Controlled Synthesis of Palladium Phosphides with Tunable Crystal Phases and Their Sulfur-Tolerant Performance

The development of sulfur-tolerant heterogeneous noble metal catalysts with high activity and stability has always been a major challenge. Herein, PdmPn/carbon nanofiber (CNF) catalysts with tunable crystal phases (Pd6P, Pd3P, Pd5P2, and PdP2) were synthesized by a simple impregnation-pyrolysis method, and their sulfur resistance was systematically studied in hydrogenation of sulfur-containing substrates and hydrogenation of aromatic compounds in a sulfur-containing system. Structural analysis showed that all PdmPn/CNF catalysts with phase-pure phosphides exhibited a similar morphology and physicochemical properties. However, the obvious differences in hydrogenation activity (Pd > Pd6P > Pd3P > Pd5P2 > PdP2) and anti-sulfur (Pd < Pd6P < Pd3P < Pd5P2 < PdP2) performance are presented. Overall, Pd5P2/CNFs exhibited the optimal catalytic hydrogenation activity of sulfur-containing substrates and aromatic compounds in sulfur-containing systems, and its turnover frequency (1043.9 h(-1)) was 20 times higher than that of Pd/CNFs (51.8 h(-1)) in 4-nitrothioanisole hydrogenation. Combining systematical characterizations with density functional theory calculation, the enhanced resistance to sulfur poisoning of PdmPn/CNFs is attributed to the reduced adsorption strength and adsorption energy of the sulfur-containing molecules. Meanwhile, when sulfur-containing molecules are adsorbed on PdmPn/CNFs, the weakened Pd-S and enhanced C-S bond suggest that it is more difficult to form palladium sulfide from being poisoned. This work was valuable for guiding the design and manipulation of noble metal-based anti-sulfur catalysts.