Cyanamide route to calcium–manganese oxide foams for water oxidation

Abstract

aIn nature, photosynthetic water oxidation is efficiently catalysed at a protein-bound μ-oxido Mn 4 Ca cluster. This cluster consists of earth abundant, non-toxic elements and serves as a paragon for development of synthetic catalysts. In this study we developed porous calcium-manganese oxides with a unique foam-like nanostructure prepared via a facile and robust synthetic route using cyanamide as a porogen.A series of such oxide foams annealed at different temperatures was characterized by TEM, SEM, XRD, N 2 physisorption, and X-ray absorption spectroscopy (XAS) in order to correlate crystallinity, atomic structure, surface area and oxidation state of the materials with catalytic activity. Some of the resulting Ca-Mn oxides show high activity as catalysts for water oxidation in the presence of cerium(IV) ammonium nitrate as a non-oxo transfer oxidant. An amorphous calcium-manganese-oxide foam with 130 m 2 g −1 surface area and Mn oxidation state of +3.6 was identified to be most active; its activity is superior to previously reported Ca-Mn oxides. At the atomic level, this material shares structural motifs with the biological paragon as revealed by dual-edge XAS at the Mn and Ca K-edge. Rather than nanostructure and surface area, the atomic structure of the Ca-Mn oxide and the extent of structural order appear to be crucial determinants of catalytic activity. Fully disordered low-valent Mn materials as well as high-valent but crystalline Mn-Ca oxides are unreactive. Highly disordered variants of layered manganese oxide with Ca and water molecules interfacing layer fragments are most reactive.

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

“…8b) normalized with respect to SSA) decreases with increasing SSA. A similar feature has been observed for calcium manganese oxides with different SSA [26]. Proportionality between activity and SSA, even coarse, is expected, if the surface given rise by ball milling does not substantially differ from the pristine one: this seems to be the case of Mn 3 O 4 .…”

Uncovering Structure–Activity Relationships in Manganese‐Oxide‐Based Heterogeneous Catalysts for Efficient Water Oxidation

“…[28,36,37,44,45] The surface of manganese oxides is always covered either with hydroxyl groups or water molecules, whereas in layered manganese oxides, the interstitial spaces are also occupied by water molecules. [28,36,37,44,45] The surface of manganese oxides is always covered either with hydroxyl groups or water molecules, whereas in layered manganese oxides, the interstitial spaces are also occupied by water molecules.…”

“…[45] The formation of oxygen from the catalyst structure and even by breaking MnÀO [36,37,45] Nevertheless, the structural changes of the catalyst during chemical/photochemical water oxidation have not been well studied so far. [44] The energetic basis of the catalytic activity of layered calcium manganese oxides for water oxidation has been studied by Birkner et al [46] Materials having a layered structure with considerable thermodynamic stability, high surface area, and low surface energy showed better catalytic activity in light-driven water oxidation. [24,28,37] In all above-mentioned cases, a serious disadvantage is the use of sacrificial agent to accept the photogenerated electrons that are instantly consumed within first few minutes of the reaction.…”

Spin-Coated vs. Electrodeposited Mn Oxide Films as Water Oxidation Catalysts

“…In some cases, the incorporation of Ca into the structure has the aim of mimicking the metal core ( i.e. , Mn 4 CaO 5 ) that catalyzes WO reaction in photosynthetic systems [7,9,10]. …”