The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by the acidity/ alkalinity of the support but are relatively independent of the metal (Pd or Pt) or the type of support (zeolite LTL or SiO 2 ). As the alkalinity of the support increases, the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a decrease in the XPS binding energy and a shift from linear to bridge bonded CO in the IR spectra. Analysis of the shape resonance in XANES spectra indicates that in the presence of chemisorbed hydrogen the difference in energy between the Pt-H antibonding orbital and the Fermi level decreases as the alkalinity of the support increases. Based on the results from the IR, XPS, and shape resonance data a new model is proposed in which the interaction between the metal and support leads to a shift in the energy of the metal valence orbitals. The EXAFS structural analysis indicates that the small metal particles are in contact only with the oxide ions of the support. Finally, a new spectroscopic characterisation, Atomic XAFS, is presented which provides new insights into the origin of the electronic changes in the metal. As the alkalinity of the support increases, there is decrease in the metal ionisation potential. The primary interaction is a Coulomb attraction between metal particle and support oxygen ions, which affects the metal interatomic potential. This model for the metal-support interaction explicitly excludes the need for electron transfer, and it can account for all observed changes in the catalytic, electronic, and structural properties of the supported metal particles induced by support acidity ranging from acidic to neutral to alkaline.

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Four highly dispersed and fully reduced rhodium on alumina catalysts with different particle sizes in the range 6–12 Å were investigated with the EXAFS technique in order to derive information about the structure of the metal–support interface. This information can only be obtained when the signal-to-noise ratio of the experimental EXAFS data is high enough and accurate reference compounds and a modified way of data analysis are used. With the aid of phase and amplitude corrected Fourier transforms it was possible to detect a small additional signal which could be ascribed to a Rh–O bond. Since the catalysts were fully reduced and since the intensity of the small signal increased with decreasing particle size, the oxygen neighbor was assigned to be originated from the metal–support interface. From the intensity of the Rh–O bond it was estimated that, on the average, each interfacial rhodium atom is surrounded by 2–3 oxygen ions of the support. The detected Rh–O bond has a coordination distance of 2.7 Å which is about 0.6 Å larger than the first coordination distance in Rh2O3 (2.05 Å). The coordination distance of 2.7 Å can be explained by assuming an interaction between metallic rhodium (atomic radius 1.34 Å) and ionic oxygen belonging to the support (ionic radius 1.4 Å). This would possibly imply an ion-induced dipole bonding between the metal particle and the support.

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X-ray absorption spectra characterizing the metal-support interface in supported metal complexes and supported metal catalysts are summarized and evaluated. Single-metal-atom transition metal complexes on non-reducible metal oxide supports are bonded with metaloxygen bonds with metal-oxygen distances of approximately 2.15 A; the bonding distance is only weakly sensitive to the oxidation state of the metal. Nearly this same metal-oxygen distance is characteristic of the metal-support interface in metal-oxide-supported metal clusters following high temperature reduction in H a (HTR: T > 450~ The metals at the interface may be polarized sufficiently that they bond with the oxygen of the support much as the cations in mononuclear complexes bond with it. When the supported metals are treated in H a at low temperatures (LTR: T < 350~ or are prepared under He with partially hydroxylated supports, a longer metal-support oxygen distance is observed, typically 2.5-2.7 A. This distance is suggested to characterize interactions between zero-valent metals and support oxygen. Changes in the performance of supported metal catalysts resulting from differences in the temperature of pretreatment in H 2 are attributed to changes in the electronic properties and/or morphology of the metal clusters, which are suggested to be related to the concomitant changes in the structure of the metal-support interface.

The structure of the cobalt present in carbon-supported Co and Co-Mo sulfide catalysts was studied by means of X-ray absorption spectroscopy at the Co K-edge and by X-ray photoelectron spectroscopy (XPS). Thiophene hydrodesulfurization activities were used to measure the catalytic properties of these catalysts. By comparison of the EXAFS and XANES spectra of the catalysts with those of c 0 9 s S and Cos2 model compounds, it was concluded that all Co atoms in a catalyst prepared with nitrilotriacetic acid as complexing agent were in the "Co-Mo-S" state, while the Co atoms in a conventionally prepared catalyst were partly present in a CO$8-like structure and partly in a "Co-Mo-S" structure. The Co atoms in the To-Mc-S" state have a distorted 5-to 6-fold sulfur coordination, and on the average, every Co atom is in contact with two Mo atoms at a distance of 2.80 A. On the basis of these data, the most likely position for the Co atoms is in front of the square sulfur faces of the MoS6 trigonal prisms along the edges of the MoS, crystallites with two additional sulfur atoms or H2S molecules attached. The Co atoms in the sulfided Co/C catalyst have Co-S and Co-Co coordinations as in c 0 9 s g , although the sulfur coordination number is higher. IntroductionCobalt-or nickel-promoted molybdenum sulfide catalysts supported on alumina are extensively used in the hydrotreatment of petroleum feedstocks. The increasing need for efficient removal of sulfur, nitrogen, and metal contaminants has led to a continuous drive to clarify the structure and the related catalytic activity of these complex catalyst systems. Especially the role and the chemical state of the promoter cobalt and nickel atoms in the sulfided catalysts is a subject of great interest, and numerous studies have been devoted to it.'v2 The introduction of in situ Mossbauer emission spectroscopy (MES) provided for the first time direct information regarding the nature of the cobalt phases present in a working Co-Mo hydrodesulfurization (HDS) catalyst. With the use of MES, Topsere et al.3 and Wivel et aL4 showed that most of the cobalt atoms are situated at MaS2 crystallite edges in a so-called "Co-Mo-S" structure and that this structure governs almost completely the HDS activity. However, the precise local structure of the cobalt promoter atoms is still unknown. Also, the high specific activity of the "Co-Mo-S" structure is not understood. In this respect, it has not been established whether the cobalt atoms are the active sites or whether the neighboring molybdenum atoms also play a direct role in the catalytic activit y . 5-7Detailed information on the chemical state of the cobalt atoms was obtained by Ledoux et al. with the use of 59C0 NMR.8 They argued that the promotion effect of cobalt was correlated with the concentration of cobalt sites having a distorted tetrahedral symmetry and that these sites were stabilized by so-called "rapid octahedral" cobalt atoms acting as a glue between the tetrahedral cobalt sites and the MoS, phase. Although the "rapid octahe...

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

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