⺰ᢥࡃ࠽ࠫ࠙ࡓ⴫㕙ߦ߅ߌࠆ᳓⚛ߩ⸃㔌෻ᔕㅦᐲߩ
ࡃ࡞ࠢਛ㉄⚛Ớᐲଐሽᕈ
ᵄᄙ㊁㓶ᴦ1, Alexander Livshits2, Andrei Busnyuk2, Vasily Alimov2, ᧻ጊ ᡽ᄦ11ንጊᄢቇ᳓⚛ห૏૕⑼ቇ⎇ⓥ䉶䊮䉺䊷
䇾930-8555 ንጊᏒ੖⑔ 3190
2
Bonch-Bruyevich University,
61 Moika, St. Petersburg 191186, Russia

Sticking of H2 on Oxygen-Covered Surface of Vanadium Doped with
Oxygen to Various Bulk Concentrations
Yuji Hatano1, Alexander Livshits2, Andrei Busnuk2, Vasily Alimov2 andMasao Matsuyama1
1

Hydrogen Isotope Research Center, University of Toyama,
Toyama 930-8555, Japan
2
Bonch-Bruyevich University,
61 Moika, St. Petersburg 191186, Russia(Received January 31, 2012; accepted March 7, 2012)

Abstract
The sticking probability of H2 on the surface of polycrystalline V covered withoxygen was examined as a function of oxygen concentration in the bulk metal (0.4 to
3.3 at.%) at specimen temperatures from 200 to 1000 ºC. At all temperatures examined,
the sticking probability was strongly dependent on the oxygen concentration in the bulk.
Such correlation between the reaction rate of H2 on the surface and the oxygen
concentration in the bulk was attributed to the dependence of concentration of oxygen
vacancies (holes in surface oxygen coverage) on the oxygen bulk concentration. The
values of sticking probability were relatively high even at the oxygen bulk

1

concentration of 3.3 at.%; the oxygen coverage on vanadium was rather “holey” even at
the oxygen concentration close to the solubility limit. The activation energy for H2
dissociation on such holes in the coverage was evaluated to be 10 kJ/mol H2.

1. Introduction
Understanding of reaction mechanism of hydrogen on metal surfaces isimportant for the development of catalysts, hydrogen permeation membranes andstorage materials, non-evaporable getters, low-outgassing materials and so on. The
surfaces of these materials, however, are not necessarily clean but covered with
nonmetallic impurities such as oxygen, sulfur and carbon in most cases. Therefore, oneof the key issues to understand the reaction mechanism is to clarify the reaction centerson the metal surfaces covered with nonmetallic impurities.
If temperature of metal surface, TS, is sufficiently high, the equilibrium isattained between the impurity on the surface and in the bulk. According toLangmuir-McLean equation [1], the correlation between the surface impurity coverage,
θO, and the impurity concentration in the bulk, CO, is described as ∆H seg θO ,= k O CO exp −1−θ O RTS (1)

where ∆Hseg is the heat of surface segregation of the impurity, and kO is the constantcoefficient. Usually, ∆Hseg takes large negative value, and hence θO is comparable tounity even at very low CO. Nevertheless, a certain portion of the surface, 1-θO, still
remains free from the impurity at any TS and CO. In other words, even a monolayer
coverage (θO ≈ 1) contains a definite number of stochastically distributed holes (the2

surface sites free from the impurity). One can consider two different concepts withregard to the reaction centers on the metal surface covered with nonmetallic impurities.
1. The “Holes in the coverage” concept: the surface sites free of impurity (holes
in coverage) form active centers. This mechanism appears to be dominant at higher TS
where, according to Eq. (1), a considerable portion of clean surface opens up. Since
1-θO depends on CO, bulk impurity concentration will govern reaction kinetics in thiscase as demonstrated by Hörz and Fromm [2,3] who reported the clear influence of thebulk concentrations of oxygen and nitrogen on the reactivity of polycrystalline Nb and
Ta towards the decomposition of CnHm molecules at TS >1200 ºC. According to Eq. (1),however, the fraction of coverage holes, 1-θO, is negligible at lower TS. For example,
1-θO should be in a range of 10-5 at TS ~ 200 ºC in the O–Nb system even at CO as low
as 0.1 at.% (with |∆Hseg| = 71 kJ/mol [4]).
2. The “Holes in the barrier concept”: here, holes are regarded to be inherent to
the regular surface sites covered with impurity [5], i.e. the holes are not the “stochastic”
sites with the lack of impurity atoms but such area in each surface unit cell where thebarrier is lower than in other parts for an incident molecule with particular orientationand internal state. For example, Eibl and Winkler [5,6] who measured the sticking
coefficient of D2 molecules on V(111) covered with oxygen and sulfur at TS = 77 K and
Tg = 300 K, attributed rather large value of sticking coefficient, 10-4, to such holes inbarrier (Tg indicates gas temperature).
In order to understand the type of dominant reaction centers at relatively low TS
where one should expect a virtually saturated coverage (θO ≈ 1), the sticking probabilityof H2, αH2, on O-covered Nb surface, was measured as a function of CO in a wide
temperature range including room temperature [7]. The sticking probability of H2, αH2,3was clearly dependent on CO at all TS examined. Namely, αH2 was proportional to 1/CO2.
Therefore, it was concluded that two adjacent oxygen vacancies played the role ofreaction centers in H2 sticking on the O-covered Nb surface even at low TS. Suchdependence on CO has been also observed for the rate constant of recombinativedesorption of D2 from Nb [8].
Preliminary investigation on V containing oxygen showed that αH2 on the V
surface is also dependent on CO [9], but details of H2 reaction have not been understood.
In the present study, αH2 on the V surface is examined in wide ranges of CO (0.4 to 3.3at.%) and TS (200 to 1000 ºC) by permeation techniques [7], and the mechanism
underlying H2 sticking is discussed by comparing the obtained results with those for Nb
acquired in the previous study [7].

2. Experimental
The specimen used was a membrane of polycrystalline V (0.1 mm thickness) intubular shape fabricated from a sheet supplied by ESPI Metals Co., USA. Nominal
purity of the specimen was 99.9 mass%.
The permeation experiments were performed in an ultra-high vacuum (UHV)
apparatus separated into two chambers by the tubular membrane [7,10]. Both chambers
were independently pumped by turbo-molecular pumps and sputter-ion pumps, and thepressure of residual gas was about 10-8 Pa. The outside surface of the tubular membrane
was exposed to H2 gas, H atoms or hydrogen plasma, depending on experimental
requirements. In the case of exposure to H2 gas, the temperature of the gas was virtuallyclose to room temperature due to collision with the chamber walls. The permeation fluxof hydrogen was measured with a calibrated ionization gauge and a quadrupole mass

4

analyzer. The specimen was heated resistively, and its temperature was measured by athermocouple inside the tubular membrane.
After the installation in the apparatus, the specimen surface was sputtered byhydrogen plasma at TS in the range from 600 to 1000 ºC. The surface analysis by Auger
electron spectroscopy (AES) carried out in a different UHV apparatus showed that
sulfur was a dominant segregated surface impurity in this temperature range before thesputtering. Nevertheless, long-term sputtering (15 hours) in this range of TS resulted in
the removal of sulfur, and oxygen became sole surface impurity. The oxygen
concentration in the bulk of the specimen was 0.4 at.% after the sputtering treatment.
Then oxygen was uniformly introduced into the specimen in a step-by-step manner up
to 3.3 at.% by a gas absorption method and subsequent diffusion annealing in UHV. At
each step of the oxygen doping, the permeation experiments were carried out by fillingthe outside chamber with H2 gas (10-3 to 10 Pa) at TS from 200 to 1000 ºC. Note that at
TS and CO utilized in this work, oxygen was in solid solution phase [11].
When the upstream and downstream surfaces of membrane are identical, thepermeation rate of H2, φH2, in steady state is expressed as
φH2 =1
α H2φinc ,2(2)

where αH2 is the sticking probability of H2, and φinc is incident H2 flux onto the
upstream surface. The incident flux is expressed as φinc = P / 2π mkTg where P is thepressure of H2 gas in the outside chamber, m is the mass of a H2 molecule, and k is
Boltzmann constant. In this work, identity of upstream and downstream surfaces was
confirmed by separate measurements of αH2 for upstream and downstream surfaces with5

the H2 absorption technique mentioned in Ref. [7]; the values of αH2 on upstream anddownstream surfaces were always comparable with each other after oxygen doping.

3. Results
At all exposure conditions, the change in the permeation rate in a transient state
after H2 introduction in the upstream side showed that the permeation rate wascontrolled by surface processes (dissociative absorption and recombinative desorption)and not by bulk diffusionprocess. The diffusivity of hydrogen in V is rather high evenat low TS. For example, the diffusion coefficient is estimated to be 1.0 × 10-8 m2/s at
200 ºC from the data recommended by Völkl and Alefeld [12]. Hence, the steady stateshould be attained within a time scale of few seconds provided that the permeation iscontrolled by the diffusion process. It took, however, several minutes or more to attainthe steady state in the present study. Therefore, it was possible to evaluate the sticking
probability, αH2, from the permeation rate in the manner described in Section 2.
Fig. 1 shows the Arrhenius plot of αH2 at various oxygen bulk concentrations,
CO. In this case, the specimen was first heated to 1200 ºC, and then TS was reduced in a
step-by-step manner; the specimen was kept at each temperature for 10 min. It should
be noted that αH2 clearly decreased with increasing CO. On the other hand, no
significant change with CO was observed in the temperature dependence; twocharacteristic bends in the plot were observed at around 500 and 300 ºC at all CO
examined. The sticking probability αH2 showed weak temperature dependence in arange from 300 to 500 ºC, and the slope in this region corresponded to the activationenergy of 10 kJ/mol H2. A stronger temperature dependence was observed in theregions of TS > 500 ºC and TS < 300 ºC, corresponding to the activation energy of 30 –6

35 kJ/mol (Fig. 1). Similar correlation between αH2 and CO was observed also for adifferent membrane prepared from a sheet with different nominal purity (99.35 mass%).
It was therefore concluded that the dependence of αH2 on CO is a general property of
V-O systems.
In order to check whether the data on αH2 shown in Fig. 1 pertain to definite
values determined by TS and CO, the influence of thermal history of the specimen on
αH2 was examined. That is definitely so in the range of sufficiently high TS and CO
where there is an equilibrium between the surface and bulk oxygen. The establishmentof such equilibrium, however, may be very slow at lower TS and CO, and it is practicallynever reached. In this range, oxygen coverage, and correspondingly αH2, may bedifferent at the same TS and CO, depending on thermal history. Figure 2 shows
temperature dependencies of αH2 at CO = 0.4 and 1.3 at.% for two different
measurement modes. Mode 1 corresponds to the method by which the data shown in

10

10

-2

1000

700 600 500

400

1.1 %
1.3 %
1.8 %
3.3 %

0.4 %
0.5 %
0.7 %
1.0 %

35
kJ
/m
ol

-3

°C

200

300

ol

㩷

αH2

10 kJ/m

10

-4

35

P = 0.4 Pa
10

kJ

/m
ol

-5

0.5

1.0

1.5

2.0

2.5

-1

1000 / TS (K )Fig. 1 Temperature dependence of sticking probability αH2 on V surface at
different oxygen bulk concentration CO indicated in the figure.
Pressure of H2 gas in upstream side was 0.4 Pa.

7

Fig. 1 were acquired; the specimen was kept at each temperature for 10 min after
heating in vacuum at 1200 ºC. In mode 2, the specimen was kept at each temperature formuch longer time (more than 1 h). No significant difference between the modes 1 and 2
was observed at low CO (as shown in Fig. 2 for CO = 0.4 at.%). At 1.0 at.% ≤ CO ≤1.3 at.% and TS ≥ 800 ºC, however, the permeation rate first reached “quasi” steadystate value within 10 min, and then gradually decreased with time to reach “new”
stationary rate. Consequently, the value of αH2 measured in the mode 2 was smaller thanthose obtained in the mode 1 by ca. factor of 2 as shown in Fig. 2. Such gradual change
in αH2 can be ascribed to gradual increase in the surface oxygen coverage by the
segregation from the bulk. The sets of data obtained by the modes 1 and 2, however,showed similar temperature dependence. The difference between the modes 1 and 2 was
not observed at CO = 1.8 at.% and higher concentrations. It was therefore concluded that㩷

CO = 0.4 at%

αH2

1E-3

mode 1

1E-4

㩷mode 2
CO = 1.3 at%mode 1

1E-5
0.5

mode 2

1.0

1.5

-1

2.0

2.5

1000 / TS (K )Fig. 2 Influence of thermal history of specimen on αH2. Solid symbols refer to
αH2 measured by keeping specimen at given temperatures for 10 min(mode 1), whereas open symbols refer to αH2 obtained by holding
specimen for sufficiently long time (mode 2). Pressure of H2 gas in
upstream side was 0.4 Pa.
8the data shown in Fig. 1 are definite values determined by TS and CO at 1.0 at.% > COand CO ≥ 1.8 at.%, and the values at 1.0 at.% ≤ CO ≤ 1.3 at.% correspond to thesurface states before the completion of oxygen segregation but show similar
temperature dependence as “definite” values. The values of oxygen diffusivity in V
reported in Ref. [13] indicated that the segregation should be completed within 1 min at
CO = 1.0 at.% and TS = 800 ºC. The rate-limiting step of oxygen surface segregation
under the present conditions is not clarified at present.
The sticking probability αH2 at various TS is plotted against CO in Fig. 3. No
systematic difference was observed at different TS, and αH2 was roughly proportional to
1/CO. Figure 4 shows comparison of αH2 between V and Nb at selected TS. The valuesof αH2 for V are significantly higher than those for Nb. Namely, the effects of oxygen
were much weaker for V than for Nb. In the case of the Nb-O system, αH2 was
proportional to 1/CO2 at TS ≥ 1000 ºC and to 1/CO1.5 at TS below 1000 ºC [7], while

10

㩷

-2

2

1/CO
o

-4

10

㩷㩷㩷TS ( C)

1000
900
800
700
600
500
400
300
250
200

㩷

10

αH2

10

-3

1/CO

-5

0.1

1

10

CO (at%)Fig. 3 Dependence of αH2 on CO at various TS. Pressure of H2 gas in
upstream side was 0.4 Pa.

9

10

㩷

-2

10

-3

10

-4

10

-5

1/C O2

㩷

αH2

1/CO

o

-6

10
0.01

V, 1000 CoV, 500 CoNb, 1000 CoNb, 500 C

0.1

1

10

C O (at%)Fig. 4 Comparison of αH2 between V and Nb [7].

that on V was proportional to 1/CO as mentioned above. Namely, the dependence of αH2
on CO observed for the V-O system was also significantly weaker than that for the Nb-O
system.

4. Discussion
The clear dependence of αH2 on CO indicates that surface defects related tooxygen vacancies play dominant roles of active centers in H2 dissociation on the V
surface covered with oxygen. Properties of such defects can be understood through thedetailed analysis of dependence of αH2 on TS and CO. The Arrhenius plot of αH2 (Fig. 1)
showed characteristic bends at 500 and 300 ºC as mentioned above. Change in slope
similar to the bend at 500 ºC was also observed in the Arrhenius plot of αH2 for the Nbsurface (Fig. 3 in Ref. [7]). Hence, the mechanism underlying the high temperature bendin Fig. 1 can be understood by the approach made in the previous study [7].

10According to Eq. (1), θO ≈ 1 at any CO if TS is sufficiently low. At certain TS,
θO should start to decrease with increasing temperature. Here, the temperature where θO
starts to decrease, TS*, rises as CO increases. In reality, situation when θO ≈ 1 is notnecessarily reached even at sufficiently low TS when CO is low. The previous studies for
the Nb-O [4, 7, 14] and the Ta-O [14] systems showed that the saturation level of θO at
sufficiently low TS decreased with decreasing CO. Nevertheless, at higher TS, θO started
to decrease with increasing TS as expected from Eq. (1), and TS* rose with CO.
The sticking probability αH2 can be expressed as
αH2 = (0.1 – 1) σ exp( −

2 Eb
),
kT

(3)

where a multiplier factor of (0.1 - 1) is a typical range of kinetic coefficient (conditioned,e.g., by the preferential sticking of molecules having certain orientations [15]), σ is theactive portion of the surface, and 2Eb is the activation energy for sticking on the active
centers. Here, T indicates gas and/or surface temperature controlling the reaction rates,
and T = TS in the present case. If the active portion of surface, σ, is determined by thesites free from impurity (oxygen in the present case), σ cannot be independent of TS: thenumber of oxygen-free sites, 1-θO, depends on TS (and CO) as described above.
Arrhenius plot of αH2 for the Nb-O system demonstrated the bend coincidedwith TS* [7]. Therefore, the sharp increase in αH2 with TS at TS > TS* was attributed toincrease in σ. At TS > TS*, αH2 was proportional to 1/CO2 as shown in Fig. 4. Thisobservation indicates that the active centers for H2 sticking are two adjacent oxygen
vacancies whose concentration is expressed as (1-θO)2. Namely, σ = (1-θO)2 in this case.
Substitution of Eq. (1) in Eq. (3) by assuming θO / (1 − θ O ) ≈ 1/ (1 − θ O ) gives theactivation energy for sticking as 2|∆Hseg| + 2Eb. On the other hand, the oxygen coverage

11was constant at TS < TS*, and the activation energy observed in this region corresponded
to 2Eb. The mechanism underlying the correlation of αH2 ∝ 1/CO1.5 at TS < TS* wasdiscussed in Ref. [7] and is not repeated here.
Although detailed evaluation of the oxygen coverage for the V-O system by
AES was impossible due to overlapping of oxygen peak (510 eV) with vanadium peak
(511 eV), analogy to the Nb-O system indicates that TS* ≈ 500 ºC for the V-O system.
The activation energy at TS > TS* (30 – 35 kJ/mol) is determined by |∆Hseg| and Eb, andthat at TS < TS* (10 kJ/mol H2) corresponds to 2Eb. This value of 2Eb is close to theactivation energy for H2 sticking on active centers on the O-covered Nb surface [7].
Reduction in αH2 with increasing CO at 300 ºC < TS < 500 ºC can be ascribed to anincrease in the saturation level of θO with CO. It is implausible that the steeper slope at
TS > 500 ºC is due to the activation of H2 sticking on regular surface sites covered withoxygen because αH2 was clearly dependent on CO even in high TS region. In addition,
rapid cooling from high temperature (1200 ºC) resulted in significantly larger values ofαH2 at sufficiently low TS (e.g. 200 ºC). Such change in αH2 is possible when theconcentration of active centers increased at high TS by reduction in θO and then it wasfrozen by rapid cooling.
The Arrhenius plot of αH2 for Nb showed only one bend, and the slope wasconstant at TS < TS* down to room temperature. Namely, the bend at 300 ºC in Fig. 1
was observed only for the V-O system. In order to understand the mechanism
underlying this low temperature bend, the pressure dependence of αH2 was examined.
As shown in Fig. 5, no noticeable pressure dependence of αH2 was observed at
TS > 300 ºC. At TS below 300 ºC, however, αH2 clearly decreased with increasing H2pressure. These observations suggest that the sticking of H2 at the holes in oxygen

12

coverage was affected by adsorbed hydrogen or impurities contained in H2 gas at
TS < 300 ºC. Details mechanisms underlying this pressure dependence, however, havenot been clarified yet.
The sticking probability αH2 on the Nb surface was proportional to 1/CO2 at
TS > TS*, and this observation indicated that two adjacent oxygen vacancies make up
active centers for H2 dissociation. In contrast to Nb, αH2 on V was proportional to 1/CO.
This correlation suggests that αH2 was in proportion to (1 - θO) (Eqs. (1) and (3)).
Namely, σ = (1 - θO). Such correlation may mean that hydrogen molecules dissociates
on a single oxygen vacancy. Alternative explanation is that oxygen vacancies on the V
surface form clusters, and the total area of those clusters is proportional to (1 - θO).
Further investigation is required to derive conclusion on this point.
Since αH2 was totally determined by the dissociation on active centers, nodirect information was available from the present results on the dissociation on regularsurfaces sites covered with oxygen atoms. Nevertheless, the minimum value of

10

-3

10

-4

1000 700

500 400

300

200

P
0.001 Pa
0.1 Pa

°C

㩷

-2

αH2

10

0.5 Pa
5.0 Pa
10

-5

0.5

1.0

1.5

2.0

2.5

-1

1000 / TS (K )Fig. 5 Temperature dependence of αH2 on V surface at various H2 pressures(CO = 0.4 at.%).
13

activation energy can be evaluated. Beutl et al. [16] measured the sticking probability ofH2 and D2 on the clean V surface and reported that the probability was 0.35 formolecules with translational energy corresponding to room temperature. By taking thisvalue as kinetic coefficient in Eq. (3) and σ ≈ 1, the sticking probability on regularreg
reg
surface sites, α H2
, can be expressed as α H2
= 0.35exp(−

2Ebreg) , where 2Ebreg is thekTS

activation energy for the dissociation on regular surface sites. The value of αH2 at TS =1000 ºC and CO = 3.3 at.% is 4.5 × 10-4. Since αH2

reg
α H2
, 2Ebreg should besignificantly larger than 70 kJ/mol H2. The activation energy for H2 sticking on
O-covered sites is larger than that on active centers by at least 7 times.
Eibl and Winkler [5] examined angular and energy distributions of D2 flux
desorbing from V(111) covered with oxygen and sulfur at TS = 950 K, and reported that
desorption flux contains both thermal and hyperthermal contributions. They ascribed
this observation to a corrugated potential energy surface. The presence of coverageholes, however, can be alternative interpretation of their results.

5. Conclusions
The sticking probability of H2, αH2, on the V surface covered with segregatedoxygen was examined as functions of the oxygen bulk concentration, CO, and thespecimen temperature, TS. At all TS examined, αH2 decreased with increasing CO. Thisobservation indicates that the main channels of H2 dissociation was oxygen-free sites
inherent to the segregated oxygen coverage. The activation energy for the H2 sticking on

14

such active centers was evaluated to be 10 kJ/mol H2. The sharp increase in αH2 with TS
was observed at TS > 500 ºC due to increase in portion of oxygen-free sites. The effectsof oxygen presently observed for V were much smaller than that for Nb examined in theprevious study [7].

Acknowledgements
This study has been supported in part by Grant-in-Aid for Scientific Research
on Priority Areas, 476, Tritium Science and Technology for Fusion Reactorfrom The
Ministry of Education, Culture, Sports, Science and Technology.

References[1]

D. McLean, Grain Boundaries in Metals, Clarendon Press, Oxford (1957) p. 116.

[2]

G. Hörz, H. Kanbach, H. Vetter, Mater. Sci. Eng. 42 (1980) 145.

[3]

G. Hörz, E. Fromm, Vacuum, 33 (1983) 265.

[4]

H. H. Farrell, H. S. Isaaks, M. Strongin, Surf. Sci. 38 (1973) 31.

[5]

C. Eibl, A.Winkler, J. Chem. Phys. 117 (2002) 834.

[6]

C. Eibl, A. Winkler, Surf. Sci. 482-485 (2001) 2002.

[7]

Y. Hatano, K. Watanabe, A. Livshits, A. Busnyuk, V. Alimov, Y. Nakamura, K.
Hashizume, J. Chem. Phys. 127 (2007) 204707.

[8]

R. Hayakawa, A. Busnyuk, Y. Hatano, A. Livshits, K. Watanabe, Phys. Scr. T103
(2003) 113.

[9]

Y. Hatano, A. Busnyuk, A. Livshits, Y. Nakamura, M. Matsuyama, Fusion Sci.
Technol. 54 (2008) 526.

15

[10] A. Livshits, Y. Hatano, V. Alimov, M. Matsuyama, J. Nucl. Mater. 363-365(2007) 801.
[11] H. Okamoto, Desk Handbook of Phase Diagrams for Binary Alloys, ASM
International, Materials Park (2000) P. 634.
[12] J. Völkl, G. Alefeld, Diffusion of Hydrogen in Metals, in: Hydrogen in Metals I,
G. Alefeld, J. Völkl (Eds.), Springer-Verlag, Berlin Heidelberg (1978) p. 333.
[13] R. W. Powers, M. V. Doyle, J. Appl. Phys. 30 (1959) 514.
[14] A. Joshi, M. Strongin, Scr. Metall. 8 (1974) 413.
[15] M. J. Murthy, A. Hodgson, J. Chem. Phys. 108 (1998) 4199.
[16] M. Beutl, J. Lesnik, E. Lundgren, C. Konvicka, P. Varga, K. D. Rendulic, Surf.
Sci. 447 (2000) 245.

16