The decomposition of formaldehyde on Pt(111): A TPD and hreels study

206

Surface Science 188 (1987) 206-218 North-Holland, Amsterdam

THE DECOMPOSITION OF FORMALDEHYDE ON Pt(lll): A TPD AND HREELS STUDY * M.A. HENDERSON, G.E. M I T C H E L L and J.M. W H I T E Department of Chemistry, University of Texas at Austin, Austin, TX 78712, USA

Received 30 December 1986; accepted for publication 30 April 1987

The decomposition of formaldehyde on Pt(lll) was studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). Upon adsorption at 105 K, doses of formaldehyde below 0.2 ML decompose to CO(a) and H(a). Polymerization occurs at higher coverages. HREELS results indicate that the surface polymer decomposes at 235 K and TPD shows the concomitant desorption of small amounts of CH 4 and H20. The major TPD products, irrespective of H2CO coverage, are CO and H 2.

1. Introduction This study of H2CO adsorption and decomposition on Pt(111) was motivated by our interest in the surface chemistry of molecules containing C - O bonds. The recent thrust of these efforts has been the chemistry of ketene (CH2CO) on P t ( l l l ) [1,2]. Under certain conditions, ketene decomposition leads to methane and ethylene, but there is no evidence for C - O bond cleavage. Since products requiring cleavage of C - O bonds have been observed in the decomposition of formaldehyde [3], it is of interest to establish how ketene and formaldehyde chemistry differ. We can use what is known about H2CO surface chemistry on a variety of substrates [4-14]. This includes HREELS studies on Ru(001) [4,12], Ni(100) [5], Ag(ll0) [13] and Zn(0001) [14] but not Pt(lll). The HREELS and TPD results we report here indicate that polymeric forms of adsorbed formaldehyde are responsible for products other than CO and H 2. These polymeric species readily account for the small amounts of C H 4 , H 2 0 and residual surface carbon which we observe at high formaldehyde coverages. This work extends, through the use of HREELS, the TPD and * Supported in part by the Robert A. Welch Foundation.

0039-6028/87/$03.50 9 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

M.A. Henderson et al. / TPD and H R E E L S study of H2CO on Pt(111)

207

AES work of Abbas and Madix [3]. Except for the kind and the distribution of minor products, our results agree; we find no CO2, CH3OH or CH3OCHO. The differences may be due to the purity of the formaldehyde source, particularly its water content.

2. Experimental The ultrahigh vacuum chamber used in this study and the methods of data collection have been detailed elsewhere [1]. The system was ion-pumped with a working base pressure of 2 x 10-10 Torr. The system's gas handling line was evacuated by a 170 d/s turbomolecular pump. Cutting, mounting, and cleaning of the P t ( l l l ) crystal have also been described previously [1]. The crystal was cooled to 105 K by conduction to a liquid nitrogen reservoir. Temperature was monitored with a chromel-alumel thermocouple spotwelded to the back of the sample. Sample cleaning was performed by Ar § bombardment and by oxidation at 800 K to remove carbon. The former was occasionally necessary to remove impurities such as K, Si, and A1. The latter treatment was generally sufficient to clean the surface between experiments. Cleanliness was confirmed by Auger electron spectroscopy (AES). HzCO was dosed onto the P t ( l l l ) crystal through a 1 / 5 " ID stainless steel tube terminating 3 / 8 " from the crystal surface. Controlled exposures were obtained by means of a leak valve and were based on dosing time at a reproducible H2CO flux which resulted in a 1 • 10 -l~ Torr pressure rise at the chamber's ion gauge [1]. Backfilling the chamber with H2CO was abandoned because HREELS data showed that substantial amounts of CO were " b u r p e d " from the ion pump during exposure. Using the doser avoided this problem. Gaseous H2CO was generated by heating solid paraformaldehyde (Aldrich) to 375 K. As received, the paraformaldehyde contained a significant amount of water which was removed by continual evacuation with periodic heating for several days on the turbomolecular-pumped gas handling system. The presence/absence of a 175 K H : O TPD peak was used as a test of the presence/absence of water in the formaldehyde source. Residual gas analysis (RGA) was also used to verify the purity of the gaseous H2CO. In agreement with previous work [15], pure H2CO gave large positive ion signals at m/e = 30, 29 and 28 in the ratio of 85 : 100 : 52. No role values corresponding to H20, CH3OH, polymeric forms of H2CO or any other common gaseous impurity were detected. Additional care was taken to avoid contamination in the gas handling system by preparing a fresh sample of gaseous formaldehyde from paraformaldehyde for each dose. The HREELS spectrometer was a: 127 ~ cylindrical sector type with a stationary monochromator and analyzer defining a total scattering angle of 120 ~ All spectra were taken in the specular direction with a primary beam

208

M.A. Henderson et al. / TPD and HREELS study of H2CO on Pt(l l l )

energy of 6.9 + 0.3 eV and resolution of 10 mV full-width-at-half-maximum. The temperature ramp rate for TPD experiments was 12 K/s.

3. Results 3.1. TPD

Fig. 1 shows the TPD spectra for a multilayer exposure (300 s) of H2CO on Pt(111). The major TPD products are desorption limited H 2 (fl], 266 K) and CO (400 K). There is an additional broad, high temperature reaction-limited H 2 peak (f12, 520 K) which comprises 25% of the desorbed molecular hydrogen. Minor decomposition products, CH 4 and H20 , desorbed with peaks at 250 + 10 K. H2CO desorbed in two peaks, the second coincident in tempera-

400

.,co)I

H20

~

^ 248

~

=

~

200

400

600

x

10 800

T E M P E R A T U R E (K)

Fig. l. TPD of saturation H2CO on P t ( l l l ) at 105 K.

M.A. Henderson et al. / TPD and H R E E L S study of H2CO on Pt(l l l)

209

0.5

~ , ~ 0.4

_

So D a~D~

0

~

0

~

CO H2 TOTAL

/O t--~

~ 0.3 w >-

[]

r'l-- ~t H2

.J

0 Z 0

I

0.2

0.1 /

(~O . , ~ 9 1 4 9 - ~

0

A

9 ,

I

100

200

i

i

CH 4 i

300

H2CO EXPOSURE (SEC)

Fig. 2. T P D product yields as a function of exposure for H2CO on Pt(111).

ture with the desorption of C H 4 and H 2 0 . At H2CO doses of more than 90 s another H2CO TPD peak was observed at 110 to 125 K. This peak did not saturate with increasing exposure, indicating multilayers [4,5,13]. Other desorption products, particularly CH3OH, CO2, CHaOCHO and C2 hydrocarbons, were not observed. A small amount of carbon, but no oxygen, was observed by AES after the TPD temperature ramp to 1000 K. Using TPD peak area calibration factors obtained for saturation coverages of CO (0.68 ML [16]) and H 2 (0.5 ML of molecular H 2 [17]) on the clean surface, absolute coverages for CO and H 2 (ill, f12 and total) were calculated as a function of H2CO exposure (fig. 2). (We define 1 ML as one adsorbed species per surface Pt atom.) Absolute coverages of C H 4 (fig. 2) were calibrated using known CH 4 yields from P(CH3) 3 decomposition on P t ( l l l ) [17]. At H2CO exposures below 10 s only fl] H2 and CO were observed in the TPD in agreement,with previous studies [3-11]. For a 10 s exposure, 0.2 ML of both are formed. As the H2CO exposure increases, the coverages of CO and /31 H 2 level off, and the miaor decomposition products appear in the TPD. At high exposure, /31 t-I2 actually drops slightly while CO rises slightly. Through the first layer, the TPD areas for HzO and molecular H2CO , although not calibrated, follow th~ trends of C H 4 and /32 H2. It is important to note that these four species all begin to appear near the same exposure, but /32 H2

210

M.A. Henderson et al. / TPD and HREELS study of H2CO on Pt(l l l) 0.25

0.2 m=CH/CH

4

---5.2

0.15 o x

"1-~r O tO nI.LI >..
/

0.1

O Z

0.05

A

v

i

0.05

0.1

MONOLAYERS

i

0.15

i

0.2

0.25

~2 H ATOMS

Fig. 3. Methane TPD yield versus H atom yield from CH decomposition (f12,520 K) for various exposures of H2CO on Pt(111).

desorption grows more rapidly than C H 4. When the first layer saturates, the absolute CO, H 2 and CH 4 yields are 0.42 +_0.02, 0.40 _ 0.02 and 0.023 _ 0.002 ML, respectively. Fig. 3 establishes the correlation between the yields of CH 4 and f12 H2. Except at low yields, these two are linearly related. 3.2. HREELS

The HREELS results for three H2CO exposures at 105 K are shown in fig. 4. At a 10 s H2CO dose (which yields only desorption limited H E and CO in the TPD) only losses due to linear CO at 2080 and 475 cm-1 were observed, verifying that HECO decomposes upon adsorption at 105 K. Several new losses appear after a 30 s HECO exposure (fig. 4b), and these can be correlated with the appearance of CH 4, HECO, and H20 in TPD.

M.A. Henderson et al. / TPD and HREELS study of H2CO on Pt(111)

211

H2CO/Pt(111) 965

.

1120

\ \ \

2960 I 585

~1450 ~ 1745

I

/~ /\

C

x 2O 9851085

~ 1230 585

A

1>. I--

2925

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x100

z

2080 475

x500 i

0

I

1000 ELECTRON

I

I

2000

I

I

I

3000

E N E R G Y L O S S (cm-1)

Fig. 4. HREELS coverage set of HzCO on P t ( l l l ) : (a) 10 s dose, (b) 30 s dose and (c) 300 s dose.

Besides those of atop CO, new peaks are present at 2925, 1770, 1380, 1230, 1085, 985, and 585 cm -1. Any exposure greater than 90 s gives the HREEL spectrum of fig. 4c. The losses at 2960, 1450, 1120, 965 and 565 cm -1 dominate. The growth of these modes is accompanied by an attenuation of the elastic peak by over an order of magnitude (compared to fig. 4a), indicating increased disorder in the adsorbed layer. Attempts to study multilayers of H2CO were also made. However, in the course of tuning the HREELS optics (requiring about 2 min), peaks due to the multilayer attenuate and the resulting spectra are identical to fig. 4c. TPD following multilayer exposures of H2CO after an HREELS measurement showed no multilayer HzCO peak at 122 K, but were otherwise identical to fig. 1. While this prevented accumulation of detailed spectra, the first few HREELS scans of a multilayer showed intense modes at 200 and 410 cm -1. To correlate TPD and HREELS, the first layer was saturated, was warmed to a preselected temperature corresponding to a TPD peak, and then was

212

M.A. Henderson et al. / TPD and HREELS study of H2CO on Pt(111)

H2CO/Pt(111) 620

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_A 2930

v >Z gJ I-

,

/',

~

z

920 1110

AA

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2055

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~

235,.

bI

X50

,

0

~

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~

20100

~x'50 I

3000

ELECTRON ENERGY LOSS (cm-1)

Fig. 5. HREELSannealing set of saturation H2CO on Pt(111).The sample was heated from 105 K to the indicated temperatureand then was immediatelycooledto 105 K for HREELS. recooled at 105 K for measurement of the vibrational spectrum. The HREELS of the saturated first layer at 105 K is repeated in fig. 5a. Upon annealing to 200 K (fig. 5b) the 2760 cm - t band disappeared and the 1450 cm -1 band shifted to 1395 cm -1. Two additional bands appeared at 615 and 300 cm -1. The count rate of the elastic peak increased by a factor of 3, indicating ordering of the adsorbed layer. Heating to 235 K (the onset temperature of CH4, H 2 0 and H2CO desorption) gives intense CO bands at 2055 and 470 cm -1, and a weaker band (bridge bonded) at 1820 cm -1 (fig. 5c). The 1770 cm -1 loss disappears. These changes are accompanied by increased intensity and sharpening of the elastic peak. Heating to 260 K (beyond the desorption temperature for the minor TPD products) redistributes the vibrational structure (fig. 5d). Intensity is lost in the

M.A. Henderson et al. / TPD and HREELS study of H2CO on Pt(lll)

213

900-1200 cm -1 regime. CO losses remain at 2055, 1820, and 465 cm -1 and there are losses at 2930, 1080 and 680 cm -1. Heating to 420 K (fig. 5e) removes adsorbed CO (the losses at 2070 and 490 cm-1 are due to readsorption), and leaves bands at 2955 and 740 cm -1. The loss at 1110 cm -1 is similar to that observed previously for ketene/Pt(lll) [1] and results from either a small amount of oxidizable impurity or an unidentified CxHyspecies. 4. Discussion

To begin our discussion, we assign the vibrational spectra and correlate them with the TPD observations. A 10 s dose of H2CO at 105 K is dissociative. The HREELS spectrum (fig. 4a) shows only bands due to linear CO (2080 and 475 cm -1) and only H 2 and CO appear in TPD (fig. 2). Larger doses at 105 K (fig. 4 and fig. 2) furnish evidence for non-dissociative adsorption, polymer formation and the opening of new product desorption channels. Table 1 summarizes the vibrational spectra observed here and in a number of other relevant studies including polymerized formaldehyde on O/Ag(ll0) [13], Ni(ll0) [5] and Zn(0001) [14], paraformaldehyde [18] and trioxane [19]. On Pt(lll), the frequencies assigned to the CH 2 stretch and scissors modes and the OCO symmetric and asymmetric stretches agree well with those of other systems, and are good diagnostics for polymerization. The two losses at 2760 and 1745 cm -1 in fig. 4c (not shown in table 1) are assigned to aldehydic p(CH) and p(CO) bands even though the latter is about 30 cm -1 higher than for solid H2CO [4,5,13]. Further evidence for solid formaldehyde is provided by the intense, briefly present modes at 200 and 410 cm-1. These are characteristic of solid H2CO lattice vibrations [5]. Table 1 Vibrational spectra (cm- 1) H2CO// Pt(lll) a) 105 K

H2CO/O / Ag(110) b) 200 K

H2CO / Ni(111) e) 95 K

H2CO / Zr(0001) a) 220 K

(H2CO) n e) solid

Trioxane r) solid

v(CH2)

2960

2960

2967

2870

B(CH2)

1450

1430

1489

1467

2978 2919 1471

~(OCO)

1120

1100

1077

1096

~(OCO)

965

960

948

950

~(OCO)

NR g)

620

613

610

3031 2887 2883 2807 1494 1483 1419 1383 1242 1222 1152 1122 744 521

~(PtO)

565

1097 1091 932 903 630

a) This work. b) Ref. 13. c) Ref. 5. d) Rr.f. 14. e) Ref. 18. r) Ref. 19. g) Not resolved.

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M.A. Henderson et al. / TPD and H R E E L S study of H2CO on Pt(111)

Annealing a layer to either 200 or 235 K desorbs considerable monomer (fig. 1) and leaves a vibrational spectrum (figs. 5b and 5c) very typical of (HECO)n. In particular, the two bands at 615 and 300 cm -1 are assigned to the 3(OCO) and "ra modes of polymerized formaldehyde [5]. After the 235 K anneal, no new species obviously leading to CH4 and H20 were identifiable. There is a huge increase in the relative intensity of the C - O stretch. Part of this is due to ordering of the adlayer, so the extent of (H2CO)n decomposition to CO(a) and H(a) cannot be determined. Between 235 and 260 K, there is desorption of fll H2, CH4, H20 and H2CO (fig. 1). The HREELS spectrum (fig. 5d) reflects this. The losses due to (HECO)n are eliminated. Those due to linear and bridged CO (2055, 1820 and 465 cm-1), and CxHy(2930, 1080 and 680 cm -1) remain [1,3]. Further heating to 420 K removes most of the CO (fig. 1 and fig. 5e) leaving only f12 HE and carbon. In fig. 5e, there is clear evidence for CH (2955 and 740 cm -1) [1,20,21]. Since heating through the/32 H2 peak leaves no detectable oxygen (AES) we conclude that no C - O bonds remain after the desorption of CO (fig. 1). With these assignments and correlations in mind, we turn to a discussion of reaction pathways. At 105 K and low coverages, H2CO dissociates upon adsorption, as commonly observed in transition metals [3-11]. In the simplest mechanism, HECO lies flat (i.e. 0r-bonds) on the surface for the C - H bonds to break. As the coverages of CO and H increase, the availability of such decomposition sites decreases and, as observed, molecular adsorption occurs, leading to a mixed overlayer containing CO(a), H(a) and H/CO(a). Polymerization requires the coupling of neighboring molecules. We propose the structure shown in scheme 1 for (HECO)n in the first layer. This structure is consistent with the strong dipole-allowed va (OCO) mode (1120 cm -1) in fig. 5b as well as the other observed modes. It is also consistent with bonding of H2CO through the oxygen lone pair as commonly found for aldehyde and ketones [22-24]. We now turn to the decomposition of the polymer. Since the losses due to (H2CO), appear at the same HECO exposure that first gives CH 4 and H20 in the TPD, and since the disappearance of these modes occurs at the same temperature that CH 4 and H20 desorb, we propose that the polymerized HECO is the source for CH 4 and H20 formation. While much of the following discussion focuses on CH 4 and H20 formation, it is important to keep in mind that H 2 and CO dominate here just as on other metals [3-11].

H ...HH ...H I

I

///////2 Scheme 1.

M.A. Henderson et al. / TPD and H R E E L S study of H2CO on Pt(l l l) ,2H

o

O +

/q

235 K

.H / .

9. HzO . . . . yr /

~, HzO

/

j': .n/.a, OH /

§

CH 3, / 1

9 H //:/1 /

(H2C 0)~

:/ \

~

215

~

CH~ -

~, C H 4

-....

//

~//./

\

C H 2 "~- \ 9 H "\ \,

>400 K

CH

....

)

C+H

z

+

HzC 0 / Hz+ C 0 ADSORBEDSPECIES S c h e m e 2.

The reaction shown in scheme 2 is proposed, where CH 2 dehydrogenation and hydrogenation compete. This scheme involves several assumptions. The observable species in the pathway involving CH 2 are (HzCO), by HREELS, CH by HREELS (quantified by TPD) and CH4 by TPD. The CH is assumed to be stable enough at 250 K that it neither hydrogenates nor dehydrogenates. Under our conditions, methane formation is irreversible and CH3 hydrogenation must compete effectively with CH 3 dehydrogenation. This is consistent with C H 4 formation as the dominant pathway for CH3I decomposition at 280 K on Pt(111) [25]. The C H 4 TPD peak temperature is coverage independent, implying first-order kinetics in the rate-limiting step of methane formation. Utilizing the method of Chan et al. [26], we calculate an activation energy for methane formation of 9.5 +_ 0.5 kcal/mol and preexponential of (3.5 + 0.6) • 108 s -1. According to scheme 2, the branching ratio between hydrogenation and dehydrogenation of methylene is given by the correlation of fig. 3 and is about 5.2 at high coverages. The nonlinear character at low coverages (favoring CH) probably reflects~ a stron~ dependence on the coverage of H(a). A similar mechanism has been proposed for methane formation at 260 K from decomposition of di-o boaded ketene on P t ( l l l ) [1,2]. Since the decomposition of alcohols on P t ( l l l ) does not result in CH 4 formation [27-29], we can assume

216

M.A. Henderson et al. / TPD and HREELS study of H2CO on Pt(l l l)

that any surface methoxy formed by hydrogenation of the carbon of (H2CO)n does not play a role in the methane formed here. Apparently the C=O double bond of formaldehyde is completely broken before hydrogenation to methane occurs. This is indicated in scheme 2 by the formation of CHz(a ) and either O(a) or OH(a). It is well known that in the presence of H(a), both O(a) and OH(a) rapidly form H20 on Pt(111) at temperatures above the water desorption point (175 K) [30]. Since we find water desorbing at 253 K (fig. 1), it is clearly reaction limited. The availability of oxygen is the key since H(a) is formed during adsorption. The small decrease in the amount of fll H2 desorption (fig. 2) with increasing H 2 0 and CH 4 desorption may be due to hydrogenation of O and CH 2. CO 2 formation is not observed since CO oxidation cannot compete with water formation below 273 K [31]. According to scheme 2, the m i n i m u m amount of polymerized H2CO at monolayer saturation is equal to the number of H atoms appearing in r2 H2 (hence, the CH coverage) plus the amount of CH 4. We calculate this minimum to be 0.12 ML. The HECO that desorbs at 250 K (not calibrated) and any decomposition of the polymer to CO and H will increase this amount. Abbas and Madix [3] have studied the decomposition of H2CO on clean Pt(111) and sulfur covered Pt(111) using TPD and AES. Although the same general findings were made (that H2CO decomposes primarily to CO and H2, both of which appear as desorption-limited states in the TPD, that H2CO desorbs at 235, that a small amount of H 2 desorbs in a higher temperature state at 520 K, and that carbon is left on the surface after TPD) there are some discrepancies involving the identity of minor decomposition products. They observed small amounts of CH3OH (255 and 304 K), CH3OCHO (338 K) and CO z (389 K), but no CH4, desorbing in the TPD. An additional H2CO state was also seen at 293 K. One experimental difference between the two works was the dosing temperature, with Abbas and Madix dosing H2CO with the Pt(lll) crystal at 195 K while we dosed at 105 K. We were unable to reproduce their results when our crystal was dosed at 200 K (only CO and H 2 (/31 and /32) were observed in the TPD). We believe, on the basis of the following points, that the paraformaldehyde source used by Abbas and Madix may have been contaminated with small amounts of CH3OH a n d / o r CH3OCHO: (1) Sexton [28] has shown that methoxy is unstable on Pt(111) and O / P t ( l l l ) and forms CO and H at temperatures above 170 K. Methanol f o r m a t i o n is thus unlikely on P t ( l l l ) above 200 K. (2) Our observation of CH 4 formation is consistent with results from k e t e n e / P t ( l l l ) [1,2]. C H 4 is formed at 260 K from ketene decomposition to methylene and CO, and (3) CH3OH and CH3OCHO are common impurities in paraformaldehyde [6]. On Ru(001), Anton et al. [4,12] observed C - O stretching frequencies from H2CO ascribable to ~2(C, O) H2CO and HCO (980 and 1180 cm -1, respectively) and ~/1(O) H / C O (1660 cm-1). These species were either not present on

M.A. Henderson et al. / TPD and H R E E L S study of H2CO on P t ( l l l )

217

P t ( l l l ) or were unobservable due to the intense OCO stretches between 965 and 1120 cm -1. While the 1770 and 1745 cm -a losses in figs. 4b and 4c are in the region typical of organic carboxyl groups, they are 35 to 60 cm -1 higher than for solid HECO at 1710 cm -1 [32]. For this reason, we believe they are probably from CO moved from atop sites to hollow sites. Such site changes in CO adlayers are quite common [33,34]. When (HECO)n decomposes, these CO species return to their favored atop sites (figs. 5c and 5d). O(a) is required for polymerization on Ag(110) [13], but not on Ni(110) [5] or Zn(001) [14]. The proposal that HECO polymerizes in the form of trioxane on Zn(0001) [14] cannot be carried over to Pt(111) because of significant disagreements in the vibrational frequencies (table 1). Anton et al. [4] have suggested that the formation of polymerized HECO on single crystal surfaces may result from traces of H 2 0 in the formaldehyde source material. In our work, extensive effort was made to remove H 2 0 from the source paraformaldehyde (see experimental section) and no HREELS losses ascribable to adsorbed water were observed. TPD of various exposures of H2CO from submonolayer to multilayer showed only reaction limited H 2 0 [30]. Thus, the formation of polymerized HECO on Pt(111) is not due to a n 2 0 impurity. Compared to ketene on Pt(111) [1,2], the major differences are (1) CO bond cleavage for formaldehyde, which we have attributed to the formation of (H2CO)n and (2) the absence of C2H 4. Ketene decomposition leads to ethylene only when the coverage is high such that large local methylene concentrations can be present transiently. This is never possible starting with formaldehyde. The CO bond cleavage channel is probably not competitive in the case of ketene because its bonding geometry (,/2(C, C)) is not conducive to polymer formation.

5. Summary During adsorption of low doses at 105 K, H2CO decomposes on clean Pt(111) to give adsorbed CO and H. TPD of H2CO on Pt(111) gives primarily desorption limited CO and H 2. At higher coverages a polymeric form of H2CO is detected by HREELS. The polymer decomposes above 250 K, leading to small amounts of HECO , CH4, r2 H2 (520 K), and H 2 0 in addition to the major products, desorption limited fll H2 and CO. No CO 2, CH3OH, or CH3OCHO were observed. Vibrational spectra of the decomposed polymer provide evidence for Cxi-Iy, CO and CH, depending on the temperature reached. The appearance of CH 4 and C H requires CO bond cleavage which we attribute to the decomposition of polymerized formaldehyde to give methylene. The latter either hydrogenates to CH 4 or dehydrogenates to CH.

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M.A. Henderson et al. / TPD and H R E E L S study of HeCO on Pt(111)

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