Effect of Quadrupole of Nitrogen, as a Probe Molecule for Surface Area Estimation: XRD and HRMC Investigation

Abstract

To obtain the properties of nanopores solids, N2 is usually used as probe molecule, however recently, IUPAC recommends using Ar instead. This study used experimental X-ray diffraction (XRD) analysis, Monte Carlo (MC), and hybrid reverse MC (HRMC) simulations to obtain information on the intermolecular structures of adsorbed N2 and Ar. Our results indicate that the quadrupole interaction between N2 molecules is relatively weak, and the N2 gives fairly good assessments as with Ar for porous carbons with low functional groups.

Formation of amorphous and quasi-two-dimensional microcrystalline structures of CO2 in activated carbon pores at low temperatures

“…For the calculation of OP, a vector on each adsorbate molecule was fixed (Figure 1, blue arrow), and the following equation was used (Eq. 21) [12,18]: Where θz is the angle between N2 vector and the Z-axis, θ is the rotation angle between the vectors of two neighbors N2 molecules. The brackets represent the average of the calculated molecules.…”

“…The adsorbents were inserted in a slit-shaped Cu sample cell, with 2 mm thickness (Figure 2B). This sample cell was inserted in the sample cell chamber, which has Be windows [12,18]. The sample cell chamber was filled with the desired adsorbate (CO2, N2, or Ar) using the adsorption line (Figure 2C), until the desired filling pressure.…”

“…After the samples reached the equilibrium pressure, the temperature was changed at a rate of ±1 K min -1 using a He compressor, until the desired T for the experiments (300 K, 100 K, 77 K or 50 K). The XRD analysis was made using a Mo radiation source (Mo Kα, λ = 0.07107 nm, 50 kV, 30 mA), from 3 o to 90 o , conducted for 12 h [12,18]. In order to obtain the XRD information of only the gas adsorbed in the ACF, it was necessary to obtain the XRD scattering of i) background (only sample cell, without sample, under vacuum), ii) only the ACF, under vacuum, and iii) gas adsorbed on the desired ACF, all of them at the same desired temperature.…”

“…An HRMC simulation is considered in good agreement with the experimental results when the exp SF and the SF obtained by the simulation (sim SF) present similar peak shape, position, and number. Besides, the HRMC programming gives a fitting percentage, which also is a parameter to be observed and check if the HRMC simulation and experiment are in good agreement, so it is known that fitting percentages lower than 20% indicates that the structure obtained at the simulation is the most probably one to be found in reality inside the pores [12].…”

“…However, much work remains to be done to fully understand how the molecules are arranged inside the porous structures.Analysis as in situ X-ray diffraction (XRD) can provide useful information on how the molecules are arranged inside nanoporous structures [1,8,10]. From this analysis is possible to obtain information as the Radial Distribution Function (RDF) and the Structure Function (SF), which can give information of the packing structure of the adsorbate inside the pores and also be used as input data for the Hybrid Reverse Monte Carlo (HRMC) simulations [11,12], which is a derivate analysis from the Monte Carlo (MC) simulation [13,14]. The simulation results can give more visual and precise information of the molecular arrangement and potential, giving a more complete understanding of the whole system [13].…”