Electrocatalysis of Ethylene Glycol Oxidation on Bare and Bi-Modified

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Electrocatalysis of Ethylene Glycol Oxidation on Bare and Bi-Modified Pd Concave Nanocubes in Alkaline Solution: An Interfacial Infrared Spectroscopic Investigation Han Wang, Bei Jiang, Ting-Ting Zhao, Kun Jiang, YaoYue Yang, Jiawei Zhang, Zhaoxiong Xie, and Wen-Bin Cai ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03108 • Publication Date (Web): 01 Feb 2017 Downloaded from http://pubs.acs.org on February 3, 2017

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Electrocatalysis of Ethylene Glycol Oxidation on Bare and Bi-Modified Pd Concave Nanocubes in Alkaline Solution: An Interfacial Infrared Spectroscopic Investigation Han Wang,† Bei Jiang,† Ting-Ting Zhao,† Kun Jiang,† Yao-Yue Yang,‡ Jiawei Zhang,§ Zhaoxiong Xie,§ Wen-Bin Cai*, † †

Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation

Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China; ‡

College of Chemistry and Environmental Protection Engineering, Southwest University for

Nationalities, Chengdu 610041, China §

State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of

Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China

Abstract: Electrocatalysis of ethylene glycol oxidation (EGO) on shape-controlled Pd nanocrystals is of great interest in the pursuit of efficient biomass fuel utilization and nanomaterial application. The present work is aimed at mechanistic study of electrocatalytic EGO in alkaline media on surface-cleaned high-index Pd concave nanocubes (Pd CNCs) with and without surface Bi modification. CO-adsorption displacement effectively removes the surfactants on as-synthesized Pd CNCs, facilitating controlled Bi adatoms formation. EGO on the Pd CNCs is notably enhanced as a result of Bi modification, with the activity peak at a Bi coverage of ca. 0.31 in terms of apparent and specific oxidation current densities. Internal (ATR-SEIRAS) and external (IRRAS) reflection modes of in situ infrared spectroscopy have been used to probe the EGO process at molecular level. High surface sensitivity ATR-SEIRAS enables to identify readily the formation and removal of CO and 2-hydroxyacetyl surface species during EGO on Pd CNCs and Bi-modified Pd (Bi/Pd) CNCs. Compared to that on bare Pd CNCs, the COad band is significantly stronger on Bi/Pd CNCs, suggestive of a promoted C-C bond cleavage. IRRAS results further reveal that glycolate and glyoxal are the main products of EGO on both pristine and Bi/Pd CNCs. In addition, formations of glyoxal, CO and CO2 on Bi/Pd CNCs are relatively enhanced, as compared to those on bare Pd CNCs. Based on the comprehensive spectral results and literature reports, relevant reaction pathways are proposed for EGO at Pd CNCs in alkaline media. Keywords: electrocatalysis, ethylene glycol oxidation, surface infrared spectroscopy, Pd concave nanocubes, Bi modification, surfactant removal

1.Introduction The efficient oxidation of renewable biomass fuels by means of electrocatalysis at Pt-group metal surfaces has drawn a great deal of attention in recent years, since it is closely relevant to the anode process of direct liquid feed fuel cells.1-3 Ethylene glycol which could be produced from cellulose, is an attractive energy carrier. Its highest boiling point 198℃ together with an energy density (4800 Ah/L) makes it a promising and safe candidate alternative to methanol and ethanol to

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be fed in direct alcohol fuel cells.3-4 Researches so far conducted on ethylene glycol oxidation (EGO) are largely limited to Pt surfaces, including the synthesis of Pt-based catalysts as well as the mechanistic study. Roughly two pathways were proposed5-7 for EGO at Pt electrodes in acidic media, as shown in Scheme 1, despite partial uncertainties and disputes. In considering the high cost and insufficient supply of Pt, high-performance non-Pt catalysts and related fundamental EGO studies are highly demanded. Among possible alternatives, Pd is very competitive for anode catalysts because it possesses a remarkable activity towards EGO in alkaline media but at a much less cost, as compared to Pt. Unfortunately, far less effort has been devoted to the EGO at Pd-based catalysts in alkaline media, especially regarding new Pd-based catalyst design and in-depth mechanistic understanding.6-9

Scheme 1. Proposed reaction pathways for EGO on Pt electrode in acidic media.

It is known that the reactivity of an electrocatalyst towards a target reaction is highly dependent on its surface structures.10-14 In this context, facet control and surface modification are two widely used effective approaches. Firstly, much higher activities usually appear on high-index surfaces than on low-index ones in the oxidation of alcohols.15-16 Specifically, the Pd concave nanocubes (CNCs) capable of exposing high-index Pd {hk0} facets are thus expected to be a promising Pd nanocatalyst for EGO.15 To our knowledge, no prior investigation has been carried out on EGO at shapecontrolled Pd nanocrystals. Notably, a big challenge in dealing with electrocatalysis on these Pd nanocrystals is to remove the surfactants in order to obtain the reliable electrochemical responses.1718 Secondly, modification by exotic adatoms may improve the electrocatalysis of small organic molecules on Pt-group metals.13-14 Specifically, Bi modification was reported to alter the reaction pathways during EGO on Pt (111) single crystal electrode in acidic media14 as well as glycerol oxidation on Pd nanocrystals enclosed with low-index facets in alkaline media19. It was suggested that Pd surface decorated with Bi adatoms may change the electronic state of Pd and facilitate OHad formation during the reaction.19-20 Nevertheless, whether and how the Bi modification on high-index Pd nanocrystals may impact the EGO activity and selectivity in alkaline media yet remain unknown and the relevant mechanistic understanding is deficient for the design of advanced catalysts. In situ surface infrared spectroscopy is one of the most powerful tools for exploring the interfacial electrochemistry, enabling to correlate the activity and selectivity of an electrocatalyst with the evolution of interfacial species at molecular level.5, 8, 21-25 Two reflection-absorption modes are widely used in surface infrared spectroscopy: the first is referred to external reflection with thin layer structure (often abbreviated as IRRAS or IRAS), the second is the internal reflection or attenuated total reflection (ATR-IR or ATR-SEIRAS).25-30 The former is more sensitive to species trapped in the thin-layer, while the latter is more sensitive to surface species, especially when significant surface enhanced infrared absorption (SEIRA) effect occurs.26-27 The preliminary IRRAS


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on EGO at Pd bulk electrode in electrolytes of different pHs was reported by Wang et al.8 without the detection of surface species. Lin et al. investigated23 the effect of ethylene glycol concentration on product distribution of EGO at Pd bulk electrode in alkaline media. Schnaidt and co-workers 6-7 used ATR-IR to study the EGO on Pt film electrode in acid media and observed the potential dependent behavior of adsorbed species such as CO and 2-hydroxyacetyl. No ATR-SEIRAS spectroscopy has been applied to investigate the EGO on Pd electrode in alkaline media. Furthermore, in principle, the application of the two reflection modes of electrochemical IR spectroscopy shall reveal comprehensively how one electrocatalyst behaves differently from the other in EGO. In this work, we aim to explore comparatively the EGO at bare and Bi-modified high-index Pd CNCs in alkaline media in terms of both electrocatalytic performance and mechanistic aspect. The Pd CNCs were synthesized, and the surfactant residues were effectively cleaned and characterized prior to the surface modification with Bi and the subsequent measurement on electrocatalysis of EGO. In situ electrochemical ATR-SEIRAS and IRRAS were combined to probe the interfacial species and relative product distribution during EGO at both types of nanocatalysts. The effect of Bi adatoms on EGO at Pd CNCs was discussed.

2. Experimental The Pd concave nanocubes (Pd CNCs) was synthesized according to a previous report by the coauthors.15 Briefly, 5 mL of 0.6 mM H2PdCl4 was mixed with 5 mL of 10 mM CTAC + CTAB at the ratio of CTAC/CTAB = 4:1 in a 30 °C water bath, after which 100 uL of 100 mM ascorbic acid was added. 7 hours later, the Pd CNCs was collected by centrifugation (10000 rpm, 10 min), washed with ultrapure water two times, and finally redispersed in ultrapure water to form a 150-uL viscous solution with a concentration of 1.75 mg/mL Pd as determined by ICP-AES. This solution was directly used as an ink solution for further electrochemical treatments by drop casting it on Au substrates. The Pd nanocubes was synthesized according to our previous work.31 Briefly, 500 uL of 10 mM H2PdCl4 was mixed with 10 mL of 12.5 mM CTAB solution and heated at 95 °C in an oil bath. Then, 25 uL of 1 mM KI was added, followed by adding 80 uL of 100 mM ascorbate acid solution. The solution was kept in the oil bath with magnet stirring at ca. 300 rpm for 30 min. Afterwards, the Pd nanocubes was collected and preliminarily cleaned by centrifugation (8000 rpm, 10 min) and redispersed in ultrapure water for three times and finally concentrated to a viscous solution of ca. 200 uL. This solution was directly used as ink solution for further electrochemical treatments by drop casting it on Au substrates. The surface cleaning of Pd CNCs on Au substrate was carried out in a 0.5 M H2SO4 solution at 0.15 V vs RHE with CO bubbled in at ca. 40 mL/min for 20 min. Afterwards, the working electrode was removed from the solution and rinsed with ultra-pure water for further measurements. Cyclic voltammetry was conducted in a typical three-compartment glass cell. Pd CNCs coated on Au disk (Φ=3 mm) serves as the working electrode. The Pd coating with a loading of 14 µg Pd CNCs was made by first transferring 8 uL of the concentrated ink solution, dried and further cleaned according to the procedures as described above. The resulting working electrode ended up with a shiny Pd metallic appearance. A Pt mesh (12 cm) was used as the counter electrode. A Hg/Hg2SO4 (K2SO4 sat.) and a Hg/HgO (1 M KOH) served as the reference electrodes in acidic and alkaline solutions, respectively. Nevertheless, all potentials were eventually referred to the RHE (reversible


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hydrogen electrode). Same counter and reference electrodes were used in the following in situ spectroscopic measurements. For ATR-SEIRAS measurement, 30 uL of the Pd CNCs ink solution (corresponding to a loading of 52.5 µg Pd CNCs) was transferred to the central area (confined by an O-ring with Φ= 6 mm) of a Au film chemically deposited on the reflecting plane of a hemicylindrical Si prism. Detailed description27 of chemical deposition of the Au film on Si as well as the setup for ATRSEIRAS with a face-up electrode configuration can be found elsewhere. The ATR-SEIRAS measurement was run on a Nicolet IS50 FTIR spectrometer equipped with a MCT-A detector at a bottom-up incidence angle of ca. 65o. For IRRAS measurement, the above ink solution was casted on an Au disk electrode (Φ=10 mm), corresponding to a loading of 52.5 µg Pd CNCs. The IRRAS measurement was conducted with the same optical setup as the ATR-SEIRAS except that a Si prism was replaced with a hemicylindrical CaF2 prism, and a thin layer of electrolyte located in between the CaF2 and the working electrode. The bottom–up incidence angle was tuned at ca. 45o from the surface normal of the basal plane of the CaF2 prism For both ATR-SEIRAS and IRRAS measurements, the spectra were calculated and shown in absorbance defined as −log(I/I0 ), where I and I0 represent the sample and reference singlebeam spectra, respectively. All spectra were collected with p-polarized IR radiation by using a builtin ZnSe polarizer and the spectral resolution was 6 cm-1. A CHI 440C electrochemistry workstation (CH Instruments, Inc.) was used for potential control. Transmission IR spectra were collected by holding a thin layer of the sample solution between two pieces of 2-mm thick CaF2 disks with a DTGS detector and non-polarized IR radiation at a spectral resolution of 6 cm-1. 3. Results and discussion Shown in Figure 1A and Figure S1 are the transmission electron microscope (TEM) images of the as-prepared Pd concave nanocubes (Pd CNCs). The inhibited electrochemical response in Figure 1B indicates the presence of CTAB/CTAC capping agents on the as-prepared Pd CNCs, which may prevent a reliable spectroelectrochemical investigation of EGO at bare and Bi-modified Pd CNCs. In order to remove the CTAC and CTAB surfactants on Pd CNCs, herein the as-synthesized Pd CNCs-coated Au disk electrode was kept in a CO-saturated 0.5 M H2SO4 at 0.15 V RHE for 20 min by taking the advantage of a stronger CO adsorption on Pd surfaces. After being rinsed with copious amount of Milli-Q water, the working electrode was then placed in a freshly prepared and deaerated 0.5 M H2SO4 electrolyte for electrochemical measurement. Figure 1B compares cyclic voltammograms recorded at 50 mV s-1 in 0.5 M H2SO4 for the Pd CNCs without and with the COadsorption displacement cleaning. The cleaned Pd CNCs exhibit characteristic peaks for Pd oxidation and PdO reduction at 0.95 and 0.72 V, respectively, together with the hydrogen and (bi)sulfate adsorption/desorption peaks around 0.2-0.25 V. These peak potentials for the cleaned Pd CNCs to some extent resemble those seen on Pd nanocubes31 and bulk Pd (100)32-33, and the peak broadening may be due to the presence of {100} terraces and {110} steps on the Pd CNCs. The displacement of the surfactants on as-synthesized Pd CNCs by CO could be tracked dynamically by using real-time ATR-SEIRAS during the gentle CO bubbling at a gas flow of ca. 5 mL/min. As shown in Figure 1C, in addition to the increase of linearly (~2052 cm-1) and bridge (~1958 cm-1) bonded CO bands (directed upwards), the downward bands indicate the decrease of


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C-H stretching at 2957, 2923, and 2852 cm-1 from surface CTA+ species. The adsorbed CO on bare Au substrate yielded a νCO band at ca. 2112 cm-1, which disappeared on the Pd CNCs-covered Au electrode. This result also suggests that a compact overlayer of Pd CNCs forms on the Au substrate. Figure 1D shows that the intensity of adsorbed CO band is largely stabilized after 400 s while the ν(C-H) band decreases until 900 s. The time delay may be attributed to the slower movement of CTA+ out of the interface. With the desorption of CTA+ which contains hydrophobic hydrocarbon chain, the interfacial water may rearrange as evidenced by the increasing O-H stretching band at ca. 3200-3600 cm-1 and H-O-H scissoring vibration (δ(HOH)) band at ca. 1650 cm-1. In addition, the sharp minor peak at 3650 cm-1 should be attributed to the O-H stretching of isolated H2O coadsorbed with COad.34 The removal of surface Cl- species was confirmed by the substantially weakened Cl 2p peaks and Pd 3d peaks from PdCl2 in Figure S2 and Figure S3, respectively, consistent with the above ATR-SEIRAS and electrochemical results. Notably, the surface cleaning procedure can be extended to other Pd and Pt nanocrystals. In fact, a quick comparison of EGO at surface cleaned Pd CNCs and Pd nanocubes (see Figure S4) indicates that the former displays an electrocatalytic current twice as the latter, in other words, high-index facets indeed favor the electrocatalytic oxidation of ethylene glycol on Pd surfaces in alkaline media. This observation inspired us to focus on the EGO at bare and Bi-modified Pd CNCs. All the electrochemical and spectroscopic measurements hereafter were conducted on surface cleanned Pd CNCs.

Figure 1. (A) TEM image of a typical Pd CNC. The inset corresponds to a selected-area electrondiffraction pattern. (B) Cyclic voltammograms for Pd CNCs covered on Au film in 0.5 M H2SO4 before and after CO adsorption- displacement treatment at 50 mV s-1. (C) Real time ATR-SEIRAS spectra for the Pd CNCs and the bare Au substrate in 0.5 M H2SO4 at 0.15 V vs RHE during bubbling CO with the reference spectrum collected prior to bubbling CO at the same potential. Each single beam spectrum was averaged of 128 scans. (D) Integrated peak intensities of adsorbed CO and ν(C-H) vs. time. The inserted diagram illustrates the displacement of CTAB due to CO adsorption.


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Controlled irreversible adsorption of Bi on Pd surfaces was enabled by cycling the Pd-CNCs electrode in a 0.1 mM Bi3+ + 0.5 M H2SO4 solution between 0.13-0.78 V vs RHE. As shown in Figure 2A, the hydrogen and (bi)sulfate adsorption/desorption peaks from 0.13 to 0.30 V shrink gradually with increasing number of cycles, indicating increasing surface blockage with Bi adatoms. Cyclic voltammograms of the Pd CNCs without and with Bi modification were recorded in a fresh 0.5 M H2SO4, as shown in Figure 2B. The Bi coverage (θ𝐵𝐵𝐵𝐵 ) is estimated based on the charges corresponding to hydrogen and (bi)sulfate adsorption/desorption peaks between 0.13-0.3 V using the following equation: θ𝐵𝐵𝐵𝐵 = 1 − 𝜃𝜃𝐻𝐻 = 1 −

𝐵𝐵𝐵𝐵 𝑄𝑄𝐻𝐻

(1)

0 𝑄𝑄𝐻𝐻

where 𝑄𝑄𝐻𝐻𝐵𝐵𝐵𝐵 is the charge for hydrogen and (bi)sulfate adsorption/desorption on the Bi modified Pd CNCs (Bi/Pd CNCs), and 𝑄𝑄𝐻𝐻0 is the charge on bare Pd CNCs. In the present work, three electrodes of Pd CNCs with Bi coverages of ca. 0.18, 0.31 and 0.42 were examined in terms of performance as EGO catalysts. Cyclic voltammograms (CVs) for EGO at the Pd CNCs and Bi/Pd CNCs electrodes were respectively measured in a 1 M NaOH + 1 M EG solution as shown in Figure 2C. CVs for the bare Au and Bi-modified Au substrates in otherwise same conditions were also recorded for comparison. It can be seen that in the potential region of interest, the Au and Au/Bi surfaces are virtually inert in EGO. In contrast, typical voltammetric features corresponding to the electrocatalysis of EGO at Pd and Bi/Pd-coated Au substrates can be found. Furthermore, each of the three Bi/Pd CNCs samples exhibits a higher peak current than unmodified Pd CNCs, indicating that the Bi adatoms increase the EGO activity on Pd surfaces. The oxidation peak current (defined as the apparent activity, or AA) culminates at θ𝐵𝐵𝐵𝐵 =0.31, as can be seen from Figure 2D that shows the plot of oxidation current versus θ𝐵𝐵𝐵𝐵 . To make a more reasonable comparison, the specific activity (SA) of EGO after calibrating the exposed surface Pd sites is defined as: SA =

AA

1−θ𝐵𝐵𝐵𝐵

(2)

The plot of the specific activity versus θ𝐵𝐵𝐵𝐵 also yields an activity peak around θ𝐵𝐵𝐵𝐵 = 0.31 , implying the optimal promotion effect lies in a relatively low Bi coverage, in contrast to a monotonic increase of activity with Bi coverage in formic acid oxidation at Bi-modified high-index Pt nanocrystals.13 Therefore, the Bi/Pd CNCs electrode with optimal θ𝐵𝐵𝐵𝐵 exhibits an apparent activity 1.7 times and a specific activity 2.44 times as high as the Pd CNCs electrode towards EGO in alkaline media. The morphology of Bi/Pd CNCs after relevant electrochemical measurements remains largely intact, indicating the good resistance to ligand cleaning and reaction procedures (Figure S5).


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Figure 2. (A) Cyclic voltammograms (CVs) showing the increasing Bi modification on Pd CNCs from 0.5 M H2SO4 + 0.1 mM Bi3+ solution with increasing potential cycles. The dash curve corresponds to the CV for the same electrode in 0.5 M H2SO4 before addition of Bi3+. (B) CVs for the Bi-modified Pd CNCs in 0.5 M H2SO4 used to evaluate different coverages of Bi adatoms. (C) CVs for various relevant electrodes in 1 M NaOH+ 1 M EG solution. (D) Plots of the EGO peak currents vs. Bi coverage. The same scan rate 50 mV s-1 was applied in all measurements.

OHad is regarded as a reactive pair in electrocatalytic oxidation of small organic molecules in alkaline media.22, 35-38 The promoted EGO on Bi/Pd CNCs may be attributed to the modification of electronic state of Pd by oxophilic Bi adatoms, which facilitates the formation of OHad.26 Comparative XPS measurements on Pd CNCs and Bi/Pd CNCs were conducted to clarify the oxidation states of surface Pd and Bi (see supporting information). The Bi 4f doublets for Bi(0) and Bi(III) are observed on Bi/Pd CNCs in Figure S6, suggesting a larger portion of Bi(0) existing on the surface with our potential controlled modification protocol. Furthermore, our XPS result clearly demonstrates the change in electronic state of Pd upon modification of Bi adatoms, as can be seen in Figure S3. The Pd(0) 3d doublets for Bi/Pd CNCs shift positively by ca. 0.6 eV, as compared to those for Pd CNCs, indicating an appreciable electron transfer between Bi and Pd. According to the detailed explanation of XPS results on bimetallic surfaces by Watanabe’ group39, the positive-shift of Pd XPS doublet peaks may correspond to the obtainment of partial electron by Pd from Bi, in contrary to the regular explanation on the XPS peak shifts for different valences of metals. This electron transfer direction is also in agreement with the work function order, i.e., 4.22 eV for Bi and 5.12 eV for Pd, as well as the electronegativity order in Pauling scale, i.e., 2.02 for Bi and 2.20 for Pd. According to the literature reports2, 40 the cleavage of the C-C bond in the C2 intermediates for the ethanol oxidation at Pt surfaces is favored at negative potentials. Along this line, the electronrich Pd surfaces by Bi modification presumbly benefit the breaking of the C-C bond during EGO.


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Indeed, this assumption is corroborated by the following ATR-SEIRAS result. In situ potentiodynamic ATR-SEIRAS spectra for EGO on Pd CNCs and Bi/Pd CNCs were respectively measured in a 0.5 M NaOH + 0.5 M EG solution by applying cyclic voltammetry at 10 mV s-1. The results are shown in Figure 3 with the band assignments listed in Table 1. Note that the reference spectra were acquired on Pd CNCs and Bi/Pd CNCs at either the open circuit potential (OCP, see Figure S7) of ca. 1.02 V in 0.5 M NaOH or at the upper potential limit (1.2 V). in 0.5 M NaOH + 0.5 M EG. On both Pd CNCs and Bi/Pd CNCs, a major ν(COB) band at 1840-1900 cm-1 and a minor ν(COL) band around 2000 cm-1 were observed in ATR-SEIRAS. The vibrational frequencies of COad are lower than those in Figure 1C, due to a lower CO coverage and a higher electrolyte pH. The ν(COB) band was used for a reliable intensity-potential analysis The ν(COB) band intensity is plotted in Figure 3C or 3F as a function of potential together with the corresponding CV. Notably, this band intensity on Bi/Pd CNCs is about 30% higher than that on Pd CNCs at E < 0.8 V in the forward scan, confirming a promoted cleavage of C-C bond on Bi/Pd CNCs. In response to the oxidation current peaks, the ν(COB) band starts to decrease at ca. 0.70 V with its intensity decreasing steeply around 0.90 V and down to zero at ca.1.1 V on Pd CNCs. In contrast, on Bi/Pd CNCs the COad oxidation proceeds through two-stages: one fourth of COad is gradually oxidized between 0.70 and 1.1 V with a sort of band intensity plateau from 0.9 to 1.1 V ; the rest of COad is quickly oxidized between 1.1-1.2 V. To understand this behavior, potential dependent ATR-SEIRAS spectra were comparatively recorded during anodic stripping of a neat CO adlayer on Bi/Pd CNCs electrode in 0.5 M NaOH, revealing that the CO adlayer could be essntially removed at ca. 0.9-1.0 V without the two- stage profile in the ν(COB) band intensity vs. potential plot (Figure S8). Hence, it is reasonable to deduce that the plateau over 0.9-1.1 V on Bi/Pd CNCs in Figure 3F arises from the interplay between the oxidative CO removal and the conversion to CO from the COHx fragment at relatively high potentials (note: the cleavage of the C-C bond of C2 intermediates at relatively low potentials leads to the CO and COHx fragments). In fact, conversion of the CHx fragment to CO species at relatively high potentials was noted for ethanol oxidation at Pd electrode in alkaline media27 as well as at Pt electrode in acidic media.41 Thus, Bi modification promotes not only the C-C bond cleavage but also the conversion of COHx to CO on Pd CNCs, with the newly formed CO being further oxidized to CO2 at even higher potentials. Another piece of evidence in support of the above proposal comes from the similar CO stripping curves on both Pd CNCs and Bi/Pd CNCs, in which both CO adlayers are oxidatively removed at 0.9-1.0 V (see Figure S9). In the backward scan, the COad band reappears on both electrodes in response to the recurring of EGO currents with the COad band being relatively stronger on Bi/Pd CNCs at lower potentials, suggesting a fairly stable Bi adsorption despite the high potential excursion. Notably, our electrochemical ATR-SEIRAS results indicate that Bi modification promotes both EGO activity and COad formation on Pd CNCs, arguing against that Bi adatoms enhance the EGO via the inhibition of CO formation42. In other words, the COad formed during the EGO process should not be simply defined as a poisoning species, it also plays the role of an active intermediate to be converted to CO2 (HCO3- and CO32- ). Herein, more COad formation on Bi/Pd CNCs may increase the oxidation pathway to CO2 at sufficiently higher potentials, which will be further demonstrated by IRRAS results (vide infra). In Figure 3A and 3D, the upward and downward interfacial water bands (δ(HOH)43 at ~1650 cm-1) were detected on Pd CNCs and on Bi/Pd CNCs, respectively, by taking the single beam


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spectrum at open circuit potential in 0.5 M NaOH solution as the reference spectrum. The δ(HOH) band on Pd CNCs could be mostly subtracted by using the single-beam spectrum at 1.2 V as the reference spectrum, with a clear new peak showing up at 1620 cm-1 in Figure 3B, attributable to adsorbed 2-hydroxylacetyl species.6-7 In contrast, the downward δ(HOH) band feature largely remains on Bi/Pd CNCs in Figure 3E. The unique spectral feature of the δ(HOH) band in Figure 3D and 3E may be caused by the interfacial structural transition due to the oxidation of Bi-OHad to Bi2O3 from 1.0 V to 1.1V (Figure S9).20, 44

Figure 3. In situ ATR-SEIRAS spectra collected on Pd CNCs electrode (A, B) and Bi/Pd CNCs electrode (D, E) in 0.5 M NaOH + 0.5 M EG solution with a time resolution of 5 s, using single beam spectrum at open circuit potential at ca. 1.02 V in 0.5 M NaOH solution (A, D) and at 1.2 V in 0.5 M NaOH + 0.5 M EG solution (B, E) as the reference spectrum, respectively. Each single beam spectrum was averaged of 18 scans. (C, F) Corresponding cyclic voltammograms (dotted line) of Pd CNCs and Bi/Pd CNCs in 0.5 M NaOH + 0.5 M EG solution at 10 mV s-1, together with the potential dependent peak intensities of COad.

On the basis of the previous spectroscopic studies on ethanol oxidation on Pd film electrode in alkaline medium27 and EGO on Pt film electrode in acidic medium involving isotope labeling6-7, the 1620 cm-1 bands in Figure 3B should be mainly assigned to the ν(C=O) vibration of 2-hydroxyacetyl. In both spectral experiment6-7 and computational simulation45, the 2-hydroxyacetyl surface specie is considered as an active intermediate of EGO at Pt electrode in acidic media. To the best of our knowledge, the adsorbed 2-hydroxyacetyl species has been detected for the first time on Pd surfaces in alkaline media. This species is stable at lower potentials, and can be oxidized when the potential is raised high enough to initiate the EGO current peak. Due to the severe interference from interfacial H2O band, the 2-hydroxyacetyl adsorbed on Bi/Pd CNCs is seen as a relatively weak bipolar peak at lower potentials, as shown in Figure 3E. On Pd CNCs and Bi/Pd CNCs, the weak bands at 1580, 1411 and 1320 cm-1 due to the νas(OCO) and νs(OCO) of interfacial glycolate species show up at potentials between 0.9-1.2 V, which are otherwise much stronger in IRRAS from 0.6 to


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1.2 V (vide infra). The glycolate may come from the direct oxidation of adsorbed 2-hydroxyacetyl species or from the partial conversion of glyoxal through Canizzaro reaction46 in alkaline solution.5

Table 1 Peak assignments for the infrared spectra of EGO on Pd in alkaline media acquired by two reflection modes. Wavenumbers (cm-1)

Assignments

1076 1107 1236 1309 1326 1361 1411 1580 1620 1650 1840-1900 2343

Aldehyde stretch (glyoxal, glycolate)5, 8, 24, 42 Typical peak for glyoxylate5 (unseen in this work) C-O stretch of glycolate42 feature peak for oxalate8, 24, 47 (unseen in this work) Symmetric stretch of COO- in glycolate8, 24 Glycolate, HCO3- 8, 26 Symmetric stretch of COO- in glycolate5, 24 Asymmetric stretch of COO- in glycolate8, 42 ν(C=O) of adsorbed 2-hydroxyacetyl6-7 δ(HOH) of interfacial water43 Adsorbed CO species26-27 Asymmetric CO2 stretch26, 42

Figure 4 shows spectral results of multi-step IRRAS measurements on Pd CNCs and Bi/Pd CNCs electrodes at different potentials in 0.5 M NaOH + 0.5 M EG. Much weaker COad band was barely observed at 1860-1900 cm-1 on both samples, which is the only distinguishable surface species relevant to EGO by IRRAS, demonstrating the advantage of ATR-SEIRAS in detecting surface species. Nevertheless, IRRAS provided much stronger signals for dissolved intermediates or products trapped in the thin layer cell. The band at 1580 cm-1 rises up at 0.6 V, attributable to the formation of glycolate and/or glyoxal. The other bands for these two species are observed at 1411, 1361, 1326, 1236 and 1076 cm-1 only at potentials positive of 0.7 V. Given that the proposed C2 pathway leads to oxalic acid and glyoxylic acid for EGO on Pt electrode in acidic media (Scheme 1), one may consider that oxalate47 or glyoxylate5 is among the main products of EGO on Pd electrode in alkaline media. However, neither the 1307 cm-1 band characteristic of oxalate nor the 1107cm-1 band characteristic of glyoxylate could be seen with confidence on the two catalysts. Therefore, oxalate and glyoxylate formation can be neglected if any under present conditions. Notably, in a previous IRRAS measurement21, no oxalate was detected on a Pd electrode in alkaline media when the EG concentration was higher than 0.1 M. As the potential moves positively to 1.0 V, the CO2 band at 2343 cm-1 shows up with a relatively strong peak intensity on Bi/Pd CNCs. In other words, the C1 pathway is promoted due to the Bi modification on Pd CNCs.


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Figure 4. In situ IRRAS spectra of Pd CNCs (A) and Bi/Pd CNCs (B) electrodes in 0.5 M NaOH + 0.5 M EG solution at different given potentials. Reference spectrum was taken at 0.15 V in the same electrolyte. Multi-step potential control was used for IRRAS. Each spectrum was averaged of 256 scans.

To compare the relative product distribution for EGO on Pd CNCs and Bi/Pd CNCs, the intensity ratios of the 2343, 1580 and 1076 cm-1 bands obtained respectively on the two catalysts at 1.2 V are replotted in Figure 5 from original spectral data of Figure 4, together with the ratio of the total oxidation charges passed during the two sets of IRRAS measurements (Figure S10). With an increase of total oxidation charge by ca. 7%, the relative intensity of the CO2 band increases by 30%, suggesting again an enhancement towards complete oxidation of EG on Bi/Pd CNCs. The 1580 cm1 band intensity decreases by 1% while the 1076 cm-1 band increases by 34%. To understand the opposite intensity evolution of these two bands, IR transmission spectra of 1.0 M sodium glycolate and 1.2 M glyoxal in 0.5 M NaOH solution (freshly prepared) were shown in Figure 5B, together with the IRRAS spectra collected on bare Pd CNCs and Bi/Pd CNCs at 1.2 V. The transmission spectra indicate that the 1580 cm-1 band intensity is much stronger than the 1076 cm-1 band for glycolate and the opposite order is for glyoxal in alkaline media. Therefore, the observation of relatively decreased 1580 cm-1 band and increased 1076 cm-1 band can be explained by more glyoxal and less glycolate produced on Bi/Pd CNCs during the IRRAS measurement. Further quantification of the products in the electrolyte was performed with 1H NMR measurements in Figure S11. The apparent faradaic efficiency of glycolate is 76% on Pd CNCs, and 55% on Bi/Pd CNCs, calculated from the concentration of glycolate in the electrolyte after cyclic voltammetry measurements for EGO. Lower yield of glycolate on Bi/Pd CNCs reasonably corroborates enhanced formation of glyoxal, CO and CO2 on Bi/Pd CNCs as indicated by IRRAS.


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Figure 5. (A) The band intensity ratios calculated from IRRAS spectra at 1.2 V and the total oxidation charge ratio during the IRRAS measurements on Pd CNCs and Bi/Pd CNCs. (B) Transmission spectra of 1.0 M sodium glycolate and 1.2 M glyoxal (freshly prepared) in 0.5 M NaOH solution. The IRRAS spectra for Pd CNCs and Bi/Pd CNCs at 1.2 V are shown for comparison.

A previous DFT calculation result45 suggested that the cleavage of the C-C bond may proceed via HOCHCO, OCCHO, HOCCO, and/ or OCCO transition intermediates. Notably, these species have not been confirmed experimentally, due possibly to the short-lived nature. Nevertheless, these transition species may form via the dehydrogenation of 2-hydroxyacetyl or glyoxal rather than glycolate. Presumably, based on our comprehensive spectral results and the literature reports5-7, 45, simplified reaction pathways of EGO at bare and Bi-modified Pd CNCs in alkaline media may be depicted in Scheme 2. Herein, EG was firstly dehydrogenated on surface sites, forming adsorbed 2hydroxyacetyl species, which may serve a kind of pivotal intermediate to be further oxidized to glycolate and glyoxal. To some extent, the 2-hydroxyacetyl acts similarly the role of adsorbed acetyl in ethanol oxidation reaction at Pd electrode in alkaline media. Glyoxal is not stable in strong alkaline media, and would be partly converted to glycolate or get decomposed to C1 species. The as-formed C1 species are further oxidized to CO2 or its derivatives HCO3- and CO32- products.

Scheme 2. Reaction pathways suggested for EGO at Pd CNCs in alkaline media.

4. Concluding Remarks Motivated by the efficient utilization of a biomass fuel in energy conversion and the potential application of high-index faceted non-Pt nanocatalysts in the anode of direct alcohol fuel cells, we have investigated ethylene glycol oxidation (EGO) at pristine and Bi-modified Pd concave nanocubes (Pd CNCs) in alkaline media by applying two reflection modes of electrochemical surface infrared spectroscopy. To facilitate the surface modification of Bi adatoms as well as to obtain convincing spectroelectrochemical results, the shape-controlling surfactants on Pd CNCs are removed effectively through the CO-adsorption displacement surface cleaning procedure. The Pd CNCs with a controlled Bi coverage of 0.31 show the most enhanced EGO activity among all the samples examined. The electronic state of Pd is significantly tuned by Bi modification. The electrochemical ATR-SEIRAS provides strong evidences of the COad and 2-hydroxyacetyl (HOCH2-C*O) formation on bare and Bi-modified Pd CNCs, suggesting that Bi adatoms promote the cleavage of the C-C bond. Complementarily, the electrochemical IRRAS and product quantification with 1H NMR indicates the formation of glycolate, glyoxal and CO2 /HCO3-, and Bi modification enhances the reaction pathways leading to glyoxal and CO2 and inhibits partly the formation of glycolate. A simplified scheme is proposed to show the reaction pathways to form C1 and C2 products in which the adsorbed HOCH2-C*O acts the role of a pivotal intermediate. The


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present study may provide an insight into the mechanistic understanding of interfacial electrocatalytic processes on faceted nanocrystals at molecular level. It should be pointed out that thorough disclosing a reaction mechanism is generally a progressive process, as already demonstrated in the history of electrocatalysis. More comprehensive understanding of how Bi modification affects the activity and selectivity of EGO on Pd CNCs requires a great deal of concerted efforts, including the thermodynamic and kinetic DFT calculations on EGO and in situ XANE characterization of Bi/Pd CNCs at different potentials, which are under plan and may be reported in due course. Supporting Information Detailed description of materials, Figures S1-S11 are included. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements: This work is supported by the 973 Program (No. 2015CB932303) of MOST and NSFC (No. 21473039 and 21273046). Corresponding Author E-mail: [email protected]

Notes The authors declare no competing financial interest. References 1.

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47. Berná, A.; Delgado, J. M.; Orts, J. M.; Rodes, A.; Feliu, J. M. Langmuir 2006, 22, 7192-7202.

TOC


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Table of contents 132x61mm (96 x 96 DPI)


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Scheme 1. Proposed reaction pathways for EGO on Pt electrode in acidic media. 316x107mm (96 x 96 DPI)


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Scheme 2. Reaction pathways suggested for EGO at Pd CNCs in alkaline media. 316x92mm (96 x 96 DPI)


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Figure 1. (A) TEM image of a typical Pd CNC. The inset corresponds to a selected-area electron-diffraction pattern. (B) Cyclic voltammograms for Pd CNCs covered on Au film in 0.5 M H2SO4 before and after CO adsorption- displacement treatment at 50 mV s-1. (C) Real time ATR-SEIRAS spectra for the Pd CNCs and the bare Au substrate in 0.5 M H2SO4 at 0.15 V vs RHE during bubbling CO with the reference spectrum collected prior to bubbling CO at the same potential. Each single beam spectrum was averaged of 128 scans. (D) Integrated peak intensities of adsorbed CO and ν(C-H) vs. time. The inserted diagram illustrates the displacement of CTAB due to CO adsorption. 455x316mm (96 x 96 DPI)


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Figure 2. (A) Cyclic voltammograms (CVs) showing the increasing Bi modification on Pd CNCs from 0.5 M H2SO4 + 0.1 mM Bi3+ solution with increasing potential cycles. The dash curve corresponds to the CV for the same electrode in 0.5 M H2SO4 before addition of Bi3+; (B) CVs for the Bi-modified Pd CNCs in 0.5 M H2SO4 used to evaluate different coverages of Bi adatoms. (C) CVs for various relevant electrodes in 1 M NaOH+ 1 M EG solution. (D) Plots of the EGO peak currents vs. Bi coverage. The same scan rate 50 mV s-1 was applied in all measurements. 365x268mm (96 x 96 DPI)


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Figure 3. In situ ATR-SEIRAS spectra collected on Pd CNCs electrode (A, B) and Bi/Pd CNCs electrode (D, E) in 0.5 M NaOH + 0.5 M EG solution with a time resolution of 5 s, using single beam spectrum at open circuit potential at ca. 1.02 V in 0.5 M NaOH solution (A, D) and at 1.2 V in 0.5 M NaOH + 0.5 M EG solution (B, E) as the reference spectrum, respectively. Each single beam spectrum was averaged of 18 scans. (C, F) Corresponding cyclic voltammograms (dotted line) of Pd CNCs and Bi/Pd CNCs in 0.5 M NaOH + 0.5 M EG solution at 10 mV s-1, together with the potential dependent peak intensities of COad. 455x278mm (96 x 96 DPI)


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Figure 4. In situ IRRAS spectra of Pd CNCs (A) and Bi/Pd CNCs (B) electrodes in 0.5 M NaOH + 0.5 M EG solution at different given potentials. Reference spectrum was taken at 0.15 V in the same electrolyte. Multistep potential control was used for IRRAS. Each spectrum was averaged of 256 scans. 518x201mm (96 x 96 DPI)


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Figure 5. (A) The band intensity ratios calculated from IRRAS spectra at 1.2 V and the total oxidation charge ratio during the IRRAS measurements on Pd CNCs and Bi/Pd CNCs. (B) Transmission spectra of 1.0 M sodium glycolate and 1.2 M glyoxal (freshly prepared) in 0.5 M NaOH solution. The IRRAS spectra for Pd CNCs and Bi/Pd CNCs at 1.2 V are shown for comparison. 458x186mm (96 x 96 DPI)


Electrocatalysis of Ethylene Glycol Oxidation on Bare and Bi-Modified

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