A Ternary Zn−Al−Ir Hydrotalcite‐Like Compound Exhibiting High Efficiency and Recyclability as a Water Oxidation Catalyst

Abstract

Herein it is shown that hydrotalcite‐like compounds (HTlcs), doped with a suitably reactive metal, can efficiently act as water oxidation catalysts. As a case study, a ternary HTlc of formula [Zn0.667Al0.306Ir0.027(OH)2]Cl0.333⋅0.6 H2O, in which iridium is homogeneously distributed into the octahedral sites of brucite‐type sheets, was prepared and tested as a heterogeneous water oxidation catalyst, using NaIO4 as a sacrificial oxidant. This HTlc showed excellent performance in terms of turnover frequency (up to 113 min−1), turnover number (>11 900), stability and recyclability.

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“…[24] Finally,i tw as shown that complex 4 (2.5 mm solution)g enerates IrO x NPs when reactedw itha250 mm NaIO 4 solution. [56,68] This points to the conclusion that, whatever the real active speciesi s, it musth ave ad imension smallert han that of 11 NPs (mean diameter of 2nm) and larger or equal than that of 10,s oi th as to be molecular in nature. [6,41,[61][62][63][64][65][66][67] Although NPso f11 could not be properly tested owing to their transformation back to 10 under our experimental conditions, the results herein reportedc learly indicatet hat their activity is smaller than that of molecular[ Ir(OH) 6 ] 2À (10), consistent with previously reported results indicatinga ni mprovement of performance when the dimensionality of the active species is decreased.…”

The Middle‐Earth between Homogeneous and Heterogeneous Catalysis in Water Oxidation with Iridium

“…At the same time, the catalyst recovered after the first catalytic run exhibits essentially the same catalytic activity than in the first run (Figure , bottom). The recyclability of Ir‐doped ZnAl LDH WOC was further explored conducting up to ten consecutive runs without noticing any drop of activity, always obtaining a production of O 2 very close to 100 % compared to that expected based on the amount of used NaIO 4 …”

“…Down: comparison between the catalytic activity of Run 1 (black line), Run 2 (red line) and supernatant (blue line) ([Ir] = 50 µM, [IO 4 – ] = 0.03 m , pH 7.7 by phosphate buffer). Adapted from ref …”

“…Another strategy that combines some advantages of homogeneous and heterogeneous catalysis, at the same time minimizing the utilization of noble metals, involves the exploitation of layered materials doped with a small amount of active centers. [106] Under the assumption that the latter are reachable by the substrates to be activated, and this should be the case for water, it is presumable that all sites are active, as it is in principle true in homogeneous catalysis. Clearly the fine modulation of the electronic and steric features of active sites is more difficult than in molecular catalysts.…”

“…Among the different layered materials, [15,[107][108][109] double hydroxides (LDHs), or hydrotalcite like materials,, [110][111][112] have attracted particular attention as WOCs. [106,[112][113][114][115] They can be prepared rather easily, even with nanometric dimensions, for a large variety of metals and exhibit remarkable catalytic performance. [116][117][118][119][120] In a certain sense, because they are intrinsically constituted by M 1 -OH functionalities, it is reasonable to believe that further oxidation could straight fully lead to M 1 = O which is considered the key species in water oxidation catalysis.…”