Engineering Surface Properties of CuO/Ce_0.6Zr_0.4O₂ Catalysts for Efficient Low-Temperature Toluene Oxidation

Abstract

The development of superior low-temperature catalytic performance and inexpensive catalysts for the removal of volatile organic compounds (VOCs) is crucial for their industrial application. Herein, CuO/Ce_0.6Zr_0.4O₂ catalysts calcinated at different temperatures (Cu/CZ-X, X represented calcination temperature) were prepared and used to eliminate toluene. It can be found that Cu/CZ-550 presented the highest low-temperature catalytic activity, with the lowest temperature (220 °C) 50% conversion of toluene, the highest normalized reaction rate (3.1 × 10⁻⁵ mol·g⁻¹·s⁻¹ at 180 °C) and the lowest apparent activation energy value (86.3 ± 4.7 kJ·mol⁻¹). Systematically, the surface properties analysis results showed that the optimum redox property, abundant oxygen vacancies, and plentiful surface Ce³⁺ species over Cu/CZ-550 were associated with the strong interaction between Cu and support could significantly favor the adsorption and activation of toluene, thus resulting in its superior catalytic performance.

1. Introduction

Volatile organic compounds (VOCs), as representative air pollutants, are mainly emitted from industrial processes and transportation [1,2], including toluene, benzene and alkanes, which significantly harm human health and the environment [3,4]. Catalytic combustion has received increasing attention due to its high efficiency and low energy consumption compared to other approaches used for VOC degradation [5,6,7]. However, the low-temperature activity and stability of catalysts do not satisfy the requirements for practical industrial applications and need to be further improved.

Active components are considered to be the key factor in determining catalytic performance. Noble catalysts (Pt, Au, Pd) exhibit high activity and stability, but wide utilization is significantly limited due to their high price and small reserves [8,9,10]. Recently, transition metal oxides (MnO [6,11,12], CuO [13,14,15], CoO [8,16,17]) have received considerable attention worldwide owning to their low price, good reducibility, and strong oxygen storage [18,19], but their low-temperature catalytic efficiency means that this issue is an open question [4,20]. Therefore, it is crucial to optimize transition metal oxides in terms of their superior catalytic activity at low temperatures and with high efficiency. Cu oxides in Cu-based catalysts are the most promising catalysts in terms of VOC combustion [13,14,21]. The mechanism of oxidation is determined by the oxidation state of Cu, which plays a vital part. The diffusion of lattice oxygen in CuO to the surface limits the rate of oxidation [3]. Additionally, the nature of the support plays a critical role in the catalytic activity of supported metal catalysts. The electronic state of the active metal can be modified or determined by the support through the interaction between the active components and the support [22,23], and, in some cases, the support is able to participate in the reaction, facilitating the kinetic process [24,25,26]. Cerium dioxide (CeO₂) has abundant oxygen vacancies and outstanding redox properties as the catalyst support, which was closely related to the reversible Ce⁴⁺ ↔ Ce³⁺ redox reaction [27,28,29]. It has been shown that during the oxidation of CeO₂, the lattice oxygen atoms are directly consumed in the reaction, forming oxygen vacancies [30]. The oxygen vacancies can adsorb and activate gaseous O₂, forming adsorbed oxygen species that favor oxygen mobility [28,30,31]. However, the modification of pure CeO₂ with other substances is essential on account of the thermal instability of CeO₂ [32,33]. ZrO₂, which has superior stability, in combination with CeO₂, can form Ce_xZr_1-xO₂ solid solutions that extremely improve the thermal stability [34,35,36,37]. Sara et al. found that the Zr⁴⁺ cations added to the lattice of CeO₂ exhibited significant improvement in their oxygen storage capacity (OSC) and thermal stability and enhanced its catalytic activity at a low temperature [38]. In addition, Zr⁴⁺ replaced Ce³⁺ generates distortion due to the difference of Zr⁴⁺ and Ce³⁺ atomic sizes in Ce_xZr_1-xO₂ solid solution, increasing oxygen vacancies and oxygen mobility [30].

In recent years, Cu-based catalysts have been considered to be the most promising catalysts for VOC combustion, and their surface properties are modulated to enhance the catalytic performance [15,39]. Song et al. synthesized a series of different molar ratio Cu-Ce binary oxides via the co-precipitation method and applied them to catalytic toluene oxidation [40]. They found that the catalysts that had a strong interaction between Cu and Ce exhibited excellent catalytic performance, with a toluene complete conversion temperature (T₁₀₀) of 230 °C. In order to strengthen the synergistic interaction between CuO and CeO₂, Li et al. synthesized a series of hollow-structured CuCeO_x via a hydrothermal approach [17]. The review illustrated that the hollow structure induced the electron transfer between CuO and CeO₂, which improved the adsorption of toluene and activation capacity with a T₁₀₀ of 280 °C. Lately, Zeng et al. further modified and synthesized the CuO-CeO₂ catalysts via a novel double redox method, which greatly enhanced the concentration of Ce³⁺ and oxygen vacancies and then promoted the activation capacity of toluene catalytic oxidation with a T₁₀₀ of 250 °C [15]. Wang et al. reported that toluene catalytic oxidation performance was closely related to the synergistic interaction between CuO and Ce-ZrO_x in recent years [41]. By comparing CuO/ZrO₂, CeO₂/ZrO₂, and CuO-CeO₂/ZrO₂, their surface properties and the interaction between Cu-CeO_x and ZrO₂ made the catalysts exhibit good toluene oxidation performance, with a T₁₀₀ of 350 °C. Additionally, an accepted catalytic activity over CuO/Ce_xZr_1-xO₂, with a T₁₀₀ lowered to 270 °C was reported, which mainly resulted from the interfacial interaction between CuO and Ce_xZr_1-xO₂, the highly dispersed CuO, and the nature of the support. However, the mechanism of the enhancement was still not very clear and needs to be further investigated.

In the present work, the surface properties of CuO/Ce_0.6Zr_0.4O₂ were engineered by varying the calcination temperature (450 °C, 550 °C, 650 °C, and 750 °C). The surface properties of the catalysts were investigated systematically using a series of measurements, such as Raman, TEM, XPS, H₂-TPR, and the relationship with the catalytic performance was also explored.

2. Results and Discussion

2.1. Catalytic Activity and Kinetic Study

The catalytic performance of toluene oxidation is presented in Figure 1a. The temperature required for a 50% conversion of toluene (T₅₀) is usually used to compare the low-temperature performance of catalysts. It could be clearly seen from Figure 1a that the T₅₀ of Cu/CZ-450, Cu/CZ-550, Cu/CZ-650, and Cu/CZ-750 was 230 °C, 220 °C, 237 °C and 272 °C, respectively. Obviously, with the increasing calcination temperature from 450 °C to 750 °C, T₅₀ initially decreased but gradually raised. Typically, Cu/CZ-550, with the lowest value of T₅₀, presented the highest low-temperature activity among these samples. The comparison of the toluene catalytic activity on a series of catalysts in the summarized literature, is listed in Table 1. The range of gas hourly space velocity (GHSV) values was 15,000 h⁻¹–75，000 h⁻¹ and 20,000 mL·g⁻¹·h⁻¹–66,000 mL·g⁻¹·h⁻¹. The T₅₀ range present in the mentioned literature in Table 2 was 200–300 °C, and the T₅₀ value of Cu/CZ-550 was 220 °C, which was lower than most of the catalysts in the summarized literatures. Therefore, the catalytic performance over Cu/CZ-550 may be superior to most of those in the literature. The preparation methods used in terms of catalysts in the summarized literature were not the same. Typically, in terms of two different types of catalysts, CuCeO_x was prepared via a co-precipitation approach, and Cu/CZ was synthesized using an impregnation method. It was obvious that the calcination temperature had a certain effect on the catalytic activity. The optimization of the calcination temperature to engineer the surface was essential for obtaining superior catalytic performance.

In order to further compare the catalytic performance, the normalized reaction rate was calculated, and it is listed in Figure 1b. It could be found that the reaction rate of toluene oxidation at 180 °C over Cu/CZ-550 was up to 3.1 × 10⁻⁵ mol·g⁻¹·s⁻¹, which was almost 1.7 times, 3.1 times, and 6.7 times higher than those of Cu/CZ-450 (1.8 × 10⁻⁵ mol·g⁻¹·s⁻¹), Cu/CZ-650 (9.8 × 10⁻⁶ mol·g⁻¹·s⁻¹) and Cu/CZ-750 (4.6 × 10⁻⁶ mol·g⁻¹·s⁻¹), respectively. A similar trend was observed at 220 °C and 240 °C. The results confirmed the superior catalytic activity of Cu/CZ-550.

To further explore the difference in catalytic activity, the kinetic analysis was conducted by plotting the Arrhenius curves. The toluene conversions were kept below 20% at a high GHSV of 80,000 mL·g⁻¹·h⁻¹ to minimize the effect of internal and external diffusion [42]. As shown in Figure 1c, the coefficients (R²) for all catalysts were at least 0.99. The apparent activation energy (E_a) was calculated from the slope of the Arrhenius plot [43]. A relatively lower E_a value was observed for Cu/CZ-550 (86.3 ± 4.7 kJ·mol⁻¹) compared to Cu/CZ-450 (105.5 ± 6.0 kJ·mol⁻¹), Cu/CZ-650 (112.3 ± 4.5 kJ·mol⁻¹), and Cu/CZ-750 (146.5 ± 5.4 kJ·mol⁻¹), respectively. Generally, the catalysts with a low value could activate the reactants more easily, which resulted in excellent catalytic performance over Cu/CZ-550, which is in line with the results of the catalytic activity test.

Table 1. A comparison on catalytic activity of literature data on catalytic oxidation of toluene.

Samples	Calcination Temperature, °C	Reaction Mixture	T50, °C	T90, °C	Reference
Cu0.4Ce0.6Oy	500	500 ppm toluene 20 vol.% O2/N2 GHSV: 50,000 h−1	230	245	[15]
5 wt% CuO/CeO2	400	500 ppm toluene 20vol.% O2/N2 GHSV: 75,000 h−1	210	235	[19]
Cu1Ce3Oy	500	1000 ppm toluene 21 vol.% O2/N2 GHSV: 60,000 h−1	200	210	[40]
CuO/CeO2-ZrO2 (nCe/nCu mole ratio = 5 and nCe/nZr mole ratio = 9)	400	1500 ppm toluene GHSV: 24,000 h−1	300	350	[44]
Cu0.15Ce0.85Oy	550	600 ppm toluene 20 vol.% O2/He GHSV: 50,000 h−1	240	250	[45]
Cu6Ce4Ox (Cu/Ce atomic ratio = 6:4)	550	1000 ppm toluene 20 vol.% O2/N2 GHSV: 36,000 mL·g−1·h−1	240	260	[39]
UiO-66-CeCu (nCe/nCu mole ratio = 3)	—	500 ppm toluene 21 vol.% O2/N2 GHSV: 30,000 mL·g−1·h−1	—	220	[13]
6% CuO/CeO2-ZrO2	700	1000 ppm toluene 30 vol.% O2/Ar	277	310	[41]
8 wt% CuO/Ce0.8Zr0.2Oy	400	4400 ppm toluene GHSV: 33,000 mL·g−1	210	230	[46]
Cu1Ce3Oy (nCe/nCu mole ratio = 3)	500	1.0 vol.% toluene GHSV: 66,000 mL·g−1·h−1	223	225	[47]
Cu0.13Ce0.87Oy	600	1000 ppm toluene GHSV: 15,000 h−1	470	490	[48]
8 wt% CuO/Ce0.6Zr0.4Oy	550	600 ppm toluene 21 vol.% O2/N2 GHSV: 20,000 mL·g−1·h−1	220	240	In this study

Table 1. A comparison on catalytic activity of literature data on catalytic oxidation of toluene.

Samples	Calcination Temperature, °C	Reaction Mixture	T50, °C	T90, °C	Reference
Cu0.4Ce0.6Oy	500	500 ppm toluene 20 vol.% O2/N2 GHSV: 50,000 h−1	230	245	[15]
5 wt% CuO/CeO2	400	500 ppm toluene 20vol.% O2/N2 GHSV: 75,000 h−1	210	235	[19]
Cu1Ce3Oy	500	1000 ppm toluene 21 vol.% O2/N2 GHSV: 60,000 h−1	200	210	[40]
CuO/CeO2-ZrO2 (nCe/nCu mole ratio = 5 and nCe/nZr mole ratio = 9)	400	1500 ppm toluene GHSV: 24,000 h−1	300	350	[44]
Cu0.15Ce0.85Oy	550	600 ppm toluene 20 vol.% O2/He GHSV: 50,000 h−1	240	250	[45]
Cu6Ce4Ox (Cu/Ce atomic ratio = 6:4)	550	1000 ppm toluene 20 vol.% O2/N2 GHSV: 36,000 mL·g−1·h−1	240	260	[39]
UiO-66-CeCu (nCe/nCu mole ratio = 3)	—	500 ppm toluene 21 vol.% O2/N2 GHSV: 30,000 mL·g−1·h−1	—	220	[13]
6% CuO/CeO2-ZrO2	700	1000 ppm toluene 30 vol.% O2/Ar	277	310	[41]
8 wt% CuO/Ce0.8Zr0.2Oy	400	4400 ppm toluene GHSV: 33,000 mL·g−1	210	230	[46]
Cu1Ce3Oy (nCe/nCu mole ratio = 3)	500	1.0 vol.% toluene GHSV: 66,000 mL·g−1·h−1	223	225	[47]
Cu0.13Ce0.87Oy	600	1000 ppm toluene GHSV: 15,000 h−1	470	490	[48]
8 wt% CuO/Ce0.6Zr0.4Oy	550	600 ppm toluene 21 vol.% O2/N2 GHSV: 20,000 mL·g−1·h−1	220	240	In this study

Table 2. Physical parameters, the ratio of I_D/I_F2g, and the value of E_a over all catalysts.

Samples	Surface Area (m2·g−1)	Pore Volume (cm3·g−1)	Average Pore Size (nm)	Average Ce0.6Zr0.4O2 Crystal Sizes a (nm)	Average CuO Crystal Sizes b (nm)	ID/IF2g c	Ea d (kJ·mol−1)
Cu/CZ-450	57.6	0.15	8.4	4.9	- e	0.50	105.5 ± 6.0
Cu/CZ-550	50.8	0.16	10.4	5.3	- e	0.61	88.3 ± 4.7
Cu/CZ-650	42.0	0.18	12.8	5.7	- e	0.55	112.3 ± 4.5
Cu/CZ-750	9.5	0.06	13.8	10.1	22.3	0.30	146.5 ± 5.4

^a From Ce_0.6Zr_0.4O₂ (111) facet of the XRD results. ^b From the CuO (111) facet of XRD results. ^c From the Raman results. ^d From the kinetic analysis results. ^e Represents not detected.

Table 2. Physical parameters, the ratio of I_D/I_F2g, and the value of E_a over all catalysts.

Samples	Surface Area (m2·g−1)	Pore Volume (cm3·g−1)	Average Pore Size (nm)	Average Ce0.6Zr0.4O2 Crystal Sizes a (nm)	Average CuO Crystal Sizes b (nm)	ID/IF2g c	Ea d (kJ·mol−1)
Cu/CZ-450	57.6	0.15	8.4	4.9	- e	0.50	105.5 ± 6.0
Cu/CZ-550	50.8	0.16	10.4	5.3	- e	0.61	88.3 ± 4.7
Cu/CZ-650	42.0	0.18	12.8	5.7	- e	0.55	112.3 ± 4.5
Cu/CZ-750	9.5	0.06	13.8	10.1	22.3	0.30	146.5 ± 5.4

^a From Ce_0.6Zr_0.4O₂ (111) facet of the XRD results. ^b From the CuO (111) facet of XRD results. ^c From the Raman results. ^d From the kinetic analysis results. ^e Represents not detected.

2.2. Textural Characteristics

The textural properties of these catalysts were measured using N₂ adsorption–desorption measurements. As shown in Figure 2a, all the Cu/CZ-X displayed a typical type-IV adsorption and desorption curve and H₃-type hysteresis loop [44,47]. This was attributed to the capillary condensation of mesoporous materials, indicating the presence of irregular porosity [2]. Figure 2b shows the distribution of the pore size over the samples. The pore sizes of Cu/CZ-450, Cu/CZ-550, and Cu/CZ-650 were mainly concentrated at 5, 6, and 7 nm, respectively. However, the pore size of Cu/CZ-750 is located mainly around 30 nm, indicating that its texture was significantly impacted by the calcination temperature. The specific surface area, pore volume, and average pore size ae listed in Table 2. It was noticeable that the specific surface area of the catalysts progressively decreased from 57.6 m²·g⁻¹ to 9.5 m²·g⁻¹ when the calcination temperature increased from 450 °C to 750 °C. It confirmed the pore collapse over Cu/CZ-750, which is in line with its extremely low pore volume (Table 2). As the surface area increased, more surface-active centers were exposed to reactants, usually promoting catalytic activity [49]. Therefore, the Cu/CZ-750 exhibited inferior activity, which was closely related to the small specific surface area.

2.3. Structural Characteristics

XRD measurements of Cu/CZ-X were implemented to confirm the crystal phase of the samples, and these are shown in Figure 3. For all samples (Figure 3a), the main diffraction peaks at 29.2°, 33.8°, 48.5°, and 57.6° can be attributed to the (111), (200), (220), and (311) planes of Ce_0.6Zr_0.4O₂ solid solution (PDF#38-1439), respectively, which is in good agreement with the ratio of the experimental preparation. It was clearly seen that the diffraction peaks at 29.8°, 49.7°, and 59.2° were attributed to the (003), (104), and (113) planes of ZrO₂ (PDF#37-0031), respectively, when the calcination temperature was 450 °C, 650 °C, 750 °C. That was due to the separation of the Ce_0.6Zr_0.4O₂ phase to produce ZrO₂. However, Cu/CZ-550 did not have contributions from ZrO₂ probably because of a slight difference in synthesis. The diffraction peak centered at 35.6° and 38.7° could be assigned to CuO (PDF#48-1548). It appeared to suggest that the intensity of the peak towards CuO increased with increasing calcination temperature (Figure 3b). However, almost no peak corresponding to CuO was observed in terms of Cu/CZ-550, indicating its high dispersion of copper species on the surface and the presence of a strong interaction between these species and Ce_0.6Zr_0.4O₂ [2]. Ce_0.6Zr_0.4O₂ and CuO crystal sizes were estimated using Scherrer’s formula from the Ce_0.6Zr_0.4O₂ (111) and CuO (111) facets of XRD, respectively (Table 2). The support crystal sizes of Cu/CZ-450, Cu/CZ-550, Cu/CZ-650, and Cu/CZ-750 were, respectively, 4.9, 5.3, 5.7, and 10.1 nm, which indicated that the agglomeration at the high calcination temperature led to larger sizes. Indeed, the CuO crystal size of Cu/CZ-750 was 22.3 nm as a result of agglomeration.

The structure of the catalysts was further investigated using Raman measurements. As displayed in Figure 4a, the two peaks at 462 and 598 cm⁻¹ were observed over all of the samples [28], which were attributed to the vibration model of octahedral local symmetry from the CeO₂ lattice (F_2g) and the defect-induced mode (D mode), respectively [27]. A blue shift of the feature (472 to 460 cm⁻¹) toward the F_2g mode was seen as increasing the calcination temperature, indicating the different structures present in these samples (Figure 4b). The D mode was attributed to the formation of oxygen vacancies, which were derived from structural defects [28]. The oxygen vacancy concentrations on the samples were assessed by estimating the ratio of the intensities in the D band and F_2g band (I_D/I_F2g) [50]. Notably, as presented in Table 2, the ratio of I_D/I_F2g was remarkably higher in Cu/CZ-550 (0.61) than in Cu/CZ-450 (0.50), Cu/CZ-650 (0.55), and Cu/CZ-750 (0.30), respectively. This illustrated the highest oxygen vacancy concentration in Cu/CZ-550 than in the other samples. The high oxygen vacancy concentration was conducive to the absorption and activation of gaseous O₂ and promoted the diffusion of reactant molecules, which is in agreement with the catalytic activity.

The structures of all of the samples were investigated using TEM. As displayed at Figure 5, it was easily observed that all samples presented clear lattice fringe distances of 0.31 nm and 0.27 nm, corresponding to CeO₂ (111) and CuO (11-1), respectively. In addition, Cu/CZ-750 had a lattice fringe distance of 0.19 nm which was attributed to ZrO₂ (104). The mean grain sizes and the distributions of grain size over the Cu/CZ-X samples were calculated using the TEM images (Figure 6), and they are listed in

Table 3. Surface information and grain sizes of Cu/CZ-X catalysts.

Samples	Cu2+ BE (eV)	Cu2+/(Cu+ + Cu2+)	Osur/(Osur + Olatt)	Ce3+/(Ce3+ + Ce4+) a	Grain Sizes b (nm)
Cu/CZ-450	935.2	0.54	0.27	0.19	5.3
Cu/CZ-550	934.8	0.58	0.28	0.22	5.9
Cu/CZ-650	935.2	0.52	0.26	0.18	6.3
Cu/CZ-750	935.4	0.35	0.27	0.17	10.8

^a Calculated from (S_v2 + S_u2)/(S_v1 + S_v2 + S_v3 + S_v4 + S_u1 + S_u2 + S_u3 + S_u4).^b Obtained from the TEM results.

. The average grain sizes (d) of Cu/CZ-450, Cu/CZ-550, Cu/CZ-650, and Cu/CZ-750 were 4.9 nm, 7.3 nm, 9.2 nm, and 10.8 nm, respectively. Noteworthily, the grain sizes increased with the increasing calcination temperature.

2.4. Chemical State

The surface composition and chemical state of the samples were examined using XPS measurements. In Figure 7a, the peak centered at 933.1–933.8 eV corresponded to Cu⁺, while the peak at 935.4–935.9 eV and the two satellite peaks belonged to Cu²⁺ species [51,52,53]. As displayed in Table 3, the binding energy (BE) of Cu²⁺ over Cu/CZ-550 (934.8 eV) was quite different from those in Cu/CZ-450 (935.2 eV), Cu/CZ-550 (935.23 eV), and Cu/CZ-750 (935.4 eV), respectively. The results indicated that the chemical environments of Cu²⁺ over these samples were different. The ratio of the surface Cu²⁺/(Cu²⁺ + Cu⁺) is listed in Table 3. It is worth noting that the ratio of Cu²⁺/(Cu²⁺ + Cu⁺) gradually increased when increasing the calcination temperature to 550 °C. Nevertheless, the values of Cu²⁺/(Cu²⁺ + Cu⁺) decreased rapidly from 0.58 to 0.35 when further increasing the calcination temperature from 550 to 750 °C. Thus, Cu/CZ-550 possessed the highest concentration of surface Cu²⁺ species. Generally, the Cu²⁺ component was considered to be the catalytically active site involved in toluene oxidation [40]. Thus, Cu/CZ-550 would show accepted catalytic activity, which is in line with the results of the performance tests.

For the O 1s XPS spectra (Figure 7b), the binding energy of 529.6 eV, 531.4–531.5 eV, and 532.9 eV corresponded to the surface lattice oxygen (O_latt), surface oxygen (O_sur), and other weakly bound oxygen species (O_abs), such as carbonate (CO₃²⁻), adsorbed molecular water, and hydroxyl (OH⁻), over the samples, respectively [54,55]. The concentration of surface oxygen species could be estimated, and the results are summarized in Table 3. The contents of these surface oxygen over all samples were almost the same. The peaks (Figure 7c) at 182.1 eV and 184.5 eV were indexed to Zr 3d_3/2 and Zr 3d_5/2, which were ascribed to Zr⁴⁺ [56,57,58]. For all of the samples, there were no obvious differences in terms of Zr binding energy.

As described in Figure 7d, Ce 3d XPS spectra 3d_3/2 and 3d_5/2 were marked as u and v, respectively. The peaks at 917.3 eV (u₄), 908.5 eV (u₃), 900.8 eV (u₁), 898.3 eV (v₄), 889.6 eV (v₃), and 882.7 eV (v₁) were assigned to surface Ce⁴⁺ species, and the peaks at 903.5 eV (u₂) and 885.2 eV (v₂) corresponded to surface Ce³⁺ species. A higher ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) (0.22) was found over Cu/CZ-550 in comparison to those over Cu/CZ-450 (0.19), Cu/CZ-650 (0.18), and Cu/CZ-750 (0.17), respectively. Ce³⁺ species were normally associated with the formation of oxygen vacancies, which could favor oxygen mobility [27,28]. Thus, Cu/CZ- might have more oxygen vacancies, which is in good agreement with Raman spectrum results, showing excellent catalytic activity.

2.5. Reducibility

The reducibility of these samples was analyzed using H₂-TPR measurements, and the profiles of all of the samples are depicted in Figure 8. The reduction peaks located at 50–275 °C could be divided into three peaks α, β, and γ. Peak α, β, and γ were ascribed to the reduction of well dispersed copper species [19,59], the reduction in copper incorporated into CeO₂ [15,60], and the reduced interaction of CuO with the support [40,59], respectively. Compared with other catalysts, the reduction peak position of peak α and β in Cu/CZ-550 shifted to a lower reduction temperature. It revealed that Cu/CZ-550 had a superior redox property. In order to further compare the reducibility of these samples, the H₂ consumptions were calculated from the peak area and calibrated using CuO as an internal standard. As listed in

Table 4. H₂ Consumption Quantification Results and kinetic features of toluene conversion on the as-prepared catalysts.

samples	Peak α H2 Consumption a (μmol·gcat−1)	Peak β H2 Consumption a (μmol·gcat−1)	Peak γ H2 Consumption a (μmol·gcat−1)	Total H2 Consumption a (μmol·gcat−1)
Cu/CZ-450	27.1	53.4	23.3	103.7
Cu/CZ-550	14.0	100.6	6.0	120.6
Cu/CZ-650	9.5	60.4	42.0	111.9
Cu/CZ-750	5.6	65.5	16.0	86.1

^a Calculated from the H₂-TPR results.

, the hydrogen consumption of Cu/CZ-550 was up to 120.6 μmol·g_cat⁻¹, which was higher than those of Cu/CZ-450 (103.7 μmol·g_cat⁻¹), Cu/CZ-650 (111.9 μmol·g_cat⁻¹), and Cu/CZ-750 (86.1 μmol·g_cat⁻¹). The result indicated the highest concentration of surface-reducible oxygen over Cu/CZ-550 and proved that Cu/CZ-550 had outstanding reducibility. This considerable low-temperature reducibility could contribute to enhancing catalytic performance [61]. Thus, Cu/CZ-550 presented appropriate catalytic efficiency.

3. Materials and Methods

3.1. Chemicals

Cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O), zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O), and copper nitrate trihydrate (Cu(NO₃)₂·3H₂O) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ammonia water (NH₃·H₂O) and ammonium carbonate ((NH₄)₂CO₃) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China) and Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China), respectively.

3.2. Preparations of Catalysts

3.2.1. Synthesis of Ce_0.6Zr_0.4O₂ Supports

A ceria–zirconia support was prepared via the co-precipitation approach, and the molar ratio of Ce and Zr was 1.5. 7.351 g Ce(NO₃)₃·6H₂O and 7.268 g Zr(NO₃)₄·5H₂O, which were, respectively, dissolved into 50 mL of deionized water to form transparent solutions and were then mixed by stirring for 10 min at room temperature. The saturated (NH₄)₂CO₃ (120 mL) solution was added dropwise to the liquid mixture while stirring, which was followed by adding the right amount of NH₃·H₂O solution steadily under constant stirring at room temperature until the pH of the solution reached about 9–10. Then, the resulting turbid solution was aged at 80 °C for 6 h, filtrated, and washed with deionized water until reaching pH = 7. The resulting solid was dried at 80 °C for 12 h to obtain powders and was transferred into a muffle furnace to calcinate at 550 °C for 3 h. The obtained solid was the Ce_0.6Zr_0.4O₂ support.

3.2.2. Synthesis of CuO/Ce_0.6Zr_0.4O₂ Catalysts

CuO/Ce_0.6Zr_0.4O₂ were synthesized using an impregnation method. Concretely, excessive NH₃·H₂O solution was dropped into 0.486 g Cu(NO₃)₂·3H₂O under stirring until Cu(NO₃)₂·3H₂O was dissolved completely at room temperature. Subsequently, 1.840 g of the ceria–zirconia support was added to the resulting mixture and stirred vigorously. The product obtained was dried at 80 °C overnight and divided into four parts to calcinate at varied temperatures of 450 °C, 550 °C, 650 °C, and 750 °C for 3 hm with a heating rate of 2 °C·min⁻¹ in air. The final obtained catalysts are named as follows: Cu/CZ-X, where X represents the calcination temperature. For example, the catalyst calcinated at 450 °C is named Cu/CZ-450. The normalized amount of CuO was 8%.

3.3. Characterization of Catalysts

Nitrogen adsorption–desorption isotherms were performed at liquid N₂ temperature (-196 °C) on a Micromeritics TriStar II 3020 instrument and were applied to determine the specific surface areas (SSAs) using the Brunauer–Emmett–Teller (BET) method. Before the analysis, the samples were degassed at 300 °C for 4 h to clean the surfaces. The pore volume and pore size distribution were discovered through the desorption branch of the N₂ adsorption isotherm based on the Barrett–Joyner–Halenda (BJH) method. Wide-angle X-ray diffraction (XRD) experiments were carried out using a Rigaku Ultima IV at 40 kV and 40 mA by means of Cu-Kα (λ = 0.15406 nm) radiation. The diffraction was recorded from 5° to 85°, with a rate of 8°/min. Raman spectroscopy was obtained via a Renishaw Via Reflex 2000 microscopic Raman spectrometer equipped with a Leica microscopy system and a microscope (50×) using laser excitation (λ = 532 nm). Additionally, the Raman spectra were calibrated using a silicon wafer before the experiment at 520.5 cm⁻¹. Transmission electron microscopy (TEM) images were taken using the Tecnai F30 device. The specimens were pretreated by ultrasonically suspending the samples in ethanol, and, subsequently, the suspension droplets were cast on a carbon-coated copper grid. The corresponding copper species sizes and ceria–zirconia solution sizes of the samples were estimated using Nano Measurer software. X-ray photoelectron spectroscopy (XPS) analyses were collected on a ThermoFischer ESCALAB 250Xi spectrometer with an Al Kα radiation source (hv = 1486.6 eV). The C 1s photoemission line was used for the binding energy (BE) calibration based on the peak position at 284.8 eV. H₂ temperature programmed reduction (H₂-TPR) experiment was undertaken on a TP-5080-D fitted with TCD. Prior to the experiments, 50 mg Cu/CZ-X samples were pretreated under N₂ flow (30 mL/min) to remove surface impurities at 400 °C for 30 min. After cooling to room temperature, the TPR test changed the gas flow to a H₂/Ar stream (30 mL·min⁻¹), which was introduced in the feed stream instead of the N₂. The catalysts were measured continuously with a range from 50 °C to 850 °C at a heating rate of 8 °C/min. The H₂ consumption amount was quantified through calibration with the standard sample of CuO (99.99%).

3.4. Catalytic Measurements

The activity tests of different catalysts were carried out on the fixed bed reaction (i.d. = 6 mm) and filled with 300 mg of samples on a sieve fraction of a 40–60 mesh. The feed-stock gas mixture was made up of 600 ppm toluene and 20% O₂, balanced by N₂ with a continuous gas flow maintained at 100 mL·min⁻¹, corresponding to a GHSV of 20,000 mL·g⁻¹·h⁻¹. Before the reaction, the catalysts in the fixed bed reaction were activated by air, which was passed for 30 min. The catalytic activity was tested at a temperature range of 180–340 °C, with an interval of 20 °C, and each test temperature was held for at least 30 min. During the tests, the inlet and outlet toluene gas concentrations were detected using a gas chromatograph (Fuli 9790) equipped with a flame ionization detector (FID) in real time. Toluene conversion was calculated using Equation (1).

$${\mathrm{X}}_{\mathrm{toluene}}(\%)=\frac{{\left[\mathrm{Toluene}\right]}_{\mathrm{in}}-{\left[\mathrm{Toluene}\right]}_{\mathrm{out}}}{{\left[\mathrm{Toluene}\right]}_{\mathrm{out}}}\times 100\%$$

(1)

where [Toluene]_in and [Toluene]_out represent the toluene concentration measured in the inlet gas and the outlet gas, respectively.

The reaction rates (r, mol·g⁻¹·s⁻¹) were obtained by following Equation (2) [28].

$$\mathrm{r} \left(\mathrm{mol}·{\mathrm{g}}^{-1}·{\mathrm{s}}^{-1}\right)=-\frac{{\mathrm{C}}_{\mathrm{inlet}}\times \mathrm{F}}{{\mathrm{m}}_{\mathrm{cat}}}·\ln (1-{\mathrm{X}}_{\mathrm{toluene}})$$

(2)

where C_inlet (ppm) refers to the toluene concentration in the inlet gas, F (mol·s⁻¹) is the flow rate, m_cat (g) is the mass of catalyst.

3.5. Kinetic Analysis

The Kinetic study was carried out at a high GHSV of 80,000 mL·g⁻¹·h⁻¹ and 75 mg samples. Additionally, the activation energy (E_a) of toluene oxidation was calculated according to the Arrhenius equation. The toluene concentration of the inlet gas was 600 ppm and was controlled to below 20%, where the effect of internal and external diffusion was negligible. The activation energy was determined according to the Arrhenius equation, as in Equation (3). [42]

$$\mathrm{r} \left(\mathsf{\mu }\mathrm{mol}·{\mathrm{L}}^{-1}·{\mathrm{s}}^{-1}\right)=-\mathrm{A} \exp [\frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{RT}}]\mathrm{C}$$

(3)

where r represents the reaction rate (μmol·L⁻¹·s⁻¹), A is the pre-exponential factor (s⁻¹), R is the molar gas constant, and T refers to the reaction temperature (K). In this study, the temperature was between 488 K and 558 K.

4. Conclusions

A series of Cu/CZ--X (X = 450 °C, 550 °C, 650 °C, 750 °C) prepared using an impregnation method were used to explore the effect of calcination temperature on the toluene removal catalytic activity. Cu/CZ-550 exhibited excellent low-temperature catalytic activity, with a 50% toluene conversion temperature of 220 °C. In addition, Cu/CZ-550 presented a high normalized reaction rate (3.1 × 10⁻⁵ mol·g⁻¹·s⁻¹ at 180 °C) and a relatively low apparent activation energy value (86.3 ± 4.7 kJ·mol⁻¹). Systematically, the characterization results revealed that the optimum redox property, abundant oxygen vacancies, and a large number of surface Ce³⁺ species were positively correlated with its superior catalytic performance. On the contrary, low specific surface area caused by structure collapse and large grain sizes were responsible for the inferior catalytic activity of Cu/CZ-750. This work will potentially guide the design of highly efficient and outstanding low-temperature activity catalysts for VOC degradation.