(284d) Confinement Effect on Chemical Reaction Yield: The Nitric Oxide Dimer Reaction

1. Introduction

It is well established that confinement within a nano-porous material, such as an activated carbon,

carbon nanotube, or porous oxide can affect reaction yield, reaction rate and even the mechanism of

the reaction [1,2,3,4]. These effects arise from the strong intermolecular forces between the various

reacting species, including the activated state, and the pore walls, but are poorly understood.

Understanding from experiment alone is difficult to achieve due to many competing effects, but

complementary experimental and molecular simulation studies may help.

2. Motivation and Background

In this study, we report a molecular simulation study of the effects of confinement within a nanoporous

carbon material on the equilibrium yield of the nitric oxide dimerization reaction, 2NO=(NO)2.

This reaction is chosen for study both because of its importance in atmospheric chemistry and

biology, and because experimental data is available for the effect of confinement within carbon

materials. Thus Kaneko and coworkers [3] have used magnetic susceptibility measurements to

determine the equilibrium yield of the dimer in activated carbon fibers having slit-shaped pores of 0.8

nm width, while Yates and coworkers [5] have used FTIR spectra to find the yield in single-walled

carbon nanotubes of diameter 1.35 nm. At the experimental conditions the yield of dimer in the gas

phase in equilibrium with the pore phase was less than 1 mol %, whereas the yield within the pores

was 100 mol % within the accuracy of the experiment. Monte Carlo simulations reported [6] in 2001

found a large increase in the yield within slit-shaped pores due to the confinement, but the calculated

yield was well below the experimental value.

3. Conclusion

In this work, we report a Reactive Monte Carlo study of this reaction in slit-shaped carbon pores of

various widths and over a range of temperatures. We examine several approximations made in earlier

molecular simulation studies, in particular the dimensionalities of the molecular partition functions 

and the force fields used, and show that the latter have a large effect on the calculated yield.

References

1 C.H. Turner, J.K. Brennan, M. Lisal, W.R. Smith, J.K. Johnson and K.E. Gubbins, “Simulation of

Chemical Reaction Equilibria by the Reaction Ensemble Monte Carlo Method: A Review”, Molecular

Simulation, 34, 119-146 (2008).

2 C.H. Turner, J.K. Brennan, J.K. Johnson and K.E. Gubbins, "Effect of Confinement by Porous

Materials on Chemical Reaction Kinetics", Journal of Chemical Physics, 116, 2138-2148 (2002).

3 K. Kaneko, N. Fukuzaki, K. Kakei, T. Suzuki and S. Ozeki, Langmuir, “Enhancement of NO

Dimerization by Micropore Fields of Activated Carbon Fibers”, 5, 960-965 (1989).

4 M.K. Kostov, E.E. Santiso, A.M. George, K.E. Gubbins and M. Buongiorno Nardelli, “Dissociation

of Water on Defective Carbon Substrates”, Physical Review Letters, 95, 136105 (2005).

5 O. Byl, P. Kondratyuk and J. T. Yates, “Adsorption and Dimerization of NO inside Single-Walled

Carbon Nanotubes – An Infrared Spectroscopic Study”, J. Phys. Chem. B, 2003, 107, 4277-4279.

6 C. H. Turner, J. K. Johnson and K. E. Gubbins, "Effect of Confinement on Chemical Reaction

Equilibria: The Reactions 2NO = (NO)2 and N2+3H2 = 2NH3 in Carbon Micropores", J. Chem. Phys.,

114, 1851-1859 (2001)