Simon R. Bare

Morphology-dependent zeolite intergrowth structures leading to distinct internal and outer-surface molecular diffusion barriers

Zeolites play a crucial part in acid-base heterogeneous catalysis. Fundamental insight into their internal architecture is of great importance for understanding their structure-function relationships. Here, we report on a new approach correlating confocal fluorescence microscopy with focused ion beam-electron backscatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped MFI-type zeolite crystals differing in their morphology and chemical composition. This powerful combination demonstrates a unified view on the morphology-dependent MFI-type intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion. It has been found that internal-surface barriers originate not only from a 90 degrees mismatch in structure and pore alignment but also from small angle differences of 0.5 degrees-2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nm.

Generation of atomic oxygen on Ag(111) and Ag(110) using NO2: a TPD, LEED, HREELS, XPS and NRA study

Clean atomic oxygen overlayers have been produced on Ag(111) and Ag(110) surfaces by dosing with NO2 between 500–520 K in UHV. These were studied using TPD, LEED, XPS, HREELS, AES and NRA. The high coverage atomic oxygen structures, Ag(111)-(4 × 4)O and Ag(110)-c(6 × 2)O are easily formed in UHV using moderate dosing pressures. The spectroscopic data indicate that these NO2-produced overlayers have the same structure as those produced using oxygen at high pressures. The absolute coverage of the Ag(111)-(4 × 4)O is determined to be 7.07(±0.55) × 1014 O atoms cm−2, or 0.51 ± 0.04 ML. This state is characterized by a single O 1s XPS peak at 528.2 eV. The NO2-produced Ag(111)-(4 × 4)O structure is suggested to be composed of two parts. The outer layer of this phase is composed of a coincidence mesh of a single layer of the (111) plane of Ag2O, which itself is comprised of a Ag plane sandwiched between two oxygen planes. Between this coincidence mesh and the Ag(111) surface is a partial monolayer of chemisorbed oxygen atoms. The c(6 × 2)O structure is characterized by two vibrational loss peaks at 298 and 339 cm−1, a single O 1s XPS peak at 528.5 eV, and a coverage of close to 1 ML.

Sensitivity of Pt x-ray absorption near edge structure to the morphology of small Pt clusters

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 shrinkag...

Electronic properties and charge transfer phenomena in Pt nanoparticles on γ-Al2O3: size, shape, support, and adsorbate effects.

This study presents a systematic detailed experimental and theoretical investigation of the electronic properties of size-controlled free and γ-Al(2)O(3)-supported Pt nanoparticles (NPs) and their evolution with decreasing NP size and adsorbate (H(2)) coverage. A combination of in situ X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculations revealed changes in the electronic characteristics of the NPs due to size, shape, NP-adsorbate (H(2)) and NP-support interactions. A correlation between the NP size, number of surface atoms and coordination of such atoms, and the maximum hydrogen coverage stabilized at a given temperature is established, with H/Pt ratios exceeding the 1 : 1 ratio previously reported for bulk Pt surfaces.

The New MRCAT (Sector 10) Bending Magnet Beamline at the Advanced Photon Source

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.

Determining the location and nearest neighbours of aluminium in zeolites with atom probe tomography

Zeolite catalysis is determined by a combination of pore architecture and Brønsted acidity. As Brønsted acid sites are formed by the substitution of AlO4 for SiO4 tetrahedra, it is of utmost importance to have information on the number as well as the location and neighbouring sites of framework aluminium. Unfortunately, such detailed information has not yet been obtained, mainly due to the lack of suitable characterization methods. Here we report, using the powerful atomic-scale analysis technique known as atom probe tomography, the quantitative spatial distribution of individual aluminium atoms, including their three-dimensional extent of segregation. Using a nearest-neighbour statistical analysis, we precisely determine the short-range distribution of aluminium over the different T-sites and determine the most probable Al–Al neighbouring distance within parent and steamed ZSM-5 crystals, as well as assess the long-range redistribution of aluminium upon zeolite steaming.

Chapter 6 Characterization of Catalysts in Reactive Atmospheres by X‐ray Absorption Spectroscopy

Abstract X‐ray absorption spectroscopy (XAS) is a powerful method for probing the average local electronic and geometric structures of catalysts in the working state. Element‐specific data can be obtained over a wide range of temperatures (room temperature to >1000 K), and pressures (from subambient to well more than 100 bar). Because the specimen can be investigated under such a wide range of conditions, XAS has become one of the most frequently applied techniques for structural characterization of working catalysts. Often there is no other way to obtain the structural information provided by this technique that allows one to develop specific structure‐activity relationships in catalysis. XAS is applicable to a broad range of elemental concentrations (from tens of ppm to wt% levels), and therefore is used to characterize both high‐surface‐area supported catalysts and bulk catalysts (e.g., oxides). This review is focused on the application of XAS to the investigation of catalysts in the working state. The designs of the experimental reaction cells, which are essential for the XAS analysis of catalysts, are comprehensively reviewed. Specific examples illustrate the application of the technique to the investigation of catalysts at both steady‐state and under dynamic conditions. The examples are chosen to demonstrate the wide variety of catalysts that can be investigated. Although XAS has been used for almost 40 years to characterize catalysts in the working state, methodology, equipment, and applications are still being advanced. Some recent major developments (as a result of improvements in optics of the X‐ray beam lines, new synchrotron radiation sources, advanced detectors, and software) are therefore highlighted.

Chemistry of chloroethylenes on Cu(100): bonding and reactions

The bonding and reactions of chloroethylenes (vinyl chloride, trans- and cis-dichloroethylene, trichloroethylene) on a Cu(100) surface have been investigated by temperature-programmed desorption and reaction, near-edge X-ray absorption fine structure (NEXAFS) studies and measurements of changes of the surface work function. The monolayer molecules adsorb molecularly intact at 95 K via π coordination. The molecules orient with their molecular planes parallel to the surface with little increase (<0.02 A) in the CC bond distance compared with the gas phase values. The degree of thermal dissociation increases with the extent of halogenation, and no chlorine is present in the hydrocarbon reaction products liberated from the surface. Both cis- and trans-1.2-chloroethylene produce acetylene and a small amount of benzene, while dissociation of trichloroethylene generates acetylene and deposits a carbon residue on the surface. Vinyl chloride desorbs from Cu(100) without reaction.

Simple flow through reaction cells for in situ transmission and fluorescence x-ray-absorption spectroscopy of heterogeneous catalysts

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 deg. 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.

Theoretical Interpretation of XAFS and XANES in Pt Clusters

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 Pt6 cluster within a zeolite-LTL pore.