Quantitative room-temperature mineralization of airborne formaldehyde using manganese oxide catalysts

Abstract

Manganese oxide-based catalysts have been synthesized and tested for the abatement of formaldehyde, an ubiquitous indoor pollutant which is not effectively eliminated by most air cleaning technologies. Catalysts were prepared by co-precipitation of MnSO4 and NaMnO4 followed by curing at 100, 200 and 400°C. Characterization was performed using X-ray diffractometry (XRD), porosimetry, scanning electron microscopy (SEM), and inductively coupled plasma-mass spectrometry (ICP-MS). Diffractograms of samples treated at 100 and 200°C matched those of nsutite and cryptomelane/manjiroite structures, with high BET surface area (up to 149m2g−1) and small particle size (<50nm), while curing at 400°C yielded pyrolusite with lower effective surface area. Room temperature catalytic oxidation of airborne formaldehyde was studied by supporting the catalyst on a particulate filter media placed in a flow system, under stable upstream formaldehyde concentrations between 30 and 200ppb. Two different face velocities (v=0.2 and 50cms−1) were studied to evaluate the oxidation efficiency under different flow regimes using formaldehyde-enriched laboratory air at 25–30% relative humidity. Results showed consistent single-pass formaldehyde oxidation efficiency greater than 80% for the synthesized catalysts, which remained active over at least 35 days of continuous operation at v=0.2cms−1 and were able to process up to 400m3 of air at v=50cms−1 without appreciable deactivation. Operation under high relative humidity (>90% RH) produced only a small reversible reduction in formaldehyde removal. Most significantly, 100% mineralization yields were verified by quantifying CO2 formation downstream of the catalyst for upstream formaldehyde concentrations as high as 6ppm and a face velocity of v=13cms−1. In contrast, a filter loaded with commercially available MnO2 did not remove appreciable amounts of formaldehyde at v=50cms−1, and yielded <20% initial removal when operated at a very low face velocity (v=0.03cms−1). Due to the relatively low costs of synthesis and deployment of these catalysts, this technology is promising for maintaining low indoor formaldehyde levels, enabling energy-saving reductions of building ventilation rates.