Learning from Nature: Charge Transfer and Carbon Dioxide Activation

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C: Physical Processes in Nanomaterials and Nanostructures

Learning from Nature: Charge Transfer and Carbon Dioxide Activation at Single, Biomimetic Fe Sites in Tetrapyrroles on Graphene Manuel Corva, Fatema Mohamed, Erika Tomsic, Matteo Rinaldi, Cinzia Cepek, Nicola Seriani, Maria Peressi, and Erik Vesselli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11871 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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The Journal of Physical Chemistry

Learning from Nature: Charge Transfer and Carbon Dioxide Activation at Single, Biomimetic Fe Sites in Tetrapyrroles on Graphene Manuel Corva,†,‡ Fatema Mohamed, †,# Erika Tomsic,† Matteo Rinaldi,† Cinzia Cepek,‡ Nicola Seriani,# Maria Peressi,†,‡ Erik Vesselli†,‡,*

†

Physics Department, University of Trieste, via Valerio 2, 34127, Trieste (Italy)

‡CNR-IOM,

#The

Area Science Park, S.S. 14 km 163.5, 34149 Basovizza, Trieste (Italy)

Abdus Salam Centre for Theoretical Physics, Strada Costiera 11, 34151, Trieste (Italy)

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ABSTRACT: Nature determines selectivity and activity in biological reactive centers, based on single metal atom macrocycles, by properly tuning the primary coordination sphere and the surrounding protein scaffold. In a biomimetic approach, we show that activation of carbon dioxide at a 2D crystal of phthalocyanines supported by graphene can be controlled by chemical tuning of the position of the Dirac cones of the support through oxygen adsorption. The room temperature stabilization of the CO2-Fe chemical bond, detected in situ and confirmed by computational density-functional theory simulations, is obtained by governing the charge transfer across the graphene-metalorganic layer interface upon oxidation of graphene at close-to-ambient conditions. In this way, we are able to turn a weakly binding site into a strong one in an artificial structure that mimics many features of complex biological systems.

1. INTRODUCTION Carbon dioxide is a greenhouse gas produced in large amounts by human activity, and one of the key challenges humankind has to face is that of becoming able to recycle it on a massive scale through chemical conversion.1–4 The closed shell nature of the molecule makes it however poorly reactive, with small binding energies at catalytic centers for its conversion.5,6 Peculiar molecular activation mechanisms are associated with a substrate-to-adsorbate charge transfer, which yields a stable CO2- reactive precursor,7–11 like for example in the case of chemisorption at surfaces.12–16 For this reason, developing materials for conversion of carbon dioxide is a central issue. On the other side, in Nature selected living organisms have developed strategies to deal with this problem efficiently, through the production of specialized biomolecules. This has spurred considerable interest in the design, synthesis, and control of 2 ACS Paragon Plus Environment

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biomimetic systems, which take advantage of chemical strategies present in natural systems, but can be handled and controlled in a lab environment. Interesting approaches may be therefore based on biomimetic principles, thus taking inspiration from the architecture of reactive sites present in biomolecules widespread in Nature.17 Acetogenic bacteria exploit for example dedicated enzymes for the fixation and conversion of CO2 at nanostructured reaction sites.18 In a challenging perspective, catalytic cages in enzymes may be tuned or modified to yield specific selectivity or kinetic properties, in the view of merging chemical and biological catalysis.19 Simple (electro)-catalysts for carbon dioxide conversion engineered on metallorganic macrocycles and porphyrin-like structures have already proven enhanced efficiency, stability, and selectivity with respect to conventional materials.20,21 Recently, more complex, bioinspired self-assembled artificial nanostructures consisting of metal-organic frameworks have appeared as promising catalyst candidates, with the drawback of a yet poor engineering and predictable control on selectivity and conversion efficiency. 22 We propose here a method to tune the properties of a 2D metallorganic crystal with respect to carbon dioxide activation and base our approach on the fast developing physics of heterostructures, consisting in the stacking of two-dimensional materials with peculiar, selected properties.23 The results imply applicative potentiality for the CO2 conversion processes based on (electro)-catalytic approaches.24 We combine a thorough surface science, single atom detail approach in order to shed light into the fundamental mechanisms involved, and exploit an experimental technique based on non-linear optics that allows extension of the investigation range to the near-ambient pressure regime, thus close to biomimetic and operative conditions. In the specific, we show how control of the charge transfer from a 2D substrate (graphene GR) to a biomimetic molecule (iron phthalocyanine - FePc) can be exploited to tune the adsorption properties of the metallorganic layer within the picture of the trans-effect.25 The



mechanism is similar to the selectivity tuning of heme-like proteins in Nature and to wellknown charge transfer and doping phenomena occurring in thin films and heterostacks.17,23

2. METHODS 2.1 Sample Preparation. An Ir(111) single crystal disc (8 mm diameter, Mateck) was suspended with Ta wires to allow for resistive heating. The sample was cleaned by standard cycles of Ar+ sputtering and annealing in Ultra-High Vacuum (UHV), alternated with oxygen treatments. Temperature was monitored by means of a K-type thermocouple. Monolayer graphene was obtained by chemical vapor deposition and thermal cracking cycles of ethylene on the basis of established procedures.26,27 Quality of graphene was checked by LEED, while integrity of the sheet was checked by CO adsorption experiments performed at room temperature. CO does not stick on a complete graphene sheet and intercalation occurs at mbar pressure only if defects are present.27 In the latter case, CO molecules adsorb at the Ir(111) surface and contribute with a very intense feature in the InfraRed-Visible Sum Frequency Generation (IR-Vis SFG) spectra. FePcs were purchased from TCI Europe (I0783-1G, 132-16-1, purity 98%) and evaporated from a quartz crucible at a background pressure in the low 10-10 mbar. 2.2 IR-Vis Sum Frequency Generation Spectroscopy. IR-Vis SFG vibronic spectroscopy measurements were performed in a dedicated setup.28 A UHV system with a base pressure of 5 × 10-11 mbar hosts standard surface science preparation and characterization techniques and is directly connected to a high-pressure cell for in situ IR-Vis SFG spectroscopy. The reactor is equipped with a gas system to handle the reactants’ pressure in the 10-9-10+2 mbar range. The inlet and outlet of the laser beams are provided by UHV-compatible BaF2 windows. The Ir(111) disc was mounted on Ta wires, used also for resistive heating. The 4 ACS Paragon Plus Environment

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excitation source (Ekspla) delivers a 532 nm (2.33 eV, 30 ps, 50 Hz, 0.01 mJ/pulse at 1% power) visible beam and tunable IR radiation in the 1000-4500 cm-1 range. The SFG spectra were normalized by the non-resonant signal generated by a GaAs crystal in order to account for modulations in the IR intensity, both intrinsic and due to gas phase adsorption along the optical path. The data were then analyzed by least-squares fitting to a parametric, effective expression of the nonlinear second-order susceptibility.28,29 The expression (reported here below) well reproduces the observed lineshapes, accounting for the resonant IR-Vis vibronic transitions and for the nonresonant background, and describing all the interference terms: 𝐼𝑆𝐹𝐺(𝜔𝐼𝑅) 𝐼𝑣𝑖𝑠𝐼𝐼𝑅(𝜔𝐼𝑅)

|

∝ 𝐴𝑁𝑅𝑒𝑠 +

∑𝜔 𝑘

|

𝐴𝑘𝑒𝑖∆𝜑𝑘

𝐼𝑅

2

― 𝜔𝑘 + 𝑖Γ𝑘

ANRes and Ak account for the amplitudes of the nonresonant and kth-resonant contributions, respectively; k is the phase difference between the kth-resonant and nonresonant signals; k is the energy position of the line; k is the Lorentzian broadening, related to the dephasing rate, which in turn stems from the energy lifetime and from the elastic dephasing of the excited vibronic state.30 In the manuscript’s figures we plot the normalized IR-Vis SFG signal intensity together with the best fit (blue lines). We also plot (color-filled curves) the intensity of selected resonances and their interference with the nonresonant background by calculating, with the parameters obtained from the fitting procedure, the following quantity: 𝐼𝑆𝐹𝐺,𝑘(𝜔𝐼𝑅) 𝐼𝑣𝑖𝑠𝐼𝐼𝑅(𝜔𝐼𝑅)

|

∝ 𝐴𝑁𝑅𝑒𝑠 +

𝐴𝑘𝑒𝑖∆𝜑𝑘

|

2

𝜔𝐼𝑅 ― 𝜔𝑘 + 𝑖𝛤𝑘

These plots directly put in evidence the amplitude and the relative phase for each of the resonances. Further details can be found in our previous work.28 In the present study, all spectra were collected in the ppp polarization configuration (SFG-visible-infrared).



2.3 X-Ray Photoelectron Spectroscopy. X-Ray Photoelectron Spectroscopy (XPS) data were collected in a dedicated setup under UHV conditions and at room temperature, at normal emission and exploiting a conventional Mg K X-ray source (h = 1253.6 eV) combined with a hemispherical electron energy analyzer. The overall energy resolution was 0.8 eV. 2.4 Numerical simulations. All calculations were performed in the framework of spinpolarized Density Functional Theory (DFT)31 as implemented in the Quantum Espresso suite,32 with a plane-wave basis set and Vanderbilt ultrasoft pseudopotentials.33 Energy cutoffs of 30 and 300 Rydberg were employed for wavefunctions and electron density, respectively. Exchange and correlation were described by the functional of Perdew, Burke and Ernzerhof (PBE),34 together with a Hubbard correction (DFT+U),35 acting on the 3d states of iron. Using the formulation of Dudarev et al.36 A Hubbard parameter U = 1 eV was chosen, in order to reproduce experimental photoemission spectra of FePc molecules by Brena et al.37 and the electronic states around the Fermi energy, a condition that was checked by comparing the observed and simulated Scanning Tunneling Microscopy (STM) images of the molecular arrays. Moreover, van der Waals interactions were included using the Grimme potential approach.38 Methfessel-Paxton broadening was used for the electronic occupation,39 with a smearing of 0.02 Ry. For the array of FePc molecules an orthorhombic unit cell with sides about of 14.8 Å was considered as initial configuration. The optimized structure was obtained via the variable-cell method and by the minimization of both atomic forces and total energy, giving in-plane dimensions of 14.87 Å and 14.76 Å, respectively (Figure S4, Tables S1 and S2), close to what determined in the literature based on experimental information.40 A (2 × 2 × 2) Monkhorst and Pack grid of k-points was used for the Brillouin zone integration.41 Relaxation was performed until forces were smaller than 0.004 eV/Å.


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The substrate, which is not commensurate with the FePc monolayer, was not taken into account. The negative doping, experimentally obtained by oxidizing the graphene sheet, was described by adding two electrons to each FePc. Calculations for the positively doped FePc molecule, described by subtracting electrons, were also performed. The adsorption energy of the CO2 molecule on the FePc was calculated as Eads = Echtot - ECO2 - EchFePc, where Echtot is the total energy of the system, ECO2 of a free standing CO2 molecule, and EchFePc of the FePc. The superscript “ch” is referred to the charge state of the system, i.e., 0 for neutral systems and +2e- for negatively charged systems. The isolated CO2 molecule was always considered neutral. The phonon calculations were performed allowing to move only the CO2 molecule and the Fe atom, and without the Hubbard correction. However, since the equilibrium geometry was not affected, we do not expect important variations on the vibrational frequencies with/without the Hubbard correction.

3. RESULTS AND DISCUSSION 3.1 Experimental We grow a single monolayer, self-assembled 2D crystal of FePc molecules on a single GR sheet. The growth of the metallorganic framework on GR yields a regular array of single metal atom reactive sites (the Fe centers), distributed on an almost square lattice with a Fe-Fe distance of about 15 Å. As determined by STM, the actual cell measures 14.1 × 13.6 Å2, with an internal angle of 87°, and is rotated by 13° with respect to the underlying graphene lattice, while the FePc molecules are rotated by 18° with respect to the FePc lattice.40 Each FePc corresponds to about 40 GR cells. The underlying graphene layer acts both as a support and as a tuning knob. We obtain the almost decoupled monolayer GR by chemical vapor deposition 7 ACS Paragon Plus Environment


and decomposition of ethylene on the Ir(111) single crystal surface,42,43 showing only slight pdoping, as Dirac points are found at a binding energy of only 0.067 eV above the Fermi level.42– 44

It is known that under model UHV conditions a gap can be artificially induced, associated

with a shift of the Dirac cones below or above the Fermi level, by oxidizing GR,42,45,46 or by intercalating oxygen beneath it,43 thus by n- or p-doping it, respectively. We show here that we can oxidize GR with molecular oxygen under near-ambient pressure conditions, thus n-doping the FePc-graphene heterostack. The charge transfer at the organic/inorganic interface to the FeN4 centers activates the framework towards CO2 adsorption that becomes favored. Consequently, carbon dioxide binds chemically to Fe already at 0.05 mbar in a “V” shaped, highly reactive configuration, at variance with the uncharged system at which only the physisorbed, linear CO2 configuration is stable and no adsorbed state is detected, at least up to two orders of magnitude higher pressure. The IR-Vis Sum-Frequency Generation (IR-Vis SFG) vibronic spectrum of the as grown FePc/GR heterostack on Ir(111) shows under UHV the predominant presence of the GR optical G mode at 1614 cm-1 (Figure 1A). At lower energy, two internal modes of the FePc are detected, associated with the benzene scissoring at 1335 cm-1 and with the B2 mode, involving the tetrapyrrole ring and the isoindoles, at 1398 cm-1.47 Upon exposure of the system at room temperature to 5 mbar CO2, no significant modifications of the vibronic spectrum of the FePc are detected in situ, and we do not observe any vibrational fingerprint associated with adsorbed carbon dioxide (Figures 1B’ and S1). The very weak CO2-Fe interaction yields no significant adsorption at 5 mbar at room temperature. When we expose the FePc/GR system to 100 mbar O2, we observe a strong increase in the amplitude of both the non-resonant background and of the resonance associated with the GR phonon that also undergoes a redshift from 1614 to 1609 cm-1 (Figures 1B and 2, Figures S2 and S3, Table S3). X-Ray Photoelectron Spectroscopy (XPS) core level spectra collected in UHV (Figure 3) after oxidation of bare graphene show a 8 ACS Paragon Plus Environment

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broad O 1s component at 532.5  0.1 eV. Concerning the C 1s region, the spectral feature associated with the pristine GR sheet slightly shifts from 284.01  0.05 to 284.09  0.05 eV, while new components appear at 284.67  0.05, 285.50  0.05, and 286.19  0.05 eV. Based on literature, reporting UHV oxidation of GR/ Ir(111) by means of atomic oxygen beams,42,45,46 we can definitely conclude that graphene oxide (GRO) is obtained in this way. We associate the new core level components at 532.5, 285.50, and 286.19 eV with the predominant presence of epoxy species at the “pores” and “wires” of a highly corrugated GRO sheet,45,48 and the peak at 284.67 to other O-induced structures and defects.45,46 The higher corrugation of the GRO sheet with respect to the pristine GR and a strong charge transfer account for the increased SFG signal amplitude (Figures 1A-B, 2 and S2A-B). Indeed, a more corrugated structure yields a larger dipole component in the direction of the surface normal, actually contributing to the SFG intensity.49 Moreover, the proportionality between the non-resonant and the resonant parts of the spectra (Figure 2) suggest the electronic origin of this effect. Upon oxidation of the GR sheet we also observe a phase change of the internal vibronic mode of the adsorbed FePcs (Figure 2), a further indirect proof of a modified electronic configuration of the system in proximity of the Fermi level. The phase shift of -60  5° accounts for the shape change of the IR-Vis SFG spectral contribution of the FePcs (Figure 1A-B, left, brownish deconvolution). These observations fit with the n-doping of GR induced by the oxidation process that introduces a pronounced energy bandgap at the Fermi level (> 0.35 eV), a deformation of the dispersion from linear to parabolic, and a downshift of the valence band below the Fermi edge.42 This effect is associated with the sp3-like character of the C atoms involved in the formation of the epoxy groups at the expense of the sp2 graphene hybridization. Oxygen intercalation at the GR/Ir(111) interface is excluded, since the effect would be opposite, contributing with a p-doping of GR. In particular, in the latter case an upward shift of the GR Dirac cone across the Fermi level by 0.6 eV would be expected. Moreover, oxygen would 9 ACS Paragon Plus Environment


decouple GR from the surface, thus lifting the sheet corrugation. Finally, the C 1s peak associated with GR would shift to lower binding energy (283.60 eV) and an associated O 1s core level component due to atomic oxygen adsorbed at the Ir(111) metal surface would appear at 529.8 eV.43 When we expose to 0.05 mbar of carbon dioxide the FePc/GRO heterostack, obtained by room temperature self-assembly and near-ambient pressure oxidation, several spectral modifications are observed in situ. In the specific, IR-Vis SFG reveals (Figure 1C) the growth of two peaks at 1535 and 1729 cm-1 that, on the basis of literature data, can be associated, respectively, with an internal mode of the FePc (stretching of the porphyrazine),44 and with the antisymmetric internal stretching mode of an activated, adsorbed carbon dioxide species.6,24,50 The latter is expected to bind in a bent, “V”-shaped configuration at the Fe metal center, in association with a charge transfer from the metal to the molecule.8,24,50 Adsorption of the ligand occurs at the macrocycle and this is confirmed by the SFG spectral features associated with the internal FePc modes (Figure 1C, left). Indeed, we assist to a general increase of the SFG signal amplitudes (Figures 1 and 2), related with a further deformation of GR and of the tetrapyrrole macrocycle, with a reorientation of the benzene rings, and with the charge transfer from the macrocycle to the ligand. We also observe a further phase change of the vibronic fingerprint of the macrocycle mode, shifting by -260  5° with respect to the pristine FePc/GR heterostack and by -200  5° with respect to the modified FePc/GRO (Figure 2). The lineshape change appears clearly in the deconvolution (brownish) of the spectra shown in Figure 1A-B-C. Furthermore, the process is reversible: when pumping out the CO2 gas phase and recovering UHV conditions, CO2 desorption occurs (Figure 1C-D) and all spectral features are accordingly restored. 3.2 Theoretical DFT calculations confirm and support the whole picture. We performed simulations of CO2 interacting with the FePc monolayer in order to investigate adsorption geometries, binding 10 ACS Paragon Plus Environment

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energies, and charge redistribution effects. The FePc system was modelled in a periodically repeated cell, whose optimization gave a planar molecular superstructure with an almost square periodicity, as observed in the experiments. In good agreement with the experiment, the highsymmetry molecular axes form an angle of about 26° with respect to the FePc lattice (details in the SI). The GR support was not explicitly considered, but the effects of oxidation or possible oxygen intercalation were modelled by varying the electronic charge on the FePc. We studied the neutral (Figure S5), the positively charged (-2e-), and the negatively charged (+2e-) FePc systems (Figure 4) to mimic the as-grown, oxygen intercalated (p-doped), and oxidized (ndoped) GR support, respectively. We estimate that two electrons can represent the maximum amount of charge transferred between GRO and each FePc, considering the shift of the GR Dirac cone across the Fermi level by 0.6 eV.42,43 Following,51 we assume a linear behavior of the variation δ(Δπ ) of the π-band amplitude, and consequently of the Dirac cone shift, with the number ne of extra electrons per GR unit cell: |δ(Δπ )| = 9 ne . In the self-assembled FePc layer, each FePc corresponds to about 40 GR cells, therefore a shift of |δ(Δπ )| = 0.6 eV, which gives ne = 0.06 electrons, implies about 2 electrons in the GR area underlying each FePc. From the calculations we find that carbon dioxide chemically binds to Fe only in the n-doping case, in agreement with our experimental results and interpretation, whereas only physisorbed, linear CO2 configurations are found in the neutral (Figure S5) and positively charged cases. By ndoping the stack, CO2 adsorption is steered to the bent, “V”-shaped configuration (CÔC = 144°), in agreement with the general trend of CO2 adsorption on metal surfaces,50 and the molecule is stabilized via the carbon atom to the central Fe atom of the phthalocyanine at a distance of 2.26 Å (Figure 4, left panel). The calculated adsorption energy is 0.28 eV. The additional electronic charge available on the FePc thanks to the n-doping is initially distributed preferentially on the Fe atom, on the macrocycle, and, to a less amount on the remaining atoms (Figure 4, central panel). Upon formation of the CO2-Fe bond, a dipole forms across the 11 ACS Paragon Plus Environment


macrocycle plane and some electronic charge is transferred from the latter to the ligand (Figure 4, right panel). A molecular trans-effect is therefore involved in the process,52 where charge transfer can be chemically governed by oxidation of the underlying GR.

4. CONCLUSIONS We have shown that we are able to tune adsorption and activation of carbon dioxide on a 2D bioinspired artificial system by chemical methods, i.e. by controlling the amount of oxygen present in the system. In this way, a weakly adsorbing system is turned into a system that strongly binds and chemically activates carbon dioxide. Oxidation of the GR substrate leads to a charge transfer to the 2D system, which is responsible for the enhanced adsorption properties. Carbon dioxide chemisorbs in an activated (“V”-shape) configuration, thus lowering the barrier for further reaction.8,53 The stabilization of the CO2-Fe chemical bond is observed in situ at room temperature and at close-to-ambient pressure conditions. DFT simulations confirm the experimental picture, showing that both adsorption and activation are obtained by governing the charge transfer across the graphene-metalorganic layer and exploiting a molecular trans-effect. The ability to turn a weakly adsorbing system into one that strongly adsorbs and activates carbon dioxide shows that bioinspired principles can be successfully applied to artificial systems, opening a venue for technological sequestration and recycling of the greenhouse gas carbon dioxide.

ASSOCIATED CONTENT


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Supporting Information. Complementary IR-Vis SFG spectra, position of the resonance associated with the GR optical G mode, DFT model of the FePc monolayer, adsorption configurations of the CO2 molecule at the FePc sites, optimized DFT parameters, computed CO2 vibration modes, best IR-Vis SFG fitting parameters. AUTHOR INFORMATION Corresponding Author *Email:

[email protected].

ACKNOWLEDGMENTS F.M., M.C., and E.V. acknowledge financial support from the University of Trieste through the FRA2016 project. Financial support from Beneficentia Stiftung is also acknowledged. Computational resources have been obtained from the CINECA Supercomputing Centre through the ISCRA initiative and the agreement with the University of Trieste.

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Figure Captions

Figure 1. IR-Vis SFG spectra including the range of the FePc intramolecular modes, of the optical G mode of graphene, and of the internal CO2 stretches. The spectra were collected in situ sequentially from (a) to (d): (a) FePc 2D crystal on GR/Ir(111) as prepared, under UHV conditions; (b’) in 5 mbar CO2; (b) after oxidation in 100 mbar O2 at room temperature; (c) FePc 2D crystal on GRO/Ir(111) in 0.05 mbar CO2; (d) FePc 2D crystal on GRO/Ir(111) after evacuation, back to UHV. The low wavenumber part of the spectra (left) has been magnified for clarity. The best fit curve of the data, according to the lineshape described in the text, is also shown (cyan line), together with the deconvolution (coloured, filled curves) of the FePc internal modes (see text for details). Polarization: ppp.

Figure 2. Evolution of the SFG signal. Red: evolution of the phase with respect to the nonresonant background of the IR-Vis SFG resonance at 1398 cm-1, associated with the internal FePc modes, showing strong electronic and geometric effects associated with both oxidation and adsorption of CO2. Cyan: evolution of the non-resonant background and of the resonant signal originating from the graphene G mode at 1608-1615 cm-1, associated with a progressive increase of the corrugation of the sheet and related with electronic effects originating from the charge transfer.

Figure 3. X-ray photoelectron spectroscopy data. (bottom) bare graphene on Ir(111) and (top) GRO, after oxidation at room temperature in 100 mbar O2. The O and C 1s core level signals are shown (left and right, respectively) together with the best fit (continuous line) and the deconvolution in the single core level shifted C 1s components (filled curves). The spectra were collected ex situ at room temperature in UHV (h = 1253.6 eV).


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Figure 4. DFT results for the negatively charged FePc (+2e) and its interaction with adsorbed CO2 molecule. (A) side and top views of the stick-and-ball model of the “V”-shaped CO2 molecule adsorbed on FePc (left). (B) spatial distribution of the additional electronic density in the charge FePc molecule in absence of CO2: side and top views of isosurfaces of electron density difference calculated from the charged FePc minus the neutral FePc (isosurfaces at ± 0.002 |e|/a03, red/blue for +/-). To avoid spurious effects in the difference, the geometry of the charged FePc is the same of the neutral molecule, although the addition of two electrons induces a small bending. (C) charge transfer from the FePc to the adsorbed CO2 molecule. Side and top view of isosurfaces of electron density difference calculated from the charged complex CO2 on FePc minus the neutral CO2 and the charged FePc separately considered, in the geometry of the complex (isosurfaces at ± 0.002 |e|/a03, red/blue for +/-). Top right inset: profile of the electron density difference averaged on planes parallel to FePc, with distances referred to the plane of the macrocycle, and the positions of Fe atom (brown) and of C (yellow) and O (red) atoms of the ligand shown for reference.



Figure 1 170x172mm (300 x 300 DPI)

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Figure 2 80x45mm (300 x 300 DPI)



Figure 3 109x94mm (300 x 300 DPI)


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Figure 4



TOC Figure 49x38mm (300 x 300 DPI)


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Learning from Nature: Charge Transfer and Carbon Dioxide Activation

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