Hydrogen Exchange through Hydrogen Bonding between Methanol and Water in the Adsorbed State on Cu(111)

The interaction between submonolayers of methanol and water on Cu(111) is studied at 95–160 K temperature range with surface-sensitive infrared spectroscopy using isotopically labeled molecules. The initial interaction of methanol with the preadsorbed amorphous solid water at 95 K is through hydrogen-bonding with the dangling hydroxyl groups of water. Upon increasing the temperature up to 140 K, methanol and deuterated water form H-bonded structures which allow hydrogen–deuterium exchange between the hydroxyl group of methanol and the deuterated water. The evolution of the O–D and O–H stretching bands indicate that the hydrogen transfer is dominant at around 120–130 K, slightly below the desorption temperature of methanol. Above 140 K, methanol desorbs and a mixture of hydrogen-related water isotopologues remains on the surface. The isotopic composition of this mixture versus the initial D2O:CH3OH ratio supports a potential exchange mechanism via hydrogen hopping between alternating methanol and water molecules in a hydrogen-bonded network.

This is a thorough study of hydrogen exchange that occurs due to the co-adsorption of methanol and water, with this film supported on an underlying Cu(111) substrate. The authors present good evidence that isotope exchange occurs between these co-adsorbed species. This is accomplished by viewing IR spectra that focus on the OH/OD signatures for various species in the exchange medium. My first point is therefore that the demonstration of isotope exchange in sequential temperature regimes seems quite convincing. My second point focuses on what is missing: The authors use their data to propose an exchange mechanism that involves hoping between alternating methanol and water molecules in a hydrogen-bonded network. While this is reasonable, the point is not proven. It would have been informative if the authors had also pre-formed crystalline ice and then dosed the methanol. This would create a different surface presentation to help test the proposed mechanism. If the authors do this, or have done this, I would support JPC/L publication without any further inquiries. While this would require some more data to be presented (not just discussed), it would further enrich the Letter. The Letter now is fine, but the suggested addition would enhance its utility to the community.
Author's Response to Peer Review Comments: We have revised the manuscript in full accordance with the reviewers' comments. For your convenience, all essential corrections and additions in the revised manuscript are marked in red.
We would like to sincerely thank the reviewers for their insightful comments, which helped us to improve our manuscript.
We hereby provide an explicit answer to each comment of the reviewers.

Reviewer #1
Comments: 1. What is the major advance reported in the paper?
Accurate measurements of H-D exchange between methanol and water on a well-defined Cu(111) surface are made in which the hydrogen bonding between molecules and proton hoping can be quantified with temperature controlled IR measurements in situ.

What is the immediate significance of this advance?
Experimental data like these provide good benchmarks for ab initio calculations of these H transfer events, which are ubiquitous in chemistry and biology.
We would like to thank the reviewer for their comments. We are happy to hear that the reviewer finds our experimental study accurate and valuable for future theoretical studies.

Comment 1
Technical suggestions: This is a high-quality study and a well written paper. Given that this data will probably be modeled by other groups it is important that the measurements be as well

Answer to comment 1
We added the calculated surface coverages (in monolayers) of water/methanol in the relevant parts in the text. The calculation is based on the kinetic theory of gases, assuming a sticking coefficient of ~1 at 95 K (the dosing temperature) and using the theoretical atomic density of the Cu (111)  It is worth noting that for both dosage (in Langmuir) and coverage (in ML) calculations the dosing pressure, measured by the hot cathode ionization gauge (calibrated for N 2 ), was corrected using the corresponding gas correction factors for methanol (1.85) and water (1.12). The gas correction factors take into account the different ionization cross section and fragmentation patterns. We mentioned this technical issue in the methods section: "While dosing, the pressure reading of the hot cathode ionization gauge was corrected using the corresponding gas correction factor for water (1.12) and methanol (1.85)."

Also, on page 7 -is it fair to compare the intensities of the O-H and O-D stretches as a way to quantify the number of molecules the exchange has occurred in? Dipole coupling effects in IR spectroscopy
can make this tricky in many systems so this should be expanded upon.

Answer to comment 2
Using

Comment 3
Is there any justification of the chain model on page 8, Figure 2? Be clear on this in the paper as it will be the first model that is use in simulations by others.

Answer to comment 3
Our justification for the exchange mechanism through a chain-like H-bonded structures of alternating CH 3 OH and D 2 O molecules is given by the PM-IRRAS experiments with different initial CH 3 OH:D 2 O ratios ( Figure 3 in the manuscript). Moreover, this model is supported by previous studies of water-ethanol adsorption on Au(111) and water-methanol interface with graphene. We tried to emphasize these points in the following sentences: "In this model for T < 140 K (Figure 2a), we present one of the possible pathways for a directional H-D exchange in an alternating CH 3 OH-D 2 O H-bonded network. Later, we provide further evidence for such a mechanism by modifying the initial D 2 O:CH 3 OH ratio (Figure 3). This mechanism is also supported by the previous TDS study of DePonte et al. 20 for the H-D exchange between water and ethanol on Au(111), and the linear structure of the water-methanol network is in line with the AFM work of Voïtchovsky et al. 9

."
Comments: This is a thorough study of hydrogen exchange that occurs due to the co-adsorption of methanol and water, with this film supported on an underlying Cu(111) substrate. The authors present good evidence that isotope exchange occurs between these co-adsorbed species. This is accomplished by viewing IR spectra that focus on the OH/OD signatures for various species in the exchange medium. My first point is therefore that the demonstration of isotope exchange in sequential temperature regimes seems quite convincing.
We would like to thank the reviewer for their comments. We are pleased to hear that the reviewer finds our results and proposed mechanism convincing.

My second point focuses on what is missing: The authors use their data to propose an exchange mechanism that involves hoping between alternating methanol and water molecules in a hydrogenbonded network. While this is reasonable, the point is not proven. It would have been informative if
the authors had also pre-formed crystalline ice and then dosed the methanol. This would create a different surface presentation to help test the proposed mechanism. If the authors do this, or have done this, I would support JPC/L publication without any further inquiries. While this would require some more data to be presented (not just discussed), it would further enrich the Letter. The Letter now is fine, but the suggested addition would enhance its utility to the community.

Answer to comment 1
Indeed, the results presented in the manuscript refers to methanol (CH 3 OH or CH 3 OD) interaction with pre-adsorbed amorphous solid water (ASW) of D 2 O, which is the phase that obtained at low surface temperatures (95 K). The idea of studying the interaction of methanol with a crystalline (ordered) layer of D 2 O is very interesting since this phase (presumably a double bilayer) is expected to be more stable than the amorphous phase which is clearly metastable. Hence, following this important comment, we performed an additional experiment of CH 3 OH adsorption on a crystalline D 2 O layer on a Cu(111). We produced this layer by annealing the ASW phase (deposited at 95 K) above the crystallization onset and cooling back to 95 K before CH 3 OH dosing. The results of this experiment are presented and discussed in the Supporting Information (see Section S2) we added.
The results clearly demonstrate the occurrence of H-D exchange between CH 3 OH and crystalline D 2 O, as both O-H and O-D bands remain in the PM-IRRAS spectra following methanol desorption (T = 140-150 K). Moreover, we examined the case of opposite dosing ordered, i.e., while CH 3 OH is dosed before D 2 O, which gives another surface presentation that supports and generalizes the H-D exchange mechanism (Section S1 in the Supporting Information). 5 We also referred to the new results in the main text: "The phenomenon of hydrogen exchange between methanol and water in the adsorbed state is not limited to the case of methanol interaction with pre-adsorbed ASW. In the Supporting Information (SI) we demonstrate that the H-D exchange also takes place both in the case of opposite dosing order (i.e., when CH 3 OH is dosed prior to D 2 O) and when CH 3 OH is co-adsorbed onto a surface covered with a crystalline layer of D 2 O rather than amorphous."