Intricate role of doping with d0 ions (Zr4+, V5+, Mo6+, W6+) on cryptomelane (K-OMS-2) performance in the catalytic soot combustion in presence of NO and SO2

Abstract

Two series of cryptomelane doped with d0 ions (Zr4+, V5+, Mo6+, W6+) were synthesized by impregnation and coprecipitation methods. The obtained samples were characterized in detail by XRD, RS, TEM/EDX, XRF, XPS, O2-TPD and by the work function measurements. Catalytic tests were performed in tight and loose contacts using various reaction mixtures (O2, O2 + NO, O2 + NO + SO2). The role of dopants was examined and discussed in terms of their influence on the principal catalysts’ functionalities such as catalytic performance, structural and thermal stability and resistance to SO2. The work function was found to be a prime descriptor used for the explanation of the catalytic performance in soot oxidation, taking into account the nature of dopants, the way they were introduced, and the composition of the reaction mixture. Due to the formation of a hot ring at the catalyst-soot interface, the oxidation of soot and NO are coupled and significantly influenced by the presence of SO2. It was found that the catalytic activity of the impregnated samples results from the opposite effects of the work function on soot combustion in NO and on sulfur poisoning of the beneficial NO to NO2 oxidation. It leads to an optimal work function value of 4.04 eV observed for the W-doped catalyst. For the coprecipitated samples, due to the non-monotonous and opposite character of those processes, the effect of doping on the cryptomelane catalytic performance in the presence of SO2 is smeared out.