*4.2. Catalyst Preparation*

Firstly, Mn-Zr hybrids catalysts with different ratios were prepared by conventional optimized co-precipitation routes [24,25]. In a typical preparation, manganese nitrate

solution (50 wt.%) (3.58, 7.16, 7.16, 10.74, and 10.74 g, respectively) and zirconium (IV) oxynitrate hydrate (7.48, 7.48, 4.99, 4.99, and 2.49 g, respectively) were added into 100 mL deionized water and vigorously stirred for 2 h to form homogeneous solution, respectively. Then ammonia was added dropwise until PH = 9 (Scheme 1a). The obtained precipitates are filtrated and washed until pH = 7. After that, the precipitate was dried in an oven at 90 ◦C for 5 h, followed by calcination at 550 ◦C for 5 h under an air atmosphere. Herein the ratios of Mn/Zr were listed as 1/3, 2/3, 1/1, 3/2, and 3/1, marked as CP = Mn1Zr3, CP = Mn2Zr3, CP = Mn1Zr1, CP = Mn3Zr2, and CP = Mn3Zr1, respectively. lution (50 wt.%) (3.58, 7.16, 7.16, 10.74, and 10.74 g, respectively) and zirconium (IV) oxynitrate hydrate (7.48, 7.48, 4.99, 4.99, and 2.49 g, respectively) were added into 100 mL deionized water and vigorously stirred for 2 h to form homogeneous solution, respectively. Then ammonia was added dropwise until PH = 9 (Scheme 1a). The obtained precipitates are filtrated and washed until pH = 7. After that, the precipitate was dried in an oven at 90 °C for 5 h, followed by calcination at 550 °C for 5 h under an air atmosphere. Herein the ratios of Mn/Zr were listed as 1/3, 2/3, 1/1, 3/2, and 3/1, marked as CP = Mn1Zr3, CP = Mn2Zr3, CP = Mn1Zr1, CP = Mn3Zr2, and CP = Mn3Zr1, respectively.

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 12 of 15

Firstly, Mn-Zr hybrids catalysts with different ratios were prepared by conventional optimized co-precipitation routes [24,25]. In a typical preparation, manganese nitrate so-

*4.2. Catalyst Preparation*

**Scheme 1.** The synthesis of catalyst by coprecipitation (**a**), and optimized coprecipitation (**b**). **Scheme 1.** The synthesis of catalyst by coprecipitation (**a**), and optimized coprecipitation (**b**).

The TP-Mn2Zr3 catalysts were prepared by two-step precipitation strategies. Specifically, manganese nitrate solution (50 wt.%) (7.16 g) and zirconium (IV) oxynitrate hydrate (7.48 g) were added into 50 mL deionized water and vigorously stirred for 2 h to form a homogeneous solution, respectively. Then ammonia was added dropwise into the two solutions until pH = 9 (Scheme 1b). We mixed the two solutions under vigorous stirring for 2 h. The washing, filtering, drying and calcining processes were the same procedures for CP-Mn2Zr3. Additionally, the final product is recorded as TP-Mn2Zr3. For comparison, MnO<sup>x</sup> and ZrO<sup>2</sup> catalysts were prepared by the above routes. Besides, the MP-Mn2Zr3 sample was prepared by mechanical ball mill of a mixture of MnO<sup>x</sup> and ZrO2. The ball milling experiment was carried out on a planetary ball mill (XQM-0.4). Actually, the stainless-steel balls with different diameters (15, 12, 10, 8, and 5 mm) were mixed and added into a ball milling tank (50 mL) and then stirred for 2 h at 400 r/min. In addition, the total mass of stainless-steel balls was 150 g, and the mass of Mn2O<sup>3</sup> and ZrO<sup>2</sup> was 1.84 g and 2.16 g, respectively, to keep the molar ratio of Mn/Zr at 2/3. The TP-Mn2Zr3 catalysts were prepared by two-step precipitation strategies. Specifically, manganese nitrate solution (50 wt.%) (7.16 g) and zirconium (IV) oxynitrate hydrate (7.48 g) were added into 50 mL deionized water and vigorously stirred for 2 h to form a homogeneous solution, respectively. Then ammonia was added dropwise into the two solutions until pH = 9 (Scheme 1b). We mixed the two solutions under vigorous stirring for 2 h. The washing, filtering, drying and calcining processes were the same procedures for CP-Mn2Zr3. Additionally, the final product is recorded as TP-Mn2Zr3. For comparison, MnO<sup>x</sup> and ZrO<sup>2</sup> catalysts were prepared by the above routes. Besides, the MP-Mn2Zr3 sample was prepared by mechanical ball mill of a mixture of MnO<sup>x</sup> and ZrO2. The ball milling experiment was carried out on a planetary ball mill (XQM-0.4). Actually, the stainless-steel balls with different diameters (15, 12, 10, 8, and 5 mm) were mixed and added into a ball milling tank (50 mL) and then stirred for 2 h at 400 r/min. In addition, the total mass of stainless-steel balls was 150 g, and the mass of Mn2O<sup>3</sup> and ZrO<sup>2</sup> was 1.84 g and 2.16 g, respectively, to keep the molar ratio of Mn/Zr at 2/3.

### *4.3. Catalyst Activity Evaluation 4.3. Catalyst Activity Evaluation*

The catalytic degradation of toluene was assessed in a fixed-bed stainless reactor (id = 7 mm). Prior to the activity test, the catalyst (0.20 g, 40–60 mesh) and quartz sands (0.30 g, 40–60 mesh) were well mixed and then loaded into the center of the reactor and activated at 300 °C for 1 h with a 60 mL/min air flow rate. After that, the temperature was dropped to 150 °C, and 1000 ppm toluene gas was continuously fed into the fixed-bed reactor; the total flow is controlled at 50 mL/min to correspond to a weight hourly space velocity (WHSV) of 15,000 mL gcat−<sup>1</sup> h−<sup>1</sup> . In addition, the catalytic activity of TP-Mn2Zr3 was tested under different WHSVs. The exhausted gas from a fixed-bed reactor was analyzed using a gas chromatography device (SP-7890 PLUS, Rui Hong Co, Ltd., Tengzhou, China) with a flame ionization detector (FID) and equipped polyethylene glycol capillary column and a thermal conductivity detector (TCD). The activity of the catalyst was measured by the toluene conversion (), which can be calculated as follow: The catalytic degradation of toluene was assessed in a fixed-bed stainless reactor (id = 7 mm). Prior to the activity test, the catalyst (0.20 g, 40–60 mesh) and quartz sands (0.30 g, 40–60 mesh) were well mixed and then loaded into the center of the reactor and activated at 300 ◦C for 1 h with a 60 mL/min air flow rate. After that, the temperature was dropped to 150 ◦C, and 1000 ppm toluene gas was continuously fed into the fixedbed reactor; the total flow is controlled at 50 mL/min to correspond to a weight hourly space velocity (WHSV) of 15,000 mL gcat <sup>−</sup><sup>1</sup> h −1 . In addition, the catalytic activity of TP-Mn2Zr3 was tested under different WHSVs. The exhausted gas from a fixed-bed reactor was analyzed using a gas chromatography device (SP-7890 PLUS, Rui Hong Co, Ltd., Tengzhou, China) with a flame ionization detector (FID) and equipped polyethylene glycol capillary column and a thermal conductivity detector (TCD). The activity of the catalyst was measured by the toluene conversion (*Xcon*), which can be calculated as follow:

$$X\_{\rm out} = \frac{\mathbb{C}\_{\rm in} - \mathbb{C}\_{\rm out}}{\mathbb{C}\_{\rm in}} \tag{1}$$

(1)

where *Cin* stand for the inlet toluene concentration, and *Cout* represent the outlet toluene concentration after 30 min reaction.
