TDS and XPS studies of the adsorption of O2 on electrolytic silver

444

Surface Science 163 (191,,;5) 444 456 North-Holland. Amsterdam

T D S A N D XPS S T U D I E S OF T H E A D S O R P T I O N OF Oz ON ELE(q'ROLYTIC S l l N E R BA() X i n h e a n d D E N G J i n g f a Department of ( "hemt.~'t:T, t"udan L.'mver~:tv. Nhane,hat. Peoph"~ Rep ,,1 ( "hma and

I)ONG

Shuzhong

In.~tltute ~{[ M~,~h,rn Ph.wwv l"ud.n { nit,er~iO. Shan,tIhat. l'e,q~lc~v R~7~. ,)I ("htm:

Recei,.cd 14 February 1985: accepted for publication 9 .lul\ 1985

"l'hermal dexorption spectroscop5 (TI)S) and X-ra,, photoelectron spectroscopy (XPS~ ha,.e been emplo.~ed to stud,, the adsorption of ox~.gcn on electrol~,tic sil'.er. In the pressure range of I I0 s "l'orr. se',eral kinds of o~ygen species have been observed on the surface: (A~ Adsorbed molecular oxygen (/:'1 -- l()l kJ/nrol, s..- 4 x lO II s I ). (B) surface bound at~mlic oxygen ~t-,i 134 kJ,'mol. ;, 4.7 × 1()1~ s I) and (C) sub-surface atomic oxsgen. Results ~f XPS shov, that the binding energies of ls orbitals of these species of oxygen on silver arc 528.3. 5297. 530.2 and 532.2 eV. F,.,,o bands centered at 2.8-3.0 and 90 eV below the Fermi le'~el appear on the difference spectrum of the valence hand v,'hich again indicates that both atomic and molecular ox~.gen e,cist on the surface of sil,.er.

I. Introduction Silver is a useful c a t a l y s t in s o m e i n d u s t r i a l processes, e.g. the e p o x i d a t i o n of e t h y l e n e [1] a n d the p r o d u c t i o n of a l d e h y d e f r o m a l c o h o l [2,3]. E x t e n s i v e r e s e a r c h a n d e x p e r i m e n t s h a v e b e e n p e r f o r m e d in o r d e r to e x p l o r e the m e c h a n i s m s of these c a t a l y t i c r e a c t i o n s [1]. C z a n d e r n a a n d K o l l e n [4,5] a n d Kilt,, et al. [6] s t u d i e d the c h e m i s o r p t i o n of o x y g e n a n d the r e a c t i o n of e t h y l e n e \vith p r e - a d s o r b e d o x y g e n on silver p o w d e r . T h e y c l a i m e d that there are three types o f o x y g e n species on silver, that is, n o n - a c t i v a t e d d i s s o c i a t i v e , a c t i v a t e d n o n d i s s o c i a t i v e a n d a c t i v a t e d d i s s o c i a t i v e species. M a d i x a n d c o w o r k e r s [7] s t u d i e d the a d s o r p t i o n a n d r e a c t i o n s of o x y g e n on s u r f a c e s of silver single crystals, a n d r e p o r t e d that m o l e c u l a r and a t o m i c o x y g e n w e r e o b s e r v e d on the A g ( l l 0 ) s u r f a c e at a d s o r p t i o n t e m p e r a t u r e b e l o w 150 K. W o r k s o n X P S , U P S [8,9] a n d E P R [10] also s u g g e s t e d that b o t h m o l e c u l a r and a t o m i c o x y g e n exist on p o l y c r y s t a l foil a n d s u p p o r t e d silver c a t a l y s t s u n d e r low t e m p e r a t u r e a n d high pressure.

0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 " Elsevier S c i e n c e P u b l i s h e r s B.V. ( N o r t h - t l o l l a n d Physics P u b l i s h i n g D i v i s i o n )

Bao Xinhe et al. / TDS and X P S of O., on electrolytic Ag

445

Electrolytic silver has been used as a catalyst in manufacturing formaldehyde from methanol for more than fifty years because of its high reactivity and selectivity. But, the mechanism of the reaction has not yet been well understood. The TDS and XPS studies outlined in this paper were carried out to help elucidate the interaction of oxygen with electrolytic silver.

2. Experimental 2.1. Sample preparation and purification of gases Sih, er samples used for the experiments were prepared by means of electrolytic refining for three times. It has a surface area of 0.1 m2/g and a purity of 99.999%. The sample was heated to 650°C and reacted with 02 ( 1 0 3 Torr) to get rid of the surface carbon [4,11], The annealing and oxidation procedure were repeated until the desorption peaks of CO and CO_, reachcd a steady minimum value. I~()2 (99.99%) was purified by passing through a liquid-nitrogen trap and a drying tube filled with 5A molecular sieve, lSO~ (99%) was obtained from Amersham Co, and used without further purification. Carbon monoxide used in this experiment was prepared by the decomposition of formic acid. After cooling, drying and deoxygenation by a No. 401 molecular sieve, the final CO gas was tested by a quadrupole mass spectrometer; no impurities were observed.

2.2. TDS experiment The experiment was conducted in a UHV chamber equipped with a quadrupole mass spectrometer. The chamber was pumped by a titanium sputtering pump (300 g/s), lowering the background pressure in the chamber to about (5-8) × 10- ~0 Torr. The main components of the residual gas were CO and H 2. After several adsorption and desorption cycles, CO~ and H 2 0 were observed among the residual gases in addition to CO and H2, and at the same time the background pressure rose to (2-5) x 10 -~ Torr. The silver sample was put in a quartz tube directly connected to the chamber. The temperature-programmed system had a good linearity within the heating rate of 1.0 to 320 K / m i n . A quadrupole mass spectrometer was employed to monitor the desorbed gases and reaction products.

2.3. XPS experiment The XPS system (VG ESCALAB) used had a background pressure of 2 x 10- ~0 Torr in the testing chamber. Silver samples used in the experiments

44t~

Ba~) X m h e et aL ,' 17).Y and .k'l'S ,,.[ O, ml ele~tr~*&ttc .4g

were cleaned by argon b o m b a r d m e n t and oxidation annealing cycles in the sample preparation chamber with a background pressure of 7.5 × 10 m "lorr. Binding energy measurements were calibrated by' using Ag3,t, , = 368.(I eV. In our experiments, The angle between incident X-ray and sample normal is 60 °. the angle between acceptance direction and sample normal is 0 °. Thus, the a m o u n t s of oxygen adsorbed on silver surface n,. (atomsflcm 2 ) can be estimated by the following equation [12]: ,t~, = }"off ~;%'pk/)i.~P-(),l.

( 1)

where )~), 11.\~ and if(), ff.~ are the total no energy loss yields of photoelectrons and the mass absorption coefficients at the excitation vsavelength (Mg Keq.:) of adsorbed oxygen and clean silver, fro-- 2533 c m : / g and t*,~,~= 4254 cmZ/g [13]: ;k is mean free path of photoelectron in silver, when ~, -- 12 A [14]: ,5 is Avogadro's number: p and ,4 are the substrate density ( g / c m ~) and the atomic weight. The total yield ratio of photoelectrons }]~/)~x~ can be obtained from the XPS intensity' ratio lol~/Ix~3j, ,. when the relative cross sections for photoionizing electrons from different orbits of an atom are known [15]. B\ assuming the atomic density of silver on the surface to be approximately 1 × 1015 a t o m s / c m 2, the coverage of oxygen on silver can be calculated.

3. Results

3.1. Sticking coefficients By varying the amounts of oxygen exposure m the range of 5 to 10" I,, isothermal adsorption curves were obtained at room temperature (fig. 1). The behavior of oxygen adsorption on silver seemed to be quite different from that of other metals. The sticking coefficient of oxygen on silver reaches a maxim u m at about 0 -- 0.5 rather than 0 = 0. The values of the sticking coefficients estimated from the amount of adsorbed oxygen on silver measured by XPS arc no less than 10 5 when n, is below 1.5 × 10 I"~ atoms/cruZ, and are sensitive to the cleaning procedure of the surface.

3.2. "I'DS results A series of thermal desorption spectra of O, have been obtained by varying the exposure of oxygen. Three peaks were observed in the spectra at oxygen exposure less than 90L (fig. 2): (a) an asymmetric low-temperature peak a at 460 +_ 10 K, (b) a symmetric peak fl~ at 620 K, and (c) a high-temperature dispersed peak /:/2 at 870 +_ 10 K. As shown in fig. 2, increasing the oxygen exposure l'ronl 5 to 90 L, the intensity of peak ,8~ increased and its ~, shifted toward lower temperature, but

Bat) Xinhe et aL / TDS and X P S of O. on electrolytic Ag

447

1.0

IM

~0.5 w > 0

0

I01

10 2

EXPOSURE

10 3

10 4

(L)

Fig. l. ('overage versus exposure profiles for oxygen on silver at 295 K. peak a did not change much and its Tp shifted slightly to higher temperature. When the exposure increased to greater than 90 L, peak a was overlapped by peak fl~. It was found that there was a break on the peak fl] when the oxygen coverage approached 0.2-0.6. This p h e n o m e n o n was also observed on silver powder by Czanderna [4] and Smeltzer [16], who suggested that it might be due to a transition from dissociative to non-dissociative adsorption. What we think may happen is that the adsorbed oxygen atoms rearrange in order to get a suitable position and fulfill a stronger bonding. The n u m b e r of neighboring atoms may change during the rearrangement, and thus the desorption activation energy and desorption temperature would be affected. It is also very interesting to note that the sticking coefficient reached a m a x i m u m in the same coverage range. As the exposure increased further, the intensity of peak a grew very fast and its T o shifted to lower temperature in contrast to the results at exposure less than 90 L, and at the same time the intensity of peak fl~ decreased. 3.3. X P S results

The XPS spectra of oxygen on electrolytic silver showed that at room temperature at least three different oxygen states existed on the sample with an exposure of 2 × 10 2 L ( P o , = 1.2 × 10 -7 Torr). The binding energies of the ls

Ba() X m h c et al

44~

/

7"/).S"

a n d .gl'?," ,,J 0., ,m oh,( tr,,Ivttc

,.I.t:

(] 90L

c

°/ )h-

,

~ z

40o

6oo TEMPERATURE

450L

aoo (K)

~0

"

600

TEMPERATURE (K)

b

Z I m ne

>I--Z

~~ [ .

j

-40o

.......

~6o-

TEMPERATURE

coo (K)

Fig. 2. thermal dcsorplion spectra for adsorbed oxygen on ~,ilverat 295 K. fl - 32 K mm

o r b i t a l of these o x y g e n species were a b o u t 528, 530 a n d 532 eV r e s p e c t i v e l y (fig. 3). T h e X P S s u b t r a c t i o n s p e c t r a b e t w e e n two d i f f e r e n t e x p o s u r e s are given in fig. 4. It is seen that the p e a k with Eb = 528.3 eV grew as the o x y g e n

Bao Xinhe et al. / TDS and XPS of O: on electrolytic Ag

449

IOIL

o; >I.Z LU

Z

Q_ X

I

i

524 528 5:32 BINDING ENERGY

536 (eV)

Fig. 3. O ls spectra of XPS at different exposure of oxygen.

exposure increased from 102 to 103 L, but it decreased and a new peak at 529.7 eV appeared simultaneously when the exposure increased further to 10 ~ L (po,=6×10 5 Torr). Then, at an exposure of 106 L ( p o = 7 . 5 x 1 0 -4 Torr), the 530.2 eV peak with a shoulder at 532.2 eV appeared, and finally, for even higher exposure, the peak at 530.2 eV increased in intensity. 3. 4. The

identification of adsorption species

By using the isotope exchange method, molecular and atomic oxygen can be readily distinguished. In our experiment, 1602 and mO 2 were adsorbed simultaneously, and the desorption spectra were recorded at m / e = 32, 34 and 36 respectively. Fig. 5 is the desorption spectrum of a sample preadsorbed with the gas mixture (1802 : 1602 = 5.0 : 3.2, 0 = 0.032). A number of experiments [17] has proven that at room temperature CO can react with atomic oxygen on a silver surface to form CO 2, but not with molecular oxygen. In this experi-

450

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," 7"1).~," a m l

X/',S" o f 0_, ~m e h ' c t r n l v m

..! e.

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uJ n u3 i.i (,3 Z UJ

12z hl i.

U_

524 528 532 BINDING ENERGY I i g . 4. l ) i f f e r c n c c

,536

(eV)

s p e c t r a of ( ) 1~, for d i f f e r e n t cxposure~, (in [.) o f ox~,gen: (a) 4 ~." l ( ) ' to 2 ,~ II)-':

(h) 1.1 × H)" to 4 × 1()~: (c) 1 . 4 × I() ~' to 1.1 × 1( "" (d) 1.7 ~ 1()" to 1.4 ,," 1() ~'

ment, (,), was p r e a d s o r b e d on silver. A f t e r p u m p i n g out the residual oxygen gas. pure ( ' O was leaked into the system to react with the a d s o r b e d oxygen. The e x p o s u r e of C O was 180 I, which was a p p r o x i m a t e l y 5 times that of oxygen. Then, the residual C O was p u m p e d out and the d e s o r p t i o n spectra of oxygen were recorded (fig. 6), which showed that the peak at high t e m p e r a t u r e d i s a p p e a r e d after reacting with ( ' O . Therefore, it m a y be c o n c l u d e d that the peak at low d e s o r p t i o n t e m p e r a t u r e c o r r e s p o n d s to m o l e c u l a r oxygen and the peak at high t e m p e r a t u r e to dissociative a t o m i c oxygen. In the XPS spectra, the peak at 532.2 eV is assigned to m o l e c u l a r oxygen on the surface [8,9], the peak at 528.3 eV may be a d s o r b e d atomic oxygen. T h e fact that the peak at 532.2 eV r e m a i n e d u n c h a n g e d when the peak at 528.3 eV dirninished and the peak at 529.7 eV a p p e a r e d indicates that the a d s o r b e d a t o m i c oxygen does not tend to r e c o m b i n e to form m o l e c u l a r oxygen but rather forms a second phase of a t o m i c oxygen as the coverage increases. This is

Bat) Xinhe et al. / TDS and X P S of O: on elertrol)'tic Ag

451

rr

f

y

Z

Z (.9 (/) o3 o3 =E

M'36 i

400

600 TEMPERATURE

800 (K)

Fig. 5. Isotope-mixing by surface oxygen dosing gas, ~60 2 •"~sO2 = 5.0 : 3.2. coverage: 0.032.

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I

8oo TEMPERATURE

(K)

Fig. 6. Thermal desorption spectra for oxygen on silver: (a) 35 L of 02 at 295 K: (b) 180 L of CO after 35 L of 02 at 295 K.

452

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ENERGY

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I0

(eV)

Fig 7. l)ifferencc spectra of the ',alence hand of the surface hcl'ore and after ad~,orptnm of ox?.gcn.

consistent with the results of ( ' z a n d e r n a [181. ( ' o n t i n u o u s increase of the oxygen exposure e n h a n c e d the peak lit 530.2 eV even at high exposure. This implies that the oxygen species c o r r e s p o n d i n g to 530.2 eV pertain,,, to subsurface a t o m i c oxygen [81. The position, line shape a n d F W H M of the Ag~ a peak all r e m a i n e d the s a m e as before oxygen a d s o r p t i o n , even at high exposure. This means that lhe d e e p l y a d s o r b e d oxygen a t o m s did not react chemically with the silver a t o m s to form hulk oxide but o c c u p i e d the sites between silver a t o m s to form c h a r g e d species such as O a (8 < 2). The diffusion and solubility of oxygen in thc hulk of silver were r e p o r t e d in previous p a p e r s [4.6.19,20]. By c o m p a r i n g lhc T D S a n d XPS results, it is c o n c l u d e d that the d e s o r p t i o n peak at 870 K (fig. 2) is due to a t o m i c oxygen diffusing in the bulk of silver. F r o m results of isothermal d e s o r p t i o n , it seems that the diffusion of oxygen in silver strongly d e p e n d s on the a d s o r p t i o n pressure and temperature. The XPS difference s p e c t r u m of the valence b a n d showed two b a n d s c e n t e r e d at 2.8-3.0 and 9.0 eV below the Fermi level (fig. 7) similar to the results of UPS [8,9], which suggests thal the atomic a n d m o l e c u l a r species of oxygen coexist on electrolytic silver surface.

Bao Xinhe et al. / TDS and XPS of 02 on electrolyttc Ag

453

3. 5. The m e a s u r e m e n t o f some kinetic p a r a m e t e r s o f the desorption In the t h e r m a l d e s o r p t i o n e x p e r i m e n t , the n u m b e r of m o l e c u l e s w h i c h d e s o r b f r o m the s u r f a c e per unit t i m e ( d N / d t ) can be related to an a c t i v a t i o n e n e r g y for d e s o r p t i o n a n d a p r e - e x p o n e n t i a l f a c t o r [21,5,22,23]:

- d N/dt

{2)

= v 0 " exp{ - E d / R T ).

T h e d e s o r p t i o n rate for a c e r t a i n species is p r o p o r t i o n a l to its signal on the m a s s s p e c t r a I; w h e n the e f f e c t i v e p u m p i n g s p e e d of the s y s t e m is large e n o u g h , then,

- d N/dt

(3}

= K I = vO ~ e x p ( - E d / R T ),

w h e r e K is a c o n s t a n t . T h e r e f o r e , In I = In( v O ~ / K ) - E d / R T .

(4)

If the d e p e n d e n c e o f K on T is n e g l e c t e d , the p l o t of In 1 versus 1 / T o can be obtained, where -Eo/R a n d I n ( v S ' ~ K ) are the s l o p e a n d i n t e r c e p t o f thc curve, respectively. T h i s m e t h o d is i n d e p e n d e n t o f the h e a t i n g rate (/3). H o w e v e r , in o r d e r to get a c c u r a t e kinetic p a r a m e t e r s , /3 s h o u l d be c h a n g e d by a f a c t o r of 100 [23]. In o u r e x p e r i m e n t s , / 3 was c h a n g e d f r o m 8 to 320 K / m i n . T h e results o b t a i n e d are listed in table 1. T a b l e 1 also i n c l u d e s the results f r o m the p l o t of ln(Tp2//3) versus l / T o. T h e d i f f e r e n c e of the t w o sets of results is w i t h i n e x p e r i m e n t a l error. It is clear t h a t E d d o e s n o t c h a n g e m u c h at two d i f f e r e n t c o v e r a g e s , but the d i f f e r e n c e s in E d a n d the p r e - e x p o n e n t i a l factors o f d e s o r p t i o n b e t w e e n the two states are r e m a r k a b l e , w h i c h i n d i c a t e s that the t w o states are v e r y d i f f e r e n t f r o m e a c h other.

Table 1 Desorption parameters of oxygen on silver surface Tp (K)

Kinetic parameters Ej ~ (kJ/mol)

508

References v b} (s- t )

168.17 134.10 -

460-480 `} 600.- 620 ¢} 600-620 0}

Ea b} (kJ/mol)

93 130 135

144.15 101 134 129

~'} Calculated by In I versus l/Tp. b} Calculated by ln(To2/fl) versus 1/T o. o 0 = 0.015. d} 0 = 0.63.

2× 10 II -2 × 10 }4 4×10 }l 4.7× 1013 2 × 10 }2

Rovida [24] Sandier and Durigon [25] Kollen and Czanderna 15] This paper This paper This paper

454

Bao Xmhe et al. / 'II).Y and

XI'.Sol O,

,m electr, dvtic A.V

4. D i s c u s s i o n

It is commonly accepted that atomic oxygen exists on silver surfaces. The kinetic parameters for desorption measured by different research groups were in good agreement [11,24]. It is still disputable, however, whether molecular oxygen exists on silver surfaces. Results published often varied with experimental conditions and systems. For supported silver [26] and silver powder, a molecular oxygen state was observed in UIIV sy.stem. For single-crystal silver [1 ] and silver rod [27], no molecular oxygen was found. Eveken and C/anderna [27] studied the adsorption of oxygen on thermally etched and polycrystallme silver, and claimed there are two oxygen states on polycrystalline silver, but onl\' one kind on thermally etched silver. As they explained, it was because the polycrystalline substrate was contaminated by' C, S, ('1. etc., but the thermall\ etched silver (processed at 1000°(') was relatively clean. They, thought thal the existence of the molecular state probably was due to the adsorption of oxygen on those contaminates. 111 our experinlent, a small amount of (" was foulld ol3 the surface of the silver sample after cleaning by. AES. It is likely' that thc molecular oxygen stale we observed at low exposure is due to the effect of surface impurities. As the exposure increases, it is possible that some impurities are introduced into the system. The sticking coefficients of those impurities are much large than that of oxygen, and they may occupy the adsorption sites where atomic oxygen adsorption would occur. This explains the fact that the molecular adsorption increased and atomic adsorption diminished with increasing oxygen exposure (of. fig. 2c). There are lsvo other facts supporting the above statements: (1) At the beginning of the treatment of the sample, molecular oxygen was observed. However. after repetition of the treatment, the molecular adsorption state became less prominent, while the atomic adsorption increased. (2) At large exposure of oxygen, the desorption peaks of C() and ( ' ( ) 2 became greater as well as the growth of molecular state. l h e increased ('() a n d ( ' 0 2 signal was presumably due to the oxidation of surface carbon present in the sample. In the XPS experiments, the concentration of the molecular species on the surface seemed not to he as large as that m the I D S experiments which could be explained by. the difference in cleaning procedures. According to the thermal desorption theory, for sccond-ordcr dcsorption, the temperature Tp al which the desorption rate is at a maximum will shift to lower temperature with increasing coverage, but 71, is independent of coverage for first-order desorption. In realistic systems, the shift of T~, may also be caused by' the inhomogeneitics of the surface and the interaction between the adsorbates. Usually', the large scale shift (60 100 K) of T~, can be considered as a criterion o f the kinetics of desorption. In this experiment, the shift of Tp of atomic oxygen is about 70 K, which indicates that the desorption was a second-order process. This conclusion is consistent with the results of isotope exchange, ('O titration experiments and XPS. For the molecular oxygen state,

Bao Xinhe et aL / TDS and X P S of O, on electrolytic Ag

455

on increasing the exposure, Tr shifted unusually to higher temperatures by about 20 K at low oxygen exposure, but to lower temperature when the exposure was higher. Rovida [24] had observed the same shift and he explained that the shift was a result of the lateral interactions between adsorbates on the surface. The results of our experiments show that the interaction depends on the coverage, i.e. attractive at low coverage and repulsive at high, which is probably caused by the difference in molecular adsorption states at different coverages. The pro-exponential factor can be expressed as follows: v = (kuT/h)

exp(alS~/R).

(5)

By using the data in table 1 (0 = 0.015), the difference in AS " between atomic and molecular states was obtained, which was 40 J / m o l . K. Engelhardt and Mcnzel [28] mentioned that if the gas molecules lost one translational degree of freedom, the change in entropy would be 46 J / m o l . K. We postulate that the molecular oxygen state has less translational freedom than atomic oxygen. This was also corroborated by the calculation of diffusion coefficients D0 using the following equation [29]: Do = aev/2,

(6)

where a is the crystal lattice constant (2.5 ,~, for Ag). The values of D, were readily calculated from eq. (6): 1.3 × 10 - 4 c m / s for the molecular adsorption state and 1.5 × 10- 3 c m / s for the atomic adsorption state. Apparently, there is a large difference between the two states.

5. Conclusions (1) The adsorption of oxygen at pressures between 10 --s and 1 Tort leads to three different types of adsorbed oxygen species on the surface of electrolytic silver. They are non-disscxziative molecular oxygen, dissociative atomic oxygen and sub-surface atomic oxygen. (2) The kinetic parameters of desorption are independent of the coverages of the adsorbed oxygen, but change considerably between two different states. The results are in agreement with other studies. (3) The sticking probability of oxygen on electrolytic silver is different from other metals. The maximum sticking probability occurs at about half a monolayer, 0 = 0.5 instead of at 8 = 0. (4) When the coverage of the adsorbed oxygen is between 0.2 and 0.6 a "break" on the peak of atomic oxygen species in TDS has been observed, which may be attributed to the rearrangement of adsorbed atomic oxygen on the silver surface.

456

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