The synthesis of ammonia from hydrogen and atomic nitrogen on the Rh(110) surface

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Siemsen, Linda
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Robert S. Hansen
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The Department of Chemistry seeks to provide students with a foundation in the fundamentals and application of chemical theories and processes of the lab. Thus prepared they me pursue careers as teachers, industry supervisors, or research chemists in a variety of domains (governmental, academic, etc).

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Catalytic synthesis of ammonia from hydrogen and atomic nitrogen was demonstrated on polycrystalline platinum and single crystal face Rh(110) catalysts. The atomic nitrogen was prepared by a microwave discharge in a nitrogen-hydrogen gas mixture and quantified by gas phase titration with nitric oxide. The afterglow from the microwave discharge was characterized spectroscopically; no NH, NH2, or NH3 species were found, and excited nitrogen species other than atomic nitrogen had sufficiently short half lives that a delay system permitted their complete decay before the afterglow reached the catalyst. Atomic nitrogen was therefore the only excited nitrogen species reaching the catalyst. No ammonia was formed in the absence of a catalyst. The rate of ammonia synthesis over Rh(110) was measured as a function of the partial pressures of N2, H2, and N over pressure ranges 100 < PN2 < 570 Pa, 110 PH2 < 1000 Pa, 13 < P N < 160 Pa, 330 < P Total < 1200 Pa. The Reynolds number was on the order of 10[superscript]-3. Most measurements were made at 500°C, the turnover number for ammonia synthesis on Rh(110) varying from 0.0079 to 0.073 molecules site[superscript]-1 sec[superscript]-1. A study of the dependence of synthesis rate on Rh(110) on temperature at fixed reactant partial pressures covered the range 400° to 750°C;Over the partial pressure ranges covered at 500°C on Rh(110) the ammonia synthesis rate could be represented within experimental error by a model assuming it to be proportional to the nitrogen adatom coverage [theta] N only. The latter was interpreted through a steady state model in which the atomic nitrogen adsorption rate, proportional to P N and to (1 - [theta] N, was exactly balanced by rates loss of nitrogen adatoms due to ammonia synthesis (proportional to [theta] N) and to recombination and desorption as N2 (proportional to [theta]N[superscript]2). Variations in PN2 and PH2 only affected the synthesis rate insofar as they affected P N. At fixed microwave discharge power, however, P N varied considerably with PH2 and PN2, being approximately proportional to PH2 at fixed PN2 and for fixed PH2, going through a maximum as PN2 increased. Qualitative interpretations of this behavior are presented. For fixed partial pressures, the ammonia synthesis rate decreases as the temperature increases, i.e., the apparent activation energy is negative. Several possible interpretations of this behavior are presented. ftn[superscript]1DOE Report IS-T-1363. This work was performed under Contract W-7405-Eng-82 with the Department of Energy.

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