NO2 adsorption mechanism on TiO2: An in-situ transmission infrared spectroscopy study

SSCI-VIDE+ING+DBIInternational audienceThe adsorption of NO2 on oxidized TiO2 surface, under dark condition at 296 K, has been investigated by in-situ transmission Fourier Transform Infrared Spectroscopy (trans-FTIR) as a function of time. It enabled the determination of detailed NO2 reactive adsorption mechanisms on TiO2. It was evidenced that, as soon as NO2 molecules adsorb on TiO2 surface it dimerize to adsorbed N2O4 species. The strongly adsorbed N2O4 undergoes intramolecular disproportionation reaction and produces: (i) weakly adsorbed monodentate nitrate (m-NO3−) species and, (ii) highly reactive NO+ and/or N2O3 species on Ti4+ sites and O2− sites, respectively. The NO+ species reacts with surface lattice oxygen (O2−) and produces more stable NO2− on Ti4+ sites. Then, the NO2− undergoes intermolecular disproportionation reaction with another strongly adsorbed N2O4 molecule and produces strongly adsorbed bidentate nitrate (b-NO3−) species on Ti4+ sites and releases NO in the gas phase. It was also noticed that, as adsorption time increases, the weakly adsorbed m-NO3− species are converted into strongly adsorbed b-NO3− species. The intramolecular disproportionation reaction rate depends on NO2 partial pressure, whereas the intermolecular disproportionation reaction rate depends on the coverage of NO2− species and the number of available Ti4+ sites. This mechanism is assessed for different NO2 partial pressures ranging from 25 to 100 Pa. This study reveals that the configuration and the amount of the N-containing species on activated TiO2 surface depend on the NO2 concentration and the contact time