Dissociative Adsorption of Benzoic Acid on Well-Ordered Cobalt Oxide

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Dissociative Adsorption of Benzoic Acid on WellOrdered Cobalt Oxide Surfaces: The Role of the Protons Matthias Schwarz, Chantal Hohner, Susanne Mohr, and Joerg Libuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09426 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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The Journal of Physical Chemistry

Dissociative Adsorption of Benzoic Acid on Well-Ordered Cobalt Oxide Surfaces: The Role of the Protons

Matthias Schwarz1, Chantal Hohner1, Susanne Mohr1, Jörg Libuda1,2*

1

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany 2

Erlangen Catalysis Resource Center and Interdisciplinary Center Interface Controlled

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

*Corresponding author: [email protected]

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Abstract: We performed a surface science model study to reveal the role of the protons which are released upon linking of organic molecules to oxide surfaces via carboxylic acid anchor groups. Specifically, we studied the adsorption, dissociation and thermal stability of deuterated benzoic acid (C6H5COOD, d1-BA) on three different atomically-defined cobalt oxide surfaces, namely (i) Co3O4(111), (ii) CoO(111), and (iii) CoO(100). All surfaces were prepared in form of thin films grown on Ir(100). d1-BA was deposited at 300 K via physical vapor deposition (PVD). The interfacial chemistry and film formation was monitored in-situ by isothermal time resolved infrared reflection-absorption spectroscopy (TR-IRAS) under ultra-high vacuum (UHV) conditions. For all three surfaces, we monitored the surface carboxylate and the surface hydroxyl groups as a function of coverage. The thermal stability of the films was probed by temperature programmed IRAS (TP-IRAS). The comparison between the three surfaces reveals pronounced structure sensitivity. (i) On Co3O4(111), d1-BA binds via a chelating and symmetric carboxylate, strongly tilted with respect to the substrate. The surface hydroxyl groups give rise to a broad vibrational band indicating their involvement into hydrogen bonds. The coadsorbate layer is stable up to 400 K. Above this temperature, hydroxyl desorbs as water, leading to oxygen depletion most likely, followed by decomposition of the benzoate between 420 and 560 K, leaving behind aromatic residues on the surface. (ii) On the oxygen terminated CoO(111) surface, d1-BA forms of a slightly distorted and slightly tilted bridging carboxylate. Similar as on Co3O4(111), the surface hydroxyl groups form hydrogen bonds. The film is stable up to 420 K. At higher temperature, the surface benzoates decompose slowly over a large temperature range partly, most likely via CO2 release and formation of aromatic residues. The surface hydroxyl groups are stable up to higher temperatures 2 ACS Paragon Plus Environment

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(480 K) as compared to Co3O4(111). (iii) On CoO(100) a completely different behavior is observed. The surface benzoate forms a well-defined mixed coadsorbate layer with free surface hydroxyl groups, which are not involved in hydrogen bonds. The orientation of the carboxylate is strongly coverage dependent. At low coverage, the benzoate is tilted with respect to the surface, whereas a fully perpendicular orientation is adopted at high coverage. This film shows the lowest thermal stability. Above 345 K the surface benzoate and protons of the nearby hydroxyl group recombine, leading to desorption of intact d1-BA. Only a small amount of surface benzoate remains, an effect that we mostly attribute to defect sites.



1. Introduction Organic films on oxides find applications in sensor technology1-2, solar energy conversion3-4, catalysis5-6 and molecular electronics7-8. Typically, the organic-oxide interface plays a critical role in the functionality of these materials. Recently, an increasing number of model studies contributed to the fundamental understanding of such hybrid interfaces.9-17 Among the various oxides that are relevant to such applications, cobalt oxide is a particularly interesting case due to its magnetic

18-19

and catalytic properties.20-23 In our model study we

simulate the surface properties of real cobalt oxide materials using well-ordered thin films of different stoichiometry, structure, or orientation. Specifically, we used Co3O4(111), CoO(100), and CoO(111) films of few nanometer thickness prepared on Ir(100).24. At this thickness, the surface is chemically decoupled from the substrate, while the application of surface science techniques is simplified by the underlying metal.24-25 The surface structures of the aforementioned films have been determined previously by low energy electron diffraction (LEED) I-V analysis and STM.24 The Co3O4(111) and CoO(100) thin-films are bulk-terminated. The Co3O4(111) spinel structure is terminated by tetrahedrally coordinated Co2+ ions, and the CoO(100) rocksalt structure exposed both Co2+ and O2- ions. The CoO(111) is a polar surface and stabilized by reconstruction of the first layer to a wurtzite structure which is oxygen terminated.26 It can be regarded as a model for wurtzite CoO known from nanomaterials.27 For details on the structural properties we refer to previous publications.24, 26, 28-30 Typically, stable oxide-organic interfaces are formed by chemical anchoring of the functional molecule. Carboxylic acid are among the most common anchors.31-34 To achieve insight into the interactions of carboxylic acid anchors with cobalt oxide surfaces, we recently performed a number of model studies by infrared reflection-absorption spectroscopy (IRAS) combined with 4 ACS Paragon Plus Environment

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density functional theory (DFT) and scanning tunneling microscopy

9, 15, 35-37

. In all cases, we

observed deprotonation and formation of carboxylates. For phthalic acid (PA) on the above mentioned cobalt oxide films, we could show that the nature of the surface species formed depends on the structure of the oxide surface.9, 35 For different carboxyl-functionalized organic molecules we showed that their orientation depends on the coverage,15, 36-37 typically with more flat-lying molecules at low coverage and upright standing species at saturation. For benzoic acid (BA) on Co3O4(111), we previously observed formation of a chelating carboxylate, with an orientation that is independent of coverage.36 Whereas all of the above and most other studies focused on the formation of the carboxylate, very little is known on the role of the proton that is released in the anchoring reaction. Typically it is assumed the proton resides on the surface at Brønstedt base sites (O2-), but little experimental evidence is available that could shine light on the nature of these sites. This is due to the low sensitivity or limited surface sensitivity of many methods to protons or surface hydroxyl groups (in the presence of a large amount of oxygen ions). Combining IRAS and temperature programmed desorption (TPD), Domen, Hirose and coworkers identified surface hydroxyl species originating from deprotonation of formic acid on NiO(111) and Ni(111).38-39 Aizawa et al. investigated the decomposition of formic acid on rutile TiO2(110) and imaged the split-off proton by scanning tunneling microscopy (STM) in combination with DFT.40 For formic acid on cerium oxide, Gordon et al. found that the stability of the surface hydroxyls depends on the oxidation state of the surface, i.e. the density of oxygen vacancy sites.41 The only study which reported IRAS data on the OH bands formed by anchoring of larger organic molecules (benzoic acid (BA) and terephthalic acid) were performed by Wöll, Wang and coworkers on



rutile TiO2(110). The authors also found a coverage dependent adsorption geometry and which also affects the adsorption site of the split-off proton.10 To date, no work has been published which addresses the role of the oxide surface structure in the formation of the hydroxyl-carboxylate coadsorbate structure that is formed upon anchoring. Here, we present a comparative IRAS study of benzoic acid deuterated at the acidic proton (C6H5COOD, d1-BA) on the three aforementioned cobalt oxide surfaces. We show that very different OH species can be formed and their nature is critical for the stability and thermal behavior of the anchored film.

2. Experimental All measurements were conducted in an UHV system (base pressure of 1.0×10-10 mbar), which was described in detail elsewhere 42. In short, the system is equipped with all required sample preparation and characterization methods as well as several evaporator sources, two quadrupole mass spectrometers and a vacuum Fourier-transform infrared (FTIR) spectrometer (Bruker Vertex 80v). Dosing of d1-BA: d1-BA (Sigma-Aldrich, 98%; contains small amounts of non-deuterated BA) was dosed via a stainless steel valve from a stainless steel reservoir. Doser and reservoir were pumped separately via a high vacuum line. Prior to first usage the doser was baked out overnight. In all experiments, d1-BA was dosed at pressures of 1-2 x 10-8 mbar. The d1-BA was dosed at a sample temperature of 300 K and during the subsequent temperature ramp up to 600 K (see below).


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IRAS Measurements. All IR spectra were acquired with a liquid nitrogen cooled mercury cadmium telluride (LN-MCT) detector at a spectral resolution of 4 cm-1. Previous to the deposition, a reference spectrum of the clean sample was recorded at the respective adsorption temperature. For the time-resolved (TR-)IRAS experiments, spectra were recorded continuously with an acquisition time of 60s using a mid-band LN-MCT detector, while the surface was exposed to d1-BA at a pressure of 1x10-8 mbar (at 300 K for 35 min). After the time-resolved experiment, another spectrum was recorded with an acquisition time of 10 min. For the temperature-programed (TP-)IRAS experiments first a reference spectrum of the clean surface was recorded. Then the surface was pre-saturated at 330 K with d1-BA.Subsequently the saturated d1-BA film was heated at a rate of 1 K/min up to 600 K while IR spectra were recorded with a narrow-band LN-MCT detector with an acquisition time of 300 s/spectrum. During the whole TP-IRAS experiment, a continuous d1-BA background pressure was maintained. The d1BA serves as a D source, to prevent exchange of OD on the surface by H2O originating from the chamber walls. For analysis of the TP-IRAS data, we applied the procedure proposed by Xu et al.35 In brief: The temperature increase reduces the reflectivity of the sample, which leads to attenuation in the intensity of the single channel spectra. When referred to the reference spectrum recorded on the clean surface, this would lead to an artificial decrease in band intensity with increasing temperature. This effect was compensated by a scaling of each single channel spectrum to the initial spectrum at the starting temperature. After this correction, the spectra are referred to clean surface spectrum acquired on the pristine surface. Preparation of Co3O4(111)/Ir(100). The Co3O4(111) thin film was prepared via evaporation of Co metal in O2 atmosphere based on the method described in literature

43

with minor

modifications. First, a clean Ir(100)-(5×1) surface was produced by Ar+ sputtering (1.8 keV,



300 K, 1 hour; Ar, Linde, 6.0) of the Ir(100) single crystal (MaTeck), followed by annealing (1400 K, 3 min) until a clear LEED pattern could be seen. Secondly, the Ir(100)-(5×1) surface was heated to 1300 K in 5×10-8 mbar O2 (Linde, 5.0) for 3 min and cooled down in O2 atmosphere to 350 K. This led to an Ir(100)-(2×1)O reconstruction, showing a clear (2×1) pattern in LEED. Starting with the Ir(100)-(2×1)O, Co was evaporated onto the surface at a sample temperature of 260 ±5 K using a commercial electron beam evaporator (Focus EFM3, 2 mm Co rod, Alfa Aesar 99,995%) in an atmosphere of 1.0 ×10-6 mbar O2 for 18 min. The evaporation rate of Co metal was determined to be 2 Å/min by means of a quartz micro-balance. After growth, the film was annealed in O2 (1.0 ×10-6 mbar) to 530 K for 2 min and, subsequently, in UHV to 700 K for 5 min. The quality of the film was checked by the LEED pattern and qualitatively by comparing to LEED I-V curves from literature (compare Figure 1S in the supporting information).43 Preparation of CoO(111)/Ir(100). The CoO(111) film was prepared starting from the Co3O4(111) film which was prepared as described above. The Co3O4(111) film transforms into CoO(111) film during annealing because of loss of oxygen.30 In our experiments, the Co3O4(111) thin films (equivalent of 36 Å Co metal) were heated at 870 K for 5 min. The quality of the CoO(111) thin film was checked by LEED (compare Figure 2S in the supporting information).30 Preparation of CoO(100)/Co/Ir(100): For the CoO(100) 44 films, first the Ir(100)-(5×1) surface and the Ir(100)-(2×1)O reconstructed surface were prepared following the procedure described above. The latter was then reduced at 550 K in 1×10-7 mbar H2 (Linde, 5.3) for 1 min and, subsequently, heated in UHV at 550 K for 1 min before cooling down to 320 K. This procedure yields a Ir(100)-(1×1) surface characterized by a sharp (1×1) pattern in LEED. At a temperature of 320 K, a metallic Co buffer layer was then deposited onto the surface at a rate of 2 Å/min for


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6 min. On top of the Co metal layer Co was deposited reactively (2 Å/min) in 4×10-7 mbar O2 for 3 min at a sample temperature of 215±5 K. The thin cobalt oxide film was annealed to 370 K for 1 min resulting in an ordered CoO(100) structure. To obtain thicker and better ordered CoO(100) films, a second reactive deposition of Co in O2 was performed for 18 min at a sample temperature of 215±5 K. Finally, the CoO(100) film was annealed at 900 K for 10 min. The asprepared CoO(100) thin film shows a sharp (1×1) pattern in LEED and a qualitative comparison to LEED IV data from literature enables clear distinction from Ir(100)-(1×1) (compare Figure 3S in the supporting information).24, 45

3. Results and Discussion 3.1 d1-BA Adsorption on Co3O4(111) at 300 K First we investigated the coverage dependent behavior of a d1-BA monolayer on Co3O4(111) at room temperature. In a TR-IRAS experiment, we followed the deposition of d1-BA for 35 min at 300 K (Figure 1a). In Figure 1b, the integrated peak intensities of selected features are plotted as function of deposition time. To resolve the very weak bands in the OD-region, an IR spectrum of the saturated layer was recorded with an acquisition time of 10 min under continuous d1-BA pressure of 2×10-8 mbar (Figure 1c). The IR-bands of BA and other carboxylic acids anchored to oxide and metal surfaces have been discussed in detail previously 9-10, 35, 46-53. In specific, we have recently studied the adsorption of non-deuturated BA on Co3O4(111).36 A full list of all bands together with their assignment and the notation used in this work is given in Table 1. For all coverages, the spectra are dominated by the symmetric carboxylate stretching band, νs(OCO), at 1419 cm-1. Weaker features are centered at 1603, 1549, and 714 cm-1. The first can be assigned to the in-plane carbon-carbon stretching 9 ACS Paragon Plus Environment


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mode, ν(CC), of the phenyl ring. The weak and broadened band at 1549 cm-1 is assigned to the asymmetric carboxylate stretching mode, νas(OCO). The CH out-of-plane bending mode of the phenyl ring, γoop(CH) is observed at 714 cm-1. This mode is polarized perpendicular to the molecular plane. As a result of the metal-surface-selection-rule (MSSR), it provides information on the orientation of the aromatic ring. The MSSR states that only dynamic dipole moments perpendicular to the surface can be probed by IRAS (both for metal surfaces and for thin oxide layers on metal supports).54 Therefore, the γoop(CH) band is most intense for flat-lying BA and forbidden for the upright standing molecule. As discussed previously,

36

BA forms a symmetric

chelating carboxylate. This can be deduced from the dominance of the νs(OCO) mode and the low intensity of the νas(OCO) band. The absence of the C=O stretching band of the free acid at 1700 cm-1 confirms that no unbound species are present at room temperature. The time dependent behavior is shown in Figure 1b. The doser was opened at 5 min, the growth of BA layer started immediately and saturation was observed after 20 min. We observe that the νs(OCO) and the γoop(CH) band grow in parallel, indicating that the molecular orientation is independent of the coverage. The low intensity of the γoop(CH) band shows that the molecule is oriented nearly but not fully perpendicular to the surface. This behavior suggests the formation of densely packed islands of strongly tilted benzoate species with respect to the substrate plane. The formation of a surface carboxylate must be accompanied by deprotonation and formation of surface hydroxyl groups. The stretching mode of the surface OD is expected in the wavenumber region between 2700 and 2450 cm-1.55-57 In previous studies, the very weak (and sometimes broad) OD band could not be detected however.36 Also, in the TR-IRAS experiment in Figure 1a no OD bands could be observed. Therefore we performed additional measurements with long acquisition times (10 min) under steady state conditions (see Figure 1c). In these spectra, it was 10 ACS Paragon Plus Environment

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for the first time possible to detect characteristic features in the OD stretching frequency regions. Specifically, we observe a very weak and broad asymmetric band around 2620 cm-1 (Figure 1c). The large peak width of around 150 cm-1 indicates that the protons are not free but are involved in hydrogen bonds with their environment, either with other surface oxygen ions or with the benzoate species themselves. Note that in another study, we have investigated the interaction of water with the Co3O4(111) surface and found several sharp bands instead of the broad band observed here (data not shown). Based on this observation, we tentatively assign the broadening to H-bond formation of the OD groups with the neighboring benzoate species rather than the surface oxygen ions.

3.2 d1-BA Adsorption on CoO(111) at 300 K Next we turn to the adsorption of d1-BA on CoO(111). All experiments were performed under identical conditions as previously for Co3O4(111). Figure 2a shows the corresponding TR-IRA spectra during deposition and the time dependent evolution of selected band intensities is shown in Figure 2b. Figure 2c shows a single spectrum recorded with an acquisition time of 10 min recorded at a d1-BA pressure of 2x10-8 mbar. In general we find that the spectra of d1-BA on CoO(111) and Co3O4(111) are quite similar (see Table 1 for peak positions and assignment). The νs(OCO) band at 1427 cm-1 dominates the spectrum and indicates formation of the surface carboxylate. The low intensity of the νas(OCO) band (1547 cm-1) reveals that the carboxylate is rather symmetric and the ratio of γoop(CH) to νs(OCO) shows that its orientation is nearly upright (similar as for Co3O4(111)). The binding mechanism on CoO(111) is more difficult to understand. The surface is terminated by oxygen ions with the Co ions located slightly below (~ 0.6 Å).26 This hinders binding of some



adsorbates such as CO phthalic acid

9, 35

58

, but carboxylic acids still form carboxylates as previously shown for

and 4,4′-biphenyl dicarboxylic acid

37

. Possibly, the surface can undergo a

rearrangement to enable binding to the Co2+ ions; an assumption that is supported by the fact that the clean surface also undergoes a temperature dependent reconstruction at 320 K.24 However, we assume that not all ions can be accessed as the band size is nearly equal to the νs(OCO) band on Co3O4(111) (compare Figure 3). Concerning molecular orientation, the weak γoop(CH) band at 717 cm-1 indicated that the benzoate adopts a strongly tilted, but not perfectly perpendicular orientation with respect to the surface. Similar as on Co3O4(111), the orientation is independent of the coverage as can be seen from the similar development of the γoop(CH) and the νs(OCO) bands as a function of coverage (Figure 2b). Again, our focus is on the detection of the OD band. As found for Co3O4(111), the band for d1BA on CoO(111) can again only be resolved with long acquisition times, recorded under steady state conditions on the fully saturated surface. Indeed, we identify a broad asymmetric feature under these conditions which is located around 2640 cm-1 and has a half width of approximately 150 cm-1. Again, this observation suggests that the OD groups are forming hydrogen bonds, either involving the coadsorbed carboxylate or the surface oxygen ions.

3.3 d1-BA Adsorption on CoO(100) at 300 K In Figure 4, we show the corresponding dataset recorded for adsorption of d1-BA on the CoO(100) surface at 300 K. All experimental parameters were identical the experiments previously described for Co3O4(111) and CoO(111). Regarding the band position and their assignment, we again refer to Table 1. Similar as on the other two oxide films, the dominating band is the symmetric OCO stretching feature, νs(OCO), at 1433 cm-1. However the band is


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much sharper and more intense. Again, no C=O stretching band of the free acid is found, indicating that the surface benzoate is the only stable species at 300 K. To follow the time dependent development, we plotted the integrated intensities of selected bands as function of deposition time in Figure 4b. The νs(OCO) band starts to grow after opening the doser (at 2 min) and reaches a saturation level after approximately 12 min. Noteworthy, the νas(OCO) band at 1566 cm-1 is barely visible during the whole deposition, showing that the carboxylate species formed is nearly perfectly symmetric. Following our previous discussion for phthalic acid on CoO(100),9, 35 we propose that the d1-BA is bound in form of bridging bidentate carboxylate (note that the distance between the Co2+ ions on CoO(100) is only 3 Å and, therefore allows formation of a bridging species). In sharp difference to the other two oxide surfaces, the γoop(CH) band at 717 cm-1 shows a very characteristic coverage dependent behavior. Initially, it grows rapidly with coverage, but it reaches a maximum after 6 min of deposition and, subsequently, decreases to nearly zero at full saturation. Taking into account the perpendicular orientation of the dynamic dipole with respect to the aromatic ring, we conclude that orientation of the anchored benzoate changes with coverage. In the low coverage limit, the benzoate adopts a tilted orientation, while at higher coverage the tilting angle changes and the benzoate adopts an almost perfectly perpendicular orientation at saturation coverage. The most remarkable difference between the IR spectra for d1-BA on CoO(100) and the other two oxides, however, is found in the ν(OD) region. The ν(OD) band appears as a very sharp and comparable intense peak at 2558 cm-1, which can already be identified in the TR-IRAS data in Figure 4a. In Figure 4c, the corresponding spectrum is displayed recorded with long acquisition time (10 min) at saturation (in d1-BA at 2×10-8 mbar). In sharp contrast to the other two oxide surfaces, the OD band shows a FWHM of only 15 cm-1. The high intensity and sharpness



suggests the formation of free upright standing OD groups which are not involved in hydrogen bonds. Noteworthy, exposure of CoO(100) to water does not lead to the formation of stable hydroxyl groups at 300 K (data not shown). Therefore, we conclude that upon deprotonation of benzoic acid, a well-defined mixed coadsorbate is formed consisting of free upright standing bridging benzoate and free hydroxyl groups in between that are stabilized in this coadsorbate structure. It is noteworthy that the free hydroxyl species is formed at high coverage only. This can be concluded from the time-resolved data shown in Figure 3b. The sharp OD band appears only, once the behavior of the νs(OCO) band indicates the onset of the coverage dependent reorientation of the surface benzoate. At lower coverage, a much broader feature around 2580 cm-1 is observed, rather indicating the presence hydrogen bonded or tilted OD species in the initial phase of the film growth.

3.4 TP-IRAS of d1-BA on Co3O4(111) In a second set of experiments we studied the temperature dependent behavior of the anchored d1-BA films, again starting with d1-BA on Co3O4(111) (Figure 5). IR spectra were recorded as a function of temperature between 330 K and 600 K while dosing d1-BA at a pressure of 2x10-8 mbar. The initial spectrum at 330 K is shown below the temperature series (Figure 5a). In Figure 5b, we show the integrated peak area in the OD and the νs(OCO) region. In line with our previous findings for non-deuterated BA 36, no spectral changes take place up to 390 K. At higher temperature, the OD peak starts to decrease continuously (Figure 5b) until it vanished completely at around at 480 K. The intensity behavior of the νs(OCO) band is different from the OD feature. Up to 420 K, it shows constant intensity, a value that is slightly higher than


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in our previous experiments.36 We attribute this difference to the fact that in the present experiments d1-BA was dosed continuously. Above 420 K, the νs(OCO) band decreases in intensity and disappears completely around 560 K, while a drastic change in the spectral shape is observed. Noteworthy, the aromatic in-plane C=C stretching band at 1604 cm-1 remains hardly unchanged up to 590 K. The different temperature dependent development of the ν(OD) and the νs(OCO) band indicates that the majority of the d1-BA does not desorb via a recombinative pathway. The anchored d1BA film is stable up to around 390 K. At higher temperature we suggest that OD desorbs by water formation involving surface oxygen. It has been shown previously, that loss of oxygen is facile on Co3O4(111).58-59 Regarding the loss of the benzoate, only a minor fraction can desorb recombinatively, as the νs(OCO) bands still shows 85% of its initial intensity once the OD signal has vanished at 480 K. Thus we have to invoke decomposition. A possible pathway involves decarboxylation. This hypothesis is supported by the observation that the aromatic in-plane ν(CC) band does not decrease in intensity up to 590 K, suggesting that a phenyl-containing species resides up to this temperature. We suggest that phenoxy species are formed via linkage of the phenyl ring to surface oxygen. A schematic summary of the temperature dependent behavior



is provided in

Scheme 1. 3.5 TP-IRAS of d1-BA on CoO(111) The data for an equivalent TP-IRAS experiment for d1-BA on CoO(111) is shown in Figure 6. Up to 420 K, no change occurs in the spectra. Above 420 K, the νs(OCO) band decrease, whereas the peak shape does not show significant changes. At 480 K, the decrease becomes slower and is not completed at 600 K. Simultaneously, the νs(OCO) band becomes broader and develops a shoulder at 1400 cm-1 at higher temperatures. The OD peak shows a slight increase and shift to higher wavenumbers (2643 to 2675 cm-1) up to 480 K and disappears rapidly at higher temperature (up to 530 K). Noteworthy, the in-plane ν(CC) stretching band of the aromatic ring remains nearly unchanged during the whole temperature ramp. 16 ACS Paragon Plus Environment

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Similar as for Co3O4(111), the OD band and the νs(OCO) band show a completely different behavior as a function of temperature. Therefore, we conclude that the d1-BA does not desorb recombinatively. The partial loss of the carboxylate (as indicated by the νs(OCO) band) which does not affect the phenyl groups (as indicated by the ν(CC) band) suggests decomposition, for instance via loss of CO2 and formation of a surface phenoxy species, similar as on the Co3O4(111). However, some carboxylates reside on the surface up to higher temperature. We attribute the increasing intensity of the OD band during the carboxylate loss (420 to 480 K) to the formation of an increasing number of hydroxyl groups. The latter may originate from the fact that d1-BA is continuously dosed in the experiment and additional adsorption sites become available in the reaction. Finally, we attribute the decrease of the OD band above 480 K to the formation of water with surface oxygen.

3.6 TP-IRAS of d1-BA on CoO(100) Finally, we performed an identical TP-IRAS experiment on a d1-BA layer on CoO(100) (Figure 7). Here, be observe a behavior which is differs drastically from the two other oxide surfaces. In fact, we observe that the ν(OD) and νs(OCO) bands show a very similar development (Figure 7b). Both bands decrease rapidly at temperatures above 345 K. Simultaneously, the OD band shifts slightly to higher wavenumbers (2562 to 2583 cm-1). Also a second weak OD band at 2625 cm-1 is observed for temperatures between 370 K and 420 K. At 420 K, the OD peak disappears completely. Simultaneously the carboxylate band reaches a plateau at very low intensity (approximately 7% of its original value). All other bands behave similar. At



temperatures above 500 K, the remaining intensity in the νs(OCO) region disappears, while some weaker bands such as the CC stretching band of the phenyl ring increase in intensity again. We attribute the decrease of the OD and the carboxylate band between 345 and 420 K to recombinative desorption of molecular d1-BA from the fixed coadsorbate layer. More than 90% of the benzoates desorb via this mechanism. The blue-shift of the OD during desorption equals the shift observed in the uptake experiment. This supports the suggestion that the dissociative adsorption process is reversible. The remaining intensity in the νs(OCO) region shows that a minority species resides on the surface at higher temperature. Such a species might also be associated to defect sites (either because the benzoates are more strongly bound or because the defects open a desorption channel for protons via water formation). The increase of the CC stretching band of the phenyl ring around 530 K may indicate the formation of upright standing decomposition products, such as phenoxy species discussed above. Higher coverages of such species might be formed if we take into account that the d1-BA is continuously dosed and assume that the CoO(100) becomes reactive at higher temperatures. 3.7 Comparison As expected, d1-BA binds dissociatively on all three cobalt oxide surfaces forming a surface benzoate (Figure 3, ). The anchoring and desorption processes are visualized in Scheme 1. The models correspond to the structures determined previously24 and the bond lengths reflect typical values in Co-carboxylates (~1.9 Å)60. The saturation coverages can be estimated as follows: For Co3O4(111), we assume that all surface Co2+ ions (3.6×1014 cm-2) bind to one carboxylate at saturation coverage (an assumption that is corroborated by the observation of similar band intensities for other carboxylic acids, see e.g. 9, 35). The saturation coverage for the other surfaces can be roughly estimated from the relative intensities of the νs(OCO) bands (Co3O4(111) : 18 ACS Paragon Plus Environment

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CoO(111) : CoO(100) = 4 : 5 : 6) yielding a saturation coverages of 4.5×1014 cm-2 for CoO(111) and 5.4×1014 cm-2 for CoO(100). Our study shows that surface structure has a dramatic influence on the properties of the anchored monolayer. In fact, the surface structure affects not only the binding geometry of the surface carboxylate, as we have already shown in our previous publications,9, 35 but also the hydroxylate groups formed upon deprotonation. The most surprising finding is that on CoO(100), the benzoate and the hydroxyl form a very well defined coadsorbate layer, as indicated by the very sharp and intense ν(OD) and νs(OCO) bands. Their orientation is strongly coverage dependent – tilted for low coverages and fully perpendicular for saturation coverage. In contrast d1-BA on Co3O4(111) and CoO(111) forms less well-defined structures. Whereas free OD groups are formed on CoO(100), the protons are incorporated into hydrogen bonding interactions on Co3O4(111) and CoO(111), as indicated by a very broad ν(OD) band. Most likely, these hydrogen bridges also involve interaction with the oxygen atoms of the surface carboxylate itself. Furthermore, the orientation of the surface carboxylate is coverage independent on the two latter surfaces. This observation suggests that islands are formed already at low coverage, in which the intermolecular interaction between the carboxylate species is similar as at higher coverage. We assume that the protons released are incorporated into these islands, as a phase separation would be disadvantageous from the point of view of electrostatics. The oxide surface structure also has a strong influence on the thermal stability of the anchored film. For both Co3O4(111) and CoO(111) the d1-BA monolayers are stable up to around 400 K. Above this temperature, the hydroxyl groups and the surface benzoate are lost; however, recombination is not the primary reaction channel. On Co3O4(111) first the hydroxyl groups desorb before the carboxylate, most likely via formation of water involving surface oxygen. On 19 ACS Paragon Plus Environment


CoO(111), we observe a partial loss of the carboxylate followed by desorption of the surface hydroxyl groups above 480 K. For both surfaces, decomposition of the surface benzoate is observed at higher temperature, most likely via decarboxylation and formation of a surfacebound species containing a phenyl rings, possibly a phenoxy species. The lower thermal stability of the hydroxyl on Co3O4(111) is associated with the higher oxidation state of cobalt spinel, containing both Co2+ and Co3+ ions. The compound readily releases oxygen leading to the formation of water. On the already reduced rocksalt CoO, the formation of oxygen vacancies is more difficult and water is formed at higher temperatures only. Noteworthy, d1-BA on CoO(100) shows the lowest thermal stability, with desorption starting already at 345 K. The facile desorption results from the fact that desorption occurs via recombination on this surface. In other words, CoO(100) is the only surface on which the adsorption process is reversible. The low recombination temperature indicates a low activation barrier, possibly due to the low stability of the free hydroxyl group which is not stabilized by additional hydrogen bonds. Further, the welldefined nature of the hydroxide-benzoate coadsorbate layer may help in the recombination process. Only at higher temperature and continuous exposure to d1-BA, a similar decarboxylation reaction is observed as on the other surfaces. 4. Conclusion We have compared the dissociative adsorption and stability of d1-BA deposited by PVD on atomically defined Co3O4(111), CoO(111), and CoO(100) films on Ir(100). The nature of the surface formed species and the interfacial reactions upon annealing were investigated by timeresolved and temperature-programed IRAS. In particular, we could observe the surface hydroxyl species that are formed upon of dissociation of the carboxylic acid group acid for all three cobalt oxides surfaces. 20 ACS Paragon Plus Environment

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(1) d1-BA dissociates on Co3O4(111) at 300 K forming a nearly symmetric chelating carboxylate. The orientation is independent of the coverage, strongly tilted with respect to the surface, but not fully perpendicular. The surface hydroxyl groups are incorporated into the surface carboxylate layer and stabilized by additional hydrogen bonds. The adsorbate layer of benzoates and hydroxyl groups is stable up to 400 K. Above this temperature, first the hydroxyl groups leave the surface via formation of water formed with surface oxygen. The benzoate decomposes between 420 and 560 K through a decarboxylation reaction in which the phenyl rings remain on the surface, most likely as a phenoxy species. (2) On the oxygen terminated CoO(111), d1-BA binds dissociatively, forming a nearly symmetric carboxylate. The orientation of the benzoate is tilted with respect to the surface. Similar as for Co3O4(111), the surface hydroxyls groups are stabilized by hydrogen bonds. Water formation is less facile on CoO. Therefore, the first thermally activated decomposition involved the benzoate at temperatures above 420 K. In contrast to Co3O4(111), we observe partial decarboxylation to phenoxy only and some carboxyl species reside on the surface up to 600 K. The surface hydroxyl groups are stable up to temperatures of around 480 K. At higher temperature desorption is observed, most likely via formation of water. (3) On CoO(100), d1-BA forms a well-defined mixed coadsorbate layer at saturation coverage which consists of surface hydroxyl groups and surface benzoates oriented perpendicular with respect to the surface. The bridging carboxylate adsorbs in a highly symmetric geometry. Its orientation, however, strongly depends on the surface coverage with a tilted adsorption geometry adopted at low coverage and an upright geometry



adopted at high coverage. Whereas the hydroxyl groups may be involved in hydrogen bonding at low coverage, free hydroxyl species are formed at saturation coverage showing no indication of hydrogen bonding at all. The anchored d1-BA layer on CoO(100) shows the lowest thermal stability, with a desorption onset at 345 K. The low desorption temperature is the result from a different desorption mechanism which involves recombination of proton and benzoate. Only a small fraction of surface bound benzoates are observed at higher temperature which then undergo decarboxylation, similar as on the other two surfaces.

Acknowledgements: This project was funded by the Deutsche Forschungsgemeinschaft (DFG) within Research Unit FOR 1878 “funCOS − Functional Molecular Structures on Complex Oxide Surfaces”. Additional support is acknowledged from the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative.

Supporting Information LEED images of Co3O4(111)/Ir(100) recorded at the indicated electron energies (Figure 1S); LEED images of CoO(111)/Ir(100) recorded at the indicated electron energies (Figure 2S); LEED images of CoO(100)/Co/Ir(100) recorded at the indicated electron energies (Figure 3S); comparison of the IR spectra of saturated d1-BA (C6H5COOD) monolayers at 300 K on Co3O4(111)/Ir(100), CoO(111)/Ir(100), and CoO(100)/Co/Ir(100) (Figure 4S).


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Figure 1: a) TR-IRAS recorded during exposure of Co3O4(111)/Ir(100) to d1-BA (C6H5COOD) at 300 K; b) integrated peak intensities at 1419 (νs(OCO)), and 714 cm-1 (γoop(CH)) as function of deposition time. c.) Surface IR spectrum of a saturated d1-BA monolayer recorded with high signal-to-noise ratio.



Figure 2: a) TR-IRAS recorded during exposure of CoO(111)/Ir(100) to d1-BA (C6H5COOD) at 300 K; b) integrated peak intensities at 1427 (νs(OCO)), and 717 cm-1 (γoop(CH)) as function of deposition time. c.) Surface IR spectrum of a saturated d1-BA monolayer recorded with high signal-to-noise ratio.


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Figure 3: Comparison of the spectra of saturated d1-BA (C6H5COOD) monolayers at 300 K on Co3O4(111)/Ir(100), CoO(111)/Ir(100), and CoO(100)/Co/Ir(100).



Figure 4 a) TR-IRAS recorded during the exposure of CoO(100)/Co/Ir(100) to d1-BA (C6H5COOD) at 300 K; b) integrated peak intensities at 2558 (ν(OD)), 1433 (νs(OCO)), and 717 cm-1 (γoop(CH)) as function of deposition time. c.) surface IR spectrum of a saturated d1-BA monolayer recorded with high signal-to-noise ratio.


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Figure 5: a) Temperature-programmed IRAS for a d1-BA (C6H5COOD) monolayer on Co3O4(111)/Ir(100) displayed as contour plot. The spectra were recorded at a continuous d1-BA pressure of 2x10-8 mbar. b) integrated peak intensities at 1421 ((νs(OCO)), and 2612 cm-1 (ν(OD)) as function of temperature. 27 ACS Paragon Plus Environment


Figure 6: a) Temperature-programmed IRAS for a d1-BA (C6H5COOD) monolayer on CoO(111)/Ir(100) displayed as contour plot. The spectra were recorded at a continuous d1-BA pressure of (2x10-8 mbar). b) integrated peak intensities at 1425 ((νs(OCO)), and 2643 cm-1 (ν(OD)) as function of temperature. 28 ACS Paragon Plus Environment

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Figure 7: a) Temperature-programmed IRAS for a d1-BA (C6H5COOD) monolayer on CoO(100)/Co/Ir(100) displayed as contour plot. The spectra were recorded at a continuous d1BA pressure of 2x10-8 mbar). b) integrated peak intensities at 1430 ((νs(OCO)), and 2562 cm-1 (ν(OD)) as function of temperature. 29 ACS Paragon Plus Environment


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Scheme 1: Schematic representation of the adsorption, desorption and decomposition reactions observed

for

d1-BA

(C6H5COOD)

on

Co3O4(111)/Ir(100),

CoO(100)/Co/Ir(100).


CoO(111)/Ir(100),

and

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Table 1: Wavenumbers and band assignement for d1-BA (C6H5COOD) on Co3O4(111)/Ir(100), CoO(111)/Ir(100), and CoO(100)/Co/Ir(100). Co3O4(111)

CoO(111)

CoO(100)

assignment

714

717

717

γ(CH)oop

841

842

840

γ(CH)ring

1026

1027

1030

ν(CC)ring+δ(CH)

1070

1070

1070

ν(CC)ring+δ(CH)

1142

1146

1144

ν(CC)ring+δ(CH)

1182

1182

1183

ν(CC)ring+δ(CH)

1419

1427

1434

νs(OCO)

1449

1449

1494

1495

1495

δ(CH)

1536

1536

1535

ν(CC)ring+δ(CH)

1549

1547

1566

νas(OCO)

1603

1603

1603

ν(CC)ring

2617

2639

2558

ν(OD)surface

3062

3061

3059

ν(CH)

δ(CH)



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Dissociative Adsorption of Benzoic Acid on Well-Ordered Cobalt Oxide

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