Controllable Synthesis Of Noble Metal Nanomaterials For Catalytic Reduction Of Hexavalent Chromium

Chromium and its compounds are widely applied in industrial processes such as metal processing,electroplating,leather tanning,pigment,metallurgy and organic synthesis,and the waste water,waste gas and waste residue from these factories have become the main source of Cr(?)contamination in the environment.It is noteworthy that Cr(?)can induce severe environmental problems if these industrial waste water is not treated properly.As it is known,Cr(?)is one of the most toxic water contaminant and highly mobile in water,while Cr(?)is less toxic and tends to form insoluble hydroxides at neutral or alkaline solutions.Therefore,the target of the research is how to transform Cr(?)to Cr(?).Many feasible methods have been developed to remove Cr(?)from environment,including traditional redox,electrochemical,photocatalytic,and biological methods.Recently,noble metals(e.g.,Pd,Pt,Ag,and Au)have received significant attention for their remarkable catalytic properties.Meanwhile,scientists from all over the world also developed a series of catalysts with enhanced catalytic activity for Cr(?)reduction.Therefore,we proposed some novel strategies to construct shape-controlled noble metal catalysts for improving their catalytic performances.In this paper,we synthesized four different noble metal nanomaterials through one-pot method with the assistance of various structure-directing agents(minoxidil,Trajenta and poly-L-lysine).The structure features and formation mechanisms of the fabricated nanomaterials were examined by a series of characterization technologies containing transmission electron microscopy(TEM),high-angle annular diffraction field-scanning transmission electron microscopy-energy dispersive X-ray spectroscopy(HAADF-STEM-EDS),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),thermogravimetry(TGA),and Raman and Fourier transform infrared(FT-IR)spectroscopy.Besides,the prepared four nanocatalysts were explored for the catalytic reduction of Cr(?)to Cr(?),which all showing the greatly enhanced catalytic activity and stability for the Cr(VI)reduction.Specific contents are follows:(1)Simple fabrication of AuPd@Pd core-shell nanocrystals for effective catalytic reduction of hexavalent chromiumHerein,we developed a facile one-pot wet-chemical coreduction method for synthesis of core-shell AuPd@Pd nanocrystals(AuPd@Pd NCs),using polyvinylpyrrolidone(PVP)and minoxidil as the dispersing and growth-directing agents,respectively.The fabricated nanocrystals were mainly characterized by TEM,STEM,XPS and XRD in details.The architectures were explored for the catalytic reduction of Cr(?)to Cr(?)by employing formic acid(HCOOH)as the reducing agent,showing enhanced catalytic activity of AuPd@Pd NCs in contrast with commercial Pd black catalyst.Additionally,the formation mechanism of AuPd@Pd NCs and the catalytic reduction mechanism of Cr(?)were illustrated and discussed in some detail,respectively.(2)Uniform Pt@Pd nanocrystals supported on N-doped reduced grapheme oxide as catalysts for effective reduction of highly toxic chromiumIn this work,we developed a facile one-pot co-reduction method for fabrication of spherical porous Pt@ultra-thin Pd skin core-shell nanocrystals supported on N-doped reduced graphene oxide(Pt@Pd NCs/N-rGO).Trajenta(Tra),a kind of dipeptidyl peptidase-4 inhibitors for treatment of diabetes,served as a new structure-directing agent for Pt@Pd NCs and N-doping resource for rGO via ?-?stacking interactions.The product was mainly characterized by microscopic analysis,XPS,XRD,TGA,FT-IR,and the formation mechanism was discussed in some detail.By taking HCOOH as the reductant,the Pt@Pd NCs/N-rGO nanocomposite showed significantly enhanced catalytic ability and improved repeatability for the reduction of highly toxic Cr(?)compared with commercial Pt/C(20 wt.%)and Pd/C(20 wt.%).(3)Shape-controlled synthesis of well-dispersed Pt nanocubes supported on g-C3N4 as an advanced visible-light-driven catalyst for rapid photoreduction of Cr(VI)at room temperatureHerein,we developed a facile hydrothermal method for shape-controlled synthesis of well-dispersed Pt nanocubes(NCs)by using poly-L-lysine(PLL)as the eco-friendly growth-directing agent,followed by dispersing them uniformly on graphitic carbon nitride(g-C3N4).The morphology,crystal structure,chemical composition,and formation mechanism of the fabricated Pt nanocubes supported on g-C3N4 nanocomposite(Pt NCs/g-C3N4)were mainly characterized by TEM,XPS,XRD and other techniques.The hybrid nanocomposite was explored for the photocatalytic reduction of Cr(?)to Cr(?)by employing HCOOH as the reducing agent under visible light at room temperature,showing the greatly enhanced photocatalytic activity as compared to blank g-C3N4 and home-made Pt black/g-C3N4,along with the significantly improved reusability of the Pt NCs/g-C3N4 catalyst.The growth mechanism and photocatalytic mechanism of Pt NCs/g-C3N4 were investigated in details.(4)Pd nanocones supported on g-C3N4:Grain boundary engineered catalysts for facet-dependent enhanced reduction of hexavalent chromium under visible-light irradiationRecent catalytic researches have highlighted the essential role of grain boundaries(GBs)engineering in scalable construction of free-standing highly active or supported on carbon-derivated materials due to the high energy of atoms at confined defect sites.Herein,a facile,large-scaled and poly-L-lysine(PLL)-mediated one-pot hydrothermal approach was developed for synthesis of Pd nanocones enclosed by the {111} facets,followed by uniformly dispersing them on graphitic carbon nitride(g-C3N4).The Pd nanocubes/g-C3N4 demonstrated the dramatic enhancement for the photocatalytic transformation of Cr(?)to Cr(?)via the dehydrogenation pathway under visible-light irradiation,which outperforms the {100}facets enclosed Pd nanocubes/g-C3N4,Pd black/g-C3N4,and bare g-C3N4 catalysts.Meanwhile,the Pd nanocones/g-C3N4 nanocomposite showed the enhanced conversion up to 99.9%and improved reusable ability via the recycle experiments.Herein,the GBs engineering was effective to modulate the catalytic ability of the g-C3N4 supported Pd catalysts(nanocones,nanocubes and nanoparticles).It provides a feasible strategy for synthesis of g-C3N4 supported novel Pd nanocatalysts with tailored catalytic ability via morphology-and composition-engineering.