Surficial engineering of active hydroxyls for ambient formaldehyde oxidation via enhanced Lewis acidity over Zr-doped cryptomelane materials

Abstract

Lewis acids of solid catalysts have been featured for a pivotal role in promoting various reactions. Regarding the oxidation protocol to remove formaldehyde, the inherent drawback of the best-studied MnO₂ materials in acidic sites has eventually caused deficiency of active hydroxyls to sustain low-temperature activity. Herein, the cryptomelane-type MnO₂ was targeted and it was tuned via incorporation of Zr metal, exhibiting great advances in not only the complete HCHO-to-CO₂ degradation but also cycling performance. Zr species were existent in doping state in the MnO₂ lattice, rendering lower crystallinity and breaking the regular growth of MnO₂ crystallites, which thereby tripled surface area and created larger volume of smaller mesopores. Meantime, the local electronic properties of Mn atoms were also changed by Zr doping, i.e., more low-valence Mn species were formed due to the electron transfer from Zr to Mn. The results of infrared studies demonstrate the higher possession of Lewis acid sites on ZrMn, and this high degree of electrophilic agents favored the production of hydroxyl species. Furthermore, the reactivity of surface hydroxyls, as investigated by CO temperature programmed reduction and temperature programmed desorption of adsorbed O₂, was obviously improved as well after Zr modification. It is speculated jointly with the characterizations of the post-reaction catalysts that the accelerated production of active hydroxyls helped rapidly convert formaldehyde into key intermediate-formate, which was then degraded into CO₂, avoiding the side reaction path with undesired intermediate-hydrocarbonate-over the pristine MnO₂, where active sites were blocked and formaldehyde oxidation was inhibited. Additionally, Zr decoration could stabilize Lewis acidity to be more resistant to heat degeneration, and this merit brought about advantageous thermal recyclability for cycled application.

Keywords: Air purification; Catalytic oxidation; Formaldehyde; Hydroxyl; Manganese oxide; Structure—Performance relation.