Catalytic Descriptors for the Design of Ziegler–Natta Catalysts

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Catalytic Descriptors for the Design of Ziegler−Natta Catalysts Revealed by the Investigation of the Cl−Ti(0001) Interaction by Density of States Calculations Emmanouil Symianakis,*,†,¶ Stavros Karakalos,‡,⊥ and Spyridon Ladas§ †

Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, United Kingdom FORTH/ICE-HT, P.O. Box 1414, GR-26504 Rion, Patras, Greece § Surface Science Laboratory, Department of Chemical Engineering, University of Patras, GR-26504 Rion, Patras, Greece ‡

ABSTRACT: The deposition of MgCl2 on Ti(0001) followed by annealing above 450 °C has been previously reported to result in the desorption of the Mg atoms, the formation of a Cl/Ti(0001) interface, and a new contribution at binding energy (BE) 3.5 eV. We present density of states results from DFT calculations obtained for the Cl/Ti(0001) interface and compare them with reported spectra obtained by photoelectron spectroscopy with synchrotron radiation. Our results suggest that the new contribution at BE 3.5 eV is a Cl 3p peak originating from Cl atoms relaxing at long distances from the surface, whereas the Cl 3p peak at BE 7.0 eV should be attributed only to Cl atoms strongly adsorbed at either one of the hollow sites of the Ti(0001) surface. Our calculations also support an interpretation for published experimental observations on the valence band of TiCl4 adsorbed on a Au surface. The comparisons of DFT calculations with experimental results highlight significant features of the Ti−Cl interaction at distances longer than the usual Ti−Cl bond, the high sensitivity of the BE of Cl 3s and Cl 3p peaks to the distance between Ti and Cl, as well as the influence of the interaction of the Cl atom with the substrate on the Ti− Cl bond and consequently on the Ti−C bond which determines the olefin polymerization. These observations provide an explanation for many features of the Ziegler−Natta heterogeneous catalysts, identify two catalytic descriptors, Ti−Cl bond length and the BE of the Cl 3p peak, and provide the basis for the development of future Ziegler−Natta catalytic systems engineered at the atomic level. experimental results reported by Karakalos et al.,14 Ri et al.,15 and Mousty-Desbuquoit et al.16 These comparisons allow us to gain new insight into the chemical interactions and the structural characteristics of the Cl/Ti interface. It has been reported14 that annealing of MgCl2/Ti(0001) deposits above 450 °C leads to desorption of Mg and formation of a Cl/Ti(0001) interface, while a peak at ∼3.5 eV BE was observed for the first time in the valence band spectra, suggesting a previously unreported interaction between Cl and Ti. The good agreement of DOS obtained from a range of Cl/ Ti(0001) systems with spectra from the valence band, including the Cl 3p region, suggests a new interpretation of the Cl 3p peak, normally found at a binding energy around 7 eV, and also an explanation for the appearance of the new contribution observed at ∼3.5 eV which is that it is also a Cl 3p peak originating from Cl atoms at large distances from the Ti(0001) surface. The structural information obtained by our DFT results is in good agreement with the interface description that was

I. INTRODUCTION Ziegler−Natta catalysts provide the technological solution for industrial polyolefin production1−3 by providing high yield and stereoregularity.4−6 However, it has been recognized that the optimization based on empirical data for heterogeneous olefin polymerization has reached a limit.6 Instead, the design of further improved Ziegler−Natta catalytic systems requires an understanding of the fundamental interactions that can be obtained through a surface science approach, by the study of well-defined model heterogeneous catalysts in controlled environments.6 In addition, the use of density functional theory (DFT) calculations is a complementary tool for the investigation of the fundamental interactions that lead to olefin polymerization. DFT calculations have been used to study the interaction of TiCl4 with a variety of MgCl2 surfaces7 and also surfaces with Cl defects,8,9 while the role of internal and external donors with respect to reactivity, stereoregularity,10−13 and the influence of defects13 has been also investigated. However, the fundamental interaction of Cl with Ti has only received sporadic attention14−16 and to our knowledge has never been investigated by means of DFT. In this work, we present density of states (DOS) results from the Cl/Ti(0001) interface obtained by DFT calculations and compare them to © XXXX American Chemical Society

Received: July 15, 2017 Revised: August 15, 2017 Published: September 10, 2017 A

DOI: 10.1021/acs.jpcc.7b06980 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C obtained by Ri et al.15 by means of diffuse low-energy electron diffraction (DLEED) from a disordered 0.2 ML chlorine overlayer. Furthermore, our results support an interpretation of the Cl 3s and Cl 3p splitting, observed by Mousty-Desbuquoit et al.16 from a system of TiCl4 molecules adsorbed on Au, to be due to an interaction of the TiCl4 with the substrate which results in modification of the Cl−Ti distances, giving also rise to the splitting of the Cl 3p peak. This sensitivity of the Ti−Cl bond length and the associated BE position of the Cl 3s and Cl 3p peaks to the interaction with their substrate can provide a fundamental explanation why TiCl4 and TiCl3 are not catalytically active without the presence of a support such as MgCl2, why the relatively less coordinated (110) and (104) surfaces are the relevant active surfaces for propylene polymerization,10,17 and also the role of surface steps and defects on the MgCl2 surfaces.8,9 Finally this work proposes two catalyst identifiers that can be used in the design of future Ti-based catalysts such as Ziegler−Natta, one being the Ti−Cl bond length and the other the position of the Cl 3p and Cl 3s contributions both accessible by DFT calculations and also by experimental methods such as EXAFS and SRPES.

III. RESULTS AND DISCUSSION III.1. Ti Bulk and (0001) Surface and Cl2 Molecule. Density of states (DOS) calculation has been performed for the relaxed bulk of Ti, and it is compared to DOS calculations previously reported for Ti32 but also to VB spectra obtained14 by X-rays with photon energy (PE) 60 eV (Figure 1). The

II. COMPUTATIONAL DETAILS The self-consistent density functional theory (DFT) calculations18,19 on the Ti bulk, Ti(0001) surface, and Cl/Ti(0001) interface systems were carried out with the exchange component of Perdew and Wang’s 199120 (PW91) generalized gradient approximation of the exchange-correlation functional21−24 within the Projector Augmented Wave pseudo potentials (PAW) scheme as implemented by VASP25−28 while the used pseudopotentials treat the Ti 3s and Cl 2s electrons as valence. The Kohn−Sham single electron wave function was expanded by plane waves with an energy cutoff of 450 eV which has been tested to converge for the bulk system of Ti. The surface slabs with (0001) orientation are produced by Material Studio visualizer29 and VESTA.30 The application of periodic boundary conditions in all three dimensions and an empty space with thickness of more than 10 Å and up to 34 Å is introduced perpendicular to the surface which was sufficient to avoid energy convergence slowdown due to the presence of the dipole moment of the Ti−Cl bond. The gamma scheme is selected with 25 × 25 × 1 k-points grid where convergence has been checked to be achieved for the case of Ti hcp bulk with 2 atoms and 25 × 25 × 25 k-points grid. The sigma factor is optimized at a value of 0.2. All calculations converged successfully using high-precision criteria as described in the manual of VASP. In our calculations, the Ti(0001) substrate is modeled by a slab of nine layers31 in order to obtain convergence on the properties of the surface. Up to six Cl atoms have been relaxed on the Ti(0001) surface, but relaxation of the sixth Cl atom resulted in lattice distortions deeper than the fourth Ti layer suggesting that our model’s predictions are reliable for up to five Cl atoms over the nine-layer Ti surface. However, those calculations correspond to experiments taking place at 0 K, a temperature at which Cl2 is expected to be in the liquid phase. Consequently, in our discussion and when comparing with experimental results taken at room temperature, we use the DFT results obtained from the models with up to three Cl atoms over the Ti(0001) surface. Due to the periodic boundary conditions, this corresponds to three monolayers of Cl over the Ti(0001) surface.

Figure 1. Comparison of the VB14 from a clean Ti(0001) surface using X-rays with 60 eV to the calculated DOS of the bulk Ti.

density of states obtained from a 9-layer slab is respectively compared to a VB spectrum obtained14 with PE 50 eV (Figure 2). The good agreement of the produced DOS to the valence band spectra suggests that our model can provide a reliable representation of the actual system.

Figure 2. Comparison of the VB14 from a clean Ti(0001) surface using 50 eV X-rays to the calculated DOS of the Ti(0001) surface as produced from a 9-layer slab shown on the right.

III.2. Cl2 Molecule. Since we investigate the relaxation and desorption of Cl from the Ti(0001) surface, we performed an additional calculation of the Cl2 molecule in order to obtain a reference point. The relaxation of the two Cl atoms is performed in an empty box with dimensions 40 Å × 40 Å × 40 Å. The Cl atoms distance is found to be 2.00 Å while the total energy is found to be −3.540 eV or −1.770 eV per Cl atom. III.3. Cl Relaxation on the Ti(0001) Surface. The process of Cl relaxation is started by relaxing a single Cl atom in the unit cell of the Ti(0001) surface using a variety of initial sites. These sites are the “on-top”, “hollow”, and “hollow-top”, as shown in Figure 3, as well as the bridge positions of the unit B

DOI: 10.1021/acs.jpcc.7b06980 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The two Cl atoms system has been relaxed starting from the following combinations, hollow-top and on-top, hollow and ontop, and hollow and hollow-top, Figure 5, and their reversed

Figure 3. Top and side views of the on-top, hollow-top, and hollow relaxed positions from right to left. Blue and green balls are used to represent Ti and Cl atoms, respectively.

cell. The Cl atom starting from the bridge position was found to consistently relax in the hollow position; therefore, bridge positions are not considered further in this study. From the total energy calculations and the relaxation distance from the Ti surface (Table 1), it can be concluded that Cl will preferentially

Figure 5. Top and side views of the (a) hollow and hollow-top, (b) hollow and on-top, and (c) hollow-top and on-top, the relaxed positions from right to left. Blue and green balls are used to represent Ti and Cl atoms, respectively.

Table 1. Result of the Total Energy Calculation in eV (Tot. En.),a Distance of the Cl Atoms from the Surface Layer (D), and Energy per Cl Atom in eV for Each Case of Cl Relaxation Presented in Figure 3 tot. en./eV D/Å energy per Cl atom/eV

slab

on-top

hollow-top

hollow

−68.143

−71.915 2.186 −3.772

−73.092 1.727 −4.949

−73.182 1.714 −5.039

height configurations. The total energy calculations indicate that each additional Cl atom adds to the total energy around −1.3 eV (Table 2). Since the energy per atom in the Cl2 Table 2. Result of the Total Energy Calculation in eV (Tot. En.), Distance of the Outermost Cl Atoms from the Surface Layer (D), Cl−Cl Distance (D′ ), and Additional Energy Due to the Presence of the Second Cl Atom in eV (En. per Cl2nd)

a

The case of the slab without Cl atoms corresponds to the nine Ti atoms forming the (0001) surface.

be found at the hollow and hollow-top positions where Cl is associated stronger. The DOS calculations, performed on the relaxed configurations, suggest a relation between the distance of the adsorbed Cl atoms to the Ti surface layer and the binding energy of the Cl 3s and Cl 3p peaks. The Cl 3s and Cl 3p peaks of hollow and hollow-top atoms have higher binding energies by ∼2 eV when compared to the on-top atom (Figure 4).

relaxation

tot. en./eV

D Cl2nd/Å

D′/Å

en. per Cl2nd/eV

hollow, hollow-top hollow, top hollow-top, top

−74.498 −74.497 −74.390

5.604 4.915 5.047

4.252 3.642 3.753

−1.316 −1.315 −1.298

molecule is −1.770 eV, it can be concluded that these additional Cl atoms will combine to form free Cl2 molecules if enough kinetic energy is provided to overcome the associated energy barriers. The DOS calculations predict that the Cl 3s and Cl 3p peaks should have two separate contributions at different binding energies. Each contribution originates from a Cl atom while proximity of each atom to the Ti surface seems to be the determining factor for the position of those peaks (Figure 6). Further continuation following the two Cl atoms runs is performed with the addition of a third atom over the remaining adsorption site results to the relaxation runs with three Cl atoms while reversal of the height in pairs was performed to enrich the number of starting positions of the relaxation runs. Following this relaxation, a fourth and a fifth atom are added and relaxed to establish the relation between the binding energy position of the Cl 3p peak and the distance of the relaxed atoms to the Ti surface layer. The energy contribution of each additional Cl atom is consistently higher than −1.770 eV, suggesting that these Cl atoms are thermodynamically available for the formation of Cl2 molecules that are expected to desorb upon formation. Furthermore, the Cl atoms that have relaxed at heights above 5 Å appear to have their Cl 3s and Cl 3p orbitals at binding energies around 3.5 eV closer to the Fermi level as shown by the DOS calculations in Figure 7. Finally the distance between the Cl layers is found to be 3.5−4.5 Å, which is twice the distance of 2.0 Å calculated for the Cl2 bond. This result implies

Figure 4. DOS comprised by the sum of spin up and spin down in the total density of states: (A) total DOS as calculated for one Cl atom at the on-top position. The individual components of Cl 3s and Cl 3p are also presented at (B) on-top, (C) hollow-top, and (D) hollow positions.

Continuation of the runs after the relaxation of the single chlorine atoms is performed with the addition of a second Cl atom over one of the other sites. In order to explore the consistency of our relaxation approach and also the effect of various configurations on the DOS calculations, the relaxations are also performed after reversing the initial distance of the Cl atoms to the Ti surface. C

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ment of Cl atoms over the Ti(0001) surface might differ if a larger supercell is employed in the calculation, which would allow more free relaxation of the Cl atoms over the Ti(0001) surface, we do not expect that such a calculation would affect the structural results obtained for the first Cl atom which forms a direct Ti−Cl bond. This expectation is reinforced by the good agreement of our results with the work of Ri et al,15despite the fact that we expect that the second and third Cl layers would be affected by the additional freedom and result in more complex arrangements. This is also supported by the LEED measurements performed by Karakalos et al.14 who reported that there is no observable structure on the Cl/Ti(0001) interface studied in their experiments where more than one monolayer of Cl is found on the surface. However, calculations with an extended supercell would obtain a small number of the possible configurations expected to be found on a liquid−solid interface such as the Cl/Ti(0001) when at 0 K. Those configurations, however, would not necessarily be more representative of the actual Cl/Ti(0001) system and would not provide any additional information on the Cl−Ti interaction which is the main subject of this work; consequently, they will not be considered further in this study. Both of the Cl atoms found at the hollow sites will present a Cl 3p peak at binding energy of 7 eV, according to our model. Any additional Cl atoms will adsorb at a higher distance from the Ti surface, around 5 Å for the second monolayer (Table 2), with the total energy calculations suggesting that the additional energy per atom is around −1.3 eV, higher than the −1.7 eV per atom found in the Cl2 molecule. In this case the Cl 3p peak is predicted by the DOS calculations to appear at 3.5 eV lower binding energy at around 3.5 eV. This binding energy coincides with the new contribution identified by Karakalos et al.14 Furthermore, according to our calculations, all additional Cl atoms at distances higher than 5 Å from the surface present their Cl 3p and Cl 3s contributions at around 3.5 eV lower binding energy. The results of Karakalos et al. indicate that upon annealing of the sample the quantity of the Cl from the surface is reduced, according to the Cl 2p core peak, and our proposed model claims that the Cl 3p at binding energy of 7 eV is produced only by the strongly absorbed Cl atoms at the two hollow sites. In order to test this claim, we measured the intensities ratio of the Cl 3p/Cl 2p spectra during annealing, which are reported in the publication of Karakalos et al.14, and the intensities ratio is plotted versus each annealing in Figure 8. The plot suggests

Figure 6. (A) Total DOS comprised by the sum of spin up and spin down as calculated from two Cl atoms at the hollow and hollow-top positions of the Ti (0001) surface, Figure 7a. The individual components of Cl 3s and Cl 3p are presented below for the cases of (B) hollow and hollow-top, Figure 7a. (C) Hollow and on-top, Figure 7b. (D) Hollow-top and on-top, Figure 7c. The numbers 1 and 2 signify the proximity of the Cl atoms to the Ti surface with 1 being the closest and 2 the furthest.

Figure 7. (A) Spectrum14 obtained with 50 eV PE after annealing at 450 °C; (B), total DOS of 3 Cl atoms on Ti(0001); and (C, D, and E) individual components of Cl 3s and Cl 2p for each Cl atom, comprised by the sum of spin up and spin down in the total density of states, with C being the closest to the surface and E the farthest.

that Cl molecules are formed by association of Cl atoms of the same layer possibly due to thermal vibrations parallel to the surface. The good agreement of the DOS obtained from a 3 Cl/ Ti(0001) system to a spectrum obtained with 50 eV after annealing of the sample to 450 °C14 (Figure 7) suggests that our model can provide a plausible explanation for the appearance of the new peak at binding energy of 3.5 eV. Although our model predicts the new contribution reported by Karakalos et al.,14 it is reasonable that we should attempt to validate farther our proposed interpretation by comparison of our model’s predictions to additional experimental results that are available in the publications of Ri et al.15 and Karakalos et al.14 According to the proposed model, one Cl atom per surface unit area of the Ti(0001) will absorb strongly, with energy around −5 eV per Cl atom, at either of the hollow positions at a distance around 1.7 Å. This result is in agreement to the findings of Ri et al.15 who studied the deposition of 0.2 ML of Cl over the Ti(0001) surface by means of DLEED. The only disagreement appears to be with their estimation of the Ti−Cl interlayer distance of 2.5 ± 0.7 Å which is considerably higher than that of 1.7 Å (Table 1) that we report. However, they used a semi-empirical method for their calculation that could only provide a rough approximation at best. Although the arrange-

Figure 8. Cl 3p/Cl 2p intensities ratio vs annealing temperature after annealing at temperatures up to 730 °C using X-rays with PE 140 eV. Cl 2p is obtained with X-rays of PE 350 eV. D

DOI: 10.1021/acs.jpcc.7b06980 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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provides a reasonable explanation for the new peak observed at BE of 3.5 eV. In order to strengthen the reliability of our interpretation for the new peak at BE of 3.5 eV, we compared some of our models predictions against additional experimental results. Specifically we attempted to verify the prediction of our model that the Cl 3p peak found at BE of 7 eV originates only from Cl atoms that are strongly bounded in one of the hollow positions, and we investigated the evolution of the Cl 3p(7 eV)/Cl 2p intensities ratio, calculated from the spectra published by Karakalos et al.,14 during the annealing cycles. The resulting plot, Figure 8, suggests that the Cl 3p(7 eV)/Cl 2p intensities ratio evolves during annealing while the absolute value of the Cl 3p(7 eV) peak remains unchanged, in agreement with the predictions of our proposed interpretation. This description also provides a reasonable interpretation for the observations of Mousty-Desbuquoit et al. regarding the TiCl4/Au interface and the Cl 3s and Cl 3p splitting and attributes them to the interaction of the Cl atoms to the Au surface.

that the Cl 3p and Cl 2p do not have a steady ratio until the final annealing at 730 °C. However, at 730 °C the peak at 3.5 eV has been reduced below the detection limit of the analyzer for the used conditions. This result supports the proposed model which predicts that the Cl atoms at the hollow sites are strongly bonded and would not desorb at any annealing below the melting temperature of metallic Ti. Finally our model allows for an interpretation of the Cl 3s and Cl 3p splitting of around 3 eV in both cases, observed by Mousty-Desbuquoit et al.16 in an investigation of TiCl4 molecules adsorbed on gold-plated surface. The observed Cl 3s and Cl 3p splitting can be attributed according to MoustyDesbuquoit et al. to one of three factors, either a Cl−Cl interaction that may occur within the TiCl4 molecules, a Jahn− Teller effect due to an electronic asymmetry caused by the photoelectric phenomenon, or finally due to a solid state effect. Our proposed model supports the interpretation of a solid state effect, and specifically it allows us to visualize the pyramidshaped molecules to have three Cl atoms in contact with the gold surface and the fourth one far from the surface and above the Ti atom. The expected charge transfer between the Cl atoms and the gold surface, which back in 1983 was not thought possible to occur, would result in a modification of the Cl−Ti bonds compared to that of the fourth Cl atom and according to the predictions of our models to a shift of the position of the binding energies of the Cl 3s and Cl 3p peaks, giving rise to the observed split. This new interpretation provides not only a validated explanation for the experimental observations made by Karakalos et al. and Mousty-Desbuquoit et al. but also demonstrates the most critical factors affecting the success of the Ziegler−Natta catalysts, that is, the sensitivity of the Ti−Cl bond length, of the TiCl4 and TiCl3, and the associated BE position of the Cl 3s and Cl 3p peaks to the interaction with their substrate. It is the relaxation of the Ti−Cl bonds in the TiCl4 or TiCl3 molecules that determines the Ti−C interaction which, among other factors such as the presence of electron donors, controls the polymerization of olefins.

V. CONCLUSIONS Our model prediction of the DOS is in good agreement with previous DOS calculations of Ti(0001) found in the literature and to experimental valence band measurements. The structural information on the Cl/Ti(0001) interface predicted by our model is in good agreement with experimental DLEED results reported by Ri et al.15 The above observations suggest that our model is a reliable tool for the investigation of the Cl/ Ti(0001) interface. The proposed model suggests that Ti−Cl interactions occur even at distances larger than the typical Ti− Cl bond length while the BE positioning of the related Cl 3s and Cl 3p peaks is very sensitive within the distance range between 1.7 and 5.0 Å. This interpretation provides a plausible explanation for all the available experimental results as discussed above but also provides an explanation for the Cl 3s and Cl 3p splitting reported by Mousty-Desbuquoit et al.16 who investigated the adsorption of TiCl4 molecules on a goldplated surface by XPS and UV spectra. This new interpretation demonstrates the most critical factor affecting the success of the Ziegler−Natta catalysts, that is, the sensitivity of the Ti−Cl bond length and the associated BE position of the Cl 3s and Cl 3p peaks to the interaction with their substrate. It is the relaxation of the Ti−Cl bonds in the TiCl4 or TiCl3 molecules that determines the Ti−C interaction which, among other factors such as the presence of electron donors, controls the polymerization of olefins. This interpretation provides an explanation as to why TiCl4 and TiCl3 are not catalytically active without the presence of a support such as MgCl2 that can play the role of the regulator by modifying the Ti−Cl bond length and the BE of the Cl 3s peak toward the catalytically active values, why the relatively less coordinated (110) and (104) surfaces are the relevant active surfaces for propylene polymerization,10,17 and also the role of surface steps and defects on the MgCl2 surfaces.8,9 Finally this work proposes two catalyst identifiers that can be used in the design of future Ziegler−Natta catalysts, one being the Ti−Cl bond length and the other the position of the Cl 3p contributions, both accessible by DFT calculations and also by experimental methods such as EXAFS and SRPES.

IV. SUMMARY Annealing at 450 °C of a sample comprised of MgCl2 that was deposited on a Ti(0001) substrate resulted in the removal of Mg from the surface and the appearance of a new peak contribution at BE of 3.5 eV. This observation motivated us to perform DOS calculations in order to establish a plausible explanation for the observed peak. The DFT calculations yielded a good reproduction of the valence band region of the clean Ti(0001) surface, X-rays with 50 eV, while spectra obtained with higher energy of X-rays, with PE 60 eV, gave a good agreement with the DOS of the bulk of pure metallic Ti. Relaxation calculations of Cl atoms on the Ti(0001) surface suggest that the Cl atoms will preferentially relax in one of the hollow positions, in agreement with the experimental results reported by Ri et al.15 who used the DLEED technique to investigate 0.2 ML of Cl on Ti(0001). Furthermore, our results suggests that the position of the Cl 3s and Cl 3p peaks is affected by the proximity of the Cl atoms to the Ti surface in the range of distances between 1.7 and 5.0 Å. Specifically, the Cl atoms relaxed at 1.7 Å above the Ti surface have their Cl 3p photopeak at BE of 7 eV, and those relaxed over 5 Å present their Cl 3p peak at around 3.5 eV lower BE while the same trend is observed for the Cl 3s peaks. This result E

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. ORCID

Emmanouil Symianakis: 0000-0003-4028-0484 Stavros Karakalos: 0000-0002-3428-5433 Present Addresses ¶

Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, Patras GR 265 04, Greece. ⊥ Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, SC 29208, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support and scientific advice of Prof. Nickolas Harrison and Prof. Anthony Kucernak during this project, both from the Department of Chemistry, Imperial College London.

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ABBREVIATIONS DFT, density functional theory; DOS, density of states; XPS, Xray photoelectron spectroscopy; DLEED, diffuse low-energy electron diffraction; SRPES, synchrotron radiation photoelectron spectroscopy

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REFERENCES

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DOI: 10.1021/acs.jpcc.7b06980 J. Phys. Chem. C XXXX, XXX, XXX−XXX