Catalysis of water oxidation in acetonitrile by iridium oxide nanoparticles

Abstract

Water oxidation catalysed by iridium oxide nanoparticles (IrO₂ NPs) in water-acetonitrile mixtures using [Ru^III(bpy)₃]³⁺ as oxidant was studied as a function of the water content, the acidity of the reaction media and the catalyst concentration. It was observed that under acidic conditions (HClO₄) and at high water contents (80% (v/v)) the reaction is slow, but its rate increases as the water content decreases, reaching a maximum at approximately equimolar proportions (≈25% H₂O (v/v)). The results can be rationalized based on the structure of water in water-acetonitrile mixtures. At high water fractions, water is present in highly hydrogen-bonded arrangements and is less reactive. As the water content decreases, water clustering gives rise to the formation of water-rich micro-domains, and the number of bonded water molecules decreases monotonically. The results presented herein indicate that non-bonded water present in the water micro-domains is considerably more reactive towards oxygen production. Finally, long term electrolysis of water-acetonitrile mixtures containing [Ru^II(bpy)₃]²⁺ and IrO₂ NPs in solution show that the amount of oxygen produced is constant with time demonstrating that the redox mediator is stable under these experimental conditions.

Fig. 1. Highlighting the increased reactivity of water molecules in a non-aqueous environment. UV/vis spectra of both phases in a biphasic system composed of a water-rich phase (green in colour) and an organic-rich phase (orange in colour) after the addition of small amounts of Ru^III(bpy)₃(PF₆)₃ (for details see the text and Movie S1, ESI†).

Fig. 2. The influence of “acidity regulators”. Real time absorbance measurements for [Ru^III(bpy)₃]³⁺ solutions after the addition of a mixture containing water, acetonitrile, buffer, and IrO₂ nanoparticles in different concentrations. The acidity regulators used were (A) 20 mM HClO₄ and (B) 4.3 mM NaHCO₃–Na₂SiF₆. In both cases the absorbances were plotted as the difference between the values at 673 nm and 900 nm. The final content of water was 10% (v/v). ln[absorbance] vs. time plots derived from the data in (A) for 20 mM HClO₄ and (B) for 4.3 mM NaHCO₃–Na₂SiF₆ are shown in (C) and (D), respectively. The inset in each case represents the plot of the first order constant determined as the slope of the curve vs. the concentration of catalyst.

Fig. 3. The influence of water content on the kinetics of the WOR. (A and B) Real time absorbance measurements for [Ru^III(bpy)₃]³⁺ solutions after the addition of a mixture containing water, acetonitrile, HClO₄ and IrO₂ nanoparticles. The final concentration of water was varied in each experiment and the concentrations of HClO₄ and IrO₂ were fixed at 20 mM and 27 μM, respectively. (C) Normalized second order constant (k ₂) for [Ru^III(bpy)₃]³⁺ reduction vs. the total content of water expressed both as water percentage (v/v) and water mole fraction (X _H2O).

Scheme 1. Proposed formation of reactive water overlayers on IrO₂ nanoparticles. Ru^III = [Ru^III(bpy)₃]³⁺, Ru^II = [Ru^II(bpy)₃]²⁺ and B^– = proton acceptor species (in the absence of any additional basic species B = H₂O).

Fig. 4. Plots of the current (black line) and charge (blue line) during bulk electrolysis of a solution containing 2 mM Ru(bpy)₃(PF₆)₂, 31 μM IrO₂ NPs and 4.3 mM buffer NaHCO₃–Na₂SiF₆, in a water–ACN mixture (2.5 : 7.5 (v/v)). For comparison, the plot of charge during bulk electrolysis of 2 mM Ru(bpy)₃(PF₆)₂ (blue dotted line) in dry acetonitrile was included. In each case the supporting electrolyte was 0.1 M TBAPF₆. The electrode was RVC (for details see Experimental section).