Ammonia Synthesis On Ru Supported On Carbon Nanotubes And TAP Study Of Ammonia Decomposition

Supported Ru is the second-generation, after fused-iron, catalyst forammonia synthesis. Studies on this catalyst will have an important impact oncatalytic science and technology. This thesis contains research on ammoniasynthesis that is made up of two parts.In the first part, the catalyst preparation and reaction test for ammoniasynthesis, Ru supported on carbon nanotubes (CNTs), is reported. The effectsof the support, metal particle size, and promoter on catalytic activity wereinvestigated in detail. The catalytic activity of Ru supported on carbonnanotubes is higher than that of Ru supported on activated carbon. This isattributed to the high purity, unique structure and superior electron transferproperties of CNTs. Catalysts with the different promoters, Ba, K and Cs,show large differences in activity for ammonia synthesis. The catalystpromoted with Ba (Ba/Ru=0.2 molar ratio) was the most active, whereas thatwith a K promoter was the least active. Methanation on the Ru/CNT catalyst isvery much inhibited compared with that on a Ru/AC catalyst. Highgraphitization of the CNTs is the main the reason for the high resistance tomethanation. The power law rate expression for ammonia synthesis onRu-Ba/CNTs catalysts is: r=kP_NH3^-0.4 P_N2^0.8 P_H2^-0.7 . This indicates that H2 inhibitsN₂ activation on the catalyst.In the second part of the thesis, the decomposition of ammonia wasstudied using a temporal-analysis-of-products (TAP) reactor. Desorption ofhydrogen and nitrogen from ammonia decomposition and the effect ofadsorbed oxygen on Ru on the rate of ammonia decomposition wereinvestigated. The adsorption of NH3 is dissociative on reduced Ru sites and therate of nitrogen desorption is the rate determining step. Hydrogen spillover isdetected on the supported Ru catalyst and this spillover is slowed down by theaddition of a Ba promoter. The rate of nitrogen desorption is inhibitedsignificantly by adsorbed hydrogen and is related to the concentration ofhydrogen on the surface. The rates of ammonia decomposition and nitrogendesorption are promoted by adsorbed oxygen on Ru. This is attributed tooxygen participation in the ammonia decomposition step, i.e. NHx* + O* →NHx-1* + OH* (x=1-3). Oxygen participation is not favorable for theactivationof nitrogen. Ammonia decomposition on Pt was also investigated using a TAP reactor.The catalytic activity of Pt is lower than that of Ru. Ammonia decompositionis considerably faster with adsorbed oxygen on Pt. The product distribution isinfluencedby the amounts of adsorbed oxygen and the reaction temperature.