Synthesis of high entropy, bidimensional mixed oxides from multimetallic layered double hydroxide precursors and application in photocatalytic N2 hydrogenation to ammonia

A multimetallic layered double hydroxide containing seven metals has been prepared by coprecipitation in DMF/ H2O at pH 10 and converted into a seven metal mixed oxide. This high-entropy mixed metal (HE-MO) inherits from the LDH precursor the hexagonal sheet morphology. High resolution transmission electron microscopy, and EDS elemental mapping convincingly proves the absence of segregation of some elements in a different phase. Remarkably, neither noble metals nor lanthanides are used during the synthesis, and no reduction processes are needed to obtain the HE-MO. The resulting HE-MO promotes under continuous flow the photocatalytic nitrogen hydrogenation at atmospheric pressure and 200 degrees C with UV-vis light (108 mW cm- 2) at a rate of 20 mu mol NH3 center dot g- 1 center dot h- 1, lower or higher temperatures resulting in lower rates. This NH3 rate formation is much higher than that of a CoFe mixed oxide reference or than values reported in the literature for noble metal-free photocatalysts. The use of isotopically labelled 15N2 firmly proves this gas is the origin of the formed NH3. Influence of the N2 and H2 proportion shows that the rate determining step is the generation of active H species on the photocatalyst surface, while the photoresponse derives from the UV region. Photoluminescence measurements indicate the existence of a transient state of charge separation responsible for the photocatalytic reaction.