Catalytic Reduction of Aqueous Chlorate With MoOx Immobilized on Pd/C

Critical Review of Pd-Catalyzed Reduction Process for Treatment of Waterborne Pollutants

Catalyzed reduction processes have been recognized as important and supplementary technologies for water treatment, with the specific aims of resource recovery, enhancement of bio/chemical-treatability of persistent organic pollutants, and safe handling of oxygenate ions. Palladium (Pd) has been widely used as a catalyst/electrocatalyst in these reduction processes. However, due to the limited reserves and high cost of Pd, it is essential to gain a better understanding of the Pd-catalyzed decontamination process to design affordable and sustainable Pd catalysts. This review provides a systematic summary of recent advances in understanding Pd-catalyzed reductive decontamination processes and designing Pd-based nanocatalysts for the reductive treatment of water-borne pollutants, with special focus on the interactions and transformation mechanisms of pollutant molecules on Pd catalysts at the atomic scale. The discussion begins by examining the adsorption of pollutants onto Pd sites from a thermodynamic viewpoint. This is followed by an explanation of the molecular-level reaction mechanism, demonstrating how electron-donors participate in the reductive transformation of pollutants. Next, the influence of the Pd reactive site structure on catalytic performance is explored. Additionally, the process of Pd-catalyzed reduction in facilitating the oxidation of pollutants is briefly discussed. The longevity of Pd catalysts, a crucial factor in determining their practicality, is also examined. Finally, we argue for increased attention to mechanism study, as well as precise construction of Pd sites under batch synthesis conditions, and the use of Pd-based catalysts/electrocatalysts in the treatment of concentrated pollutants to facilitate resource recovery.

Enhancing Aqueous Chlorate Reduction Using Vanadium Redox Cycles and pH Control

Chlorate (ClO3-) is a toxic oxyanion pollutant from industrial wastes, agricultural applications, drinking water disinfection, and wastewater treatment. Catalytic reduction of ClO3- using palladium (Pd) nanoparticle catalysts exhibited sluggish kinetics. This work demonstrates an 18-fold activity enhancement by integrating earth-abundant vanadium (V) into the common Pd/C catalyst. X-ray photoelectron spectroscopy and electrochemical studies indicated that VV and VIV precursors are reduced to VIII in the aqueous phase (rather than immobilized on the carbon support) by Pd-activated H2. The VIII/IV redox cycle is the predominant mechanism for the ClO3- reduction. Further reduction of chlorine intermediates to Cl- could proceed via VIII/IV and VIV/V redox cycles or direct reduction by Pd/C. To capture the potentially toxic V metal from the treated solution, we adjusted the pH from 3 to 8 after the reaction, which completely immobilized VIII onto Pd/C for catalyst recycling. The enhanced performance of reductive catalysis using a Group 5 metal adds to the diversity of transition metals (e.g., Cr, Mo, Re, Fe, and Ru in Groups 6-8) for water pollutant treatment via various unique mechanisms.

Pd sub-nanolayer on Au core for enhanced catalytic hydrogenation reduction of oxyanions pollutants: Synergistic effect of Pd and Au

Oxyanion pollutants in industrial wasterwater, such as (Cr(VI)), BrO3- (Br(V)) and SeO32- (Se(IV)) have detrimental or toxic effects on individual health when their concentrations accumulated to a certain level. The conversion of these oxyanions into harmless/industrial-valuable products or removal from wastewater is of significance. Herein, we designed Pd sub-nanolayer on Au core catalysts supported on Al2O3 (sub-Pd-Au/Al2O3) for highly effective catalytic hydrogenation reduction of oxyanions under ambient conditions. The sub-Pd(0.049)-Au(0.927)/Al2O3 catalyst exhibited the highest catalytic activity and TOF value for Cr(VI), Br(V) and Se(IV) reduction, respectively, by optimizing the Pd loading amount. The synergistic effect between Pd sub-nanolayer and Au core enhanced catalytic activity by regulating the Pd dispersion and site property, according to thorough characterizations that included high-angle annular dark-field transmission electron microscopy (HAADF-TEM) image, in-situ CO-IR adsorption, CO chemisorption, and X-ray photoelectron spectroscopy (XPS). This work might provide some new lights on design of highly efficient catalysts for the elimination of oxyanion pollutants.

Efficient purification of a nitrate and chlorate mixture in water via photoredox activated intermediate coupling-decoupling pathway

Nitrate (NO3-) is a widespread contaminant that threatens human health and ecological safety. Meanwhile, the disinfection byproducts chlorate (ClO3-) is generated inevitably in conventional wastewater treatment. Therefore, the contaminants mixture of NO3- and ClO3- are universal in common emission units. Photocatalysis technology is a feasible approach for the synergistic abatement of contaminant mixture, where matching suitable oxidation reactions is a potential strategy to improve the photocatalytic reduction reactions. Herein, formate (HCOOH) oxidation is introduced to facilitate the photocatalytic reduction of the NO3- and ClO3- mixture. As a result, high purification efficiency of NO3- and ClO3- mixture are achieved, evidenced by 84.6% e--dependent removal of the mixture at a reaction time of 30 min, with 94.5% N2 selectivity and 100% Cl- selectivity, respectively. Specifically, by the close combination of in-situ characterizations and theoretical calculations, the detailed reaction mechanism is revealed, in which the intermediate coupling-decoupling route from NO3- reduction and HCOOH oxidation is established by the chlorate-induced photoredox activation, leading to the significantly enhanced efficiency for the wastewater mixture purification. The practical application of this pathway is established for simulated wastewater to show its wide applicability. This work provides new insights into photoredox catalysis technology for its environmental application.

Preparation and Synergy of Supported Ru0 and Pd0 for Rapid Chlorate Reduction at pH 7

Chlorate (ClO₃^-) is a common water pollutant due to its gigantic scale of production, wide applications in agriculture and industry, and formation as a toxic byproduct in various water treatment processes. This work reports on the facile preparation, mechanistic elucidation, and kinetic evaluation of a bimetallic catalyst for highly active ClO₃^- reduction into Cl^-. Under 1 atm H₂ and 20 °C, Pd^II and Ru^III were sequentially adsorbed and reduced on a powdered activated carbon support, affording Ru⁰-Pd⁰/C from scratch within only 20 min. The Pd⁰ particles significantly accelerated the reductive immobilization of Ru^III as >55% dispersed Ru⁰ outside Pd⁰. At pH 7, Ru-Pd/C shows a substantially higher activity of ClO₃^- reduction (initial turnover frequency >13.9 min^-1 on Ru⁰; rate constant at 4050 L h^-1 g_metal^-1) than reported catalysts (e.g., Rh/C, Ir/C, Mo-Pd/C) and the monometallic Ru/C. In particular, Ru-Pd/C accomplished the reduction of concentrated 100 mM ClO₃^- (turnover number > 11,970), whereas Ru/C was quickly deactivated. In the bimetallic synergy, Ru⁰ rapidly reduces ClO₃^- while Pd⁰ scavenges the Ru-passivating ClO₂^- and restores Ru⁰. This work demonstrates a simple and effective design for heterogeneous catalysts tailored for emerging water treatment needs.

Accelerating Catalytic Oxyanion Reduction with Inert Metal Hydroxides

Adding CrIII or AlIII salts into the water suspension of platinum group metal (PGM) catalysts accelerated oxyanion pollutant reduction by up to 600%. Our initial attempts of adding K2CrVIO4, K2CrVI2O7, or KCrIII(SO4)2 into Pd/C enhanced BrO3- reduction with 1 atm H2 by 6-fold. Instrument characterizations and kinetic explorations collectively confirmed the immobilization of reduced CrVI as CrIII(OH)3 on the catalyst surface. This process altered the ζ-potentials from negative to positive, thus substantially enhancing the Langmuir-Hinshelwood adsorption equilibrium constant for BrO3- onto Pd/C by 37-fold. Adding AlIII(OH)3 from alum at pH 7 achieved similar enhancements. The Cr-Pd/C and Al-Pd/C showed top-tier efficiency of catalytic performance (normalized with Pd dosage) among all the reported Pd catalysts on conventional and nanostructured support materials. The strategy of adding inert metal hydroxides works for diverse PGMs (palladium and rhodium), substrates (BrO3- and ClO3-), and support materials (carbon, alumina, and silica). This work shows a simple, inexpensive, and effective example of enhancing catalyst activity and saving PGMs for environmental applications.

Chlorate induced false reduction in chemical oxygen demand (COD) based on standard dichromate method: Countermeasure and mechanism

Deliberate addition of mildly oxidative chlorate (ClO₃^-), so-called "chemical oxygen demand (COD) remover", into wastewater in China or electrochemical production of ClO₃^- from Cl^- induces the false COD reduction, which would bring about false appearance of effluents meeting the COD discharge standards. In this study, an easy sulfite-based reduction method was developed for the first time to remove ClO₃^- from the water samples before COD determination to eliminate this interference of ClO₃^-. In this reaction system, keeping the reaction temperature of sulfite reducing ClO₃^- at 60 ^°C was crucial for fast ClO₃^- removal rate, fixed molar [sulfite]_ini/[chlorate]_ini ratio value and the synchronous exhaustion of sulfite and ClO₃^-, which were of great significance for the real application of this improved COD determination method. The ClO₃^- interference on COD determination could be successfully eliminated after 20 min reduction of ClO₃^- by sulfite at pH_ini 4.0∼6.0 with the molar [sulfite]_ini/[chlorate]_ini ratio value in the range of 5∼6 when concentration of ClO₃^- was below 5 mM. Despite of the involvement of SO₄^·- in the sulfite reducing ClO₃^- system, the degradation of organic matters by SO₄^·- could be greatly impeded due to the lessened dissolved oxygen for SO₄^·- production at high reaction temperature and the scavenging of SO₄^·- by sulfite. In this reaction system, ClO₂, ClO₂^- and ClO^- were also generated and could be further reduced by sulfite stoichiometrically via oxygen transfer process with Cl^- as the final product. In general, this study pioneered an effective, fast and convenient method for COD determination of the ClO₃^--laden wastewaters and evaluating the real electrochemical wastewater treatment performance in terms of COD removal.

Inspired by Nature-Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

Throughout the previous ten years many scientists took inspiration from natural molybdenum and tungsten-dependent oxidoreductases to build functional active site analogues. These studies not only led to an ever more detailed mechanistic understanding of the biological template, but also paved the way to atypical selectivity and activity, such as catalytic hydrogen evolution. This review is aimed at representing the last decade’s progress in the research of and with molybdenum and tungsten functional model compounds. The portrayed systems, organized according to their ability to facilitate typical and artificial enzyme reactions, comprise complexes with non-innocent dithiolene ligands, resembling molybdopterin, as well as entirely non-natural nitrogen, oxygen, and/or sulfur bearing chelating donor ligands. All model compounds receive individual attention, highlighting the specific novelty that each provides for our understanding of the enzymatic mechanisms, such as oxygen atom transfer and proton-coupled electron transfer, or that each presents for exploiting new and useful catalytic capability. Overall, a shift in the application of these model compounds towards uncommon reactions is noted, the latter are comprehensively discussed.

Ultrasound-mediated synthesis of nanoporous fluorite-structured high-entropy oxides toward noble metal stabilization

High-entropy oxides (HEOs) are an emerging class of advanced ceramic materials capable of stabilizing ultrasmall nanoparticle catalysts. However, their fabrication still relies on high-temperature thermal treatment methodologies affording nonporous architectures. Herein, we report a facile synthesis of single-phase, fluorite-structured HEO nanocrystals via an ultrasound-mediated co-precipitation strategy under ambient conditions. Within 15 min of ultrasound exposure, high-quality fluorite-structured HEO (CeHfZrSnErO_x) was generated as ultrasmall-sized particles with high surface area and high oxygen vacancy concentration. Taking advantage of these unique structural features, palladium was introduced and stabilized in the form of highly dispersed Pd nanoclusters within the CeHfZrSnErO_x architecture. Neither phase segregation of the CeHfZrSnErO_x support nor Pd sintering was observed under thermal treatment up to 900°C. The as-afforded Pd/CeHfZrSnErO_x catalyst exhibits good catalytic performance toward CO oxidation, outperforming Pd/CeO₂ of the same Pd loading, which highlights the inherent advantage of CeHfZrSnErO_x as carrier support over traditional oxides.

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Single-phase, fluorite-structured high-entropy oxides nanocrystals was synthesized

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An ultrasound-mediated co-precipitation strategy under ambient conditions was used

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CeHfZrSnErO_x exhibited high surface area and high oxygen vacancy concentration

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Pd nanoclusters within the CeHfZrSnErO_x architecture can be stabilized

Development of Nickel- and Magnetite-Promoted Carbonized Cellulose Bead-Supported Bimetallic Pd-Pt Catalysts for Hydrogenation of Chlorate Ions in Aqueous Solution

Cellulose grains were carbonized and applied as catalyst supports for nickel- and magnetite-promoted bimetallic palladium- and platinum-containing catalysts. The bimetallic spherical aggregates of Pd and Pt particles were created to enhance the synergistic effect among the precious metals during catalytic processes. As a first step, the cellulose bead-based supports were impregnated by nitrate salts of nickel and iron and carbonized at 973 K. After this step, the nickel was in an elemental state, while the iron was in a magnetite form in the corresponding supports. Then, Pd and Pt particles were deposited onto the supports and the catalyst surface; precious metal nanoparticles (10–20 nm) were clustered inside spherical aggregated particles 500–600 nm in size. The final bimetallic catalysts (i.e., Pd–Pt/CCB, Pd–Pt/Ni–CCB, and Pd–Pt/Fe₃O₄–CCB) were tested in hydrogenation of chlorate ions in the aqueous phase. For the nickel-promoted Pd–Pt catalyst, a >99% chlorate conversion was reached after 45 min at 80 °C. In contrast, the magnetite-promoted sample reached an 84.6% chlorate conversion after 3 h. Reuse tests were also carried out with the catalysts, and in the case of Pd–Pt/Ni–CCB after five cycles, the catalytic activity only decreased by ~7% which proves the stability of the system.

A Bioinspired Molybdenum Catalyst for Aqueous Perchlorate Reduction

Perchlorate (ClO₄^-) is a pervasive, harmful, and inert anion on both Earth and Mars. Current technologies for ClO₄^- reduction entail either harsh conditions or multicomponent enzymatic processes. Herein, we report a heterogeneous (L)Mo-Pd/C catalyst directly prepared from Na₂MoO₄, a bidentate nitrogen ligand (L), and Pd/C to reduce aqueous ClO₄^- into Cl^- with 1 atm of H₂ at room temperature. A suite of instrument characterizations and probing reactions suggest that the Mo^VI precursor and L at the optimal 1:1 ratio are transformed in situ into oligomeric Mo^IV active sites at the carbon-water interface. For each Mo site, the initial turnover frequency (TOF₀) for oxygen atom transfer from ClO_x^- substrates reached 165 h^-1. The turnover number (TON) reached 3840 after a single batch reduction of 100 mM ClO₄^-. This study provides a water-compatible, efficient, and robust catalyst to degrade and utilize ClO₄^- for water purification and space exploration.