CO oxidation over Cu-Ce binary oxide prepared by solvothermal method: Effects of cerium precursors on the properties and catalytic behavior

doi:10.21203/rs.3.rs-2216323/v1

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CO oxidation over Cu-Ce binary oxide prepared by solvothermal method: Effects of cerium precursors on the properties and catalytic behavior

https://doi.org/10.21203/rs.3.rs-2216323/v1

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Cu-Ce binary oxides were prepared by 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 physico-chemical characteristics of the catalysts were characterized by 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), temperature-programmed reduction with H₂ (H₂-TPR), and temperature-programmed desorption of adsorbed O₂ (O₂-TPD). 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, more 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.

Solvothermal method

CuO-CeO2 catalyst

Cerium precursor

CO oxidation

Low temperature

CO2 resistance

With the rapid development of modern industry, transportation, and energy consumption, the problem of environmental pollution is becoming more and more serious. Air pollution is one of the most concerning global environmental problems. There are about 10 kinds of air pollutants that can harm the environment and human beings, among which CO is a widespread and universal pollutant. The elimination of CO is of great practical significance, and the catalytic oxidation is currently considered to be a direct, simple, economical, and efficient CO removal method (Feng et al. 2021; Ghiassee et al. 2021).

Up to now, the most widely investigated catalysts for CO oxidation are supported noble metals and supported or mixed transition metal oxides. Because of the drawbacks of precious metal catalysts with limited resources and high cost, transition metal oxides, especially Cu-Ce composite oxides, have been widely studied due to their high catalytic activity in CO oxidation reaction, low price, and abundant resources (Zheng et al. 2016; Chen et al. 2020). Although much research has been done on the Cu-Ce composite oxide catalyst and some progress has been made over the past decades, it still has the disadvantages of low activity and poor resistance to water or CO₂ at low temperatures, which limit its practical application (Zhang et al. 2019; Cui et al. 2019).

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 (Waikar et al. 2020); alkali earth metal Mg (Waikar et al. 2021); transition metals such as Zr (Wang et al. 2021), Co (Liu et al. 2014), Mn (Lin et al. 2018) and Fe (Ma et al. 2011); rare earth metals such as La, Pr, Sm (Reddy et al. 2012), precious metals such as Ag (Kim et al. 2019), Ru (Cui et al. 2014), Pd (Cwele et al. 2016) and Pt (Wu et al. 2018), etc. For example, Kim et al. (2019) reported that the addition of Ag reduced the temperature for complete conversion of CO (T₁₀₀) of Cu-Ce mixed oxide from 120 to 100 ^oC. Similarly, Wu et al. (2018) found that the addition of Pt decreased the T₁₀₀ of Cu-Ce mixed oxide for the catalytic oxidation of CO from > 100 ^oC to 85 ^oC.

The optimization of catalyst preparation methods and conditions is another efficient approach to enhance the performance of CO oxidation catalyzed by CuO-CeO₂ (Yang et al. 2019; Du et al. 2021). Up to now, the main preparation methods for CuO-CeO₂ catalysts are impregnation method (Sun et al. 2015a), deposition-precipitation method (Gurbani et al. 2009), co-precipitation method (Jung et al. 2004), sol-gel method (Pu et al. 2008), solvothermal method (Liu et al. 2020), solution combustion method (Cam et al. 2020), photodeposition method (Pitman et al. 2020), MOF precursor method (Zhang et al. 2018a) and spray pyrolysis method (Li et al. 2019), etc. Shang et al. (2017) studied the influence of three preparation methods (impregnation, co-precipitation, and mechanical mixing) on the catalytic CO oxidation activity of the CuO/CeO₂ catalysts, and found that CuO was more dispersed and the interaction between CuO and CeO₂ was stronger in the catalyst prepared by the impregnation method, which promoted the reduction of CuO to Cu⁺ species on Cu-Ce interface and thus resulting in the highest catalytic activity. The solid solution structure of the catalyst prepared by co-precipitation inhibits the chemisorption of CO, thus showing low catalytic performance. The catalyst prepared by the mechanical mixing method has the lowest activity among the three catalysts due to the separation of CuO and CeO₂ phases and the absence of the Cu-Ce interface. Zhan et al. (2017) prepared a CuO-CeO₂ catalyst with wide pore channel and large specific surface area by mechanochemical grinding, whose catalytic activity for CO oxidation reaction was comparable to that of the catalyst prepared by sol-gel method, but better than that prepared by the impregnation method. Zheng et al. (2016) prepared the CuO/CeO₂ catalyst by a solvothermal deposition method with CeO₂ as the carrier and found that its catalytic activity for CO oxidation was significantly better than that of the corresponding catalysts prepared by impregnation method and deposition-precipitation method. Very recently, Liu et al. (2020) used a one-pot solvothermal method to prepare the CuO-CeO₂ catalyst for low-temperature CO oxidation and found that compared with the CuO/CeO₂ catalyst prepared by the solvothermal deposition method previously reported (Zheng et al. 2016), the CuO-CeO₂ catalyst prepared by 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 (Sun et al. 2015a; Wang et al. 2017; Guo et al. 2020; Jin et al. 2022), but only a few studies on the influence of Ce precursor. Qi et al. (2012) used a surfactant (CTAB) assisted co-precipitation method to prepare 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. (2020) synthesized CeO₂ carrier by precipitation method and then loaded the active component Cu by impregnation method to obtain the CuO/CeO₂ catalyst, and investigated the effects of cerium sources (Ce(NO₃)₃, CeCl₃, Ce(NH₄)₂(NO₃)₆, 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 by a solvothermal method has not been reported.

In this paper, two different CuO-CeO₂ catalysts were prepared by the one-pot solvothermal method using cerium nitrate and cerium ammonium nitrate as cerium precursors, respectively, and the effects of different cerium precursors on the physico-chemical properties and catalytic activity of the prepared CuO-CeO₂ catalysts for low-temperature CO oxidation were investigated. The relationship between catalytic activity and physico-chemical properties of catalysts was constructed. In addition, the water or CO₂ resistance of the prepared catalysts was investigated comparatively.

Catalyst preparation

The CuO-CeO₂ catalyst was prepared by a one-pot solvothermal method, in which the Cu content was fixed at 20 at% according to our previous study (Liu et al. 2020). The typical procedures were as follows: a certain amount of cerium salt precursors, namely cerium nitrate hexahydrate (Ce(NO₃)₃⋅6H₂O, Adamas-Beta, AR), cerium ammonium nitrate (Ce(NH₄)₂(NO₃)₆, Sinopharm, AR) and copper nitrate trihydrate (Cu(NO₃)₂⋅3H₂O, Adamas-Beta, AR) were dissolved in ethylene glycol ((CH₂OH)₂, General-Reagent, AR) and then poured into a PTFE-lined stainless steel reactor. The reaction mixture was stirred and heated at 180 ^oC for 3 h. After the reaction, the precipitate was filtered, washed three times with water and ethanol, dried at 120 ^oC overnight, and then calcined in a muffle furnace for 4 h at 450 ^oC. After cooling down to room temperature naturally, the desired catalysts were obtained and named as CC-X, where X represents different cerium salt precursors (N for cerium nitrate and NH for cerium ammonium nitrate), i.e., CC-N and CC-NH, respectively.

Catalyst characterization

The physico-chemical characteristics of the catalysts were characterized by TG-DSC, XRD, Raman spectroscopy, N₂ adsorption-desorption, ICP-AES, XPS, in-situ DRIFTs, H₂-TPR, and O₂-TPD techniques following previously reported procedures (Liu et al. 2023).

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 (Liu et al. 2023).

Analysis of thermal decomposition of catalyst precursors

Figure 1 shows the TG-DSC curves of the different catalyst precursors after drying at 120 ^oC, from which it can be seen that both TG and DSC curves are very similar for the two catalyst precursors. Specifically, the TG curve has two weight loss steps, the first one at < 200 ^oC, corresponding to the removal of water adsorbed on the catalyst surface, and the second one with a significant weight loss accompanied by a large exothermic peak at 200–400 ^oC, corresponding to the decomposition of the precursor hydroxide. From the above experimental results, it is clear that the precursors of both catalysts have decomposed completely at > 400 ^oC, so in this paper, 450 ^oC was selected as calcination temperature for the precursors to prepare the CuO-CeO₂ catalysts.

Characterization of catalysts

Structural and textural properties

Figure 2 shows the XRD spectra of the CuO-CeO₂ catalysts prepared from different cerium salts. It can be seen from the figure that strong characteristic diffraction peaks of CeO₂ are present at 2θ = 28.5 ^o, 33.0 ^o, 47.4 ^o, and 56.3 ^o for both samples (Jin et al. 2022). In addition, characteristic diffraction peaks of CuO are also present at 2θ = 35.5 ^o and 38.7 ^o (Jin et al. 2022). The results obtained in combination with those of the thermal analyses (Fig. 1) indicate that the precursors of the catalysts have decomposed to oxides under calcination at 450 ^oC.

The average particle sizes of CuO and CeO₂ were calculated based on the diffraction peaks of the two crystalline planes of CuO and the diffraction peaks of the crystalline plans of CeO₂(111) using the Scherrer formula, and the results are listed in Table 1. As can be seen from Table 1, the CeO₂ crystallite sizes of the two catalysts are very close to each other, but the particle size of CuO of the CC-N catalyst is significantly larger than that of CC-NH.

Table 1

Structural and textural properties of the different CuO-CeO₂ catalysts
Sample	Cu/(Cu + Ce)^a /mol%	D_CeO2^b /nm	D_CuO^c /nm	V_p^d /(cm³/g)	D_p^d /(Å)	S_BET /m²/g)	Lattice constant^b /nm
CC-N	18.9	9.35	19.0	0.12	76.5	61	0.5382
CC-NH	18.0	9.98	7.4	0.08	78.9	38	0.5472
^a Copper content obtained from ICP-AES tests.
^b Crystal size and lattice constant of CeO₂ were obtained from CeO₂(111) plane.
^c Crystal size of CuO is based on CuO(002) and CuO(111) planes.
^d V_p (pore volume) and D_p(pore diameter) were measured from N₂ adsorption.

In addition, the lattice constants of CeO₂ calculated by the Bragg equation based on CeO₂(111) plane are also listed in Table 1. As can be seen, the lattice constant of CeO₂ of CC-NH is larger than that of CC-N. Generally, two factors can affect the lattice constants of CeO₂. On the one hand, the radius of Cu²⁺(0.072 nm) is smaller than that of Ce⁴⁺(0.097 nm), which leads to lattice contraction when Cu²⁺ replaces part of Ce⁴⁺ into the lattice (Zheng et al. 2016). 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) will have the opposite effect on the lattice of CeO₂ (Hossain et al. 2018). These two opposing effects together determine the lattice constants of CeO₂.

The Raman spectra of CuO-CeO₂ catalysts prepared from different cerium precursors are shown in Fig. 3. It can be seen from the figure that both samples have a strong peak at ~ 450 cm^− 1 ascribed to the F_2g characteristic peak of cubic CeO₂ (Jin et al. 2022). In addition, there is a very weak peak at ~ 600 cm^− 1 corresponding to the characteristic peak of oxygen vacancy due to lattice distortion of CeO₂ (Jin et al. 2022), which is generally believed to be generated due to the entrance of Cu²⁺ into the CeO₂ lattice, leading to its lattice contraction. Comparing the two catalysts prepared from different cerium precursors, there is little difference in their F_2g characteristic peaks, indicating that the valence state of the cerium ion does not have much influence on its F_2g characteristic peak of CeO₂. The ratio of the two peak areas at 600 and 450 cm^− 1 (A₆₀₀/A₄₅₀) is commonly used to calculate the relative concentration of oxygen vacancies in catalysts (Liu et al. 2020; Jin et al. 2022), but in this paper, it is difficult to use this method to accurately determine the relative concentration of oxygen vacancies in catalysts because the peak at 600 cm^− 1 is too weak and too broad (Jin et al. 2022).

The textural properties of CuO-CeO₂ catalysts prepared with different cerium precursors are also presented in Table 1. As can be seen, the specific surface area (S_BET) and pore volume (V_p) of CC-N are significantly larger than those of CC-NH, but their pore sizes (D_p) are similar.

The actual Cu content in the catalyst was measured by the ICP-AES, and the results are collected in Table 1. Clearly, the actual Cu content of CC-N and CC-NH samples is close to the preseted value (20%).

Surface Characterization (XPS)

The CuO-CeO₂ catalysts prepared with different cerium precursors were characterized by XPS to observe the state of metal oxides on the surface and the distribution of surface components. The obtained results are presented in Fig. 4 and Table 2. As can be seen from Table 2, the Cu/Ce and Ce/O atomic ratios on the surface of the CC-NH catalyst are slightly higher than those of the CC-N catalyst (0.66 vs. 0.63, 0.29 vs. 0.25, respectively), indicating that the Cu and Ce contents on the surface of CC-NH catalyst are relatively higher. In addition, compared with the results of ICP-AES analysis (Table 1), it can be seen that the Cu content on the surface of the two catalysts is significantly higher than that of the bulk composition, indicating that Cu species are enriched on catalyst surface because the surface energy of Cu is lower than that of Ce (Hu et al. 2010).

Table 2

XPS data measured for the different CuO-CeO₂ catalysts
Sample	Atomic ratio		O_A ^a/%	Ce^3 + b/%	Cu^+ c/ %
Sample	Cu/Ce	Ce/O	O_A ^a/%	Ce^3 + b/%	Cu^+ c/ %
CC-N	0.63	0.25	24.8	15.5	21.0
CC-NH	0.66	0.29	22.7	14.4	18.2
^a Area ratio of O_A peaks to the entire O 1s peaks.
^b Area ratio of peaks attributed to Ce³⁺ to all Ce 3d peaks.
^c Area ratio of peaks attributed to Cu⁺ to all Cu 2p peaks.

According to the Cu 2p spectra of different catalysts (Fig. 4(A)), both catalysts have a strong main peak at 933.7 eV, and a satellite peak appears at 941.2-941.8 eV, indicating that Cu species on the surface of catalysts mainly exist as Cu²⁺ (Liu et al. 2020; Jin et al. 2022). In addition, a shoulder peak representing Cu⁺ or Cu⁰ species at 932.0 eV was also observed in the spectra of the two catalysts (Liu et al. 2020; Jin et al. 2022). At this time, it is necessary to further distinguish with the help of the Cu LMM spectra (Fig. 4(B)). It can be seen from Fig. 4(B) that there is a peak representing the presence of Cu⁺ at 914.5 eV for both catalysts. In order to explain the reduction of Cu species on the catalyst more clearly, the obtained Cu LMM spectra were peak-separated, and the relative content of Cu⁺ in different catalysts was expressed as Cu⁺(%) by the ratio of the area belonging to the Cu⁺ peak to the total area of all Cu species. According to the calculated data (Table 2), the relative content of Cu⁺ on CC-NH is less than that of CC-N, which corresponds to 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 will partially reduce and generate a small amount of Cu⁺, which has been reported to have a better ability to adsorb and activate CO (Liu et al. 2020; Jin et al. 2022).

Figure 4(C) shows the Ce 3d spectra of the different catalysts, where it can be seen that both samples contain eight peaks attributed to four pairs of double spins, with Ce⁴⁺ 3d_3/2 peaks labeled as u, u'', u''' and Ce⁴⁺ 3d_5/2 peaks labeled as v, v'', v''', while u' and v' are used to denote the two electron arrangement configurations of the Ce³⁺ species (Fu et al. 2015). This observation shows that both Ce⁴⁺ and Ce³⁺ species are present on the surface of both catalysts. The ratio of the sum of u' and v' peak areas to the total area of all peaks is usually used to estimate the relative content of Ce³⁺ species in the catalysts (Ce³⁺/(Ce⁴⁺+Ce³⁺)) (Sun et al. 2015b), and the Ce³⁺(%) data for both catalysts are listed in Table 2. From Table 2, it can be found that the Ce³⁺ species are present on both catalysts and the relative content of Ce³⁺ on CC-NH is less than that on CC-N. According to the literature (Guo and Zhou 2016), the relative content of Ce³⁺ in the catalyst represents the strength of the Cu-Ce interaction in the catalyst and the concentration of the oxygen deficiency, and the higher the relative content of Ce³⁺, the higher the concentration of the oxygen deficiency. Additionally, it is well known that the presence of Ce³⁺ can promote the electron transfer process Ce³⁺ + Cu²⁺→ Ce⁴⁺ + Cu⁺, i.e., more Cu⁺ is generated (Lu et al. 2021; Wang et al. 2022), which is consistent with the previous results obtained by Cu LMM.

It can be seen from the O 1s spectra of both catalysts in Fig. 4(D) that there are three peaks in the spectra of both catalysts at 528.9-529.2 eV, 529.5-529.9 eV, and 531.2-531.4eV. They belong to lattice oxygen (O_L), surface adsorbed oxygen (O_A), and hydroxyl oxygen (O_OH) (Zhao et al. 2019; Jin et al. 2022). The ratio of the area of the O_A peak to the total area of the three peaks, designated as O_A(%), represents the relative concentration of adsorbed oxygen in catalysts. Therefore, it can be seen from the results in Table 3 that the relative concentration of adsorbed oxygen on the surface of CC-N is greater than that of CC-NH. This result indicates that the concentration of oxygen deficiency on the surface of CC-N is greater than that of CC-NH, which is consistent with the oxygen deficiency concentration inferred from the Ce³⁺ content above. In addition, the binding energy of lattice oxygen of CC-N is significantly greater than that of CC-NH, indicating that its lattice oxygen has better mobility and higher activity (Zhao et al. 2019).

The above results show that different cerium precursors lead to differences in the contents of Cu⁺, Ce³⁺, and oxygen vacancies on the surface of CuO-CeO₂ catalysts, and consequently resulting in different catalytic performance in CO oxidation at low-temperatures.

CO - IR analysis

In order to further study the existence status of Cu species in catalysts, in-situ DRIFTs characterization of the adsorbed CO on both catalysts was carried out. As shown in Fig. 5, there is a strong absorption peak at 2103 cm^− 1 on the DRIFT spectra of both catalysts, which can be ascribed to the stretching vibration peak attributable to the linear adsorption of CO on Cu⁺ (Cu⁺-CO) (Dong et al. 2016), and the intensity of this peak for CC-NH catalyst is weaker than that for CC-N catalyst. This is consistent with the relative content of Cu⁺ obtained by the above XPS analysis (Table 2).

It can also be seen from Fig. 5 that DRIFT spectra of the two samples in the range of 1290 to 1598 cm^− 1 have many peaks belonging to formate and carbonate adsorbed on the CeO₂ surface. According to the literature (Sun et al. 2015a; Zhang et al. 2018b), the peak around 1598 cm^− 1 belongs to carbonate or formate adsorbed on CeO₂, and the peak around 1470 cm^− 1 belongs to polydentate or monodentate carbonates, and the peaks around 1386 cm^− 1 and 1293 cm^− 1 belong to bidentate carbonate. After adding CO at 30 ^oC, obvious stretching vibration peaks of formate and carbonate appeared on the surface of both samples, and these peaks were 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 stretching vibration peak of carbonate species of CC-N is stronger than that of CC-NH, indicating that CO adsorbed on its surface is more likely to react with oxygen species on the surface of the catalyst (Sun et al. 2015b).

Therefore, it can be seen from the above analysis results that Cu⁺ species formed under the promotion of CuO-CeO₂ interaction have better CO adsorption capacity than Cu²⁺, and CC-N has greater CO adsorption capacity than CC-NH.

Reducibility of catalyst (H₂-TPR)

As shown in Fig. 6, three hydrogen consumption peaks appear on the H₂-TPR curves of both catalysts: the low-temperature α peak corresponds to the reduction of finely dispersed CuO with strong interaction with CeO₂; the medium temperature β peak is attributed to the reduction of CuO entering into the CeO₂ lattice, and the high-temperature γ peak is attributed to the reduction of bulk CuO with weak or no interaction with CeO₂ (Zheng et al. 2016). The temperature and area of each peak of the catalysts are summarized in Table 3.

Table 3

Quantitative data of the H₂-TPR tests over the different CuO-CeO₂ catalysts
Catalyst	Temperature of peaks / ^oC			Area of peaks / (a.u.) and relative intensities^* / %
Catalyst	T_α	T_β	T_γ	A_α	A_β	A_γ
CC-N	165	192	217	126(27)	195 (42)	146 (31)
CC-NH	169	195	205	91 (20)	219(49)	136 (31)
* It is calculated according to the proportion of each peak in the whole reduction peak.

It can be observed from Table 3 that the T_α and T_β of the CC-N catalyst are slightly lower than those of the CC-NH catalyst, suggesting that CC-N has stronger reducibility. However, the T_γ of the CC-N catalyst is higher than that of the CC-NH catalyst. Based on the fact that the larger the grain size of CuO crystallites is, the higher the reduction temperature is (Zheng et al. 2016), it can be judged that the size of CuO crystallites in the bulk phase of the CC-N catalyst is larger than that of the CC-NH catalyst, which is consistent with the previous XRD characterization results (Table 1). In addition, it can be seen from Table 3 that the fraction of A_γ in the two catalysts is the same (both of 31%), indicating that the relative content of CuO in the bulk phase is the same. However, the fraction of A_α in CC-N was higher than that in CC-NH (27 vs 20%), but the fraction of A_β was lower than that in CC-NH (42 vs 49%). These observations suggest that the relative content of highly dispersed CuO with strong interaction with CeO₂ in CC-N was greater than that in CC-NH, and the relative amount of CuO entering into the lattice of CeO₂ is less than that of 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.

Oxygen adsorption analysis (O₂-TPD)

To further investigate the defects on the catalyst surface and the mobility of adsorbed oxygen, O₂-TPD characterization was performed. In general, the oxygen species adsorbed on the catalyst surface undergo the following transformation process with increasing electron content: O₂(ad) → O₂⁻(ad) → O⁻(ad) → O²⁻(ad/lattice). O₂(ad) refers to physically adsorbed oxygen, which is usually purged out by helium flow before the desorption temperature rises. The adsorbed oxygen species O₂⁻(ad) and O⁻(ad) have weak bonding on the catalyst surface and are therefore easily desorbed. O²⁻(ad/lattice) is surface or bulk phase lattice oxygen and is difficult to be desorbed. According to the literature (He et al. 2022), the desorption peaks below 350 ^oC generally originate from the surface adsorbed O₂⁻(ad) and/or O⁻(ad), which are surface oxygen species associated with surface defects of the catalyst, and the peaks above 350 ^oC from the desorption of surface lattice oxygen and bulk phase lattice oxygen. Therefore, it can be seen from Fig. 7 that the adsorbed oxygen species of CC-N are more active than those of CC-NH.

Reaction Performance Of Catalysts

Activity of catalysts

Figure 8 presents the activity of CuO-CeO₂ catalysts prepared by different cerium precursors for CO oxidation at low temperatures. Obviously, CO conversion of both samples gradually increased with the elevation in reaction temperature. In addition, the CO conversion of CC-N is evidently higher than that of CC-NH at all temperature 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 are noticeably lower than those of CC-NH, indicating that CC-N prepared by cerium nitrate shows better catalytic activity for CO oxidation at low temperatures.

Table 4

T₃₀, T₅₀ and T₉₀ of various catalysts for CO oxidation
Sample	T₃₀ / ^oC	T₅₀ / ^oC	T₉₀ / ^oC
CC-N	59	68	86
CC-NH	76	84	100

Kinetic test of catalysts

The apparent activation energies (E_a) of the CC-N and CC-NH catalysts were estimated on the base of CO oxidation results at low temperatures (< 55^oC) by using an Arrhenius plot (Fig. 9), which are 61.3 and 63.8 kJ/mol, respectively. The E_a data are consistent with the activity test results of both catalysts (Fig. 8), since lower E_a corresponds to higher activity of the catalyst.

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 by 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 (Zhao et al. 2018; Xie et al. 2020). 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 (Fig. 8 and Table 4) could be attributed to its larger specific surface area and pore volume to some extent.

(2) It is well accepted that the activity of CuO-CeO₂ catalyst for CO catalytic oxidation is closely related to the CuO species on its surface. It is generally believed that the highly dispersed CuO is the active species, while the bulk CuO contributes little to the activity or even has negative effects (Yang et al. 2014; Zheng et al. 2016). According to the TPR characterization results (Table 3), although the relative amount of bulk CuO in the CC-N and CC-NH is the same, the amount of highly dispersed CuO in CC-N is significantly higher than that in CC-NH. Therefore, the existence of more highly dispersed CuO species on the surface of CC-N is an important factor for its higher activity.

(3) It is well known that the adsorption of CO as one of the reactant molecules on the Cu⁺ species of CuO-CeO₂ catalyst is an important step in CO oxidation. Therefore, more Cu⁺ on the surface of the catalyst will be conducive to the adsorption of CO, which is beneficial to the improvement of catalytic activity (Sun et al. 2015b; Zhang et al. 2018b). Therefore, the higher activity of CC-N can be attributed to the presence of more amount of Cu⁺ species on its surface, which can be confirmed by Cu LMM (Fig. 4(B) and Table 2) and in-situ DRIFTs analysis results (Fig. 5).

(4) According to the mechanism of CO oxidation reaction on CuO-CeO₂ catalyst, that is, CO adsorbed by Cu⁺ reacts with oxygen species on the catalyst surface to generate CO₂ (Lykaki et al. 2018; Zhang et al. 2020), 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 larger number and stronger reactivity of oxygen species on its surface, which were evidenced by the results obtained from XPS analysis (Fig. 4) and O₂-TPD analysis (Fig. 7), respectively.

Anti-toxicity Of Catalysts

As we know, water or CO₂ resistance of a catalyst is very important indexes for evaluating its CO oxidation performance, especially at low temperatures. Therefore, the water or CO₂ resistance of the CuO-CeO₂ catalysts prepared by different cerium precursors was investigated here.

As shown in Fig. 10, the activity of the two catalysts under moisture conditions with different contents of water vapor (0.6vol% and 4.2vol%) is almost the same as that under a dry atmosphere, indicating that both catalysts have preeminent water resistance.

As shown in Fig. 11, the CC-N catalyst achieved 99% CO conversion at 110 ^oC with 10 vol% CO₂ in the reaction gas, which is 10 ^oC higher than the T₉₉ for the reaction gas without CO₂ addition; while the T₉₉ of the CC-NH catalyst increased by 30 ^oC with the addition of CO₂ to the reaction gas. These results clearly indicate that the CC-N catalyst has better resistance to CO₂ than CC-NH. It is generally believed that the reduction 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. (2015) 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. (2021) claimed that Cu⁺ sites preferentially adsorb CO compared to CO₂, so more 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, is 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 is larger than in CC-NH, as shown by the characterization results of XPS (Table 2) and CO-IR (Fig. 5). Furthermore, the results of N₂ physical adsorption/desorption characterization (Table 1) indicate that the specific surface area and pore volume of CC-N are 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 also can 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 more amounts of highly dispersed CuO species in close contact with CeO₂ and Cu⁺ species.

In this paper, CuO-CeO₂ catalysts were prepared by a direct 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, more number of surface oxygen species, and stronger reactivity of the surface oxygen species. Moreover, the CC-N catalyst has better resistance to CO₂ poisoning compared to the CC-NH catalyst, which is 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.

Acknowledgements

The authors gratefully acknowledge the financial support received from the National Natural Science Foundation of China under the grant number of 21273150.

Ethics approval and consent to participate Not applicable.

Consent for publication Not applicable.

Availability of data and materials Not applicable.

Competing interests The authors have no competing interests to declare.

Authors' contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Yanmin Liu and Wen Jin. The first draft of the manuscript was written by Wen Jin and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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

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CO oxidation over Cu-Ce binary oxide prepared by solvothermal method: Effects of cerium precursors on the properties and catalytic behavior

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Catalyst characterization

Evaluation of catalytic performance

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Characterization of catalysts

Structural and textural properties

Surface Characterization (XPS)

CO - IR analysis

Reducibility of catalyst (H₂-TPR)

Oxygen adsorption analysis (O₂-TPD)

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CO oxidation over Cu-Ce binary oxide prepared by solvothermal method: Effects of cerium precursors on the properties and catalytic behavior

Status:

Version 1

Abstract

Figures

Introduction

Experimental

Catalyst preparation

Catalyst characterization

Evaluation of catalytic performance

Results And Discussion

Analysis of thermal decomposition of catalyst precursors

Characterization of catalysts

Structural and textural properties

Surface Characterization (XPS)

CO - IR analysis

Reducibility of catalyst (H2-TPR)

Oxygen adsorption analysis (O2-TPD)

Reaction Performance Of Catalysts

Activity of catalysts

Kinetic test of catalysts

Correlation Between Physicochemical Properties And Catalytic Activity

Anti-toxicity Of Catalysts

Conclusions

Declarations

References

Status:

Version 1

Reducibility of catalyst (H₂-TPR)

Oxygen adsorption analysis (O₂-TPD)