富山大学水素同位体科学研究センター研究報告26 : 1 ‑12, 2006.

論文

Kinetics of Methane Decomposition over Zr‑Ni Alloys
K･ Watanabe, W･ Shu* and M. Matsuyama
Hydrogen Isotope Research Center, University of Tbyama
Gofuku 3190, Toyama 93か8555,.Japan* Present AfBliation: Tritium Process Laboratory, JAERI, Japan

Received May 18th, 2007; accepted June 29th, 2007

ABSTRAC甘The decomposition of methane over Zr, Zr4Ni and Zr2Ni was investigated to develop highly
active materials for capttLring tritiated methane inevitably formed in tritium handling sys‑
tems･ The entire decomposition or absorption curves, however) Could not been described byany simple kinetic equations appearlng ln the literature. The present paper describes veri&‑cation of plmsible kinetic equations reproducing the observed absorption curves) assuminga reaction mechanism consisting of a progressive removal of hydrogen atoms according toCH4(9) ‑ CII3(a) ‑ CH2(a) ‑ CH(a) ‑ C(a) alld its modi丘cation including carbon seg‑regation schemes by solving a set of kinetic eqllations by means offinite diqerence method,by talking into account of ab‑initio calculations of potential energy surra･ces by Gaussian 03.
It was found that a step‑by‑step H‑deletion model could not generate the experimentallyobserved absorption cllrVeS, btlt they collld be reproduced qlliLe well ill the whole reaction

range by a modiGed reaction scheme assuming coagulation of a carbon residue like CH2(a)or cH(a) to carbonaceous deposits, described as CH｡(9) ii CH3(a)曳cH(a)曳C‑deposits.

It was conchlded that thefinalthird step (with a rate constant of k3) governs the overall

absorption reactivity (methalle COnSunlption beyond 90%) of the materials.

1. Introduction
In tritium handling facilities including those processmg tritium for thermonuclear reactors,
tritium gas is contaminated by many lmpurity gases gel‑e一ated during tritium handling･ Typicalgaseous impurities found in various tritium systems are CQ4, CO, CO2, 02, N2, NQ3 and Q20,
where Q denotes H, D or T atom･ They should be removed from the gaseous stream and tritium
recovered from the impurity gases should be made available for reuse or stored in stable form.

Metallic compounds consisting of Zr and some transition elements have been proved to be quite
promising for storage, supply and recovery of tritium in tritium handling systemslll. They caneasily absorb tritium not only in the elemental form but also in form of waf,er, ammonia andgaseous hydrocarbons as well as non‑radioactive impurity species such as CO, CO2, 02 andN2･ Among them it is most difBcult to recover tritium from hydrocarbons, especially methane･In general, tritium included in hydrocarbons could be reco､rered by catalytic isotope exchange
reactionsI but the procedur岱are rather complicated･ Some of metallic compounds consistingof Zr and transition metals (hereafter denominated as Zr‑alloys) can beused to decompose
hydmcarbons and capture tritium at the same time･ Tb decompose hydrocarbons and to
capture tritium bound to carbon) however) it is required to heat up the metallic compounds

渡辺凶昭･洲亘･松山政夫

to rather high temperature･ Among the hydrocarbons, methane is most inactive with these
metallic compounds and requires higher temperature than the others[2, 3】.

The present authors have investigated the usefulness of Zr‑alloys for handling tritium gas
containing gaseous impuritiesll]and found that Zr‑Ni alloys are most promising for removing
tritium from tritiated methanel1, 4, 5]. According to the studies, Zr4Ni is most active fordecomposing methane among other alloys as Zr2Ni, ZrNi and Zr7Nill and pure Zr･ In thesestudies, however) the activity was evaluated from the linear part of ln P vs time plots of theabsorption curves･ But the absorption curves themselves were quite complicatedand did notobey the丘rst order kinetics nor other analytically known kinetic equations･ It is considerably

important, however, to analyze the absorption curves as low as 0.1% of the initially loaded
amount of methane or below to predict the behavior of methane decomposition and the remol′al
to a trace level, b∝ause the permissible level of tritated methane in the environment is verymuch lower than that of other chemical forms of tritium.
The present paper describes a model fわr analyzlng the absorption curves of methane bv
ZトNi alloys to obtain relevant kinetic parameters. Simulated absorption curves by use or thesekinetic parameters were compared with the observed ones.

2. Experimentals

Althoughthe experimentalapparatus, procedures and results obtained with some Zr‑Nialloys have been d6Cribed in previous papers【1, 4, 5j, a brief description of experimental

procedures is glVen below for readers convenience.

The sample materials used were Zr4Ni, Zr2Ni, ZrNi and Zr7Nill and pure Zr･ X‑ray diffrac‑tion measurements showed that while Zr2Ni, ZrNi and Zr7Nill Were Single‑phase metallic com‑pounds? Zr4Ni consisted of a mixture of Zr2Ni and Zr･ They were used as powder of 200

mesh (below 74 pm)･ According to BET measurements using Kr, the specific surface area was
O･30, 0･24 and O･10 m2/g fわr Zr7Nill, ZrNi and Zr2Ni, respectively. Decomposition/absorption

curves of methane over these materials were measured by constant volume method, and theabsorption curves weregiven by the pressure as a function of time, P(i) ‑ i, where 0.5 gramsof powder were used fわr all the samples. Prior to the absorption measurements, the sample

powder was heated at 423 K for 2 hours under vacuum and then at 873 K for 2 hours as thestandard activation procedures in the apparatus whose residual pressure was routinely below
2.67× 10‑6 pa.

Methalle absorption/decomposition curves were measured by constant volume method, where
the initial pressure was set at 13.3 Pa in the reaction cell of 173 cc.

3. Results and Discussion

3.1･ Methane Absorption Curves

2

Kinetics of Methane Decomposition over Zr‑NiAlloys

Figure 1 shows the absorption curves of

10･5畑m防

一〇∈∈＼ueL1 0∈101unOEV

methane for Zr, as example, at temperatures

I‑､

箸

■､

from 350 to 550 C, where the abscissa shows■rj+ 凵｡lthe reaction time in second and the ordinate

0

ﾂ

0.,0
ノl

ﾈ

(

ｲ

JT

the number or methane molecules in mol in●

the reaction vessel of 173 cc. As seen in the僂0
一=一= 凵戟R

figures, the number of methane molecules in‑t,he reaction chamber decreasedwith reac̲

0

辻

5∝)

1000

1500

2000

2500

30∝)

3500

Reaction time /stion time. It is evident, however, that theabsorption curves do not obey simple first●■■

Fig. 1. Absorption curves of methane for Zr at
350, 400, 450, 500 and 550 C

order kinetics. It is also seen that the charg‑ed methane amounting to 1 mmol almost disappeared from the gas phase at hightemperaturesin these runs. Thus, the total amount of methane reduction exceeded the total number ofsurface site on the Zr sample. For this condition it was assumed that the density of active sites

is of the order or 1014 sites/cm2. This indicates that methane is decomposed to hydrogen andcarbon or carbon residues. As will be shown below, whereas hydrogen is absorbed in the bulkof the sample, carbon remains on the surface. This is also true for the other sample alloys.
The feLCt that the absorption curves do not obey simplefirst order kinetics suggests thatthe decomposition of methane takes place in a series of consecutive reactions. Accordingly theabsorption curves should be described by a succession of elementary reactions･

3･2. Elementary Reactions on the Surface
The elementary reactions are considered to consist or a dissociative adsorption of methane

and a subsequent dehydrogenation or the adsorbates. R)r example, a methane molecule isexpected to be adosrbed on Zr‑surface throughthe followlng elementary reactions as:(Zr U lH'川U dZuLU

CH4+4Zr→

ZトCH+3ZトH

CH4+3Zr→

ZトCH2+2ZトH

CH4+2Zr‑

Zr‑CH3+ZトH

1 2 3
ーヽノ

ー

With respect to the subsequent steps, the elementary reactions described below should betaken into account;

ZトCH+ZrーZトC+Zr‑H

Zr‑CH2+2Zr‑

ZトC+2ZトH,

or ZトCH2+Zr‑

ZトCH+ZトH

ZトCH3+2Zr‑

ZトCH+2ZトH,

or ZトCH3+Zr‑

ZトCH2+ZトH

45678( ー (((

3)) ー ー )渡辺国昭･洲亘･松山政夫

Namely, the kinetic parameters of the reactions from (1) to (8) are required to analyze themethane absorption curves･ Tb the author's knowledge, however, no such kinetic parameters

have been reported so far･ Accordingly the preference of competitive elementary reactions
such as (1) ,(2) and (3), for example, was examined by at,inito calculation of potential energy
surface by using Gaussian O3[6】, and it was assumed that the most likely reaction is theelementary reaction having the smallest activation energy･ The calculation was carried out by
uslng Small clusters of Zr hcp crystal, where the details of the calculation have been described
elsewhere【7, 8】.

3･3･ Potential Energy Surfhcesfor methane adsortion3

vJoc9.0+lエ6!￠〓

5

vJoc9.0+lL16!aH

つ山

7ー 】一

1

1.5

2

2.5

3

′b

4

1

3.5

1.5

2

2.5

3

3.5

rXH/A

rHXノA

Fig. 2. Cluster model and contour map ofFig. 3. Cluster model and contollr map Of

PES for Zr4‑CH4

PES for Zr3‑CH4

The potential energy surface (PES) for theadsorption of methane was calculated by

3 2 2 2 2 ｢▲23 8 64 2 2 8

adopting small clusters fわr modeling the Zr(1000) surface, where a set of four Zr atoms
was selected for reaction (1), three Zr‑atoms
for reaction (2) and two atoms for reaction

≦〓LNL

(3).

Figrues 2, 3 and 4 show the cluster models
and contour maps of PES fわr Zr4‑CH4, Zr㌻
CH4 and Zr2‑CH4, respectively. The丘rst one

corresponds to elementary reaction (1), the1

1.5

second to reaction (2)and the third to reac‑tion (3). The model cluster for reaction (1)consists of four Zr atoms, where one Zr atom

4

2

2.5

3

3.5

rCH /A

Fig. 4. Cluster model and contour map of
PES for Zr2‑CH4

Kinetics of Methane Decomposition over ZrNi Alloys

corresponds to the center atom of the (0001) plane of hcp‑Zr, and the other three atoms are
selected from every other site of the hexagonal･ For reaction (2), the Zr‑cluster was made by
selecting three Zr atoms from a basal axis of the plane, for example l11雪o]. with respect to thelast one, a set of two Zr atoms of nearest neighbor on (0001) plane forms the adsorption site.
PES surfaces for reactions (1)and (2) were calculated by changing the distance between the
dummy atom (shown by the purple sphere) and Zrl atom (to be denoted as height hereafter),and that betw能n the dummy and a hydrogen atom (H3 for reaction (1) and lilO for reaction(2))･ As for reaction (3), PES was calculated by changing the distances of CZrand (}H. In
the PES diagraIm Of Figs･2 and 3, the abscissa represents the separation between the dummy

and the hydrogen atoms (denoted as rXH below), and the ordinategives the distance between
C and Zr atoms, where the unit of both the abscissa and the ordinate is given in A. In thesecases, the distance between C and Zrl atoms is given by the height + 0.630 A Concernlng
the PES in FigA, the abscissa shows the bond distance of C‑H, and the ordinate that of CIZr.
It is seen in the contour maps that no apparent potential well appears fわr methane toapproach the surface for reactions (1) and (2), or the potentialwell is very shallow as seen in Fig.

2. These results suggest that no stable adsorbed state of CH4 is formed. Methane molecules
approaching the surface are reflected or surmount the potential barrier to be dissociatively
adsorbed･ The potential barriers fわr the dissociative adsorptioI1 0f CH4 Were evaluated to be
2･35, 3･72 and 2･59 eV fらr reactions (1), (2) and (3), respectiヽ℃ly. Similar calculations werecarried out for reactions (4) ‑ (8) and gave the activation energies of lA5 eV for reaction (4),3115 eV for reaction (5), 1･14 eV for reaction (6), 1.41 eV for reaction (7) and 2.07 eV forreaction (8).

3･4･ Analysts Of Absorption Curves
3･4･1･ Examination of kinetic equations
According to the results described above, a reaction scheme consisting of reactions (1) and(4) Was considered to be probable. Accordillgly the methalle absorptioll by Zr Would proceed
via the followlng ProcessescH｡(9)+4S

土.

S‑CH(a) + 3 S‑H(a)ncTin(CH4)

S‑cH(a) + S

一生

(9)

nli

S‑C(a) + S‑H(a)

(10)

nC

sIC(a) dlffusi:n‑into btdk

C(soln)(ll)

S‑H(a) diM空n‑Ttobtdk H(･50ln)(12)where S denotes an active site for adsorption, and (g) and (a) represent gaseous and adsorbed

states, respectively, The final products of methane decomposition, i.e. C(a) and H(a), aresupposed to diffuse into the bulk.

The diffusion constant of carbon into Zr in the temperature range adopted in the present5渡辺陸川73 ･洲亘･松山政夫

study is, however, rather small lg]. Therefore the diffusion of carbon into bulk should play noimportant role. In addition, the diffusion of hydrogen is also considered to play a negligible
role on the absorption kinetics, because it is extremely fast under the present experimental

conditionsl10]. Consequently, the following equations were expected to describe the methane
absorption curv錯by Zr:(笠) ‑ klnCH4(so‑ncH‑nC)4(%) ‑ klnCH4(so‑ncH‑nC)4 ‑k2nCH(so‑ncH ‑nC)(Z) ‑ k2nCH(so‑ncH‑nC)
where so represents the number of active sites on the surface and nx denotes the number ofmolecules of species X (X ‑ CH4, CH, or C)･ Absorption cu‑S wereanalyzed by solving theabove simultaneous di鮎rential equations by finite difference scheme throughtrial‑and‑error

curve fitting, where kl COuld be determined from the slope of the very initial part of respective
absorption curv6 and k2 Was treated as afitting parameter･ It was found, however) t･hat theabove scheme could not reproduce any of the absorption curves･ On account of the requirementof a four‑fold site, Zr4, for the above scheme, this is not unexpected･

According to the PES calCulations, the followlng reaction schemecH｡(a)+2S土もS‑cH3(a)+S‑H(a)
ncH3

nH

S‑cH3(a)+2S曳S‑cH(a)+2S‑H(a)nCHsICH(a)+S

曳S‑C(a) +S‑H(a)ncis also considered plausible･ Consequently the methane absorption process is now described by‑(%) ‑ klnCH4(30‑ncH3‑nCH‑nC)2(19)
ilU iZq ntJU(警) ‑ (klnCH4 ‑k2nCH3)(so‑ncH3 ‑nCH ‑nC)2

し‖llnu IH"ml Hu lMrhu

2 2 2O 1 2(器) ‑ k2nCH3(so‑ncH3 ‑nCH‑nC)2 ‑k3nCH(so ‑ncH3 ‑nCH‑nC)(33) ‑ k3nCH(30‑ncH3‑nCH‑nC)
These equations, however, could notgive any meaningfull solution becau.se of too many un‑
known parameters.

Tb reduce the number of unknown parameters, it was assumed that CH(a) reacts with eachother to form carbonaceous deposits on the surfaJ=e･ In addition) the surface concentration ofCH(a) was assumed to be negligibly small in comparison to that of the others･ Under these6Kinetics of Methane Decomposition over ZrNiAlloys

assumptions, the reaction scheme can be simplified tocH｡(g) +2S生S‑cH3(a) +S‑H(a)n(CH3)n(H)S‑cH3(a) +2S曳S‑cH･(a) +2S‑H(a)n(CH)

2S‑CH(a) A S‑CHn(a)n(cd)

and solved by the fblloⅥng equations‑(管) ‑ klnCH4(so‑ncH3 ‑nd)2(警) ‑ (klnCH. ‑k2nCH3)(so‑ncH3 ‑nd)2農) ‑ k2nCH3(so ‑ncH3 ‑nd)2 lk3nbH

It was found, however, that the above equations could not befitted to any of the experimen‑

tally observed methane absorption curves･ The above equations gave simulation curves whichshowed monotonical reduction of the absorption rate with time; the absorption curves asymp
to乞ically approaching a gl､′en level at an early stage or absorption･ They thus did not reproduce

the latter half of the curves, where the absorption was accelerated. This aJ:Celeration of at>sorption Ⅵ,as typically observed ill the last half stage of the absorption at hightemperaturt器･

3A.2. Modified kinetic modelWith respect to the appearance of the accelerated absorption, it should be mentioned herethat growth of carbonaceous deposits of three‑dimensional forms has been observed for catalytic
reactions of hydrocarbons on metal surfaces【11トOIl account Of such observations, growthof carbonaceous deposits was considered to take place similarly fわr the methane absorption

process on Zr‑Ni alloys. Accordingly it was assumed that the carbonaceous deposits are formed
from CH(a) and C(a) and they take part in the reduction of surface coverage by growing
inthree dimensions. Providing ned be the number of molecule of the carbonaceous deposits
growing Semispherically, its surface coverage should be proportional to n慧2 because the numberof adsorbed and/Or deposited molecules are assumed to be proportional to the area coveredby them ill the above reaction schemes･ The modified equations under these assumptions is
described a5

‑(箸)

(管)

‑

‑

klnCH4(so‑ncH,(klnCH｡‑‑nSi2)2

k2nCH3)(so

‑ncH3

(29)

‑nSi2)2(器) ‑ 【k2nCH,(so ‑ncH3 ‑nS2) ‑ k3nCH](so ‑ncH3 ‑nS2)

(30)

(31)

for the real:tion scheme described by reactions (23), (24) and (25).

7渡辺国昭･洲Figure 5 compares calculated absorption

亘/･松山政夫

一〇∈∈＼OueLJlOE101unOEV

⊥ュ

⊥

⊥

curves for Zr with the observed ones, where

■■ヽ.

‑.｣

the closed circles represent observed data

〇

‑

■●

ﾈ

2

〇

and solid lines show absorption curves simu‑

▼

I

〇

〇

〇

lated from the evaluated rate constants, kl,
1

k2 and k3. The rate constant, kl could bei0determined from the slope of the initial partIof observed absorption curves and the other

0

500

1000

rate constants, i.e. k2 and k3, Were evaluatedby trial‑and‑error fitting. It is seen in this

1500

2000

2500

3000

台500

RQction time /seeFig･ 5･ ComparisoI1 0f simulated absorption

figure that the calculated absorption curves

curveswith observed ones for Zr at 350, 400, 450,agree very well with the observed ones, in‑

500 and 550 C (from upper to lower)

dicating that the above reaction scheme isadequate fわr describing the kinetics of methane

absorption for Zr･ It should be mentioned here that the simulation could also reproduce weu
the convex part or the absorption curⅥ∋s appearlng ill the latter half of the absorption curヽ℃s

at hightemperatures･ According to the above‑mentioned scheme, the acceleration of theabsorption is attributed to the n慧2 term, which is assigned to the growth of the carbonaceous

deposits on the surface iIl three dimensions.

With respect to the methane absorption by Zr4Ni and Zr2Ni, the active cent,ers for theabsorption should not be Zr alone but they are considered to consist of Zr alld Ni atoms･

Nevertheless it Was assumed that the same reaction scheme as the above is valid in these cases.

一OE∈＼Out2LJLOE101unOEV

1U750･∬皿防

A

鳴

【

鳴

i

1

一〇∈∈＼￠Ut2LlIOuJ lOIUnOuJVo o･.o▼■■ヽ､ヽ

'≠●.､ 冓Z

【

≠㌧

●●

o c; C!

O

一一̲̲｣

】

ヽ

1

【

白

苓

一､＼l

ｲ

､ヽ

l

唏

鳴

爾

鼎

l

00
lー

(

ｲ

白

t

0

500

＼
l

0

20

0

40

0

600

Reaction time /sec

∝)0

1000

0

50

1∝)

150

＼
200

250

300

Reaction time /seeFig･ 6･ Comparison of simulated absorption Fig･ 7･ Comparison of simulated absorption

curves with observed ones for Zr,Ni CurVeSwith observed ones for Zr4Ni
Figures 6 and 7 show the results of simulations fわr the absorption by Zr4Ni and Zr2Ni,

respectively, where the procedures to obtain simulation curves were the same aS those adopted

for Zr･ It was found that the kinetic equations from (29) to (31) could also reproduce quite
well the observed methane absorption curves for Zr4Ni and Zr2Ni at different temperatures,

8

Kinetics of Methane Decomposition over Zr‑NiAlloys

3A･3･ Temperature dependence of the rate constants
Figure 8 shows the temperature depen‑

1

●

l

dence of the rate constants, kl, k2 aIld k3,■‑■■‑̲

ー一､■■̲.̲

一一一一

椋‑b

0.1

evaluated from the data analysis fわr theー̲●methane absorption by Zr. All of them

0.01

ー▼

■ヽ

ーrt′■ ｲ

exhibited fairly well linear straight lines
against 1/T, from which the respective acti‑
vation energleS Were Calculated as 70.9, 58,5

㌔

0.001

0.0001

and 125･5 kJ/mol for kl, k2 and k3, reSPeC‑

0.00120.00130.0014

tively･ Although these values are far smaller
than those estimated from PES calculations

0.00150.0016

1/T /k‑1

Fig･ 8･ Arrhellius plot of the rate constants, kl,

by Gaussian‑03, a trend indicating that the

k2 and k3, evaluated for Zr: the dimeIISions areactivation energy of kl is larger than that of

1/mmo12･sec for kl and k2, and 1/mmol.see for k3

k2 is apparent･ The large discrepancy bet‑

ween the activation energies determined from experimental results and from PES calculations
can be ascribed to over‑simplification of cluster models for PES calculations.
⊥̲l ト､‑±70J 冲l .F
o･ 2ぷo･ 1∬巧 8位E]m

ら̲!

冤l

i

l 】 】 免ﾂro o o o 0.‑0 0.

uaS･zJO∈∈＼L＼LJl

UOS.N10uJuJ L＼
‑‑

#

‑‑

"

(+

■｣

‑LN 白
■‑M
】‑I̲Ltl

IL.4l

ll

鳴

000

｢

冓

1､､一

/r 白

葡
｢ゝ.

▼ヽr､

5 つん ⊥

凵RL

峰ﾇ"

1｣､̲一､1.1

i

‑'2
(

B

､､一､ 銅H

論運

*"

唏爾

ヽ ヽヽ

亦

0

rR

辻

l

▼▲

iN o･05
Jl!

】 】 仏"

＼ 鳴

ﾂ

1

0.02
0.COコ2

0.CO130.CX)140.α)150.∝)160.00170.00180.0012

ヽL‑‑

､ヽ

迫Dﾂ

0.00160.0017】}t
0.∝)13

1/T /K‑1

0.00140.0015

1/T /K‑1

Fig･ 9･ Arrhenius plot of kl evaluated for Zr,

Fig･ 10･ ArrhenillS plot of k2 evaluated for Zr,

Zr4Ni

and

alld

Zr2Ni

Zr4Ni

Zr｡Ni

Figures 97 10 and ll show the temperature dependence of the rate coIIStantS, kl, k2 andk3, respectively, that were evaluated for Zr, Zr2Ni and Zr4Ni. It is seen in Fig. 9, that although
Zr2Ni has the largest kl, the values of kl are not SO much different for the various materials(see Figs･5, 6 and 7)･ This is contradictory to the results reported iI" PreViouspaperl1], inwhich a large difference of absorption rate constants was reported for Zr, Zr4Ni, and Zr2Ni.

The large difference is also apparent from Figs･5) 7 and 6, which show the time required for

90% reduction of the charged methane to be approximately 2500, 110 and 550 Seconds at 400oC
for Zr) Zr4Ni and Zr2Ni) respectively･ In addition, according to the present data analysis) the

activation energy for dissociative adsorption of CH4 differs among the three materials; it wasdetermined to be 70･9, 37･2, and 55･3 kJ/mol fらr Zr, Zr4Ni, and Zr2Ni, respectively. These9渡辺国昭･洲ItjI ･松山政夫

results are contradictory to the previous paper回, where the activation energy fわr methane

absorption was evaluated to be around 49 kJ/mol for all of these materials･ This is because
the apparent absorption rate constants, k, Were determined from the slopes of the first partof Imp vs i plots in the previouspaper, and accordingly detailed difference among different
materials in their Arrhenius plots are considered to be averaged out,giving similar values fordifferent materials.
The activation energy for k2 Was evaluatedto be 58･5, 42･3, and 67･7 kJ/mol and that
●

ﾂ

I

ヽ

釘

for k3 Was 125.5, 534 and 74.8 kJ/mol. forlZr, Zr4Ni, and Zr2Ni, respectively. It should‑ ヽ

be mentioned that the largest value of k2 Was

observed for Zr2Ni. and Zr4Ni gave the small‑】0.00120.00130.00140.00150.α)16est
one.0.0皿7This is not consistent with the ac‑tivity order ofZr4Ni > Zr2Ni > Zr･ On the1汀/K･1other hand, Zr4Ni gave the largest k3 Value

Fig. ll. Arrhenius plot of k3 eVahlatCd for Zr,

and Zr the smallest value; this feature is the

ZT4Ni and Zr2Nisame as the activity order of Zr4Ni > Zr2Ni> Zr. These observations illdicate that the overall activity for the decomposition/absorption of
methane is determined by k3: kl and k2 Play minor roles for absorption of methane, especiallymethane removalof more than 90%. It can be concluded that the aggregatioI1 0f carbonaceous
residues responsible fわr the three‑dimensional groWtI1 0f carbollaCeOuS deposits oIl the surface

governs the activity for methane absorption as a whole･

4. Conclusions

The kinetics of methane absorption by Zr, Zr2Ni, and Zr4Ni was amilyzed over the entire
range of the absorption curves by taking account of plausible elementary reactions) whichwere inspected by potentialenergy surfaces(PESs) of relevant reactions calculated with theGaussian 03 package. The kinetic analysts and PES calculation revealed that the absorp‑
tion/decomposition of methane proceedsvia the reactions CH｡(9)生cH3(a)曳cH(a)聖Cdeposits. The absorption curves down to 99.9% Consumption of the initially loaded amountof methane could be reproduced uslng the rate constants, kl, k2 and k3, Which were evalu‑

ated from curve‑fitting of the observed absorption curves by considering a series of differential
equations describing the elementary reactions･It was fわund that the values of the three rate constants are dependent on the materials,

but only the order of magnitude of k3 is consistentwith the experimentally observed orderof activity for the absorption beyond 90% Consumption of the initially loaded methane･ This
indicates that the first and second step of the reaction series described above play only minor

10Kinetics of Methane Decomposition over ZrNiAlloys

roles; the overall absorption reactivity (methane consumption beyond 90%) of the materials isgoverned by the丘nal third step.

Reference sll] K･ Watanabe, W･ M･ Shu, M･ Motohashi, and M. Matsuyama, Fusion Engn. Design,
39/40 (1998) 1055 ‑1060[2] L･ C. Emerson, a. J. Knize, 3. L. Cecchi, and 0. Auciello, J. Vac. Sci. TechnoE., A4(1986) 297‑299[3] L･ C･ Emerson, R･ J･ Knize, and J. L. Cecchi, J. Vac. Sci. Technol., A5 (1987) 2584‑2586[41 W･ M･ Shu, M･ Matsuyama, and K. Watanabe, Ann. Kept. HRC., Toyama Univ., 16(1996) 59‑づ8[5] M･ Matsuyama, E･ Motohashi, W. M. Shu, and K. Watanabe, Ann. Kept. HRC., Toyama
Univ., 17 (1997) 73‑84[6] MJ･ Frisch, GI W･ Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robband J. R. Cheesemanand
J･ A･ Montgomeryand Jr.and T. Vrevenand K. N. Kudinand ∫. C. Burantand J. M. Millamand

SI S･ Iyengarand I. Tomasiand V. Baroneand B. Mennucciand M. Cossiand G. Scalmaniand N.
Regaand G･ A･ Peterssonand H･ Nakatsujiand M. Hadaand M. Eharaand K. Toyotaand R. Fuku‑
daand J･ Hasegawaand M･ Ishidaand T. Nakajimaand Y. Hondaand 0. Kitaoand H. Nakaiand M.
Kleneand X. Liand ∫. E. Knoxand H. P. 班ra.tcbianaIld J. B. Crossand C. Adamoalld J. Jaramil̲loand R･ Gompertsand R･ E･ StratmannaIld 0. Yazyevand A. J. Austinand a. Cammiand C.

Pomeuiand J･ W1 0chterskiand P. Y. Ayalaand K. Morokumaand G. A. Vothand P. Salvadorand

JJ･ Dannenbergand V･ GI Zakrzewskiand S. Dapprichand A. D. Danielsand M. C. Strainand
O･ Farkasand D･ K･ Ma,lickand A･ D･ RAbuckand K･ Raghavachariand JI B･ Foresmanand J･

V. Ortizand Q. Cuiand A. G. Babouland S. Cliffordand J. Cioslowskiand B. B. Stefanovand
G. Liuand A. Liashenkoand P. Piskorzand I. Komaromiand R. L. Martinand D, I. Foxand T.
Keithand MI A･ ALLahamand C. Y. Pengand A. Nanayakkaraand M. Challacombeand P. M.

W･ Giuand B･ Jolmsonand W･ Chenand M. W. Wongand C. Gonzalezand, and J. A. Popleand,GaussianO3, Rel,isionB･ 03･ Gaussian lnc.and Pittsburgh, 2003.
[7] L･ Wan, R･ Hayakawa, Y. Hatano, and K. Watanabe, Ann. Repl. HRC. Toya7na Unil'.,Japan, 23 (2003) 29‑41[8] Y･ Jim, L･ Wan, M･ Hara, Y･ Hatano,and K. Watanabe, Mater. Trans., 48 (2007) 560‑565
t9】 The Japan Institute of Metals, Metal Databook, 2nd Ed. (in Japanese), Muruzen, Tokyo,

Japan, 1984ll

渡辺国昭･洲亘･松山政夫[10] Y･ Fukai, K･ Tanaka and H･ Uchida, Hydrogen and Metals (in Japanese), Uchida
Roukakuho, Japan, 1998[11] R･ T･ K･ Baker and R･ J･ Waite, J. Catal., 37 (1975) 101‑10512