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

Abstract

Nitrate and nitrite reduction reactions (NO₃RR and NO₂RR, respectively) are important processes in water treatment as well as model processes in surface science. The sluggish kinetics observed for NO₃RR on Pt electrodes is usually explained by the difficulty in removing NO_ad. However, NO₂RR 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_ad. Both NO₃RR and NO₂RR were studied on Pt(111) and Pt(111) modified with cyanide ions (Pt(111)-CN). While NO₂RR depicted lower activity on Pt(111)-CN than on Pt(111), NO₃RR was completely inhibited. Regardless of the presence of cyanide, the DFT-based analysis revealed that both NO₃ and NO₂ adsorption could occur on the bidentate form. However, after this step, extra contiguous sites should be provided to ensure NO_3ad 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 NO₃RR, and the presence of NO_ad (produced as intermediate during the NO₃RR) does not favor this step once NO_ad also limits the adsorption site size.