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Selective Electro-Oxidation of Glycerol into Dihydroxyacetone by PtAg Skeletons Yongfang Zhou, Yi Shen, Jingyu Xi, and Xuanli Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09431 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Materials & Interfaces

Selective Electro-Oxidation of Glycerol into Dihydroxyacetone by PtAg Skeletons Yongfang Zhou,† Yi Shen,†, ⤉*, Jingyu Xi‡, and Xuanli Luo× †

School of Food Science and Engineering, South China University of Technology, Guangzhou, 510640, China. ⤉ Overseas

Expertise Introduction Center for Discipline Innovation of Food Nutrition and Human Health (111 Center), Guangzhou, 510640, China

‡

Institute of Green Chemistry and Energy, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China.

× Advanced

Materials Research Group, Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD, UK

Abstract Developing high-performance electrocatalysts for the selective conversion of glycerol into value-added chemicals is of great significance. Herein, threedimensional nanoporous PtAg skeletons were studied as a catalyst for the electro-oxidation of glycerol. The structural features of the PtAg skeletons were revealed by electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and UV-vis spectroscopy. The electrochemical activity of the catalysts was examined by cyclic voltammetry, linear sweeping voltammetry, and chronoamprometry. The resulting PtAg skeletons exhibit a peak current density of 7.57 mA cm-2, which is 15.4-fold higher than that of the Pt/C, rendering the PtAg skeletons as one of the best electrocatalysts for glycerol oxidation. High performance liquid chromatography results show that the PtAg skeletons yield a remarkable dihydroxyacetone selectivity of 82.6%, which has so far been the second largest value reported in the literature. The superior activity and selectivity of the PtAg skeletons are ascribed to the large surface area and abundant Pt (111) facets. Additionally, the effects of glycerol and KOH concentrations, and reaction time on the product selectivity were investigated. Keywords: PtAg, glycerol electro-oxidation, selectivity, dihydroxyacetone, seedmediated synthesis

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1. Introduction Being a valuable biomass, glycerol (GLY) has a great potential to transform into many valuable products, such as lactic acid (LA), glycolic acid (GA), tartronic acid (TA), dihydroxyacetone (DHA), glyceraldehyde (GALD), and glyceric acid (GLA).1-8 Among these products, DHA is of particular interest for its widespread application in cosmetics, which is an ingredient for the synthesis of sunless tanning lotions.9 Currently, the transformation of GLY into DHA is mainly implemented via a biological fermentation route. Unfortunately, a typical fermentative process is characteristic of low efficiency, thus leading to a long operating time. Besides, the subsequent extraction procedures are tedious and time-consuming, which prohibits the scalable production.10 Apart from fermentative processes, the production of DHA from GLY can be fulfilled by heterogeneous catalytic processes. In a conventional catalytic process, GLY oxidation is performed using strong oxidants, which always results in poor selectivity. Alternatively, electrochemical production of DHA from GLY represents a promising approach because its high efficiency and environmental friendliness.8 Importantly, the efficiency of this process can be greatly enhanced by engineering catalyst structures and optimizing applied potentials.11-12 The feasibility of DHA production by GLY electro-oxidation relies on the activity and selectivity of the catalyst. Up to now, Pt-based materials have been extensively studied for GLY oxidation. Since Pt is expensive and easily poisoned by CO, the Pt loading should be reduced and its durability should be improved.


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To this end, many studies have been conducted to search for efficient Pt-based catalysts which possess excellent activity and product selectivity. Especially, the introduce of heterogeneous metals into Pt lattice can significantly enhance the performance of GLY oxidation. Among the doped metals, Ag is beneficial for the growth of Pt due to its similar lattice constant to Pt. The presence of Ag in Pt lattice can change the structures of Pt, thus improving the catalytic activity and selectivity of GLY oxidation.13 Unfortunately, the abundant Ag sites on Pt surface could impede the catalytic activity because of the limited activity of Ag on GLY oxidation.14-16 Therefore, the structures of PtAg catalyst should be delicately engineered to expose more Pt sites. To date, one of the most attractive methods is to fabricate Pt-Ag bimetallic catalysts with a hollow structure. Compared with the solid nonporous Pt catalysts, hollow structured catalysts can significantly enhance the utilization efficiency of the Pt because of the larger surface areas and abundant unsaturatedly coordinated Pt atoms, which would expose more effective Pt active sites for the electro-oxidation of GLY. Ag nanoparticles (NPs) have been used as sacrificial seeds in the process of galvanic replacement reactions, which has been demonstrated as a facile method to synthesize nanomaterials with tunable nanostructures.

17

In this work, three-dimensional

(3D) porous PtAg skeletons were fabricated via galvanic replacement reactions in combination with an acid etching procedure, and further studied as catalysts for the selective electro-oxidation of GLY. A remarkable peak current density of 7.57 mA cm-2 and DHA selectivity of 82.6% are obtained from the PtAg skeletons. Such outstanding activity and selectivity render the PtAg skeletons to be a competitive electrocatalyst in the field of biomass-derived GLY conversion.

2. Experimental Details ACS Paragon Plus Environment


2.1 Material synthesis Ag seeds were synthesized via a hydrothermal method recorded by Chang et al18, and served as sacrificial seeds in the process of PtAg skeletons synthesis. PtAg skeletons were fabricated via galvanic replacement reactions in combination with an acid etching procedure.19-20 Briefly, 500 mg of citric acid and 100 mg of polyvinyl pyrrolidone were added into 24 mL of distilled water. After completely dissolved, 15 mL of the as-prepared Ag seeds were slowly added. The above mixed solution was heated by oil bath at 90°C for 10 min. Subsequently, 5 mL of 10 mM H2PtCl6 solution was added dropwise with a syringe. The reaction temperature was kept at 90°C for 3 h. Afterwards, the suspension was centrifuged at 10000 rpm for 20 min. The precipitate was collected and treated with concentrated NH3·H2O at ambient temperature for 24 h. Finally, the product was thoroughly cleaned with deionized water and ethanol and stored in deionized water. 2.2 Physicochemical characterization A transmission electron microscopy (TEM) equipped with an electron energy loss spectroscopy (EELS) analyzer was used to observed the morphology of the sample. The crystalline structure of the sample was revealed using X-ray diffraction (XRD) measurements. X-ray photoelectron spectroscopy (XPS) was employed to investigate the composition and valence states of the catalyst. A UVvis spectrometer was used to examine the optical properties of the sample. 2.3 Electrochemical characterization The electrochemical measurements were performed at ambient temperature in a typical three-electrode cell using a potentiostat (CHI 660E). A


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Pt plate, Hg/HgO electrode, and glassy carbon (diameter = 3 mm) electrode were used as the counter, reference, and working electrodes, respectively. The set-up for product analysis and details of the measurements were previously reported.6, 21-23

All the potentials reported in this study were referenced to a reversible

hydrogen electrode (RHE). 2.4 Chromatographic analysis of products The product of GLY oxidation was detected by high performance liquid chromatography (HPLC). The details of the measurements were previously reported.22-23

3. Results and Discussion 3.1 Structural characterizations The PtAg skeletons were fabricated via the galvanic replacement reaction in combination with an acid etching procedure, in which Ag NPs with sizes of 4050 nm (see Figure S1) were used as seeds and subsequently etched by concentrated ammonia solutions. The morphology of the PtAg skeletons is show in Figure 1a-c. The resulting PtAg NPs are quite uniform and the majority of the NPs have diameters of ca. 55-60 nm, which is slightly larger than those of the Ag seeds. The PtAg NPs are characteristic of a 3D skeleton-like structure, consisting of numerous interconnected networks. Notably, the resulting Pt@Ag skeletons are hollowed out and transformed into porous crystalline shells. It is noteworthy that the presence of a myriads of interconnected pores could significantly increase the surface area of the PtAg NPs and facilitate the diffusion of the reactants and intermediates.24 Figure 1d-f show the high-resolution TEM images of the PtAg NPs. It clearly shows that the PtAg NP consists of ultrafine frames



with sizes of ca. 2 nm. The insets shown in Figure 1d-f are the corresponding magnified view of the spots. The d-spacing of the adjacent fringes is 0.226 nm, which can be ascribed to interplanar distance of (111) facets of Pt crystals.25 A close inspection could reveal that abundant steps are present in the frames. High-angle annular dark-field scanning TEM (HAADF-STEM) images show in Figure 1 g and h vividly reveal the porous skeleton-like architectures. It should be pointed out that the generation of such a skeleton-like structure is related to the orderly deposition of the Pt atoms during galvanic replacement reactions and the etching of Ag seeds by concentrated ammonia solution.26 The distribution of Pt and Ag elements are clearly recorded in the electron energy-loss spectroscopy (EELS) elemental mapping images. The signal of Ag (2.4%) is quite weak while that of Pt (97.6%) is strong as shown in Figure 1i and k, respectively, which indicates the selective removal of Ag by ammonia etching.27


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Figure 1. Morphological characterization of the as-prepared PtAg skeletons. (a-c) Different magnification TEM images, (d-f) HR-TEM images (the insets are corresponding magnified view), (g-h) HAADF-STEM images and (i, k) EELS elemental mapping micrographs of Pt@Ag NPs. The XRD patterns of the samples are displayed in Figure 2. The XRD pattern of the Pt/C shows five peaks at 2θ = 39.5, 46.2, 67.3, 81.7, and 85.9, corresponding to the (111), (200), (220), (311), and (222) lattice planes of the crystalline Pt, respectively. 28 For comparison, the diffraction pattern of the Ag is also recorded as shown in Figure 2. The peaks of the PtAg skeletons slightly shifted to low 2θ angles, suggesting the formation of an alloy structure. To further reveal the composition of the PtAg skeletons, XPS measurements were carried out. Figure 3a and b show the high-resolution of Pt 4f and Ag 3d spectra



of the PtAg skeletons. The Pt 4f spectrum consists of two doublets of 7/2 and 5/2. Each doublet can be deconvoluted into two peaks. The peaks at 71.1 and 74.5 eV are assigned to Pt0 while those located at 71.9 and 75.2 eV are ascribed to Pt2+.29 The Ag 3d spectrum can be deconvoluted into two peaks located at 373.9 and 367.9 eV, corresponding to the doublets of 5/2 and 3/2, respectively. The binding energies of Ag indicates that the major atoms are mainly in the zero valence states.30 To further explore the structures of the PtAg skeletons, UV-vis spectroscopy was performed as shown in Figure S2. The spectrum of the Ag NPs shows an absorption peak at 420 nm while that of the Pt NPs have no clear absorption in this region. The absorption of the PtAg skeletons is similar to that of the Pt NPs, indicating that the Ag NPs are absent.

Figure 2. XRD patterns of the PtAg skeletons, Pt/C and Ag seeds.


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Figure 3. XPS spectra of (a) Pt 4f and (b) Ag 3d in PtAg skeletons. 3.2 Electrochemical characterization Since the adsorption-desorption of hydrogen on Pt is sensitive to its surface structures,31 the voltammogram measurements were first conducted in 1 mol L-1 KOH as shown in Figure 4a. A commercial Pt/C with diameters of 2.6 ± 0.4 nm was also examined as shown in Figure S3. The voltammogram of the PtAg skeletons was recorded in a wide potential range of 0.1~1.6 V since the redox of Ag0/AgI occurs at high potentials. However, no peaks associated with the redox of Ag0/AgI is noted from the voltammogram, which can be attributed to the limited Ag content and the enrichment of Pt atoms in the skeleton.32 Further inspection can reveal that the peaks related to hydrogen adsorption-desorption in the voltammogram of the PtAg skeletons are less defined compared with those of the Pt/C, which could be probably due to the synergistic effect of Pt and Ag.30 Notably, a pronounced peak located at ca. 0.73 V is observed from the positive scan of the voltammogram of the PtAg skeletons, which is attributed to the oxidation of Pt (111) facets.11 The linear sweep voltammetry (LSV) was conducted to examine the onset potential of GLY oxidation as shown in Figure 4b. The onset potential of the PtAg



skeletons is determined to be 0.51 V, which is smaller than that of the Pt/C (0.59 V). The PtAg skeletons exhibit a peak current density of 0.29 mA cm-2 at 0.91 V, which is 3.63 times that of the Pt/C (0.08 mA cm-2). More interestingly, a second peak located at 1.28 V is noted from the voltammogram of the PtAg skeletons. The cyclic voltammogram (CV) measurements of the catalysts were conducted in 0.1 M KOH + 1 M GLY as shown in Figure 4c. The Pt/C catalyst possesses a typical voltammogram which displays one peak in the positive scan and another one in the negative scan. In contrast, the voltammogram of the PtAg skeletons shows four peaks. Apart from a strong peak located at 0.96 V, an additional peak is noted at 1.25 V in the forward scan, suggesting that the PtAg skeletons maintain considerable activity at high potentials. Such prominent features of the voltammogram are attributed to the inherent properties of Pt (111) facets as reported by Fernández et al.33 The first peak was associated with the transformation of GLY into intermediates while the second one located at a higher potential was arisen from further oxidization of the intermediates to C=O containing species and carbonates.33 It was reported that Pt (111) facets had better resistance to poisoning species compared with the Pt (100) and Pt (110) facets because the weaker binding strength with the intermediates.34 Additionally, for the PtAg skeletons, the interaction of Pt with Ag could possibly weaken the adsorption energy of the adsorbates in Pt active sites, which facilitates their desorption and thereby refreshes the Pt surface, leading to increases in current density at high potentials.35 Notably, the PtAg skeletons possess a peak current density of 7.57 mA cm-2, which is 15.4 times that of the Pt/C catalyst. Such a large peak current renders the PtAg skeletons as one of the best electrocatalysts for GLY oxidation as shown in Table S1.


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Figure 4. Voltammograms of the catalysts. (a) CV curves recorded in 1 M KOH with a scan rate of 50 mV s-1, (b) LSV curves recorded in 0.1 M KOH and 1 M GLY with a scan rate of 1 mV s-1 and (c) CV curves recorded in 0.1 M KOH + 1 M GLY with a scan rate of 50 mV s-1. Figure S4 shows the chronoamperometric curves of the catalysts. Similar to the LSV and CV results, the PtAg skeletons exhibit the higher current densities. In addition, after two-hour chronoamperometric tests, the final current densities of the PtAg skeletons are 70, 58.3, 52.3 and 53.1% of the initial current densities, which are larger than those of 20, 44.8, 42.9 and 50% obtained from the Pt/C at potentials of 0.7, 0.9, 1.0 and 1.3 V, respectively, manifesting better durability of the PtAg skeletons. On the basis of the electrochemical results, it can be concluded that the assynthesized PtAg skeletons exhibit outstanding activity for GLY oxidation. The superior activity of the PtAg skeletons is ascribed to the unique structural properties. The PtAg skeletons consist of ultrafine frames with dominant Pt (111) facets, which could afford plenty of active sites for GLY oxidation. It was reported that compared with Pt (100) and Pt (110) facets, Pt (111) facets were more active for GLY oxidation and showed better resistance towards poisoning species such as CO-like intermediates.36 The presence of abundant steps and a trace of Ag atoms in the frames could also play a critical role in GLY oxidation.37-39 In addition, the interconnected porous structure is beneficial for the diffusion of



GLY and products24, and effectively prevents agglomeration, thus improving the stability of the catalyst.40 3.3 Product distribution The product distribution of GLY oxidation was investigated by HPLC, and the HPLC curves are shown in Figure S5a, b. Ten compounds including LA, GLY, oxalate acid (OA), TA, glyoxylic acid (GLOA), GLA, GALD, GA, formic acid (FA) and DHA were identified and their concentrations were well calibrated by HPLC. As shown in Figure 5, the product distribution of GLY oxidation was recorded at 0.7, 0.9, 1.0 and 1.3 V. Table S2 shows the concentration of the products. The Pt/C yields seven products including DHA, GLA, GALD, GA, OA, TA, and GLOA as shown in Figure 5a. The selectivity of DHA decreases with increasing applied potential, leading to a maximum value of 50.4% at 0.7 V. GLA selectivity increases with increasing potential from 0.7 to 1.0 V, resulting in maximum value of 37.4% at 1.0 V. Specially, no GALD is observed at 1.0 V. Compared to Pt/C, the PtAg skeletons show a stronger tendency to produce DHA. Notably, a maximum DHA selectivity of 82.6% is detected at 0.7 V as shown in Figure 5b. Such a remarkable value has so far been the second largest, only next to that of a PtBi catalyst reported by Koper group as shown in Table S3. When the potential is increased from 0.7 to 0.9 V, GALD selectivity increases from 7.6% to 20.7%. For a clear view, the product selectivities of the two catalysts are comparatively shown in Figure S6. It clearly indicates that compared with the Pt/C catalyst, the PtAg skeletons yield more DHA, but less GLA. The PtAg skeletons yield DHA selectivities of 82.6, 61.5, 79.2 and 79.8% at potentials of 0.7, 0.9, 1.0 and 1.3 V, which are 1.64, 1.46, 2.10 and 2.12 times those of the Pt/C, respectively. It is noteworthy that no GA is obtained from the PtAg skeletons at the four potentials.


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In contrast, considerable GA is produced from the Pt/C. OA is obtained from both PtAg skeletons and Pt/C, but its selectivity is less than 8% as shown in Figure S6. The selectivities of TA and GLOA are also very limited (

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