Atmospheric gas and heating TEM with water vapor

Abstract text (incl. figure legends and references)

Water"s negative effects on metal corrosion and catalyst deactivation are well known and under study for decades. On the positive side, water can act as reactants in hydrogen production reactions, like water-gas shift reaction (product in reverse water-gas shift reaction (RWGS)). With the introduction of aberration correctors, in-situ and operando gas and heating TEM can reveal structure and composition down to atomic resolution in their working status. However, the investigations on water"s influence inside a TEM are limited due to limited control over the water vapor flow and fear of potential leak to contaminate the TEM columns. In this work, we will present three application examples of our recently developed vaporizer for MEMS-based gas and heating TEM. One is the corrosion of FeCrAl alloy, one is the reversible growth and dispersion of the Au nanoparticles on support materials and another is the oscillatory reduction and oxidation of NiAu nanoparticles.

Material and Methods

DENSsolutions" Climate G+ system, including 3 feeds GSS, TEM sample holder, vaporizer componnets between GSS and TEM holder, gas and heating nano-reactor and Impulse software, DENSsolutions gas analyzer, and a FEI Titan (TEM) operated at 300 kV were used in this work. FeCrAl alloy was firstly cut and thinned by focused ion beam and then transferred to Climate nano-reactor. Au nanoparticles on support materials and NiAu nanoparticles were firstly dissoved in ethanol and then transferred to the Climate nano-reactor by drop-casting.

Results

Interrupted cooling water cycle could cause nuclear cladding material Zr to react much faster under water vapor condition than in vacuum or in dry oxygen. This can cause Zr break and release a lot of explosive hydrogen gas. As an alternative material to Zr, the FeCrAl alloy"s performance under water vapor heating and vacuum heating were investigated in this work.

Au dispersed on reducible support materials has important applications in low temperature water-gas shift reactions. Research on deactivation mechanism of such material under realistic gas environment can help to find more resistant ones. In our work, we visualized the agglomeration of Au nanoparticles from CeZrO4 support under wet gas environment and afterwards redispersion under dry oxygen environment. The results show complex dynamics and possible regeneration of Au nanoparticles under proper conditions and thus gives practical applications better scopes.

For NiAu nanoparticles, we firstly investigated three extreme situations, i.e. reduction condition without vapor (H2 & He), reduction condition with highest humidity vapor (H2, H2O & He) and only vapor at highest humidity level (H2O & He), as shown in Figure 1. The particle retained in a structure of Ni core and Au shell in H2 & He. Loose NiO structure formed outside the particle upon introduction of high humidity vapor (H2, H2O & He), and then condensed upon disappearance of H2 in the inlet gas (H2O & He).Then we varies humidity level from 0 to ~50% with H2 and without H2. We found an interesting oscillative transition between loose and condensed NiO layer formation, whose periodicity influenced by humidity level.

More experimental details and interpretations will be presented on-site.

Conclusion

Development and application of such vaporizer are expected to help design more sustainable metal materials and catalysts from fundamental failure mechanism studies.