Kinetic Investigation of CO 2 Reforming of CH 4 over Ni Catalyst Deposited on Silicon Wafer Using Photoacoustic Spectroscopy

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1 Kinetics fr CO 2-CH 4 Reactin ver Ni/Si Wafer Bull. Krean Chem. Sc. 2010, Vl. 31, N DOI /bkcs Kinetic Investigatin f CO 2 Refrming f CH 4 ver Ni Catalyst Depsited n Silicn Wafer Using Phtacustic Spectrscpy Jin-Hyuck Yang, Ji-Wng Kim, Yung-Gil Ch, Hng-Lyul Ju, Sung-Han Lee,,* and Jng-Gill Chi * Department f Chemistry, Ynsei University, Seul , Krea. * jgchi@ynsei.ac.kr Department f Chemistry, Ynsei University, Wnju , Krea. * shl2238@ynsei.ac.kr Department f Physics, Ynsei University, Seul , Krea Received January 20, 2010, Accepted March 12, 2010 The CO 2-CH 4 reactin catalyzed by Ni/silicn wafers was kinetically studied by using a phtacustic technique. The catalytic reactin was perfrmed at varius partial pressures f CO2 and CH4 (50 Trr ttal pressure f CO2/CH4/N2) in the temperature range f C in a static reactr system. The phtacustic signal that varied with the CO 2 cncentratin during the catalytic reactin was recrded as a functin f time. Under the reactin cnditins, the CO 2 phtacustic measurements shwed the as-prepared Ni thin film sample t be inactive fr the reactin, while the CO 2/CH 4 reactins carried ut in the presence f the sample pre-treated in H2 at 600 C were assciated with significant time-dependent changes in the CO 2 phtacustic signal. The rate f CO 2 disappearance was measured frm the CO 2 phtacustic signal data in the early reactin perid f sec t btain precise kinetic data. The apparent activatin energy fr CO 2 cnsumptin was determined t be 6.9 kcal/ml frm the CO 2 disappearance rates. The partial reactin rders, determined frm the CO2 disappearance rates measured at varius PCO2's and PCH4's at 600 C, were determined t be 0.33 fr CH 4 and 0.63 fr CO 2, respectively. Kinetic data btained in these measurements were cmpared with previus wrks and were discussed t cnstruct a catalytic reactin mechanism fr the CO 2-CH 4 reactin ver Ni/silicn wafer at lw pressures. Key Wrds: Ni/silicn wafer catalyst, CO 2 refrming f CH 4, Phtacustic spectrscpy Intrductin The CO 2 refrming reactin f CH 4 can be utilized t make a synthesis gas with a lw H 2/CO rati that is suitable fr the prductin f xygenated chemicals, such as aldehydes, methyl alchl, and acetic acid. A nickel catalyst is prved t be effective fr the reactin, but it is easily deactivated by carbn depsitin n the surface during the reactin. The cking tendency f a nickel catalyst largely depends n the type f a supprt s that many supprted Ni catalysts have been studied in the cntext f the CO 2 refrming reactin f CH 4. Hwever, these studies have been mainly fcused n the develpment f cking-resistant catalysts and little kinetic investigatin has been perfrmed fr the Ni-catalyzed CO 2-CH 4 reactin. 1 Accrdingly, there still exists a cntrversy t assign a detailed mechanism fr this imprtant reactin. The researches n the catalytic activity f the reactins invlve the quantitative studies f reactin rate. Fr the kinetic experiments, thereby, it is desirable t measure the cncentratins f reactants and/r prducts f interest as a functin f time s that an inflectin pint r a shrt inductin perid can be clearly bserved frm the kinetic curves. Althugh absrptin spectrscpic techniques are generally emplyed t the direct measurement f cncentratins, their sensitivities may be diminished due t the difficulty in the bservatin f differences between the incident and transmitted intensities f radiatin. Mrever, these techniques have ften limited success in measuring tempral changes precisely in the cncentratins at shrt reactin times where the rates are large. In cntrast, the phtacustic spectrscpic (PAS) technique measures the acustic waves generated frm the absrptin f ptical energy directly, aviding these limitatins. Nte als that its highly selective detectin and extremely lw mlecular gas level detectin limit can prvide an acustic signal with enugh intensity fr even time-reslved experiments. 2,3 In additin, a further advantage f this technique is t measure the cncentratins f analytes withut utilizing any sphisticated sampling techniques which are inevitable in the cnventinal techniques such as gas chrmatgraphy r mass spectrscpy. Since the PAS technique can directly measure the absrptin characteristics f the samples f interest with pssessing inherent high sensitivity and practical advantages, the technique is cnsidered t be suitable fr in situ mnitring f the catalytic reactins. In the previus reprts, 3-7 we have demnstrated that the PAS technique can be utilized t btain the precise kinetic data f the catalytic reactins in which CO 2 is invlved as reactants r prducts. As mentined earlier the mst cnsideratin f previus studies f Ni-catalyzed CO 2-CH 4 reactin has been related t the develpment f cking-resistant nickel catalysts and kinetic data available fr these reactins have been mstly btained fr metal xide-supprted catalysts. Mrever, little kinetic data have been btained fr the reactin prcess 1 and thse reprted have been determined fr the mst part by mnitring the rates f CH 4 cnsumptin. 8,9 In this wrk, a kinetic study f the catalytic CO 2 -CH 4 reactin was perfrmed by measuring the rates f CO 2 cnsumptin by the use f a thin Ni metal depsited n silicn wafer which has nt been yet examined as a catalyst fr the CO 2-CH 4 reactin. The tempral variatins f CO 2 cncentratin during the reactin were mnitred by in situ PAS and a suitable differential phtacustic cell in the temperature range f C. The apparent activatin energy and reactin rders were calculated by the phtacustic data determined frm the

2 Bull. Krean Chem. Sc. 2010, Vl. 31, N. 5 rates f CO2 disappearance in the early reactin stage. The kinetic results btained in these experiments were discussed and cmpared with thse f previus wrks t infer a mechanism fr the Ni-catalyzed CO2-CH4 reactin at lw pressures. Experimental A nickel thin film depsited n silicn (100) wafer substrate with a native xide layer was prepared by a DC-magnetrn sputtering system frm a Ni target (99.99% purity Ni disc f 5.1 cm in diameter). The chamber was evacuated at a pressure f Trr r belw befre the depsitin. During depsitin, 3 the depsitin pressure in the chamber was 5 10 Trr in flwing argn (99.99% purity) gas with a flw rate f 50 cm3/min and the sputtering pwer was 50 W. FE-SEM (field emissin-scanning electrn micrscpy) images f samples were btained with a Hitachi SU70. An X-ray diffractin (XRD) analysis f samples was perfrmed at rm temperature by using a Philips PW-1710 diffractmeter with Cu-Kα radiatin. The details f the experimental arrangement fr the PAS detectin t mnitr the catalytic reactins are described in previ3,4 us papers. Phtacustic measurements were perfrmed by using a differential phtacustic cell cnsisted f tw cmpartments, i.e. a reference cell and a sample cell, separated frm each ther by a ZnSe windw. Each phtacustic cell was a Helmhltz resnatr f 1.9 cm in diameter and 3.3 cm in length with an adjining tube f 1.0 cm in diameter and 10 cm in length. The utput beam f a cw CO2 laser (Synrad Series ) perating in multiline f 10.6 µm was mdulated at 25 Hz. The nnresnant cnditin was used in rder t prevent the change f the signal due t the variatin in the resnance frequency fllwed by the temperature change in the Helmhltz resnatr. The phtacustic signals detected by the micrphnes in the sample (signal A) and reference (signal B) cell were amplified by a lckin amplifier (EG & G Princetn Applied Research Mdel 5210) and the signal rati (A/B) was recrded by a persnal cmputer as a functin f time. The sensitivity f the phtacustic signal generally increases with decreasing the ttal pressure f gas medium. Hence, the ttal pressure f gaseus reactants in the reactr was kept at 50 Trr filled with N2 as a buffer gas in these measurements. The reference cell was filled with a gaseus mixture f CO2 (0.2 Trr) and N2 (49.8 Trr) t btain the signal B. The sample cell was directly cnnected t a quartz micrreactr with 21 cm3 vlume apprximately 15 cm away by an adjining tube. The fragment f Ni thin film with a size f 10 mm 10 mm was used as catalyst fr the reactin. The catalyst laded in the reactr was typically treated in a flw f H2 at 600 C fr 1 h prir t each kinetic measurement. After the pre-treatment f the catalyst, the reactr was cled dwn t rm temperature and helium gas was passed t remve hydrgen gas remaining in the micrreactr. The reactin mixture cntaining methane and carbn dixide was then admitted int the micrreactr at a given temperature in the range f C. The purity f CH4, CO2, and N2 gas was greater than 99.99%. The gas pressure was mnitred with a capillary silicn il manmeter (1/13 Trr precisin) and a pirani gauge. Jin-Hyuck Yang et al. Results It is nted that the PAS technique is applicable t the quantitative analysis by mnitring the prgress f reactins, since the phtacustic signal is directly prprtinal t the cncentratin f analyte in a given cnditin. The experiments in this wrk were perfrmed in the linear respnse range f the CO2 phtacustic signal with respect t the CO2 partial pressure f belw 10 Trr and the pwer f the incident laser beam f less than 8 W. A blank test, perfrmed using CO2/CH4/N2 (8 Trr/8 Trr/34 Trr) mixture in the absence f catalyst, shwed n variatin f the CO2 phtacustic signal in the temperature range f C. It was als fund that the silicn wafer itself used as a substrate in this wrk was nt active fr the reactin. Fig. 1 shws the surface mrphlgy and the crss sectin SEM images f the as-prepared sample. The Ni thin film with a thickness f ~ 60 nm was cmpsed f the small crystalline particles which were unifrmly distributed all ver the surface f the substrate. Fig. 2 exhibits the XRD pattern f the as-prepared Ni film. The XRD peak lcated at 39.2 crrespnds t the (010) planes f hexagnal Ni metal and the ther small peaks are at- Figure 1. SEM images f Ni thin film depsited n silicn wafer. (100) Intensity (arb. units) 1296 (220) (200) θ Figure 2. X-ray diffractin pattern f Ni thin film depsited n silicn wafer. Ni (JCPDS n ); NiOOH (JCPDS n ).

3 Kinetics fr CO2-CH4 Reactin ver Ni/Si Wafer Bull. Krean Chem. Sc. 2010, Vl. 31, N Ni As-prepared sample H2-treated sample Intensity (arb. units) PA signal (arb. unit) NiSi2 (111) (200) (220) inductin perid Time (sec.) tributed t nickel xyhydrxide which is frmed during the expsure t air prir t the XRD analysis. In Fig. 3 is displayed the variatins f the CO2 phtacustic signal with time fr the CO2-CH4 reactin at 500 C ver bth the as-prepared sample and the sample treated in H2 fr 1 h. The reactin ver the as-prepared sample reveals a very slight decrease in the CO2 phtacustic signal, while the reactin ver the H2treated sample demnstrates a significant decrease in the signal. It is als bservable that the CO2 phtacustic curve fr the H2-treated sample shws t have an initial inductin perid. The SEM and XRD results f the H2-treated sample are shwn in Figs. 4 and 5, respectively. The SEM result shws that the Ni film structure is brken during the pre-treatment. The XRD pattern exhibits the (111) preferential plane f cubic Ni metal, in which nickel xyhydrxide and nickel silicides are bserved as XRD detectable phases. The XRD result als indicates that Ni metal structure is changed frm hexagnal t cubic during the pre-treatment. The effect f temperature n the CO2 disappearance rate fr the CO2-CH4 reactin ver the H2-treated catalyst was inves tigated in the temperature range f C. Fig. 6 shws the variatins f the CO2 phtacustic signal with time at vari- 80 Figure 5. X-ray diffractin pattern f Ni/silicn wafer treated in H2 at 600 C fr 1 h. Ni(JCPDS n ); NiOOH(JCPDS n ); Nickel silicide(jcpds n ). 500 C 550 C 600 C inductin perid 0 Figure 4. SEM image f Ni/silicn wafer treated in H2 at 600 C fr 1 h. 70 2θ PA signal (arb. unit) Figure 3. CO2 phtacustic signals fr CO2-CH4 reactin as a functin f time at 500 C n as-prepared Ni/silicn wafer and H2-treated Ni/ silicn wafer; CO2(g)/ CH4(g)/N2(g) (8/8/34 in Trr) reactin mixture C Time (sec.) Figure 6. Variatins f CO2 phtacustic signal with time at varius temperatures fr CO2-CH4 reactin ver H2-treated Ni/silicn wafer; CO2(g)/CH4(g)/N2(g) (8/8/34 in Trr) reactin mixture. us temperatures, exhibiting the presence f initial inductin perid in each curve. The rates f CO2 disappearance were determined frm the slpe f curves in the perid f sec. The rates were then pltted as a functin f reciprcal temperature accrding t the Arrhenius equatin, as shwn in Fig. 7. The apparent activatin energy frm the Arrhenius plt was calculated t be 6.9 kcal/ml. The PCO2 and PCH4 dependences f the CO2 disappearance rate were measured at varius partial pressures f CO2 and CH4. The reactin rders were then determined frm the best fit f the rate data fr CO2 cnsumptin t the pwer rate α β law, rate = kpco2 PCH4. Fig. 8 shws the PCO2 and PCH4 dependences f the CO2 cnsumptin rate fr the CH4-CO2 reactin n the H2-treated catalyst. The reactin rders determined frm the slpes f curves were 0.63 with respect t CO2 and 0.33 with respect t CH4. In Fig. 9 is displayed the XRD pattern f the used catalyst, in which nickel xide and nickel xyhydrxide are bserved as XRD detectable phases. The result indicates that nickel catalyst has been xidized in part during the catalytic reactin.

4 1298 Bull. Krean Chem. Sc. 2010, Vl. 31, N. 5 Jin-Hyuck Yang et al. In rate (Trr/s) / T(K) Figure 7. Rate f CO 2 disappearance vs. 1/T fr CO 2-CH 4 reactin ver H 2-treated Ni/silicn wafer. Lg rate (Trr/s) Lg PCO Lg PCH4 Figure 8. (A) PCO2 dependence and (B) PCH4 dependence f CO 2 disappearance rate fr CO 2-CH 4 reactin ver H 2-treated Ni/silicn wafer at 600 C. Intensity (arb. Unit) Ni NiO NiOOH Figure 9. X-ray diffractin pattern f H 2-treated Ni/silicn wafer after catalytic CO 2-CH 4 reactin. Ni (JCPDS n ); NiOOH (JCPDS n ); NiO (JCPDS n ). 2θ Discussin As shwn in Fig. 3, the H 2-treated Ni/silicn wafer appreciably prmted a change in the CO 2 phtacustic signal fr the CO 2-CH 4 reactin, while the as-prepared Ni film failed t exhibit a change in the signal, indicating an imprtance f the H 2- treatment n the catalytic activity fr the reactin. The XRD pattern f the as-prepared sample shwed the (010) preferential plane f hexagnal Ni and that f the H 2-treated sample shwed the (111) preferential plane f cubic Ni, as revealed in Figs. 2 and 4. It is cnsidered frm the results that the Ni (111) plane is active fr the reactin, but the Ni (010) plane is nt active. Since catalytic prperty f nickel thin film r nickel/silicn wafer fr the CO 2 refrming f CH 4 has nt been reprted yet, it is nt pssible t cmpare directly the present data with thers. Mrever, kinetic data reprted fr the catalytic CO 2-CH 4 reactin have been determined fr the mst part by mnitring the rate f CH 4 cnsumptin. 8 It is interesting t cmpare the earlier data with thse determined frm the rate f CO 2 cnsumptin in the CO 2- CH 4 reactin ver Ni catalyst. Accrding t kinetic data f the Ni-catalyzed CO 2-CH 4 reactin, the apparent activatin energies fr CH 4 cnsumptin r CO 2 cnsumptin vary with the supprts used and are in the range f 7-28 kcal/ml. 4 Bradfrd and Vannice 8 determined the apparent activatin energy fr CO 2 cnsumptin t be 19 kcal/ml fr the Ni/SiO 2 -catalyzed CO 2 -CH 4 reactin, which is greater than 6.9 kcal/ml btained in this wrk. It is nted that they perfrmed the reactin in a flw reactr at a ttal pressure f 740 Trr in the temperature range f C, while ur experiments were carried ut in a static reactr at a ttal pressure f 50 Trr in the temperature range f C. Takan et al. 9 studied the CO 2 refrming f CH 4 ver Ni/Al 2 O 3 catalyst at a ttal pressure f 63 Trr and reprted the apparent activatin energy fr CH 4 cnsumptin t be 10.4 kcal/ml. In general, the apparent activatin energy determined frm the CH 4 cnsumptin in the CO 2 refrming f CH 4 is smewhat higher than that frm CO 2 cnsumptin. Frm the cnsideratin, the present value is believed t be in reasnable agreement with theirs. The reactin rders btained in present wrk are als difficult t cmpare directly with thers. In this experiment, the reactin rders were determined t be 0.33 with respect t CH 4 and 0.63 with respect t CO 2. In ur previus studies, 6 the reactin rders determined frm the rate f CO 2 disappearance in the CO 2-CH 4 reactin ver Ni/SiO 2 catalyst at a ttal pressure f 40 Trr have been fund t be 0.32 and 0.65 with respect t CH 4 and CO 2, respectively, which are nearly equal t thse f current wrk. Bradfrd and Vannice 8 als reprted very similar results t ur values such that the reactin rders determined frm the CO 2 cnsumptin rate were 0.27 with respect t CH 4 and 0.64 with respect t CO 2. As described early, catalytic prperties f Ni catalyst fr the CO 2-CH 4 reactin can be varied with the type f supprt used. When a reducible metal xide like TiO 2 is applied as a supprt fr Ni catalyst, the supprt itself participates the catalytic CO 2- CH 4 reactin, in which xygen vacancies frmed during the pre-reductin can act as active sites fr dissciative adsrptin f CO 2. 8,10 On the ther hand, when an irreducible xide supprt like silica is used, the CO 2 dissciatin is prmted by the H

5 Kinetics fr CO 2-CH 4 Reactin ver Ni/Si Wafer Bull. Krean Chem. Sc. 2010, Vl. 31, N (ads) riginating frm the CH 4 dissciatin which can be assisted by xygen atms n the supprt. 11 The silicn wafer, used as a supprt in this wrk, cntains neither xygen vacancies nr xygen atms in it. Additinally, the Ni (111) plane is knwn t be inactive fr the dissciative adsrptin f CO Thus ther prcess fr the CO 2 dissciatin n the present catalyst must be cnsidered. The Ni (111) plane is nt active fr the CO 2 dissciatin, 12 but active fr the CH 4 dissciatin. 13 Hence, the CH 4 dissciatin is believed t ccur prir t the CO 2 dissciatin ver the present catalyst. Althugh there are sme disagreements in the reactin mechanism f catalytic CO 2-CH 4 reactin, it is generally accepted that methane is dissciatively adsrbed n the surface f the metal catalyst t frm bth CH x fragment and H(ads) 1,14 : CH 4 (g) CH x (ads) + (4-x) H (ads). It has been als prved that a reverse water-gas shift reactin, CO 2 (g) + H 2 (g) CO (g) + H 2O (g), as a side reactin is invlved in the reactin mechanism. The reverse water-gas shift reactin can ccur in the temperature range f C, explred in these measurements, and the reactin n the catalyst surface culd be represented as CO 2 (ads) + H (ads) CO (ads) + OH (ads), which means that the CO 2 dissciatin is prmted by the H (ads). The OH (ads) grups are evlved frm the surface as H 2O (g) t leave O (ads) n the surface: 2OH (ads) H 2O (g) + O (ads). If the reverse water-gas shift reactin is favrable under these cnditins, the rate f CO 2 disappearance wuld increase with increasing temperature in the given regin. As a result, the bservatin f high apparent activatin energy fr the CO 2 cnsumptin is expected. Hwever, the apparent activatin energy btained in this wrk, 6.9 kcal/ml, is rather lw, which enables us t cnsider that ther side reactins may ccur in this catalytic reactin. Bth the CO disprprtinatin (2CO (g) CO 2 (g) + C (s)) and the methane cracking (CH 4 (g) 2H 2 (g) + C (s)) can be cnsidered as side reactins under the reactin cnditins. In bth the side reactins prducing slid carbn, the methane cracking wuld be mre preferable at lw pressures. Carbn species frmed frm the methane cracking, which may be regarded as intermediates in the reactin, are knwn t have high reactivity. 8,15 When the active carbn species react with water vapr frmed in the catalytic reactin, CO, CO 2, and H 2 can be prduced accrding t the prcesses: C (s) + H 2O (g) CO (g) + H 2 (g) r C (s) + 2H 2O (g) CO 2 (g) + 2H 2 (g). Nte that bth the reactins are cnsidered t be favrable at lw pressures 3 and the ccurrence f the reactin can suppress the depsitin f slid carbn n the catalyst surface. The CO 2 (g) prductin frm the side reactin wuld give rise t a little change in the ttal CO 2 phtacustic signal during the reactin and, accrdingly, the lw apparent activatin energy fr the CO 2 cnsumptin wuld be bserved. It is nticeable that the CO 2 phtacustic curves in Fig. 6 reveal an initial inductin perid. The bservatin f the initial inductin perid implies that intermediates may be invlved in the reactin mechanism r ther active sites may be generated in the early reactin stage. Many mechanistic studies fr the CO 2 refrming f CH 4 ver supprted Ni and Pt catalysts have been prpsed that surface CH x fragments prduced frm the dissciative adsrptin f CH 4 react with either xygen atms r hydrxyl grups t frm CH xo intermediates: CH x + O CH xo, CH x + OH CH xo + H. 8,16-18 The CH xo intermediates are subsequently decmpsed int bth CO and H 2 prducts: CH xo(ads) CO(ads) + H x(g). In rder t frm the CH xo intermediates, the CO 2 dissciatin must be ccurred prir t the reactin f CH x fragments, suggesting that the CO 2 dissciatin depends n the CH 4 dissciatin int CH x fragments and H (ads). If CH xo is an intermediate in the reactin mechanism, the rate f CO 2 cnsumptin shuld be dependent n the partial pressure f methane, as was bserved in this study. As shwn in Fig. 9, the XRD pattern f the catalyst after the reactin revealed the presence f nickel xide and nickel xyhydrxide phases. The result indicates that Ni metal was xidized during the catalytic reactin, in which xygen atms shuld be prvided frm CO 2. The reactin f CO 2 (ads) with H (ads) generates bth CO and OH (ads) and the OH (ads) grups are evlved frm the surface such as H 2O t leave O atm n the surface, thereby resulting the xidatin f nickel. It is expected that the reactin f CH x fragments with either O atms r OH grups t frm CH xo intermediates can ccur favrably in the nickel-nickel xide interfacial regin. When the cncentratin f CH xo intermediates rises t a maximum, the CO 2 cncentratin wuld begin t decrease gradually with increasing the cncentratins f CO and H 2 prducts. Cnsequently, a gradual decrease in the CO 2 phtacustic signal alng with time wuld be bserved. The initial inductin perid, bserved in the current wrk, is cnsidered as the time required t reach the maximum in the cncentratin f CH xo intermediate with the frmatin f nickel xide. In this reprt, the kinetic analysis f the CO 2-CH 4 reactin catalyzed by nickel metal depsited n silicn wafer was perfrmed at a ttal pressure f 50 Trr in the temperature range f C by using the CO 2 laser-based phtacustic methd. An initial inductin perid was bserved frm the CO 2 phtacustic curves recrded as a functin f time fr the catalytic reactin. The bservatin f initial inductin perid is cnsidered t be clsely related t the frmatins f nickel xide and CH xo intermediates during the reactin. It is als suggested that in the nickel-nickel xide interfacial regins, the CH x fragments riginated frm CH 4 react with OH grups r O atms t frm CH xo intermediates which subsequently decmpse int bth CO and H 2 prducts. The lw apparent activatin energy fr the CO 2 disappearance bserved in this wrk, 6.9 kcal/ml, seems t be resulted frm the ccurrence f the side reactins between slid carbn and water vapr t prduce CO 2. It shuld be pinted ut that since these measurements were cnducted at lw pressures, the kinetic data btained may be different frm thse measured at high pressures. Nevertheless, the phtacustic technique and its capability f lw mlecular level detectin at the early reactin stage enable us t btain precise and useful kinetic infrmatin abut the reactin mechanism. Acknwledgments. This wrk was supprted by grant N frm the Basic Research Prgram f the Krea Science & Engineering Fundatin and by the Brain Krea 21 Prject f the Ministry f Educatin, Science and Technlgy f Krea. Authrs thanks KBSI Gangneung Center fr FE-SEM measurements.

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