Formation of 6H-Ba3Ce0.75Mn2.25O9 during Thermochemical Reduction of 12R-Ba4CeMn3O12: Identification of a Polytype in the Ba(Ce,Mn)O3 Family.

The resurgence of interest in a hydrogen economy and the development of hydrogen-related technologies has initiated numerous research and development efforts aimed at making the generation, storage, and transportation of hydrogen more efficient and affordable. Solar thermochemical hydrogen production (STCH) is a process that potentially exhibits numerous benefits such as high reaction efficiencies, tunable thermodynamics, and continued performance over extended cycling. Although CeO2 has been the de facto standard STCH material for many years, more recently 12R-Ba4CeMn3O12 (BCM) has demonstrated enhanced hydrogen production at intermediate H2/H2O conditions compared to CeO2, making it a contender for large-scale hydrogen production. However, the thermo-reduction stability of 12R-BCM dictates the oxygen partial pressure (pO2) and temperature conditions optimal for cycling. In this study, we identify the formation of a 6H-BCM polytype at high temperature and reducing conditions, experimentally and computationally, as a mechanism and pathway for 12R-BCM decomposition. 12R-BCM was synthesized with high purity and then controllably reduced using thermogravimetric analysis (TGA). Synchrotron X-ray diffraction (XRD) data is used to identify the formation of a 6H-Ba3Ce0.75Mn2.25O9 (6H-BCM) polytype that is formed at 1350 °C under strongly reducing pO2. Density functional theory (DFT) total energy and defect calculations show a window of thermodynamic stability for the 6H-polytype consistent with the XRD results. These data provide the first evidence of the 6H-BCM polytype and could provide a mechanistic explanation for the superior water-splitting behaviors of 12R-BCM.

Influence of the Rare Earth Cation on the Magnetic Properties of Layered 12R-Ba4M4+Mn3O12 (M = Ce, Pr) Perovskites.

Material design is increasingly used to realize desired functional properties, and the perovskite structure family is one of the richest and most diverse: perovskites are employed in many applications due to their structural flexibility and compositional diversity. Hexagonal, layered perovskite structures with chains of face-sharing transition metal oxide octahedra have attracted great interest as quantum materials due to their magnetic and electronic properties. Ba4MMn3O12, a member of the "12R" class of hexagonal, layered perovskites, contains trimers of face-sharing MnO6 octahedra that are linked by a corner-sharing, bridging MO6 octahedron. Here, we investigate cluster magnetism in the Mn3O12 trimers and the role of this bridging octahedron on the magnetic properties of two isostructural 12R materials by systematically changing the M4+ cation from nonmagnetic Ce4+ (f0) to magnetic Pr4+ (f1). We synthesized 12R-Ba4MMn3O12 (M= Ce, Pr) with high phase purity and characterized their low-temperature crystal structures and magnetic properties. Using substantially higher purity samples than previously reported, we confirm the frustrated antiferromagnetic ground state of 12R-Ba4PrMn3O12 below TN ≈ 7.75 K and explore the cluster magnetism of its Mn3O12 trimers. Despite being atomically isostructural with 12R-Ba4CeMn3O12, the f1 electron associated with Pr4+ causes much more complex magnetic properties in 12R-Ba4PrMn3O12. In 12R-Ba4PrMn3O12, we observe a sharp, likely antiferromagnetic transition at T2 ≈ 12.15 K and an additional transition at T1 ≈ 200 K, likely in canted antiferromagnetic order. These results suggest that careful variation of composition within the family of hexagonal, layered perovskites can be used to tune material properties using the complex role of the Pr4+ ion in magnetism.

Formation of Ba3Nb0.75Mn2.25O9-6H during thermo-chemical reduction of Ba4NbMn3O12-12R.

The resurgence of inter-est in hydrogen-related technologies has stimulated new studies aimed at advancing lesser-developed water-splitting processes, such as solar thermochemical hydrogen production (STCH). Progress in STCH has been largely hindered by a lack of new materials able to efficiently split water at a rate comparable to ceria under identical experimental conditions. BaCe0.25Mn0.75O3 (BCM) recently demonstrated enhanced hydrogen production over ceria and has the potential to further our understanding of two-step thermochemical cycles. A significant feature of the 12R hexa-gonal perovskite structure of BCM is the tendency to, in part, form a 6H polytype at high temperatures and reducing environments (i.e., during the first step of the thermochemical cycle), which may serve to mitigate degradation of the complex oxide. An analogous compound, namely BaNb0.25Mn0.75O3 (BNM) with a 12R structure was synthesized and displays nearly complete conversion to the 6H structure under identical reaction conditions as BCM. The structure of the BNM-6H polytype was determined from Rietveld refinement of synchrotron powder X-ray diffraction data and is presented within the context of the previously established BCM-6H structure.

A High Entropy Oxide Designed to Catalyze CO Oxidation Without Precious Metals.

The chemical complexity of single-phase multicationic oxides, commonly termed high entropy oxides (HEOs), enables the integration of conventionally incompatible metal cations into a single-crystalline phase. However, few studies have effectively leveraged the multicationic nature of HEOs for optimization of disparate physical and chemical properties. Here, we apply the HEO concept to design robust oxidation catalysts in which multicationic oxide composition is tailored to simultaneously achieve catalytic activity, oxygen storage capacity, and thermal stability. Unlike conventional catalysts, HEOs maintain single-phase structure, even at high temperature, and do not rely on the addition of expensive platinum group metals (PGM) to be active. The HEOs are synthesized through a facile, relatively low temperature (500 °C) sol-gel method, which avoids excessive sintering and catalyst deactivation. Nanostructured high entropy oxides with surface areas as high as 138 m2/g are produced, marking a significant structural improvement over previously reported HEOs. Each HEO contained Ce in varying concentrations, as well as four other metals among Al, Fe, La, Mn, Nd, Pr, Sm, Y, and Zr. All samples adopted a fluorite structure. First row transition metal cations were most effective at improving CO oxidation activity, but their incorporation reduced thermal stability. Rare earth cations were necessary to prevent thermal deactivation while maintaining activity. In sum, our work demonstrates the utility of entropy in complex oxide design and a low-energy synthetic route to produce nanostructured HEOs with cations selected for a cooperative effect toward robust performance in chemically and physically demanding applications.

Reduction-induced CO dissociation by a [Mn(bpy)(CO)4][SbF6] complex and its relevance in electrocatalytic CO2 reduction.

[Mn(bpy)(CO)3Br] is recognized as a benchmark electrocatalyst for CO2 reduction to CO, with the doubly reduced [Mn(bpy)(CO)3]- proposed to be the active species in the catalytic mechanism. The reaction of this intermediate with CO2 and two protons is expected to produce the tetracarbonyl cation, [Mn(bpy)(CO)4]+, thereby closing the catalytic cycle. However, this species has not been experimentally observed. In this study, [Mn(bpy)(CO)4][SbF6] (1) was directly synthesized and found to be an efficient electrocatalyst for the reduction of CO2 to CO in the presence of H2O. Complex 1 was characterized using X-ray crystallography as well as IR and UV-Vis spectroscopy. The redox activity of 1 was determined using cyclic voltammetry and compared with that of benchmark manganese complexes, e.g., [Mn(bpy)(CO)3Br] (2) and [Mn(bpy)(CO)3(MeCN)][PF6] (3). Infrared spectroscopic analyses indicated that CO dissociation occurs after a single-electron reduction of complex 1, producing a [Mn(bpy)(CO)3(MeCN)]+ species. Complex 1 was experimentally verified as both a precatalyst and an on-cycle intermediate in homogeneous Mn-based electrocatalytic CO2 reduction.