Selective catalytic NOx reduction by H2 in excess O2 over Pt/zirconium phosphate nanosheets

Abstract

Two-dimensional nanomaterials have attracted attention over the past several decades in the field of catalyst materials chemistry because of their intrinsic anisotropic crystal structure and large specific surface area. In particular, transition-metal phosphate (TMP) nanosheets have great potential as supporting materials for platinum-group metal (PGM) catalysts for the conversion of NO via selective catalytic reduction (SCR). A previous report concluded that the Rh/ZrP₂O₇ catalyst showed good catalytic activity in the SCR reaction with C₃H₆ due to the strong anchoring effect of Rh–O–P linkages and the electrophilic nature of PO₄, which surpasses sintering of RhO_x nanoparticles and promotes the rapid redox reactions of Rh species. In the present study, we demonstrated the catalytic activity toward NO conversion via SCR with H₂ (H₂-SCR) using Pt nanoparticles supported on a monolayer zirconium phosphate (ZrP) nanosheet. The monolayer ZrP nanosheet was obtained by delaminating layered alpha-zirconium phosphate (α-Zr(HPO₄)₂·H₂O) in 0.25 M ethylamine solution (50% ethanol in water). The crystal structure of the ZrP nanosheet was thermally stable upto 500 °C after the Zr(HPO₄)₂ was transformed into ZrP₂O₇, which was a sufficiently high temperature for operation as an H₂-SCR catalyst (RT-400 °C). The 0.49 wt% Pt/ZrP nanosheet showed high NO conversion (89%) and moderate N₂ selectivity (83%) at 150 °C under a NO (200 ppm)–H₂ (5000 ppm)–O₂ (10%)–He (balance) atmosphere. The Pt/ZrP nanosheet catalyst maintained its catalytic activity when the H₂-SCR test was repeated for three cycles. Further investigation of the particle size (HAADF-STEM) and electronic state (XPS) of the Pt suggested that the small Pt particles (diameter: ∼1 nm) with a high loading amount on the ZrP nanosheet structure were key to achieving a high NO conversion rate with a low yield of NO_x byproducts.