Single-Atomic Cu with Multiple Oxygen Vacancies on Ceria

Transcription

1 Supporting Information Single-Atomic Cu with Multiple Oxygen Vacancies on Ceria for Electrocatalytic CO 2 Reduction to CH 4 Yifei Wang, 1, Zheng Chen, 1, Peng Han, 1, Yonghua Du, 2 Zhengxiang Gu, 1 Xin Xu, 1, * and Gengfeng Zheng 1, * 1 Department of Chemistry, Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai, , China. 2 Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island, , Singapore *Address correspondence to: xxchem@fudan.edu.cn (X.X.) and gfzheng@fudan.edu.cn (G.Z.) Y.W., Z.C. and P.H. contributed equally to this work.

2 Computational Description: All density functional theory (DFT) calculations were performed by using the Vienna ab initio simulation package (VASP). [1-3] The 5s, 5p, 5d, 4f, 6s electrons in cerium, the 2s, 2p electrons in oxygen and the 3d, 4s electrons in copper were treated as valence electrons, while the kinetic energy cut-off for the plane wave basis sets was set to be 400 ev. The remaining core electrons were described by the projector augmented-wave (PAW) method. [4] The surface Monkhorst Pack meshes [5] of k-point sampling in the surface Brillouin zone were employed in all calculations. A 2 3 supercell of five atomic layers was used, where the bottom three layers of atoms were fixed in their optimized bulk positions, while the top two layers, as well as the adsorbate, were allowed to fully relax. After the convergence criterion for optimizations has been met, the largest remaining force on each atom is less than 0.02 ev A -1. For all calculations, the spin polarized generalized gradient approximation (GGA) of the Perdew Burke Ernzerhof (PBE) functional [6] was used. As the standard DFT functionals tended to over-delocalize electrons, DFT+U [7] was employed with an effective U value of 5.0 ev for both Ce 4f-orbitals and Cu 3d-orbitals. [8,9] The contribution of dispersive interactions are accounted by DFT+D3 method with Becke-Jonson (BJ) damping. [10,11] The CeO 2 (110) surface was chosen as the base. To model the Cu-doped CeO 2 (110) surfaces at low Cu content, one pair of Ce 4+ O 2- on the top layer was replaced by a Cu 2+ ion, leading to the spontaneous formation of the first oxygen vacancy (V O ): [12] [2Ce 4+,O 2- ] [Ce 4+,Cu 2+,V O ] + 1/2 O 2 (S1) The as-formed (110) surface was labelled as CuCe x-1 O 2x-1 and depicted in Figure 1a. It is well-known that the formation of a V O is accompanied by the release of 2 electrons, which, in turn, results in two nearby Ce 4+ being reduced to 2 Ce 3+ : [13] [2Ce 4+,O 2- ] [2Ce 3+,V O ] + 1/2 O 2. E pure f 1 (V O ) = E t (Ce x O 2x-1 ) + 1/2 E t (O 2 ) E t (Ce x O 2x ), (S2) (S3)

3 where E t (Ce x O 2x-1 ) and E t (Ce x O 2x ) are the total energies of the optimized supercell with and without V O vacancy, respectively, while E t (O 2 ) is the total energy of a gas-phase O 2. The formation energy for one Vo on a pure CeO2(110) was calculated to be 1.74 ev, which is in good agreement of the literature number of 1.78 ev. [13] The concentration of Ce 3+ is often related to the concentration of V O. As compared Eq. S1 with Eq. S2, it is clear, however, that Cu-doping may decrease the Ce 3+ /(Ce 3+ + Ce 4+ ) ratio without affecting the V O concentration as a result of charge balance: [2Ce 3+,V O ] [Ce 4+,Cu 2+,V O ]. (S4) The second V O formation energies, E dop f 2 (V O ), from CuCe x-1 O 2x-1 were calculated as: E dop f 2 (V O ) = E t (CuCe x-1 O 2x-2 ) + 1/2 E t (O 2 ) E t (CuCe x-1 O 2x-1 ), (S5) where E t (CuCe x O 2x-1 ) and E t (CuCe x O 2x-2 ) are the total energies of the optimized supercell with the first and second V O vacancy, respectively. Figure 1b depicted the structure with the lowest E dop f 2 (V O ), where a dimer vacancy, V O -2, formed, leading to a four-coordinated Cu site. The formation energy for a Vo-2 was calculated to be 1.82 ev. The relative energies for some representatives of the CuCe x-1 O 2x-2 surfaces with an energy span up to 1.08 ev for different configurations are shown in Figure S1. The third V O formation energies, E dop f 3 (V O ), from CuCe x-1 O 2x-2 were calculated as: E dop f 3 (V O ) = E t (CuCe x-1 O 2x-3 ) + 1/2 E t (O 2 ) E t (CuCe x-1 O 2x-2 ), (S6) where E t (CuCe x O 2x-3 ) is the total energy of the optimized supercell with the third V O vacancy. Figure 1c depicted the structure with the lowest E dop f 3 (V O ), where a trimer vacancy, V O -3, formed, leading to a three-coordinated Cu site. The formation energy for a Vo-3 was calculated to be 1.65 ev. As shown in Figure S2 (a), three Vo s in close vicinity of Cu ion (i.e., Vo-3) is more stable than two Vo s in close vicinity of Cu ion to form a dimer vacancy (Vo-2) plus a nearby Vo associated with 2 Ce 3+. It can be estimated that a Vo-3 is more stable than Vo-2 plus an isolated Vo associated with 2 Ce 3+ by ( = 0.09 ev): Vo-2 + Vo Vo ev (S7)

4 On the other hand, as shown in Figure S2 (b), Vo-3 plus a nearby Vo is more stable than four Vo s in close vicinity of Cu ion to form a tetramer vacancy (Vo-4). Therefore, on the singleatomic Cu-substituted CeO 2 where there are rich oxygen vacancies, the surface with multiple oxygen vacancies such as Vo-3 is the most stable. The adsorption energies of CO 2 on the V O site of the Cu-doped CeO 2 (110) surfaces were calculated by E dop ad (CO 2 ) = E t (CO 2 /CuCe x-1 O 2x-y ) E t (CO 2 ) E t (CuCe x-1 O 2x-y ), (S8) where E t (CO 2 /CuCe x-1 O 2x-y ) and E t (CO 2 ) are, respectively, the total energies of the adsorption systems with y-v O s and a gas-phase CO 2 molecule. The structures for CO 2 adsorptions on the four-coordinated and three-coordinated Cu sites were depicted in Figure 1d and Figure 1e, respectively. It is to shown that the adsorbed CO 2 on the Cu site with three Vo s exhibits a bended structure (Fig. 1e), suggesting efficient activation toward CO 2.

5 Experimental Section: Synthesis of CeO 2, Cu-CeO 2 -x% (x = 2, 4, 10): A hydrothermal method was used to synthesize CeO 2 nanorods. Typically, 0.88 g of Ce(NO 3 ) 3 6H 2 O (Sinopharm Chemical Reagent Co., China) was dissolved in 20 ml of deionized (DI) water, and 8.44 g of NaOH (Sinopharm Chemical Reagent Co., China) was dissolved in 15 ml of water. The NaOH solution was added dropwise into the Ce(NO 3 ) 3 solution under stirring at room temperature. The mixed solution was adequately stirred for additional 30 min at room temperature and then transferred into a 50-mL Teflon bottle. The Teflon bottle was tightly sealed and hydrothermally treated in a stainless-steel autoclave at 100 C for 24 h. After cooling, the obtained white precipitate was collected, washed with water and ethanol for several times, and then dried in oven at 60 C for 16 h. The obtained pale yellow powder was calcined in a tube furnace under Ar atmosphere at 500 C for 4 h to get CeO 2. The CeO 2 was then used as ingredient to prepare Cu-CeO 2 -x%. For instance, to prepare Cu-CeO 2-4%, 400 mg of CeO 2 was dispersed in 10 ml of deionized water with ultrasonication, and 63.6 mg of Cu(NO 3 ) 2 3H 2 O (99% purity, Sinopharm Chemical Reagent Co., Ltd., China) was added to reach 4 weight% nominal loading of Cu. After stirring for 10 min, 5 ml of aqueous solution containing 250 mg of Na 2 CO 3 (Sinopharm Chemical Reagent Co., Ltd) was added. The obtained suspension was then stirred at room temperature for 2 h, filtered, thoroughly washed with hot deionized water, dried at 60 C in an oven overnight and finally calcined in H 2 and Ar mixture (H 2 ratio: 5%) at 250 o C for 1 h (ramping rate: 5 o C/min). Different Cu contents were achieved by changing the weights of Cu(NO 3 ) 2 3H 2 O added. Synthesis of Cu nanoparticles: 63.6 mg of Cu(NO 3 ) 2 3H 2 O (99% purity, Sinopharm Chemical Reagent Co., China) was dissolved into 10 ml of deionized water. After stirring for 10 min, 5 ml of aqueous solution containing 250 mg of Na 2 CO 3 (Sinopharm Chemical

6 Reagent Co., China) was mixed with Cu(NO 3 ) 2 solution. The obtained solution was dried at 60 C in an oven overnight and finally calcined in 5% H 2 in Ar mixture at 250 o C for 1 h (ramping rate: 5 o C/min). Characterization: The X-ray adsorption near edge structure (XANES) measurement of Cu-CeO 2 -x% was performed at XAFCA beamline of the Singapore Synchrotron Light Source (SSLS) [14]. The content of Cu in each sample was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Hitachi P4010 plasma spectrometer). Electrochemical measurements: Electrochemical tests were carried out in a custommade H-shaped cell in a three-electrode system under room temperature. Different sample mixtures were prepared by dispersing 5 mg of samples into 1 ml of ethanol (Sinopharm Chemical Reagent Co., China) and 120 µl of Nafion (perfluorosulfonic acid-ptfe copolymer, 5% w/w solution, Alfa Aesar, USA), and were then dropped on carbon paper (Toray TGP-H-060) that was cut into 0.5 cm 0.5 cm used as working electrode. An Ag/AgCl electrode was used as a reference electrode and a Pt wire as counter electrode. For CO 2 reduction experiments, cyclic voltammetry (CV) was carried out in a CO 2 -saturated 0.1 M KHCO 3 aqueous solution (ph 6.8). All the electrochemical measurements were controlled by an Autolab electrochemical workstation (PGSTAT204, Sweden) at room temperature. The gas products were analysed with a gas chromatograph (Shanghai Ramiin GC 2060, China) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The liquid products were quantified by 1 H nuclear magnetic resonance spectroscopy (Bruker AVANCEAV III HD 500). 1 ml of deuterated water (D 2 O) was added into 5 ml of electrolyte. Then 0.5 μl of dimethyl sulfoxide (DMSO, 99.99%, Alfa Aesar, USA) was added as the internal standard. The pre-saturation method was used to suppress water peak.

7 Supporting Figures Figure S1. The relative energies for some representatives of the CuCe x-1 O 2x-2 surfaces with an energy span up to 1.08 ev for different configurations. The red fonts indicated the energy reference where both Vo s are located around a single Cu site. Figure S2. The relative energies of (a) CuCe x-1 O 2x-3 and (b) CuCe x-1 O 2x-4 with different configurations. The red fonts indicated the energy reference where all Vo s are located around a single Cu site.

8 Figure S3. Comparison of CO 2 adsorptions on the CuCe x-1 O 2x-δ surfaces, where δ = 1, 2, 3 indicate the number of Vo vacancies as shown in (a c), respectively. CO 2 prefers to adsorb linearly for δ = 1 and 2 as shown in (d) and (e), while is easily bended for δ = 3 as shown in (f). A large deformation energy is required for transforming linear CO 2 to bent CO 2 as shown in (g). The formation energy for 1 Vo on a pure CeO 2 (110) is calculated to be 1.74 ev, indicating that the 3 Vo-bound structure (c) is the most stable for the single-atomic Cu site on CeO 2.

9 Figure S4. XRD spectra of samples: Cu-CeO 2-8% (red curve), Cu-CeO 2-6% (green curve) and CuO (blue curve). Figure S5. Energy dispersive X-ray spectroscopy (EDS) mapping images of Cu-CeO 2-4%: Cu (yellow), Ce (green) and O (red).

10 Figure S6. XPS of Ce 3d spectra in (a) CeO 2, (b) Cu-CeO 2-2% and (c) Cu-CeO 2-10%.The calculation of Ce 3+ ratio was also listed out (Table S2).

11 Figure S7. XPS of Cu 2p spectra of (a) Cu-CeO 2-2% and (b) Cu-CeO 2-10% nanorods. Figure S8. EXAFS R-space fitting curves of Cu-CeO 2-4% nanorods.

12 Figure S9. The CO 2 temperature programmed desorption profiles for CeO 2 and Cu-CeO 2-4% nanorods. Figure S10. (a) TEM and (b) HRTEM images of Cu nanoparticles.

13 Figure S11. Representative gas-chromatography data of gas products of Cu-CeO 2-4%. Figure S12. Representative NMR spectra for liquid-phase products of Cu-CeO 2-4%.

14 Table S1. Cu content in different samples determined by ICP-AES. Sample ID Reported Conc (Samp) Samp Units % Cu-CeO 2-2% mg/l 2.29 Cu-CeO 2-4% mg/l 4.05 Cu-CeO 2-6% mg/l 5.98 Cu-CeO 2-8% mg/l 7.63 Cu-CeO 2-10% mg/l 7.86 Cu standard mg/l 100 *This results was lower than the nominal 10%, as the content of Cu may be limited by the capacity of ceria to substitute Ce with Cu atoms.

15 Table S2. Ce 3d XPS results. The listed-out figures are the bind energies (BE) and the area of each peak. The ratio of Ce 3+ /Ce 4+ were calculated to show the content of Vo near surface. Binding Energy (ev) CeO 2 Cu-CeO 2-2% Cu-CeO 2-4% Cu-CeO 2-10% μ μ μ μ μ o ν ν ν ν ν o Ce Ce Ce 3+ / Ce Peaks μ, μ o, ν and ν o belong to Ce 3+.

16 Table S3. Fitting results of Cu-CeO 2-4%. Cu-O CN R (Å) δ 2 (Å 2 ) R-factor 4.8± ± ± Table S4. Literature comparison of electrochemical performances of ECR producing CH 4 on different Cu-based catalysts conducted in H-cells at room temperature. Electrode Electrolyte Applied potential (V vs. RHE) Current density (ma cm -2 ) FE (%) Reference Cu-CeO 2-4% 0.1 M KHCO ~ 56 ~58% This work Polished Cu foil 0.3 M KI M KHCO ~ 12 ~56% * ACS Catal. 2016, 6, fold Twinned Cu Nanowires 0.1 M KHCO ~ 8 55% Nano Lett. 2017, 17, 1312 Pd-decorated Cu 0.5 M KHCO ~ 60 ~50% Angew.Chem. Int.Ed. 2017, 56, Cu-porphyrin complex 0.5 M KHCO ~ 8 48% J. Am. Chem. Soc. 2016, 138, *FE of CH 4 was not specified in the article. This figure was obtained from the line graph.

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