DFT and RAIRS Investigations of Methanol on Cu(110) - American

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J. Phys. Chem. C 2008, 112, 14034–14040

DFT and RAIRS Investigations of Methanol on Cu(110) and on Oxygen-Modified Cu(110) P. Singnurkar,† I. Bako,‡ H. P. Koch,†,‡ E. Demirci,† A. Winkler,† and R. Schennach*,† Institute of Solid State Physics, Graz UniVersity of Technology, Austria 8010, and Chemical Research Centre, Hungarian Academy of Sciences, Budapest, Hungary 1025 ReceiVed: March 21, 2008; ReVised Manuscript ReceiVed: June 30, 2008

The adsorption and subsequent reaction of methanol on Cu(110) and on the oxygen stripe phase on Cu(110) was investigated using reflection absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD) and density functional theory (DFT) calculations. It was shown that during high methanol exposures water can coadsorb and it is incorporated into the methanol rows formed in the chemisorbed layer. The RAIR spectra of adsorbed methanol are very similar with and without coadsorbed oxygen. This is due to the fact that the ν(CO) vibration is at the same frequency for both methanol and methoxy adsorbed in the more stable short-bridge site. However, methoxy adsorbed in the long-bridge site shows the ν(CO) vibration at lower wavenumbers and is found with increasing surface temperature. With coadsorbed oxygen the reaction products are formaldehyde, H2, and CO2. DFT and RAIRS results suggest that the intermediate leading to CO2 is an η2-formaldehyde and OH species on the surface, rather than formate. Introduction The interaction of methanol with copper surfaces has been studied intensively in the past, because copper plays an important role as a catalyst in the synthesis of methanol from carbon monoxide and hydrogen and in the oxidation of methanol to carbon dioxide and hydrogen. In spite of the simplicity of methanol, its oxidation process on catalytic surfaces exhibits a large complexity with several possible reaction routes. Due to the technical importance of Cu/ZnO catalysts in the oxidation and formation of methanol, the methanol-copper system is one of the best studied systems in surface science. Most studies focused on two different Cu surfaces, namely, the Cu(110) and the Cu(100) surfaces; less amount of work was done on Cu(111).1,2 Experimental studies on the Cu(110) surface were done using temperature-programmed desorption (TPD),3-5 temperatureprogrammed reaction spectroscopy (TPRS),6 X-ray photoelectron spectroscopy (XPS),6-8 ultraviolet photoelectron spectroscopy (UPS),4,6 scanning tunneling microscopy (STM),9-11 and electron energy loss spectroscopy (EELS),4 as well as different combinations of these methods. More recently, a detailed density functional theory (DFT) study of this system has been published.12-15 These studies give a good description of the adsorption and reaction of methanol on Cu(110) and on oxygenprecovered Cu(110). However, to the best of our knowledge only ref4 did vibrational spectroscopy on this surface. On the Cu(100) surface more studies using refelction absorption infrared spectroscopy (RAIRS) have been published.16-18 While most of the assigned frequencies are similar on Cu(110) and on Cu(100), there is a rather large difference in the ν(CO) frequency of methoxy between the two surfaces (980 cm-1 on Cu(100)16 and 1020 cm-1 on Cu(110)4). Building on this large body of previous work, a detailed TPD and angular resolved TPD investigation of the reaction of methanol on Cu(110) and on oxygen precovered Cu(110) has * To whom correspondence should be addressed. E-mail: robert.schennach@ tugraz.at. † Graz University of Technology. ‡ Hungarian Academy of Sciences.

been done.19 In this paper it was shown that large methanol exposures on clean Cu(110) lead to similar TDS results as have been found with preadsorbed oxygen. This finding led to a more detailed RAIRS investigation using methanol exposure dependent and temperature dependent RAIRS. The results of these investigations together with density functional theory calculations are presented in this paper. Experimental Section The experiments were done in an ultrahigh vacuum (UHV) apparatus equipped with an Auger electron spectrometer (AES), an X-ray photoelectron spectrometer (XP spectrometer), and a quadrupole mass spectrometer (QMS; Balzers Prisma). An argon ion gun, an electron beam evaporator, and a quartz microbalance are mounted on the system. The base pressure of the system is normally in the 2 × 10-10 mbar range. Thermal desorption experiments were done with a heating rate of 2 K s-1, and the desorbing molecules were detected with the quadrupole mass spectrometer. During TPD detection is possible in line of sight with the QMS ion source and out of line of sight. The system is also equipped with a capillary array detector for angular resolved thermal desorption experiments. RAIRS measurements were done using a Bruker IFS66v/S FTIR spectrometer and an external liquid N2 cooled mercurycadmium-telluride (MCT) detector attached to the same UHV system, as described earlier.20 A grazing incidence angle of about 83° and a scan time of 15 min were used with a resolution of 4 cm-1, unless otherwise stated in the text. The Cu (110) crystal had a surface area of 0.78 cm2 and was cleaned by several cycles of argon sputtering and annealing up to 900 °C. The cleanliness of the surface was checked by XPS. Methanol (Chromasolv, Riedel-de Hae¨n, p.a.) and oxygen (Messer, 5.0) were brought into the chamber using two different leak valves. The methanol was cleaned by several freeze and thaw cycles. The cleanliness of the two gases was checked with the quadrupole mass spectrometer. The exposures were monitored by recording both the total pressure in the system (using a Leybold Ionivac IM520) and the corresponding QMS signal.

10.1021/jp802488n CCC: $40.75  2008 American Chemical Society Published on Web 08/15/2008

Methanol on Cu(110) and Oxygen-Modified Cu(110)

J. Phys. Chem. C, Vol. 112, No. 36, 2008 14035

TABLE 1: Selected Geometric, Energetic, and Vibrational Parameters for the Investigated Molecules on the Cu(110) Surfacea methanol 1 (OtHlb) methanol 2 (OtHsb) methanol 3 (OtHsb) water (Ot) methoxy Ab methoxy A1 methoxy Bc methoxy B1 methoxy Cd methoxy De CH2OH (η2(CO)) H2CO (η2(CO)) H2COO HCOO di-σ(OO) HCO OOH

-∆E (eV)

M · · · O/M · · · C (Å)

0.27 0.29 0.30 0.31 2.35 2.11 2.41 2.12 2.64 2.67 1.35 0.31 3.61 3.17 1.47 1.53

2.19, 3.07 2.23, 3.34 2.19, 3.08 2.23 1.92, 3.03 2.02, 3.05 1.94, 3.00 2.01, 3.06 2.02, 3.06 2.04, 3.01 2.10, 2.00 1.96, 2.19 1.84 1.97, 2.84 2.11, 1.92 1.91

ν (CO) or ν (OO) (cm-1) 987 991 982 1062 1007 1032 1008 1005 1006 837 1311 990, 935 1321 1521 672

ν (OH) (cm-1)

ν (CHX,deform) (cm-1)

3675 3646 3663 3695, 3515

1423, 1411, 1428 1408, 1437, 1445 1427, 1451, 1458

3672

1426, 1435 1410, 1435 1424, 1441 1415, 1434 1416, 1445 1414, 1441 1397 1475 1357 1328

3633

M · · · O, M · · · C are the metal oxygen and metal carbon distances. The adsorption energy was calculated at 0.166 monolayer molecule coverage. In methanol 1-3, t is the on top site; lb, the long bridge; and sb, the short bridge. b CO bond perpendicular to the surface. c CO bond tilted from the surface. d CO bond perpendicular to the surface on added row Cu in long-bridge site. e CO bond tilted to the surface on added row Cu in the long-bridge site; A1 and B1 are the structures on the long-bridge site. a

TABLE 2: Selected Geometric, Energetic, and Vibrational Parameters for the Investigated Dimer and Chain Structures on Cu(110) -∆E (eV) methanol dimer (0.33 monolayer) methanol chain (0.5 monolayer) methanola-water dimer (0.33 monolayer) Watera-methanol (0.33 monolayer) 2 methanol-1water chainb (0.5 monolayer) a

0.55 1.02 0.80 0.74 1.28

M · · · O/OsO (Å) 2.39, 2.79 2.62, 2.72 3.05, 2.16, 2.74 2.21, 2.44, 2.76 2.82, 2.76

ν (CO) (cm)

ν (OHm), ν (OHw) (cm-1)

999, 982 1015, 1017, 1020 999 982 1017, 1012

3637, 3579 3377, 3322, 3220 3648, 3658, 3405 3704, 3404, 3203 3369, 3536, 3410

H-bonded donor. b Structural unit in the chain consists of 2 methanol and 1 water.

Computational Details All adsorption energies as well as the electronic and vibrational properties were calculated using the Vienna ab initio simulation package (VASP),21,22 which is a DFT code with a plane wave basis set. Electron-ion interactions were described using the projector-augmented wave (PAW)23 method, which was expanded within a plane wave basis set up to a cutoff energy of 400 eV. Electron exchange and correlation effects were described by the PerdewsBurkesErnzerhof (PBE)24 GGA type exchange-correlation functional. The basis set dependence was evaluated by varying the energy cutoff Ecut from 300 to 600 eV for the free molecule in gas phase. The total energy converged to within 0.01 eV for Ecut greater than 400 eV with no significant changes in the optimized geometry of the molecule for cutoffs above 400 eV (Supporting Information Table 1). The lattice constant for bulk Cu is predicted to be 3.631 Å, in good agreement with the experimental value of 3.614 Å.25 Geometry optimizations were performed on a supercell structure using periodic boundary conditions. The Cu substrate is modeled by a slab of five layers that is separated by at most 15 Å vacuum. In all calculations the two uppermost Cu layers are fully relaxed. The chemisorbed species and the atoms in the top metal layers were relaxed until the residual forces were less than 0.02 eV/Å. Most results reported here have been obtained for 3 × 2 unit cells. The Brillouin zone integration was performed using a 7 × 7 × 1 MonkhorstsPack26 grid and a MethfesselsPaxton27 smearing of 0.2 eV. In all cases convergence of the total energy with respect to the k-point set (Table 2 of the Supporting Information) and with respect to the number of metal layers included is confirmed. The adsorption of molecules on the oxygen-covered surface was modeled by a 2 × 1 added row reconstruction on a 2 × 2 unit cell with a slab of 5 + 1 layers. The reference energies of the isolated

TABLE 3: Selected Geometric, Energetic, and Vibrational Parameters for Investigated Species on the Cu(110) + O Surface -∆E (eV) M · · · O (Å) ν(CO) (cm-1) ν(OHm) (cm) methanol methoxy OHsH2CO

0.15 1.33 1.42

2.30 2.1 3.3

1033 1028 1130

3658 3387

gas-phase species (molecule and radicals) were calculated by placing them in a cubic cell with 12 Å sidelength. Harmonic vibrational frequencies of adsorbed species were calculated by applying the finite-difference method to create the Hessian matrix, which was diagonalized to obtain the characteristic frequencies of the system. For this purpose we used an atomic displacement of 0.02 Å in the three directions of space to maintain the harmonic vibration approximation for the adsorbate and the first metal layer. We calculated the dipole activity, which is related to the intensity obtained in RAIRS and HREELS spectra. This is proportional to the square of the dipole moment variation with respect to the surface normal for the corresponding mode.28-30 The simulated spectra were calculated with a 10 cm-1 Gaussian broadening. Our calculated vibrational frequencies of the investigated molecules in the gas phase are always in good agreement (difference is less than 2-4%) with experimental or more sophisticated quantum chemical calculations. Geometric, energetic, and vibrational information for the most favorable configurations of the investigated systems is presented in Tables 1-3. Sakong and Gross12,15 have already presented adsorption energies for some of the molecules presented in this paper using the same DFT functional. Our results differ for the closed shell species by about 0.03-0.05 eV from Sakong and Gross12 The different adsorption energy data could be explained by the fact that we use a different molecular coverage in our calculations (0.16 and 0.25 mono-

14036 J. Phys. Chem. C, Vol. 112, No. 36, 2008

Figure 1. (a) IR spectra of methanol after adsorption of 10 langmuirs of methanol at 100 and 200 K on Cu(110). (b) IR spectra of methanol after adsorption of 12 langmuirs of methanol at 100 and 200 K on 0.25 monolayer of oxygen on Cu(110). The positions of the CO and CH vibrations are given in the spectra.

layer). In the case of water our calculations show that the most stable position on this surface is the top site and not the bridge site as determined by Sakong and Gross.12,15 Results and Discussion In this section the experimental results will be shown and discussed with the help of the results from the density functional theory calculations. First the results for methanol adsorption on Cu(110) and on oxygen-precovered Cu(110) will be presented, followed by the data obtained from methanol desorption and reaction. The partially oxygen-covered Cu(110) surface has been shown to consist of stripes of adsorbed oxygen and stripes of clean Cu(110).31 The oxygen exposures in this work were done at a surface temperature of 550 K, which leads to the ordered Cu/CuO stripe phase on the surface prior to the methanol exposure.19 Because the UHV chamber used in this study is not equipped with an STM, the stripe phase was not characterized. However, previous studies showed that this phase can be formed with good reproducibility, when the oxygen is dosed at 550 K.31,19 Figure 1a shows the RAIRS spectra of adsorbed methanol on Cu(110) at two different adsorption temperatures. The lower curve is found after adsorption of 10 langmuirs (1 langmuir is

Singnurkar et al. 1 × 10-6 torr s) of methanol at 100 K. From thermal desorption measurements it is known that this leads to the formation of a multilayer. The band at 1050 cm-1 stems from the ν(CO) stretching vibration of the methanol molecule, and the two bands at 2948 and 2832 cm-1 are the symmetric and asymmetric stretching vibrations of the CH3 group, these assignments agree with.4 The bands at 1120 and 1450 cm-1 have also been reported before4 and were assigned to the methyl rocking and methyl bending vibrations, respectively. The upper curve was obtained after 10 langmuirs of methanol adsorption at 200 K. This leads to a sub-monolayer of methanol on the Cu(110) surface, because the sticking coefficient is very small according to ref 19. One can again see the ν(CO) frequency, which is shifted down to 1042 cm-1, and the ν(CH3) frequencies are at the same positions as in the multilayer. The ν(CO) vibration of methanol is in the same frequency range on Cu(110) as on Rh(111)32 and Pd(111).33 The CO stretching frequency for gas-phase methanol is at about 1010 cm-1 from our DFT calculations. This is about 2-3% smaller than the experimental value for the free methanol molecule.34 DFT calculations of methanol adsorbed on the Cu(110) surface show some changes in the IR spectra going from single methanol molecules to dimers and methanol chains, which corresponds to going from low coverages to high coverages. There are three stable structures according to DFT for single methanol molecules on the Cu(110) surface (see Table 1, methanols 1-3), which are shown in Figure 2a. In all of these structures the methanol adsorbs via the oxygen atom and the methyl group is tilted toward the surface. In methanol 1 the axis of the methanol molecule is perpendicular to the copper rows, in methanol 2 the axis of the molecule is parallel to the rows, and in methanol 3 the axis is again parallel to the Cu rows, but the tilt angle away from the surface is higher. The corresponding calculated IR spectra are shown in Figure 2b, and the vibrations are assigned. The ν(CO) vibrations for the adsorbed methanol monomers are just below 1000 cm-1 (see Table 1 and Figure 2b). These results reveal a small red shift of the adsorbed methanol CO stretching frequency compared to the gas-phase methanol molecule. However, the comparison with the experimentally observed spectra of a monolayer of methanol should rather be made with higher coverages. The structure of a methanol dimer is shown in the left side of Figure 3a along with the structure of a methanol chain (right side in Figure 3a). In both cases there are hydrogen bonds between the individual methanol molecules on the surface, similar to the case of methanol on Pd(110).35 The corresponding calculated IR spectra with the assigned vibrations can be seen in Figure 3b. Here again the ν(CO) is in the 1000 cm-1 region as in the experiments (see Table 2). The ν(CO) frequency of the chain with 1020 cm-1 is the better fit to the experiment with 1042 cm-1 compared to the dimer with 982 cm-1. One has to keep in mind that IR spectra calculated from DFT usually do not give the exact frequencies. Nevertheless, trends obtained from the calculations are in many cases found in experiments.36,37 Another important result from DFT is the fact that substituting methanol with water in the chain structure does not lead to a change in the ν(CO) frequency (see Table 2); however, a slight increase (0.26 eV) in the binding energy of the chain structure is found. This suggests that the similarities found in TPD in ref19 between the adsorption of 50 langmuirs of methanol on clean Cu(110) and 5 langmuirs of methanol on an oxygenprecovered Cu(110) surface could be due to coadsorbed water from the residual gas during the long exposure.


Figure 2. (a) Side view of the three stable geometries of methanol molecules on Cu(110) (see Table 1 for details). Cu atoms are blue, carbon is green, oxygens are red, and hydrogens are light gray. (b) Calculated IR spectra of the three geometries shown in a. The positions of the CO vibrations and CH vibrations are indicated in the spectrum.

When oxygen is preadsorbed on the Cu(110) surface before methanol adsorption, a much higher sticking coefficient for methanol is found.19 This has been interpreted in terms of a much faster methoxy formation by a proton transfer from methanol to coadsorbed oxygen. However, when one compares the RAIRS spectra of methanol on Cu(110) in Figure 1a to methanol on oxygen-precovered Cu(110) in Figure 1b, one can clearly see that the ν(CO) vibrations are the same in both cases, suggesting that the methanol on the oxygen-covered Cu(110) surface does not react immediately to methoxy at the adsorption temperature. After adsorption at 100 K, which leads to a methanol multilayer, the RAIR spectra are the same on both surfaces. However, at an adsorption temperature of 200 K (meaning in the monolayer regime) there is a slight difference, as the CH3 frequencies at 2830 and 2948 cm-1 are much less intense on the oxygen-precovered surface compared to the clean Cu(110) surface. Therefore, a detailed theoretical study of adsorbed methoxy on the copper surface has been done. DFT results show that methoxy is more stable when bound on the short-bridge site of the clean copper (110) surface (methoxy A perpendicular to the surface and methoxy B tilted to the surface in Table 1 and left side of Figure 4), compared to the longbridge site (methoxy A1 perpendicular to the surface and methoxy B1 tilted toward the surface in Table 1 and right side


Figure 3. (a) Stable geometries of methanol dimers (left) and chains (right) on Cu(110) (see Table 2 for details). The upper part is the side view; the lower part is the top view. Cu atoms are blue, carbons are green, oxygens are red, and hydrogens are light gray. (b) Calculated IR spectra of the dimers and chains shown in a. The positions of the CO vibrations, OH vibrations, and CH vibrations are indicated in the spectrum.

of Figure 4). The corresponding calculated IR frequencies are between 1005 and 1062 cm-1 wavenumbers depending on the exact geometry (see Table 1, methoxy A-D). A comparison of Figures 3a and 4 shows that the C-H bonds of the CH3 group are rather parallel to the surface in case of methoxy, and more perpendicular to the surface in case of methanol. This difference would explain the different intensities of the CH3 frequencies observed in Figure 1a,b, when the surface selection rule is taken into account. This means that by looking at the ν(CO) frequency in IR alone one cannot distinguish between adsorbed methanol and methoxy on the clean Cu(110) surface. However, when the Cu surface shows an added row reconstruction, as can be found with preadsorbed oxygen, the binding energy on the long-bridge site parallel to the added row increases (methoxy C and D in Table 1). The calculated IR frequencies for the now more stable long-bridge sites for methoxy are at about 1005 cm-1. This means one can interpret Figure 1a,b at 200 K as adsorbed methanol on the oxygen-free case and adsorbed methoxy on the long-bridge sites parallel to the oxygenscopper added row on the oxygen-precovered surface. In ref 19 it was shown that the TPD after adsorption of 50 langmuirs of methanol on clean Cu(110) looks the same as after 5 langmuirs of methanol adsorption on an oxygen-precovered


Figure 4. Four stable geometries of the methoxy species on Cu(110) (see Table 1 for details). A1 is methoxy on the long-bridge site perpendicular to the surface, A is methoxy on the short-bridge site perpendicular to the surface, B1 is methoxy on the long-bridge site tilted to the surface, and B is methoxy on the short-bridge site tilted to the surface. Cu atoms are blue, carbons are green, oxygens are red, and hydrogens are light gray.

Figure 5. Thermal desorption spectra obtained after adsorption of 5 langmuirs of methanol at 180 K on oxygen precovered Cu(110). The heating rate was 2 K s-1. Mass 2 is hydrogen, mass 18 is water, mass 28 is carbon monoxide, mass 30 is formaldehyde, mass 31 is methanol, and mass 44 is carbon dioxide.

Cu(110) surface. The latter case is shown in Figure 5. This experimental result most likely is due to coadsorbed water from the residual gas during the long exposure. As the water is incorporated in the methanol structure (see above), partial dissociation of the water can take place during the long exposure time, leading to OH and maybe even O on the surface, which then has a marked influence on the desorption, as has been shown in ref 19. Therefore, the case of preadsorbed oxygen will be used in the following, as in this case a more defined starting point for the reaction is given compared to a layer with coadsorbed water and methanol. The latter gave very similar results, which are not presented here. To get more insight into the reactions going on, the oxygen-precovered copper surface experiments were designed to measure IR spectra as a function of surface temperature. After the adsorption of methanol on the 0.25 monolayer oxygenprecovered Cu(110) surface at a temperature of 100 K, IR spectra were recorded in situ during a following thermal desorption experiment with a heating rate of 0.1 K s-1 up to a temperature of 600 K. The measuring time for one spectrum was 70 s correspond-

Singnurkar et al.

Figure 6. IR band intensities as a function of temperature from temperature-programmed RAIRS experiments obtained with a heating rate of 0.1 K s-1 after the adsorption of 12 langmuirs of methanol at 100 K on oxygen precovered Cu(110). The intensities of the ν(CO) frequency of methanol and methoxy (squares) and the vibrational features at 1020 (stars) and 1098 cm-1 (triangles) are shown as a function of the surface temperature. The insert shows three IR spectra at three different temperatures.

ing to 390 added scans. Hence, every 7 K a new spectrum was recorded. As a reference the same experiment was done with the clean Cu surface. In this way a reference spectrum at the right surface temperature was available for each methanol spectrum. The results are shown in Figure 6. Due to the adsorption temperature of 100 K one finds a quite large ν(CO) band, which stays constant up to about 125 K, when it starts to decrease rapidly up to about 175 K. This is due to the desorption of the methanol multilayer. With desorption of the multilayer the ν(CO) signal shifts from about 1050 to 1040 cm-1 (see Figure 1). The ν(CO) signal of the methanol monolayer seems to be constant from 175 up to about 275 K (see Figure 6). However, one has to keep in mind that methoxy on the short-bridge site has the same ν(CO) as methanol. This fact together with the decrease of the CH3 frequencies shown in Figure 1 suggests that in this temperature region the reaction starts by methoxy formation from methanol. In addition, at about 210 K a shoulder appears in the IR at approximately 1020 cm-1 (see Figure 6). This vibrational feature can be assigned to methoxy bound on the long-bridge site according to the DFT results (methoxy C and D in Table 1). With increasing surface temperature the ν(CO) signal of both methoxy species in the IR decreases,and it disappears around 330 K (see Figure 6). At temperatures above 350 K a new small and broad vibrational feature at about 1098 cm-1 is found, which shows a constant small intensity up to about 430 K and then decreases again until it also disappears at about 450 K. The end of the methoxy signal in the RAIRS experiments (330 K) is at lower temperatures than the end of the thermal desorption of methanol and formaldehyde presented in Figure 5 (375 K). This difference is due to the two different heating rates (2 K s-1 in TDS and 0.1 K s-1 in the temperature-programmed RAIRS experiments). One can see that the vibration at 1098 cm-1 seen in Figure 6 is found in the temperature region after the formaldehyde desorption is finished (compare with Figure 5). Therefore, it is unlikely that this vibration is due to formaldehyde. According to our calculations the CO stretching vibration of formaldehyde would be around 1311 cm-1 (see



Figure 8. Calculated geometry of the η2-formaldehyde and OH species on an oxygen-precovered Cu(110) surface (see Table 3 for details). Left is the side view looking perpendicular to the CusO row; right is the top view with the CusO row on the left. Cu atoms are blue, carbons are green, oxygens are red, and hydrogens are light gray. This species seems to be the surface intermediate leading to CO2 formation (see text for details).

Figure 7. (a) Geometry of a formate species on Cu(110) in the side view. Cu atoms are blue, carbon is green, oxygens are red, and hydrogen is light gray. (b) Calculated IR spectra of formate and formaldehyde on Cu(110). The CO vibrations and CH vibrations are indicated in the spectrum.

Figure 7b and Table 1), but we do not see a vibrational feature in this wavenumber range in our experiments. From comparison with the TPD data and the literature it would be more likely that the vibration at 1098 cm-1 is associated with a formate group as CO2 desorption has been attributed to come from formate formed on the surface. The calculated structure for the formate species on the Cu(110) surface (see Table 1) is shown in Figure 7a together with the calculated IR spectrum (Figure 7b). Here the strongest vibration would be the OCH bending and the symmetric stretch of the C-O bond at 1321 cm-1. This value is about 200 cm-1 higher than the observed vibration, making an assignment of the observed vibration to a formate species unlikely. To find the intermediate with a weak vibration at about 1098 cm-1 several possible intermediates were calculated (see Table 1 for details). Formaldehyde bound via the oxygen atom would give a frequency of 1311 cm-1. Hydroxymethyl gives a vibration at 837 cm-1 very similar to η2-formaldehyde. Dioxomethylene would be around 990 cm-1, an HCO species would be around 1521 cm-1, and an OOH species should be found at about 672 cm-1. An intermediate in the form of η2-formaldehyde associated with an OH group on the oxygen precovered surface should give a very weak band around 1130 cm-1 according to the DFT results (see Table 3). In this case the formaldehyde molecule is far from the surface (dCu-C ) 4.1 Å), so the interaction can be described as a H-bonded interaction between the OH group on the surface and the formaldehyde molecule. This would fit to the experimental data. The geometry of this species is shown in Figure 8. In the side view one can see that the OH group is part of the CusO row and the formaldehyde species is bound to the OH via a hydrogen bond (left side of Figure 8). The right side in Figure 8 shows the top view with the CusO row on the left. One can clearly see that in this species most bonds are parallel to the surface, meaning that their vibrations will not give a RAIRS signal due to the surface selection rule. However,

an O-C-H wagging vibration has a small dipole moment perpendicular to the surface, according to the calculations leading to the observed small and broad vibrational band at 1098 cm-1. This intermediate then will further react to carbon dioxide and hydrogen on the surface, both of which will desorb at these temperatures. The calculated adsorption energy of this species is 1.42 eV (see Table 3). This would approximately correspond to a desorption temperature of about 500 K according to the Redhead formula.38 This means that this species is stable enough on the surface to be an intermediate in the CO2 formation. η2Formaldehyde has also been found in a RAIRS study of methanol reaction on an oxygen-precovered Ru(001) surface, giving a similar vibrational feature.39 In Table 3 also the values for methanol and methoxy species adsorbed on the oxygencovered Cu surface are given for comparison. Both species have a ν(CO) frequency around 1030 cm-1, meaning that they cannot explain the vibration measured at 1098 cm-1. If the η2-formaldehyde and OH intermediate leads to the formation of CO2, one should be able to increase the CO2 desorption by forming more of this species. In addition, one should be able to detect additional hydrogen desorption in the temperature region where CO2 desorbs, as this intermediate contains three hydrogen atoms. To do this experimentally, methanol was dosed at 270 K on the oxygen striped phase and then a TPD experiment was done from 270 up to 500 K. From Figure 5 one can see that adsorption at 270 K will lead to methanol and water desorption, leading to methoxy species adsorbed on the surface. Figure 9a shows that the subsequent TPD experiment reduces the amount of formaldehyde desorbing, while the amount of CO2 desorption hardly changes. This indicates that the intermediate leading to CO2 formation is stabilized by this experiment. In Figure 9b the same experiment was done, but there was a waiting period of 1 h at 270 K in UHV between methanol adsorption at 270 K and the TPD experiment. In this case the formaldehyde desorption is further suppressed and the amount of CO2 formed increased slightly. In addition, one could detect a small hydrogen peak and some water desorption in the temperature range of the CO2 desorption peak. This points to an increased formation of the η2formaldehyde OH species on the surface which leads to increased CO2, hydrogen, and water formation. Conclusions The adsorption and reaction of methanol on Cu(110) and on an oxygen stripe phase on Cu(110) was studied using RAIRS, TPD, and DFT. RAIRS measurements show very similar spectra


Singnurkar et al. Acknowledgment. This work was supported by the Austrian “Fonds zur Fo¨rderung der wissenschaftlichen Forschung (FWF)”. I.B. is indebted to the Hungarian Supercomputer Center (NIIF) for computer resources used during this work and to the Hungarian OTKA for support. The help of Martin Kornschober of the machine shop at the Institute of Solid State Physics is greatly appreciated. Supporting Information Available: Total energy of free molecules calculated with different density cutoff, adsorption energies of species with different Brillouin zone sampling on the Cu(110) surface, and the characteristic frequencies of the investigated species from different calculations, from our calculations and from experiment. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 9. (a) Thermal desorption spectra obtained after adsorption of 5 langmuirs of methanol at 270 K on oxygen-precovered (0.25 monolayer) Cu(110). The heating rate was 2 K s-1. (b) Thermal desorption spectra obtained after adsorption of 5 langmuirs of methanol at 270 K on oxygen-precovered (0.25 monolayer) Cu(110). After the adsorption a waiting period of 1 h at 270 K was used before desorption. The heating rate was 2 K s-1. Mass 2 is hydrogen, mass 18 is water, mass 28 is carbon monoxide, mass 30 is formaldehyde, mass 31 is methanol, and mass 44 is carbon dioxide.

for a monolayer of methanol with and without preadsorbed oxygen. DFT calculations showed that this is due to the fact that one cannot distinguish adsorbed methanol and methoxy adsorbed on the more stable short-bridge site by looking at the ν(CO) frequency. However, methoxy adsorbed in the longbridge site shows the ν(CO) vibration at lower wavenumbers, and it is found with increasing surface temperature. From the TPD results and the DFT calculations one can see that adsorbing 50 langmuirs of methanol on oxygen-free Cu(110) leads to coadsorption of water, which can be incorporated in the methanol rows formed on the surface. This coadsorbed water leads to a very similar TPD of this adsorbed layer as methanol adsorbed on the oxygen stripe phase. The formation of carbon dioxide during thermal desorption seems to take place via a previously unknown η-formaldehyde and OH intermediate on the surface that was detected in temperature-programmed RAIRS experiments and characterized by DFT. TPD experiments indicate that this species can be populated by methanol adsorption at elevated temperatures.

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JP802488N

DFT and RAIRS Investigations of Methanol on Cu(110) - American

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