Following the introduction of closed-cell in-situ TEM, there has been growing attention placed on the methodology for mixing and delivering gases to the holder tip, which encloses the sample. This article outlines the mixing method employed in the Atmosphere gas system, along with its benefits and the extra analytical abilities it supports.
Gas Mixing: Volumetric Blending
The method known as volumetric blending is regularly employed in the production of industrial gases, from breathing to welding gas. The process is initiated with a pure gas, or established gas blend, and other identified gases are incorporated in sequence. The gases mix together through entropy, making an accurate, consistent gas blend.
The mixture’s composition is managed by determining the partial pressure of each gas as it is incorporated into the mixture, ensuring that the overall make-up can be determined as per Dalton’s Law of Additive Pressures and the Ideal Gas Law.
As this methodology only necessitates control and pressure readings of each component gas, an arbitrary gas mixture can be produced to high accuracy, independent of the gas species. This includes pure vapor, such as H2O and vapor mixtures, establishing volumetric blending as a greatly adaptable approach to mixing.
Figure 1. A) Schematic of a three-component mixture being created with Atmosphere 210. B) Volumetric blending is the mixing method used by gas manufacturers to produce multi-component gas standards and application specific specialties like breathing and welding.
Benefits of Volumetric Blending
The Atmosphere gas system allows for a trio of gas inputs, which are designated by the user as per their needs. The input gases can be pure, blended, multi-component, or complex gases which mimic real-world environments.
Each of these gases is then impelled into the manifold and mixed to the desired ratios, while sensitive pressure gauges record the partial pressure of each gas across each stage of the sequential mixing. With these three gas inputs, operators can produce various gas mixtures:
- Mixtures of pure gases, including H2, He, O2, N2, Ar and more
- Mixtures of multi-component gases, including CO2, CH4, CO, NO2, NH3, CO and more
- Mixtures of real-world, complex gases, including exhaust or reaction products
Pressure and Flow Control
As the Atmosphere 210 employs volumetric blending – a partial pressure blending process that creates no requirement for MFCs (Mass Flow Controllers), there is no environmental reliance on gas mass, flow and pressure.
Atmosphere 210 manages the gas pressures with the help of sensitive gauges, able to create gases with an accuracy of 0.001 Torr, meaning that dilute 1% mixtures can be produced at any pressure needed, for example, 99.0% Ar; 1.0% H2.
Additionally, Atmosphere 210 engages a software-controlled variable leak valve that can be modified to produce gases at flow rates ranging from 0.005 ml/min to 1.000 ml/min, contingent on the pressure of the gas delivered.
This enables an increase in the ‘residence time’ of the gas exposed to users’ samples, which can be an essential function for catalysis testing. Through the combination of volumetric blending’s adaptability, alongside the use of independent pressure and flow control systems, operators can enjoy a number of advantages.
- Create dilute mixtures down 1% composition
- Control gas ‘residence time’ via independent flow control
- Deliver gases at pressures down to 0.3 Torr
With the use of volumetric blending, the Atmosphere gas system can create mixtures in just five minutes, while transferring between mixtures needs only a further five minutes. Throughout the process, the concentration of intermediate steps’ composition is always ascertainable. This offers support for multiple varieties of gas study, beyond just mixing capabilities.
Figure 2. A) Volumetric blending is a robust technique that enables mixing of a wide variety of gasses independent of the mass of each constituent. B) Using the high-speed pumps in the atmosphere manifold, new gasses can be created in just minutes. Two different mixtures can be stored at any time, allowing the user to switch between two different compositions virtually instantaneously.
Analyzing Reactions with Mass Spectrometry (MS)
Mass Spectrometers and related apparatuses, like Residual Gas Analyzers (RGA), offer corresponding information to in situ environmental TEM. It is possible to monitor the composition of the gas in real time with the use of the RGA port fixed directly to the gas outlet on the Atmosphere 210 holder.
This kind of test can be used to observe reaction by-products developing in the sample throughout the study – a highly useful function for catalysis studies. Through simply joining an RGA to the gas outlet, as demonstrated in Figure 3A, any lab can enhance their in situ study with chemical monitoring.
To illustrate, NiO catalysts employed for methanation convert CO2 gas to CH4 fuel via a complex exothermic reaction. Users could engage the Atmosphere 210 to directly image the structural and morphological changes happening at the catalyst sample, while following the methanation in real time, as detailed in Figure 3B.
With such observations, it is possible to gain a more quantitative and holistic understanding of the primary dynamics underneath catalytic reactions. Additionally, RGAs allow for users to monitor the accuracy of the gas blend inside the usual instrument prediction error level of the spectrometer (3%).
The Atmosphere 210’s mixing precision was verified by comparing RGA spectra against a known reference gas. The experiment incorporated a multitude of measurements at varying pressures and flowrates, with an example shown in Figure 3C.
After testing Atmosphere 210 in a number of other conditions, down to 100x lower pressure (7 Torr), it was determined that, although still functioning, the RGA prediction error grew, particularly at lower concentrations.
Combining MS with in situ environmental TEM studies delivers fresh insight into sample reactions, and Atmosphere 210 capitalizes on these benefits:
- Simply attach a commercial RGA using a standard KF-16 port
- Track chemical changes of the sample environment in real-time
- Correlate sample changes with reaction conditions
Figure 3. A) Schematic showing how an RGA can be connected to an in situ system. B) RGA monitoring of a NiO catalyst showing the consumption of CO2 and H2 in the production of CH4 and H2O at various temperatures. Arrows indicate the formation of Ni nanoparticles within the sample during the catalysis reactions. Courtesy Mounib Bahr institut de Physique et Chimie des Materiaux de Strasbourg. C) RGA analysis of a 2.5% CO / 17.8% CH4 / 79.7% Ar calibration gas. Both the calibration gas and volumetrically blended gas composition were measured three different times at 2 different pressures. Line graphs show average composition over time for each test while bar graphs show the average composition recorded over all three tests.
Volumetric blending is often employed by industrial gas providers to create gases of arbitrary make-up and pressure, making this an established approach for preparing gases for in situ TEM.
The structure of the gas mixtures developed using the Atmosphere gas system has been tested against standard gasses and calibrated RGA mass spectrometry, and demonstrated levels of accuracy within the detection error of the spectrometer at low and high pressure, and concentration.
Volumetric blending applied to the Atmosphere in-situ gas system affords the user:
- Compatibility with multiple gas species, vapors and even complex mixtures of which the system does not support production
- Wide variety of operational pressure, flow, molecular weight and concentration of the gas species
- Accurateness of produced mixes within the detection error of mass spectrometry residual gas analysis
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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