Electrostatic aspects of adsorption on silica and titania surfaces
The influence of outer electric field of the ionic crystal surface layer on the structure of adsorption complexes is examined and their role in the run of physicochemical processes at the solid-gas interface is studied. Taking into account of the subsurface electric field has been shown to allow to...
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| Дата: | 2001 |
|---|---|
| Автори: | , |
| Формат: | Стаття |
| Мова: | Англійська |
| Опубліковано: |
Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine
2001
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Репозитарії
Surface| _version_ | 1869291128111497216 |
|---|---|
| author | Lobanov, V. V. Chuiko, A. A. |
| author_facet | Lobanov, V. V. Chuiko, A. A. |
| author_institution_txt_mv | [
{
"author": "V. V. Lobanov",
"institution": "Інститут хімії поверхні НАН України"
},
{
"author": "A. A. Chuiko",
"institution": "Інститут хімії поверхні НАН України"
}
] |
| author_sort | Lobanov, V. V. |
| baseUrl_str | |
| collection | OJS |
| datestamp_date | 2018-11-27T09:42:39Z |
| description | The influence of outer electric field of the ionic crystal surface layer on the structure of adsorption complexes is examined and their role in the run of physicochemical processes at the solid-gas interface is studied. Taking into account of the subsurface electric field has been shown to allow to predict the structure of primary adsorption complexes, to estimate the energetics of the transformations of molecules at solid surfaces, and to determine properties of the surface compounds formed. |
| first_indexed | 2025-09-24T17:44:37Z |
| format | Article |
| fulltext |
ЕLЕCTROSTATIC AsPЕCTs oF ADsoRРTIoN oN
SILIсA AND TITANIA sURFAсЕs
V.V.Lobanov and A.A.Chuiko
Institute of Surfаcе Chemisф, Nаtionаl Аcаdеmу of Sciences
Gen, Nаumov str. 17, Kуiv.I64, 03680 UKRАINE
Abstrасt
The influеnсe of outeг еleсtriс field of the ionic сrystal suгfaсе layеr on thеl
struсtuгe of adsorption сomplexes is еxamined and thеir гole in thе run of physiсо.
сhеmiсal proсеsses at thе solid-gas intегfaое is studied. Taking into aссount oг tье
subsuгfaсe еleсtгiс fiеld has been shown to allow to prediсt the struсturе of primary
adsоrption сomplеxes, to estimate the enrгgеtiсs of the transformations of moleiules й
solid.surfacеs, and to determine pгopегtirs of the surfaсe сompounds formеd.
tn а priori analysis on the reaсtivity and peсuliarities of сhemiсal behavioг of
moleсulеs is a rathеr diffiсult but quit solvablе task of thеoretiсal сhemistгy. If moleсulеs
interaсt with solid suгfaоe, the сomplexity of its solution inсreases гepйedly' This is
оa-usr Ъy thе сirсumstanсes as follows: firstly, an inteгaсtion oссurs betweеn two systems
ofdifferent naturе _ moleсule and suгfaсе that сшr bе сonsidered to be endless at thЪ sсale
of partneг; seоondly, it is diffrсult to simulate a surfaсr adequatеly that is a macгodefесt of
thе crystal peгiodiс struсtuгe. Мorеovец a dеfinitе grade of amoгphization of suгfaсe layer
is a сharacteгistiс of еvеn typiсal сrystals [lJ. Taking into acоount probablе геlaxation ind
reсonstruсtion of rеal surfaсe as compared with ideal one, obtaining valid struсtural
information on surfaсe and subsuгfaсe layer of solids seems to be rathЬг pгoblematiс. A
сlustеr modеl of solid and its surfaсе that is natuгal for сhеmists operating terms of loсal
сhemiсal bonds (despite- it is not quite suitable fоr the systems with сovalеnt bond) may
bе оonsidегed to be fit foг the objeсts with ioniс bonds that arе objeсts of oй
investigation.
In the lattег сase we havе a system of altеrnating сharge dеnsities what prеsses to
take into aссount long.rangе eleсtrostаtic intсraсtions сausф by сhaгges at dots of thе
сrystalline lattice of support as wеll as by those of the atoms limiting its suгfaсe. Suсh a
сounting ofelеоtrostatiс еffeсts is not сorliinon usеd foг desсription Ьfthе rеaсtivity and
сhеmiсal transfoгmations of admolecules within subsurfacе layеr what еffеоts on thе
сoпeсtness of the rеsults obtained.
Frоm thе сonсеpt asprсt, a miсrosсopiс desсription of thе геaсtions at solid
suгfaсе with ioniс bond typе should inсludе the following moments:
l. An analysis of elесtгoniс and nuсlear subsystems of the moleсulе.rеagent;
2, An eхamination of еlесtroniс and nuсlear subsystems of solid what is possibte
provided subsurfaсe layer is simulated at appropriate level;
3. A сalсulation of the intеraсtion bеtwern them at thе aссount of surfaсe eleсtrostatiс
field (oгientation and polarization of admolесulе, drformation of admoleсulе and local
suпounding of rеactivе site of thе suгfасe;
4. The final stagе is ai act of сhemiсal tгansformation аt solid suгfaсe what assumеs
knowledgе on multipimrnsional surfaсе of potеntial rneгgy.
This program сan bе rea|ized, in prinсiple, at two levels: the non-empirical onе
foresеes сalсulations of admolесule and small оlusteц i.e. badly simulatеs the surГaсе
itsel{ and the semiempiriсal one allows to eхamine, besides admoleсule, a|arge сlustеrs
that pегfoгms the struсtuге of the most сharaсteristiс suгfaсe parts in a propeг way.
6
G-
dr
I
-
r
I
G!
iшс
Neverthеless, the approach desсribеd сan not be realized praсtiсally in the puгe form in
case of the systems interesting foг сhemistry, and so a neсеssity aгisеs of thе adequatе
taking into aссount of еleсtrostatiс field and potential of solid objeсt. A problem of
expanding procedurе ofthe сalсulation ofеlесtrostatiс potential acоeptеd in the thеory of
moleсulе reaсtivity [2] to thе interaсtion of admoleсulеs with solid surfaсe is rather
сompliсatеd and labor-сonsuming but we wеre a suссess in its solution and in devеloping
еffесtivе sсhеme of оalсulation for the сasе of ioniс оrystals' Taking silica surfaсr as an
ехamplе, lеt us demonstratе thе importanсe and fтuitfulnеss of сounting elесtrostatiс fiеld
of solid support.
A fragment of hydroxylatеd Sioz surfaсе is peгformеd in the fig. 1 that has a struсturе
of p.сrystobalite (1ll) faсe, and thе еlесtrostatiс potrntial distribution itsеlf within
subsurfaсe layег is given in the fig. 2 |31. It is sеen from this figurе that therе is a lосal
minimum p within thе planе of silanol group at thе distanсe of about l A from its oхy$еn
atom, and сontinuous regions of positive potеntial are сharaсtеristiс of hydrogеn and
siliсon atom surroundings. It allows to assumе that small polar moleсulеs (foг examplе,
Hzo, [IF) arе сapable of penetration into сoordination spherе of hydгoxylatеd siliсon
atom. A region of high positivе p value in the hеxagonal сavity of surfaое layeг and near
rntranсe into it forming the most сonsiderable adsoгption potential for binding anions and
small еlеоtron donoг molесules is of partiсular inteгest. A rеgion with little nеgative
potential value stationed above entranсе into thе сavity seems to determine relativеly low
potential baгrieг for penetration of anions and small elrсtron donor molecules into thе
сavity. It should bе noted that the qualitative сonсlusions on thr pесuliarities of thе p
spatial distгibutiоn obtained for model systems arе appliсablе also to dispersе siliсas
(siliсagеls and aeгosil) with thе suгfaсе rеsembling struсturally thе сrystallographiс faсеs
of p-сrystobalitе.
Fig. 1. A sсhсmatiс rеpге.
sеntation of thе struсturе of
silica surfaсе similar to thе
B.сrystоbalite facе (1 1 1)
oo Фs i о t{
Chеmiсal modifiсation is known to be сapable to сhange substantially sorptivе
pгopеrties of siliоa surfaсе [a]' Dur to identity of suppoгt for a hydroхylated and
Ьhemiсallу modifiеd siliсas сhangеs in sorptivе рroperties of diffеrеnt samples arе to be
сonnеcted natuгatly with сorresponding pесuliaritiеs of p within thе surгounding of
functional gгoups' Thus in сasе of сhlorinatеd silica (fig. 3) it should bе notiсеd that p has
substantialiy lessе\hеterogеneity above surfaсе nеar a funсtionalgгoup as сompared with
that of hydroxylated..Sioz [5]. The smallеr negativе value of potential nеаr сhlorine atom
as сomparеd with that thе oхygen atom of silаnol group as wеll as negativе values abovе
hexagonal сavity indiсatе a сonsiderablе гise of potential Ьarrier foг embеdding small
Iigands into coordination spherе of thе chlorsilyl group siliсon atom. Pгobablу just this
I
гaЁt саn еxрlain an initial hvdrорhоbity and rеduсеd spееd оf the primary stagеs оf
йсoni.y*i* *r ,ь* сhiоrinatеj Sidz suriaсе' An analуsis оf thе elесtгоstatiс potential
i';fii*; цеaг thе сlrlоrinated, a,тiinatеd (fig 4}, and hуdrogenated {fig, 5,) siliсa surfасe
disсоr,еrs а riistinст dеpеndenсe оn thе nature оf funсtional grоups and altrows tо dеsсribe
;i;i, ооmparativе aсtivitу оf thesе groups in thе proсеsses оf polaг сompounds
adsофi*rr [6l.
r : r ; J - :a :
i : . . . . . т .
i:r:.д '-
I
--.--t -\ ;
z - \ - ;
/ \ i u-F*
-5 -.'"---."
' .. 11 --..-' r i
i I . r
Fig. 2. Тhe profilе of
еiесtгоstatlс pоtеntial (in
lтlnrоli in thе surгoundings of
fl.сrystоbalitе fbсе (i l1)
Fic. 3" T.hе prоfilе оf
*il;lс.trоst'atiс роtеntial (in
jtnlо'ij lEr tlie sшrrоundings of
l , - ' з ! l . , сhlоrinatеd
11 .;ryst*.bаiitr surГaсе faсе
{ .l ; -i, i.
A,n anaiysis оf th* ресuliaritiеs оf еlесtгоstаtiс роtеntial. sрatial distгibutiог wr*.hln
sшbsul.tbсе laуer сan gi;Ь-;" hpоrtant infогшatioг on r.lnitial stagrs.1r siliса
1u11;1
hyd'rаtlоn аnd оn pеойiaгitiеs оf сatiоn sоrрtiоn ivithout сarrying .ut iз.bО..,Г-sonsi]mlng
сomРLitatlons оn pоrеnti'al еnеrgy suгfaсe [?]. It is known that bеsiij*ll L. l.::Очt',] grouрs
lll.r1tоьt alrд",а:ls thегe аr* watегЪоleсulеs оn sitiсa surfaсе bс'.unri.+'; :; ll l'a't'lОds way'
Aссогdiпg i0 thе litcratt]rе data thе main fеatuгеs оf liуdrat;"': i",{n!,сi lii thе rеgions of
isоlatgd hуdlоxуl g'ffi
-*u*
ь. dеsсribеd within fгar.:nеrчi:кs оf lhrее modеls' A
formаtiоn r:f, hуdrоgетn bоnds trеtrveеп surfaсе silanо! grоr:р a.nri il:О пlolесule is a
оharaсt*гisiiс, оithе rnoсiels I and Il (fig. 6 and fig" ?)' Тlre first rnоiеl has watеr molесule
as protOtl dоn*r ivhегеas thе sесоnd.оne has silйоl grоup' \i.ji:nln inоdеl I (tig. 6) thеrе
arе nо;:еgiсlns оf the p nеgativе r,aluеs nеar оxуgen atопr оi'stianоi srcuр. Such a region
is рrеsеrit u.u, o'уg"n *'*,* *г admolесulе, thЪ minimum bеiIlg гaтhеr сonsidеrablе and
Ы;;;g tо -26s i"йс1. Thе a*сеss оf,сatiоn,s lntо thеsе ;:еgiotls tr.о;r sciuttоn oг Vacuum
is оut оf iзatгisr. l'е t,}rtir tra-iесtсriеs со.mрl*tе!у aa,s* thiоugh tnе геgions оf nеgativе
Ьйi-i our***. it is fbllоws frоm this figtrrе t}'at whеn sоr.Ьing. the оations firstlу arе
a
loсalizеd within thе rеgions of minimal p valuеs nеaг thе watеr moleсule oхygеn atоm
followеd by ехсhange with thе nrarеst pгoton that doеs not takе paгt of hydгogеn bоnd
foгmation. Analogous сonоlusions arе true also for the modеl II.
t
\ r s
Fig. 4. Thе pгofilе of
еleсtrostatiс poteпtial (in
J/mol) in thе surгoundings of
totally aminatеd p-сrystoba-
litе surfaсe faсe (1 l 1)
/ - z э \
\
,/too---\(,Gо
I | ( o )
\ I \_,/
Тhе modеl III (fig. 8) assumes a сooгdinativе bond bеtrlrеen the Hzo molесule oxygrn
atom and suгfaсe hydroxylatеd siliсon atom and is сommon usеd fоr an ехplanatiоn of thе
firm keeping of watеr with dispersе siliсa surfacrs. Thе presenсe of one сompaсt rеgion
of thе negativе potential values is a сharaсteristic of it. Similar to thе modеls I and II thе p
minima arе nеar thе water mоleсulе oхygen atoms and сorrеsponds to _240 kJ/mol.
Nеvеrthеlеss in this сase thе differеnсе in the potеntial values nеar oxygen atoms bеtwееn
silanol group and сoordinativеly bonded water is not suсh сonsidеrablе as in thе models
ехaminеd above but does not eхсludе an opportunity of thе initial loсalization of thе
сations sorbed in thеsе minima.
Thus the data on thе p distribution сonfirm the initial stage of сation soгption with
siliсa surfaсе to oссur aссompaniеd by thе moleсulеs of sorbеd wateг rather than by
stгuсtural silanol groups.
Fig. 5. The profile of еlесtгo-
static potential (in kJlmоl) in
the surгoundings of totally
hydridesilyliс B.сrystobalitе
suгfaсе faсе (l 1 l)
Fig. 6. Thо eleсtrostatic
potеntial distribution for
the model I of hydratеd
state of silica suгГaсe.
Thе p distribution vrithin
thе plane peгpеndiсulaг
to thе water moleсule
planе is given in uppeг
left part of the piсture.
Isopotential linеs
сorгespond to p values in
kJ/mol !
Fig. 7. The distribution
of еlесtrostatiо potеntial
for the model II of siliсa
surfaсе hydrated statе.
Isopotential linеs
сопespond to thе p
values in kJ/mol
Use of idea on thе role of eleсtrostatiо potential in oxidе surfaсe сhеmistry allows also
to simulateJhе change in suгfaсe propеrtiеs in сlear way wlrgn various admiхture ions are
йffi;;;".-"-iЪ_*"гix [.9]. Foг eхamplе, modifying siliсas is probable via
10
-50
L|гl
f t
Ч..т!i
Ц'rbr
s'_tr
lсr
br&a
bdr
l i ldo
Fi. h r.
frlсi s
ffп
Ёr.фd ril
fftsrr
:
. - . l ъ 2 .
/ - \ ( -r\
l t
/ /О
,-{ t"ry
тъз 6g l]
р:rсnoе of tL
&rrrсriliс o
tсщэaш
Ц!i! оf фэ p
!.rinЦБ 'ti
LтiGr fш tЬ l
осdiпзсd Дl
шiс fr.lgпа
F.ш rссCpш
llol.
тhс Gшпin
Оscп r cшo
looстЕte aolB
-itп !лd !hл
lsD of .fumiп
Glg/ of tЬ li
t mrin сotrl
-fiсс е|ссrrц
tхimum of r
сrс9ondф t
isomoгphous substitutioп оf aluminum atoms foг some quantity of siliсon atoms. A rathеr
сonsiderаble part of disperse siliсa surfaсеs, in particulaг thosе of aеrosil and siliсa gels,
bеing сhaгaсterizеd by thе struсtural еlemеnts that arе typical of thе B-сrystobalitе (l l l)
faсe, it is naturally to assumе the model with structural paramеters of this faсe that is
slightly distoгted by introduсing aluminum atoms into siliоatе matrix to bе suitablе for
aluminosiliсates. To dеsсribr thе prоpеrtiеs of aluminosiliсa surfaсе morе сomplеtеly,
two modеls should be еxamined that simulate thе struсturе of its most impoгtant seсtions.
In the modеl Iv (fig' 9) an aluminum atom substitutes a hydroхylated siliсon atom in the
initial model of thе p-сrystobalite (l1l), and in the model v (fig. 10) an aluminum atom is
insегtеd in plaсe of subsurfaсe siliсon atom bound with four siloхane bounds to
nеighboring Si atoms. Тhe topology of thе p distгibution within subsurfaсr rеgion of
aluminosiliсas is morе сompliсated than that of initial hydroхylated siliсa what is
сonneсted with the greatеr variеty of aсtivе sitеs (three-сoordinated siliсon atoms, bгidgе
hуdrogеn atoms, and silanol groups).
/ .0\ г .5
/ \r-s
Fig. 8. The distribution of
electrostatiс pоtential for the
modеl III of siliсa surfaсе
hydratеd statе. Isopotеntial
linеs сorrеspond to thе p
valuеs in kJ/mol/
a-too/
Thе fig. 11 pегforms thе p distriЬution near 3-сooгdinated aluminum atom. Thе
prеsеnсе of the region of nеgative potential valuеs adjoining the bridge oxygen atom is
сhaгactеristiс of it what tеstifу its proton aссeptor propеrties. Suсh regions arе absеnt for
thе oхygen atom binding two neighbouгing siliсon atoms in B-crystobalitе. Thе сompaсt
геgion of thе p positive values neighbouring with the aluminum atom is suгrounded with
сontinuous region of negativе potential valuеs what сгеatеs a rathег perсеptible potеntial
baгriеr for thе penetration of elесtron donor molecules to the Lеwis aсidiс sitе that is 3-
сooгdinatеd aluminum atom in the сasе given. For the сlustеr V thе potеntial nеar 4-
atоmiс fragment of =Al-oH-Si= has only positivе values (fig. 12)' i.e. the aссеss of
ргoton aссrptor molеcules or anions to thе Bronstеd aсidiс sitе is praсtiсally out of baгrier
[ 1 0 ] .
Тhе еxamination of thе ioоal state dеnsitiеs (LsD) of the systеms undeг сonsidеration
сan sеrve as one morе еxample of obtaining usеful infoгmatiоn on thе propeгtiеs of
сonсretе atоms in surfaсe struсtures [l1]. Thе fig. 13 pегfoгms thesе statе dеnsities of
siliсon and aluminum atoms сalсulatеd for the сlustеr IV. Thus, thе onе of thе maхima of
LSD of aluminum atom is stationеd within the enеrgy region from_1 to *l еV, and thе
еnегgy of thе highеst oссupiеd orbital is of _8.4 еV. Тhe 3p, aluminum atom orbital pays
thе main contribution to this maхimum what givеs grounds to assume a localization of the
suгfaсе eleсtroniс statе at this atom displaying as an еleсtron donor sitr. Thе prinсipal
maximum of a hydroxylated surГaсe siliсon atom is loсalizеd within еnergy region
согrеsponding to the highеst oссupied orbitals, and thеre is a maximum within *l to +l
l 1
-50
еV testiфing the eleсtron aссeptoг pгopегtiеs of this atom. A сoordinativе bond of
еlесtron donor molеj"' *itь ьyЪ.oxytateо suгfaсе silicon atom sеems to be гealizеd duе
iй. p,",.nсe of thе lsп -a*im.,m within thе region of vaсant levels. For a subsurfaсe
siliсоn atom, all ttre maхima are within thе regionЪf loweт.еnеrgiеs. Analogous features
of thе mutuat stationing-tье iso maxima of virious type siliсon atoms arе сharaсtегistics
of thе B-сrystobalitе.
О o O s i . I ] Од
Fig. 9. A sсhеme of the cluster
Iv modеling a paгt of
aluminosiliсa surface realizеd
due to the сhange of
hуdroxylated siliсon atom by
aluminum onе within surfaсе
layer of p-сrystobalite. Thе
atoms numbеring is givеn in
depеndenсе on their distanсes
from aluminum one. The atoms
arе labeled with ..f' letter that
kept thеir positions undеr
optimization unсhangеd and
thе samе as thosе in
p-сrystobalitе
Fig. 10" A sсhemе of the
сlustеr V modеling a part of
aluminosiliсa surfaсе rеalized
duе to the сhange of bulk
subsurfaсе siliсon atom by
aluminum oпе within surfaсe
layer of p-сrystobalitе' The
atoms numbering is given in
dеpendеnсе on thеir distanсеs
from aluminum onе. Thr atoms
arе labеlеd with ..f' lеtter that
kеpt their positions undеr
oрimization unсhangеd and
the same as those in
B.сrystobalitе
=Si.OH + ХY -+ =Si.oХ + HY
Basing on idеas on thr importanсе of taking the еlесtrostatiо potеntial within
subsurfaсе layеr of o*idеs into aсiount, lеt us е*amйe the pеculiarities of the elеmentary
aсts of thе сhеmiсat tйformation at ihe surfaсе. Both a modifiсatoг molесule and thе
suгfaсе rеaсtivе sitе wй nеighboгing atoms undеrgo to dсformation сhangеs in thе
proсеssrs of the сhеmic;i *;riyi"g oF surfaсе. It wai shown in thе litегaturе [4] that thе
mаin оontribution tO thе aоtivation еnеrgy оi the rеaction of еlесtrophiliс substitution of
thе silanol group рroton
l s ? а : : : i ' . . : (
s.rо.;]:
.э<
а:.::
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' , / - -
Т; еs:.
.tсоrсl:; :с
сr .с i . l а : rэ i s с j
tiх сс:J-::..:a:t^
!а сl'!а:1gе ::.}t s
iсirrtа::с :-:l
соrпэir;аlес :. l
оf : i .с ::r:... lе::.
Ьэзэgеn'- -s с.
Т:е sс:
с .с . .1 : ; la t : ; : ] t
s.eпl. .:;,'.u:
.Еп!е:е: ::3г.-i::
tl!сs с: r:с aс:
tPfr.:j Ёr..-: з
a. .еаз.е - s: .е
:с:а:ес:з. s-г
irг.а. ;.:n:.-::.
t2
is paid by the dеformation proсess in admolесulе
should bе anotherthan thosе ofisolatеd state.
and its сharaсtеristiсs near surfaсе
Fig. 11. The distribution of
eleсtrostatiс potential for the
сlustег IV within thе planе
peгpendiсulaг to thе planе of
the АI[o(l)si(2)Ь fragmеnt
and сrossing the si(z)' o(l),
Al' and s(0 atoms. Тhе
potential valuеs arе givеn in
kJlmol
AI o(l) s(2)
Fig. 12. The distribution of
еIeсtrostatiс potentiаl for thе
сluster V within thе planе of
(нo)s(2)o(*)Alo(D. Тhе
potential valuеs are given in
kJ/mol
To estimatr this diffеrеnсe, gеnerally spеaking, two approaсhеs сan be usеd.
Aссording to the first onе thе геsults should bе сompared of quantum сhеmiсal
саlculations of isolated molеоulе with analogous data foг the same moleсule nеar surfaсе,
thе сonfiguгation of transition state bеing applied. Thе sеоond approach makes it possible
to сhange the study of moleсule near surfaсе with its examination within somе effеоtivе
eieсtrostatic field [l2, l3]. of сourse thе total сalсulаtion on thе effесt of the field of
сonrplicatеd distribution is impossible practiсally, but general оonformities to natural laws
of the influеnсе of surfaсе on admoleсule сan be eluсidatеd within the approximation of
homogenous eleсtriс field.
Thе sесond appгoaсh bеing apрliеd, lеt us ехaminе what is thе role of subsurfaсe
еleсtrostatiс field in the rеaсtions of elесtrophiliс substitution of thе pгoton of isolatеd
silanol group with tгimethylsilylic radiоals _SiRr&R.3. Thеsе rеaсtions run yja four.
сеnterеd tгansition states [4] (seе fig. l4). A defoгmation is еssential for suсh tгansition
statеs of fhe admoleculе silicon atom tetгahеdral surrounding up to planarity of the ХRrRz
atomic group es well as a trigonal.bipyгamidal сonftguгation of the system: mоdifiсator
molесulе + silanol group oxygеn atom. A dеgrее of distoпion of thе сеntral siliсon atom
tеtгahеdгal suгroundings is сontrollеd by angles с,1 and сt2 that aгe еqual to 90o foг a
planaг confoгmation of the group mentionеd.
13
(A
o()Р
!ч
q(н
o
0)
Fig. 13. The loсal state
dеnsitiеs of the Si(o)' Si(h)
and Al atoms сalсulatеd for
the сlustег IV. The values of
statе dеnsities are given in
rеlativе units
0 Е , eV
Thе plots of thе heat of formаtion on anglе C[1 aГe givеn.in thе fig 14 сalсulatеd for
the isolatеd.Ngzsi(сI{3)3 moleсule at two values of outеr field and without it. onе сan
makе two important сonсlusion fгom this figuге. Firstly, 'Фing outer fiеld into acсount
.i"ut"а by latiiсe and surface atoms in the region rryhеrе modifiсator molесulе is stationed
for the transition state structurе гesults in deсreаse of deformation rnеrgy Еа"г with гise of
field. Sеcondly, if thе Еa*г maximum is aсhiеvеd at с[t = 90o for zero field then at F =-
0.02 a.u. this maximum сolTrsponds to or = 92o aлd at F = O.04 a.u. thе following гisе of
сrr to 95o oссurs' Calсulating subsurfaсr fiеld lеads to thr situation that the transition state
of exothermiс rеaсtions сonsiderеd is aсhievеd at thе сrr angle values (ш1 сontrols moving
on along гeaсtion сoordinate) that preсede the planar struсturr of NHzSi(CHз)3 fragmеnt,
i.e" thе transition state struсturе at zero field is nearer to that of initial reagеnts as
comparеd with the situation for zeгo field. Thе values of Ес.я and of angle с1
oo,,Ёsponaing to its maximum for the molесules of some eleоtrophillic reagrnts are given
in thе iablе, Ii is sеen from it fiгst of all that thе maхimum of Е6"9 valuе is aсhieved at the
с[1 an$lе va1uеs < 90o for thе endothermiс rеaсtions of trimеthylpseudohalogensilanеs
ён,шш" (сI{з)зsiNCS, and (CIIзЬSiNCO intеraсtion with silanol groups. This
i.'tiii.' a
"on'iсe.uьlе
distortion of moleculе struсtures in transition states.
Thus, applying idea on subsurfaсe еlectrostatiс fiеld allows to dеsсribe moге
сoгreсtly tьеse rЬactiJns within the fгamеwork of thе dеformation model fоr aсtivation
barгiеrs, of thе rеaсtions of еleсtrophilliс substitution of silanol group protons.^and to
eluсidate faсtors dеtermining a dеgree оf distortion of thе spatial structurе of modifiсatoг
moleсulеs nеar adsorbent surfaсe.
Thе advantages of this approaсh сonsist in simpliсity (thеre is no neсessity to
ехaminе interaсtion of admoleсuies with solid surfaсe that appears only as effесtive
еlе"шоstatiс field сreatеd by it and lattiсe), in гefusal from standaгd supеrmoleсular
;;*.;йii;. within thе fгamейorks of usual approaсhes of zегo differеntial ovегlap that in
;;i;;фi. fail in dеscгiption of potеntial barriеrs. Also an opportunф aгises to сompare
i.u.iЁity of molесules in thе rеiсtions of proton еleсtrophilliс substitution not only in the
row of rеlated сompоunds (foг ехample mеthylсhlorsilanеs) but of those сontaining
variоus funсtional groups.
T4
iГ
тL&
bJз
iErЪьчь
ЕЬsnхс
Еьsгrct
ФЬrc
(CtъЬsв.
(clъЬsJ
Ьш
b*r
gldPhуй
llrl ш
rrrfrпir;сs t
il.:I шdl
I--эd ф r
шfuсd d
-t;: algrrr
Ertшtr
сd ?idin
.1 . $Гdd
CH.
.71s'-
o , / l \ o '
/ s i \Уr'\y
N}I2 /
-сFl'
CHз
Fic. 14. The dеformation
еnеrgies of tгimethylamino-
silanе molеоulе in zеro fiеld
and those in thе fiеIds of
F:0.02 and 0.04 a.u.
o
Е . 1 0 0
-t
:
' r50
Thе dеformation еneгgiеs, Е6.1(kJ/mol), heat effесts, Q (kJ/mol), and angles
сt."*(degгees) of thе reaсtiоn of silylation
F = 0.00 F: 0 .02 F : 0 . 04
Мolесulе Еа"t с!l** Еа"г сLl.o* Еа.r с[l**
(CIr3ЬsiN3
(CHз)зSiNCS
(CI{3)3siNсo
сIr3)3siсl
(CHз)зSiBг
(CHз)зSiJ
1 51
t75
t72
r62
140
t?6
120
t:
90
90
90
90
9o
90
r4l
l6з
164
151
128
108
88
86
85
88
98
100
130
148
153
148
rt4
86
85
83
80
86
102
105
In сonсlusion let us. сonsider problеms on the formation of hydгoхyliс оoveг on
Гтеsh-made surfaсes. The eteсtгostatiс fields сreatеd by latticе ions nёaг su'fu.. limitin!
сrystal play the most important and dеtеrmining role in its formation. Depеnding on oхidБ
сhеmical сomposition, definitе factoгs arе promotrd to the foregгound, bu:t general
соnfoгmities to natural laws of hydroxyliс сover foгmation сan be Jbservеd usin! some
simplest models. Thus, the fig. 15 dеmonstrates a frеsh.madе suгface of p.сryйbalitе
obtained by ruptuгe of сrystallinе lattiсе aссording to thе (1ll) inсline planЬ. Threе.
сoоrdinatеd siliсon atoms aг€ stationed in plane of this faсe,
.гows
of singlЪ-сoordinй
oxygеn atoms bеing гisеn abovе them. As the valеnсies of surfaсe siliс-on anа o*yju,
аtoms are totally unsaturated' stron-gly hеtеrogenrous eleсtrostatiс fields of high,t..,,Ъ,.
сгеated within thеir surгoundings. Gradients ofthеsе fiеlds сan be judged гroйлg. tв.
t 5
fi? t !4 fl t r08 105 t02 99 S6 e3 90
i ,."-"
1 . ! {
\i I *
\ \i 4io\Ч
1 . \
r \ i
i t i
l l l
Г-*j с ц} f;i
. . ' l Г l . * .
--'-) iiO *.i rln
t v u
\
r... l
ф ;.l
* T l
i' iЁ' li ,c. tresh-
т**.d. iiiifla('е оf
t,i"i,st-*1lirtе t]-
сryst*tiaiit*
Fig. 16. Тhе distribu.
tiоn оf,еlесtrоstatiс
рotential nеаr fгeslr*
rnadе surfaсе of
p.сrystоbalite
Fig 17' Watеr molе.
оulе nеar fтеsh-made
surfaсе оf p-сrystо-
bal!tе
L - : 1 "
I ; L
l . -
i , l -
: r a .
:с.
'rl
t r ' i
- ! : ' -
:+:
, \ -jiu
i /', I
+ o
+5fi i -, r1 -
i r
i !
li
i
"
i -'.,
i:' '
-'--1О.]-.r. ...i' f
/ ' r . , i l
,' 1. 1
' - t
-"'\',i - t. 1
,""'\ \ i
; ' i i i
t r 1T \ \ i !' t' :.',/
*t
I*
П
д
i r : : r , , , 1
t.,,. 1
t !
(
l l t /
l 6
Whеn hitting hollows at surfaсе of the faсе disсussed (fig. 19) a watеr moleсulе (thеre is
an eхсrss of it always and еverywhеrе) takes its beaгings suоh that its oхygen atom is
stationеd near a siliсon one -and its hydгogеn atoms are stationed n.a. oiygen o,",'
Calсulations indiсate that an еleоtrostatiс fiеld of about 0.02 a.u. is сreated [12j йthin thе
rеgion of thе water molесulе oxygеn atom loсalization. When in suсh fiЬlds, watеr
molесule eleсtroniс shells arе so deformеd that its ionization takes plaсе following by a
moleсular ion Hzo*fоrmation (fig.20). Еffесt of fiеld on this ion results in its оonsБquЬnt
deсomposition into oЕf radiсal and H*. A hydroxyl radiсal is bound with threе.
сoordinatеd siliсon atom and proton migratеs into neaгest minimum of negative potrnt;al
nеar oхygеn atom' As a rеsult two surfасе hydrохyl groups arе foгmed- duе to
dесomposition of onе molесular ion.
2,6 R(oн.rD'A
Hzo" +H. +o}Г
I
I
I
I
!
I
II
ЕIэo..+tг+oн.
Hzo + I{. +oн.
Fig. 18. A sсhematiо pеrformance of the proсеssеs of ionizing and decomposition of
watеr molесule as wеll as of its molесular ion in zero fiеld and in thе fiеld of 0.b2 a.u.
Nеveгthеlеss suсh a mесhanism of thе formation of hуdroхyliс сovеr on frеsh.
madе B-сгistobalitе (1l1) faсe is not univегsal one. Thus, in оase of rutile (сrystalline
modifiсation of titania) surfaсе (l10) faсе besidеs еlесtrostatiс fiеld a mutual eхсйange of
еlесtгon dеnsity betwеen admolеоule and surfaоe (fig. 2l) plays impoпant role;Ъs a
геsult dесomposition of water molесulе and hydroхyl сovеr formation oссur.
Some сonсlusion саn be madе Гтоm thЬ data pгеsеntеd, in paгtiсular:
1' Thеrе is a сlеar and sinonymous сonсretizing sitеs of polar molесulе primary
adsoгption.
t'7
A hetеrol}гсiо сharaсtеr of сhеmiсal rеaсtions оn Sioz surfaсе follows from
"i""i."''.'i"
potеntial distribution пeaг siliсa surfaсe with polar funсtional groups. An
еxistеnсe оf potеntial i*gion. with соnsidеrablе valuеs of opposite sign nеar thesе
gгoups at thе distanсes оiabout сhemiсal bond length indiсatоd strong polarization of
Гь",Ь""ti.," sitе of геaсting mo1есulе as wеll as inсrеasе in bond polarity.
ёь*й l" nature of йсtional grоups, hydration- of -.silanоl.
groups' insегting
admi*urе atoms into siliсa matrix-,.s,,lt in еssеntial гrdistribution Of rleоtrostatiс
i*й
",с
in rеspесtivе vaгying adsoгbеnt suгfacе propеrtiеs.
It should bе notеd that thе iiformation еxaminеd on aсtivе sitеs natuге was obtainеd
vrithоut сarrying ; соmpliсatеd quantum сhеmiсal сalоulatiоns within
suреrmolеоutu. uрp.oa"ь" lt is йpоrtant that a сlear analysis is pгobablе on this basis
оf thg сhеmiсal .on.*tu"n.*, oi uny сhangе in сompоsition and surfaсе and bulk
strшсtuге of oxidеs unГiь*i,
"o*po,itions
(from ioniс impiantation to destruсtion of
thе sувtеm simulatеd) wlrat allows to tесognize сalсulations with rеal сounting
*leсtrоstatiс fiеld tо b. un .ffe.tivе toоl of Ъomputing ехpеrimеnt and thеoretiсal
iu'ig' of sоiid sуstеms with thе ргоpегtiеs assignеd.
Fig. 19. The struсture of a
paгt of hydroxylatеd rutilе
(1 10) faсе
Fig. 20. A schematiс
nеrГormanсе of the orbital
intеraсtions rеsulting in
dissосiativе оhemisorption
of watеr at the (110) faсe
of rutilе surfaсе: a) ovеrlap
of thе 3d,2 orbital of a
surfaсе titanium atom with
the EIoМo of water
molесulе; b) ovеrlap of the
same atom 3d*, orbital
with the Lt]Мo of water
molесulе. The dirесtion of
an еlесtron dеnsity transfег
is dеpiсtеd with aгrows
Rсfсгепсеs
] Bс:sс::
цц Е.есl
] P э.::zс: j
с.с:]с..:s
j L;:а:^: i
:: :з дjs
.i --.:т.l* э:
i l: R;s
l С=:r: .{
li6. =JсL-
J L.^.}з.:.; ъ
:tв с.+..::
.:a'-::a* -':
Ё L;.зал:i
l : ,
L-r"*' . i -
P:.r s.:д (
L;еl..ъ,:..
;: :..s с,3t
R-sr'д::
l- rэ:;t
\ \ Srс-
.\\ sssR,
}.'с ::-с
\ \ ' ;е*
\ ' _ t \ e L
]оiд.ъ:..
rtr ](.тt
rl:l R *r::t
fu1д.L-: :
1 L 1 . 4 . ! . - - r
Lсtа:,:.
с. € i : - . : . . :
Еlslgi:r
Ciс:т" .яr
т} ; : i
\ \ ]с . :д
D.Э.д .
O
tz i nH
p-Оk]5о,ьzз
o *oн
fr
-0,400
П
t l
l l
t )
0,402
- - - H
ri(5) (a) Тi(5) (б)
18
Rеfеrеnсеs
1. Bеоhstedt F., Еnderlеin R' Semiсonduсtor Surfaсеs and Interfaсеs' Their Atomiс
and Еleсtroniо Struсtures. - Bеrlin: Aсadеmie.Verlag, 1988' _ 488 p.
2' Politzer P. // Homoatomiс rings, сhains, and macromoleсulеs of maingrouр
elеments. Еd. A.L.Rheingo1d. * Amsterdam: Еlsеvier, 19,77, -P.95 _ 125.
3. Lobanov V.V., Gorlov Yu'I. The eleсtгostatiс potential of Sioz suгfaоe and naturе
of its adsorption sites // Khimiya, Fizika i Tekhnologiya Poverkhnosti [Russian
Jouгnal of Surfaсe Chemistry, Physiсs, and TeсhnologуJ, -1992. _ Issuе 1. _ P.з.
8. (In Russian).
4, Chuiko A.A., Gorlov Yu.I. Thе Chеmistry of Silica: struсturе of surfaсе, aсtive
sitеs, meсhanisms of sorption. _ Kyiv.: Naykova Dymka, |992' _248 p.
5. Lobanov V.V., Gorlov Yu.I The effeсt of natuгe surfaсe of funсtional gгoups on
the elесtrostatiс potеntial of Sioz surfaсе ll Zhurna| Fiziсhеskoi Khimii [Russian
Jouгnal of Physiсal Chemistry]. - 1995. _ v. 69, N 4. - Р.407-418. (In Russian).
6. Lobanov V.V., Gorlov Yu.I The strength of eleсtrostatic field in suгroundings of
silanol group on Sioz suгfaсe l| Zhurna| Fizicheskoi Khimii [Russian Jouгnal of
Physiсal Chеmistry]. * l995. - v' 69, N4. -P.652_654' (InRussian).
7 ' Lobanov V.V. Thе soгption of сations on hydrated Sioz surfaсе and thе сharaсtеr
of its elесtrostatiс potеntial. - Kyiv: |997 ' - 14 p. (Preprint NTUU ..KPI',). (In
Russian).
8' Тhe soгption of tгansition and rarе еarth ions on сalсium form of siliсa gеl /
V.V.Strelko, D.N.Stгazhesko, N.I.Soloshenko, S.K.Rubanic, A.A.Bеran i/ Dokl.
AN sssR. - |969._ v. 86, N 6. _ P, |з62 _ |з64, (In Russian).
9. Thе role of eleсtrostatiс interaсtions in adsoгption on solid oxide surfaоеs /
V.V.Lobanov, Yu.I.Gorlov, A.A.Chuiko, V.М.Pinсhuk, Yu.I.Sinеkop,
Yu.I.Yakimenko llKyiv, ''Vес*'' Ltd., 1999. - 2З1 p' (In Russian).
l0. Lobanov V.V., Terez М.I., Gorlov Yu.I. The struсture of aluminosiliсa surfaсе
and propегtirs of its aсtivе sitеs. - Kyiv: |997. - 22 p. prepnnt NTUU ..KPI',).
(In Russian).
ll. Salahub D.R., Меssmeг R.P. Мolесular-orbital study of aluminum сlustеrs
сontaining up to 43 atoms // Phys. Rеv. B, - |977 ' - v. l6, N 6. _ P ,252615з6.
12. Lobanov V.V., Alеksankin М.М. The influеnсe of eхternal elеctгiс fiеld on thе
еlесtroniс struсture of molесulеs and theiг сation.radiсals // Tеorеtiсhеskaya i
Еkspеrimentalnaya Khimia [Russian Journal of Thеoгеtiсal and Ехpeгimеntal
Chеmistry]. - l980. - v. 16, N 4. - P, 477486, (In Russian).
13. The physiсal foundations oГ thе field mass-spесtrometry / Е.N.Korol,
V.V.Lobanov, V.A.Nazarenko, V.A.Pokovsky / Еd. Е.N.Korol _ Kyiv: Naykova
Dumka, 1978. - 194 p. Qn Russian).
l 9
|
| id | oai:ojs.pkp.sfu.ca:article-45 |
| institution | Surface |
| keywords_txt_mv | keywords |
| language | English |
| last_indexed | 2025-09-24T17:44:37Z |
| publishDate | 2001 |
| publisher | Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine |
| record_format | ojs |
| resource_txt_mv | surfacezbircomua/25/46e68f9d839b910977647928d9a9be25.pdf |
| spelling | oai:ojs.pkp.sfu.ca:article-452018-11-27T09:42:39Z Electrostatic aspects of adsorption on silica and titania surfaces Electrostatic aspects of adsorption on silica and titania surfaces Electrostatic aspects of adsorption on silica and titania surfaces Lobanov, V. V. Chuiko, A. A. The influence of outer electric field of the ionic crystal surface layer on the structure of adsorption complexes is examined and their role in the run of physicochemical processes at the solid-gas interface is studied. Taking into account of the subsurface electric field has been shown to allow to predict the structure of primary adsorption complexes, to estimate the energetics of the transformations of molecules at solid surfaces, and to determine properties of the surface compounds formed. The influence of outer electric field of the ionic crystal surface layer on the structure of adsorption complexes is examined and their role in the run of physicochemical processes at the solid-gas interface is studied. Taking into account of the subsurface electric field has been shown to allow to predict the structure of primary adsorption complexes, to estimate the energetics of the transformations of molecules at solid surfaces, and to determine properties of the surface compounds formed. The influence of outer electric field of the ionic crystal surface layer on the structure of adsorption complexes is examined and their role in the run of physicochemical processes at the solid-gas interface is studied. Taking into account of the subsurface electric field has been shown to allow to predict the structure of primary adsorption complexes, to estimate the energetics of the transformations of molecules at solid surfaces, and to determine properties of the surface compounds formed. Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine 2001-06-10 Article Article application/pdf https://surfacezbir.com.ua/index.php/surface/article/view/45 Surface; No. 4-6 (2001): Chemistry, Physics and Technology of Surface; 6-19 Поверхность; № 4-6 (2001): Химия, физика и технология поверхности; 6-19 Поверхня; № 4-6 (2001): Хімія, фізика та технологія поверхні; 6-19 3154-8091 3154-8083 en https://surfacezbir.com.ua/index.php/surface/article/view/45/44 Авторське право (c) 2001 V.V. Lobanov, A.A. Chuiko |
| spellingShingle | Lobanov, V. V. Chuiko, A. A. Electrostatic aspects of adsorption on silica and titania surfaces |
| title | Electrostatic aspects of adsorption on silica and titania surfaces |
| title_alt | Electrostatic aspects of adsorption on silica and titania surfaces Electrostatic aspects of adsorption on silica and titania surfaces |
| title_full | Electrostatic aspects of adsorption on silica and titania surfaces |
| title_fullStr | Electrostatic aspects of adsorption on silica and titania surfaces |
| title_full_unstemmed | Electrostatic aspects of adsorption on silica and titania surfaces |
| title_short | Electrostatic aspects of adsorption on silica and titania surfaces |
| title_sort | electrostatic aspects of adsorption on silica and titania surfaces |
| url | https://surfacezbir.com.ua/index.php/surface/article/view/45 |
| work_keys_str_mv | AT lobanovvv electrostaticaspectsofadsorptiononsilicaandtitaniasurfaces AT chuikoaa electrostaticaspectsofadsorptiononsilicaandtitaniasurfaces |