Dynamics of bound states of dihydrogen at Cu(I) and Cu(II) species coordinated near one and two zeolite framework aluminium atoms: A combined sorption, INS, IR and DFT study

Ambient conditions sorption isotherms of dihydrogen in a series of various levels of Cu exchanged ZSM-5 zeolites, with two different Si/Al ratios, namely 11.5 and 25, show the presence of different amount of Cu centres able to strongly bind H-2. Although the isosteric heats of adsorption derived from these isotherms are rather similar, of the order of 30 kJ/ mol H-2, Inelastic Neutron Scattering (INS) of adsorbed dihydrogen and Fourier Transformed Infra-Red (FTIR) spectroscopy measurements of adsorbed CO and NO reveal that copper is encountered in two oxidation states. At least two types of Cu(I) ions are clearly detected as well as some heterogeneity of the Cu(II) species. The number of these Cu species is different in the two investigated ZSM-5 materials and depends on the Cu exchange level. With the aid of DFT model cluster calculations we find that under different coordination environments, determined by the Al distribution, both mono-and divalent Cu ions could bind H-2 with a different strength. Unprecedentedly, we found that Cu-ions compensating two Al atoms, i.e. formally Cu(II) species, relatively far apart from eachother, may behave very similarly to the monovalent Cu-species or alternatively viewed as Cu(I) species that compensate for two framework Al-atoms. Such Cu-species also form stable 712 dihydrogen complexes. (C) 2021 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

Automated summary

The study reveals differences in the binding form, quantity, and stability of copper with dihydrogen in different ZSM-5 zeolites, influenced by the copper exchange level and aluminum distribution.