Nonclassical Adsorption of Methanol on Palladium: The Competition

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Non-Classical Adsorption of Methanol on Palladium: The Competition between Adsorption of Single Molecules and Clusters Vasily V Kaichev, Aleksandra Selivanova, Anna Tsapina, Andrey Aleksandrovich Saraev, and Valerii I. Bukhtiyarov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00840 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 9, 2019

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The Journal of Physical Chemistry

Figure 1. PM IRRAS spectra of Pd(111) obtained after dosing 50 L methanol at 80, 90, and 100 K, respectively. 177x78mm (300 x 300 DPI)

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Figure 2. Evolution of in situ PM IRRAS spectra of a Pd(111) surface obtained during exposure 50 L methanol at 80 K and following heat in vacuum to 120 K. 177x71mm (300 x 300 DPI)


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Figure 3. The structures of linear dimer (1), cyclic dimer (2), and cyclic trimer (3) of methanol. 82x44mm (300 x 300 DPI)



Figure 4. Evolution of the ν(CO) and ν(OH) spectra of the Pd(111) surface obtained in situ during exposure in methanol at 100 K. The spectra were recorded one after another without delay; the time scale of the spectra was 1 min. 118x78mm (300 x 300 DPI)


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Figure 5. Evolution of the integrated intensity of the ν(CO) and ν(OH) bands during the stepwise heating of Pd(111) after dosing methanol at 100 K. 82x97mm (300 x 300 DPI)



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Non-Classical Adsorption of Methanol on Palladium: The Competition between Adsorption of Single Molecules and Clusters Vasily V. Kaichev,1,2,* Aleksandra V. Selivanova,1 Anna M. Tsapina,1 Andrey A. Saraev,1,2 and Valerii I. Bukhtiyarov1,2 1

Boreskov Institute of Catalysis, Novosibirsk 630090, Russia 2

Novosibirsk State University, Novosibirsk 630090, Russia

ABSTRACT: The adsorption of methanol on the Pd(111) surface at 80, 90, and 100 K has been studied by polarization modulation infrared reflection-absorption spectroscopy. It is found that the adsorption of methanol on the palladium surface does not proceed via the layer-by-layer mechanism. At temperatures below 100 K methanol adsorbs molecularly to form clusters containing several hydrogen-bonded CH3OH molecules. These clusters have low thermal stability and decompose even at 100 K to form isolated methanol molecules bonded with palladium atoms. As a result, at a temperature between 100 and 120 K methanol adsorbs intact to produce adsorbed isolated molecules. These adsorbed species fast desorb at a temperature above 120 K. The dehydration of methanol is not observed under used conditions.

1. INTRODUCTION


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The adsorption of various molecules on single-crystal surfaces was intensely studied in the second half of the 20th century. The focus was on the study of adsorption of small molecules such as O2, H2, CO, and NO because this is the initial step in many practical-important catalytic reactions.1-6 The adsorption of other molecules such as formaldehyde, methanol, ethanol, etc. has also been studied but to a lesser extent. In full agreement with Brunauer-Emmett-Teller theory many researchers observed that at low temperatures the molecules adsorb intact to form isolated species or ordered structures over solid surfaces. Because the adsorbent-adsorbate interaction is usually stronger than the adsorbate-adsorbate interaction, at first the adsorption proceeds with formation of only one layer of the adsorbate on the adsorbent surface (the monolayer adsorption) and then condensation (the multilayer adsorption) starts when the molecules from gas phase interact with the adsorbed species to form second, third, fourth, etc. layers. One of the most studied systems is the adsorption of CO on metal surfaces. It was shown that CO can adsorb to on-top, 2-fold bridge, and 3-fold hollow positions only to form different ordering surface phases on the Pd(111) and Pt(111) surfaces.2,7-8 At low coverages (θ), CO adsorbs on Pd(111) to 3-fold hollow positions to form the (3×3)R30º structure when the CO coverage achieves 0.33 monolayer (ML). Increasing the CO coverage leads to occupation of the 2-fold bridge positions and at θ = 0.5 ML another c(4×2) structure is observed. The saturation coverage (θ = 0.75 ML) corresponds to the (2×2) structure in which CO occupies the on-top and 3-fold hollow positions. The coverage increases with pressure or with decreasing the adsorption temperature. However, even at high coverages there are no "compression" structures or "floating" phases, and CO molecules occupy high symmetry sites only on the palladium surface. The CO condensation over solid surfaces proceeds at extremely low temperatures; this complicates studying the multilayer CO adsorption. In contrast, other molecules such as water or methanol condense even at the


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liquid nitrogen temperature. In particular, Christmann and Demuth studied the adsorption of methanol on Pd(100) and observed the multilayer adsorption at 77 K.9 They postulated that methanol adsorbs through a classical layer-by-layer growth mechanism. The first layer is formed by chemisorbed molecules bonded with palladium atoms through the oxygen lone-pair orbital. There is no long-range surface ordering, possibly owing to the low symmetry of the CH3OH molecules. The formation of the first layer of physisorbed molecules over the layer of chemisorbed methanol is not accompanied by the formation of hydrogen bonds. Only the subsequent layers do exhibit properties expected for hydrogen bonding. Afterwards, the same model was suggested by Gates and Kesmodel for the adsorption of methanol on Pd(111).10 It should be stressed that this model was based on the hypothesis that molecules of methanol exist in the gas phase as monomers only. However, there are many reports indicating that due to strong H-bonds, water, methanol, formaldehyde, etc. exist not only in the liquid phase, but also in the gas phase as clusters.11-16 These clusters usually consist of two, three, four and more molecules linked in chains, rings, or even 3D structures. It is very likely that these clusters can adsorb intact at least at low temperatures, thereby breaking the layer-by-layer growth of the adsorption layer. To elucidate the adsorption mechanism we performed an in situ study of the adsorption of methanol on Pd(111) at temperatures between 80 and 120 K using polarization modulation infrared reflection-absorption spectroscopy (PM IRRAS). This technique is quite suitable for the analysis of species residing at gas-solid and gas-liquid interfaces because at a high (grazing) angle of incidence, the absorption of p-polarized IR radiation by thin films on metal surfaces is enhanced so that even adsorbed species with a concentration of several hundreds of monolayer can be observed in the p-polarized infrared reflection spectrum (see Ref. (17) and refs therein). Because the contribution of the absorbance of the adsorbed species to the


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s-polarized infrared reflection spectrum is negligible, the difference between s- and preflectivities yields surface species information. The ratio between the difference and sum reflectivity can be used to compensate the gas phase absorbance, hence yielding a vibrational surface spectrum. This technique becomes especially popular in the field of in situ studies of the adsorption of various molecules on atomic-smooth metal surfaces in a wide pressure range from ultrahigh vacuum (UHV) to near ambient pressures (NAP). 2. EXPERIMENTAL METHODS The experiments were carried out in a custom-designed UHV/NAP instrument that consists of two stainless steel chambers. The first chamber denoted as the UHV chamber was used for preparation and characterization of the samples under study. The UHV chamber was equipped by a PHOIBOS-150-MCD-9 hemispherical energy analyzer, an XR-50 X-ray source with a dual Al/Mg anode, and an IQP-10/63 ion source (SPECS Surface Nano Analysis GmbH). The based pressure was 2×10-10 mbar provided by a turbomolecular pump coupled with a scroll-pump. The UHV chamber was also equipped by a xyz manipulator with a sample holder that provided transfer of the sample under study from the UHV chamber to the second chamber denoted as the catalytic cell. The catalytic cell was pumped separately by another turbomolecular pump coupled with a scroll-pump. The based pressure in the catalytic cell was 5×10-9 mbar. The UHV chamber and the catalytic cell are connected via a double-differentially pumped sliding seal.17 The catalytic cell was attached to a VERTEX 80v FTIR spectrometer (Bruker Optic GmbH) via two differentially pumped BaF2 windows for input and output IR radiation. The spectrometer was equipped by a mercury-cadmium telluride detector and a photoelastic modulator PEM-100 (Hinds instruments, Inc.) which provided a modulation of the polarization of IR radiation with


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frequency of 84 kHz. Sum (p + s) and difference (p - s) signals collected by the detector were processed in real time. A Pd(111) single crystal (disc with 8 mm diameter and 2 mm thickness) prepared by standard cutting and polishing techniques was used for the experiments. The crystal was set on the sample holder which contained two electrical power feedthroughs (3 mm diameter Mo rods) and a liquid N2 cooling system. The crystal was mounted between the Mo rods by two 0.2 mm diameter W wires and could be resistive heated from 80 to 1300 K. The temperature was measured with a Ktype thermocouple spot-welded to the crystal edge. Before each adsorption experiment, the Pd(111) surface was cleaned by a sequence of ion etching by 3 keV Ar+-ion beam at 300 K for 2 min, heating to 1270 K, oxidation during cooling in 10-6 mbar O2 between 1200 and 600 K, and final flash to 1000 K in UHV. After several cycles, no contaminants were detected by XPS. The cleaned Pd(111) single crystal was transferred to the catalytic cell with surface plane near horizon that provided the incident angle of the IR radiation of approximately 86°. Methanol was introduced to the catalytic cell through a variable leak valve. Before the experiments methanol was cleaned by freeze-thaw cycles. Pressure within the catalytic cell was measured with a cold cathode ionization gauge. For calculation of a methanol exposure in Langmuir (1 L = 10-6 Torr·s), the pressure indicated by the ionization gauge was corrected by the gauge sensitivity factor for CH3OH equal to 1.9. The IR spectra were recorded within a range of 6000-400 cm-1 at resolution of 4 cm-1. By polarization modulation of the incident infrared light and by monitoring the sum and difference interferograms, a surface vibrational spectrum (ΔR/R)=(Rp−Rs)/(Rp+Rs) was obtained. Before each experiment, a spectrum of the clean Pd(111) surface was obtained and all following spectra were divided by this background spectrum. By this method, the curvature in the initial spectrum


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due to the second-order Bessel function originating from the photoelastic modulation process was removed.18 Such processing allowed us to obtain high-quality absorption spectra of adsorbates within a range of 4000-900 cm-1. 3. RESULTS AND DISCUSSION Figure 1 shows the PM IRRAS spectra of the Pd(111) surface obtained after exposure in methanol at 80, 90, and 100 K. The exposure was performed at 5×10-7 mbar CH3OH during 130 s. One can see that the spectrum obtained at 100 K consists of sharp peaks at 3309, 2959, 2832, 1473, 1144, and 1029 cm-1. The presence of the OH stretching band near 3300 cm-1, the CH3 stretching bands near 2960 and 2830 cm-1, the characteristic CH3 symmetric bending near 1450 cm-1, and the intensive band of the CO stretching mode near 1030 cm-1 together indicates that the adsorption is molecular. Frequency values and their assignment performed on the basis of spectroscopic characteristics of methanol adsorbed and condensed on Pd(111) and Rh(100)10,19 are given in Table 1 along with corresponding data on gaseous, liquid, and crystalline methanol.20 The spectra obtained at lower temperatures consist of the same vibrational features; however, the absorption peaks are shifted and broadened. The main distinction is observed in the OH stretching mode which is very sensitive to hydrogen bonding. At 100 K the OH stretching mode is characterized by the sharp peak at 3309 cm-1 whereas at lower temperatures two bands are observed at 3210-3216 and 3300-3303 cm-1. Comparing the ν(OH) frequencies for gaseous, liquid, and crystalline methanol20 we can conclude that OH-bond formation leads to shift the ν(OH) band from 3682 to 3337 or even 3284 cm-1. Hence, we assign the bands at 3300-3309 cm1

to isolated chemisorbed CH3OH molecules, while the band at 3210-3216 cm-1 could be

attributed to CH3OH molecules linked by hydrogen bonds with other CH3OH molecules. A


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similar shift of the ν(OH) band was observed by Parmeter with co-workers,19 who assigned the bands at 3335 and 3245 cm-1 to methanol chemisorbed and condensed on Rh(100), respectively. Christmann and Demuth9 attributed the ν(OH) bands at 3345 and 3285 cm-1 to methanol chemisorbed and condensed on Pd(100). In contrast, according to data by Gates and Kesmodel,10 methanol molecules chemisorbed and condensed on Pd(111) are characterized by almost the same ν(OH) frequencies equal 3355 and 3320 cm-1, respectively. The small shift in the latter case may indicate that the methanol molecules do not form hydrogen bonds in the condensed state.

Figure 1. PM IRRAS spectra of Pd(111) obtained after dosing 50 L methanol at 80, 90, and 100 K, respectively.


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Table 1. Vibrational frequencies (cm-1) and mode assignments for chemisorbed and condensed methanol on Pd(111) and Rh(100) as well as gas phase, liquid, and crystalline CH3OH. CH3OH20

assignment

chemisorbed

chemisorbed

a

19

chemisorbed on Pd(111)10

multilayers on Pd(111)10

3245

3355

3320

2980

2950

2950

2855

2825

2835

gas

liquid

crystalline

on Pd(111)

ν(OH)

3682

3337

3284

3309

νa(CH3)

2973

2934

2955

2959

νs(CH3)

2844

2822

2829

2832

δs(CH3)

1455

1455

1445

1473

1455

1475

1440

1445

ρ(CH3)

1116

1114

1142

1144

1135

1180

1110

1120

ν(CO)

1034

1029

1029

1029

1005

1040

1010

1010

a

on Rh(100)

multilayers on Rh(100)19

3335 2960

Spectrum was observed after dosing 50 L methanol at 100 K.

ν – stretching mode, δ – bending mode, ρ – rocking mode, s – symmetric, a – asymmetric.

Another distinction in our spectra concerns the most intensive bands corresponded to the ν(CO) stretching mode (Figure 1). One can see that at 100 K the band contains the peak at 1029 cm-1 with a weak shoulder at 1035 cm-1. In contrast, at lower temperatures this band contains two peaks at 1035 and 1045 cm-1. According to the literature,20 this band is also sensitive to hydrogen bonding, therefore, we attribute the peak at 1029 cm-1 to isolated chemisorbed molecules, and the peaks at 1035 and 1045 cm-1 to hydrogen-bonded species. The difference in the structure of adsorbed layers formed at 80-90 K and at 100 K is also confirmed by changes in bands near 1500, 1470 and 1440 cm-1. The spectrum obtained at 100 K contains sharp peaks at 1511, 1473, and 1144 cm-1 with the full width at half maximum (FWHM) equal to approximately 24, 11, and 7 cm-1, respectively (Figure 1). According to Falk and Whalley,20 these bands correspond to the δas(CH3) and δs(CH3) bending and ρ(CH3) rocking vibrations, respectively. It should be noted that in the previous studies9-10,19 the authors used


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electron energy loss spectroscopy, and the rocking vibration mode was not observed in the spectra of adsorbed methanol due to poor spectral resolution. At lower temperatures, the peaks are broader and shifted to the low-frequency region (Figure 1). For example, the maximum of the ρ(CH3) band is shifted to 1133 cm-1 in the spectrum obtained at 80 K. In addition, the band has two shoulders at 1124 and 1113 cm-1, which leads to increasing FWHM to approximately 30 cm1

. The δas(CH3) and δs(CH3) bands have maximums at 1500 and 1459 cm-1, respectively. This

means that the local environment of the methyl groups depends on the adsorption temperature, that is, the structures of the adsorption layers formed at 80-90 K and at 100 K are different. In order to elucidate the adsorption mechanism we studied the formation and subsequent transformation of the adsorption layer on Pd(111) during methanol exposure at 80 K and following heating to 120 K. In this experiment we obtained a series of cascade spectra; the spectra were recorded without delays; the time scale of the spectra was approximately 1 min. The results are present in Figure 2. One can see that the first spectrum recorded in UHV at 80 K before dosing methanol does not contain any peaks. The next three spectra were recorded during dosing 50 L methanol at 80 K. As a result, at first a ν(CO) band at 1045 cm-1 with a shoulder at 1032 cm-1 develops indicating the formation some hydrogen-bonded species on the palladium surface. The following heat to 90 K leads to a shift of the ν(CO) bands to 1039 and 1029 cm-1. At 100 K the position of the bands is the same. Only at 120 K the first band shifts to 1034 cm-1. The heating also leads to changes in the relative intensity of the bands (Table 2). The contribution of the peak at 1029-1032 cm-1 to the total intensity of the ν(CO) spectrum increases from 27% to 68% when Pd(111) is heated from 80 to 120 K. After heating to 115 K the bands start to decrease in intensity, then at 120 K their intensities drops down to zero due to desorption of methanol.


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Figure 2. Evolution of in situ PM IRRAS spectra of a Pd(111) surface obtained during exposure 50 L methanol at 80 K and following heat in vacuum to 120 K. Table 2. Observed wavenumbers of the peaks in the ν(CO) and ν(OH) spectra obtained during exposure 50 L methanol at 80 K and following heating in vacuum to 120 K. T, K

ν(CO), cm-1

ν(OH), cm-1

peak 1

peak 2

peak 1

peak 2

80

1032 (27%)

1045 (73%)

3210 (26%)

3300 (74%)

90

1029 (32%)

1039 (68%)

3193 (30%)

3302 (70%)

100

1029 (45%)

1039 (55%)

3194 (28%)

3305 (72%)

105

1029 (48%)

1039 (52%)

3195 (28%)

3306 (72%)

110

1029 (51%)

1039 (49%)

3196 (29%)

3306 (71%)

115

1029 (57%)

1038 (43%)

3198 (37%)

3306 (63%)

120

1029 (68%)

1034 (32%)

3200 (39%)

3306 (61%)

The ν(OH) spectra also depend on temperature (Figure 2). A ν(OH) band at 3300 cm-1 with a shoulder at 3210 cm-1 are observed in the spectra at 80 K. However, after heating to 100 K, the ν(OH) band transforms to a well-defined doublet containing the peaks at 3305-3306 and 3194-


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3196 cm-1 (Table 2). The relative intensity of these peaks is weakly dependent on temperature. Again, at 120 K the peaks disappear due to desorption of methanol. It is very important to note that both the ν(CO) and ν(OH) spectra observed at 100 K (Figure 2) differ significantly from the spectra shown in Figure 1 obtained after methanol exposure at 100 K. This indicates that the structures of the adsorption layers formed at 80-90 K and at 100 K are different, and the full realignment of the adsorbed methanol molecules at 100 K is slow. We can interpret these data as the formation of adsorbed methanol clusters containing several hydrogen-bonded CH3OH molecules at 80-90 K and prevalent adsorption of isolated CH3OH molecules at 100 K. According to Huisken and Stemmler,11 monomers, trimers, and tetramers of methanol in the gas phase are characterized by the CO stretching vibrations at 1030, 1042, and 1044 cm-1. The simplest structures of linear dimers and cyclic dimers and trimers are presented in Figure 3. The ν(OH) stretching vibrations are more sensitive to the H-bond formation and the frequency shift can exceed 400 cm-1. Thus, Han with co-workers15 found that the ν(OH) stretching vibration in methanol clusters (CH3OH)n in the gas phase decreases from 3683 to 3240 cm-1 when n increases from 1 to 5. The formation of the hydrogen-bonded cluster of methanol was also detected in the solid state. Doroshenko with co-workers21 used a matrix-isolation technique to show that the IR spectrum of methanol trapped in an Ar matrix at 15 K contains a narrow band with a frequency 3667 cm-1 and several weak wide bands in the range of 3200-3600 cm-1. The narrow band was assigned to the vibration of the hydroxyl group, which is not involved in the formation of H-bonds and belongs to the methanol monomer. This band decreased with temperature and even dropped to zero at 40 K when the Ar matrix began to destroy. In contrast, the intensity of the other bands increased with temperature. Based on DFT calculations, the authors assigned the band in the frequency range of 3480-3525 cm-1 to methanol


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dimers, the band in the frequency range of 3370-3430 cm-1 to trimers, and the band in the frequency range of 3250-3350 cm-1 to tetramers. The most red-shifted bands were assigned to the largest clusters.

Figure 3. The structures of linear dimer (1), cyclic dimer (2), and cyclic trimer (3) of methanol. Based on these data we can assume that the adsorbed methanol molecules having interaction directly with palladium atoms are characterized by the ν(CO) and ν(OH) stretching bands at approximately 1030 and 3309 cm-1. Most likely the molecules adsorb mainly to the terminal position; the 2D ordering hydrogen-bonded structures are not formed due to the low symmetry of the CH3OH molecules. These are isolated adsorbed species. These stretching vibrations shift to 1045 and 3210 cm-1, respectively, in the methanol molecules that formed the adsorbed clusters due to strong H-bonding. The structure of these clusters may have 3D character. The adsorbed molecules that hydrogen-bonded with other CH3OH molecules can be characterized by middle values of the ν(CO) and ν(OH) frequencies near 1039 and 3195 cm-1, respectively. We can speculate that the adsorption of methanol on the palladium surface at 80 K does not proceed via the layer-by-layer mechanism. The methanol molecules adsorb at once as clusters containing several hydrogen-bonded molecules. However, the adsorbed clusters are not stable and can be transformed to chemisorbed species at higher temperatures. Indeed, we found that adsorption of methanol at 100 K starts from the formation of the methanol clusters which fast


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decompose to form chemisorbed species. Figure 4 shows a series of in situ cascade ν(CO) and ν(OH) spectra of the Pd(111) surface during adsorption of methanol at 100 K. The main spectral features are summarized in Table 3. The first spectra were recorded in UHV before dosing methanol and did not contain any peaks. The next spectra were recorded during dosing 5×10-7 mbar methanol for 130 s. One can see that the peak at 1044 cm-1 with a weak shoulder at 1029 cm-1 appears in the second ν(CO) spectrum. Then the peak at 1044 cm-1 slightly increases and stabilizes whereas the shoulder increases strongly. In the final spectrum the main peak is observed at 1029 cm-1 with a shoulder at 1042 cm-1. In the second ν(OH) spectrum two peaks at 3310 and 3230 cm-1 are observed; the ratio of their intensities is 66:34. Then both peaks increase in intensity and slightly shift. In the third spectrum the peaks lie at 3306 and 3222 cm-1 and the ratio is 77:23. In the fourth spectrum the peaks lie at 3306 and 3204 cm-1 and the ratio is 93:7. The last spectrum consists of a single peak at 3306 cm-1. These findings indicate that methanol adsorbs on the Pd(111) surface at 100 K at least in two different states. The first one is characterized by the ν(CO) and ν(OH) bands at 1042-1044 and 3204-3230 cm-1 and could be attributed to adsorbed methanol molecules hydrogen-bonded with other methanol molecules. The second one is characterized by the bands at 1029 and 3306-3310 cm-1, respectively. We attribute these bands to adsorbed isolated molecules of methanol.


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Figure 4. Evolution of the ν(CO) and ν(OH) spectra of the Pd(111) surface obtained in situ during exposure in methanol at 100 K. The spectra were recorded one after another without delay; the time scale of the spectra was 1 min.

Table 3. Wavenumbers and relative intensities of the peaks in the ν(CO) and ν(OH) spectra presented in Figure 4. time, min

ν(CO), cm-1

ν(OH), cm-1

peak 1

peak 2

peak 1

peak 2

1

–

–

–

–

2

1029 (27%)

1044 (73%)

3230 (34%)

3310 (66%)

3

1029 (45%)

1044 (55%)

3222 (23%)

3306 (77%)

4

1029 (67%)

1043 (33%)

3204 (7%)

3306 (93%)

5

1029 (74%)

1042 (26%)

–

3306 (100%)

The evolution of total intensity of the ν(CO) and ν(OH) spectra obtained during the stepwise heating of Pd(111) after dosing methanol at 100 K are presented in Figure 5. One can see that the


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total intensity of both bands does not change between 100 and 110 K. At 115 K a slow decrease of both signals is observed. Following heating to 120 K leads to drop out the absorbance signal due to desorption of methanol. Since we did not observed an appearance of the strong ν(CO) band of adsorbed CO in a range 1800-2000 cm-1, the dehydrogenation of methanol to CO does not proceed at the temperatures used. This means that the activation energy of desorption is lower than the activation energy for dehydrogenation. It should be noted that Christmann and Demuth9 used a temperature desorption technique and observed the sharp desorption peak of physisorbed methanol at 142 K; they estimated of the activation energy for desorption to be 7.2 kcal/mol on Pd(100). In contrast, Gates and Kesmodel10 observed the desorption of the physisorbed phase from Pd(111) even at 160 K. Most likely, these contradictions are associated with errors in measuring the temperature or difference in the heating rate.

Figure 5. Evolution of the integrated intensity of the ν(CO) and ν(OH) bands during the stepwise heating of Pd(111) after dosing methanol at 100 K.


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Hence, our data indicate that at 100 K methanol adsorbs on the Pd(111) surface mainly as isolated molecules. At lower temperatures methanol forms hydrogen-bonded clusters pinned on the palladium surface. We cannot exclude that these clusters form in the gas phase and attach intact to the palladium atoms after colliding with the surface. The temperature of 80-90 K is too low to activate decomposition of the clusters and following surface migration of methanol molecules; these processes start at approximately 100 K. These clusters may be dimers or other ring/chain polymeric structures containing several hydrogen-bonded CH3OH molecules (Figure 3). Such structures had been observed for water adsorbed on metal surfaces. For example, Benndorf and Madey22 found that molecules of water adsorbed on the Ni(110) surface form a c(2×2) bilayer structure when θ > 0.5-1 ML; at lower coverages, H2O dimers bonded via oxygen lone-pair orbitals to Ni substrate atoms exist on the surface. Afterwards, it was showed that water adsorption on close packed surfaces such as Pt, Ru, Ni, Pd and Rh, has been based on the formation of a hydrogen-bonded two dimensional ice film similar in structure to bulk ice. This model, which has been discussed in detail by Thiel and Madey23 and Henderson24, involves formation of a buckled ―bilayer‖ of ice, analogous to the (0001) basal plane of hexagonal ice or (111) plane of cubic ice. Recently, the formation of closed-loop structures comprising five or more water molecules at 25 K was observed by Liriano with co-workers25 who studied the adsorption of water on the atomically flat Cu(111) surface by scanning tunneling microscopy (STM). In this model, water forms a network of interlinked hexamers, in which each water molecule has three hydrogen bonds to its neighbors. Half of the water molecules are bonded to the surface via electron donation from the oxygen atom to the metal, with alternate water molecules adsorbed between these, forming the upper half of a bilayer structure to complete the hydrogen-


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bonded network. The review of experimental and theoretical studies of adsorbed water can be found elsewhere.26 A similar adsorption model can be developed for methanol. According to DFT calculations,27 the lowest energy structure of small free methanol clusters, (CH3OH)n, n = 2-5, is markedly similar to the water cluster counterparts. Methanol dimer exhibits a linear hydrogen bond, while trimer, tetramer, and pentamer are ring structures with dangling methyl groups oriented alternately ―up‖ and ―down‖ from the ring formed by the oxygen atoms. Stable non-cyclic structures were found for n > 5.28 The structure of oligomers is complicated by their size. For example, the methanol octamer may have a layered structure with two 4-rings connected in a twisted fashion or even may be like a distorted cuboid. To elucidate the structure of adsorbed methanol clusters, additional studies are needed engaging the structure sensitive methods such as STM, low energy electron diffraction, or X-ray photoelectron diffraction. It should be noted that cluster adsorption can occur not only with methanol, but also with other molecules prone to the formation of hydrogen bonds. We can speculate that at elevated pressure such adsorbed clusters can form even at ambient temperatures and take part in some catalytic reactions as a precursor state. Because this precursor state aggregates several hydrogen-bonded molecules, the rate of some bimolecular reactions such as, for example, condensation reactions may be accelerated on the catalyst surface. 4. CONCLUSIONS We briefly summarize the main conclusions which result from our data for methanol adsorption on Pd(111). The adsorption of methanol on the palladium surface at 80-90 K does not proceed via the layer-by-layer mechanism. At temperatures below 100 K methanol adsorbs molecularly to form clusters containing several hydrogen-bonded CH3OH molecules. These clusters have low thermal stability and decompose even at 100 K to form isolated chemosorbed


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methanol molecules. This competition between adsorption of isolated single molecules and hydrogen-bonded clusters indicates that the energy of H-bonding between methanol molecules is slightly higher than the energy of bonding methanol with palladium atoms at temperatures below 100 K.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by Budget Project No. АААА-А17-117041710078-1 for Boreskov Institute of Catalysis. REFERENCES 1.

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20. Falk, M.; Whalley, E., Infrared Spectra of Methanol and Deuterated Methanols in Gas, Liquid, and Solid Phases. J. Chem. Phys. 1961, 34, 1554-1568. 21. Doroshenko, I.; Pogorelov, V.; Sablinskas, V.; Balevicius, V., Matrix-Isolation Study of Cluster Formation in Methanol: O–H Stretching Region. J. Mol. Liq. 2010, 157, 142-145. 22. Benndorf, C.; Madey, T. E., Adsorption of H2O on Clean and Oxygen-Preposed Ni(110). Surf. Sci. 1988, 194, 63-91. 23. Thiel, P. A.; Madey, T. E., The Interaction of Water with Solid Surfaces: Fundamental Aspects. Surf. Sci. Rep. 1987, 7, 211-385. 24. Henderson, M. A., The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited. Surf. Sci. Rep. 2002, 46, 1-308. 25. Liriano, M. L.; Gattinoni, C.; Lewis, E. A.; Murphy, C. J.; Sykes, E. C. H.; Michaelides, A., Water–Ice Analogues of Polycyclic Aromatic Hydrocarbons: Water Nanoclusters on Cu(111). J. Am. Chem. Soc. 2017, 139, 6403-6410. 26. Hodgson, A.; Haq, S., Water Adsorption and the Wetting of Metal Surfaces. Surf. Sci. Rep. 2009, 64, 381-451. 27. Rai, D.; Kulkarni, A. D.; Gejji, S. P.; Pathak, R. K., Methanol Clusters (CH3OH)n, n = 3– 6 in External Electric Fields: Density Functional Theory Approach. J. Chem. Phys. 2011, 135, 024307. 28. Kazachenko, S.; Bulusu, S.; Thakkar, A. J., Methanol Clusters (CH3OH)n: Putative Global Minimum-Energy Structures from Model Potentials and Dispersion-Corrected Density Functional Theory. J. Chem. Phys. 2013, 138, 224303.


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For Table of Contents Only


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Nonclassical Adsorption of Methanol on Palladium: The Competition

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