Min Suk Choi,
Hojin Jeong and
Hyunjoo Lee*
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, South Korea. E-mail: azhyun@kaist.ac.kr; Tel: +82-42-350-3922
First published on 13th January 2021
The Pd/CeO2 catalyst, which is highly active catalyst in automobile emission control especially for CO oxidation, often suffers from severe sintering under harsh condition, specifically hydrothermal treatment. Here, we report re-dispersion of Pd-based bimetallic (Pd–Fe, Pd–Ni, and Pd–Co) catalysts deposited on ceria by hydrothermal treatment at 750 °C using 10% H2O. The re-dispersion was confirmed by various characterization techniques of transmission electron microscopy, CO chemisorption, CO-diffuse reflectance infrared Fourier transform, CO-temperature programmed desorption, and X-ray absorption spectroscopy. The dispersion of Pd increased significantly after hydrothermal treatment, resulting in improved CO oxidation activity. The presence of secondary transition metals enhanced the CO oxidation activity further, especially hydrothermally treated Pd–Fe bimetallic catalyst showed the highest activity for CO oxidation.
In particular, CO oxidation is an important reaction in an automobile emission control system.10,11 Emissions in gasoline engine and diesel engine vehicles contain substantial CO, NOx, particulate matter (PM), and various hydrocarbons (CxHy). These pollutants should be converted into CO2, N2, and H2O through the catalytic reactions. However, a large amount of pollutants are discharged at cold-start condition because the temperature of the engine is lower, resulting in lower temperatures in exhaust after-treatment system, often lower than the operation temperature of the catalyst.12 Therefore, it is necessary to improve the catalytic activity at low temperature. Typically, precious metals such as Pt, Pd, and Rh have been used for pollution remediation in automobile exhaust. Especially, supported Pd catalysts have attracted a lot of attention for various chemical reactions such as oxidation, hydrogenation, dehydrogenation,13–16 automobile emission control system.17,18 However, because of the scarcity and high cost, the use efficiency and catalytic activity of Pd should be improved significantly.
To increase the utility of precious metals, the re-dispersion, which indicates the size decrease of metal nanoparticles after the certain treatment, has been reported. Nagai et al. showed that the oxidative treatment induced re-dispersion of Pt nanoparticles supported on CeZrY mixed oxide for automobile after-treatment.19 Kaneko et al. reported that the effect of pretreatment conditions on the dispersion of Pt/SiO2.20 Dispersion of Pt species depends on calcination temperature of supported Pt precursor. Re-dispersed PtCl2 species were obtained from decomposition of PtCl4 by calcination at 375 °C. Jeong et al. reported the re-dispersion of Pd/CeO2 catalyst after hydrothermal treatment, resulting in enhanced catalytic activity for CO oxidation.21 The re-dispersion process using hydrothermal treatment is a non-toxic treatment process, and has the advantage of having similar condition with hydrothermal aging durability test in automobile catalysts.
To improve the activity and minimize the use of precious metals, various studies on bimetallic catalysts with non-precious transition metals have been performed. Yamamoto et al. reported Pt bimetallic catalyst using Cu and Ni on γ-Fe2O3 support. Pt78Cu22 and Pt85Ni15 showed the mostly improved CO oxidation activity.22 Liu et al. reported computational DFT study about CO oxidation pathways when various second metals were added to CeO2 supported Pd.23 When Ag or Cu was added to Pd/CeO2, CO was adsorbed on Pd site, but O2 was adsorbed on Ag or Cu site. The adsorption sites of CO and O2 were separated, resulting in enhanced CO oxidation activity. Wu et al. reported monodispersed CoPd bimetallic nanoparticle catalysts for CO oxidation.24 Co-existed Pd and CoOx on the surface promoted the CO oxidation activity because lattice oxygen atoms which are provided from CoOx can react with adsorbed CO.
In this work, we report that Pd-based bimetallic catalysts were synthesized by deposition–precipitation method with addition of transition metals of Fe, Ni, and Co. The Pd-based bimetallic catalysts were significantly re-dispersed by hydrothermal treatment. The hydrothermally treated Pd-based bimetallic catalysts were employed to CO oxidation. The effects of the hydrothermal treatment and secondary metal species on Pd-based bimetallic catalysts were investigated using various analyses.
Pd-based bimetallic catalysts (PdFe/CeO2, PdNi/CeO2, and PdCo/CeO2) were synthesized by deposition–precipitation method. H2MCl4 (M = Pd, Fe, Ni, or Co) precursor solution was prepared with HCl solution. 2.5 mg of PdCl2 (99%, Sigma-Aldrich), 3.5 mg of FeCl2 (98%, Sigma-Aldrich), 6.1 mg of NiCl2·6H2O (ReagentPlus® grade, Sigma-Aldrich), or 6.2 mg of CoCl2·6H2O (98%, Sigma-Aldrich) was dissolved in 1 mL of DI water and mixed with HCl with a 1:2 molar ratio of metal to HCl. H2PdCl4 solution was homogeneously mixed with H2FeCl4, H2NiCl4, or H2CoCl4 solution to make Pd-based bimetallic catalysts. Na2CO3 solution was prepared using 530 mg of Na2CO3 powder and 20 mL of DI water. Ceria powder 300 mg was dispersed in 5 mL of DI water, and it was mixed with bimetallic metal precursor solution. Then Na2CO3 solution were added to the ceria solution simultaneously until pH reached 9. The solution was stirred at 800 rpm for 2 h and aged without stirring for 2 h. Then, the solution was filtered and washed with DI water and dried in convection oven at 80 °C for 5 h. Finally, dried powder was pulverized using ceramic mortar. For single phase Pd/CeO2 catalyst, H2PdCl4 solution was introduced without mixing with other metal precursor solution. Other synthetic procedures were the same as the method for the bimetallic catalysts.
For re-dispersion of the Pd/CeO2 and Pd-based bimetallic catalysts, hydrothermal treatment was performed for each catalyst. The catalyst powder was treated in a U-tube quartz cell with 144.5 sccm of air containing 10% H2O at 750 °C for 25 h. After hydrothermal treatment, each catalyst powder was collected and denoted as Pd/CeO2-HT, PdFe/CeO2-HT, PdCo/CeO2-HT, and PdNi/CeO2-HT.
Temperature-programmed desorption (TPD) of CO and CO2 were performed using a BELCAT-B (BEL, Japan) equipped with a thermal conductivity detector. Each catalyst was loaded and treated with He flow at 200 °C for 2 h. The catalyst was cooled to room temperature under He flow, and then exposed to 10% CO/He flow for 1 h. After physically adsorbed CO was purged out by He flow at room temperature for 1 h, the sample was heated to 800 °C at a ramping rate of 10 °C min−1. CO2-TPD was conducted with the same pre-treatment method, and then pure CO2 gas was introduced instead of 10% CO gas. Other methods were the same between CO-TPD and CO2-TPD.
Metal dispersion was determined by modified CO chemisorption method of Takeguchi et al.25 First, 50 mg of catalyst was heated under He gas at 200 °C for 10 min, and then the sample was reduced under a 4.9% H2/Ar flow to 200 °C. After the sample was cooled to 50 °C, it was exposed to the following gas flow: (1) He (5 min); (2) 3.5% O2/He (5 min); (3) CO2 (10 min); (4) He (20 min); (5) 5% H2/Ar (5 min). Then, CO pulse was injected every 1 min until the adsorption of CO onto the sample was saturated. Over-estimating the metal dispersion, which might be caused by CO adsorption on a ceria support as a carbonate, was prevented by CO2 flow.
CeO2 | Pd/CeO2 | PdFe/CeO2 | PdNi/CeO2 | PdCo/CeO2 | ||
---|---|---|---|---|---|---|
Metal content (wt%) | Pd | — | 0.4 | 0.4 | 0.4 | 0.4 |
Fe/Ni/Co | — | — | 0.5 | 0.5 | 0.5 | |
BET surface area (m2 g−1) | As-made | 58.0 | 56.9 | 51.2 | 61.8 | 60.5 |
After HT | 20.0 | 24.6 | 22.2 | 28.5 | 20.8 | |
Dispersion (%) | As-made | — | 45.6 | 61.3 | 55.9 | 64.9 |
After HT | — | 75.1 | 86.7 | 87.4 | 88.0 | |
Pd size (nm) | As-made | — | 2.4 | 1.8 | 2.0 | 1.7 |
After HT | — | 1.5 | 1.3 | 1.3 | 1.3 |
CO-DRIFTS experiments were carried out to investigate the surface metal structures over Pd/CeO2 and Pd-based bimetallic catalysts. Fig. 1 shows the DRIFT spectra of adsorbed CO on as-made and hydrothermally treated Pd-based catalysts. CO is adsorbed linearly on an atop site of single Pd atom (2000–2200 cm−1), and also on ensemble Pd sites with a bridge mode (1900–2000 cm−1) and a threefold hollow mode (1800–1900 cm−1).26 The as-made Pd/CeO2 and Pd-based bimetallic samples mainly exhibited hollow CO peak and bridge CO peak. However, the peak for linear CO adsorption increased significantly after hydrothermal treatment over all the samples, indicating the re-dispersion of metal species. CO-DRIFT experiments were also carried out using CeO2 supported catalysts with only transition metals, and any peak for CO adsorption was not found on the transition metal.
Fig. 1 CO-DRIFT spectra using as-made and hydrothermally treated (a) Pd, (b) PdFe, (c) PdNi, and (d) PdCo catalysts. |
As shown in the high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) mapping images (Fig. 2 and S2–S4†), Pd and transition metal species in as-made catalysts were highly dispersed on CeO2 supports. In addition, hydrothermally treated catalysts also showed the highly dispersed Pd and transition metal species, similar to the initial state of metal distribution without any aggregated particles. Only the domain size of CeO2 increased slightly after hydrothermal treatment.
XRD patterns (Fig. S5†) of all as-made and hydrothermally treated Pd based catalysts in the range of 20°–80° were similar and matched well with the standard peak of CeO2 (ICDD file no. 01-080-5548). There was no peak from metal particles, indicating that the metal species were highly dispersed in a small size on CeO2 supports. The peak of CeO2 became sharper after hydrothermal treatment, indicating that the CeO2 domain size increased. These results are consistent with the BET surface area and TEM results. Overall, Pd-based bimetallic catalysts supported on CeO2 are re-dispersed by hydrothermal treatment.
The catalytic repeatability was tested for PdFe/CeO2-HT catalyst. The light-off experiment was repeated 4 times (Fig. S7†), and the catalytic activity decreases by shifting to the right. The spent catalyst was regenerated by the hydrothermal treatment again at 750 °C for 25 h using 10% H2O/air flow. The activity of the regenerated catalyst was restored to a similar level of the initial activity.
The adsorption strength of CO and CO2 on the as-made and hydrothermally treated catalysts was estimated by CO-TPD and CO2-TPD. The CO-TPD results of the Pd/CeO2 and Pd-based bimetallic catalysts were shown in Fig. 4. The temperature of CO desorption decreased after hydrothermal treatment, indicating that the interaction of CO molecules with ceria surface became weaker, probably resulting from the re-dispersion of Pd species.27 In particular, for PdFe/CeO2 catalysts, the temperature of CO desorption showed the lowest value of 91 °C after hydrothermal treatment. CO2-TPD was also conducted and the temperature of CO2 desorption also decreased after hydrothermal treatment as shown in Fig. 5. In the CO oxidation mechanism, facile CO2 desorption is an important step to achieve the high activity at low temperature.28 The CO2 desorption temperature of PdFe/CeO2-HT catalyst showed the lowest value of 87 °C. In addition, the peak area of CO2 desorption was much smaller after the hydrothermal treatment in all cases. Facile CO2 desorption after hydrothermal treatment is an important factor to improve the CO oxidation activity.
Fig. 4 CO-TPD results using as-made and hydrothermally treated (a) Pd/CeO2, (b) PdFe/CeO2, (c) PdNi/CeO2, and (d) PdCo/CeO2 catalysts. |
Fig. 5 CO2-TPD results using as-made and hydrothermally treated (a) Pd/CeO2, (b) PdFe/CeO2, (c) PdNi/CeO2, and (d) PdCo/CeO2 catalysts. |
XANES analysis was conducted over the Pd/CeO2 and Pd-based bimetallic catalysts to determine the change in Pd oxidation state before and after hydrothermal treatment. As shown in Fig. 6a, all of the as-made and hydrothermally treated samples had similar Pd oxidation state of the intermediate stage between Pd foil and PdO references. Pd K edge EXAFS data of Pd/CeO2 and PdFe/CeO2 before and after hydrothermal treatment were described with PdO and Pd foil (Fig. 6b), and their fitting results are shown in Table 2. The coordination number of Pd–Pd and Pd–O–Pd decreased from 0.8 and 0.9 to 0.0 and 0.2, respectively after hydrothermal treatment in the case of Pd/CeO2. On the other hand, the coordination number for Pd–O–Ce increased from 1.6 to 2.7 after hydrothermal treatment. It indicates that the Pd species were re-dispersed, and the coordination of Pd with CeO2 increased after hydrothermal treatment. For PdFe/CeO2, the coordination number of Pd–Pd and Pd–O–Pd decreased from 0.4 and 0.5 to 0.0 and 0.3, respectively, after hydrothermal treatment. Notably, the interaction of Pd–Fe newly appeared with coordination number of 1.3 after hydrothermal treatment. PdNi/CeO2 and PdCo/CeO2 also had the coordination number of Pd–Ni (1.1) and Pd–Co (0.9) after hydrothermal treatment, as shown in Fig. S8 and S9 and Table S1.† The hydrothermal treatment induced the metal re-dispersion, and also made the interaction of Pd-transition metal in Pd-based bimetallic catalysts.
Sample | Path | Coordination number (R) | Interatomic distance (Å) |
---|---|---|---|
Pd/CeO2 | Pd–O | 2.4 ± 0.4 | 1.98 ± 0.02 |
Pd–Pd | 0.8 ± 0.2 | 2.72 ± 0.01 | |
Pd–O–Ce | 1.6 ± 0.5 | 3.20 ± 0.03 | |
Pd–O–Pd | 0.9 ± 0.2 | 3.48 ± 0.04 | |
Pd/CeO2-HT | Pd–O | 3.0 ± 0.3 | 1.99 ± 0.03 |
Pd–Pd | 0.0 ± 0.0 | 2.72 ± 0.02 | |
Pd–O–Ce | 2.7 ± 0.3 | 3.22 ± 0.02 | |
Pd–O–Pd | 0.2 ± 0.2 | 3.47 ± 0.01 | |
PdFe/CeO2 | Pd–O | 2.3 ± 0.4 | 2.00 ± 0.02 |
Pd–Fe | 0.0 ± 0.0 | 2.62 ± 0.02 | |
Pd–Pd | 0.4 ± 0.1 | 2.74 ± 0.02 | |
Pd–O–Ce | 1.5 ± 0.5 | 3.22 ± 0.03 | |
Pd–O–Pd | 0.5 ± 0.0 | 3.48 ± 0.01 | |
PdFe/CeO2-HT | Pd–O | 2.5 ± 0.3 | 1.99 ± 0.02 |
Pd–Fe | 1.3 ± 0.2 | 2.62 ± 0.03 | |
Pd–Pd | 0.0 ± 0.0 | 2.74 ± 0.01 | |
Pd–O–Ce | 1.7 ± 0.4 | 3.21 ± 0.03 | |
Pd–O–Pd | 0.3 ± 0.1 | 3.48 ± 0.02 |
CO was adsorbed on Pd sites in bimetallic catalysts, as confirmed by CO-DRIFT in Fig. 1. In general, it is assumed that CO oxidation follows Mars–van Krevelen mechanism, by which CO directly reacts with the surface oxygen, producing CO2 and leaving behind surface oxygen vacancies which are then filled with gaseous O2.29 The transition metal sites in bimetallic catalysts have the high oxygen affinity and possibly act as O2 dissociation sites to enhance CO oxidation reaction.30 The activity of PdFe/CeO2-HT catalyst for CO oxidation was compared with literature values in Table S2,† indicating that the PdFe/CeO2-HT showed good activity compared to other works.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra09912k |
This journal is © The Royal Society of Chemistry 2021 |