A static SIMS study of superficial reactions (O2, (CN)2) on silver

A static SIMS study of superficial reactions (O2, (CN)2) on silver

surface science ELSEVIER Surface Science 395 (1998) 356 362 A static SIMS study of superficial reactions (02, (CN)2) on silver S. Bourgeois *, R...

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surface science ELSEVIER

Surface Science 395 (1998) 356 362

A static SIMS study of superficial reactions (02, (CN)2)

on

silver

S. Bourgeois *, R. Gouttebaron, M. Perdereau Laboratoire de Recherches sur la R~actitit~; des So~ides, UMR 5613 CNRS/'Unicersit~; de Bourgogne, UFR Sciences et Techniques, BP 400, 21011 Dijon (k'dex, France Received 6 December 1996; accepted for publication 5 August 1997

Abstract

The exposure of silver surfaces to oxygen under about 10 6 m b a r at room temperature was studied mainly by secondary ion mass spectrometry (SIMS) used in a static mode. No reactivity of oxygen appeared under these exposure conditions in accordance with previous works. No modifications in the AES spectra, but an unexpected and huge increase in the intensities of secondary ions such as CN , C N O , A g ( C N ) 2 , Ag2CN * were observed. Different experiments were performed in order to specify the origin of this unexpected reaction in presence of pure oxygen. Moreover, exposures to pure (CN)2 and to a mixture of cyanogen and oxygen were performed in order to compare the reactivity of silver towards cyanogen when oxygen was present or not. It was shown that the presence of oxygen enhanced the quantity of adsorbed species and modified the adsorption mode of cyanide species on silver. It was also shown that when the temperature increased, these species rearranged on the surface to a polymerized form of cyanogen molecules. ~7 1998 Elsevier Science B.V.

Keywords. Cyanogen: Oxygen: Secondary ion mass spectroscopy; Silver; Surface chemical reaction

I. Introduction Silver is known to be a very good catalyst for oxidation reactions such as the epoxidation of ethylene [1], which is a key industrial process. However, it has been shown that organic molecules such as hydrocarbons or epoxides do not adsorb on silver surfaces at temperatures higher than 150 K [2,3]. As pure silver is inert towards the adsorption of a lot of molecules, the high catalytic efficiency of silver can be accounted for only by the presence of oxygen, which modifies drastically the reactivity of silver surfaces; for example, a 13-fold enhancement in the adsorption of ethylene

* Corresponding author. Fax: (4-33) 3 80396132: e-mail: [email protected] 0039-6028/98/$19.00 ~; 1998 Elsevier Science B.V. All rights reserved. PH S 0 0 3 9 - 6 ( I 2 8 ( 9 7 ) 0 0 6 4 0 - 7

on Ag (110) at 133 K has been shown due to the presence of adsorbed oxygen [4]. This is the reason for the numerous studies that have been performed on the system of adsorbed species on silver for more than a decade: a wide range of surface science techniques and methods have been used in order to specify the nature, the geometry, the site, and the electronic structure of the adsorbed molecules; for example Refs. [5 14]. Despite this large number of studies, the role of oxygen in the reactivity of silver surfaces is not yet fully understood. The sticking coefficient of oxygen on silver at room temperature is extremely low [10,15,16] [in t h e 10 - 3 range for A g ( l l 0 ) and even lower for the ( 111 ) surface], and high exposures are required to form detectable overlayers; this makes the experiments difficult to perform with surface

S. Bourgeois et aL ' Surfiwe Science 395 (1998) 356 362

science techniques. Despite the interest in the study of the reactivity of cyanide species, little surface studies have been performed on the cyanogensilver system. The conclusion of a TDS study [ 17] is that cyanogen is adsorbed in a molecular form on Ag (110). In this work, the adsorption on silver surfaces has here been studied by secondary ion mass spectrometry (SIMS). SIMS, when used in a static, low damaging mode, can provide information on the species present on the surface, with a particularly high sensitivity. Moreover, by means of the observed molecular ions, information on the adsorption mode can be obtained [18].

2. Experimental

2.1. Methods The experimental ultra-high vacuum ( U H V ) apparatus has been described in detail elsewhere [19]. The basic pressure in the apparatus obtained by an ion-pump unit was in the 10 10 mbar range during the experiments. Auger experiments were performed with a Riber OPC 105 CMA analyser (primary energy, 3 keV). The Auger intensities at 273 eV (C KLL), 351 and 356 eV (Ag), and 381 eV (N KLL) and 510eV (O KLL) were followed during the preparation of the samples and during the gas exposures. They were recorded in the derivative mode. The generally accepted sensitivity for this kind of Auger experiments is in the order of one tenth of a monolayer. SIMS experiments were carried out with a Riber Q 156 quadrupolar mass spectrometer. SIMS was used in a static, low damaging mode: primary argon ions had low energy (1 keV) and low density (6 x 101° ions c m - 2 s 1). In these conditions, the mean lifetime of one monolayer was about 200 min, the total time necessary for an experiment being about 5 min, SIMS is definitely used in a static mode. Furthermore, the total energy bandpass of the selection-detection system was selected in order to promote the superficial ions [20]. Only secondary ionic intensities higher than five times the noise were taken into account.

357

2.2. Preparation of the samples The samples used were silver 5 N polycrystalline sheets 10x 1 0 x 2 . 5 m m supplied by Johnson Matthey. After mechanical polishing, they were submitted to an heating treatment under UHV at 1000 K in order to achieve a recrystallization of the samples. Different kinds of subsequent polishing, either chemical or electrochemical, were then tested, the best results being obtained by an electrochemical polishing using a cyanide-based bath including AgCN, KCN and K_,CO 3. A heating treatment at 900 K under an atmospheric pressure of A r - H 2 was then achieved before introducing the sample in the experimental set-up. Before exposure, samples were submitted to in situ classical preparations consisting of repetitive cycles of Ar ion sputtering (500eV, 1 btAcm -2) and heating at 700 K. The purity of the surface was checked by AES and SIMS. In particular, no significant cyanide containing secondary ionic intensities were observed in SIMS spectra. The exposures of the surfaces to either O2 or CaN _, were performed at room temperature unless otherwise specified. Oxygen gas, supplied by Air Liquide, was of N48 purity. Cyanogen was prepared in situ by thermal decomposition of AgCN. The maximum pressure in the reacting species was 10 ~ mbar as measured with an ionization gauge located near the ion pump of the experimental vessel. According to the Pact that gases were introduced in the UHV chamber just in front of the sample by means of a leak valve followed by a capillary, this pressure value was underestimated by about one order of magnitude. Nevertheless, the SIMS or AES intensities were recorded as functions of the exposures expressed in Langmuir, the pressures being measured by the gauge: that is to say the exposure values were underestimated.

3. Results and discussion

3. I.

0 2

exposures

No ions involving both oxygen and silver such as AgO, AgO 2 or Ag202 can be demonstrated by static SIMS under the maximum oxygen exposures

358

S. Bourgeois el al.

Sur/ace Science 395 [ 1998) 356- 362

that have been performed. Moreover, the AES spectra after exposures were always those of pure silver: no adsorption at all is shown by AES. However, in SIMS spectra, unexpected cyanide based ions appear such as C N - , Ag(CN)2 and AgzCN + The values of some secondary ionic intensities before and after exposures at 1200 L are presented in Table I. These values are expressed in impulsionss l nA 1 in order to work with intensities referred to the same primary ion current. Positive and negative SIMS spectra after exposures to 1200 L of oxygen are presented in Figs. 1 and 2, respectively. In order to try to understand the origin of the presence of cyanide species on the surface, different experiments were carried out. At first, in order to rule out a pollution of the experimental set-up, SIMS experiments were carried out in another vessel. The results were the same as the results obtained by introducing oxygen by another gas Table I Characteristic positive and negative secondar~ ionic intensities of the silver sample before and after 1200 L of oxygen exposure Ions

Before exposure: intensity (impulsionss ~nA ~)

Alter 1200 L oxygen: intensity (impulsionss ~nA -~)

O CN Ag(CN) 2 AgAg 2 Ag=CN +

40 260 0 30 0 0

40 56 800 2155 4380 800 1930

4 3 8 0 imp / s n A

Ag +

800 imp !s n A Ag2 + 1930 tmp s n A (Ag: ,2~) 1

[

Fig. 1. Positive SIMS spectrum of the silver sample after 1200 L of oxygen exposure•

[Ag +52)

580 imp / s n A

2155 imp / s n A 4256800 imp / s n A

35 imp / s n.A (Ag + 26)

CN40 imp / s n A , O-

J

JI I r] i

~290imp/sn

A

50 -~[ 128 imp / s.n A

J

q46

I

;]i !i : ,,

'iI

Fig. 2. Negative SIMS spectrum of the silver sample after 1200 L of oxygen exposure.

introduction system. Another hypothesis was a pollution of the samples related to their preparation: the electrochemical bath which was chosen for the polishing was a cyanide-based bath. However, oxygen exposures of silver samples submitted to an ex situ preparation with only alumina powders gave the same cyanide-based ions. Moreover, SIMS spectra of samples just after their introduction in the UHV chamber without any further preparation exhibited no cyanide species. The hypothesis of a pollution related to the electrochemical treatment of the sample can thus be also ruled out, as well as a pollution coming from the bulk of the sample: the same cyanide species were observed as well for polycrystalline samples from different origins as for a monocrystalline sample. The last hypothesis was a pollution of the oxygen gas. In the chemical analysis of oxygen N48 provided by the manufacturer, no presence of cyanide species was specified, Gas phase analysis of the oxygen was performed by mass spectrometry. During the introduction of oxygen, a small peak can be evidenced at m/e equal to 52, corresponding to cyanogen, its intensity increasing with increasing oxygen pressure. Thus, cyanogen appeared in the experimental set-up in relation to the introduction of oxygen. After calibration, the quantity of cyanogen which is present in oxygen can be estimated to 0.03% in pressure. This volumic concentration is much higher than the values given by the manufacturer: one possible explanation is that cyanogen results from a reaction in the ion pump when oxygen is pumped in

S. Bourgeois et al. SutJace Science 395 (109~') 356 362

high quantities in both experimental vessels we use, the pumping units were ion pumps. Only a total change of the pumping units should allow to confirm this explanation. 3.2. C2N 2 exposures, pure or in the presence o[ OXI'~['ll

The exposure of silver to 900 L of pure cyanogen and to 900 L of cyanogen under an oxygen pressure of 10 ~ mbar was performed in order to study the influence of the presence of oxygen on the reactivity of silver towards cyanogen. In Fig. 3, the evolutions of the SIMS intensities CN , A g ( C N ) , and C2N2 are presented as functions of the exposure to pure cyanogen, and in Fig. 4 a comparison of the intensities of these ions for a cyanogen exposure of 900 L when oxygen was present or not is provided. The nature of the ions which are observed in the two cases of cyanogen exposure, that is to say without and with oxygen being present, leads us to think that molecular adsorption of cyanogen takes place on the silver surface. The non-dissociative character of this adsorption is evidenced by the fact that no ions involving Ag and C or N

C2N 3

Ag(CN) 2

1000

0-

go0

0-

600

,0 -

CN"1.5 104

. °•• " ' ' ' ' . . . .

"-~

°°

< =-

.'

4(10

/

- 1.0 10a

J

.

0-

I a

~ -5.0 103

i :oo o

"////

/,, o

I 200

I 400

I 600

I 800

1000

Exposure to cyanogen(L~

Fig. 3. E v o l u t i o n of the s e c o n d a r y ionic intensities C N . Ag(CN)_, and CzN 3 as functions of the exposure to pure cyanogen.

359

35000.

700 ~

25000"

500

15000.

~

400]

300

.|

,ooo.

CN-

Ag(CN)2

C2N3

Exposure to 900L of cyanogen under 10-8 mbar of oxygen Exposure to 900L of pure cyanogen Fig. 4. C o m p a r i s o n of the intensities of C N , Ag(CN)_, and C , N 3 when the exposures to 900 L of cyanogen are performed in the presence of oxygen or not.

such as for example AgC±, AgN ~, Ag2C-... can be observed by SIMS [18]. Moreover two other remarks have to be made. ( 1 ) The intensities of CN - and Ag(CN ), are higher when oxygen is present. In order to verify that this fact is not related to an enhancement of the ion yields due to the presence of oxygen, samples were submitted to an oxygen exposure after exposure to pure C2N2, No variations in the secondary ionic intensities can be observed. The higher values which are observed can thus be related to a higher coverage in cyanide species when the surface is exposed to a mixture of cyanogen and oxygen. (2) In addition to this fact, it can be observed in Fig. 4 that the ionic intensity corresponding to C,N 3 shows a special behaviour: it is higher when exposures are performed under pure cyanogen. This kind of molecular ion can be related to the presence on the surface of special cyanide species. Moreover, these species are weakly bound on the surface: the secondary intensity of the only ion involving both silver and cyanogen, Ag ( C N ) 2 , is small when the surface is exposed to pure cyanogen. The result that these species are weakly bound is also demonstrated by the behaviour of C2N 3 as a function of exposure shown in Fig. 3: this secondary intensity increases up to about 500 L of cyanogen exposure and decreases alter. This decrease is related to an increase in C4 and C3N intensities. The presence of these

360

S. Bourgeois et aL/ Sur/dce Science 395 (1998) 356-362

ions can be explained by a fragmentation-recombination process of CzN 3. A discussion of our results in comparison with those previously obtained is difficult to perform because of the lack of papers dealing with the adsorption of cyanogen on silver, only one paper having been found in the literature [17]. The different adsorption mode of cyanogen on silver when oxygen is present can be described in the general framework of acid-base chemistry [1]: oxygen atoms present on the surface having a high electronegativity may induce electron-deficient sites on the adjacent surface metals atoms. These electron-withdrawing effects of oxygen on silver give to the surface a Lewis acid character. These Lewis acid sites are able to interact with Lewis bases. In this picture, a bond can thus be created between one or two silver atoms with a Ag a+ character and either the free electron doublet of nitrogen atoms of (CN)2 or the 7t-electrons of the cyanogen molecule. When no oxygen is present on the surface, such an interaction is not able to occur. Such a description in terms of formation of Lewis sites could be a possible explanation of the fact that cyanogen adsorbs on the surface with a higher coverage and in a stronger manner in the presence of oxygen.

--_

C4N 3 1.5104

80O

600

...... ...... -....._,,

-1.0 104

E

_+

400

II

I1

",

-5.0 103 200

I 300

400

I

I

500

600

0 700

Temperature (K)

Fig. 5. Evolutions of CN C 2 N 3 and C4N3 intensities as functions of temperature after an exposure to 900 L of pure cyanogen.

- - - C4N3 _ _

3.3. hTJtuence o f thermal treatments

In order to go further in this discussion on the nature of cyanide species present on silver, surfaces which have been exposed to pure cyanogen or to cyanogen plus oxygen, as has been presented in the last paragraph, are submitted to a temperature increase up to 800 K, the characteristic secondary ionic intensities being recorded as functions of temperature. The evolutions of C N - , CzN3 and C4N3 are presented in Figs. 5 and 6. In Fig. 5, the results for a pure cyanogen exposure are shown, and in Fig. 6, the results for the exposure of cyanogen in presence of oxygen are shown. The variations of Ag(CN )_~ are not presented for sake of clarity. Its intensity is continuously decreasing in both cases, becoming equal to zero for temperatures above 650 K. A fact which is interesting to note besides the

"••"CN

C2N 3

C2N3

CN-

......

2000

6.0 104

f\ I

1500-

.- ...... •.

° +°

X

1-.,

I

...°

i •, ~

I

i

'3 1

I

1000-

-4.0 104

I I

I'

I

-2.0 104

500-

I 0

300

I',

I

[

I

I

400

500

600

700

Temperature (KI

Fig. 6. Evolutionsof CN , CzN3 and C4N 3 intensities as functions of temperature after an exposure to 900 L of cyanogenin the presence of oxygen.

S. Bourgeois et ell. / Surlace Science 395 (1998) 356 362

4. Conclusion

C4N; C2N; N

Ill

c

i

/ N

C

ca N

Ill c i C

f; N

N

c

c

Ili

: C

361

Ul I

C

/ \

........ N.?,2.,,. E ...........

Fig. 7. Possible formula for a polymerized form of cyanogen molecules able to give rise to the observed ions.

decrease o f CN for temperatures above 600 K is the presence in the temperature range of 450 to 650 K o f C2 N 3 and C4N 3 ions, the intensities of which are maxima at about 550 K. These new ions can be accounted for by a reorganization of the cyanide species present on the surface when the temperature increases. In the case of adsorption o f pure cyanogen, Fig. 5, the species being weakly b o u n d on the surface, a rearrangement begins just above r o o m temperature: CzN 3 decreases and C4N 3 increases from 300 K, which is not the case when oxygen is present. A b o v e 450 K, both secondary ionic intensities increase whether oxygen is present or not. In this temperature range, a new cyanide species is present on the surface. It can be accounted for by a reorganization of the initial species to a polymerized form o f cyanogen molecules, such as "paracyanogen". One o f the structure o f this polymerized species is given in Fig. 7. In this structure, it is possible to obtain the observed ions C 2 N 3 and C4N 3 by breaking a m i n i m u m n u m b e r of bonds, as indicated on Fig. 7. Although the same ions CzN 3 and C4N3 are observed when oxygen is present in the initial adsorption stage or not, the fingerprint o f this new species is not exactly the same in the two cases: the intensity of C4N 3 is relatively higher in Fig. 6 than in Fig. 5. This fact can be accounted for by the higher coverage when oxygen is present leading to a slightly different polymerized species or to a higher probability for the ion emission o f bigger fragments.

In this work, the great influence o f oxygen on the reactivity of silver surfaces has been evidenced in the case o f cyanogen adsorption: the presence o f oxygen enhances the quantity o f adsorbed species and modifies the adsorption m o d e on the surface. Moreover, a reorganization o f the adsorbed species has been shown to occur when the sample is submitted to a thermal treatment. This rearrangement could lead to polymerized forms of cyanogen molecules on the surface. In order to further the understanding of the adsorption m o d e o f cyanide species on silver surfaces and on the enhancing role of oxygen, other experimental techniques such as UPS should be used, but these experiments are difficult to perform in view o f the very small coverages of adsorbed species. It has been shown in this study that SIMS is an extremely sensitive technique: it allows the detection and the characterization o f adsorbed species resulting from the adsorption of cyanide impurities present in the introduced oxygen, while other surface techniques like AES give only spectra characteristic o f a clean surface. However, this advantage is lessened by the problem o f the mechanism o f secondary ions formation which is not truly understood,

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[13] L.H. Tjeng. M.B.J. Meinders, G.A. Sawatzky, Surf. Sci. 236 (1990) 341. [14] T. Hashizume. M. Taniguchi. K. Motai, Hua Lu. K. Tanaka, T. Sakuri, Surf. Sci. 266 (1992) 282. [15] R.B. Grant. R.M. Lambert, Surf. Sci. 146 (19841 256. [16] C.T. Campbell, Surf. Sci. 157 (1985) 43. [17] M.E. Bridge, R.A. Marbrow. R.M. Lambert, Surf. Sci. 57 (1976) 415.

[18] A. Brown, J.C. Vickerman, Surf. Sci. 151 (19851 319. [19] S. Bourgeois, M. Perdereau, J. Spectrosc. Electron. 6 (1981) 335. [20] S. Bourgeois. M. Perdereau, Le Vide, Les Couches Minces 38{1983) 183.