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1 Proc. Ntl. Acd. Sci. USA Vol. 74, No. 3, pp , Mrch 1977 Chemistry Surfce, ctlytic, nd mgnetic properties of smll iron prticles: The effect of chemisorption of hydrogen on mgnetic nisotropy* (superprmgnetism/mgnetic nisotropy/chemisorption/supported ctlysts/mossbuer effect) M. BOUDARTt, J. A. DUMSICt, AND H. TOPS Stuffer Lbortories of Chemistry nd Chemicl ngineering, Stnford University, Stnford, Cliforni 9435 Contributed by Michel Boudrt, November 11, 1976 ABSTRACT The superprmgnetic behvior of very smll prticles of metllic iron (c 1.5 nm), with bout hlf of their toms t the surfce, is chnged reversibly by dsorption nd desorption of hydrogen below the superprmgnetic trnsition temperture. The chnge fter dsorption implies lowering of the nisotropy energy brrier for the mgetic relxtion of iron nd is scribed to chnge in c tl ine shpe. No such chnges re observed for lrger prticles of iron (c 8 nm) with bout 1% of their toms t the surfce. We hve prepred nd chrcterized smll prticles of metllic iron supported on mgnesium oxide (1). The rte of mmoni synthesis on these iron prticles ws shown to increse with increse in their size (2). The behvior of the smller prticles ws scribed to the lower reltive concentrtion of certin surfce toms with low coordintion number; chnges in the mgneticlly split Mossbuer spectr fter nitrogen dsorption indicted tht the surfce concentrtion of these prticulr sites ws pprently incresed by chemisorption of nitrogen (3). But it ws not estblished unmbiguously whether the chnges in superprmgnetic behvior indicted by the Mossbuer effect were due to nitrogen dsorption itself or to chnge in surfce structure brought bout by nitrogen dsorption. The present work ws initited to resolve this question. Hydrogen ws selected s n dsorbte, nd mgnetic susceptibility ws used in ddition to Mossbuer spectroscopy to provide the complementry nd, s it turned out, decisive missing informtion. XPRIMNTAL The Mossbuer spectrometer, the cells, nd the gs hndling systems hve been described elsewhere (1, 4). The essentil feture is the cpbility of collecting Mossbuer spectr with the smple in flowing plldium-diffused hydrogen or purified helium between 3 nd 8 K within ±1 K. Helium ws used s convenient, noncontminting lterntive to high vcuum Ṫhe mgnetic susceptibility dt were obtined by the Frdy method, with n pprtus described elsewhere (5). In brief, wter-cooled electromgnet provided vrible fields up to 3 ka-m- (1 G = 13 A-m-') nd field grdients up to 3 MA-m-2 (1 Oe = 79.6 A-m-2). The smple ws suspended in qurtz bucket from Chn electroblnce enclosed in Abbrevitions: D, dispersion; M, mgnetiztion (ka m'1); HIT, field divided by temperture (A-m-l-K-1); TB, temperture t which hlf the metllic iron prticles re superprmgnetic. * This is pper no. IV in series. Ppers nos. I, II, nd III re refs. 1, 2, nd 3, respectively. t To whom queries should be ddressed. t Present ddress: Deprtment of Chemicl ngineering, University of Wisconsin, Mdison, Wisc Present ddress: Hldor Topsoe Reserch Lbortories, Lyngby, Denmrk. multipurpose gs-hndling vcuum system tht llowed gssing with H2 or He nd evcution to O-4 P between 77 nd 72 K. For purifiction, the H2 ws pssed through Deoxo unit, nd the He ws pssed through copper turnings t 5 K. Both gses were subsequently pssed through seprte moleculr sieve (Linde 13X) trps t 77 K. The smples, prepred nd chrcterized s described elsewhere (1), were 1%, 3%, nd 8% iron (wt/wt) supported on mgnesium oxide with metllic iron dispersions of.5,.2, nd.1, respectively (dispersion, D, of the metl is defined s the frction of the iron toms tht re surfce toms). The corresponding surfce verge prticle sizes were 1.5, 4., nd 8. nm, respectively. The frction of the iron present s 57Fe for these three smples ws.26,.32, nd.22, respectively. For convenience these three smples will be referred to s Fe(D =.5), Fe(D.2), nd Fe(D = =.1), respectively. RSULTS Mossbuer Spectroscopy. A known quntity of the smple Fe(D =.5) ws reduced in flowing H2 ccording to the previously described reduction schedule (1). M6ssbuer spectr were then collected in flowing H2 t 298 nd 683 K (Fig. 1) nd in flowing He t 683 K (Fig. 2). The qulittive fetures of these spectr revel two different sttes of iron in the reduced smple: metllic iron giving rise to the mgneticlly split spectrl component, nd Fe2+ in MgO which is responsible for the centrl spectrl doublet (1). In the present study, it is the metllic iron species tht is of interest. It represents bout hlf of the totl iron in ll the reduced smples. The prticle size dependence on the bove effect of the gseous tmosphere over the smple ws studied by obtining spectr of smple Fe(D =.2). This smple ws reduced, nd spectrum ws tken t 39 K in flowing H2 with the velocity offset method, which llows the scnning of the two metllic iron peks tht re locted the furthest from the zero of velocity (6). This method provides greter sensitivity to chnge in the metllic iron spectrum thn tht possible with the constnt ccelertion mode used to collect the dt of Figs. 1 nd 2. The smple ws then heted to 683 K in flowing H2 followed by switching to flowing He. After 1 h t 683 K in flowing He, the smple ws gin cooled to 39 K nd second spectrum ws tken t this temperture in flowing He. The two spectr re shown in Fig. 3. For smple Fe(D =.5), the constnt ccelertion velocity mode ws used in order to record simultneously the chnges in the different iron spectrl components. Since this method does not hve the sensitivity of the offset method, computer nlysis of the spectr shown in Figs. 1 nd 2 ws undertken. The mechnics nd rtionle of the computer nlysis hve been described elsewhere (1), but the essentil fetures of the computer fitting will be outlined here. The mgneticlly split metllic iron component ws fitted with six peks, the two peks 86

Chemistry: 1..98.96 94.92 9.88.86 ry ( b -6-4-2 2 4 6 VLOCITY, mm s-' FIG. 1. M6ssbuer spectr of 1% Fe/MgO smple in H2. () t 673 K; (b) t 298 K.

2 Chemistry: ry ( b VLOCITY, mm s-' FIG. 1. M6ssbuer spectr of 1% Fe/MgO smple in H2. () t 673 K; (b) t 298 K. closest to the zero of velocity being constrined to be equl. The Fe2+ component ws fitted with two peks of equl dip nd width, nd the superprmgnetic metllic iron component, hidden by the Fe2+ spectrum, ws tenttively given single, unconstrined pek. From the position nd temperture-dependence of the unconstrined pek, this spectrl component cn be ssigned to superprmgnetic metllic iron (1). It should be noted here tht the superprmgnetic spectrl component could hve been fitted with more thn one pek-e.g., n symmetry doublet (6). However, the ddition of more peks to the lredy complex centrl region of the spectrum is not wrrnted. The results of the computer nlysis for the spectr shown in Figs. 1 nd 2 re presented in Tble 1. The spectrl res hve been corrected for the nonresonnt bckground count rte ccording to the procedure described by Housley et l. (7), the vrition in these corrections not exceeding 1% from spectrum to spectrum. The spectr of Fig. 3 showed 6% increse in the mgneticlly split re with the switch from H2 to He. Additionl experiments showed tht this chnge disppered completely with the switch bck, from He to H2. The sme remrk pplies to chnges recorded in Tble 1. In other words, ll chnges in spectr cused by switching from H2 to He re reversible. As to smple Fe(D =.1), the metllic iron spectrl prmeters remined unchnged to within 1% s the gs flowing over the smple ws switched from H2 to He between 3 nd 683 K Z CO Q88 - Q86 (.1) 1. Q98 _ Q Boudrt et l _ VLOCITY, mm-s-i FIG. 2. M6ssbuer spectr of 1% Fe/MgO smple t 683 K. () in H2; (b) in He. f i '11'i-.' -V I.-I-4.tPor". ; 1 -P. ** VI1%, T % I, '11-. '\ r-a. '. ; A: i :i I :. b -4 Proc. Ntl. Acd. Sci. USA 74 (1977) 87 Tble 1. Metllic iron spectrl prmeters of Fe(D =.5) In H2 In H2 In He Prmeter t 298 K t 683 K t 683 K Mgneticlly split re (mms-'), ± Superprmgnetic re (mm-s-), ± Totl re (mm-s-1), ± Frctionl superprmgnetic re, ± Internl mgnetic field* (MA-m-), ± Isomer shiftt for mgneticlly split line (mm-s1), +± * Compred to tht of NBS Fe foil: I.2 MA-m-1 t 298 K. t Isomer shifts reported with respect to metllic iron t 298 K. Mgnetic Susceptibility. Smple Fe(D =.5) ws reduced in flowing H2, fter which mgnetic susceptibility dt were collected t tempertures of 71, 74, 65, 49, 386, 299, nd 77 K in sttic H2 (Fig. 4A). A density equl to 3.58 g-cm-3 for the smples ws used in the clcultion of the mgnetiztion (M). Above 299 K, the metllic iron prticles behved superprmgneticlly, s evidenced by the superposition of curves showing M versus field divided by temperture (HIT) (8). The minor devition from complete superposition (in the low mgnetiztion direction) s the temperture is incresed is due to the temperture-dependence of the spontneous mgnetiztion nd does not represent devition from superprmgnetic behvior. Quite clerly, however, the mgnetic behvior t 77 K does not fll in superposition with the M-versus-H/T curves obtined t higher tempertures, nd this devition must be ttributed to deprture from complete superprmgnetic behvior. An dditionl experiment, with results not shown in Fig. 4A, showed tht M-versus-H/T curve of dt t 193 K could be superposed with those obtined t higher tempertures. L 1. I.. -, i,...%.92 I z 1. z.84- Cn O &O VLOCITY, mm-s-' FIG. 3. M6ssbuer spectr of smple Fe(D =.2) t 39 K. () in H2; (b) in He. b

3 88 Chemistry: Boudrt et l. Proc. Ntl. Acd. Sci. USA 74 (1977) 4y 3 A [.h B IA. 1o 6A B ~~~~~~~AA S o^ ozo A o A /? / & HrT, A-m-'. K-' H/T, A-m-1-K-' FIG. 4. Mgnetiztion (M) versus field/temperture (H/T) in H2. (A) Fe(D =.5).,71 K;, 74 K; O.,65 K; &, 49 K; v, 386 K; 299 K;@,77K.(B)Fe(D =.1).,71K;,536K;A,31K;@,77K. Before proceeding, it should be mentioned tht no hysteresis effects were found t those tempertures for which M-versus- HIT superposition ws observed. However, devitions from M-versus-H/T superposition were ccompnied by hysteresis effects when the temperture ws sufficiently lowered, nd the dt reported in these cses were collected by strting t the highest field strength nd progressively recording dt t lower field strengths. Upon completion of the mgnetic susceptibility dt t 77 K, the smple ws heted to 74 K in flowing H2; He ws then flowed over the smple for 1 hr t 74 K, nd dt were collected in sttic He, first t 73 K nd subsequently t 33 nd 77 K. A comprison of these M-versus-H/T dt in H2 nd He is shown in Fig. 5A nd B. The sme experimentl protocol ws followed to gther similr dt for smple Fe(D =.1) t 71, 536, 31, nd 77 K in sttic H2 (Fig. 4B) nd in both H2 nd He t 31 nd 77 K (Fig. 6A nd B). DISCUSSION Superprmgnetic behvior of smll iron prticles A smll prticle of iron below its Curie point-i.e., single mgnetic domin-possesses single mgnetic moment nd behves like free superprmgnet t sufficiently high tempertures. As temperture is decresed, nisotropies in the mgnetic energy with respect to the mgnetiztion direction of the prticles become more nd more importnt, compred to the therml energy kt (6). Indeed, if represents n nisotropy energy brrier, the relxtion time r for the prticle mgnetiztion depends exponentilly on temperture: T = ro exp(/kt) [1] in which To is constnt. As the temperture is decresed sufficiently so tht r pproches the chrcteristic time for the experiment, the prticle mgnetiztion does not rech therml equilibrium with the pplied field during the time of the experiment. This results in devition from superprmgnetic behvior in the lowmgnetiztion direction for rndomly oriented prticles since mgnetic nisotropy tends to prevent the prticle mgnetic moment from ligning with the pplied field. A useful feture of combining Mossbuer spectroscopy nd mgnetic susceptibility for studying superprmgnetic behvior is tht the criticl time scle is pproximtely 1-8 sec for Mbssbuer spectroscopy but of the order of 12 sec for mgnetic susceptibility (9). In prticulr, the temperture (TB) 1 2 H/T, A-m-'-K-' H/T, A-m-'-K-' FIG. 5. Mgnetiztion (M) versus field/temperture (H/T) for Fe(D =.5) bove nd below superprmgnetic trnsition temperture (TB). (A) Above TB., in H2 t 299 K;, in He t 33 K. (B) Below TB., in H2 t 77 K;, in He t 77 K.

Chemistry: Boudrt et l. Proc. Ntl. Acd. Sci. USA 74 (1977) 89 3- A ie 2o 1-1 2 3 4 5 6 7 8 H/T, A-m-1-K-' H/T, A-m-' K-' FIG. 6. Mgnetiztion (M) versus field/temperture (H/T) for Fe(D =.

4 Chemistry: Boudrt et l. Proc. Ntl. Acd. Sci. USA 74 (1977) A ie 2o H/T, A-m-1-K-' H/T, A-m-' K-' FIG. 6. Mgnetiztion (M) versus field/temperture (H/T) for Fe(D =.1) ner nd below superprmgnetic trnsition temperture, (TB). (A) Ner TB-, in H2 t 31 K; o, in He t 31 K. (B) Below TB., in H2 t 77 K; 3, in He t 77 K. t which hlf of the metllic iron prticles re superprmgnetic must be very different for both types of experiments. With reference to the Mossbuer dt of Tble 1, TB cn be estimted to be pproximtely 8 K for smple Fe(D =.5). Following the ides of Kundig et l. (1, 11) nd McNbb (12), the mgnetiztion relxtion time t TB for prticles with size equl to the volume verge dimension of the prticle size distribution is pproximtely equl to the nucler Lrmor precession time, TL. Tht is, TI= KTOexp ['TB [2] in which K is n nisotropy energy brrier constnt for mgnetic relxtion, nd (V) is the verge prticle volume. An nisotropy energy brrier constnt of pproximtely 1 J.cm-3 cn be thereby clculted for the smll iron prticles. In single crystls of iron, the nisotropy of the mgnetic energy with respect to different crystllogrphic directions (mgnetocrystlline nisotropy) cn be mesured, nd n order of mgnitude vlue of the energy brrier constnt is 1-2 to 1-1 JIcm (13). Clerly, this vlue is too smll to explin the present results. If the smple of iron is under stress, n dditionl mgnetic nisotropy my be induced due to mgnetostriction effects. A vlue for this type of nisotropy effect is hrder to estimte, but it hs been found tht this energy brrier constnt is roughly equl to 1-1 J-cm-3 (14) for iron films on MgO supports. Demgnetiztion effects crete mgnetic nisotropy in certin directions in ccordnce with the shpe of the prticle, nd this type of effect is clled "shpe nisotropy." For iron, the energy brrier constnt for shpe nisotropy cn be estimted to be bout 1 J-cm-3 for prticle with n spect rtio of.5 (15). For smll prticles, the effect of the surfce must be tken into ccount, nd this could be the origin of yet nother type of mgnetic nisotropy. A phenomenologicl theory of surfce nisotropy hs been proposed by NMel (16), nd vlue for this nisotropy energy brrier cn be estimted to be 1-7 J-cm-2 (17). The conversion of this vlue to volume-normlized energy involves the surfceto-volume rtio of the prticle (since the nisotropy energy brrier is now proportionl to the prticle surfce re insted of the prticle volume), nd for 2.5-nm prticle the energy brrier constnt corresponds to c 1 J-cm-3. From the Mossbuer spectroscopy vlue of TB = 8 K for smple Fe(D =.5), nd from the time scles of 1-8 nd 12 sec for the Mossbuer event nd mgnetic susceptibility experiment, respectively, q. 1 gives TB = 9 K for the ltter. This is in excellent greement with the observtion recorded bove, ccording to which the M-versus-H/T curve t 193 K cn still be superposed with the other curves t higher temperture but not with tht obtined t 77 K. Hence, from direct mgnetic susceptibility dt, TB must be between 77 nd 193 K. Let us note tht, for smple Fe(D =.1), devitions from complete superprmgnetic behvior re observed t 31 K. But these devitions re smll, so tht TB for this smple must be slightly higher, sy round 35 K. In summry, superprmgnetic behvior of the iron prticles studied in this work is clerly indicted in self-consistent mnner by the mgnetic informtion obtined by techniques with criticl times differing by 11. Let us now exmine chnges in superprmgnetism brought bout by chemisorption. ffect of hydrogen chemisorption on mgnetic nisotropy Upon switching from H2 to He there ws 12% increse in the re of the mgneticlly split spectrl component of metllic iron for Fe(D =.5), 6% increse in the re of the outer peks of the sme component for Fe(D =.2), nd no observble chnge for Fe(D =.1). Also, upon switching from H2 to He, there ws no chnge in the mgnetiztion curves for Fe(D =.5) bove TB (Fig. 5A) nd for Fe(D =.1) bove nd below TB (Fig. 6A nd B). But there ws chnge in the direction of lower mgnetiztion for Fe(D =.5) below TB (Fig. SB). All these dt indicte tht chemisorbed H2 decreses the mgnetic nisotropy brrier of smll (D =.5, D =.2) iron prticles but hs no observble effect on the superprmgnetic behvior of lrger ones. One possible explntion is tht chemisorbed H2 might weken the exchnge interction between metl toms t the surfce nd below the surfce, thereby forming mgneticlly smller prticle. This explntion hs been proposed to explin chnges in mgnetic properties of nickle fter chemisorption of oxygen (18). But for the system H2-iron, this explntion is in conflict with the fct tht, for the smllest prticles (D =.5), neither the internl mgnetic field nor the isomer shift were ffected by

5 81 Chemistry: Boudrt et l. H2 chemisorption (Tble 1), nd no effect of H2 chemisorption on mgnetiztion ws detected bove TB (Fig. SA). Possible rtefcts due to ccidentl oxygen chemisorption or interction with the MgO support cn lso be discounted by these observtions s well s by the constncy in totl metllic spectrl re (Tble 1) when H2 ws replced by He. The chnge of mgnetic nisotropy by chemisorption hs been reported in the cse of H2 on nickel (19) nd, more recently, in the cse of dsorbed orgnic molecules on smll finite prticles (2, 21). The lst question concerning the H2-iron system is: If H2 chemisorption does decrese the mgnetic nisotropy brrier of prticles of iron with D =.2 or.5, how does it do it? The best explntion seems to be tht bsed on Neel's concept of surfce mgnetic nisotropy. The vlue of the nisotropy brrier constnt for Fe(D =.5) is comptible with either Neel's estimtes of surfce mgnetic nisotropy or shpe nisotropy. It is perhps impossible to distinguish between these effects for prticles with dimeters s smll s 1.5 nm. Indeed, if chemisorption induces chnges in surfce structure, it must lso chnge the shpe of these very smll prticles. Such chnges re not expected for lrger prticles, t lest t moderte tempertures nd during reltively short periods of observtion. This is indeed wht ws observed in the present work. Conclusion A combintion of observtions by Mossbuer spectroscopy nd conventionl mgnetic studies hs led to the conclusion tht chemisorbed hydrogen decreses the mgnetic nisotropy brrier of very smll metllic iron prticles (c 1.5 nm) but does not do so in ny observble mnner for lrger prticles (c 8 nm). This effect is ttributed to chnges in surfce structure nd/or shpe of the smller prticles. Stimulting discussions with Drs. J. H. Anderson nd J. H. Sinfelt re hertily cknowledged; we wish to thnk B. Turnhm for vlued help. Support of this work by Ntionl Science Foundtion Grnt GK Proc. Ntl. Acd. Sci. USA 74 (1977) 17451X is grtefully cknowledged. J.A.D ws Ntionl Science Foundtion Fellow during the course of this work. Thnks re lso due to xxon Reserch nd ngineering Co. for.generous finncil help. 1. Boudrt, M., Delbouille, A., Dumesic, J. A., Khmmoum, S. & Topsoe, H. (1975) J. Ctl. 37, Dumesic, J. A., Topsoe, H., Khmmoum, S. & Boudrt, M. (1975) J. Ctl. 37, Dumesic, J. A., Tops6e, H. & Boudrt, M. (1975) J. Ctl. 37, Topsoe, H., Dumesic, J. A. & Boudrt, M. (1973) J. Ctl. 28, Turnhm, B. (1974) Ph.D. Disserttion, Stnford University. 6. Dumesic, J. A. & Topsoe, H. (1976) Adv. Ctl Housley, R., rickson, N.. & Dsh, J. D. (1964) Nucl. Instrum. Methods 27, Selwood, P. W. (1962) Adsorption nd Collective Prmgnetism (Acdemic Press, New York), Chp. 3, pp Ben, C. P. & Livingston, J. D. (1959) J. Appl. Phys. 3,12 S-129 S. 1. Kundig, W., Bommel, H., Constbris, G. & Lindquist, R. H. (1966) Phys. Rev. 142, Kfundig, W., Ando, K. J., Lindquist, R. H. & Constbris, G. (1966) Czech. J. Phys. 17, McNbb, T. K. (1968) Ph.D. Disserttion, University of Western Austrli. 13. Morrish, A. H. (1965) The Physicl Principles of Mgnetism, (John Wiley nd Sons, New York), p Kirenski, L. V., Pynko, G. P. & Pynko, V. G. (1975) J. Appl. Phys. 39, Morrish, A. H. (1968) J. Appl. Phys. 39, NMel, L. (1954) J. Phys. Rdium 15, Jcobs, I. S. & Ben, C. P. (1963) in Mgnetism, eds. Rdo, G. T. & Suhl, H. (Acdemic Press, New York), Vol. III. 18. Geus, J. W. & Nobel, A. P. P. (1966) J. Ctl. 6, Dietz, R.. & Selwood, P. W. (1961) J. Chem. Phys. 35, Berkowitz, A.., Lhut, J. A., Jcobs, I. S., Levinson, L. M. & Forester, D. W. (1975) Phys. Rev. Lett. 34, Morup, S., Tops6e, H. & Lipk, J. (1976) Intl. Conf. Mossbuer Spect. (Corfu, Greece).

CHEMICAL KINETICS

CHEMICAL KINETICS Long Answer Questions: 1. Explin the following terms with suitble exmples ) Averge rte of Rection b) Slow nd Fst Rections c) Order of Rection d) Moleculrity of Rection e) Activtion Energy