The thermal stability and catalytic application of MnO_x-ZrO₂ oxide powders

MnO_x-ZrO₂ mixed oxide is an active catalyst for combustion, oxidation, and oxygen storage applications. MnO_x-ZrO₂ mixture also has large reversible adsorption capability for NO_x, which makes it a promising candidate for NO_x abatement in automobile emission control. However, MnO_x-ZrO₂ mixed oxide has not been used extensively because the processing and the thermal stability of resulting powders have not been studied systematically. It is critical to have thermally stable catalytic material because the application temperature can reach as high as 1000°C during service. In this study, we focused on improving the thermal stability of oxide powders, such as MnO_x, ZrO₂, and MnO_x-ZrO₂, by controlling the processing methods and parameters. For pure MnO_x made from the precipitation method using Mn(NO₃)₂ aqueous solution and ammonium hydroxide, we found that lower concentration of Mn(NO₃)₂ solution and larger amount of ammonium hydroxide resulted in higher surface area powders. For pure ZrO₂, we found curing hydrous zirconia in the mother liquid produced ZrO₂ powders with larger pore volume and pore size. The specific surface area was also significantly enhanced by curing for the synthesized powders before calcination or after low temperature calcinations, and this improvement could be preserved to high temperatures if SiO₂ was doped in ZrO₂. A Monte Carlo simulation model examining the effect of primary particle packing on the specific surface area was used to explain the curing result. MnO_x-ZrO₂ mixtures had higher surface area than the single component oxide at 500 and 700 °C because composite powders sintered less. The sintering behavior of composite powders at 900 °C was opposite to that at 500 °C and the specific surface area of MnO_x-ZrO₂ decreased drastically at 900 °C. Curing ZrO₂ first or using La dopant could significantly enhance the specific surface area of MnO_x-ZrO₂ at 900 °C. Through the tests of the redox property and NO storage capability we found a close relationship between the enhanced thermal stability and better catalytic performance.