**1. Introduction**

Nowadays, the quality of air, both in indoor and outdoor environments, is an extremely important concern. Furthermore, the COVID-19 emergency has pointed to the necessity of clean air to discourage virus infection. Among the air pollutants, volatile organic compounds (VOCs) include many of the most dangerous substances for both human health and the environment. Different strategies were employed to remove VOCs from the air, and an innovative and sustainable solution is represented by solar photocatalytic or photothermo-catalytic oxidation [1,2]. Compared to the most used catalytic or non-catalytic VOCs combustion, the photocatalytic process allows one to exploit solar irradiation with green advantages to work at milder conditions using renewable energy [2]. However, the performance of the photocatalysts is much lower compared to the catalysts employed for the thermocatalytic removal of VOCs [3], and for these reasons, the multi-catalytic approach of the photothermo-catalysis is a fascinating way to obtain high VOCs removal values of thermocatalysis but at lower temperatures, increasing, at the same time, the energy efficiency of the process. To design performing photothermo catalysts, different properties are required [4,5]. Analogously to photocatalysis, it is necessary to have a semiconductor material that, after solar irradiation, is able to generate photoelectrons and photoholes in its conduction (CB) and valence (VB) bands, respectively. It should have redox properties activated with the temperature; in this way, the superficial/mobile oxygens of the catalyst or of the support can participate in the oxidation of VOCs increasing the overall activity [3,6]. Finally, the photothermo catalysts should be resistant to both long-time irradiation and heating. The preparation of mixed oxides or composites is the

**Citation:** Fiorenza, R.; Farina, R.A.; Malannata, E.M.; Lo Presti, F.; Balsamo, S.A. VOCs Photothermo-Catalytic Removal on MnOx-ZrO<sup>2</sup> Catalysts. *Catalysts* **2022**, *12*, 85. https://doi.org/10.3390/catal 12010085

Academic Editor: Xintong Zhang

Received: 27 December 2021 Accepted: 12 January 2022 Published: 13 January 2022

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**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/).

best and easiest way to combine all of these features. Indeed, with the formation of a suitable heterojunction, it is possible to exploit solar irradiation, decreasing the bandgap (Eg) of the main semiconductor oxide, profiting from both the photocatalytic activity of the principal oxides and the thermocatalytic activity of the hosted oxide. Moreover, the introduction of host ions in the lattice of the main oxide allows one to create defects and oxygen vacancies that favour VOCs oxidation [7,8].

The TiO2-CeO<sup>2</sup> composites showed promising performance in the photo-thermal approach for both VOCs removal and CO<sup>2</sup> reduction [3,9]; however, one of the side effects of the current pandemic situation is the crisis of raw materials exportation, and as a consequence, in 2020, titanium featured in the EU critical raw materials list [10]. Considering that up until now, TiO<sup>2</sup> is the most studied and applied semiconductor, both in academia and in industrial research focused on photocatalytic applications, the exploration of unconventional non-critical (photo)catalysts is highly required.

In this work, we have investigated the photothermo-catalytic properties of MnOx-ZrO<sup>2</sup> mixed oxides, with the aim of finding new and sustainable alternatives to the most common TiO2-based photocatalysts, and without the addition of noble metal co-catalysts, usually used in the catalytic and photocatalytic removal of VOCs [11], to obtain even more environmentally friendly catalysts, in the end.

Manganese oxide exists in four stable forms (MnO, MnO2, Mn3O<sup>4</sup> and Mn2O3), and all of them own a semiconductor electronic structure characterized by the partially filled d orbitals which permit the electronic d-d transitions under UV or visible light irradiation [12]. Based on the preparation method, it is common to obtain a non-stoichiometric oxide or a mixture of different MnO<sup>x</sup> oxides with the +II, +III and/or +IV oxidation states. The high mobility/reducibility of manganese oxide lattice oxygens is particularly useful for VOCs removal [13,14], whereas the redox properties of MnO<sup>x</sup> can be particularly advantageous for the photothermo-catalytic oxidation of VOCs, as well as its low bandgap (in the range 2.0–3.5 eV depending of the crystalline structure [12,15]) that can allow a more efficient use of solar radiation.

Zirconium oxide (zirconia) was largely used as a support of several noble metalbased catalysts used for the thermocatalytic oxidation of VOCs, due to its high stability, thermal resistance, and ionic conductivity [16,17]. Furthermore, it is a large bandgap semiconductor (E<sup>g</sup> of about 5.0 eV [18] or lower depending to the zirconia synthesis). Therefore, its coupling with a lower bandgap semiconductor (as MnOx) can be a performing and fascinating strategy to reduce the odds of charge recombination (a common reason for photocatalysts deactivation) and to synergistically exploit both the thermal stability and the redox properties of MnO<sup>x</sup> and ZrO<sup>2</sup> [19,20] together with their photocatalytic features.

We have also determined the chemico-physical and the photocatalytic, thermocatalytic and photothermo-catalytic activities of MnOx-ZrO<sup>2</sup> oxides in the oxidation of toluene and ethanol, chosen as VOCs models, due to the high toxicity nature of toluene and to the wide use of ethanol as a solvent in many industrial processes and as an octane booster in combustion engines, whose incomplete oxidation can give the emission of dangerous compounds, as acetaldehyde, in the environment [21].
