Effect of Zn/Co ratio in MOF-74 type materials containing exposed metal sites on their hydrogen adsorption behaviour and on their band gap energy

Simultaneous incorporation of Zn and Co ions into MOF-74 during the crystallization process has been studied, covering the whole Zn/Co concentration range (0–100% Co). The characterization techniques used, including X-ray diffraction, DR-UV-visible spectroscopy, N2 adsorption isotherms and thermogravimetric analysis, strongly evidence the successful incorporation of both cations into the material framework, producing the crystallization of MOF-74 with 100% Co and MOF-74 with 100% Zn starting from Zn-free and Co-free initial mixtures, respectively, under the same conditions. H2, CH4 and CO2 uptakes of MOF-74 type materials generally increase with framework Co content at any pressure, suggesting a relevant role of cobalt in the adsorption process. As expected, the comparison between the gas adsorption behavior of Co-substituted MOF-5 and Co-containing MOF-74 materials shows that exposed metal sites play a key role in MOF performance in adsorption. On the other hand, the good agreement between the evolution of the experimental band gap values as a function of the Co content and the computational-based predictions found in the literature further supports the coexistence of Zn2+ and Co2+ ions forming the MOF-74 metal clusters. Finally, we found that variations of both isosteric heat of hydrogen adsorption and band gap energy with the metal cluster composition show a parallel trend, although it is not systematic in the whole range of Zn/Co ratio. Indeed, a minimum band gap energy value and a maximum isosteric heat of H2 adsorption value were found to appear simultaneously for Co-rich samples still having some Zn rather than for all-Co samples.