Formic acid as the in-situ hydrogen source for catalytic reduction of nitrate in water by PdAg alloy nanoparticles supported on amine-functionalized SiO2

•PdAg/SiO2-NH2 catalyst for catalytic nitrate reduction with HCOOH as reducing agent.•The closed reaction system enhanced catalytic nitrate reduction activity.•Surface -NH2 modification enhanced nitrate reduction efficiency.•PdAg/SiO2-NH2 demonstrated better nitrate reduction efficiency than PdCu/SiO2-NH2.•Different optimal Ag contents for catalytic HCOOH decomposition and nitrate reduction.

Nitrate pollution in water is becoming a severe problem all over the world, and the catalytic reduction of nitrate by reducing agents had been considered as one of the most promising methods because it could convert nitrate to harmless nitrogen gas with a high efficiency. In this work, the PdAg/SiO2-NH2 catalyst was developed by firstly loading Pd onto NH2 surface-modified SiO2 catalyst support, followed by a controlled surface reaction to load Ag and create PdAg alloy nanoparticles on SiO2-NH2 catalyst support. For the first time, the PdAg/SiO2-NH2 catalyst was found to be able to effectively reduce nitrate with HCOOH as the reducing agent precursor. Its enhanced nitrate reduction performance could be attributed to the combination effects from the surface NH2 modification and the better electron transfer from Ag to Pd than from other metals due to their larger difference of work function, both of which were beneficial for the catalytic HCOOH decomposition to provide the in situ sources of the reducing agent of H2 and buffer of CO2 for the catalytic nitrate reduction process. To optimize the nitrate reduction performance of the PdAg/SiO2-NH2 catalyst, Ag content was carefully modulated and the Pd:Ag molar ratio of 1:0.5 was found to have the most efficient nitrate reduction capability.

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