Local Coordination and Reactivity of a Pt Single-Atom Catalyst as Probed by Spectroelectrochemical and Computational Approaches
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-12-30 |
| Journal | CCS Chemistry |
| Authors | Kun Jiang, Xian‐Yin Ma, Seoin Back, Jiajun Zhao, Fangling Jiang |
| Institutions | Sogang University, Shanghai Institute of Ceramics |
| Citations | 34 |
Abstract
Section titled “Abstract”Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2021Local Coordination and Reactivity of a Pt Single-Atom Catalyst as Probed by Spectroelectrochemical and Computational Approaches Kun Jiang, Xian-Yin Ma, Seoin Back, Jiajun Zhao, Fangling Jiang, Xianxian Qin, Junliang Zhang and Wen-Bin Cai Kun Jiang Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Fuel Cells, Interdisciplinary Research Center, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Xian-Yin Ma Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200438 , Seoin Back Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 04107 , Jiajun Zhao Institute of Fuel Cells, Interdisciplinary Research Center, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Fangling Jiang State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899 , Xianxian Qin Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200438 , Junliang Zhang Institute of Fuel Cells, Interdisciplinary Research Center, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240 and Wen-Bin Cai Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200438 https://doi.org/10.31635/ccschem.020.202000667 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The interaction between isolated transition-metal atoms and neighboring dopants in single-atom catalysts (SACs) plays a key role in adsorption strength tuning and catalytic performance engineering. Clarifying the local coordination structures of SACs is therefore of great importance and yet very challenging at the atomic level. Here, we employ a SAC with isolated Pt species anchored on nitrogen-doped carbon as a prototype and investigate the local coordination environment around Pt sites with the CO probe molecule by combined electrochemical infrared (IR) spectroscopy and density functional theory calculations. Two types of Pt coordination structures are clearly revealed, involving a Pt-C moiety with weak CO binding and a pyrrolic Pt-N-C moiety with strong CO binding, highlighting the Pt local coordination structure-dependent CO binding strength. This inventory then allows a comparative COad electrooxidation mechanism study on an identified pyrrolic Pt-N-C moiety of the Pt SAC and a bulk Pt surface toward the feedback loop between theory and experiment. A much higher coupling barrier for COad and OHad is noted on the simulated Pt-N-C site than that on Pt(111), making the Langmuir-Hinshelwood pathway kinetically unfavorable on the Pt SAC. In contrast, a lower theoretical limiting potential is predicted for CO oxidation on pyrrolic Pt-N-C with free H2O in favor of the Eley-Rideal pathway and qualitatively agrees with the experimental kinetics. The present methodology may promote the understanding of the SAC structure-activity relationship and the discovery of novel SACs. Download figure Download PowerPoint Introduction Transition-metal single-atom catalysts (TM-SACs) are well-known for their maximum atom efficiency, superb specific activity, and distinctive catalytic performance, as well as their low-coordinated nature for surface chemistry studies.1-3 Actually, the past 10 years have seen a rapid expansion in the number of publications regarding the synthesis, characterization, and application of TM-SACs in the vast realm ranging from thermal catalysis to electrocatalysis. Due to the high surface free energy of these isolated TM atoms, strong metal-support interactions are needed for anchoring or dispersing these TM atoms individually onto a support to prevent aggregation.4-7 Metal oxide supports like FeOx8 and CeO29 are usually employed in thermal catalysis, and heteroatom-doped carbon supports are widely used in electrocatalysis because of their large surface area, high electron conductivity, and chemically inert nature.10-12 Moreover, due to the abundant defects in a carbon support11,13 that is used to host these isolated metal species, a variety of tuning structures exist, for both metal atom centers and adjacent coordinative dopants, toward desired catalytic pathway manipulation. For example, Fe-N-C moieties have been intensively investigated as an alternative to Pt for oxygen reduction reaction (ORR), in which pyridinic (Pyd) and quaternary Fe-Nx-C groups are identified as the surface active sites for the 4e− pathway in acid and alkaline media.14,15 In contrast, the recent report demonstrates that by switching the neighboring dopant N to O, the 2e− pathway is dominant for ORR on the Fe-O-C moiety with the H2O2 selectivity >95% in both neutral and alkaline media owing to the change of active sites from Fe in the Fe-N-C moiety to C in the Fe-O-C moiety.16 Resolving the specific atomic configuration and exploring the delicate coordination environment of SACs is quite valuable yet very challenging. So far, aberration-corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption fine structure (XAFS) spectroscopy are the two most widely used techniques to probe the structural information of SACs.3,17,18 The former directly captures images at atomic resolution within a selected area, typically at a scale of a few nanometers, to demonstrate the transition-metal dispersion. The latter provides averaged information of the electronic structure from near-edge and the coordination structure from extended-edge over a bulk TM-SAC sample. Although XAFS could be operated at operando electrochemical conditions to track structural evolution during reactions, it is hard to distinguish the fine metal-metalloid coordination like M-C/N/O, due to their similarity in scattering paths and scattering intensities.16,19 As an alternative, vibrational spectroscopy with selected probe molecules20-22 could be a convenient and sensitive tool to differentiate active motifs under operando conditions as well as to probe the dynamic interfacial chemistry during (electro-)catalysis on SACs. On the one hand, the vibrational frequency and intensity of a surface probe molecule like CO are closely related to the coordination nature of surface metal sites, which is associated with their dispersion and electronic structures.23 On the other hand, the kinetics study of such an adsorbate oxidation removal could in turn deepen the understanding of the local coordination environment in a SAC. Actually, the reactivity of the surface probe on the isolated metal atoms is largely dependent on the surrounding coordination environment, thus serving as a more relevant indicator of catalytic behavior.20 In recent thermocatalysis reports, the CO adsorption and oxidation have been applied to investigate the Pt-O-Cu24 and Pt-O-Ce25 SAC moieties at the solid and gas interface. Unfortunately, no such attempt has been fulfilled yet under electrochemical conditions at the SAC and electrolyte interface, given the complex coordination environment of heteroatom-doped carbon support as well as the limited surface sensitivity of the conventional reflection absorption infrared (IR) technique. In this work, we employ isolated Pt atoms anchored on nitrogen-doped carbon nanotubes (Pt-CNT) as a model SAC and demonstrate clearly the presence of Pt-C and Pt-N-C coordination moieties in the SAC by combined spectroelectrochemical and computational approaches involving the surface CO probe. During COad electrooxidation, the dynamic interfacial chemistry is probed by in situ attenuated total reflection IR (ATR-IR) at the molecular level and the reaction energetics is interpreted from density functional theory (DFT) calculations. In contrast to the predominant Langmuir-Hinshelwood (L-H) mechanism for COad oxidation on bulk Pt surface, the Eley-Rideal (E-R) mechanism is more favorable on the pinpointed pyrrolic (Pyr) Pt-N-C moiety. Experimental and Computational Methods Material synthesis The Pt-CNT catalyst was prepared by the impregnation and reduction method.16 Briefly, a 7.2 mM Pt(II) stock solution was first prepared by dissolving K2PtCl4 salt (AR; Sinopharm, Shanghai, China) into Millipore water (18.2 MΩ·cm). A carbon suspension was prepared by mixing 40 mg multiwalled carbon nanotubes (Carbon Nanotubes Plus GCM389, used as received) with 30 mL of Millipore water, and tip sonicated (JY92-IIN; Scientz, Ningbo, Zhejiang Province, China) for 30 min till a homogeneous dispersion was accomplished. Then 400 μL of Pt2+ solution, given a raw atomic ratio of Pt:C to be ∼0.08 atom %, was dropwise added into CNT solution under vigorous stirring, followed by quick freezing in liquid nitrogen. The as-prepared K2PtCl4/CNT powder was mixed with urea at a mass ratio of 1:10, then heated in a tube furnace to 800 °C under a gas flow of 100 sccm Ar (99.999%; Wetry-SH, Shanghai, China) for 60 min, and kept at the same temperature for another 60 min before cooling to room temperature. N-CNT was prepared by the same thermo annealing method without the K2PtCl4 preadsorption and freeze-drying steps. Material characterization The morphology of Pt-CNT catalyst was characterized by bright-field transmission electron microscopy (TEM) using a JEM-2010 microscope (JEOL, Akishima Shi, Japan). Dark-field STEM characterization was carried out using a JEM ARM200F aberration-corrected transmission electron microscope (JEOL) under 200 kV. Drift correction was applied during acquisition. X-ray photoelectron spectroscopy (XPS) was obtained with a AXIS Ultra DLD spectrometer (Kratos Analytical, Trafford Park, Manchester, UK), using monochromatic Al Kα radiation (1486.6 eV) and a low energy flood gun as neutralizer. The binding energies were calibrated by referencing the C 1s peak at 284.8 eV. The Casa XPS program was employed for surface componential content analysis. The XAFS data at Pt L3-edge (11,564 eV) of the Pt-CNT sample and PtO2 and Pt foil references were collected at room temperature at the beamline BL14W1 of Shanghai Synchrotron Radiation Facility (SSRF; XAFS station) using a Si(311) double-crystal monochromator. The synchrotron radiation was operated at an energy of 3.5 GeV and a current between 150 and 210 mA. The obtained data were processed and fitted using the IFEFFIT-based Athena and Artemis programs with the theoretical standards calculated using FEFF8.26 The photon energy was calibrated using the first inflection point of the Pt L3-edge in Pt metal foil. The K range was 3.1-13.8 Å−1 for Pt foil and PtO2, and 3.1-10.6 Å−1 for Pt-CNT sample. The fitting analysis of Pt-CNT extended XAFS (EXAFS) was carried out with either Pt-C, Pt-N, or Pt-C and Pt-N paths using an iterative least-squares technique. Appropriate X-ray absorption near-edge structure (XANES) references (Pt foil and PtO2) were used for linear combination fitting (LCF) of the Pt-CNT sample. Spectroelectrochemical measurements A CH Instruments 440 A workstation (Shanghai, China) was employed to record the electrochemical response. Certain amounts of HClO4 (ACS Grade, 70%; Sigma-Aldrich, Darmstadt, Germany) were dissolved in Millipore water to prepare the 0.1 M electrolyte. All electrolytes were deaerated with Ar prior to and throughout the measurements. In situ electrochemical ATR-IR measurements were run on a Pt-CNT catalyst layer- or Pt overlayer-covered Au film on a hemicylindrical Si prism using a Nicolet iS50 spectrometer (Thermo Fisher Scientific, Carlsbad, California, USA) equipped with an MCT detector at a spectral resolution of 4 cm−1 with an p-polarized IR radiation at an incidence angle of ca. 70°. A graphite rod (99.995%; Sigma-Aldrich) was used as the counter electrode and a CH Instruments saturated calomel electrode (SCE) as the reference electrode. All potentials measured against SCE were converted to the RHE scale using the relationship E (vs RHE) = E (vs SCE) + 0.241 V + 0.0591 × pH, where the pH value of 0.1 M HClO4 was determined to be ∼1.0. All the spectra are shown in the absorbance unit as −log(I/I0), where I and I0 represent the intensities of the reflected radiation of the sample and reference spectra, respectively. To prepare the Pt-CNT cast working electrode, 2 mg of as-prepared Pt-CNT catalyst was mixed with 1 mL of ethanol and 10 μL of Nafion 117 solution (5%; Sigma-Aldrich), and sonicated for 30 min to get a homogeneous catalyst ink. Approximately 50 μL of catalyst ink was further diluted into 1 mL of ethanol and air-brushed onto Au/Si within an area of ∼0.64 cm2. Electrochemical deposition was used to prepare the Pt film working electrode on Au/Si, by cycling the potential from 0.45 to −0.2 V versus SCE for 15 cycles in 0.1 M HClO4 + 10 mM H2PtCl6.27 Other ATR-IR experimental details regarding the electroless deposition of the Au film and the spectral cell setup can be found elsewhere.28 Computational details We performed DFT calculations using the Vienna Ab initio Simulation Package (VASP)29,30 with BEEF-vdW exchange-correlation functional31 and projector augmented wave (PAW) method.32 To calculate energetics and vibrational frequencies, we performed tight geometry optimizations with energy and force criteria for self-consistent iteration and ionic relaxation set to 10−8 eV and 0.01 eV/Å, respectively. All plane waves with a kinetic energy smaller than 500 eV are included. Using these computational settings, we first optimized bulk structures of graphene and Pt, which resulted in C-C distance of 1.423 Å and Pt lattice parameter of 3.993 Å. We then constructed an (8 × 8) supercell of graphene consisting of 72 carbon atoms and modeled various single Pt atom-incorporated systems, as shown in the Supporting Information. For these systems, we used Gamma point (single k-point) sampling. For the bulk Pt system, we modeled a (3 × 3) Pt (111) surface with four layers, where the bottom two layers were fixed during all geometry optimizations with (3 × 3 × 1) k-points. In all cases, we added a sufficient vacuum (∼15 Å) to avoid imaginary interactions between repeating atomic structures in the z-direction. The vibrational frequencies were calculated by a finite difference of the Hessian matrix as implemented in atomic simulation environment (ASE),33 where the adsorbed CO molecule was displaced by 0.005 Å in x-, y-, and z-directions. The energies of gas-phase CO2, HCOOH, H2, and adsorbed COOH were corrected by +0.41, +0.20, +0.09, and +0.20 eV, respectively, to account for inaccurate descriptions of the C=O double bond and H-H bond in BEEF-vdW functional.34,35 The energies of COOH, CO, and OH* were further corrected by adding solvation corrections.36 To construct free-energy diagrams of CO electrooxidation, we converted electronic energies into free energies by adding free-energy corrections for adsorbates (COOH*, CO*, and OH*) and gaseous molecules (CO, H2, CO2, and H2O). The free-energy correction values are summarized in Supporting Information Table S1. The free energies of the potential-dependent reaction steps involving a proton-electron pair (H+ + e−) were treated using the computational hydrogen electrode (CHE) method.37 The effect of the electrode potential was applied by shifting the electron free energy by −eUelec, where e and Uelec are the elementary charge of electron and the electrode potential, respectively. For example, for the COOH* → * + CO2 + (H+ + e−) reaction, the free energy at the potential Uelec was calculated as ΔG (U = Uelec) = ΔG (U = 0) − eUelec. Thus, for CO electrooxidation, the largest Uelec that makes all electrochemical reaction steps energetically downhill is defined as the “theoretical onset potential” or “limiting potential (UL),” which corresponds to the experimental onset potential to achieve the arbitrarily set current density. Results and Discussion To prepare atomically dispersed Pt SAC, diluted K2PtCl4 stock solution was first dispersed with commercial surface-functionalized CNTs in Millipore water. Due to the large surface area and the carboxylate functional groups, the CNTs possess a high adsorption capacity to Pt cations in aqueous solution. The mixture was freeze-dried overnight and the obtained powder was thermo annealed with urea as the N source under Ar flow at 800 °C. No metal nanoparticles or clusters were observed in the bright-field TEM image (Figure 1a and Supporting Information Figure S1), while isolated Pt atoms with high z-contrast were found to be homogeneous over the CNTs, as shown in the AC-STEM image of Figure 1b. Figure 1 | Physical characterizations of Pt-CNT catalyst. (a) High-resolution TEM image, (b) AC-STEM image, (c) Pt L3-edge EXAFS spectra in the R space, and (d) corresponding XANES spectrum of Pt-CNT in comparison with Pt foil and PtO2 references. (e) Simulated charge density distribution of the Pt single-atom confined in graphene vacancies with or without neighboring N dopant. Blue and yellow represent electron depletion and accumulation, respectively. Download figure Download PowerPoint Ex situ X-ray radiated characterizations were carried out to study the average dispersion information and the electronic structure of Pt single-atom sites. Figure 1c shows the k2-weighted Fourier-transform EXAFS of Pt-CNT at the Pt L3-edge, where bulk Pt foil and PtO2 are plotted as model references. The major feature of the Pt-CNT SAC in the R space locates at 1.84 Å, which is largely different from the Pt-Pt coordination at Å or the bond at Å thus the isolated distribution of Pt single atoms on the CNT carbon vacancies with or without neighboring dopant could atomic Pt this scattering feature could be to either Pt-C or Pt-N in the first EXAFS in Supporting Information Figure The XPS and Pt of Pt-CNT are plotted in Supporting Information Figure The Pt atomic was determined to be atom in with from In the Pt binding energy at eV for which was higher than eV of Pt thus a Pt electronic structure information can be by for which the intensity of the Pt L3-edge peak at eV to the level of in Pt Figure shows the XANES spectrum of where the peak between and A further analysis performed on the Pt-CNT XANES spectrum and in Supporting Information Figure a of which from the strong Pt-C or Pt-N that from the metal centers to the neighboring C and N sites (Figure To further the local coordination environment of Pt atoms, an in situ ATR-IR study of the CO adsorption on Pt-CNT was carried out as a model due to the nature of the frequency on metal in 0.1 M HClO4 were prior to the spectroelectrochemical measurements. hydrogen evolution was seen on Pt-CNT V versus RHE without conventional Pt Supporting Information Figure with the dominant presence of isolated Pt CO was the electrolyte at a fixed potential of V to CO The ATR-IR spectra are shown in Figure consisting of COad at and respectively. of these from the CO which is most for the latter two during the 1 Figure the spectra of COad on Pt, and Au where ATR-IR spectra on can be found in Supporting Information The peak in Figure can thus be to on Au on Pt sites or large Pt were with the isolated nature of Pt atoms in the Moreover, the frequencies of the on the Pt-CNT are of CO the of interactions from neighboring COad the atomic dispersion of isolated Pt sites. Figure 2 | (a) ATR-IR spectra during CO adsorption at V RHE on (b) the comparison of COad on bulk Pt film electrode, and bulk Au at V in 0.1 M HClO4 (c) of ATR-IR spectra for Pt-CNT measured during Ar and (d) simulated binding free energy and vibrational frequency of on the most Download figure Download PowerPoint 1 CO an Ar flow was into the electrolyte to dissolved During the Ar the cm−1 peak largely the same while the cm−1 peak over (Figure the of two different CO binding sites on the To the IR analysis of the CO binding over various Pt coordination moieties ranging from single carbon to four carbon sites was carried out using DFT calculations Supporting Information Figure and Table We the energy of Pt single-atom in carbon with or without neighboring of and species, and the most moieties are in Figure for both Pt-C and Pt-N-C coordination The model surface was employed as a with a simulated of eV and a calculated of cm−1 without a which calculated and the experimental value of the metal or is a widely used model moiety in SAC with or Fe it was noted that the carbon double anchored configuration was energetically favorable for CO binding with from the which largely to the observed IR A much CO adsorption strength was on as with Pt-C with a of simulated frequency on the former Pt-N-C In contrast, CO adsorption with a was observed for Pt-C coordination and both the sensitive ATR-IR characterization of COad and the theoretical the cm−1 the weak CO adsorption at the Pt-C coordination sites and the cm−1 the strong CO adsorption at the Pt-N-C coordination sites. In other the coordination structures around Pt atoms of a SAC can be by electrochemical ATR-IR To further investigate the effect of coordination structure on catalytic activity, we COad electrooxidation on the identified Pt-N-C moiety of the Pt-CNT versus the bulk Pt and of IR spectra during the potential on the Pt-CNT and the Pt respectively. The frequency of from to cm−1 on Pt-CNT during the potential from to that of on bulk Pt from to cm−1 at V and then to cm−1 at more potential the electrochemical effect is for the observed on both Pt-CNT and bulk Pt, which is to the surface and directly related to the applied COad oxidation the frequency for bulk Pt is associated with the largely interaction from COad that the In contrast, the interaction is for isolated Pt sites, the frequency with potential due to the On the bulk Pt surface, COad oxidation at ca. V as from both the onset potential in Figure and the of the intensity in Figure In contrast, the oxidation removal of on the Pt-CNT at V with a oxidation current due to the low Pt As noted in Figure and Supporting Information Table the CO binding strength on the Pt-N-C moiety than on the bulk Pt surface at for this CO oxidation kinetics. To the different COad oxidation DFT calculations were carried out to study the reaction energetics from the two COad electrooxidation Figure 3 | (a) of ATR-IR spectra of COad electrooxidation on Pt-CNT and (b) bulk Pt film electrode in 0.1 M HClO4 at a of 10 with (c) the corresponding linear and (d) selected potential-dependent intensities of Download figure Download PowerPoint In the mechanism is widely for electrooxidation on Pt sites with neighboring OH* species from adsorption of C * + * → C * → * + CO 2 + + + e − mechanism on isolated Pt sites, the mechanism is in which H2O molecule directly with the adsorbed to COOH* on an isolated Pt C * + 2 → C * + + + e − → * + CO 2 + 2 + + 2 e − E R mechanism where * the active site (Figure diagrams of these two are plotted in and for and the respectively, with free-energy corrections summarized in Supporting Information and The limiting potentials are as and the for coupling are as We noted that eV barrier kinetics at room The pathway was found to on the surface with a low coupling barrier of eV and a theoretical onset potential of with the This is in with that neighboring OH* species from H2O could largely in COad on the Pt Figure 4 | (a) of COad electrooxidation steps and corresponding free-energy diagrams on (b) Pt (111) surface and (c) moiety as strong binding site either the mechanism or mechanism without the applied potential, = configuration of + OH*) and relevant energy barrier of the coupling to COOH* in are shown as the theoretical limiting potential that makes all electrochemical reaction steps energetically downhill in Download figure Download