Atomically Ordered PdCu Electrocatalysts for Selective and Stable Electrochemical Nitrate Reduction

Electrochemical nitrate reduction (NO3 RR) has attracted attention as an emerging approach to mitigate nitrate pollution in groundwater. Here, we report that a highly ordered PdCu alloy-based electrocatalyst exhibits selective (91% N2), stable (480 h), and near complete (94%) removal of nitrate without loss of catalyst. In situ and ex situ XAS provide evidence that structural ordering between Pd and Cu improves long-term catalyst stability during NO3RR. In contrast, we also report that a disordered PdCu alloy-based electrocatalyst exhibits non-selective (44% N2 and 49% NH4+), unstable, and incomplete removal of nitrate. The copper within disordered PdCu alloy is vulnerable to accepting electrons from hydrogenated neighboring Pd atoms. This resulted in copper catalyst losses which were 10× greater than that of the ordered catalyst. The design of stable catalysts is imperative for water treatment because loss of the catalyst adds to the system cost and environmental impacts.


Synthesis of ordered PdCu bimetals.
The procedure to synthesize the ordered structure of PdCu comprises two steps.First, we prepared randomly disordered PdCu bimetallic nanoparticles supported by Vulcan XC-72R carbon (D-PdCu/C) by means of simple one-pot method.The D-PdCu/C catalyst was synthesized without using any capping agents and surfactants.In detail, 15.2 mg of Pd (acac) 2 , 26.6 mg of Cu (acac) 2 , 71.2 mg of L-ascorbic acid (AA), 13 mL of oleylamine (OA), and 40 mg of Vulcan XC-72R were used as Pd and Cu precursors, reducing agent, solvent, and carbon support.After a solvothermal reduction process at 200 • C for 12 h, the disordered PdCu alloy catalyst was obtained and the bimetal NPs were uniformly dispersed on the carbon support.We conducted various heat treatments from 200 to 500 • C for 1 h under the reductive atmosphere (4 % H 2 , and 96 % Ar flow) to search the atomically ordered nanoparticles of Pd and Cu.

Synthesis of Cu nanoparticles on carbon (Cu NP/C)
For the procedure to synthesize the Cu nanoparticles on carbon support, 20 wt% targeted amount of Cu (acac) 2 , 71.2 mg of L-ascorbic acid (AA), 13 mL of oleylamine (OA), and 40 mg of Vulcan XC-72R were used as Cu precursors, reducing agent, solvent, and carbon support.After a solvothermal reduction process at 200 • C for 12 h, the Cu NPs were uniformly dispersed on the carbon support, and we collected the samples by centrifugation (Supplementary Fig. 13).

Characterization of ordered PdCu catalysts.
X-ray powder diffraction (XRD) patterns were collected using PANalytical Empyrean XRD system for crystalline identification with Cu Kα radiation (λ = 1.54051Å).TEM images were obtained by FEI Tecnai F20 at 200 kV operation.The aberration corrected high-angle annular dark-field scanning TEM (HAADF-STEM) combined with complementary energy dispersive X-ray spectroscopy (EDS) was performed on a JEOL NEOARM operated at 200 kV.All TEM grid samples were prepared by drop casting on a gold grid (Electron Microscopy Science Co.), and then dried under ambient conditions.Inductively coupled plasma mass spectrometer (ICP-MS, PerkinElmer Elan DRC) was used to determine the ratio of Pd and Cu atoms, and measure the leaching concentrations of Pd and Cu after long-term electrolysis.The X-ray absorption spectroscopy (XAS) spectra at Pd k-edge and Cu k-edge were measured from 8-ID (ISS) Beamline National Synchrotron Light Source II at Brookhaven National Laboratory, using a Si (111) double crystal monochromator and a passivated implanted planar silicon fluorescence detector at room temperature, with energy calibrated using Pd foil and Cu foil.The catalyst samples were sealed in Kapton films for ex-situ XAS measurements and our customizing designed cell was used for in-situ XAS measurements.All X-ray absorption near edge structure (XANES) and extended X-ray absorption fine struc-ture (EXAFS) data were analyzed using the Athena and Artemis software for conversion of raw data to µ(E) spectra, background subtraction and normalization, Fourier transformation and plotting, and fitting in k-space and R-space.

ECSA calculation.
The ECSA P dO values were calculated from the integration of the Pd oxide reduction region around 0.6 V RHE in Fig. 2c.This is the most extended approach to determine ECSA of Pd-based catalysts. 1,2We also estimated the ECSAs for all catalysts measured by TEM measurement.We counted the diameter of nanoparticles (d i ) over 200 individual nanoparticles and measured the number(l i )-averaged particle sizes, d N .
We used d S for the surface averaged diameter and metal density (ρ metal ) of Pd (12.02 g cm −3 ) and Cu (8.96 g cm −3 ) to calculate ECSA T EM , assuming all nanoparticles have a spherical shape.* The value of S 2 0 determined for the Pd foil was estimated as 0.