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The thermostabilizing effect of lanthanum on γ-Al2O3 studied by low-energy ion scattering
Applied Surl;,c¢ Science 55 (19q2) I I - 1~ Nt)rlh-] I,)lland
applied
surface
science
The thermostabilizing effect of lanthanum on T-AI=O.~ studied by low-energy ion scattering G.C. van Leerdam, H.H. Brongcrsma l'a~ tilt)" o/ PIl~ ~t~ ~ and Sthint ht~ttttdc o! ~'atah'~t~, l'tndhot ~'n I_hut eritty oJ' 7~'c'hnoh&,y. I~ 0 Bo~ 51.t. 5000 MR Em~fllm en. Ncttwrhmd~
I.I.M. Tijburg and J.W. G e u s l)cpartotcnt ~Jl hlor~,,ttnic (Ttl'mi~tr~. [hut ~'rwty ~1 Utrecht. Sorhonnehlun lb. 35b~4 ('4 U/n'cht. Nefll<.rlaltd~ Received 25 March Itlt~l: accepted for puhlication 2h S e p t e m b e r 19ql
Lo'.,.-t'ncrgy ion sc:~nering has been tl~etl I',) Mtldy the ntlrfac¢ ol I;inlhanum Ihcrn'Jost~lbilized 7 . A I 2 0 ~ during several Mages (If the ,,intcring proton, ., Dcpo,,it.on o l l:.nlhlmum onto the ~uppt~rt by ",p¢cific ad',¢~rptitm :ff ~1[I,a(EDTA)I complex lurm, out It) be ho.mlgcnc~tP., After calcin;,tit'm ;.t 55{1"C La_,O~ p~lrtlclc,, h~t'.c been f~rmed on the ,,ur|itcc. "~.hdc LEIS re,,ults after ~.intering lit 11)5(I°( . ;n.licatL: (he I't~rm;ition of a lill3thanum,altlminal¢ ~.ur[;ic¢ layer, T h e prc~,cncc of U;inth;tnum prevent,, Ir;ln~rorl~ilti~.)rt of 7 - A I : O ~ to rr-AI,O~ ;It 10511"('.
I. Introduetion Many industrially important chemical reactions arc economically feasible only by the use of a catalyst. Utilization of solid catalysts predominates in the chemical industry, since they elm bc readily separated from the reaction products. To provide a thcrmostable, active catalyst, the catalytically active component is usually applied to a highly porous, inert support. Generally used supports arc silica, alumina and active carbon. The surhlce area of these supports varies from about 100 to 5(11) m 2 per gram. In combustion catalysis, the support, material has to mcct severe requirements, since it must withstand high temperatures in the presence of water vapour. The best support to cope with these conditions is ",/-alumina of acicular elementary particles [I]. However, cvcn this support material sinters at temperatures above 60(.I o C, which causes a considerable drop in thc surface arcv. At
temperatures above IO[)O°C the thermodynamically most stable alurnina, c~-Al:O~, is formed. The transformation to ~ - A I : O ~ involves rccrys-
tallization of the oxygen lattice frem cubic to hexagonal close packing [2]. "121stabilizc 7-A120 ~ against sintcring the support material has been doped with elements such as manganese, chromium, zirconium and lanthanum [3]. Among these elements lanthanum appears to be most effective. Schaper et al. [4] attribute the stabilizing effcct of lanthanum to the formation of a surface lanthanum aluminatc (LaAIO0, which dccreascs the rate of surfacc diffusion. Vcrcshchagin ct al. [5], however, observed an accelerated transformation to a - A I ~_03. For samples prcparcd by coprceipitation Matsuda c t a l . [6] report ihc fermati~m of lanthanum-,Baluminate (LaeO 3 • 11AI20 3) for low La contents. and LaAIO 3 for higher contents. Recently, it has been demonstrated that low loadings of lanthanum can be sufficient to pre-
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(;.( ". ~a~ I.c~'r~htm ~'t al. , lilt' tlwrmo.~tabih:m~, cl¢i'o ~tf hmtl.mum ott y-..ll : O t
pare thcrmoF,tablc supportF', prtwidcd tile lanthan||hi waF' deposited Olltt~ the Stll}polt by specific adF'orptitm of the [La(I-DTA)I complex [ I]. An X-ray p hotoc lectmn spect roF'copy (X PS ) study indicated that lanthanum remains distribuled unili~rmly twcr the .~upport after several stages of the sintc,ing prt~css. Although XPS is a surfaccF'cnF,itivc analysis technique, it averagcs the inlormatitm +tlxltlt the chemical compoF'itioll and state over a number of atomic htycrF'. One of the v¢13, few F'urfacc analyF,iF, techniques which is only sensitive to the outcrmoF't atomic layer is hlw-cncrgy ion scattcl'ing (I.EIS). Therefore. thi.~ tcclmiquc |'an I"J12 :l vallla['~lc tool Io ;.isstL'~F'inore at'¢tll'alely the posiliun of the stahilizing lanthanttna F,pccicF,. In a typical LI:!IS cxr~crimcnl monocncrgetic noble gl,s ions al't2 baekscatlcrcd by Ih¢ | ; r e e l atoms. Who,1 it~llF, t~l"~mly a few kcV lind a large seal|crane angle arc used. the b;lekscattelcd ions resuh fmnl ~,lile collision of each ion with a serbia?t: atom. Then the energy .~pe¢trunl of Ih¢ baekscattered ions iF, cquivalcnl 1o a Illa,~s spl2cIrurn of the surface alOnls [7-~)]. The high stn'lacc sensitivity iF' ntainly due to tile effeclive neulrali/alison of those it~llS ~vhieh penetrale the litrgcl I~eylllld tile tltl|;.'l'lllt)S| ;110111i¢ laly¢r, since OllJy Jams atre detected, l'he nalurc of Ihis teehnitluC JlllpJJes that lltl hll~.)rtllatJUll is obtained on the CIlelllJt.';.tl static OI the Slll'l;lfe :ttOllls. t\r~alll fltllll inltitlllalJon al',otll the Slll'lalCC CIHllpositiIH1. :liSt) inlornlation ahtlttt the depth 1.tistril~utitm o[ II1,.: different e~llstitttelll~ Cilll I'ICgained by use o f tile sputtering action of Ihe primnry beam. In the ideal ea,~e this result.,, in a laycr-by-laycr removal td tile stiFf;ICe eonMJ!tlenlF'. I:or several sltpllOl'led himetallic calal.vsts. Ill,.? intqucnec of the Setlucncc of [nll~regnatit~n . of lhc t.';.tlcinatJoll teml'ler;tttH.¢ ;1111.1 tinle t)n the distribtttion o1" tht.' stn'l'aec iill)~ns has thus been chtcidatcd [t)_12]. The application to'~,al'dr, Inonttnletal[i¢ cataJ}sts ofl'Cl'~., el'eat| per",pcctives in tile study t~l" the spreading behavior t;I tile tnctal tlvcr tile ~,uppt~rl [13]. Recently. a detailed stud~, ol tile h}cation of Mo tan 7 - A I , O t ha', I~c, .alrl'll.'d t'~tll [I.ll. "1"o obtain retire inlilrnlalion al'~oLIt the diMribulion ~ll lanth;tllllnl thtth]g tile sintclJllg process ol the tllernmstabiltzcd F'Upl'~t~rts"ptepau'ed I'Ll.• the spe¢ilic :tdF'tnplitm technique, a I.I'IS invesliga-
lion haF, bccn performed. Samples with variouF' Ioading~, of lanthanum have been investigated.
2. Experimental
2. I. Sample preparotion "File samples F'lodied arc g - A I 2 0 ~ (AI 4172. Engclhard Chemic b.v. De Meern, The Netherlands) loaded by different a m o u n t s of lamhanunl (I.6, 2.3 and 2.(~ w t q I.a/. The support is prepared by dehydration of pscudo-btlchmitc at filll)"(" and has a BET surlacc area of 265 m - ' / g and a pore volun'lc of 1.14 c m t / g . The hmthanum has been applied by F'pccific adsorption of a [I.a(EDTA)] complex according to the proccdurc dcscribcd in detail in rcf. [I]. By calcination of the thuF' loaded alumina all 550°C. Ihc [I.a(EDTA)] complex decomposes and lanthanum oxide iF' left on the F'mfacc. After calcinalion for 5 h at 55(1°( ", samples of 2 g ~cre loaded into alumina crucibles, and placed in an electrically heated furnace fro" standard F,intcr cxpcriinents at 111511"(." dr, ring 23 and i45 h in stagnant air. The freshly prepared samples were dried ;.it (ill' (" overnight prior to analysis. l ; h r a p u r c ¢t-Al,()~ (Fluka At1) with it BI"I" Mlrla¢¢ atoll tlf 5.5 nl"/g w;is almlyzcd a~, a referell ft.' ~ittllple.
2.2.
l:quipmcm
A descril',lion of lhc basic experimenlal set-up of lilt: ion scattering apparatus (NODI.!S) has been published belore [15]. Nov,'ada~n, mtnment'lgetic ion.', are produced m a I.cybold ion source tlypc I O | - 12/3S1 alld suhncquentJy mass-selected ill it WJen filler. FIle ions scatttett.'d over lit1 itng[c of 142 " ate cnerg.v-analyzed in a kind of cylindrical nlirl',,~r analyzer. Ftmr stages of differential I+umping ensure that the pressure in the Ut.IV ehanlJ'~cr increases only ttom 4 x Ill ~ to t) "-. 111 s I)a x~,'h,dn the prinlal3: heHnl JF, tl.ltned till. The sinlldtanct+us storage of t'~,clvc different samples tm a carotlseJ Jn the U I I \ ; chamber alJlY~VS;ttlltlysis tinder idelltical experimental eLiildJ-
(i.('./ill! I.l'l'rihllll i'1 ol / Thl' IIh'ntli+it(lhlh2ln.l~ cllc<; o! /'+illl/llllll+lll till )<-.'lid,l+
lions. The targets were m o u n t e d o n t o it s a m p l e h o l d e r which can bc insurtcd onto the cl, rouscl out of a n adjacent t r a n s f e r vessel by m e a n s of a load lock. Powdered m a t e r i a l s such as catalyst.,, are pressed into a t a n t a l u m holder before m o u n t ing on the sample holder. During ion h~mbardmerit of insulating materials charging effects may occur. Surface c h a r g i n g can be effectively eliminated by fltmding the surfitce with low-energy electrons. Tn o v e r c o m e local charging by g e o m e t ric shadowing, it r i n g - s h a p e d neutralizing system wan constructed which e n s u r e s flcK~ding from all sides. Even with e l e c t r o n s having e n e r g i e s a:+ low its It| eV. c o m p l e t e c h a r g e neutralization is achieved lhr rough and r ~ r o u s systems such its these catalysts. C o n t a m i n a t i o n of the filaments b y s p u t t e r e d particles is p r e v e n t e d by phtcing the f i l a m e n t s out of the line-of-sight of the target. T h e e m i t t e d electrons are directed t o w a r d s the silmple by applying the appropriate voltages to deflection plates. By thin configuration h e a t i n g of the s a m p l e by irradiation ix also minimized. Typical m,.21l
3. Results I.EIS spcctta have b c c n acquired lor the lrcstd.v p r e p a r e d , calcined and s i n ! t r o d s a m p l e s of lant h a n u m loaded "y-AI.O~. In fig. I I.l:.lS spectra ate shown of tile I.a-loaded support (2.3 ~t<~)
0
AI
;a
' -
l.
II II I I I i
~
,s
[
I
,
,'2"0~ ~000 tevl Fig I l.l.l~ ,,p.++utlaill t .i.to.i.+l,.:d~Upl~-t ,lltt.t dct',+nlr~Istlh+t+ ~iI flu.+1 I')I %.,.ompl~+', .it ~,'~ll ( { . . . . ) .nid .llh;l ~ml+.:ling .It I{151l (" 1) lh¢ l'"m.iP. ~.c,g~. +,l th.g ' l h ' u,n, i, tlRRI i.'~ ff
after decompo,d|um of the F.I)IA cumplc,, at 55(I~(" and alter nintcring ;it 1050 (" for 145 11 "l'hc pcakn ol oxyecn, aluminium and lanthanum arc ch.'arly ru.,,uk~..ll.The.' crurg:, range from 22U0 to 2qlltl c V ha', I',cu'rl ,,canm.'d v, ith a lilth ol thu' speed o l Ihu' rcnlanling pa"l ot I|1c" np¢¢lrum Io +..:'lhiH~c+. lh+d dclu'chon of 1.1111hilnu111. lNpcciall$ in 111+.",,pc,+'lra ol lhc Itcnhh pl+..'palcd ",amph:, a u'h.'.il lhlorhl,.." pu'.ik i', ~,l~,,,:e.~:d. Ninc~: l]tlnrinc+ cotdd ab, t+ l'~c th:tcctcd ten hc unlo,tdcd "qlplltlrt l'e,+ 1'~olJt IF.IN ,lild ~I'+%. ,hi "+, clement i', mmt pmb,dd) intloduc.'d +,11.11in~ ,q. nth~."~r,. Since. aIr tatg~.t- ha~c bc,_'n +tlldl)/'Jd Illldti M e t r i c a l experimental con,lit!ont, lhc a1",solut,.; signals o f lh,.: dilh.'r,.:nl saml+l,¢~, c'a11 be culnpar,.:d v, ilhoui r¢,,trictitm+ l.or a o'qiManl loading, l+,oth thu" o',u'rall inlcn,it.x in the." uncrg3 rang,..' bciv,~.'cn f~lll~, antl l~tltl ¢\" .'nld the La intcnsily change as d lunclion o l the thermal tr,,:alnlcnt (see lig. I). ]'h+ AI intcn,,it.~, m gcm.'lal, dUCl,.glr, u',, ,it higilu'r cak+~ natK;n t+.+'nlpu'litltlt++.'ntel longer tiln~?n..2%It'+,.'I lb'+.' dcct+mpt)+,ition of the 1:I)'I,.\ colnplcx at 55(I (', h)l cxanlph.', tl1~.' AI hltu'nsit', ol th,.: 2.3 v.t +'< +,.:::!pie h.~,'- ,.!L',:r,_':t,,_'d h~, I 7" ; Nmlcring .tl Ii1511 (" l¢,r 23 and t45 h r,~,,ult., in an addHumal d¢clca'.¢ |'~3 7', and 1,';';. tc,,pccltx~:l~, Ih~." l.,~ intcnsil), hox~.u'xcr, ha', dc'crCil'+.cd I',~. 41l+,• altL.'t calcination ill .~511 ('. hl omtra',t, the l a +,igndl inct~.'ilsu'% b~, 711+; and 7 ( f ; alter ,qlltcrirlg at lllSll+'(" h n 23 and 145 h. I CSl'~CCti',u'.).
( L ( " t ittt I.c't'rdam i't al. / 77h Hlf'l.tO~lal~ill.'ltlg ~'11~'~t C~] I . I l t h t l l t U m Oil y ' . ' l l ' ( } l
I-I
-I
I •
60 oC
A Sh 550°C O/., o 23h I050°C
=
.f
& It.Sh 10S0O.f
/
j02
0 La
I I
• AI ....... 0
I
I . . . . . . . . 20
2
La- conlent (~t~'=)
Fig. 2. I)L'pU'zltlcncc I)t Ih¢ I.EIS I . ; I / , \ l ratio tm thu' thurnl;ll Ir~:;llnl,dnl ~11 Vilritltl~, htlJk l.;I ConlCllls.
"1"o summarize the results (11"the different l,a contents, the La/AI signal ratio after the dillcrcnt thermal trcatmcnts is plottcd in fig. 2 as a function of Lit loading. From this figure it follows that the La/AI ratio for the as-prepared samples increases linearly with the lanthanum content. It should bc mentioned that these samples differ from the other samples, since the lanthanum i~ns arc thought to bc still embedded in thc El T A complex on the surface. After calcination: at 55(1"(" the La/AI ratio hits decreased and the linear dependence on the l,'l content has d~ ,Lppcarcd. After the sintcring experiment', at 1(15()"C. however, the La/AI ratio increases ~tnd is again proportional to the La content. In ov'dcr to obtain more complete int~)rm~ion about the bchaviour of the lanthanum during the thermal trcatmcnts, sputter profiles wcrc also measured for slmlplcs containing 2.3 wt";~ l,zt. Gcncr~dly, thc ratio of the metal to support is plot!cd a..., a function (11"ion flux. This approach is zlmt)ngst others based on the assumption that both the support and the La arc equally covered witll the s:.lmc contaminatkm. In the case of the freshly prepared samples, however, this approach bcconlcs invalid since all La ions arc supposed to bc embedded in the EDTA complex, while this complex only partially covers the 7-AI,O 3 support. In addition, on the surfacc of the alumina support terminal - O H groups will bc present, wl,;,:h may decrease the detectability of the aluminium atoms. In fig. 3 it can bc sccn that for the
bombardment
"0
hme ;~,n)
I:iL~. 3 S p u t t e r prolilc Ior tilt: dried ~,tmplc cont~tming 2.3 ','.tg: I .;t,
freshly prepared sample the La-signal decreases with increasing bombardment time (ion flucncc). apart from the first point ¢1t"the series. The La sputter profiles o|" the calcined and sintcrcd samples arc combined in fig. 4. For the calcincd sample the Lit intensity hardly decreases throughout the ion bombardment. In contrast. 1(11' the sample sintcrcd at 11150°C Ibr 145 h the La signal decreases exponentially from the beginning. The shape of the profile of the sample sintcrcd l~.)r 23 h is intermediate between that of
the two other prot'ilcs, l:¢~r all samples, the AI intensity slowly increases during the sputtering. periods applied. To investigate whether the I.a-loadcd support transforms, to ~-AI_,O~ at sintcring temperatures
m
: i
0
O_
__
10
Oe
20 bombardmont
i
30 hme (ram)
|:ig. 4. L[intJl~llltlln ~,])HI(L'F iln)li[C ", h)r the ~,ilnlpl¢~, Con|;iHlillg ~.~ '~'.11; |.;I ;IIlCt L';I]CiN[I(i(Itl :LI ~.~11"(" ~llld ~.illtu'rirlg ;It lglSO" (',
r;.C t ,tl l.ccrdam ct aL / "lhc thermmtahlh:ing c/Jet t o] h m t h a n u m on ),.,.11:() ~ "
rm - m
I
:
i
'I
AI
l I
: ,
o
17 i Lt_
600
1200
~8oo
Ef
2~oo
(evl
Fig. 5. Typical LEIS ,.pcclrum o l .'r-Al:O~ (111c'. primary energy tff 31ltll)eV).
of 1(15(I° C. a reference spectrum of a-Al_~O~ was recorded (fig. 5). The A I / O signal ratio of all lanthanum loaded supports sintered for 145 h at 11,151)o C is found to be 75 + 4% of that of a-Al~O~ (2.67). For the other samples, the A I / O ratio depends on the La content and the thermal treatment. Generally, this ratio is smaller than 2.
4. Discussion As a result o f the sintering experiments, a surface urea loss can be expected fi)r the Laloaded y-Al20~ particles, which is indeed confirmed by BET yurfacc area measurements. The BET surlacc ,trca for the samples calcined at 551J°C amc:ants to 2211 m-'/g, while for the sampies calci,lcd for 23 h and for 145 h at 105tI°C it amounts ',o only l(l(I :md 91) m2/g [I]. respcctivcly. The effect of sintering on the absolute intensity of the LEIS spectra will dcpcnd on several factors. On a microscopic scale many orientations of the support surface exist with respect to the incident ion beam. For a given beam diameter the number of surface atoms which can be hit increases when changing thc angle of incidence from perpendicular to more grazing angles. However. for very ~,mall angles of incidence mutual shadowing of atoms in one surface plane may occur. Although the incident ion beam may hit effectively many surface atoms, the detected
amount of backseattered ions may bc much less due to blocking effects. Espccmlly for surfaces with deep and small pores a considerable fraction of the backscattercd ions will bc prevented from detection. The combination of thcse effects will determine whcthcr an incrcase or a decrease of the signal intensity is observed. According to Margraf ct al. [16]o the AI intensity of single crystalline a-Al~O~ is five times larger than i'or powdered y-AI,Oa, which indicates that the steric effects dominate. Thc present support, however. consists of ncedles which are esscntially non-porous. TEM micrographs of the sintcred support show that the surface of the -/-AI:O 3 parh=les has flattened [I]. Apparently. this flattering during sintering causcs thc decrease of the ovcrall intensity in the LEIS spectra. However, after termination of the sintering experiments the BET measurement indicate a 6tic/e, surface area loss, whereas the concomitant LEIS AI-intensity loss is only 18%. Apart from the already mentioned reasons this discrepancy may be due to the fact that LEIS probes only the visiblc part of the elementary particles, while in the BET method thc total surface area is measured. In the spectra of the freshly prepared samples a clear La peak is observed already for such low ion doses that hardly any damage to the surface can have occurred. One way to account for this observation is that the EDTA complex has decomposed after insertion into the UHV system. Another possibility is that the surrounding ligands shield the central lanthanum atom so poorly that scattering is still possible to some extent, Recently, this weak shielding has been observed for a number of oxides [14]. The proportionality between the La/AI ratio and the lanthanum loading for the freshly prepared samples (fig. 2) can be interpreted either as a monolaycr formation of the La or as the deposition of cqui-sized lanthanum-containing particles. To distinguish between these possibilities, the shape of the La sputter profile can be used. If. for instance. La would be present as a monolayer an initial exponential decrease of the La intensity with sputter time is expected. In the case of three-dimensional La-containing particles, however, the intensity is assumed to remain initially constant. Experimcn-
16
( i C. t a n la'~'rtho~l ~'t t d / 7"h~"therolo~tahilizttlg ~'[]i'~'t o f lanthctnutn *m ]t-Al.,O ¢
tally, an exponential decrease of the La intensity is found (fig. 3), which indicates that the lanthanum compound is distributed uniformly over the surface when the specific adsorption technique is used as a preparation procedure. For the freshly prepared catalyst a loading of 2.3 wt% La is estimated to correspond to a fractional coverage of 4%-8%. The. cnnsiderable drop in the La intensity after calcination at 55{1"C (fig. 2) indicates that lanthanum coagulation has occurred during the thermal treatment, most probably to form La_,O3 particles on the support. Diffusion of the lanthanum into the support seems unlikely at this temperature duc to its relatively large atomic radius. The non-linearity between the La loading and the La/AI ratio (fig. 2) also suggests a multilayer model for La,O~. Further support for the lowered La dispersion is given by the shape of the sputter profile (fig. 4), which shows a hardly decreasing La intensity for increasing bombardment times. Coagulation of metal-oxide particles on the support is a rather common phenomenon in catalysis. The eventual occurrence depends on the relative values of the specific surface free energies of the active oxide and the support, and on the specific free interface energy between the oxides [17,18]. The net result of this energy balance determines whether spreading of the active oxide over the support occurs or coagulation. Approximate values of surface free energies of several oxides have been compiled in the literature [19]. Unfortunately, no data exist for the specific surface free energy of La,O 3, nor for the free interface energy of the mixed system. In most cases, however, three-dimensional particles are formed on the support [18]. Wetting of the support by the active oxide is observed for MoO~ and WO 3 on ~/-AI_,Os and for V_~Os on TiO 2 [13]. if calcination temperatures are sufficiently high. a solid-state reaction between the support and the active oxide may take place. The shape of the La,O 3 particles will also be determined by thermodynamics [18]. However, if it is assumed that spherical La_,O~ particles of equal size are formed, an attempt can be made to estimate the size of the particles. After calcina-
tion at 55(1°C the absolute La intensity has decreased by 40%. Assuming that this decrease is purely a result of the sintering of lanthanum, thi. reduction corresponds with La203 particles containing approximately 13 La atoms. In this model it is supposed that the density of La in the surface layer of the La203 aggregates is representative for the bulk density. The diameter of the La20 3 clusters will then be about 1'~ am. However, for other shapes of the La20 3 aggregates the thickness of the particles will also be about 2 or 3 layers. The increased La/AI ratio for the samples sintered during 145 h at 1050°C (fig. 2) i.~ combination with the exp~mential decrease of the La signal in the sputter profile (fig. 4) indicates that redispersion of the coagulated partit, les occurs during sintering at I(I50°C. At first sight, this redispersion seems illogical, since in the preceding part of the discussion it has just been shown that the originally uniform La distribution after deposition has disappeared. The redispersion after calcination at 1050 °C, however, can be explained by a solid-state reaction between the support and the lanthanum oxide. Due to this reaction the La ion will be present b~ the surface plane, instead of o n the surface plane of the support. Since the rate of a solid-state reaction is much slower than surface diffusion, the relatively long sintcring time ( > 23 h) to obtain the mixed surface layer is also comprehensible. The formation of a mixed surface layer is in agreement with the phase diagram of lanthana-alumina, which indicates the formation of lanthanum-/3-aluminate in dilute lanthanaalumina mixtures [20]. The A I / O signal ratio of La-added "y-AI.,O~ after calcination at 1050°C appears to be three fourth of the signal ratio for ~-AI20 3. One possibility to explain this effect would be that one fourth of the AI atoms in the surface of a-AhO~ has been replaced by La atoms. However, for the highest La content the number of lanthanum atoms on the surface of the alumina can be calculated to be only one half of the amount required to replace the desired number of AI atoms. This indicates that the surface of La-added ~,-AI,O.~ has not bccn transformed to a-AI~O~
( i C'. t a n I . t ' c r d t t m c! t/I / The t h c r m t J ~ t a h t h . z m g
during calcination at 1050 o C, due to the incorporation of hmthanum in the surface layer. In a recent study of the surface structure of -y-AI20.; [21] strong indications have been found that the elementary alumina particles prclerentially exposes one of the two cleavage planes of the (ll0) face (the D-layer). The A I / O signal ratio of the latter alumina appears to be identical to the A I / O ratio for the present La-added sup-
c]lt,t t ~J] h m t l l a n l m l
t m 7..41_,0 ~
60Oc
[-~
550°C
(~
10500C
port. Possibly, the thermostabilizcd support also preferentially exposes the forementioned surface plane. In that case, the lanthanum atoms should
occupy originally vacant interstices. However, other models for the surface structure of the present support can be imagined in which the incorporation of the lanthanum is accounted for more plausibly, Several surface structures exist for y-AI203 which have a higher cation surface density than the forementioned plane. If it is assumed that the lanthanum atoms partially occupy the surface tetrahedral sites and the aluminium atoms the octahcdral interstices, two possibilities exist. The surface planes of the thermostabilized y-AI:O.~ which are in agreement with the experiments are then the C-layer of the (110) plane and the E-layer of the (100) plane [21]. Re~.cntly, another example of the modifying effect of additives on the surface structure of
',/-AI20.~ has been observed [14]. The addition of less than l(le/t of a monolayer of Mo already induced a y-AI20 ~ surface reconstruction at a relatively low calcination temperature of approximately 5(1(1°C. However. the addition of Mo stimulated the increase of the AI surface density. while the addition of La appears to block the increase of the AI surface density.
17
O
lanthanum
i J,0r support
Fig. 6. Schematic representation of the di,,Iribution of hmthanum twcr the 7-AI,()~ ,,upport after Ihe different thermal Ircammcnl~,.
support is observed. Decomposition of the complex by calcination at 55(1 ° C results in dewetting of the support to give coagulated LazO 3 particles. These small particles redisperse after sintering at 1050°C, which is accompanied by a solidstate reaction with the support to form a lanthanum-aluminate surface layer. The formation of this stable surface layer blocks the transformalion to a - A l 2 0 3.
Acknowledgement The investigations were supported in part by the Netherlands' Foundation of Chemical Reseumch (SON~ with fimmcial aid from the Netherlands" Organization for the Advancement o f Scientific Research (NWO).
5. Conclusion Low-energy ion scattering has been used to investigate the distribution of lanthanum on the surface of 7-A1_~O3. The temperature-dependent bchaviour of lanthanum is schematically depicted in fig. 6. After deposition of u [La(EDTA)]complex onto the support by specific adsorption, it unil0rm distribution of this complex over the
References [I] I.I.M, Tijhurg. Phi)"rh¢.,i,,. UIr¢chl tlOXg). [2] B.('. Lippum, and J.J. St,,:ggcrda. m: Pby',ic;d and C'hcmieal A,,pccts of Ad',orhun!', ~md ('alaly',l',. Ed. B.(;. l.im, en (Ac;Jdcmic Pro',',. New York. 197(I) p. 171. [3] II. Schapcr and E L van Rcycn. in: Prt~¢. 5th Int. Round l";lhl¢ Conference on Sinlcring. Porlorez. l¢)tll.
G.C t an l.eerdanl ct al. / The thernlo~tabih:i,k, effect n! hn~than,,i
14] 11. Schapcr, E.B.M. Docsburg and L L win Rcycn, Appl. (_'itlill. 7 (1983) 21 I. [5] V.I. Vcrc~;hchagin. V.Yu. Zelinskii. T.A. Kh;ib~ts and N.M. Kolova. J. ApDI. (?hem. USSR 55 (It~82). [6J S. Matsuda. A. K~tto. M. Milsumol~ and tt. Yitm~Jshita. in: Pr¢~c. 8th Int. ('ongr. on C';nl;dysis. Berlin. Ig84. p. 87tL [7] t1.11. Br,.mgcrsm;~ und P.M. Mul. Chem. Phys. l.ctt. 14 (Itl721 380. [bi] E. Tuglauer. Appl. Phys. A 3~ I.l~t,i.~l t~';l. [9] I:I.A. Ilorrel and D . L ¢,_',.lckc. ('alaL Roy. Sci. Eng. 2t~ (1987) 447. [10] I1 Kn~izinger. tl, Jczi~.~row~,ki arid E Taglaucr. Slud. Surf. Sci. ('~taL 7 (Pt..g.. Nev,, Ih~riz. (?atal.) (It.~81) fllbl. [I II II. Jezior~,wski. i.t. Kn~izingcr. E. Taglaucr and C. Vogdt. J. C~tzd. ~,11(1983) 28fi. [12] R.L Chi. and D.M. Ilerculcs. J. C;~tu!. 74 (ItJS2) 121. [13] J. Le~,rcr. R. Margr~tf. tS "]'i=gl;tucr and |1. Kn~izingcr. Surf. Sci. 2(11 (1088) 6(13. [14] II.H. Brongcrsma lind G.C. vim Lcerd;M]. in: Fundamcn-
n,
y..41:O ,
lal AspCt'Is ~,~f||¢;urogcll¢oun ('al~dynis SludiCd by PurUc'Ic B¢~tnls. [:.iln. l l l l . Brongcrsma and R.A. van Sanl'~ll. NATO-ASI Scr. B2.'35(Plenum. LondCm. Iggl) p. 283. [ 15] II.I I. llr~mgcr~ma. N. I lazcwindus. J.M. v~m Nicuwl~md. A.M.M. Ollcn ~tnd A.J. Snle¢Is, Rcv. S¢i. Inslr. 4g (Ig781 7()7, [16] R. IMargr~ff. I I Kniizingcr and [-. Taglaucr. Surf. Sci. 211/212 (l'gff'~) 11)83. I17] K. l:oBur, in: Cillalynis. Science and Technology ~. Edn. JR. Andcrs~..n and M. l~ludarl (Springer. Berlin. ll~ff4)p. 262. [IN] J. ll~d~u.r, in: (.'al~dysi~,. SciL..n¢c ;rod Technology 2. Eds. J.R. ,..ndcr~,cn ~ind M. l]oud~irl (Springer. Berlin, It,~81)p. 13. I I~l] 8.11. Ovcrbury. P.A. Berlr;tnd and G.A. Somorjui, Chem. Rev. 75 (1t~75) 547. [20] M.K. Reset. Ed.. Pha~,¢ I)i~grums fi~r Ceramints (The Americ~m Ccr;Jmic Sl~cicty. Columbus. Otl. 1964) p. 122. [21] (i.('. van Lccrdam, Phi) Thesis. Eindh~.wcn (lt~t)l).
applied
surface
science
The thermostabilizing effect of lanthanum on T-AI=O.~ studied by low-energy ion scattering G.C. van Leerdam, H.H. Brongcrsma l'a~ tilt)" o/ PIl~ ~t~ ~ and Sthint ht~ttttdc o! ~'atah'~t~, l'tndhot ~'n I_hut eritty oJ' 7~'c'hnoh&,y. I~ 0 Bo~ 51.t. 5000 MR Em~fllm en. Ncttwrhmd~
I.I.M. Tijburg and J.W. G e u s l)cpartotcnt ~Jl hlor~,,ttnic (Ttl'mi~tr~. [hut ~'rwty ~1 Utrecht. Sorhonnehlun lb. 35b~4 ('4 U/n'cht. Nefll<.rlaltd~ Received 25 March Itlt~l: accepted for puhlication 2h S e p t e m b e r 19ql
Lo'.,.-t'ncrgy ion sc:~nering has been tl~etl I',) Mtldy the ntlrfac¢ ol I;inlhanum Ihcrn'Jost~lbilized 7 . A I 2 0 ~ during several Mages (If the ,,intcring proton, ., Dcpo,,it.on o l l:.nlhlmum onto the ~uppt~rt by ",p¢cific ad',¢~rptitm :ff ~1[I,a(EDTA)I complex lurm, out It) be ho.mlgcnc~tP., After calcin;,tit'm ;.t 55{1"C La_,O~ p~lrtlclc,, h~t'.c been f~rmed on the ,,ur|itcc. "~.hdc LEIS re,,ults after ~.intering lit 11)5(I°( . ;n.licatL: (he I't~rm;ition of a lill3thanum,altlminal¢ ~.ur[;ic¢ layer, T h e prc~,cncc of U;inth;tnum prevent,, Ir;ln~rorl~ilti~.)rt of 7 - A I : O ~ to rr-AI,O~ ;It 10511"('.
I. Introduetion Many industrially important chemical reactions arc economically feasible only by the use of a catalyst. Utilization of solid catalysts predominates in the chemical industry, since they elm bc readily separated from the reaction products. To provide a thcrmostable, active catalyst, the catalytically active component is usually applied to a highly porous, inert support. Generally used supports arc silica, alumina and active carbon. The surhlce area of these supports varies from about 100 to 5(11) m 2 per gram. In combustion catalysis, the support, material has to mcct severe requirements, since it must withstand high temperatures in the presence of water vapour. The best support to cope with these conditions is ",/-alumina of acicular elementary particles [I]. However, cvcn this support material sinters at temperatures above 60(.I o C, which causes a considerable drop in thc surface arcv. At
temperatures above IO[)O°C the thermodynamically most stable alurnina, c~-Al:O~, is formed. The transformation to ~ - A I : O ~ involves rccrys-
tallization of the oxygen lattice frem cubic to hexagonal close packing [2]. "121stabilizc 7-A120 ~ against sintcring the support material has been doped with elements such as manganese, chromium, zirconium and lanthanum [3]. Among these elements lanthanum appears to be most effective. Schaper et al. [4] attribute the stabilizing effcct of lanthanum to the formation of a surface lanthanum aluminatc (LaAIO0, which dccreascs the rate of surfacc diffusion. Vcrcshchagin ct al. [5], however, observed an accelerated transformation to a - A I ~_03. For samples prcparcd by coprceipitation Matsuda c t a l . [6] report ihc fermati~m of lanthanum-,Baluminate (LaeO 3 • 11AI20 3) for low La contents. and LaAIO 3 for higher contents. Recently, it has been demonstrated that low loadings of lanthanum can be sufficient to pre-
(Hfiq-4332/q2/S()5Jl(I , Iqq2 - El,,cvicr Science Publi.,hcr,, B.V All ri,:ht!, rc~,cr~.cd
(;.( ". ~a~ I.c~'r~htm ~'t al. , lilt' tlwrmo.~tabih:m~, cl¢i'o ~tf hmtl.mum ott y-..ll : O t
pare thcrmoF,tablc supportF', prtwidcd tile lanthan||hi waF' deposited Olltt~ the Stll}polt by specific adF'orptitm of the [La(I-DTA)I complex [ I]. An X-ray p hotoc lectmn spect roF'copy (X PS ) study indicated that lanthanum remains distribuled unili~rmly twcr the .~upport after several stages of the sintc,ing prt~css. Although XPS is a surfaccF'cnF,itivc analysis technique, it averagcs the inlormatitm +tlxltlt the chemical compoF'itioll and state over a number of atomic htycrF'. One of the v¢13, few F'urfacc analyF,iF, techniques which is only sensitive to the outcrmoF't atomic layer is hlw-cncrgy ion scattcl'ing (I.EIS). Therefore. thi.~ tcclmiquc |'an I"J12 :l vallla['~lc tool Io ;.isstL'~F'inore at'¢tll'alely the posiliun of the stahilizing lanthanttna F,pccicF,. In a typical LI:!IS cxr~crimcnl monocncrgetic noble gl,s ions al't2 baekscatlcrcd by Ih¢ | ; r e e l atoms. Who,1 it~llF, t~l"~mly a few kcV lind a large seal|crane angle arc used. the b;lekscattelcd ions resuh fmnl ~,lile collision of each ion with a serbia?t: atom. Then the energy .~pe¢trunl of Ih¢ baekscattered ions iF, cquivalcnl 1o a Illa,~s spl2cIrurn of the surface alOnls [7-~)]. The high stn'lacc sensitivity iF' ntainly due to tile effeclive neulrali/alison of those it~llS ~vhieh penetrale the litrgcl I~eylllld tile tltl|;.'l'lllt)S| ;110111i¢ laly¢r, since OllJy Jams atre detected, l'he nalurc of Ihis teehnitluC JlllpJJes that lltl hll~.)rtllatJUll is obtained on the CIlelllJt.';.tl static OI the Slll'l;lfe :ttOllls. t\r~alll fltllll inltitlllalJon al',otll the Slll'lalCC CIHllpositiIH1. :liSt) inlornlation ahtlttt the depth 1.tistril~utitm o[ II1,.: different e~llstitttelll~ Cilll I'ICgained by use o f tile sputtering action of Ihe primnry beam. In the ideal ea,~e this result.,, in a laycr-by-laycr removal td tile stiFf;ICe eonMJ!tlenlF'. I:or several sltpllOl'led himetallic calal.vsts. Ill,.? intqucnec of the Setlucncc of [nll~regnatit~n . of lhc t.';.tlcinatJoll teml'ler;tttH.¢ ;1111.1 tinle t)n the distribtttion o1" tht.' stn'l'aec iill)~ns has thus been chtcidatcd [t)_12]. The application to'~,al'dr, Inonttnletal[i¢ cataJ}sts ofl'Cl'~., el'eat| per",pcctives in tile study t~l" the spreading behavior t;I tile tnctal tlvcr tile ~,uppt~rl [13]. Recently. a detailed stud~, ol tile h}cation of Mo tan 7 - A I , O t ha', I~c, .alrl'll.'d t'~tll [I.ll. "1"o obtain retire inlilrnlalion al'~oLIt the diMribulion ~ll lanth;tllllnl thtth]g tile sintclJllg process ol the tllernmstabiltzcd F'Upl'~t~rts"ptepau'ed I'Ll.• the spe¢ilic :tdF'tnplitm technique, a I.I'IS invesliga-
lion haF, bccn performed. Samples with variouF' Ioading~, of lanthanum have been investigated.
2. Experimental
2. I. Sample preparotion "File samples F'lodied arc g - A I 2 0 ~ (AI 4172. Engclhard Chemic b.v. De Meern, The Netherlands) loaded by different a m o u n t s of lamhanunl (I.6, 2.3 and 2.(~ w t q I.a/. The support is prepared by dehydration of pscudo-btlchmitc at filll)"(" and has a BET surlacc area of 265 m - ' / g and a pore volun'lc of 1.14 c m t / g . The hmthanum has been applied by F'pccific adsorption of a [I.a(EDTA)] complex according to the proccdurc dcscribcd in detail in rcf. [I]. By calcination of the thuF' loaded alumina all 550°C. Ihc [I.a(EDTA)] complex decomposes and lanthanum oxide iF' left on the F'mfacc. After calcinalion for 5 h at 55(1°( ", samples of 2 g ~cre loaded into alumina crucibles, and placed in an electrically heated furnace fro" standard F,intcr cxpcriinents at 111511"(." dr, ring 23 and i45 h in stagnant air. The freshly prepared samples were dried ;.it (ill' (" overnight prior to analysis. l ; h r a p u r c ¢t-Al,()~ (Fluka At1) with it BI"I" Mlrla¢¢ atoll tlf 5.5 nl"/g w;is almlyzcd a~, a referell ft.' ~ittllple.
2.2.
l:quipmcm
A descril',lion of lhc basic experimenlal set-up of lilt: ion scattering apparatus (NODI.!S) has been published belore [15]. Nov,'ada~n, mtnment'lgetic ion.', are produced m a I.cybold ion source tlypc I O | - 12/3S1 alld suhncquentJy mass-selected ill it WJen filler. FIle ions scatttett.'d over lit1 itng[c of 142 " ate cnerg.v-analyzed in a kind of cylindrical nlirl',,~r analyzer. Ftmr stages of differential I+umping ensure that the pressure in the Ut.IV ehanlJ'~cr increases only ttom 4 x Ill ~ to t) "-. 111 s I)a x~,'h,dn the prinlal3: heHnl JF, tl.ltned till. The sinlldtanct+us storage of t'~,clvc different samples tm a carotlseJ Jn the U I I \ ; chamber alJlY~VS;ttlltlysis tinder idelltical experimental eLiildJ-
(i.('./ill! I.l'l'rihllll i'1 ol / Thl' IIh'ntli+it(lhlh2ln.l~ cllc<; o! /'+illl/llllll+lll till )<-.'lid,l+
lions. The targets were m o u n t e d o n t o it s a m p l e h o l d e r which can bc insurtcd onto the cl, rouscl out of a n adjacent t r a n s f e r vessel by m e a n s of a load lock. Powdered m a t e r i a l s such as catalyst.,, are pressed into a t a n t a l u m holder before m o u n t ing on the sample holder. During ion h~mbardmerit of insulating materials charging effects may occur. Surface c h a r g i n g can be effectively eliminated by fltmding the surfitce with low-energy electrons. Tn o v e r c o m e local charging by g e o m e t ric shadowing, it r i n g - s h a p e d neutralizing system wan constructed which e n s u r e s flcK~ding from all sides. Even with e l e c t r o n s having e n e r g i e s a:+ low its It| eV. c o m p l e t e c h a r g e neutralization is achieved lhr rough and r ~ r o u s systems such its these catalysts. C o n t a m i n a t i o n of the filaments b y s p u t t e r e d particles is p r e v e n t e d by phtcing the f i l a m e n t s out of the line-of-sight of the target. T h e e m i t t e d electrons are directed t o w a r d s the silmple by applying the appropriate voltages to deflection plates. By thin configuration h e a t i n g of the s a m p l e by irradiation ix also minimized. Typical m,.21l
3. Results I.EIS spcctta have b c c n acquired lor the lrcstd.v p r e p a r e d , calcined and s i n ! t r o d s a m p l e s of lant h a n u m loaded "y-AI.O~. In fig. I I.l:.lS spectra ate shown of tile I.a-loaded support (2.3 ~t<~)
0
AI
;a
' -
l.
II II I I I i
~
,s
[
I
,
,'2"0~ ~000 tevl Fig I l.l.l~ ,,p.++utlaill t .i.to.i.+l,.:d~Upl~-t ,lltt.t dct',+nlr~Istlh+t+ ~iI flu.+1 I')I %.,.ompl~+', .it ~,'~ll ( { . . . . ) .nid .llh;l ~ml+.:ling .It I{151l (" 1) lh¢ l'"m.iP. ~.c,g~. +,l th.g ' l h ' u,n, i, tlRRI i.'~ ff
after decompo,d|um of the F.I)IA cumplc,, at 55(I~(" and alter nintcring ;it 1050 (" for 145 11 "l'hc pcakn ol oxyecn, aluminium and lanthanum arc ch.'arly ru.,,uk~..ll.The.' crurg:, range from 22U0 to 2qlltl c V ha', I',cu'rl ,,canm.'d v, ith a lilth ol thu' speed o l Ihu' rcnlanling pa"l ot I|1c" np¢¢lrum Io +..:'lhiH~c+. lh+d dclu'chon of 1.1111hilnu111. lNpcciall$ in 111+.",,pc,+'lra ol lhc Itcnhh pl+..'palcd ",amph:, a u'h.'.il lhlorhl,.." pu'.ik i', ~,l~,,,:e.~:d. Ninc~: l]tlnrinc+ cotdd ab, t+ l'~c th:tcctcd ten hc unlo,tdcd "qlplltlrt l'e,+ 1'~olJt IF.IN ,lild ~I'+%. ,hi "+, clement i', mmt pmb,dd) intloduc.'d +,11.11in~ ,q. nth~."~r,. Since. aIr tatg~.t- ha~c bc,_'n +tlldl)/'Jd Illldti M e t r i c a l experimental con,lit!ont, lhc a1",solut,.; signals o f lh,.: dilh.'r,.:nl saml+l,¢~, c'a11 be culnpar,.:d v, ilhoui r¢,,trictitm+ l.or a o'qiManl loading, l+,oth thu" o',u'rall inlcn,it.x in the." uncrg3 rang,..' bciv,~.'cn f~lll~, antl l~tltl ¢\" .'nld the La intcnsily change as d lunclion o l the thermal tr,,:alnlcnt (see lig. I). ]'h+ AI intcn,,it.~, m gcm.'lal, dUCl,.glr, u',, ,it higilu'r cak+~ natK;n t+.+'nlpu'litltlt++.'ntel longer tiln~?n..2%It'+,.'I lb'+.' dcct+mpt)+,ition of the 1:I)'I,.\ colnplcx at 55(I (', h)l cxanlph.', tl1~.' AI hltu'nsit', ol th,.: 2.3 v.t +'< +,.:::!pie h.~,'- ,.!L',:r,_':t,,_'d h~, I 7" ; Nmlcring .tl Ii1511 (" l¢,r 23 and t45 h r,~,,ult., in an addHumal d¢clca'.¢ |'~3 7', and 1,';';. tc,,pccltx~:l~, Ih~." l.,~ intcnsil), hox~.u'xcr, ha', dc'crCil'+.cd I',~. 41l+,• altL.'t calcination ill .~511 ('. hl omtra',t, the l a +,igndl inct~.'ilsu'% b~, 711+; and 7 ( f ; alter ,qlltcrirlg at lllSll+'(" h n 23 and 145 h. I CSl'~CCti',u'.).
( L ( " t ittt I.c't'rdam i't al. / 77h Hlf'l.tO~lal~ill.'ltlg ~'11~'~t C~] I . I l t h t l l t U m Oil y ' . ' l l ' ( } l
I-I
-I
I •
60 oC
A Sh 550°C O/., o 23h I050°C
=
.f
& It.Sh 10S0O.f
/
j02
0 La
I I
• AI ....... 0
I
I . . . . . . . . 20
2
La- conlent (~t~'=)
Fig. 2. I)L'pU'zltlcncc I)t Ih¢ I.EIS I . ; I / , \ l ratio tm thu' thurnl;ll Ir~:;llnl,dnl ~11 Vilritltl~, htlJk l.;I ConlCllls.
"1"o summarize the results (11"the different l,a contents, the La/AI signal ratio after the dillcrcnt thermal trcatmcnts is plottcd in fig. 2 as a function of Lit loading. From this figure it follows that the La/AI ratio for the as-prepared samples increases linearly with the lanthanum content. It should bc mentioned that these samples differ from the other samples, since the lanthanum i~ns arc thought to bc still embedded in thc El T A complex on the surface. After calcination: at 55(1"(" the La/AI ratio hits decreased and the linear dependence on the l,'l content has d~ ,Lppcarcd. After the sintcring experiment', at 1(15()"C. however, the La/AI ratio increases ~tnd is again proportional to the La content. In ov'dcr to obtain more complete int~)rm~ion about the bchaviour of the lanthanum during the thermal trcatmcnts, sputter profiles wcrc also measured for slmlplcs containing 2.3 wt";~ l,zt. Gcncr~dly, thc ratio of the metal to support is plot!cd a..., a function (11"ion flux. This approach is zlmt)ngst others based on the assumption that both the support and the La arc equally covered witll the s:.lmc contaminatkm. In the case of the freshly prepared samples, however, this approach bcconlcs invalid since all La ions arc supposed to bc embedded in the EDTA complex, while this complex only partially covers the 7-AI,O 3 support. In addition, on the surfacc of the alumina support terminal - O H groups will bc present, wl,;,:h may decrease the detectability of the aluminium atoms. In fig. 3 it can bc sccn that for the
bombardment
"0
hme ;~,n)
I:iL~. 3 S p u t t e r prolilc Ior tilt: dried ~,tmplc cont~tming 2.3 ','.tg: I .;t,
freshly prepared sample the La-signal decreases with increasing bombardment time (ion flucncc). apart from the first point ¢1t"the series. The La sputter profiles o|" the calcined and sintcrcd samples arc combined in fig. 4. For the calcincd sample the Lit intensity hardly decreases throughout the ion bombardment. In contrast. 1(11' the sample sintcrcd at 11150°C Ibr 145 h the La signal decreases exponentially from the beginning. The shape of the profile of the sample sintcrcd l~.)r 23 h is intermediate between that of
the two other prot'ilcs, l:¢~r all samples, the AI intensity slowly increases during the sputtering. periods applied. To investigate whether the I.a-loadcd support transforms, to ~-AI_,O~ at sintcring temperatures
m
: i
0
O_
__
10
Oe
20 bombardmont
i
30 hme (ram)
|:ig. 4. L[intJl~llltlln ~,])HI(L'F iln)li[C ", h)r the ~,ilnlpl¢~, Con|;iHlillg ~.~ '~'.11; |.;I ;IIlCt L';I]CiN[I(i(Itl :LI ~.~11"(" ~llld ~.illtu'rirlg ;It lglSO" (',
r;.C t ,tl l.ccrdam ct aL / "lhc thermmtahlh:ing c/Jet t o] h m t h a n u m on ),.,.11:() ~ "
rm - m
I
:
i
'I
AI
l I
: ,
o
17 i Lt_
600
1200
~8oo
Ef
2~oo
(evl
Fig. 5. Typical LEIS ,.pcclrum o l .'r-Al:O~ (111c'. primary energy tff 31ltll)eV).
of 1(15(I° C. a reference spectrum of a-Al_~O~ was recorded (fig. 5). The A I / O signal ratio of all lanthanum loaded supports sintered for 145 h at 11,151)o C is found to be 75 + 4% of that of a-Al~O~ (2.67). For the other samples, the A I / O ratio depends on the La content and the thermal treatment. Generally, this ratio is smaller than 2.
4. Discussion As a result o f the sintering experiments, a surface urea loss can be expected fi)r the Laloaded y-Al20~ particles, which is indeed confirmed by BET yurfacc area measurements. The BET surlacc ,trca for the samples calcined at 551J°C amc:ants to 2211 m-'/g, while for the sampies calci,lcd for 23 h and for 145 h at 105tI°C it amounts ',o only l(l(I :md 91) m2/g [I]. respcctivcly. The effect of sintering on the absolute intensity of the LEIS spectra will dcpcnd on several factors. On a microscopic scale many orientations of the support surface exist with respect to the incident ion beam. For a given beam diameter the number of surface atoms which can be hit increases when changing thc angle of incidence from perpendicular to more grazing angles. However. for very ~,mall angles of incidence mutual shadowing of atoms in one surface plane may occur. Although the incident ion beam may hit effectively many surface atoms, the detected
amount of backseattered ions may bc much less due to blocking effects. Espccmlly for surfaces with deep and small pores a considerable fraction of the backscattercd ions will bc prevented from detection. The combination of thcse effects will determine whcthcr an incrcase or a decrease of the signal intensity is observed. According to Margraf ct al. [16]o the AI intensity of single crystalline a-Al~O~ is five times larger than i'or powdered y-AI,Oa, which indicates that the steric effects dominate. Thc present support, however. consists of ncedles which are esscntially non-porous. TEM micrographs of the sintcred support show that the surface of the -/-AI:O 3 parh=les has flattened [I]. Apparently. this flattering during sintering causcs thc decrease of the ovcrall intensity in the LEIS spectra. However, after termination of the sintering experiments the BET measurement indicate a 6tic/e, surface area loss, whereas the concomitant LEIS AI-intensity loss is only 18%. Apart from the already mentioned reasons this discrepancy may be due to the fact that LEIS probes only the visiblc part of the elementary particles, while in the BET method thc total surface area is measured. In the spectra of the freshly prepared samples a clear La peak is observed already for such low ion doses that hardly any damage to the surface can have occurred. One way to account for this observation is that the EDTA complex has decomposed after insertion into the UHV system. Another possibility is that the surrounding ligands shield the central lanthanum atom so poorly that scattering is still possible to some extent, Recently, this weak shielding has been observed for a number of oxides [14]. The proportionality between the La/AI ratio and the lanthanum loading for the freshly prepared samples (fig. 2) can be interpreted either as a monolaycr formation of the La or as the deposition of cqui-sized lanthanum-containing particles. To distinguish between these possibilities, the shape of the La sputter profile can be used. If. for instance. La would be present as a monolayer an initial exponential decrease of the La intensity with sputter time is expected. In the case of three-dimensional La-containing particles, however, the intensity is assumed to remain initially constant. Experimcn-
16
( i C. t a n la'~'rtho~l ~'t t d / 7"h~"therolo~tahilizttlg ~'[]i'~'t o f lanthctnutn *m ]t-Al.,O ¢
tally, an exponential decrease of the La intensity is found (fig. 3), which indicates that the lanthanum compound is distributed uniformly over the surface when the specific adsorption technique is used as a preparation procedure. For the freshly prepared catalyst a loading of 2.3 wt% La is estimated to correspond to a fractional coverage of 4%-8%. The. cnnsiderable drop in the La intensity after calcination at 55{1"C (fig. 2) indicates that lanthanum coagulation has occurred during the thermal treatment, most probably to form La_,O3 particles on the support. Diffusion of the lanthanum into the support seems unlikely at this temperature duc to its relatively large atomic radius. The non-linearity between the La loading and the La/AI ratio (fig. 2) also suggests a multilayer model for La,O~. Further support for the lowered La dispersion is given by the shape of the sputter profile (fig. 4), which shows a hardly decreasing La intensity for increasing bombardment times. Coagulation of metal-oxide particles on the support is a rather common phenomenon in catalysis. The eventual occurrence depends on the relative values of the specific surface free energies of the active oxide and the support, and on the specific free interface energy between the oxides [17,18]. The net result of this energy balance determines whether spreading of the active oxide over the support occurs or coagulation. Approximate values of surface free energies of several oxides have been compiled in the literature [19]. Unfortunately, no data exist for the specific surface free energy of La,O 3, nor for the free interface energy of the mixed system. In most cases, however, three-dimensional particles are formed on the support [18]. Wetting of the support by the active oxide is observed for MoO~ and WO 3 on ~/-AI_,Os and for V_~Os on TiO 2 [13]. if calcination temperatures are sufficiently high. a solid-state reaction between the support and the active oxide may take place. The shape of the La,O 3 particles will also be determined by thermodynamics [18]. However, if it is assumed that spherical La_,O~ particles of equal size are formed, an attempt can be made to estimate the size of the particles. After calcina-
tion at 55(1°C the absolute La intensity has decreased by 40%. Assuming that this decrease is purely a result of the sintering of lanthanum, thi. reduction corresponds with La203 particles containing approximately 13 La atoms. In this model it is supposed that the density of La in the surface layer of the La203 aggregates is representative for the bulk density. The diameter of the La20 3 clusters will then be about 1'~ am. However, for other shapes of the La20 3 aggregates the thickness of the particles will also be about 2 or 3 layers. The increased La/AI ratio for the samples sintered during 145 h at 1050°C (fig. 2) i.~ combination with the exp~mential decrease of the La signal in the sputter profile (fig. 4) indicates that redispersion of the coagulated partit, les occurs during sintering at I(I50°C. At first sight, this redispersion seems illogical, since in the preceding part of the discussion it has just been shown that the originally uniform La distribution after deposition has disappeared. The redispersion after calcination at 1050 °C, however, can be explained by a solid-state reaction between the support and the lanthanum oxide. Due to this reaction the La ion will be present b~ the surface plane, instead of o n the surface plane of the support. Since the rate of a solid-state reaction is much slower than surface diffusion, the relatively long sintcring time ( > 23 h) to obtain the mixed surface layer is also comprehensible. The formation of a mixed surface layer is in agreement with the phase diagram of lanthana-alumina, which indicates the formation of lanthanum-/3-aluminate in dilute lanthanaalumina mixtures [20]. The A I / O signal ratio of La-added "y-AI.,O~ after calcination at 1050°C appears to be three fourth of the signal ratio for ~-AI20 3. One possibility to explain this effect would be that one fourth of the AI atoms in the surface of a-AhO~ has been replaced by La atoms. However, for the highest La content the number of lanthanum atoms on the surface of the alumina can be calculated to be only one half of the amount required to replace the desired number of AI atoms. This indicates that the surface of La-added ~,-AI,O.~ has not bccn transformed to a-AI~O~
( i C'. t a n I . t ' c r d t t m c! t/I / The t h c r m t J ~ t a h t h . z m g
during calcination at 1050 o C, due to the incorporation of hmthanum in the surface layer. In a recent study of the surface structure of -y-AI20.; [21] strong indications have been found that the elementary alumina particles prclerentially exposes one of the two cleavage planes of the (ll0) face (the D-layer). The A I / O signal ratio of the latter alumina appears to be identical to the A I / O ratio for the present La-added sup-
c]lt,t t ~J] h m t l l a n l m l
t m 7..41_,0 ~
60Oc
[-~
550°C
(~
10500C
port. Possibly, the thermostabilizcd support also preferentially exposes the forementioned surface plane. In that case, the lanthanum atoms should
occupy originally vacant interstices. However, other models for the surface structure of the present support can be imagined in which the incorporation of the lanthanum is accounted for more plausibly, Several surface structures exist for y-AI203 which have a higher cation surface density than the forementioned plane. If it is assumed that the lanthanum atoms partially occupy the surface tetrahedral sites and the aluminium atoms the octahcdral interstices, two possibilities exist. The surface planes of the thermostabilized y-AI:O.~ which are in agreement with the experiments are then the C-layer of the (110) plane and the E-layer of the (100) plane [21]. Re~.cntly, another example of the modifying effect of additives on the surface structure of
',/-AI20.~ has been observed [14]. The addition of less than l(le/t of a monolayer of Mo already induced a y-AI20 ~ surface reconstruction at a relatively low calcination temperature of approximately 5(1(1°C. However. the addition of Mo stimulated the increase of the AI surface density. while the addition of La appears to block the increase of the AI surface density.
17
O
lanthanum
i J,0r support
Fig. 6. Schematic representation of the di,,Iribution of hmthanum twcr the 7-AI,()~ ,,upport after Ihe different thermal Ircammcnl~,.
support is observed. Decomposition of the complex by calcination at 55(1 ° C results in dewetting of the support to give coagulated LazO 3 particles. These small particles redisperse after sintering at 1050°C, which is accompanied by a solidstate reaction with the support to form a lanthanum-aluminate surface layer. The formation of this stable surface layer blocks the transformalion to a - A l 2 0 3.
Acknowledgement The investigations were supported in part by the Netherlands' Foundation of Chemical Reseumch (SON~ with fimmcial aid from the Netherlands" Organization for the Advancement o f Scientific Research (NWO).
5. Conclusion Low-energy ion scattering has been used to investigate the distribution of lanthanum on the surface of 7-A1_~O3. The temperature-dependent bchaviour of lanthanum is schematically depicted in fig. 6. After deposition of u [La(EDTA)]complex onto the support by specific adsorption, it unil0rm distribution of this complex over the
References [I] I.I.M, Tijhurg. Phi)"rh¢.,i,,. UIr¢chl tlOXg). [2] B.('. Lippum, and J.J. St,,:ggcrda. m: Pby',ic;d and C'hcmieal A,,pccts of Ad',orhun!', ~md ('alaly',l',. Ed. B.(;. l.im, en (Ac;Jdcmic Pro',',. New York. 197(I) p. 171. [3] II. Schapcr and E L van Rcycn. in: Prt~¢. 5th Int. Round l";lhl¢ Conference on Sinlcring. Porlorez. l¢)tll.
G.C t an l.eerdanl ct al. / The thernlo~tabih:i,k, effect n! hn~than,,i
14] 11. Schapcr, E.B.M. Docsburg and L L win Rcycn, Appl. (_'itlill. 7 (1983) 21 I. [5] V.I. Vcrc~;hchagin. V.Yu. Zelinskii. T.A. Kh;ib~ts and N.M. Kolova. J. ApDI. (?hem. USSR 55 (It~82). [6J S. Matsuda. A. K~tto. M. Milsumol~ and tt. Yitm~Jshita. in: Pr¢~c. 8th Int. ('ongr. on C';nl;dysis. Berlin. Ig84. p. 87tL [7] t1.11. Br,.mgcrsm;~ und P.M. Mul. Chem. Phys. l.ctt. 14 (Itl721 380. [bi] E. Tuglauer. Appl. Phys. A 3~ I.l~t,i.~l t~';l. [9] I:I.A. Ilorrel and D . L ¢,_',.lckc. ('alaL Roy. Sci. Eng. 2t~ (1987) 447. [10] I1 Kn~izinger. tl, Jczi~.~row~,ki arid E Taglaucr. Slud. Surf. Sci. ('~taL 7 (Pt..g.. Nev,, Ih~riz. (?atal.) (It.~81) fllbl. [I II II. Jezior~,wski. i.t. Kn~izingcr. E. Taglaucr and C. Vogdt. J. C~tzd. ~,11(1983) 28fi. [12] R.L Chi. and D.M. Ilerculcs. J. C;~tu!. 74 (ItJS2) 121. [13] J. Le~,rcr. R. Margr~tf. tS "]'i=gl;tucr and |1. Kn~izingcr. Surf. Sci. 2(11 (1088) 6(13. [14] II.H. Brongcrsma lind G.C. vim Lcerd;M]. in: Fundamcn-
n,
y..41:O ,
lal AspCt'Is ~,~f||¢;urogcll¢oun ('al~dynis SludiCd by PurUc'Ic B¢~tnls. [:.iln. l l l l . Brongcrsma and R.A. van Sanl'~ll. NATO-ASI Scr. B2.'35(Plenum. LondCm. Iggl) p. 283. [ 15] II.I I. llr~mgcr~ma. N. I lazcwindus. J.M. v~m Nicuwl~md. A.M.M. Ollcn ~tnd A.J. Snle¢Is, Rcv. S¢i. Inslr. 4g (Ig781 7()7, [16] R. IMargr~ff. I I Kniizingcr and [-. Taglaucr. Surf. Sci. 211/212 (l'gff'~) 11)83. I17] K. l:oBur, in: Cillalynis. Science and Technology ~. Edn. JR. Andcrs~..n and M. l~ludarl (Springer. Berlin. ll~ff4)p. 262. [IN] J. ll~d~u.r, in: (.'al~dysi~,. SciL..n¢c ;rod Technology 2. Eds. J.R. ,..ndcr~,cn ~ind M. l]oud~irl (Springer. Berlin, It,~81)p. 13. I I~l] 8.11. Ovcrbury. P.A. Berlr;tnd and G.A. Somorjui, Chem. Rev. 75 (1t~75) 547. [20] M.K. Reset. Ed.. Pha~,¢ I)i~grums fi~r Ceramints (The Americ~m Ccr;Jmic Sl~cicty. Columbus. Otl. 1964) p. 122. [21] (i.('. van Lccrdam, Phi) Thesis. Eindh~.wcn (lt~t)l).