Kinetics of hydrogen adsorption on thin iron films with preadsorbed hydrogen sulfide

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  • React. Kinet. Ca~al. Lett., Vol. 27, No. 1, 77~81 (1985)

    KINETICS OF HYDROGEN ADSORPTION ON THIN IRON FILMS WITH PREADSORBED HYDROGEN

    SULFIDE

    P. Nowacki* [ , W. Lisowski and R. Dug

    Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland

    * Space t~eseareh Centre, Polish Academy of SCiences, ul. Ordona 21, 01-237 Warszawa, Poland

    l~eeeived March 5, 1984 Accepted April 18, 1984

    The influence of H~S preadsorbed on thin iron filnq surface on the character of TD spectra and on sticking probability dependence on population for hydrogen adsorption was studied. Kisliuk's model for adsorption with the precursor state was examined.

    IdCCJ~e/IOBaH~ Boag~Rge npe~aneop6u~u H2S Ha TOHKHX HHeHKax ~

  • NOWACKI et al.: THIN IRON FILMS

    ffl

    c

    c~

    z3

    i

    0~, ' ~ -~

    0 r ' -~

    Tempero~ur% K

    Fig. 1. I-Iydrogen TD spectra from thin iron films, clean (a) and preeovered with H~St at 195 K (b) and at 298 K (c)

    I t can be seen in Fig. 1 that H2S preadsorbed under the conditions described above, does not change the main character of hydrogen deposit. Two desorption peaks, characterized by activation energies of desorption 90 kJ/mol and 67 kJ/mol were observed, similarly to previous reports for the hydrogen- iron system [3]. However, the amount of hydrogen adsorbed depends strongly on the character of HaS deposit. I-I2S preadsorbed at 195 K significantly dimi- nishes hydrogen adsorption. The ratio of the amount of hydrogen adsorbed on ir~on film precovered with I.I~S at 195 K to the amount adsorbed on a clean surface was found to be 0.5 and 0.4 on the basis of TDS and volumetric cal- culations, respectively. On the other hand, It2S preadsorbed at 298 K increases the amount of hydrogen consumed. The ratio mentioned above reaches now 1.7 and 1.6, respectively.

    We suppose that molecularly adsorbed tteS poisons the thin iron film surface. for hydrogen adsorption, occupying the most active sites, while dissoeiatively adsorbed H2S, changing the character of hydrogen adatom polarization from electronegative to eleetropositive [6], promotes the incorporation of hydrogen below the surface. Drastic increase of permeability of iron foils for hydrogen in the presence of HaS [7] confirms this assumption.

    The same conclusion can be drawn on the basis of sticking probability (S), dependence on hydrogen population N H for clean and HaS precovered thin iron films (Fig. 2). In Fig. 2 curve A corresponds to hydrogen adsorption or~

    78

  • NOWACKI et al.: THtN IRON FILaMg

    clean iron surface, while B and C are connected with adsorption on iron surface precovered with 2.49 H.~S molecules/cm '~ and 2.77X10 i4 H~S mole- eules/cm 2, respectively. One can notice that a small coverage of H~S such as 4 -a (curve B Fig. 2) influences significantly the S (N~) behavior. The curve C in Fig. 2 resembles the S (N~) relation for the process of hydride for- mation associated with hydrogen incorporation below the surface [1, 8].

    h

    $

    i~ - \ . _

    ~d 3 I

    4 i

    i 4

    A ~B C

    -SL_.__ T ! I [ -,. ~0 0 i 2 4 5

    Fig. 2. Sticking probability dependence on hydrogen population for adsorption on clean (A), and H2S preeovered thin iron films (C and ]3)

    It was interesting to examine the validity of Kisliuk's model [ 9] for adsorption with the precursor state for the curves A, B and C in Fig. 2, as was done pre- viously for the hydrogen-iron system [10]. According to this model [9]

    S 1 -O - - - 0 0 2 ( ] ) S~ 1-t 1 -0 K+I -~-O -S~

    lim where O is hydrogen coverage, S O = 0 -~0 S, and K is a parameter independent of O, showing the relation between the probability of adsorption, migration and desorption of the precursor state.

    71L

  • NOWACKI et aL: TH IN IRON F ILMS

    The question arises how to express O in the presence of H2S adspecies and hydrogen penetration below the surface. One can express 0 in two ways :

    (i) 0 = NHmax(Fe)

    Where NH max (~e) is the maximal uptake of hydrogen on a clean iron surface. With this relation for curves B and C, O can be higher than 1.

    NH {ii) O~ =

    Nltmax(z)

    Where Nm~,~(z) is the maximal uptake of hydrogen in every individual experi- ment. The results are shown in Fig. 3 (parts I and II). Points correspond to the experimental results, lines are calculated using eq. 1. The continuous line cor- responds to adsorption on a clean iron surface, dashed lines to adsorption on the surface with preadsorbed It2S. Similarly as was found previously [10], hydrogen adsorption on a clean thin iron film surface fits Kisliuk's model well. In contrast to that, preadsorption of H2S at 298 K changes the character of the kinetics of hydrogen adsorption so strongly that eq. 1 no more describes this process.

    t

    -4, f

    1o II- I Part '~

    ! __ i I I [ ~d 0.2 0.4 o.e o.8 to ~.2 i.4 1.6 L8

    e

    ]

    S

    1(; 3

    ~5

    ~x, x

    - ",',. \

    "I Part I1

    I 1 I t '1 0.2 0.4 0.6 0.8 7.0

    e=

    Fig. 3. Exanf inat ion of Kisliuk's mode l for hydrogen adsorption on clean (continuous line) and ~[2S preeovered (dashed line) thin iron films for S (o) (Part I) and S (o z) (Part It). Points correspond to the exper imental results

    ,~0

  • NOWACK[ et al.: THIN IRON FILMS

    REFERENCES

    1. P. Nowaeki, 1~. Dug: Polish J. Chem. 55, 2387 (198l). 2. D. O. t tayward, N. Taylor: J. Sei. Instr., 41, 327 (1967). 3. E. Nowicka, W. Lisowski, R. Dus: Surface Sei. (in press). 4. D. Brennen, D. O. I-[ayward, B. M. W. Trapnell: Proe. 1%oy. Soe. (London) A 256~

    81 (1960). 5. I. E. Den Besten, P. W, Selwood: J. Catai, 1, 93 (1962). 6. I. Szymerska, W. Palczewska: Rocz, Chem., 45, 435 (1971). 7. W. Palczewska, I. 1%atajczykowa: Buil. Acad. Polon. Sci. Set. Sci. Chim., 9, 267

    (~961). 8. P. Nowacki, 1%. Dug: 1%ocz. Chem., 51, 103 (1977). 9. P. Kisl iuk: J. Phys. Chem. Solids, 5, 78 (1958).

    10. P. Nowacki, W. Lisowski, 1%. Dug: React. Kinet. Catal. Left., 26, 297 (1984).

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