I. Interaction of ammonia with single crystal rhodium catalysts. II. Hydrogen and nitrogen adsorption on a W(111) surface: a theoretical molecular orbital approach

Technical Report · Mon Jan 01 04:00:00 EST 1979

DOI:https://doi.org/10.2172/6461544· OSTI ID:6461544

Vavere, A.

Rates of ammonia decomposition on (110), (100), and (111) single crystal faces of rhodium were measured at 580 to 725/sup 0/K and 10/sup -3/ to 500 x 10/sup -3/ torr. The decomposition rates were proportional to P/sub NH/sub 3//sup/1/2/ and P/sub NH/sub 3// at low and high hydrogen pressures, respectively. The H/sub 2/ kinetic order varied from 0 (low P/sub H/sub 2//) to -1.0 (high P/sub H/). The rate was independent of N/sub 2/ pressure. NH/sub 3/ decomposes about 1.5 times faster than ND/sub 3/ on the (110) and (111) faces. Rates on the (110) surface are over 10 times as rapid as on the (111). LEED, Auger, and flash desorption experiments indicated that boron was a significant surface poison and that the Rh(110) surface was essentially nitrogen-free. A rate expression is derived from a model involving surface species Rh/sub 2/NH, RhH, and RhN on a nearly bare RH surface. The rate limiting process involves the concurrent dehydrogenation of Rh/sub 2/NH and desorption of N/sub 2/. A decreasing NH/sub 3/ order (< 1/2) at high P/sub NH/sub 3// and low T is due to buildup of surface intermediates. The relative bonding energies of hydrogen and nitrogen chemisorbed at three sites on a W(111) surface were obtained via the extended Hueckel molecular orbital theory. The preferred site for both H and N chemisorption was determined as the TOP position, i.e., a single coordination site on top of a protruding W atom. The W(111) surface was simulated by truncated arrays of seven tungsten atoms. The basis set for the calculations included the tungsten valence orbitals plus the filled 5p orbitals needed for repulsion at small internuclear distances. N adsorption in the three-fold holes available on the W(111) lattices used disrupted the W--W bonds sufficiently to cause the overall bond energy to be less than for the single coordination site. The dissymmetry between the three-fold lattices and the four-fold W d orbitals may also be a contributing factor.

• Research Organization: Ames Lab., IA (USA)
• DOE Contract Number: W-7405-ENG-82
• OSTI ID: 6461544
• Report Number(s): IS-T-849
• 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
BINDING ENERGY
BORON
CATALYSIS
CATALYTIC EFFECTS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHEMISORPTION
CRYOGENIC FLUIDS
CRYSTALS
DATA
DATA FORMS
DECOMPOSITION
DEUTERIUM
ELEMENTS
ENERGY
EXPERIMENTAL DATA
FLUIDS
HYDRIDES
HYDROGEN
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
INFORMATION
ISOLATED VALUES
ISOTOPE EFFECTS
ISOTOPES
KINETICS
LIGHT NUCLEI
METALS
MOLECULAR ORBITAL METHOD
MONOCRYSTALS
NITROGEN
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
NONMETALS
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
PLATINUM METALS
PRESSURE DEPENDENCE
REACTION KINETICS
REFRACTORY METALS
RHODIUM
SEMIMETALS
SEPARATION PROCESSES
SORPTION
SORPTIVE PROPERTIES
STABLE ISOTOPES
SURFACE PROPERTIES
SURFACES
TRANSITION ELEMENTS
TUNGSTEN