Acid Etching-Induced In Situ Growth of λ-MnO₂ over CoMn Spinel for Low-Temperature Volatile Organic Compound Oxidation

Abstract

Surface lattice oxygen is crucial to the degradation of volatile organic compounds (VOCs) over transition metal oxides according to the Mars–van Krevelen mechanism. Herein, λ-MnO2 in situ grown on the surface of CoMn spinel was prepared by acid etching of corresponding spinel catalysts (CoMn-Hx-Ty) for VOC oxidation. Experimental and relevant theoretical exploration revealed that acid etching on the CoMn spinel surface could decrease the electron cloud density around the O atom and weaken the adjacent Mn–O bond due to the fracture of the surface Co–O bond, facilitating electron transfer and subsequently the activation of surface lattice oxygen. The obtained CoMn-H1-T1 exhibited an excellent catalytic performance with a 90% acetone conversion at 149 °C, which is 42 °C lower than that of CoMn spinel. Furthermore, the partially maintained spinel structure led to better stability than pure λ-MnO2. In situ diffuse reflectance infrared Fourier transform spectroscopy confirmed a possible degradation pathway where adsorptive acetone converted into formate and acetate species and into CO2, in which the consumption of acetate was identified as the rate-limiting step. This strategy can improve the catalytic performance of metal oxides by activating surface lattice oxygen, to broaden their application in VOC oxidation.

Elimination of NH3 by Interfacial Charge Transfer over the Ag/CeSnOx Tandem Catalyst

“…In addition, the cause of phase separation was experimentally demonstrated, as shown in Figure S7. The elevated temperature due to the exothermic combustion of AgNO 3 during calcination, the acidity of the AgNO 3 solution, and the secondary calcination after impregnation with AgNO 3 would cause the phase separation in the CeSnO x . − …”

Boosting the Catalytic Performance of Volatile Organic Compound Oxidation Over Platelike MnO2/CoAlO Catalyst by Weakening the Co–O Bond and Accelerating Oxygen Activation

“…The reducible species of the prepared catalysts were investigated by H 2 -TPR (Figure a). The peaks at around 212–280, 270–450, and 450–710 °C could be attributed to the reduction of Co 3+ to Co 2+ or the combined reduction processes of Mn 4+ to Mn 3+ and Co 3+ to Co 2+ , and Mn 3+ to Mn 2+ , and Co 2+ to Co 0 , respectively. ,, MnO 2 /CoAlO-F presented a higher reduction temperature at 229 °C for the first reduction peak in comparison to that for MnO 2 /CoAlO-P, at 212 °C . According to previous reports, the reduction peaks for the adsorbed oxygen species on oxygen vacancies generally appear below 200 °C. , Moreover, no O 2 desorption signals were detected at 100–250 °C (Figure S15).…”

“…This suggests that the first reduction peak in H 2 -TPR is due to the reduction of oxygen species from Mn–O or Co–O bonds rather than those adsorbed oxygen species on the oxygen vacancies. According to Hooke’s law, the strength of the Mn–O bond could be evaluated through the calculated bond force constant ( k ) ω = 1 2 π c k μ where ω is the Raman shift (cm –1 ) of Mn–O, c is the velocity of light, and μ is the effective mass. The smaller the k value is, the more easily the Mn–O bond will break.…”

“…This suggests that the first reduction peak in H 2 -TPR is due to the reduction of oxygen species from Mn−O or Co−O bonds rather than those adsorbed oxygen species on the oxygen vacancies. According to Hooke's law, 26 the strength of the Mn−O bond could be evaluated through the calculated bond force constant (k)…”

“…In our previous research, it was found that the Co 3+ species of CoAlO and oxygen species of MnO 2 could facilitate VOC oxidation. 25,26 The reductive Co 2+ species on CoAlO could be oxidized into Co 3+ species, accompanying MnO 2 , via an in situ redox reaction between reductive Co 2+ and oxidative MnO 4 − . Hence, platelike and flowerlike CoAlO and MnO 2 /CoAlO catalysts were prepared by a hydrothermal method in this work (Scheme S1).…”