Using the site-knockout strategy to understand the low activity of the nitrate electro-reduction reaction on Pt(111)

Nitrate and nitrite reduction reactions (NORR and NORR, respectively) are important processes in water treatment as well as model processes in surface science. The sluggish kinetics observed for NORR on Pt electrodes is usually explained by the difficulty in removing NO. However, NORR shares this same intermediate and depicts higher activity under the same experimental conditions. Herein, we employed the site block strategy to show that the nitrate requires contiguous Pt sites to be converted into nitrite and then to NO. Both NORR and NORR were studied on Pt(111) and Pt(111) modified with cyanide ions (Pt(111)-CN). While NORR depicted lower activity on Pt(111)-CN than on Pt(111), NORR was completely inhibited. Regardless of the presence of cyanide, the DFT-based analysis revealed that both NO and NO adsorption could occur on the bidentate form. However, after this step, extra contiguous sites should be provided to ensure NO proceeds with the reduction reaction, which are not available on Pt(111)-CN. These results bring experimental evidence that nitrate to nitrite conversion is an important bottleneck in NORR, and the presence of NO (produced as intermediate during the NORR) does not favor this step once NO also limits the adsorption site size.