Abstract
Layered double hydroxides (LDHs) are an ideal platform to host catalytic metal centers for water oxidation (WO) owing to the high accessibility of water to the interlayer region, which makes all centers potentially reachable and activated. Herein, we report the syntheses of three iridium-doped zinc–aluminum LDHs (Ir-LDHs) nanomaterials (1–3, with about 80 nm of planar size and a thickness of 8 nm as derived by field emission scanning electron microscopy and powder X-ray diffraction studies, respectively), carried out in the confined aqueous environment of reverse micelles, through a very simple and versatile procedure. 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, turnover number up to ∼10,000). Nanomaterials 1–3 display the highest ever reported turnover frequency values (up to 402 min–1) for any heterogeneous and heterogenized catalyst, comparable only 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 1 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. X-ray absorption 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 obtained 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 engineer efficient WOCs, minimizing the amount of iridium used, consistent with the principle of the noble-metal atom economy.
Highlights
The development of an efficient catalytic system for the oxidation of water (WO) to molecular oxygen is one of the most demanding challenges that the scientific community is facing nowadays
As for the dilution, which is extremely important considering the low abundance of iridium, we show that already 3.0 mol % mol is enough to reach top performances (TOF up to 402 min−1)
The possibility of using Ir-diluted materials probably derives from having most of iridium centers reachable by water and the oxidant, owing to the nanometric dimensions of Ir-doped Layered double hydroxides (LDHs) materials, which causes a consistent enhancement of the surface area and by their implicit layered structure that allows substrate diffusion
Summary
The development of an efficient catalytic system for the oxidation of water (WO) to molecular oxygen is one of the most demanding challenges that the scientific community is facing nowadays. A comparison of XANES, as well as k2-weighted EXAFS spectra, with those of the reference species, suggests that the octahedral Ir−O environment present in Ir-LDHs after catalysis is different from that of IrO2, despite the similarity in fitted Ir− O bond distance (1.98 Å for 2 after catalysis vs 1.99 Å for IrO2; Figures S9 and S10; entries 6 vs 7 Table 2). This indicates that the noble metal centers remain highly dispersed in the brucitelike scaffold of LDHs during the catalysis. It can be seen that the time necessary to obtain the highest TOF with 3 is ≤5 min, which is slightly smaller
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