Surface Science 144 (1984) 665-678

North-Holland. Amsterdam

665

THE ADSORPTION KINETICS OF WEAKLY BOUND HYDROGEN ON THIN IRON FILM SURFACES

E. NOWICKA

Space Research Cenrre. Polish Academ.v of Scrences, cl. Ordona 21. 01-237 Wursaw, Polund

and

R. DUS

Itrstirure o/ Ph.vsrcul Chem’srry, Pohsh Academ.v oj Sciences, Ul. Kasprraku 44/52. 01 - 237 Wursow.

Poland

Received 6 February 1984; accepted for publication 8 May 1984

Three adsorption states of hydrogen fis , /I,, and a ; on a thin iron film surface were

distinguished on the basis of surface potential, and volumetric measurements carried out simulta-

neously. The sticking probability for the selected weakly bound hydrogen adsorption state p& was

determined. and its variation with coverage and temperature was shown, monitoring hydrogen

population by means of surface potential measurements. Consideration of adsorption-desorption

phenomena which occur under isothermal conditions leads IO the conclusion that the weakly

adsorbed state /Ii arises due to induced heterogeneity.

1. Introduction

Studies on hydrogen interaction with iron surface have resulted in the following conclusions:

(i) Two or three desorption states of hydrogen exist on thin film [1,2], polycrystalline foils [3], and single crystal [3,4] iron surfaces.

The strongly and the weakly adsorbed states of hydrogen are characterized by an average activation energy of desorption 88-96 and 59-67 kJ/mol. respectively.

As hydrogen adsorbs on clean Fe films, the electrical resistance first increases and then decreases, and this has been interpreted as a proof of two states of the adsorption (5-71. On the other hand, UPS data lead to the conclusion that hydrogen adsorption on polycrystalline and single crystal iron surface gives rise to a single resonance level at 6 eV below the Fermi level. The bonding level was derived from coupling the H 1s state to the valence states of metal [4,8].

0039-6028/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

666 E. Nowicka, R. DUE / Ad.rorptton krnerics of hydrogen on iron

Hydrogen adatoms are electronegatively polarized on thin iron film surface leading to a decrease of surface potential (sp) [2,9-111. Electronegatively polarized deposit of hydrogen exists also on (100) and (111) single crystal iron surfaces, while an unexpected increase of surface potential was observed at the beginning of hydrogen adsorption on the surface of (110) plane [4].

(ii) The kinetics of hydrogen adsorption on iron cannot be described in a simple way.

Rapid uptake of hydrogen at the beginning of adsorption is followed by a slow preocess. It was suggested by some scientists [3,12] that the incorporation of hydrogen below the surface could be the reason for that phenomenon, in contradistinction with the opinion of others who suggested that surface migra- tion of hydrogen adatoms to the sites of higher adsorption potential should be considered f 13 -- 151.

It was also reported, that the simple Langmuir model well describes the sticking probability dependence on coverage S(e), on iron single crystal planes

[41. On the other hand, it was announced [16] that a precursor state of hydrogen

adsorption on thin iron films was observed, and it was found that Kisliuk’s model [18], for adsorption with the precursor state, fitted well S(B) for the strongly adsorbed state of hydrogen [17].

It seems that a lot of these discrepancies could be removed by considering the sticking probability dependence on coverage for the selected strongly and weakly bound adspecies. However, there is a lack of data for adsorption kinetics of the weakly bound state. The aim of this work was to fill this gap, distinguishing accurately between the two states of hydrogen adsorbed, and determining .S(e) for the weakly adsorbed adspecies.

It can be expected [2] that the weakly bound state of hydrogen will be formed with a reasonable efficiency at pressures of the order of 10-4-10-2 Torr in the temperature range 78-300 K. For that reason the classical flow method of S(e) determination on the basis of measurements of gas phase density changes cannot be applied. To get reliable data, one should determine directly the changes of weakly bound adspecies population on the surface during adsorption. To distinguish between the two states of hydrogen deposit on thin iron film surface, and to determine their population, we applied measurements of surface potential changes dsp during adsorption.

2. The method of sticking probability determination

The effective isothermal adso~tion rate dil’,/dt at pressure P is described by the well known equation

dN.,‘dt=AS(t’)Z(P-I+‘,,) (1)

E. Nowicka, R. Dti / Adsorption kinetics of hydrogen on iron 667

where N, is the uptake of the adsorbate, A the real area of the adsorbent, s(e) the coverage dependent sticking probability cofficient, Z the collision factor, and Peq the equilibrium pressure over the adsorbate.

In the present work, the adsorption rate was determined by measurements of the rate of surface potential changes dAsp/dt during adsorption. To transform dAsp/dt into dN,/dt, the relation Asp = cp(N,) should be experi- mentally obtained. That can be done in the course of static experiments, when gas is introduced in small, calibrated, successive doses into the capacitor cut off from the pumps, recording simultaneously Asp and pressure P. Steady values of Asp and P registered as the result of every successive dose introduc- tion can be used to get the Asp = ‘p( N,) relation, on the basis of volumetric calculations. The steady value of pressure in this procedure can be taken as P,,(N,). Carrying out the adsorption at sufficiently low temperature (e.g. 78 K), up to sufficiently high pressure, one can expect, that a monolayer of the adsorbate can be reached with total uptake denoted N,,,,,. Further, taking a proper stoichiometry of adsorption, the real area of the adsorbent, A, can be calculated:

N A _ amax number of hydrogen adatoms adsorbed on the film

na.max (hydrogen adatoms)/cm* ’ (4

where n,_,,, is the maximal population per square centimeter in the adsorbate layer. For the hydrogen-iron system we take na.max equal to the average density of surface atoms [4] of thin film, e.g. 1.63 X 10” atoms/cm* [24]. Carrying out dynamic experiments, with a constant gas flow through a calibrated capillary into capacitor cut off from the pumps, with simultaneous registration of Asp =f(t) and P =f,(t) the relation dAsp/dt =f2(P) can be obtained.

Thus on the basis of the results of static and dynamic experiments carried out under isothermal conditions, all variables in the relation (1) can be measured and all parameters determined. The sticking probability coefficient S can then be calculated together with its dependence on coverage 0 for selected states of the adsorbate.

3. Experimental

The experiments were carried out using an entirely glass UHV system working at a pressure of the order of lo-” Torr. A modified version of the previously described static capacitor [19] was applied. Our capacitor, shown in fig. 7, is used in the course of the static experiments when gas is introduced in the successive doses, and in the course of the dynamic experiments at steady, continuous gas flow through the capillary into the condenser cut off from the

668 E. Nowicka, R. DuS / Adsorption kinems of hydrogen on u-on

pumps. The reference electrode of the capacitor was prepared by coating the outer wall of the inner cylinder with conducting (SnO + Sb,O,) layer. The active electrode was the iron film deposited on the outer cylinder maintained at 78 K by evaporation of iron wire (Johnson-Matthey grade I) from a tungsten heater. To avoid alloy formation, iron wire was never melted during the deposition.

The geometrical area of the films was 135 cm2, and their thickness - lo-’ m. The films were sintered at 320 K for 30 min under UHV conditions. During the deposition the pressure measured by means of a Groszkowski type ioniza- tion gauge with modulation did not exceed 1.5 x lo-” Torr.

When the thin film was ready for measurements, to avoid atomization of hydrogen and its uncontroled adsorption, the ionization gauge was turned off, and cut off from the capacitor by means of a greaseless, ground Dekker valve.

Fig. 1. Static capacitor used in the course of the dynamic and static experiments. (1) The adsorbent _ thin iron film. (2) Movable reference electrode. (3) Electrical contact to the adsorbent. (4) Electrical contact to the conducting coating of the reference electrode. (5) Holes in the reference electrode for symetrical gas introduction to the adsorbent surface. (6) Tungsten heater with iron wire wound around. (7) Tube joining the capacitor with pumping line. The ultrasensitive Pirani gauge was enclosed into this tube. (8) Glass coated iron slugs for moving of the reference electrode by means of magnet. (9) Capillary for gas flow in the course of dynamic experiments; the capillary

was removed when static experiments were performed. (10) Gas inlet. The outer walls of the

capacitor are painted with liquid bright platinum.

E. Nowicka, R. Du.4 / Adsorption kinetics of hydrogen on iron 669

We observed that the ionization gauge influenced the Asp = q$ N,) relation, even at an emission current as low as 4 x lo-’ A. For that reason the hydrogen pressure during adsorption was measured by means of an ultrasensi- tive, short-response time, Pirani gauge capable of working within 1O-5-1O-’ Torr. Application of the Pirani gauge excludes the possibility of sticking probability determintaion at low coverages, when the pressure is below low5 Torr; however, it allows us to determine reliable S( 0) values at high coverages not accurately known before.

In static experiments, when the pressure was below the lower limit of the Pirani scale, the amount of hydrogen in the gas phase was 0.1% of the introduced dose content, so it could be assumed that the adsorption of gas from this dose was complete.

The electronic circuit of the new construction was applied to measure surface potential changes [20]. The overall response time of the system was lob3 s, at a sensitivity of 0.1 mV and average noise level of 0.2 mV. This allows us to observe changes of coverage as small as 10e3 of the monolayer of hydrogen on iron.

Spectroscopically pure hydrogen, purified additionally by diffusion through a palladium thimble, was used.

Each experiment was repeated two or three times. The features of the Asp( N,) and the Asp(t) graphs were exactly the same. The reproducibility of Asp values at the characteristic points was 5 15 mV. The experiments were carried out at 78, 195 and 298 K.

4. Results and discussion

4.1. The nature of hydrogen adsorbed on thin iron ~I~ and sticking probabili~ of weakly adsorbed species

Surface potential changes caused by hydrogen adsorption on thin iron films at 78, 195 and 298 K registered in the course of static and dynamic experi- ments showing Asp = q$N,) and Asp =f( t) relations are presented in figs.

The corresponding increase of hydrogen pressure during adsorption, P = fi(t), and the change of the pressure and surface potential during isothermal evacuation of the system are also shown.

The characteristic features of Asp at the beginning of isothermal desorption at 185 and 298 K are accurately demonstrated in the enlargements.

Similarly as was previously reported [2], one can deduce from the Asp = q~( N,) and Asp = f( t) relations that three negatively charged adsorption states of hydrogen exist on thin iron film surface.

The j3s state arises at the beginning of adsorption. We suppose [2] that this

670 E. ~o~fick~, R. BUS / Adsorption kinetics of hydrogen on iron

30 --- N,d'[molec]

!

p [%I

0 500 1000 1500 ’ [W

Fig. 2. Surface potential changes in the course of static experiments (calibration curve) Asp = v( Na).

and dynamic experiments, Asp = f(t), caused by hydrogen adsorption on thin iron film at 78 K.

The increase of hydrogen pressure at the &,, state formation is shown.

c ASP ImV

t

-100

-200

-300

-100

10 2p 3p - N,x K?[molec]

0 500 t [set]

, ASP id b-4 .16' -I -350 -360

.104 -370

i 0 1 2 3 tIsec1 $1

Fig. 3. Surface potential changes in the course of static experiments (calibration curve) Asp = vp( N, ),

and dynamic experiments (dynamic curve) Asp =f(l), obtained as the result of hydrogen

adsorption on thin iron film at 195 K. The sp changes during isothermal desorption caused by the

evacuation are shown in the graph, and in the enlargement. The increase of hydrogen pressure at

the fi& state formation is also presented.

E. Nowicka, R. Dti / Adsorption kinetics oj hydrogen on iron 671

state corresponds to the TDS peak characterized by an average activation energy of desorption of 89.8 kJ/mol.

Within the /3; state, nonlinearity of the Asp = cp(N,) relation caused by a depolarization effect [2] is observed, particularly at 78 K. The distinct kink in the Asp = ‘p( N,) and Asp =f( t) graphs accompanied by a quick increase of hydrogen pressure indicates a new state of hydrogen adspecies formation which we call &.

The j3; state is characterized by an average activation energy of desorption of 67.8 kJ/mol [2]. In the coordinate system l/N, versus (1/P,,)‘/2, a straight line is obtained within a reasonable approximation, while l/N, versus l/P,,

does not yield a straight line. This indicates the atomic character of the /3~ adspecies [2]. However, this is no proof of a constant heat of adsorption since a Temkin type isotherm also fits the Pes = t+b(Na) relation. The end of the j?;

formation is seen as a sharp bend in the sp-isotherms. Starting from this point, volumetric measurements indicated that some further adsorption with very small surface potential change (- 7 mV) occurred as results of the successive hydrogen doses were introduced. At this step of adsorption, the Henry type isotherm for molecular, mobile adspecies was valid [2]. We call these adspecies

The existence of the (~6 adspecies can be directly deduced from the features

20 30 - N;10q7 [mole;

t

P [Trl ASP

Id Lmvl -1%

.ld -160

_- callbratlon c&e

-200 Asp= q’(N,I

0 500 1000 1500 t [set]

Fig. 4. Surface potential changes in the course of static experiments (calibration curve); Asp = cp(N,), and dynamic experiments (dynamic curve); Asp = j(t) obtained as a result of hydrogen

adsorption on thin iron film at 298 K. The sp changes during isothermal desorption caused by the

evacuation are shown in the graph and in the enlargement. The increase of hydrogen pressure at

the /I& state formation is also presented.

672 E. Nowicka, R. DuS: / Adsorption kinetrcs of hydrogen on iron

of the Asp(t) curves at isothermal desorption at 195 and 298 K (figs. 3 and 4). In the enlargements one can notice a rapid increase of Asp at the beginning of desorption, corresponding to the removal of the (YE adspecies, followed by a slower change of Asp due to desorption of the /I& state.

At 78 K the (YE state was stable within the evacuation period of - lo3 s. For such a long time of life on the surface, the activation energy of desorption has to be higher than 25.1 kJ/mol. Shanabarger [16] estimated that the activation energy of desorption of the molecular precursor state for hydrogen adsorption was 28.5 kJ/mol.

Some penetration of hydrogen adspecies below the surface of thin film cannot be excluded, remembering that imperfections of crystals strongly increase the solubility of hydrogen in iron [21-231.

One can notice in figs. 2-4 that a decrease of adsorption temperature significantly increases the hydrogen adspecies population for the /3s state, and the total population for both the j3s and /?w states. A similar observation was previously reported [2,5,25].

We suppose that the sharp bend in the sp-isotherm at 78 K corresponds to the complete monolayer of the ps and the /I& adspecies on thin iron film surface. That amount of the adsorbate we denoted previously N,,,,,. With this

Fig. 5. Sticking probability for hydrogen adsorption in the PW state on thin iron film. Points represent experimental results, while continuous lines correspond to the calculations based on the proposed model.

E. Nowicka, R. Dti / Adsorptron kinetics of hydrogen on won 673

assumption the adspecies are supposed to form a second layer. Knowing the N,.,,,, value one can express the coverage 8 at every step of the adsorption

process as:

0 = lv.,/Na.,,,, . (3)

The graphs shown in figs. 2 - 4 provide the possibility to distinguish precisely between the & and the 8, states and to gather all variables required to calculate sticking probability dependence on coverage for the /3w state of hydrogen using eq. (1). The results of these calculations are shown in fig. 5 (points). The values of sticking probability at the beginning of the /3, state formation are close to those characteristic of the end of the ps state adsorp- tion. determined by means of the classical Wagener method [17].

At the end of the fib state formation the sticking probability coefficients are

of the order of 10m9.

4.2. The model o/hydrogen adsorption on rhin iron films

4.2.1. The isothermul adsorption Having in mind Shanabarger’s results [16], and the fitting of Kishuk’s

precursor state model [18] to the observed rate of the & adspecies formation [17], we suppose that the a; adspecies can play the role of the precursor state for hydrogen adsorption on thin iron films. The transformation at; + /3; is probably nonactivated. Long-range order lateral interaction of the electrical nature exists within the & state, leading to the observed nonlinear character of the Asp = q( N,) relation due to the depolarization [2].

Assuming that hydrogen adspecies on thin iron film do not form an ordered structure, one can put:

* = l/a2n,.,,,.,,. where u is the average distance between the hydrogen adspecies.

The coverage and the average distance between adatoms corresponding to the maximal population in the /3s state, we denote by 8, and uM, respectively.

In table 1, calculated (using eqs. (3) and (4)) tiM and uM values for various temperatures of adsorption are shown.

Table 1

Adsorption

temperature

(K)

BP.4 or.4 (1W” m)

78 0.89 2.62

195 0.69 2.96

298 0.56 3.30

674 E. Nowicka, R. Dti / Adsorprion kinetics of hydrogen on iron

The & adspecies appears when the ,f3s state is completed. The rate of the pw state formation does not fit Langmuir’s or Kisliuk’s model.

The rate of the isothermal desorption of the adspecies monitored by sp changes (see figs. 3 and 4) is not described by the second order kinetic equation.

We suppose that a strong short-range order interaction in the adsorbate layer arising at 13 2 8, induces a characteristic behaviour of hydrogen ad- species, observed as the properties of the pw state. The induced heterogeneity of hydrogen adsorbate on iron was earlier suggested by Bozso et al. [4] and Madix and Benziger [25].

The short-range order interaction can be expressed as a function of the relative change in the average distance between adatoms in the adsorbate layer

(ah4 - u)/uM. This interaction leads to an activation barrier for the ai + /3w transformation. We suppose that the height of this barrier can be expressed as the Morse type potential function:

or, having in mind (4)

where c and y are parameters independent of 0. Thus the sticking probability for the /3w state formation will be described

by the equation:

S=S,(1-0)2exp -FT , l i

where

S, = lim S. e-8,

(8)

In fig. 5, the continuous lines represent the dependence of sticking probabil-

ity on coverage for the pw state, calculated using eq. (7). It can be seen that the agreement with the experimental results is very good. In the coordinate system

S

ln(be)* versus {I - ex,[,iI - fij]j2.

straight lines are obtained with the correlation factor 0.99. Taking y = 6, as it is usually done for the condensed system, the values of the parameter c at 298, 195 and 78 K were calculated: 11.5, 13.2 and 12.3 kJ/mol, respectively. Within

E. Nowicka, R. Dti / Adsorption kinetics of hydrogen on iron 615

the experimental error the values of e are then constant, as it should be, if the model is valid. The dependence of the barrier U, on the average distance between hydrogen adspecies on iron surface is presented in fig. 6.

It is assumed on the basis of our previus work [2] that at 0 < eM an attractive long-range order interaction of the electrical nature within the adsorbate layer exists; a similar idea was previously presented by Christmann and Ertl [26]. The relation between the attractive interaction value and the average distance a cannot be estimated in this work and is symbolically presented in fig. 6 (dashed line). The parameter e represents the attractive interaction between adatoms at 0 = 8,. The repulsive short-range order inter- action U arising at 8 > eM is described by eq. (5).

The U value increases quickly with diminishing of the average distance a between hydrogen adspecies, lowering the sticking probability. It can be extrapolated, using eq. (7), that the sticking probability reaches 10m2’ at B = 0.9 at 298 K. If the barrier U, did not exist, the sticking probability would be of the order of 10e6. ‘.

~[kllmole]

120 I

100~ I

I 80. I

I I

60, I

-loo- I\ \\ 78K 195K 298K

Fig. 6. The dependence of the potential energy of the adsorbate U on average distance between hydrogen adatoms according to the proposed model. The attractive interaction for 0 < 8, is the

dashed line. The repulsive interaction at 298, 195 and 78 K for 0 > 6, is calculated using Morse type potential (eq. (5)) on the basis of the experimental results (points), and extrapolated to 0 = 1.

4.2.2. The isothermul desorption

The observed activation energy of desorption of the pw state Ed,@, can be expressed in the form:

Ed& = . 0 t d.11, - u,,, (9)

where

E” d.CIw = lim Ed,,j, . O-~,,

(10)

and U,, is the decrease of binding energy of hydrogen deposit caused by the short-range order interaction within an adsorbate layer, expressed, similarly as

above, as the Morse type potential function:

.,,-,(l -exp[yjl - pj$. (11)

The equation describing isothermal desorption rate of the adspecies will be now

Examining the rate of the isothermal desorption shown in figs. 3 and 4. in the coordinates system

straight lines with correlation factor 0.985 were obtained. That confirmes the validity of the model. Adsorption and desorption phenomena of the weakly bound hydrogen adspecies on iron are governed by the same short-range order interaction within the adsorbed layer.

On the base of eq. (12) the parameters x and u can be calculated, assuming that at 8 = eM, L’:$, approaches activation energy of desorption characteristic of the & adspecies, that means 90 kJ/mol and y = 6. At 195 K, when the rate of desorption is not influenced by the pumping speed of our system, we found Y = 3 x 10” s ’ and x = 7 kJ/mol. One can expect that the values of the parameters c and x should be close. We suppose that the difference is the result of an experimental error during the observation of the adsorption and desorp- tion phenomena. It was recently reported 1271 that the vibration frequency of the adsorbate can depend on coverage. In the light of our results this

dependence seems to be not very strong in the hydrogen-iron system, since v is a constant.

We conclude that two atomic states of hydrogen adspecies on iron thin film surface appear as the result of an induced heterogeneity. Studying hydrogen

E. Nowcka, R. DUE / Adsorpmn kinetrcs of hydrogen on iron 677

adsorption on thin nickel films, we have found that the same model is valid

P81.

5. Conclusions

(1) Surface potential measurements in the course of the static and dynamic experiments carried out simultaneously with the determination of gas phase pressure allow us to distinguish between three states of hydrogen adsorbed on iron: the atomic /3s and /?, states and the molecular ai state.

(2) The sticking probability for the selected & state of hydrogen on iron is as low as 10-.4-10-9, because of a barrier for adsorption arising as the result of strong short-range order interactions within the adsorbate layer at high coverages.

(3) Two atomic states of hydrogen on iron, /3s and pk. appear as a result of the induced heterogeneity.

Acknowledgements

The authors have pleasure in acknowledging the master glass-work done by Mr. J. Biechohski and Mr. R. Bojarski, which made it possible to carry out our studies.

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