Noble-metal alloys are widely used as heterogeneous catalysts. However, due to the existence of scaling properties of adsorption energies on transition metal surfaces, the enhancement of catalytic activity is frequently accompanied by side reactions leading to a reduction in selectivity for the target product. Herein, we describe an approach to breaking the scaling relationship for propane dehydrogenation, an industrially important reaction, by assembling single atom alloys (SAAs), to achieve simultaneous enhancement of propylene selectivity and propane conversion. We synthesize γ-alumina-supported platinum/copper SAA catalysts by incipient wetness co-impregnation method with a high copper to platinum ratio. Single platinum atoms dispersed on copper nanoparticles dramatically enhance the desorption of surface-bounded propylene and prohibit its further dehydrogenation, resulting in high propylene selectivity (~90%). Unlike previous reported SAA applications at low temperatures (<400 °C), Pt/Cu SAA shows excellent stability of more than 120 h of operation under atmospheric pressure at 520 °C.
Two-dimensional (2D) metallic transition metal dichalcogenides (MTMDs) have recently drawn increasing interest for fundamental studies and potential applications in catalysis, charge density wave (CDW), interconnections, spin-torque devices, as well superconductors. Despite some initial efforts, the thickness-tunable synthesis of atomically thin MTMDs remains a considerable challenge. Here we report controlled synthesis of 2D cobalt telluride (CoTe2) nanosheets with tunable thickness using an atmospheric pressure chemical vapor deposition (APCVD) approach and investigate their thickness-dependent electronic properties. The resulting nanosheets show a well-faceted hexagonal or triangular geometry with a lateral dimension up to ∼200 μm. Systematic studies of growth at varying growth temperatures or flow rates demonstrate that nanosheets thickness is readily tunable from over 30 nm down to 3.1 nm. X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution scanning transmission electron microscope (STEM) studies reveal the obtained CoTe2 nanosheets are high-quality single crystals in the hexagonal 1T phase. Electrical transport studies show the 2D CoTe2 nanosheets display excellent electrical conductivities up to 4.0 × 105 S m–1 and very high breakdown current densities up to 2.1 × 107 A/cm2, both with strong thickness tunability.
Abstract It is of great significance to reveal the detailed mechanism of neighboring effects between monomers, as they could not only affect the intermediate bonding but also change the reaction pathway. This paper describes the electronic effect between neighboring Zn/Co monomers effectively promoting CO 2 electroreduction to CO. Zn and Co atoms coordinated on N doped carbon (ZnCoNC) show a CO faradaic efficiency of 93.2 % at −0.5 V versus RHE during a 30‐hours test. Extended X‐ray absorption fine structure measurements (EXAFS) indicated no direct metal–metal bonding and X‐ray absorption near‐edge structure (XANES) showed the electronic effect between Zn/Co monomers. In situ attenuated total reflection‐infrared spectroscopy (ATR‐IR) and density functional theory (DFT) calculations further revealed that the electronic effect between Zn/Co enhanced the *COOH intermediate bonding on Zn sites and thus promoted CO production. This work could act as a promising way to reveal the mechanism of neighboring monomers and to influence catalysis.
Nanostructures in silicon (Si) induced by phase transformations have been investigated during the past 50 years. Performances of nanostructures are improved compared to that of bulk counterparts. Nevertheless, the confinement and loading conditions are insufficient to machine and fabricate high-performance devices. As a consequence, nanostructures fabricated by nanoscale deformation at loading speeds of m/s have not been demonstrated yet. In this study, grinding or scratching at a speed of 40.2 m/s was performed on a custom-made setup by an especially designed diamond tip (calculated stress under the diamond tip in the order of 5.11 GPa). This leads to a novel approach for the fabrication of nanostructures by nanoscale deformation at loading speeds of m/s. A new deformation-induced nanostructure was observed by transmission electron microscopy (TEM), consisting of an amorphous phase, a new tetragonal phase, slip bands, twinning superlattices, and a single crystal. The formation mechanism of the new phase was elucidated by ab initio simulations at shear stress of about 2.16 GPa. This approach opens a new route for the fabrication of nanostructures by nanoscale deformation at speeds of m/s. Our findings provide new insights for potential applications in transistors, integrated circuits, diodes, solar cells, and energy storage systems.
Abstract The electrochemical N 2 fixation to produce ammonia is attractive but significantly challenging with low yield and poor selectivity. Herein, we first used density function theory calculations to reveal adjacent bi-Ti 3+ pairs formed on anatase TiO 2 as the most active electrocatalytic centers for efficient N 2 lying-down chemisorption and activation. Then, by doping of anatase TiO 2 with Zr 4+ that has similar d -electron configuration and oxide structure but relatively larger ionic size, the adjacent bi-Ti 3+ sites were induced and enriched via a strained effect, which in turn enhanced the formation of oxygen vacancies. The Zr 4+ -doped anatase TiO 2 exhibited excellent electrocatalytic N 2 fixation performances, with an ammonia production rate (8.90 µg·h −1 ·cm −2 ) and a Faradaic efficiency of 17.3% at −0.45 V versus reversible hydrogen electrode under ambient aqueous conditions. Moreover, our work suggests a viewpoint to understand and apply the same-valance dopants in heterogeneous catalysis, which is generally useful but still poorly understood.
Open AccessCCS ChemistryRESEARCH ARTICLE1 Oct 2019Unraveling the Reactivity and Selectivity of Atomically Isolated Metal–Nitrogen Sites Anchored on Porphyrinic Triazine Frameworks for Electroreduction of CO2 Ying Hou, Yuan-Biao Huang, Yu-Lin Liang, Guo-Liang Chai, Jun-Dong Yi, Teng Zhang, Ke-Tao Zang, Jun Luo, Rui Xu, Hua Lin, Su-Yuan Zhang, Hui-Min Wang and Rong Cao Ying Hou State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007 (China) , Yuan-Biao Huang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Yu-Lin Liang University of the Chinese Academy of Sciences, Beijing 100049 (China) Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800 (China) , Guo-Liang Chai State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Jun-Dong Yi State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Teng Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Ke-Tao Zang Center for Electron Microscopy, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials, Tianjin University of Technology, Tianjin, 300384 (China) , Jun Luo Center for Electron Microscopy, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials, Tianjin University of Technology, Tianjin, 300384 (China) , Rui Xu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Hua Lin State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Su-Yuan Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) , Hui-Min Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) and Rong Cao *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 (China) University of the Chinese Academy of Sciences, Beijing 100049 (China) https://doi.org/10.31635/ccschem.019.20190011 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Electroreduction of CO2 (CO2RR) to value-added chemicals offers a promising approach to balance the global carbon emission, but still remains a significant challenge due to high overpotential, low faradaic efficiency, and poor selectivity of electrocatalysts systems. Thus the key point is to develop low-cost, highly efficient, and durable electrocatalysts for CO2RR. To benefit from their exposed active sites and to maximize atomic efficiency, single-metal atom catalysts that usually show high activities are required. Herein, we unravel the trends in the reactivity and selectivity of atomically isolated M–N4 (M = Ni, Cu, Fe, and Co) sites within porous porphyrinic triazine framework (metal single atoms/PTF) for the electroreduction of CO2 to CO. We found that NiSAs/PTF exhibited the highest faradaic efficiency (98%) at a mild potential of −0.8 V versus reversible hydrogen electrode and the highest turnover frequency of 13,462 h−1 for the production CO at an applied potential of −1.2 V. The relations of catalytic performance of CO2 to CO over the different active M–N4 sites were unraveled by the combination of density functional theory calculations and experiments. This work gives an extensive mechanistic understanding of the selectivity of CO2 to CO from the M–N4 sites at an atomic scale, thus it will bring new inspiration for the design of highly efficient CO2RR. Download figure Download PowerPoint Introduction The large-scale use of fossil fuels has led to a substantial increase in atmospheric carbon dioxide concentration, hence causing environmental problems, including global warming and sea-level rise.1–3 Electrochemical reduction of CO2 (CO2RR) to value-added chemicals and fuels using renewable electricity is a promising strategy to balance the global carbon emission.4–6 Among the electroreduction products, CO is considered one of the most valuable industrial feedstocks, which can be used for the catalytic Fischer–Tropsch synthesis to produce hydrocarbon liquid fuels.7,8 Fortunately, compared with other CO2RR processes, the reduction of CO2 to CO [CO2 + 2H++ 2e− → CO + H2O, −0.11 V vs reversible hydrogen electrode (RHE)] needs only two electron/proton transfers,9 which makes it an essentially less-hindered pathway.10 Nevertheless, due to the low energetic efficiency, sluggish electron transfer kinetic, poor selectivity, and low stability of electrode materials significant challenges remain that impede the practical use and technological commercialization result from the inert chemical bond of CO2 (C = O, 806 kJ·mol−1)11 and the accompanied competing hydrogen evolution reaction (HER).12,13 Furthermore, the high cost and scarcity of noble metal catalysts, such as Au- and Ag-based catalysts, limit their large-scale applications.14,15 Therefore, there is an urgent need to develop highly efficient earth-abundant metal-based electrocatalysts specifically for CO2 to CO conversion with long durability to meet the practical application. Single-atom catalysts (SACs) with atomically isolated metal atoms anchored on supports usually show high activity and selectivity toward various catalytic performances because of their maximum atom efficiency, exposed active sites, and tunable coordination microenvironment.16–18 Thus, SACs anchored on conductive materials such as porous carbons and graphene have displayed great potential to enhance their performance in CO2RR.19–22 However, the preparation of stable and highly efficient SACs remains a great challenge because of the high surface energy of their single atoms and easy formation of inactive aggregates.23,24 Moreover, mechanistic understanding of the relationships between the CO2RR reactivity and the structures of SACs is still elusive, which impedes further designing of highly efficient SACs for the CO2RR.25–27 Porous covalent triazine frameworks constructed by 1,3,5-triazine linkers have attracted much attention in gas adsorption and heterogeneous catalysis because of their high specific surface area, high nitrogen contents, and remarkable acid/base stability.28–30 Recently, our group reported that the porphyrin architectures stabilized Fe/Co single atoms within porphyrinic triazine frameworks (PTF), which can behave as accessible active sites for highly efficient electrocatalysis for oxygen reduction reaction and HER.31,32 Herein, a series of atomically isolated M–N4 sites anchored on highly porous porphyrinic triazine framework [metal single atoms (MSAs)/PTF, M = Ni, Cu, Fe, and Co] were prepared and employed as electrocatalysts for CO2RR. Among these single-atom metal elements, Ni material, NiSAs/PTF, was found to have a very high faradaic efficiency (FE), between 94% and 98% for CO generation over a wide range of applied potential (−0.6 to −1.2 V) versus RHE (RHE scale was used as a reference for all applied potential scales). Impressively, NiSAs/PTF reached a high turnover frequency (TOF) of 13,462 h−1 at a mild applied potential of −1.2 V. Density functional theory (DFT) calculations revealed that NiSAs/PTF and FeSAs/PTF have more positive estimated electrochemical limiting potentials than those of CoSAs/PTF and CuSAs/PTF. The rate determining step for NiSAs/PTF, CoSAs/PTF, and CuSAs/PTF is the *COOH intermediate formation, whereas, with FeSAs/PTF, it is the last step of CO desorption that seemed to be rate determining. Furthermore, the CO2RR occurs easier than the HER over the coordinately unsaturated Ni–N4 site in NiSAs/PTF. Results and Discussion The atomically isolated Ni–N4 and Cu–N4 sites anchored by porphyrin moieties within porous porphyrinic triazine framework (denoted as NiSAs/PTF and CuSAs/PTF, respectively) were prepared via a one-pot ionothermal trimerization reaction of the corresponding 5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato]-M (M-TPPCN, M = Ni, Cu), catalyzed by the molten zinc chloride as Lewis acid at 600 °C for 40 h (Figure 1). After washing with diluted hydrochloric acid for NiSAs/PTF or nitric acid for CuSAs/PTF, followed by deionized water and ethanol, black powder samples were obtained after oven drying in vacuum at 120 °C for 12 h. The FeSAs/PTF and CoSAs/PTF materials containing single-atom sites of Fe–N4 and Co–N4, were prepared respectively, according to a previously reported method.31,32 The polymerization reaction and formation of polytriazine networks were confirmed by Fourier-transform infrared analysis (). The disappearance of the carbonitrile stretching band at ca. 2225 cm−1 demonstrated that the conversion of the monomers M-TPPCN (M = Ni, Cu) was completed successfully. Meanwhile, a strong absorption band at 1587 cm−1 assigned to triazine rings was observed, indicating the success of the trimerization reaction.33,34 Figure 1 | Schematic illustration of the fabrication of MSAs/PTF (M = Fe, Co, Ni, Cu). Download figure Download PowerPoint The powder X-ray diffraction (PXRD) patterns of NiSAs/PTF and CuSAs/PTF (Figure 2a) are similar to those of Fe- and CoSAs/PTF,31,32 indicating that all the MSAs/PTF have amorphous structures. A prominent wide peak, centered at ca. 25.6° was apparent, which could be assigned to the amorphous (001) reflection of aromatic sheets.35 In addition, a weak peak at ca. 43.0° corresponding to the appearance of (101) reflection of graphitic carbon, and also, indicated that their graphitization occurred at 600 °C. Notably, there was no evidence of an appearance of diffraction peak relative to metal-based particles or carbides. Strikingly, the results obtained from the inductively coupled plasma atomic emission spectroscopy () revealed that the samples NiSAs/PTF, CuSAs/PTF, FeSAs/PTF, and CoSAs/PTF have high corresponding metal contents of 1.11, 0.82, 2.60, and 0.85 wt%, respectively. Thus, we deduced that these metals may possess an atomic-level distribution and that single-atom metals were successfully embedded in the PTFs. The Raman spectra (Figure 2b) also showed no peak of metal nanoparticles (NPs) in NiSAs/PTF and CuSAs/PTF, as they were similar to the Fe- and CoSAs/PTF samples.31,32 Moreover, these samples have high IG/ID ratios (where IG and ID represent the G-band and D-band intensity, respectively) and a broad weak two-dimensional (2D) peak at ca. 2800 cm−1, suggesting that partial graphitization phenomenon had occurred and graphitic layered structures were presented in the catalysts. The Nyquist plots from the electrochemical impedance spectroscopy (EIS) measurements for the MSAs/PTF (M = Ni, Cu, Co, and Fe) further demonstrated that these materials have small semicircles (Figure 2c and ), and that the partial graphitized structures are beneficial for improving the electron transfer rate between the interfaces of the catalyst and the electrolyte, thus enhancing CO2RR activity. Figure 2 | (a) PXRD patterns, (b) Raman spectra of NiSAs/PTF and CuSAs/PTF, (c) Nyquist plots, and (d) CO2 adsorption–desorption isotherms at 273 K for FeSAs/PTF, CoSAs/PTF, NiSAs/PTF, and CuSAs/PTF. Download figure Download PowerPoint The nitrogen adsorption measurements revealed that all the MSAs/PTF (M = Ni, Cu, Fe, and Co) samples have high N2 adsorption uptake and large Brunauer–Emmett–Teller surface areas in the range of 454–873 m2 g−1 (). Moreover, distinct hysteresis loops were observed at the P/Po range of 0.45–1.0, indicating meso- and/or macropores were formed in all the MSAs/PTF materials, attributed to the porogenic agent zinc chloride.36 The pore size distributions based on the nonlocal DFT method () for the MSAs/PTF family further revealed that their pores are in the range of 0.7–9.9 nm. The hierarchically porous structure could imply that the single-atom metal active sites are highly accessible to the reactive species and electrolytes, thus facilitating mass transportation and improving energy conversion efficiency. The CO2 adsorption isotherms further revealed that all the MSAs/PTF materials have large adsorption capacities, ranging from 52–97 cm3 g−1 at 273 K (Figure 2d), illustrating that there was a strong interaction between these nitrogen-rich MSAs/PTF materials and CO2 molecules, which, in turn, enhanced the electrocatalytic activity of CO2RR. Transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and aberration-corrected HAADF-STEM with subangstrom resolution measurements were further used to confirm the atomic levels of the metal sites. As shown in the TEM (Figure 3a and ) and HAADF-STEM (Figure 3d and ) images of the NiSAs/PTF and CuSAs/PTF, no obvious NPs were observed, a finding which is consistent with their PXRD (Figure 2a) and Raman results (Figure 2b). Notably, the TEM element mapping images (Figure 3c and ) show that Ni and Cu elements are uniformly distributed at atomic-level throughout the architecture of NiSAs/PTF and CuSAs/PTF, respectively. In addition, a large number of bright spots related with the heavier Ni and Cu elements with atomic dispersion could be seen in the aberration-corrected HAADF-STEM images for NiSAs/PTF (Figure 3b) and CuSAs/PTF (), respectively.37–42 These results suggested that single-atom metal sites within NiSAs/PTF and CuSAs/PTF have been fabricated successfully. Figure 3 | (a) TEM images of NiSAs/PTF. (b) Aberration-corrected HAADF-STEM images showing the atomic dispersion of Ni in NiSAs/PTF. HAADF image (c) and corresponding C, N, and Ni EDS elemental mapping. Download figure Download PowerPoint In order to confirm the local chemical environment of single-atom metals, X-ray photoelectron spectroscopy (XPS) was first employed. As shown in , the N 1s signal band of all the MSAs/PTF materials split into three-type configurations, originating from triazinic N (398.5 eV), porphyrinic-like metal-coordinated M–Nx (399.7 eV), and graphitic N (401.1 eV) species.43,44 The high-resolution metal 2p XPS spectra of MSAs/PTF (M = Ni, Cu) are displayed in . As shown in the fitted Ni 2p spectrum, two dominated peaks were centered at 855.4 eV (2p3/2) and 872.5 eV (2p1/2), similar to those of Ni2+ in Ni-TPPCN (855.3 and 872.6 eV, ), but higher than the Ni(0) metal (854.3 and 871.9 eV).45 The Fe 2p3/2 and Co 2p3/2 are 709.6 and 780.6 eV, which are near to those of Fe2+ and Co2+, respectively.31,32 As for CuSAs/PTF, the dominating signals at binding energies of 935.8 eV (2p3/2) and 955.9 eV (2p1/2) are originated from the positive Cu–N bond.46,47 These results revealed that all the metals are significantly positive, implying that no metal NPs were detected,48 which are in good agreement with PXRD (Figure 2a) and the TEM results (Figure 3a). Synchrotron-based X-ray absorption spectroscopy was further performed to confirm the coordination environment of these single-atom metals in MSAs/PTF. As shown in the X-ray absorption near edge structure (XANES) of the Ni K-edge in NiSAs/PTF (Figure 4a), both of the pre-edge and absorption edge energy position of NiSAs/PTF are located in between Ni foil and NiO, indicating that the Ni single atoms carried positive charges between metallic Ni(0) and the fully oxidized states Ni(II),49 consistent with the X-ray photoelectron spectroscopy (XPS) () and electron paramagnetic resonance results ().50 Moreover, a weak pre-edge peak (A position) at ca. 8330 eV appeared in NiSAs/PTF, and the precursor Ni-TPPCN was attributed to the dipole-forbidden but quadrupole-allowed transition (1s → 3d), which suggested that hybridization of 3d and 4p orbitals of the Ni central atoms had occurred.9 Additionally, a strong peak at ca. 8336 eV (peak B) was observed in both of NiSAs/PTF and the precursor Ni-TPPCN, which was due to the 1s → 4pz transition. These transitions serve as fingerprints for square-planar configuration with D4h centrosymmetry, and confirm that the NiSAs/PTF contains Ni–N4 porphyrin-like moieties originating from the Ni-TPPCN monomer. Furthermore, the relatively larger intensity ratio for peaks C and D than Ni-TPPCN, in which the two peaks are associated with the 1s → 4px,y transitions and multiple scattering processes, respectively, are expected to show good electrocatalytic performances. In addition, the Ni L-edge XANES spectrum () of NiSAs/PTF displayed a distinctive single peak related to 3d-electrons at the L3-edge, which is similar to Ni rather than Ni-based oxides.51 The N K-edge of the XANES spectrum of NiSAs/PTF () shows a resonance peak of the π*-transition pyrrolic-N that bonded to Ni atoms to form Ni–Nx species. The formation of Ni–Nx was confirmed further by the observation of weak π* peak assigned to sp2-hybridized carbon of C–N–Ni in C K-edge of NiSAs/PTF (). Figure 4 | (a) Normalized Ni K-edge XANES spectra of the Ni-TPPCN, NiSAs/PTFs, NiO, and Ni foil. (b) Normalized Cu K-edge XANES spectra of the CuSAs/PTFs, Cu-TPPCN, Cu2O, CuO, and Cu foil. (c) Fourier-transform EXAFS spectra of Ni-TPPCN, NiSAs/PTFs, and Ni foil. (d) Fourier-transform EXAFS spectra of CuSAs/PTFs, Cu-TPPCN, and Cu foil. The corresponding EXAFS fitting curves of (e) NiSAs/PTF-600 and (f) CuSAs/PTF-600. Download figure Download PowerPoint For the XANES spectrum of CuSAs/PTF, its absorption edge energy was positioned between Cu foil and Cu(II)-TPPCN (Figure 4b), suggesting partial Cu atoms in CuSAs/PTF had been reduced to Cu(I). A similar phenomenon was also found in FeSAs/PTF, in which the Fe atoms have been reduced to Fe(II) from Fe(III) of Fe-TPPCN at high synthesis temperature.31 Moreover, the pre-edge peak at 8985 eV was similar to the monomer Cu-TPPCN, which is associated with 1s → 4pz shakedown transition feature for a square-planar configuration with high D4h symmetry structure. These results indicated that Cu–N4 porphyrin-like structure predominates in CuSAs/PTF, which is similar to that of NiSAs/PTF, FeSAs/PTF, and CoSAs/PTF.31,32 Concurrently, in the Fourier-transform, k3-weighted χ(k) function of the extended X-ray absorption fine structure (EXAFS) spectra for NiSAs/PTF (Figure 4c) exhibited a strong major peak at 1.44 Å associated with Ni–N/O, which is similar to the monomer Ni-TPPCN (1.47 Å), suggesting that the Ni atoms coordinated with N elements. Furthermore, no obvious peak at the positions of Ni foil was observed for NiSAs/PTF sample, indicating that no Ni–Ni bond was presented in NiSAs/PTF. These results suggested that no Ni NP was formed, and all Ni sites were in atomically isolated form, which is also consistent with the PXRD and aberration-corrected HAADF-STEM results. In addition, the fitting results from EXAFS analysis in Figure 4e showed that the coordination number of Ni species in NiSAs/PTF is ca. 4.0 (), suggesting that the Ni porphyrin-like structure was retained, which agreed well with the XANES results (Figure 4a). The same analysis for CuSAs/PTF (Figure 4d) also revealed that Cu NPs were absent, and the single copper atoms adopted identical coordination configuration to NiSAs/PTF with Cu–N4 structure (Figure 4f and ), which was similar to the Fe–N4 and Co–N4 in FeSAs/PTF and CoSAs/PTF.31,32 Inspired by the unique structures of these MSAs/PTF (M = Ni, Cu, Fe, and Co) materials, their activities relative to CO2RR were then evaluated by observing their performance in a gas-tight H-type electrochemical cell, separated by a Nafion-117 proton exchange membrane. The comparison of the CO2RR activities of these materials is summarized in Figure 5. As shown in the linear sweep voltammetry (LSV) curves in Figure 5a, it was apparent that at CO2-saturated (1 atm), 0.5 M KHCO3, all the MSAs/PTF materials exhibited much more positive onset potentials and higher current densities than those in N2-saturated, 0.5 M KHCO3, suggesting that their activities might have originated from CO2 reduction. CoSAs/PTF exhibited a small onset potential of −0.109 V, which was much more positive than that of FeSAs/PTF (−0.112 V), NiSAs/PTF (−0.332 V), and CuSAs/PTF (−0.436 V) (Figure 5a). Figure 5 | (a) LSV curve in the CO2-saturated and N2-saturated 0.5 M KHCO3 electrolyte at a scan rate of 10 mV−1. (b) CO current density of MSAs/PTF at various potentials. (c) FECO at various applied potentials. (d) FEH2 at various applied potentials. Download figure Download PowerPoint The reduced products and FE for these catalysts at different potentials within 0.5 h are displayed in Figure 5b–d. Notably, no liquid product was found in off-line 1H NMR spectroscopy and ion chromatography, whereas the main products CO and H2 were detected by gas chromatography in the applied potential range between −0.6 and −1.2 V versus RHE. Only a trace of methane was found when using FeSAs/PTF electrode (). As shown in Figure 5b, NiSAs/PTF has the largest CO current density in a wide potential range, compared with the other three single-atom catalysts. Particularly, as shown in Figure 5c, NiSAs/PTF showed best activity for CO2 to CO with high FE over 94% in the wide potential ranges from −0.6 to −1.2 V and achieved a maximum FE of 98% at −0.80 V with a high CO partial current density of 16.28 mA cm−2 (Figure 5b). As for FeSAs/PTF, it exhibited the maximum FE (67%) for CO with a current density of 4.61 mA cm−2 at an applied potential of −0.60 V. Particularly, the FECO decreased (Figure 5c), but the FEH2 increased (Figure 5d) with an increasing applied potential. Unlike the NiSAs/PTF and FeSAs/PTF catalysts, H2 was the major product for CuSAs/PTF and CoSAs/PTF (Figure 5c,d), indicating that HER is the dominant process during the CO2–CO conversion over the applied potential range. Accordingly, the difference presented by the CO2RR performance of the four MSAs/PTFs indicated that the FE of CO is strongly dependent on the nature of the active metal sites. Further, we calculated CO TOF per metal site. As shown in (see "Electrochemical measurements" part,9), when a potential at −0.8 V was applied, the CO TOF followed the order of NiSAs/PTF (5571 h−1) >CoSAs/PTF (1547 h−1) > FeSAs/PTF (853 h−1) >CuSAs/PTF (163 h−1). Furthermore, the maximum TOF of 13,462 h−1 was achieved over NiSAs/PTF at −1.2 V, which outperforms most of the reported catalysts ().3,5,6,12,45 By pulling our above discussion together regarding CO2RR behaviors, we inferred that Ni is the most efficient metal in MSAs/PTF for the reduction of CO2 to CO. Noteworthy, in N2-saturated 0.5 M KHCO3, no CO and other carbon-based products were detected for NiSAs/PTF under identical reaction conditions, suggesting that the CO was indeed derived from CO2. In order to confirm whether single-atom Ni was actually the active site for CO2RR, a series of control experiments were conducted. The Ni-free catalysts were also fabricated by de novo synthesis from 5,10,15,20-tetraki(4-cyanophenyl)porphyrin (TPPCN) at 600 °C (denoted as PTF) or removal of Ni species from NiSAs/PTF, using trifluoromethanesulfonic (TFMS) acid as etching reagent, [labeled as (NiSAs)-PTF]. The Ni NPs encapsulated on PTF-600 were also prepared (), which is denoted as NiNPs/PTF. It is worth mentioning that the pure carbon paper showed high negative onset potential, and almost no CO2RR occurred (Figure 6a), which excluded that the CO originated from carbon paper. As shown the LSV results in Figure 6a, NiSAs/PTF displayed an onset potential of −0.332 V, which is much more positive than those of Ni-free samples (PTF and NiSAs/PTF) and NiNPs/PTF, demonstrating the superiority of single-atom Ni active sites in driving the CO2RR. Figure 6 | (a) LSV curves of NiSAs/PTF, (NiSAs)/PTF, NiNPs/PTF, PTF, and pure carbon paper. (b) FE of PTF without metal loading, NiNPs/PTF, and (NiSAs)/PTF. (c) LSV curves of NiSAs/PTF before and after the addition of 0.2 M NaSCN in 0.5 M KHCO3. (d) Stability of NiSAs/PTF at a potential of −0.9 V versus RHE for 10 h. Download figure Download PowerPoint Moreover, by comparing the NiSAs/PTF achievement of maximum FECO of 98% at −0.8 V and at a large current density of 14.4–mA cm−2 with FECO of PTF and NiNPs/PTF, both only exhibited maximum of 44% and 4.36%, respectively (Figure 6b), demonstrating a significant suppression of HER ability for the atomic isolated Ni−N4 active sites. To further verify the Ni atom role, a poisoning experiment was conducted, whereby, the atomic isolated Ni sites were poisoned by reactive sulfur cyanide (SCN−) ion, in which a distinct depression of catalytic activity for NiSAs/PTF was observed (Figure 6c), which was evidenced by a much more negative Eonset (−0.9 V). This result can be ascribed to that Ni–N4 atoms being blocked by SCN− in the two axial positions, thus confirming the active site role of the Ni−N4 structure. These observations indicated the high CO2RR activity of NiSAs/PTF originated from Ni–N4 active sites. In addition, the NiSAs/PTF exhibited long-term stability of 10 h of operation at −0.9 V, during which no obvious decay in FECO and current density was detected (Figure 6d), demonstrating a good electrochemical stability of the Ni–N4 active sites. The TEM image for NiSAs/PTF after catalysis showed no appreciable Ni NP, demonstrating that the Ni–N4 structure remained intact (). Theoretical simulations were employed to investigate the CO2 to CO process further to understand the mechanisms. The simulation model of MSAs/PTF (M = Fe, Co, Ni, and Cu) is shown in Figure 7a. The metal sites selected are putative active sites. The calculated CO2RR free energy variations for MSAs/PTF are shown in Figure 7b–d and . During the CO2RR process, *COOH, *CO intermediates are formed, and the corresponding electrochemical limiting potentials can be obtained by the free energy variations calculated by first-principles simulations, as described in the and previous articles.52,53 From Figure 7 and , we can see the calculated electrochemical limiting potentials for NiSAs/PTF, FeSAs/PTF, CoSAs/PTF, and CuSAs/PTF are −0.96, −0.35, −2.28, and −3.03 V versus RHE, respectively. The electrochemical limiting potentials for CoSAs/PTF and CuSAs/PTF are too negative for CO2RR process. The rate determining step for NiSAs/PTF, CoSAs/PTF, and CuSAs/PTF is the first electron step for forming *COOH intermediate, whereas, with regards to FeSAs/PTF, it is the last step of CO desorption. Although FeSAs/PTF showed a relatively reasonable electrochemical limiting potential of −0.35 V, the CO desorption barrier was as high as 1.23 eV. Note here that the HER overpotential is only 0.26 V for FeSAs/PTF, as shown in . Thus the FeSAs/PTF should be more favorable for HER, compared with CO2RR. For NiSAs/PTF, the electrochemical limiting potential is −0.96 V, and the overpotential is 0.85 V for the process of converting CO2 to CO. However, the HER overpotential for NiSAs/PTF, is as high as 1.30 eV. These results indicate that the NiSAs/PTF should show high selectivity for CO rather than H2 during the electrochemical reduction process, which was in agreement with the experimental results, revealing that, NiSAs/PTF catalyst exhibited high FE (96%) at a mild potential of −0.9 V. Figure 7 | (a) The atomic structure of MSAs/PTF (M = Fe, Co, Ni, and Cu). The pink, white, gray, and blue spheres denote M, H, C, and N atoms, respectively. The free energy variations for (b) NiSAs/PTF, (c) FeSAs/PTF, and (d) CoSAs/PTF during the CO2RR process under
Supported platinum-group metal (PGM) catalysts are widely used in many important industrial processes. Metal-support interaction is of great importance in tailoring their catalytic performance. Here, we report the first example of oxidative strong metal-support interactions (OMSIs) between PGM and hydroxyapatite (HAP) which can be extended to PGM and ZnO. It occurred under high-temperature oxidation conditions accompanied by the encapsulation of PGM by HAP and electron transfer between PGM and HAP. With this OMSI, the aggregation and leaching of PGMs were significantly inhibited, resulting in an excellent catalytic stability and much improved reusability of supported Pt and Pd catalysts, respectively. This is the first time to find that PGMs can manifest OMSI which benefits the stabilization of PGM catalysts under oxidative reaction conditions. This new type of SMSI not only contributed to a deeper understanding of SMSI but also provided a new way to develop new stable PGM catalysts.
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