**1. Introduction**

Volatile organic compounds (VOCs) have been considered important harmful pollutants and can be transferred into secondary aerogel and ozone via complex photochemical reactions in the atmosphere, which are threatening the ecological environment and human health [1–4]. It is urgent to adopt effective routes to control VOCs emissions. In particular, the toluene with toxicity and carcinogenicity is one of the most common VOCs, be discharged from the petrochemical industry, and be normally selected as the target VOCs to test the activity of catalyst.

Up to now, several techniques, such as adsorption and absorption, thermal incineration, plasma, membrane separation, biological treatment, and catalytic oxidation, have been widely reported for the degradation of VOCs [5]. Among them, catalytic oxidation has been considered one of the most promising technologies because it can completely convert VOCs under relatively low temperatures (<400 ◦C) into harmless CO<sup>2</sup> and H2O with low energy consumption, which has gained a lot of attention either in scientific and industrial fields. The key issue of catalytic oxidation is to design catalysts with high catalytic activity and low cost [6,7].

Given the active components of the VOCs catalysts, catalysts can be divided into noble metal catalysts and non-noble metal catalysts. There is no doubt that the supported noble metal catalysts have higher activity at lower temperatures. However, the disadvantages of

**Citation:** Huang, X.; Li, L.; Liu, R.; Li, H.; Lan, L.; Zhou, W. Optimized Synthesis Routes of MnOx-ZrO<sup>2</sup> Hybrid Catalysts for Improved Toluene Combustion. *Catalysts* **2021**, *11*, 1037. https://doi.org/10.3390/ catal11091037

Academic Editor: Roberto Fiorenza

Received: 2 August 2021 Accepted: 25 August 2021 Published: 27 August 2021

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**Copyright:** © 2021 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/).

noble metal catalysts also need to be taken into consideration, such as high price, scarce resources, poor thermal stability, and sulfur resistance, etc., which fades their industrial application [8–10]. Therefore, it is necessary to develop alternative catalysts with high catalytic activity. The transition metal oxides have many advantages, such as relatively low price, good activity, abundant content in the earth, and environmental friendliness, which have attracted great interest [11]. For example, manganese-based oxides have been confirmed as one of the most potential alternates to noble metal catalysts and exhibited high catalytic activity due to the high mobility of lattice oxygen and the existence of transformation of unstable valence states, such as Mn4+ <sup>↔</sup> Mn3+ <sup>↔</sup> Mn2+ [12–15]. In addition, the physical and chemical characteristics of the catalysts, such as morphology [8,16], specific surface area [17], oxygen vacancy [18], reducibility, and reactivity of lattice oxygen [19], oxygen migration rate [14], also determine the efficiency of the catalytic oxidation of VOCs. However, it is difficult to efficiently remove certain VOCs for a single phase of MnO<sup>x</sup> due to the poor thermal stability [12,20]. Some studies have reported that the synthesized Mn-M (M = Co [21], Cu [22], Ce [15,23], Zr [24,25]) composite metal oxides can improve catalytic activity towards VOCs abatement through the strong interaction between the parent material and the modifier. Among them, ZrO2-based materials show excellent corrosion resistance, high stability, and ionic conductivity, and the ZrO2-based solid solutions keep high catalytic activity [26]. For instance, MnOx-ZrO<sup>2</sup> bimetal oxides catalysts present a good catalytic activity due to their excellent redox property and thermal stability [27,28].

Choudhary et al. [29] reported that the reactivity of the lattice oxygen of Mn-doped ZrO<sup>2</sup> catalysts depended on the Mn/Zr ratio and calcination temperature; the Mn-doped ZrO<sup>2</sup> (cubic) with the Mn/Zr ratio of 0.25 prepared at a calcination temperature of 600 ◦C processed the best methane combustion activity. Gutie'rrez-Ortiz et al. [24] reported that Mn-Zr mixed oxides exhibited better catalytic activity for 1,2-dichloroethane (DCE) and trichloroethylene (TCE) than that of pure zirconia and manganese oxide, which was attributed to the modification effects of surface acid sites coupled with the readily accessible active oxygen. Zeng et al. [30] successfully manufactured an MnZrO<sup>x</sup> catalyst with a three-dimensional microporous (3DM) structure, which hold a better reducibility and oxygen mobility, accompanied by higher Mn4+/Mn3+ and Oβ/Oα, and the special structure significantly promoted the adsorption of reactant molecules and the mobility of lattice oxygen and propane oxidation. Compared with the B-Mn0.6Zr0.4 catalyst prepared by traditional sol–gel methods, the activation energy of propane combustion on 3DM MnZrO<sup>x</sup> decreased from 156.2 kJ·mol−<sup>1</sup> to 105.0 kJ·mol−<sup>1</sup> . Moreover, Yang et al. synthesized a series of impurity MnxZr1−xO<sup>2</sup> catalysts, and it was found that low-valent manganese (Mn2+) can enter zirconium lattice by substituting Zr4+ and inducing the formation of oxygen vacancy, and its concentration hinges on the doping content, which is conducive to the activation of oxygen on the surface of Mn-doped c-ZrO2(111) crystal plane, thereby improving the catalytic performance for toluene oxidation [31].

Recently, we found that preparation process would play a critical role in adjusting the structural properties of nanocatalysts in oxidation reaction [32,33]. Particularly, the precipitation sequence determined the structural, redox properties, and textural stability of ZrO2-based catalysts [34]. Inspired by those points, we constructed Mn-Zr bimetal catalysts based on the controllable catalytic active centers of MnxZr1−xO<sup>2</sup> solid solution via a modulated precipitation sequence and further evaluated the catalytic capacity of the combustion of toluene model molecules, and the synergistic effect between ZrO<sup>2</sup> and manganese oxide was also analyzed.
