Effect of ball milling on the catalytic activity of cryptomelane for VOC oxidation

ABSTRACT

Cryptomelane-type manganese oxides prepared by a solvent-free method were evaluated as catalysts for the oxidation of ethyl acetate, ethanol and toluene. The original catalyst (K-OMS-2) presented high catalytic activity for ethyl acetate and ethanol oxidation, achieving 90% conversion into CO₂ around 200°C for both pollutants. Toluene was much harder to oxidize, requiring a temperature near 270°C for the same conversion. The original catalyst was mechanically treated in a ball mill at different intensities, in order to decrease the particle size for subsequent impregnation onto structured supports, as small particle sizes are usually recommended. The catalytic activity of the materials decreases with the increase in the severity of this treatment, which is related to the decrease of the surface area of the catalysts, since the other properties (phase purity, thermal stability, surface oxygen, average oxidation state and reactivity of the oxygen species) are similar among the catalysts with different ball milling treatments. For comparison, a platinum-based catalyst (1%-Pt/Al₂O₃) was also tested, which exhibited a high activity for toluene, but much lower activities for the two other volatile organic compounds tested. A long-term experiment, using ethanol as model pollutant, showed that the cryptomelane catalyst was stable for more than 100 h.

GRAPHICAL ABSTRACT

KEYWORDS:

• Manganese oxides
• cryptomelane
• low-temperature catalytic oxidation
• volatile organic compounds

Acknowledgements

The authors also acknowledge Dr. Carlos M. Sá (CEMUP) for assistance with XPS and SEM analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Diogo F.M. Santos http://orcid.org/0000-0003-2009-1889

Olívia S.G.P. Soares http://orcid.org/0000-0002-9015-1237

José L. Figueiredo http://orcid.org/0000-0002-0395-8199

Manuel Fernando R. Pereira http://orcid.org/0000-0002-5447-2471

Additional information

Funding

This work is a result of project ‘AIProcMat@N2020 – Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020’, with the reference NORTE-01-0145-FEDER-000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF) and of Project POCI-01-0145-FEDER-006984 – Associate Laboratory LSRE-LCM funded by ERDF through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) – and by national funds through FCT – Fundação para a Ciência e a Tecnologia. Diogo F. M. Santos acknowledges grant received from FCT (PD/BD/105983/2014).