Solvent-Mediated Reduction of Carbon Dioxide in Anionic Complexes

Article pubs.acs.org/JPCA

Solvent-Mediated Reduction of Carbon Dioxide in Anionic Complexes with Silver Atoms Benjamin J. Knurr and J. Mathias Weber* JILA and Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder Colorado, 80309, United States ABSTRACT: The development of efficient routes toward sustainable fuel sources by electrochemical reduction of CO2 is an important goal for catalysis research. While these processes usually occur in the presence of solvent, solvation effects in catalysis are largely not understood or even characterized. In this work, mass-selected clusters of silver anions with CO2 serve as a model system for reductive activation of CO2 by a catalyst in the presence of a well-controlled number of solvent molecules. Vibrational spectroscopy and electronic structure calculations are used to obtain molecular-level information on the interaction of solvent with the catalyst−CO2 complex and the effects of solvation on one-electron reductive activation of CO2. Charge transfer from the silver catalyst to CO2 increases with increasing cluster size. We observe the coexistence of catalyst−ligand complexes with CO2 monomer and dimer anions, indicating that CO2-based charge carriers can exist in the presence of a silver atom.

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INTRODUCTION Reduction of CO2 is a potential avenue toward a carbon neutral fuel cycle. This approach promises to be environmentally benign and sustainable, because CO2 as the basic feedstock for fuel production is regenerated by combustion of the same fuel. The first step in this conversion requires reduction of CO2 to produce feedstock for further processing, e.g., carbon monoxide, formate anion (HCOO−), methanol, or methane. These reactions, which involve the transfer of electrons and protons to CO2, are endothermic. In addition, there is a kinetic barrier that is thought to be associated with the formation of a CO2− intermediate.1 Forming CO2− from CO2 in the absence of solvent is endothermic by ca. 0.6 eV.2 In electrochemical cells, the barrier associated with this step necessitates applying an overpotential to drive the reduction reaction, which increases the energy cost of the conversion process and contributes to making CO2 reduction economically less attractive than continued use of fossil fuels at present. Although the thermochemical cost to go from CO2 to reduced products is unavoidable, the barrier associated with CO2− formation can be suppressed using heterogeneous or homogeneous catalysts. A possible mode of operation of such a catalyst is the transfer of negative charge to a CO2 molecule through a covalent bond, leading to reductive activation of the CO2 moiety. Different approaches have been used to achieve CO 2 reduction. Polycrystalline metal surfaces have been widely studied as catalysts, but they generally suffer from low conversion efficiency.3 Recently, Kanan and co-workers4−6 studied oxide-derived metal nanoparticles that were found to be very efficient and selective electrocatalysts, albeit of limited lifetime. Carpenter and co-workers7 recently reported a homogeneous amine-based catalyst which can be coupled to photochemical water splitting and is renewed during this process. Bocarsly and co-workers8 used an electrochemical © 2013 American Chemical Society

approach to convert CO2 to methanol in a six-electron reduction process, where pyridine-based homogeneous catalysts facilitate charge transfer and reductive activation. Some very interesting approaches to catalytic reduction of CO2 involve metal atoms embedded in an organic framework9 or surfacesupported metal catalysts,10−13 where the ultimate limit for such applications is a single-atom catalyst. The molecular-level details involved in the function of suitable catalysts for this process are not well understood, hindering the rational design of catalyst materials. In addition to support effects, solvent effects are very important for catalyst function, because the solvent environment is always present and changes the properties of the solute. It is also relevant for technical catalysts, because the choice of solvent and chemical environment can be crucial for efficient catalyst function. Solvent can have a strong influence on the electronic structure of solute molecules, particularly ions.14−16 In the context of CO2 reduction, ionic liquids as solvents have in fact been implicated as cocatalysts for conversion to CO at low overpotentials,1 even though the details of their interaction with the activated CO2 molecule remain unexplored so far. Spectroscopic characterization of solvent effects is difficult to achieve in situ, because fluctuations in the solvation environment and the sheer number of different species will often lead to a very congested spectroscopic response where the relevant spectroscopic signatures are not immediately recognized. These problems can be circumvented by using mass-selected cluster ions in vacuo as model systems, affording precise control over the composition and size of the solvation environment. Vibrational spectroscopy offers a particularly suitable probe of Received: July 31, 2013 Revised: September 23, 2013 Published: September 27, 2013 10764

dx.doi.org/10.1021/jp407646t | J. Phys. Chem. A 2013, 117, 10764−10771

The Journal of Physical Chemistry A

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solvent and solute structure15,17−21 and can serve to unravel the molecular-level mechanisms involved in catalytic reductive activation of CO2. A recent study from our laboratory focused on the effects of solvent CO2 molecules in clusters of the form [Au(CO2)n]−. We found that while initially enhancing reductive activation of the CO2 ligand bound to the metal (up to ca. 80% charge transfer for n = 2−9), a large enough solvent shell ultimately diminished the reductive activation of the CO2 molecule (n = 10−13). As this study showed, understanding the interplay between the activated species and the solvent is critical to fully understand this activation process and probably most catalytic processes. Silver surfaces and nanoparticles have been used as catalysts for electrochemical reduction of CO2 with varying degrees of success.22−27 The electronic structure of atomic silver with its (4d)10(5s)1 configuration is similar to that of gold. However, there are also significant differences between these two noble metals due to smaller relativistic effects in silver compared to gold. This results in rather different energetics, for example in electron affinity (1.303 eV in Ag vs 2.309 eV in Au).28,29 Initial inspection of the electronic properties of Ag and Au suggests that silver anions should behave similarly in the overall interaction with CO2 but perhaps be better suited to facilitate CO2 reduction due to a lower electron affinity. In this paper, we investigate reductive activation of CO2 by anionic silver and its dependence on solvation by employing infrared spectroscopy of [Ag(CO2)n]− clusters. Although this does not represent a practical heterogeneous catalyst (after all, there is no support material), this model system highlights the effects of solvation on the charge distribution between catalyst and CO2 substrate. We discuss the charge transfer onto CO2 on the basis of the observed trends in the experimental spectra, using density functional theory (DFT) calculations and comparisons with [Au(CO2)n]− clusters to interpret the experimental results.

the signal-to-noise ratio. The experimental repetition rate was 20 Hz.

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COMPUTATIONAL DETAILS We performed DFT calculations using the TURBOMOLE V. 6.2 suite of programs34 to determine possible structures of [Ag(CO2)n]− clusters (n = 1−5). The B3-LYP35,36 functional was used for all calculations with def2-TZVPP37 basis sets for all atoms. Vibrational spectra were calculated with the AOFORCE program.38,39 All of the reported calculated vibrational frequencies have been scaled by 0.9380 to account for anharmonicity. The scaling factor was obtained by comparing the calculated value of the antisymmetric CO2 stretch in AuCO2− with the predicted value from high-quality calculations by Boese et al.40 Partial charges were calculated using natural population analysis.41 To estimate the effect of solvation beyond the first solvation shell, a conductor-like screening model (COSMO) was employed,42 using a dielectric constant of 1.6 (corresponding to saturated liquid CO2 at 291 K43) and a molecular-shaped cavity with the default radii for carbon (2.0 Å), oxygen (1.72 Å) and silver (2.223 Å). The COSMO results in Figure 7 are shown without zero-point corrections.

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RESULTS AND DISCUSSION There are two limits for the spectral signature of a CO2 molecule undergoing reduction. The CO2 molecule can be neutral, in which case the antisymmetric stretch (ν3) will be at (or very close to) its value44 for free, neutral CO2 (2349 cm−1). In the limit of a CO2− anion, ν3 is shifted down to 1656 cm−1 due to a weakening of the CO bonds and a concomitant change away from linearity.45 Infrared fragment action spectra monitoring the loss of a single CO2 solvent molecule from [Ag(CO2)n]− are displayed in Figure 1. There are three distinct groups of features in these spectra. A set of vibrational bands is observed starting with a single peak at 1865 cm−1 in [Ag(CO2)2]−, splitting into a doublet for some cluster sizes and monotonically shifting to lower wavenumbers as the cluster size increases. A second group of features is found at 1660 cm−1 for n = 3−9, showing minimal shifts for all spectra. Finally, we observe peaks between 2320 and 2350 cm−1, close to the antisymmetric stretching mode of free CO2. All of these features correspond to excitations of the antisymmetric stretching modes of CO2 molecules (or functional groups) in varying states of reduction where their wavenumbers reflect their structural nature within the [Ag(CO2)n]− clusters. These three sets of features afford detailed insight into a rich bonding and charge localization behavior and will be discussed individually in the following sections. Region of 1680−1870 cm−1: AgCO2− Ions. The features in the range 1680−1870 cm−1 are signatures of a partially reduced CO2 molecule, in analogy to similar features in [Au(CO2)n]− clusters.46 The structural motif giving rise to these features is a [AgCO2]− core ion. Here, the excess electron is shared between the silver atom and a CO2 molecule, in a complex that is structurally similar to a formate ion where the H atom has been replaced with a silver atom (Figure 2 I). In terms of molecular orbital theory, the Ag−C bond is a rather weak σ bond with a calculated bond dissociation energy of ca. 220 meV and a Ag−C distance of 246 pm, similar in nature to the Au−C bond in the AuCO2− binary complex.40 The argentyl formate ion is solvated by the additional CO2 molecules, and

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EXPERIMENTAL SECTION The experimental apparatus has been described in detail previously30,31 and is based on a design by Lineberger and coworkers.32 Briefly, [Ag(CO2)n]− clusters were produced by employing the third harmonic of a nanosecond-pulsed Nd:YAG laser (355 nm) to create silver vapor by laser ablation from a rotating silver target. The metal vapor was entrained31 in a pulsed supersonic expansion of CO2 (stagnation pressure 5.5 bar) and the high-density region of the expansion was bombarded with a high-energy (800 eV) electron beam to form various cluster species by adiabatic cooling and condensation, resulting in [Ag(CO2)n]− as well as other species, e.g., (CO2)n− clusters. The anions from this beam were then accelerated into and mass-selected by a Wiley−McLaren time-of-flight mass spectrometer. The mass-selected clusters of interest were irradiated by the tunable output of an optical parametric converter (LaserVision) with a bandwidth of 2 cm−1 and 7 ns pulse duration. Irradiation occurred in a multipass cell based on a design of Liu and co-workers.33 We tested for and did not observe any multiphoton effects. The formation of fragment ions was monitored using a reflectron as a secondary mass analyzer. Photodissociation experiments were performed over the wavenumber range 1600−2400 cm−1. The resulting fragment action spectra were corrected for laser fluence and multiple spectra from different days were averaged to improve 10765



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the additional CO2 molecules solvate the CO2 moiety of the [AgCO2]− core, polarizing the negative charge into the partially reduced CO2 ligand. In addition to carbon−oxygen interactions between solvent and the partially reduced CO2, quadrupole interactions between the solvent molecules favor solvation of the carboxylic group of the argentyl formate complex over solvation of the metal. The spectra of [Ag(CO2)3]− and [Ag(CO2)4]− show clear doublets in the νF region. The lower energy feature of these doublets, starting with [Ag(CO2)3]−, follows a sequential red shift as cluster size increases up to [Ag(CO2)11]−. The higher energy feature of these doublets (which disappears at n = 5) follows a similar trend for [Ag(CO2)2]− to [Ag(CO2)4]−. Following the arguments above on the origin of these features, these two νF peaks must arise from two different levels of CO2 reduction. We performed DFT calculations on a number of different structural conformers of [Ag(CO2)1−5]−. In complete agreement with the empirical interpretation offered above, the lowest energy conformers exhibited solvation around the partially reduced CO2 moiety in all cases (Figure 2). Within the formate-like structural motif, two unique solvation positions for smaller clusters (n = 2−3) were recovered. In the first, a solvent CO2 is located between the silver atom and reduced CO2 (“side position”, Figure 2 II-A). This results in modest reductive activation (and concomitant observed red shift). In the other motif a solvent CO2 docks directly behind the partially reduced CO2 (“terminal position”, Figure 2 II-B). A solvent molecule in this position polarizes the excess charge more strongly into the reduced ligand than a solvent molecule occupying the side position. Both of these conformers are found in the calculations with little energy difference, and the predicted infrared spectra of conformers II-A and II-B confirm our empirical analysis (Figure 3), showing that solvation in the terminal position causes a greater red shift of νF than solvation in the side position. We assign conformer II-A as the structure that gives rise to the feature at 1865 cm−1 in [Ag(CO2)2]−. We also note that although there is a clear νF signature in [Ag(CO2)2]−, it is

Figure 1. Infrared action spectra of [Ag(CO2)n]− (n = 2−11) monitoring the loss of CO2. (Right traces) signatures of the antisymmetric stretching mode νS of nearly unperturbed solvent CO2 molecules centered around 2349 cm−1. (Left traces) signatures of reduced CO2 ligands: νF (red dashed line) and νA (blue dash-dotted line). Numbers denote the number of CO2 molecules present in the cluster. Each trace is individually normalized, so left and right traces are on different scales.

Figure 2. Calculated minimum energy structures for [Ag(CO2)n]− (n = 1−3). (I) n = 1. (II-A) shows solvation at the “side” position (II-B) at the terminal position for n = 2. (II-C) shows symmetric solvation of a Ag− ion. (III-A), (III-B), and (III-C) show side/side, side/terminal, and terminal/terminal conformers for n = 3. Relative energies for each cluster size are indicated.

the red shift of its characteristic signature (νF) encodes the amount of partial charge on the CO2 moiety.46 Because the red shift of νF increases with cluster size, the addition of CO2 solvent molecules increases the negative partial charge on the partially reduced ligand. This is only possible if

Figure 3. Comparison of calculated infrared spectra in the νF region (lower two traces) of the lowest energy isomers of [Ag(CO2)2]− (left) and [Ag(CO2)3]− (right) with experimental spectra (top traces). We note that the calculated spectrum of conformer (III-C) is nearly identical to that of (III-B). 10766



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not the dominant structural motif of [Ag(CO2)2]− as will be discussed in the next section. The overall relative orientation of the solvent molecules is governed by optimization of the electrostatic interactions between their atomic partial charges and those in the charge carrier, resulting in arrangements where solvent CO2 molecules are preferentially oriented perpendicular to the Ag−CO2 plane. Similar solvation motifs are at play in larger clusters, as can be seen from the case of [Ag(CO2)3]− structures III-A and III-B (Figures 2 and 3). Again, there is excellent agreement between the calculated and experimental frequencies for both observed peaks, where the lower wavenumber peak is the signature of a conformer with the terminal position occupied, whereas the higher wavenumber component indicates the presence of a conformer where the terminal position is empty. Because both features are present in the spectrum of [Ag(CO2)3]−, and because the two conformers are isoenergetic within the accuracy of the calculations, we assume that a mixture of isomers III-A and III-B is present in the experiment. For large clusters, there are many possible conformers to survey that are less easily distinguished by their spectra. However, the smooth trend in the size evolution of the spectra (Figure 4) suggests

Figure 5. Calculated partial charge on the reduced CO2 ligand versus antisymmetric stretching frequency νF in both [Ag(CO2)2−5]− and [Au(CO2)2−5]− clusters (see text for discussion on structural assignments). Solid circle: antisymmetric stretching frequency of CO2− in a neon matrix.45 Blue triangles: data points for [Ag(CO2)2−5]−. Orange squares: data points for [Au(CO2)2−5]−.46 Dotted lines: maximum calculated charge transfer for [Ag(CO2)11]− (0.9 e−) and [Au(CO2)9]− (0.79 e−) based on the most red-shifted νF features observed.

assignments up to n = 4 and 5 in the case of both Au and Ag are based on calculated structures predicted by the side and terminal solvation motif, which are consistent with the dominant structural motifs for these cluster sizes. The fact that the data points for Ag and Au both lie on the same curve suggests that the linear relationship is independent of the metal in the region of 0.4−1 e− of charge on the reduced CO2, if the charge carrier is a formate-like complex. We caution that this linear trend breaks down and becomes apparently cubic in nature as the donated charge approaches zero. Region above 2300 cm−1: Solvent Molecules. The antisymmetric stretching frequency ν3 of free CO2 is measured at 2349 cm−1.44 Consistent with previous work on [Au(CO2)n]−, we assign features in this region of the spectra (righthand side of Figure 1) to the antisymmetric stretching mode of unperturbed or weakly perturbed solvent CO2 molecules (νS). Because many of the νS peaks are located below 2349 cm−1, it is clear by the arguments presented in the previous section and by comparison with other anion−CO2 complexes30,40 that the CO2 molecules in the solvent shell are slightly distorted by electrostatic interaction with the charge carrier and even take up a small fraction of the excess charge. In some instances, multiple peaks are clearly present, implying different coexisting ion−solvent interaction motifs. The solvent features in the spectrum of [Ag(CO2)2]− deviate the most from the antisymmetric stretching vibration in free CO2, with one broad peak centered at 2325 cm−1. Both [Ag(CO2)3]− and [Ag(CO2)4]− also exhibit a peak showing νS at relatively low wavenumbers, centered around 2330 and 2335 cm−1, respectively, in addition to a peak at 2349 cm−1. A nearly identical spectroscopic signature has been observed before in the Cl−·CO2·Ar cluster.30 The cause was determined to be a combination of a small amount of charge transfer from the halide onto the CO2 and deformation of the CO2 ligand by electrostatic interaction with the ion, resulting in the roughly ∼30 cm−1 red shift. In [Ag(CO2)2]−, the red shift of νS is due to conformers where the two CO2 solvent molecules compete

Figure 4. Peak positions in the lower energy region (left panel in Figure 1) plotted versus cluster size. The open squares represent structures where the terminal position is vacant. Solid squares represent structures where the terminal position is filled. Solid circles indicate when the charge carrier is CO2− (see text).

that the side/terminal solvation motifs persist until the terminal position is always filled. This situation occurs for n ≥ 5, because no doublet is observed in the νF region any more. If we combine all computational and experimental data on the νF signature, we can connect the predicted partial charge on the reduced CO2 moiety with the observed position of νF (Figure 5). On the basis of our experimental peak positions, the calculated partial charges for those conformers we can assign (i.e., for n = 2−5), and the antisymmetric stretching frequency of CO2−,45 we find there is a linear relationship between the antisymmetric stretching frequency and partial charge on the CO2 ligand in this wavenumber range (Figure 5). On the basis of this relationship, we can interpolate the amount of the excess charge on the partially reduced CO2 ligand as a function of νF to larger cluster sizes. We conclude that ca. 0.9 e− has been transferred to the activated CO2 moiety in [Ag(CO2)11]−. As was mentioned earlier, the same νF feature and a similar red shift trend has been observed in [Au(CO2)n]− clusters. The 10767



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for access to the negative charge on a Ag− ion, and neither forms a strongly bound complex, very different from conformations giving rise to the νF feature (Figure 2 II-C). The calculated red shift for this motif is ca. 25 cm−1. Consequently, there is no substantial activation of either CO2 molecule (and no feature in the νF region) but rather a ∼25 cm−1 red-shifted νS for both. The same feature is observed in [Ag(CO2)3]− and [Ag(CO2)4]− and can be attributed to similar solvation motifs where all of the CO2 molecules interact nearly equivalently with a Ag− core. Increasing the number of solvent molecules around a Ag− ion also explains the slight shift of the νS signature from 2325 cm−1 in [Ag(CO2)2]− to 2330 cm−1 in [Ag(CO2)3]− to 2335 cm−1 in [Ag·(CO2)4]− as charge transfer to each CO2 will slightly diminish with increasing cluster size. Taking a look at the relative intensities of the 2349 cm−1 features vs the 2330 cm−1 features as cluster size increases, it is apparent that this symmetric solvation motif becomes less favorable with increasing size and disappears altogether for n ≥ 5. Although the discussion above explains the νS features for the small cluster sizes, this is clearly not the only structure at play as the presence of the νF signature discussed further above demonstrates. The observation of the νF feature in the spectrum of [Ag(CO2)2]− indicates that structure II-A is populated as well. This conformer should give rise to a feature close to 2349 cm−1, contrary to our observations. Because the calculated intensities of νS for all conformers calculated for [Ag(CO2)2]− are comparable in magnitude, we postulate that the absence of such a feature is due to a significantly smaller population of conformation II-A. Though it is difficult to quantitatively report relative experimental intensities for the νF and νS regions, we note that the νF signature for [Ag(CO2)2]− is rather weak, consistent with this idea. We assume that the νS signature from conformer II-A is too weak relative to the strong feature arising from conformer II-C and is buried in the highwavenumber slope of the latter. Features at 1660 and 1850−1950 cm−1: CO2-Based Charge Carriers. The third set of features observed is located around 1660 cm−1 in all [Ag(CO2)n]− spectra for n = 3−9. These features are very close to the antisymmetric stretching signature of the CO2− monomer anion νA, observed at 1658.3 cm−1 in matrix isolation experiments45 and in the spectrum of (CO2)7−,47 where the charge carrier is a solvated CO2− monomer anion.48 Figure 6 shows a comparison of the IR spectra of (CO2)4,7− clusters with those of [Ag(CO2)4,5]−, the latter acquired by monitoring the loss of a neutral silver atom. The comparison conclusively shows that the νF signatures (1750−1810 cm−1 for these sizes) are no longer observed for this loss channel, but the feature at 1660 cm−1 persists. We assign this feature to a fully reduced CO2− anion, solvated by the remaining cluster constituents, including a neutral silver atom. Interestingly, another group of three peaks is observed for n = 4 around 1900 cm−1, which has been previously assigned to a solvated (CO2)2− dimer anion,47 again suggesting that the excess charge localization no longer significantly involves the Ag atom. Direct comparison with the spectrum of (CO2)4− confirms this assignment (Figure 6). We note that there are additional vibrational signatures in the region 2800−3800 cm−1, which encode information on the bending and symmetric stretching modes of CO2− and (CO2)2−.48 However, we refrained from taking spectra there, because combination bands and overtones are usually weaker than fundamentals, and the antisymmetric stretching mode

Figure 6. Infrared photodissociation spectra of (CO2)7− (A) and (CO2)4− (B) compared to [Ag·(CO2)]5− (C) and [Ag·(CO2)]4− (D) monitoring loss of Ag0.

already yields much information on the details of structure and charge distribution in these clusters. The evidence discussed above clearly demonstrates that AgCO2− and (CO2)1,2− can be charge carriers in [Ag(CO2)n]− clusters for n = 3 − 9, and that silver can act as solvent in this size regime. This observation is intriguing, because it suggests that complete charge transfer and with it the first step of a catalytic reduction of CO2 can be achieved in the presence of Ag. To further explore this possibility, we performed DFT calculations to assess the barrier for charge carrier conversion from [AgCO2]− to (CO2)−2 in [Ag(CO2)4]− clusters. The calculations were carried out by scanning the OCO bond angle of the reductively activated CO2 molecule while letting the remaining degrees of freedom relax. The resulting energy as a function of the fixed OCO bond angle is shown in Figure 7. As predicted from the experimental spectra, two stable minima were found, one for each type of charge carrier. Assuming a CO2 solvent binding energy of roughly 150 meV, based on the observation that at most two CO2 molecules

Figure 7. Potential energy as a function of OCO angle of the reduced ligand in [Ag(CO2)4]− without (red, filled, full line) and with dielectric (black, filled, dashed line). The blue, open data points (blue dotted line) represent a calculation with dielectric, but without the two CO2 solvent molecules in the side position. The data points are the calculated values, the line is to guide the eye. 10768



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could be evaporated at 2349 cm−1, both charge carriers should be present in our experiment because the internal energy of the clusters (which is of the order of the binding energy of a solvent molecule49) is insufficient to overcome the barrier between the conformers, freezing out the structural conformers in local minima as previously observed in other cluster systems.50 Simulating additional solvation using COSMO resulted in a lowering of the barrier and the second minimum. Another simulation with dielectric, but without the two CO2 solvent molecules in side positions shows that the molecular nature of the solvent plays an important role and cannot be taken into account with a dielectric continuum model like COSMO. Let us revisit our statement above that reduction of CO2 can be achieved in the presence of Ag. On the basis of the potential energy curve in Figure 7, we judge that those clusters exhibiting CO2-based charge carriers are likely formed in the ion source, where they are formed sufficiently cold to remain in a robust higher lying minimum analogous to that shown in Figure 7. A possible pathway to such species is the attachment of a neutral silver atom to preformed (CO2)n− cluster ions during cluster growth. Similar adducts have been observed by Johnson and coworkers in expansions of CO2 containing pyridine.47 It is noteworthy that a Ag atom can be a weakly bound spectator (in contrast to Au46), so its presence does not preclude the formation of CO2-based charge carriers. However, we cannot claim that the presence of the Ag ion in fact leads to complete reduction. Comparison of [Au(CO2)n]− vs [Ag(CO2)n]−. There are several differences between [Au(CO2)n]− vs [Ag(CO2)n]− cluster ions that warrant mention. First, the maximum amount of charge transferred to the reduced CO2 moiety by solvation of the metal−CO2 complex (Figure 5) is greater for silver (0.9 e−) than for gold (0.79 e−).46 This can be easily explained by the greater electronegativity of Au than of Ag (2.4 vs 1.93 on the Pauling scale),51 allowing a more polar metal−carbon bond for Ag than for Au. Another difference in behavior is the appearance of CO2based charge carriers in [Ag(CO2)n]−, whereas they are absent in [Au(CO2)n]−. Consistent with this observation, Figure 8 shows the potential energy of [Au(CO2)4]− as a function of the OCO angle of the reduced CO2 ligand. Different from the case of silver complexes (Figure 7), there is no local minimum corresponding to a (CO2)2− charge carrier. Instead of

electronegativity, this may be more appropriately rationalized by the (related) difference in electron affinity between AuCO2 and AgCO 2 as compared to those of CO 2 clusters. Unfortunately, there are no well-determined experimental values available for any of these three species. Estimated values for CO2 clusters52 are in the same range as Au (2.309 eV), but significantly larger than of Ag (1.303 eV).28,29 Assuming that the electron affinities of AuCO2 and AgCO2 track those of the metals, the lower electron affinity of AgCO2 is likely responsible for the observation of (CO2)1,2− charge carriers. Finally, the size evolution of νF in [Au(CO2)n]− shows a blue shift beginning at n = 10, which is due to the fact that the immediate vicinity of the reduced CO2 moiety is filled and solvation has to proceed by adding solvent molecules to the Au moiety in the AuCO2− charge carrier.46 Although there is no blue shift of νF in [Ag(CO2)n]− at n = 10, this is the cluster size where the signature of CO2− charge carriers disappears. This suggests that solvation at the metal atom does not strongly withdraw electron density from the CO2 moiety of the AgCO2− ion, but that solvation of the Ag atom precludes formation of CO2− ions as charge carriers. All these observations point toward a very delicate balance between electron binding energies and electron localization trends that highlight the intricate problems in choosing an appropriate material and a suitable solvent for electrochemical or photoelectrochemical reduction of CO2.

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CONCLUSION Our observations show that upon formation of a complex between a CO2 molecule and an anionic silver model catalyst, the interaction of solvent molecules with the [AgCO2]− intermediate drives the reduction of the molecular ligand. The linear correlation between charge on the CO2 ligand and its νF frequency can be a very useful tool to look for reduced CO2 intermediates on the surface of real catalysts in electrochemical cells, and to characterize such intermediates in situ. The coexistence of [AgCO2]− and (CO2)1,2− as charge carrying species indicates that reduced CO2 can exist in the immediate vicinity of a single silver atom, and that the tendency toward electron transfer to CO2 is higher than in gold. It is an open question how the interaction with other solvents (e.g., water) changes the behavior of the [AgCO2]− intermediate, and this issue will be addressed in future studies. The importance of molecular-level solvation effects (as opposed to effects of a continuum dielectric) highlights the importance of understanding the details of the solvation of reactive intermediates in catalysis. Together with support properties, the interaction with solvent molecules is a large factor for the suitability of a catalyst for CO2 reduction.

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AUTHOR INFORMATION

Corresponding Author

*J. M. Weber: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We gratefully acknowledge support from the National Science Foundation through Grant Number CHE-0845618. We thank Professor John F. Stanton for helpful discussions on technical aspects of the calculations.

Figure 8. Potential energy as a function of OCO angle of the reduced ligand in [Au(CO2)4]− . The data points are the calculated values; the lines are to guide the eye. 10769



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Solvent-Mediated Reduction of Carbon Dioxide in Anionic Complexes

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