Combined EC-NMR and In Situ FTIR Spectroscopic Studies of

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Research Article pubs.acs.org/acscatalysis

Combined EC-NMR and In Situ FTIR Spectroscopic Studies of Glycerol Electrooxidation on Pt/C, PtRu/C, and PtRh/C Long Huang,† Jia-Yu Sun,† Shuo-Hui Cao,‡ Mei Zhan,† Zu-Rong Ni,‡ Hui-Jun Sun,‡ Zhong Chen,‡ Zhi-You Zhou,† Eric G. Sorte,§ YuYe J. Tong,§ and Shi-Gang Sun*,† †

State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡ Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, 361005, China § Department of Chemistry, Georgetown University, 37th and O Streets, NW, Washington, DC 20057, United States S Supporting Information *

ABSTRACT: Glycerol, a byproduct of biodiesel production, is an industrial waste because of its excess yield. Electrooxidation of glycerol is a promising way to utilize glycerol through harvesting electric energy as fuels in a fuel cell or hydrogen as sacrificial agent in electrolysis cellwhile generating valuable chemicals. Here, we report a detailed mechanistic study of the glycerol electrooxidation reaction (GOR) on a series of Pt/C, PtxRuy/C, and PtxRhy/C nanocatalysts synthesized by NaBH4 reduction. The EC cyclic voltammetry characterization indicates that alloying Ru with Pt greatly enhanced the GOR activity, especially at low potential, but not as much with alloying Rh, as compared with Pt/C. In situ FTIR and 13C NMR spectroscopies were used to investigate the GOR mechanism at a molecular level. The results demonstrate that the selectivity of products depends on the type of catalysts and the oxidation potential. Although both PtRu/C and PtRh/C could accelerate the oxygen insertion reactions that led to higher selectivity of carboxylic acids, tartronic acid was more favored at high potential on the PtRh/C surface. KEYWORDS: glycerol electrooxidation, in situ FTIR, in situ 13C NMR, nanocatalyst, electrocatalysis

1. INTRODUCTION Recently, biodiesel has become an important supplement fuel to the fossil-fuels-based market due to its sustainability and cleanness. The biodiesel is produced by a transesterification reaction between triglycerides and alcohol, usually methanol, which generates glycerol as a coproduct. About 1 kg of glycerol is generated for every 10 kg of biodiesel production. The worldwide production of glycerol is growing thanks to a rapidly increased demand for biodiesel, leading to an overproduction of the former and thus the generation of an industrial waste. Therefore, new strategies for repurposing glycerol are necessary. In fact, glycerol is a key platform chemical for producing valuable chemicals: many of the glycerol oxidation products, such as glyceric acid, glycolic acid, dihydroxyacetone (DHA), tartronic acid, and so on, are useful intermediates or high value-adding fine chemicals.1 For example, a carboxylic acid like glyceric acid and tartronic acid, can be converted into various market products (e.g., polymers or biodegradable emulsifiers).2 Tartronic acid can also be used as a drug-delivery agent, increasing the blood absorption of tetracycline antibiotic. DHA is widely used in the cosmetic industry as sunless tanning lotion or as a monomer for making polymeric biomaterials. © XXXX American Chemical Society

However, the process of glycerol oxidation is quite complex (Scheme 1), and many of its products can interconvert one another via redox reactions, which usually leads to poor selectivity of the desired products. In industry, stoichiometric oxidants (e.g., permanganate, nitric acid or chromic acid) are used to oxidize glycerol to the desired products, but these reactions are not only toxic but also less selective, producing large amounts of undesired byproducts. Glycerol electrooxidation reaction (GOR) represents a promising alternative to generating valuable chemicals while harvesting electric energy or hydrogen, depending on the electrochemical (EC) reactor configuration (fuel cell or electrolysis cell, respectively). Moreover, the use of environmentally unfriendly oxidants can also be avoided by oxidative conversion of glycerol in EC reactors. The GOR in an acidic environment is much less studied as compared to other C1 and C2 molecules (e.g., formic acid,3−6 methanol,7,8 and ethanol).9−11 This may be due to its sluggish Received: July 25, 2016 Revised: September 15, 2016

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DOI: 10.1021/acscatal.6b02097 ACS Catal. 2016, 6, 7686−7695

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ACS Catalysis Scheme 1. Main Intermediates/Products of Glycerol Oxidation

(FTIR) spectroscopy and in situ high-resolution EC NMR to analyze, both qualitatively and quantitatively, the GOR products on home-synthesized Pt/C, PtxRuy/C, and PtxRhy/C nanocatalysts.

kinetics, complex oxidation mechanisms, and diverse products, which make the mechanistic studies very challenging.12−14 Gomes et al.13 studied the GOR on polycrystalline Pt and Au surface combined with in situ FTIR in 0.1 M H2SO4. They suggested that the GOR on Pt leads to the formation of tartronic acid, glycolic acid, glyoxylic acid, formic acid, and CO2 in acidic environment. Over Au in acidic medium, the GOR formed tartronic acid, formic acid, and CO2. Using a combined spectroelectrochemical DEMS/ATR-FTIRS setup, Behm et al.15 investigated the adsorption/oxidation of glycerol on a Pt thin film electrode. Their data show that glyceraldehyde was an intermediate to COad formation, and glyceric acid was a deadend production in the GOR. Koper et al.14 studied the mechanism of GOR on Pt and Au as a function of pH by combining online high-performance liquid chromatographic (HPLC) with online EC mass spectrometric studies. Their results indicate that on Pt, the main stable product was glyceraldehyde at relatively low potential in acidic media, which was converted further to formic acid and CO2 as the potential increased. On the other hand, no activity of GOR was observed on the Au under the studied conditions. In addition to the monometallic catalysts, bimetallic, and trimetallic catalysts have also been studied for the GOR. Koper et al.16 studied GOR on Pt/C in a Bi3+-saturated acidic solution. They observed that glycerol could be oxidized to DHA with almost 100% selectivity at low potential. It was hypothesized that spontaneous adsorption of Bi3+ on the Pt surface would block the active sites for the oxidation of the primary alcohol but not the secondary alcohol, resulting in high selectivity toward DHA. Kim et al.17 observed that ternary PtRuSn/C catalysts could enhance substantially the electrocatalytic activity for GOR, resulting in larger peak currents and lower onset oxidation potential. The authors speculated that the synergistic effect of Ru and Sn in PtRuSn/C catalysts helped complete oxidation of CO-like species on the Pt sites and make them free for GOR. Despite that the above briefly discussed previous works have advanced our general qualitative understanding of GOR on an electrode surface, its detailed reaction mechanism on Pt-based catalysts is still far from clear. Due to the intrinsic complexity of GOR, we believe that achieving a better quantitative analysis of the GOR products is a key step toward untangling the complex reaction mechanism. We report herein such an attempt by combining complementary in situ Fourier transform infrared

2. EXPERIMENTAL SECTION Synthesis of Nanocatalysts. The Pt/C, PtxRuy/C, and PtxRhy/C nanocatalysts were synthesized by a sodium borohydride (NaBH4) reduction method. Briefly, 50 mg of XC-72 carbon black was dispersed in water and mixed with a desired amount of metal salts and 95 mg of sodium citrate in a round-bottom flask in an ultrasonic ice bath for 30 min. The desired metal loading is 20 wt %. For the Pt/C, 1.66 mL of 38.6 mM H2PtCl6 solution was added as the Pt precursor. For PtxRuy/C and PtxRhy/C, an appropriate amount of RuCl3 or RhCl3, calculated according to the desired Pt:Ru or Pt:Rh atomic ratio of 5:1, 3:1, 2:1, and 1:1, respectively, was added to the reaction solution that had a total final volume of 50 mL. Then 5 mL of reduction solution containing 20 mM NaBH4 and 0.1 M NaOH was added drop by drop to the roundbottom flask that was placed in the ultrasonic ice bath. The amount of NaBH4 was 5 equiv of the metal salt. After that, an appropriate amount of 1 M H2SO4 solution was added the reaction solution to adjust the pH value to about 2−3. Then, the synthesized nanocatalysts were washed with water by centrifugation. Finally, the nanocatalysts were dried at 80 °C under vacuum overnight. The synthesized samples are designated as Pt5Ru1/C, Pt3Ru1/C, Pt2Ru1/C, Pt1Ru1/C and Pt5Rh1/C, Pt3Rh1/C, Pt2Rh1/C, Pt1Rh1/C, respectively. Physical Characterizations. Transmission electron microscopic (TEM) images were taken on a TECNAI F20 (FEI Co., U.S.A.) operating at 200 kV. The bulk composition of the assynthesized nanocatalysts was analyzed by energy dispersive Xray spectroscopy (EDX) on an s4800 (Hitachi co. Japan) operating at 15 kV. The XRD measurements were performed with a Rigaku Ultima IV diffractometer (Rigaku Co. Japan) by using a Cu Kα (λ = 1.5405 Å) radiation source operating at 40 kV and 30 mA. The X-ray photoelectron spectroscopy (XPS) measurements were performed with a PHI quantum 2000 spectrometer (Physical Electronics Co., U.S.A.) to determine the elemental oxidation state of the as-synthesized nanocatalysts. The position of the C 1s peak (284.4 eV) was used to 7687

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chokes were used to isolate NMR detection from any electrical interference from the potentiostat (CHI660D). All NMR measurements were carried out on a hybrid Bruker/P-B-GCYJ500 500 MHz spectrometer using a standard 5 mm probe. The EC-NMR cell was placed in the magnet, and the position of the electrode assembly was adjusted carefully to just above the NMR detection region to minimize its interference of NMR detection as described in our previous work.20 13C-labeled glycerol 13CH2OH13CH(OH)13CH2OH (99%, Cambridge Isotope) was used in the study. For the 13C{1H} NMR spectra of 0.1 M GOR related compounds, 1H decoupling sequence was used to collect the 13C spectra over a period of near 14 h with 8192 scans for each spectrum. For ex situ 13C NMR analysis of GOR products, a given as-synthesized nanocatalysts was first dispersed in ethanol containing 10 wt % Nafion, which was then drop-casted onto the carbon fiber. The final loading of nanocatalysts was 2 mg for each measurement. The GOR was conducted in the EC-NMR cell at 60 °C for 8 h at different potentials: 0.25, 0.35, and 0.45 V, respectively. After the GOR, the electrode assembly was removed, and 13C{1H} NMR spectra were acquired using 1H decoupling sequence for 64 scans.

calibrate the XPS binding-energy scale for all nanocatalysts for possible charge transfer effects. Electrochemical Measurements. The electrochemical (EC) measurements were performed in a conventional threeelectrode EC cell with a CHI631A potentiostat (Chenhua Co. China). A Pt plate and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. All potentials are referred to the SCE in this paper. The working electrode was a glassy carbon (GC, Φ = 5 mm) electrode embedded in a Teflon sleeve. Prior to an EC measurement, the GC electrode was mechanically polished using α-Al2O3 powder of size 5, 1, and 0.3 μm successively. It was then cleaned in an ultrasonic bath and drop-casted with the as-synthesized nanocatalysts. The nanocatalyst-modified GC electrode was wetted with 2 μL Nafion diluents (0.25 wt % Nafion in ethanol) and air-dried. The cyclic voltammetry (CV) cleaning and COad stripping voltammograms were recorded in a 0.1 M H2SO4 and 0.1 M HClO4, respectively. The solution was deaerated by ultrapure nitrogen for 15 min prior to any measurements. For COad stripping voltammograms, the EC cell was first purged with CO for 15 min and then followed by N2 purging for another 20 min to remove the excess CO in electrolyte during which the electrode potential was held at 0.01 V. COad stripping voltammograms was scanned between −0.25 to 1.0 V at 50 mV/s. The EC active surface area (ECSA) of the nanocatalysts were determined from the charge of COad stripping according to the literature.18 The GOR activity was measured in solution containing 0.1 M glycerol and 0.1 M HClO4 at 60 °C, with the temperature controlled by a water bath. In Situ FTIR Study of GOR. In situ FTIR reflection spectroscopic studies were carried out on a Nexus 870 FTIR spectrometer (Nicolet) equipped with a liquid-nitrogen-cooled MCT-A detector and an EverGlo IR source. A thin-layer IR cell with a CaF2 planar window was employed. The in situ FTIR spectra of GOR on the as-synthesized nanocatalysts were recorded at electrode potentials varied from −0.2 to 0.6 V in a mixture of 0.1 M glycerol and 0.1 M HClO4 with a potential interval of 0.1 V. The reference spectrum was taken at −0.25 V. The resolution is 8 cm−1, and 400 spectra were averaged for each potential. It took 30 min to obtain a whole set of spectra. All in situ FTIR experiments are performed at room temperature. EC-NMR Study of GOR. The design of the EC-NMR cell was modified from the Dunsch group’s and our previous work.19,20 In brief, it was based on a 5 mm standard commercial NMR tube used in routine solution NMR measurements, as illustrated in Scheme S1. It consisted of a carbon fiber bundle as the working electrode (WE), a Pt wire as the counter electrode (CE), and a Pd wire as the reference electrode (RE). Before the GOR measurements, the Pd wire was immersed in 0.1 M H2SO4, holding the potential at −0.30 V for 60 min to form PdH, which was used as the reference electrode. The open circuit potential of the as-made PdH was −0.25 V vs SCE. The carbon fiber was adjoined to a capillary-housed copper wire by conducting adhesive. The carbon-fiber-protruding end of the capillary was sealed by hydrogen flame annealing so that only carbon fiber could contact the electrolyte. To quantify the reaction species, we used an internal reference consisting of a short capillary sealed with chloroform or hexamethyldisiloxane that was placed inside the NMR tube as indicated in Scheme S1. Their integrated NMR signal at 77.2 or 2.2 ppm was used to normalize all the NMR signals reported here. Radio frequency

3. RESULTS AND DISCUSSION The Pt/C, PtxRuy/C, and PtxRhy/C nanocatalysts synthesized by NaBH4 reduction were characterized by TEM. Figure 1

Figure 1. Representative TEM images of the as-synthesized (a) Pt/C, (b) Pt3Ru1/C, (c) Pt2Rh1/C nanocatalysts (scale bars correspond to 20 nm).The measured size distributions based on 500 counts of nanoparticles are presented in panels d−f for the Pt/C, Pt3Ru1/C, and Pt2Rh1/C nanocatalysts, respectively.

shows typical TEM images of the three nanocatalysts: Pt/C, Pt3Ru1/C, and Pt2Rh1/C, respectively. The metallic nanoparticles were evenly dispersed on carbon support, with only slight agglomerations. Their average nanoparticle sizes were measured over 500 counts to be 5.4 ± 1.4 nm, 3.9 ± 1.1 nm, and 4.9 ± 1.5 nm, respectively. The compositions of the assynthesized nanocatalysts were characterized by EDX. The atomic ratios of Pt:Ru in the PtxRuy/C and Pt:Rh in the PtxRhy/C and the corresponding metal loadings are listed in Table S1. The metal loadings and the atomic ratios are close to what expected from the initial amounts and ratios of the metal salts, indicating a total reduction of the metal precursors. The XRD patterns of all the synthesized nanocatalysts are shown in Figure 2. For the Pt/C, the diffraction peaks at 2θ = 39.8°, 46.2°, 67.5°, 81.3° are characteristic for (111), (200), (220), and (311) crystalline planes of Pt. For the PtxRuy/C and 7688

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Figure 2. XRD patterns of the as-synthesized (a) Pt/C, PtxRuy/C, and Ru/C and (b) Pt/C, PtxRhy/C, and Rh/C nanocatalysts.

PtxRhy/C, similar diffraction patterns were observed. That no individual Ru or Rh diffraction peaks were observed indicates the formation of PtRu and PtRh alloy nanoparticles with the face-centered-cubic (fcc) structure. By alloying either Ru or Rh with Pt, the diffraction peaks shifted to higher angles, consistent with the lattice contraction caused by the substitution of smaller Ru or Rh atoms. The compressive strain would result in the down shift of the d-band center and therefore weaken the CO adsorption on Pt surface. The latter is expected to enhance CO tolerance on the PtxRuy/C and PtxRhy/C nanocatalysts.21 The diffraction peaks shift further to higher angles as the content of Ru or Rh increases. Figure 3 displays the XPS spectra of the Pt 4f core level region for all the nanocatalysts. The binding energies of all the

Figure 4. CV curves of GOR on the as-synthesized (a) Pt/C, PtxRuy/ C, (b) Pt/C, PtxRhy/C, and (c) Pt/C, Pt3Ru1/C, Pt2Rh1/C nanocatalysts in 0.1 M glycerol + 0.1 M HClO4 solution at 60 °C, scan rate: 50 mV·s−1. (d) CO stripping curve on Pt/C, Pt3Ru1/C, Pt2Rh1/C nanocatalysts in 0.1 M HClO4 solution, scan rate: 50 mV· s−1.

shown in Figure 4a. As the Ru content increased, the activity of GOR as measured by the reaction current at low potential region on the PtxRuy/C increased initially, peaked at the Pt:Ru = 3:1, and then decreased thereafter. For the PtxRhy/C nanocatalyst, it seems that alloying Rh had much smaller effect on the GOR on the PtxRhy/C nanocatalyst as reflected by the CVs in Figure 4b. The onset potential of GOR on the PtxRhy/C was almost the same as that on the Pt/C, but the reaction current was slightly larger on the former. The CV curves of GOR on the PtxRhy/C nanocatalysts with Pt:Rh ratios of 5:1, 3:1, and 2:1, respectively, are almost the same, with the reaction current on the Pt2Rh1/C being slightly higher. Further increasing the Rh content lowered the GOR activity as in the case of PtxRuy/C nanocatalysts. To highlight the differences just discussed, we compare the GOR CVs obtained on the Pt/C, Pt3Ru1/C, and Pt2Rh1/C, respectively, in Figure 4c. The onset potential of GOR on the Pt3Ru1/C was ∼200 mV more negative than that on the Pt/C and Pt2Rh1/C. As very similar strain and electronic effects by incorporation of Ru vs Rh into the Pt lattice are expected, as indicated by the XRD results in Figure 2 and the XPS results in Figure 3, respectively, we attribute the much improved GOR activity on the Pt3Ru1/C to a bifunctional mechanism. It is well-known that Ru can activate adsorbed water molecule to generate OHad at a much lower potential and therefore facilitate the oxidation of the adsorbed species including COad.26,27 Indeed, as shown in Figure 4d, the CO stripping was much easier on the Pt3Ru1/C than on the Pt/ C or Pt2Rh1/C nanocatalysts. The onset and peak potentials of CO stripping on the Pt3Ru1/C were 170 mV and 180 mV, respectively, more negative than those on the Pt/C. For Pt2Rh1/C, the onset potential of CO stripping is the same as that of Pt/C, while the peak potential is negatively shifted by ∼40 mV. The superior COad stripping activity resulted in a much faster GOR on the Pt3Ru1/C nanocatalyst at low potential region, likely via the bifunctional mechanism.26,27 The similar behavior of GOR and COad stripping on the Pt/C and Pt2Rh1/C indicate that the CO-poisoning is likely one of the

Figure 3. XPS Pt 4f spectra of the as-synthesized (a) Pt/C, PtxRuy/C and (b) Pt/C, PtxRhy/C nanocatalysts.

peaks were calibrated by setting the C 1s value to 284.4 eV. The PtxRuy/C and PtxRhy/C show higher Pt 4f binding energies as compared to the Pt/C, which is due to the different work function among Pt, Ptx Ru y and Pt x Rh y nanocatalysts, accompanied by rehybridization of the d-band as well as the sp-band, as proposed by Watanabe and other groups.22−24 The work function change leads to the reference level (EF) shift in photoelectron measurement. The positive shift of the Pt 4f peak would imply a similar shift of Pt’s d orbitals, therefore a lowering of the d band center and weakening of the CO adsorption, leading to an enhanced CO tolerance on the PtxRuy/C and PtxRhy/C nanocatalysts.22,25 COad species are generated when small organic molecule adsorbs dissociatively on a Pt-based surface. By alloying Ru or Rh with Pt, the induced electronic and strain effects could act synergistically in enhancing small organic molecule electrooxidation. Increasing Ru or Rh content in the corresponding nanocatalysts resulted in higher Pt 4f binding energy, indicating a stronger electronic effect. It was observed that the positive shift of binding energy was larger on the PtxRuy/C than on the PtxRhy/C nanocatalysts. Figure 4a−c compare the GOR activity of different nanocatalysts. According to the CVs, the onset potential for GOR on the Pt/C was ∼0.3 V. By alloying Ru, the onset potential for GOR on the PtxRuy/C was negatively shifted as 7689

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glyceraldehyde and glyceric acid, respectively. These two products are generated by the oxidation of the terminal hydroxymethyl (−CH2OH) group in glycerol, losing 2 and 4 electrons, respectively. In the in situ FTIR spectra, the appearance of glyceric acid on the Pt3Ru1/C and Pt2Rh1/C is at 0.2 V, earlier than that on the Pt/C (0.3 V). The downward peaks at 1736 cm−1 on the Pt3Ru1/C and 1740 cm−1 on the Pt/ C and Pt2Rh1/C correspond to the generation of species containing a carbonyl group, such as aldehydes, ketones, and carboxylic acids. Because almost all the GOR products contain a carbonyl group, it is very difficult, if not impossible, to distinguish each of them by their IR peaks. Similarly, the downward peaks located between 1200 and 1300 cm−1, which corresponds to carboxylic acids, are also hardly useful to help discern the GOR products quantitatively, even if we compare them with the standard spectra,13 because this spectral regions are highly crowded due to those compounds share similar molecular structure and functional groups. Still, qualitative information can be gleaned by inspecting the subtle band shape changes, as shown in Figure 5d. For the Pt2Rh1/C, the band shape between 1200 and 1300 cm−1 shows two reasonably strong peaks at 1234 and 1273 cm−1. For the Pt/C, as the overall band intensity decreases substantially, the highfrequency of the band is suppressed much more than the low-frequency side. As for the Pt3Ru1/C, the two peaks merge into a single hump with the maximum intensity at 1250 cm−1. The varying shape of the band between 1200 and 1300 cm−1 clearly indicates the different product selectivity of GOR on the different nanocatalysts. We will discuss this in more detail with the help of NMR measurements below as NMR has the capability of distinguishing chemicals of very similar molecular structures as long as the sensitivity is not an issue. We discuss first here the ex situ NMR results that were obtained without active potential control. Standard 13C NMR spectra of all available chemicals that are believed to be present during GOR were taken and shown in Figure S1. The assignments of the corresponding peaks are listed in Table S2. Figure S2 compares the 13C NMR spectra before and after GOR on the Pt/C at 0.45 V in a 5 mm NMR tube at 60 °C for 8 h. The total volume of the solution was 800 μL, containing 0.1 M 13C-label glycerol as the initial reactant, 0.1 M HClO4 as supporting electrolyte, and water with D2O:H2O = 1:4 as solvent. The addition of D2O was for field locking during NMR measurements. The black curve in Figure S2 is the 13C NMR spectrum before the GOR. Three peaks are clearly seen. The doublet with peaks at δ = 62.7 ppm and δ = 62.3 ppm corresponds to carbon atoms in terminal hydroxymethyl (−*CH2OH) group; the triplet with peaks at δ = 71.8 ppm, δ = 72.1 ppm, and δ = 72.4 ppm corresponds to carbon atoms in hydroxyl methylene (−*CHOH) group; and the singlet peak at δ = 2.2 ppm corresponds to carbon atoms of methyl (−*CH3) in hexamethyldisiloxane, sealed in a capillary and used as a standard for chemical shift reference and products quantification. The doublet and triplet come from the 13C coupling with adjacent 13C atoms. After the GOR on the assynthesized Pt/C nanocatalysts at 60 °C for 8 h, another 13C NMR spectrum was recorded, as the red curve in Figure S2. Many new peaks, some of which overlapped the others at first glance, appeared beside the three peaks observed before the GOR. However, most of the seemingly overlapping peaks can be adequately resolved by zooming in the spectra, as illustrated in Figure 6a,b. The doublets with peaks at δ = 59.1 and 59.5 ppm, δ = 51.9 and 62.3 ppm, δ = 63.1 and 63.4 ppm, and δ =

possible reasons why the GOR was sluggish at low potential region. To further illustrate the mechanism of GOR at a molecular level, electrode-potential-dependent in situ FTIR spectra were recorded during GOR, as presented in Figure 5. The peak

Figure 5. (a)−(c) In situ MS-FTIR spectra for GOR on as-synthesized (a) Pt/C, (b) Pt3Ru1/C, (c) Pt2Rh1/C nanocatalysts, in 0.1 M glycerol + 0.1 M HClO4 solution; (d) in situ FTIR spectra of [email protected] V, zoom in from (a)−(c). The reference spectra were acquired at −0.25 V.

Table 1. Assignments of the In Situ FTIR Spectrum of GOR wavenumbers/cm−1

peak assignment

2343 ∼2050 ∼1740 1238−1281 1126 1100 1010

νas(OCO)CO2 ν(CO)COad ν(CO)carboxylic acids, ketones, and aldehydes ν(O−C−O), δ(O−H) carboxylic acids ν(C−O)glyceraldehyde37,42,43 ClO4− ν(C−O)glyceric acid37

assignments of the spectra are listed in Table 1. The downward peak at 2343 cm−1 can be assigned to CO2, which indicates that glycerol molecule could be fully oxidized on all three types of nanocatalysts in which C−C bonding cleavage is a prerequisite for the generation of CO2. While the onset potential for an observable CO2 appearance on all three nanocatalysts was 0.1 V, the amplitude was much larger on the Pt3Ru1/C than that on the other two nanocatalysts, confirming the much higher activity of the former. Notice that the CO2 appeared at a potential that was even more negative than the generation of other partially oxidized products such as aldehydes and carboxylic acids. This indicates that the fresh Pt, PtRu, and PtRh surfaces were highly active for C−C bond cleavage.28 The clearly observable bipolar peaks at around 2050 cm−1 in Figure 5a indicate the formation of COad, another species formed by the glycerol dissociative adsorption. However, the intensity of this peak is much lower on the Pt2Rh1/C and almost not observable on the Pt3Ru1/C, as shown in Figure 5b,c. This suggests considerably reduced CO poisoning on the Pt3Ru1/C but more active non-COad generation reaction pathways on the Pt2Rh1/C than that on the Pt/C. The two downward peaks at 1126 cm−1 and 1010 cm−1 correspond to the formation of 7690

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the atmosphere and reach equilibrium with the unlabeled CO2 in atmosphere. Therefore, a quantitative determination of CO2 generated during GOR is impossible. The peaks at δ = 171.3 and 171.7 ppm, δ = 172.3, 172.9, and 173.9 ppm, δ = 175.5 and 175.9 ppm, δ = 176.1 and 176.6 ppm can be assigned to carbon atoms in carboxylic acid groups (−*COOH) of tartronic acid, glyoxylic acid, glyceric acid, and glycolic acid, respectively. By careful comparisons with the standard 13C NMR spectra in Figure S1, the peaks in Figure 6 can all be matched with them, enabling the identification of the corresponding species. Moreover, with those peaks convincingly identified, we can now analyze quantitatively the products of GOR by 13C NMR using the aforementioned internal standard (i.e., the peak integral of hexamethyldisiloxane at 2.2 ppm). The centration of a given product, c, can be calculated by c = [(A /S)/A G] × 100

where A is the peak integral of the product, S the sensitivity factor, AG is the peak integral of glycerol before GOR starts, and 100 represents the initial concentration of glycerol: 100 mM. The sensitivity factor of glycerol at δ = 62.5 ppm was set be 1, and other sensitivity factors were listed in Table S3. Specifically, peaks at δ = 74.3 ppm, δ = 63.5 ppm, δ = 59.5 ppm, δ = 173.3 ppm, δ = 163.0 ppm, δ = 65.0 ppm, δ = 166.0 ppm, and δ = 171.7 ppm are chosen to calculate the concentrations of glyceraldehyde, glyceric acid, glycolic acid, glyoxylic acid, oxalic acid, DHA, formic acid, and tartronic acid, respectively. A sample calculation for c is given in SI. The selectivity toward the liquid products on the Pt/C, Pt3Ru1/C and Pt2Rh1/C at different oxidation potentials are presented in Figure 7 and Table S4. The order in selectivity toward glyceraldehyde was Pt/C > Pt3Ru1/C > Pt2Rh1/C, which is interesting because glyceraldehyde is the redox intermediate between glycerol and glyceric acid, as illustrated in Scheme 1. As the surface elemental composition changed, the selectivity toward each product changed accordingly. On the PtRh surface, glyceraldehyde had the smallest fraction among the final products, regardless the electrode potentials. Most glyceraldehyde were probably further oxidized to glyceric acid once they were formed on the PtRh surface. On the other hand, the GOR on the Pt/C produced the highest fraction of glyceraldehyde among the three nanocatalysts, suggesting that it is a relatively poor electrocatalyst for oxygen insertion during GOR. The selectivity toward glyceric acid was relatively high under all studied conditions, ranging from 26.1% to 44.3%, and it was one of the major products. The glyceric acid can be formed via oxidation of the terminal hydroxymethyl group (−CH2OH), losing 2 × 2 electrons sequentially. It was observed that increasing the electrode potential generally decreased the selectivity toward it. As GOR can have multiple reaction pathways and glyceric acid can be further oxidized to other downstream products such as tartronic acid, glycolic acid, and/ or formic acid, as proposed by Kwon et al.14 and illustrated in Scheme 2, the above-observed potential-dependent selectivity toward glyceric acid can be rationalized by either that increasing electrode potential would accelerate other reaction pathways more or/and that further oxidation of glyceric acid would be accelerated more than its production. Except for the GOR on the Pt2Rh/C at 0.45 V (Figure 7c), the selectivity toward tartronic acid on the three nanocatalysts was much lower than toward the sum of glycolic acid and glyoxylic acid, though it increased with the addition of Ru or Rh

Figure 6. 13C{1H} NMR spectra before and after GOR on the assynthesized Pt/C nanocatalyst at 0.45 V, in 0.1 M 13C-labeled glycerol + 0.1 M HClO4 solution (D2O:H2O = 1:4) and at 60 °C.

64.6 and 65.0 ppm correspond to (−*CH2OH) carbon atoms in glycolic acid, glyceraldehyde, glyceric acid, and DHA, respectively. The small peaks around δ = 65 ppm can all be attributed to the carbon atoms of DHA (−*CH2OH) and its ketal and hemiketal derivatives, as ketone can coexist with ketal and hemiketal in an acidic environment by establishing equilibria with water or alcohol.29 The triplets with peaks at δ = 71.0, 71.3, and 71.4 ppm, and δ = 70.6, 71.1, and 71.5 ppm correspond to hydroxyl methylene group (−*CHOH) carbon atoms in glyceric acid and tartronic acid, respectively. The peaks at δ = 73.7, 74.0, 74.1, 74.5 ppm can be assigned to hydroxyl methylene group (−*CHOH) carbon atoms in glyceraldehyde and its ketal and hemiketal derivatives. In fact, aldehyde is more apt than ketone to react with water or alcohol to form hemiacetal and acetal. This can be proved by analyzing the standard 13C NMR spectrum (black curve) in Figure S1. In the spectrum, the characteristic peak at ∼200 ppm of the aldehyde group (−CHO) is absent; instead, the peak corresponding to acetal at 90.0 ppm is seen, confirming that in an acidic environment, glyceraldehyde exists mostly in the form of acetal by reacting with water molecule. The crowded peaks at around 86 and 90 ppm can be assigned to the hemiacetal and acetal derivatives of glyoxylic acid and glyceraldehyde, respectively, while the peaks at around 95 ppm are assigned to the hemiketal and ketal derivatives of DHA, by comparing them to the standard spectra in Figure S1. The crowded peaks arose from the diversity of the reacting environment: in principle, any compound having hydroxyl group (−OH) in the GOR process can react with ketone and aldehyde, leading to peak crowding in this region of the 13C NMR spectra. Fortunately, each of the three compounds (i.e., glyoxylic acid, glyceraldehyde, and DHA) has other characteristic peaks that do not overlap one another. The sharp singlet at δ = 124.6 ppm can be assigned to the carbon atom in CO2. To our best knowledge, this represents the first time that CO2 was detected as a reaction product in situ by 13C NMR spectroscopy. Due to the low solubility of CO2 in water, most of the 13CO2 generated during the GOR would diffuse to 7691

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oxidation reaction (EOR) on PtRu in which the non C−C bond breaking partial oxidation to products such as acetic acid is more favored.30,31 Two possible reasons might account for this difference in selectivity toward C−C bond breaking. One reason could be that ethanol has a hydrophobic −CH3 group that makes breaking the C−C bond more difficult and acetic acid a dead-end product of EOR.32 However, for glyceric acid, the presence of the two hydroxyl groups makes breaking the C−C bond easier, similar to oxidation of glycol.33 The other reason might be that we conducted the GOR at 60 °C, and higher temperature could enhance the catalytic C−C bond breaking.34−36 However, one exception to this higher selectivity toward C−C bond breakage was the GOR on the Pt2Rh1/C at 0.45 V at which tartronic acid became the dominant species with a selectivity as high as ∼40%. Kwon et al.14 studied glycerol electrooxidation on Pt as a function of solution pH. In alkaline media, tartronic acid was detected as one of the products, which was thought to be generated through the oxidation of glyceric acid. Although tartronic acid was not detected in an acidic environment, the authors still proposed that tartronic acid could be generated in a similar fashion as in alkaline media, which is through the oxidation of glyceric acid. However, our control experiments with pure glyceric acid, as shown in Figure S3, suggest that its oxidation on the three Pt-based nanocatalysts was relatively slow and thus is highly unlikely to account for the dominant presence of tartronic acid mentioned above. This is further confirmed by 13C NMR analysis of the products of glyceric acid electrooxidation on the Pt/C nanocatalyst, as presented in Figure S4. When glyceric acid was oxidized at 0.45 V and at 60 °C for 8 h, no peaks corresponding to tartronic acid were observed. Instead, a peak at 59.6 ppm corresponding to glycolic acid was seen, confirming that tartronic acid could not be generated via glyceric acid oxidation as proposed by Kwon et al.14 On the other hand, Fernández et al.37 proposed that tartronic acid could be generated directly from the non C−C bond breaking oxidation of glycerol in which oxygen insertion to glycerol’s two terminal carbon atoms were involved. Our results are consistent with this mechanism and show that the addition of Rh or Ru enhanced the oxygen insertion reactions in GOR. It is interesting to note that although Rh has long been thought to promote the C−C bond cleavage in EOR,11,38,39 it

Figure 7. Liquid products selectivity of GOR on the as-synthesized (a) Pt/C, (b) Pt3Ru1/C, (c) Pt2Rh1/C nanocatalysts as a function of electrode potential, in 0.1 M 13C-labeled glycerol + 0.1 M HClO4 solution (D2O:H2O = 1:4, v/v), at 60 °C for 8 h. Left to right: blue bar, glyceraldehyde; red bar, glyceric acid; black bar, glycolic acid; green bar, glyoxylic acid; yellow bar, tartronic acid; aqua bar, DHA.

and with the increase of electrode potential. This suggests that the C−C bond cleavage was generally more favored than the oxidation of the other terminal hydroxymethyl group (−CH2OH) of glyceric acid. That the C−C bond cleavage is more favored in GOR is opposite to the ethanol electroScheme 2. Schematic of Glycerol Electrooxidation Mechanism

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ACS Catalysis enhanced instead the oxygen insertion reactions in GOR, like converting glyceraldehyde to glyceric acid and glycerol to tartronic acid. Ru could also enhance the oxygen insertion reaction, as shown in Figure 7 and Table S4: the conversion of glycerol to glyceric acid and the oxidation of glycolic acid to glyoxylic acid were both enhanced on the Pt3Ru1/C, which was also the only nanocatalyst on which the generation of tartronic acid was observed at 0.25 V (Figure 7b). However, the PtRh surface showed higher EC oxygen insertion activity at 0.35 and 0.45 V (Figure 7c). The selectivity toward DHA on all three catalysts was similar but lower than other products via the terminal hydroxymethyl group (−CH2OH) oxidation or C−C bond cleavage pathway. It also decreased as the electrode potential increased. Overall, oxidation of terminal carbons in glycerol was generally favored on all nanocatalysts studied in the paper, which is consistent with previous work.14 To further explore the GOR mechanism, in situ EC−13C NMR spectroscopy was used. The 13C NMR spectra were recorded during GOR under active potential control as a function of time in an in situ NMR cell, as illustrated in Scheme S1. The time-resolved 13C NMR spectra so recorded are presented in Figure 8. Quantitative analyses of the products

Figure 9. Time-dependent concentration of (a) glyceraldehyde, (b) glyceric acid, (c) tartronic acid, and (d) DHA during GOR on the assynthesized Pt/C (blue bar), Pt3Ru1/C (green bar), and Pt2Rh1/C (red bar) nanocatalysts as a function of reaction time at 0.45 V, in 0.1 M 13C-labeled glycerol + 0.1 M HClO4 solution (D2O:H2O = 1:4, v/ v), and at 60 °C over 8 h.

on the Pt2Rh1/C. On the other hand, the concentration of glyceraldehyde on the Pt/C was much higher than that on the Pt3Ru1/C, whereas the concentration of glyceric acid on the Pt3Ru1/C was similar to that on the Pt/C, as showed in Figure 9b and Figure S5, indicating a faster conversion rate of glyceraldehyde to glyceric acid and to other downstream species on the Pt3Ru1/C, in line with the analysis of ex situ 13C NMR as described above. Notice that at low potential, the concentration of glyceraldehyde increased monotonously as the GOR continued (Figure S5). However, at 0.45 V, a different trend was observed (vide supra), indicating that increasing electrode potential could accelerate the electrooxidation rate from glyceraldehyde to glyceric acid. The Pt/C nanocatalyst showed the poorest EC oxygen insertion activity among the three catalysts, leading to the highest glyceraldehyde selectivity, as summarized in Scheme 2. Therefore, high selectivity of glyceraldehyde could be achieved on the Pt/C at low potential with shorter reaction time. The glyceric acid is the dominant product on all three catalysts under study as shown in Figures 9b and S5, similar to the results of ex situ 13C NMR experiments discussed above. That glyceric acid appeared first under all reaction conditions indicates that oxidation of the terminal hydroxymethyl group was easier than the oxidation of the middle hydroxyl methylene group (−CHOH). The glyceric acid can be further converted to glycolic acid by C−C bond cleavage of the vicinal diol group in glyceric acid. The concentration histograms of glycolic acid were similar on all three nanocatalysts and increased as the reaction potential or reaction time increased (Figure S5). These results indicate that the Pt/C, Pt3Ru1/C, and Pt2Rh1/C nanocatalysts all possessed electrocatalytic activity for oxidizing the initial reactant glycerol and intermediate product glyceric acid to glycolic acid by C−C bond cleavage. The signals of glyoxylic acid on all three catalysts could only be observed at 0.45 V with the ex situ 13C NMR. This was probably due to the worse signal-to-noise ratio caused by the conducting electrode and catalytic current during the in situ 13C NMR measurements, as well as the relatively poor 13C NMR sensitivity of glyoxylic acid due to its longer spin−lattice relaxation under our experiment condition, as indicated in Table S3.

Figure 8. 13C{1H} NMR spectra during GOR on the as-synthesized (a) Pt/C, (b) Pt3Ru1/C, and (c) Pt2Rh1/C nanocatalysts at different potential in 0.1 M 13C-labeled glycerol + 0.1 M HClO4 solution (D2O:H2O = 1:4) and at 60 °C.

were conducted, and the results are shown in Figure S5. Notice that no results on the Pt/C at 0.25 V are shown, because a very poor signal-to-noise ratio did not warrant meaningful integrations of peaks. As can be observed in Figure S5, the concentration of glycerol decreased continuously as a function of time, signifying the ongoing consumption of glycerol as the starting reactant. For helping the discussion on illustrating the GOR process and products selectivity, Figure 9 illustrates the concentration histograms of the four characteristic products of GOR on the three different nanocatalysts at 0.45 V as a function of reaction time. The concentration of glyceraldehyde shows a similar trend on the Pt/C and Pt3Ru1/C (Figure 9a): it increased quickly at the early stage, reached a maximum at ∼4 h, and then decreased thereafter. However, it remained much less variant 7693

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resulting in a similar trend of glycolic acid in Figure 7 and Figure S5. That no formic acid was observed by either ex situ or in situ 13C NMR may be due to the high activity of formic acid oxidation on the Pt-based catalysts, especially at relatively high overpotential and temperature.41 Third, the addition of Ru or Rh to Pt also accelerated the transformation of glycolic acid to glyoxylic acid. As demonstrated in Figure 7, glyoxylic acid was detected at 0.25 V on the Pt2Rh1/C, on the Pt3Ru1/C at 0.35 V, and on the Pt/ C at 0.45 V, respectively. Glyoxylic acid can be further oxidized to oxalic acid and CO2. However, no oxalic acid was detected by 13C NMR, probably due to its low concentration or oxalic acid decomposition at elevated temperature. Fourth, the Pt2Rh1/C showed superior selectivity toward tartronic acid in GOR, by simultaneously oxidizing two primary hydroxyl groups in glycerol, as proposed by Fernández et al.37 As the potential increased, the glycerol could react via its terminal carbons with two Pt−O, thus generating tartronic acid. In our cases, the water molecule could be activated easier at elevated temperature, and the addition of Ru or Rh atoms could further activate a water molecule at low potential to form adsorbed oxygen-containing species. The potential at which tartronic acid was observed by ex situ 13C NMR on the Pt3Ru1/ C was as low as 0.25 V, confirming the importance of surface oxygen-containing species to the formation of tartronic acid. The Pt2Rh1/C showed the highest selectivity toward tartronic acid, which might be due to that the PtRh surface could activate water better than Pt but worse than PtRu in terms of electrode potential, and a better type of oxygen-containing species (vide infra) would make the PtRh surface have the highest selectivity of glycerol oxidation toward tartronic acid. As discussed above, the selectively of GOR is highly sensitive to the surface properties of catalysts, the surface with high oxygen insertion catalytic activity might selectively convert glycerol to carboxylic acid such as glyceric acid and tartronic acid. On the other hand, if the aldehyde is the desired product, the catalytic surface should be highly active in dehydrogenation but less active in oxygen insertion reaction like pure Pt on low potential.

The Pt2Rh1/C demonstrated much higher electrocatalytic activity for oxidizing glycerol to tartronic acid, as shown in Figures 9c and S5. The generation of tartronic acid is through the simultaneous oxidation of the two hydroxyl methyl groups (−CH2OH) in glycerol as discussed above. Another interesting result is that the concentration of DHA increased as the electrode potential increased on all three catalysts during the in situ measurements, Figure S5, but the results from Figure 7 indicate the selectivity of DHA decreased as the electrode potential increased. This can be understood by realizing that the values presented in Figure 7 are relative fractions of the solution products, whereas those in Figure 9 are the absolute product concentrations. Increasing electrode potential is expected to increase a product’s concentration as a result of increased reaction kinetics. However, if the latter is still slower than those of the reactions that lead to other coexisting products, then its relative fraction would decrease as the electrode potential increases. We believe that this is what happened to DHA here. The results of in situ 13C NMR analysis at 8 h are also compared with the ex situ 13C NMR analysis. The results are presented in Figure S6, which show similar trends in both the ex situ and in situ 13C NMR analyses. The in situ and ex situ 13 C NMR analyses are complementary to each other. The in situ 13C NMR analysis gives time-dependent information on the products and reactant for better understanding the GOR process, while the ex situ 13C NMR analysis is more accurate in determining the concentration and selectivity of the liquid products. On the basis of the above observations and discussion, we propose an overall reaction mechanism of GOR on the Pt/C, Pt3Ru1/C, and Pt2Rh1/C nanocatalysts, as summarized in Scheme 2. First, the primary alcohol is easier to be oxidized than the secondary alcohol, which may be due to the reduced steric hindrance of the terminal primary hydroxyl groups.40 The oxidation of the terminal hydroxymethyl groups leads to glyceraldehyde and glyceric acid but glyceraldehyde electrooxidation was relatively sluggish on Pt surface, in agreement with the results reported by Kwon et al.14 Kwon et al. studied the GOR on Pt electrode in acidic environment at room temperature, and they found that glyceraldehyde was the dominant products when the potential was lower than 1.1 V (vs RHE). Glyceraldehyde can only be quickly converted to glyceric acid and other products when the surface Pt−O species are formed. In our cases, the selectivity of glyceraldehyde was much lower than reported by Kwon et al., because we studied the GOR at a higher temperature (60 °C), which caused faster reaction kinetics in both glyceraldehyde electrooxidation and water activation on Pt. By alloying Ru or Rh with Pt, the electrooxidation process could be further enhanced, probably via the bifunctional mechanism: the oxygen-containing species could be preferentially formed on the adjacent Ru or Rh sites, accelerating the electrooxidation of glyceraldehyde. The selectivity of glyceraldehyde on the Pt2Rh1/C is the lowest among the three nanocatalysts, indicating the highest oxygen insertion activity. Second, glyceric acid can be further oxidized to glycolic acid via C−C bond breaking of the vicinal diol group in glyceric acid, leading to formic acid. The glycolic acid can also be generated via oxidation of the C2 fragment generated by dissociative adsorption of glycerol molecule on the catalytic surface. The three nanocatalysts possess the similar catalytic ability to break the C−C bond in glycerol and glyceric acid,

4. CONCLUSIONS In summary, by combining conventional EC methods with in situ IR and 13C NMR spectroscopies, we reported a detailed mechanistic study of GOR on the home-synthesized, wellcharacterized Pt/C, PtxRuy/C, and PtxRhy/C nanocatalysts of different atomic ratios and proposed an overall reaction mechanism, as illustrated in Scheme 2. We observed that pure Pt was the poorest oxygen insertion catalyst among the three studied for glycerol to tartronic acid and glyceraldehyde to glyceric acid reactions in GOR, likely due to its poorest ability to bring activated oxygen-containing species to surface. However, alloying Rh and Ru with Pt could enhance these two oxygen insertion reactions in GOR, more on PtRh than on PtRu, particularly at higher electrode potential (0.45 V). We speculate that such a difference might have to do with the different ability to bring O (Rh) vs OH (Ru) to the surface, and the former might facilitate the oxygen insertion reactions.37 That increasing electrode potential enhanced more the oxygen insertion reactions is consistent with this hypothesis as more metal−O would appear on the surface at higher electrode potential, Moreover, the PtRh favored dominantly more oxygen insertion reaction toward tartronic acid than C−C bond breaking reaction in GOR at 0.45 V. This is in great contrast to 7694

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(15) Schnaidt, J.; Heinen, M.; Denot, D.; Jusys, Z.; Jürgen Behm, R. J. Electroanal. Chem. 2011, 661, 250−264. (16) Kwon, Y.; Birdja, Y.; Spanos, I.; Rodriguez, P.; Koper, M. T. M. ACS Catal. 2012, 2, 759−764. (17) Kim, H. J.; Choi, S. M.; Green, S.; Tompsett, G. A.; Lee, S. H.; Huber, G. W.; Kim, W. B. Appl. Catal., B 2011, 101, 366−375. (18) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz, P.; Brijoux, W.; Bönnemann, H. Langmuir 1997, 13, 2591−2595. (19) Klod, S.; Ziegs, F.; Dunsch, L. Anal. Chem. 2009, 81, 10262− 10267. (20) Huang, L.; Sorte, E. G.; Sun, S. G.; Tong, Y. Y. J. Chem. Commun. 2015, 51, 8086−8088. (21) Tsuda, M.; Kasai, H. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 155405. (22) Wakisaka, M.; Mitsui, S.; Hirose, Y.; Kawashima, K.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2006, 110, 23489−23496. (23) Jiang, K.; Cai, W.-B. Appl. Catal., B 2014, 147, 185−192. (24) Jiang, K.; Chang, J.; Wang, H.; Brimaud, S.; Xing, W.; Behm, R. J.; Cai, W.-B. ACS Appl. Mater. Interfaces 2016, 8, 7133−7138. (25) Igarashi, H.; Fujino, T.; Zhu, Y.; Uchida, H.; Watanabe, M. Phys. Chem. Chem. Phys. 2001, 3, 306−314. (26) Watanabe, M.; Motoo, S. J. Electroanal. Chem. Interfacial Electrochem. 1975, 60, 275−283. (27) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617−625. (28) Leung, L. W. H.; Weaver, M. J. J. Phys. Chem. 1988, 92, 4019− 4022. (29) McMurry, J. Organic chemistry, 7th ed; Cengage Learning: Boston, MA, 2008; pp 705−707. (30) Camara, G. A.; de Lima, R. B.; Iwasita, T. J. Electroanal. Chem. 2005, 585, 128−131. (31) Colmenares, L.; Wang, H.; Jusys, Z.; Jiang, L.; Yan, S.; Sun, G. Q.; Behm, R. J. Electrochim. Acta 2006, 52, 221−233. (32) Prieto, M. J.; Tremiliosi-Filho, G. Electrochem. Commun. 2011, 13, 527−529. (33) Schnaidt, J.; Heinen, M.; Jusys, Z.; Behm, R. J. J. Phys. Chem. C 2012, 116, 2872−2883. (34) Ghumman, A.; Li, G.; Bennett, D. V.; Pickup, P. G. J. Power Sources 2009, 194, 286−290. (35) Ghumman, A.; Vink, C.; Yepez, O.; Pickup, P. G. J. Power Sources 2008, 177, 71−76. (36) Sun, S.; Halseid, M. C.; Heinen, M.; Jusys, Z.; Behm, R. J. J. Power Sources 2009, 190, 2−13. (37) Fernández, P. S.; Martins, M. E.; Camara, G. A. Electrochim. Acta 2012, 66, 180−187. (38) Sen Gupta, S.; Datta, J. J. Electroanal. Chem. 2006, 594, 65−72. (39) Rao, L.; Jiang, Y.-X.; Zhang, B.-W.; Cai, Y.-R.; Sun, S.-G. Phys. Chem. Chem. Phys. 2014, 16, 13662−13671. (40) González-Cobos, J.; Baranton, S.; Coutanceau, C. J. Phys. Chem. C 2016, 120, 7155−7164. (41) Jiang, J.; Kucernak, A. J. Electroanal. Chem. 2002, 520, 64−70. (42) Martins, C. A.; Fernández, P. S.; Troiani, H. E.; Martins, M. E.; Camara, G. A. J. Electroanal. Chem. 2014, 717−718, 231−236. (43) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271−340.

PtRh-catalyzed EOR, in which adding Rh favors the latter reaction instead. We also observed that except for the GOR on the PtRh at 0.45 V, the C−C bond breaking reaction from glyceric acid to glycolic/glyoxylic acid was more favored in general for GOR on the Pt-based electrocatalysts studied here, particularly as compared to EOR. The last but certainly not the least is that our NMR data show clearly that glyceric acid could not be electrooxidized to tartronic acid, offering a strong experimental evidence supporting the proposed direct oxygen insertion reaction to the two terminal carbons in glycerol as proposed previously.37

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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/acscatal.6b02097. EDX analysis of nanocatalysts, schematic of EC-NMR cell, 13C NMR spectra and sensitivity factors of GOR related compounds, time-resolved 13C NMR analysis of GOR, 13C NMR analysis of ex situ glycerol electrooxidation and glyceric acid electrooxidation (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 was supported by grants from NSFC (21229301, 21321062, and 21361140374). The work done by the Georgetown coauthors is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Catalysis program, under Award Number DE-FG0207ER15895.

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REFERENCES

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