Ir0/graphdiyne atomic interface for selective epoxidation

ABSTRACT The development of catalysts that can selectively and efficiently promote the alkene epoxidation at ambient temperatures and pressures is an important promising path to renewable synthesis of various chemical products. Here we report a new type of zerovalent atom catalysts comprised of zerovalent Ir atoms highly dispersed and anchored on graphdiyne (Ir0/GDY) wherein the Ir0 is stabilized by the incomplete charge transfer effect and the confined effect of GDY natural cavity. The Ir0/GDY can selectively and efficiently produce styrene oxides (SO) by electro-oxidizing styrene (ST) in aqueous solutions at ambient temperatures and pressures with high conversion efficiency of ∼100%, high SO selectivity of 85.5%, and high Faradaic efficiency (FE) of 55%. Experimental and density functional theory (DFT) calculation results show that the intrinsic activity and stability due to the incomplete charge transfer between Ir0 and GDY effectively promoted the electron exchange between the catalyst and reactant molecule, and realized the selective epoxidation of ST to SO. Studies of the reaction mechanism demonstrate that Ir0/GDY proceeds a distinctive pathway for highly selective and active alkene-to-epoxide conversion from the traditional processes. This work presents a new example of constructing zerovalent metal atoms within the GDY matrix toward selective electrocatalytic epoxidation.

Materials. All of the chemicals used were of analytical grade and used directly without any further purification. The carbon cloth was thoroughly cleaned by washed by nitric acid and deionized water before use.
Synthesis of 3D GDY electrodes. The freshly cleaned 3D carbon cloth (3 cm  4 cm) was added to a 50 mL Teflon-lined stainless-steel autoclave containing 30 mL pyridine solution of hexacetylenebenzene (0.3 mg mL 1 ). and two pieces of Cu foils. Cu foils act as the catalyst for the growth of GDY nanosheets. The autoclave was then kept at 110 °C for 12 h under the protection of Ar. After the completion of the reaction, the 3D GDY electrode was obtained. Prior to uses, the freshly-synthesized 3D GDY electrodes were washed by 3 M HCl, hot DMF, acetone, and 3 M HCl subsequently for at least three times to remove the possible copper residues.
Synthesis of Ir 0 /GDY. The zero-valent Ir 0 /GDY were synthesized by a facile electrochemical reduction method using a standard three-electrode system, in which the 3D GDY electrode (1 cm  2 cm), carbon rod and saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. The dilute solution of sulfuric acid (0.5 M) of iridium chloride (5 mM) was used as the electrolyte. During the anchoring of Ir atoms, the GDY electrode was directly immersed in the electrolyte, followed by the in-situ adsorption and reduction of Ir at 10 mA cm -2 for 200 s. The obtained Ir 0 /GDY was washed by 0.5 M H 2 SO 4 , and deionized water subsequently for three times, and used for electrocatalysis immediately.
Morphological and structural characterizations. Scanning electron microscope (SEM; Apreo, Thermo Scientific), transmission electron microscope (TEM; Talos F200X G2 TEM, Thermo Scientific), high-resolution TEM (HRTEM) were used to characterize the morphologies of the samples. Energy-dispersive X-ray spectroscopy (EDX) was collected with an energy-dispersive X-ray detector in Apreo SEM and Talos F200X G2 TEM. HAADF-STEM images were taken from an aberration-corrected cubed FET Titan Cubed Themis G2 300 or JEM-ARM200F (JEOL, Tokyo, Japan). The metal content was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent ICPOES730). (In-situ) Raman spectra was characterized through the LabRAM HR Evolution spectrometer (the excitation laser source is 473 nm), the sample was scanned with a static raster scan type and a 600 lines per millimeter (L/mm) grating at 50 times magnification. Exposure time of 10 s and two accumulative cycles of 3.2% laser power were used for in situ spectral acquisition. For the in-situ Raman characterization, an electrochemical workstation was attached to the sample cell, the Ir 0 /GDY, Pt wire and Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. A mixed solvent of deuterium dimethyl sulfoxide and deuteroxide (3:1, ~0.5 mL) which containing 0.25 M styrene and 0.25 M sodium bromide was used as electrolyte. The in situ electrolysis was conducted at a constant current of 5 mA cm -2 for continuous electrocatalysis, the spectrum was collected at 2-minute intervals during the electrocatalysis. X-ray photoelectron spectroscopy (XPS) measurements were carried through a Thermo Scientific Nexsa instrument with monochromatic Al Kα X-ray radiation, the XPS spectra during the electrocatalysis were collected through characterzing the sample with different electrocatalysis time. Nuclear magnetic resonance spectroscopy ( 1 H NMR, AVANCE NEO) was used to determine the yield and purity of final products. Two-dimensional wide-angle X-ray scattering (2D GIWAXS) experiments were performed at SAXSpoint 5.0 (Anton Paar, Austria).
Electrochemical measurements. The electrochemical measurements were recorded from a CHI 660E electrochemical workstation (Chenhua, Shanghai), equipped with an undivided electrolytic 6 cell. The Ir 0 /GDY or IrNP/GDY, carbon rod and Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. A mixed solvent of 1.5 mL deuterium dimethyl sulfoxide and 0.5 mL deuteroxide which containing 0.5 mmol styrene and 0.5 mmol sodium bromide was used as electrolyte. All potentials were recorded against the Ag/AgCl. Where n SO is the molar number of styrene oxide, F is the Faradaic constant (96485 C mol -1 ), Q is the total charge passing the electrode.
XAFS measurements. The X-ray absorption find structure spectra were collected at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The storage rings of BSRF was operated at 2.5 GeV with an average current of 250 mA. Using Si(111) double-crystal monochromator, the data collection were carried out in transmission/fluorescence mode using ionization chamber. All spectra were collected in ambient conditions. XAFS analysis. The acquired EXAFS data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages. The L3-weighted EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, L3-weighted χ(k) data of Ir L-edge were Fourier transformed to real (R) space using a hanning windows (dk=1.0 Å -1 ) to separate the EXAFS contributions from different coordination shells. To obtain the quantitative structural parameters around central atoms, least-squares curve parameter fitting was performed using the ARTEMIS module of IFEFFIT software packages.
The L 3 weighting, and R range of 1.0 -2.5 Å were used for the fitting. The four parameters, coordination number, bond length, Debye-Waller factor and E0 shift (CN, R, σ2, ΔE 0 ) were fitted without anyone was fixed, constrained, or correlated.
Calculation setup. All the density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) 1 . To consider the interaction between core electrons and nucleus with valence electrons, the projector augmented plane-wave (PAW) method was employed 2, 3 , and the energy cutoff for plane-wave was set to 400 eV. The generalized gradient approximation proposed by Perdew-Burke-Ernzerhof (GGA-PBE) was adopted to consider the exchange-correlation effects 4 . The structure relaxation was finished until the energy was less than 10 −5 eV and the residual forces on the atoms was less than 0.01 eV/Å. The 3 × 3 × 1 K-points were sampled in the Monhorst-Pack grid for the first Brillouin zone integration. For the electronic structure calculations, the much mesh 5 × 5 × 1 K-points were sampled. To avoid the interaction between periodic repeated slabs, a vacuum layer of 20 Å is added perpendicular to the sheet along the z-direction. To describe the long-range van der Waals interaction, the DFT-D3 approach was used in the study 5 . During the geometry optimizations, all atoms were allowed to relax. To model the atom catalyst of Ir/GDY, a single layer 22 graphdiyne (GDY) supercell with 72 carbon atoms was chosen as the support, and one metal Ir atom was loaded on GDY surface for optimization. The