The reduction of NO with CO and H2 is shown to
comprise two basic reactions: a surface oxygen
abstraction
by the reducing agent and a reoxidation of the surface by NO. The
former reaction step has been demonstrated
by transient CO2 formation during CO exposure of oxidized
CaO surfaces, while the latter was demonstrated
by N2 and/or N2O transient formation during NO
exposure of a prereduced CaO surface. It was shown
that
at low temperatures (between room temperature and 500 °C) both
N2 and N2O were formed, but at
temperatures
above 500 °C only N2 was observed. The activation
energies of the respective steps have been determined
using temperature-programmed reaction experiments. The activation
energy of the surface oxygen abstraction
was determined to be 25 kcal/mol and is similar to the apparent
activation energy of the overall reaction.
The activation energy of the NO bond breakage was determined to be
maximum 10 kcal/mol as measured by
N2O formation. The importance of an
N2O2
- or
N2O2
2- intermediate
in the formation of N2O will be discussed,
and the importance of N2O decomposition in forming
N2 at temperatures above 500 °C will be
compared
with a N surface diffusion mechanism.
The heterogeneous reduction of NO by H2 and CO over
different CaO materials is investigated. The
dependence of the specific NO reduction rate on the impurity content is
demonstrated for both reducing
species. The roles of two specific impurities, i.e., Na and Fe, as
well as their combined effect are investigated.
The apparent activation energies for the NO + CO and NO +
H2 reactions are determined for three
different
calcium oxides. Values between 26 and 28 kcal/mol are obtained.
The influence of impurity content is
found in the preexponential factor of the Arrhenius equation. A
reaction mechanism based on a rate-determining
surface-oxygen-abstraction step is suggested. This mechanistic
understanding is explored to compare the
activities of other alkaline-earth oxides. Particularly, a linear
correlation between the apparent activation
energy and the lattice parameter is observed.
The effect of O 2 on the reduction of NO over prereduced CaO surfaces is investigated. The experimental results suggest the existence of at least three different reaction channels, of which two are related to the high-temperature reduction of the CaO surfaces and involve the use of extra electrons in breaking the NO bond. The third reaction channel does not employ extra electrons for bond breaking, but the activity is affected by the amount of adsorbed surface oxygens. The difference between the former two reaction channels is found in the temperature needed for an observable activity. The reaction channel which is already active at low temperatures is described by a model based on F-centers, whereas the one which needs elevated temperatures involves a hole transport through the bulk. The activation energy for this transport is determined experimentally using a temperature-programmed reaction technique as well as theoretically by means of ab initio quantum chemistry calculations. Room-temperature exposure to O 2 is suggested to result in a poisoning of the F-centers, but has only a minor effect on the reaction channel proposed for high temperatures. Effects on the reduction of NO of time as well as temperature for the O 2 exposure step are also investigated and found to be consistent with an understanding based on the coexistence of different reaction channels.
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