Soot deep oxidation catalyzed by molybdena and molybdates: a thermogravimetric investigation

Molybdena (MoO₃) and molybdates of bismuth (Bi₂Mo₃O₁₂), chromium (Cr₂Mo₃O₁₂), barium (BaMoO₄), manganese (MnMoO₄) and copper (Cu₃Mo₂O₉) were synthesized and characterized by X-ray powder diffractometry and infrared spectroscopy. They were then assessed as ‘loose contact' catalysts for soot deep oxidation (combustion) in air by thermogravimetry. A similar assessment was carried out on commercial chromia (Cr₂O₃) and tungsta (WO₃). Observed high oxidation activity of MoO₃, as compared to both Cr₂O₃ and WO₃, is attributed to the higher volatility (mobility) of MoO_x species. On similar grounds, observed high activity of MoO₃ and Cu₃Mo₂O₉, as compared to the other test molybdates, is explained. Relatively speaking, however, a higher activity was observed for Cu₃Mo₂O₉ than MoO₃, whereby soot ignition temperature decreased from 571°C (uncatalyzed oxidation) to 430°C, to occur within the temperature range of diesel exhaust (200–450°C). This observation is ascribed to copper-promoted redox conduct of Mo(VI) in the oxidation reaction of soot. Kinetics of the reaction was studied non-isothermally, and the kinetic parameters (A, k, ΔE and the reaction order) were calculated.