Vanadium oxide monolayer catalysts. 3. A Raman spectroscopic and temperature-programmed reduction study of monolayer and crystal-type vanadia on various supports Roozeboom, F.; Mittelmeijer-Hazeleger, M.C.; Moulijn, J.A.; Medema, J.; de Beer, V.H.J.; Gellings, P.J. Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the 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. 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. Link to publication• 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 ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Vanadium(V) oxide supported on 7-A1203, GO2, Cr2O3, Si02, Ti02, and Zr02 was studied by X-ray fluorescence, by X-ray diffraction, and especially by the combination of Raman spectroscopy and temperature-programmed reduction (TPR) for qualitative and quantitative structural analysis, respectively. Catalysts were prepared via ion-exchange and wet-impregnation methods. The V contents ranged from -1 to 40 wt % V. At low surface concentrations only surface vanadate phases of two-dimensional character are observed for all carriers. According to Raman and TPR data the structure of these surface vanadate species is independent of the preparation technique. At medium and high surface concentrations, the webimpregnated samples already contain crystalline V20b At equal surface concentrations the ion-exchanged catalysts contain no V205 crystallites. An exception is Si02 on which also crystalline V206 is formed in both preparation techniques. Monolayer stability toward thermal treatment decreases in the order AZO3 > Ti02 > Ce02, whereas on heating ion-exchanged V/Si02 the crystalline V206 spreads out over the silica surface. The reducibilities of the ion-exchanged catalyats, as measured by TPR, can be used as a measure for the contact interaction between vanadia and the carrier oxides. At temperatures of 500-800 K, this interaction ranges fr...
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...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.