3.1. Ir L3-edge EXAFS studies
Figures 1(a) and S1(a) show the Fourier transform (EXAFS-FT, radial distribution functions) and k3χ(k) data of the Ir L3-edge EXAFS data of Ir-NPs supported on MgO, Al2O3, and SiO2, followed by thermal treatment at 1073 K, respectively. The data obtained by the curve-fitting analysis for Ir-NP/MgO are listed in Table 1. The EXAFS-FT analyses of Ir-NP/Al2O3 and Ir-NP/SiO2 completely agreed with that of IrO2, indicating that the oxidation of Ir0-NPs and the formation of agglomerated IrO2 proceeded on these supports. In EXAFS-FT of Ir-NP/MgO, two peaks appeared at 1.7 and 2.7 Å; the feature was considerably different from those of Ir-NP/Al2O3 and Ir-NP/SiO2. The peak at 1.7 Å could be straightforwardly assigned to the Ir − O bond on comparing with the spectrum of IrO2. The second peak at 2.7 Å was assigned to the nearest neighboring Ir − Mg bond on the basis of curve-fitting analysis (Table 1). The Pt − Mg bond distance was calculated to be 2.98 Å, which almost agreed well with that of the nearest neighboring Mg − Mg bond (3.01 Å) in MgO with a salt rock structure. The possibility of the formation of the cubic iridate with an ilmenite structure (MgIrO3) could be excluded because the bond distance of the nearest neighboring Ir − Mg (3.40 Å) of MgIrO3 was much longer than that of Ir − Mg found in the Ir-MgO solid solution [15]. Furthermore, diffraction other than that assignable to the MgO crystal was found in the XRD pattern of Ir-NP/MgO, which is discussed later. The coordination number (CN) of the Ir − Mg bond was calculated to be 9.7 ± 0.9, which was smaller than that of the nearest neighboring Mg−(O) − Mg bond (CN = 12), probably because the Ir ions in the solid solution were located close to the surface of MgO. The curve-fitting analysis was improved after incorporating the second neighboring Ir − O bond with 3.55 Å in distance (Table 1, Figure S2(a)). This distance was slightly shorter than that of the second neighboring Ir − O bond (3.69 Å).
Table 1
Curve-fitting analysis of Ir L3-edge EXAFS data measured at room temperature for Ir NPs and Ir(OAc)3 loaded on MgO treated at 1073K in air
Sample | Scatterer | CNa | R/Åb | ΔE0/eVc | DW/Åd | Rf/%e |
Ir-NP/MgO | O | 6.5 ± 0.6 | 1.99 ± 0.01 | 1.9 | 0.078 | 1.1 |
Mg | 9.7 ± 0.9 | 2.98 ± 0.01 | 4 | 0.069 |
O | 7.9 ± 2.7 | 3.55 ± 0.02 | -1 | 0.070 |
Ir(OAc)3/MgO | O | 6.5 ± 1.5 | 1.98 ± 0.01 | 0 | 0.074 | 1.5 |
Mg | 10.0 ± 2.1 | 2.98 ± 0.01 | 3 | 0.066 |
O | 9.1 ± 1.5 | 3.56 ± 0.02 | 3 | 0.071 |
MgOf | O | (6) | (2.11) | | | |
Mg | (12) | (3.01) | | | |
O | (8) | (3.69) | | | |
acoordination number, bbond distance, cdifference in the origin of photoelectron energy between the reference and the sample, dDebye-Waller factor, eresidual factor, fdata of X-ray crystallography. Fourier transform range: 30-130nm− 1. Fourier filtering range: 1.3–3.5Å. |
Figure 1(b) shows the EXAFS-FT analyses of Ir-NP/MgO treated at 573–1273 K, unloaded Ir-NP, and the Ir powder. The corresponding k3χ(k) data are shown in Figure S1(b). The EXAFS-FT of the unloaded Ir-NP was close to that of metal Ir powder; the peak assignable to the Ir − Ir bond was observed at 2.6 Å (phase shift uncorrected), except for the difference in intensity. For the sample heated at 573 K, new peaks assignable to the Ir − O bonds appeared at 1.7 Å owing to the partial oxidation of Ir-NPs. With a further increase in the thermal treatment temperature to 973 K, the Ir − Mg bond appeared at 2.7 Å. On further increasing the temperature to 1273 K, the intensity of the Ir − Mg peak slightly decreased.
Figure 2(a) shows the CNs of the Ir − O and Ir − Mg bonds of Ir-NP/MgO plotted as a function of the thermal treatment temperature. Ir − O and Ir − Mg bonds appeared in the sample treated at 673 K. The CN (Ir − Mg) increased up to 973 K; the value gradually declined with a further increase in the temperature up to 1273 K. It is likely that the segregation of Ir-MgO occurred in this temperature range. Apart from the CNs, the distances of the Ir − O and Ir − Mg bonds of Ir-NP/MgO are plotted as a function of the thermal treatment temperature in Fig. 3(a). The distance did not change with the treatment temperature. The bond distances of the nearest-neighboring Ir − O bond (1.99 Å) were notably shorter than that of the Mg − O bond (2.11 Å); a similar phenomenon was found in the Pt-MgO solid solution [16]. This is because Ir3+, which has a higher valence than Mg2+, attracted the O2− anion, leading to the shrinkage of the Ir3+–O2− bond in the Ir-MgO solid solution.
Figure 1(c) shows the EXAFS-FTs of the as-prepared sample and Ir(OAc)3/MgO treated at 573–1273 K. The corresponding k3χ(k) data are provided in Figure S1(c). In the EXAFS-pristine Ir(OAc)3, a single peak assignable to hexa-coordinated Ir − O bonds appeared at 1.7 Å (phase shift uncorrected), which arose from the three acetate (OAc-) ligands coordinated to the Ir center. The feature of Ir(OAc)3/MgO treated at 673 − 1273 K were similar to those of Ir-NP/MgO when the treatment temperature was the same. This similarity indicated that an Ir-MgO solid solution was also formed at 773–1273 K in the case of Ir(OAc)3/MgO. The curve-fitting data for Ir(OAc)3/MgO treated at 1073 K are included in Figure S2(b) and Table 1. The bond distances in the Ir-MgO solid solution prepared using Ir(OAc)3 as the Ir precursor were close to that of Ir-NP/MgO, wherein the distances of short and long Ir − O and Ir − Mg bonds were observed to be 1.98 and 2.98 Å, respectively. The temperature-dependent changes in the CNs shown in Fig. 2(b) were similar to those of Ir-NP/MgO (Fig. 2(a)). The Ir − O and Ir − Mg bond distances of the solid solution of Ir(OAc)3/MgO (Fig. 3(b)) were consistent with those of Ir-NP/MgO (Fig. 3(a)).
3.2. Ir L3-edge XANES studies
X-ray absorption near edge structure (XANES) was utilized in order to obtain information about the valence or electronic state and symmetry of absorbing atoms (Ir) [17]. Figure 4 shows the Ir L3-edge XANES profiles of Ir-NP/MgO treated at 1073 K and the reference samples. The formation of a solid solution in the samples treated at 1073 K was confirmed using Ir L3-edge EXAFS, as described above. The inflection points and peak maximum energy of the white line tended to shift to a higher energy, accompanied by an increase in the valence of Ir in the reference compounds (Ir0 powder, Ir2O3, and IrO2). This tendency is consistent with that previously reported for these samples [18]. Here transition from the Ir 2p to unoccupied Ir 5d states is the origin of the Ir L3-edge white line. The energy of the peak maximum of the white line in the XANES profile of Ir-NP/MgO agreed with that of Ir2O3 (11219 eV), indicating that the valence of Ir cations in Ir-NP/MgO was 3+, whereas the peak energies of Ir-NP/Al2O3 and Ir-NP/SiO2 agreed with that of IrO2 (Figure S3). The ionic radius of Ir3+ has been reported to be 0.68 Å, which is comparable to that of hexacoordinated Mg2+ (0.720 Å) [19]. It could be assumed that the close value of the ionic radii between Mg2+ and Ir3+ resulted in the formation of an Ir3+-MgO solid solution. The difference in the ionic radii was 7%, which is within the general criteria for the formation of a solid solution (Hume–Rothery's rules [20]). In the case of Ir4+, the difference in the ionic radii was 13%, while that of Ir4+ was 0.625 Å. This larger difference of ionic radii between Mg2+ and Ir4+ may be the reason why Ir4+ was not included in the solid solution. Apart from the edge energy, a characteristic peak appeared in the Ir L3-edge XANES profile at 11228 eV in Ir-NP/MgO (Fig. 4). Similar peaks were observed in the Ni K-edge XANES spectra of the NiO-MgO and Pt-MgO solid solutions [21] but were not found for the reference compounds, including Ir0 powder, Ir2O3, and IrO2. This difference suggested that the characteristic peak arises from the interference of photoelectrons caused by Mg2+ ions neighboring Ir3+ ions. It should be noted that the intensity of the white line of Ir-NP/MgO was higher than that of Ir2O3, probably because the Ir-MgO solid solution in which Ir3+ was coordinated with six O2− anions had high ionicity [22].
3.3. Ir loading
The Ir loading of these samples is plotted as a function of the treatment temperature in Fig. 5. The loading was measured based on the edge jump of the Ir L3-edge XANES and the sample weight using reference samples. Accompanied by an increase in temperature from 1073 to 1273 K, the Ir loading in the Ir-NP-loaded samples gradually decreased. A marked decrease in the Ir loading was particularly observed in Ir-NP/Al2O3 and Ir-NP/SiO2 (ca. 0.5 wt% at 1273 K), probably owing to the sublimation of the generated IrO2 from the surface of these oxides [23]. The extent of the decrease in the Ir loading was suppressed in the MgO-supported Ir-NP and Ir(OAc)3 samples, probably due to the formation of solid solutions, as was described already.
3.4. TEM study
Figure 6 show TEM images of the as-prepared and thermally treated Ir-NP/MgO samples. Well-dispersed Ir particles of ca. 1–4 nm diameter were observed in the TEM image of as-prepared Ir-NP/MgO (Fig. 6(a)), while Ir particles could not be found in the TEM image of Ir-NP/MgO thermally treated at 1073 K because of the formation of an Ir-MgO solid solution (Fig. 6(b)). Ir particles were not observed in the TEM image of Ir-NP/MgO treated at 1273 K (Fig. 6(c)). This is probably because most of the Ir was present as a solid solution with MgO, even after treatment at 1273 K, as evident in Ir L3-edge EXAFS analysis (Fig. 1(b)).
3.5. XRD patterns
Figure 7(a) shows the expanded XRD patterns of Ir-NP/MgO treated at different temperatures. Original images of the XRD patterns are displayed in Figure S4. The XRD patterns of the as-prepared Ir-NP/MgO agreed with that of the pristine (as-received) MgO mixed with a small amount of Mg(OH)2. The XRD patterns of Ir-NP/MgO treated between 673 and 1273 K agreed with that of MgO, and no new peaks assignable to Ir2O3, IrO2, nor metallic Ir were found [24]. This feature is different from that of Ir-NP/Al2O3 and Ir-NP/SiO2 treated at 1073 K, in which intense diffraction assignable to IrO2 appeared, as shown in Fig. 7(b). No other diffraction assignable to MgO was found in Ir-NP/MgO with 1.1 and 3.0 wt% Ir, while small diffraction assignable to IrO2 appeared in the 4.6 wt%-Ir samples (Figure S5), indicating that Ir was well dispersed in MgO up to 3.0 wt%-Ir in MgO. Figure 8(a) shows the intensity of the diffraction assignable to the MgO (200) facet appearing at 42.8° in Ir-NP/MgO and MgO, plotted as a function of the treatment temperature. The intensity of the diffraction peak of MgO (200) in Ir-NP/MgO was larger than that of MgO when the comparison was made at the same treatment temperature, probably due to the replacement of three Mg2+ ions in MgO with two Ir3+ ions which led to enhanced X-ray diffraction efficiency of the MgO lattice. An enhancement of the diffraction intensity was observed in diffraction other than the MgO (200) facet. This phenomenon is similar to that observed for the Pt-MgO solid solution [16]. In the XRD patterns of the as-prepared Ir(OAc)3/MgO, peaks assignable to a mixture of Mg(OH)2 and MgO were observed (Figures S6). This was likely due to the formation of Mg(OH)2 during the impregnation of MgO with Ir(OAc)3, which was caused by the hydration of MgO, considering that aqueous impregnation was carried out in a water bath using boiling water [25]. The intensity of the Mg (200) facet of Ir(OAc)3/MgO together with boiling water-treated MgO is plotted as a function of the treatment temperature in Fig. 8(b). The intensity of the diffraction assignable to Mg (200) in Ir(OAc)3/MgO was larger than that of MgO treated with a boiling water, regardless of the Ir loading in a similar way found in the XRD patterns of Ir-NP/MgO.
3.6. Nitrogen adsorption isotherms
The N2 adsorption isotherms of MgO and boiling water-treated MgO are displayed in Figures S7. The boiling water-treated MgO showed much higher isotherms than those of untreated MgO owing to the formation of Mg(OH)2 [26]. The N2 adsorption isotherms of Ir-NP/MgO and Ir(OAc)3/MgO are displayed in Figures S8 and S9. The specific surface areas and pore volumes were calculated from the N2 adsorption isotherms, and the data were plotted against the treatment temperature in Fig. 9. A gradual decrease in the specific surface area was observed for the MgO treated with boiling water and Ir(OAc)3/MgO samples except for 4 wt%-Ir(OAc)3/MgO, which showed an enhanced specific surface area and pore volume when the treatment was carried out at 773 K. Perturbation caused by Ir3+ probably results in structural disturbance and enhancement of the pore volume of MgO. However, the specific surface area and pore volume of Ir-NP/MgO did not change in the temperature range between 673 and 1073 K, probably because the Ir-NPs were impregnated on MgO at a lower temperature (~ 300 K) using an evaporator, which prevented the hydration of MgO to form Mg(OH)2, which had high surface area.
3.7. TG-DTA analysis
Figure 10 shows the TG-DTA curves of unloaded and Ir-loaded MgO. In the TG-DTA curve of the as-received MgO, an endothermic peak in the DTA curve appeared at 625 K. A weight loss of 10% was observed in the same temperature range, which was caused by the dehydration of Mg(OH)2 mixed with MgO. The extent of weight loss of boiling water-treated MgO increased to 27% owing to the partial hydration of MgO that progressed during preparation. In the DTA curves of Ir(OAc)3/MgO and Ir-NP/MgO, a small exothermic peak was observed between 535 and 595 K, which was caused by the combustion of the OAc- ligand and PVP polymer, respectively. The curves suggested that the organic moiety present in the Ir precursors was almost completely removed at temperatures up to 673 K.
3.8. H2-TPR analysis
Figure 11 presents the H2-TPR plots of Ir-NP loaded on Al2O3, SiO2, and MgO, and Ir(OAc)3/MgO thermally treated at 1073 K, together with that of pristine IrO2. A H2 consumption peak appeared at 570 K in the H2-TPR of IrO2, which could be attributed to the reduction of Ir4+ to Ir0. A similar profile was reported in the H2-TPR of rutile-type IrO2 [27]. The H2-TPR profiles of Ir-NPs loaded on Al2O3 and SiO2 were similar to that of IrO2, in which the formation of IrO2 was observed in the Ir L3-edge EXAFS. For Ir-NP/MgO and Ir(OAc)3/MgO treated at 1073 K, a major reduction peak appeared at ca. 660 and 770 K, respectively, at a temperature much higher than that of Ir/Al2O3 and Ir/SiO2. This is probably because of the strong interaction between Ir and MgO to form solid solutions, which retarded the reduction of Ir cations. In the H2-TPR profile of Ir-NP/MgO and Ir(OAc)3/MgO treated at different temperatures, the peak at 770 K was observed when the samples were treated at 1173 and 1273 K (Figure S10), probably due to the migration of Ir3+ ions deep inside the MgO crystal to the MgO surface, as observed in the CNs change of Ir-Mg bond in Ir L3-edge EXAFS.
3.9. Dispersion of Ir measured through CO adsorption
Figure 12 shows the dispersion values of Ir in Ir-NP/MgO, Ir/Al2O3, and Ir/SiO2, together with that of Ir(OAc)3/MgO, as a function of the thermal treatment temperature. The dispersion of Ir-NPs loaded on Al2O3 and SiO2 was less than 10%; it was lower than those of Ir-NP/MgO and Ir(OAc)3/MgO when the comparison was made at 1073–1273 K. The low dispersion of Ir-NPs loaded on Al2O3 and SiO2 indicated severe aggregation of Ir, as confirmed by the appearance of peaks assignable to IrO2 in the XRD patterns. In the case of Ir-NP/MgO, two peaks appeared at 873 K and 1173 K, together with the appearance of a two-spike pattern. It is likely that the migration of Ir3+ to the MgO surface resulted in the reduction of Ir3+ to Ir0 with H2, such that the adsorption of CO was promoted in the sample treated at 1173 K (the second peak), which was supported by the Ir L3-edge EXAFS. The H2-TPR profile of Ir(OAc)3/MgO was similar to that of Ir-NP/MgO in that the second peak appeared in the sample treated at 1173 K, suggesting the formation and segregation of Ir-MgO solid solution occurred commonly in Ir-NP/MgO and Ir(OAc)3/MgO.