Diek C. Koningsberger

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mossbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Bronsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination. uf6d9 2002 Elsevier Science (USA). All rights reserved.

A New Method for Parameterization of Phase Shift and Backscattering Amplitude

Abstract Parameterization of phase and backscattering amplitude with cubic splines is described. Using the cubic spline, the analytical partial derivatives of the plane wave EXAFS function can be calcalated. The use of analytical partial derivatives decreases the CPU time needed for a refinement by over 60% for a three shell system compared to a refinement with partial derivaties calculated with the finite difference method.

Remarkable spreading behavior of molybdena on silica catalysts: an in situ EXAFS-Raman study

In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K. The increase of the peak in the XANES region (Is -→ 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo=O bond from 944 to 986 cm−1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm−1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.

Deactivation processes of homogeneous Pd catalysts using in situ time resolved spectroscopic techniquesElectronic supplementary information (ESI) available: EXAFS data and Fourier Transform. See http://www.rsc.org/suppdata/cc/b2/b206758g/

UV-Vis, combined with ED-XAFS shows, for the first time, the evolution of inactive Pd dimers and trimers, that are a possible first stage in the deactivation process of important palladium catalysed reactions, leading to larger palladium clusters and eventually palladium black.

Sulfur Poisoning of a Pt/BaK-LTL Catalyst: A Catalytic and Structural Study Using Hydrogen Chemisorption and X-ray Absorption Spectroscopy

The sulfur poisoning of a Pt/BaK-LTL catalyst has been studied with X-ray absorption spectroscopy and hydrogen chemisorption. The fresh catalyst contained highly dispersed platinum inside the zeolite pores. EXAFS analysis determined a Pt-Pt coordination number of 3.7, suggesting an average platinum cluster size of 5 or 6 atoms, consistent with the TEM and chemisorption data (H/Pt = 1.4). The catalyst was poisoned with H2S until the dehydrocyclization activity of n-hexane decreased to 30% of fresh activity. The first-shell Ptue5f8Pt coordination number increased to 5.5, indicating a growth of the average platinum cluster size to 13 atoms. Hydrogen chemisorption measurements of the poisoned catalyst show a decrease in the H/Pt value to 1.0. The EXAFS data also provide evidence for the presence of sulfur adsorbed on the surface of the platinum particles with a Ptue5f8S bond distance of 2.27 A. The high sensitivity of the Pt/LTL catalyst to poisoning by very low levels of sulfur is attributed to the loss of active platinum surface by adsorption of sulfur and the growth of the platinum clusters. Much of the available platinum surface was found to be capable of chemisorbing hydrogen, but with no activity for dehydrocyclization. Growth of the platinum particle was sufficient to block the pore. In the sulfur-poisoned catalyst, only the sulfur-free platinum atoms exposed through the pore windows remain active. The evidence suggests the location of the sulfur was at or near the metal-zeolite interface. Since both high activity and selectivity require extremely small platinum particles, regeneration of sulfur-poisoned catalysts will require removal of the adsorpted sulfur and restoration of the original particle size.

Unique Structural Properties of the Mg-Al Hydrotalcite Solid Base Catalyst : An In Situ Study using Mg and Al K-edge XAFS during Calcination and Rehydration

The changes in the layered structure of Mg-Al hydrotalcite (Mg/ Al = 2) during heat treatment have been investigated by using in situ XAFS simultaneously at the Mg and Al K-edges. The development of unique in situ instrumentation allowed the coordination environments at both the Mg and Al centers to be monitored as a function of the temperature and heat treatment. The results of this study show that the hydrotalcite structure is highly flexible, and should lead to the further development of hydrotalcites as new solid basic catalysts. Moreover, the Mg and Al cations in the cation layers show different behavior as a function of temperature. The coordination of some octahedral Al ions decreases already at a temperature of 425 K, whereas the coordination about Mg does not show any modification at this temperature. However, hydrotalcite treated at 425 K, followed by cooling down to room temperature resulted in a complete reversal to the original octahedral Al coordination. It is proposed that Al-OH bond breakage occurs at 425 K, without the evolution of H2O. This bond is restored after cooling to room temperature. The actual dehydroxylation of hydrotalcite commences between 425 and 475 K, as indicated by a change in coordination of both the Mg and Al centers. This is accompanied by the evolution of H2O molecules and the changes are hence irreversible without the presence of excess water. Heat treatment at 725 K leads to the development of an MgO-like phase (octahedral Mg) and a mixed octahedral/tetrahedral Al phase. A subsequent rehydration at room temperature entirely restores the original coordination about the Al and Mg centers of hydrotalcite to a distance of 15 A, to which XAFS spectroscopy is sensitive.

Coordination of palladium on carbon fibrils as determined by XAFS spectroscopy

To investigate the anchoring of precious metals onto carbon fibrils Pd/C-fibril samples were investigated in both the precursor and the reduced state by X-ray absorption fine structure (XAFS) spectroscopy. Analysis of the XAFS data of the freshly prepared catalyst showed a palladium–tetraamine complex in interaction with carbon. The proposed model involves [Pd(NH3)4]2+ coordinated to the carbon fibril surface, most probably stabilised by the carboxylic groups (through O–H bridging and charge balance) and the π-electron system of the support. After insitu reduction, analysis pointed to small palladium particles of ca. 15 A present on the fibrils with a significant Pd–C interaction.

Understanding the influence of support alkalinity on the hydrogen and oxygen chemisorption properties of Pt particles

The influence of the alkalinity of the support on the chemisorption of oxygen on Pt particles has been studied using Pt L3 and L2 X-ray absorption near edge data from Pt/Al2O3 doped with F or Rb. The O binding site is determined by using a procedure similar to that previously utilised for H chemisorption. The Pt L3 X-ray absorption edge data of the sample in vacuum is subtracted from the data obtained after chemisorption of oxygen. The signature of this difference spectrum is compared with that resulting from a similar procedure applied to theoretical data, the latter obtained with the help of full multiple scattering ab initio calculations on small model Pt4 clusters using the FEFF8 code. After O2 exposure of the F doped alumina, oxygen is found in hcp/sub-surface binding sites, whereas in the Rb doped case oxygen is present in the bridged/fcc sites. These results imply that the properties of the support have a strong effect on the oxygen chemisorption properties of small Pt particles. Since O is known to bond in a bridged position near step edges or corners at low coverage, followed by 3-fold fcc sites on flat faces or terraces at higher coverage, and then finally at hcp sites and/or in the subsurface region, these data indicate that the Pt–O bond strength and hence coverage increases with support acidity. This is in contrast to that found previously for the Pt–H bond, which decreases in strength with support acidity. Consistent with these results, it is concluded from density functional theory (DFT) calculations on model Pt4 clusters that the isolated Pt–O bond strength is indeed significantly altered by the support and in the opposite direction to that reported previously for Pt–H bonding. The calculations also show that the Pt–O bond ionic character is dependent on the properties of the support. The increased Pt–O ionic character in a basic support gives rise to a stronger O–O lateral repulsion. This lateral interaction, along with the decreased Pt–O bond strength, decreases the O coverage for basic supports. A survey of the recent literature reporting DFT and experimental results on H and O adsorption on Pt(111) and Pt clusters is given.

Determination of Metal Particle Size of Highly Dispersed Rhodium, Iridium and Platinum Catalysts by Hydrogen Chemisorption and EXAFS

Chemisorption of gaseous molecules, especially hydrogen, is extensively used to estimate the dispersion of group VIII (groups 8-10) metal catalysts. Chemisorption methods are especially important for highly dispersed catalysts, because of the difficulty to establish their dispersion by other techniques, such as X-ray diffraction or electron microscopy. When hydrogen chemisorption data are used to calculate metal surface areas, often a hydrogen-to-metal stoichiometry of one is assumed. However, H/M values exceeding unity have been obtained for supported Pt, Rh, and Ir systems. In these cases the dispersions cannot be calculated straight forwardly from the chemisorption results, because of the uncertainty in the adsorption stoichiometry. To solve this problem the authors used the EXAFS technique to determine the average metal-metal coordination number in the metal particles, which is related to the particle size. From the combined results of the chemisorption and EXAFS measurements they have been able to conclude that the high H/M values are due to the fact that more than one hydrogen atom can be attached to a surface metal atom and that the trend in stoichiometry is analogous to that observed in metal polyhydride complexes.

On-line characterization by EXAFS of tin promoted platinum graphite catalysts in the aqueous phase

The structure of tin promoted graphite supported platinum catalysts has been studied with extended X-ray absorption fine structure spectroscopy (EXAFS). A newly developed EXAFS cell allows on-line characterization avoiding contact to ambient or drying. Hereto catalyst samples are transferred from a slurry reactor to the EXAFS cell forming a ‘‘bed’’ of catalyst particles in the EXAFS cell. The cell design was based on considerations concerning possible mass transport limitations while performing reactions in the liquid phase. The structures of the tin promoted platinum catalysts were investigated directly after preparation, drying, treatments with hydrogen (363 K) and oxygen (RT) in aqueous phase and a hydrogen gas treatment at 573 K at both the Pt LIII and the Sn K-edge. After preparation, under aqueous hydrogen, reduced platinum can be detected with three coordinations: Pt‐Pt, Pt‐C and Pt‐Sn. Tin appears to be partly oxidic showing a Sn‐O and a Sn‐Pt coordination. A treatment with aqueous oxygen or exposure to ambient leads to oxidized platinum and tin. At the Pt LIII-edge only a Pt‐Pt and Pt‐O coordination for platinum are detected. At the Sn K-edge tin has only a Sn‐O coordination. An aqueous treatment with hydrogen at 363 K reduces platinum showing, however, different coordination numbers for the Pt‐Pt and Pt‐Sn coordination. Tin only shows a Sn‐O coordination. A treatment with hydrogen at 573 K reduces both the platinum and the tin. Platinum shows a Pt‐Pt, Pt‐C and Pt‐Sn coordination. Tin shows a Sn‐Pt and Sn‐O coordination indicating tin deposition on the platinum, tin being bonded via oxygen to the graphite support. Reductive treatments in the aqueous phase appear to reduce platinum and only the tin deposited on the platinum. The effects of drying and consecutive reductive treatments could only be studied since the developed EXAFS cell allowed catalyst preparation and treatments avoiding contact to ambient. # 1998 Elsevier Science B.V.