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Thermal activation and reaction of allyl alcohol on Ni(100)
Surface Science 605 (2011) 1236–1242
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Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
Thermal activation and reaction of allyl alcohol on Ni(100) Qing Zhao a,c,⁎, Rongping Deng b, Francisco Zaera c a b c
Key Laboratory of Cluster Science, Ministry of Education of China, Department of Physics, School of Science, Beijing Institute of Technology, Beijing 100081, PR China Science Division, Beloit College, Beloit, WI 53511, USA Department of Chemistry, University of California Riverside, Riverside, CA 92521, USA
a r t i c l e
i n f o
Article history: Received 25 January 2011 Accepted 4 April 2011 Available online 13 April 2011 Keywords: Allyl alcohol Nickel single crystal surface Thermal activation Surface reaction Temperature programmed desorption X-ray photoelectron spectroscopy
a b s t r a c t The thermal chemistry of allyl alcohol (CH2CHCH2OH) on a Ni(100) single-crystal surface was studied by the temperature programmed desorption (TPD) and the X-ray photoelectron spectroscopy (XPS). The allyl alcohol adsorbs molecularly on the metal surface at 100 K. Intact molecular desorption from the surface occurs at temperatures around 180 K, but some molecules exhibit chemical reactivity on the surface: activation of the O\H, C_C, and C\O bonds produces η1(O)-allyloxy CH2_CHCH2O(a), η2(C, C) allyl alcohol (C(a)H2C(a)HCH2OH), and η3(C, C, O)-alkoxide (C(a)H2\C(a)CH2 O(a)) intermediates. Further thermal activation of allyl alcohol on the surface yields propylene (CH2CHCH3), 1-propanol (CH3CH2CH2OH), propanal (CH3CH2CHO), and combustion and dehydrogenation products (H2O, H2, and CO). Propylene desorbs from the surface at temperatures of around 270 K. Hydrogenation to the η3(C, C, O)-alkoxide intermediate leads to the production of propanal which desorbs from the surface around 320 K, while hydrogenation of the η2(C, C) allyl alcohol intermediate produces 1-propanol, which desorbs at around 310 K. The co-adsorption of hydrogen atoms on the surface enhances the formation of the saturated alcohol, while co-adsorption of oxygen enhances the formation of both the saturated alcohol and the saturated aldehydes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Both unsaturated alcohols and unsaturated aldehydes contain more than one functional group. The selective hydrogenation and oxidation of these unsaturated molecules has important applications in chemical processes, and therefore has been a central interest in the surface chemistry. Many surface-science techniques have been employed to investigate the reactions of these molecules on single-crystal surfaces of, for instance, Ag(110) [1], Ag(111) [2], Pt(111) [3–6], Rh(111) [7], Cu(110) [8] and Pd(111) [9,10]. Those studies indicate that these molecules interact with the metal surface through both their oxygen and their C_C bonds. The surface chemistry of these unsaturated species exhibits diverse reaction pathways: dehydration, deoxydation, decarbonylation, hydrogenation, and combustion may take place on surface, to yield hydrocarbons, saturated alcohols, aldehydes, water, CO and hydrogen products. Past studies have shown that alkyl, alkoxide, and oxametallacycle intermediates form during the thermal activation of such molecules. Those intermediates can have different chemical configurations on the surface depending on the nature of the metal. Chemical selectivity is determined by the specific details of the intermediates involved, and thus the reaction pathways of the alcohol reactions and the aldehyde reactions vary from metal to metal. ⁎ Corresponding author at: Key Laboratory of Cluster Science, Ministry of Education of China, Department of Physics, School of Science, Beijing Institute of Technology, Beijing 100081, PR China. Tel./fax: + 86 10 68918710. E-mail address: [email protected] (Q. Zhao). 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.04.008
Allyl alcohol in particular is involved in many chemical processes. The surface chemistry of allyl alcohol has yet not been well studied. There are only a few experimental reports on the reactivity of this unsaturated alcohol, on Cu(110) [8], Ag(110) [1], Rh(111) [7] and Cu2O(100) [11]. Since the O\H bond in alcohols is in general quite weak, their thermal conversion on transition metal surfaces is typically initiated by the scission of that bond, and is then followed by a combination of C\H, C\O, C\C bond scission steps [12–17]. The surface chemistry of allyl alcohol may take this initial reaction pathway by undergoing activation of the O\H bond in the hydroxyl group to form an allyloxy intermediate CH2_CHCH2O(a). However, the C_C bond in the vinyl group makes the molecule more prone to react further compared to the behavior seen with the saturated alcohols. Accordingly, besides the allyloxy intermediate, allylic and oxametallacycle species could form on the surface, and oxidation, decarbonylation, hydrogenation, and isomerization reactions may all occur. It has been suggested that on Cu(110) [8] the partial hydrogenation of the allyloxy intermediate leads to the formation of C(a)H2CH3CH2O(a) and CH3C(a)HCH2O(a) oxametallacycles. On Ag (110), hydrogenation of the carbon double bond in allyl alcohol has not been seen [1], but evidence has been provided that the rehybridization of the C_C bond in the allylloxy intermediate produces an η3(C, C, O)-alkoxide intermediate on Rh(111) [7]. The hydrogenation to the η3(C, C, O)-alkoxide intermediate could produce the saturated alcohol or the saturated aldehyde. The allylic and allyloxy intermediates, which are typical intermediates in the surface reaction of allyl alcohol, are also key
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2. Experimental The experiments were conducted in an ultrahigh vacuum chamber described in detail in previous reports [22–24]. The base pressure of the chamber was maintained under 1 × 1010 Torr. The chamber was equipped with appropriate instrumentation for temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), secondary ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). TPD experiments were carried out by using an Extrel quadrupole mass spectrometer (QMS) capable of detecting signals in a mass range between 1 and 800 amu. Up to 15 individual masses could be detected simultaneously in each TPD experiment thanks to the use of an interfaced personal computer and home-written software. The ionizer of this spectrometer was located inside an enclosed compartment with a 7-mm diameter frontal aperture used for gas sampling. In the experiments reported here, the Ni crystal was placed within 1 mm of the front aperture in order to detect the molecules that desorbs from the front surface of the crystal. A linear heating rate of 10 K/s was used in all experiments. The choice of the specific masses to follow the evolution of the different desorbing products and the analysis of those signals to account for the potential cracking pattern overlaps were done following criteria discussed in detail elsewhere [25–28]. Molecules of reagents and potential products were tested individually with the same experimental condition to measure their cracking patterns and the instrumental sensitivities. The XPS spectra were obtained using a 50-mm radius hemispherical electron energy analyzer set at the constant pass energy of 50 eV, which corresponds to a total resolution of approximately 1.2 eV fullwidth-at-half-maximum (FWHM). An aluminum anode was used as the X-ray source. The Cu 2p3/2 at 932.7 eV, Ni 2p3/2 at 852.7, and Ni 3p3/2 at 66.2 eV peaks were used for energy scale calibration [29]. The Ni(100) crystal was cut and polished using standard procedures, and spot-welded to two tantalum rods attached to a manipulator capable of cooling down to liquid nitrogen temperature and of resistively heating to temperatures above 1300 K. Prior to each experiment, cycles of oxygen treatment, ion sputtering, and annealing were performed to clean the crystal surface until no impurities were detected by XPS and until standard CO and H2 TPD could be reproduced. The allyl alcohol (CH2CHCH2OH) was purchased from Aldrich (purity 99%), and subjected to several freeze–pump–thaw cycles before being introduced into the vacuum chamber. Hydrogen (purity 99.99%) and oxygen (purity 99.99%) were purchased from Matheson and used as supplied. The purity of all gases was periodically checked by mass spectrometry. No impurities were detected in the mass spectra. All gas exposures were measured in Langmuir (1 L = 10−6 Torr·s), uncorrected for ion gauge sensitivities.
3. Results 3.1. TPD experiments The reactivity of allyl alcohol was first investigated by TPD. The following set of masses were used to best represent the desorption molecules: m/z = 31, 41, 56, 57, and 58 for 1-propanol, propylene, acrolein, allyl alcohol and propanal, respectively [1,6,8]. Fig. 1 shows hydrogen, water, CO, propylene, 1-propanol, propanal, and allyl alcohol desorption from Ni(100) after the clean surface is dosed with 3.0 L of allyl alcohol at 100 K. The molecular desorption of allyl alcohol is observed with a peak position around 180 K, most likely from the monolayer; the 3.0 L is not sufficient to initiate multilayer condensation. The desorption of other species indicates that some allyl alcohol molecules are thermally activated and undergo surface reactions. H2, CO, and H2O are typical products of alcohol decomposition on metal surfaces, and are seen here as well. The H2 peak is asymmetric with a shoulder extending to 500 K, which indicates that multiple decomposition steps are involved during the conversion process. The decomposition reaction of allyl alcohol also produces water. There are two water desorption peaks, the low-temperature peak one (at 150 K) is clearly due to the background water. It was determined that only the high-temperature peak at 300 K grows with increases in the dose of the allyl alcohol, as shown in Fig. 2. The growth of the water peak and the growth of the hydrogen peak correlate well with the allyl alcohol dose. CO desorption, from alcohol decarbonylation, occurs around 380 K. The removal of the hydroxyl group from alcohols produces water and hydrocarbons. The dehydration reaction of allyl alcohol may produce propadiene (CH2_C_CH2), but the TPD experiment does not show clear evidence for that species. Instead, the removal of the hydroxyl group from allyl alcohol is indicated by the production of propylene, which is observed at 280 K (Fig. 1). The complexity of the cracking patterns of the main desorbing species prevent us from unambiguously identifying smaller hydrocarbons such as ethylene (C2H4) and ethane (C2H6), but a small amount of 1-propanol (CH3CH2CH2OH) desorption is seen at 310 K, from self-hydrogenation of either the adsorbed allyl alcohol or an intermediate species. No observable desorption of propanal (CH3CH2CH_O) could be detected
x2 Partial Pressure (arb. units)
intermediates in propylene oxidation [18] and acrolein hydrogenation [2,5] reactions. Allyl alcohol may in fact be produced in these reactions. Therefore, further study on the surface chemistry of allyl alcohol is also important to better understand the reactions of unsaturated alcohols and many other chemical processes. As an effort to understand the surface chemistry of alcohols on metal surfaces, we have studied the thermal conversion of saturated alcohols on Ni(100) [19–21]. In this study, we investigate the reaction of allyl alcohol on Ni(100). We also study the selectivity in bond activation promoted by surface oxygen and surface hydrogen and the corresponding effects of that selectivity on the relative yields of the different possible reaction products. TPD and XPS experiments were conducted to identify the intermediates and the reaction products. We also discuss the reaction mechanisms of the allyl alcohol on Ni (100), and compare that with similar reactions on other metal surfaces.
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1-Propanol
H2
x0.1
Allyl Alcohol H2O
x2 Propylene x2
CO
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Propanal
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Temperature (K) Fig. 1. TPD spectra from 3.0 L of allyl alcohol adsorbed on clean Ni(100) at 100 K. The spectra show hydrogen (m/z = 2), water (m/z = 18), CO (m/z = 28), allyl alcohol (m/ z = 57), propylene (m/z = 41), 1-propanol (m/z = 31), and propanal (m/z = 58) desorption from the surface.
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Partial Pressure (arb. units)
H2
x0.1
H2O
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3.0 L
2.0 L
2.0 L
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400
1.0 L
1.0 L
0.5 L
0.5 L
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Temperature (K) Fig. 2. TPD spectra for water and hydrogen desorption from 0.5, 1.0, 1.5, 2.0, and 3.0 L of allyl alcohol adsorbed on clean Ni(100) at 100 K.
after a 3.0 L allyl alcohol dose on clean Ni(100), but was seen on surfaces with co-adsorption of oxygen (see below). The TPD spectra of allyl alcohol adsorption on hydrogen pre-dosed Ni(100) show a similar molecular desorption behavior as on the clean Ni(100). Fig. 3 shows a comparison of the reaction products for 3.0 L of allyl alcohol dosed at 100 K on clean Ni(100) versus a surface predosed with 50 L of H2. There is no observable change in the amount of propylene desorption, but the production of 1-propanol increases by about a factor of two, and the production of both water and CO decreases by about 40%. The CO desorption peak also shifts to higher temperatures, to 430 K. Since the reduction in production of water and CO correlates with the increase in 1-propanol production, combustion of the alcohol is likely to be suppressed by the predosed hydrogen on the surface. Also, the molecular desorption peak is more intense in the hydrogen-predosed surface, and there is more hydrogenation to the surface intermediate species to 1-propanol.
H/Ni(100)
Fig. 4 shows hydrogen, water, CO, propylene, 1-propanol, and propanal desorption traces for 3.0 L of allyl alcohol dosed at 100 K on a Ni(100) surface predosed with 1.0 L of O2. The oxygen was dosed at 100 K and then flashed to 300 K to produce atomic oxygen on the surface. More water desorbs from this surface, and at a lower desorption temperature, than from the clean Ni(100). The H2 desorption peak shifts to 340 K, a temperature about 40 K higher than that on the sample without oxygen co-adsorption. Water formation is complete by temperatures of about 300 K. CO desorption increases by about 30%, with its peak shifting to 360 K, 20 K lower than that of the sample without oxygen co-adsorption. Propylene formation is also detected, evidence for a C\O bond scission reaction. Additional CO formation may be accounted for by reaction of the hydrocarbons with the surface oxygen atoms. Although propylene desorption is similar on clean and oxygenpredosed surfaces, oxygen co-adsorption significantly enhances the formation of propanal and 1-propanol. Indeed, the desorption of 1propanol increases by about 170%, and propanal is not even observed on the clean or hydrogen pre-dosed surfaces but it is clearly seen after oxygen predosing. In that case it desorbs at 320 K, a temperature similar to that for 1-propanol. However, although both propanal and 1-propanol desorption peaks occur at a similar temperature, their intensities are not correlated, indicating that they may be produced by different pathways. We did not observe the formation of acrolein (CH2_CHCH_O) in any of the experiments carried out in this study, with clean, hydrogen pre-dosed, or oxygen pre-dosed surfaces. 3.2. XPS experiments Fig. 5 shows O 1s XPS data for the thermal conversion of 3.0 L allyl alcohol, initially dosed at 100 K, on clean Ni(100). A binding energy of 532.8 eV is seen at 100 K, characteristic of oxygen atoms in the hydroxyl group in alcohol. After annealing at 180 K, the peak shifts to 531.7 eV, indicating the scission of the weak O\H bond in the alcohol and the formation of an alkoxide intermediate on the surface [12–17]. After annealing at 300 K, the O 1s peak shifts to 530.2 eV, a value typical of atomic oxygen on the surface, and its intensity is reduced, likely because of the desorption of water (see above). The evolution of the chemical state of carbon is shown by the C 1s XPS spectra in Fig. 6. The broad peak around 285.0 eV in the 100 K
O/Ni(100)
x3 H2O
Allyl Alcohol H2
Propylene
1-Propanol
x0.1
Partial Pressure (arb. units)
H2O
Partial Pressure (arb. units)
x3
Allyl Alcohol H2
Propylene
x0.1 1-Propanol CO
CO x0.1
100
Propanal
200
300
400
500
100
200
300
400
500
Temperature (K) Fig. 3. Allyl alcohol, propylene, 1-propanol, propanal, water, hydrogen, and CO TPD spectra from 3.0 L of allyl alcohol adsorbed at 100 K on clean (dashed gray lines) and Hpredosed (solid color lines) Ni(100) surfaces. The spectra of water and H2 are scaled down by a factor of 1/10.
x0.1
100
Propanal
200
300
400
500
100
200
300
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500
Temperature (K) Fig. 4. Allyl alcohol, propylene, 1-propanol, propanal, water, hydrogen, and CO TPD spectra from 3.0 L of allyl alcohol adsorbed at 100 K on clean (dash gray lines) and O-predosed (solid color lines) Ni(100) surfaces. The spectra of water and H2 are scaled down by a factor of 1/10.
Q. Zhao et al. / Surface Science 605 (2011) 1236–1242
O 1s
C 1s
532.8
285.5
283.8 100 K
100 K 531.7
140 K
140 K XPS Intensity
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160 K XPS Intensity
160 K 530.2
200 K
282.5 200 K
240 K
240 K
300 K 300 K
524
526
528
530
532
534
536
538
540
542
400 K
Binding Energy (eV) Fig. 5. O 1s XPS spectra from 3.0 L of allyl alcohol adsorbed on clean Ni(100) at 100 K and after annealing at 100, 140, 160, 200, 240, and 300 K.
274 276 278 280 282 284 286 288 290 292 294
Binding Energy (eV) spectrum corresponds to a mixture of the states of C in the vinyl group \CH_CH2 and the C bonded to the oxygen in the hydroxyl group [17,26]; the subtle difference of the carbon states in these groups cannot be resolved due to the low signal-to-noise level and the resolution limits of our instrument. Heating of the adsorbed allyl alcohol to 160 K decreases the 285.5 eV peak intensity, and a new peak at 283.8 eV fully develops by 240 K. The 285.5 eV peak vanishes in the 300 K spectrum, and a lower energy peak at 282.5 eV becomes dominant in the 400 K spectrum. This latter peak has been identified to be due to surface carbon on Ni(100) [17], produced after the full decomposition of the hydrocarbons. The 283.8 eV peak seen in the 160–300 K is associated with some allylic and/or alkoxide intermediates. 4. Discussion Although the signal-to-noise level of the XPS spectra from the submonolayer coverages of the molecules generated in this study is in general low, some features assignable to the molecular conversion are clearly observable. The broad and asymmetric shape of the O 1s peaks in the XPS spectra in Fig. 5 indicates that there is more than one oxygen chemical state on the surface. The peak seen at 100 K could be fit with two components, a dominant peak at 532.8 eV and a minor peak at 531.7 eV. The 532.8 eV peak is characteristic of alcohols in there molecular state, whereas the 531.7 eV peak we have identified in early reports as associated with the oxygen atom in an alkoxide species [19,20]. The peak intensity of this latter feature grows while the 532.8 eV peak decreases as a function of temperature, as seen in particular in the 140 and 160 K spectra. The 531.7 eV component does in fact become the dominant one in the 160 K spectrum. It indicates the scission of the O\H bond in the original molecule and of bonding of the remaining moiety, the allyloxy intermediate CH2_CHCH2O(a),
Fig. 6. C 1s XPS spectra from 3.0 L of allyl alcohol adsorbed on clean Ni(100) at 100 K and after annealing at 100, 140, 160, 200, 240, and 300 K.
to the surface through its oxygen atom. This is consistent with the reported formation of a similar allyloxy intermediate by conversion of allyl alcohol on Rh(111) [7] and Cu(100) [8]. A third O 1s XPS peak shows up at 530.2 eV in the 200 K spectrum. This peak, which becomes dominant in the 240 and 300 K spectra, is centered at a binding energy between those of CO, reported at 531.1 eV on Ni(111) [30], and nickel oxide, at 529.5 eV [23,31–34]. The peak in our spectra could therefore originate from a mixture of those two chemical states. The C 1s XPS spectrum obtained after adsorption at 100 K is dominated by a 285.3 eV feature. This is a value previously reported for hydroxyl groups in alcohol adsorbed on Ni(100) [35], and also close to those for the carbon atoms in the C_C bond of acrolein on Ag (111) [2]. Accordingly, the species on the surface at this stage can be speculated to be mainly the molecular allyl alcohol, possibly in conjunction with some of the allyloxy intermediate, CH2_CHCH2O(a). A change in the C 1s binding energy is seen in the temperature range from 160 to 300 K, indicating the formation of certain new intermediates on the surface. Since by 240 K the 285.5 eV peak becomes almost insignificant and the 283.8 eV feature becomes dominant, we assume that the C_C bond may possibly undergo rehybridization around those temperatures. Combining the information from the O 1s and C 1s binding energy changes seen here, we conclude that the newly formed intermediates may bind to multiple metal sites, through both the oxygen and the carbon atoms in the hydroxyl and the vinyl functional groups, respectively. There have been extensive studies on the rehybridization of the C_C bond in unsaturated hydrocarbons on metal surfaces. The rehybridization of the C_C double bond in olefin occurs readily on transition metal surfaces upon adsorption of the olefin molecule
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below the room temperature. For example, the rehybridization of the C_C bond in ethylene leads to the formation of a di-σ bonded species on Pt(111) [36,37], Ni(111) [38], Ni(100) [39,45] and other metal surfaces [40]. The same occurs with propylene on Pt(111) [41] and Ru(0001) [42], and with methylacetylene and 2-butenes on Pt(111) [43]. On the other hand, an early TPD and RAIRS study has shown that C3 allylic intermediates derived from allyl iodine and allyl bromide on Pt(111) have planar structures and bind to the surface in a η3 configuration [44]. On Ni(100), the C 1s binding energy of a di-σ bonding configuration for olefins has been determined to be around 283.7 eV [46,47]. Highresolution XPS experiments on the adsorption of ethylene on Ni(100) have shown that, in fact, this feature consists of two components at 283.74 and 284.07 eV [48]. The 283.8 eV peak in our XPS spectra for allyl alcohol conversion in the temperature range 160–300 K is consistent with the same type of π to di-σ rehybridization. The allyl group in the allyl alcohol is similar to that in propylene (CH2_CHCH3), and, therefore, the chemical bonding of the allyl group in the allyl alcohol on metal surfaces may adopt a similar configuration. The adsorption of the propylene on Ni(100) has been determined by high resolution XPS to be molecular, via a π bond of the C_C double bond to the surface, at 105 K [49]. However, by 150 K a rehybridization is seen to a di-σ bonded configuration. The C 1s binding energies of the two carbon atoms are slightly different, about 283.5 and 284.0 eV, in both bonding configurations. An UPS experiment also corroborated that the π-bonded propylene is converted to di-σ bonded propylene at 150 K on Ni(100) [45]. By analogy, we suggest that the intermediate derived from the alkoxide intermediate at temperatures above 150 K may probably adopt a di-σ bonding on the Ni(100) surface. In fact, intermediates with either η2 or η3 bonding configurations may form on the nickel surface, and their hydrogenation lead to the production of the saturated alcohol and saturated aldehydes seen in TPD. The reactivity of the C_C double bond in unsaturated alcohols and unsaturated aldehydes has also been investigated, on the surfaces of Ag(110) [1], Ag(111) [2], Pt(111) [3–5], Pt(100) [5], Pd(111) [9], Ni (111) [6] and Cu(110) [8]. Those studies found that this double bond can be hydrogenated in the presence of coadsorbed hydrogen to form saturated alcohols and saturated aldehydes. Hydrogen coadsorption also affects the molecular geometry of adsorption: A NEXAFS experiment on Ag(111) indicated that the co-adsorbed hydrogen largely reduces the tilted angle of the C_C bond in the adsorbed allyl alcohol [2]. This makes this double bond more kinetically adept to adopt a η2(C, C) configuration, which may be hydrogenated to produce the saturated alcohol more easily. Such argument may explain the enhanced yield for 1-propanol seen in our TPD experiments with hydrogen co-adsorption. In the conversion of saturated alcohols on metal surfaces, their further conversion following the formation of the alkoxide intermediate is largely determined by the regio-selectivity of the next dehydrogenation step [50]. In many cases, β-hydrogen elimination is favored, as, for example, has been indicated in a past study with ethanol on Rh(111). This hydride elimination is a key for the formation of an oxametallacycle intermediate [51,52]. In such cases, hydrogen elimination at the γ carbon seems kinetically less favorable, although that may still occur at higher temperatures [14,19]. It is worth pointing out that the three carbon atoms in the allyloxy intermediate are hydrogen deficient, so hydrogen elimination at these carbons is not kinetically favorable. The formation of cyclic alkoxide intermediates may therefore occur through C_C double bond rehybridization instead. The saturated alcohols and the saturated aldehydes may be produced through these intermediates. The C_O bond in acrolein can be readily hydrogenated to form allyl alcohol on several metal surfaces [2,5,6]. The oxidation of allyl alcohol, on the other hand, produces acrolein, as observed on Ag(110) [1] and Cu(110) [8]. Our TPD experiments on the conversion of allyl alcohol on Ni(100) do not show any acrolein desorption from the
surface. In an additional reference TPD experiment with acrolein, we found that the molecular desorption of acrolein occurs below 200 K on Ni(100), and that its decomposition starts above 200 K (data not shown). Therefore, since decomposition dominates at low coverages, even if acrolein forms from oxidation of allyl alcohol on Ni(100), that intermediate could decompose upon formation. There is also no evidence for the production of allyl alcohol back from conversion of any of the surface intermediates; the TPD data for allyl alcohol from our experiments show only a single peak at 185 K, from desorption of the intact allyl alcohol. The oxidation of allyl alcohol can be enhanced by co-adsorbing surface oxygen, as also previously demonstrated in experiments with allyl alcohol on Ag(110) [1] and on Cu(110) [8]. The surface oxygen is expected to promote hydrogen extraction from the allyl alcohol, typically from its hydroxyl group, to produce more alkoxide intermediates. As the result of this hydrogen extraction and a subsequent rehybridization of the C_C double bond in the vinyl group, a η3(C, C, O)-alkoxide intermediate may form on surface. The saturated alcohols and the saturated aldehydes, the 1-propanol and propanal detected in our TPD data (Fig. 4), can then be produced via hydrogenation of those. The activation of the C\O bond in the allyl alcohol adsorbed on Ni (100) occurs at 160 K, as indicated by the reduced C 1s XPS intensity at 285.3 eV in Fig. 6. The deoxygenation reaction may produce an allylic intermediate, and propylene may form this way. Propylene desorbs at temperatures above 200 K, with a peak temperature at 265 K, as we observed in our TPD experiments (Fig. 1). The reactivity of allylic intermediates has been studied with the TPD and vibrational spectroscopy on a few metal surfaces. It was reported that the activation of allyl bromide on Cu(100) produces an η3-allyl intermediate, which is capable of C\C coupling to form 1,5-hexadiene [53]. The allylic intermediate derived from allyl chloride on Ag(110) shows a similar coupling reaction [54], but this coupling reaction is not favored on other metal surfaces. The allylic intermediates derived from allyl chloride on Ni(100) [55] and from allyl bromide on Al(100) [56] and Pt(111) [44,57] display two configurations, with η1-allyl and η3-allyl coordination to the surface. The η1-allyl intermediate has a tilted geometry and undergoes hydrogenation to propylene, whereas the flat η3-allyl intermediate mostly decomposes to smaller hydrocarbons. In the conversion of allyl alcohol on Ni(100), the C\O bond scission in the η1-allyloxy likely produces an η1-allyl intermediate, which is then readily hydrogenated to propylene. Any η3-allyl intermediate is likely to decompose if it forms on surface. Although the activation of the C\O bond can easily occur on early transition metal surfaces [12,58], the activation is kinetically less favored on later transition metal surfaces. On the other hand, that C\O scission can be promoted by co-absorbing oxygen atoms. For instance, C\O bond scission in tert-butanol has been shown to be promoted by coadsorbed oxygen on Rh(111) [14]. However, our TPD data (Fig. 5) do not show any obvious increase in the propylene yield. It is realized that the surface oxygen not only promotes the C\O bond scission in alcohols, it also promotes its oxidation, as indicated by the observed CO desorption. The reaction of allyl alcohol on Ni(111) film prepared on Pt(111) was studied with TPD and HREEL [59]. The surface structure of the Ni(111) film is considered to be similar to the Ni(111) single crystal surface. The Ni(111) close-packed surface contains more reaction sites comparing to the Ni(100) surface that we used in our experiment; therefore, molecules are more strongly bonded to the Ni(111) surface. The desorption of allyl alcohol on Ni(100) single crystal surface in our experiment shows a desorption peak at 180 K, while the desorption peak of allyl alcohol was observed at 261 K on Ni(111) film. Both the experiment on Ni(111) film and our experiment on Ni(100) crystal surface show the evidence of 1-propanol production due to the allyl alcohol self-hydrogenation. Due to the stronger bonding on Ni(111) film, the 1-propanol desorption also occurs at higher temperature on the Ni(111) film than on the Ni(100) surface. The stronger surface bonding
Q. Zhao et al. / Surface Science 605 (2011) 1236–1242
Allyloxy
Propylene + H, - O
CH2
CH
CH CH2
O
CH
CH
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CH2 H
Propanal
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CH2
Ally Alcohol
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OH
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CH2
CH2
CH
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240 K η3(C, C, O)-alkoxide
CH2 OH CH2
CH2
+ 2H
CH
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240 K η 2(C, C)- Allyl Alcohol
CH3
OH CH2
1-Propanol
Fig. 7. Proposed reaction pathways for the formation of propylene, propanal, and 1-propanol during the thermal conversion of allyl alcohol adsorbed on a Ni(100) surface.
also enhances the reactivity of chemical decomposition. The C\O scission occurs at lower temperature on the Ni(111) film which leads to the propylene production at around 190 K, while propylene is produced at higher temperature on Ni(100) in our experiment. For the same reason, the productions of large molecules of 1-propanol and propanal may have lower rates on Ni(111) than on Ni(100). Based on the above experimental results and in our analysis and discussion of those data for the activation and reaction of allyl alcohol on Ni(100), we propose that 1-propanol forms in this system through the hydrogenation of a η2(C, C) allyl alcohol intermediate, and that propanal forms through a η3(C, C, O)-alkoxide intermediates instead. Propylene is also produced, via a C\O bond scission in an allyloxy surface intermediate. These reaction pathways are summarized schematically in Fig. 7. 5. Conclusions The thermal chemistry of allyl alcohol on Ni(100) follows a reaction mechanism that starts with the activation of both the O\H bond in the hydroxyl group and the rehybridization of the C_C bond in the allyl moiety. While the activation of the O\H bond to form the allyloxy intermediate is facile for allyl alcohol on the surface, the further activation of the C_C bond through the rehybridization on surface is the key for the production of the saturated alcohol (1-propanol) and the saturated aldehyde (propanal). Co-adsorption of hydrogen in this system enhances the allyl alcohol hydrogenation on surface and increases the yield of 1-propanol. On the other hand, co-adsorption of oxygen atoms enhances the yields of both 1-propanol and propanal. The acrolein formation through the allyl alcohol oxidation on surface was not observed in our experiments, instead, the C_C bond activation is more favored, leading to the 1-propanol and the propanal productions. The activation of the C\O bond in the adsorbed allyl alcohol competes with other reactions, which leads to the propylene production. Acknowledgment This work was funded by a grant from the US National Science Foundation, and by the Ministry of Science and Technology of China
(2009IM033000). Additional support was provided by the National Natural Science Foundation of China (50935001).
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Contents lists available at ScienceDirect
Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
Thermal activation and reaction of allyl alcohol on Ni(100) Qing Zhao a,c,⁎, Rongping Deng b, Francisco Zaera c a b c
Key Laboratory of Cluster Science, Ministry of Education of China, Department of Physics, School of Science, Beijing Institute of Technology, Beijing 100081, PR China Science Division, Beloit College, Beloit, WI 53511, USA Department of Chemistry, University of California Riverside, Riverside, CA 92521, USA
a r t i c l e
i n f o
Article history: Received 25 January 2011 Accepted 4 April 2011 Available online 13 April 2011 Keywords: Allyl alcohol Nickel single crystal surface Thermal activation Surface reaction Temperature programmed desorption X-ray photoelectron spectroscopy
a b s t r a c t The thermal chemistry of allyl alcohol (CH2CHCH2OH) on a Ni(100) single-crystal surface was studied by the temperature programmed desorption (TPD) and the X-ray photoelectron spectroscopy (XPS). The allyl alcohol adsorbs molecularly on the metal surface at 100 K. Intact molecular desorption from the surface occurs at temperatures around 180 K, but some molecules exhibit chemical reactivity on the surface: activation of the O\H, C_C, and C\O bonds produces η1(O)-allyloxy CH2_CHCH2O(a), η2(C, C) allyl alcohol (C(a)H2C(a)HCH2OH), and η3(C, C, O)-alkoxide (C(a)H2\C(a)CH2 O(a)) intermediates. Further thermal activation of allyl alcohol on the surface yields propylene (CH2CHCH3), 1-propanol (CH3CH2CH2OH), propanal (CH3CH2CHO), and combustion and dehydrogenation products (H2O, H2, and CO). Propylene desorbs from the surface at temperatures of around 270 K. Hydrogenation to the η3(C, C, O)-alkoxide intermediate leads to the production of propanal which desorbs from the surface around 320 K, while hydrogenation of the η2(C, C) allyl alcohol intermediate produces 1-propanol, which desorbs at around 310 K. The co-adsorption of hydrogen atoms on the surface enhances the formation of the saturated alcohol, while co-adsorption of oxygen enhances the formation of both the saturated alcohol and the saturated aldehydes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Both unsaturated alcohols and unsaturated aldehydes contain more than one functional group. The selective hydrogenation and oxidation of these unsaturated molecules has important applications in chemical processes, and therefore has been a central interest in the surface chemistry. Many surface-science techniques have been employed to investigate the reactions of these molecules on single-crystal surfaces of, for instance, Ag(110) [1], Ag(111) [2], Pt(111) [3–6], Rh(111) [7], Cu(110) [8] and Pd(111) [9,10]. Those studies indicate that these molecules interact with the metal surface through both their oxygen and their C_C bonds. The surface chemistry of these unsaturated species exhibits diverse reaction pathways: dehydration, deoxydation, decarbonylation, hydrogenation, and combustion may take place on surface, to yield hydrocarbons, saturated alcohols, aldehydes, water, CO and hydrogen products. Past studies have shown that alkyl, alkoxide, and oxametallacycle intermediates form during the thermal activation of such molecules. Those intermediates can have different chemical configurations on the surface depending on the nature of the metal. Chemical selectivity is determined by the specific details of the intermediates involved, and thus the reaction pathways of the alcohol reactions and the aldehyde reactions vary from metal to metal. ⁎ Corresponding author at: Key Laboratory of Cluster Science, Ministry of Education of China, Department of Physics, School of Science, Beijing Institute of Technology, Beijing 100081, PR China. Tel./fax: + 86 10 68918710. E-mail address: [email protected] (Q. Zhao). 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.04.008
Allyl alcohol in particular is involved in many chemical processes. The surface chemistry of allyl alcohol has yet not been well studied. There are only a few experimental reports on the reactivity of this unsaturated alcohol, on Cu(110) [8], Ag(110) [1], Rh(111) [7] and Cu2O(100) [11]. Since the O\H bond in alcohols is in general quite weak, their thermal conversion on transition metal surfaces is typically initiated by the scission of that bond, and is then followed by a combination of C\H, C\O, C\C bond scission steps [12–17]. The surface chemistry of allyl alcohol may take this initial reaction pathway by undergoing activation of the O\H bond in the hydroxyl group to form an allyloxy intermediate CH2_CHCH2O(a). However, the C_C bond in the vinyl group makes the molecule more prone to react further compared to the behavior seen with the saturated alcohols. Accordingly, besides the allyloxy intermediate, allylic and oxametallacycle species could form on the surface, and oxidation, decarbonylation, hydrogenation, and isomerization reactions may all occur. It has been suggested that on Cu(110) [8] the partial hydrogenation of the allyloxy intermediate leads to the formation of C(a)H2CH3CH2O(a) and CH3C(a)HCH2O(a) oxametallacycles. On Ag (110), hydrogenation of the carbon double bond in allyl alcohol has not been seen [1], but evidence has been provided that the rehybridization of the C_C bond in the allylloxy intermediate produces an η3(C, C, O)-alkoxide intermediate on Rh(111) [7]. The hydrogenation to the η3(C, C, O)-alkoxide intermediate could produce the saturated alcohol or the saturated aldehyde. The allylic and allyloxy intermediates, which are typical intermediates in the surface reaction of allyl alcohol, are also key
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2. Experimental The experiments were conducted in an ultrahigh vacuum chamber described in detail in previous reports [22–24]. The base pressure of the chamber was maintained under 1 × 1010 Torr. The chamber was equipped with appropriate instrumentation for temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), secondary ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). TPD experiments were carried out by using an Extrel quadrupole mass spectrometer (QMS) capable of detecting signals in a mass range between 1 and 800 amu. Up to 15 individual masses could be detected simultaneously in each TPD experiment thanks to the use of an interfaced personal computer and home-written software. The ionizer of this spectrometer was located inside an enclosed compartment with a 7-mm diameter frontal aperture used for gas sampling. In the experiments reported here, the Ni crystal was placed within 1 mm of the front aperture in order to detect the molecules that desorbs from the front surface of the crystal. A linear heating rate of 10 K/s was used in all experiments. The choice of the specific masses to follow the evolution of the different desorbing products and the analysis of those signals to account for the potential cracking pattern overlaps were done following criteria discussed in detail elsewhere [25–28]. Molecules of reagents and potential products were tested individually with the same experimental condition to measure their cracking patterns and the instrumental sensitivities. The XPS spectra were obtained using a 50-mm radius hemispherical electron energy analyzer set at the constant pass energy of 50 eV, which corresponds to a total resolution of approximately 1.2 eV fullwidth-at-half-maximum (FWHM). An aluminum anode was used as the X-ray source. The Cu 2p3/2 at 932.7 eV, Ni 2p3/2 at 852.7, and Ni 3p3/2 at 66.2 eV peaks were used for energy scale calibration [29]. The Ni(100) crystal was cut and polished using standard procedures, and spot-welded to two tantalum rods attached to a manipulator capable of cooling down to liquid nitrogen temperature and of resistively heating to temperatures above 1300 K. Prior to each experiment, cycles of oxygen treatment, ion sputtering, and annealing were performed to clean the crystal surface until no impurities were detected by XPS and until standard CO and H2 TPD could be reproduced. The allyl alcohol (CH2CHCH2OH) was purchased from Aldrich (purity 99%), and subjected to several freeze–pump–thaw cycles before being introduced into the vacuum chamber. Hydrogen (purity 99.99%) and oxygen (purity 99.99%) were purchased from Matheson and used as supplied. The purity of all gases was periodically checked by mass spectrometry. No impurities were detected in the mass spectra. All gas exposures were measured in Langmuir (1 L = 10−6 Torr·s), uncorrected for ion gauge sensitivities.
3. Results 3.1. TPD experiments The reactivity of allyl alcohol was first investigated by TPD. The following set of masses were used to best represent the desorption molecules: m/z = 31, 41, 56, 57, and 58 for 1-propanol, propylene, acrolein, allyl alcohol and propanal, respectively [1,6,8]. Fig. 1 shows hydrogen, water, CO, propylene, 1-propanol, propanal, and allyl alcohol desorption from Ni(100) after the clean surface is dosed with 3.0 L of allyl alcohol at 100 K. The molecular desorption of allyl alcohol is observed with a peak position around 180 K, most likely from the monolayer; the 3.0 L is not sufficient to initiate multilayer condensation. The desorption of other species indicates that some allyl alcohol molecules are thermally activated and undergo surface reactions. H2, CO, and H2O are typical products of alcohol decomposition on metal surfaces, and are seen here as well. The H2 peak is asymmetric with a shoulder extending to 500 K, which indicates that multiple decomposition steps are involved during the conversion process. The decomposition reaction of allyl alcohol also produces water. There are two water desorption peaks, the low-temperature peak one (at 150 K) is clearly due to the background water. It was determined that only the high-temperature peak at 300 K grows with increases in the dose of the allyl alcohol, as shown in Fig. 2. The growth of the water peak and the growth of the hydrogen peak correlate well with the allyl alcohol dose. CO desorption, from alcohol decarbonylation, occurs around 380 K. The removal of the hydroxyl group from alcohols produces water and hydrocarbons. The dehydration reaction of allyl alcohol may produce propadiene (CH2_C_CH2), but the TPD experiment does not show clear evidence for that species. Instead, the removal of the hydroxyl group from allyl alcohol is indicated by the production of propylene, which is observed at 280 K (Fig. 1). The complexity of the cracking patterns of the main desorbing species prevent us from unambiguously identifying smaller hydrocarbons such as ethylene (C2H4) and ethane (C2H6), but a small amount of 1-propanol (CH3CH2CH2OH) desorption is seen at 310 K, from self-hydrogenation of either the adsorbed allyl alcohol or an intermediate species. No observable desorption of propanal (CH3CH2CH_O) could be detected
x2 Partial Pressure (arb. units)
intermediates in propylene oxidation [18] and acrolein hydrogenation [2,5] reactions. Allyl alcohol may in fact be produced in these reactions. Therefore, further study on the surface chemistry of allyl alcohol is also important to better understand the reactions of unsaturated alcohols and many other chemical processes. As an effort to understand the surface chemistry of alcohols on metal surfaces, we have studied the thermal conversion of saturated alcohols on Ni(100) [19–21]. In this study, we investigate the reaction of allyl alcohol on Ni(100). We also study the selectivity in bond activation promoted by surface oxygen and surface hydrogen and the corresponding effects of that selectivity on the relative yields of the different possible reaction products. TPD and XPS experiments were conducted to identify the intermediates and the reaction products. We also discuss the reaction mechanisms of the allyl alcohol on Ni (100), and compare that with similar reactions on other metal surfaces.
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Temperature (K) Fig. 1. TPD spectra from 3.0 L of allyl alcohol adsorbed on clean Ni(100) at 100 K. The spectra show hydrogen (m/z = 2), water (m/z = 18), CO (m/z = 28), allyl alcohol (m/ z = 57), propylene (m/z = 41), 1-propanol (m/z = 31), and propanal (m/z = 58) desorption from the surface.
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Temperature (K) Fig. 2. TPD spectra for water and hydrogen desorption from 0.5, 1.0, 1.5, 2.0, and 3.0 L of allyl alcohol adsorbed on clean Ni(100) at 100 K.
after a 3.0 L allyl alcohol dose on clean Ni(100), but was seen on surfaces with co-adsorption of oxygen (see below). The TPD spectra of allyl alcohol adsorption on hydrogen pre-dosed Ni(100) show a similar molecular desorption behavior as on the clean Ni(100). Fig. 3 shows a comparison of the reaction products for 3.0 L of allyl alcohol dosed at 100 K on clean Ni(100) versus a surface predosed with 50 L of H2. There is no observable change in the amount of propylene desorption, but the production of 1-propanol increases by about a factor of two, and the production of both water and CO decreases by about 40%. The CO desorption peak also shifts to higher temperatures, to 430 K. Since the reduction in production of water and CO correlates with the increase in 1-propanol production, combustion of the alcohol is likely to be suppressed by the predosed hydrogen on the surface. Also, the molecular desorption peak is more intense in the hydrogen-predosed surface, and there is more hydrogenation to the surface intermediate species to 1-propanol.
H/Ni(100)
Fig. 4 shows hydrogen, water, CO, propylene, 1-propanol, and propanal desorption traces for 3.0 L of allyl alcohol dosed at 100 K on a Ni(100) surface predosed with 1.0 L of O2. The oxygen was dosed at 100 K and then flashed to 300 K to produce atomic oxygen on the surface. More water desorbs from this surface, and at a lower desorption temperature, than from the clean Ni(100). The H2 desorption peak shifts to 340 K, a temperature about 40 K higher than that on the sample without oxygen co-adsorption. Water formation is complete by temperatures of about 300 K. CO desorption increases by about 30%, with its peak shifting to 360 K, 20 K lower than that of the sample without oxygen co-adsorption. Propylene formation is also detected, evidence for a C\O bond scission reaction. Additional CO formation may be accounted for by reaction of the hydrocarbons with the surface oxygen atoms. Although propylene desorption is similar on clean and oxygenpredosed surfaces, oxygen co-adsorption significantly enhances the formation of propanal and 1-propanol. Indeed, the desorption of 1propanol increases by about 170%, and propanal is not even observed on the clean or hydrogen pre-dosed surfaces but it is clearly seen after oxygen predosing. In that case it desorbs at 320 K, a temperature similar to that for 1-propanol. However, although both propanal and 1-propanol desorption peaks occur at a similar temperature, their intensities are not correlated, indicating that they may be produced by different pathways. We did not observe the formation of acrolein (CH2_CHCH_O) in any of the experiments carried out in this study, with clean, hydrogen pre-dosed, or oxygen pre-dosed surfaces. 3.2. XPS experiments Fig. 5 shows O 1s XPS data for the thermal conversion of 3.0 L allyl alcohol, initially dosed at 100 K, on clean Ni(100). A binding energy of 532.8 eV is seen at 100 K, characteristic of oxygen atoms in the hydroxyl group in alcohol. After annealing at 180 K, the peak shifts to 531.7 eV, indicating the scission of the weak O\H bond in the alcohol and the formation of an alkoxide intermediate on the surface [12–17]. After annealing at 300 K, the O 1s peak shifts to 530.2 eV, a value typical of atomic oxygen on the surface, and its intensity is reduced, likely because of the desorption of water (see above). The evolution of the chemical state of carbon is shown by the C 1s XPS spectra in Fig. 6. The broad peak around 285.0 eV in the 100 K
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Temperature (K) Fig. 3. Allyl alcohol, propylene, 1-propanol, propanal, water, hydrogen, and CO TPD spectra from 3.0 L of allyl alcohol adsorbed at 100 K on clean (dashed gray lines) and Hpredosed (solid color lines) Ni(100) surfaces. The spectra of water and H2 are scaled down by a factor of 1/10.
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Temperature (K) Fig. 4. Allyl alcohol, propylene, 1-propanol, propanal, water, hydrogen, and CO TPD spectra from 3.0 L of allyl alcohol adsorbed at 100 K on clean (dash gray lines) and O-predosed (solid color lines) Ni(100) surfaces. The spectra of water and H2 are scaled down by a factor of 1/10.
Q. Zhao et al. / Surface Science 605 (2011) 1236–1242
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Binding Energy (eV) Fig. 5. O 1s XPS spectra from 3.0 L of allyl alcohol adsorbed on clean Ni(100) at 100 K and after annealing at 100, 140, 160, 200, 240, and 300 K.
274 276 278 280 282 284 286 288 290 292 294
Binding Energy (eV) spectrum corresponds to a mixture of the states of C in the vinyl group \CH_CH2 and the C bonded to the oxygen in the hydroxyl group [17,26]; the subtle difference of the carbon states in these groups cannot be resolved due to the low signal-to-noise level and the resolution limits of our instrument. Heating of the adsorbed allyl alcohol to 160 K decreases the 285.5 eV peak intensity, and a new peak at 283.8 eV fully develops by 240 K. The 285.5 eV peak vanishes in the 300 K spectrum, and a lower energy peak at 282.5 eV becomes dominant in the 400 K spectrum. This latter peak has been identified to be due to surface carbon on Ni(100) [17], produced after the full decomposition of the hydrocarbons. The 283.8 eV peak seen in the 160–300 K is associated with some allylic and/or alkoxide intermediates. 4. Discussion Although the signal-to-noise level of the XPS spectra from the submonolayer coverages of the molecules generated in this study is in general low, some features assignable to the molecular conversion are clearly observable. The broad and asymmetric shape of the O 1s peaks in the XPS spectra in Fig. 5 indicates that there is more than one oxygen chemical state on the surface. The peak seen at 100 K could be fit with two components, a dominant peak at 532.8 eV and a minor peak at 531.7 eV. The 532.8 eV peak is characteristic of alcohols in there molecular state, whereas the 531.7 eV peak we have identified in early reports as associated with the oxygen atom in an alkoxide species [19,20]. The peak intensity of this latter feature grows while the 532.8 eV peak decreases as a function of temperature, as seen in particular in the 140 and 160 K spectra. The 531.7 eV component does in fact become the dominant one in the 160 K spectrum. It indicates the scission of the O\H bond in the original molecule and of bonding of the remaining moiety, the allyloxy intermediate CH2_CHCH2O(a),
Fig. 6. C 1s XPS spectra from 3.0 L of allyl alcohol adsorbed on clean Ni(100) at 100 K and after annealing at 100, 140, 160, 200, 240, and 300 K.
to the surface through its oxygen atom. This is consistent with the reported formation of a similar allyloxy intermediate by conversion of allyl alcohol on Rh(111) [7] and Cu(100) [8]. A third O 1s XPS peak shows up at 530.2 eV in the 200 K spectrum. This peak, which becomes dominant in the 240 and 300 K spectra, is centered at a binding energy between those of CO, reported at 531.1 eV on Ni(111) [30], and nickel oxide, at 529.5 eV [23,31–34]. The peak in our spectra could therefore originate from a mixture of those two chemical states. The C 1s XPS spectrum obtained after adsorption at 100 K is dominated by a 285.3 eV feature. This is a value previously reported for hydroxyl groups in alcohol adsorbed on Ni(100) [35], and also close to those for the carbon atoms in the C_C bond of acrolein on Ag (111) [2]. Accordingly, the species on the surface at this stage can be speculated to be mainly the molecular allyl alcohol, possibly in conjunction with some of the allyloxy intermediate, CH2_CHCH2O(a). A change in the C 1s binding energy is seen in the temperature range from 160 to 300 K, indicating the formation of certain new intermediates on the surface. Since by 240 K the 285.5 eV peak becomes almost insignificant and the 283.8 eV feature becomes dominant, we assume that the C_C bond may possibly undergo rehybridization around those temperatures. Combining the information from the O 1s and C 1s binding energy changes seen here, we conclude that the newly formed intermediates may bind to multiple metal sites, through both the oxygen and the carbon atoms in the hydroxyl and the vinyl functional groups, respectively. There have been extensive studies on the rehybridization of the C_C bond in unsaturated hydrocarbons on metal surfaces. The rehybridization of the C_C double bond in olefin occurs readily on transition metal surfaces upon adsorption of the olefin molecule
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below the room temperature. For example, the rehybridization of the C_C bond in ethylene leads to the formation of a di-σ bonded species on Pt(111) [36,37], Ni(111) [38], Ni(100) [39,45] and other metal surfaces [40]. The same occurs with propylene on Pt(111) [41] and Ru(0001) [42], and with methylacetylene and 2-butenes on Pt(111) [43]. On the other hand, an early TPD and RAIRS study has shown that C3 allylic intermediates derived from allyl iodine and allyl bromide on Pt(111) have planar structures and bind to the surface in a η3 configuration [44]. On Ni(100), the C 1s binding energy of a di-σ bonding configuration for olefins has been determined to be around 283.7 eV [46,47]. Highresolution XPS experiments on the adsorption of ethylene on Ni(100) have shown that, in fact, this feature consists of two components at 283.74 and 284.07 eV [48]. The 283.8 eV peak in our XPS spectra for allyl alcohol conversion in the temperature range 160–300 K is consistent with the same type of π to di-σ rehybridization. The allyl group in the allyl alcohol is similar to that in propylene (CH2_CHCH3), and, therefore, the chemical bonding of the allyl group in the allyl alcohol on metal surfaces may adopt a similar configuration. The adsorption of the propylene on Ni(100) has been determined by high resolution XPS to be molecular, via a π bond of the C_C double bond to the surface, at 105 K [49]. However, by 150 K a rehybridization is seen to a di-σ bonded configuration. The C 1s binding energies of the two carbon atoms are slightly different, about 283.5 and 284.0 eV, in both bonding configurations. An UPS experiment also corroborated that the π-bonded propylene is converted to di-σ bonded propylene at 150 K on Ni(100) [45]. By analogy, we suggest that the intermediate derived from the alkoxide intermediate at temperatures above 150 K may probably adopt a di-σ bonding on the Ni(100) surface. In fact, intermediates with either η2 or η3 bonding configurations may form on the nickel surface, and their hydrogenation lead to the production of the saturated alcohol and saturated aldehydes seen in TPD. The reactivity of the C_C double bond in unsaturated alcohols and unsaturated aldehydes has also been investigated, on the surfaces of Ag(110) [1], Ag(111) [2], Pt(111) [3–5], Pt(100) [5], Pd(111) [9], Ni (111) [6] and Cu(110) [8]. Those studies found that this double bond can be hydrogenated in the presence of coadsorbed hydrogen to form saturated alcohols and saturated aldehydes. Hydrogen coadsorption also affects the molecular geometry of adsorption: A NEXAFS experiment on Ag(111) indicated that the co-adsorbed hydrogen largely reduces the tilted angle of the C_C bond in the adsorbed allyl alcohol [2]. This makes this double bond more kinetically adept to adopt a η2(C, C) configuration, which may be hydrogenated to produce the saturated alcohol more easily. Such argument may explain the enhanced yield for 1-propanol seen in our TPD experiments with hydrogen co-adsorption. In the conversion of saturated alcohols on metal surfaces, their further conversion following the formation of the alkoxide intermediate is largely determined by the regio-selectivity of the next dehydrogenation step [50]. In many cases, β-hydrogen elimination is favored, as, for example, has been indicated in a past study with ethanol on Rh(111). This hydride elimination is a key for the formation of an oxametallacycle intermediate [51,52]. In such cases, hydrogen elimination at the γ carbon seems kinetically less favorable, although that may still occur at higher temperatures [14,19]. It is worth pointing out that the three carbon atoms in the allyloxy intermediate are hydrogen deficient, so hydrogen elimination at these carbons is not kinetically favorable. The formation of cyclic alkoxide intermediates may therefore occur through C_C double bond rehybridization instead. The saturated alcohols and the saturated aldehydes may be produced through these intermediates. The C_O bond in acrolein can be readily hydrogenated to form allyl alcohol on several metal surfaces [2,5,6]. The oxidation of allyl alcohol, on the other hand, produces acrolein, as observed on Ag(110) [1] and Cu(110) [8]. Our TPD experiments on the conversion of allyl alcohol on Ni(100) do not show any acrolein desorption from the
surface. In an additional reference TPD experiment with acrolein, we found that the molecular desorption of acrolein occurs below 200 K on Ni(100), and that its decomposition starts above 200 K (data not shown). Therefore, since decomposition dominates at low coverages, even if acrolein forms from oxidation of allyl alcohol on Ni(100), that intermediate could decompose upon formation. There is also no evidence for the production of allyl alcohol back from conversion of any of the surface intermediates; the TPD data for allyl alcohol from our experiments show only a single peak at 185 K, from desorption of the intact allyl alcohol. The oxidation of allyl alcohol can be enhanced by co-adsorbing surface oxygen, as also previously demonstrated in experiments with allyl alcohol on Ag(110) [1] and on Cu(110) [8]. The surface oxygen is expected to promote hydrogen extraction from the allyl alcohol, typically from its hydroxyl group, to produce more alkoxide intermediates. As the result of this hydrogen extraction and a subsequent rehybridization of the C_C double bond in the vinyl group, a η3(C, C, O)-alkoxide intermediate may form on surface. The saturated alcohols and the saturated aldehydes, the 1-propanol and propanal detected in our TPD data (Fig. 4), can then be produced via hydrogenation of those. The activation of the C\O bond in the allyl alcohol adsorbed on Ni (100) occurs at 160 K, as indicated by the reduced C 1s XPS intensity at 285.3 eV in Fig. 6. The deoxygenation reaction may produce an allylic intermediate, and propylene may form this way. Propylene desorbs at temperatures above 200 K, with a peak temperature at 265 K, as we observed in our TPD experiments (Fig. 1). The reactivity of allylic intermediates has been studied with the TPD and vibrational spectroscopy on a few metal surfaces. It was reported that the activation of allyl bromide on Cu(100) produces an η3-allyl intermediate, which is capable of C\C coupling to form 1,5-hexadiene [53]. The allylic intermediate derived from allyl chloride on Ag(110) shows a similar coupling reaction [54], but this coupling reaction is not favored on other metal surfaces. The allylic intermediates derived from allyl chloride on Ni(100) [55] and from allyl bromide on Al(100) [56] and Pt(111) [44,57] display two configurations, with η1-allyl and η3-allyl coordination to the surface. The η1-allyl intermediate has a tilted geometry and undergoes hydrogenation to propylene, whereas the flat η3-allyl intermediate mostly decomposes to smaller hydrocarbons. In the conversion of allyl alcohol on Ni(100), the C\O bond scission in the η1-allyloxy likely produces an η1-allyl intermediate, which is then readily hydrogenated to propylene. Any η3-allyl intermediate is likely to decompose if it forms on surface. Although the activation of the C\O bond can easily occur on early transition metal surfaces [12,58], the activation is kinetically less favored on later transition metal surfaces. On the other hand, that C\O scission can be promoted by co-absorbing oxygen atoms. For instance, C\O bond scission in tert-butanol has been shown to be promoted by coadsorbed oxygen on Rh(111) [14]. However, our TPD data (Fig. 5) do not show any obvious increase in the propylene yield. It is realized that the surface oxygen not only promotes the C\O bond scission in alcohols, it also promotes its oxidation, as indicated by the observed CO desorption. The reaction of allyl alcohol on Ni(111) film prepared on Pt(111) was studied with TPD and HREEL [59]. The surface structure of the Ni(111) film is considered to be similar to the Ni(111) single crystal surface. The Ni(111) close-packed surface contains more reaction sites comparing to the Ni(100) surface that we used in our experiment; therefore, molecules are more strongly bonded to the Ni(111) surface. The desorption of allyl alcohol on Ni(100) single crystal surface in our experiment shows a desorption peak at 180 K, while the desorption peak of allyl alcohol was observed at 261 K on Ni(111) film. Both the experiment on Ni(111) film and our experiment on Ni(100) crystal surface show the evidence of 1-propanol production due to the allyl alcohol self-hydrogenation. Due to the stronger bonding on Ni(111) film, the 1-propanol desorption also occurs at higher temperature on the Ni(111) film than on the Ni(100) surface. The stronger surface bonding
Q. Zhao et al. / Surface Science 605 (2011) 1236–1242
Allyloxy
Propylene + H, - O
CH2
CH
CH CH2
O
CH
CH
280 K
CH2 H
Propanal
CH3
CH2
Ally Alcohol
1241
CH3
O CH2
OH
CH2
+H
160 K Allyloxy
CH2
CH2
CH
320 K O
240 K η3(C, C, O)-alkoxide
CH2 OH CH2
CH2
+ 2H
CH
310 K
240 K η 2(C, C)- Allyl Alcohol
CH3
OH CH2
1-Propanol
Fig. 7. Proposed reaction pathways for the formation of propylene, propanal, and 1-propanol during the thermal conversion of allyl alcohol adsorbed on a Ni(100) surface.
also enhances the reactivity of chemical decomposition. The C\O scission occurs at lower temperature on the Ni(111) film which leads to the propylene production at around 190 K, while propylene is produced at higher temperature on Ni(100) in our experiment. For the same reason, the productions of large molecules of 1-propanol and propanal may have lower rates on Ni(111) than on Ni(100). Based on the above experimental results and in our analysis and discussion of those data for the activation and reaction of allyl alcohol on Ni(100), we propose that 1-propanol forms in this system through the hydrogenation of a η2(C, C) allyl alcohol intermediate, and that propanal forms through a η3(C, C, O)-alkoxide intermediates instead. Propylene is also produced, via a C\O bond scission in an allyloxy surface intermediate. These reaction pathways are summarized schematically in Fig. 7. 5. Conclusions The thermal chemistry of allyl alcohol on Ni(100) follows a reaction mechanism that starts with the activation of both the O\H bond in the hydroxyl group and the rehybridization of the C_C bond in the allyl moiety. While the activation of the O\H bond to form the allyloxy intermediate is facile for allyl alcohol on the surface, the further activation of the C_C bond through the rehybridization on surface is the key for the production of the saturated alcohol (1-propanol) and the saturated aldehyde (propanal). Co-adsorption of hydrogen in this system enhances the allyl alcohol hydrogenation on surface and increases the yield of 1-propanol. On the other hand, co-adsorption of oxygen atoms enhances the yields of both 1-propanol and propanal. The acrolein formation through the allyl alcohol oxidation on surface was not observed in our experiments, instead, the C_C bond activation is more favored, leading to the 1-propanol and the propanal productions. The activation of the C\O bond in the adsorbed allyl alcohol competes with other reactions, which leads to the propylene production. Acknowledgment This work was funded by a grant from the US National Science Foundation, and by the Ministry of Science and Technology of China
(2009IM033000). Additional support was provided by the National Natural Science Foundation of China (50935001).
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