Questions? Ask Prof. Herz, General Classification of adsorption

Transcription

1 Questos? Ask rof. Herz, Geeral Classfcato of adsorpto hyscal adsorpto - physsorpto - dsperso forces - Va der Waals forces - weak - oly get hgh fractoal coerage of surface at low temperatures - ot selecte - same order of magtude attracto betwee all molecules ad surfaces - use physsorpto of a molecule to measure total surface area of hgh surface area materals - may play a role catalyss by affectg ketcs of etry ad et from chemsorpto state as "precursor state" Chemcal Adsorpto - chemsorpto - chemcal bods are formed betwee adsorbed molecule ad surface atoms - strog - ca get hgh fractoal coerage of surface at relately hgh temperatures - selecte - bods oly form betwee certa molecules ad certa surfaces - use chemsorpto to measure areas of specfc materals surfaces wth comple composto Adsorpto measuremets Ths s a schematc of oe erso of a adsorpto apparatus. The crcle labeled s a capactace maometer pressure gauge whch has small teral olume. The sample s represeted as the shaded crcles the sample cell. After troducto of a kow mass of sample, the sample s degassed to remoe water by pumpg wth the acuum pump ad heatg the sample cell. For chemsorpto measuremets oer supported metal catalysts, other pretreatmets such as cleag ad reducto pretreatmets ca be performed. The olume V s kow. The ukow olumes V 2, whch cludes the teral olume of the pressure gauge, ad V 3, whch s the od olume the sample cell after the sample s loaded, are determed by fllg V wth helum to kow pressure, the epadg the helum, measurg the fal pressure ad usg the deal gas law. Helum wll ot chemsorb ad wll ot physsorb to a apprecable etet ecept at temperatures ear absolute zero. The the sample cell s eacuated ad brought to the desred temperature. For physsorpto measuremets performed at lqud troge temperature, a dewar cotag lqud troge s rased to coer the sample cell but ot the ale aboe the sample cell. By opeg ad closg ales the correct order, the gas to be adsorbed ca be admtted to olume V 2, the pressure recorded ad the deal gas law used to determe the moles of gas cotaed V 2. The the ale

2 betwee V 2 ad V 3 s opeed ad tme for equlbrato s allowed. The the pressure s recorded ad the kow olumes ad temperatures ad the deal gas law ca be used to determe the moles of gas remag the gas phase. The dfferece betwee the tal moles of gas V 2 ad the moles of gas remag s the moles of gas molecules that hae bee adsorbed oer the sample's surfaces. The ale betwee V 2 ad V 3 s closed, more gas s admtted to V 2 ad the the process s repeated wth creasg pressures. Commercal strumets are aalable whch are automated. ome strumets hae mass flow cotrollers ad leak gas at a kow, slow rate to (or out of) the sample cell as the pressure s cotuously recorded. Whe the flow rate s suffcetly slow, adsorpto equlbrum s closely approached durg the epermet. Chemsorpto measuremets ad the Lagmur sotherm ce chemsorpto s selecte, chemsorpto of arous gases ca be used to get formato about the surface areas cotrbuted by the dfferet compoets of a comple materal. For eample, chemsorpto of CO, H 2 ad/or O 2 s ofte used to estmate metal surface area supported metal catalyts. The "sotherm" measured at costat temperature wll look smlar to ths: ce oly oe layer of molecules ca bod to the surface atoms, the amout of gas adsorbed approaches a mamum at relately hgh pressure. Ofte we are oly terested the mamum amout adsorbed. Kowg ths amout, the rato of adsorbed molecules to surface atoms at mamum coerage, the dameter of the surface atoms, ad a assumed surface structure, we ca compute the surface area of the materal m 2 /g. There are a arety of models of chemsorpto ad assocated sotherm equatos whch ca be used to ft all of the epermetal data oer the full pressure rage measured. urfaces are hae a rage of stes wth dfferet chemsorpto bodg stregths, ad there s a arety of sotherms whch descrbe such adsorpto, cludg: Eloch, Temk, Freudlch, etc. For surfaces wth relately uform propertes, the Lagmur adsorpto sotherm ca be used. The assumptos oled the derato of the Lagmur sotherm are: - the surface s composed of a array of detcal adsorpto stes - adsorbed molecules are dstrbuted radomly oer the surface - there are o lateral (parallel to surface) teractos betwee stes or adsorbed molecules

3 The last assumpto meas that the chemsorpto bodg eergy ad the heat (ethalpy) of adsorpto are costat wth coerage (amout adsorbed). At equlbrum, the rate of adsorpto s equal to the rate of desorpto: k = k ads des The arable s the cocetrato of surface stes coered by adsorbed molecules (umber per amout of surface or adsorbet). = + tot o The total ste cocetrato ca be determed from the mamum amout that ca adsorb. The fractoal surface coerage, θ, s defed by θ = tot m where s the amout adsorbed terms of the equalet gas olume at stadard temperature ad pressure per gram of sample ad m s the mamum amout whch ca adsorb. tot ads ( θ ) = ( θ ) k = k des θ k K k ads des θ = K + K Ths equato s the Lagmur sotherm. We ca estmate m ad K by plottg the erse of the amout adsorbed s. the erse pressure ad fttg a straght le to the data: = + m mk By performg chemsorpto measuremets ad determg K alues at dfferet temperatures, we ca estmate the heat (ethalpy) of adsorpto by plottg lk s /T. k k e k k K = = = e = e k k e k k Eads / RT ads ads ads ( Eads Edes )/ RT ads ΔHads / RT Edes / RT des des des des Δ kads Hads l K = l kdes R T

4 Usually E ads < E dess such that ΔH ads <. That s, chemsorpto s usually eothermc, ad the amout adsorbed at a costat pressure decreases wth creasg temperature. The mamum amout adsorbed, m, should rema costat, although the pressure at whch t wll be closely approached wll crease wth creasg temperature. Ofte Eads =. Edothermc adsorpto ca occur assocato wth large etropy chages, e.g., whe there are large chages prote cofgurato durg prote adsorpto o surfaces. Measuremet of total surface area usg the BET method The BET method s amed after ts deelopers, tephe Bruauer, aul Emmett, ad Edward Teller [J. Am. Chem. oc., ol. 6, p. 39 (938)]. Ths method s used eery day, all oer the world to measure the total area of hgh surface area materals such as pat pgmets, sols, food (breakfast cereals that sap, crackle ad pop), adsorbets ad catalysts. I the BET method, a gas, ofte troge, s physsorbed oer a hgh surface area materal at the ormal bolg pot temperature of lqud troge (77 K). The adsorpto sotherm, or plot of amout adsorbed s. gas pressure at costat temperature, ofte looks lke ths: s the gas pressure ad o s the apor pressure of the gas molecule at the measuremet temperature. There are aratos o ths plot, ad ths shape of sotherm s classfed as a Type II sotherm. The objecte s to aalyze these data order to determe the amout of gas that would be adsorbed oer the surface oe complete moolayer. Wth moolayer coerage, the surface s completely coered wth oe ad oly oe layer of adsorbed molecules. I a physsorbed moolayer, the molecules are closely packed together. Oce we kow the moolayer coerage amout, e.g., moles of N 2 adsorbed a moolayer per gram of sample, we ca use the Va der Waals dameter of N 2 ad a assumed close-packed arragemet to compute the surface area occuped by a adsorbed N 2 molecule ad, fally, the total surface area of the materal m 2 /g. Wth physsorpto, the attracte forces betwee the adsorbed molecules ad the surface are usually oly a lttle stroger tha betwee the adsorbed molecules whe they are the lqud phase. Thus, multlayer adsorpto occurs oer parts of the surface before a complete moolayer of adsorbed molecules s formed.

5 I the schematcs aboe, the surface s show as a flat plae but we kow that t s really a layer of surface atoms of the sold materal. The arable s the cocetrato of surface stes coered by adsorbed molecules (umber per amout of surface or adsorbet). Our goal s to determe the amout of gas adsorbed a moolayer oer our sample but the adsorpto does't occur that way. Thus, we eed a model of multlayer adsorpto ad some math. Here are the equatos that lead to deelopmet of the BET sotherm equato. At equlbrum, the rate of adsorpto from the gas to stes of type equals the rate of desorpto from stes of type : a = be Q RT / where a ad b are the adsorpto ad desorpto rate coeffcets, respectely, ad Q s the heat of adsorpto of molecules drect cotact wth the surface. For molecules adsorbed to hgher layers: a be = where Q s the heat of aporzato of the lqud phase of the adsorbg molecules. The ma deas oled here are: - all surface stes are detcal - there are o lateral teractos that affect adsorpto ad desorpto - the attracte force betwee molecules drect cotact wth the surface s dfferet (stroger) tha the force betwee molecules the frst layer ad the secod layer, the secod layer ad the thrd, etc. - the attracte force betwee molecules the multlayers (betwee frst layer ad secod layer, betwee secod layer ad thrd, etc.) are the same as betwee these molecules whe they are the lqud phase. Defe the arables, y, ad c. a 2 3 = = = = 2 b e Q / RT y a = = b e Q / RT y ab = = ab ( ) ( ) Q Q / RT Q Q / RT c e e = e.g., = = = = = y = c

6 The umber cocetrato of total molecules adsorbed,, dded by the umber cocetrato wth oly a hypothetcal moolayer adsorbed, m, s: m = = c c + c ( ) 2 c = = c m + ( ) + ( c ) ( ) Ths s the BET sotherm equato. We ca measure as a fucto of gas pressure at costat temperature. We wat to use these measuremets to fd m. The theory ow relates the arable to the gas pressure dded by the apor pressure of the gas molecules at the measuremet temperature: = How do we get ths? From aboe: a = = b e a = e b a = be Ths says that, at equlbrum, the rate of adsorpto from the gas to the surface of a lqud of the adsorbg molecules s equal to the desorpto rate from the lqud surface. Rearragg the sotherm equato, c = + ( ) cm cm we see that we ca plot measuremets of /((-)) s. = / o ad estmate alues of c ad m from the slope ad tercept of a straght-le ft to the data. The ft usually apples oly the the pressure rato rage / o from.5 to.3. Oe reaso s that at hgher pressure ratos, codesato ad fllg of small pores wll start to occur. The umber cocetrato of gas molecules adsorbed s proportoal to the olume of gas at stadard temperature ad pressure that s adsorbed per gram of sample,. We usually see plots of s. / o.

7 For c alues of order magtude ad the pressure rato rage / o equato prodes a reasoable ft: from.5 to.3, the followg m + m lot measuremets of s./(-) ad ft a straght le through the org. The slope of the le prodes a estmate of m. The "sgle-pot method" uses a measuremet of at oe alue of = / o. Ths s coeet for a apparatus whch uses flowg N 2 (or other gas) ophyssorbg He at oe mole fracto at atmospherc pressure. Ths type of apparatus s less epese that the apparatus show aboe whch ca measure may pots o a sotherm. The sample s cotaed a glass U-tube. After a cleag pretreatmet, the U-tube s mmersed lqud troge ad the sample s equlbrated a flow of the gas. The let ad outlet gas flow through the two sdes of a thermal coductty detector, such as are foud gas chromatographs. The the sample s rapdly warmed ad the N 2 that desorbs s measured as a peak by the detector. The area of the peak that s recorded s proportoal to the amout of N 2 that desorbs, ad ca be determed by calbrato of the detector. By usg a adsorbg gas wth a lower apor pressure at lqud troge temperature, such as Ar or Kr, oe ca get greater sestty. Ths s because a smaller mole fracto of that gas He ca be used, thus prodg a larger sgal from the thermal coductty detector whe desorpto occurs.