l. [2000]. In this sense, the K 1 vlues pre-set in the determintion of the kinetic prmeters t 500 o C nd bove tempertures nd the resultnt k 2 my be thought s fesible. The reltively high vlue of H 1 indictes the tendency of N dsorption less fvored t higher tempertures. The exothermic het of dsorption of N onto Cu 2+ in the oxidized Cu 2+ -dimer is estimted s 27.8 kcl/mol, nd tht of dissocitive dsorption of 2 onto the two reduced Cu + -dimers is 27.4 kcl/mol. From the viewpoint of dsorption equilibrium, lrger het of dsorption for n dsorbte mens its higher tendency to desorb from the dsorbent s temperture increses. So, the therml desorption of N 2 in sequence, with pek tempertures 375 nd 395 o C, respectively, during the N-TPSR shown in Fig. 6(A) of the previous pper [Lee, 2004] is closely relted to these enthlpy vlues of dsorption. In Eq. (8), the Cu-dimer frctions of [N 2 *] nd [*] re clculted by K 3 K 9 PN P 2/D nd K9 P2/D, respectively. Their temperture dependences re therefore relted to the mgnitude of enthlpies, ( H 3 )+( H 9 )/2 nd ( H 9 )/2, respectively, ech giving 41.5 nd 13.7 kcl/mol. Such enthlpy difference is well reflected by the sequentil desorption of N 2 with incresing temperture with pek temperture difference bout 20 o C in Fig. 6(A) of the previous pper [Lee, 2004]. 3. Kinetic Effects of Temperture, P 2 nd P N Fig. 4 shows the chnge in the frctionl coverge of d-species on the Cu-dimer sites s [*], [N*], nd [N 2 *] including the Fig. 4. Chnge of the frction coverge of the Cu-dimer sites with temperture pressures (A): 0.01, (B): 0.1, nd (C): 1.0 kp under constnt pressure of N t 1.0 kp. Koren J. Chem. Eng.(Vol. 23, No. 4)

6 552 D. K. Lee vcnt [*] with temperture t P N =1 kp nd different levels of P 2. As expected by the enthlpy difference between [N 2 *] (41.5 kcl/mol) nd [N*] (34.1 kcl/mol), the coverge of the former decreses more rpidly thn tht of the ltter s temperture increses. This is consistent with the result of in-situ IR study for the N-TPSR over Cu-ZSM-5 by Konduru nd Chung [1999], reporting tht the IR intensity of Cu 2+ (N) disppered rpidly with concurrent desorption of N 2 s temperture incresed. As listed in Tble 5, free energies clculted from the dsorption equilibrium constnts t different tempertures indicte tht the dsorption of N onto [Cu + --Cu + ] nd [Cu Cu 2+ ] is not thermodynmiclly fvored Tble 5. Free energy in the dsorption equilibri over Cu-dimers Temperture, G, kcl/mol-dsorbte o C Step 1 Step 3 Step 9 (reversed) G= RT lnk, where K is equl to K 1, K 3 or K 9 1 in Tble 3. t tempertures bove from 500 nd 450 o C, respectively, while the dissocitive dsorption of 2 onto [Cu + --Cu + ] is highly fvored ll over the temperture. Therefore, even under the presence of 2 in very low pressure like P 2 =0.01 kp, s shown in Fig. 4(A), the site-frction of [*] is predominnt over the others mostly throughout the temperture experimented. The presence of 2 in higher pressures leds to higher site-frction of [*], s shown in Fig. 4(B) nd 4(C). In this regrd, severe 2 inhibition reported [Modén et l., 2002; Li nd Hll, 1991] on the rte of the N decomposition to over Cu-ZSM-5 ctlysts cn be well understood. The rte of clculted by Eq. (6) t conditions including the P N nd P 2 exmined in Fig. 4 is shown in Fig. 5(A). The rte of ppers to be highly inhibited by 2 even t very low pressure. Fig. 5(A) demonstrtes lso tht the clculted rte of is mximized t temperture round 500 o C. Such temperture dependence of the ctlytic ctivity ws commonly observed in the N decomposition rection over Cu-ZSM-5 ctlysts [Modén et l., 2002; Li nd Hll, 1991; Yokomichi et l., 2000; Tomsic et l., 1998]. In view of the bove discussion on N dsorption, it cn be thought tht this unusul ctivity pttern with incresing temperture is ttributed to the reltively lrge enthlpy of dsorption N onto the [Cu + --Cu + ] site s compred to the rection ctivtion energy in step 2, s the vlues given in Tble 4. As result, pprent ctivtion energy (E A_pp = 14.6 kcl/mol) ppers negtively in the N decomposition rection, suggesting tht the pprent rte constnt given by Eq. (7) decreses with the increse of temperture. Fig. 5(B) shows the dependence of the clculted rte of on the pressure of N nd temperture t P 2 =0.1 kp. The pprent rection order with respect to the P N ws clculted to 1.75, 1.91, 1.97, 1.99 nd 2.0 t tempertures 400, 450, 500, 550 nd 600 o C, respectively. This indictes tht t higher tempertures bove 500 o C, the dimer site-coverge of [N*] becomes negligible to hve no contribution to the denomintor term given by Eq. (8). 4. Activtion Energy of Dissocitive Adsorption of 2 onto the Reduced Cu-Dimers Activities of the ctlysts of Tble 1, determined in the experiments using the multitube rector, showed the sme temperture Fig. 5. (A) The dependence of the clculted rte of on temperture pressure t P N =1 kp, nd (B): tht on temperture nd N pressure t P 2 =0.1 kp. Fig. 6. Rte of in terms of the TF bsed on the Cu d of the ctlysts. July, 2006

7 Thermodynmic fetures of the Cu-ZSM-5 ctlyzed N decomposition rection 553 dependency s observed in Fig. 5(A). Temperture dependence of the rte of in terms of the TF bsed on totl mount of Cu loded ws shown in Fig. 1 of the previous pper [Lee, 2004]. Fig. 6 shows the rte in terms of the TF bsed on the Cu d of the ctlysts. It is interesting tht the ctlysts showing different ctlytic ctivities mong them in the TF bsed on the totl Cu cn be clssified into two groups of the sme ctivity in the TF bsed on the Cu d : ne group of Cu(116)-Z(14.7) nd Cu(262)-Z(29), nd the other of Cu(76)-Z(14.7) nd Cu(156)-Z(29), s shown in Fig. 6. The two ctlysts in ech group re similr to ech other in their mol mounts of Cu d per grm ctlyst, s given in Tble 1, tking it for grnted tht their Cu d -normlized TFs pper nerly sme when considering the Cu-dimers s ctive centers. Ctlysts in the former group re bout twice more thn the ltter in the mounts of Cu d /g. Another interesting point in Fig. 6 is tht the ctlysts with higher mounts of Cu d /g re less ctive t lower temperture thn those with lower mounts of Cu d /g. This my result from the difference between the two groups of ctlysts in the degree of 2 -inhibition effect t low tempertures. In recent pper [Lee, 2005], it ws reveled tht the ctivtion energy (E A_2_des) for the therml 2 desorption vi recombintion of tomic oxygen from the Cu Cu 2+ sites decresed linerly with incresing Cu d /g, s listed in Tble 6. With the vlues of E A_2_des, it is possible to obtin the ctivtion energy (E A_2_ds) for the dissocitive dsorption of 2 onto the two vicinl [Cu + --Cu + ] sites using the following reltionship: E A_2 _ds=e A_2 _des ( H 9) (9) As listed in Tble 6, the ctlysts with higher mounts of Cu d /g need much less ctivtion energies for the dissocitive dsorption of 2 thn those with lower mounts of Cu d /g. Accordingly, t low tempertures round 450 o C, the former ctlysts re expected to severely suffer from higher dimer site-frction of [*] in comprison with the ltter, by which the former ctlysts pper less ctive thn the ltter, s shown in Fig. 6. At higher tempertures where the difference in the E A_2_ds between the two groups of ctlysts does not bring ny significnt effect, the former ctlysts with lower E A_2_des become more ctive thn the ltter with higher E A_2_des. n the mechnism proposed in Tble 2, there re three different wys of tomic oxygen removl from the [Cu Cu 2+ ]: step 8, step 9, nd the step 10 through steps 3-4. From the TF profile of N 2 desorption in the N-TPSR shown in Fig. 6(A) of the previous pper [Lee, 2004], the ctivtion energy (E A_N2_des) for N 2 desorption ws obtined s 22.3 kcl/mol t the temperture spn ( o C) of initil desorption. And, the ctivtion energy (E A_2_form) for 2 in the -TPSR shown in Fig. 5(A) of the previous pper [Lee, 2004] ws 18.9 kcl/mol t temperture region o C where re evolved in the sme TF. These lower ctivtion energies over the ctlyst of Cu(116)-Z(14.7) s compred to tht (35.3 kcl/mol) for the therml recombined desorption of 2 indictes the rediness of the tomic oxygen removl vi step 8 nd step 10 reltive to vi step 9. Considering the previling inhibition effect by gs phse oxygen on the kinetic rte during N decomposition, step 9 should be regrded s n equilibrted route for the dissocitive dsorption of 2 onto the Cu + --Cu + sites, wheres the reduction of oxidized Cu 2+ -dimers to Cu + --Cu + is mostly crried out vi step 8 nd step 10. SUMMARY Cu d -normlized rection rte of in the Cu-ZSM-5 ctlyzed N decomposition ws well represented by the proposed rection mechnism-bsed kinetic model. The numertor term in the kinetic eqution ws expressed s k pp P 2 N by ssuming the second N dsorption onto molecule of N-dsorbed Cu + -dimer, [N*], s the rte-determining step for the, nd the denomintor consisted of the dsorption terms of [*], [N*], [N 2 *], nd [*] s the vcnt Cu + -dimer. It ws chrcteristic for the N decomposition rection over Cu-ZSM-5 tht the enthlpy of the dsorption of N ( H= 34.1 kcl/mol) onto the [*] site ws higher thn tht ( H= 27.8 kcl/mol) onto the [*], or tht ( H= 27.4 kcl/mol) of the dissocitive dsorption of 2 onto the two [*] sites in neighbor. The dsorption of N onto the sites of [*] nd [*] ws not thermodynmiclly fvored t tempertures bove 500 nd 450 o C, respectively. However, the dissocitive dsorption of 2 onto the [*] site ws lwys fvored throughout the temperture of o C, nd resultntly, the rte of ppered to be highly inhibited by 2 even t very low pressure. The unusul ctlytic behvior showing mximum ctivity round 500 o C ws ttributed to the reltively lrge enthlpy of N dsorption onto the [*] s compred to tht of the rection ctivtion energy (19.5 kcl/mol), resultntly giving the negtive pprent ctivtion energy (E A_pp = 14.6 kcl/mol). The ctlysts with higher mounts of Cu d /g were nlyzed to need much less ctivtion energies ( kcl/ mol) for the dissocitive dsorption of 2 thn those ( kcl/ mol) with lower mounts of Cu d /g. It ws believed tht the [*] on the ctlyst mintined in equilibrted stte by the dissocitive dsorption of 2 onto the [*] sites while the reduction of [*] to [*] seemed to be mostly crried out vi the rection of with [*] nd the dsorption of N onto the [*] followed by the desorption of N 2. ACKNWLEDGMENT The uthor expresses his deep thnks to Professor Enrique Iglesi nd Mr. Björn Modén in the Ph. D course t the Deprtment of Chemicl Engineering, University of Cliforni, Berkeley for their Tble 6. Activtion energy involved in the desorption/dsorption of 2 over Cu-dimers Cu(116)-Z(14.7) Cu(76)-Z(14.7) Cu(262)-Z(29) Cu(156)-Z(29) E A_2_des, kcl/mol E A_2_dsb, kcl/mol Activtion energy for the therml recombined desorption of 2 from the two [Cu Cu 2+ ] species, from the reference [Lee, 2005]. b Activtion energy for the dissocitive dsorption of 2 onto the two [Cu + --Cu + ] species, clculted by Eq. (9). Koren J. Chem. Eng.(Vol. 23, No. 4)

8 554 D. K. Lee kind help nd discussion. It is cknowledged tht the rw dt of this pper were obtined from the uthor s experimentl work in the Professor Iglesi s lbortory. REFERENCES Anpo, M., Mtsuok, M., Hno, K., Mishim, H., no, T. nd Ymshit, H., Photoctlytic decomposition of on Cu + /Y-zeolite ctlysts prepred by ion-exchnge, Koren J. Chem. Eng., 14, 498 (1997). Beutel, T., Srkny, J., Lei, G.-D., Yn, J. Y. nd Schtler, W. M. H., Redox chemistry of Cu/ZSM-5, J. Phys. Chem., 100, 845 (1996). D Cost, P., Modén, B., Meitzner, G. D., Lee, D. K. nd Iglesi, E., Spectroscopic nd chemicl chrcteriztion of ctive nd inctive Cu species in N decomposition ctlysts bsed on Cu-ZSM-5, Phys. Chem. Chem. Phys., 4, 4590 (2002). Groothert, M. H., Lievens, K., Leemn, H., Weckhuysen, B. M. nd Schoonheydt, R. A., An operndo opticl fiber UV-Vis spectroscopic study of the ctlytic decomposition of N nd over Cu- ZSM-5, J. Ctl., 220, 500 (2003). Konduru, M. V. nd Chung, S. C., Investigtion of sdorbte rectivity during N decomposition over different levels of copper ion-exchnged ZSM-5 using in situ IR technique, J. Phys. Chem. B, 103, 5802 (1999). Kurod, Y. nd Iwmoto, M., Chrcteriztion of cuprous ion in high silic zeolites nd rection mechnisms of ctlytic N decomposition nd specific dsorption, Topics in Ctl., 28, 111 (2004). Kurod, Y., Kumshiro, R., Yoshimoto, T. nd Ngo, M., Chrcteriztion of ctive sites on copper ion-exchnged ZSM-5-type zeolite for N decomposition rection, Phys. Chem. Chem. Phys., 1, 649 (1999). Lee, D. K., Quntifiction nd redox property of the oxygen-bridged Cu 2+ dimers s the ctive sites for the N decomposition over Cu- ZSM-5 ctlysts, Koren J. Chem. Eng., 21, 611 (2004). Lee, D. K., Kinetic evlution of mechnistic models for 2 relese from ZSM-5-supported [Cu Cu 2+ ] ions by therml reduction or chemicl interction with impinging molecules, Ctl. Lett., 99, 215 (2005). Lei, G. D., Adelmn, B. J., Srkny, J. nd Schtler, W. M. H., Identifiction of Copper(II) nd Copper(I) nd their interconversion in Cu/ ZSM-5 De-Nox ctlysts, Appl. Ctl. B: Env., 5, 245 (1995). Li, Y. nd Hll, W. K., Ctlytic decomposition of nitric oxide over Cu-zeolites, J. Ctl., 129, 202 (1991). Modén, B., D Cost, P., Fonfe, B., Lee, D. K. nd Iglesi, E., Kinetics nd mechnism of stedy-stte ctlytic N decomposition rections on Cu-ZSM5, J. Ctl., 209, 75 (2002). Modén, B., D Cost, P., Lee, D. K. nd Iglesi, E., Trnsient studies of oxygen removl pthwys nd ctlytic redox cycles during N decomposition on Cu-ZSM5, J. Phys. Chem. B, 106, 9633 (2002b) Skny, J., d Itri, J. nd Schtler, W. M. H., Redox chemistry in excessively ion-exchnged Cu/N-ZSM-5, Ctl. Lett., 16, 241 (1992). Tomsic, V., Gomzi, Z. nd Zrncevic, S., Ctlytic reduction of N x over Cu/ZSM-5 ctlyst, Appl. Ctl. B: Env., 18, 233 (1998). Yn, J. Y., Lei, G.-D., Schtler, W. M. H. nd Kung, H. H., Dectivtion of Cu/ZSM-5 ctlysts for len Nx reduction: chrcteriztion of chnges of Cu stte nd zeolite support, J. Ctl., 161, 43 (1996). Yokomichi, Y., Ymbe, T., Kkumoto, T., kd,., Ishikw, H., Nkmur, Y., Kimur, H. nd Ysud, I., Theoreticl nd experimentl study on metl-loded zeolite ctlysts for direct N x decomposition, Appl. Ctl. B: Env., 28, 1 (2000). July, 2006