Unraveling the coordination structure-performance relationship in Pt1/Fe2O3 single-atom catalyst

Heterogeneous single-atom catalyst (SAC) opens a unique entry to establishing structure–performance relationship at the molecular level similar to that in homogeneous catalysis. The challenge lies in manipulating the coordination chemistry of single atoms without changing single-atom dispersion. Here, we develop an efficient synthetic method for SACs by using ethanediamine to chelate Pt cations and then removing the ethanediamine by a rapid thermal treatment (RTT) in inert atmosphere. The coordination chemistry of Pt single atoms on a Fe2O3 support is finely tuned by merely adjusting the RTT temperature. With the decrease in Pt-O coordination number, the oxidation state of Pt decreases, and consequently the hydrogenation activity increases to a record level without loss of chemoselectivity. The tunability of the local coordination chemistry, oxidation states of the metal, and the catalytic performance of single atoms reveals the unique role of SACs as a bridge between heterogeneous and homogeneous catalysis.

at the beamline 14W of Shanghai Synchrotron Radiation Facility (SSRF) in China. The output beam was selected by Si(111) monochromator, and the energy was calibrated by Pt foil. Before measurement, the samples were diluted by boron nitride and tableted. The data were collected at room temperature under fluorescence mode by using Lytle detector. Athena software package was employed to process the XAS data.
In the XAFS results, the contribution of Pt-C arises from the (Pt(en)2) 2+ complex, as shown in Figure 3d The C species is located on the Pt second shell. After adsorption of the (Pt(en)2) 2+ complex on the FeOOH support, the Pt-C contribution still existed, but it was absent from the samples after the RTT treatment at high temperatures owing to the decomposition of the en ligand. Therefore, in the EXAFS spectra (Figure 3a), only the two samples, Pt-en (liquid) and the Pt1/FeOOH-RT, present the Pt-C contributions at around 2.5 Å, which coincidently overlaps with the Pt-Pt contribution. But there aren't any Pt-Pt contributions in both samples X-ray photoelectron spectroscopy (XPS) spectra were obtained on a Thermo ESCALAB 250 X-ray photoelectron spectrometer equipped with Al Kα excitation source and with C as internal standard (C 1s = 284.6 eV).
H2-microcalorimetric measurement was performed by a BT2.15 heat-flux calorimeter, which was connected to a gas handling and a volumetric system employing MKS Baratron Capacitance Manometers for precision pressure measurement. The ultimate dynamic vacuum of the microcalorimetric system was 10 -7 Torr by calculation. First, the fresh sample was treated in a special cell in high pure He at 120 o C to eliminate the adsorption. Then, the sample was transferred and sealed in a Pyrex capsule without exposure to Air. After that, the capsule was placed into the the high vacuum system and stabilized for (6-8 h). After thermal equilibrium was reached, the capsule was broken by a vacuum feedthrough. And the H2-microcalorimetric data was collected by sequentially introducing small doses (10 -6 mol) of H2 into the system until it became saturated (5-6 Torr). Simultaneously, the differential heat versus adsorbate coverage plots and adsorption isothermals can be obtained after a typical H2-microcalorimetric experiment.
In the computational details section, all the calculations were performed using periodic DFT methods as implemented in the Vienna ab-initio simulation package (VASP). 1,2 Projector augmented wave (PAW) method was used for the interaction between the atomic cores and valence electrons. 3 The valence orbitals of Fe (3d, 4s), O (2s, 2p) and H (1s) were described by plane-wave basis sets with cutoff energies of 400 eV. The exchange-correlation energies were calculated via the generalized gradient approximation (GGA) with the PBE functional. 4 Gaussian smearing method with a width of 0.05 eV was used. Spin-polarized DFT+U calculations 5,6 with a value of Ueff = 3.0 eV 7,8 was used to describe the localized Fe 3d states. The Brillouin zone was sampled at the Γ-point.
The convergence criteria for the energy and force were set to 10 -5 eV and 0.02 eV Å -1 . The transition state (TS) of the surface reaction was searched using the dimer method. 9 Vibrational analysis was further used to confirm the transition states with only one imaginary frequency. The energy barrier (Ea) was determined as the energy difference between the corresponding transition-and initial-states.
The adsorption energies was calculated according to the supplementary equation (1).
Supplementary Equation (1) In supplementary equation (1), E(slab + adsorbate), E(slab), and E(adsorbate) are the energies of species adsorbed on the surface, the bare surface, and the gas-phase molecule, respectively. The reaction energy was calculated by supplementary equation (2).

ΔE = E(products) − E(reactants)
Supplementary Equation (2) Atomic charges were computed using the atom-in-molecule (AIM) scheme proposed by Bader. 10 The α-Fe2O3-p(4 × 4) (0001) surface was modeled by p(4 × 4) supercells containing 12 layers of Fe atoms and 7 atomic layers of O3 to model the O-terminated surface. The 10 top-layer slabs of the surface were allowed to relax while the other layers beneath the surface were frozen during the geometry optimizations. The vacuum gap was set as ∼15 Å to avoid the interaction between periodic images. All supercell slabs were periodically repeated with a 15 Å vacuum layer between surfaces in the direction of the surface normal. Antiferromagnetic properties of α-Fe2O3 were represented by a (+ --+) magnetic configuration, which was proven to be the most energetically favorable for [a] The Pt loading was determined by ICP-AES.
[b] The Pt species content error is approximately 3%. 12 [c] For XPS analysis, the average oxidation state of Pt is calculated by supplementary equation (3):  [a] The KIE value was calculated using supplementary equation (6):