Mechanism of Ammonia Formation on Rh(111) Studied by Dispersive Near-Edge X-ray Absorption Fine Structure Spectroscopy

Abstract

The reaction mechanism of ammonia formation on a Rh(111) surface was investigated by means of dispersive near-edge X-ray absorption fine structure (dispersive-NEXAFS) spectroscopy. Nitrogen-covered Rh(111) surfaces were exposed to gaseous hydrogen (1.0−5.0 × 10−7 Torr) at constant surface temperatures (330−390 K) to form ammonia which desorbs immediately from the surface. Continuous data acquisition of nitrogen K-edge NEXAFS spectra enables us to monitor coverage changes of surface species during the progress of the reaction. The obtained NEXAFS spectra were well reproduced by summation of the standard spectra for N and NH. We found that the NH species, which is considered as a stable intermediate, is not only hydrogenized to NH3, but also decomposed into N and H under H2-lacking conditions. The reaction-order analyses indicate that the NH decomposition occurs at periphery of 2-dimensional islands of NH while the NH3 formation takes place most likely over the entire NH islands. We propose a two-step reaction mechanism: (1) formation of NH islands and (2) hydrogenation of the NH species to NH3. The former process reaches equilibrium with decomposition after a short period.

Tuning Metal–Metal Interactions through Reversible Ligand Folding in a Series of Dinuclear Iron Complexes

“…This work makes use of 3 PDI 2 , a tert -butylated version of a macrocyclic, pyridyldiimine-based scaffold (Figure ) − that we recently employed for mediating the putative formation of transient dicobalt bridging nitrides . The Co 2 N complexes parallel proposed surface nitrides that develop during bulk Rh/Ir-catalyzed NH 3 decomposition/oxidation chemistry. , Variations in the reaction conditions resulted in a wide array of products, suggesting that an improved understanding of the factors that govern changes in the ligand geometry will lead to enhanced reaction selectivity. The present report describes a series of diiron complexes in which we are able to observe geometric changes as a function of charge, spin state, and the identity of ancillary ligands.…”

In Situ Observation of Model Catalysts under Reaction Conditions Using X‐ray Core‐Level Spectroscopy

“…Under this pressure range of reactant gas, we could observe the time evolution of coverages of surface reactants and products from analyses of the continuously measured data set and then obtain kinetics information under various reaction conditions with different pressures and temperatures. This spectroscopic in situ observation of surface reaction kinetics has been applied to mechanistic studies on CO oxidation, water formation, NO reduction and NH 3 formation . In particular, by comparing the obtained time evolution of surface coverages with simulations using the kinetic Monte Carlo method, we proposed a microscopic reaction model that can explain the observed kinetics well .…”

Development of Fluorescence-yield Wavelength-dispersive Soft X-ray Absorption Spectroscopy for Real-time Observation of Surface Chemical Reaction

“…In addition, the L edges for the 3d transition metals are also in the soft X-ray region. The soft Xray wavelength-dispersive XAS technique has been applied to the real-time observation of surface chemical reactions owing to its small probing depth [3][4][5][6][7][8][9]. However, the available pressure for the reaction gas is limited due to the short attenuation length of the electrons in gas.…”