Tailored Functional Oxide Nanomaterials by Chiara Maccato & Davide Barreca

Author:Chiara Maccato & Davide Barreca [Maccato, Chiara & Barreca, Davide]
Language: eng
Format: epub
Published: 2022-03-01T15:07:18+00:00

Figure 7.9 The conversion pathways for the NO adsorbed on the catalyst surface in the absence of light.

Source: L. Sánchez.

Once the catalyst is irradiated, the nitrogen species originated by the photochemical process are observed in the DRIFTS study. The nitrate and nitrite species are the major final products obtained in the oxidation process participated by ˙OH and ˙O2− radicals [21, 69, 123, 181]. However, reaction intermediates such as NO+, N2O3, NO2, and NO2+ are also observed [123]. The presence of N2O, together with N2O3, has also been observed, due to the coupling reaction between NO and NO2, or the NO with N atoms from the catalyst framework [21]. Also, N2O3 could be formed as a result of the direct reaction of NO with oxidant species [83, 89]. Regarding the nitrate species, its appearance as monodentate nitrate is transformed with time into more stable bidentate and bridged linkages [182]. In a similar way, the nitrite ion − which occurs when NO2 receives an electron − usually occurs in bridging form [102]. In some cases, it is observed that both NO and NO2 molecules can obtain electrons from the catalyst to form NO− and N2O22− [21]. Moreover, NO− also occurs from the disproportionation of NO adsorbed on the surface of the photocatalyst [85, 88]. It has also been reported for some catalysts that NO, once it makes contact with the surface, tends to lose electrons and form active NO+ [178, 180, 181, 183]. Finally, Araña et al. [184] reported the formation of stable [(NO3−)(H2O)n(NO2)] complexes, explaining a non‐efficient photocatalytic conversion of the NO2 gas. Moreover, the presence of this complex also inhibits the renoxification reaction of NO with NO3− to form NO2.

The multiple photochemical oxidation pathways for NOx molecules present on the catalyst surface under light irradiation are summarized in Figure 7.10.

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