The effects of preadsorbed oxygen on the adsorption and decomposition of methanol on Ni(110)

Surface Science 155 (1985) L281-L291 North-HolIand, Amsterdam

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SURFACE SCIENCE LETTERS THE EFFECTS OF PREADSORBED OXYGEN ON THE ADSORPTION AND DECOMPOSITION OF METHANOL ON Ni(ll0) Simon R. BARE *, Joseph A. STROSCIO and W. HO Lahratoty ofAtomic end Solid State Phvfics and Materials Science Center, Cornell University, Ithaca, New York 14853, USA Received 18 December 1984; accepted for publication 5 February 1985

Tbe adsorption and decomposition of methanol on a partially oxidized nickel surface, the Ni(llO)-(2x1)0 structure, has been studied using high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). Methanol adsorbs molecularly at 80 K and multilayers of methanol can be condensed at this temperature. Molecular methanol is thermally unstabie above -150 K and decomposes to a methoxy intermediate. Half of the chemisorbed methoxy desorbs by r~nmb~ation in three desoxption states with peak temperatures at 200,240 and 300 K. Most of the remaining CH,O decomposes to CO and H, between 300 and 350 K, while a small amount is stabilized as a surface formate. HCOO, species. The formate decomposes into CO2 and H, at 385 K. The presence of the surface oxygen affects the decomposition of methanol on Ni(ll0) in two ways: it stabilizes the methoxy group to higher temperatures, and induces the formation of a second intermediate, HCOO, above the methoxy decomposition temperature.

The chemical properties of a surface can be modified by the presence of adatoms which can have a profound effect on both the reactivity and selectivity of the surface in catalytic reactions. We have chosen an ordered overlayer of oxygen atoms as a modifier on the Ni(ll0) surface, and have investigated the reaction of methanol on such a surface. Methanol decomposition is an interesting reaction to follow since it decomposes via a methoxy (CH,O) intermediate to yield CO and H, on a number of transition metal surfaces [l] while on Cu [2,3] and Ag[4] it is oxidized to primarily formaldehyde (H&O), also via a methoxy intermediate, with some oxidation to a formate species (HCOO) which subsequently decomposes to evolve CO2 and H,. We have recently studied in detail the adsorption and decomposition of methanol on a clean Ni(ll0) surface [1], while a summary of the results of the reaction on the (2 x 1)0 overlayer have been presented briefly elsewhere [5]. Here we report a detailed study on the modified surface, along with a comparison of the results for the clean Ni(ll0) surface. * Present address: Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720, USA.

~39-6028/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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The results obtained from the study of methanol decomposition on the clean Ni(ll0) surface can be summarized briefly as follows. It was found that methanol adsorbs molecularly at 80 K but above - 150 K molecular methanol is unstable and it decomposes to adsorbed methoxy and hydrogen. Further heating resulted in 20% of the saturated chemisorbed phase desorbing as methanol by recombination. The remaining CH,O decomposes between 260-300 K to CO(a) and H(a) which desorbs as CO and H, in desorption rate limited processes. There was no evidence for the formation of any other intermediates, nor the desorption of other reaction products. The experiments were performed in two-level ultrahigh vacuum system which has been described previously [6]. Both monochromator and the analyzer of the HREEL spectrometer consist of the double pass 127’ cylindrical electrostatic deflectors. The HREEL spectra were recorded using an incident beam energy of 3.5 eV at an overall spectrometer resolution of 4-7 meV (32-56 cm-‘). All spectra were recorded in the specular direction with an angle of reflection of 60” measured from the crystal normal. The TDS was performed using a Spectramass SMlOO quadrupole mass spectrometer fitted with an apertured nozzle to enhance the direct line of sight desorption signal and multiplexed to an LSI 11/23 computer enabling simultaneous measurement of up to 10 masses along with the crystal temperature. The Ni crystal was cleaned by repeated cycles of Ne ion bombardment and annealing. Methanol used was research grade CH,OH (99.9%, Mallinkrodt), and oxygen used was research grade 1602 (99.995%, Matheson) and i80Z (99% 1802, MSD Isotopes). In all cases the (2 x 1)0 structure was produced by exposing the clean Ni(ll0) crystal to 5 x 10e9 Torr 0, with the crystal at a constant temperature of 335 K. An exposure of 0.6 L (1 L = 1 X 10e6 Torr s) oxygen resulted in the formation of a well-ordered (2 X 1) structure as observed with low energy electron diffraction (LEED). The crystal was then cooled to 80 K before exposing to methanol vapor. All exposures quoted are uncorrected for ion gauge sensitivity. The notation A(a)/B refers to the desorption of the ~1 peak of gas A following adsorption of gas B. Fig. 1 shows a thermal desorption spectrum for all of the observed reaction products after a 5 L exposure of CH,OH on the Ni(llO)-(2 X 1)0 surface. This exposure is sufficient to condense multilayers of methanol, as evidenced by the narrow methanol desorption peak at 150 K Gvhich does not saturate with increased exposures. Only four gaseous products were detected and the corresponding signals for the four masses are shown in fig. 1: 31 (methanol fragment), 28 (CO), 2 (HZ), and 44 (CO,). Four desorption states are observed for CH,OH/CH,OH on Ni(llO)-(2 X l)O, as shown in more detail in fig. 2a. In addition to the multilayer peak at 150 K, the other states are observed at - 200, - 240 and - 300 K. As a

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Fig. 1. Thermal desorption spectrum showing all the observed reaction products following a 5 L at 80 K. Heating rate used is 7.3 K s-t. Signals from the four masses are presented with the same sensitivity scale. exposure of CH,OH

function of increasing exposures the states of 240 and 300 K populate first, followed by that at 200 K and finally the 150 K state. As is demonstrated later, the HREELS results are indicative that above - 180 K the remaining methanol on the surface is dissociated into CH,O(a) and H(a). Thus it is concluded that the three CH,OH desorption states at higher temperatures are due to the recombination of CH,O(a) + H(a). It is estimated that the amount of CH,OH desorbing by recombination is 4.2 x 1014 molecules cm-*, which corresponds to approximately 0.4 monolayers (ML) as referenced to a surface density of 1.14 x 1015 Ni atoms cm-*. On the clean surface only about half as much methanol was seen to desorb by recombination in two states at - 200 and - 258 K. In fact, above - 260 K the methoxy began to decompose into CO(a) and H(a) on the clean surface, and by 300 K there was no evidence for any remaining CH,O(a). Thus, on the (2 x 1)0 pre-covered surface the oxygen must have a stabilizing effect on the CH,O(a), thereby allowing the formation of the 300 K desorption state. In fact, in other methanol adsorption studies, for example on Cu [2,3], Ag [4], Pt [7], and Pd [8], the presence of surface oxygen has also been shown to stabilize the methoxy intermediate. The thermal desorption of H,/CH,OH on Ni(llO)-(2 X 1)0 shows two peaks, as illustrated in fig. 2b for increasing initial methanol exposures. The much larger /I state is at 325 K, with the smaller y state at 385 K. The temperature maximum (T,) of the y peak decreases slightly as a function of

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Fig. 2. Effect of initial exposures of CH,OH at 80 K on the desorption of (a) CH,OH, (b) H,, (c) CO,, and (d) CO. The desorption curves for each maw are for increasing exposures of 0.25, 0.5, 1.0, 2.0, and 5.0 L. Not all the higher exposures are shown. The heating rate used is 7.3 K s-‘.

increasing coverage, while that of the fi state shifts slightly to lower temperatures. Both states saturate after a 2 L CH,OH exposure. Thermal desorption spectra of Hz/H, on clean Ni(ll0) show two states for H coverages less than one monolayer, p, at - 295 K and & at - 345 K. Although H, does not dissociate and adsorb on the Ni(llO)-(2 X 1)0 surface [9], the B state is attributed to the recombination of hydrogen atoms from both the dissociative decomposition of methanol and methoxy. In addition to the expected decomposition products of H, and CO, we also observed the desorption of CO, in a single peak at 385 K as shown in figs. 1 and 2c. As a function of increasing CH,OH exposures, Tp does not shift and the peak area saturates at a methanol exposure of 2 L. Carbon dioxide does not adsorb or dissociate on the clean and oxygen precovered Ni(ll0) surface

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[9]. Thus it is concluded that the CO, evolution is reaction rate limited. In order to determine if any of the surface oxygen was directly involved in the CO, formation, a (2 x 1)0 layer was prepared using 1802, and the thermal desorption spectrum was monitored for masses 48 (C’80’80), 46 (C’80160), and 44 (C’60160). It was found that all of the CO, evolved as mass 46, implying that the surface oxygen is furnishing one of the oxygen atoms. The amount of CO, desorbing is estimated to be 2.3 X 1013 molecules cmW2. The coincident desorption of yH, and CO, at 385 K suggests that they are evolved from the decomposition of the same intermediate, the most likely one being the formate intermediate, HCOO, which is known to be stable up to these temperatures on this surface and which decomposes into CO, and H,

WI. The CO/CH,OH desorption spectra are shown in fig. 2d. The major peak is at 430 K with a smaller shoulder at - 370 K. Neither state shifts with increasing coverage, the shoulder appears to saturate after a 0.5 L CH,OH exposure while the 370 K state saturates at 2 L. The CO spectra does not resemble those from CO adsorption on either the clean surface or the (2 X 1) oxidized surface [9,11]. In the case of CO/CH,OH on Ni(ll0) the evolution was desorption rate limited with the thermal desorption spectrum being identical to that of the same coverage of CO/CO on Ni(llO), indicating that the CO was adsorbed on the surface in the same binding state immediately prior to desorption [l]. This is not the case for CO/CH,OH on Ni(llO)-(2 X l)O. It is noted that a similar effect was observed by Johnson and Madix during the decomposition of formic acid on the oxygen pre-treated Ni(ll0) surface [9]. They concluded that the decomposition of the formic acid caused a reduction of the partially oxidized Ni(ll0) surface thereby creating new CO binding states. A similar effect could be occurring for methanol decomposition. On the i802 pre-covered surface there was no evidence for the desorption of any mass 30 (C180) for any CH,OH exposure. The production of small amounts of water during the CH,OH decomposition was inconclusive due to the coadsorption of small traces of water vapor from the background. However, there were no signs of the production of any formaldehyde H,CO (mass 30). Thus, it was not possible to oxidize the methanol to formaldehyde on the Ni(llO)-(2 X 1)0 surface in contrast to Cu and Ag surfaces, although it is possible to stabilize a formate intermediate. The HREELS results are summarized in figs. 3-5. The bottom curve in fig. 3 shows the vibrational spectrum for the (2 X 1) oxygen structure on Ni(ll0). Two loss peaks are observed at 240 and 400 cm-‘. Although the structure of the (2 x 1)O surface has been much debated in the past, recent HREELS [12], ion induced Auger electron spectroscopy [13], and scanning tunneling microscopy [14] measurements give consistent results, and favor a sawtooth model with the oxygen atoms occupying long bridge sites on a reconstructed

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Fig. 3. HREEL spectra for the Ni(llO)-(2 x 1)0 structure at 300 K and the same surface with a saturated overlayer of chemisorbed CH,OH at 80 K.

surface. Our vibrational results are in good agreement with those of Bar6 and 0116 [12]; the 400 cm-’ loss is due to the symmetric Ni-0 stretch. However, we are also able to resolve an intense mode at 240 cm-’ which we assign as an oxygen derived surface phonon occurring below the maximm bulk phonon frequency of Ni. Also shown in fig. 3 is the vibrational spectrum after adsorbing 2 L CH,OH on the (2 x 1)0 surface at 80 K. This exposure corresponds to saturation coverage of the chemisorbed layer, i.e. before the onset of the multilayer peak in TDS. In order to assign the vibrational losses we rely on a comparison between this spectrum and that of CH,OH on clean Ni(ll0) and the infrared absorption spectra of gas, liquid and solid methanol [I]. The following modes are readily identifiable: the intense mode at 1040 cm-’ is the C-O stretch, the CH, rock is at 1150 cm-‘, the CH, deformation at 1440 cm-‘, and the symmetric and asymmetric CH, stretch at 2820 an 2955 cm-‘, respectively. The modes due to the OH group are broad; the OH stretch appears as a tail on the CH, stretch and is centered at - 3175 cm-*, and the OH bend is at - 750 cm-‘. Both of these modes become relatively more pronounced as additional layers of methanol are condensed. Their presence at 80 K indicates that the adsorption is molecular. The most striking observation in the low frequency region is the lack of a vibrational mode at 400 cm-’ due to the Ni-0 stretch of

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Fig. 4. HREEL spectra depicting the thermal decomposition of CH,OH on Ni(llO)-(2x l)O. Spectra are shown at 80 K and after heating momentarily to 180, 300, 350, and 450 K. All spectra were recorded after quenching to 80 K.

the oxygen atoms, which appears in the lower curve of fig. 3. Instead vibrational modes at 300 and 515 cm-’ are observed. This observation can be interpreted by considering the behavior of oxygen alone on Ni(ll0) [12]. Here two Ni-0 stretch frequencies are observed at 420 and 530 cm-’ and their relative intensity is dependent upon the oxygen coverage. Both are due to the perpendicular vibration of 0 atoms in long bridge sites. The 530 cm-’ mode is

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Fig. 5. A comparison of HREEL spectra recorded after heating a saturated overlayer of chemisorbed CH,OH on Ni(llO)-(2 x 1)O to 350 K with that for the same exposure of HCOOH on clean Ni(ll0) after heating to 300 K. Both spectra recorded after quenching to 80 K.

observed at low oxygen coverages on the unreconstructed Ni surface, whereas at higher coverges the 420 cm-’ mode is observed on the reconstructed (2 X 1) Ni surface. The nature for the softening of the Ni-0 frequency on the reconstructed surface is not fully understood [12]. Thus assigning the mode at 515 cm-’ to be due to the N-O (atom) vibration implies that the adsorption of methanol on the (2 X 1)0 surface inhibits the softening of the Ni-0 vibration. The mode at 300 cm-’ is then assigned to the Ni-0 (methanol) stretch vibration. Further evidence for this assignment is obtained in the thermal decomposition spectra shown in fig. 4. The vibrational spectra and frequencies of CH,OH on the partially oxidized and the clean Ni(ll0) surfaces are very similar. This suggests that the methanol molecules are still bound to the nickel atoms, and that the oxygen has little overall effect on methanol adsorption at 80 K. Fig. 4 shows a sequence of HREEL spectra depicting the decomposition of CH,OH on the partially oxidized Ni surface. In each case the sample was heated to the temperature indicated and retooled to 80 K for the recording of the spectrum. The spectrum at 180 K is characterized by a significant decrease

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of the OH modes, indicating that as on clean Ni(ll0) molecular methanol is thermally unstable above - 150 K and decomposes to methoxy and hydrogen. The methoxy group has the following observable vibrational modes: 1025 cm-l C-O stretch, 1150 cm-’ CH, rock, 1440 cm-’ CH, deformation, and 2955 cm-’ CH, asymmetric stretch. The modes in the lower frequency region are not fully resolvable. However, the broad peak can be deconvoluted into a Ni-OCH, stretch at - 400 cm-’ and a Ni-0 (atom) stretch at - 500 cm-‘. The upward shift of the Ni-OCH, frequency from that of molecular methanol is similar to that seen on the clean surface [l]. Again, with the exception of the mode at 500 cm-r, the vibrational spectrum of methoxy on the partially oxidized surface is essentially the same as that on the clean surface. One point to note is that there was no evidence for the formation of free OH groups by the reaction of O(a) f H(a) (expected frequency > 3500 cm-‘) either during the adsorption of methanol at 80 K or on formation of methoxy on this partially oxidized Ni surface. On heating to below - 300 K the spectrum is characterized by a gradual loss in intensity of the modes associated with the methoxy group. This is in agreement with the TDS where a continuous evolution of methanol is seen up to - 300 K. A spectrum after briefly heating to 300 K is shown in fig. 4. The only methoxy mode clearly observable is the C-O stretch at 990 cm-‘. The downward shift in frequency of this mode with reduced methoxy coverage was also observed previously on the clean Ni(ll0) and is ascribed predominantly to dipole-dipole interactions [l]. However, it is to be noted that by 300 K on the clean surface, methoxy is fully decomposed to CO and H whereas on the partially oxidized surface there is still an appreciable coverage of methoxy species. Also observable in the 300 K spe&um are the oxygen derived surface phonon at 240 cm-‘, the Ni-0 (atom) stretch at 440 cm-‘, and the CO stretch of the methoxy decomposition product carbon monoxide at 2020 cm-‘. On further heating to 350 K two new modes at 775 and 1335 cm-’ are observed. The remaining modes are associated with the Ni-0 (atom) stretch at 400 cm-‘, the surface phonon at 240 em-‘, the CO stretch of the decomposition product carbon monoxide at 1900 and 2020 cm-’ and a small recant C-O stretch of CH,O at 970 cm-‘. The two losses at 1900 and 2020 cm-r are attributed to stretching vibration of CO in the bridge and on-top sites respectively. The modes at 775 an 1335 cm-’ are assigned to the OCO bending and OCO symmetric stretching modes of a formate intermediate, respectively. The observation of these modes confirms the presence of formate species, which is consistent with results from the TDS. The agreement of these frequencies with those of the formate species on Ni(ll0) can be seen in fig. 5 which shows the spectrum from heating a saturated overlayer of chemisorbed methanol on the partially oxidized Ni(ll0) surface to 350 K and that from heating a formic acid overlayer (adsorbed at 80 K) on clean Ni(ll0) to 300 K. The latter spectrum is in good agreement with that of Madix et al. [lo].

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The final spectrum shown in fig. 4 is after heating to 400 K. In agreement with results from the TDS there is only a small amount of carbon monoxide remaining on the surface, otherwise the spectrum is identical to that for the (2 X 1)0 in fig. 3. The presence of surface oxygen has been found to affect the decomposition of methanol on Ni(ll0) in two major respects: (1) it stabilizes the CH,O group to higher temperatures, and (2) it induces the formation of a second intermediate, HCOO, above the methoxy decomposition temperature. The stabilizing -effect has been observed previously on the noble metals, but in these cases it is due to the reaction of surface oxygen with the methanol. On Cu(llO), for example, the reactions are [2] CH,OH(g) + O(a) -+ CH,O(a) + OH(a), CH,OH(g)

+ OH(a) + CH,O(a) + H,O(g).

Thus after heating an oxygen pre-covered surface exposed to methanol on the noble metals, no oxygen is left on the surface at the end of the decomposition; all of the oxygen is used in the initial hydroxyl H abstraction. This is clearly not the case for Ni(llO)-(2 X 1)O. There is no conclusive evidence for any water formation in the results from HREELS or TDS, no evidence for OH groups in the vibrational spectra, and at the end of the decomposition a vibrational spectrum closely resembling the original (2 x 1)0 overlayer is obtained. Only a small fraction of the surface oxygen reacts with the methoxy, forming the formate intermediate which then decomposes into CO* and H, at higher temperatures. Further work would have to be done in order to identify conclusively the role of the oxygen atoms in the adsorption and decomposition of methanol on the partially oxidized Ni surface. However, it is to be noted that the vibrational spectra of both methanol and methoxy on this surface are virtually identical to those on the clean surface. Thus it appears that any electronic effect is small, compared to the effect of coadsorbed alkaki metals such as potassium on CO adsorption. It is possible that there is some localized charge redistribution at the surface with the presence of oxygen. This effect together with a site blocking mechanism can lead to the stabilization of the methoxy on Ni(ll0). This work is supported by the Office of Naval Research under Grant N00014-81-K-0505. In addition, the support to one of us (SRB) and the use of facilities/equipment provided by the Cornell Materials Science Center through the National Science Foundation under Grant DMR-8217227-A02 also are acknowledged. References [l] S.R. Bare, J.A. Stroscio and W. Ho, surface Sci. 150 (1985) 399, and references therein. [2] I.E. Wachs and R.J. Madix, J. Catalysis 53 (1978) 208.

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