Mechanistic details of the heterogeneous decomposition of ammonia on platinum
Absolute reaction rates have been measured for the catalytic decomposition of NH/sub 3/ and ND/sub 3/ and for the NH/sub 3/ + D/sub 2/ exchange reaction over a polycrystalline platinum wire at pressures between 5 x 10/sup -7/ and 0.5 torr and temperatures between 400 and 1200 K in a continuous flow microreactor. At relatively low pressures and/or high temperatures, a primary isotope effect was observed for the decomposition of ND/sub 3/. Under these conditions, the order of the decomposition reaction is unity with respect to ammonia pressure with an apparent activation energy of 4.2 kcal/mol. As coverages increase, corresponding to relatively high pressures and/or low temperatures, the order of the decomposition reaction is zero with respect to ammonia, and the reaction rate becomes controlled by nitrogen desorption, with an activation energy of the decomposition reaction of 22 kcal/mol. The kinetics of the NH/sub 3/ + D/sub 2/ exchange reaction have been used, together with data concerning the adsorption-desorption parameters of NH/sub 3/, H/sub 2/, and N/sub 2/ as well as the reaction intermediates NH and NH/sub 2/, to develop a mechanistic model which describes the reaction rate over a wide range of experimental conditions. 35 references, 6 figures.
- Research Organization:
- California Institute of Technology, Pasadena
- OSTI ID:
- 6341641
- Journal Information:
- J. Phys. Chem.; (United States), Journal Name: J. Phys. Chem.; (United States) Vol. 89:15; ISSN JPCHA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400201* -- Chemical & Physicochemical Properties
ACTIVATION ENERGY
ADSORPTION
AMMONIA
CATALYSIS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
DATA
DECOMPOSITION
DESORPTION
DEUTERIUM
ELEMENTS
ENERGY
EXPERIMENTAL DATA
HIGH TEMPERATURE
HIGH VACUUM
HYDRIDES
HYDROGEN
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
INFORMATION
ISOTOPE EFFECTS
ISOTOPES
ISOTOPIC EXCHANGE
KINETICS
LIGHT NUCLEI
MATHEMATICAL MODELS
MEDIUM VACUUM
METALS
NITROGEN
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
NONMETALS
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
PLATINUM
PLATINUM METALS
REACTION KINETICS
SORPTION
STABLE ISOTOPES
TRANSITION ELEMENTS
VERY HIGH TEMPERATURE