Surface Chemistry of 2-Iodoethanol on Pd(111) - American Chemical

Jan 13, 2012 - Department of Chemical and Biological Engineering, University of Colorado Boulder, Campus Box 424, Boulder, Colorado 80309,. United Sta...
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Surface Chemistry of 2-Iodoethanol on Pd(111): Orientation of Surface-Bound Alcohol Controls Selectivity Michael B. Griffin, Simon H. Pang, and J. Will Medlin* Department of Chemical and Biological Engineering, University of Colorado Boulder, Campus Box 424, Boulder, Colorado 80309, United States ABSTRACT: Temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) were used to investigate the thermal surface chemistry of 2-iodoethanol (ICH2CH2OH) on Pd(111). 2-Iodoethanol undergoes C−I scission upon adsorption at low temperatures, resulting in 2-hydroxyethyl (−CH2CH2OH) formation. At low coverage, this intermediate decomposes to form carbon monoxide and hydrogen during TPD. At higher coverage, hydroxyethyl reacts to produce ethanol, ethylene, water, 2-iodoethanol, and acetaldehyde, with nearly complete suppression of the decarbonylation pathway. DFT and HREELS results indicate that the coverage dependence of the reaction pathways is correlated with a change in the adsorption geometry of hydroxyethyl. At low coverage the C−O bond lies parallel to the surface, while at higher coverage it is shifted into a perpendicular position. Adsorbed iodine is observed to play a significant role in blocking sites and favoring the upright adsorption geometry. These results have important implications for the use of catalyst modifiers in controlling selectivity in reactions of functional alcohols.

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INTRODUCTION Growing concerns over the use of traditional fossil fuels has led to an increased interest in the development of renewable technologies. Included in these technologies is the conversion of plant biomass to fuels, pharmaceuticals, and chemicals.1−3 Plant biomass is an abundant and diversified feedstock, but its high level of functionality leads to some refining challenges.4 Overcoming these challenges requires the development of novel catalytic systems. Using surface science to study how biomass derivatives like polyols and bifunctional alcohols interact with catalytic surfaces can elucidate reaction mechanisms and lead to the improvement of industrial catalysts.5,6 One particularly useful probe molecule is 2-iodoethanol (ICH2CH2OH). Halogenated compounds such as 2-iodoethanol have been extensively used to synthesize surface intermediates and elucidate reaction mechanism on a number of catalytic surfaces.7−11 The ease of C−I bond activation makes 2-iodoethanol well-suited for providing insight into the chemistry of alcohols that form intermediates with surface bound carbon atoms. This type of chemistry is particularly applicable to the aqueous phase reforming of biomass derivatives like ethylene glycol (HOCH2CH2OH) which can be converted into hydrogen via decomposition to CO and H2 followed by the water-gas shift reaction.12,13 Additionally, upon scission of the C−I bond, iodine becomes strongly adsorbed and acts as a site blocker.14 Restriction of the density of available surface sites can have a significant effect on reaction selectivity. This is illustrated by the selective oxidation of glycerol to dihydroxyacetone (DHA) in which bismuth has been proposed to limit available metal ensemble sizes on a © 2012 American Chemical Society

supported platinum catalysts in order to increase selectivity to a desired product.15,16 The surface chemistry of 2-iodoethanol has been studied on Ag(111),7 Ag(110),8 Ni(100),9 Rh(111),11 and Cu(100).10 On all of the surfaces 2-iodoethanol undergoes C−I scission at low temperatures, forming 2-hydroxyethyl (−CH2CH2OH). On Ag(111),7 the hydroxyethyl species reacts at 255 K to form ethylene, acetaldehyde, and a surface oxametallacycle (−CH2CH2O−). The oxametallacycle is stable until 320 K, at which point it decomposes to acetaldehyde. A similar mechanism is observed for Ag(110),8 in which decomposition of 2-hydroxyethyl occurs at 263 K, producing acetaldehyde, ethylene, water, and surface oxametallacycle. The oxametallacycle remains to 340 K and then forms acetaldehyde, ethylene, water, ethanol, and γ-butyrolactone. On Ni(100)9 both 2hydroxyethyl (∼20%) and −O(H)CH2CH2− (∼80%) exist at low temperature. At 160 K, the hydroxyethyl decomposes to form ethanol and vinyl alcohol, while the −O(H)CH2CH2− intermediate dehydrogenates to an oxametallacycle. The remaining hydroxyethyl and oxametallacycle species tautomerize to acetaldehyde at 210 and >250 K, respectively; acetaldehyde either desorbs or decomposes to carbon monoxide and hydrogen. On Rh(111),11 2-iodoethanol also forms 2-hydroxyethyl, which dehydrogenates to surface acetaldehyde at ∼225 K. The acetaldehyde decomposes further to evolve methane, carbon monoxide, and hydrogen. On Cu(100),10 2-iodoethanol forms 2-hydroxyethyl at low temperReceived: November 22, 2011 Revised: January 6, 2012 Published: January 13, 2012 4201

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10−8 Torr of O2 between 400 and 900 K. Mild sputtering with Ar+ ions (1−3 keV) and annealing in UHV were also utilized. The Pd(111) crystal was mounted onto a 1.5 mm tantalum disk and held onto a copper stage within each chamber using two metal clips. In both chambers temperature was measured using a thermocouple spot-welded to the tantalum stage that held the sample. 2-Iodoethanol, stabilized with copper, was obtained at >99% purity from Sigma-Aldrich and purified using repeated freeze−pump−thaw cycles. Ultrahigh-purity H2, O2, CO, D2, and Ar were obtained from Matheson Trigas. DFT calculations were performed using the Vienna Ab-Initio Simulation Package.19−22 A 3 × 3 Pd surface unit cell with a thickness of four atomic layers was employed to simulate low coverage (1/9 monolayer) conditions, while a 2 × 2 Pd surface unit cell with identical thickness was employed to simulate high coverage (1/4 monolayer) conditions. The bottom two layers were held fixed in the bulk-optimized positions while the top two layers and the adsorbate were allowed to relax. The vacuum space between the top of an adsorbed molecule and the bottom of the next slab was at least 11 Å in all cases. The exchangecorrelation energy was calculated within the generalized gradient approximation using the revised Perdew−Burke− Ernzerhof (rPBE) functional.23,24 Electron−ion interactions were described by the projector augmented wave method.25,26 The Brillouin zone was sampled with a 7 × 7 × 1 k-point grid generated automatically using the Monkhorst−Pack scheme.27 The plane-wave cutoff energy was 360 eV. Hydroxyethyl adsorption energies were calculated with respect to the relaxed clean surface and relaxed gas phase hydroxyethyl. Vibrational frequencies were calculated using density functional perturbation theory to determine the Hessian matrix. Relative vibrational intensities are proportional to the squared dynamic dipole moment for a given vibration:18,28

ature, which decomposes to evolve ethylene in two peaks (∼180 and ∼230 K) for doses 1.5 langmuirs. No evidence of an oxametallacycle intermediate is observed on Cu(100) or Rh(111). These prior studies provide insight into how 2-iodoethanol behaves on a number of transition metal surfaces. However, no investigations have been carried out on palladium, which is used for a number of important catalytic reactions including the selective oxidation of ethane to acetaldehyde, vinyl acetate synthesis, and a wide range of hydrogenation reactions.17 Comparisons between Pd and its neighbors in the periodic table provide an opportunity to explore catalytic trends. However, even seemingly small electronic and geometric changes can have large effects on catalytic performance, as can be seen by the differences in the reactivity of 2-iodoethanol on Ag(111), Ag(110), and Cu(100). In this work we use highresolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and density functional theory (DFT) to provide a systematic experimental and theoretical approach to understand how 2-iodoethanol adsorbs and reacts on Pd(111). A better understanding of how 2-iodoethanol interacts with palladium can provide insights toward the development of selective Pd-based catalysts for the upgrading of biomass derivatives.

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EXPERIMENTAL METHODS All experiments were conducted in one of two ultrahighvacuum chambers described previously.18 HREELS experiments were conducted in a stainless steel ultrahigh-vacuum chamber with a LK5000 high-resolution electron energy loss spectrometer (LK Technologies) and a model 981-2046 sputter gun for cleaning (Varian). All HREELS experiments were conducted at a specular angle of 60° with respect to the surface normal and beam energy of 6.32 eV. HREELS peak positions are reported to the nearest 5 cm−1. To study thermal chemistry, the crystal was annealed to various temperatures and then cooled before collecting spectra. All HREEL spectra are normalized to the elastic peak height. TPD experiments were conducted in a separate chamber equipped with a Smart-IQ+ quadrupole mass spectrometer (VG Scienta) and a model NGI3000-SE sputter gun for cleaning (LK Technologies). Fragmentation patterns were obtained by backfilling the TPD chamber to a pressure of ∼10−9 Torr. Reaction products were identified by comparing the fragmentation patterns obtained by backfilling the chamber with those obtained from experiments. To ensure that all products were properly identified, mass/ charge ratios spanning from 2 to 200 amu were monitored during TPD, and analysis of these spectra using fragmentation patterns was employed to deconvolute mass traces that result from multiple products. In both chambers cooling below room temperature was accomplished using a liquid nitrogen reservoir located in contact with the sample, and 2-iodoethanol was dosed using lines facing the sample except where otherwise noted. While direct dosing decreases background adsorption and sample contamination, it makes direct comparison of exposures difficult between different chambers. In these experiments different surface coverage was obtained by varying the pressure in the dosing manifold and monitoring the increase in chamber pressure. Both chambers had a base pressure of ∼10−10 Torr. The polished Pd(111) crystals (Princeton Scientific) were cleaned primarily through cycles of cooling and heating in 5.0 ×

Ii ∝

∂μz

2

∂Q i

where μz is the dipole moment in the direction normal to the surface, Qi is the magnitude of the vibration displacement, and ∂μz/∂Qi is the dynamic dipole moment. Derivatives were evaluated numerically to estimate relative vibrational intensities.

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RESULTS TPD. Temperature-programmed desorption experiments were carried out for a range of submonolayer to multilayer exposures of 2-iodoethanol on Pd(111). Figure 1 shows the results after a direct exposure at 150 K. The major desorption features were identified as hydrogen (m/z = 2), carbon monoxide (m/z = 28), ethanol (m/z = 31), acetaldehyde (m/z = 29), ethylene (m/z = 27), 2-iodoethanol (m/z = 127), and water (m/z = 18). Hydrogen and carbon monoxide are evolved in desorption-limited peaks centered at 350 and 475 K, respectively. Water desorbs in a peak at 275 K with a broad shoulder extending to 375 K. Ethanol is evolved in a sharp peak with a maximum desorption temperature of 240 K. Acetaldehyde desorbs in two peaks at 275 and 375 K. Ethylene is produced in a broad peak centered at 375 K, and a small amount of 2-iodoethanol is produced in a broad peak centered at 375 K. Similar reaction products have been reported in previous studies of 2-iodoethanol on Ag(111),7 Ag(110),8 Ni(100),9 Cu(100),10 and Rh(111).11 Carbon monoxide desorbs from 4202

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Figure 2 shows the effect of varying 2-iodoethanol exposure sizes on carbon monoxide, hydrogen, ethanol, 2-iodoethanol, acetaldehyde, and ethylene desorption rates. The smallest dose (a) was obtained by filling the dosing manifold with 0.025 Torr of 2-iodoethanol and resulted in 0.05 ML of carbon monoxide desorption. This exposure led to small desorption features for acetaldehyde, ethanol, ethylene, and 2-iodoethanol. However, significant yields of the complete decomposition products carbon monoxide and hydrogen are produced, suggesting that dehydrogenation and decarbonylation reactions are favored at low coverage. Increasing the pressure in the dosing manifold to 0.050 Torr (b) leads to larger desorption features for acetaldehyde, ethylene, ethanol, and 2-iodoethanol, but hydrogen desorption does not noticeably increase and carbon monoxide desorption actually decreases. The decrease in CO yield indicates that the decarbonylation pathway is suppressed at large exposures. The largest dose (c) was obtained by filling the dosing manifold to 0.075 Torr and resulted in a slight increase in the ethanol, ethylene, and acetaldehyde peaks, while hydrogen desorption decreases and carbon monoxide desorption drops to 0.03 ML. This number is likely overestimated due to background CO desorption, and in reality the decomposition pathway is almost completely suppressed at this exposure as demonstrated below. A similar shift from the production of carbon monoxide and hydrogen to other products at high coverage was also observed on Ni(100).9 The 2-iodoethanol trace shows an increase in the broad peak at 375 K and the appearance of an additional desorption feature at 250 K. This new feature does not saturate with increasing exposure and is attributed to multilayer desorption from the surface. Previous investigations show 2-iodoethanol multilayer desorption from Ag(110) at 225 K, Ag(111) at 255 K, Cu(100) at ∼200 K, Ni(100) at 210 K, and Rh(111) at 188 K.

Figure 1. Major TPD traces from a direct dose of 2-iodoethanol on Pd(111) at 150 K.

Rh(111) at 440 K and from Ni(100) at 380 K. Hydrogen desorbs from Rh(111) in two peaks at 272 and 386 K and from Ni(100) in three peaks at 300, 370, and 420 K. The desorption of water is observed on Ag(111), Ag(110), and Ni(100) with peak temperatures of 230, 340, and 160 K, respectively. Ethanol desorbs from Ag(110) in two peaks at 263 and 340 K and from Ni(100) in a single peak at 210 K. Acetaldehyde is evolved from Ag(111) at 260 and 315 K, from Ag(110) at 263 and 340 K, and from Ni(100) at 210 and ∼325 K. Coverage-dependent ethylene desorption peaks at 176 and 228 K (0.5 langmuir) and 210 and 223 K (15 langmuirs) are observed on Cu(100). Ethylene desorbs from Ni(100) at 210 K, from Ag(110) at 263 and 340 K, and from Ag(111) at 255 and 320 K.

Figure 2. TPD spectra for carbon monoxide (m/z = 28), hydrogen (m/z = 2), ethanol (m/z = 31), 2-iodoethanol (m/z = 127), acetaldehyde (m/z = 29), and ethylene (m/z = 27) desorption formed from the decomposition of 2-iodoethanol on Pd(111) as a function of dose size. In each case, the crystal was exposed to a direct dose at 150 K. Dosing line pressure corresponding to each exposure is (a) 0.025, (b) 0.05, and (c) 0.075 Torr, and CO desorption in ML is (a) 0.05, (b) 0.04, and (c) 0.03. 4203

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cm−1 suggest that the C−H bonds are also intact. However, noticeably absent on Pd(111) are the modes associated with the C−I bond. This is not surprising since cleavage of the relatively weak C−I bond (53 kcal/mol) is expected to occur below the adsorption temperatures of 173 K.7 The OH stretching mode at ∼3300 cm−1 is also unresolved at this exposure level, but it was only detected with very low intensity for condensed multilayers (see below) and was also not detected at all on Rh(111).11 The absence of C−C−O ring deformation modes and surface-oxygen vibrations suggest that oxametallacycle (−OCH2CH2−) formation has not occurred.7 For this reason, we hypothesize that the O−H bond remains intact at these conditions but cannot rule out the possibility that the O−H bond of hydroxyethyl has been cleaved, producing a surface oxametallacycle. The crystal was then sequentially annealed to progressively higher temperatures to track changes in adsorbate structure. Heating to 200 K (b) leads to a slight decrease in the relative intensities of the CH rocking, wagging, and stretching modes at 800, 1390, and 3020 cm−1, likely signaling the onset of dehydrogenation. At 225 K (c), these modes are almost indiscernible, and a peak at 1830 cm−1 becomes apparent. This new feature is attributed to the CO stretch of carbon monoxide. The formation of CO is a result of decarbonylation and dehydrogenation, a pathway that has been shown to be a highly favorable for many oxygenates (including alcohols) on Pd(111).29,30 At 350 K (d) the only discernible peak is at 1830 cm −1 , signaling complete decomposition to carbon monoxide. Note that this result is in agreement with the TPD results presented above, where CO and hydrogen were the dominant products after low 2iodoethanol exposures. Figure 4a shows the HREEL spectrum collected following a large 2-iodoethanol dose at 173 K. The major vibrational mode

In order to further investigate the scavenging of surface hydrogen, experiments were carried out in which the Pd(111) surface was precovered with deuterium. Any hydrogenation reaction would then be observable as a mass shift in the desorption product. These experiments resulted in the detection of m/z = 47. This mass trace is indicative of CH2DCH2OH desorption, which is formed from the direct hydrogenation of hydroxyethyl. The preadsorption of deuterium also leads to an increase in ethanol production relative to acetaldehyde production at a low initial coverage of 2iodoethanol. However, acetaldehyde desorption is not completely eliminated even when excess D is present on the surface, as evidenced by an m/z = 4 peak in the TPD spectra, indicating that acetaldehyde formation and ethanol formation are not closely coupled processes. Similar results were observed on Ni(100).9 HREELS. HREELS was used to characterize surface intermediates and gain insight into the surface chemistry of 2-iodoethanol on Pd(111). The spectrum in Figure 3a was

Figure 3. HREEL spectra of 2-iodoethanol on Pd(111) obtained by exposing the surface to a 0.1 langmuir direct dose at (a)173 K and briefly annealing to (b) 200, (c) 225, and (d) 350 K.

collected after an exposure producing submonolayer coverage at 173 K. The major vibrational mode assignments are shown in Table 1. The strong C−C stretch, C−O stretch, and C−C−O Table 1. Low-Coverage 2-Iodoethanol Vibrational Mode Assignments (cm−1)

a

mode

Pd(111) submonolayera

C−I O−C−C str, CH2 rock C−C str C−O str C−C−O deform CH2 wag CH2 bend C−H str

800 1015 1015 1015 1260 1390 3020

IR liquidb 607 795 1014 1033

Figure 4. HREEL spectra of 2-iodoethanol on Pd(111) obtained by exposing the surface to a 1.0 langmuir direct dose at (a)173 K and briefly annealing to (b) 200, (c) 225, and (d) 275 K.

1265 1415 2866

assignments are shown in Table 2. Comparison of the lowtemperature spectrum with liquid phase IR data for 2iodoethanol shows many similarities, suggesting nondissociated multilayers are condensed on the surface. Because of poor resolution from multilayer condensation, the C−I modes at 500−700 cm−1 are unresolved from the broad peak in Figure 4a. At 200 K (b) the multilayers begin to desorb as indicated by the improved resolution and a decrease in the relative intensity

From this work. bFrom ref 31.

deformation modes at 1015 cm−1 indicate that the molecular backbone remains intact upon adsorption, in agreement with previous studies on Ag(111)7 and Cu(100).10 The CH2 rocking, wagging, and bending modes at 800, 1260, and 1390 4204

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Table 2. Multilayer 2-Iodoethanol Vibrational Mode Assignments (cm−1) mode O−C−C str, CH2 rock C−C str C−O str C−OH deform CH2 wag CH2 wag C−H str O−H str a

Pd(111) multilayera 800 1025 1025 1190 1465 2980 3325

Ag(111) multilayerb

IR liquidc

750−840

795−905

1050−1100 1050−1100 1132−1180 1270 1400−1500 2890−3050 3250−3370

977 1068 1147−1174 1265 1414−1455 2871−2961 3360−3580

From this work. bReference 7. cReference 31.

of the loss peaks. This is in agreement with the TPD data, which shows the onset of multilayer desorption at ∼195 K. After annealing at 225 K (c) multilayer desorption is complete, and a strong signal at 1115 cm−1 is revealed. This mode is attributed to a mixture of C−O stretching, C−C−O deformation, and C−OH deformation. Additional modes that become apparent are CH2 rocking at 615 cm−1, O−C−C deformation at 830 cm−1, CH2 wagging at 1260 cm−1, CH2 bending at 1390 cm−1, C−H stretching at 2985 cm−1, and O− H stretching at 3400 cm−1. The CH2 rocking, C−OH deformation, and O−H stretching modes are particularly interesting because they are unobserved under low coverage conditions. Density functional theory calculations, discussed in detail in a later section, explain this discrepancy as a difference in adsorption geometry due to increased surface coverage. Heating the surface corresponding to Figure 4a above 225 K reveals additional differences in the surface chemistry of the two exposures. No peak at 1830 cm−1 is observed, confirming that appreciable amounts of CO are not produced from hydroxyethyl at saturation coverage. Instead, there is little loss intensity after annealing to 275 K (d), consistent with the observation that most products have desorbed by this temperature in TPD. Additional experiments were carried out in which the surface was exposed to a 1.0 langmuir dose of 2-iodoethanol at 173 K, resulting in the formation of condensed multilayers. The sample was then annealed to 500 K in order remove all adsorbed intermediates except iodine, which does not desorb from Pd(111) until above 850 K.14 Subsequent direct exposures of 0.1 and 0.4 langmuir did not produce any loss peaks. Repeating this experiment with an initial direct dose of 0.1 langmuir produced similar results, suggesting that iodine and surface-bound carbon have the potential to populate the surface and block further adsorption. DFT Calculations. Density functional theory calculations were used to identify the most stable hydroxyethyl intermediates resulting from C−I bond scission. Figure 5 shows hydroxyethyl in a bidentate configuration at 1/9 and 1/4 monolayer coverage. Hydroxyethyl adsorbs in these configurations with adsorption energies of −179.2 and −171.0 kJ/ mol, respectively. The primary interaction with the surface is through the terminal carbon, and the molecular backbone lies parallel to the surface, but there is also significant interaction between the oxygen and the surface at 1/9 ML coverage. The low-coverage structure is similar to the structure of oxametallacycle intermediates characterized on Ag(111),7 except that an O−H bond is still intact in the structure considered here. For

Figure 5. Hydroxyethyl adsorbed in bidentate configuration. Left: 1/9 monolayer coverage; right: 1/4 monolayer coverage. Red denotes O atoms, gray denotes C, blue denotes Pd, and white denotes H.

the purposes of the discussion below, however, it is worth pointing out that O−H scission would lead to production of an oxametallacycle intermediate, and the presence of such an intermediate cannot be excluded based on the spectroscopic data, since no O−H stretching or bending modes are resolved in the data. In the high-coverage case, the oxygen atom is further away from the surface. Figure 6 shows hydroxyethyl in a monodentate configuration at the two coverages; for this geometry, the adsorption energy of −173.2 kJ/mol does not vary for the two surface coverages considered here. Although the energy differences are not large, the results of these calculations suggest a possible transition from a flat-lying to upright hydroxyethyl geometry as coverage is increased. The transition of hydroxyethyl species from a bidentate to monodentate geometry with increasing coverage is supported by the agreement between vibrational spectra calculated using density functional perturbation theory and experimental spectra collected using HREELS, as shown in Figure 7. The theoretical vibrations have been broadened by a 60 cm−1 Voigt function to mimic the resolution of the HREELS data.31 The intensities of CH2 and OH stretches are typically overestimated by theoretical techniques; for clarity, the intensities of these modes have been scaled down as indicated in the figure.32 Vibrational mode assignments, shown in Table 3 for the monodentate intermediate and Table 4 for the bidentate intermediate, are made based on literature but were confirmed using visualization of the forces on each ion for each frequency. The agreement between experimental and theoretical vibrational spectra for the bidentate hydroxyethyl is observed to be better than that for the monodentate structure. This may be due in part to the fact that the adsorbate−adsorbate interactions for species present at high coverage (including 4205

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Figure 6. Hydroxyethyl adsorbed in monodentate configuration. Left: 1/9 monolayer coverage; right: 1/4 monolayer coverage. Red denotes O atoms, gray denotes C, blue denotes Pd, and white denotes H.

Figure 7. Calculated (red) and experimental (black) vibrations for hydroxyethyl in the (a) bidentate and (b) monodentate orientation. The theoretical vibrations have been broadened by a 60 cm−1 Voigt function to mimic the resolution of the HREELS data,31 and the CH2 and OH stretches have been decreased in intensity.

iodine) are not well-captured by the current model. Others have shown that including combinations of adsorbed structures can increase the accuracy of the predicted vibrational structure possibly due to the presence of multiple adsorbed structures on the surface and interactions between adsorbed molecules, though even in these studies the spectra at high coverage do not match perfectly.33,34 Furthermore, the geometry for the bidentate species is more constrained, perhaps reducing the flatness of the potential energy surface. The dominant modes seen for both the bidentate and monodentate species are the CH2 stretching around 3020 and 2985 cm−1, respectively, and various C−C−O deformations around 1015 cm−1. Many of the differences in the computed spectra are subtle. However, a major distinguishing feature is present in the experimental HREELS data at 615 cm−1, which is observed as a strong peak for the high-coverage hydroxyethyl species but is not observed at low coverage. This frequency has been associated with a CH2 rocking motion out of the surface plane for the upright hydroxyethyl adsorption geometry;7 furthermore, it is identified as a strong mode in the DFT calculations for monodentate hydroxyethyl as well. In the bidentate hydroxyethyl configuration, the dynamic dipole moment for this mode is oriented parallel to the surface and is thus not expected to be observed as an intense mode in HREELS. In addition, the observation of an O−H stretching mode at 3300 cm−1 in the high coverage spectrum (not observed in the low-coverage spectrum) suggests the presence of an intact O−H bond that is at least partly oriented in the direction of the surface normal. The fact that the O−H bond is still intact is also supported by the fact that ethanol-d1 produced during TPD of 2-iodoethanol on Dprecovered Pd(111) is observed not to be deuterated at the

Table 3. Monodentate Hydroxyethyl Vibrational Mode Assignments (cm−1) hydroxyethyl on Pd(111)a

mode I−Ag str C−C−O deform CH2 rock O−C−C deform, CH2 rock C−C twist C−O stretch C−C−O deform C−OH deform CH2 wag CH2 bend C−H str O−H str a

hydroxyethyl on Ag(111)b 265 468, 522

615 830

1115 1115 1115 1260 1390 2985 3400

820 910 980 1070 1070 1267 1420−1440 2830−3060 very small

From this work. bReference 7.

hydroxyl position, indicating that the O−H bond has remained uncleaved at high coverage.

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DISCUSSION On the basis of insights gained from the TPD, HREELS, and DFT investigations, we propose in Scheme 1 a reaction mechanism for the adsorption and thermal decomposition of 2iodoethanol on Pd(111). Our work suggests that on Pd(111) the C−I bond of 2-iodoethanol is broken upon adsorption at 173 K. The resulting hydroxyethyl (−CH2CH2OH) inter4206

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Pd(111) and Ni(100).9 A mixture of monodentate and bidentate intermediates is observed on each of these surfaces, resulting in the formation of ethylene, ethanol, acetaldehyde, water, carbon monoxide, and hydrogen. However, on Ni(100) the different intermediates appear to coexist, while on Pd(111) there is a coverage dependent shift from one structure to the other. The DFT calculations show that adsorbate−adsorbate interactions play a role in restructuring of hydroxyethyl, but it is important to note that they do not consider the fact that larger doses result in greater deposition of iodine, which can decrease the availability of surface sites. HREELS experiments discussed in the Results section provide direct evidence of this phenomenon by showing that the adsorbed iodine from a 1.0 langmuir direct exposure is sufficient to block subsequent adsorption. It can be expected that as iodine coverage increases and neighboring sites become unavailable, hydroxyethyl will adopt a more upright geometry which requires fewer surface sites. Iodine may also exert electronic effects, but previous studies suggest that charge transfer between adsorbed iodine and palladium is small.36,37 We hypothesize that the coveragedependent surface chemistry of 2-iodoethanol on palladium is thus likely due to crowding from iodine and other coadsorbates. The blocking of active sites and corresponding shift in geometry is particularly interesting because it hints at an element of tunability for the reactivity of these types of multifunctional molecules. If interest lies in the complete decomposition of multiple functional groups, such as in aqueous phase re-forming of polyols,12,13 then it may be desirable to run reactions under conditions in which significant metal atom ensemble sizes are readily available. This allows space for multiply coordinated intermediates to undergo dehydrogenation and decarbonylation reactions. However, at high coverage, restriction of the ensemble size hypothetically results in the production of monodentate species. When selective reaction of a single functional group is the goal, it may be desirable to carry out reaction under crowded surface conditions, perhaps through the modification of the catalyst with site-blocking species. Achieving such conditions has been shown to be possible through the use of strongly adsorbed site blockers or self-assembled monolayers.38 This is illustrated by the selective oxidation of glycerol to DHA in which bismuth is utilized as a site blocker on supported platinum catalysts in order to increase selectivity to reaction at a single hydroxyl group.15,16 For 2-iodoethanol on Pd(111) it is likely the similarity in adsorption energy between the two hydroxyethyl structures that enables the clear observation of a shift from monodentate to bidentate hydroxyethyl as coverage is increased. At low coverage the bidentate intermediate is only favored by 8.2 kJ/ mol. This energy difference is small and apparently can be overcome by the effects of surface crowding. By comparing the relative adsorption energies of the hydroxyethyl geometries on other surfaces and alloys, it may be possible to predict the resulting selectivity.

Table 4. Bidentate Hydroxyethyl Vibrational Mode Assignments (cm−1) mode Ag−O, Ag−C O−C−C str, CH2 rock C−O str C−C−O deform CH2 wag CH2 bend C−H str a

−O(H)CH2CH2− on Pd(111)a 800 1015 1015 1260 1390 3020

oxametallacycle on Ag(111)b 344 793 996 1060 1218, 1278 1446 2990

From this work. bReference 7.

Scheme 1. Proposed Reaction Mechanism for 2-Iodoethanol on Pd(111)

mediate adopts one of two geometries depending on the surface coverage. At low coverage hydroxyethyl assumes a bidentate configuration with the C−O bond nearly parallel to the surface. DFT calculations confirm that this structure is the most favorable, likely due to the stabilizing effect from interaction between the oxygen lone pair and the surface, similar to the interaction seen between alcohols and Pd(111).29 Additionally, good agreement between vibrational spectra from DFT and HREELS confirms the identity of this species. In this configuration hydroxyethyl undergoes complete decomposition to carbon monoxide and hydrogen as temperature is increased. At higher coverage, hydroxyethyl adopts a monodentate configuration, with the C−O bond roughly perpendicular to the surface. This change in geometry is supported by the DFT calculations which show that increasing coverage destabilizes the bidentate intermediate and causes the monodentate intermediate to have competitive favorability. The monodentate hydroxyethyl intermediate can undergo a number of different reactions. The major pathways are dehydrogenation to form acetaldehyde and hydrogenation to form ethanol. At higher temperature minor pathways lead to the production of ethylene, 2-iodoethanol, and acetaldehyde. These findings are in agreement with previous investigations of alkyl halides on Pt(111).35 In these prior studies, vibrational spectroscopy selection rules were used in a similar way to that described above to track how adsorbate geometries vary with coverage. These experiments showed that the alkyl intermediate from the adsorption of methyl chloride, methyl iodide, ethyl bromide, and ethyl iodide undergo a shift from a flat-lying to a more perpendicular geometry at roughly half saturation.35 There are also similarities between the reactivity of 2-iodoethanol on

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CONCLUSIONS 2-Iodoethanol undergoes C−I scission upon adsorption at low temperatures, resulting in hydroxyethyl (−CH2CH2OH) formation. At low coverage, this intermediate decomposes to form carbon monoxide and hydrogen. Higher coverage leads to the production of ethanol, ethylene, water, and acetaldehyde. 2Iodoethanol is also formed from recombination with surface 4207

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The Journal of Physical Chemistry C

Article

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iodine. The different reaction pathways are attributed to two separate adsorption geometries. At low coverage the C−O bond lies parallel to the surface in a bidentate configuration, while at higher coverage it is shifted into a monodentate perpendicular position.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Ph: 303-492-2418; Fax: 303-492-4341.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation for funding this research (Award CBET-0828767). This work used the Extreme Science and Engineering Discovery Environment (XSEDE) under Grant TGCHE040023N, which is supported by National Science Foundation Grant OCI-1053575.

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dx.doi.org/10.1021/jp211259h | J. Phys. Chem. C 2012, 116, 4201−4208