Toward an Effective Control of the H2 to CO Ratio of Syngas through

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Toward an Effective Control of the H2 to CO Ratio of Syngas through CO2 Electroreduction over Immobilized Gold Nanoparticles on Layered Titanate Nanosheets Filipe Marques Mota, Dang Le Tri Nguyen, Ji-Eun Lee, Huiyan Piao, Jin-Ho Choy, Yun Jeong Hwang, and Dong Ha Kim ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00647 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Catalysis

Toward an Effective Control of the H2 to CO Ratio of Syngas through CO2 Electroreduction over Immobilized Gold Nanoparticles on Layered Titanate Nanosheets

Filipe Marques Mota,†,§ Dang Le Tri Nguyen,‡,◊,§ Ji-Eun Lee,† Huiyan Piao,†,± Jin-Ho Choy,†,± Yun Jeong Hwang,‡,◊,* and Dong Ha Kim†,#,* †

Department of Chemistry and Nano Science, Division of Molecular and Life Sciences,

College of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea ‡

Clean Energy Research Center, Korea Institute of Science and Technology (KIST), 5,

Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ◊

Division of Energy and Environmental Technology, KIST School, Korea University of

Science and Technology (UST), 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ±

Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano

Science, Division of Molecular and Life Sciences, College of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea #

Division of Chemical Engineering and Materials Science, College of Engineering, Ewha

Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea

*

To whom correspondence should be addressed:

E-mail: [email protected]; Fax: +82-2-958-5809; Tel.: +82-2-958-5227 E-mail: [email protected]; Fax: +82-2-3277-4546; Tel.: +82-2-3277-4517 §

These authors contributed equally to this work.

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ABSTRACT In recent years, the electroreduction of CO2 to valuable products has emerged as a rational answer to uprising CO2 emissions and a strategic approach to incorporate renewable electricity from intermittent sources (e.g. wind and solar) into the global energy supply. The reduction of CO2 to CO has been highlighted in the widely explored industrial conversion of syngas (CO and H2) to fuels. Herein, we report a promising electrocatalyst incorporating well-dispersed gold nanoparticles (Au NPs) on ultra-thin titanate nanosheets (TiNS). By tuning the contents of Au (in the ranges of 0 to 93 wt.% Au) in the hybrid Au/TiNS architecture, CO product selectivity was effectively controlled (in the range of CO Faradaic efficiency from 3 to over 80%) with the sole additional formation of H2, which is of pronounced industrial interest. Most importantly, a control of both component amounts was suggested to result in a variation of corresponding electronic properties based on the interaction between Au NP and TiNS substrate, dictating the stabilization of formed reaction intermediates and resulting product selectivity. In addition, our Au/TiNS achieved optimally high CO and H2 production current densities, with 73 wt.% Au at the low cathodic potential region (-0.6 to -0.9 VRHE). The suggested synergetic effect between both catalytic components underlines the promising character of this hybrid system and is expected to significantly add to the strategic production of syngas for following applications.

KEYWORDS CO production; CO2 reduction reaction; Electrocatalysis; Au catalyst; Titanate nanosheets; Syngas conversion


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INTRODUCTION Increasing anthropogenic CO2 emissions leading to rising atmospheric concentrations, and derived environmental concerns have underlined the urgency for a gradual shift from fossil fuels to energetic strategies based on renewable sources.1 Fixation of CO2 by chemical reaction rather than capture and storage has gathered major interest for energy storage and carbon recycling. To date the thermochemical, electrochemical and photochemical-driven transformation of CO2 toward a sustainable energy circle to targeted fuels and chemicals has been largely explored.2 The electrochemical reduction of carbon dioxide (CO2RR) to carbon-based products (such as carbon monoxide, formate, methane, ethylene, methanol, and ethanol)3-8 is a potential solution to quickly incorporate renewable electricity from intermittent sources (e.g., wind and solar) into the global energy supply. Continuous exploratory works have paid heightened attention to the synthesis of oxygenates and hydrocarbons to partially replace crude oil-derived transportation fuels, while serving previously established infrastructures in a global-scale demand.9 Among all value-added products derived from CO2RR, CO formation, involving the transfer of only two electrons and protons, exhibits the highest Faradaic efficiencies (typically above 90%) at the lowest overpotentials.10 Obtained mixtures of CO and its main side product H2 can be subsequently converted to alcohols and hydrocarbons of longer chain through appropriate well-developed technologies. In the selective synthesis of CO, Au, Ag, and Zn have been underlined as metal electrocatalysts of choice,11-13 with alternative strategies being scarce to date.14-15 Through a preferential stabilization of the intermediate over the final product, Au features the highest current density and the lowest onset potential toward CO.11, 16 Pursued efforts using creative nanostructures have shown that even minor changes in size, morphology, or surface elements of the metal nanoparticles (NPs) strongly influence the CO2RR activity.13, 17-21 In particular, size-dependence on the selectivity has been ascribed to the coexistence of competing active centers for CO2RR and HER.


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In the evaluation of metal NP as CO2RR cathode catalysts, selected support/substrate e.g. conductive carbon black powder, metals, or metal oxides have been assessed in the literature.12, 22-23 The intrinsic properties and the interaction between the loaded catalyst and the support can remarkably determine the resulting catalytic activity of the binary system. Porous or nanostructured architectures providing high surface area for CO2RR, and enhanced conductivity of the support/substrate hindering resistance losses, reflect well-known strategies toward enhanced electrocatalytic performances. An activity enhancement facilitated by constructing the metal-metal oxide interface has been a widely recognized strategy for some popular heterogeneous reactions,24-26 and recently applied toward an electrochemical CO2RR.27 A charge transfer between the catalytic metal and the substrate can occur due to their electronic difference.28-30 The interaction between the catalyst and the support has been shown to dictate the electronic properties on the surface for CO2RR catalyst, which is related to a change of electronic structure, the stabilization of intermediates, and consequentially the product selectivity.31-32 In addition, the synergic interaction of a metal and its substrate is proposed to further activate the resulting catalytic properties, which are different from those of the metallic catalyst or substrate alone.31 In the production of syngas for subsequent transformation into alcohols or hydrocarbons, it has been considered more efficient to optimize the electrolysis cell toward the production of CO, with suppressed HER activities. Practically, however, it may be unreasonable to continuously aim for complex catalytic systems with extremely low H2 yields when additional hydrogen is necessary to attain specific syngas composition ranges demanded in subsequent industrial processes.33-36 Moreover, to date additional H2 is mostly derived from hydrocarbon based feedstocks e.g. steam reforming of CH4 (48%), heavy oils and naphtha reforming (30%), and coal gasification (18%), leading to additional CO2 formation.37


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In this sense and as recently underlined by Chen’s group, we believe that current efforts may be eventually skewed from suppressing the HER activity of materials of choice to rationally designing a catalytic library capable of effectively tune the relative activities of CO2RR and HER.38 To tune the H2 to CO ratio in the production of syngas is an unexplored path, and a valuable sidestep to additional H2 sources. In this work, we report a binary system incorporating well-dispersed Au NPs as a CO2RR catalyst on titanate nanosheets (TiNS) as the substrate and a secondary catalyst, which can affect the HER activity of the catalytic system. The morphology of these exfoliated TiNS with ultrathin thickness has been shown highly advantageous in a wide range of catalytic reactions and was here considered ideal to incorporate well-dispersed Au NPs. In addition, the negative surface of the TiNS was found suitable to investigate the effect of an eventual electronic reconstruction in the resulting binary system. The binary catalytic system was shown to enable a control of the wide-ranging H2 to CO ratio by simply changing the incorporated Au amount on the TiNS. The electrochemical performance and corresponding characterization of interest have been detailed in this contribution.


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EXPERIMENTAL SECTION Fabrication of Au nanoparticles on titanate nanosheets. TiNS were fabricated as reported elsewhere.39-40 Pristine cesium titanate Cs0.67Ti1.83□0.17O4 was prepared by solid-state reaction by calcining a stoichiometric mixture of Cs2CO3 and TiO2 at 800°C. Proton exchange was carried out by reaction of the cesium titanate powder with 1 M HCl aqueous solution at room temperature for 4 days, the HCl solution being replaced with a fresh one each day. The exfoliation of the layered titanate lattice was achieved by the intercalation of tetrabutylammonium cation (TBA+) into the protonic titanate for more than 10 days. For an effective exfoliation, the amount of TBA hydroxide was adjusted to yield the equivalent of exchangeable protons in H0.67Ti1.83□0.17O4·H2O. After the reaction, a small fraction of incompletely exfoliated particles was eliminated through centrifugation at 12,000 rpm for 10 min, and the obtained colloidal suspension was used for subsequent experimental works. The exfoliated TiNS have been reported to retain its original composition of Ti1.83□0.17O4-0.67 with negative charge. After washing the TiNS were finally dispersed in water. The fabrication of Au NPs on TiNS was achieved through an ion-exchange method. A positive [Au(en)2]Cl3 complex solution was prepared by mixing HAuCl4 (2 mL, 0.05 M) with ethylenediamine (100 µL) in 48 mL of water.41 TiNS dispersed in water (5 mL, 2x10-3 M) were mixed with varying volumes of the prepared Au solution. To achieve equilibrium of ion-exchange the mixture was stirred overnight. Under vigorous stirring NaBH4 was added (20 mL, 0.1 M). The yellow powder immediately changed to dark violet. The Au/TiNS was centrifuged and extensively washed with water. Characterization Techniques. X-ray diffraction (XRD) assessed under Ni filtered Cu-Kα radiation (λ=1.5418 Å) using a Rigaku Dmax 2000 diffractometer in a 10° to 70° 2θ region revealed pertinent information on the crystallinity and phase structure of the synthesized materials. The surface morphology was characterized using scanning electron microscopy


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(SEM; JEOL JSM6700-F) and transmission electron microscopy (TEM; JEOL JSM-2100F operated at 200 kV). The average particle size of these TiNS was determined by dynamic light scattering. Corresponding atomic force microscopy (AFM) images were obtained using a Dimension 3100 scanning force microscope in tapping mode (Digital Instrument). UV-vis absorbance was measured using a Varian Technologies Cary 5000 spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were measured on a Thermo Scientific K-Alpha XPS, using a dual beam source and ultra-low energy electron beam for charge compensation. The actual Au and Ti amounts in the prepared materials were measured by an inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer, OPTIMA 8300). The nature of the chemical bonding of the hybrid films was investigated with micro-Raman spectroscopy. CO2 electroreduction reaction measurement. Prepared catalysts were dispersed into a mixture of isopropanol and Nafion solution (5 wt.%, Aldrich) in a volume ratio of 5:5:1. The prepared catalyst ink was sonicated prior to measurement for 20 min. A glassy carbon plate (Alfa Aesar) was used as the electrode substrate (GCE) after mechanically polishing and subsequent cleaning with deionized water. In each case the catalyst ink solution (20 µL) was deposited onto the glassy carbon plate by drop casting. The active area of the electrode was 0.5 cm2. Experimentally the prepared ink loaded in the GCE for each sample contained a relatively similar amount of TiNS, with increasing corresponding Au contents. Platinum and Ag/AgCl (3 M NaCl) were used as the counter electrode and the reference electrode, respectively. In light of recent reports evidencing electrochemical and chemical dissolution of Pt in both acidic and alkaline electrolytes and a subsequent redeposition on the working electrode,42-43 collected electrolytes and Au/TiNS working electrodes were characterized following evaluation under the selected CO2RR operating conditions (Figure S1). Pt


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dissolution and an impact in the electrocatalytic performance were however confirmed to remain negligible. Electrochemical measurements were performed by using a potentiostat (CHI Instruments) in a two-compartment electrochemical cell, and a Nafion 117 membrane was used to separate the catholyte and the anolyte. The electrolyte solution (0.5 M KHCO3; Sigma-Aldrich, ≥99.95%) was purged with 20 sccm high-purity CO2 (99.999%) gas for at least 1 h until saturated (pH 7.2) and the CO2 flow was continuously flowed during the CO2RR. The applied potentials for CO2RR activity measurement were compensated for iR loss and converted versus the reversible hydrogen electrode (RHE) according to Equation (1). (. RHE) = (. Ag/AgCl) + 0.197V + (0.0591 × pH)

(1)

The CO2RR activity was measured by using chronoamperometry at each fixed potential, and gaseous products H2, CO, and CH4 quantified using a gas chromatography (GC) system (Younglin, Model 6500 GC) equipped with a capillary column (Restek, RT-Msieve 5A) and a pulsed discharge detector (PDD). Ultrahigh purity helium (He, 99.9999%) was used as a carrier gas. The partial current density of products ( ) and corresponding Faradaic efficiency (F. E. ) were obtained following the methodology outlined in some our previous studies as follows: = " × # × F. E. =

*+ ,- ./ *0,012

$%&' ()

× 100

(2) (3)

where " is the volume concentration of H2 or CO measured by the GC, # is the flow rate measured by a universal flow meter (Agilent Technologies, ADM 2000) at the exit of the cathode in the electrochemical cell, 3345 is the total current density, 6 is the Faradaic constant, 78 is pressure, 9 is the ideal gas constant, and : is the temperature. Liquid products were also analyzed by ion chromatography (IC, DIONEX IC25A).


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The electroactive surface area of the Au/TiNS samples was compared by lead underpotential deposition (UPD). Characterization was conducted using a potentiostat (CH Instrument, CHI 660) with a Ag/AgCl (3 M KCl) reference electrode, and a Pt-foil counter electrode. The drop-casted inks prepared in similar fashion as above described were cycled from 0.1 V to -0.8 VAg/AgCl at a scan rate of 10 mV/s in 0.1 M NaOH electrolyte containing 1 mM Pb(CH3CO2)2 saturated in N2 for 30 min prior to measurement.


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RESULTS AND DISCUSSION Au NPs were incorporated in ultra-thin TiNS through ion-exchange and subsequent reduction in presence of NaBH4, in an attempt to unveil a potential mechanistic cooperation between both materials in the CO2RR. A simplified schematic diagram of the experimental procedure has been included in Figure S2. Characterization of the synthesized materials. TiNS were synthesized in a series of synthesis steps outlined in the Experimental Section. The X-ray diffraction (XRD) patterns of the prepared material are in accordance with those previously reported.39-40 The XRD peaks are relatively broad, which reflects the nature of these two-dimensional nanomaterials (Figure 1a). The structural stacking derived from the presence of tetrabutylammonium cations intercalating the thin sheets agrees with reflections found at ca. 5° and 14.5°. UV-Vis absorption spectra disclosed a sharp absorbance peak centered at a wavelength of 267 nm (Figure 1b). Scanning electron microscopy (SEM) images confirmed the synthesis of welldispersed exfoliated sheets (Figure 1c), whereas the atomically thin-layered two-dimensional structure could be further evidenced by atomic force microscopy (AFM) measurements. From the collected AFM images, the thickness of these ultrathin TiNS lied in a 1-2 nm range (Figure S3). Its average size was found to be within 50-200 nm as determined by dynamic light scattering (Figure 1d). The TiNS displayed an average zeta potential of -40.8 mV, underlining the negatively charged surface of these materials. The adopted methodology for Au incorporation was believed ideal to avoid the propensity of titanate phase transformation with temperature.44 Characteristic metallic Au diffraction peaks detected in the hybrid materials agreed with face-centered cubic (fcc) crystalline of those referenced in the literature (Figure S4). The peak ascribed to the (020) plane of the titanate sheets agrees with prior characterization depicted in Figure 1a. Inductively coupled


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plasma (ICP) results conveniently summarized in Table S1 confirmed a continuous increase of the Au content incorporated in the TiNS. The observation agreed with collected SEM images in Figure 2. No distinct morphological evolution occurred with Au incorporation. In all cases a systematic good Au nanoparticle dispersion in the Ti layers could be noticed, even at high Au/Ti ratios (Figure S5). Au incorporation was further probed with UV-Vis spectroscopic analysis to measure the plasmonic absorption by small sized Au NPs.45-46 Corresponding optical absorption spectra yielded peaks with increasing intensities at higher Au loadings. The intense absorbance peaks were conspicuously found in the 528 nm to 537 nm range, suggesting a rather similar average particle size in each case (Figure 3a). A close inspection of representative transmission electron microscopy (TEM) images confirmed an intimate coupling between well-dispersed and small sized Au NPs and TiNS for all samples (Figure 3b and S6). In agreement with Figure S5, the loaded nanosheets remain well dispersed. Au/TiNS with weight percent of Au of 18, 37, 60, 73, 82, and 93% showed comparable particle size distributions and average diameters (nm) of 5.6±1.4, 6.6±2.2, 6.9±2.0, 6.7±2.2, 7.5±3.0, and 7.8±3.4, respectively (Figure 3c and S7). In good agreement, using Scherrer’s equation based on the predominant XRD peak at 2θ = 37.9°, the average crystal size was found to be ca. 7.3 nm for the incorporated AuNP for AuNP/TiNS-37% Au. The presence of relatively large Au particles remained negligible even on Au/TiNS-93% Au. The average Au particles size range is in line with previous reports when the reduction was executed by NaBH4, in comparison with procedures involving reduction steps with molecular hydrogen at higher temperature levels. High-resolution TEM depicts the lattice fringes of the immobilized Au in the TiNS, corroborating the presence of metallic Au in agreement with a d-spacing of ca. 2.4 Å (Figure 3d).


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To investigate the chemical interaction between Au and these TiNS high-resolution X-ray photoelectron spectroscopy (XPS) was assessed for all samples (Figure 4). The C 1s line position was used as a reference. Representative results obtained with our TiNS revealed Ti 2p signals at 457.72 and 463.43 eV corresponding to Ti 2p3/2 and Ti 2p1/2, which are attributed to the Ti4+ state of the TiNS (Figure 4a).47-48 Two small peaks at relatively lower binding energies i.e., 456.50 and 461.72 eV, respectively, were ascribed to the presence of reduced Ti3+ species, related to oxygen defects.48-49 Upon Au incorporation, all signals were progressively skewed toward higher binding energies up to a ca. 1.10 eV shift with increasing Au content. In all cases, the presence of Ti3+ species was accounted to remain ca. 3%. Conversely, the O 1s XPS spectrum of our TiNS yields two distinguished signals at 529.48 and 531.65 eV ascribed to lattice O2− species and hydroxyl group, respectively (Figure 4b).48, 50-52

Comparatively, the spectra of the binary samples yielded an asymmetrical peak with a

tail extending towards higher energies, well known in Au/TiO2 catalytic systems and herein considered as being composed of three components.53-55 With all samples the Au 4f core-level XPS spectra showed two single peaks corresponding to Au 4f7/2 and Au 4f5/2 (Figure 4c). The deconvoluted peaks indicated a primary presence of metallic Au (Au0), whereas the presence of oxidized Au species (Au3+ and Auδ+) could also be distinguished.56-58 In all samples the amount of minor oxidized Au species was accounted to be below 10%. Interestingly, a relatively similar (Au3++Auδ+)/Au0 ratio could be achieved within a wide Au loading range, which concurs with a similar Au particle size in all samples. Most importantly, the relatively high content of metallic Au0 agrees with the use of NaBH4 as a strong reducing agent. Compared with other synthesis methods, in the reduction of metal ions to synthesize metal NPs NaBH4 reduces the positively charged gold from the ion exchange positions to metallic states.44 A steeping decrease of the binding energy of Au 4f with decreasing concentration of Au in the prepared samples was


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further noted. Metallic Au shifts up to ca. 0.95 eV between samples Au/TiNS-18% Au and Au/TiNS-93% Au.

CO2 electroreduction. The electrochemical CO2RR performed using the prepared Au/TiNS electrodes showed enhanced catalytic activity for CO production compared with the bare TiNS sample, which suggests Au is the active catalyst for CO2RR. Samples were prepared with relatively similar amounts of TiNS with increasing Au contents. Figure S8 compares cyclic voltammograms obtained under Ar and CO2-bubbled conditions. Corresponding electrocatalysts were evaluated in a varying potential range from -0.3 to -1.0 VRHE (Figure 5). Total current densities per mass of catalyst (Au/TiNS) were depicted in Figure 5a. Compared with all Au/TiNS materials, bare TiNS showed a different profile trend with an onset potential in a much more negative range (ca. -0.7 VRHE), and a relatively sharp increase of the current density at limiting potential ranges. With a higher Au content, the evaluated binary systems revealed an enhanced total current density per mass of catalyst following the order Au/TiNS-73% Au > Au/TiNS-60% Au > Au/TiNS-37% Au > Au/TiNS18% Au. The total current density of Au/TiNS-73% Au showed the highest values at the low cathodic potential region (-0.6 to -0.9 VRHE) underlining the promising character of this hybrid system. With a continuous increase of loaded Au, the limiting current density at -0.95 VRHE decreased for both Au/TiNS-82% Au (-28.8 mA mgcat-1) and Au/TiNS-93% Au (-10.9 mA mgcat-1), when compared with Au/TiNS-73% Au (-46.5 mA mgcat-1). Figure 5b shows the iR-corrected potential-dependent CO Faradaic efficiency (F.E.CO) profiles for all samples. Product analysis assessed by gas chromatography confirmed the sole formation of H2 and CO. The result is consistent with previous studies on gold-based electrocatalysts for CO2RR,59-60 with the formate Faradaic efficiency being negligible under the detection limit of ion chromatography (IC). When individually evaluated, our bare TiNS


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produced CO solely at a minor extent (Figure 5b), suggesting the occurrence of a quasiexclusive hydrogen evolution reaction (HER) catalyst these two-dimensional materials. With a small incorporation of Au, Au/TiNS-18% Au unveiled a maximum F.E.CO at -0.75 VRHE of 29.9%. The enhanced CO selectivity over HER became progressively more prominent with a continuous Au content increase attaining the maximum F.E.CO of 81.9% with Au/TiNS-93% Au at -0.65 VRHE. With a relatively facilitated CO2 activation and a strong binding energy to intermediates, these findings agree with reports of CO2RR over Au metallic surfaces, on which mild binding with CO leads to its facilitated desorption. We have assessed the catalytic properties of Au NPs incorporated on Ketjen Carbon Black (CB) in a similar fashion. The characterization of the prepared reference material including SEM and TEM images, absorbance, and ICP analysis is detailed in Figure S9 and Table S1. Well-dispersed Au nanoparticles were confirmed by SEM and TEM photographs. The Au/CB sample revealed Au particle size distribution with an average diameter of 5.9±3.2 nm comparable to those found for the TiNS-loaded counterparts. With a reported negligible catalytic contribution of the carbon material in CO2RR,12 the well-dispersed Au NPs evidenced a maximal F.E.CO of ca. 95% in the -0.6 to -0.7 VRHE range. These findings again indicate the capability of Au NPs to exhibit a quasi-exclusive CO2 reduction reaction to CO production activity in these potential ranges (Figure S10). Previously reported Au NP sizes have been found at similar ranges as the ones herein reported toward an optimized CO selectivity.11, 19 This has been ascribed to the fraction of available surfaces showing a higher activity to CO2RR compared to HER. Faradaic Efficiency of the prepared hybrid materials does not necessarily lie in between those found for Au/CB and the TiNS. The lower onset potential for CO production of all Au/TiNS samples (-0.35 VRHE) compared with that value of Au/CB (-0.4 VRHE) implies an interaction between Au and TiNS and promising aptitude for an enhanced activation of CO2RR.


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To better understand the activity trend of the binary Au/TiNS catalysts, the F.E.CO was compared at a fixed potential (at -0.65 VRHE) in function of the wt.% of Au in each sample (Figure 5c). At -0.65 VRHE, Au NP surface exhibited quasi-exclusive CO2RR and effectively suppressed HER, agreeing with representative high Faradaic efficiency results obtained with Au/CB. Notwithstanding the apparent linearity found between the evaluated binary systems, a discrepancy could be denoted upon comparison with the individually tested TiNS and Au (herein represented by the catalytic properties of Au/CB) materials. The applied methodology to modify the product selectivity in presence of a catalyst/substrate binary system unlocked a relatively facile control of the H2 to CO ratios in a -0.45 V to -0.95 VRHE range (Figure 5d). Syngas composition ratios were found in a range from 0.22 to 4.93. Previously reported CO2RR catalysts have shown limited variation in H2 to CO product ratio through a tentative selection of the applied potential.19 By adjusting the quantity of Au in Au/TiNS binary catalysts, the syngas compositions were suitably controlled for specific industrial applications (cf. shaded area in Figure 5d). Our novel Au/TiNS showed tunable H2 to CO syngas ratios for e.g., the emerging biotechnological method of high CO concentration syngas fermentation (0.3 to 1), or Fischer-Tropsch synthesis to produce fuels and chemicals as hydrocarbons form (0.5 to 2), alcohols (1 to 2) or in methanol synthesis (1), and methanation of syngas (3).33-36

CO and H2 partial current densities. For an in-depth discussion of the obtained results, Figure 6 shows the iR-corrected potential dependent CO and H2 partial current densities measured at the steady-state current density from chronoamperometry for each binary system. Based on the initial assumption that HER primarily occurs on the surface of TiNS, and CO2RR over Au active centers, the partial current densities of H2 and CO were rationally


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calculated per mass of Ti and Au, respectively (Figure 6c and d). The CO partial current density rationalized per mass of Au in the Au/CB reference is depicted in Figure S10b. CO partial current density. Comparable profile trends for CO formation were observed with all catalysts (Figure 6a). Calculated overpotentials considering that the thermodynamic reduction potential for CO2 to CO is -0.11 VRHE are in accordance with previous reports for Au NP of similar size. The CO current density of Au/TiNS-73% Au showed the highest values at the low cathodic potential region (-0.6 to -0.9 VRHE). For Au/TiNS-18% Au, Au/TiNS-37% Au, Au/TiNS-60% Au, and Au/TiNS-73% Au, the CO partial current density per mass of Au at -0.65 VRHE sees a variation in the -22.4 to -29.9 mA mgAu-1 range (Figure 6c). The rather quasi-steady-linearity for the Au weight rationalized current corroborates the assumption that CO is primarily formed over metallic Au. In accordance, an increasing number of Au active centers lead to an increase of the overall reaction rate (Figure 5a) and to a nearly proportional increase in CO production rate. Particularly surprising is the trend found for Au/TiNS-82% Au and Au/TiNS-93% Au, with CO partial current densities per mass of Au following a marked decline down to -13.35 and -6.21 mA mgAu-1, respectively. These findings were not reflected in corresponding CO partial current density per surface area of the electrode (Figure S11a). In the latter case, the CO partial current density increases up to ca. 1.7 mA cm-2 with Au/TiNS-73% Au, with both Au/TiNS-82% Au and Au/TiNS-93% Au revealing a similar limiting current density value. In CO2RR the catalytic properties of Au are known to notably depend on the size and morphology of these NPs.19 However, in this study Au has been shown to exhibit a relatively similar average size in a 5.6 nm to 7.8 nm range. Additionally, we have compared the electrochemically active Au surface area (ECSA) by under-potential deposition (UPD) of Pb using lead acetate. Although Pb UPD is diagnostic for the relative population of low index


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Au facets, the methodology was assumed to reflect a qualitative representation of the available surface area in each case. For Au/TiNS-73% Au, Au/TiNS-82% Au, and Au/TiNS93% Au, corresponding inks prepared as above described were cycled with a KOH 0.1 M electrolyte containing 1 mM Pb(OAc)2 (Figure S12). The notable increase of the available number of (111) and (110) surfaces hinted to a remarkable enhancement of the available ECSA of Au with increasing metal content. These findings confirm the expected increase of the available number of active centers for CO2RR to CO toward higher Au loadings. Nonetheless with a continuous decrease of the CO partial current densities per mass of Au with both Au/TiNS-82% Au and Au/TiNS-93% Au, an increasing fraction of Au is evidenced to not participate in the CO2RR. In line with these findings we have considered the possibility of CO2 diffusional limitations in the electrolyte. Diffusion limitations emerge if the electrochemical reaction is sufficiently fast, the diffusion of CO2 becoming the rate-limiting step of the overall catalytic process. Under selected operating conditions a scarce supply of CO2 is presumed to diffuse to the surface of the electrocatalyst where adsorption occurs. H2 partial current density. Based on the assumption that HER could be primarily driven by the presence of TiNS, H2 partial current densities have been rationalized per mass of Ti. Surprisingly, all Au/TiNS unveiled a dramatic upswing toward H2 production in the less cathodically biased region, when compared with individually tested TiNS (Figure 6b). In particular, with increasing Au loading, the H2 partial current density over TiNS (particularly inactive at -0.65 VRHE, -5.4 mA mgTi-1) increased up to -37.2 mA mgTi-1 with Au/TiNS-73% Au (Figure 6d). Contrarily to our initial expectation, HER is suggested to not exclusively occur over the TiNS. These findings hint to an effect of the incorporation of Au on the production of H2 at low potential ranges, despite the representative H2 Faradaic Efficiency of Au/CB below 5% at -


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0.65 VRHE. The continuous increase witnessed by the H2 current density per mass of TiNS suggests that either Au itself has higher HER activity upon incorporation on the TiNS substrate, or the interaction between Au and TiNS leads to a HER enhancement. Either case underlines however a cooperative synergy between the AuNP and TiNS, which effectively contributes to wide variations in H2 to CO product ratio. The dramatic decrease in H2 partial current with both Au/TiNS-82% Au and Au/TiNS-93% Au seems, however, to corroborate the main generation of hydrogen on the surface of these layered titanate materials due to high coverage of Au on TiNS surface. Indeed, it is presumed that with a continuous increase of Au loading in the ultra-thin Ti-based materials a reduction of the number of available active centers in the two-dimensional sheets for HER may occur.

Electronic reconstruction. A key point in the characterization of these materials is underlined in the found trends in the nature of the chemical interactions between Au and the TiNS (Figure 4), which can cause the catalytic activity changes in CO2RR according to the amount of Au loading on the TiNS surface. In detail, XPS spectra unveiled a dramatic increase of the binding energy of all Au 4f signals with increasing concentration of Au. The Au 4f7/2 and 4f5/2 binding energies of all Au/TiNS samples were skewed to lower values compared to the reported metallic bulk Au references (ca. 84.0 and 87.7 eV, respectively),56 with a larger shift occurring with lower Au contents. These findings were confirmed to agree with several previous studies,55, 61-63 particularly on Au NPs with an average size below 10 nm supported on TiO2, in which the binding energy of the Au 4f7/2 and 4f5/2 electrons was below corresponding values of metallic bulk Au.55, 61 It is well known that Au 4f7/2 electrons of surface Au atoms exhibit binding energies lower than that of a bulk atom due to the lower coordination number of surface atoms on the surface.64 Here, however, the variation of particle size in an average 5.6 nm to 7.8 nm range


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is not believed to be accountable for the ca. 0.95 eV shift in Au 4f7/2 binding energies between Au/TiNS-18% Au and Au/TiNS-93% Au, as reported elsewhere.61 It is reasonable to assume that a strong electronic interaction induced by the negatively charged surface of TiNS to the Au site increases due to the higher population of Au NPs, demonstrated by the shift of O 1s to higher binding energies (Figure 4b). Corroborative data has evidenced a charge transfer from oxygen species from the substrate to Au, with a resulting shift of the binding energy value for O 1s toward higher levels as a result of the low negative charge.53-54 Collected Ti 2p XPS spectra has shed light in this assumption (Figure 4a). A shift of the Ti 2p peaks toward higher binding energy for 1.25 eV between the TiNS and Au/TiNS-93% Au was positively correlated with the increase of the loaded amount of Au on TiNS. These findings suggest that the proposed electronic interaction intensively augmented with the increasing proportion of deposited Au. They further agree with recent reports on zeolitic imidazolate framework-8 (ZIF-8) nanocrystals on the surface of analogously synthesized layered TiNS for gas adsorption, hinting to the occurrence of charge transfer from the latter to ZIF-8.49

Mechanistic discussion. In previous sections, a synergism between the TiNS and the Au NPs has been claimed based on the analysis of the electrocatalytic results and considering the suggested electronic reconstruction between the components of these hybrids. Cases of clear evidence have been detailed above e.g., (1) the Faradaic Efficiency of the prepared hybrid materials was found to not necessarily lie in between those of both Au/CB and the TiNS, being in fact strikingly superior at low and high potential ranges (e.g., -0.4 VRHE), and (2) the dramatic increase of the rationalized H2 current density per mass of TiNS, superior to those found for the individual materials. In this optic, the impact of Au incorporation on the resulting performance was assessed below.


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The electronic reconstruction of the active sites of both components is expected to directly impact the adsorption strength of reaction intermediates, which differs in the range of the prepared hybrid nanostructures. This assumption reflects the synergism between both components and the divergence to expected electrocatalytic properties from a simple physical mixture of the two materials. Induced by the presence of titanate nearby centers, the chemical bond between metallic Au surfaces and adsorbates is proposed to be strengthened since their repulsion force decreases by an enlarged bond length.30, 65 The adsorption strength of active sites on Au NPs with reaction intermediates can in this optic be increased with an enhancement of the amount of deposited Au on TiNS. A subsequent stabilization of intermediates states on Au/TiNS may be ascribed to the slight enhancement in partial current density per mass of Au for Au/TiNS18% Au, Au/TiNS-37% Au, Au/TiNS-60% Au, and Au/TiNS-73% Au (Figure 6c). Most importantly it may unlock the reason behind a fair increase of the partial current density of H2 per mass of Ti at -0.65 VRHE for the above listed samples (Figure 6d). These findings could not be directly correlated with the poor activity of the individually evaluated TiNS at -0.65 VRHE, neither by the highly CO-selective Au active sites as suggested by attained F.E.CO of ca. 95% with our Au/CB reference. In the current study, the increase of partial current density of H2 per mass of Ti at -0.65 VRHE of the catalysts from Au/TiNS-18% Au to Au/TiNS-73% Au may therefore be attributed to a modification of the adsorption strength of proton over these Au/TiNS following a change of the electronic structure of Au and the TiNS. It is well known that surface defects such as oxygen vacancies are among the most reactive sites on the surfaces of metal oxides, which can modify the structure and change the electronic and chemical properties of the semiconductor surface. Surface oxygen vacancies have been proposed to play important roles in governing the adsorption and activation of reactants such as CO2 molecules. This is particularly relevant as CO2 can act as an electron


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acceptor when the electrophilic C atom interacts with surface electron centers or Lewis basic sites. In agreement, the introduction of functional basic sites has been developed in recent years to enhance the CO2 activation during the CO2RR. In the current study, the negatively charged surface of our TiNS containing a relatively high number of defects and irregularities is susceptible to suitably activate CO2. In addition, the reconstruction of the electronic structure caused by Au and TiNS hybridization is expected to alter the adsorption bonding of catalytic surfaces of the titanate sheets and adsorbates, not only for the intermediates of CO2RR, but also HER. The present discussion unlocks a key-point toward the activation of CO2 following the transfer of a proton-electron couple as the widely proposed rate-determining step of the CO2RR. Whereas the involvement of Au in the transformation of CO2 appears to be well established in the above discussion, it may seem plausible to consider a facilitated activation of CO2 over the TiNS. The assumption follows previous reports on CO2ads activation over TiO2 for further reduction over neighboring Ag metal centers. According to said claims TiO2 can activate the first step for CO2RR through the adsorption and stabilization of the intermediate on the surface following the transfer of one electron.66 The proposed reaction pathway of the reduction of intermediate on this Ag/TiO2 catalyst suggests subsequent reaction to form CO on adjacent Ag particles. Similarly, the redox features of the CO2RR intermediate were observed in the cyclic voltammetry of bare TiNS (Figure S8a). In the past, zirconium complex supported on layered tinanate has been studied as an effective CO2 adsorbent and photocatalyst.67 Afterwards, intermediates can be catalyzed to form CO product only by active Au NPs. These findings may be further correlated with the decrease in the CO partial current density with both Au/TiNS-82% Au and Au/TiNS-93% Au (Figure 5c). Whereas said phenomena was a priori ascribed to a CO2 mass transport


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limitation, it is reasonable to assume that the observed trend could find its origin in a limited TiNS surface where CO2 activation may occur. A reconstruction of the electronic structure following the hybridization of TiNS with Au NP is in conclusion expected to modify the adsorption bonding of the involved catalytic surfaces and adsorbates for the intermediates of CO2RR and HER. Above drawn conclusions underlining the effect of a synergism originated from the intimate hybridization between Au and layered TiNS may be further extended to the varying activities found for the evaluated electrocatalysts with different Au contents. In accordance the Au/TiNS-73% Au catalyst unveiled an optimum Au loading on these TiNS among the six evaluated binary systems. The sample yielded higher current density in the formation of both CO and H2. As a final remark, the present work and the proposed mechanistic insight reflect a valuable addition to current metal/metal oxide hybrid architectures toward enhanced electrocatalytic properties. Here, Au as a precious metal exhibits, as demonstrated with the reference Au/CB sample, the highest selectivity toward CO. Nonetheless, this study reflects a strategy to attain equally high selectivity ranges in less expensive designed hybrid architectures incorporating metals and metal oxides as potential cost-effective catalysts for CO2 reduction. The approach is believed to find equally suitable application in other domains but is of prime importance in the electrochemical synthesis of syngas, which has largely relied on expensive Au and Ag-based catalysts in the past years.


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CONCLUSION A nanostructured electrocatalyst fabricated by loading Au NPs on layered titanate nanosheets to produce syngas within a wide H2 to CO ratio range is reported in this study for the first time. Au NPs with an average 6 to 8 nm diameter were synthesized by ion-exchange for the reduction of gold complex solution and well dispersed on ultra-thin titanate nanosheets. Both components provided enhanced surface area for the selected reactions to occur. By adjusting the Au NPs loading on the TiNS, the F.E.CO could be selectively tuned from 3 to 82%, with corresponding H2 to CO ratios in a 0.3 to 3 range found suitable application as feedstock syngas for several reactions in the industry. XPS analysis suggested a change in the electronic structure of the individual components of our hybrid materials, due to a strong interfacial electronic interaction between the negatively charged TiNS and the incorporated Au NPs. In accordance, electronic structural changes were proposed to unlock a synergism by directly strengthening the adsorption of reaction intermediates and subsequently tuning the CO and H2 partial current densities. By tuning the Au loading on TiNS the stabilization of intermediates was proposed to dictate the selectively of CO and H2 in these hybrid structures. We believe that our novel hybrid nanostructure and its underlined synthesis strategy reflect a simple yet promising route to produce syngas with tunable H2 to CO ratios and provide a practical platform for industrial applications to reconvert the atmospheric CO2 as a renewable energy resource into useful chemicals.


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ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures S1-S12 address additional characterization of interest and details regarding the photocatalytic tests, including SEM and EDX characterization following catalytic assessment, AFM analysis of synthesized titanate nanosheets, XRD patterns and additional SEM and HRTEM images of the prepared Au/TiNS samples, Au particle size distribution, electrochemical surface area (ECSA) qualitative assessment recorded in 0.1 KOH containing 1 mM Pb(OAc)2 electrolyte, cyclic voltammetry measurements, characterization of the prepared Au/CB reference sample and corresponding electrocatalytic performance, and CO partial current density per surface area and mass of Au. Table S1 provides detailed ICP results for all evaluated samples.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Y.J.H.), [email protected] (D.H.K.). Author Contribution §

These authors contributed equally to this work.

ORCID Filipe Marques Mota: 0000-0002-0928-3583 Dang Le Tri Nguyen: 0000-0002-7933-4294 Ji-Eun Lee: 0000-0002-1800-7986 Huiyan Piao: 0000-0001-9998-3411 Jin-Ho Choy: 0000-0002-4149-7100


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Yun Jeong Hwang: 0000-0002-0980-1758 Dong Ha Kim: 0000-0003-0444-0479 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea Grant, funded by the Korean Government (2017R1A2A1A05022387; 2011-0030255). F. Marques Mota acknowledges the support by the Korea Research Fellowship Program through the National Research

Foundation

of

Korea

(NRF)

funded

by

the

Ministry

of

Science

and ICT (2017H1D3A1A02054206). D. L. T. Nguyen and Y. J. Hwang acknowledge the support from the Korea Institute of Science and Technology (KIST) institutional program.

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FIGURES

Figure 1. Characterization of synthesized titanate nanosheets. (a) XRD patterns for Cesium, TBA intercalated and protonated nanosheets, (b) absorbance, (c) representative SEM image (scale bar = 1 µm) and (d) particle size distribution.


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Figure 2. Representative SEM images of prepared Au/TiNS electrodes with (a) 18, (b) 37, (c) 60, (d) 73, (e) 82, and (f) 93 wt.% Au (scale bar = 100 nm).


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Figure 3. (a) Absorbance spectra for the prepared Au/TiNS samples with increasing Au:TiNS ratios. (b) TEM image for the sample Au/TiNS-60% Au (scale bar = 50 nm) and (c) corresponding Au particle size distribution. (d) HR-TEM images of incorporated Au particles in the prepared Au/TiNS samples (scale bar = 2 nm).


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Figure 4. Deconvoluted (a) Ti 2p, (b) O 1s, and (c) Au 4f XPS spectra of TiNS and of all prepared Au/TiNS samples with increasing Au amount.


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Figure 5. (a) Total current density, (b) CO Faradaic efficiency, (c) the relation weight percent of Au vs F.E.CO at -0.65 VRHE, and (d) the range of H2 to CO ratio for various applications.


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Figure 6. (a) CO partial current density, (b) H2 partial current density, and (c, d) the relation of weight percent of Au vs. partial current density of CO and H2 at -0.65 VRHE.


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TABLE OF CONTENTS


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Toward an Effective Control of the H2 to CO Ratio of Syngas through

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