Catalysis over molybdenum containing nitride materials.
PhD thesis, University of Glasgow.
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Reactions involving nitrogen transfer/ fixation are of great industrial interest, as novel processes could help industries meet their increasingly challenging economic and environmental targets. An example of this could be the direct synthesis of aniline from benzene, which would avoid the lengthy, uneconomic, and environmentally unfriendly process currently employed. This work explores the reactivity of bulk and supported transition metal nitride catalysts, with particular emphasis on the possible reactivity of lattice nitrogen within bulk nitride catalysts. The experimental work has focussed on two main objectives: determining the most active transition metal nitride catalyst for ammonia synthesis, and hydrogenating these materials to in the absence of N2 to produce reactive NHx species. These materials were then tested for the possible direct synthesis of aniline from benzene by entrapment of reactive NHX species. The second objective was to achieve nitrogen reactivity with nitride catalysts akin to the Marsvan Krevelen mechanism observed in oxidation catalysis. It has been shown that the binary nitrides, γMo2Nand βMo2N0.78, have comparable ammonia synthesis activities although measurements indicate that βMo2N0.78 may have a much greater activity on a surface area normalised basis. Meanwhile the δMoNphase has an intermediate surface area, but very low activity. The influence of morphology in the ammonia synthesis reaction was investigated by testing nanorod forms of βand γphase molybdenum nitrides and comparing their ammonia synthesis activities with molybdenum nitride powders. Morphology was found to have little effect on the reaction and the influence of structure sensitivity is thought to be limited in this case. What was apparent, was that the highly specific temperature programmed reaction synthesis required to prepare γMo2Nproduced an ammonia synthesis catalyst with no catalytic advantage over one that is prepared in mixtures of H2/N2 (βMo2N0.78). The influence of preparation on the ammonia synthesis activity of ternary nitride catalysts was also investigated by preparing materials in NH3 or H2/N2 atmospheres. Treatment of iron and cobalt molybdenum oxide under H2/N2 was not sufficient to yield a pure phase nitride, however NiMoO4 was fully reduced to Ni2Mo3N. Co3Mo3N, prepared using NH3, was the
most active of the ternary nitride catalysts tested, and preparing the materials in H2/N2 failed to increase the activity, with the exception of Ni2Mo3N. Reaction of Co3Mo3N with H2/Ar significantly decreased the nitrogen content of the material, and it is believed that a previously unknown eta12 Co6Mo6N phase has been formed as a result of the nitrogen removal. Hydrogen was shown to be essential to induce this change, despite the fact that most of the eliminated N ended up in the form of N2. Prolonged treatment with H2/Ar at elevated temperature did not remove any additional nitrogen. It is believed that the incomplete loss of nitrogen is a direct consequence of the migration of nitrogen between crystallographic sites as the stoichiometry is reduced. In the case of iron and nickelmolybdenum nitrides the loss of nitrogen was evidenced by combustion analysis, however no new phases of material were formed. Similar experiments, with conducted with different molybdenum nitride polymorphs have shown the removal of nitrogen with a mixed phase of constituent metal and nitrided species, with only βMo2N0.78 fully decomposing to the pure metal. The loss of nitrogen, and hence its potential for reaction, is evident. However, in all cases the predominant form of lost nitrogen is N2, which is believed to be a consequence of the thermodynamics of ammonia decomposition at higher temperatures. Restoration of stoichiometry by treatment with H2/N2 has been observed for a number of materials, i.e. Co3Mo3N, γMo2N. In the case of Co6Mo6N, the NH3 synthesis activity has been found to be comparable with Co3Mo3N. HZSM5 supported nitride catalysts were also tested for ammonia synthesis and it was observed that the introduction of iron as a dopant has significant promotional effects. XPS evidence confirmed the presence of Fe0 in the material, in addition to the molybdenum (oxy)nitride species observed for MoO3/HZSM5. FTIR spectroscopy was used to conduct isotopic nitrogen exchange experiments over nitrided HZSM5 and MoO3/HZSM5. Under the conditions of the experiment, it was shown that the presence of “molybdenum nitride”
facilitated the exchange of 15N2 with zeolite framework NHx species. This shows that supported γMo2Nspecies can be a source of reactive and mobile Nspecies, potentially opening up possibilities for its application as a source of spillover nitrogen. A potential route for the direct synthesis of aniline from benzene by hydrogenating ternary nitrides with benzene in the feed, trapping the possible reactive nitrogen species, was investigated. GCMS data showed the no reaction occurred, as only benzene was found in the David Mckay Abstract iv
product condensate. In all cases a significant amount of carbon was incorporated/ deposited on the catalysts. In the case of the cobalt molybdenum sample, the XRD data confirmed the conversion of nitride to the carbide, however postreaction XRD of the iron and nickel samples did not indicate carbide formation.
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