Highly active copper-intercalated weakly crystallized δ-MnO2 for low-temperature oxidation of CO in dry and humid air

Abstract

Copper intercalated birnessite MnO₂ (δ-MnO₂) with weak crystallinity and high specific surface area (421 m²/g) was synthesized by a one-pot redox method and investigated for low-temperature CO oxidation. The molar ratio of Cu/Mn was as high as 0.37, which greatly weakened the Mn-O bond and created a lot of low-temperature active oxygen species. In situ DRIFTS revealed strong bonding of copper ions with CO. As-synthesized MnO₂-150Cu achieved 100% conversion of 250 ppm CO in normal air (3.1 ppm H₂O) even at −10 °C under the weight-hourly space velocity (WHSV) of 150 L/(g·h). In addition, it showed high oxygen storage capacity to oxidize CO in inert atmosphere. Though the concurrent moisture in air significantly inhibited CO adsorption and its conversion at ambient temperature, MnO₂-150Cu could stably convert CO in 1.3% moisture air at 70 °C owing to its great low-temperature activity and reduced competitive adsorption of water with increased temperature. This study discovers the excellent low-temperature activity of weakly crystallized δ-MnO₂ induced by high content intercalated copper ions.