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CO chemisorption on PdCu(110): a work function study
Vacuum/volume 48/number 3/4/pages 187 to 190/1997 0 1997 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon PII: SOO42-207X(96)00252-7
CO chemisorption
0042-207x/97
$17.00+.00
on PdCu( 1 IO): a work function study
A Hammoudeh,“b J Loboda-Cackovic,” M S Mousa,“,’ J H Block, ‘* “Fritz-Haber-lnstitut der Max-PlanckGesellschaft, Berlin-Dahlem, Germany; bDepartment of Chemistry, Yarmouk University, Irbid, Jordan; “Department of Physics, M&ah University, PO Box 7, Al-Karak, Jordan
The work function change (A@) that occurs upon CO chemisorption on PdCuf 170) single crystal was studied using the Kelvin probe technique. The A@ measurements were carried out for various surface compositions ranging from 100% Pd to 700% Cu, which were prepared by sputtering and annealing the sample at different conditions. Auger Electron Spectroscopy (AES) and Thermal Desorption Spectroscopy (TDS) were used to characterise the surface before and after each ACD measurement. The results obtained in this study prove that the Aa technique is very sensitive to changes in surface composition and can be therefore successfully used to determine quantitatively the top layer composition of PdCu(1 IO) single crystal alloy. The AQ, results reported here indicate a charge transfer from Pd to Cu in PdCu alloys. As a result of this transfer process, the back donation of electrons from Pd to adsorbed CO is reduced and the dipole moment of adsorbed CO on Pd becomes smaller. 0 1997 Published by Elsevier Science Ltd. All rights reserved
Introduction From a theoretical point of view, the Pd-Cu system is interesting because these alloys form a continuous series of solid solutions above a critical temperature of 773 K and also form two ordered phases (Cu,Pd and CuPd).’ On the other hand and in many cases the presence of Cu was found to improve the catalytic function of some Pd based catalysts. For example, intermediate loadings of Cu were found to increase the methane and methanol yields in the Pd catalyscd CO hydrogenation.2 Pd, sCuo,s(l I I) alloys were found to be active in the NO reduction by CO with Turn Over Frequencies (TOF) higher than those for I’d3 and fairly close to those on Rh/A120~.4 The CO-oxidation was also investigated on PdCu( I IO) single crystal.‘.(’ A hysteresis in the CO-oxidation rate was obtained indicating that this system could display surface kinetic oscillations. Relatively high oxygen pressures (> 10. ’ Torr) were needed to start these kinetic oscillations on Pd surfaces,‘-’ where close values would be also expected for PdCu(l IO). Under such conditions the measurement of the work function change using the Kelvin probe technique was successfully used to monitor kinetic oscillations.‘O.” However, before applying this technique to CO-oxidation on PdCu(l IO), the work function change due to the adsorption of the pure reactants (CO and oxygen) should be examined in order to confirm that the difference in the work function between the CO covered and oxygen covered surface is large enough to allow eventual oscillations. The aim of this work was to study the work function change upon CO chemisorption
_--*Passed
away on 23rd July 1995.
on PdCu(ll0). The dependence of Act, on both position and temperature was investigated.
surface
com-
Experimental Theexperiments were performed in an ultra, high vacuum (UHV) chamber that had a base pressure of 8 x IO-” mbar. The techniques employed in this study included Auger Electron Spectroscopy (AES), the Kelvin probe technique for A@ measurements and Thermal Desorption Spectroscopy (TDS) using a differentially pumped mass spectrometer. The PdCu single crystal alloy had a Pd/Cu atomic concentration ratio of I in the bulk and was prepared as the (I lO)plane. The sample surface was prepared by carrying out cycles of sputtering and annealing until no contaminants (specially sulphur) could be detected. Furthermore, this procedure was repeated until the required surface composition (i.e. ratio of Pd:Cu) was obtained (for details see Ref. 12). The CO-TDS was developed as a reliable technique for the determination of the top layer composition, whereas the surface composition as determined by AES corresponded on average to the first four layers (referred to as surface region).” The Kelvin probe technique with a S mm’ gold ring reference electrode was used to measure the work function change (A@) upon CO adsorption on PdCu( I IO). Kesults To charactcrise the different CO species populated on the PdCu(l10) alloy surface, that may have different contributions to the work function change of this system, TDS investigation 187
A Hammoudeh
et al: CO chemisorption
on PdCu(1 IO)
was undertaken before and after each A@ measurement. Figure I shows a CO desorption spectrum from PdCu( 110) with a Cu/Pd ratio of 0.65 in the surface region. This surface was saturated with CO (10 L) at 115 K. Three different CO desorption states with maxima at 160 K, 240 K and 400 K could he well distinguished and identified as the t(, /I? and /If4states, respectively. as assigned in the recent comprehensive study done by Mousa et (Il.‘2 The /I4 desorption state coincides with the desorption peak obtained in the CO desorption from pure Pd(ll0)” and corresponds therefore, in the first approximation to CO dcsorption from ‘pure’ Pd atoms on the alloy surface. The /I? state is characteristic of the alloy and is believed to arise from CO molecules sitting on top of Pd atoms with high concentrations of Cu in subsurface sites ” and/or from combined Cu Pd adsorption sites. This type of Pd atom will be referred to as ‘Pd’ in this article. The maximum of the 3: state is at a somewhat lower tempcraturc (160K) than that of CO on pure Cu(ll0) (218K).14 As a result of this lowering of the CO-dcsorption temperatures, the Cu atoms are believed to be not completely saturated with CO at the adsorption temperature of I I5 K. This expectation was sustained by (A@) mcasuremcnts that showed further CO uptake by Cu rich surfaces under high CO pressures (reversible adsorption). The Cu fraction in the top layer as determined by TDS is, therefore, underestimated. The error is however very small and can be neglected, since for the very Cu rich surfaces (Cu!Pd = 4.0) the further work function change due to reversibly adsorbed CO at 115 K and 5 x IO 4 Torr CO is below 20 mV. Figure 2 shows the work function change of PdCu(ll0) saturated with CO at 1 I5 K, upon heating from 115 K up to 510 K with a constant heating rate of 2.5 K/s. Curves aA in this figure were obtained for various surface compositions. From these curves a general trend can be observed (with the exception of curve d, the very Cu rich surface). With increasing desorption temperatures the work function continuously increases and reaches a maximum at temperatures around 300 K, and then it decreases until CO is completely desorbcd. The value of the work function of the clean surface a,,,,, is reached at 7’ ~~450-500 K. It will be easy to interpret these results if it is remembered that low CO coverages on Cu cause a negative work function change “X while those on Pd result in a positive work function change.“.‘”
!3WX!3!
Cu/Pd
( LN)_ 0
C
c
Ino
200
300
400
ncg.A@,,,,
Figure 1. Thermal face with a Cu;Pd 188
desorption spectrum ratio = 0.65.
(K) of CO saturated
PdCu(l
IO) sur-
1.3
so0
f
0
The increase (decrease) in the work function at low (high) tcmperatures corresponds to CO dcsorption from ‘pure’ Cu (‘pure’ Pd). However, careful comparison of Figures 1 and 2 shows clearly that the CO desorption from ‘pure’ Cu (a state) should ” bc complete at ~200-250 K, which is a general behaviour that is not restricted to the surface composition shown in Figure 1. However, the increase in the work function continues until 7‘~ 300.-350 K. Therefore, the further increase in the work function for T > 250 K should correspond to CO desorption from ‘Pd’ (/I2 state). Considering the work function of the CO saturated surface at 115 K ((I),,), the work function of the clean surface at 500 K (m,,,,,,) and the maximum work function of the CO:‘PdCu(l IO) system at T=:300K (m,,,), the following quantities can be identified:
pos.A@,,,,
360
0
Figure 2. Work function change of CO saturated PdCu(ll0) surfaces with dikrent Cu/Pd ratios during the CO desorption process. The heating rare was 2.5 K/s. Cu/Pd = (a) 0.3, (h) I .3 (c) I .Xand (d) 3.9.
change caused by a saturated
= A@,,,,
work
= OsaI- CD,,,
CO
function
change
work function
change
(2)
- Q’c,can
3. neg.A@,,,, as the maximum negative caused by CO adsorption, where
Temperature
h
%G
T/K
as the maximum positive 2. pos.AO,,, caused by CO adsorption. where
200
1.8
0
l
1. A@‘,, as the work function adlayer at 115 K, where
)
3.9
d
(3)
With increasing CuiPd ratio the following trends arc obvious from Figure 2: (i) a decrease in A@,,, (ii) a decrease in pos.A@,,,, and (iii) a shift of the work function maximum to lower desorption temperatures. For ncg.AcD,,;,, no simple relationship can bc recognised indicating the complexity of factors determining this quantity.
A Hammoudeh
et al: CO chemisorption
on PdCu(l10)
CO/ PdC
800
T=llSK
Pd % Figure 3. A(1),, as a functton of Pd content in the surlacc region, as determined by AES. Values of A@,, for 100% Pd and 0% Pd were taken from Kefs 17 and 15. corresponding to CO saturated layers on Pd( I IO)
and Cu(l1 I). respecuvely.
in the Figure 3 shows A@,.,, as a function of Pd composition surface region as determined by AES. A non linear relationship was found indicating non-addiditive adsorptive properties of the alloy components. Such a behaviour is well known in literature: c.g. for AgPd films” and CuNi alloys.*” The strong decrease in A@‘,,, with increasing Cu concentration may have two causes:
the entire composition range. As expected, a linear relationship was not obtained between pos.AcD,,, and the total Pd content in the surface region SR (curve b) or in the top layer TL (curve a). A linear dependence of pos.A@,,,, was obtained only if the fraction of ‘pure’ Pd in the top layer (/I4 state) was taken in consideration (curve c). indicating that only CO adsorbed in the /I., state causes a positive work function change. However. the decrease in pos.A@,,,,, with decreasing ‘pure’ Pd concentrations is stronger than that expected for the case where this decrease is caused only by a decrease in total CO coverages (curved), proving that the CO dipole moment must also have decreased. This decrease in the CO dipole moment becomes larger with increasing Cu concentrations. For ‘pure’ Pd concentrations below 22% the CO dipole moment even becomes negative, causing a negative work function change due to CO adsorption on ‘pure’ Pd. as was also shown in Figure 2, curve d. It was concluded from Figure 2 that a negative work function change is not only caused by CO adsorbed on Cu but also by CO adsorbed on ‘Pd.. The dependence of neg.A@,,, on the Cu and ‘Pd’ content is represented in Figure 5. Obviously ncg.AQ),,, reacts more strongly to changes in the ‘Pd’ content (Figure 5(b)) than to changes in the Cu content (Figure 5(a)). and a linear relationship between ncg.A@,,,, and ‘Pd’ content is obtained. The direct contribution of ‘pure’ Cu to ncg.A@,,, can therefore be neglected. As a result eqn (4) for Am,, (in mV) can be written as: A@,,, = a[‘Pd ‘I+ b[Pd] + c
I. The maximum coverage decrcascs: Mousa et 01.” found that very small amounts of Cu, even in subsurface sites, wcrc sufficient to suppress the building of the p5 state (Ts4SO K) of CO adsorbed on PdCu( I 10). Similar results were obtained by Noordermer et al.,2’ who reported a drastic decrease in the CO uptake by PdCu( I I 1) and PdAg( I I I) in comparison to pure Pd( 11 I). 2. The dipole moment produced by CO-adsorption on the metallic surface becomes smaller as will bc shown in this work. A@,,, can bc divided into two contributions A@,,, = neg.W,,,,
low “pd” content
given by
+ ~os.A%~
(4)
Figure 4 shows pos.AU+,,,, as a function
of the Pd content
high “pd” content
across
-Sld
I
60
40
20
80
cu c/o 1000 -
co/.‘dCu(!
10)
I””
r
(b) CO/_PdCu_(no,
>
E -200 > zz E
2 p -300 -4001 n
8
20
’ 40
’ 60
.
’ 80
’ 100
1
Pd %J Figure 4. Pos.A@,,,, as a function of’: (a) I’d content in the top layer (determined by TDS). (b) Pd content in the surface region (determined by A ES), (c) fractwn of ‘pure’ Pd in the top layer (/L state in TDS). Curve d rcprcscnts the expected dependence of pos.A@,,,, on the CO coverage only.
-4001 0
’ IO
’ 20
’ ’ ’ ’ 40 50 30 “Pd” c/c as a function of the Cu content in the surface Figure 5. (a) Neg. A(D,,,, region (b) Ncg.AQ,,,, as a function of the ‘Pd’ content (fil state in TtX). 189
A Hemmoudeh
et al: CO chemisorption
on PdCu(l10)
with a = -4.8 and b = 14.3 (the slopes of the curve in Figure 5(b) and of the curve c in Figure 4) and c = -444.8. [Pd] and [‘Pd.] are the contents of ‘pure’ Pd and ‘Pd’ in the top layer in percent.
Discussion
It is generally accepted that the bond between chemisorbed CO and transition metals is formed by electron transfer from the 5, orbital of CO to unoccupied metal orbitals (donation of electrons to the metal) accompanied by back donation of electrons from occupied d-orbitals into the unoccupied 2rr* orbital of CO. The magnitude of these two contributions to the metal-CO bond is reflected in the work function change, that indicates the net charge transfer between CO and the metal. The correlation between A@ and the extent of donation and back donation was discussed by Nieuwenhuys.” Electron donation from CO to the metal is expected to decrease the work function. whereas the back donation from the metal to CO increases the work function. Accordingly, metals with positive work function change upon CO adsorption, such as Pd,” N?’ and RuTz5 are characterised by a strong back donation. that overcompensates the electron donation, leading lo a positive dipole moment. Calculations support this conclusion. where the population of the 27n* orbital in Ni(CO), and for CO adsorbed on Pd was found to be 0.44 ” and 0.4-0.6,‘72n respectively. On the other hand. CO chemisorption on Cu”~j” decreases the work function (strictly speaking, in the low coverage range), indicating that the back donation effect is negligible. The dipole moment of CO on Cu is, therefore, rcvcrscly oriented IO that of CO on Pd. In this work alloying Pd with Cu was found to decrease the dipole moment of CO on Pd, indicating that the back donation of metal electrons into the 2n* orbital of CO must have dccrcased. This depression of back donation must correspond to a reduction in the clcctron density on Pd. This conclusion is in agreement with UPS studies showing a depletion of the Pd electron density near the Fermi level for supported Pd monolayers on W(1 IO).” Ta( I IO)‘* and Nb( I lo).” Our A@ results are therefore consistent with an electron transfer taking place from Pd to Cu supporting the elcctronegativity scale presented in Ref. 34, which is completely contrary to that proposed for bulk metals. Another effect of alloying Pd with Cu is the decrease in the CO adsorption energy, AHddr(CO). in comparison to that on pure Pd. This decrease in AH,,,,(CO) is reflected in shifting the thermal desorption maxima to lower dcsorption temperatures. as was also shown by Mousa er al. in Ref. 12. Accordingly, the observed shift in the work function maximum with increasing Cu;Pd ratios to lower temperatures (Figure 2) can bc explained as follows: In the presence of high Cu concentrations the CO dcsorption from ‘pure’ Pd atoms will now be more signilicant at low temperatures due to the lowered AH,,,(CO). However, this decrease in AH&CO) is not so large and cannot, therefore, be related to the observed strong reduction in the back donation. The TD spectra obtained in this study show clearly that even though the dipole moment of CO on ‘pure’ Pd in the Cu rich surfaces of PdCu(1 IO) alloy becomes negative. this CO (/II4state) can still be considered to be strongly adsorbed. since it is complctcly desorbed at T 2 400 K. Obviously, there is, in addition lo the back bonding. another factor that determines the strength of the CO-metal bond. This factor is probably the dipole-dipole interaction between CO and the clcctron distribution in Pd, which increases with the charge transfer from Pd to Cu leading to a stronger inductive bond with CO. 190
Conclusion In this work the AQ, method was shown to be very sensi to changes in surface composition. It was successfully use( determining the top layer composition of PdCu( 110) single cry and a quantitative relationship between A&, and top layer cc position was established. In PdCu alloys a charge transfer from Pd to Cu was obser\ As a result the back donation from Pd to adsorbed CO is rcdu and the dipole moment of CO becomes smaller. However, decrease in the CO metal bond strength due to reduced b donation is partially compensated probably by increased dips dipole interaction between CO and the metal. Acknowledgements
The authors thank W Feigc for technical assistance. The lowship by the Alexander von Humboldt Stiftung is grate!’ acknowlcdgcd by M S Mousa. References M. and Anderko. K.. Comrrrurion of Rimq A/, I. Hansen, McGraw-JIill, New York, 1958, p. 612. 2. Leon y Leon. C.A. and Vannice, MA., .4pp/. C‘u/cr/.. 1991. 69. 3 3. Debauge. Y.. Ru17, Ph.. Rochefort, A., Abon. M.. Massardie and Bertolini, J.C.. Pnvate Communication. 4. Oh, Se H. and Carpentler. J.E., .I. Cutal.. 1986, 101. 114. J. and BI 5. Mousa, MS., Ilammoudch, A.. Loboda-Cackovic, J.H., J. M&c. Curd.. 1995. 96. 271. J.. Hammoudeh. A., Mousa. M.S. and Bl 6. Loboda-Cackovic, J.H., Chcuum. 1995. 46. 41 I. K. and BI 7. Ehsasl, M., Berdau, M., Karpowicy. A., Christmann, J.H.. Proc. 10th Inr. Congress WI Curd.. Budapest. Elsevler. Am dam. iYY2. p. 321. 8. Ehsasi, M.,-Berdau. M . Rebitzki, T.. Charle. K.P., Christmanr and Block. J.H.. J. Chem. Phvs.. 1993, 98, 0177 9. Bassett. M.R. and Imbihl, R.: J. Chrm. Phy.r.. ‘1’990.93. 8 I I. 10. Ehsasi. M.. Scidcl, C., Ruppender, H.. Drachsel. W. and Block, J sur/: sci.. lY8Y. 210. L198. A., Engel. W., Christmanr 11. Berdau, M.. Ehsasi. ,M.. Karpowlcr. and Block. J.H.. pacuum. 1994.45.271. J. and Block. J.H.. Vacuum, I 12. Mousa. M.S.. Loboda-Cackovic. 46, 117. J.. Grunre. M., Loboda-Cackovic, J. and Block, ! 13. Goschnik, Surface Science. 189-l!%J, 1987, 137. 14. Fu, S.S. and Smorjai. G.A.. Surf. %i., 1992, 262. 68. 15. Kcrstem, W., Krucgcr. 1). and Thlemc, t‘.. Surf. Sci.. 1986. 176, 16. Judd, K.W.. Hollins, 1’. and Pritchard, J., Sztr/. Sci., 1986. 171. 17. He. J.W. and Norton, P.R., J. Chem. Phj.r.. 1988. 89. 1170. 18. Berdau, M., Ph D Thesis, Frcic University, Berlin. 1993. IY Christmann. K. and Ertl, G., Surf. Sci., 1972. 33. 254. 20. Ertl, G. and Kucppcrs. J.. J. Vuc. Sci. Technol.. 1972, 9. 829. 21. Noordermeer. A., Kok. G.A. and Nicuwenhuys, B.E., Sur/. 1986. 172. 349. 22. Nieuwenhuys, B.E.. .Sur/: Sci.. 1981, 10% 505. 23. Ertl, G. and Koch, J., Z. Xarurforschtmnq. 1970. 2%. IYO6. 24. ChrIstmann. K.. Schoher, 0. and Ertl. G.. J. Chrm. Phps., 1974 4719. 25. Madey, J. and Menxl. D.. J. Appl. Phvs Suppi. 2. 2. 1974. 229. 26. Bacrands. E.J. and Ros, I’., iclol. Phys., 1975. 30. 2940. 27. Ellis, D.T... Baerands. E.J., Adachi. II. and Avcrill. F.W.. Sur/. 1977.64.649. 28. Bagus. P.S. and Roob, B.O.. J. Chern. Phgs.. 1981. 75. 5961. 29. Tracy. J.C.. J. Chem Phys.. lY72, 56, 2748. 30. Ho&, K.. Ilussain. M. and Pritchard. J.. SurJ Sci.. 197Y. 63, 2~ 31. Graham, G.W., J. Vat. Sci. Technol.. 1986, A4. 760. 32. Keel. B.E.. Smith. R.J.. Berlowitl. Sur/. Sri.. 1990, 231, 325. 33. El-Batanouny. X4.. Strongin. M. and Williams, G.P., Phys. , 1983.827.4580. 34. Kuhn, W.K.. Campbell, R.A. and Goodman, D.W.. in 7hr Cher Physics of Solid Surfaces. Vol. 6. eds D.A. Kin8 and D.P. Wood Elserier. Amsterdam, 1993, p. 157.
Pergamon PII: SOO42-207X(96)00252-7
CO chemisorption
0042-207x/97
$17.00+.00
on PdCu( 1 IO): a work function study
A Hammoudeh,“b J Loboda-Cackovic,” M S Mousa,“,’ J H Block, ‘* “Fritz-Haber-lnstitut der Max-PlanckGesellschaft, Berlin-Dahlem, Germany; bDepartment of Chemistry, Yarmouk University, Irbid, Jordan; “Department of Physics, M&ah University, PO Box 7, Al-Karak, Jordan
The work function change (A@) that occurs upon CO chemisorption on PdCuf 170) single crystal was studied using the Kelvin probe technique. The A@ measurements were carried out for various surface compositions ranging from 100% Pd to 700% Cu, which were prepared by sputtering and annealing the sample at different conditions. Auger Electron Spectroscopy (AES) and Thermal Desorption Spectroscopy (TDS) were used to characterise the surface before and after each ACD measurement. The results obtained in this study prove that the Aa technique is very sensitive to changes in surface composition and can be therefore successfully used to determine quantitatively the top layer composition of PdCu(1 IO) single crystal alloy. The AQ, results reported here indicate a charge transfer from Pd to Cu in PdCu alloys. As a result of this transfer process, the back donation of electrons from Pd to adsorbed CO is reduced and the dipole moment of adsorbed CO on Pd becomes smaller. 0 1997 Published by Elsevier Science Ltd. All rights reserved
Introduction From a theoretical point of view, the Pd-Cu system is interesting because these alloys form a continuous series of solid solutions above a critical temperature of 773 K and also form two ordered phases (Cu,Pd and CuPd).’ On the other hand and in many cases the presence of Cu was found to improve the catalytic function of some Pd based catalysts. For example, intermediate loadings of Cu were found to increase the methane and methanol yields in the Pd catalyscd CO hydrogenation.2 Pd, sCuo,s(l I I) alloys were found to be active in the NO reduction by CO with Turn Over Frequencies (TOF) higher than those for I’d3 and fairly close to those on Rh/A120~.4 The CO-oxidation was also investigated on PdCu( I IO) single crystal.‘.(’ A hysteresis in the CO-oxidation rate was obtained indicating that this system could display surface kinetic oscillations. Relatively high oxygen pressures (> 10. ’ Torr) were needed to start these kinetic oscillations on Pd surfaces,‘-’ where close values would be also expected for PdCu(l IO). Under such conditions the measurement of the work function change using the Kelvin probe technique was successfully used to monitor kinetic oscillations.‘O.” However, before applying this technique to CO-oxidation on PdCu(l IO), the work function change due to the adsorption of the pure reactants (CO and oxygen) should be examined in order to confirm that the difference in the work function between the CO covered and oxygen covered surface is large enough to allow eventual oscillations. The aim of this work was to study the work function change upon CO chemisorption
_--*Passed
away on 23rd July 1995.
on PdCu(ll0). The dependence of Act, on both position and temperature was investigated.
surface
com-
Experimental Theexperiments were performed in an ultra, high vacuum (UHV) chamber that had a base pressure of 8 x IO-” mbar. The techniques employed in this study included Auger Electron Spectroscopy (AES), the Kelvin probe technique for A@ measurements and Thermal Desorption Spectroscopy (TDS) using a differentially pumped mass spectrometer. The PdCu single crystal alloy had a Pd/Cu atomic concentration ratio of I in the bulk and was prepared as the (I lO)plane. The sample surface was prepared by carrying out cycles of sputtering and annealing until no contaminants (specially sulphur) could be detected. Furthermore, this procedure was repeated until the required surface composition (i.e. ratio of Pd:Cu) was obtained (for details see Ref. 12). The CO-TDS was developed as a reliable technique for the determination of the top layer composition, whereas the surface composition as determined by AES corresponded on average to the first four layers (referred to as surface region).” The Kelvin probe technique with a S mm’ gold ring reference electrode was used to measure the work function change (A@) upon CO adsorption on PdCu( I IO). Kesults To charactcrise the different CO species populated on the PdCu(l10) alloy surface, that may have different contributions to the work function change of this system, TDS investigation 187
A Hammoudeh
et al: CO chemisorption
on PdCu(1 IO)
was undertaken before and after each A@ measurement. Figure I shows a CO desorption spectrum from PdCu( 110) with a Cu/Pd ratio of 0.65 in the surface region. This surface was saturated with CO (10 L) at 115 K. Three different CO desorption states with maxima at 160 K, 240 K and 400 K could he well distinguished and identified as the t(, /I? and /If4states, respectively. as assigned in the recent comprehensive study done by Mousa et (Il.‘2 The /I4 desorption state coincides with the desorption peak obtained in the CO desorption from pure Pd(ll0)” and corresponds therefore, in the first approximation to CO dcsorption from ‘pure’ Pd atoms on the alloy surface. The /I? state is characteristic of the alloy and is believed to arise from CO molecules sitting on top of Pd atoms with high concentrations of Cu in subsurface sites ” and/or from combined Cu Pd adsorption sites. This type of Pd atom will be referred to as ‘Pd’ in this article. The maximum of the 3: state is at a somewhat lower tempcraturc (160K) than that of CO on pure Cu(ll0) (218K).14 As a result of this lowering of the CO-dcsorption temperatures, the Cu atoms are believed to be not completely saturated with CO at the adsorption temperature of I I5 K. This expectation was sustained by (A@) mcasuremcnts that showed further CO uptake by Cu rich surfaces under high CO pressures (reversible adsorption). The Cu fraction in the top layer as determined by TDS is, therefore, underestimated. The error is however very small and can be neglected, since for the very Cu rich surfaces (Cu!Pd = 4.0) the further work function change due to reversibly adsorbed CO at 115 K and 5 x IO 4 Torr CO is below 20 mV. Figure 2 shows the work function change of PdCu(ll0) saturated with CO at 1 I5 K, upon heating from 115 K up to 510 K with a constant heating rate of 2.5 K/s. Curves aA in this figure were obtained for various surface compositions. From these curves a general trend can be observed (with the exception of curve d, the very Cu rich surface). With increasing desorption temperatures the work function continuously increases and reaches a maximum at temperatures around 300 K, and then it decreases until CO is completely desorbcd. The value of the work function of the clean surface a,,,,, is reached at 7’ ~~450-500 K. It will be easy to interpret these results if it is remembered that low CO coverages on Cu cause a negative work function change “X while those on Pd result in a positive work function change.“.‘”
!3WX!3!
Cu/Pd
( LN)_ 0
C
c
Ino
200
300
400
ncg.A@,,,,
Figure 1. Thermal face with a Cu;Pd 188
desorption spectrum ratio = 0.65.
(K) of CO saturated
PdCu(l
IO) sur-
1.3
so0
f
0
The increase (decrease) in the work function at low (high) tcmperatures corresponds to CO dcsorption from ‘pure’ Cu (‘pure’ Pd). However, careful comparison of Figures 1 and 2 shows clearly that the CO desorption from ‘pure’ Cu (a state) should ” bc complete at ~200-250 K, which is a general behaviour that is not restricted to the surface composition shown in Figure 1. However, the increase in the work function continues until 7‘~ 300.-350 K. Therefore, the further increase in the work function for T > 250 K should correspond to CO desorption from ‘Pd’ (/I2 state). Considering the work function of the CO saturated surface at 115 K ((I),,), the work function of the clean surface at 500 K (m,,,,,,) and the maximum work function of the CO:‘PdCu(l IO) system at T=:300K (m,,,), the following quantities can be identified:
pos.A@,,,,
360
0
Figure 2. Work function change of CO saturated PdCu(ll0) surfaces with dikrent Cu/Pd ratios during the CO desorption process. The heating rare was 2.5 K/s. Cu/Pd = (a) 0.3, (h) I .3 (c) I .Xand (d) 3.9.
change caused by a saturated
= A@,,,,
work
= OsaI- CD,,,
CO
function
change
work function
change
(2)
- Q’c,can
3. neg.A@,,,, as the maximum negative caused by CO adsorption, where
Temperature
h
%G
T/K
as the maximum positive 2. pos.AO,,, caused by CO adsorption. where
200
1.8
0
l
1. A@‘,, as the work function adlayer at 115 K, where
)
3.9
d
(3)
With increasing CuiPd ratio the following trends arc obvious from Figure 2: (i) a decrease in A@,,, (ii) a decrease in pos.A@,,,, and (iii) a shift of the work function maximum to lower desorption temperatures. For ncg.AcD,,;,, no simple relationship can bc recognised indicating the complexity of factors determining this quantity.
A Hammoudeh
et al: CO chemisorption
on PdCu(l10)
CO/ PdC
800
T=llSK
Pd % Figure 3. A(1),, as a functton of Pd content in the surlacc region, as determined by AES. Values of A@,, for 100% Pd and 0% Pd were taken from Kefs 17 and 15. corresponding to CO saturated layers on Pd( I IO)
and Cu(l1 I). respecuvely.
in the Figure 3 shows A@,.,, as a function of Pd composition surface region as determined by AES. A non linear relationship was found indicating non-addiditive adsorptive properties of the alloy components. Such a behaviour is well known in literature: c.g. for AgPd films” and CuNi alloys.*” The strong decrease in A@‘,,, with increasing Cu concentration may have two causes:
the entire composition range. As expected, a linear relationship was not obtained between pos.AcD,,, and the total Pd content in the surface region SR (curve b) or in the top layer TL (curve a). A linear dependence of pos.A@,,,, was obtained only if the fraction of ‘pure’ Pd in the top layer (/I4 state) was taken in consideration (curve c). indicating that only CO adsorbed in the /I., state causes a positive work function change. However. the decrease in pos.A@,,,,, with decreasing ‘pure’ Pd concentrations is stronger than that expected for the case where this decrease is caused only by a decrease in total CO coverages (curved), proving that the CO dipole moment must also have decreased. This decrease in the CO dipole moment becomes larger with increasing Cu concentrations. For ‘pure’ Pd concentrations below 22% the CO dipole moment even becomes negative, causing a negative work function change due to CO adsorption on ‘pure’ Pd. as was also shown in Figure 2, curve d. It was concluded from Figure 2 that a negative work function change is not only caused by CO adsorbed on Cu but also by CO adsorbed on ‘Pd.. The dependence of neg.A@,,, on the Cu and ‘Pd’ content is represented in Figure 5. Obviously ncg.AQ),,, reacts more strongly to changes in the ‘Pd’ content (Figure 5(b)) than to changes in the Cu content (Figure 5(a)). and a linear relationship between ncg.A@,,,, and ‘Pd’ content is obtained. The direct contribution of ‘pure’ Cu to ncg.A@,,, can therefore be neglected. As a result eqn (4) for Am,, (in mV) can be written as: A@,,, = a[‘Pd ‘I+ b[Pd] + c
I. The maximum coverage decrcascs: Mousa et 01.” found that very small amounts of Cu, even in subsurface sites, wcrc sufficient to suppress the building of the p5 state (Ts4SO K) of CO adsorbed on PdCu( I 10). Similar results were obtained by Noordermer et al.,2’ who reported a drastic decrease in the CO uptake by PdCu( I I 1) and PdAg( I I I) in comparison to pure Pd( 11 I). 2. The dipole moment produced by CO-adsorption on the metallic surface becomes smaller as will bc shown in this work. A@,,, can bc divided into two contributions A@,,, = neg.W,,,,
low “pd” content
given by
+ ~os.A%~
(4)
Figure 4 shows pos.AU+,,,, as a function
of the Pd content
high “pd” content
across
-Sld
I
60
40
20
80
cu c/o 1000 -
co/.‘dCu(!
10)
I””
r
(b) CO/_PdCu_(no,
>
E -200 > zz E
2 p -300 -4001 n
8
20
’ 40
’ 60
.
’ 80
’ 100
1
Pd %J Figure 4. Pos.A@,,,, as a function of’: (a) I’d content in the top layer (determined by TDS). (b) Pd content in the surface region (determined by A ES), (c) fractwn of ‘pure’ Pd in the top layer (/L state in TDS). Curve d rcprcscnts the expected dependence of pos.A@,,,, on the CO coverage only.
-4001 0
’ IO
’ 20
’ ’ ’ ’ 40 50 30 “Pd” c/c as a function of the Cu content in the surface Figure 5. (a) Neg. A(D,,,, region (b) Ncg.AQ,,,, as a function of the ‘Pd’ content (fil state in TtX). 189
A Hemmoudeh
et al: CO chemisorption
on PdCu(l10)
with a = -4.8 and b = 14.3 (the slopes of the curve in Figure 5(b) and of the curve c in Figure 4) and c = -444.8. [Pd] and [‘Pd.] are the contents of ‘pure’ Pd and ‘Pd’ in the top layer in percent.
Discussion
It is generally accepted that the bond between chemisorbed CO and transition metals is formed by electron transfer from the 5, orbital of CO to unoccupied metal orbitals (donation of electrons to the metal) accompanied by back donation of electrons from occupied d-orbitals into the unoccupied 2rr* orbital of CO. The magnitude of these two contributions to the metal-CO bond is reflected in the work function change, that indicates the net charge transfer between CO and the metal. The correlation between A@ and the extent of donation and back donation was discussed by Nieuwenhuys.” Electron donation from CO to the metal is expected to decrease the work function. whereas the back donation from the metal to CO increases the work function. Accordingly, metals with positive work function change upon CO adsorption, such as Pd,” N?’ and RuTz5 are characterised by a strong back donation. that overcompensates the electron donation, leading lo a positive dipole moment. Calculations support this conclusion. where the population of the 27n* orbital in Ni(CO), and for CO adsorbed on Pd was found to be 0.44 ” and 0.4-0.6,‘72n respectively. On the other hand. CO chemisorption on Cu”~j” decreases the work function (strictly speaking, in the low coverage range), indicating that the back donation effect is negligible. The dipole moment of CO on Cu is, therefore, rcvcrscly oriented IO that of CO on Pd. In this work alloying Pd with Cu was found to decrease the dipole moment of CO on Pd, indicating that the back donation of metal electrons into the 2n* orbital of CO must have dccrcased. This depression of back donation must correspond to a reduction in the clcctron density on Pd. This conclusion is in agreement with UPS studies showing a depletion of the Pd electron density near the Fermi level for supported Pd monolayers on W(1 IO).” Ta( I IO)‘* and Nb( I lo).” Our A@ results are therefore consistent with an electron transfer taking place from Pd to Cu supporting the elcctronegativity scale presented in Ref. 34, which is completely contrary to that proposed for bulk metals. Another effect of alloying Pd with Cu is the decrease in the CO adsorption energy, AHddr(CO). in comparison to that on pure Pd. This decrease in AH,,,,(CO) is reflected in shifting the thermal desorption maxima to lower dcsorption temperatures. as was also shown by Mousa er al. in Ref. 12. Accordingly, the observed shift in the work function maximum with increasing Cu;Pd ratios to lower temperatures (Figure 2) can bc explained as follows: In the presence of high Cu concentrations the CO dcsorption from ‘pure’ Pd atoms will now be more signilicant at low temperatures due to the lowered AH,,,(CO). However, this decrease in AH&CO) is not so large and cannot, therefore, be related to the observed strong reduction in the back donation. The TD spectra obtained in this study show clearly that even though the dipole moment of CO on ‘pure’ Pd in the Cu rich surfaces of PdCu(1 IO) alloy becomes negative. this CO (/II4state) can still be considered to be strongly adsorbed. since it is complctcly desorbed at T 2 400 K. Obviously, there is, in addition lo the back bonding. another factor that determines the strength of the CO-metal bond. This factor is probably the dipole-dipole interaction between CO and the clcctron distribution in Pd, which increases with the charge transfer from Pd to Cu leading to a stronger inductive bond with CO. 190
Conclusion In this work the AQ, method was shown to be very sensi to changes in surface composition. It was successfully use( determining the top layer composition of PdCu( 110) single cry and a quantitative relationship between A&, and top layer cc position was established. In PdCu alloys a charge transfer from Pd to Cu was obser\ As a result the back donation from Pd to adsorbed CO is rcdu and the dipole moment of CO becomes smaller. However, decrease in the CO metal bond strength due to reduced b donation is partially compensated probably by increased dips dipole interaction between CO and the metal. Acknowledgements
The authors thank W Feigc for technical assistance. The lowship by the Alexander von Humboldt Stiftung is grate!’ acknowlcdgcd by M S Mousa. References M. and Anderko. K.. Comrrrurion of Rimq A/, I. Hansen, McGraw-JIill, New York, 1958, p. 612. 2. Leon y Leon. C.A. and Vannice, MA., .4pp/. C‘u/cr/.. 1991. 69. 3 3. Debauge. Y.. Ru17, Ph.. Rochefort, A., Abon. M.. Massardie and Bertolini, J.C.. Pnvate Communication. 4. Oh, Se H. and Carpentler. J.E., .I. Cutal.. 1986, 101. 114. J. and BI 5. Mousa, MS., Ilammoudch, A.. Loboda-Cackovic, J.H., J. M&c. Curd.. 1995. 96. 271. J.. Hammoudeh. A., Mousa. M.S. and Bl 6. Loboda-Cackovic, J.H., Chcuum. 1995. 46. 41 I. K. and BI 7. Ehsasl, M., Berdau, M., Karpowicy. A., Christmann, J.H.. Proc. 10th Inr. Congress WI Curd.. Budapest. Elsevler. Am dam. iYY2. p. 321. 8. Ehsasi, M.,-Berdau. M . Rebitzki, T.. Charle. K.P., Christmanr and Block. J.H.. J. Chem. Phvs.. 1993, 98, 0177 9. Bassett. M.R. and Imbihl, R.: J. Chrm. Phy.r.. ‘1’990.93. 8 I I. 10. Ehsasi. M.. Scidcl, C., Ruppender, H.. Drachsel. W. and Block, J sur/: sci.. lY8Y. 210. L198. A., Engel. W., Christmanr 11. Berdau, M.. Ehsasi. ,M.. Karpowlcr. and Block. J.H.. pacuum. 1994.45.271. J. and Block. J.H.. Vacuum, I 12. Mousa. M.S.. Loboda-Cackovic. 46, 117. J.. Grunre. M., Loboda-Cackovic, J. and Block, ! 13. Goschnik, Surface Science. 189-l!%J, 1987, 137. 14. Fu, S.S. and Smorjai. G.A.. Surf. %i., 1992, 262. 68. 15. Kcrstem, W., Krucgcr. 1). and Thlemc, t‘.. Surf. Sci.. 1986. 176, 16. Judd, K.W.. Hollins, 1’. and Pritchard, J., Sztr/. Sci., 1986. 171. 17. He. J.W. and Norton, P.R., J. Chem. Phj.r.. 1988. 89. 1170. 18. Berdau, M., Ph D Thesis, Frcic University, Berlin. 1993. IY Christmann. K. and Ertl, G., Surf. Sci., 1972. 33. 254. 20. Ertl, G. and Kucppcrs. J.. J. Vuc. Sci. Technol.. 1972, 9. 829. 21. Noordermeer. A., Kok. G.A. and Nicuwenhuys, B.E., Sur/. 1986. 172. 349. 22. Nieuwenhuys, B.E.. .Sur/: Sci.. 1981, 10% 505. 23. Ertl, G. and Koch, J., Z. Xarurforschtmnq. 1970. 2%. IYO6. 24. ChrIstmann. K.. Schoher, 0. and Ertl. G.. J. Chrm. Phps., 1974 4719. 25. Madey, J. and Menxl. D.. J. Appl. Phvs Suppi. 2. 2. 1974. 229. 26. Bacrands. E.J. and Ros, I’., iclol. Phys., 1975. 30. 2940. 27. Ellis, D.T... Baerands. E.J., Adachi. II. and Avcrill. F.W.. Sur/. 1977.64.649. 28. Bagus. P.S. and Roob, B.O.. J. Chern. Phgs.. 1981. 75. 5961. 29. Tracy. J.C.. J. Chem Phys.. lY72, 56, 2748. 30. Ho&, K.. Ilussain. M. and Pritchard. J.. SurJ Sci.. 197Y. 63, 2~ 31. Graham, G.W., J. Vat. Sci. Technol.. 1986, A4. 760. 32. Keel. B.E.. Smith. R.J.. Berlowitl. Sur/. Sri.. 1990, 231, 325. 33. El-Batanouny. X4.. Strongin. M. and Williams, G.P., Phys. , 1983.827.4580. 34. Kuhn, W.K.. Campbell, R.A. and Goodman, D.W.. in 7hr Cher Physics of Solid Surfaces. Vol. 6. eds D.A. Kin8 and D.P. Wood Elserier. Amsterdam, 1993, p. 157.