An oxygen chemisorption method has been developed for measuring the active surface area of supported and unsupported VzOs following reduction in hydrogen. It is shown that to achieve complete reduction of the vanadia surface without reducing the bulk, reduction must be carried out at 640 K. Oxygen uptakes of unsupported samples reduced at close to this temperature yield an oxygen atom site density of 3.2 X 10l8 a value near that expected for a monolayer. The same oxygen chemisorption technique is applied to silica-supported V205. Laser Raman spectroscopy confirms that, near 640 K, oxygen chemisorbs primarily at the surface of the dispersed vanadia but does not exchange with the bulk of the oxide. For very low weight loadings, a limiting stoichiometry of one adsorbed oxygen atom per vanadium atom is obtained. This stoichiometry is used to calculate dispersions ranging from 93% to 50% for supported VzOs samples of 0.3-9.8% weight loading.

A series of moderate surface area transition metal carbides and nitrides of molybdenum, tungsten, vanadium, niobium, and titanium were prepared by temperature-programmed reaction of the oxide precursor with a reactant gas (20% CHdH2 for the carbides and 100% NH3 for the nitrides). The phase purity and composition of the samples were established by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy ( X P S ) , while surface properties were determined by NZ BET and CO chemisorption measurements. The catalysts were tested in a three-phase trickle-bed reactor for their activity in hydrodenitrogenation (HDN), hydrodesulfurization, and hydrodeoxygenation, with particular emphasis on HDN. The catalytic tests were carried out using a model liquid feed mixture containing 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt % aromatics (tetralin), and balance aliphatics (tetradecane).The carbides and nitrides were found to be active for HDN of quinoline with activity following the order group 6 > group 5 > group 4. Notably, Mo2C showed superior areal HDN activity than a commercial sulfided Ni-Mo/AlzO3 catalyst (Shell 324). The XRD analysis of the spent catalysts indicated no change in the bulk structure, while XPS results showed little incorporation of sulfur in the surface region of the catalysts, suggesting that these materials are tolerant of sulfur.

Molybdenum oxide catalysts (1 wt %) supported on SiO 2 (L-90, 95 m 2 /g), SiO 2 (EH-5, 350 m 2 /g), Al 2 O 3 (96 m 2 /g), and TiO 2 (52 m 2 /g) were characterized using Raman spectroscopy and near-edge and extended X-ray absorption fine structure (NEXAFS and EXAFS, respectively) spectroscopies. The structural possibilities (tetrahedral and distorted octahedral) for the Mo active center were explored using ab initio calculations through the Hartree-Fock method using a 3-21G(d) basis set. The Raman vibrational frequencies and bond lengths obtained from EXAFS were compared to ab initio calculations to arrive at the likely structures for the isolated Mo centers on each support. The studies revealed a mixture of tetrahedral Mo sites along with crystalline MoO 3 for the MoO 3 /SiO 2 (L-90) catalyst. The Mo sites for the MoO 3 /SiO 2 (EH-5) catalyst were found to be a mixture of isolated, tetrahedral, and distorted octahedral sites. The MoO 3 /Al 2 O 3 catalyst was found to have isolated, tetrahedral sites. Finally, the MoO 3 /TiO 2 catalyst was found to have distorted octahedral Mo active centers. Complementary information from Raman, NEXAFS, and EXAFS spectra and ab initio calculations were required to arrive at the structural assignments for the Mo centers in the present study. Importantly, however, the calculations indicate the possible existence of other stable geometries and help explain the many diverging conclusions in the literature concerning the structure of supported molybdenum oxide catalysts.

Samples of unsupported and silica-supported molybdenum oxide were characterized by oxygen chemisorption, X-ray diffraction, and laser Raman spectroscopy. In the supported materials, the interaction between molybdenum oxide and silica was weak, and for high-concentration samples, the molybdenum oxide was mostly in the form of small crystallites of bulk structure. At very low concentrations, however, laser Raman spectroscopy indicated the presence of a highly dispersed, monomeric species. The structure of this surface molybdate is probably that of a distorted tetrahedron with two short M=0 bonds and two long M-O bonds. Oxygen chemisorption at 630 K gave correct values of dispersion in both the low and high ranges of molybdenum concentration. It indicated close to 100% dispersion for the surface species and a rapid dropoff in dispersion in the higher concentration samples, consistent with X-ray diffraction results. Thus, the chemisorption method developed here can be used to titrate surface molybdenum centers in unsupported and silica-supported Mo03.

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High surface area carbides and nitrides have been synthesized and tested for hydrodenitrogenation activity. Some of the novel materials, particularly Mo2C and Mo2N, provided hydrodenitrogenation activity of the same magnitude as commercial sulfided Ni-Mo/Al203.