Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-04-07 |
| Journal | CCS Chemistry |
| Authors | Max J. Hülsey, Sambath Baskaran, Shipeng Ding, Sikai Wang, Hiroyuki Asakura |
| Institutions | National University of Singapore, Tsinghua University |
| Citations | 38 |
Abstract
Section titled “Abstract”Open AccessCCS ChemistryRESEARCH ARTICLE3 Oct 2022Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation Max J. Hülsey, Sambath Baskaran, Shipeng Ding, Sikai Wang, Hiroyuki Asakura, Shinya Furukawa, Shibo Xi, Qi Yu, Cong-Qiao Xu, Jun Li and Ning Yan Max J. Hülsey Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 , Sambath Baskaran Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055 , Shipeng Ding Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 , Sikai Wang Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 Joint School of National University of Singapore, Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207 , Hiroyuki Asakura Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510; 615-8245 Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8245 , Shinya Furukawa Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8245 Institute for Catalysis, Hokkaido University, Sapporo 001-0021 , Shibo Xi Institute for Chemical and Engineering Sciences, Agency for Science, Technology, and Research in Singapore, Singapore 138634 , Qi Yu Shaanxi Key Laboratory of Catalysis, Shaanxi University of Technology, Hangzhou 723001 Department of Chemistry, Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084 , Cong-Qiao Xu Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055 , Jun Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055 Department of Chemistry, Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084 and Ning Yan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 https://doi.org/10.31635/ccschem.022.202201914 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Fundamental knowledge of structure-activity correlations for heterogeneous single-atom catalysts (SACs) is essential in guiding catalytic design. While linear scaling relations are powerful for predicting the performance of traditional metal catalysts, they appear to fail with the involvement of SACs. Comparing the catalytic CO oxidation activity of different atomically dispersed metals (3d, 4d, and 5d) in conjunction with computational modeling enabled us to establish multiple scaling relations between the activity and simply calculated descriptors. Through these efforts, we found that the thermodynamic driving force for the oxygen vacancy formation needed to be considered in addition to the adsorption energies of substrates (in particular CO). Our approach was to reduce the computational requirements in determining better CO oxidation catalysts using a few key thermodynamic descriptors. This work presents one of the first successful approaches for re-establishing scaling relations for catalytic reactions by SACs with potentially broad implications for catalytic processes actively involving this support. Download figure Download PowerPoint Introduction The design of improved catalysts from the vast chemical space relies on the discovery of computationally inexpensive parameters that describe catalytic activity adequately.1,2 Among others, scaling relationships linking properties like adsorption and reaction energies with activation barriers have been developed for a range of reactions and surface morphologies but are so far predominantly focused on extended metal surfaces. While parameters like adsorption energies could be determined relatively quickly for large materials’ libraries with high accuracy and precision, activation energies are significantly more challenging to calculate. For the case of CO oxidation on extended fcc(111) surfaces and transition metal (TM) nanoparticles, linear correlations between the adsorption energies of O atoms, O2, and CO molecules, as well as the activation barriers, were predicted.3 For nanoparticles supported on reducible oxides, the CO oxidation commonly follows a Mars-van Krevelen (MvK) mechanism with the support actively contributing to the catalytic reaction by donating and accepting O atoms. For this reason, the CO oxidation activity was found to correlate well with the reducibility of the support.4,5 For metal oxides, scaling relations, in particular, for reactions involving C-H activation6-10 had been assessed previously. However, parameters at the interface of metal species and oxide supports are rarely discussed, although they are known to be essential for heterogeneous catalysis and are present in virtually all industrial catalysts.11-14 Single-atom catalysts (SACs) have recently been shown to be active and selective for a variety of oxidation, reduction, and coupling reactions.15-28 Density functional theory (DFT) calculations based on structures obtained via various tools such as X-ray absorption spectroscopy (XAS) serve as a powerful tool in revealing activity-structure correlations.15,16,27 Although CO oxidation is among the earliest and most thoroughly investigated reactions, performed experimentally and by computational modeling,29-33 clear guidelines for designing SACs are lacking. Obvious differences exist between catalytic surfaces and SACs because of the different electronic states, intimate metal-support interactions, the absence of adjacent TM atoms, and the absence of electronic bands forcing the treatment of scaling relations for SACs in alternative ways. In fact, many reports have shown that linear scaling relations do not apply to SACs supported on metal oxides or in host metals in so-called single-atom alloys.34-37 This has been explained previously by the unique electronic structure of SACs,36,38 geometric dynamics beyond what is achievable with nanoparticle catalysts,35 and ensemble effects with host metals in the case of single-atom alloys.34,37 Without discovering simple physical parameters of the catalysts governing catalytic activity, efficient screening of materials’ libraries is severely prohibited by computational expenses.39 Herein, we describe the synthesis of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, and Pt SACs supported by phosphomolybdic acid (PMA). We present evidence for atomic dispersion by XAS and CO diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the uniform catalyst structure and morphology determined by N2 sorption, scanning electron microscopy (SEM), Raman, and IR spectroscopies, as well as the determination of identical active site structural arrangement of the exposed four-fold hollow site surface area by all Keggin-structured polyoxometalates (POMs) (Figure 1).40,41 Despite their structural similarities, the light-off temperatures for the CO oxidation reaction differed by as much as 400 °C for the SACs. DFT calculations further confirmed the stability of the active site structure and the dominance of the MvK mechanism of the reaction kinetics. It was further used to predict the adsorption energies of the substrates and the product, along with the formation energy of the oxygen vacancy (OV). Scaling relations between each of the adsorption energies and the light-off temperatures did not well match the catalytic activity, but after including the OV formation energy, we understood the trends of the catalytic performance. Figure 1 | Schematic depiction of the SACs investigated in this study. Different atomically dispersed TMs (Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, and Pt) are anchored on POMs composed of a central PO4 unit with 12 surrounding edge-sharing MoO6 units. Download figure Download PowerPoint Experimental Methods Catalyst synthesis and characterization An aqueous solution of appropriate amounts of PMA in 30 mL of deionized water was cooled to 0 °C. Under stirring and cooling, a solution of 365 mg cesium nitrate (MilliporeSigma, Burlington, MA, USA) and the appropriate amount of chromium nitrate nonahydrate, manganese hydrate tetrahydrate, iron nitrate nonahydrate, cobalt nitrate hexahydrate, nickel nitrate hexahydrate, copper nitrate hemi(pentahydrate), zinc nitrate hexahydrate, ruthenium nitrosyl nitrate solution, rhodium nitrate, palladium nitrate dihydrate, and silver nitrate (all from MilliporeSigma, Burlington, MA, USA) dissolved in 30 mL deionized water with 5 drops of concentrated nitric acid was added gradually over 30 min period. The solution was aged for 5 h under constant stirring and ice cooling. The formed solid was then separated by centrifugation (5 min, 8000 g) and washed twice with 30 mL of deionized water. After freeze-drying overnight, the catalysts were used as obtained. Metal contents were determined using inductively coupled plasma-optical emission spectrometry (ICP-OES; iCAP 6000 series instrument; Thermo Fisher Scientific, Waltham, MA, USA) with calibration curves obtained from solutions with the pure metal salts. The catalysts were dissolved in aqua regia under heating at 80 °C for 4 h, and then the filtered solutions were diluted with an appropriate volume of deionized water before measurement. Attenuated total reflection (ATR-IR) spectroscopy was performed using a Thermo Scientific Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) in the range of 525-4000 cm−1 with a spectral resolution of 4 cm−1. Raman spectromicroscopy was performed using a Horiba Yvon Modular Raman spectrometer (Horiba, Kyoto, Japan) with a 532 nm excitation laser, 1% filter, a grating with 1200 mm−1 and a x100 objective. Calibration was done using the 521 cm−1 vibrations of a silicon wafer. Field emission scanning electron microscopy (FESEM) was performed using a JEM-6700F (JEOL, Ltd., Tokyo, Japan) microscope. Computational modeling Vienna Ab initio simulation package (VASP; version 5.3.5)42-44 was used to perform spin-polarized DFT calculations. The projector augmented plane wave (PAW) pseudopotentials were employed to describe the electron-ion interaction.45 The generalized gradient approximation (GGA) with the Perdew-Wang 91 (PW91) exchange-correlation functional was used,46 and widely applied for single-atom supported polyoxometalate clusters by different research groups.47-49 The plane wave cutoff energy was set to 400 eV in all calculations. A 20 × 20 × 20 Å3 cubic box was used to avoid interactions between the periodic images, as reported by Macht et al.,47 Yu et al.,48 and Wang et al.49 The Γ point was used for Brillouin zone center sampling. The total energy was converged to 10−4 eV by applying the Gaussian smearing method with a width of 0.05 eV. All the ions were allowed to relax until the maximum atomic forces became less than 0.05 eV/Å. The dimer method50-52 was used to determine the transition states, followed by vibrational frequency calculations to confirm saddle points with one imaginary frequency. Following the convention of thermodynamics, the adsorption energy (Eads) of O2, CO, or CO2 on the [email protected] surface was calculated as E ads = E total − E TM @ PMA − E adsorbate where Etotal, E[email protected], and Eadsorbate correspond to the electronic energies of adsorbed species on the [email protected], [email protected], and free adsorbates (CO, O2, and CO2), respectively. The OV formation energy (Evac formation) of [email protected] was calculated as E vac formation = − ( E CO - TM / PMA + E CO 2 − E 2 CO - TM / PMA ) where ECO-TM/PMA, ECO2, and E2CO-TM/PMA correspond to the calculated electronic energies of adsorption structure with one CO molecule, gas-phase CO2, and adsorption structure with two CO molecules, respectively. CO DRIFTS Approximately 100 mg catalyst powder was loaded in a Harricks HV-DR2 (Harrick Scientific Products Inc., Pleasantville, New York, United States) reaction cell covered by a Praying Mantis high-temperature reaction chamber with ZnS windows in a Thermo Scientific Nicolet iS50 FT-IR spectrometer; Thermo Fisher Scientific, Waltham, MA, USA) with a mercury-telluride (MCT) detector. The chamber was closed gastight, and background scans were recorded under 40 mL/min nitrogen gas flow (Air Liquide purity, 99.9995%) at room temperature. 40 mL/min 5% carbon monoxide in argon gas (Air Liquide) were introduced at 50 °C for 30 min, whereupon the diluted carbon monoxide gas flow was replaced with a gas flow of 40 mL/min nitrogen gas. During the gas treatments, spectra were collected regularly. CO oxidation reaction For the CO oxidation reaction, appropriate amounts of the catalysts were loaded into a stainless-steel tubular plug flow reactor, fixed with quartz wool, and heated using a tube furnace with an external thermocouple. Before the reaction, catalysts were activated in a flow of 5% O2 (balance Ar) for 60 min at 250 °C with a heating rate of 5 °C min−1. Then 2.5% CO and 2.5% O2 (balance Ar) with a total flow rate of 80 mL min−1 were introduced into the reactor set at room temperature. Subsequently, the temperatures were increased, and the reactor efflux was analyzed with an Agilent 7890B gas chromatograph (Agilent Technologies, Santa Clara, CA, USA) with a thermal conductivity (TCD) detector. Conversions and yields were determined after 30 min when a steady-state was reached. X-ray absorption spectroscopy Self-supported undiluted pellets of 200 mg sample were pressed before measurements. X-ray absorption spectroscopy (XAS) measurements at the Cr, Mn, Fe, Co, Ni, and Cu K edges were performed using the X-ray absorption fine structure for catalysis (XAFCA) beamline detector at the Singapore Synchrotron Light Source (SSLS; Buona Vista, Singapore).53 The storage ring of the SSLS was operated at 0.7 GeV with a maximum current of 200 mA. Data were collected at fluorescence mode while the respective metal foils were measured in transmission mode simultaneously using a Si(111) double crystal monochromator. The average of 3-5 spectra was calculated to reduce noise. XAS measurements of the Ru, Rh, Pd-edges and Pt L3-edge were performed at BL37XU (Ru, Rh, Pd) and BL11XU (Pt) at SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan), operated at 8.0 GeV with a constant current of 99.5 mA. The XAS spectra were collected in fluorescence mode, while the standard samples were measured in transmission mode. For the Ru K-edge or Rh K-edge XAS measurement of the Ru/CsPMA or Rh/CsPMA, we exploited the fluorescence lines at Ru Kβ or Rh Kβ lines because the Mo Kβ1 lines (19606 and 19962 eV) significantly overlapped with the Ru Kα1 (19279 eV) or Rh Kα1,2 lines (20216, 20074 eV), measured with a Ge solid-state detector (Mirion Technologies, Inc., Atlanta, GA, USA). For the Pd K-edge XAS measurement of Pd/CsPMA, we measured the XAS spectra typically using the Pd Kα line. For the Pt L3-edge XAS measurement, we employed a high-energy resolution fluorescence detection (HERFD) method with a Si(733) spherically bent analyzer (XRS TECH LLC., Freehold, NJ, Unite States). The Pt L3-edge extended X-ray absorption spectroscopy (EXAFS) spectrum was broadened with a Gaussian function by 5 eV in width to make the EXAFS amplitude comparable to the EXAFS spectra measured in conventional transmission mode. We reported some of the details in a previous paper.54 Data analysis was done with the Demeter software package.55 σ2 values were determined from commercial metal oxide samples with a known crystal structure. X-ray absorption near-edge spectra (XANES) simulations were performed using the CASTEP ab initio quantum mechanical program code56 with the PW91 exchange-correlation functional46 based on the GGA. The plane-wave basis set was truncated at a kinetic energy of 400 eV. Wavelet transformation analysis was performed using the Morlet wavelet transform procedure with values for κ and σ of 5 and 1, respectively.57 Results and Discussion Active site structure of M1/CsPMA The single-atom M1/CsPMA catalysts were synthesized by a coprecipitation method starting with cesium nitrate, metal nitrate salts, and PMA, as reported previously.40,58 Then the dried catalysts were used for analysis and catalytic testing without further modification. Atomic dispersion of the TMs was confirmed by XAS, as shown in Figure 2a. XANES revealed that all SACs were highly charged (2+, 3+, or 4+), and EXAFS confirmed that none of the catalysts exhibited metal-metal bond scattering, with all of them showing significant metal-oxygen scattering contributions (Figure 2b, Table 1, Supporting Information Table S1, and Figures S1-10). EXAFS fitting was consistent with the formation of the active site, which displayed metal atoms in the four-fold hollow sites of the Keggin structure with adsorption of one O2 molecule, except for Cu1/CsPMA and Pd1/CsPMA, which showed no adsorbed O2. This was consistent with DFT calculated O2 adsorption energies (vide infra). For 4d elements, the XAS analysis was more complicated due to the strong X-ray fluorescence of Mo close to the absorption edges of Ru, Rh, and Pd. Nevertheless, Morlet wavelet transformation analysis suggested that the SAC only exhibited first shell coordination to light scattering atoms like oxygen and none to heavier atoms like TMs (Figure 2c and Supporting Information Figures S11-S19). To develop an appropriate catalyst model for atomistic simulations, we ensured that all the catalysts exhibited the same known active site structure. XANES simulations based on DFT-calculated catalyst structures could help identify the exact local coordination environment. Metal oxide samples were used as a reference to confirm the validity of the employed level of theory ( Supporting Information Figure S20). For the predicted active site structures, an almost ideal overlap between simulated and experimental spectra confirmed the formation of active site structures, as discussed above (Figure 2d and Supporting Information Figure S20). This is in accordance with our previous findings, where the active sites of POM-supported Rh1 and Pd1 catalysts were elucidated in detail.58,59 Figure 2 | Confirmation of the SAC structure of M1/CsPMA. (a) K edge XANES spectra of ten different M1/CsPMA catalysts and their respective metallic and oxide reference materials. (b) R-space EXAFS spectra of different M1/CsPMA catalysts (colored, solid lines) and their respective metal foils (black, dashed lines). (c) Morlet wavelet transformation analysis of Cr foil, Cr2O3, and Cr1/CsPMA. (d) Experimental and simulated XANES spectra for different M1/CsPMA catalysts; the active site structure considered for XANES simulations are shown. Additional data are shown in the supporting information. Download figure Download PowerPoint Table 1 | EXAFS Fitting Parameters of Some M1/CsPMA Additional in the Supporting Information σ2 0.7 We employed the DRIFTS as approach to confirm the single-atom using CO as a We that without metal CO with two broad bands at and a one at to and of the SACs based on TMs showed CO adsorption which was in with their CO be the of CO adsorbed on or charged metal sites such as by our For some of the heavier elements, strong CO adsorption could be in particular, for Rh, Ag, and For two CO were adsorbed and the and vibrations were at and to previous reports on Rh CO with an IR at than gas-phase CO, the of a as commonly for like In the CO spectrum of a at cm−1 was of a highly charged Pt in a oxidation A at cm−1 was to no in the range of cm−1 were the absence of metallic nanoparticles in all the catalysts (Figure A of oxidation for the different M1/CsPMA catalysts based on CO DRIFTS and XANES is in Table Figure | CO DRIFTS spectra for and different M1/CsPMA catalysts under a 5% CO Download figure Download PowerPoint Table 2 | Oxidation of M1/CsPMA on CO DRIFTS and XAS Cr Cu Ru Rh Pd Pt Oxidation based on CO DRIFTS Oxidation based on EXAFS For the of the structure be the synthesis and catalytic The stability and of the Keggin structure after the adsorption of TMs were confirmed by Raman and infrared For and the M1/CsPMA the vibrational for PMA were no overlap with the spectrum of or oxides and The surface determined from by N2 and values were for POMs and were not significantly by the of atomically dispersed of the different M1/CsPMA catalysts showed a consistent morphology with between nm (Figure and Supporting Information Figure the surface of to be with a few in ( Supporting Information Figure Figure 4 | of the structure and morphology of M1/CsPMA. (a) Raman spectra of M1/CsPMA with of the surface determined by analysis of N2 adsorption (b) spectra of (c) of different catalysts; the has a of 2 Download figure Download PowerPoint Scaling relations for the CO oxidation activity of M1/CsPMA CO oxidation light-off curves were measured with a reactor at temperatures to °C. the temperatures for the CO oxidation reaction were on different significant differences were between some of which exhibited values above 400 as Rh the most active SAC with a of followed by Pd, and Ru with values of and (Figure The differences were much less for although SACs to be active for CO oxidation with a of almost metals analyzed were Cr, Mn, Cu, Fe, Zn, and Ni, with values of and respectively. to be a case where the coordination of CO in the to have the background activity of the support ( Supporting Information Figure the the SAC was from further analysis of scaling Raman and IR spectroscopy confirmed the stability of the catalyst support after the reaction had to °C ( Supporting Information Figure This is in accordance with our previous in which the stability of the POM-supported Rh SACs at high reaction temperatures was Figure 5 | CO oxidation activity and reaction of different M1/CsPMA (a) curves of M1/CsPMA catalysts Supporting Information Figure for The at CO to the (b) CO and O2 adsorption and OV formation energies on M1/CsPMA = 4d The more of the two different O2 adsorption and were (c) O2 reaction for different M1/CsPMA catalysts at different Download figure Download PowerPoint In to correlate the catalytic CO oxidation activity with simple properties of the we DFT calculations on To confirm the validity of the catalyst model used for the simulations, of the reaction mechanism for were The predicted mechanism the MvK mechanism previously determined by and kinetic with support as a ( Supporting Information Figures and We SACs to have reaction barriers or For computational screening of catalyst was to identify simple thermodynamic parameters that could be determined with computational scaling relationships are by between adsorption energies of different substrates and the SACs were different from extended metal surfaces because they did not adjacent metal active effects and interactions between adsorbates did not a metal in SACs with the the of the of the support the catalytic reaction in of the adsorption energies of CO and O2 ( Supporting Information Table and Figure on all TM revealed clear trends the periodic ( Supporting Information Figure trends among different were comparable with adsorption energies for heavier TMs due to differences were when of TMs with CO but high O2 energies were in adsorption energies for substrates in and O2 adsorption coupled with high CO adsorption energies in The formation of oxygen was for and then in energy to but was less for these became that TMs were the OV formation and CO while TMs did not CO and their high OV formation energies to reaction energy It that only catalysts with adsorption and OV formation energies were in the oxidation of CO (Figure and Supporting Information Figure Although O2 adsorption in the oxygen metal-support interface in the MvK we found that these adsorption energies were almost with on the metal site ( Supporting Information Figure we to on the O2 adsorption energies on the metal sites as the more To all the SACs followed the same we measured the reaction O2 for different to be a linear between the O2 and the reaction that the reaction was 1 the (Figure the were