Vacuum/volume 41 /numbers 7-g/pages 1766 to 1768/l 990 Printed in Great Britain

0042-207x/90$3.00 + .oo @ 1990 Pergamon Press plc

Weakly bound hydrogen state on thin iron films E Nowicka, Z Wolfram and R DuS, Institute of Physical Chemistry, Polish Academy of Sciences, ul Kasprzaka 44, 01-224 Warszawa, Poland

Hydrogen and deuterium adsorption on thin iron films deposited under uhv conditions was studied over the pressure range 1 O-lo- 10-l torr within the temperature interval 78 K - 298 K, measuring simultaneously surface potential and pressure. A rapidly recording (response time 1 ms) static capacitor system and ultrasensitive Pirani-type gauge were applied to avoid atomic hydrogen presence in the gas phase. The weakly adsorbed /I; state of the adsorbate following the strongly adsorbed l(; state was clearly distinguished. lsosteric heat of adsorption, differential entropy, spreading pressure and sticking probability for the /I; state are determined as a function of coverage. Isotope effect is demonstrated.

Introduction Experimental

Thermal desorption studies of hydrogen adsorption on iron revealed two or three peaks in the spectrum for a single crystal adsorbenti.2 as well as for thin iron films3,4 or polycrystalline foi1s2. The average activation energy of desorption for the strongly bound state reaches 88-96 kJ mol-‘, while that for the weakly bound state 59-67 kJ mol-‘. It has been sug- gested’. ’ that in a hydrogen-iron system weakly bound adspe- ties appear as a result of coverage (0) induced transformation of the whole adsorbate into a new state. This weakly bound state appears rather as a result of coverage induced binding energy change between the adsorbate and adsorbent than be- cause of phase transition associated with a small energy change (of several kJ mol-i)6.7.

The experiments were carried out using a glass uhv system capable of routinely reaching (l-2) x 10-‘” torr. Thin iron films were deposited at this pressure on the active plate of the static capacitor made of Pyrex glass’, kept at 78 K, by evapora- tion of iron wire (Johnson-Matthey, grade I).

Thin films were sintered at 320 K for 30 min under uhv conditions, and next the temperature required for the ex- periments was maintained. The geometrical area of the films was N 135 cm2, their thickness N 1000 A, and the rough- ness factor 18 f 25. Spectroscopically pure hydrogen, purified additonally by diffusion through a palladium thimble, was applied.

It has been calculated that the H-Fe distance strongly influences hydrogen adsorbate binding’. The change of this distance affects the average dipole moment of the adsorbate, and should be observed through measurements of hydrogen adsorption induced surface potential changes. There is still a lack of basic understanding of surface potential-coverage effects since they are influenced by all structural and electronic changes of both the adsorbent and the absorbate.

However, it has been observeds,9%‘0 that an increase in the dipole moment at high coverage can correspond to a decrease in the heat of adsorption. The weakly bound state of hydrogen on iron has not been widely studied since its equilibrium pressure at commonly applied temperatures exceeds lo-’ torr, and in consequence instruments operated on the basis of elec- tron beam application cannot be used.

Hydrogen adsorption induced surface potential changes ASP, and hydrogen pressure P over thin iron film were mea- sured simultaneously. The surface potential measurements were performed using the static capacitor method” with the cell described previously’ and an electronic systemi of short overall response time ( N 1 ms), high sensitivity (0.1 mV) and low noise level (0.2 mV). This made it possible to study surface phenomena occurring within a short time interval e.g. during fast isothermal desorption of weakly adsorbed species.

We have found that surface potential measurements by means of a rapidly recording static capacitor system, carried out simultaneously with pressure detection in a non-disturbing manner (without production of atomic hydrogen) can be suc- cessfully applied to study weakly adsorbed hydrogen state on thin iron films. Additional information can be obtained from studying isotope effects for deuterium adsorption.

To avoid hydrogen atomization on the hot filament and the pumping effect of the ionization gauge disturbing the adsorp- tion process, in the course of the experiments hydrogen pres- sure was measured using ultrasensitive, short response time (1 decade SC’) Pirani type gauge, working within pressure interval 1 x 1O-6-1 x 10-l torr. Two types of experiments were performed:

(i) static experiments with the introduction of hydrogen in the successive calibrated doses into the capacitor cut off from pumps; (ii) dynamic experiments with a constant flow of hydrogen into the capacitor cut off from pumps.

1766

E Nowicka ef al: Weakly bound hydrogen on iron films

From the static experiments the thermodynamic and SP

isotherms are obtained:

Peq =fi(G and ASP =fi(n,)r (1)

where Peq is the equilibrium pressure over the adsorbate con- taining n, hydrogen atoms (the n, value is determined volumet- rically).

From the dynamic experiments one obtains:

P =,f;(t)., and ASP =f4(t)T (2)

where P is the hydrogen pressure in the static capacitor at time I of flow.

Combining equations (1) and (2) the effective adsorption rate can be determined:

dn. 4 = S(n,).A .Z.[P - Peq(n,)] dt

(3)

where S(n,) is the coverage dependent sticking probability, A is the true area of the then film, and Z is the collision factor.

Assuming that at n, maximal (at 78 K) a complete monolayer

of the adsorbate is formed A = nyx’ma’/Ns, where N, is the surface atom density for a thin iron film13. This method offers

the possibility of calculating S(n,) for weakly adsorbed states arising e.g. at pressure as high as lop2 torr, since the change of population is monitored directly on the surface measuring ASP,

instead of being deducted from the gas phase density change. Monitoring ASP as a function of time during isothermal evacu-

ation of the static capacitor resulting in desorption of weakly bound hydrogen adspecies, one obtains the relation:

ASP =,f;(t)T. (4)

Combining this relation with ASP =f2(n,&, the kinetics equa- tion for isothermal desorption is obtained and can be solved, thus giving the energy activation for the fi; desorption, close

to the heat of adsorption. The experiments were carried out within the temperature interval 78 K-298 K, at the pressure range 1 x IO- ‘O-1 x IO-’ torr.

Results and discussion

Hydrogen adsorption induced surface potential changes and

pressure increase in the course of the static experiment are shown in Figure 1 as a function of coverage. As previously5, in order to calculate coverage 0 we assumed that at hydrogen

uptake corresponding to the end of distinct surface potential change at 78 K (poin fIN in Figure l), at the equilibrium pressure of 1 .I x lo-’ torr, the complete monolayer is attained (0 = I). The increase in the dipole moment at coverage 8, and

the accompanying rapid pressure increase is clearly seen. These features were always observed within the whole ap-

plied temperature interval 78-298 K. As previously’, we shall call hydrogen adspecies adsorbed

before the dipole moment increase (0 < 0 < 0,) the /I;, and those adsorbed after (0, < 0 < 0,) the /J;. It can be seen in Figure 1 that lowering the pressure caused quick isothermal desorption of the /?; adspecies at temperatures 195-298 K. Redosing of hydrogen always resulted in the readsorption of the /I; state. At 78 K, however, the 8; species are stable on the surface. On the basis of the time of life consideration this phenomenon indicates that the lower limit of the energy bind- ing interval for the p; state is x24 kJ mol-‘. Features similar

A SPt,

[mvl -LO

-80

-120

-160

-200

-240

-280

-320

-360

-ioc

-LLC

0 0.2 0.i 0.6 G.8 1.0

- hydrogen

. deuterium

P

[Tor r]

10-Z

10-3

Figure 1. Surface potential isotherms for hydrogen and deuterium adsorption on thin iron films at 78 K and 298 K. Thermodynamic isotherms P,(0) for the weakly adsorbed hydrogen (deuterium) B; adspecies at 298 K are shown.

-+80

-+60

--60

,[-*0

06 07 08

8

Figure 2. Coverage dependent isosteric heat of adsorption and differen- tial entropy for hydrogen and deuterium in the /?; state on thin iron film. Triangles A correspond to activation energy of desorption Ed calculated on the basis of the isothermal desorption.

1767

E Nowicka et al: Weakly bound hydrogen on iron films

0.5 0.6 0.7 0.8

I A9 [10v7 l/cm*]

i

0 Figure 2 shows the coverage dependence of the isosteric heat of adsorption qst for the fi; state calculated according to the

50 298 K

Clausius Clapeyron equation, and differential entropy calcu- lated for the /I state according to the well known relation:

Ss=&-Rln[P-($)I (5)

where So is the entropy of gas in the standard state. The binding energy for adsorption can be calculated indepen-

dently on the basis of the examination of the /I; adspecies isothermal desorption rate at several temperatures monitoring this process by means of ASP measurements according to equation (4). The calculated values were close to qsl, as can be seen in Figure 2.

The collected data permit the calculation of the spreading pressure for the 8; state, giving a possibility of obtaining the equation of state.

The well known equation describing the spreading pressure Figure 3. The relative spreading pressure Acp for hydrogen and deu- dependence on equilibrium gas phase pressure can be rewritten terium at 298 K. in the form of two terms for the /?, and the fl; states:

298 K

l-~ln[P]=RT~Tdln[P+RT~p~Tdln(P)],

(6)

and further:

Aq = cp - ‘pp, = RT l-d ln[P], (7)

N, is the number of moles of the adsorbate, P, is the pressure at coverage BM, A is the effective surface area, and here Acp is the relative spreading pressure. The coverage dependence of Acp for hydrogen and deuterium at 298 K is shown in Figure 3.

The effective sticking probability for the /?; adspecies of hydrogen and deuterium, calculated according to equation (3) is shown in Figure 4.

10-1, 0

0.5 CL6 0.7 08

Figure 4. Sticking probability for hydrogen and deuterium adsorption on thin iron film in the 8; state at 298 K.

Acknowledgements

This work was carried out within the research project CPBP 01-08.

to those for the hydrogen-iron system were also observed for deuterium adsorption. This is presented in Figure 1.

It can be seen that &, and ON increases with adsorption temperature decrease. The 0, for deuterium at a given temper- ature is smaller than for hydrogen. We have observed that the temperature dependence of QM and ON for hydrogen and deu- terium within the applied temperature interval is almost linear. It has been suggested5 that the fi; state appears as a result of coverage induced binding energy change of the adsorbate. That should be associated with a strong decrease in the heat of adsorption within the /?; state. The increase in entropy as a result of the increase in the adsorbate mobility, and the possib- lity of some association within the adsorbate at high coverage, can also be expected. Indeed, such behavior of the /3; state is observed for hydrogen and for deuterium.

References

’ F Bozso, G Ertl, M Grunze and M Weiss, Appl Surface Sci, 1, 103 (1977). *J C Cavalier and E Chornet, Surface Sci, 60, 125 (1976). 3 G Wedler and D Borgmann, Ber Phys Chem, 78, 67 (1974). 4 E Nowicka, W Lisowski and R DuS, Surface Sci, 137, 85 (1984). 5 E Nowicka and R DuS, Surface Sci, 144, 665 (1984). 6 R Imbihl, R J Behm, K Christmann, G Ertl and T Matsushima, Surfice Sci, 117, 257 ( 1982). ’ W Kinzel, W Selke and K Binder, Surface Sci, 121, 13 (1982). s J P Muscat, Surface Sci, 118, 321 (1982). 9 R J Behm, K Christmann and G Ertl, Surface Sci, 99, 320 ( 1980). ” M Ehsasi and K Christmann, Surface Sci, 194, 172 (1988). ” T Delchar, A Eberhagen and F C Tompkins, J Scient Instrum, 40, 105 (1963). ‘*A Bachtin, Vacuum, 12, 519 (1985). I3 D Brennan, D 0 Hayward, B M W Trapnell, Proc R Sot, A256, 81 (1960).

1766