Hierarchical carbon nanotube membrane-supported gold nanoparticles for rapid catalytic reduction of p -nitrophenol

Gold nanoparticles (AuNPs) have attracted increasing attention as catalysts for pollutant degradation because of their unique reactivity. Direct use of gold nanoparticles in water treatment faces prohibitive challenges from nanoparticle aggregation and post-treatment separation. To prevent nanoparticles from aggregating and eliminate the need for separation, we affixed AuNPs on hierarchical carbon nanotube membrane (HCNM) that was approximately 50 μm thin with 10 μm × 10 μm openings as pores for water passage. HCNM was fabricated by growing vertically aligned carbon nanotube (CNT) arrays on stainless steel mesh. Using p-nitrophenol (PNP) as model pollutant, we showed that in batch experiments HCNM-supported AuNPs retained 78% of their catalytic capability compared to suspended AuNPs. The slight reduction in reactivity was attributed to the blockage of part of the gold surface at the AuNP-CNT juncture. When the membrane was used in continuous flow-through operation, HCNM-supported AuNPs achieved 71% of the maximum catalytic ability measured in batch. The rapid kinetics obtained with HCNM-supported AuNPs was in great contrast to the slow kinetics that one would expect for a rigid membrane of similar configuration. For a rigid membrane, water passing through microscopic pores was confined as laminar flow and thus would not mix well with catalysts affixed on pore walls. For HCNM, CNTs aligning pore walls were flexible so that they could move vigorously to create a chaotic mixing condition and promote AuNP-catalyzed PNP reduction.