TiC: Enhanced Activity Due to Metal

Feb 3, 2017 - Sun et al. reported the highly selective electrocatalytic reduction of CO2 to CO; this was controlled by introducing a high edge-to-corn...
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CO2 Electroreduction on Au/TiC: Enhanced Activity due to Metal-Support Interaction Jun-Hyuk Kim, Hyunje Woo, Jihwan Choi, Hyun-Woo Jung, and Yong-Tae Kim ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03706 • Publication Date (Web): 03 Feb 2017 Downloaded from http://pubs.acs.org on February 5, 2017

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CO2 Electroreduction on Au/TiC: Enhanced Activity due to Metal-Support Interaction Jun-Hyuk Kim,†+ Hyunje Woo,†+ Jihwan Choi,‡ Hyun-Woo Jung,‡ and Yong-Tae Kim‡,* †

‡

Hybrid Materials Solution National Core Research Center (NCRC) and Department of Energy System, School of Mechanical Engineering, Pusan National University, Busan, 609-735, Korea + These authors contributed equally to this work Supporting Information Placeholder

ABSTRACT: CO2 electroreduction technology is considered an important example of efficient carbon-containing energy sources. Herein, we first introduce the metal-support interaction effect with a TiC support for Au/TiC electrocatalysis, which exhibits considerably enhanced activity and selectivity for electroreduction of CO2 to CO, while suppressing H2 evolution. With this catalyst, an important electronic effect for CO2 electroreduction was clearly elucidated. Local sp-band charge transfer and d-band shifts play an important role in bonding with both CO and COOH adsorbates. Furthermore, the ideal surface interface between Ti and Au could inevitably maximize the electronic effect, thereby enhancing the catalytic activity of Au/TiC and subsequent CO production.

Carbon dioxide emissions have drastically increased annually, which might lead to environmental concerns.1 Development of the carbon capture and sequestration (CCS) technology can reduce CO2 emissions and help reuse CO2.2 CO, which has a higher reactivity than CO2, can be formed by recycling carbon dioxide.3 Furthermore, syngas, a mixture of H2 and CO, can be used to produce a wide variety of fuels and chemical products. Thus, the conversion of CO2 to CO has been widely investigated,4 one method being electrochemical CO2 reduction. Many trials have evaluated the electrochemical CO2 reduction reaction with metals such as Cu,5 Sn,6 Pb,7 Ag,8 and Au.9 Of these, Au sits near the top of the volcano curve for CO2 reduction to CO, demonstrating a high selectivity in theoretical calculations10 and a high experimental CO conversion rate.11 Until now, studies with Au have been conducted in order to increase the maximum CO conversion rate. Sun et al. reported the highly selective electrocatalytic reduction of CO2 to CO; this was controlled by introducing a high edge-tocorner ratio in ultrathin Au nanowires9 and size-controlled Au nanoparticles.12 Kanan et al. reported the highly selective CO2 reduction to CO in water with oxide-derived Au nanoparticles at overpotentials as low as 140 mV,13 and Jaramillo et al. reported AuPd alloys for synergistic electrocatalytic CO2 reduction.14 Using a different approach, Yang et al. recently attempted to enhance the activity and selectivity with bimetallic Au–Cu alloy nanoparticles, which tune the adsorbate strength by modifying the electronic structure.15-16 There has been clear progress in the fundamental understanding of the catalyst electronic structure variation after alloying.15 This investigation could also be viewed as introducing a combination of metal and support.17 Although the adsorption strength on the Au surface could be positively tuned by

changes in the electronic structure due to the large interaction between Au and Ti,18-20 there has been no attempt to precisely understand its CO2 electroreduction activity by changing the support materials. For this reason, we first designed Au/TiO2, Au/TiO2-x, Au/TiN, and Au/TiC catalysts on a polycrystalline Ti platform, not only to optimize catalytic performance, but also to elucidate the relation between the electronic effect and the CO2 reduction activity. Au nanoparticles were readily deposited onto the support by sputtering. Particularly, the relative surface coverage (interface) of Au and Ti played an important role in revealing the electronic effect. These findings serve as a good practical approach to designing ideal catalysts for the electroreduction of CO2 to CO. The CO2 electroreduction activity to CO was evaluated with Au/Ti, Au/TiO2, Au/TiO2-x, Au/TiN, and Au/TiC catalysts (details in Figure S1). Au/TiO2-x performed better than Au/TiO2 due to its improved conductivity. It is known that both TiN and TiC have a higher conductivity than TiO2,20-22 particularly Au/TiC, which shows a higher rate of CO evolution than Au/TiN; thus, Au/TiC was selected for a representative study. For systematic investigation, we prepared five catalysts of Au/TiC and named samples according to the Au thickness (e.g., Au/TiC(4) denotes a Au/TiC sample with 4 nm thick Au). Transmission electron microscopy (TEM) was used to investigate the morphology, particle distribution, and size of Au in Au/TiC. In order to confirm the Au dispersion behavior, we prepared a sample by Au sputtering on a carbon-coated Cu grid, instead of Au/TiC, which was too thick to study with TEM because Au was sputtered onto a sintered polycrystalline TiC pellet. The grid and TiC were sputtered with Au in one-pot to eliminate any variables. As depicted in Figures 1a-e, Au clusters formed over isolated interconnected islands. When the sputtering time was increased, the surface coverage also increased, bringing gold clusters closer to each other23 before being mostly covered in the end (Figure 1e). A focused ion beam (FIB) technique, for general preparation of cross-section samples for TEM, was used to observe the crosssection of the Au layer on polycrystalline TiC (TEM sample preparation process using FIB and elemental mapping images are presented in Figures S2 and S3). Figures 1f-j display the magnified FIB cross-sectional HRTEM images of Au/TiC. The trend in increasing Au coverage with increasing sputtering time is the same. After 5 min of Au sputtering, a vertical thickness of ca. 13 nm was observed for the Au/TiC lamella, and the value increased in Au/TiC(43). The TiC was formed on a Ti pellet with ~30 µm thickness, as confirmed by GDS depth profile analysis (Figure S4). The same trend in increasing Au coverage on GC with increasing

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Figure 1. TEM image of C-coated Cu grid with Au sputtering for (a) 3 min, (b) 5 min, (c) 7 min, (d) 9 min, and (e) 11 min. Magnified FIB cross-sectional HRTEM image of TiC with Au sputtering for (f) 3 min, (g) 5 min, (h) 7 min, (i) 9 min, and (j) 11 min. Insets in (a-e): magnified TEM images. sputtering time was confirmed, indicating geometrically similar Au structures with Au/TiC (Figure S5). SEM and elemental mapping images also shows similar Au interconnected islands on GC because the surface diffusion of Au is limited at low temperature (RT), resulting in no potential geometric effects (Figure S6).24 The crystal structure of Au/TiC was precisely measured on the 5D beamline at the Pohang Light Source (PLS) and determined using Rietveld refinement (Figure S7). No peaks related to surfacedispersed Au were observed by XRD analysis due to its limitation as a bulk technique, although Au was clearly confirmed by TEM (see HRTEM images of Au in Figure S8). The calculated lattice parameter for TiC (space group, Fm-3m) was 4.327 Å, which was in good agreement with experimental values (4.33 Å25 and 4.328 Å26). The current density of Au/TiC was subsequently measured with linear sweep voltammetry (LSV) over a range of potentials, and compared to Au/glassy carbon (GC) and its substrates (GC and TiC), as depicted in Figure 2. All samples were evaluated under the same electrochemical conditions with an i–R compensation of 21 ohms, and then corrected using the calculated active surface area. The specific current density, Js, was calculated by dividing the current density per geometric area (J) by the roughness factor (RF) of the surface, as shown below:27 ‫ܬ‬ ‫ܬ‬௦ ൌ ܴ‫ܨ‬ where RF is the roughness factor, calculated by taking the active surface area and dividing it by the geometric area of the electrode (diameter, 1/4"), 0.3165 cm2 (RF details in Figure S9). An exponential increase in current density was observed for TiC, Au/GC, and Au/TiC, but not GC, which was hardly active for reduction

Figure 2. Comparison of the current density profiles (LSV) for GC, TiC, Au/GC, and Au/TiC in a CO2 purged environment.

reactions. It was clear that electroreduction started at an earlier potential in all Au/TiCs than in Au/GC, and exhibited a higher current density over the same potential range. Interestingly, Au/TiC(13) displayed the highest current density over the same potential range. At -1.0 V, the current density of Au/TiC was ca. 7 mA/cm2, which is close to the highest reported value from Au3Cu in previous work on Au-Cu bimetallics.15 Since Au has been reported to mostly form CO and H2 in the potential range from –0.6 V to –1.2 V,9, 12-13, 15 and the standard equilibrium potential of CO2/CO (–0.11 V vs. RHE)11 is close to that of H2/H+(0 V vs. RHE), the competitive reduction reaction would occur in a similar potential range. In order to distinguish the two main currents attributable to CO and hydrogen evolution reactions, the chronoamperometry (CA) test was conducted with gas chromatography (GC). The average current density at each potential (–0.6, –0.8, –1.0, and –1.2 V) for Au/GC and Au/TiC was plotted as a function of time, as illustrated in Figure 3a. This demonstrates an obvious increase in magnitude of the current density with increasing potential. The current densities in Au/TiC were higher than those in Au/GC with the same reduction potential, as expected based on the trend in LSV. The current density was roughly unchanged, but could fluctuate with time in longperiod experiments (see Figure S10) due to rising cell temperature and/or evolution of bubbles on an electrode surface.28 The faraday efficiencies of major products using Au, such as CO and H2, were calculated from in-situ gas captured by the GC at a potential of –1.0 V after 300 s during the CA test, as illustrated in Figure 3b. Distinct differences in major products between samples with and without Au were observed: GC and TiC predominantly generated H2 (99 % and 78 %, respectively), while CO was the major product for both Au/GC (CO: 62 %, H2: 37 %) and Au/TiC (Au/TiC(4): 52 % and 38 % for CO and H2, respectively; Au/TiC(13): 68 % (yield of CO: 8.5∙10-8 mol/h) and 26 % for CO and H2, respectively; Au/TiC(23): 62 % and 31 % for CO and H2, respectively; Au/TiC(33): 61 % and 36 % for CO and H2, respectively; and Au/TiC(43): 55 % and 35 % for CO and H2, respectively). Combined current efficiencies above 90 % were observed with a low percentage of other products; however, relatively high percentages of H2 and other products were confirmed on Au/TiC(4) because the relative Ti surface coverage was higher than that of other Au/TiC catalysts. Au/TiC(13) exhibited the highest CO selectivity, while drastically suppressing H2 formation. Trends in CO Faraday efficiency for Au/GC and Au/TiC(13) at each potential between –0.6 V and –1.2 V (Figure S11). Both samples showed similar trends; when the potential was more negatively polarized, the CO Faraday efficiency improved, reaching minimum values at –0.6 V and maximum values above –1.2 V. Interestingly, all values for Au/TiC(13) were higher than those for Au/GC in the measured potential range.

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Figure 3. (a) Chronoamperometric curves in terms of current densities of Au/GC and Au/TiC at various potentials between -0.6 V and -1.2 V. (b) Distribution of faradaic efficiency in CO2 reduction for CO and H2. (c) CO faradaic current density of GC, TiC, Au/GC, and Au/TiC at -1.0 V. The partial current density of CO can be obtained by multiplying ty towards CO2 reduced products and suppress the hydrogen evothe total current density at each potential by the CO faradaic effilution reaction, the proton-catalyst interaction at the catalytic surciency. Figure 3c displays the CO current density for GC, TiC, face should be prevented and, simultaneously, the binding Au/GC, and Au/TiC. Noticeably, the value of Js in Au/TiC leads strength of the intermediates (COOH and CO) in this pathway to a volcano-shaped dependence of the amount of Au surface should be optimized using an electronic effect.15 According to the coverage with the highest current density observed in Au/TiC(13), kinetic volcano and free energy diagrams for CO evolution sug3.17 mA cm–2, which is more than 2.6 times that of Au/GC. gested by Norskov et al., Au can be more favorable on the activity for CO2 to CO reduction reaction when the adsorption strength of To understand the origin of this high CO selectivity and activity intermediates on Au becomes stronger.16 on Au/TiC, we mainly focused on the electronic effect, which is an important factor when changing the chemical properties of Several surface science investigations have shown changes in the catalyst. The CO2 to CO reduction mechanism follows:16 chemisorption properties of a supported metal upon interaction with the substrate,30-31 which can induce charge redistribution.32 CO2(g) + * + H+(aq) + e– ↔ COOH* (1) Therefore, we studied the core levels and valence bands of the COOH* + H+(aq) + e– ↔ CO* + H2O(l) (2) samples to elucidate the enhanced CO selectivity mechanism with X-ray photoelectron spectroscopy (XPS) measurements, which CO* ↔ CO(g) + * (3) were performed using the 8A1 undulator beamline of the PLS. where * denotes a free step site. In order to acquire high selectivi-

Figure 4. XPS Au 4f core level signals for (a) Au/TiC(4), (b) Au/TiC(13), (c) Au/TiC(23), (d) Au/TiC(33), (e) Au/TiC(43), and (f) Illustration of the effect of the change in surface electron density on the bond distance for different substrates, modified from Linic et al.29

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Figures 4a-e display a set of selected XPS spectra from the Au 4f core level region for Au/GC and Au/TiC. The peaks at around 84.0 eV and 87.7 eV were attributed to Au0 4f7/2 and Au0 4f5/2, respectively.33 The figures distinctly reveal that the XPS spectra of Au/TiCs shifted toward a higher binding energy when compared to Au/GC, except for Au/TiC(43). This finding was consistent with previous results that indicated the Au 4f binding energy was slightly larger in Au on a Ti substrate, corresponding to bulk Auo.34-35 This might be a consequence of Au-Ti electron structure perturbation at their interface.36 This resulted in a shift towards higher core-level binding energy caused by dhybridization loss compensation. However, the 4f binding energy was lowered due to the increasing s electron charge at the Au atom, regardless of which atomic species comprised the alloy partner.36 This assumption in sp-electron change was verified by DFT calculations, and the result is displayed in Table 1 (details in Figure S12). The Au configuration is 5d106s1, and the charges are averaged for the total atoms. Mulliken charges are obtained by projecting the total charge onto the s, p, and d atomic orbitals of the Au atoms in Au/C and Au/TiC. Mulliken analysis provides useful information on changes in population of the charge around atoms.37 Most of the charge was transferred to the sp-band, rather than to the d-band, in Au/TiC. The sp-electron change in the local electron density affected the

the ideal interface coverage of Au/TiC(13), the electronic effect decreased as the interface of Au-Ti decreased, and it influenced the performances of Au/TiC(23) and Au/TiC(33). When Au with bulk characteristics almost covers the catalytic surface, there is no

Table 1. Mulliken charge analysis of Au atoms, in electron charge units, for Au/C and Au/TiC.

significant difference between Au/TiC(43) and Au/GC(43). In this study, we adopted TiC as a representative support with the metal-support interaction effect for Au/TiC, and through a systematic investigation, we demonstrated considerably enhanced activity and selectivity for CO2 to CO reduction. This improved performance was related to an electronic structure change in Au. The local sp-band charge transfer and d-band shift played an important role in the bonding with CO and COOH adsorbates. Furthermore, a balanced surface interface of Ti and Au could inevitably lead to maximum CO production activity for Au/TiC. We believe that this approach to systematic catalyst design will enable us to attain the ultimate goals of CCS technology.

Atom/orbital

s

p

d

f

Au (Au/C)

0.97

0.06

9.80

0

Au (Au/TiC)

1.23

0.30

9.77

0

adsorbate-substrate bond length at the local site. When spelectrons were transferred to the Au site, the adsorbate moved further away and, consequently, strengthened the bond,28 which represents a positive approach to designing optimal Au for CO2 to CO conversion10, as illustrated in Figure 4f. Furthermore, one can see another electronic structure change through valence band analysis. Measurement of the d-band also directly provides specific information on the change in electronic structure, which correlates to the catalyst adsorption energy.38-39 Norskov et al. reported that scaling relations associated with the d-band theory of adsorption suggest there may be strong correlations within carbon-bound and oxygen-bound species due to the coupling of adsorbate states to the metal d-band.10, 40 From this model, a higher d-band center, with respect to the Fermi level (EF=0 eV), resulted in stronger bonding with adsorbates and vice versa. The d-band center can be identified by measuring the valence band spectrum. The valence band spectra of Au/GC and Au/TiC was recorded using 130 eV incident photons generated by the PLS, as shown in Figure S13. The main spectral features remained the same in both samples. However, the calculated d-band center shifted towards the EF, from 4.6 eV in Au/GC(13) to 4.34 eV in Au/TiC(13), denoting stronger bonding to adsorbates. This might also inevitably lead to higher CO production from CO2 reduction. Notably, the electronic effect can be altered by the surface environment. It has a trade-off point between the minimum loading of Au and the maximum electronic effect. We proved the optimal Au film thickness with Au/TiC(13) for the high performance in CO2 electroreduction (Figure 5); Au/TiC(4) was less active due to the low surface coverage of Au, while Au/TiC(13) maximized CO2 electroreduction through a balanced surface coverage of Ti and Au (Figure S14), thereby maximizing the electronic effect. After

Figure 5. Correlation between Au coverage and metal-support electronic effect for optimal CO2 reduction performance. The height of bar means faraday efficiency of CO, H2, others.

ASSOCIATED CONTENT Supporting Information. Additional characterization of TiC and Au/TiC, electrochemical data of Au/Ti, Au/TiO2, Au/TiO2-x, Au/TiN, Au/TiC, and DFT calculation results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by grant of National Research Foundation (NRF) (2015R1A2A1A10056156, 2014M1A8A1049348, 2015M1A2A2056556), Korea Institute of Energy Technology Evaluation and Planning (KETEP) (20153030031510) and NanoConvergence Foundation (R201500910).

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