Mechanistic Insights into the Enhanced Activity and Stability of

Dec 10, 2015 - Practices for the collection and reporting of electrocatalytic performance and mechanistic information for the CO 2 reduction reaction...
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Mechanistic Insights into the Enhanced Activity and Stability of Agglomerated Cu Nanocrystals for the Electrochemical Reduction of Carbon Dioxide to n‑Propanol Dan Ren,†,‡ Nian Tee Wong,† Albertus Denny Handoko,†,‡ Yun Huang,† and Boon Siang Yeo*,†,‡ †

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 The Singapore-Berkeley Research Initiative for Sustainable Energy, CREATE Tower 1, Create Way, #11-00, Singapore 138602

‡

S Supporting Information *

ABSTRACT: The reduction of carbon dioxide (CO2) to n-propanol (CH3CH2CH2OH) using renewable electricity is a potentially sustainable route to the production of this valuable engine fuel. In this study, we report that agglomerates of ∼15 nm sized copper nanocrystals exhibited unprecedented catalytic activity for this electrochemical reaction in aqueous 0.1 M KHCO3. The onset potential for the formation of n-propanol was 200− 300 mV more positive than for an electropolished Cu surface or Cu0 nanoparticles. At −0.95 V (vs RHE), n-propanol was formed on the Cu nanocrystals with a high current density (jn‑propanol) of −1.74 mA/cm2, which is ∼25× larger than that found on Cu0 nanoparticles at the same applied potential. The Cu nanocrystals were also catalytically stable for at least 6 h, and only 14% deactivation was observed after 12 h of CO2 reduction. Mechanistic studies suggest that n-propanol could be formed through the C− C coupling of carbon monoxide and ethylene precursors. The enhanced activity of the Cu nanocrystals toward n-propanol formation was correlated to their surface population of defect sites.

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inefficiently with the simultaneous formation of many other compounds.7−9 High overpotentials, most likely caused by the high energy requirements needed to form critical reaction intermediates, such as the CO2 anion radical (CO2•−), COH or CHO, are also required.10,11 In their pioneering CO2 reduction studies using Cu single-crystal substrates, Hori et al. reported that n-propanol could be produced on Cu(100) surfaces with a faradic efficiency (FE) of 1.5%.9 Because a constant current electrolysis at −5 mA/cm2 was performed, the partial current density of n-propanol (jn‑propanol) was −0.08 mA/cm2. Further improvements were made when CO2 reduction was carried out using stepped Cu(100) surfaces. For example, the jn‑propanol increased to −0.23 mA/cm2 (FE = 4.6%) on the high-index Cu(S)-[4(100)×(111)] surface.9 These results indicate that surface features such as atomic steps aid in the formation of C3 compounds, most probably by stabilizing and allowing the chemisorbed C1 and C2 intermediates to undergo intermolecular C−C coupling. The use of single-crystal substrates is, however, not practical for industrial scale catalytic processes. Recently, Kanan and co-workers reported that CO could be reduced to n-propanol on roughened Cu nanoparticles with a maximum jn‑propanol of −0.08 mA/cm2 (FE = 10.0%) at −0.4 V.12,13 Defects such as grain boundaries and steps were also invoked as catalytic sites. Strongly chemisorbed CO species

arbon dioxide reduction to alcohols has the potential of providing a sustainable supply of valuable fuels for our industries and energy needs.1 Of the C1−C3 alcohols, npropanol has the highest energy−mass density (30.94 KJ/g) and octane number (research octane number, 118).2,3 With these properties, the fuel efficiency of n-propanol approaches that of gasoline.4 Being an oxygenate, it can also be blended with gasoline to deliver a cleaner burning fuel with significantly lower CO and hydrocarbon emissions.5 The current high market value of n-propanol (compared to other common CO2 reduction products such as CO and CH4) also adds value to its production from CO 2 and makes the process more commercially attractive. Current industrial production of npropanol involves a two-step process: ethylene is first hydroformylated to propionaldehyde in the presence of cobaltor rhodium-based catalysts; the propionaldehyde is then reduced to n-propanol.2 The formation of n-propanol from the anaerobic fermentation of sugars obtained from biomass has also been explored using microorganisms such as recombinant Escherichia coli.6 However, these bacteria are affected adversely by the n-propanol and tend to die before they are able to produce it in large quantities. As compared to the above two methods, the electrochemical reduction of CO2 to n-propanol using water and renewable electricity could be a cost-effective method to produce this valuable commodity: 3CO2 + 13H2O + 18e− = C3H7OH + 18OH− (Eo = 0.21 V; all potentials in this work are referenced to the reversible hydrogen electrode (RHE)).7 To date, only copper (Cu) surfaces are known to catalyze this process, albeit © 2015 American Chemical Society

Received: November 14, 2015 Accepted: December 7, 2015 Published: December 10, 2015 20

DOI: 10.1021/acs.jpclett.5b02554 J. Phys. Chem. Lett. 2016, 7, 20−24

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The Journal of Physical Chemistry Letters were identified on the surfaces of these nanoparticles through temperature-programmed desorption studies. This strengthens the postulate that the role of the defects is to stabilize the critical reaction intermediates such as CO. While the low overpotential required to produce n-propanol is highly commendable, a scale-up of the current density is necessary to make this process more applicable for industry. On the basis of the above discussions, we posit that a viable method to enhance the formation of n-propanol from CO2 reduction would be to use Cu catalysts with a high surface population of defects. In this work, we prepared these catalytically active Cu nanocrystals by electroreducing a Cu2O/Cu(OH)2 film. Because of volume changes in the Cu films during the reduction process, material strains occur. These are known to be relieved by the formation of cracks and hence produce defect sites on the films.14 The population of these defects was estimated using cyclic voltammetry and was found to correlate linearly with the product yield of n-propanol. The pathway of how n-propanol is formed from CO2 is also explored and discussed. Cu nanoparticles (Cu-NP) were prepared by pulse electrodeposition onto pristine Cu discs (Supporting Information, section S1).15 Cu nanocrystals (Cu-NCX, where X represents anodization time in unit of minutes) were synthesized by first anodizing Cu-NP at +1.5 mA/cm2 in 3 M NaOH electrolyte for 5, 10, 20, or 40 min. This was then followed by reduction at −0.8 V in 0.1 M KHCO3 for 300 s. Electropolished Cu surfaces (Cu-EP) were employed as reference catalysts. X-ray diffraction and electron microscopy (as well as in situ Raman spectroscopy and double-layer capacitance) were used to characterize these catalysts (Supporting Information, section S2). The Cu-NP was composed of only metallic Cu0 (Figure 1A). After anodization in 3 M NaOH, these Cu0 particles oxidized to Cu(OH)2 and Cu2O. The prereduction step then reduced these Cu oxides back to Cu0. Electron microscopy images of the catalysts are presented in Figure 1B−F. The surface of the Cu-EP is atomically smooth, while the Cu-NP are mainly ∼317 ± 122 nm in size. After anodization, these particles partially fragment to ∼100 nm nanoparticles (particle size, 102 ± 71 nm). However, the greatest structural transformation to the particles was observed after the prereduction step. Not only did they break up to give agglomerates of 15 ± 13 nm Cu nanocrystals, but also various crystal orientations were found within the individual Cu nanocrystals, suggesting the presence of grain boundaries. These morphological changes can be attributed to structural modifications that occurred in order to relieve mechanical stress caused by volume changes when Cu2O and Cu(OH)2 are reduced to Cu0 during the reduction step (Cu2O, Cu(OH)2, and Cu0 have different crystal structures and lattice constants).16 The electrochemical reduction of CO2 was performed on these catalysts, and the process was characterized by chronoamperometry and cyclic voltammetry (Supporting Information, sections S3 and S4). Online gas chromatography and 1H nuclear magnetic resonance spectroscopy were also used to identify and quantify the 12 observed CO2 reduction products, which include carbon monoxide, methane, ethylene, ethanol, n-propanol, etc. Current densities measured during chronoamperometry at different potentials are presented in Figure 2A. At a representative potential of −0.95 V, Cu-NC20, Cu-NC10, Cu-NP, and Cu-EP gave currents of −24.7, −19.9, −4.0, and −2.1 mA/cm2, respectively. The current density exhibited by the Cu-NC20 is thus 1 order of magnitude higher

Figure 1. (A) X-ray diffractograms of Cu nanoparticles and representative Cu nanocrystals before prereduction (B.R.) and after prereduction (A.R.). The diffractograms are compared with standards from the ICDD. The Cu0 XRD peaks found in the Cu nanocrystal samples are from the underlying Cu substrate. Scanning electron microscopy (SEM) images of (B) electropolished Cu, (C) Cu nanoparticles, (D) Cu nanocrystals-20 before prereduction, and (E) Cu nanocrystals-20 after prereduction. The insets in panels C, D, and E show the respective transmission electron microscopy (TEM) images of the samples. (F) High-resolution TEM image of a Cu nanocrystal (Cu-NC20)

than Cu-EP, which can be partially attributed to differences in their electrochemically active surface areas. The maximum faradic efficiency for n-propanol formation on the Cu-EP, CuNP, Cu-NC10, and Cu-NC20 were ∼1.5, ∼5.1, ∼8.8, and ∼10.6%, respectively (Supporting Information, section S4). Here, we also use the partial current density of n-propanol (jn‑propanol), which is a more practical figure-of-merit, to gauge the efficiency of our catalyst for producing this valuable commodity.17 At −0.95 V, an unprecedentedly high yield of npropanol, jn‑propanol = −1.63 to −1.74 mA/cm2, was achieved on the Cu nanocrystals (Figure 2B). This is 23−25× higher than the amount of n-propanol that could be formed using the CuNP (jn‑propanol = −0.07 mA/cm2) at the same potential. No npropanol could be detected using the electropolished Cu at this potential. The high partial current density of n-propanol exhibited by the Cu-NC compares favorably to reported values from other Cu-based catalysts (−0.08 to −0.26 mA/ cm2).7−9,12,18 Furthermore, Cu-NC20 starts to electrocatalyze the reduction of CO2 to n-propanol at a more positive potential of −0.75 V (jn‑propanol = −0.19 mA/cm2), as compared to the Cu-EP (jn‑propanol = −0.23 mA/cm2 at −1.05 V) and Cu-NP (jn‑propanol = −0.07 mA/cm2 at −0.95 V). It is also noteworthy that the Cu-NC reduced CO2 to significant amounts of ethylene and ethanol, while CH4 production was practically suppressed (Supporting Information, section S4). This 21

DOI: 10.1021/acs.jpclett.5b02554 J. Phys. Chem. Lett. 2016, 7, 20−24

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Figure 2. (A) Total current density measured during CO2 reduction (including partial current densities of H2) and (B) partial current density of n-propanol formation on electropolished Cu, Cu nanoparticles, and Cu nanocrystals (Cu-NC10 and Cu-NC20), respectively, at different potentials in 0.1 M KHCO3. (C) Total current density during CO2 reduction and partial current density of n-propanol formation on Cu-NC10 at −0.95 V in 0.1 M KHCO3 as a function of electrolysis time. (D) Chronoamperogram and partial current densities of major gaseous products during 6 h of electrolysis on Cu-NC10 at −0.95 V in 0.1 M KHCO3.

Figure 3. Turnover frequency (TOF) of ethylene, ethanol, carbon monoxide, and n-propanol production as a function of potential on (A) electropolished Cu, (B) Cu nanoparticles, (C) Cu nanocrystals-10 (Cu-NC10), and (D) Cu nanocrystals-20 (Cu-NC20).

potential in which n-propanol formation was maximized. For the Cu-NC20, this potential shifts positive toward −0.85 to −0.95 V. These observations suggest strongly that C2H4 and CO are reaction precursors to n-propanol. To further test this proposition, we electroreduce a 1:1 mixture of C2H4 and CO over Cu-NC20 catalyst. n-Propanol could be detected in significant amounts (Supporting Information, section S6). In contrast, reducing CO or C2H4 alone over Cu-NC20 gave little or no detectable quantities of n-propanol, respectively. These experiments demonstrate that the facile formation of npropanol on the Cu nanocrystals is intimately connected to the formation of CO and C2H4 precursors. A mechanism by which CO2 is reduced to n-propanol is hereby proposed: CO2 is first reduced to C1 intermediates such as CO.10 These can then be liberated as CO gas or be further reduced to CH4. Alternatively, these C1 intermediates could dimerize to C2 species. These adsorbed C2’s could hydrogenate to either C2H4 or C2H5OH, with the former being the kinetically favored product on Cu surfaces.25 This explains why these two molecules usually form in tandem, with a higher product yield for ethylene (Figure 3).7,10,16 An adsorbed C2 intermediate such as C2H4 could also undergo intermolecular C−C coupling with an adjacent C1 intermediate (for example, CO), followed by proton/electron transfers to form propionaldehyde (CH3CH2CHO).24 Propionaldehyde is then reduced on the Cu sites to n-propanol (Supporting Information, section S7). Control experiments showed that this reduction could not occur on a noncatalytic surface such as glassy carbon at the same applied potential. Our proposed reaction mechanism is reminiscent of the thermal formation of propionaldehyde through the hydroformylation of ethylene using carbon monoxide and hydrogen.2 Defect sites on Cu-based electrocatalysts are generally agreed to be important for reducing CO2 to higher C2 and C3 compounds because they are good for binding CO adsorbates.13,15,16,20 These features can be identified through

phenomenon is similar to previous observations made on roughened Cu nanoparticles.15,16,19−23 The catalytic activity of the Cu nanocrystals was remarkably stable (Figure 2C,D). At −0.95 V, n-propanol could be continuously produced over 6 h at a jn‑propanol of ∼ −1.74 mA/ cm2, and this decreased by only ∼14% to −1.50 mA/cm2 after 12 h (Supporting Information, section S5). The partial currents of the other CO2 reduction products such as CO and C2H4 also remained stable during the electrolysis. In stark contrast, the jn‑propanol on Cu-EP decreased by 70% after 6 h from −0.27 to −0.08 mA/cm2 (E = −1.15 V), and the jn‑propanol exhibited by Cu-NP decreased by 41% over the same time period from −0.81 to −0.48 mA/cm2 (E = −1.05 V). The long-term stability of the Cu-NC as compared to Cu-EP and Cu-NP could be explained by the former’s low propensity toward CH4 formation. According to Kas et al., intermediates formed during CH4 production (like COHads) could decompose into graphitic carbon.22 The suppression of this route would thus minimize the poisoning of the catalyst. Inspired by the mechanism proposed by Hori. et al. for the formation of n-propanol during CO reduction over polycrystalline Cu, we hypothesize that the C−C coupling of CO(ads) and C2H4(ads) intermediates could be a key step in n-propanol formation.24 This proposition could be verified by analyzing the yields of carbon monoxide, ethylene, and n-propanol as a function of electrochemical potentials (Figure 3). We express the yields here as turnover frequencies because this figure-ofmerit gives the number of molecules formed. Irrespective of the catalyst used, the formation of n-propanol was found to be optimized at those potentials in which both C2H4 and CO were formed at reasonably high rates. For example, on Cu-NP, the optimum potential for the formation of both C2H4 and CO is at approximately −1.05 V. This potential matches well with the 22

DOI: 10.1021/acs.jpclett.5b02554 J. Phys. Chem. Lett. 2016, 7, 20−24

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The Journal of Physical Chemistry Letters their voltammetric signatures.26 Thus, we record the cyclic voltammograms of the catalysts used in this work (Figure 4A,

miscible with water and cannot be extracted as such. These have to be separated using other methods such as distillations, which require an additional energy input. Herein, we demonstrate that simple modifications to the structure of a Cu catalyst could lead to remarkable efficiency of n-propanol formation, in terms of current density, stability, and onset potential. The onset potential for the formation of npropanol in 0.1 M KHCO3 electrolyte using Cu nanocrystals was 200−300 mV more positive than for an electropolished Cu surface or Cu0 nanoparticles. At −0.95 V (vs RHE), n-propanol was formed on the Cu nanocrystals with an unprecedentedly high and stable jn‑propanol of −1.74 mA/cm2, which is ∼25× larger than that found on Cu0 nanoparticles at the same applied potential. The mechanism and active sites of n-propanol formation from CO2 using the Cu catalysts were also presented. Based on these findings, a practical approach to enhance the reduction of CO2 to n-propanol would be to promote the density of defects and agglomeration of the Cu nanocrystals. This study serves to narrow the gap between CO2 reduction to synthetic propanol in the laboratory and application of this process in industry.

Figure 4. (A) Cyclic voltammograms of electropolished Cu, Cu nanoparticles, and representative Cu nanocrystals in 0.1 M KHCO3 (saturated with N2). Scan rate: 10 mV/s. (B) A plot of the partial current density of n-propanol at −0.95 V vs RHE vs charges beneath defect peaks.

from −0.8 → 1.2 → −0.8 V). All the catalysts exhibited two major reduction peaks at 0.50 and 0.25 V, which can be assigned to Cu2+ → Cu+ and Cu+ → Cu0, respectively.20 The charges beneath these peaks increase with the electrochemically active surface areas of the catalysts (Figure 4A). However, these charges could not be correlated with the amount of n-propanol formed. For example, Cu-NC40 has a 1.5× higher surface area compared to Cu-NC20, but is a relatively inferior catalyst for reducing CO2 to n-propanol. This indicates that the catalytic selectivity of the Cu-NC for n-propanol cannot be ascribed totally to their large surface areas. Interestingly, a pair of reduction peaks at ∼0 V and ∼−0.25 V were identified on the Cu nanocrystals (indicated by the bracket inserted in Figure 4A). Considering the highly corrugated morphology of the CuNC catalysts as indicated by SEM and TEM, and the mismatch of their voltammograms with those of Cu(100), Cu(111), and Cu(110) single-crystal surfaces (Supporting Information, section S8), it is reasonable to assign these two peaks to the reduction of oxidized Cu defect sites.26 Significantly, we found that the amount of charges beneath these defect peaks correlates linearly to the yield of n-propanol exhibited by the catalysts at −0.95 V (Figure 4B). This suggests strongly that the active catalytic sites for the formation of n-propanol are the surface defects. The role of these defects might be to stabilize the C1 and C2 intermediates, and to allow them to trimerize to a C3 compound such as n-propanol.20,27,28 It is noteworthy that the catalytic activity of our ∼15 nm (±13 nm) sized Cu nanocrystals is different from those of isolated ∼5−25 nm-sized nanoparticles, which has been shown by Strasser, Alivisatos, and co-workers to be efficacious for reducing CO2 to carbon monoxide or methane.29,30 This difference could be attributed to the dense aggregates of the Cu-NC, which must bear numerous particle−particle junctions and defect sites. We also note that the high current densities exhibited by the Cu-NC would give rise to a high local pH at its surface. This will increase the formation of ethylene intermediates, which will in turn promote n-propanol production.8,21 The extraction of liquid CO2 reduction products dissolved in an aqueous electrolyte must be made before they can be used as fuels. Using 2-pentanol as the extracting solvent for liquid− liquid separation, we found that n-propanol could be facilely extracted from aqueous KHCO3 standard solution with >99% efficiency (Supporting Information, section S9). Note that lowcarbon alcohols such as methanol and ethanol are highly

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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/acs.jpclett.5b02554. Detailed experimental procedures, materials characterization results, and electrochemistry data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by research grants from the National University of Singapore (R143-000-515-133 and R-143-000587-112).

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REFERENCES

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