A PtPdCoCuNi high-entropy alloy nanocatalyst for the hydrogenation of nitrobenzene

High-entropy alloys (HEAs) with multiple elements in near-equiatomic proportions hold great promise in heterogeneous catalysis because of their exceptional physicochemical properties governed by synergy. Herein, we prepared PtPdCoCuNi HEA nanoparticles via a one-step colloid-based route and tested their catalytic performance for nitrobenzene hydrogenation to aniline. The SiO2 supported PtPdCoCuNi displays 93.9% yield of aniline at 80 °C, which is 2.11 times that of PtPd/SiO2. Even at room temperature, a 47.4% yield of aniline is attained with the PtPdCoCuNi/SiO2 catalyst. DRIFTS experiments indicate formation of isolated Pt and Pd sites after alloying the transition metals and evidence a stronger interaction between the HEA catalyst and nitrobenzene. Both XPS data and DFT calculations disclose charge transfer to Pt and Pd species, which eventually enhance the interaction between nitrobenzene and the isolated metal sites and the hydrogenation activity as well. The experimental and theoretical results shed light on mechanistic understanding of the unique catalytic performance of the HEA nanocatalyst and pave a new avenue to realize the high catalytic performance of nitrobenzene hydrogenation over well-isolated noble metal sites with specific geometries.

and 100 mg PVP were all dissolved in 10 mL ethylene glycol with stirring at 70 °C.
The resulting homogeneous solution was transferred to a Teflon-lined steel autoclave.
The sealed vessel was then heated at 200 °C for 12 h in the oven before it was cooled to room temperature. The black products were obtained by centrifugation and washed with anhydrous ethanol. Then 10 mL ethanol, 1 mL oleylamine, and 8 mL cyclohexane were added to the resulting products. The mixture was refluxed at 70 °C for 12 h. The PtPdCoCuNi nanoparticles were obtained by centrifugation and subsequently washed with mixture of ethanol-cyclohexane (1:1). The obtained PtPdCoCuNi nanoparticles were dispersed in 10 mL cyclohexane for further experiments. For the syntheses of Pt, Pd, Co, Cu, Ni, PtPd, PtPdCo, PtPdCoCu, PtPd(CoCuNi) 1.5 , PtPd(CoCuNi) 2 , and PtPd(CoCuNi) 4 nanoparticles, all of the conditions were similar to those of PtPdCoCuNi, except for the amounts and the kinds of metal precursors. The total metal precursors were kept to 0.05 mmol.

Materials characterization
BET surface area and pore distribution were tested on a micromeritics ASAP 2420 instrument. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance diffractometer using Cu Kα radiation operating at 40 kV and 40 mA. Transmission electron microscopy (TEM) images were obtained using JEOL JEM 2100 at 200 kV with a thermionic LaB 6 source. Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) mapping were acquired with Hitachi HF5000 at 200 kV using a cold field emission gun. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Kratos Axis Ultra DLD using Al Kα as the excitation source. The elemental analyses were determined with inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a PerkinElmer Avio 500 instrument. 1 H NMR and 13 C NMR spectra were recorded on a Bruker 500 Nuclear Magnetic Resonance spectrometer operating. Bruker Vertex 70 with an MCT detector was used to collect diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The sample was pretreated with Ar flow (30 mL/min) at 150 °C for 1 h, and the background was recorded after cooling to room temperature. The sample was then exposed to CO flow (30 ml/min) for 30 min. The Ar flow (30 mL/min) was introduced to remove physically adsorbed species, and the CO-DRIFTS spectrum was collected.
For nitrobenzene-DRIFTS, 30 mL/min of Ar carrying the volatilized nitrobenzene was continuously introduced into the cell for saturated adsorption followed by Ar purge.
Temperature programmed desorption of nitrobenzene (nitrobenzene-TPD) was conducted on a Thermo Scientific Lindberg Bule M instrument with connecting mass spectrometry (Thermostar). The sample was purged with Ar (30 mL/min) for 1 h at 150 °C and then cooled to room temperature. The sample was adsorbed with nitrobenzene for 3 h, and the remaining nitrobenzene was purged by Ar for another 3 h. Finally, the desorption of nitrobenzene was carried out by heating to 400 °C at a rate of 5 °C/min under Ar atmosphere. The signals were measured using mass spectrometry. The DRIFTS and TPD results were normalized.

Catalytic reaction
Nitrobenzene hydrogenation reactions were performed in a 50 mL stainless steel autoclave with a magnetic stirrer. In each reaction, 50 mg catalyst, 30 mL cyclohexane, 2.04 mL (20 mmol) nitrobenzene, and 2.94 mL (13 mmol) dodecane (used as internal standard) were transferred into the autoclave. Before reaction, the reactor was purged with N 2 several times in order to remove the air. The system was then raised to the target temperature and the vessel was pressurized by 3 MPa H 2 . After the reaction, the products were collected by centrifugation and analyzed by an Agilent 7890B gas chromatograph. Witout other treatment.
The stability of PtPdCoCuNi/SiO 2 catalyst was investigated by cycling for five times under the same conditions. The catalyst after each reaction run was recovered by centrifugation, then washed with ethanol and used for next run after drying without other treatment.

Computational method
All first-principles data were obtained using the density functional theory (DFT) method implemented in the Vienna Ab initio simulation package (VASP) 1, 2 and periodic slab models. The projector-augmented-wave (PAW) 3 method was employed, which is an all-electron DFT technique to present electron-ionic interactions. The electron exchange correlation was treated by the well-known generalized gradient approximation in the Perdew-Burke-Ernzerhof form (GGA-PBE) 4 . For the sake of providing an exact description of the magnetic properties, calculations of spinpolarization were carried out, which are also essential to the adsorption energy calculations. In order to ensure that the energy errors were within a reasonable range, an energy cutoff of 500 eV and the second order Methfessel-Paxton 5 electron smearing with σ = 0.05 eV were used. The vacuum layer between periodically repeated slabs was set as 10 Å to avoid interactions among slabs.
The adsorption energy (E ads ) of one nitrobenzene molecule is defined as E ads = E nitrobenzene/slab − [E slab + E nitrobenzene ], where E nitrobenzene/slab is the total energy of the slab with one nitrobenzene adsorption, E slab is the total energy of the bare slab and E nitrobenzene is the total energy of a free nitrobenzene molecule in gas phase, and a more negative E ads indicates a stronger adsorption.