Decomposition of ammonia on rhodium crystals

Journal Article · Fri May 01 04:00:00 EDT 1981 · J. Catal.; (United States)

OSTI ID:6324348

Vavere, A; Hansen, R S

The rates of ammonia decomposition of (110), (100), and (111) single-crystal faces of rhodium were measured over the temperature range 580 to 725 K for ammonia pressures ranging from 1 x 10/sup -3/ to 500 x 10/sup -3/ torr and hydrogen and nitrogen pressures varying from 1 x 10/sup -3/ to 150 x 10/sup -3/ torr and 1 x 10/sup -3/ to 250 x 10/sup -3/ torr, respectively. The decomposition rates were proportional to P/sup 1/2//sub NH3/ and P/sub NH3/ at low and high hydrogen pressure, respectively. The H/sub 2/ kinetic order varied from 0 (low P/sub H2/) to -1.0 (high P/sub H2/). The rate was independent of nitrogen pressure. Isotope studies indicated that NH/sub 3/ decomposes about 1.5 times faster than ND/sub 3/ on the (110) and (111) faces. A large face specificity is evident with the rates on the (110) surface over 10 times as great as those on the (111) surface. LEED, AES, and flash desorption experiments indicated that boron (B) was a significant surface poison and that the Rh(110) surface under reaction conditions might contain at most moderate coverages of nitrogen adatoms, but that coverages of other species were very small. The LEED/AES characterizations were done in a different system from that used in the kinetic and thermal desorption experiments. Rate expressions consistent with the observed kinetics are derived from a model involving surface species Rh/sub 2/NH, RhH, and RhN, with only the RhN coverage being appreciable under the experimental conditions used. The kinetic data for the Rh (110) face are semiquantitatively fit to these rate expressions via a superposition technique and also to an empirical rate expression which functionally is similar to the mechanism rate equation. A decreasing NH/sub 3/ order (< 1/2) at high P/sub NH3/ and low T is attributed to the buildup of surface nitrogen. Kinetic orders and activation energies obtained are generally consistent with most other literature data.

• Research Organization: Iowa State Univ., Ames
• DOE Contract Number: W-7405-ENG-82
• OSTI ID: 6324348
• Journal Information: J. Catal.; (United States), Journal Name: J. Catal.; (United States) Vol. 69:1; ISSN JCTLA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical &amp; Physicochemical Properties
400202 -- Isotope Effects
Isotope Exchange
&amp; Isotope Separation
AMMONIA
AUGER ELECTRON SPECTROSCOPY
CATALYTIC EFFECTS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CRYSTALS
DATA
DECOMPOSITION
DESORPTION
DEUTERIUM
ELECTRON SPECTROSCOPY
ELEMENTS
EXPERIMENTAL DATA
HIGH TEMPERATURE
HYDRIDES
HYDROGEN
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
INFORMATION
ISOTOPE EFFECTS
ISOTOPES
KINETICS
LIGHT NUCLEI
METALS
MONOCRYSTALS
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
NONMETALS
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
PLATINUM METALS
PRESSURE DEPENDENCE
REACTION KINETICS
REFRACTORY METALS
RHODIUM
SPECTROSCOPY
STABLE ISOTOPES
TEMPERATURE DEPENDENCE
TRANSITION ELEMENTS