The adsorption and reaction of 2-iodoethanol on Ag(111)

Transcription

1 Surface Science 463 (000) The adsorption and reaction of -iodoethanol on Ag(111) G. Wu, D. Stacchiola, M. Kaltchev, W.T. Tysoe * Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 5311, USA Received 9 February 000; accepted for publication 5 May 000 Abstract Oxametallacycles have been previously grafted onto Ag(110) by adsorbing -iodoethanol to mimic possible intermediates in the catalytic epoxidation of ethylene. Since Ag(111) has been shown to be a good model catalyst for this reaction under realistic pressures of ethylene and oxygen, this work was extended to studying the chemistry of -iodoethanol on Ag(111) using temperature-programmed desorption and reflection absorption infrared spectroscopies. It was found that iodoethanol reacts at relatively low temperatures to form -hydroxyethyl species and co-adsorbed iodine. Hydroxyethyl species react above ~150 K to eliminate water and evolve equimolar amounts of ethylene. The remaining fragment with stoichiometry C H 4 O reacts with the surface to form predominantly oxametallacycles which thermally decompose at ~60 K to yield acetaldehyde. A minority species is also identified which thermally decomposes to yield acetaldehyde at ~315 K and may be due to an oxametallacycle which adopts a different orientation as a function of temperature and coverage or a -hydroxyethylidene species. 000 Elsevier Science B.V. All rights reserved. Keywords: Alcohols; Infrared adsorption spectroscopy; Reflection spectroscopy; Silver; Thermal desorption spectroscopy 1. Introduction this has been suggested to facilitate the nucleophilic addition of ethylene to adsorbed oxygen [5]. The pathway to the catalytic formation of ethythat However, the nature of the surface intermediate lene oxide from ethylene and oxygen has been is formed from the interaction of ethylene with studied extensively over the past 0 years [1 4]. adsorbed atomic oxygen and which yields ethylene The results of this work have demonstrated that a oxide (as well as being proposed to oxidize further Ag(111) single crystal can catalyze the formation forming CO and H O) yet remains to be identified. of ethylene oxide with kinetics that resemble those There are two obvious candidates for this intermedi- of supported catalysts [5 7]. It has been further ate species. The first forms when both p orbitals of shown that the active form of adsorbed oxygen for the ethylenic p orbital simultaneously interact with both selective and total oxidation is adsorbed adsorbed oxygen to form adsorbed ethylene oxide atomic oxygen, not dioxygen species [5]. Subsurface directly. This either would desorb yielding partial oxygen, which is also present during the reaction, oxidation products or undergo further oxidation on is also proposed to be a spectator species although the surface yielding CO and H O [5]. Alternatively, the p orbitals can interact with both adsorbed atomic oxygen and the silver surface forming an * Corresponding author. Fax: oxametallacycle [8,9]. This would undergo a reduc- address: wtt@uwm.edu ( W.T. Tysoe) tive elimination to form ethylene oxide /00/$ - see front matter 000 Elsevier Science B.V. All rights reserved. PII: S ( 00 )

2 8 G. Wu et al. / Surface Science 463 (000) 81 9 Oxametallacycles have been grafted onto angle of ~80 and the reflected light steered onto Ag(110) by reaction with -iodoethanol [8,9]. The the detector of a liquid-nitrogen-cooled, mercury chemistry in this case exploits results from previous cadmium telluride detector. The complete light work on iodoalkanes on silver where they react by path is enclosed and purged with dry, CO -free removal of the iodine to deposit an alkyl species air. The spectrometer operated at 4 cm 1 resolution and adsorbed iodine. Hydrogen abstraction from and data were typically collected for 1000 the hydroxyl species in silver is generally induced scans. by the presence of atomic oxygen [10]. The formation The infrared cell was attached to the main 1 in of an oxametallacycle from -iodoethanol on diameter ultrahigh vacuum chamber and the Ag(110) was confirmed from its high-resolution sample could be moved from the cell into the main electron energy loss ( HREELS) spectrum where chamber by means of a transfer rod. This chamber the fingerprint spectrum was calculated using density was equipped with a single-pass, cylindrical-mirror functional theory [9]. Interestingly, this was analyzer which was used to collect Auger spectra found to decompose yielding ethylene, water, acetaldehyde of the sample. and ethanol and also dimerized on the Temperature-programmed desorption data were surface yielding c-butyrolactone. However, no collected using a heating rate of ~3 Ks 1 and ethylene oxide formation was found. Since the desorbing species detected using a Dycor quadrupole majority of the previous mechanistic work has mass spectrometer located in the main ultramajority been carried out on Ag(111), the chemistry of high vacuum chamber and interfaced to a PC -iodoethanol is investigated on this surface using allowing five masses to be monitored sequentially a combination of temperature-programmed during the same desorption sweep. desorption and reflection absorption infrared The sample was cleaned using a standard pro- ( RAIRS) spectroscopies. It is found that, on clean cedure [1,13] which consisted of bombarding with Ag(111), adsorbed -hydroxyethyl species react to argon ions (1 kev, ma cm ) at 300 K and then evolve equimolar amounts of water and ethylene annealing to 1000 K in vacuo to remove any and form a species with a stoichiometry C H O. remaining surface species. The -iodoethanol used 4 This appears to form two distinct surface species for the experiments (Aldrich, 99% purity) was both of which thermally decompose to yield acetaldehyde. transferred to glass vials, attached to the gas- One of these is suggested to consist of an handling line of the vacuum system, and further oxametallacycle and the other, either an oxametallacycle purified by repeated freeze pump thaw cycles and adopting a different orientation or an its cleanness monitored mass spectroscopically. adsorbed -hydroxylethylidene species. 3. Results. Experimental Fig. 1 shows a series of temperature-programmed The experiments were carried out in a stainless desorption spectra collected following steel, ultrahigh vacuum chamber operating at a adsorption of -iodoethanol (3 L, 1 L=1 base pressure of ~ Torr following 10 6 Torr s) on Ag(111) at 80 K, monitoring vari- bakeout and which has been described in detail ous masses. In contrast to the behavior found on elsewhere [ 11]. Infrared data were collected from Ag(110), no c-butyrolactone desorption is found. an Ag(111) single crystal sample mounted in a Signals are found at all of the other masses (18, modified.75 in six-way cross equipped with infrared-transparent 7, 9, 45 and 31 amu). All of the spectra (except KBr windows. The sample could that at 18 amu) exhibit a relatively sharp peak at be resistively heated to 1000 K, or cooled to 80 K 05 K. This peak increases in intensity with using liquid nitrogen. Light from a Bruker Equinox increasing iodoethanol exposure without saturating infrared spectrometer passes through a polarizer and is assigned to multilayer desorption (see and is focused onto the sample at an incidence Fig. 4). Similar multilayer desorption was found

3 G. Wu et al. / Surface Science 463 (000) at 60 and 315 K where, at this exposure, the 60 K feature is significantly more intense than that at 315 K. This contrasts the behavior on Ag(110) where the 9 amu signal was detected simultaneously with ethylene and water in two states at ~340 and 63 K [8]. Note that both ethylene oxide and acetaldehyde have significant mass spectrometer ionizer fragments at 9 amu. However, the 9 amu species was unequivocally identified as acetaldehyde, not ethylene oxide, by carefully measuring the mass spectrometer ionizer fragments for acetaldehyde and ethylene oxide at various masses and by comparing these with the temperature-programmed desorption spectra taken at these masses. Note that acetaldehyde desorbs from Ag( 111) at substantially lower temperatures (160 K, [14]) so that the desorption states measured here represent a reaction- rather than desorption-rate-limited process. Finally, no other higher molecular weight species, in particular due to c-butyrolactone ( Fig. 1), which was found on Ag(110) [8], were detected. Fig. shows the evolution of the 9 amu desorption spectra with exposure where the exposures in Langmuirs are marked adjacent to the correspond- ing spectra. Note that the sharp peak at 05 K for exposures larger than L are due to the desorption of iodoethanol multilayers as discussed above. At low exposures (~1 L), both the low- and hightemperature acetaldehyde desorption states are evident with relatively equal intensities. The high- temperature (315 K ) state saturates at an exposure of ~1 L whereas the low-temperature (35 60 K) state continues to grow with increasing exposure. In addition, the peak temperature of the Fig. 1. Temperature-programmed desorption spectra of iodoethanol (3 L) adsorbed on Ag(111) at 80 K, monitoring 18 amu (H O), 7 amu (ethylene), 9 amu (acetaldehyde), 45 amu (iodoethanol ), 31 amu (ethanol ) and 86 amu (c-butyrolactone). The masses and the desorbing species are marked adjacent to the corresponding spectrum. at a slightly higher temperature (5 K) on Ag(110) [8]. In addition, the relative intensities at each mass correspond well to the mass spectrometer ionizer fragmentation pattern of iodoethanol in agreement with this conclusion. Both water and ethylene desorb simultaneously in a peak centered low-temperature state increases slightly with at ~30 K. This will be discussed in greater detail increasing exposure so that at low exposures below. The water desorption peak is slightly (0.5 L) it is centered at ~35 K and shifts monotonically broader than that for ethylene which may reflect to higher values with increasing temperbroader the slower pumping speed for the former molecule. ature to ~60 K for exposures of L and greater. A small amount of iodoethanol (45 amu) desorbs This effect may be due to the influence of some from the adsorbed overlayer at 45 K in contrast co-adsorbed iodoethanol and has been observed to the behavior on Ag(110) where no molecular by others [15]. desorption was detected, except when multilayers The corresponding exposure effect on the ethylene had formed. Essentially no ethanol (31 amu) (7 amu) desorption profile is shown in Fig. 3 desorption is detected since the 05 K peak in the where again the 05 K feature that appears at spectrum collected at this mass is due to iodoethanol larger exposures is due to multilayer iodoethanol fragmentation. Two 9 amu states are detected desorption. Two peaks are seen at low exposures

4 84 G. Wu et al. / Surface Science 463 (000) 81 9 Fig.. 9 amu (acetaldehyde) temperature-programmed desorption spectra collected as a function of iodoethanol expo- sure at 80 K. The exposures (in Langmuirs) are marked adjacent to the corresponding spectra. Fig amu (ethylene) temperature-programmed desorption spectra collected as a function of iodoethanol exposure at 80 K. The exposures (in Langmuirs) are marked adjacent to the corre- sponding spectra. (0.5 L) at 05 and 5 K. The high-temperature peak of a similar two-peaked spectrum on Ag( 110) function of iodoethanol exposure in Fig. 5. In this was ascribed to the effect of surface defects [8]. case, desorption yields are calculated from the The ethylene desorption yield and peak temper- integrated intensity under the desorption profile. ature increase with increasing exposure with the This yield is converted to relative molar quantities peak temperature varying from 05 K at low expo- by measuring the intensity at each of the masses sures (0.5 L) to 30 K at high (3 L). detected in the desorption spectra (9 amu for The desorption spectra for iodoethanol acetaldehyde, 7 amu for ethylene and 18 amu for (45 amu) are displayed as a function of exposure water) for known pressures of each of these gases. in Fig. 4 where the growth of the multilayer These pressures were corrected for ionization (05 K) feature is clearly evident. A peak also gauge sensitivities. It is clear from these data that appears at 45 K due to the desorption of iodo- essentially equimolar amounts of ethylene, water ethanol adsorbed on the (111) surface. A Redhead and acetaldehyde are evolved for all iodoethanol analysis of the desorption state [16], assuming a exposures. Similar results were obtained for iodoethanol pre-exponential factor of s 1, yields a adsorbed on Ag( 110) where equimolar desorption activation energy for this state of ~63 amounts of water and ethylene desorbed in both kj mol 1. the 63 and the 340 K desorption states [8]. The relative molar desorption yields for ethy- However, in contrast to the results obtained here lene, water and acetaldehyde are plotted as a on Ag( 111), approximately mol of acetaldehyde

5 G. Wu et al. / Surface Science 463 (000) Fig amu (iodoethanol ) temperature-programmed desorption spectra collected as a function of iodoethanol exposure at 80 K. The exposures (in Langmuirs) are marked adjacent to the corresponding spectra. Fig. 5. Plot of the relative molar desorption yield of water, ethylene and acetaldehyde in temperature-programmed desorption as a function of iodoethanol exposure. is dosed at 185 K but the yield increase substantially when the sample is dosed at 15 K. Note that multilayer desorption is completely suppressed by dosing the sample at 185 K even at higher iodoethanol exposures. were formed for each mole of water or ethylene on Ag(110). Temperature-desorption spectra were also collected as a function of dosing temperature. In this The RAIRS spectra associated with these experiment, the sample was held at various temperatures desorption data are displayed in Figs. 7 9 which (80, 185 and 15 K ) and dosed with 1.5 L display the spectral region between 700 and of iodoethanol. The sample was allowed to cool 000 cm 1. The low sensitivity of the mercury to 80 K following which the temperature-pro- cadmium telluride detector precluded collecting grammed desorption spectrum was collected. This meaningful data at higher frequencies. These data procedure was adopted to ensure that each of the were collected by dosing iodoethanol (3 L) at desorption spectra were collected under identical various temperatures (80 K, Fig. 7; 185 K, Fig. 8; conditions. The results for acetaldehyde (9 amu) 15 K, Fig. 9). The temperature evolution was desorption are displayed in Fig. 6. The desorption followed by annealing to the temperature indicated yield from the high-temperature state is only mar- adjacent to each spectrum. The sample was then ginally affected by varying the dosing temperature, allowed to cool to the dosing temperature and the the most marked effect being a broadening and spectrum collected. The spectra obtained by dosing shift to high temperature when iodoethanol is at 80 K are shown in Fig. 7. Clearly evident after dosed at 15 K. Similarly, the low-temperature dosing at 80 K and also after annealing to 185 K (50 K) state changes slightly when iodoethanol are a relatively large number of peaks, the majority

6 86 G. Wu et al. / Surface Science 463 (000) 81 9 Fig amu (acetaldehyde) temperature-programmed desorption spectra collected following a 1.5 L exposure of iodoethanol on Ag(111) as a function of dosing temperature. The dosing temperatures are marked adjacent to the corresponding spectrum. Fig. 7. RAIRS spectra of iodoethanol (3 L) adsorbed on Ag(111) at 80 K between 700 and 000 cm 1 as a function of annealing temperature. Annealing temperatures are marked adjacent to the corresponding spectra. desorb ( Fig. 1). Heating to 40 K essentially completes of which disappear on annealing to 15 K. This ethylene and water desorption ( Fig. 1) and temperature corresponds to multilayer iodoethanol the infrared spectrum consists of two rather intense desorption ( Figs. 1 4) suggesting that these features features at 1069 and 97 cm 1 with weaker peaks are predominantly due to condensed iodoe- at ~1164 and 795 cm 1. Further heating to 75 K thanol. This conclusion is confirmed by the data causes both the 1069 and 97 cm 1 modes to in Table 1 which compares the observed frequen- decrease substantially in intensity while acetaldehyde cies with those for liquid [18] and matrix-isolated desorbs from the surface (Figs. 1 and ). The [17] iodoethanol. The frequencies are also com- relationship between these infrared features and pared with those for multilayers of iodoethanol the desorption of acetaldehyde is further confirmed condensed on Ag( 110) [9] collected using electron by heating to 345 K since all infrared spectral energy loss spectroscopy, where the agreement features are absent (Fig. 7) and acetaldehyde between the two sets of data is good. desorption is complete (Figs. 1 and ). The detection Heating to 15 K, as noted above, removes the of decomposition products (ethylene and iodoethanol multilayer, and leads to a rather weak water) at 15 K ( Fig. 1) suggests that a portion of spectrum consisting of a peak at 1070 cm 1 and the adsorbed iodoethanol has reacted on the sur- broad, rather low-intensity features centered at face by this temperature. This reaction is clearly ~975 and 1175 cm 1 ( Fig. 7). Simultaneously at not complete since some iodoethanol still desorbs this temperature, ethylene and water start to above this temperature (Fig. 1). It is suggested

7 G. Wu et al. / Surface Science 463 (000) Fig. 8. RAIRS spectra of iodoethanol (3 L) adsorbed on Ag(111) at 185 K between 700 and 000 cm 1 as a function of annealing temperature. Annealing temperatures are marked adjacent to the corresponding spectra. Fig. 9. RAIRS spectra of iodoethanol (3 L) adsorbed on Ag(111) at 15 K between 700 and 000 cm 1 as a function of annealing temperature. Annealing temperatures are marked adjacent to the corresponding spectra. that this intermediate species yields the relatively weak infrared spectrum found after annealing to heated to the same temperature ( Fig. 7) with a 15 K shown in Fig. 7 consisting of features at peak evident at 795 cm 1 which is detected as ~975, 1070 and 1175 cm 1. a weak feature in the spectra of Fig. 7 collected The yield of acetaldehyde changes with dosing after annealing to 40 K. These spectral changes temperature ( Fig. 6) and the corresponding infra- further suggest that different surface species are red spectra are displayed in Figs. 8 and 9 for formed at 15 and 40 K. These are again com- samples dosed at 185 and 15 K respectively. The pletely removed by heating to 340 K confirming RAIRS spectrum formed by dosing at 185 K that the surface species that give rise to these ( Fig. 8) shows a broad feature at ~150 cm 1, a modes thermally decompose to form acetaldehyde small peak at ~1175 cm 1, a broad, more intense (Figs. 1 and ). peak at ~1075 cm 1, a small feature at Features are detected at 1164, 1069, 97 and ~975 cm 1 and a weak, rather sharp peak at 80 cm 1 for a sample dosed at 15 K (Fig. 9) ~800 cm 1. These resemble the peaks found after and their evolution with heating agrees well with dosing at 80 K and annealing to 15 K (Fig. 7) that seen for the sample dosed at 185 K (Fig. 9) but are more clearly seen in Fig. 8. Heating the and which disappear as acetaldehyde desorbs sample dosed at 185 K to 40 K (Fig. 8) produces (Figs. 1 and ) the features at 1164, 1069 and 97 cm 1 seen in As noted above, the acetaldehyde desorption the spectrum for the sample dosed at 80 K and yield varies with iodoethanol adsorption temper-

8 88 G. Wu et al. / Surface Science 463 (000) 81 9 Table 1 Assignments of the vibrational spectrum in the region cm 1 of iodoethanol adsorbed on Ag(111) by comparison with liquid and matrix-isolated iodoethanol. Shown also for comparison are the frequencies for iodoethanol adsorbed on Ag(110) using electron energy loss spectroscopya Mode Frequency (cm 1) Multilayer on Ag(111) Gas matrix [17] Liquid [18] Multilayer on Ag(110) [9] r(ch ) n(cmc) n(cmo) d(cmoh) d(cmoh) w(ch ) / w,b(ch ) w,b(ch ) a b=bend; n=stretch; r=rock; d=deformation; w=wag. ature ( Fig. 6). This effect is mirrored in the inten- 4. Discussion sity of the 1069 cm 1 RAIRS feature as a function of dosing temperature. The peak absorbance of Iodoethanol adsorbs at 80 K on Ag(111) to this feature, when dosing at 80 K, is after form a condensed layer that desorbs at ~05 K annealing the sample to 40 K ( Fig. 7). When the ( Fig. 1 and Table 1). Dosing at 185 K completely sample is dosed at 185 K and annealed to 40 K, suppresses multilayer condensation and yields this increases to ( Fig. 8), and at a dosing infrared features at ~800, 975, 1075, 1175 and temperature of 15 K, after annealing to 40 K, it 150 cm 1 (seen most clearly in Fig. 8). These has grown further to ( Fig. 9). features can be assigned to the presence of either The intensity of the 97 cm 1 peak compared molecular iodoethanol or an adsorbed hydroxy with that at 1069 cm 1 also varies with dosing ethyl species and the peak positions and assignments temperature so that after dosing at 80 K and are summarized in Table. The frequencies annealing to 40 K, the ratio I(1069)/I(97), agree slightly better with those for -hydroxyethyl where I refers to the peak height of the respective than for iodoethanol, but both sets of frequencies features, is 3. ( Fig. 7), when dosing at 185 K it are sufficiently close that they could equally well is 5.9 (Fig. 8), and increases further to 7.1 when be assigned to either. Certainly, in view of the fact the sample is dosed at 15 K ( Fig. 9). This implies that iodoethanol desorbs from the surface ( Figs. 1 either that the geometry of the species changes and 4), the presence of adsorbed iodoethanol with dosing temperature or that the features at cannot be ruled out. In general, alkyl iodides 1069 and 97 cm 1 correspond to two different adsorbed on silver react rapidly to form adsorbed surface species. The appearance of two different alkyl species and iodine where, for example, this acetaldehyde desorption states ( Figs. 1 and ) is reaction occurs at ~10 K for methyl iodide on in accord with the presence of two distinct surface Ag(111) [19]. Clearly, however, this reaction is species. Note finally, that the 1164 cm 1 feature likely to be inhibited by the presence of a hydroxyl persists relatively unattenuated up to an annealing group. However, the surface species initially reacts temperature of 75 K ( Fig. 8) suggesting that it is to desorb equimolar amounts of water and ethyl- associated with the high-temperature (315 K) acet- ene ( Fig. 5) at ~180 K with a maximum rate at aldehyde desorption state. ~30 K ( Fig. 1). Adsorbed hydroxyl species react

9 G. Wu et al. / Surface Science 463 (000) Table Comparison of the infrared modes for iodoethanol adsorbed on Ag(111) at 185 K (Fig. 8) with the frequencies of iodoethanol and an adsorbed hydroxyethyl species on Ag(110) [9]a Mode Frequency (cm 1) Iodoethanol on Ag(111) at 185 K (Fig. 8) -hydroxyethyl on Ag(110) [9] Liquid iodoethanol r(ch ) n(cmc) n(cmo) d(cmoh) / w(ch )Q a b=bend; n=stretch; r=rock; d=deformation; w=wag. to form water on Ag(111) at 300 K [0,1], sub- with this suggestion. A final possibility, that cannot stantially higher than the water desorption temperature be ruled out, is a reaction between an adsorbed found following iodoethanol adsorption iodoethanol and a hydroxyethyl species which ( Fig. 1). Furthermore, adsorbed ethylene desorbs would yield either ethylene and iodoethoxy or a from Ag(111) at ~145 K [] so that ethylene species with stoichiometry C H O and an adsorbed 4 desorption at 30 K is clearly limited by the rate iodoethyl species. The equimolar amounts of ethyl- that it is formed during reaction. This suggests ene and water formed in temperature-programmed that water formation is not mediated by the surface desorption (Fig. 5) argues against this reaction. but that two alcohol groups react directly to evolve The resulting species with stoichiometry C H O 4 water and ethylene. Elimination of water from desorb as acetaldehyde in two states at 60 and alcohols is well known in organic chemistry to 315 K (Fig. 1). Again, the amount of acetaldehyde form ethers [3]. An analogous dehydration reaction formed compared to ethylene and water ( Fig. 5) for adsorbed molecular iodoethanol would is in accord with this suggestion. yield (bis)-(-iodoethyl ) ether (ICH MCH MOM This proposed reaction pathway is summarized CH MCH I) and water but no ethylene. Neither in Scheme 1. was (bis)-(-iodoethyl )ether detected in temperature-programmed Alternative dehydrogenation pathways of a precur- desorption. In the case of sor with stoichiometry C H O (formed by remov- 5 adsorbed -hydroxyethyl species, an analogous ing iodine from iodoethanol ) would involve elimination of water would yield equimolar cleavage of a CMH or OMH bond. Adsorbed amounts of ethylene and a species of stoichiometry alkyl species do not tend to dehydrogenate on C H O. The formation of equimolar amounts of silver and rather dimerize to yield an alkane [15]. 4 water and ethylene evolved in temperature-programmed In addition, OMH bond cleavage is only induced desorption ( Fig. 5) is certainly in accord on silver by the presence of adsorbed oxygen Scheme. 1.

10 90 G. Wu et al. / Surface Science 463 (000) 81 9 whereas reaction, in this case takes, place on clean Table 3 Ag(111) [10]. In addition, such direct abstraction Relative intensities of the infrared modes measured from the peak heights (I ) taken from the infrared spectra displayed in pathways would be accompanied by hydrogen Fig. 8 for iodoethanol adsorbed on Ag(111) at 185 K desorption. Small amu (H ) signals are detected in temperature-programmed desorption (not Frequency (cm 1) shown) but these can be entirely ascribed to the fragmentation of desorbing water, ethylene and acetaldehyde. Broadly similar reaction products I(75)/I(40) 0.3 ± ± ± ±0. are found on Ag(110) except that ethylene, water and acetaldehyde all desorb simultaneously [8] so that subsequent reaction of the species of stoichiometry I(345)/I(40) C H O appears to be rather rapid on features to become more clearly evident (Fig. 9). 4 Ag(110). Almost equimolar amounts of water and The relative intensity changes of these features ethylene desorb from Ag(110). However, the indeed suggest that more than one type of surface amount of acetaldehyde from Ag(110) is approximately species is present. This is illustrated in Table 3 twice that predicted by Scheme 1. This taken from the data shown in Fig. 8 which displays implies that there may be an additional dehydrogenation the intensities of these features at various annealing pathway available on Ag(110) not seen on temperatures relative to the intensity measured at Ag(111). 40 K. These data were measured by integrating We turn our attention now to the nature of the the area under each peak for several spectra. All species of stoichiometry C H O formed on the of the infrared features decrease to zero intensity 4 surface as a result of the reaction depicted in on heating to 340 K corresponding to the desorption Scheme 1. Temperature-programmed desorption of all acetaldehyde from the surface ( Figs. 1 data suggest that there are two distinct species and ). The 1164 cm 1 peak maintains its intensity formed on the surface that react to form acetaldehyde on heating to 75 K suggesting that it is associated at 60 and 315 K (Figs. 1 3). Note that with the high-temperature (315 K ) acetaldehyde acetaldehyde adsorbed on Ag( 111) desorbs at desorption state. Both the 1069 and 795 cm K indicating that these desorption states are states decrease to ~30% of the intensity found at indeed surface-reaction-rate limited [14]. The relative ~40 K implying that they are associated with the proportion of these two states and the total same surface species and the decrease in intensity yield depend on the temperature at which iodoethanol of the 97 cm 1 mode is intermediate between adsorbs ( Fig. 6). The intensity of the these two values and suggests that it is coinciden cm 1 vibrational mode also increases as a tally common to both species. This conclusion is function of dosing temperature ( Figs. 7 9) corre- in accord with the variable relative intensity of sponding to the increase in acetaldehyde desorption this feature compared with the 1069 cm 1 peak as yield. When the sample is dosed at 80 K, the a function of dosing temperature ( Figs. 7 9). RAIRS spectrum found on heating to ~40 K These results imply the presence of one surface exhibits sharp features at 97, 1069 and species having infrared frequencies of 1069, cm 1 ( Fig. 7). Dosing the sample at 185 K and 795 cm 1 which is associated with the lowand annealing to ~40 K increases the intensity temperature ( 60 K) desorption state ( Fig. ). The of the 1069 cm 1 peak, decreases the relative intensity other species is characterized by frequencies of 97 of the 97 cm 1 feature, and an additional and 1164 cm 1 and decomposes at 315 K to yield peak becomes evident at 795 cm 1 ( Fig. 8) which acetaldehyde. is weakly visible in the data of Fig. 7. Adsorbing We turn our attention initially to the majority at 14 K and annealing to ~40 K causes the species exhibiting infrared frequencies at 795, 97 intensity of the 1069 cm 1 peak to increase further, the relative intensity of the 97 cm 1 feature to diminish somewhat and the 1164 and ~795 cm 1 and 1069 cm 1 which desorbs as acetaldehyde at 60 K. Frequencies close to these for iodoethanol and hydroxyethyl on Ag(110) [ 9] are assigned to

11 G. Wu et al. / Surface Science 463 (000) Table 4 the temperature-programmed desorption experi- Comparison of the frequencies assigned to a metallacycle on ments, an alternative possibility is that it is due to Ag(110) measured using high-resolution electron energy loss with those formed by adsorbing iodoethane on Ag(111) and adifferent species of formula CH CH OH(ads), x y heating to 40 K measured using infrared spectroscopya where x+y=3. A similar intermediate has been proposed on Ag(110) as an enol (with x= and Assignment Frequency (cm 1) y=1) [9]. This would presumably be produced from the CH CH O species formed in Scheme 1 Oxametallacycle/ Species formed Ag(110) [9] from iodoethanol on by a hydrogen transfer to the oxygen from the Ag(111) (this work) CH adjacent to the oxygen. This would form an OH species but leave a carbon carbon double n (OCC ) s bond. This is likely to be only weakly adsorbed n (OCC) a n (OCC) on silver. In addition, the presence of a mode for a tw(ch ) 118 this species at 97 cm 1 suggest that it contains a w(ch ) 173 carbon carbon single bond. Another possibility is CH sciss 1446 that hydrogen is transferred from the CH species more remote from the oxygen to yield adsorbed a sciss=scissor; n=stretch; tw=twist; d=deformation; w= wag. -hydroxyethylidene (HOMCH MCHN(ads)). Alkylidenes dimerize on Ag(111) to form alkenes although no 1,4-dihydroxybutene (HOCH - CH rocking modes, and CMC and CMO stretching modes respectively ( Table ) although the sur- grammed desorption. However, such dimerization CHCHCH OH) is detected in temperature-pro- face species on Ag( 111) apparently contains no reactions are more rapid for alkyl species but no OH groups. The most likely candidate is a surface analogous dimerization reactions are found for oxametallacycle and the experimental frequencies adsorbed hydroxyethyl species so that the presence are compared with those assigned to an oxametal- of a hydroxyl group apparently inhibits this reaction lacycle on Ag(110) [9] in Table 4, where the higher pathway. Since the 1164 cm 1 mode persists frequency CH modes are not detected since they on annealing to 75 K (Table 3), this species is are relatively weak. The agreement between the proposed to react forming acetaldehyde at 315 K two sets of data is reasonably good. The major in temperature-programmed desorption. The relatively discrepancy is that the 996 cm 1 peak is the most low desorption yield from this state ( Figs. 1 intense in the electron energy loss data for Ag(110) and ) implies that it is present on the surface as [9] while the 1069 cm 1 peak is the most intense a minority species. in the infrared spectra for Ag(111). There may be There are, in principle, a number of possible two possible causes for these intensity differences. decomposition pathways for an oxametallacycle. First, the HREELS data may also contain contri- It can undergo a reductive elimination reaction to butions from impact scattering and second, the yield ethylene oxide, undergo hydrogen transfer to oxametallacycle structure on Ag(111) may be yield acetaldehyde, or can decompose to form different to that on Ag(110). ethylene and adsorbed oxygen or formaldehyde There are two possible explanations for the and adsorbed methylene species [ 4]. The temperature-programmed second, minority species. The first is that this is desorption data indicate that also due to the presence of an oxametallacycle the hydrogen transfer reaction to form acetaldehyde which adopts a different orientation as a function is the favored decomposition pathway. of temperature and coverage. However, the pres- However, ethylene oxidation takes place on a silver ence of a mode at 1164 cm 1 for the second species surface that includes both adsorbed and subsurface implies that this has a COH functionality since oxygen so that this may affect the reactivity of the this frequency is characteristic of a CMOH deformation oxametallacycle. Note, however, that acetaldehyde mode (see Tables 1 and ). Since there is is significantly more thermodynamically stable no evidence of any CMC bond cleavage in any of than ethylene oxide (DH0(ethylene oxide(g))= f

12 9 G. Wu et al. / Surface Science 463 (000) kj mol 1; DH0(acetaldehyde(g))= kj [] D.J. Hucknell, Selective Oxidation of Hydrocarbons, Aca- f demic Press, New York, Chapter. mol 1 [5,6]) indicating that the thermal decom- [3] R.A. van Santen, C.P.M. de Groot, J. Catal. 98 (1986) 530. position of a surface species of stoichiometry [4] C.T. Campbell, M.T. Paffett, Surf. Sci. 139 (1984) 396. C H O is likely thermodynamically to favor the 4 [5] R.B. Grant, R.M. Lambert, J. Catal. 9 (1985) 364. formation of acetaldehyde. [6] V.I. Bukhtiyarov, A.I. Boronin, V.I. Savchenko, J. Catal. 150 (1994) 6. [7] V.I. Bukhtiyarov, A.I. Boronin, I.P. Prosvirin, V.I. 5. Conclusions Savchenko, J. Catal. 150 (1994) 68. [8] G.S. Jones, M.A. Barteau, J. Vac. Sci. Technol. A 15 (1997) Iodoethanol adsorbs on Ag(111) at 80 K and [9] G.S. Jones, M. Mavrikakis, M.A. Barteau, J.M. Vohs, J. Am. Chem. Soc. 10 (1998) is proposed to react at relatively low temperatures [10] Q. Dai, A.J. Gellman, Surf. Sci. 37 (1991) 103. to form -hydroxyethyl species and to deposit [11] G. Wu, M. Kaltchev, W.T. Tysoe, Surf. Rev. Lett. 6 iodine onto the surface. These react to eliminate (1999) 13. water and simultaneously desorb ethylene to leave [1] J. Pawela-Crew, R.J. Madix, N. Vasquez, Surf. Sci. 340 (1995) 119. a surface fragment of stoichiometry C H O. 4 [13] W.S. Sim, P. Gardner, D.A. King, J. Am. Chem. Soc. 118 Surface infrared data suggest that two surface (1996) species are formed, the majority species being an [14] G. Wu, D. Stacchiola, M. Collins, M. Kaltchev, W.T. oxametallacycle which thermally decomposes at Tysoe, Surf. Rev. Lett. (in press). ~60 K to desorb acetaldehyde. The other may [15] X.L. Zhou, J.M. White, J. Phys. Chem. 95 (1991) [16] P.A. Redhead, Vacuum 1 (196) 03. be an oxametallacycle adopting a different orienta- [17] E. Wyn-Jones, J. Orville-Thomas, J. Mol. Struct. 1 tion or a -hydroxyethylidene species which also (1967) 79. reacts to desorb acetaldehyde at ~315 K. [18] L. Homamen, Spectrochim. Acta A 39 (1983) 77. [19] X.L. Zhou, F. Solymosi, P.M. Blass, K.C. Cannon, J.M. White, Surf. Sci. 19 (1989) 94. Acknowledgements [0] M.A. Barteau, R.J. Madix, Surf. Sci. 115 (198) 355. [1] E.M. Stuve, R.J. Madix, B.A. Sexton, Surf. Sci. 13 (198) 491. We gratefully acknowledge support of this work [] G. Wu, D. Stacchiola, M. Kaltchev, W.T. Tysoe, in preparation by the Donors of the Petroleum Research Fund, [3] F.A. Carey, R.J. Sundberg, Advanced Organic Chemistry, administered by the American Chemical Society. Plenum, New York, [4] R.M. Lambert, R.M. Ormerod, W.T. Tysoe, Langmuir 10 (1994) 730. [5] K.B. Wiberg, L.S. Crocker, K.M. Morgan, J. Am. Chem. References Soc. 113 (1991) [6] M.W. Chase Jr., NIST-JANAF Themochemical [1] W.M.H Sachtler, C. Backx, R.A. van Santen, Catal. Rev. TablesJ. Phys. Chem. Ref. Data, Monograph 9 (1998) Sci. Eng. 3 (1981)