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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Surface-Templated Assembly of Molecular Methanol on the Thin Film “29” Cu(111) Surface Oxide Andrew J. Therrien, Alyssa J. R. Hensley, Ryan T. Hannagan, Alex C. Schilling, Matthew D. Marcinkowski, Amanda M. Larson, Jean-Sabin McEwen, and E. Charles H. Sykes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10284 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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The Journal of Physical Chemistry

Surface-Templated Assembly of Molecular Methanol on the Thin Film “29” Cu(111) Surface Oxide Andrew J. Therrien,1 Alyssa J. R. Hensley,2 Ryan T. Hannagan,1 Alex C. Schilling,1 Matthew D. Marcinkowski,1 Amanda M. Larson,1 Jean-Sabin McEwen,2,3,4,5,6* and E. Charles H. Sykes1* 1Department

of Chemistry, Tufts University, Medford, MA 02155

2The

Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164 3Department

of Physics and Astronomy, Washington State University, Pullman, WA 99164

4Department

of Chemistry, Washington State University, Pullman, WA 99164

5Department

of Biological Systems Engineering, Washington State University, Pullman, WA

99164 6Institute

for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352

*Correspondence to: [email protected] (J.-S. M.), [email protected] (E. C. H. S.)

Abstract Identifying and characterizing the atomic-scale interaction of methanol with oxidized Cu surfaces is of fundamental relevance to industrial reactions, such as methanol steam reforming and methanol synthesis. In this work, we examine the adsorption of methanol on the well-defined “29” Cu oxide surface using a combination of experimental and theoretical techniques, and elucidate the atomic-scale interactions that lead to a unique spatial ordering of methanol on the oxide thin film. We determine that the methanol chain structures form firstly due to epitaxy with the underlying “29” oxide surface. Specifically, the geometry of the “29” oxide is such that there are spatially adjacent Oδ− sites, in the form of O adatoms, and Cuδ+ species, within the Cu2O-like rings, which allow for methanol to simultaneously bond to the surface via an Omethanol-Cuδ+ dative bond and an OHmethanol-Oδ− hydrogen bond. The methanol-oxide bond strength outweighs the strength of the methanol-methanol hydrogen bonds on the “29” Cu oxide, unlike methanol assembly on bare coinage metal surfaces on which hydrogen bonding between adjacent molecules leads to ordered arrays. Secondly, weaker long-range interactions lead to the formation of chains of only even numbers of methanol molecules. Together, this work reveals that, unlike on metal surfaces, the corrugation of the oxide surface drives methanol adsorption to preferred binding sites, preventing intermolecular hydrogen bonding and dictating the adsorption geometry. 1 ACS Paragon Plus Environment

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Introduction Cu-based heterogeneous catalysts are extensively used in industry for economically important reactions such as water-gas shift, alcohol oxidation, methanol synthesis, and methanol reforming.1–7 Due to the wide range of applications for methanol reactions on Cu, there have been many model studies to understand the behavior of methanol on Cu surfaces.8–17 The balance between methanol-methanol and methanol-surface interactions leads to a wide variety of observed coverage dependent structures.18 In general, methanol adsorbs non-dissociatively to coinage metal surfaces by dative bonding atop a metal atom through a lone pair of electrons on the O atom,18–20 which imparts a chirality to the adsorption geometry.21 This binding motif has also been observed for larger alcohols.22 Hydrogen bonding between methanol molecules leads to the formation of a variety of hexamer and chain structures, which are related to the coverage and symmetry of the surface.18 Given that the reactivity of methanol in surface science studies has been shown to depend heavily on O coverage,10,23–27 it is also of interest to understand how the lateral interactions between methanol molecules are affected when adsorbed on an oxidized Cu surface. The oxidation of Cu has been heavily studied, and a number of oxide structures have been discovered.28,29 On a Cu(111) substrate, a variety of complex Cu2O(111)-like islands and films have been reported.28–36 Bulk Cu2O(111) is made up of buckled hexagonal rings with linear OCu-O bonds, resulting in upward and downward pointing O atoms. In the center of these rings are the upward pointing O atoms of the next layer.36,37 The “29” oxide has been thoroughly characterized in previous studies and is a Cu2O(111)-like single layer oxide film distorted such that it is commensurate with the Cu(111) substrate, as shown in Figure S1.33–36 The “29” oxide is a surface-saturated oxide, which has the highest density of oxygen preceding bulk oxidation, but a very limited number of examples exist regarding its interactions with molecules. A single unit cell of the “29” oxide structure consists of six buckled hexagonal rings made of linear O-Cu-O bonds. Inside five of these oxide rings reside O adatoms bound directly with the Cu(111) substrate below. With the structural details of the surface well described, the “29” oxide surface serves as an ideal model surface for understanding the interaction of methanol with oxidized Cu surfaces. The self-assembly of methanol on flat metal surfaces is dominated by hydrogen bonding. However, this intermolecular hydrogen bonding can be disrupted on oxide surfaces, where the structure and energetics of hydrogen bonded molecular clusters can be very different. This is seen for the case of water,38 which forms hydrogen bonded ice clusters on metals at cryogenic temperatures,38–41 whereas water hexamers have been shown to be surprisingly stable on partially oxidized Cu,42 and form monomers and chain structures on other oxide surfaces.43–49 Herein, using temperature programed desorption (TPD), scanning tunneling microscopy (STM), reflection absorption infra-red spectroscopy (RAIRS), and density functional theory (DFT), we show in detail how the geometry and chemical properties of surface sites on the “29” oxide surface template highly ordered molecular methanol adsorption. We have also previously 2 ACS Paragon Plus Environment

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characterized the defects on the “29” oxide surface,50 which occur at a very low density and therefore allows for the careful examination of the interaction of methanol on the defect-free “29” oxide surface. Methanol desorbs reversibly from the “29” oxide surface, but unlike for metal surfaces, the arrangement of methanol on the oxide is driven by adsorption at specific sites on the oxide. These sites allow for methanol to simultaneously bond to the surface via both a dative bond and a hydrogen bond, as compared to metals on which maximizing hydrogen bonding between molecules dominates their assembly on the surface. Materials and Methods 1.1.

Temperature Programed Desorption (TPD)

TPD experiments were carried out in an ultra-high vacuum (UHV) chamber with a base pressure of < 1 × 10−10 mbar. The chamber was equipped with a quadrupole mass spectrometer (Hiden) and the Cu(111) crystal was able to be cooled to 85 K with liquid nitrogen and resistively heated to 750 K. The Cu(111) crystal was cleaned by Ar+ sputtering and annealing to 750 K. The “29” oxide film was formed by exposure to O2 gas at a pressure of 5 × 10−6 mbar for 3 minutes at a sample temperature 650 ± 20 K. The formation of the “29” oxide was verified by LEED (OCI Vacuum Microengineering). Methanol (Alfa Aesar, ultrapure HPLC grade 99.8%) and methanol-d4 (Aldrich, 99.8%) were further purified via freeze-pump-thaw cycles prior to deposition via a high precision leak valve. One monolayer (ML) of methanol is defined as the saturation coverage of the first surface layer. TPD experiments were performed with a linear heating ramp of 1 K s−1. 1.2.

Scanning Tunneling Microscopy (STM)

For the STM experiments, the sample was prepared in a preparation chamber (P = 2 × mbar). The Cu(111) crystal was cleaned by Ar+ sputtering and annealing to 750 K. The “29” oxide film was formed by exposure to O2 gas (Airgas, USP grade) at a pressure of 5 × 10−6 mbar for 3 minutes at a sample temperature 650 ± 20 K. The sample was then transferred in UHV to the STM chamber (P = 1 × 10−11 mbar) and into the pre-cooled STM stage at 5 K for methanol (Alfa Aesar, ultrapure HPLC grade 98.8%) experiments. After anneals the sample was cooled back to 5 K before imaging was conducted using a low-temperature Omicron NanoTechnology STM. Images were acquired using an etched W tip, and biases are reported with respect to the sample. 10−10

1.3.

Reflection Absorption Infra-Red Spectroscopy (RAIRS)

RAIRS experiments were performed with an out-of-vacuum Bruker Fourier transform infra-red source (Tensor II) and liquid nitrogen cooled MCT detector, with a reflection angle of 5° to the sample. The sample was in UHV and IR light traveled in and out of UHV through ZeSe windows enclosed in a dry-gas purge. All sample preparation followed the same procedures as above. The background for reported RAIRS data was the bare “29” oxide surface at 85 K. 3 ACS Paragon Plus Environment


1.4.

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Density Functional Theory (DFT)

The DFT calculations performed here utilized the Vienna Ab Initio Simulation Package (VASP) code51,52 where the core electrons were treated with the projector augmented wave (PAW) method.53,54 The PAW potentials updated in 2007 were used here. The valence electrons for all systems were described using the generalized gradient approximation (GGA) with the optB88-vdW functional.55,56 The energy cutoff for the plane wave basis set was set to 500 eV, and the electron smearing was described by the Gaussian smearing method with a width of 0.2 eV. All surface calculations were performed using a (1 × 2 × 1) Monkhorst-Pack k-points mesh.57 Each ground-state optimization calculation was considered converged when the total energy changed by less than 10–6 eV, and the forces between atoms were smaller than 0.02 eV/Å. The DFT-based STM images were generated using the method discussed in our previous work.33,34 The “29” oxide surface structure was modeled by putting a Cu2O-like layer on a 4 layer thick Cu(111) surface with a 13𝑅46.1° × 7𝑅21.8° supercell, as shown in Figure S1. The bottom 2 layers of the slab were kept fixed in their bulk positions, with a lattice constant of 3.629 Å. The Cu2O-like layer is made from fused hexagonal rings, each with 6 Cu atoms and 6 O atoms. There are 6 of these hexagonal rings per “29” oxide unit cell, which has 18 Cu oxide atoms and 12 O oxide atoms in total. There are also O adatoms in the center of 5 of the 6 rings, which adsorb at hollow sites of the Cu(111) surface where they are bound most strongly.58 The details of the construction and verification of the surface model of “29” oxide support was thoroughly investigated elsewhere, and the resulting “29” oxide support model is used here.33,34 The DFT-based IR spectra were calculated using density functional perturbation theory (DFPT)59 in order to simultaneously generate the Born effective charges and vibrational modes for the adsorbed methanol species. The IR intensities were then calculated according to:

𝐼discrete =

∑ |∑ 𝛼

𝑠,𝛽

∗ ( ) ( │ ) 𝑍𝛼𝛽 𝑠 𝑒𝛽 𝑠 𝜐

|

2

(1)

∗ where 𝑍𝛼𝛽 = ∂𝑃𝛽 ∂𝑅𝛼 and is the Born effective charge tensor for the sth atom, Pβ is the polarization, Rα is the ionic coordinate, 𝑒𝛽is the normalized vibrational eigenvector for the υth mode, and α and β are the Cartesian polarization directions.60,61 Only the surface normal components of the Born effective charge tensors were used here in order to account for this surface specific selection rule. Once the discrete, surface normal active IR intensities were calculated, DFT-based spectra were generated by fitting a Gaussian function to each pair of intensities and frequencies using:

4 ACS Paragon Plus Environment

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𝐼spectra =

1

―

𝑒 2𝜋𝑤2

(𝜐 ― 𝜐0)2

(2)

2𝑤2

where w is the distribution width (chosen here to be 20 cm-1), υ is the vibrational frequency value, and υ0 is the vibrational frequency around which the distribution is centered. Finally, the spectra of the individual vibrational modes are summed in order to generate the overall vibrational spectra for each structure of interest. The electronic adsorption energy was calculated according to: 𝐸ads = 𝐸CH3OH Oxide ― 𝐸Oxide ― 𝑁CH3OH𝐸CH3OH

(3)

where 𝐸CH3OH/Oxide, 𝐸Oxide, and 𝐸CH3OH are the total, DFT-calculated energies for the methanol adsorption surface, the “29” oxide alone, and gas phase methanol; 𝑁CH3OH is the number of adsorbed methanol. Average adsorption energy was calculated as Eads normalized by the number of methanol adsorbed in a particular system. In order to investigate the thermodynamic cause behind methanol dimer formation on the “29” Cu oxide, we have calculated the interaction energy between adsorbed methanol according to:

𝐸int =

Isolated 𝐸Mix ads ― ∑Sites𝑁CH3OH𝐸ads

∑

(4)

Sites𝑁CH3OH

Isolated where 𝐸Mix are the adsorption energies of the mixture of methanol and isolated ads and 𝐸ads methanol. The summation in Equation 4 runs over each site occupied in the “mixed” system. The phase stability of methanol configurations on the “29” Cu oxide was determined by using the formation energy, which allows for easy comparison of relative 𝐸Mix ads values and is calculated according to:

(

𝐸form = 𝐸Mix ads (𝑁CH3OH) ―

𝑁CH3OH Max 𝐸Mix ads (𝑁CH3OH) 𝑁Max CH3OH

)

(5)



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Results and Discussion

Figure 1. Series of intact methanol TPD traces from the “29” Cu2O/Cu(111) oxide surface.

Methanol desorption from the “29” oxide surface was probed by temperature programed desorption (TPD). A series of TPD curves of intact methanol desorption are shown in Figure 1. The monolayer desorption peak at 171 K does not shift with respect to methanol coverage, indicative of a non-dissociative first-order desorption process. As the methanol coverage is increased, the monolayer peak saturates and a desorption feature around 138 K is seen, consistent with desorption of methanol multilayers.16 Using a Redhead analysis with a pre-exponential factor of 1015 s−1,62,63 we obtain a binding strength to the surface of −0.53 eV when the methanol coverage is less than one monolayer. The desorption temperature of methanol from the “29” oxide surface (171 K) is slightly higher in temperature than methanol desorption from the Cu(111) terraces, which is around 163 K,16 suggesting that methanol is bound slightly more strongly to the oxide surface. There is an additional subtle difference between the nature of methanol desorption from the “29” oxide as compared to Cu(111). On both surfaces, methanol desorption is first-order up to a coverage of one monolayer. On Cu(111) the temperature of the methanol desorption peaks decrease as the methanol coverage is increased, indicative of lateral repulsions.8,16 However, there is no observed shift in the peak maximum on the “29” oxide, indicating that adsorbed methanol has negligible lateral interactions.


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Figure 2. DFT-calculated top (A.1, B.1, C.1) and side (A.2, B.2, C.2) views of the three binding motifs for a single intact methanol molecule on the “29” oxide: (A) dative bonding between the Omethanol and the Cuoxide, (B) hydrogen bonding between the hydroxyl H and the Ooxide, and (C) dual dative and hydrogen bonding sites. The adsorption energies for each conformation is given in the text and Table S1. Larger-scale views of all tested molecular methanol adsorption sites within the “29” oxide unit cell are shown in Figures S2-S4.

With an accurate structural model for the “29” oxide previously determined,33 DFT-based simulations were performed to test all possible adsorption sites for a single methanol molecule. (see Figures S2-S4 and Table S1 for details). There are three general adsorption motifs for molecular methanol on the “29” oxide, which are dative bonding between the Omethanol lone electron pair and a Cuoxide cation (Figure 2A), hydrogen bonding between an Ooxide anion and the methanol OH group (Figure 2B), and dual dative and hydrogen bonding by adsorption of the methanol perpendicular to the surface within a Cu2O-like ring (Figure 2C). The hydrogen bonding to an Ooxide anion can be seen more clearly in Figure 2B.2. Overall, a total of 37 adsorption sites were tested, 20 Cuoxide dative bonding sites (Figure S2), 12 Ooxide hydrogen bonding sites (Figure S3), and 5 dual dative and hydrogen bonding sites (Figure S4) across the model surface of the “29” oxide unit cell. The most favorable one was found to be the dual dative and hydrogen bonding site type, with an adsorption energy of −0.74 eV. The pure datively bonded methanol sites were the next most favorable with an average adsorption energy of −0.52 eV followed by the pure hydrogen bonded sites with an average adsorption energy of −0.31 eV. For the Cu(111) surface we calculated the methanol binding energy to be −0.51 eV, which is 7 ACS Paragon Plus Environment


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consistent with previous experimental and theoretical results.16,18 These calculations agree with the experimental TPD in that the “29” oxide binds methanol more strongly that bare Cu(111) surface. The details for the tested adsorption sites and their adsorption energies are shown in Figures S2-S4 and Table S1. The possibility that methanol would dissociate at its preferred site on the “29” oxide into a H adatom bonding to either the Oadatom or an adjacent Ooxide and methoxy bonding to a Cuoxide cation was tested, as shown in Figure S5. DFT calculations of the dissociated methanol structures were either not stable and the system relaxed back to the molecularly adsorbed species or the reaction energy was very large, ranging from 0.7–1.4 eV (Table S2 and Figure S5). The energetic preference for molecularly adsorbed methanol on the intact “29” oxide is consistent with the first-order desorption of methanol from the “29” oxide surface seen by the TPD data in Figure 1. The endothermic reaction energies for the dissociation of surface bound molecular methanol makes the desorption of methanol more favorable than dissociation. Taken together, the DFT calculations and TPD experiments strongly suggest that methanol is non-reactive on the defect-free “29” oxide surface.


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Figure 3. (A) Top and (B) side views of the differential charge density for methanol in its most favorable site on the “29” oxide surface as calculated using DFT. The yellow and blue volumes show areas of charge gain and loss, respectively, upon methanol adsorption, and the sphere colors are identical to Figure 2. The isosurface level was set at 0.005 electrons/Bohr3. (C) PDOS for methanol before and after adsorption in the most favorable site as well as the PDOS for the methanol bound surface Cu and O species (denoted in Figure S6).

In order to evaluate the electronic interactions between the “29” oxide and adsorbed methanol we calculated the differential charge density and projected density of states (PDOS) for the most favorable methanol adsorption site, which is the dual dative and hydrogen bonding structure shown in the right-hand panels of Figure 2. The differential charge density for methanol adsorbed with the dual bonding motif confirms the participation of both hydrogen bonding to the Oadatom and dative bonding to the Cuoxide species. In general, there is an electron density gain at 9 ACS Paragon Plus Environment


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the interacting Oadatom (black spheres in Figure 3) and an electron density loss at the most adjacent Cuoxide cation (silver spheres in Figure 3). However, integrating the differential charge density over the entire unit cell shows that only ~0.4 electrons worth of charge has been transferred between the surface and methanol during adsorption, which is comparable to other weakly adsorbing systems.64–66 This relatively small amount of electronic interaction between the adsorbed methanol and the “29” oxide surface is further evident in the PDOS, shown in Figure 3C, as both the surface and the adsorbate electronic states are relatively unchanged by the adsorption process. The lowest energy state for gas phase methanol was aligned with that of adsorbed methanol at approximately −21 eV, as this orbital should be low enough in energy to remain unaffected by adsorption. Only the two highest occupied orbitals in methanol see any significant change upon adsorption as these states partially overlap with the oxide surface species. Some hybridization of these methanol states is seen, however, there is a minimal shift in the orbital energies suggesting a small degree of overlap. Overall, this electronic characterization shows that methanol does not undergo a large degree of electronic hybridization upon adsorption to the “29” oxide, which is consistent with its weak binding energy and being non-reactive.

Figure 4. (A) 5 K STM image of methanol molecules on the “29” oxide after a 130 K anneal. Imaging conditions of −0.05 V and 0.05 nA. (B) STM image of the surface before applying 2 V pulses over the points marked with a star. (C) STM image of surface after pulses, in which the circles highlight the pulse location area. Imaging conditions of −0.5 V and 0.05 nA.

A STM image of a submonolayer methanol coverage on the “29” oxide after a 130 K anneal is shown in Figure 4A. This anneal temperature is just below the leading edge of the methanol monolayer desorption peak, shown in Figure 1, and therefore allows methanol to find its most stable binding site but not desorb. Interestingly, the methanol STM images feature twolobed protrusions, which will be referred to as dimers. Dispersed dimers and empty sites of the 10 ACS Paragon Plus Environment

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“29” oxide are seen across most of the image in Figure 4A; however, there are several instances where the dimers line up into chains. These even-numbered chain structures are multiple repeating units of the dimer structure and align with the lattice of the underlying “29” oxide. In fact, the periodicity of the methanol chains is consistent with the unit cell of the “29” oxide itself, suggesting that the chains are composed of methanol dimer units in adjacent substrate unit cells. On (111) metal surfaces of Cu and Au, methanol arranges itself into hydrogen bonded chains.18– 20 However, on the “29” oxide the methanol chains are spaced too far apart to be a continuous hydrogen bonded network. The lobes of the dimer unit are approximately 0.5 nm apart, while optimal hydrogen bonding distances are around 0.28 nm,18,45 indicating that the observed chains are not a one-dimensional hydrogen bonded network. However, it is clear that the spatial distribution of dimers is not random, and that there must be a driving-force making it more favorable for methanol molecules to exist in dimers. In order to further investigate the nature of the observed methanol dimer features and demonstrate that they are indeed two-molecule dimers, STM pulsing experiments were performed. Shown in Figure 4B is an STM image containing six dimers. The open stars represent the location of four separate 2 V STM tip pulses applied after image collection. Shown in Figure 4C is an STM image of the same area after the pulses, and the open black circles highlight the location of the pulses, which are also the location of the original four dimers shown in Figure 4B. Over each of the four pulse locations, the pre-existing dimer feature no longer occupies the same area. In general, STM tip pulses can induce desorption, diffusion, or fragmentation of adsorbates.67 The STM tip pulsing experiments allows for evaluation of the binding site of methanol molecules, but the resulting chemical identities of surface species resulting from the tip pulsing were not evaluated. At pulse site number 1 only one bright lobe remains, at site 2 the dimer unit stayed intact but diffused to a similar binding site one unit cell away, and sites 3 and 4 have broken into scattered fragments. The changes from before (Figure 4B) and after (Figure 4C) STM tip pulsing reveals that the 2 V pulses were sufficient in energy to disrupt the dimer units. An important result arising from these STM tip pulsing experiments is that all the original dimer features occupy intact regions of the “29” oxide and not defect sites because the pulsing experiments reveal the perfect “29” oxide surface beneath their original adsorption locations so we can rule this out as a mechanism for the unique ordering observed.



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Figure 5. Identification of CH3OH dimer adsorption site location on the “29” oxide unit cell. (A) STM image of separated dimer features, imaging conditions of −0.5 V and 0.05 nA, and the unit cell is 17.96 Å × 9.25 Å. (B) Top view of CH3OH adsorbed in the two most favorable sites (O13 and O17 of Figure S3) of the “29” oxide, where the sphere colors are identical to Figure 3. (C) DFT-based simulated STM image of the structure in A with a bias of -0.5 V. The unit cell and dimer location from the experimental STM image is overlaid.

The nature of the dimer species on the “29” oxide was investigated using DFT-based calculations and benchmarked against the experimental imaging as spectroscopy data. Candidates involving two co-adsorbed intact methanol molecules were the focus. The possibility that the dimer structure was composed either partially or entirely of surface methoxy species was eliminated as the dissociation of methanol was found to be thermodynamically unfavorable with an energy cost of around 0.7–1.4 eV (Table S2). This is primarily due to the highly unfavorable binding of methoxy on the “29” oxide (Table S3 and Figure S7). Therefore, the best candidate for the dimer structure are the conformations with two co-adsorbed molecular methanol species. Using the single methanol molecule adsorption DFT calculations (Figure 2), the dimer structure was modeled from the co-adsorption of the two most favorable binding sites in a single “29” oxide unit cell. This is a logical approach because STM images suggest that the methanol molecules are too far apart to be hydrogen bonding to each other. The location of the methanol features in the experimental STM image of the dimer structure is shown in Figure 5A. The proposed structure of this feature is shown in Figure 5B, in which two methanol molecules are bound per “29” oxide unit cell. Both molecules take advantage of the dual bonding motif, with dative bonding between the Omethanol and a Cuoxide species and hydrogen bonding between an Oadatom species and the methanol OH group. A DFT-based simulated STM image of this structure 12 ACS Paragon Plus Environment

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is shown in Figure 5C. The location of the methanol molecules in the image aligns well with the protrusions seen in the experimental STM image, both with respect to each other and with respect to their position on the “29” oxide substrate. The simulated STM image of repeating dimer units in adjacent “29” oxide unit cells also replicates the zigzag pattern of larger methanol chain structures observed experimentally (as seen in Figure 4A).

Figure 6. (A) Histogram of methanol ensemble sizes in the experimental STM images at 5K. (B) Adsorption energy for methanol in all tested combinations of the 2 dimer sites in a supercell of the “29” Cu oxide where the short axis has been doubled (configurations shown in Figure S8). In this supercell, the full chain structure occurs when 4 𝐂𝐇𝟑𝐎𝐇 for this study is 4. The dotted line shows the “ideal heat of mixing” methanol molecules are adsorbed, so 𝑵𝐌𝐚𝐱 between the adsorbed methanol and the empty sites. (C) Formation energy for the methanol configurations from (B) calculated as the deviation of the methanol adsorption energies from the “ideal heat of mixing”. The configurational ground states are connected by dashed lines.



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While it is clear that the adsorbed methanol on the “29” Cu oxide does not form hydrogen bonded chains like those observed on Cu(111),18 the formation of the methanol dimer and chain structures at submonolayer coverages on the oxide suggests the presence of attractive lateral interactions between the adspecies. In order to investigate this further by DFT, the short axis of the “29” Cu oxide unit cell was doubled and all possible combinations of methanol (1, 2, 3, and 4 molecules) in the two dimer adsorption sites was calculated, as shown in Figure S8. Due to the large distances between the co-adsorbed methanol molecules, the lateral interactions between adspecies were weak, ranging from −22 to +37 meV/methanol (Table S4). These lateral interactions are the average effects over each configuration of methanol (Figure S8) without distinction with respect to particular two- and three-body interactions. The only combination that had an attractive averaged lateral interaction (−22 meV/methanol) was an isolated dimer structure. This is in very good agreement with an analysis of the experimental STM ensemble sizes, as displayed in the histogram in Figure 6A, which shows that methanol dimers make up ~70% of the observed features. The additional species observed experimentally–methanol monomers and chains of 4 and 6 molecules (Figure 6A)–can be rationalized by examining the phase stability of each combination of methanol in the two dimer sites. Figure 6B shows the adsorption energy as a function of the number of methanol adsorbates, while Figure 6C shows the formation energy, i.e. the deviation from the “ideal heat of mixing” where adsorption can be seen as a “mixing” of empty sites and fully occupied sites, for the configurations in Figure 6B. Together, these graphs provide two crucial pieces of information: (1) the methanol chemical potential required to move between the different configurational ground states on the “29” Cu oxide and (2) the ground state configurations at a given number of adsorbed methanol.68 Ground states are those that are stable against phase separation, and the chemical potential at 0 K is obtained from the slopes that 𝑑𝐸 connect these ground states (𝜇 = ads 𝑑𝑁CH3OH).69 As methanol is sequentially added to the “29” Cu oxide surface, the chemical potential required to adsorb the first methanol molecule is −0.777 eV/methanol, while the chemical potential required to reach the most stable twomethanol configuration is −0.767 eV/methanol. This shows that if a gas phase methanol chemical potential is sufficient to form monomers on the surface, there is only a 10 meV/methanol energy cost to then also form dimers on the surface. Furthermore, from Figure 6C, it is clear that it is less energetically costly to go from methanol dimers (2 methanol adsorbates) to the full chain (4 methanol adsorbates) than to stop at a 3-methanol chain structure (μ2→4 = −0.658 eV/methanol versus μ2→3 = −0.640 eV/methanol). This is reflected in the fact that the 3-methanol structure is not a ground state and will phase separate into dimers and full chains. Put in another way, if the applied chemical potential is sufficient to add the third methanol, there will always be sufficient chemical potential to then add the fourth methanol molecule. In fact, from the observed STM images (Figure 4A and 6B), it can be posited that the chemical potential responsible for the adsorbate structures is likely somewhere between that of the 2 and 4 methanol systems, forming a co-existence region between these two ground state 14 ACS Paragon Plus Environment

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configurations of methanol. The weak nature of the lateral interactions and limited degree of charge transfer between methanol and surface upon adsorption (Figure 3) make it difficult to fully deconvolute and electronic and/or geometric effects that may be at play. Overall, the DFTbased results of the methanol configurations shown here indicate that the experimentally observed dimer and chain structures on the “29” Cu oxide are formed due to the weak attractive lateral interactions between two methanol molecules in one particular dimer structure, which thermodynamically favors even numbered methanol chains. Another interesting aspect of methanol adsorption is the associated surface-bound chirality, as dictated by the number of different substituents around the O atom of methanol in its adsorbed state. In the case of alcohol adsorption on bare metal surfaces, which has been reported previously, the two possible adsorption chiralities are energetically degenerate, although the interplay of hydrogen bonding can lead to both long-range homo and hetero chiral structures.18– 22,70 However, methanol on the “29” oxide is forced to adsorb into a particular chirality in order to satisfy the dual bonding motif, as shown in Figure 2. While the adsorption chirality of methanol is not determined experimentally, the DFT model clearly shows that methanol has a favored adsorption chirality dictated by the geometry of the binding pocket based on the geometry of the dual hydrogen bonding and dative bonding adsorption motif. However, within a given unit cell of the "29" oxide there are some binding pockets that facilitate one methanol adsorption chirality and others that facilitate the opposite adsorption chirality, which has been investigated by DFT (Figure S4). In particular, for the dimer structure proposed in Figure 5A, the two methanol molecules at different binding sites are of opposite chirality. This shows that a particular binding site on the “29” oxide pre-determines the adsorption chirality of methanol, although favorable binding sites for both adsorption chiralities exist.



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Figure 7. RAIRS data for 0.2 ML methanol on the “29” oxide at 85 K (black) and DFT generated surface normal IR spectra (red). Data is included for both (A) CH3OH and (B) CD3OD.

RAIRS provides an additional characterization method and for a quantitative comparison to theory, a summary of which is shown in Figure 7. The experimental RAIRS data for 0.2 ML of methanol adsorbed on the “29” oxide at 85 K is shown in black in Figure 7A. There are several IR bands observed upon methanol adsorption at 1029, 1445, 2797, 2976, and 3224 cm−1, which are assigned to the C-O stretch, CH3 bending, symmetric CH3 stretch, asymmetric CH3 stretch, and O-H stretch, respectively.24–26 The spectrum is in good agreement with that of intact methanol on Cu surfaces,24–26 and is not consistent with methoxy formation as also ruled out earlier. 24–27 These RAIRS data which contain all expected visible modes for the methanol molecule, in conjunction with the TPD and STM data, definitively show that methanol adsorbs non-dissociatively on the “29” oxide. In general, the C-O stretch is sensitive to the binding strength of methanol to the surface, as stronger binding via charge transfer from the oxygen to the surface weakens the C-O bond, therefore shifting the C-O stretch frequency to a lower wavenumber.71 For methanol adsorption on the “29” oxide this stretch is observed at 1029 cm−1, which is red-shifted by about 20 cm−1 with respect to methanol adsorption on bare and partially oxidized Cu surfaces.24–26 This finding is consistent with the TPD results suggesting the methanol is bound slightly more strongly on the “29” oxide than on Cu(111). But it is interesting to note that the C-O stretch frequency of methanol is red-shifted on the “29” oxide, signifying stronger binding despite being non-reactive yet, as compared to partially oxidized Cu, on which methanol is reactive.10 This suggests that the reactivity of methanol on oxidized Cu does not require strong binding, but rather the presence of an active site geometry that can facilitate dissociation. 16 ACS Paragon Plus Environment

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DFT-based simulations were used to generate an IR spectrum from the optimized DFT structures, as shown in red in Figure 7A. In the simulated spectrum, only vibrations with a component along the surface normal were included, as this is an important selection rule in RAIRS. The agreement between experiment and theory in the IR spectra is very good, as the reported vibrational bands are generally within about 150 cm−1 (∼0.02 eV). RAIRS data was also collected for deuterated methanol and is shown in Figure 7B. A benefit to using deuterated methanol is to assure no band overlap with the O-H stretch of background water. The shifts in the IR band positions of the different isotopes is well captured by the theory. Once again, with deuterated methanol there is excellent agreement between experiment and theory, with the measured vibrations within about 40 cm−1 of the DFT-predicted values. Overall, the agreement between experimental and theory from TPD, STM, and RAIRS data is evidence toward the accuracy of the proposed dimer structure. It is interesting to note that the molecular chains formed by methanol on the “29” oxide is not due to hydrogen bonding between the adsorbed molecules, as on metal single crystal surfaces. This is analogous to the self-assembly of water on metals being very different from the ice structures formed on oxide surfaces.42–49 Instead, methanol adsorption is governed here by the surface-molecule interactions, which comes from a combination of dative bonding with Cuδ+ species and hydrogen bonding with surface Oδ- species. Therefore, unlike the hydrogen bonding driven chains observed on metals, it is actually the structure of the underlying oxide structure that drives the unique evennumbered ordered structures to form. Conclusions The interaction of methanol on the “29” oxide surface was characterized by TPD, STM, RAIRS, and DFT. Methanol was shown to adsorb to and desorb from the oxidized Cu surface reversibly and non-dissociatively, as evidenced by the first-order desorption state of methanol and characteristic IR bands. This is because the “29” oxide is a complete Cu2O-like layer, which lacks oxide-metal interface sites that can readily dissociate methanol.10 A point of similarity between the “29” oxide versus Cu(111) is seen in the adsorption of methanol and CO. In both cases the molecules are adsorbed slightly more strongly on the oxide surface, but saturate at slightly lower coverage,34 which for methanol is based on the fact that molecular chains are more closely packed on Cu(111) due to hydrogen bonding.20 The energy landscape of the “29” oxide is much more corrugated, and hence there are some stronger adsorbing sites, but many weaker ones also. Through DFT calculations, we found that the optimal binding geometry for molecular methanol is by dual Omethanol-Cuδ+ dative bonding and OHmethanol-Oδ− hydrogen bonding. Interestingly, intermolecular hydrogen bonding between methanol molecules was not observed, as STM images show that the methanol molecules are spaced too far apart. Instead, a dimer species was observed, and an extensive survey by our DFT calculations indicates that the structure is made up of two methanol molecules adsorbed to nearby sites on the surface that have relatively weak attractive lateral interactions. These attractive lateral interactions within the dimer configuration lead to the preferential formation of chain-like structures with even numbers 17 ACS Paragon Plus Environment


Page 18 of 24

of methanol on the “29” Cu oxide. The nature of the dual binding motif also leads to site specific adsorption chirality of the methanol molecule in its adsorbed state. We propose that ability of the oxide to accept hydrogen bonds allows the surface structure and energetic corrugation of the “29” oxide surface to, in contrast to bare metal Cu surfaces, dictate the self-assembly of methanol into unique even-numbered chain structures. Supporting Information for Publication Structure of the “29” oxide model surface (Figure S1); methanol adsorption energies at all tested sites (Table S1); top views of DFT optimized structures for methanol adsorption at all tested sites (Figures S2-S4); reaction energies for methanol dissociation (Table S2); top views of DFT optimized structures for methanol dissociation (Figure S5); top view of DFT optimized most favorable site for methanol adsorption with the most electronically affected “29” Cu oxide surface species highlighted (Figure S6); DFT-calculated adsorption energies for methoxy on the “29” oxide surface (Table S3); top views of DFT optimized structures for methoxy adsorption (Figure S7); top views of DFT optimized structures for all possible combinations of the two methanol molecules bound at the identified dimer sites within a (12) “29” oxide supercell (Figure S8); summary of DFT-calculated adsorption, interaction, and formation energies for the methanol configurational tests for two methanol molecules bound at the identified dimer sites within a (12) “29” oxide supercell (Table S4). Acknowledgements All the experimental work was supported by the Catalysis Science Program at the Department of Energy BES under grant No. DE-FG02-05ER15730. M. M. thanks Tufts Chemistry for an Illumina Fellowship. Financial support for the theory work at WSU was provided by the National Science Foundation CAREER program under contract No. CBET1653561. A portion of the computer time for the computational work was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL. PNNL is a multi-program national laboratory operated for the US DOE by Battelle. The authors thank Greg Collinge for his helpful comments and discussions.


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â29â Cu(111) Surface Oxide - ACS Publications - American Chemical

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â29â Cu(111) Surface Oxide - ACS Publications - American Chemical