The interaction of methanol with modified Ru(001) surfaces: The effects of oxygen and potassium

Surface Science 166 (1986) 361-376 North-Holland, Amsterdam

361

THE INTERACTION OF METHANOL WITH MODIFIED Ru(OO1) SURFACES: THE EFFECTS OF OXYGEN AND POTASSIUM Jan HRBEK

*, Robert

DE PAOLA

** and Friedrich

M. HOFFMANN

Exxon Research and Engineering Company, Corporate Research Science Laboratories, Clinton Township, Route 22 East, Annandale, New Jersey 08801, USA

Received

12 May 1985; accepted

for publication

19 September

1985

The adsorption and decomposition of methanol on a Ru(OO1) surface modified by preadsorbed potassium and oxygen has been investigated with electron energy loss spectroscopy (EELS) and multiple mass thermal desorption spectroscopy (TDS). On ruthenium precovered with a low coverage of atomic oxygen (0, = 0.25), methoxy formation is promoted with respect to the clean surface and is found to occur at 85 K. In contrast, on ruthenium modified by “ionic” potassium (Bk = O.lO), methoxy formation is inhibited, i.e., oxygen-hydrogen bond breaking does not occur until temperatures reach 240 K. On both surfaces, the decomposition of methanol proceeds via O-H and C-H bond breaking and ultimately leads to the formation of CO and H,. The results differ from those obtained on the clean Ru(OO1) surface, where methoxy formation is observed at 85 K only when exposed to small amounts of methanol, and where a significant C-O bond-breaking decomposition channel leading to the formation of water and surface carbon is also present. The differences in methanol reactivities on these low-coverage potassiumand oxygen-modified surfaces can be attributed largely to electronic modifications of the metal substrate. At higher coverages of the surface additives, a different behavior is encountered. Oxygen at /lo = 0.60 completely inhibits the formation of methoxy due to physical site blocking, while metallic potassium (0, = 0.33) reacts directly with methanol to form potassium methoxide.

1. Introduction Because the chemistry of methanol (CH,OH) on transition [l-7] and noble metals [8-111 is characterized by a variety of reaction products, the interaction of this molecule with metals has frustrated past attempts to clarify the role of metal selectivity in its decomposition. Methoxy mediates methanol decomposition on most metal substrates and its reactivity with a particular substrate has been found to control the ultimate decomposition products. On most noble metals (e.g. Cu and Ag), methoxy is found to decompose to formaldehyde * Permanent address: Chemistry New York 11973, USA. ** Permanent address: Department sylvania 19104, USA.

Department of Physics,

555, Brookhaven University

National

of Pennsylvania,

0039-6028/86/%03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

Laboratory, Philadelphia,

Upton, Penn-

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J. Hrhek er al. / Interaction of methanol with modified Ru(001)

(CH,O) [S-lo], while on transition metals it typically decomposes directly to CO + H, [l-7]. Modification of metal surfaces with additives, however, alters this behavior. Oxygen has been found to increase both the rate of formation and the stability of methoxy on Cu [8,9], Ag [lo], Pt [l], Pd [5] and Rh [12]; only on MO is the formation of methoxy inhibited by preadsorbed oxygen [3]. Sulfur has also been found to increase the stability of methoxy in the case of a Ni(lOO) substrate [13]. Thus far, electropositive coadsorbates such as alkali metals have not been employed in studies of methanol decomposition. Surface additives can modify chemical decomposition through (1) the electronic modification of the metal substrate, (2) by physical site blocking or (3) through a direct reaction with the adsorbate. In order to discriminate the electronic modification (metal selectivity) from other contributions it is necessary, then, to use both electropositive and electronegative additives. We have chosen, therefore, to modify the Ru(001) surface with potassium (electropositive) and oxygen (electronegative) in order to study methanol decomposition. Ru(001) is a particularly attractive substrate for the investigation of the effects of surface modifiers on reaction selectivity not only due to the extensive methanol decomposition study previously performed on the clean surface [7,14], but also because of the ability of this surface to cleave both the C-O and O-H bonds of methanol.

2. Experimental The experiments have been performed in a three-level stainless-steel UHV chamber (10-t’ Torr range) equipped with high resolution electron energy loss spectroscopy (EELS), LEED, Auger electron spectroscopy (AES), multiple mass thermal desorption spectroscopy (TDS) and a Kelvin probe (for workfunction measurements). Experimental details are described elsewhere [7]. Oxygen (research grade from Matheson) was exposed to the surface by backfilling the chamber. The adsorbed overlayer was characterized with LEED, EELS and AES. A coverage of 0, = 0.25 (~(2 X 2) LEED pattern) was chosen for the modification studies. Potassium was evaporated from a commercial dispenser (SAES Getters) which was mounted in a retractable bakeable housing. Overlayers of potassium were characterized by AES, LEED and TDS for coverage and cleanliness as described in detail elsewhere [15]. Two coverages of potassium were chosen for the modification studies, 8, = 0.10 and 8, = 0.33. Both were prepared by annealing a potassium multilayer to specific temperatures (to ensure a uniform distribution and coverage reproducibility) and characterized by LEED. These two potassium overlayers differ in their electronic adsorption state. At low coverages (0, = O.lO), the potassium adatom is partly ionic due to charge transfer to the metal surface as inferred from the strong decrease in work

J. Hrbek et al. / Interaction of methanol with modified Ru(OO1)

363

function [15] (this modified surface will be referred to as K+-Ru). At a monolayer coverage (e, = 0.33), characterized by a (fi x fi)R30” LEED pattern, the potassium adatoms are depolarized and exhibit a metallic character [15]. This surface will be referred to as K”-Ru. After preparation and characterization, the modified surfaces were exposed to methanol either through capillary beam dosers (at both TDS and EELS levels) or via backfilling of the chamber (for calibration purposes). For further experimental details see ref. [7].

3. Results 3.1. Methanol a&orbed on clean Ru(OO1) The adsorption of methanol on clean Ru(001) has been previously studied in detail [7,14] and can be summarized as follows: (1) Methanol adsorbs at 85 K dissociatively at low coverages and non-dissociatively at high coverages. (2) There are two reaction channels for dissociation: (a) O-H bond breaking resulting in the formation of a methoxy intermediate, (b) C-O bond splitting which produces water, hydrogen and carbon (fig. la). (3) The methoxy intermediate either recombines with hydrogen and desorbs as methanol between 220 and 250 K (fig. la) or decomposes above 220 K to form adsorbed carbon monoxide (fig. lb) and hydrogen. 3.2. Methanol adsorbed on oxygen-precovered

Ru(001)

The adsorption of methanol on a Ru(OO1) surface which has been precovered with a p(2 X 2) layer of oxygen (0, = 0.25) [16] produces thermal desorption spectra which are distinctly different from those obtained from a clean surface. As is evident from fig. 2a, water (mass 18, 19 and 20) was not detectable among the desorption products, thus implying that the C-O bondbreaking reaction channel is inhibited by preadsorbed oxygen (see the discussion section for a more complete treatment). As in methanol adsorption on clean Ru(OOl), EELS identifies methoxy as a decomposition intermediate. Its characteristic vibrations can be identified as v(C-0) 1015 cm-‘, def. (CH,) 1470 cm-‘, v(CH,) 2985 cm-’ and Y(Ru-OCH,) 320 cm-‘. The absence of characteristic methanol features v(O-H) = 3200 cm-’ and S(O-H) = 700 cm-’ has been confirmed with deuterated methanol CH,OD (not shown) thus enabling an unambiguous identification of the methoxy species. There is, however, a significant difference in methoxy formation on 0-Ru as compared to clean Ru. On 0-Ru, methoxy intermediates are formed independently of the initial methanol exposure, while on the clean surface they

364

J. Hrbek

et al. / Interaction

ofmethanolwiih

modified

Ru(OOl)

(a) CH30D/Ru(001) T, = 85K p = 10K.s-

100

200

300

400

500

600

Temperature [K]

form only at low exposures. This observation leads us to conclude that oxygen acts as a promoter for methoxy formation. Heating to higher temperatures (> 200 K) converts methoxy to carbon monoxide (fig. 2b) and hydrogen. Both decomposition products are observed in the thermal desorption spectra (fig. %a). Finally, heating to 500 K leaves only the oxygen-covered surface as observed by the Ru-0 stretch (535 cm-‘) in fig. 2b. In light of the absence of any methanol vibrational losses at 85 K,

J. Hrbek et al. / Interaction

of methanol with modified Ru(OOl)

365

(b)

X3000

480

1990 CHJoHIRu(OO1) Eo=sev; e,=q

35OK

Lclun

DO

x 3000

190 K x1000

320

1005

85 K

1000

2ooo

3000

Energy Loss km-11 Fig. 1. (a) Multiple mass thermal desorption spectra of methanol (CHsOD) adsorbed at clean Ru(OO1) monitoring methanol (mass 32), water Da0 (mass 20), HDO (mass hydrogen (mass 2). All spectra are equally scaled, but not corrected for different pumping and ionization efficiencies. The heating rate is 10 K s-‘. (After ref. [7].) (b) Vibrational from a submonolayer of methoxy obtained after exposure of methanol (CH,OH) to clean at 85 K. Annealing to temperatures above 230 K results in decomposition of methoxy into H2.

85 K on 19) and speeds spectra Ru(001) CO and

366

J. Hrbek et al. / Interaction of methanol with modified Ru(O01)

130

I

210 m/e 33

28

/ 325

20

I

I

I

I

I

I

I

100

200

300

400

!xo

800

Temperature [1(1

the presence of the methanol peak in the TDS spectrum (210 K) suggests that a portion of the methoxy recombines with hydrogen to desorb as methanol. This CH,O + H recombination takes place, however, at a lower temperature than on the clean surface (250 K). We conclude by noting, in the case of high precoverages of oxygen (0, = 0.6) a totally different adsorption behavior. Preliminary EELS data indicate a complete suppression of methoxy formation. The spectra are characteristic of undissociated methanol and exhibit no evidence of Ru-OCH, stretching vibrations as in figs. 1 and 2. This suggests the influence of site blocking at saturation coverages of oxygen. Wachs and Madix [8] have observed a similar effect: the inhibition of CH,O production from methanol due to the physical site blocking of coadsorbed oxygen adatoms on Cu(ll0). 3.3. Methanol a&orbed

on K +-Ru (6, = 0. IO)

We have precovered Ru(OO1) with a low coverage of potassium in order to observe the effects of differing surface electronic environments on methanol

J. Hrbek et al. / Interaction of methanol with modified Ru(OO1)

r

-Y

367

CH,OH/Ru(OOl)-p(2x2)-0

I

80=0.25 E,,=!W

d

; 8,=0,=60"

(b) mx

47s

4ooK

D so0

1000

1500

2Ow

2500

3ooo

J

Eners~b(m-'l Fig. 2. Methanol adsorbed on a Ru(OOl)/O-p(2 X 2) surface (tie = 0.25) at 85 K: (a) Multiple mass thermal desorption spectra of CH,OD. (b) Vibrational spectra taken after annealing to indicated temperatures (CH,OH).

decomposition. This surface has been well characterized previously [15] and it has been demonstrated in CO coadsorption studies that while physical siteblocking effects are minimal, the highly ionic alkali atoms act as strong electronic modifiers of the metal surface. This modification is manifested in dramatic C-O orientational changes and redshifts of the C-O vibrational frequency [15]. The adsorption of methanol is also substantially altered on this

J. Hrbek et al. / Interaction of methanol with modified Ru(001)

368

CH,OD/Ru(OO1)-K+ &=O.l $= lolw

I

100

1

200

I

I

I

I

I

300

400

500

600

700

Temperature [K)

surface. As is evident from the vibrational spectra in fig. 3b, methanol remains undissociated even after heating to 240 K, i.e. far above the desorption temperature of the multilayer (130 K). This implies that the presence of potassium inhibits O-H bond breaking and thus methoxy formation. The observation of undissociated methanol is unambiguous: the vibrational spectrum of a CH,OD submonolayer exhibits characteristic features of the undissociated molecule (fig. 3b, 240 K). The modes at 2315 cm-’ (O-D stretch) and 525 cm-’ (out-of-plane O-D bend) have been observed previously only for undissociated methanol [7,14]. Other vibrational features, Y(C-0) 1030 cm-‘, def. (CH,) 1475 cm-’ and v(CH,) 3010 cm-’ confirm this assignment. The spectrum taken at 85 K, which exhibits several other features will be discussed later. Heating above 240 K produces the same chemistry observed on Ru and 0-Ru. Some methanol desorbs (fig. 3a, 270 K peak), while the remaining portion dissociates to form carbon monoxide and hydrogen. Both products are observed in the thermal desorption spectra (fig. 3a) with their desorption peaks shifted to higher temperature as compared to clean Ru; an effect which must be attributed to the presence of coadsorbed potassium [15]. This result is also observed in the vibrational spectrum of fig. 3b (obtained after annealing to 285

J. Hrbek et al. / Interaction

of methanol with modified Ru(OO1)

369

CH,OD I Ru (001)- K+ O,=O.l EpSeV

PI

6,=6,=60°

265K

240K

65K

I

0

500

1000

1500 Energy

2wo Loss

I

2500

1

3000

[cm-l]

Fig. 3. Methanol adsorbed on a K+-Ru(OO1) surface (6’, = 0.10): (a) Multiple mass desorption spectra. (b) Vibrational spectra after annealing to indicated temperatures.

thermal

K). The CO which remains on the surface exhibits a strongly redshifted C-O stretch (1500 cm-‘) due to the coadsorbed alkali metal in agreement with previous work [15]. Although the peak shape of the methanol desorption trace at 270 K (fig. 3a) resembles that of recombined methanol from the clean surface (cf. ref. [2] and fig. la), we have been unable to find vibrational evidence for methoxy or other intermediates resulting from the decomposition of methanol. This is probably due to the narrow temperature range (240-270

370

J. Hrhek et al. / Interaction

of methanolwith

modified Ru(001)

K) over which methanol decomposes thus making detection of intermediates difficult. The vibrational spectra contain several features which must be discussed. The first pertains to the question of whether the methanol molecules interact directly with the potassium atom. The vibrational spectra obtained after annealing to 240 K clearly exclude this possibility: we find no evidence for K-O related vibrations at low frequencies (200 to 300 cm-‘). The only low frequency vibration observed at temperatures below 500 K (i.e. prior to methanol desorption) is the 140 cm-’ Ru-K mode [15]. K-O vibrations, which are typically very intense, are absent (K-OH (220 to 275 cm-‘) [18] or K-OCH, (180 to 200 cm-‘) [19]). The only noticeable difference in the vibrational spectrum of CH,OD/(K+-Ru) and the unperturbed CH,OD multilayer on clean ruthenium is a downshift of the v(O-D) mode from 2460 cm-’ on clean Ru(001) [7] to 2315 cm-’ (fig. 3b). Both the frequency shift and the high intensity of the O-D stretch (2315 cm-‘) compared to clean Ru(001) (2460 cm-‘) [7] indicates bond weakening due to interaction with the potassium-modified surface. It is unclear whether this interaction occurs through hydrogen bonding with the surface (as discussed later) or via oxygen-surface bonding of the methanol. Bonding of the methanol to the metal surface via the oxygen end has been suggested earlier [2,7,20] and could cause the observed perturbation of the O-H bond. This bonding mode would certainly be sensitive to electronic modification of the metal surface; such modifications, then, should perturb both desorption energetics and the O-H bond stretching frequency. We note that upon annealing to 200 K a marked increase in the intensity of the O-D stretch (2325 cm-‘) and the C-O stretch (1030 cm-‘). This is likely related to the desorption of the multilayer and subsequent ordering of the first chemisorbed layer. The assignment of the modes at 2035 and 430 cm -’ remains unclear. Carbon monoxide, which possesses C-O and Ru-CO stretches in those frequency ranges on the clean surface is not likely to produce these modes on a potassium-precovered surface. C-O stretches on Kf-Ru typically are found below 1600 cm-’ for &.,, < 0.25 (cf. fig. 3b) and it is difficult to imagine that the coadsorption of physisorbed methanol (which desorbs at 130 K) causes such large frequency shifts. A possible explanation for the 2035 cm-’ mode is that it results from a strongly shifted O-D stretch of disordered or physisorbed methanol. Similarly low values have been observed in organometallic hydroxides [21] and in the interaction of water with Pt(ll1) [22] and Ru(001) [23]. 3.4. Methanol

adsorbed on Kci-Ru

(0, = 0.33)

Higher precoverages of potassium (6, > 0.15) are characterized by alkali atoms which exhibit more metallic character. Saturation of the first potassium

J. Hrbek et al. / Interaction of methanol with modified Ru(#I)

371

layer (8, = 0.33) produces a (fi x fi)R30” LEED pattern. Strong repulsion among the adatoms [15] leads to a large reduction in adsorption energy as well as to a depolarization of the alkali atoms. As in the case of CO [24], adsorption of methanol on this K”-Ru surface is characterized by a substantial reduction in the sticking coefficient (by a factor of ten). Large exposures of methanol, however, produce vibrational spectra attributable to K-OCH, which results from the direct interaction between methanol and potassium. Since, in the present paper, we are interested principally in the interaction of methanol with the modified metal surface, further details of this reaction will be given in a separate communication [19].

4. Discussion Both preadsorbed oxygen and potassium strongly affect the desorption and decomposition of methanol on ruthenium as demonstrated by the changes in the vibrational and thermal desorption spectra. The principal results are presented in table 1 and can be summarized as follows: Table 1 Decomposition

pathways

of methanol

on clean and modified

Ru(OO1)

Clean Ru(001) CH30H \=

“7 CH30a

+ H,

H20a

+ C,

25OK/\>25oK

CH30Hg

Ru(OOl)/O-p(2

CO,

4 l

H20g

H,

+ H,

CH30H 185 K

X 2)

CH,O, Z'OK/

+ H, \23OK

CH30Hg K+-Ru(OO1)

CO,

l

H,

CH30H 85K

1 CH30H,

‘“;/ CH30Hg

YK CO, + H,

H,

l

2101.

+ C,

372

J. Hrbek et al. / Interaction

of methanolwith

modified Ru(OO/)

(1) Preadsorption of oxygen promotes methoxy formation. Methoxy is present at 85 K independent of exposure. Furthermore, oxygen lowers the temperature of methoxy + methanol recombination. (2) Preadsorption of low coverages of potassium inhibits methoxy formation. Methanol is adsorbed undissociated and neither C-O nor O-H bond splitting is observed up to 240 K. No evidence for a direct K-CH,OH interaction is observed. The final decomposition products at T > 240 K are CO + H, (as for 0-Ru). (3) Surface modification with either 0 or K inhibits the C-O bond breaking observed on the clean surface. (4) Preadsorption of high coverages of oxygen or potassium results either in the suppression of methoxy formation (oxygen) or in the direct reaction of methanol with the surface additive (potassium). 4.1. Methanol

decomposition

Methoxy formation is observed in almost all cases where methanol dissociates on clean or modified metal surfaces. As in the case of Ru(001). oxygen has been found to promote methoxy formation on Pt [l], Ag [lo], Cu [9] and Pd [5,25]. On the other hand we find in this study that on Ru(OOl), methoxy formation is inhibited by potassium at low precoverages. These data, then, suggest in the present case that an electronegative additive (oxygen) promotes methoxy formation, while an electropositive additive (potassium) inhibits methanol O-H bond breaking. The applicability of this hypothesis to other metal surfaces. however, must be tested. An attempt to rationalize this model can be made if one assumes that the bonding strength of the methanol molecule to the metal substrate is related to its tendency towards 0-H bond breaking, i.e. methoxy formation. Methanol is believed to act as a weak charge donor upon adsorption to metals via its oxygen lone-pair orbital [2]. If we further assume that increasing charge transfer facilitates 0-H bond breaking, then the chemical effect of surface modifiers can either promote (0) or inhibit (K) O-H bond breaking [38]. In examining the effect of surface modifiers on the chemistry of coadsorbed species, one must also consider the possibility of the active participation of the modifier in the reaction. Examples of this participation can be found in hydrogen scavenging by coadsorbed oxygen/methanol on palladium as observed by Gates and Kesmodel [5] and by Madix and coworkers [25]. In these instances, oxygen accelerates the dehydrogenation of methanol due to the direct interaction with the surface hydrogen atoms evolved from the conversion of methanol to the methoxy intermediate. This reaction leads to the formation of water. Such a process is not observed for the O-Ru(001) surface. We find neither evidence for H,O in thermal desorption (fig. 2a) nor a measurable decrease in the oxygen coverage which would result from water formation. The reason that such a process appears not to be accessible on Ru(001) is likely due

J. Hrbek et al. / Interaction of methanol with modified Ru(OOI)

313

to the substantially higher energy of the Ru-0 bond ( EA = 80 kcal/mol [35]) as compared to the Pd-0 bond ( EA 5: 55 kcal/mol [36]). Indeed, experiments performed on the Ru(OOl)/(O + H) system under relatively severe conditions (heat treatments from 200 to 400 K of Ru(OOl)/@, = 0.6 under lo-’ Torr H,) failed to produce detectable amounts of evolved water. Another example of direct participation is the reaction of metallic potassium atoms with methoxy to form K-OCH, which has been mentioned earlier [19]. Additionally, at high coverages, the surface modifier can act as a site blocker. Examples can be found in the case of the oxygen-saturated Ru surface and in the previously mentioned case of O/MO [26,3], where oxygen acts as a site-blocking agent for methoxy formation, and for formaldehyde production on O/Cu(llO) [8]. C-O bond breaking resulting from methanol adsorption has been observed on clean Ru(001) via desorption of water (fig. la) and the formation of surface carbon [7]. This is the only reported case to date where C-O bond breaking has been observed in studies of methanol decomposition on group VIII metal surfaces. Both oxygen and potassium inhibit this reaction channel. On these surfaces TDS measurements detect no traces of evolved water or of surface carbon (via titration with exposed oxygen and subsequent monitoring of CO thermal desorption traces [7]). The only products detected are CO (vibrational spectroscopy and thermal desorption) and H, (thermal desorption). It appears that for this reaction, both oxygen and potassium act as site blockers, since they both occupy threefold hollow sites and thus could block these and adjacent sites which may be required for C-O bond breaking. Should this be the case, our findings would lead to the interesting conclusion that, in contrast to O-H bond breaking (methoxy channel) where no site blocking at this oxygen coverage is observed, C-O bond breaking is sterically inhibited. This may, then, be an example of a promoter exhibiting a dual functionality by promoting one reaction channel (O-H bond breaking) via electronic surface modification, while inhibiting another channel (C-O bond breaking) via site-blocking effects. 4.2. Methoxy

rehydrogenation

and decomposition

On clean Ru(OOl), a kinetic analysis [7] indicates that a portion of the methoxy is rehydrogenated and desorbs via second order kinetics, while the remainder decomposes to CO and H,. The similar TDS line shapes and coverage-dependent peak shifts observed on the 0-Ru surface argue for the same rehydrogenation kinetics. This also appears to be the case for the K+-Ru surface, though we have not been able to identify methoxy intermediates with EELS. (The line shape of the methanol desorption trace from this surface is similar to that of the rehydrogenated methoxy on the clean surface and the same CO + H, decomposition products are evolved.) It appears that methanol

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J. Hrbek et al. / Interaction of methanol with modified Ru(GO1)

desorption (via methoxy recombination) and methoxy decomposition are related. Both processes are in all cases observed to occur over the same temperature range. At high methoxy coverages recombination is favored, while at low coverages, the decomposition pathway predominates [7]. A straightforward explanation is the availability of hydrogen for the recombination process at higher coverages. Ho and coworkers [34] have observed a similar effect in the case of Ni(llO), where the onset of hydrogen desorption coincides with a decrease in the recombination channel. As a result, the decomposition of methoxy accelerates after the partial removal of hydrogen. The question remains whether the recombination of methoxy is determined solely by a steric effect (proximity of hydrogen), or by an additional chemical effect due to varying surface coverages of methoxy and its decomposition products (CO and Hz). Our data suggest that chemical effects may play an important role as they determine both M-H and M-OCH, binding energies and hence the activation energy for recombinant desorption. When comparing the recombination temperatures of the methoxy intermediate, we find the order 0-Ru < Ru < K+-Ru. In order to determine the activation energies for recombination we consider both the bonding strength of hydrogen and methoxy, i.e., ERU_u and ERu_OCH,. From our TDMS results we obtain for ERu_u the order 0-Ru < Ru < K+-Ru (cf. figs. l-3). If we agree with Upton [28] that ERU_OCH3 is similar to that of ERUmO), then we can expect the binding energy of methoxy to increase in the order 0-Ru < Ru < K+-Ru [29]. Therefore, we can conclude that these binding energies determine the experimentally observed trend in the rehydrogenation temperatures for the modified- surfaces. We note, however that, for noble metals (and Pt, Pd) the situation may be different. There, the binding energy for oxygen is much lower than on Ru and oxygen thus can act as a hydrogen scavenger as pointed out by Madix and coworkers [25]. Consequently, oxygen will tend, on these substrates, to reduce the surface rehydrogenation as hydrogen is removed in the form of water via the 2H + 0 -+ H,O reaction. The stability of the methoxy species with respect to decomposition into CO + H, also determines the reaction channel, i.e. whether rehydrogenation or decomposition of methoxy dominates. This stability also is expected to be affected by electronic modification of the surface. Unfortunately, the available data do not allow us to determine methoxy decomposition temperatures precisely for the various modified surfaces. However, we can infer the presence of an electronic effect on the methoxy stability through an examination of the C-H stretching frequencies on Ru and 0-Ru (figs. lb and 2b). The CH stretches on Ru (2890 to 2910 cm-‘) are lower than on 0-Ru (2945 to 2985 cm-‘), thus indicating a somewhat more stable C-H bond on the oxygenpromoted surface. As suggested by Upton [28], the electronic effect of an electronegative coadsorbate (which depletes surface electron density) should result in less charge transfer from the metal surface into the methoxy and thus

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375

greater methoxy C-H bond strengths and stability. This stabilization of the methoxy species has also been observed with sulfur on Ni(100) where c(2 x 2)-S overlayers raise the decomposition temperature of methoxy to 440 K [13]. The relevance of chemical effects can be demonstrated by noting the dependence of the methoxy decomposition rate on methoxy coverage. As noted above, methoxy decomposition is preferred at lower methoxy coverage [7]. Ho and coworkers [34] have demonstrated with TPEELS (temperature programmed EELS) that the methoxy decomposition temperature actually drops at low methoxy coverage. This indicates that the availability of hydrogen for rehydrogenation is not the only factor determining the reaction channel. Since the chemical effect of high methoxy coverage is similar to that of oxygen (also an electronegative coadsorbate), we arrive at the same conclusion as above. Coadsorbed methoxy as well as oxygen tend to stabilize neighboring methoxy species with respect to decomposition into CO + H,.

Acknowledgements The authors gratefully acknowledge Paul, J. Gland and R. Hall.

useful

discussions

with T. Upton,

J.

References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [II] [12] [13] [14] [15] 1161 [17]

[18] [19]

B.A. Sexton, Surface Sci. 102 (1981) 271. K. Christman and J.E. Demuth, J. Chem. Phys. 76 (1982) 6308, 6318. S.L. Miles, S.L. Bernasek and J.L. Gland, J. Phys. Chem. 87 (1983) 1626. J.E. Demuth and H. Ibach, Chem. Phys. Letters 60 (1979) 395. J.A. Gates and L.L. Kesmodel, J. Catalysis 83 (1983) 437. P.H. McBreen, W. Erley and H. Ibach, Surface Sci. 133 (1983) L469. J. Hrbek, R.A. de Paola and F.M. Hoffmann, J. Chem. Phys. 81 (1984) 2818. I.E. Wachs and R.J. Madix, J. Catalysis 53 (1978) 208. B.A. Sexton, Surface Sci. 88 (1979) 299. I.E. Wachs and R.J. Madix, Surface Sci. 76 (1978) 531. T.E. Felter, W.H. Weinberg, G.Y:Latushkina, P.A. Zhdan, G.K. Boreskov and J. Hrbek, Appl. Surface Sci. 16 (1983) 351. F. Solymosi, T.L. Tamoczi and A. Berko, J. Phys. Chem. 88 (1984) 6170. S. Johnson and R.J. Madix, Surface Sci. 103 (1981) 361. J. Hrbek, R.A. de Paola and F.M. Hoffmann, J. Vacuum Sci. Technol. Al (1983) 1222. R.A. de Paola, J. Hrbek and F.M. Hoffmann, J. Chem. Phys. 82 (1985) 2484. We have used this coverage which is about one third of the saturation coverage to prevent site blocking due to the oxygen atoms. The terms K+ and K” are used to denote low coverage (0, = 0.10) and high coverage (8, = 0.33) potassium adlayers. This nomenclature was chosen for convenience and does not describe quantitatively the state of ionization of the potassium atom. P.A. Thiel, J. Hrbek, R.A. de Paola and F.M. Hoffmann, Chem. Phys. Letters 108 (1984) 25. R.A. de Paola, J. Hrbek and F.M. Hoffmann, submitted.

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