Model of atomic hydrogen adsorption on thin gold film surface

Vacuum/volume45/numbers 2/3/pages 299 to 301/1994

0042-207X/94S6 00+ 00 © 1993 Pergamon Press Ltd

Printed m Great Bntam

Model of atomic hydrogen adsorption on thin gold film surface l S t o b i f i s k i a n d R D u ~ , Instttute ofPhystcal Chemmtry, Pohsh Academy of Sciences, 01-224 Warszawa,

Kasprzaka 44/52, Poland

A model for hydrogen adsorptton on surface of thin gold him was proposed on the basts of measured parameters (0 mttml heat of adsorptton, (H) activation energy for desorptton at low coverages (0 ~ 0 01), (tit) heat of hydrogen solutton m the bulk of thm gold him, and (tv) calculated acOvatton energy for dtssocmtwe H2 adsorpbon

I. Introduction It has been well estabhshed experimentally ~ 6 and explained theoretically 7 ' that molecular hydrogen adsorption on noble metals is strongly activated For that reason only H_~ molecules of high translational energy can be dlssoclatwely adsorbed ~° ~: On the other hand, atomic hydrogen can be easily adsorbed on noble metal surfaces even at low temperatures ~ 246 The value of ach~atlon energy for H2 dissocmtlve adsorption on gold surface Is unknown up to now Since the associative desorptlon of hydrogen deposit from noble metals occurs 6 1 ~ then knowing the ~alues of heat of adsorption and activation energy for desorptlon, the height of the acUvatlon barrier for Ha d~ssoclatlve adsorption E,~, ,d can be estimated Estimatmn of Eacta d w a s the aim of this work

2. Experimental and the method of calculations A Pyrex glass uh~ system capable of routinely reaching (13) x 10 TM torr has been applied as was prevmusly described ~ The apparatus was eqmpped with a mass spectrometer, a Schulz Ionization gauge working within the pressure interval 10 5-10 torr and a cell allowing one to deposit m ,ttu thin gold films under uhv condltmns and to generate atomic hydrogen The spherical cell (qb = 7 x 10 ~" m) with two tungsten filaments (~bv~ = 2 5 x l0 4 m) was apphed One of the filaments was used as a heater for gold wire (qSa. = 1 x 10 4 m, Johnson-Matthey. grade I) evaporation and the second one was apphed as an atomic hydrogen generator Temperature distribution along this second filament as a function of heating current was carefully determined by means of an opUcal nucropyrometer with accuracy _+ 5 K Thus the efficiency of hydrogen atomization could be controlled ~4 Thin gold film (0 020 g Au) was deposited on the inner wall of the cell malntamed at 78 K The rate of the gold deposition was ~ 1 x 10 ~ g Au min- ~ Non-transparent thin film of golden colour (average geometrical area was ~ 1 5 x 10 2 m-' and a~erage thickness was ~ 70 nm) was obtained The film was sintered at 420 K under uhv conditions for ~ 3 0 mm Spectroscopically pure hydrogen purified additionally by diffusion through a palladium thimble was used In the course of the expemments with atomic hydrogen generation, the cell was d~sconnected from the diffusion pumps by means of a glass greaseless valve The temperature of the thin gold film T ~ was determined usmg three chromel-constantan thermocouples kept

m touch with the outer wall of the cell (at the top, in the middle and at the bottom) The difference in temperature reading was a maximum of 2 K The purity of the thin gold film surface was occasionally examined in a separate uhv apparatus equipped with a C M A Auger spectrometer It was found that thin gold film surfaces were free of any metallic ~mpurltles which could d~ssocmtlvely adsorb molecular hydrogen The structure of some thin gold films was also analysed by means of the X-ray method The reflexes (111) were dominating, showing textured structure This observation agrees well with the result reported by von Gelger et al ~s It has been shown 6 that at exposure of thin gold films to (H + H 2) atmosphere of low concentration of atomic hydrogen ( p . of the order of 10 9 10-0 torr), adsorption on the surface occurs without slgmficant solubility In the bulk The solublhty is usually observed as a high temperature diffusion tall in the T D spectrum N o diffusion tall was registered under the above condltlons 6 ~6 To determine the heat of hydrogen adsorption on a thin Au film, isotherms showing coverage 0 in eqmhbrlum with atomic hydrogen in the gas phase at pressure PR were obtained Atomic hydrogen pressure was calculated 6 on the basis of measurements of the rate of H adsorption on thin Au film at 78 K The rate of H adsorption dN~a/dt linearly decreases with increase of hydrogen adatoms populatmn N,d on the adsorbent surface as It is shown in Figure 1, and can be described by the equation (1) dN.d/dt

= JHS0( 1 -- 0) -- JH)'O

( 1)

where JH lS the total atomic hydrogen stream Impinging the thin gold surface per second, So IS the initial sticking probability for H on the thin gold film surface at 78 K 6, )' IS the probablhty for recombination via the Eley-Rideal mechanism 6, 0 is the hydrogen coverage on the thin gold film surface (0 is defined usually as 0 = N~d/N,j~x, where NMAXlS the value determined previously 6) Parameters of the equation (1) are directly determined by the slope and Intercept of the straight line in Figure 1 It was found previously 6 that So reaches 6 x 10 ~ Atomic hydrogen pressure p , can be expressed ~7 as vu[cm-2s pu[torr] =

I ] ( T H [ K ] M H [ g m o 1 1])l,2 3 51 x 1022

(2)

where VH = (JR[at H s ~]/AA.[cm2])--atomlc hydrogen colhsion frequency with gold surface, AAo--the real surface area of thin 299

L Stob#rfsk# and R Dug H adsorption on Au

I

dNa¢ dt

eeq

,101415

05-

13 0411 9

0,3-

7 02-

5 3

01-

1 12

24

36

46

60

72

84

96 108

0

120 Ned[otH] .1016

Figure 1 The rate of atomic hydrogen adsorption on thin Au film surface dN.a,,'dt at 78 K as a function of H~d population N,d and atomic hydrogen pressure pH(~), where 6 ~ - 1 for P i t ~ 5 x l 0 s tort. ~ = 2 for P i t ~ 2 x l 0 7 t o r r . ~ = 3 f o r p ) l ~ - 6 x I 0 -7torr,-, = 4 f o r p n ~ 3 X l 0 6 torr

gold film 6, T . - - a v e r a g e t e m p e r a t u r e of H atoms, M . - - m o l e c ular mass o f atomic h y d r o g e n Activation energy for h y d r o g e n desorpUon was calculated on the basis of T D (thermal desorptaon) spectra The h y d r o g e n pressure PH~ a n d the t e m p e r a t u r e TA~ was measured simultaneously as a function of time t K n o w i n g the course of the functions PH. = J~ (t) a n d Ta. = [At) the h y d r o g e n a d a t o m population was calculated a n d the time dependence of coverage 0 = J 3 ( t ) m the desorptlon process was determined By solving the kinetic e q u a t i o n for desorption d O / d t = 0%,exp ( - - E d / R T A ~ )

-20

-19

-15

-14 In ['PH(Torr)7

78 88 1 2 8 = 0 6 1 0 6 9 l , w h l l e w e o b s e r v e d 0 5 8 0 8 3 1 T h u s we suppose t h a t we o b t a m e d T l o m k i n ' s type isotherm indeed Calculated initial heat of a d s o r p t i o n Q O (where 0 --* 0) is equal to 21 kJ m o l - ~ H > assuming t h a t ~ ~ 1 (as it is usually taken) The spectrum o f t h e r m a l desorption of h y d r o g e n a d s o r b e d at 78 K o n thin gold film surface at low p o p u l a t i o n (0 _~ 0 01) is presented m Figure 3 The spectrum consists of the one symmetrical peak well described by the kinetic e q u a t i o n (3) when

,lO" 4

(3) 3 "C"

1

The i s o t h e r m for atomic h y d r o g e n a d s o r p t i o n on a thin gold film surface at 78 K is s h o w n in Figure 2 It can be seen t h a t the isotherm obeys a n e q u a t i o n of the type

i/

~ ;}5

1'5

L___ 175

2

225

2'5

2;75 r('K ) ,10 2

Figure 3. TD spectrum for thermal hydrogen desorptlon from thin Au film surface Hydrogen deposit corresponds to 0 ~ 0 01

(4)

where A and B are c o n s t a n t s This could be T i o m k l n ' s type isotherm of the form ~9

2H

Ep

.°,o,

I G°°

I -r

(5)

(where m . - - m a s s of h y d r o g e n atom, k - - B o l t z m a n n c o n s t a n t ) if the criterion for its ~alidity is fulfilled It can be seen from equation (5) that the slope o f the T l o m k i n ' s isotherm must increase p r o p o r t i o n a l l y with increase of a d s o r p t i o n t e m p e r a t u r e T o examine w h e t h e r o u r relation 0 = I ( P . ) c o r r e s p o n d s to T i o m k i n ' s i s o t h e r m we carried o u t a d s o r p t i o n at 78, 88 a n d 128 K The ratio of the slopes o f the isotherms is expected to be

300

-16

surface and the equlhbrlum atomic hydrogen pressure pH at 78 K

3. Results and discussion

0=arA"ln~ ltQ~°~°a7In ,,(2.m.fTA.)": +; p"

-17

Figure 2 The relation between hydrogen coverage 0 on thin Au film

the order of the desorption reaction n. the activation energy for desorption Ea a n d the pre-exponentlal factor v were f o u n d All the functions a n d p a r a m e t e r s m e n t i o n e d a b o v e were calculated by means of ' T D M S - 1 9 9 2 ' p r o g r a m elaborated by Z K a s z k u r et al ~~ The heat of atomic h y d r o g e n solution Q,o~ in thin gold film was determined previously to be ~ 9 kJ mol ~ H ~ ~6

0 = A In (BpH)

-18

~__]_

__~_

:"'

Q~o:18

"

'/ I ,",L/i Q~d-2~

]__ L ~ _ T _

E<.-.>o

~

QP ~ ~J-

"2

-

~

i

-

Ep-0

-

r

Figure 4. The model for dissociative H 2 adsorption on thin Au him surface

L Stobtfisk~ and R Du,4 H adsorption on Au

n = 2 Activation energy for thermal hydrogen desorptlon is calculated as Ed = 57 kJ mol ~ H : ActlvaUon energy for dissociative molecular h y d r o g e n adsorpUon on thin Au film surface can be expressed as E.~t.a = Ed-- Q O O n the basis o f the above results we can calculate activation energy for H2 d~ssoclauve adsorptxon E ~ t ~ = 36 kJ tool ~ H2 K n o w i n g the values Q o, E~, E~t.,~ and Q.o~ we can p r o p o s e a model for hydrogen a d s o r p t i o n on thin gold film which is presented m F~gure 4

4. Conclusions H : molecules o f temperature below ~ 3000 K do not adsorb on thin Au film surface because o f high a d s o r p t i o n activation energy ( 3 6 k J m o l iH2)

References ] C S Alexander and J Pntchard, J Chem Soc Fa~ada~ Tram I, 68, 202 (1972)

: F Greuter and E W Plummer~ Sohd St Commun, 48, 37 (1983) ~E M McCash, S F Parker, J Pntchard and M A Chesters, SurJa~e 5ct, 215, 363 (1989) 4X L Zhou, J M White and B E Koel, Surface Set, 218, 201 (1989) ~A G Sault. R J Madlx and C T Campbell. SurJaee Sol, 169, 347 (1986) 6L Stoblfiskl and R Dus, SurJace Scz, 269]270, 383 (1992) J Hams, Appl Ph~ ~. A47, 63 (1988) s J Hams, Sur]ace Sol, 221,335 (1989) D Halstead and S Hollowa2¢. J Chem Phy~. 93, 2859 (1990) " M Balooch. M J Cardfllo, D R Mdler and R E Sttckney. SurJace S~t, 46, 358 (1974) I IG Anger. A Wlnkler and K D Renduhc Sur[ace Sc~, 220, 1 (1989) ~2H F Berger, M Lelsch, A Wmkler and K D Renduhc, Chem Phw Lett, 175, 425 (1990) ~ G Comsa and R David. Surface Set. 117, 77 (1982) ~4D Brennan and P C Fletcher, Proc R Soc. A250, 389 (1959) ~ H von Gelger~ K Haupl, P Wtssmann and E Wlttmann, I~ak Te¢h. 34, 135 (1985) ~6L Stobmskl and R Dus, 4ppl Sur/a~e S~t, 62, 77 (1992) ~'S Dushman, ScwntdT~ Foundatton~ oj Vacuum Techmque Wdey, New York (1965) ~ Z Kaszkur. B Mlerzwa and L Stoblfiskl. to be pubhshed ~gG Welder, Chemtsorptton An E.~pnmental 4pproa~h Butterworths, London (1976)

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