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The reactivity of the graphite overlayer on Fe(110) towards oxygen
Surface Science 194 (1988) 559-566 North-Holland, Amsterdam
559
THE REACTIVITY OF THE GRAPHITE OVERLAYER ON Fe(ll0) TOWARDS OXYGEN T.J. VINK, M. BOLECH, O.L.J. GIJZEMAN and J.W. GEUS Van 't Hoff Laborator); University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands Received 7 July 1987; accepted for publication 25 September 1987
The gasification by oxygen of a carbon adlayer on Fe(ll0) was studied by AES and LEED. The carbon layer was obtained by thermal decomposition of ethylene, and LEED measurements revealed that graphite islands were formed, in agreement with former studies. It appeared that at temperatures below 573 K only part of the carbon layer could be removed by oxygen. This phenomenon is explained by the fact that certain parts of the graphite overlayer matrix are better matched to the substrate than other parts, which leads to a lower reactivity towards oxygen.
1. Introduction Iron is a particularly reactive metal for CO dissociation [1-4] and as a consequence of this high reactivity a process of excess carbon deposition during CO hydrogenation is often observed [5,6]. An investigation by Krebs et al. [6] showed evidence for a correlation between the chemical form of ttds deposited carbon and catalytic activky. The same authors [7] characterized these various forms of carbon deposits, using AES and XPS. The present paper is aimed at elucidath~g further the reactivity of different chemical forms of carbon on iron towards oxygen. Recently we k~ave studied the gasification of a carbide layer on Fe(100) by oxygen [8]. This carbide layer consists of carbon atoms which are situated in a c(2 x 2) superstructure. It was found that during the reaction the carbon adatoms are replaced by oxygen atoms exclusively, while CO desorbs. The cm'bon adlayer on Fe~llO) shows quite different characteristics, since Grabke et al. [9] found that c:wbon extended study by Bezuidenhout et al. [10] revealed tha' graphke ~a'ye; ~s formed uninfluenced by the geometry of the Fe(110) sub°. ,,e around nucl~eation centres and results in the formation of islands of nearly random orientation. This variety in cherrfical nature of carbo~, on both h-on single crystal surfaces prompted our present study, where it will be shown that flaese surfaces differ considerably in their reactivity towards oxygen. 0039-6028/88/$03.50 © Elsevier Science Publishers KV. ~No~h-~~oh,~r~ Physics Pub!ishing DbAsio~)
560
T.J. Vink et al. / Reactivity of graphite on Fe(l lO) towards oxygen
2. Experimental The investigations vcere carried out on a Fe single crystal, mounted in a Varian UHV system, equipped with LEED, AES, ion bombardment and ell~psometrie facilities. The base pressure of the system, during the experiments, being pumped by a turbomolecular pump, was - 5 x 10 -~° Torr. Auger spectra were recorded in a derivative mode ( E d N ( E ) / d E ) using an on-axis cylindrical mirror analyser. The iron single crystal was oriented by means of Laue back reflection X-ray analysis mad spark-cut within 0.5 ° of the (110) orientation. The crystal was ground and finally mechanically polished. The standard cleaning procedure consisted of Ar + sputtering (400 eV Ar+; I = 2 x 10 -2 A m-2; 30 min, 295 K; 20 min, 670 K; 20 rain, 470 K) and annealing at 470 K, producing a well ordered and clean surface. The surface concentrations of the remaining contaminants, being carbon and oxygen, were reduced to about 3 at% (relative to one monolayer ire,n) in this way. The gases used for the experiments described here, i.e. Ar (99.999%), 02 (99.995%), C2H4 (99.95%), H 2 (99.9995%) were supplied by l'Air Liquide.
3. Results After cleaning the crystal, a carbon layer on the Fe(110) surface was obtained by thermal dehydrogenation of ethylene at a crystal temperature of 523 K. Exposing the crystal surface to 20 L of ethylene (40 s at a pressure of 5 x 10 .7 Torr; 1 L = 1 x 10 -6 Tort-s) at the given crystal temperature, resulted in a saturation of the carbon coverage. The Auger ratio of the carbon and iron peak heights, defined as hc(272 eV)/hF~(703 eV), equals 0.194 + 0.017 at saturation and with LEED a so-called "carbon ring" could be seen [9,10]. The observed Auger ratio is in agreement with measurements of Brod6n et al. [4], who deposited carbon on Fe(110) by thermally dehydrogenating acetylene at 550 K. An absolute coverage calibration has been obt~fined for carbon on Fe(100), where a simple c(2 x 2) LEED pattern was obselved [8], corresponding to about half a monolayer of carbon. A comparison of the hC/hFe values on both planes yields, after correction for the different atomic densities of the planes, a calibration factor for carbon on Fe(110): Th~s it can be concluded that the deposiled carbon layer contains 0.45 ~2 0.04 carbon per iron atom. In this calibration it is assumed r.hat the Fe(703 eV) Auger signal is equal for both Fe planes. This was confirmed by measu~ng the absolute peak heights of Ib.e Fe(703 eV) signal for both the (100) and (110) planes.
T.Z I,qnk et al. / ReactiviO, of graphite on Fe(110) towards oxygen
561
Although the pretreatment of the surface may have some influence on the carbon deposition, our experimental method (i.e. the cleaning p-ocedure described earlier) resulted in reproducible carbon coverages. According to
Bonzel and Krebs [7] the observed shape of the carbon Auger spectra indicates a carbidic nature of our carbon layer. In the same report it is suggested that a carbon layer with such Auger characteristics, may contain some hydrogen, and indeed the presence of CH fragments cannot be excluded [11]. On the other hand, according to Yoshida and Somorjai [12] the temperature u~ed for the deposition makes the presence of these hydrogen species unlikely. The observed LEED pattern (the carbon ring) is believed to be due to the presence of graphitic carbon islands with a very slight preference for certain orientations [10]. Thus there exists a contradiction between the carbidic nature of carbon indicated by the shape of the Auger spectra and the graphitic nature suggested by Grabke et al. The absence of a relatively simple LEED pattern, however, reduces the probability of an ordered carbidic overlayer too. From the foregoing we conclude that the electronic structure of the carbon present in the graphitic carbon islands is carbide-like and therefore points to a relatively strong interaction with the substrate. After keeping the crystal at 573 K for half an hour, a decrease of the relative Auger ratio hc/hFe was noticed. This phenomenon is due to diffusion of carbon into the crystal and was noticed earlier on carbided Fe(100) [8]. Therefore the oxygen exposures were started immediately after reaching the desired crystal temperatures, while the overall reaction time in one run, consisting of several exposures was kept as short as possible in order te keep the diffusion at a negligible level. As mentioned above, the deposited carbon layer on the iron surface was exposed to discrete oxygen exposures, at crystal temperatures ranging from 473 to 573 K. The applied oxygen pressures varied from 5 x 10 -~ to 4 x 10Torr. After each exposure the system was evacuated and the Auger signals hc(272 eV), ho(510 eV) and hFe(703 eV) were recorded. The time needed for evacuation ( p ( O 2 ) < 10 -1° Tarr) was instantaneous when compared to the exposure time. An advantage of this method is that the influence of the electron beam on the reaction is minimised. It appeared that the reaction scales in the product of pressure end time ( = exposure). The admittance of oxygen in the vacuum chamber gave rise te a considerable increase of "mass-28"°pressare as was shown en a ARGA residual gas analyzer. This increase is probably due to desorption of c~hylene, used for depositing the carbon ~ayer on the Fe(ll0) surface. Therefore the oxygen partial pressure was measured on ~he gas anatyzero The effect of the increase of the mass-28 pressure, ~hat occmrcd mo~ notably (p2s/..'p32~1) during the firs~ oxygen exposures, on the ra~e of gasification of the carbon adlayer with oxygen was studied as foHew~. After bake-out of the UHV system the initial ratio of the ethylene arid ~xygen
T.J. Vink et at / Reactivity of graphite on Fe(l lO) towards oxygen
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Fig. 1. Ratio of the carbon hc(272 eV) (a) and oxygen ho(510 eV) (b) Auger signals to the iron Auger signal h Ee(703 eV) versus the oxygen exposure (in Langmuir units) of a carbonaceous layer on a Fe(ll0) surface at different crystal temperatures. "
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Fig. 2. The sum of the carbon and oxygen coverage on Fe(llO) versus the oxygen exposure at the crystal temperatures given.
T.J. Vink et al. / Reactivity of graphite on Fe(l It)) towards oxygen
563
partial pressure was reduced to 0.2. In a separate experiment the carbon adlayer was also exposed deliberately to a relatively high pressure ratio of pEs//P32 = 4. N o influence of this strong variation of the ethylene partial pressure on the initial reaction rate could be detected. The reaction curves are depicted in fig. 1, where h c/hF~ and h o/hFc as function of the oxygen exposure are given for different crystal temperatures. The amount of carbon which can be removed by oxygen, strongly depends on the crystal temperature. At 573 K all carbon can be removed while the overall coverage of carbon and oxygen decreases, as given in fig. 2. This is in contrast with the observations made at a reaction temperature of 473 K. Here a significant amount of carbon is left on the surface while at the same time the oxidation of the Fe(110) substrate is observed, resulting in an increase in 0c + 0o during the reaction. The amount of carbon which can be removed by oxygen is !"nfluenced by the partial pressure of ethylene during that particular reaction stage. At P2JP32= 4, more carbon is left on the surface, fortunately during our standard experiments this pressure ratio decreased from 1 to 0.1, and therefore an effect on the reaction rate seems very unlikely.
4. Discussion From the preceding section we concluded that the graphite islands on F e ( l l 0 ) are only partially removed by oxygen at reaction temperatures beiow 573 K. Our main objective in this discussion is to focus on this aspect in particular and therefore the following experiments were done• A reaction curve ( ~ ) at 473 K is given in fig. 3. At point (a) the system was evacuated and the crystal temperature was subsequently raised to 573 K. With AES a decrease in the oxygen and carbon coverage as a function of time was found. This is indicated by the arrows in fig. 3, obtained after 30 s. Apparently the adsorbed species are thermally activated to form CO. It is relevant to state here that diffusion of both oxygen and carbon cannot account for the observed reductions of the Auger signals. In fig. 3 another reaction curve is given (- - -) which can give insight into the reaction mechanism involved. In this c,ase the applied carbon coverage is identical to the amount of carbon which is left after reaction at 473 K (compare both curves ( ~ ) and (- - -)). In this case however chernisorbed ~4
reduction of the carbon coverage. It seems therefore plausible to suggest that a certain amount of oxygen, which adsorbs curing the rea::t~on, inhibits the further reaction with carbon at 473 K. However it canno~ be understood how at higher temperatures the reactivity of this residual carbon can be reduced by only a fairly low surface oxygen concentration as can be concluded from figs.
564
T.J. Vink et al. / Reactivity of graphite on Fe(l lO) towards oxygen
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Fig. 3. The carbon (descending curves) and oxygen (ascending curves) to iron Auger ratios versus oxygen exposure. Note the different scales for carbon (left) and oxygen (right) Auger ratios. The upper carbon curve and its corresponding oxygen curve are the same as those given in fig. 1 at 473 K. By interrupting the exposure afte~ 35 L of oxygen (point (a)) and heating to 573 K, an almost instantaneous decrease of carbon as well as oxygen coverage was observed (as depicted by the arrows). Also a reaction curve starting at a lower carbon coverage is shown.
1 and 2. In order to find out whether the interpretation of these reaction curves is correct, the following experiment was done. When the carbon coverage was reduced to point (a), as given in fig. 3, part of the oxygen was removed by hydrogen (also a slight decrease in carbon coverage was observed). On re-exposing this surface to oxygen no reaction was observed except for the oxidation of the surface. Since, despite the low oxygen coverage, the carbon remains inactive the proposed reaction mechanism, i.e. blocking of sites by adsorbed oxygen, can be excluded. It is now more likely to postulate that upon reaction with oxygen the carbon islands can be reduced to a certain size only, which apparently is a function of temperature. This model can be made more plausible by considermg the epitaxy of the graphite islands and the F e ( l l 0 ) surface. Before going into more detail, we first return to the reaction curve ( - - - ) starting at low carbon coverage, as depicted in fig. 3. At this initial coverage tt~e cart:on islands are not identical to those present after reaction at point (a) (although the amount of carbon present on the surface is the same) since then no
T.J. Vink et al. / Reactivity of graphite on Fe(l lO) towards oxygen
565
reaction would be observed. We assume that after dehydrogenation of ethylene graphite islands of nearly uniform size are present on the (110) substrate of iron, independent of total carbon coverage. Thus at an intermediate carbon coverage, as given in fig. 3, curve ( - - -), there are merely fewer islands when •
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Fig. 4. (a) Schematic representation of the graphite overlayer on Fe(ll0) as proposed by Bezuidenhout et al. [10]. (b) Graphic presentation of a possible distribution of the ada~0m-substrate interaction of the graphite island on Fe(110). Only a quarter of the island is shown.
T.J, Vink et al. / Reactivity of graphite on Fe(l lO) toward~ oxygen
566
compared to the saturation coverage, and upon reaction with oxygen their size is reduced to a defined level at that particular temperature. With the schematic presentations in figs. 4a and 4b an attempt is made to elucidate the relation between size and reactivity of the graphite islands. From fig. 4a obtained from ref. [10], it can be seen that the graphite island is not randomly oriented on the Fe(110) surface. This arises from the small mismatch in the [001] direction of about 1%, as was concluded by Bezuidenhout et al. [10]. Since some preferential orientation of the graphite islands is thus observed, we assume that the adatom-substi'ate interaction is a function of the surface direction. Fig. 4b visualizes this "interaction energy", based on the misfit of the islands on the substrate, as a function of the surface direction. In the [110] orientation carbon can easily be removed by oxygen which is reflected in fig. 1, where the initial decrease in carbon coverage is independent of temperature. Carbon closely aligned along the [001] orientation needs to be thermally activated to form CO upon reaction with oxygen, which accounts for the observed "saturation" levels in fig. 1. We finally emphasize that this model only gives a qualitative interpretation of the reaction curves given and that no mathematical model can be given to describe the kinetics accurately. However, our proposed model does not contradict the observation of both graphitic carbon (LEED structure) and carbidic carbon (Auger peak shapes), since not all carbon atoms in the adlayer are in equivalent positions with respect to the iron surface. Thus structurally (C-C distances) a graphite-like overlayer is formed, whereas electronically most carbon atoms behave as if they are part of a carbide structure.
Acknowledgement The investigations were supported by the Netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).
References [11 K. KJshi and M.W. Roberts, J. Chem. Soc. Faraday Trans. 1, 71 (1975) 1715. [21 C.R. Brundle, IBM J. Res. Develop. 11 (1978) 235. I31 T.N, Rhodin an~:~C.F. Brucker, Solid State Commun. 23 {1977) 275. [a! c,. Brod6n, G. Gafner and H.P. Bop!e!, App!. Phys. !3 ~,1977) ~3. [5] D.J. Dwyer and G.A. Somorjai, ,L Catalysis 52 (1978~ 291. I6] H.L Krebs, H.P. Bonzel ard G. Garner, Surface Sci. 8,~ (t07q} 269. t7] HP. Bonzet and HA. K~e~,s, Surface Sci. 9t {1980) 499. T3. V~ak, ,g.FG.LL Sp~o~ck, O.U,I. Gijzema~ and J.W. Ge~s, Surface ~ . 175 ~1986) 177.
[9] H.L Grabke, G. Tauber aad H. ¢icJhaus, ~fipta Met. 9 {19~57 ~ . [101 F. BezuidenhouL J. du Plcg~,~sand P.E. Viljcea, Surface ScL 171 (1986} 392, W. E~ey, A.M. Baro a~ ~ H. lbach, Surface Sci. 120 {1982) 273 K~ Y~sh~da and G.A, ~ ~moq~a, Surface Sci. 75 ~1978} 46.
559
THE REACTIVITY OF THE GRAPHITE OVERLAYER ON Fe(ll0) TOWARDS OXYGEN T.J. VINK, M. BOLECH, O.L.J. GIJZEMAN and J.W. GEUS Van 't Hoff Laborator); University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands Received 7 July 1987; accepted for publication 25 September 1987
The gasification by oxygen of a carbon adlayer on Fe(ll0) was studied by AES and LEED. The carbon layer was obtained by thermal decomposition of ethylene, and LEED measurements revealed that graphite islands were formed, in agreement with former studies. It appeared that at temperatures below 573 K only part of the carbon layer could be removed by oxygen. This phenomenon is explained by the fact that certain parts of the graphite overlayer matrix are better matched to the substrate than other parts, which leads to a lower reactivity towards oxygen.
1. Introduction Iron is a particularly reactive metal for CO dissociation [1-4] and as a consequence of this high reactivity a process of excess carbon deposition during CO hydrogenation is often observed [5,6]. An investigation by Krebs et al. [6] showed evidence for a correlation between the chemical form of ttds deposited carbon and catalytic activky. The same authors [7] characterized these various forms of carbon deposits, using AES and XPS. The present paper is aimed at elucidath~g further the reactivity of different chemical forms of carbon on iron towards oxygen. Recently we k~ave studied the gasification of a carbide layer on Fe(100) by oxygen [8]. This carbide layer consists of carbon atoms which are situated in a c(2 x 2) superstructure. It was found that during the reaction the carbon adatoms are replaced by oxygen atoms exclusively, while CO desorbs. The cm'bon adlayer on Fe~llO) shows quite different characteristics, since Grabke et al. [9] found that c:wbon extended study by Bezuidenhout et al. [10] revealed tha' graphke ~a'ye; ~s formed uninfluenced by the geometry of the Fe(110) sub°. ,,e around nucl~eation centres and results in the formation of islands of nearly random orientation. This variety in cherrfical nature of carbo~, on both h-on single crystal surfaces prompted our present study, where it will be shown that flaese surfaces differ considerably in their reactivity towards oxygen. 0039-6028/88/$03.50 © Elsevier Science Publishers KV. ~No~h-~~oh,~r~ Physics Pub!ishing DbAsio~)
560
T.J. Vink et al. / Reactivity of graphite on Fe(l lO) towards oxygen
2. Experimental The investigations vcere carried out on a Fe single crystal, mounted in a Varian UHV system, equipped with LEED, AES, ion bombardment and ell~psometrie facilities. The base pressure of the system, during the experiments, being pumped by a turbomolecular pump, was - 5 x 10 -~° Torr. Auger spectra were recorded in a derivative mode ( E d N ( E ) / d E ) using an on-axis cylindrical mirror analyser. The iron single crystal was oriented by means of Laue back reflection X-ray analysis mad spark-cut within 0.5 ° of the (110) orientation. The crystal was ground and finally mechanically polished. The standard cleaning procedure consisted of Ar + sputtering (400 eV Ar+; I = 2 x 10 -2 A m-2; 30 min, 295 K; 20 min, 670 K; 20 rain, 470 K) and annealing at 470 K, producing a well ordered and clean surface. The surface concentrations of the remaining contaminants, being carbon and oxygen, were reduced to about 3 at% (relative to one monolayer ire,n) in this way. The gases used for the experiments described here, i.e. Ar (99.999%), 02 (99.995%), C2H4 (99.95%), H 2 (99.9995%) were supplied by l'Air Liquide.
3. Results After cleaning the crystal, a carbon layer on the Fe(110) surface was obtained by thermal dehydrogenation of ethylene at a crystal temperature of 523 K. Exposing the crystal surface to 20 L of ethylene (40 s at a pressure of 5 x 10 .7 Torr; 1 L = 1 x 10 -6 Tort-s) at the given crystal temperature, resulted in a saturation of the carbon coverage. The Auger ratio of the carbon and iron peak heights, defined as hc(272 eV)/hF~(703 eV), equals 0.194 + 0.017 at saturation and with LEED a so-called "carbon ring" could be seen [9,10]. The observed Auger ratio is in agreement with measurements of Brod6n et al. [4], who deposited carbon on Fe(110) by thermally dehydrogenating acetylene at 550 K. An absolute coverage calibration has been obt~fined for carbon on Fe(100), where a simple c(2 x 2) LEED pattern was obselved [8], corresponding to about half a monolayer of carbon. A comparison of the hC/hFe values on both planes yields, after correction for the different atomic densities of the planes, a calibration factor for carbon on Fe(110): Th~s it can be concluded that the deposiled carbon layer contains 0.45 ~2 0.04 carbon per iron atom. In this calibration it is assumed r.hat the Fe(703 eV) Auger signal is equal for both Fe planes. This was confirmed by measu~ng the absolute peak heights of Ib.e Fe(703 eV) signal for both the (100) and (110) planes.
T.Z I,qnk et al. / ReactiviO, of graphite on Fe(110) towards oxygen
561
Although the pretreatment of the surface may have some influence on the carbon deposition, our experimental method (i.e. the cleaning p-ocedure described earlier) resulted in reproducible carbon coverages. According to
Bonzel and Krebs [7] the observed shape of the carbon Auger spectra indicates a carbidic nature of our carbon layer. In the same report it is suggested that a carbon layer with such Auger characteristics, may contain some hydrogen, and indeed the presence of CH fragments cannot be excluded [11]. On the other hand, according to Yoshida and Somorjai [12] the temperature u~ed for the deposition makes the presence of these hydrogen species unlikely. The observed LEED pattern (the carbon ring) is believed to be due to the presence of graphitic carbon islands with a very slight preference for certain orientations [10]. Thus there exists a contradiction between the carbidic nature of carbon indicated by the shape of the Auger spectra and the graphitic nature suggested by Grabke et al. The absence of a relatively simple LEED pattern, however, reduces the probability of an ordered carbidic overlayer too. From the foregoing we conclude that the electronic structure of the carbon present in the graphitic carbon islands is carbide-like and therefore points to a relatively strong interaction with the substrate. After keeping the crystal at 573 K for half an hour, a decrease of the relative Auger ratio hc/hFe was noticed. This phenomenon is due to diffusion of carbon into the crystal and was noticed earlier on carbided Fe(100) [8]. Therefore the oxygen exposures were started immediately after reaching the desired crystal temperatures, while the overall reaction time in one run, consisting of several exposures was kept as short as possible in order te keep the diffusion at a negligible level. As mentioned above, the deposited carbon layer on the iron surface was exposed to discrete oxygen exposures, at crystal temperatures ranging from 473 to 573 K. The applied oxygen pressures varied from 5 x 10 -~ to 4 x 10Torr. After each exposure the system was evacuated and the Auger signals hc(272 eV), ho(510 eV) and hFe(703 eV) were recorded. The time needed for evacuation ( p ( O 2 ) < 10 -1° Tarr) was instantaneous when compared to the exposure time. An advantage of this method is that the influence of the electron beam on the reaction is minimised. It appeared that the reaction scales in the product of pressure end time ( = exposure). The admittance of oxygen in the vacuum chamber gave rise te a considerable increase of "mass-28"°pressare as was shown en a ARGA residual gas analyzer. This increase is probably due to desorption of c~hylene, used for depositing the carbon ~ayer on the Fe(ll0) surface. Therefore the oxygen partial pressure was measured on ~he gas anatyzero The effect of the increase of the mass-28 pressure, ~hat occmrcd mo~ notably (p2s/..'p32~1) during the firs~ oxygen exposures, on the ra~e of gasification of the carbon adlayer with oxygen was studied as foHew~. After bake-out of the UHV system the initial ratio of the ethylene arid ~xygen
T.J. Vink et at / Reactivity of graphite on Fe(l lO) towards oxygen
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Fig. 1. Ratio of the carbon hc(272 eV) (a) and oxygen ho(510 eV) (b) Auger signals to the iron Auger signal h Ee(703 eV) versus the oxygen exposure (in Langmuir units) of a carbonaceous layer on a Fe(ll0) surface at different crystal temperatures. "
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Fig. 2. The sum of the carbon and oxygen coverage on Fe(llO) versus the oxygen exposure at the crystal temperatures given.
T.J. Vink et al. / Reactivity of graphite on Fe(l It)) towards oxygen
563
partial pressure was reduced to 0.2. In a separate experiment the carbon adlayer was also exposed deliberately to a relatively high pressure ratio of pEs//P32 = 4. N o influence of this strong variation of the ethylene partial pressure on the initial reaction rate could be detected. The reaction curves are depicted in fig. 1, where h c/hF~ and h o/hFc as function of the oxygen exposure are given for different crystal temperatures. The amount of carbon which can be removed by oxygen, strongly depends on the crystal temperature. At 573 K all carbon can be removed while the overall coverage of carbon and oxygen decreases, as given in fig. 2. This is in contrast with the observations made at a reaction temperature of 473 K. Here a significant amount of carbon is left on the surface while at the same time the oxidation of the Fe(110) substrate is observed, resulting in an increase in 0c + 0o during the reaction. The amount of carbon which can be removed by oxygen is !"nfluenced by the partial pressure of ethylene during that particular reaction stage. At P2JP32= 4, more carbon is left on the surface, fortunately during our standard experiments this pressure ratio decreased from 1 to 0.1, and therefore an effect on the reaction rate seems very unlikely.
4. Discussion From the preceding section we concluded that the graphite islands on F e ( l l 0 ) are only partially removed by oxygen at reaction temperatures beiow 573 K. Our main objective in this discussion is to focus on this aspect in particular and therefore the following experiments were done• A reaction curve ( ~ ) at 473 K is given in fig. 3. At point (a) the system was evacuated and the crystal temperature was subsequently raised to 573 K. With AES a decrease in the oxygen and carbon coverage as a function of time was found. This is indicated by the arrows in fig. 3, obtained after 30 s. Apparently the adsorbed species are thermally activated to form CO. It is relevant to state here that diffusion of both oxygen and carbon cannot account for the observed reductions of the Auger signals. In fig. 3 another reaction curve is given (- - -) which can give insight into the reaction mechanism involved. In this c,ase the applied carbon coverage is identical to the amount of carbon which is left after reaction at 473 K (compare both curves ( ~ ) and (- - -)). In this case however chernisorbed ~4
reduction of the carbon coverage. It seems therefore plausible to suggest that a certain amount of oxygen, which adsorbs curing the rea::t~on, inhibits the further reaction with carbon at 473 K. However it canno~ be understood how at higher temperatures the reactivity of this residual carbon can be reduced by only a fairly low surface oxygen concentration as can be concluded from figs.
564
T.J. Vink et al. / Reactivity of graphite on Fe(l lO) towards oxygen
l~
I
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I
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20
I
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o
I
I
30 40 50 > 0 2 Exp.(L)
Fig. 3. The carbon (descending curves) and oxygen (ascending curves) to iron Auger ratios versus oxygen exposure. Note the different scales for carbon (left) and oxygen (right) Auger ratios. The upper carbon curve and its corresponding oxygen curve are the same as those given in fig. 1 at 473 K. By interrupting the exposure afte~ 35 L of oxygen (point (a)) and heating to 573 K, an almost instantaneous decrease of carbon as well as oxygen coverage was observed (as depicted by the arrows). Also a reaction curve starting at a lower carbon coverage is shown.
1 and 2. In order to find out whether the interpretation of these reaction curves is correct, the following experiment was done. When the carbon coverage was reduced to point (a), as given in fig. 3, part of the oxygen was removed by hydrogen (also a slight decrease in carbon coverage was observed). On re-exposing this surface to oxygen no reaction was observed except for the oxidation of the surface. Since, despite the low oxygen coverage, the carbon remains inactive the proposed reaction mechanism, i.e. blocking of sites by adsorbed oxygen, can be excluded. It is now more likely to postulate that upon reaction with oxygen the carbon islands can be reduced to a certain size only, which apparently is a function of temperature. This model can be made more plausible by considermg the epitaxy of the graphite islands and the F e ( l l 0 ) surface. Before going into more detail, we first return to the reaction curve ( - - - ) starting at low carbon coverage, as depicted in fig. 3. At this initial coverage tt~e cart:on islands are not identical to those present after reaction at point (a) (although the amount of carbon present on the surface is the same) since then no
T.J. Vink et al. / Reactivity of graphite on Fe(l lO) towards oxygen
565
reaction would be observed. We assume that after dehydrogenation of ethylene graphite islands of nearly uniform size are present on the (110) substrate of iron, independent of total carbon coverage. Thus at an intermediate carbon coverage, as given in fig. 3, curve ( - - -), there are merely fewer islands when •
O
o
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Fig. 4. (a) Schematic representation of the graphite overlayer on Fe(ll0) as proposed by Bezuidenhout et al. [10]. (b) Graphic presentation of a possible distribution of the ada~0m-substrate interaction of the graphite island on Fe(110). Only a quarter of the island is shown.
T.J, Vink et al. / Reactivity of graphite on Fe(l lO) toward~ oxygen
566
compared to the saturation coverage, and upon reaction with oxygen their size is reduced to a defined level at that particular temperature. With the schematic presentations in figs. 4a and 4b an attempt is made to elucidate the relation between size and reactivity of the graphite islands. From fig. 4a obtained from ref. [10], it can be seen that the graphite island is not randomly oriented on the Fe(110) surface. This arises from the small mismatch in the [001] direction of about 1%, as was concluded by Bezuidenhout et al. [10]. Since some preferential orientation of the graphite islands is thus observed, we assume that the adatom-substi'ate interaction is a function of the surface direction. Fig. 4b visualizes this "interaction energy", based on the misfit of the islands on the substrate, as a function of the surface direction. In the [110] orientation carbon can easily be removed by oxygen which is reflected in fig. 1, where the initial decrease in carbon coverage is independent of temperature. Carbon closely aligned along the [001] orientation needs to be thermally activated to form CO upon reaction with oxygen, which accounts for the observed "saturation" levels in fig. 1. We finally emphasize that this model only gives a qualitative interpretation of the reaction curves given and that no mathematical model can be given to describe the kinetics accurately. However, our proposed model does not contradict the observation of both graphitic carbon (LEED structure) and carbidic carbon (Auger peak shapes), since not all carbon atoms in the adlayer are in equivalent positions with respect to the iron surface. Thus structurally (C-C distances) a graphite-like overlayer is formed, whereas electronically most carbon atoms behave as if they are part of a carbide structure.
Acknowledgement The investigations were supported by the Netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).
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[9] H.L Grabke, G. Tauber aad H. ¢icJhaus, ~fipta Met. 9 {19~57 ~ . [101 F. BezuidenhouL J. du Plcg~,~sand P.E. Viljcea, Surface ScL 171 (1986} 392, W. E~ey, A.M. Baro a~ ~ H. lbach, Surface Sci. 120 {1982) 273 K~ Y~sh~da and G.A, ~ ~moq~a, Surface Sci. 75 ~1978} 46.