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Soft X-ray Spectroscopic Properties of Ruthenium Complex Catalyst under CO Electrochemical Reduction Conditions: A First-principles Study 2

Rocío Sánchez-de-Armas, Barbara Brena, Ivan Rivalta, and Carlos Moyses Graça Araujo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05626 • Publication Date (Web): 16 Sep 2015 Downloaded from http://pubs.acs.org on September 22, 2015

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The Journal of Physical Chemistry

Soft X-ray Spectroscopic Properties of Ruthenium Complex Catalyst under CO2 Electrochemical Reduction Conditions: A First-principles Study Rocío Sánchez-de-Armas1*, Barbara Brena1, Ivan Rivalta2 and C. Moyses Araujo1 1

Materials Theory Division, Department of Physics and Astronomy, Uppsala University, P.O

Box 516, S75120, Uppsala, Sweden. 2

École Normale Supérieure de Lyon, CNRS, UMR 5182, Laboratoire de Chimie, 46, Allée

d'Italie, 69364 Lyon, cedex 07, France.

ABSTRACT. Solar fuel production through photoelectrochemical reduction of CO2 is a promising route to popularize the use of solar energy. However, the underlying mechanisms of these complex reactions are not yet fully resolved, hindering the rational design of novel photoelectrocatalysts. To shed light on this challenging problem, the X-ray photoelectron spectroscopy (XPS) and the near edge X-ray absorption fine structure (NEXAFS) of a number of molecular systems have been calculated using first-principles theory. First, it was found that both XPS and NEXAFS display specific features that correlate with the complex charge state and the coordination number of Ru atom. Furthermore, through the analysis of C1s and N1s XPS and

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NEXAFS spectra of key intermediates, we have identified clear fingerprints for metal-hydride and Ru-CO2 chemical bonding formation, two alternative pathways for catalytic CO2 reduction. These results indicate that the understanding of the electrochemical properties of the electrocatalyst, as well as the reaction pathways, could be significantly advanced through operando X-ray spectroscopy experiments based on synchrotron radiation. We expect that these theoretical findings will be the basis of and motivate future experimental initiatives.

1. INTRODUCTION The photoelectrochemical reduction of CO2 to high-energy products like methanol and methane is being considered as a promising route to enable a sustainable world growth with reduced impact on the environment.1-2 In the ideal systems, water, CO2 and sunlight would be combined to form O2 and fuels mimicking the natural photosynthesis. The process would actually take place in distinct steps. The sunlight is absorbed at a photosensitizer creating electron-hole pairs and triggering multiple charge transfer events. The holes move toward the oxidative catalysts sites where water is oxidized to O2 (2H2O O2 + 4H+ + 4e-) while the reducing equivalents (H+ + e-) move toward reductive catalysts sites where CO2 is reduced to solar fuels (e.g. HCOOH, CH3OH, CH4, etc). Although this is a simple concept with strong potential for producing a clean energy matrix, there are many hurdles to be overcome in order to scale up such technology. Electrochemical reduction of CO2 to methanol has been achieved by Cole et al.3 at low overpotentials in a 10 mM aqueous solution of pyridine (Pyr) at pH 5.3 using a Pt disk electrode. This result has brought significant advance in the field and has prompted a vivid discussion


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about the reaction mechanism.3-7 Hybrid semiconductor-enzyme systems have also been developed to successfully promote the reduction of CO2 to formic acid, isocitric acid and CO.8-9 However, these systems were unstable over long time periods, due to enzyme detachment from the semiconductor surface. Another alternative approach has been proposed to immobilize the coordination complexes and polymers (instead of enzymes) on the semiconductor surface.10-13 For instance, Sato et al.11,

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have designed a successful Z-scheme device with Ru-complex

adsorbed on N-doped Ta2O5 compound with high selectivity toward CO2 reduction. These uphill reactions are totally photon-driven, which is a key feature for the application in solar fuel production. More recently, other successful systems based on Ru-complex have been reported but using carbon nitride as the photosensitizer.14-16 The underlying mechanisms of the electron-transfer reactions that take place during the conversion of CO2 to solar fuels in these systems are not fully resolved yet, hindering the rational design of novel photoelectrocatalysts made of Earth-abundant elements and displaying suitable properties. These processes are very sensitive to the chemical environment, proton source and electrochemical conditions giving rise to a number of possible reaction pathways. For instance, in coordination complexes like Ir(PCP)H2(MeCN)

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and Ir(PNP)H3

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it is proposed that the

reaction mechanism involves the interaction of CO2 with hydride species, while the electrochemical reduction of CO2 on metallic electrode surfaces involves a proton-coupled electron transfer mechanism.19 Proton-coupled hydride transfer6 has also been proposed to explain the experimental findings of Cole et al.3 In the case of hybrid semiconductorcoordination photocatalysts, as e.g. in the experiment of Sato et al.,11, 13 the mechanism is even more complicated due to eventual interface effects. In this scenario, novel sophisticated


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experimental techniques need to be employed to shed light on such complex physical-chemical processes. One approach that stands up on the fundamental understanding of reaction mechanisms is the implementation of operando X-ray spectroscopy techniques, e.g. photoelectron spectroscopy (PES) and Near Edge X-ray Absorption Fine Structure (NEXAFS), using synchrotron facilities. Such techniques have undergone significant developments in the past few years reaching more realistic physical-chemical conditions at higher pressures and temperatures.20-21 Soft X-ray synchrotron based techniques characterized by element selectivity and high resolution, give a unique picture of the electronic structure of the molecules at different chemical environments.2223

For example, X-ray photoelectron spectroscopy (XPS) can investigate the core level region,

while X-ray absorption spectroscopy (XAS) looks into the unoccupied region. Novel developments are coming from the latest generation sources, like the ultrafast pump probe experiments, which can shed light in chemical reaction pathways. Photoelectron spectroscopy experiments generally require ultra-high vacuum conditions, due to the strong attenuation of intensity of the photoemitted electron beam traveling in a medium, due to inelastic scattering processes. However, due to the strong interest in expanding the field of applications of XPS to liquid samples, like aqueous solutions, methodologies exploiting liquid microjets24 or differential pumping systems25 have been successfully developed in recent years. We are thus moving toward the operando X-ray spectroscopy of solid-liquid interfaces and solvated coordinated complexes, which will in turn play an important role in the energy related challenges. Bearing this in mind, we have employed first-principles theory to analyze whether soft X-ray spectroscopies based on the ionization/excitation of core electrons, can be used as a tool to determine the structure of complexes, the oxidation states and the reaction intermediates that


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have been predicted theoretically. We have focused on the Ru-complex electrocatalysts employed by Sato et al., see Figure 1, simulating C 1s and N 1s X-ray spectroscopy (XPS) and C K-edge NEXAFS of these complexes in different oxidation states and considering different reaction pathways for the CO2 reduction. The computed XPS spectra in the gas-phase show large differences in the electronic structure of the complexes, illustrating how X-ray based experimental techniques could be employed to fingerprint the electrochemical properties and reaction mechanism. A similar approach, for example, has been successfully used to study the shifts in the Mn K-edge NEXAFS spectra as a function of the oxidation state of the Mn atoms in natural photosynthesis, shedding light on the structure and functionality of PSII.26 2. COMPUTATIONAL DETAILS All geometry optimizations were carried out within the framework of Density Functional Theory (DFT), using the hybrid density functional B3LYP27-29 as implemented in the Jaguar 7.7 software package30 with LACV3P effective core potential and basis set (ECP).31 The computation of the ionization potentials (IPs) and the near edge X-ray absorption fine structure (NEXAFS) has been performed using the gradient-corrected DFT program StoBe,32 with the exchange functional by Becke27 and the correlation functional by Perdew.33 The ionization potential (IP) of the 1s electronic level of each carbon and nitrogen atom was obtained independently, as the energy difference between the ground state and the core excited state (full core), according to the ∆Kohn–Sham method.34 To describe the core-excited C or N atoms, the IGLOO-III triple ζ basis of Kutzelnigg, Fleischer and Schindler35 were used and effective core potentials of 4 or 5 electrons provided by the StoBe package were chosen for the remaining carbon or nitrogen atoms, respectively.36 For Ru, an effective core potential of 14 electrons


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available in the StoBe package was employed. The rest of the species (hydrogen, oxygen as well as carbon in the nitrogen IP calculations and nitrogen in the carbon IP calculations) were described by a double ζ plus valence polarization basis set, also provided by the StoBe package. The total calculated XPS spectrum was obtained as the sum spectrum of all calculated IP values, convoluted by Gaussian curves in order to facilitate comparison with experiment, with a fixed full width at half maximum (FWHM) of 0.6 eV. The C K-edge NEXAFS spectrum was generated for each carbon atom. The computation of the excitation energies and of the transition moments was performed in StoBe using the same basis sets as for the IPs. The excited 1s state was represented as a half core–hole according to the transition potential method.34 The energy positions of the single-atom NEXAFS curves were corrected according to the corresponding computed IP (∆KS-SCF correction). The spectral intensities were generated from the computed dipole transition probabilities and the results were then convoluted by Gaussian curves. For each carbon separately, a Gaussian broadening with a fixed full width at half maximum (FWHM) was used in the low energy part of the spectrum (0.5 eV), while, starting from the IP, the FWHM was linearly increased from the initial value of 0.5 eV up to 5 eV in order to better reproduce the experimental continuum region. The total calculated NEXAFS spectrum for each complex was obtained by summing up all the single-atom 1s theoretical spectra. Most of the studied structures are singlets in their ground state. However, for those structures that have spin multiplicity 2, we have considered the excitation of both alpha and beta core electrons. These methods have been widely used to calculate the XPS and NEXAFS spectra of different molecular systems in very good agreement with experimental findings.26, 37-39


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In order to validate the employed method for this kind of complexes, the C1s XPS, N1s XPS and N K-edge NEXAFS have been calculated for the well-studied [Ru(bpy)3]2+ complex, and they have been compared to both experimental spectra and previous calculations.40 We have found an excellent agreement between theory and calculation, which indicates that our method is suitable to describe k-edge NEXAFS, as well as to reproduce the shape (the relative position and the relative intensity of the different peaks) of the C1s XPS of this kind of Ru complexes. This is very important on the design of theoretical tools, based on spectroscopic properties, to support the identification of charge-transfer chemical reaction fingerprints to resolve reaction mechanisms. Detailed information about this comparison can be found in SI file (Figures S1 and S2). 3. RESULTS AND DISCUSSION 3.1. Effect of the ligand environment We first assess the spectroscopic properties of three different ruthenium complexes that have been investigated by Sato et al.,11 [Ru(bpy)2(CO)2]2+, [Ru(dcbpy)(bpy)(CO)2]2+ and [Ru(dcbpy)2(CO)2]2+ (named RuII-bpy-2CO, RuII-dcb-2CO and RuII-2dcb-2CO, respectively), where bpy=2,2’-bipyridine and dcbpy=4,4’-dicarboxy-2,2’-bipyridine. The three complexes, illustrated in Figure 1, differ only by the number of carboxylic groups in the ligand environment. The formal oxidation state of ruthenium in these complexes is Ru(II) and the complexes are more stable in their singlet state. XPS is a powerful technique, which has the ability to distinguish atoms of the same species in different chemical environments. Therefore, we have analyzed the effect of different substituents on the C1s and N1s XPS spectra.


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Figure

1.

Ruthenium

complex

catalysts

[Ru(bpy)2(CO)2]2+

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(RuII-bpy-2CO),

[Ru(dcbpy)(bpy)(CO)2]2+ (RuII-dcb-2CO) and [Ru(dcbpy)2(CO)2]2+ (RuII-2dcb-2CO) involved in visible-light-induced selective CO2 reduction to methanol on semiconductor support. The C1s XPS spectra of the three complexes, shown in Figure 2 (a), have a similar profile, where three different peaks can be distinguished. The peak at lower binding energy, around 296.1 eV, is the most intense one and can be attributed to carbon atoms bound to other two carbon atoms in the aromatic rings (-CH=CH-CH=). A second broad peak centered at 297.3 eV appears in the spectra, arising from carbon atoms bound either to nitrogen or to a carbon of a carboxylic group contribute. Finally, the peak at higher binding energy, at around 298.9 eV, has contributions from the carbon atoms bound to oxygen (both terminal C-O ligands and COOH groups of dcbpy). The position of these three peaks remain unaltered when passing from RuIIbpy-2CO to RuII-2dcb-2CO, but the different substituents produce differences in the relative intensity of the peaks. As one could expect, adding -COOH groups to the complex produces a decrease of the peak at lower energy (there are less carbon atoms bound only to carbon atoms) while the intensities of the second and third peak increase (more details about the assignment of the C1s XPS spectrum are given in figure S3 in the SI file). The shape and the relative energy position of the different peaks of the calculated C1s XPS spectra are in good agreement with experimental spectra of Ru complexes with similar structures.41 For instance, the energy


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difference between the peaks attributed to pyridine and carboxyl are 2.8, 2.5 and 2.6 eV for Ru 535, Ru 455 and Ru 470 complexes, respectively. Small differences with our complexes can be attributed both to the different ligands of the complexes and to the fact that the experimental spectra are registered with the complexes adsorbed on a TiO2 surface. The N1s XPS spectra are displayed in Figure 2 (b). All of them present one peak at 411.1 eV with the same intensity. The effect of adding -COOH is to slightly shift the peak to lower energies, inducing however only a tiny change. In fact, this first assessment of the spectroscopic properties indicates that the addition of –COOH to the bipyridine ligands may display a minor influence on the spectroscopic fingerprints of intermediate species formed during the CO2 reduction reaction. Therefore, we will carry on further analysis only on the smallest complex, RuII-bpy-2CO.


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Figure 2. Theoretical C1s XPS (a) and N1s XPS spectra (b) for RuII-bpy-2CO (blue full line), RuII-dcb-2CO (red dashed line) and RuII-2dcb-2CO (green dash-dot line) complexes.

3.2. Two-electron reduction process The reduction of CO2 to formic acid involves a two-electrons charge transfer chemical reaction. We have therefore studied the two-electron reduction of [Ru(bpy)2(CO)2]2+ to [Ru(bpy)2(CO)2]+ (named RuI-bpy-2CO) and [Ru(bpy)2(CO)2] (named Ru0-bpy-2CO), taking into consideration whether the successive reduction processes from Ru(II) to Ru(0) formal oxidation states involve reduction of the metal center or the ligands. The computed C1s and N1s XPS spectra for the complex in the different oxidation states are shown in Figures 3(a) and 3(b)


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respectively. The C1s XPS spectra present a very similar profile for the three complexes, where three different peaks can be distinguished. But the main effect is that the reduction of the complex entails a considerable shift of the whole XPS spectra to lower binding energies. In fact, after the first reduction process the binding energies are shifted by about 4.8 eV and after the two-electron reduction related to the RuII-bpy-2CO species by about 8.7 eV. Moreover, the second peak of the spectrum, corresponding to carbon atoms bound to nitrogen, is strongly modified after the first reduction, changing from a broad peak for RuII-bpy-2CO to a narrow and more intense peak for RuI-bpy-2CO and Ru0-bpy-2CO. Therefore, an experimental resolution of 0.5 eV would be enough to distinguish between the different oxidation states of the catalyst. In analogy to the C1s spectra, the N1s binding energies are similarly shifted by 4.5 eV to lower energies after the first reduction and 8.2 eV to lower energies after the two-electron reduction.


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Figure 3. Theoretical C1s XPS (a) and N1s XPS (b) spectra for RuII-bpy-2CO (blue full line), RuI-bpy-2CO (red dashed line) and Ru0-bpy-2CO (green dash-dot line) complexes.

We have performed further investigations on this complex in its different oxidation states, by computing C K-edge NEXAFS spectra (Figure 4). Element specific information about the unoccupied states of a molecule, like on the local chemical bonding as well as on the charge transfer, can be extracted from the NEXAFS spectra analysis. Four peaks are observed in the C K-edge NEXAFS spectrum of the Ru(II) complex at photon energies below 288 eV, which have been denoted with the roman numbers I (284.5 eV), II (285.2 eV), III (285.7 eV) and IV (287.8 eV). These peaks undergo significant changes after the first reduction process. Peak number I almost disappears after reduction, becoming a quite small shoulder and the intensity of peak number III also decreases. Moreover, the whole spectrum is shifted by about 0.6 eV to lower photon energies. In contrast, the second reduction process produces only slight changes in the C K-edge NEXAFS spectrum: the spectrum of the Ru(0) species, in fact, presents the 4 first peaks in the same position and almost with the same relative intensity as in the case of the Ru(I) species. This could indicate that the first reduction process involves a reduction of the ligands while the second one involves a reduction of the metal center thus affecting the ligands less. This is consistent with the calculated spin density of Ru(I) state shown in Figure S4. To further understand the origin of the change in the spectrum after the one-electron reduction process, we have investigated the type of carbon atoms that contribute to each peak of the spectra, summing up the individual NEXAFS spectra corresponding to a specific kind of carbon atom. Only those carbon atoms that are bound solely to other carbon atoms contribute to the


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peaks I and II. Peak number III corresponds to carbon atoms bound to nitrogen atoms while the main contributions to peak IV come from the carbon atoms bound to oxygen (see Figures S5, S6 and S7 in the SI file). Therefore, the changes in the NEXAFS spectrum after the first reduction process are mainly a consequence of modifications in the electronic structure of the carbon atoms that are bound to other carbons.

Figure 4. Calculated C K-edge NEXAFS spectra for RuII-bpy-2CO (blue), RuI-bpy-2CO (red) and Ru0-bpy-2CO (green) complexes.

3.3. CO release It has been speculated that the one-electron-reduced complex (RuI-bpy-2CO) undergoes some structural change before acceptance of the second electron and this change has been proposed to be the release of one CO ligand (Figure 5(a)).11 This would create an open coordination site in the Ru complex favoring the hydrogenation of the metal center or the coordination of a CO2 molecule. To investigate the existence of spectroscopic fingerprints for CO extrusion upon first


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reduction, we have compared the C1s XPS spectra of the Ru(I) complex with two or one CO ligand (RuI-bpy-2CO and RuI-bpy-CO, respectively), as showed in Figure 5(b). The release of CO produces important variations in the C1s XPS spectrum. The two first peaks, at lower binding energy, corresponding to carbon bound to carbon or nitrogen, respectively, are shifted about 0.5 eV to higher binding energies. Moreover, the second peak, which is a clear sharp peak in the complex with two CO ligands, becomes broader and less intense upon CO release. Finally, the peak at higher binding energy, arising from carbon bound to oxygen, disappears after CO dissociation since the binding energy of the C1s in the remaining CO ligand is shifted from 294.1 eV to 293.8 eV, contributing to the second peak of the spectrum. Such correlation between the spectroscopic properties and the coordination structure is an important result, which demonstrates a way of addressing experimentally the problem of CO-dissociation before the second reduction process.


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Figure 5. Structures (a) and C1s XPS spectra (b) for the RuI-bpy-2CO and RuI-bpy-CO complexes. 3.4. Fingerprinting possible intermediates Finally, we have evaluated the XPS (C1s and N1s) and the C K-edge NEXAFS spectra of doubly-reduced Ru complexes representing possible reactions intermediates involved in different mechanisms of catalytic CO2 reduction. Figure 6 displays five different molecular structures that have been analyzed, covering three relevant scenarios: (i) hydride formation on the metal center (Ru0-bpy-CO-H), (ii) electrophilic attack leading to hydride-CO2 interaction (with and without the effect of a proximal proton source, Ru0-bpy-CO-H-CO2-H+B and Ru0-bpy-CO-H-CO2 respectively), and (iii) direct interaction between CO2 and the metal center (Ru0-bpy-CO-CO2) with an inner charge transfer reaction leading to CO2 activation forming Ru-C chemical bond. The latter is predicted to take place in the CO2 to CO reduction pathway in Mn and Re complexes.42 Furthermore, with the purpose of establishing a reference for the analysis, we have considered the complex containing two monodentate CO ligands (Ru0-bpy-2CO), of which C1s XPS, N1s XPS and C K-edge NEXAFS spectra have been described in detail previously, and also the HCOOH, CO and CO2 molecules in the gas-phase. We are then investigating two main reaction pathways. The first one is characterized by the hydride formation with the protonation of the coordination complex following the second electron reduction of the Ru complex. Here, we have investigated the spectroscopic properties of three complexes, viz. Ru0-bpy-CO-H, Ru0-bpy-CO-H-CO2 and Ru0-bpy-CO-H-CO2-H+B. In the latter, we have introduced the effect of a proton source, in this case chosen to be a pyridinium ion. Our choice is based on the fact that in the experiments of Sato et al. the proton source is not


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yet clearly resolved, while it has been proposed by Ertem et al.6 that pyridinium ion could play a relevant role in the pre-activation of CO2 during the electrophilic attack and thus it can be considered a good proton donor during CO2 reduction. The second pathway is characterized by the inner charge transfer process and formation of metal-carbon chemical bond. Here, we have investigated the spectroscopic properties of the coordination complex Ru0-bpy-CO-CO2. It should be highlighted that all spectroscopic calculations have been carried out on the optimized structures of the molecular complexes. Furthermore, our thermodynamic analyses indicate that these are indeed feasible pathways to explain the CO2 reduction on such Ru complex. The results of this study, which is out of the scope of the current paper, will be published in due course.

Figure 6. Structure of the different Ru(0) species which could be possible intermediates in the catalytic cycle. Figure 7 (a) presents the C1s XPS spectra of all investigated systems, viz. five Ru-complexes, CO2 and HCOOH molecules in the gas phase. As one can notice, the spectra of the neutral complexes (Ru0-bpy-CO-CO2 and Ru0-bpy-2CO) are located in the interval 287-291 eV with


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significant overlap. However, the distinctive features, with the main peaks separated by about 1.5 eV, could allow the identification of different coordination species on the metal center, either CO or CO2. We have shown previously that the XPS measurement could be a good probing technique to resolve the change in the coordination number following the reduction reactions. Here, it shows to have sufficient sensitivity to distinguish between CO and CO2 coordination. A more significant modification occurs, actually, on the spectra of the coordination complexes with different charge states. There is a shift on the spectra of around 4 eV to higher energies when increasing the charge from 0 (Ru0-bpy-CO-CO2 and Ru0-bpy-2CO) to +1 (Ru0-bpy-CO-H and Ru0-bpy-CO-H-CO2) and then to +2 (Ru0-bpy-CO-H-CO2-H+B). These results indicate that an operando XPS experiment could clearly identify reaction pathways and shed light on the reaction mechanisms, indicating for instance the formation of the hydride states, which is considered to be relevant for the catalytic selectivity toward CO2 reduction. The same trend has also been found in the N1s XPS spectra, see Figure 7(b), and also on the C1s energy level of CO ligand that is present in all calculations (see Table 1). The fingerprint on the N1s spectra for the CO release, without charge state change, is related to the fact that N atoms are also coordinating the Ru metal center and the modification of coordination shell affects the chemical environment leading to alteration of the features of N1s.


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Figure 7. Computed XPS spectra for the different Ru(0) species: Ru0-bpy-2CO (black), Ru0bpy-CO-CO2 (red), Ru0-bpy-CO-H (purple), Ru0-bpy-CO-H-CO2 (orange) and Ru0-bpy-CO-HCO2-H+B (green). C1s XPS spectra (a), and N1s XPS spectra (b). In the C1s XPS spectra free CO2 (blue) and free HCOOH (brown) spectra have been included for comparison Figure 8 displays the C1s energy level of CO2 molecule in different chemical environments. The calculated energy level of free CO2 is 295.1 eV in good agreement with the experimental findings (293.5 eV).43 This energy is shifted to 297.6 eV when CO2 is coordinated to the hydride specie (Ru0-bpy-CO-H-CO2), which is closer to the C1s energy level of HCOOH in the gasphase, found to be 297.0 eV. It should be pointed out that in Ru0-bpy-CO-H-CO2, CO2 still displays sp-hybridization with linear geometry (O-C-O angle of 180°) according to our geometry


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optimizations. However, the interaction with the proton donor favors the sp to sp2 transition changing the C-O-C bond angle to 135.4º. This result is consistent with previously reported important impact of proton sources in CO2 activation. However, such geometry modification does not affect much the spectroscopic properties, with C1s energy level shifted only by 0.6 eV to 298.2 eV. Thus, the main effect on the C1s core level shift is the local charge state dictated by the formation of the hydride rather than the hybridization change. Finally, when CO2 is directly bound to Ru (Ru0-bpy-CO-CO2), its C1s binding energy is 289.8 eV, far from both free CO2 and free HCOOH. We have, thus, clear fingerprints in the XPS spectra, which could help to resolve the details of the CO2 activation mechanisms.

Table 1. C1s binding energies for carbon atoms belonging to CO and CO2 ligands of the different catalyst complexes, as well as free CO, CO2 and HCOOH molecules (eV). CO

CO2

CO

296.001

-

CO2

-

295.114

HCOOH

-

297.022

290.246 Ru0-bpy-2CO

290.159

Ru0-bpy-CO-CO2

290.335 289.809

Ru0-bpy-CO-H

294.144

Ru0-bpy-CO-H-CO2

294.178 297.564

Ru0-bpy-CO-H-CO2-H+B

297.118 298.176

-


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The identification of different coordination species on the metal center (CO or CO2) is also possible from the C K-edge NEXAFS spectrum, as depicted in Figure 9. As one could expect, the intensity of the forth peak in Ru0-bpy-2CO spectrum, which has mainly contributions from the carbon atoms from carbonyl group, decreases after CO2 coordination, but also the profile of the lower energy peaks is very different for the two complexes. The effect of the complex charge state in the C K-edge NEXAFS less pronounced than in XPS, and an important shift of the spectra is not observed. The spectra for the two structures with charge 1, Ru0-bpy-CO-H and Ru0-bpy-CO-H-CO2, are very similar, and only an increase of the intensity of the peak at 289.2 eV is observed. The reason is that this peak has a big contribution of the CO2 carbon (Figure S8, SI). Nevertheless, the interaction with the proton donor produces noticeable differences in the NEXAFS spectra. The first peak, which has almost the same intensity as the second one for Ru0bpy-CO-H-CO2, becomes less intense in the case of Ru0-bpy-CO-H-CO2-H+B. Moreover, the peak at 289.2 eV also decreases its relative intensity with respect to the two first peaks, one of the reasons being the shift of the photon energy, associated with CO2 carbon atom, to lower energies. The CO ligand that is common in every intermediate, contributes to the peak that appears in the spectra at around 287.2 eV and its position is almost unaltered in the different complexes.


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Figure 8. Binding energy diagram for the C1s level of carbon atom in different chemical environments. From top to bottom Ru0-bpy-CO-H-CO2-H+B, Ru0-bpy-CO-H-CO2, CO, HCOOH, CO2, and Ru0-bpy-CO-CO2. Yellow, grey, red, white and blue spheres represent Ru, C, O, H and B (a non-specific base), respectively.

Figure 9. Calculated C K-edge NEXAFS spectra for the different Ru(0) complexes: Ru0-bpy2CO (black), Ru0-bpy-CO-CO2 (red), Ru0-bpy-CO-H (green), Ru0-bpy-CO-H-CO2 (blue) and Ru0-bpy-CO-H-CO2-H+B (orange).


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It is necessary to point out that the presence of the solvent around the Ru catalysts, with the possible formation of hydrogen bonding, could have an important role in the reported spectroscopic results, and in which way it could affect the spectra. For instance, the effect of the hydrogen bonding has been thoroughly analysed in soft X-ray emission spectra of water (see for example ref. 44, and more recently ref. 45). However, in a paper by Ottosson et al. of XPS of the O 1s core level, in a water jet, it is argued that the measured spectra could be well reproduced by theoretical calculations of the molecules in the gas phase.46 Notably, a similar conclusion comes also from the work by Brena et al.,26 where a study of PSII, the shifts in the Mn K edge as a function of the oxidation state of the Mn atoms was studied, and where the single molecule calculations were able to reproduce also quantitatively the experimental spectra. Therefore, the reported outcome definitely encourage new experimental spectroscopic investigations, which will strongly benefit of the theoretical assignments presented here.

4. CONCLUSIONS In summary, we have carried on a comprehensive theoretical study based on first-principles spectroscopy calculations, which identified the correlation between the spectra features and electrochemical properties of the Ru-complex catalyst under CO2 reduction conditions. These results demonstrate how X-ray spectroscopy experiments could discriminate between still unknown reaction mechanisms involving either Ru-hydride-CO2 or Ru-CO2 intermediates. First, it is shown that the two-electron reduction process give rise to a shift of about 8.7 eV in the C1s XPS spectra leading to some changes in their features. Similar shift was found in the N1s XPS spectra. Also the NEXAFS spectra undergo significant modifications following the


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decrease of the complex charge state from +2 to 0. These results together indicate that the ligand is involved in both reduction steps. Furthermore, we demonstrate that it would be possible to confirm experimentally the dissociation of CO, after the first reduction step from XPS spectroscopy. Finally, we have evaluated the C1s and N1s XPS and the C K-edge NEXAFS spectra of reduced Ru-complexes investigating possible reactions intermediates involved in different mechanisms of catalytic CO2 reduction. We have covered three scenarios, namely (i) hydride formation on the metal center, (ii) metal hydride-CO2 interaction and (iii) direct interaction of CO2 with the metal center. It was found that the distinctive features in the XPS spectra, with the main peaks separated by about 1.5 eV, could allow the identification of different coordination species on the metal center, either CO or CO2. Even larger shifts are observed for complexes with different charge states, due to different binding energies of the C1s electron. Such spectroscopic fingerprints indicate that operando XPS spectroscopy could clearly identify reaction pathways and shed light on the reaction mechanism. We expect that these theoretical findings will be the basis motivating future experimental initiatives.

ASSOCIATED CONTENT Supporting Information. Detailed information about the assignment of the different peaks of XPS and NEXAFS spectra, spin density surface and cartesian coordinates for all calculated species. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *[email protected] +46 184717308 [email protected] [email protected] [email protected]

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by Vatenskapsrådet (VR), Swedish Energy Agency, STandUP for Energy and the Carl Tryggers Foundation. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at PDC Center for High Performance Computing and National Supercomputer Center at Linköping University (triolith). IR gratefully acknowledges the support of the École Normale Supérieure de Lyon (Fonds Recherche 900/S81/BS81-FR14).

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Soft X-ray Spectroscopic Properties of Ruthenium ... - ACS Publications

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