Guangming Jiang

Tuning Nanoparticle Structure and Surface Strain for Catalysis Optimization

Controlling nanoparticle (NP) surface strain, i.e. compression (or stretch) of surface atoms, is an important approach to tune NP surface chemistry and to optimize NP catalysis for chemical reactions. Here we show that surface Pt strain in the core/shell FePt/Pt NPs with Pt in three atomic layers can be rationally tuned via core structural transition from cubic solid solution [denoted as face centered cubic (fcc)] structure to tetragonal intermetallic [denoted as face centered tetragonal (fct)] structure. The high activity observed from the fct-FePt/Pt NPs for oxygen reduction reaction (ORR) is due to the release of the overcompressed Pt strain by the fct-FePt as suggested by quantum mechanics-molecular mechanics (QM-MM) simulations. The Pt strain effect on ORR can be further optimized when Fe in FePt is partially replaced by Cu. As a result, the fct-FeCuPt/Pt NPs become the most efficient catalyst for ORR and are nearly 10 times more active in specific activity than the commercial Pt catalyst. This structure-induced surface strain control opens up a new path to tune and optimize NP catalysis for ORR and many other chemical reactions.

Removal of chromium(VI) from wastewater by nanoscale zero-valent iron particles supported on multiwalled carbon nanotubes.

For the first time, nanoscale zero-valent iron (nZVI)-multiwalled carbon nanotube (MWCNT) nanocomposites were adopted to remove Cr(VI) from wastewater. Such composites were prepared through depositing nZVI particles onto MWCNTs by in situ reduction of ferrous sulfate and then characterized by TEM, SEM and XRD. The results showed that nZVI particles could disperse on the surface or into the network of MWCNTs. Compared to bare nZVI or nZVI-activated carbon composites, the nZVI-MWCNT nanocomposites exhibited around 36% higher efficiency on Cr(VI) removal. The mass ratio of nZVI to MWCNTs was optimized at 1:2, at ionic strength of 0.05M NaCl. The reaction followed a pseudo first-order model under different initial Cr(VI) concentrations and pHs. Low pH and initial Cr(VI) concentration could increase both removal efficiency and rate constants. Anions, such as SO(4)(2-), NO(3)(-) and HCO(3)(-), exhibited negative effects on the removal of Cr(VI), while the effects of PO(4)(3-) and SiO(3)(2-) were insignificant. Overall, nZVI-MWCNT nanocomposites offer a promising alternative material for the removal of Cr(VI) ions from wastewater.

Highly active nanoscale zero-valent iron (nZVI)–Fe3O4 nanocomposites for the removal of chromium(VI) from aqueous solutions

For the first time, nanoscale zero-valent iron (nZVI)-Fe(3)O(4) nanocomposites, prepared by an in situ reduction method, are employed for chromium(VI) removal in aqueous environment. 96.4% Cr(VI) could be removed by these novel materials within 2h under pH of 8.0 and initial Cr concentration of 20 mg L(-1), compared with 48.8% by bare nFe(3)O(4) and 18.8% by bare nZVI. Effects of several factors, including mass composition of nZVI-Fe(3)O(4) nanocomposites, initial pH and Cr(VI) concentration, were evaluated. The optimal ratio of nFe(3)O(4) to nZVI mass lies at 12:1 with a fixed nZVI concentration of 0.05 g L(-1). Low pH and initial Cr(VI) concentration could increase both the Cr(VI) removal efficiency and reaction rate. Corresponding reaction kinetics fitted well with the pseudo second-order adsorption model. Free energy change (ΔG) of this reaction was calculated to be -4.6 kJ mol(-1) by thermodynamic study, which confirmed its spontaneous and endothermic characteristic. The experimental data could be well described by the Langmuir and Freundlich model, and the maximum capacity (q(max)) obtained from the Langmuir model was 100 and 29.43 mg g(-1) at pH 3.0 and 8.0, respectively. The reaction mechanism was discussed in terms of the mutual benefit brought by the electron transfer from Fe(0) to Fe(3)O(4).

Nanoscale Zero-Valent Iron (nZVI) assembled on magnetic Fe3O4/graphene for Chromium (VI) removal from aqueous solution

Nanoscale Zero-Valent Iron (nZVI) assembled on magnetic Fe3O4/graphene (nZVI@MG) nanocomposites was synthesized for Cr(VI) removal from aqueous solution. nZVI particles were perfectly dispersed either among Fe3O4 nanoparticles (Fe3O4 NPs) or above the basal plane of graphene. This material shows Cr(VI) removal efficiency of 83.8%, much higher than those of individuals (18.0% for nZVI, 21.6% for Fe3O4 NPs and 23.7% for graphene) and even their sum of 63.3%. The removal process obeys pseudo-second-order adsorption model, suggesting that adsorption is rate-controlling step. Maximum Cr(VI) adsorption capacity varies from 66.2 to 101.0 mg g(-1) with decreasing pH from 8.0 to 3.0 at 30°C. Negative ΔG and ΔH indicate spontaneous tendency and exothermic nature. Robust performance of nZVI@MG arises from the formation of micro-nZVI-graphene/nZVI-Fe3O4 batteries and strong adsorption capability of broad graphene sheet/Fe3O4 surfaces. Electrons released by nZVI spread all over the surfaces of graphene and Fe3O4, and the adsorbed Cr(VI) ions on them capture these floating electrons and reduce to Cr(III). Fe3O4 NPs also served as protection shell to prevent nZVI from agglomeration and passivation.

Core/Shell Face-Centered Tetragonal FePd/Pd Nanoparticles as an Efficient Non-Pt Catalyst for the Oxygen Reduction Reaction.

We report the synthesis of core/shell face-centered tetragonal (fct)-FePd/Pd nanoparticles (NPs) via reductive annealing of core/shell Pd/Fe3O4 NPs followed by temperature-controlled Fe etching in acetic acid. Among three different kinds of core/shell FePd/Pd NPs studied (FePd core at ∼8 nm and Pd shell at 0.27, 0.65, or 0.81 nm), the fct-FePd/Pd-0.65 NPs are the most efficient catalyst for the oxygen reduction reaction (ORR) in 0.1 M HClO4 with Pt-like activity and durability. This enhanced ORR catalysis arises from the desired Pd lattice compression in the 0.65 nm Pd shell induced by the fct-FePd core. Our study offers a general approach to enhance Pd catalysis in acid for ORR.

Fe0-Fe3O4 nanocomposites embedded polyvinyl alcohol/sodium alginate beads for chromium (VI) removal

In this study, Fe(0)-Fe3O4 nanocomposites embedded polyvinyl alcohol (PVA)/sodium alginate (SA) beads were synthesized, which exhibited an excellent physical properties and catalytic reactivity, and a robust performance of post-separation (complete separation using a simple grille) and reusability (efficiency of 69.8% after four runs) in Cr(VI) removal. 5.0 wt% PVA with 1.5 wt% SA was the optimal proportion for beads molding, and the followed acidification and reduction treatments were critical to ensure high mechanical strength and high Cr(VI) removal ability of beads. Effects of Fe(0) and Fe3O4 mass fraction, initial pH and Cr(VI) concentration on final removal efficiency were also evaluated. Merely 0.075 wt% Fe(0) together with 0.30 wt% Fe3O4 was sufficient to deal with 20 mg L(-1) Cr(VI) solution. The efficiency decreased from 100 to 79.5% as initial Cr(VI) increased from 5 to 40 mg L(-1), while from 99.3 to 76.3% with increasing pH from 3.0 to 11.0. This work provides a practical and high-efficient method for heavy metal removal from water body, and simultaneously solves the problems in stabilization, separation and regeneration of Fe(0) nanoparticles.

Highly Efficient Performance and Conversion Pathway of Photocatalytic NO Oxidation on SrO-Clusters@Amorphous Carbon Nitride

This work demonstrates the first molecular-level conversion pathway of NO oxidation over a novel SrO-clusters@amorphous carbon nitride (SCO-ACN) photocatalyst, which is synthesized via copyrolysis of urea and SrCO3. The inclusion of SrCO3 is crucial in the formation of the amorphous carbon nitride (ACN) and SrO clusters by attacking the intralayer hydrogen bonds at the edge sites of graphitic carbon nitride (CN). The amorphous nature of ACN can promote the transportation, migration, and transformation of charge carriers on SCO-ACN. And the SrO clusters are identified as the newly formed active centers to facilitate the activation of NO via the formation of Sr-NOδ(+), which essentially promotes the conversion of NO to the final products. The combined effects of the amorphous structure and SrO clusters impart outstanding photocatalytic NO removal efficiency to the SCO-ACN under visible-light irradiation. To reveal the photocatalytic mechanism, the adsorption and photocatalytic oxidation of NO over CN and SCO-ACN are analyzed by in situ DRIFTS, and the intermediates and conversion pathways are elucidated and compared. This work presents a novel in situ DRIFTS-based strategy to explore the photocatalytic reaction pathway of NO oxidation, which is quite beneficial to understand the mechanism underlying the photocatalytic reaction and advance the development of photocatalytic technology for environmental remediation.

Enhanced plasmonic photocatalysis by SiO 2 @Bi microspheres with hot-electron transportation channels via Bi–O–Si linkages

ABSTRACT The semimetal Bi has received increasing interest as an alternative to noble metals for use in plasmonic photocatalysis. To enhance the photocatalytic efficiency of metallic Bi, Bi microspheres modified by SiO2 nanoparticles were fabricated by a facile method. Bi–O–Si bonds were formed between Bi and SiO2, and acted as a transportation channel for hot electrons. The SiO2@Bi microspheres exhibited an enhanced plasmon-mediated photocatalytic activity for the removal of NO in air under 280 nm light irradiation, as a result of the enlarged specific surface areas and the promotion of electron transfer via the Bi–O–Si bonds. The reaction mechanism of photocatalytic oxidation of NO by SiO2@Bi was revealed with electron spin resonance and in situ diffuse reflectance infrared Fourier transform spectroscopy experiments, and involved the chain reaction NO → NO2 → NO3− with •OH and •O2− radicals as the main reactive species. The present work could provide new insights into the in-depth mechanistic understanding of Bi plasmonic photocatalysis and the design of high–performance Bi-based photocatalysts.

Identification of Active Hydrogen Species on Palladium Nanoparticles for an Enhanced Electrocatalytic Hydrodechlorination of 2,4-Dichlorophenol in Water

Clarifying hydrogen evolution and identifying the active hydrogen species are crucial to the understanding of the electrocatalytic hydrodechlorination (EHDC) mechanism. Here, monodisperse palladium nanoparticles (Pd NPs) are used as a model catalyst to demonstrate the potential-dependent evolutions of three hydrogen species, including adsorbed atomic hydrogen (H*ads), absorbed atomic hydrogen (H*abs), and molecular hydrogen (H2) on Pd NPs, and then their effect on EHDC of 2,4-dichlorophenol (2,4-DCP). Our results show that H*ads, H*abs, and H2 all emerge at -0.65 V (vs Ag/AgCl) and have increased amounts at more negative potentials, except for H*ads that exhibits a reversed trend with the potential varying from -0.85 to -0.95 V. Overall, the concentrations of these three species evolve in an order of H*abs < H*ads < H2 in the potential range of -0.65 to -0.85 V, H*ads < H*abs < H2 in -0.85 to -1.00 V, and H*ads < H2 < H*abs in -1.00 to -1.10 V. By correlating the evolution of each hydrogen species with 2,4-DCP EHDC kinetics and efficiency, we find that H*ads is the active species, H*abs is inert, while H2 bubbles are detrimental to the EHDC reaction. Accordingly, for an efficient EHDC reaction, a moderate potential is desired to yield sufficient H*ads and limit H2 negative effect. Our work presents a systematic investigation on the reaction mechanism of EHDC on Pd catalysts, which should advance the application of EHDC technology in practical environmental remediation.

Unraveling the mechanisms of visible light photocatalytic NO purification on earth-abundant insulator-based core-shell heterojunctions

Earth-abundant insulators are seldom exploited as photocatalysts. In this work, we constructed a novel family of insulator-based heterojunctions and demonstrated their promising applications in photocatalytic NO purification, even under visible light irradiation. The heterojunction formed between the insulator SrCO3 and the photosensitizer BiOI, via a special SrCO3-BiOI core-shell structure, exhibits an enhanced visible light absorbance between 400-600 nm, and an unprecedentedly high photocatalytic NO removal performance. Further density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) analysis revealed that the covalent interaction between the O 2p orbital of the insulator (SrCO3, n-type) and the Bi 6p orbital of photosensitizer (BiOI, p-type) can provide an electron transfer channel between SrCO3 and BiOI, allowing the transfer of the photoexcited electrons from the photosensitizer to the conduction band of insulator (confirmed by charge difference distribution analysis and time-resolved fluorescence spectroscopy). The •O2- and •OH radicals are the main reactive species in photocatalytic NO oxidation. A reaction pathway study based on both in situ FT-IR and molecular-level simulation of NO adsorption and transformation indicates that this heterojunction can efficiently transform NO to harmless nitrate products via the NO → NO+ and NO2+ → nitrate or nitrite routes. This work provides numerous opportunities to explore earth-abundant insulators as visible-light-driven photocatalysts, and also offers a new mechanistic understanding of the role of gas-phase photocatalysis in controlling air pollution.