Development of Metal Oxide Materials for Rapid and Reversible DeNOx by Absorption into Solid Phase

The scope of this research is to develop a new solid absorbent capable for rapid and reversible deNOx of exhaust gases containing dilute nitrogen oxides in the presence of coexisting gases such as O_2, CO_2, and H_2O at high concentrations.We have discovered excellent ability of MnO_y-ZrO_2 and Pt-ZrO_2-Al_2O_3 mixed metal oxides as solid absorbents for deNOx in the presence of a large amount of oxygen. The highest capability for NOx absorption was found on MnO_y-ZrO_2, which demonstrated complete elimination of NOx as dilute as 75 ppm.DeNOx was rapid and reversible, being easily repeated with high reproducibility by heating or changing the atmosphere from oxidizing to reducing. The plausible mechanism of NOx absorption is oxidation of NO at the Mn site followed by absorption of NO_2 into solid phase via formation of metal (Zr) nitrate. Because the mixed oxide contains no alkali or alkaline earth metals, coexisting CO_2 and H_2O had no influence on the deNOx ability of the material. The major species desorbed from MnO_y-ZrO_2 were NO and NO_2, while they also contained a substantial amount of N_2 and N_2O when exposed to the reducing atmosphere by use of reducing agents such as C_3H_8 and C_2H_5OH.Moreover, Pt-ZrO_2-Al_2O_3 was also found to be capable for reversible NOx absorption. Desorbed species from Pt-ZrO_2-Al_2O_3 under the reducing atmosphere was revealed to consist of as much as 70% of N_2, indicating that selective reduction of desorbing NOx is significant on the Pt catalyst supported on the absorbent. Furthermore, Pt-ZrO_2-Al_2O_3 was revealed to be highly tolerant to SOx in contrast to MnO_y-ZrO_2, whose deNOx capacity gradually decreased by repeated operation in the presence of SO_2. In conclusion, the metal oxide-based absorbents investigated in the present research were confirmed to be capable for rapid and completely reversible absorption of dilute NOx into solid phase at relatively low temperature such as 300ﾟC in the presence of O_2, CO_2, and H_2O.The sorption-desorption of NOx can be cycled by elevating temperature or changing the atmosphere from oxidizing to reducing, resulting in releasing NOx having concentrated in the solid absorbents.