Water Dissociative Adsorption on NiO(111): Energetics and Structure

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Water Dissociative Adsorption on NiO(111): Energetics and Structure of the Hydroxylated Surface Wei Zhao, Michal Bajdich, Spencer J. Carey, Aleksandra Vojvodic, Jens K. Norskov, and Charles T. Campbell ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01997 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016

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Water Dissociative Adsorption on NiO(111): Energetics and Structure of the Hydroxylated Surface Wei Zhao†, Michal Bajdich‡, Spencer Carey†, Aleksandra Vojvodic‡, Jens K. Nørskov‡ and Charles T. Campbell†* †

Department of Chemistry, University of Washington, Seattle Washington 98195-1700, USA ‡

SUNCAT Center for Interface Science and Catalysis, Chemical Engineering, Stanford

University, Stanford, California 94305, and SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

Abstract The energetics of the reactions of water with metal oxide surfaces are of tremendous interest for catalysis, electrocatalysis, and geochemistry yet the energy for the dissociative adsorption of water was only previously measured on one well-defined oxide surface, iron oxide. In the present paper, the enthalpy of the dissociative adsorption of water is measured on NiO(111)-2×2 at 300 K using single-crystal adsorption calorimetry. The differential heat of dissociative adsorption decreases with coverage from 172 kJ/mol to 119 kJ/mol in the first 0.25 ML of coverage. Water adsorbs molecularly on top of that, with a heat of ~94 kJ/mol.

Density functional theory (DFT)

calculations reproduce the measured energies well (all within 15 kJ/mol), and provide insight to the atomic-level structure of the surfaces studied experimentally. They show that the oxygen-terminated O-octo(2×2) structure is the most stable NiO(111)-2×2 termination, and give reaction energies with water that are more consistent with the calorimetry results than the metal-terminated surface. They show that water adsorbs dissociatively on this (2×2)-O-octo surface to produce a hydroxyl-covered surface with a heat of adsorption of 171 ± 5 kJ/mol in the low-coverage limit (very close to 172 kJ/mol experimentally), and an integral heat that decreases by 14 kJ/mol up to saturation (compared to ~30 kJ/mol experimentally). Sensitivity of this reaction’s energy to choice of DFT method is tested using variety of different exchange correlation functionals, including HSE06, and found to be quite weak.

* Corresponding author: [email protected], tel. = 206-616-6085 1 ACS Paragon Plus Environment

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Introduction Late transition metal oxides play a very important role in catalytic oxidation and steam reforming reactions, where surface hydroxyl groups are key intermediates.

Yet very little is

known about the structure or energetics of surface –OH groups on any oxide surface.

Indeed,

much less is understood about the chemistry of oxide surfaces than metal surfaces.

A recent

comprehensive review of adsorption energies on oxide surfaces1 pointed out that while the adsorption energies are known for many species which are molecularly absorbed on well-defined, single-crystal oxide surfaces, only a few values have been reported for dissociatively adsorbed species. In all of those cases, the adsorption energy was estimated based on temperature programmed desorption of the reverse process (associative desorption), and a pre-exponential factor was assumed for the desorption rate constant.1

In only one case reported

there, the dissociative adsorption of water on α-Fe2O3(012), was this prefactor actually determined from the experimental data.1-2 Since that review, there has also been one direct calorimetric measurement of the adsorption energy of dissociative adsorption process on any well-defined, single-crystalline oxide surface, water dissociating on Fe3O4(111).3 In other words, there are only two prior measurements of the energies of adsorption reactions that produce well-defined adsorbed molecular fragments on any oxide surface.

We report here calorimetric

measurements of the heats of reaction for the dissociative adsorption of water on NiO(111) to produce two different types of surface –OH groups, and complimentary DFT calculations to better define their surface structures. These results add substantially to our limited knowledge of the energy and structure of surface –OH groups on oxide surfaces. We also report here DFT calculations of the energies of this reaction using several different exchange correlation functionals, and compare these to the calorimetric results to validate their energy accuracies. This comparison to experimental benchmarks adds new confidence in the energy accuracy of some of these DFT methods as applied to transition metal oxide surfaces.

2 ACS Paragon Plus Environment

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Nickel oxide is used as a main catalyst or co-catalyst in a wide range of important reactions, e.g., partial oxidation of methane,4-5 ethene dimerization,6 and hydrogen evolution electrocatalysis.7 Surface hydroxyls and molecularly adsorbed water are very important intermediates in most of these reactions, and in a large number of catalytic and electrocatalytic reactions on transition metal and oxide surfaces in general. Thus, the reaction between water and NiO surfaces, and its adsorbed hydroxyl products, have attracted considerable attention for decades. As reported previously, water vapor dissociatively adsorbs rapidly on the NiO(111) surface.8-9

Upon dissociation, it is reasonable to assume that it forms two types of surface –OH

groups, one whose oxygen atom came from the water molecule, i.e., an adsorbed hydroxyl, OHad, bound to a coordinatively-unsaturated Ni atom, and one where the H atom lost from this water bonds to an oxygen atom that was already part of the NiO(111) surface, so it might be considered instead an adsorbed H atom, Had, although it is also part of another surface hydroxyl. This picture is confirmed by the density functional theory (DFT) calculations below, which further show that the two –OH groups thus formed can be structurally identical or nearly identical, depending upon the starting NiO surface structure. Thus, this reaction may be described by either of the following shorthand expressions: H2Og OHad + Had

(1)

H2Og + NiO(111) HO-NiO-H

(2)

In contrast, the NiO(100) surface was found to be much less reactive in dissociatively adsorbing water.8-9 Here we report the first calorimetric measurement of the heat of reaction for the dissociative adsorption of water on NiO(111)-2×2 as a function of coverage. This provides an experimental measure of the heat of formation of the resulting surface hydroxyl groups. These measurements provide an important benchmark for validating computational estimates of adsorption energies of molecular fragments on correlated metal-oxides such as NiO and for oxide surface chemistry in general, which is more challenging in this respect than for metal or wide-gap semiconductor surfaces.10-13


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Experimental Methods The experiments were performed in an ultrahigh vacuum (UHV) chamber (base pressure 99.9%) was degassed by five freeze-pump-thaw cycles after putting into its reservoir connecting the vacuum chamber. We define the coverage of D2O molecules which adsorb onto the surface irreversibly in units of monolayers (ML) where 1 ML = 1.33×1019 atoms/m2, the density of O atoms in a (hypothetically) unreconstructed NiO(111) surface18). A typical dose is 0.02 ML (∼3.5 × 1012


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molecules within the beam diameter of ~4 mm) per D2O pulse. A detailed description of the experimental principles and implementation of the molecular beam flux, sticking probability and heat measurements can be found elsewhere.14-15, 20-21 To accurately calibrate the adsorption heat for D2O on NiO(111) at 300 K, we measured the heat signal from a pulsed, collimated He-Ne laser pulse as described previously.14-15, 20-21 The optical reflectivity of the NiO-coated Ni(111) sample relative to the clean Ni(111) sample was determined by measuring the ratio of the heat signals for the same laser intensity at 300 K, averaged over several measurements. The optical reflectivity of Ni(111) was obtained from literature reports,22 but we measured it with an integrating sphere to verify that value. Absolute calibration is estimated to be accurate within 3%.23 Theoretical Methods To calculate the bulk and surface energetics of NiO, we employ density functional theory (DFT) calculations within the Vienna ab-initio simulation package (VASP)24-25 together with the projector augmented wave (PAW) potentials.26 We adapt the PBE27 functional together with the Hubbard-U method28 applied for the d-electrons of Ni atoms, which is referred to as PBE+U in the text. The value of the effective Hubbard-U parameter, Ueff=6.45 eV, is taken from previous studies on bulk NiO.29 The application of our PBE+U method to bulk NiO results in the optimized lattice constant of a0=4.181 Å, optical band gap Eg=3.1 eV and a magnetic moment on the Ni ions of M=1.76 µB which compares favorably with the experimental values of a0=4.177 Å,30 Eg=4.0–4.3 eV31-32 and M=1.77–2.26 µB33-34. Additionally, we test the effect of varying Ueff from 4.0 to 8.0 eV and performance of other exchange correlation functionals such as BEEF-vdW+U,35 PBEsol+U36 and hybrid HSE0637-38 on the adsorption energetics. All slab calculations are performed with an energy cutoff of 550 eV and an approximately constant k-point mesh of 8×8×1 points per p(1×1) unit cell. The fully symmetric slabs of at least 4 NiO layers with adsorbates on both sides were relaxed below the threshold force of 0.03 eV/Å, while at least 15 Å of vacuum was used to separate the slabs. The magnetic ordering of Ni atoms of


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the lowest energy structures is always of type-II antiferro-magnetic (AFM) order along the [111] direction.39 The reported surface energies, ∆γ, of neutral surfaces are calculated relative to the surface energy of the oxygen-terminated octopolar-(2×2) [O-octo-(2×2)] surface and chemical potentials of water, µH2O, and bulk NiO, µNiO. Conventionally, these chemical potentials are simply taken as DFT energies of water in the gas phase and bulk energy of NiO. To compare calculated DFT energies (∆EDFT) with the measured enthalpy (∆H) of the dissociative adsorption of water, we add the zero-point energy change (∆ZPE) and thermal energy change (T∆Cp) as calculated within the harmonic approximation. The final comparison to experimental enthalpy also includes a pressure-times-volume work change for the adsorption of D2O, which based on the ideal gas law equals to -RT (see Ref. 40 and 41). As a result, the overall calculated correction at 300 K to heat of adsorption (negative of the enthalpy change) is: HEATDFT = -∆HDFT = -∆EDFT - 6.2 kJ per mol of D2O. See Table S2 of the SI for a detailed breakdown of all the corrections.

Experimental Results Preparation of the octo-(2×2) NiO(111) surface Previous studies showed that oxidation of a clean Ni(111) surface at 300 K leads to the growth of 3 ~ 4 monolayers (ML) (0.724 ~ 0.965 nm) of NiO(111) at saturation (oxygen exposure >160 L).9, 16-18, 42 We used that synthesis method to prepare the NiO(111) surface studied here, and made sure that very little hydrogen and water was in the chamber during preparation. The unreconstructed NiO(111) surface is a polar surface and thus it is thermodynamically unstable.13 Wolf et al.43 proposed a stabilizing p(2×2) octopolar reconstruction of such polar surfaces with rock-salt crystal structure, in which 3/4 of the atoms in the first layer and 1/4 of the atoms in the second layer are missing, shown in Figure 1, for both the oxygen-terminated and metal-terminated surfaces, which we will refer to as the O-octo(2×2) and M-octo(2×2) surfaces, respectively. Experimentally for NiO(111)/Ni(111), Rohr et al.44 observed a (2×2) reconstruction with LEED and Okazawa et al.18 also suggested the (2×2)


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octopolar reconstruction. Ventrice et al.45 observed the octopolar reconstruction with STM for NiO(111) films grown on Au(111), and Barbier et al.46 found it on a (111) surface of a NiO crystal. We also find a LEED pattern for the NiO(111) surface studied here which is consistent with a octo-(2×2) reconstructed surface. As discussed below, our DFT results find the O-octo(2×2) surface to be slightly more stable and to give energies for dissociative water adsorption that are more consistent with our calorimetry results than the M-octo(2×2) surface, so we conclude that we have prepared the O-octo(2×2) surface here.

Sticking Probability As described before,20 two types of sticking probabilities are measured in our SCAC experiments: the long-term sticking probability, S∞, and the short-term sticking probability, S102 ms.

The long-term sticking probability is the probability that a gas molecule strikes the sample

surface, sticks, and remains until the next gas pulse starts ∼5 s later, which is used to calculate the adsorbate coverage remaining at the start of the next gas pulse. The short-term sticking probability is the probability that a gas molecule strikes the sample surface, sticks, and remains at least throughout the time window of our heat measurement (i.e., the first 102 ms). This is used to calculate the number of moles of gas-phase reactant that contribute to the measured heat of adsorption. Figure 2 shows both of these sticking probabilities for D2O versus coverage on NiO(111) at 300 K. The long-term sticking starts at 0.82 and increases to 0.94 in the coverage range 0.05-0.18ML. It then drops sharply to ~0 at ~0.25 ML. In contrast, the short-term sticking starts at 0.96 and approaches unity, dropping slightly to 0.92 at 0.25 ML. That shows that D2O sticks to NiO surface with a high probability and saturates at 0.25 ML, and still adsorb with nearly unit probability after that, but with such weak binding that it desorbs again before the next pulse arrives. The saturation coverage of 0.25 ML is consistent with one water molecule adsorbing (dissociatively) per unit cell on the p(2×2) octopolar surface structure of NiO(111) studied here. The short-term sticking probability stays high (~0.92) after saturation of D2O dissociative adsorption (0.25 ML) for hundreds of pulses, but simultaneously long-term sticking


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stays at ~0, indicating that D2O molecule sticks to the hydroxyl-saturated NiO(111) surface (after 0.25 ML), but desorbs again before the next pulse. Figure 3 shows the normalized mass spectrometer signal versus time during the 5000 ms pulse cycle of D2O gas impinging on the hydroxyl-saturated NiO(111) surface at 300 K (the red curve), averaged over the 80 pulses after the saturation of D2O dissociative adsorption on NiO (> 0.25 ML). For reference, the mass spectrometer signal of a pulse of D2O gas impinged on an Au flag at room temperature, where D2O desorbs very rapidly, is also shown as the blue curve. The decay of the mass spectrometer signal for water pulses after saturation (> 0.25 ML) was well fitted by an exponential decay with an average surface residence time (t) of ~786 ms, shown as the black curve. This corresponds to the inverse of the first-order rate constant for D2O desorption (kdes), giving ~1.27 s-1. Assuming a pre-exponential factor for desorption of 1015 s-1,1 this gives an activation energy for desorption (Edes) of this weakly-adsorbed water on the hydroxyl-saturated NiO surface of ~86 kJ/mol, close to the value of 94 kJ/mol measured after saturation by calorimetry (see below). Assuming instead a prefactor of 2 × 1016 s-1 would give 93 kJ/mol and perfect agreement (remembering that ½ RT must be added to Edes to compare to heats of adsorption47). For comparison, a prefactor of 1017 s-1 was measured from TPD for the associative desorption of dissociatively adsorbed water on Fe2O3(012).2 Such large prefactors have also been reported for molecularly adsorbed species at defects (where it has limited mobility).48

Heat of Water Adsorption In this paper, we define the term heat of adsorption as the negative of the differential standard molar enthalpy change for the adsorption reaction, with the gas and the sample surface being at the same temperature. As described previously, this requires a small correction on the gas temperature compared to the actual experimental molecular beam conditions.20 Water is known to dissociatively adsorb on NiO(111) at 300 K, forming surface hydroxyls, reactions (1) and (2). Figure 3 shows the corresponding heat of adsorption vs D2O coverage until


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saturation at ~0.25 ML, where each point represent a single gas pulse averaged over five identical calorimetry experiments. The heat drops with coverage (Θ, in ML), from an initial value of ~170 kJ/mol down to a final value of ~94 kJ/mol. After saturation, water molecules continue to adsorb with high probability but desorb again slowly but before the next gas pulse (see Figure 2 and Figure 3). The slope of the initial heat-versus-time signal at each gas pulse still gives an approximately correct heat of adsorption for such transiently-adsorbed species.20 The heat value we measured in this way for the transiently-adsorbed D2O after saturation is 94 ± 4 kJ/mol. This heat is very similar to the final value measured just before saturation of 96 kJ/mol (Figure 4), suggesting that both measurements probe water in the same state.

Based on DFT calculations below, we attribute

this state to water which is transiently molecularly adsorbed on hydroxyl-saturated parts of the surface. Above 0.18 ML in Fig. 4, interpretation of heat versus coverage is complicated by the fact that this heat measures both the molecules which adsorb permanently and those which desorb again with a slow time constant like in Fig. 3. The coverage axis reflects only those that stick permanently. The fraction of molecules which stick permanently is given by the long-term sticking probability, S∞, fitted in this coverage range by the straight line Sdiss in Fig. 2. The fraction which desorb slowly is 1-Sdiss. We assume here that these have a heat of adsorption which is the same as after saturation (94 kJ/mol), and attribute these to water which is transiently molecularly adsorbed on hydroxyl-saturated parts of the surface as above. The surface residence time for these was also measured with the QMS at ~0.22 ML and seen to be similar to that after saturation (Fig. 3), but with much poorer signal-to-noise ratio due to the inability to signal-average here many pulses as we did after saturation to make Fig. 3.

The black line in Fig.

4 shows the heat expected for this two-state model, assuming a value of ∆Hdiss = 119 kJ/mol for those that stick permanently. It fits the data very well. The red line reflects the (constant) heat of adsorption versus coverage for the dissociatively adsorbed water that makes surface hydroxyls within this model. This flat line, combined with the heat measurements below 0.18 ML in Fig. 4,


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gives an integral (i.e., average) heat up to 0.25 ML for the dissociatively adsorbed water of 140 ± 4 kJ/mol. Such error bars here represent the standard deviation over 5 individual experiments. The integral heat of adsorption is defined here as the total heat per mole released when adsorption occurs on a clean surface up to a fixed coverage. We also measured the heat of adsorption of water on this hydroxyl-saturated NiO(111) surface at 100 K by SCAC. (This surface was pre-saturated with hydroxyl at 300 K.)

The heat

of adsorption of water was found to start at ~74 kJ/mol and drop smoothly to below 55 kJ/mol by ~0.5 ML of additional water, asymptotically approaching ~47 kJ/mol at high coverage. This final value is very close to the reported heat of sublimation of amorphous ice multilayers21. Some water probably adsorbed during sample cooling, so the true initial value is probably higher than the 74 kJ/mol seen in this experiment. Based on Figs. 3 and 4 above, one would expect the initial value to be closer to 94 kJ/mol. We attribute this difference to water that adsorbed from background gases during the long time required to cool the surface and sample holder from 300 K to 100 K, which probably populated the stronger binding sites with heats between 94 and 74 kJ/mol.

Theoretical Results Previously, Ciston et.al.49 and Ebensperger and Meyer13 have extensively studied the hydroxylation of the NiO(111) surface theoretically. Our calculated surface energetics (see Figure 1) closely follows their results for non-polar surfaces. Under water-poor conditions (i.e., in the absence of water, on the hydroxyl-free surfaces): the oxygen-terminated O-octo-(2×2) and metal-terminated M-octo-(2×2) are the most stable terminations (structures shown in Figures 1a and 1b). The M-octo-(2×2) is marginally less stable than O-octo-(2×2) by 0.03 eV per (1×1) unit cell. All other neutral, H-free surface terminations we tested, such as O-missing-row-(2×1) and R3t-type structures 13, were found to be less stable by more than 0.25 eV per (1×1) (not shown). The fully hydroxylated (1×1)-OH termination is the most stable surface. Because the experiment was conducted at UHV conditions, in the rest of the paper we will limit our discussion of water


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adsorption only to the O-octo-(2×2) and M-octo-(2×2) surfaces. The relative energies of all surfaces are listed in Table S1 of SI.

Calculated Energetics of D2O adsorption In Figure 5, we compare the calculated reaction energies (-∆E) for D2O adsorption on both the O-octo and the M-octo type surfaces together with their lowest-energy optimized structures for low- and high-coverage regimes, defined as 1/16 ML and 0.25 ML, respectively. We have considered 3 different structural models for the starting NiO surface: i) symmetrically terminated O-octo surfaces (Figures 5a and 5d), ii) 3 layers of O-octo(4×4) supported on top of Ni(111)(5×5) in the unit cell of Ni(111), which has 5% expanded lattice compared to the unsupported NiO (Figures 5b and 5e) and iii) symmetrically terminated M-octo surfaces (Figures 5c and 5f). The reason behind the proposed structure ii) is twofold: First, note that the experimentally prepared sample was known to have only 2-4 complete monolayers of NiO on top of Ni(111), and second, the NiO(4×4)/Ni(111)(5×5) represents simulation cell with the reasonably small lattice mismatch, which is still tractable by DFT methods. We checked the effect of this 5% lattice expansion in structure ii) on the surface energies of the M and O terminated octo surfaces. Under this expansion (on the underlying Ni(111) slab), the relative surface energy of the M-octo is -0.018 eV/(1x1) more stable than the O-octo surface, while for the equilibrium NiO(111) slab in the absence of Ni(111), the M-octo was 0.029 eV /(1x1) less stable than O-octo surface. While this expansion very narrowly reverses the ordering of the surfaces, in both cases the energy difference is far too small to confidently predict their relative stabilities. In the low coverage regime (1/16 ML), dissociative adsorption has -∆E =168 kJ/mol of H2O for O-octo (Fig. 5a), -∆E =177 kJ/mol of H2O for supported truncated O-octo (Fig. 5b) and to -∆E =140 kJ/mol of H2O for M-octo (Fig. 5c). For the first case, we have also tested the effect of decreasing the coverage to 1/36 ML, which resulted in no further change of ∆E. For the O-octo terminated surfaces, the dissociative adsorption of H2O is similar in character, whereby a single Ni atom splits the H-OH bond in H2Oads, the H then attaches to the exposed O-site, while the


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remaining OHads fills the vacancy at the hollow Ni-Ni-Ni site (Figure 5a). The Ni(111) support leads to a slightly expanded NiO lattice, which in turn allows for easier movement of the OHads. and results in the Ni-O-Ni bridge like geometry (Figure 5b). Note that the starting surface for Fig. 5b before water adsorption was the same NiO(111)-(2×2)-O-octo structure as in Fig. 1 except that it had a 5% expanded lattice in the xy surface plane (and consequently a slight contraction in the z direction). The adsorption on the M-octo surface is governed by the strong preference to fill O-vacancies by OHads, while the remaining Hads bonds to the existing O-site of the octopolar termination (Fig. 5c). This mechanism is different than adsorption on the octahedral-Fe site of Fe3O4,, where the OHads binds directly to an exposed metal atom 3. At the high coverage regime of 0.25 ML, dissociative adsorption is more exothermic for unsupported O-octo: -∆E =186 kJ/mol (Fig. 5d), less exothermic for supported truncated O-octo: -∆E =163 kJ/mol (Fig. 5e) and is basically unchanged for the M-octo surface: -∆E =136 kJ/mol (Fig. 5f) when compared to low coverage results (see also Table 1). These results can be understood in terms of OHads and O+Hads structures, which are formed at 0.25 ML coverage, which are either complete symmetric hexagonal rings on an unsupported O-octo surface (Figure. 5d), distorted hexagonal rings or lines on the supported xy-expanded O-octo surface (Figure. 5e) and lines of OHads. and O+Hads with alternating orientations for the M-octo surface (Figure. 5f), with calculated energies also summarized in Table 1. The two resulting –OH groups produced upon water adsorption are identical in Fig. 5d and nearly identical in Fig. 5a.

While this is consistent with the shorthand notation of reactions (1)

and (2) as defined, it highlights the danger of over-interpreting these shorthand reaction equations as written.

The two –OH groups produced upon water adsorption differ more

obviously on the expanded NiO(111) lattice of Figs. 5b and 5e. To produce these structures, we started from initial geometries that resembled Figs. 5a and 5d, respectively, but these spontaneously relaxed the structures shown, thus breaking the initial hexagonal symmetry in the case of Fig. 5e to achieve ~10 kJ/mol stabilization. Any energy costs associated with the accompanying lattice-atom rearrangements apparently were more than compensated by the extra


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stabilization achieved in this barrier-less process. Next, we have calculated single water molecular adsorption on top of this surface precovered with 0.25 ML of dissociated water fragments. We obtain -∆E =50 kJ/mol for unsupported

O-octo(2×2)

and

-∆E

=103

kJ/mol

for

Ni(111)

supported

truncated

O-octo(4×4)/Ni(111)(5×5) (see Table 1). The later structure is stabilized via visible formation of H2O-OH hydrogen-bonded bridging (see Figure S1), as has been observed on other oxide surfaces, e.g., Fe3O4(111),3 RuO250 and PdO(101)51. We also note that the -∆E =103 kJ/mol is in good agreement with the measured ∆H=94 kJ/mol for molecular adsorption at 0.25 ML. Additionally,

we

studied

molecular

water

adsorption

onto

pre-coverages

of

dissociatively-adsorbed water below 0.25 ML, and found that molecular water is always less stable than its further dissociative adsorption. Molecular adsorption is possible below 0.25 ML only for the M-octo surface (where it gives -∆E = 72 kJ/mol of H2O, see Table 1), since there is no barrier for its dissociation on the O-octo surfaces.

Table 1: Summary of the calculated energies (-∆E) for dissociative and molecular adsorption of water on three considered systems discussed in the text. The reported values are for the PBE+U(6.45 eV) functional and 550 eV energy cutoff. The corresponding structures are shown in Figures 5 and S1. Coverage (cell)

-∆E [kJ/mol]

-∆E [kJ/mol]

O-octo(2×2)

O-octo(4×4)/ M-octo(2×2)

-∆E [kJ/mol]

Ni(111)(5×5) 1/16 ML dissociated

168.2

177.4

140.1

¼ ML dissociated

185.9

162.9

136.0

¼ ML molecular on ¼ ML dissociated

50.0

103.3

-

unstable

-

71.9

¼ ML molecular only

For the case of the symmetric O-octo surface on the free-standing NiO(111) sheet (i.e., without Ni(111) below), we also performed a sensitivity analysis of the calculated energies at low and high coverage with respect to the type of DFT approach used, as described in the 13 ACS Paragon Plus Environment

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computational methods. Varying the value of the Hubbard U parameter within the range 4 ~ 8 eV, or changing the DFT GGA functional from PBE to BEEF-vdW to PBEsol or adding screened exact-exchange as HSE06 (high coverage only) shows relatively weak dependence in the obtained dissociative adsorption energies (see Figure 6). The biggest outliers are the values for the PBE+U = 4 eV and PBEsol+U=6.45 eV exchange correlation functionals. The over-binding of adsorbates by PBEsol+U is well known and has been observed also previously for other systems (e.g., cases of CO and NO on fcc(111) metals35). We also note that the energy for the simultaneous adsorption of OH- and H+ (which is the net result of dissociative water adsorption, and gets more stable as U increases) has the opposite dependence on Hubbard-U term as the separate adsorption of OH and H (see Figure S1), which gets less stable with increasing U value, in agreement with previously reported studies on metal-oxides.10,

52-53

The reason for such

dependence must be due to the fact that simultaneous adsorption leads to almost no change in oxidation state of Ni centers or polarity of the surface, while both of the two separate adsorptions alter these properties. Discarding the PBE+U = 4 eV and PBEsol+U=6.45 eV functionals, all other functionals predict energies within a narrow range (within ±5 kJ/mol), including more expensive HSE06 calculations. The finding that the differences between low and high coverage values are also fairly constant gives a confidence in the results as being functional-independent except for the above-mentioned two outliers. Since the experimental accuracy is also at the ±4 kJ/mol level, any of these functionals can be taken as an equally good representation of the experiment.

Discussion Comparison to Results from TP-XPS It is interesting to compare the energetics of water reacting with NiO(111) measured by SCAC in this work to previous experimental studies. A temperature-programmed XPS (TP-XPS) study using a heating rate (β) being 1.7 K/s showed that hydroxyls desorb from NiO(111)/Ni(100) in the temperature range of ~400 to ~600 K.16 We simulated the corresponding TP-XPS


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spectrum using the experimental heating rate of 1.7 K/s (shown as Figure S2 in SI) based on our calorimetric adsorption enthalpies (∆Had, Fig. 4). Assuming a negligible activation energy for adsorption (which is consistent with the large sticking probability at 300 K in Fig. 2), this gives a desorption activation energy of Edes = ∆Had – ½ RT,54-55, where ∆Had varies with coverage as ∆Had = 172 – 282Θ while Θ < 0.18 ML (obtained by linearly fitting the dissociative adsorption heats before 0.18 ML) and ∆Had = 119 kJ/mol while Θ ≥ 0.18 ML (see Fig. 4). To simulate the TP-XPS, we assumed first-order desorption kinetics with a prefactor of 1017 s-1, based on the value estimated for the same process on α-Fe2O3(012).2 Starting at 200 K with an initial water coverage (Θ) of 0.25 ML of dissociated water (the saturated coverage at 300 K found here, which was used in that TP-XPS experiment), the simulated TP-XPS spectrum exhibits a broad desorption temperature range of around 320 K ~ 500 K, in fairly good agreement with the range of 400-600 K reported from TP-XPS.

Discussion of Theory vs. Experiment The measured differential heat of dissociative H2O adsorption varies nearly linearly from 172 kJ/mol to 119 kJ/mol of H2O within 0 to 0.18 ML coverage (Fig. 4). The dissociative H2O adsorption on the O-octo NiO(111) surface supported on Ni(111) gives a calculated heat of -∆H=171(±5) kJ/mol at the lowest coverage (Fig. 5a and 5b), remarkably close to the experimental value of -172 kJ/mol in the limit of low coverage. On the other hand, none of the calculated M-octo energies (Fig. 5c and 5f) are more exothermic than ~ 140 kJ/mol of H2O, which suggests that M-octo is unlikely to represent the NiO(111) surface studied experimentally. The experimental dependence of coverage (shown in Figure 4) indicates ~50 kJ/mol decrease in the differential heat with coverage from 0 to 0.25 ML, and ~30 kJ/mol decrease in the integral heat. As we have discussed above, only the O-octo(2×2) NiO(111) supported on Ni(111) has –∆E which decreases with increasing coverage. The calculated decrease in the integral heat is ~14 kJ/mol, about half that observed. This agreement is quite good given our lack of experimental knowledge of the atomic-level structure of the starting surface or the surface


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after water adsorption. Thus, we interpret the measured integral heats of adsorption in Fig. 5 as reflecting the dissociative adsorption of water on the NiO(111)-(2×2)-O-octo surface (like shown in Fig. 1a except that it has a 5% expanded lattice in the xy surface plane due to the NiO(111) film being supported on top of Ni(111) with (4x4):(5×5) periodicity). Water dissociatively adsorbs via reactions (1)/(2) to produce the structures of coadsorbed –OH plus –H shown in Fig. 5b at low coverage (1/16 ML, integral heat = 168 kJ/mol) and Fig. 5e at high coverage (1/4 ML, integral heat = 140 kJ/mol). At intermediate coverages, the heat reflects some linear combination of the energies for these two separate adlayers, presumably in separate domains covering the surface. (This avoids populating less stable periodic structures which we calculated at intermediate coverages but are not described here due to their poor stability.) At coverages slightly greater than ¼ ML, water adsorbs molecularly and reversibly in the structure of Fig. S1 (differential heat = ~94 kJ/mol). Conclusions The energetics of water interacting with NiO(111)-2×2 was directly measured by single crystal adsorption calorimetry, showing the differential heat of dissociative adsorption at 300 K decreases from 172 kJ to 119 kJ/mol with coverage up to 0.25 ML of dissociated water, the saturation coverage in UHV. These energetics are consistent with previous TP-XPS experiments.16 Our DFT+U calculations reveal that only truncated O-octo NiO(111) supported on Ni(111) has H2O adsorption which is fully consistent with experiments. At low coverage, we obtain value of -∆H=171(±5) kJ/mol, which is very close to a measured value and has the characteristic OHads. and O+Hads formation near the metal centers. At higher coverage (up to 0.25 ML), the interaction of adsorbed hydroxyl groups decreases the integral heat of dissociative adsorption as is also observed in the experiment. Additional molecular adsorption above 0.25ML coverage is also well described by our calculations and shows stabilization via H2O-OH hydrogen-bonded bridging, consistent with the calorimetric experiments at 100 K. Finally, the calculated values are shown to be fairly insensitive to the choice of the DFT+U functional and


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are particularly well described within the surface specific BEEF-vdW functional with added Hubbard-U correction.

Supporting Information.

Calculated structure of molecularly adsorbed on top of 0.25 ML of

dissociated water, simulated TP-XPS spectrum, dependences of the energies of dissociative H2O, H and OH adsorption on the Hubbard-U parameter, tables of calculated relative surface energies of different surfaces and calculated zero-point energy and heat capacity corrections to water adsorption enthalpies.

Acknowledgements This work has been supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences through grants to SUNCAT and through the Materials Genome Initiative. MB and AV would like to acknowledge the use of the computer time allocation for the “Computational search for highly efficient 2D & 3D nano-catalysts for water splitting” at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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Figure 1. Top and side views of the two most stable surfaces under ultrahigh vacuum (water poor conditions): a) oxygen-terminated O-octo(2×2) and b) metal-terminated M-octo(2×2) reconstructions. The green (red) spheres represent Ni (O) atoms respectively and for clarity only the top most layers are shown as opaque. c) Calculated relative surface energies, ∆γ per (1×1) surface area, of the most stable neutral NiO(111) surfaces, relative to the O-octo-(2×2) surface, which an absolute surface energy of γ = 0.860 eV = 1.821 J/m2 (PBE+U functional).


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Figure 2. Long-term (S∞: in red) and short-term (S102 ms: in blue) sticking probability of D2O on NiO(111) as a function of the total D2O coverage that adsorbed (irrespective of whether it dissociated or not) at 300K. The line Sdiss shows that the long-term sticking points decreases nearly linearly with coverage (Θ) from essentially unity at 0.18 ML to 0 at 0.25 ML.


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Figure 3. Normalized mass spectrometer signal versus time during the 5000 ms pulse cycle of D2O gas impinging on the hydroxyl-saturated NiO(111) surface at 300 K, averaged over the 80 pulses after the saturation of D2O adsorption on NiO (> 0.25 ML). The 102 ms pulse strikes the surface from ∼460 to 562 ms on this scale. The slow desorption of D2O from the hydroxyl-saturated NiO(111) surface is apparent in the broad tail of the mass spectrometer response after this (red trace), which is well fit by an exponential decay with a 786 ms time constant (smooth black curve). For reference, the signal from the same 102 ms pulses of D2O after impinging on a room-temperature Au flag, where the molecules desorb very rapidly, is also shown (blue trace).


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Figure 4. Differential heats of dissociative adsorption of D2O on NiO(111) at 300 K versus the total coverage of dissociatively adsorbed D2O. The red curve through red data points at 0 to 0.18 ML and the flat red line from 0.18 to 0.25 ML represent the heat of dissociative adsorption (see below). Each data point represents a pulse of ∼0.022 ML of D2O gas and is the result of averaging five experimental runs. One ML is defined as 1.33 × 1019 (dissociated) D2O molecules per m2. Above 0.18 ML, two lines are shown to reflect data interpretation, necessitated by the fact that this heat measures both the molecules which adsorb permanently and those which desorb again with a slow time constant, whereas the coverage axis is only for those that stick permanently. The black line assumes a two-state model for the heat here, with a value of ∆Hdiss = 119 kJ/mol for those that stick permanently and 94 kJ/mol for those that desorb again slowly, with the fraction of each given by Sdiss and (1-Sdiss), respectively. The value of Sdiss is taken from the long-term sticking probability measured in Fig. 2. The flat red line thus reflects the heat of adsorption for the dissociatively adsorbed water that makes surface hydroxyls in this coverage range (0.18 ~0.25 ML). The molecules which desorb slowly with a heat of adsorption of 94 kJ/mol are attributed to water which is transiently molecularly adsorbed on hydroxyl-saturated parts of the surface (see text). Within this model, the integral heat for the dissociatively adsorbed water is plotted as water coverage (the blue curve), giving 140. ± 4 kJ/mol at 0.25 ML. The two green points are the integral heats of dissociative adsorption calculated with DFT at 0.0625 ML and 0.25 ML for the model of the 3 layers of O-octo(2×2) surface of NiO(111) supported on Ni(111) (shown in Figure 5).


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Figure 5: Calculated structures for the dissociative adsorption of water, and corresponding energies per single H2O molecule, on the NiO(111)-(2x2)-O-octo surface (left), the NiO(111)-(2x2)-O-octo surface supported on Ni(111) (middle) and the NiO(111)-(2x2)-M-octo surface (right) at low coverage (1/16 ML) (a-c) and high coverage (0.25 ML) (d-f). The energies listed are for the PBE+U(6.45 eV) functional. The color scheme for atoms is identical to Fig. 1, except the added gray spheres represent H atoms.

Figure 6. Comparison of several DFT functionals for the total energy change upon dissociative adsorption of water onto the (2×2)-O-octo surface structure at low coverage (1/16 ML) and high coverage (1/4 ML) at less expensive 400 eV energy cutoff. The experimental line refers to the – ∆E value from Figure 4, where –∆H was extrapolated to zero coverage as 172 ± 4 kJ/mol, and then adding 6.2 kJ/mol to correct for ZPE and pV terms (see above), to give –∆Eexp = 178.2± 4 kJ/mol.


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Water Dissociative Adsorption on NiO(111): Energetics and Structure

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