Directed thermocatalytic CO2 reduction over NiAl4 layered double hydroxide precursors − activity and selectivity control using different interlayer anions

The use of rare nickel-poor and aluminum-rich layered double hydroxides (LDHs) as catalyst precursors in CO2 hydrogenations has revealed the key importance of chemical quality of the interlayer anions for the achievable CO2 conversion and CH4/CO selectivity. At atmospheric pressures between 300 and 700 °C, the obtained CH4 and CO formation rates varied widely, reaching up to the remarkable 76.8 (LDH-NO3) and 47.7 μmol/gcats (LDH-NH2SO3), respectively, competing with Pt and Co containing catalysts. Depending on thermal stability of the interlayer anions, their partial or complete removal resulted in significant differences in LDH transformation and thus in reducibility of Ni(II) and specific surface area values of the resulting metal oxides. Moreover, clear correlation was found between the conversion, selectivity and the abundance, strength of base and acid sites presented in the calcined LDHs related to the nature of starting interlamellar anions. Incorporation of nitrate and perchlorate ions into the precursors helped the formation of weak basic sites, while sulphate and sulphamate ions resulted in a large number of strong acid and base sites. The high-resolution quasi in situ X-ray photoelectron spectra revealed the effect of the anions on chemical composition of the forming catalytic interfaces. Meantime, the in situ diffuse reflectance infrared spectroscopy measurements indicated multifarious CO2 hydrogenation pathways via formate (LDH-NO3, -Br, -I), carboxylate (LDH-Cl, -ClO4) intermediates and CO2 dissociation (LDH-SO4, -NH2SO3).