Understanding the activation mechanism induced by NOx on the performances of VOx/TiO2 based catalysts in the total oxidation of chlorinated VOCs

A previous investigation of the chlorobenzene combustion activity of VOx/TiO2, VOx-WO/TiO2 and VOx-MoOx/TiO2 catalysts in the presence of NO pointed out the activation effect of NO. The suggested three-step mechanism based on catalytic performances data only was: (1) chlorobenzene is oxidized on the surface of the VOx phase (as described by Mars-van Krevelen), (2) NO gets oxidized to NO2, mainly on WOx and MoOx and (3) the in situ produced NO2 assists O-2 in the reoxidation of the VOx phase thus speeding up the oxidation step of the Mars-van Krevelen mechanism. The latter effect macroscopically corresponds to the observed increase of chlorobenzene conversion. This contribution aims at validating this hypothetical mechanism by pointing out the favourable occurrence of an oxidation of NO to NO2 on the WOx and MoOx phases and by pointing out the higher efficiency of NO2 than O-2 to reoxidize the reduced VOx sites. In addition, the present contribution clearly demonstrates that, in the absence of NO, the chlorobenzene total oxidation occurred following the Mars-van Krevelen mechanism. Moreover, a thorough characterization of the oxidation state of the vanadium proving that the improvement of the catalyst activity brought by the simultaneous presence of NO and O-2 is linked to the stronger reoxidation of the VOx active phase. Furthermore, plotting all the catalytic activity data versus the mean vanadium oxidation level clearly depicts, for the first time, the strong dependence between them. Under a mean vanadium oxidation level of 4.82 the catalyst is inactive while above 4.87 the activity is stabilized at a high level of conversion independent of the vanadium oxidation level. (c) 2006 Elsevier B.V. All rights reserved.