Method of H2 Transfer Is Vital for Catalytic Hydrodefluorination of Perfluorooctanoic Acid (PFOA).

The efficient transfer of H2 plays a critical role in catalytic hydrogenation, particularly for the removal of recalcitrant contaminants from water. One of the most persistent contaminants, perfluorooctanoic acid (PFOA), was used to investigate how the method of H2 transfer affected the catalytic hydrodefluorination ability of elemental palladium nanoparticles (Pd0NPs). Pd0NPs were synthesized through an in situ autocatalytic reduction of Pd2+ driven by H2 from the membrane. The Pd0 nanoparticles were directly deposited onto the membrane fibers to form the catalyst film. Direct delivery of H2 to Pd0NPs through the walls of nonporous gas transfer membranes enhanced the hydrodefluorination of PFOA, compared to delivering H2 through the headspace. A higher H2 lumen pressure (20 vs 5 psig) also significantly increased the defluorination rate, although 5 psig H2 flux was sufficient for full reductive defluorination of PFOA. Calculations made using density functional theory (DFT) suggest that subsurface hydrogen delivered directly from the membrane increases and accelerates hydrodefluorination by creating a higher coverage of reactive hydrogen species on the Pd0NP catalyst compared to H2 delivery through the headspace. This study documents the crucial role of the H2 transfer method in the catalytic hydrogenation of PFOA and provides mechanistic insights into how membrane delivery accelerates hydrodefluorination.

Hydrodehalogenation of Trichlorofluoromethane over Biogenic Palladium Nanoparticles in Ambient Conditions.

Among a number of persistent chlorofluorocarbons (CFCs, or freons), the emissions of trichlorofluoromethane (CFCl3, CFC-11) have been increasing since 2002. Zero-valent-Pd (Pd0) catalysts are known to hydrodehalogenate CFCs; however, most studies rely on cost-inefficient and eco-unfriendly chemical synthesis of Pd0NPs and harsh reaction conditions. In this study, we synthesized Pd0 nanoparticles (Pd0NPs) using D. vulgaris biomass as the support and evaluated hydrodehalogenation of CFC-11 catalyzed by the biogenic Pd0NPs. The presence of D. vulgaris biomass stabilized and dispersed 3-6 nm Pd0NPs that were highly active. We documented, for the first time, Pd0-catalyzed simultaneous hydrodechlorination and hydrodefluorination of CFC-11 at ambient conditions (room temperature and 1 atm). More than 70% CFC-11 removal was achieved within 15 h with a catalytic activity of 1.5 L/g-Pd/h, dechlorination was 50%, defluorination was 41%, and selectivity to fully dehalogenated methane was >30%. The reaction pathway had a mixture of parallel and sequential hydrodehalogenation. In particular, hydrodefluorination was favored by higher H2 availability and Pd0:CFC-11 ratio. This study offers a promising strategy for efficient and sustainable treatment of freon-contaminated water.

Bio-Pd(0) diverting electron from CoQ-long chain to FDH/Hase-short chain during sulfamethoxazole degradation.

Microbial electron output capacity is critical for organic contaminants biodegradation. Herein, original C. freundii JH could oxidate formate in anaerobic respiration, but lack the ability to degrade sulfamethoxazole (SMX). While the incorporation of Pd(0) could effectively improve the electron output via improving the combination between flavins and c-type cytochromes (c-Cyts), increasing the activities of key enzymes (formate dehydrogenase, hydrogenase, F0F1-ATPases), etc. More importantly, the presence of Pd(0) caused the NADH dehydrogenase (complex I) nearly in idle, and triggered the decrease of NADH/NAD+ ratio and increase of H+-efflux transmembrane gradient, eventually resulting in the electrons diverting from CoQ-involved long respiratory chain (decreasing from 91.67% to 36.25%) to FDH/Hases-based hydrogen-producing short chain (increasing from 22.44% to 84.88%), which further intensified the electron output. Above changes effectively launched and guaranteed the high-level SMX degradation by palladized C. freundii JH, alleviating the ecotoxicity of SMX in aquatic and terrestrial environments. These conclusions provided the new view to regulate the microbial electron output behaviors.

Prompting the FDH/Hases-based electron transfers during Pt(IV) reduction mediated by bio-Pd(0).

Due to the excellent hydrogen affinity and high conductivity, palladium nanoparticles (Pd NPs) were considered as a potential strategy to regulate bacterial electron transfer and energy metabolism. Herein, Citrobacter freundii JH, capable of in-situ biosynthesizing Pd(0) NPs, was employed to promote Pt(IV) reduction. The results showed that the Pt(IV) reduction to Pt(II) was accomplished mainly via the flavins-mediated extracellular electron transfer (EET) process, while Pt(II) reduction to Pt(0) was limit step, and proceeded via two intracellular respiratory chains, including FDH/Hases-based short chain (S-chain) and typical CoQ-involved long respiratory chain (L-chain). Noteworthily, the incorporation of Pd(0) NPs mainly diverted the electrons to S-chain (as high as 71.7%-73.4%) by improving the hydrogenases (Hases) activity. Furthermore, Pd(0) NPs could stimulate the secreting of flavins and the combination between flavins and cytochrome c (c-Cyt), which converted electron transfer manner of L-chain. Additionally, Pd(0) NPs might also act as alternative proton channels to improve the energy metabolism. These findings provided significant insights into the promotion by Pd(0) NPs in terms of electron generation, electron consumption and proton translocation.