Catalytic Reduction of Aqueous Chlorate with MoO _xImmobilized on Pd/C

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 MoOx-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 MoOx 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.