Thermal desorption spectroscopy of CO adsorption on epitaxial Cu(111)-Pd(111) thin film surfaces

Vacuum~volume41/numbers 1-3/pages 227 to 229/1990 Printed in Great Britain

0042-207X/9053.00+ .00 © 1990 Pergamon Press plc

Thermal desorption spectroscopy of CO adsorption on epitaxial Cu(1 1 1 )-Pd(11 1 ) thin film surfaces B Oral and R W Vook,

Laboratory for Sofid State Science and Technology, Physics Department, Syracuse,

NY 13244-1 130, USA

Thermal desorption spectroscopy experiments of CO adsorbates on vacuum deposited, epitaxial (111)Pd and (1 11)Cu-(11 1)Pd bilayer films were carried out under uhv conditions. Auger electron spectroscopy and LEED measurements showed that Cu grows epitaxially on (1 11)Pd films formed on mica in layer growth mode. CO adsorption on (11 1)Cu-Pd bilayers occurs in decreasing amounts as the Cu overlayer thickens. Beyond approximately two monolayers (ML) Cu, no CO adsorbs. The saturation CO coverages decrease linearly by approximately 25% for every 1/2 ML Cu deposited. The desorption energy corresponding to the peak of the main desorption line decreased as the Cu and/or CO coverages increased. A simple relationship between Cu coverage and the temperature at the peak of the main CO desorption line for different CO coverages was derived from the data. The results show that thin Cu overlayers on Pd significantly affect the adsorption and desorption characteristics of CO on these thin film bilayers.

Introduction

Results and discussion

CO adsorption on catalytic surfaces has been widely investigated I-3. It is also well known that transition metals adsorb CO and this makes them suitable for reactions such as methanation and oxidation 4'5. Recent studies have also shown that ultra-thin metal overlayers on another metal substrate can drastically alter the chemical reactivity of the substrate s u r f a c e 6'7. Often the metal overlayer in bulk form may not be active itself for that particular reaction, yet have a significant effect as a thin film deposited on another material surface 6. In this work, the adsorption of CO on a well defined epitaxial Pd(111) thin film surface and various bilayer Cu(lll)-Pd(111) surfaces was studied using the methods of Thermal Desorption Spectroscopy (TDS).

The TDS results show that the maximum saturation CO adsorption takes place on the P d ( l l l ) surface and that it decreases linearly by approximately 25~ for every 1/2 monolayer (ML) Cu deposited. 1 ML Cu(l 11) equals 2.09 A. Beyond approximately 2 ML Cu, no CO adsorbs. The results also show that CO adsorption on Pd(111) and Cu(l 11)-Pd(111) bilayers takes place in two steps: the high binding energy sites (f12) are filled first during early exposure and then one or more low energy sites are filled to saturation. The desorption energy corresponding to the peak of the main desorpti0n line decreased as the Cu and/or CO coverage increased. For a given sample, the peak temperature of the TDS curves decreased linearly with increasing CO coverage and did so more rapidly with increasing Cu coverage (Figure 1). A simple relationship between Cu coverage and the temperature at the peak of the main CO desorption line for different CO coverages was derived from the data. In Figure 1 the solid lines represent the Least Squares Analysis (LSA) fit to the data. The dashed lines were derived from the following analysis and are superimposed on the figure. The derivation of the dashed lines was carried out as follows. From Figure 1, the slopes of the solid lines a to d (in units of degree per Langmuir exposure), $, were plotted as a function of Cu thickness, hc° (Figure 2). Then a least squares quadratic relation was fitted to the points with h in ~,:

Experimental details The epitaxial Pd film was deposited by evaporation on an air cleaved mica surface at 250°C in uhv. The base pressure in the system was 2.5 x 10 -1° torr and rose to approximately 6 x 10 - 9 torr during deposition. The deposition rate 0.2 ]k s - ~. After the Pd film was formed, ultra-thin Cu layers were deposited at room temperature at a rate of 0.05 A s - i. The thicknesses of the films were measured with a quartz crystal microbalance having a ___0.1 A sensitivity. The surfaces were exposed to a CO partial pressure of 2 x 10 - 9 torr at 25°C. After the exposure, the sample was rotated to face the Quadrupole Mass Spectrometer (QMS) and then heated at a linear rate of 4°C s - 1. A shutter with a window was placed between the sample and the QMS in order to let CO molecules coming only from the center of the sample surface reach the QMS. The temperature of the surface was measured by a 2 mm diameter chromel-alumel thermocouple which was pasted on the front surface of the substrate using a dot of silver paint. The CO intensity ( A M U 28) was recorded on an X-Y recorder.

S(h) = - 5 . 4 5 5 - 8.855h - 5.725h 2.

(1)

Similarly a least squares quadratic for the intercepts of the solid lines a to d in Figure 1 at zero CO coverage, 7~° in °C, was derived for h in ~,:

T°(h) = 156.97 + 0.082h - 3.97h 2.

(2)

The experimental values for T O along with equation (2) are plotted in Figure 2. Finally, the peak temperature, Tp in °C, as a

227

B O r a l a n d R W Vook: TDS of CO adsorption on Cu-Pd surfaces

200 180 ,18o

F.--

120

i~c

100 8~_ n 1,0

,

V II e 0.0

a

I

I

I

1.0 2.0 3.0 CO EXPOSURE,L

=

I

~

4.0

I

5.0

Figure 1. Dependence of TDS peak temperature, Tp on CO exposure for various epitaxial Cu overlayer thicknesses: (a) Pd(lll); (b) I A CuPd(l 11); (c) 2 A Cu-Pd(l 11); (d) 3 ,~ Cu-Pd(! 11); (e) 4 ,~ Cu-Pd(111). The solid lines represent the least squares analysis fits to the data. The dashed lines represent equation (3) for the corresponding Cu thicknesses.

180 140

- - U ~ u ~ 9

~

U

~

100 =d h-

_J

60 at:

20

0

-40 -6o

-80

-1°°o

3

4

hcu, A

Figure 2. Slopes of the solid lines, S(h), intercepts of the solid lines, T°, from Figure 1, as function of Cu thicknesses,he,. The solid lines represent the least squares quadratic fit.

function of CO exposure, e, in Langmuir, is given by:

Tp = T°(h) + S(h)e.

From the above equations, one can predict that 0.1 L CO exposure at 2 x 1 0 - 9 torr on an ~6.2A, (2.5 ML) thick CuPd(111) surface at room temperature (28°C) would not show any TDS peak because the peak temperature itself is going to be 28°C. This result agrees with the work function measurements s which have shown that work function change saturates with deposition of 2.5 ML Cu on Pd(111) at room temperature. This example shows the utility of equations (1-3) in predicting CO adsorption behavior for different CO exposures and Cu overlayer thicknesses. AES and LEED studies have shown that Cu grows epitaxially on P d ( l l l ) with a layer growth model t. LEED patterns of Cu overlayers have shown that a (1 x 1) Cu superstructure forms on the P d ( l l l ) surface. These results indicated that Cu grows as monolayer high, two-dimensional islands until it completes the first monolayer. The second monolayer starts growing in the same two dimensional fashion until it is completed and so on. Our present results showed two important features. First, the shift of the desorption peak temperature to lower temperatures is linear with increasing CO coverages for Cu overlayer thicknesses of less than 2 ML (See Figure 1) Second, the rate at which the peak temperature decreases with increasing CO exposure, S(h), decreases linearly, but with different slopes, in the one and two monolayer region, Figure 3. We propose the following simple model to explain the results shown in Figure 3. During the growth of the first monolayer, Cu forms 1 ML thick islands on P d ( l l l ) in the form of patches. CO adsorption takes place on both Pd(111) and Cu(111 ) surfaces but with different ( ~ 50% less on Cu) coverages and desorption energies (as evidenced by the shift in the peak temperature). Under these circumstances, the overall CO desorption behavior appears to become a linear combination of CO desorption from the Cu islands and from the Cu-free Pd(111) surfaces. This desorption regime continues until the first Cu monolayer is complete. CO desorption behaviour follows essentially the same course during the growth of the second monolayer. However, in this case the underlayer is no longer Pd(l 11) but 1 ML of Cu(l 11) on Pd(111), and the amount of CO that adsorbs on 2 ML high Cu islands is almost zero. Therefore, the slope of the curve in Figure 3 changes when the second layer starts growing. Again the linearity of this decrease suggests that the results are due to a linear combination of the adsorption/desorption phenomena which are dependent only on the relative Cu coverages of the surface.

(3)

hcu,

Equation (3) is plotted as dashed lines in Figure 1. All five lines intersect or almost intersect at a point in the negative s region. The slopes and intercepts of the solid lines and the dashed lines are compared in Table 1.

00

1

2

3

4

-40 (D

Table 1. Slopes and intercepts of solid (measured) and dashed (calculated) lines in Figure 1 hcu A

Measured

0 1 2 3 4

156.5 154.5 139.8 122.0 --

228

T°, °C Calculated 156.97 153.08 141.25 121.48 93.78

S, °C L - 1 Measured Calculated - 4.6 - 22.6 -43.5 - 84.4 -

- 5.45 - 20.04 -46.07 - 83.5 - 132.47

u) -80

-120 0

I

0.5

I

1.0 hcu, ML

I

1.5

2.0

Figure 3. Change of S(h) with increasing Cu thickness on Pd(111). [] are the measured slopes and O is the calculated slope as given in Table 1.

B Oral and R W Vook: TDS of CO adsorption on Cu-Pd surfaces

Acknowledgement T h e a u t h o r s would like to t h a n k the U S Dept of Energy for financial s u p p o r t on g r a n t N o D E - F G 0 2 - 8 4 E R 4 5 1 3 9 .

References i B Oral and R W Vook, J Vac Sci Technol, In press (1990). 2T Engel and G Ertl, Adv Catal, 12, 1 (1979).

3 K A Trush and J M White, Appl Surface Sci, 24, 108 (1985). 4 G A Somorjai, Chemistrv in Two Dimensions, Surfaces, p 526. Corn¢ll Univ Press, Ithaca, NY (1981). 5 I M Campbell, Catalysis at Surfaces, p 117. Chapman and Hall, London (1988). 6 j W A Sachtler, J P Biberian and G A Somorjai, Surface Sci, 110, 43 (1981). 7 M W Ruckman and M Strongin, Phys Rev, 1329, 7105 (1984). s R W Vook, T J Swirbel and J V Bucci, J Vac Sci Technol, A6 (3), 1710 (1988).

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