On the dependence of the catalytic activity of nickel ferrite nanoparticles in the oxidative dehydrogenation of 2-propanol on the crystallite size

Nickel ferrite spinel nanoparticles of different crystallite sizes were prepared in a glycine assisted sol-gel autocombustion reaction and characterised by powder x-ray diffraction, attenuated total reflection infrared spectroscopy, near ambient pressure x-ray photoelectron spectroscopy, nitrogen physisorption, hydrogen and carbon monoxide temperature programmed reduction and oxygen and carbon dioxide temperature programmed desorption. A different distribution of Ni2+ cations in the tetrahedrally and octahedrally coordinated sites of the spinel lattice and an increased reducibility of the smaller crystallite size sample were identified as the main impacts of different crystallite size. Their catalytic activity in the oxidative dehydrogenation of 2-propanol was investigated by temperature programmed reaction studies using different ratios of 2-propanol:O2 as well as of possible parallel and consecutive reactions at atmospheric pressure and maximum 400 °C. Operando-DRIFTS-MS studies at different 2-propanol:O2 ratios were carried out under continuous-flow conditions at atmospheric pressure as well. Thereby, the increased reducibility of the small crystallite size sample could be linked to an unselective activity for dehydrogenation yielding acetone and hydrogen, partial oxidation of 2-propanol and acetone, and total combustion leading to a complex network of reactions going on, being further pushed by an excess of oxygen. Ex-situ x-ray diffraction measurements were performed following the temperature programmed reaction experiments. The large crystallite size sample was found to be generally less active, but more selective towards non-oxidative dehydrogenation. Ex-situ x-ray diffraction measurements performed following the temperature programmed reaction experiments confirmed the increased reducibility of the smaller crystallite size sample. In the operando IR studies, 2-propoxide, adsorbed acetone, carbonates and acetates were identified as species occurring in the reaction.