(299g) Confinement Effect on Chemical Reaction Yield: The Nitric Oxide Dimer Reaction

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]. 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.

Here 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 [4] 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 [5] 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.

There has also been evidence that nanophases can exhibit high pressures when adsorbed within

nanoporous materials due to confinement effects [6,7]. While the pressure in the bulk is a scalar

quantity, the pressure inside a pore is a second-order tensor with both normal and tangential

components, with respect to the wall. Molecular simulations for simple fluids in pores has shown the

presence of high local tangential pressures, even at lower bulk pressures.

In this work, we report a Reactive Monte Carlo study of the nitric oxide dimerization 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 flexibility of the bonds

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

particular, we show that the force fields used for both the monomer-wall and dimer-wall interactions

were in error and this was a primary cause of the discrepancy between the simulations and experiment.

Using ab initio calculations at the MP2 level we report revised force fields for these interactions, and

show that the use of these leads to agreement with experimental results. In addition, we also investigate

the relationship between the high in-pore tangential pressures and the higher reaction conversion yields

under confinement.

References

[1] C.H. Turner, J.K. Brennan, M. Lisal, W.R. Smith, J.K. Johnson and K.E. Gubbins, â??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, â??Enhancement of NO Dimerization by

Micropore Fields of Activated Carbon Fibers�, 5, 960-965 (1989).

[4] O. Byl, P. Kondratyuk and J. T. Yates, â??Adsorption and Dimerization of NO inside Single-Walled Carbon Nanotubes â??

An Infrared Spectroscopic Study�, J. Phys. Chem. B, 2003, 107, 4277-4279.

[5] 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).

[6] Yun Long, Jeremy C. Palmer, Benoit Coasne, MaÅ?gorzata Å?liwinska-Bartkowiak and Keith E. Gubbins, â??Pressure

enhancement in carbon nanopores: A major confinement effect�, Physical Chemistry Chemical Physics, 13, 17163-17170

(2011).

[7] Yun Long, Jeremy C. Palmer, Benoit Coasne, MaÅ?gorzata Å?liwinska-Bartkowiak, George Jackson, Erich A. Müller and

Keith E. Gubbins, â??On the Molecular Origin of High Pressure Effects in Nanoconfinement: Effects of Surface Chemistry

and Roughness�, Journal of Chemical Physics, 139, 144701 (2013)