- Email: [email protected]
Adsorption isotherms for oxygen chemisorbed on silver powder
Thomochism'ca Ac~a, 24 (1978) 359-367 ~) Elsevier Sclentif~ Pub~hi~g C o m p l y , ~
AI~ORPTIOH POWDER*
. _-
-- Printed in The Netlzxtax~
ISOTHERMS FOR OXYGEN
CHEMISORBED
ON- SILVER
A . W . C Z A N D E R N A ** Department o f P h y s ~ and Im~il~te o f Colloid and Sarfac~ ..~iem~, ~ Potsdam, N Y 13676 ( U-S_A.)
College o f Technology,
A I ~ I tO, CT
A d s o r p t i o n isotherms h a v e been m e a s u r e d o n cleaned silver p o w d e r f r o m 178 to 339cC a t oxygen pr__e~_ur~ o f 0 3 2 6 P a to 40 k P a using a v a c u u m u l t r a m i c r o balance. A d s o r p t i o n equilibrium was f o u n d a t all t e m p e r a t u r e s a n d pressures studied_ T h e surface was p r e p a r e d f o r the reproducible c h e m i s o r p t i o n studies using a n established m e t h o d o f cyclic outgassing, o x y g e n a d s o r p t i o n a n d r e d u c t i o n in c a r b o n monoxide_ Seven isotherm-~ were m e a s u r e d t h a t sl~nne~i fractional surface c o v e r a m ~ f r o m 0.17 t o 1.1. T h e isosteric h e a t o f a d s o r p t i o n q was d e t e r m i n e d a t c o n s t a n t values o f 0. A f t e r decreasing f r o m 42 t o 17.7 kcal t o o l - 1 a t the l o w e r coverages, q r e m a i n s c o n s t a n t at 18.4 __+ 0.8 kcal m o l - ~ f r o m 0 o f 0.33 to a b o u t 0.90 a n d t h e n decreases to zero a t the highest coverages a n d temperatures. T h e initial d r o p in q is a t t r i b u t e d to the f o r m a t i o n o f islands o f a two-dimensional surface silver oxide. T h e c o n s t a n t value o f q results t h e n f r o m the c o m p l e t i o n o f the oxide layer a n d m o l e o d a r a d s o r p tion o n a n d / o r t h r o u g h t h e oxide. T h e decrease in q to zero a t t h e highest coverages results f r o m repulsions in the a d l a y e r at T <_ 275°C a n d a b s o r p t i o n into silver a t T > 302cC. INTRODUCTION
T h e m a g n i t u d e o f t h e isosteric h e a t o f a d s o r p t i o n q is a n i m p o r t a n t q,l~qtity in atl adsorption. T h e functional relationship o f q(0) w~th surface c o v e r a g e a n d / o r surface s t r u c t u r e m a y p r o v i d e i n f o r m a t i o n a b o u t the ~ - s u r f a c e b o n d , t h e factors responsl-ble f o r high catalytic activity, a n d the n a t u r e o f t h e c h e m i ~ o r b e d specie~ Silver is u n i q u e in its ability t o catalyze t h e oxidation o f ethylene to e t h y l e n e oxide a n d , in c o m m e r c i a l reactors, o x y g e n partial pressures o f t h e o r d e r o f 40 k P a (300 T o r r ) a r e used. T h u s , t h e d e t e r m i n a t i o n o f a reliable h e a t o f a d s o r p t i o n is i m p o r t a n t f o r
P~Xat ~ r e ~ : Solar:Ene~ ~
X n U i ~ 1536 C_~ mvd. Getd~-COS0401. US.A.
360 u n d e r s t a n d i n g the silver--oxygen a d s o r b a t e - a d s o r b e n t s y s t e m as well as f o r the e p o x i d a t i o n o f ethylene. M e a s u r e d values o f the h e a t o f a d s o r p t i o n o f oxygen o n silver 67(16) ~, 7 1 - 1 0 5 (17-25) z. 75--79(18-19) 3. a n d 67(16) 4 kJ(kcal) m o l - E a n d e a l e u l - t e d ranges o f 6 7 - 1 4 2 (16--34) s arid 4 6 - 8 8 ( 1 1 - 2 1 ) 6 Icl(kcal) m o l - ~ h a v e b e e n reported_ T h e range in these values c o u l d result f r o m m e a s u r e m e n t s m a d e a t different coverages, o n samples o f varying surface purity, o r a c o m b i n a t i o n o f b o t h o f these factors. W h e r e a d s o r b a t e a d s o r b e n t equilibrium exists, the isosteric h e a t o f a d s o r p t i o n can b e readily determined. T h e mass m o f the gas a d s o r b e d per unit a r e a o f the p u r e a d s o r b e n t d e p e n d s o n the pressure, t e m p e r a t u r e a n d , o n s o m e systems, the surface structure. F o r a particular stable surface structure, q m a y be o b t a i n e d at a c o n s t a n t a m o u n t o f gas a d s o r b e d f r o m the relationship (d In P/dT),= = q / R T z. I f the value o f m can be related t o the surface area, the q(0) can be determined where 0 is the fractional coverage. A l t h o u g h the specific surface a r e a o f cleaned silver is small, the v a c u u m u l t r a m i c r o b a l a n c e ~ can b e used t o measure the a d s o r p t i o n equilibrium f r o m 10 - 3 T o r t t o s u p e r a t m o s p h e r i c pressures. It is particularly suitable f o r s t u d y i n g the silveroxygen system where w e a k e h e m i s o r p t i v e b o n d s are o b t a i n e d a t higJa a d s o r b a t e coverages a n d less than a t m o s p h e r i c pressures o f oxygen. In fact, an ultramicrobalance, with U H V capabilities is o n e o f the few m e a s u r i n g m e t h o d s in surface science t h a t m a y b e applied in situ f o r s t u d y i n g c h e m i s o r p t i o n o n surfaces f r o m the " i d e a l " c o n d i t i o n s in U H V t o regions o f practical interest. EXPERIM~'TAL
Silver p o w d e r , s u s p e n d e d f r o m a v a c u u m ultramicrobalance, was e x p o s e d t o oxygen at pressures o f 0.266 t o 4 x 104 Pa (2 m T to 300 T o n ' ) a n d t e m p e r a t u r e s o f 178 to 339°C. Details a b o u t the purity o f the silver p o w d e r s. 9 a n d the p r o c e d u r e f o r processing the p o w d e r to a cleaned surface ~o. ~ ~ with r e p r o d u c i b l e c h e m i s o r p t i v e b e h a v i o r s - ~ ~ a n d a c o n s t a n t B E T surface area ' o h a v e been published. In this study, a 0.75 g silver s a m p l e h a d a surface area o f 920 c m 2 g - ~ based o n nitrogen a d s o r p t i o n a t 78 K. T h e u l t r a m i c r o ~ a v i m e t r i c a p p a r a t u s s. E2, the v a c u u m system s, the p r e p a r a t i o n o f the gases 8 - ~ ~, the m e a s u r e m e n t a n d c o n t r o l o f the t e m p e r a t u r e s, the d e t e r m i n a t i o n o f the surface a r e a ~, a n d the specialized p r o c e d u r e f o r o b t a i n i n g a d s o r p t i o n isotherms 13 a t the t e m p e r a t u r e s a n d pressures o f ~hi~ s t u d y h a v e been reported. T h e a d s o r p t i o n i s o t h e r m s w e r e o b t a i n e d in p u r e oxygen a b o v e 1.33 k P a ; b e l o w thi~ pre~sure, o x y g e n nitrogen mixtures v-ere used ~be~_use K n u d s e n forces decrease the p , ~ s i o n o f t h e m e a s u r e d m a s s t'~_ A d s o r p t i o n isotherms were m e a s u r e d a t t e m p e r a t u r e s o f 178, 199, 217, 245, 275, 302 a n d 339°C_ T h e m a s s ~ained a t equilibrium w a s m ~ . b - u r ~ a t t h e p r e ~ u r e intervals (1.33, 2.37, 4.12 a n d 7.98) in each d e c a d e o f pressure o v e r the range f r o m 0 9"76 t o 4 × 104 Pa. T h e a~,l~! o x y g e n pressure was m e a s u r e d t o an accurac3f o f 5 p e r c e n t a t the lowest pressures a n d t o 0.05 p e r c e n t at the h i o h ~ t pressure. Equilib-
361 r i u m w a s ~ e n e r a l l y a t t a i n e d in less t h a n t e n m i n u t e s a s d e d u c e d f r o m o b s e r v i n g t h e time base chart recording of the automatically operated ultramicrobalance. Adsorption isobars were obtained over the same temperature and pressure re~ons as for the isotherms. After the amount adsorbed was monitored at 339cC. t h e t e m p e r a t u r e w a s d e c r e a s e d in s e v e r a l d e c a e m e n t s t o 1 7 8 ° C a n d r e h e a t e d i n inc r e m e n t s t o 339 ~C. C o r r e c t i o n s f o r b u o y a n c y effects w e r e m a d e f r o m s i m i l a r e x p o s u r e s t o n i t r o g e n a n d a r g o n . W h e n a c o n s t a n t c o v e r a g e w a s o b t a i n e d in o x y g e n f o r a t I c a ~ four hours at any temperature, the temperature was changed to a new value. The cycle, 3 3 9 - 1 7 8 - 3 3 9 ° C , w a s r e p e a t e d a t least t h r e e t i m e s a t e a c h p r e s s u r e . A d s o r p t i o n i s o b a r s w e r e n o t m e a s u r e d b e l o w !.33 P a b e c a u s e t h e p a r t i a l p r e s s u r e o f o x y g e n in t h e closed system could be altered by the amount adsorbed or desorbed from the sample. A d s o r p t i o n i s o b a r s w e r e m e a s u r e d f r o m 1.33 P a t o 1.33 k P a o f o x y g e n in o x y g e n n i t r o g e n m i x t u r e s ' z. RESULTS The amount adsorbed on increasing or decreasing the pressure or temperature w a s r e p r o d u c i b l e a n d reversible, a s e x p e c t e d f o r e q u i l i b r i u m a d s o r p t i o n . T h e d a t a f r o m t h e a d s o r p t i o n i s o b a r s w e r e r e p l o t t e d a s i s o t h e r m s since re(P, T ) h a s o n l y t w o i n d e p e n d e n t variables. T h e detail o f t h e c o m p o s i t e i s o t h e r m s is s h o w n a t t h e h i g h e r p r e s s u r e s in Fig. 1 a n d a t t h e l o w e r p r e s s u r e s o n t h e l o g - l o g p l o t in Fig. 2. T h e b e s t smooth curves are plotted to avoid confusion. Typical scatter of the data points a r o u n d a given i s o t h e r m , a s s h o w n in Fig_ 1 f o r t h e a d s o r p t i o n a t 2 4 5 ¢ C , w a s a l w a y s within a p r e c i s i o n o f ~ 0.25 n g c m - z o r b e t t e r t h a n +__ 1 ~/o o f a m o n o l a y e r ~ A l t h o u g h t h e s c a t t e r o f t h e d a t a b e t w e e n 0.078 a n d 7.8 T o r t (10 t o 1000 P a ) ~ s a t t h e limits o f u n c e r t a i n t y , t h e i s o t h e r m s in this regi'on a r e n e a r l y linear o n a l o g P v e r s u s m p l o t a n d w e r e s m o o t h e d easily b y u s i n g a m o d i f i e d least s q u a r e s m e t h o d . T h e v a l u e s f r o m [-
T-c
w I X
o
|
O
•
•
•
I
!00
•
!
o
o
•
!
200
PRESSURE (~o~-)
F'tS- 1. Adsorption isotherms for o ~ on sDver pov.~ler. The surface cover~£~ 0 were c~tc~l~tFd an the ~ silver arums are in a < l l l ~ ~ - - a t l o n _ -
362
2C
0~'i
g 3
~
i
o i LOG Po= (tocr)
z
3
Fig. 2. L o g - l o g p l o t o f the i s o ~ s h o w n i n Fig. 1 s h o w i n g low-pressure detail. T h e low c o v e r a ~ p a r t o f the i s o t h e r m s e x t r a p o l a t e to a c o m m o n p o i n t o f 17.8 n g crn -~- a n d ~ PaL.
TABLE I I..~O:Si ~ L I C H E A E
][:OR O X Y G ~
~RBED
O . ~ S I L V E R A T V A R I O U ~ ~tYRFACE C O V E R A G E S
Maxxodsorbed q! (n~ era-'-) (kcal mol-~)
Trange o f yah'cliO" o f qt
q!
( ~kr~zlmol-i)
Trar.ge o f v~,llrl;t.v o f q±
6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
7"> 260 T> ~ T > 260 7"> ~ 7"> 260 178-339 i 78-339 178-339 7"> 302 7"> 302 7"> ~ 7"> 302 199
--41.0-43.0 40.0--43.5 2.8_0--M.0 ~ 17.6-17.8 17.6-17.8 17.6-18.7 18.3-19.1 18.3-2D.3 18_7-19.5 17.6--19.9 13.0--15.0b 9-4-1 L 4 b 4 . 0 - 6.0 b
~ -T<245 7"< 245 7<245 178--339 178-339 178-339 178-302 178-302 178-302 178--302 3 " < ! 9 9b 7"<199 ~ T<199 b
40.5--43.5 31-5--35.5 24.2-26.2 17.8-18.5 17.9-18.5 17.6-17.8 ! 7.6--17.8 17.6-18.7 13.5--15-0 9.5--i 1.5 8.0-10.0 4 . 0 - 5_5 18.6-18.8 s 17-6"-18-5s 12.5-13.5s
= D o u b l e v a l u e s i n q here m o r e likely a c o n t i n t m ~ n o f q~. b D o u b l e values i n q here m o r e likely r e s u l t f r o m a "q~".
01H. surJ~r_t coreroge for
0. ! 62
0-216 0--~'/0 0-324 0_378 0.43~_ 0.486 0.540 0.595 0.649 0_703 0.757 0.811 0.865 0.919
363 t h e s m o o t h e d curves were used f o r calculating q a t e a c h m a s s o f oxy~en a d s o r b e d . T h e q a t a n y 0 was o b t a i n e d f r o m t h e d a t a u s e d t o c o n s t r u c t Figs_ 1 a n d 2 by p l o t t i n g In P vs. 1 / T a t a c o n s t a n t m a s s a d s o r b e d , e.g., a c o n s t a n t 0. T h e r e s u l t i n g slope is q l R as d e d u c e d f r o m t h e defining r e l a t i o n s h i p *. T h e values o f q o b t a i n e d at various masses o f o x y g e n a d s o r b e d a r e given in T a b l e 1. T h e ranges give t h e m a x i m u m v a r i a t i o n in q f o r d a t a p o i n t s falling + 0.25 n g c r n - 2 a w a y f r o m t h e s m o o t h e d curve. W h e n t h e d e t e r m i n a t i o n f o r q i n c l u d e d d a t a f r o m all seven i s o t h e r m s , a n d w a s single v a l u e d ( T a b l e 1, a t 16, 17, 18, 19 a n d 20 n g c m - z o f o x y g e n a d s o r b e d ) , t h e d e v i a t i o n s in t h e slope led t o uncertainties o f + 0.2 kcal t o o l - t o r a p p r o x i m a t e l y + 1 ~ o f t h e m e a s u r e d value. L a r g e r u n c e r t a i n t i e s necessarily resulted w h e n q's h a d to b e determ i n e d f r o m o n l y t w o o r t h r e e available i_,othern~ a t a c o n s t a n t ~ adsorbed_ T h e increased u n c e r t a i n t y results, in part, f r o m t h e e r r o r in t h e m e a s u r e d t e m p e r a t u r e as well as f r o m t h e p r o b a b l e c h a n g e s i n t h e a d s o r p t i o n . T h e r a w d a t a used f o r t h e plots in Figs. 1 a n d 2 a r e available o n r e q u e s t f r o m t h e a u t h o r . A surface s t r u c t u r e m u s t be a s s u m e d t o o b t a i n fractional coverages 0 f r o m t h e m a s s o f o x y g e n a d s o r b e d . I f it is a s s u m e d o n e o x y g e n a t o m will be associated with o n e silver a t o m a t 0 ----- 1, t h e n for t h e ( l l i ) , <100> a n d ( 1 1 0 > surfaces, 37, 32, a n d 22.7 n g o f o x y g e n p e r c m 2 o f silver surface will c o m p r i s e a m o n o l a y e r , c o r r e s p o n d i n g to 7.2, 8_31 a n d 11.75 A 2 f o r t h e average area o f a silver a t o m o n these t h r e e surfaces. T h e O's c o m p u t e d are also listed in T a b l e 1 a n d t h e a p p r o p r i a t e c o o r d i n a t e labels a r e a t t a c h e d to Figs. 1 a n d 2. DISCUSSION
T h e t e x t b o o k i n t e r p r e t a t i o n o f t h e q(0)'s in T a b l e 1 c o u l d b~ as follows: T h e " l i n e a r " decrease o f q u p to m = 12 n g crn - 2 is characteristic o f a T e m k i n i s o t h e r m , with o r w i t h o u t dissociative a d s o r p t i o n , o n a h e t e r o g e n e o u s surface t s . A c o n s t a n t value o f q f r o m 12 t o 32 n g c m - z is a d e q u a t e l y described as a n a d s o r p t i o n w i t h o u t d ~ o n , w i t h o u t lateral interactions, w i t h surface u n i f o r m i t y a n d with an i m m o b i l e layer, e.g., a s i m p l e L a n g m u i r i n t e r p r e t a t i o n . T h e decrease in q t o zero a t t h e hi~aer coverages a n d / o r h i g h e r t e m p e r a t u r e s results f r o m a m o b i l e a d s o r b e d species o r w e a k lateral interactions. T h e complexities o f t h e s i l v e r - o x y g e n s y s t e m a r e i g n o r e d by t h e t e x t b o o k i n t e r p r e t a t i o n . N u m e r o u s investigators h a v e c o n c l u d e d t h a t o x y g e n a d s o r b s o n silver in b o t h atomic and molecular forms z-5, s-~, as is a d e q u a t e l y referenced in recent reviewst~, ~ s D i r e c t c o n f i r m a t i o n o f t h e existence o f t h e m o l e c u l a r f o r m h a s been d e m o n s t r a t e d b y C l a r k s o n a n d CiriHo u s i n g electron s p i n r e s o n a n c e s9 a n d b y Kilty e t a l 5 u s i n g i n f r a r e d s p e c t r o s c o p y . R o v i d a et al.2 °" 2~ h a v e p r o v i d e d evidence f o r t h e existence o f a n Ordered a t o m i c species, u s i n g l o w e n e r g y e l e c t r o n diffractioIL They_ p o s t u l a t e d 2t t h a t a t w o - d i m e n s i o n a l surface oxide o f (111 > A g 2 0 is f o r m e d o n ( 1 1 1 ) A g t o explain 4. x 4 L E E D p a t t e r n s , b a s i n g t h e i r i n t e r p r e t a t i o n o n t h e lattice c o n s t a n t s o f A g 2 0 a n d A g . - E v i d e n c e f o r t h e w e l l - k n o w n a b s o r p t i o n o f o x y g e n i n t o silver h a s b e e n d e t e c t e d a t t e m p e r a t u r e s as l o w as 100°C with F E M z 2 150-200~C
364 microgravimetricallys, a n d 200°C by thermal d e s o r p t i o n z°. T h e s e thresholds o f absorption are considered to be small a m o u n t s t h a t p e n e t r a t e o n l y the first few a t o m i c layers. T h u s , oxygen a d s o r b e d o n silver includes oxygen adions a n d admolecule ions, where the adions m a y f o r m a n o r d e r e d two-dimensional "oxide'" o r diffuse into the bulk at higher temperatures. T h e likely possibilities t h a t c o u l d unravel the complexities o f q ( m ) in T a b l e 1, will n o w be a t t e m p t e d . T h e d r o p in the isosteric heat f r o m 42 t o i 8 kcal t o o l - t at low coverages is associated with the f o r m a t i o n o f a two-dimensional surface oxide r a t h e r t h a n surface heterogeneity. A d r o p in q can result f r o m c h a r g e d ions i f they a r e close t o g e t h e r ~6. This c o u l d be accomplished by the surface diffusion o f oxygen a d i o n s t o f o r m twodimensional islands o f silver oxide. Large stable nuclei o f A g : O o n silver h a v e been r e p o r t e d a t s u p e r - a t m o s p h e r i c pressures23; the partially ionic c h a r a c t e r o f oxygen in built silver oxide is well k n o w n . I f the d r o p in q results f r o m repulsions o f " o x i d e i o n s " in a two-dimensional AgzO, is it then reasonable also to a s s u m e t h e surface o f the silver p o w d e r is u n i f o r m ? Excellent a m,e e m e n t exists between the surface coverages given b y R o v i d a et al. z~ for a (1 ! 1 ) single crystal (Fig. 4, ref. 2 1 ) a n d those listed in T a b l e i, which were calculated o n the a s s u m p t i o n t h a t the silver p o w d e r is c o m p o s e d primarily o f (111 ) microfacets. W h e n e v e r quantitative c o m p a r i s o n s have been possible, the results o b t a i n e d on sing/e crystals -'°- ±~ a n d O A O R cycled silver p o w d e r s. ~o have s h o w n ~ood a~oreement. O n e is led to the conclusion that either the a d s o r p t i o n o f oxygen is n o t sensitive to surface structure o r the initially polycrystalline p o w d e r s a s s u m e low-index plane microfacets. In either case, the decrease in q with the a m o u n t a d s o r b e d should n o t be attributed to surface heterogeneity. F u r t h e r m o r e , additional c o m p a r i s o n s between the d a t a on silver powders a n d s i n ~ e crystals c a n be m a d e t h a t d o n o t a p p e a r to be especially speculative_ Rovida et al. -'°" -"~ r e p o r t e d the u p p e r b o u n d s o f stability o f a 4 x 4 superstructure r a n g e f r o m a b o u t 200 to 2600C between 10-"- to 1 0 - ' T o n ' , respectively, e . g , the 4 x 4 is stable a t hi mher pressures (higher coverages) a n d l o w e r t e m p e r a t u r e s . T h u s , the d r o p in q a t T < 245cC a n d m f r o m 10-16 ng c m - z c o u l d be associated with the f o r m a t / o n o f o r d e r e d 4 x 4 arrays o f silver oxide; t h e d r o p in q a t T > 260~C a n d m f r o m 6-12 ng e r a - z then c o u l d be associated with the f o r m a t i o n o f islands o f silver oxide n o t detectable b y L E E D . I f this assignment is correct, t h e mass o f oxygen a d s o r b e d a l o n g the b o u n d a r y o f 4 x 4 stability is a b o u t 13 ng ¢m - z o r 0t t ~ = 0.35. Additional a r g u m e n t s can be a d v a n c e d to s u p p o r t t h e hypothesis f o r the f o r m a t / o n o f a surface silver oxide. T h e plots o f log m versus log P, s h o w n in Fig. p r o d u c e straight lines, a t the lower pressures, that c o n v e r g e to a p o i n t w h e r e the mass gain is 17.8 ± 0.8 n g c m - 2 a n d the pressure is 266 Pa. ( A t higher pressures, the l o g - l o g plot does n o t exhibit a discernible convergence.) T h e point o f c o n v e r g e n c e o f isotherms o n l o g - l o g plots can be interpreted as the limiting coverage o f a p a r t i c u l a r c h e m i s o r b e d state z6. I f this state is the surface oxide AgzO, as is suggested b y c o m paring this w o r k with that o f R o v i d a et al. zz, the limiting coverage should be 0 =
365 0.50; m ----- 17.8 n g c m - z c o r r e s p o n d s t o 0 ---- 0.48 f o r t h e m o d e l o f a ~111~> surface, w h i c h is in g o o d a g r e e m e n t w i t h a 0 o f 0.5 f o r a surfac~ A~zO. Finally, g o v i d a e t al. 20 r e p o r t e d t h e 4 x 4 s t r u c t u r e is f o r m e d o n e x p o s u r e to 0.01 T o r t o f O 2 a t 2 0 0 ° C f o r I0 m i n a n d t h a t t h e o x y g e n c o v e r a g e f o r this e.~posnre o f t h e A g <~111 ~>is a b o u t h a l f t h e m a x i m u m v a l u e t h e y o b t a i n s ! by e x p o s i n g a t ! T o r r for 10 m i n a t --~0¢C. A n e q u i l i b r i u m coverage c a n be o b t a i n e d in 10 rain a t 2 0 0 ° C based o n t h e kinetic studies o f C z a n d e r n a s, so f u r t h e r c o m p a r i s o n s are n o t unre~_ sonable. I n Fig. 1, 22.2 n g c m - 2 o f o x y g e n will be o b t a i n e d at 1 T o r r a n d 199°C; h a l f this a m o u n t , 1 !. 1 n g c m - z s h o u l d c o r r e s p o n d to the l o w e r b o u n d w h e r e t h e 4 x 4 c o u l d be f o r m e d a n d is in r e a s o n a b l y g o o d a g r e e m e n t w i t h t h e l o w e r b o u n d o f a b o u t ! 3 n g c m - z d e d u c e d a b o v e f r o m t h e t e m p e r a t u r e a n d pressure limits o f stability o f the4
x
4_
I n t e r p r e t a t i o n o f q(m) f o r m exceeding 12-16 n g c m - 2 is r a t h e r s t r a i g h t f o r w a r d o n c e t h e h y p o t h e s i s o f f o r m i n g a surface oxide is accepted. T h e " c o n s t a n t ' " v a l u e o f 1 7 . ' / k c a l tool -~ ( m f r o m 12-19 n g c m - 2 ) is associated w i t h t h e h e a t o f f o r m a t i o n o f t h e surface oxide a t t h e lower coverages a n d t h e value o f 19 kcal t o o l - t at hi pAaer coverages is a t t r i b u t e d to t h e h e a t o f a d s o r p t i o n o f m o l e c u l a r l y a d s o r b e d o x y g e n o n t o t h e surfa_ee oxide. T h e g r a d u a l d r o p in q at t h e lower t e m p e r a t u r e s a n d hig~lest coverages is ascribed t o repulsive i n t e r a c t i o n s b e t w e e n t h e c h a r g e d ud~.orbed molecules. T h e a p p a r e n t d r o p in q at T > 3 0 2 ° C results f r o m m ~ s u r a b l e a m o u n t s o f o x y g e n a b s o r p t i o n i n t o t h e s u b s u r f a c e regions. F o r a b s o r p t i o n , t h e a m o u n t o f u p t s k e increases with increasing t e m p e r a t u r e , w h i c h is a n e n d o t h e r m i c r a t h e r t h a n a n e x o t h e r m i c process. T h e t h r e s h o l d d e v i a t i o n s f r o m e a c h i s o t h e r m f r o m t h e c o n v e r g i n g linear plots ( d a s h e d lines, Fig. 2) p r o v i d e detectability limits f o r a b s o r p t i o n i n t o silver at pressures o f 266, 53 a n d 20 Pa for 275, 302, a n d 3 3 9 ° C , respectively. A l t h o u g h direct evidence f o r f o r m i n g ( I 1 ! ) microfacets (~- 2 0 - 3 5 A in size) has n o t been p r o d u c e d , Kollen 2 . d i d s h o w facets, several h u n d r e d a n g s t r o m s in size, c a n d e v e l o p o n a small p a r t o f t h e surface d u r i n g O A O R cycling. Recently, E k e r n 2s d e m o n s t r a t e d t h e activation e n e r g y f o r d e s o r p t i o n o f o x y g e n f r o m t h e r m a l l y e t c h e d (facetted) a n d polyerystalline wire b e c o m e identic~a! w h e n t h e latter is subjected to o u t g a s s i n g a n d o x y g e n a~L~orptioncycling, similar to t h a t used o n single crystals -'°" : * F u r t h e r m o r e , R o v i d a et al. 2° r e p o r t e d t h e s a m e a d s o r p t i o n is o b t a i n e d w h e n silver single crystals are cleaned by cyclic o u t g a s s i n g a n d o x y g e n a d s o r p t i o n o r by i o n b o m b a r d m e n t a n d a n n e a l i n g . T h u s , t h e r e is persuasive evidence n o w available, in a d d i t i o n to p r e v i o u s a r g u m e n t s a o t h a t a d d s c o n s i d e r a b l e credibility to t h e suggestion t h a t ( ! i I ) m i c r o f a c e t s f o r m d u r i n g t h e O A O R cycling o f silver p o w d e r . T h e s u m o f q -k EA s h o u l d equal ED, w h e r e E A a n d ED are t h e uetivation e n e r ~ e s o f a d s o r p t i o n a n d d e s o r p t i o n , if e n t r o p y effects are negligible. By a d d i n g C z a n d e r n a ' s values o f 2 2 - 2 4 kcal m o I - * f o r E^ w i t h t h e q's r e p o r t e d in T a b l e I a t n o r m a l i z e d coverages, a n ED o f 41 __+ 3 kcal t o o l - 1 is o b t a i n e d . T h i s value c o m p a r e s favorably with r e p o r t e d values o f ED o f 41.8 ~_ 3.5, 37.8, a n d 35 to 45 kcal m o l - * given f o r silver p o w d e r s 9, filaments zs a n d single crystals z°, respectively. A n E D o f 41 + 3 kcal
366 t o o l - ~ is also q u i w interestingly within t h e p r o b a b l e u n c e r t a i n t y o f t h e a e r a t i o n e n e r g y f o r t h e dissociation o f silver o r / d e 2s o f 36 kcal t o o l - ~. For the m o d e l given in this p a p e r , the ED w o u l d result f r o m t h e d e s o r p t i o n o f m o l e c u l a r o x y g e n a t h i g h e r coverages a n d t h e d~=,sociation o f silver oxide a t lower coverages. A t t h e lower coverages, EA m u s t decline t o w a r d s z e r o as q(m) rises to 42 kcal t o o l - j to m a i n t a i n a c o n s t a n t E o o f --- 4! kcal t o o l - t A n E A o f 3 kcal t o o l - 1 h a s been r e p o r t e d at n o r m a l i z e d coverages b e l o w 0 ---- 0.4 b u t these were m e a s u r e d below 65~'C. ODNCLUSIONS
In s u m m a r y , a d s o r p t i o n e q u i l i b r i u m was f o u n d f o r t h e silver-oxygen system f r o m 178 t o 339 ¢C a t pressures o f 0.226 to 40,000 Pa. F a v o r a b l e c o m p a r i s o n s between t h ~ w o r k , w h e r e silver p o w d e r s were used, a n d t h a t o n sin~Ie crystals 2°- 2~ p r o v i d e a basis f o r a s s u m i n g a (11 ! ) facetted surface is d e v e l o p e d o n t h e p o w d e r d u r i n g extensive a n d repeated o u t ~ . ~ n g a n d oxygen a d s o r p t i o n c y c l i n ~ T h e maxs a d s o r b e d ~ias t h e n regated t o surface coverages o n ( I i 1 ) silver. A __decrease in t h e isosteric h e a t f r o m 42 to 17.7 kcal t o o l - • at c o v e r a e e s f r o m 0. ! 7 t o 0.33 is a t t r i b u t e d t o t h e f o r m a t i o n o f / s l a n d s o f a t w o - d i m e n s i o n a l surface oxide c o m p o s e d o f c h a r g e d ~nlons. A t h i g h e r coverages, t h e variation o f t h e h e a t f r o m 17.7 to 19 kcal t o o l - t is a t t r i b u t e d to ~m'owth o f the surface oxide to f o r m a m o n o l a y e r o f A g 2 0 a n d to a d s o r p t i o n o f m o l e c u l a r o x y g e n by a n d / o r t h r o u g h t h e o r / d e . R e p u l s i o n s in t h e m o l e c u l a r l y a d s o ~ oxygen a t t h e coverages o f a b o u t 0.9 p r o d u c e s a decrease in t h e isosteric h e a t a t l o w t e m p e r a ° tures; at h i g h e r t e m p e r a t u r e s , a decrease in q at 0 o f 0.6 results f r o m t h e o n s e t o f a b s o r p t i o n o f o x y ~ - n into t h e silver. ACKNOWLEIX/M~W
T h e a u t h o r is pleased to t h a n k U n i o n C a r b i d e C o r p o r a t i o n f o r partial financial s u p p o r t a n d f o r the specialized e q u i p m e n t used f o r this study. REFERENCES ! 2 3 4 5 6 7
A- F- Benton and L C. Drake, d. A m . Chem...~c., 56 (1934) 255. W_ W_ Smeltzet. E_ L T o ! b f c o n a n d A- C.asnbroth C'~z..I. Ckem., 34 (1956) 1046. C- B_ Mr:Carry, Ph.D. ~r/ws/s. Purdue Univexsity, I961, Univ. Microfilms 61-5741o Y . L Sandier and D_ D- Durigon. J. Phys. Clwm~ 69 (19K5"34201_ P_ A - Kff~.. N_ C R o l and W . M . H . Sar.htler, Fifth Int. Cong. on Cam/ys/s, N o r t h HoP!and, A m s t e r d a m , 1972. G . Rovkta, E Prazesi, M. Maglietta a n d E. Fe~roni, Surface S¢/., 43 0 9 7 4 ) 230. A. ~ in S. P. W o l s k y and E. J. 7_danuk (Eds.), Ultra M i o v - W e ~ g h t D e t e r m h ~ i o n in C o n t r o I l e d E m ~ o n n u m s , WRey, N e w Y o r k , 1969., p. 389.
s A.w. ~ - - ~
Jr. PATx. Chem.. 64(1964) 2765.
9 I0
W . K o l k n arid A . Czalldetna. J. Colloid Interface Sd_. 38 0 9 7 2 ) 152_ A . W . C~,~derna, Jo Pky'.r. Cicero., 66 (1966) 2120.
It
A. W_ C z a n d e r ~ in G. Goldfmgcr fed.), Clean ~arfacea, Degger, New York, 1969, p. 133.
367 I2 13 14 15
A . W . C 2 ~ d e r n a , in P. M- W~t~ys (Ed.), Vacuum Microbalance Tectmiqur_x, Vol. 4, Plenmn, N e w Y o r k , 1965, p. 5_/. A . W . C2andcxna, in P. M. Waters (lEd.), Vacmzm M / c r o b a / a n ~ Tec/zn/~ex, Vol. 4, Plenum, I~Iew Y o r k , 1965, p. 69. E. Roberts. in A_ W . Czandetna OEd.), Vaezasm Micz,obalance Techniques, Vol. 8, Plenum, N e w Y o r k , 1971, p. 73. F o r sample loads o f 10-20 g that can n o w be a c c o ~ o n the balance described b y Czanderaa eA al., .!. Vac. S d . Tedmol., 13 (1976~ 556, a n o r d e r o f magnitucte ~m4~ovezrte~t
(0A ~ o f 0) appears realizable_ 16 17 18 19 20 21 22 23 24 25 26
D . O . H a ~ - a r d a n d B. M. W. Tragmell, C_hemi.corptian, Bollexworrbe ~ n , 1964, p_ 159 fl~ H . H . Vo~e a n d C- R . A d a m s , ~ / r . Cata/., 17 (1967) 151. W . M . 14- Sac.htler, Cata/. Rev., 4 (1970) 27. R . B _ C l a x k _ ~ n a n d A . Cuillo, J r . , J o Vac. S c L T e c ~ . o L , 9(1972) 1073. G . Rovida, E. Fefroni, M. Maglietta a n d E Pratesi, in 1=. Ricca (Ed.), Adsorption-De.sorption Phenomer--,, Academic P re~¢. N e w Y o r k , 1972, p. 417_ G_ Ro-cida. F_ pr~te~i M . Magiietta a n d != Ferroni, 5arface ~c/., 43 (1974) 230. A . W . Czanderna, O. Frank and W . A . S c . h ~ d t , Surface So/., 38 (1973) 129. C_ Menzel a n d E. Men2ei-Kopp, ~urface Sci., 2 (19(34) 402_ W . Kollen, Ph.D. The.~, C'h.t'kson College o f Tech., 1969. R . l~kca-a~ Ph.D. The:~.f, ~ C~:~11¢~ o f Tech., 1974. J. A . ~ Atz~f . J. Chem., 13 (1960) 431.
AI~ORPTIOH POWDER*
. _-
-- Printed in The Netlzxtax~
ISOTHERMS FOR OXYGEN
CHEMISORBED
ON- SILVER
A . W . C Z A N D E R N A ** Department o f P h y s ~ and Im~il~te o f Colloid and Sarfac~ ..~iem~, ~ Potsdam, N Y 13676 ( U-S_A.)
College o f Technology,
A I ~ I tO, CT
A d s o r p t i o n isotherms h a v e been m e a s u r e d o n cleaned silver p o w d e r f r o m 178 to 339cC a t oxygen pr__e~_ur~ o f 0 3 2 6 P a to 40 k P a using a v a c u u m u l t r a m i c r o balance. A d s o r p t i o n equilibrium was f o u n d a t all t e m p e r a t u r e s a n d pressures studied_ T h e surface was p r e p a r e d f o r the reproducible c h e m i s o r p t i o n studies using a n established m e t h o d o f cyclic outgassing, o x y g e n a d s o r p t i o n a n d r e d u c t i o n in c a r b o n monoxide_ Seven isotherm-~ were m e a s u r e d t h a t sl~nne~i fractional surface c o v e r a m ~ f r o m 0.17 t o 1.1. T h e isosteric h e a t o f a d s o r p t i o n q was d e t e r m i n e d a t c o n s t a n t values o f 0. A f t e r decreasing f r o m 42 t o 17.7 kcal t o o l - 1 a t the l o w e r coverages, q r e m a i n s c o n s t a n t at 18.4 __+ 0.8 kcal m o l - ~ f r o m 0 o f 0.33 to a b o u t 0.90 a n d t h e n decreases to zero a t the highest coverages a n d temperatures. T h e initial d r o p in q is a t t r i b u t e d to the f o r m a t i o n o f islands o f a two-dimensional surface silver oxide. T h e c o n s t a n t value o f q results t h e n f r o m the c o m p l e t i o n o f the oxide layer a n d m o l e o d a r a d s o r p tion o n a n d / o r t h r o u g h t h e oxide. T h e decrease in q to zero a t t h e highest coverages results f r o m repulsions in the a d l a y e r at T <_ 275°C a n d a b s o r p t i o n into silver a t T > 302cC. INTRODUCTION
T h e m a g n i t u d e o f t h e isosteric h e a t o f a d s o r p t i o n q is a n i m p o r t a n t q,l~qtity in atl adsorption. T h e functional relationship o f q(0) w~th surface c o v e r a g e a n d / o r surface s t r u c t u r e m a y p r o v i d e i n f o r m a t i o n a b o u t the ~ - s u r f a c e b o n d , t h e factors responsl-ble f o r high catalytic activity, a n d the n a t u r e o f t h e c h e m i ~ o r b e d specie~ Silver is u n i q u e in its ability t o catalyze t h e oxidation o f ethylene to e t h y l e n e oxide a n d , in c o m m e r c i a l reactors, o x y g e n partial pressures o f t h e o r d e r o f 40 k P a (300 T o r r ) a r e used. T h u s , t h e d e t e r m i n a t i o n o f a reliable h e a t o f a d s o r p t i o n is i m p o r t a n t f o r
P~Xat ~ r e ~ : Solar:Ene~ ~
X n U i ~ 1536 C_~ mvd. Getd~-COS0401. US.A.
360 u n d e r s t a n d i n g the silver--oxygen a d s o r b a t e - a d s o r b e n t s y s t e m as well as f o r the e p o x i d a t i o n o f ethylene. M e a s u r e d values o f the h e a t o f a d s o r p t i o n o f oxygen o n silver 67(16) ~, 7 1 - 1 0 5 (17-25) z. 75--79(18-19) 3. a n d 67(16) 4 kJ(kcal) m o l - E a n d e a l e u l - t e d ranges o f 6 7 - 1 4 2 (16--34) s arid 4 6 - 8 8 ( 1 1 - 2 1 ) 6 Icl(kcal) m o l - ~ h a v e b e e n reported_ T h e range in these values c o u l d result f r o m m e a s u r e m e n t s m a d e a t different coverages, o n samples o f varying surface purity, o r a c o m b i n a t i o n o f b o t h o f these factors. W h e r e a d s o r b a t e a d s o r b e n t equilibrium exists, the isosteric h e a t o f a d s o r p t i o n can b e readily determined. T h e mass m o f the gas a d s o r b e d per unit a r e a o f the p u r e a d s o r b e n t d e p e n d s o n the pressure, t e m p e r a t u r e a n d , o n s o m e systems, the surface structure. F o r a particular stable surface structure, q m a y be o b t a i n e d at a c o n s t a n t a m o u n t o f gas a d s o r b e d f r o m the relationship (d In P/dT),= = q / R T z. I f the value o f m can be related t o the surface area, the q(0) can be determined where 0 is the fractional coverage. A l t h o u g h the specific surface a r e a o f cleaned silver is small, the v a c u u m u l t r a m i c r o b a l a n c e ~ can b e used t o measure the a d s o r p t i o n equilibrium f r o m 10 - 3 T o r t t o s u p e r a t m o s p h e r i c pressures. It is particularly suitable f o r s t u d y i n g the silveroxygen system where w e a k e h e m i s o r p t i v e b o n d s are o b t a i n e d a t higJa a d s o r b a t e coverages a n d less than a t m o s p h e r i c pressures o f oxygen. In fact, an ultramicrobalance, with U H V capabilities is o n e o f the few m e a s u r i n g m e t h o d s in surface science t h a t m a y b e applied in situ f o r s t u d y i n g c h e m i s o r p t i o n o n surfaces f r o m the " i d e a l " c o n d i t i o n s in U H V t o regions o f practical interest. EXPERIM~'TAL
Silver p o w d e r , s u s p e n d e d f r o m a v a c u u m ultramicrobalance, was e x p o s e d t o oxygen at pressures o f 0.266 t o 4 x 104 Pa (2 m T to 300 T o n ' ) a n d t e m p e r a t u r e s o f 178 to 339°C. Details a b o u t the purity o f the silver p o w d e r s. 9 a n d the p r o c e d u r e f o r processing the p o w d e r to a cleaned surface ~o. ~ ~ with r e p r o d u c i b l e c h e m i s o r p t i v e b e h a v i o r s - ~ ~ a n d a c o n s t a n t B E T surface area ' o h a v e been published. In this study, a 0.75 g silver s a m p l e h a d a surface area o f 920 c m 2 g - ~ based o n nitrogen a d s o r p t i o n a t 78 K. T h e u l t r a m i c r o ~ a v i m e t r i c a p p a r a t u s s. E2, the v a c u u m system s, the p r e p a r a t i o n o f the gases 8 - ~ ~, the m e a s u r e m e n t a n d c o n t r o l o f the t e m p e r a t u r e s, the d e t e r m i n a t i o n o f the surface a r e a ~, a n d the specialized p r o c e d u r e f o r o b t a i n i n g a d s o r p t i o n isotherms 13 a t the t e m p e r a t u r e s a n d pressures o f ~hi~ s t u d y h a v e been reported. T h e a d s o r p t i o n i s o t h e r m s w e r e o b t a i n e d in p u r e oxygen a b o v e 1.33 k P a ; b e l o w thi~ pre~sure, o x y g e n nitrogen mixtures v-ere used ~be~_use K n u d s e n forces decrease the p , ~ s i o n o f t h e m e a s u r e d m a s s t'~_ A d s o r p t i o n isotherms were m e a s u r e d a t t e m p e r a t u r e s o f 178, 199, 217, 245, 275, 302 a n d 339°C_ T h e m a s s ~ained a t equilibrium w a s m ~ . b - u r ~ a t t h e p r e ~ u r e intervals (1.33, 2.37, 4.12 a n d 7.98) in each d e c a d e o f pressure o v e r the range f r o m 0 9"76 t o 4 × 104 Pa. T h e a~,l~! o x y g e n pressure was m e a s u r e d t o an accurac3f o f 5 p e r c e n t a t the lowest pressures a n d t o 0.05 p e r c e n t at the h i o h ~ t pressure. Equilib-
361 r i u m w a s ~ e n e r a l l y a t t a i n e d in less t h a n t e n m i n u t e s a s d e d u c e d f r o m o b s e r v i n g t h e time base chart recording of the automatically operated ultramicrobalance. Adsorption isobars were obtained over the same temperature and pressure re~ons as for the isotherms. After the amount adsorbed was monitored at 339cC. t h e t e m p e r a t u r e w a s d e c r e a s e d in s e v e r a l d e c a e m e n t s t o 1 7 8 ° C a n d r e h e a t e d i n inc r e m e n t s t o 339 ~C. C o r r e c t i o n s f o r b u o y a n c y effects w e r e m a d e f r o m s i m i l a r e x p o s u r e s t o n i t r o g e n a n d a r g o n . W h e n a c o n s t a n t c o v e r a g e w a s o b t a i n e d in o x y g e n f o r a t I c a ~ four hours at any temperature, the temperature was changed to a new value. The cycle, 3 3 9 - 1 7 8 - 3 3 9 ° C , w a s r e p e a t e d a t least t h r e e t i m e s a t e a c h p r e s s u r e . A d s o r p t i o n i s o b a r s w e r e n o t m e a s u r e d b e l o w !.33 P a b e c a u s e t h e p a r t i a l p r e s s u r e o f o x y g e n in t h e closed system could be altered by the amount adsorbed or desorbed from the sample. A d s o r p t i o n i s o b a r s w e r e m e a s u r e d f r o m 1.33 P a t o 1.33 k P a o f o x y g e n in o x y g e n n i t r o g e n m i x t u r e s ' z. RESULTS The amount adsorbed on increasing or decreasing the pressure or temperature w a s r e p r o d u c i b l e a n d reversible, a s e x p e c t e d f o r e q u i l i b r i u m a d s o r p t i o n . T h e d a t a f r o m t h e a d s o r p t i o n i s o b a r s w e r e r e p l o t t e d a s i s o t h e r m s since re(P, T ) h a s o n l y t w o i n d e p e n d e n t variables. T h e detail o f t h e c o m p o s i t e i s o t h e r m s is s h o w n a t t h e h i g h e r p r e s s u r e s in Fig. 1 a n d a t t h e l o w e r p r e s s u r e s o n t h e l o g - l o g p l o t in Fig. 2. T h e b e s t smooth curves are plotted to avoid confusion. Typical scatter of the data points a r o u n d a given i s o t h e r m , a s s h o w n in Fig_ 1 f o r t h e a d s o r p t i o n a t 2 4 5 ¢ C , w a s a l w a y s within a p r e c i s i o n o f ~ 0.25 n g c m - z o r b e t t e r t h a n +__ 1 ~/o o f a m o n o l a y e r ~ A l t h o u g h t h e s c a t t e r o f t h e d a t a b e t w e e n 0.078 a n d 7.8 T o r t (10 t o 1000 P a ) ~ s a t t h e limits o f u n c e r t a i n t y , t h e i s o t h e r m s in this regi'on a r e n e a r l y linear o n a l o g P v e r s u s m p l o t a n d w e r e s m o o t h e d easily b y u s i n g a m o d i f i e d least s q u a r e s m e t h o d . T h e v a l u e s f r o m [-
T-c
w I X
o
|
O
•
•
•
I
!00
•
!
o
o
•
!
200
PRESSURE (~o~-)
F'tS- 1. Adsorption isotherms for o ~ on sDver pov.~ler. The surface cover~£~ 0 were c~tc~l~tFd an the ~ silver arums are in a < l l l ~ ~ - - a t l o n _ -
362
2C
0~'i
g 3
~
i
o i LOG Po= (tocr)
z
3
Fig. 2. L o g - l o g p l o t o f the i s o ~ s h o w n i n Fig. 1 s h o w i n g low-pressure detail. T h e low c o v e r a ~ p a r t o f the i s o t h e r m s e x t r a p o l a t e to a c o m m o n p o i n t o f 17.8 n g crn -~- a n d ~ PaL.
TABLE I I..~O:Si ~ L I C H E A E
][:OR O X Y G ~
~RBED
O . ~ S I L V E R A T V A R I O U ~ ~tYRFACE C O V E R A G E S
Maxxodsorbed q! (n~ era-'-) (kcal mol-~)
Trange o f yah'cliO" o f qt
q!
( ~kr~zlmol-i)
Trar.ge o f v~,llrl;t.v o f q±
6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
7"> 260 T> ~ T > 260 7"> ~ 7"> 260 178-339 i 78-339 178-339 7"> 302 7"> 302 7"> ~ 7"> 302 199
--41.0-43.0 40.0--43.5 2.8_0--M.0 ~ 17.6-17.8 17.6-17.8 17.6-18.7 18.3-19.1 18.3-2D.3 18_7-19.5 17.6--19.9 13.0--15.0b 9-4-1 L 4 b 4 . 0 - 6.0 b
~ -T<245 7"< 245 7<245 178--339 178-339 178-339 178-302 178-302 178-302 178--302 3 " < ! 9 9b 7"<199 ~ T<199 b
40.5--43.5 31-5--35.5 24.2-26.2 17.8-18.5 17.9-18.5 17.6-17.8 ! 7.6--17.8 17.6-18.7 13.5--15-0 9.5--i 1.5 8.0-10.0 4 . 0 - 5_5 18.6-18.8 s 17-6"-18-5s 12.5-13.5s
= D o u b l e v a l u e s i n q here m o r e likely a c o n t i n t m ~ n o f q~. b D o u b l e values i n q here m o r e likely r e s u l t f r o m a "q~".
01H. surJ~r_t coreroge for
0. ! 62
0-216 0--~'/0 0-324 0_378 0.43~_ 0.486 0.540 0.595 0.649 0_703 0.757 0.811 0.865 0.919
363 t h e s m o o t h e d curves were used f o r calculating q a t e a c h m a s s o f oxy~en a d s o r b e d . T h e q a t a n y 0 was o b t a i n e d f r o m t h e d a t a u s e d t o c o n s t r u c t Figs_ 1 a n d 2 by p l o t t i n g In P vs. 1 / T a t a c o n s t a n t m a s s a d s o r b e d , e.g., a c o n s t a n t 0. T h e r e s u l t i n g slope is q l R as d e d u c e d f r o m t h e defining r e l a t i o n s h i p *. T h e values o f q o b t a i n e d at various masses o f o x y g e n a d s o r b e d a r e given in T a b l e 1. T h e ranges give t h e m a x i m u m v a r i a t i o n in q f o r d a t a p o i n t s falling + 0.25 n g c r n - 2 a w a y f r o m t h e s m o o t h e d curve. W h e n t h e d e t e r m i n a t i o n f o r q i n c l u d e d d a t a f r o m all seven i s o t h e r m s , a n d w a s single v a l u e d ( T a b l e 1, a t 16, 17, 18, 19 a n d 20 n g c m - z o f o x y g e n a d s o r b e d ) , t h e d e v i a t i o n s in t h e slope led t o uncertainties o f + 0.2 kcal t o o l - t o r a p p r o x i m a t e l y + 1 ~ o f t h e m e a s u r e d value. L a r g e r u n c e r t a i n t i e s necessarily resulted w h e n q's h a d to b e determ i n e d f r o m o n l y t w o o r t h r e e available i_,othern~ a t a c o n s t a n t ~ adsorbed_ T h e increased u n c e r t a i n t y results, in part, f r o m t h e e r r o r in t h e m e a s u r e d t e m p e r a t u r e as well as f r o m t h e p r o b a b l e c h a n g e s i n t h e a d s o r p t i o n . T h e r a w d a t a used f o r t h e plots in Figs. 1 a n d 2 a r e available o n r e q u e s t f r o m t h e a u t h o r . A surface s t r u c t u r e m u s t be a s s u m e d t o o b t a i n fractional coverages 0 f r o m t h e m a s s o f o x y g e n a d s o r b e d . I f it is a s s u m e d o n e o x y g e n a t o m will be associated with o n e silver a t o m a t 0 ----- 1, t h e n for t h e ( l l i ) , <100> a n d ( 1 1 0 > surfaces, 37, 32, a n d 22.7 n g o f o x y g e n p e r c m 2 o f silver surface will c o m p r i s e a m o n o l a y e r , c o r r e s p o n d i n g to 7.2, 8_31 a n d 11.75 A 2 f o r t h e average area o f a silver a t o m o n these t h r e e surfaces. T h e O's c o m p u t e d are also listed in T a b l e 1 a n d t h e a p p r o p r i a t e c o o r d i n a t e labels a r e a t t a c h e d to Figs. 1 a n d 2. DISCUSSION
T h e t e x t b o o k i n t e r p r e t a t i o n o f t h e q(0)'s in T a b l e 1 c o u l d b~ as follows: T h e " l i n e a r " decrease o f q u p to m = 12 n g crn - 2 is characteristic o f a T e m k i n i s o t h e r m , with o r w i t h o u t dissociative a d s o r p t i o n , o n a h e t e r o g e n e o u s surface t s . A c o n s t a n t value o f q f r o m 12 t o 32 n g c m - z is a d e q u a t e l y described as a n a d s o r p t i o n w i t h o u t d ~ o n , w i t h o u t lateral interactions, w i t h surface u n i f o r m i t y a n d with an i m m o b i l e layer, e.g., a s i m p l e L a n g m u i r i n t e r p r e t a t i o n . T h e decrease in q t o zero a t t h e hi~aer coverages a n d / o r h i g h e r t e m p e r a t u r e s results f r o m a m o b i l e a d s o r b e d species o r w e a k lateral interactions. T h e complexities o f t h e s i l v e r - o x y g e n s y s t e m a r e i g n o r e d by t h e t e x t b o o k i n t e r p r e t a t i o n . N u m e r o u s investigators h a v e c o n c l u d e d t h a t o x y g e n a d s o r b s o n silver in b o t h atomic and molecular forms z-5, s-~, as is a d e q u a t e l y referenced in recent reviewst~, ~ s D i r e c t c o n f i r m a t i o n o f t h e existence o f t h e m o l e c u l a r f o r m h a s been d e m o n s t r a t e d b y C l a r k s o n a n d CiriHo u s i n g electron s p i n r e s o n a n c e s9 a n d b y Kilty e t a l 5 u s i n g i n f r a r e d s p e c t r o s c o p y . R o v i d a et al.2 °" 2~ h a v e p r o v i d e d evidence f o r t h e existence o f a n Ordered a t o m i c species, u s i n g l o w e n e r g y e l e c t r o n diffractioIL They_ p o s t u l a t e d 2t t h a t a t w o - d i m e n s i o n a l surface oxide o f (111 > A g 2 0 is f o r m e d o n ( 1 1 1 ) A g t o explain 4. x 4 L E E D p a t t e r n s , b a s i n g t h e i r i n t e r p r e t a t i o n o n t h e lattice c o n s t a n t s o f A g 2 0 a n d A g . - E v i d e n c e f o r t h e w e l l - k n o w n a b s o r p t i o n o f o x y g e n i n t o silver h a s b e e n d e t e c t e d a t t e m p e r a t u r e s as l o w as 100°C with F E M z 2 150-200~C
364 microgravimetricallys, a n d 200°C by thermal d e s o r p t i o n z°. T h e s e thresholds o f absorption are considered to be small a m o u n t s t h a t p e n e t r a t e o n l y the first few a t o m i c layers. T h u s , oxygen a d s o r b e d o n silver includes oxygen adions a n d admolecule ions, where the adions m a y f o r m a n o r d e r e d two-dimensional "oxide'" o r diffuse into the bulk at higher temperatures. T h e likely possibilities t h a t c o u l d unravel the complexities o f q ( m ) in T a b l e 1, will n o w be a t t e m p t e d . T h e d r o p in the isosteric heat f r o m 42 t o i 8 kcal t o o l - t at low coverages is associated with the f o r m a t i o n o f a two-dimensional surface oxide r a t h e r t h a n surface heterogeneity. A d r o p in q can result f r o m c h a r g e d ions i f they a r e close t o g e t h e r ~6. This c o u l d be accomplished by the surface diffusion o f oxygen a d i o n s t o f o r m twodimensional islands o f silver oxide. Large stable nuclei o f A g : O o n silver h a v e been r e p o r t e d a t s u p e r - a t m o s p h e r i c pressures23; the partially ionic c h a r a c t e r o f oxygen in built silver oxide is well k n o w n . I f the d r o p in q results f r o m repulsions o f " o x i d e i o n s " in a two-dimensional AgzO, is it then reasonable also to a s s u m e t h e surface o f the silver p o w d e r is u n i f o r m ? Excellent a m,e e m e n t exists between the surface coverages given b y R o v i d a et al. z~ for a (1 ! 1 ) single crystal (Fig. 4, ref. 2 1 ) a n d those listed in T a b l e i, which were calculated o n the a s s u m p t i o n t h a t the silver p o w d e r is c o m p o s e d primarily o f (111 ) microfacets. W h e n e v e r quantitative c o m p a r i s o n s have been possible, the results o b t a i n e d on sing/e crystals -'°- ±~ a n d O A O R cycled silver p o w d e r s. ~o have s h o w n ~ood a~oreement. O n e is led to the conclusion that either the a d s o r p t i o n o f oxygen is n o t sensitive to surface structure o r the initially polycrystalline p o w d e r s a s s u m e low-index plane microfacets. In either case, the decrease in q with the a m o u n t a d s o r b e d should n o t be attributed to surface heterogeneity. F u r t h e r m o r e , additional c o m p a r i s o n s between the d a t a on silver powders a n d s i n ~ e crystals c a n be m a d e t h a t d o n o t a p p e a r to be especially speculative_ Rovida et al. -'°" -"~ r e p o r t e d the u p p e r b o u n d s o f stability o f a 4 x 4 superstructure r a n g e f r o m a b o u t 200 to 2600C between 10-"- to 1 0 - ' T o n ' , respectively, e . g , the 4 x 4 is stable a t hi mher pressures (higher coverages) a n d l o w e r t e m p e r a t u r e s . T h u s , the d r o p in q a t T < 245cC a n d m f r o m 10-16 ng c m - z c o u l d be associated with the f o r m a t / o n o f o r d e r e d 4 x 4 arrays o f silver oxide; t h e d r o p in q a t T > 260~C a n d m f r o m 6-12 ng e r a - z then c o u l d be associated with the f o r m a t i o n o f islands o f silver oxide n o t detectable b y L E E D . I f this assignment is correct, t h e mass o f oxygen a d s o r b e d a l o n g the b o u n d a r y o f 4 x 4 stability is a b o u t 13 ng ¢m - z o r 0t t ~ = 0.35. Additional a r g u m e n t s can be a d v a n c e d to s u p p o r t t h e hypothesis f o r the f o r m a t / o n o f a surface silver oxide. T h e plots o f log m versus log P, s h o w n in Fig. p r o d u c e straight lines, a t the lower pressures, that c o n v e r g e to a p o i n t w h e r e the mass gain is 17.8 ± 0.8 n g c m - 2 a n d the pressure is 266 Pa. ( A t higher pressures, the l o g - l o g plot does n o t exhibit a discernible convergence.) T h e point o f c o n v e r g e n c e o f isotherms o n l o g - l o g plots can be interpreted as the limiting coverage o f a p a r t i c u l a r c h e m i s o r b e d state z6. I f this state is the surface oxide AgzO, as is suggested b y c o m paring this w o r k with that o f R o v i d a et al. zz, the limiting coverage should be 0 =
365 0.50; m ----- 17.8 n g c m - z c o r r e s p o n d s t o 0 ---- 0.48 f o r t h e m o d e l o f a ~111~> surface, w h i c h is in g o o d a g r e e m e n t w i t h a 0 o f 0.5 f o r a surfac~ A~zO. Finally, g o v i d a e t al. 20 r e p o r t e d t h e 4 x 4 s t r u c t u r e is f o r m e d o n e x p o s u r e to 0.01 T o r t o f O 2 a t 2 0 0 ° C f o r I0 m i n a n d t h a t t h e o x y g e n c o v e r a g e f o r this e.~posnre o f t h e A g <~111 ~>is a b o u t h a l f t h e m a x i m u m v a l u e t h e y o b t a i n s ! by e x p o s i n g a t ! T o r r for 10 m i n a t --~0¢C. A n e q u i l i b r i u m coverage c a n be o b t a i n e d in 10 rain a t 2 0 0 ° C based o n t h e kinetic studies o f C z a n d e r n a s, so f u r t h e r c o m p a r i s o n s are n o t unre~_ sonable. I n Fig. 1, 22.2 n g c m - 2 o f o x y g e n will be o b t a i n e d at 1 T o r r a n d 199°C; h a l f this a m o u n t , 1 !. 1 n g c m - z s h o u l d c o r r e s p o n d to the l o w e r b o u n d w h e r e t h e 4 x 4 c o u l d be f o r m e d a n d is in r e a s o n a b l y g o o d a g r e e m e n t w i t h t h e l o w e r b o u n d o f a b o u t ! 3 n g c m - z d e d u c e d a b o v e f r o m t h e t e m p e r a t u r e a n d pressure limits o f stability o f the4
x
4_
I n t e r p r e t a t i o n o f q(m) f o r m exceeding 12-16 n g c m - 2 is r a t h e r s t r a i g h t f o r w a r d o n c e t h e h y p o t h e s i s o f f o r m i n g a surface oxide is accepted. T h e " c o n s t a n t ' " v a l u e o f 1 7 . ' / k c a l tool -~ ( m f r o m 12-19 n g c m - 2 ) is associated w i t h t h e h e a t o f f o r m a t i o n o f t h e surface oxide a t t h e lower coverages a n d t h e value o f 19 kcal t o o l - t at hi pAaer coverages is a t t r i b u t e d to t h e h e a t o f a d s o r p t i o n o f m o l e c u l a r l y a d s o r b e d o x y g e n o n t o t h e surfa_ee oxide. T h e g r a d u a l d r o p in q at t h e lower t e m p e r a t u r e s a n d hig~lest coverages is ascribed t o repulsive i n t e r a c t i o n s b e t w e e n t h e c h a r g e d ud~.orbed molecules. T h e a p p a r e n t d r o p in q at T > 3 0 2 ° C results f r o m m ~ s u r a b l e a m o u n t s o f o x y g e n a b s o r p t i o n i n t o t h e s u b s u r f a c e regions. F o r a b s o r p t i o n , t h e a m o u n t o f u p t s k e increases with increasing t e m p e r a t u r e , w h i c h is a n e n d o t h e r m i c r a t h e r t h a n a n e x o t h e r m i c process. T h e t h r e s h o l d d e v i a t i o n s f r o m e a c h i s o t h e r m f r o m t h e c o n v e r g i n g linear plots ( d a s h e d lines, Fig. 2) p r o v i d e detectability limits f o r a b s o r p t i o n i n t o silver at pressures o f 266, 53 a n d 20 Pa for 275, 302, a n d 3 3 9 ° C , respectively. A l t h o u g h direct evidence f o r f o r m i n g ( I 1 ! ) microfacets (~- 2 0 - 3 5 A in size) has n o t been p r o d u c e d , Kollen 2 . d i d s h o w facets, several h u n d r e d a n g s t r o m s in size, c a n d e v e l o p o n a small p a r t o f t h e surface d u r i n g O A O R cycling. Recently, E k e r n 2s d e m o n s t r a t e d t h e activation e n e r g y f o r d e s o r p t i o n o f o x y g e n f r o m t h e r m a l l y e t c h e d (facetted) a n d polyerystalline wire b e c o m e identic~a! w h e n t h e latter is subjected to o u t g a s s i n g a n d o x y g e n a~L~orptioncycling, similar to t h a t used o n single crystals -'°" : * F u r t h e r m o r e , R o v i d a et al. 2° r e p o r t e d t h e s a m e a d s o r p t i o n is o b t a i n e d w h e n silver single crystals are cleaned by cyclic o u t g a s s i n g a n d o x y g e n a d s o r p t i o n o r by i o n b o m b a r d m e n t a n d a n n e a l i n g . T h u s , t h e r e is persuasive evidence n o w available, in a d d i t i o n to p r e v i o u s a r g u m e n t s a o t h a t a d d s c o n s i d e r a b l e credibility to t h e suggestion t h a t ( ! i I ) m i c r o f a c e t s f o r m d u r i n g t h e O A O R cycling o f silver p o w d e r . T h e s u m o f q -k EA s h o u l d equal ED, w h e r e E A a n d ED are t h e uetivation e n e r ~ e s o f a d s o r p t i o n a n d d e s o r p t i o n , if e n t r o p y effects are negligible. By a d d i n g C z a n d e r n a ' s values o f 2 2 - 2 4 kcal m o I - * f o r E^ w i t h t h e q's r e p o r t e d in T a b l e I a t n o r m a l i z e d coverages, a n ED o f 41 __+ 3 kcal t o o l - 1 is o b t a i n e d . T h i s value c o m p a r e s favorably with r e p o r t e d values o f ED o f 41.8 ~_ 3.5, 37.8, a n d 35 to 45 kcal m o l - * given f o r silver p o w d e r s 9, filaments zs a n d single crystals z°, respectively. A n E D o f 41 + 3 kcal
366 t o o l - ~ is also q u i w interestingly within t h e p r o b a b l e u n c e r t a i n t y o f t h e a e r a t i o n e n e r g y f o r t h e dissociation o f silver o r / d e 2s o f 36 kcal t o o l - ~. For the m o d e l given in this p a p e r , the ED w o u l d result f r o m t h e d e s o r p t i o n o f m o l e c u l a r o x y g e n a t h i g h e r coverages a n d t h e d~=,sociation o f silver oxide a t lower coverages. A t t h e lower coverages, EA m u s t decline t o w a r d s z e r o as q(m) rises to 42 kcal t o o l - j to m a i n t a i n a c o n s t a n t E o o f --- 4! kcal t o o l - t A n E A o f 3 kcal t o o l - 1 h a s been r e p o r t e d at n o r m a l i z e d coverages b e l o w 0 ---- 0.4 b u t these were m e a s u r e d below 65~'C. ODNCLUSIONS
In s u m m a r y , a d s o r p t i o n e q u i l i b r i u m was f o u n d f o r t h e silver-oxygen system f r o m 178 t o 339 ¢C a t pressures o f 0.226 to 40,000 Pa. F a v o r a b l e c o m p a r i s o n s between t h ~ w o r k , w h e r e silver p o w d e r s were used, a n d t h a t o n sin~Ie crystals 2°- 2~ p r o v i d e a basis f o r a s s u m i n g a (11 ! ) facetted surface is d e v e l o p e d o n t h e p o w d e r d u r i n g extensive a n d repeated o u t ~ . ~ n g a n d oxygen a d s o r p t i o n c y c l i n ~ T h e maxs a d s o r b e d ~ias t h e n regated t o surface coverages o n ( I i 1 ) silver. A __decrease in t h e isosteric h e a t f r o m 42 to 17.7 kcal t o o l - • at c o v e r a e e s f r o m 0. ! 7 t o 0.33 is a t t r i b u t e d t o t h e f o r m a t i o n o f / s l a n d s o f a t w o - d i m e n s i o n a l surface oxide c o m p o s e d o f c h a r g e d ~nlons. A t h i g h e r coverages, t h e variation o f t h e h e a t f r o m 17.7 to 19 kcal t o o l - t is a t t r i b u t e d to ~m'owth o f the surface oxide to f o r m a m o n o l a y e r o f A g 2 0 a n d to a d s o r p t i o n o f m o l e c u l a r o x y g e n by a n d / o r t h r o u g h t h e o r / d e . R e p u l s i o n s in t h e m o l e c u l a r l y a d s o ~ oxygen a t t h e coverages o f a b o u t 0.9 p r o d u c e s a decrease in t h e isosteric h e a t a t l o w t e m p e r a ° tures; at h i g h e r t e m p e r a t u r e s , a decrease in q at 0 o f 0.6 results f r o m t h e o n s e t o f a b s o r p t i o n o f o x y ~ - n into t h e silver. ACKNOWLEIX/M~W
T h e a u t h o r is pleased to t h a n k U n i o n C a r b i d e C o r p o r a t i o n f o r partial financial s u p p o r t a n d f o r the specialized e q u i p m e n t used f o r this study. REFERENCES ! 2 3 4 5 6 7
A- F- Benton and L C. Drake, d. A m . Chem...~c., 56 (1934) 255. W_ W_ Smeltzet. E_ L T o ! b f c o n a n d A- C.asnbroth C'~z..I. Ckem., 34 (1956) 1046. C- B_ Mr:Carry, Ph.D. ~r/ws/s. Purdue Univexsity, I961, Univ. Microfilms 61-5741o Y . L Sandier and D_ D- Durigon. J. Phys. Clwm~ 69 (19K5"34201_ P_ A - Kff~.. N_ C R o l and W . M . H . Sar.htler, Fifth Int. Cong. on Cam/ys/s, N o r t h HoP!and, A m s t e r d a m , 1972. G . Rovkta, E Prazesi, M. Maglietta a n d E. Fe~roni, Surface S¢/., 43 0 9 7 4 ) 230. A. ~ in S. P. W o l s k y and E. J. 7_danuk (Eds.), Ultra M i o v - W e ~ g h t D e t e r m h ~ i o n in C o n t r o I l e d E m ~ o n n u m s , WRey, N e w Y o r k , 1969., p. 389.
s A.w. ~ - - ~
Jr. PATx. Chem.. 64(1964) 2765.
9 I0
W . K o l k n arid A . Czalldetna. J. Colloid Interface Sd_. 38 0 9 7 2 ) 152_ A . W . C~,~derna, Jo Pky'.r. Cicero., 66 (1966) 2120.
It
A. W_ C z a n d e r ~ in G. Goldfmgcr fed.), Clean ~arfacea, Degger, New York, 1969, p. 133.
367 I2 13 14 15
A . W . C 2 ~ d e r n a , in P. M- W~t~ys (Ed.), Vacuum Microbalance Tectmiqur_x, Vol. 4, Plenmn, N e w Y o r k , 1965, p. 5_/. A . W . C2andcxna, in P. M. Waters (lEd.), Vacmzm M / c r o b a / a n ~ Tec/zn/~ex, Vol. 4, Plenum, I~Iew Y o r k , 1965, p. 69. E. Roberts. in A_ W . Czandetna OEd.), Vaezasm Micz,obalance Techniques, Vol. 8, Plenum, N e w Y o r k , 1971, p. 73. F o r sample loads o f 10-20 g that can n o w be a c c o ~ o n the balance described b y Czanderaa eA al., .!. Vac. S d . Tedmol., 13 (1976~ 556, a n o r d e r o f magnitucte ~m4~ovezrte~t
(0A ~ o f 0) appears realizable_ 16 17 18 19 20 21 22 23 24 25 26
D . O . H a ~ - a r d a n d B. M. W. Tragmell, C_hemi.corptian, Bollexworrbe ~ n , 1964, p_ 159 fl~ H . H . Vo~e a n d C- R . A d a m s , ~ / r . Cata/., 17 (1967) 151. W . M . 14- Sac.htler, Cata/. Rev., 4 (1970) 27. R . B _ C l a x k _ ~ n a n d A . Cuillo, J r . , J o Vac. S c L T e c ~ . o L , 9(1972) 1073. G . Rovida, E. Fefroni, M. Maglietta a n d E Pratesi, in 1=. Ricca (Ed.), Adsorption-De.sorption Phenomer--,, Academic P re~¢. N e w Y o r k , 1972, p. 417_ G_ Ro-cida. F_ pr~te~i M . Magiietta a n d != Ferroni, 5arface ~c/., 43 (1974) 230. A . W . Czanderna, O. Frank and W . A . S c . h ~ d t , Surface So/., 38 (1973) 129. C_ Menzel a n d E. Men2ei-Kopp, ~urface Sci., 2 (19(34) 402_ W . Kollen, Ph.D. The.~, C'h.t'kson College o f Tech., 1969. R . l~kca-a~ Ph.D. The:~.f, ~ C~:~11¢~ o f Tech., 1974. J. A . ~ Atz~f . J. Chem., 13 (1960) 431.