Adsorption of hydrogen on thin iron films

Surface Science 137 (1984) L85-L91 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS A D S O R P T I O N OF H Y D R O G E N O N THIN IRON FILMS

E. NOWICKA *, W. LISOWSKI and R. DUE Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Ol- 224 Warszawa, Poland Received 28 June 1932; accepted for publication 9 December 1983

Hydrogen adsorption on thin iron films was studied through the application of surface potential measurements by means of a rapidly and continuously recording static capacitor system and by TDS experiments. Three electronegatively polarized states of the adsorbate were detected: (i) strongly bound fls adatoms, (ii) weakly bound flw atomic adspecies, and (iii) molecular deposit denoted a E . Thermal disorder on the surface diminishes the average dipole moments and the depolarization effect within the/~s adspecies.

Investigation of hydrogen adsorption on iron showed that at least two states of adspecies exist on thin iron films or polycrystalline surfaces [1-9], as well as on (110), (100) and (111) iron single crystal planes [10,11]. The strongly bound, electronegatively polarized state [11-14] arising due to dissociative adsorption [1-4,10,11] is characterized by an activation energy of desorption of 20.3 kcal/mol [15] to 23 kcal/mol [9,11]. There are few data concerning the nature of the weakly bound state. The activation energy of desorption of this state was found to be - 1 8 kcal/mol [9,11]. Some investigators suggested that the weakly bound adspecies of hydrogen on iron are of atomic character [2-4,9,11], while others rather expected a molecular deposit for this state [8]. An electropositive polarization of the weakly bound hydrogen adspecies on the iron surface was predicted [4,8]. The aim of the present work was to recognize experimentally the character of the weakly bound hydrogen adspecies on iron thin film surfaces, distinguishing between them and the strongly bound state not only in the course of thermal desorption, but also during adsorption. We expected that measurements of the surface potential (s.p.) carried out by means of a rapidly and continuously recording static capacitor method [16] together with volumetric measurements on the one hand and observation of TDS spectra on the other hand would give us these possibilities. * Space Research Centre, Polish Academy of Sciences, Ordona 21, 01-237 Warszawa, Poland.

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

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E. Nowicka et al. / Hydrogen on thin iron films

The s.p. experiments were carried out in a glass UHV system. Iron films deposited at 78 K on the outer electrode of the cylindrical capacitor by means of evaporation of a thin iron wire (Johnson-Matthey grade I) from a tungsten heater at pressure not exceeding 3 × 10-10 Torr, were sintered at - 330 K for 30 min. The geometrical area of the films was 135 cm 2, their thickness - 1000 ,~. The reference electrode was made of Pyrex glass coated with SnO. The overall response time of the electronic circuit of the static capacitor was less than 0.1 s and its sensitivity + 1 mV. This gave us the possibility to register s.p. changes during the adsorption, or desorption processes. During adsorption, the pressure of hydrogen was measured by means of a specially constructed, ultrasensitive Pirani-type gauge characterized by a short response time (to be published), to avoid atomization or pumping effects which could be caused by hot filaments of the ionization gauge or mass spectrometer. Spectroscopically pure hydrogen, additionally purified by diffusion through a palladium thimble, was used. Two types of experiments were performed: (i) A static experiment, with hydrogen introduced in calibrated successive doses to the capacitor, which was cut off from the pumping system, until resulting s.p. changes were unmeasureable, and the appropriate increase of pressure (in the 10 1 Torr range) indicated that the adsorption was finished. (ii) A dynamic experiment, with a constant flow of hydrogen through a calibrated capillary into the capacitor, which was cut off from the pumping system, with continuous s.p. and pressure recording (up to - 10-1 Torr). TDS experiments were performed in a separate glass UHV system equipped with a mass spectrometer gauge (Topatron, Leybold Co.). In these experiments, hydrogen was introduced in calibrated doses into the spherical cell, the walls of which were coated with a thin iron film deposited under conditions similar to those described above. This allowed us to measure volumetrically the amount of hydrogen adsorbed and to compare this amount with that desorbed as calculated on the basis of the TDS spectra. The agreement was better than 5%. During desorption, the continuously pumped cell was immersed into an appropriate bath with effective stirring to keep the temperature even all along the walls. The hydrogen pressure PH2 and the temperature I were measured simultaneously as functions of time t. Knowing the course of the functions PH2 = f l ( t ) and T=f2(t), we can solve the kinetic equation of desorption. The dependence of the s.p. of iron thin films on the amount of hydrogen adsorbed (s.p. isotherms) at 78, 195 and 298 K is shown in fig. 1. The shape of s.p. isotherms clearly indicates that at least two adsorption states of hydrogen on iron exist. The first state which appears at the beginning of adsorption we shall call fls, the second state following the fls we shall call flw. The first kink in the s.p. isotherms (fig. 1) indicates the end of the fls formation and the beginning of adsorption of the flw adspecies. The second kink corresponds to the end of the flw formation. Starting from that point successive typical doses

E. Nowicka et al. / Hydrogen on thin iron films

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Fig. 1. Surface lbotential isotherms showing the dependence of the surface potential of thin iron film on the amount of hydrogen adsorbed at 78, 195 and 298 K. The equilibrium pressure at some population of the adsorbate is shown (in Torr).

of hydrogen introduced into the system did not result in any measurable change of s.p. However, the volumetric measurements indicated that some further adsorption occurred. Introduction of several large H 2 doses (hundred times larger than a typical dose) caused an additional small increase of s.p. ( - 10 mV). We suppose that this step of adsorption corresponds to the formation of the third, weakly adsorbed, and very slightly polarized hydrogen adspecies on the iron thin film surface. We shall call this state a E . It should be emphasized that at all temperatures only electronegatively polarized hydrogen adspecies; the fls, flw and a E , were formed on the surface of thin iron films, in contradistinction to hydrogen adsorption on palladium [17], niobium [18], nickel [19], cobalt [20] and platinum [21]. The existence of the three adsorption states of hydrogen on the thin iron film surface can be clearly observed in the course of the dynamic experiments, when hydrogen flows continuously through the capillary into the capacitor which was cut off from the pumping system. The results of these experiments (s.p. versus time) are shown in fig. 2. We can see that the fls state formed at the beginning of adsorption is followed by the flw state. At the end of adsorption, a quick increase of the hydrogen pressure was accompanying very small changes of s.p. Isothermic evacuation of the system (called isothermic desorption) at 298 and 195 K caused a characteristic change of s.p. as shown in the enlargements in fig. 2. A rapid increase of s.p. by - 8 mV a t the beginning of the isothermic desorption, was followed by a slower increase of s.p. The rapid increase of s.p.

L88 sp.

E. Nowicka et aL

/ Hydrogen on thin iron films

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Fig. 2. Rate of s.p. variations recorded during the dynamic experiments with a continuous hydrogen flow into the capacitor at 78, 195 and 298 K. The effect of the evacuation of the system on the s.p. is shown. In the enlarged portions of the graph the result of the evacuation at 195 and 298 K is shown. The rapid change of s.p. due to the desorption of the aft state can be clearly distinguished from the increase of s.p. caused by the slower desorption of the Bw state.

is, in our opinion, associated with the desorption of electronegatively polarized aft adspecies, while the slower increase of s.p. corresponds to the desorption of the flw adspecies. The isothermic evacuation was carried out during 1000 s. At 298 K, complete desorption of the aft and the flw adspecies and the removal of a small amount of the /Is adspecies was observed, while at 195 K complete desorption of the aft adspecies and partial desorption of the flw adspecies was registered. All deposit of hydrogen was stable at 78 K within the accuracy of our measurements. This means that the heat of adsorption of a E adspecies should be higher than 5 kcal/mol. The existence of the a E adspecies on thin iron films precovered with the fls and the flw adspecies was also indicated by isotherms showing the relation between the equilibrium pressure of hydrogen over the adsorbate, and the amount adsorbed, measured in the course of the static experiments. During the formation of the fls state, the equilibrium pressure was below the readings of our Pirani gauge, however, for the flw state at 298 K and partially at 195 K the pressure could be registered. The flw state is well described by the Langrnuir isotherm for dissociative and mobile adsorption, or by the Tiomkin isotherm, while the Henry-type isotherm corresponds to the aft state, indicating its molecular character and mobility on the surface.

E. Nowicka et al. / Hydrogen on thin iron films

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In fig. 3 are presented TDS spectra of hydrogen adspecies on a thin iron film surface, obtained when: (i) the adsorption occurred at 78 K, and (ii) the adsorption was carried out at 298 K and next the system was rapidly cooled down to 78 K without evacuation. Two desorption peaks denoted by A and B, characterized by activation energies of desorption of 16.2 kcal/mol and 21.4 kcal/mol, correspondingly were observed. The populations of the adspecies corresponding to the peaks A and B we shall call r/A and n a. The ratio r / A / n B increases with the increase of the adsorption temperature. The same phenomenon can be seen in fig. 1 for the flw and the fls states. We noticed that n g / n B ~ n#,,,//n#s. We suppose that the peak A corresponds to the adsorption state flw and the peak B to flsIn fig. 1 it can be clearly seen that there is a nonlinear dependence of s.p. on n#s at 78 K. This nonlinearity is less pronounced at higher temperatures. At low temperatures, the deposit is undoubtedly less mobile, and we suppose that the depolarization effect due to an interaction in a planar network of dipoles can be responsible for the nonlinearity of s.p. Indeed, the observed phenome200

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Fig. 3. TDS spectra of hydrogen adsorbed on iron thin films at a final pressurepH 2 of the order of 10 -2 Torr. Spectrum I corresponds to hydrogen adsorption carried out at 78 K. Spectrum II corresponds to hydrogen adsorption carried out at 298 K, followed by a rapid freezing of the system down to 78 K, at a hydrogen pressure of the order of 10 - 2 Torr.

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E. Nowicka et a L / Hydrogen on thin iron films

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Fig. 4. The dependence of 1/(s.p.) versus 1/n~s (Helmholtz relation with the Topping correction for depolarization) for hydrogen adsorption on thin iron film at 78 K. The 1 / T dependence of/t o showing the influence of the thermal disorder on the calculated dipole moment of the fls adspecies is presented in the insert.

non is well described by the Helmholtz equation for s.p. (s.p. =f(nads) ) corrected by the well known Topping relation taking into account the depolarization effect caused by an interaction planar network of immobile dipoles [22], as it is shown in fig. 4. The existence of an ordered structure of hydrogen adspecies on surfaces of some single crystal planes of iron has been previously confirmed by way of LEED pattern findings by Bozso et al. [11]. On the basis of the corrected Helmholtz equation, the polarizability of the fls adspecies at 78 K was calculated to reach 1.6 × 10 -24 c m 3. In agreement with our above assumption, one can see in fig. 1 that the average value of the (perpendicular to the surface) component of the dipole moment of the fls adspecies, #0, at coverage close to zero decreases with the increase of temperature. In the coordinate system/% versus 1/T a straight line was obtained (see fig. 4). According to the Langevin equation, a thermal disorder on the surface is responsible for this phenomenon. This thermal disorder can also diminish the depolarization effect.

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E. Nowieka et al. / Hydrogen on thin iron films

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