Formation of Ir–MgO Solid Solutions Analyzed with X-ray Absorption Spectroscopy

Thermal treatment of MgO-loaded Ir nanoparticles or Ir(OAc)3 formed Ir–MgO solid solutions. The valence of Ir in the Ir–MgO solid solution was 3 +, as evidenced by Ir L3-edge XANES combined with XPS analysis. A slight contraction of the Ir–O bond distance was observed compared to that of the nearest neighboring Mg–O bond in MgO. Ir–MgO dispersion exhibited a two-spike pattern depending on the treatment temperature owing to the formation and successive segregation of the solid solutions.


Introduction
Supported metal catalysts have been widely used in such as petroleum refining, environmental remediation, and fine chemical synthesis, thus the demand for efficient supported metal catalysts is increasing progressively. In order to increase the surface area of the supported metal species, it is desirable to support noble metal active species in a highly dispersed manner. The highly dispersed catalyst particles often agglomerate at high temperatures, which leads irreversible deactivation. One possible way to overcome this problem is to utilize the strong interaction between the metal or metal cations and the oxide support. Such strong metal-support interactions have been applied to obtain renewable dispersed palladium supported on perovskite oxides, which have been commercialized for the purification of automobile exhaust emissions [1]. However, the interaction mechanism between various metal elements and MgO has not been studied in detail, particularly little is known about the solid solution formation of MgO and sixth period elements. Among these elements, much attention has recently been paid to Ir from the viewpoint of obtaining single-site active sites because atomically dispersed supported metal catalysts realize maximized efficient metal-active centers [2]. In fact, the single-site Ir complexes have been utilized for electrocatalytic water splitting [3]. Atomically dispersed Ir species on MgO (111) nanosheets are active in benzene-ethylene coupling with styrene [4]. Atomically dispersed Ir was observed using scanning transmission electron microscopy [5]. Single-site Ir atoms doped with nitrogen-doped carbon were active in the decomposition of formic acid [6]. Moreover, Ir dimers have been studied in detail: Ir dimers (pair-sites) prepared with Ir 2 (μ-OMe) 2 (COD) 2 were found to be more active and resistant to CO poisoning than analogous single-site catalysts for ethylene hydrogenation [7]. In addition to the monomer and dimer species, Ir clusters supported on MgO have been extensively studied [8]. Most atomically dispersed or cluster-like Ir species have been prepared using Ir complexes such as Ir 4 (CO) 12 and Ir(CO) 2 (acac) [9,10]. However, it is desirable to obtain well-dispersed Ir species using metal Ir precursors in order to reveal direct interaction between metal species and MgO. For this purpose, Ir metal nanoparticles (NPs) were loaded onto MgO here. To monitor the formation of Ir-MgO solid solutions, polyvinylpyrrolidone (PVP) polymer-protected Ir-NPs were employed as the precursor for Ir-MgO in this study. Polymer-protected NPs are advantageous due to their narrow size distribution [11]. Commercially available PVP-protected Ir-NPs were used as precursors to prepare supported Ir samples. Ir(OAc) 3 was employed as another Ir precursor to reveal 1 3 the influence of the precursor type on the formation process of Ir-MgO solid solutions. The MgO supports loaded with Ir-NPs and Ir(OAc) 3 are denoted as Ir-NP/MgO and Ir(OAc) 3 /MgO, respectively. The electronic state and local structure of Ir in these samples were primarily analyzed using X-ray adsorption spectroscopy (XAS) coupled with other technique [12].

Sample Preparation
The suspension of PVP-protected Ir-NPs dispersion (Renaissance Energy Research Co.) was diluted with 2-propanol/water [2-propanol: water = 2: 8 (vol.)] and mixed with MgO (JRC-MGO-4 500A, Catalysis Society of Japan), followed by evaporation on an evaporator maintained at 313 K. The obtained powder sample was thermally treated in air at 573-1273 K for 3 h. The PVP-protected Ir-NPs were also loaded on Al 2 O 3 (JRC-ALO-7, Catalysis Society of Japan) and SiO 2 (Q-10, Fuji Silysia Co.) using the same procedure. The initial loading of Ir was 1 wt% when the Ir-NPs were loaded on MgO, Al 2 O 3 , and SiO 2 . Ir(OAc) 3 (Wako Chemical Co.) was impregnated on MgO from an Ir(OAc) 3 aqueous solution in hot (boiling) water. The Ir loading of Ir(OAc) 3 / MgO was 2-4 wt%. The obtained powder was thermally treated in air in the same manner as that used for preparing Ir-NP/MgO. The nominal (initial) loadings of Ir in Ir(OAc) 3 / MgO was 2 wt% unless otherwise stated.

Ir L 3 -Edge EXAFS Measurements and Analyses
The Ir L 3 -edge EXAFS data of the Ir-NPs-and Ir(OAc) 3loaded supports were collected using synchrotron radiation. XAS data were recorded at the PF-9C beamline with the approval of the Photon Factory of the High Energy Accelerator Research Organization (KEK-PF). The data was collected in quick scan mode within 5 min using an Si (111) monochromator. The beam size was 2.0 mm (horizontal) × 0.8 mm (vertical). The Ir L 3 -edge extended X-ray absorption fine structure (EXAFS) was analyzed by extracting oscillations using a spline smoothing method. The Fourier transform (FT) of the k 3 -weighted EXAFS oscillations and k 3 χ(k) from k-space to r-space was conducted in the range of 3-13/Å for curve-fitting analysis. The EXAFS data were analyzed using the REX software (Rigaku Co.). The parameters for the analysis of the Ir-O bond was extracted from Ir L 3 -edge EXAFS of IrO 2 according to the literature [13]. Those for the analysis of the Ir-Ir bond was extracted from Ir L 3 -edge EXAFS of metal Ir powder. For the analysis of the Ir-Mg bond, parameters were obtained using the FEFF8.0 code [14].

Sample Characterizations Other Than XAFS Technique
X-ray photoelectron spectroscopy (XPS) data were collected on a JEOL JPS-9030 spectrometer with a Mg Kα emission line (hν = 1253.6 eV). Sample charge compensation was controlled by referencing the C 1 s line at 284.8 eV. Transmission electron microscopy (TEM) images of the Ir-MgO samples were obtained using a JEM-2100 microscope (JEOL Co.). The operating voltage was 200 kV. The sample was prepared after dropping an ethanol suspension of the sample onto a Cu grid coated with a C-coated porous membrane followed by dryness. X-ray diffraction (XRD) patterns of the powder samples were obtained using a MiniFlex X-ray diffractometer (Rigaku Co.) using Cu Kα radiation in the 2θ range of 20° to 90° with 10°/min scanning speed. N 2 adsorption isotherms were recorded on a BELSORP-mini-X (Microtrac Bel Co.) instrument. The samples were dehydrated in vacuum at 573 K prior to the measurements. Thermogravimetry-differential thermal analysis (TG-DTA) data were collected using a DTG-60 analyzer (Shimadzu Co.) at a heating rate of 10 K/min under an air flow. Temperatureprogrammed reduction of the samples with H 2 (H 2 -TPR) was performed using BELCATII (Microtrac Bel Co.) instrument using 5% H 2 /Ar (50 mL/min flow rate) without pretreatment. The samples were heated at a rate of 10 K/min from 300 to 923 K using a thermocouple detector (TCD) to monitor the H 2 concentration in the flowing gas. The dispersion of Ir on the supports was measured using BELCATII equipment (Microtrac BEL Co.). The samples were treated with H 2 at 773 K for 1 h prior to the measurement. The dispersion value of Ir was measured via CO adsorption at 323 K using a TCD detector. The dispersion value was calculated assuming CO/ Ir ratio of 1.0.  [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  Fig. S2a). This distance was slightly shorter than that of the second neighboring Ir-O bond (3.69 Å). Figure 1b shows the EXAFS-FT analyses of Ir-NP/MgO treated at 573-1273 K, unloaded Ir-NP, and the Ir powder. The corresponding k 3 χ(k) data are shown in Fig. S1b. 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, the new peak assignable to the Ir-O bond 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.    [16]. This is because Ir 3+ , which has a higher valence than Mg 2+ , attracted the O 2− anion, leading to the shrinkage of the Ir 3+ -O 2− bond in the Ir-MgO solid solution. Figure 1c shows the EXAFS-FTs of the as-prepared sample and Ir(OAc) 3 /MgO treated at 573-1273 K. The corresponding k 3 χ(k) data are provided in Fig. S1c. In the EXAFS-pristine Ir(OAc) 3 , the single peak assignable to the 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. Formation of Ir-MgO solid solutions proceeded via Ir oxide at 673 K, which was evidenced by the appearance of Ir-Ir bond (oxide) at the temperature. The feature of Ir(OAc) 3 /MgO treated at 773 − 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 curvefitting data for Ir(OAc) 3 /MgO treated at 1073 K are included in Fig. S2b 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 2.08 and 2.98 Å, respectively. The temperature-dependent changes in the CNs shown in Fig. 2b were similar to those of Ir-NP/MgO (Fig. 2a). The Ir-O and Ir-Mg bond distances of the solid solution of Ir(OAc) 3 /MgO (Fig. 3b) were consistent with those of Ir-NP/MgO (Fig. 3a).

Ir L 3 -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 L 3 -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 L 3 -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 (Ir 0 powder, Ir 2 O 3 , and IrO 2 ). 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 L 3 -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 Ir 2 O 3 (11,219 eV), indicating that the valence of Ir cations in Ir-NP/MgO was 3 +, whereas the peak energies of Ir-NP/Al 2 O 3 and Ir-NP/SiO 2 agreed with that of IrO 2 (Fig. S3). However, the intensity of white line observed in Ir-NP/MgO was larger than that of Ir 2 O 3 . In general, the white line intensity of L 3 -edge XANES is related to the density of unoccupied d-orbital. Nevertheless, the shape and intensity of L 3 -edge XANES vary with the degree of covalent and ionic nature of Ir [17]. It was likely that the strong ionicity and Oh symmetry around the Ir cation caused the enhancement of the white line intensity. This was because such an enhancement of white line intensity in comparison with corresponding simple oxides was commonly observed not only in Ir-MgO, but also Pt  (Fig. S4). The ionic radius of Ir 3+ has been reported to be 0.68 Å, which is comparable to that of hexacoordinated Mg 2+ (0.720 Å) [19]. It could be assumed that the close value of the ionic radii between Mg 2+ and Ir 3+ resulted in the formation of an Ir 3+ -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 Ir 4+ , the difference in the ionic radii was 13%, while that of Ir 4+ was 0.625 Å. This larger difference of ionic radii between Mg 2+ and Ir 4+ may be the reason why Ir 4+ was not included in the solid solution. Apart from the edge energy, the characteristic peak appeared in the Ir L 3 -edge XANES profile at 11,228 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 Ir 0 powder, Ir 2 O 3 , and IrO 2 . This difference suggested that the characteristic peak arises from the interference of photoelectrons caused by Mg 2+ ions neighboring Ir 3+ ions. It should be noted that the intensity of the white line of Ir-NP/MgO was higher than that of Ir 2 O 3 , probably because the Ir-MgO solid solution in which Ir 3+ was coordinated with six O 2− anions had high ionicity [22]. Due to the similarity of the phenomena observed in Ir-MgO and Pt-MgO, we speculated that the charge compensation mechanism for Ir 3+ is a structural defect, which was proved by DFT calculations of the Pt-MgO solid solution [16]. Figure 5 shows the XPS spectra of Ir-NP/MgO and Ir(OAc) 3 /MgO treated at 1073 K. XPS of Ir-NP/MgO and Ir(OAc) 3 /MgO showed peaks at 63.7 eV and 66.5 eV, which could be assigned to the Ir 4f 7/2 and 4f 5/2 respectively. The binding energies of these peaks were close to those of Ir(NH 3 ) 6 Cl 3 (Ir: 3 +), whereas the spectra of IrO 2 (Ir: 4 +) shifted the peak energies by − 3.7 eV. The XPS data showed that the valence of Ir in the Ir-MgO solid solution is 3 +, as supported by the XANES data.

Ir Loading
The Ir loading of these samples is plotted as a function of the treatment temperature in Fig. 6. The loading was measured based on the edge jump of the Ir L 3 -edge XANES and the sample weight using reference samples. The reason for the small increase in the Ir loading up to 973 K may be attributed to the decrease in weight of the support (MgO), which was observed after the large weight loss between 580 and 710 K in the TG curve (Fig. S5). The gradual weight loss may be caused by the dehydration of Mg-OH groups present in MgO. Accompanied by an increase in temperature from 1073 to 1273 K, the Ir loading in the Ir-NPs-loaded samples gradually decreased. A marked decrease in the Ir loading was particularly observed in Ir-NP/Al 2   (ca. 0.5 wt% at 1273 K), probably owing to the sublimation of the generated IrO 2 from the surface of these oxides [23]. The extent of the decrease in the Ir loading was suppressed in the MgO-supported Ir-NPs and Ir(OAc) 3 samples, probably due to the formation of solid solutions, as was described already. Figure 7 shows 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. 7a), 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. 7b). Ir particles were not observed in the TEM image of Ir-NP/MgO treated at 1273 K (Fig. 7c). 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 L 3 -edge EXAFS analysis (Fig. 1b). Figure 8a shows the expanded XRD patterns of Ir-NP/ MgO treated at different temperatures. Original images of the XRD patterns are displayed in Fig. S6. 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 Ir 2 O 3 , IrO 2 , nor metallic Ir were found [24]. This feature is different from that of Ir-NP/Al 2 O 3 and Ir-NP/ SiO 2 treated at 1073 K, in which intense diffraction assignable to IrO 2 appeared, as shown in Fig. 8b. The mean crystalline sizes of IrO 2 in Ir-NP/Al 2 O 3 and Ir-NP/SiO 2 were estimated to be 30 and 20 nm, respectively. The values were calculated based the diffraction appeared at 34.8° (the (101) facet) using the Scherrer equation. 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 IrO 2 appeared in the 4.6 wt%-Ir samples (Fig. S7), indicating that Ir was well dispersed in MgO up to 3.0 wt%-Ir in MgO. Figure 9a 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 Mg 2+ ions in MgO with two Ir 3+ 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 asprepared Ir(OAc) 3 /MgO, the peaks assignable to the mixture of Mg(OH) 2 and MgO were observed (Fig. S8). 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. 9b. 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.

Nitrogen Adsorption Isotherms
The N 2 adsorption isotherms of MgO and boiling watertreated MgO are displayed in Fig. S9. The boiling watertreated MgO showed much higher isotherms than those of untreated MgO owing to the formation of Mg(OH) 2 [26]. The N 2 adsorption isotherms of Ir-NP/MgO and Ir(OAc) 3 / MgO are displayed in Figs. S10 and S11. The specific surface areas and pore volumes were calculated from the N 2 adsorption isotherms, and the data were plotted against the treatment temperature in Fig. 10. 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 Ir 3+ 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. Figure 11 shows the TG-DTA curves of unloaded and Irloaded 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. Figure 12a presents the H 2 -TPR plots of Ir-NPs loaded on Al 2 O 3 , SiO 2 , and MgO, and Ir(OAc) 3 /MgO thermally treated at 1073 K, together with that of pristine IrO 2 . An H 2 consumption peak appeared at 570 K in the H 2 -TPR of IrO 2 , which could be attributed to the reduction of Ir 4+ to Ir 0 . A similar profile was reported in the H 2 -TPR of rutile-type IrO 2 [27]. The H 2 -TPR profiles of Ir-NPs loaded on Al 2 O 3 and SiO 2 were similar to that of IrO 2 , in which the formation of IrO 2 was observed in the Ir L 3 -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-NP/Al 2 O 3 and Ir-NP/SiO 2 . 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 H 2 -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  (Fig. 12b). The curve showed a U shape centered at 973 K. The change suggested reduction of the Ir cation was suppressed in the samples treated around 973 K because Ir 3+ located deep inside the MgO, while Ir located close to the surface of MgO in the sample treated at lower (< 873 K) and higher (1273 K) temperatures, so that the reduction of Ir cations were promoted to give Ir 0 in the corresponding temperature ranges. Figure 13 shows

Conclusions
The structural change in an Ir-MgO solid solution depending on the thermal treatment temperature was analyzed in detail using Ir L 3 -edg EXAFS together with other techniques. Formation of a stable Ir-MgO solid solution was observed when Ir-NPs and Ir(OAc) 3 were employed as the Ir precursors after thermal treatment. The formation of the solid solution progressed up to 873 K, whereas the segregation of the solid solutions gradually occurred up to 1273 K in both cases. In the case of 4 wt%-Ir(OAc) 3 /MgO, an enhancement of the high surface area pore volume was observed when the sample was treated at 773 K. The behavior of Ir indicated that the location of Ir and physical properties of the solid solution changes depending on the thermal treatment temperature and preparation conditions. The dissolution in MgO and segregation behaviors of Ir found here offer novel means to obtain atomically dispersed Ir-supported catalysts.