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Interaction of ethylene and acetylene with oxygen on An Ir(111) surface
Surface Science 184 (1987) 359-373 North-Holland, Amsterdam
359
INTERACTION OF ETHYLENE AND ACETYLENE WITH OXYGEN O N AN I r ( l l l ) S U R F A C E K.L. K O S T O V a n d Ts.S. M A R I N O V A Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria Received 20 July 1986; accepted for publication 15 January 1987
Investigations have been carried out using HREELS and XPS to study the interaction of C 2 H4 and C2H ~ with oxygen on Ir(lll). In the presence of atomically adsorbed oxygen, ethylene is adsorbed on Ir(lll) at 180 K molecularly and is bonded to the surface by a ~r-bond. At this temperature acetylene is adsorbed on an oxygen-covered Ir(lll) surface in a similar way as on a clean surface. The oxygen coverage hinders the dehydrogenation of ethylene to a higher degree than in the case of acetylene. At 300 < T ~ 400 K, CzH 2 and C2H 4 react with adsorbed oxygen atoms forming CO, CO2 and H20. Decomposition products of C2H 2 and C2H 4, which are observed on a clean surface, are also found on the oxygen-coverediridium surface. An oxidation mechanism of CzH 2 and C2H 4 via an intermediate product, CCH, is proposed. Above 450 K both hydrocarbons are completely oxidized to CO2 and H20. Atomically adsorbed oxygen is needed for the oxidation of the hydrocarbon species on Ir(lll).
1. Introduction The a d s o r p t i o n of C2H 2 a n d C2H 4 a n d their interaction with oxygen o n clean metal surfaces is of interest for u n d e r s t a n d i n g the m e c h a n i s m of catalytic oxidation of more complex h y d r o c a r b o n molecules. The surfaces of group V I I I metals are of special interest in this respect. The investigations have shown that C2H 2 a n d C2H 4 interact differently with clean a n d with oxygen-covered surfaces of these metals. F o r instance, o n a clean Pt(111) surface at temperatures below 220 K the ethylene forms a di-o b o n d with the surface. I n the presence of atomically adsorbed oxygen, ethylene molecules f o r m i n g a g - b o n d with the surface are observed [1] in a d d i t i o n to the di-o b o n d e d CEH 4 molecules. A t 300 K ethylene is adsorbed on a clean surface as stable ~ C C H 3 species [1,2], whereas o n an oxygen-covered Pt(111) surface C 2 H 4 is completely converted to C O 2 a n d H 2 0 [1]. A more recent s t u d y [3] b y the SIMS m e t h o d has shown the presence of a small a m o u n t of adsorbed ~ C C H 3 species even with a saturated oxygen coverage on Pt(111). Megiris et al. [4] have used the thermal desorption method for the study of the interaction between C2H 2 a n d an oxygen-covered Pt(111) surface. They have observed oxidation products (CO, CO2 a n d H : O ) in a d d i t i o n to the 0 0 3 9 - 6 0 2 8 / 8 7 / $ 0 3 . 5 0 9 Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
360
K . L Kostov, Ts.S. Marinova / Interaction of C 2 H ~ and C2H e with 0 on lr(111)
products of acetylene decomposition characteristic of a clean surface. The authors have assumed the first step of C2H 2 oxidation to be the formation of an intermediate product (CH) as in the case of C2H 4 oxidation [5]. Contrary to the case of a clean surface, the C2H 4 dehydrogenation on a Pd(100) surface with preliminary adsorbed oxygen atoms is hindered [6,7]. The same conclusion has also been drawn concerning a F e ( l l l ) surface where the presence of adsorbed oxygen atoms leads to a new molecular adsorption state of C 2 H 4 but does not substantially affect the adsorption of C : H 2 [8]. The oxygen coverage on Ru(001) also hinders the dehydrogenation of ethylene which is adsorbed as molecules alone forming a ~r-bond with the surface. At higher temperatures it is desorbed without reacting with oxygen [9]. In previous studies by the XPS, UPS and AES methods we have paid attention to the reaction of oxygen with C2H2 [10] and C 2 H 4 [11] on a polycrystaUine Ir surface. The present paper deals with the interaction of C2H 2 and C2H4 with oxygen on an I r ( l l l ) surface using the H R E E L S and XPS methods. The same methods were applied to the adsorption of C2H 2 and C2H 4 [12] and O2113 ] on a clean I r ( l l l ) surface. At 180 K C2H 4 is dehydrogenated to ethylidyne ( ~ C C H 3 ) while at 300 K C C H species are additionally formed. At 180 K C2H 2 is adsorbed as C C H and ~ C C H 3 species. After C2H 2 or C2H4 adsorption at 500 K there are mainly C C H species on the Ir surface.
2. Methods The investigations were carried out with an Escalab II ( V G S c i e n t i f i c Ltd.) electron spectrometer at a residual gas pressure of 1 • 1'0 -8 Pa. The XPS measurements were made using a M g K excitation source (hv = 1253.6 eV), the total instrumental resolution being - 1 eV as measured with the F W H M of the Ag 3d5/2 photoelectron peak. The corresponding resolution of the H R E E L S was 10-12 meV (80-96 cm -1) as the measurements were taken in the direction of specular scattering (45 o with respect to the surface normal) with a kinetic energy of electrons of 3 eV.
Acetylene and ethylene of 99.95% purity (Messer Griesheim) and oxygen of 99.95% purity (Merck) were used for the experiments. The Ir single crystal (purity, 99.998%, Metal Crystals) had a thickness of 1 ram, a diameter of 7 m m and was oriented and cut with an accurary of about 1 ~ in the (111) direction. A clean I r ( l l l ) surface was obtained after heating the sample at 1100 K under an oxygen pressure of 1 • 10 -5 Pa followed by heating up to 1900 K in vacuo of - 10 - s Pa. The experimental technique and the cleaning procedure of the sample have been described in detail in previous papers [12,13].
K.L. Kostov, Ts.S. Marinova / Interaction of C2H4 and C2H2 with 0 on Ir(lll)
361
3. Results 3.1. Interaction between C 2 H 4 and oxygen on I r ( l l l )
The experiments were carried out on a clean Ir(111) surface pre-covered with one of the reagents at a given temperature. After that, the coverage was exposed to the other reagent at the same temperature. The surface species resulting from the proceeding interaction were investigated by the XPS and H R E E L S methods. 3.1.1. Interaction o f C2H 4 with an oxygen-covered Ir(l l l ) surface
Fig. l a shows the H R E E L spectrum of a saturated oxygen coverage on I r ( l l l ) followed by a C z H 4 exposure of 2 • 10 -4 Pa s. In addition to an intense peak at 550 cm -1 characteristic of t,(Ir-O) vibrations of oxygen adsorbed in an atomic form, peaks at 1010, 1220, 1480 and 3010 cm - I are also discernible. The spectrum differs from that after ethylene adsorption at 180 K on a clean I r ( l l l ) surface where only ethylidyne species producing loss peaks at 457, 986, 1165, 1400 and 2940 cm -1 are formed [12]. In fig. la, a weak CO peak is observed at 2000 c m - a which is due to the adsorption of the residual gases in the spectrometer. The scanning time of the H R E E L spectrum is about 600 s, and hence, at a pressure of about 10 -8 Pa, the CO concentration is below 1% of the saturated coverage [14]. The CO observed is not a product of C2H4 oxidation since the same CO amount is formed on a clean I r ( l l l ) surface during scanning at 180 K. At 300 K the H R E E L spectrum differs from that at 180 K (fig. lb). After C 2 H 4 exposures of 2 • 10 4 Pa s, products of the decomposition of ethylene to ethylidyne (with characteristic peaks at 970, 1160, 1390 and 2970 c m - 1 [12]) and a product of the oxidation (CO, with a p ( I r - C ) peak at 500 cm -1 and a ~,(C-O) peak at 2020 cm -1) are observed on an oxygen-covered I r ( l l l ) surface. In addition to CO, the XP spectra show, depending on the C2H 4 exposures of the oxygen-covered surface (fig. 2a), the presence of oxygen adsorbed dissociatively with a binding energy of the O(ls) line at 529.8 eV. The total area of the O(ls) peak decreases with increasing C 2 H 4 exposures, which probably indicates oxidation of the hydrocarbons to CO 2 and H~O which are desorbed from the surface. At 340 K, after C2H 4 exposures of 2 • 10 _4 Pa s, the H R E E L spectrum of an oxygen-covered I r ( l l l ) surface (fig. lc) shows, in addition to the ethylidyne peaks, another peak at 810 cm -1 which is interpreted in ref. [12] as a ~ ( C - H ) m o d e of C C H species. At this temperature there is again interaction between the hydrocarbons and the adsorbed oxygen atoms, which is evidenced by the presence of an intense CO peak at 2020 cm-1. The XP spectra (fig. 2b) show that after C2H 4 exposures of 1 • 10 -4 Pa s, no atomically adsorbed oxygen is present on the surface. There is only CO producing an O(ls) peak with a
362
K . L Kostoo, Ts.S. Marinooa / Interaction of C2H4 and C2H2 with 0 on Ir(111)
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Fig. 1. HREEL spectra of an oxygen-covered I r ( l l l ) surface after 02 exposures of 3 0 x 10 -4 Pa s followed by C2H4 exposures of 2 • 10 -4 Pa s at (a) 180 K; (b) 300 K; (c) 340 K; (d) 450 K.
binding energy of 532 eV. The area of the O(ls) peak of the initially adsorbed oxygen is larger than the area of the O(ls) peak after C2H 4 exposures of 1 • 10 -4 Pa s. This is probably an indication that the oxygen atoms participate not only in the formation of CO but also in the oxidation of the
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Fig. 2. O(ls) peaks of an oxygen-covered I r ( l l l ) surface after 02 exposures of 30• followed by different C2H 4 exposures at (a) 300 K; (b) 340 K; (c) 450 K.
-4 Pa s
K . L Kostot), Ts.S. Marinova / Interaction o f C 2 n 4 and C 2 H 2 with 0 on Ir(l 11)
363
hydrocarbons to CO 2 and H 2 0 which are then desorbed from the surface. An analogous interaction is observed between C 2 H 4 and an oxygen-covered Ir(111) surface at 400 K. At T>~450 K, C 2 H 4 exposures of an oxygen-covered I r ( l l l ) surface probably lead to the complete oxidation of ethylene to CO 2 and H20. This is evidenced by the O(ls) peak of adsorbed oxygen (fig. 2c). With increasing ethylene exposure, the area of this peak diminishes and at 1 x 10 - 4 Pa s the peak disappears. Under these conditions the H R E E L spectra have peaks at 830, 1300 and 3020 cm -1 only (fig. ld) which are interpreted in ref. [12] as modes on C C H species.
3.1.2. Interaction of oxygen with a CeH4-exposed I r ( l l l ) surface A clean I,r(lll) surface was exposed to C z H 4 and the adsorption of oxygen in the presence of the hydrocarbon coverage was studied. At 180 K ethylene is adsorbed on a clean I r ( l l l ) surface as ethylidyne species. With hydrocarbon coverages lower than the saturated one the oxygen is adsorbed at 180 K in the form of atoms to which a vibrational frequency at 550 cm i corresponds. The oxygen does not interact with adsorbed hydrocarbon species: the H R E E L spectrum does not change with increasing oxygen exposure (fig. 3a). At 300 K C z H 4 exposures of 0.8 x 10 - 4 Pa s lead to the formation on I r ( l l l ) of a C C H coverage which corresponds to 0.4 C atoms per one Ir atom (in a saturated coverage there is one C atom per one Ir atom [12]). Subsequent oxygen
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364
K . L Kostov, Ts.S. Marinova / Interaction of C2 H4 and C 2 H 2 with 0 on Ir(111)
I O(ls}
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Fig. 4. O(ls) peaks after different oxygen exposures of a hydrocarbon coverage on Ir(lll) obtained upon C2H4 exposures of (a) 0.8 • 10-4 Pa s at 300 K; (b) 0.8 • 10-4 Pa s at 340 K; (c) 1.5 x 10-4 Pa s at 450 K. exposures of 30 x 1 0 - 4 Pa s lead to a reaction associated with the formation of CO ( p ( C - O ) at 2020 cm -1) (fig. 3b). This is also observed with the C(ls) and O(ls) peaks. The form of the C(ls) peak negligibly changes due to the formation of a small amount of CO. A small peak at 532 eV characteristic of CO appears near the peak of adsorbed oxygen (fig. 4a). At T = 340 K the reaction is more intense and leads to the formation of CO; p(Ir-C) and ~,(C-O) being at 490 and 2020 cm -I, respectively (figs. 3c and 4b). Probably CO 2 and H 2 0 are also formed because the area of the C(ls) peak decreases with increasing oxygen exposure. At 450 K increasing oxygen exposures up to 30 x 10 -4 Pa s of the hydrocarbon coverage (0.6 atoms per 1 Ir atom) lead to the disappearance in the H R E E L and XP spectra of the peaks for the hydrocarbon species. Instead of this, a characteristic peak of oxygen adsorbed in the atomic form is visible at 550 cm -1 (fig. 3d) as well as an O(ls) peak at 529.8 eV (fig. 4c).
3.2. Interaction between acetylene and oxygen on lr(111) 3. 2.1. Interaction of C21-12 with an oxygen -covered Ir(111) surface At 180 K, C2H ~ exposures of 2 x 10 -4 Pa s lead to the appearance, in the H R E E L spectrum of a saturated oxygen-covered I r ( l l l ) surface, of the same electron loss peaks (fig. 5a) which are observed during C2H 2 adsorption on a clean I r ( l l l ) surface [12]. They are characteristic of C C H species (the 8 ( C - H ) mode at 810 cm - ] ) and ethylidyne (the modes at 1000, 1170 and 1410 cm -1 [12]). Along with the vibrations characteristic of the hydrocarbon species, a
K.L. Kostov, Ts.S. Marinova / Interaction of C e l l 4 and C2H 2 with 0 on Ir(11l)
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Fig. 5. H R E E L spectra of an oxygen-covered I r ( l l l ) surface after a n 0 2 exposure of 30 • 10-4 Pa s followed by C2H 2 exposures of 2 x 10 - 4 Pa s at (a) 180 K; (b) 300 K; (c) 340 K; (d) 450 K.
peak of p ( I r - O ) vibrations of atomically adsorbed oxygen is observed at 550 cm -1 as well as a smaller p ( C - O ) peak indicating a negligible amount of CO. The latter does not essentially change with increasing acetylene exposure which means that it is not a product of the interaction of C2H2 with adsorbed oxygen.
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Fig. 6. O(ls) peaks of an oxygen-covered l r ( l l l ) surface after an 02 exposure of 3 0 x l0 4 Pa s followed by different C2H 2 exposures at (a) 300 K; (b) 340 K; (c) 450 K.
366
K . L Kostov, Ts.S. Marinooa / Interaction of Cell 4 and C e l l 2 with 0 on Ir(111)
At 300 K the CO amount significantly increases with the C 2 H 2 exposures of an oxygen-covered I r ( l l l ) surface, and peaks at 810, 980, 1160 and 1380 cm-~ characteristic of CCH and ethylidyne species are observed in addition to the p(C-O) vibrations in the H R E E L spectrum (fig. 5b). The XP spectra for increasing acetylene exposures are shown in fig. 6a. A new peak at 532 eV characteristic of CO is observed along with a peak at 529.8 eV of oxygen adsorbed dissociatively. The total area of the O(ls) peaks diminishes with higher acetylene exposures which indicates that, in addition to CO, CO 2 and H 2 0 are also products of the oxidation reaction. The reaction between adsorbed oxygen and C2H 2 proceeds similarly at 340 K, but the amount of CO is considerably larger, whereas the amount of oxygen adsorbed in the atomic form drops with increasing C2H 2 exposures (figs. 5c and 6b). At 450 K no oxygen is registered after acetylene exposures of 2 x 10 - 4 Pa s on the oxygen-covered I r ( l l l ) surface. Only products of the hydrocarbon decomposition (CCH species) are found, similarly to the case of a clean surface (figs. 5d and 6c) [12]. Therefore, an intensive reaction of complete oxidation of hydrocarbons to CO 2 and H 2 0 proceeds, the latter desorbing under these conditions.
3.2.2. Interaction of oxygen with a C2H2-exposed Ir(l l l) surface The adsorption of oxygen on an I r ( l l l ) surface partially covered hydrocarbon species after C2H 2 exposure has been investigated. At 180 K oxygen 50
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Energy Loss (crn-~) Fig. 7. H R E E L spectra after 02 exposures of 30 • ]0 -4 Pa s of a hydrocarbon coverage on |r(]11) obtmned after C2H 2 exposures of (a) 0.6 C atoms pet 1 Ir atom at 180 K; (b) 0.4 C atoms per 1 Ir atom at 300 K; (c) 0.4 C atoms per ] Ir atom at 340 K; (d) 0.6 C atoms per I [r atom at 450 K.
K.L. Kostov, Ts.S. Marinova / Interaction of C2H 4 and C2H2 with 0 on Ir(l l l)
367
O(ls) O~ds.
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B inding Energy (eV) Fig. 8. O(ls) peaks after different 02 exposures of a hydrocarbon coverage on I r ( l l l ) obtained after C2H 2 exposures: (a) 0.4 C atoms per 1 Ir atom at 300 K; (b) 0.4 C atoms per 1 lr atom at 340 K; (c) 0.6 C atoms per 1 Ir atom at 450 K.
exposures of a hydrocarbon coverage (0.6 C atoms per 1 Ir atom) lead to the appearance in the H R E E L spectrum of a peak of adsorbed oxygen atoms at 550 cm -1 (fig. 7a). The hydrocarbon species observed are characteristic of a clean surface [12]. No oxidation products are observed. At 300 K, a hydrocarbon coverage of 0.4 C atoms per 1 Ir atom, and subsequent oxygen exposures of 30 • 10 -4 Pa s, the H R E E L spectrum has an intensive CO peak at 2030 cm -1 and peaks of CCH species at 820, - 1280 and 3000 cm ~ (fig. 7b). In the XP spectrum there are peaks of CO and adsorbed oxygen atoms (fig. 8a). Similar results are obtained at 340 K (figs. 7c and 8b). Similarly t o C2H4, at 450 K complete oxidation of the hydrocarbon coverage occurs after the subsequent oxygen exposures of 30 x 10-4 Pa s, and the H R E E L and XP spectra show oxygen adsorbed in the atomic form alone (figs. 7d and 8c).
4. Discussion 4.1. Interaction between C 2 H 4 and oxygen on I r ( l l l )
The adsorption of C 2 H 4 o n an oxygen-covered I r ( l l l ) surface at 180 K proceeds differently from that on a clean surface. The frequencies from fig. l a
368
K . L Kostov, Ts.S. Marinova / Interaction of C2H 4 and C2H 2 with 0 on Ir(l l l)
Table 1 Comparison of the vibration frequencies (in cm -1) from HREELS spectrum of adsorbed C2H 4 on the oxygen-covered surfaces Ir(lll) at 180 K (fig. la), Pd(100) at 80 K [6], Ru(001) at 170 K [9] with those from the IR spectrum of K[Pt(CI)3C2H4] [15] and C2H4 (gas phase) [16] Mode
Ir(111)+O 180 K
CH 2 deformation
1010
C--C stretch CH 2 scissor CH 2 stretch
1220 1480 3010
C2H 4 gas phase 826 949 1023 1623 1444 2989 3106
K[Pt(C1)3C2H4 ]
841 975 1243 1515 3013 3079
Pd(100)+ O 80 K
Ru(001)+O 170 K
940
990
1510 3020
1230 1510 3010
are c o m p a r e d in table 1 with those of the I R spectrum of K[Pt(CI)3C2H4] [15] and the H R E E L spectrum of C / H 4 adsorbed on oxygen-covered surfaces: Pd(100) at 80 K [6], and Ru(001) at 170 K [9], where C z H 4 forms a it-bond with the metals. W e assume that the b r o a d intensive peak at 1010 c m -a consists of the peaks characterizing C H 2 deformation modes. The peaks at 1220 and 1480 cm -1 are ascribed to C---C stretch and C H 2 scissor modes, respectively. The b r o a d peak at 3010 cm -1 characterizes ~'(CH2)s and u ( C H 2 ) ~ modes whose peaks are not resolved in the spectrum. These results show that at 180 K C z H 4 is adsorbed on an oxygen-covered Ir(111) surface as molecules forming a ~r-bond with the surface contrary to the case of a clean surface where ethylene is dehydrogenated to ethylidyne [12]. N o species resulting from the interaction of C2H 4 with the oxygen at 180 K are observed on Ir(111) in addition to the ethylene b o n d e d to the surface by a ~r-bond. The results are similar to those concerning C2H 4 adsorption on oxygencovered P t ( l l l ) [1], Pd(100) [6,7], Ru(001) [9] and F e ( l l l ) [8] surfaces at temperatures below 250 K where C2H 4 is adsorbed in a molecular state forming a ~r-bond with the surface. With increasing temperature up to 300 K, C 2 H 4 exposures of 2 • 10 -4 Pa s of a clean I r ( l l l ) surface lead to ethylene decomposition to C C H and ~ C C H 3 species [12]. Our results indicate that on an oxygen-covered I r ( l l l ) surface the decomposition of C2H 4 is accompanied by oxidation of part of the hydroc a r b o n species. Figs. l b and 2a show that after ethylene exposures of a saturated oxygen coverage on I r ( l l l ) , there are adsorbed oxygen, ethylidyne ( ~ C - C H 3 ) and C O species. The total area of the O(ls) peak decreases with increasing C 2 H 4 exposure (fig. 2a). This m a y be ascribed to the oxidation of ethylene to C O 2 and H / O which are desorbed. The desorption of CO 2 and H 2 0 already at 300 K is established by Hagen et al. [14]. Complete oxidation of C z H 4 to C O 2 and H 2 0 on an oxygen-covered P t ( l l l ) surface is observed by Steininger et al. [1]. A more recent investigation on this adsorption system
K.L. Kostov, Ts.S. Marinova / Interaction of C 2H4 and C 2H 2 with 0 on It(111)
369
has also revealed the presence of a small amount of adsorbed ethylidyne ( ~ C - C H 3 ) [31. After complete oxidation of part of the incident ethylene molecules, C2H 4 is adsorbed on the oxygen-flee I r ( l l l ) surface upon decomposing to CCH and ~-C-CH 3 species as in the case of a clean surface [12]. As is evident from fig. lb, the 6 ( C - H ) vibration characteristic of CCH species (at 825 cm -1) [12] is not observed. Therefore, these species participate in the oxidation reaction forming CO and probably also CO 2 and H20. The reaction might be presented by the following scheme: C2H4(gas ) + O(ads)
+o
) CCH(ads) + CCHa(ads ) + CO2(gas ) + H20(gas )
to
CO2(gas ) + H20(gas ) + CO(ads). After C 2 H 4 exposures of 0.8 • 10 - 4 Pa s, mainly CCH species (a 8 ( C - H ) mode at 810 cm 1) and a small amount of ethylidyne (a 6(CH3) mode at 1400 cm -1 as well as a I,(C-C) mode at 1150 cm 1 [12]) are formed on the clean I r ( l l l ) surface. Subsequent oxygen exposures lead to the appearance of adsorbed oxygen atoms (a u(Ir-O) mode at 520 cm -1) and CO (a p(C-O) mode at 2020 cm-1) (figs. 3b and 4a). Maybe at 300 K the presence of other particles, CO and CCH 3, causes a decrease in mobility of unreacted oxygen atoms and CCH species on the surface. This makes their interaction difficult and explains their simultaneous presence on the surface (figs. 3b and 4a). The conclusions drawn are similar to those concerning the oxidation of C2H 2 and C2H 4 on an oxygen-covered Pt(111) surface [4,5] where the reaction proceeds in the same way but via an intermediate product (CH). On a Pd(100) surface ethylene is oxidized to CO, CO 2 and H 2 0 which are formed during the interaction of oxygen with the products of C2H 4 dehydrogenization [6,7]. The oxygen coverage might hinder the dehydrogenation of C 2 H 4 t o CCH species. In this case, it would proceed till the formation of stable ethylidyne particles. The CCH species may appear only on the oxygen-free part of the Ir(111) surface after the complete C 2 H 4 oxidation. The fact that the dehydrogenation of ethylene to CCH is restricted explains the small increase in CO amount with increasing C z H 4 exposures (fig. 2a). At 340 K the oxygen coverage is consumed in the oxidation of ethylene after C z H 4 exposures of 1 • 10 - 4 Pa s (fig. 2b). At higher ethylene exposures, CCH species with a /~(C-H) mode at 820 cm -1 and ethylidyne species are formed on the surface (fig. lc), similarly to the case of a clean I r ( l l l ) surface [12]. The oxidation proceeds faster at 340 K due to the more pronounced decomposition of ethylene to CCH species at this temperature. Figs. 3c and 4b show that with increasing oxygen exposures of the hydrocarbon coverage at 340 K up to 30 • 10 4 Pa s, no adsorbed oxygen atoms are observed on the surface. This indicates that the oxygen atoms participate in the formation of oxidation products by the above mechanism.
370
K . L Kostov, Ts.S. Marinova / Interaction of C2H 4 and C2H2 with O on lr(l l l)
At 450 K C2H 4 is directly oxidized to CO/ and H 2 0 without formation of CO since figs. l d and 2c show no peaks for CO. After the consumption of the oxygen coverage during the oxidation, C2H 4 is adsorbed on the clean surface as CCH species [12] (fig. ld). Exposure of the hydrocarbon coverage at 450 K to 02 leads to its complete oxidation (figs. 3d and 4c), after which atomically adsorbed oxygen alone is observed on the clean Ir(111) surface. This indicates that at 450 K the CCH species are oxidized to CO 2 and H20. It is established that up to 400 K the incident oxygen molecules do not interact with the saturated hydrocarbon coverage on Ir(111). This is confirmed by the fact that with increasing oxygen exposures no O(ls) peak is seen and the H R E E L spectra do not change. The reaction proceeds at hydrocarbon coverages lower than the saturated coverage and subsequent oxygen exposures permitting adsorption of oxygen in the atomic form. Figs. 1 and 2 show that CxH 4 interacts with an oxygen-presaturated Ir(111) surface. Obviously, the presence of adsorbed oxygen atoms on the Ir(111) surface is an indispensable condition for the proceeding of a reaction between C2H 4 and the oxygen. 4.2. I n t e r a c t i o n b e t w e e n C2H2 a n d o x y g e n on an I r ( l l l )
surface
On a clean I r ( l l l ) surface acetylene is adsorbed at 180 K upon dehydrogenizing to C C H and ~ C C H 3 species [12]. For C C H species on a clean Ir(111) surface it is assumed that at 180 K the C - C axis is parallel to the surface and the 6 ( C - H ) mode is at 780 cm -1. With increasing temperature these species are deformed, the C - C axis is inclined and a new v ( C - C ) peak aplSears at 1280 cm -1, whereas the ~ ( C - H ) mode has a frequency of 820-830 cm -1 (table 2) [12]. Fig. 5a shows a peak at 810 cm -1 (/J(C-H) mode) and peaks at 1000, 1170 and 1410 cm -~ which are characteristic of ethylidyne. The change in frequency of the 8 ( C - H ) vibration shows that in the presence of adsorbed oxygen atoms the C - C axis of the CCH species is probably inclined with respect to the surface already at 180 K. Table 2 Comparison between the vibration frequencies (in cm-1) of adsorbed C2H 2on the oxygen-covered surface Ir(111) at 180 K (this work), and on the clean surface Ir(111) at 180, 300 and 500 K [12] As~gnment
Ir(111)+O 180K
Clean I(111) 180K
Clean Ir(111) 300K
v(CM) B(CH) p(CH3) v(CC) 8(CH3) v(CH3) v(CH)
810 1000 1170 1410 2980
430 780 986 1160 1410 2950
460 820 970 1152 1385 2960
Clean Ir(111) 500K 830 1280 3020
K . L Kostoo, Ts.S. Marinova / Interaction of C2H 4 and C2H2 with 0 on Ir(l l l)
371
On an oxygen-flee I r ( l l l ) surface the 3 ( C - H ) mode peak intensity for C C H species considerably exceeds those of the loss peaks of ethylidyne [12]. However, on an oxygen-covered I r ( l l l ) surface the intensities are commensurable (fig. 5a). Hence, at 180 K the oxygen coverage hinders the dehydrogenation of ethylene and the amount of the C C H species formed is smaller than on a clean surface. The results indicate that, similarly to the case of C2H 4, no acetylene oxidation takes place at 180 K. At 300 K C2H 2 is adsorbed on an oxygen-covered I r ( l l l ) surface as on a clean surface (fig. 5b) [12]. Therefore, contrary to the case of C2H4, the oxygen coverage has no strong effect on C2H 2 adsorption. This result coincides with the conclusion drawn concerning acetylene adsorption on oxygencovered F e ( l l l ) [8] and N i ( l l ) [17] surfaces. The oxygen atoms block only part of the sites for C 2 H 2 adsorption. However, the decrease of the total area of the O(ls) peak with increasing C2H 2 exposures (fig. 6a) and the presence of CO (figs. 5b and 6a) show that the reaction between the acetylene and the oxygen proceeds on I r ( l l l ) by a mechanism similar to that in the case of C 2 H 4. At 340 K the reaction takes place in a similar way (figs. 5c and 6b). The results on the interaction of oxygen with an I r ( l l l ) surface partially covered with hydrocarbons after C2H 2 exposures (figs. 7b, 7c and 8a, 8b) show a similarity with those on C2H 4 and are explained in an analogous way. It should be taken into account that at low exposures acetylene is mainly adsorbed as C C H species on a clean surface [12]. This can also be seen in fig. 7 where these particles are characterized by 3 ( C - H ) modes at 810-820 cm -1, v ( C - C ) modes at 1280-1300 cm -1 and ~,(C-H) modes at - 3010 cm 1. At 450 K, similarly to the case of C a l l 4 again, C2H 2 is completely oxidized to CO 2 and H 2 0 (figs. 5d and 6c). The adsorbed C C H species are also oxidized at this temperature upon 02 exposures forming CO 2 and H 2 0 (figs. 7d and 8c). The above results indicate that together with the oxidation product (CO), there are products of the decomposition of C2H 2 which are also observed on a clean Ir(111) surface (table 2). Hence, at T>__300 K, when a saturated oxygen coverage is exposed to C2H2, the adsorbed oxygen atoms react with the incident acetylene molecules and form CO2 and H20, thus cleaning part of the surface for subsequent CzH 2 adsorption. As a result, the area of the O(ls) peak decreases with increasing C z H 2 exposures. Maybe the hydrocarbon species already adsorbed interact with the nearest oxygen and CO is formed on the Ir surface. The carbon monoxide decreases the mobility of adsorbed species and makes further interaction of adsorbed hydrocarbons with the oxygen atoms difficult. This is confirmed by the simultaneous existence of atomic oxygen and hydrocarbon species on an oxygen covered Ir(111) surface after C2H 2 exposures up to 2 • 10 - 4 Pa s (figs. 6a and 6b). These conclusions also concern the interaction of C z H 4 with an oxygen-covered I r ( l l l ) surface. Similar results are obtained on the reaction of acetylene with an oxygen-
372
K.L Kostov, Ts.S. Marinooa / Interaction of C2H 4 and C2H2 with 0 on lr(111)
covered P t ( l l l ) surface [4]. The oxidation products found are CO, C O 2 and H20. Products of the decomposition of C2H 2 characteristic of a clean surface are also established. In the same paper it is noticed that the hydrocarbon species interact with periphery atoms of the islands of adsorbed oxygen atoms. Oxygen exposures of a saturated hydrocarbon coverage lead to no oxidation up to about 400 K. This is confirmed by the fact that the area of the C(ls) peak in the XP spectrum shows no alteration and the H R E E L spectra demonstrate no changes with higher oxygen exposures. However, similarly to C2H4, the acetylene reacts with a saturated coverage of oxygen atoms. Hence, a necessary condition for the reaction between oxygen and the hydrocarbon species on I r ( l l l ) is the presence of adsorbed oxygen atoms.
5. Conclusion
The results show that on an oxygen-covered I r ( l l l ) surface C2H 4 is adsorbed at 180 K in a molecular form (contrary to the case of a clean surface [12]), as a result of which a 7r-bond with the surface appears. At this temperature the oxygen coverage hinders the dehydrogenation of ethylene. In contrast to ethylene, acetylene is adsorbed on an oxygen-covered I r ( l l l ) surface at 180 K in a similar way as on a clean surface. However, the oxygen coverage makes acetylene dehydrogenation difficult, and at 180 K it forms a smaller amount of C C H species than on a clean I r ( l l l ) surface. It is assumed that the C - C axis of the C C H particles is inclined to the oxygen-covered I r ( l l l ) surface already at 180 K, contrary to the case of a clean surface. At higher temperatures up to 450 K the interaction between C2H 4 or C2H 2 and the oxygen coverage leads to oxidation products (CO,' CO2 and H 2 0 ) and adsorbed species resulting from the decomposition of the two hydrocarbons similarly to the case of a clean I r ( l l l ) surface. A mechanism is proposed according to which on an oxygen-covered I r ( l l l ) surface the two hydrocarbons are oxidized via an- intermediate product ( C C H species). There is some analogy between the results on C2H2 and C2H 4 oxidation over oxygen-covered P t ( l l l ) and I r ( l l l ) surfaces, and on the adsorption of these gases on clean surfaces. At high temperatures acetylene and ethylene are decomposed to C H species on a clean P t ( l l l ) surface [4,18], while the oxidation of the two gases on oxygen-covered P t ( l l l ) is assumed to occur via an intermediate product (CH) [4]. On a clean I r ( l l l ) surface, C2H 2 and C a l l 4 decompose to C C H particles [12] at T>~ 450 K. The same species participate as an intermediate product in the oxidation of both hydrocarbons on an oxygen-covered I r ( l l l ) surface. Therefore, the dehydrogenation products of these hydrocarbons take part in their oxidation on the I r ( l l l ) surface. Kesmodel et al. [19] have suggested the products of C2H 2 and C2H 4 decompositions on P t ( l l l ) to be CCH, and not C H species. Then, the interaction of
K.L. Kostov, Ts.S. Marinova /Interaction of C2H ~ and C2H 2 with 0 on Ir(111)
373
acetylene and ethylene with an oxygen-covered Pt(lll) surface should proceed p r o b a b l y via i n t e r m e d i a t e C C H species as in the case o f I r ( l l l ) . A c o n c l u s i o n is a r r i v e d at t h a t t h e p r e s e n c e o f a d s o r b e d o x y g e n a t o m s o n I r ( l l l ) is a n i n d i s p e n s a b l e c o n d i t i o n f o r t h e p r o c e e d i n g of o x i d a t i o n o f t h e h y d r o c a r b o n species.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
H. Steininger, H. Ibach and S. Lehwald, Surface Sci. 117 (1982) 685. L.L. Kesmodel, L.H. Dubois and G.A. Somorjai, J. Chem. Phys. 70 (1979) 2180. J.R. Creighton and J.M. White, Surface Sci. 129 (1983) 327. C.E. Megiris, P. Berlowitz, J.B. Butt and H.H. Kung, Surface Sci. 159 (1985) 184. P. Berlowitz, C. Megiris, J.B. Butt and H.H. Kung, Langmuir 1 (1985) 206. E.M. Stuve and R.J. Madix, and C.R. Brundle, Surface Sci. 152/153 (1985) 532. E.M. Stuve and R.J. Madix, Surface Sci. 160 (1985) 293. U. Seip, M.-C. Tsai, J. Kiippers and G. Ertl, Surface Sci. 147 (1984) 65. M.A. Barteau, J.Q. Broughton and D. Menzel, Appl. Surface Sci. 19 (1984) 92. Ts.S. Marinova and K.L. Kostov, Reaction Kinet. Catalysis Letters 30 (1986) 253. K.L. Kostov and Ts.S. Marinova, Reaction Kinet. Catalysis Letters, in press. Ts.S. Marinova and K.L. Kostov, Surface Sci. 181 (1987) 573. Ts.S. Marinova and K.L. Kostov, Surface Sci., submitted. D.I. Hagen, B.E. Nieuwenhuys, G. Rovida and G.A. Somorjai, Surface Sci. 57 (1976) 632. J. Hiraishi, Spectrochim. Acta 25A (1969) 749. G. Herzberg, Infrared and Raman Spectra, Vol. 2 (Van Nostrand, New York, 1954). J.E. Demuth, Surface Sci. 69 (1977) 365. A.M. Baro and H. Ibach, J. Chem. Phys. 74 (1981) 4194. L.L. Kesmodel, G.D. Waddill and J.A. Gates, Surface Sci. 138 (1984) 464.
359
INTERACTION OF ETHYLENE AND ACETYLENE WITH OXYGEN O N AN I r ( l l l ) S U R F A C E K.L. K O S T O V a n d Ts.S. M A R I N O V A Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria Received 20 July 1986; accepted for publication 15 January 1987
Investigations have been carried out using HREELS and XPS to study the interaction of C 2 H4 and C2H ~ with oxygen on Ir(lll). In the presence of atomically adsorbed oxygen, ethylene is adsorbed on Ir(lll) at 180 K molecularly and is bonded to the surface by a ~r-bond. At this temperature acetylene is adsorbed on an oxygen-covered Ir(lll) surface in a similar way as on a clean surface. The oxygen coverage hinders the dehydrogenation of ethylene to a higher degree than in the case of acetylene. At 300 < T ~ 400 K, CzH 2 and C2H 4 react with adsorbed oxygen atoms forming CO, CO2 and H20. Decomposition products of C2H 2 and C2H 4, which are observed on a clean surface, are also found on the oxygen-coverediridium surface. An oxidation mechanism of CzH 2 and C2H 4 via an intermediate product, CCH, is proposed. Above 450 K both hydrocarbons are completely oxidized to CO2 and H20. Atomically adsorbed oxygen is needed for the oxidation of the hydrocarbon species on Ir(lll).
1. Introduction The a d s o r p t i o n of C2H 2 a n d C2H 4 a n d their interaction with oxygen o n clean metal surfaces is of interest for u n d e r s t a n d i n g the m e c h a n i s m of catalytic oxidation of more complex h y d r o c a r b o n molecules. The surfaces of group V I I I metals are of special interest in this respect. The investigations have shown that C2H 2 a n d C2H 4 interact differently with clean a n d with oxygen-covered surfaces of these metals. F o r instance, o n a clean Pt(111) surface at temperatures below 220 K the ethylene forms a di-o b o n d with the surface. I n the presence of atomically adsorbed oxygen, ethylene molecules f o r m i n g a g - b o n d with the surface are observed [1] in a d d i t i o n to the di-o b o n d e d CEH 4 molecules. A t 300 K ethylene is adsorbed on a clean surface as stable ~ C C H 3 species [1,2], whereas o n an oxygen-covered Pt(111) surface C 2 H 4 is completely converted to C O 2 a n d H 2 0 [1]. A more recent s t u d y [3] b y the SIMS m e t h o d has shown the presence of a small a m o u n t of adsorbed ~ C C H 3 species even with a saturated oxygen coverage on Pt(111). Megiris et al. [4] have used the thermal desorption method for the study of the interaction between C2H 2 a n d an oxygen-covered Pt(111) surface. They have observed oxidation products (CO, CO2 a n d H : O ) in a d d i t i o n to the 0 0 3 9 - 6 0 2 8 / 8 7 / $ 0 3 . 5 0 9 Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
360
K . L Kostov, Ts.S. Marinova / Interaction of C 2 H ~ and C2H e with 0 on lr(111)
products of acetylene decomposition characteristic of a clean surface. The authors have assumed the first step of C2H 2 oxidation to be the formation of an intermediate product (CH) as in the case of C2H 4 oxidation [5]. Contrary to the case of a clean surface, the C2H 4 dehydrogenation on a Pd(100) surface with preliminary adsorbed oxygen atoms is hindered [6,7]. The same conclusion has also been drawn concerning a F e ( l l l ) surface where the presence of adsorbed oxygen atoms leads to a new molecular adsorption state of C 2 H 4 but does not substantially affect the adsorption of C : H 2 [8]. The oxygen coverage on Ru(001) also hinders the dehydrogenation of ethylene which is adsorbed as molecules alone forming a ~r-bond with the surface. At higher temperatures it is desorbed without reacting with oxygen [9]. In previous studies by the XPS, UPS and AES methods we have paid attention to the reaction of oxygen with C2H2 [10] and C 2 H 4 [11] on a polycrystaUine Ir surface. The present paper deals with the interaction of C2H 2 and C2H4 with oxygen on an I r ( l l l ) surface using the H R E E L S and XPS methods. The same methods were applied to the adsorption of C2H 2 and C2H 4 [12] and O2113 ] on a clean I r ( l l l ) surface. At 180 K C2H 4 is dehydrogenated to ethylidyne ( ~ C C H 3 ) while at 300 K C C H species are additionally formed. At 180 K C2H 2 is adsorbed as C C H and ~ C C H 3 species. After C2H 2 or C2H4 adsorption at 500 K there are mainly C C H species on the Ir surface.
2. Methods The investigations were carried out with an Escalab II ( V G S c i e n t i f i c Ltd.) electron spectrometer at a residual gas pressure of 1 • 1'0 -8 Pa. The XPS measurements were made using a M g K excitation source (hv = 1253.6 eV), the total instrumental resolution being - 1 eV as measured with the F W H M of the Ag 3d5/2 photoelectron peak. The corresponding resolution of the H R E E L S was 10-12 meV (80-96 cm -1) as the measurements were taken in the direction of specular scattering (45 o with respect to the surface normal) with a kinetic energy of electrons of 3 eV.
Acetylene and ethylene of 99.95% purity (Messer Griesheim) and oxygen of 99.95% purity (Merck) were used for the experiments. The Ir single crystal (purity, 99.998%, Metal Crystals) had a thickness of 1 ram, a diameter of 7 m m and was oriented and cut with an accurary of about 1 ~ in the (111) direction. A clean I r ( l l l ) surface was obtained after heating the sample at 1100 K under an oxygen pressure of 1 • 10 -5 Pa followed by heating up to 1900 K in vacuo of - 10 - s Pa. The experimental technique and the cleaning procedure of the sample have been described in detail in previous papers [12,13].
K.L. Kostov, Ts.S. Marinova / Interaction of C2H4 and C2H2 with 0 on Ir(lll)
361
3. Results 3.1. Interaction between C 2 H 4 and oxygen on I r ( l l l )
The experiments were carried out on a clean Ir(111) surface pre-covered with one of the reagents at a given temperature. After that, the coverage was exposed to the other reagent at the same temperature. The surface species resulting from the proceeding interaction were investigated by the XPS and H R E E L S methods. 3.1.1. Interaction o f C2H 4 with an oxygen-covered Ir(l l l ) surface
Fig. l a shows the H R E E L spectrum of a saturated oxygen coverage on I r ( l l l ) followed by a C z H 4 exposure of 2 • 10 -4 Pa s. In addition to an intense peak at 550 cm -1 characteristic of t,(Ir-O) vibrations of oxygen adsorbed in an atomic form, peaks at 1010, 1220, 1480 and 3010 cm - I are also discernible. The spectrum differs from that after ethylene adsorption at 180 K on a clean I r ( l l l ) surface where only ethylidyne species producing loss peaks at 457, 986, 1165, 1400 and 2940 cm -1 are formed [12]. In fig. la, a weak CO peak is observed at 2000 c m - a which is due to the adsorption of the residual gases in the spectrometer. The scanning time of the H R E E L spectrum is about 600 s, and hence, at a pressure of about 10 -8 Pa, the CO concentration is below 1% of the saturated coverage [14]. The CO observed is not a product of C2H4 oxidation since the same CO amount is formed on a clean I r ( l l l ) surface during scanning at 180 K. At 300 K the H R E E L spectrum differs from that at 180 K (fig. lb). After C 2 H 4 exposures of 2 • 10 4 Pa s, products of the decomposition of ethylene to ethylidyne (with characteristic peaks at 970, 1160, 1390 and 2970 c m - 1 [12]) and a product of the oxidation (CO, with a p ( I r - C ) peak at 500 cm -1 and a ~,(C-O) peak at 2020 cm -1) are observed on an oxygen-covered I r ( l l l ) surface. In addition to CO, the XP spectra show, depending on the C2H 4 exposures of the oxygen-covered surface (fig. 2a), the presence of oxygen adsorbed dissociatively with a binding energy of the O(ls) line at 529.8 eV. The total area of the O(ls) peak decreases with increasing C 2 H 4 exposures, which probably indicates oxidation of the hydrocarbons to CO 2 and H~O which are desorbed from the surface. At 340 K, after C2H 4 exposures of 2 • 10 _4 Pa s, the H R E E L spectrum of an oxygen-covered I r ( l l l ) surface (fig. lc) shows, in addition to the ethylidyne peaks, another peak at 810 cm -1 which is interpreted in ref. [12] as a ~ ( C - H ) m o d e of C C H species. At this temperature there is again interaction between the hydrocarbons and the adsorbed oxygen atoms, which is evidenced by the presence of an intense CO peak at 2020 cm-1. The XP spectra (fig. 2b) show that after C2H 4 exposures of 1 • 10 -4 Pa s, no atomically adsorbed oxygen is present on the surface. There is only CO producing an O(ls) peak with a
362
K . L Kostoo, Ts.S. Marinooa / Interaction of C2H4 and C2H2 with 0 on Ir(111)
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Fig. 1. HREEL spectra of an oxygen-covered I r ( l l l ) surface after 02 exposures of 3 0 x 10 -4 Pa s followed by C2H4 exposures of 2 • 10 -4 Pa s at (a) 180 K; (b) 300 K; (c) 340 K; (d) 450 K.
binding energy of 532 eV. The area of the O(ls) peak of the initially adsorbed oxygen is larger than the area of the O(ls) peak after C2H 4 exposures of 1 • 10 -4 Pa s. This is probably an indication that the oxygen atoms participate not only in the formation of CO but also in the oxidation of the
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K . L Kostot), Ts.S. Marinova / Interaction o f C 2 n 4 and C 2 H 2 with 0 on Ir(l 11)
363
hydrocarbons to CO 2 and H 2 0 which are then desorbed from the surface. An analogous interaction is observed between C 2 H 4 and an oxygen-covered Ir(111) surface at 400 K. At T>~450 K, C 2 H 4 exposures of an oxygen-covered I r ( l l l ) surface probably lead to the complete oxidation of ethylene to CO 2 and H20. This is evidenced by the O(ls) peak of adsorbed oxygen (fig. 2c). With increasing ethylene exposure, the area of this peak diminishes and at 1 x 10 - 4 Pa s the peak disappears. Under these conditions the H R E E L spectra have peaks at 830, 1300 and 3020 cm -1 only (fig. ld) which are interpreted in ref. [12] as modes on C C H species.
3.1.2. Interaction of oxygen with a CeH4-exposed I r ( l l l ) surface A clean I,r(lll) surface was exposed to C z H 4 and the adsorption of oxygen in the presence of the hydrocarbon coverage was studied. At 180 K ethylene is adsorbed on a clean I r ( l l l ) surface as ethylidyne species. With hydrocarbon coverages lower than the saturated one the oxygen is adsorbed at 180 K in the form of atoms to which a vibrational frequency at 550 cm i corresponds. The oxygen does not interact with adsorbed hydrocarbon species: the H R E E L spectrum does not change with increasing oxygen exposure (fig. 3a). At 300 K C z H 4 exposures of 0.8 x 10 - 4 Pa s lead to the formation on I r ( l l l ) of a C C H coverage which corresponds to 0.4 C atoms per one Ir atom (in a saturated coverage there is one C atom per one Ir atom [12]). Subsequent oxygen
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364
K . L Kostov, Ts.S. Marinova / Interaction of C2 H4 and C 2 H 2 with 0 on Ir(111)
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Fig. 4. O(ls) peaks after different oxygen exposures of a hydrocarbon coverage on Ir(lll) obtained upon C2H4 exposures of (a) 0.8 • 10-4 Pa s at 300 K; (b) 0.8 • 10-4 Pa s at 340 K; (c) 1.5 x 10-4 Pa s at 450 K. exposures of 30 x 1 0 - 4 Pa s lead to a reaction associated with the formation of CO ( p ( C - O ) at 2020 cm -1) (fig. 3b). This is also observed with the C(ls) and O(ls) peaks. The form of the C(ls) peak negligibly changes due to the formation of a small amount of CO. A small peak at 532 eV characteristic of CO appears near the peak of adsorbed oxygen (fig. 4a). At T = 340 K the reaction is more intense and leads to the formation of CO; p(Ir-C) and ~,(C-O) being at 490 and 2020 cm -I, respectively (figs. 3c and 4b). Probably CO 2 and H 2 0 are also formed because the area of the C(ls) peak decreases with increasing oxygen exposure. At 450 K increasing oxygen exposures up to 30 x 10 -4 Pa s of the hydrocarbon coverage (0.6 atoms per 1 Ir atom) lead to the disappearance in the H R E E L and XP spectra of the peaks for the hydrocarbon species. Instead of this, a characteristic peak of oxygen adsorbed in the atomic form is visible at 550 cm -1 (fig. 3d) as well as an O(ls) peak at 529.8 eV (fig. 4c).
3.2. Interaction between acetylene and oxygen on lr(111) 3. 2.1. Interaction of C21-12 with an oxygen -covered Ir(111) surface At 180 K, C2H ~ exposures of 2 x 10 -4 Pa s lead to the appearance, in the H R E E L spectrum of a saturated oxygen-covered I r ( l l l ) surface, of the same electron loss peaks (fig. 5a) which are observed during C2H 2 adsorption on a clean I r ( l l l ) surface [12]. They are characteristic of C C H species (the 8 ( C - H ) mode at 810 cm - ] ) and ethylidyne (the modes at 1000, 1170 and 1410 cm -1 [12]). Along with the vibrations characteristic of the hydrocarbon species, a
K.L. Kostov, Ts.S. Marinova / Interaction of C e l l 4 and C2H 2 with 0 on Ir(11l)
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peak of p ( I r - O ) vibrations of atomically adsorbed oxygen is observed at 550 cm -1 as well as a smaller p ( C - O ) peak indicating a negligible amount of CO. The latter does not essentially change with increasing acetylene exposure which means that it is not a product of the interaction of C2H2 with adsorbed oxygen.
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Fig. 6. O(ls) peaks of an oxygen-covered l r ( l l l ) surface after an 02 exposure of 3 0 x l0 4 Pa s followed by different C2H 2 exposures at (a) 300 K; (b) 340 K; (c) 450 K.
366
K . L Kostov, Ts.S. Marinooa / Interaction of Cell 4 and C e l l 2 with 0 on Ir(111)
At 300 K the CO amount significantly increases with the C 2 H 2 exposures of an oxygen-covered I r ( l l l ) surface, and peaks at 810, 980, 1160 and 1380 cm-~ characteristic of CCH and ethylidyne species are observed in addition to the p(C-O) vibrations in the H R E E L spectrum (fig. 5b). The XP spectra for increasing acetylene exposures are shown in fig. 6a. A new peak at 532 eV characteristic of CO is observed along with a peak at 529.8 eV of oxygen adsorbed dissociatively. The total area of the O(ls) peaks diminishes with higher acetylene exposures which indicates that, in addition to CO, CO 2 and H 2 0 are also products of the oxidation reaction. The reaction between adsorbed oxygen and C2H 2 proceeds similarly at 340 K, but the amount of CO is considerably larger, whereas the amount of oxygen adsorbed in the atomic form drops with increasing C2H 2 exposures (figs. 5c and 6b). At 450 K no oxygen is registered after acetylene exposures of 2 x 10 - 4 Pa s on the oxygen-covered I r ( l l l ) surface. Only products of the hydrocarbon decomposition (CCH species) are found, similarly to the case of a clean surface (figs. 5d and 6c) [12]. Therefore, an intensive reaction of complete oxidation of hydrocarbons to CO 2 and H 2 0 proceeds, the latter desorbing under these conditions.
3.2.2. Interaction of oxygen with a C2H2-exposed Ir(l l l) surface The adsorption of oxygen on an I r ( l l l ) surface partially covered hydrocarbon species after C2H 2 exposure has been investigated. At 180 K oxygen 50
550
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200 9
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Energy Loss (crn-~) Fig. 7. H R E E L spectra after 02 exposures of 30 • ]0 -4 Pa s of a hydrocarbon coverage on |r(]11) obtmned after C2H 2 exposures of (a) 0.6 C atoms pet 1 Ir atom at 180 K; (b) 0.4 C atoms per 1 Ir atom at 300 K; (c) 0.4 C atoms per ] Ir atom at 340 K; (d) 0.6 C atoms per I [r atom at 450 K.
K.L. Kostov, Ts.S. Marinova / Interaction of C2H 4 and C2H2 with 0 on Ir(l l l)
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O(ls) O~ds.
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B inding Energy (eV) Fig. 8. O(ls) peaks after different 02 exposures of a hydrocarbon coverage on I r ( l l l ) obtained after C2H 2 exposures: (a) 0.4 C atoms per 1 Ir atom at 300 K; (b) 0.4 C atoms per 1 lr atom at 340 K; (c) 0.6 C atoms per 1 Ir atom at 450 K.
exposures of a hydrocarbon coverage (0.6 C atoms per 1 Ir atom) lead to the appearance in the H R E E L spectrum of a peak of adsorbed oxygen atoms at 550 cm -1 (fig. 7a). The hydrocarbon species observed are characteristic of a clean surface [12]. No oxidation products are observed. At 300 K, a hydrocarbon coverage of 0.4 C atoms per 1 Ir atom, and subsequent oxygen exposures of 30 • 10 -4 Pa s, the H R E E L spectrum has an intensive CO peak at 2030 cm -1 and peaks of CCH species at 820, - 1280 and 3000 cm ~ (fig. 7b). In the XP spectrum there are peaks of CO and adsorbed oxygen atoms (fig. 8a). Similar results are obtained at 340 K (figs. 7c and 8b). Similarly t o C2H4, at 450 K complete oxidation of the hydrocarbon coverage occurs after the subsequent oxygen exposures of 30 x 10-4 Pa s, and the H R E E L and XP spectra show oxygen adsorbed in the atomic form alone (figs. 7d and 8c).
4. Discussion 4.1. Interaction between C 2 H 4 and oxygen on I r ( l l l )
The adsorption of C 2 H 4 o n an oxygen-covered I r ( l l l ) surface at 180 K proceeds differently from that on a clean surface. The frequencies from fig. l a
368
K . L Kostov, Ts.S. Marinova / Interaction of C2H 4 and C2H 2 with 0 on Ir(l l l)
Table 1 Comparison of the vibration frequencies (in cm -1) from HREELS spectrum of adsorbed C2H 4 on the oxygen-covered surfaces Ir(lll) at 180 K (fig. la), Pd(100) at 80 K [6], Ru(001) at 170 K [9] with those from the IR spectrum of K[Pt(CI)3C2H4] [15] and C2H4 (gas phase) [16] Mode
Ir(111)+O 180 K
CH 2 deformation
1010
C--C stretch CH 2 scissor CH 2 stretch
1220 1480 3010
C2H 4 gas phase 826 949 1023 1623 1444 2989 3106
K[Pt(C1)3C2H4 ]
841 975 1243 1515 3013 3079
Pd(100)+ O 80 K
Ru(001)+O 170 K
940
990
1510 3020
1230 1510 3010
are c o m p a r e d in table 1 with those of the I R spectrum of K[Pt(CI)3C2H4] [15] and the H R E E L spectrum of C / H 4 adsorbed on oxygen-covered surfaces: Pd(100) at 80 K [6], and Ru(001) at 170 K [9], where C z H 4 forms a it-bond with the metals. W e assume that the b r o a d intensive peak at 1010 c m -a consists of the peaks characterizing C H 2 deformation modes. The peaks at 1220 and 1480 cm -1 are ascribed to C---C stretch and C H 2 scissor modes, respectively. The b r o a d peak at 3010 cm -1 characterizes ~'(CH2)s and u ( C H 2 ) ~ modes whose peaks are not resolved in the spectrum. These results show that at 180 K C z H 4 is adsorbed on an oxygen-covered Ir(111) surface as molecules forming a ~r-bond with the surface contrary to the case of a clean surface where ethylene is dehydrogenated to ethylidyne [12]. N o species resulting from the interaction of C2H 4 with the oxygen at 180 K are observed on Ir(111) in addition to the ethylene b o n d e d to the surface by a ~r-bond. The results are similar to those concerning C2H 4 adsorption on oxygencovered P t ( l l l ) [1], Pd(100) [6,7], Ru(001) [9] and F e ( l l l ) [8] surfaces at temperatures below 250 K where C2H 4 is adsorbed in a molecular state forming a ~r-bond with the surface. With increasing temperature up to 300 K, C 2 H 4 exposures of 2 • 10 -4 Pa s of a clean I r ( l l l ) surface lead to ethylene decomposition to C C H and ~ C C H 3 species [12]. Our results indicate that on an oxygen-covered I r ( l l l ) surface the decomposition of C2H 4 is accompanied by oxidation of part of the hydroc a r b o n species. Figs. l b and 2a show that after ethylene exposures of a saturated oxygen coverage on I r ( l l l ) , there are adsorbed oxygen, ethylidyne ( ~ C - C H 3 ) and C O species. The total area of the O(ls) peak decreases with increasing C 2 H 4 exposure (fig. 2a). This m a y be ascribed to the oxidation of ethylene to C O 2 and H / O which are desorbed. The desorption of CO 2 and H 2 0 already at 300 K is established by Hagen et al. [14]. Complete oxidation of C z H 4 to C O 2 and H 2 0 on an oxygen-covered P t ( l l l ) surface is observed by Steininger et al. [1]. A more recent investigation on this adsorption system
K.L. Kostov, Ts.S. Marinova / Interaction of C 2H4 and C 2H 2 with 0 on It(111)
369
has also revealed the presence of a small amount of adsorbed ethylidyne ( ~ C - C H 3 ) [31. After complete oxidation of part of the incident ethylene molecules, C2H 4 is adsorbed on the oxygen-flee I r ( l l l ) surface upon decomposing to CCH and ~-C-CH 3 species as in the case of a clean surface [12]. As is evident from fig. lb, the 6 ( C - H ) vibration characteristic of CCH species (at 825 cm -1) [12] is not observed. Therefore, these species participate in the oxidation reaction forming CO and probably also CO 2 and H20. The reaction might be presented by the following scheme: C2H4(gas ) + O(ads)
+o
) CCH(ads) + CCHa(ads ) + CO2(gas ) + H20(gas )
to
CO2(gas ) + H20(gas ) + CO(ads). After C 2 H 4 exposures of 0.8 • 10 - 4 Pa s, mainly CCH species (a 8 ( C - H ) mode at 810 cm 1) and a small amount of ethylidyne (a 6(CH3) mode at 1400 cm -1 as well as a I,(C-C) mode at 1150 cm 1 [12]) are formed on the clean I r ( l l l ) surface. Subsequent oxygen exposures lead to the appearance of adsorbed oxygen atoms (a u(Ir-O) mode at 520 cm -1) and CO (a p(C-O) mode at 2020 cm-1) (figs. 3b and 4a). Maybe at 300 K the presence of other particles, CO and CCH 3, causes a decrease in mobility of unreacted oxygen atoms and CCH species on the surface. This makes their interaction difficult and explains their simultaneous presence on the surface (figs. 3b and 4a). The conclusions drawn are similar to those concerning the oxidation of C2H 2 and C2H 4 on an oxygen-covered Pt(111) surface [4,5] where the reaction proceeds in the same way but via an intermediate product (CH). On a Pd(100) surface ethylene is oxidized to CO, CO 2 and H 2 0 which are formed during the interaction of oxygen with the products of C2H 4 dehydrogenization [6,7]. The oxygen coverage might hinder the dehydrogenation of C 2 H 4 t o CCH species. In this case, it would proceed till the formation of stable ethylidyne particles. The CCH species may appear only on the oxygen-free part of the Ir(111) surface after the complete C 2 H 4 oxidation. The fact that the dehydrogenation of ethylene to CCH is restricted explains the small increase in CO amount with increasing C z H 4 exposures (fig. 2a). At 340 K the oxygen coverage is consumed in the oxidation of ethylene after C z H 4 exposures of 1 • 10 - 4 Pa s (fig. 2b). At higher ethylene exposures, CCH species with a /~(C-H) mode at 820 cm -1 and ethylidyne species are formed on the surface (fig. lc), similarly to the case of a clean I r ( l l l ) surface [12]. The oxidation proceeds faster at 340 K due to the more pronounced decomposition of ethylene to CCH species at this temperature. Figs. 3c and 4b show that with increasing oxygen exposures of the hydrocarbon coverage at 340 K up to 30 • 10 4 Pa s, no adsorbed oxygen atoms are observed on the surface. This indicates that the oxygen atoms participate in the formation of oxidation products by the above mechanism.
370
K . L Kostov, Ts.S. Marinova / Interaction of C2H 4 and C2H2 with O on lr(l l l)
At 450 K C2H 4 is directly oxidized to CO/ and H 2 0 without formation of CO since figs. l d and 2c show no peaks for CO. After the consumption of the oxygen coverage during the oxidation, C2H 4 is adsorbed on the clean surface as CCH species [12] (fig. ld). Exposure of the hydrocarbon coverage at 450 K to 02 leads to its complete oxidation (figs. 3d and 4c), after which atomically adsorbed oxygen alone is observed on the clean Ir(111) surface. This indicates that at 450 K the CCH species are oxidized to CO 2 and H20. It is established that up to 400 K the incident oxygen molecules do not interact with the saturated hydrocarbon coverage on Ir(111). This is confirmed by the fact that with increasing oxygen exposures no O(ls) peak is seen and the H R E E L spectra do not change. The reaction proceeds at hydrocarbon coverages lower than the saturated coverage and subsequent oxygen exposures permitting adsorption of oxygen in the atomic form. Figs. 1 and 2 show that CxH 4 interacts with an oxygen-presaturated Ir(111) surface. Obviously, the presence of adsorbed oxygen atoms on the Ir(111) surface is an indispensable condition for the proceeding of a reaction between C2H 4 and the oxygen. 4.2. I n t e r a c t i o n b e t w e e n C2H2 a n d o x y g e n on an I r ( l l l )
surface
On a clean I r ( l l l ) surface acetylene is adsorbed at 180 K upon dehydrogenizing to C C H and ~ C C H 3 species [12]. For C C H species on a clean Ir(111) surface it is assumed that at 180 K the C - C axis is parallel to the surface and the 6 ( C - H ) mode is at 780 cm -1. With increasing temperature these species are deformed, the C - C axis is inclined and a new v ( C - C ) peak aplSears at 1280 cm -1, whereas the ~ ( C - H ) mode has a frequency of 820-830 cm -1 (table 2) [12]. Fig. 5a shows a peak at 810 cm -1 (/J(C-H) mode) and peaks at 1000, 1170 and 1410 cm -~ which are characteristic of ethylidyne. The change in frequency of the 8 ( C - H ) vibration shows that in the presence of adsorbed oxygen atoms the C - C axis of the CCH species is probably inclined with respect to the surface already at 180 K. Table 2 Comparison between the vibration frequencies (in cm-1) of adsorbed C2H 2on the oxygen-covered surface Ir(111) at 180 K (this work), and on the clean surface Ir(111) at 180, 300 and 500 K [12] As~gnment
Ir(111)+O 180K
Clean I(111) 180K
Clean Ir(111) 300K
v(CM) B(CH) p(CH3) v(CC) 8(CH3) v(CH3) v(CH)
810 1000 1170 1410 2980
430 780 986 1160 1410 2950
460 820 970 1152 1385 2960
Clean Ir(111) 500K 830 1280 3020
K . L Kostoo, Ts.S. Marinova / Interaction of C2H 4 and C2H2 with 0 on Ir(l l l)
371
On an oxygen-flee I r ( l l l ) surface the 3 ( C - H ) mode peak intensity for C C H species considerably exceeds those of the loss peaks of ethylidyne [12]. However, on an oxygen-covered I r ( l l l ) surface the intensities are commensurable (fig. 5a). Hence, at 180 K the oxygen coverage hinders the dehydrogenation of ethylene and the amount of the C C H species formed is smaller than on a clean surface. The results indicate that, similarly to the case of C2H 4, no acetylene oxidation takes place at 180 K. At 300 K C2H 2 is adsorbed on an oxygen-covered I r ( l l l ) surface as on a clean surface (fig. 5b) [12]. Therefore, contrary to the case of C2H4, the oxygen coverage has no strong effect on C2H 2 adsorption. This result coincides with the conclusion drawn concerning acetylene adsorption on oxygencovered F e ( l l l ) [8] and N i ( l l ) [17] surfaces. The oxygen atoms block only part of the sites for C 2 H 2 adsorption. However, the decrease of the total area of the O(ls) peak with increasing C2H 2 exposures (fig. 6a) and the presence of CO (figs. 5b and 6a) show that the reaction between the acetylene and the oxygen proceeds on I r ( l l l ) by a mechanism similar to that in the case of C 2 H 4. At 340 K the reaction takes place in a similar way (figs. 5c and 6b). The results on the interaction of oxygen with an I r ( l l l ) surface partially covered with hydrocarbons after C2H 2 exposures (figs. 7b, 7c and 8a, 8b) show a similarity with those on C2H 4 and are explained in an analogous way. It should be taken into account that at low exposures acetylene is mainly adsorbed as C C H species on a clean surface [12]. This can also be seen in fig. 7 where these particles are characterized by 3 ( C - H ) modes at 810-820 cm -1, v ( C - C ) modes at 1280-1300 cm -1 and ~,(C-H) modes at - 3010 cm 1. At 450 K, similarly to the case of C a l l 4 again, C2H 2 is completely oxidized to CO 2 and H 2 0 (figs. 5d and 6c). The adsorbed C C H species are also oxidized at this temperature upon 02 exposures forming CO 2 and H 2 0 (figs. 7d and 8c). The above results indicate that together with the oxidation product (CO), there are products of the decomposition of C2H 2 which are also observed on a clean Ir(111) surface (table 2). Hence, at T>__300 K, when a saturated oxygen coverage is exposed to C2H2, the adsorbed oxygen atoms react with the incident acetylene molecules and form CO2 and H20, thus cleaning part of the surface for subsequent CzH 2 adsorption. As a result, the area of the O(ls) peak decreases with increasing C z H 2 exposures. Maybe the hydrocarbon species already adsorbed interact with the nearest oxygen and CO is formed on the Ir surface. The carbon monoxide decreases the mobility of adsorbed species and makes further interaction of adsorbed hydrocarbons with the oxygen atoms difficult. This is confirmed by the simultaneous existence of atomic oxygen and hydrocarbon species on an oxygen covered Ir(111) surface after C2H 2 exposures up to 2 • 10 - 4 Pa s (figs. 6a and 6b). These conclusions also concern the interaction of C z H 4 with an oxygen-covered I r ( l l l ) surface. Similar results are obtained on the reaction of acetylene with an oxygen-
372
K.L Kostov, Ts.S. Marinooa / Interaction of C2H 4 and C2H2 with 0 on lr(111)
covered P t ( l l l ) surface [4]. The oxidation products found are CO, C O 2 and H20. Products of the decomposition of C2H 2 characteristic of a clean surface are also established. In the same paper it is noticed that the hydrocarbon species interact with periphery atoms of the islands of adsorbed oxygen atoms. Oxygen exposures of a saturated hydrocarbon coverage lead to no oxidation up to about 400 K. This is confirmed by the fact that the area of the C(ls) peak in the XP spectrum shows no alteration and the H R E E L spectra demonstrate no changes with higher oxygen exposures. However, similarly to C2H4, the acetylene reacts with a saturated coverage of oxygen atoms. Hence, a necessary condition for the reaction between oxygen and the hydrocarbon species on I r ( l l l ) is the presence of adsorbed oxygen atoms.
5. Conclusion
The results show that on an oxygen-covered I r ( l l l ) surface C2H 4 is adsorbed at 180 K in a molecular form (contrary to the case of a clean surface [12]), as a result of which a 7r-bond with the surface appears. At this temperature the oxygen coverage hinders the dehydrogenation of ethylene. In contrast to ethylene, acetylene is adsorbed on an oxygen-covered I r ( l l l ) surface at 180 K in a similar way as on a clean surface. However, the oxygen coverage makes acetylene dehydrogenation difficult, and at 180 K it forms a smaller amount of C C H species than on a clean I r ( l l l ) surface. It is assumed that the C - C axis of the C C H particles is inclined to the oxygen-covered I r ( l l l ) surface already at 180 K, contrary to the case of a clean surface. At higher temperatures up to 450 K the interaction between C2H 4 or C2H 2 and the oxygen coverage leads to oxidation products (CO,' CO2 and H 2 0 ) and adsorbed species resulting from the decomposition of the two hydrocarbons similarly to the case of a clean I r ( l l l ) surface. A mechanism is proposed according to which on an oxygen-covered I r ( l l l ) surface the two hydrocarbons are oxidized via an- intermediate product ( C C H species). There is some analogy between the results on C2H2 and C2H 4 oxidation over oxygen-covered P t ( l l l ) and I r ( l l l ) surfaces, and on the adsorption of these gases on clean surfaces. At high temperatures acetylene and ethylene are decomposed to C H species on a clean P t ( l l l ) surface [4,18], while the oxidation of the two gases on oxygen-covered P t ( l l l ) is assumed to occur via an intermediate product (CH) [4]. On a clean I r ( l l l ) surface, C2H 2 and C a l l 4 decompose to C C H particles [12] at T>~ 450 K. The same species participate as an intermediate product in the oxidation of both hydrocarbons on an oxygen-covered I r ( l l l ) surface. Therefore, the dehydrogenation products of these hydrocarbons take part in their oxidation on the I r ( l l l ) surface. Kesmodel et al. [19] have suggested the products of C2H 2 and C2H 4 decompositions on P t ( l l l ) to be CCH, and not C H species. Then, the interaction of
K.L. Kostov, Ts.S. Marinova /Interaction of C2H ~ and C2H 2 with 0 on Ir(111)
373
acetylene and ethylene with an oxygen-covered Pt(lll) surface should proceed p r o b a b l y via i n t e r m e d i a t e C C H species as in the case o f I r ( l l l ) . A c o n c l u s i o n is a r r i v e d at t h a t t h e p r e s e n c e o f a d s o r b e d o x y g e n a t o m s o n I r ( l l l ) is a n i n d i s p e n s a b l e c o n d i t i o n f o r t h e p r o c e e d i n g of o x i d a t i o n o f t h e h y d r o c a r b o n species.
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