Versatile Homebuilt Gas Feed and Analysis System for Operando TEM of Catalysts at Work

Abstract

Understanding how catalysts work during chemical reactions is crucial when developing efficient catalytic materials. The dynamic processes involved are extremely sensitive to changes in pressure, gas environment and temperature. Hence, there is a need for spatially resolved operando techniques to investigate catalysts under working conditions and over time. The use of dedicated operando techniques with added detection of catalytic conversion presents a unique opportunity to study the mechanisms underlying the catalytic reactions systematically. Herein, we report on the detailed setup and technical capabilities of a modular, homebuilt gas feed system directly coupled to a quadrupole mass spectrometer, which allows for operando transmission electron microscopy (TEM) studies of heterogeneous catalysts. The setup is compatible with conventional, commercially available gas cell TEM holders, making it widely accessible and reproducible by the community. In addition, the operando functionality of the setup was tested using CO oxidation over Pt nanoparticles.

Quo Vadis Micro-Electro-Mechanical Systems for the Study of Heterogeneous Catalysts Inside the Electron Microscope?

“…6, which illustrates the formation and removal of an SMSI induced overlayer on Pt and Pd nanoparticles deposited on TiO 2 supports under different reaction conditions. It should be noted that in all of these works, with the exception of a few operando CO oxidation studies [23,24,100,105,106], catalytic activity is assumed but has not been demonstrated, i.e. catalytic conversion has not been proven in parallel to TEM observations.…”

“…These CO oxidation studies achieved the simultaneous acquisition of conversion rates and TEM measurements. This is done using a mass spectrometer (MS) connected to the gas outlet of the in situ TEM holder [106] that allows a direct correlation of structure and reactivity and facilitates insights into local structure-function relationships [23,24,100,105,106]. Conversion can also be tracked via EELS analysis of the gas composition as demonstrated by Crozier et al [107,108].…”

“…(i) Inlet gas flow Although the inlet gas flow of gas cell TEM holders can be adjusted from 5 mL/min to 1 µL/min [106], the gas flow is extremely fast relative to flows in FBRs due to the small volume of the nanoreactor. As a result the residence time of the gas mixture in the nanoreactor is in the millisecond regime [106], which is three orders of magnitude lower than typical values of FBRs. This is indicative of significantly different mass transport conditions.…”

“…The coverage is sparse, as particle have to be clearly separated for TEM observation. The total amount of catalyst does not exceed 1 µg [106] and gases mostly diffuse over the catalyst bed rather than through it. Studies of other MEMS-based microreactors that feature similar beds of catalyst particles along the reactor walls found that catalytic processes were dominated by mass diffusion, owing to the inability of the reactant stream to adequately wet the catalyst bed [34].…”

“…Although several methodology and instrumentation papers have been published on in situ MEMS-based cells [106,157], only a few have specifically discussed the role of SiN x windows, which are the cornerstones of MEMS-based nanoreactors. In this section, we discuss why SiN x materials are best for this application, their corrosion under particular operating conditions, and their role as catalyst supports when catalyst particles are directly deposited on SiN x windows.…”