Introduction

Air pollution caused by automobile exhaust, waste gas of coal mines and incomplete combustion of hydrocarbons becomes increasingly serious nowadays [1]. The air pollutants include SO2, NOx, CO and suspended particulate matter. Among these, CO is the most dangerous one because its affinity with hemoglobin is over 200 times than oxygen [2], thus easily causes human death. Hence, the elimination of CO is of great significance for protecting the environment and human health.

Recently, catalytic oxidation method is widely employed for CO elimination due to its high conversion and environmentally friendliness. Catalysts commonly used for CO catalytic oxidation include noble metal catalysts and non-noble metal catalysts [3, 4]. Although noble metal catalysts have good catalytic performance, the disadvantages of their high cost, high sintering temperature and low poison tolerance restrain their application [5, 6]. Non-noble metal catalysts have become promising catalysts because of their low cost and potential applications in various fields. Among non-noble metal catalysts, CeO2 has attracted much attention due to its excellent oxygen storage capacity (OSC), inherent Ce3+/Ce4+ redox electron pair and rich oxygen vacancy [7,8,9]. Although a large amount of research has been devoted to the preparation of cerium-based catalysts, a challenging problem which arises is that CO cannot be completely oxidized unless at relatively high temperatures.

A series of transition metal oxides have been introduced to the cerium-based catalysts to improve their CO catalytic performance at low temperature [10,11,12]. Doping CeO2 with transition metals could generate oxygen vacancies or surface defects in the ceria lattice, which are essential to CO oxidation [13, 14]. Lykaki et al. [13] reported the effect of ceria nanoparticles shape and Cu doped ceria on CO oxidation. They found that Cu/CeO2 nanorods had an excellent catalytic performance of complete CO conversion at 150 °C compared with Cu/CeO2 nanopolyhedra and Cu/CeO2 nanocubes. Besides, Mn-CeO2, Co-CeO2 and Ni-CeO2 nanorods also show high catalytic activity compared with pure CeO2 nanorods [14]. These results indicate transition metal doped ceria catalysts exhibit higher CO oxidation activity compared with pristine CeO2 nanorods, but Cu doped ceria catalysts show the best catalytic effect for CO oxidation.

In this work, Cu loaded on nanorod-like, 3D flower-like and gear-like CeO2 were synthesized and the CO catalytic performance were tested. Nanorod-like CeO2 with excellent CO catalytic performance was applied to investigating the effects of Cu load, and different ratios of Cu was loaded on CeO2 nanorods with different ratios by wet impregnation method [13, 15]. We report syntheses, surface structure, chemisorption property, and CO catalytic activity (including the influence of water and CO2) of a serious of Cu/CeO2 nanorods and have found an optimal Cu load to CeO2. Our results indicate that the morphology CeO2 and Cu load play vital role in CO catalytic performance. From a micro point of view, structural defects and active sites dominate the surface catalyst reaction of copper-ceria binary oxides.

Experimental

Synthesis of CeO2 and Cu/CeO2 nanorods

Synthesis of nanorod-like CeO2

All chemicals were commercially purchased and used without further purification. CeO2 nanorods were synthesized by a hydrothermal method following the reported procedures [16], with minor modifications. Ce(NO3)3·6H2O (4.5 mmol) was dissolved in aqueous KOH solution (90 mL, 6 M) in a Teflon bottle and stirred at room temperature for 30 min, then the mixture was transferred into a stainless-steel autoclave, sealed tightly and maintained at 100 °C for 12 h. After cooling to room temperature, the precipitate obtained was collected by filtration and washed with deionized (DI) water. The final precipitate was dried in vacuum at 80 °C for 4 h and calcined at 400 °C in air for 4 h (heating ramp 3 °C/min).

Synthesis of 3D flower-like CeO2

3D flower-like CeO2 was synthesized by the reported procedures [17]. 1.5 g CeCl3·6H2O, 2.2 g urea and 6 g tetrabutylammonium bromide (TBAB) were added to 150 mL ethylene glycol in a 250-mL round flask. The obtained solution was stirred with a magnetic stir bar and was heated to 180 °C. 30 min later, the reaction was stopped and the mixture was cooled to room temperature. The precipitate as ceria precursor was collected by centrifugation and washed with ethanol for 4 times. Ceria was from the as-prepared precursor via calcinations in air at 450 °C for 2 h (heating ramp 3 °C/min).

Synthesis of gear-like CeO2

Gear-like CeO2 was synthesized by the reported procedures [18]. 4 mmol cetyltrimethyl ammonium bromide (CTAB) and 40 mmol NH4HCO3 were dissolved in 20 mL distilled water under vigorous stirring for 30 min. 8 mmol Ce(NO3)3·6H2O were dissolved in 20 mL distilled water under vigorous stirring for 30 min. Then 20 mL Ce(NO3)3·6H2O aqueous solution were added to 20 mL CTAB and NH4HCO3 aqueous solution under continuous stirring for 30 min, forming a homogeneous solution. The mixed solution was transferred into a 50 mL Teflon-lined autoclave and heated at 180 °C for 12 h. After the autoclave was cooled to room temperature naturally, fresh precipitates were washed with distilled water and ethanol for three times, and then dried at 80 °C overnight. CeO2 microstructures were obtained by calcinating at 400 °C for 5 h (heating ramp 3 °C/min), accompanied by a color change from white to slight yellow.

Synthesis of Cu/CeO2 catalysts

Cu/CeO2 catalysts were synthesized by wet impregnation method, using an aqueous solution of Cu(NO3)2·3H2O to obtain different Cu/(Cu + Ce) atomic ratios of 0.05, 0.1, 0.15 and 0.2. The suspensions were heated under stirring to remove excess water. Finally, the obtained precipitate was dried at 80 °C for 4 h and calcined at 400 °C in air for 4 h (heating ramp 3 °C/min). The obtained catalysts were denoted as xCu/CeO2, in which x represented Cu/(Cu + Ce) atomic ratios.

Catalysts characterization

SEM, TEM and HRTEM analysis

The scanning electron microscope (SEM) images were collected by HITACHI SU8010 scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) characterizations was examined by using a TECNAI G2 F20 instrument operated at 100 kV. 400 mesh copper grid covered by ultrathin carbon film (Ted Pella) was used. To prepare the TEM samples, the catalyst powder samples were first dispersed in ethanol and then sonicated for 30 min. One or two drops of the sample suspension from the sonicated solution using a small pipette were poured on the copper grid.

Textural characterization

The measurement of the surface area (SBET), average pore size and total pore volume of the catalysts were operated on a Quantachrome autosorb iQ ASIQMU0001000-7 with N2 adsorption isotherms at − 196 °C. The surface area (SBET) was calculated using the Brunauer–Emmett–Teller (BET) method, the pore size distribution curves were determined from the adsorption branches calculated by the Barrett-Joyner-Halenda (BJH) method and the total pore volume was calculated based on the adsorbed nitrogen at the relative pressure of 0.995. Prior to the measurements, the samples were degassed at 300 °C for 3 h.

X-ray diffraction

The crystalline structure of the catalysts was performed by a Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation (λ = 0.154 nm), which with accelerating voltage of 40 kV and emission current 40 mA. The scan range (2θ) is between 10 and 80° with a scan rate of 1° min−1. JADE software was used to calculate the average crystallite size using the Scherrer equation.

XPS studies

The surface composition and chemical valence state of the samples was analyzed by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific, Escalab 250Xi spectrometer, and the energy calibration was performed using contaminated carbon (C 1 s, BE = 284.8 eV) as standard.

TPR and TPD analysis

Temperature programmed reduction (TPR) experiments were performed by using Micromeritics Autochem™ II 2920 chemisorption analyzer to determine the reduction temperature and amount of hydrogen consumption. The samples (50 mg) were reduced with a mixture of 1.53 V% H2/Ar (30 mL/min) and the temperature was increased from ambient temperature to 800 °C at the rate of 10 °C/min. Prior to the TPR experiment, the sample was treated by heating from ambient temperature to 150 °C for 30 min under pure argon flow, in order to clean the sample surface. The consumption of H2 was detected by a TCD detector. CO-TPD measurement was conducted at the same instrument as H2-TPR. The sample was pretreated under He flow at 150 °C for 30 min. After cooling to room temperature in He flow, 5%CO/He gas was flown at 50 mL/min through the sample for 30 min. The sample was reheated up to 450 °C at 10 °C/min under He gas and the desorption behavior of CO can be analyzed at elevated temperature.

Catalytic activity measurements

The catalytic evaluation was performed in a quartz fixed-bed reactor with 10 mm internal diameter, loaded with 200 mg of catalyst. The reactant gas composed of 3 V% gaseous CO balanced with 15 V% O2 in N2 was flowed into the reactor at a flow rate of 100 cm3/min. Catalytic evaluation measurements were carried out by increasing the temperature by 10-degree steps from 30 °C up to 150 °C. The catalyzed gas were measured by an online gas chromatography. The CO catalytic activity was evaluated by using the following equations:

$$X_{{CO}} \left( \% \right) = \frac{{CO_{{inlet}} - CO_{{outlet}} }}{{CO_{{inlet}} }} \times 100\%$$

Here XCO, COinlet and COoutlet are the conversion of CO, CO concentration (ppm) in the inlet and outlet gas streams.

Results and discussion

CO oxidation

Fig. 1 shows the CO catalytic performance of nanorod-like 0.10Cu/CeO2, 3D flower-like 0.10Cu/CeO2 and gear-like 0.10Cu/CeO2. Nanorod-like 0.10Cu/CeO2 performs the best catalytic activity with a 90% CO catalytic conversion at 96 °C. While for 3D flower-like 0.10Cu/CeO2 and gear-like 0.10Cu/CeO2, the temperatures required to achieve 90% CO catalytic conversion are 139 °C and 135 °C. The possible reasons of the catalytic differences will be described in SEM below. In view of the excellent catalytic performance of the nanorod-like Cu/CeO2 catalyst, we choose the nanorod-like Cu/CeO2 catalyst to explore the influence of different copper loading on the CO catalytic performance, and the influence of CO2 and water on the catalytic performance of the catalyst are also explored.

Fig. 1
figure 1

CO conversion as a function of temperature for nanorod-like 0.10 Cu/CeO2, 3D flower-like 0.10Cu/CeO2 and gear-like 0.10Cu/CeO2 (CO/O2/N2 = 3/15/82, the flow rate is 100 cm3/min, mcat = 0.2 g)

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Fig. 2 displays the catalytic activity of CO oxidation over CeO2 and Cu/CeO2 nanorods. The conversion curves of CO show S-shaped growth with the increase of temperature. Pure CeO2 nanorods perform a poor CO catalytic activity, they can achieve a complete CO conversion until 350 °C. The addition of copper to the CeO2 nanorods greatly improves the catalytic efficiency. CO catalytic conversion does not increase monotonously with the copper loading, instead there is an optimal copper loading. Before reaching the optimal copper loading, the catalytic efficiency increases gradually with copper loading. While when the copper loading exceeds the optimal copper loading, the catalytic efficiency begins to decrease. The possible reason is that when the copper loading exceeds the optimum value, agglomeration will occur on the nanorods surface. Agglomeration of the nanorods will lead to the enlargement of the nanorods, the coverage of the active sites of copper and the reduction of oxygen vacancies, thus reducing the catalytic efficiency, as will be discussed below. As shown in Fig. 2, 0.15Cu/CeO2 nanorods performs the best catalytic activity with a 50% CO catalytic conversion at 68 °C and a 90% CO catalytic conversion at 87 °C. Besides, when the temperature is higher than 100 °C, the catalytic CO conversion is greater than 99%. The conversions of CO over these nanorods are in the following order: 0.15Cu/CeO2 > 0.10Cu/CeO2 > 0.05Cu/CeO2 > 0.20Cu/CeO2 > CeO2, and the complete CO conversion is achieved at 120 °C, 140 °C, 150 °C, 170 °C, 350 °C.

Fig. 2
figure 2

CO conversion as a function of temperature for CeO2 and Cu/CeO2 nanorods (CO/O2/N2 = 3/15/82, the flow rate is 100 cm3/min, mcat = 0.2 g)

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In order to investigate the effect of CO2 and water vapor on nanorods, water and CO2 were added to the reaction gas to test the catalytic efficiency of 0.15Cu/CeO2 nanorods. As displayed in Fig. 3, when 5 V% CO2 was introduced, CO2 inhibit the CO oxidation due to the competitive adsorption between CO and CO2 on copper sites, but the temperature required for complete conversion of the nanorods was almost unchanged. However, when 9.4 V% H2O was injected into the reaction gas, the catalytic activity was greatly affected and the complete conversion has reached to 160 °C. When 5 V% CO2 and 9.4 V% H2O were added to the reaction gas at the same time, the catalytic efficiency further decreased, and CO could be completely oxidized at 190 °C. The main reason why water reduces the catalytic activity is that water molecules will be adsorbed on the active sites, affecting CO adsorption on the surface of nanorods. Therefore, a conclusion can be drawn that CO2 has less influence while water has greater influence on the catalytic efficiency of the nanorods.

Fig. 3
figure 3

Effect of 5 V% CO2 and 9.4 V% H2O on CO conversion (a) in the absence of CO2 and H2O, (b) in the presence of 5 V% CO2, (c) in the presence of 9.4 V% H2O, (d) in the presence of 5 V% CO2 and 9.4 V% H2O (CO = 3%, O2 = 15%, the flow rate is 100 cm3/min, mcat = 0.2 g)

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Morphological characterization (SEM and TEM)

The SEM images of CeO2 samples with different morphological characterization are shown in Fig. S1. The nanorod-like morphology of CeO2, and many nanorod-like CeO2 precursors form sheet-like structures (Fig. S1a). 3D flowerlike micro/nanostructure can be seen from Fig. S1b, the SEM image indicates the entire structure of the architecture was built from micropetals which connected with each other forming the cured 3D flowerlike micro/nanostructure by self-assembly. Fig. S1c shows the gear-like microstructures of 1 mm in diameter. From the comparison of the above three morphological characterizations, the nanorod-like CeO2 has smaller size. And due to CeO2 nanorods have larger specific surface areas than other morphological characterization of CeO2 and preferentially expose certain specific crystal planes with high catalytic activity, thus exhibiting higher CO catalytic activity [19].

In order to further understand the structural characteristics of nanorod-like CeO2, the morphological characterization of pure CeO2 and 0.15Cu/CeO2 nanorods were investigated by transmission electron microscopy (TEM). The pure CeO2 nanorods show rod-like morphology with the diameter of 7.6 nm and length of 30–60 nm (Fig. 4a). When copper is doped on CeO2 nanorods, the diameter of the nanorods were reduced to 7.1 nm (Fig. 4c), which may increase the specific surface areas of the nanorods. Fig. 5b and d show the HRTEM images of the nanorods. The lattice fringes are clearly visible with the spacings of 0.311, 0.271, 0.194 nm (Fig. 4b), corresponding to the (111), (200) and (220) crystalline planes of CeO2 nanorods [20]. The lattice fringes with distances of 0.311 nm and 0.231 nm (Fig. 4d) are in good agreement with the (111) plane of CeO2 and (111) plane of CuO [21].

Fig. 4
figure 4

TEM images of the obtained nanorods a TEM image of the pure CeO2 nanorods, b HRTEM image of the pure CeO2 nanorods, c TEM image of the 0.15Cu/CeO2 nanorods, d HRTEM image of the 0.15Cu/CeO2 nanorods

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Fig. 5
figure 5

XRD patterns of the obtained CeO2 and Cu/CeO2 nanorods

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Crystal structures, sizes, and surface areas

The crystal structure of the obtained CeO2 and Cu/CeO2 nanorods is determined by XRD (Fig. 5). The main XRD peaks of all the Cu doped CeO2 samples centered at 2θ = 28.8°, 33.4°, 47.9°, 56.9°, 59.7°, 69.7°, 77.0° and 79.2°, which can be indexed to (111), (200), (220), (311), (222), (400), (331) and (420) planes of pure fluorite type cubic CeO2 (JCPDS 43-1002) [22], indicating that the Cu doping does not change their crystal structure. In addition, two diffraction peaks for CuO with a monoclinic structure were observed at 2θ = 35.8° and 39.0°, which correspond to (-111) and (111) planes (JCPDS 45-0937) [23], respectively, suggesting that the formation of bulk CuO on the surface of CeO2. The average crystallite diameter of CeO2 phases calculated by the Scherrer equation is also summarized in Table 1. The diameters of CeO2 for the pristine CeO2 and Cu/CeO2 nanorods are in the range of 6.9–8.8 nm, among them, the 0.15Cu/CeO2 nanorod has the smallest crystal size. It is also found that when Cu/(Cu + Ce) atomic ratios increase from 0.05 to 0.15, the crystallite sizes of CeO2 decrease gradually, but when Cu/(Cu + Ce) atomic ratios increase to 0.20, the crystallite sizes of CeO2 begin to increase, indicating that the addition of appropriate Cu species can reduce the sizes of crystallite.

Table 1 Physicochemical properties of CeO2 and Cu/CeO2 samples
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The obtained CeO2 and Cu/CeO2 nanorods are characterized by the N2 adsorption and desorption profiles (Table 1). The BET surface area of the undoped CeO2 nanorods was measured to be 102.6 m2/g, while the surface area increased slightly upon Cu doping except 0.20Cu/CeO2 nanorods. And the 0.15Cu/CeO2 nanorods present the highest BET surface area of 120.9 m2/g. Interestingly, when Cu/(Cu + Ce) atomic ratios increase from 0.05 to 0.15, the BET surface area of CeO2 phases follows the reverse order of average crystallite diameter, implying that the addition of appropriate copper species will increase the dispersion of CeO2 crystallite and reduce the agglomeration of CeO2 crystallite. The same trend was also observed by Lykaki et al. [13] and Zabilskiy et al. [24] for Cu/CeO2 catalysts.

Surface analysis (XPS)

Fig. S2A shows the Ce 3d XPS spectra of the obtained CeO2 and Cu/CeO2 nanorods, which can be composed of eight components associated with the four pairs of Ce3d spin–orbit doublets. The peaks labeled as v (v–v‴) and the others marked as u (u–u‴) correspond to the 3d5/2 and 3d3/2 spin–orbit components, respectively. The main peaks at 882.0 eV (v), 888.6 eV (v″), 897.9 eV (v‴), 900.4 eV (u), 907.5 eV (u″) and 916.4 eV (u‴) are ascribed to Ce4+, and the rest of two peaks at 884.4 eV (v′) and 902.8 eV (u′) are characteristics of the Ce3+ [25, 26]. It was reported that the presence of Ce3+ is beneficial to the formation of oxygen vacancy defects, and the high ratio of Ce3+/(Ce4+ + Ce3+) can accelerate the oxygen migration rate, thus improving the catalytic activity of the catalyst [26, 27]. The fraction of Ce3+ and the concentration of oxygen vacancy of the obtained nanorods have been calculated, which are shown in Table 2. The order of the surface Ce3+ content and the concentration of oxygen vacancy about the obtained samples perform the same trend: 0.15Cu/CeO2 > 0.10Cu/CeO2 > 0.05Cu/CeO2 > 0.20Cu/CeO2 > CeO2, which is perfectly matched to the order of their CO catalytic performance, indicating that Ce3+ species and oxygen vacancy are very likely to be related to catalysis of CO.

Table 2 XPS results of CeO2 and Cu/CeO2 samples
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Fig. S2(B) shows the Cu 2p XPS spectra of Cu/CeO2 nanorods in which the component centers at binding energy corresponds to Cu 2p3/2, and the component centers at binding energy corresponds to Cu 2p1/2. The shake-up satellite between 937.7 and 946.7 eV is the signal of the existence of Cu2+ ions. The components of Cu 2p3/2 and Cu 2p1/2 can be decomposed into two peaks at 932.1 eV and 933.9 eV, 951.8 eV and 953.7 eV. The peaks centered at 932.1 eV and 951.8 eV could be assigned to Cu+, while the peaks centered at 933.9 eV and 953.7 eV are the characteristics of Cu2+, indicating that the coexistence of two copper species of the Cu/CeO2 nanorods. The existence of reduced species can be further confirmed by the intensity of the shake-up satellite contribution at high binding energy together with the intensity of the main Cu 2p3/2 peak (denoted as Isat/Imp). The Isat/Imp ratios of Cu/CeO2 nanorods are shown in Table 2, all the Isat/Imp values of them are lower than 0.55, indicating that the presence of reduced copper (Cu+) in Cu/CeO2 nanorods [28, 29]. The following order in terms of Isat/Imp values of them is as follows: 0.15Cu/CeO2 < 0.10Cu/CeO2 < 0.05Cu/CeO2 < 0.20Cu/CeO2 < CeO2, and the Cu+/Cu2+ values follows the reverse order, which is perfectly consistent with their CO catalytic activity. Furthermore, it can be also inferred that the more reducing copper species (Cu+) are, the higher the activity of the catalyst is. And a copper-ceria interaction of Ce3+  + Cu2+  ↔ Ce4+  + Cu+ probably happens on the surface of the Cu/CeO2 nanorods [13, 30].

Fig. S2(C) depicts the O 1s XPS spectra of the CeO2 and Cu/CeO2 nanorods, where two peaks (Olatt and Oads) are clearly resolved. The low binding energy peak (denoted as Olatt) at 529–530 eV is the characteristics of the lattice oxygen (O2−) and the high binding energy of 531–533 eV might be attributed to low coordination surface oxygen species, surface oxygen defects, as well as the surface adsorption of oxygen ions (denoted as Oads) [31, 32]. The relative amount of Olatt and Oads species are presented in Table 2. It is obvious that Cu/CeO2 nanorods have a higher concentration of Oads species in the range of 37.72–44.01% compared to pure CeO2 (34.23%). And the 0.15Cu/CeO2 nanorods perform the highest population of Oads species. It was reported that Oads species are more active and significant for the CO catalytic activity [15]. Hence, we calculated their relative ratios to the surface lattice oxygen which were shown in Table 2. The corresponding order is as follows: 0.15Cu/CeO2 (0.786) > 0.10Cu/CeO2 (0.667) > 0.05Cu/CeO2 (0.635) > 0.20Cu/CeO2 (0.606) > CeO2 (0.520), which is in great agreement with their CO catalytic activity, suggesting that adsorbed oxygen performs the key role on CO catalysis.

H2-TPR

H2-TPR was carried out to determine the amount of H2 consumption and reduction temperature of each peak. Fig. 6 depicts H2-TPR profiles of CeO2, 0.05Cu/CeO2, 0.10Cu/CeO2, 0.15Cu/CeO2 and 0.20Cu/CeO2 nanorods. Pure CeO2 nanorods (Fig. 6a) show two broad peaks located at 411.9 °C and 709.6 °C, which are ascribed to the reduction of surface oxygen and bulk oxygen of CeO2 [33, 34].

Fig. 6
figure 6

H2-TPR profiles of the obtained CeO2 and Cu/CeO2 nanorods: (a) CeO2, (b) 0.05Cu/CeO2, (c) 0.10Cu/CeO2, (d) 0.15Cu/CeO2, (e) 0.20Cu/CeO2

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As shown in Fig. 6, doping Cu into CeO2 results in significant modifications in the H2-TPR profiles. All Cu/CeO2 nanorods exhibit three reduction peaks, the first two peaks centered at temperature range of 130–142 °C (denoted as α) and 152–171 °C (denoted as β) are assigned to the reduction of finely dispersion copper oxide clusters interacting with the support of ceria and larger CuO particles non-associated with ceria [35]. The last peak located at ~ 721 °C is ascribed to the reduction of Ce4+  → Ce3+ [36]. It is obvious that the reduction peaks of Cu/CeO2 nanorods shift to much lower temperatures compared to pure CeO2 nanorods. What’s more, with the increase of Cu doping, the reduction temperature of all Cu/CeO2 nanorods (Table 3) is as the following trend: 0.15Cu/CeO2 < 0.10Cu/CeO2 < 0.05Cu/CeO2 < 0.20Cu/CeO2 < CeO2, corresponding to the reduction ability of all nanorods follows the reverse trend: 0.15Cu/CeO2 > 0.10Cu/CeO2 > 0.05Cu/CeO2 > 0.20Cu/CeO2 > CeO2, which is greatly agree with the XPS results and their CO catalytic activity.

Table 3 H2 consumption amount and reduction temperature of Cu/CeO2 samples
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H2 consumption of peak α, peak β and CeO2 peak of the Cu/CeO2 nanorods were also listed (Table 3). H2 consumption at α peak increased with the increase of Cu/(Cu + Ce) atomic ratios from 0.05 to 0.15. H2 consumption at α peak (0.389 mmol/g) of the 0.20Cu/CeO2 nanorods was lower than that (0.538 mmol/g) of the 0.15Cu/CeO2 nanorods, which was due to the presence of more CuO particles covered up CeO2 surface [12]. The XRD (Fig. 5) could also prove that there were more bulk CuO on the surface of 0.20Cu/CeO2 samples. The results of H2 consumption at α peak indicate that 0.15Cu/CeO2 nanorods perform the highest reducibility which could improve CO catalytic activity greatly.

CO-TPD

CO‐TPD was used to obtain information on the CO adsorption and CO2 desorption ability of the samples. The CO-TPD spectra of 0.15Cu/CeO2 and pure CeO2 nanorods are displayed in Fig. 7. Desorption of CO2 during the heating progress is observed for the two samples, indicating that the absorbed CO is completely oxidized into CO2 by lattice oxygen in the samples. Obviously, three main peaks are observed for the 0.15Cu/CeO2 nanorods while only two peaks are observed for pure CeO2 nanorods. The peak around 85 °C may be corresponded to desorption of CO2 which resulted from the reaction between adsorbed CO and surface oxygen. The peak at higher temperature (~ 267 °C) can be attributed to lattice oxygen which oxidizes CO to CO2 [37]. As shown in Fig. 7, the introduction of Cu to CeO2 nanorods leaded to a shift to a lower temperature of CO2 desorption peak (~ 85 °C) by 7 °C, implying that the surface lattice oxygen and produced carbonate species of 0.15Cu/CeO2 nanorods can be reduced and desorb more easily compared with pure CeO2 nanorods. Moreover, the extra peak of 0.15Cu/CeO2 nanorods at 154 °C, indicating that there are more active sites on the surface for CO adsorption [38]. The results of CO-TPD indicated that the addition of Cu to CeO2 nanorods could increase the amount of active sites and make CO absorb on the active sites more easily, which was an important factor to improve the catalytic efficiency.

Fig. 7
figure 7

CO-TPD profiles of 0.15Cu/CeO2 and CeO2 nanorods

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Conclusions

In summary, CeO2 nanorods have smaller size and larger specific surface areas than other morphological characterization of CeO2. 3D flower-like 0.10Cu/CeO2, gear-like 0.10Cu/CeO2 and xCu/CeO2 nanorods (X = 0.05, 0.10, 0.15, 0.20) were synthesized by hydrothermal method and wet impregnation method for low temperature CO oxidation. The experimental results show that nanorod-like 0.10Cu/CeO2 performs the best catalytic activity than 3D flower-like 0.10Cu/CeO2 and gear-like 0.10Cu/CeO2, the addition of copper can greatly improve the catalytic efficiency of CO, 0.15Cu/CeO2 nanorods perform the best catalytic activity with a 99% CO conversion at 100 °C. The characterization by various instruments indicates 0.15Cu/CeO2 nanorods have smaller crystallite sizes, higher relative ratio of Ce3+, more oxygen vacancies, higher dispersion of Cu species together with more Cu+ species, which should be responsible for the highest CO conversion at low temperature. Moreover, we have also investigated the influence of water vapor and CO2 on the catalytic activity of 0.15Cu/CeO2 nanorods. It is found that the catalyst has better resistance to CO2 but worse resistance to water vapor.