The Nitric Oxide Dimer Reaction in Carbon Nanopores Deepti Srivastava, † C. Heath Turner, ‡ Erik E. Santiso, † and Keith E. Gubbins* ,† † Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, United States ‡ Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, Alabama 35487-0203, United States ABSTRACT: When confined within nanoporous carbons (activated carbon fibers or carbon nanotubes) having pore widths of about 1 nm, nitric oxide is found to react completely to form the dimer, (NO) 2 , even though almost no dimers are present in the bulk gas phase in equilibrium with the pore phase. Moreover, the yield of dimer is unchanged upon varying the temperature over the range studied in the experiments. Earlier molecular simulation studies showed a significant increase in dimer formation in carbon nanopores, but the dimer yield was considerably less than that found in the experiments, and decreased rapidly as the temperature was raised. Here, we report an ab initio and molecular simulation study of this reaction in both slit-shaped pores and single-walled carbon nanotubes. The ab initio calculations show that the nitric oxide dimer forms a weak chemical bond with the carbon, and the bonding energy is more than 20 times stronger than the van der Waals energy assumed in the previous studies. When this is accounted for, the predicted dimer yield is in good agreement with the experimental values, as is its temperature dependence. We also report results for the pressure tensor components for this confined reactive mixture. Local tangential pressures near the pore walls are as high as millions of bar, reflecting the strong nanoscale forces. 1. INTRODUCTION When carried out in a nanoporous catalyst or catalyst support, heterogeneous reactions are often strongly affected by the pore shape and width, surface heterogeneity, and the interactions of the various reactants, products, and activated species with the pore walls. These factors can have a large influence on the reaction yield, 1−4 the rate, 5 and the activation barriers 6−8 and in some cases can alter the reaction mechanism 6,8,9 or produce new product phases. 9−11 Other considerations in such heterogeneous reactions include surface defects in the solid substrate, which can form highly reactive sites, 6,12 and high tangential pressures due to the strong force field from the solid substrate, thus promoting high pressure reactions 9,13 and shape-catalytic effects. 8 The latter effect occurs when the pore size and shape become similar to those of the molecules. For such reactions, when the transition state theory is appropriate, the pore size can enhance or hinder the reaction if the shape of the transition state is similar to that of the pore, leading to dramatic effects of pore size. Small changes in the pore width then have a large effect on the potential energy surface of the reaction and on the reaction rate, as shown by a study of the rotational isomerism of C 4 hydrocarbons in carbon pores. 8 In this paper, we report a study of the nitric oxide dimerization reaction, 2NO ↔ (NO) 2 , in carbon nanopores. This reaction is of interest for several reasons: (a) reliable experimental data are available for this reaction in well-defined nanoporous carbons; (b) the results of these experimental studies are surprising, showing complete conversion to the dimer in the nanopores, although there are almost no dimers in the bulk gas phase that is in equilibrium with the porous material; and (c) the failure of theoretical studies so far to provide a satisfactory explanation of these results. In 1989, Kaneko et al. 1 reported a study of the nitric oxide dimer reaction in NO-adsorbed activated carbon fibers, for the temperature range 273−423 K under a nitric oxide gas pressure of 80 kPa. The carbon fibers, which have slit-shaped pores, had a relatively uniform pore width of 0.8−0.9 nm. The composition of the confined nanophase was determined using magnetic susceptibility measurements (while the monomer is paramagnetic, the dimer is diamagnetic), and it was found that more than 98 mol % of the molecules consisted of (NO) 2 dimers, even though the fraction of dimers in the bulk phase in equilibrium with the pore was of the order a few mole % or less. This surprising result was confirmed in later experiments carried out by Byl et al., 2 who studied the reaction in (10,10) single-walled carbon nanotubes having a diameter of 1.36 nm over the temperature range 103−136 K, under nitric oxide gas pressures up to a few torr. Equilibrium compositions within the SWNTs were determined using Fourier transform infrared spectroscopy, and the mole fraction of dimers was shown to be Special Issue: Benjamin Widom Festschrift Received: November 3, 2017 Revised: December 14, 2017 Published: December 14, 2017 Article pubs.acs.org/JPCB Cite This: J. Phys. Chem. B XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.jpcb.7b10876 J. Phys. Chem. B XXXX, XXX, XXX−XXX