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Article

CO Oxidation over Cu/Ce Binary Oxide Prepared via the Solvothermal Method: Effects of Cerium Precursors on Properties and Catalytic Behavior

by
Wen Jin
,
Yanmin Liu
,
Hongyan Xue
*,
Jun Yu
and
Dongsen Mao
*
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(12), 856; https://doi.org/10.3390/catal14120856
Submission received: 22 October 2024 / Revised: 16 November 2024 / Accepted: 22 November 2024 / Published: 25 November 2024
(This article belongs to the Special Issue Featured Papers in “Environmental Catalysis” Section)
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Abstract

:
Cu/Ce binary oxides were prepared via the one-pot solvothermal method, and the effects of different cerium precursors (cerium nitrate and cerium ammonium nitrate) on the catalytic activity and resistance to water vapor or CO2 of the prepared samples for low-temperature CO oxidation reaction were investigated. The physicochemical characteristics of the catalysts were characterized via thermal analyses (TG-DSC), X-ray diffraction (XRD), Raman spectroscopy, N2 adsorption/desorption, inductively coupled plasma-atomic emission spectrometry (ICP-AES), X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs), and temperature-programmed reduction with H2 (H2-TPR). The results indicated that the CuO/CeO2 catalyst (CC-N) prepared with cerium nitrate showed higher activity for low-temperature CO oxidation, which can be ascribed to its larger specific surface area and pore volume, higher amounts of highly dispersed CuO species with strong interaction with CeO2, Cu+ species, and more active surface oxygen species, compared with the counterpart prepared with cerium ammonium nitrate (CC-NH). Furthermore, the CC-N catalyst also exhibited better resistance to CO2 poisoning than CC-NH.

1. Introduction

The catalytic oxidation of CO over catalysts such as Cu/CeO2 has attracted considerable attention due to its potential for efficient CO removal in various environmental applications [1,2]. Although much research has been completed on the Cu/Ce composite oxide catalyst and some progress has been made over the past decades [3,4], it still has the disadvantages of low activity and poor resistance to water or CO2 at low temperatures, which limit its practical application [5,6].
To improve the activity of Cu/Ce composite oxide catalyst, the addition of a third component has been usually adopted, such as alkali metal Cs [7]; alkali earth metal Mg [8]; transition metals including Zr [9], Co [10], Mn [11], and Fe [12]; rare earth metals such as La, Pr, and Sm [13], precious metals such as Ag [14], Ru [15], Pd [16], and Pt [17], etc. For example, Kim et al. [14] reported that the addition of Ag reduced the temperature for complete conversion of CO (T100) of Cu/Ce mixed oxide from 120 to 100 °C. Similarly, Wu et al. [17] found that the addition of Pt decreased the T100 of Cu/Ce mixed oxide for the catalytic oxidation of CO from >100 °C to 85 °C.
The optimization of catalyst preparation methods and conditions is another efficient approach to enhance the performance of CO oxidation catalyzed by CuO/CeO2 [18,19]. Up to now, the main preparation methods for CuO/CeO2 catalysts are the impregnation method [20], deposition/precipitation method [21], co-precipitation method [22], sol-gel method [23], solvothermal method [24], solution combustion method [25], photodeposition method [26], MOF precursor method [27], and spray pyrolysis method [28], etc. Shang et al. [29] studied the influence of three preparation methods (impregnation, co-precipitation, and mechanical mixing) on the catalytic CO oxidation activity of the CuO/CeO2 catalysts. The results showed that CuO was more dispersed and the interaction between CuO and CeO2 was stronger in the catalyst prepared via the impregnation method, which promoted the reduction of CuO to Cu+ species on Cu/Ce interface, therefore achieving the highest catalytic activity. The solid solution structure of the catalyst prepared via co-precipitation inhibits the chemisorption of CO, resulting in low catalytic performance. The catalyst prepared via the mechanical mixing method has the lowest activity due to the separation of CuO and CeO2 phases and the absence of the Cu/Ce interface. Zhan et al. [30] prepared a CuO/CeO2 catalyst with a wide pore channel and large specific surface area via mechanochemical grinding. The catalytic activity for CO oxidation reaction was comparable to that of the catalyst prepared via the sol-gel method, but better than that prepared via the impregnation method. Zheng et al. [3] prepared the CuO/CeO2 catalyst via a solvothermal deposition method with CeO2 as the carrier and found that the catalytic activity for CO oxidation was significantly better than that of the corresponding catalysts prepared via the impregnation method and deposition/precipitation method. Very recently, Liu et al. [24] used a one-pot solvothermal method to prepare the CuO/CeO2 catalyst for low-temperature CO oxidation. Compared with the CuO/CeO2 catalyst prepared via the solvothermal deposition method previously reported [3], the CuO/CeO2 catalyst prepared via the one-pot solvothermal method had higher activity as well as the merits of high efficiency, simplicity, and energy saving.
On the other hand, it has been well documented that different metal salt precursors usually have an important effect on the catalytic performance of the prepared catalysts. Concretely, for the Cu/Ce mixed oxide catalysts, there have been many studies on the influence of Cu precursors [20,31,32,33], but only a few studies on the influence of Ce precursor. Qi et al. [34] used a surfactant (CTAB)-assisted co-precipitation method to prepare the CuO/CeO2 catalyst and investigated the influence of cerium precursors (cerium nitrate and ammonium cerium nitrate) on its activity for CO oxidation. Wang et al. [35] synthesized CeO2 carrier via the precipitation method and then loaded the active component Cu via the impregnation method to obtain the CuO/CeO2 catalyst, and investigated the effects of cerium sources (Ce(NO3)3, CeCl3, Ce(NH4)2(NO3)6, and Ce(SO4)2) on its performance for hydrogen production from methanol steam reforming. However, the effect of cerium precursors on the CO oxidation activity of the CuO/CeO2 catalyst prepared via a solvothermal method has not been reported.
In this paper, the CuO/CeO2 catalysts with different cerium precursors were prepared via the one-pot solvothermal method. The effect of cerium nitrate and ammonium ceric nitrate on the physicochemical properties and catalytic activity of the prepared CuO/CeO2 catalysts for low-temperature CO oxidation was investigated by using copper nitrate as the precursor of copper. The relationship between catalytic activity and physicochemical properties of catalysts was constructed. In addition, the resistance of the prepared catalysts to H2O or CO2 was investigated comparatively.

2. Results and Discussion

2.1. Analysis of Thermal Decomposition of Catalyst Precursors

Figure 1 presents the TG-DSC curves of the catalyst precursors after drying at 120 °C. The curves of TG and DSC for both samples exhibited similar trends. Specifically, the TG curve reveals two weight loss stages. The first stage at <200 °C corresponds to the removal of water adsorbed on the catalyst surface. The second stage with a significant weight loss accompanied by a large exothermic peak at 200–400 °C, was ascribed to the decomposition of the precursor hydroxide. Based on the results, the precursors of both catalysts were fully decomposed at >400 °C. Therefore, 450 °C was selected as the calcination temperature for the precursors to prepare the CuO/CeO2 catalysts in this work.

2.2. Characterization of Catalysts

2.2.1. Structural and Textural Properties

Figure 2 shows the XRD spectra of the CuO/CeO2 catalysts synthesized from different cerium salts. The prominent characteristic diffraction peaks of CeO2 were observed at 2θ = 28.5°, 33.0°, 47.4°, and 56.3° for both samples [33]. Additionally, characteristic diffraction peaks of CuO were also present at 2θ = 35.5° and 38.7° [33]. The results, combined with the results of the thermal analyses (Figure 1), confirm that the precursors of the catalysts decompose into oxides under calcination at 450 °C. Notably, the diffraction peaks of CeO2 were weakened and broadened after the incorporation of Cu species as shown in Figure S1, indicating that the crystallinity degree and crystallite size of CeO2 had decreased [36].
The average particle sizes of CuO and CeO2 were calculated using the Scherrer formula, based on the diffraction peaks of two crystalline planes of CuO and the (111) plane of CeO2. The results are listed in Table 1. The CeO2 crystallite sizes for both catalysts were nearly identical, while the particle size of CuO in the CC-NH catalyst is larger than in the CC-N catalyst.
In addition, the lattice constants of CeO2, calculated via the Bragg equation based on the (111) plane of CeO2, are also listed in Table 1. The lattice constant of CeO2 in CC-NH was higher than that of CC-N. Generally, the lattice constants of CeO2 were influenced by the following factors: On the one hand, the radius of Cu2+ (0.072 nm) was smaller than that of Ce4+ (0.097 nm), leading to lattice contraction when Cu2+ replaces part of Ce4+ in the lattice [3]. On the other hand, the formation of oxygen vacancies due to the substitution of Ce4+ by Cu2+ or the formation of Ce3+ (0.103 nm) exerted an opposite effect on the lattice of CeO2 [37]. These two opposing effects together determined the lattice constants of CeO2.
The Raman spectra of CuO/CeO2 catalysts synthesized from different cerium precursors are shown in Figure 3. Both samples exhibited an obvious peak at ~450 cm–1, ascribed to the F2g characteristic peak of cubic CeO2 [33]. Additionally, a very weak peak at ~600 cm–1 corresponded to the characteristic peak of oxygen vacancy resulting from lattice distortion of CeO2 [33]. This case is generally believed to be caused by the entrance of Cu2+ into the CeO2 lattice, leading to its lattice contraction. Comparing the two catalysts derived from different cerium precursors, there was minimal difference in their F2g characteristic peaks, indicating that the valence state of the cerium ion did not significantly influence its F2g characteristic peak of CeO2. The ratio of the peak areas at 600 and 450 cm–1 (A600/A450) is commonly used to calculate the relative concentration of oxygen vacancies in catalysts [24,33]. The A600/A450 value of CC-N catalyst was 0.29, while that of CC-NH catalyst was 0.16, indicating that the relative concentration of oxygen deficiency in the CC-N catalyst was greater than that in the CC-NH catalyst.
The textural properties of CuO/CeO2 catalysts prepared with different cerium precursors are also presented in Table 1. The specific surface area (SBET) and pore volume (Vp) of CC-N were significantly larger than those of CC-NH, but their pore sizes (Dp) were similar.
The actual Cu content in the catalyst was measured via the ICP-AES, with the results compiled in Table 1. Notably, the actual Cu content of CC-N and CC-NH samples was close to the reported value of 20%.

2.2.2. Surface Characterization (XPS)

The CuO/CeO2 catalysts prepared with different cerium precursors were characterized via XPS to examine the state of metal oxides on the surface and the distribution of surface components. The obtained results are presented in Figure 4 and Table 2. As can be seen, the molar content of Cu (Cu/(Cu + Ce)) on the surface of both catalysts was similar (within experimental error). Additionally, compared with the results of ICP-AES analysis (Table 1), the Cu content on the surface of both catalysts was significantly higher than that of the bulk composition. These results suggest that Cu species are enriched on the catalyst surface due to the lower surface energy of Cu compared to that of Ce [38].
According to the Cu 2p spectra of different catalysts (Figure 4A), both catalysts exhibited a main peak at 933.7 eV, with a satellite peak observed at 941.2–941.8 eV, indicating that Cu species on the surface of catalysts mainly exist as Cu2+ [24,33]. Moreover, a shoulder peak representing Cu+ or Cu0 species at 932.0 eV was also noted in the spectra of the two catalysts [24,33]. In the Cu 2p spectra, the peak area (A) belonging to Cu 2p3/2 and its satellite peak area (B) were utilized to calculate the Cu+ content in the catalyst prepared via different methods. The calculation formula is Cu+(%) = (A − (A1/B)B)/(A + B) × 100, where A1/B = 1.89 [39], and the results are shown in Table 2. The relative content of Cu+ on CC-NH was less than that of CC-N, which correlated with the relative content of Ce3+ on the two catalysts obtained later. Under the influence of CuO/CeO2 interaction, the Cu species on the surface of the catalyst were partially reduced, resulting in the formation of a small amount of Cu+, which has been reported to have a better ability to adsorb and activate CO [24,33].
Figure 4B shows the Ce 3d spectra of the different catalysts. Both samples contained eight peaks attributed to four pairs of double spins, with Ce4+ 3d3/2 peaks labeled as u, u″, u‴ and Ce4+ 3d5/2 peaks labeled as v, v″, v‴. The peaks labeled u′ and v′ are used to denote the two electron arrangement configurations of the Ce3+ species [40]. This observation confirms the presence of both Ce4+ and Ce3+ species on the catalyst surface. The ratio of the sum of u′ and v′ peak areas to the total area of all peaks is commonly used to estimate the relative content of Ce3+ species in the catalysts (Ce3+/(Ce4+ + Ce3+)) [41], and the Ce3+ (%) data for both catalysts are provided in Table 2. As can be seen, the Ce3+ species were present on both catalysts and the Ce3+ content for both catalysts were comparable within experimental uncertainty. According to the literature [42,43], the presence of Ce3+ can promote the electron transfer process Ce3+ + Cu2+→ Ce4+ + Cu+, leading to the generation of more Cu+. In this case, however, the Cu+ content of the CC-N catalyst was significantly larger than that CC-NH, which was inconsistent with the trend of the Ce3+ content. A similar phenomenon was also reported previously by other researchers [44].
The O 1s spectra of both catalysts in Figure 4C revealed three distinct peaks at 528.9–529.2 eV, 529.5–529.9 eV, and 531.2–531.4eV, corresponding to lattice oxygen (OL), surface adsorbed oxygen (OA), and hydroxyl oxygen (OOH), respectively [33,45]. The ratio of the area of the OA peak to the total area of the three peaks, designated as OA (%), represents the relative concentration of adsorbed oxygen in catalysts. As shown in Table 2, the relative concentration of adsorbed oxygen on the surface of CC-N was greater than that of CC-NH. This result indicates that the concentration of oxygen deficiency on the surface of CC-N was greater than that of CC-NH, which was consistent with the oxygen deficiency concentration inferred from the Ce3+ content mentioned earlier. Furthermore, the binding energy of lattice oxygen in CC-N (529.3 eV) was greater than that in CC-NH (528.9 eV), suggesting that its lattice oxygen in CC-N exhibited better mobility and higher activity [45].
The above results demonstrate that different cerium precursors lead to variations in the surface contents of Cu+ and oxygen vacancies on CuO/CeO2 catalysts, which in turn result in different catalytic performance for CO oxidation at low temperatures.

2.2.3. CO-IR Analysis

To further investigate the state of Cu species in catalysts, in situ DRIFT characterization of the adsorbed CO on both catalysts was carried out. As shown in Figure 5, a strong absorption peak at 2103 cm–1 appears in the DRIFT spectra of both catalysts, ascribing to the stretching vibration peak associated with the linear adsorption of CO on Cu+ (Cu+–CO) [46]. Specially, the Cu+ species were likely generated from the reduction of highly dispersed CuO species and stabilized by interactions with the CeO2 surface [29]. The relative peak areas at 2103 cm⁻1 were 43.1 (a.u.) for CC-NH and 88.6 (a.u.) for CC-N, indicating that CC-N had a greater CO adsorption capacity, which aligns with the relative content of Cu+ determined via the XPS analysis (Table 2).
Figure 5 also shows that the DRIFT spectra of both catalysts in the range of 1290 to 1598 cm–1 exhibited several peaks belonging to formate and carbonate adsorbed on the CeO2 surface. According to the literature [20,47], the peak around 1598 cm–1 belongs to carbonate or formate adsorbed on CeO2, and the peak around 1470 cm–1 is attributed to polydentate or monodentate carbonates, and the peaks around 1386 cm−1 and 1293 cm–1 correspond to bidentate carbonate. After introducing CO at 30 °C, obvious stretching vibration peaks of formate and carbonate appeared on the surface of both samples, mainly caused by the adsorption of CO in the gas phase or CO2 generated by the reaction between CO and the sample on the surface of CeO2. Moreover, the intensity of the carbonate stretching vibration peak was stronger for CC-N than for CC-NH, suggesting that CO adsorbed on the surface of CC-N is more reactive with surface oxygen species [41].
In summary, the analysis indicates that Cu+ species, formed through the interaction between CuO and CeO2, exhibited superior CO adsorption capacity compared to Cu2⁺. Additionally, the CO adsorption capacity of the CC-N catalyst was greater than that of the CC-NH catalyst.

2.2.4. Reducibility of Catalyst (H2-TPR)

As shown in Figure 6, three H2 consumption peaks are observed in the H2-TPR curves of both catalysts. The low temperature peak (α) corresponds to the reduction of finely dispersed CuO with strong interaction with CeO2. The medium temperature peak (β) was attributed to the reduction of CuO incorporated into the CeO2 lattice, while the high temperature peak (γ) was ascribed to the reduction of bulk CuO with weak or no interaction with CeO2 [3]. The reduction temperature and H2 consumption amounts of each peak of the catalysts are summarized in Table 3.
As shown in Table 3, the reduction temperatures of the CC-N catalyst were slightly higher than those of the CC-NH catalyst. However, the fraction of Aα in CC-N was significantly higher than that in CC-NH (27% vs. 16%), indicating that the relative content of highly dispersed CuO which strongly interacts with CeO2 in CC-N was more than that in CC-NH. A similar tendency was observed on Aβ of CC-N and CC-NH, illustrating that relative amount of CuO entering into the lattice of CeO2 was higher than that of CC-NH. Notably, the fraction of Aγ in CC-N was lower than that in CC-NH (41% vs. 58%), revealing more bulk CuO in CC-NH. These results clearly indicate the distinction in the distribution of Cu species in different CuO/CeO2 catalysts prepared with cerium nitrate and ammonium cerium nitrate as precursors.

2.3. Reaction Performance of Catalysts

2.3.1. Activity of Catalysts

Figure 7 presents the activity of CuO/CeO2 catalysts prepared via different cerium precursors for CO oxidation at low temperatures. Obviously, CO conversion of both samples gradually increased with the increase in reaction temperature. Significantly, the CO conversion of CC-N was evidently higher than that of CC-NH at all temperatures investigated here. Specifically, T30, T50, and T90 (corresponding to the temperature at which the CO conversion equals to 30, 50, and 90%, respectively) of the catalysts are collected in Table 4. Clearly, the T30, T50, and T90 values of CC-N were noticeably lower than those of CC-NH, indicating that CC-N prepared via cerium nitrate showed better catalytic activity for CO oxidation at low temperatures. Additionally, compared the performance of the catalysts reported in the literature [48,49,50,51,52] as shown in Table S1, the CC-N catalyst exhibited excellent catalytic activity at a relatively high space velocity.

2.3.2. Kinetic Test of Catalysts

The apparent activation energies (Ea) of the CC-N and CC-NH catalysts were estimated on the base of CO oxidation results at low temperatures (<55 °C) by using an Arrhenius plot (Figure 8), which were 61.3 and 63.8 kJ/mol, respectively. The similar apparent reaction energies suggest that the nature of active sites in both catalysts were basically the same and the number of active sites was the primary determinant of the CO catalytic performance, which is discussed in detail in Section 2.3.3.

2.3.3. Correlation Between Physicochemical Properties and Catalytic Activity

Based on the results of characterization and activity testing, the correlation between physicochemical properties and catalytic activities of the CuO/CeO2 catalysts prepared via different cerium precursors can be obtained as follows:
(1)
Generally speaking, the larger specific surface area and pore volume of catalysts can provide more active sites for adsorption and activation of reactants, which is conducive to the improvement of catalytic activity [53,54]. N2 adsorption/desorption characterization results (Table 1) indicated that the specific surface area and pore volume of CC-N were significantly larger than those of CC-NH, so the higher activity of CC-N (Figure 7 and Table 4) could be attributed to its larger specific surface area and pore volume to some extent. To further assess the contribution of specific surface area to the catalytic activity, specific surface area-normalized CO conversion for CC-N and CC-NH were calculated and the results are shown in Figure S2. Clearly, the specific surface area-normalized CO conversion of both catalysts was not identical across the tested temperature range, suggesting that factors beyond specific surface area alone contribute to the catalytic activity.
(2)
The adsorption of CO on Cu+ species within the CuO/CeO2 catalyst is widely recognized as a crucial step in the CO oxidation reaction. Thus, an increased presence of Cu+ on the catalyst surface enhances CO adsorption, which, in turn, boosts catalytic activity [41,45]. Additionally, highly dispersed CuO species indirectly contribute to this process by serving as precursors to active Cu+ sites. Through interaction with CeO2, these CuO species facilitate both the formation and stabilization of Cu+, thereby enhancing overall catalytic performance [24,29,33]. Based on the TPR characterization results (Table 3), the amount of highly dispersed CuO in CC-N was substantially higher than in CC-NH, favoring Cu+ formation. The data in Table 2, alongside in situ DRIFT analysis results shown in Figure 5, further confirm that CC-N had a higher concentration of Cu+ species on its surface, which accounted for its superior catalytic activity.
(3)
According to the mechanism of CO oxidation reaction on CuO/CeO2 catalyst (CO adsorbed by Cu+ reacts with oxygen species on the catalyst surface to generate CO2 [55,56]) it can be inferred that the CO oxidation activity of CuO/CeO2 catalyst may be related to the number and reactivity of oxygen species on the catalyst surface. Therefore, the higher activity of CC-N can be attributed to the higher amount oxygen species on its surface, which were evidenced by the results obtained from XPS analysis (Figure 4 and Table 2).

2.3.4. Anti-Toxicity of Catalysts

As we know, H2O or CO2 resistance of a catalyst are very important indexes for evaluating its CO oxidation performance, especially at low temperatures. Therefore, H2O or CO2 resistance of the CuO/CeO2 catalysts prepared via different cerium precursors were investigated here.
As shown in Figure 9, the activity of both catalysts under moisture conditions with different contents of H2O (0.6% and 4.2%) was almost the same as that under a dry atmosphere, indicating that both catalysts had preeminent resistance to H2O.
As shown in Figure 10, the CC-N catalyst achieved 99% CO conversion at 110 °C with 10 vol% CO2 in the reaction gas, which was 10 °C higher than the T99 for the reaction gas without CO2 addition. However, the T99 of the CC-NH catalyst increased by 30 °C with the addition of CO2 to the reaction gas. These results clearly indicate that the CC-N catalyst had better resistance to CO2 than CC-NH. It is generally believed that the decrease in the activity of CuO/CeO2 catalysts caused by CO2 is mainly due to competitive adsorption at the active site and the formation of carbonates that inhibit oxygen mobility. Cecilia et al. [57] suggested that the highly dispersed CuO species in close contact with CeO2 are the active sites for CO oxidation reaction but weakly react with CO2, so the higher the content of the highly dispersed CuO species, the better the resistance to CO2 poisoning. In addition, He et al. [58] claimed that Cu+ sites preferentially adsorb CO compared to CO2, so a higher amount of Cu+ in the catalyst is favorable for enhancing its resistance to CO2. In this work, the amount of finely dispersed CuO, which strongly interacts with CeO2, was significantly larger in CC-N than in CC-NH, as evidenced by the characterization results of TPR (Table 3). In addition, the amount of Cu+ in the CC-N catalyst was larger than in CC-NH, as shown by the characterization results of XPS (Table 2) and CO-IR (Figure 5). Furthermore, the results of N2 physical adsorption/desorption characterization (Table 1) indicate that the specific surface area and pore volume of CC-N were significantly larger than those of CC-NH, and the larger specific surface area and pore volume of the catalyst not only provide more active sites for adsorption and activation of reactant molecules but can also accommodate more deposition of the resulting carbonate. Therefore, we suggest that the better resistance of CC-N to CO2 poisoning can be attributed to its larger specific surface area and pore volume, and the presence of higher amounts of highly dispersed CuO species in close contact with CeO2 and Cu+ species.

3. Experimental Section

3.1. Catalyst Preparation

(1)
Materials
Cu(NO3)2·3H2O (Adamas-Beta, AR) served as the copper precursor, while Ce(NO3)3∙6H2O (Adamas-Beta, AR) with a + 3 valence state of Ce species and Ce(NH4)2(NO3)6 (Sinopharm, AR) with a + 4 valence state of Ce species were used as the precursors of cerium salts. Ethylene glycol ((CH2OH)2, General-Reagent, AR) was employed as the solvent for the solvothermal reaction. All chemical reagents were of analytical grade and used without further purification in this work.
(2)
Synthesis
The CuO/CeO2 catalyst was prepared via a one-pot solvothermal method, with the molar ratio of Cu/Ce maintained at 1:4 as determined in our previous study [24]. The typical procedures were as follows: a certain amount of Ce(NO3)3∙6H2O and Cu(NO3)2·3H2O were dissolved in (CH2OH)2 and then poured into a PTFE-lined stainless steel reactor. The mixture was reacted in a rotary oven at 180 °C for 3 h, then naturally cooled to room temperature. The resulting precipitate was filtered, washed three times with water and ethanol, then dried at 120 °C overnight. Finally, the sample was calcined in a muffle furnace at 450 °C for 4 h to yield the desired catalyst, which was labeled CC-N. A second catalyst was synthesized using the same method, except that Ce(NH4)2(NO3)6 was used as the cerium precursor. The catalyst obtained was labeled CC-NH.

3.2. Catalyst Characterization

TG-DSC measurements of the catalyst precursors after drying at 120 °C were performed using a NETZSCH STA-499F3 thermal analyzer made in Germany, under an air stream (50 mL/min) from 30 to 900 °C. The heating rate was 10 °C/min.
Raman spectra of all calcined samples at 450 °C were obtained at room temperature on a DXR-Raman microscope (Thermo Fisher Scientific, Waltham, MA, USA) using the 532 nm exciting line (20 mW beam), with 5 scans for each spectrum.
XPS data for all calcined samples at 450 °C without pretreatment were obtained on a Kratos Axis Ultra DLD spectrometer, using Al Kα radiation. The data were calibrated according to the standard C1s peak at 284.6 eV. The experimental uncertainties for XPS determinations were estimated to be ±5%.
H2-TPR experiments were conducted in a quartz microreactor. A TCD monitored the consumption of H2. Approximately 20 mg of the catalyst was pretreated at 400 °C in N2 (50 mL/min) for 1 h. Then the sample was heated to 500 °C at a rate of 10 °C/min under a flow of H2 (10 vol%)/N2.
The physicochemical characteristics of the catalysts were also characterized via XRD, N2 adsorption/desorption, ICP-AES, and in situ DRIFTs following previously reported procedures [36].

3.3. Evaluation of Catalytic Performance

The catalyst activity and apparent activation energy for the CO oxidation reaction was determined using a fixed-bed microreactor combined with a GC2060 gas chromatograph under atmospheric pressure following previously reported procedures [36]. The experimental error in the CO conversion was within ±3%.
The reaction rates in terms of the weight of the catalyst were calculated and the apparent activation energies were derived when the CO conversions were controlled below 15% [36]. A total of 100 mg of catalyst (40–60 mesh) diluted with silica sand (600 mg) were loaded into the reactor (i.d. = 5 mm) at atmospheric pressure and pretreated at 200 °C for 1 h under N2 (50 mL/min). Then a mixed gas including 1.0 vol% CO, 2.5 vol% O2, and balance N2 was fed to the reactor at a total flow rate of 30 mL/min, and the space velocity was 1800 L/(kg∙h).

4. Conclusions

In this paper, CuO/CeO2 catalysts were prepared via a one-pot solvothermal method, and the effect of different cerium precursors on their catalytic CO oxidation performance was investigated. The results show that the cerium precursors have an important influence on the catalytic performance of the prepared CuO/CeO2 catalysts. The catalytic performance of the CC-N catalysts prepared using cerium nitrate was better, with T50 and T90 of only 68 and 86 °C, respectively. The high activity of the CC-N catalyst was mainly attributed to its larger specific surface area and pore volume, stronger CO adsorption capacity, and higher number of surface oxygen species. Moreover, the CC-N catalyst had better resistance to CO2 poisoning compared to the CC-NH catalyst, which was mainly attributed to its larger specific surface area and pore volume, the presence of more highly dispersed CuO species with strong interaction with CeO2 and Cu+ species.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14120856/s1, Figure S1. XRD patterns of the various CuO-CeO2 catalysts; Figure S2. Activity normalized to the surface area of the various CuO-CeO2 catalysts; Table S1. Comparison of CuO-CeO2 catalysts in CO oxidation prepared by different methods.

Author Contributions

Conceptualization, W.J. and H.X.; methodology, W.J. and Y.L.; software, Y.L. and H.X.; validation, H.X., J.Y. and D.M.; formal analysis, W.J. and Y.L.; investigation, Y.L. and W.J.; resources, D.M.; data curation, J.Y.; writing—original draft preparation, W.J.; writing—review and editing, H.X. and D.M.; visualization, J.Y.; supervision, J.Y.; project administration, D.M.; funding acquisition, D.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (21273150).

Data Availability Statement

Data are contained within the article or Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. TG-DSC curves of the various CuO/CeO2 precursors in the air.
Figure 1. TG-DSC curves of the various CuO/CeO2 precursors in the air.
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Figure 2. XRD patterns of the various CuO/CeO2 catalysts.
Figure 2. XRD patterns of the various CuO/CeO2 catalysts.
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Figure 3. Raman spectra of the various CuO/CeO2 catalysts.
Figure 3. Raman spectra of the various CuO/CeO2 catalysts.
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Figure 4. XPS spectra of the different CuO/CeO2 catalysts. (A) Cu 2p; (B) Ce 3d; (C) O 1s.
Figure 4. XPS spectra of the different CuO/CeO2 catalysts. (A) Cu 2p; (B) Ce 3d; (C) O 1s.
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Figure 5. DRIFT spectra of CO adsorbed on the different CuO/CeO2 catalysts at 30 °C.
Figure 5. DRIFT spectra of CO adsorbed on the different CuO/CeO2 catalysts at 30 °C.
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Figure 6. H2-TPR profiles of the different CuO/CeO2 catalysts.
Figure 6. H2-TPR profiles of the different CuO/CeO2 catalysts.
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Figure 7. The catalytic activity of the different CuO/CeO2 catalysts for CO oxidation.
Figure 7. The catalytic activity of the different CuO/CeO2 catalysts for CO oxidation.
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Figure 8. Arrhenius plots for CO oxidation over the various CuO/CeO2 catalysts.
Figure 8. Arrhenius plots for CO oxidation over the various CuO/CeO2 catalysts.
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Figure 9. The effect of different contents of water vapor on catalytic activity of different CuO/CeO2 catalysts: (a) CC-N and (b) CC-NH.
Figure 9. The effect of different contents of water vapor on catalytic activity of different CuO/CeO2 catalysts: (a) CC-N and (b) CC-NH.
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Figure 10. The effect of CO2 (10 vol%) on catalytic activity of different CuO/CeO2 catalysts.
Figure 10. The effect of CO2 (10 vol%) on catalytic activity of different CuO/CeO2 catalysts.
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Table 1. Structural and textural properties of the different CuO/CeO2 catalysts.
Table 1. Structural and textural properties of the different CuO/CeO2 catalysts.
SampleCu/(Cu + Ce) a
/mol%		DCeO2 b
/nm		DCuO c
/nm		Vpd
/(cm3/g)		Dpd
/(Å)		SBET
/(m2/g)		Lattice Constant b
/nm
CC-N18.98.116.80.1276.5610.5447
CC-NH18.07.919.90.0878.9380.5472
a Copper content was obtained via ICP-AES tests. b Crystal size and lattice constant of CeO2 were obtained from CeO2 (111) plane. c Crystal size of CuO was based on CuO (002) and CuO (111) planes. d Vp (pore volume) and Dp (pore diameter) were measured from N2 adsorption.
Table 2. XPS data measured for the different CuO/CeO2 catalysts.
Table 2. XPS data measured for the different CuO/CeO2 catalysts.
SampleCu/(Cu + Ce)/at%OA a/%Ce3+ b/%Cu+ c/%
CC-N38.624.814.021.3
CC-NH39.822.713.417.1
a Area ratio of OA peaks to the entire O 1s peaks. b Area ratio of peaks attributed to Ce3+ to all Ce 3d peaks. c The peak area ascribed to Cu 2p3/2 is A and its satellite peak area is B, A1/B = 1.89, Cu+(%) = (A − (A1/B) B)/(A + B) × 100.
Table 3. Quantitative data of the H2-TPR tests over the different CuO/CeO2 catalysts.
Table 3. Quantitative data of the H2-TPR tests over the different CuO/CeO2 catalysts.
CatalystTemperature of Peaks/°CH2 Consumption Amounts (mmol/g) and Relative Ratio */%
TαTβTγAαAβAγ
CC-N1651912123.16 (27)3.68 (32)4.81 (41)
CC-NH1601862001.90 (16)2.97 (26)6.80 (58)
* It is calculated according to the proportion of each peak in the whole reduction peak.
Table 4. T30, T50, and T90 of various catalysts for CO oxidation.
Table 4. T30, T50, and T90 of various catalysts for CO oxidation.
SampleT30/°CT50/°CT90/°C
CC-N596886
CC-NH7684100
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Jin, W.; Liu, Y.; Xue, H.; Yu, J.; Mao, D. CO Oxidation over Cu/Ce Binary Oxide Prepared via the Solvothermal Method: Effects of Cerium Precursors on Properties and Catalytic Behavior. Catalysts 2024, 14, 856. https://doi.org/10.3390/catal14120856

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Jin W, Liu Y, Xue H, Yu J, Mao D. CO Oxidation over Cu/Ce Binary Oxide Prepared via the Solvothermal Method: Effects of Cerium Precursors on Properties and Catalytic Behavior. Catalysts. 2024; 14(12):856. https://doi.org/10.3390/catal14120856

Chicago/Turabian Style

Jin, Wen, Yanmin Liu, Hongyan Xue, Jun Yu, and Dongsen Mao. 2024. "CO Oxidation over Cu/Ce Binary Oxide Prepared via the Solvothermal Method: Effects of Cerium Precursors on Properties and Catalytic Behavior" Catalysts 14, no. 12: 856. https://doi.org/10.3390/catal14120856

APA Style

Jin, W., Liu, Y., Xue, H., Yu, J., & Mao, D. (2024). CO Oxidation over Cu/Ce Binary Oxide Prepared via the Solvothermal Method: Effects of Cerium Precursors on Properties and Catalytic Behavior. Catalysts, 14(12), 856. https://doi.org/10.3390/catal14120856

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