Citation: Todorova, S.; Naydenov, A.; Shopska, M.; Kolev, H.; Yordanova, I.; Tenchev, K. Pt-Modified Nano-Sized Mn 2 O 3 Oxide Prepared from the Mn 3 O 4 Phase with Tetragonal Symmetry for CO Oxidation. Symmetry 2022, 14, 2543. https:// doi.org/10.3390/sym14122543 Academic Editor: Edwin Charles Constable Received: 30 October 2022 Accepted: 15 November 2022 Published: 1 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). symmetry S S Article Pt-Modified Nano-Sized Mn 2 O 3 Oxide Prepared from the Mn 3 O 4 Phase with Tetragonal Symmetry for CO Oxidation Silviya Todorova 1, * , Anton Naydenov 2 , Maya Shopska 1 , Hristo Kolev 1 , Iliyana Yordanova 1 and Krasimir Tenchev 1 1 Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Block 11, 1113 Sofia, Bulgaria 2 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bldg 11, 1113 Sofia, Bulgaria * Correspondence: todorova@ic.bas.bg; Tel.: +359-29792576 Abstract: One of the current problems in the environmental catalysis is the design of an effective and less costly catalytic system for the oxidation of CO. The nano-sized α-Mn 2 O 3 oxide has been prepared and modified with 0.5 wt.% Pt. The catalysts have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), temperature- programmed reduction (TPR) and diffuse-reflectance infrared spectroscopy (DRIFTS). Finely divided PtO and Pt(OH) 2 are being formed on the Mn 2 O 3 surface as a result of the strong interaction between platinum and the nano-oxide. Based on DRIFTS investigations and the model calculations, a Langmuir–Hinshelwood type of mechanism is supposed for CO oxidation on Pt/Mn 2 O 3 . The CO and oxygen are adsorbed on different types of sites. The Mars–van Krevelen mechanism is the most probable one over pure Mn 2 O 3 , thus suggesting that CO 2 is adsorbed on the oxidized sites. The CO adsorption in the mixture CO + N 2 or in the presence of oxygen (CO + N 2 +O 2 ) leads to a partial reduction in the Pt + surface species and the formation of linear Pt 1+ −CO and Pt 0 −CO carbonyls. Both of them take part in the CO oxidation reaction. Keywords: Pt-Mn 2 O 3 catalyst; nano-sized Mn 2 O 3 Mn 3 O 4 phase; tetragonal symmetry; CO oxidation; reaction mechanism; in-situ DRIFTS of CO oxidation 1. Introduction The main global environmental problems include air, water and soil contamination. Water pollution is caused by oil spills, acid rain and urban waste [1–3]. Air pollution is caused by various gases and toxins that are released by the industry and by burning fuels. The main air pollutants are PM (particulate matter), SO 2 , NH 3 , NOx, CO, volatile organic compounds (VOCs), and CH 4 . The abatement of volatile organic compounds (VOCs) and carbon monoxide (CO) in the waste gases is an important task in the areas of environmental protection and odor control. One of the most dangerous air pollutants, CO, is due to its high toxicity. Long-term exposure should not exceed 25 ppm in 8 h or 50 ppm in 4 h [4]. Catalysis plays a key role in solving environmental problems by creating highly active and efficient catalytic systems for environmental protection and reducing greenhouse gas emissions from the industry. Designing an efficient and cheaper catalytic system for CO oxidation is an important problem of actual environmental catalysis. Low-temperature CO oxidation has attracted considerable attention due to its wide application in exhaust gas treatment, automotive emission control, and the preferential oxidation of CO (PROX) for proton exchange membrane fuel cell application [5,6]. According to the literature data, a single α-Mn 2 O 3 phase has been demonstrated to display remarkable catalytic activity in different reactions, such as ethylene oxidation [7], methane oxidation [8,9] and carbon monoxide oxidation [10–12]. It has been established Symmetry 2022, 14, 2543. https://doi.org/10.3390/sym14122543 https://www.mdpi.com/journal/symmetry