For many applications, oxygen transport at solid-solid, solid-adsorbate, or solid-gas interfaces underpin many interactions taking place during processes of relevance to energy conversion. In non-stoichiometric oxides, creation and annihilation of oxygen vacancies is key to technologically relevant functionalities impacting fields such as catalysis, oxygen ion conductors and photochemical reactions. Environmental transmission electron microscopy (ETEM) allows gas-solid and solid-solid interactions to be explored in oxides with atomic resolution under in situ and operando conditions [1-3]. The range of stimuli available in the ETEM continues to grow and at ASU we are able to perform experiments in gas, heat and light. The ability to extract structural and bonding information with advanced imaging and electron energy-loss spectroscopy allows the dynamic changes taking place inside and on reducible oxide nanoparticles to be explored. This provides a powerful approach for elucidating the atomic-level view of the reactions taking place on oxide surfaces.
The oxygen exchange reaction is a fundamental process taking place on oxide surfaces involving the creation and annihilation of oxygen vacancies. We have investigated the structural rearrangements taking place on different surfaces of CeO2 and TiO2 nanoparticles. With negative Cs aberration corrected electron microscopy, oxygen columns may be directly visualized under favorable conditions especially when direct exposure detectors are employed. The creation of oxygen vacancies can also result in local cation relaxations, which are often easier to detect during in situ experiments. The interplay between oxygen vacancy formation energy, cation displacements and the oxygen exchange reaction will be explored and discussed. The chemical reactions associated with oxygen vacancies is a strong function of the ambient gas environment. In the presence of water vapor, surface hydroxylation can take place leading to an order-to-disorder transformation. Vacancies may also play a major role in oxidation catalysis where oxygen transfers to gas adsorbates. This process may be critical in the Mars van Krevelen mechanism for reactions such as CO oxidation. The interplay between lattice oxygen, gas adsorbates and metal nanoparticles can have a major impact on catalyst selectivity. This is highlighted in hydrocarbon reforming where oxygen vacancy formation may play a decisive role in carbon deposition, leading to catalysts deactivation.
1. F. Tao, and P.A. Crozier, Chemical Reviews, 2016. 116(6): p. 3487-3539.
2. S. Chenna and P.A Crozier, ACS Catal. 2 (2012) 2395.
3. B.K Miller et al, Ultramicroscopy 2015. 156: p. 18-22.
4. The support from the National Science Foundation (NSF-CBET 1604971, DMR-1308085, CHE-1508667), US Department of Energy (DE-SC0004954) and the use of TEMs at the John M. Cowley Center for High Resolution Microscopy at Arizona State University are gratefully acknowledged.