Local atomic structure in disordered and

# by Oldenbourg Wissenschaftsverlag, Mu¨nchen

Local atomic structure in disordered and nanocrystalline catalytic materials Wojtek Dmowski*, I, Takeshi Egami I, II, III, Karen E. Swider-LyonsIV, Wen-Fu YanIII, Sheng DaiIII and Steven H. OverburyIII I II III IV

Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Code 6113, Naval Research Laboratory, Washington, DC 20375, USA

Received March 23, 2007; accepted June 18, 2007

Pair distribution function / X-ray scattering / Nanocrystalline materials / Catalysts Abstract. The power of the atomic pair density function method to study the local atomic structure of dispersed materials is discussed for three examples (I) supercapacitor hydrous ruthenia, (II) electroctalyst platinum-iron phosphate and (III) nanoparticle gold catalyst. Hydrous ruthenia appears to be amorphous, but was found to be nanocomposite with RuO2 nanocrystals supporting electronic and hydrous boundaries protonic conductivity. A platinum-iron phosphate electrocatalyst, that exhibits activity for the oxygen reduction reaction has platinum in a nonmetallic state. In catalysts comprised of gold nanoparticles supported on TiO2, atomic correlations in the second atomic shell were observed suggesting interaction with the support that could modify gold chemical activity.

yond the nearest neighbor atoms, up to tens of nanometers, while other local structural probes, such as the EXAFS or NMR, can resolve only the first or second coordination shell. The PDF method has long been used exclusively for the study of glasses and liquids, but recently its use has been extended to crystalline and nanocrystalline materials because of the advent of synchrotron based modern radiation sources. The reduced pair distribution function G(r) is obtained by the direct Fourier-transformation of the total scattering function including both the Bragg peaks and diffuse scattering intensities, as given in Eq. (1), where scattering vector Q ¼ 4p sin q=l, and q is the scattering angle and l denotes wavelength of the probe. 1 ð 2 Q½SðQÞ
1 sin ðQrÞ dQ : ð1Þ GðrÞ ¼ p 0

Introduction Determination of atomic structure is the first step in understanding the physical and chemical properties of materials. Much progress in materials science has been made through the crystallographic studies of crystal structure using diffraction. However, as we focus more and more on nanotechnology and nano-scale materials manipulation the traditional approaches of crystallography are being challenged. For instance diffraction peaks from nano-scale materials are broad, making rigorous structural characterization difficult. There is apparent need of a structural local probe that does not depend on the periodicity of the structure and achieves nano-crystallography. An attractive alternative to the crystallographic method is the atomic pair-density function (PDF) method for studying the local and medium-range atomic arrangements. In the PDF method, we measure both Bragg and diffuse diffraction intensities, and the structural information is obtained by a Fourier transformation to real space. This technique can identify local atomic order much be* Correspondence author (e-mail: [email protected])

The atomic pair density function r(r) (PDF) can be obtained from G(r): GðrÞ ð1aÞ rðrÞ ¼ r0 þ 4p r where r0 is atomic density. The PDF is related to the pair distribution function g(r), r(r) ¼ g(r) r0 . The atomic PDF gives direct information, albeit one dimensional due to spherical averaging, about the distribution of the interatomic distances. Because no assumptions are made about translational or other symmetries during data processing, the PDF can describe periodic as well as aperiodic structures. The theoretical and experimental procedures are described elsewhere [1]. The reduced PDF is obtained directly from (1) and is convenient to use especially for multiphase materials or if density is not well known. The r(r) obtained through eq. 1a represents atomic density correlations and can be used to evaluate coordination numbers. We may note that since r(r) is calculated from G(r) by dividing by r the error becomes enlarged at small distances. The PDF can be obtained by using X-ray, neutron or electron scattering, provided that the energy of the scattering probe is high enough to extend integration in Eq. (1) to high Q, since Q < 4p=l. Each scattering probe has its merits. X-ray and neutrons are commonly employed be-

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Z. Kristallogr. 222 (2007) 617–624 / DOI 10.1524/zkri.2007.222.11.617

W. Dmowski, T. Egami, K. E. Swider-Lyons et al.

cause of efficient powder averaging during experiment and easy sample preparation. Neutron scattering lengths does not vary much among atoms, and for a given atom it does not depend on Q, which is advantageous; on the other hand weak scattering power necessitates use of large volume of samples. X-ray scattering amplitude depends on number of electrons; therefore heavy atoms are more weighted in the X-ray scattering intensity. Recent advances in synchrotron sources and the use of area detectors allow studying very small samples with a high data acquisition rate. In this article we describe the use of X-ray scattering in the study of the local atomic structure and nano scale ordering and its relation to the chemical and physical properties in various nano-scale catalytic materials.

Results and discussion Hydrous ruthenia Ruthenium is a rare metal that is used in many catalysts, for example together with platinum as an anode in the direct methanol fuel cells. Its dioxide, RuO2 (ruthenia) is an example of a highly metallic oxide with high electronic conductivity and Pauli paramagnetism. Ruthenium dioxide is commonly used in the semiconductor and chemical industries as a resistor thin film and a catalyst for chlorine production. It has a body-centered-tetragonal unit cell (a ¼ 4.4906
A, b ¼ 3.1064
A) [2], with Ru atoms occupying the corners and the body center. A Ru atom is coordinated with 6 oxygen atoms forming a distorted octahedron. There are four short and two long Ru––O distances at 1.94
A and 1.98
A, receptively. The oxygen octahedra share edges along the c-axis and vertices in the basal plane. The RuO6 octahedron at the corner of the unit cell is rotated by 90 degrees with respect to the one at the body-center. The nearest distance between two Ru atoms along the c-axis is 3.1
A. The distance from the corner Ru to the body centered Ru is 3.54
A. It has been observed that hydrous form of ruthenia, RuO2  xH2O, has ability to store charge, making it attractive media for supercapacitors [e.g. 3]. In addition the presence of hydrous ruthenia has been observed together with ruthenium in the Pt––Ru electrodes used in methanol fuel cells [4]. Hydrous ruthenium dioxide is a mixed electronic-protonic conductor [5, 6]. The electrochemical properties of this material depend on the amount of water in its structure. Optimum pseudocapacitance is achieved when RuO2  xH2O is heated at  150 ºC to yield x  0.5 mole of H2O [3] and its protonic and electronic conductivity is optimized. The optimized  0.5 mole water-content sample is usually obtained by selected heat treatment of hydrous samples containing more than 2.5 mole of water, as described in detail elsewhere [7]. The structural origin of the maximum in the specific capacitance and crossover in the type of conductivity were difficult to explain because of presumed amorphous structure of the hydrous ruthenia. This is illustrated in the Fig. 1, which shows typical inhouse X-ray diffraction results of hydrous ruthenia undergone heat treatment to adjust water content.

Fig. 1. X-ray diffraction patterns of hydrous ruthenia using Ka radiation. Sample with x ¼ 0.48 and x ¼ 0.02 were obtained by heating in air at 170 and 400  C respectively. The x ¼ 2.5 sample was purchased from Alfa Aesar. The water content was determined by weight loss.

The XRD patterns are shown for three samples ruthenias with different water contents (x ¼ 0.02, 0.48, heated at 400, 169  C respectively, and x ¼ 2.5, as received) measured with CuKa radiation. The broadness of the peaks suggest that the samples heated below 200  C are amorphous, while the sample annealed at 400  C with Bragg peaks that match well to anhydrous RuO2, has crystalline order. Thus the diffraction patterns appear to be either crystalline or amorphous, suggesting extreme changes in the atomic ordering. Based on such differences in the diffraction patterns, it could be surmised that the high concentration of water in the metal-oxides leads to a collapse of the crystalline order and formation of an amorphous structure. Using the synchrotron X-ray PDF methodology [7] we have shown that this view is incorrect and in fact all samples are nanocrystalline. The measurement was carried out at the National Synchrotron Light Source (NSLS), beamline X7A, using a Ge solid state detector and incident energy of 21 keV, with more experimental details in [7]. Figure 2a presents pair distribution functions in the first 7
A range for samples heated at different temperatures to control the water content (from x ¼ 0.02 to 0.84). The PDF analysis shows that short range atomic order in all of the hydrous RuO2 samples is equivalent to that of the anhydrous one. The first four Ru––Ru distances corresponding to the rutile structure are well preserved in each PDF, and they are sufficient to reproduce the rutile unit cell. Therefore, none of the samples is amorphous but all are nanocrystalline. Preserving the nano-crystalline state must be crucial in maintaining metallic conductivity since A RuO2 is a metallic oxide. The PDFs in the range 7–20
in Fig. 2b show that peak amplitudes decrease and peak widths increase for samples with increasing water content. This signifies decreasing nanocrystal size and lost longerrange correlations and surface defects. The broadening and damping of the PDF peaks (Fig. 2b) suggest that the ruthenium-oxide nanocrystals are smaller and more dispersed for samples with high water contents, and can be as small as 10
A. The presence of small PDF peaks at large distances r indicates size distribution of the ruthenium-oxide grains in

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 a

We are able to show with PDF analysis that all hydrous ruthenia samples are nanocrystalline and their local atomic structure is independent of water content. This nanocrystalline model of the structure of RuO2  xH2O involves definite boundaries between rutile-like nanocrystals and structural water. The hydrous RuO2 could be described as a complex nanocomposite with nanocrystals surrounded by hydrous layers, or electrolyte. These solidliquid interfaces can serve as Farady layers that support capacitive, charge storing capabilities of the material. In the mixed-conducting material, electron (metallic) conduction is supported by the RuO2 nanocrystals, while proton conduction is supported by the hydrous boundaries between the nanocrystals. Due to the dispersive nature of this nanocomposite structure, separate percolation paths must be present between the RuO2 nanocrystals and the hydrous regions to provide routes for long-range metallic and protonic conduction, respectively. The optimum electrochemical performance is obtained if the resulting structure supports both the high metallic and protonic transport. We also have carried out X-ray absorption near-edge structure (XANES) studies of the LIII Ru absorption edge in hydrous and single crystal ruthenia samples. The XANES spectra were measured at the beamline X19A of the NSLS. In this experiment empty electronic states are probed by the quadrupolar transition from 2p to 4d. Because Ru atoms are in the octahedral environment, the 4d energy levels are split into eg and tg levels by a crystal field. There is additional split of the eg and tg levels due to spin-orbit coupling and small tetrahedral Jahn-Teller distortion. For RuO2, with 4 electrons in 4d orbitals, within the single particle model, two peaks are expected at the LIII edge [e.g. 8]. Figure 3 shows the obtained spectra. The spectra were normalized to the peak height to emphasize interesting feature that we observed for the nano-

 b

Fig. 2. PDF of hydrous ruthenia with different water content obtained by annealing in air for 18 hrs from 100 to 400  C. Note that we were not able to obtain PDF for x ¼ 2.5 because of partial dehydration during experiment. (a) PDF in the range up to 7
A, (b) PDF in the range 5–20
A. Figures are adapted from [7].

the hydrous samples. For example, there is a small peak close to 10.5
A in Fig. 2b for the RuO2  xH2O samples with both x ¼ 0.84 (heated at 100  C) and x ¼ 0.02 H2O sample (heated at 400  C). This shows that, in some parts of samples, atomic correlations at this distance are preserved, so there is a distribution of small and large crystals. Heating the hydrous RuO2 below 300  C results in better defined short-range PDF peaks. It indicates that the nanocrystal size changes in that temperature range with some minor coalescence and improved surface structure due to dehydration. Above 300  C there is clear crystal growth due to improved atomic mobility.

Fig. 3. XANES spectra obtained at the LIII Ru absorption edge. Sample denoted as s.c. (single crystal) is a reference RuO2. Other samples were obtained by heating the hydrous ruthenia to change its water content. The data is normalized to the peak height, not to the step function as is usually done, to emphasize change in spectra width. It should be noted that sample heated at 400  C has average crystallite size 60
A.

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Local atomic structure in disordered and nanocryst

W. Dmowski, T. Egami, K. E. Swider-Lyons et al.

crystalline samples. The curve denoted as single crystal is a reference RuO2. It is in good agreement with other published spectra [8]. The position of the high energy shoulder, the 2p to the empty 4d eg states transition, at 2837 eV is the same in all of the materials. The most intriguing is the narrowing of the edge spectra, with the full width at half maximum decreasing with increasing water content, or decreasing size of the nanocrystalline ruthenium oxide clusters. This narrowing most likely reflects the ultra small crystalline size and surface effect on the width of a 4d band in nanocrystalline ruthenia through the loss of crystal-field splittings.

Pt––FePO electrocatalysts Proton-exchange membrane fuel cells (PEMFC) use hydrogen and oxygen to produce electric power, heat and water. At the heart of the electrochemical conversion lie two chemical reactions: the oxygen reduction reaction (ORR) at the fuel cell cathode and the hydrogen oxidation reaction (HOR) at the anode. Both the ORR and HOR reactions have activation energy barriers that are reduced by adding catalyst to the electrode assemblies. The 2-electron HOR is very fast, however the 4-electron ORR exhibits a slow and complicated kinetic behavior [9–11]. So far, the best catalysts for low temperature acid-based fuel cells are platinum metal and its alloys [12]. The high cost of platinum and platinum-based catalysts is motivating extensive efforts to reduce Pt loading. The catalytic efficiency per unit weight of Pt is improved by decreasing the particle size of the catalysts, as smaller particles exhibit larger catalytic surface area per unit volume. However, the catalytic activity drops with decreasing particle size below 4 nm, because active Pt crystal facets disappear [13]. In addition, the surface energy increases with the decrease in particle diameter and the reactants are more strongly absorbed and not easily released. Hence, Pt nanoparticles below 4 nm exhibit lower activity than expected from their catalytic surface area [14] and further gains from smaller sizes are limited. We have observed high catalytic activity for the ORR in a group of phosphate-based materials, including a hydrous platinum-iron phosphate (Pt––FePO4  xH2O) [15, 16]. Pt is dispersed in hydrous iron phosphate (nominally FePO4  xH2O) via a sol-gel method. Spectroscopic data show that Pt is in a non-metallic state, and the electrochemical data suggest that the activity of the Pt ions is strongly coupled with the support material [15]. The structure of both the hydrous iron phosphate (blank support material) and the Pt––FePO4x  H2O have been investigated using X-ray scattering and the PDF method. The data were collected at the NSLS beamline X7A and APS BM-30 using 24 keV X-rays in a transmission geometry. The scattered X-ray intensity was measured using a Ge-solid state detector coupled with a fast Canberra amplifier with built in three SCAs (0.8 ms measured dead-time). Statistics was better than 6  105 counts per
A
1. The data were corrected for background, absorption, Compton scattering, multiple scattering and normalized. The PDF, r(r), was obtained using Eq. (1) (using A
1) and 1a. In addition crystallized FePO4 Qmax ¼ 19


Fig. 4. PDF of a glassy hydrous FePO4 and sample annealed at 650  C for 24 hrs in the air. The first peak corresponds to tetrahedral coordination of the phosphorus, second to tetrahedral coordination of the iron. Crystalline sample has a a-quartz structure with alternating P and Fe tetrahedra. The mineral name is berlinite.

was measured at the NPDF pulsed neutron diffractometer at LANSE, Los Alamos Natl. Lab. Its structure was refined using standard Rietveld method. The diffraction pattern of hydrous FePO4  xH2O is consistent with those expected for glassy random-network materials, such as silica or borosilicate glasses. We resolved the local atomic structure of the amorphous FePO  xH2O support by comparing its PDF with that of crystallized FePO4 after annealing at high temperature (650  C). The structure of the crystallized sample was characterized by the standard Rietveld refinement method. This structure was identified as berlinite FePO4, which is isostructural with AlPO4 and aquartz [17]. Berlinite FePO4 is an example of a phosphate compound having only tetrahedrally coordinated Fe. Figure 4 compares PDFs of the crystalline FePO4 and hydrous FePO4  xH2O. The short range PDF peaks match very well, indicating that hydrous iron phosphate is a random-network polyhedral glass with tetrahedraly coordinated Fe and P. The water present in amorphous iron phosphate modifies the oxidation state of the Fe. A charge transfer from water to some iron atoms is evident from the sizable change in the average Fe––O distance. In crystalline FePO4, the average A, while in the hydrous Fe3þ ––O bond length is 1.87
FePO the Fe––O peak is seen at 1.96
A. Tetrahedrally coordinated Fe3þ and Fe2þ ions have ideal bond lengths of 1.89
A and 2.04
A [18]. If we assume that the average bond distance of 1.96
A in the hydrous FePO4  xH2O is the average of the weighting of the Fe3þ ––O and Fe2þ ––O bond lengths, then approximately half of the iron atoms are in the 2þ oxidation state. Thus the amorphous, hydrous catalyst support contains active Fe2þ/Fe3þ redox couples, which also may be contributing to the electrocatalytic performance of the material. The addition of Pt causes a small structural modification of the support material. This observation is estab-

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hydrous Pt––FePO4  xH2O catalyst (dashed line) is characterized by a very strong white line, typical for a high Pt oxidation state, and strongly damped EXAFS oscillations compared to those of the Pt metal. This implies that Pt is dispersed as ions and only coordinated with oxygen and hydroxyl groups, and the Pt ions are embedded into hydrous support framework in a similar way as glass modifier additives to borosilicate glasses. This finding is important as only metallic Pt particles are typically considered for the electrocatalytic application. It is clear that Pt introduced during sol-gel synthesis has never been fully reduced. Because of high oxidation state of Pt atoms the catalytic mechanism may be entirely different from the one for the Pt metal. The catalyst likely operates in a bifunctional mechanism with the support, whereby the oxygen chemisorbs and possibly dissociates on the FePO4, and then the Pt combines the oxygen radicals with protons to form water [15]. Fig. 5. PDFs of glassy hydrous iron phosphate compared with two samples containing dispersed Pt at 3 and 6 wt% loading. The typical nearest neighbor Pt––Pt distance at 2.8
A is not observed in the data.

lished by examining the PDFs of the FePO and 3 and 6 wt% Pt––FePO, shown in Fig. 5. A In Pt––FePO4  xH2O, the cation–cation peak at 3.2
is broader and the resolution of peaks beyond 3.5
A is poorer than in FePO4  xH2O. The PDF of 3- and 6-% Pt––FePO4  xH2O shows no evidence of any Pt––Pt nearest neighbor at 2.78
A, typical for f.c.c. metallic platinum, suggesting that there are no detectable metallic clusters of Pt. The oxidized state of Pt atoms in the catalysts was confirmed with X-ray absorption spectroscopy of the near edge structure (XANES). The XANES spectra of hydrous Pt––FePO4  xH2O, crystalline Pt––FePO4 and Pt metal are shown in the Fig. 6. The spectra clearly show that after heating to 650  C, the crystallized Pt––FePO4 is nearly equivalent to the Pt metal standard. In contrast, the XANES spectrum of the

Fig. 6. XANES spectra of Pt––FePO4  xH2O compared with Pt standard foil and sample annealed at 650  C for 24 hrs in the air. Apart from clear edge shift very strong white line peak is observed for asprepared sample indicating oxidized state of Pt atoms.

Gold nanoparticle catalysts Bulk metallic gold is chemically inert. However, its chemistry drastically changes when gold is deposited on metal oxides as nano-sized particles with diameters smaller than 5 nm. Oxide-supported gold catalysts have been shown to exhibit exceptional catalytic activity in many important reactions [19]: combustion of CO and saturated hydrocarbons, oxidation-decomposition of amines and organic halogenated compounds, partial oxidation of hydrocarbons, reduction of nitrogen oxides. It is believed that the specific environment of highly dispersed nanoscale Au particles in some way contributes to the catalytic activity. However, many aspects of the interface structure and chemical nature of gold particles are not well known. Catalytic activity of highly dispersed Au particles for several reactions [19, 20] has been established. However, the morphology and chemical state of the active Au species is strongly disputed. Some attribute the high reactivity of Au in CO oxidation to the properties of the Au-oxide interface, in particular because of the dependence on the Au particle shape or perimeter, the Au-oxide contact area [21], and the metal oxidation state [22]. The support effects [23] have also been proposed to account for the special catalytic properties of nanosized Au particles. Some experimental evidence shows that the most active structures of Au consist of bilayer islands that have distinctive electronic and chemical properties compared to bulk Au [24]. Conversely it has been speculated that Au ions (Au
or Auþ) may be catalytic centers or that the oxide support is playing a minority role. Au nano-particles, depending on the specific oxide support (e.g. TiO2, ZnO, CeO2, Al2O3, SiO2), show different gas molecule adsorption properties. According to ref. 19 this markedly enhanced catalytic activity cannot be fully explained by the size effect of Au particles. For example the catalytic activity per unit surface area of Au particles is about 100 times smaller than that of Au/TiO2. It is observed that Au supported on reducible oxides, such as TiO2, has a less tendency to suffer deactivation compared to Au on irreducible oxides, such as MgO and SiO2. Because titania plays a special role as a gold support, if other porous particles, such as alumina or silica, are

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Local atomic structure in disordered and nanocryst

used, they are covered with few atomic layers of TiO2 before gold deposition. The role of titania could be due to its wetting properties, but also there could be some intricate interaction with gold making it catalytically active. For example the nearest-neighbor coordination sphere of the surface gold atoms may include also Ti and O neighbors, and not only gold atoms. This may explain why the reactivity of the dispersed nanoparticle atoms differs from that of bulk gold. To investigate if there are any structural features of a gold-support interaction we examined the structure of nanoparticle catalysts with gold dispersed directly on titania, silica and alumina covered with a tiatnia layer [25]. The Au loading was 12 wt% on TiO2 and 4.1 wt% on TiO2/alumina and TiO2/silica. We carried out high-energy X-ray PDF measurements at the ID-6 (MUCAT) beamline at the Advanced Photon Source (APS) with the incident energy 87 keV and the image plate (MAR-345 IP) as an area detector. The main advantage of a 2D detector is its ability to cover a large portion of Q-space. For an isotropic sample integrating over the azimuth angle boosts statistics, allowing for short exposure times. In the case of 12 wt% sample most of the scattering comes from gold because gold has a high atomic number (Z ¼ 79) and X-ray scattering intensity scales with Z2. In addition to the loaded catalyst we examined a blank substrate. Figure 7 shows the PDF of a catalyst and titania substrate. The catalyst PDF is clearly dominated by gold. The only distinctive feature that would suggest atomic correlations between gold and titania is a broad peak in the range 3.5–3.8
A. Unfortunately, there is a main peak in the titania PDF at 3.76
A, which makes this conclusion uncertain. In principle we could subtract one PDF from the other, however the process of forming gold nanoparticles involves heat treatment and gold reduction, which may modify the substrate. Also samples with lower Au loading have much stronger signal coming from the oxide support making the subtraction unreliable. We have employed the resonant scattering method at the gold LIII absorption edge to resolve the support issue.

W. Dmowski, T. Egami, K. E. Swider-Lyons et al.

The measurement was carried out at the beamline BM-30 (UNICAT) at the Advanced Photon Source using an energy-sensitive Si Vortex detector. Typical energy resolution of 200 eV for this detector allows elimination of the La fluorescence and a significant part of the Compton scattering from the measured signal. The Lb contribution was subtracted by measuring the La intensity and the ratio of Lb=La. The resonant scattering experiment is performed at few incident energies with the highest energy being few eV below the edge [26]. This type of experiment is difficult and not commonly carried out. Data corrections depend on the type of monochromator used, X-ray optics (mirrors, focusing), type of detector or analyzer and determination of anomalous factors [27–32]. Also Qmax is limited by the energy of the available absorption edge. In this method the incident X-ray energy is tuned to the vicinity of the absorption edge, where the atomic scattering factor depends strongly on the X-ray energy E, f ðQ; EÞ ¼ f0 ðQÞ þ f 0 ðEÞ þ if 00 ðEÞ :

This energy dependence of the scattering factor is known as the anomalous dispersion, and originates from the resonance of the X-ray with the excitation of electrons in the core of an atom. Near the absorption edge the scattering power from that particular element varies strongly with energy while the scattering from other elements remain almost constant. By taking the derivative of a signal with energy measured at two or more energies near the absorption edge we can eliminate correlations not involving that particular element allowing us to obtain the differential structure function. It is important to note that if there is bonding of Au with a substrate atoms this correlation is preserved. For example, let us consider two elements A and B. Then intensity is proportional to: I  c2A  fA2 SAA þ 2cA cB fA fB SAB þ c2B  fB2 SBB

ð3Þ

where cA and fA represent atomic concentration and scattering form factor for element A or B; SAB, SAA, SBB represent atomic correlations between A and B and so forth. If experiment is done at the absorption edge of A then derivative of I with respect to energy will eliminate last term because fB does not depend on energy: @I @fA ðcA  fA SAA þ cB fB SAB Þ :  2cA @E @E

Fig. 7. Reduced PDFs (G(r) ¼ 4prr0(g(r)
1)) of 12 wt% Au on titania support, blank support and PDF of gold. We did not attempt to model nanocrystalline size effect, the Au PDF is shown for a reference only.

ð2Þ

ð4Þ

If there are no atomic correlations between A and B, for example bonding between gold and substrate element B, then SAB ¼ 0 and the substrate intensity is eliminated from the analysis. If there is a correlation then SAB 6¼ 0 and gold substrate interaction is observed. We have measured X-ray scattering from the 4.1 wt% Au samples on TiO2/alumina TiO2/silica supports at 11.6 and 11.9 keV. Figure 8 presents the differential reduced PDFs obtained from the difference in the X-ray intensity with respect to energy together with the G(r) of polycrystalline gold. Additional pair correlations apparently not associated with Au––Au pairs are observed in the PDFs in the range 3.5–3.8
A. While peaks below 2.5
A are due to termination errors of the integration in Eq. (1), the termination errors diminish quickly with increasing r. In our judgment

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References

Fig. 8. Reduced differential PDFs (DG(r)) of 4.1%wt Au on TiO2/ alumia and TiO2/silica support and reduced PDF of gold. The gold PDF is only for comparison and was scaled down.

the additional peak in the range 3.5–3.8
A is real, and represents the correlation between nanocrystalline gold and the TiO2 support. These extra bonding appear as the second nearest neighbors to Au atoms. Thus it may as well modify the chemical state of Au resulting in, for example, preferred binding of CO and facilitating CO oxidation. We plan to extend our study to planar supports for further investigating the special properties of gold nanoparticles on titania.

Conclusions The PDF method is a useful technique that can determine the local atomic structure of disordered and nanocrystalline materials. When applied on materials for catalysis, it facilitates understanding of the mechanism of these materials for catalytic reaction. Electronic conductivity in hydrous ruthenia is supported by well defined nanocrystals with the rutile structure, while protonic conductivity is supplied by a hydrous boundary and structural water. Platinum in Pt––FePO4  xH2O is distributed in ionic state despite activity of the electrocatalyst towards oxygen reduction reaction. Gold atoms in nanoparticles supported on titania appears to have atomic correlations with substrate atoms, which may enhance gold activity in CO oxidation reaction. Acknowledgments. This work was supported by the National Science Foundation through grant DMR04-04781 and in part by the Office of Naval Research. WY, SD and SHO are supported by the Office of Basic Energy Sciences, U.S. Department of Energy. We would like to thank D. Robinson (APS, MUCAT) and J. Karapetrova (APS, UNICAT), W. Calibe (NSLS, X19A) for the help with experimental setup. The operation of the National Synchrotron Light Source is supported by the Department of Energy, Division of Materials Sciences and of Chemical Sciences. Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The Midwest Universities Collaborative Access Team (MUCAT) sector at the APS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through the Ames Laboratory under Contract No. W-7405-Eng-82.

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