Sample Characterization of Palladium Supported on Tetraphenylborate

M. C. Duff, D. B. Hunter, and J. Urbanik-Coughlin
Savannah River Ecology Laboratory
The University of Georgia
Aiken, SC 29803

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Palladium-XAFS spectroscopic analyses were conducted by Martine C. Duff (the PI), Douglas B. Hunter (the Co-PI) and Jessica Urbanik-Coughlin (technician) at SREL on several of Pd- and Hg-containing samples (after reaction with TPB) and a range of Hg and Pd reference materials that were provided by SRTC. The intent of these analyses was to provide as much characterization information about these samples as possible. In particular, information such as the average oxidation state, their coordination geometry and cluster size of the Hg and the Pd were to be determined. Spectral information from the reference samples and the published literature on Hg- and Pd-XAFS spectra were used to infer the characteristics of the samples and reference materials provided by SRTC. The deliverable for this work is a final report (including the raw spectra, method of data analysis, spectra interpretation and spectra modeling) detailing the characterization of the Hg- and Pd-containing materials. Other deliverables include a literature review on material that is relevant to the data presentation and experiments associated with the safety aspects of running HLW samples at NSLS—including work to determine the experimental needs associated with the detection of low levels of noble metals (e.g., Pd, Rh) in the HLW sludges.

Although Ru- and Rh-XAFS studies were not part of this proposed study, additional samples and reference materials that contained Rh and Ru were also characterized with XAFS. To avoid confusion among the Pd and the Rh/Ru results in this presentation, the results for Rh and Ru will be presented after the Pd and Hg results. Other studies that were not initially part of this proposed ERDA study included HR-TEM, FE-SEM, SAED, and acidic digestions of some of the samples (followed by ICP-MS determination of the Pd and Hg contents in the digest solutions). These studies were done at the request (verbal or written) of Reid A. Peterson, Mark J. Barnes and Samuel D. Fink at SRTC.

The primary objective of this research was to use XAFS spectroscopic techniques such as XANES and EXAFS to obtain information on Pd and Hg in samples that were potentially supported on KTPB and had been reacted with dissolved TPB and TPB decomposition products (tri-, di- and mono- phenylborate and benzene). Palladium- and Hg-XAFS spectroscopic analyses were performed on several Pd and Hg-containing samples that had been equilibrated under heat (~45^oC) with KTPB, dissolved TPB and TPB decomposition. For the Pd studies, eight samples containing known amounts of added starting reagents were prepared by SRTC to examine the influence of:

on the behavior of Pd and Hg in the solid phases produced after the incubation of these materials. Five of the samples were equilibrated for 24 hours. Three of the five samples contained added Pd:Hg mole ratios of 3.4 (performed in duplicate) or 1.7 (not duplicated). The forth sample had a starting Pd:Hg mole ratio of 3.4 and was irradiated and the fifth sample had a starting Pd:Hg mole ratio of 3.4 and contained Pd that had been reduced onto alumina. The three remaining samples were equilibrated for 12 hours and had starting Pd:Hg mole ratios of 34, 3.4 and 0.34.

Information on the speciation of Pd and Hg in the samples, such as the average oxidation state, coordination geometry, near and next-nearest neighbor environment and cluster size of the supported Pd and Hg were determined. For comparison, several Pd and Hg standards (provided by SRTC or by NSLS) in addition to several forms of supported Pd reference materials were examined with the XAFS techniques.

Palladium-XANES data were collected for the sample treatments in addition to several reference materials: Pd metal foil and Pd supported on alumina, silica and SrCO₃. The studies indicate that the Pd supported on alumina was mostly metallic Pd whereas the Pd on silica and SrCO₃ was at least 50 to 100 % Pd(II). The XANES spectra for the Pd on SrCO₃ indicate Pd is present as a mixture of Pd metal and Pd(II), whereas the Pd on silica is probably mostly Pd(II). The Pd-XANES spectra obtained for the Pd(II) hydrous oxide solid had a small white-line feature.

Quantitative analysis of the Pd-XANES spectra for the samples incubated for 24 hours (with starting Pd:Hg mole ratios of 3.4 and 1.7) yielded somewhat ambiguous information about the average oxidation state of Pd. However, qualitative features in the Pd-XANES spectra for these samples resembled that of the Pd foil. The amplitude of the oscillations in the XANES spectra for the samples was reduced relative to that of the Pd foil, which suggests the Pd is present as very small clusters. A white-line feature was not observed in the spectra for the Pd-containing slurry samples that had been incubated for 24 hours.

Palladium-XANES studies with samples that were incubated for 12 hours (with starting Pd:Hg mole ratios of 34, 3.4 and 0.34) possessed a lower quality signal relative to the samples incubated for longer periods of time due to their low Pd concentrations. The spectra for samples that were equilibrated for 12 hours with initial added Pd:Hg mole ratios of 34 and 3.4 did not resemble that of the foil. The Pd-XANES edge of the spectra for these two samples was at a higher energy than that of the foil and the high energy side of the Pd absorption edge for the sample spectra contained a small white-line feature. These findings suggest that the Pd in these two samples was primarily oxidized [Pd(II)]. In contrast, the Pd in the sample with the starting Pd:Hg mole ratios of 0.34 (the high Hg treatment) appeared to contain mostly metallic Pd.

Palladium-EXAFS data were collected for the sample treatments and reference materials. Average structural information on bond lengths, cluster size, near neighbors and coordination environment were obtained by modeling the EXAFS spectra and comparing the spectra with known and simulated spectra. The interpretations of the EXAFS spectra for the supported Pd-containing materials were consistent with that of the XANES observations. Specifically, the EXAFS studies indicated that the Pd supported on alumina was mostly metallic Pd and the other forms of supported Pd had less metallic Pd bonding and more bonding between Pd and a light atomic weight atom such as O.

Model fits of the Pd EXAFS data confirm that the five Pd samples that were equilibrated for 24 hours (with starting Pd:Hg mole ratios of 3.4 and 1.7) contain FCC Pd metal with Pd-Pd first shell bond lengths of 2.75 to 2.77 Å. The data also indicated that the samples contain an average CN of 5 to 7 for Pd-Pd metal bonds. Because FCC Pd metal in an infinite lattice contains a CN of 12, the smaller CN (for first shell Pd metal) obtained from the analysis of the samples allowed an estimate of cluster size to be made. Based on maximum mnp site indices, Pd clusters with an average metal CN of 5 and 7 would have an average radius ~2 Å and 4.65 Å respectively. This would correspond to Pd being present as a nanocluster, a characteristic most pronounced in the samples that were equilibrated for 24 hours. Fits of the EXAFS data for these samples show that on average each Pd in the cluster is coordinated to one or more low molecular weight atoms (such as O, C or Na) with a bond length of ~2 Å, but an identity of the light atom or atoms could not be made for all of the samples. Some of these samples appeared to have a significant Pd-O interaction in the first shell.

Two of the samples that were equilibrated for 12 hours (at starting Pd:Hg ratios of 3.4 and 0.34) had a mixture of Pd-Pd and Pd-Hg bonding in the first shell. The total first shell CN for these two samples did not exceed 12. The third sample with an added Pd:Hg mole ratio of 34 contained no Pd-Hg interactions in the first coordination shell and the CN for the Pd in was 6.7—suggesting that Pd clusters were forming. No model fits of the Pd-EXAFS for the light atomic weight atoms in the three samples that were equilibrated for 12 hours were performed. Additionally, a dark color associated with the samples that were equilibrated for 24 hours was not present in the samples that were equilibrated for 12 hours.

The Hg L₃-edge XANES studies indicated that Hg is present as Hg metal in the Pd-containing samples that had been equilibrated for 24 hours. This observation was based on the qualitative features of the Hg-XANES spectra for the samples and by comparison with spectra for a HgO standard and spectra for Hg metal in the literature. The Hg L₃-edge EXAFS analyses indicated that Pd was the only detectable atom in the first coordination shell (of Hg) in all sample treatments that were equilibrated for 24 hours. For these samples, the structure of the Hg was determined to be FCC, the average Hg-Pd metal bond length was 2.77 Å and the Hg had a CN of 12 (the highest CN possible)--with the exception of the treatment that was gamma-irradiated, which had a CN of 9.74. In contrast, the Hg-EXAFS spectra for the treatments that were equilibrated for 12 hours with the Pd:Hg starting mole ratios of 0.34 and 3.4 had both Pd and Hg in the first coordination shell of the Hg. Hg-EXAFS spectra simulations of diphenylmercury (DPM) also indicated that a substantial amount of DPM was present in the two samples that were equilibrated for 12 hours. The analyses of the Hg-EXAFS data predicted bond lengths for Hg-Pd in these samples were in good agreement with the bond lengths for Pd-Hg that were obtained from the Pd-EXAFS data analysis. Due to the poor quality of data, no model fits for Hg were obtained for the one sample that was equilibrated for 12 hours at a starting Pd:Hg ratio of 34.

No Pd-Hg bonds were observable in the Pd-EXAFS for the samples that were equilibrated for 24 hours. It is difficult with EXAFS to identify 2 sub-shells if one shell is less than 10 % of the other. Hence, we conclude that Hg constitutes less than 10 % of the cluster by mole %. If it is assumed that all Hg is bound to Pd, which is present in the form of a nanocluster, Pd would be on the outside of the particles and Hg would be on the inside of the particles. This is because the value of the first shell CN of Hg is nearly twice that of the first shell CN for Pd. Additionally, if Pd is in a nanocluster form and the Hg has a full CN of 12, the size of the HgPd clusters would be constrained by the size of the Pd nanoclusters. Our analyses of the EXAFS data indicated the structure of the metals was FCC structure, the analyses did not permit a determination of the identity of the atoms in the outer coordination shells.

Furthermore, EXAFS analyses give average information on atomic distances and coordination environment and cannot discern between two or more populations of clusters. A multi-method approach using microscopic techniques such as HR-TEM and SAED in addition to EXAFS is needed for the most accurate interpretations of the presented EXAFS data. HR-TEM and SAED studies on one of the samples (which had a Pd:Hg ratio of 3.4 and had been aged at room temperature for 2 months) indicate two populations of particles are present. These HR-TEM studies indicate that the Pd is present as small 5- to 10-nm sized Hg-free nanoclusters and that the Pd is also present as coarser-grained Hg-rich particles, which occur as 100-nm thick metallic films on KTPB solids. The SAED studies indicate that the coordination environment of the metals in the sample is similar (i.e. it is FCC), which is in complete agreement with the EXAFS data. The TEM studies indicate that the two populations of particles were usually found in close proximity to each other. Lattice fringe measurements indicate that the particles were fairly consistent in size (5 to 10 nm). However, these studies were performed on two-month old samples and additional studies on freshly prepared samples are recommended.

The second objective of this research was to use XANES and XRF spectroscopic techniques to determine the feasibility and experimental conditions required for the XAFS characterization of Pd, Hg, Ru and Rh in HLW sludge materials at NSLS. These studies determined that two types of detectors are required for the proposed HLW work and that embedding the samples in a plastic resin did not pose a significant problem. However, due to the perceived difficulties of analyzing these metals in such a complex waste form, which is difficult to simulate, the true test will ultimately be with the actual materials.

A third objective of this research was to use XAFS spectroscopic techniques such as XANES and EXAFS to obtain information on Ru in one sample that had been reacted with DPM, dissolved TPB, the TPB decomposition products and KTPB at 45^oC for several days. Similar studies were done with Rh (instead of Ru) but the equilibration was conducted at 70^oC for 12 hours. After the equilibrations, Ru-, Rh- and Hg-XAFS analyses were performed on the solid phases in these samples. The results of the analyses indicate that the Ru and Rh were not present as metallic species. However, the results indicate that bonding between Ru and Hg occurred (as with the Pd studies but a longer equilibration time may be required before cluster formation can be observed. In contrast to the Pd-containing samples that had metallic Hg and Pd and were dark gray in color, these samples were light in color.

Removal of radiocesium by precipitation with dissolved tetraphenylborate (TPB) from High-Level Radioactive Waste (HLW, the waste product associated with the dissolution of spent fuel rods for the recovery of plutonium) is being considered as a waste treatment strategy at the U.S. Department of Energy's (DOE) Savannah River Site (SRS). Approximately 200 million L of this extremely radioactive material reside in carbon steel subsurface tanks awaiting treatment at the SRS. Previous attempts to treat HLW tanks at the SRS with dissolved TPB resulted in the evolution of levels of benzene (a product of TPB decomposition), which did not permit the safe operation of the facility at design throughput rates (Walker et al., 1996). Although low levels of benzene from the radiolytic and thermal decomposition of TPB were anticipated, the amount of benzene was underestimated. The HLW tanks contain most every element in the periodic chart (the stable and radioactively unstable products of nuclear fission) and are thought to contain catalytic metals that are capable of decomposing TPB.

Tests with HLW tank sludge materials amended with solutions of dissolved TPB resulted in the rapid decomposition of the added TPB after an induction period of 2 months (Hobbs et al. 1998). Additional tests were conducted to identify potential catalysts in high pH (0.5 M NaOH) HLW simulants. Dissolved forms of the metals Cu, Pd, Pt, Rh and Ru [added as Cu(II), Pd(II), Pt(II), Rh(III) and Ru(III) or as the zero-valent metal species] were each found to catalyze the decomposition of dissolved TPB (Hsu and Ritter, 1996; Barnes and Peterson, 1998; Crawford et al. 1999, Y. Su, EMSL, PNNL, Richland, WA). In these systems, the reaction required the presence of one or more of the TPB decomposition intermediates (tri-, di- and mono-phenylborate), in addition to benzene, Hg and a noble metal such as Pd, which was identified to be the most reactive metal of all metals tested (Barnes and Peterson, 1998). A series of studies were conducted at high pH with TPB and TPB decomposition products, dissolved Pd(II) at 25^oC and at 55^oC, in N₂ or air in the absence of Hg (King and Bhattacharyya, 1998). These reactions resulted in the formation of a black precipitate and the loss of dissolved Pd(II) from solution. It was then concluded that phenylborate compounds can reduce Pd(II) to Pd(0) and that elevated temperatures increased the rate of Pd precipitation in these studies. In high pH HLW simulants, reducing conditions are known to favor the reduction of Pd(II) to Pd(0) metal (Oda et al. 1996). Little information was available on the form of the Pd catalysts in these HLW simulants or in the HLW tank waste prior to this study.

Metallic nanoclusters consist of particles less than 10 nm in diameter (Schmid et al. 1999). These particles are often too small to possess the same characteristic properties of their bulk materials—they may differ with respect to their magnetism, lattice size parameter, melting point and superconductivity properties (Schmid et al. 1999; Coq and Figueras, 1998; Schmid, 1992). Due to the small size of these particles or colloids, many of the atoms are present at the particle surface and the clusters contain can contain a sizable portion of atoms with incomplete coordination shells (Schmid et al. 1999). Monometallic nanoclusters can form from many metals such as Pd, Hg, Pt, Cu, Ni, Au, Rh, Ru, Ag, Ir, Os, Re, Co (Catalano et al. 1995; Aiken and Finke, 1999a;b and references therein; Rademann et al. 1992; Moiseev and Vargaftik, 1998; Haberland et al. 1993). The experimental conditions by which these materials have been made vary greatly. For example, nanoclusters can be made under such conditions as: in organic and inorganic solvents; below, at, and above room temperature; in H₂ or in air; by chemical and electrochemical reduction; laser ablation, and thermal evaporation in vacuum among other methods (many general methods are described in (Aiken and Finke, 1999a; Bönnemann et al. 1994). Methods for synthesizing nanoclusters do not always result in the formation of highly stable products (discussed in Aiken and Finke, 1999a and references therein). Consequently, this instability has had a negative impact on potential industrial use of these materials.

The reason that nanoclusters tend to be unstable in solution is that they have a pronounced tendency to coalesce and form larger particles. However, this aggregation process can be stabilized by three experimental conditions: electrostatic stabilization, ligand stabilization and a combination of electrostatic and ligand stabilization (Aiken and Finke, 1999a,b; Bönnemann et al. 1994; Schmid, 1992). For example, nanoclusters can be electrostatically stabilized in high ionic strength solutions (e.g., by Na⁺ ions in pH 14, NaOH-rich solutions). Addition of NaOH to nanoclusters in aqueous solutions has been known to enhance catalytic reaction rates (Hirai et al. 1985). A variety of organic compounds with bulky functional groups can stabilize nanoclusters such as surfactants, polymers, and solvents (Schmid et al. 1999; Ciebien et al. 1998; Schmid, 1992) and polyoxoanions of tungstonate also promote nanocluster stabilization (Aiken and Finke, 1999). Nanoclusters do not necessarily require solid supports to be stabilized or catalytically active; however, they have been known to be supported on the traditional solid supports used for larger clusters such as graphite, alumina, magnesium oxide(100), silica gel, zeolites, and titanium oxide (Hansen et al. 1999; Bönnemann et al., 1992; Ali et al. 1997; Calais et al. 1998; Schneider et al. 1999; Goyhenex et al. 1994; Schmid et al. 1999; Schmid, 1992 and many others). These solid supports precluded cluster aggregation.

Bimetallic catalysts can form from metals that are normally immiscible in the bulk (i.e., from metals that do not form alloys, such as Cu and Ru; Sinfelt, 1983). Additionally, bimetallic clusters of immiscible metals can exhibit surface properties that reflect considerable interaction between the metals (Sinfelt, 1983 and references therein). Bimetallic nanoclusters have been synthesized with Au and Cu, Au and Pd, Pd and Pt, Pd and Pb, Pd and Ag, Pd and Fe, Pt and Ru, and other metals (Bazin et al. 1995; Berry et al. 2000; Davis and Boudart, 1994 and ref. therein; Couves and Meehan, 1995; Harada et al. 1992; Klabunde and Li, 1993: Berry et al. 2000; Schmid et al. 1996; Lee et al. 1995; Nashner et al. 1997; Tröger et al. 1997; Sinfelt, 1983 and references therein). Ligand-stabilized trimetallic clusters containing Pd, Au and Hg and Pt, Au and Hg have also been produced (Pignolet et al. 1995; Gould and Pignolet, 1994; Gould et al. 1995) but no information on their reactivity is known. During synthesis, atmospheric conditions such as the presence or absence of O₂ can influence the degree of preference that one metal has over the other metal for the outer surface (Davis and Boudart, 1994; Sinfelt, 1983).

The presence of a second type of metal in a nanocluster can increase the versatility and functionality of the catalyst (Tröger et al. 1997; Sinfelt, 1983 and references therein). The increase in reactivity behavior has been attributed to charge transfer (ligand effects) and the modification of multi-atom surface sites (ensemble effects) (Sornorjai, 1994). The structural incorporation of atoms with a significantly larger or smaller radius than that of the host atoms may promote the formation of surface defects, which can serve as highly reactive surface sites (Gleiter, 1992). Gleiter (1992) further describes bimetallic nanocrystalline materials as being highly defective solids. These materials were described as containing numerous defects at grain interfaces, dislocations and phase boundaries within the crystallites. Clusters with many defects are likely to have more surface area for reactions than clusters with few defects. However, reaction products can occlude surface sites and result in decreased cluster reactivity. The inhibition of bimetallic catalysts has usually been attributed to the buildup of surface carbonaceous material and the addition of too much of one metal to the bimetallic catalysts and the addition of sulfur are other common processes which decrease reaction rates (Klabunde and Li, 1993 and references therein).

Table 1. First coordination shell Pd-Pd fit literature data for Pd metal foil and some
examples of Pd-containing nanoclusters with a FCC structure. EXAFS information on
Au presented in Table 2. The magnitude of the Debye-Waller Factor [represented in
Tables 1 and 2 as s ²[Å]²] indicates the variation of the bond length determination.

Sample Type	Pd-Pd Bond Distance r[Å]	N	s ²[Å]²
Foils
Pd metal foil @300^oC¹	2.75	12	0.0044
Pd metal foil²	2.75	12	0.0044
Pd metal foil³	2.75	12	0.0032
Pd-Pd Bonds in Nanoclusters
Pd 561 atom cluster¹	2.74	6.8	0.0073
Pd 7 to 8 shell cluster¹	2.75	7.5	0.0066
Pd cluster²	2.76	6.2	0.0055
Pd/Au clusters, H₂, 300^oC, 1.3:1, pH 7⁴	2.80	2.4	0.0032
Pd/Au clusters, H₂, 300^oC, 1.3:1, UHV⁴	2.78	2.4	0.0021
Pd/Au clusters, H₂, 350^oC, 1.3:1, pH 7⁴	2.81	4.3	0.0065
Pd/Au clusters, H₂, 350^oC, 1.3:1, UHV⁴	2.81	4.2	0.0052
Pd/Au clusters⁵	2.77	2.5	0.0075

Table 2. First coordination shell fit literature data for the interactions between
Au-Au in Au-foil and Au-Au and Au-Pd interactions in PdAu nanoclusters.

Sample Type	Bond Distance r[Å]	N	s ²[Å]²
Foils
Au¹	2.86	10.5	ND
30 to 70 Å Au cluster particles¹	2.86	10.5	ND
25 to 35 Å Au cluster particles²	2.87	9.5	0.0027
Au-Au Bonds in Bimetallic Nanoclusters
Pd/Au clusters, H₂, 300^oC, 1.3:1, pH 7²	2.83	6.8	0.0044
Pd/Au clusters, H₂, 300^oC, 1.3:1, UHV²	2.83	6.6	0.0048
Pd/Au clusters, H₂, 350^oC, 1.3:1, pH 7²	2.81	4.3	0.0065
Pd/Au clusters, H₂, 350^oC, 1.3:1, UHV²	2.81	4.2	0.0052
Pd/Au clusters³	2.77	2.5	0.0075
Au-Pd Bonds in Nanoclusters
Pd/Au clusters, H₂, 300^oC, 1.3:1, pH 7¹	2.77	3.5	0.0045
Pd/Au clusters, H₂, 300^oC, 1.3:1, UHV¹	2.78	3.8	0.0065
Pd/Au clusters, H₂, 350^oC, 1.3:1, pH 7¹	2.81	4.3	0.0065
Pd/Au clusters, H₂, 350^oC, 1.3:1, UHV¹	2.81	4.2	0.0052
Pd/Au clusters²	2.77	2.5	0.0075

2.4. Use of XANES Spectroscopy for the Characterization of Noble Metal Nanoclusters.

Slurry samples were prepared by addition of dissolved Pd(II)-nitrate solution, addition of dissolved Ru(III) solution, or addition of dissolved Rh(III) solution to DPM so that the starting mole ratios of the slurries were 34, 3.4, 1.7 and 0.34 for the Pd:Hg treatments, 3.4 for the Ru:Hg treatments and 3.4 for the Rh:Hg treatments (Table 3). These noble metal- and Hg-containing solutions were added to a mixture of 125 mg L^-1 each of the TPB decomposition products (tri-, di- and mono-phenylborate) and 820 mg L^-1 of benzene (Table 3). Additionally, 1 wt. % solid phase potassium-TPB (KTPB) was added as a potential support to each of the treatments. The sample mixtures were heated to 45^oC in a water bath (with the exception of the Rh Sample V, which was heated to 70^oC) in sealed Teflon polybottles as described in Table 3 and filtered with a 1-mm polycarbonate filter in an inert N₂atmosphere. Preparation of Samples 1, 2, 3, V, Aged Ru Sample 1, X, Y, and Z are described in Table 3. The filter cake solids were prepared for the XAFS measurements in an inert N₂atmosphere by placing the solids in 2- and 5-mm thick plastic mounts with polypropylene windows. The samples were placed in vacuum-sealed plastic bags and put in sealed Mason jars until the EXAFS analysis. With the exception of the metal foils, all standard and reference materials (ranging from 0.5 to 1 % by wt. noble metal) were prepared as finely ground powders by mounting them in 2-mm thick plastic mounts with Kapton tape windows.

Additional samples were provided by SRTC that served as HLW sludge surrogates (Table 4). These samples were to be embedded by SREL researchers in clear casting resin. The embedded samples were taken to NSLS (beamlines X23a2 and X26a) for studies to determine if HLW studies were feasible for embedded samples (Fig. 1) at the concentrations of metals that were expected to be in actual HLW sludge samples.

The Pd- and Hg-XAFS data were collected at the Pd K-edge (24.35 keV) and Hg L₃-edge (12.284 keV) on the filtered solids (as previously described in Table 3), 25-mm thick Pd metal foil and on Pd and Hg reference materials. The Ru- and Rh-XAFS data were collected at the Ru K-edge (22.12 keV) and Rh K-edge (23.22 keV) on filtered solids (Table 3), with a Ru (-325 mesh) metal powder and a 25-mm thick Rh metal foil. All XAFS data were collected on beamline X23a2 at the National Synchrotron Light Source (NSLS, Brookhaven National Laboratory, Upton, NY). The XAFS data were collected in transmission (Pd, Ru, Rh) and fluorescence (Pd, Ru, Rh, and Hg) mode using an unfocussed X-ray beam and a fixed-exit Si(311) monochromator. Ion chambers were used to collect incident (Io), transmission (It) signals and reference (Ir) signals. Gas ratios for the Hg data collection in Io were 80% Ar and 20% He, whereas the gas ratios for the other elements were almost 100% Ar. A Lytle detector was used to collect fluorescence X-rays (If), using an Al foil (for Ru-, Rh- and Pd-XAFS) to reduce the background fluorescence counts. The monochromator energy was maximized using a piezo stack feedback energy stabilization system, with a settling time of 0.3 seconds per change in monochromatic energy. An X-ray beam size of 2 by 14 mm² was used. Samples were placed in the Lytle detector with a steady purge of

He to help maintain a low O₂ environment during the analyses. To enhance fluorescence detection, argon or krypton gas was used in the Lytle detector chamber.

For the feasibility tests with polyester resin embedded HLW simulants, "quick" Pd-, Hg-, Rh- and Ru-XANES scans were performed to determine the potential of using the Lytle detector for future XAFS measurements of Hg, Ru, Rh and Pd in actual HLW samples. The Lytle detector at beamline X23a2 was found to be inappropriate for the detection of Pd and Rh, therefore another detector was used to determine its suitability for these proposed HLW measurements (to be described). For these studies, the synchrotron hard X-ray fluorescence microprobe (beamline X26a) at the NSLS was used (Duff et al. 1999 and references therein). Tantalum shutters with a 350 by 350 mm² aperture were used to produce a small polychromatic X-ray beam with a total flux of about 10¹⁰ photons s^-1 and an energy cut off of 60 keV (Duff et al. 1999 and references therein). The embedded simulant samples were mounted on an automated, digital x-y-z stage at 45^o to the beam. Fluorescent X-rays were detected with a Si(Li) energy dispersive (EDS) detector (30 mm² area) mounted at 90^o to the incident beam and about 1 cm from the sample.

The background contribution to the EXAFS spectra was removed using an algorithm (AUTOBK) developed by Newville et al. (1993), which minimizes R-space values in low k-space Each chi data set was read into the WINXAS analysis package (version 1.3, Ressler, 1999). Replicate scans were co-added to improve S/N. After background subtraction and normalization, the XANES spectra were compared with spectra from the Hg, Rh, Ru and Pd standards and spectra from the literature. For the samples, the Pd-EXAFS spectra were analyzed from 2 to 11 Å^-1, the Ru-EXAFS data were analyzed from 2 to 12.5 Å^-1, the Rh-EXAFS data were analyzed from 2 to 10 Å^-1, and the Hg-EXAFS spectra were analyzed from 2 to 16 Å^-1 (unless otherwise noted). For the reference materials, the Pd-EXAFS spectra were analyzed from 2 to 11 Å^-1, the Ru-EXAFS data were analyzed from 2 to 15 Å^-1, the Rh-EXAFS data were analyzed from 2 to 15 Å^-1, and the Hg-EXAFS spectra were analyzed from 2 to 16 Å^-1 (unless otherwise noted). The k²-weighted chi data were Fourier-transformed to yield R-space or pseudo Radial Distribution Function (RDF) plots as in Sayers and Bunker (1988). The EXAFS spectra that were acquired for the Pd and Rh metal foils, the Ru metal powder and Hg metal reference data were used to estimate phase shifts, coordination environment and radial bond distances. Simulated EXAFS spectra were also generated based on the documented crystallographic properties for Pd metal using ab initio based theory, using FEFF 7.2, a program created by researchers at the Univ. of Washington (Mustre de Leon et al. 1991; Rehr et al. 1991; 1992; Rehr and Albers, 1990; Stern et al.1995).

Samples 1 through 5 were analyzed by FE-SEM techniques for micro-morphological features and for elemental distributions and co-associations. These analyses were performed at the Univ. of GA Athens campus using a Leo 982 Field Emission SEM at the Center for Ultrastructural

Research. Samples were mounted by dusting them on adhesive on carbon discs. No further sample preparation, such as coating with a conductive surface, was necessary to prevent excessive charging. All images were acquired at 20 keV at a working distance of 11 mm. The beam size was less than 3 nm (Lercel et al. 1996) with a resolution of 1.2 nm at 20 keV. BSE images were taken in addition to SE images to highlight areas with significantly different densities. Energy dispersive X-ray spectroscopy was used to confirm the elemental composition at regions of interest (data not shown).

A subset of these five samples was digested in aqua regia to determine the Pd:Hg ratios in the solid phase of the filter cake and the elemental concentrations of Pd and Hg were determined by ICP-MS. These data were used to calculate the Pd:Hg mole ratios in the samples. HR-TEM, LR-TEM, FE-TEM and SAED were performed on one of the samples, Sample 1, which had a starting Pd:Hg mole ratio of 3.4. However, this sample was over 2 months old and more work is needed to confirm the findings that will be presented in this report. A description of the methodology for this work is not included in this report. The TEM analyses were kindly performed on short notice by Dr. John Bradley of Georgia Tech. in Atlanta, GA and MVA.

Table 3. Reference materials and sample that were supplied to SREL for analyses.

Treatment or Reference Material	Noble Metal Added	Noble Metal:Hg Mole Ratio	Specifics
Pd on Alumina Pellet	Pd(II)	NA	Sample was ground before analysis.
Pd on Alumina Powder	Pd(II)	NA	Used as is.
Pd on Silica	Pd	NA	Used as is.
Pd on SrCO₃	Pd	NA	Used as is.
Sample 1	Pd(II)	3.4	Equilibrated for 45^oC for 24 hours and filtered.
Sample 2	Pd(II)	3.4	Replicate treatment of Sample 1.
Sample 3	Pd(II)	1.7	Equilibrated for 45^oC for 24 hours and filtered.
Sample 4	Pd(II)	3.4	Pd reduced onto alumina, equilibrated for 45^oC for 24 hours and filtered.
Sample 5	Pd(II)	3.4	Equilibrated for 45^oC overnight, irradiated, heated and filtered.
Sample X	Pd(II)	3.4	Equilibrated for 45^oC for 12 hours and filtered.
Sample Y	Pd(II)	34	Equilibrated for 45^oC for 12 hours and filtered.
Sample Z	Pd(II)	0.34	Equilibrated for 45^oC for 12 hours and filtered.
Ru on Alumina	Ru	NA	Used as is.
Aged Ru Sample 1	Ru(III)	3.4	Equilibrated for 45^oC overnight for five days and filtered.
Rh on Alumina	Rh	NA	Used as is.
Sample V	Rh(III)	3.4	Equilibrated for 70^oC for 12 hours and filtered.

In our previous report in March 2000, we concluded that the Pd-XANES studies with the SRTC samples did not provide information on the average oxidation state of Pd. This assumption was based on the Pd-XANES edge shift data. Because the XANES information was inconclusive, we obtained preliminary EXAFS data to determine whether EXAFS could help elucidate the structure of the Pd- and Hg-containing materials. The preliminary findings are presented in our earlier March 2000 report.

The Pd metal foil spectra is compared with spectra for the slurries that were equilibrated for 24 hours (Samples 1 through 5) in Figure 2. The similarities in the samples may be due to their similar oxidation state and coordination environment. The amplitude of the oscillations in the post edge XANES spectra for the samples are dampened relative to that of the foil. The spectra suggest the oxidation state of Pd in the samples is Pd(0). Spectra for the slurries that were equilibrated for less than one day (with Pd:Hg ratios of 34, 3.4 and 0.34) are shown in Figure 3. Based on a shift in the Pd -XANES edge toward higher energy relative to Sample 1, which contained Pd metal, Sample Z was more oxidized. Sample X was slightly more reduced than Sample Z. The Pd in the Sample Y treatment with the highest amount of added Hg was more reduced than the other two similarly treated slurry samples. Although the Pd levels in the slurries were quite low and thus had a low S/N, the Pd-XANES spectra for Sample X and Z have a small white-line feature, which was also observed for Pd(II)-containing samples (Fig. 4). This white-line feature, which in the spectra for Samples Z and X and some of the supported Pd reference materials is probably indicative of O in the first Pd coordination shell (Figs. 3 and 4). The Pd in the alumina-supported references were more reduced than the Pd supported on silica and SrCO₃ (Fig. 4).

Due to excessive X-ray scattering from the embedding material, the high florescence background from the samples and low levels of Pd in the HLW sludge simulants, the detection of Pd by the Lytle detector was not satisfactory (see example XANES scan of Tank 15H simulant in Fig. 5). Therefore, the embedded samples were taken to beamline X26a to determine whether another type of detector, such as an energy dispersive Si(Li) detector, would be more suitable than a Lytle detector. [The Lytle detector was our first choice because it has faster electronics than that of a Si(Li) or Ge detector.] At beamline X26A, a sufficient signal was detected for Pd as demonstrated by the SXRF signal obtained for the Pd Ka emission at 21.18 keV (Fig. 6.). The energy cut off of the monochromator at X26a is ~20 keV, making Pd-XANES (which involves scanning the monochromator to obtain monochromatic light over the 24.3 to 24.5 keV range) at the beamline impossible under its current platform. However, the amount of detectable Pd in the samples was sufficient for XANES studies at beamline X23a2 and possibly for EXAFS, if a Si(Li) or Ge detector were used.

Fig. 2. Step-normalized Pd-XANES spectra for Pd foil and Pd slurry samples
that were equilibrated for 24 hours with Pd:Hg mole ratios of 3.4 and 1.7.

Fig. 3. Step-normalized Pd-XANES spectra for Pd slurry samples that were equilibrated
12 hours at Pd:Hg mole ratios of 34 (low Hg), 3.4 (medium Hg) and 0.34 (high Hg).
Sample 1, which had a Pd:Hg ratio of 3.4 was equilibrated for 24 hours at 45^oC before analysis at NSLS.

Fig. 4. Step-normalized Pd-XANES spectra for the Pd foil and the supported Pd references.

Fig. 5. Raw Pd-XANES spectra for HLW Sludge No. 6 material
as detected with a Lytle detector at beamline X23a2.

The Hg-XANES spectra for Samples 1 through 5 resembled that of published spectra for metallic Hg (Fig. 7; San Miguel et al. 1995). In contrast, the spectra for the samples that were equilibrated for less than 1 day indicate that most of the Hg is present as Hg(II) and is probably present as unreacted DPM (Fig. 8). The XANES spectra for the Hg(II) oxide standard, which is shown for reference in Figure 7, has a pre-edge feature in the spectra (at 0.5 of the normalized fluorescence counts), which complicates the interpretation of the Hg L₃-XANES edge. If one examines the position of the small whiteline feature, there is shift in the XANES edge. A DPM standard was not examined with XANES techniques. Figure 9 shows the Hg-XANES spectra for the Ru and Rh TPB slurries that had been equilibrated for less than one day. The spectra do not resemble that of Hg metal and it is quite likely that the spectra are indicative of (unreacted) DPM (San Miguel et al. 1995). The EXAFS analyses for the Pd, Ru and Rh samples indicate that DPM is present as the dominant Hg species in these slurry samples as will be discussed.

The Hg-XANES studies with the HLW simulants (Table 4) indicated that there would be few problems in detecting the XAS of the Hg at the L₃ absorption edge even if a Lytle detector were used (Fig. 10). Collectively, these studies indicate that suitable Hg-XANES and Hg-EXAFS spectra could be acquired with the Lytle detector for the HLW samples.

The Ru-XANES studies for Aged Ru Sample 1 (the Ru-containing slurry equilibrated for 5 days) indicate that the Ru is present as oxidized Ru (Fig. 11). It is probably present as Ru(III) but more XANES analyses with Ru(II, III and IV) standards would be required to confirm this observation. The average oxidation state of the Ru in the Ru supported on alumina reference material appeared to lower than the oxidation state of the Ru in the aged Ru TPB treatment but higher than that of the Ru metal in the Ru powder reference. More Ru-XANES studies would be needed if average oxidation state information of the Ru in these materials were desired. However, the Ru-EXAFS data for these solids provides additional characterization information (to be discussed).

Due to insufficient time, fairly noisy Ru-XANES spectra were acquired for the simulants (Fig. 12) and better quality spectra are obtainable. The data demonstrate that Ru-XAFS studies are possible with the simulants.

The Rh-XANES studies for Sample V (the Rh-containing slurry that was equilibrated for less than 24 hours) indicate that the Rh is present as oxidized Rh (Fig. 13). It is probably present

Fig. 6. White light SXRF plot of HLW Simulant 6 taken at beamline X26A with a Si(Li) detector.

Fig. 7. Step-normalized Hg-XANES spectra for the five Pd slurry samples
(Samples 1 through 5) that were equilibrated for 24 hours at a Pd:Hg mole ratios of 3.4 and 1.7.

Fig. 8. Step-normalized Hg-XANES spectra for Hg-Pd slurry samples that were equilibrated
for 12 hours at Pd:Hg mole ratios of 34 (Sample Y), 3.4 (Sample X) and 0.34 (Sample Z).

Fig. 9. Normalized Hg-XANES spectra for Aged Ru Sample 1 and 12-hour Rh "Sample V".

Fig. 10. Normalized Hg-XANES spectra for HLW Simulants 5 and 6
as detected with a Lytle detector at beamline X23a2.

Fig. 11. Normalized Ru-XANES spectra for the Ru powder,
Ru slurry treatment and Ru supported on alumina.

Fig. 12. Normalized Ru-XANES spectra for HLW Simulants 5 and 6
as detected with a Lytle detector at beamline X23a2.

Fig. 13. Normalized Rh-XANES spectra for the Rh Sample V and the Rh supported on alumina.

as Rh(III), but more XANES analyses with Rh(II, III and IV) standards would be required to confirm this observation. The average oxidation state of the Rh in the Rh supported on alumina reference material appeared to lower than the oxidation state of the Rh in Sample V but higher than that of the Rh metal in the Rh foil reference. More XANES studies are needed if the average oxidation state of the Rh in these materials is to be determined. However, the EXAFS data for these solids provides additional characterization information (to be discussed).

As with the HLW simulant studies that involved the detection of Pd, there were considerable problems associated with using the Lytle detector to detect the levels of Rh in the simulants. Therefore, studies were performed at beamline X26a and these studies demonstrated that a Si(Li) detector would be more appropriate for these studies than a Lytle detector and that suitable XANES and EXAFS data could be acquired on the HLW samples (Fig. 6).

The k²-weighted chi data (the plot of the wavevector in reciprocal space) for the Pd foil standard, the Pd(II) standard, the alumina- and silica-supported Pd reference materials are shown in Figures 14 through 20. The raw EXAFS data (showing a acquisition glitch in the software that limited our ability to use all of the spectra) and k²-weighted chi data for the Pd supported on SrCO₃ are shown in Figures 21 and 22. In simple terms, these chi data plots show the oscillation patterns (both constructive and destructive interference patterns) of the atoms in the neighbor environment of the Pd—part of the photoelectron wave that can be defined by the EXAFS equation (Teo, 1986; Prins and Koningsberger, 1988 and references therein; Stern, 1974). The EXAFS equation is shown in a highly simplified form below (see list of definitions for explanation of equation terms):

At the Pd K-edge, a glitch created a small erroneous signal in the data at around 11.5 to 13 Å^-1, which was most prevalent in the Pd slurry samples due to their lower signal relative to the Pd metal foil (as shown at high k-space in Fig. 16). Due to these glitches, the spectral analyses for all of the Pd-containing samples, standards and references were performed consistently for 0 to 11 Å^-1 in chi space.

The chi data for the alumina-supported Pd reference materials (Figs. 18, 19 and 22) all Pd-containing slurry samples (Figs. 23-31, to be discussed) indicate there are light and heavy mass

Fig. 14. The full collected spectrum of the k²-weighted chi data for the Pd metal foil standard.

Fig. 15. The k²-weighted chi data for Pd(II) hydrous oxide standard (data from 01/00).

Fig. 16. The energy spectrum of the k²-weighted Pd chi data collected for Sample 1.
A glitch at about 11.5 to 13 Å^-1 made the remaining high-energy portion of the spectra unusable.
Therefore, the energy region between 0 and 11 Å^-1 was used for the EXAFS data analyses.

Fig. 17. The portion of the energy spectrum of the k²-weighted chi data for the
Pd metal foil standard that was used to fit the chi data of the Pd-containing samples.

Fig. 19. The k²-weighted chi data for (ground) Pd supported on alumina pellet.

Fig. 21. The raw Pd-EXAFS data for Pd supported
on SrCO_3¾showing portion of usable data.

elements in the first coordination shell of the Pd. This is observable by comparing the "envelope" or the amplitude of the chi data with respect to reciprocal distance for the foil, the Pd(II) hydrous oxide spectra and the samples. For the foil, the amplitude increases with greater reciprocal energy space (at ~8 Å^-1), which is generally indicative of a Pd-Pd interaction (Fig. 14). When a light atom in the first shell and a heavy atom are nearby, the amplitude first increases, decreases and then increases with increasing reciprocal energy. The samples and Pd foil also have a small "beat" pattern near 10 Å^-1 (Figs. 14 and 16). These features are possibly due to the presence of two or more different bond distances in a first coordination shell or multiple scattering (Sayers and Bunker, 1988; Teo, 1986). The chi data for Pd on alumina pellet has a stronger contribution of a light atomic weight atom (most likely O or Al) than the Pd supported on alumina powder relative to that of the envelope for the heavier mass element, which is most likely Pd (Figs. 18-19). The local environment of the Pd in the silica-supported Pd reference material is dominated by one or more light atomic weight atoms in the local coordination environment—most likely O or Si (Fig. 20). There is little contribution of a heavy atom in the chi data for this material. Collectively, these data for the Pd on silica suggest Pd is not present as metallic Pd. The k²-weighted chi data for Pd supported on SrCO₃indicate that a light atom (such as O or C) and a heavy atom (possibly Sr and/or Pd) are present in the local neighbor environment around the Pd (Fig. 22).

The k²-weighted chi data for the Pd Slurry Samples 1-5, X, Y, and Z (described in Table 3) are shown in Figures 23 through 30. As with the Pd on alumina references, there appears to be a light atomic weight atom and a heavy atomic weight atom in the local environment around the Pd atom for all of these samples. The chi data for Samples 1 through 5 are very similar whereas the chi data for Samples X, Y, and Z differ greatly. The Pd concentrations in Samples X, Y, and Z were considerably lower than that of Samples 1 through 5 (based on the total absorption signal, which is related to the Pd concentration)—as indicated by the low S/N in the chi data. An overlay (Fig. 31) of the chi data for Samples X, Y, and Z shows the differences in the chi data, which may be due to the influence of the starting DPM concentrations.

The pseudo radial distribution function (RDF) distribution plots for the standards and Pd reference materials are shown in Figures 32 through 36. In simpler terms, these plots give radial information about the atom "density" around the Pd in the sample. Generally, the height of each FT magnitude peak is indicative of the number of bonds and the position of each peak centroid is indicative of the bond length. However, these relationships vary with distance in R-space. These data are uncorrected for phase shift--a phase correction (~0.5 Å) must be applied to the FT data to account for the interference between the incoming and outgoing photoelectron wave. However, not all of the peaks in the RDF plots are what can be referred to as "real" in the spectra. For example, the peak at near 2 Å in Figure 32 is probably the result of destructive interference.

The RDF data for Pd in the foil show a strong first shell Pd-Pd interaction (Fig. 32). A fit of the first shell fit for Pd-Pd metal interactions indicates that Pd is in a FCC structure with a CN of ~12 and that the Pd-Pd bond length is about 2.763 Å (Fig. 32).

Fig. 32. Fourier-transformed chi data (RDF) and first shell Pd-Pd model fit data
for the Pd metal foil standard—uncorrected for phase shift.

Fig. 33. Pd RDF data for the Pd supported on alumina powder—uncorrected for phase shift.

Fig. 34. Pd RDF data for the Pd supported on ground alumina pellet—uncorrected for phase shift.

Fig. 35. Pd RDF data for the Pd supported on silica—uncorrected for phase shift.

FEFF fits were not performed on the supported Pd materials but several observations could be made comparing the RDF data for the reference materials with the RDF data for the foil. In contrast to the foil RDF data, the RDF data for Pd on alumina powder show considerably more atom density at shorted R distances than in the RDF data for the Pd foil (Fig. 33). This is most likely due to bonding between O and or Al. The Pd-EXAFS data for Pd supported on alumina pellet (Fig. 34) contains a considerably greater contribution between Pd and a light atomic weight atom than that observed for Pd on (ground) alumina powder. These observations are consistent with that of the Pd chi data that was presented in the previous section. The two Pd supported on alumina reference materials also contain a Pd-Pd interaction. The Pd on silica RDF spectra indicate that most of the Pd is bound to a light molecular weight atom (at a bond length of less than 2.5 Å) and a small contribution of Pd-Pd bonding is present (Fig. 35). The Pd RDF data for Pd on SrCO₃ indicate there is some Pd-Pd metal bonding (Fig. 36).

4.3.1.4. Fourier-Transformed Chi and First Shell Fits for a Pd Interaction with One or More High Mass Elements.

Plots of the Pd RDF data and the corresponding output of the FEFF model fits of the first shell Pd-Pd interactions for Samples 1-5 are shown in (Figs. 37-41 and Table 5). The RDF data and model fits for Samples 1 through 5 were fairly similar and Pd was the only high atomic mass atom identified in the first shell for these samples. Based on model comparisons of the RDF spectra for metallic FCC Pd in the Pd foil (Fig. 32), it was evident that first shell Pd-Pd bonds, with average Pd-Pd bond distances of 2.77 Å existed in Samples 1 through 5 (Figs. 37-41 and Table 5). The average CN for the Pd clusters (determined to be FCC) in Samples 1 through 5 ranged from 5 to 7. Typically, EXAFS has an associated error of ± 1 for an absolute coordination number (N) determination (Sayers and Bunker, 1988). Because the CN for first shell Pd was considerably less than 12, the Pd in these samples was most likely present as a cluster form, which has a substantial amount of Pd atoms on the surface relative to Pd atoms in the bulk.

Based on the mnp site indices, the clusters (if spherical) probably range between 18 and 42 Pd atoms in size (see Table 6). Based on maximum mnp site indices, a cluster that has an average CN of 6.3 to 7.4 would have shell radii of 3.8 to 4.65 Å. If the CN were less than 6.3 (which is the case for Samples 2, 3 and 5), the cluster (if spherical) would be small enough for the total number of Pd less 1 to be on the exterior of the cluster. The calculated radial Pd-Pd distances in all the samples are >0.01 Å longer than in Pd metal (2.76 Å, Table 5).

Higher (near neighbor) shell Pd-Pd bonds are observed at 3.8 Å and 4.76 Å for Samples 1 through 5 (Pd radial distances in FCC crystal structure: 2.75, 3.89, 4.76, 5.50, 6.15, 6.74, 7.28, 8.25 Å). Higher shell fits for heavy atoms in a FCC structure were possible--indicating that higher coordination shells existed beyond the first coordination shell for the Pd but an identification of the atoms in those shells could not be made (data not shown).

The plots in Figures 42-45 represent the Pd RDF and FEFF model fit data for Pd in Samples X, Y, and Z, which were equilibrated for about 12 hours. Unlike the Pd-EXAFS data for Samples 1-5, the Pd-EXAFS data for Samples X and Y contained both Pd and Hg in the first coordination shell (Table 5). The large difference in the atomic weight of Hg and Pd greatly facilitates the use of EXAFS to differentiate between these two atoms in the Pd structure. The total CN for Pd in these two samples did not exceed 12, which is consistent with a close-packed environment. These data reflect some of the changes that were taking place upon cluster formation. The presence of Pd–Pd bonds and Pd-Hg bonds in the first coordination shell of the Pd at the initial stage of the reaction indicates that the Pd may be nucleating around the Hg. The Pd-EXAFS data for Sample Z, which had the lowest initially added Hg concentration (Fig. 44), did not have Hg in the first coordination shell of the Pd. Figure 45 shows an overlay of the RDF data for Samples 1, X, Y, and Z. It is evident that these four samples have unique structural chemistries.

To determine which light atomic weight atom or atoms are present in the first coordination shell, Pd-EXAFS simulations were performed for interactions between Pd and one of the following atoms: Be, B, C, O, Na, Al and P for Samples 1-5 (Fig. 46, fits for Samples X, Y, and Z not performed). Although some of these atoms are not likely to be in the TPB system that we studied, the fits demonstrate that a unique identification of the atom or atoms in the first shell was not possible for many of the samples, despite the high quality of the Pd-EXAFS data. Samples 3 and 5 appeared to have a good degree of fit for a Pd-O interaction in the first coordination shell (Fig. 46). The bond length for Pd and the light molecular atom or atoms was about 2 Å. No fits were performed for the three samples that were equilibrated for less than 24 hours (i.e., Sample X, Y, and Z).

The analyses of the Hg-EXAFS spectra for Samples 1 through 5 were conducted over a wider energy range (2 to 16 Å^-1) than that of the Pd EXAFS studies (2 to 11 Å^-1). Samples X, Y and Z had considerable noise so the usable EXAFS data were confined to 2 to 14 Å^-1 in k-space for that Hg-EXAFS data set. As with the Pd chi data for the samples, the Hg chi data for the samples showed the presence of both low and high atomic weight atoms in the first coordination shell around the Hg (Fig. 47-53). However, there were substantial differences in the Hg-EXAFS data for Samples 1 through 5 and Samples X and Y in that there appears to be a considerable contribution of a low atomic weight atom in the chi data for Sample Y. An overlay of Samples 1, X and Y (Fig. 54) shows that a low weight atom dominates the Hg-EXAFS. However, this dominance does not preclude the presence of Hg-metal first shell bonding (to be discussed).

The Hg-EXAFS data for the Ru and Rh samples had considerable noise due to the low Hg concentrations that were present. Therefore, the analysis of the Hg-EXAFS for Aged Ru Sample 1 was conducted over 2 to 12.5 Å^-1 and the analyses for Rh Sample V were confined to 2 to 11 Å^-1 in k-space. The Hg chi data for the samples provided little information other than that there was probably some low atomic weight atom in the local Hg environment for these two samples (Figs. 55 and 56).

Fig. 36. Pd RDF data for Pd supported on SrCO₃ reference—uncorrected for phase shift.

Fig. 37. Pd RDF and first shell Pd-Pd model fit data for Sample 1—uncorrected for phase shift.
Vertical dashed lines denote region selected for Fourier filtering and FEFF fits for the Pd Samples.

Fig. 38. Pd RDF and first shell Pd-Pd model fit data for Sample 2—uncorrected for phase shift.

Fig. 39. Pd RDF and first shell Pd-Pd model fit data for Sample 3—uncorrected for phase shift.

Fig. 40. Pd RDF and first shell Pd-Pd model fit data for Sample 4—uncorrected for phase shift.

Fig. 41. Pd RDF and first shell Pd-Pd model fit data for Sample 5—uncorrected for phase shift.

Fig. 42. Pd RDF and first shell model fit data for Sample X—uncorrected for phase shift.

Fig. 43. Pd RDF and first shell model fit data for Sample Y—uncorrected for phase shift.

Fig. 44. Pd RDF and first shell Pd-Pd model fit data for Sample Z—uncorrected for phase shift.

Fig. 45. Overlay of Pd RDF data for Samples 1, X, Y, and Z—uncorrected for phase shift.

Fig. 46. First coordination shell fit residual information for
Pd interactions between a light atom for the Pd-TPB slurry samples.

Table 5. First coordination shell Pd-Pd and Pd-Hg fit data for Pd metal foil
and the Pd-containing unknowns that were equilibrated with TPB, DPM
and TPB decomposition products—based on the Pd-EXAFS analysis.

Sample	First Shell Neighbor Interaction	Bond Distance r[Å]	N	s ²[Å]²
Pd metal foil	Pd-Pd	2.76	12.02	0.00313
Sample 1	Pd-Pd	2.77	7.01	0.00643
Sample 2	Pd-Pd	2.77	6.16	0.00562
Sample 3	Pd-Pd	2.77	5.08	0.00689
Sample 4	Pd-Pd	2.77	6.38	0.00579
Sample 5	Pd-Pd	2.77	5.55	0.00733
Sample X	Pd-Pd	2.76	4.66	0.00269
	Pd-Hg	2.75	6.30	0.00873
Sample Y (high Hg added)	Pd-Pd	2.75	1.52	0.00393
	Pd-Hg	2.81	10.1	0.01369
Sample Z (low Hg added)	Pd-Pd	2.75	6.70	0.00393
	Pd-Hg	Not detected	Not detected	Not detected

Table 6. Average numbers of first shell neighbors
with maximum mnp indices (Kodre et al. 1999).

mnp	Number of Atoms in Cluster	Average Number of First Shell Pd Neighbors	mnp-shell Radius (Å)
000	1	0	-
011	12	6.0	2.69
201	18	6.3	3.80
102	42	7.4	4.65
031	54	8.0	5.37
220	78	8.6	6.01

No reference Hg metal solid was available to serve as a suitable Hg metal EXAFS reference. Simulations of rhombohedral Hg metal with a CN of 6 are possible and Hg-EXAFS data for metallic Hg exist in the literature (San Miguel et al. 1995). The Hg-EXAFS RDF data for Samples 1 through 5 indicate that no Hg-Hg bonding is present in the first coordination shell, the structure is FCC, and only Pd is present as shown in Figures 57 through 61 and Table 7. The fit data for the determined that CN for the Hg was about 12 for Samples 1 through 4. Unlike Samples 1 through 4, Sample 5 (gamma irradiated) had a CN of 9.76. The bond distance for the Hg-Pd interaction was near 2.79 Å for Samples 1 through 5 (Table 7). Higher shell fits for FCC metal were possible as shown for Sample 1 in Figure 62. However, no identifications of second shell atom interactions were made, such as Hg-Pd-Hg. This type of interaction is possible but due to the complexity of such fits, an identification of second shell elements was not made.

First shell metal FEFF fits were conducted on Samples X and Y but not on Sample Z—due to the low amount of Hg in the sample (Figs. 63-66). The fits for a single interaction for a heavy metal in the first shell for a Hg-Pd interaction were not as good as fits for a Hg-Pd and a Hg-Hg interaction in the first shell (compare Figs. 63 and 64 with 65 and 66). An overlay of the Hg RDF data for Samples 1, X and Y is shown (Fig. 67) to demonstrate that there are substantial differences in the Fourier-transformed data for these samples. In comparison to the Hg-EXAFS data for Sample 1, the Hg-EXAFS data for Samples X and Y have a more significant interaction between Hg and a low atomic weight atom (Fig. 67). Figure 68 shows a FEFF simulation of DPM along with an overlay of spectra for Samples X, Y, and 1. The centroid of the second peak in the Fourier transformed chi data at 1.6 Å (uncorrected for phase shift) is the position of the Hg-C peak in the simulation. The length of the Hg-C bond in DPM is 2.0 Å (McAuliffe, 1977) and with the phase shift, the centroid of the second peak in the RDF data for the Pd samples would be about is 2.0 Å, which is consistent with the simulation. From these data, it is quite possible that some DPM is in the filter cake.

The peak at 1 Å is not representative of the first shell coordination environment of the Hg because the bond length would be too short. It is possible that this feature in low R-space is due Atomic X-ray Absorption Fine-Structure oscillations (AXAFS). AXAFS oscillations are usually related to the scattering associated with the periphery of the absorbing atom. AXAFS can also include the interaction between the absorbing atom and hydrogen (Koningsberger and Prins, 1988; De Groot et al. 2000). However, although AXAFS has been applied to clusters, it is our opinion that AXAFS studies are under considerable development and therefore any observations made about them should be treated with some caution.

Fig. 57. Hg RDF data and first shell model fits for Sample 1—uncorrected for phase shift. .

Fig. 58. Hg RDF data and first shell model fits for Sample 2—uncorrected for phase shift. .

Fig. 59. Hg RDF data and first shell model fits for Sample 3—uncorrected for phase shift. .

Fig. 60. Hg RDF data and first shell model fits for Sample 4—uncorrected for phase shift. .

Fig. 61. Hg RDF data and first shell model fits for Sample 5—uncorrected for phase shift. .

Fig. 62. Hg RDF data and higher shell model fits for Sample 1—uncorrected for phase shift..

Fig. 63. Hg RDF data with fit data for Hg-Pd for Sample X—uncorrected for phase shift. .

Fig. 64. Hg RDF data with fit data for Hg-Pd for Sample Y—uncorrected for phase shift. .

Fig. 65. Hg RDF data for Sample X with Hg-Hg and Hg-Pd fit data—uncorrected for phase shift. .

Fig. 66. Hg RDF data for Sample Y with Hg-Hg and Hg-Pd fit data—uncorrected for phase shift. .

Fig. 67. Overlay of Hg RDF data for Samples 1, X and Y with Hg-Hg and Hg-Pd fit data—uncorrected for phase shift. .

Fig. 68 Overlay of Hg RDF data for the DPM simulation and Samples X, Y and 1—uncorrected for phase shift. .

The RDF data for Hg in the Ru and Rh treatments are shown in Figures 69 and 70. Due to the poor quality of the data (as a result of low sample Hg concentrations), no fits were performed on the EXAFS spectra. However, some conclusions can be made. For the spectra in Figs. 69 and 70, the first peak in the Fourier transform is most likely due to AXAFS and the second peak is most likely due to an interaction between Hg and C. The length of the Hg-C bond in DPM is 2.0 Å (McAuliffe, 1977) and with the phase shift, the centroid of the second peak in the Hg RDF data for would be at 2.0 Å, which is consistent with the data from the FEFF simulation of DPM (Fig. 68). The remaining peaks in the RDF data may be representative of bonding between 2 types of heavy atoms but the quality of the data limits our ability to successfully determine the identity of these atoms and their radial distances from the absorbing Hg atom.

The chi data for Ru in the metal powder and for the Ru supported on alumina are shown in Figures 71 and 72. As can be expected, the Ru in the metal powder (Fig. 71) is clearly dominated by a heavy atom (Ru) and the chi spectra have several "beat" patterns at 12, 13 and 14.5 Å ^-1. The chi data for the Ru supported on alumina (Fig. 72) show that light and heavy atoms are present in the local coordination environment around the Ru and several beat patterns are also present as in the Ru metal standard. The noise in the chi data for the Aged Ru Sample 1 (Fig. 73) demonstrates that the Ru concentration was quite low. However, the spectra show a greater contribution of a low mass atom in the local environment around the Ru than that of Ru in the alumina-supported material (Fig. 72). The crystal structural information for Ru metal is needed to interpret and fit the EXAFS data from unknowns. The structure of Ru metal is thought to be HCP, with a space group of P63/mmc but our FEFF fits for the Ru powder standard suggest the structure of Ru in the material is somewhat like FCC, with a space group of Fm-3m. Figure 74 shows an overlay of the chi data for HCP Ru metal FCC Ru metal and the chi data for the Ru metal powder. However, since the FEFF fits for FCC Ru and HCP Ru are based on an amplitude reduction factor of 1.0 and the amplitude reduction factor for Ru in the spectra for the Ru metal powder was 0.58, the amplitude of the model and real chi data for Ru metal differ. The chi data for Ru metal powder resemble both FCC and HCP. Nevertheless, from this information it is not clear what the structure of the Ru standard is so no fits were performed to describe the data for the Ru metal powder.

The RDF data for the Ru metal standard and the Ru supported on alumina were fairly similar--suggesting that the Ru supported on alumina contains mostly metallic Ru (Figs. 75-76). No FEFF fits were performed for the Ru supported on alumina. The first shell FEFF fits for the Ru in the Aged Ru Sample 1 indicate that the first coordination shell contains both Ru at 2.52 Å with a CN of 2.2 (DW of 0.00599) and Hg at 2.94 Å with a CN of 4.11 (DW of 0.00267). Model fits were not performed for the light atomic weight atom because of the high probability that an exact determination of the atom identity was not possible.

Fig. 74. The overlay of k²-weighted Ru chi data for Ru metal powder, HCP Ru and FCC Ru.

Fig. 76. Ru RDF data for Ru supported on alumina—uncorrected for phase shift.

Fig. 77. Ru RDF and first shell metal fit data for Ru supported on alumina—uncorrected for phase shift.

The chi data for Rh in the metal foil and for Rh supported on alumina are shown in Figures 78 and 79. As can be expected, the Rh in the metal foil (Fig. 78) is dominated by a heavy atom (Rh) and the chi spectra for the Rh foil have several "beat" patterns at 10, 13, and 15 Å ^-1. The chi data for the Rh supported on alumina show that light and heavy atoms are present in the local coordination environment around the Rh, in addition to several beat patterns. In comparison to the chi data for Ru supported on alumina (Fig. 72), the chi data for the Rh on alumina contains more bonding to one or more light atomic weight atoms (Fig. 79). The high noise in the chi data for the Rh Sample V (Fig. 80) demonstrates that the Rh concentration was quite low and few conclusions can be made about the chi spectra.

The crystal structural information for Rh metal is needed to interpret and fit the EXAFS data for the unknowns. The structure of Rh metal is thought to be FCC, with a space group of Fm-3m. Figure 78 shows an overlay of the chi data for FCC Rh metal and the chi data for Rh in the Rh metal foil. From this information the structure of the Rh standard appears to be FCC but due to the low S/N, no fits were performed for the Rh in Sample V.

4.4.4.2. Fourier-Transformed Chi Data and First Shell Metal Fits for Rh Reference and the Rh-containing TPB Slurry.

The RDF data for the Rh metal standard and the Rh supported on alumina were somewhat similar--suggesting that the Rh supported on alumina contains some metallic Rh (Figs. 81-82). A fair amount of bonding between a low molecular weight atom such as O or Al is likely in the supported Rh reference and O bonding is likely in the Rh Sample V (Fig. 83). First shell FEFF fits for a metal in the Rh-EXAFS spectra for the Rh on alumina were not made. Fits for Rh in Sample V were not possible due to the poor quality of the data but the data provide little evidence of cluster formation.

Fig. 78. The k²-weighted Rh chi data for Rh foil and for a FEFF simulation of Rh FCC metal.

Fig. 82. Rh RDF data for Rh supported on alumina¾ uncorrected for phase shift.

Figures 84 through 86 show several FE-SEM photomicrographs for Samples 1, 4 and 5 (A SEM BSE photomicrograph for Sample 3 and energy-dispersive X-ray spectroscopic data for not shown). The images contain what appear to be "fractal" arrangements of Na-, Pd- and Hg-rich clusters that are intimately associated with the KTPB material that was added to the reaction initially. The fractal nature of these particles is most clearly seen in Figure 84f. Unlike Samples 1, 3 and 4, Sample 5 appeared to have rounded particles, which may have formed as a result of the gamma irradiation (Fig. 86).

The highest degree of magnification used with the FE-SEM was 50000X. At this magnification, instrumental vibrations exert a negative influence on the photomicroscopy by producing images of marginal resolution. These analyses provide micro-morphological information about particles that are much larger than in size than that of nanoclusters. The photomicrographs suggest aggregates of Pd and Hg are present. However, a higher resolution

Technique, such as TEM is needed to determine whether nanoclusters are present (to be discussed). This information will help confirm the EXAFS observations that suggest Pd nanoclusters are present.

The results of the digestions of the filter cake solids that were used in the EXAFS studies are shown in Table 8. These numbers indicate that some loss of Hg occurred upon filtering. The results also indicate that the ratio of Pd to Hg in the filter cakes is roughly proportional to the Pd:Hg ratio that was initially added (compare Samples 1, 2, 4 and 5 with Sample 3 in Table 8). The data also suggest that there is not enough Hg in the solid phase to have Hg-Hg bonding in the first coordination shell. This is based on calculating how much Hg can be doped into the FCC structure of with the formation of Hg-Pd-Hg bonds and without the formation of Hg-Hg bonds. We calculated that at least 25 mole % Hg could be put into the FCC Pd structure without the formation of Hg-Hg bonds.

The FE-TEM, LR-TEM, HR-TEM and SAED results (Figs. 87-90) indicate there are two populations of Pd-containing species. One of which is a nanocluster form of Pd that ranged between 5 and 10 nm in diameter. The second type of species consists of a coarse 100-nm thick film that contains Pd and Hg. This material appears to be coating the KTPB solids.

The SAED results indicate that the crystal size of the Pd nanoclusters is smaller than beam size and that the crystal is not oriented (i.e., it is powder-like) and diffraction pattern resembles FCC Pd. FCC Pd metal has a diffraction pattern with the following d-spacing values of 2.246, 1.945, 1.375, 1.173, 1.123, 0.973, 0.892 and 0.870 Å with the corresponding intensities (in %) of 100, 60, 42, 55, 15, 13, 40 and 11. The Hg-rich Pd material had a crystal size that was larger than the beam size and had a diffraction pattern that strongly resembled FCC Pd.

Table 8. Ratios of Pd and Hg in that were in Samples 1 through 5
as determined by acidic digest ¾ -followed by ICP-MS quantification.

Fig. 84. SEM photomicrographs of Sample 1 showing a) SE image with
b) complementary BSE image, c) SE image with
d) complementary BSE image and e) SE image with
f) complementary BSE image with noted magnifications.

Fig. 85. SEM photomicrographs of Sample 4 showing a) SE image with b) complementary BSE image,
c) SE image with d) complementary BSE image with noted magnifications (rectangle in "d" denotes the region
where energy-dispersive X-ray spectra were taken). Center particle in a) and b) is magnified in c) and d).

Fig. 86. SEM photomicrographs of Sample 5 (irradiated) showing a) SE image with
complementary b) BSE image, c) SE image with d) complementary BSE image, and
e) SE image with f) complementary BSE image with noted magnifications.

Fig. 89. FE-TEM images of Sample 1 (aged 2 months in air) a) lattice fringe and b) dark field.

Fig. 90. SAED images of Sample 1 (aged 2 months in air) for
the a) Pd nanoclusters and b) the PdHg coarse material.

Palladium K-edge XANES are not effective for determining average oxidation state information but these studies can be used to qualitatively demonstrate that Pd is metallic and present as a cluster. The XANES studies provide a foundation of spectral information that will be used to support the proposed XANES studies with actual HLW. The forthcoming XANES studies with HLW should yield some chemical information about the oxidation states of the Hg, Ru, Rh and Pd in the HLW.

EXAFS spectra can estimate size of very small clusters that are present in the sample and TEM studies confirm the presence of clusters. There were slight differences in average cluster size for Pd coordination for the samples that had been equilibrated for 24 hours. The number of atoms that can make a 100 nm-thick HgPd film is considerably greater than the cluster size that was determined for Pd. It is possible that the film is porous and contains nanostructural features that result in a significantly high fraction of Pd atoms at grain boundaries. Löffler and Weissmüller (1995) present X-ray scattering data and empirical models on the association of Pd atoms at grain boundaries of fine particles of Pd clusters. They provide evidence that large conglomerates of nanoparticles of Pd may have a low average CN due to the high fraction of atoms at grain boundaries. This would explain how the average first shell CN of the FCC Pd in Samples 1 through 5 is less than 12, even though size of the HgPd coarse material greatly exceeds the size of nanoclusters.

According to the interpretations of the Pd-XANES and Pd-EXAFS data, the supported Pd materials were different with respect to the amount of Pd-Pd metallic bonding and Pd-light atomic weight atom associations and their average oxidation state. These differences may relate to their observed catalytic reactivities toward TPB degradation (Barnes, M. J. personal communication).

EXAFS can be used determine the influence of Hg on the reaction. Cluster formation is not greatly influenced by radiation, Hg level or the presence an alumina support. However, cluster formation may not always equate to high catalytic activity. For example, if too much of an inert metal (such as Hg) were present on the outside of the nanoclusters, there may be a loss in reactivity at low initial Pd:Hg ratios. This has been observed with studies at SRTC (Barnes, M. J. personal communication).

The authors wish to thank J. Woicik (NIST), A. Ackerman (BNL), W. Rao, C. Strojan (SREL), R. A. Peterson, M. J. Barnes, S. D. Fink, F. Fondeur, S. Serkiz, W. R. Wilmarth, D. D. Walker, W. Tamosaitis (SRTC), J. Boncella (Univ. of Florida), and R. B. King (UGA) for their assistance, support and bright ideas.

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Sludge Simulant		Descriptions:

Base Sludge 1: High Fe/Low Al/Low Bi (U surrogate)
Base Sludge 2: Low Fe/High Al/Low Bi (U surrogate) - Tank 15H simulant
Base Sludge 3: High Fe/Low Al/High Bi (U surrogate)
Base Sludge 4: Low Fe/High Al/High Bi (U surrogate)

			Target Concentration

Sludge	Base		Pd	Rh	Ru	Hg
Sample	Sludge ID		(wt %)	(wt %)	(wt %)	(wt %)

5	1		0.00047	0.014	0.059	3.32
6	2		0.00047	0.014	0.059	3.32
7	3		0.00047	0.014	0.059	3.32
8	4		0.00047	0.014	0.059	3.32
9	2		0.000235	0.007	0.0295	1.66
10	2		0.00094	0.028	0.118	6.64

Sample	First Shell Neighbor	Bond Distance r[Å]	N	s ²[Å]²
Hg metal foil*	Hg-Hg	3.01	6.00	Not applicable
Sample 1	Hg-Pd	2.78	11.95	0.00889
Sample 2	Hg-Pd	2.79	12.24	0.00829
Sample 3	Hg-Pd	2.79	11.97	0.00817
Sample 4	Hg-Pd	2.79	12.07	0.00819
Sample 5	Hg-Pd	2.80	9.76	0.00813
Sample X	Hg-Hg	2.77	4.94	0.02538
	Hg-Pd	2.74	5.21	0.01399
Sample Y (high Hg)	Hg-Hg	2.90	5.20	0.01569
	Hg-Pd	2.79	3.81	0.00985
Sample Z (low Hg)	Hg-Hg	No data	No data	No data
	Hg-Pd	No data	No data	No data