Synthesis and CO Oxidation Activity of 1D Mixed Binary Oxide CeO2-LaO x Supported Gold Catalysts
© The Author(s). 2017
Received: 21 August 2017
Accepted: 24 October 2017
Published: 2 November 2017
One-dimensional (1D) Ce-La nanorods with different La contents (Ce and La in the molar ratio of 1:0, 3:1, 1:1, 1:3, and 0:1) were synthesized by hydrothermal process. Au/Ce-La nanorod catalysts were obtained by a modified deposition-precipitation method. The samples were characterized by N2 adsorption-desorption (BET), ICP, X-ray diffraction (XRD), SEM, TEM, EDX, X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), and temperature-programmed reduction (H2-TPR). It revealed that La existed as LaO x in the 1D nanorods. The catalysis results demonstrated that the mixed binary Ce-La nanorod oxides could be a good support for gold catalysts. The contents of La had an important influence on the catalytic performance of Au/Ce-La nanorod catalysts. Among the catalysts, when the Ce/La molar ratio was 3:1, the 1.0%Au/Ce0.75-La0.25 nanorods pretreated at 300 °C showed the best activity among the catalysts for CO oxidation, which could convert CO completely at 30 °C. The catalysts also performed high temperature resistance and good stability for CO oxidation at the reaction temperatures of 40, 70, and 200 °C.
As a very harmful gas, CO can strongly binds to the iron atom in blood hemoglobin preventing the release of oxygen. So, its presence indoors can even cause the death of human beings and animals in the short time. It has become an increasingly severe problem on air pollution. Catalytic CO oxidation has been one of the most effective solutions for CO removal to solve such serious environmental problem [1–8]. It has also received a great deal attention recently by the scientific community in the fields of the pollution control devices for vehicle exhaust purification, indoor air cleaning, and low-temperature CO sensors [6–10]. In many cases, the precious Au dispersed on specific metal oxides with high oxygen storage capacity such as CeO2, TiO2, and Fe2O3 are highly effective candidates towards the CO oxidation [11–13]. Over the past decades, studies on the supported gold catalysts for CO oxidation at low temperatures have resulted in unexpected observations. It is generally accepted that the catalytic activities of Au catalysts depend strongly on the nature of Au nanoparticles and properties of the supports, such as the gold particle size, the Au metal-support interaction and the reducibility of the support [14–18].
As one of the most important rare earth oxides, CeO2 has been widely used in three-way catalysts as an efficient catalyst support due to its unique physical and chemical properties [6, 8, 15, 17]. CeO2 has an excellent oxygen storage and release capacity due to the ability to switch Ce4+/Ce3+, which makes CeO2 become an active oxide component of various oxidation catalysts used in diverse redox catalytic reactions [17–32]. Surface areas, mesoporous structures, lattice defects, and synergistic effects with other dopants can all promote the catalytic properties of ceria nanomaterials [3, 22]. To further improve the performance of Au-CeO2 catalysts for CO oxidation reaction, many strategies have been tried, such as preparation methods including deposition-precipitation, coprecipitation, and urea-gelation coprecipitation, which has been used to control and optimize the interaction of the Au-O-Ce structure, as well as the size and shape of ceria [33–35]. Attempts have been also made by the surface modification of support [4, 5, 22, 24, 26, 36–38]. It has been found that the use of binary mixed oxides as support could provide a good solution for the stabilization of gold nanoparticles. Moreover, the promotion by noble or transition metal enhances ceria reducibility and facilitates the formation of surface oxygen vacancies. Meanwhile, doping with transition metal cations has been proved to be an effective method to promote the physicochemical properties of one-dimensional (1D) nanostructured nanomaterials, such as catalytic activity [38–40]. Wang et al.  modified the surface of Au/CeO2 with highly dispersed CoO x and demonstrated excellent catalytic activity in low-temperature CO oxidation. Ma et al.  reported that CaO, NiO, ZnO, Ga2O3, Y2O3, ZrO2, and rare earth additives to gold-titania catalyst are beneficial for CO oxidation, and the doped catalysts could show significant activity at ambient temperature after 500 °C aging. Park et al.  reported that CeO x modified TiO2 support is a good catalyst for water gas shift reaction. There have been lots of studies about mixed metal oxides for CO catalytic oxidation. These doped metal ions are either deposited on the surface of the support in the form of oxide particles or into lattice of the support, which could not form a separate oxide phase. The goal of this research is to prepare 1D binary Ce-La nanorods, which is non-perovskite or solid solution type mixed oxide. That is, in the 1D nanorod structure, the two metal oxides coexist combining the merits of the two compositions to maximize the synergistic effect. Due to potential technological applications, a lot of 1D nanomaterials including nanorods, nanowires, and nanotubes have been extensively investigated during the past years [2, 4, 41, 42]. These 1D nanostructured materials, especially 1D nanorod materials, have been studied as important supports or active components in the field of catalysis, optics, and electrochemistry, such as well-controlled silicon nanowires used in solar cells . It has been found that the properties of 1D structure materials such as catalytic activity are often closely related to their crystal structure and shape. As a consequence, the development of 1D nanorod materials to tailor their electronic and catalytic properties proves to be intriguing and valuable.
Herein, we report a simple solvothermal strategy to prepare a series of mixed Ce-La nanorod composites. In the synthesis process, the LaO x and CeO2 could grow together in one rod. The morphology of the final products was not influenced. The XRD and TEM results show that the La cations have existed in the form of LaO x . It was found that the dopant of LaO x showed a positive effect on the activity of gold-ceria catalysts. Au/Ce0.25-La0.75 nanorods exhibited excellent catalytic activity for CO oxidation.
All chemicals in this paper were of analytical grade, and they were used as received without any purification.
The Ce-La nanorods were synthesized by conventional hydrothermal method. In a typical synthesis, solutions of NaOH (9 mol/L) and Ln(NO3)3 (Ln = Ce, La, 0.8 mol/L) were mixed and maintained vigorous stirring for 30 min at room temperature. The resulting suspension was poured into a Teflon-lined stainless steel autoclave. The autoclave was sealed and kept at 110 °C for 14 h and then air-cooled to room temperature. The resulting products were filtered, washed with deionized water and absolute alcohol, dried at 80 °C for 12 h, and then calcined at 400 °C in air with a heating rate of 5 °C min−1 before supporting gold nanoparticles. The final products with different La contents (Ce and La in the molar ratio of 1:0, 3:1, 1:1, 1:3, and 0:1) were denoted as Ce nanorods, Ce0.75-La0.25 nanorods, Ce0.50-La0.50 nanorods, Ce0.25-La0.75 nanorods, and La nanorods.
A deposition-precipitation process was carried out to prepare Au/Ce-La nanorod catalysts. Briefly, the required amount Ce-La nanorods were dispersed in 100 mL deionized water, and then mixed with a certain amount 0.01 mol/L HAuCl4 solution. As the pH of final HAuCl4 solution was about 7, which was related to the basicity of the support and acidity of HAuCl4, pH of the solution would be not adjusted. The suspension was keeping stirring for 12 h and refluxed at 100 °C for 4 h. After the deposition-precipitation procedure, the precipitate was centrifuged, washed with water to remove Cl− ions, and dried at 80 °C under air for 12 h. The concentrations of gold were expressed as percent by mass content.
Gold loadings of Au/Ce-La nanorod catalysts were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-9000, USA Thermo Jarrell–Ash Corp). The Brunauer–Emmett–Teller (BET) surface areas of Ce-La nanorod samples were measured by nitrogen adsorption at − 196 °C using a Micromeritics Tristar II 3020 apparatus. The XRD study was carried out on a Rigaku D/Max-2500 X-ray diffractometer (Kα λ = 0.154 nm) in the 2θ range of 3–80°. Uv-visible DRS of the catalysts were collected on a UV–vis NIR spectrophotometer (JASCO Corp V–570). TEM observations and energy dispersive X-ray analysis (EDX) were obtained with a JEM-2100 transmission electron microscope operating at 200 kV. SEM data and element mapping images were obtained with a JSM-7500F scanning electron microscope operating at 15 kV. XPS were recorded to identify the chemical composition and the oxidation state of the catalysts on a Kratos Axis Ultra DLD X-ray photoelectron spectrometer using a monochromated Al Kα source operated at 150 W. The binding energies were calibrated using the C 1s peak located at 284.6 eV. Temperature-programmed reduction (H2–TPR) was performed on a PX200 apparatus to measure H2 consumption. Prior to H2-TPR analysis, the samples were pretreated in He flow at 300 °C for 1 h. After cooled to 50 °C, the catalyst was reduced with 10 vol% H2/Ar gas flow by heating up to 900 °C at a rate of 10 °C/min.
Catalytic Activity Test
Results and Discussion
Characterization of Au/Ce-La Nanorod Catalysts
Gold loading of the Au/Ce-La samples with different supports
Nominal gold loading (%)
Actual gold loading (%)
BET specific surface area of the Ce-La nanorod samples with different La contents
BET surface Area (m2/g)
Average pore size (nm)
Pore volume (cm3/g)
SEM and TEM
EDS results of Ce-La nanorods with different La contents
Effect of La Content
In consideration of the preparation methods, gold loadings, gold particle size and distribution on different Ce-La nanorods supports, XRD, TEM and XPS data showed that all the catalysts should have the same number and type of active Au sites. So this high activity of the Au/Ce0.75-La0.25 nanorods catalysts correlates well with the reducibility data discussed above. H2-TPR results indicated that Au/Ce0.75-La0.25 nanorods has the lowest reducibility temperature and highest reducibility in the region of 50–400 °C, especially in the region of 50–150 °C, which could exactly approach the region of reaction temperature. In the process of reaction, the Ce0.75-La0.25 nanorod support served as oxygen carrier. The reducibility of Ce0.75-La0.25 nanorods could promote the formation of active oxygen. That is to say high reducibility of the catalyst, good activity the catalyst has. Au/Ce0.75-La0.25 nanorod catalyst subsequently has the best activity.
Effect of Gold Content
Effect of Calcination Temperature
Reaction Mechanism Speculate
In summary, a series of mixed Ce-La nanorods with various amounts of La was prepared via a simple hydrothermal reaction at high concentration of NaOH and without surfactant. Gold was loaded by deposition-precipitation. After La doping, the composite could retain the initial rod morphology. As a result, Ce-La nanorods with 25 at.% La maintained the optimal nanorods with the length of 0.6 um and the diameter of 3–5 nm. Gold particles were dispersed well on the support. The reducibility of Ce-La nanorods could be affected significantly by LaO x doping. The deposition of gold had important influence on the reducibility of catalyst. Thus, the CO oxidation activity of Au/Ce-La nanorods was essentially changed in comparison with pure Au/CeO2 and Au/La nanorods. One percent Au/Ce0.75-La0.25 nanorods could convert CO to CO2 completely at 30 °C. Further increase in La content results in decreased activity due to the decrease in reducible oxygen sites. The Au/Ce0.75-La0.25 nanorod catalyst with low gold concentration also showed high activity. With the increase of gold content, there is an increase in the activity. The stability test of 0.3% Au/Ce0.75-La0.25 nanorods indicated that the catalyst not only kept 100% conversion after continuous operation for 10 h under 70 °C but also showed no deactivation after 10 h on stream at 40 °C. As expected, the activity of 0.3% Au/Ce0.75-La0.25 nanorods also retained a 100% CO conversion during 50 h at 200 °C. The results revealed that LaO x as the dopant could grow together with CeO2 in one rod. The 1D binary mixed Ce-La nanorods could be a good support for precious metal group catalysts with low content of gold.
This work was supported by the National Natural Science Foundation of China (nos. 21271110, 21373120, and 21271107) and MOE Innovation Team of China (IRT13022).
HHY, WPH, BLZ, and SMZ had conceived and designed the experiments. HHY performed the experiments. SYZ and HHY synthesized and characterized the reported materials. HHY wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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