Langmuir 1994,10,3615-3620
3615
Coadsorbate Induced Adsorption of CO2 on Ag(ll0): CO2 Interactions with Cs/Ag(llO)and with O/Cs/Ag(110) J. M. Campbell, S. Reiff, and J. H.Block* Fritz-Haber-Institut der Mm-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin (Dahlem), Germany Received January 6, 1994. In Final Form: July 27, 1994@ The adsorption and reaction of COz were examined on Ag( 110)precovered with submonolayer amounts of cesium and precovered with cesium and oxygen. Cesium promotes the adsorption of C02 at 325 K but not to the extent found in earlier studies for oxygen-precovered Ag(ll0). The adsorbed species is most likely a surface cesium carbonate, which decomposes a t higher temperatures than the surface carbonate formed on oxygen-precovered Ag(ll0). When both cesium and oxygen are preadsorbed, C02 adsorption increases dramatically, where again the adsorbed species is a carbonate. An increase in the corrugation of the electrical field near the surface is proposed as a reason behind the synergetic C02 adsorption on the cesium- and oxygen-precovered Ag(l10) surface.
1. Introduction The activation of C02 on metal surfaces has received sufficient attention to warrant a review artic1e.l For silver surfaces in particular, C02 adsorption was studied because of its relevance in the epoxidation of ethylene, where C02 is produced from the unwanted complete oxidation reaction. These studies found that C02 is physisorbed on the clean surface, but that preadsorbed oxygen promotes the chemisorption of C02. 1-6 For the oxygen-covered silver surface, CO2 adsorbs via the surface reaction
to produce the surface carbonate.1-6a This work examines the effects on C 0 2 adsorption over Ag(ll0) of an electropositive coadsorbate, cesium, which has been shown to increase selectivity in the ethylene epoxidation reaction over Ag(llO)' and of a mixed layer of electropositive and electronegative additives, cesium plus oxygen. I n addition, the interaction of 0 2 with cesium-precovered Ag(110) is also studied. 2. Experimental Section The ultrahigh vacuum system and the preparation of the Ag(110)crystal are described elsewhere.s The apparatus has the capabilitiesfor thermal desorption spectroscopy(TDS),low energy electrondiffraction (LEED),Auger electron spectroscopy (AES), argon ion sputtering,work function measurements via a Kelvin probe, and second harmonic generation (SHG). The sample was sputter-cleaned every five or six TDS cycles using 500-eVargon ions and briefly annealed t o 950 K to remove defects. Cleanliness was confirmed by LEED and by AES.2)339 Because 950 K is close to the transition temperature for Ag roughening,lO the annealing time was kept short (55 min) to
Abstract published in Advance ACS Abstracts, September 15, 1994. (1)Solymosi, F.J.Mol. Catal. 1991,65,337. @
(2)Campbell, C. T.; Paffett, M. T. Surf. Sci. 1984, 143,517. (3)Bowker, M.; Barteau, M. A.; Madix, R. J. Surf. Sci. 1980,92,528. (4)B a c k , C.;De Groot, C. P. M.; Biloen, P.; Sachtler, W. M. H. Surf. Sci. 1983,128, 81. (5)Stuve, E. M.; Madix, R. J.;Sexton, B. A. Chem. Phys. Lett. 1982, 89,48. (6) (a) Czanderna, A. W. J. Colloid Interface Sci. 1966,22,482.(b) Czanderna, A. W.; Biegen, J. R. J. Vac. Sci. Technol. 1971, 8,594. (7)(a)Campbell, C. T. J.Phys. Chem. 1986,89,5789.(b) Grant, R. B.; Lambert, R. M. Langmuir 1985, I , 29. (8)Reiff, S; Drachsel, W.; Block, J. H. Surf. Sci. 1994,304,L420. (9)Engelhardt, H. A.;Menzel, D. Surf. Sci. 1976, 57,618. (10)Bracco, G.; Malo, C.; Moses, C. J.;Tartarek, R. Surf. Sci. 1993, 2871288,871.
0743-7463/94/2410-3615$04.50/0
preclude significantsurface degradation. Surfacesannealed to only 800 Kexhibited defect-related features in the TDSof cesium. Cesium was deposited to the front of the sample by a lineof-sight doser using an SAES Cs getter zeolite. In all the experiments presented here, cesium coverages were 50.25 of a monolayer. Here, a monolayer is defined as the number of topmost atoms of the unreconstructed Ag(ll0) surface, 8.45 x 1014atoms cm-2. Reproduciblecoveragesof cesiumwere obtained by first dosing 0.3-0.4 monolayer of Cs, then annealing to the appropriate temperature as indicated by the Cs TDS.11 The cesium coverages were calibrated by LEED and by using work function measurements, where the minimum ofthe typical alkali work function curve was set at Oca = 0.25.11J2 The cesium coverageswere obtained by annealing to temperaturesin excess of 500 K, which exceeds the transition temperature for the long) at Oca z 0.1.11-14The presence of range ~ ( 1 x 2reconstruction the Cs-inducedp(l x2) reconstructionfor these Cs coverages was confirmedby LEED. At lower Cs coverages,alocalreconstruction is thought to occur;llJ4 therefore, all experiments using cesium were performed on initially reconstructed surfaces. COz (299.995%purity) and 0 2 (299.998%purity) were used as received from Messer-Griesheim and were dosed via backfilling. Gas dosingwas usually performedat 325 K, a temperature that could be rapidly attained afier annealing the Cs/Ag(llO) sample. TDS measurements were taken with an -4.8 Ws linear temperature ramp. A Balzers Quadrupole MS housed in a separate, differentiallypumped arm of the chamber was used for detection. Ametal cone normal t o the sample extending from the MS arm was placed within 2 mm of the front of the Ag(ll0) surface. 3. Results 3.1. C02/0/&(110). The adsorption of 0 2 onAg(ll0) and the coadsorption of C02 and 0 2 have been thoroughly ~ J ~adinvestigated by a number of r e ~ e a r c h e r s . ~ -The sorption of C02 on clean Ag surfaces is negligible when dosed above 100 K.3-6 On the oxygen-pretreated Ag(ll0) surface, however, COZis adsorbed at room temperature via a reaction with surface oxygen. The coadsorption of C02 and 0 2 on clean Ag(ll0) has been shown to produce a surface carbonate, COg,a,+1-6aAt room temperature, the reaction proceeds via a Langmuir-Hinshelwood mechanism where, first, oxygen is adsorbed and dissociated, then (11)Dohl-Oelze, R.; Stuve, E. M.; Sass, J. K. Solid State Commun. _1986. _ _ _ 67. - - 323. (12)Hayden, B.E.; Prince, IC C.; Davie, P. J.;Paolucci, G.; Bradshaw, A. M. Solic State Commun. 1983,48,325. (13)Reiff, S. Ph.D. Dissertation, TechnischeUniversitat, Berlin, 1993. (14)Barnes, C. J.;Lindroos, M.; Holmes, D. J.; King, D. A. Surf. Sci. 1989,219,143. (15)Barteau, M. A,; Madix, R. J. In The Chemical Physics ofSolid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1982;Vol. 4,p 95. 7
I
1994 American Chemical Society
Campbell et al.
3616 Langmuir, Vol. 10, No. 10, 1994 1
5
~
~
~
~
~
"
-
.-C
v)
a
.
10
. II
a
E -
300
400
500
600
700
800
900
1000
Temperature / K Figure 1. C02 ( d e = 44)thermal desorption spectra for various (a-e) COZexposures onAg(ll0)precoveredwith0.25monolayer of cesium. weakly adsorbed COZ reacts with the adsorbed atomic oxygen to form the ~ a r b o n a t e .The ~ carbonate can be thermally decomposed, liberating C02 a t -460 K and leaving behind O a d , which subsequently may be associatively desorbed a t -580 K.2-5J6 The adsorbed carbonate species has been shown to have three structurally similar oxygen atoms,2J6-18with the plane ofthe C-0 bonds parallel to the surface ~ f A g ( l l O ) . ' ~The J ~ formation of the surface carbonate produces ~ ( 1 x 2 LEED ) dom a i n ~ , ~which , ' ~ may be caused by a surface reconstruction.lg The surface carbonate resembles a bulk ionic carbonate, Cos2- 17~18,20and has a decomposition energy of approximately 27 kcal/m01.~?~ Since C02 does not adsorb on Ag(ll0) a t 325 K, experiments where C02 was dosed a t 325 K to the Ag(110) surface prior to exposure to 0 2 produced no adsorbed carbonate. Reversing the dosing procedure, however, did produce the adsorbed carbonate species as evidenced by the TDS peaks of COZa t 445 K and of 0 2 at 565 K. Previous experiments have shown that the saturation amount of C02 adsorbed is proportional to the O a d coverage, with a saturation carbonate coverage nearly equal to the oxygen coverage when 60 is less than 0.25.2,3J6 The saturation carbonate coverage becomes lower than the oxygen coverage when 60 is greater than 0.25 because of lateral repulsion effect^.^,^,'^ At a n oxygen coverage of 60 = 0.10, the integrated C02 TDS peaks from carbonate decomposition were nearly linear with increasing C02 exposure. At this oxygen coverage, the maximum carbonate coverage was achieved with approximately 100 langmuirs (1langmuir = 1L = Torrs) exposure of CO2, in accordance with previous results.16 3.2. CO&s/Ag(llO). Figure 1 shows the thermal desorption spectra for various doses of C02 a t 325 K on a n Ag(110) surface precovered with cesium. The cesium coverage is a t the minimum in the cesium-induced work function change, i.e., 8cs = O.25.l1-l3 I t can be seen that C02 adsorbs in a t least two adsorption states, which desorb a t -675 and -750 K, respectively. The less stable 675 K state is formed first a t lower coverages and is diminished (16)Barteau, M. A.; Madix, R. J. J . Chem. Phys. 1981, 74, 4144. (17) Madix, R. J.;Solomon, J. L.; Stohr, J. Surf.Sei. 1988,197, L253. (18)Prince, K. C.; Bradshaw, A. M. Surf. Sci. 1983, 126, 49. (19) Bader, M.; Hillert, B.; Puschmann, A,; Haase, J.; Bradshaw, A. M. Europhys. Lett. 1988, 5, 443. (20) Barteau, M. A.; Madix, R. J. J . Electron Spectrosc. Rel. Phemm. 1983, 31, 101.
'
~ expense ~ ~ stable ~ 750~ K state " with~ higher ' a t the of~the more doses. For the spectra between the 25 langmuir CO2 and 100 langmuir C02 doses, the C02 coverage remains approximately the same, about 0.04;much higher doses of C02 (-500 L) were needed to appreciably increase the coverage of adsorbed C02. The shape of the TDS curves changes although the coverage does not vary substantially here. This suggests that the adsorption of C02 causes some ordering of the cesium layer; however, the ~ ( 1 x 2 ) LEED pattern of the Cs-induced reconstruction remained unchanged except for higher background with COZexposure. The desorption peaks in Figure 1are due to CO2 assigned to the decomposition of a surface carbonate ( C O s , a d ) , associated with cesium. C02 is not known to adsorb or decompose on clean Ag(110);2-5J6however, a carbonate is formed upon dosing C02 to matrix-isolated Cs,2l to bulk alkali m e t a l ~ , 2and ~ , to ~ ~alkali-covered metal surface^.^^-^^ Two mechanisms have been proposed to account for the formation of this In each case, the mechanism proceeds via the adsorbed C02- anion formed from charge transfer from the alkali. The carbonate can be formed via an oxalate intermediate, which thermally decomposes
Alternatively, the carbonate can be formed via disproportionation
~
C02
(3) The surface carbonate on clean Ag(ll0) decomposes to adsorbed oxygen and COz, which immediately d e ~ o r b s ; ~ - ~ J ~ therefore, the formation of a similar surface carbonate upon adsorption of C02 on Cs/Ag(llO) can be checked by the presence of surface oxygen after the desorption of C02. Following the exposure of Cs/Ag(llO) to 25 langmuirs of C02, as in spectrum lb, the TD spectrum of m/e = 32 (not shown) reveals the desorption of oxygen a t 800 K, consistent with the desorption of highly stabilized oxygen. Following exposures of 100 langmuirs of C02, lower temperature oxygen desorption, starting at 650 K, also occurs. No m/e = 28 (Le., CO) peaks that could not be attributed to dissociation of C02 in the ionizer of the mass spectrometer were observed. This implies that CO produced via eq 2 or 3 is immediately desorbed a t our dosing temperature of 325 K. Transient adsorption of CO, as a result of carbonate formation (eq 2 or 3) or from impurities in the gas feed, however, may cause the apparent ordering of the adsorbate layer. A caveat is in order here. The coverages of carbonate are very small here, on the order of 4-5% of a monolayer, so that surface defects and slight oxygen impurities in the (21) Kafafi, Z. H.; Hauge, R. H.; Billups, W. E.; Margrave, J. L. Inorg. Chem. 1984,23, 177. (22) Paul, J . ; Hoffman, F. M.; Robbins, J. L. J . Phys. Chem. 1988,92, 6967. (23) Shao, Y.; Paul, J.;Axelsson, 0.; Hoffman, F. M. J . Phys. Chem. 1993,97,7432. (24) Solymosi, F.; Berko, A. J. Catul. 1986, 101,458. Furadu-y Trans. 1 1987, (25) Solymosi, F.; Buwi, L. J . Chem. SOC., 83, 2015. (26) Kiss, J.; Revesz, K.; Solymosi, F. Surf. Sci. 1988,207, 36. (27) Liu, Z. M.; Zhou, Y.; Solymosi, F.; White, J. M. J . Phys. Chem. 1989,93,4383. (28) Liu, Z. M.; Zhou, Y.; Solymosi, F.; White, J. M. Surf. Sei. 1991, 245. 289. (29) Rodriguez, J. A.; Clendening, W. D.; Campbell, C. T. J . Phys. Chem. 1989,93,5238.
__
~
Adsorption of CO, on Ag(ll0)
Langmuir, Vol. 10, No. 10, 1994 3617
23
2
6oo
.
m
0.7
500 7
:0.5
.@ 400 { v)
C
-
0.4
300 :0.3
FJ
200 1 -
s! E
i 0)
2 a 0
N
I1
&
100 6)
0.2
E 52 x
0
:0.1
a
= L
07 0
I
I
300
500
400
600
700
800
900
1000
Temperature / K Figure 2. 0 2 (mle = 32) thermal desorption spectra for 100 langmuir exposures of oxygen on Ag(ll0) for various precoverages of cesium. gas feed or during Cs dosing could influence the observed desorption. However, because no COZ adsorption nor dissociation products were observed after dosing CO2 to clean Ag(llO), impurities could only be expected from the cesium doser, which was thoroughly outgassed prior to use. The Cs (m/e = 133)TD spectra were very sensitive to impurities, and no gas dosing was performed until a n impurity-free and defect-free spectrum could be reproduced. Surface oxygen might be produced from the reaction COZ CO Oad. If so, the reaction must proceed a t a very low rate, probably defect mediated, because the reaction to form carbonate, as in eq 1, proceeds rapidly on the cesium-doped surface as seen below. From the coverages of COZmeasured here, it can be said that the sticking probability for C02 on Cs/Ag(llO) a t 325 K is very low, 0.02 and 8cs 0.25. At 8cs = 0.25, the minimum of the work function, there is a low temperature peak a t -555 K accompanied by resolved high temperature peaks at 720 K, 785 K, and a small peak a t 910 K, a s well a s shoulders a t 690 K and 850 K. The uptake of 0 2 by the Ag(ll0) surface after 100 langmuir 0 2 doses increases as the cesium precoverage is increased as shown in Figure 3, which plots the integrated area of the 0 2 desorption peaks against the cesium precoverage. The apparent enhancement of 0 2 adsorption evident in Figure 3 is most drastic a t low cesium coverages but levels off a t Bcs x 0.10. For lower oxygen exposures, the initial sticking probability was proportional to the cesium coverage, but showed two regimes.13 At a coverage of 8cs = 0.12, where the Cs-induced (1x2)
-
0.05
0.1
0.15
0.2 0.25 0.3
Cesium Coverage Figure 3. Uptake of 100 langmuirs oxygen by CdAg(ll0)for various precoverages of cesium. Oxygen coverages were determined by the area ofthe oxygen thermal desorption curves calibrated for known coverages of oxygen. ~ " " ' " ' " " " " ' " ' ' " ' " " " ' ' c
50 40
98 L
30
20
+
3.2 L
10
1.1 L
s!
E
O
0.0L 1 ,
~~
~~~~~~
,
I
.
,
.
,
I
25 L
.
,
/ . .
, . I , , . . ,
, , . ,
,.,..
I . . - -
~
(30) Becker, L.;Aminpirooz, S.;Schmalz, A.; Hillert, B.; Pedio, M.; Haase, J. Phys. Rev. B 1991,44, 13655. (31) Yang, L.;Rahman, T. S.;Bracco, G . ;Tartarek, R. Phys. Rev. B 1989. 40. 12271. _._. -~~ (32) Taniguchi, M.; Tanaka, K.; Hashizume, T.; Sakurai, T. Chem. Phys. Lett. 1992,192, 117. (33) Bradshaw, A. M.; Engelhardt, A.; Menzel, D. Ber. Bunsen-Ges. Phys. Chem. 1972, 76,500. I
- - 7
~
Campbell et al.
3618 Langmuir, Vol. 10, No. 10, 1994
'5 20
.
0,Dose
15
x
-
v)
5
10-
C A
I,,,,
I
300
400
500
600
700
800
900 1000
Temperature / K Figure 5. COz (mle = 44) thermal desorption spectra for coadsorptionof 10 langmuirsof COz with various 02 exposures on Ag(110)precovered with 0.25 monolayer of cesium. Oxygen was dosed first at 325 K t o Cs/Ag(llO),followed by exposure t o 10 langmuirs of COz. muirs), almost all lower-temperature cesium desorption is suppressed in favor of the 780 K desorption with another small peak appearing a t -920 K. It might be added that mle = 149 (CsO+) was also monitored during TDS; however, any desorption of a CsO-containing species was below the detection limits of the mass spectrometer, implying that the desorption of oxygen and of cesium are separate events. Thus, as one component is desorbed, the other loses its stabilization and desorbs almost simultaneously. 3.4. COz/O/Cs/Ag(llO). Having seen that COZwill adsorb on the Ag(ll0) surface in the presence of oxygen and of cesium, the effect of a coadsorbed layer of OKs on Ag(ll0) was also investigated. In the experiments of Figure 5, an excess of cesium was deposited to the clean Ag(ll0)surface, and then annealed to 550 K to reduce the cesium coverage to Bcs = 0.25." The cesium-covered Ag(110) surface was exposed at 325 K to oxygen first, then to COZ. After a n oxygen dose of 0.52 langmuir, the COz TDS exhibits two peaks a t 765 K and 650 K. These two peaks resemble the peaks present when COz alone is dosed to the Cs precoveredAg(ll0)surface (see Figure 11,except that the peak temperatures have been shifted and that the intensity of COZ is much higher for the oxygen-treated surface. It is possible that the 650 K peak could be a new desorption state induced by the adsorbed oxygen and that the 675 K state exhibited in Figure 1,ifpresent, is masked in Figure 5 by the decay of this new state and by the onset of the 765 K desorption. Increasing the oxygen coverage causes the suppression of the 765 K state and destabilization of the 650 K state to -600 K as well as the appearance of a new state at -480 K, which becomes the main desorption feature. This 480 K state is very close to the desorption temperature exhibited by the TDS of COZ for the decomposition of carbonate on the cesiumfree Ag(110)~ u r f a c e . ~ -At ~ Jthe ~ highest oxygen coverages shown here, additional features in the high temperature tail are exhibited, along with the broadening of the main desorption peak toward lower temperatures. A procedure similar to that used to generate Figure 5 was used to produce the spectra of Figure 6, except that the cesium adlayer was annealed to 800 K to yield a cesium coverage of 0.02.11 0 2 is not adsorbed as readily a t this cesium coverage as a t Bcs = 0.25, as shown in Figure 3, and consequently, neither is COZ, so that the TD spectrum taken after a 0.52 langmuir dose ofoxygenshows
300
. , , ,
400
, , , ,
500
, , , ,
700
600
, . , ,
, , , ,
,,,,I
900 1000
800
Temperature / K Figure 6. COz (mle = 44) thermal desorption spectra for coadsorptionof 10langmuirsof COz with various 0 2 exposures onAg(ll0)precovered with 0.02 monolayer of cesium. Oxygen was dosed first at 325 K to Cs/Ag(llO),followed by exposure to 10 langmuirs of COz.
-
.-C v)
a
500
I C02/0/Cs/Ag(110) C02 Dose = 10 L
.
/&
a
= 0.25
- 0.20 - 0.15
'
e,, =0.02
-
0.10
5
6$ mC 0
em
- 0.05 0
9 E
r 0.00
P -
L 1 0
C
5
10
15 20
40
60
80 100 120
O2 Exposure / L Figure 7. Uptake of 10 langmuirs of COz by O/Cs/Ag(llO)for various exposures of oxygen and for three precoverages of cesium. Oxygen was dosed first to the Cs/Ag(llO) surface. Carbonate coverages were determined from the area under the COz thermal desorption curves calibrated for k n o w n coverages.
very little evidence of C02 adsorption. With a slightly higher oxygen dose, a peak appears a t -600 K. The effect of oxygen and cesium on the adsorption of COZon Ag(ll0) is depicted graphically in Figure 7 for 10 langmuir exposures of C02 to the Ag(ll0) surface as treated above. It can be seen that COz adsorption increases with oxygen exposure for the three cesium coverages shown. The enhancement of C02 adsorption is also dramatically affected by the amount of cesium precoverage. This is partly caused by the increase in oxygen adsorption with Cs coverage. At Bcs = 0.02 and with a 100 langmuir 0 2 exposure, the oxygen coverage reaches -0.22 (see Figure 3). With the 10 langmuir COz exposure, the carbonate coverage reaches -0.11, or about half the saturation coverage of carbonate if the reaction COZ Oad COS,^^ is considered the sole mechanism of formation. For a similar oxygen coverage on Ag(ll0) without cesium, more than 75 langmuirs of COz would be required to achieve the 0.11 carbonate coverage.16 For Bcs = 0.25, the 10 langmuir COz dose is sufficient to saturate the carbonate coverage for all 0 2 exposures shown. It is evident that the adsorption of COz is much more facile with the mixed OlCs adlayer than with either preadsorbed species alone. The LEED patterns for all 0 Cs coverage regimes
+
-
+
Adsorption of CO2 on Ag(ll0) exhibited faint p(1x 1)patterns after adsorption of C02. The (2 x 1)reconstruction induced by oxygen exposures of 1.100 langmuirs (see above) is apparently lifted by C02 adsorption and streaking in the [OOlI direction occasionally appears. The streaking probably represents partial reconstruction induced by the carbonate, which has been reported for the carbonate on clean Ag(l10).19
4. Discussion 4.1. C02/Cs/Ag(110).The adsorption of COZon Cs/ Ag(ll0) at 325 K most likely involves adsorption through a n activated, bent C02- anion as opposed to the gaseous, linear form. The COZ-is formed through charge transfer from Cs to CO2, possibly via a transient physisorbed state. Studies of C02 adsorption on Cs-dosed Ag films have shown the presence of an electronically unperturbed, physisorbed C02 and a n activated, bent form, assigned to COz-, a t 50 K,34which was not observed on the clean Ag film. COzwas also identified after the reaction of COZ and Cs in nitrogen and argon matrices a t low temperatures.21 In subsequent thermal reaction, the carbonate (assigned to Cs2C03)was preferentially formed along with the oxalate (CszC204)by 300 K. At 100 K on bulk Na films, there is evidence for a n oxalate (Na2C204,*d)species which decomposes to the carbonate between 150 and 360 K, probably via eq 2b.22 Similarly, detailed analysisz3of data for COZadsorbedonK/Rh(111)26a n d ~ n W P t ( l l lassigns )~~ a mechanism whereby adsorbed COZ- forms a n oxalate that decomposes to the adsorbed carbonate by 300 K. These studies indicate that adsorbed C02 (COz-) or surface oxalate ( C Z O ~ ~ are - ~probably ~) unstable at 325 K. Thus, it is most likely a surface carbonate that is formed at our dosing temperature of 325 K. Indeed, for C02 dosed to the Cs/Cu(llO) surface, it was shown that carbonate is formed a t as low as 100 K.29 The oxalate mechanism was also proposed for other alkali-covered metal surfaces, even a t less than monolayer coverage^.^^-^^ The studies above also suggested that carbonate formation might also occur through disproportionation of two COz molecules, for instance, via eq 3; however, it is indistinguishable in our experiments from a mechanism with a n oxalate intermediate. For bulk potassium, a t least, there is no thermodynamic preference; A H is approximately -112 kcaVmol for reactions 2 and 3.35 Note that neither mechanism requires adsorbed oxygen to produce the carbonate, although the net reaction appears to involve the dissociation of C02. Assuming that the desorption of COz coincides with the decomposition of the carbonate, carbonate decomposition is stabilized by more than 190 K on Cs/Ag(llO) compared to the carbonate formed from 0 2 and COz on the clean Ag(l10) surface. Considering that the oxygen remaining on the surface after carbonate decomposition is also stabilized against desorption until -800 K, this suggests that the carbonate is intimately associated with cesium, most likely as a cesium carbonate. For comparison, the peak temperatures of the spectra in Figure 1are similar to the peak temperatures for carbonate decomposition from multilayer cesium on Cu(llO), which were also assigned to a bulklike cesium carbonate.29 4.2. OZ/Cs/Ag(llO).For Cs coverages less than 0.25, the majority of the oxygen desorption in Figure 2 occurs in the low temperature peak after oxygen exposures of 100 langmuirs and is little different in peak temperature for oxygen on a n otherwise clean Ag(ll0) surface. In a simple model for adsorption, the high temperature tail (34) (a) Maynard, K. J.;Moskovitz, M. J.Chem.Phys. 1989,90,8668. (b) Maynard, K. J.; Moskovitz, M. Surf. Sci. 1990,225, 40. (35) CRC Handbook of Chemistry and Physics, 55th ed.; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1974.
Langmuir, Vol. 10, No. 10, 1994 3619 present when cesium is predosed represents desorption of oxygen more intimately adsorbed with cesium, while the low temperature desorption peak is indicative of oxygen adsorbed on clean patches of silver. Indeed, the feature at 785 K that is relatively prominent in the spectra for a cesium precoverage of 0.25 correlates well with an oxygen desorption a t 800 K following the decomposition of carbonate formed from dosing COZto Cs/Ag(llO) as described above, which must originate in the cesium layer. The main peak temperature trend to lower temperatures with increasing cesium coverage is a consequence of the increasing oxygen coverage. With increasing oxygen coverage, more oxygen on clean silver patches becomes shielded from the long-range attractive effects of cesium and subject to repulsive lateral interactions. This combination of effects may also explain the broadening of the main peak for the intermediate Cs coverages. Note the sudden prominence of high temperature features and the narrowing of the main peak a t 8cS = 0.25, at the minimum ofthe Cs work function. The abrupt onset of 4s-like resonances in EELS36and a n increase in the Cs-Ag interatomic distance37have been observed at the minimum of the work function for Cs on silver single crystal surfaces. Rhead has proposed that a nucleation of alkali chains into dense island structures occurs a t this coverage.38 Modesti et al. have shown that potassium will compress into dense two-dimensional islands on Ag(100)well before saturation of a condensed overlayer and Islanding have claimed similar results for Cs 0nAg(lO0).~~ might also be induced by the adsorption of oxygen which would relieve Cs-Cs dipole repulsions. This phenomenon could explain the narrowing of the main peak because the number of oxygen atoms adjacent to alkali chains would decrease as islands coalesce. Within these islands, adsorbed oxygen would be stabilized by the increase in nearest neighbor alkali atoms and the closer packing. Of course, this assumes that the dense island structure is not greatly perturbed by the adsorption of oxygen. The stabilization of alkali metals upon coadsorption of oxygen has also been observed on other metal surf a c e ~ For . ~oxygen ~ ~ coadsorbed ~ ~ ~ ~ with ~ Cs on Ag(llO), the interpretation of UPS results has indicated the formation of a surface oxide, or oxide complex.42Even a t low oxygen exposures, most cesium could exist as a n oxide, but a t the cesium coverage of 0.25 in Figure 4, the dense island structure (see above) would cause the completion of the oxide to proceed slowly. The separate but nearly simultaneous desorption of oxygen and cesium also suggests the decomposition of a surface compound with mutual stabilization. Albano has modeled the stabilization of K on Fe by coadsorbed ~ x y g e n ,where ~ ~ , ~atomic ~ oxygen forms a pair with an alkali atom. This alkalioxygen pair shows attractive interactions to other pairs, thereby stabilizingboth the oxygen and the alkali. Similar proposals have been made by others for oxygen and alkali coadsorption on silver,44but in more general terms. 4.3. COdO/Cs/Ag( 110). Figures 5-7 illustrate that COZ adsorption is considerably enhanced by the exposure (36) Matthew, J. A. D.; Netzer, F. P.; Astl, G. Surf. Sci. 1991,259, -L'15'1. --(37)-&tmble,-G.M.; Brooks, R. S.;King, D. A.; Norman, D. Phys. Rev. Lett. lwu3, til, 111z. (38)Rhead, G. E. Suq. Sci. 1988,203, L663. (39) Modesti, S.;Chen, C. T.; Ma, Y.; Meigs, G.; Rudolf, P.; Sette, F. Phys. Rev. B 1990,42, 5381. (40) (a) Garfunkel, E. L.; Somojai, G. A. Surf. Sci. 1982, 115, 441. (b) Pas, Z.; Ertl, G.; Lee, S. B. Appl. Surf. Sci. 1981, 8, 231. (41) (a) Albano. E. V. Surf. Sci. 1989,215,333. (b) Albano, E. V. J. Chem. Phys. 1986,85, 1044. (42) Prince, K. C.; Kordesch, M. E. Appl. Surf. Sci. 1986,22/23,469. (43)Albano. E. V . ADDLSurf. Sci. 1982. 14. 183. (44) Heskek, D.; Ti&, D.; Shi, X.; Tsuei, K.-D. Chem. Phys. Lett. 1992,199, 138.
3620 Langmuir, Vol. 10, No. 10, 1994 ofthe Cs/Ag(llO) surface to even small amounts ofoxygen. It is known that small amounts of oxygen lift the Csinduced (1x2) reconstructionl1Jz and can eliminate metallike bands in the EELS of C S / A ~ Oxygen . ~ ~ also provides another pathway for carbonate formation via eq 1. While SERS and EELS experiments a t 50 K indicate that the carbonate precursor-adsorbed, activated COZ(assigned t o COz-)-is quickly saturated at low COZexposures ( < 7 l a n g m u i r ~ ) Figure ,~~ 1 shows a very slow buildup of adsorbed carbonate with increasing COz exposure. At the 325 K dosing temperature, reaction 1 should have a much higher rate than oxalate formatiorddecomposition (eq 2) or COZ disproportionation (eq 3)mechanisms, which require the bimolecular reaction of short-lived COZ-ad precursors. Thus, a major factor in the apparent increase of COZ adsorption on O/Cs/Ag(llO) as opposed to Cs/Ag(110) is the ability of oxygen to trap activated COz. On the oxygen-precovered Ag(ll0) surface, carbonate formation also proceeds by reaction 1,yet, as noted earlier, 275 langmuirs of COZare needed to reach 80 = 0.1 on Ag(110),16whereas only 10 langmuirs are required to achieve oxygen coverages of up to 0.33 on Cs/Ag(110). This synergy toward COzadsorption may have several reasons. The interaction of COz with oxygen-predosed WPt(ll1) has been investigated by Liu et alez8 They observed enhanced carbonate formation a t 200 K with HREELS, which was attributed to scavenging of oxygen by COz-. Enhanced CO adsorption was explained through increased charge on K induced by electronegative oxygen.28 Those experiments were performed a t K coverages greater than 3 times the coverage of the work function minimum. In fact, oxygen adsorption has been observed to lower the work function on transition metals for alkali coverages beyond the alkali-induced minimum of the work function.38~40,43~44 The lower work function, of course, means a lower energy barrier for electron transfer from the surface; thus, upon COz dosing, the population of adsorbed COZ- should increase. The Cs coverages in this work, however, are a t or below the work function minimum, and oxygen adsorption raises the work function slightly. This does not rule out the influence of the work function, because one effect of oxygen adsorption onto Cs/Ag(110) is lifting the cesium-induced (1x 2) reconstruction.'lJZ The dipole moment for cesium on the unreconstructed surface is some 3 D larger than on the (1x2) surface (10.9 D vs 7.9 D).ll This larger dipole moment, which may be considered a lower local work function of Cs on the Ag(110)-(1x 1)surface, may enhance charge transfer to COZ, which is more polarizable than Oz.35COz might therefore be expected to be activated to a greater extent than 0 2 , and, as discussed above, 0 2 adsorption is greatly enhanced by the presence of cesium. What may be more important than the absolute value of the dipole moment is the electric field corrugation near the surface. Adsorbed Cs is thought to be slightly embedded in the troughs of the (1x2) reconstructed Ag(110)surface.llJ4 Such a n arrangement would screen the Cs dipole moment and smooth out the electric field near the surface. With oxygen lifting the reconstruction, the dipoles would no longer be screened by the Ag surface atoms, resulting in an electrically more corrugated surface. The perturbation of the electronic structure of the COZ molecule by the surface field corrugation should lower the barrier to the COZ bending vibration. The increased lifetime of the bent COz conformation would consequently increase the probability of charge transfer from the surface. In Albano's alkali-oxygen pairing the electric field corrugation would be even more pronounced between alternating cesium and oxygen dipoles. Some support for this model comes from EELS measurements, which show
Campbell et al. that metallic 4s-like resonances are negated by oxygen a d ~ o r p t i o nand , ~ ~from a UPS study, which shows that Cs is in close proximity to adsorbed oxygen.42 As the oxygen exposure is increased in Figure 6, the COz desorption occurs a t increasingly lower temperatures, although the total amount of COZdesorption increases (see Figure 7). At such a low cesium coverage, very little islanding can occur except for isolated chain formation, so the absence ofhighly stabilized desorption states is not surprising. What is puzzling is the increasing instability of the high temperature desorption states as the coverage increases, which is also exhibited in Figure 5 to some degree. Barteau and Madix have suggested that compression of the carbonate adlayer hinders further COZ adsorption on O/Ag(110).l6 However, significant effects were not observeduntil the carbonate coverage approached 0.2518and desorption was unaffected despite the proposed increase in repulsive interactions a t high coverages. In this case, the carbonate coverage is only -0.11 for the topmost spectrum in Figure 6, whereas all spectra show coverage effects in desorption. This suggests that the local coverage of carbonate near adsorbed cesium atoms may be high enough to induce significant lateral repulsions manifested in a lowering of decompositiorddesorption temperature. Evidently, the oxygen left behind after carbonate decomposition is sufficiently immobile to cause the loss of higher temperature COZdesorption states. This may be related to results of plasma dosing and of highpressure dosing experiments, which have assigned the oxygen associated with the 545 K oxygen TDS peak (see Figure 2, bottom) to or to subsurface46oxygen rather than chemisorbed surface oxygen. If the adsorption site for oxygen is subsurface, one would expect higher diffusional barriers than for surface oxygen. E M S has only shown that the oxygen lies very close to the Ag surface, 0.2 f 0.2 A above or below the topmost Ag plane of the reconstructed surface.30 Because of the many unknowns in the mechanism of ethylene epoxidationover silver,it may seem unreasonable to speculate on the selectivity enhancement seen in ethylene epoxidation when cesium is present.' Cesium promotes both complete and partial oxidation of ethylene; however, there is a selectivity enhancement because complete oxidation is promoted less e f i ~ i e n t l y . The ~ stabilization of carbonate by cesium suggests that the higher coverage of carbonate during catalytic reaction could inhibit complete oxidation after the formation of the epoxide by a n ensemble effect. Adsorbed oxygen, which might otherwise be hydrogenated by the epoxide,7b would be bound up in the carbonate. Another suggestion is that the increased carbonate coverage promotes the adsorption of dioxygen, which is then responsible for ethylene epoxidation.l* In summary, the presence of cesium enables the adsorption of COZon Ag(ll0) a t 325 K. COZuptake on Cs/Ag(llO) is limited by the low probability of forming a stable surface species, which is assigned to the surface COZ uptake is enhanced by the presence carbonate, of o a d on Cs/Ag(llO),which provides a n alternate pathway for carbonate formation. COZadsorption is also enhanced by the corrugation of the electric field near the surface when oxygen and cesium are coadsorbed on Ag(ll0). Acknowledgment. J.C. wishes to thank the MaxPlanck-Gesellschafi for fellowship support. We thank Professor Dr. Jurgen Sass for helpful discussion and Frau Beran for preparing the silver sample. (45) Bowker, M.Su$. Sei. 1986,155, L276. (46) Rehren, C.;Issac, G.;Schlogl, R.; Ertl, G. Cutul. Lett. 1991,11, 253.
