Chlorate (ClO3 –) is an undesirable byproduct in the chlor-alkali process. It is also a heavily used chemical in various industrial and agricultural applications, making it a toxic water pollutant worldwide. Catalytic reduction of ClO3 – into Cl– by H2 is of great interest to both emission control and water purification, but platinum group metal catalysts are either sluggish or severely inhibited by halide anions. Here, we report on the facile preparation, robust performance, and mechanistic investigation of a MoO x –Pd/C catalyst for aqueous ClO3 – reduction. Under 1 atm H2 and room temperature, the Na2MoO4 precursor is rapidly immobilized from aqueous solution onto Pd/C as a mixture of low-valent Mo oxides. The catalyst enables complete reduction of ClO3 – in a wide concentration range (e.g., 1 μM to 1 M) into Cl–. The addition of Mo to Pd/C not only enhances the catalytic activity by >55-fold, but also provides strong resistance to concentrated salts. To probe the reaction mechanisms, we conducted a series of kinetic measurements, microscopic and X-ray spectroscopic characterizations, sorption experiments, tests with other oxyanion substrates, and a comparative study using dissolved Mo species. The catalytic sites are the reduced MoO x species (primarily MoIV), showing selective and proton-assisted reactivity with ClO3 –. This work demonstrates a great promise of using relatively abundant metals to expand the functionality of hydrogenation catalysts for environmental and energy applications.

The detection of perchlorate (ClO4 − ) on and beyond Earth requires ClO4 − reduction technologies to support water purification and space exploration. However, the reduction of ClO4 − usually entails either harsh conditions or multi-component enzymatic processes. We developed a heterogeneous Mo−Pd/C catalyst from sodium molybdate to reduce aqueous ClO4 − into Cl − with 1 atm H2 at room temperature. Upon hydrogenation by H2/Pd, the reduced Mo oxide species and a bidentate nitrogen ligand (1:1 molar ratio) are transformed in situ into oligomeric Mo sites on the carbon support. The turnover number and frequency for oxygen atom transfer from ClOx − substrates reached 3850 and 165 h −1 on each Mo site. This simple bioinspired design yielded a robust water-compatible catalyst for the removal and utilization of ClO4 − .

We have recently developed a highly active ligand-enabled (L)­Mo–Pd/C catalyst (L = 4,4′-diamino-2,2′-bipyridine) for aqueous perchlorate (ClO4 –) reduction with 1 atm H2 at room temperature. This study reports on a series of satisfactory properties of this catalyst closely relevant to ClO4 – treatment in waste brines resulting from ion-exchange resin regeneration. In the presence of concentrated salts and humic acid, the catalyst experienced limited inhibition but completed ClO4 – reduction in a few hours with an adjustable loading between 0.2 and 2 g/L. The catalyst was not deactivated by the high oxidative stress from multiple spikes of 100 mM ClO4 –. The challenge of deactivation by nitrate was solved by pretreating the brine with In–Pd/Al2O3. The loss of activity upon ligand hydrogenation was overcome by regenerating the Pd/C at pH 12. We also optimized the catalyst formulation and saved 70% of Pd without sacrificing the activity. The substantially enhanced performance and lowered adverse environmental impacts of (L)­Mo–Pd/C make the catalytic treatment competitive to microbial reactors for ClO4 – reduction. We showcase the power of coordination chemistry in environmental technology innovation and expect this catalyst to promote the reuse of ClO4 –-selective resins for sustainable water treatment.