ESD and TDS study of the adsorption of methanol on Nb(110)

Vacuum~volume41/numbers 1-3/pages 54 to 56/1990

0042-207X/90$3.00 + .00

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© 1990 Pergamon Press plc

ESD and TDS study of the adsorption of methanol on Nb(110) Laboratorio de Fisica de Superficies, Instituto de Ciencia de Materiales, CSIC, Serrano 144, 28006 Madrid, Spain

S R e y , I C o l e r a a n d J L de S e g o v i a ,

The adsorption of methanol on the Nb(110) surface has been studied by the electron stimulated desorption (ESD) and thermal desorption spectrometry (TDS) techniques, electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). The main ESD product is H + irrespective of the surface temperature and coverage of the surface. The thermally desorbed gases from a layer adsorbed at a temperature of 240 K are CH 4, H 2 and CO. The ion energy distribution (lED) reveals a most probable kinetic energy of 8 eV that shifts to 5.6 eV at higher exposures. The initial layer is mainly formed by methoxy species, that are dissociated into an intermediate with an energy of 71 kJ mo1-1

1. Introduction

The adsorption and decomposition of methanol on metal surfaces have been studied for a long time using most of the modern surface science techniques in addition to the basic technique of thermal desorption spectrometry (TDS) T M , due to its importance in catalysis studies ~3. One of the reasons to study the methanol dissociation rather than the methanol synthesis is due to the low yield of this reaction in vacuum for most of the surfaces studied. Main findings are that the initial dissociated layer of methoxy and hydrogen species are thermally desorbed mainly as methanol, hydrogen, water and carbon monoxide. As far as we know methane has been reported to be desorbed from the AI(II1) surface only in small amounts 9 and from the W(II1) 3 although in a smaller intensity. The aim of the present work has been to study the reactivity of methanol on the Nb(110) surface by using the ESD and TDS techniques. The complementary AES and ELS techniques have been used. 2. E x p e r i m e n t a l

The experiments were carried out in an uhv system previously described ~4 with a base pressure of 10- ~0 mbar (N 2 equivalent). The Nb(110) samples was cleaned either by ohmic heating up to 2000 K or by electron bombardment on the back of the sample up to 2300 K. The residual O contamination of 0.05 ML was determined from the AES O - N b intensity ratio. Methanol was vacuum degassed by repetitive freezing and heating cycles. During the experiments the H 2 contamination was 4 ~ with respect to the methanol pressure. The adsorption experiments were performed at constant methanol pressure and sample temperature of 240 K. 3. Results

Figure 1 shows the variation of the H ÷ ESD surface ion current vs methanol exposure. The surface was under electron bombardment during the time of mass recording, in order to avoid the electron damage of the layer. The curve shows a maximum at methanol exposure of about 2 L (1 L = 106 mbar.s) and the 54

surface ion current remains almost constant beyond 10 L. H ÷ was the only ESD ion surface species recorded. The result shows the same behavior as that from water 14 and hydrogen ~5'16 layers, but different from methanol on Ti(0001) 5. The lED curves for the H + ESD surface ion species at the indicated methanol exposure are shown in Figure 2. The H ÷ lED from H20 adsorbed on the same surface 18 has also been represented for comparison. The most probable kinetic energy at 8 eV for the lowest exposure shifts to lower energies on increasing methanol exposures. The lowest measured value at 5.6 eV energy for a methanol exposure of 20 L corresponds to the saturation. This low value is surprisingly higher than, for example the 3.8 eV for H ÷ from water, and 2.8eV for H + from methanol on the Ti(001) surface s. The thermal treatment experiments are shown in Figure 3. Figure 3(a) shows the O and C AES peak-to-peak intensities as a function of the surface temperature after a methanol exposure of 10 L at 240 K. Figure 3(b) shows the spectra of the thermally induced desorbed species at the same exposure. The main desorbed product is methane, followed by hydrogen. The carbon

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25

Figure I. Adsorption of methanol on the Nb(110) surface at 240 K. H +

ESD surface ion current as a function of the methanol exposure (1 L = 10-6 mbar.s)

S Rey et al: Adsorption of methanol on Nb(110)

[ • • •

with the first desorption structure at T < 400 K. In the second region at T > 400 K, the H + surface current signal decreases accordingly with the desorption rate of the desorbed molecules CH4, H2 and CO. The extinction of the C signal revealed by AES is also in agreement with the extinction of species containing C. The O AES peak-to-peak intensity, however, decreases slightly up to a temperature of ~ 1000 K and then it remains almost constant. The O was removed at temperatures of about 2000 K to the initial contamination of 0.05 ML. While the CH 4 and CO species do not show any significant increase at temperatures higher than 1000 K, the H 2 presents a third desorption state (not shown in the figure) with a maximum desorption rate at a temperature of about 1350 K.

CH30H/Nb(110)

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2 4 6 8 10 12 14 ION KINETIC ENERGY (eV) Figure 2. Adsorption of methanol on the Nb(! 10) surface at 240 K. Ion energy distribution (lED) ofH ÷ ESD surface ions, at indicated methanol exposures. The IED of H ÷ from adsorbed water is also shown for comparison.

monoxide is desorbed with less intensity. However, at exposures less than 2 L the H 2 intensity was higher than the methane intensity. The spectra show a shoulder in the desorption curve at about 400 K and a well defined peak at 600 K for all the gases. Figure 3(c) shows the variation of the ESD H ÷ surface ion current as a function of the temperature. This curve shows two clearly differentiated regions. In the first region the intensity of the ESD H ÷ surface ion current increases up to a temperature of about 400 K. It is remarkable the coincidence of this increase

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3. Adsorption on the Nb(110) surface at 240 K. (a) Oxygen and carbon AES intensities. (b) Thermal desorption spectra of CH4, H2 and CO. (c) H + ESD surface ion current as a function of the temperature.

4. Discussion Preliminary results of the reactivity of methanol with the N b ( l l 0 ) surface at 240K by using the basic ESD and TDS techniques have been presented. The initial layer formed by methoxy species as revealed by the EELS 17 spectra, yields only ESD H ÷ surface ions. The decrease of this H + surface current after its maximum at 2 L of methanol exposure is a consequence of the adsorbate-adsorbate interaction tg. This decrease of the ion yield is supported by both models of the ejection mechanism, the MenzeI-Gomer-Redhead z°'2~ and the Feibelman-Knotek2z. In the first one the lifetime of the de-excitation event depends on the position of the adsorbate relative to the surface, that increases with coverage. In the second model, the localization of the two holes (2h) final state will be also affected by the adsorbate-adsorbate interaction. The lED is also affected by the adsorbateadsorbate interaction. In the MGR theory the lED curve is a reflection of the wave-function of the fundamental or ground state in the antibonding part of the M - H + ionic potential curve. If the slope of the antibonding part curve decreases as a consequence of the lateral interactions or due to an increase of the internuclear distance, the most probable kinetic energy will decrease at increasing coverages. In those systems where the MGR mechanism applies the ions are created with a relatively low energy, as for example in the case of H 2 adsorption where ions are created with 3 eV most probably of kinetic energy z°'zl. However, in the case of the 2h final state the high kinetic energy as well as the width in the lED is justified as consequence of the strong Coulomb repulsion of the two hole. As far as we know, this is the first time that this high kinetic energy of the H ÷ surface ions ranging from 8 to 5.5 eV has been reported for a complex molecule, and it should be explained by a 2h final state located in the molecular orbital of the C - H bond in a similar way as in the O - H radical 1s'23. The thermal desorption experiments jointly with the variation of the ESD H ÷ surface ion current as a function of the temperature show very interesting properties. The increase of this surface current at T < 400 K has a good correlation with the shoulder in the desorption peak of methane, hydrogen and carbon monoxide, and with the slight decrease of the O and C AES signals in this temperature range. An Arrhenius plot of the ESD H ÷ ion current difference lma~ (H +) - I ( H +) in this region yields an energy of 15 kJ mol-1 24 corresponding to the dissociation as methoxy and hydrogen of some molecularly adsorbed methanol. This molecular methanol would shield the methoxy species yielding H + surface ions with a decrease in the ion yield. The most important desorption structure at T > 400 K is characterized by a decrease of the H + surface ion current that is quite well 55

S Rey et al: Adsorption of methanol on Nb(11 O)

correlated with the desorption to the gas phase of CH,,,H 2 and CO. O n the basis that the H + surface ions have their origin in the methoxy species, there is a useful way to determine the energy of the formation of the intermediate species responsible for the formation of products to the gas phase. An Arrhenius representation of this surface ion current yields an energy coverage dependent with a value of 71 kJ mol -~ at zero coverage. The temperature of the maximum of the dissociation rate of methoxy species at 600 K (determined from d l ( H ÷ ) / d T ) agrees quite well with the maximum of the desorption rate of the species to the gas phase. This means that the intermediate is not 'stable'. If one assumes a pre-exponential factor of 10 -13 s -1 and first-order reaction, the desorption energy to the gas phase is 146 kJ m o l - ~. The C and O AES intensities agree with the previous discussion. In the range 400 > T > 700 K, the O intensity decreases and then remains constant. The C intensity also decreases but some remaining C disappears between 700 and 1400 K. The remaining O is desorbed at 2000 K. After this temperature the oxygen a m o u n t corresponds to the initial 0.05 M L contamination of the surface. As was mentioned, the hydrogen presented a desorption peak at 1350 K, due to the H diffusion-desorption process from the bulk. This was proved by interrupting the desorption process at T = 1000 K. When the surface cools down to a temperature of 400 K the ESD H ÷ surface ion current begins to increase very fast. But when the temperature is higher than 1400 K, in order to have surface ion current it is necessary to expose the surface to methanol.

5. Conclusions The E S D of methanol adsorbed on Nb(110) at 240 K yields only H ÷ surface ions. The efficiency of the ion production and the most probable kinetic energy at 8 eV decrease due to the adsorbate-adsorbate interaction. The methoxy species are thermally transformed into an 'intermediate' with 71 kJ mo1-1 energy at zero coverage. This intermediate yields methane, hydrogen and carbon monoxide to the gas phase. Hydrogen and

56

oxygen are diffused to the bulk and they finally desorb at 1400 and 2000 K respectively.

Acknowledgements This work has been supported under grant N o CCA8510/051 of the U S - S p a i n Joint Committee for Scientific and Technological Cooperation and Comisi6n Asesora de Investigaci6n Cientifica y Trcnica (PB85-0144).

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