Surface Reaction of CO on Carbide-Modified Mo ... - ACS Publications

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Surface Reaction of CO on Carbide-Modified Mo(110) Christoph Gleichweit,† Christian Neiss,‡ Sven Maisel,‡ Udo Bauer,† Florian Spaẗ h,† Oliver Höfert,† Andreas Görling,‡ Hans-Peter Steinrück,†,§ and Christian Papp*,† †

Lehrstuhl für Physikalische Chemie II Friedrich-Alexander-Universität Erlangen-Nürnberg Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Lehrstuhl für Theoretische Chemie Friedrich-Alexander-Universität Erlangen-Nürnberg Egerlandstrasse 3, 91058 Erlangen, Germany § Erlangen Catalysis Resource Center Friedrich-Alexander-Universität Erlangen-Nürnberg Egerlandstrasse 3, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: The adsorption and reaction of CO on a monolayer carbide and a bulk carbide, prepared on Mo(110), was studied with synchrotron-based XPS, TPD, and densityfunctional calculations using slab models. In the experiments on the monolayer carbide, we find two CO species at 140 K, with a saturation coverage of ∼0.7 ML, while on the bulk carbide, Mo2C, three molecular adsorption states are found, showing a similar total coverage of ∼0.7 ML at saturation. In addition, CO partly dissociates on both surfaces (monolayer carbide: 7%, bulk carbide: 15%). The calculations on the monolayer carbide show that the adsorption of CO on Mo sites is most stable. At increased coverages, several different adsorption sites on the monolayer carbide become possible. From the core level shifts, an assignment to the experimentally found species becomes available. Upon heating, we find on both carbides the competing processes of desorption, interconversion of different CO species, and dissociation of CO. The detailed quantitative analysis of these processes shows that desorption and dissociation to atomic oxygen and carbon is completed at ∼400 K on the monolayer carbide and ∼450 K on the bulk carbide; in both cases, about 35% (0.25 ML) of the initially adsorbed CO decomposes upon heating. Above 800 K, atomic carbon and oxygen desorb associatively, and at 1200 K the carbide surfaces are restored.

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INTRODUCTION The surface chemistry of transition metal carbides (TMCs) has been studied in the past both in experiment and theory.1−6 These carbides have been suggested as alternative catalysts to the Pt-group metals.7 This is due to their similar catalytic activity for a number of chemical reactions including (de)hydrogenation and hydrogenolysis of hydrocarbon molecules.3 However, for important other chemical reactions, where the reactivity toward oxygen-containing molecules is relevant such as those relevant for fuel cells and biomass conversion, and for the water gas shift reaction, the behavior of TMCs is different compared to the Pt-group metals.4 In particular, the reactivity toward CO is of interest, since resistance to poisoning plays a critical role in many of these applications. In general, TMCs show a lower desorption temperature of CO than the Pt group metals, which suggests a higher resistance to CO poisoning.3,4 We aim at a detailed understanding of the adsorption properties and reactivity of TMCs on a microscopic level, demanding for a highly defined nature of the system. One way of providing such a system is the use of carbide-modified metal single crystals, e.g., Mo(110). They offer the advantage that the C/Mo ratio can be tuned, and the corresponding changes in reactivity can be studied.8,9 © XXXX American Chemical Society

The reaction of small molecules on carbide modified-surfaces on Mo(110) single crystals has been addressed in a number of studies in the past; see refs.3,4 and references therein. To the best of our knowledge, the reactivity for different C/Mo ratios has only been studied so far for ethylene10−12 and CO.12,13 However, the exact structure of the carbide surfaces was not known at that time, and thus, conclusions on the adsorption sites from these previous experiments were not possible. Recently, we performed a detailed study on the formation and structure of carbide-modified Mo(110) surfaces14 that showed that the surface structure of the monolayer carbide is different to that of the bulk carbide, even though both surfaces show a (4 × 4) pattern in LEED. In other studies concerning CO adsorption,15,16 the thickness of the carbide and related possible changes in its surface structure were not taken into account; only the (4 × 4)-C/Mo(110) superstructure was considered. Herein, we investigate the differences in the adsorption and reaction behavior of CO for the monolayer carbide and the bulk carbide on Mo(110). We conducted in situ experiments Received: November 28, 2016 Revised: January 11, 2017 Published: January 12, 2017 A

DOI: 10.1021/acs.jpcc.6b11950 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. XP spectra of the adsorption of CO on the monolayer carbide on Mo(110). On the left-hand side, (a) selected C 1s spectra are shown and (c) the spectrum at saturation is depicted including the fits. (b and d) respective O 1s spectra.

scatter in these data. The binding energy axis has been calibrated by the Fermi edge at low temperature. During the experiments, no synchrotron-radiation-induced effects were found. The Mo(110) crystal (MaTecK, 99.99%) was spot-welded to Ta-wires and type K thermocouples were attached to the sample. Cleaning was performed by sputter−anneal cycles using Ar+ bombardment at 1 kV (4 × 10−6 mbar). The preparation is detailed in a paragraph below. Carbon monoxide (CO) (3.5) was purchased from Westfalen AG. The C 1s coverages were calibrated using the known c(4 × 2) superstructure of CO on Pt(111);19 the O 1s coverages were calibrated by comparison to the C 1s data. For quantitative analysis, the spectra were fitted using Doniach−Sunjic20 functions convoluted with a Gaussian after subtraction of a linear background. The TPD experiments were carried out in a different UHV setup.21,22 It is equipped with a simple XPS setup (Al Kα X-ray source and single-channel electron energy analyzer), a sputter gun, LEED optics, and a quadrupole mass spectrometer (Pfeiffer QME 200) in a “Feulner cup”23 arrangement. The preparation of the monolayer and the bulk carbide are described in details in ref 14. For the surface carbide a partly oxygen-covered Mo(110) surface is exposed to ethylene at 1200 K. After removal of the oxygen, which is observed in situ with XPS, ∼1.5 L are needed to form the surface carbide. The bulk carbide is prepared similarly that is at 1200 K from a partly oxygen-covered surface, but with ethylene exposures that are orders of magnitude higher, typically several thousand Langmuir were used that lead to saturation, as monitored by in situ XPS. Computational Details. DFT calculations were performed using the Vienna ab initio program VASP24,25 adopting a planewave basis set within the projector augmented wave (PAW) approach.26 We used the exchange-correlation functional by Perdew, Burke and Ernzerhof (PBE).27 The SCF convergence criterion was 10−6 eV, and geometries were relaxed until the nuclear gradient became smaller than 0.01 eV/Å. Geometry

using synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS). On both the monolayer carbide and the bulk carbide, C 1s and O 1s spectra were collected during exposure to CO. To gain some insight about possible adsorption sites and structural details, the measured data are put into relation to computed adsorption geometries and computed core level energies (within the framework of DFT). Subsequently, heating experiments were conducted to gain information on the reaction paths of CO at elevated temperatures. Moreover, TPD experiments were performed to complete the picture by monitoring the desorbing molecules in the gas phase. We show that the adsorption and the reactivity of the two molybdenum carbide surfaces toward CO shows distinct differences.

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EXPERIMENTAL SECTION The HR-XPS measurements were conducted at the beamline U49/2-PGM1 of BESSY II of Helmholtz-Zentrum Berlin. We use a transportable ultrahigh vacuum (UHV) setup18 that consists of two chambers: the analysis chamber (base pressure 2 × 10−10 mbar) houses an electron energy analyzer (EA 125 HR U7), a quadrupole mass spectrometer (QMS) and is directly connected to a supersonic molecular beam. The preparation chamber is equipped with a two-grid LEED optics, evaporators, and a sputter gun. Liquid nitrogen cooling allows reaching temperatures of approximately 120 K, and by resistive heating temperatures up to 1400 K can be achieved. A filament at the back of the sample enables heating to 600 K, while continuously measuring spectra (typical acquisition time ∼10 s/spectrum) without disturbing electric or magnetic fields. The spectra for the monolayer carbide were taken at photon energies of 380 eV for the C 1s region and 780 eV for the O 1s region, at resolutions of 180 and 300 meV, respectively. The electron emission angle was 0° with respect to the surface normal. For the bulk carbide the photon energy for the O 1s region was 650 eV, with a resolution of 200 meV. Note that beam instabilities, due to low-α mode operation during the O 1s experiment of the bulk carbide, lead to increased noise and B


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The increase of COM2 and COM3 is accompanied by a decrease of the monolayer carbide signal by 25%, due to damping by the adsorbate. In addition, the carbide peak shifts from 282.8 to 282.9 eV. This shift allows distinguishing the clean carbide (dark green) from the CO covered carbide (dark brown) in our fits. In Figure 1c, the spectrum at 2.6 L including the fits is shown. The fit shows an additional peak at 283.3 eV, which is attributed to carbon from dissociated CO. Please note that a similar species is also found after the dehydrogenation of ethylene10 and upon CO dissociation on pristine Mo(110).17 In the O 1s region in Figure 1b, we find the growth of two peaks at 531.5 (COM2) and 532.5 eV (COM3). The peaks shift to higher binding energy at higher coverages, ending up at 532.0 and 533.0 eV at saturation at ∼2 L. An additional peak at 530.3 eV is assigned to atomic oxygen; this peak is attributed to CO dissociation during adsorption at 140 K. This assignment is further confirmed by a separate oxygen adsorption experiment (see Supporting Information). Figure 1d shows the last spectrum of the adsorption experiment of Figure 1b at 2.4 L, along with the corresponding fit. By fitting all spectra, we obtain quantitative information on the coverage of the different surface species. Figure 2a and b

optimizations were carried out using PAW pseudopotentials from the VASP library (PAW_PBE 08Apr2002 potentials) with 6/4/6 explicit electrons for Mo/C/O, respectively, and with a plane wave cutoff of 430 eV. The monolayer carbide was modeled by a carbon covered four-layer (4 × 4)-Mo(110) slab where the two bottom-most layers were kept fixed at optimized Mo bulk structure during geometry optimizations. A vacuum of ∼10 Å between the slabs was added yielding a cell dimension of 10.955 Å × 10.955 Å × 20.0 Å. In case of the bulk carbide, the slab model consists of six Mo2C layers, separated by ∼18 Å vacuum, where the three lowest layers were fixed at the optimized bulk Mo2C geometry, which resulted in an orthorhombic 6.063 Å × 10.448 Å × 29.2 Å unit cell. The first Brillouin zones of the monolayer carbide and bulk carbide were sampled by 7 × 7 × 1 and 14 × 7 × 1 Monkhorst−Pack k-point grids,28 respectively (which was shifted in the bulk carbide to contain the Γ-point). Methfessel-Paxton smearing29 of order 1 and width σ = 0.2 eV was applied throughout, extrapolated energies for σ → 0 were used for evaluation. The energy of a single CO molecule was calculated by relaxing CO in a large box at the Γ-point. Adsorption energies are defined as the energy gain upon adsorption. Core level binding energies were calculated in the final state approximation as described in ref 14. For these calculations harder pseudopotentials (PAW Mo_sv_GW 23Mar2010, PAW C_GW_new 19Mar2012, PAW O_GW 19Mar2012) with 14/ 6/4 explicit electrons for Mo/O/C, respectively, were used, and the cutoff energy was increased to 550 eV. Since the core states are frozen within the PAW method, this approach cannot account for screening effects of core electrons, and the absolute core level energies will have substantial systematic errors, which are, however, expected to cancel approximately for core level energy differences leading to relative errors smaller than 0.2 eV according to our experience.

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RESULTS AND DISCUSSION CO Adsorption on the Monolayer Carbide. We begin with discussing the adsorption of CO on the monolayer carbide. The surface was monitored in situ, by continuously recording XP spectra during exposure to 5 × 10−9 mbar CO at 140 K. The corresponding data for the C 1s and O 1s core levels, collected in separate experiments, are shown in Figure 1a and b, respectively. The blue spectra in Figure 1a and b reflect the situation before adsorption, that is, at an exposure of 0 Langmuir (L). In the C 1s region (Figure 1a), we find the sharp, asymmetric peak of the monolayer carbide at a binding energy of 282.8 eV. The peak area corresponds to 0.43 ± 0.06 ML of C atoms per Mo atom, in line with our previous work.14 The O 1s region shows no intensity before exposure to CO. During CO exposure, in the C 1s region first a small peak appears at 284.7 eV denoted “COM1”. At higher exposures, at least two broad, asymmetric contributions at 285.5 (COM2) and 286.0 eV (COM3) rise, while the peak at 284.7 eV disappears above 0.25 ML. A similar behavior was found for CO adsorption on pristine Mo(110), and was attributed to steric effects leading to a change of the tilt angle of CO (with respect to the surface normal) depending on coverage.17 In our calculations on the monolayer carbide (see below), such a change in the tilt angle is, however, not found.

Figure 2. Quantitative analysis of the XP spectra of Figure 1, collected during adsorption of CO and successive heating, on the monolayer carbide on Mo(110). (a and c) Analysis of the C 1s spectra during adsorption and heating, respectively, and (b and d) the corresponding analysis of the O 1s spectra.

show the analysis in the C 1s and O 1s core levels, respectively. For both core levels, we deduce (within the margin of error) the same coverages of the different species, namely ∼0.40 ML for the COM2 species, ∼ 0.25 ML for the COM3 species, and ∼0.05 ML of atomic oxygen and carbon. These values show that the dissociated CO species amounts to 7% of the total coverage. Similar to our results, the dissociation of CO on Mo(110) also occurs already at low temperatures leading to a C 1s component at ∼283 eV.17

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THEORETICAL RESULTS AND INTERPRETATION OF THE ADSORPTION OF CO ON THE MONOLAYER CARBIDE We performed DFT calculations using a slab model for the monolayer carbide surface. As we have argued in ref 14, the C


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adsorption geometries. In general, CO adsorbs more or less on-top of Mo surface atoms in a tilted geometry, always binding via its C atom to the surface. The tilt angle with respect to the surface normal is ∼25°. (Although the CO molecule is always shifted toward a hollow site or a bridge site, there is always one Mo surface atom that is closer than the other ones.) In case of sites A and D, we also found energetic minima with CO binding in upright configuration directly over Mo. For site A, the upright adsorption geometry is practically iso-energetic with the tilted one (Eads = 1.75 eV); for site D, the adsorption energy is somewhat smaller than in the tilted geometry (Eads = 1.60 eV). No such structures were found for the remaining sites, which are in direct contact with surface carbon. Together with the observation that the CO molecules, if adsorbed to a Mo atom that is in contact with a surface C2-unit, always point away from this unit and move to the opposite side of the Mo atom (see Figure S4 in the Supporting Information). These findings indicate a repulsion between surface carbon and CO. Nonetheless, according to our calculations, there is also a local energetic minimum with CO binding directly to the Cdimers on the surface, but as the adsorption energy of this structure is very low (0.49 eV), we do not expect that this adsorption site is significantly populated and it is not considered further. In the following, we will discuss the calculated core level shifts (CLS) for the various CO sites and CO surface structures and compare them to the results obtained in the experiment. The experiment though does not show distinct, sharp contributions as is the case e.g. in the adsorption of CO on Pt(111) or CO on Rh(111) examined with high resolution synchrotron-based XPS. In the experimental data for CO on the monolayer carbide, multiple adsorption sites with very similar CLS, denoted, e.g., COM1, COM2, and COM3 are observed. The small differences in the calculated CLS of several adsorption sites therefore is reflected in the high full width at half-maximum (fwhm) found in the experiment. At first, we regard the CLS of only one CO molecule in the unit cell: For the most stable adsorption sites A and D, we calculate CL shifts in the C 1s region of 1.9 and 1.7 eV relative to the surface carbon, respectively, while the CLSs of sites B, C, and F are somewhat higher (2.0 eV), see Table S1. The range of calculated CLSs leads to a rather broad peak in a simulated XP spectrum (see Figure S6), especially if only the three most stable adsorption geometries A, B, and D are considered. Note that these three configurations have adsorption energies that are the same within 0.1 eV, which is within of the expected accuracy of the calculations. In the adsorption experiment, the simulated coverage should correspond to the species COM1. The disappearance of COM1 is due to adsorbate−adsorbate interactions that lead to a higher CLS, that is, these CO molecules are not disappearing but become part of COM2. In the experiment, when increasing the coverage, we find the decrease of the COM1 species and the increase of COM2 and COM3 species at higher binding energies, as compared to COM1. Similarly, the number of adsorbed CO molecules was increased in the calculations. We considered models for CO distributions on the surface by selecting low-energy sites or a roughly homogeneous distribution of molecules over the surface in the start geometries and subsequent geometry optimization. An energetically favorable structure with a CO coverage of 5/16 ML is shown in Figure 3c. (see Figure S5 in the Supporting Information for another structure) We note that at this coverage the initially most stable CO adsorption sites are still

most likely structure for this surface can be described as the pristine Mo(110)-surface and thereon adsorbed C2-units with a coverage of 3/16 ML, see Figure 3a) for a sketch of the unit cell. In the following, this surface model will be the basis for our considerations.

Figure 3. (a): Surface model of the monolayer carbide with labeling of surface Mo atoms. (b−d) Low-energy adsorption geometries of CO for different coverages. In all cases top and side views are provided. The unit cell is indicated by dashed lines. For additional structures see Supporting Information, Figures S4 and S5 (carbon: brown; oxygen: red; molybdenum: mauve).

At low coverage (1/16 ML), there are six nonequivalent adsorption sites in one unit cell which we label A to F in the following, see Figure 3a). In all cases, we found adsorption geometries representing local energetic minima. The most stable adsorption sites are A (Eads = 1.78 eV) and D (Eads = 1.73 eV), i.e. those where CO is farthest away from the carbide C2 units. While site B has a similar adsorption energy (Eads = 1.71 eV), the stabilities clearly decrease for sites E (Eads = 1.60 eV), F (Eads = 1.48 eV), and C (Eads = 1.31 eV), see the Supporting Information for graphical representations of the different D


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Figure 4. Selected XP spectra of the heating experiment (TPXPS) of CO on the monolayer carbide of Mo(110) for the (a) C 1s and (b) O 1s core levels.

stable species on the surface is in line with the calculations: According to their CLS, sites A and D, that are preferentially populated at low coverages, are also the most stable ones during annealing before CO either desorbs or dissociates on the surface. For the oxygen core levels we find that, similar to the C 1s region, the spectral components shift on average to higher binding energies with increasing coverage, and broaden at the same time in the experiment. The broadening of the core level is also due to the overlap of several differently adsorbed CO molecules. As for carbon, we simulated the XP spectra for oxygen based on the calculated core level binding energies, see Figure S7. Similar to the carbon CLS, site D exhibits a lower O 1s binding energy and a higher adsorption energy than the other adsorption sites. On the basis of the experimental electron binding energies, this again leads to the assumption that species COM1 corresponds to CO bound at site D. Going to higher coverages, one observes similar trends as for the carbon shifts, however, the spread is larger. This is in turn consistent with the broader peak shape observed in experiment. As mentioned before, additional species emerge due to CO dissociation. As atomic carbon is expected to have a clear negative shift (calculated carbon CLS for atomically dissociated CO is −1.1 eV), the peak at 283.3 eV in the carbon XP spectra probably should not be attributed to atomic carbon. It is much more likely that dissociation of CO leads to additional carbon dimers on the surface. This is supported by the fact that the formation of carbon dimers on the surface is accompanied by an energy gain of about 0.1 eV: the calculated adsorption energy per CO is 2.95 eV if carbon dimers are formed vs 2.85 eV in case of atomic carbon. Oxygen, on the other hand, shows no preference to form dimers on the surface according to our calculations. The predicted shift of atomic oxygen is approximately −1.5 eV compared to oxygen in adsorbed CO molecules. Thus, the measured peak at 530.3 eV is attributed to atomic oxygen. To conclude, we interpret the experimental data as follows: at low temperature and low coverage, CO molecules bind to Mo surface atoms preferentially at sites not in contact with surface carbon dimers (sites A and D). Increasing CO coverage is accompanied by a calculated shift toward higher core level

occupied but their CLSs have increased at increased CO coverage, i.e., to 1.9−2.0 eV (cf. Tables S1 and S2). This is also nicely reflected in the simulated XP spectrum at this elevated coverage, see Figure S6 in the Supporting Information. In agreement to the calculations, in the experiments the contribution COM1 at lower C 1s binding energies is not found any more at higher coverages. This enables us to assign, according to the CLS, that the species COM1 is CO binding at site D. Since COM1 is the dominant species at low coverage in the experiment, site D may actually be more stable than site A. Going to even higher coverages of 10/16 ML (see Figure 3d and Figure S5 for a possible structure), the range of computed core level binding energies increases even more up to ∼1.9−2.3 eV (cf. Table S3), i.e., the XP spectrum exhibits a further shift of the CO-carbon peak, and moreover a broadening or shoulder at the high-energy edge emerges (see Figure S6). By comparison with experiment, we assign this high-energy contribution to species COM3. Structurally, it corresponds to CO molecules binding close to surface C-dimers at high coverage. In passing, we note that it has been previously suggested for CO on clean Mo(110) that core level binding energies are affected by the CO tilt angle with respect to the surface.17 At least, for the surface discussed here, the tilt angle, however, does not have a strong influence on the carbon core level energies. The average adsorption energy per CO molecule decreases with increasing CO coverage, e.g., from 1.79 (1/16 ML) to 1.55 (5/16 ML) to 1.24 eV (10/16 ML) if one considers the most stable configurations in this contribution. Our attempts to increase the CO coverage even more showed that it is not possible to get higher values than 11/16 ML of directly surface bound CO due to the increasing repulsion of neighboring CO molecules. Geometry optimizations of 12/16 ML CO and higher always led to maximally 11/16 ML surface-bound CO with the remaining CO molecules being squeezed to above the first CO adsorption layer. Moreover, already the binding of the additional CO from 10/16 to 11/16 ML is practically energyneutral, i.e., the surface is saturated with molecular CO at approximately 10/16 ML, which resembles the experimental value found during adsorption at low temperatures (sum of COM2 and COM3). Also that we find COM2 being the more E


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The Journal of Physical Chemistry C binding energies in agreement with experimental findings with CO showing a tendency to cluster around sites A and D. At higher CO coverages also other adsorption sites become more and more populated, and, again in agreement with experimental data, the calculated carbon CLSs become even more positive on average due to an increasing number of neighboring CO molecules and a decreased distance to surface carbon. Thus, the CO CLS seems to be related both to the number of adjacent CO molecules and surface C-dimers. From a structural point of view, CO molecules are pushed closer to surface carbon dimers by adjacent adsorbed CO molecules, and the tilt angle becomes smaller on average. The experimentally found species COM1, COM2, and COM3 can be identified, with COM1 being found at low coverages on sites located as far away from the surface carbon dimers as possible in the unit cell (sites A and D; by comparison with experiment, site D seems to be higher populated), the species COM2 is attributed to an ensemble of different adsorption sites at different coverages. Species COM3 can be assigned to CO at higher coverages close to surface carbon. Adsorbate−adsorbate interactions lead to a change in the CLS of COM1. In the experiment these molecules then become part of COM2. Besides that, it is experimentally observed that dissociation of CO occurs, which seems to require some activation energy, as spontaneous CO decomposition was not observed in the calculations (which refer to 0 K). CO Reaction on the Monolayer Carbide. To follow the thermal evolution of the adsorbates, we performed temperature-programmed XPS (TPXPS), that is, we continuously measured XP spectra, while heating to 550 K with a constant rate of 0.5 K/s. Above 550 K, flashes to higher temperatures were performed in steps of 50 K, after which the spectra were taken (during cool down of the sample). Selected spectra in the C 1s and O 1s regions are shown in Figure 4a and b. The corresponding quantitative analyses are depicted in Figure 2c and d, respectively. Upon heating, we immediately find a decrease of the COM3 species in both the C 1s and O 1s regions. Up to 300 K, COM3 vanishes, which is assigned to molecular desorption. Around 260 K, when most of the COM3 has disappeared, also the COM2 species start to decrease. This decrease is accompanied by an increase of the dissociated CO species (from ∼0.05 to ∼0.20 ML, see Figure 2c and d), due to the thermally activated dissociation of the COM2 species. The quantitative analysis shows that parallel to dissociation also desorption of the COM2 species occurs, in line with the TPD results, which will be discussed below. Moreover, the damping of the carbide peak at 282.8 eV is reduced due to desorption of CO. After the COM2 species has vanished above 400 K, the monolayer carbide surface (∼0.43 ML of carbide) is covered by ∼0.2 ML oxygen and ∼0.2 ML carbon. The total surface coverage is therefore ∼0.83 ML. The atomic carbon and oxygen species show almost no changes in intensity up to 800 K. Above that temperature, both decrease to finally vanish at 1150 K. This decrease is accompanied by the emergence of the clean carbide signal in the C 1s region at 282.8 eV at the cost of the CO-covered carbide signal. Above 1050 K, the original state of the carbide is restored, and only the clean carbide signal is observed in the C 1s region at 283.8 eV. To further investigate the desorption behavior of CO, we conducted TPD experiments. CO exposure was again carried out at 140 K, and the desorption of mass m/e = 28 was followed using a QMS, with a heating ramp at a rate of 2 K/s

up to 1200 K. Note that the experiments were performed in a different chamber than the one used for the XPS measurements; therefore, the exposure values are not comparable. Figure 5a shows selected TPD spectra for exposures from 0.25

Figure 5. Temperature-programmed desorption (TPD) spectra (m/e = 28) for various exposures of CO adsorbed on (a) the monolayer carbide and (b) the bulk carbide on Mo(110).

to 5 L. At exposures up to 0.5 L, we find one peak at 360 K and second peak at ∼860 K, with a broad shoulder at 890 K. At intermediate exposures (0.75−1.0 L), a third feature is found at 290 K, while the peaks at 360 K and above 800 K are saturated. Above 1 L, a very sharp additional peak appears at 260 K, with a weaker lower temperature shoulder at ∼210 K. We interpret these results as follows: The peaks 260 K and the shoulder at 210 K are assigned to the COM3 species, since this species decreases in the XP spectra of the saturated surface, while that of the COM2 species shows no decrease in this temperature range. From XPS, we deduce that the amount of desorbing CO from COM2 is ∼50% of the total desorbing CO. This corresponds to the peaks at 290 and 360 K, as determined from the integration of the TPD spectra (see Supporting Information). The peaks above 800 K are due to associative desorption of the adsorbed atomic oxygen and carbon,15 in agreement with the XPS results. CO Adsorption on the Bulk Carbide. Next, we discuss our findings for adsorption on the bulk carbide, Mo2C, which was prepared as described previously.14 Selected spectra of the adsorption experiment are shown in Figure 6a and b, and the quantitative analysis is depicted in Figure 7a and b, respectively. Note that compared to the monolayer carbide, we find a somewhat larger scatter of our data, which is due to the operational mode of the synchrotron with lower flux and less F


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Figure 6. XP spectra of adsorption of CO on a bulk carbide on Mo(110). On the left-hand side (a) selected C 1s spectra are shown and (c) the spectrum at saturation is shown including the fits. (b and d) Respective O 1s spectra.

during CO exposure on the monolayer carbide, the intensity of the initially appearing CO species, COB1, decreases at higher exposures, here from ∼0.22 ML at 0.5 L to ∼0.07 ML at saturation. This behavior hints at strong lateral interactions between the surface species. Moreover, during adsorption the surface carbide peak shifts to higher binding energy, ending up at 283.2 eV at saturation (see Figure 6c). As for the monolayer carbide, the shift reflects the interaction with the adsorbed species. Note that because of the similar binding energy of the bulk carbide at 283.4 eV and a possible atomic carbon peak due to dissociated CO (expected at 283.3 eV, see above), we cannot clearly separate the two signals. Consequently, in the quantitative analysis of the C 1s region in Figure 7a, we only consider the molecular CO species. At saturation above 2 L, we find a coverage of ∼0.07 ML for COB1 and ∼0.3 ML for both COB2 and COB3. In the O 1s region, initially a peak at 531.5 eV (COB1) grows and reaches its maximum at 0.5 L with a higher maximum coverage of ∼0.30 ML compared to the C 1s region. At higher exposures, it also decreases, while the COB2 and COB3 species at 532.4 and 533.2 eV grow. The total surface coverage of molecular CO as deduced from the O 1s data is 0.6 ± 0.1 ML, which is lower than that found in the C 1s spectra (0.7 ± 0.1 ML, Figure 7a). These differences in coverage, in particular also for the COB1 species, are attributed to differences in photoelectron diffraction for the two core levels at the low kinetic energies used. In addition to the peaks due to molecularly adsorbed CO, a peak assigned to atomic oxygen grows at 530.4 eV, similar as in the experiment on the monolayer carbide. From the quantitative analysis in Figure 7b, we find that ∼0.1 ML (∼15%) of CO dissociates during adsorption. The higher reactivity toward dissociation hints at an

Figure 7. Quantitative analysis of the XPS experiments with CO on the bulk carbide shown in Figure 6. (a and c) Analysis of the C 1s spectra of the adsorption and reaction and (b and d) the respective analysis of the O 1s region.

stable conditions (see Experimental Section). The solid lines in Figure 7 are a guide to the eye. The blue spectra in Figure 6a and b, were taken before adsorption. In the C 1s region, we find two sharp, asymmetric peaks of the bulk carbide: the surface peak at 283.0 eV, and the bulk peak at 283.4 eV. The third smaller peak at 282.4 eV is probably due to defects. Upon exposure to CO, in the C 1s region, first a peak at 285.2 eV (COB1) appears that reaches its maximum at 0.5 L, with a coverage of ∼0.2 ML. Note that the peaks show a lower full width half-maximum (fwhm) compared to the peaks of the monolayer carbide. Above 0.5 L, two additional peaks rise, at 285.9 eV (COB2) and 286.2 eV (COB3). Similar to the findings G


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Figure 8. Selected XP spectra of the heating experiment (TPXPS) of CO on the bulk carbide of Mo(110) for the (a) C 1s and (b) O 1s core levels.

K as compared to the COM1 species that vanishes at ∼400 K (cf. Figure 2 and 7). In the C 1s region, we find an increase in intensity at 283.4 eV, approximately at the position of the bulk carbide peak. This is due to formation of adsorbed carbon from the dissociation of CO. We cannot quantify the changes in the C 1s region, but a precise value is obtained from the O 1s region, where no overlapping signals are present. Between 500 and 800 K, the atomic oxygen signal (∼0.25 ML) stays constant. Above 800 K, it drops and vanishes around 1050 K. At the same time, in the C 1s region the surface carbide peak shifts to lower binding energy; at 1200 K it has reached its original position at 283.0 eV and the defect peak has reappeared at 282.4 eV. This shows that, as for the monolayer carbide, the adsorbed atomic carbon and the oxygen desorb associatively until the bulk carbide is restored to its original state before the adsorption experiment. In addition, we performed TPD experiments, shown in Figure 5b, to investigate the desorbing species. At 0.25 L, peaks at 420, 860, and 920 K are found. For exposures from 0.50 to 1.00 L, a second peak below the one at 420 K grows, which shifts to lower temperatures and is found at 310 K at saturation. Above 1.0 L, a third peak appears at 260 K, followed by a fourth one at 220 K above 2.0 L. In the Supporting Information, we present the integrated TPD spectrum for 10 L from Figure 5b, together with the O 1s data in the range of 140−500 K. In XPS, we find that COB3 desorbs in the range of 200−260 K, and we thus assign the TPD peak at 220 K to this species. The peak at 260 K is attributed to desorption of COB2, since COB1 shows no desorption in XPS below 260 K and COB3 already vanished at that temperature. The broad peak at 310 K is then due to desorption of both COB2 and COB1, since both species decrease in the O 1s spectra in this temperature range; COB2 does not vanishes until 380 K and a ∼20% decrease is found for COB1 in the range from 260 to 340 K. Finally, the peak at 420 K is unambiguously assigned to COB1. It is relatively small, because only 30−40% of COB1 desorb up to 460 K, while the other part dissociates. The two peaks at higher temperatures at 860 and 920 K are both attributed to associative CO desorption. The peak at 860 K is similar to the one of the monolayer carbide; the one at 920 K only exists for the bulk carbide.

increased amount of reactive sites for dissociation as compared to the monolayer carbide (∼7%). As in ref14., we considered a carbon-terminated quasihexagonal surface of orthorhombic Mo2C as structure model for the bulk carbide. The calculation of the CLS of the atomic surface carbon yields ∼−1.5 eV relative to the bulk carbon atoms, i.e. approximately a factor of 4 larger than in experiment, see Figure 6a. This may point to a problem with our structural model. For example, the experimentally studied surface may feature surface reconstructions. From these differences, the need for further theoretical investigations is obvious; they are, however, out of the scope of the present work. CO Reaction on the Bulk Carbide. The TPXPS measurements up to 1200 K were performed similar to the monolayer carbide. Selected spectra are shown in Figure 8a and b, and the corresponding quantitative analysis is shown in Figure 7c and d, respectively. Above 200 K, in the O 1s and C 1s region we observe a decrease of COB2 and COB3, accompanied by an increase of COB1. At 260 K, COB3 vanishes, while COB1 reaches a maximum at 0.25 ML in the O 1s and 0.16 ML in the C 1s region; the larger value in the O 1s region is again attributed to pronounced photoelectron diffraction effects for the COB1 species (see above). At the lower coverage, the suppression of the COB1 species by lateral interactions is reduced, leading to the increase of this peak, which is the reverse process to what was found during adsorption. Interestingly, in the C 1s region, after the disappearance of the COB3 species, the carbide surface peak is again clearly separable from the bulk carbide peak, as indicated in the spectrum at 261 K in Figure 8a with a red arrow. This hints at that the COB3 species more strongly interacts with the carbon atoms of the bulk carbide surface than COB1. Between 260 and 340 K, a decrease of COB1 by 20% occurs, while COB2 drops rapidly. Upon further heating to 380 K, the COB2 species vanishes, leaving ∼0.15 ML of both COB1 and atomic oxygen on the surface (note that the corresponding atomic carbon peak is at the same binding energy as the bulk carbide peak at 283.4 eV). The adsorbed oxygen species rises when COB1 decreases, with ∼0.1 ML (∼60−70%) of COB1 dissociating. This behavior is similar to what we found for the monolayer carbide. However, on the bulk carbide the process is shifted by ∼50 K to higher temperatures as compared to the monolayer carbide, that is, the COB1 species vanishes at ∼450 H


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CONCLUSION We performed an in situ XPS study of the adsorption and reaction of CO on a monolayer carbide and on a bulk carbide, both prepared on Mo(110) and carried out DFT slab calculations. The reactivity of both carbide surfaces is found to be similar. However, there are some interesting differences in adsorption behavior. At 140 K, on the monolayer carbide at low exposures one species, COM1, is found in XPS that is suppressed at high coverage by two other species, COM2 and COM3. On the bulk carbide, the behavior is similar: three different species (COB1−3) are observed; again the species initially observed at low coverages, COB1, is suppressed at high coverage by the other two species, probably due to lateral interactions. The total coverages of molecular CO are very similar, with ∼0.7 ± 0.1 ML and ∼0.6 ± 0.1 ML on the monolayer and bulk carbide, respectively. On both surfaces, a fraction of CO dissociates during adsorption at 140 K, with the bulk carbide showing a higher activity (15%, ∼ 0.1 ML) compared to the monolayer carbide (7%, ∼ 0.05 ML). Upon heating, most of the CO desorbs molecularly. On the monolayer carbide, COM3 desorbs up to 250 K, while ∼70% of COM2 desorbs between 250 and 400 K, and ∼30% dissociates. In the range from 400 to 800 K, the surface is only covered by the dissociation products, atomic carbon and oxygen, each at a coverage of roughly 0.20 ML. On the bulk carbide, the COB2 and COB3 species desorb up to 380 K. In contrast to the monolayer carbide, where the COM1 species observed is only found at low coverage and not during heating, the COB1 species increases on the bulk carbide to ∼0.25 ML at 260 K. This behavior indicates that lateral interactions are reduced, leading to the increase of the COB1 peak, which is the reverse process to what was found during adsorption. Between 300 and 460 K, ∼ 60−70% of COB1 dissociate and ∼30−40% desorb. Similar to the monolayer carbide, only atomic oxygen (and carbon, each ∼0.25 ML) are adsorbed between 460 and 800 K. Starting at 800 K, we find a decrease of the overall surface coverage on both carbides, which is due to the associative desorption of carbon and oxygen. This leads to a “clean” carbide surface as prepared before the adsorption experiments.

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support by the Fonds der Chemischen Industrie and the DAAD. Special thanks goes to the Helmholtz-Zentrum Berlin for allocation of synchrotron beam time and to the BESSY II staff for support during beam time.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11950. Additional XPS data, comparison of XPS and TPD data, calculated adsorption geometries, simulated XPS spectra, and calculated core level shifts (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(C.P.) E-mail: [email protected]. ORCID

Hans-Peter Steinrück: 0000-0003-1347-8962 Christian Papp: 0000-0002-1733-4387 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) within the Cluster of Excellence “Engineering of Advanced Materials”. Further, we acknowledge I


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