Sammendrag
Ruthenium based catalysts for ammonia synthesis have been studied extensively following
the industrial adoption of a promoted carbon supported ruthenium catalyst in
the Kellogg Advanced Ammonia Process (KAAP). Nevertheless, there are still fundamental
aspects such as the dissociative adsorption of nitrogen—generally regarded
as the rate determining step of NH3 synthesis—and the influence of the barium promoter,
not fully explored. In the present work, the activation of nitrogen on barium
promoted ruthenium is elucidated through a combined experimental and computational
approach.
Nitrogen dissociation and association on a clean and barium promoted Ru(0001)
step were investigated through DFT based calculations using VASP. Unpromoted dissociation
was found to proceed with an energy barrier of 51 kJmol−1, with a N-N
distance of the transition state (TS) of 1.864 Å. The calculated activation barrier for
association was 135 kJmol−1, which increased to 161 kJmol−1 when diffusion of atomic
nitrogen along the terrace was considered. Upon promotion by a unit of atomic Ba,
BaOH and BaO at the step, the dissociation barrier decreased rather similarly by 21,
18 and 18 kJmol−1, respectively. The chemical state of the promoting unit was determined
to have a larger effect on the association barrier, which decreases by 19, 10
and 5 kJmol−1, respectively. A previously not reported local energy minimum state
with one nitrogen atom adsorbed on the b5-hcp site and the other at a step-bridge site
was identified. A significant stabilization of the local minimum state is observed upon
promotion: the N-N distance of the initial and final state of dissociation increased
(which can be associated with a weakening of the N-N bond), while it decreased for
the TS and the local minimum state. The promoting effect decreases rapidly with
increasing distance to the dissociating nitrogen, indicating that the interactions were
of electronic nature.
Powdered catalyst samples of Ru-Ba/AC were prepared by Ba(NO3)2 wet-impregnation
of 5 wt% Ru on activated carbon. The nitrogen isotope exchange (IE) reaction 14,14N2
+ 15,15N2 = 214,15N2 on Ru-Ba/AC was investigated in the temperature range 320–
750°C at N2 pressures of 20–230mbar, by means of gas-phase analysis with mass
spectrometry (GPA-MS). Apparent isotope exchange activation energies in range 162–
178 kJmol−1 were obtained below 425°C. This is in good agreement with the literature
and the present computational results. At higher temperatures the apparent activaiii
Abstract
tion energy abruptly decreases to 64–88 kJmol−1. It is suggested that the change in
temperature dependence is due to limitations by pore diffusion at higher temperatures.
In the presence of 1mbar water vapor in the temperature range 575–625°C, the isotope
exchange rate was significantly reduced compared to under dry conditions, and
the apparent activation energy increased from 88 ± 2 kJmol−1 to 126 ± 12 kJmol−1.
When water vapor was introduced, evolution of H2 was observed, indicating that oxidation
of partially reduced Ba occurred in the presence of H2O. Isotherms of the
isotope exchange rate showed reaction orders with respect to nitrogen partial pressure
of 0.83 ± 0.05 and 0.88 ± 0.03 at 625°C and 700°C, respectively, and 1.1 at 450°C. All
of which are in good agreement with values reported in literature for NH3 synthesis.
Deactivation of the catalyst was observed at temperatures above 500°C, resulting
in a significantly decreasing IE rate with time. In accordance with reports from literature,
and the computational and experimental results, it is proposed the isotope
exchange rate and activation energy are highly dependent on the chemical state of the
barium promoter, which is further dependent on the environmental conditions, such
as temperature and the presence of water vapor.
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