For many materials, nanoparticles show enhanced catalytic activity and selectivity compared to their bulk state [1]. While catalytic nanoparticles have many industrial, economic and environmental benefits, they also present new challenges, the most significant of which is to characterise the intermediate phases and structural changes to the active catalyst species under reaction conditions. TEM is uniquely suited to the study of solid-state heterogeneous catalysis as it allows us to directly characterise the catalyst with regard to its nanostructure on the atomic scale. Complementary spectroscopy techniques such as EDS and EELS can be used in tandem with imaging to add chemical data to the structural observations. The in-situ capabilities of the E(S)TEM allow for intermediate catalyst phases and transformations to be observed, some of which are only present under reaction conditions and would be lost using ex-situ methodologies [2].

Here we present a study of the dynamic nature of Ni based catalysts under redox conditions relevant to industrial processes [3, 4]. The increased activity of finely divided Ni means that it is important to directly observe the Ni nanoparticles rather than interpreting models based on bulk or ex-situ techniques. Many chemical processes that rely on Ni catalysts involve exposure to both oxidising and reducing gas in the reaction environment. Under reducing and oxidising conditions the structure of Ni nanoparticles is dynamic and involves transitions between single crystal Ni, core-shell Ni/NiO and hollowed Kirkendall-like structures [5, 6]. These changes are dependent on the material properties (size, shape and support interaction) and reaction parameters (temperature, gas type and pressure). Furthermore, these dynamic structure/shape changes influence the stability of the catalyst, which in turn is controlled by the reaction environment. As such, this area of research is crucial for understanding the conditions necessary for attaining and maintaining a particular catalytic species and designing the reaction routes required to reactivate spent Ni catalysts.

Environmental TEM/STEM has been used to probe the dynamic oxidation of Ni nanoparticles. The particle size, reaction time and temperature dependencies of the oxidation process have been investigated using both model and industrial Ni catalysts. Furthermore, we have applied the same in-situ techniques to reveal the conditions needed for complete reformation/reactivation of the original Ni species.

[1] B.R. Cuenya, Thin Solid Films, 518 (2010) 3127-3150.

[2] E.D. Boyes, M.R. Ward, L. Lari, P.L. Gai, Annalen der Physik, 525 (2013) 423-429.

[3] P.M. Mortensen, J.-D. Grunwaldt, P.A. Jensen, A.D. Jensen, Catalysis Today, 259 (2016) 277-284.

[4] S. Hu, M. Xue, H. Chen, Y. Sun, J. Shen, Chinese Journal of Catalysis, 32 (2011) 917-925.

[5] S. Chenna, P.A. Crozier, Micron, 43 (2012) 1188-1194.

[6] R. Nakamura, J.G. Lee, H. Mori, H. Nakajima, Philosophical Magazine, 88 (2008) 257-264.

Acknowledgements: The EPSRC (UK) is supporting the AC ESTEM development and continuing applications at York under strategic research grant EP/J018058/1.

Figures:

Solid Single crystal Ni nanoparticles.

ESTEM-HAADF image showing size dependence of the oxidation process.

Core-shell NI/NiO nanoparticle showing the presence of poly-crystalline NiO.

Complete oxidation results in a hollow sphere of poly-crystalline NiO.

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EMC Abstracts - https://emc-proceedings.com/abstract/dynamic-oxidation-and-reduction-of-catalytic-nickel-nanoparticles-using-estem/