Effects of composition and structure on the activity of heterogeneous molybdenum catalysts
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.
0542: Chemical engineering