Inhibition by hydrogen of the heterogeneous decomposition of ammonia on platinum

Journal Article · Thu Nov 07 04:00:00 EST 1985 · J. Phys. Chem.; (United States)

DOI:https://doi.org/10.1021/j100269a009· OSTI ID:6126923

Tsai, W; Vajo, J J; Weinberg, W H

Absolute reaction rates have been measured in a continuous flow microreactor for the inhibition by hydrogen of the heterogeneous decomposition of ammonia over a polycrystalline platinum wire at pressures between 10 T and 0.6 torr, with ammonia to hydrogen partial pressure ratios varying from 1:1 to 1:4 and temperatures between 400 and 1200 K. Inhibition of the decomposition is observed at relatively high total pressures (0.2-0.6 torr) for all temperatures studied. The reaction orders with respect to hydrogen and ammonia are -1.5 and 1.0, respectively, with an apparent activation energy of 38 kcal/mol. For relatively low total pressures (10 T-5 x 10 T torr), inhibition is only observed at temperatures below 650 K, where the activation energy varies from 33 to 55 kcal/mol and the reaction order with respect to hydrogen approaches -1.5 at low temperatures. The decomposition becomes uninhibited at high temperatures and the activation energy is 4.5 kcal/mol. The kinetics of this reaction as well as previous results for the NH3 + D2 exchange reaction are described quantitatively by a mechanistic model employing independently measured adsorption-desorption parameters of NH3, N2 and H2, where the rate coefficient for hydrogen desorption is a function of the fractional surface coverage of nitrogen adatoms. The hydrogenation of NH2(a) to produce molecularly adsorbed ammonia is predicted to be the dominant factor in the inhibition of the ammonia decomposition. 37 references, 5 figures, 1 table.

• Research Organization: California Institute of Technology, Pasadena
• OSTI ID: 6126923
• Journal Information: J. Phys. Chem.; (United States), Journal Name: J. Phys. Chem.; (United States) Vol. 89:23; ISSN JPCHA
• Country of Publication: United States
• Language: English

Related Subjects

090320 -- Inorganic Hydrogen Compound Fuels-- Preparation-- (1976-1989)
10 SYNTHETIC FUELS
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
AMMONIA
ARRHENIUS EQUATION
CATALYSIS
CATALYTIC EFFECTS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
DATA
DECOMPOSITION
ELEMENTS
EQUATIONS
EXPERIMENTAL DATA
HIGH TEMPERATURE
HYDRIDES
HYDROGEN
HYDROGEN COMPOUNDS
INFORMATION
KINETICS
MATHEMATICAL MODELS
MEDIUM VACUUM
METALS
NITROGEN
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
NONMETALS
NUMERICAL DATA
PLATINUM
PLATINUM METALS
PRESSURE DEPENDENCE
QUANTITY RATIO
REACTION KINETICS
SORPTIVE PROPERTIES
SURFACE PROPERTIES
TEMPERATURE DEPENDENCE
TRANSITION ELEMENTS
VERY HIGH TEMPERATURE