Plasmonic photosynthesis of C₁–C₃ hydrocarbons from carbon dioxide assisted by an ionic liquid

Abstract

Photochemical conversion of CO₂ into fuels has promise as a strategy for storage of intermittent solar energy in the form of chemical bonds. However, higher-energy-value hydrocarbons are rarely produced by this strategy, because of kinetic challenges. Here we demonstrate a strategy for green-light-driven synthesis of C₁–C₃ hydrocarbons from CO₂ and H₂O. In this approach, plasmonic excitation of Au nanoparticles produces a charge-rich environment at the nanoparticle/solution interface conducive for CO₂ activation, while an ionic liquid stabilizes charged intermediates formed at this interface, facilitating multi-step reduction and C–C coupling. Methane, ethylene, acetylene, propane, and propene are photosynthesized with a C₂₊ selectivity of ~50% under the most optimal conditions. Hydrocarbon turnover exhibits a volcano relationship as a function of the ionic liquid concentration, the kinetic analysis of which coupled with density functional theory simulations provides mechanistic insights into the synergy between plasmonic excitation and the ionic liquid.

While light-driven conversion of CO₂ and H₂O directly into fuels affords an attractive means to store sunlight in chemical bonds, few systems produce high-value hydrocarbons. Here, authors show gold nanoparticles to reduce CO₂ to multi-carbon products using visible light, ionic liquids, and H₂O.

Introduction

Carbon dioxide (CO₂) fixation is recognized to be a much-needed component of a carbon-neutral energy strategy^1–4. Although CO₂ is relatively unreactive, various catalytic processes triggered by heat (thermochemical)^5–8, electricity (electrochemical)^9–17, and light (photochemical)^18–28 are being explored for activating CO₂ and recycling it back to valuable petrochemicals. Sunlight-driven conversion of CO₂ to fuels is particularly attractive as a means to store intermittent solar energy in the form of C–C and C–H bonds. Semiconductor and metal-catalyzed photoelectrolytic reduction of CO₂ has shown promise; however, these processes have often required ultraviolet (UV) light and/or considerable electrical energy input, or they do not favor energy-rich hydrocarbon products. Longer-chain hydrocarbons possess higher energy densities. Moreover, hydrocarbons in the liquid state are easier to transport^29,30. However, the formation of longer-chain hydrocarbons from CO₂ requires multiple electron (e^–) and proton (H⁺) transfer steps, as well as C–C bond formation^9,31,32, which pose major kinetic bottlenecks.

Here we demonstrate a visible-light-driven route for the conversion of CO₂ and H₂O into C₁–C₃ hydrocarbons. The scheme does not involve the application of an electrochemical potential, UV light, high temperatures, hydrogen gas, or a sacrificial agent. It uses green light as the sole energy input and driving agent. The strategy employs plasmonic Au nanoparticles (NPs) of a pseudospherical shape and an average diameter of ~12 nm, as characterized previously²⁸. Au NPs are known from electrochemical studies³³ to activate CO₂. The choice of Au NPs was further driven by the relative chemical stability of Au against bulk oxidation and photocorrosion; the other two common plasmonic metals, Ag and Cu, while electrocatalytically active for CO₂ reduction, are prone to oxidation in air, water, and/or light excitation. The Au NPs possess a strong localized surface plasmon resonance (LSPR) band centered around 520 nm (Fig. 1a), which enables strong, resonant absorption of green light. The LSPR excitation of the NPs yields energetic electron–hole (e^––h⁺) carriers via Landau damping. These e^––h⁺ carriers were shown in recent studies to drive redox conversions^28,34–36, especially the conversion of CO₂ to methane and ethane under blue–green light²⁸. However, in this past demonstration, isopropanol was used as a sacrificial h⁺ scavenger to facilitate e^––h⁺ pair separation; otherwise, unproductive e^––h⁺ recombination dominated. Thus, isopropanol served as the H⁺ source in this CO₂ reduction scheme, which posed a major limitation for net energy storage.

Fig. 1.

Ionic-liquid-promoted CO₂ reduction to C₁–C₃ hydrocarbons using a plasmonic Au nanoparticle (NP) photocatalyst. a UV−vis extinction spectrum of a colloid of the Au NPs used for preparation of the photocatalyst film. The spectrum exhibits a localized surface plasmon resonance (LSPR) band centered around 520 nm, as indicated by the dotted line. b Scheme for CO₂ conversion on plasmon-excited Au NPs promoted by an ionic liquid, EMIM-BF₄. A continuous-wave (CW) laser of a wavelength of 532 nm and intensity of 1 W cm^–2 was used as the light source for photoexcitation of Au NPs. EMIM-BF₄ stabilizes CO₂ and resulting adsorbates/intermediates on the photoexcited Au surface. c Turnover frequencies of hydrocarbon products formed in the CO₂RR plotted as a function of the EMIM-BF₄ concentration (mol%). The CO₂ conversion activity peaks at 5 mol% of EMIM-BF₄. d Hydrocarbon product selectivity as a function of EMIM-BF₄ concentration (mol%). Each data point in c and d is the average of results from three identical trials and the error bar represents the SD of these measurements

The present strategy overcomes this drawback and uses water as the H⁺ source and does not require a sacrificial h⁺ scavenger, thus constituting a truly fuel-forming reaction. The enhanced reactivity was enabled by the use of an ionic liquid (IL) medium, specifically comprised 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄). Our choice was motivated by examples from electrocatalytic CO₂ reduction reaction (CO₂RR) where the EMIM-BF₄ electrolyte, owing to its highly ionic character, stabilizes the high-energy CO₂^•− radical anion intermediate formed in the reaction and decreases the overpotential needed for CO₂RR^37–42. In addition, EMIM-BF₄ has a wide electrochemical window and high thermal stability^43,44. In our photocatalytic scheme, the EMIM-BF₄, as we find from kinetic analysis and density functional theory (DFT) simulations, promotes e^– transfer at the interface of the photoexcited Au NP and adsorbed CO₂ (Fig. 1b), obviating the need for a h⁺ scavenger or applied potential for e^––h⁺ separation.

Results

IL-mediated plasmonic CO₂ reduction

The photocatalyst had the form of a substrate-supported film of Au NPs immersed in an aqueous solution of EMIM-BF₄ saturated with CO₂ and contained inside a glass reactor (Supplementary Methods). The light excitation source comprised a continuous-wave (CW) laser of a wavelength of 532 nm light and an intensity of 1 W cm⁻². Under CW excitation, the steady-state temperature of the reaction medium got moderately elevated to ~48 °C. Hydrocarbon products collected in the reactor headspace were measured (Supplementary Figs. 1−11) using a gas chromatograph (GC) equipped with a flame ionization detector. The EMIM-BF₄ concentration was varied from 0 to 100 mol%, to find optimal conditions for CO₂RR. In 1–10 mol% EMIM-BF₄, the products of plasmon-excitation-driven CO₂RR were found to be C₁ (CH₄), C₂ (C₂H₄ and C₂H₂), and highly reduced C₃ (C₃H₆ and C₃H₈) hydrocarbons (Fig. 1c, d and Supplementary Note 1). This product profile is quite striking when one considers that the major product in electrochemical CO₂RR is carbon monoxide (CO) formed by 2e^––2H⁺ reduction of CO₂ (refs. ^13–17). On the other hand, propane (C₃H₈), formed in our scheme, requires an overall 20e^––20 H⁺ reduction and coupling of three CO₂ molecules. Such generation of C₃ hydrocarbons by artificial photosynthesis is challenging and therefore rare.

The CO₂RR activity depends on the IL concentration (Fig. 1c). In pure water the activity was nil, whereas in 1 mol% EMIM-BF₄ solution the generation of C₁, C₂, and C₃ hydrocarbons was observed. The CO₂RR activity, as quantified by turnover frequencies (TOFs) of the hydrocarbon products, increased dramatically with an increase in the EMIM-BF₄ concentration. The highest activity was found at 5 mol% EMIM-BF₄. Increasing the EMIM-BF₄ concentration further resulted in a sharp drop in the CO₂RR activity. In 100 mol% EMIM-BF₄ solution, the activity was nil, similar to that in pure water. Thus, the CO₂RR activity exhibits a volcano relationship as a function of the EMIM-BF₄ concentration (Fig. 1c). At all EMIM-BF₄ concentrations, where C₁, C₂, and C₃ hydrocarbons were produced, the product selectivity was found to follow the order: C₁ > C₂ > C₃. The selectivity for C₂₊ production is ~50% in 1−10 mol% EMIM-BF₄ solution (Fig. 1d).

Non-hydrocarbon products were also characterized by a GC equipped with a thermal conductivity detector (TCD) (Supplementary Figs. 12–15). Considerable hydrogen (H₂) production was measured (Supplementary Fig. 12), the TOF of which was 138.2 NP⁻¹ h⁻¹ in 5 mol% EMIM-BF₄ solution, the IL concentration where CO₂RR activity is the highest. The H₂ likely originates from the competing reduction of H⁺ in the reaction medium (Supplementary Eq. (6)). In the GC-TCD measurements, there were no detection of CO (Supplementary Fig. 15), otherwise known to be a major product in electrocatalytic CO₂RR on Au (refs. ^13–17). Of the possible oxidation products, there was no measurable production of O₂ (see Supplementary Information). H₂O₂ was detected (Supplementary Figs. 16–18) by the fluorogenic test employing a amplex red and horseradish peroxidase reagent⁴⁵. Thus, the oxidation of H₂O to H₂O₂ and H⁺ (2H₂O → H₂O₂ + 2H⁺ + 2e⁻) is the likely oxidation half-reaction that consumes the photogenerated h⁺.

Control studies were performed, one without Au NPs, another without light, and a third without CO₂. The conditions were otherwise maintained the same as those in the photoreaction tests and a 5 mol% EMIM-BF₄ solution, found to be most optimal in the photoreaction tests, was employed. The control studies showed that the absence of any one of the components Au NPs, green light illumination, or CO₂ resulted in nil hydrocarbon production, despite the use of 5 mol% EMIM-BF₄ solution (Supplementary Fig. 19a–c). Thus, it is confirmed that the hydrocarbon production originates from green-light-driven CO₂ reduction on Au NPs. The control study without light excitation was performed at an elevated temperature of 50 °C so as to mimic the steady-state bulk solution temperature of the reaction mixture in the photoreaction tests. The lack of CO₂RR activity in this dark control study demonstrates that the CO₂RR activity in the photoreaction tests does not originate from simply a photothermal effect of the light excitation. Rather a photoredox process facilitated by the Au NPs and the IL is responsible for the conversion of CO₂ to hydrocarbons.

The plasmonic catalyst also exhibited stability and recyclability under the photoreaction conditions and IL media subjected on the catalyst. We tested the same substrate-supported Au NP film immersed in 5 mol% EMIM-BF₄ over multiple cycles, each consisting of a 10 h photoreaction. The CO₂RR activity and product selectivity, as determined from the TOFs of the hydrocarbon products, was maintained over the course of this multi-cycle test (Supplementary Fig. 20). As the NP film or EMIM-BF₄ solution were not replenished between cycles, the maintenance of CO₂RR activity over multiple cycles suggests that Au and EMIM-BF₄ were not consumed, at any discernible levels, in the photoredox reaction.

The origin of products

Given the hydrocarbon profile of the product mixture, it was necessary to go beyond the control studies described above and confirm more directly that CO₂, rather than carbon contamination or photolysis of the EMIM-BF₄, was the source of the hydrocarbon products. For this confirmation, ¹³C isotope labeling was employed (Fig. 2 and Supplementary Figs. 21 and 22). In this labeling study, ¹³CO₂ was employed as the reactant instead of ¹²CO₂, whereas all other conditions were kept the same as those in other photoreaction tests. GC-mass spectrometry (GC-MS) was used for identification of the hydrocarbon products generated in the photoreaction (Fig. 2a). The GC-MS analysis confirmed the presence of ¹³CH₄ (Fig. 2b) and ¹³C₂H₂ (Fig. 2c), manifested by their characteristic mass fragmentation patterns, shifted to higher m/z compared with reference fragmentation patterns of ¹²CH₄ and ¹²C₂H₂, respectively. Thus, isotope labeling confirms CO₂ to be the origin of hydrocarbon products.

Fig. 2.

¹³CO₂ isotopolog confirmation of CO₂RR. a Total ion chromatogram (TIC) of the gaseous products from a 240 h long Au NP-photocatalyzed ¹³CO₂RR in 5 mol% EMIM-BF₄ solution under CW irradiation of 532 nm light (1 W cm^–2). Peaks in the TIC appearing at retention times of 4.6 min and 8.2 min correspond to ¹³CH₄ and ¹³C₂H₂. The basis for this assignment is provided in Supplementary Fig. 22. b Mass fragmentation pattern (red bars) acquired at a retention time of 4.6 min of the TIC shown in a. For comparison, a reference fragmentation pattern (gray bars) of ¹²CH₄ from the National Institute of Standards and Technology (NIST) Chemistry WebBook is shown. Fragments at m/z = 14 and 18 in the experimental pattern were suppressed to remove the mass peaks contributed by N₂ and moisture, respectively. c Mass fragmentation pattern (blue bars) acquired at a retention time of 8.2 min of the TIC shown in a. For comparison, a reference fragmentation pattern (gray bars) of ¹²C₂H₂ from the NIST Chemistry WebBook is shown. The fragment at m/z = 28 in the experimental pattern has relatively high abundance as compared with that of the reference fragmentation pattern due to the contribution of N₂ from the atmosphere. Relative abundances in b and c were obtained from the measured abundances shown in Supplementary Fig. 21a, b, respectively. It is noteworthy that ¹³C₂H₄ was not resolved by GC-MS due to the likely overlap of the ¹³C₂H₄ peak with the broad, intense ¹³CO₂ peak in the TIC (Supplementary Fig. 22b)

The role of the IL

We attempted to gain a mechanistic understanding of this catalytic scheme focusing on the question of how the IL promotes CO₂RR activity. It was observed that the presence of EMIM-BF₄ in the aqueous medium results in a considerably acidic pH (Supplementary Fig. 23): the 5 mol% EMIM-BF₄ solution has a pH of 2.95. To determine whether this acidity is responsible for the enhanced CO₂RR activity in a EMIM-BF₄ solution, we performed a photoreaction in deionized water containing no EMIM-BF₄ but with a pH of 2.93 achieved using acid (Supplementary Fig. 19d). All other conditions were kept the same as in the photoreactions in EMIM-BF₄ solutions. In this EMIM-BF₄-free photoreaction, no products were observed, which demonstrated that the high acidity or H⁺ concentration, [H⁺], of the EMIM-BF₄-containing medium is not the sole cause of the enhanced CO₂RR activity. EMIM-BF₄ plays other role(s). It is possible, in principle, for EMIM-BF₄, instead of H₂O, to serve as the h⁺ acceptor; however, if this were the case, then the CO₂RR activity would have been enhanced at higher EMIM-BF₄ concentrations, in line with a study of a different plasmon excitation-catalyzed redox reaction³⁶. Instead, we observed peak activity at a EMIM-BF₄ concentration of 5 mol%, above which the activity drops steeply reaching nil in pure EMIM-BF₄ wherein H₂O is not available.

We hypothesized that the strongly ionic character of EMIM-BF₄ plays a role in the activation of CO₂, which is otherwise fairly redox inactive. CO₂, however, is highly polarizable, as indicated by its quadrupole moment of −4.3 D Å (ref. ⁴⁶). The interaction of EMIM-BF₄ and CO₂ was simulated by DFT. A past study suggests that CO₂ can undergo complexation with the N-heterocyclic carbene, EMIM^*, formed from EMIM⁺ by H⁺ loss⁴². We investigated using DFT the structure of such a [EMIM*-CO₂] complex (Fig. 3a). The complex exhibits binding between the C atom of the CO₂ and the C₂ atom of the imidazole ring with an energy of intermolecular interaction, E_m-m, of −0.36 eV. This interaction is stronger than, for instance, the interaction of an H₂O molecule and CO₂ (Fig. 3b). Unlike the latter case, complexation with EMIM^* leads to considerable restructuring of the CO₂ moiety. The CO₂ moiety adopts a bent configuration with an O=C=O angle of 133.7° and C=O bonds lengthened to 1.24 Å. In fact, the geometry of the CO₂ moiety in the complex closely mirrors that of the CO₂^•− anion radical, which has a bond angle of 137.8° and bond length of 1.23 Å (Supplementary Fig. 24). Moreover, from Mulliken charge partitioning analysis (Supplementary Fig. 25), the CO₂ moiety in the [EMIM*-CO₂] complex is found to have a net charge of −0.73, which indicates its partial anionic character.

Fig. 3.

The role of the ionic liquid in Au NP-photocatalyzed CO₂RR. a,b DFT-optimized geometries of [EMIM*-CO₂] (a) and [H₂O-CO₂] (b) complexes. C, H, O, and N atoms are depicted by gray, white, red, and blue spheres, respectively. Key bond lengths, bond angles, and the energy of intermolecular interaction, E_m-m, are indicated for each of the complexes. c–e DFT-computed free energy cost, ∆G, of formation of the 1e⁻ adduct of CO₂ (c), 1e⁻ adduct of [EMIM*-CO₂] (d), and 1e⁻ adduct of CO₂ in the presence of EMIM⁺ (e). In the latter case, the 1e⁻ adduct of CO₂, CO₂^•−, is stabilized by complexation with EMIM⁺ as described by the net process: EMIM⁺ + CO₂ + e⁻→ [EMIM-CO₂]^•. In c–e, the free energy of each species is indicated in parentheses. Scale bars are 1 eV in length

It is known that the energetic cost of the drastic structural reorganization from linear CO₂ to the bent CO₂^•− anion radical poses a major barrier for e⁻ acceptance by CO₂ (refs. ^37–42). However, our DFT calculations show that in its complex with EMIM*, the CO₂ moiety is structurally pre-configured for e⁻ acceptance. Consistent with this finding, 1e^– addition to [EMIM*-CO₂] is much more favorable as compared with 1e⁻ addition to CO₂ (Fig. 3c, d). Thus, it appears that EMIM-BF₄ can promote the transfer of photogenerated e^– from the Au NP to adsorbed CO₂, which is otherwise a major kinetic bottleneck in the photocatalytic reduction process. Furthermore, it is plausible that the CO₂^•− anion radical formed on the Au surface by photo-initiated e^– transfer process has an enhanced lifetime due to solvation or complexation by EMIM⁺ (Fig. 3e). A longer lifetime of this reactive intermediate would increase the probability of C–C coupling between the intermediates.

Empirical kinetic model

Although the DFT computations provide insight into the central role of EMIM-BF₄ in CO₂ activation, the volcano-type dependence of the CO₂RR activity on the IL concentration deserves an explanation. From the hydrolysis of EMIM-BF₄ known from past studies^47–50:

$${\mathrm{EMIM\; -\; BF}}_4+x{\mathrm{H}}_2\mathrm{O}\to {\mathrm{EMIM}}^{+}+{\left[{\mathrm{BF}}_{4-x}{\left(\mathrm{OH}\right)}_x\right]}^{-}+x\mathrm{HF}$$ 1

where x = 1–4 and the complexation of CO₂ with EMIM⁺ predicted in DFT simulations:

$${\mathrm{EMIM}}^{+}+{\mathrm{CO}}_2\to \left[{\mathrm{EMIM}}^{*}{\mathrm{-CO}}_2\right]+{\mathrm{H}}^{+}$$ 2

we postulate a rate determining step in the reaction of CO₂ and H₂O:

$${\mathrm{EMIM\; -\; BF}}_4+{\mathrm{CO}}_2+x{\mathrm{H}}_2\mathrm{O}\to \left[{\mathrm{EMIM}}^{*}{\mathrm{-CO}}_2\right]+{\left[{\mathrm{BF}}_{4-x}{\left(\mathrm{OH}\right)}_x\right]}^{-}+\left(x+1\right){\mathrm{H}}^{+}+x{\mathrm{F}}^{-}$$ 3

From this reaction equation, the concentration of the activated CO₂ complex, [EMIM*-CO₂], is expected to be directly proportional to [H⁺]^x+1. Therefore, the [H⁺] determined from the measured pH of the EMIM-BF₄ solution (Supplementary Fig. 23) serves as a proxy for the concentration of [EMIM*-CO₂], based on which the [EMIM*-CO₂] concentration is expected to be the highest in the EMIM-BF₄ concentration range around 5 mol%. The higher the concentration of the activated [EMIM*-CO₂] complex, the greater is the rate of CO₂ conversion and also the higher the likelihood of C–C coupling required for C₂₊ production. Therefore, both the overall activity and the selectivity in favor of C₂₊ products are favorable in the 3–7 mol% EMIM-BF₄ range, with the most optimal performance achieved at 5 mol% EMIM-BF₄. On the other hand, the activated complex has zero concentration in pure water on one extreme and in pure EMIM-BF₄ on the other extreme, which explains the nil turnover at these conditions. An additional reason for the drop in activity at higher EMIM-BF₄ concentrations may be that the adsorption of BF₄^– to the Au NP surface (Supplementary Fig. 26) dominates at these concentrations to such an extent that the adsorption of CO₂ and/or [EMIM*-CO₂] to the Au surface is largely inhibited and so is the e⁻ transfer to CO₂.

The CO₂RR activity depends on the concentration of this activated complex to a high reaction order. This is best exemplified by the plots of TOF for each hydrocarbon as a function of the [H⁺] (Fig. 4a–e), which as explained above, serves as a proxy for the concentration of [EMIM*-CO₂]. The pseudo-reaction order, n, is found to be 1.9 for C₂H₄, 2.5 for C₂H₂, 3.7 for C₃H₆, and 4.0 for C₃H₈. The fit for the CH₄ TOF has a relatively high χ²-value, so the n of 2.7 estimated for CH₄ has a lower confidence. In general, the pseudo-reaction order is higher for the longer hydrocarbons, which perhaps captures the need for multiple activated complexes to be available for undergoing coupling to C₂ and C₃ fragments. The high pseudo-reaction order for the C₃ products goes hand-in-hand with an apparent threshold in [H⁺] below which the TOF is zero or below the detection limit (Fig. 4d, e). For each of the hydrocarbon products, the [H⁺] raised to the power of the corresponding n follows a volcano trend with respect to the EMIM-BF₄ concentration, mirroring closely the trend in the TOF for that hydrocarbon (Fig. 4f–j).

Fig. 4.

Empirical kinetic model for the CO₂RR. a–e For each of the hydrocarbon products, the TOF is plotted as a function of the H⁺ concentration, [H⁺], which serves as a proxy for the concentration of the [EMIM*-CO₂] complex. A fit to a power-law function y = axⁿ (dashed line) yields the fit parameter n, which represents the apparent reaction order in [H⁺]. The best-fit equation is indicated for each plot along with the values of χ² and R², which serve as metrics of the goodness-of-fit. f–j For each hydrocarbon product, the TOF plotted as a function of the EMIM-BF₄ concentration follows a similar trend as the [H⁺]ⁿ, where n is the corresponding reaction order obtained from the plots in a–e. Each data point in a–j is the average of results from three identical trials and the error bar represents the SD of these measurements

Thus, we reported the green-light-driven synthesis of C₁–C₃ hydrocarbons from CO₂ and water on plasmonic Au NPs in an IL medium. The resonant green light absorption of the plasmonic NPs and their ability to sustain electrostatically charged surfaces under resonant CW excitation are at the heart of the observed photoreactivity. The IL plays a synergistic role due to its complexation with the CO₂, which preconfigures the CO₂ for accepting e^– from photoexcited Au NPs. The enhanced reactivity of CO₂ in the presence of the IL obviates the need for an applied potential or a sacrificial scavenger. Although hydrocarbon production yields in the reaction need further optimization, the generation of propane by overall 20e⁻–20H⁺ reduction and coupling of three CO₂ molecules is both striking and mechanistically rich. The precise intermediates and reaction pathways, including C–C coupling and dehydrogenation steps, which yield each of the hydrocarbons, deserve further elucidation. Beyond CO₂ conversion studied here, ILs may have promise in other photocatalytic schemes where activation of relatively inert substrates and stabilization of high-energy charged intermediates is desirable.

Author contributions

S.Y. performed all experimental studies, DFT computations, and data-analysis, and co-wrote the manuscript. P.K.J. conceived project and designed studies, analyzed results, and co-wrote the manuscript.

Data availability

All raw images and source data are available from the authors upon reasonable request.

Competing interests

The authors declare no competing interests.

Supplementary information

Supplementary Information accompanies this paper at 10.1038/s41467-019-10084-5.