Experimental and Theoretical
Investigation of Homogeneous Gaseous Reaction of CO<sub>2</sub>(g)
+ <i>n</i>H<sub>2</sub>O(g) + <i>n</i>NH<sub>3</sub>(g) → Products (<i>n</i> = 1, 2)
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Abstract
Decreasing CO<sub>2</sub> emissions into the atmosphere
is key for reducing global warming. To facilitate the CO<sub>2</sub> emission reduction efforts, our laboratory conducted experimental
and theoretical investigations of the homogeneous gaseous reaction
of CO<sub>2</sub>(g) + <i>n</i>H<sub>2</sub>O(g) + <i>n</i>NH<sub>3</sub>(g) → (NH<sub>4</sub>)HCO<sub>3</sub>(s)/(NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>(s) (<i>n</i> = 1 and 2) using Fourier transform infrared attenuated total reflectance
(FTIR-ATR) spectroscopy and ab initio molecular orbital theory. Our
FTIR-ATR experimental results indicate that (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>(s) and (NH<sub>4</sub>)HCO<sub>3</sub>(s) are formed
as aerosol particulate matter when carbon dioxide reacts with ammonia
and water in the gaseous phase at room temperature. Ab initio study
of this chemical system suggested that the reaction may proceed through
formation of NH<sub>3</sub>·H<sub>2</sub>O(g), NH<sub>3</sub>·CO<sub>2</sub>(g), and CO<sub>2</sub>·H<sub>2</sub>O(g)
complexes. Subsequent complexes, NH<sub>3</sub>·H<sub>2</sub>O·CO<sub>2</sub> and (NH<sub>3</sub>)<sub>2</sub>·H<sub>2</sub>O·CO<sub>2</sub>, can be formed by adding gaseous reactants
to the NH<sub>3</sub>·H<sub>2</sub>O(g), NH<sub>3</sub>·CO<sub>2</sub>(g), and CO<sub>2</sub>·H<sub>2</sub>O(g) complexes,
respectively. The NH<sub>3</sub>·H<sub>2</sub>O·CO<sub>2</sub> and (NH<sub>3</sub>)<sub>2</sub>·H<sub>2</sub>O·CO<sub>2</sub> complexes can then be rearranged to produce (NH<sub>4</sub>)HCO<sub>3</sub> and (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> as
final products via a transition state, and the NH<sub>3</sub> molecule
acts as a medium accepting and donating hydrogen atoms in the rearrangement
process. Our computational results also reveal that the presence of
an additional water molecule can reduce the activation energy of the
rearrangement process. The high activation energy predicted in the
present work suggests that the reaction is kinetically not favored,
and our experimental observation of (NH<sub>4</sub>)HCO<sub>3</sub>(s) and (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>(s) may be attributed
to the high concentrations of reactants increasing the reaction rate
of the title reactions in the reactor