Low-Temperature Adsorption of H2 and D2 on Dehydrated and Water Precovered CPO-27-Ni

Metal-organic frameworks (MOFs) possessing open metal sites (e.g., from the CPO-27 series) are a promising class of materials for hydrogen storage. However, there is still no consensus on the vibrational signatures of H-2 adsorbed on different sites. In this work we report results of a combined Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) study on H-2 adsorption on dehydrated and D2O-precovered CPO-27-Ni. For unambiguous interpretation of the results adsorption of CO was also studied. Low-temperature CO adsorption on dehydrated CPO-27-Ni results in formation of Ni2+-CO adducts stabilized by pi-back-donation and characterized by a CO stretching frequency at 2182 cm(-1) (low coverage). With D2O-precovered a sample CO replaces part of the preadsorbed D2O molecules, and in this case the nu(CO) is significantly lowered (2172 cm(-1)). When H-2 is adsorbed at 100 K on dehydrated sample, complexes involving Ni2+ sites are formed and characterized by a nu(H-H) band at 4031 cm(-1). A satellite band (Q(trans) mode) is detected at 4249 cm(-1). Similar results were obtained after D-2 adsorption. However, D-2 was more strongly adsorbed than H-2 and the Qtrans mode was of lower relative intensity. Only at high H-2/D-2 equilibrium pressures (>= 50 mbar) occupation of secondary sites was clearly detected by a nu(H-H) band at 4118 cm(-1) or nu(D-D) band at 2964 cm(-1). No significant differences in the strength of adsorption of H-2 and D-2 were detected in this case, suggesting the mode of adsorption is different from that realized for the complexes involving Ni2+ sites. Progressive filling of the Ni2+ sites by D2O leads to a strong decrease in intensity of the H-2/D-2 bands associated with Ni2+ sites and red shift of their frequencies indicating a decrease of the interaction strength. On the contrary, the bands due to H-2/D-2 interaction with secondary sites appear with enhanced intensities and are blue-shifted, which suggests an increase of the interaction strength. These results were confirmed by DFT calculations showing an increase of the H-2 adsorption enthalpy on secondary sites (identified as framework oxygen atoms) in the presence of water due to attractive interaction of H-2 with the nearby water molecule. Spectral evidence of direct interaction of adsorbed H-2/D-2 with adsorbed water was also found. Our results allow confirming that H-2 adsorbed on open Ni2+ sites is characterized by a stretching frequency lower than 4100 cm(-1) and some proposed values of Mn+-H-2 adducts above 4100 cm(-1) are in fact due to H-2 interacting with secondary sites affected by H2O located in the vicinity.