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Adsorption and dissociation of oxygen molecules on palladium (110) surfaces at low temperatures
155
Surface Science 217 (1989) 155-166 North-Holland, Amsterdam
ADSORPTION AND DISSOCIATION OF OXYGEN MOLECULES ON PALLADIUM (110) SURFACES AT LOW TEMPERATURES Tatsuo MATSUSHIMA Research Institute for Catalysis, Hokkaido
University, Sapporo 060, Japan
Received 9 December 1988; accepted for publication 3 February 1989
The adsorption and dissociation of oxygen molecules were studied on palladium (110) surfaces at low temperatures by using thermal desorption combined with isotope tracer and low energy electron diffraction. Around 110 K, part of the admolecules dissociates and yields sites for molecular adsorption. The degree of both dissociations during exposure at 110 K and subsequent thermal desorption were estimated. New molecular adsorption was found on surfaces covered with a c(2 X 4)-O lattice.
1. Introduction Oxygen is adsorbed molecularly and dissociatively on noble metal surfaces around 100 K. The dissociation energy of the O-O bond is significantly reduced on metal surfaces from that of the double bond in free 0, [1,2]. The degree of dissociation of admolecules depends strongly on the structure of the surface plane [3,4]. This molecule may play a role as a probe in examining the surface reactivity. However, this degree of dissociation was determined on only a few metals [3,5]. In simple thermal desorption, dissociation of admolecules during heating procedures cannot be excluded. The titration of adatoms by CO [5] is likely to be obscured by a sharp angular distribution of CO, desorption flwc and high pumping rate of cooled portions of the sample holder. We previously proposed thermal desorption combined with isotope tracer which allows a differentiation between both dissociations [3]. Dissociation of oxygen admolecules was not detected on Pd(ll1) around 100 K [3,6], whereas a large fraction of admolecules was converted into the atomic form on Pd(lOO) [7,8]. On Pd(llO), Nishijima et al. have concluded the dissociation at 100 K through EELS measurements [9]. They have also suggested the presence of admolecules at this temperature. Recently, Norton et al. have confirmed the presence of admolecules at 100 K, using thermal desorption combined with isotope tracer [lo]. In this paper we will report the degree of dissociation at the adsorption temperature as a function of coverage and new molecular adsorption found 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
0, on Pd(l IO)at low temperatures
156
possible on surfaces covered by a c(2 X 4)-O lattice. The degree of dissociation was estimated through the difference of the fraction of isotope oxygen between O,(a) and O(a) when the surface was sequentially exposed to 1802 and 1602.
2. Experimental The experimental apparatus and procedures were reported previously [3]. The apparatus consisted of a reaction chamber with LEED-AES, a collimator, and an analysis chamber. Thermal desorption spectra were recorded with a mass spectrometer in the reaction chamber. The signal (angle-integrated) involves the contributions from the sides of the crystal as well as well-defined faces. In the isotope experiments, the mass spectrometer was controlled by a microcomputer to monitor the change in the signals due to 160,, 160180 and 180, simultaneously versus either time or temperature. Another mass spectrometer in the analysis chamber was used to confirm that the above desorption signal was truly due to the sample surface. The crystal was a disk-shaped slice (10 mm diameter X 0.9 mm thickness). It was cut with spark erosion from a rod supplied by Metal crystal Ltd UK. Both faces were polished with standard metallographic techniques. The sample was mounted on a specimen manipulator via spot-welding it to Ta wires used to heat the crystal. It was cleaned by prolonged Ar+ bombardment in the range of room temperature to 1200 K, and heated in 1 x 10e8 Torr oxygen around 800 K. After the crystal was flashed in vacua up to 1373 K, no impurity was detected with AES. AES is insensitive to carbon on Pd, since the signal is overlapped by that of the substrate. Finally, the sample was exposed to oxygen at about 200 K and heated to remove trace surface carbon which would desorb as CO. This procedure was repeated until no further CO was produced. The LEED pattern showed a sharp (1 X 1) structure at this stage. The temperature was monitored by using a chromel-alumel thermocouple spot-welded to the side of the crystal.
3. Results 3. I. Desorption
spectra
and exposure
Thermal desorption of oxygen after the adsorption on clean surfaces at 110 K shows peaks at 130 K (a) and above 800 K (/3), as shown in fig. la. As shown below, a is due to the desorption from the molecular adsorption state, and /3 to the recombination of oxygen adatoms [lo]. The desorption peak areas were plotted against the exposure in fig. lb. The coverage, do, is the value relative to the maximum total peak area. Below 0.4 L (1 Langmuir
157
(a) Pd(llO), IIOK
T/lOOK
Fig. 1. (a) Resorption spectra of oxygn after exposure to 0.37 and 0.74 t “%& at 110 K. The beating rate was about 4 K/s below 300 K and 8 K/s beyond this temperature_(b) Desorptian yield from the mokc&r and atom& adsorptionstates. These are representedas valuesrekive to tbe maximumtotal peak area.
ulIit=1x10-
6 Ton - s), the EYpeak was negligible. It increased above 0.5 L and became saturated around 1 L. On the other band the /3 peak increased linearly with increasing exposure below 0.6 L, and was nearjy saturated around 0.8 L. This peak attained about 75% of the total amount of oxygen adsorbed. Above this exposure, both peaks increased very slowly. The maximum amauat of atomic oxygen around 1.0 L has recently been confirmed to
158
0, on Pd(1 IO) at low temperatures
be half a monolayer [ll]. The amount in our experiments was probably underestimated somewhat due to a portion of adatoms being subject to removal as CO, in the presence of background CO. Desorption after a few Langmuir units of 0, exposure at room temperature yields an additional peak around 700 K. As this is due to the desorption of oxygen absorbed beneath the surface [lo], it is not discussed in this work. 3.2. Isotope studies In order to examine dissociation at the adsorption temperature, the isotope distribution in (Y-and p-0, was monitored when the surface was sequentially exposed to ‘*Oz and 1602. Typical desorption spectra of isotopic oxygen were
L
16 02
lxlo-‘2E
10s
Atomic
I
I.1
I
1.5
I
2.0
I
Alll
2.5
”
5
6
7
I
I
8
9
1
T/lOOK Fig. 2. Thermal
desorption The heating
spectra after exposure to 0.3 L 180z followed by 1.3 L 160, rate was increased from about 4 K/s to 8 K/s at 300 K.
at 110 K.
0, on Pd(ll0)
at lowtemperatures
159
reproduced in fig. 2. The surface was first exposed to 0.3 L **02 at 110 K and then to 1.3 L 1602. ~0, involves only 160, and ‘*02 and not 160180. This indicates that it is directly desorbed from the molecular adsorption state [lo]. On the other hand p-0, is due to the recombination of adatoms, since a random distribution of the isotope in 160,, 160180 and 1802 was confirmed. The larger atomic fraction of ‘*O in p-0, than that in (~-0~ suggests that the dissociation takes place at 110 K prior to any treatment [3]. Pre-dosed oxygen would be promoted to the atomic state when the degree of dissociation is higher at lower coverages. On the other hand, the “0 fraction would be equal in each state, when the dissociation occurs only during heating procedures with equal probability for 1802(a) and 1602(a) [3]. Furthermore, when the surface is covered to saturation during the sequential exposure, the IgO fraction would increase linearly with increasing “0 pre-coverage. For this examination, the total exposure was kept at 1.6 L. The results over the range of pre-coverages are summarized in fig. 3a. The dashed line indicates the ideal dissociation. The pre-coverage of 180 (8,) was calculated from the relation of the total amount of adsorbed oxygen versus exposure in fig. lb. The “0 fraction was determined from the desorption peak areas. The fraction in the atomic state is consistently above the dashed line, and that in the molecular state, consistently below. The difference in the “0 fraction between (Y-and p-0, will be discussed later to estimate the degree of dissociation at the adsorption temperature. 3.3. Effect of pre-adsorbed
oxygen
The desorption spectra from the molecular adsorption state depended strongly on the amount of pre-adsorbed atomic oxygen and also on the heat treatment temperature. The desorption was clearly split into two peaks at 130 K ((or) and 165 K ((Y& when the pre-adsorbed oxygen layer was heated above 350 K. The effect of this is represented in fig. 4a. The surface was first exposed to 0.9 L 0, at 110 K and heated to a desired temperature, TP. It was cooled again to 110 K and exposed to 0.2 L 0,. No significant change was seen in the desorption spectra below TP = 350 K. Above this heat treatment temperature, the (or peak shifted somewhat to higher temperatures and increased with TP. It was accompanied with the (Y* peak. This peak increased rapidly with an increase in TP. Even without this heat treatment, a small amount of (Y*is seen as a high temperature tail. At small post-dosages of O,, only the (Ye peak appeared with TP = 520 K. The (pi peak height exceeds (Yeat higher exposures. These spectra for TP = 520 K are quite similar to those from Pd(ll1) surfaces without pre-dosed oxygen [3]. During heat treatment of surfaces, the amount of adsorbed oxygen was kept constant. This large annealing effect must be caused by a structure change in the adsorption layer of oxygen adatoms.
160
0, on Pd(I 10) at low temperatures
I
(a)
aL ‘*OZ + bL 1602, IIOK (o+b = 1.6)
I .C 80,
relative
“0
precoveroge
Fig. 3. (a) The atomic fraction of “0 in the low and high temperature desorption peak. The surface was heated after exposure, at 110 K, to various amounts of “0, followed by saturation with 1602. (b) Degree of dissociation, X and Y, during pre-exposure and post-exposure. The amount of dissociated oxygen during the pre-exposure, eoX, is also shown.
This effect became significant when the pre-exposure of oxygen was more than 0.3 L. The effect on the desorption spectra due to the amount of oxygen for Tp = 520 K is shown in fig. 5. Both q and (Y* increased sharply above 0.3 L, which yielded an absolute adatom coverage (0,) of 0.3. The saturation point of oxygen adatoms was previously found to be at 8, = 0.5 [ll]. The structure of the oxygen lattice changed from (2 X 3)-1D to c(2 x 4) above this exposure. The results suggest again that the structure of the oxygen adlayer plays an important role in producing new adsorption states.
161
0, on Pd(ll0) at low temperatures
Al
I
7
0.9L 02 -+Tp,
0.2OL IIOK
L200 -0 I IO
~
02
-I
1
I
I
(b)
0.9L 02, I IOK
’ 150
200 T/K
-
400
600
813(1
Tp /K
Fig. 4. Effect of pre-adsorbed atomic oxygen on the thermal desorption from the molecular states. (a) The surface was first exposed to 0.9 L Oz at 110 K and heated to T,. It wasfurther exposed to 0.2 L Oz at 110 K. Curve (1) represents the desorption after exposure to 0.5 L. (b) Variation of the spot intensity of the LEED pattern of a c(2 X 4)-O and a quasi-stable structure with heat treatment temperature. The intensity was measured at 190 K. (c, d) Part of both LEED patterns and the accelerating voltage. (0) Substrate spot, and (0) superstructure spot.
For a discussion of the degree of dissociation, the transformation between ai and a2 was examined on surfaces covered fully by the c(2 X 4)-O lattice. The surface was first covered by oxygen 16 to saturation (0.9 L) and annealed to 520 K. After cooling to 110 K, it was exposed to 1802 and then to 1602. The subsequent thermal desorption showed that pre-dosed “02 was enriched in a2 and post-dosed i602 in al. Desorption traces of 1802 and 1602 are presented in fig. 6a. In this desorption the content of I80 in p-0, was less than 5%. The dissociation of oxygen on the oxygen-precovered surface was negligible. The heterogeneity of the isotope distribution became clear when 160, was dosed after the surface covered by 1802 was heated to 145 K, leaving only a*-0,. Typical spectra are reproduced in fig. 6b. This heterogeneity indicates that
162
0, on Pd(il0)
/
/
1
‘602 + 520K,
P E
al C 4
0.54L
‘80z/ I IOK
ai
I Q z??
at low lemperatures
a2
h
(Lf 0.45
‘602
-.-
iA
2 .P I--“(0
0.32
_-_-_---
0” m
0.36
.-_-_-._
I
I
I
I
I IO
150
200
250
T/K
Fig. 5. Effect of oxygen adatoms on desorption spectra from admolecules. The surface was first exposed to various amounts of 0, and annealed to 520 K. it was further exposed to 0.54 L ‘*02 at 110 K. The region of the LEED patterns observed after annealing to 520 K is also shown.
“Oz is first adsorbed on sites with higher binding energies, and the displacement hardly occurs with post-dosed 1602 during heating procedures. On the other hand, the heterogeneity in the isotope distribution mostly disappeared when the surface is heated to 200 K after pre-exposure to Oz (fig. 6~). The amount of a2-Oz is very small and the isotope distribution in al-O, is fairly homogeneous. Thus, no heterogeneous distribution is expected for 180*(a) and 160,(a) after seq uential exposures of clean surfaces to isotope oxygen at 110 K. 3.4, LEED studies Oxygen adatoms on Pd(l10) form a (2 x 3)-1D (or distorted (1 X 3) [5]) lattice and then a c(2 x 4) lattice with increasing coverage [12-141. Exposure to 0.9 L 0, is enough to saturate the surface and produce a c(2 X 4)-O lattice. This lattice does not appear until the heat treatment, after adsorption, is above 350 K [13-X5]. The intensity of LEED spots due to the ~(2 X 4)-O lattice was monitored by using a spot-photometer (fig. 4b), after the surface was sequentially heated to 670 K. The intensity was always recorded at 190 K, as it
0, on Pd(ll0)
at low temperatures
I
I
0.9L ‘60 2
+TP
163
I
0.13~ “02 +0.40!?02/‘l~OK (a)
Tp -520K
(b) 520K
145 /I
[cl
200K
I
I
I (0
150
I
200 T/K
Fig. 6. Transformations between oxygen admolecules. (a) The surface was first exposed to 0.9 L 0% and annealed to 520 K, to produce the c(2 X4)-0 lattice. It was sequentially exposed to 0.13 L “Oz and 0.4 L I64 at 110 K. (b) The surface was once heated to 145 K after l8O2 exposure in the above procedure, and further exposed to 0.4 L 1602. (c) The surface was annealed to 200 K, and then exposed to I6 Oz and I6 4.
maintained a nearly steady value below this temperature. A sharp increase in the intensity was found around TP= 400 K, where both 01~and 01~increased rapidly (fig. 4a). This struckre must yield new molecular adsorption sites. During this heating, another diffraction was observed below TP= 400 K. Extra spots were seen at (f l/2, + 5/6) and (0, k l/2) at an accelerating voltage of 42 eV. The intensity of the spot at (I/2,5/6) was recosded (fig. 4d). This structure appeared predominantly around TP= 300 K, and only in the
164
0, on Pd(I 10) at low temperatures
course of temperature increase. The (2 X 3)-1D lattice was observed at higher temperatures, Tp> 700 K, where oxygen begins to desorb, or at small exposures with Tp> 400K. These observations are in good agreement with work reported previously [14]. However, neither structure seems to contribute to the new sites for molecular adsorption. The amount of molecular adsorption increased rapidly above Tp= 400 K (fig. 4), and at levels of pre-dosed oxygen exceeding 0.3 L (fig. 5), where both of the structures ceased to appear. 4. Discussion We will discuss the degree of dissociation of oxygen admolecules during exposure at low temperatures and subsequent heating. The results of the isotope experiments shown in fig. 3a are analyzed in a way similar to that reported previously [3]. The pre-dosed isotope (180) is enriched in p-0,. This suggests that a portion of the oxygen dissociates at the adsorption temperature and the degree is higher at lower coverages. In fact the dissociation at 100 K has been confirmed by EELS measurements [9], as a significant signal due to the Pd-0 vibration was found on Pd(ll0) after 0, exposure without subsequent annealing. Isotope experiments indicate that oxygen adatoms can recombine only above 600 K. Further, the desorption and dissociation of admolecules are complete below 200 K. No reverse process is seen in the dissociation pathway below 600 K, i.e., -200 0, (a) i
-2002
O(a) -O,(g) (g) .
A simple approximation presents a means of estimating the degree of dissociation, X, Y, and 2, during 180, pre-exposure, 160, post-exposure and subsequent thermal desorption, i.e., an equal probability of the dissociation or the desorption of each admolecule is assumed, irrespective of the adsorption sequence. The pre-dosed isotope molecules adsorb preferentially on sites with higher binding energies, when the surface is heated above 350 K after 0, exposure. This heterogeneity disappears on unheated oxygen-covered surfaces, although a small fraction of a-0, is seen as CQ-O~in the high temperature tail (fig. 6~). The present surface is quite similar to this case, since a large fraction of admolecules is dissociated at the adsorption temperature. It is expected that ‘*02 remaining molecularly until the subsequent exposure has a probability of dissociation and desorption similar to that for post-dosed 1602. The ratio of 180, to 160, in a-0, equals that in the admolecules, which is the ratio of the amount of ‘a02 remaining molecularly to that of 1602 adsorbed. This ratio, A, is given as A = f?,(l - x)(1 - Y,)(l - Z,)/(l- e,)(l
- Y*)(l - z,),
(1)
0, on Pd(ll0)
at low temperatures
165
where 8, is the amount of ‘*02 pre-adsorbed. The total amount of ‘*02 and 1602 is kept at saturation. Yr and Y, are the degrees of dissociation of ‘*O,(a) and 1602(a) during 1602 exposure. Z, and Z, are similar degrees during subsequent thermal desorption. Thus, using the approximations, (1 - Y,)/(l - Y,) = 1, and (1 - Z,)/(l - Z,) 5: 1, the following equation for the degree of dissociation during ‘*01 pre-exposure can be derived. x= 1 -A(1
-6),)/e,.
(2)
The values of X calculated in this way are summarized in fig. 3b. The dissociation decreases almost linearly with increasing ti,, as expected. The right-hand side of eq. (2) yields the upper limit of X, when pre-dosed oxygen admolecules still have a probability of dissociation higher than that for post-dosed oxygen, i.e., Y, > Y,. On the other hand, it gives the lower limit when predosed admolecules are less dissociative, i.e., Y, < Y,. The value of X approaches 0.2-0.3 at high values of 0,. Yl and Y, should be less than these values, since the degree of dissociation is lower at higher coverages. Therefore, no significant error in the estimation of X is likely to be introduced by the above approximation, i.e., (1 - Y,)/(l - Y,) = 1. A similar situation is expected in the approximation for the Z value. The figure shows the amount of dissociated oxygen during ‘*O, exposure, 0,X. It approaches 0.35 at high values of 6,. The amount of dissociation during 160, exposure equals the difference between 0.35 and 0,X. The value of Y (= Y, = Y,) was thus estimated. It decreased from 0.35 to zero as shown in the figure. The value of Z can be estimated in a similar way: B = [0,X+
/](l -
e,(l - X)Yl + 0,(1 -X)(1
eo)y, + (I - eo)(I -
- Y,)Z,]
r,b~l v
(3)
where B is the ratio of ‘*O to 160 in p-0,. Considering Y -=z1, the following equation is obtained:
z,
or
z, = e,x/[ B(i - eo) - e,(i - x)] .
(4)
The value of Z (= Z, = Z,) was estimated to be 0.5 f 0.1 over the whole range of e,. From the foregoing results, it was concluded that (1) the degree of dissociation during exposure at 110 K is high and decreases linearly with increasing coverage, and (2) about one third of the admolecules at saturation dissociates during exposure. During subsequent heating procedures, half of the remnant desorbs from the molecular adsorption state and the other half dissociates. The validity of above treatment is limited by the approximations used for Yl and Y,, and also Z, and Z,. The approximation of (1 - Y,)/(l - Y,) = 1 and (1- Z,)(l - Z,) = 1 for the evaluation of X is justified when each oxygen admolecule has the same probability of dissociation during the post-exposure and subsequent heating procedures. This becomes plausible when the ad-
166
0, on Pd(l IO) at low temperatures
sorbed state of pre-dosed oxygen is affected by the post-adsorption and homogenized over all admolecules. On the other hand, the situation of Yi > Y, and 2, > Zz may be expected in the case that pre-dosed Oz adsorbs on sites with higher binding energies and higher dissociation probabilities. This is likely to occur in the present experiments. In such cases the values in fig. 3b are the upper limit of X. However, the deviation from the value estimated without approximation would not be significant as discussed above. For the values of Y and Z, the estimation is presumably obscured in more steps of approximation. Large scattering of Y values estimated for 8, > 0.5, and also of Z values, suggests that experimental errors become more serious than the approximation. Molecular oxygen is unstable on clean Pd(ll0) surfaces at 110 K, since the value of X is close to unity at small coverages. It is stabilized on surfaces covered by atomic oxygen and subsequently annealed above 400 K. This surface is covered by a c(2 x 4)-O lattice. A correlation was found between the amount of admolecules and the c(2 X 4)-O formation. It should be noticed that the presence of oxygen adatoms at saturation is not enough to stabilize the admolecule. The stabilization cannot be explained by a simple charge transfer model from the metal to the electronegative oxygen. The mechanism should be found in the structure of the c(2 x 4)-O lattice produced by high temperature annealing.
Acknowledgement
This work was supported in part by Grant-in-Aid from the Ministry of Education, No. 62540274.
for General Scientific
References [l] [2] [3] [4] [5]
J.L. Grand, Surface Sci. 93 (1980) 487; 95 (1980) 587. H. Steininger, S. LehwaId and H. Ibach, Surface Sci. 123 (1982) 1. T. Matsushima, Surface Sci. 157 (1985) 297, and references therein. C.T. Campbell, Surface Sci. 157 (1985) 43. J. Goschnick, M. Wolf, M. Grunze, W.N. Unertl, J.H. Block and J. Joboda-Cackovic, Surface Sci. 178 (1986) 831. [6] R. Imbihl and J.E. Demuth, Surface Sci. 173 (1986) 359. [7] C. Nyberg and C.G. TengstaI, Surface Sci. 126 (1983) 163. [8] E.M. Stuve, R.J. Madix and C.R. Brundle, Surface Sci. 146 (1984) 155. [9] M. Nishijima, M. Yo, Y. Kuwahara and M. On&i, Solid State Commun. 60 (1986) 257. [lo] J.W. He and P.R. Norton, Surface Sci. 204 (1988) 26. [ll] J.W. He, U. Memmert, K. Griffith% W.N. Lennard and P.R. Norton, Surface Sci. 202 (1988) L555. [12] G. Ertl and P. Rau, Surface Sci. 15 (1969) 443. [13] M. Jo, Y. Kuwahawa, M. On&i and M. Nishijima, Chem. Phys. Letters 131 (1986) 106. [14] J.E. Hulse, K. Wandelt, J. Ktippers and G. Ertl, in: Proc. 14th Intern. Conf. on Solid Surfaces and ECOSS-3, Cannes, France, 1980, Vol. 1, p. 108.
Surface Science 217 (1989) 155-166 North-Holland, Amsterdam
ADSORPTION AND DISSOCIATION OF OXYGEN MOLECULES ON PALLADIUM (110) SURFACES AT LOW TEMPERATURES Tatsuo MATSUSHIMA Research Institute for Catalysis, Hokkaido
University, Sapporo 060, Japan
Received 9 December 1988; accepted for publication 3 February 1989
The adsorption and dissociation of oxygen molecules were studied on palladium (110) surfaces at low temperatures by using thermal desorption combined with isotope tracer and low energy electron diffraction. Around 110 K, part of the admolecules dissociates and yields sites for molecular adsorption. The degree of both dissociations during exposure at 110 K and subsequent thermal desorption were estimated. New molecular adsorption was found on surfaces covered with a c(2 X 4)-O lattice.
1. Introduction Oxygen is adsorbed molecularly and dissociatively on noble metal surfaces around 100 K. The dissociation energy of the O-O bond is significantly reduced on metal surfaces from that of the double bond in free 0, [1,2]. The degree of dissociation of admolecules depends strongly on the structure of the surface plane [3,4]. This molecule may play a role as a probe in examining the surface reactivity. However, this degree of dissociation was determined on only a few metals [3,5]. In simple thermal desorption, dissociation of admolecules during heating procedures cannot be excluded. The titration of adatoms by CO [5] is likely to be obscured by a sharp angular distribution of CO, desorption flwc and high pumping rate of cooled portions of the sample holder. We previously proposed thermal desorption combined with isotope tracer which allows a differentiation between both dissociations [3]. Dissociation of oxygen admolecules was not detected on Pd(ll1) around 100 K [3,6], whereas a large fraction of admolecules was converted into the atomic form on Pd(lOO) [7,8]. On Pd(llO), Nishijima et al. have concluded the dissociation at 100 K through EELS measurements [9]. They have also suggested the presence of admolecules at this temperature. Recently, Norton et al. have confirmed the presence of admolecules at 100 K, using thermal desorption combined with isotope tracer [lo]. In this paper we will report the degree of dissociation at the adsorption temperature as a function of coverage and new molecular adsorption found 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
0, on Pd(l IO)at low temperatures
156
possible on surfaces covered by a c(2 X 4)-O lattice. The degree of dissociation was estimated through the difference of the fraction of isotope oxygen between O,(a) and O(a) when the surface was sequentially exposed to 1802 and 1602.
2. Experimental The experimental apparatus and procedures were reported previously [3]. The apparatus consisted of a reaction chamber with LEED-AES, a collimator, and an analysis chamber. Thermal desorption spectra were recorded with a mass spectrometer in the reaction chamber. The signal (angle-integrated) involves the contributions from the sides of the crystal as well as well-defined faces. In the isotope experiments, the mass spectrometer was controlled by a microcomputer to monitor the change in the signals due to 160,, 160180 and 180, simultaneously versus either time or temperature. Another mass spectrometer in the analysis chamber was used to confirm that the above desorption signal was truly due to the sample surface. The crystal was a disk-shaped slice (10 mm diameter X 0.9 mm thickness). It was cut with spark erosion from a rod supplied by Metal crystal Ltd UK. Both faces were polished with standard metallographic techniques. The sample was mounted on a specimen manipulator via spot-welding it to Ta wires used to heat the crystal. It was cleaned by prolonged Ar+ bombardment in the range of room temperature to 1200 K, and heated in 1 x 10e8 Torr oxygen around 800 K. After the crystal was flashed in vacua up to 1373 K, no impurity was detected with AES. AES is insensitive to carbon on Pd, since the signal is overlapped by that of the substrate. Finally, the sample was exposed to oxygen at about 200 K and heated to remove trace surface carbon which would desorb as CO. This procedure was repeated until no further CO was produced. The LEED pattern showed a sharp (1 X 1) structure at this stage. The temperature was monitored by using a chromel-alumel thermocouple spot-welded to the side of the crystal.
3. Results 3. I. Desorption
spectra
and exposure
Thermal desorption of oxygen after the adsorption on clean surfaces at 110 K shows peaks at 130 K (a) and above 800 K (/3), as shown in fig. la. As shown below, a is due to the desorption from the molecular adsorption state, and /3 to the recombination of oxygen adatoms [lo]. The desorption peak areas were plotted against the exposure in fig. lb. The coverage, do, is the value relative to the maximum total peak area. Below 0.4 L (1 Langmuir
157
(a) Pd(llO), IIOK
T/lOOK
Fig. 1. (a) Resorption spectra of oxygn after exposure to 0.37 and 0.74 t “%& at 110 K. The beating rate was about 4 K/s below 300 K and 8 K/s beyond this temperature_(b) Desorptian yield from the mokc&r and atom& adsorptionstates. These are representedas valuesrekive to tbe maximumtotal peak area.
ulIit=1x10-
6 Ton - s), the EYpeak was negligible. It increased above 0.5 L and became saturated around 1 L. On the other band the /3 peak increased linearly with increasing exposure below 0.6 L, and was nearjy saturated around 0.8 L. This peak attained about 75% of the total amount of oxygen adsorbed. Above this exposure, both peaks increased very slowly. The maximum amauat of atomic oxygen around 1.0 L has recently been confirmed to
158
0, on Pd(1 IO) at low temperatures
be half a monolayer [ll]. The amount in our experiments was probably underestimated somewhat due to a portion of adatoms being subject to removal as CO, in the presence of background CO. Desorption after a few Langmuir units of 0, exposure at room temperature yields an additional peak around 700 K. As this is due to the desorption of oxygen absorbed beneath the surface [lo], it is not discussed in this work. 3.2. Isotope studies In order to examine dissociation at the adsorption temperature, the isotope distribution in (Y-and p-0, was monitored when the surface was sequentially exposed to ‘*Oz and 1602. Typical desorption spectra of isotopic oxygen were
L
16 02
lxlo-‘2E
10s
Atomic
I
I.1
I
1.5
I
2.0
I
Alll
2.5
”
5
6
7
I
I
8
9
1
T/lOOK Fig. 2. Thermal
desorption The heating
spectra after exposure to 0.3 L 180z followed by 1.3 L 160, rate was increased from about 4 K/s to 8 K/s at 300 K.
at 110 K.
0, on Pd(ll0)
at lowtemperatures
159
reproduced in fig. 2. The surface was first exposed to 0.3 L **02 at 110 K and then to 1.3 L 1602. ~0, involves only 160, and ‘*02 and not 160180. This indicates that it is directly desorbed from the molecular adsorption state [lo]. On the other hand p-0, is due to the recombination of adatoms, since a random distribution of the isotope in 160,, 160180 and 1802 was confirmed. The larger atomic fraction of ‘*O in p-0, than that in (~-0~ suggests that the dissociation takes place at 110 K prior to any treatment [3]. Pre-dosed oxygen would be promoted to the atomic state when the degree of dissociation is higher at lower coverages. On the other hand, the “0 fraction would be equal in each state, when the dissociation occurs only during heating procedures with equal probability for 1802(a) and 1602(a) [3]. Furthermore, when the surface is covered to saturation during the sequential exposure, the IgO fraction would increase linearly with increasing “0 pre-coverage. For this examination, the total exposure was kept at 1.6 L. The results over the range of pre-coverages are summarized in fig. 3a. The dashed line indicates the ideal dissociation. The pre-coverage of 180 (8,) was calculated from the relation of the total amount of adsorbed oxygen versus exposure in fig. lb. The “0 fraction was determined from the desorption peak areas. The fraction in the atomic state is consistently above the dashed line, and that in the molecular state, consistently below. The difference in the “0 fraction between (Y-and p-0, will be discussed later to estimate the degree of dissociation at the adsorption temperature. 3.3. Effect of pre-adsorbed
oxygen
The desorption spectra from the molecular adsorption state depended strongly on the amount of pre-adsorbed atomic oxygen and also on the heat treatment temperature. The desorption was clearly split into two peaks at 130 K ((or) and 165 K ((Y& when the pre-adsorbed oxygen layer was heated above 350 K. The effect of this is represented in fig. 4a. The surface was first exposed to 0.9 L 0, at 110 K and heated to a desired temperature, TP. It was cooled again to 110 K and exposed to 0.2 L 0,. No significant change was seen in the desorption spectra below TP = 350 K. Above this heat treatment temperature, the (or peak shifted somewhat to higher temperatures and increased with TP. It was accompanied with the (Y* peak. This peak increased rapidly with an increase in TP. Even without this heat treatment, a small amount of (Y*is seen as a high temperature tail. At small post-dosages of O,, only the (Ye peak appeared with TP = 520 K. The (pi peak height exceeds (Yeat higher exposures. These spectra for TP = 520 K are quite similar to those from Pd(ll1) surfaces without pre-dosed oxygen [3]. During heat treatment of surfaces, the amount of adsorbed oxygen was kept constant. This large annealing effect must be caused by a structure change in the adsorption layer of oxygen adatoms.
160
0, on Pd(I 10) at low temperatures
I
(a)
aL ‘*OZ + bL 1602, IIOK (o+b = 1.6)
I .C 80,
relative
“0
precoveroge
Fig. 3. (a) The atomic fraction of “0 in the low and high temperature desorption peak. The surface was heated after exposure, at 110 K, to various amounts of “0, followed by saturation with 1602. (b) Degree of dissociation, X and Y, during pre-exposure and post-exposure. The amount of dissociated oxygen during the pre-exposure, eoX, is also shown.
This effect became significant when the pre-exposure of oxygen was more than 0.3 L. The effect on the desorption spectra due to the amount of oxygen for Tp = 520 K is shown in fig. 5. Both q and (Y* increased sharply above 0.3 L, which yielded an absolute adatom coverage (0,) of 0.3. The saturation point of oxygen adatoms was previously found to be at 8, = 0.5 [ll]. The structure of the oxygen lattice changed from (2 X 3)-1D to c(2 x 4) above this exposure. The results suggest again that the structure of the oxygen adlayer plays an important role in producing new adsorption states.
161
0, on Pd(ll0) at low temperatures
Al
I
7
0.9L 02 -+Tp,
0.2OL IIOK
L200 -0 I IO
~
02
-I
1
I
I
(b)
0.9L 02, I IOK
’ 150
200 T/K
-
400
600
813(1
Tp /K
Fig. 4. Effect of pre-adsorbed atomic oxygen on the thermal desorption from the molecular states. (a) The surface was first exposed to 0.9 L Oz at 110 K and heated to T,. It wasfurther exposed to 0.2 L Oz at 110 K. Curve (1) represents the desorption after exposure to 0.5 L. (b) Variation of the spot intensity of the LEED pattern of a c(2 X 4)-O and a quasi-stable structure with heat treatment temperature. The intensity was measured at 190 K. (c, d) Part of both LEED patterns and the accelerating voltage. (0) Substrate spot, and (0) superstructure spot.
For a discussion of the degree of dissociation, the transformation between ai and a2 was examined on surfaces covered fully by the c(2 X 4)-O lattice. The surface was first covered by oxygen 16 to saturation (0.9 L) and annealed to 520 K. After cooling to 110 K, it was exposed to 1802 and then to 1602. The subsequent thermal desorption showed that pre-dosed “02 was enriched in a2 and post-dosed i602 in al. Desorption traces of 1802 and 1602 are presented in fig. 6a. In this desorption the content of I80 in p-0, was less than 5%. The dissociation of oxygen on the oxygen-precovered surface was negligible. The heterogeneity of the isotope distribution became clear when 160, was dosed after the surface covered by 1802 was heated to 145 K, leaving only a*-0,. Typical spectra are reproduced in fig. 6b. This heterogeneity indicates that
162
0, on Pd(il0)
/
/
1
‘602 + 520K,
P E
al C 4
0.54L
‘80z/ I IOK
ai
I Q z??
at low lemperatures
a2
h
(Lf 0.45
‘602
-.-
iA
2 .P I--“(0
0.32
_-_-_---
0” m
0.36
.-_-_-._
I
I
I
I
I IO
150
200
250
T/K
Fig. 5. Effect of oxygen adatoms on desorption spectra from admolecules. The surface was first exposed to various amounts of 0, and annealed to 520 K. it was further exposed to 0.54 L ‘*02 at 110 K. The region of the LEED patterns observed after annealing to 520 K is also shown.
“Oz is first adsorbed on sites with higher binding energies, and the displacement hardly occurs with post-dosed 1602 during heating procedures. On the other hand, the heterogeneity in the isotope distribution mostly disappeared when the surface is heated to 200 K after pre-exposure to Oz (fig. 6~). The amount of a2-Oz is very small and the isotope distribution in al-O, is fairly homogeneous. Thus, no heterogeneous distribution is expected for 180*(a) and 160,(a) after seq uential exposures of clean surfaces to isotope oxygen at 110 K. 3.4, LEED studies Oxygen adatoms on Pd(l10) form a (2 x 3)-1D (or distorted (1 X 3) [5]) lattice and then a c(2 x 4) lattice with increasing coverage [12-141. Exposure to 0.9 L 0, is enough to saturate the surface and produce a c(2 X 4)-O lattice. This lattice does not appear until the heat treatment, after adsorption, is above 350 K [13-X5]. The intensity of LEED spots due to the ~(2 X 4)-O lattice was monitored by using a spot-photometer (fig. 4b), after the surface was sequentially heated to 670 K. The intensity was always recorded at 190 K, as it
0, on Pd(ll0)
at low temperatures
I
I
0.9L ‘60 2
+TP
163
I
0.13~ “02 +0.40!?02/‘l~OK (a)
Tp -520K
(b) 520K
145 /I
[cl
200K
I
I
I (0
150
I
200 T/K
Fig. 6. Transformations between oxygen admolecules. (a) The surface was first exposed to 0.9 L 0% and annealed to 520 K, to produce the c(2 X4)-0 lattice. It was sequentially exposed to 0.13 L “Oz and 0.4 L I64 at 110 K. (b) The surface was once heated to 145 K after l8O2 exposure in the above procedure, and further exposed to 0.4 L 1602. (c) The surface was annealed to 200 K, and then exposed to I6 Oz and I6 4.
maintained a nearly steady value below this temperature. A sharp increase in the intensity was found around TP= 400 K, where both 01~and 01~increased rapidly (fig. 4a). This struckre must yield new molecular adsorption sites. During this heating, another diffraction was observed below TP= 400 K. Extra spots were seen at (f l/2, + 5/6) and (0, k l/2) at an accelerating voltage of 42 eV. The intensity of the spot at (I/2,5/6) was recosded (fig. 4d). This structure appeared predominantly around TP= 300 K, and only in the
164
0, on Pd(I 10) at low temperatures
course of temperature increase. The (2 X 3)-1D lattice was observed at higher temperatures, Tp> 700 K, where oxygen begins to desorb, or at small exposures with Tp> 400K. These observations are in good agreement with work reported previously [14]. However, neither structure seems to contribute to the new sites for molecular adsorption. The amount of molecular adsorption increased rapidly above Tp= 400 K (fig. 4), and at levels of pre-dosed oxygen exceeding 0.3 L (fig. 5), where both of the structures ceased to appear. 4. Discussion We will discuss the degree of dissociation of oxygen admolecules during exposure at low temperatures and subsequent heating. The results of the isotope experiments shown in fig. 3a are analyzed in a way similar to that reported previously [3]. The pre-dosed isotope (180) is enriched in p-0,. This suggests that a portion of the oxygen dissociates at the adsorption temperature and the degree is higher at lower coverages. In fact the dissociation at 100 K has been confirmed by EELS measurements [9], as a significant signal due to the Pd-0 vibration was found on Pd(ll0) after 0, exposure without subsequent annealing. Isotope experiments indicate that oxygen adatoms can recombine only above 600 K. Further, the desorption and dissociation of admolecules are complete below 200 K. No reverse process is seen in the dissociation pathway below 600 K, i.e., -200 0, (a) i
-2002
O(a) -O,(g) (g) .
A simple approximation presents a means of estimating the degree of dissociation, X, Y, and 2, during 180, pre-exposure, 160, post-exposure and subsequent thermal desorption, i.e., an equal probability of the dissociation or the desorption of each admolecule is assumed, irrespective of the adsorption sequence. The pre-dosed isotope molecules adsorb preferentially on sites with higher binding energies, when the surface is heated above 350 K after 0, exposure. This heterogeneity disappears on unheated oxygen-covered surfaces, although a small fraction of a-0, is seen as CQ-O~in the high temperature tail (fig. 6~). The present surface is quite similar to this case, since a large fraction of admolecules is dissociated at the adsorption temperature. It is expected that ‘*02 remaining molecularly until the subsequent exposure has a probability of dissociation and desorption similar to that for post-dosed 1602. The ratio of 180, to 160, in a-0, equals that in the admolecules, which is the ratio of the amount of ‘a02 remaining molecularly to that of 1602 adsorbed. This ratio, A, is given as A = f?,(l - x)(1 - Y,)(l - Z,)/(l- e,)(l
- Y*)(l - z,),
(1)
0, on Pd(ll0)
at low temperatures
165
where 8, is the amount of ‘*02 pre-adsorbed. The total amount of ‘*02 and 1602 is kept at saturation. Yr and Y, are the degrees of dissociation of ‘*O,(a) and 1602(a) during 1602 exposure. Z, and Z, are similar degrees during subsequent thermal desorption. Thus, using the approximations, (1 - Y,)/(l - Y,) = 1, and (1 - Z,)/(l - Z,) 5: 1, the following equation for the degree of dissociation during ‘*01 pre-exposure can be derived. x= 1 -A(1
-6),)/e,.
(2)
The values of X calculated in this way are summarized in fig. 3b. The dissociation decreases almost linearly with increasing ti,, as expected. The right-hand side of eq. (2) yields the upper limit of X, when pre-dosed oxygen admolecules still have a probability of dissociation higher than that for post-dosed oxygen, i.e., Y, > Y,. On the other hand, it gives the lower limit when predosed admolecules are less dissociative, i.e., Y, < Y,. The value of X approaches 0.2-0.3 at high values of 0,. Yl and Y, should be less than these values, since the degree of dissociation is lower at higher coverages. Therefore, no significant error in the estimation of X is likely to be introduced by the above approximation, i.e., (1 - Y,)/(l - Y,) = 1. A similar situation is expected in the approximation for the Z value. The figure shows the amount of dissociated oxygen during ‘*O, exposure, 0,X. It approaches 0.35 at high values of 6,. The amount of dissociation during 160, exposure equals the difference between 0.35 and 0,X. The value of Y (= Y, = Y,) was thus estimated. It decreased from 0.35 to zero as shown in the figure. The value of Z can be estimated in a similar way: B = [0,X+
/](l -
e,(l - X)Yl + 0,(1 -X)(1
eo)y, + (I - eo)(I -
- Y,)Z,]
r,b~l v
(3)
where B is the ratio of ‘*O to 160 in p-0,. Considering Y -=z1, the following equation is obtained:
z,
or
z, = e,x/[ B(i - eo) - e,(i - x)] .
(4)
The value of Z (= Z, = Z,) was estimated to be 0.5 f 0.1 over the whole range of e,. From the foregoing results, it was concluded that (1) the degree of dissociation during exposure at 110 K is high and decreases linearly with increasing coverage, and (2) about one third of the admolecules at saturation dissociates during exposure. During subsequent heating procedures, half of the remnant desorbs from the molecular adsorption state and the other half dissociates. The validity of above treatment is limited by the approximations used for Yl and Y,, and also Z, and Z,. The approximation of (1 - Y,)/(l - Y,) = 1 and (1- Z,)(l - Z,) = 1 for the evaluation of X is justified when each oxygen admolecule has the same probability of dissociation during the post-exposure and subsequent heating procedures. This becomes plausible when the ad-
166
0, on Pd(l IO) at low temperatures
sorbed state of pre-dosed oxygen is affected by the post-adsorption and homogenized over all admolecules. On the other hand, the situation of Yi > Y, and 2, > Zz may be expected in the case that pre-dosed Oz adsorbs on sites with higher binding energies and higher dissociation probabilities. This is likely to occur in the present experiments. In such cases the values in fig. 3b are the upper limit of X. However, the deviation from the value estimated without approximation would not be significant as discussed above. For the values of Y and Z, the estimation is presumably obscured in more steps of approximation. Large scattering of Y values estimated for 8, > 0.5, and also of Z values, suggests that experimental errors become more serious than the approximation. Molecular oxygen is unstable on clean Pd(ll0) surfaces at 110 K, since the value of X is close to unity at small coverages. It is stabilized on surfaces covered by atomic oxygen and subsequently annealed above 400 K. This surface is covered by a c(2 x 4)-O lattice. A correlation was found between the amount of admolecules and the c(2 X 4)-O formation. It should be noticed that the presence of oxygen adatoms at saturation is not enough to stabilize the admolecule. The stabilization cannot be explained by a simple charge transfer model from the metal to the electronegative oxygen. The mechanism should be found in the structure of the c(2 x 4)-O lattice produced by high temperature annealing.
Acknowledgement
This work was supported in part by Grant-in-Aid from the Ministry of Education, No. 62540274.
for General Scientific
References [l] [2] [3] [4] [5]
J.L. Grand, Surface Sci. 93 (1980) 487; 95 (1980) 587. H. Steininger, S. LehwaId and H. Ibach, Surface Sci. 123 (1982) 1. T. Matsushima, Surface Sci. 157 (1985) 297, and references therein. C.T. Campbell, Surface Sci. 157 (1985) 43. J. Goschnick, M. Wolf, M. Grunze, W.N. Unertl, J.H. Block and J. Joboda-Cackovic, Surface Sci. 178 (1986) 831. [6] R. Imbihl and J.E. Demuth, Surface Sci. 173 (1986) 359. [7] C. Nyberg and C.G. TengstaI, Surface Sci. 126 (1983) 163. [8] E.M. Stuve, R.J. Madix and C.R. Brundle, Surface Sci. 146 (1984) 155. [9] M. Nishijima, M. Yo, Y. Kuwahara and M. On&i, Solid State Commun. 60 (1986) 257. [lo] J.W. He and P.R. Norton, Surface Sci. 204 (1988) 26. [ll] J.W. He, U. Memmert, K. Griffith% W.N. Lennard and P.R. Norton, Surface Sci. 202 (1988) L555. [12] G. Ertl and P. Rau, Surface Sci. 15 (1969) 443. [13] M. Jo, Y. Kuwahawa, M. On&i and M. Nishijima, Chem. Phys. Letters 131 (1986) 106. [14] J.E. Hulse, K. Wandelt, J. Ktippers and G. Ertl, in: Proc. 14th Intern. Conf. on Solid Surfaces and ECOSS-3, Cannes, France, 1980, Vol. 1, p. 108.