Abstract/Details

Effects of composition and structure on the activity of heterogeneous molybdenum catalysts


2011 2011

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Abstract (summary)

This dissertation investigates the use of heterogeneous molybdenum catalysts for use in water-gas shift (WGS) and methane dehydroaromatization (MDA) chemistry. The WGS reaction, CO + H2O [special characters omitted] H2 + CO2, is catalyzed by unsupported molybdenum carbide (Mo2C). The MDA reaction, 6 CH4 [special characters omitted] C6H6 + 9 H2, is catalyzed by a zeolite-supported molybdenum catalyst, Mo/HZSM-5. Both reactions involve Mo2C, hydrogen production, and are industrially relevant.

The composition of the near-surface carbide phase in a variety of unsupported Mo2C powders was investigated using high-resolution XPS. Mo 2C powders with elevated carbon contents were prepared via temperature programmed carburization of a molybdate, and were handled with or without air exposure. Binding energies, peak widths and line shapes were investigated for six carbon environments. The XPS of some catalysts suggest that non-Mo 2C carbide phases are present near the surface, e.g., α-MoC 1-x and MoOxCy. The catalytic performance of Mo 2C for water-gas shift catalysts were evaluated in a flow reactor by FTIR, GC, and MS. Catalyst activities were enhanced by reduction in hydrogen gas at temperatures between 450 and 850°C. High CO conversion and low lightoff temperature (< 250°C) were achieved when the catalysts were activated at 600°C. Catalysts activated at 825°C had higher lightoff temperatures (>300°C). The activity of catalysts with high graphite content were unaffected by H2-TPR at 600°C. The activity of these catalysts increased after H2-TPR at 825°C. Catalysts with higher concentrations of MoOxCy did not perform better than those with low levels. The MoOxCy phase diminished upon activation at 600°C.

The local structure of molybdate ions in the crystalline materials MgMoO 4 and MgMo2O7 and in Mo/HZSM-5 (Mo wt% = 2, 4, and 6) and (Si/Al = 15, 25, and 39) was explored by EXAFS at the Mo K-edge, using both Fourier and Wavelet Transforms. Curvefitting analysis of the FT-EXAFS of MgMoO4 and MgMo2O7 reveals the need to include several single- and multiple-scattering paths in order to locate the Mo-Mo paths. Curvefitting of the FT-EXAFS of in Mo/HZSM-5 is unable to distinguish between possible mono-molybdate and di-molybdate structures. The coordinates of the WT-EXAFS maxima provide strong evidence for the presence of dimolybdates, especially at low Mo loading and high Si/Al ratios.

Indexing (details)


Subject
Inorganic chemistry;
Chemical engineering
Classification
0488: Inorganic chemistry
0542: Chemical engineering
Identifier / keyword
Applied sciences; Pure sciences; Catalysts; Exafs; Molybdenum carbide; Wavelet transform; Zeolites
Title
Effects of composition and structure on the activity of heterogeneous molybdenum catalysts
Author
Savinelli, Robert Oak
Number of pages
266
Publication year
2011
Degree date
2011
School code
0035
Source
DAI-B 72/08, Dissertation Abstracts International
Place of publication
Ann Arbor
Country of publication
United States
ISBN
9781124657622
Advisor
Scott, Susannah L.
Committee member
Moskovits, Martin; Seshadri, Ram; de Vries, Mattanjah
University/institution
University of California, Santa Barbara
Department
Chemistry
University location
United States -- California
Degree
Ph.D.
Source type
Dissertations & Theses
Language
English
Document type
Dissertation/Thesis
Dissertation/thesis number
3456416
ProQuest document ID
872072188
Copyright
Database copyright ProQuest LLC; ProQuest does not claim copyright in the individual underlying works.
Document URL
http://search.proquest.com/docview/872072188
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