DEMS and RAMAN study of the monatomic hydrogen adsorption during electro-reduction of NO₃- and NO₂- at Pt nanoparticles supported at W₁₈O₄₉-ZrO₂-C nanocomposite

A multifunctional hybrid material composed of W₁₈O₄₉ nanobelts (NBs), zirconium oxide, and Vulcan carbon (W₁₈O₄₉-ZrO₂-C) has been used as support for platinum nanoparticles (PtWZC). For comparison, platinum supported on Vulcan carbon (PtC) was also synthesized. These catalysts were evaluated in the NO₃- and NO₂- (NO_x) reduction reactions. The addition of W₁₈O₄₉ and ZrO₂ to the carbon improved the size and distribution of the platinum nanoparticles. The electrochemical results showed that enhanced electrocatalytic activity for the NO_x reduction was obtained on the PtWZC catalyst compared to PtC synthesized by the same technique and PtEtek from a commercial source. Our findings suggest that the adsorbed monatomic hydrogen (H_ads) over platinum inhibits the reduction of nitrates and nitrites. Therefore, these reactions need a material that removes H_ads from the surface or modifies it for the NO_x reduction. The WZC support adsorbed monatomic hydrogen from water to form active bronze species (H_x-W₁₈O₄₉) that interacted with the NO₃- and NO₂- ions enabling their reduction, in accord with the results from in-situ Raman spectroscopy and Tafel slopes. N₂, NO, NH₃, and N₂H₄ products were detected in nitrite ion reduction by the Differential electrochemical mass spectrometry (DEMS) technique on the PtWZC catalyst. The production of N₂ dominated in the cathodic and anodic scan. Hence, the novel catalyst can follow the indirect pathway, where the reduction process involves adsorption of NH_x and adsorption of NO. The higher electrocatalytic activity obtained by the PtWZC catalyst was attributed to the uniform distribution of platinum on the support, the increased electrical conductivity produced by the W₁₈O₄₉ nanobelts, and the modulation of H_ads on the surface.