TiO2 Catalyst Reduced

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High Resolution HAADF Characterization of Ir/TiO Catalyst Reduced at 500 C: Intensity Profile Analysis °

Orlando Hernández-Cristóbal, Jesús A. Arenas-Alatorre, Gabriela Díaz, Daniel Bahena Uribe, and Miguel José-Yacamán J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01923 • Publication Date (Web): 30 Apr 2015 Downloaded from http://pubs.acs.org on May 1, 2015

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The Journal of Physical Chemistry

High Resolution HAADF Characterization of Ir/TiO2 Catalyst Reduced at 500°C: Intensity Profile Analysis

Orlando Hernández-Cristóbal1,2, Jesús Arenas-Alatorre1*, Gabriela Díaz1*, Daniel Bahena3# and Miguel J. Yacamán3

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Instituto de Física, Universidad Nacional Autónoma de México, Circuito de la

Investigación Científica s/n, Cd. Universitaria, México, DF 04510, México.

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Escuela Nacional de Estudios Superiores, Universidad Nacional Autónoma de México.

Antigua carretera a Pátzcuaro 8701 Colonia San José de la Huerta, Morelia, Michoacán, 58089, México.

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Physics and Astronomy Department, University of Texas at San Antonio, Texas USA.

# Current Address: LANE-CINVESTAV, Av. IPN 2508, México D.F. 07360, México.

ACS Paragon Plus Environment

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*Corresponding authors:

Dr. Jesús Arenas-Alatorre Phone: +525556225193 Fax: +525556225008 [email protected]

Dr. Gabriela Díaz Phone: +525556225097 Fax: +525556225008 [email protected]


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ABSTRACT A comparison is presented between experimental and simulated High Resolution-High Angle Annular Dark Field images (HR-HAADF) of Ir/TiO2 catalyst reduced at 500°C where classical strong metal-support interaction develops. The analysis is based on integrated intensity profiles of the atomic columns of iridium nanoparticles on the TiO2 support. The results revealed as expected, that when Ir/TiO2 catalyst is reduced at high temperature, partially reduced species from the support (TiOx) migrate to the surface of iridium nanoparticles. Two very important facts can be highlighted; one is that the Ti atoms sited on atomic columns in the [1 1 0] direction preserve the atomic ordering of the metallic particle (epitaxial growth) while those located in atomic columns at perpendicular directions to the observation axis are disordered and as a consequence, they appeared as an amorphous thin film. Finally, changes in the (111) interplanar spacing of TiO2-rutile structure could be due to an enrichment of Ti3+ species at the rutile surface.

KEYWORDS:

Iridium, TiO2, Strong Metal-Support Interaction, HR-HAADF, Intensity

Profile, AC-STEM


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Introduction Supported metal catalysts are constituted of metal nanoparticles dispersed on the surface of an oxide support. They are important for many technological applications, from oil refining to environmental protection. Metal-support interactions are key factors determining the properties and performance of supported catalysts. The term, strong metal-support interaction (SMSI) coined by Tauster et al.1 in 1978, was used to describe the modifications in the adsorption properties of noble metals of group VIII supported on TiO2, when submitted to high temperature reduction. The ability to chemisorb H2 or CO was strongly inhibited or vanished entirely when these metals were supported on TiO2 and activated at 500°C. As particle size was about the same, this feature could not be ascribed to a simple decrease of adsorption sites due to sintering. One of the models proposed to explain the behavior of the catalyst is geometrical in nature, where TiOx moieties issued from partial reduction of the support migrate to the surface of the metal particle. Such structural modifications affect parameters as the availability of adsorption sites and consequently modification in the catalytic properties of supported catalysts. Spectroscopic technics such as X-ray photoelectron spectroscopy (XPS); X-ray excited Auger electron spectroscopy (XAES), secondary ion mass spectrometry (SIMS) and ion scattering spectroscopy (ISS) have been used to study catalysts at this particular state. Pesty et al.2 by XPS analysis identified Tin+ (1≤n≤3) species over Pt nanoparticles with an increase of Ti3+ species at the surface of the support. Aizawa et al.3 mentioned that the encapsulation process and the enrichment of Ti3+ in surface could be two key features to show the strong metal-support interaction.


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Concerning electron microscopy techniques, until now most of the HRTEM observations are limited to show an amorphous thin film decorating the metal particles as in Rh, Ru and Ag nanoparticles supported on TiO2.4-6 The appearance of this thin film is commonly associated with reduction at high temperature, however reports indicate that at temperatures as low as 200 °C this phenomenon initiates in the case of Ag/TiO2.7 These HRTEM observations are very important; however this technique is not able to reveal information about the atomic species contained in atomic columns.

Recently,

Transmission Electron Microscopy in the STEM mode combined with sub-Angstrom resolution (STEM-AC) has been a useful tool in the detailed study of the structure of nanoparticles. Due to the fact that the technique is based on the elastic dispersed electrons (Rutherford dispersion) collected in a High Angle Annular Dark Field detector (HAADF), the intensity in HAADF images varies approximately proportional to Z1.7 (Z atomic number), so the intensity is greater in the case of heavy atoms.8,9 In this kind of images there is a linear dependence of intensity with the number of atomic columns (i.e. the size of a cluster).10 Therefore, the use of integrated intensity profiles in HR-HAADF images is an ideal method to quantify atomic columns in nanoparticles of catalysts which contain few atoms. In addition, simulation of STEM-HAADF images is important to compare with experimental data. Due to the importance of metal-support interactions in heterogeneous catalysis, it is relevant to develop analysis methodologies to understand the structure of supported metal nanoparticles. From a structural point of view, at present time no focused analysis has been developed in this direction.


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A system with important catalytic properties as hydrogenation catalyst among others is iridium supported on titanium oxide (Ir/TiO2). It has been reported that Ir/TiO2 reduced at 500°C develops classical strong metal-support interaction.11,12 In this work, we show that STEM-HAADF images give significant information of the structure of nanoparticles. Analysis of the integrated intensities of iridium nanoparticles supported on TiO2 (Ir/TiO2) reduced in hydrogen at 500°C using STEM-HAADF images is presented. In this case, the contrast in STEM-HAADF images is given from the atomic number of the iridium (Z=77) and the atoms contained in the support, Ti(Z=22) and O (Z=8).

Experimental Section. Sample preparation. Ir/TiO2 catalyst (1 wt.% iridium nominal) was prepared by deposition–precipitation with urea (DPU) as reported in the literature.12 After the deposition–precipitation procedure, the sample was washed with deionized water, then centrifuged, dried under vacuum and finally calcined at 400°C. Reduction was performed at 500°C under hydrogen flow for 1h. The sample will be identified as Ir/TiO2 HTR. Characterization STEM images were acquired in a TEM JEM-2010 FEG and a TEM JEM-ARM 200 electron microscopes, both with an acceleration voltage of 200 kV. The morphology of the nanoparticles was characterized by transmission electron microscopy (TEM) and high


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resolution transmission electron microscopy operated at 200 kV with a 0.19 nm point resolution. The scanning transmission electron microscopy (STEM) images were recorded in a probe Cs-corrected Microscope operated at 200 kV. High angle annular dark field (HAADF) STEM images were obtained with a convergence angle of 26 mrad and the collection semi-angles from 50 to 180 mrad. These variations in semi-angles satisfy the conditions set forth for the detectors to eliminate contributions from unscattered and low-angle scattered electron beams. The probe size used was about 0.09 nm with the probe current of 22 pA. In addition, bright field (BF) STEM images were recorded by using a collection semi-angle of 11 mrad. Standard grid preparation for observation of the sample was employed. Simulation of STEM-HR images was done by using the linear approximation model (LAM) proposed by Kirkland

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implemented in the STEM_Cell software.14 The LAM is

based on the multislice theory which first takes a division of the nanoparticle in cuts perpendicular to the incident electron beam, and afterwards the contribution of each generated cross section cut is calculated. These values were compared with experimental results and possible atomic combinations present in the atomic columns of the nanoparticle were obtained. Using the software STEM_CELL, it is possible to simulate the intensity of atomic columns constituted by a variable number of atoms, of the same kind or different nature.

Results and discussion Figure 1 shows representative images of Ir/TiO2 sample reduced at 500°C. A homogeneous distribution of iridium particles is obtained and when observed in high


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resolution mode, Figure 1 (a), besides nanoparticles and clusters of few atoms, isolated iridium atoms are observed over the TiO2 support. Particle size histogram taking into account nanoparticles and clusters only, issued from HR-HAADF images (in the inset), gives an average particle size of 1.2 nm. Isolated deposited metal atoms have been reported in the Ir/MgO system by Aydin et al. ,15 where as a function of temperature in presence of hydrogen or under the influence of the electron beam they aggregate to form small Ir4 clusters and eventually stable nanoparticles of a critical size around 1 nm average size. At this stage no presence of isolated atoms was detected in the sample. This behavior was observed also for alumina and zeolite supported iridium catalysts. The intrinsic resistance to sintering of iridium already pointed out by Aydin et al. was corroborated, but in contrast, to their observations remaining isolated iridium atoms in our sample showed no tendency to aggregate under the electron beam indicating great stability. Single iridium atoms were reported also to be present in Ir/FeOx catalysts.16 This suggests that special anchoring sites are available on this type of supports which provide enhanced stability to single iridium atoms. As shown in Fig. 1 (b), HRTEM images revealed the presence of a thin layer (indicated by the arrow) covering the surface of the iridium particles. The adsorption properties of the sample reduced at 500 °C decreased dramatically indicating a classical strong metalsupport effect.12 Most of the nanoparticles had irregular shapes as observed in Figure 2, where three examples are shown. In Figure 2 (a) and (c), the particles are surrounded by species that have not the same ordering than that of the atoms at the center of the particle and in


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Figure 2(b) a particle with cubic geometry is shown. Intensity profiles of selected areas revealed important differences indicating that the atomic columns are composed by different number of atoms.

Figure 1. Typical electron microscopy images of Ir/TiO2 HTR catalyst. a) HR-HAADF and HAADF images with particle size histogram in the inset. The structure of the sample includes nanoparticles, small clusters and isolated iridium atoms on the surface of TiO2 support. b) HRTEM image showing a thin layer covering iridium nanoparticles.


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Figure 2. HR-HAADF images showing various iridium nanoparticles shapes (a), (b) and (c) present in the Ir/TiO2 HTR sample. Intensity profiles (d), (e) and (f) corresponding to the enclosed atomic rows above.

Simulation To carry out the study of the intensity of atomic columns, we will consider the effective intensity of an iridium atom, Ieff, which is defined as the difference between the intensity of a column of atoms that contains one iridium atom at the surface and the intensity of columns composed only by titanium atoms. The main purpose of the above is to remove the intensities contribution originated from the support; and considering that if it is feasible to get atomic resolution of a nanoparticle without superposition with the lattice of the support, then, structural information of the nanoparticle only, can be obtained. This can be achieved by considering the effective intensity of an isolated iridium atom near to the nanoparticle. To calculate relative


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intensities, the obtained values of intensity profiles related to each column were referred to Ieff giving rise to relative intensities Irel defined as:

(1)

Where n represents the row of atoms and x the number in the column.

Figure 3 shows a HR-HAADF image with atomic resolution. As observed in the FFT, the support was identified as rutile and the metal nanoparticle as iridium, both the support and the particle (enclosed by squares) are oriented in the 1 1 0] direction.

Figure 3. HR-HAADF image of Ir/TiO2 HTR catalyst. The FFTs show that the metal iridium nanoparticle and the TiO2-rutile crystalline phase are oriented in the 1 1 0] direction.


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The effective intensity of an iridium atom was calculated considering the selected area showed in Figure 4(a), where three atomic columns labelled as L1, C and L2 (enclosed in yellow) were chosen. As observed, in row C the central atomic column is more intense than the lateral ones. If an iridium atom (Z=77) occupies the site in the upper position of the central column, then the difference of intensities gives the effective intensity associated to one iridium atom. The calculated Ieff value was 38 (arbitrary units). To confirm the presence of one iridium atom in the central column, the intensity profile of the contiguous columns was considered (denoted by L1 and L2). As it could be observed in Figure 4(b), L1 and L2 rows have intensity profiles with the same intensities but different from that observed in row C.

Figure 4. Intensity profile comparison of atomic rows. (a) Three rows were chosen (L1, C, L2) where the central one shows in the middle a more intense column of atoms. (b) Comparison of intensity profiles of rows labeled L1, L2 and C.


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To develop the analysis, the atomic columns were labeled with the purpose of calculating the experimental relative intensity between columns and the effective intensity of the iridium atom, Figure 5. The maximum value of the calculated relative intensity was 5.05, indicating that the number of atoms contained in a column constituted only by iridium atoms is 5. Taking a maximum value of 6 atoms by column, 27 combinations can be generated, 12 for monometallic (only iridium or titanium atoms) and 15 for bimetallic (iridium + titanium atoms). Standard deviation of the error calculated from the difference between experimental and simulated intensity profiles values is 0.07. Based on HAADF intensities of simulated atomic columns the relative intensities were calculated and results are shown in Table 1. Relative experimental intensities associated with atomic columns of the particle shown in Figure 4(b) are presented in Table 2. Comparing with simulated relative intensities (in brackets), we found that most of them can be described as atomic combinations of iridium and titanium atoms. To show this, consider Row D in the particle. Figure 6 shows the comparison between experimental and simulated intensities associated to this column. It can be observed a very good agreement between them, corroborating the mixed composition in the column (Ir + Ti). Now, taking into account the observed peripheral intensities (columns I-VIII), quantification indicates that some of them are composed only by Ti atoms. Figure 7 shows the comparison between the experimental and simulated image of the iridium nanoparticle (a, b).


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Figure 5. Filtered image (left) of the iridium nanoparticle shown in Figure 4(a) where labels a, b, c, d, e, f and I-VIII roman numerals indicate, respectively, atomic columns and peripheral atoms (right). The number identifies a particular column.

Table 1. Relative intensities from HAADF simulated images.


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Table 2. Experimental and simulated relative intensities of atomic columns and atomic compositions of rows designated with A-F and I-VIII Roman numerals in Figure 5.


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Figure 6. Comparison between experimental and simulated intensity values of Row D in Figure 5. (a) Experimental intensity profile, (b) atomic combination of columns extracted from Table 2, and (c) simulated intensity profile using the atomic combination related to Row D.

Figure 7. Experimental and simulated HR-HAADF of the iridium nanoparticle presented in Figure 5. (a) Experimental filtered image (b) simulated HR-HAADF image.


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Figure 8. HR-HAADF filtered image showing disordered Ti atoms over (220) and (111) faces of an iridium nanoparticle. Dashed line shows the epitaxial growth of the iridium nanoparticle along [110] direction of the support.

Two very important facts can be highlighted; first, the Ti atoms in atomic columns in the [1 1 0] direction preserve the atomic ordering (epitaxial growth) of the metallic particle, while those located in atomic columns in perpendicular directions to the observation axis are disordered and as a consequence they are observed as an amorphous thin film as shown in Figure 8. As it was mentioned above, the amorphous film is only observed in perpendicular directions to the electron beam, that is, in (111) and (220) crystalline faces. Datye et al.

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reported the same behavior in a Pt/TiO2 system. Pt particles were covered

with an amorphous film depending on the exposed crystalline face. Thus, over (111) crystalline planes, the TiOx film is ordered while on other surfaces, it is not. Thus in our case, it can be concluded that over (110) faces of iridium metal nanoparticles, TiOx species preserve the crystalline structure of the iridium particle and while those in (111) and (220) faces they are disordered.


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A 3-D atom density profile10,18 of Ir/TiO2 area shown in Figure 8 is presented in Figure 9; disordered Ti atoms are displayed in the periphery of the Ir nanoparticle (enclosed in circles).

Figure 9. Three-dimensional atom density profile of Ir/TiO2 reduced at 500°C.

The other important finding is the expansion of the (100) interplanar distance of TiO2rutile (0.324 nm) in regions near the iridium nanoparticle, where about 3% increase is observed (0.336 nm). Sun et al.19 observed a similar phenomenon in the case of Pd nanoparticles supported on CeO2/ZrO2 when reduced at high temperature (700°C). They argued that the surface enrichment in Ce3+ gives rise to the expansion of the lattice parameters. In the case of TiO2 based systems, in agreement with the description given by Xiong et al.,20 Ti3+ cation is localized in the nearest position to an oxygen vacancy Ov generated by the reduction treatment. On the other hand, a XPS study of the Ir/TiO2


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system reduced at 500ºC showed enrichment of Ti3+ species at the surface, which was an indicative of TiOx migration toward the surface or iridium nanoparticles.21 In our case, the TiO2 support reduced at 500°C would give rise to Ti3+ whose ionic radius is 0.067 nm, bigger than the ionic ratio of Ti4+ (0.061 nm). This fact hinders the diffusion of Ti3+ into the bulk generating a higher concentration of it in the surface, mechanism similar to that proposed by Sun et al. A question could arise about the position of oxygen atoms when the migration of TiOx species takes place. We addressed this point in a qualitative way by considering a support crystal identified as TiO2-rutile in the 1 1 0] direction and simulate the HRHAADF image of such crystal. In the HR-HAADF simulated image presented in Figure 10, intensities associated with oxygen atomic columns are not observed.

Figure 10.

Comparison between experimental and simulated HR-HAADF images of

TiO2-rutile in the 1 1 0] direction. (a) Experimental image, (b) Filtered image, (c) Simulated HR-HAADF image, and (d) Model of the TiO2-rutile crystal (Titanium atoms blue balls, oxygen atoms red balls).


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As it can be appreciated in the proposed model, oxygen atoms are located in columns alternating positions with Ti atoms and in between the columns containing it. When experimental and simulated image are compared, intensities related with columns composed only by oxygen atoms are not observed. This finding can be well understood, by taking in consideration the atomic number of oxygen (Z=8), extremely low compared with that of titanium (Z=22). Taking into account this result, it is not possible to determine the oxygen positions in the TiOx entities covering the iridium nanoparticles. However, by changing the arrangement of the HAADF detector to smaller angles, intensities of lighter atoms as oxygen could be observed.

Conclusions We used HR-HAADF images and the linear approximation model to analyze the structure of Ir/TiO2 catalyst reduced at 500°C. From the analysis based on integrated intensity profiles, two very important facts can be highlighted; one is that the Ti atoms sited in atomic columns in the [1 1 0] direction preserves the atomic ordering (epitaxial growth) of the metallic particle while those located in atomic columns in perpendicular directions to the observation axe are disordered and as a consequence they are observed as an amorphous thin film. On the other hand, the enrichment of Ti3+ species at the surface of TiO2-rutile generates an increase in the d100(TiO2-rutile) which is observed in regions near to iridium nanoparticles. This quantitative HR-HAADF analysis can be very useful to explain the modified catalytic properties of metal nanoparticles when supported on TiO2 and reduced at high temperature.


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Acknowledgments To LCM-IFUNAM for the electron microscopy images. OHC is grateful to CONACYT and Red Nano-CONACYT for financial support. This work was supported by grants from the National Center for Research Resources (5 G12RR013646-12) and the National Institute on Minority Health and Health Disparities (G12MD007591) from the National Institutes of Health and the Welch Foundation Agency (project AX-1615).

Supporting Information Figure 1. Verification of linearity between intensity and number of atoms of Ti or Ir in an atomic column using the STEM_CELL Code. Figure 2. Intensity profiles comparison of atomic columns in Row D of iridium particle shown in Figure 5.

This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information *Corresponding Authors Email: [email protected]

Email: [email protected]


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Rojas, H.; Borda, G.; Reyes, P.; Martínez, J.J.; Valencia, J.; Fierro, J.L.G. Citral Hydrogenation over Ir/TiO2 and Ir/TiO2/SiO2 Catalysts. Catal. Today 2008, 133– 135, 699–705.


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Caption for Figures Figure 1. Typical electron microscopy images of Ir/TiO2 HTR catalyst. a) HR-HAADF and HAADF images with particle size histogram in the inset. The structure of the sample includes nanoparticles, small clusters and isolated iridium atoms on the surface of TiO2 support. b) HRTEM image showing a thin layer covering iridium nanoparticles.

Figure 2. HR-HAADF images showing various iridium nanoparticles shapes (a), (b) and (c) present in the Ir/TiO2 HTR sample. Intensity profiles (d), (e) and (f) corresponding to the enclosed atomic rows above

Figure 3. HR-HAADF image of Ir/TiO2 HTR catalyst. The FFTs show that the metal iridium nanoparticle and the TiO2-rutile crystalline phase are oriented in the 1 1 0] direction.

Figure 4. Intensity profile comparison of atomic rows. (a) Three rows were chosen (L1, C, L2) where the central one shows in the middle a more intense column of atoms. (b) Comparison of intensity profiles of rows labeled L1, L2 and C.

Figure 5. Filtered image (left) of the iridium nanoparticle shown in Fig. 4(a) where labels a, b, c, d, e, f and I-VIII roman numerals indicate, respectively, atomic columns and peripheral atoms (right). The number identifies a particular column.

Figure 6. Comparison between experimental and simulated intensity values of Row D in Figure 5. (a) Experimental intensity profile, (b) atomic combination of columns extracted from Table 2, and (c) simulated intensity profile using the atomic combination related to Row D.


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Figure 7. Experimental and simulated HR-HAADF of the iridium nanoparticle presented in Figure 5. (a) Experimental filtered image, (b) simulated HR-HAADF image.

Figure 8. HR-HAADF filtered image showing disordered Ti atoms over (220) and (111) faces of an iridium nanoparticle. Dashed line shows the epitaxial growth of the iridium nanoparticle along [110] direction of the support.

Figure 9. Three-dimensional atom density profile of Ir/TiO2 reduced at 500°C. Figure 10.

Comparison between experimental and simulated HR-HAADF images of

TiO2-rutile in the 1 1 0] direction. (a) Experimental image, (b) filtered image, (c) Simulated HR-HAADF image, and (d) model of the TiO2-rutile crystal (Titanium atoms blue balls, oxygen atoms red balls).

List of Tables

Table 1. Relative intensities from HAADF simulated images.

Table 2. Experimental and simulated relative intensities of atomic columns and atomic compositions of rows designated with A-F and I-VIII Roman numerals in Figure 5.


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TOC Graphic


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Figure 1 140x93mm (265 x 265 DPI)


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Figure 2 106x84mm (242 x 242 DPI)



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Figure 3 128x90mm (241 x 241 DPI)


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Figure 5 85x42mm (300 x 300 DPI)


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Figure 10 148x41mm (300 x 300 DPI)



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Table 1 87x94mm (300 x 300 DPI)


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Table 2 136x127mm (300 x 300 DPI)



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TiO2 Catalyst Reduced

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