Plasma-Catalytic CO2 Hydrogenation over a Pd/ZnO Catalyst: In Situ Probing of Gas-Phase and Surface Reactions

Enhancing the Reaction of CO2 and H2O Using Catalysts within a Nonthermal Plasma

The direct conversion of emitted and captured carbon dioxide into usable fuels remains a significant challenge and is a key element in the transition to net zero. Herein, we examine the reaction of CO2 and H2O over Ni- and Cu-based catalysts combined with nonthermal plasma (NTP) technology. The catalysis under NTP conditions enabled significantly higher CO2 conversion and product yield, which was almost six times higher than that of the plasma-only system. A maximum H2 concentration of ∼2500 ppm was achieved for the Cu/ZSM5 catalyst at 17% CO2 conversion. Comprehensive catalyst characterization together with the reaction performances reveals that Cu in a reduced state promotes both the CO2 and H2O conversion leading to H2 formation. In situ diffuse reflectance infrared spectroscopy (DRIFTS) coupled with mass spectrometry (MS) analysis of the gas phase products confirms that CO is the major active species to drive the water gas shift reaction to form H2 in addition to the direct CO2 and H2O interaction. It also explains how the different metal support interactions influence the CO adsorption and its interaction with water. Among the catalysts studied, ZSM5-supported Cu catalysts were found to be the most effective in facilitating the CO2 and H2O reaction to produce H2.

Imaging Gas-Involved Structural Dynamics by Environmental Electron Microscopy

Understanding gas-involved physicochemical reactions is undoubtedly one of the most significant challenges in the modern chemical industry. To clarify how those reactions precede requires deep insights into the real-time visualization of reaction dynamics within a gas environment. The emergence and rapid development of in situ environmental electron microscopy (EEM) including scanning electron microscopy (ESEM) and transmission electron microscopy (ETEM) have enabled multiscale observation of dynamic gas-involved physicochemical reactions. This review examines the state-of-art EEM technologies, categorizing those gas reactions into various physical and chemical processes and detailing the corresponding dynamic behaviors. It begins by reviewing the state-of-the-art EEM techniques and is followed by detailing their application in typical physical processes. It clarifies physical vapor condensation, deposition, and geometric reshaping with gaseous involving. More importantly, all the gas-involved chemical reactions into electrochemical reactions, thermochemical reactions, chemical crystal growth, and catalytic reactions are thoroughly explored and categorized. Finally, the review highlights the technical challenges and valuable perspectives provided by in situ EEM for addressing critical gas-involved issues. Overall, this article offers a multiscale and comprehensive understanding of the physicochemical origins associated with gas-involved reactions, envisioning fundamental strategies for designing high-performance gas-involved functional materials.

Characterizing the stability of ultra-thin metal oxide catalyst films in non-thermal plasma CO2 reduction reactions

The use of metal oxide catalysts to enhance plasma CO2 reduction has seen significant recent development towards processes to reduce greenhouse gas emissions and produce renewable chemical feedstocks. While plasma reactors are effective at producing the intended chemical transformations, the conditions can result in catalyst degradation. Atomic layer deposition (ALD) can be used to synthesize complex, hierarchically structured metal oxide plasma catalysts that, while active for plasma CO2 reduction, are potentially vulnerable to degradation due to their high surface area and nanoscopic thickness. In this work, we characterized the effects of extended non-thermal, glow discharge plasma exposure on ALD synthesized, ultra-thin film (<30 nm) TiO2 and ZnO catalysts. We used X-ray diffraction, reflectivity, and spectroscopy to compare films exposed to a CO2 plasma to ones exposed to an Ar plasma and to ones annealed in air. We found that the CO2 plasma exposure generated some surface reduction in TiO2 and increased surface roughening by a small amount, but did not initiate any phase changes or crystallite growth. The results suggest that ALD-deposited metal oxide films are robust to low pressure CO2 plasma exposure and are suitable as catalysts or catalyst supports in extended reactions.

Plasma-Derived Atomic Hydrogen Enables Eley-Rideal-Type CO2 Methanation at Low Temperatures

Activating H2 molecules into atomic hydrogen and utilizing their intrinsic chemical reactivity are important processes in catalytic hydrogenation. Here, we have developed a plasma-catalyst combined system that directly provides atomic hydrogen from the gas phase to the catalytic reaction to utilize the high energy and translational freedom of atomic hydrogen. In this system, we show that the temperature of CO2 methanation over Ni/Al2O3 can be dramatically lower compared to thermal catalysis. Using a detailed mechanistic study with kinetic studies, laser plasma diagnostics, in situ plasma surface characterization, and theoretical calculations, we revealed that plasma-derived atomic hydrogen (PDAH) plays a crucial role in reaction promotion. In particular, PDAH effectively lowers the energy barrier of bidentate formate hydrogenation by translating from the Langmuir-Hinshelwood to the Eley-Rideal-type reaction.

Unveiling the Mechanism of Plasma-Catalytic Low-Temperature Water-Gas Shift Reaction over Cu/γ-Al2O3 Catalysts

The water-gas shift (WGS) reaction is a crucial process for hydrogen production. Unfortunately, achieving high reaction rates and yields for the WGS reaction at low temperatures remains a challenge due to kinetic limitations. Here, nonthermal plasma coupled to Cu/γ-Al2O3 catalysts was employed to enable the WGS reaction at considerably lower temperatures (up to 140 °C). For comparison, thermal-catalytic WGS reactions using the same catalysts were conducted at 140-300 °C. The best performance (72.1% CO conversion and 67.4% H2 yield) was achieved using an 8 wt % Cu/γ-Al2O3 catalyst in plasma catalysis at ∼140 °C, with 8.74 MJ mol-1 energy consumption and 8.5% H2 fuel production efficiency. Notably, conventional thermal catalysis proved to be ineffective at such low temperatures. Density functional theory calculations, coupled with in situ diffuse reflectance infrared Fourier transform spectroscopy, revealed that the plasma-generated OH radicals significantly enhanced the WGS reaction by influencing both the redox and carboxyl reaction pathways.

Unveiling the Mechanism of Plasma-Catalyzed Oxidation of Methane to C2+ Oxygenates over Cu/UiO-66-NH2

Nonthermal plasma (NTP) offers the potential for converting CH4 with CO2 into liquid products under mild conditions, but controlling liquid selectivity and manipulating intermediate species remain significant challenges. Here, we demonstrate the effectiveness of the Cu/UiO-66-NH2 catalyst in promising the conversion of CH4 and CO2 into oxygenates within a dielectric barrier discharge NTP reactor under ambient conditions. The 10% Cu/UiO-66-NH2 catalyst achieved an impressive 53.4% overall liquid selectivity, with C2+ oxygenates accounting for ∼60.8% of the total liquid products. In situ plasma-coupled Fourier-transform infrared spectroscopy (FTIR) suggests that Cu facilitates the cleavage of surface adsorbed COOH species (*COOH), generating *CO and enabling its migration to the surface of Cu particles. This surface-bound *CO then undergoes C-C coupling and hydrogenation, leading to ethanol production. Further analysis using CO diffuse reflection FTIR and 1H nuclear magnetic resonance spectroscopy indicates that in situ generated surface *CO is more effective than gas-phase CO (g) in promoting C-C coupling and C2+ liquid formation. This work provides valuable mechanistic insights into C-C coupling and C2+ liquid production during plasma-catalytic CO2 oxidation of CH4 under ambient conditions. These findings hold broader implications for the rational design of more efficient catalysts for this reaction, paving the way for advancements in sustainable fuel and chemical production.

Isomorphous Substitution in ZSM-5 in Tandem Methanol/Zeolite Catalysts for the Hydrogenation of CO2 to Aromatics

Intensified reactors for conversion of CO2 to methanol (via hydrogenation) using metal oxide catalysts coupled with methanol conversion to aromatics in the presence of zeolites (e.g., H-ZSM-5) in a single step are investigated. Brønsted acid sites (BAS) in H-ZSM-5 are important sites in methanol aromatization reactions, and correlations of the reactivity with zeolite acid properties can guide reaction optimization. A classical way of tuning the acidity of zeolites is via the effect of the isomorphous substitution of the heteroatom in the framework. In this work, H-[Al/Ga/Fe]-ZSM-5 zeolites are synthesized with Si/T ratios = 80, 300, affecting the acid site strength as well as distribution of Brønsted and Lewis acid sites. On catalytic testing of the H-[Al/Ga/Fe]-ZSM-5/ZnO-ZrO2 samples for tandem CO2 hydrogenation and methanol conversion, the presence of weaker Brønsted acid sites improves the aromatics selectivity (CO2 to aromatics selectivity ranging from 13 to 47%); however, this effect of acid strength was not observed at low T atom content. Catalytic testing of H-[B]-ZSM-5/ZnO-ZrO2 provides no conversion of CO2 to hydrocarbons, showing that there is a minimum acid site strength needed for measurable aromatization reactivity. The H-[Fe]-ZSM-5-80/ZnO-ZrO2 catalyst shows the best catalytic activity with a CO2 conversion of ∼10% with a CO2 to aromatics selectivity of ∼51%. The catalyst is shown to provide stable activity and selectivity over more than 250 h on stream.

Plasma-Enabled Selective Synthesis of Biobased Phenolics from Lignin-Derived Feedstock

Converting abundant biomass-derived feedstocks into value-added platform chemicals has attracted increasing interest in biorefinery; however, the rigorous operating conditions that are required limit the commercialization of these processes. Nonthermal plasma-based electrification using intermittent renewable energy is an emerging alternative for sustainable next-generation chemical synthesis under mild conditions. Here, we report a hydrogen-free tunable plasma process for the selective conversion of lignin-derived anisole into phenolics with a high selectivity of 86.9% and an anisole conversion of 45.6% at 150 °C. The selectivity to alkylated chemicals can be tuned through control of the plasma alkylation process by changing specific energy input. The combined experimental and computational results reveal that the plasma generated H and CH3 radicals exhibit a "catalytic effect" that reduces the activation energy of the transalkylation reactions, enabling the selective anisole conversion at low temperatures. This work opens the way for the sustainable and selective production of phenolic chemicals from biomass-derived feedstocks under mild conditions.

Unraveling Temperature-Dependent Plasma-Catalyzed CO2 Hydrogenation

Hydrogenation of carbon dioxide to value-added chemicals and fuels has recently gained increasing attention as a promising route for utilizing carbon dioxide to achieve a sustainable society. In this study, we investigated the hydrogenation of CO2 over M/SiO2 and M/Al2O3 (M = Co, Ni) catalysts in a dielectric barrier discharge system at different temperatures. We compared three different reaction modes: plasma alone, thermal catalysis, and plasma catalysis. The coupling of catalysts with plasma demonstrated synergy at different reaction temperatures, surpassing the thermal catalysis and plasma alone modes. The highest CO2 conversions under plasma-catalytic conditions at reaction temperatures of 350 and 500 °C were achieved with a Co/SiO2 catalyst (66%) and a Ni/Al2O3 catalyst (68%), respectively. Extensive characterizations were used to analyze the physiochemical characteristics of the catalysts. The results show that plasma power was more efficient than heating power at the same temperature for the CO2 hydrogenation. This demonstrates that the performance of CO2 hydrogenation can be significantly improved in the presence of plasma at lower temperatures.

Facile Synthesis of Cu-Doped ZnO Nanoparticles for the Enhanced Photocatalytic Disinfection of Bacteria and Fungi

In this study, Cu-doped ZnO was prepared via the facile one-pot solvothermal approach. The structure and composition of the synthesized samples were characterized by XRD (X-ray diffraction), TEM (transmission electron microscopy), and XPS (X-ray photoelectron spectroscopy) analyses, revealing that the synthesized samples consisted of Cu-doped ZnO nanoparticles. Ultraviolet-visible (UV-vis) spectroscopy analysis showed that Cu-doping significantly improves the visible light absorption properties of ZnO. The photocatalytic capacity of the synthesized samples was tested via the disinfection of Escherichia coli, with the Cu-ZnO presenting enhanced disinfection compared to pure ZnO. Of the synthesized materials, 7% Cu-ZnO exhibited the best photocatalytic performance, for which the size was ~9 nm. The photocurrent density of the 7% Cu-ZnO samples was also significantly higher than that of pure ZnO. The antifungal activity for 7% Cu-ZnO was also tested on the pathogenic fungi of Fusarium graminearum. The macroconidia of F. graminearum was treated with 7% Cu-ZnO photocatalyst for 5 h, resulting in a three order of magnitude reduction at a concentration of 105 CFU/mL. Fluorescence staining tests were used to verify the survival of macroconidia before and after photocatalytic treatment. ICP-MS was used to confirm that Cu-ZnO met national standards for cu ion precipitation, indicating that Cu-ZnO are environmentally friendly materials.

Plasma-Catalytic CO2 Reforming of Toluene over Hydrotalcite-Derived NiFe/(Mg, Al)O x Catalysts

The removal of tar and CO₂ in syngas from biomass gasification is crucial for the upgrading and utilization of syngas. CO₂ reforming of tar (CRT) is a potential solution which simultaneously converts the undesirable tar and CO₂ to syngas. In this study, a hybrid dielectric barrier discharge (DBD) plasma-catalytic system was developed for the CO₂ reforming of toluene, a model tar compound, at a low temperature (∼200 °C) and ambient pressure. Periclase-phase (Mg, Al)O _x nanosheet-supported NiFe alloy catalysts with various Ni/Fe ratios were synthesized from ultrathin Ni-Fe-Mg-Al hydrotalcite precursors and employed in the plasma-catalytic CRT reaction. The result demonstrated that the plasma-catalytic system is promising in promoting the low-temperature CRT reaction by generating synergy between DBD plasma and the catalyst. Among the various catalysts, Ni4Fe1-R exhibited superior activity and stability because of its highest specific surface area, which not only provided sufficient active sites for the adsorption of reactants and intermediates but also enhanced the electric field in the plasma. Furthermore, the stronger lattice distortion of Ni4Fe1-R provided more isolated O^2- for CO₂ adsorption, and having the most intensive interaction between Ni and Fe in Ni4Fe1-R restrained the catalyst deactivation induced by the segregation of Fe from the alloy to form FeO _x . Finally, in situ Fourier transform infrared spectroscopy combined with comprehensive catalyst characterization was used to elucidate the reaction mechanism of the plasma-catalytic CRT reaction and gain new insights into the plasma-catalyst interfacial effect.

Plasma-Catalytic Reforming of Naphthalene and Toluene as Biomass Tar over Honeycomb Catalysts in a Gliding Arc Reactor

Biomass gasification is a promising and sustainable process to produce renewable and CO2-neutral syngas (H2 and CO). However, the contamination of syngas with tar is one of the major challenges to limit the deployment of biomass gasification on a commercial scale. Here, we propose a hybrid plasma-catalytic system for steam reforming of tar compounds over honeycomb-based catalysts in a gliding arc discharge (GAD) reactor. The reaction performances were evaluated using the blank substrate and coated catalytic materials (γ-Al2O3 and Ni/γ-Al2O3). Compared with the plasma alone process, introducing the honeycomb materials in GAD prolonged the residence time of reactant molecules for collision with plasma reactive species to promote their conversions. The presence of Ni/γ-Al2O3 gave the best performance with the high conversion of toluene (86.3%) and naphthalene (75.5%) and yield of H2 (35.0%) and CO (49.1%), while greatly inhibiting the formation of byproducts. The corresponding highest overall energy efficiency of 50.9 g/kWh was achieved, which was 35.4% higher than that in the plasma alone process. Characterization of the used catalyst and long-term running indicated that the honeycomb material coated with Ni/γ-Al2O3 had strong carbon resistance and excellent stability. The superior catalytic performance of Ni/γ-Al2O3 can be mainly ascribed to the large specific surface area and the in situ reduction of nickel oxide species in the reaction process, which promoted the interaction between plasma reactive species and catalysts and generated the plasma-catalysis synergy.