Iridium-Doped Nanosized Zn-Al Layered Double Hydroxides as Efficient Water Oxidation Catalysts.

Layered double hydroxides (LDHs) exhibit some key features, particularly the high accessibility of small molecules (as water) to the interlayer region, which make them an ideal platform to host catalytically active metal centers for water oxidation (WO). This has been demonstrated herein by synthesizing three iridium-doped zinc-aluminum LDHs (Ir-LDHs) nanomaterials (1-3, width and thickness of about 75 nm and 15 nm, respectively) in the confined aqueous environment of reverse micelles, through very simple and versatile synthetic procedures. These materials exhibit excellent catalytic performances in WO driven by NaIO4 at neutral pH and 25 °C, with an iridium content as low as 0.5 mol% (~0.8 wt%), leading to quantitative oxygen yields (based on utilized NaIO4, TON up to ~10,000). Nanomaterials 1-3 display TOF values (up to 402 min-1) comparable to those of the most efficient molecular iridium catalysts, tested under similar reaction conditions. The boost in activity can be traced to the increased surface area and pore volume (> 5 times and one order of magnitude, respectively, higher than those of micrometric materials of size 0.3-1 µm) estimated for the nanosized particles, which guarantee higher noble metal accessibility. Extensive X-ray adsorption spectroscopy (XAS) studies suggest that 1-3 nanomaterials, as-prepared and after catalysis, contain a mixture of isolated, single octahedral Ir(III) sites, with no evidence of Ir-Ir scattering from second-nearest neighbors, excluding the presence of IrO2 nanoparticles. The combination of the results coming from XAS, elemental analysis and ionic chromatography strongly suggests that iridium is embedded in the brucite-like structure of LDHs, having four hydroxyls and two chlorides as first neighbors. These results demonstrate that nanometric LDHs can be successfully exploited to engineering efficient WOCs, minimizing the amount of iridium used, consistently with the principle of the noble metal atom economy.