Barium-promoted Ru/carbon catalyst for ammonia synthesis: State of the system when operating

The subjects of the research were barium-promoted ruthenium catalysts for ammonia synthesis supported on graphitized carbon. The purpose of this work was to study in detail the process of an active Ba–Ru/C catalyst formation. Another goal was to characterize the active state of the Ba promoter, that is the state corresponding to ammonia synthesis conditions. In situ XRD and TPR–MS techniques were applied to monitor the changes in the Ba–Ru/C specimens when heating in hydrogen (or H2 + N2) and H2 + Ar mixtures, respectively. The post-activation state of the catalyst was characterized chemically via interaction of the reduced samples with water vapour at 50 °C and also via interaction with oxygen at 0 °C. The above mentioned experiments were supplemented with those of ammonia synthesis. It was shown that ruthenium facilitates decomposition of the promoter’s precursor (Ba(NO3)2) deposited onto the surface of Ru/C catalysts when heating the specimens in a hydrogen-containing stream. The Ba(NO3)2/C reference materials, which do not contain ruthenium, are stable in a flowing H2 + Ar mixture up to about 400 °C, whereas the Ba(NO3)2 decomposition starts at 100–120 °C in the Ba(NO3)2–Ru/C systems (XRD, TPR–MS). The decomposition of Ba(NO3)2 in hydrogen leads to barium oxide (BaO) and metallic barium. Under steady-state conditions BaO is the only Ba-containing phase detected by the X-ray diffraction technique. Characterization of the post-activation catalysts showed that barium is partially reduced during the aforementioned operations and that these catalysts react with oxygen and water vapour. Based on the comparison of the O2 consumption and H2 evolution data one may deduce that the active form of the promoter is a mixture: Ba0 + BaO. It can be stated that the temperature and content of the promoter (CBa) have a significant influence on NH3 formation. The shape of reaction rate vs. barium content function is assumed to be an outcome of the promoter distribution on the active carbon surface and ruthenium surface. The trend of the integral reaction rate clearly reflects that of the Ru coverage by the barium-containing species, which is controlled by the heats of adsorption on ruthenium and carbon, respectively.

Barium-promoted ruthenium catalysts were studied in the ammonia synthesis: N2+3H2=2NH3N2+3H2=2NH3. Under steady-state ammonia synthesis conditions the surface of the catalysts is probably partly covered with Ba0 + BaO.Figure optionsDownload high-quality image (46 K)Download as PowerPoint slide