Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Highly dispersed supported platinum clusters have been extensively studied because of their many uses as the active metallic component in heterogeneous catalysts, including catalytic reforming. EXAFS modelling has been used to determine the average coordination number for the first through fifth Pt shells and thereby determine the average cluster size and shape. Several studies have shown that Pt clusters on gamma-Al2O3 are truncated to form plate-like clusters in the hexagonal (111) plane as evidenced by an overly reduced second shell coordination number, which would be zero for a truly 2-dimensional cluster. In our system of Pt on gamma-Al2O3 the average Pt morphology is determined, on the average, to be small similar to 13 atom clusters that are 2-diminsional in the hexagonal (111) plane morphology as determined by first and, nearly zero, second nearest neighbour coordination numbers of 4.4 +/- 0.6 and 0.5 +/- 0.3, respectively.

The Materials Research Collaborative Access Team (MRCAT) has completed construction of its bending magnet beamline at the Advanced Photon Source, with commissioning ongoing since October 2008. Full operations including General User access will begin in January 2010. The beamline is designed to operate in two distinct modes: pink beam for lithography, photochemistry and tomography; and monochromatic beam for x-ray absorption spectroscopy and tomography. Pink beam is obtained by means of a 880 mm water cooled Pt mirror combined with filters, while monochromatic beam is selected using a water-cooled double-crystal Si(111) monochromator, providing an energy range from below 4 keV to greater than 33 keV. Switching between modes is accomplished in under one hour.

A four-channel ionization chamber has been designed, constructed and tested. This ionization chamber allows X-ray absorption spectra to be collected in transmission from up to four samples simultaneously. This results in spectra that are free of systematic uncertainty in relative energy alignment introduced by scan-to-scan stability of the monochromator or of numerical uncertainty associated with a post-processing alignment algorithm, allowing, in a single shot, an absolute measure of edge shift between four samples of different valence. As four samples can be measured in parallel, the time expended over the course of an experiment to cycle the measurement environment between its rest state and the measurement condition is substantially reduced. The ionization chamber is simple in design and could be implemented at virtually any XAFS beamline with a horizontal fan of radiation such as that provided by a bend magnet or wiggler.

The EXAFS spectra of Cu and Pd foil from many different beamlines and synchrotrons are compared to address the dependence of the amplitude reduction factor (S-0(2)) on beamline specific parameters. Even though S-0(2) is the same parameter as the EXAFS coordination number, the value for S0 2 is given little attention, and is often unreported. The S0 2 often differs for the same material due to beamline and sample attributes, such that no importance is given to S-0(2)-values within a general range of 0.7 to 1.1. EXAFS beamlines have evolved such that it should now be feasible to use standard S0 2 values for all EXAFS measurements of a specific elemental environment. This would allow for the determination of the imaginary energy (Ei) to account for broadening of the EXAFS signal rather than folding these errors into an effective S-0(2)-value. To test this concept, we model 11 Cu-foil and 6 Pd-foil EXAFS spectra from around the world to compare the difference in S-0(2)- and Ei-values.

Extract Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

We report on the design of both transmission and fluorescence x-ray-absorption spectroscopy cells suitable for in situ characterization of heterogeneous catalysts. The heart of both cells is a quartz tube used to house the catalyst sample. Both cells allow in situ x-ray-absorption fine-structure (XAFS) data to be recorded from −196 to 825 °C using a wide range of gas flows at atmospheric pressure. Excellent temperature control is demonstrated with both designs. XAFS data can be recorded over a wide x-ray energy range (2.1–29 keV). These designs are simple, robust, relatively low cost, and, moreover, are reliable and easy to operate. All of the critical components of the transmission reactor can be purchased commercially, with little machining required. The design of the fluorescence reactor requires access to a skilled glass blower.

The Sn silicate zeolite, Sn-beta, has been shown to be an efficient, selective heterogeneous catalyst for Baeyer-Villiger oxidations. Using primarily a multishell fit to extended X-ray absorption fine structure (EXAFS) data, we show that the Sn does not randomly insert into the beta-zeolite structure but rather occupies identical, specific, crystallographic sites. These sites are the T5/T6 sites in the six-membered rings. Moreover, the Sn is substituted in pairs on opposite sides of these six-membered rings. We believe that it is the specific, uniform crystallographic location of the Sn in the beta crystal structure that leads to sites with uniform catalytic activity, and consequently to the high chemical selectivity demonstrated for this catalyst. This manifests itself in the almost enzyme-like selectivity of this catalyst in Baeyer-Villiger oxidations. This uniform site distribution of the Sn suggests that there is likely a symbiotic relationship between the structure-directing agent in the zeolite synthesis and the Sn heteroatoms during the framework formation.

The tin zeolite Beta (Sn-Beta = zeolite Beta structure with framework incorporated tin) is synthesized and used as a heterogeneous catalyst for the Baeyer-Villiger (BV) reaction with hydrogen peroxide. Cyclic ketones are transformed into the corresponding lactones with very high selectivity using hydrogen peroxide as a replacement of the environmentally unfriendly peracetic acid. This new oxidation system can also be used for unsaturated ketones, which are oxidized to the corresponding unsaturated lactones with very high chemoselectivity. In some BV reactions the selectivity obtained is similar to that in the equivalent enzyme reaction. The catalytic Sri sites have been characterized using a combination of characterization techniques including TEM, FTIR, XPS and in-situ EXAFS. The most selective Sn-Beta sample, activated using a wet air calcination, shows high weak Lewis acidity and high framework tin. The data from this detailed characterization study provides a scientific basis for a deeper understanding of this fascinating catalyst system.

A theoretical study of the sensitivity of Pt L3 x-ray absorption near edge structure (XANES) to the size and shape in small Ptn clusters is reported. Calculations, based on a full multiple scattering, self-consistent field, real-space Green’s function approach implemented in the ab initio FEFF8 code, show that XANES provides a characteristic signature of cluster shape. For example, the calculated white line intensity exhibits a large variation for small cluster sizes and geometry, but becomes independent of cluster size for large clusters. A strong polarization dependence of the white line is predicted for two-dimensional clusters. For three-dimensional clusters the polarization dependence is smaller, but can be used as a measure of the “flatness” of a cluster. A series of semirelativistic all-electron, full potential density functional calculations was also performed for several Ptn clusters. These calculations show the existence of intrinsic static disorder in these clusters due to nonisotropic shrinkage. There is an expansion along the (111) direction, and a contraction in the other directions. Additional calculations for a model Pt6 cluster in zeolite-LTL pore show that, except for additional broadening of the white line, all XANES features of the free clusters are qualitatively preserved.

The synthesis and analysis of inorganic material combinatorial libraries by the split-pool bead method were demonstrated at the proof-of-concept level. Millimeter-size spherical beads of porous gamma-alumina, a commonly used support material for heterogeneous catalysts, were modified with Al(13)O(4)(OH)(24)(H(2)O)(12)(7+) cations in order to promote irreversible adsorption of the anionic fluorescent dyes Cascade Blue, Lucifer Yellow, and Sulforhodamine 101. The compositions of individual beads were easily determined through three split-pool cycles using a conventional fluorescence plate reader. Small split-pool material libraries were made by adsorbing noble metal salts (H(2)PtCl(6), H(2)IrCl(6), and RhCl(3)) into the beads. Analysis of these beads by micro-X-ray fluorescence showed that quantitative adsorption of metal salts without cross-contamination of beads could be achieved at levels (0.3 wt % metal loading) relevant to heterogeneous catalysis. The method offers the potential for synthesis of rather large libraries of inorganic materials through relatively simple benchtop split-pool chemistry.

In-situ X-ray absorption near-edge structure (XANES) experiments were performed on a working reformate-air fuel cell to study the structure of carbon supported Pt−Ru anode electro-catalyst. The fuel cell was operated in a normal mode without the use of supplemental electrolytes. A fresh membrane electrode assembly (MEA) and a conditioned MEA were studied at different operating conditions of the fuel cell and different feed (pure H2 or H2/CO [100 ppm CO]) at the anode. The in-situ Pt LIII-edge and Ru K-edge XANES of the fuel cell MEAs showed metallic characteristics under all operating conditions. These results demonstrate that, under the reducing conditions of normal fuel cell operation, the Pt−Ru catalyst exists as a metallic phase(s).

This paper first briefly summarizes the dramatic progress over the past decade both in fundamental theory and in the interpretation of XAFS and XANES. These developments have led to several ab initio codes such as FEFF for simulating XAFS and XANES, together with compatible analysis codes which permit an interpretation of the spectra in terms of geometrical and electronic properties of a material. As an example of relevance to catalysis, we discuss recent work which interprets the Pt L-edge XANES of PtX clusters based on the self-consistent FEFF8 code. For pure Pt clusters, we find that self-consistency is important in determining the variation of XANES with cluster size. For PtCl clusters, we show that the presence of a Cl–Pt bond leads to a “hybridization peak,” i.e. , a peak in the Cl d-density of states (dDOS) mixed with Pt d-states, which can be used as a measure of Cl content. For Pt–H clusters, we show that hydrogen addition is well correlated with the growth of a broad shoulder above the white line. We find that this feature can be attributed largely to AXAFS, i.e. , to a change in the atomic background absorption. We also analyze the effect of a support, in terms of model calculations for a realistic Pt 6 cluster within a zeolite-LTL pore.