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A study of the initial reaction of water vapor with Fe(001) surface
Surface Science 64 (1977) 617-632 @North-Holland Publishing Company
A STUDY OF THE INITIAL REACTION OF WATER VAPOR WITH Fe(001) SURFACE
Daniel J. DWYER and G.W. SIMMONS Center for Surface and CoatingsResearch, Lehigh University,Bethlehem, Pennsylvanti 18015, USA
R.P. WE1 Department ofMechanical Engineering and Mechanics, Lehigh University,Bethlehem, Pennsylvania18015. USA Received 18 November 1976
LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(OO1) and water vapor at temperatures from 298 to 473 K. A disordered c(2 X 2) structure was formed at all temperatures, and only 80% of the total number of sites were filled at saturation. The initial sticking coefficient was 0.56 f 0.03, and the reaction rate increased with increasing temperature. A model was proposed that successfully accounted for these experimental observations. Irreversible chemisorption of water is proposed to take place via a precursor of physically adsorbed water molecules. The precursor, which is adsorbed on both bare surface and surface covered by chemisorbed species, is mobile and retains most of its degrees of rotational freedom. Water molecules in the precursor state can either desorb or dissociate, and the difference in activation energies for these reactions was found to be 5.7 f 0.5 kcal/mol. Only 80% of the available c(2 X 2) sites are filed and the surface layer is disordered because the chemisorbed species are immobile, and because each one blocks four nearest neighbor sites for further adsorption. The chemisorbed species occupy the fourfold symmetric sites either above the iron atoms or above the interstitial “holes” between iron atoms.
1. Introduction
The development of detailed mechanism for reaction of water and water vapor with iron is of fundamental importance to the understanding of corrosion and passivation of iron and steels, and of the enhancement of subcritical crack growth in low alloy steels by environments containing water. Experimental methods for investigating reactions in aqueous environments are limited to electrochemical and ellipsometric techniques, and these methods have been used extensively to characterize the corrosion and passivation behavior of iron and steel. Further details of 617
618
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
the reaction of water with iron and steel surfaces, however, can be derived from studies of water vapor using a number of other experimental techniques. Only a few of these studies have been reported thus far. The reaction of water vapor with evaporated iron films has been studied through electrical resistivity, work function, and photoelectric measurements [l-3]. The relative amounts of reversibly and irreversibly adsorbed water on reduced iron powder have been measured by a volume-manometric method [3]. Adsorption of water vapor on evaporated iron fnms and initial oxidation have been examined by a quartz microbalance method [4]. Studies of the hydroxylation and oxidation of evaporated iron films in water vapor by X-ray photoelectron spectroscopy have also been reported [5]. Fundamental mechanisms for the water/metal reactions have been proposed on the basis of these studies [ l-51. There is, however, considerable uncertainty regarding these proposed mechanisms because, in many cases, iron surfaces of unknown structure and composition were used. Experiments with better defined surfaces are therefore needed to confirm and to refine the mechanisms derived from these earlier studies. The objective of this investigation is to further develop the mechanisms for iron-water surface reaction from measurements of the reaction kinetics and from determinations of changes in surface structure. A clean Fe(OO1) surface was selected for study. Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) techniques were used in the present studies in conjunction with ultra-high vacuum. A model for the gas-metal surface reaction was developed to explain both the observed kinetic data and changes in surface structure.
2. Experimental The instrumentation, as well as many of the experimental procedures used in this study, have been described already in connection with previous studies of the reaction of oxygen with Fe(OO1) surface [6]. Only those additional experimental details relating to the studies of reactions with water vapor are described here. The rates of water vapor adsorption on Fe(OO1) surface were measured by monitoring the oxygen Auger electron intensity as a function of exposure (pressure X time) and temperature. Corresponding changes in the surface structure were followed by LEED. Exposures were made for pressures of lop7 to 10V5 Torr and at temperatures from room temperature to 2OO’C. The specimen was heated by radiation from a heated tungsten filament, and the temperature was measured by an iron-constantan thermocouple spot welded to the back surface of the specimen. Water vapor was admitted to the system through a variable leak from a reservoir of triply distilled water (conductivity of 10 -6 a-’ cm). The water was degassed by alternately freezing and melting, and by pumping on the reservoir when the water was frozen. The adsorption of water vapor was found to be inhibited by the presence of sur-
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
619
face impurities. Procedures described in ref. [6] were therefore used to minimize possible contamination of the specimen surface by impurities (specifically, sulfur, chlorine, carbon and nitrogen). Because of apparent interactions of the adsorbate with the electron beam, reproducible results could be obtained only when exposures were made with the accelerating voltage for the electron beam switched off. In addition, it was essential to re-evacuate the system to pressures near the background value (
3. Results The oxygen coverage on clean Fe(OO1) following exposures to water vapor at 298, 329, 413 and 473 K are shown as a function of exposure in fig. 1, where coverage is defined as the fraction of the available surface sites that are occupied. For the c(2 X 2) surface structure that is formed, it is assumed that there is one site for every two surface iron atoms. Since the reaction was found to be irreversible, it is assumed that the oxygen Auger electron signal originated from chemically adsorbed hydroxyl group or from chemically adsorbed oxygen. Molecular water
620
D.J. Dwyer et al. f Initial reaction of water vapor with Fe(OO1) surface
0.8 -
(4
*+*
;
0.6 -
+
lb)
m
Y
d w 0.2
-
B 0 A 0.0
a (cl E 0.8 -
(d) $-WC=== .A
Ox)0
I
I
I
234501234s
I
I
I
I
I
I
I
I
J
EXPOSURE ( Langmuirs 1 Fig. 1. Chemisorption of water vapor as a function of exposure at (a) 298 K, (b) 329 K, (c) 413 K, and (d) 473 K. (1 Langmuir = 10” Torr set).
is assumed to make little contribution to the oxygen signal, since water either dissociates or is removed from the surface upon re-evacuation of the system. The absence of significant changes in the low-energy iron Auger electron peaks furthermore suggests that bulk oxide phase did not form at these temperatures and exposures [6 1. The reaction approached completion for each temperature at a surface coverage (based on the experimental calibration of the oxygen Auger signal) equivalent to 80% of the total available c(2 X 2) adsorption sites. No appreciable increase in oxygen coverage beyond this level was observed at any of the temperatures, for water vapor exposures up to lo3 Torr sec. The effect of the initial chemisorption on further reaction of the surface with water vapor differs from the results for reaction of oxygen with clean Fe(OO1) [6]. In the latter case, oxidation of the surface was found to follow immediately after the formation of the c(2 X 2) oxygen layer, and to form several layers of oxide for oxygen exposures as low as 10V4 Torr sec. The initial reaction rate of water vapor with iron is relatively fast. A sticking coefficient of 0.56 f 0.03 was calculated from the linear part of the kinetic data at 473 K. The reaction rate increases with temperature, and the initial portion of the
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
621
b Fig. 2. (a) LEED patterns of Fe(001) surface after exposure to water vapor at 298 K. (b) Optical Fourier transform and (c) calculated Fourier transform of proposed surface structure shown in fii. 3.
coverage versus exposure curves becomes more linear with increasing temperature. The LEED patterns that were observed as a function of exposure at each temperature showed the development of a c(2 X 2) surface structure. The relatively large and diffuse “half order” reflections from the overlayer (as shown in fig. 2a) suggest that there is relatively little long range order in the surface structure. Similar diffuse diffraction features, however, were not found around any of the integral order reflections.
4. Discussion In the following sections an adsorption model that is consistent with both the kinetic and electron diffraction data is developed. A model for the reaction of water vapor with Fe(OO1) at these temperatures and exposures must account for the following: (a) the shape of coverage versus exposure curves at the different temperatures, (b) the saturation value of coverage equal to 80% of that for a complete c(2 X 2) structure, and (c) the lack of long range order in the c(2 X 2) structure.
4.1. Kinetic model 4.1.1. Chemisorption via a precursor A model based on the formation of a molecularly adsorbed precursor, which subsequently either desorbs or decomposes, was found to give a satisfactory explanation for the requirements listed previously. Since kinetic models of chemisorption based on a precursor step have been derived and discussed in detail elsewhere [9-l 11, only the essential features of this model used to describe the reaction of water with iron are presented.
622
D.J. Dwyer et al. /Initial reaction of water vapor with FefOOI) surface
The dissociation reaction detected by the AES and LEED techniques to take place according to the following reaction:
is assumed
H 0
H
P201 g =[H20],+-Fe-Fe-T--je--Le--, kd or 0
[H201g~IH~Olp + - Fe - Fe - -----+-Fe-Fe+H2. k, Since no evidence of bulk oxide was found, the oxygen exists on the surface as chemisorbed oxygen or as a chemisorbed hydroxyl group. The ex~rimental measurements reflect only the rate of change in surface concentration of one of these chemisorbed species. It was assumed that the precursor rapidly reaches a steady state concentration and that this concentration is small. This ~sumption is necessary to eliminate the precursor concentration (a quantity that cannot be readily measured) from the expression for the rate of chemisorption. It is further assumed that the precursor is physically adsorbed and that the adsorption of the precursor is non-activated. Hydrogen from the decomposition is assumed either to desorb or to have suf~cient mobility that it does not block sites for water dissociation. The rate of formation of the precursor and its rate of desorption are as follows: d fH,O],/dt -d[H20]n/dt
= Z&Jr,(@), = k&-120]p.
(1) (2)
Z is the arrival rate from the gas phase and is equal to P/d-, where P is the vapor pressure of water. S, is the sticking probab~ity for adsorption of the precursor, and its value is determined by the degrees of freedom lost by the precursor during its formation. f,(0) is the functional dependence of the rate of precursor adsorption on coverage of the surface by chemically adsorbed species, 0. According to the proposed surface reaction, the rate of chemisorption (dissociation) is given below: d[OH]/dt
= k,[H201p%f,(~).
(3)
Ne is the total number of sites per unit area initially available; and f,(Q) is the functional dependence of the chemisorption rate on surface coverage (6). The product N&,(e) is the concentration of sites available for adsorption after the reaction reaches a coverage (0). The net rate of formation of the precursor is equal to the rate of precursor for-
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
mation minus the rates of desorption
(dFWl,ld~h
=z$&@‘>-
623
and chemisorption:
kd[H2Olp-
kW,Ol.fcW
Since it has been assumed that (d[H20]n/dQnet mined from expression (4):
(4)
= 0, then [Ha01 n can be deter-
[HzOl,=ZS,f,(e)l[k,f,(e) +kil.
(9
The rate of chemisorption (or the measured rate of change of the oxygen Auger signal) is obtained by substituting the precursor concentration from expression (5) into expression (3): dWH1 ldt = ~$&WW,f&‘)/
tN,kfcW
+ kdl.
(6)
The sticking coefficient (that is conventionally used for describing the change in rate of adsorption with coverage) can be obtained directly from expression (6). The sticking coefficient for chemisorption of water, S,, is defined as the ratio of the rate of chemisorption, d[OH]/dt, and the rate of arrival of water from the gas phase,Z: d [OH] /dt s, = Z
Ncl &f,(e) =Spfp(Q
Ar,k,f,(8)
+ kd 3 ’
Before the kinetic data can be used to check the proposed model (represented by expressions (6) and (7)), explicit expressions for f,(e) and fde) must be determined. 4.1.2. Determination of functions fc(0)and f.,(e) An expression for f,(e) can be determined from consideration of the manner in which the surface sites are filled. The lack of long range order in the c(2 X 2) layer, as indicated by the LEED results, and the saturation value for coverage at 80% of that for a complete c(2 X 2) layer suggest that the chemisorbed species are immobile and that, at high coverages, more than two surface sites are blocked for every occupied site. (Positions directly above the iron atoms have been arbitrarily chosen as the adsorption sites for the present argument.) In the formation of a c(2 X 2) surface structure each occupied site excludes the four nearest neighbor sites for adsorption. It has been shown that a full close-packed c(2 X 2) layer cannot be obtained by randomly filling the sites with immobile species, fig. 3a [12]. The change in the fraction of available sites as a function of coverage, fde), in this case, is different from (1 - e), because each additional adsorbed species at a given coverage can block from 2 to 5 sites depending on the location of the adsorption site with respect to those sites that are already occupied. Analytical expressions for f,(O) were obtained from Monte Carlo simulations of the chemisorption reaction, using a method similar to that described by Ertl and Ktippers [ 131. For convenience the fractional coverage, 8, is defined relative to the close-
624
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
packed c(2 X 2) structure: 6 = 2na/Ntot .
(8)
n, is the number of adsorbed species and N tot is the total number of adsorption sites on the Fe(OO1) surface. The average number of surface sites blocked per adsorbed species (mean occupation number, $0)) is defined as follows: C(0) = n&r,
= 2nr,/BNt0,.
(9)
nb is the total number of blocked sites at a coverage 13.u ranges from 5 at low values of 8 to 2 as saturation of the c(2 X 2) layer is approached. The fraction of available sites as a function of coverage is, therefore, given by expression (10): f,(e) = 1 - tQ,/&
= 1 - @c(o).
(10)
The mean occupation number, E, as a function of coverage was determined by randomly filling a 100 X 100 square array. To simulate the formation of a c(2 X 2) structure, only those positions that had empty next-nearest neighbors were allowed
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Fig. 3. (a) Model of proposed c(2 X 2) surface structure showing site exclusion, antiphase domains and incomplete filling of sites. (b) Structure formed by Monte Carlo simulation of chemisorption of water vapor. Only fffled sites are shown.
D.J. Dwyer et al. / Initinl reaction of water vapor with Fe(OO1) surface
625
1
I 0.2
2L 0
I
I 0.4
I
I 0.6
I
I 0.6
I 1.0
8 Fig. 4. Mean occupation number, g versus coverage as determined from Monte Carlo simulation of adsorption process.
to be filled. A typical surface structure resulting from this Monte Carlo simulation is shown in fig. 3b. The lack of long range order and the incomplete filling of the c(2 X 2) sites are natural consequences of the random processes of adsorption and of the exclusion of next-nearest neighbor sites, and are demonstrated by the simulations. The average value of saturation coverage determined from the simulations was 8 = 0.76 + 0.02, and is consistent with the observed value of 0 = 0.8 for the water reaction. The values of u for seven separate simulations are plotted as a function of coverage in fig. 4. Since ii versus 0 is approximately linear, an analytical expression for F(0) was obtained from a linear regression analysis of the data: Tj(e)= 5.0 - 3.11 8.
(11)
The function f,(0) given by expression (10) can now be written as follows: f,(e)= i.o- 2set
1.55 e2,
(12)
or, f,(e)= (1 - 1.25 ej2.
(13)
Note that f,(e) approaches zero as the coverage approaches the saturation value of 0.8. The rate of chemisorption (which can be defined here in terms of the rate of change in coverage) is obtained by substitution of the expression for f,(e) into expression (6): (14)
626
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
Now, f,,(0) may be readily evaluated after considering the temperature dependence of the bracketed term in expression (14). Assuming that desorption of the precursor and the chemisorption step are thermally activated, the temperature dependence of the reaction is determined by the temperature dependence of the rate constants k, and kd. Since kd = kb exp(-Ed/kT)
and
k, = kk exp(-E,/kT),
the term kd/No k, in expression (14) is defined below: h/kc
= (khlk’,) exp [(EC - &)/kT]
.
(15)
The increase in reaction rate with temperature for the interaction of water with iron suggests that E, > Ed. The fact that the initial rate of reaction, dtI/dt, at 473 K is constant for coverages up to approximately 0.5, suggests that kd/N,-,k, is much less than unity at high temperatures. The constant initial rate of reaction at 473 K, furthermore, indicates that the&(e) term must be approximately unity. Hence, the precursor is able to adsorb on both the bare surface and on the surface covered with chemisorbed species. 4.1.3. Analysis of kinetic data in teems of model Taking fp(0) s 1, expression (14) is then integrated to give the change in coverage of the chemisorbed species as a function of time, expression (18): 0 < 8 < 0.8.
et(&o)(~lLY.25e=Zf’
(16)
This expression was used to analyze the kinetic data shown in fig. 1. S, was first calculated from the linear portion of the rate data taken at 473 K, and a value of 0.56 f 0.03 was found. S, was assumed not to be strongly dependent upon specimen temperature. Values of (l/l .25 No) (k d /k c) were then determined so as to provide the best fit of expression (16) to the kinetic data for each of the experimental temperatures ?. These results are summarized in table 1. The oxygen coverage as a function of exposure was calculated from expression (16) for the value of S, and each of the values of (kd/k,)*. These results are plotted as a solid line along with the experimental data in fig. 1. Since the analysis provides an adequate tit for expression (16) to the kinetic data, the assumption that S, is independent of temperature can be considered as a reasonable approximation. 4.1.4. Difference in activation energies EC - Ed The value for the difference in activation energies E, - Ed was obtained the temperature dependence of (kd/kC)*: ln(kd/k,)*
= [(E, - Ed)/kl (l/T) + ln(khlkk)*.
t Henceforth, the term (l/l 25 No) (I&c)
is referred to as @d/kc)*.
from (17)
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
627
Table 1 Experimental values for (/cd/kc) * Temperature
#d/kc)*
D
298
0.5
329 413 473
0.19 0.05 0.01
kO.1 to.02 to.0 1 rO.O1
Q estimated 90% confidence limit: (kd/kC)* = (l/No X 1.25) kd/k,.
A plot of ln(k&,)* versus (l/T) is given in fig. 5, and a value of 5.7 f 0.5 kcal/mol for E, - Ed was determined from a linear regression analysis of this data. 4.1.5. Sticking coefficient versus coverage The sticking coefficient for chemisorption as a function of coverage can be obtained by setting fp(0) 2 1 and substituting expression (13) for f,(O) into expression (7): & =s,
N&,(1
- 1.25 8)‘/[&&(1
- 1.258)* +k,j].
(18)
The change in adsorption rate as a function of the coverage at each of the experimental temperatures can be conveniently summarized by plots of sticking coefficient , S,, versus coverage. The value for S, and the values for (k&)* were used in expression (18) to obtain these graphical representations, and the results are shown in fig. 6. The shape of the curve at each temperature reflects the relative rates of desorption and dissociation of the precursor. At high temperatures the rate of dissociation is much faster than the rate of desorption, and the sticking coefficient for
- I .o + 72 y
-2o-
\ a? c -
-3.0
-
2.0
2.3
2.6
2.9
3.2
:
Fig. 5. Arrhenius plot of the ratio of rate constant for desorption and the rate constant for dissociation of the precursor, (k,j/kC)*.
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
628
0.0 0.0
I
I
0.2
I
I
I
I
0.4 FRACTIONAL
0.6
0.8
1.0
COVERAGE,Q
Fig. 6. Calculated sticking coefficient, S, as a function of coverage for the values of kdkd obtained at each of the experimental temperatures (a) 298 K, (b) 329 K, (c) 413 K, and (d) 473 K. The bars represent estimates of the errors in the calculated values of Sc from the propagation of the errors in the experimental values of S, and kdk,.
chemisorption, in this case, is determined essentially by the sticking coefficient for precursor formation. At low temperatures the rates of desorption and dissociation of the precursor are comparable. Hence, the initial sticking coefficient for chemisorption is smaller at low temperatures. Dissociation of the precursor becomes more rate controlling at lower temperatures, and since this rate is proportional to the concentration of available adsorption sites, SC decreases as a function of coverage . 4.2. Model for the precursor state Further physical insight into the mechanism for the water reaction is obtained from theoretical consideration of the value of the sticking coefficient S,. According to the theory of absolute reaction rates, the value of the sticking coefficient is determined by the loss of degrees of freedom during adsorption [ 141. For a general case the sticking coefficient S is given by the following expression:
[J%Ol,[~of@N f*
s=z h
P/(2mnkT)‘i2
(19)
FTexp
Since P = [Hz01 &kT, expression (19) can be written as follows: (2nmkT)L’2
S=
(20)
h
fT0) is considered
as a general case of adsorption.
Fg and f’ are the partition
func-
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
629
Table 2 Calculated sticking coefficient for different types of adsorbed layers Type of layer
Degrees of freedom lost
Maximum sticking coefficient at 298 K
Immobile
3 translation ’ 3 rotation d+ 1 translation, 3 rotation 1 translation, rotation about x-axis f 1 translation, totation about y-axis f 1 translation, rotation about z-axisf 1 translation
6.9 x lo-’
Mobile Mobile Mobile Mobile Mobile
[ 1,2]
2.0 x 10-2 0.22 0.28 0.38 1.0
a Maximum sticking coefficient for h’ = 0 in expression (20). b No = 0.61 X lots sites/cm* (number of sites in a Fe(001) c(2 X 2) structure). c Translational partition function (d2z/h)“, n degree of translational freedom. d Rotational partition function (6/Z) (8n2/,k~/~r2)1~2(8n21,kr/h2)1~2(8~2~~~~/h2)1~2. eI,=2.94X1040gcm2;ly=1.02X1040gcm2;Iz=1.92X1040gcm2. tions of the gas phase and the activated complex respectively. When the adsorbed species are immobile, the sticking coefficient is a function of concentration of surface sites, [Nofle)]. For the case of mobile adsorbed phase, on the other hand, the sticking coefficient is independent of the concentration of surface sites, and the _[Nef(e)] term would therefore not appear in expressions (19) and (20). The calculated sticking coefficients for adsorption into different types of layers at room temperature are summarized in table 2. The calculated upper limit for sticking coefficient for the adsorption of water directly from the gas phase into an immobile layer is 6.9 X lo-’ as compared to a value of 0.56 found experimentally. This difference indicates that the chemisorption step must be preceded by physical adsorption of water (precursor) which retains most of its degrees of freedom. The measured sticking coefficient of the precursor, S, = 0.56 f 0.03, is higher than any of the calculated values for mobile layers with losses of rotational freedom. Hence, the precursor must be essentially a freely rotating and mobile water molecule. The fact that the experimental value for S, is less than unity suggests a partial loss of rotational freedom (assuming that systematic errors in the determination of S,, such as differences in the measured pressure and true pressure at the specimen, were small).
4.3. Surface structure The observed changes in LEED patterns as a function of water vapor exposure are consistent with the proposed model for the reaction of water vapor with clean
630
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
WOOI) surface. The short range order of the c(2 X 2) structure results from the random filling of sites, the immobility of the adsorbed species, and the exclusion of nearest neighbor sites by filled sites. The diffuse diffraction features in the LEED patterns, fig. 2a, indicate that the surface structure consists of c(2 X 2) domains with an average size smaller than the coherence width of the incident electron beam, which has been estimated to be on the order of 120 to 300 A [ 151. The average domain width formed in the Monte Carlo simulations of adsorption, fig. 3, is less than 20 8. Two methods were used to ascertain whether the width of the “half order” reflections relative to the integral order reflections is consistent with the degree of disorder simulated by the Monte Carlo experiment. An optical technique and a computer method were used to obtain Fourier transforms of the model surface structure. Optical Fourier transforms of the structure shown in fig. 3b were obtained by procedures described elsewhere [ 16,17]. The 50 X 50 array, representing an area well within the coherence width of the electron beam, was photographically reduced to produce a negative with a spot separation of approximately 6pm. The optical Fourier transform of this grating was produced with a He-Ne laser light, and the result is shown in fig. 2b. The size of the “half order” reflections in the optical transform are comparable to that observed in the LEED patterns. A more rigorous evaluation of the electron diffraction line broadening that would be expected from the proposed disordered c(2 X 2) surface structure was made by computing the Fourier transform. The intensity of the diffracted waves from a two-~mensional array of scattering centers is given by the following expression 1131: I(zr,za)=
i Cfn,m
n,m
cos2n(nzr
+mzz)f2+
[ Cf,,,,sin2n(nzr+mzz)]2. n,m
(21)
zr and 22 are coordinates in reciprocal space and fn,mis the scattering factor of the point in the array with coordinates n,m. Summations were carried out for all points in a 30 X 30 array of the simulated structure. A smaller array than that used in the optical method was chosen to reduce computation time. The scattering factor, f n,m,was set equal to one for points in the array that were tilled and.&, was set equal to zero for points that were unoccupied. The distribution of intensities in two dimensional reciprocal space was obtained by drawing contour lines between points of equal intensity, Z(.zr, z2), and the results are shown in fig. 3b. The widths of the half order reflections are in good agreement with those observed in the LEED patterns. The differences in phases of electrons scattered from subdomains in the c(2 X 2) structure give rise to the diffuse diffraction features observed in the LEED patterns. The diffraction reflections that are affected by disorder in the adsorbed layers depends upon the symmetry of the adsorption sites. The Fe(001) surface has two types of four-fold symmetric sites, one directfy above the iron atoms and the other above the interstitial “holes” between the iron atoms. In addition, the Fe(001) iron
D.J. Dwyer et al. /Initial reaction
of mter vapor with Fei~~l~ surface
631
surface has two-fold symmetric sites between pairs of iron atoms. When c(2 X 2) subdomains form by adsorption on the four-fold symmetric sites, the phase differences of electrons scattered from these sites produces a broadening of the beams that satisfy the following condition [ 181: 2h + k = (2n f 1)/2,
n = integer.
(22)
Of the possible reflections kk = m, /2, m2/2 (ml, m2 both odd or both even) only the “extra” (i.e. fractional order) spots are broadened. The integral order spots are unaffected. If, on the other hand, the subdomains of the c(2 X 2) layer consist of adsorbed species on the two-fold symmetric sites, the diffraction beams affected by the disorder would satisfy the following conditions [ 18 ] : h + 2k = (2n + 1)/2,
(23)
3h+k=2n+l,
(24)
h+k=2n+l.
(25) In this case, therefore, all of the diffraction beams are affected by the disorder. Since only the fractional order beams for the c(2 X 2) layer formed by the water reaction are broadened, the surface sites with four-fold symmetry are occupied by the adsorbed species. Unfortunately, it is not possible, with this analysis, to distinguish between adsorption on the two different adsorption sites that have four-fold symmetry. The four-fold symmetric sites directly above the iron atoms were chosen as adsorption sites in the Monte Carlo simulations described earlier. The Fourier transforms from the simulated c(2 X 2f structures further indicated that the integral reflections are not affected by the disorder in the layer when adsorption occurs on the four-fold symmetric sites.
5. Summary The initial stages for the reaction of water vapor with single crystal Fe(OOI) surface for temperatures between 298 and 473 K and for exposure to 1O3Torr set was studied by Auger electron spectroscopy and low energy electron diffraction. A model was proposed for this reaction that successfully accounts for the measured sticking coefficient, reaction kinetics (including temperature dependence), and surface structure. The irreversible dissociation (chemisorption) of water takes place via a precursor of physically adsorbed water molecules. Adsorbed water in the precursor state is mobile and retains most of its degrees of rotational freedom. The precursor is adsorbed on both the bare surface and on the surface covered by chemisorbed species. Water molecules in the precursor state can either desorb or dissociate, and the difference in activation energies between these two reactions is 5.7 + 0.5 kcal/mol.
632
D.J. Dwyer et al. /Initial
reaction
of water vapor with Fe(OO1) surface
Dissociation with formation of either chemisorbed hydroxyl group or oxygen takes place randomly on the four-fold symmetric sites either directly above the iron atoms or above the interstitial “holes” between iron atoms. The chemisorbed species are immobile, and each chemisorbed species blocks the four nearest-neighbor sites for further chemisorption. Only 80% of the total available sites are filled as a result of the site exclusion and immobility of the chemisorbed species. The chemisorbed layer effectively passivates the iron surface toward further reaction with water vapor for exposures as high as 1O3 Torr sec.
Acknowledgement Support of this work by the Office of Naval Research, under Contract N0001475-C-0543, NR 036-097, is gratefully acknowledged.
References ]l] R. Suhrmann, J.M. Heras, L. Viscid0 De Heras and G. Wedler, Ber. Bunsenges. Physil. Chem. 72 (1968) 855. [2] D. Lazarov and G, Bliznakov, Z. Physik. Chem. 233 (1966) 255. [3] G.M. Kornacheva, R.Kh. Burshtein and N.A. Shurmovskaya, Elektrokhimiya 9 (1973) [4] ?&rang and W.H. Wade, J. CoUoid Interface Sci. 34 (1970) 413. [5] K. Kishi and S. tkeda, Bull. Chem. Sot. Japan 46 (1973) 341. [6] G.W. Simmons and D.J. Dwyer, Surface Sci. 48 (197.5) 373. [ 71 M. Perdereau, Surface Sci. 24 (1971) 239. [S] J.W. Ridgway and D. Haneman, Surface Sci. 24 (1971) 451. (91 D.A. King and M.G. Wells, Proc. Roy. Sot. (London) A339 (1974) 245. [lo] P. Kisliuk, J. Phys. Chem. Solids 3 (1957) 95. [ 1l] G. Ehrlich, J. Phys. Chem. 59 (1955) 473. [12] I. Langmuir, J. Chem. Sot. (1940) 511. [ 131 G. ErtJ and J. Kiippers, Surface Sci. 21 (1970) 61. 1141 S. Glasstone, K.J. Laidler and H. Eyring, The Theory of Rate Processes (McGraw-HilI, New York, 1941) pp. 339-399. [15] R.L. Park, in: The Structure and Chemistry of Solid Surface, Ed. G.A. Somorjai (Wiley, New York, 1969) pp. 28-1,28-17. [ 161 B.D. Campbell and W.P. Ellis, Trans. Am. Cryst. Assoc. 4 (1968) 97. [ 171 D.G. Fedak, T.E. Fischer and W.D. Robertson, J. Appl. Phys. 39 (1969) 5658. 1181 P.J. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1.
A STUDY OF THE INITIAL REACTION OF WATER VAPOR WITH Fe(001) SURFACE
Daniel J. DWYER and G.W. SIMMONS Center for Surface and CoatingsResearch, Lehigh University,Bethlehem, Pennsylvanti 18015, USA
R.P. WE1 Department ofMechanical Engineering and Mechanics, Lehigh University,Bethlehem, Pennsylvania18015. USA Received 18 November 1976
LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(OO1) and water vapor at temperatures from 298 to 473 K. A disordered c(2 X 2) structure was formed at all temperatures, and only 80% of the total number of sites were filled at saturation. The initial sticking coefficient was 0.56 f 0.03, and the reaction rate increased with increasing temperature. A model was proposed that successfully accounted for these experimental observations. Irreversible chemisorption of water is proposed to take place via a precursor of physically adsorbed water molecules. The precursor, which is adsorbed on both bare surface and surface covered by chemisorbed species, is mobile and retains most of its degrees of rotational freedom. Water molecules in the precursor state can either desorb or dissociate, and the difference in activation energies for these reactions was found to be 5.7 f 0.5 kcal/mol. Only 80% of the available c(2 X 2) sites are filed and the surface layer is disordered because the chemisorbed species are immobile, and because each one blocks four nearest neighbor sites for further adsorption. The chemisorbed species occupy the fourfold symmetric sites either above the iron atoms or above the interstitial “holes” between iron atoms.
1. Introduction
The development of detailed mechanism for reaction of water and water vapor with iron is of fundamental importance to the understanding of corrosion and passivation of iron and steels, and of the enhancement of subcritical crack growth in low alloy steels by environments containing water. Experimental methods for investigating reactions in aqueous environments are limited to electrochemical and ellipsometric techniques, and these methods have been used extensively to characterize the corrosion and passivation behavior of iron and steel. Further details of 617
618
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
the reaction of water with iron and steel surfaces, however, can be derived from studies of water vapor using a number of other experimental techniques. Only a few of these studies have been reported thus far. The reaction of water vapor with evaporated iron films has been studied through electrical resistivity, work function, and photoelectric measurements [l-3]. The relative amounts of reversibly and irreversibly adsorbed water on reduced iron powder have been measured by a volume-manometric method [3]. Adsorption of water vapor on evaporated iron fnms and initial oxidation have been examined by a quartz microbalance method [4]. Studies of the hydroxylation and oxidation of evaporated iron films in water vapor by X-ray photoelectron spectroscopy have also been reported [5]. Fundamental mechanisms for the water/metal reactions have been proposed on the basis of these studies [ l-51. There is, however, considerable uncertainty regarding these proposed mechanisms because, in many cases, iron surfaces of unknown structure and composition were used. Experiments with better defined surfaces are therefore needed to confirm and to refine the mechanisms derived from these earlier studies. The objective of this investigation is to further develop the mechanisms for iron-water surface reaction from measurements of the reaction kinetics and from determinations of changes in surface structure. A clean Fe(OO1) surface was selected for study. Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) techniques were used in the present studies in conjunction with ultra-high vacuum. A model for the gas-metal surface reaction was developed to explain both the observed kinetic data and changes in surface structure.
2. Experimental The instrumentation, as well as many of the experimental procedures used in this study, have been described already in connection with previous studies of the reaction of oxygen with Fe(OO1) surface [6]. Only those additional experimental details relating to the studies of reactions with water vapor are described here. The rates of water vapor adsorption on Fe(OO1) surface were measured by monitoring the oxygen Auger electron intensity as a function of exposure (pressure X time) and temperature. Corresponding changes in the surface structure were followed by LEED. Exposures were made for pressures of lop7 to 10V5 Torr and at temperatures from room temperature to 2OO’C. The specimen was heated by radiation from a heated tungsten filament, and the temperature was measured by an iron-constantan thermocouple spot welded to the back surface of the specimen. Water vapor was admitted to the system through a variable leak from a reservoir of triply distilled water (conductivity of 10 -6 a-’ cm). The water was degassed by alternately freezing and melting, and by pumping on the reservoir when the water was frozen. The adsorption of water vapor was found to be inhibited by the presence of sur-
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
619
face impurities. Procedures described in ref. [6] were therefore used to minimize possible contamination of the specimen surface by impurities (specifically, sulfur, chlorine, carbon and nitrogen). Because of apparent interactions of the adsorbate with the electron beam, reproducible results could be obtained only when exposures were made with the accelerating voltage for the electron beam switched off. In addition, it was essential to re-evacuate the system to pressures near the background value (
3. Results The oxygen coverage on clean Fe(OO1) following exposures to water vapor at 298, 329, 413 and 473 K are shown as a function of exposure in fig. 1, where coverage is defined as the fraction of the available surface sites that are occupied. For the c(2 X 2) surface structure that is formed, it is assumed that there is one site for every two surface iron atoms. Since the reaction was found to be irreversible, it is assumed that the oxygen Auger electron signal originated from chemically adsorbed hydroxyl group or from chemically adsorbed oxygen. Molecular water
620
D.J. Dwyer et al. f Initial reaction of water vapor with Fe(OO1) surface
0.8 -
(4
*+*
;
0.6 -
+
lb)
m
Y
d w 0.2
-
B 0 A 0.0
a (cl E 0.8 -
(d) $-WC=== .A
Ox)0
I
I
I
234501234s
I
I
I
I
I
I
I
I
J
EXPOSURE ( Langmuirs 1 Fig. 1. Chemisorption of water vapor as a function of exposure at (a) 298 K, (b) 329 K, (c) 413 K, and (d) 473 K. (1 Langmuir = 10” Torr set).
is assumed to make little contribution to the oxygen signal, since water either dissociates or is removed from the surface upon re-evacuation of the system. The absence of significant changes in the low-energy iron Auger electron peaks furthermore suggests that bulk oxide phase did not form at these temperatures and exposures [6 1. The reaction approached completion for each temperature at a surface coverage (based on the experimental calibration of the oxygen Auger signal) equivalent to 80% of the total available c(2 X 2) adsorption sites. No appreciable increase in oxygen coverage beyond this level was observed at any of the temperatures, for water vapor exposures up to lo3 Torr sec. The effect of the initial chemisorption on further reaction of the surface with water vapor differs from the results for reaction of oxygen with clean Fe(OO1) [6]. In the latter case, oxidation of the surface was found to follow immediately after the formation of the c(2 X 2) oxygen layer, and to form several layers of oxide for oxygen exposures as low as 10V4 Torr sec. The initial reaction rate of water vapor with iron is relatively fast. A sticking coefficient of 0.56 f 0.03 was calculated from the linear part of the kinetic data at 473 K. The reaction rate increases with temperature, and the initial portion of the
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
621
b Fig. 2. (a) LEED patterns of Fe(001) surface after exposure to water vapor at 298 K. (b) Optical Fourier transform and (c) calculated Fourier transform of proposed surface structure shown in fii. 3.
coverage versus exposure curves becomes more linear with increasing temperature. The LEED patterns that were observed as a function of exposure at each temperature showed the development of a c(2 X 2) surface structure. The relatively large and diffuse “half order” reflections from the overlayer (as shown in fig. 2a) suggest that there is relatively little long range order in the surface structure. Similar diffuse diffraction features, however, were not found around any of the integral order reflections.
4. Discussion In the following sections an adsorption model that is consistent with both the kinetic and electron diffraction data is developed. A model for the reaction of water vapor with Fe(OO1) at these temperatures and exposures must account for the following: (a) the shape of coverage versus exposure curves at the different temperatures, (b) the saturation value of coverage equal to 80% of that for a complete c(2 X 2) structure, and (c) the lack of long range order in the c(2 X 2) structure.
4.1. Kinetic model 4.1.1. Chemisorption via a precursor A model based on the formation of a molecularly adsorbed precursor, which subsequently either desorbs or decomposes, was found to give a satisfactory explanation for the requirements listed previously. Since kinetic models of chemisorption based on a precursor step have been derived and discussed in detail elsewhere [9-l 11, only the essential features of this model used to describe the reaction of water with iron are presented.
622
D.J. Dwyer et al. /Initial reaction of water vapor with FefOOI) surface
The dissociation reaction detected by the AES and LEED techniques to take place according to the following reaction:
is assumed
H 0
H
P201 g =[H20],+-Fe-Fe-T--je--Le--, kd or 0
[H201g~IH~Olp + - Fe - Fe - -----+-Fe-Fe+H2. k, Since no evidence of bulk oxide was found, the oxygen exists on the surface as chemisorbed oxygen or as a chemisorbed hydroxyl group. The ex~rimental measurements reflect only the rate of change in surface concentration of one of these chemisorbed species. It was assumed that the precursor rapidly reaches a steady state concentration and that this concentration is small. This ~sumption is necessary to eliminate the precursor concentration (a quantity that cannot be readily measured) from the expression for the rate of chemisorption. It is further assumed that the precursor is physically adsorbed and that the adsorption of the precursor is non-activated. Hydrogen from the decomposition is assumed either to desorb or to have suf~cient mobility that it does not block sites for water dissociation. The rate of formation of the precursor and its rate of desorption are as follows: d fH,O],/dt -d[H20]n/dt
= Z&Jr,(@), = k&-120]p.
(1) (2)
Z is the arrival rate from the gas phase and is equal to P/d-, where P is the vapor pressure of water. S, is the sticking probab~ity for adsorption of the precursor, and its value is determined by the degrees of freedom lost by the precursor during its formation. f,(0) is the functional dependence of the rate of precursor adsorption on coverage of the surface by chemically adsorbed species, 0. According to the proposed surface reaction, the rate of chemisorption (dissociation) is given below: d[OH]/dt
= k,[H201p%f,(~).
(3)
Ne is the total number of sites per unit area initially available; and f,(Q) is the functional dependence of the chemisorption rate on surface coverage (6). The product N&,(e) is the concentration of sites available for adsorption after the reaction reaches a coverage (0). The net rate of formation of the precursor is equal to the rate of precursor for-
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
mation minus the rates of desorption
(dFWl,ld~h
=z$&@‘>-
623
and chemisorption:
kd[H2Olp-
kW,Ol.fcW
Since it has been assumed that (d[H20]n/dQnet mined from expression (4):
(4)
= 0, then [Ha01 n can be deter-
[HzOl,=ZS,f,(e)l[k,f,(e) +kil.
(9
The rate of chemisorption (or the measured rate of change of the oxygen Auger signal) is obtained by substituting the precursor concentration from expression (5) into expression (3): dWH1 ldt = ~$&WW,f&‘)/
tN,kfcW
+ kdl.
(6)
The sticking coefficient (that is conventionally used for describing the change in rate of adsorption with coverage) can be obtained directly from expression (6). The sticking coefficient for chemisorption of water, S,, is defined as the ratio of the rate of chemisorption, d[OH]/dt, and the rate of arrival of water from the gas phase,Z: d [OH] /dt s, = Z
Ncl &f,(e) =Spfp(Q
Ar,k,f,(8)
+ kd 3 ’
Before the kinetic data can be used to check the proposed model (represented by expressions (6) and (7)), explicit expressions for f,(e) and fde) must be determined. 4.1.2. Determination of functions fc(0)and f.,(e) An expression for f,(e) can be determined from consideration of the manner in which the surface sites are filled. The lack of long range order in the c(2 X 2) layer, as indicated by the LEED results, and the saturation value for coverage at 80% of that for a complete c(2 X 2) layer suggest that the chemisorbed species are immobile and that, at high coverages, more than two surface sites are blocked for every occupied site. (Positions directly above the iron atoms have been arbitrarily chosen as the adsorption sites for the present argument.) In the formation of a c(2 X 2) surface structure each occupied site excludes the four nearest neighbor sites for adsorption. It has been shown that a full close-packed c(2 X 2) layer cannot be obtained by randomly filling the sites with immobile species, fig. 3a [12]. The change in the fraction of available sites as a function of coverage, fde), in this case, is different from (1 - e), because each additional adsorbed species at a given coverage can block from 2 to 5 sites depending on the location of the adsorption site with respect to those sites that are already occupied. Analytical expressions for f,(O) were obtained from Monte Carlo simulations of the chemisorption reaction, using a method similar to that described by Ertl and Ktippers [ 131. For convenience the fractional coverage, 8, is defined relative to the close-
624
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
packed c(2 X 2) structure: 6 = 2na/Ntot .
(8)
n, is the number of adsorbed species and N tot is the total number of adsorption sites on the Fe(OO1) surface. The average number of surface sites blocked per adsorbed species (mean occupation number, $0)) is defined as follows: C(0) = n&r,
= 2nr,/BNt0,.
(9)
nb is the total number of blocked sites at a coverage 13.u ranges from 5 at low values of 8 to 2 as saturation of the c(2 X 2) layer is approached. The fraction of available sites as a function of coverage is, therefore, given by expression (10): f,(e) = 1 - tQ,/&
= 1 - @c(o).
(10)
The mean occupation number, E, as a function of coverage was determined by randomly filling a 100 X 100 square array. To simulate the formation of a c(2 X 2) structure, only those positions that had empty next-nearest neighbors were allowed
@FILLED SITES 0 UNFILLED SITES . EXCLUDED SITES
......::. :::.. :.:.:::: .,:, .: .,I...‘.: .
. . . . . . . .,..
.
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.:.I
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.
..
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.:
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.:.:. :
::::
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.,...,.
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:. ,...,.’
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::::::.
.
.
:::::.::::‘.‘. :
:.:.:
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:.. ,,,:::.:.,.
. .., 1.1.: .:.‘.‘.:.:.:.‘....,.,...,
. ..:.,
. . . . . .. . . . . .
.
..‘.‘..~....~.:~:~.,.~,~.~.‘.~. .,..
:;
..;::::,:::
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. .
...
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.... ,. ,.,
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.‘.‘.‘,‘. .:
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1::b
Fig. 3. (a) Model of proposed c(2 X 2) surface structure showing site exclusion, antiphase domains and incomplete filling of sites. (b) Structure formed by Monte Carlo simulation of chemisorption of water vapor. Only fffled sites are shown.
D.J. Dwyer et al. / Initinl reaction of water vapor with Fe(OO1) surface
625
1
I 0.2
2L 0
I
I 0.4
I
I 0.6
I
I 0.6
I 1.0
8 Fig. 4. Mean occupation number, g versus coverage as determined from Monte Carlo simulation of adsorption process.
to be filled. A typical surface structure resulting from this Monte Carlo simulation is shown in fig. 3b. The lack of long range order and the incomplete filling of the c(2 X 2) sites are natural consequences of the random processes of adsorption and of the exclusion of next-nearest neighbor sites, and are demonstrated by the simulations. The average value of saturation coverage determined from the simulations was 8 = 0.76 + 0.02, and is consistent with the observed value of 0 = 0.8 for the water reaction. The values of u for seven separate simulations are plotted as a function of coverage in fig. 4. Since ii versus 0 is approximately linear, an analytical expression for F(0) was obtained from a linear regression analysis of the data: Tj(e)= 5.0 - 3.11 8.
(11)
The function f,(0) given by expression (10) can now be written as follows: f,(e)= i.o- 2set
1.55 e2,
(12)
or, f,(e)= (1 - 1.25 ej2.
(13)
Note that f,(e) approaches zero as the coverage approaches the saturation value of 0.8. The rate of chemisorption (which can be defined here in terms of the rate of change in coverage) is obtained by substitution of the expression for f,(e) into expression (6): (14)
626
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
Now, f,,(0) may be readily evaluated after considering the temperature dependence of the bracketed term in expression (14). Assuming that desorption of the precursor and the chemisorption step are thermally activated, the temperature dependence of the reaction is determined by the temperature dependence of the rate constants k, and kd. Since kd = kb exp(-Ed/kT)
and
k, = kk exp(-E,/kT),
the term kd/No k, in expression (14) is defined below: h/kc
= (khlk’,) exp [(EC - &)/kT]
.
(15)
The increase in reaction rate with temperature for the interaction of water with iron suggests that E, > Ed. The fact that the initial rate of reaction, dtI/dt, at 473 K is constant for coverages up to approximately 0.5, suggests that kd/N,-,k, is much less than unity at high temperatures. The constant initial rate of reaction at 473 K, furthermore, indicates that the&(e) term must be approximately unity. Hence, the precursor is able to adsorb on both the bare surface and on the surface covered with chemisorbed species. 4.1.3. Analysis of kinetic data in teems of model Taking fp(0) s 1, expression (14) is then integrated to give the change in coverage of the chemisorbed species as a function of time, expression (18): 0 < 8 < 0.8.
et(&o)(~lLY.25e=Zf’
(16)
This expression was used to analyze the kinetic data shown in fig. 1. S, was first calculated from the linear portion of the rate data taken at 473 K, and a value of 0.56 f 0.03 was found. S, was assumed not to be strongly dependent upon specimen temperature. Values of (l/l .25 No) (k d /k c) were then determined so as to provide the best fit of expression (16) to the kinetic data for each of the experimental temperatures ?. These results are summarized in table 1. The oxygen coverage as a function of exposure was calculated from expression (16) for the value of S, and each of the values of (kd/k,)*. These results are plotted as a solid line along with the experimental data in fig. 1. Since the analysis provides an adequate tit for expression (16) to the kinetic data, the assumption that S, is independent of temperature can be considered as a reasonable approximation. 4.1.4. Difference in activation energies EC - Ed The value for the difference in activation energies E, - Ed was obtained the temperature dependence of (kd/kC)*: ln(kd/k,)*
= [(E, - Ed)/kl (l/T) + ln(khlkk)*.
t Henceforth, the term (l/l 25 No) (I&c)
is referred to as @d/kc)*.
from (17)
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
627
Table 1 Experimental values for (/cd/kc) * Temperature
#d/kc)*
D
298
0.5
329 413 473
0.19 0.05 0.01
kO.1 to.02 to.0 1 rO.O1
Q estimated 90% confidence limit: (kd/kC)* = (l/No X 1.25) kd/k,.
A plot of ln(k&,)* versus (l/T) is given in fig. 5, and a value of 5.7 f 0.5 kcal/mol for E, - Ed was determined from a linear regression analysis of this data. 4.1.5. Sticking coefficient versus coverage The sticking coefficient for chemisorption as a function of coverage can be obtained by setting fp(0) 2 1 and substituting expression (13) for f,(O) into expression (7): & =s,
N&,(1
- 1.25 8)‘/[&&(1
- 1.258)* +k,j].
(18)
The change in adsorption rate as a function of the coverage at each of the experimental temperatures can be conveniently summarized by plots of sticking coefficient , S,, versus coverage. The value for S, and the values for (k&)* were used in expression (18) to obtain these graphical representations, and the results are shown in fig. 6. The shape of the curve at each temperature reflects the relative rates of desorption and dissociation of the precursor. At high temperatures the rate of dissociation is much faster than the rate of desorption, and the sticking coefficient for
- I .o + 72 y
-2o-
\ a? c -
-3.0
-
2.0
2.3
2.6
2.9
3.2
:
Fig. 5. Arrhenius plot of the ratio of rate constant for desorption and the rate constant for dissociation of the precursor, (k,j/kC)*.
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
628
0.0 0.0
I
I
0.2
I
I
I
I
0.4 FRACTIONAL
0.6
0.8
1.0
COVERAGE,Q
Fig. 6. Calculated sticking coefficient, S, as a function of coverage for the values of kdkd obtained at each of the experimental temperatures (a) 298 K, (b) 329 K, (c) 413 K, and (d) 473 K. The bars represent estimates of the errors in the calculated values of Sc from the propagation of the errors in the experimental values of S, and kdk,.
chemisorption, in this case, is determined essentially by the sticking coefficient for precursor formation. At low temperatures the rates of desorption and dissociation of the precursor are comparable. Hence, the initial sticking coefficient for chemisorption is smaller at low temperatures. Dissociation of the precursor becomes more rate controlling at lower temperatures, and since this rate is proportional to the concentration of available adsorption sites, SC decreases as a function of coverage . 4.2. Model for the precursor state Further physical insight into the mechanism for the water reaction is obtained from theoretical consideration of the value of the sticking coefficient S,. According to the theory of absolute reaction rates, the value of the sticking coefficient is determined by the loss of degrees of freedom during adsorption [ 141. For a general case the sticking coefficient S is given by the following expression:
[J%Ol,[~of@N f*
s=z h
P/(2mnkT)‘i2
(19)
FTexp
Since P = [Hz01 &kT, expression (19) can be written as follows: (2nmkT)L’2
S=
(20)
h
fT0) is considered
as a general case of adsorption.
Fg and f’ are the partition
func-
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
629
Table 2 Calculated sticking coefficient for different types of adsorbed layers Type of layer
Degrees of freedom lost
Maximum sticking coefficient at 298 K
Immobile
3 translation ’ 3 rotation d+ 1 translation, 3 rotation 1 translation, rotation about x-axis f 1 translation, totation about y-axis f 1 translation, rotation about z-axisf 1 translation
6.9 x lo-’
Mobile Mobile Mobile Mobile Mobile
[ 1,2]
2.0 x 10-2 0.22 0.28 0.38 1.0
a Maximum sticking coefficient for h’ = 0 in expression (20). b No = 0.61 X lots sites/cm* (number of sites in a Fe(001) c(2 X 2) structure). c Translational partition function (d2z/h)“, n degree of translational freedom. d Rotational partition function (6/Z) (8n2/,k~/~r2)1~2(8n21,kr/h2)1~2(8~2~~~~/h2)1~2. eI,=2.94X1040gcm2;ly=1.02X1040gcm2;Iz=1.92X1040gcm2. tions of the gas phase and the activated complex respectively. When the adsorbed species are immobile, the sticking coefficient is a function of concentration of surface sites, [Nofle)]. For the case of mobile adsorbed phase, on the other hand, the sticking coefficient is independent of the concentration of surface sites, and the _[Nef(e)] term would therefore not appear in expressions (19) and (20). The calculated sticking coefficients for adsorption into different types of layers at room temperature are summarized in table 2. The calculated upper limit for sticking coefficient for the adsorption of water directly from the gas phase into an immobile layer is 6.9 X lo-’ as compared to a value of 0.56 found experimentally. This difference indicates that the chemisorption step must be preceded by physical adsorption of water (precursor) which retains most of its degrees of freedom. The measured sticking coefficient of the precursor, S, = 0.56 f 0.03, is higher than any of the calculated values for mobile layers with losses of rotational freedom. Hence, the precursor must be essentially a freely rotating and mobile water molecule. The fact that the experimental value for S, is less than unity suggests a partial loss of rotational freedom (assuming that systematic errors in the determination of S,, such as differences in the measured pressure and true pressure at the specimen, were small).
4.3. Surface structure The observed changes in LEED patterns as a function of water vapor exposure are consistent with the proposed model for the reaction of water vapor with clean
630
D.J. Dwyer et al. /Initial reaction of water vapor with Fe(OO1) surface
WOOI) surface. The short range order of the c(2 X 2) structure results from the random filling of sites, the immobility of the adsorbed species, and the exclusion of nearest neighbor sites by filled sites. The diffuse diffraction features in the LEED patterns, fig. 2a, indicate that the surface structure consists of c(2 X 2) domains with an average size smaller than the coherence width of the incident electron beam, which has been estimated to be on the order of 120 to 300 A [ 151. The average domain width formed in the Monte Carlo simulations of adsorption, fig. 3, is less than 20 8. Two methods were used to ascertain whether the width of the “half order” reflections relative to the integral order reflections is consistent with the degree of disorder simulated by the Monte Carlo experiment. An optical technique and a computer method were used to obtain Fourier transforms of the model surface structure. Optical Fourier transforms of the structure shown in fig. 3b were obtained by procedures described elsewhere [ 16,17]. The 50 X 50 array, representing an area well within the coherence width of the electron beam, was photographically reduced to produce a negative with a spot separation of approximately 6pm. The optical Fourier transform of this grating was produced with a He-Ne laser light, and the result is shown in fig. 2b. The size of the “half order” reflections in the optical transform are comparable to that observed in the LEED patterns. A more rigorous evaluation of the electron diffraction line broadening that would be expected from the proposed disordered c(2 X 2) surface structure was made by computing the Fourier transform. The intensity of the diffracted waves from a two-~mensional array of scattering centers is given by the following expression 1131: I(zr,za)=
i Cfn,m
n,m
cos2n(nzr
+mzz)f2+
[ Cf,,,,sin2n(nzr+mzz)]2. n,m
(21)
zr and 22 are coordinates in reciprocal space and fn,mis the scattering factor of the point in the array with coordinates n,m. Summations were carried out for all points in a 30 X 30 array of the simulated structure. A smaller array than that used in the optical method was chosen to reduce computation time. The scattering factor, f n,m,was set equal to one for points in the array that were tilled and.&, was set equal to zero for points that were unoccupied. The distribution of intensities in two dimensional reciprocal space was obtained by drawing contour lines between points of equal intensity, Z(.zr, z2), and the results are shown in fig. 3b. The widths of the half order reflections are in good agreement with those observed in the LEED patterns. The differences in phases of electrons scattered from subdomains in the c(2 X 2) structure give rise to the diffuse diffraction features observed in the LEED patterns. The diffraction reflections that are affected by disorder in the adsorbed layers depends upon the symmetry of the adsorption sites. The Fe(001) surface has two types of four-fold symmetric sites, one directfy above the iron atoms and the other above the interstitial “holes” between the iron atoms. In addition, the Fe(001) iron
D.J. Dwyer et al. /Initial reaction
of mter vapor with Fei~~l~ surface
631
surface has two-fold symmetric sites between pairs of iron atoms. When c(2 X 2) subdomains form by adsorption on the four-fold symmetric sites, the phase differences of electrons scattered from these sites produces a broadening of the beams that satisfy the following condition [ 181: 2h + k = (2n f 1)/2,
n = integer.
(22)
Of the possible reflections kk = m, /2, m2/2 (ml, m2 both odd or both even) only the “extra” (i.e. fractional order) spots are broadened. The integral order spots are unaffected. If, on the other hand, the subdomains of the c(2 X 2) layer consist of adsorbed species on the two-fold symmetric sites, the diffraction beams affected by the disorder would satisfy the following conditions [ 18 ] : h + 2k = (2n + 1)/2,
(23)
3h+k=2n+l,
(24)
h+k=2n+l.
(25) In this case, therefore, all of the diffraction beams are affected by the disorder. Since only the fractional order beams for the c(2 X 2) layer formed by the water reaction are broadened, the surface sites with four-fold symmetry are occupied by the adsorbed species. Unfortunately, it is not possible, with this analysis, to distinguish between adsorption on the two different adsorption sites that have four-fold symmetry. The four-fold symmetric sites directly above the iron atoms were chosen as adsorption sites in the Monte Carlo simulations described earlier. The Fourier transforms from the simulated c(2 X 2f structures further indicated that the integral reflections are not affected by the disorder in the layer when adsorption occurs on the four-fold symmetric sites.
5. Summary The initial stages for the reaction of water vapor with single crystal Fe(OOI) surface for temperatures between 298 and 473 K and for exposure to 1O3Torr set was studied by Auger electron spectroscopy and low energy electron diffraction. A model was proposed for this reaction that successfully accounts for the measured sticking coefficient, reaction kinetics (including temperature dependence), and surface structure. The irreversible dissociation (chemisorption) of water takes place via a precursor of physically adsorbed water molecules. Adsorbed water in the precursor state is mobile and retains most of its degrees of rotational freedom. The precursor is adsorbed on both the bare surface and on the surface covered by chemisorbed species. Water molecules in the precursor state can either desorb or dissociate, and the difference in activation energies between these two reactions is 5.7 + 0.5 kcal/mol.
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D.J. Dwyer et al. /Initial
reaction
of water vapor with Fe(OO1) surface
Dissociation with formation of either chemisorbed hydroxyl group or oxygen takes place randomly on the four-fold symmetric sites either directly above the iron atoms or above the interstitial “holes” between iron atoms. The chemisorbed species are immobile, and each chemisorbed species blocks the four nearest-neighbor sites for further chemisorption. Only 80% of the total available sites are filled as a result of the site exclusion and immobility of the chemisorbed species. The chemisorbed layer effectively passivates the iron surface toward further reaction with water vapor for exposures as high as 1O3 Torr sec.
Acknowledgement Support of this work by the Office of Naval Research, under Contract N0001475-C-0543, NR 036-097, is gratefully acknowledged.
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