Effective formaldehyde elimination over pyrolusite-manganite hybrid catalysts promoted by Keggin acid decoration: Tungsten doping and chemically adsorbed active oxygen

A series of pyrolusite-manganite composites with five levels of phosphotungstic acid additions (HPWxMn, x indicating the added mass of phosphotungstic acid) were applied in the catalytic oxidation of 330-350mg/m3 formaldehyde under ambient conditions. Different techniques were used to characterize the pristine composite and the best-performed HPW0.2Mn catalyst. Compared to the effects of hydrogen (replacing a small part of structural K+ ions) and phosphorus (not detected in the HPW0.2Mn catalyst) elements in phosphotungstic acid, the doping of tungsten would prefer to occur via forming W-O-Mn species, which significantly altered the crystallite growth and induced numerous defects. As to formaldehyde degradation, the best 91% conversion and 72% CO2 yield were achieved on HPW0.2Mn. The uniqueness of tungsten doping on boosting formaldehyde oxidation was expounded from the angles of intimate elemental interaction (XPS), redox behavior and molecular O2 activation (CO-TPR, UV-Vis DRS and O2-TPD). The incorporation of tungsten led to higher surface Mn3+ concentration, higher content of surface oxygen vacancies, better low-temperature reducibility and abounding chemically adsorbed oxygen species (They were proposed to be the dominant oxidants for formaldehyde oxidation in this study), which were well correlated with the sequence of catalytic activity. Finally, possible reaction pathways were examined by coupling the characterizations of used catalysts.