Browsing by Author "Guo, Sujin"

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    Advanced Reduction of Nitrogen-Oxyanions Using Precious Metal-Based Model Catalysts
    (2020-08-14) Guo, Sujin; Wong, Michael S
    Nitrate (NO3−) is a contaminant detected globally in surface water and underground aquifers. Nitrate pollution occurs due to the overuse of nitrogen-rich agriculture fertilizers, wastewater discharge, and contaminant leaching from landfills. This anion, in addition to its partially reduced form, nitrite (NO2−), can cause adverse health effects in humans including methemoglobinemia (blue baby syndrome), and is a suspected carcinogen. Pd-based catalytic reduction of nitrate and nitrite to nontoxic dinitrogen has emerged as an advanced treatment technology for drinking water decontamination. The primary goal of this work is to better understand the catalytic mechanisms of nitrate reduction using structure-controlled model palladium (Pd)-based catalysts to catalytically remove NO3−/NO2− from drinking water. The effects of surface coverages of metal promoter, catalyst support, and other promoting metals were explored. This work provides new insights into the reaction mechanism, and the design of catalysts with enhanced activity and selectivity in addition to deactivation resistance in model drinking water. Bimetallic Pd-based catalysts have been found to be promising for treating NO3−/NO2− contaminated waters. Those containing indium (In) are unusually active, but the mechanistic explanation for catalyst performance remains largely unproven. Different surface coverages of In deposited on Pd nanoparticles (NPs) (“In-on-Pd NPs”) exhibited room-temperature nitrate catalytic reduction activity that varies with a volcano-shape dependence on In surface coverage. The most active catalyst had an In surface coverage of 40%, whereas monometallic Pd NPs and In2O3 have nondetectable activity for nitrate reduction. X-ray absorption spectroscopy (XAS) results indicated that In is oxidized in the as-synthesized catalyst; reduces to zerovalent metal in the presence of H2, and reoxidizes following exposure to NO3−. Density functional theory (DFT) simulations from collaborators suggested that sub-monolayer coverage amounts of metallic In provide strong binding sites for nitrate adsorption and lower the activation barrier for the nitrate-to-nitrite reduction step. This improved understanding of the In active site expands the prospects of improved denitrification using metal-on-metal catalysts. The use of magnetic iron oxide (Fe3O4) support was also used to explore the recyclability and reusability of Pd-In nanoparticles. Magnetic catalysts offer the possibility of rapidly eliminating NO3−, without generating a secondary waste stream, and easily reusing for multiple reactions. In order to evaluate the function of Fe3O4 magnetic core, a four-component catalyst (Pd-In/nFe3O4@SiO2) was synthesized and NO3− reduction reaction was conducted in both clean water and simulated drinking water (SDW). The magnetically recoverable bimetallic Pd-In material exhibits excellent chemical stability, reusability, and high nitrate removal efficiency. The Pd-In/Fe3O4@SiO2 contains nanocrystalline magnetite with a silica shell upon which indium-decorated palladium nanoparticles were attached. The SiO2 shell slowed down iron leaching from Fe3O4 and the bimetallic nano-domains showed nitrate reduction activity in deionized (DI) water without obvious deactivation through multiple recovery and reuse cycles. This magnetically responsive reusable catalyst, which retained activity in simulated drinking water, can serve as a design basis for materials to degrade other oxyanion water contaminants. Lastly, the promotional effect of gold in trimetallic InPdAu was explored for nitrate hydrogenation. A range of mixed alloy PdAu nanoparticles (NPs) were synthesized with varying Pd:Au atomic ratios (90:10 to 10:90), before depositing submonolayer amounts of In metal. The resulting series of In-on-PdAu NPs especially Pd-rich samples had higher activity than In-on-Pd NPs for nitrate hydrogenation, due to optimized electronic and ensemble effects between Pd and Au that resulted in acceleration of the intermediate reduction of the overall hydrogenation reaction. The Au-rich NPs had lower activity, likely due to over-dilution of Pd surface that resulted in unfavorable hydrogen and nitrate/nitrite binding energies. In-on-PdAu generally showed higher N2 selectivity than In-on-Pd, respectively. In-decorated mixed PdAu alloy structure further enhances nitrate reduction performance and expands the prospects of improved denitrification using metal-on-metal catalysts. In summary, Pd-based catalysts can be tailored for enhanced activity, selectivity, longevity and reusability, and catalytic treatment holds the promise for advanced nitrogen-oxyanions treatment.
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    Design of a Pd–Au Nitrite Reduction Catalyst by Identifying and Optimizing Active Ensembles
    (American Chemical Society, 2019) Li, Hao; Guo, Sujin; Shin, Kihyun; Wong, Michael S.; Henkelman, Graeme
    Nitrate (NO3–) is a ubiquitous contaminant in groundwater that causes serious public health issues around the world. Though various strategies are able to reduce NO3– to nitrite (NO2–), a rational catalyst design strategy for NO2– removal has not been found, in part because of the complicated reaction network of nitrate chemistry. In this study, we show, through catalytic modeling with density functional theory (DFT) calculations, that the performance of mono- and bimetallic surfaces for nitrite reduction can be rapidly screened using N, N2, and NH3 binding energies as reactivity descriptors. With a number of active surface atomic ensembles identified for nitrite reduction, we have designed a series of “metal-on-metal” bimetallics with optimized surface reactivity and a maximum number of active sites. Choosing Pd-on-Au nanoparticles (NPs) as candidate catalysts, both theory and experiment find that a thin monolayer of Pd-on-Au NPs (size: ∼4 nm) leads to high nitrite reduction performance, outperforming pure Pd NPs and the other Pd surface compositions considered. Experiments show that this thin layer of Pd-on-Au has a relatively high selectivity for N2 formation, compared to pure Pd NPs. More importantly, our study shows that a simple model, based upon DFT-calculated thermodynamic energies, can facilitate catalysts design relevant to environmental issues.
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    Room-Temperature Catalytic Treatment of High-Salinity Produced Water at Neutral pH
    (American Chemical Society, 2020) Yin, Y. Ben; Coonrod, Christian L.; Heck, Kimberly N.; Said, Ibrahim A.; Powell, Camilah D.; Guo, Sujin; Reynolds, Michael A.; Wong, Michael S.
    Produced waters from hydraulic fracturing (HFPW) operations greatly challenge traditional water treatment technologies due to the high concentrations of total dissolved solids (TDS), highly complex and variable water matrices, and significant residual hydrocarbon content. We recently reported the unusual ability of a PdAu catalyst to degrade phenol in simulated HFPW at room temperature by generating H2O2 in situ from formic acid and air. Phenol removal occurred at TDS levels as high as ∼10 000 ppm (ionic strength I = 0.3 M), but the catalytic reaction required pH < 4 to proceed. Here, we find that PdAu, Pd, and Au degraded phenol in the pH 5–8 range by using hydroxylamine as the hydrogen source in place of formic acid. Pd exhibited the highest activity, and Au the least. Activity of the monometallic catalysts decreased >70% as TDS increased from 0 to ∼100 000 ppm (I = 3 M), whereas the PdAu was comparatively less affected (∼50% activity decrease). All catalysts remained active at TDS levels as high as 100 000 ppm. The majority of the hydroxylamine formed N2, however this reaction generated additional nitrite/nitrate anion byproducts with nitrogen selectivities ranging from 0.5% to 11.5%, depending on the catalyst identity and reaction salinity. To demonstrate one possible flow treatment process concept, we constructed and tested a recirculating trickle bed reactor that removed 28% phenol from simulated HFPW over 48 h. These results show the potential of oxidation catalysis as a treatment approach for produced water and other high-salinity industrial wastewaters.
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    Titanium oxide improves boron nitride photocatalytic degradation of perfluorooctanoic acid
    (Elsevier, 2022) Duan, Lijie; Wang, Bo; Heck, Kimberly N.; Clark, Chelsea A.; Wei, Jinshan; Wang, Minghao; Metz, Jordin; Wu, Gang; Tsai, Ah-Lim; Guo, Sujin; Arredondo, Jacob; Mohite, Aditya D.; Senftle, Thomas P.; Westerhoff, Paul; Alvarez, Pedro; Wen, Xianghua; Song, Yonghui; Wong, Michael S.; Center for Nanotechnology Enabled Water Treatment
    Boron nitride (BN) has the newly-found property of degrading recalcitrant polyfluoroalkyl substances (PFAS) under ultraviolet C (UV-C, 254 nm) irradiation. It is ineffective at longer wavelengths, though. In this study, we report the simple calcination of BN and UV-A active titanium oxide (TiO2) creates a BN/TiO2 composite that is more photocatalytically active than BN or TiO2 under UV-A for perfluorooctanoic acid (PFOA). Under UV-A, BN/TiO2 degraded PFOA ∼ 15 × faster than TiO2, while BN was inactive. Band diagram analysis and photocurrent response measurements indicated that BN/TiO2 is a type-II heterojunction semiconductor, facilitating charge carrier separation. Additional experiments confirmed the importance of photogenerated holes for degrading PFOA. Outdoor experimentation under natural sunlight found BN/TiO2 to degrade PFOA in deionized water and salt-containing water with a half-life of 1.7 h and 4.5 h, respectively. These identified photocatalytic properties of BN/TiO2 highlight the potential for the light-driven destruction of other PFAS.
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    Treating Water by Degrading Oxyanions Using Metallic Nanostructures
    (American Chemical Society, 2018) Yin, Yiyuan B.; Guo, Sujin; Heck, Kimberly N.; Clark, Chelsea A.; Coonrod, Christian L.; Wong, Michael S.; Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment
    Consideration of the water–energy–food nexus is critical to sustainable development, as demand continues to grow along with global population growth. Cost-effective, sustainable technologies to clean water of toxic contaminants are needed. Oxyanions comprise one common class of water contaminants, with many species carrying significant human health risks. The United States Environmental Protection Agency (US EPA) regulates the concentration of oxyanion contaminants in drinking water via the National Primary Drinking Water Regulations (NPDWR). Degrading oxyanions into innocuous compounds through catalytic chemistry is a well-studied approach that does not generate additional waste, which is a significant advantage over adsorption and separation methods. Noble metal nanostructures (e.g., Au, Pd, and Pt) are particularly effective for degrading certain species, and recent literature indicates there are common features and challenges. In this Perspective, we identify the underlying principles of metal catalytic reduction chemistries, using oxyanions of nitrogen (NO2–, NO3–), chromium (CrO42–), chlorine (ClO2–, ClO3–, ClO4–), and bromine (BrO3–) as examples. We provide an assessment of practical implementation issues, and highlight additional opportunities for metal nanostructures to contribute to improved quality and sustainability of water resources.