The low temperature oxidation of carbon monoxide (CO) has gained much attention owing to its numerous applications such as in CO gas sensors [1] and automobile exhaust purification [2]. Supported noble metals (e.g., Ag, Au, Pd, Pt) have been considered as efficient catalysts for CO oxidation processes [3, 4, 5, 6, 7]. Among them, supported Pd catalysts are considered to be promising owing to their excellent activity. Though Ag and Au catalysts can convert CO at room temperature or below, the melting points of Ag (960 °C) and Au (1063 °C) are lower than that of Pd (1550 °C). Consequently, Ag and Au nanoparticles are more prone to sintering during the process, thereby lowering the performance of the catalyst. Similar to Pd catalysts, Pt catalysts feature a high melting point (1769 °C); however, Pt supported catalysts are more expensive than Pd catalysts.
Much effort has been devoted in understanding the catalytic oxidation of CO over Pd catalysts. Currently, it is widely believed that this reaction follows a Langmuir-Hinshelwood mechanism, involving the reaction between CO adsorbed onto the Pd species and oxygen from ceria at the metal-ceria interface [8]. However, to date, the nature of the active species for this reaction remains unclear and a challenging research topic. For example, both Pd and PdO species were identified as the active sites for CO oxidation by Venezia [9]. Other research studies reported that the enhanced performance of Pd supported on CeO2 catalyst could be due to the ionic dispersion of Pd on CeO2 that leads to a strong metal-ceria interaction [10]. Furthermore, as demonstrated, the high dispersion of active species promotes the reaction over Pd catalysts [11].
Additionally, the morphology, exposed crystal facets, oxygen vacancies, and pore structure of CeO2 play vital roles in determining the catalyst activity. Various CeO2 nanostructures, such as nanorods, nanowires, and nanocubes, were recently synthesized and investigated as supports in CO oxidation [12]. The results demonstrated that the catalyst activity toward CO oxidation was greatly dependent on the shape of CeO2. Yuan and his co-workers [13] prepared a series of Au supported on CeO2 catalysts, and experimentally observed that CeO2 nanorods featured a considerably higher oxygen storage capacity and exhibited superior activity toward CO oxidation when compared with CeO2 nanocubes. Furthermore, it was found that the preferentially exposed crystal planes (100) and (110) of CeO2 nanorods were more active toward CO oxidation than the crystal plane (111) of the corresponding nanoparticles [14]. Zhu [15] showed that the catalytic activity of Pd supported on CeO2 catalysts could be improved by employing a pre-reduction treatment, particularly a low-temperature reduction pre-treatment, which promotes oxygen vacancy formation and enhances metal-support interactions. This interaction was also considered by many other researchers as discussed. As reported, cationic Pd species were formed owing to the strong interaction between Pd and CeO2 [16, 17], and accordingly Pd2+ ions in the CeO2 matrix were suggested as the active sites of the low-temperature oxidation of CO [18]. In another study, Tang [19] examined the effect of the pore structure of CeO2 on CO oxidation; three types of CeO2 nanomaterials with different pore structures were synthesized i.e., mesoporous, microporous, and nanoparticle CeO2. The results showed that Pd supported on mesoporous CeO2 exhibited the highest catalytic activity among the prepared catalysts. The superior activity was ascribed to the mesoporous structure, large surface area, and abundant surface oxygen species of the mesoporous CeO2 support.
Based on the above analyses, CeO2 nanotubes could be considered as potential supports for CO oxidation owing to their advantageous properties such as high surface area and tubular morphology [20]. Thus, in this work, we prepared CeO2 nanotube-supported Pd catalysts for application in CO oxidation at low temperatures. To achieve a high dispersion of metal particles, a novel alcohol reduction method [21, 22] was used to stabilize the Pd nanoparticles on the support. A series of catalysts with different Pd contents were prepared and characterized by Nitrogen sorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and CO-temperature-programmed desorption (CO-TPD) techniques. The relationship between the structure and activity of the catalysts was explored to contribute to ongoing discussion regarding the role of active Pd species in CO oxidation.
CeO2 nanotubes were prepared by the hydrothermal method using poly(ethylene glycol)-block-poly(propylene glycol)- block-poly(ethylene glycol) (PEO-PPO-PEO; P123) as the surfactant [23].
Poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Pd nanoparticles (NPs) were synthesized using an alcohol reduction method. First, H2PdCl4 aqueous solution (2.0 mmol/L) was prepared by dissolving the precursor PdCl2 solution (Sinopharm Chemical Reagent Co., Ltd.) in HCl aqueous solution. Then, in a typical synthesis, H2PdCl4 aqueous solution (45 mL), ethanol (60 mL), H2O (40 mL), and PVP (0.4 g) were refluxed at 90 °C for 3 h. The as-prepared Pd NPs were collected by vacuum rotary evaporation and then re-dispersed and stored in ethanol (40 mL).
For the preparation of Pd supported on CeO2 nanotubes (Pd/CeO2-nanotube), the following procedure was employed. CeO2 nanotubes (1 g) were added to a required amount of the above Pd NPs ethanolic solution. The mixture was stirred for 24 h to obtain a homogeneous suspension. Then, the suspension was evaporated in a rotary evaporator under vacuum conditions and dried in an oven at 110 °C, followed by calcination at 500 °C in air for 2 h. Three Pd/CeO2-nanotube catalysts with varying Pd contents of 0.3, 0.6, and 0.9 wt% were prepared, and are referred as 0.3Pd/CeO2-nanotube, 0.6Pd/CeO2- nanotube, 0.9Pd/CeO2-nanotube, respectively. Additionally, for comparison, commercial CeO2 (Sinopharm Chemical Reagent Co., Ltd.) was used as a support to prepare Pd supported on CeO2 with a Pd content of 0.9 wt%, and referred as 0.9Pd/CeO2. The same preparation procedure as that used for preparing Pd/CeO2-nanotube materials was employed.
The catalytic activity tests were performed in a continuous flow fixed bed reactor. A stainless steel tube with an inner diameter of 8 mm was used as the reactor. The catalyst (100 mg) with a diameter of 20-40 mesh was introduced into the reactor. The reaction gas mixture consisting of CO (1 vol%), O2 (4 vol%), and He (95 vol%) was passed through the catalyst bed at a total flow rate of 20 mL/min. The composition of the influent and effluent gases was determined using an online GC-9860 gas chromatograph equipped with a thermal conductivity detector. The CO conversion was evaluated based on the CO concentration difference between the inlet and outlet.
Nitrogen sorption analysis was performed on a Micromeritics ASAP 2020 to determine the BET specific surface area and BJH pore diameter distribution of the samples. Powder X-ray diffraction (XRD) was performed on a Shimadzu XRD-6000 to identify the crystallographic phase of the samples. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical states of the atoms on the sample surface, as performed on a Thermo Scientific Escalab 250Xi. The XPS data were calibrated using the binding energy of C 1s (284.6 eV) as standard. Microscopic structure information was obtained via transmission electron microscopy (TEM) performed on a FEI Tecnai G2 F20. CO-TPD was carried out on a Micromeritics Auto Chem II 2920. For analysis, the sample (100 mg) was reduced with 5 vol% of H2 diluted with Ar at 300 °C for 1 h and kept under a stream of Ar for 0.5 h. After cooling the sample to room temperature, CO was supplied for 1 h and the excess CO on the sample was removed with a flow of Ar for 0.5 h. The signal was tested via online mass spectrometry. For the measurements, the samples were heated from 50 to 700 °C at a heating rate of 10 °C/min.
Figure 1 shows the N2 adsorption-desorption isotherms and BJH pore size distributions of representative samples 0.9Pd/CeO2 and 0.9Pd/CeO2-nanotube. More specifically, 0.9Pd/CeO2-nanotube displayed a Type IV adsorption- desorption isotherm with an H3 hysteresis loop, which is characteristic of mesoporous solids [24]. The average pore diameter of 0.9Pd/CeO2-nanotube was ~20 nm, which is similar to the inner diameter of CeO2 nanotubes reported in some literature [23]. The surface area of the prepared samples was calculated using the BET method (Table 1). As observed, CeO2-nanotube support featured a surface area of 58.0 m2/g, which is considerably higher than that of pure CeO2 (2.0 m2/g). Impregnation of the CeO2-nanotube support with a Pd NPs ethanolic solution resulted in a decrease in the specific surface area that may be due to pore blocking by the Pd species that reduces the accessibility of the nitrogen molecules to the pores.
Figure 2 shows the XRD patterns of the CeO2 support, CeO2-nanotube support, and corresponding Pd-containing catalysts. The samples all displayed distinct diffraction peaks at 28.6 °, 33.1 °, 47.5 °, 56.3 °, 59.1 °, 69.4 °, 76.7 °, and 79.1 °, which could be indexed to the (111), (200), (220), (311), (222), (400), (331), and (420) crystal planes of cubic fluorite-structured CeO2 (JCPDS No. 34-394). Diffraction peaks corresponding to Pd species were not observed in the XRD patterns, indicative of the high dispersion of the Pd species with a low mass content on the supports [25, 26].
TEM images of the PVP-stabilized Pd nanoparticles, CeO2-nanotube support, 0.9Pd/CeO2, and 0.9Pd/CeO2- nanotube are shown in Fig. 3. As reported, Pd nanoparticles are single crystallites with a 5-shell structure [22]. Based on the statistical analysis of ~100 metal nanoparticles, the average particle size of the obtained PVP-stabilized Pd was calculated as 2.06 ± 0.56 nm (inset in Fig. 3(a)). As observed in Fig. 3(b), CeO2-nanotube support featured a uniform tubular morphology with tube outer diameters of ~40-60 nm. After the calcination, the Pd species were well dispersed on the CeO2-nanotube support and their size was estimated as 3-5 nm (Fig. 3(c)). In contrast, the degree of Pd dispersion on the CeO2 support was lower and the Pd species were larger than 5 nm (Fig. 3(d)). However, because of the high electron density of polycrystalline CeO2 [27], and the high dispersion and small size of the Pd nanoparticles, it was difficult to resolve the interface between the Pd nanoparticles and CeO2 nanotubes. Accordingly, the Pd nanoparticles in the TEM images in Fig. 3(c) and (d) could not be distinctly determined. Such a phenomenon has also been reported previously [28]. The dispersion and size of Pd will be discussed further in the subsequent CO-TPD analysis section.
CO-TPD is a useful tool for obtaining information on the CO adsorption ability of the samples. The CO-TPD spectra of 0.9Pd/CeO2 and Pd/CeO2-nanotube samples are shown in Fig. 4. The CO2 signal was detected following CO adsorption onto the samples, thereby reflecting the amount of CO oxidized by the surface oxygen. The Pd/CeO2-nanotube samples all displayed a strong and sharp peak at ~110 °C, a broad band at ~370 °C, and a small peak in the range of 500-700 °C, indicative of their strong CO adsorption ability. In contrast, 0.9Pd/CeO2 did not exhibit a significant CO2 desorption peak because of the low surface area of the sample (1.3 m2/g; Table 1) [29].
CO can also be used as a probe molecule to determine the noble metal dispersion and metal particle size on the catalyst. The adsorption of CO onto metal Pd proceeded via an irreversible linear single point attachment fashion. Because the amount of CO adsorbed irreversibly (COirr) corresponds to the number of Pd atoms, the Pd dispersion and Pd crystallite size can be estimated accordingly [30]. The calculated data are displayed in Table 1. Generally, as observed, with increasing Pd contents, COirr and DPd (Pd crystallite size) increased, whereas Pd dispersion decreased. Furthermore, 0.9Pd/CeO2 exhibited a considerably lower COirr (13.2 μmol/g) than 0.9Pd/CeO2-nanotube (149 μmol/g). The calculated Pd dispersion for 0.9Pd/CeO2 (2.13%) was significantly lower than that for 0.9Pd/CeO2-nanotube (23.3%). And the Pd crystallites on the CeO2 support were larger (>10 nm) than those on the CeO2-nanotube support (7.3 nm). Therefore, these results showed that a high dispersion of Pd particles on CeO2 nanotube could be obtained using the present ethanol reduction method.
The XPS spectra of the Pd/CeO2-nanotube samples and 0.9Pd/CeO2 are given in Fig. 5. The peaks of 0.9Pd/CeO2 were fitted with one set of spin-orbit doublets of 3d5/2 (336.9 eV) and 3d5/2 (342.3 eV), indicating that Pd was present as Pd2+ as bulk PdO [31]. The peaks of the Pd/CeO2-nanotube samples at Pd 3d5/2 (337.6-338 eV) and Pd 3d3/2 (342.9-340.3 eV) indicate that Pd was mostly present as Pd2+. However, the Pd2+ XPS signals were stronger than that of bulk PdO. This result was attributed to the formation of Pd-O-Ce bonds on the catalyst surface after calcination as reported previously [32, 33]. Pd2+ was stabilized on Ce3+ sites, leading to strong interactions via Pd-O-Ce bonding [34].
As observed in Fig. 5(b), the Ce 3d XPS spectra featured nine peaks labeled as V, V0, V1, V2, V3, V4, V5, V6, V7, whereby V, V2, V3, V4, V6, V7 are characteristic of the Ce4+ 3d final states and the remaining peaks are characteristic of Ce3+. These results suggest the co-existence of Ce3+ and Ce4+ species on the surface of the catalysts [19]. Ce3+ sites in ceria are involved in the balance of electrostatic equilibrium and production of oxygen vacancies. The relative content of Ce3+ in each sample is listed in Table 1. As deduced, the relative content of Ce3+ decreased in the following order as 0.9Pd/CeO2-nanotube (12.6%) > 0.6Pd/CeO2- nanotube (12.0%) > 0.3Pd/CeO2-nanotube (10.4%) > 0.9Pd/ CeO2 (7.6%). It was evident that the relative content of Ce3+ on the Pd/CeO2-nanotube catalysts increased with increasing contents of Pd. Accordingly, 0.9Pd/CeO2-nanotube is expected to provide a higher number of oxygen vacancies when compared with 0.9Pd/CeO2 owing to the higher content of Ce3+ on the Pd/CeO2-nanotube surface.
Figure 6 shows the light-off curves of CO oxidation over 0.9Pd/CeO2 and Pd/CeO2-nanotube catalysts. As observed, with increasing reaction temperatures, the catalytic activities toward CO oxidation improved for all catalysts studied. CeO2-nanotube, 0.3Pd/CeO2-nanotube, 0.6Pd/CeO2-nanotube, 0.9Pd/CeO2-nanotube, and 0.9Pd/CeO2 respectively yielded T50 values (temperature corresponding to 50% CO conversion) of 275, 118, 96, 62, and 152 °C, and T100 values (temperature corresponding to 100% CO conversion) of 320, 180, 140, 100, and 200 °C. The results showed that the Pd/CeO2-nanotube catalysts exhibited considerably higher activities than 0.9Pd/CeO2. Among the prepared catalysts, 0.9Pd/CeO2-nanotube showed the highest activity.
To gain insights into the intrinsic activities of the CeO2-supported Pd catalysts, the turnover frequency (TOF) was calculated based on the conversion level of CO and metal dispersion [35]. The results are listed in Table 1. The TOF values over 0.9Pd/CeO2, 0.3Pd/CeO2-nanotube, 0.6Pd/CeO2-nanotube, 0.9Pd/CeO2-nanotube at 100 °C were 0.26, 0.34, 0.53, and 0.63 s−1, respectively. The Pd/CeO2-nanotube catalysts displayed higher TOFs than 0.9Pd/CeO2. Among all samples, 0.9Pd/CeO2-nanotube exhibited the highest TOF. The TOF results were consistent with those of the light-off curves shown in Fig. 6.
Arrhenius plots for the different catalysts were derived using a method reported in the literature [36], and are shown in Fig. 7. The activation energy (Ea) values for 0.9Pd/CeO2, 0.3Pd/CeO2-nanotube, 0.6Pd/CeO2-nanotube, and 0.9Pd/CeO2- nanotube were 58.6, 36.9, 32.3, and 26.5 kJ/mol, respectively. With increasing Pd contents from 0.3 to 0.9 wt% on the CeO2-nanotube support, Ea decreased slightly, suggesting that CO oxidation over these catalysts proceeded via similar reaction pathways [36]. In contrast, 0.9Pd/CeO2 displayed a considerably higher Ea than the Pd/CeO2-nanotube catalysts, thereby suggesting that CO oxidation over this catalyst proceeded via a different reaction pathway. Regardless, it could be deduced that the oxidation of CO over 0.9Pd/CeO2-nanotube was the most favorable.
It is generally believed that the interface between the metal and oxide support provides the active sites for a given catalytic reaction. Also, CO oxidation over supported Pd catalysts follows a Langmuir-Hinshelwood mechanism. More specifically, the reaction proceeds as follows [15, 19]: (1) CO adsorbs onto the surface of Pd species; (2) the adsorbed CO reacts with surface active species oxygen, including active lattice oxygen on PdO or CeO2 and adsorbed oxygen, to produce CO2; and (3) the catalyst generates new oxygen vacancies, which are promptly filled with O2 from the reaction gas to subsequently complete the redox recycle.
Based on the above-discussed results and proposed mechanism, it is clear that the metal size, structure of the support, CO adsorption ability, and metal-ceria interaction are the key factors that influence the catalytic performance. According to the characterization results, relative to the Pd/CeO2 catalyst, the Pd/CeO2-nanotube catalysts exhibit larger BET surface areas (Table 1), which favor Pd dispersion. It is in agreement of dispersion and size results obtained by CO-TPD (Table 1) that Pd/CeO2-nanotube has higher metal dispersion and smaller particle size than those of Pd/CeO2. The higher CO adsorption ability (Fig. 4) is beneficial to Step (1) in the Langmuir-Hinshelwood mechanism. From the XPS results, the Pd/CeO2-nanotube catalysts can provide more oxygen vacancies owing to the higher content of Ce3+ (Table 1) on the surface. The better interaction, enhanced by Pd-O-Ce bonding (Fig. 4), between the metal and support is favorable to Step (2). Furthermore, the mesoporous structure (Fig. 1) of the Pd/CeO2-nanotube catalysts favors the adsorption and diffusion of reactant and product gases [37]. The lower activation energies of the Pd/CeO2-nanotube catalysts (Fig. 7) prove that the catalytic reaction over the catalysts proceeds via a different pathway (which is favorable toward CO oxidation) from that occurring over the Pd/CeO2 catalyst. Accordingly, the Pd/CeO2-nanotube catalysts displayed better redox abilities and better catalytic activities toward CO oxidation (Fig. 6) than the Pd/CeO2 catalyst.
In this work, we have prepared CeO2-supported Pd nanoparticles and investigated their catalytic performance toward CO oxidation. Compared with pure CeO2, the mesoporous and tubular structure of the CeO2-nanotube support improved the dispersion of Pd species and accordingly enhanced CO adsorption. The abundant amounts of Ce3+ on the nanotube surface promoted the formation of oxygen vacancies that are important for oxygen activation. The Pd-O-Ce interactions on the nanotube are responsible for the reaction.
2016, Vol. 37 Issue (1): 83-90
PDF (850 KB)
2016, Vol. 37 Issue (1): 83-90
PDF (850 KB)