FT-IR study of the nature and stability of NOX surface species on ZrO2, VOx/ZrO2 and MoOx/ZrO2 catalysts

The adsorption of NO, NO/O-2 mixtures and NO2 on pure ZrO2 and on two series of catalysts supported on ZrO2, one containing vanadia and the other molybdena (ZV and ZMo, respectively), has been investigated. The V and Mo surface contents of the latter were less than or equal to 3 atoms nm(-2) and less than or equal to 5 atoms nm(-2), respectively. All samples had been previously submitted to a standard oxidation treatment. On all samples, only extremely minor amounts of NOx surface species are formed by NO interaction at room temperature (RT). NOx surface species are formed in greater amounts on pure ZrO2 when NO and O-2 are coadsorbed at RT; they are mainly nitrites, small amounts of nitrates, and small amounts of (O2NO-H)(delta-) species; when ZrO2 is warmed to 623 K in the NO/O-2 mixture, nitrites decrease, nitrates and (O2NO-H)(delta-) species increase. The same NOx species as on the ZrO2 surface free from V (or Mo) are formed on ZV (or ZMo) samples with surface V (or Mo) density < 1.5 atoms nm(-2), however, they occur in decreased amount with increasing V (or Mo) coverage. On ZV samples with a surface V density of 1.5-3 atoms nm(-2) (or ZMo samples with a surface Mo density of 1.5-5 atoms nm(-2)) when NO and O-2 are coadsorbed at RT, there is formation of small amounts of nitrites, nitrates (both on ZrO2 surface free from V (or Mo) and at the edges of V- or Mo-poryoxoanions) and NO2delta+ species, associated with V5+ (or Mo6+) of very strong Lewis acidity; when samples are warmed up 623 K in the NO/O-2 mixture, nitrites disappear, nitrates increase, NO2delta+ species remain constant or slightly decrease. When NO, is allowed into contact at RT with oxidized samples, surface situations almost identical to those obtained for each sample warmed to 623 K in NO/O-2 mixture is reached. The NOx surface species stable at 623 K,the temperature at which catalysts show the best performance in the selective catalytic reduction (SCR) of NO by NH3, are nitrates, both on ZrO2 and on polyvanadates or polymolybdates at high nuclearity. On the contrary, nitrites and NO2delta+ species are unstable at 623 K.