Elucidating the Role of Surface Ce⁴⁺ and Oxygen Vacancies of CeO₂ in the Direct Synthesis of Dimethyl Carbonate from CO₂ and Methanol

Abstract

Cerium dioxide (CeO₂) was pretreated with reduction and reoxidation under different conditions in order to elucidate the role of surface Ce⁴⁺ and oxygen vacancies in the catalytic activity for direct synthesis of dimethyl carbonate (DMC) from CO₂ and methanol. The corresponding catalysts were comprehensively characterized using N₂ physisorption, XRD, TEM, XPS, TPD, and CO₂-FTIR. The results indicated that reduction treatment promotes the conversion of Ce⁴⁺ to Ce³⁺ and improves the concentration of surface oxygen vacancies, while reoxidation treatment facilitates the conversion of Ce³⁺ to Ce⁴⁺ and decreases the concentration of surface oxygen vacancies. The catalytic activity was linear with the number of moderate acidic/basic sites. The surface Ce⁴⁺ rather than oxygen vacancies, as Lewis acid sites, promoted the adsorption of CO₂ and the formation of active bidentate carbonates. The number of moderate basic sites and the catalytic activity were positively correlated with the surface concentration of Ce⁴⁺ but negatively correlated with the surface concentration of oxygen vacancies. The surface Ce⁴⁺ and lattice oxygen were active Lewis acid and base sites respectively for CeO₂ catalyst, while surface oxygen vacancy and lattice oxygen were active Lewis acid and base sites, respectively, for metal-doped CeO₂ catalysts. This may result from the different natures of oxygen vacancies in CeO₂ and metal-doped CeO₂ catalysts.

1. Introduction

The emission of CO₂ into the earth’s atmosphere is continuously increasing with the growing burning of fossil fuels and industrial activities. For example, about 33,890.80 million tons of CO₂ was emitted into the atmosphere in 2018. The global CO₂ concentration in the atmosphere reached 407.65 ppm in September 2019, which is about 1.2 times as high as that of the past 40 years [1]. According to the earth’s CO₂, the concentration of CO₂ further increased to 416.47 ppm on 30 May 2020 [2]. The dramatically increased CO₂ emission results in severe climate change, which brings about a series of issues in terms of ecology, economy, and the environment. Taking these facts into consideration, it is necessary to develop effective approaches to mitigate CO₂ emission.

As an important part of carbon dioxide capture, utilization, and storage (CCUS), the chemical transformation of CO₂ to fuels and value-added chemicals can not only alleviate CO₂ emission but also reduce the dependence on fossil resources, which is of great significance for the sustainable development of energy, the environment, and the economy. Dimethyl carbonate (DMC), as one of the downstream products of methanol, is considered a green chemical, which has been widely applied as an intermediate for organic synthesis, an electrolyte for lithium-ion batteries, and a raw material for the synthesis of polycarbonate [3]. Among various synthesis approaches, the direct synthesis of DMC from CO₂ and methanol is deemed a green route due to the beneficial environment impact and high atomic use [4].

Although this reaction suffers from low DMC yield due to the thermodynamic limit, a significant endeavor has been made to develop effective catalysts, such as supported catalysts, alkali carbonate catalysts, ionic liquid catalysts, heteropoly acid catalysts, and transition metal oxide catalysts [4,5]. Transition metal oxides, especially ZrO₂ and CeO₂, have attracted tremendous attention in recent years due to their promising catalytic activity. Fujimoto et al. [6] first reported that ZrO₂ can catalyze CO₂ and methanol to DMC, and the catalytic activity was closely related to the surface basic and acidic sites.

Bell et al. [7] further studied the reaction mechanism over ZrO₂ using in situ infrared spectra. The results revealed that surface coordinately unsaturated Zr⁴⁺ cations, such as Lewis acid sites, promote the dissociation of methanol to methoxide (–OCH₃) and a proton, which can react with the surface hydroxyl group to produce water. Next, bidentate carbonates were formed by the interaction of C and O atoms in CO₂ with surface Lewis acid–base pairs (Zr⁴⁺–O²⁻), which were subsequently inserted into –OCH₃ and formed monodentate methyl carbonate (MMC) intermediate species. Finally, the MMC species reacted with methyl (–CH₃) from the dissociation of methanol on Lewis base sites (O²⁻) to produce DMC [7]. In this regard, the improvement of surface acidic and basic sites is beneficial for the catalytic activity. For example, Xiao et al. [8] found that the doping of Fe into ZrO₂ can significantly improve the surface acidic and basic sites, and the catalytic activity was positively correlated with surface acidic and basic sites.

Compared with ZrO₂, CeO₂ shows much higher catalytic activity and thus receives much more attention. For example, Marin et al. [9] studied the kinetic and reaction mechanism over a CeO₂ catalyst, and the results showed that the reaction follows the Langmuir–Hinshelwood mechanism, in which the adsorption of CO₂ is considered the rate-determining step. Wang et al. [10] reported that the catalytic activity of CeO₂ is closely related to the corresponding morphology, and CeO₂ nanorods showed much higher catalytic activity than that of the cube and octahedron due to the exposure of more acidic and basic sites. A further study of the morphology effect was performed with in situ infrared spectra. The results indicated that the (110) facet of CeO₂ nanorods promotes the formation of bidentate carbonates that can react with –OCH₃ to form MMC. However, other carbonates, such as monodentate carbonates, bicarbonates, and bridged carbonates, cannot accomplish that process, and therefore, the bidentate carbonates were identified as active species in terms of CO₂ adsorption [11]. In addition, doping the metal cation of Ca²⁺ [12], Zn²⁺ [13,14], Mn²⁺ [15], Ti⁴⁺ [16], Fe³⁺ [17], Co⁴⁺ [18], and Zr⁴⁺ [19,20] into the lattice of CeO₂ promotes the formation of oxygen vacancies (O_V) and is also proven to be an efficient strategy to improve catalytic activity. The results of in situ infrared spectra over the Zr–CeO₂ catalyst demonstrated that O_V promote the formation of bidentate carbonates via the interaction of C and O atoms in CO₂ with surface Lewis acid–base pairs (O_V–O²⁻), in which one of the O atoms in CO₂ is inserted into O_V [19]. Therefore, both the amount of adsorbed CO₂ and the catalytic activity increase with the increasing concentration of O_V [12,13,14,15,16,17,18,19,20].

It is obvious that two different Lewis acid–base pairs, Zr⁴⁺–O²⁻ and O_V–O²⁻, are considered active sites in ZrO₂ and metal-doped CeO₂ catalysts, respectively. However, of Ce⁴⁺ and O_V, which one is catalytically active in CeO₂ catalysts? Is the formation of O_V in CeO₂ catalysts beneficial to the catalytic activity as in metal-doped CeO₂ catalysts? The answers to these two questions are of prominent significance to the design of an effective catalyst. Inspired by this, the surface concentrations of Ce⁴⁺ and O_V were regulated via reduction and reoxidation of CeO₂ under different conditions in this work. Combining XRD, TEM, XPS, CO₂-TPD, NH₃-TPD, and CO₂-FTIR results, the role of surface Ce⁴⁺ and O_V in the adsorption of CO₂ and the catalytic activity for DMC synthesis from CO₂ and methanol were deeply elucidated. The possible influence of two types of O_V on the adsorption and activation of CO₂ was also proposed.

2. Results

2.1. Textural Properties of Catalysts

The N₂ adsorption–desorption isotherms of CeO₂ treated under different conditions are shown in Figure 1. All the samples showed the type IV isotherm with a distinct hysteresis loop, indicating the presence of a mesopore.

The detailed texture parameters are listed in

Table 1. Textural parameters of catalysts.

Catalysts	SBET (m2·g−1)	Vp (cm3/g)	Pore Size (nm)
CeO2	130.3	0.65	13.8
CeO2–H450	120.9	0.59	14.6
CeO2–H550	106.2	0.57	14.6
CeO2–H650	72.9	0.47	16.9
CeO2–H750	28.2	0.27	23.1
CeO2–H800	18.6	0.24	30.7
CeO2–H550–O350	107.3	0.59	14.8
CeO2–H550–O450	109.2	0.64	14.9
CeO2–H550–O550	107.8	0.52	12.3
CeO2–H750–O350	23.1	0.23	25.1
CeO2–H750–O450	26.2	0.27	25.6
CeO2–H750–O550	24.5	0.24	23.9

. As seen in Table 1, the specific surface area and pore volume of CeO₂ gradually decreased with an increase in the reduction temperature; however, the pore size of CeO₂ presented the opposite trend, which suggests that the mesopore originated from the piled pore. This phenomenon may arise from the aggregation of CeO₂ particles induced by high temperature [21]. In contrast, as CeO₂–H550 and CeO₂–H750 were reoxidized under an air atmosphere at 350 °C, 450 °C, and 550 °C, the texture parameters did not change so much.

2.2. XRD Analysis of Catalysts

XRD characterization was performed to analyze the crystal structure of CeO₂, and the results are presented in Figure 2. All the samples displayed the characteristic diffraction peaks of a cubic fluorite structure (JCPDS no. 65-5923), and the peaks at 28.5°, 33.0°, 47.5°, 56.5°, 59.4°, and 69.6° were ascribed to the (111), (200), (220), (311), (222), and (400) crystal facets of CeO₂, respectively [19].

As seen in Figure 2A, the intensity and width of the characteristic diffraction peaks of CeO₂ did not change obviously as the reduction temperature decreased below 550 °C, indicating the similar crystal size of CeO₂, CeO₂–H450, and CeO₂–H550. However, as the reduction temperature increased beyond 550 °C, the peak intensity was gradually enhanced and the peak width gradually narrowed, indicating the gradually increasing crystal size. The crystal size of each catalyst is presented in

Table 2. Crystal and particle size of catalysts.

Catalysts	dCeO2 (nm) a	Catalysts	dCeO2 (nm) a
CeO2	7.1	CeO2–H550–O350	7.6
CeO2–H450	7.4	CeO2–H550–O450	7.3
CeO2–H550	7.4	CeO2–H550–O550	7.5
CeO2–H650	12.9	CeO2–H750–O350	32.1
CeO2–H750	25.5	CeO2–H750–O450	29.7
CeO2–H800	46.6	CeO2–H750–O550	31.6

^a Crystal size calculated from the XRD diffraction peaks of CeO₂ (111) based on the Scherrer equation.

, which was calculated using the Scherrer equation as follows: D = Kλ/[(β − b)cosθ], where K is the Scherrer constant (0.89), λ is 0.15414 nm (the wavelength of Cu Kα radiation), β is the full width at half-maximum of the CeO₂(111) diffraction peak, b is the corrected full width at half-maximum of the instrument, and θ is the diffraction angle [22].

As seen in Table 2, similar crystal sizes of about 7~8 nm were observed for CeO₂, CeO₂–H450, and CeO₂–H550. However, the crystal sizes significantly increased from 7.4 to 46.6 nm with an increase in the reduction temperature from 550 °C to 800 °C, which resulted from the aggregation of CeO₂ particles [21,23,24]. In addition, the crystal sizes of CeO₂–H550 remained basically about 7~8 nm after reoxidation treatment, while those of CeO₂–H750 remained basically about 30 nm after reoxidation treatment. These results revealed that reoxidation treatment does not obviously affect the crystal sizes of CeO₂.

2.3. TEM of Catalysts

TEM was used to characterize the morphology of catalysts. As seen in Figure 3, the morphology of catalysts was irregular, including being polyhedron- and rodlike. The particle size of CeO₂, CeO₂–H450, CeO₂–H550, CeO₂–H550–O350, CeO₂–H550–O450, and CeO₂–H550–O550 seemed almost the same. The possible reason may be that the starting material of CeO₂ was already calcinated at 550 °C for 5 h in a muffle furnace before reduction and reoxidation treatment. However, the particles significantly aggregated with increasing reduction temperature as the reduction temperature increased beyond 550 °C. In addition, reoxidation treatment did not obviously affect the particle sizes of the catalysts. This was consistent with the results of XRD.

HRTEM was used to further characterize the exposed facet of catalysts, as shown in Figure 4. The observed lattice fringes of about 0.31 and 0.27 nm corresponded to the (111) and (200) facets of CeO₂. Obviously, the main exposed facet was (111), and the (200) facet only appeared in CeO₂–H650 and CeO₂–H750–O550, which was in line with our previous studies [15,19]. These results also indicated that the reduction temperature and reoxidation treatment do not apparently affect the exposed facet of catalysts.

2.4. Surface Composition Analysis of Catalysts

XPS characterization was performed to analyze the chemical state of the surface Ce and O of catalysts, as shown in Figure 5. After gaussian fitting, the Ce 3D spectra could be fitted into eight peaks: U‴(~916.3 eV), U″(~907.4 eV), U′(~903.3 eV), U(~900.6 eV), V‴(~898.0 eV), V″(~888.4 eV), V′(~884.9 eV), and V(~882.1 eV) [16].

In the O 1s spectra, the peaks at ~529.0 eV, ~530.5 eV, and ~531.5 eV were assigned to the surface lattice oxygen (O_L), oxygen vacancies (O_V), and chemisorbed oxygen species (O_C), respectively [25,26]. The surface concentration of O_V was calculated based on the peak area from gaussian fitting, and the results are listed in

Table 3. Surface concentration of Ce cations and oxygen vacancies determined with XPS.

Catalysts	Ce Atom Ratio (%)		O Atom Ratio (%)
	Ce3+	Ce4+	OV
CeO2	13.5	86.5	5.3
CeO2–H450	15.7	84.3	6.9
CeO2–H550	16.2	83.8	7.6
CeO2–H650	16.7	83.3	7.8
CeO2–H750	16.8	83.2	8.1
CeO2–H800	17.7	82.3	8.5
CeO2–H550–O350	15.7	84.3	7.1
CeO2–H550–O450	15.3	84.7	6.6
CeO2–H550–O550	15.2	84.8	6.7
CeO2–H750–O350	15.8	84.2	7.2
CeO2–H750–O450	15.2	84.8	6.5
CeO2–H750–O550	14.9	85.1	6.1

.

As seen in Table 3, the concentration of Ce³⁺ gradually increased from 13.5 to 17.7% as the reduction temperature increased to 800 °C, while that of Ce⁴⁺ gradually decreased from 86.5 to 82.3%. Moreover, the concentration of O_V also increased with an increase in the reduction temperature. These results were consistent with the work reported by our group and Chen et al. that the percentage of Ce³⁺ is positively correlated with that of O_V [16,19]. In addition, the concentrations of Ce³⁺ and O_V decreased for CeO₂–H550 and CeO₂–H750 after reoxidation treatment.

The concentration of the surface O_V of CeO₂ is closely related to the calcination atmosphere, oxygen partial pressure, and calcination temperature [27]. The formation of O_V is accomplished through the diffusion of lattice oxygen to the environment, where the content of oxygen is low. Under these circumstances, an oxygen vacancy is a kind of anionic defect. When CeO₂ was exposed to the reductive H₂ atmosphere, the lattice oxygen reacted with H₂ and formed H₂O and therefore promoted the formation of O_V; moreover, the promoting effect gradually strengthened with increasing reduction temperature. It should be noted that two surplus electrons would be retained in the crystal structure of CeO₂ accompanied with the formation of O_V, and the two surplus electrons can bind with two adjacent Ce⁴⁺, resulting in the conversion of Ce⁴⁺ to Ce³⁺. Moreover, the two electrons were mobile that could migrate between two adjacent Ce ions. In other words, the two quasi-free mobile electrons were trapped around the anionic defect (O_V), which could migrate between adjacent Ce ions and cause electron conduction. A material with this kind of defect is an N-type semiconductor [28]. The O atoms were reinserted back into O_V and formed O_L when CeO₂–H550 and CeO₂–H750 were reoxidized under an air atmosphere. In this process, the trapped electrons around O_V were transferred to O atoms, and the conversion of Ce³⁺ to Ce⁴⁺ occurred simultaneously. Singhal et al. [29] also found that the conversion between O_V and O_L is reversible to some extent.

Furthermore, as shown in Figure S1, the color of CeO₂ gradually changed from yellow to brown and finally to gray-black with the increase in the reduction temperature, and the color changed back to yellow to a certain degree when CeO₂–H550 and CeO₂–H750 were reoxidized under an air atmosphere. This is because the trapped electrons around O_V can absorb light with a certain wavelength and the absorption capacity is gradually enhanced with the increasing number of electrons or O_V [21,29]. This result also indicated that reduction treatment is favorable for the formation of O_V, while reoxidation treatment plays the opposite role.

2.5. Surface Acidity and Basicity Analysis of Catalysts

The surface acidity and basicity of catalysts were characterized using NH₃-TPD and CO₂-TPD, as shown in Figure 6 and Figure 7, and the detailed results are listed in

Table 4. Adsorption amount of CO₂ and NH₃ determined using TPD profiles.

Catalysts	CO2 Adsorption (mmol/g)			NH3 Adsorption (mmol/g)
	Weak (<200 °C)	Moderate (200~400 °C)	Strong (>400 °C)	Weak (<200 °C)	Moderate (200~400 °C)	Strong (>400 °C)
CeO2	0.607	0.206	0.039	0.592	0.158	0.040
CeO2–H450	0.533	0.147	0.025	0.472	0.124	0.046
CeO2–H550	0.525	0.136	0.020	0.586	0.134	0.043
CeO2–H650	0.361	0.074	0.015	0.384	0.084	-
CeO2–H750	0.142	0.023	0.002	0.199	0.018	-
CeO2–H800	0.113	0.019	0.006	0.117	0.031	-
CeO2–H550–O350	0.525	0.130	0.024	0.586	0.143	0.040
CeO2–H550–O450	0.538	0.154	0.028	0.416	0.143	0.037
CeO2–H550–O550	0.590	0.156	0.028	0.598	0.152	0.041
CeO2–H750–O350	0.183	0.023	0.004	0.227	0.022	-
CeO2–H750–O450	0.192	0.024	0.006	0.232	0.023	-
CeO2–H750–O550	0.205	0.039	0.007	0.240	0.027	-

. The desorption peaks in the temperature region of 50~200, 200~400, and 400~600 °C in CO₂-TPD and NH₃TPD profiles were attributed to the weak acidic/basic, moderate acidic/basic, and strong acidic/basic sites, respectively [30]. Studies have reported that weak basic sites are mainly ascribed to surface –OH groups, which react with CO₂ and form bicarbonates [31,32]. The moderate basic sites were derived from the Ce–O_L or O_V–O_L ion pairs, which combined with CO₂ and promoted the formation of bridged and bidentate carbonates [8,19,32], while the strong basic sites were mainly assigned to the low-coordination oxygen anions [8,31,32], which resulted in the formation of stable monodentate carbonates [33].

Overall, the surface acidic sites with different natures all decreased with an increase in the reduction temperature and increased with an increase in the reoxidation temperature (Figure 6 and Table 4). A similar trend was observed for the surface basic sites under various treatment conditions (Figure 7 and Table 4). A possible reason for the change in surface acidity and basicity is discussed in Section 3.2.

2.6. Measurement of Catalytic Activity

The DMC yields of CeO₂ treated under different conditions are displayed in Figure 8. As seen in Figure 8A, the DMC yield of CeO₂ gradually decreased from 9.02 to 1.75 mmol/g as the reduction temperature increased to 800 °C. However, as seen in Figure 8B,C, the DMC yield of CeO₂–H550 and CeO₂–H750 gradually increased from 6.94 and 2.85 to 7.98 and 4.21 mmol/g, respectively, as the reoxidation temperature increased to 550 °C. These results indicated that reduction treatment is detrimental to the improvement of catalytic activity but reoxidation treatment is beneficial.

3. Discussion

3.1. The Relationship between Catalytic Activity and Surface Acidity/Basicity

Surface basicity and acidity play a significant role in determining the catalytic activity of CeO₂-based catalysts. Wang et al. [11] found that only bidentate carbonates can react with surface –OCH₃ originating from the dissociation of methanol and form MMC, which is considered an active intermediate species for the formation of DMC, while monodentate carbonates, bicarbonates, and bridged carbonates cannot accomplish this process. Inumaru et al. [34] also found that the catalytic activity of ZrO₂ nanocrystals is closely related to the surface bidentate carbonate species. As discussed in Section 2.4, bidentate carbonates were derived from the interaction of CO₂ with moderate basic sites. In addition, surface acidic sites significantly affected the adsorption and activation of methanol. Studies have reported that methanol is adsorbed and dissociated on acid sites and forms –OCH₃ groups, which participate in the formation of MMC [7,19,35]. Xiao et al. [8] revealed that catalytic activity is positively correlated with the moderate surface acidic/basic sites of Fe–Zr composite catalysts. Taking these findings into account, it is reasonable to build a relationship between catalytic activity and moderate surface acidic/basic sites. As shown in Figure 9A,B, catalytic activity was positively correlated with the number of moderate surface acid and basic sites (determined from NH₃-TPD and CO₂TPD profiles) of CeO₂ catalysts.

3.2. Deep Insight into the Role of Ce⁴⁺ and O_V

Marin et al. [9] reported that conversion of CO₂ and methanol to DMC over a CeO₂ catalyst follows a Langmuir–Hinshelwood mechanism, with the CO₂ adsorption step being rate controlling. However, there are two different theories about the adsorption and activation of CO₂ for CeO₂-based catalysts. One is that surface coordination unsaturated Ce⁴⁺ ions and O_L serve as Lewis acid–base pairs to promote the adsorption and activation of CO₂ [7,36,37]. The other is that surface O_V and O_L serve as Lewis acid–base pairs to promote the adsorption and activation of CO₂. Especially for metal-doped CeO₂ catalysts, it was found that one of the O atoms of CO₂ can be inserted into O_V and thus promotes the adsorption and activation of CO₂, and therefore, the adsorption of CO₂ improves with an increase in surface O_V [12,15,19]. Obviously, the focus was that of Ce⁴⁺ or O_V, which Lewis acid sites determine the adsorption and activation of CO₂ for CeO₂ catalysts?

XPS results showed that reduction treatment results in a decrease in surface Ce⁴⁺ and an increase in O_V. In contrast, reoxidation treatment resulted in a decrease in surface O_V and an increase in Ce⁴⁺. Combining XPS and CO₂-TPD results, it was apparent that the adsorption amount of CO₂ and the concentration of Ce⁴⁺ showed a similar trend under various treatment conditions. Moreover, combining XPS and NH₃TPD results, we found that the adsorption amount of NH₃ and the concentration of Ce⁴⁺ shows a similar trend under various treatment conditions. These results indicated that surface Ce⁴⁺ serves as active Lewis acid sites to adsorb and activate CO₂ and methanol molecules.

Nevertheless, it should be noted that the particle size of CeO₂ can also affect the adsorption of CO₂ and NH₃. For example, as the reduction temperature increased beyond 550 °C, the aggregation of CeO₂ particles occurred (Table 2 and Figure 3), which can also result in decreased adsorption of CO₂ and NH₃.

The profiles of the DMC yield for catalysts treated under different conditions with a change in the concentration of Ce⁴⁺ and O_V are presented in Figure 10 and Figure 11, respectively.

It was obvious that the catalytic activity increased with the increasing concentration of Ce⁴⁺, while it decreased with the increasing concentration of O_V. Nevertheless, as discussed before, treatment conditions greatly affected the particle size of CeO₂, as presented in Table 2 and Figure 3. Generally, a particle with a smaller size is conducive to the exposure of more active sites and thus improves the adsorption of the reactant and catalytic activity [38]. To eliminate the particle size effect, CeO₂, CeO₂–H450, CeO₂–H550, CeO₂–H550–O350, CeO₂–H550–O450, and CeO₂–H550–O550 catalysts (with similar crystal size of about 7~8 nm and basically an unchanged particle size) were selected to correlate the number of moderate basic sites as well as catalytic activity with the concentration of Ce⁴⁺ and O_V. As shown in Figure 12, the number of moderate basic sites as well as catalytic activity were positively correlated with the concentration of surface Ce⁴⁺ but negatively correlated with that of O_V.

To further confirm the role of surface Ce⁴⁺ in the formation of active bidentate carbonates, FTIR spectra of CO₂ adsorption for CeO₂, CeO₂–H550, and CeO₂–H550–O550 were developed, and the results are displayed in Figure 13. The bands at 1215, 1393, and 1610 cm⁻¹ were associated with bicarbonates, while the bands at 856, 1033, 1293, and 1581 cm⁻¹ were ascribed to bidentate carbonates [11,19]. In addition, the small bands at 1242 and 1652 cm⁻¹ were attributed to bridged carbonates, while the small bands at 1356 and 1466 cm⁻¹ were assigned to monodentate carbonates [11,19]. Obviously, the peak intensity and area of bidentate carbonates decreased after reduction treatment and increased after reoxidation treatment. These results strongly supported the fact that the surface Ce⁴⁺ of CeO₂ serves as active Lewis acid sites and promotes the adsorption of CO₂ and the formation of bidentate carbonates and thus catalytic activity for the synthesis of DMC from CO₂ and methanol.

Why O_V played an opposite role in the adsorption of CO₂ for pure CeO₂ and metal-doped CeO₂ catalytic systems is thought provoking. This may be because O_V has a different nature in pure CeO₂ and metal-doped CeO₂ catalysts. As discussed in Section 2.3, the O_V formed by H₂ reduction are a kind of anionic defect, and a material with this kind of defect is an N-type semiconductor [28]. The possible reason is that the quasi-free mobile electrons around O_V impede the adsorption and activation of CO₂, and therefore, the catalytic activity decreases with an increase in O_V. This was consistent with the results reported by Tan et al. that the catalytic activity of CeO₂ prepared under different atmospheres follows the order CeO₂–O₂ > CeO₂–Ar > CeO₂–air > CeO₂–H₂; especially, the catalytic activity of CeO₂–O₂ was 2.5 times as high as that of CeO₂–H₂ [27]. However, for metal-doped CeO₂ catalysts, the O_V derived from the incorporation of metal ions (for example, Ca²⁺, Zn²⁺, Mn²⁺, Ti⁴⁺, and Zr⁴⁺) into the lattice of CeO₂ is a kind of cationic defect, and a material with this kind of defect is a P-type semiconductor[39,40]. This kind of cationic defect (O_V) is beneficial for the adsorption and activation of CO₂, and thus, the catalytic activity increases with an increase in O_V [12,13,15,16,19]. The possible influence of two types of O_V on the adsorption and activation of CO₂ is shown in Figure 14.

It is interesting that similar phenomena have also been observed during the conversion of syngas to light olefins over ZrCeZnO_x catalysts [41,42]. It was found that the incorporation of Ce³⁺ into Zn–Zr oxide improves the percentage of O_V and thus catalytic activity [42]; however, the catalytic activity of ZrCeZnO_x decreased after reduction with H₂, although the percentage of O_V improved significantly [41]. This indicated that the types of O_V remarkably affects the adsorption of CO and therefore catalytic activity.

4. Materials and Methods

4.1. Preparation of CeO₂

First, 3.47 g of Ce(NO₃)₃·6H₂O was dissolved in 30 mL of deionized water. Next, 100 mL of NaOH solution (2 mol/L) was added to the solution of Ce(NO₃)₃ with a constant-flux pump under stirring. After aging for 1 h at ambient temperatures, the slurry was filtered and washed with deionized water and ethanol several times until it became neutral. Finally, the products were dried for 12 h at 80 °C and then calcinated for 5 h at 550 °C in a muffle furnace. The resultant catalyst was labeled as CeO₂ and used as a starting material for further treatment.

4.2. Reduction Treatment of CeO₂

Typically, 0.25 g of CeO₂ was placed in a tube furnace, and then, the temperature was increased to 450, 550, 600, 650, 700, 750, and 800 °C under 10a % H₂–N₂ atmosphere for 4 h at a heating rate of 5 °C/min. The obtained catalysts were named as CeO₂–HT₁, where T₁ is the reduction temperature.

4.3. Reoxidation Treatment of CeO₂–H550 and CeO₂–H750

Typically, 0.2 g of CeO₂–H550 or CeO₂–H750 was placed in a muffle furnace, and then, the temperature was increased to 350, 450 and 550 °C under an air atmosphere for 4 h at a heating rate of 2 °C/min. The obtained catalysts were named as CeO₂–H550–OT₂ or CeO₂–H750–OT₂, where T₂ is the reoxidation temperature.

4.4. Characterization of Supports and Catalysts

N₂ physisorption was performed on a Beishide 3H-2000PS2 instrument at −196 °C using N₂ as an adsorbate. The sample was outgassed at 250 °C for 4 h in vacuum to remove the physisorbed impurities before measurement. The specific surface area and pore volume were calculated using the BET and BIH methods, respectively.

XRD characterization was carried out on a Rigaku SmartLab SE diffractometer (40 kV, 40 mA) with a diffraction angle (2θ) from 10 to 80° and a scanning rate of 10°/min.

TEM images were taken with FEI Tecnai F30 at 200 kV. Before measurement, the sample was first dispersed in ethanol, then ultrasonicated for 30 min, and finally deposited on a carbon-coated copper film.

TPD characterization was performed on Micromeritics Autochem 2920. First, 100 mg of the sample was degassed for 30 min at 250 °C with N₂ (30 mL/min) to remove the physisorbed impurities. After the temperature was cooled down to 50 °C, 15% NH₃ in He (30 mL/min) or pure CO₂ (30 mL/min) was introduced and maintained for 30 min to allow saturated adsorption of NH₃ and CO₂. Subsequently, the physisorbed substance was removed with He (30 mL/min) at 50 °C for 30 min. Thereafter, the temperature was gradually increased to 800 °C at a heating rate of 10 °C/min with He as a carrier gas (30 mL/min), and the desorbed NH₃ and CO₂ were detected with TCD.

XPS spectra were analyzed with an ESCALAB 250Xi (Thermo Scientific, Waltham, MA, USA) photoelectron spectrometer under AlKα radiation at 300 W. The binding energies were referred to the C1s line from adventitious carbon at 284.6 eV.

CO₂-FTIR spectra were recorded using a Bruker Tensor II instrument equipped with an MCT detector by collecting 64 scans at a resolution of 4 cm⁻¹. About 20 mg of the sample was loaded into the cell and then pretreated under He flow (30 mL/min) at 400 °C for 1 h to remove the physisorbed impurities. After the temperature dropped down to 140 °C (reaction temperature), pure CO₂ (30 mL/min) was introduced and maintained for 30 min to allow saturated adsorption of CO₂. Subsequently, the physisorbed substance was removed with He (30 mL/min) for 30 min, and then, FTIR spectra were recorded.

4.5. Catalytic Evaluation

The catalytic activity of catalysts for DMC synthesis from CO₂ and methanol was investigated in a 250 mL high-pressure slurry-bed reactor equipped with a mechanical stirrer. Typically, 0.2 g of a catalyst and 30 mL of methanol were placed into the reactor, which was subsequently sealed. Next, the reactor was filled with CO₂ several times to check the tightness and remove air. Thereafter, the reactor was pressurized with CO₂ to 3 MPa at ambient temperatures, and then, the reaction temperature was increased to 140 °C and maintained for 4 h. After the reaction finished, the reactor was depressurized and cooled down to room temperature. The liquid product was analyzed with n-propanol as an internal standard substance using a gas chromatograph (FID-GC, O Hua 9160). It should be noted that no by-products were formed except DMC, and the yield of DMC was defined as follows:

$$Y_{\mathrm{DMC}}\left(\mathrm{mmol}/\mathrm{g}\right)=n_{\mathrm{DMC}}\left(\mathrm{mmol}\right)/m_{\mathrm{catalyst}}\left(\mathrm{g}\right)$$

(1)

5. Conclusions

(1)

The concentrations of surface Ce⁴⁺ and oxygen vacancies of CeO₂ were regulated with reduction and reoxidation treatments under different conditions. The reduction treatment promoted the conversion of Ce⁴⁺ to Ce³⁺ and improved the surface concentration of oxygen vacancies, while the reoxidation treatment favored the conversion of Ce³⁺ to Ce⁴⁺ and decreased the concentration of oxygen vacancies.

(2)

Catalytic activity was positively correlated with the number of moderate surface acidic/basic sites of catalysts. Moreover, the number of moderate basic sites and catalytic activity were positively correlated with the surface concentration of Ce⁴⁺ but negatively correlated with that of oxygen vacancies.

(3)

The surface Ce⁴⁺ rather than oxygen vacancies served as active Lewis acid sites, and lattice oxygen served as active Lewis base sites to adsorb and activate CO₂, promoting the formation of active bidentate carbonates species and DMC.

(4)

The cationic oxygen vacancy was beneficial but the anionic oxygen vacancy was detrimental to the formation of active bidentate carbonates species and DMC, which sheds light upon the active sites for CeO₂ and metal-doped CeO₂ catalysts and provides evidence for the design of efficient catalysts.