Low Temperature Production of Formaldehyde from Carbon Dioxide and Ethane by Plasma-Assisted Catalysis in a Ferroelectrically Moderated Dielectric Barrier Discharge Reactor

Removal and Oxidation of Low Concentration tert-Butanol from Potable Water using Nonthermal Plasma Coupled with Metal Oxide Adsorption

Taste and odor are crucial factors in evaluating the quality of drinking water for consumers. Geosmin is an example of a pollutant commonly found in potable water responsible for earthy and musty taste, and odor even at low concentrations. We have investigated the use of a hybrid two-step adsorption-mineralization process for low-level volatile organic compounds removal from potable water using dielectric barrier discharge over common metal oxides (MO). The system proposed is a proof of principle with tert-butanol (TBA) used as a model compound for geosmin removal/degradation during wastewater treatment when combined with an appropriate metal oxide adsorbent. Initial assessments of the adsorption properties of titania by density functional theory (DFT) calculations and experimental tests indicated that the adsorption of geosmin and TBA with water present results in only weak interactions between the sorbate and the metal oxide. In contrast, the DFT results show that alumina could be a suitable adsorbent for these tertiary alcohols and were reinforced by experimental studies. We find that while there is a competitive effect between the water and TBA adsorption from gaseous/liquid feed, the VOC can be removed, and the alumina will be regenerated by the reactive oxygen species (ROS) produced by a dielectric barrier discharge (DBD). The use of alumina in conjunction with NTP leads to efficient degradation of the adsorbate and the formation of oxygenated intermediates (formates, carbonates, and carboxylate-type species), which could then be mineralized for the regeneration of the adsorbent. A reaction mechanism has been proposed based on the in-situ infrared measurements and DFT calculations, while the removal of TBA with conventional heating is indicative of a gradual desorption process as a function of temperature rather than the destruction of the adsorbate. Furthermore, steady performance was observed after several adsorption-regeneration cycles, indicating no alteration of the adsorption properties of alumina during the NTP treatment and demonstrating the potential of the approach to be applied in the treatment of high throughput of water, without the challenges faced by the biocatalysts or formation of toxic byproducts.

Utilizing CO2 as a Reactant for C3 Oxygenate Production via Tandem Reactions

One possible solution to closing the loop on carbon emissions is using CO2 as the carbon source to generate high-value, multicarbon products. In this Perspective, we describe four tandem reaction strategies for converting CO2 into C3 oxygenated hydrocarbon products (i.e., propanal and 1-propanol), using either ethane or water as the hydrogen source: (1) thermocatalytic CO2-assisted dehydrogenation and reforming of ethane to ethylene, CO, and H2, followed by heterogeneous hydroformylation, (2) one-pot conversion of CO2 and ethane using plasma-activated reactions in combination with thermocatalysis, (3) electrochemical CO2 reduction to ethylene, CO, and H2, followed by thermocatalytic hydroformylation, and (4) electrochemical CO2 reduction to CO, followed by electrochemical CO reduction to C3 oxygenates. We discuss the proof-of-concept results and key challenges for each tandem scheme, and we conduct a comparative analysis of the energy costs and prospects for net CO2 reduction. The use of tandem reaction systems can provide an alternative approach to traditional catalytic processes, and these concepts can be further extended to other chemical reactions and products, thereby opening new opportunities for innovative CO2 utilization technologies.

Nonthermal plasma catalysis for toluene decomposition over BaTiO3-based catalysts by Ce doping at A-sites: The role of surface-reactive oxygen species

The insights on the primary surface-reactive oxygen species and their relation with lattice defects is essential for designing catalysts for plasma-catalytic reactions. Herein, a series of Ba_1-xCe_xTiO₃ perovskite catalysts with high specific surface areas (68.6-85.6 m² g^-1) were prepared by a facile in-situ Ce-doping strategy and investigated to catalytically decompose toluene. Combining the catalysts with a nonthermal plasma produced a significant synergy effect. The highest decomposition efficiency (100%), CO_x selectivity (98.1%), CO₂ selectivity (63.9%), and the lowest O₃ production (0 ppm) were obtained when BC₄T (Ce/Ti molar ratio = 4:100) was packed in a coaxial dielectric barrier discharge reactor at a specific input energy of 508.8 J L^-1. The H₂-TPR, temperature-programmed Raman spectra, EPR and OSC results suggested that superoxides (^•O₂^-) were the primary reactive oxygen species and were reversibly generated on the perovskite surface. Molecular O₂ was adsorbed and activated at the active sites (Ti³⁺-V_O) via an electron transfer process to form ^•O₂^-. Surface-adsorbed ^•O₂^- had a greater effect on the heterogeneous surface plasma reactions than the dielectric constant, and enhanced the toluene decomposition and intermediate oxidation. A possible reaction path of toluene decomposition was also proposed.

Isotope Labelling for Reaction Mechanism Analysis in DBD Plasma Processes

Dielectric barrier discharge (DBD) plasmas and plasma catalysis are becoming an alternative procedure to activate various gas phase reactions. A low-temperature and normal operating pressure are the main advantages of these processes, but a limited energy efficiency and little selectivity control hinder their practical implementation. In this work, we propose the use of isotope labelling to retrieve information about the intermediate reactions that may intervene during the DBD processes contributing to a decrease in their energy efficiency. The results are shown for the wet reforming reaction of methane, using D2O instead of H2O as reactant, and for the ammonia synthesis, using NH3/D2/N2 mixtures. In the two cases, it was found that a significant amount of outlet gas molecules, either reactants or products, have deuterium in their structure (e.g., HD for hydrogen, CDxHy for methane, or NDxHy for ammonia). From the analysis of the evolution of the labelled molecules as a function of power, useful information has been obtained about the exchange events of H by D atoms (or vice versa) between the plasma intermediate species. An evaluation of the number of these events revealed a significant progression with the plasma power, a tendency that is recognized to be detrimental for the energy efficiency of reactant to product transformation. The labelling technique is proposed as a useful approach for the analysis of plasma reaction mechanisms.

A review of plasma-assisted catalytic conversion of gaseous carbon dioxide and methane into value-added platform chemicals and fuels

CO2 and CH4 contribute to greenhouse gas emissions, while the production of industrial base chemicals from natural gas resources is emerging as well. Such conversion processes, however, are energy-intensive and introducing a renewable and sustainable electric activation seems optimal, at least for intermediate-scale modular operation. The review thus analyses such valorisation by plasma reactor technologies and heterogeneous catalysis application, largely into higher hydrocarbon molecules, that is ethane, ethylene, acetylene, propane, etc., and organic oxygenated compounds, i.e. methanol, formaldehyde, formic acid and dimethyl ether. Focus is given to reaction pathway mechanisms, related to the partial oxidation steps of CH4 with O2, H2O and CO2, CO2 reduction with H2, CH4 or other paraffin species, and to a lesser extent, to mixtures' dry reforming to syngas. Dielectric barrier discharge, corona, spark and gliding arc sources are considered, combined with (noble) metal materials. Carbon (C), silica (SiO2) and alumina (Al2O3) as well as various catalytic supports are examined as precious critical raw materials (e.g. platinum, palladium and rhodium) or transition metal (e.g. manganese, iron, cobalt, nickel and copper) substrates. These are applied for turnover, such as that pertinent to reformer, (reverse) water-gas shift (WGS or RWGS) and CH3OH synthesis. Time-on-stream catalyst deactivation or reactivation is also overviewed from the viewpoint of individual transient moieties and their adsorption or desorption characteristics, as well as reactivity.

One-Step Reforming of CO2 and CH4 into High-Value Liquid Chemicals and Fuels at Room Temperature by Plasma-Driven Catalysis

The conversion of CO₂ with CH₄ into liquid fuels and chemicals in a single‐step catalytic process that bypasses the production of syngas remains a challenge. In this study, liquid fuels and chemicals (e.g., acetic acid, methanol, ethanol, and formaldehyde) were synthesized in a one‐step process from CO₂ and CH₄ at room temperature (30 °C) and atmospheric pressure for the first time by using a novel plasma reactor with a water electrode. The total selectivity to oxygenates was approximately 50–60 %, with acetic acid being the major component at 40.2 % selectivity, the highest value reported for acetic acid thus far. Interestingly, the direct plasma synthesis of acetic acid from CH₄ and CO₂ is an ideal reaction with 100 % atom economy, but it is almost impossible by thermal catalysis owing to the significant thermodynamic barrier. The combination of plasma and catalyst in this process shows great potential for manipulating the distribution of liquid chemical products in a given process.

Low-temperature H2-plasma-assisted NOx storage and reduction over a combined Pt/Ba/Al and LaMnFe catalyst

With the assistance of H₂ plasma in rich phase, higher NO_x conversions could be obtained over the PBA+LMF sample over a wide temperature range (200–350 °C).

Synergistic effects of plasma–catalyst interactions for CH4 activation

Plasma-assisted catalysis populates vibrationally excited CH₄ interacting with catalyst, leading to small energy barriers and enhanced rates to activate CH₄.

Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Plasma-based conversion of CO2: current status and future challenges

This paper discusses our recent results on plasma-based CO₂ conversion, obtained by a combination of experiments and modeling, for a dielectric barrier discharge (DBD), a microwave plasma and a packed bed DBD reactor. The results illustrate that plasma technology is quite promising for CO₂ conversion, but more research is needed to better understand the underlying mechanisms and to further improve the capabilities.