The characteristic responses of ammonia synthesis reaction rates of the traditional Temkin [3] and modified Temkin [1] models are investigated in view of catalyst poisoning under periodic operation due to the competitive adsorption of N2 and H2[2]. Poisoning of surface at high hydrogen pressures was a natural consequence of the models. The surface started to be populated by nitrogen when hydrogen pressures are 250 times lower than the nitrogen pressure. Furthermore, it is noted that even the slightest abundance of ammonia decreases the reaction rate sharply and the production stops in a time scale of 0.01 s at low hydrogen pressures. The performance enhancement via a time interrupted periodic operation over Ru/SBA-15 catalysts were tested in situ in a DRIFTS reactor. The catalysts were reduced in situ under hydrogen flow at 300 °C for 2 hours, cooled and purged by Ar. The flow of N2 was resumed at room temperature to observe ammonia formation at ambient conditions via DRIFTS, a result is given in Figure-1, during nitrogen flow. Present work include the optimization of the species flow rates, exposure times and operating temperatures. Hydrogen is supplied by the amounts spilt-over in the previous cycle, the results will be matched with a model incorporating the rate kinetics used above with a Fickian surface diffusion model. A preliminary model prediction incorporating a surface diffusion coefficient and a first order rate parameter which lumps solid-state and bulk diffusion fits the experimentally measured hydrogen uptake curves very well, albeit qualitatively. Further study will estimate the coefficients of the model will be determined for hydrogen on SBA-15 supported Ru catalysts, quantitatively. [1] I. Rossetti, et.al. Ind.Eng. Chem. Res. 45 (2006), 4150. [2] S.E. Siporin and R. Davis, J. Catal., 225 (2005), 359 [3] M.I. Temkin and V. Pyzhev, Acta Physicochim. U.S.S.R., 12 (1940), 327.

Time interrupted behavior of ammonia synthesis over ruthenium based catalysts

249th National Meeting (2015)

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