In an article recently published in the journal ACS Engineering Au, researchers discussed the utility of in situ/operando transmission infrared spectroscopy to examine the plasma-catalyst interface.
Study: Interrogation of the Plasma-Catalyst Interface via In Situ/Operando Transmission Infrared Spectroscopy. Image Credit: IvaFoto/Shutterstock.com
Non-thermal plasmas (NTPs) are partially ionized gases that differ in temperature from low-energy ions and neutrals and high-energy electrons. Many different applications have made use of low-temperature plasma-induced interactions. Over the past ten years, there has been a tremendous increase in research on plasma-assisted catalysis.
However, due to the intricacy of plasma-phase interactions with reactive surfaces, there is little knowledge of the fundamental elementary processes taking place at the plasma-catalyst interface, leading to a large number of empirical research on the performance of various catalytic materials. Therefore, by directly monitoring how plasma stimulation impacts the evolution of surface speciation, a shift toward operando and in situ characterization methodologies can advance the research.
Identification of adsorbates on a metal surface when it is stimulated by plasma, as well as the correlation of plasma input parameters with observations, have been the main focus of recent advancements in this field. Other methods of observing the plasma-catalyst interface have taken advantage of Fourier transform infrared (FTIR) spectroscopy's accessibility and adaptability to find functional groups on solid surfaces. Although the infrared-based designs have been effective in a substantial portion of their attempts to produce relevant data regarding the plasma-surface interaction, the sample set-up and metal enclosure of cells still pose extra engineering difficulties.
In recent years, several designs that alter conventional transmission cells have been put into practice. However, molecular gases require designs that either boost the applied voltage, reduce the electrode gap, or use vacuum pressure to generate a stable discharge.
About the Study
In this study, the authors discussed a straightforward and flexible design for a dielectric barrier discharge (DBD) plasma cell that could interface with FTIR, mass spectrometry (MS), and optical emission spectroscopy (OES) to characterize the surface, gas phase, and plasma phase simultaneously. Two example applications, i.e., plasma oxidation of primary amine functionalized SBA-15 and catalytic low-temperature nitrogen oxidation, were used to illustrate the technology.
The team selectively oxidized the amino groups to nitro groups without changing the alkyl tether. The results from the first application directly demonstrated how a 1% O2/He plasma interacted with the amino silica surface. The second application utilized plasma stimulation to track the evolution of NOX species bound to platinum and silica surfaces. The combined experimental findings indicated the variety of potential uses for the proposed device and supported its potential as a key instrument for the study of plasma-surface connection.
The researchers offered a simple alternative design for a plasma-transmission IR cell that could perform surface-sensitive in situ and operando measurements under a variety of plasma circumstances. The cell's speedy sample preparation, construction costs, and capacity to simultaneously interface with widely used analytical techniques encouraged its adoption as a popular tool for the quick accumulation of significant amounts of in situ plasma-catalysis data. The design and functionality of the cell were comparable to commonly used dielectric barrier discharge (DBD) plasma-catalytic reactors. Both the low-temperature plasma-assisted nitrogen oxidation over a Pt/SiO2 catalyst and the plasma-assisted oxidation of 3-aminopropyl SBA-15 (AP-SiO2), which established plasma-surface contact, were used to illustrate the effectiveness of the cell.
The three primary characteristics of the IR spectra were located around 1700–1800, 1653, and 1580 cm-1. After the plasma was turned off and the cell was cleansed with He, all peaks remained unaltered. After 30 minutes of temperature exposure, there were no discernible changes in the catalyst surface's IR spectra.
The data from MS, FTIR, and OES taken together showed that plasma-generated reactive oxygen derivatives specifically converted primary amines to nitro groups without further oxidizing the secondary amines linker to the support made of hydrocarbons. This outcome demonstrated the ability to draw trustworthy inferences about plasma-surface interactions.
The proposed design could also be easily modified to meet particular requirements by using a vacuum manifold for controlled dosing, in-plasma versus post-plasma configurations, and integrating step-scan for microsecond resolution. It could also be combined with additional analytical and characterization methods like MS and OES. The silicon nitrates created by interaction with NOX and plasma-generated oxygen vacancies on the silica surface were recognized by IR results from the support-only experiment. Both bending and linear NO species were adsorbed on several surfaces on Pt/SiO2. The cell was able to monitor the waxing and waning of these species on the catalyst surface over time owing to Pt facets.
In conclusion, this study proposed a straightforward design for an IR transmission cell that could probe a surface while being stimulated by plasma. The thermal control of the cell was tested at varying pressure of 0.3 and 0.1 atm and feed gases, N2 and He. It was discovered that the set-point temperature was consistently repeatable to within ±10 °C under all circumstances. The oxidation of primary amine-functionalized SBA-15 was carried out as an example application of the cell to confirm the capability to both induce and recognize plasma effects on a surface. A Pt/SiO2 catalyst for plasma-assisted nitrogen oxidation was employed as a second example application, and the cell was used to monitor surface species on the catalyst.
The authors mentioned that since the building ingredients for the IR cell are widely available and affordable, any researcher interested in investigating plasma-surface coupling can use the proposed technology. They believe that the findings of this study demonstrated the usefulness of this plasma cell and its capability to investigate plasma catalysis in situ.
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Clarke, R. J., Hicks, J. C., Interrogation of the Plasma-Catalyst Interface via In Situ/Operando Transmission Infrared Spectroscopy. ACS Engineering Au (2022). https://pubs.acs.org/doi/10.1021/acsengineeringau.2c00026