Applied Science and Convergence Technology 2019; 28(3): 35-40
Published online May 31, 2019
Copyright © The Korean Vacuum Society.
Department of Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea
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The introduction of guest metal ions significantly change the nature of bare support materials. In this study lanthanide, Ln, (La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) ions were loaded into a cubic phase CeO2 support by a hydrothermal method. Their fundamental characteristics were examined by scanning electron microscopy, transmission electron microscopy, optical microscopy, X-ray diffraction crystallography, and Raman spectroscopy. Their CO oxidation performances temperature were measured by temperature-programmed reaction mass spectrometry. It was found that the fundamental natures of the Ln-loaded catalysts were all dependent on the ionic radii of guest lanthanide metal ions. Higher CO oxidation activities were obtained for lanthanide metal ions with ionic radii closer to that of the host Ce4+ ion. The present results highlight that defects, particles sizes and surface active sites were affected from the matching of the host and guest-guest ionic radii.
Keywords: CeO2, Lanthanide (III) ions, CO oxidation, Temperature programmed reduction
Heterogeneous catalytic air purification such as catalytic carbon monoxide (CO) oxidation has extensively been researched using a model catalyst of cerium (IV) oxide (CeO2) [1–7]. To achieve higher catalytic activity and understand the catalytic pathways many efforts have been devoted to develop various CeO2 catalysts, test their catalytic activities, and employ density functional theory calculations . CeO2 shows convertible two oxidation states (Ce4+/Ce3+) and good oxygen release-acceptance ability over the surface under CO and O2 environments [8–12]. Doping and loading guest metals (or their ions) into (or onto) have widely been employed to show significant alternation of the catalytic activity of a bare CeO2 support [1–12]. Ha et al. prepared Au/CeO2 cubes with exposed (100) surface and Au/CeO2 octahedra with exposed (100) surface and showed a higher catalytic activity for Au/CeO2 cubes, attributed to oxygen release capacity at the interface of Au and CeO2(100) via the Mars-van Krevelen mechanism . Papavasiliou et al. prepared CuO-CeO2 catalysts, treated under nitric acid solutions, and observed higher CO oxidation activity, attributed to higher oxygen vacancies, Cu+ species, and improved dispersion of active Cu species and reducibility of CeO2 catalyst . Vacancy formation is also known to be highly dependent on the dopant ionic radius [15–17]. Chen et al. employed diffuse reflectance infrared Fourier transformed spectroscopy to study surface species formed during CO oxidation over Au/CeO2 catalysts and concluded that the chemisorbed CO was not greatly dependent on the Au particle size, whereas the carbonate, bicarbonate, and formate species were found to be strongly dependent on the size .
For the literature survey of lanthanide metal loaded CeO2 catalysts [18–28], Zhang et al. prepared Pr-doped CeO2 by a co-precipitation method, tested a Prins condensation–hydrolysis reaction of isobutene with HCHO, and observed a dependency of catalytic performance on Pr-doping induced oxygen vacancy . Sutradhari et al. synthesized Sm-doped CeO2 nanoparticle and applied to allylic oxidation of cyclohexene . Hernandez et al. reported CO oxidation onset at around 300 °C for Ce1–
In this paper, lanthanide metal ions were first loaded into CeO2 support to examine the effects of ionic radius on the fundamental natures and a catalytic activity. X-ray diffraction crystallographic data, Raman spectra, temperature-programmed reduction and CO oxidation profiles consistently showed that ionic radii of host-guest ions were very important for designing a catalyst.
For loading lanthanide (III) (La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) ions into CeO2 nanoparticles, 10 mol% of Ln (III) (vs. Ce) was dissolved in 20.0 mL of 0.05 M cerium (III) nitrate hexahydrate (99%, Sigma-Aldrich) solution. La (III) nitrate hexahydrate (99%, Sigma-Aldrich), Pr (III) nitrate pentahydrate (99%, Sigma-Aldrich), Nd (III) nitrate (99%, Sigma-Aldrich), Sm (III) sulfate octahydrate (99.9%, Sigma-Aldrich), Eu (III) nitrate pentahydrate (99%, Sigma-Aldrich), Gd (III) nitrate hexahydrate (99%, Sigma-Aldrich), Tb (III) nitrate hydrate (99%, Sigma-Aldrich), Dy (III) nitrate hydrate (99%, Sigma-Aldrich), Ho (III) nitrate pentahydrate (99%, Sigma-Aldrich), Er (III) nitrate solution (0.1 M), Tm (III) nitrate pentahydrate (99%, Sigma-Aldrich), and Yb (III) nitrate pentahydrate (99%, Sigma-Aldrich) were used as received. Afterwards, 1.0 mL of 30% ammonia solution was added to obtained precipitates. After this process, the solution in a 100 mL Teflon-lined stainless autoclave was placed in an oven at 120 ºC for 12 h. Upon finishing the reaction, the obtained powder samples were fully washed by ethanol and deionized water repeatedly and dried in an oven setting at 80 ºC. The crystal phases of bare and Ln-loaded CeO2 catalysts were examined using a PANalytical X’Pert Pro MPD diffractometer with Cu K
Figure 1 displays the X-ray diffraction (XRD) patterns of unloaded CeO2 and Ln-loaded CeO2 powder samples. Several peaks were commonly found at around 2
Figure 2 displays the XRD 2
Figure 3 shows the photo images of all the powder samples, the selected SEM and TEM images of unloaded CeO2 nanoparticles. For unloaded CeO2, the size distributions were found to be 10–50 nm with an average size of ~23 nm. The average value is fairly close to the calculated particle size of ~20 nm. The corresponding HRTEM image shows clear lattice fringes with an interplanar distance of 0.31 nm. This distance is in good agreement with the (111) plane of cubic phase CeO2 . The unloaded powder sample is in yellowish white color. Based on the optical microscope images, the color of CeO2 was changed very slightly upon Ln (III)-doping. However, the colors of the Tb-loaded and Pr-loaded samples were dramatically changed to light brown and reddish brown colors, respectively. The change in color was plausibly due to change in oxidation state from 3+ to 4+ for both metal ions [32,33]. For Pr oxide, Kang et al. observed brown color, attributed to an oxidation state of Pr (IV) . For Tb oxide, Sohn reported brown color, attributed to charge transfer absorption of Tb (IV) . In Fig. 2, Pr and Tb-loaded samples showed larger deviations from the linearity. This could be due to much smaller crystal ionic radii of Pr and Tb upon changing oxidation state from 3+ to 4+.
Figure 4 displays the CO oxidation profiles with temperature for unloaded and Ln-loaded CeO2 catalysts. The CO oxidation onset was commonly observed at above 400 °C, but the onset position was dependent on the loaded lanthanide ions. For bare CeO2, the onset was observed at 445 and 420 °C for the 1st and 2nd runs, respectively. The onset in the 2nd run was 25 °C lower than that in the 1st run. The enhancement of the catalytic activity was plausibly due to increase in crystallinity and exposure of active sites upon thermal CO oxidation reaction to 700 °C during the 1st run [9–11]. Most of the Ln-loaded CeO2 samples (exception of Pr, Gd, and Ho) showed an enhancement in catalytic activity during the 2nd run, compared with that observed during the 1st run. The Nd-loaded CeO2 showed the highest catalytic activity and the oxidation onset was observed at 424 and 410 °C for the 1st and 2nd runs, respectively. Hernandez et al. prepared La-, Eu-, and Gd-loaded Ce0.9M0.1O2–
TPR experiments were performed to verify surface and bulk oxygen reduction states. Figure 5 shows TPR profiles of unloaded and Ln-loaded CeO2 catalysts. Two broad peaks were commonly observed at around 600 and 950 °C. The former peak at 600 °C and the latter peak at 970 °C were generally attributed to the reduction of surface oxygen and bulk CeO2 [9–11]. It was clearly seen that Pr-CeO2 showed the lowest surface reduction peak at 530 °C. Above the atomic number (59) of Pr, the surface reduction peak position was observed at higher temperatures. Based on the calculated particle sizes (Fig. 2), the catalyst with larger particles sizes showed a higher temperature surface reduction peak. For Sm-loaded CeO2, the surface reduction peak was dramatically diminished, instead a strong sharp peak was observed at 800 °C, plausibly due to reduction of isolated Sm species as discussed above.
Figure 6 displays the Raman spectra of bare and Ln-loaded CeO2 samples. A strong peak was commonly observed around 457–463 cm−1 for all the samples. This has generally been assigned to F2g vibration mode of the cubic fluorite-type structure . For bare CeO2, a strong peak was observed at 462 cm−1. For Pr-CeO2, the F2g peak was found at 457 cm−1, the lowest wavenumber compared with those for other samples. As discussed above, the Pr-CeO2 sample showed the lowest surface reduction peak. At above the atomic number (59) of Pr, the F2g peak was found to be shifted to a higher wavenumber with increasing the atomic number. This could be related with the ionic radius dependent CeO2 crystal lattice [17,34]. Upon metal ion loading, additional Raman peaks were commonly observed between 500 and 650 cm−1, attributed to oxygen vacancies (and/or crystal lattice distortions) created by metal doping [14,16,20,30]. Bare CeO2, the defect-related Raman peak was found to be extremely weak. Mostly, two extra peaks were observed at 540 and 600 cm−1. For Ho-, Eu-, and Pr-loaded CeO2 samples, defect-related peaks were observed at 530 and 570 cm−1 and much stronger than other samples. Interestingly, for only Tb- and Pr-loaded CeO2 samples the defect related peaks were found at mid-position between the two defect peaks. This could be related with an oxidation state of +4. For Ce0.9Pr0.1O2–
In summary, lanthanide ions were loaded into CeO2 support to show the effects of ionic radius on the fundamental characteristics and catalytic CO oxidation properties. The XRD patterns (including calculated particle sizes), Raman peak profiles, TPR and CO oxidation profiles were found to be all dependent on the lanthanide metal ions. The lanthanide ions with ionic radius closer to that of Ce4+ ion commonly showed higher CO oxidation activity. The present systematic research results highlight that matching of host-guest ionic radius was important to determine defects, particles sizes, and surface active sites.
This work was supported by research fund of Chungnam National University research grant (Project No. 2018-1099-01).