Dual immobilization of Pd-Cu nanoparticles on halloysite nanotubes by CTAB and PVP for automobile exhaust elimination

摘要

Low thermal stability and metal sintering are considered the reasons for the deactivation of three-way catalysts, especially on transition metals. Herein, using a versatile clay halloysite nanotubes (Hal) as a support and two surfactants, cetyltrimethylammonium bromide (CTAB) and polyvinylpyrrolidone (PVP), modified Hal with hy-drophobic external surfaces and Pd-PVP colloids with various particle sizes were synthesized by adjusting the ratios of surfactants. As a result, modified Hal could make CuO nanoparticles coordinate with unsaturated Al in the internal lumen of Hal, which is favored to increase the CuO nanoparticle dispersion and inhibit their migration. In particular, Cu@HC-2 (Hal/CTAB weight ratio = 2) exhibited a higher n(Cu)/n(Al) ratio, surface active oxygen species and CuO species, which endowed better catalytic performance. The T-50 of CO conversion decreased from 250 degrees C (unmodified Cu/Hal catalyst) to 180 degrees C. However, the NOx reduction is less effective due to the limited reducibility of transition metal. The reduced Pd-PVP nanoparticles with the molar ratio of PVP/Pd = 4 were immobilized on the Cu-based catalyst, which contributes to the smaller Pd-0 nanoparticles and the maximum NOx conversion. Unexpectedly, excessive PVP-Pd may block the gas reaction with CuO species and weaken the CO and C3H8 oxidation.