HREELS and ARUPS investigation of the coadsorption of CO and K on Cu(111)

Surface Science 271 (1992) 513-518 North-Holland

surface science

HREELS and A R U P S investigation of the coadsorption of CO and K on Cu(111) * S. Bao, R. Xu, C.Y. Xu, H.Y. Li, L. Z h u a n d Y.B. Xu Physics Department, Zhejiang Unicersity, Hangzhou 310027, People~ Republic of China and Laboratory, for Surface Physics, Academia Sinica, Beijin,q 100080, People~ Republic of China Received 17 October 1991- accepted for publication 21 January 1992

The CO + K / C u ( I I I ) system at potassium precoverages below 0.18 ha., been investigated using H R E E L S and ARUPS. Two molecular adsorption states ot CO were found at low temperature. One with lower C - O stretch frequency, named at. has a stronger interaction between the adsorbed CO molecule and coadsorbed K, while the other with higher C - O stretch frequency, named a2, has a weaker interaction. The a~ and a , states are occapied sequentially during the exposure to CO. At room temperature, only the o~Z state was found. The loxv 4o-/(5o- + 1~-) intensity ratio suggests that the o¢~-CO molecules do not stand upright.

I. Introduction Alkali coadsorption with carbon monoxide on metal has attracted much attention in surface science due to its role as a promoter in severa' catalytic reactions. A lot of studies show th~,t coadsorbed potassium enhances the CO-iV, e bonding, weakens the C - O bonding and pipmotes the CO dissociation [1-7]. In contrast to adsorption on nickel or iron substrates, CO adsorbs rather weakly on copper, and the orde" of the binding energies of the 5o-, 17r and 4o- mclecular orbitals of CO is not the same as that ~)n a variety of other metals. On copper, the I~- or3ital has a higher binding energy, than 5o-, just as that of gas phase CO [8]. The desorption temperature of CO on copper is around 200 K [9] and there is normally no molecular adsorption state of CO on copper at room temperature. Like in ..... u~c cas,e of K promotion on other metals, the H R E E L S and TDS results of the coadsorption of CO "v[th K on the Cu(llO) and Cu(lO0) surface [i0,11] show * Project supported by the National Science Founda, ion of China. 01)39-6028/92/${15.0I)

that the coadsorbed K leads to a shift in the C - O stretch frequency from 259 to 191 meV and to an increase in the CO desorption temperature ~two CO desorption peaks at 476 and 545 K). An angle resolved photoemission study, using He 11 14(t.8 cV) radiation, of the CO + K/Cu(10(I) system [12] shows an increase in the 5~r binding energy of 0.7 cV. The ARUPS u,dng polarized light ~vith 35 eV photon e n e r ~ shows a more important effect on CO, i.e., the splitting of the 17r orbitals caused by the coadsorbed K [13]. Our H R E E L S and A R U P S experiments on the CO + K/Cu(111 ) system show similar results. However, we notice that the experiments on Cu(100) were taken at a K precoverage of (I.3, near 1 ML, where only one CO adsorption state, considered as strongly interacting with K atom, could be fould. Our experiments on the CO + K/Cu(11J ........ t.,t.on ,,t lawer K c~werat~cs and at different CO exposures. .....

•

2. Experimental The Cu samp!e was a single-crystal slab approximately 11 mm in diameter and 1 mm thick,

~ 1992 - Elsevier Science PuFlishcrs B.V. All rights rescr~'cd

514

S. Bao et aL / Coadsorption of CO and Kon Cu~lll)

oriented such that the largest face was parallel to (111). The surface was mechanically and chemically polished and then put into the UHV system. in which the base pressure was 1 x 10 -~° Tort. The C u ( l l l ) surface was cleaned by continuous Ar + ion bombardmer, t during alternate cycles of heating (900 K) and cooling. The clean and well ordered surface was checked by AES and L E E D . Potassium was deposited from a thoroughly degassed SAES dispenser. The K coverage was determined by tee change in work function. The CO for exposures was released from research grade gas and adsorbed at temperatures 150 K and 300 K. The ARUPS work was performed on an ADES400 angle-resolved electron spectrometer, and the t t R E E L S experiment was carried out in a two-stage cylindrical H R E E L S system (ELS-22 spectrometer).

°° (AI- , ~

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100 150 200 250 ENERGY LOSSImeVI Fig. 1. HREEL spectra of CO adsorbed on C u ( l l l ) with K coverage of 0.12 at 150 K. Spectrum (A) is taken after spectrum (B), but annealed to 220 K.

3. Results

3.1. H R E E L S Fig. 1 shows the H R E E L S spectra after different CO exposures at 150 K with a K coverage of 0.12. At lower CO exposure, a loss peak assigned to a C - O stretching vibration, labeled with a~, appears around 193 meV. The large downward shifts in the C - O stretch frequency with the addition of K atoms are similar to the shifts reported on the CO + K/Cu(100) system at a K precoverage of 0.3 (1 ML) by Heskett et al. [14]. Since our spectra were taken at the K precoverage of 0.12, much lower than 1 ML, some more information was obtained with higher CO exposures. During tLe exposure to CO, the C - O stretch frequency shifts to higher values and a new ')ss peak, lobeled a 2, alzpears around 255 me'v'. The ~2-CO which has a stretch peak close to thai of CO adsorbed at the on-top site of the clean Cu(I ll) surface must be only weakly perturbed by K atoms, while the a~-CO which has the weaker C - O bond must be strongly interacteo by the coadsorbed K atoms. With further CO exposure, the C - O stretching frequencies of both c~-CO and c~2-CO shift to higher values.

The two different adsorption states of CO should have different values of adsorption heat. Since a CO molecule is mobile in the initial period of adsorption, it can most probably reach and adsorb in a state with the lowest energy. According to the sequential appearance of the adsorption state, ~]-CO must have a larger heat of adsorption than az-CO has. Fig. 2 shows the H R E E L S spectra taken after saturation exposure to CO with a variety of K precoverage at 150 K. Two loss peaks of the C - O stretch frequency could be seen except for OK = 0 or 0 K = 0.18. With increasing K precoverage, both the two peaks shift to lower frequencies, the relative intensity of peak a 2 to a~ decreases in general, and peak ~2 almost disappears at 0~ = 0.18. The curve (A) of fig. 1 was taken after llllll~¢i.lllll~ LII~.4 %.I~L~LGILI (.ILIJli.~¥1k,¢ di.*lJlk.f g ~lill i'~n'fll~n~Ft'~s'~ DfllUl f l L~.,,U CO coverage at 150 K. It shows that the peak ~2 disappears completely and peak c~ decreases as a distinct shoulder at 200 meV. A broad feature appears around 180 meV. The disappearance of peak a 2 suggests that the desorption temperature of weakly perturbed CO is close to that of CO on the clean Cu(111).

S. Bao et al. / Coadsorption of CO and K on Cu(I l l)

515

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,.z Z

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ENERGY LOSS lmeVI

Fig. 2. HREEL spectra of saturated CO adsorbed on Cu(l l l ) with different K coverages at 150 K.

The results of CO coadsorbed on Cu(111) with potassium at room temperature, shown in fig. 3, is similar to that shown in curve (A) of fig. 1. The lower freqcency EELS peaks were also found b

179 x 256

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Fig. 4. ARUPS of He II taken at surface normal with photons incident at 60° for: (a) 5 L CO exposure on the clean C u ( l l l ) at 150 K; (b) and (c) 0.2 L and 5 L CO exposure on the C u ( l l l ) with a K coverage of 0.1 at 150 K, respectively; (d) 100 L CO exposure on the C u ( l l l ) with a K coverage of 0.1 at room temperature. The dashed line is the result of 5 L O expor",'e on the same surface with (b) and (c).

Heskett et al. [i4] in the case of coadsorption on Cu(100) after annealing to 300 K. They pointed out that these EELS peaks may be due to different CO states, but other explanations cannot be ruled out. In order to clarify the nature of the loss peak around 180 meV, we have also taken spectra for oxygen coadsorbed with K on the Cu(111) surface. The result is shown as the dashed curve in fig. 3, which has the same peak position around 180 meV. This peak cannot be from any CO molecular adsorption state. Since the loss peak for adsorbed oxygen atoms should appear around 60 meV, the peak around 180 meV should be assigned to the -ibration of a K - O complex.

20 3.2. A R U P S

Fig. 4 shows a series of photoemission spectra I'~

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ENERGY LOSS lmeV} Fig. 3. HREEL spectra of CO adsorbed on C U ( ] ] ] ) w i t h K coverages of 0.16 at 300 K.

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emission with photon energy h v = 40.8 eV incident at 60 ° from thc ~urfacc rormal. The lowest spectrum (a) in fig. 4 was taken at low temperature (150 K) with a saturation dose of CO on a clean C u ( l l l ) surface. It shows the 5o-/lrr levels at a binding energy of 8.5 eV and the 4or level at 11.7 eV.

S. Bao et al. / Coadsorption of CO and K on Cu(l l l)

516

The spectrum (b) in fig. 4 was taken at low temperature with 0.2 L dose of CO on a Cu(111) surface precovered with K (Or( = 0.1). According to the H R E E L S results, only a~-CO molecules adsorbed. Two significant differences compared to spectrum (a) were found. One is an increase in 5o. binding energy of 0.4 eV and the other is the low 4 o ' / ( l r r + 5o') peak intensity ratio in spectrum (b). They show the difference of electronic structure between a v C O and the CO on clean Cu(111 ) surfac.~. For higher CO exposure (5 L), an additional peak was found at 8.1 eV, and there is a drastic increase in the 4 o . / ( l z r + 5 o . ) intensity ratio (shown in spectrum (c)). The change could be considered as the appearance of teE-CO. The spectrum (d) was taken at room temperature with a saturation CO dose on the C u ( l l l ) surface precovered with potassium (0 K = 0.1). The intensity ratio and energy positions of the 5o'/113and 4o- peaks are similar to that of low CO exposure on the same surface at I50 K, suggesting that only a I-CO exists. A new peak with lower binding energy appears at 5.5 eV in spectrum (d). We have also taken an UP spectrum for oxygen coadsorbed with K on the C u ( l l l ) s u r f a c e (shown in the dashed spectrum of fig. 4). The O(2p) derived level situates exactly at 5.5 eV. This shows that the coadsorbed K promotes the CO dissociation at room temperature. The curves (a) and (b) of fig. 5 are the H c l (21.2 eV) A R U P spectra with normal light incidence and a collection angle of 40 ° from surface normal taken at low temperature (150 K) with a saturation CO dose on a clean and a K-covered (OK = 0.1) Cu(111) surface respectively. The spectrum (a) shows that the lrr/5o- peak is located at 8.9 eV for CO on the clean C u ( l l l ) surface, with a lower energy than that found in spectrum (a) of fig. 4. T h j UPS studies of CO adsorption have shown that the o- symmetry valence levels of CO hnvo higher o~,-,~_~o,;,.,..~+ +~,.~ He T~ ..u..+.... .... -energy and the lrr level has higher cross-section for He I light [15]. Hence our data mean that 50has a lower binding energy than l r , in accord with the view that the order of the binding energies of the CO molecular orbitals is not the same as that on a variety of other metals [8]. Under the ',,,,, l v

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Fig. 5. ARUPS of He I taken at 40 ° emission with photons incident along the surface normal for 5 L CO on the (a) clean Cu(l 11 ); (b) Cu(111)with K coverage of 0. I.

influence of preadsorbed K, the l~r/5o, peak, mainly contributed by lrr, is found at 8.5 eV as shown in spectrum (b) of fig. 5. The peak is quite broad, but no splitting could be observed clearly.

4. Discussion

As reported above, we have ebserved some significant changes of CO under the influence of coadsorbed K. Comparing with other systems, such as CO + K / F e ( l l 0 ) [2], CO + K/Ni(100) [6] and CO + K/Ru(0001)[1], the adsorption state of CO in the CO + K / C u ( l l l ) system is quite simple. There are only one molecular state of CO, namely, a~-CO at room temperature and two molecular states o~-CO and a2-CO at low temperature ( < 200 K). For the O~l-CO, strongly interacted by K, the stretch frequency is much lower than that of CO adsorbed on the clean copper surface. This can be accounted for by the enhancement of electron donation into the 2rr UIUII.~II

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~k..l~...I.

A further question is: how do coadsorbed K atoms enhance electron backdonation? The continuous shift of the stretch frequency with the CO and K coverages suggests that the indirect interaction model is favored. In the direct interaction model [16], one CO molecule and one K atom

517

s. Bao et al. / Coadsorption of CO and K on Cu( l l l '

form a strongly interacting cluster. When more CO molecules are adsorbed, the additional CO will not join the original clusters and their influence on the preadsorbed CO will be relatively weak. The large continuous shift of the C - O stretch, more than 20 meV, could not be well explained. Moreover, our H R E E L S results show no vibration modes attributable to any bond between CO and K. In the indirect interaction model the coadsorbed K causes a redistribution of electron density in K with more density near the interface [17,18] which can enhance the donation to the 2~- orbital of a neighboring CO. The adsorbed CO consumes some of these electrons and surface electrons causing less donation to other CO molecules. In this model, the strong influence of K atoms exists in a relatively large region on the metal surface. Any change in the number of CO molecules and K atoms on the copper surface will cause a change of back donation. Hence the indirect model seems to be more likely. At low temperature (150 K), the He lI UP spectrum of saturated CO dosed on Cu(111) at a K precoverage of 0.1 shows an additional peak near the 5o-/l~r peak tat 8.1 eV). A similar result was also observed on the CO + K/Cu(100) system at a high K coverage of 0.3 by Surman et al. [13]. They pointed out that the two peaks around the 5o'/17r binding energy level were a splitting of the l~r orbitals cause by the CO symmetry lowering. Our H R E E L S data, however, show that there may be two different CO adsorption states in the case of coadsorption on Cu(111) with lower K coverage. For low CO exposure only c~-CO exists, and there is no additional UPS peak near the 5 o ' / l z r peak. Thus, the two peaks around 5o-/1~- binding energy level for low K and high CO coverages in our case are due to different CO states rather than ~ splitting of the 1~- orbitals. But other explanations cannot be ruled out. Our UP spectrum is taken at a photon energy of 40.8 eV, different from that of ref. [13]. The lack of the peak at 8.1 eV could be due to a cross section effect: it is too weak to be observed. We note that there is only one adsorption state of CO in the CO + K/Cu(100) system at a K coverage of 0.3 with saturated CO [14]. In this case. the two

peaks found around the 5o-/lzr binding energy level cannot be ascribed to different CO states and should be resulted from a splitting of the l~r orbitals. If there is indeed no splitting of 1~, orbitals for a~-CO in our ease, the difference between our case and the case of [13] may originate from the quite different adsorption orientation of CO in the two cases. At low K coverage of 0.1, the low 4o'/(1~-+ 5o-) intensity ratio of at-CO, shown in the UP spectrum with low CO exposure, is indicative of an electronically different bonding mechanism. Although the photoemission cross section of the 4o" orbital may decrease with the presence of K, a previous investigation has shown that this decrease amounts to only less than 30% at a photon energy of 40.8 eV (see figs. 4 and 6 of ref. [14]). So the drastic decrease in our data should be ascribed to an alternative origin, i.e., the strong tilt of CO molecules. The CO ( a 3) molecules on Fe(100) surface are inclined strongly as indicated by a decrease of 4o- peak [19] and confirmed by NEXAFS [20]. The measurements of the dependence of the CO 4o- peak intensity on the incidence angle show some differences for CO adsorption on the clean and K-precovered Cu( 111 ) surface. The dots in fig. 6 show the 40- peak area with He II light and normal emission for CO on a clean surface, while the triangles are that on the surface with a K coverage of 0.1. The data are normalized at

z

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Fig. 6. Total area of 4~r peak (||ell) at normal emission versus incident angle for 5 L CO exposure at 15(1K.The dots are the results on the clean Cu(l 11) xvhile the triangles on the Cu( i 11) with a K coverage of 0.1.

518

S. Bao et al. / Coadsorption of CO and K on Cu(l l l)

maximum intensity. The result for the clean surface agrees with the thin solid curve obtained by calculation assuming CO standing upright without K on the surface [21], but that for the precovered surface does not. This is quite different from the case of CO + K/Fe(110) [2] where the incidence angle dependence of the 4o- peak area agrees with the calculated curve despite the presence of K atoms. The different incidence angle dependence also suggests that the a~-CO molecules on the Cu(111) surface are inclined strongly. However, this suggestion needs to be confirmed by other structural determination techniques. The final question is on the CO dissociation. Both the UPS and EELS data at room temperature show that coadsorbed K promotes CO dissociation. The HREELS result suggests that the effect of K is not to form K - C O complexes indicated by Yates and co-workers [22,23] but to form K - O complexes. This was also suggested from the TDS results by Solymosi et al. [24]. According to our H R E E L S result at room temperature, the loss peak of K - O complexes at 180 meV cannot be found at lower K coverage (O K < 0.04). It i,~ generally believed [25,26] that the K adsorbed on metal surfaces is polarized due to t~le hybridization of adatom and substrate orbitals when OK is small, and the depolarization of K on the surface begins with 0~ increasing to some extent [17]. O,:~v i-iREELS results suggest that the polarized K and depolarized K are two different states of K on , . metal surfaces and the K - O complexes can be formed only from the depolarized K state.

Ackr~wledgement The authors would like to thank Z.G. Ji, D.S. Jt.~t,.av, x...J.a._.,llA ~,L.,II~.,JIOtlI,~ U l l l Y ~ l b l t y J

d l l l d ~..J.L. L l l i A l l g

(Academia Sinica, Beijing) for technical supoort.

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