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FIELD-EMISSION STUDIES OF THE ADSORPTION OF MERCURY ON TUNGSTEN AT 295 K
J. Saleh
To cite this version:
J. Saleh. FIELD-EMISSION STUDIES OF THE ADSORPTION OF MERCURY ON TUNGSTEN AT 295 K. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-111-C7-115.
�10.1051/jphyscol:1986720�. �jpa-00225913�
FIELD-EMISSION STUDIES OF THE ADSORPTION OF MERCURY ON TUNGSTEN AT 295 K
J . M . SALEH
Department of Chemistry, C o l l e g e o f S c i e n c e , U n i v e r s i t y o f Baghdad, Baghdad, J a d i r i y a , Republic of I r a q
The i n t e r a c t i o n of mercury vapeur with tunqsten has been studied by f i e l d - emission microscouy. Adserption of mercury a t 295 K occurred on (1 11 ) planes and on regkons 2 r r o u n d i n q (001 1 , (112) and (011 ) planes. The adserption a t a pressure of 10- N m w a s complete within 2 minutes. An increase i n t h e average work function of 0.182 v o l t has been estimated f o r t h e process. 01 heatinq a s u r f a c e s a t u r a t e d with mercury a t 295 K, s u r f a c e miqration began a t temperatures >750 K t o and over t h e c e n t r a l (01 1 ) lanes a l o n j t h e zones l i n k i n g it w i t ? t h e o t h e r (01 1 ) planes ; t h e a c t i v a t i o n energy (El of t h e process w a s 45-50 k J mol- . Desorption of t h e absorbed mercury s t a r t e d above 1200 K and t h e values of E f o r s u r f a c e migration and de-l sorption increased with temperature a t t a i n i n g a constant value of 120 k J mol- a t temperatures > 1700 K. The s u r f a c e ~ o t e n t i a l changes throughout adsorption, s u r f a c e migration and desorption have been determined.
Mercury is known t o show a s&ong tendency f o r adsorption on various (1 - 7) surfaces. Due t o such a behaviour, it is considered as a ~ o i s o n f o r many c a t a l y t i c r e a c t i o n s ( 1 , 4 ) . Mercury displaces adsorbed hydroaen from metal s u r f a c e s , and t h e mercury t h a t is adsorbed after hydrcuen displacement is shown t o be present as physically adsorbed l a y e r on t o p of t h e chemisorbd mercury l a y e r ( 1 , 4 ) . Paunev ard Mikhailev ( 6 ) have u t i l i z e d t h e f i e l d emission microscope t o i n v e s t i g a t e multilayer adsorption of mercury on tungsten a t 150 K.
I h e present work involves f i e l d emission microscope s t u d i e s of t h e adsorption of mercury on tungsten a t 245 K. me i n v e s t i g a t i o n covered many o t h e r aspects t h a t have been ignored i n t h e previous s t u d i e s .
EXPERIMENTAL
Ihe apparatus and t h e experimental technique have already been described ( 8 , 9 ) . Triply d i s t i l l e d mercury was used a f t e r thorough freezing and p n p i n g opera- t i o n s . Activation energies (E) f o r various processes, occurring throughout heating of a mercury s a t u r a t e d t i p t o temperatures higher than t h a t of adsorption, have been determined as explained previously ( 8 ) . The work functions (@) were derived from t h e current-voltage c h a r a c t e r i s t i c s which obeyed Fawler-Nordheim equation (10). The surface p o t e n t i a l ( v ) , characterizing any adsorption s t a t e , w a s defined by:
v = u clean - ads. (1
where 9 is t h e work function of t h e clean tungsten (8.0eV) s u r f a c e and gads is t h e worfC1=tion of t h e same s u r f a c e a f t e r mercury adsorption.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986720
C7-112 JOURNAL DE PHYSIQUE
RESULTS A M ) DISCUSSION
When mercury is adsorbed a t a pressure of N m-2 on a tungsten t i p which w a s kept a t 295 K p r e f e r e n t i a l adsorption occurs on t h e (111) planes and around
(001 ) , (112) and (011 planes as indicated i n at terns 2 t o 9 of p l a t e 1. Surface s a t u r a t i o n with Hq a t 295 K occurs i n 2 min ( p a t t e r n 9 ) and t h e i n t e n s i t y d i s t r i - bution p a t t e r n of t h e s a t u r a t e d s u r f a c e does not chanqe t h e r e a f t e r with time.Ad- sorption of Hg a t 295 K increases t h e average work function of tungsten as idi- cated by t h e s u r f a c e p o t e n t i a l . P o t e n t i a l ( v ) changes with t i m e i g fig,l ( A ) . Adsorption of Hg on tungsten a t 295 K and under a pressure of 10- N m was l i k e l y t o r e s u l t i n a chemisorbed mercury l a y e r on t h e t i p surface. The f a c t t h a t p a t t e r n 9 of p l a t e 1 was s t a b l e over t h e t e m v r a t u s e range 295 - 750 K even when t h e mercury pressure was reduced down t o 10- N m excludes t h e p o s s i b i l i t y of physical adsorption on t h e t i p s u r f a c e o r on t h e top of t h e chemisorbed mercury l a y e r . Physical adsorption of mercury on t o p of chemisorked mercury l a y e r was reported ( 1 ) t o occur on N i f i l m when t h e v a m r pressure (p) of mercury r e l a t i v e t o its s a t u r a t e d vapour pressure ( p O ) , t h a t is p/pO, was considerably higher than 0.1 ; t h e value o f p/pO i n t h e present work w a s only 0.01 . Mercury whiskers have been r e m r t e d (13 - 16) t o g r p on metal surfaces a t temperatures .h 195 K under pressures of 10- - 1 o - ~ N m- . Th? growtq of s 1 u r whiskers on a W t i p a t 295 K occured (8) under pressures of 10- - 1 N rnY.Pho such qmwth was observed i n t h e presegt i n v e s t i g a t i o n as t h e experimental pressure was always f a r lower than 1 0-4 N m- . A t lower pressures it would be u n l i k e l y t o observe whiskers during adsorption on a W t i p s i n c e a t low supersaturation t h e formation of h y p e r c r i t i c a l nuclei i s d i f f i c u l t because evaporation e x c e d e ---krth.
When t h e mercury s a t u r a t e d t i p is qeated2under a pressure o f 10- N m t o temtxratures
750 K t h e adsorption imaae ( p a t t e r n 9) is a l t e r e d consider- ably and t h e adsorbed mercury underqoes s u r f a c e migration and eventually is desorked by way of many c h a r a c t e r i s t i c s i n between steps ( p a t t e r n 10-32 of p l a t e 2 ) . The s u r f a c e p o t e n t i a l v a r i a t i o n s throughout migration and desorption a r e indicated i n fig.1 (B). The sequence of events durinq heating t h e t i p subsequent
t o s u r f a c e s a t u r a t i o n with Hg a t 295 K may be resolved i n t o t h r e e d i s t i n c t stages.
1 - Stage I , covering p a t t e r n s 10-l-ate 2, which occurs with a constant energy of a c t i v a t i o n of 45 t o 50 k J mol- l and t h i s value is smaller than t h e h e a t o f eva r a t i o n of mercury
of 61.5 k J molq. Paunev and P
Mikhailev ( 6 ) have found t h a t t h e heat of evaporation of H q from multiadsorbed mercury l a y e r s on W a t 150 K converged
with t h e l a t e n t heat of subli- P l a t e 1 - P a t t e r n 1 t o 9 which have
mation of bulk Hg. Such a low been obtained throughout
energy of a c t i v a t i o n which is adsorption of mercury vapour measured f o r t h i s s t a g e is u n d y a p y ~ ~ m r e of
l i k e l y t o correspond t o purely 10- N m- on W a t 295 K.
Hg migration on t h e W surface.
( i ) t h e decrease of emission a r o u d (111) region as shown i n p a t t e r n s 10 and 11 of p l a t e 2, and t h e extension of t h e dark (112) region towards (113), (123) and (223) planes, thus enlarqina a t t h e exoense of t h e (001 and (111) regions.
(ii) the emission increases once again from t h e (111) reqion i n p a t t e r n s 12 - 17 of p l a t e 2. I n t h e meantime, t h e dark a r e a around (011) decreases considerably. It is knum t h a t c e n t r a l (011) plane and t h e s p e c i a l zones l i n k i n g i t t o o t h e r (011) planes c o n s t i t u t e a network of lm-impedance paths f o r t h e d i f f u s i n g atoms. The Hg atoms w i l l t h e r e f o r e migrate rapidly t o and over t h e c e n t r a l (011) plane of t h e t i p , but w i l l be trapped on t h e rough s u r f a c e a t its edge. Mimation of chemisor- bed CO on W was found (17 - 19) t o s t a r t round 700 K and w a s rapid over t h e (011) plane and t h e zones connecting it with (111) planes: t h e a c t i v a t i o n enerqy of such process was estimated t o be 88 k J mol- .
2 - Staqe 11, covering p a t t e r n s 18 - 28 of p l a t e 2, which occurs on heatinq t h e t i p t o t e m ~ e r a t u r e s 1200 - 1700 K involvinq probably both s u r f a c e migration and desorption processes. q e energy of a c t i v a t i o n a t t h i s s t a g e increased gradually from 50 t o 110 k J mol- as indicated i n fig.2. me major events of t h i s s t a g e are :
( i ) t h e continuous growing of t h e (112) region a t t h e expense of (111) and (001) regions. Some of t h e Hg which is removed from t h e s e reqions may be deposited on t h e (111) region from t h e t h r e e nearest (112) planes. 'Ihese accumulations o r pro- trusions above t h e hemispherical s u r f a c e produce e x t r a l o c a l f i e l d and show up as regions of increased emission.
Fig. 1
-
Energies of a c t i v a t i o n (E) f o r s u r f a c e migration and desorption as a function of temperature (TI.
Fig. 2
I h e changes of s u r f a c e p o t e n t i a l ( v ) against ( A ) t i m e of ad- sorption ( t ) ; and (B) temperature
(T) throughout heating of a mercury;
s a t u r a t e d t i p t o 750 - 1900 K.
Y '3
40
C7-114 JOURNAL DE PHYSIQUE
In patterns 27 - 28 of p l a t e 2, t h e Hg adsorbates a r e not shown t o be uniformly d i s t r i b u t e d on t h e (111) region but appear t o be concentrated i n t h e form of
"clusters". The c l u s t e r s a r e shown i n patterns 27 - 28 t o be surrounded by neigh- bouring regions having considerably smaller concentrations and consequently d i f f e r - ent i n t e n s i t i e s of electron emission. Some of t h e adsorbed Hg on ( 1 1 1 ) and ( 1 1 2 ) regions may d i f f u s e i n t o (011) plane, along t h e edges of t h e l a t t e r plane, from which desorption is l i k e l y t o take place.
(ii changes occurring around (01 1 ) plane. The central dark rectangle appears most c l e a r l y i n pattern 18 with its boundary l i n e s a t t h e edges of t h e (011) plane.
This is interpreted t o mean t h a t t h e adsorbed Hg can migrate along t h e (112) -
(011 zone concentrating on t h e edge of t h e (011) plane, then migrating t o t h e surface of t h e (011) plane from which desorption begins.
3 - Stage 111, involving myrely desorption which occurs with a constant enerqy of activation of 11 0 k J mol- ffig.2), covering patterns 29 - 32. This s t a q e begins a t temuerature z 1700 K i n which l e s s Hg remains on t h e (112) and (111) regions and consequently t h e (112) planes shrink t o t h e normal s i z e ( w t t e r n 32). A t tem- peratures a1700 K , t h e removal of Hg atoms takes place from (1231, (1341, (213) and (223) planes on which Hg atoms make strong bonds. The (001) regions a l s o re- s t o r e t h e i r o r i g i n a l shapes.
The applied voltage increased throughout adsorption (pattern 2 - 9) but de-
creased smoothly on desorption ( p a t t e r n 10 - 32). The d e t a i l s of t h e changes i n t h e values of 0 and v due t o Hg adsorption and subsequent migration and desorption processes a r e indicated i n t a b l e 1 .
P l a t e 11 - Patterns 10 t o 33, which have been obtained on heating mercury- saturated tips t o temperatures ,750 K.
Pattern tem/K 0 / e V - v /e V Pattern t m / K 0 / e v - v / e v
2 295 4.535 0.035 16 1050 4.632 0.132
3 2 95 4.573 0.073 17 1100 4.621 0.121
4 2 95 4.587 0.087 18 1200 4.61 1 0.111
5 295 4.61 1 0.111 19 1300 4.606 0.106
6 295 4.629 0.129 20 1375 4.608 0.103
7 2 95 4.646 0.146 2 1 1420 4.598 0.098
8 2 95 4.665 0.165 22 1480 4.587 0.087
9 ,295 4 -682 0.182 23 1550 4.587 0 -087
10 750 4 -684 0.184 24-28 1550-1 700 4.573 0.073 1 1 78 0 4.676 0.176 29-30 1 700-1 750 4 -555 0.055
12 850 4.669 0.169 31 1800 4.535 0.035
13 900 4.656 0.156 32 1870 4.518 0.01 8
14 940 4.646 0.146
15 980 4.639 0.139
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