This project explored using earth-abundant metals for the reductive treatment of oxyanions (e.g., perchlorate, chlorate, and bromate). They are toxic water pollutants from industrial wastes, agricultural applications, drinking water disinfection, etc. Earlier studies used palladium (Pd) catalysts to activate hydrogen gas (H₂) and reduce the oxyanions into harmless forms (e.g., chloride). However, Pd showed limited activity and often required acidic pH. In this project, we hypothesized that Earth-abundant elements in Group 6 (molybdenum and chromium), Group 5 (vanadium), and Group 8 (ruthenium) could accelerate oxyanion reduction by (1) participating in the redox cycle or (2) enabling novel mechanisms.

First, we developed a MoO_x–Pd/C catalyst with facile preparation and robust performance for chlorate (ClO₃^–) reduction. Under 1 atm H₂ and room temperature, Na₂MoO₄ (a typical micronutrient fertilizer) is rapidly immobilized from aqueous solution onto Pd/C as low-valent Mo oxides. The catalyst completely reduced ClO₃^– in a wide concentration range (1 μM to 1 M) into Cl^– (Fig. 1). The Mo species not only enhanced the activity by 55-fold but also exhibited strong resistance to concentrated salts.

We further added a nitrogen ligand (L) to the MoO_x–Pd/C catalyst to reduce the more inert perchlorate (ClO₄^–) and obtained the highest activity among all abiotic (non-microbial/enzymatic) catalysts. The Mo^VI precursor and L formed in situ into oligomeric Mo^IV active sites at the carbon–water interface (Fig. 2). The (L)MoO_x–Pd/C reduced ClO₄^– at 0.01–100 mM in the presence of various anions as a water-compatible, efficient, and robust catalyst to degrade and utilize ClO₄^– for water purification and space exploration.

We further evaluated the (L)MoO_x–Pd/C catalyst in conditions relevant to ion-exchange resin regeneration waste brines (Fig. 3). It showed limited inhibition by concentrated salts and humic acid and was not deactivated by the high oxidative stress from five spikes of 100 mM ClO₄^–. Deactivation by nitrate was solved by pretreating the brine with another catalyst, In–Pd/Al₂O₃. The loss of activity upon ligand hydrogenation was overcome by recovering the Pd/C at pH 12. We optimized the catalyst formulation and saved 70% of Pd without sacrificing the activity. Results show that (L)Mo–Pd/C is competitive to microbial ClO₄^– reduction.

Motivated by the success of Mo studies, we further explored Cr. Adding K₂Cr^VIO₄ and KCr^III(SO₄)₂ into Pd/C enhanced bromate (BrO₃^–) reduction by 6-fold. The Cr^VI was reduced and immobilized as Cr^III(OH)₃ on the catalyst surface and altered the ζ-potentials from negative to positive, thus enhancing the adsorption equilibrium constant for BrO₃^– by 37-fold. Adding Al^III(OH)₃ from alum at pH 7 achieved similar enhancements (Fig. 4). The Cr–Pd/C and Al–Pd/C showed top-tier catalytic performance among all reported Pd catalysts. The strategy of adding inert metal hydroxides works for multiple oxyanion pollutants and support materials by accelerating the reaction by up to 600%. This is a simple, inexpensive, and effective way to enhance catalyst activity and save precious metals for environmental applications.

Our further explorations of Group 5 vanadium (V) obtained an 18-fold activity enhancement by integrating V precursors into Pd/C (Fig. 5). V^V and V^IV were reduced to V^III in the aqueous phase (rather than immobilized on the support). The V^III/IV and V^IV/V redox cycles allowed ClO_x^– reduction. To capture the potentially toxic V metal from the treated solution, adjusting the pH from 3 to 8 after the reaction could completely immobilize V^III onto Pd/C for catalyst recycling.

Besides incorporating OAT metals, we established a convenient approach to preparing and optimizing Pd catalysts. Na₂PdCl₄ in water was quickly adsorbed by activated carbon within 5 min and reduced into Pd⁰ nanoparticles in situ within another 5 min under 1 atm H₂ at 20 °C. The Pd catalysts prepared with the new method showed no significant difference from benchmark commercial Pd catalysts. Using the new method, we quickly elucidated the relationships among the Pd content, Pd⁰ particle size, and catalytic activity. We further showcased that the precious metals in previously reported catalysts can be saved up to 80% without sacrificing the activity.

We further prepared a bimetallic palladium-ruthenium catalyst for highly active ClO₃^– reduction at pH 7 (Fig. 6). Pd^II and Ru^III were sequentially adsorbed and reduced on a carbon support, affording Ru⁰–Pd⁰/C from scratch within only 20 min. At pH 7, Ru–Pd/C shows a substantially higher activity of ClO₃^– reduction than all previous catalysts. In particular, it fully reduced 100 mM ClO₃^–, whereas Ru/C was quickly deactivated. In the bimetallic synergy, Ru⁰ rapidly reduces ClO₃^– while Pd⁰ scavenges the Ru-passivating ClO₂^– and restores Ru⁰.

In summary, this project substantially expanded the knowledge and toolbox of using transition metals for environmental applications. The outcomes have led to two patents. The project also trained multiple undergraduate and high school students who continued their careers in environmental chemistry-related fields.

Last Modified: 12/10/2023
Modified by: Jinyong Liu