The 2020 plasma catalysis roadmap Bogaerts, Annemie; Tu, Xin; Whitehead, J Christopher ...
Journal of physics. D, Applied physics,
10/2020, Letnik:
53, Številka:
44
Journal Article
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Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO2 conversion into value-added chemicals and fuels, CH4 activation into hydrogen, higher hydrocarbons ...or oxygenates, and NH3 synthesis. Other applications are already more established, such as for air pollution control, e.g. volatile organic compound remediation, particulate matter and NOx removal. In addition, plasma is also very promising for catalyst synthesis and treatment. Plasma catalysis clearly has benefits over 'conventional' catalysis, as outlined in the Introduction. However, a better insight into the underlying physical and chemical processes is crucial. This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling. The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment. Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial. All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges.
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•Nonthermal plasma enables mode-selective surface chemistry.•Vibrationally excited CH4 is the key to enhancing the strong C–H bond breaking.•Co-operative activation of CO2 is ...necessary to promote overall reforming performance.•Cumulative population of bending mode CH4 is highly anticipated in 100 kHz DBD.•Kinetic parameters of plasma-enabled promotion of CH4 dry reforming is clarified.
A mode-selective control of the surface reaction is expected to be a promising approach in the heterogeneous catalysis. Nonthermal plasma is a vital solution for the generation of vibrationally excited molecules, thereby enhancing mode-selective surface chemistry. Especially, plasma-enabled promotion of heterogeneous catalysis for CH4 conversion attracts keen attention because the strong C–H bond breaking is possible via vibrational excitation of CH4 at low temperature. Similarly, vibrational excitation of CO2 possesses unique reactivity in heterogeneous catalysts. Herein, we provide a rigorous determination of kinetic parameters of CH4 dry reforming to elucidate the drastic reaction promotion mechanism enabled by plasma-catalyst interaction. Lanthanum-modified Ni/Al2O3 catalyst was combined with dielectric barrier discharge (DBD) at 5 kPa and 400–700 °C without dilution gas. Reaction order for CH4 and CO2 were determined respectively as 0.68 and −0.17; these values were kept unchanged by DBD, indicating the surface coverage of CH4 and CO2 was not influenced by nonthermal plasma. The Arrhenius plot for forward CH4 rate constant revealed that 12 kHz DBD hybrid reaction is characterized as mixed catalysis where plasma and thermal catalysis are not decoupled. The apparent activation energy was influenced only slightly by the specific energy input (SEI, eV/molecules) and gaseous hourly space velocity (GHSV, h−1), because the electrical properties of streamer swarm are not influenced to a large extent by either SEI or GHSV at fixed frequency. In contrast, 100 kHz DBD yielded significant improvement of CH4 and CO2 conversion via vibrational excitation. Activation energy decreased from 91 kJ/mol to 44.7 kJ/mol which was well correlated with the state-specific gas-surface reactivity of vibrationally excited CH4 on Ni surfaces.
Abstract
There is urgent need for spintronics materials exhibiting a large voltage modulation effect to fulfill the great demand for high-speed, low-power-consumption information processing systems. ...Fcc-Co (111)-based systems are a promising option for research on the voltage effect, on account of their large perpendicular magnetic anisotropy (PMA) and high degree of freedom in structure. Aiming to observe a large voltage effect in a fcc-Co (111)-based system at room temperature, we investigated the voltage-induced coercivity (
H
c
) change of perpendicularly magnetized Pt/heavy metal/Co/CoO/amorphous TiO
x
structures. The thin CoO layer in the structure was the result of the surface oxidation of Co. We observed a large voltage-induced
H
c
change of 20.2 mT by applying 2 V (0.32 V/nm) to a sample without heavy metal insertion, and an
H
c
change of 15.4 mT by applying 1.8 V (0.29 V/nm) to an Ir-inserted sample. The relative thick Co thickness, Co surface oxidation, and large dielectric constant of TiO
x
layer could be related to the large voltage-induced
H
c
change. Furthermore, we demonstrated the separate adjustment of
H
c
and a voltage-induced
H
c
change by utilizing both upper and lower interfaces of Co.
The voltage-controlled magnetic anisotropy (VCMA) effect is a key to realising high-speed, ultralow-power consumption spintronic devices. The fcc-Co-(111)-based stack is a promising candidate for the ...achievement of large VCMA coefficients. However, only a few studies on the fcc-Co-(111)-based stack have been reported and the VCMA effect has not been well understood. Previously, we observed a significant increase in the voltage-controlled coercivity (VCC) in the Pt/Ru/Co/CoO/TiO
structure upon post-annealing. However, the mechanism underlying this enhancement remains unclear. This study performs multiprobe analyses on this structure before and after post-annealing and discusses the origin of the VCMA effect at the Co/oxide interface. X-ray magnetic circular dichroism measurement revealed an increase in the orbital magnetic moment owing to post-annealing, accompanied by a significant increase in VCC. We speculate that the diffusion of Pt atoms into the vicinity of Co/oxide interface enhances the interfacial orbital magnetic moment and the VCMA at the interface. These results provide a guideline for designing structures to obtain a large VCMA effect in fcc-Co-(111)-based stacks.
Localized surface plasmon resonance (LSPR) of doped Si nanocrystals (NCs) is critical to the development of Si-based plasmonics. We now experimentally show that LSPR can be obtained from both B- and ...P-doped Si NCs in the mid-infrared region. Both experiments and calculations demonstrate that the Drude model can be used to describe the LSPR of Si NCs if the dielectric screening and carrier effective mass of Si NCs are considered. When the doping levels of B and P are similar, the LSPR energy of B-doped Si NCs is higher than that of P-doped Si NCs because B is more efficiently activated to produce free carriers than P in Si NCs. We find that the plasmonic coupling between Si NCs is effectively blocked by oxide at the NC surface. The LSPR quality factors of B- and P-doped Si NCs approach those of traditional noble metal NCs. We demonstrate that LSPR is an effective means to gain physical insights on the electronic properties of doped Si NCs. The current work on the model semiconductor NCs, i.e., Si NCs has important implication for the physical understanding and practical use of semiconductor NC plasmonics.
The plasma catalytic valorization of gases, particularly CH4 and CO2, has gained increasing attention. Value‐added chemicals, such as syngas and ethene, can be formed under mild conditions when ...temperature‐decoupled plasma activation and multistep feasible catalytic conversion are combined. In this sense, efficient plasma–catalyst interaction is of key importance, for which, however, plasma catalysis, as an emerging technology, is still poorly studied, where new catalyst design and investigation took up the most effort. In this perspective work, the challenging but equally important plasma–catalyst interaction is discussed, comparatively analyzing which type of plasma, catalyst bed, is the most promising. Representative plasma catalytic systems with their characteristic features are summarized, where the intrinsic capability of fluidized‐bed dielectric barrier discharge (FB‐DBD) reactor to maximize the plasma–catalyst interaction is highlighted. Furthermore, ongoing research on FB plasma catalysis is reviewed, based on which the superiority of FB‐DBD to other candidates, especially the most widely used packed‐bed DBD reactor, is critically evaluated. In addition, the perspectives of FB‐DBD, including challenges and development potential, are discussed.
Herein, we focused on the challenging but critically important plasma–catalyst interaction, comparatively analyzing the candidate plasma catalytic technologies in terms of both catalyst bed and plasma characteristics, by which, the intrinsic superiority of fluidized‐bed dielectric barrier discharge (FB‐DBD) reactor was clearly confirmed: the FB with extended powdered catalyst surface area, efficient plasma generation and high transfer of heat, combined with DBD with direct catalyst‐oriented coupling, highly nonthermal properties and appropriate ionization, can cooperatively contribute to a maximized plasma–catalyst interaction.
The voltage‐controlled magnetic anisotropy (VCMA) effect in ferromagnets is of crucial interest for next‐generation non‐volatile magnetic memory technologies since it enables spin manipulation with ...low power consumption in addition to high‐speed operation and high writing endurance. Although intense efforts are made in the past decade to increase the efficiency of the purely electronic VCMA effect, the physical origin of this phenomenon is still elusive, particularly for interface‐engineered ferromagnetic materials. Here, a pathway is proposed to tune the VCMA effect by exploiting a combination of the insertion of heavy metals and underlayers. Focusing on Co/MgO junction deposited on Os underlayer, a large VCMA effect of −100 fJ V−1 m−1 is demonstrated with Pt insertion, which is ≈50% greater than the case deposited on Pt underlayer. On the other hand, smaller VCMA effect is observed with Ir insertion, and even a positive VCMA effect is observed with Os insertion, which are vastly different from the case deposited on Pt underlayer. The systematic variation of VCMA coefficient depending on the underlayer materials implies the effect of electron depletion from underlayers. The concepts demonstrated here can be applicable for other spintronic materials and may open a way to interfacial spin‐orbit engineering for VCMA effect.
A method is proposed to significantly tune the voltage‐controlled magnetic anisotropy (VCMA) effect in ultrathin ferromagnetic films by optimizing adjacent heavy metal layers. With focusing on Os and Pt underlayers, the systematic variation of VCMA coefficient depending on the combination of insertion and underlayer materials is found, suggesting the possible contribution of electron depletion from underlayers.
•Non-thermal plasma and catalyst hybrid reaction enhances CH4 and CO2 conversion with reduced coke deposition.•Pulsed reforming enables continuous operation without serious coking problem.•Pulsed CH4 ...injection produces H2 (higher H2/CO ratio) selectivity.•Carbon removal reaction (Boudouard reaction) is promoted by non-thermal plasma.•Optical emission of C2 high pressure Swan system is associated with the solid carbon removal via Boudouard reaction.
Pulsed dry methane reforming (DMR) in dielectric barrier discharge (DBD) and 12wt.% Ni/Al2O3 catalyst hybrid reaction was investigated, aiming for efficient conversion of greenhouse gas (CH4, CO2) into syngas (H2, CO) at low temperature. CO2 was continuously supplied, while CH4 was introduced intermittently for 1min at constant interval of 3min. Although solid carbon was deposited during the reforming reaction, carbon was almost fully removed by turning off CH4 flow and applying CO2-fed DBD. Pulsed transient analysis revealed that CH4 dehydrogenation and subsequent reverse water–gas-shift reaction is sufficiently fast with and without DBD, producing syngas with the H2/CO ratio of 0.8–0.9. In contrast, carbon removal reaction, i.e. Boudouard reaction, is promoted clearly by DBD hybridization. Radical injection is primarily important step. Besides, selective surface heating by DBD such as charge recombination on the catalysts is anticipated to promote carbon diffusion through Ni catalyst particles and subsequent oxidation by adsorbed CO2. DBD and catalyst hybrid reaction enabled higher CH4 and CO2 conversion without having serious coking problem.
Using nonthermal plasma (NTP) to promote CO2 hydrogenation is one of the most promising approaches that overcome the limitations of conventional thermal catalysis. However, the catalytic surface ...reaction dynamics of NTP-activated species are still under debate. The NTP-activated CO2 hydrogenation was investigated in Pd2Ga/SiO2 alloy catalysts and compared to thermal conditions. Although both thermal and NTP conditions showed close to 100% CO selectivity, it is worth emphasizing that when activated by NTP, CO2 conversion not only improves more than 2-fold under thermal conditions but also breaks the thermodynamic equilibrium limitation. Mechanistic insights into NTP-activated species and alloy catalyst surface were investigated by using in situ transmission infrared spectroscopy, where catalyst surface species were identified during NTP irradiation. Moreover, in in situ X-ray absorption fine-structure analysis under reaction conditions, the catalyst under NTP conditions not only did not undergo restructuring affecting CO2 hydrogenation but also could clearly rule out catalyst activation by heating. In situ characterizations of the catalysts during CO2 hydrogenation depict that vibrationally excited CO2 significantly enhances the catalytic reaction. The agreement of approaches combining experimental studies and density functional theory (DFT) calculations substantiates that vibrationally excited CO2 reacts directly with hydrogen adsorbed on Pd sites while accelerating formate formation due to neighboring Ga sites. Moreover, DFT analysis deduces the key reaction pathway that the decomposition of monodentate formate is promoted by plasma-activated hydrogen species. This work enables the high designability of CO2 hydrogenation catalysts toward value-added chemicals based on the electrification of chemical processes via NTP.
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