Hydroxylamine electrochemistry at polycrystalline platinum in acidic media: a voltammetric, DEMS and FTIR study

摘要

The electrochemical activity of hydroxylamine on polycrystalline Pt in acidic media has been characterized using cyclic voltammetry, on-line differential electrochemical mass spectrometry (DEMS) and in situ FTIR. The electrochemistry of hydroxylamine (HAM) is essentially controlled by other species that interact strongly with the electrode surface. Therefore, quite moderate current densities, both in oxidation and reduction, are observed in a wide potential window between ca 0 and 1 V. The HAM electroreduction is a slow process and is masked by the Hupd. No formation of gaseous products was detected in this region; hence, ammonia must be the main product of HAM reduction. The HAM electro-oxidation is strongly influenced by the adsorption of its products, as well as their chemical transformations in solution. The key intermediate in HAM oxidation is NO, based both on voltammetric and spectroscopic evidence. Nitric oxide forms an adsorbed layer, stable over a wide potential region between ca. 0.55 and 0.75 V. At higher potentials NOads is oxidized to form (adsorbed) HNO2. At potentials above ca. 0.9 V the accumulation of HNO_2 in solution, accompanied by its partial oxidation to NO_2, is postulated. N_2O formation, observed in the potential region between ca. 0.5 and 1 V, has multiple sources. The most important source of N_2O is a homogeneous reaction between HNO_2 and HAM. The Tafel slope analysis suggests the second electron transfer to be the rate-determining step in HAM oxidation to NOads. A tentative mechanism for this reaction is proposed.