The adsorption and decomposition of methanol on … files/134_ss_249_91_44.pdf · Surface Science...

Surface Science 249 (1991) 44-60

Nosh-Holland

The adsorption and decomposition of methanol on copper on the Rh( 100) surface

Xudong Jiang, John E. Parmeter, Cesar A. Estrada and D. Wayne Goodman * D~part~nent of Chemisrv, Texas A&hf University, Cokge Station, TX 77843-3255. USA

Received 3 August 1990; accepted for publication 17 December 1990

overlayers

The adsorption and decomposition of methanol on copper overlayers on the Rh(100) surface have been studied using thermal

desorption mass spectrometry (TDMS) and high-resolution electron energy loss spectroscopy (HREELS). The reaction scheme of methanol on this model bimetallic catalyst is similar to that on the clean Rh(lO0) surface, but with a number of important

differences. First, the presence of a monolayer or more of copper on the Rh(100) surface greatly inhibits methanol decomposition and

also reduces the binding energy of molecularly adsorbed methanol. That portion of the methanol monolayer that desorbs reversibly

desorbs near 200 K on a Rh(100) surface witb a pseudomorphic copper monolayer and at - 380 K when two or more monolayers of

copper are present, compared with approximately 210 K on clean Rh(100). Second, formation of methoxy via OH bond cleavage of

the adsorbed methanol occurs on the copper overlayer as on the clean surface. but to a reduced extent. Compared with methoxy

formed on clean ~(I~), which decomposes completely by 320 K, methoxy formed on the copper overlayers is substantially more

stable. On a pseudorno~~~ copper monolayer, the methoxy is stable up to - 370 K; when two or more monolayers of copper are

present, the methoxy decomposition is complete only near 400 K. ,Unlike the case of methoxy on clean Rh(lOO), decomposition of

methoxy on the copper overlayers is accompanied by formaldehyde evolution. Third, the methoxy on the copper overlayers exhibits a

metal-oxygen stretching frequency that is substantially reduced compared with the frequency of this mode on clean Rh(100). These

observations are discussed in relation to the chemistry of methanol on various copper surfaces, and in terms of the electronic and

structural effects which may lead to such changes.

1. Introduction

Bimetallic systems have been used successfully in commercial catalytic applications because, through careful preparation, they provide catalytic activity and selectivity superior to those of either component metal [1,2]. Surface science studies have modelled the bimetallic catalysts by thermal evaporation of one metal onto the surface of another, usually a single crystal [3-151. This al- lows the various surface analysis methods to be used to characterize the structural, electronic, and chemical properties of the model catalysts. Reac- tion kinetics at elevated pressures and temperatures have also been measured on these model catalysts [l&20] and correlated with their struct-

* To whom correspondence should be addressed.

ural and electronic properties. Studies on these model catalysts have led to a better understanding of the mechanisms of reactions catalyzed by bimetallic systems.

We are interested in the ~opper/rhodium system since copper is a major ingredient in many highly active methanol synthesis catalysts [21-241. Whether Cu(1) cations on the surface or metallic copper clusters are the active site is still a subject of intense debate [25-291. Copper is also capable of removing a hydrogen atom from methanol without breaking the C-O bond; therefore, it serves as a selective oxidation catalyst in the conversion of methanol to formaldehyde [30]. Rhodium can also catalyze methanol synthesis 124,311. Previous studies on copper overlayers on ~(100) 1321 show that the copper overlayer growth follows a layer-by-layer mechanism up to at least the third layer. The first layer grows pseudomor-

0039-6028/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

X. Jiang et al. / Adsorption and decomposition of methanol on Cu / Rh(lO0) 45

phically and is therefore strained with respect to spectroscopic studies on the adsorption and de its bulk lattice ~nfi~a~on, having an atomic composition of methanol on copper overlayers on density ~smatch of 10% [33]. One purpose of this the ~(1~~ surface. Comparison is made with the study is to see how this strain affects the reactivity results on the clean Rh(lO0) surface and on vari- of this overlayer towards methanol. ous copper surfaces.

Reaction of methanol with bulk copper has been studied previously [34-451. Madix and co- workers [34,35] investigated the adsorption and oxidation of methanol on clean and oxygen dosed Cu(l10) surfaces and concluded that methanol was dissociated on both surfaces to form a methoxy species at 270 K. The main difference was the production of large amounts of I&CO at 370 K on oxygen-dosed Cu(ll0). Sexton [37] found that a surface methoxy species, stable to approximately 370 K, was formed upon adsorption of methanol on both clean and pre-oxidized Cu(100) surfaces. The concentration of this methoxy species was relatively small on the clean surface and, at about 370 K, this species decom- posed to yield gaseous products and a clean surface. On the other hand, Russell et al. [44] found that on the clean Cu(ll1) surface, molecularly adsorbed methanol, the only stable surface species identifi~ on this surface, desorbed at about 210 K. Pre-adsorbed oxygen served as an acceptor of the methanol hydroxyl hydrogen, thus allowing the formation of a methoxy species which decomposes to give formaldehyde at about 400 K.

2. Experimental

The apparatus used in this study, the Rh(100) single-crystal preparation and cleaning procedure, and the calibration of methanol exposures have been described in detail in a previous publication

WI- Methanol exposures were carried out at a sub-

strate temperature of 115 K unless otherwise stated. All exposures quoted are given in units of Langmuirs (I;, 1 L = 1 x 10e6 Torr . s) and without ionization gauge correction. A linear heating rate of 10 K/s was used in the thermal desorption experiments.

The resolution of the HREEL spectra presented in this paper varies from 7-12 meV. Spec- tra were obtained with electron collection in the specular direction, with nominal beam energies of - 0.5-2 eV.

In a recent publication [46], we reported a study of the adsorption and decomposition of methanol on the Rh(100) surface. It was shown that below 200 K, methanol is molecularly adsorbed and bonds to the surface via the oxygen atom. At 200-220 K, a saturated methanol layer undergoes two competing reactions: desorption and OH. bond cleavage to form an oxygen-bonded methoxy species. The methoxy species is stable to approximately 250 K. Between 250 K and 320 K, a fraction of the methoxy species decomposes to form coadsorbed CO and hydrogen adatoms while the remainder recombines with hydrogen adatoms to desorb as molecular methanol. Recombinative desorption of H, between 270 and 400 K and desorption of molecular CO near 500 K regener- ates the clean Rh(100) surface.

In this paper, we report the results of thermal desorption and ~~-resolution electron energy loss

Copper was evaporated onto the Rh(lO0) single-crystal surface at a substrate temperature of 115 K by resistively heating a tantalum wire wrapped with high-purity copper wire [32]. The sample was then annealed to 750 K, 50 K higher than the maximum temperature used in the TDMS experiments. The coverage of the copper overlayer was determined by comparing the Cu(60 eV)/ Rh(302 eV) Auger ratio with a previously defined calibration curve [32]. In the case of the multilayer copper films, copper in the amount of ten equiv- alent layers was evaporated onto the Rh(lO0) surface, and the sample was then annealed to 750 K. A previous study [32] has shown that upon annealing multilayer copper films, the first two layers are stable up to the desorption temperature which is above 1000 K. Multilayer copper films beyond the second layer nucleate to form three-di- mensional clusters, starting at temperatures as low as 300 K. Therefore, multilayer copper films prepared in this study will consist of three-dimen- sional clusters on top of a bilayer copper film.

HREEL spectra of these annealed, copper- covered surfaces often have a weak shoulder near 300 cm-‘. This suggests that some adatom impurity could have been present on these surfaces in an amount (- 0.01 ML) near the limit of Auger detectability. It appears that even very small impurity concentrations can affect dramatically the chemistry of methanol on copper surfaces and overlayers, particularly if the impurity is a proton scavenger that can enhance OH bond cleavage. For example, Paul and Hoffmann [15] found that methanol does not dissociate at all on clean copper overlayers on Ru(OOOl), but that the presence of 0.03 ML of oxygen adatoms on the surface led to the formation of substantial amounts of methoxy. It is therefore quite possible that minute impurity concentrations might be enhancing methoxy formation in the HREELS results of the present study, although if this is the case their effect on the subsequent chemistry of methoxy, as in the ~u/Ru(OOOl) study, should be small.

3. Results

3. I. TDMS

The major thermal desorption product from methanol adsorbed on copper overlayers on a Rh(100) surface at 115 K was CH,OH. Other products detected were CO (for copper coverage < 1 ML), H, and H&O. The results for monolayer (or sub-monolayer) and multilayer copper films will be presented for each product separately.

Fig. 1 shows the signal for mass 31 (the strongest fragment in the cracking pattern of CH,OH) of the thermal desorption spectra of CH,OH on a 0.86 monolayer (ML) copper covered Rh(100) surface at various exposures. At very low coverage, only a single peak (a,) appears at 257 K, and shifts to lower temperatures with increasing exposures. A new state (8,) centered at 202 K starts to develop at 0.90 L exposure, and shows no shift with the increase of the exposure. While the j_& state was saturated at - 16 L, the & state was not filled until - 28 L methanol exposure. Shortly after the /3i state appears, a new peak ((~i) devel-

a,- Pt 0.86 ML Cu/Rh(lOOj 202K / p2

!O.cJ6 L I

r257 K

io.03 L / j # -.

100 150 200 250 300 350

TEMPERATURE (K) Fig. 1. Mass 31 sipal of the thermal desorption spectra of methanol on a 0.86 ML copper-covered Rh(100) surface at

various exposures. Adsorption temperature: 115 K.

ops at 151 K. This peak grows and shifts to higher temperatures with increasing exposure and does not show any sign of saturation at large methanol exposures. It exhibits zero-order kinetics and can be identified as being due to condensed methanol. On the other hand, the & peak has a fixed peak maximum temperature, and the peak shape is asymmetric with respect to the peak maximum, suggesting first-order kinetics with coverage-inde- pendent activation energy and the desorption of a molecularly adsorbed species. In contrast, the & peak has a symmetric peak shape and shows a large shift in peak temperature from 257 K at low coverage to below 225 K at saturation coverage, suggesting second-order kinetics and that the desorption product is probably due to recombination of two adsorbed species. By referring to previous studies on the adsorption and decomposition of methanol on the Rh(100) surface [46], we assign the & state to be due to the desorption of

X. Jiang et al. / Adwrption and decomposition of methanol on Cu/Rh(IOO) 47

a,1 1.85 PIlL Cu/Rh(lOO)

B, 82

100 150 200 250 300 350

TEMPERATURE (K)

Fig. 2. Mass 31 signal of the thermal desorption spectra of methanol on a 1.85 ML copper-covered Rh(lO0) surface at

various exposures. Adsorption temperature: 115 K.

molecularly adsorbed methanol and the /$ state, the desorption of methanol resulting from the recombination of adsorbed methoxy (CH30) and hydrogen adatoms. This assignment is supported by the HREELS results which will be presented in section 3.2.

In fig. 1, the shoulder on the ~~-temperat~e side of the & peak is the ~nt~bution of that portion of the Rh(lOO} surface not covered by copper and was saturated at an early stage of the exposure. This state also originated from the recombination of adsorbed methoxy and hydrogen adatoms and was discussed in detail in ref. [46].

Fig. 2 displays a series of mass 31 thermal desorption spectra for methanol adsorbed on a 1.85 ML copper-covered Rh(100) surface at various exposures. These are similar to the spectra in fig. 1. At low exposures, only a weak & feature is present. At higher exposures, the much stronger /3,

feature develops, and the al peak appears at a subtly higher exposure. In addition to the simi.lar- ities, there are also some differences between these two sets of spectra. First, while the a, peak appears at the same temperature, the j$ and & states in fig. 2 have peak temperatures about 20 K lower than those in fig. 1. This means that the molecularly adsorbed methanol desorbs and the adsorbed methoxy rehydrogenation occurs at lower temperatures on bilayer copper films than on monolayer copper-covered Rh(100). Second, the & peak in fig. 2 is substantially weaker and less well resolved than that in fig. 1. This indicates that less methoxy recombines with hydrogen adatoms and desorbs from bilayer copper than from monolayer copper films, and probably indicates that a smaller concentration of the methoxy species is formed.

The mass 31 signal of the thermal desorption spectra of methanol on multilayer copper films on Rh(100) (not shown) does not show any substantial differences from the spectra in fig. 2. This suggests that copper bilayers and multilayers on Rh(100) have very similar chemical properties with respect to adsorbed methanol. The HREELS results, which will be presented later, lead to the same conclusion.

Fig. 3a displays the mass 28 signal from the thermal desorption spectra of 0.39 L CH,OH adsorbed on 0.95 ML copper-covered Rh(100). For different methanol exposures, the peak around 472 K follows first-order kinetics and can be identified as due to desorption of CO associatively adsorbed on the parts of the Rh(100) surface not covered with copper. This CO results from the de~m~sition of methoxy on the ~(1~) surface, the methoxy having been formed from the decomposition of methanol 1461. This peak was saturated at a very low methanol exposure (< 0.4 L). Com- paring the saturated peak area with that obtained following the separate adsorption of CO at saturation coverage on a clean Rh(100) surface reveals that - 0.005 ML CO, i.e. 6.9 X 10” CO molecules/cm’, desorbed from this state, considering that the saturation coverage of CO on the Rh(100) surface is 0.60 ML [47,48]. This is - 2.3% of that desorbed from a saturated layer of methanol on a clean ~(1~) surface, giving further support to

48 X. Jiang et al. / Ahorption and decomposition of methanol on Cu/ Rh(lO0)

0.39 L CHcjOH

CLEAN Rh(100)

ki 240 i0.95ML 1

K _

irx ;

I 100 200 300 400 500 600

TEMPERATURE (K)

Fig. 3. Mass 28 signal of the thermal desorption spectra of 0.39

L methanol on (a) a 0.95 ML copper-covered Rh(100) surface,

and (b) a clean Rh(100) surface. Adsorption temperature: 115

K.

the assumption that this CO is a decomposition product of methanol on the small areas of the Rh(100) surface not covered with copper.

At first glance, the peak at 248 K in fig. 3a could be assigned to the mass 28 fragment of methanol desorbed from the & state. But after comparing it with the mass 31 signal in fig. 1 at similar exposure, and taking into account the fact that in the cracking pattern of CH,OH, the intensity of the mass 28 signal is about an order of magnitude smaller than that of mass 31, it is found that the fragmentation of methanol desorbed from the & state can only account for - l/10 of the peak area. The major portion of this peak is due to CO generated from a reaction rate-limited process-methoxy decomposition. Calculation from this peak area reveals that - 0.017 ML CO, i.e. 2.3 x 1013 CO molecules/ cm*, were released in this process at 0.39 L methanol exposure. Increasing the methanol exposure caused two more desorption peaks to appear near 200 K and 153 K successively. They are the counterparts of the p, and q states in fig. 1, and are due to the fragmentation of methanol

desorbed from these two states. For comparison, fig. 3b shows the mass 28 signal of the thermal desorption spectra of 0.39 L methanol on a clean Rh(100) surface where - 0.043 ML CO is generated as a result of methoxy decomposition.

The peak at 378 K in fig. 3a coincides in shape and maximum temperature with the mass 29 signal in fig. 5b, and is therefore identified as a fragment of formaldehyde, which will be discussed later.

No CO desorption is observed from copper multilayers on Rh(100) following a saturation methanol exposure at 115 K. In the mass 28 thermal desorption spectra, there are only weak features due to cracking fragments of other species and desorption from the crystal sides. This is shown in fig. 4. The peaks near 179 and 190 K are due to cracking of methanol desorbed from the & and & states. Their peak areas can be entirely accounted for by the fragmentation of methanol. The peak due to a cracking fragment of formaldehyde has upshifted from 378 K in fig. 3 to 407 K, consistent with the peak temperature in fig. 5a. Finally, the small feature around 520 K can be identified as CO desorbed from the edges of the Rh(100) crystal. (In fig. 3a, there is not such a feature because of the low methanol exposure of 0.39 L in that case.) Thus copper multilayers in-

100 150 200 250 300 350 400 450 500 550

TEMPERATURE (K)

Fig. 4. Mass 28 signal of the thermal desorption spectra of 11.1 L methanol on multilayer copper films on a Rh(100) surface.

Adsorption temperature: 115 K.

X. Jiang et al. / Adsorption and decomposition of methanol on Cu/ Rh(lO0) 49

hibit completely the decomposition of methanol to CO on Rh(100).

Very small amounts of H, were desorbed following methanol adsorption on copper-covered (both monolayer and multilayer) Rh(lO0) surfaces with a peak temperature in the range of 350-400 K. Based on the integrated peak intensities, less than 0.1 monolayer of hydrogen adatoms were produced in all cases. This would correspond to 0.025-0.05 monolayers of methanol decomposition, depending on the degree to which CO and formaldehyde are formed as the other desorption product. The desorbed hydrogen arises from two

375 K

I CH30H EXPOSURE

(plYi_ 1 1.0 ML CulRh(100) + PREDOSED

@)------- 1.0 ML CuiRh(100

100 zoo 300 400 t

TEMPERATURE (K)

Fig. 5. Mass 29 signal of the thermal desorption spectra of

methanol on copper overlayers on Rh(lW) surface. The peaks below 300 K are due to fragmentation of methanol and the

peaks above 300 K are due to fragmentation of formaldehyde. (a) On multilayer copper films; methanol exposure: 1.0 L;

adsorption temperature: 115 K. (b) Copper overage: 1.0 ML,

methanol exposure: 1.5 L; adsorption temperature: 115 K. (c) Copper coverage: 1.0 ML, methanol exposure: 3.1 L; adsorption temperature: 300 K. (d) Copper coverage: 1.0 ML; predosed with submonolayer oxygen; methanol exposure: 1.2 L;

adsorption t~~at~e: 115 K.

distinct sources. First, a small amount of reaction-cited hydrogen desorption must accom- pany fo~~dehyde desorption (next paragraph), since the HREELS results (cf. section 3.2) indicate that formaldehyde desorbs as a result of methoxy decomposition. Second, as in the case of CO thermal desorption, small amounts of H, are expected to desorb as the result of methanol decomposition at defects in the overlayer structure or on the edges of the crystal. This is supported by the fact that the desorption of H, occurred in a range typical of desorption of H, from rhodium, rather than in the range of appro~~tely 200 to 350 K [49,503 typical of bulk copper.

Thermal d~omposition of methanol on copper overlayers on Rh(lO0) also produces formaldehyde. Fig. 5 displays the signal for mass 29, the strongest fragment in the cracking pattern of formaldehyde. As can be seen from this figure, on monolayer and multilayer copper films, the formaldehyde has desorption peak temperatures around 372 K and 405 K, respectively. Fig. 5c shows that when the methanol exposure was performed at a substrate temperature of 300 K, which is above the temperature for methanol desorption in the & state (cf. fig_ l), there was still an appreciable amount of formaldehyde desorption detectable. Predosed submonolayer oxygen greatly enhances the formaldehyde formation, as can be seen in fig. 5d.

3.2. HREELS

As discussed earlier, experiments employing electron energy loss spectroscopy were performed on Rh(100) with a pseudomo~~c monolayer of pre-adsorbed copper, ~~1~~ with two layers of pre-adsorbed copper, and ~(1~) with multilayers of pre-adsorbed copper. The experimental results obtained for methanol adsorption and decomposition on 1 ML Cu/Rh(lOO) and on Rh(100) with higher copper coverages will be discussed separately below.

3.2.1. Methanol adsorption and decomposition on Rh(lO0) with a pseudomorphic monolayer of pre-adsorbed copper

A Rh(100) surface with a pseudomorphic copper monolayer was prepared as described in

50 X. Jiung et al. / Aakorption a~~~ecQ~o~ifion of methanol on Cu/~h~i~~

the experimental section. Exposing copper-covered rhodium surfaces to methanol exposures in excess of 3 L at temperatures below approximately 155 K led to condensation of methanol multilayers; these multilayers resemble closely those formed on clean Rh(100) [46] and will not be discussed in this paper. Annealing to slightly above 155 K to cause multilayer desorption, or adsorbing methanol with the surface temperature slightly greater than 155 K, resulted in EEL spectra characteristic of monolayer methanol on 1 ML Cu/~(l~).

Fig. 6 shows representative EEL spectra for monolayer methanol on 1 ML Cu/Rh(lOO). All are for surfaces that were saturated at the given adsorption temperature; lower coverages were not

(c) 1 ML Cu/Rh(lOO) 30 L CDsOD at 147 K Anneal to 177 K

i

’ 1090 r

~~ I

r260

II x 3.65

‘Y25

(b) 1 ML Cu/Rh(lOO) 20 L CH30H at 151 K

’ I Anneal to 177 K

(a) 1 ML Cu/Rh(iOO) 30 L CH30H at 166 K

160

256o 2935 I

\ 1 \ I\ 1’74o / 1 ~3270

i 0 1000 2000 3000

ENERGY LOSS {cm-‘) Fig. 6. The HREZEL spectra that result from 1 ML Cu/~(l~) surface following: (a) a 30 L exposure of CHsOH at 166 K, (Jr) a 20 L exposure of CH,OH at 151 K annealed to 177 K, (c) 30 L exposure of CDsOD at 147 K annealed to 177 K. These spectra are characteristic of molecularly chemisorbed methanol.

Table 1 Vibrational frequencies and mode assignments for methanol molecularly adsorbed on 1 ML Cu/Rh(lOO), after annealing to 177 K (data for gas phase and liquid CHsOH [51] are given for comparison)

1 ML Cu/Rh(lOO) CH,OH CH,OH

Mode CH,OH CDsOD H/D ratio (gas) (liquid)

@HI 3245 2455 1.32 3681 3328

~~~~~~ 2920 2185 -1.34 3000 2980

5 _ > 2080 -1.40 2844 2834

6,(CH,) n.r. n.r. _ 1477 1480 6,(CH,) 1455 1090 1.33 1455 1450 &OH) n.r. n.r. _ 1345 7418 &CHa) nr. nr. - 1060 1115

GOI 1025 975 1 .a5 1033 1030

n(OH) 795 605 1.31 655 v(Cu0) 260 265 0.98 - -

n.r. = not resolved.

investigated. Mode assignments are given in table 1 and are verified easily by a comparison with data for liquid and gas phase methanol and by the observed frequency shifts upon deuteration. As on clean Rh(100) [46], the high-coverage low-temperature state of fig. 6a closely resembles the multilayer state in its vibrational spectrum. In particu- lar, the OH bending [r(OH)], CO stretching [v(CO)], and the CH, symmetric deformation [S,(CH,)] modes are all of comparable intensity and the CH [v,(CH,) + v,(CH,), overlapping] and OH [ v(OH)] stretching modes are less intense by a factor of about three. Annealing to 177 K changes the spectrum rather ~~atic~ly, so that Y(CO) is now clearly the most intense loss feature in the spectrum and the copper-oxygen stretching vibra- tion is resolved near 260 cm-‘. The fact that the methanol is still adsorbed molecularly is confirmed by the substantially weakened but clearly present v(OH) and rr(OH) loss features. Both of these loss features are stronger in the corresponding loss spectrum of CD,OD shown in fig. 6c, with the different intensities resulting from the frequency shifts and differing amounts of coupling between the various modes. The changes observed in going from spectrum 6a to 6b are similar to those observed upon annealing monolayer methanol on clean Rh(lOO), or in going from a low to a high methanol coverage at low tempera-

X. .I iang et 01. / Adrctrption and exposition of methanol on Cu / I#@ 00) 51

ture on clean Rh(lO0). These changes may result both from an ordering of the methanol adlayer at higher temperatures, and from a reduced coverage at the higher annealing temperature resulting e.g. in a loss of hydrogen-bonding interactions. Since these results closely parallel those we obtained on clean Rh(lOO), we do not discuss this in any further detail here.

Annealing the surface to temperatures slightly in excess of 200 K resulted in the d~omposition of chemisorbed rne~~ol via OH bond cleavage to form an adsorbed methoxy species. (As discussed in section 3.1, the thermal desorption results indicate additionally that some methanol desorbs near 200 K in the & peak.) Some typical EEL spectra are shown in fig. 7. As can be seen in fig. 7a, no v(OH) loss feature remains after annealing a methanol (CH,OH) saturated surface to 208 K. The relative intensity of the loss feature due to n(OH) near 775 cm-’ is also greatly reduced, further supporting cleavage of most (if not all) of the methanol OH bonds [52]. In addition, the metal-ligand stretching loss feature upshifts to 305 cm-’ and appears to increase in intensity. Such a frequency upshift in v(M-0) is typical for the conversion of adsorbed methanol to methoxy [53]. These spectral changes occur on 1 ML Cu/Rh(lOO) at a temperature lo-15 K lower than on clean Rh(100). These EELS results thus corre- late well with the mass 31 thermal desorption results, which show that the & (molecular) desorption peak on 1 ML Cu/Kh(lOO) is approximately 10 K lower than on clean Rh(100). On both surfaces competing desorption and decomposition of adsorbed methanol appear to occur simultaneously at the /3i desorption temperature; it is not clear whether desorption triggers decomposition or vice versa.

Further annealing removes the loss feature near 775 cm-’ completely, producing virtually pure methoxy overlayers. Such spectra are shown in fig. 7b for CH,OH annealed to 266 K, and in fig. 7c for CD,OD annealed to 269 K. The characteristic loss features of the methoxy species in fig. 7b are Y(CH& 2895 cm-‘; 6&H,), 1445 cm-‘; v(CO), 1035 cm-‘; and v(Cu-0CH3), 325 cm-‘. The very weak loss features at 1920 and 2050 cm-r are due to bridge-bonded (1920 cm-*) and on-top

1035 (a) 1 ML Cu/Rh(lOO) 20 L CHaOH at 151 K Anneal to 206 K

{c) t ML Cu/Rh(lOO) 30 L CDJOD at 147 K Anneal to 269 K

(b) 1 ML CufRh(Y 00) 30 L CHPH at 166 K Anneal to 266 K

ENERGY LOSS (cm-‘)

Fig. 7. The HREEL spectra that result following: (a) meal& the surface of fig. 6b to 208 K, and (b) annealing the surface of fig. 6a to 266 K, (c) annealing the surface of fig. 6c to 269 K. These spectra are characteristic of adsorbed methoxy species.

(2050 cm-‘) CO adsorbed on rhodium. These trivial amounts of CO are thus bonded at defects in the copper overlayer where small amounts of rhodium are exposed. It is not clear whether this CO results from methanol decomposition or background adsorption. The loss features due to CD,0 in fig. 7c are 6,(CD,), 1075 cm-‘; v(CO), 975

-l; and Y(CU-OCD& 280 cm-‘, The 6 (CDJ) r:s featnre is much more intense than the &CH,) loss feature, because its lowered frequency results in much stronger coupling to the v(C0) mode. The loss feature at 2075 cm-’ is probably due primarily to &D,), although on-top CO on rhodium may also contribute to the observed intensity. The feature at 1910 cm-’ is again due to a

52 X. Jiang et al. / Adsorption and deco~p~s~tjon of methanol on CM / R~~l~~

Table 2

Vibrational frequencies and mode assignments for methoxy on

1 ML Cu/Rh(lOO) (this work), Rh(100) [46], and Cu(100) [37]

(all frequencies are in cm-‘)

ML Cu/Rh(lOO) Rb(100) Rb(100) al Cu(100) b’

Mode CH,O CDs0 H/Dratio CH30 CH30

- 2075 1.40 2960 2910

2890 2830

&WI,) 145 1075 1.34

p(CH,) nr. nr. -

r(CO) 1035 975 1.06

r@Q 325 280 1.16

M = metal, nr. = not resolved.

” Annealed to 254 K.

b, Annealed to 375 K.

1445 1450

1115 n.r.

1005 1010

405 290

trivial amount of bridge-bonded CO on rhodium. The very weak loss feature at 1415 cm-’ is of uncertain origin; possibly it results from a hydro- genie deformation of a small OCD,H impurity. Table 2 summarizes the vibrational spectra of CH,O and CD,0 on 1 ML Cu/~(lOO).

Since the principal focus of this study is a comparison of the methanol chemistry of copper-

covered Rh(100) with the methanol chemistry of Rh(100) and of various copper surfaces, table 2 also gives vibrational data for methoxy that were obtained in our previous EELS study on Rh(lO0) [46], along with data from Sexton’s study on the Cu(100) surface [37]. While in most regards the methoxy vibrational spectrum is not surface sensi- tive, the frequency of v(CU-OCH3) of 325 cm-’ on 1 ML Cu/Rh(lOO) is much closer to the Cu(100) value of 290 cm-’ than to the Rh(100) value of - 400 cm-’ (at a comparable annealing temperature). Thus one can argue that the rhodium surface with a copper monolayer already behaves much more like Cu(100) than like Rh(100). A similar downshift in v(Metal-0) of methoxy was observed by Paul and Hoffmann on copper overlayers on Ru(001) [15]: on clean Ru(OOl), v(Ru0) = 350 cm-“, while on 2 ML Cu/Ru(OOl), v(Cu0) = 270 cm-‘.

Further annealing causes the loss features due to methoxy to grow weaker in unison until none remains on the surface. This is illustrated in fig. 8. The intensity of the methoxy signal (i.e. the intensity of the methoxy signal normalized with respect

r--- ~. ~. ~~__ ._--... _.-____

(b) Anneal to 330 K

(a) 1 ML Cu/f?h(lOO) 30 L CHJOH at 166 K Anneal to 288 K ;


(d) Anneal to 415 K

1 - x0.84

(c) Anneal to 375 K

‘:i; k-$&+__, / / x 0.83 _J

0 1000 2000 3000


Fig. 8. The HREEL spectra that result following further annealing the surface of fig. 7b to (a) 288 K, (b) 330 K, (c) 375 K, and (d)

415 K.

X. Jiang et al. / Aa!vorption and decomposition of methanol on Cu / Rh(lO0) 53

to the elastic peak intensity) reaches a maximum in the 260-290 K bag regime; thereafter it falls rapidly and approaches zero after annealing to approximately 380 K. Since the thermal desorption results indicate clearly that the methoxy concentration starts to decrease well below 260 K due to recombinative desorption of methanol, the intensity maximum that is attained at 260-290 K reflects increased ordering of the adlayer in this temperature range rather than a maximal methoxy coverage. After annealing to 330 K the methoxy signal is reduced by approximately 50% with no new loss features appearing in the EEL spectrum. Annealing to 375 K leaves only very weak loss features at 290 cm-’ (possibly due to v(Cu0) of methoxy, but see below), 1020 cm-” (v(C0) of methoxy), 1345 cm-’ (see below), and 1925 cm-’ (v(C0) of bridge-bonded CO on rhodium). Fur- ther annealing to 415 K removes all loss features except a slight shoulder at 295 cm-‘. As discussed earlier, this shoulder was usually present on 1 ML Cu/Rh(lOO) surfaces prior to methanol adsorption and is of uncertain origin. No impurities were detected in Auger spectra to which such a loss feature could be att~buted, though it is possible that some impurity is present in a concentration below the Auger detection limit.

The weak loss feature at 1345 cm-i, in fig. 8c is of interest because it might be attributable to the v(C0) loss feature of a partially dehydrogenated intermediate in the decomposition of a small amount of methoxy. Indeed, a similar loss feature with a frequency between 1320 and 1365 cm-’ is seen between approximately 300 and 400 K in the decomposition of CD,OD on 1 ML Cu/Rh(lOO), as is illustrated in fig_ 9 for a surface mealed to 385 K. Since the frequency of this mode clearly does not shift significantly upon deuteration of the methanol, it is indeed possible that this loss feature is due to v(C0) of a small concentration of H,CO or HCO formed via methoxy decomposition. The possibility of adsorbed H,CO is especially intriguing because this loss feature disap- pears at a temperature near which formaldehyde desorption is observed in the thermal desorption spectra. However, the formaldehyde observed in the thermal desorption spectra appears to be primarily reaction-ante (from methoxy decom-

i5 / t 1 3 If--l .

Irm lrfc f295

t !I 1 ML Cu/~h(lOO) / 30 L CD30D at 147 K Anneal to 385 K

/

iij,h, L- 1 0 1000 2000 3000

ENERGY LOSS (cm-‘) Fig. 9. The HREEL spectrum that results following further

annealing the surface of fig. 7c to 385 K.

position), so one would not necessarily expect to observe an adsorbed formaldehyde species.

3.2.2. Methanol adsorption and decomposition on bilayer copper and on copper multilayers on Rh(lO0)

The EELS data obtained when methanol was adsorbed on surfaces with two layers or with multilayers of pm-adsorbed copper did not differ sig~fic~tly, and hence are discussed together here. Once again only saturation methanol coverages were investigated, and we consider here only monolayer methanol since methanol multilayers have been discussed in detail previously [46].

Compared with 1 ML Cu/Rh(lOO), the Rb(100) surfaces with higher copper coverages showed two major differences with regard to methanol chemistry. First, the OH bond cleavage of methanol to form methoxy occurred at slightly lower temperatures, consistent with the downshift of the & thermal desorption state observed in thermal de sorption spectra. Second, the methoxy was more stable, with a substantial amount still present in EEL spectra collected after annealing to - 3’70 K. In addition, the v(Cu0) vibrational feature of methoxy appears to be downshifted slightly compared with its value on 1 ML Cu/Rh(lOO), so that it is difficult to resolve well from the shoulder of the elastic peak.

Spectra illustrating these points are shown in fig. 10 for the case of copper multilayers on Rh(100). For annealing temperatures below 250 K, weak modes are present near 480 and 2040

X. Jiang et al. / Aakorption and decomposition of methanol on Cu/ Rh(I00)

(b) Anneal to 191 K

x 1.89 1 r295

(a) Multilayer Cu/Rh(lOO) ! s. 1040

i 20LCHaOHat137K , Anneal to 188 K

Fho ~ xl

I I j I 1000 2000 3000


1 (d) Anneal to 382 K 960

II Wk x 2.03 / I I

I 1000 2000 3000


1 275 L 1460 2695

x 2.97

(c) Anneal to 252 K

Fig. 10. The HREEL spectra that result following saturated CH30H exposure to a multilayer Cu/Rh(lOO) surface with subsequent annealing to (a) 168 K, (b) 191 K, (c) 252 K, and (d) 362 K.

cm -’ due to a small amount of CO adsorbed on copper from the chamber background. (The ab- sence of a bridge-bonded CO loss feature near

1900 cm-’ strongly suggests that this CO is indeed bonded to copper and not to rhodium.) Otherwise, only methanol and methoxy are identi-

fied. After annealing a saturated methanol layer to 168 K, the methanol is still primarily molecularly adsorbed as evidenced by the strong, broad OH bending loss feature near 795 cm-’ and a much weaker and broad v(OH) loss feature near 3200 cm-‘. However, virtually all of the methanol that remains on the surface undergoes OH bond cleavage upon annealing to 190 K, as evidenced by a much lowered intensity of the OH bending loss feature and the complete disappearance of v(OH). For higher annealing temperatures only vibrational loss features due to methoxy are observed. The various loss features are best resolved in spectrum (c). These are v(CuO), 295 cm-‘; v(CO), 1010 cm-‘; 6,(CH,), 1440 cm-‘; v,(CH,), 2820 cm-‘; and v,(CH,), 2945 cm-‘. Note that after annealing to 362 K, spectrum (d), a substantial

methoxy signal remains. This signal disappeared completely after an anneal to 398 K.

4. Discussion

Combining the results of the thermal desorption and high-resolution electron energy loss spectroscopic studies, we suggest the following picture of adsorption and decomposition of methanol on copper overlayers on the Rh(100) surface. The initial exposure of the substrate to methanol at temperatures lower than 150 K produces a chemisorbed layer followed by condensed multilayer methanol,

CH,OH(g) -+ CH,OH(ads)

+ CH ,OH (condensed).

In the chemisorbed layer, methanol adsorbs on the substrate molecularly via the oxygen atom. Upon warming the crystal to above 151 K, the con-

X. Jiang et al. / Adsorption and decomposition of methanol on Cu/Rh(lOO) 55

densed multilayer methanol desorbs first, giving rise to the ai state in the thermal desorption spectra. With further heating of the crystal, the chemisorbed methanol undergoes two competing processes, namely, molecular desorption in the pi state and OH bond cleavage to form an adsorbed methoxy species and hydrogen adatoms,

CH,OH(ads) --, CH,O(ads) f H(ads).

These processes occur at - 200 K on monolayer Cu/Rh(lOO) and at - 180 K on multilayer (including bilayer) Cu/Rh(lOO). On multilayer copper, methoxy formation is minimal in the ab- sence of residual ~p~ti~. When the crystal is heated to a temperature 20 to 30 degrees higher, a portion of the methoxy species recombines with hydrogen adatoms and desorbs in the & state,

CH,O(ads) + H(ads) + CH,OH(g),

and the remainder stays on the surfaces until - 370 K on monolayer copper films and 400 K on multi layer (including bilayer) copper films when it decomposes to form formaldehyde and hydro-

gen,

CH,O(ads) -+ H&O(g) + H,(g).

The above methanol adsorption and decomposition pathways are summarixed in fig. 11.

Our experimental results indicate that there are several significant differences between the methanol adsorption-desorption and decomposi-

CH30H(g)

CH3OH (ads) + CHPH (~ndens~)

c LCf%CW (gf (a,) z 1

~~QH (9) (PI) CH30(ads) t H (ads)

r---L

L t-b2 (9) + n (ads) CkWH W (82) H2CG (9) + Hi @I

Fig. 11. Adsorption and decomposition mechazkm of me&o1 on copper overlayers on Rh(lO0). Gas phase and adsorbed

species are denoted by (g) and (ads), respectively.

tion scheme on copper overlayers/~~l~) and that on the clean Rh(lO0) surface. First, methanol desorbs molecularly in the /3t state at lower temperatures on copper overlayers/ RhflOO). On monolayer Cu/Rh(lOO), this process occurs near 200 K as opposed to near 210 K on clean Rb(100) [46]. It shifts to the even lower temperature of approximately 180 K on multilayer Cu/Rh(lOO). The temperature for the adsorbed methoxy species resulting from the decomposition of methanol to recombine with hydrogen adatoms and desorb in the is, state shows the same trend. Second, the copper overlayers greatly inhibit methanol decomposition to CO and hydrogen. While on a clean Rh(100) surface saturated with methanol, 0.2 ML of methanol decomposes via a methoxy intermediate to yield ultimately CO and H2 as thermal desorption products; this amount is greatly reduced when the copper overlayer is present, As can be seen in fig. 3, following a 0.39 L methanol exposure, 0.043 ML CO desorbs from a clean Rh(100) surface; on 0.95 ML Cu/Rh(lOO), only 0.017 ML CO is released to account for the major portion of the desorption peak at 248 K. Even this CO is probably generated from the decomposition of methanol on uncovered areas of the Rh(100) surface with subsequent spilling over to the copper overlayer. Third, the methoxy species remains on copper overlayers to higher temperatures, - 370 K on monolayer and 400 K on multilayer copper films, when it decomposes to form formaldehyde, which is not seen on clean I&(100). Although the formation of methoxy and formaldehyde is minimal on clean copper overlayers, especially multilayers, predosed submonolayer oxygen greatly enhances their fo~ation.

~eth~ol, as an example of a weak electron donor ligand (Lewis base), usually adsorbs on metal surfaces via one or both of its oxygen atom electron lone pairs. This is supported by experimental evidence from photoelectron spectroscopic studies and work-function change measurements. For example, in a UPS study of methanol adsorption on Cu(ll0) [35], Bowker and Madix observed that the oxygen lone pair (201”) orbital was stabilized by - 0.7 eV relative to deeper-lying orbitals upon adsorption, indicating a surface bond via the oxygen atom. The work-lotion change associ-

ated with the adsorption of methanol at saturation coverage on Cu(l10) is - 1.35 eV [35], indicating a net electron flow from the adsorbed species to the surface. Therefore, the interaction between methanol and copper can be thought of in classic Lewis acid-Lewis base terms, with methanol acting as the Lewis base and the copper as the Lewis acid.

In general, copper tends to be more inert than rhodium. Rhodium has an electronic configura- tion of d8 and el~tronegati~ty of 2.2, while copper has an electronic config~ation of d’* and electro- negativity of 1.9. Therefore, rhodium tends to be a more effective Lewis acid than copper. This may explain why the /?t and & states have lower peak temperatures on monolayer Cu/Rh(lOO) than on clean Rh( 100).

A factor that may account for the higher /?r peak desorption temperature on monolayer Cu/ Rh(100) compared with multilayer Cu/Rh(lOO) is the effective coordination of the copper atoms in the overlayers. As pointed out before, the pseudomorphic monolayer of copper on ~(1~) has an atomic density mismatch of 10% (i.e. the atomic density of copper is 10% less than on Cu(lO!)). The Cu-Cu nearest neighbor distance of 2.69 A ts substantially larger than the value of 2.56 A observed in bulk copper. Thus, the effective coordination of copper atoms in the copper monolayer is lower. This reduction of the effective coordination may lead to a larger bonding strength with ad- sorbates, and in turn, higher desorption temperatures, When the copper overlayer grows thicker, the copper-copper distance undoubtedly approaches more nearly the bulk copper value, resulting in a higher effective coordination for indi- vidual copper atoms and hence lowered bonding strength and desorption temperature for adsorbed methanol.

Bonding geometry may also play a role in de- termining the desorption temperatures. There is some disagreement regarding the C-O bond orientation with respect to the surface and the exact chemisorption site of the methoxy species on the Cu(100) surface. In a HREELS study [37], Sexton proposed that the methoxy species is ori- ented normal to the surface by applying the surface dipole selection rule and suggested that it sat in

the fourfold hollow site. On the other hand, Ry- berg [39,41] proposed a tilted orientation based on the observation in the infrared spectra of the asymmetric C-H stretch. By means of surface extended (SEXAFS) and near-edge X-ray absorp- tion fine structure (NEXAFS) measurements above the oxygen K edge of the methoxy species on the Cu(100) surface, Madix and co-workers [54] determined that at 200 K the C-O axis in the methoxy species was tilted with respect to the surface normal and that while the exact chemisorption site of the methoxy species could not be determined, the atop site could be ruled out. By calculating the total energies for clusters repre- senting the system, Wander and Holland [5.5] suggested the possibility of a nonequilibrium phase of mixed fourfold hollow site and bridge site occupa- tion although the fourfold hollow site is energetically more favorable. They also concluded that the methoxy species is untilted. More recently, based on photoelectron diffraction and NEXAFS studies, Lindner et al. 1561 suggested that the methoxy species adsorbed almost pe~endicul~ly on a low symmetry site, about 0.9 A from the bridge site and 0.4 A from the hollow site. We tend to favor a tilted, twofold bridging methoxy on the pseudomorphic monolayer Cu/Rh(lOO) based on the same rationalization as described in ref. (461.

We also note that the CO resulting from the decomposition of methanol desorbed from Rh(100) partially covered with copper at lower temperatures (cf. peak at 472 K in fig. 3a) than from clean ~(1~) (cf. fig. 3 in ref. 1461, where CO desorbed at 530 K at the same methanol exposure). In their TDS and HREELS study of methanol adsorbed on copper overlayers on Ru(0001) [15], Paul and Hoffmann also found that at submonolayer copper coverage the desorption temperature of CO formed during the decomposition process shifted to lower temperatures with increasing copper coverage.

It has been demonstrated via HREELS that a methoxy species can be formed on copper overlayers on I&(100), just as on the clean ~(1~) surface [46]. Concerning the amounts of methoxy formed under various conditions, there appears to be some discrepancy between the HREELS and TDMS results presented here. Although it is not

possible to draw precise conclusions concerning the amount of a surface species present based purely on HREELS data, the EEL spectra presented here certainly seem to indicate a substantial methoxy concentration up to > 330 K on 1 ML Cu/Rh(lOO), and to > 360 K on multilayer Cu/Rh(lOO). In contrast, the TDMS results for temperatures above 300 K show only a rather trivial amount of formaldehyde desorption, which occurs near 370 R on 1 ML Cu/~(l~) and near 405 K for ~~~~) surfaces with copper coverages 2 2 ML. Thus while there is good agreement as to the temperature at which methoxy d~omposi~on occurs (the fo~aldehyde desorbed being the product of methoxy d~rnpositi~~~~ there is ap- parent disagreement as to the amount involved. For several reasons, it is probable that this disagreement can be accounted for by the presence of very small impurity concentrations on the copper overlayers from which the HREELS measurements were obtained. First, it has been demonstrated previously that small boots of ooad sorbed oxygen [I5,44] can dramatically enhance methoxy formation on copper surfaces, and we have confirmed this in our own case by observing an increased fo~~dehyde thermal desorption signal from copper overlayers on Rh(XO0) when oxygen is pre-adsorbed. This effect need not be limited to oxygen but rather probably occurs with any adatom that can enhance OW bond cleavage. Second, it was found on “clean” copper overlayers that the amount of formaldehyde evolved (i.e. the boot of methoxy d~mpos~g) varied consid- erably; this suggests that this formaldehyde desorption is dependent on some impurity, the con- ~~~atio~ of which on the surface is very small but variable. Finally, since the copper overlayers prepared in the EELS chamber consistently showed a weak loss feature near 300 cm-i despite the fact that no impurity could be identified in Auger spectra, it is likely that some type of impurity atom was present on these surfaces in very low concentration, effectively catalyzing methoxy formation from adsorbed methanol. It thus seems reasonable to propose that on absolutely pure copper overlayers little or no methoxy would be formed, with the possible exception of the methoxy that gives rise to the & thermal desorption feature

from I ML C~/~~I~~_ However, we agree with the point of view taken by Paul and Hoffmarm 1151 that, once methoxy is formed on a copper overlayer, its subsequent reactions will be affected very little by the small impurity concentrations that catalyze its formation. This point of view is supported by the decomposition temperature range of methoxy on our copper overlayers, which is very similar to the range of decomposition temperatures observed on bulk copper surfaces (see next p~a~aph~. We therefore move on to a discussion of the stability and reactivity of methoxy on copper overlayers on ~(1~) without further eonsidera- tion of the role of spurges.

In contrast to molecularly adsorbed methanol, methoxy on copper overlayers on Rh(100) is stabilized compared with methoxy on clean Rh(100). On a clean Rh(100) surface that has been saturated with methanol, methoxy decomposition to CO and hydrogen is complete by 320 K. On 1 ML Cu/~(l~) some methoxy remains on the surface to 370 K, and for copper coverages ex- ceeding 2 ML some remains to - 400 K. Since the first step in methoxy d~rnpo~~on (whether to form~dehyd~ or CO) must be the cleavage of a CH bond, this indicates an increased activation barrier for this process on copper as opposed to rhodium. This is not surprising given the Mgher binding energy of hydrogen adatoms on Rh(100) [57] compared with bulk copper surfaces [49,X$ It should be noted that the maximum temperatures for a stable methoxy species (i.e. the temperatures of fo~~d~hyde evolution) reported here agree very well with those reported for various copper surfaces. Specifically, methoxy d~ompositio~ accompanied by fo~~dehyde desorption occurs at 370 K on C~(l~) f373 and Cu(ll0) [34,35], while on close-packed Cu(ll1) it occurs at 410 K [44]. Thus on a pseudomorphic copper monolayer on Rh(lO0) the methoxy decomposition temperature is virtually identical to what it is on the structurally similar Cu(100) surface, while on copper multilayers on Rh(100) it is virtually identical to what it is on the Cu(ll1) surface. The latter fact su~ests that the thr~-~mension~ copper clusters formed when copper m~~ayers are present may have a local surface structure resembling the close-packed (Ill) surface. This

also is not surprising because for an fee metal the (111) surface structure is energetically the most favorable. In a methanol decomposition study on Cu/ZnO thin films, Chan and Griffin [58] also observed that formaldehyde resulting from methoxy decomposition on metallic copper clusters desorbed at 410 K.

It has been shown that methoxy on copper overlayers on Rh(100) has a metal-oxygen stretching frequency that is reduced subst~tially compared with methoxy on clean Rh(lO0). As shown in table 2, this frequency is about 400 cm-’ on clean Rh(lOO) and 325 cm-r on 1 ML Cu/Rh(lOO). When higher copper coverages are present on the Rh(100) surface, the frequency of this mode approaches its Cu(100) value of 290 cm-‘. At first glance, this trend appears to con- tradict the trend of increasing methoxy stability with increasing copper coverage: the reduced metal-oxygen stretching frequency suggests a weakened surface-methoxy bond, while the greater stability of methoxy in the presence of copper suggests stronger surface-ligand bonding. There is, however, no real contradiction; apart from the fact that a lowered stretching frequency does not necessarily indicate a weakened bond, the initial activation barrier to be overcome in the decomposition of methoxy does not involve cleavage of the metal-oxygen bond. Rather, as has already been stated, it involves CH bond cleavage (and possibly also a tilting of the methoxy group towards the surface). Cleavage of the met&oxygen bond is important only in subsequent steps in the methoxy decomposition. On copper-covered Rh(100) the next step is simply formaldehyde desorption, which appears to follow immediately upon CH bond cleavage of the methoxy (there being no HREELS evidence for the formation of any appreciable amounts of a stable, adsorbed H&O species). On clean Rh(lOO), the subsequent steps involve conversion of a short-lived, adsorbed H,CO intermediate to CO + 2H; here again, these reactions appear to follow immediately upon CH bond cleavage of methoxy. Thus on both clean and copper-covered Rh(100) surfaces the activation energy to CH bond cleavage of methoxy is higher than the activation energies of subsequent

reaction steps, and in neither case does one unam- biguously isolate adsorbed H&O.

These observations also offer an explanation for the different decomposition mechanisms of methoxy on clean and copper-covered Rh(lO0). In both cases the first step in the decomposition involves the cleavage of one CH bond of the methoxy; the activation barrier for this is lower on the clean surface, as evidenced by the lower temperature of methoxy de~mposition. The next step involves additional CH bond cleavage if adsorbed CO is to be formed, or metal-oxygen bond cleavage if formaldehyde evolution is to occur. Since clean Rh(100) has a relatively high binding energy for hydrogen adatoms it seems reasonable that cleavage of the second and third CH bonds of the methoxy would be relatively facile; this is indeed the case and adsorbed CO is the only decomposition product of methoxy observed using either HREELS or TDMS. In contrast, copper-covered Rh(100) surfaces have a relatively low binding energy for hydrogen adatoms and a rather large activation barrier for breaking the first methoxy CH bond; it is thus reasonable to expect that cleavage of the remaining CH bonds would be relatively more difficult. In addition, the lowered metal-oxygen stretch compared with methoxy on clean Rh(100) suggests a weaker metal-oxygen bond. These effects combine to make metal- oxygen bond cleavage of a short-lived adsorbed H&O species favored over CH bond cleavage, and the result is that methoxy decomposes via form~dehyde evolution on the copper overlayers.

Copper is a major component in many catalysts for methanol synthesis from CO and H, ]21-241. In this catalytic reaction, the adsorption of CO and desorption of CH,OH must be two elemen- tary steps and the methoxy species is very likely one of the intermediates. In a previous study [32], it was shown that the CO molecules bond more strongly on the monolayer or submonolayer Cu/Rh(lOO) and have desorption temperatures - 50-85 K higher than on bulk copper. In the present study, it is demonstrated that the adsorbed methanol has a higher desorption temperature on monolayer ~u/~(l~) than on multilayer copper/~(lOO), The effects of these char-

X. Jiang et al. / Adwption and&composition of methanol on Cu/Rh(100) 59

acteristics of the monolayer copper films on the methanol synthesis are not clear at this point. A kinetic study of methanol synthesis from CO and H, on this model bimet~~c catalyst is currently under way in our laboratory.

5. Conclusions

From this work, we arrive at the following conclusions :

(1) On the copper overlayers on Rh(lOOT), the adsorption and d~mposi~on of methanol follow reaction pathways similar to those on clean Rh(lOO), but with a number of important differences. At low temperatures, methanol is molecularly adsorbed and bonds to the surface via the oxygen atom. At higher temperatures, adsorbed methanol undergoes two competing reactions: desorption and OH bond cleavage to form an oxygen-bonded methoxy species. At still higher temperatures, a fraction of the methoxy species recombines with hydrogen adatoms to desorb as molecular methano& while the remainder stays on the surface until 370-400 K to form gaseous formaldehyde and hydrogen as decomposition products.

(2) On monolayer Cu,/Rh(lOO), the methanol desorption and the methoxy species recombination with hydrogen adatoms occur at - 200 K and 225-260 K, respectively. These temperatures are some 10 K lower than the corresponding temperatures on clean I&(100). This can be largely explained by the differing electronic properties of copper and rhodium.

(3) On multilayer Cu~~(I~~, the methanol desorption and the methoxy species recombination occur at even lower temperatures of - 180 K and 205-230 K, respectively. The effective coordination of the copper atoms in the overlayers and the bonding geometry of the methoxy species are invoked to account for the differences between monolayer and multilayer copper films.

(4) The copper overlayers greatly inhibit methanol decomposition to CO. Instead, methanol decomposes to form formaldehyde on copper overlayers via a methoxy intermediate. Although the formation of methoxy and formaldehyde is

minimal on clean copper overlayers, especially multilayers, predosed submonolayer oxygen or other impurities which favor OH bond cleavage will enhance their formation.

(5) The characteristics of the vibrational spectra of adsorbed methanol and methoxy species on copper overlayers/Rh(lOO) are very similar to those on clean Rh(100). While in most regards the methoxy vibrational spectrum is not surface sensi- tive, the frequency of v(Cu-OCH,) of 325 cm-i on monolayer Cu/Rh(lOO) is substantially lower than the ~(l%h-OCH,) frequency of - 400 cm-’ on clean ~(1~~ and further downshifts on multi-layer Cu/Rh(IOOf. Some methoxy remains on the surface to - 400 K on copper m~tilay~s/ Rh(100) and to 370 K on monolayer copper/ Rh(lOO), as opposed to 320 K on clean Rh(lO0).

We acknowledge with pleasure the support of this work by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sci- ences.

References

(11 G.M. Schwab, Disc. Faraday Sot. 8 (1950) 166.

12) J.H. Sinfelt, Bimetallic Catalysis: Discoveries, Concepts

and Appliclzticms (Wiley, New York, 1983). 131 K. Chriwnarm, G. Ertl and H. Shimizu, J. Catal. 61

(1980) 397. f4] H. Shimizu, K. Christmazm and G. Ertl, J. Catal. 61

(l980) 412. f5] J-C. Vickerman, K. Christmann and G. Ert& J. Catal. 71

(1981) 175. [6] J.C. Vickwman and K. Christmann, Surf. Sci. 120 (1982)

1.

[i’] J.C. Vickerman, K. Christmann, G. Erti, P. Heimarm, EY. Himpsel and DE. Eastman, Surf. Sci. 134 (1983) 367.

[8] S.-K. Shi, H.-L Lee and J.M. White, Surf. Sci. 102 (1981)

[9] ?M. Dar&l, Y. Kim, H.C. Peebles and J.M. White, Surf. Sci. 111 (1982) 189.

IlO] L. Richter, S.D. Bader and MB. Brodsky, J. Vat. Sci. Technol. 18 (1981) 578.

fll] S.D. Bader, L. Richter, P.-L. Cao, DE. Ellis and A.J. Freeman, J. Vat. Sci. Tehr&. A if1983) 1185.

fl2] J.S. Fwrd and PD. Jones, Surf. Sci. X52/153 (1985) 487.

60 X. Jiang et al. / Adsorption and &composition of methanol on Cu/ Rh(I00)

[13] J.T. Yates, Jr., C.H.F. Peden and D.W. Goodman, J. Catal. 94 (1985) 576.

[14] D.W. Goodman, Chapter Role of Ch~acte~~tion in Catalyst Development, in: ACS Symp. Ser. 1111 (1989) p. 191.

[15] J. Paul and F.M. Hoffmann, Surf. Sci. 172 (1986) 151. [16] JE. Houston, C.H.F. Peden, P.J. Feibelman and D.W.

Goodman, Ind. Eng. Chem. Fundam. 25 (1986) 58. [17] D.W. Goodman, in; Heterogeneous Catalysis (Proc.

IUCCP Conf.) Texas A&M University, 1984. [18] J.E. Houston, C.H.F. Peden, D.S. Blair and D.W. Good-

man, Surf. Sci. 167 (1986) 427. [19] J.W.A. Sachtler and G.A. Somojai, J. Catal. 89 (1984) 35. [20] C.H.F. Peden and D.W. Goodman, J. Catal. 100 (1986)

5209. 1211 E.G. Baglin, G.B. Atkinson and L.J. Nicks, Ind. Eng.

Chem. Prod. Res. Dev. 20 (1981) 87. [22] F.P. Daly, J.Catal. 89 (1984) 131. [23] G. Owen, CM. Hawkes, D. Lloyd, J.R. Jennings, R.M.

Lambert and R.M. Nix, Appl. Catal. 33 (1987) 405. [24] G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer

and K.C. Waugh, Appl. Catal. 36 (1988) 1. [25] K. Klier, Advan. Catal. 31 (1982) 243. [26] R.G. Herman, K. Klier, G.W. Simmons, B.P. Finn, J.B.

Bulko and T.P. Kobylinski, J. Catal. 56 (1979) 407. [27] M. Bowker, Vacuum 33 (1983) 669. [28] J.B. Friedrich, D.J. Young and M.S. Wainwright, J. Catal.

80 (1983) 1, 14. [29] H.H. Kung, Catal. Rev. Sci. Eng. 22 (1980) 235. [30] C.N. Satterfield, Heterogeneous Catalysis in Practice (Mc-

Graw-Hill, New York, 1980) p, 195. [31] B.J. Kip, E.G.F. Hermans and R. Prins, Appl. Catal. 35

(1987) 141. [32] X. Jiang and D.W. Goodman, Surf. Sci., submitted. [33] C. Kittel, Introduction to Solid State Physics (Wiley, New

York, 1986) p. 24. [34] I.E. Wachs and R.J. Madix, J. Catal. 53 (1978) 208. [35] M. Bowker and R.J. Madix, Surf. Sci. 95 (1980) 190. [36] J. Stohr, J.L. Gland, W. Eberhardt, D. Outka, R.J. Madix,

F. Sette, R.J. Koestner and U. Doebler, Phys. Rev. Lett. 51 (1983) 2414.

(371 B.A. Sexton, Surf. Sci. 88 (1979) 299. [3X] B.A. Sexton, A.E. Hughes and N.R. Avery, Appl. Surf.

Sci. 22/23 (1985) 404. 1391 R. Ryberg, Chem. Phys. Lett. 83 (1981) 423. [40] R. Ryberg, Phys. Rev. Lett. 49 (1982) 1579. [41] R. Ryberg, Phys. Rev. B 31 (1985) 2545. [42] R. Ryberg, J. Chem. Phys. 82 (1985) 567. [43] S. Andersson and M. Persson, Phys. Rev. B 24 (1981)

3659. [44] J.N. Russell, Jr., SM. Gates and J.T. Yates, Jr., Surf. Sci.

163 (1985) 516. [45] M.A. Chesters and E.M. McCash, Spectrochim. Acta A 43

(1987) 1625. 1461 J.E. Parmeter, X. Jiang and D.W. Goodman, Surf. Sci.

240 (1990) 85. [47] Y. Kim, H.C. Peebles and J.M. White, Surf. Sci. 114

(1982) 363. [48] D.E. Peebles, H.C. Peebles and J.M. White, Surf. Sci. 136

(1984) 463. 1491 F. Greuter and E.W. Plummer, Solid State Commun. 48

(1983) 37. [SO] G. Anger, A. Winkler and K.D. Rendulic, Surf. Sci. 220

(1989) 1. [Sl] M. Falk and E. Whalley, J. Chem. Phys. 34 (1961) 1554. [52] It is possible that the remaining intensity of this loss

feature in fig. 7a results largely or entirely either from readsorption of a small amount of molecular methanol after the flash to 208 K was made, or from the presence of a residual amount of water or some other coadsorbed impurity. See also ref. 1461.

1531 A.K. Bhattacharya, M.A. Chesters, M.E. Pemble and N. Sheppard, Surf. Sci. 206 (1988) L845.

1541 D.A. Outka, R.J. Madix and J. Stiihr, Surf. Sci. 164 (1985) 235.

[55] A. Wander and B.W. Holland, Surf. Sci. 203 (1988) L637. [56] Th. Lindner, J. Somers, A.M. Bradshaw, A.L. D. Kilcoyne

and D.P. Woodruff, Surf. Sci. 203 (1988) 333. [57] L.J. Richter and W. Ho, J. Vat. Sci. Technol. A 5 (1987)

453. [58] L. Chan and G.L. Griffin, Surf. Sci. 173 (1986) 160.

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