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Water on Atomically-Defined Cobalt Oxide Surfaces Studied by Temperature-Programmed IR Reflection Absorption Spectroscopy and Steady State Isotopic Exchange Matthias Schwarz, Susanne Mohr, Chantal Hohner, Kristin Werner, Tao Xu, and Jörg Libuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04611 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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The Journal of Physical Chemistry

Water on Atomically-Defined Cobalt Oxide Surfaces Studied by Temperature-Programmed IR Reflection Absorption Spectroscopy and Steady State Isotopic Exchange M. Schwarz1, S. Mohr1, C. Hohner1, K. Werner1,2, T. Xu1,3, J. Libuda1,4* 1

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany

2

Present address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abt. Chemische Physik, Faradayweg 4-6, 14195 Berlin

3

Present address: Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark 4

Erlangen Catalysis Resource Center and Interdisciplinary Center Interface Controlled Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany

Abstract In this work, we investigate the interaction of water with three different atomically-defined cobalt oxide surfaces under ultrahigh vacuum (UHV) conditions using time-resolved and temperature programmed infrared reflection-absorption spectroscopy (TR-IRAS, TP-IRAS) in combination with isotopic exchange experiments. The three surfaces, CoO(100), CoO(111), and Co3O4(111), are prepared in form of well-ordered films on Ir(100). Very different behavior is observed on the three surfaces, both with respect to D2O adsorption and desorption. On CoO(100), water adsorbs molecularly. It forms extended ice clusters even at low adsorption *

Corresponding Author: [email protected]

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temperature (200 K) and which desorb molecularly at 200 K (Ea,des ~ 60 kJ.mol-1). A very small amount of defect sites is observed at which D2O dissociates and forms strongly bound OD groups. On CoO(111), the interaction with ordered facets is very weak and no adsorption occurs at these sites at 200 K. The CoO(111) films expose, however, a higher density of defects as compared to the CoO(100) films, at which D2O dissociates and forms strongly bound OD species with desorption temperatures of 455 K ± 10 K and 620 K ± 10 K, respectively. In contrast to the above cases, water interacts strongly with the Co3O4(111) surface. At 200 K, D2O dissociates readily and forms a partially dissociated (D2O)n(OD)m network. With increasing temperature, the (D2O)n(OD)m network breaks up into (D2O)n(OD)m clusters the size of which decreases with increasing temperature. Desorption of molecular D2O occurs over a broad temperature range from 210 K to 470 K (Ea,des ~ 60 kJ.mol-1 to ~ 140 kJ.mol-1). Above 470 K, only isolated OD species reside on the surface which desorb at 540 K ± 20 K. Isotopic exchange experiments with D2O and H2O on Co3O4(111) show that isotopic scrambling in the (D2O)n(OD)m clusters is slow in comparison to exchange with the gas phase and that the clusters are composed of distinct species that show different exchange rates with the gas phase. The structure-dependent differences regarding the interaction with D2O are rationalized in terms of the surface termination and coordination environment of the surface ions on the three different surfaces.

1. Introduction Cobalt oxide-based materials have recently attracted considerable attention in heterogeneous catalysis1-3 and electro-catalysis4. For instance, Co3O4 is a reducible support which facilitates low-temperature Au catalysis3, but it may also become a highly active oxidation catalysts itself if prepared with the appropriate surface structure1. In electrochemistry, cobalt oxide is an efficient catalyst for the oxygen evolution reaction (OER) in water splitting.4 Furthermore, spinel cobalt oxide also efficiently catalyzes other electro-catalytic reactions, such as the oxidation of hydrocarbon oxygenates.5-6 In any of these catalytic applications, water plays an essential role. In ambient heterogeneous catalysis, gas phase water is an omnipresent modifier while in electrochemical reactions liquid water is present as a solvent or even reactant, such as in water splitting. Therefore, an in-depth understanding of the interaction of water with oxide surfaces in general, and with cobalt oxide in specific, is highly desirable. However, our present 2 ACS Paragon Plus Environment

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understanding of the water/oxide interface is still limited, in contrast to water/metal interfaces which have been studied in great detail7-8. Only very recently, a number of studies on different oxides shed some light on the fundamental interactions of water with well-defined oxide surfaces, also demonstrating the potential of surface science studies performed under well-defined ultra-high vacuum conditions (UHV). For instance, a combined scanning tunneling microscopy (STM) and density functional theory (DFT) study of water on anatase TiO2(101) by He et al. showed that water adsorbs molecularly and forms a locally ordered (2x2) superstructure.9 An STM study on the same substrate by Dahal and Dohnálek showed that water can form metastable one-dimensional chains at low substrate temperatures.10 Combining DFT, infrared reflection adsorption spectroscopy (IRAS), sum frequency generation (SFG) spectroscopy, and X-ray photoelectron spectroscopy (XPS) Włodarczyk et al. found that water partially dissociates on MgO(001)/Mo(001) forming a c(4x2) supercell at low temperature and a p(3x2) structure at elevated temperatures, both containing two dissociated water molecules per unit cell.11 A combined STM, XPS, and DFT study by Halwidl et al. on the adsorption of water on the (100) surfaces of strontium ruthenates (Srn+1RunO3n+1) also showed the dissociation of water. 12 Here, dimers of dissociated water assembled into chains and percolating networks with molecular water adsorbing in the gaps.12 A combined IRAS, XPS, STM, and DFT study on the hydroxylation of CaO(001) on Mo(001) by Fujimori et al. showed, that in the monolayer regime water can be present as monomers, small water clusters, or as onedimensional chains depending on the coverage.13 Exposure to higher water pressures induced partial transformation of CaO(001) into a Ca(OH)2-like phase. Combining STM, XPS, and DFT Wagner et al. found that water dissociates on defect free In2O3(111), but only at 5-fold coordinated In-ions, while the proton is donated to a specific type of surface oxygen only.14 The adsorption of water to Fe3O4(111), has been investigated in several surface science studies15-16 Recently, Mirabella et al. showed by IRAS, temperature programmed desorption (TPD) spectroscopy, low energy electron diffraction (LEED) and DFT, that half-dissociated dimers are formed at low coverage, which assemble into a (2x2) structure and anchor further water molecules at higher exposure, thus forming an extended hydrogen bonded network.15 For the adsorption of water on cobalt oxides, very few data from surface science studies is available. A comparative study on CoO(100), Co3O4(110), and Co3O4(111) by Petitto et al. using XPS, high-resolution electron energy loss spectroscopy (HREELS), and LEED showed water 3 ACS Paragon Plus Environment


dissociates on the spinel surfaces and defect adsorption on CoO(100).17 However, defect densities were high and information on the surface termination were lacking in this case. A promising approach to study the interaction of water with atomically-defined oxide surfaces of known structure makes use of epitaxial thin films on metallic substrate. In a combined XPS, STM, and DFT study, Fester et al. studied water on wurzite-type CoO(111) bilayer nano-islands on Au(111).18 The authors could show that water dissociates readily at low-coordinated Co2+ ions located at the island edges, while the protons travel to the terraces and form a (√3x√3) superstructure at higher exposures. While such ultra-thin islands are excellent models for cobalt oxide nanomaterials, their behavior is expected to differ dramatically from the bulk material. In this work, we investigate the adsorption and dissociation of water on epitaxial cobalt oxide films with a thickness of several nanometers. The properties of such films represent those of bulk materials and are not influenced by the underlying metal support. Specifically, we study three films, Co3O4(111), CoO(111), and CoO(100), prepared on Ir(100) under UHV conditions. All films are well-ordered and atomic surface structures have been determined previously by Hammer et al.19-23 using LEED-IV and STM. Previously, we have studied various adsorbates on these surfaces such as CO and carboxylic acids.24-29 In a short letter, we have also reported on first results on the interaction of water with some of these surfaces using IRAS and synchrotron radiation photoelectron spectroscopy (SR-PES).30 Here, we present the results of a comprehensive study on the interaction of water with all three surfaces.

2. Experimental All measurements were conducted in a UHV system (base pressure of 1.0×10-10 mbar) which was described in detail elsewhere.31 In short, the system is equipped with the required sample preparation and characterization methods, several evaporator sources, two quadrupole mass spectrometers, and a vacuum Fourier-transform infrared (FTIR) spectrometer (Bruker Vertex 80v) with an externally housed liquid nitrogen-cooled mercury cadmium telluride (LN-MCT) detector. All IR spectra were recorded at a spectral resolution of 4 cm-1. Previous to deposition, H2O and D2O were degassed by repeated freeze-pump-thaw cycles. H2O and D2O were dosed with two independent and fully automatized effusive beam sources at effective pressures at the sample surface of approximately 3.10-8 mbar. 4 ACS Paragon Plus Environment

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Experiments with isothermal D2O exposure: The sample was exposed to increasing doses of D2O at effective pressures of 3.10-8 mbar. After each dose, an IR spectrum was recorded with an acquisition time of 300 s. All spectra are referenced to a background acquired on the clean substrate previous to exposure. Temperature programed IRAS: For the temperature programmed IRAS (TP-IRAS) experiments, the surface was pre-saturated with D2O at 200 K and, subsequently, heated up to 700 K at a rate of 1 K/min (600 K for Co3O4(111)) while IR spectra were recorded continuously with an acquisition time of 5 min. During the complete experiment, a constant D2O pressure of 2.10-8 mbar was maintained to exclude exchange of deuterium with residual water from the background in the UHV chamber. For analysis of the TP-IRAS data, we applied the procedure proposed by Xu et al.27 In brief, the attenuation in the single channel spectra at elevated temperatures was compensated by scaling to the initial spectrum at the starting temperature (200 K). After this correction, the spectra were referenced to the spectrum acquired on the pristine surface previous to D2O dosage. Proton exchange experiments: We performed three types of isotopic exchange experiments, in which both H2O and D2O were dosed at an effective pressure of 3.10-8 mbar. (i) In order to obtain full exchange of H2O and D2O, the Co3O4(111) was initially saturated with H2O by exposure for 30 min at 300 K. Next, a reference spectrum was recorded in H2O atmosphere (acquisition time 300 s). Finally, D2O was dosed for 30 min and an IR spectrum was recorded in D2O atmosphere (acquisition time 300 s), which was referenced to the one obtained on the H2O treated surface. (ii) Isotopic exchange at shorter timescales was measured by alternately pulsing H2O and D2O with pulse durations of 100 s. The experiment consisted of 60 dosing cycles. During each cycle, 46 spectra were recorded with an acquisition time of 4.382 s per spectrum. The corresponding spectra from all cycles were accumulated. To avoid deviations in the first cycle because of different starting conditions, one additional dosing cycle (H2O and D2O) was applied before starting the experiment. All spectra were referenced to the 23rd spectrum, i.e. the last spectrum taken during the H2O pulse. (iii) To access shorter exchange times, an equivalent pulsing experiment was performed with pulse durations of 10 s for H2O and 10 s for D2O. In each cycle, 46 spectra were recorded with an acquisition time of 438 ms per spectrum, and the data was accumulated over 600 cycles.



Preparation of Co3O4(111)/Ir(100): The Co3O4(111) film was prepared on an Ir(100) single crystal (MaTecK, 99.99%) following a procedure described in the literature. 23 First Ir(100) was cleaned by Ar sputtering (1.8 keV, 300 K, 120 min; Ar, Linde, 6.0). Next, the clean (5x1)-Ir(100) surface was prepared by annealing for (i) 3 min at 1370 K in UHV, (ii) 5 min in O2 (1×10-7 mbar, 1270 K, Linde, 5.0), and again at 1370 K in UHV for 1 min. Formation of the (5×1) reconstruction was confirmed by LEED. To form the (2×1)−O reconstruction, the surface was oxidized for 5 min in O2 (1×10-7 mbar, 1270 K) and, subsequently, cooled to 370 K in O2 (1×107

mbar). This resulted in formation of a well-defined (2×1) superstructure as confirmed by

LEED. Onto the (2×1)−O-Ir(100) surface, Co was deposited using a commercial electron beam evaporator (Focus EFM3, 2 mm Co rod, Alfa Aesar 99,995%) in O2 atmosphere (1×10-6 mbar) at a sample temperature of 270 K. The film was annealed at 520 K in O2 (1×10-6 mbar) for 2 min and, subsequently, in UHV at 670 K for 10 min. This procedure yielded a highly ordered film with characteristic LEED pattern and thicknesses of around 7 nm as determined by a quartz crystal microbalance. Preparation of CoO(111)/Ir(100): CoO(111) films were prepared from the Co3O4(111)/Ir(100) (see above) by annealing to 830 K in UHV. The thermal treatment causes loss of oxygen and reduction of the film. The CoO(111) film formed shows a characteristic hexagonal pattern in LEED.19 Preparation of CoO(100)/Co/Ir(100): For preparation of the CoO(100) films, the (2×1)-OIr(100) surface was prepared as described above. Next, the (2×1)-O-Ir(100) surface was reduced in H2 (Linde, 5.3, 1×10-7 mbar) for 1 min at 550 K and, subsequently, annealed in UHV for 1 min at 550 K. This yields a clean (1×1)-Ir(100) surface, as verified by LEED. On the (1×1) surface, a Co-metal buffer layer of around 1 nm was deposited by physical vapor deposition of Co at 320 K. Onto this Co/Ir(100) surface a thin CoO(100) layer of 1 nm thickness was grown reactively by Co deposition in O2 atmosphere (2×10-7 mbar) at 220 K. After short annealing at 370 K, a thicker CoO(100) layer of approximately 6 nm was deposited via reactive deposition of Co in O2 (2×10-7 mbar) at 220 K. To obtain a well-ordered film, the sample was finally annealed in UHV at 870 K for 5 min. The quality and crystallinity of the CoO(100) films were verified by LEED.


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3.

Results and Discussion

In this work, we investigate the adsorption and dissociation of water on three epitaxial cobalt oxide films, i.e. Co3O4(111), CoO(111), and CoO(100). All films were prepared on Ir(100) under UHV conditions (see Experimental Part). All films are well-ordered and their atomic surface structure has been determined previously by Heinz, Hammer and coworkers,19-23 using LEED-IV and STM. The Co3O4(111) film is terminated by the tetrahedral Co2+ ions of the spinel structure. As a result, the surface exposes threefold coordinated Co2+ ions with a relatively large Co2+-Co2+ distance of 5.7 Å and O2- ions in between. The bulk-terminated rocksalt CoO(100) surface exposes Co2+ and O2- ions in fivefold coordination, with a Co2+-Co2+ distance of 3.0 Å. Finally, the (polar) CoO(111) film has rocksalt structure in the bulk, however, shows a reconstruction to wurzite structure in the top layers. It can be considered as a model for cobalt oxide nanomaterials.32 Its surface is terminated by O2- ions, while the fourfold coordinated Co2+ ions are buried 0.6 Å below the surface. In this work, we present data from three types of surface IR spectroscopy experiments on the three surfaces mentioned. The data from isothermal adsorption experiments at different temperatures are shown in Figure 1 for CoO(100), in Figure 2 for CoO(111), and in Figure 3 for Co3O4(111). Temperature programmed IR spectroscopy data on all three surfaces is provided in Figure 4. Finally, we present the results from isotopic exchange experiment in combination with time-resolved IR spectroscopy for the most complex adsorption system, i.e. D2O/Co3O4(111) (see Figures 5 and 6).

3.1. D2O on CoO(100): Isothermal Adsorption and Temperature Programmed IRAS We start our discussion with the case of water adsorption on the CoO(100) surface. The IR spectra recorded after stepwise exposure to increasing doses of D2O are shown in Figure 1. The adsorption experiment was performed at 200 K, i.e. slightly above the multilayer desorption temperature, and at 300 K. At 200 K, (see Figure 1a) we first observe growth of very small features at 2728, 2711 and 2685 cm-1 at low H2O dose (0.14 L, 1 Langmuir = 1 L = 10-6 Torr.s). At higher exposures, these bands transform into a strong and narrow feature at 2702 cm-1 and a broad and very intense band at 2585 cm-1.



In our previous letter30, we attributed the spectral features to the formation of large aggregates of molecular water. By means of synchrotron radiation photoelectron spectroscopy (SRPES) in the valence and core region we could show that water adsorbs exclusively molecularly on the CoO(100) surface. In such bigger clusters the majority of OD bonds are hydrogen bond donors and, therefore, give rise to the broad ν(OD) band at 2585 cm-1. Only a smaller fraction of OD bands at the periphery of the clusters is “dangling”, as indicated by the weaker and narrow band of free OD at 2702 cm-1. Thus the intensity ratio of the two features is an indicator for the size of the D2O aggregates that are formed. In contrast, the weak bands at 2728, 2711 and 2685 cm-1 observed at low doses indicate the initial formation of very small H2O aggregates. As individual vibrational features are visible, such aggregates can consist of few molecules only. It is possible that these aggregates are partially stabilized by interaction with defect sites, such as boundaries between CoO grains.19 The latter assignment is in line with the observation that the bands at 2711 and 2685 cm-1 remain visible at higher coverage as small shoulders on both sides of the free OD band at 2702 cm-1. When CoO(100) is exposed to D2O at 300 K (see Figure 1b) no bands evolve up to the highest exposure used in this study (46 L). This shows that water only interacts weakly with CoO(100). Our observations are in line with previous findings on UHV-cleaved CoO(100) crystals, where only defect water species were observed at 300 K by photoelectron spectroscopy.17 In our experiment, we do not even observe weak defect features at 300 K even at highest exposure of 46 L. This suggests that the thin film used in this work has much lower density of strongly adsorbing defects in comparison with the single crystals used in the above mentioned study.17 This also implies that the defect sites which give rise to the nucleation of the water clusters at 200 K bind D2O too weakly to be populated at 300 K. These observations are in line with our TP-IRAS data which is shown in Figure 4. For the TPIRAS experiments, CoO(100) was pre-saturated by D2O at a pressure of 2×10-8 mbar at 200 K and, subsequently, IR spectra were acquired while heating the sample to 700 K at a rate of 1 K.min-1. During the whole experiment, a constant D2O background pressure of 2×10-8 mbar was applied to prevent exchange with H2O from the chamber background. Right after the start of the ramp, the two dominating bands at 2702 and 2585 cm-1 decrease rapidly. The desorption peak is located at 210 ± 5 K. Taking into account the heating rate (of 0.0167 K.s-1) we obtain a rough estimate of around 63 kJ.mol-1 for the desorption activation 8 ACS Paragon Plus Environment

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energy of D2O monolayers on CoO(100) (using the Redhead formula for the temperature at which half coverage is reached and a “normal” prefactor of 1013s-1; note that similar values are obtained, when we estimate the activation energy from the adsorption isobar by assuming equilibrium with the gas phase at the D2O background pressure of 2×10-8 mbar and a desorption rate constant with a prefactor of 1013s-1). Above 225 K, only the weak features at 2728, 2711, and 2685 cm-1 remain which were already observed at low coverage during adsorption (see Figure 1). At around 240 K, these bands evolve into two very weak bands at 2720 and 2708 cm-1, which disappear between 300 K at 360 K. Their extremely low intensity (∆R/R < 0.01%) supports the assignment to defects. The corresponding desorption activation energy using Redhead’s formula is in the range between approximately 90 and 110 kJ.mol-1 (heating rate 0.0167 K.s-1, prefactor 1013s-1).

3.2. D2O on CoO(111): Isothermal Adsorption and Temperature Programmed IRAS Next we turn to the adsorption behavior of D2O on CoO(111). The corresponding adsorption data is shown in Figure 2. The measurements were conducted under conditions identical to those used for CoO(100) and the scale is chosen identical to the one in Figure 1 to facilitate direct comparison. At a sample temperature of 200 K (see Figure 2a), we observe the formation of two bands at 2710 and 2669 cm-1. Both features appear already for low D2O doses and saturate at exposure around 0.3 L, while remaining considerably weaker (∆R/R < 0.03%) than those observed on CoO(100) (see Figure 1) and Co3O4(111) (see Figure 3). At higher exposure, no further changes are observed. The low intensity of the D2O derived bands implies that they are related to adsorption at defect sites. This indicates that D2O does not adsorb at all at 200 K on the regular CoO(111) facets. From the upper threshold for the desorption temperature of 200 K, we estimate an upper threshold for the desorption activation energy for D2O monolayers on CoO(111) of approximately Edes < 60 kJ.mol-1 (Readhead equation, heating rate 0.0167 K.s-1, prefactor 1013s-1, note that at lower temperatures multilayer adsorption is observed). We attribute the weak interaction with D2O to the fact that the CoO(111) film is terminated by O2- ions only and, therefore, does not provide Lewis acid sites that could interact with the free electron pairs of oxygen. Very weak adsorption on CoO(111) was previously observed also for other adsorbates such as CO.24-25 9 ACS Paragon Plus Environment


When CoO(111) is exposed to D2O at 300 K (see Figure 2b), we observe bands at 2708 and 2661 cm-1, which are nearly identical to those seen at 200 K. However, the bands appear at higher exposures. We attribute this observation to the shorter residence time of a weakly adsorbed molecular precursor at elevated temperatures, which reduces the probability to react at the defect sites. In order to investigate the thermal stability of the defect species, we performed TP-IRAS using an experimental procedure identical to the one used for CoO(100) (see Figure 4). From the temperature dependence of the two bands, we conclude that the two defect species have different stability. The band at 2669 cm-1 disappears at 455 K ± 10 K and the band at 2710 cm-1 disappears at 620 K ± 10 K. Based on the desorption temperatures, we obtain an estimate for the desorption activation energies of approximately 140 kJ.mol-1 and 190 kJ.mol-1, respectively (Redhead equation, heating rate 0.0167 K.s-1, prefactor 1013s-1, note that the latter number is a rough estimate only as the OD desorption in principle involves a recombinative mechanism). The high activation energy for desorption suggests the two features are due to strongly and dissociatively bound D2O (hydroxyl groups) at defect sites. Both defect bands show no indication for vibrational coupling, suggesting they arise from spatially separated defect sites.

3.3. D2O on Co3O4(111): Isothermal Adsorption and Temperature Programmed IRAS Finally, we consider the adsorption of D2O on the Co3O4(111) film. The corresponding IRAS experiments are shown in Figure 3. The experiments were performed at 200 K and 300 K (see Figure 3a, 3b) following the procedure identical to the one used for CoO(100) and CoO(111). As the adsorption behavior turns out to be more complex in the case of Co3O4(111), we also show isothermal adsorption data at elevated temperature, i.e. 370 K and 500 K (see Figure 3c, 3d). At 200 K (Figure 3a) we identify two main features at 2702 cm-1 and 2530 cm-1. Following the arguments outlined for the CoO(100) (see above), the two bands are attributed to free OD (2702 cm-1) and hydrogen bonded OD donors (2530 cm-1), respectively. The intensity ratio of both bands is dramatically different from the case of CoO(100), however, with the free OD band dominating the spectrum. This observation indicates that the majority of OD bonds are free, i.e. the D2O aggregates are much smaller than in the case of the CoO(100). Previously, we studied the adsorption of H2O on Co3O4(111) using synchrotron radiation photoelectron spectroscopy (SRPES).30 Both, valence and core level spectra indicated the 10 ACS Paragon Plus Environment

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formation of partially dissociated structures consisting of both OH groups and molecular water. Recently, Mirabella et al. investigated the adsorption of water on a Fe3O4(111) film which has a similar surface structure.15 Based on LEED, TPD experiments, and DFT, the authors suggested the formation of partially dissociated water clusters and partially dissociated networks at full monolayer coverage. Their IR spectra recorded a 180 K are very similar to those observed in the present study at 200 K. Based on this comparison, we attribute the bands to the formation of a partially dissociated (D2O)n(OD)m network similar to the one proposed in the above mentioned work by Mirabella et al.15 Noteworthy, however, no coverage dependence is observed at 200 K (see Figure 3a). The spectra show essentially identical features with constantly increasing intensity from lowest exposure to saturation. This observation implies that the structure of the (D2O)n(OD)m aggregates is preserved with increasing coverage. This finding is surprising as we will show below that isolated OD is strongly bound on Co3O4(111) and remains present up to high temperature (>500 K). Still, we observe the formation of partially dissociated structures including more weakly bound molecular D2O. This implies that the formation of the (D2O)n(OD)m aggregates at low temperature is kinetically controlled, i.e. the OD groups formed by dissociative adsorption efficiently capture additional molecular water with a reaction probability that is much higher than the probability for dissociative adsorption. The spectra acquired during isothermal adsorption of D2O on Co3O4(111) at 300 K are shown in Figure 3b. At low coverage, we observe one single sharp band at 2657 cm-1. At higher exposure the band shifts slightly to the blue (2663 cm-1) and several additional features develop at 2731, 2698, 2683, 2671, 2652, 2634, 2613 and 2550 cm-1. Based on our previous studies using SRPES, we attribute the low coverage feature at 2657 cm-1 to isolated OD groups.30 This assignment is in line previous work on other oxides.13,

15-16

Consequently, the additional bands appearing at larger exposure are attributed to the formation of partially dissociated (D2O)n(OD)m aggregates. Specifically, we attribute the broad band at 2550 cm-1 to hydrogen bonded donor species and the eight sharper features between 2613 cm-1 and 2731 cm-1 to isolated OD species within the (D2O)n(OD)m aggregates. Several conclusions can be drawn from the above assignment. First, the lower relative intensity of the hydrogen bonded OD band suggests that the (D2O)n(OD)m aggregates at 300 K are smaller than those formed at 200 K. Secondly, the appearance of sharp bands (i.e. 8 isolated OH 11 ACS Paragon Plus Environment


features) suggest the formation of (D2O)n(OD)m aggregates with well-defined structure. The latter assumption is in line with DFT results reported by Mirabella et al., which showed that at lower coverage characteristic cluster-like structures are formed.15 Finally, it is noteworthy that, in sharp contrast to adsorption at 200 K, we observe the initial formation of isolated OD. This finding shows that the adsorption probability of molecular water on the OD species decreases with increasing surface temperature. We attribute this effect to the reduced residence time of the molecular D2O precursor state at higher temperature, which also reduces the probability of finding and attaching to the isolated OD species. At an adsorption temperature of 370 K (see Figure 3c) we again observe a single ν(OD) band at 2657 cm-1 at low exposure. At higher exposure we detect the same bands as for 300 K, but strongly different relative intensities. Specifically, the free OD band at 2659 cm-1 dominates the spectrum, while the features at 2731, 2629 and 2615 cm-1 show similar intensities as at 300 K. The two free OD bands at 2694 and 2667 cm-1 show strongly reduced intensity, similar as the hydrogen bond donor band at 2545 cm-1. The observations indicate the coexistence of free OD groups and smaller (D2O)n(OD)m aggregates. When D2O is dosed at 500 K, (see Figure 3d), the only band which remains is the one of the isolated surface OD species at 2552 cm-1. This shows that the stability of this species is higher than that of the molecular water and that the preferential formation of (D2O)n(OD)m species at lower temperature is a purely kinetically controlled effect. In order to obtain more detailed information on the thermal stability of the above mentioned structures, we have performed TP-IRAS experiments in between 200 K and 600 K, again under a continuous D2O pressure of 2×10-8 mbar (see Figure 4c). At 200 K we observe the two broad bands at 2702 cm-1 and 2530 cm-1, previously attributed to the free and hydrogen bonded OD in the partially dissociated (D2O)n(OD)m network. With increasing temperature, the spectrum transforms in several relatively broad steps. At 240 K the free OD band sharpens and shifts to 2714 cm-1, while a new free OD band appears around 2673 cm-1. At 280 K, the free OD region shows two prominent bands at 2702 cm-1 and 2671 cm1

. Between 350 and 450 K the free OD spectrum shows at least seven distinct features at 2731,

2698, 2683, 2671, 2663, 2635, 2616 in addition to the hydrogen-bonded band at 2550 cm-1. We attribute these changes with breaking of the (D2O)n(OD)m network into partially dissociated clusters of different size.


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At around 470 K, all bands associated with (D2O)n(OD)m clusters disappear. This implies that the molecular water desorbs from the surface over a very broad range of temperatures from 210 K to 470 K. From these temperatures, we obtain an estimate of the desorption activation energy for molecular water from the (D2O)n(OD)m clusters between approximately 60 and 140 kJ.mol-1 (Redhead equation, heating rate 0.0167 K.s-1, prefactor 1013s-1). At temperatures above 470 K, a single sharp band appears at 2652 cm-1, i.e. at a position that is similar to the one observed at low exposure in the isothermal experiments (2652 – 2657 cm-1, see Figure 3). We attribute this feature to the formation of isolated OD. We note the appearance of an additional very small feature at 2698 cm-1 between 520 and 550 K, the origin of which is not clear. It might be associated with a defect species similar to the one observed on CoO(111). At 540 K ± 20 K, the free OD band disappears, indicating complete dehydroxylation of the surface. The desorption activation energy for the isolated OH species is estimated to be around 170 kJ.mol-1 (Redhead equation, heating rate 0.0167 K.s-1, prefactor 1013s-1, note that the latter number is a very rough estimate as the OD desorption involves a recombinative mechanism the details of which are not established). In conclusion, the interaction of D2O with Co3O4(111) is much stronger than with the two other cobalt oxide surfaces discussed above. In particular, the comparison with the CoO(100) surface is of interest as both, the Co3O4(111) and the CoO(100) surface are formally terminated by the same ions, i.e. Co2+ and O2-. Although the electronic surface structure of Co3O4(111) is not fully established, the principle difference is coordination number of the Co ions, which are threefoldcoordinated by O2- in the case of Co3O4(111) and fivefold coordinated in the case of CoO(100). This finding suggests that the coordination number of the surface cations is a key parameter controlling the interaction strength with water. For the CoO(111), the weakest interaction is found as the surface is terminated by O2- only and do not provide the Lewis acid base pairs that are required for the dissociation of water.

3.4. Isotopic exchange experiment and TR-IRAS for D2O on Co3O4(111) In order to obtain more insight into the dynamic properties of the (D2O)n(OD)m clusters formed on Co3O4(111), we performed isotopic exchange experiments with H2O and D2O in combination with TR-IRAS. In Figure 5, we show the results of two TR-IRAS experiments performed at 300 K. The surface was alternately exposed to doses of H2O and D2O with a pulse length of 10 s 13 ACS Paragon Plus Environment


(Figure 5a) and 100 s (Figure 5b) while IR spectra were recorded with an acquisition time of 440 ms and 4400 ms, respectively. Data was accumulated over 60 (Figure 5a) and 600 (Figure 5b) exchange cycles to obtain a sufficient signal to noise ratio. As a reference, we use the spectrum obtained just before switching to D2O. Thus, the spectra displayed show only the changes that occur on the timescale of the pulses applied. In Figure 6, a comparison is shown of the spectra obtained at exchange times of 10 s and 100s. In addition, we show the spectrum obtained after initial exposure of the pristine Co3O4(111) film to D2O and exchange spectrum obtained after extended exposure of a H2O-saturated surface to D2O (exposure times of 30 min, 2×10-8 mbar). When we compare the spectrum obtained after exposure of the pristine surface to D2O to the one obtained after long exchange times (Figure 6, two topmost spectra), we observe nearly the same bands in the OD region. The only minor difference is found in the band at 2663 cm-1, which shows a slightly lower intensity in comparison to the other free OD features in case of the exchange experiment. We attribute this difference to the slightly higher steady state D2O coverage in case of the isotopic exchange experiment which is performed under continuous exposure to D2O. Otherwise, the close correspondence of the adsorption and exchange spectrum shows that both OD/OH and D2O/H2O are fully exchanged on the timescale and conditions applied in this experiment (300 K, 1800 s, 2×10-8 mbar). This is also confirmed, by the spectra obtained in the OH stretching frequency region (3700-3400 cm-1), where we find the same difference bands pointing upwards, i.e. indicating loss of H2O/OH (see Figure 6, left panel). The situation changes if we proceed to shorter exchange times. On the timescale of 100 s (see Figure 5b and Figure 6), we observe that the band at 2698 cm-1 is partially suppressed, similar as the two weak features at 2634 cm-1 and 2613 cm-1. If we proceed to even shorter exchange times of 10 s, the differences become even more apparent (see Figure 5a and Figure 6). The corresponding spectrum after 10 s exchange shows only three bands at 2731, 2671 and 2663 cm-1 for D2O and the corresponding negative bands at 3703, 3618 and 3608 cm-1 for H2O. In particular, the strong OD band at 2698 cm-1 (3662 cm-1 for OH) is missing completely. The finding shows that (i) the exchange with gas phase water (D2O or H2O) occurs at different timescales for the different D2O and OD species in the adsorbed clusters and (ii) that isotopic scrambling in the (D2O)n(OD)m clusters is slow on the timescale of the exchange with the gas phase. If we compare the isotopic exchange behavior to the isothermal adsorption spectra in Figure 3, it is surprising that the free OD species associated with the peak at 2698 cm-1 shows 14 ACS Paragon Plus Environment

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slowest exchange, in spite of the fact that it shows the lowest thermal stability (i.e. the band is found to be strongly suppressed in the isothermal adsorption spectra obtained at 370 K, see Figure 6). This observation can be explained, however, if we assume that the band arises from free OD inside larger (D2O)n(OD)m clusters. This hypothesis would explain the appearance of this band for lower adsorption temperatures only (300 K), while it shows slower exchange with the gas phase water as compared to the free OD at the perimeter of the islands.

4. Conclusion We investigated the adsorption of water on the three different atomically-defined cobalt oxide surfaces by temperature-programmed and time-resolved IRAS in UHV in combination with isotopic exchange experiments. The CoO(100), CoO(111), and Co3O4(111) surfaces were prepared in form of thin oxide films on an Ir(100) single crystal surface. The main findings are summarized as follows: (1) D2O on CoO(100): Water adsorbs molecularly on CoO(100) and interacts weakly with the surface. The water molecules are highly mobile and form large aggregates even at low adsorption temperature (200 K). Desorption occurs at 210 K (i.e. with a desorption activation energy of approximately of 60 kJ.mol-1). At higher temperature a very small amount of dissociated water is found to dissociate at defect sites. Desorption from these defect states occurs at temperatures between 300 and 360 K. (2) D2O on CoO(111): D2O interacts very weakly with the regular domains of oxygenterminated CoO(111). No adsorption at regular facets is observed at a temperature of 200 K, yielding an estimate for the upper limit of the adsorption energy of D2O on CoO(111) of approximately 60 kJ.mol-1. The CoO(111) films expose, however, a higher density of defects as compared to the CoO(100) films. D2O dissociates at these defects and forms strongly bound OD species. Two defect states are identified, which reside on the surface up to temperatures of 455 K ± 10 K and 620 K ± 10 K, respectively. (3) D2O on Co3O4(111): Water interacts strongly with the Co3O4(111) surface. At low adsorption temperature (200 K), D2O dissociates readily and is immediately followed by formation of a partially dissociated (D2O)n(OD)m network. At submonolayer coverage, surface OD groups anchor molecular D2O species, leading to a kinetically controlled 15 ACS Paragon Plus Environment


formation of (D2O)n(OD)m instead of isolated OD species. With increasing temperature, the (D2O)n(OD)m network breaks up into (D2O)n(OD)m clusters the size of which decreases with increasing temperature. As a consequence, molecular D2O desorbs over a very broad temperature range from 210 K to 470 K (i.e. with desorption activation energies ranging from approximately 60 to 140 kJ.mol-1). Above 470 K, only isolated OD species reside on the surface. These OD species finally desorb at 540 K ± 20 K. (4) Isotopic exchange experiments with D2O and H2O on Co3O4(111) show that the partially dissociated (D2O)n(OD)m clusters exchange completely with the gas phase at 300 K and long timescales (>1000 s). At shorter timescales (0.5 s – 100 s), two groups of bands can be identified which show different exchange rates with gas phase water. It is proposed that the corresponding species are located at the perimeter and at the center of the (D2O)n(OD)m islands, respectively. The isotopic exchange experiments also show that isotopic exchange with the gas phase is faster than isotopic scrambling within the (D2O)n(OD)m clusters.

Acknowledgments This project was financially supported by the Deutsche Forschungsgemeinschaft (DFG) within the Research Unit FOR 1878 “funCOS – Functional Molecular Structures on Complex Oxide Surfaces” and the Priority Program 1708 “Materials Synthesis near Room Temperature”. Additional support by the DFG is acknowledged from the Excellence Cluster “Engineering of Advanced Materials” (Bridge Funding).

References 1.

Xie, X.; Li, Y.; Liu, Z.-Q.; Haruta, M.; Shen, W., Low-Temperature Oxidation of CO

Catalysed by Co3O4 Nanorods. Nature 2009, 458, 746-749. 2.

Ren, Y.; Ma, Z.; Qian, L.; Dai, S.; He, H.; Bruce, P. G., Ordered Crystalline Mesoporous

Oxides as Catalysts for CO Oxidation. Catal. Lett. 2009, 131, 146-154.


Page 16 of 27

Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.

Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B., Low-

Temperature Oxidation of Co over Gold Supported on TiO2, Alpha-Fe2O3, and Co3O4 J. Catal. 1993, 144, 175-192. 4.

Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H., Co3O4 Nanocrystals

on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780-786. 5.

Lang, X. Y.; Fu, H. Y.; Hou, C.; Han, G. F.; Yang, P.; Liu, Y. B.; Jiang, Q., Nanoporous

Gold Supported Cobalt Oxide Microelectrodes as High-Performance Electrochemical Biosensors. Nat. Commun. 2013, 4, 2169. 6.

Sun, S. N.; Sun, L. B.; Xi, S. B.; Du, Y. H.; Prathap, M. U. A.; Wang, Z. L.; Zhang, Q.

C.; Fisher, A.; Xu, Z. C. J., Electrochemical Oxidation of C3 Saturated Alcohols on Co3O4 in Alkaline. Electrochim. Acta. 2017, 228, 183-194. 7.

Hodgson, A.; Haq, S., Water Adsorption and the Wetting of Metal Surfaces. Surf. Sci.

Rep. 2009, 64, 381-451. 8.

Carrasco, J.; Hodgson, A.; Michaelides, A., A Molecular Perspective of Water at Metal

Interfaces. Nat. Mater. 2012, 11, 667-674. 9.

He, Y. B.; Tilocca, A.; Dulub, O.; Selloni, A.; Diebold, U., Local Ordering and

Electronic Signatures of Submonolayer Water on Anatase TiO2(101). Nat. Mater. 2009, 8, 585589. 10.

Dahal, A.; Dohnalek, Z., Formation of Metastable Water Chains on Anatase TiO2(101).

J. Phys. Chem. C. 2017, 121, 20413-20418. 11.

Włodarczyk, R.; Sierka, M.; Kwapień, K.; Sauer, J.; Carrasco, E.; Aumer, A.; Gomes, J.

F.; Sterrer, M.; Freund, H.-J., Structures of the Ordered Water Monolayer on MgO(001). J. Phys. Chem. C 2011, 115, 6764-6774. 12.

Halwidl, D., et al., Adsorption of Water at the SrO Surface of Ruthenates. Nat. Mater.

2016, 15, 450-455. 13.

Fujimori, Y.; Zhao, X.; Shao, X.; Levchenko, S. V.; Nilius, N.; Sterrer, M.; Freund, H.-J.,

Interaction of Water with the CaO(001) Surface. J. Phys. Chem. C 2016, 120, 5565-5576. 14.

Wagner, M., et al., Resolving the Structure of a Well-Ordered Hydroxyl Overlayer on

In2O3(111): Nanomanipulation and Theory. ACS Nano 2017, 11, 11531-11541.



15.

Mirabella, F.; Zaki, E.; Ivars-Barceló, F.; Li, X.; Paier, J.; Sauer, J.; Shaikhutdinov, S.;

Freund, H.-J., Cooperative Formation of Long-Range Ordering in Water Ad-Layers on Fe3O4(111) Surfaces. Angew. Chem., Int. Ed. 2018, 57, 1409-1413. 16.

Dementyev, P.; Dostert, K.-H.; Ivars-Barceló, F.; O'Brien, C. P.; Mirabella, F.;

Schauermann, S.; Li, X.; Paier, J.; Sauer, J.; Freund, H.-J., Water Interaction with Iron Oxides. Angew. Chem., Int. Ed. 2015, 54, 13942-13946. 17.

Petitto, S. C.; Marsh, E. M.; Carson, G. A.; Langell, M. A., Cobalt Oxide Surface

Chemistry: The Interaction of CoO(100), Co3O4(110) and Co3O4(111) with Oxygen and Water. J. Mol. Catal. A: Chem. 2008, 281, 49-58. 18.

Fester, J.; García-Melchor, M.; Walton, A. S.; Bajdich, M.; Li, Z.; Lammich, L.;

Vojvodic, A.; Lauritsen, J. V., Edge Reactivity and Water-Assisted Dissociation on Cobalt Oxide Nanoislands. Nat. Commun. 2017, 8, 14169. 19.

Heinz, K.; Hammer, L., Epitaxial Cobalt Oxide Films on Ir(100)—the Importance of

Crystallographic Analyses. J. Phys.: Condens. Matter 2013, 25, 173001. 20.

Biedermann, K.; Gubo, M.; Hammer, L.; Heinz, K., Phases and phase transitions of

hexagonal cobalt oxide films on Ir(100)-(1 × 1). J. Phys.: Condens. Matter 2009, 21, 185003. 21.

Gubo, M.; Ebensperger, C.; Meyer, W.; Hammer, L.; Heinz, K., Substoichiometric cobalt

oxide monolayer on Ir(100)-(1 × 1). J. Phys.: Condens. Matter 2009, 21, 474211. 22.

Meyer, W.; Hock, D.; Biedermann, K.; Gubo, M.; Müller, S.; Hammer, L.; Heinz, K.,

Coexistence of Rocksalt and Wurtzite Structure in Nanosized CoO Films. Phys. Rev. Lett. 2008, 101, 016103. 23.

Meyer, W.; Biedermann, K.; Gubo, M.; Hammer, L.; Heinz, K., Surface Structure of

Polar Co3O4(111) Films Grown Epitaxially on Ir(100)-(1×1). J. Phys.: Condens. Matter 2008, 20, 265011. 24.

Ferstl, P., et al., Adsorption and Activation of CO on Co3O4(111) Thin Films. J. Phys.

Chem. C 2015, 119, 16688-16699. 25.

Mehl, S.; Ferstl, P.; Schuler, M.; Toghan, A.; Brummel, O.; Hammer, L.; Schneider, M.

A.; Libuda, J., Thermal Evolution of Cobalt Deposits on Co3O4(111): Atomically Dispersed Cobalt, Two-Dimensional CoO Islands, and Metallic Co Nanoparticles. Phys. Chem. Chem. Phys. 2015, 17, 23538-23546.


Page 18 of 27

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


26.

Xu, T.; Schwarz, M.; Werner, K.; Mohr, S.; Amende, M.; Libuda, J., Structure-

Dependent Anchoring of Organic Molecules to Atomically Defined Oxide Surfaces: Phthalic Acid on Co3O4(111), CoO(100), and CoO(111). Chem. - Eur. J. 2016, 22, 5384-5396. 27.

Xu, T.; Schwarz, M.; Werner, K.; Mohr, S.; Amende, M.; Libuda, J., The Surface

Structure Matters: Thermal Stability of Phthalic Acid Anchored to Atomically-Defined Cobalt Oxide Films. Phys. Chem. Chem. Phys. 2016, 18, 10419-10427. 28.

Mohr, S.; Schmitt, T.; Döpper, T.; Xiang, F.; Schwarz, M.; Görling, A.; Schneider, M.

A.; Libuda, J., Coverage-Dependent Anchoring of 4,4′-Biphenyl Dicarboxylic Acid to CoO(111) Thin Films. Langmuir 2017, 33, 4178-4188. 29.

Schwarz, M.; Hohner, C.; Mohr, S.; Libuda, J., Dissociative Adsorption of Benzoic Acid

on Well-Ordered Cobalt Oxide Surfaces: Role of the Protons. J. Phys. Chem. C 2017, 121, 28317-28327. 30.

Schwarz, M., et al., Structure-Dependent Dissociation of Water on Cobalt Oxide. J. Phys.

Chem. Lett. 2018, 2763-2769. 31.

Desikusumastuti, A.; Staudt, T.; Happel, M.; Laurin, M.; Libuda, J., Adsorption and

Reaction of NO2 on Ordered Alumina Films and Mixed Baria–Alumina Nanoparticles: Cooperative Versus Non-Cooperative Reaction Mechanisms. J. Catal. 2008, 260, 315-328. 32.

Risbud, A. S.; Snedeker, L. P.; Elcombe, M. M.; Cheetham, A. K.; Seshadri, R., Wurtzite

CoO. Chem. Mater. 2005, 17, 834-838.



Figure 1: Surface IR spectra acquired after stepwise exposure of CoO(100)/Ir(100) to D2O at (a) 200 K and (b) 300 K; the total D2O dose is given in Langmuir (1 L = 10-6 Torr.s). (Both adapted with permission from Schwarz et al. 2018 30. Copyright 2018 American Chemical Society.)


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Figure 2: Surface IR spectra acquired after stepwise exposure of CoO(111)/Ir(100) to D2O at (a) 200 K and (b) 300 K; the total D2O dose is given in Langmuir (1 L = 10-6 Torr.s).



Figure 3: Surface IR spectra acquired after stepwise exposure of Co3O4(111)/Ir(100) to D2O at (a) 200 K, (b) 300 K, (c) 370 K, and (d) 500 K; the total D2O dose is given in Langmuir (1 L = 10-6 Torr.s). ((a) and (b) adapted with permission from Schwarz et al. 2018 30. Copyright 2018 American Chemical Society.)


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(Figure 3 continued)



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Figure 4: Surface IR spectra from (a) CoO(100)/Ir(100), (b) CoO(111)/Ir(100), and (c) Co3O4(111)/Ir(100) obtained during heating in D2O at a partial pressure of of 2·10-8 mbar (heating rate 1 K/min) ((a) and (c) adapted with permission from Schwarz et al. 2018 Copyright 2018 American Chemical Society.). 24 ACS Paragon Plus Environment

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Figure 5: Time-resolved surface IR spectra acquired during isotopic exchange experiments on Co3O4(111)/Ir(100) at 300 K. The surface was exposed periodically to H2O and D2O pulses with a duration of (a) 10 s and (b) 100 s (pH20 = pD2O = 3.10-8 mbar). All spectra are referenced to the last spectrum in the previous pulse.



Figure 6: Comparison of the IR spectra recorded during isotopic exchange experiments with H2O and D2O on Co3O4(111)/Ir(100) at different pulse length. For comaprison, an IR spectrum (Adapted with permission from Schwarz et al. 2018

30

. Copyright 2018 American Chemical

Society.) is shown obtained after exposure of pristine Co3O4(111) to D2O (46 L, 1 L = 10-6 Torr.s). 26 ACS Paragon Plus Environment

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