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. 2024 Jan 25;2(3):103–111. doi: 10.1021/prechem.3c00105

Glycerol Electrooxidation over Precision-Synthesized Gold Nanocrystals with Different Surface Facets

Hansen Mou †, Fang Lu ‡,*, Zechao Zhuang †, Qiaowan Chang †,§, Lihua Zhang ‡, Xiaobo Chen ∥, Yugang Zhang ‡,*, Jingguang G Chen †,⊥,*
PMCID: PMC10966739  PMID: 38550915

Abstract

graphic file with name pc3c00105_0005.jpg

Electrochemical glycerol oxidation (EGO) emerges as a promising route to valorize glycerol, an underutilized byproduct from biodiesel production, into value-added chemicals. This study employed three types of gold (Au) nanocrystals with controlled shapes to elucidate the facet-dependent electrocatalytic behavior in EGO. Octahedral, rhombic dodecahedral, and cubic Au nanocrystals with {111}, {110}, and {100} facets, respectively, were precisely synthesized with uniform size and shape. Rhombic dodecahedra exhibited the lowest onset potential for EGO due to facile AuOH formation, while octahedra showed enhanced electrochemical activity for glycerol oxidation and resistance to poisoning. In-situ FTIR analysis revealed that Au {111} surfaces selectively favored C2 products, whereas Au {100} surfaces promoted C3 product formation, highlighting the significant effect of facet orientation on EGO performance and informing catalyst design.

Keywords: Electrochemical glycerol oxidation, Precision synthesis, Shaped nanocrystals, Au nanocrystals, SAXS, In-situ FTIR

1. Introduction

Glycerol valorization has drawn significant attention as an underutilized byproduct of global biodiesel production.1,2 Electrochemical glycerol oxidation (EGO) offers an attractive pathway as it can yield energy and value-added products simultaneously.3 Due to the variety of potential products formed from EGO, ideal catalysts should allow for the selective scission of the C–C bond, thereby enabling precise control over the selectivity of glycerol oxidation products. EGO demonstrates faster kinetics in alkaline conditions due to the base-catalyzed first deprotonation step, and precious metals such as platinum (Pt) and gold (Au) are considered among the most active electrocatalysts for this reaction.3−5 Compared to Pt, Au has been found to be more favorable for products of partial glycerol oxidation while demonstrating high catalytic activity, attributed to the greater resistance to CO poisoning and surface oxide formation.6,7

While the choice of catalyst material plays a significant role in the activity and product selectivity, previous studies have also demonstrated the dependence of electrocatalytic pathways to the size, shape, and surface structure of metal catalysts.8−13 For Au-based catalysts, these factors impact the formation of alkoxides and adsorption of OH– to form AuOH. Studies with single-crystal Au surfaces found a correlation between the onset potentials for AuOH formation and for methanol, ethanol, and ethylene glycol oxidation on Au {100}, Au {110}, and Au {111} surfaces.14−16

Although previous studies of glycerol oxidation have suggested that the facets of Au surfaces influenced not only the onset potential of EGO through AuOH formation but also EGO activity and selectivity, more insights are provided in the current work.6 For instance, studies using carbon-supported gold nanoparticles attributed the lower onset potential of smaller particles to higher fractions of Au {110} facets, while higher fractions of Au {111} facets were correlated to higher surface area-normalized activity and greater selectivity toward the C–C bond scission, leading to higher formate and glycolate selectivity.17,18 However, the selectivity comparison was confounded by the influence of mixed surface facets on the nanoparticles. Another study investigating shape-controlled Au nanoparticles reported higher electrooxidation activity for glycerol on the Au {111} surface than that on the Au{100} surface but did not discuss differences in selectivity.8 This work sought to bridge this knowledge gap with the novel approach of using Au nanoparticles with well-defined single-crystal surface facets as model systems to provide insight into the mechanisms of glycerol electrooxidation.

The preparation of single-crystal surfaces conventionally involves the aligning and cutting of large crystals, followed by the polishing and heat treating of these surfaces. Even minor misalignment during cutting can result in a significant departure from the intended single-crystal surfaces. Recent advancements in the controlled synthesis of metallic nanocrystals have introduced a new approach to producing high-quality single-crystal facets, particularly valuable for the investigation of structurally sensitive catalytic reactions.19−23 This also presents practical significance because of the large surface area offered by nanoscale materials. However, the uniformity and the postsynthesis cleanliness of the resulting surfaces present a challenge with these bottom-up synthetic methods. For instance, the removal of surface capping agents and/or stabilizers, used during synthesis, necessitates exceptional cleaning procedures. Additionally, extensive post-treatment could potentially compromise the integrity of the synthesized surface structure.24−27 Surfactants, ionic impurities, and generated byproducts can introduce additional disruptive factors that block reactant adsorption onto the catalyst surface.25 Shaped Au nanocrystals with atomically well-defined nanofacets previously demonstrated novel oxygen reduction reactions and contributed to the understanding of underlying mechanisms.20 The present study shows the feasibility of meeting both controlled facets and surface cleanliness requirements for shaped Au nanocrystals to serve as a platform for studying facet-sensitive reactions such as glycerol electrooxidation.

In this study, three monofaceted Au nanocrystals were synthesized by precisely controlling the shape of the particles through wet chemistry seed-mediated methods. Octahedra (OC), rhombic dodecahedra (RD), and cubes (CB) enclosed by {111}, {110}, and {100} surfaces, respectively, were employed to illustrate the capabilities for obtaining pristine single-crystalline surfaces. Electrochemical experiments were performed to assess the glycerol electrooxidation activity and selectivity of each type of Au nanocrystal. FTIR spectra were collected with in-situ infrared reflection absorption spectroscopy (IRRAS) to examine the intermediates formed as a function of applied potential and to compare the C–C bond scission selectivity among each type of Au nanocrystal. This work highlighted key differences in activity and selectivity for EGO attributed to the different Au single-crystalline surfaces.

2. Experimental Methods

2.1. Catalyst Synthesis and Characterization

2.1.1. Reagents

Gold(III) chloride trihydrate (HAuCl4·3H2O, 99.9+%), silver nitrate (AgNO3, 99.9999%), sodium borohydrate (NaBH4, 99.99%), l-ascorbic acid (AA, 99+%), potassium bromide (KBr), cetyltrimethylammonium bromide (CTAB, 99.9%), and cetylpyridinium chloride (CPC, 99%) were purchased from Sigma-Aldrich and used for syntheses without further purification. Milli-Q ultrapure deionized water (18.2 MΩ, Millipore UV Plus) was used in preparing all aqueous solutions.

2.1.2. Synthesis of Au Seeds

The synthesis of Au nano-octahedral seeds involved a two-step procedure. First, 3 nm Au seeds were produced by rapidly injecting 0.60 mL of ice-cold, freshly prepared 10 mM NaBH4 into a vigorously stirred solution of 0.25 mL of 10 mM HAuCl4 and 9.75 mL of 0.1 M CTAB. After stirring the seed solution for 2 min, it was left undisturbed at 25 °C for 3 h, allowing complete decomposition of the residual NaBH4. This solution was then diluted a 100-fold with 0.1 M CTAB. Second, 360 μL of this diluted seed solution was added to a mixture solution containing 4 mL of 0.3 mM HAuCl4 aqueous solution, 24 mL of 20 mM CTAB aqueous solution, and 1.8 mL of 100 mM AA aqueous solution under stirring. This mixture was then aged at 25 °C for 18 h. The resultant seeds were washed three times using 0.1 M CPC solution by centrifugation at 12 000 rpm for 10 min and finally dispersed in 1.125 mL of 0.1 M CPC solution to obtain the CPC-capped seeds.

2.1.3. Seed-Mediated Growth of Polyhedral Au Nanocrystals

Synthesis of Au Nanocrystal: Octahedra

To prepare Au nanocrystal octahedra with a 50 nm edge length, the process began with 9.7 mL of 0.1 M CPC solution, to which 200 μL of 10 mM HAuCl4 solution, 26 μL of freshly prepared 100 mM AA, and 300 μL of the CPC-capped seed solution were sequentially introduced at 28 °C. After thoroughly mixing, the reaction was left undisturbed for 3 h, allowing for the formation of nanocrystals. The reaction was stopped by centrifuging the solution at 8000 rpm for 6 min. The produced nanocrystals were then washed twice with Milli-Q water and subsequently concentrated for further analysis.

Synthesis of Au Nanocrystal: Rhombic Dodecahedra

To synthesize Au nanocrystal rhombic dodecahedra with a 30 nm edge length, 200 μL of 10 mM HAuCl4 solution was first added to 9.7 mL of 0.01 M CPC solution at 30 °C. This was followed by adding 800 μL of freshly prepared 100 mM AA and 300 μL of the CPC-capped seed solution, thoroughly mixing after each step. The solution was left undisturbed for 6 h to produce nanocrystals, which were then collected by centrifugation at 6000 rpm for 6 min. The nanocrystals were then washed twice with Milli-Q water and concentrated for examination.

Synthesis of Au Nanocrystal: Cubes

Distinct from the synthesis of octahedra and rhombic dodecahedra, the formation of cube-shaped nanocrystals required a KBr solution. For a typical synthesis of 48 nm edge length Au nanocubes, 9.7 mL of 0.1 M CPC solution was first mixed with 1000 μL of 100 mM KBr solution at 32 °C. This solution was then sequentially introduced with 200 μL of 10 mM HAuCl4 solution, 800 μL of freshly prepared 100 mM AA, and 300 μL of the CPC-capped seed solution. The solution was thoroughly mixed after each addition. The reaction was left undisturbed for 6 h and halted by centrifugation at 6000 rpm for 6 min. The obtained nanocrystals were washed twice with Milli-Q water and concentrated for analysis.

2.1.4. Characterization

The synthesized polyhedral Au nanocrystals were characterized by electron microscopy, UV–vis spectroscopy, and small-angle X-ray scattering (SAXS). Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 SEM, while transmission electron microscopy (TEM) was conducted using a JEOL JEM-2100F high-resolution analytical TEM equipped with an energy-dispersive X-ray (EDX) detector. For optical property measurement, UV–vis spectra were obtained on a PerkinElmer Lambda 35 spectrometer covering a wavelength range of 400–800 nm. The concentration of polyhedral Au nanocrystals was determined by measuring the absorbance at their surface plasmon resonance (SPR) peak in the UV–vis absorption spectra (Figure S1). Different molar extinction coefficients were applied for the respective polyhedrals: 3.2 × 1010 M–1·cm–1 at 568 nm for the ∼50 nm Au nano-octahedra, 3.4 × 1010 M–1·cm–1 at 558 nm for the 30 nm Au nanorhombic dodecahedrons, and 4 × 1010 M–1·cm–1 at 547 nm for the 48 nm Au nanocubes.

Additionally, SAXS measurements were carried out at the Complex Materials Scattering (CMS, 11-BM) beamline of the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory. The SAXS data were collected using a beam energy of 13.5 keV and a 200 × 200 μm beam size with a Pilatus 2 M area detector (Dectris, Switzerland), featuring 172 μm square pixels in a 1475 × 1679 array. The detector was placed 5 m downstream from the sample and captured two-dimensional (2D) scattering patterns, which were then converted to 1D scattering intensity, I(q), by circular average. Here, q represents wave vector transfer, calculated as q = (4π/λ)sin(θ), where λ is the wavelength of the incident X-ray beam (0.9184 Å) and 2θ is the scattering angle.

2.1.5. Modeling SAXS Profile for Shaped Au Nanoparticles

The form factor I(q) for the nanoparticles was expressed as I(q) = |F(q)|2, with the form factor amplitude F(q) given by F(q) = Δρ∫Ve–iqr dr, where the integral extends over the volume V of the particle and Δρ is the electron-density contrast between the particles and the surrounding medium.28 In the scenario of noninteracting colloidal nanoparticles, which were randomly distributed in solution without any orientation correlation, the experimentally measured quantity was I(q) = ⟨|F(q)|2⟩, representing the orientational averaged form factor.

For the present study involving cubes, octahedra, and rhombic dodecahedra, analytic equations were utilized to compute their form factor amplitude, denoted as Fs(q, L), where the subscript s refers to the shape and L represents the edge length of the three shapes.29 The polydispersity of the nanoparticles was modeled by assuming a Gaussian distribution, Inline graphic, where μ and σ are the average edge length and standard deviation, respectively. The percentage polydispersity was then given by Inline graphic. Therefore, the form factor of the shaped nanoparticles was modeled as I(q) = aG(L)·⟨|Fs(q, L)|2⟩ + b, with a and b being scalars to account for the scaling factor and scattering background, and the fitting parameters were σ, μ, a, and b.

2.2. Glycerol Electrooxidation Measurements

To prepare the working electrodes, a 15 μL suspension of 1 nM Au nanocrystals was drop casted onto a glassy carbon disk electrode and dried in a vacuum chamber at room temperature. The surface of the working electrode was then washed in deionized water and ethanol 3 times before a final rinse with deionized water to remove the surfactants capping the surfaces of the deposited shaped nanoparticles. A 3-electrode cell was used to perform the cyclic voltammetry (CV) experiments, in which a graphite rod served as the counter electrode, and the potential was measured with reference to a saturated calomel electrode. All potentials written hereafter are referenced against RHE. A 10 min argon purge was performed before CV from 0.1 to 1.6 V for 10 cycles at 50 mV/s in 0.1 M KOH electrolyte to clean and condition the deposited Au nanoparticles. This was followed by 10 cycles of CV in 0.1 M KOH and 1 M glycerol electrolyte from 0.1 to 1.6 V at 50 mV/s to assess electrocatalytic activity.30

2.3. In-Situ Infrared Reflection Absorption Spectroscopy (IRRAS)

In-situ IRRAS measurements were performed using a Nicolet IS50 spectrometer equipped with a mercuric cadmium telluride detector using a homemade 3-electrode cell setup consisting of a zinc–selenium hemispherical window with the nanocrystal-deposited glassy carbon disk working electrode, a graphite rod counter electrode, and an Ag/AgCl reference electrode on top of a purged IR beam reflection chamber. A purge with CO2- and H2O-free air was maintained to minimize signal interference from atmospheric CO2 and moisture in the chamber, and Au-plated mirrors were used to direct the IR beam to the cell and the detector. The electrolyte was purged with argon before collecting FTIR spectra. The spectra were obtained while performing linear sweep voltammetry in 0.1 M KOH and 1 M glycerol electrolyte from 0.1 to 1.6 V at a 2 mV/s scan rate. The resulting spectrum was created after adding together 127 interferograms at 4 cm–1 resolution.31 The spectra were processed by subtracting the first reference spectrum as the background.

3. Results and Discussion

3.1. Synthesis and Characterization

The wet chemistry method was employed to produce monofaceted cubes, octahedra, and rhombic dodecahedra. As illustrated in Figure 1, these nanocrystals exhibited a narrow size distribution and possessed high-quality terminating surfaces. The synthesis method in the current study yielded single-crystal nanoparticles with surfaces that could be easily cleaned through a simple washing process,19,20 thereby facilitating the accurate investigation of facet-dependent EGO reaction behaviors.

Figure 1.

Figure 1

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Uniformity of the three types of shaped gold nanocrystals: (a) octahedron, (b) rhombic dodecahedron, and (c) cube. (a1–c1) Typical large area scanning electron microscopy (SEM) images of Au nanocrystals. Scale bar: 200 nm. (a2–c2) Locally magnified SEM images of Au nanocrystals. Scale bar: 50 nm. (a3–c3) Schematic representation of nanocrystals’ polyhedral shapes. (a4–c4) Small-angle X-ray scattering (SAXS) 1D profile for form factors, I(q), with experimental data (ring-chain), fitted curve (red solid line) and 2D scattering images (insert) with color bars showing the intensity.

In the synthesis of Au shaped nanocrystals, a multistep procedure was employed to produce a variety of polyhedral nanocrystals, each with precisely controllable edge lengths. In general, the surface energies associated with different Au crystallographic facets follow a specific order: the energy increases from {111} to {100} and then to {110}.32,33 The surface energies of the metal crystal facets could be altered selectively by adsorbates, such as surfactants, polymers, small molecules, and atomic adsorbates, in solution-phase synthesis.34 It has been reported that cetylpyridinium chloride (CPC) could stabilize either the {111} or the {110} facets of Au,19,20,33 while ionic adsorbates, such as Br– ions, could stabilize the {100} facets. Thus, adjusting the makeup of CPC and Br– ions can alter the surface energies of the Au facets in a desired sequence under certain growth conditions. Therefore, by tuning the synthetic parameters, including the concentration of the surfactants and metal sources, the reaction temperature, and the reaction time, the shape-controlled growth of Au nanocrystals enclosing different facets can be achieved.

To form Au octahedra, an aqueous CPC solution was utilized, and AuCl4– ions were reduced by ascorbic acid at 28 °C. The surfactant CPC has weak adsorption, making it unnecessary to use harsh treatments for its removal,35 a process commonly applied in such syntheses. The growth passivation of the {111} facets, critical for forming Au octahedral, was ensured by utilizing a high concentration of the capping agent, CPC, coupled with a low concentration of the reducing agent, ascorbic acid. To obtain Au rhombic dodecahedra terminated with the {110} facets, a low concentration of CPC and an increased amount of the reducing agent were introduced with the reaction temperature raised to 32 °C. For the preparation of Au cubes, KBr was added. In conjunction with CPC, the cationic surfactant with Br– counterions contributes to stabilizing the Au {100} facets. This stabilizing mechanism is akin to the role played by the surfactant cetyltrimethylammonium bromide (CTAB).36

Our method produced three types of high-quality Au nanocrystals with yields exceeding 95% for each type. As shown in Figure 1, due to their high degree of uniformity in size and shape, these nanocrystals could readily self-assemble into superlattices, with the structures defined by their shapes and exhibiting long-range ordering. Accordingly, the Au octahedrons with eight {111} faces had an edge length of 50 nm with a standard deviation of 4% for their size distribution. They assembled only via incomplete face-to-face contact and formed a simple hexagonal arrangement, with the bottom layer lying on a flat substrate surface (Figure 1a1–a3). The Au rhombic dodecahedra enclosed by 12 {110} facets (Figure 1c3) had edges of 30 nm with a standard deviation of 5% and acted like spheres, thus assembling into a quasi-hexagonal packing structure within the layer (Figure 1b1–b3). For Au cubes with six {100} facets (Figure 1c3), the edge length was 48 nm with a standard deviation of 4.2% (Figure 1c2), and these Au cubes formed closely packed layers with in-plane square symmetry (Figure 1c1 and 1c2). The high uniformity of the synthesized Au nanocrystals was also confirmed by the analysis of the nanoparticles form factor, I(q), obtained through the small-angle X-ray measurement (Figure 1a4, 1b4, and 1c4). While electron microscopy observations provide real-space imaging, it only characterizes several hundreds to thousands of nanocrystals. On the other hand, SAXS can measure millions of nanocrystals illuminated in the beam path, thus offering a more robust statistical analysis. By fitting the data with the modeled I(q), as described in the Experimental Methods section, the average edge length, μ, and the corresponding size distribution standard deviation, σ, of nanocrystals with different shapes could be extracted. The Au octahedron, rhombic dodecahedron, and cube showed 51 nm and 4.5% (Figure 1a4), 29 nm and 4.9% (Figure 1b4), and 46 nm and 4.8% (Figure 1c4), respectively, which agree well with the SEM measurements.

TEM measurements and selected area electron diffraction (SAED) were used to further characterize the atomic lattice structures of these monofaceted nanocrystals. Figure 2 showcases TEM images that include both ensemble (Figure 2a1–c1) and individual (Figure 2a2–c2) particles of different Au nanocrystal shapes: octahedra (a), rhombic dodecahedra (b), and cubes (c). It also includes the SAED patterns (Figure 2a3–c3) collected from the corresponding single particles of each shape. The TEM images further corroborated the shapes of the nanocrystals (Figure 2a1, 2b1, and 2c1). The measured projected contours of each type of nanocrystals matched the profiles of their corresponding orientations (Figure 2a2–c2 and the insets), with the sharp diffraction dots in the characteristic SAED patterns confirming the single crystallinity of each type of nanocrystal (Figure 2a3, 2b3, and 2c3).

Figure 2.

Figure 2

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Structural characterization of single-crystalline gold nanocrystals with three types of shapes: (a) Au octahedra, (b) Au rhombic dodecahedra, and (c) Au cubes. (a1–c1) Transmission electron microscopy (TEM) images of Au nanocrystals. Scale bar: 50 nm. (a2–c2) TEM images of a single Au nanocrystal. (Insets) Corresponding projection schemes along the (a2) octahedral [011], (b2) rhombic dodecahedral [001], and (c2) cubic [001] axes. Scale bar: 20 nm. (a3–c3) Selected area electron diffraction (SAED) of Au nanocrystals.

3.2. Electrochemical Measurements

The CV profiles in 0.1 M KOH revealed the stabilization of the electrochemical features after 10 cycles, which showed distinct Au oxidation peaks of the different crystalline surfaces from three types of shaped Au nanocrystals (Figure 3a). The cubic nanocrystals exhibited several minor preoxidation peaks at 0.78, 0.95, and 1.1 V with two primary oxidation peaks at 1.24 and 1.32 V. The octahedral nanocrystals showed one preoxidation peak at 1.1 V attributed to anion adsorption and a feature at 1.24 V resulting from oxide formation.37 Similarly, the rhombic dodecahedral nanocrystals exhibited an oxidation feature at 1.26 V and a broader preoxidation peak starting with a small peak at 0.8 V and ending with another small peak at 1.1 V. These preoxidation peaks indicated the formation of AuOH on the nanocrystal surfaces.16 All three types of nanocrystals exhibited a primary Au–O reduction peak at 1.08 V on the reverse sweep. The features observed on the nanofacets of shaped Au nanocrystals matched the previous observation on the Au {100}, Au {111}, and Au {110} single-crystal surfaces in alkaline condition,37 verifying the high crystallinity and cleanliness of the nanocrystal samples. The charge transferred during Au–O reduction was used in conjunction with the corresponding facet-dependent QO values to determine the electrochemical surface area (ESCA) of the Au nanoparticles.38−40

Figure 3.

Figure 3

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CV profiles for shape-controlled Au nanocrystals with exposed Au {100}, Au {111}, and Au {110} surfaces. (a) Forward and reverse sweep of the 10th cycle for 0.1 M KOH normalized by the geometric surface area. (b) Forward sweep of the 3rd cycle for 0.1 M KOH + 1 M glycerol normalized by ECSA. (c) From top to bottom, top-view atomic model on the {111}, {110}, and {100} crystalline planes of Au.

With the introduction of glycerol to the electrolyte, the differences in surface facets manifested primarily in the peak heights. After normalizing by the ECSA, the forward sweeps showed that the octahedral nanocrystals exhibited the highest activity (Figure 3b), followed by the rhombic dodecahedral and the cubic nanocrystals. The rhombic dodecahedral nanocrystals exhibited the lowest onset potential, which correlated to the most prominent preoxidation peak observed in the absence of glycerol. This suggested that EGO required the oxidation of Au to AuOH for the reaction to begin, agreeing with the reported findings for the oxidation of glycerol and other organic molecules.6,15−17,41 This is due to AuOH formation decreasing the energy barrier for activating the C–H and O–H bonds in adsorbed glycerol.42 Increasing the glycerol concentration excessively has been found to result in an increased onset potential and a reduced glycerol oxidation activity due to the high surface coverage of glycerol inhibiting the formation of AuOH.43 AuOH formation has been reported to be more favorable on Au {100} than the other facets due to the 4-fold symmetry of the {100} site allowing for a strong interaction to take place between Au and OH–, whereas the smaller vacancy in the 3-fold symmetry of the {111} site reduces the extent of orbital overlap and charge transfer.44

Analysis of the ratio between the peak current of the forward scan and the reverse scan (If/Ir) has been reported as an indicator for catalyst resistance to poisoning from the oxidation intermediates.45−48 The CV profiles for the three shaped nanocrystals in the presence of glycerol revealed that the octahedral nanocrystals had the highest If/Ir ratio of 1.9 (Figure S2). In contrast, the rhombic dodecahedral and cubic nanocrystals both exhibited significantly lower If/Ir ratios close to 1. The surfaces of Au nanocrystals with different arrangements of atoms (Figure 3c) show high crystallinity and thus high fidelity in surface-dependent reactions. These results suggested that the {111} surfaces of octahedral nanocrystals were both the most active for EGO and the least susceptible to poisoning by the intermediates.

3.3. In-Situ IRRAS Measurements

The in-situ FTIR spectra revealed a multitude of peaks corresponding to various C3 and C2 products (Table 1). All three samples showed peaks at 1076, 1110, 1326, 1348, 1412, 1572, and 1730 cm–1, while the octahedral and rhombic dodecahedral samples also exhibited distinct peaks at 1310 and 1380 cm–1 (Figure 4). The peak at 1572 cm–1 can be assigned as the carboxylate group from several products, such as tartronate, mesoxalate, glycolate, glyoxylate, and formate, while the one at 1730 cm–1 can be attributed to adsorbed carbonyl groups belonging to species like tartronate.49−51 Tartronate and mesoxalate can be further distinguished by the peak at 1110 cm–1, while the peaks at 1076, 1310, 1326, and 1412 cm–1 can be attributed to C2 products like glycolate, glyoxal, glyoxylate, and oxalate.7,49,52,53 Additionally, the peak at 1348 cm–1 can be assigned to formate, while the peak at 1380 cm–1 can be assigned to both formate and glyoxylate.49,52 In general, the observed products agreed with the previously reported findings for polycrystalline Au surfaces in alkaline conditions, where tartronate, mesoxalate, glyoxylate, oxalate, glycolate, and formate were observed.7,54−56

Table 1. Characteristic Vibrational Frequencies for Various Surface Intermediates7,49−56.

species wavenumbers (cm–1)
formate (C1) 1348, 1380, 1572
glycolate (C2) 1076, 1326, 1412, 1572
glyoxal (C2) 1076
glyoxylate (C2) 1076, 1380, 1572
oxalate (C2) 1310, 1572
glycerol (C3) 993, 1040, 1110
tartronate (C3) 1110, 1572, 1730
mesoxalate (C3) 1110, 1572
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Figure 4.

Figure 4

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In-situ FTIR spectra during EGO from 0.1 to 1.6 V on shape-controlled nanoparticles with exposed (a) Au {111}, (b) Au {110}, and (c) Au {100} surfaces; (d) peak height ratios of 1076 and 1110 cm–1 comparing the selectivity of C2 to C3 products over applied potential.

The different crystalline surfaces yielded observable differences in the product selectivity from EGO. The octahedral and rhombic dodecahedral nanocrystals exhibited many of the same peaks. However, the peaks appearing for octahedral nanocrystals at around 0.9 V were similar to those for the rhombic dodecahedral nanocrystals at 1.2 V (Figure 4a and 4b), indicating the facet-dependent onset potentials for EGO. In contrast, the cubic nanocrystals showed peaks at 993, 1040, and 1110 cm–1, indicative of adsorbed glycerol, with the observed surface species evolving after 1.1 V with the appearance of peaks at 1076 and 1575 cm–1 (Figure 4c).

Further analysis was performed by using the height of the peaks at 1076 and 1110 cm–1 as descriptors for C2 and C3 products, respectively. Holding the 1110 cm–1 peak as a reference, a larger peak at 1076 cm–1 would suggest greater selectivity toward C–C bond scission to forming C2 products. Plotting the ratio of C2 to C3 peak heights demonstrated that the {100} surface of the cubic nanocrystals showed the lowest selectivity for C–C bond scission, which reflected the late development of the 1076 cm–1 peak (Figure 4d). In contrast, while the {110} surface showed higher selectivity for C2 products, the {111} surface of the octahedral nanocrystals exhibited the greatest selectivity and the lowest onset potential for C2 products of EGO. These differences may be due in part to preferential formation of the hydroperoxyl intermediates on the Au {111} surface, which has been previously attributed to enhanced C–C bond scission for glycerol on Au-based catalysts.18,57−59 These results demonstrated that different facets of nanocrystals not only result in differences in EGO activity but also play a key role in the oxidation product selectivity.

4. Conclusions

Shape-controlled Au nanocrystals of uniform size and shape were precisely synthesized to prepare well-defined {111}, {110}, and {100} crystalline facets for the study of glycerol electrooxidation. Rhombic dodecahedral nanocrystals demonstrated the lowest onset potential for EGO owing to facile AuOH formation on the Au {110} surface. However, octahedral nanocrystals exhibited the highest electrochemical activity for glycerol oxidation and greatest resistance to poisoning by oxidation intermediates. In-situ FTIR analysis revealed that the Au {111} surfaces of the octahedral nanocrystals promoted C2 product formation more selectively than the Au {110} and Au {100} surfaces, indicating a greater activity for C–C bond scission. These results provide insights into the glycerol electrooxidation performance over different Au crystalline facets, demonstrating that shaped Au nanocrystals can serve as model systems to guide the design of future catalysts for glycerol upgrading.

Acknowledgments

We acknowledge support by the Department of Energy, Division of Chemical Sciences, Geosciences, & Biosciences, Catalysis Program (grant no. DE-FG02-13ER16381). H.M. acknowledges support by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 2036197. This research was also supported in part by Brookhaven National Laboratory, Laboratory Directed Research and Development Grant No. 22-059. This research also used resources of the Center for Functional Nanomaterials and the CMS beamline (11-BM) of the National Synchrotron Light Source II, both supported by U.S. DOE Office of Science Facilities at Brookhaven National Laboratory under Contract No. DE-SC0012704.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/prechem.3c00105.

  • UV–vis absorption spectra for the three types of shaped Au nanocrystals; CV forward and reverse profiles of shaped Au nanocrystals for alkaline glycerol oxidation (PDF)

The authors declare no competing financial interest.

Supplementary Material

pc3c00105_si_001.pdf (196.8KB, pdf)

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