Water Oxidation Catalysis by Nanoparticulate Manganese Oxide Thin Films: Probing the Effect of the Manganese Precursors

Abstract

Nanoparticulate manganese oxides, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyze water oxidation under neutral pH conditions with the onset of electrocatalysis occurring at an overpotential of only 150 mV. Although XAS experiments indicate that each complex generates the same material in Nafion, the catalytic activity varied substantially with the manganese precursor and did not correlate with the amount of MnO x present in the films. The XAS and EPR studies indicated that the formation of the nanoparticulate oxide involves the dissociation of the complex into Mn(II) species followed by oxidation on application of an external bias. TEM studies of the most active films, derived from [Mn(Me3TACN)(OMe)3]+ and [(Me3TACN)2MnIII 2(μ-O)(μ-CH3COO)2]2+ (Me3TACN = N,N′,N″-trimethyl-1,4,7-triazacyclononane), revealed that highly dispersed MnO x nanoparticles (10–20 nm and 6–10 nm, respectively) were generated in the Nafion film. In contrast, the use of [Mn(OH2)6]2+ resulted in both a higher manganese oxide loading and aggregated nanoparticles with 30–100 nm approximate size, which were less effective water oxidation catalysts. Much higher turnover frequencies (TOFs) were observed for films derived from the two complexes, viz., ∼20 molecules of O2 per Mn per hour in dark and 40 molecules of O2 per Mn per hour under illumination at an overpotential of 350 mV, when compared with MnO x films made with [Mn(OH2)6]2+. This corresponds to a TOF > 100 molecules of O2 per Mn per second for a 10 nm MnO x nanoparticle. Thus, the catalytic activity is dependent on the ability to generate well-defined, dispersed nanoparticles. Electrochemical and spectroscopic methods have been used to follow the conversion of the molecular precursors into MnO x and to further evaluate the origin of differences in catalytic activity.

Effect of surface area on the rate of photocatalytic water oxidation as promoted by different manganese oxides

“…Mn oxides show good activities [6][7][8][9][10][11][12][13], are abundant and low cost. They occur, though, in a wide number of structures and oxidation states, so constituting a wide field.…”

Molecular complexes in water oxidation: Pre-catalysts or real catalysts

“…With the comprehensive characterization of Co-POM under various reaction conditions, it is suggested that Co-POM serves as the true WOC in chemical catalysis, whereas it transforms to active cobalt oxide nanoparticles in electrochemical oxidation [28,40,103,104]. As the selected element by nature, molecular manganese catalysts are surprisingly rare and easily decompose to Mn oxide in both chemical and electrochemical catalysis [21,86,87]. For iron-based catalysts, the true active species strongly depend on the catalytic conditions such as pH.…”

“…The extremely high resolution of TEM enables the detection of fine structural details, even as small as a few nanometers in size. TEM is widely used for elemental analysis and nanoparticle detection of the deposited films, but not for solution samples [21,22].…”

“…Reprinted with permission from ref[21]. Copyrights 2013 American Chemical Society.Since the first example of the molecular O 2 evolution catalyst using manganese was prepared as a manganese−terpyridine dimer, multiple homogeneous manganese catalysts containing organic or inorganic ligand have been investigated in catalytic oxidation of water.…”

The effect of different metal ions between nanolayers of manganese oxide on water oxidation

“…Mn oxides were reported as water-oxidizing catalysts for artificial photosynthetic systems [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. Among Mn oxides, nanolayered Mn oxides with different organic or inorganic ions between layers are efficient and stable in water oxidation [41].…”