Discerning structure sensitivity of Ni-core Pd-shell nanoparticles for enhanced nitrite reduction

Denitrification using Pd-based catalysts converts N-oxyanions into harmless products. However, the metal cost and scarcity are barriers to application. In this work, we synthesize water-suspended, structure-controlled Ni-core/Pd-shell nanoparticles (Ni@Pd NPs) that show Pd-catalyzed nitrite (NO₂⁻) reduction catalysis promoted by Ni. The NPs immobilized on Al₂O₃ (0.47 wt % Pd and 0.74 wt % Ni) have higher NH₄⁺ selectivity and higher catalytic activity than wet-impregnated Pd/Al₂O₃ (1.0 wt % Pd), i.e., 315.4 vs. 132.1 Lg_{surface Pd}⁻¹min⁻¹. X-ray photoelectron spectroscopy (XPS) and CO-diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) analyses show that Ni increases Pd electron density, which is proposed to lower the bond dissociation energy barrier for nitric oxide surface intermediate. Low-coordinated Pd atoms correlate to high activity and low NH₄⁺ selectivity. Using CatCost software, we estimate Ni@Pd/Al₂O₃ to have a ∼40% lower manufacturing cost compared to wet-impregnated Pd/Al₂O₃. This rationally designed catalyst illustrates how nickel promotes Pd catalysis and how Pd usage can be lowered for denitrification and other hydrogenation reactions.