The decomposition of methanol on Ni(100)

Surface Science 0 North-Holland

100 (1980) Publishing

210-224 Company

THE DECOMPOSITION OF METHANOL ON Ni( 100)

F.L. BAUDAIS, A.J. BORSCHKE, J.D. FEDYK and M.J. DIGNAM Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S IA1 Received

16 November

1979; accepted

for publication

14 February

1980

The decomposition of methanol and deuterated methanol on Ni(lOO) has been investigated using mainly IR ellipsometric spectroscopy but also thermal desorption and LEED measurements. The IR data support the existence of two intermediate states, here assigned to methoxy and formal moieties respectively. Starting from saturation conditions at -170 K and warming in 10 K steps, the initial chemisorbed state, believed to be a methoxy species, begins to decompose to a carbonylcontaining species, believed to be a formal species, at -230 K. At -260 K, the second intermediate begins to transform to adsorbed carbon monoxide with concurrent desorption of hydrogen. Totally deuterated methanol behaves similarly but shows substantial isotope effects.

1. Introduction

The mechanism of decomposition of alcohols on metals is of interest in its own right but also in relation to the Fischer-Tropsch synthesis [l]. In particular, a study of alcohol decomposition could shed some light on whether or not surface species such as CH,O play a role in the Fischer-Tropsch synthesis [ 1,2]. Investigations of the decomposition of methanol on Ni filaments and SiOz supported Ni have established CO and H, as the only detectible products [3-S]. Though the mechanism of the reaction was not established, the reaction was postulated to occur through an aldehyde-like adsorbed intermediate. In a study of the decomposition of methanol and deuterated methanol on platinum black at pressures between 95-100 Torr, McKee [6] concluded from the isotope effects that the rate-determining step was probably the decomposition of a surface methoxy species, rather than the loss of the hydroxyl hydrogen. Egelhoff et al. [7], using ultraviolet photoelectron spectroscopy (UPS) to study the properties of CHaOH and HCHO on W(100) at room temperature, found that exposures up to 1 L methanol lead to CO and H on the surface. Larger exposures, however, introduced new features in the UPS spectra, suggesting the formation of a molecular complex. Thermal desorption experiments revealed that the CO coverage was just below 0.5 monolayer when the molecular complex first appeared. Although the UPS spectrum for this complex was similar to that generated by adsorbed HCHO, the thermal desorption spectra were not the same. Nevertheless, 210

the similarity in the two UPS spectra prompted the authors to suggest that the intermediate consisted of two hydrogen atoms bonded to a CO core. Rubloff and Demuth [8], investigating the chemisorption and decomposition of CHaOH on Ni( 111) using UPS and thermal desorption measurements, showed that methanol weakly chemisorbs non-dissociatively, at temperatures near 80 K. Perturbation of the UPS band assigned to the oxygen lone pair orbital indicated that the methanol was most likely adsorbed with its oxygen atom next to the surface. Saturation of the surface was observed at approximately 6 L exposure. Upon heating the crystal to -160 K, a different UPS spectrum developed, whose appearance coincided with desorption of some CHaOH, Exposure of the surface to methanol at 300 K also led to the appearance of this same intermediate species. Further heating of the sample to 400-475 K yielded CO as the only surface species. Demuth and Ibach 193 studied the adsorption and decomposition of CH@H and CD@D on Ni(l11) using ~brat~on~ electron energy-loss spectroscopy (EELS). Their spectra confirm that a 3 L exposure of methanol near 150 K results in nondissociative adsorption, with the oxygen end of the molecule lying closest to the surface. Upon heating the sample to -180 K, the bands for the O-H (and O-D) stretching and bending modes disappeared, suggesting the formation of a methoxy intermediate. Further heating to room temperature produced CO and H as the main surface species, although some atomic oxygen was also observed. In this study, methanol was adsorbed onto a clean Ni(lO0) surface at low temperatures (-170 K) to saturation coverages, following which the crystal was heated in IO K steps. The progress of the reaction was followed by IR ellipsometric spectroscopy (IRES) in the carbonyl stretching spectral region 1750-2300 cm-‘. IRES is particularly sensitive to carbonyl~ont~ing species [lo] and the IR radiation of such a low energy and intensity that it should not bring about decomposition of intermediates. The investigation included isotope, LEED, and thermal desorption studies.

2. f. Apparatus A schematic diagram of the UHV system, showing afso the optical layout for the IRES measurements, is presented in tlg. 1. (Details of the IRES spectrometer can be found elsewhere [ 1 l-131). Not shown in this figure is a pair of effusive molecular beam dosers, positioned for line of flight dosing of the front surface of the crystal specimen, about 9 cm from the crystal. The dosers are connected through Varian model 951-5006 leak valves to two separate gas handling systems, and are terminated by collimated hole structures (overall dimensions 6 mm diameter X3 mm thickness, pore dimensions 50 lun diameter, open area 41% of the surface) provided

212

F.L. Bauduis et al. /Decomposition

of methanol on NiflOO)

E =f

.*2 P 4 I __---

_----

CL

F.L. Baudais et al. /Decomposition

of methanol on Ni(100)

213

by Brunswick Corp. Since dosing levels were determined using an ion gauge placed in an off-specular direction, the true dosing levels are anywhere from 10 to 100 times higher than those reported, the reported values acting, therefore, simply as relative measurements of dosing. The 45 cm diameter UHV chamber, LEED optics, LEED and Auger electronics, ion gauge (model VIGlO), argon ion gun (Type AC-l) and quadrupole mass spectrometer (model Q’7B) were obtained from Vacuum Generators. A pair of diametrically opposed 20 cm flanges are fitted with 3.75 cm aperture CaF, windows obtained from Harshaw Chemical. These are mounted to subtend an angle of incidence of 80” at the crystal surface. The chamber is pumped by 2 Varian MSS-100 LNa sorption pumps, a Varian 110 h’s noble gas Vat Ion pump, and a Vacuum Generator titanium subl~ation pump operating in the base of the chamber with the titanium being deposited onto a LN, cooled baffle. Pressures below 10 -11 Torr are achieved after baking at 470 K, usually overnight, but 24-36 h are required following exposure of the interior of the chamber to the atmosphere. 2.2, Materials After drying over Linde 5 a molecular sieves, ACS reagent grade methanol was vacuum distilled into a cold finger attached to a Pyrex gas-handling manifold. Successive distillations between cold fingers were employed to remove dissolved gases prior to storage in a cold finger, isolated from the manifold by a teflon valve. Before dosing the Ni crystal, the vacuum manifold, up to the leak valve, was flushed two or three times with the methanol vapour. Fully deuterated methanol, obtained from Baker Chemical in sealed glass vials (Baker Grade >99 atom% D) was introduced into the vacuum system via a magnetic break-seal, then degassed as above. High purity Hz (99.9999%), Dz (99.6%), CO (99.99%), and Ar (99.995%) were obtained in 1 1 breakneck seal flasks from Scientific Gas Products Ltd. and used without further purification. CO and H, were introduced using a second glass and stainless steel UHV gas manifold connected to the gas dosers described above. Argon for ion bombardment was introduced into the chamber via a separate leak valve. The nickel single crystal (Marz grade 99.995%, Materials Research Corp.) was cut by spark erosion, then mechanica~y polished with successively finer grades of SiC wet grinding paper, followed by metallographic diamond paste down to I pm. Finally, the specimen was chemically polished in a solution of SO ml glacial acetic acid, 30 ml cont. HNOa, 10 ml cont. H,S04, and 10 ml H3P04 [14]. Transmission X-ray diffraction measurements on the crystal indicated that the surface was oriented in the (100) direction with an accuracy of about 0.5’. The specimen, which had an ,area of nearly 2 cm2, was spot-welded to a nickel rod fastened to the specimen manipulator unit. The crystal could be heated in vacua either by electron bombardment or radiatively, and cooled to 120 K by thermal

214

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of methanol on hJi(100)

contact with a LNa reservoir via a copper braid and the nickel rod. The crystal temperature was measured and controlled using one of two chromel-alumel thermocouples spot-welded to the back of the crystal. A temperature controlling unit maintained the crystal temperature constant to within 1 K. This controller was also used to provide a linear temperature ramp accurate to 1% over a range of 500 K. Final cleaning of the crystal involved continuous argon ion bombardment in 5 X 10e6 Torr argon providing an ion flux of 0.3 PA at 450 eV, with the crystal held at 725-775 K. Only after heating the crystal and thoroughly outgassing all filaments in use was bombardment attempted. During bombardment, a fresh film of titanium was maintained on the cryo-baffle in the base of the chamber; however, the ion pump was shut off. Following bombardment, the crystal was annealed at 1100 K for lo-20 min, then cooled rapidly to room temperature or below. This procedure was repeated until sharp LEED patterns indicative of the clean (100) surface were obtained, and thermal desorption results for low coverages of CO agreed with those reported by Benziger and Madix [ 151 within experimental accuracy (i.e. within 3 K), as this has been found to be a sensitive indicator of carbon and oxygen surface contamination [ 151. 2.3. Results 2.3.1. IRES data The IRES data were obtained at a scan rate of about 1 cm-’ s-l and a resolution of about 6 cm-r, giving an inst~mental rms sensitivity -0.003% absorption. The effective or useable sensitivity is up to 10 times worse than this, however, due to irreproducibility in the optical properties of the surface, particularly when measurements are made over a range of temperatures using a single background or reference scan. The usual experimental procedure is to record two scans on the “clean” surface, then proceed with dosing and subsequent scans. The first scan is then subtracted from the subsequent scans to yield difference spectra, which are subjected to a sliding polynomial smoothing procedure designed so as not to degrade the spectral resolution. The IRES absorption data are presented in figs. 2 to 5. Fig. 2 shows spectra for a number of dosing conditions for CO on Ni(lOO), and is included to illustrate the range of spectra which can arise for various CO exposures. At low coverage, there is generally only a single C-O stretching band at about 2070 cm-’ which shifts slowly to higher frequency with increasing coverage up to about 0.5 (~(2 X 2)) coverage [16]. Above 0.5 coverage, a pair of bands grow in at about 1900 and 1960 cm-‘. At the highest coverages, only the latter of this pair remains, while the -2070 cm-’ band is replaced first by one at -2120 cm-‘, then by one at -2154 cm-‘. The three bands near 2100 cm-’ are believed to be due to CO linearly bonded to Ni atoms (i.e. to CO at “on top” sites), while those near 1950 cm-” to CO bonded to 2 Ni atoms (i.e. to CO at two-fold bridge sites) [ 161. The appearance of additional component bands with increasing CO exposure is believed to result from interac-

F.L. Baudais et al. /Decomposition of methanol on Ni(100)

2300

2200

2100

2000

WAVENUMBER

Fig. 2. IRES absorption 180 K.

1900

1800

215

1700

(a-r-‘)

spectra for CO adsorbed on Ni(lOO) for various dosing conditions at

tions between neighbouring CO molecules [ 161. Spectra arising from the successive dosing of Ni(lOO) with CHsOH at room temperature are shown in fig. 3. They show the development of a strong band at -2070 cm-‘, due presumably to linearly bonded CO, with perhaps a weak band at -1950 cm-’ for the higher two dosing conditions, due presumably to bridgebonded CO. Fig. 4 presents IRES data for CH30H adsorbed under high dose conditions on Ni(lOO) at low temperatures (-170 K). The difference spectra were obtained before and after dosing, and following warming the crystal in steps, usually of 10 K. With increasing temperature, they show the development of a strong, broad band in the range 1800-1880 cm-‘, which gives way at the higher temperatures to a strong, narrower band at -2070 cm-‘. Fig. 5 shows IRES data for’ fully deuterated methanol, CDaOD. In addition .to the bands at 1800-1880 cm-’ and -2070 cm- ‘, these spectra show a weak band at

216

F.L. Baudais et al. /Decomposition

of methanol on Ni(lO0)

4.0

L

1.0 L

1

2300

I 2200

I 2100

I 2000

0.5

L

0.1

L

I

I

I

1900

1800

1700

WAVENUMBER Fig. 3. IRES absorption spectra generated following Ni(lOO) at 293 K for the total doses indicated.

Cm-‘) successive adsorption

of CH30H

on

-2120

cm-’ which is fully developed immediately following dosing, but vanishes during the development of the above two bands. This band is more easily seen in fig. 5a.

2.3.2. Thermal desorption data Fig. 6 presents typical thermal desorption data for CHsOH and CDsOD. For both sets of data, the various mass peaks were followed simultaneously and hence apply to a single desorption run. 2.3.3. LEED data Typical LEED patterns are shown in fig. 7 for CHaOH dosed on Ni(lOO) at low temperature then warmed progressively. Upon dosing at 133 K the sharp p(1 X 1) pattern of the clean surface is transformed to one in which the (1, 1) spots remain sharp while the (1 , 0) spots become diffuse, a behaviour expected for a disordered

F.L. Baudais et al. /Decomposition

I

I

I

I

I

of methanol on Ni(lO0)

217

I

Ni(1001

333

K

313

K

303

K

333

K

313

K

303

K

293

K

283

K

273

K

263

K

253

K

243

K

293K

283

K

273

K

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253

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K L_/

233

K

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K

173

K

233K

BARE SURFACE at 173K 5,

I 22~0

I 2100

I 2000

I 1900

WAVENUMBER

Fig. 4. IRES absorption dose of 0.5 L, followed

I 1800

(cm-‘)

I 1700

223

K

173

K

BARE SURFACE Ot 173 K

_I -I

2s 30

2200

2100

2ooo

WAVENUMBER

spectra generated on adsorbing CH30H by warming in successive 10 K steps.

1900

I800

l700

(cd)

onto Ni(lOO) at 173 K, relative

overlayer consisting of species adsorbed at four-fold symmetric sites (“on top” sites or four-fold hollow sites) [17]. No change in pattern occurs until about 233 K when a faint c(2 X 2) pattern develops. No further change takes place up to 293 K.

FL.

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Baudais et al. /Decomposition

_

I

NL(100:

of methanol on Ni(lO0) -T--1-1-11 T

CD,OD

on Nt

I001

(b)

373

K

343

K

333

K

a00 2000 WAVENUMBER

1900

1800

(cm-‘)

K

333

K

323

K

K

313

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303

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313

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293

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303

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293

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253

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263

K

253

K

243

K

233

K

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K

243

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233

K

223

K K

BARE SURFACE

m

ii..

K

343

323

173

__I_.IL__ 30 2200

373

1700

1 v1-_L1

2x 10

2200

2100

x00

1900

WAVENUMBER

Fig. 5. IRES absorption spectra generated on adsorbing CDsOD dose, followed by warming in successive 10 K steps, as for fig. 4.

onto

Ni(lOO)

1800

1700

(cd)

at 173 K, 0.5 L

3. Discussion of results

3.1. Nature of intermediates The present data, along with those of Rubloff and Demuth [8] and Demuth and Ibach [9] for CHaOH on Ni(l1 l), lead to the following general outline of the processes taking place upon saturating the Ni(lOO) surface at 130 or 170 K then warming slowly or in stages to room temperature and above. At or above -160 K, CHaOH is expected to adsorb dissociatively [8,9] as CHaO-S and H-S, where S represents a surface site, and the methoxy moiety is bonded to the surface through

F.L. Baudais et al. /Decomposition of methanol on Ni(100)

1

I

273

I

I

1

373 TEMPERATURE

I 473 ( K)

I

219

I 573

Fig. 6. Mass spectrometric thermal desorption. The upper two curves are for CHsOH, thelower two curves for CDsOD for a relative dose of 0.5 L on Ni(100) at 173 K, with the measurements performed at 0.73 K s-l rate of temperature rise.

the oxygen. Ultimately the methoxy moiety must lose three hydrogen atoms to become chemisorbed CO bonded to the Ni through the carbon atom. Two possible adsorbed intermediate states in this process are HCHO-S and CHO-S. The former is either a stable molecular species (formaldehyde) or it does not contain a carbonyl group. Thus it will either give rise to no absorption band in the carbonyl stretching region of the spectrum, or it would be expected to desorb very readily at -273 K and above. We found no evidence for HCHO desorption. The latter species (CHO), on the other hand, would be expected to exist as a strongly bound, carbonyl contaming species, specifically as:

i.e., as a formal moiety. It appears to us that by far the most likely candidates for the species giving rise to the strong absorption band at 1800-1880 cm-’ are: (i) a formal moiety, as

220

F.L. Baudais et al. /Decomposition

of methanol on Ni(lO0)

Fig. 7. LEED patterns: (a) the clean Ni(lOO) surface, (b) the same surface following sure to CHsOH at 133 K, (c) the same exposed surface following heating to 233 K.

1 L expo-

above; (ii) CO bonded carbon-down in a four-fold bridging site; and (iii) CO bonded oxygen-down. The C-O stretching frequency for (iii) is completely unknown. The expected frequencies for both (i) and (ii), allowing for frequency shifts through lateral interactions, are in the range 1750-1850 cm-’ [ 18,191. The following observations, however, militate against (ii) and (iii) and in favour of (i). If the band observed at 1800-1880 cm-’ were due simply to CO bonded either carbon-down or oxygendown, its oscillator strength would be expected to be close to that for the linearly bonded CO whose band appears at -2070 cm-‘. On the contrary, however, the band which forms at 1800-1880 cm-‘, when developed to its maximum, has an area about double that for the band at 2070 cm-‘, indicating that the adsorbed intermediate has at least double the oscillator strength of the linearly bonded CO. The latter conclusion rests on the reasonable assumption that most of the intermediate is ultimately converted to CO.

F. L. Baudais et al. 1 Decomposition of methanol on Ni(lO0)

221

The second point supporting the formal moiety as the intermediate concerns the behaviour of the band at -2120 cm-’ observed for CDsOD, but not for CHsOH. We suggest this band is due to C-D stretching modes, first for the methoxy moiety, then for the formal moiety. If the methoxy group is bonded to the surface with the O-C axis normal to the surface, then only the symmetric C-D stretching mode will be infrared active, as the transition dipole moments for the degenerate asymmetric modes are oriented parallel to the surface, hence inactive [13]. By symmetry, the oscillator strength for the symmetric mode is equal to that for a single C-D bond oriented normal to the surface. If, on the other hand, the methoxy group is bonded to the surface as in fig. 8a, giving a NiOC angle -loo”, i.e. equal to the HOC angle in CHsOH, then steric considerations require one of the C-D axes to be nearly normal to the surface, the other two nearly parallel to it, again leading to an oscillator strength equal to that for a single C-D bond oriented normal to the surface. Based on the structure of molecules containing the formal moiety, we expect the formal group to bond with the surface in a trigonal planar configuration, with the C-O and C-D axes each making an angle of about 60” with the normal to the surface as in fig. 8b. This gives a normal component for the C-D oscillator strength one-half that for a single C-D bond oriented normal to the surface. Thus, in the absence of methanol desorption, transformation of the methoxy moiety to the formal moiety should be accompanied by a reduction of the C-D stretching band area by about one-half. The band should ultimately disappear, of course, with the formation of CO. The data of fig. 5 (Sa particularly) seem consistent with this picture. Thus between 173 and 253 K, the C-D stretching band (2120 cm-‘) remains, within experimental uncertainty, constant in area. Around 273 to 283 K, when the band at 1800-1880 cm-’ (tentatively assigned to formal carbonyl stretch) is near its maximum a
(a)

(bl

I

Fig. 8. Scale drawing of possible reaction model by which a methoxy moiety on Ni(100) decomposes to a formal moiety via a concerted process; Ni, open circles; H, single-hatched circles; 0, cross-hatched circles; C, solid circle).

the C-D stretching band is reduced to about half height, disappearing in the range 283 to 3 13 K as the CO band grows at the expense of the 1800-1880 cm-” band. The picture we propose is one in which the methoxy moiety decomposes in a concerted process to form a formal moiety and chemisorbed hydrogen. Under high coverage conditions, this intermediate is stable over a substantial temperature range (see fig. 4). Fig. 8a shows a drawing, approximately to scale, of a methoxy group bonded to the surface as already described. From this geometry, which simply maintains the bond geometry of the CHJOH molecule, the transition to a formal moiety and adsorbed hydrogen (fig. 8b) can be effected with minimum displacement of the atoms involved and in such a way that the metal-carbon bond is forming and the carbon-oxygen bond strengthen~g as the metal-oxygen and c~bon-hydrogen bonds are breaking. In fig. 8b, the formal moiety is shown bonded to a single metal atom (one adjacent to that to which the methoxy group was bonded) with the same bond geometry as formaldehyde. We do not wish to suggest that the methoxy moieties are necessarily all oriented as in fig. 8a, but simply that they may well react through such a conjuration. 3.2. Other fe&tdres of the decomposition process

Calibration of CO IR band areas using LEED and IRES data for CO on Ni(lOO) 1161 leads to final CO coverages -0.25 from the data of iig. 5, but to more than double that from the data of fig. 4. Coupled with the present LEED data, suggesting an initial CHaOH coverage -0.5, these results indicate that most of the methoxy moiety is ultimately converted to adsorbed CO; that is to say, there is no evidence for the decomposition process being accompanied by methanol desorption. The constancy of the C-D stretching band area (-2120 cm-’ in fig. 5) between 173 and 253 R, and its reduction by about one-half from 253 to -280 K is also consistent with this view, as are the desorption data (fig. 6). On the other hand, comparing the desorption data for CHJOW (fig. 4) with the IRES data (fig. 4) it can be seen that the onset of H, desorption occurs near the onset of formation of CO and near the development of maximum surface concentration of the intermediate. It would appear, therefore, that for the dosing conductions of fig. 4, initial coverages -0.5 in methanoi lead to intermediate conditions corresponding to coverages -0.5 in CHO-S and -1 S in H-S, and finally to coverages -0.5 in CO-S, assuming of course that the intermediate has been correctly identified. The data for CDaOD decomposition are surprising in that they differ substantially from those for CHaOH decomposition. Thus for CDsOD the various onset temperatures are displaced to higher temperature by -20 K, and both the intermediate band area and final CO band area reduced to about half of those observed for CHaOEI decomposition. While the temperature shift is presumably the result of

F.L. Baudais et al. /Decomposition of methanol on Ni(lO0)

223

a kinetic isotope effect, it is more difficult to see how the coverage change could also be an isotope effect. Note that H, desorption also begins -20 K lower for CHaOH decomposition than does Dz desorption for CDaOD decomposition (fig. 6). Desorption spectra for a clean surface dosed at 173 K with H, or Dz show no isotope effect within experimental scatter. For 0.1 to 0.5 L relative dose, both give desorption spectra close to that in fig. 6 for Dz desorption. Although higher dosing conditions led to desorption spectra showing a broad desorption feature on the low temperature side of the main peak, no dosing conditions reproduced the main features of the H, desorption peak in fig. 6. This suggests that for the CHaOH decomposition runs, H, is displaced from the surface by the reaction products at a temperature below its normal desorption temperature. This could explain the steep rise in the H, desorption spectrum in fig. 6 relative to that for H2 adsorbed on a clean surface. Although CDaOD decomposition does not give rise to Dz desorption at a significantly reduced temperature, due no doubt to the higher temperature for decomposition of its intermediate, it does give rise to a somewhat steeper rise in the D, desorption spectrum, presumably for the same reason.

4. Summary In summary, the present data, in conjunction with reported UPS and EELS data [8,9j are generally consistent with a mechanism for the decomposition of methanol on Ni(lOO) at low temperatures in which methanol is first chemisorbed as a methoxy moiety then decomposes by a concerted mechanism to a formal moiety which in turn decomposes to CO. If the surface is saturated with methanol, the second intermediate (presumed to be a formal moiety) is stable. For unsaturated conditions, upon generation, the second intermediate decomposes directly to CO and H until sufficiently high coverage conditions are achieved. At higher temperatures and for saturation conditions, the intermediate decomposes to CO concurrent with the desorption of hydrogen. A large isotope effect is observed on substituting CDaOD for CH30H in that the appearance temperature for the second intermediate and for CO are shifted to higher temperatures by -20 K. Deuterium substitution also appears to effect coverages in a manner that is difficult to explain. Further studies are required to clarify both the effects of isotope substitution and of dosing level. Of particular value for such studies would be spectral information covering the Ni-C and Ni-0 stretching frequency regions, regions inaccessible to our present spectrometer.

Acknowledgements The authors

are pleased

to acknowledge

the support

of this research by the

F.L. Baudais et al. /Decomposition

224

Natural Science and Engineering research grant from the University

of methanol on Ni(lO0)

Research Council of Toronto.

of Canada

and by a special

References [I] [2] [3] [4] [5] [6] [7] [8] [9]

[lo] [ 1 l] [ 121 [ 131 [14] [15] [16]

[ 171 [ 181 [19]

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