The Effect of Particle Shape and Electrolyte Cation on CO Adsorption

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

The Effect of Particle Shape and Electrolyte Cation on CO Adsorption to Copper Oxide Nanoparticle Electrocatalysts Matthew M Sartin, Zongyou Yu, Wei Chen, Fan He, Zhijuan Sun, Yanxia Chen, and Weixin Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08541 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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The Journal of Physical Chemistry

The Effect of Particle Shape and Electrolyte Cation on CO Adsorption to Copper Oxide Nanoparticle Electrocatalysts Matthew M. Sartin, Zongyou Yu, Wei Chen, Fan He, Zhijuan Sun, Yan-Xia Chen* and Weixin Huang* Hefei National Lab for Physical Science at the Microscale and Department of Chemical Physics, University of Science and Technology of China, Hefei, 230026, China ABSTRACT Cu2O-derived nanoparticles are efficient catalysts for the electrochemical conversion of CO and CO2 to multi-carbon products. Generation of multi-carbon products requires dimerization of adsorbed CO, which is accelerated when the coverage of CO is high. The electrolyte cation and the initially exposed crystal plane of the catalyst both affect the reaction rate, but the relation between these effects and CO coverage is unclear, especially given the surface reconstruction that occurs during reduction reactions on Cu2O. We prepared a series of shape-controlled Cu2O nanoparticles with similar sizes but different initially exposed crystal planes (cubes (100), octahedra (111), and dodecahedra (110)), and we used the IR absorption bands detected in-situ to compare the potential-dependent CO coverage on each of the nanomaterials in CO-saturated 0.1 M NaHCO3 and CsHCO3 during cyclic voltammetry. After correcting for the shape of the particle, there was less than 20% difference in the coverage of adsorbed CO on the different structures. The fact that the surface coverages are so similar may be a result of surface reconstruction occurring during the reaction. If so, the fact that it occurs so rapidly shows that the surface structure will not, in practical situations, impact the surface coverage of CO. In CsHCO3, a lower surface coverage of CO was measured, even for potentials at which CO dimerization is very slow. Although Cs+ accelerates the reduction of CO through interaction with adsorbed intermediates, its ability to adsorb to the electrode surface likely enables it to block potential adsorption sites for CO. Therefore, the effect of interaction with intermediates must have more impact than the reduced surface coverage of CO caused by cation adsorption.

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INTRODUCTION The electrochemical reduction of CO is an important intermediate step in the reduction of CO2. Adsorbed CO can be electrochemically reduced to intermediates along the C1 and C2 paths, leading to products containing a single carbon, such as CH4 and CH3OH (C1 products), or multiple carbons, such as CH2CH2, CH3CH3, and CH3CH2OH (C2 products), respectively.1-3 The C2 products are especially valued, due to their potential use as fuels.4-5 Along the C2 path, it is believed that the rate-limiting step is the dimerization of two adjacent CO molecules. Along the C1 path, it is the protonation of CO to form HCO.1 In order to generate high yields of C2 products, the C2 path must compete with the C1 path and hydrogen evolution. Hence, reaction conditions that favor the dimerization of CO are highly desirable. Copper and copper-oxide (Cu2O) derived electrodes are currently the only materials at which C2 products are generated efficiently from the electrochemical reduction of CO and CO2. Of the two, Cu2O-derived catalysts are more promising, as they produce even higher yields of C2 products than regular copper electrodes, and they generate lower yields of H2.6-10 A high CO coverage is expected to enhance the rate of dimerization of CO to produce C2 products,11 but the contribution of this effect is very complicated, because a high rate of dimerization reduces the surface coverage of CO, and the electrode surface is also affected by local pH changes and site-blocking by ions during the reaction.12 Therefore, it is important to understand how all of these factors are interrelated. To date, there are only a few cases in which the effect of solution conditions on CO coverage has been examined. In this paper, we consider the effect of the Cu2O exposed crystal plane and cation on C2 yield. Theory shows that the (111) crystal plane is the most active Cu2O surface.13-14 However, Cu2O materials are known to undergo surface reconstruction during electrochemical reduction reactions.15-17 The effect of the surface reconstruction on CO coverage and yield of C2 products is uncertain. One study found that crystal plane orientation of Cu2O electrodes had little effect on the selective reduction of CO2 to C2 products, and that the initial oxide film thickness was the most important factor. This difference from the behavior of pure copper could indicate that, due to Cu2O surface reconstruction, the surfaces of the materials studied were equivalent to one another after the reaction.18 However, the initial crystal plane of a Cu2O material affects the final structure of the material after surface reconstruction.19 Moreover, in another study, a series of shapedcontrolled Cu2O nanomaterials showed structure-dependent product selectivity, despite significant surface reconstruction that occurred during CO2 reduction.20 The authors of that study observed 2


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that, following reduction, the crystal structures of the nanomaterials had changed their preferred crystallographic orientation, which suggests that, even after surface reconstruction, there were differences between the three kinds of particles that affected the reaction. The materials exhibiting higher C2 product selectivity also showed stronger CO Raman bands, indicating a higher surface coverage of CO on those materials than others. The authors suggest that the more efficient generation of C2 products at these materials is due to the higher coverage of CO on their surfaces. However, the authors point out that the materials used in this study also had very different morphologies, and the Raman signals measured are affected by the morphology, making the results on CO coverage inconclusive. The different results concerning surface structure effects between the two studies make it difficult to understand the role CO surface coverage plays in the dimerization efficiency, and how it is affected by crystal structure and particle shape. A similar observation associating low CO surface coverage with high C2 generation efficiency was recently made in a study of the effect of cation size on CO surface coverage. In electrolyte solutions containing large cations, CO dimerization occurs more rapidly than in solutions containing smaller cations,21-22 because the larger cations are weakly hydrated and can interact more strongly with surface adsorbates. This allows them to stabilize the large dipole of the CO dimer intermediate of the C2 pathway, accelerating the rate-determining step of the reaction, resulting in higher yields of C2 products.23-24 A recent study demonstrated lower CO coverage on copper electrodes in solutions containing CsHCO3 than in solutions containing electrolytes with smaller cations.25 These authors also concluded that the accelerated dimerization rate rapidly depleted the surface coverage of CO in CsHCO3. However, while they certainly demonstrated accelerated loss of adsorbed CO from the surface at potentials were the dimerization reaction is rapid, the results at less negative potentials were not compared. Due to ambiguity in how the surface coverage of CO influences the rate of CO dimerization, a detailed analysis of the potential-dependence of CO coverage under these conditions is needed. Electrochemical FTIR spectroscopy is ideally suited to examining CO coverage in situ.26 Not only is it highly sensitive to surface-adsorbed species, but the ability to detect adsorbed species in-situ means that long product accumulation times can be avoided, minimizing the damage to the catalyst surface. Combined electrochemical-FTIR experiments have been extensively applied to studies of CO and CO2 reduction on copper to determine the effects of the solution conditions23, 25, 27-28 and electrode crystal orientation,23 as well as to identify other binding sites29-31 and adsorbed 3



intermediates.23,

32-33

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There are surprisingly few electrochemical-FTIR studies of Cu2O-based

materials. In recent years, the possible contribution of subsurface oxygen to the binding of CO,34 and the binding of the electrolyte anion and CO2 to the copper oxide surface have been examined.35 In this study, we use electrochemistry combined with FTIR spectroscopy in an attenuated total reflection configuration (ATR-FTIR) to examine how the initially exposed crystal plane of Cu2O and the cation of the electrolyte affect the potential-dependence of CO coverage. We prepared a series of Cu2O nanoparticles with typical sizes of 600-800 nm, shaped to expose different crystal planes (cube (100), octahedra (111), dodecahedra (110)) to the solution. The nanoparticles were dropcast onto the reflective surface of an ATR prism that was coated with gold to make it conductive. This surface was used as the working electrode in a 3-electrode spectroelectrochemical cell. For each type of nanoparticle, IR spectra were collected while sweeping the electrode potential. The area of the CO absorption band was used to compare the CO coverage on the respective surfaces. To avoid errors due to differences in particle geometry and sample preparation, we introduce a correction using the area of the IR band originating from CO32desorption from the nanoparticles. We also examine how the electrolyte cation affects the CO coverage over the whole potential range on these nanomaterials by carrying out the experiments in CO-saturated 0.1 M NaHCO3 and CsHCO3. A detailed description of the experimental protocols is provided in the Supporting Information. EXPERIMENTAL SECTION Materials NaHCO3 (99.998%, Puratronic, Alfa Aesar) and CsHCO3 (99.99%, Puratronic, Alfa Aesar) were used as the electrolytes for measurements. CO (99.99%), Ar (99.999%), and N2 (99.999%) were purchased from Linde. All other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. Synthesis and characterization of Cu2O nanoparticles Shape-controlled nanoparticles were prepared following previously reported procedures. The nanocubes and octahedra were prepared using the procedure of Zhang et al.36 A 5 mL solution of 2.0 M NaOH was added dropwise to a solution of 0.01 M CuCl2 in 50 mL deionized water containing different amounts of polyvinylpyrrolidone (PVP, MW = 30000) (cubic, 0 g; octahedra, 4


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4.44 g), while stirring at 55 oC. After 30 min., a 5 mL solution of 0.6 M ascorbic acid was added dropwise to the solution. The solution was stirred for different amounts of time to generate the appropriate crystals (cubic: 3 h, octahedra: 5 h). The products were then precipitated by centrifugation, washed with deionized water and ethanol, and then dried under vacuum for 12 hours at room temperature. The dodecahedra were prepared using the procedure of Liang et al.37 1 mmol CuSO4 was dissolved in 40 mL water. 4 mL oleic acid and 20 mL of absolute ethanol were successively added to the solution while stirring vigorously. The mixture was then heated to 100 C, and left to react for 30 min., after which 10 mL of 0.8 M NaOH solution was added. After 5 minutes, a 30 mL solution of 3.42 g D-(+)-glucose was added under constant stirring. The mixture was then allowed to react for another 60 min, until the solution color changed to brick-red. The product was then precipitated by centrifugation, washed with distilled water and ethanol, and dried under vacuum at room temperature for 12 hours. The residual PVP (for octahedra) or oleic acid (for dodecahedra) was removed using a controlled oxidation method,38 in which the samples were treated using a stream of C3H6/O2/N2 (C3H6:O2:N2 = 2:1:22), while heating at 200 C (for PVP) or 215 C (for oleic acid) for 30 min. The composition of the sample was confirmed using an X-ray photoelectron spectrometer (ESCALAB 250, Thermo Scientific), with monochromatized Al Kα (hν = 1486.7 eV) as the excitation source. The X-ray photoelectron spectroscopy (XPS) spectra are provided in the Supporting Information, Figure S1. The particle shape before and after the reaction were determined using a field-emission scanning electron microscope (GeminiSEM 500, Zeiss). The average particle size was 600-800 nm. . Electrochemical-ATR-FTIR experiments The electrochemical cell for the electrochemical ATR-FTIR experiments used a 3electrode configuration. The electrode potential was controlled using a potentiostat (CHI 700 or CHI 760, CH Instruments, Shanghai, China). The counter electrode was a gold wire, and the reference electrode was Ag/AgCl. The working electrode was prepared by depositing a gold film onto the flat surface of a hemicylindrical Si prism, to make the surface conductive, and then dropcasting the Cu2O nanoparticles on the gold film.

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A detailed procedure for preparing the working electrode is as follows. The flat face of each prism was polished using diamond paste (Struers, Cleveland, OH, USA) with progressively smaller particle sizes, down to 0.25 μm. The polished prisms were then cleaned by boiling in a 1:1:5 by volume solution of NH4OH : H2O2 : water for 10 min., followed by a 1:1:5 by volume solution of HCl : H2O2 : water for 10 min. Deposition of the gold film followed the procedure reported previously.39 The polished Si surface was immersed in a 17.8 M NH4F solution for about 90 s to make it hydrophobic. A gold deposition solution was prepared by mixing a solution consisting of 15 mM NaAuCl4•H2O, 150 mM Na2SO3, 50 mM Na2S2O3•5 H2O, and 50 mM NH4Cl, in a 2:1 v/v ratio with a solution of 2% HF in water. The prism was heated to 60 °C and the deposition solution was dropcast onto the flat face of the prism and left to stand for ~70 s to create a thin film of gold. The thickness of the Au films was not measured, but a resistivity of 12±4 Ω/cm gave sufficient conductivity and transmission of IR light. After assembling the electrochemical ATR-FTIR cell, a suspension of 1.5 mg/mL Cu2O nanoparticles in ethanol was sonicated for 3 min. to disperse the particles, and 10 μL of the suspension was deposited on the gold surface (0.67 mg/cm2) and allowed to dry under N2, before the cell was filled with electrolyte solution. After the electrolyte was added, the solution was bubbled with N2 for 10 min. Then two cyclic voltammograms from 0.42 V to -0.78 V vs RHE were carried out at 10 mV/s in the N2saturated electrolyte as a pretreatment to reduce surface oxides, which prevent the adsorption of CO.20, 27, 35 The electrolyte solution was then bubbled with CO for 10 minutes, after which the CO flow rate was reduced, and two cyclic voltammograms over the potential range from 0.42 V to 0.78 V vs RHE were carried out at 10 mV/s while collecting IR spectra. IR spectra were recorded using a commercially available FTIR spectrometer (FTS-7000E, Varian, Palo Alto, CA) with a liquid N2-cooled MCT detector. All spectra are constructed from the average of 16 scans recorded at 20 kHz, with 4 cm-1 resolution, giving a time-resolution of 5 seconds (50 mV) per spectrum. A spectrum accumulated for 60 scans at the initial potential of the scan was used as the reference spectrum. All spectra are reported in units of ΔmOD = -103 log(R / R0), where R and R0 are the reflected IR intensities recorded at the measured and reference potentials, respectively. The baseline tilt and fluctuations that occurred during the measurement have been removed by fitting a polynomial through the region of interest, as described in the Supporting Information, Figure S2. Plots of band area vs potential are an average of 3-4 measurements made using freshly-coated prisms. 6


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Preparation of samples for ex-situ SEM measurements To prepare samples for SEM measurements after the experiments, 25 mm2 silicon wafers were coated on all sides with gold, using the procedure described above. Then, 10 μL of a suspension of 0.2 mg/mL copper nanoparticles in ethanol was dropcast onto the gold-coated wafers. The coated wafers were placed on a large, gold electrode that was sealed face-up in an electrochemical cell. To obtain the conditions of the catalyst observed in the middle of the ATRFTIR experiments, 2 CVs were carried out in N2-saturated solution, followed by 1 CV carried out in CO-saturated solution. When the sequence was complete, the solution was drained, and the wafers were dried under N2 and stored in an N2-filled vial until they could be transferred to the SEM for imaging. RESULTS AND DISCUSSION 1. SEM characterization before and after experiments To investigate the distribution of nanoparticles on the prisms and the morphological changes that occur during the experiment, SEM measurements of the Cu2O nanoparticles deposited on gold-coated wafers were carried out before and after the potential sweep sequence used in the ATR-FTIR spectroscopy experiments, as shown in Figure 1. In all cases, the nanoparticles deposited on the prism surface form clumps on the surface with several regions of exposed gold. Most CO and CO2 reduction experiments are carried out at potentials negative of -0.7 V vs RHE.10, 17, 21

In our experiments, the potentials were kept positive of -0.8 V vs RHE to minimize surface

damage. However, although all the particles retain their basic shapes, they all show some surface roughening. The octahedra are the most stable, consistent with previous results.40 Recent in-situ AFM experiments have shown that exposing nanocubes to air after electrochemical reduction of the surface oxides causes additional surface roughening.17 As we were unable to prevent exposure to air when transferring the sample to the SEM chamber, these images may show more damage than actually occurred during the experiment. However, the result suggests that even under fairly mild reaction conditions, the initially exposed crystal plane is altered during CO reduction.

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Figure 1. SEM images of (a) cubes (b) octahedra (c) dodecahedra before and after the potential sweep sequence used in the electrochemical ATR-FTIR spectroscopy experiments in this paper. The average particle size is 600-800 nm. The 400 nm scale bar is represented by the white line in the lower-left corner of each image. 2. Particle shape effects on the IR spectra The adsorption of CO on each type of nanoparticle was examined by carrying out electrochemical ATR-FTIR spectroscopy measurements on the Cu2O nanoparticle-coated, gold film electrode surface. Since the working electrode contains several regions of exposed gold, we first discuss the potential-dependent adsorption of CO on a bare gold film electrode. The IR spectra measured at a bare gold film electrode are shown in the Supporting Information, Figure S4. Three major IR bands can be detected on both gold and copper, at approximately the same frequencies. The HOH bending vibration of water at 1650 cm-1, the negative band around 1500 cm-1 that indicates desorption of CO32- that was adsorbed to the electrode at 0.97 V vs RHE,41 and the CO band that shifts from 2098 to 2070 cm-1 as the potential is decreased from 0.47 V to 0.07 V. The HOH bending vibration detected on gold is stronger than that detected on the Cu2O nanoparticles, so it is assumed to obscure the corresponding absorption band on the Cu2O nanoparticles. Therefore, it is not discussed further. The integrals of the CO32- and CO bands on gold are presented in Figure 2, along with the faradaic current measured while scanning the electrode potential from 0.97 V to -0.23 V vs RHE. 8


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As shown in Figure 2a, as the electrode potential is scanned negatively, the IR signal from CO32on gold becomes increasingly negative as the CO32- desorbs, until it reaches a plateau at potentials below 0.5 V vs RHE, indicating complete desorption of CO32-. Since the highest potential used in the experiments on Cu2O nanoparticles is 0.42 V vs RHE, this contribution from gold can be neglected when discussing CO32- on Cu2O. Figure 2b shows that the IR band of CO adsorbed on gold reaches a maximum near 0.35 V vs RHE. CO is not reduced on gold, and it is oxidized to CO2 at potentials positive of 0.4 V vs RHE (Figure 2c). Since 0.42 V is the reference potential used for the experiments on the Cu2O nanoparticles, there is very little CO oxidation, but the desorption of CO from gold could contribute a negative signal centered at 2090 cm-1 to these measurements. As discussed below, the absorption band of CO on Cu2O shifts to 2045 cm-1 when the potential is scanned to -0.78 V vs RHE, so a negative signal at 2090 cm-1 should be clearly visible in the spectra obtained at this potential. The spectra obtained with Cu2O nanoparticles (Figure 3, Supporting Information Figure S5) do not show a negative signal at 2090 cm-1. Most likely, this means that the Cu2O nanoparticles cover enough of the gold surface that the small signal from CO on gold is below our detection limit. Therefore, we can discuss the signals measured at Cu2O without considering the absorption band from CO on gold. 0.0

-1

Band Area (mOD • cm )

(a) CO3

2-

-0.1 -0.2 0.04

(b) CO

0.03 0.02 0.01 0.00

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 0.2 0.1 0.0 -0.1 -0.2

(c)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Potential (V vs RHE)

Figure 2. IR absorption band integrals of the (a) CO32- band and (b) CO band (c), along with the faradaic current measured during a CV at a gold film electrode in CO-saturated 0.1 M NaHCO3

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at a scan rate of 10 mV/s. Arrows indicate the direction of the potential sweep. The reference spectrum was collected at 0.97 V vs RHE. The ATR-FTIR spectra obtained using Cu2O octahedra in CO-saturated, 0.1 M NaHCO3 are shown in Figure 3, referenced to the spectrum obtained at 0.42 V vs RHE. The other types of nanoparticles show qualitatively similar spectra, which are provided in the Supporting Information, Figure S5. The negative signal at 1500 cm-1 is associated with a C-O vibration originating from desorption of the CO32- that was adsorbed onto the Cu2O surface at 0.42 V vs RHE.20, 28, 32, 42 A small, positive band at slightly lower frequencies indicates that the adsorbed CO32- undergoes a Stark shift before it fully desorbs at more negative potentials. The IR band of CO bound to top sites on Cu2O is observed starting at +0.1 V vs RHE, and its peak frequency shifts from 2090 to 2045 cm-1 as the potential is made more negative. A small peak that shifts from 1845 to 1790 cm1 is also detected in this potential range. This band has previously been assigned to multiply-bonded

CO, which is the preferred configuration for CO at low surface coverages.27, 29-30, 43 However, we are unable to conclusively assign the observed band to multiply-bonded CO for several reasons. First, as shown by the spectra, the species remains on the electrode at the end of the CV, whereas previous calculations show that the energy difference between the bridge/hollow sites and the top sites is similar to kT, indicating that it should have similar stability to that of atop CO and freely interconvert between the two states.44-47 Second, a recent STM study carried out on copper only detected atop CO, although the experimental conditions are different from ours.31 Third, as shown in the Supporting Information, Figure S4, we observe a similar feature in the spectra measured on the bare gold film whose identity is also uncertain.48-49 Since we cannot find a suitable assignment for this feature, and it is difficult to measure a consistent absorbance from such a weak band, we focus our discussion on the band associated with atop CO.

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5 mOD

-0.78 V -0.68 V


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-0.58 V -0.48 V -0.38 V -0.28 V -0.18 V -0.08 V 0.02 V 0.12 V 0.22 V 0.32 V 0.42 V 2200

2000

1800

1600

1400

-1

Wavenumber (cm )

Figure 3. Selected potential-dependent ATR-FTIR absorption spectra collected at a working electrode consisting of Cu2O octahedra deposited on a gold film. The potential was scanned at 10 mV/s in CO-saturated 0.1 M NaHCO3. The reference spectrum was collected at 0.42 V vs RHE. The large, vertical arrow indicates the scan direction. The integrals of the IR bands are related to the coverage of the respective adsorbate on the surface. Therefore, in order to compare the CO coverage of each particle, the integral of the CO band is plotted as a function of potential in Figure 4a. For all three kinds of nanoparticles, the growth of the CO band, beginning around 0.2 V vs RHE (Figure 4a), coincides with the CO32band becoming more negative (Figure 4b), indicating that desorption of CO32- from the Cu2O surface is needed for CO adsorption to occur. The CO adsorption band reaches a peak at the potential Ep = -0.35 V vs RHE. The decrease in CO coverage as the potential is swept negatively from Ep has previously been attributed to the competitive binding of hydrogen to the surface and to the increasing rate of the CO dimerization reaction in this potential range.25, 32, 34, 50

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(a) CO

Cube Octahedron Dodecahedron

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Band Area (mOD • cm )

0.12 0.08 0.04 0.00 0.0 -0.1

(b) CO3

2-

-0.2

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Peak Freq. (cm )

-0.3

Current (mA)

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2085

(c)

2070 2055 2040 0

(d)

-1 -2 -3

Epeak

-4 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4


Figure 4. Results of electrochemical ATR-FTIR experiment on shape-controlled Cu2O nanoparticles in CO-saturated 0.1 M NaHCO3. IR absorption integrals of (a) the CO band and (b) CO32- band, (c) peak frequency of the CO band, (d) the faradaic current measured during a CV at a scan rate of 10 mV/s. The reference spectrum was collected at 0.42 V vs RHE. The peak frequency of the CO band is determined by the interaction of CO with the substrate and with other adsorbates, including other CO molecules. As the electrode potential is made more negative, donation of electron density into the π* orbitals of the CO weakens the bond, shifting the CO band to lower frequency.51 However, as the surface coverage of CO increases, dipole-dipole coupling between CO molecules increases,52 and the CO band becomes asymmetrical, showing more absorption on the high-frequency-side of the band. Other processes, such as coupling of the CO stretch with metal-based absorptions, multiple binding site energies,2728

Fano resonance,53 and surface reconstruction54 may also play a role in the asymmetry, but its

close correspondence with the CO band area suggests that the major factor is dipole-dipole coupling. As the potential is swept negatively towards Ep, the shift of the maximum of the CO band towards lower frequency is very slow, because the increased dipole-dipole coupling brought about by increasing surface coverage and the electronic Stark shift brought about by decreasing 12


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potential act on the band maximum in opposite directions (Figure 4c). At potentials negative of Ep, the asymmetry of the CO band decreases, shifting absorbance back towards the center of the CO band. Thus, at potentials negative of Ep, the CO band maximum shifts to lower frequencies more rapidly. This causes a sudden change in the slope of the frequency vs potential plot near the peak CO coverage. During the reverse scan, the CO coverage, and hence the asymmetry, is constant, so the shift of the band reflects the purely electronic Stark shift. The areas of the CO bands measured for each type of nanoparticle are different (Figure 4a), suggesting a different coverage of CO on each particle. However, the CO peak frequencies, which are also affected by the surface coverage of CO and the exposed crystal plane, are nearly the same (Figure 4c). The combination of the differences in the peak frequencies expected for the three initially-exposed crystal planes19 and the peak frequency shifts expected from the different apparent surface coverages cannot explain this result. The similar peak frequencies suggest similar final structures for the three types of nanoparticles and similar amounts of CO on each surface. The difference in CO absorbance between the three types of particles exists because absorbance depends not only on surface coverage, but also on the nanoparticle geometry. The nanoparticle geometry affects the surface area of the nanoparticle, the orientation of the adsorbed CO with respect to the reflection plane, and the distance of the adsorbed CO from the reflection plane of the prism. To correct for these geometrical effects, as well as run-to-run variations in the distribution and number of particles in each measurement of the same sample, we normalize the CO band area for each measurement by the area of the respective CO32- band measured at E < -0.4 V vs RHE during the reverse potential scan. At these potentials, CO32- is fully desorbed, so this absorbance is proportional to the surface coverage of CO32- at 0.42 V vs RHE, and it depends on the same geometrical factors as CO. On some metals, the CO32- binding mode varies with crystal structure, which could affect this correction procedure. We were unable to find a study on how CO32- binds to different crystal planes of Cu2O. However, variation in the binding mode of CO32to Pt can be shown by shifts in the IR frequency and the appearance of additional bands at other frequencies.55 As shown in the Supporting Information, Figure S6, the band shapes and peak frequencies for the CO32- desorption band on all three types of nanoparticles are nearly the same, suggesting that at 0.42 V vs RHE, the binding of CO32- is similar on each type of surface. For each nanoparticle shape, at least 3 measurements were made on freshly-prepared prisms. Each measurement was then normalized by its respective CO32- band integral, and the average of the 13



corrected CO band areas of each type of nanoparticle, along with the standard deviation, are presented as a function of potential in Figure 5. 0.5

Corrected CO Band Area

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cube Octahedron Dodecahedron

0.4 0.3 0.2 0.1 0.0 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4


Figure 5. Average area of the CO absorption band measured during electrochemical ATR-FTIR experiment on shape-controlled Cu2O nanoparticles in CO-saturated 0.1 M NaHCO3 after correction for particle geometry using the area of the CO32- band. The arrows indicate the scan direction. The shaded area represents the standard deviation of each measurement. The reference spectrum was collected at 0.42 V vs RHE. As shown by the error bars, when corrected for differences in geometry, the CO band areas do not show a significant dependence on the initially-exposed crystal plane. For the CO band between -0.05 V vs RHE and the peak at -0.35 V vs RHE, the standard deviation of the measurements is around 20% of the band area, and at potentials negative of Ep, it quickly rises to 40%. The CO coverage measured on a particle surface is related to the free energy of its adsorption to the surface and to the energies of its subsequent reactions. These energies can vary with different crystal planes. However, this result shows that if the initially exposed crystal plane affects these energies, the total effect on the surface coverage of CO is less than 20%. As discussed in the first section, although the reaction conditions are fairly mild, all of the Cu2O nanoparticles experienced substantial surface reconstruction during the CV. The extent of reconstruction on each particle is different, but it may be enough that the distribution of different kinds of active sites on each particle is the same. Due to the ease with which the surface structure is altered, the previously-reported variation in C2 product yield for different Cu2O materials was likely caused by the large differences in the morphology of the particles, rather than the initially exposed crystal plane.20 This can be expected for most CO and CO2 reduction experiments carried out under practical conditions on Cu2O materials.

14


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3. Cation effect on the potential-dependent CO adsorption To study the effect of cation size on the potential-dependence of CO coverage on Cu2O nanoparticles, we carried out the electrochemical ATR-FTIR experiment in 0.1 M CsHCO3 and compared the results to spectra measured in 0.1 M NaHCO3. Representative potential-dependent IR spectra obtained using the Cu2O octahedra in CO-saturated CsHCO3 and NaHCO3 solutions are shown in Figure 6 (see Supporting Information, Figure S5 for the spectra of the other particles). The major differences between the spectra obtained in CsHCO3 solution and those collected in NaHCO3 solution are as follows. First, the CO32- bleach is narrower and shifted to higher frequency compared with those measured in NaHCO3. This shift to higher frequency may indicate a different surface coverage of CO32-, or a preference for CO32- to adsorb in a different mode,55 due to different interaction of the CO32- with the Cs cation. As shown in the Supporting Information, Figure S10, the average area of the CO32- band is not affected by the electrolyte solution. Therefore, regardless of whether the binding mode of CO32- is different in each electrolyte solution, the CO32- band can be used to correct for sample-to-sample variations in CO band area for better comparison of the bands in each electrolyte. The second difference between the spectra obtained in CsHCO3 and those obtained in NaHCO3 is that the CO bands measured in CsHCO3 are weaker and shifted to lower frequency. As in NaHCO3, the nanoparticle shape does not significantly affect the peak CO coverage (Supporting Information, Figure S7), so we focus the comparison of CO adsorption in the two electrolyte solutions on the octahedra.

15



in NaHCO3 in CsHCO3

5 mOD

-0.78 V -0.68 V


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.58 V -0.48 V -0.38 V -0.28 V -0.18 V -0.08 V 0.02 V 0.12 V 0.22 V 0.32 V 0.42 V 2200

2000

1800

1600

1400

-1

Wavenumber (cm )

Figure 6. Selected potential-dependent ATR-FTIR absorption spectra collected at a working electrode consisting of Cu2O octahedra deposited on a gold film. The potential scan was carried out at 10 mV/s in CO-saturated 0.1 M NaHCO3 (red, filled spectra) and 0.1 M CsHCO3 (black lines). The reference spectrum was collected at 0.42 V vs RHE. The large, vertical arrow indicates the scan direction. The CO band areas measured during the CV carried out on the Cu2O octahedra in both electrolytes are presented in Figure 7a (See Supporting Information, Figures S9 and S10 for the other particles). The CO band detected in CsHCO3 is smaller than that in NaHCO3, consistent with measurements carried out using copper films.25 Interestingly, like the band in NaHCO3, the CO band area measured in NaHCO3 reaches a peak near -0.35 V vs RHE. The fact that Ep is the same in both electrolytes suggests that the decrease in surface coverage at more negative potentials is not caused by a cation-specific process. The faradaic current, shown in Figure 7c, is primarily related to hydrogen evolution. Ep for both bands is near the onset of the faradaic current, so the main contributor is likely competitive binding between CO and hydrogen.32, 50 Since the onset of the faradaic current is near -0.2 V vs RHE (Figure 7c), whereas the CO band intensities measured in the two electrolytes begin to differ at least as early as 0 V vs RHE, the cation effect on CO adsorption at E > Ep requires another explanation.

16


-1

Peak Freq. (cm )

0.4

(a)

NaHCO3 CsHCO3

0.3 0.2 0.1 0.0 2085

-1

24 cm /V -1

(b)

23 cm /V -1

82 cm /V -1 82 cm /V

2070

-1

58 cm /V -1 65 cm /V

2055 2040 0

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Corr. Band Area

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(c)

-1 -2 -3

EPeak

-4 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4


Figure 7. Results of electrochemical ATR-FTIR experiment on Cu2O octahedra in CO-saturated 0.1 M NaHCO3 (red) and 0.1 M CsHCO3 (blue). (a) Integrated IR absorption of the CO band corrected by dividing by the CO32- bleach (b) peak frequency of the CO band (c) faradaic current. The shaded regions represent the standard deviations of the measurements. The reference spectrum was collected at 0.42 V vs RHE. Due to the weaker hydration of Cs+, it can interact more strongly with the electrode and its adsorbates than Na+. Therefore, we now consider possible consequences of the direct interaction of the cation with the Cu2O particles and any adsorbates. One possibility is that the interaction between CO and Cs+ shifts a fraction of adsorbed CO from top sites to bridge and hollow sites. This would move absorbance from the band at ~2080 cm-1, which represents CO bound to top sites, to the region of 1800 cm-1. The Cu-CO interaction occurs primarily through π backbonding, which is stronger at bridge/hollow sites, due to the greater overlap between the CO antibonding orbital and the Cu metal.44, 56 Therefore, if polarization of the CO molecule by interaction with a cation increases the strength of the π backbonding interaction with the metal, it can further increase the preference for bridge/hollow binding sites.57 This cation effect on CO binding preference has been demonstrated at Pt electrodes.58-59 However, the CO band area measured in CsHCO3 is 50% of the corresponding band area measured in NaHCO3. Since the transition dipole moment of bridge CO is greater than or equal to that of atop CO,60 this explanation requires that the 50% loss of absorbance at ~2080 cm-1 be accompanied by a similar increase in absorbance at 1800 cm-1, and 17



Page 18 of 25

this is not observed. Moreover, Figure 7b shows that the peak frequency of CO in CsHCO3 is redshifted relative to that in NaHCO3 at all potentials, which is consistent with a lower surface coverage of CO at all potentials. It is possible to view this red shift as stabilization of the weak CO dipole via electrostatic interaction between the Cs and the adsorbed CO. However, the fact that the relation between CO band area and peak frequency holds even in the different electrolyte solutions suggests that the CO coverage in CsHCO3 is really lower. For example, at -0.1 V and -0.5 V vs RHE, the CO band areas measured during the reverse scan in NaHCO3 are the same as those measured during the forward scan carried out in CsHCO3. The peak frequencies at these positions are also the same, indicating that the peak frequency reliably reflects the surface coverage measured in both electrolytes. Thus, it is very likely that the CO coverage measured in CsHCO3 really is lower than in NaHCO3. Another interaction of Cs+ with the surface adsorbates is with the C2O2 intermediate of the CO reduction. This interaction lowers the energy of the intermediate of the C2 pathway, and is responsible for the increased generation of C2 products observed in electrolytes containing Cs+.2324

The result of this interaction is to accelerate the dimerization of CO. This could explain the

reduced surface coverage of CO observed in CsHCO3 if the dimerization reaction is accelerated sufficiently to deplete the local concentration of CO, making the transport of CO to the electrode surface diffusion-limited. However, a 50% difference in band intensities between the two electrolytes is already observed starting around 0 V vs RHE. Even in CsHCO3, the CO reduction rate is extremely low at this potential, so diffusion-limited reaction is unlikely. In addition, if CO reduction in CsHCO3 is already diffusion-limited at 0 V, its Ep should occur near 0V vs RHE. However, the CO band area measured in CsHCO3 is nearly parallel to that measured in NaHCO3 throughout the CV, and the peak is at a slightly more negative potential. Therefore, accelerated dimerization of CO cannot explain the lower CO coverage measured in CsHCO3. Having ruled out cation-induced change of the CO binding site and accelerated dimerization as the primary causes for the low CO coverage in CsHCO3, we propose instead that the weaker hydration of the Cs+ allows it to adsorb directly to the electrode surface and block adsorption sites. Obstruction of CO-binding sites by cations has previously been observed on gold, where even Na+ can displace the adsorbed CO.49 The data obtained at the bare gold film electrode shown in Figure 2 shows the cation-induced desorption of CO starting 0.35 V vs RHE. CO binds more strongly to copper than to gold, which could explain why neither cation can displace the CO, 18


Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


as it does in gold. However, the ability of Cs+ to adsorb to the electrode allows it to block adsorption sites. As the potential is made more negative, cations and CO molecules both adsorb at the electrode surface in greater numbers. This explains why the potential-dependent adsorption profiles of CO in the two electrolytes are nearly parallel throughout the potential range scanned. Thus, although dimerization of CO is accelerated by Cs+, the surface coverage of CO is kept lower by other adsorbates. The fact that more C2 products are generated in CsHCO3 means that the stabilization of the C2O2 dimer intermediate by Cs accelerates the dimerization reaction enough to enhance the C2 yield in spite of the lower surface coverage of CO. CONCLUSION In this paper, we used electrochemical ATR-FTIR spectroscopy to examine how the potential-dependent adsorption of CO on Cu2O nanoparticles is affected by the exposed crystal plane and by the electrolyte cation. We prepared a series of Cu2O nanoparticles shaped to expose different crystal planes (cube (100), octahedron (111), dodecahedron (110)). We used the IR signal from the desorption of CO32- to correct for differences in CO absorption band strength caused by particle shape. After making this correction, we find that within our 20% uncertainty, we are unable to observe any difference in the CO coverage between the three kinds of nanoparticles. SEM images taken before and after the measurement show varying degrees of surface reconstruction on each type of nanoparticle, which suggests that the surfaces of each of the three kinds of nanoparticles are no longer different from one another after the reaction. We are unable to judge whether this is really the case, or if the effect of surface structure on CO coverage is simply below our detection limit. However, even if this result is entirely caused by surface reconstruction, the fact that it is observed during the first CV indicates that the surface reconstruction is so rapid that under most practical conditions, that the CO coverage is effectively independent of the initiallyexposed crystal face for these types of Cu2O nanoparticles. To examine how the ability of large cations to accelerate the CO dimerization reaction affects the potential-dependent CO coverage, we carried out electrochemical ATR-FTIR studies in CO-saturated solutions of NaHCO3 and CsHCO3. In CsHCO3 solution, the surface coverage of CO throughout the CV is lower than that in NaHCO3, at all potentials. Although this is consistent with a mechanism in which accelerated CO dimerization results in diffusion-controlled transport of CO to the electrode surface, the fact that this difference begins at 0 V vs RHE, where C2 19



Page 20 of 25

dimerization is fairly slow, and the fact that the peak CO coverage is at the same potential in both electrolytes (-0.35 V vs RHE) suggests that another explanation is needed for the lower CO coverage in CsHCO3. We propose that the weakly hydrated Cs+ adsorbs to the electrode and blocks CO binding sites. Since Cs+ is known to enhance the rate of CO dimerization, this means that the energetic penalty for a low coverage of CO on the electrode surface must be less than the benefit of stabilization of the dimer intermediate by Cs+. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XPS spectra of nanomaterials, baseline-fitting procedure, electrochemical ATR-FTIR spectra of CO on gold, electrochemical ATR-FTIR spectra of CO adsorption on all nanoparticles in NaHCO3 and CsHCO3 with reverse potential scans, comparison of CO32- bleach spectra for each type of nanoparticle, potential-dependence band integrals of CO32- bleach in NaHCO3 and CsHCO3, IR band integrals of CO adsorption on cubes and dodecahedra, complete citation for Reference 20. AUTHOR INFORMATION Corresponding Author: *Phone: +86-551-63600035 Email: [email protected] (Y.-X.C.) *Phone: +86-551-63600435 Email: [email protected] (W. H.) Notes The authors declare no competing financial interests ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (no. 21750110437, 91545124,). MMS is supported by the Chinese Academy of Sciences President’s International Fellowship Initiative (Grant No. 2017PM0049) REFERENCES 20


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The Effect of Particle Shape and Electrolyte Cation on CO Adsorption

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