Measurement of the effective diffusivity for a vanadia-tungsta-titania/sepiolite catalyst for SCR of NOx

Experimental measurement of the effective diffusion coefficient for a vanadia-tungsta-titania/sepiolite catalyst, as employed for SCR of NOx, was carried out using a chromatographic technique in a packed column (under inert conditions) employing helium as carrier and nitrogen, argon and oxygen as tracers. Measurements were carried out at temperatures 298 and 473 K with particle sizes 0.4 and 0.5 mm. The values obtained for D-e are from 0.6 10(-6) to 0.94 . 10(-6) for the gas tracers and temperature range employed. Adsorption constant for the gas tracers above quoted have been also determined giving values of K-a around 1.3, thus, a light adsorption strength existed. Because D-e depends on the gas temperature and composition, different structure models for the solid have been applied to calculate from D-e the corresponding structural parameters: tau (in pore models) or Z (in percolation model), which only depends on the solid structure and can be employed for other gases and temperatures. The tau values obtained from the bundle pore model (around 3) are smaller than calculated applying the cross linked pore model (tau between 5 and 6). The coordination number, Z, has a meaningless value when catalyst is analyzed as unimodal (Z < 3). On the other hand, if the experimental pore size distribution was taken into account Z yields appropriate values were between 5 and 6.