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Atomic ratio effect on catalytic performance of FeW-based carbides and nitrides on thiophene hydrodesulfurization

Abstract:

(Graph Presented) Synthesis of bulk and alumina-supported, Fe-promoted W carbides and nitrides were carried out by using oxidic precursors with atomic ratios (Fe/(Fe + W)) varying from 0 to 1. These materials were prepared by temperature-programmed reaction up to 700°C of the precursors under flow of 20 vol.% CH<inf>4</inf>/H<inf>2</inf>, for carbides, and of NH<inf>3</inf> for nitrides. Crystalline structure of catalysts was established by X-ray diffraction (XRD), while textural properties were determined by N<inf>2</inf> physisorption. Catalytic tests of thiophene hydrodesulfurization (HDS) were carried out in a fixed bed reactor at 400°C and atmospheric pressure and monitored by FID-GC. The catalysts were activated in situ under a CS <inf>2</inf>/H<inf>2</inf> mixture at 300°C/2 h prior to HDS. In addition, both fresh and exhausted catalysts were analyzed by elemental chemical analysis (ECA) of C, N and S. XRD results showed a main bimetallic phase Fe <inf>2</inf>WO<inf>6</inf> for supported oxidic precursors, while bulk catalysts revealed a mixture of Fe<inf>2</inf>WO<inf>6</inf> and FeWO<inf>4</inf>. On the other hand, for supported carbides, XRD revealed the presence of the cubic Fe<inf>3</inf>W<inf>3</inf>C mixed phase, and bulk carbides showed the presence of Fe<inf>6</inf>W<inf>6</inf>C. For supported nitrides, XRD data revealed the presence of the hexagonal FeWN<inf>2</inf> mixed phase for bimetallic catalysts and W<inf>2.2</inf>N<inf>2</inf> and Fe<inf>3</inf>N for each monometallic catalyst. In contrast, for bulk monometallic W nitride it was found that W <inf>2</inf>N was the monometallic crystalline phase obtained. Texture analysis showed that specific surface area of catalyst decreased with respect to alumina, when Fe and W were incorporated and this effect increased with increasing Fe content. Catalytic tests revealed that the catalyst based on W carbide had the highest activity, followed by the carbide catalyst with atomic ratio 0.1, which turned out to be more active than W-based nitride catalyst and the nitride catalyst with atomic ratio 0.1. Finally, after HDS, catalyst showed an increase in S content due to the displacement of C or N atoms by S atom during presulfurization and HDS reaction, which was corroborated with the decrease in C and N content of exhausted catalysts in comparison with the initial contents. © 2012 Elsevier Ltd. All rights reserved.