Adsorption of CO on Fe, Cu, and Cu–Fe surface alloysO. L. J. Gijzeman, T. J. Vink, O. P. van Pruissen, and J. W. Geus Citation: Journal of Vacuum Science & Technology A 5, 718 (1987); doi: 10.1116/1.574280 View online: http://dx.doi.org/10.1116/1.574280 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/5/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Atomistic simulations of grain boundary pinning in CuFe alloys Appl. Phys. Lett. 87, 231904 (2005); 10.1063/1.2137871 Surface oxidation of Fe from a Cu(Fe) alloy observed by Mössbauer spectroscopy J. Appl. Phys. 51, 388 (1980); 10.1063/1.327384 HighField Magnetoresistance of Dilute Cu–Mn and Cu–Fe Alloys J. Appl. Phys. 40, 1472 (1969); 10.1063/1.1657726 Effects of Internal Oxidation and Heat Treatment on the Electrical Resistivity of Dilute CuMn, CuFe, and CuCoAlloys J. Appl. Phys. 31, 1730 (1960); 10.1063/1.1735435 XRay Line Broadening from Precipitation in Cu–Fe Alloys J. Appl. Phys. 24, 813 (1953); 10.1063/1.1721382

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Adsorption of CO on Fe, Cu, and Cu-Fe surface alloys o. L. J. Gijzeman, T. J. Vink, O. P. van Pruissen, and J. W. Geus Van 't HojJLaboratory, University of Utrecht, Padualaan 8, 3584 Cf{ Utrecht, The Netherlands

(Received 2 September 1986; accepted 27 October 1986)

The adsorption and decomposition of CO on Fe (100), Fe (110), Cu( 111 )-Fe, and Cue 11O)-Fe surface alloys with different amounts of iron in the surface layers has been studied with ellipsometry. Only on Fe (100) at room temperature is CO found to dissociate rapidly. On all surfaces dissociation takes place, at higher temperatures, leaving equal amounts ofC and a on the surface. On this adlayer reversible adsorption of CO takes place. The heats of adsorption have been determined from the observed isotherms. Heats of adsorption have also been determined for adlayers of only oxygen or carbon on the iron crystal faces.

I. INTRODUCTION

The adsorption and decomposition of CO on metal surfaces is an important subject in surface science. Unfortunately, many techniques that are often used in these studies are diffi­cult to apply in the case of CO adsorption, since an incoming or outgoing electron beam can lead to dissociation or stimu­lated desorption of CO. It is also rather difficult to apply the electron spectroscopies in studies of reversible adsorption (adsorption isotherms) when a high-pressure (p> 10 - (, Torr) gas phase must be present in the system. One tech­nique that can be conveniently used in this case is ellipso­metry, which uses the change in the state of polarization of reflected visible light to monitor adsorption at a surface. 1,2 It has been amply shown that this technique is capable of deter­mining submonolayer coverages of adsorbates on meta12

,3

and semiconductor surfaces.4-

6

The important quantity measured in elipsometric mea­surements is the value of~, which is defined as the difference of the relative phases of the electric field vector parallel and perpendicular to the plane of incidence for the reflected and incoming light beam. Upon adsorption the value of 6. changes due to the altered dielectric properties of the sur­face. This change, conventionally called o~, is expressed in degrees. For adsorbed layers 8~ is linearly related to the layer thickness 1 (i.e., amount adsorbed) and a linear rela­tionship is empirically found between o~ and a submono­layer surface coverage as well,2,3 although the proportional­ity constant has to be determined by independent means.

In this paper we will describe the decomposition and ad­sorption isotherms of CO on Fe ( 100) and Fe ( 110) contain­ing adlayers of carbon, oxygen, and carbon plus oxygen. We will also compare the results to isotherms measured on a Cu(lll )-Fe and Cu(1lO)-Fe surface aHoy. These alloy surfaces were prepared, with a varying amount of iron, by thermal decomposition ofFe(CO)5 on the appropriate cop­per surface.? All experiments were carried out in conven­tional ultrahigh vacuum systems, containing facilities for el­lipsometry, Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and argon ion cleaning. The cleaning procedure for Fe (100) has been de­scribed previously,S whereas the procedure used for Fe ( 110) is reported elsewhere in this journal. ')

II. RESULTS AND DISCUSSION

A. Adsorption and decomposition of CO

On pure Cu single crystals no adsorption and/or decom­position of CO takes place under our experimental condi­tions. On Fe (100), decomposition of CO is virtually instan­taneous at room temperature and above, even at pressures of 10-') Torr,81eaving a surface with a quarter monolayer cov­erage of both oxygen and carbon. On Fe (110) adsorption of CO takes place at room temperature and above as illustrated in Fig. 1. As can be seen the saturation coverage decreases with increasing temperature. This could be due to an in­creased rate of dissociation of CO, when less CO is molecu­larly adsorbed, or to a shift in the equilibrium coverage with temperature. The latter explanation is ruled out by the fact that upon increasing the CO pressure the measured value of 8~ remains constant. In order to investigate the possible oc­currence of CO dissociation at room temperature the CO­saturated surface was subsequently exposed to oxygen at a pressure of 10-7 Torr. The ellipsometric parameter de­creased somewhat and after 12 min began to increase again (see Fig. 1), indicative of the oxidation of the Fe (110) sur­face. ') The oxygen exposure was interrupted at this point and the surface composition was determined with AES. Only oxygen was found at the surface with a surface coverage of about 0.5 monolayer. Based on our work on the gasification of carbon on Fe ( 100) with oxygen, where at room tempera­ture no reaction between carbon and oxygen takes place, 10

we conclude that this amount of oxygen is insufficient to mask the presence of any carbon arising from CO dissocia­tion. Thus CO dissociation on Fe ( 110) at room temperature is very slow, at least compared to the process on Fe (100). An alternative method to establish this fact, viz., heating the CO covered surface in vacuum in order to desorb the gas, failed, because during the heating process CO dissociation takes place. 11

The initial sticking coefficient of CO on Fe (110) can be inferred from the known surface coverage at saturation, Le., 6.2 X 1014 molecules/cm2

• 12 In this way a value of So = 1 was found. The adsorption kinetics can be described by a mobile precursor model, as first described by Kohn and Gomer13

and later extended by Schonhammer.14 This model gives for the sticking coefficient as a function of coverage

718 J. Vac. Sci. Technol. A 5 (4), Jull Aug 1987 0734-2101/871040718-04$01.00 © 1987 American Vacuum Society 718

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719 Gljzeman et 81.: Adsorption of CO on Fe, Cu, and Cu-Fe surface alloys 719

0..4 co. Fe (nOi Q--i> • Q

" e ~-~-.. ~

<J <00.3 298K

r 02

----.~

)",5K

/ /

/ /

/

0.1

11 2 3 4 '0

~ Exposure (L)

( 1 )

where a is the probability for a gas phase molecule to enter the precursor state and Pa , P d, and Pd , are the probabilities for adsorption, desorption from above an empty site, and desorption from above a fined site. As shown by Schonham­mer, 14 Pd , is related via detailed balance to Pd and Fa:

Pd , = F,J(1 - Pa ). (2)

Using these equations the adsorption kinetics are described by the solid line in Fig. 1, provided Pa is taken to be larger than 0.99 and Pd less than 0.003, keeping the ratio Pdf

(1 - Pa ) fixed at 0.3. The value of a has to be unity, of course, in order to ensure a value of 1 for the initial sticking coefficient. As can be seen in Fig. 1 the observed adsorption curve can be quantitatively described by this modeL

On Cu Ol1)-Fe and Cu (1lO)-Fe surface alloys, pre­pared by the decomposition of Fe( CO) 5 on the copper sur­faces,7 CO does adsorb and decompose slowly at room tem­perature. After preparation of these alloy surfaces some carbon and oxygen are already present at the surface, so that accurate kinetic measurements are very difficult and unre-

TABLE I. Decomposition of CO 011 Fe and Fe-Cu surface alloys with differ­ent molefractions of iron in the first two atomic layers. The initial (i) and final (f) coverages of oxygen and carbon are given as a fraction of the number of surface atoms.

Cu (lll)-Fe

Cu (llO)-Fe

Fe (l00)

Fe (110)

x (Fe)

0.10 0.18 0.23 0.36

0.08 0.27 0.39

fio (i)

0.Q3 0.04 0.06 0.18

0,05 0.08 0.10

fie (i) eo (f)

0.Ql 0.04 0.02 0.09 0.03 0.10 0.12 0.20

0.05 0.05 0.09 0.12 0.10 0.15

0.25

0.25

J. Val:. Sci. Techno!. A, Vol. 5, No.4, Jul/Aug 1987

ec (0

0.05 0.07 0.10 0.19

0.05 0.19 0.22

0.25

0.25

5 10. ----'i» Time (min)

15

FIG. 1. Adsorption kinetics of CO on Fe (110) at 298 K. The amount of CO ad­sorhed (proportional to lill.) is plotted against t he exposure for pressures of • 5X 10 " Torr, /:" 9X 10 " Torr, and 0 2 X 10 -" Torr. At 315 K the apparent ad­sorption rate and satura.tion coverage are lower. The right-hand panel shows the ef­fel't of exposing a CO saturated surface to oxygen at room temperature.

liable. However, exposing these surfaces to CO at pressures of 5 X 10 -4 Torr at 400 K leads to an increase in the carbon and oxygen coverage, their final values being larger when more iron is present near the surface as shown in Table 1. Here, the average iron molefraction in the first two atomic layers x (Fe) is used to characterize the surface. 7

,1s Whereas for pure iron the final stoichiometry is Fe40,C 1• for the al­loys a value near Fe20 1C 1 is obtained, independent of the iron molefraction. Since the actual amount of iron in the surface is probably less than that given by x(Fe) as Cu en­richment of the surface takes place,16,17 it seems plausible that the stoichiometry is quite different for alloyed and pure iron.

B. Reversible adsorption of CO

Reversible adsorption of CO has been studied previously on Fe (100) containing carbon and/or oxygen adlayers at a total coverage of one-half of a monolayer. g Similar experi­ments are reported here for Fe (110). Figure 2 shows the adsorption isotherms at room temperature of CO on Fe (110) containing different amounts of carbon and oxygen.

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720 Gijzeman et al: Adsorption of CO on Fe, Cu, and Cu-Fe surface alloys 720

FIG. 3. Adsorption isotherms of CO on Fe (IIO) containing.\ monolayer of

carbon and ~ monolayer of oxygen.

For comparison the saturation adsorption on clean Fe (110) is also given. As can be seen the effect of carbon and oxygen is to decrease the amount of CO adsorbed reversibly at any pressure. Also further dissociation of CO is prevented by a C or 0 adlayer. This can of course be determined by measuring the surface carbon and/or oxygen coverage with AES before and after the determination of a set of adsorption isotherms. The effect of oxygen on the amount of CO adsorbed is larger than a corresponding amount of carbon. This is not due to a site-blocking mechanism only as the curves have different shapes and are not simply scaled down by a constant factor. Rather, the effect seems to be caused by a change in the heat of adsorption and the adsorption entropy. From a set of iso­therms measured at different temperatures for the same sur­face adlayer (as exemplified in Fig. 3 for Fe (llO)/C + 0] the isosteric heat of adsorption can be obtained from the appropriate Clausius-Clapeyron plots. The results are given in Table II, which includes data for Cu-Fe as well. In this case the experiments were performed on Cu-Fe surface al­loys with a saturation coverage of equal amounts of carbon and oxygen as given in Table I. The heat of adsorption of CO on these surfaces was independent ofthe actual iron content, although the amount of CO adsorbed at a given temperature increased with the amount of iron present.

The uncertainty in the data is much larger as the amount

of reversibly bound CO is appreciably less (compare Fig. 4). The number of CO molecules per iron atom, however, is comparable to the high coverage limit on pure Fe, as can be seen by dividing the observed values of SA by the iron mole­fraction. From Table II it follows that the heat of adsorption on Fe (llO)/C + 0 at higher coverages is equal to that on Cu (111 )-Fe and Cu (llO)-Fe within experimental error, but lower than that on Fe (100). The similarity ofCu (111)­Fe and Cu (llO}-Fe to Fe (110) is due to the fact that on both copper surfaces iron atoms occupy positions very near­ly equivalent to a Fe (110) plane. On Cu (Ill) this can be accomplished on the surface without appreciable lattice mis­match,IK on Cu (110) the surface forms facets after iron deposition with a bcc (110) orientation. From Fig. 4 it fol­lows that noticeable adsorption on Fe (110) and en-Fe starts at lower pressures than on Fe (l00). This also points to a more bcc (110) character for iron on the two copper surfaces.

The shapes of the isotherms on Fe in Figs. 2-4 as well as the varying heat of adsorption suggest that they are not easi­ly amenable to a simple theoretical description. In particu­lar, they cannot be described by a Langmuir,van cler Waals, or quasichemical isotherm equation. The surprising differ­ence between Fe (lOO)/C + 0 and Fe (llO)/C + 0 is the fact that in the latter case adsorption starts at lower pres­sures, although the heat of adsorption on this surface is actu­ally less (see Table II). This could be expiained by an in­creased adsorption entropy (or preexponential factor). A rough estimate indicates that this preexponential factor has to be increased by a factor of 105

, which must be caused by an equally large ratio of the partition functions of the adsorbed CO molecules on the two surfaces. From the published vi­brational frequencies 19.20 no such large number can reasona­bly be obtained. An alternative and more likely explanation could be the fact that on Fe (110) the initial heat of adsorp­tion at e->o increases rather steeply, which will cause ad­sorption at lower pressures than on Fe (100). This is not indicated in the trend in Table II, as the lowest coverage where a heat of adsorption can be measured is e = 0.05. As such it remains only a (plausible) hypothesis.

Finally, we comment on the absolute coverages given in

TABLE II. Observed heat of adsorption of CO on Fe and Fe-Cu surface alloys in kJ/mol. The measured parameter!56 is given in degrees and the computed coverages 8 in monoJayers. For the alloy systems the value of 6 refers to the total surlacc coverage, the iron surface coverage is larger by a factor of 3 or more.

86 <0.03 <;0.08 0.05 O. \0 0.15 0.20 0.25

() 0.07 0.13 0.20 0.27 0.33 Fe (lOO)/C + 0 g9:! 4 85 J: 4 84 ± 4 75 ± 4 Fe (IOO)/C 77:1: 4 77:t~ 4 77:L 4 77 j: 4 77 ±4 Fe (100)/0 74:±: 10 74 ± 10 74 ± 10 74± 10 74 ± 10

e 0.05 0.Q9 0.14 0.19 0.24

Fe (IIO)C + 0 80:1: 4 74±4 63 ± 4 44±4 Fe (llO)/C 83:1-.4 79:1:: 4 76 :.1: 4 75 ± 4 66±4

(j <0.07 Cu (l11)/C+O 70:t 15

B ,;;0.05

Cu (llO)/C + 0 70:1: 15

J. Vac. Sci. Technol. A. Vol. 5, No.4, Jul/Aug 1987

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721 GiJzeman et al.: Adsorption of CO on Fe, Cu, and Cu-Fe surface alloys 721

FIG. 4. Adsorption isotherms of CO on iron and copper-iron surfaces with equal coverages of carbon and oxygen. Although the total surface coverage of CO on the alloys is less than that on Fe, the CO/Fe ratio is of the same order of magnitude.

Table n as deduced from the observed values of 8t:... In gen­eral 8t:.. is linearly proportional to the number of adsorbed molecules per unit area. For the clean Fe (110) a limiting value of 8t:.. = 0.38° is found (see Fig, 1) corresponding to 6.2 X 1014 CO molecules per cm2

• Dividing this number by the appropriate surface atom density gives the coverage e, defined as the number of CO molecules adsorbed per surface metal atom. For the alloy systems this number should be divided by the (unknown)iron mole fraction in the surface layer, which is certainly less than 0.4 (cf. Table I).

J. Vac. Sci. Techno!. A, Vol. 5, No.4, JulfAug 1987

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