Invited Paper IS+AS+SS-TuA1
In Situ and Operando TEM of Thermal and Photocatalysts

Tuesday, October 20, 2015, 2:20 pm, Room 211C

Heterogeneous catalysts play a vital role in the development of energy technologies. Understanding the fundamental relationships between catalyst activity and structure at the nanoscale will enable the improved design of catalyst nanostructures. In-situ and operando environmental transmission electron microscopy (ETEM) is a powerful technique for the investigation of structure-reactivity relationships in high surface area catalysts under reaction conditions. With current instruments, atomic resolution imaging and spectroscopy can be carried out in the presence of gas, liquid, light and thermal stimuli. The combination of mass spectrometry and electron energy-loss spectroscopy allow catalytic products to be detected and quantified directly in the electron microscope. Several specific applications of ETEM instrumentation and experiments to several heterogeneous catalysts will be presented.

Photocatalytic water splitting can be accomplished by a heterostructure of several materials, including a light absorbing semiconductor and one or more co-catalysts. Our group has focused on Ni-NiO co-catalysts on both TiO₂ and Ta₂O₅. In the Ni-NiO/TiO₂ system, deactivation occurs due to this Ni dissolution into water during illumination and H₂ is only produced by the oxidation of Ni metal[1]. For the Ta₂O₅ supported catalyst, H₂ was produced predominantly by a catalytic reaction [2] and the deactivation rate was found to be inversely proportional to the initial thickness of the NiO shell. In both systems, deactivation is observed only during light illumination, so that this deactivation is properly called photocorrosion.

Our group has also been pioneering the use of operando TEM to study CO oxidation over supported Ru nanoparticles [3]. There is uncertainty and debate in the literature regarding the most active form of this catalyst. Images of the Ru nanoparticles after reduction in-situ show a clean metal surface, but after only 0.5% O₂ is introduced into the cell, a thin oxide layer forms on the surface. Similar experiments are currently being performed under operando conditions.

References:

[1] L. Zhang, et al. The Journal of Physical Chemistry C, 119, (2015), p. 7207–7214.

[2] Q. Liu, et al. Applied Catalysis B: Environmental, 172–173, (2015), p. 58–64.

[3] B.K. Miller, P.A. Crozier Microscopy and Microanalysis 20, (2014), p. 815–824.

[4] The support from the U.S. Department of Energy (DESC0004954), and the National Science Foundation (CBET-1134464), and the use of ETEM at John M. Cowley Center for HR Microscopy at Arizona State University is gratefully acknowledged.