pubs.acs.org/JPCC Article Water Interaction with Fe2NiP Schreibersite (110) Surface: a Quantum Mechanical Atomistic Perspective Published as part of The Journal of Physical Chemistry virtual special issue “Vincenzo Barone Festschrift”. Stefano Pantaleone,* Marta Corno, Albert Rimola, Nadia Balucani, and Piero Ugliengo* Cite This: J. Phys. Chem. C 2022, 126, 2243−2252 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ABSTRACT: Phosphorus is an element of primary importance for all living creatures, being present in many biological activities in the form of phosphate (PO43−). However, there are still open questions Downloaded via 62.57.40.192 on March 8, 2022 at 16:43:03 (UTC). about the origin of this specific element and on the transformation that allowed it to be incorporated in biological systems. The most probable source of prebiotic phosphorus is the intense meteoritic bombardment during the Archean era, a few million years after the solar system formation, which brought tons of iron-phosphide materials (schreibersite) on the early Earth crust. It was recently demonstrated that by simple wetting/corrosion processes from this material, various oxygenated phosphorus compounds are produced. In the present work, the wetting process of schreibersite (Fe2NiP) was studied by computer simulations using density functional theory, with the PBE functional supplemented with dispersive interactions through a posteriori empirical correction. To start disentangling the complexity of the system, only the most stable (110) surface of Fe2NiP was used simulating different water coverages, from which structures, water binding energies, and vibrational spectra have been predicted. The computed (ana- )harmonic infrared spectra have been compared with the experimental ones, thus, confirming the validity of the adopted methodology and models. 1. INTRODUCTION In 2005, Pasek et al. carried out the first water corrosion Naturally occurring iron−nickel phosphides are in the mineral experiment on a model of schreibersite, Fe3P, in which they form of schreibersite (Fe,Ni)3P, which is present as a minor demonstrated the production of different phosphorus oxy- phase of iron meteorites,1−3 whose heavy elements are the first genated compounds (phosphates, phosphites, hypophos- that condensate from solar nebulae.4 phites), and also their reaction with organic molecules.10 Phosphorus is present in living systems in the form of Recent models estimate that about 1−10% of the early Earth phosphate (PO43−) and, despite its low abundance (1%) with crust was composed by phosphide minerals and that a respect to the other macro-elements (SONCH), it is meteorite of about 60 tons should produce about 1 ton of ubiquitous, in nucleic acids (DNA and RNA), in molecules that provide energy for metabolic processes (ATP, ADP, reduced phosphorus.11 Another route recently proposed by AMP), and in phospholipids, among others. The most Hess et al. implies the in situ formation of schreibersite by abundant source of phosphorus on Earth is the mineral apatite light-activated processes in clay-rich soils.12 According to this (Ca5(PO4)3(F,OH,Cl), which, however, has extremely low study, 10−1000 kg/year of phosphide and 100−10000 kg/year solubility and reactivity, and therefore, it can hardly support of phosphite and hypophosphite could be produced, ensuring a the presence of the phosphate groups in living organisms.5 As a continuous availability of reduced phosphorus without depend- consequence, other possible sources of reduced and reactive ing on the meteor flux. Therefore, the reactivity of schreibersite phosphorus were investigated in the last decades. In 1955, Gulick postulated the release of phosphates by corrosion reactions operated by water contact with the surface Received: November 20, 2021 of schreibersite.6 Indeed, during the Archean era (4.0−3.8 Revised: January 6, 2022 billion years ago), just after the solar system formation, a large Published: January 25, 2022 quantity of small bodies (meteorites, comets, and other asteroidal bodies) hit the planet, thus, brining all the organic matter encapsulated inside.7−9 © 2022 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.jpcc.1c09947 2243 J. Phys. Chem. C 2022, 126, 2243−2252 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article toward water is a key step for biological phosphorus of the Brillouin zone was used for the k-points mesh. Shrinking incorporation. factors for the (110) surface have been set on (8 8 1) on the The first (Fe,Ni)3P water corrosion experiment by Pasek et unit cell (a = 4.374 Å, b = 6.719 Å, γ = 108.998°), while they al. encouraged many subsequent works;13−19 in some cases, were reduced to (4 8 1) and (4 4 1) for 2 × 1 and 2 × 2 organic species were added in the experiment,16 showing the supercell models (for a total number of 64 and 16 k-points, tendency of reduced phosphorus to activate phosphorylation respectively). The shrinking factor related to the nonperiodic reactions, also producing nucleosides14 and complex sugars.19 direction was always set to “1” (i.e., no sampling of the Experimentally, the wetting process of schreibersite was reciprocal space). As VASP relies on the plane waves basis set investigated by means of RAIRS (Reflection−Absorption and, accordingly, surfaces are replicated also along the InfraRed Spectroscopy) at both different temperatures (120− nonperiodic direction, the vacuum space among fictitious 298 K) and water coverages (from 1 to 100 Langmuir, 1 replicas was set to at least 20 Å to minimize the interactions Langmuir representing the monolayer).13,17 These studies among replica images. Therefore, the final c cell axis was set to showed that the H2O molecules preferentially bind to the P 40 Å. The relaxation of water adsorbed at the (110) surface atoms at the schreibersite surface through a direct P···O was carried out by moving all atoms in the unit cell while interaction, also confirmed by addition of isotopic water-18O.18 keeping the cell parameters fixed at the geometry optimized for Nevertheless, an atomistic perspective of the whole water the bare surface to enforce the rigidity due to the underneath corrosion process of schreibersite (both from the point of view bulk. of the adsorption and the reactivity) is still elusive, as, to our Adsorption energies (AE) of water adsorbed on the (110) knowledge, no computational work has been performed on this shreibersite surface (water was only adsorbed on one of the specific topic. Some studies were carried out on the “father” of two equivalent (110) faces of schreibersite) were calculated as schreibersite, Fe3P, studying the phase stability of different polymorphs in order to understand if it is present, and in which ECPLX − (nE H2O + E(110)) form, in planetary cores.20−22 AE = n A recent computational work on the bulk and surface structures and properties of schreibersite was published by where ECPLX is the energy of the complex (water adsorbed on us.23 According to that, it is unlikely that the oxygen of H2O the surface), EH2O and E(110) are the energies of the isolated (partially negatively charged) interacts with the surface P water molecule and the (110) bare surface, each one calculated atoms, which also bring a partial (close to −1e) negative at its relaxed geometry, while n is the number of adsorbed charge,23−26 thus, resulting in an electrostatic repulsion. In the water molecules per surface unit cell. The corresponding present paper, we simulate the adsorption of water on the most adsorption enthalpy (AH) and free energy (AG) were stable facet of schreibersite, namely, the {110} exploring calculated by adding the proper thermodynamic corrections. different water’s coverage regimes, up to a water multilayer. When considering water dissociation at the (110) surface, the Moreover, also the water deprotonation was considered to reaction energy is computed with respect to the most stable explore the very first step of the route toward the formation of molecular adsorption and labeled DE, as well as the phosphorus oxygenated species. The most reactive surfaces will corresponding thermic corrections to the energy (DH and be the object of future work. DG). Please note that negative values of AE and DE mean thermodynamic favorable processes. 2. COMPUTATIONAL DETAILS Vibrational frequencies were computed at the Γ point by The adsorption of water on schreibersite was studied by means numerical differentiation of the analytical first derivatives using of periodic DFT calculations carried out with the Vienna Ab- the central difference formula (i.e., two displacements of 0.02 initio Simulation Package (VASP) code,27−30 which uses Å for each atom in each x-, y-, and z-direction), in order to projector-augmented wave (PAW) pseudopotentials31 to confirm that the optimized structure is a minimum (all real describe the ionic cores and a plane wave basis set for the frequencies) and to apply thermal corrections to the calculated valence electrons. The same approach was used to characterize energies. To simulate IR spectra, the Phonopy35 code was used the pristine schreibersite.23 for both generating atomic displacements and processing Geometry optimizations and frequency calculations were VASP outputs. To plot IR spectra, the convolution of performed with the gradient corrected PBE functional,32 with a intensities was done with Lorentzian functions and a fwhm posteriori Grimme D2 correction,33 modified for solids (full width at half maximum) of 50 cm−1. Thermochemistry (D*).34 Moreover, C6 atomic coefficients related to polar- has been corrected using the quasi-harmonic approximation, izabilities on Fe and Ni metal atoms were set to 0 (i.e., no proposed by Grimme,36 in which frequencies lower than the dispersion interaction contribution from metal atoms). This 100 cm−1 are replaced by free rotor modes. This improves the setup was chosen according to the best results obtained in our calculation of the thermal corrections, which would be previous work on the bulk and bare surfaces of schreibersite.23 otherwise underestimated when considering very low fre- On O and H atoms, the original D* parameters were used. quency values. To avoid discontinuity close to the cutoff, a This method of choice is referred to as PBE-D*0 along the damping function was used to interpolate the values computed work. The cutoff energy of plane waves (which control the within the two ranges of frequencies. To recover the systematic accuracy of the calculations) was set to 500 eV. The self- error due to the methodology and to the anharmonic nature of consistent field (SCF) iterative procedure was converged to a the O−H vibration, simulated O−H harmonic stretching tolerance in total energy of ΔE = 10−5 eV for geometry frequencies were scaled for a proper factor calculated as 1 jijj νsym,exp νasym,exp yzz optimizations, while for frequency calculations, the tolerance jj zz = 0.979 νasym,comp zz was decreased to ΔE = 10−6 eV. The tolerance on gradients 2 j νsym,comp k { during the optimization procedure was set to 0.01 eV/Å for νscaled = + each atom in each direction. The Monkhorst−Pack sampling 2244 https://doi.org/10.1021/acs.jpcc.1c09947 J. Phys. Chem. C 2022, 126, 2243−2252 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Table 1. Bader Charge Analysis of All the Studied Systemsa molecular water deprotonated water atom net charge charge diff net transfer atom net charge charge diff (110) Fe/NiH-FeOH P −0.577 H−Fe_Ni −0.240 −0.824 Fe 0.327 H−OH 0.579 −0.005 Ni −0.043 O −1.023 0.145 H2O gas Fe29−H 0.420 0.093 O −1.168 Ni76−H 0.065 0.108 H 0.584 Fe28−OH 0.677 0.350 H2O−Ni 1×1 FeH-Fe/Ni−OH H 0.595 0.011 0.017 H−Fe −0.167 −0.750 H 0.629 0.045 H−OH 0.565 −0.019 O −1.206 −0.038 O −1.073 0.095 Ni 0.125 0.169 Fe28−H 0.285 −0.042 P10 −0.651 −0.074 Fe29−OH 0.591 0.265 P11 −0.638 −0.061 Ni76−OH 0.275 0.319 H2O−Ni 2×1 Fe/NiH-POH H 0.616 0.032 0.033 H−OH 0.593 0.009 H 0.613 0.029 H−Fe_Ni −0.257 −0.841 O −1.196 −0.028 O −1.302 −0.134 Ni 0.170 0.213 P−OH 0.301 0.878 P10 −0.641 −0.064 Fe41−H 0.428 0.101 P11 −0.607 −0.030 Ni82−H −0.035 0.009 H2O−Ni 2×2 Fe/NiH-POH_2 H 0.604 0.020 0.040 H−Fe −0.249 −0.833 H 0.611 0.027 H−OH 0.617 0.033 O −1.175 −0.007 O −1.298 −0.130 Ni 0.174 0.217 P−OH 0.233 0.810 P10 −0.639 −0.061 Fe29−H 0.385 0.059 P11 −0.616 −0.039 Ni76−H 0.039 0.083 H2O−Fe 1×1 Fe/NiH-POH_3 H 0.605 0.021 −0.012 H−Fe −0.242 −0.826 H 0.606 0.022 H−OH 0.608 0.024 O −1.223 −0.055 O −1.291 −0.123 Fe 0.455 0.128 P−OH 0.333 0.910 P7 −0.584 −0.007 Fe53−H 0.359 0.032 P8 −0.600 −0.022 Ni89−H 0.025 0.068 H2O−Fe 2×1 PH-Ni/FeOH H 0.604 0.020 0.010 H−P −0.325 −0.909 H 0.615 0.031 H−OH 0.568 −0.016 O −1.209 −0.041 O −1.085 0.083 Fe 0.467 0.140 P−H −0.082 0.495 P4 −0.586 −0.009 Fe40-OH 0.590 0.263 P5 −0.626 −0.049 Ni82-OH 0.157 0.201 H2O−Fe 2×2 PH-Ni/FeOH_2 H 0.597 0.013 0.014 H−P −0.393 −0.977 H 0.589 0.005 H−OH 0.597 0.013 O −1.171 −0.004 O −1.115 0.053 Fe 0.470 0.144 P−H −0.032 0.545 P4 −0.602 −0.025 Fe40−OH 0.565 0.238 P17 −0.620 −0.042 Ni82−OH 0.217 0.260 a Net charge, charge difference, and net transfer are in units of the electron charge and calculated as differences between charges of the actual system with respect to the corresponding bare system. In the “atom” column, the notation X−Y means that the reported charge is relative to atom X, which is bound to atom Y. where νsym,exp and νasym,exp are the experimental symmetric and ensure negligible lateral interactions among water replicas. The antisymmetric stretching of the isolated water molecule as final scaled water monomer stretching frequencies are 3761 taken from NIST, while νsym,comp and νasym,comp are the and 3652 cm−1, for the symmetric/antisymmetric modes. The calculated ones in the harmonic approximation with the present methodology, on a geometry optimized single water bending mode at 1585 cm−1 was obviously unscaled. For the molecule in a unit cell of a = 20 Å, b = 20 Å, and c = 20 Å, to prediction of the infrared spectra for the considered adsorption 2245 https://doi.org/10.1021/acs.jpcc.1c09947 J. Phys. Chem. C 2022, 126, 2243−2252 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure 1. PBE-D*0 optimized structures of a single water molecule (a) and (b), water monolayer (c), and water multilayer (d), in interaction with the (110) schreibersite surface. Atoms color legend: H is in white, O is in red, P is in yellow, Fe is in light gray, and Ni is in dark gray, and the unit cell in blue. In round parentheses is the adsorption energy per water molecule in kJ/mol. In square and curly parentheses are the adsorption enthalpy and free energy at 125/298 K. Italic numbers in section (d) are computed using the water bulk as a reference. Distances are in Å. cases, we ensure that the scaling was only applied to vibrational through the electrostatic complementarity principle, that is, its modes involving the OH stretching only. O (−1.178 |e|) atom will likely be adsorbed on the surface of Visualization and manipulation of the structures and figures Fe atoms through its lone pairs (electrostatic plus charge rendering have been done with the MOLDRAW,37 VMD,38 transfer), while the H atoms (0.584 |e|) will interact with P and POVRAY39 programs. atoms through a weak H-bond. 3.2. Single Water Adsorption. Figure 1 reports the 3. RESULTS structures of water adsorbed on the unit cell of the 3.1. Molecular Water Adsorption. To study the schreibersite (110) slab (a = 4.374 Å, b = 6.719 Å, γ = weathering process of schreibersite, the interaction of water 108.99°) and their corresponding adsorption energies. was only modeled on the (110) most stable schreibersite Surprisingly, the most stable adsorption mode (i.e., the one surface, as characterized in our recent work.23 The adsorption which presents the smallest adsorption energy) is for H2O−Ni of water was simulated at different surface coverage regimes, (−37.6 kJ/mol, Figure 1a), in contrast with what was expected by adopting the naivë electrostatic complementarity principle. from a single water molecule to the monolayer up to a thin liquid water multilayer. We ascribe this unexpected stabilization to the d8 electron Bader charge analysis in Table 1 reveals that, among the configuration of the atomic Ni, which allows for the electron outermost atoms of the (110) schreibersite surface, Fe is the pair donation of the water O atom toward Ni unfilled d only one with a weakly positive charge (0.327 |e|), Ni preserves orbitals, thus, almost filling up completely the d manifold. This its atomic nature (the charge being almost zero), while P justifies the additional stabilization with respect to water brings a negative charge (−0.577 |e|). In view of these results, adsorption on Fe (−29.1 kJ/mol), whose d block is still the adsorption of a water molecule is expected to take place incomplete, even after water adsorption. Table 1 shows that 2246 https://doi.org/10.1021/acs.jpcc.1c09947 J. Phys. Chem. C 2022, 126, 2243−2252 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure 2. PBE-D*0 optimized structures of isolated water molecule adsorbed on different supercell sizes and corresponding IR spectra. The experimental spectrum is also shown. Adapted with permission from ref 17. Copyright 2017 ACS. Atoms color legend: H is in white, O is in red, P is in yellow, Fe is in light gray, and Ni is in dark gray, and the unit cell is in blue. The blue spheres represent the removed water molecules to simulate the decreased coverage. In round parentheses is the adsorption energy per water molecule in kJ/mol. In square and curly parentheses are the adsorption enthalpy and free energy at 125/298 K. Distances are in Å. charge transfer effects are rather subtle to rationalize. Upon computed at the Ni site (−37.6/−41.3/−45.1 kJ/mol). As adsorption, the system does not present a significant charge already pointed out, despite the significant difference in the transfer character between the adsorbate and the surface, the O−metal bond, the effect on the O−H bond length appears to difference in the atomic charges of the adsorbed water be negligible. molecule being negligible with respect to the isolated one. In To check more carefully on the bonding effects, we contrast, the charge on the Ni atom increases by 0.17 |e|. All computed the vibrational frequencies for all the considered this means that the water molecule does not exert a direct supercell models. The IR spectra of Figure 2 show that (i) the effect on the electronic structure of schreibersite; instead, its symmetric/antisymmetric OH stretching of adsorbed water adsorption affects the coordination geometry of Ni, which suffers from a bathochromic shift by around 300 cm−1 with ultimately produces a slight rearrangement in the global respect to the gas-phase molecule (vibrating at 3761/3652 electronic structure of the surface. cm−1); and (ii) the infrared intensities are also sensitive to the 3.3. Single Water Adsorption: Coverage Effect. For a water coverage and also to the metal (please compare spectra single water molecule adsorption at the (110) schreibersite for sections a−c with those of sections d−f). When adsorbed surface, the lateral interaction with water adsorbed in the on Ni, the water frequency bands are less affected by the neighboring cells appeared to be small from the perspective of decreasing water coverage while showing a change in the the interwater distances. Nevertheless, we checked this effect relative infrared intensity. Instead, when adsorbed at Fe site, by enlarging the original adsorption cell to 2 × 1 and 2 × 2 the H2O−Fe_1×1 case (Figure 2d) shows a definite larger supercells, simulating a lower water coverage. As Figure 2 bathochromic shift of both frequencies compared to both the shows, for both adsorptions on the Ni and Fe sites, the O−H higher water coverage and when compared to the correspond- bond distances are almost unchanged as a function of the ing Ni adsorption case H2O−Ni_1×1 (Figure 2a). Inspection different surface coverages, while the O−metal bond shortens of charge transfer from data in Table 1 revealed that, while for with the decrease in water loading, with a corresponding the Ni cases water always loses electronic charge toward the decrease in the adsorption energy. This effect is less surface, for Fe cases the charge flux is reversed with respect to pronounced in the case of the adsorption on Fe (see Figure the Ni adsorption. While evaluating absolute charge transfer is 2, sections d and f) than on Ni (see Figure 2, sections a−c), dependent on the adopted methodology, the present the O−Fe bond (2.407, 2.361, 2.347 Å, for the 1 × 1, 2 × 1, comparison is meaningful, as it compares similar situations and 2 × 2 supercells, respectively) being longer than the O−Ni on closely related systems. Indeed, an increased charge transfer bond (2.165, 2.124, and 2.115 Å). The adsorption energies at toward water populates the σ*(OH) orbital, hence, increasing the Fe site (−29.1/-27.6/−30.6 kJ/mol) are less-dependent on the bathochromic shifts of the OH related stretching the water loading and more erratic compared with those frequencies, in agreement with theoretical interpretation.40 2247 https://doi.org/10.1021/acs.jpcc.1c09947 J. Phys. Chem. C 2022, 126, 2243−2252 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure 3. Experimental (red) and computed (blue) IR spectra. Black bars correspond to the isolated water molecule normal modes. Green arrows represent the motion of the vibrational modes. Atoms color legend: H is in white, O is in red. (a) The experimental spectrum (1.0 Langmuir, 125 K recorded T) is adapted with permission from ref 17. Copyright 2017 ACS. (b) The experimental spectrum (100 Langmuir, 295 K recorded T) is adapted with permission from ref 13. Copyright 2016 Royal Society of Chemistry. Figure 4. PBE-D*0 optimized structure of deprotonated water molecules adsorbed on the (110) schreibersite surface. Atoms color legend: H is in white, O is in red, P is in yellow, Fe is in light gray, and Ni is in dark gray, and the unit cell in blue. In round parentheses, the dissociation energy in kJ/mol is computed with respect to the most stable molecular adsorption case (H2O−Fe_2×1) assumed as a reference. In square parentheses the dissociation enthalpy at [125/298] K. Distances are in Å. We also see that water coverage decrease causes a energy per water molecule (−57.0 kJ/mol) compared to the hypsochromic shift of the OH vibrational bands, thus, single water adsorption case (−37.6 kJ/mol, vide supra). worsening the agreement with the experiment, particularly As the last step, we simulated a water multilayer case for the Fe adsorption. This may be taken as an indication that (starting from the optimized geometry of the monolayer) by the experimental results of ref 17 are for a water surface completely filling in with water molecules the vacuum space coverage closer to the monolayer rather than to the adsorption among the fictitious nonperiodic replicas of the schreibersite of well isolated water molecules. slab. The starting geometry of water molecules was obtained 3.4. Water Monolayer and Multilayer Adsorption. A with the Packmol program.41 The adsorption energy per water water monolayer coverage was simulated by adsorbing up to molecule (−60.5 kJ/mol) is close to that of the monolayer three water molecules on the surface unit cell, limiting our (−57,0 kJ/mol), and both are similar to the reverse of the analysis to the adsorption on the Ni site, as it is the most binding energy per water molecule for the pure amorphous favorable one (Figure 1c). Results indicate that only one out of water bulk model (AE = −62.8 kJ/mol) [AH125/298 = −54.4/− three water molecules is in direct contact with the Ni atom; the 56.1 kJ/mol] {AG125/298 = −39.1/−19.5 kJ/mol}. This proves other water molecules prefer an interwater cooperative H-bond that the effect of the schreibersite surface on a water multilayer interaction, rather than being adsorbed on the Fe atoms. The is rather moderate as their features are close to that of liquid cooperative H-bond interactions between water molecules, water models. Nevertheless, when we used as a reference the together with the contribution from the water directly amorphous water bulk instead of isolated water molecules, the adsorbed at the Ni atom, brings a more favorable adsorption adsorption process is still slightly favorable, at least for 2248 https://doi.org/10.1021/acs.jpcc.1c09947 J. Phys. Chem. C 2022, 126, 2243−2252 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure 5. IR spectra of adsorbed deprotonated water on (110) schreibersite. Green arrows represent the motion of the vibrational modes. Atoms color legend: H is in white, O is in red, P is in yellow, Fe is in light gray, and Ni is in dark gray. The experimental spectrum (1.0 Langmuir, 125 K recorded T) is adapted with permission from ref 17. Copyright 2017 ACS. enthalpy and free energy contributions (AE = 2.3 kJ/mol) disagreement with that model (vide supra), as we find that [AH125/298 = −4.2/−7.6 kJ/mol] {AG125/298 = −3.1/−10.0 kJ/ single water directly interacts with the metal atoms, a mol} (see Figure 1d, italic numbers), showing a role of the phenomenon that can be explained on a simple electrostatic surface in dictating the energetic of adsorption. base (vide supra). Any attempt to force the P−OH2 interaction Figure 3 shows the anharmonic (scaled, vide supra) by putting the water molecule close to the P atom resulted in simulated infrared spectra of both the water mono (Figure water promptly escaping from that position during geometry 3a) and multilayer cases (Figure 3b) in comparison with the optimization. The only way water will interact with the P experimental ones (taken and adapted from ref 17 for water atoms is by dissociative adsorption, in which a new P−O bond monolayer recorded at 125 K, and from ref 13 for water is formed (vide infra). multilayer recorded at 295 K). The simulated spectrum of 3.5. Dissociative Water Adsorption. Above, only the water monolayer (Figure 3a) was built up by merging the IR molecular water was considered as a possible candidate toward spectra of single water adsorptions (H2O−Ni and H2O−Fe) in schreibersite (110) surface adsorption. In this section, we addition to the water monolayer (H2O−mono) to mimic the consider the case of chemisorption, with the formation of complex experimental coverage in which different water metal−H/metal−OH and P−H/P−OH surface features by patches may populate the surface simultaneously. In contrast, water dissociative adsorption. Figure 4 shows the possible the spectrum for the water multilayer case (Figure 3b) was dissociative water adsorption modes on the 2 × 1 supercell of simulated using exclusively the water multilayer contributions, the (110) schreibersite. Dissociative adsorption represents the as there is little ambiguity in controlling the coverage from the very first step of the schreibersite corrosion process, where experiments. For the monolayer case, the agreement with the oxidized forms of each surface species are formed, the most experiment is good, when considering the spectrum recorded important for biological processes being the P−O bond at 125 K, despite some discrepancies in the IR relative formation, at the cost of heterolytically breaking one OH bond intensities related to the rocking modes of adsorbed water. In of water to give OH− and H+. ref 17, the authors hypothesized the formation of a water In this case, the supercell was used to increase the water multilayer despite the maximum coverage of 1.0 Langmuir, fragments degrees of freedom at the (110) surface. We characteristic of the water monolayer. The main difference computed the deprotonation energies (DE) shown in Figure 4 between mono- and multilayer experimental spectra is a rather by using, as a reference system, the energy of the most stable broad peak shown in Figure 3b (marked by an asterisk) at molecular single water adsorption (H2O−Ni_2×1, Figure 2b). around 2423 cm−1, which was attributed by the authors of ref In all cases, the water deprotonation at the surface is 17 to the formation of either P−H and/or P−OH bonds (i.e., endothermic with respect to the molecular adsorption case, to the water dissociative adsorption, see next section).13 At low therefore requiring relative high temperatures to occur. temperatures, the predominant interaction was attributed to The following possibilities were studied here: (i) water water molecularly adsorbed on schreibersite, through a direct deprotonation on metal atoms, that is, both the H and OH interaction of the O atoms with the P atom of the surface. groups on Ni and Fe (Figure 4a,b); (ii) OH group on However, the present atomistic simulation results are in full phosphorus and H on metal atoms (Figure 4c); and (iii) OH 2249 https://doi.org/10.1021/acs.jpcc.1c09947 J. Phys. Chem. C 2022, 126, 2243−2252 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article group on metal atoms and H on phosphorus (Figure 4d; more incipient proton transfer are difficult to be correctly treated cases in the SI). The stability ranking is clear: the most stable by the level of theory adopted here. Indeed, the PBE functional situation happens when both metal atoms are saturated (i.e., underestimates the OH bond strength in H-bonding both H and OH on Fe and Ni, and vice versa, Figure 4a,b) interactions giving too loosely bound protons and exceedingly with almost indistinguishable DE values, followed by the large bathochromic OH shifts. The extreme consequence is a hydroxylation of phosphorus (OH on P, H on Fe or Ni, Figure biased easier formation of ion pairs (H3O+/OH−) in a liquid 4c), and the least stable cases are the hydriding of phosphorus water simulation than it should be.44 To further study that (OH on Fe or Ni, H on P, Figure 4d). feature, a PBE-D*0 molecular dynamics simulation at 300 K In contrast to the adsorption of molecular water, the was performed for 1 ps and the final structure was then deprotonation leads to a strong electronic rearrangement of reoptimized followed by an harmonic frequency calculation. the surface due to the proton attachment, which reduces to a The above-mentioned feature does not disappear, meaning hydride ion, with an average charge of −0.27 |e|. Figure 4a and that, according to PBE-D*0, the formation of such ion pair b show the water dissociation at Fe and Ni to have the same (H3O+/OH−) is stable, at least within the very short MD DE (around 29 kJ/mol). In contrast, the formation of the P−H simulation (longer ones are too demanding for our computa- bonds destabilizes the systems (Figure 4d, DE = 70.5 kJ/mol), tional facilities). To clarify the dependence of the results on the while hydroxylation on P leads to moderately stable structures adopted functional, we reoptimize the PBE-D*0 optimized (Figure 4c, DE = 35.6 kJ/mol), especially when considering structure with the BLYP-D*0 functional, followed by a the remarkable strength of the P−O bond. Therefore, our harmonic frequency calculation: as expected, the BLYP results show that, at low temperatures, water deprotonation is functional increases the intermolecular H-bond distance with not thermodynamically favored. As a further confirmation of respect to the PBE (O1H1···O2 from 1.491 to 1.562 Å), while our findings, Figure 5 shows the computed spectra of all the the intramolecular OH bonds strength is shortened (O1−H1 adsorption modes with deprotonated water (as shown in decreases from 1.047 to 1.029 Å). As a direct consequence, the Figure 4) in comparison with the experimental one for the peak moves to higher wavenumbers (from 2408 to 2649 cm−1) water monolayer case. As one can see, all the computed getting closer to the main OH stretching manifold band. As a vibrational modes of deprotonated water are qualitatively further check, we also carried out BLYP optimization and different with respect to the experimental spectrum, thus, frequency calculations on the PH-Ni/FeOH structure (see indicating that it is unlikely that water undergoes deprotona- Figure 4d of the main text). The P−H stretching slightly tion at low temperatures on the (110) surface. decreases from 2205 cm−1 (PBE-D*0 value) to 2190 cm−1 This is also confirmed by the experimental spectra of Figure (BLYP-D*0 value), showing that, in contrast to the O−H 3, where in the case of the multilayer water adsorption at room stretching, the vibration of the P−H moiety is far less affected temperature (Figure 3b) a new peak rises at 2423 cm−1, which by the adopted functional, giving confidence to the PBE-D*0 is not present in Figure 3a, where the adsorption is studied at result for that specific band. In conclusion, we cannot exclude low temperature (125 K). As already pointed out, this was the experimental vibrational feature at 2423 cm−1 as due to the originally assigned to P−H/P−OH stretching, and, accord- formation of an incipient H3O+/OH− ion pair at the ingly, to water deprotonated at the surface. Considering gas schreibersite surface, in agreement with the fact that, at phase phosphonic acid (HP(O)OH2), the P−H bond vibrates variance with its absence for a water monolayer, water indeed at 2487 cm−1,42 and it could, in principle, explain the multilayers are needed to stabilize the ion pair at the peak at 2423 cm−1, while the stretching associated with P−OH schreibersite interface. However, it has to be established group falls at the lower frequency (below 1000 cm−1). whether this spectroscopic feature can be accounted for by less However, in the case of schreibersite, phosphorus has a fancy chemical situations, when water will be studied in completely different chemical environment than the simpler interaction with the most reactive (001) surface with a most phosphonic acid. According to our results, P−H vibration probable stabilization of chemisorbed water over the represented in Figure 4d is located at 2204 cm−1 (without physisorbed state. Indeed, water chemisorption may result in anharmonic correction, which would further cause a bath- a deep surface corrosion exposing moieties similar, in the ochromic shift of the peak), which is quite different with spectral character, to the phosphonic acid (HP(O)OH2), with respect to the experimental interpretation and, accordingly, can the P−H vibrating at a frequency close to that experimentally be excluded as origin of the observed band. Nonetheless, observed. Figure 3b, shows that the position and relative intensity of the 2432 cm−1 band is correctly reproduced by our simulations. A 4. CONCLUSIONS deeper analysis of the structure associated with this particular In the present work, the adsorption of water on the most stable feature indicates that it belongs to an incipient proton transfer (110) surface of schreibersite was modeled by means of between two water molecules in close contact (O1H1···O2 of periodic calculations using plane waves and the PBE-D*0 the pair is 1.491 Å, O1−H1 1.047 Å, see Figure 1d) with the approach. We consider different surface water coverage, from schreibersite surface, rather than a newly formed covalent bond single water molecule per unit cell, to monolayer (1 Langmuir) with the surface. In that configuration, a proton shuttle up to a multilayer case. For the single water molecule case, we between the two oxygen atoms causes a very large bath- reached almost zero coverage by enlarging the surface unit cell ochromic shift of the original OH stretching bands, as the area. For the multilayer, we fill in the empty space between proton is loosely bound vibrating at 2407 cm−1. This value is surface replicas (about 20 Å), reaching a water density close to also in agreement with the value of the frequency associate to 1 g/cm3. an OH bond belonging to the H3O+ moiety formed in an Adsorption energy shows that water, when molecularly acidic chabazite engaged in a hydrogen bond with a neighbor adsorbed, binds preferentially through an O−metal bond to Ni water molecule.43 We are, however, suspicious about the (−37.6 kJ/mol) and less so to Fe (−29.1 kJ/mol) atoms, both robustness of the value computed for this vibration, as sites acting as electron acceptor of the O electron pairs, 2250 https://doi.org/10.1021/acs.jpcc.1c09947 J. Phys. Chem. C 2022, 126, 2243−2252 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article stabilized by electronic (Ni reach a d10 configuration) and Authors electrostatic (Fe brings a partial positive charge) effects, Marta Corno − Dipartimento di Chimica and Nanostructured respectively. In contrast, the interaction with phosphorus is Interfaces and Surfaces (NIS) Centre, Università degli Studi through a very weak H-bond, in which water acts as a di Torino, I-10125 Torino, Italy; orcid.org/0000-0001- hydrogen donor. The IR spectra corresponding to the 7248-2705 adsorption of one water molecule per unit cell revealed a Albert Rimola − Departament de Química, Universitat weak dependence of the OH stretching bands when the water Autònoma de Barcelona, 08193 Bellaterra, Catalonia, coverage reached the limit of zero coverage. The most relevant Spain; orcid.org/0000-0002-9637-4554 change occurred for the adsorption at Fe atom on the original Nadia Balucani − Dipartimento di Chimica, Biologia e smallest unit cell, where an increased charge transfer from the Biotecnologie, Università degli Studi di Perugia, I-06123 surface toward the water molecule causes a larger OH Perugia, Italy; Osservatorio Astrofisico di Arcetri, I-50125 stretching bathochromic shift than that for the cases of smaller Firenze, Italy; Université Grenoble Alpes, CNRS, Institut de water coverage. Spectra of the monolayer case is in general Planétologie et d’Astrophysique de Grenoble (IPAG), F- good agreement with the experimental one recorded at 125 K. 38000 Grenoble, France; orcid.org/0000-0001-5121- For the multilayer case, the experimental spectrum revealed a 5683 prominent band at 2423 cm−1, which was attributed by the Complete contact information is available at: authors by water chemisorption forming either P−H or P−OH https://pubs.acs.org/10.1021/acs.jpcc.1c09947 groups. Our simulated spectrum revealed a feature at 2408 cm−1 due to the incipient (H3O+/OH−) pair, only present Author Contributions when water multilayer was considered. While we can exclude The manuscript was written through contributions of all the P−H/P−OH attribution, we cannot exclude this computed authors. All authors have given approval to the final version of feature to be due to a specific arrangement of the water the manuscript. molecules adopted to simulate the multilayer or to artifacts in the adopted PBE-D*0 functional, which tends to give a too Funding weak OH bond when engaged in a hydrogen bond. The authors acknowledge the Italian Space Agency for The deprotonation of water on the (110) surface was cofunding the Life in Space Project (ASI N. 2019-3-U.O). studied for the case of a single water molecule per unit cell: the This project has received funding from the European Research process is thermodynamically unfavorable, being endothermic Council (ERC) under the European Union’s Horizon 2020 (around 29 kJ/mol) with respect to the most stable molecular Research and Innovation Program (Grant Agreement No. adsorption case. 865657) for the Project “Quantum Chemistry on Interstellar Work is in progress to extend the study to water adsorption Grains” (QUANTUMGRAIN). Support from the Italian MUR on the most reactive (001) face, which will provide a more (PRIN 2020, “Astrochemistry beyond the second period complete picture of the role of water chemisorption, a more elements”, Prot. 2020AFB3FX) is gratefully acknowledged. robust rationalization of the 2423 cm−1 band in terms of Notes surface attached species containing the P−H feature and paths The authors declare no competing financial interest. ■ for the formation of phosphates. ■ * ASSOCIATED CONTENT sı Supporting Information ACKNOWLEDGMENTS A.R. is indebted to the “Ramón y Cajal” Program. MINECO (Project CTQ2017-89132-P) and DIUE (Project The Supporting Information is available free of charge at 2017SGR1323) are acknowledged. This paper has benefited https://pubs.acs.org/doi/10.1021/acs.jpcc.1c09947. from discussions with the members of the Marie Sklodowska- A figure with more deprotonated structures and Curie “Astro-Chemical Origins” (ACO) Project. ■ corresponding relative energy with respect to the most stable one and the coordinates of the optimized REFERENCES structures (PDF) (1) Grossman, L. Condensation in the Primitive Solar Nebula. The coordinates of the optimized structures in POSCAR Geochim. Cosmochim. Acta 1972, 36 (5), 597−619. format (ZIP) (2) Kelly, W. R.; Larimer, J. W. Chemical Fractionations in ■ Meteorites-VIII. Iron Meteorites and the Cosmochemical History of AUTHOR INFORMATION the Metal Phase. Geochim. Cosmochim. Acta 1977, 41 (1), 93−111. (3) Pasek, M. A. Phosphorus as a Lunar Volatile. Icarus 2015, 255, Corresponding Authors 18−23. Piero Ugliengo − Dipartimento di Chimica and (4) Lehner, S. W.; Buseck, P. R.; McDonough, W. F. 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