Oxygen Defects and Surface Chemistry of Reducible Oxides

Abstract

The magic reducible oxides properties likely are mainly due to the presence of oxygen defects and their rich surface chemistry, which provide a rational pathway to the emergent of entirely new properties. Although significant progress has performed in the last years, it can be stated that they are not fully understood at the nanoscale level so far. This mini-review provides a comprehensive perspective on the oxygen defects and surface chemistry of reducible oxides based on materials with two-dimensional (2D) structure. Thus, in this perspective, we intend to discuss some of the main challenges and opportunities in this important field of research in materials chemistry and engineering. From a practical standpoint, chemical insights from defect-engineering may provide vital clues for improving synthetic methods and understanding fundamental nanoscale properties, driving innovation in the field of reducible oxides based on 2D structure materials.

Copper-Containing Mixed Metal Oxides (Al, Fe, Mn) for Application in Three-Way Catalysis

“…The extra CuO in CuMn 1000 compared to CuMn2 1000 also explains the better CO conversion. The energy cost for the generation of oxygen vacancies is much lower on steps and corners than on a perfect surface of a metal oxide, so smaller crystals will have a larger surface to bulk ratio and more low-coordinated ions at edges, which explains the lower reduction temperature [45,70]. The mixture of CuO and Fe 2 O 3 in CuFe 600 did not show any improvement for CO oxidation compared to CuAl 600, which indicates there is probably also no spill-over effect of oxygen from Fe 2 O 3 to CuO that improves the reducibility [6].…”

Complexity of the Nano-Bio Interface and the Tortuous Path of Metal Oxides in Biological Systems

“…Reduction in the oxidation number of the metal (i.e., the accounting of the number of electrons a metal possess or lacks) occurs when the crystal loses an oxygen atom and forms a vacancy in the NP. Thermodynamically, any oxide is potentially reducible [ 10 ], and the distinction between reducible and non-reducible metal oxides depends on the ease with which oxygen vacancies can be formed [ 10 ]. In non-reducible metal oxides, the thermodynamic cost of formation of oxygen vacancies is high, and redox activity is absent [ 11 ].…”

“…In reducible metal oxides, oxygen vacancy formation is thermodynamically more favorable and occurs at lattice surfaces and edges where the coordination number of the surface atoms (i.e., the total number of bonds to the atom) is less than inside the crystalline structure of the oxide. The edge is also where lattice strain is highest [ 12 ]; all of which facilitates the formation of oxygen vacancies [ 10 , 13 ]. Thus, the highest enzyme-mimetic activity occurs at the surface of the nanoparticle [ 10 , 14 ].…”

“…The edge is also where lattice strain is highest [ 12 ]; all of which facilitates the formation of oxygen vacancies [ 10 , 13 ]. Thus, the highest enzyme-mimetic activity occurs at the surface of the nanoparticle [ 10 , 14 ]. Many transition-metal oxides, such as TiO 2 , MnOx, NiO, Fe 2 O 3 , and CeO 2 , are reducible because the energetic barriers to oxygen vacancy formation are low, and vacancies can occur spontaneously across the surface of the crystal.…”

“…Each adjacent STM image shows the surface structure of the metal oxide before and after the interaction with molecular oxygen and the migration of the oxygen vacancy. Used with permission from Pinto et al [ 10 ].…”