Rapid removal of nitrate with high nitrogen selectivity using formic acid as reductant over Pd-In catalyst: effect of catalyst composition and reaction conditions on activity and selectivity

Nitrate is detrimental to human health and contributes to eutrophication. This study evaluated conditions for rapid, nitrogen-selective nitrate removal using formic acid as a reducing agent with a Pd-In/TiO₂ catalyst. Compared to hydrogen, formic acid effectively and rapidly removed high concentrations of nitrate while exhibiting lower selectivity for ammonium ions; the degradation rate was 2.5 times higher, with ammonia selectivity reduced to less than one-third at a nitrate concentration of 16 mM. Catalysts with an In/Pd ratio of 0.2–0.4 displayed high decomposition rates. The In/Pd ratio notably affected the selectivity between nitrous oxide and nitrogen, with a rise in nitrogen selectivity at lower In/Pd ratios (81% at In/Pd = 0.1, 32% at In/Pd = 0.8). Nitrous oxide decomposition to nitrogen likely occurs at Pd sites, suggesting that, in addition to using catalysts with a low In/Pd ratio, increasing the catalyst amount or incorporating Pd-supported catalysts can enhance nitrogen selectivity. The pH also plays a significant role, with the highest decomposition rate observed at pH 6. Increasing the pH from 3 to 6 notably decreased nitrous oxide selectivity (from 70 to 4%) while increasing nitrogen selectivity (from 26 to 63%) and ammonium ion selectivity. Conducting the reaction at a controlled pH of 6 with a catalyst that exhibits low selectivity for ammonium ions enables rapid and nitrogen-selective removal of nitrate.