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Potassium-promoted, oxide-supported PdAu is catalytically active for the gas-phase acetoxylation of ethylene to form vinyl acetate monomer (VAM), in which the potassium improves long-term activity and VAM selectivity. The alkali metal is incorporated into the catalyst via wet impregnation of its salt solution, and it is generally assumed that this common catalyst preparation step has no effect on the catalyst structure. However, in this work, we report evidence to the contrary. We synthesized a silica-supported PdAu (PdAu/SiO2, 8 wt% Pd, 4 wt% Au) model catalyst containing Pd-rich PdAu alloy and pure Au phases. Impregnation with potassium acetate (KOAc) aqueous solution and subsequent drying did not cause XRD-detectible changes to the bimetal structure. However, DRIFTS indicated the presence of Pd3(OAc)6 species, which is correlated to up to 2% Pd loss after washing of the dried KOAc-promoted PdAu/SiO2. Carrying out the impregnation step with an AcOH-only solution and subsequent drying caused significant enlargement of the pure Au grain size and generated a smaller amount of Pd3(OAc)6. During co-impregnation of AcOH and KOAc, grain sizes were enlarged slightly, and substantial amounts of K2Pd2(OAc)6 and Pd3(OAc)6 were detected by DRIFTS and correlated to up to 32% Pd loss after washing. Synchrotron XAS analysis showed that approximately half the Pd atoms were oxidized, corroborating the presence of the Pd-acetate species. These results indicate wet-impregnation-induced metal leaching can occur and be substantial during catalyst preparation. [ABSTRACT FROM AUTHOR]

Highly ordered TiO2 nanostructures, known as nanotube arrays (NTAs), exhibit potential in various energy, chemical sensing, and biomedical applications. Owing to its simplicity and high degree of control, titanium anodization serves as the prevailing NTA synthesis method. However, the practicality of this approach is marred by sluggish and inconsistent growth rates, on the order of 10 nm min−1. Growth rates strongly depend on the electrolyte conductivity, yet most reports neglect to consider this property as a measured and controllable parameter. Here, we have systematically determined a broad set of conditions (at 60 V applied potential, elevated temperatures) that allow researchers to fabricate NTAs quickly and simply. By modulating conductivity through variation of bulk electrolyte temperature and the controlled addition of several hydroxy acid species, we achieve consistent accelerated growth up to 10 times faster than traditional methods. We find that regulating the solution conductivity within a desired region (e. g., ∼800–1000 μS cm−1) enabled the fabrication of double‐sided NTA layers of around 10 μm and 90 μm NTA in 10 and 180 min, respectively. [ABSTRACT FROM AUTHOR]

• Depositing catalysts on hydrogen gas-transfer membranes in-situ created a homogeneous bimetallic layer. • Bimetallic palladium plus indium nanoscale films on hydrogen gas-transfer membranes enabled nitrate reduction. • Nanocatalyst oxidation must be avoided to retain nitrate removal activity. • Irreversible nanocatalyst attachment was necessary for sustained nitrate removal. Catalytic hydrogenation of nitrate in water has been studied primarily using nanoparticle slurries with constant hydrogen-gas (H 2) bubbling. Such slurry reactors are impractical in full-scale water treatment applications because 1) unattached catalysts are difficult to be recycled/reused and 2) gas bubbling is inefficient for delivering H 2. Membrane Catalyst-film Reactors (MCfR) resolve these limitations by depositing nanocatalysts on the exterior of gas-permeable hollow-fiber membranes that deliver H 2 directly to the catalyst-film. The goal of this study was to compare the technical feasibility and benefits of various methods for attaching bimetallic palladium/indium (Pd/In) nanocatalysts for nitrate reduction in water, and subsequently select the most effective method. Four Pd/In deposition methods were evaluated for effectiveness in achieving durable nanocatalyst immobilization on the membranes and repeatable nitrate-reduction activity: (1) In-Situ MCfR-H 2 , (2) In-Situ Flask-Synthesis, (3) Ex-Situ Aerosol Impaction-Driven Assembly, and (4) Ex-Situ Electrostatic. Although all four deposition methods achieved catalyst-films that reduced nitrate in solution (≥ 1.1 min−1gPd−1), three deposition methods resulted in significant palladium loss (>29%) and an accompanying decline in nitrate reactivity over time. In contrast, the In-Situ MCfR-H 2 deposition method had negligible Pd loss and remained active for nitrate reduction over multiple operational cycles. Therefore, In-Situ MCfR-H 2 emerged as the superior deposition method and can be utilized to optimize catalyst attachment, nitrate-reduction, and N 2 selectivity in future studies with more complex water matrices, longer treatment cycles, and larger reactors. [ABSTRACT FROM AUTHOR]

The efficient transfer of H 2 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 H 2 transfer affected the catalytic hydrodefluorination ability of elemental palladium nanoparticles (Pd 0 NPs). Pd 0 NPs were synthesized through an in situ autocatalytic reduction of Pd 2+ driven by H 2 from the membrane. The Pd 0 nanoparticles were directly deposited onto the membrane fibers to form the catalyst film. Direct delivery of H 2 to Pd 0 NPs through the walls of nonporous gas transfer membranes enhanced the hydrodefluorination of PFOA, compared to delivering H 2 through the headspace. A higher H 2 lumen pressure (20 vs 5 psig) also significantly increased the defluorination rate, although 5 psig H 2 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 Pd 0 NP catalyst compared to H 2 delivery through the headspace. This study documents the crucial role of the H 2 transfer method in the catalytic hydrogenation of PFOA and provides mechanistic insights into how membrane delivery accelerates hydrodefluorination.

PFAAs (perfluorinated alkyl acids) have become a concern because of their widespread pollution and persistence. A previous study introduced a novel approach for removing and hydrodefluorinating perfluorooctanoic acid (PFOA) using palladium nanoparticles (Pd 0 NPs) in situ synthesized on H 2 -gas-transfer membranes. This work focuses on the products, pathways, and optimal catalyst conditions. Kinetic tests tracking PFOA removal, F - release, and hydrodefluorination intermediates documented that PFOA was hydrodefluorinated by a mixture of parallel and stepwise reactions on the Pd 0 NP surfaces. Slow desorption of defluorination products lowered the catalyst's activity for hydrodefluorination. Of the platinum group metals studied, Pd was overall superior to Pt, Rh, and Ru for hydrodefluorinating PFOA. pH had a strong influence on performance: PFOA was more strongly adsorbed at higher pH, but lower pH promoted defluorination. A membrane catalyst-film reactor (MCfR), containing an optimum loading of 1.2 g/m 2 Pd 0 for a total Pd amount of 22 mg, removed 3 mg/L PFOA during continuous flow for 90 days, and the removal flux was as high as 4 mg PFOA/m 2 /d at a steady state. The EPA health advisory level (70 ng/L) also was achieved over the 90 days with the influent PFOA at an environmentally relevant concentration of 500 ng/L. The results document a sustainable catalytic method for the detoxification of PFOA-contaminated water.