Selective Branching of Plasmonic Photosynthesis into Hydrocarbon

http://pubs.acs.org/journal/aelccp

Selective Branching of Plasmonic Photosynthesis into Hydrocarbon Production and Hydrogen Generation Sungju Yu† and Prashant K. Jain*,†,‡,§,∥ Department of Chemistry, ‡Materials Research Laboratory, §Department of Physics, and ∥Beckman Institute for Advanced Science and Technology, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States

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†

S Supporting Information *

ABSTRACT: Electrocatalytic and photocatalytic carbon fixation suffer from a major challenge: the undesirable hydrogen evolution reaction (HER) typically outcompetes the CO2 reduction reaction (CO2RR), amounting to a loss of energetic charge carriers and a reduced selectivity for hydrocarbon production. Likewise, hydrocarbon production by the CO2RR and hydrogen generation by the HER constitute competing pathways in plasmon-excitation-driven photosynthesis on Au nanoparticles promoted by an ionic liquid medium. Here, we measure the selectivity between these two reduction pathways under various reaction conditions. We show that the branching between the HER and CO2RR pathways can be tuned by controlling the proton availability in the medium. We identify conditions at which the CO2RR effectively outcompetes the HER: 84% of harvested electrons are utilized selectively for hydrocarbon production, whereas the HER branch is suppressed. The greater implication of this work is that reaction selectivity can be overturned by the utilization of plasmonic excitation and optimization of the solid−solution interfacial environment. reduction of CO2 into C1−C3 hydrocarbons. The counter reaction involves the two-hole (2h+)-mediated oxidation of H2O to H2O2 and H+,15 enabled by the energetically deep dband position of the h+ in Au. On the basis of standard free energies, the overall conversion of CO2 and H2O to hydrocarbons and H2O2 is endergonic (ΔG0 > 0), the larger implication of which is that plasmonic excitations can be harvested for performing thermodynamic work (plasmonic work), going beyond the well-established enhancement of chemical reaction kinetics by plasmonic excitation (plasmonic catalysis).30−35 Despite the promise of this scheme, there was a significant loss of plasmonically generated e− and H+ to the HER, the pathway alternative to CO2 reduction. In fact, the H2 yield was manifold that of the hydrocarbon yield. To address this major limitation in the artificial photosynthetic scheme, here we measure the selectivity toward H2 versus hydrocarbon production under various reaction conditions. Using this information, we develop microkinetic understanding and control of the elementary reactions involved in the plasmondriven CO2 reduction scheme, including the oft-neglected oxidation half-reaction.36 We find that the H2O oxidation reaction mediates the branching between the HER and CO2RR

P

roducing fuels and feedstocks through the CO 2 reduction reaction (CO2RR) is a candidate strategy for effectively closing the carbon cycle.1−5 Considerable discoveries and advances continue to be made toward catalysts that enable the reduction conversion of CO2 into hydrocarbons driven by electrical or light energy input.6−18 However, these processes still suffer from an inherent limitation: under most typical conditions, the two-electron (2e−) reduction of protons, H+, to H2 is kinetically much more facile than the reduction of CO2. As a result, a major fraction of energetic charge carriers is consumed by the dominant H+ reduction channel and the energy efficiency of the CO2RR suffers as a result. The problem stems from the large activation barrier (∼1.9 eV) associated with a transfer of 1e− to CO2 to form CO2•−,19−21 the critical intermediate in the further reduction of CO2. Furthermore, the production of higher hydrocarbons requires additional e−−H+ transfer and C−C coupling steps, further exacerbating the sluggishness of the CO2 conversion process relative to the hydrogen evolution reaction (HER). The HER effectively outcompetes the CO2RR at the consumption of valuable e− and H+. Consequently, the overall selectivity toward hydrocarbons suffers alongside the energy efficiency of the CO2RR. In a recent study, we found15 that e− generated by the plasmonic excitation of Au nanoparticles (NPs) in an aqueous medium containing the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4)22−29 drive the © XXXX American Chemical Society

Received: August 5, 2019 Accepted: August 28, 2019 Published: August 28, 2019 2295

DOI: 10.1021/acsenergylett.9b01688 ACS Energy Lett. 2019, 4, 2295−2300

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Cite This: ACS Energy Lett. 2019, 4, 2295−2300

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Figure 1. Tunability of CO2RR activity on a plasmonic Au NP photocatalyst and EMIM-BF4 by variation of [H+]. (a and c) TOFs and (b and d) selectivity of hydrocarbon products generated in the CO2RR as a function of the [H+] in (a and b) 5 mol % and (c and d) 25 mol % EMIM-BF4. The selectivity for each hydrocarbon is defined as the (TOF for that hydrocarbon) ÷ (sum of TOFs for all hydrocarbons) × 100. Each data point is the average of results from three identical trials, and the error bar represents the standard deviation of these measurements.

adsorbed H+, adsorbed CO2, or adsorbates derived from CO2, triggering the multi-e−, multi-H+ reduction of CO2 to hydrocarbons or the 2e−-reduction of H+ to H2. The d-band h+ are removed by the counter half-reaction involving the 2h+oxidation of adsorbed H2O to H2O2. The h+ removal by the difficult H2O oxidation step is expected to lag that of e− removal. This kinetic asymmetry effectively enables e−−h+ pair separation even without a cocatalyst or semiconductor in contact with the Au NP. The quantum efficiency of e−−h+ pair separation is expected to be determined by the rate of the e− transfer step relative to the rate of e−−h+ recombination, which is known to take place with a 1 ps time constant.40 The products detected by gas chromatography (GC) are C1 (CH4), C2 (C2H4 and C2H2), and C3 (C3H6 and C3H8) hydrocarbons formed by the e−-mediated reduction of CO2, as previously confirmed by a control photoreaction performed in Ar atmosphere and 13C-isotope-labeling,15 and H2 generated by the competing e−-mediated reduction of H+. Another control reaction performed without light excitation at an elevated bulk reaction medium temperature showed no detectable activity (Figure S2), ensuring that the activity observed in the photoreactions cannot be attributed to a photothermal effect. The photosynthesis of hydrocarbons from CO2 is a H+demanding reaction. For instance, CH4 formation involves an 8e−−8H+ process and C3H8 formation involves a 20e−−20H+ reaction; therefore, we hypothesized that increasing the availability of H+ in the solution would enhance the yield

pathways on the reduction side. The selectivity of H2 versus hydrocarbon production is controlled by the H+ availability in the IL medium. We identify an optimal H+ concentration, [H+], at which hydrocarbon production dwarfs the HER and 84% of productive e− are harvested in the form of C1−C3 hydrocarbons, a large advance relative to the previous demonstration.15 Taken together, these results show that by engineering of the reaction medium, conventional selectivity can be overturned in plasmonic catalysis. The CO2RR scheme employed in the studies here is based on our previous work.15 The photoreaction is carried out in 5 mol % EMIM-BF4 solution in the presence of a substratesupported film of Au NPs under continuous-wave (CW) laser illumination (532 nm wavelength, 1 W cm−2 intensity). Similar to our previous studies,14,15 the Au NPs were quasi-spherical in shape and ∼12 nm in diameter, on average, and exhibit in their aqueous colloidal state a localized surface plasmon resonance (LSPR) band centered around 520 nm (Figure S1). The LSPR excitation of the Au NPs generates energetic e−−h+ pairs via interband damping of the LSPR. These charge carriers drive redox conversions at the Au NP surface.14,15,34,35 In addition, from electrocatalytic studies, Au NPs are known to adsorb and activate CO2 for reduction.37−39 Thus, the Au NPs play the dual role of light absorber and catalyst in the photocatalytic CO2RR scheme here. The e−−h+ pairs generated in a Au NP drive a full photoredox process consisting of reduction and oxidation halfreactions.14,15,34,35 The e− are transferred from the sp-band to 2296

DOI: 10.1021/acsenergylett.9b01688 ACS Energy Lett. 2019, 4, 2295−2300

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ACS Energy Letters and rate of hydrocarbon production. A 5 mol % EMIM-BF4 solution has a baseline [H+] of 1.12 mM. We varied the [H+] over an order-of-magnitude range above this baseline value. The turnover frequencies (TOFs) of C1, C2, and C3 hydrocarbons were measured as a function of the [H+] (Figures 1a and S3−S7). The C1 and C2 TOFs increased with an increase in the [H+], reaching a maximum at [H+] = 4.68 mM. A further increase in the [H+] resulted in a decrease of the C1 and C2 TOFs. The C3 TOFs did not change appreciably with the variation in the [H+]. Possibly, at these conditions, C−C coupling influences the kinetics of C3 formation more strongly than the H+ availability. The selectivity among hydrocarbon products is in the order of C1 > C2 > C3 (C1 > C2+) at [H+] = 1.12 mM (Figure 1b). The C2 selectivity increases as the [H+] is increased above 1.12 mM, whereas the C3 selectivity decreases. The C2+ selectivity is nearly 50% at [H+] values above 1.12 mM, despite the reduced C3 selectivity. The hydrocarbon product branching changes to C1 ≈ C2 > C3 (C1 ≈ C2+) at [H+] values above 1.12 mM. Similar trends are observed in 25 mol % EMIM-BF4 solution (Figures S8−S12), which has a baseline [H+] of 0.54 mM, conditions at which only C1 and C2 hydrocarbons are observed. The C1 and C2 TOFs increased with an increase in the [H+], reaching a maximum at [H+] = 4.90 mM (Figure 1c). There is an onset of C3H6 generation at [H+] = 2.09 mM and the more reduced C3 product, C3H8, at [H+] = 4.90 mM. The C2+ selectivity increases from ∼20% at [H+] = 0.54 mM to ∼55% at [H+] = 9.12 mM (Figure 1d). The hydrocarbon product branching changes from C1 > C2 > C3 (C1 > C2+) at [H+] = 0.54 mM to C1 ≈ C2 > C3 (C1 ≈ C2+) above [H+] values of 0.54 mM. One factor responsible for the [H+]-dependent trend in the CO2RR activity may simply be a [H+]-dependent change in the concentration of dissolved CO2 or carbonates in the reaction mixture. To investigate this factor, we estimated the total carbonate concentration in an aqueous solution (Figure S13a) and found that it changes by a negligible magnitude over the [H+] range studied here. Therefore, when the measured TOFs of hydrocarbons produced in the CO2RR in 5 mol % EMIM-BF4 are normalized by the total carbonate concentration, their [H+]-dependent trend remains unaffected by the normalization (Figure S13b). In other words, the observed [H+]-dependence of the CO2RR activity is not caused, even in part, by a [H+]-dependent CO2 solubility. The HER activity in 5 mol % EMIM-BF4 solution was also characterized as a function of [H+] (Figure 2a). The amount of H2 produced was measured by a GC equipped with a thermal conductivity detector (TCD) (Figures S14−S18). The TOF for H2 decreased with an increase in the [H+] above the baseline value of 1.12 mM. The TOF reached a minimum at [H+] = 4.68 mM. At [H+] = 1.12 mM, H2 is the favored product accounting for 95% of the product moles and the hydrocarbon (C1−C3) selectivity is a mere ∼5% (Figure 2b). However, at [H+] = 4.68 mM, the selectivity is reversed and hydrocarbons account for ∼67% of the product moles. Further increase in the [H+] above 4.68 mM led to a marginal increase in the H2 TOF. Thus, the hydrocarbon selectivity is most optimal at [H+] = 4.68 mM. From the measured TOF and e− factor of each product, we determined the rates of e− harvested by the HER and CO2RR branches as a function of [H+] (Figure 3a). At the optimal [H+], 84% of harvested e− are utilized for hydrocarbon photosynthesis. Only 16% are channeled into the HER branch.

Figure 2. Tunability of HER activity on a plasmonic Au NP photocatalyst and EMIM-BF4 by variation of [H+]. (a) TOF of H2 generation and (b) selectivity toward hydrocarbons and hydrogen as a function of [H+] in 5 mol % EMIM-BF4. The selectivity toward H2 is defined as the (TOF for H2) ÷ (sum of TOFs for H2 and all hydrocarbons) × 100. The selectivity toward hydrocarbons is defined as the (sum of TOFs for all hydrocarbons) ÷ (sum of TOFs for H2 and all hydrocarbons) × 100. Each data point is the average TOF from three identical trials, and the error bar is the standard deviation of these measurements.

We did not detect any other nonhydrocarbon products such as CO (Figure S19), which is intriguing because CO is a major product of electrocatalytic CO2RR on Au.37−39 It is thought that photoexcitation of the catalyst/adsorbate system can promote alternative reaction pathways and intermediates,30−32 often leading to modified product selectivities. In fact, in a range of Au-based photoreaction schemes,13−17 the product distribution comprises substantial fractions of hydrocarbons, deviating markedly from an electrochemical scheme. There are two potential reasons for why CO is not produced at detectable levels in the plasmon-excitation-driven scheme studied here. Possibly, CO is formed as an intermediate by 2e−−2H+ conversion of the CO2, a process that may be ratelimiting in the overall reduction. The CO intermediate formed in an adsorbed state at the Au NP surface is then rapidly transformed to CH4 by subsequent, fast (nonrate-limiting) e−−H+ transfer steps.41−44 Such a scenario would be assisted by the e− and H+ rich environment at the NP−solution interface under plasmonic excitation.13 Alternatively, the reaction sequence triggered by LSPR excitation is unique from the electrocatalytic mechanism and does not go through CO formation. The adsorbed CO 2 or the 1e − −1H + intermediate HOCO radical, observed on the surface of a plasmonically excited NP under CO2RR conditions,45 is first deoxygenated to form C adsorbed at the Au NP surface, Cads. The Cads undergoes further hydrogenation in the e− and H+ rich environment to form CH4. C−C coupling steps yield C2 and C3 hydrocarbons. While outside the scope of the current study, one cannot rule out the roles of leftover polyvinylpyrrolidone (PVP) ligands or EMIM+ and BF4− adsorbed on the NP surface in facilitating charge separation,46 influencing the energetic stability of surface adsorbates, and modifying reaction selectivity. The competition between the HER and CO2RR branches is tunable by variation of the [H+]. However, the [H+]dependence of the HER activity is anomalous. One would expect HER activity to increase with an increase in [H+] because of greater H+ availability. However, the observation is in direct contrast to this expectation. An explanation of this 2297

DOI: 10.1021/acsenergylett.9b01688 ACS Energy Lett. 2019, 4, 2295−2300

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Figure 3. (a) Measured rate of e− harvested in the form of hydrocarbons (blue circles), H2 (orange squares), and both (gray rhombuses) as a function of the [H+] in 5 mol % EMIM-BF4. The e− harvesting rate for a product is given by a product of its TOF and the number of e− required for the formation of one molecule of the product (i.e., 2 for H2, 8 for CH4, 10 for C2H2, 12 for C2H4, 18 for C3H6, and 20 for C3H8). Each data point is the average of results from three identical trials performed under the same conditions, and the error bar is the standard deviation of these measurements. The gray dashed line is a guide to the eye. (b) The predicted efficiency of net e− harvesting (eq 4), likelihood of HOCO formation (eq 6), and likelihood of H2 (eq 9) formation as a function of [H+]. Note that because we are interested in only the qualitative nature of the trends, the rate constants knr, ke′, and kh′ were arbitrarily set to 1 and [H+] is on an arbitrary scale.

ke CO2(IL) + e− + H+ → HOCO

anomaly is that the CO2RR is the dominant branch for harvesting of e−−H+ and the HER is the secondary branch. In other words, the e− and H+ (or H·) available for the HER are controlled by the CO2RR. This is supported by the observation that the e− harvesting rates of the two branches are anticorrelated (Figure 3a). Second, a reaction model with the CO2RR as the dominant channel reproduces the anomalous [H+]-dependent trends (Figure 3b). As per this reaction model, e− and h+ generated in the Au NP by plasmonic excitation can readily undergo nonradiative recombination unless they are separated: k nr e− + h+ ⎯⎯→ heat

As depicted by reaction 5, the e− transfer to the IL-complexed CO2 may be H+-coupled. The capture of additional e− and H+ by HOCO to form Cads, CxHy species, and C−C coupling results in the synthesis of the C1−C3 hydrocarbons. If HOCO is the critical intermediate in the hydrocarbon synthesis, then the rate of hydrocarbon production is limited by the rate of HOCO formation, which is proportional to ke[CO2 ][H+]ηe = ke′[H+]ηe −

Reaction 1 has a rate constant knr. Because h+-mediated oxidation of H2O to H2O2 is thought to be considerably slower than the reduction reactions, the rate of net e− harvesting will be limited by the rate of h+ removal by this reaction: kh 2H 2O + 2h → H 2O2 + 2H+

k h[H 2O]2 [h+]2 /[H+]2 (3)

k h′ /[H+]2 k h′ /[H+]2 + k nr

H·+H· → H 2

(8)

(9)

This simple kinetic model qualitatively captures the [H+]dependent trends in the e− harvesting rates via the CO2RR and HER branches. The agreement between the observed and predicted trends confirms the proposition that the CO2RR is the dominant pathway for e− harvesting under our conditions. Such a scenario is plausible because the adsorption of H· is unfavorable on the Au surface unlike on hydrogenation catalysts such as Pt or Ni.47 In fact, nanostructured Au surfaces have been shown to disfavor the HER.37 Second, it is thought that in the presence of EMIM-BF4, e− transfer to CO2 has a low activation barrier.22−29 Complexation between the EMIM+ and CO2 results in the reorganization of the linear, spbonded CO2 to a bent CO2 geometry with a structure that is similar to CO2•− and therefore preconfigured for e− acceptance.15 We further verified the key feature of the reaction model, according to which the oxidation of H2O to H2O2 mediates the observed competition between the CO2RR and the HER. For this investigation, we conducted the photocatalytic reaction using the same conditions as those employed for the results in

When concentrations are combined with rate constants and because under low-quantum yield of carrier harvesting [e−] ≈ [h+], eq 3 simplifies to ηe =

(7)

(ηe − ke′[H+]ηe)2

(2)

k h[H 2O]2 [h+]2 /[H+]2 + k nr[e−][h+]

e− + H+ → H·

From eqs 6−8, the rate of H2 formation is proportional to

From the competition between the rates of reactions 1 and 2, the efficiency of net e− harvesting is given by ηe =

(6)

+

e and H not harvested by reaction 5 are channeled into the HER branch:

(1)

+

(5)

(4) −

Equation 4 predicts that the net e harvesting rate decreases with increasing [H+] (Figure 3b), which reproduces the experimental observation (Figure 3a). Essentially, as the [H+] increases, H2O oxidation becomes thermodynamically and kinetically more challenging, which results in a decrease in the rate of h+ removal and, therefore, in the e− harvesting rate. The harvested e− are channeled into two competitive branches, of which the CO2RR is the leading one: 2298

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and co-wrote the manuscript. P.K.J. conceived and designed studies, performed modeling, and co-wrote the manuscript. We thank a reviewer for advising us to compare the observed trends with those measured for photocatalytic reactions performed in the presence of methanol as a h+ scavenger.

Figure 1a, except for one change: we replaced a fraction (10 vol %) of the H2O in the reaction medium with an equivalent volume of methanol, which is considerably easier to be oxidized than H2O and is, therefore, a potent h+ scavenger. In the presence of methanol, the [H+]-dependent trends in activity are quite unlike those presented in Figure 1a: the CO2RR activity increases with an increase in the [H+] with no drop at high [H+] (Figure S20a). The HER activity increases with an increase in the [H+] (Figure S20b); there is no longer the anomaly observed in the results presented in Figure 1a. The selectivity toward hydrocarbon products decreases with increasing [H+] (Figure S20c), and the CO2RR activity and selectivity no longer peak at an intermediate [H+]. These trends indicate that the HER is the dominant reaction branch and the CO2RR is the secondary branch in the presence of methanol. Thus, a potent h+ scavenger eases the counter oxidation reaction and removes the kinetic bottleneck posed by the otherwise difficult H2O oxidation reaction, especially at large [H+]. When the counter oxidation reaction is no longer limiting, the trends are simpler: the higher the [H+], the greater the H+ availability and the higher are both the CO2RR and HER activities. In other words, the difficulty of the h+mediated oxidation of H2O to H2O2 is the primary cause of the unusual relationship observed here between the CO2RR and HER branches. In summary, the conventional relationship between HER and CO2RR pathways is overturned in plasmonic catalysis on a Au surface in an IL-containing medium. Under these conditions, the CO2RR is inherently the dominant reaction branch for harvesting of plasmonically generated e−, while the HER is the secondary branch. As a result, increasing H+ availability to an optimal value enhances hydrocarbon selectivity while suppressing the HER. This insight ought to be applicable to other catalytic schemes where the CO2RR and HER are in competition and higher hydrocarbon selectivity is desired.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b01688.

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REFERENCES

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Experimental methods, characterizations of Au NPs, control experiments, results of GC analysis of hydrocarbon and nonhydrocarbon products, and results of the photocatalytic reaction conducted with methanol as the h+ scavenger (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Prashant K. Jain: 0000-0002-7306-3972 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by the Energy & Biosciences Institute (EBI) through the EBI-Shell program. GC-TCD calibration data was obtained from Andrew J. Wilson. S.Y. performed all experimental studies, data-analysis, 2299

DOI: 10.1021/acsenergylett.9b01688 ACS Energy Lett. 2019, 4, 2295−2300

Letter

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DOI: 10.1021/acsenergylett.9b01688 ACS Energy Lett. 2019, 4, 2295−2300