Probing equilibrium of molecular and deprotonated water on TiO2(110)

Edited by Martin Gruebele, University of Illinois at Urbana–Champaign, Urbana, IL, and approved January 10, 2017 (received for review August 17, 2016)
February 6, 2017
114 (8) 1801-1805

Significance

Understanding how water binds and dissociates on surfaces has broad implications in a vast range of physical and chemical processes. The relative stability of molecularly and dissociatively bound water has been debated for decades on many oxide surfaces, but it has never been successfully measured. Our study describes unique instrumentation, direct measurements, and a state-of-the-art computation and theory approach that yield a detailed kinetic and dynamic description of water deprotonation equilibrium on TiO2(110), a prototypical surface commonly used in mechanistic studies of photocatalytic water splitting. This unique study demonstrates that the molecularly bound water on TiO2(110) is preferred over the surface-bound hydroxyls by only 0.035 eV.

Abstract

Understanding adsorbed water and its dissociation to surface hydroxyls on oxide surfaces is key to unraveling many physical and chemical processes, yet the barrier for its deprotonation has never been measured. In this study, we present direct evidence for water dissociation equilibrium on rutile-TiO2(110) by combining supersonic molecular beam, scanning tunneling microscopy (STM), and ab initio molecular dynamics. We measure the deprotonation/protonation barriers of 0.36 eV and find that molecularly bound water is preferred over the surface-bound hydroxyls by only 0.035 eV. We demonstrate that long-range electrostatic fields emanating from the oxide lead to steering and reorientation of the molecules approaching the surface, activating the O–H bonds and inducing deprotonation. The developed methodology for studying metastable reaction intermediates prepared with a high-energy molecular beam in the STM can be readily extended to other systems to clarify a wide range of important bond activation processes.
Water is ubiquitous in the environment and, as such, the nature of its interactions with interfaces can determine the outcome of a broad range of processes that include wetting, dissolution, precipitation, phase transformation, corrosion, and catalytic and environmental reactions (17). In this regard, the relative stability of molecularly and dissociatively bound species can be of critical importance with the preferred configuration being controlled by many factors including surface structure, acid/base properties, defects, impurities, water coverage, and temperature (813). For oxides in particular, the relative stability of molecularly and dissociatively bound water has been widely debated even on the simplest, low-index surfaces.
Here we focus on resolving the fundamental question of water binding on rutile TiO2(110), one of the most studied oxide surfaces, which is often used as a prototype for reducible oxide surfaces and a model for understanding photocatalytic water splitting (3, 1417). Interestingly, despite the overwhelming wealth of literature, the nature of water adsorption and dissociation on nondefect titanium sites has been disputed for decades and to date remains unsettled (3, 1417). The underpinning difficulty in resolving this debate is that it is practically impossible to prepare stoichiometric TiO2(110) surfaces. As such, bridging hydroxyl groups formed by water dissociation in oxygen vacancy defects interfere with determining the extent of dissociation on regular Ti sites (3, 1419). A number of recent studies by a variety of techniques including X-ray photoelectron spectroscopy (XPS) (20), infrared reflection absorption (21), photoelectron diffraction (PhD) (22), and scanning tunneling microscopy (STM) (2325) arrived at conflicting conclusions. Whereas the XPS and PhD studies concluded partial dissociation of water in the hydrogen-bonded chains on Ti sites at higher coverages (20, 22), others are in favor of molecular bonding (21, 25).
To address the adsorption configuration of isolated water molecules, extremely low coverages are required. As such, the ensemble-averaged methods generally do not possess sufficient sensitivity and STM is the method of choice. Although the appearance of water-related features is suggestive of molecular binding, indirect evidence strongly indicates that water monomers can easily access the dissociated configuration (18, 19). For example, in the water-assisted diffusion of bridging hydroxyl hydrogen (18) and Ti-bound oxygen adatoms (19), water monomer dissociation represents a key step in the proposed mechanism. This is further supported by the wealth of theoretical studies that yield very close adsorption energies (within ∼0.1 eV) for molecularly and dissociatively bound water monomers (1418).
To address this topic, we have constructed a unique instrument that allows for in situ molecular beam scattering studies directly under the STM tip, an instrument combination that has hitherto proven difficult to achieve (2628). In this study, we used hyperthermal water beams with variable energy and probed the probability of the formation of metastable binding configurations on TiO2(110) at low substrate temperatures (∼80 K). On metals, such molecular beam studies have proven indispensable for the understanding of dynamic factors controlling dissociation processes such as the excitation of vibrational modes, energy flow, and steering (2931). We demonstrate that the observed metastable configurations are composed of terminal and bridging hydroxyl pairs that can be converted back to molecularly bound water at higher temperatures (>100 K). In parallel, theoretical protocols were developed to reveal mechanistic details, guide experiments, and provide data interpretation. We accurately determined the equilibrium distribution and the interconversion energy barriers. The analysis reveals that the dissociated water configuration is only 0.035 eV higher in energy. A unique finding of this study is that long-range (5–10-Å) electrostatic coupling between the incident molecules and the oxide surface results in a strong steering and reorientation before collision.

Results and Discussion

The STM images of clean TiO2(110) composed of rows of topographically low-lying fivefold-coordinated titanium ions (Ti5c, imaged bright) and high-lying bridging oxygen ions (Ob, imaged dark) are shown in Fig. 1 A and C. A small fraction of Obs is missing, resulting in vacancies (VOs) (15).
Fig. 1.
Same area empty-state STM images before (A and C) and after (B and D) adsorption of 0.05 monolayer (ML) of water on TiO2(110) at 80 K with the incident energy of 0.06 (B) and 1.30 eV (D). Images A and C show clean TiO2(110) [Ti5c, bright rows; Ob, dark rows; VO defects (15%) appear as bright spots on dark Ob rows]. (B) A single type of features (labeled H2O) is observed on Ti5c rows at low-incident energy. A high-contrast image and the inset with three different line profiles along the Ti5c rows in B (Right) highlight their uniformity. (D) Adsorption of the high-incident energy water (1.30 eV; D, Left) shows two types of features, H2O and features labeled P. High-contrast image (D, Right) and the Inset further highlight their differences. The angle of incidence of the molecular beam is fixed at a polar angle of 60° with respect to the surface normal and azimuth pointing across the rows.
Fig. 1B shows the result after dosing H2O with a room-temperature effusive molecular beam (incident energy of 0.06 eV) on the TiO2(110) at 80 K. In all STM experiments, H2O coverage was kept very low (∼0.05 ML) to keep the observed molecules isolated. Only one type of feature, corresponding to water monomers centered on the Ti5c rows (17, 18), is seen. Their uniformity is demonstrated in a high-contrast image (Fig. 1B, Right) and by the reproducibility of the line profiles (Fig. 1B, Inset, Right). Under limited surface mobility conditions (<160 K), the H2O molecules do not diffuse on the Ti5c rows on an experimentally observable timescale (24). As such, they are unable to reach VO sites where they are known to dissociate and form bridging hydroxyl pairs (HObs) (3, 14, 15).
Whereas the H2O features (Fig. 1B) are centered on Ti5c sites, their appearance cannot be taken as conclusive evidence for the purely molecular binding of water. Under the equilibrium, H2O + Ob HOt + HOb, where HOt is the Ti5c-bound terminal hydroxyl, the majority species (H2O) determine the appearance. The evidence comes indirectly from studies of water reactions with O adatoms (19) and from water-assisted cross-row hydrogen transfer of HOb species (18). Recent theoretical studies predict the higher stability of molecularly bound H2O by less than 0.1 eV (13, 1618).
We further probe the preparation of metastable species by colliding water molecules of high-incident energy (1.3 eV in Fig. 1D). In contrast to the low-incident energy experiment (Fig. 1B), two distinct types of features (65% H2O and 35% P) are observed. The P features are round and also centered on Ti5c sites. High contrast is needed (Fig. 1D, Left) to clearly discern their differences from H2O. The line profiles in the inset further reveal that the P features are distinctly smaller and ∼30% less intense than H2O. Whereas in the area shown in Fig. 1B the P features are not present, they comprise ∼5% of the total population at low H2O incident energy. The large-scale images illustrating the overall distribution of the species at both low- and high-incident energies are shown in SI Appendix, section S1 and Fig. S1.
The appearance of the P features is surprising, as heuristically, pairs of two bright features (HOt and HOb) due to water dissociation are expected. To further interrogate the chemical makeup of the P species, we carry out temperature-dependent experiments for water with incident energy of 1.3 eV (Fig. 2). These experiments allow us to follow the thermal equilibration of the HOt and HOb species with H2O.
Fig. 2.
(A) Thermally induced conversion of P features to H2O following the adsorption of 0.05 ML of H2O with incident energy of 1.3 eV. STM images (Left, T = 111 K; Right, 114 K) from an extended sequence obtained during the slow temperature ramp (0.1 K/min from 110 K). (B) The fraction of P features determined from the STM images obtained ∼30 min after the H2O dose at different temperatures. The solid and dotted lines are the fits to the kinetic model. The error bars are determined based on counting statistics and at higher temperatures are within the size of the symbols used.
In the first type of experiment, we image continuously the same area while the sample temperature is increased from 111 to 114 K (Fig. 2A). The conversion of one P feature (yellow circle) to H2O (white circle) shows that the P features contain all of the atoms from the water molecule and that none of them has been irreversibly lost to the vacuum or subsurface. This evidence strongly supports that the P features comprise the HOb/HOt pairs. Additional evidence from tip-pulse manipulation experiments and STM image simulations is provided in SI Appendix, section S1 and Figs. S2–S4.
We quantify the temperature-dependent fraction of P features, FP(T), relative to the total number of H2O + P features observed on the surface, FP(T), in Fig. 2B (circles). The initial H2O adsorption temperature is stabilized between 80 and 150 K, water is dosed, and the surface is imaged. Below ∼110 K, FP remains approximately constant and equal to 0.35. Above 110 K, a sharp drop is detected and T > 130 K, FP drops to 0.05 and remains approximately constant up to 150 K.
The temperature dependence of FP allows us to determine the barrier for the H2O recombination, HOb + HOt → H2O, by first-order kinetics (SI Appendix, section S2). With a prefactor of 1 × 1012 s−1, the optimum fit to the data (solid line, Fig. 2B) yields the recombination barrier, ΔER = 0.355 eV. The dotted lines show the lower- and upper-bound values of 0.345 and 0.365 eV, respectively. Further, assuming equilibrium at 140 K, the value of FP(140K) = 0.05 reveals that H2O is only 0.035 eV lower in free energy than the HOb + HOt pair.
In Fig. 3A, we further probe the H2O dissociation barrier, H2O → HOb + HOt, in experiments where different seeding gases (neat, He, H2) and nozzle temperatures are used to vary the H2O incident energy (see Methods and further details in SI Appendix, section 3 and Fig. S5). At low-incident energies (< 0.3 eV), the FP is approximately constant (∼0.05), increases above ∼0.3 eV, and reaches ∼0.35 at 1.3 eV. The observed break at ∼0.3 eV is a consequence of H2O dissociation barrier as discussed below. Similar FP dependence is observed for the azimuth parallel to the Ti5c rows (SI Appendix, Fig. S6). This similarity along the two azimuths with very different corrugation of both physical and potential energy surfaces suggests that the energy dissipation may not be strongly dependent on the incident angle and exhibit so-called total energy scaling as seen previously on corrugated MgO(001) (32).
Fig. 3.
(A) Water incident energy dependent fraction of P features, FP, determined experimentally (blue squares) and theoretically (red squares, DFT AIMD trajectories). Seeding-gas-dependent data are coded by the square fill (neat H2O, dot; He-seeded, cross; and H2-seeded, empty). The red line is the fit to Eq. 1 yielding the dissociation barrier of 0.36 ± 0.01 eV. (Inset) Schematic shows that the water molecule is oriented with an OH bond parallel to the surface. (B) The distribution of OH-bond kinetic energies P(EOH, Ek) at the collision obtained from AIMD (black line) and a fit to a Boltzmann distribution (red). Distribution for a subset of trajectories resulting in OH bond breaking (blue). (C) The probability of OH-bond dissociation PD(EOH) as a function of OH-bond kinetic energy as estimated from an ensemble of classical MD trajectories on the DFT potential energy surface.
To understand the origin of the observed incident energy and temperature dependence of FP, we perform ab initio molecular dynamics (AIMD) studies of H2O collisions with a stoichiometric TiO2(110). We note that extensive conformational sampling of molecularly bound H2O resulted in only a single stable binding configuration indicating that the P features are not the result of an alternate metastable binding configuration. We start with a sample (100–200) of thermalized water configurations (T = 300 K) uniformly distributed at ∼7 Å above the surface (SI Appendix, Fig. S7). We add translational kinetic energy, EK = 0.1, 0.4, 0.7, and 1.3 eV with velocities in the direction concurrent with the experiment. We propagate each trajectory until the molecules collide with the surface and dissipate their excess energy; see SI Appendix, section S5 for details. This procedure reproduces both the types and relative populations of species observed in the experiments, as reported in Fig. 3A (red squares) and SI Appendix, Table S1.
From the AIMD trajectories we observe the following: (i) At all incident energies, the fate of the species is determined by its collision point: those that land on the Ti5c sites dissociate and remain adsorbed, whereas the rest are scattered. In general, at low EK there is a pronounced steering toward the Ti5c sites due to electrostatic attraction; hence, the overall number of retained species is highest at low energies. (ii) Irrespective of their fate after the collision, all of the molecules experience a reorientation as they approach the surface such that one OH bond is approximately perpendicular to the surface normal (Fig. 3A, Inset). This reorientation is the result of large, nonuniform electrostatic fields radiating out from the surface, coupling to the OH-bond dipole at distances ∼6 Å, that orient the molecules before impact. We note that this reorientation breaks the twofold degeneracy associated with the water binding configuration, as discussed in more detail in SI Appendix, section S5. (iii) The torquing of the molecules induces coupling of the translational/rotational/vibrational modes and amplifies the kinetic energy transfer to the water OH-bond–stretching mode. The number of events leading to bond breaking steadily increases with EK, and those that do break have on average a higher OH-bond kinetic energy, EOH. Even for the lowest EK = 0.1 eV, there is a finite dissociation probability (∼5%) as a consequence of the steering of molecules into the bound state.
This long-range electrostatic coupling seen here for an oxide is distinct from metals and has consequences for how one can extract meaningful chemistry from molecular beam data. To generalize the interpretation of the EK-dependent product cross-section, we note that this energy transfer effectively raises the temperature of the OH mode, and the resulting velocity distribution at the collision point can be represented by a Boltzmann distribution. AIMD trajectories resulting in OH bond breaking contribute to the long tail of P(EOH, EK) (Fig. 3B, blue line). To assess the probability, PD(EOH), of whether or not a molecularly adsorbed species with EOH will break, we fit the reaction coordinate (Fig. 2B) to a classical electrostatic potential (see SI Appendix, section S6 for details) and simulate an ensemble of events. The resulting PD(EOH) (Fig. 3C) is well-described by a Fermi function, as a result of the existence of well-defined energy barrier. We can combine these factors into a probability describing the OH-bond–breaking process:
PD(EK)=P(EOH,EK)PD(EOH)dEOH.
[1]
This model accurately reproduces the results of the AIMD trajectories as shown in Fig. 3 and SI Appendix, Fig. S17. This approach allows us to decouple the vibrational excitation probability, P(EOH, EK), from the bond-dissociation event probability, PD(EOH). Whereas P(EOH, EK) depends solely on electrostatics and is well-described by density-functional theory (DFT), the latter, PD(EOH), is associated with the potential energy landscape. Therefore, we fit the experimental data using Eq. 1 and obtain a barrier height of ED = 0.36 ± 0.01 eV (Fig. 3A, red line), only ∼0.04 eV lower than that obtained from DFT. Inclusion of quantum and isotopic effects is further discussed in SI Appendix, section S7, providing a DFT estimate of a zero-point energy-corrected barrier ED = 0.24 eV, which is below that of experiment by 0.12 eV. The analysis presented in SI Appendix, section S7 implies that the system is near the cross-over between the classical and quantum regimes at the experimental temperatures.
The complete energy diagram describing the equilibrium of water deprotonation on TiO2(110) determined from our studies is shown in Fig. 4. These unique, site-specific experiments provide a direct measure of the acid/base properties of water on an oxide surface. A combination of different computational approaches unravels the mechanism and underlying physical principles leading to water dissociation. We reveal the surprising role of long-range electrostatics in the energy transfer and redistribution before collision with the surface. We further distill a simple and meaningful theoretical framework for the interpretation of the kinetic energy dependence of scattering cross-sections. This study provides a blueprint for extracting chemical insight from supersonic beam studies of oxide surfaces and provides energetic quantification of important surface reactions relevant to catalytic materials and environmental processes.
Fig. 4.
Values determined for the water deprotonation potential energy surface (in electron volts) on TiO2(110).

Methods

Experimental Setup.

Experiments were carried out in an Omicron low-temperature scanning tunneling microscope (LT-STM) system, which consists of a preparation chamber, microscopy chamber, and molecular beam chamber. The system is equipped with LT-STM/AFM, XPS, UV photoelectron spectroscopy, and low-energy electron diffraction. The TiO2(110) sample (Princeton Scientific) was mounted on a standard Omicron Ta sample holder. The sample was cleaned by repeated cycles of Ne+ sputtering and annealing up to 900–950 K. Auger electron spectroscopy was used to determine the presence of impurities on the sample surface. The concentration of VOs on the surface determined from the images was 15%. Electrochemically etched and UHV-annealed tungsten tips were used for imaging. In all STM experiments, H2O coverage was kept very low (∼0.05 ML) to keep the observed molecules isolated. All STM images were recorded in constant-current mode at a positive sample bias of 1.2–1.6 V and tunneling currents of 5–80 pA. Based on the information from the manufacturer, the reported TiO2 sample temperatures can be up to 3 K higher during counter heating (> 80 K) than the real sample temperatures.
Deionized H2O was purified by several freeze–pump–thaw cycles using liquid nitrogen and dosed on TiO2(110) directly in the STM stage with the molecular beam incident at 60° with respect to the surface normal. Experiments were carried out at different two azimuths pointing along and across the Ti5c and Ob rows. Both quasi-effusive and supersonic molecular beams were used for water deposition. The neat H2O beam was created by expanding 20 torr of water through a 50-µm orifice. The supersonic beams were produced by passing 500 torr of either helium or hydrogen through a water bubbler at room temperature and expanding the resulting mixture through a 50-µm orifice. A heatable nozzle assembly (300–900 K) was used to vary the energy of the incident H2O. Further details are provided in SI Appendix, section S3.

Computational Details.

All DFT calculations were performed using the CP2K package (33). The exchange-correlation energy was described by the generalized gradient approximation with the spin-polarized Perdew–Burke–Ernzerhof functional (34). The wave functions were expanded in optimized double-ζ Gaussian basis sets (35) and the plane waves were expanded with a cutoff energy of 400 Ry. Core electrons have been modeled by scalar relativistic norm-conserving pseudopotentials with 12, 4, and 1 valence electrons for Ti, O, and H, respectively (36). Brillouin zone integration is performed with a reciprocal space mesh consisting of only the Γ-points. The TiO2(110)-p(6 × 2) surface was used to model the TiO2 substrate, consisting of 6 O–Ti–O trilayers (18 atomic layers), and only the bottom Ti atomic layer was frozen while the remaining layers were allowed to relax. The slab was repeated periodically with a vacuum depth of ∼20 Å in the direction of the surface normal. The dependence of water dissociation equilibrium on the parameters and methods of the calculations has been discussed in detail elsewhere (13, 37).
All AIMD simulations were performed by sampling initial conditions using a canonical [constant number, volume, temperature (NVT)] ensemble using Nosé−Hoover thermostats (38, 39) with a time step 0.5 fs. The collision simulations were performed within the microcanonical [constant number, volume, energy (NVE)] ensemble. The temperature of the TiO2 substrate is set as 10 K and the initial translational kinetic energy of water, EK, is chosen to be 0.1, 0.4, 0.7, and 1.3 eV, respectively, consistent with the range of beam energies in the experiments. To be comparable with the experiments, we started the simulation with a H2O molecule in the gas phase, initially located at ∼7 Å above the surface, with a velocity at 60° to the surface normal and perpendicular to the Ob rows. We used 208 independent runs at EK = −0.1 eV due to the relatively low percentage of dissociated water seen in the experimental studies at low EK values and 104 for EK = 0.4, 0.7, and 1.3 eV; each run was propagated for a total time of 1–3 ps. A total of ∼1 ns of combined AIMD trajectories was performed and used for the data analysis presented in SI Appendix, section S2.
For characterizing the potential energy surface of the water dissociation event, the location and energy of the transition state was performed using the climbing image nudged-elastic-band method (40) including 16 replicas. The convergence criterion for the maximum forces on the atoms is set as 2 × 10−3 a.u. The vibrational analysis was further used to confirm the transition states with only one imaginary frequency.

Acknowledgments

We thank Bruce D. Kay and Charles T. Campbell for fruitful discussions. This work was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences under Grant KC0301050-47319 and performed in Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the DOE by Battelle.

Supporting Information

Appendix (PDF)

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Information & Authors

Information

Published in

Go to Proceedings of the National Academy of Sciences
Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 114 | No. 8
February 21, 2017
PubMed: 28167775

Classifications

Submission history

Published online: February 6, 2017
Published in issue: February 21, 2017

Keywords

  1. adsorbate dynamics
  2. water
  3. dissociative adsorption
  4. titanium dioxide
  5. kinetic barriers

Acknowledgments

We thank Bruce D. Kay and Charles T. Campbell for fruitful discussions. This work was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences under Grant KC0301050-47319 and performed in Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the DOE by Battelle.

Notes

This article is a PNAS Direct Submission.

Authors

Affiliations

Zhi-Tao Wang1
Microscopy Group, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352;
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Yang-Gang Wang1
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Catalysis Group, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352;
Rentao Mu1
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Catalysis Group, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352;
Yeohoon Yoon
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Catalysis Group, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352;
Arjun Dahal
Microscopy Group, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352;
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Gregory K. Schenter
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Chemical Physics & Analysis Group, Physical Sciences Division, Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352
Vassiliki-Alexandra Glezakou
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Catalysis Group, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352;
Roger Rousseau2 [email protected]
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Catalysis Group, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352;
Igor Lyubinetsky2 [email protected]
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Catalysis Group, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352;
Present address: School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR 97331.
Zdenek Dohnálek2 [email protected]
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352;
Catalysis Group, Physical Sciences Division, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352;

Notes

2
To whom correspondence may be addressed. Email: [email protected], [email protected], or [email protected].
Author contributions: G.K.S., R.R., I.L., and Z.D. designed research; Z.-T.W., Y.-G.W., R.M., Y.Y., A.D., and R.R. performed research; Y.-G.W., R.M., G.K.S., V.-A.G., R.R., I.L., and Z.D. analyzed data; and V.-A.G., R.R., I.L., and Z.D. wrote the paper.
1
Z.-T.W., Y.-G.W., and R.M. contributed equally to this work.

Competing Interests

The authors declare no conflict of interest.

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