Evidence for carbocation formation during the coadsorption of methanol and hydrogen on Pt(110)

Surface Science 418 (1998) 329–341

Evidence for carbocation formation during the coadsorption of methanol and hydrogen on Pt(110) N. Chen, P. Blowers, R.I. Masel * Department of Chemical Engineering, University of Illinois at Urbana-Champaign, 600 South Matthews Avenue, Urbana, IL 61801, USA Received 4 June 1998; accepted for publication 2 September 1998

Abstract There are reasons to suspect that carbocations can form during hydrocarbon adsorption on metal surfaces, but so far no one has observed a carbocation on any metal surface spectroscopically. In this paper, we examine the coadsorption of methanol and hydrogen on Pt(110) to see whether a methoxonium species [CH OH ]+ can form. We find that a new species does form when we 3 2 coadsorb hydrogen and methanol on Pt(110) at 100 K and then heat to 200 K. The new species has an EELS spectrum which is consistent with the one expected for a methoxonium ion, and has the isotopic shifts expected for methoxonium. Comparison to the EELS spectrum estimated from ab initio calculations is used to verify the EELS assignment, and to rule out the possibility that a methoxonium radical or other similar species is formed instead. We also observe the chemistry expected when a carbocation forms (i.e. dehydration). These results provide strong evidence that methoxonium forms when hydrogen and methanol coadsorb on platinum. This is the first time that a carbocation has been observed spectroscopically on a metal surface. © 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Ab initio quantum mechanical calculations; Alcohols; Electron energy loss spectroscopy; Platinum

1. Introduction Over the years, people have identified a wide number of species during hydrocarbon adsorption on metal surfaces. So far, most of the species which have been observed look like neutral species covalently bound to the metal [1]. The vibrational spectra look like those of covalently bound species, and the reactions of the species follow pathways which are similar to those of radical reactions in the gas phase. The purpose of the work described in this paper * Corresponding author. Fax: +1 217 3335052; e-mail: [email protected]

was to determine whether it is possible for carbocations to also form on metal surfaces. To put this work into perspective, carbocations have never been observed on metal surfaces. Years ago, Anderson [2] mentioned the possibility that a carbocation could form on a metal surface. However, Anderson’s suggestion has not yet been confirmed experimentally. Still, several previous investigators have observed hydronium formation during water and hydrogen adsorption on certain platinum surfaces [3–6 ]. In particular, Drachsel [6 ] observed the desorption of hydronium ions from platinum surfaces. Water has a modest proton affinity compared to other Lewis bases, as indicated in Table 1. If water can pick up a proton

0039-6028/98/$ – see front matter © 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 07 4 6 -8

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Table 1 Selected bases and corresponding proton affinity Base

Proton affinity (kcal mole−1) [7]

Pyridine Ammonia Di-n-propyl ether Phenol Isopropyl alcohol Ethanol Methanol Benzene 2,2,2-Trichloro-ethanol 2-Fluoro-ethanol Water Cyclohexane Isobutane Hexafluorobenzene Propane

222.3 204.1 200.3 195.0 190.1 185.6 180.3 179.3 174.5 171.0 166.5 164.3 161.5 154.7 150.5

from a platinum surface, it seems likely that other bases could pick up protons from the surface as well. If so, it seems likely that carbocations could form on platinum surfaces. In this paper, we were looking for carbocation formation during the coadsorption of hydrogen and methanol on platinum. The idea was to see whether the reaction CH OH+H+[CH OH ]+ (1) 3 3 2 occurs on Pt(110). We chose to look at this reaction for several reasons. First, we noted that methanol has a much greater proton affinity than water [7]. If water can pick up a proton on platinum, it seems likely that methanol can too. Another interesting observation is that, in the previous literature, Kishi et al. [8,9] observed ion formation when a beam of alcohols scattered from certain metal surfaces, including ethanol scattering from a platinum surface [8]. If ions desorb from a surface, it is quite reasonable that ions can also form on the surface. Another reason to look for ion formation during the coadsorption of alcohols and hydrogen is that, in previous work [12], we found chemistry which is very characteristic of carbocation formation when investigating reactions on platinum. Recall that when hydrogen atoms and methanol [10]

react in the gas phase, the main reactions are CH OH+HH +CH OH 3 2 2 and

(2)

CH OH+HH +CH O. (3) 3 2 3 In contrast, when methanol reacts with protons in the aqueous phase, [11] the main reaction is CH OH+H+[CH OH ]+CH++H O. (4) 3 3 2 3 2 In previous work [12], we found that when methanol and hydrogen coadsorb on (1×1) Pt(110), the methanol molecule dehydrates, i.e. CH OH+H CH +H O. (5) 3 (ad) 3(ad) 2 The adsorbed methyl groups can subsequently hydrogenate and desorb, i.e. CH +HCH . (6) 3 4 Eqs. (5) and (6) represent the kind of reactions one would expect to see if a carbocation formed on the surface, and are not the kind of chemistry one would expect if there was a radical intermediate forming instead. At this point, we know ions do form when alcohols scatter from platinum surfaces, and we see characteristic chemistry one would expect if carbocations formed on metal surfaces. We also know that hydronium does form on Pt(110). Therefore, it seems possible that methoxonium would also form on Pt(110). In this paper, we have examined the coadsorption of methanol and hydrogen with EELS to see whether we can detect methoxonium formation. This measurement is feasible because, as we will see later in this paper, the EELS spectrum of the methoxonium ion is very different from the EELS spectrum of the corresponding neutral species. Physically, methanol has a proton affinity of 180.3 kcal mole−1 [7], while hydrogen atoms can only form weakly bound van der Waals complexes with alcohols. The result is that methoxonium has characteristic vibrations which can be used for species identification. Unfortunately, no complete reference spectrum for methoxonium has yet been published. Consequently, we also performed calculations using the 92 [13] suite of programs to

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determine the vibrational frequencies of all possible intermediates in the system.

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[16 ] showed that this procedure reproduces vibrational frequencies within 60 cm−1 for most simple molecules.

2. Procedures 3. Results: experiments The experiments were performed as described previously [14]. We cleaned a Pt(110) sample until no impurities could be detected by AES and a sharp (2×1) LEED pattern was obtained. We then dosed the sample with 1.0 L of hydrogen at 273 K, cooled it to 100 K, and dosed it with 0.5 L of methanol. Subsequent spectra were taken by annealing the layer to a desired temperature, cooling to 100 K and scanning an EELS spectrum. All of the other techniques were standard. The reader is referred to our previous work for more details [12,14]. The calculations were performed with the 92 suite of programs. Generally, we followed the procedures of Pople et al. [15,16 ] to calculate the vibrational spectra. First we calculated the structure of molecules at the MP2(Full )/6-31G* level of theory. Then we scaled the vibrational frequencies by 0.9427. Pople et al.

Fig. 1 shows an EELS spectrum taken after exposing a clean (2×1) Pt(110) surface to 1.0 L of hydrogen at 273 K, cooling to 100 K, exposing to 0.5 L of methanol and then scanning an EELS spectrum. Subsequent spectra were taken by annealing at elevated temperatures for 2 min before cooling to 100 K and then scanning an EELS spectrum. At 100 K, our EELS spectrum matches that of methanol ice, as described previously [21–23]. When we anneal to 200 K, three new small peaks appear in the EELS spectrum: peaks at 450 and 1550 cm−1 associated with some new species, and peaks at 1780 and 2090 cm−1, which are probably associated with background CO (note that the scale was increased by a factor of 10, so that the background CO appears to be an order of magnitude larger). We also find that the 1140 cm−1 peak for methanol grows and shifts to slightly lower frequen-

Fig. 1. EELS spectra taken on specular angle by exposing a clean (2×1) Pt(110) surface to 1.0 L of hydrogen at 273 K, to 0.5 L of methanol at 100 K, and then annealing at the temperatures indicated for 2 min.

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Fig. 2. EELS spectra taken off specular angle by exposing a clean (2×1) Pt(110) surface to 1.0 L of hydrogen at 273 K, cooling to 100 K, adding 0.5 L of methanol, then scanning and annealing at the temperatures indicated for 2 min.

cies, suggesting the possibility that there is also another new peak around 1100 cm−1. In other spectra, we found that the extra peaks started to appear in the spectrum at about 185 K and persisted to 220 K. Fig. 1 also includes an insert which shows the region of the spectrum between 1300 and 1800 cm−1 after coadsorbing methanol and hydrogen, annealing at 100 K and then annealing to 200 K. Notice that the 1550 cm−1 peak is fairly distinct in the 200 K spectrum. However, no similar peak was seen with methanol alone when we were careful to keep background hydrogen off the surface. We also performed LEED measurements during the experiment above. We began with a sample which had a sharp (2×1) LEED pattern, but when we exposed the sample to hydrogen at 273 K, the (2×1) features became less intense. We believe that we are partially reconstructing the surface into a (1×1) configuration. However, the LEED patterns were not sharp, so it is difficult to be sure. Fig. 2 repeats the data in Fig. 1, except that we

tilted the EELS so that we could measure 4° off axis. Notice that the 1550 cm−1 peak becomes more distinct as we tilt off axis. We also repeated these same measurements where we dosed methanol but not hydrogen. In those cases, we still observe a CO peak at 2060 cm−1, but do not observe the other new peaks in the spectrum [12]. Therefore, we conclude that a new species forms when methanol and hydrogen coadsorb on (2×1) Pt(110) which does not form during the adsorption of methanol alone. Fig. 3 shows some additional data we took after adsorbing 1.0 L of H at 273 K, cooling to 100 K, 2 adding 0.9 L of methanol, heating to 190 K for 2 min and cooling to 100 K before scanning. We counted longer for Fig. 3 than for Fig. 2, and filtered the data with a RAZOR filter. The filtering process substantially improved the signal-to-noise ratio, so the peaks can be seen more clearly. There are distinct peaks at 450, 810, 970, 1130, 1430, 1550, 1740, 1950, 2060, 2850, 2930, 3110, 3190, 3320 and 3420 cm−1. The 1950 and 2060 cm−1 peaks are seen on the ‘‘clean’’ surface, so they are

N. Chen et al. / Surface Science 418 (1998) 329–341

Fig. 3. EELS spectrum obtained by exposing a clean (2×1) Pt(110) surface to 1.0 L of hydrogen at 273 K, cooling to 100 K, adding 0.5 L of methanol, annealing to 190 K for 2 minutes, cooling to 100 K and then scanning an EELS spectrum. Unlike the other figures in this paper, the spectrum was filtered by a RAZOR filter.

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probably associated with background CO. The 1740 cm−1 peak is also sometimes seen in our background spectra, although we do not know its origin. The rest of the peaks appear to be real and associated with the species which form when methanol and hydrogen coadsorb on platinum. Fig. 4 shows a series of spectra taken when coadsorbing deuterium and fully deuterated methanol and heating as indicated. The 100 K spectrum matches that of deuterated methanol except that there are two small peaks at 2990 and 3300 cm−1 associated with some exchange during dosing. When we anneal to 200 K, there are distinct new peaks at 350 and 1180 cm−1. The 1180 cm−1 peak seems to be a shifted version of the 1550 cm−1 peak, while the 350 cm−1 peak seems to be a shifted version of the 450 cm−1 peak. There are also shifts and growth in the region of the spectrum around 900 cm−1, suggesting that a new peak forms at about 980 cm−1. This seems to be a shifted version of the 1130 cm−1 peak. Finally, Fig. 5 shows a reference spectrum taken

Fig. 4. EELS spectra taken by exposing a clean (2×1) Pt(110) surface to 1.0 L of deuterium at 273 K, to 0.5 L of d -methanol at 4 100 K, and then annealing at the temperatures indicated for 2 min.

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Fig. 5. EELS spectra taken by exposing a clean (2×1) Pt(110) surface to 3.0 L of hydrogen at 273 K, to 0.5 L of water at 100 K, and then annealing at the temperatures indicated for 2 min.

by adsorbing hydrogen at 273 K and then adsorbing water at 100 K on to a clean Pt(110) surface, scanning, annealing to higher temperatures, and then scanning again. The 100 K spectrum matches that of water. When we anneal to 140 K, we observe a shoulder at 1150 cm−1 which previous investigators have associated with hydronium ion formation. Importantly, however, the hydronium species decomposes below 180 K. At 200 K, there is no hydronium or water left on the surface. In other measurements, we also measured the hydronium ion concentration on this surface using techniques developed by previous investigators [3]. We infer a hydronium concentration of about 25% of a monolayer, corresponding to a pH of −1.1.

4. Results: calculations The results so far show that we can form a new species when hydrogen and methanol coadsorb on (2×1) Pt(110). We observe new peaks in the EELS spectrum which are not observed with methanol alone. However, the identity of this species is

not completely clear. We were expecting a methoxonium ion, [CH OH ]+. There is a partial reference 3 2 spectrum for a methoxonium ion in the literature, but we needed a more complete spectrum. Also, we needed reference spectra for several other compounds to be able to rule out their presence. Fortunately, the IR spectra of simple gas-phase molecules can be calculated routinely using ab initio methods [15,16 ]. Generally, one can calculate a gas-phase IR spectrum which is accurate to within 60 cm−1 using commercial software and obtain reasonable intensity predictions (see Refs. [15,16 ] for details). In this work, we calculated a reference spectra for a methoxonium radical, CH OH , a methoxo3 2 nium ion, [CH OH ]+, and a methoxonium dimer 3 2 [(CH OH ) H ]+. We also calculated the IR 3 2 spectrum for methanol so that we could assess the reliability of the calculational methods. Table 2 shows our results for gas-phase methanol. We find that our calculated geometry for methanol closely matches the previous experimental observations. Our calculated vibrational frequencies are within 30 cm−1 of those which have

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N. Chen et al. / Surface Science 418 (1998) 329–341 Table 2 A comparison of the geometry and vibrational frequencies of methanol calculated in the gas phase at thw MP2/6-31G* level to the experimental values [24,25] Geometry parameter

Calculated

˚) C–H (A ˚) O–H (A ˚) C–O (A %COH (°) %HCH (°) Frequency mode C–O stretch Methyl rock Methyl bend

C–H stretch

O–H stretch

Experimental gas phase

1.094 0.970 1.423 109.3 108.8

1.094 0.945 1.425 108.5 108.6

Calculated (cm−1) 1022 1337 1452 1476 1489 2901 2964 3038 (w) 3578

Experimental (cm−1) 1029 NO 1420 1455 1480

2934 3681

been measured in the gas phase, except for the O–H stretching which is off by 100 cm−1. We have also performed calculations for various clusters. Table 3 shows the geometries we predict for [CH OH ]+, CH OH , the CH O–H complex 3 2 3 2 3 2 and the CH –OH complex. Our calculations show 3 2 that methoxonium ions are stable species in the gas phase. In contrast, CH OH is not a stable 3 2 molecule. Instead, it decomposes into either a more weakly bound CH O–H complex or a weakly 3 2 bound CH –OH complex. These geometries are 3 2 given in Table 3.

Table 4 shows the spectra we calculated for these species. The IR spectrum of methoxonium ions should be similar to that of methanol. However, there should be extra peaks at 490 and 1631 cm−1 associated with a OH scissor mode 2 and CH deformation. We also find that the methyl 3 rock at 1180 cm−1 should shift to lower frequencies since the ion pulls charge out of the bond. We have also calculated the spectrum of methoxonium radicals. The calculations indicate that methoxonium radicals are not stable in the gas phase. Instead, the radicals decompose to either methoxy and H , or methanol and hydrogen 2 atoms. The spectra of the CH –OH and 3 2 CH O–H complexes look like the spectra of the 3 2 isolated radicals. The metastable CH OH state 3 2 has an OH stretching frequency of 2105 cm−1. 2 We have also calculated a spectrum of a neutral species at the geometry of the methoxonium ions. This species has a OH scissor mode of 2 2898 cm−1. Finally, we calculated spectra of [(CH OH ) H ]+. This spectra looks like the meth3 2 oxonium spectrum, except that the OH stretch shifts to 3300 cm−1.

5. Discussion: species identification The results above show that a new species forms when hydrogen and methanol coadsorb on (2×1) Pt(110). We observe extra peaks in the 200 K EELS spectrum which are not seen with methanol alone. The peaks grow as we tilt the EELS off axis to enhance impact scattering and shift by a factor

Table 3 The geometry of the CH OH+ complex, CH O–H complex and CH OH metastable species calculated at the MP2/6-31G* level 3 2 3 2 ˚ , angles3 in 2degrees) (bond lengths in A Geometry parameter C–H C–O O–H O–H∞ H–H %HCO %HOC %H∞OC

CH OH+ complex 3 2 1.087 1.481 1.140 1.14 107.0 108.9

CH O–H complex 3 2 1.094 1.387 2.644 0.738 110.1 107.4

CH OH metastable species 3 2 1.084 1.701 0.982 1.213 105.5 104.2 156.5

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Table 4 MP2/6-31G* frequencies for the CH OH+ complex, CH O–H complex and CH OH metastable species (cm−1) 3 2 3 2 3 2 Vibrational mode Rotational motions

CH OH + complex 3 2 231

H–H twist O-H out of plane bend O–C stretch Methyl rock

771 904 1137 1239 (w)

Methyl bend

1239 1427 1442 1452 1631 2993 3125 (w) 3136 (w)

O–H bend 2 C–H stretch

O–H stretch

CH O–H complex 3 2 27 47 323 330 860 944 1077 1380 944 1398 1490

2875 2957 2990 170

CH OH metastable species 3 2 228

847 919 919 1109 1346 1460 1473 2105 2973 3072 (w) 3090 (w) 1089

3382 H–H out of plane bend

of 1.4 as we deuterate the molecule. These results suggest that the new species has hydrogen atoms in a different state than those in methanol. We had to do some detective work to identify the species. First we note that in our previous work we examined the decomposition of methanol on (1×1) and (2×1) Pt(110). On (1×1) Pt(110), the methanol dehydrates to produce water desorption at 180 K, while on (2×1) the methanol decomposes by the following mechanism: first, the methanol loses a hydrogen to produce methoxy at about 180 K. Then, the methoxy decomposes via formaldehyde and formyl intermediates to yield CO and hydrogen. Note that one would not expect to observe a 1600 cm−1 peak in the hydrogen spectrum, or a 1190 cm−1 peak for the deuterated spectra from methoxy or formyl. Consequently, our 200 K spectrum cannot be simply explained by the fact that hydrogen is stabilizing a methoxy or formyl intermediate on the surface. Formaldehyde does have a CO stretching mode in the right range. However, if we formed formaldehyde, the 1590 cm−1 peak would not shift to 1190 cm−1 when we deuterated the hydroxyl hydrogen.

4259

Consequently, we can rule out the peak being associated with formaldehyde. Another interesting observation is that the species decomposes at 210 K. We have taken TPD spectra and only observed CO, hydrogen, water and methane desorption above 200 K. There is no evidence for CO , which one might expect to form 2 if we were forming an (H CO) polymer on this 2 x surface. It is interesting to consider just what it is that we are forming. First, we note that the new species must contain one carbon, one oxygen and an undetermined number of hydrogens, since we do not see any other species with more than one carbon or oxygen desorbing from the surface. There are eleven stable species containing one carbon, one oxygen and an undetermined number of hydrogens: CH OH, [CH OH ]+, CH O, 3 3 3 [CH O]+, [CH OH ]+, CH O, [CH O]+, 3 3 2 2 2 CH OH, [CH OH ]+, CHO and [CHO]+. We 2 2 observe a 1550 cm−1 peak in the spectrum of our species and an 1180 cm−1 peak in the spectrum of the analog with a deuterated hydroxy group. The methoxonium ion [CH OH ]+ would have a peak 3 2 around 1631 cm−1 in the 1H spectrum and a peak

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around 1207 cm−1 for the deuterated spectrum, as is observed. However, one would not observe a 1550 cm−1 peak from CH OH, [CH OH ]+, 3 3 CH O, [CH O]+, CH OH, [CH OH ]+, CHO or 3 3 2 2 [CHO]+. No 1180 cm−1 peak is expected in the spectrum of either CH O or [CH O]+ produced 2 2 from CH OD. Consequently, we can rule out the 3 new species being any of these other species. We can also rule out some sort of neutral hydrogen methanol cluster. Methanol has a proton affinity of 181.3 kcal mole−1 [7]. However, methanol does not form any highly stable adducts with hydrogen. In our ab initio calculations, we have found that hydrogen and methanol can form a weak van der Waals complex with a binding energy of 1 kcal mole−1. There are no neutral methanol hydrogen adducts with vibrational frequencies near 1550 cm−1. One does have to consider whether the 1550 cm−1 peak could be associated with a water impurity. However, when we coadsorb water and methanol, the water desorbs below 190 K. There is no evidence for methanol coadsorbed with water on the surface to form water–methyl complexes at 210 K. Further, water shows a peak at 1620 cm−1, not at 1550 cm−1 as we observe in Fig. 3: background water would not give a peak which shifts with deuteration. Therefore, it does not appear that the 1550 cm−1 peak is associated with a water impurity. Of the species we have considered, only methoxonium ions (CH OH )+ have a 1550 cm−1 peak 3 2 and a 1180 cm−1 peak in the hydroxy-hydrogen deuterated analog. Therefore, the formation of

methoxonium is the most likely explanation for our EELS spectra. Measurements were performed where we coadsorbed either CD OH or CH OD, and either 3 3 hydrogen or deuterium, in order to verify our EELS assignment. Table 5 summarizes these results. We find that the 1550 cm−1 peak shifts to 1180 cm−1 for the spectrum of coadsorbed CH OD and deuterium. No shift is seen in the 3 spectrum of coadsorbed CD OH and hydrogen. 3 These results suggest that the 1550 cm−1 peak is associated with a motion of the OH in methanol. Further, the 1550 cm−1 peak is suppressed for the CD OH and deuterium spectrum. The latter result 3 suggests that the 1550 cm−1 mode is associated with a motion of at least two hydrogens, as for a OH scissor mode. For the later case, in principle, 2 the peak should disappear, but it does not disappear because of exchange. Similarly, the 450 cm−1 peak shifts to 350 cm−1 when we deuterate the CH group in 3 methanol. However, it is unaffected by both deuteration of the OH and the surface deuterium. This result shows that the 450 cm−1 peak is associated with the motion of the CH group. 3 The only species which we can find with a CH 3 group, a OH group and only one carbon and 2 oxygen atom is methoxonium [CH OH ]+. 3 2 Therefore, this suggests that we are forming methoxonium when hydrogen and methanol coadsorb on Pt(110). Further evidence for the methoxonium assignment comes from a comparison of the vibrational

Table 5 Vibrational frequencies (cm−1) of H+CH OH, H+CD OD, D+CH OD and D+CD OD on (2×1) Pt(110) after annealing the 3 3 3 3 surface at 200 K for 2 min H+CH OH 3 M–O stretch Methyl rotation O–H out of plane bend MO–C stretch Methyl rock Methyl bend OH scissor 2 CH/CD stretch OH/OD stretch

330 450 810 970 1130 1430 1550 2930, 3100 3320, 3420

H+CD OH 3 330 800 950 1100 1580 2270 3310

D+CH OD 3

D+CD OD 3

320 450 590 980 980 1440 1140 2980 2460

350 590 980 980 1090 1180 2280 2460

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Table 6 Vibrational frequencies calculated for CH OH+ and its deuterated analogs 3 2 CH OH+ 3 2 M–O stretch Methyl rotation OH out of plane bend

231 771

O–C stretch Methyl rock

904 1137 1239 (w)

Methyl bend

1447 1442 1453 1631 2992 3125 3136 3382 3469

OH , OD scissors 2 2 CH/CH stretch OH/OD stretch

CD OH+ 3 2

CH OD+ 3 2

CD OD+ 3 2

191 555

791 760 886 1050 1068

166 543.7 645 961 1052 1068

Combination 1121 1181 1206 1428 1442 1450

1107

1628 2136 2328 2331 3381 3469

1042

1198 2136 2327 2337 2437 2551

2993 3126 3136 2437 2555 (w)

Table 7 Frequency comparison between the experimental spectra of the new species on Pt(110) and the calculated spectra of CH OH+ ions 3 2 and CH OH radicals 3 2 Assignment

Experimental vibrational frequencies on Pt(110) (cm−1) during methanol and hydrogen coadsorption

M–O stretch Methyl rotation O-H out of plane bend O–C stretch Methyl rock

330 450 800 970 1130 1300 (w) 1430

Methyl bend

C OH scissor 2y 2 Methyl C–H stretch

O–H stretch

1550 2930 3110 (w) 3320 3420

spectrum calculated for a methoxonium ion to the spectrum observed experimentally. Tables 6 and 7 compare these results. Note that our experimental spectra are in reasonable agreement with the spectra expected for a methoxonium ion. There are some small shifts, but these are to be expected if the methoxonium forms in a methanol cluster

Calculated CH OH+ ion 3 2 frequencies (cm−1)

Calculated CH OH 3 2 radical frequencies (cm−1)

771 904 1137 1239 1447 1442 1453 1631 2992 3125 3136 3382 3469

847 919 1109 1346 1460 1473

(w)

(w) (w)

2105 2973 3072 (w) 3090 (w) 1089

(w)

and not in the gas phase. We have also calculated the spectrum of a methoxonium radical, and did not find good agreement with the experimental values. So far, the only explanation for our data is the formation of a methoxonium ion. Therefore, we conclude that we must be forming methoxonium.

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There is one other species which we cannot rule out completely, which is a methoxonium–methanol dimer, i.e. [(CH OH ) H ]+. Tarakanova [17] esti3 2 mated the IR spectrum of the dimer, and it looks similar to the spectrum we observe. In particular, Tarakanova reported a peak at 2850 cm−1 which would not be expected for a bare methoxonium species. We do observe the 2850 cm−1 peak which Tarakanova associates with the dimer. So, based on Tarakanova’s analysis, we should be forming a dimer and not a monomer. However, in our ab initio calculations we found that a methoxonium ion surrounded by two or three methanols also shows a 2850 cm−1 peak. Therefore, we cannot tell whether we are forming a methoxium dimer or a methoxonium surrounded by many methanols. At this point, the best explanation of our data on Pt(110) is that we are probably forming methoxonium with some unknown number of methanols.

6. Implications Next, it is useful to consider the implications of our results. In the previous literature, people [8,9] have observed the desorption of carbocations from metal surfaces, but our results are the first spectroscopic observation of a carbocation on a metal surface. Our results are a strong indication that ionic species can form on metal surfaces, as previously suggested by Anderson [2], Ponec [19] and Gault [20]. Still, the formation of a methoxonium ion when methanol and hydrogen coadsorb on Pt(110) is exactly what one would expect from previous work. Recall that previous investigators have also observed hydronium ion formation during water and hydrogen coadsorption. However, Drachsel et al. [6 ] have observed desorption of the hydronium ions from a platinum field emitter tip. The desorption of hydronium in this case is strong evidence that hydronium actually does form on the surface. When we coadsorb water and hydrogen on (2×1) Pt(110) we also observe an EELS spectrum which has been widely associated with hydronium ions [3], as shown in Fig. 4. The previous findings that hydronium ions can

339

form on platinum makes the idea that methoxonium ions also form quite reasonable. After all, methanol has a higher proton affinity than water [7]. If water can grab a proton from a platinum surface to form a hydronium ion which desorbs from the surface, it is reasonable to assume that methanol can also grab a proton to form a methoxonium ion. Another supporting piece of evidence for the formation of methoxonium is that the thermodynamics seem favorable. Let us consider the heat of the reaction for the reaction H+CH OH[CH OH ]++e− (7) 3 3 2 (ab) in a methanol cluster on Pt(110), where e− is an (ab) electron deposited into the metal. One can calculate the heat of Eq. (7) in the gas phase using the simple thermodynamic path HH++e−,

(8)

H++CH OH[CH OH ]+, (9) 3 3 2 e−e− . (10) (ab) Previous data shows that the gas-phase reaction ( Eq. (8)) is 13.6 eV endothermic, while Eq. (9) is 7.87 eV exothermic [7] (i.e. the proton affinity of methanol is 7.87 eV ). The heat of reaction for Eq. (10) is harder to measure, but on a clean Pt(110) surface the work function is 5.8 eV. If we assume that we gain back the clean surface work function when we put an electron into the metal, we find that Eq. (7) is 0.07 eV exothermic in the gas phase. Therefore, it could occur. Now consider what happens if Eq. (7) occurs in a small methanol cluster on a platinum surface. If we run Eq. (7) in a cluster, we lose the heat of adsorption of hydrogen atoms (2.60 eV ) and gain back the heat of solvation of a methoxonium ion in methanol. Marcus [18] reported a value of 2.97 eV for this. Therefore, we conclude that if Eq. (1) occurred in a small methanol cluster on a Pt(110) surface, the reaction should be 0.43 eV exothermic, and this is sufficient to let the reaction happen. Next, it is important to note that we do see chemistry characteristic of methoxonium formation on platinum. In Section 1 we noted that when hydrogen atoms and methanol react in the gas

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phase, the main reactions are CH OH+HH +CH OH, 3 2 2 and

(11)

CH OH+HH +CH O. (12) 3 2 3 In contrast, when methanol reacts with protons, the main reaction is

nium radicals or other likely species. Further, we provide arguments which indicate that it is thermodynamically favorable to form methoxonium ions on Pt(110), and demonstrate that the surface chemistry is as expected if methoxonium forms. This is the first time that a carbocation has been observed during hydrocarbon adsorption on a metal surface.

CH OH+H+[CH OH ]+CH+ +H O. 3 3 2 3 2 (13)

Acknowledgements

In previous work [12] we have found that when methanol and hydrogen are coadsorbed on (1×1) Pt(110), the methanol dehydrates, i.e.

This work was supported by the National Science Foundation Grant CTS 96-10115.

CH OH+H CH +H O. (14) 3 (ad) 3(ad) 2 The adsorbed methyl group can subsequently hydrogenate and desorb, i.e.

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CH +HCH . (15) 3 4 Eq. (14) is the kind of chemistry which one would expect to see if methoxonium ions form. We have observed a new species when methanol and hydrogen coadsorb on Pt(110). We have shown that the new species has an EELS spectrum consistent with that of methoxonium, and we have observed the chemistry which one would expect to see if a methoxonium species forms. We have also shown that the formation of methoxonium is thermodynamically feasible. At this point we have not been able to find a reasonable explanation of our data if we do not assume that methoxonium forms. Therefore, we conclude that methoxonium does indeed form during the coadsorption of methanol and hydrogen on Pt(110).

7. Conclusions In summary, in this paper we provide EELS evidence that a methoxonium ion forms when methanol and hydrogen coadsorb on Pt(110). Our data shows that a new species forms during this reaction. The EELS spectrum of the species is consistent with that expected for methoxonium ions, and is not consistent with either methoxo-

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