Temperature-dependent XANES spectra. For this study, Pt nanoparticles were synthesized on titania-incorporated fumed silica (Ti-FS) supports with and without H2O2 treatment [9,10], as summarized in Fig. 1. The size and the distribution of Pt nanoparticles were examined by energy dispersive spectroscopy (EDS) and TEM measurements, as shown in Fig. 2. X-ray absorption near edge structure (XANES) is sensitive to the chemical valence state as well as the geometry of the nearest neighboring atoms around a probing atom [37,38]. Temperature-dependent XANES at the Pt L3 edge shows a dramatic change in the white line intensity during heating, as shown in Figs. 3 (a) and (b). Changes in the Pt white line are known to be directly related to Pt oxidation [11,39,40]. Jeong et al. clarified that the white line area of Pt L3 edge directly corresponds to the coordination number of oxygen atoms bonding to Pt atoms . When a Pt atom bonds with oxygen atoms, the empty state density of the Pt 5d orbitals increases because the electrons in the Pt 5d orbitals transfer to the oxygen atoms. The strong intensity of the white line and the slight shift toward a higher energy of the Pt absorption edge of both Pt/Ti-FS and H2O2-Pt/Ti-FS at the RT indicate a high oxidation of Pt atoms compared to those of a Pt foil, as shown in Figs. 3 (a) and (b). The white line features strongly suggest that the Pt atoms on Ti-FS supports with no matter of the H2O2 treatment at RT are oxidized with PtOx. The intensity of the Pt white line of H2O2-Pt/Ti-FS at the RT is considerably stronger than that of Pt/Ti-FS at RT, as shown in Figs. 3 (a) and (b), respectively. This indicates that the mean oxidation state of Pt atoms of H2O2-Pt/Ti-FS is higher than that of Pt atoms of Pt/Ti-FS, which is attributed to the H2O2 treatment. When heated up to 250℃, the intensities of the white lines of both Pt/Ti-FS and H2O2-Pt/Ti-FS decrease dramatically, and the absorption edges shift toward a lower energy, which is the nearly same as the absorption edge of a Pt foil at RT. The temperature-dependent behavior of the white line strongly implies that the Pt atoms of both Pt/Ti-FS and H2O2-Pt/Ti-FS are rapidly reduced in an H2 environment during heating, and that most of the oxygen atoms dissociate from the Pt nanoparticles at 250oC.
The chemical valence state and the local structural properties around the Ti atoms of Pt/Ti-FS and H2O2-Pt/Ti-FS supports are also changed with those around the Pt atoms during the synthesis and heating processes. A change around the Ti atoms of Ti-FS supports is crucial for understanding the dispersion mechanism of Pt nanoparticles on the supports. Figures 3 (c) A – C show XANES spectra of Ti-FS supports at the Ti K edge during the temperature change of RT → 500oC → RTc in an H2 environment. The XANES spectra of the Ti-FS supports at RT and RTc are nearly identical, implying a lack of changes in the local structural and chemical properties around the Ti atoms of Ti-FS supports for the calcining process. The XANES of H2O2-Ti-FS reveals that H2O2 treatment significantly affects the local structural and chemical properties around the Ti atoms of Ti-FS, as shown in Fig. 3 (c) D. The increased intensity of the white line in Fig. 3 (c) D indicates that an H2O2 treatment affects the oxidation of the Ti atoms of the supports. To elucidate the dispersion mechanism of Pt nanoparticles on Ti-FS supports, it is important to have a direct comparison of the XANES spectra of Pt/Ti-FS with and without H2O2 treatment at the Ti K edge as well as at the Pt L3 edge. Figures 3 (c) E–G and H–J, respectively, show the XANES spectra of Pt/Ti-FS without and with H2O2 treatment at the Ti K edge. The XANES spectra of Pt/Ti-FS and H2O2-Pt/Ti-FS are nearly identical to each other while being substantially different from those of Ti-FS and H2O2-Ti-FS, as shown in Figs. 3 (c) A–J. This finding strongly suggests that the local structural and chemical properties around the Ti atoms of Ti-FS supports are significantly affected by a Pt impregnating process during the synthesis of Pt/Ti-FS. The XANES spectra show that the local structure around the Ti atoms of Pt/Ti-FS and H2O2-Pt/Ti-FS is close to an anatase TiO2. Pt atoms might serve as a catalyst to the Ti atoms, as a TiO2 was formed when tetra amine platinum nitrate was applied to Ti-FS during the synthesis. The structural properties around the probing Pt and Ti atoms of Pt/Ti-FS systems at the Pt L3 and Ti K edges, respectively, can be more clearly seen in more detail in extended XAFS (EXAFS), which shows small oscillations above the absorption edges.
Temperature-dependent local structural properties. EXAFS can be used to quantitatively determine the local structural properties around a selected species atom of compounds [41-43]. After atomic background function was determined using the AUTOBK code , EXAFS data was extracted from XAFS and Fourier transformed to the r-space, as shown in Fig. 4. EXAFS was quantitatively analyzed with the IFEFFIT package  using standard analysis procedures [43,46]. The peak positions of EXAFS data correspond to the atomic shell distances from a probing atom. The peak positions of the EXAFS data are approximately 0.3 Å shorter than the true distances of atomic pairs because the phase shift of back-scattered photoelectrons by neighboring atoms has not yet to be considered. The temperature-dependent EXAFS of Pt/Ti-FS with and without H2O2 treatment reveals that the local structure around Pt atoms is significantly changed by heating, as shown in Figs. 4 (a) and (b). At RT the first and second peaks of ~1.7 Å and 2.2 Å respectively correspond to O and Pt atoms . The first peak intensity gradually weakens during heating, and it virtually disappears at 250oC. This indicates that Pt atoms initially bond with oxygen atoms as well as Pt atoms, and that there is a lack of Pt-O bonds at high temperatures. After being heated up to 500oC and cooled down to RT (RTc), the local structures around the Pt atoms of both Pt/Ti-FS and H2O2 Pt/Ti-FS at RTc are nearly identical to those above 250oC, strongly implying that Pt nanoparticles have a stable structure due to the calcining process. This result is consistent with the XANES measurements of Pt/Ti-FS and H2O2 Pt/Ti-FS. EXAFSs at the Pt L3 edge of both Pt/Ti-FS and H2O2 Pt/Ti-FS show prominent peaks at approximately 2.2 Å and 2.8 Å when heated above 200oC; these peaks are expected to be Pt-Pt pairs. Above 200oC, the lack of any change in the temperature-dependent EXAFS of Pt-Pt pairs strongly implies a stable structure of Pt atoms in particular in H2O2-Pt/Ti-FS. EXAFS measurements of H2O2-Pt/Ti-FS at the Pt L3 edge reveal that oxygen atoms wrapping the tops of Pt nanoparticles are mostly dissociated, while Pt atoms form into stable Pt nanoparticles, tightly bonded to the Ti-FS supports, when heated up to 250℃.
EXAFS of Ti-FS at the Ti K-edge shows the first and second peaks at 1.7 Å and 2.5 Å, respectively, as shown in Figs. 4 (c) A and C. These peaks respectively correspond to the Ti-O and Ti-Ti pairs of a TiO2 structure [46,47]. The distance of the Ti-Ti pairs is expanded due to the H2O2 treatment, as shown in Fig. 4 (c) D. The EXAFS of Ti-FS at the Ti K edge indicates that the TiO2 of the Ti-FS has a quite unstable structure, because the peak positions and shapes of EXAFS substantially depend on the H2O2 treatment. The local structure around the Ti atoms of Pt/Ti-FS is considerably different from that before the Pt precursor is impregnated on Ti-FS, as shown in Fig. 4 (c). EXAFS of H2O2-Pt/Ti-FS shown in Figs. 4 (c) H–J clearly shows three peaks at ~1.5 Å, ~2.3 Å, and ~3.2 Å. The local structure around the Ti atoms of Pt/Ti-FS is substantially different from that of H2O2-Pt/Ti-FS, particularly at 250oC, as shown in Figs. 4 (c) G and J. The EXAFS peak positions of H2O2-Pt/Ti-FS at the Ti K edge show a lack of changes, whereas they exhibit some changes in Pt/Ti-FS during heating from RT to 250oC. This finding strongly suggests that the local structure around the Ti atoms of H2O2-Pt/Ti-FS is quite stable and analogous to an anatase TiO2. The quantitative local structural properties can be obtained by fitting the EXAFS data to the EXAFS theoretical calculations using a structural model .
Quantitative analysis of local structural properties. Using the standard fitting procedures, EXAFS data in the r-space were fitted to EXAFS theoretical calculations with different structural models at the Pt L3 and Ti K edges [43,46]. The structural models of EXAFS theoretical calculations were designed based on the measured XANES and EXAFS data. PtOx and Pt foil structures were initially modeled for the low and high temperatures of both Pt/Ti-FS and H2O2-Pt/Ti-FS, respectively. At intermediate temperatures, a mixture structure of PtOx and Pt foil was used to fit EXAFS data. For the EXAFS data fitting of the Ti K edge, structural models were selected based on the XANES and EXAFS data of Ti-FS and Pt/Ti-FS. In the fitting of the EXAFS data of H2O2-Pt/Ti-FS, a distorted-anatase TiO2 structure was used for the EXAFS theoretical calculations. EXAFS theoretical calculations were done using the FEFF8 code , and EXAFS data was fitted using the IFEFFIT package . In the fittings, the distance, the coordination number, and the Debye-Waller factors ( , including thermal vibration and static disorder) of each atomic shell were varied. Only single-scattered paths were included in the fittings; this decision was made because, due to the particle size and the structural disorder, the EXAFS signal of a multiple-scattered path of nanoparticles is much weaker than that of a single-scattered path. A k-weight fit was used to reduce the correlation between and coordination number .
Figure 5 shows representative EXAFS data and the best fits. The quantitative structural properties around the Pt and Ti atoms of Pt/Ti-FS and H2O2-Pt/Ti-FS were obtained from the goodness fits of the EXAFS data. The results of the best fits are summarized in Tables 1–3. EXAFS at the Pt L3 edge reveals that a Pt atom of Pt/Ti-FS and H2O2-Pt/Ti-FS initially bonds with approximately five and six oxygen atoms, respectively. Pt atoms of Pt/Ti-FS initially have the second and third neighbors of ten Pt atoms at the respective distances of ~2.8 Å and ~3.1 Å. This indicates that Pt atoms are partially oxidized and have Pt-Pt bonds at RT. When heated above 250oC, the Pt atoms of both Pt/Ti-FS and H2O2-Pt/Ti-FS form into Pt nanoparticles with a substantial amount of structural disorder, compared to that of the Pt foil. When H2O2 treatment is applied on Pt/Ti-FS at RT, Pt atoms have the first and second neighbors of six O and eight Pt atoms at the respective distances of ~2.0 Å and ~3.28 Å. Due to the H2O2 treatment, the distance of Pt-Pt pairs is elongated and the coordination number of oxygen atoms around Pt atoms is increased, compared to those of Pt/Ti-FS. This finding indicates that, due to the H2O2 treatment, oxygen atoms penetrate into Pt nanostructures, thus cracking Pt-Pt bonds at RT. The O atoms between Pt atoms play a decisive role in a high dispersion of Pt nanoparticles; meanwhile, their existence at the interface of Pt nanoparticles and Ti-FS supports assists a strong bond between Pt atoms and TiO2 supports at high temperatures. When heated above 250oC, the coordination number of Pt atoms of Pt/Ti-FS is gradually increased to be ~10 at 500oC, whereas that of H2O2-Pt/Ti-FS shows a lack of changes in the temperature range of 250 – 500oC. This result strongly implies that, at high temperatures, the Pt atoms of Pt/Ti-FS and H2O2-Pt/Ti-FS move to become lumpy and are pinned on the supports, respectively. A large σ2 value of the Pt-Pt pairs of Pt/Ti-FS indicates a less stable structure of Pt nanoparticles than that of H2O2-Pt/Ti-FS, particularly at RTc. When cooled down to RT from 500oC, the local structural properties around Pt atoms of both Pt/Ti-FS and H2O2-Pt/Ti-FS are nearly the same as those at 500oC, except for the σ2 values of Pt-Pt pairs due to the thermal effect. Jeong and co-workers demonstrated using EXAFS measurements that the Pt nanoparticles of H2O2-Pt/Ti-FS likely have a pancake shape on TiO2 supports . In this case, Pt nanoparticles stably bond to the supports with a high catalysis efficiency. At RTc, the bond lengths of the Pt-Pt pairs of both Pt/Ti-FS and H2O2-Pt/Ti-FS are shorter than that of a Pt foil due to the effects of nanoparticle boundaries with dangling bonds .
The EXAFS data of Ti-FS and Pt/Ti-FS with and without H2O2 treatment at the Ti K edge were also analyzed in the same manner as the EXAFS data analysis at the Pt L3 edge, and the best fit results are summarized in Table 3. The best fit of EXAFS data at the Ti K edge suggests that the Ti atoms of Ti-FS form Ti-O complexes in the temperature range of RT – 250oC. When an H2O2 treatment is applied on Ti-FS, the local structures around Ti atoms are substantially changed; however, they do not form into a crystalline structure. The EXAFS data of Pt/Ti-FS and H2O2-Pt/Ti-FS at the Ti K edge shows the Ti-O(1), Ti-O(2), Ti-Ti(1), and Ti-Ti(2) pairs. This means that the Ti atoms of Pt/Ti-FS and H2O2-Pt/Ti-FS form a TiO2 crystalline structure, although there is still a substantial amount of structural distortion in the atomic pairs. In an anatase TiO2, a Ti atom consists of a Ti-O octahedron with six O atoms. The slightly low coordination of O atoms of Pt/Ti-FS and H2O2-Pt/Ti-FS indicates the existence of some vacancies on the O sites of the octahedrons. The coordination numbers of both the O and Ti atoms of the Pt/Ti-FS and H2O2-Pt/Ti-FS remain constant during heating. For the Pt/Ti-FS, the low coordination of Ti atoms and the large σ2 value of Ti-Ti pairs, particularly at 250oC, indicate an incomplete TiO2. It is worth noting that σ2 values can be gradually increased at temperatures above 100 K due to the thermal vibration of atomic pairs [37,46]. Compared to that at RT, the σ2 value of the Ti-Ti pairs of H2O2-Pt/Ti-FS decreased when heated up to 250oC. This finding strongly suggests that the TiO2 particles of H2O2-Pt/Ti-FS form into a more stable crystalline structure at 250oC. The distance change of the Ti-O and Ti-Ti pairs of H2O2-Pt/Ti-FS serves as further evidence of TiO2 crystallization at 250oC, compared to that at RT. The EXAFS measurements at the Ti K edge reveal that the Ti atoms form a distorted-anatase TiO2 in H2O2-Pt/Ti-FS, whereas the Ti atoms do not form a stable crystalline structure in Pt/Ti-FS. The in-situ EXAFS measurements indicate that the Pt precursor and the H2O2 treatment in succession assist the Ti atoms of Ti-FS in forming into a crystalline structure.