Rod-Like Nanoporous CeO2 Modified by PdO Nanoparticles for CO Oxidation and Methane Combustion with High Catalytic Activity and Water Resistance
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A PdO/CeO2 composite with a rod-like nanoporous skeletal structure was prepared by combining the dealloying of Al-Ce-Pd alloy ribbons with calcination. For CO oxidation and CH4 combustion, the nanoporous PdO/CeO2 composite exhibits excellent catalytic activity, and the complete reaction temperatures of CO and CH4 are 80 °C and 380 °C, respectively. In addition, the composite possesses excellent cycle stability, CO2 toxicity, and water resistance, and the catalytic activity hardly decreases after 100 h of long-term stability testing in the presence of water vapour (2 × 105 ppm). The results of a series of characterizations indicate that the enhanced catalytic activity can be attributed to the good dispersion of the PdO nanoparticles, large specific surface area, strong redox capacity, interaction between PdO and CeO2, and more surface active oxygen on PdO. The results of the characterization and experiments also indicate that the PdO nanoparticles, prepared by combining dealloying and calcination, have a stronger catalytic activity than do Pd nanoparticles. Finally, a simple model is used to summarize the catalytic mechanism of the PdO/CeO2 composite. It is hoped that this work will provide insights into the development of high-activity catalysts.
KeywordsAl-Ce-Pd alloy ribbons Dealloying Nanoporous PdO/CeO2 CO oxidation CH4 combustion
Energy dispersive spectrometer
Energy-dispersive X-ray spectra
Flame ionization detector
Hydrogen temperature-programmed reduction
Specific surface areas
Scanning electron microscope
Transmission electron microscope
X-ray photoelectron spectroscopy
At present, an increasing number of people are paying attention to environmental issues and focusing on mitigating several important environmental issues, such as exhaust emissions and global warming [1, 2]. In particular, the elimination of toxic CO and the greenhouse gas CH4 is the focus of a plethora of research. Among such investigations, low-temperature catalysis has proven to be an effective way to eliminate these polluting gases [3, 4, 5, 6].
Although many studies have proven that cheap metals and their metal oxides (e.g. transition metals and oxides, and rare earth metals and oxides) can be used as catalysts for CO oxidation and CH4 combustion, it is undeniable that the use of noble metals usually significantly improves the catalytic performance [7, 8]. In recent years, Pd and PdO catalysts have been extensively studied and are considered to be some of the most effective catalysts for CO oxidation and CH4 combustion. They exhibit not only low volatility at high temperatures but also high catalytic activity at low temperatures [9, 10].
However, from an application point of view, since the abundance of precious metals in the earth is relatively low, the Pd and PdO catalysts that are generally used in practical industrial applications are loaded onto supports such as metal oxides, zeolites, carbon materials, and metal-organic frameworks. This configuration is also in line with the trend of developing sustainable catalysis by conserving noble metals and using the support-noble metal interaction to improve the catalytic activity [11, 12]. Among the types of supports, CeO2 is considered to be a promising support due to its strong oxygen storage/release properties and excellent thermal stability. For example, MacLachlan et al. used a combination of incipient wetness impregnation and surface-assisted reduction to prepare a nanostructured PdO/CeO2 composite, which exhibited a good activity as a catalyst for methane combustion after calcination . Luo et al. reported the preparation of a PdO-CeO2 catalyst by a solution combustion method and proved that the synergistic effects of PdO and CeO2 are the reason for the enhanced catalytic activity .
Although many good results have been achieved, there are still some challenges. For example, many organic chemicals or surfactants may contaminate the nanomaterials, resulting in an insufficient catalytic activity, which is common with wet chemistry . Furthermore, the process of preparing a catalyst based on the method of liquid precursor ageing is complicated, and the yield is low . Therefore, the development of non-polluting, high-yield, and high catalytic activity materials remains a challenge.
In this work, we developed a method for the preparation of PdO/CeO2 composites by dealloying Al-Ce-Pd alloy ribbons and then calcinating. The preparation method is simple, the structure of the material is easy to control, and no organic reagents are needed [17, 18], which is particularly suitable for large-scale industrial production and a sustainable future [19, 20]. However, as far as we know, there have been no literature reports on the use of dealloying to prepare the catalysts for methane combustion. Therefore, it is hoped that this work can provide insights into and help with the synthesis and preparation of nanomaterials.
All chemicals and metals were used as received of analytical grade without further purification. Pure Al (99.90 wt%), pure Ce (99.90 wt%), and pure Pd (99.90 wt%) were from Sino-Platinum Metals Co., Ltd. Granular NaOH (AR) was from Shanghai Aladdin Biochemical Technology Co., Ltd. High-purity argon was from Xi’an Jiahe Co., Ltd.
Synthesis of PdO/CeO2 Composite
Al92−XCe8PdX (X = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.1) precursor alloys were prepared by arc melting pure Al, pure Ce, and pure Pd under an argon atmosphere. An obtained Al-Ce-Pd precursor alloy was re-melted by high-frequency induction heating in a quartz tube under argon protection. The molten alloy was blown onto a high-speed rotating copper roll by argon for rapid solidification, and an Al-Ce-Pd alloy ribbon with a width of approximately 3 to 4 mm and a thickness of approximately 20 to 30 μm was obtained.
X-ray diffraction (XRD) patterns were obtained using a Shimadzu XRD-6100 diffractometer with Cu Kα radiation (20 kV and 40 mA). The morphology and elemental composition of the samples were obtained from a JSM-7000F scanning electron microscope (SEM) and an INCA X-Sight Oxford energy dispersive spectrometer (EDS). Transmission electron microscope (TEM) images, high-resolution transmission electron microscope (HRTEM) images, and scanning transmission electron microscopy images coupled with energy-dispersive X-ray spectra (STEM-EDX) were recorded on a JEOL JEM-200 electron microscope. The specific surface area (SBET), pore size (Dp), and pore volume (Vp) of the samples were determined with a Micromeritics ASAP 2020 apparatus at 77.4 K. The X-ray photoelectron spectroscopy (XPS) analysis was performed with a multifunctional spectrometer model Axis Ultra Kratos. Hydrogen temperature-programmed reduction (H2-TPR) was carried out on a Quantachrome Autosorb-iQC-TPX, in which a 50-mg sample was heated from 50 to 800 °C at a ramp rate of 10 °C/min in 10 vol% H2/Ar mixture gas flowing at a velocity of 40 mL min−1.
where X represents the conversion of CO or CH4, Cin represents the inlet concentration of CO or CH4, and Cout represents the outlet concentration of CO or CH4.
where rCO/CH4 represents the reaction rate of CO or CH4; the concentration of CO or CH4 is expressed as CCO/CH4 in the feed gas; the conversion of CO or CH4 is expressed as XCO/CH4; P is the atmospheric pressure, which is 101.3 KPa; V is the total flow rate; mcat is the mass of the catalyst in the reactor; WPd is the loading of Pd; R is the molar gas constant, which is 8.314 Pa m3 mol−1 K−1; and T is the ambient temperature (293 K).
Results and Discussion
Specific surface area (SBET), pore size (Dp) and pore volume (Vp) of the dealloyed Al91.3Ce8Pd0.7 ribbons calcined at different temperatures
Calcination temperature (°C)
SBET (m2 g−1)
Vp (cm3 g−1)
Calcined at 300 °C
Calcined at 400 °C
Calcined at 500 °C
Calcined at 600 °C
Ratios of Ce, Pd, and O in different states for different catalysts, as obtained from the XPS results
Ce3+/(Ce3+ + Ce4+) (%)
Pd2+/(Pd0 + Pd2+) (%)
Osur/(Olat + Osur + OH2O) (%)
Calcined dealloyed Al92Ce8
Calcined dealloyed Al91.3Ce8Pd0.7
To further investigate the effects of calcination on the surface PdO nanoparticles, the Pd 3d XPS spectra of the dealloyed Al91.3Ce8Pd0.7 sample and the calcined dealloyed Al91.3Ce8Pd0.7 sample are shown in Fig. 6b. There are two forms of Pd in the calcined dealloyed Al91.3Ce8Pd0.7 ribbon; the strong peaks at 336.8 eV and 342.2 eV can be attributed to PdO (Pd2+) , and the weak peaks at 335.4 eV and 341.0 eV can be attributed to metallic Pd (Pd0) . Table 2 shows that the concentrations of Pd2+ and Pd0 were approximately 91.25% and 8.75%, respectively. However, the analysis results for the dealloyed Al91.3Ce8Pd0.7 sample are the opposite, and the concentrations of Pd2+ and Pd0 were approximately 6.45% and 93.55%, respectively. This finding indicates that Pd is present in the form of metallic Pd in the dealloyed sample, whereas after calcination, Pd was oxidized into PdO and uniformly dispersed on the surface of CeO2, which is consistent with the results of the HRTEM images.
It is well known that surface active oxygen (Osur) is usually an active oxygen species for catalytic reactions. Figure 6c shows the O 1s XPS spectra of the two catalysts. For the calcined dealloyed Al91.3Ce8Pd0.7 sample, the peaks at 528.9 eV, 530.6 eV, and 532.1 eV correspond to the lattice oxygen (Olat), surface active oxygen (Osur), and weakly adsorbed H2O (OH2O), respectively [25, 26]. The ratio of Osur (Osur/(Olat + Osur + OH2O)) was calculated and is listed in Table 2. The ratios of Osur for the dealloyed Al91.3Ce8Pd0.7 sample and the calcined dealloyed Al91.3Ce8Pd0.7 sample were approximately 16.2% and 29.3%, respectively, indicating the presence of more surface active oxygen species in the calcined dealloyed Al91.3Ce8Pd0.7 sample. The concentrations of Ce3+ in these two catalysts were similar (Fig. 6a), suggesting that PdO has a stronger ability to adsorb and activate O2 than do the metallic Pd nanoparticles. A separate experiment (Additional file 1) was designed to eliminate the interference of thermal activation on the experimental results, as shown in Additional file 1: Figure S4, and the results obtained also support the above conclusion.
Based on the above characterization results, a possible formation mechanism for the PdO/CeO2 composites is proposed (Fig. 1). First, in the NaOH solution, Al is dissolved, and Ce reacts with OH− at 80 °C to form the rod-like nanoporous Ce(OH)3 skeletal structure due to the anisotropy of Ce(OH)3 growth. At the same time, Pd atoms diffuse to the surface of the Ce(OH)3 nanorods. Because Ce(OH)3 is extremely unstable, it is easily dehydrated and oxidized to CeO2 during drying in air. After calcination under O2, most of the Pd nanoparticles on the surface of the CeO2 nanorods were oxidized to form PdO (Fig. 6b) and were partially embedded into the CeO2 nanorods (Fig. 4) at the high temperature to produce a strong metal-oxide-support interaction. As a result, the PdO/CeO2 composites were formed.
Catalytic Activity Test
The CO and CH4 conversions as functions of the reaction temperature over dealloyed Al91.3Ce8Pd0.7 ribbons calcined at different temperatures are shown in Fig. 8c and d. The dealloyed ribbon (no calcination) exhibited a poor CO catalytic activity compared to that of the calcined samples, as shown in Fig. 8c. Combined with the XPS analysis, these results indicate that PdO supported on CeO2 nanorods exhibits a better CO catalytic activity than that of Pd, which is consistent with the H2-TPR analysis. Below 400 °C, the catalytic activity towards CO increased gradually with the calcination temperature; however, when calcined at temperatures greater than 400 °C, the catalytic activity towards CO decreased with increasing calcination temperature. For the CH4 combustion, similarly, the sample calcined at 400 °C exhibited the best catalytic activity, as shown in Fig. 8d. However, the dealloyed ribbon exhibited a light-off temperature and T50 similar to those of the calcined sample, and the conversion of CH4 was always lower than 93%. According to the experimental results and analysis of the catalytic activity of the dealloyed ribbon and the calcined dealloyed ribbon under different O2 atmospheres (Additional file 1: Figure S5), the reasons for this phenomenon may be because the light-off temperature for CH4 was high (> 240 °C) and a portion of the Pd had been oxidized into PdO; thus, the sample exhibits a good CH4 catalytic activity. However, because the composite is not calcined in a pure O2 atmosphere, the oxidation was insufficient to the extent that it is unable to fully convert CH4. For the dealloyed Al91.3Ce8Pd0.7 calcined at different temperatures, the order of catalytic activity towards CH4 is as follows: dealloyed sample (no calcination) < calcined at 600 °C < calcined at 500 °C < calcined at 300 °C < calcined at 400 °C. The experimental results show that the calcination temperature has an important influence on the catalytic activity of the sample.
Generally, for catalysts with practical application value, they must be stable in the presence of CO2 and H2O. Reaction gases containing CO2 or H2O were passed over the catalyst to examine the CO2 and H2O tolerances of the dealloyed Al91.3Ce8Pd0.7 ribbons calcined at 400 °C, as shown in Figs. 9c–f. Compared with the response in the absence of CO2 in the reaction gas, the addition of 15 vol% CO2 reduced the activity of the catalyst towards CO oxidation, as shown in Fig. 9c, with a T50 and T99 of 80 °C and 130 °C, respectively. However, upon further increasing the CO2 to 30 vol%, the activity of the catalyst towards CO oxidation only slightly reduces, and the T50 and T99 are 88 °C and 140 °C, respectively. For methane combustion, the presence of 15 vol% CO2 in the reaction gas has little effect on the catalytic activity, and the T50 and T99 increased by only 5 °C and 30 °C, respectively, compared to those in the absence of CO2, as shown in Fig. 9d. When the concentration of CO2 was doubled (30 vol% CO2), the catalytic activity continues to decrease, with a T50 and T99 of 350 °C and 460 °C, respectively. Thus, in cases where the reactant concentration is constant, increasing the concentration of the CO2 products will form a strong competitive relationship with CO and CH4 for adsorption on the PdO nanoparticles and at its interfaces, thereby reducing the amount of CO or CH4 adsorbed per unit time and, consequently, the conversion rate. However, due to the higher reaction temperature required for methane combustion, the desorption of CO2 is enhanced such that the CO2 effect on methane combustion seems to be weaker than the effect on CO oxidation.
The long-term stability and water resistance tests of the catalyst are shown in Figs. 9e and f. For CO oxidation, the catalytic activity hardly decreases after 100 h of testing, regardless of the presence or absence of a high concentration of water vapour (20 vol%), indicating that the catalyst has excellent long-term stability and water resistance for CO oxidation. For methane combustion at high conversion (99%), the catalyst possesses similar properties with respect to CO oxidation. At the same time, the effect of water vapour on methane combustion at low conversion (30%, 50%, and 85%) is also discussed in Additional file 1: Figure S6. The effect of water vapour on the catalytic activity at low conversion is greater than that at high conversion. Detailed descriptions and discussions are presented in Additional file 1: Figure S6. The conclusions obtained are similar to those reported by Burch et al. . By comparison with the water resistance of some recently reported Pd-based catalysts for methane combustion (Additional file 1: Table S2), the PdO/CeO2 catalyst prepared in this study retains a relatively excellent catalytic activity after a higher water vapour concentration (20 vol%) and a longer reaction time (100 h), which is very helpful to further the practical application of methane combustion.
Next, the effect of the O2 concentration in the reaction gas on the catalytic activity of the catalyst was investigated. For CO oxidation, as shown in Fig. 10c, when under oxygen-rich conditions (10 vol% O2), the CO conversion was maintained at 99%, and as the O2 concentration suddenly decreased to 0 (anaerobic conditions), the CO conversion decreased rapidly before eventually stabilizing at approximately 12%. The reason for this phenomenon was that the surface lattice oxygen participated in the oxidation reaction of CO. Generally, the CO oxidation pathway involving lattice oxygen on the surface of the support is slow and inefficient compared to the direct adsorption activation of the O2 molecule . Therefore, the CO conversion remained at a lower level under the anaerobic conditions in this study. This result also indicated that the CeO2 carrier has a strong ability to store/release oxygen. Subsequently, 0.5% O2 was introduced into the reaction gas, and the CO conversion rapidly recovered to 90%. As the O2 concentration continues to increase, the CO conversion eventually reached the initial 99%, and a new steady state was established. For methane combustion (Fig. 10d), a similar result to that of CO oxidation was observed but with two different points. The first point was that when in an anaerobic environment, the conversion of CH4 finally stabilized at 25%, higher than that of the anaerobic conversion of CO, which indicated that a high reaction temperature could accelerate the migration of surface lattice oxygen thereby improving the conversion efficiency. The second point was that as the O2 concentration increased, the increasing rate of CH4 conversion and the final establishment of a steady state were slower than those in the CO conversion, which may be due to the incomplete combustion of methane under oxygen-poor conditions (0.5~2 vol% O2). This result also shows that methane combustion is a more complicated and difficult reaction compared to CO oxidation.
First, the CO and CH4 molecules in the reaction gas are adsorbed onto the surface of PdO, reacting rapidly with the adsorbed and activated oxygen on the surface of PdO, and then CO2 and H2O are produced and desorbed. The active sites become available again, and a high reaction rate for CO oxidation and CH4 combustion is maintained. It is worth noting that the catalytic oxidation reaction can still proceed slowly under anaerobic conditions, which is shown to be related to the participation of the surface lattice oxygen of the nanorods in the catalytic reaction (Figs. 10c, d), as shown in Fig. 11.
A large number of experimental results indicated that the PdO/CeO2 catalyst prepared by dealloying combined with calcination exhibited excellent catalytic activities towards CO oxidation and methane combustion and possesses outstanding cycle stability, resistance to CO2 toxicity, and water resistance. In addition to its inherent simplicity, the “green” preparation method of dealloying can effectively avoid the contamination of nanomaterials by organic chemicals and other surfactants, which are common to wet chemical synthesis methods. In addition, the PdO/CeO2 catalyst prepared by dealloying combined with calcination exhibits excellent reproducibility, and the repeated experiments detailed in Additional file 1: Figures S7–S10, Tables S3 and S4 proved this point very well. Therefore, this work can provide insight into the preparation of other new catalysts.
In summary, a simple method of dealloying an Al-Ce-Pd ribbon combined with calcination has been developed for the preparation of a PdO/CeO2 rod-like nanoporous composite. The experimental results indicate that the sample prepared by the dealloying of an Al91.3Ce8Pd0.7 ribbon in 20 wt% solution and then calcining at 400 °C showed the best catalytic activities towards CO oxidation and methane combustion, and the reaction temperatures for the complete conversions of CO and CH4 are 80 °C and 380 °C, respectively. The high catalytic activities could be attributed to the good dispersion of the PdO nanoparticles (having a large specific surface area of 102 m2 g−1), a strong redox capacity, the interaction between PdO and CeO2, and more surface active oxygen on PdO. In addition, the catalyst also exhibited excellent cycle stability, resistance to CO2 toxicity, and water resistance, where after 100 h of testing, the catalytic activity hardly decreased in the presence of H2O. Furthermore, the catalytic reactions can occur even under anaerobic conditions. These results demonstrate the feasibility of the combined dealloying calcination method for the preparation of new catalysts. It is expected that the method can be applied to the preparation of similar composite materials.
The authors thank Ms. Jiamei Liu and Ms. Jiao Li at the Instrument Analysis Center of Xi’an Jiaotong University for their assistance with the XPS and TEM-EDX mapping.
DD, CH, LW, WS, HW, and GH carried out the experiments and analysis. DD, LG, and ZS participated in the experimental design and drafted the manuscript. All authors read and approved the final manuscript.
National Natural Science Foundation of China (Grant No. 51771141, 51371135, and 51671155).
The authors declare that they have no competing interests.
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