Structural Effects and Interactions of Carbon Dioxide Molecules Adsorbed on Ni, Zn, and Cd Nitroprussides

Abstract

A thorough structural characterization of the synthesized Ni-, Zn-, and Cd-nitroprussides (NPs) with X-ray diffraction (XRD), thermogravimetric analysis, diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), Mössbauer spectroscopy, and magnetic measurements was performed. However, the innovative part of the research was the study of the structural effects and the interactions of the CO2 molecule with the Ni-, Zn-, Cd-NP frameworks. DRIFTS of adsorbed molecules, high pressure adsorption, and in situ XRD adsorption experiments were performed. The DRIFTS spectra displayed peaks assigned to CO2 physical adsorption and the formation of adducts (M2+···O═C═O). The fitting of the adsorption data to the Dubinin–Radushkevich equation and a Langmuir-type equation for volume filling allowed the calculation of the micropore volume and the isosteric heats of adsorption. The calculated parameters indicated an unusual behavior of the adsorption process in Cd-NP at high pressure. This fact was caused by the interaction of CO2 molecules with the framework cations and the small adsorption space of Cd-NP. The Ni- and Zn-NPs behaved normally. The Pawley fitting of the XRD profiles of the dehydrated materials and under CO2 adsorption indicated that in both cases Ni-NP, Zn-NP, and Cd-NPs displayed the Fm3̅m, R3̅, and Pnma space groups, respectively. The dehydrated samples demonstrated a change in the cell parameters. However, only Cd-NP presented a noticeable variation of its cell parameters under CO2 adsorption. This fact was linked to the unusual behavior of the adsorption process in Cd-NP. Additionally, was shown that dehydrated Ni-, Zn-, and Cd-NPs can store 27, 22, and 15 wt % of CO2 at 298 K and 9 atm., respectively. Then, Ni- and Cd-NPs are excellent for CO2 storage, and Cd-NP is good for gas cleaning.