Kinetics of hydrogen adsorption and desorption on thin platinum films

Kinetics of hydrogen adsorption and desorption on thin platinum films

Applied Surface Science 31 (1988) 451-459 North-Holland. Amsterdam 451 KINETICS OF HYDROGEN ADSORPTION ON THIN PLATINUM FILMS AND DESORPTION W. LI...

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Applied Surface Science 31 (1988) 451-459 North-Holland. Amsterdam

451

KINETICS OF HYDROGEN ADSORPTION ON THIN PLATINUM FILMS

AND DESORPTION

W. LISOWSKI Institute of Physical Chemistry 01.224 Warsznwa, Poland

Received

22 October

Polish Academy

1987; accepted

of Sciences, ul. Kasprzaka 44/52,

for publication

4 January

1988

The nature of hydrogen adsorption on thin platinum films at 78,195, 273 and 298 K has been studied by means of sticking probability measurements and thermal desorption mass spectrometry (TDMS). A high value of the initial sticking coefficient (Se = 0.95) was found at all temperatures. The behaviour of the sticking probability S as a function of hydrogen coverage B has been analyzed. Three TDMS peaks of hydrogen were detected on Pt films when the adsorption was carried out at 195 K. The TDMS analysis carried out for hydrogen adsorbed at 78 K, revealed the existence of a molecular form of hydrogen deposit.

1. Introduction The interaction of hydrogen with platinum - one of the most important catalysts of hydrogenation reactions, has been extensively studied in the past. The results of investigations depend strongly on the type of Pt surface and its preparation [l-4]. Thermal desorption (TD) studies indicated the existence of several TD peaks as well on Pt single crystal surfaces [2-91 as on polycrystalline Pt surfaces [lo-131. Results on the adsorption kinetics confirm the complexity of hydrogen adsorption on platinum. Reports in the literature concerning the magnitude of the initial sticking probability S, differ considerably. For polycrystalline Pt surface S, has been reported to be between 0.10 and 0.16 [13-151 and 0.0045 [16]. The same differences were observed for platinum single crystal surfaces [3-6,17-201. Very few authors tried to analyze the dependence of the sticking probability coefficient on coverage [3]. In this work the results of an investigation of the adsorption kinetics of hydrogen on and its thermal desorption from thin platinum films are presented. The behaviour of the sticking probability as a function of coverage is carefully analyzed. 0169-4332/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

2. Experimental

All experiments were carried out in a glass UHV apparatus capable of reaching routinely pressures of (l-2) X lo- “’ Torr. Platinum films were deposited on Pyrex glass supports maintained at 78 K by evaporation of a thin, 1 X 1O-4 m diameter Pt wire (Johnson Matthey. grade I) coiled on a 3 X 10 4 m diameter tungsten heater, at pressures lower than 5 x lo-- ‘(’ Torr (1 Torr = 133.3 Pa). After evaporation the films were sintered at - 330 K for - 30 min. The investigations of the adsorption kinetics (AK) and TDMS were carried out using separate cells described previously [21]. The geometrical areas of Pt films used in the AK and TDMS experiments were 480 and 160 cm?, whereas their thickness was - 600 and - 1000 A respectively. Several independent experiments were carried out to estimate the roughness factor of the applied platinum films. As for Pd films [22], hydrogen-oxygen titration with the application of surface potential changes [23.24] was performed for this purpose. The roughness factor of routinely obtained Pt films used in AK experiments was within the limit 4 f 1. Spectroscopically pure hydrogen, purified additionally by diffusion through a palladium thimble, was used. The adsorption of hydrogen was performed at temperatures of 78. 195, 273 and 298 K. The sticking probability coefficient was measured using the method described by Hayward and Taylor [25], but a corrected version of the equation for the sticking probability was applied [26]:

s(e) =

L-

(V/kT)

L+zA(P-P,,)

dP/dt 5

(1)

where L is the flux through the capillary, P the hydrogen pressure at time t, Peq an equilibrium pressure for a given hydrogen coverage. A the area of the film. V the volume of the cell, T temperature, k Boltzmann’s factor and Z the collision factor. During the AK experiments, the pressure was measured continuously by means of the previously calibrated Groszkowski-type ionization gauge up to - 1 X 10e4 Torr. The TDMS measurement technique and peak analysis method were the same as described previously [27,28]. The mass spectrometer (Topatron. Leybold) was used for hydrogen pressure determination during the thermal desorption process. The temperature was measured by means of three separate chromel-constantan fine thermocouples kept in contact with the outer wall of the cell.

W. Lisowski / Hydrogen aa?orprion /desorprion on thin Pi films

453

3. Results and discussion 3. I. The analysis of the thermal desorption spectra

Fig. la shows the TDMS spectrum for hydrogen from platinum film obtained in adsorption at 78 K up to the final equilibrium pressure of H, (- 1 X 10P3 Torr). The TDMS spectrum is characterized by the existence of three distinct desorption peaks. The first peak denoted I, with a broad maximum at the maximum temperaa superposition of two peaks. That can be ture T,, = 205 K is probably suggested on the basis of an analysis the results of which are presented in the

Fig. 1. TDMS spectra of hydrogen from platinum film. (a) Adsorption was carried out at 78 K and the final equilibrium hydrogen pressure was - 1 X 10e3 Torr. The test of the kinetic equation of desorption for first- and second-order desorption (n) of peak I is presented in the insert. (b) Adsorption was carried out at 195 K and the final equilibrium hydrogen pressure was - 1 X 10 ~’ Torr.

insert to fig. la. The first-order kinetic equation describes the low temperature part of peak I, and may point to a molecular state of the hydrogen adsorbate. The activation energy of desorption Ed,,, = 10.8 kJ/mol estimated for this component of peak I fits this suggestion. The second part of peak I (T,,,, at higher temperature) is well described by the second-order kinetic equation for desorption. This may correspond to dissociative hydrogen adsorption. The estimated activation energy for desorption (51.6 f 2.5 kJ/mol) agrees well with the one reported by Stephan et al. [lo] for Pt films (50.2 kJ/mol). The peak denoted II. with a maximum at 7;,,,, = 370 K is well described by the second-order kinetic equation for desorption. The estimated Edc. = 89.9 _t 2 kJ/mol is similar to that published by Stephan et al. [lo] (87.9 kJ/mol) and corresponds to the initial heat of hydrogen adsorption on Pt film measured calorimetrically by Cerny et al. [29] (88.9 i S kJ/mol). The peak denoted III, with a maximum at 7;,,;1, = 430 K is also well described by the second-order kinetic equation for desorption. It can be supposed that the creation of the adsorption state of hydrogen corresponding to this peak is caused by some defects on the surface of the Pt films. A high temperature TDMS hydrogen peak of similar nature was observed by Christmann et al. [3] on the sputtered “imperfect” Pt( 111) surface, by Collins and Spicer [2] on the stepped Pt 6( 111) X (100) and Pt 6( 111) x (111) surfaces. and by Davis and Somorjai [9] on the Pt(12,9.8) and Pt(lO.8.7) surfaces. Fig. 1 b shows the TDMS spectrum for hydrogen on Pt film after H, adsorption at 195 K. Three TDMS peaks are detected at T,,,i,y = 242, 365 and 420 K respectively. All peaks are well described by the second-order kinetic equation for desorption. Estimated activation energies for desorption are: 51.3, 89.9 and 103.4 kJ/mol respectively. The presence of a molecular hydrogen state on the Pt films was not observed at this temperature. 3.2. The meusurements

of sticking prohahili~~~

The dependence of the sticking probability on coverage for hydrogen adsorption on platinum films at 78, 195, 273 and 298 K is presented in fig. 2. The initial sticking probability SC,is independent of temperature, within the range studied. and indicates a nonactivated process of adsorption. The sticking coefficient remains constant until the coverage reaches a saturation level of about 50% at 195-298 K and of about 80%’ at 78 K. This behaviour may correspond to a precursor state for dissociative adsorption of hydrogen on Pt at these temperatures [30]. Similar relations between sticking probability S and coverage were reported for various temperatures by Thrush and White (121 for Pt foil, and also by Breakspere et al. [13] and Norton and Richards [15] for polycrystalline Pt wires. The initial sticking coefficient S,, = 0.95 determined in this work is significantly higher than the values reported in the literature for polycrystalline Pt

W. Lisowski

/ Hydrogen adrorptron /desorption

on thin Pi films

455

78K

0 Fig. 2. Sticking

0.l

probability

0.2

03

dependence

OL

0.5

0.6

07

on coverage for hydrogen 78,19X273 and 298 K.

08

09

adsorption

10

-Q

on platinum

films at

(0.10-0.16 [13,15]), and single Pt crystals (0.10-0.35 [3,4,6,17,20]). It is well known, however, that clean surfaces of platinum films prepared by deposition at 78 K exhibit remarkable hydrogen adsorption characteristics, different from those of films deposited at temperatures > 273 K [24]. Recently Thrush and White [12] pointed out the important role of the way of polycrystalline Pt preparation on the nature of hydrogen adsorption. A significant influence of defects on the initial sticking coefficient for hydrogen adsorption on single Pt crystals was also reported [19,20]. It can be seen in fig. 2 that the increase of the adsorption temperature decreases the hydrogen uptake. This is reasonable in the light of the TDMS results described above. The character of the decrease of sticking probability as a function of coverage is carefully analyzed. Results are presented in figs. 3 and 4. In fig. 3a are shown the experimental sticking probability data (points) for hydrogen adsorption at 298 K on Pt films versus coverage. The analysis results of the relation S =f(0) are presented in figs. 3b and 3c. It can be seen (fig. 3b) that the decrease of S with the increase of coverage is well described by the simple Langmuir relation: S=S,(l

-e)‘,

which indicates TDMS results.

(2) second order The behaviour

of hydrogen adsorption, of S =f( f?) for higher

in agreement with coverage is however

456

W. Lisowskl / H_ydrogen aabrpiron

s I 1

I

/desorpt~on

on thin Pt films

.

J...,

a)

b]

l-10

cl

1.10

1.10 I

0

01

02

0.3

Ok

-t 3

033+

- y&7

Fig. 3. (a) Sticking probability dependence on coverage for hydrogen adsorption on platinum film at 298 K. The arrow marked by L indicates the beginning of the validity of the Langmuir model for adsorption. The arrow marked by E indicates the beginning of the validity of the Elovich equation for hydrogen adsorption. (b) Examination of the Langmuir model for hydrogen adsorption on Pt film at 298 K. The test includes the experimental data points marked by arrow L on fig. 3a. (c) Examination of the Elovich equation for hydrogen adsorption on Pt film at 298 K. The test includes the experimental data points marked by arrow E on fig. 3a.

described by the Elovich equation, energy for adsorption on coverage: S = A(1 - 8)‘exp(

-aB/RT),

with a linear

dependence

of the activation

(3)

where A and (Y are constants. This is shown in fig. 3c. Examination of the corresponding part of the experimental dependence S = f( 8) in the coordinate system ln[S/(l - 8)2] versus 6 gives a straight line. Fig. 4 shows the results of the S =f(B) analysis for hydrogen adsorption at 273 and 195 K on Pt films. At the beginning, at lower coverages, the sticking probability decreases with an increase of the hydrogen population according to the Elovich equation for both temperatures (fig. 4a). This description is however inaccurate at higher coverages. It can be expected on the basis of the analysis of TDMS data for hydrogen adsorption on Pt at 195 K (fig. lb) that at 195 and 273 K at high population a new form of hydrogen on Pt film appears. This expectation is confirmed by the results of kinetics investigations and particularly by the course of S = f( 0) at 273 K (fig. 4a).

W. Lisowski

s I

/ Hydrogen adsorption /desorption

al

1 .. .... . .. I .. . . ,. I. ... , . . . . . . . iI E/ : 273K

M;’

on thin Pt films

457

195 K

.,

Fig. 4. (a) Sticking probability dependence on coverage for hydrogen adsorption on platinum film at 195 and 273 K. The arrows marked by E indicate the beginning of the validity of the Elovich equation for hydrogen adsorption. The arrows marked by M indicate the beginning of the validity of the Morse-type potential. (b) Examination of the Morse-type potential application for the description of the influence of the repulsive interaction in the adsorbate layer on sticking probability for hydrogen adsorption at 195 and 273 K.

It can be supposed that the new form of hydrogen deposit at high coverage arises as a result of the induced heterogeneity. The repulsive interaction that occurs at high population in the adsorbed layer, forms a potential barrier AU for further adsorption. Such a model has been proposed for hydrogen adsorption on thin iron films [31,32]. The height of this barrier can be expressed as a Morse-type potential function. This leads to the following equation for s(O): S = S,(l

- 0)2exp( -AU/RT),

(4)

where

ACT=+

-exp[y(l

-/m)]}‘,

and S,,, c and B,

(5)

are parameters

that are experimentally

is taken 6 as it is usually done for condensed The results of the application adsorption

at 273

and

195

determined

while y

systems.

of eq. (4) for the analysis

K are presented

in fig. 4b.

of S = f‘( 0) for H1 In the coordinate

system: ln[S/(l straight

- S)l]

versus (1 - exp[ y(1 - @M/B)]

iL

lines were obtained.

The calculated values of the parameter c at 273 and 195 K are 0.40 and 0.42 kJ/mol respectively. The parameter L being independent of temperature fits the applied model. This model can explain the existence of peak TDMS spectrum for hydrogen adsorption at 195 K (fig. lb). The interpretation is difficult surface.

of the S = f( 0) relation

because

of the coexistence

Nevertheless,

the linear

for hydrogen

of several

character

hydrogen

of the S =f(s)

I in the

adsorption states

at 78 K

on the Pt

function

(fig.

2)

seems to be indicative of a dominant role of the molecular adsorption for higher coverage. This behaviour is in agreement with the TDMS data (fig. la).

4. Conclusions (1) Hydrogen adsorbs dissociatively on Pt films in the range of temperature 78-298 K via a precursor state. The initial sticking probability S,, = 0.95 remains constant at these temperatures. (2) The decrease of the sticking coefficient

S with an increase

of coverage

for hydrogen adsorption on thin platinum films at 298 K can be described by the Langmuir and the Elovich equations. At high population arising of a new form of hydrogen

deposit,

The S =f(f3) function barrier for adsorption. (3) The characterized equation

TDMS

spectrum

by three

describes

as the result of induced

can be described

peaks.

desorption

of hydrogen Analysis

adsorbed shows

of the whole

state arises as the result of molecular

heterogeneity,

is observed.

then using the Morse-type

that

deposit.

hydrogen

potential

at 195 K on Pt film the second-order

is

kinetic

At 78 K an additional

adsorption.

Acknowledgements The author is grateful to Dot. dr. hab. R. DuS for valuable discussions and remarks, and to Z. Wolfram for his experimental contribution to this work. This work was carried out within the Research Project 12.2.94 of the Polish Academy of Sciences.

W. Lisowski

/ Hydrogen adsorption /desorption

on thin Pt films

References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] (271 [28] [29] [30] [31] [32]

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