Revealing Lattice Oxygen Exchange During
CO
Oxidation over 6
H
‐
B
a
F
eO3‐δ Catalyst Nanoparticles by Diffraction and Atomic Resolution Techniques
Authors: Gómez‐Recio Isabel , Hadri Achraf El , Cortes‐Gil Raquel , Varela Aurea , Parras Marina , Gutierrez Angel , García‐García F. Javier , Rio Eloy Del , Calvino Jose Juan , Hernadez Juan Carlos , Delgado Juan Jose , Pérez‐Omil Jose Antonio , Gonzalez‐Calbet Jose Maria 
Published: December 2016

Eliminating or reducing the amount of noble metals and rare earths in catalysts is a primary issue to reach the goal of sustainable development. The substitution of noble metals by 3d metals is receiving increasing attention. Related to this strategy, ABO
3
perovskite oxides (A=lanthanide and B=Fe, Co, Mn) are being essayed as catalysts in many reactions. Substitution of La by an alkaline earth has been reported to modify the catalytic activity of such perovskites.
[1]
To balance the charge deficiency produced by the incorporation of divalent ions into the A sublattice, either the B element increases its oxidation state or oxygen vacancies are created or, even more, both processes occur simultaneously. In general terms, a positive effect on the catalytic activity is observed on methane, propene and hydrogen oxidation reactions within the whole temperature range, after the addition of barium in Mn, Fe and Co perovskites. On the basis of these ideas, we have considered of interest exploring the catalytic behavior of a lanthanide‐free, BaFeO
3‐δ
perovskite in CO oxidation, putting emphasis on the influence of redox pre‐treatments on catalytic activity and on the possible involvement of lattice oxygen in the process. This particular process was firstly of interest in automotive pollution control devices but more recently for cutting‐edge technologies related to production of hydrogen for fuel cells. Though it is admitted that the catalytic behavior of perovskites is intimately related to the nature of their defects, studies focused on the detailed analysis of the nature of such defects at atomic scale are systematically lacking. Therefore, this contribution aims at illustrating the large potential of diffraction data combined with AC‐STEM to establish structure‐function correlations. As an illustration, we have explored the involvement of lattice oxygen in the catalytic activity of BaFeO
3‐δ
perovskite for the CO oxidation.

BaFeO
3‐δ
(0.2<δ<0.4) depicts a 6H perovskite hexagonal structural type, with Fe both in (III) and (IV) oxidation states and oxygen nonstoichiometry accommodated by random distribution of anionic vacancies.
[2]
Analysis of the redox properties of 6H‐BaFeO
2.78
reveals the involvement of two types of lattice oxygen atoms in the CO oxidation over BaFeO
3‐δ
. On its hand, an exhaustive study combining different diffraction techniques (
in‐situ
high temperature SAED, X‐ray diffraction, powder neutron diffraction) and aberration‐corrected STEM techniques, both in imaging and spectroscopic modes, have provided us with an atomically resolved picture of the accommodation of oxygen‐non‐stoichiometry. Thus, mapping of Ba, Fe and O atomic columns, Figure 1, have allowed us identifying the distribution of all the elements in the sample in the …cchcch… sequence expected for the 6H‐polytype. No systematic differences have been observed between the hexagonal and cubic layers in BaFeO
3‐δ
this meaning, in agreement to NPD and ABF results, that there is not a preferential distribution of the oxygen vacancies along the different layers. From the whole set of results obtained it has been possible to propose the role of the oxygen deficiency responsible for the low temperature CO oxidation activity of BaFeO
3‐δ
.