The DeNO$_x$ process and NO$_2$ adsorption in MOF74*

Due to the harmful character of NO$_2$ and its slow decomposition rate, the use of catalytic materials for its removal (DeNO$_x$ process) has attracted a lot of attention. The high porosity and highly reactive uncoordinated metal centers of MOF74 have led us to investigate the use of Mg- and Zn-MOF74 as materials for trapping NO$_2$ with resistance to poisoning by SO$_2$. In this combined theoretical and experimental study, we investigate the interaction between the unsaturated metal centers of the MOF and the NO$_2$ guest molecules. For our theoretical modeling we use ab initio calculations at the DFT level, utilizing vdW-DF to capture the significant van der Waals component of the interaction between NO$_2$ and the MOF. We present detailed first-principle results concerning the adsorption energies and geometries, as well as vibration frequencies of the NO$_2$ molecule adsorbed in the MOF. Our experimental efforts (IR and Raman spectroscopy) have shown a blue shift to 1684 cm$^{-1}$ in the vibration stretching mode of the NO$_2$ upon adsorption and a thermal stability up to 150$^{\circ}$C. Our first-principle calculations and experimental results show a remarkable agreement, allowing us to give a complete picture of the adsorption of NO$_2$ molecules in the MOF74 structure.