Structure-Dependent Dissociation of Water on Cobalt Oxide - The

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Structure-Dependent Dissociation of Water on Cobalt Oxide Matthias Schwarz, Firas Faisal, Susanne Mohr, Chantal Hohner, Kristin Werner, Tao Xu, Tomáš Skála, Nataliya Tsud, Kevin Charles Prince, Vladimír Matolín, Yaroslava Lykhach, and Jörg Libuda J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01033 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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The Journal of Physical Chemistry Letters

Structure-Dependent Dissociation of Water on Cobalt Oxide Matthias Schwarz,† Firas Faisal,† Susanne Mohr,† Chantal Hohner,† Kristin Werner,†,§ Tao Xu,†,$ Tomáš Skála,‡ Nataliya Tsud,‡ Kevin C. Prince,# Vladimír Matolín,‡ Yaroslava Lykhach,†,* Jörg Libuda†,&,* †

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

‡

Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovičkách 2, 18000 Prague, Czech Republic

#

Elettra-Sincrotrone Trieste SCpA, Strada Statale 14, km 163.5, 34149 Basovizza-Trieste, Italy &

Erlangen Catalysis Resource Center and Interdisciplinary Center Interface Controlled

Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany §

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

$

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

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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected].


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Abstract

Understanding the correlation between structure and reactivity of oxide surfaces is vital for the rational design of catalytic materials. In this work, we demonstrate the exceptional degree of structure sensitivity of the water dissociation reaction for one of the most important materials in catalysis and electrocatalysis. We have studied H2O on two atomically-defined cobalt oxide surfaces, CoO(100) and Co3O4(111). Both surfaces are terminated by O2- and Co2+ in different coordination. By infrared reflection absorption spectroscopy and synchrotron radiation photoelectron spectroscopy we show that H2O adsorbs molecularly on CoO(100), while it dissociates and forms very strongly bound OH and partially dissociated (H2O)n(OH)m clusters on Co3O4(111). We rationalize this structure dependence by the coordination number of surface Co2+. Our results show that specific well-ordered cobalt oxide surfaces interact very strongly with H2O whereas others do not. We propose that this structure dependence plays a key role for catalysis with cobalt oxide nanomaterials.

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Understanding water-oxide interfaces is vital for the advance of renewable energy technologies specifically for the concepts of energy conversion and storage in an electrochemical cycle.1-3 Among the most efficient catalysts, cobalt oxides, cobalt hydroxides, and cobalt phosphates have recently attracted much attention because of their high activity in different heterogeneously4-5 and electrochemically6-8 catalyzed reactions. The catalytic activity of these materials is often discussed in terms of the oxidation state and the local coordination environment of cobalt cations, however, the origins of their reactivity are still under debate.7-12 In spite of the pivotal importance of these materials in reactions such as water splitting,13 studies of water interaction with atomically defined cobalt oxide surfaces are rather scarce.8 Only very recently, studies of water in the form of monomers, clusters, films and liquid bulk phase on atomically-defined oxide surfaces have started to shed light on the interaction mechanisms, energetics and structure formation at the interface.14-23 While the formation of very stable OH was often associated with the presence of defects, recent studies demonstrated that water may dissociate readily on wellordered films of titanium oxide,14-15 iron oxide,24-25 and zinc oxide.26-28 These findings suggest that the specific coordination at the adsorption site is responsible for the dissociation of a water molecule. In those cases where the metal cations were considered to be the active sites, the coordination of the metal cations was tetrahedral.24-28 So far it was, however, not possible to verify the reactivity of tetrahedral sites because the corresponding studies on defect-free wellordered oxide surfaces which expose metal cations in the same oxidation state but exclusively in octahedral coordination are largely missing. For instance, in the case of iron oxide, the reactivity of different facets terminated with octahedral cations was studied in the presence of defects.29-30 Importantly, cobalt oxide crystallizes in two stable phases, i.e. spinel Co3O4 and rocksalt CoO. The Co3O4 structure contains Co2+ and Co3+ cations in tetrahedral and octahedral coordination,


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respectively. The CoO structure contains Co2+ exclusively in octahedral coordination. Heinz, Hammer and co-workers31-33 demonstrated that both phases can be prepared in the form of wellordered Co3O4(111) and CoO(100) films on an Ir(100) substrate. Both surfaces are terminated by Co2+ ions, however, the coordination environment of the surface Co2+ ions is different in these two cases. Thus, the octahedral Co2+ ions on CoO(100) are five-fold coordinated by O2-, whereas the tetrahedral Co2+ ions on Co3O4(111) are coordinated by three O2- ions only (see Figure 1 ab). In this work we present a comparison between the reactivities of two surfaces with different local coordination of surface Co2+ cations using defect-free and atomically-defined cobalt oxide films. We show that water adsorbs weakly in molecular form on one surface, whereas very stable hydroxyl groups are formed on the other. This archetypical example illustrates that it is the local coordination environment that controls the stability of the hydroxyl groups and, thereby, the dissociation reaction. Our present results are of particular importance in view of the fact that cobalt oxide catalysts show pronounced structure dependencies5 and a strong effect of humidity on activity.34 This suggests that water has to be considered as an omnipresent catalytic modifier that selectively interacts with specific sites of the cobalt oxide nanomaterial.



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Figure 1. Structure models of the Co3O4(111) (a) and the CoO(100) (b) films on Ir(100). Surface IR spectra obtained from Co3O4(111) (c, e) and CoO(100) (d, f) during stepwise adsorption of D2O at 200 K (c, d) and 300 K (e, f). The total D2O dose is given in Langmuir (1 L = 10-6 Torr×s).

In the first step, we studied the adsorption of water on Co3O4(111) and CoO(100) surfaces by infrared reflection absorption spectroscopy (IRAS). Both surfaces were prepared in situ in 6 ACS Paragon Plus Environment

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ultrahigh vacuum (UHV) in the form of highly-ordered films on an Ir(100) single crystal by means of physical vapor deposition of cobalt in an oxygen atmosphere following the multistep procedure suggested by Heinz, Hammer and co-workers31-33 (see Supporting Information for more detail). The thicknesses of both films were approximately 7.0 nm. The IR spectra obtained from Co3O4(111) and CoO(100) films upon adsorption of D2O at 200 K and 300 K are shown in Figure 1c-f. For water adsorption at 200 K, two OD stretching bands, ν(OD), emerge in pairs at 2702 and 2530 cm-1 and at 2702 and 2584 cm-1 on Co3O4(111) and CoO(100), respectively. These spectra are characteristic for the formation of water clusters.35 The broad ν(OD) band at lower wavenumber is assigned to hydrogen-bonded donor species in water droplets or clusters, while the band at 2702 cm-1 is attributed to free OD groups at the cluster-vacuum interface. Note that the experiments were performed at 200 K, i.e. above the desorption temperature of multilayer water (170 K).17,

25

Therefore, the water clusters consist of strongly bound

chemisorbed water molecules. Interestingly, the spectral shape is very different in both cases. On Co3O4(111), the high intensity of the band at 2702 cm-1 indicates that the concentration of free OD is high and its large width results from different local environments in the D2O clusters of different structure. In general, the much higher intensity of the band at 2702 cm-1 in comparison to this at 2530 cm-1 suggests that the size of D2O clusters is rather small. We note that due to broad structure and low intensity of the band at 2530 cm-1, the shifts, typically observed as a function of water coverage, can hardly be detected in the present study. In sharp contrast, the free OD band at 2702 cm-1 is sharp on CoO(100) while the band at 2584 cm-1 dominates the spectrum. This indicates formation of larger clusters with most D2O molecules incorporated into the hydrogen bonded network. Further analysis of the water adsorption based on the bending



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vibrations of D2O was not possible due to low intensity and substantial broadening of the corresponding band at about 1200 cm-1 presumably due to low adsorbate coverage.36 To identify the chemical nature of the surface species formed upon water adsorption, we performed synchrotron radiation photoelectron spectroscopy (SRPES). The evolution of O 1s spectra upon stepwise adsorption of water at 150 K on both surfaces is shown in Figure 2a-b.


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Figure 2. O 1s core level (a, b) and valence band (VB) (c, d) spectra obtained from Co3O4(111) (a, c) and CoO(100) (b, d) upon stepwise adsorption of H2O at 150 K followed by annealing in UHV. The total H2O dose is given in Langmuir (1 L = 10-6 Torr×s).



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Prior to H2O exposure, the O 1s spectra are dominated by the contribution from lattice oxygen (1) at 529.5 on Co3O4(111) and 530.1 eV on CoO(100). These are accompanied by the corresponding small peaks (2) at 531.3 and 532.2 eV due to traces of water adsorbed from the residual atmosphere of the vacuum chamber. Upon stepwise exposure to water, the intensities of the peaks (2) increase on both Co3O4(111) and CoO(100) films. At higher exposure, a third component (3) emerged in the O 1s region at 532.5 eV on Co3O4(111) and at 533.7 eV on CoO(100) which increased in intensity with increasing H2O exposure. To identify the nature of the surface species formed, we analyzed the binding energy separations between the features (2) and (3) and the lattice oxygen peak (1). The corresponding separations are labelled ∆1 and ∆2 in Figure 2 a-b. For Co3O4(111) (Figure 2a), we determined a binding energy separation ∆1 of 1.3 eV and ∆2 of 3.0 eV. On CoO(100) (Figure 2b), we found a value of 2.1 eV for ∆1 and of 3.7 eV for ∆2. Typically, spectral contributions from hydroxyl groups and molecular water are shifted towards higher binding energy with respect to the lattice oxygen by 0.8‒2.3 eV and 2.4‒4.0 eV, respectively (see Supporting Information for references). Accordingly, we assign species (3) to molecular water physisorbed on both surfaces. Based on the binding energy separation ∆1 of 1.3 eV, species (2) on Co3O4(111) is identified as surface hydroxyl groups. A similar ∆1 value of 1.5 eV was found by Petitto et al.37 for water adsorption on the same surface. It must be noted that peak (2) likely contains a contribution from strongly bound chemisorbed water which cannot be resolved in O 1s spectra. This assumption is consistent with the observed shift of peak (2) towards lower binding energy with increasing water dose (Figure 2a). Similar shifts were previously observed for hydroxyl groups in the presence of co-adsorbed molecular water on other surfaces.38-39


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In view of the binding energy separation ∆1 of 2.1 eV, species (2) on CoO(100) could be assigned either to hydroxyl groups or molecular water. In order to aid the assignment, we studied the valence band spectra upon adsorption of water on both surfaces (Figure 2c-d). Before water adsorption, the valence band spectra of Co3O4(111) and CoO(100) show contributions from the Co 3d states at 0‒4.0 eV and from the O 2p states at 5.0‒9.0 eV.40 On CoO(100), a broad satellite feature at 11 eV arises from unscreened photoemission from Co 3d states.41 Molecular water gives rise to three features in the valence band, associated with the molecular orbitals 1b1, 3a1, and 1b2, while hydroxyl groups give rise to two features, 1π and 3σ.42-43 In the valence band spectra of Co3O4(111), we already observed weak features from hydroxyl groups at 5.3 eV (1π) and 9.7 eV (3σ) prior to water exposure (Figure 2c), due to adsorption from the residual gas in the UHV chamber. With increasing exposure, these features increase in intensity and additional peaks from molecular water emerge at 11.8 eV (1b2) and 5.9 eV (1b1). Noteworthy, emission from the 3a1 orbital shows low intensity due to the formation of hydrogen bonds.44 For water adsorption on CoO(100) at 150 K, we find no evidence for the formation of hydroxyl groups. Instead, we observe the features of molecular water at 7.3 eV (1b1), 9.0 eV (3a1), and 13.3 eV (1b2), only (see Figure 2d). The assignment to molecular water is supported by the binding energy separation between 1b1 and 3a1 emissions of 1.7 eV. Typically, this separation is around 2 eV for molecular water44 while the separation between 1π and 3σ states of surface hydroxyl is 3 to 4 eV.42 Similar O 1s and valence band spectra were observed for water adsorption on CoO(100) at 200 K (see Supporting Information, Figure S1). However, the relative intensity of peak (3) was



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lower, suggesting a lower amount of multilayer water and a larger contribution of chemisorbed molecular water. Our findings are in agreement with previous studies which reported formation of hydroxyl groups on CoO(100) in the presence of oxygen vacancies only.41 Based on the work of these authors, we would expect emission from the 1π states of surface hydroxyl at 7.3 eV and from the 3σ states at 10.6 eV.41 Clearly, we did not observe these features on CoO(100) and, therefore, we can exclude the dissociation of H2O, both at 150 and 200 K. Noteworthy, we observed a gradual shift of the O 1s contribution from lattice oxygen as a function of water dose on CoO(100) (see Figure 2b). The shift of the corresponding peak (1) towards lower binding energy is consistent with the decrease of work function in the presence of molecularly adsorbed water.25, 42 In contrast, formation of hydroxyl groups on Co3O4(111) results in an increase of work function42 and, respectively, in the shift of the peak (1) towards higher binding energy as a function of water dose (Figure 2a). The constant binding energies of peak (2) and (3) on CoO(100) (Figure 2b) as well as the shifts of the corresponding peaks on Co3O4(111) (Figure 2a) can be rationalized in terms of surface potential changes within the adsorbate clusters as a function of OH/H2O ratio. Next, we considered the adsorption of water at room temperature and above. IR spectra obtained for D2O adsorption at 300 K are shown in Figure 1e-f. For Co3O4(111), only one sharp OD band was observed at 2658 cm-1 in the limit of low exposure. Its frequency is consistent with isolated hydroxyl groups on other oxides.17, 45 In addition, SRPES both in the valence band and the O 1s region shows the formation of surface hydroxyl species (Figure 2a and 2c). Accordingly, we assign this feature to isolated hydroxyl groups. With increasing coverage a variety of additional sharp bands develop at 2731, 2698, 2671, 2634 and 2613 cm-1 accompanied


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by a broad feature at 2550 cm-1. Following our previous arguments, the bands at higher frequency are assigned to free OD groups and the broad band at 2550 cm-1 to bridge-bonded OD. Accordingly, we attribute the spectrum to partially dissociated (D2O)n(OD)m clusters, similar as in the recent work of Mirabella et al.24 The highly structured IR band in the region of free OD suggests that these clusters are very small, i.e. they consist of few water molecules only. In sharp contrast, no adsorbed water is observed under identical conditions on CoO(100) at 300 K (Figure 1f and 2d). The absence of any water-related features at room temperature shows that water binds only weakly to CoO(100). To further investigate the difference in adsorption strength on both surfaces, we performed temperature-programmed IRAS (TP-IRAS). In these experiments, the surface was exposed to D2O (2×10-8 mbar), while the temperature was ramped at a rate of 1 K.min-1 and IR spectra were recorded continuously between 200 and 600 K (Figure 3).



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Figure 3. Temperature programmed surface IR spectra obtained from (a) Co3O4(111) and (b) CoO(100) during heating of the samples at a constant partial pressure of D2O of 2×10-8 mbar. The IR spectra in (a) and (b) are plotted on the same scale.

For Co3O4(111), we observed various bands in the ν(OD) region which change continuously with temperature. At 200 K, the features at 2702 and 2530 cm-1 correspond to those observed in the isothermal experiments (Figure 1c). With increasing temperature, the bands decrease in intensity and the features in the free-OD region gradually develop the same complex structure that is observed in the isothermal experiment at 300 K (Figure 1e). We attribute these changes to partially dissociated (D2O)n(OD)m clusters, the size of which decreases with increasing 14 ACS Paragon Plus Environment

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temperature due to the loss of molecular water. Noteworthy, the intensities of the corresponding ν(OD) bands at 300 K in Figures 3a and 1e are similar. This suggests that the degree of hydroxylation does not depend on the adsorption temperature. Above 400 K, the band associated with hydrogen bonded donors (2530 cm-1) decreases and the spectrum in the free OD region simplifies. Both effects indicate the formation of very small aggregates, i.e. dimers or trimers. Above 500 K, the spectrum is dominated by a single sharp feature at 2652 cm-1 which we attribute to isolated OD groups. These OD groups are the most strongly bound species and reside on the surface up to 570 K. TP-IRAS data is in line with results of the SRPES experiments shown in Figure 2. In particular, we found that the intensity ratio of peak (2) to peak (1) at 500 K is consistent with full hydroxylation of surface Co2+ cations by isolated OH groups. The TP-IRAS data are much simpler for CoO(100) (Figure 3b). The intensive bands at 2702 and 2584 cm-1 observed at 200 K vanish almost completely at around 210 K. This shows that the molecular water on CoO(100) is bound weakly. Only traces of water associated with weak features at 2720 and 2708 cm-1 (Figure 3b) and a minor shoulder in O 1s spectra (Figure 2b) remain visible at higher temperature due to adsorption at defect sites. After desorption of water we found no indication of restructuring in both Co3O4(111) and CoO(100) films by means of LEED. However, the comparison of Co 2p spectra before and after the adsorption/desorption experiment revealed slight reduction of Co3O4(111) films (data are not shown). In principle, this could lead to a change in the reactivity of the cobalt oxide with respect to water dissociation. However, a systematic investigation of this effect is hindered by a rapid adsorption of water from the residual atmosphere in both SRPES and IRAS setups. In conclusion, we investigated the interaction of water with two different well-ordered cobalt oxide surfaces, CoO(100) and Co3O4(111). Both surfaces are terminated by O2- ions and Co2+



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ions in different coordination environment. Combining IRAS and SRPES in the core level and valence regions we show that water adsorbs weakly and molecularly on CoO(100), which exposes fivefold coordinated Co2+ ions. On Co3O4(111), which exposes threefold coordinated Co2+ ions, water dissociates and forms strongly bound hydroxyl groups. These surface hydroxyl groups anchor partially dissociated (H2O)n(OH)m clusters with an average size that is continuously decreasing with increasing temperature. Above 500 K, only isolated OH groups reside on the surface. Our results demonstrate the exceptional structure sensitivity of the water dissociation reaction on different oxide surfaces with well-defined atomic structure. We believe that these findings are of particular relevance for understanding the catalytic properties of cobalt oxide materials. The strong structure dependencies in heterogeneous catalysis observed for cobalt oxide catalysts can be understood only when taking into account water as an omnipresent catalytic modifier that interacts selectively with specific surface facets. With respect to the activity of cobalt oxides, hydroxides, and phosphates in an electrochemical environment, a consistent picture emerges regarding the reactivity of tetrahedrally coordinated cobalt ions towards water dissociation. We believe that our findings may help to expedite the rational design of cobalt oxide-based materials for catalysis and electrocatalysis.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.


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Experimental details, synchrotron radiation photoelectron spectroscopy data for water adsorption at 200 K (PDF) Notes The authors declare no competing financial interests. Acknowledgment 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) and within the DACH Project “COMCAT”. The authors acknowledge the CERIC-ERIC Consortium for the access to experimental facilities and financial support. References (1)

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Figure 1. Structure models of the Co3O4(111) (a) and the CoO(100) (b) films on Ir(100). Surface IR spectra obtained from Co3O4(111) (c, e) and CoO(100) (d, f) during stepwise adsorption of D2O at 200 K (c, d) and 300 K (e, f). The total D2O dose is given in Langmuir (1 L = 10-6 Torr×s). 154x172mm (300 x 300 DPI)

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Figure 2. O 1s core level (a, b) and valence band (VB) (c, d) spectra obtained from Co3O4(111) (a, c) and CoO(100) (b, d) upon stepwise adsorption of H2O at 150 K followed by annealing in UHV. The total H2O dose is given in Langmuir (1 L = 10-6 Torr×s). 191x231mm (300 x 300 DPI)



Figure 3. Temperature programmed surface IR spectra obtained from (a) Co3O4(111) and (b) CoO(100) during heating of the samples at a constant partial pressure of D2O of 2×10-8 mbar. The IR spectra in (a) and (b) are plotted on the same scale. 125x94mm (300 x 300 DPI)


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Table of Content graphic 49x49mm (300 x 300 DPI)


Structure-Dependent Dissociation of Water on Cobalt Oxide - The

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