Immobilized Pd-Ag bimetallic nanoparticles on polymeric nanofibers as an effective catalyst: effective loading of Ag with bimetallic functionality through Pd nucleated nanofibers

Tuning Pd-to-Ag Ratio to Enhance the Synergistic Activity of Fly Ash-Supported PdxAgy Bimetallic Nanoparticles

Fly ash (FA)-supported bimetallic nanoparticles (PdxAgy/FA) with varying Pd:Ag ratios were prepared by coprecipitation of Pd and Ag involving in situ reduction of Pd(II) and Ag(I) salts in aqueous medium. All the supported nanoparticles were thoroughly characterized with the aid of powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), electron microscopy (field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM)), and elemental analyses, which include inductively coupled plasma-optical emission spectroscopy (ICP-OES) and energy-dispersive X-ray spectroscopy (EDS). A gradual broadening and shifting of PXRD peaks, ascribable to Ag, to higher angles with an increase in the Pd:Ag ratio affirms the alloying of interface between Pd and Ag nanoparticles. The coexistence of Pd and Ag was further confirmed by EDS elemental mapping as well as by the presence of bimetallic lattices on the FA surface, as evident from the high-resolution TEM analysis. The dependency of crystallite size and average size of bimetallic nanoparticles on Ag loading (mol %) was elucidated with the help of a combination of PXRD and TEM studies. Based on XPS analysis, the charge transfer phenomenon between contacting Pd-Ag sites could be evident from the shifting of 3d core electron binding energy for both Pd and Ag compared with monometallic Pd and Ag nanoparticles. Following a pseudo-first-order reaction kinetics, all the nanocatalysts were able to efficiently reduce 4-nitrophenol into 4-aminophenol in aqueous NaBH4. The superior catalytic performance of the bimetallic nanocatalysts (PdxAgy/FA) over their monometallic (Pd100/FA and Ag100/FA) analogues has been demonstrated. Moreover, the tunable synergistic effect of the bimetallic systems has been explored in detail by varying the Pd:Ag mol ratio in a systematic manner which in turn allowed us to achieve an optimum reaction rate (k = 1.050 min-1) for the nitrophenol reduction using a Pd25Ag75/FA system. Most importantly, all the bimetallic nanocatalysts explored here exhibited excellent normalized rate constants (K ≈ 6000-15,000 min-1 mmol-1) compared with other supported bimetallic Pd-Ag nanocatalysts reported in the literature.

AgPd nanocages sandwiched between a MXene nanosheet and PDA layer for photothermally improved catalytic activity and antibacterial properties

In this work, a MXene@AgPd/polydopamine (PDA) nanosheet with excellent photothermal conversion efficiency was successfully synthesized by a simple redox-oxidative polymerization method. Interestingly, AgPd bimetallic nanocrystals sandwiched between a MXene nanosheet and PDA layer have cage-like nanostructure, which is favorable for high catalytic efficiency and antibacterial performance. Importantly, the MXene@AgPd/PDA nanosheet exhibits good catalytic activity for the reduction of 4-nitrophenol (1.2 min^-1 mg^-1) and the catalytic dynamics can be improved by about 1.2 times under NIR (near-infrared light, 808 nm, and 2.5 W cm^-2) irradiation. As the PDA shell is well protected, the MXene@AgPd/PDA nanosheet retained more than 90% catalytic activity after 6 cycles. In addition, due to the presence of the Ag component, the MXene@AgPd/PDA nanosheet exhibited good antibacterial activity against both Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria. Under near-infrared light irradiation, its antibacterial activity was further enhanced due to the NIR photothermal effect.

Atomic layer deposition of palladium nanoparticles on a functional electrospun poly-cyclodextrin nanoweb as a flexible and reusable heterogeneous nanocatalyst for the reduction of nitroaromatic compounds

We here show a rational approach for the fabrication of a flexible, insoluble catalytic electrospun nanoweb of cross-linked cyclodextrin (CD) for the reduction of nitroaromatics. CD nanofibers were produced by electrospinning an aqueous HP-β-CD solution containing a multifunctional cross-linker (i.e., 1,2,3,4-butanetetracarboxylic acid, BTCA) and were subsequently cross-linked by heat treatment, which led to an insoluble electrospun poly-CD nanoweb. The poly-CD nanoweb was decorated with Pd nanoparticles (Pd-NPs) by atomic layer deposition (ALD) technique over 20 cycles to give rise to a catalytic electrospun nanoweb (i.e., Pd@poly-CD). The formation of the Pd-NPs on the poly-CD nanofiber surface was clearly evidenced by TEM and STEM imaging, which displayed the homogeneously distributed Pd-NPs with a mean size of 4.34 nm. ICP-MS analysis revealed that the Pd content on the Pd@poly-CD nanoweb was 0.039 mg per mg of nanoweb. The catalytic performance of the Pd@poly-CD nanoweb was tested for the reduction of a nitroaromatic compound (i.e., 4-nitrophenol (4-NP)), and high catalytic performance of the Pd@poly-CD nanoweb was observed with a corresponding TOF value of 0.0316 min-1. XPS was used to explore the oxidation state of Pd atoms before and after the catalytic reduction of 4-NP, and no significant change was observed after catalytic reactions. In brief, the Pd@poly-CD nanoweb having handy, flexible, structural stability and reusability can be effectively used in environmental applications as a heterogeneous nanocatalyst for the reduction of toxic nitroaromatics.

Ultra-fine Pd nanoparticles confined in a porous organic polymer: A leaching-and-aggregation-resistant catalyst for the efficient reduction of nitroarenes by NaBH4

Porous organic polymers (POPs) containing nitrogenous substituents have potential practical applications as heterogeneous catalysts based upon controlled porous structure and surface-anchored noble metal nanoparticles (NMNPs). In this work we prepared a POP material from piperazine and cyanuric chloride starting materials (PC-POP). The PC-POP material contains numerous triazinyl moieties, thus rendering the pores hydrophobic. Subsequently, by means of a novel reverse double-solvent approach (RDSA), microdroplets of Pd(AcO)₂/CH₂Cl₂ were introduced into the hydrophobic pores of PC-POP in an aqueous environment; Pd(II) was rapidly reduced by NaBH₄ to form ultra-fine Pd NPs and confined within the pores of PC-POP at high dispersity. The extensive porosity and dispersity of the Pd NPs made the active sites readily accessible, and led to efficient mass transfer. Thus, Pd@PC-POP exhibits superior catalytic performance in catalytic reduction of various nitroarenes. Furthermore, Pd@PC-POP has excellent recyclability, without significant loss of activity nor leaching of Pd active sites during 10 successive reaction cycles. This work points to a practical and cost-effective approach to preparation of POP materials, and also for confining ultra-fine NMNPs in POPs for use as catalysts.