Rational Design and Precise Synthesis of Single‐Atom Alloy Catalysts for the Selective Hydrogenation of Nitroarenes

Abstract Single‐atom alloys (SAAs) have gained increasing prominence in the field of selective hydrogenation reactions due to their uniform distribution of active sites and the unique host‐guest metal interactions. Herein, 15 SAAs are constructed to comprehensively elucidate the relationship between host‐guest metal interaction and catalytic performance in the selective hydrogenation of 4‐nitrostyrene (4‐NS) by density functional theory (DFT) calculations. The results demonstrate that the SAAs with strong host‐guest metal interactions exhibit a preference for N─O bond cleavage, and the reaction energy barrier of the hydrogenation process is primarily influenced by the host metal. Among them, Ir1Ni SAA stands out as the prime catalyst candidate, showcasing exceptional activity and selectivity. Furthermore, the Ir1Ni SAA is subsequently prepared through precise synthesis techniques and evaluated in the selective hydrogenation of 4‐NS to 4‐aminostyrene (4‐AS). As anticipated, the Ir1Ni SAA demonstrates extraordinary catalytic performance (yield > 96%). In situ FT‐IR experiments and DFT calculations further confirmed that the unique host‐guest metal interaction at the Ir‐Ni interface site of Ir1Ni SAA endows it with excellent 4‐NS selective hydrogenation ability. This work provides valuable insights into enhancing the performance of SAAs catalysts in selective hydrogenation reactions by modulating the host‐guest metal interactions.

the lattice constants of Cobalt (Co), Nickel (Ni), and Copper (Cu) and the results are summarized as Table S3.Evidently, the findings underscore that the lattice constants derived from PBE-D3 align more closely with experimental values.Hence, we assert that among these three functionals, PBE-D3 emerges as the optimal choice for the meticulous computation of M1Co, M1Ni and M1Cu SAAs.
Secondly, we conducted an assessment of the binding energies of 4-nitrostyrene on Ru1Co(111), Ru1Ni(111), and Ru1Cu(111) surfaces.The results show that compared with the calculation results of PBE-D3 functional, the PBE functional consistently displays a propensity to underestimate binding energies.Conversely, the PBEsol functional tends to magnify the binding energy of 4-nitrostyrene on Ru1Co(111) and Ru1Ni(111) surfaces, while concurrently exhibiting an inclination to underestimate the binding energy on Ru1Cu(111).Even though the absolute value changes, the trend are exactly same that the 4-nitrostyrene adsorbs strongest on Ru1Co(111) and weakest on Ru1Cu(111), as shown in Figure S31.All three of these functionals showed consistent trends in predicting binding energies.
While PBE-D3 demonstrating superior accuracy in determining lattice constants.Therefore, the PBE-D3 functional was chosen for the subsequent calculations.A 331 Monkhorst-Pack k point mesh and 400 eV cutoff energy for the plane-wave basis were employed for the geometry optimizations.The convergence criterion for the total energy self-consistent iterations was set to 10 -4 eV, and the geometry optimization stops when the total force was less than 0.05 eV/Å.The energy barriers of transient states were determined using the climbing image nudged elastic band (CI-NEB) method 8 , or the DIMER method, 9 and vibrational frequency analysis was conducted to confirm only one imaginary frequency in each transition state.Co(111), Ni(111) and Cu(111) surfaces were represented by p(5×5) periodic unit cells with three atomic layers, and a vacuum space of 15 Å was added in the z direction to minimize the interaction between the periodically repeated slabs or adsorbates in the direction normal to the slab.The SAA surfaces were built by substituting one host atom (Co, Ni or Cu) on the topmost layer with one noble metal atom.During all the optimizations, the bottommost layer of SAA was fixed, while the top two layers were fully relaxed to participate in reactions.
The binding energy (Eb) of 4-NS on SAAs surfaces and the activation energy barrier (Ea) of 4 where ε is the energy with respect to the Fermi level, and nd(ε) is the electronic density of states.
The Bader Charge analysis was performed via Henkelman programme based on near-grid algorithm with refine-edge method 11−13 The charge density differential analysis was performed by using visualization software VESTA 14 .

Chemicals and materials.
Analytical reagents used in the experiments were bought from Sigma

Synthesis of catalysts.
As a precursor, hierarchical NiAl-LDHs was synthesized by in situ growth method reported previously by our group 15 .Afterwards, NiAl-LDHs (0.3 g) was reduced in a H2/N2 (10/90, v/v; 35 mL•min −1 ) stream at 500 C for 4 h (heating rate: 2 C•min -1 ) to prepare amorphous Al2O3 supported monometallic Ni sample (Ni/Al2O3).The supported M1Ni bimetallic samples were synthesized by a galvanic replacement method.Taking Ir1Ni SAA as an example, the fresh Ni/Al2O3 sample (0.2 g) was dispersed in 30 mL purified water, followed by slowly adding H2IrCl6 solution (0.07 mmol•L −1 ) and stirring vigorously for 60 min under the protection of a N2 atmosphere.The obtained precipitation was centrifugated, washed with purified water, and dried for 24 h in vacuum oven at 50 °C to obtain 0.6% Ir1Ni sample.The Ru1Ni, Rh1Ni, Pd1Ni and Pt1Ni samples were prepared via the same method with tuning the desired amount of corresponding metal precursor solution.Before the catalytic evaluation, the as-synthesized samples were pre-reduced in a H2/N2 flow (10/90, v/v) at 300 °C (heating rate: 2 C•min -1 ) for 1 h, followed by cooling to the room temperature in N2.As references, Ir/Al2O3 samples were prepared by a deposition precipitation method and reduced at 300 °C (H2/N2 flow: 10/90, v/v; heating rate: 2 C•min -1 ) for 3 h.
Characterizations.The X-ray diffraction (XRD) experiments were carried out on Bruker DAVINCI D8 ADVANCE diffractometer with a Cu Kα radiation source (40 kV and 40 mA).Scanning electron microscope (SEM) images were displayed by using a Zeiss Supra 55 electron microscope.
Transmission electron microscopy (TEM) characterizations were performed on a JEOL JEM-2010 high-resolution transmission electron microscope.FEI Titan Cube Themis G2 300 and JEOL JEMARM200F instruments with a spherical aberration corrector and energy-dispersive X-ray spectroscopy (EDS) system were adopted to perform aberration-corrected high angle annular darkfield scanning transmission electron microscopy (AC−HAADF−STEM) and EDS mapping measurements.X-ray absorption fine structure spectroscopy (XAFS) at Ir K-edge and Ni K-edge were measured at the beamline 1W1B and 1W2B of the Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS).A Ir foil reference was scanned simultaneously for energy calibration.The raw data were processed using the Athena interface of the Demeter software package. 16Wavelet transformation for the Ir L3-edge XAFSsignals was employed based on Morlet wavelets. 17For the CO-DRIFTS experiment, about 50 mg of sample was carefully put into the support sink of diffuse reflectance cell firstly.Subsequently, the sample was prereduced in a H2/He flow (1/19, v/v; 30 mL•min −1 ) at 300 °C (heating rate: 5 C•min -1 ) for 1 h, followed by cooling to the room temperature in a high purity He stream, and collecting background signal.
Afterwards, the CO/He (1/19, v/v; 30 mL•min −1 ) was purged into the cell, and then DRIFTS spectra were collected until the adsorption spectrums kept unchanged.Finally, the gas flow was switched to a pure He stream to collect CO chemisorption spectra.In situ FT-IR measurements of 4-NS adsorption and surface reaction were performed using a transmission reactor.The sample (20 mg) was pressed into self-supporting wafer with a diameter of 13 mm, followed by a pretreatment under the same conditions.After the sample was cooled down to 50 °C in He stream, 4-NS was introduced into the reactor for 30 min; and He was purged to remove the physically adsorbed molecule followed by collection of IR signals.Subsequently, the spectra for hydrogenation process were collected per 60 s after the introduction of H2 (flow rate: 30 mL•min −1 ).
Catalytic test.Firstly, substrate (4-NS, 1 mmol), solvent (ethanol, 8 ml) and catalyst (0.03 g) were carefully added to a 25 mL stainless-steel autoclave.Subsequently, the reactor was purged completely with 2.0 MPa hydrogen (>99.999%) for 5 times, followed by pressurized and sealed with H2 to 1.0 MPa.The reaction was carried out at 50 C with a constant stirring speed of 700 rpm.After the reaction is over, the resulting products were identified by GC-MS, and quantitatively analyzed using a Shimadzu GC−2014C gas chromatograph system outfitted a GSBP−INOWAX capillary column (30m×0.25mm×0.25mm)and an FID detector.The conversion of 4-nitrostyrene and the selectivity of products were determined as follows:

Figure S6
Figure S6 Schematic diagram of the adsorption configuration of 4-nitrostyrene on the surfaces of SAA.

Figure S7
Figure S7 Adsorption configurations and corresponding binding energies of 4-NS on Ru1Co SAA.

Figure S9
Figure S9 Adsorption configurations and corresponding binding energies of 4-NS on Ru1Cu SAA.

Figure S10
Figure S10The adsorption configuration and corresponding binding energy of vinyl adsorbed on single atom sites.

Figure S16
Figure S16The structures of intermediates and transition state for the 4-NS reduction on Ru1Cu(111).

Figure S17
Figure S17 Potential energies profiles and corresponding optimized structures for C=C hydrogenation over Co(111) and Ru1Co(111) surface.

Figure S18
Figure S18 Potential energies profiles and corresponding optimized structures for C=C hydrogenation over Ni(111) and Ru1Ni(111) surface.

Figure S19
Figure S19 Potential energies profiles and corresponding optimized structures for C=C hydrogenation over Cu(111) and Ru1Cu(111) surface.

Figure S20
Figure S20 Product distribution in the presence of monometallic Ni, Ir and Ir1Ni catalysts.

Figure S22
Figure S22 XRD patterns of pristine Ni, pristine Ir and Ir1Ni SAA samples.

Figure S23
Figure S23 XRD pattern of the fresh and the used Ir1Ni SAA catalyst after 5 cycle times.

Figure S24
Figure S24Element EDS mapping images of used Ir1Ni SAA sample.

Figure S28
Figure S28 Potential energies profiles and corresponding optimized structures for C=C hydrogenation over Ir1Ni(111) surface.

Figure S29
Figure S29 In situ FT-IR spectra of 4-NS hydrogenation in the presence of pure Ni catalyst.

Figure S30
Figure S30 Potential energy profiles and corresponding optimized structures for C=C hydrogenation in styrene and N-O scission in nitrobenzene over Ir1Ni SAA.
,   ,   and   represent the energies of adsorbed system, substrate, adsorbate

Table S1
Binding energy and N-O1 bond length, M1-O bond length.