Metal oxides as heterogeneous catalysts for oxygen evolution under photochemical conditions

Abstract

Metal oxides, in the form of dispersed powders, have been tested as potential catalysts for the four-electron oxidation of water to 0, under photochemical conditions. The most efficient catalysts were found to be IrO,, Co,O,, RuO,, NiCo,O,, Rh,O, and Mn,O, and, in particular, high activity was observed with IrO,. Comparison of the oxide structure with its observed rate of 0, generation under standard conditions has allowed formulation of a few general requisites for an effective catalyst. Samples of iridium oxide deposited onto the surface of a second (inert) oxide were tested for their 0,-evolving capability. The efficiency of the system depended markedly upon the nature of the support. Materials that favour formation of small deposits of iridium oxide (e.g. ZnO, MgO, TiO,) are the best supports, whilst 0, production is almost completely inhibited with acidic supports. Many metal oxides can be prepared in the form of hydrates of variable composition. These materials also function as 0,-evolving catalysts, the efficiency of the process depending upon any thermal pretreatment. This finding is explained in terms of changes in structure and composition of the oxide that occur upon heating.As part of a protracted research programme, concerned with building a photosystem capable of storing solar energy via the photodissociation of water, a range of 0,-evolving catalysts has been developed. 1-6 Mostly, these catalysts have contained ruthenium dioxide in some form, and all have been prone to corrosion under high anodic bias. homogeneous,'^ colloidal5* and heterogeneou~l.~ Ru0,-based catalysts have been tested for their 0,-evolving capacities under photochemical conditions. In certain cases, quite high efficiencies have been reported. Despite such detailed studies, the poor performance of the catalyst remains as the most serious obstacle in developing a suitable anodic branch of a solar-energy storage device and, if a satisfactory catalyst is to be identified, attention should be given to materials other than RuO,. Indeed, it has been shown that Ir0,,lo Pt0210 and MnO," powders will function as heterogeneous catalysts for water oxidation in related model systems.Using a well established photochemical test system,6 we have evaluated the 0,-evolving capabilities of a range of metal oxides. These materials have been used in the form of powders suspended in aqueous solution. Most of the materials were found to be inferior to RuO, powders, but a few oxides, notably IrO,, were effective catalysts for water oxidation and demonstrated much higher levels of corrosion resistance than the catalysts used previously. Several of these oxides exist in the form of hydrates for which the 0,-evolving capability is increased upon dehydration. In order to extend the scope of the work, a series of catalysts was prepared in which IrO, was deposited onto the surface of a second (inert) oxide. By this means, the effect of the supporting agent on the 2795

Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex

“…In the artificial transposition of the natural process, the attention is actually focused on metal-oxide heterogeneous catalysts, which conjugate robustness and efficiency, with the most prominent examples being RuO 2 , IrO 2 , or cobalt phosphate (7,8). In particular, RuO 2 -based materials for electrocatalytic water oxidation have been the subject of intense investigation, including single-or polycrystalline systems (4, 9, 10), compact RuO 2 films, and composite oxides (4,11,12).…”

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

“…Because the capability for efficient water oxidation is unique to photosystem II among all biological photosystems (6, 7), these CaMnO materials that mimic the elemental composition, manganese oxidation state, and particle size of the photosynthetic water oxidation center are of special interest (8)(9)(10)(11)(12). Although a number of other metal compounds function as water-oxidizing catalysts (13), many contain rare and expensive metals like iridium and ruthenium; the advantage of manganese oxides (Mn-oxides) is that they are earth-abundant, inexpensive, and environmentally friendly (1)(2)(3)(4)(5)14).CaMnO phases have short-range order structure and lamellar morphology (12), which are hallmarks of phyllomanganates (classically known as hydrous Mn-oxides), and they possess a layered structure (15)(16)(17)(18)(19)(20). In this family of structures, manganese octahedra (and vacancies) form the layers, and charge-balancing cations (i.e., alkali and alkaline earth ions, protons) and water occupy the interlayer space.…”

“…Because the capability for efficient water oxidation is unique to photosystem II among all biological photosystems (6, 7), these CaMnO materials that mimic the elemental composition, manganese oxidation state, and particle size of the photosynthetic water oxidation center are of special interest (8)(9)(10)(11)(12). Although a number of other metal compounds function as water-oxidizing catalysts (13), many contain rare and expensive metals like iridium and ruthenium; the advantage of manganese oxides (Mn-oxides) is that they are earth-abundant, inexpensive, and environmentally friendly (1)(2)(3)(4)(5)14).…”

“…nanomaterials | thermochemistry R esearch on calcium manganese oxides (CaMnOs) has been inspired by the water-oxidation centers in photosystem II (1)(2)(3)(4)(5), which is a manganese-calcium cluster Mn 4 CaO 5 (H 2 O) 4 supported by a protein environment. Because the capability for efficient water oxidation is unique to photosystem II among all biological photosystems (6,7), these CaMnO materials that mimic the elemental composition, manganese oxidation state, and particle size of the photosynthetic water oxidation center are of special interest (8)(9)(10)(11)(12).…”

The enduring relevance and academic fascination of catalysis

“…Moreover, the catalyst needs to operate close to the thermodynamic potential of the redox reaction so that a maximum fraction of the solar photon energy is converted to chemical energy. Stability considerations favour all-inorganic oxide materials, and previous work 35,36 points to IrO 2 nanoclusters (approximately 2 nm diameter) as a very good candidate. However, Ir is the least abundant metal on Earth.…”