富山大学水素同位体科学研究センター研究報告28 : 9‑22, 2008.

Preliminary Study of H20 Adsorption on Cr203and Al203 Surfaces
by Ab Znitio Cluster Calculations

K. Watanabe, Y. Torikai, M. Hara and Y. Hatano
Hydrogen isotope Research Center, University of Tbyama

Gofuku 3190, Tbyama 930‑8555, Japan(Received Dec･ 26th, 2008: Accepted Feb･ 23th, 2009)
ABSTRACT
Adsorption and desorption of water over Cr‑OXide and alumina are of great concern withthe contamination of stainless steel and gas chromatographic isotope separation･ However,
the mechanisms of those phenomena have not been fully clarified･ A preliminary ab inito
study on the behavior of water molecules on the (001) surfaces of α‑Cr203 and α‑A1203 Was

carried out for small clusters of Me404 (Me ‑ Cr or Al) by use of the Gaussian 03 package.
It was found that water molecules can be adsorbed molecularly and dissociatively on thesesurfaces･ Molecular adsorption of a H20 molecule takes place on a metal atom of the cluster

as Mel0H2, With the bond length of about 2.0 A. Dissociative adsorption occurs by forming
Me‑OH (on‑top site) or Me2‑OH (bridge site) and 0‑H bonds･ There appeared a featurethat the bond length of Me2‑0H is greater than that of Me‑OH･
Calculated adsorption energies were examined by comparing with the values estimatedfrom the thermal desorption spectra of water for α‑Cr203 and 7‑alumina･ The ab initio cal‑culations coincided quite well with the observed thermal desorption spectrum for α‑Cr203,
whereas they did not for l′‑alumina･ Further studies are required to clarify the structures ofadsorbed species, adsorptir)n energleS and kinetic processes･

1. 1mtroduction

Adsorption of water vapor on oxides such as Cr203 andAl203 is an important problem todevelop tritium‑handling techniques. The former is concerned with tritium handling systems,
where stainless steel such as SS316 is commonly used aS Structural material･ Since the surfaceof stainless steel is usually covered with native oxides such as Cr203[1, 21, behavior of wateron Cr203 Surface is of great importance for tritium contamination and decontamination･ The
latter, Al203, has been used for gas chromatic separation of hydrogen isotopes as a support
material of the working material such as Pd and/or Pd‑Pt alloy particles[3, 4トAdsorption ofwater vapor significantly affects the performance of the isotope separationl5, 6] ･As for the tritium‑contamination of stainless steel, it has been revealed that the contami‑
nation can be divided into two types: one is the tritium solution in the bulk and the othertritium capturing by a thin sub‑surface layerl7, 8]･ It should be noticed that the tritium con‑centration in the sub‑su血ce layer is considerably higher than that in the bulk【7, 8, 9]･ It

wasalSo found that tritium captured in the bulk can be released rather easily to gas phase byheating in vacuum and/or in inert gas atmosphere, whereas the removal of tritium captured

K

Watanabe,Y.

Torlkal,

M.

Har乱

and

Y

Hatanoby the sub‑surface layer is much more difncult[7, 8]･ The tritiuln in the bulk is attributed Lot,Tit,ium solvedinthe interstitialsite of the bulk structure.As for the tritium captured in the,sub‑surface layer, it is considered to be the one captured in the native oxide layer of stainlesssteel comprising Cr203[7, 8]. From these viewpoints, extensive studies of the contaminationand decontamination of the stainless steel have been carried out but no further details of

capturing by the sub‑surface layer have not been clarified so far. For example, it should be

mentioned here that tritium is predominantly released as elemental form. HT, to gas phase

under evalulation, whereas it is essentially in the form of HTOininert gas atmosphere. It has
been also found that the release of tritium solved in the bulk can be explained by diffusion

models. The tritium release from the sub‑surface layer, on the other hand, Cannot be describedby simple diffusion mechanisms. This is complicated by the concurrence and/or competitionof the diffusion in the sub‑surface layer and surface reactions, and then it is still under rigorous
investigat ions ･Asfor A1203, it has been observed that the performance of gas chromatographic hydrogenisotope separation system is significantly impeded by adsorbed water, in other words under thepresence of moisture in the isotopic mixture gasl5, 6]. This is considered due to the presence ofOH一groups adsorbed on the carrier (commonly A1203 powder) of the working materials such as

Pd‑Pt alloy particles【5, 6トIn the separation column hydrogen isotopes in the loading mixture
gas are丘rstly dissociatively absorbed by the working material and then released to gas phaseas H2, D2 and T2 according to thermodynamic isotope cffcct. Under the presence of OH‑groups
on the supporting A1203 powder, however, the dissolved hydrogen isotope atoms such as H,

D, and T are spilled over to the support surface, where isotope replacement occurredwith
native H atoms of residual 0H‑groupsl6] A This phenomenon caused severe interference on theisotope effect of hydrogen absorption/desorption by the working materials as Pd, Pd‑Pt andso onl3, 4, 6]. Detailed mechanisms of the disturbance, however, have not been clarified sofar. Since tlle presence Of OH‑groups on A1203 powder are unavoidable because the powder
are exposed to theair even after rigorous out‑gasslng after the preparation, understanding ofadsorption of water is also an important problem from this point of view.

As afirst step to understand the tritium capture and release by/from the sub‑surface layer
as well as the supporting material, ab initio calculations were carried out for adsorption of

H20 0n the surfaces of Cr203 and A1203, Which were modelled by Me404 (Me ‑ Cr or Al)
clusters, byusing Gaussian 03 packagel10]. The aim of the present preliminary study is to
丘gure out adsorption sites and species formed on the surfaces of Cr203 and A1203 exposed to

H20vapor.

2. Modeling of Oxide Surfaces
Chromium oxide, Cr203, and aluminum oxide, α‑A1203, belong to the tetragonaLhexagonalgroup, belonging to the space group of R言C (#167) structure, conventionally called as corundum10

prellminary Study of H20 Adsorption on Cr203 and A1203 Surfaces by Ab Init10 Cluster CalculatlOnS

i;

･了己.､

Fig･

1･

A

part

of

corundum

unit

cell)

rep‑

Fig･

2･

Structure

of

the

model

clusterforresenting Me404 cluster frame ; dark balls Me404 (Me‑Cr, Al); dark balls represent Me
represent Me atoms and light ones 0 atoms atoms and light ones 0 atoms

structure. The lattice parameters are a ‑ 4.9607 and c ‑ 13.599A for Cr203[11], and a ‑4.7589 and c ‑ 12.991 Åfor α‑A1203【12]. Figure 1 demonstrates a part of the unit cellof corundum, for the sake of simplicity, where the blue balls represent metal atoms and thepink ones oxygen atoms･ For inspecting the adsorption of H20 0n those oxide surfaces, it was

required to simplifythe structure for the sake of computing time by Gaussian O3･ In the present

study, small clusters consisting eight atoms were adopted for the structural optimization ofadsorbed H20. Figure 2 shows the cluster model of Me203 COmprising four Me‑atoms (Me‑Al or Cr) and four 0‑atoms, where the atoms numbered 2, 5, 6 and 7 represent Me‑ atoms,
and those of 1, 3, 4 and 8 are 0‑atoms･ This is a characteristic con五guration of atoms in the(ool) palne of corundum structure as shown in f'ig･1･ Tables 1 and 2 show the locations｡f atoms in the cartesian coordinate for Cr404 and A1404 clusters, respectively, where x, y

and z values are in A unit. These structures were fixed during the calculationsof structure
optimization for adsorption of H20, by assumlng that the surfaces of those oxides are notsubjected to relaxation owlng tO H20 adsorption･

3. Structure Optimi2:ation
The ab initio structural optimization by Gaussian 03 was carried out by adding a H20
molecule at an appropriate position of the Cr404 or A1404 cluster･ In these cases, the cluster
structures were fixed as mentioned above and the oxygen and two hydrogen atoms of watermolecule were freed to move to find an optimum structure having minimum energy･ The
model chemistry and basis functions adopted for the structural optimization were UB3LYP

of the density functionalmethods and Lan12dzfor Cr (or Al) and Owith 6‑31G(d,p) forhydrogen atoms, respectively･ Owing to rather slow converging speed by personal computers
used, the optimization started in low convergence criterion with the SCF option of conver‑4llTable 1. Location of Cr alld 0 atoms of Cr404

Table 2. Location of Al and 0 atoms of A1404

cluster ir上 Cartesian coordinate;X, y and z are

clusterincartesian coordinate;Ⅹ, y aIld z are

given in A

given in A

and proceeded further with the option of conver‑8[13]. The optimizationgives the structure ofadsorbed species (adsorption sites, bond lengths and angles among respective atoms) and thetotal energy of the system. In the case of H20 adsorption, the adsorption energy is evaluatedaS

Eads ‑ Etol(Me404‑H20) ‑ Efot(Me404 + H20),

い)where Etot(Me404‑H20) is the total energy of the system obtained by the structure optimiza‑tion and Blot(Me404+H20) the total energy of Me404‑H20 (Me‑Cr,Al) without optimization,
where the H20 molecule locates at 7 A apart from a Me‑atom, corresponding H20 gas in the
Vacuum.

4. Results and Discussion
4.1. Adsorbed States of H20 for Cr404
Several kinds of adsorbed species were found andthey could be roughly divided into five groups

with respect to adsorption energy. The mostweakly adsorbed species, H20(α), showed the ad‑sorption energy of ‑92.9 kJ/mol. , whose structul･eis schematically sllOWn in Fig.3, where the atoms
numbered 1, 3, 4 and 8 are oxygen atoms, andthose indicated as 2, 5, 6 and 7 chromium atomsof the Cr404 cluster; 0(ll), H(9) and H(10) con‑stitutes H20 moleclule in this case. The oxygenFig. 3. Molecularly adsorbcd spccics of H20

atom of H20 molecule binds to the Cr(5) atom offor Cr404 cluster (Cr4041tyPell)the cluster. The bond length of Cr(5)‑0(ll) is

2.002 A. This is slightly longer than the bond length of Cr(5)‑0(1), 1.9458 A, of the cluster
frame.As for adsorbed H20 molecule, the bond lengths of 0(ll)‑H(9) and 0(ll)‑H(10) are12

prellmlnary Study of H20 AdsorptlOn On Cr203 and A1203 Surfaces by Ab In山o Cluster CalculaLIOnS

1.005 and 0.965 A and the angle ∠H(9)0(ll)H(19) is 109.6 0･ Althoughthe structure ofadsorbed H20(a) was slightly deformed, the bond length and the bondangle of the adsorbed
H20 molecule was almost the same as those of free H20 molecule (0.967 A and 108.34 0),

indicating that the interaction between Cr(5) and 0(ll) is weak enough. This type of adsorbed
species is denoted as Cr404‑type‑1.

Fig･ 4･ Dissociatively adsorbed H20I form‑

Fig･ 5･ Dissociatively adsorbed li20, form‑

ing Cr(6)‑OH and 0(3)‑H bonds (Cr404‑ ing Cr(5)‑OH and 0(8)‑H bonds (Cr404‑
type‑2)

Fig･

6･

type‑3)

Dissociatively

adsorbed

H20,

Fig･

7･

Dissociatively

adsorbed

H20,

form‑

forming Cr(5)Cr(7)‑OH and 0(8)‑H bonds ing Cr(7)‑OH and 0(8)‑H bonds (Cr404‑(cr404‑type‑4)

type‑5)Figure 4 illustrates schematic views of dissociatively adsoibed H20 on the Cr4041Cluster. rtshows the formation of Cr(6)‑0(9)H(10) and 0(3)‑H(ll) bonds･ In this case, 0(9) corresponds
to the oxygen atom of H20 and 0(8) does the one of Cr404 cluster･ The bond length of Cr(6)‑o(9) is 1.774 A. The bond length of 0(9)‑H(10) is O･967 and that of 0(8)‑H(ll) 0･972 Å･

The energy of adsorption was evaluated to be ‑168･2 kJ/moll This type of adsorbed species
is denoted as Cr404‑type‑2･ Figure 5, which is denominated as Cr404‑type‑3 hereafter, also
shows dissociatively adsorbed H20･ In this case, the OH
fragment of H20 bound to the Cr(5) atom. The bond length of Cr(5)10(10) is l･850 Å･ Those

13

Table 3. Location of H and 0 atoms for Cr404‑H20 system in Z‑matrix form
type/energyF

ﾖﾂ(kJ/mol)剪Cr404‑type‑1 儉r

‑92.9

冗(9)

ﾆﾂ

atom2 免V誣F

atom3

中FVw&VR
0(ll)

Cr(5)

ﾆﾂ

ｳ田R

1.005

"

Cr(5)
"コ

誚ﾆR

atom4 芳昧VG&

ﾂ

中FVw&VR
#R縱R

81.91

0(1) 都R紊B

0(1) 蔦

‑1.84

Cr(2) 蔦Sゅ

Cr404‑type‑2 狽0(9)纉cr Cr(6)#"縱B 0(4) 免ﾂ b0(9)"ッ 1.774
ィ 108.94
モ ‑127.95‑168.2 狽 ﾆﾂ 0(3)纉s" Cr(6)r緜r 0(4) 蔦 C偵湯

Cr404‑type‑3 儉rヲ

0(10)

纉c2

Cr(5)

3

0(10)
"コ
ﾆﾂ 0(8)1.850
纉cr

129.91Cr(7)2

Cr404‑type‑4 狽ヲ 0(ll)
0(ll)
"コ

纉sb
1.923

Cr(5)

‑259.8 狽‑297.9緜""0(1) 塔B緜

‑158,630(4) 都2緜"經2 0(1) 蔦3B貳ﾂ
105.47"‑70.04

ﾆﾂ Cr(7)ヲ Cr(5 鼎 緜H(10)モ 0.967"ビ 114.84

Cr404‑type‑5 狽0(9)纉cb Cr(7)偵0(9)"ビ 1.837
モ 111.04‑338.1 狽 ﾆﾂ 0(8)
纉cr 0(3)經r0(3) 蔦s""0(8)147.400(4) 蔦

s偵S"

of 0(10)‑H(9) and 0(8)‑H(ll) are is 0.963 and 0.967 A, respectively. The adsorption energy
wasgiven as ‑259･8 kJ/mol･ The thrid type of dissociatively adsorbed scecies is shown in Fig･6,
where the oxygen atom of water binds to both Cr(5) and Cr(7) atoms･ The bond lengths ofo(ll)‑Cr(5) and 0(ll)‑Cr(7) 1.923 and 2.089 A, respectively. They are only slightly longer

than the respective bond length of Cr404‑type‑3, 1.94580 A.As for the lenghof OH bond, it
was o.976for 0(ll)‑H(9) and 0.967 A for 0(8)‑H(10). The adsorption energy for this type
was ‑297.9 kJ/moll This type will be denoted as Cr4041type‑4･ The last type shown in Fig･7
was found to be most stable with the adsorption energy of ‑338･l kJ/mol･ The oxygen atom ofwater binds to the Cr(7)with the bond length of 1.837 A for 0(9)‑Cr(7), and those of 0(10)‑H(9) and 0(8)‑H(ll) are 0.9665 A and 0.9671 A, respectively. Tllis type is denominaLed asCr404‑type‑5. The structures of adsorbed species for Cr404 clllSter are Sllmmarized in Table3.

4.3. Adsorbed St,ates of H20 for A1404

Adsorbed species of water for A14O4 cluster can be roughly divided into four groups withrespect to adsorption energy. Molecularly adsorbed species showed the smallest adsorptionenergy around ‑84 kJ/mol. Figure 8 shows an example of molecularly adsorbed species･ The

oxygen atom of water molecule binds to Al(5) of the A1404 cluster, where the bond length ofo(ll)‑Al(5) is 2.026 A. This is considerably larger than that of 0‑Al (1.85270 A) of A1404
cluster. In addition, the bond lengths of 0(10)‑H(9) and 0(10)‑H(ll) in the adsorbed watermolecule are 1.001 and 0.968 A, and the angle ∠H(9)0(10)H(ll) is 111.450. Althoughthesebond length andangle are slightly larger thanthose of free water molecule (0.967 A and

14

Prelimlnary Study of H20 AdsorptlOn On Cr20i and A1203 Surfaces by Ab Initio Cluster Calculations

lO8･340), Llle bollding beLween 0(ll) and Al(5) does I10te affect noticeably to the structureof H20(a) and t･hen this collld be recognized as molecularly adsorbed spcies. The energy ofadsorption was calculated to be ‑91･6 kJ/moll This species will be denoted as A1404‑typell.
The followlngfigures are illustrations of dis‑sociatively adsorbed species. They could be
divided into three groups with respect to ad‑
sorption energy. Figures 9 and 10 s血ow

typical examples of dissociatively adsorbed
H20･ The丘rst type is the adsorption form‑

ing Al(7)Al(5)‑OH (α)十0(3)‑H (α) (Fig.9),

and the second one Al(5)‑OH (a)+0(3)‑H(α) (Fig･10), These species are denoted

Fig･ 8･ Molecularly adsorbed species of H20

as A1404‑type‑2 and A1404‑type‑3, respec‑

for A1404 cluster (A1404ltyPe‑1)

tively･ For A1404‑type‑2, the bond length ofFig･ 10･ Dissociatively adsorbed H20, form‑
ing Al(5)‑OH and 0(3)‑H bonds (A1404‑
type‑3)

Fig･ 9･ Dissociatively adsorbed H20,

forming Al(5)Al(7)10H and 0(3)‑H bonds(A1404‑type‑2)

Al(7)‑0(10) (Fig･9) is l･746 and that of 0(10)‑H(9) 0･963 A. The 0(10) atomalso binds toAl(5) with slightly larger distance (2･221 A) than that of Al(7)‑0(10)･ The length of 0(3)‑H(ll) is O･976 A･ The adsorption enegy was evaluated to be ‑206.7 kJ/mol. The A1404‑type13

is different from A1404‑type‑2 in the adosprion site (Fig･10),althoughthe adsorption energyof ‑209･2 kJ/mol was obtained for this species, beingalmost the same as that of A14041tyPe12.

In this case, the 0(ll) atom of OH group makes bonding only with Al(5) with the lengthof l1692 A for 0(10)‑Al(5). With respect to H(ll)‑0(3) bond, the distance between theseatoms is O･976 Å･ Figure ll shows another kind of bridged type dissociative adsorption ofwater･ The 0(10) atom of the OH‑group also binds to Al(7) and Al(5), but the bond lenghtsof 0(10)‑Al(7) and 0(1叶Al(5) are l･819and 2･036 A, respectively, begin different from those15

ofA14041tyPe12. In addition, the H atom of water binds to 0(1) with the bond length of 0.967

A. The adsorption energy wasgiven as ‑254A kJ/mol. This type will be called as A14041tyPe1
4. Figure 12 is the schematic representation of the dissociatively adsorbed species having
larger adsorption energy of ‑369･g kJ/moll In this case the oxygen atom of water binds toAl(7) with the length of 1.690 A and the length of 0(10)‑H(ll) is 0.960 A. The dissociatedH atom binds to 0(8) atom of the length at 0.965 A. This type is designated as A1404‑type‑
5. Those structures of adsorbed species obtained for A1404 cluster are summarized in Table 4.

Dissociatively adsorbed H20,

Fig. 12. Dissociatively adsorbed H20, form‑

forming Al(5)Al(7)‑OHand 0(1)‑H bonds

ing Al(7)‑OH and 0(8)‑H bonds (A1404‑(A1404‑type‑4)

type‑5)

Fig. ll.

TAble 4. Location of li and 0 atoms for A14041H20 system in Z‑matrix form
type/enel.gy

F(kJ/mol)ﾓ

at,om2 免V誣F剪

A1404‑type‑1 狽 ﾆﾂ
H(9)
‑91.6

atom3

中FVw&VR0(10)ﾃ田 Al(5)
1.001
ﾂコ

Al(5)

#b

0(1)

A1404‑type‑2 狽ヲ
0(10) 0(10)
ﾂビ

ﾃ田2
1,746

Al(7)
ﾂコ

‑206.7

ｆﾆﾂ Al(5) 0(3)A1404‑type‑3 狽ヲ 0(10)0(10)
ﾂコ‑209̲2 狽 ﾆﾂ 0(3)A1404‑type‑4 儉rヲ
0(10)

‑254A

0(10)
ビ

A15
班(ll)

#

纉c
1.692
纉sb

0.967

atom4 芳昧VG&#b經 0(1)
87.17

都偵3

0(4)

3b52.76" Al(5)ィ纉sb 1(7) 0(8)

ﾂ

ゅs"
3.93
x

ｳS

S"緜B
158.79ゅsb コ縱B 0(4)

sR紊R

Al(5)
#b纉B 0(1) 澱 B
ィ 126.55
148.470(4)CR紊B 0(8) 蔦2緜b

纉cB
1.819 A1(7)
ﾂコ

3b

誚ﾆR

中FVw&VR

A17 鼎
ﾂコ

3"繝"
Al(5)
SR
46.84
ィ 156.53
緜0(4) 蔦
C偵

120.60Al404‑type‑5 狽 ﾆﾂ 0(10)
纉c
Al(7)
3
0(10)
ﾂビ 1.690
モ 133.47‑369.9 狽ヲ 0(8)
纉cb 0(3)"纉r0(8) 鉄" 2ィ ‑139.03
0(4) 蔦 sゅ3B

4.4. Inspection of ab initio calculations by TDS spectra of H20 for Al203

16

Prellmlnary St,udy of H20 AdsorptlOn On Cr20! and Al203 Surfaces by Ab ZnlLio Clust,er Calculat,10nS

TllC ab miLJLO CalculaLions described above showed that several different types of adsorption

are possible for both Cr203and A1203. However, there appeared no simple straightforward
relations between the energy of adsorption and structure of adsorbed speciesI It appears, es‑
pecially for dissociative adsorption, that the adsorption energy largely change with the siteof binding of oxygen in dissociated OH‑group and that of fragemiH adsorption･ This fea‑
ture might be due to the cluster models as well as the model chemistry and basis functions
used adopted in the present study. With this respect, the above mentioned ab iniiio calcula一
七ions should be examined with experimental results to inspect the reliability of calculalations･

Thermal desorption spectra (TDS) are useful for this purpose･
Desorption of an adsorbed species from a surface under continuous pumplng Can be described
aS

･(i)

‑

V(響)

+

sp(i),(2)

where N(i) is the desorption rate and P(i) the pressure in the system having the volume of V
at time i; S is the pumping speed of the system. In the extreme case of V(dP/dt) <{ SP, theequation can be simplified as
N(i)‑SP(f),(3)and then P(i) is proportionalto N(i), providing the change in N(i) with time･ The desorption
rate, N(i), is described asdn(i)N(i) ‑ ‑｢㌃ ‑ u n(i)a expl‑Ed/W],(4)

where n(i) is the number of adsorbed

molecules at time i, u the frequency factor,
Ed the activation energy for desorption, α theorder of reacLion, 1mlle gas COnStant and T
the temperaT,llre. Thermaldesorption spec一もra are observed when the sample is heatedwith a function of time. In the case of linear
temperature ramp, i.e., T ‑ To + βt, where

To is the starting temperature, P the rate of400

5∝) 6CX)

700

8CO gce 1(0011(刀temperaturerise, and i the time, a desorp‑

Fig. 13. Thermal desorption spectrum oftion peak appears at Tp that is glVen aS

H20 from as‑recieved Tlalumina; the boldline demonstrates the observed and thinline does t,he first order calculated spec‑

REip2 ‑ (芸,expl‑藷(5,‑ (管)exp上蓋] (6)trum for the first peakwith u ‑ 2.0 ×

108/see and Ed ‑ 91.6 kJ/mol.

Tp is the peak temperature and no the number of adosrbed molecule initially present. Eqs･(5)and (6) are valid for the arst Order and the second order desorptJion, respectively･17

Figure 13

shows a desorptioII SpeCtrullof water observed for as‑recieved l′‑alumina

with a temperature ramp of 10 K/min.囲,

where the abscissa represents the tempera‑
vJau qsIP(OL)〇･(S)lV

ture in K, and the ordinate the signal inten‑
sity of m/e ‑18 0f mass spectrometer in ar‑bitrary unit. The circles and bold line show

the observed desorption cllrVe Of H20･ There
乱rised three peaks about 500, 600 and 900

K. Since details of the measurement, i.e., the
09

1

ll

0(1 0)‑H(9) distance/A

Fig. 14. PES (PotentialEIlergy Surface) for
H201A1404‑type‑1; the difference in the levelof countour lines is 0.1 eVvolume and pumplng Speed, are not known,

it is assumed here that the pumping speed

was large enoughto validate Eq.(3). AL
though this should not be true, qualitative

analysis could be done under this assump‑tion. On account of the ab initio calculations, the丘rst peak ascribed to moleculary adsorbed
water and then this should obey the丘rst order desorption kinetics･ Accordingly the activationenergy, Ed, could be estimated from Eq.(5) by adopting the frequency factor to be u ‑ 1013 ‑

1014/see as commonly accepted･ The value thusobtained was 130 ‑ 140 kJ/moll The thinline in Fig.13 is the desorption spectrum丘tted to the observed one by by setting α ‑ 1,
i, ‑ 2.0 × 108/sec, T ‑ 293+ lot and Ed‑ 91.6 kJ/mol. for Eq･(4)･ In this case, the activationenergy for desorption was assumed to be the same as the energy of molecular adsorption

evaluated the ab inilio calculation. To examine the validity of this assumption, PES (Potenrial

Energy Surface) was calculated by varying the distance of 0(10)‑H(9) and that of 0(10)‑Al(5)Corredponding to Fig･8･ The result is shown in Fig･14, where the abscissa represents the

distance of 0(10)‑H(9) and the ordinate that of 0(10)‑Al(5) in A unit･ There appeared no
noticeable suddle point. Hence the activation energy for desorption can be safely assumed
the same as the adsorption energy for A1404‑type‑1. However】 the values 9116 kJ/moll and

2.0 x 108/see. for Edand I/, respectively, are too small incomparison with those obtained fromthe conventional TDS analsys. In addition, the calculated desorption peak is too narrower

than the observed first peak. With respect to the considerabley narrower peak, it should be
mentioned that other types of molecularly adsorbed water with di鮎rent adsorption energies

can be exist on different crystalplanes. It should be also mentioned that the peak boardening
could arise from the VdP/dt term in Eq.(2), which causes a desorption peak in "N(i)一七" tobe weakened and broadened, especially at the tailing part of "P(i) ‑ i" spectrum･

Asfor the fairly small value of u for calculated desorption spectrum arises from the smallvalue of Ed. It appears due to the model chemistryand basis functions used as well as cluster
size. tt should be also mentioned that the observed spectrum was obtained for l′‑alumina,

18

Preliminary Study of H20 Adsorption on Cr203 and A1203 Surfaces by Ab ZnLtio Cluster CalculatlOnS

whereas Llle CalculaLioII Was dolle for α‑alumia. Further examinations accounting those factors

are needed in additi()a to much well de丘ned TDS measurements.

Figure 15 is a desorption spectrum of water from部 の Fi a 1

α‑Cr203 Surface (001) at a temperature ramp of

2 K/see after the water exposure of 8.9 xIO14
molecules/cm2 at 120 K, where the dots were thedata points read from the paper by Hernderson
and Chambersl14]. But the abscissa is in arbi‑
trary unit, since the paper gives it as signal in‑tensity of m/e‑18 0f mass spectrometer. The
L●●竺≡≡｣ peak temperature, however, was invariant with
250

300

350

4CO

450 the amount of exposure,and hence the desorp‑

Fig･ 15･ Thermal desorption spectrun of Lion obeys the first order kineticsI The best val‑
H20 from α‑Cr203; the dots were read丘･om ues to reproduce the desorption peaks shown inHenderson's reportl14]and thin line are the their paper were reported to be i, ‑ 1012/see andfirst order calculated spectrawith u = 3 ×

Ed ‑ 89 kJ/moll The adsorption energy of‑92･9

1 ′ヽ111013 and Ed ‑ 92･g kJ/mol.,respectively.
kJ/mol, Obtained in the present study for themolecular adsorption is quite close to the value of 89 kJ/mol. for Ed. Then the peak wasanalyzed by applying Ed ‑ 92.9 kJ/mol. in the same way as the first peak of water foralumina
as mentioned above. The solid line in the figure is the calculated spectrum assuming that thefrequency factor and activation energy for desorption are 3 x lO12/see and 92.9 kJ/mol,
respectively, where the activation energy was assumed to be the same as the adsorption energy

for molecular species evaluated for Cr4041Cluster (see Table.3). According to a preliminay
calculation of vibration, the stretching vibration of the H20 molecule corresponding to thedesorption'was evaluated to be around 1012/see for this species. The figure shows that thecalculated spectrum coincided unexpectedly well with the observed one. It should be mentioned
here that the values of 89 kJ/mol (Ed) and u ‑ 7 × 1012 reported in their paper[14】almost
exactly overlaps with the solid line in this丘gure. The slight disagreement at the front part of thepeal( is due to the presence of another kind of desorption speciesl14]. As for the disagreementin the tailing part, it should be ascribed to the contribution of V(dp/dt) term in Eq.(2),althoughthe V/S ratio appears quite small in this case. These results indicate that the abinitio calculations for small clusters are valid as far as the molecular adsorption of water on(001) surface of α‑Cr203 COnCerned. With respect to other species appeared in their paper,

inspection of calculated absorption energies could not be done owing to the lack of data suchas the H20 pressure, volumeand pumping speed of the system as well as the number ofmolecules desorbed. Iiowever, the calculated adsorption energies for other adsorbed species

are considered meaningfull.
Asfor tlle Calculated results for α‑alumina, calculations for slnall clusters are consideredalso

19meaningfull by accouning that the bond strengtl"f Cr10 is not so much different from that of

A1‑0[15], suggesting that the adsorption energies for Cr203 do not differ much from those for
A1203, being consistent with the results that similar adsorption energies were obtained between

Cr404 and A1404 clusters. More detailed ab inilio studies, however, should be carried out
for adsorption/desorption kinetics along with observations of adsorption/desorption processes･

Concerning the relation bctwcen the structure of adsorbcd species and the adsorption cncrgy,

effects of different model chemistry and basis functions should be examined in detail･ In
addition, studies on larger clusters arealso required on account of the size effect on this kindof calculations[16, 17, 18, 19], by which the structure as well as the energy of the system vary
witII Cluster size.

5. Conclusions
A prelimiary ab initio study was carried out for adsorption of water on Cr203and A1203

surfaces by adopting small clusters as Me404 (Me‑Cr,Al) by use of Gaussian 03 package･
The Me404 clusters were embraced by assuming (001) surfaces of Cr203 and A1203･ It was
found for both of the clusters that molecluar and dissociative adsorption take place, where

the molecular adsorption preferentially occurs on a Me atom and the dissociative adsorption

takes place through forming Me'(or Me2)‑OH + 0‑H bonds･ In the latter case, one kind of‑OH bonding lS an On‑top type and the other is a bridged type･ Adsorbed water could beroughly divided into five groups for the Cr404 cluster and four groups for A1404 cluster withrespect to the adsorption energy. There appeared a feature that the bond length between Me

atom(S) and 0 atoms of water increases in the order of Me10H (on‑top) < Me2‑OH (bridged)< Me‑OH2 (molecular adsoption). But no straighforward simple relations could not be foundbetween the structure of adsorbed species and adsorption energy.

Adsorption energiesgiven by the ab mitio calculations were examined throughthe analysisof thermal desorption spectra of H20 for l′‑alumina and (001) surface of α‑Cr203･ Although

the TDS spectra foralumina could not be well explained by the cluster calculations, it canbe ascribed to the fact that the TDS spectrum was observed for l′‑alumina, whereas thecalculation was done for α‑alumina. In addition, it is considered that the model chemistry

and basis functions as well as the cluster size were inadequate for this system. On the other
hand, the TDS specrum for α‑Cr203 could be quite well reproduced by use of the calculated

adsorption energies.

However, detailed features could not be consistent between the calculated energleS and theresults of TDS analysis. One of the reasons is the TDS spectra used, which were not N(i) ‑ i
curves but P(i) ‑ i curves, On accounting the signal intensity of m/e ‑18 to be proportional towater pressure. Another reason might be the calculations that were carried out for small cluster
as Me404 (Me ‑ Cr or Al). Effects of model chemistry and basis function should be examined
andalSo the size effect on this kind of calculatioIIS Should be studied to clarify relations between

20

prellmlnary Study of H20 Adsorption on Cr203 and A1203 Surfaces by Ab Initio Cluster Calculations

sLrucLurc of adsorbed species alld adsorptioll energy aS Well as kinetic processes･

Ack nowledge me nt
This work was partially supported by a Grant‑in‑Aid for Scientific Reserach on Priority

Areas, 476, nirium Scienceand Technology for Fusion Reactors, The MEXT･
Referenc esl1] Y. Hatano, T. Maetani, and M. Sugisaki, Fuswn Technol･, 28 (1955) 1182
[2] J. E. Castle and CI R･ Clayton, Com10Sion Sci･, 17 (1997) 7[3] K. Watanabe, M･ Matsuyama, T･ Kobayashi, and A･ Sakamoto, Ann･ Repl･ Hydrogen
Isol. Rea一 Ctr･, 16 (1996) 33[4] K. Watanabe, M･ Matsuyama, T･ Kobayashi,and W M･ Shu, Fusion Engn･ Design, 39‑40(1998)1001[5〕 Y. Sakamoto, BS thesw, Department of Chemwtry, Fucaluty of Science, Toyama Univ･,

1992[6] T. Kobayashi, MS Thesis, Department of Chemistry, Fucaluly of Science, Toyama Univ･,
1993[7] Y. Torikai, R‑D･ Penzhorn, M･ Matsuyama, and K･ Watanabe, Fusior7J Sci･ Technol･, 48(2005)177[8] Y･ Torikai, M･ Murata, RID･ Penzhorn, K･ Akaishi, K･ Watanabe, and M･ Matsuyama, J･
Nucl. Maier., 363‑365 (2007) 462[9] R.‑D. Penzhorn, Y･ Torikai, M･ Matsuyama, and K･ Watanabe, J･ Nucl･ Mater･, 353(2006)66[101 M･ J･ Frisch, G･ W･ Trucks, Il･ B･ Schlegel, G･ E･ Scuseria, M･ A･ Robb, J･ a Cheese‑man, Jr･ J･ A･ Montgomery, T･ Ⅵeven, K･ N･ Kudin, J･ C･ Burant, J･ M･ Millam, S･ S･

Iyengar, JI Tomasi, V･ Barone, BI Mennucci, M･ Cossi, G･ Scalmani, N･ Rcga, G･ A･ Pc‑
tersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R･ Fukuda, J･ Hasegawa, M･ Ishida,
T. Nakajima, Y. Honda, 0. Kitao, H. Nakai, M. Klene, X･ Li, J･ EI Knox, HI P･ Hratchian,J. B. Cross, C. Adamo, ∫. Jaramillo, a Gomperts, R･ ll Stratmann, 0･ Yazyev, A･ J･

Austin, a. Cammi, CI Pomelli, J･ W･ Ochterski, P･ Y･ Ayala, K･ Morokuma, G･ A･ Voth,p. Salvador, J･ J･ Dannenberg, Ⅴ･ G･ Zakrzewski, S･ Dapprich, A･ D･ Daniels, M･ C･ Strain,

0. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J･ BI Foresman, J･ V･ Ortiz,Q. Cui, A. G. Baboul, S. Clifford, ∫. Cioslowski, B･ B･ Stefanov, G･ Liu, A･ Liashenko,

21

P. Piskorz, I. Komaromi, 氏. L Martin, D. J. R)X, T. Keith, M. A. Al‑Lallam, C･ Y･ Peng,

A･ Nanayakkara, M･ Challacombe, P･ M･ W･ Gill, B･ Johnson, W･ Chen, M･ W･ Wらng,

C. Gonzalez, and J. A. Pople, GaussianO8, RevisionB.03, Gaussian lnc･and Pittsburgh,
2003[11] JCPDSIJCDD, pdf184‑1616[12] JCPDSIJCDD, pdf‑81‑2267[13日Eleen Frisch, Micahel J･ Frisch,and Gary W･ Trucks, GaussianO3 User's Referlencc･

GallSSian lnc. Carnegie, PA 15106 USA, 2003[14] Michael A･ Hendersonand Scott A･ Chambers, Surface Sci･, 449 (2000) 135[151 1n R. C. Weast, editor, CRC Handbook of Chernislry and Physics, 62nd ed･ The Chemical
Rubber C0., 1971[16] R. Broer, I･ P･ Bartra, and P･ S･ Gagus, Ph%l･ Mag･ B, 51 (1985) 243[17] E. M･ Fernarndez, R･ I･ Eglitis, G･ Borstel, and L･ C･ Balbas, Comp･ Mater･ Sci･, 39(2007) 587[18] E. Penev, P･ Kratzer, and M･ Schemer, J･ Chem･ Phys., 110 (1999) 3986
[19] S. Erkoc, T･ Bastug, M･ Hirata, and SI Tachimori, Chem･ Phys･ Letters, 321 (2000) 321

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