Insight into the size effect of Pd nanoparticles on the catalytic reduction of nitrite in water over Pd/C catalysts

Abstract

Catalytic denitrification over Pd-based catalysts under mild conditions has received significant attention as an emerging technology. However, the Pd particle size effect has been much less systematically investigated. Herein, carbon-supported Pd nanoparticles (NPs) with particle sizes ranging from 2.1 to 22.1 nm were synthesized and characterized using high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), CO-chemisorption, and X-ray photoelectron spectroscopy (XPS). Catalytic nitrite reduction experiments were conducted in a semi-batch reactor at room temperature. The results showed that the highest mass catalytic activity of 603.6 mg g_Pd⁻¹ min⁻¹ was obtained at the Pd size of 2.7 nm. The turnover frequency (TOF) and the N₂ selectivity increased gradually with the increasing Pd NP size. Considering both the mass catalytic activity and N₂ selectivity, the Pd catalyst with a size of approximately 5 nm had a better comprehensive performance. Mechanism studies showed that the enhanced TOF of larger Pd NPs could be ascribed to the formation of more β-hydride, which has a higher reducing activity than α-hydride. Compared with the terraces, the edges and corners of Pd NPs are the major active sites where ammonia is generated, most likely because of their low-coordination and electron-deficient properties. This indicated that the NO₂⁻ catalytic reduction is a size sensitive reaction in terms of both activity and selectivity, and may provide new insight into tailoring Pd NP size of Pd-based catalysts to improve catalytic activity and selectivity for nitrate and nitrite reduction in water.