Catalyst Preparation with Plasmas: How Does It Work? - ACS Catalysis

Review Cite This: ACS Catal. 2018, 8, 2093−2110

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Catalyst Preparation with Plasmas: How Does It Work? Zhao Wang,† Yao Zhang,† Erik C. Neyts,‡ Xinxiang Cao,† Xiaoshan Zhang,† Ben W.-L. Jang,§ and Chang-jun Liu*,† †

Tianjin Co-Innovation Center of Chemical Science & Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China ‡ Department of Chemistry, Research Group PLASMANT, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium § Department of Chemistry, Texas A&M University-Commerce, 2600 South Neal Street, Commerce, Texas 75429-3011, United States ABSTRACT: Catalyst preparation with plasmas is increasingly attracting interest. A plasma is a partially ionized gas, consisting of electrons, ions, molecules, radicals, photons, and excited species, which are all active species for catalyst preparation and treatment. Under the influence of plasma, nucleation and crystal growth in catalyst preparation can be very different from those in the conventional thermal approach. Some thermodynamically unfavorable reactions can easily take place with plasmas. Compounds such as sulfides, nitrides, and phosphides that are produced under harsh conditions can be synthesized by plasma under mild conditions. Plasmas can produce catalysts with smaller particle sizes and controllable structure. Plasma is also a facile tool for reduction, oxidation, doping, etching, coating, alloy formation, surface treatment, and surface cleaning in a simple and direct way. A rapid and convenient plasma template removal has thus been established for zeolite synthesis. It can operate at room temperature and allows the catalyst preparation on temperature-sensitive supporting materials. Plasma is typically effective for the production of various catalysts on metallic substrates. In addition, plasma-prepared transition-metal catalysts show enhanced low-temperature activity with improved stability. This provides a useful model catalyst for further improvement of industrial catalysts. In this review, we aim to summarize the recent advances in catalyst preparation with plasmas. The present understanding of plasma-based catalyst preparation is discussed. The challenges and future development are addressed. KEYWORDS: plasma, photocatalyst, electrocatalyst, hydrogen, carbon, zeolite, platinum, oxidation

1. INTRODUCTION Catalysts play an important role in the chemical industry. With the increasing energy and environmental concerns, one can expect a further rapidly expanding market for catalysts. However, there are still some challenges in catalyst preparation with controllable size and structure. Some catalyst preparation methods are time consuming or complex. Most of the present catalyst preparation methods are not environmentally friendly. The conventional thermal treatment procedure sometimes induces sintering and encounters difficulties in handling some new supporting materials, such as metals, soft materials, nanosized porous materials, and ultrahigh-surface-area carbon. Innovation in the catalyst preparation approach is thus immediately needed. The use of various plasmas for catalyst preparation to better control size and structure to improve catalytic properties has thus received increasing interest since the 1990s1,2 with a rapidly increasing number of publications.3−10 A plasma is a partially ionized gas, consisting of electrons, ions, molecules, radicals, photons, and excited species. It is thus a highly reactive mixture, which makes it different from conventional gaseous mixtures. Under the influence of plasma, the resulting catalysts can be very different from those prepared by conventional thermal means. This is the major reason that plasmas have been extensively applied for catalyst preparation. © 2018 American Chemical Society

Depending on their energy level, temperature, and ionic density, plasmas are usually classified as high-temperature (equilibrium) plasmas (for nuclear applications) and lowtemperature (nonequilibrium) plasmas (including thermal and nonthermal plasmas). In thermal plasmas, the gas bulk temperature is close to the electron temperature (up to several tens of electron volts; 1 eV = 11605 K). On the other hand, the bulk temperature in nonthermal plasmas can be as low as room temperature (or even below), while the electron temperature can reach as high as 10000−100000 K (1−10 eV). Nonthermal plasmas are also called cold plasmas, if the gas temperature is close to room temperature. For the use of catalyst preparation, the bulk temperature for nonthermal plasmas normally ranges from room temperature to several hundred degrees Celsius, depending on the operation period, pressure, external heating or cooling, etc. For plasma operations under vacuum, the bulk temperature can remain near the starting point, if no external heating is applied. This can effectively avoid the effects from thermal heating. For some operations at atmospheric pressure or under vacuum with high energy input, the bulk temperature can be up to several hundred degrees Celsius if no external Received: October 31, 2017 Revised: January 25, 2018 Published: January 29, 2018 2093

DOI: 10.1021/acscatal.7b03723 ACS Catal. 2018, 8, 2093−2110

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ACS Catalysis cooling is applied. If thermal effects are not needed or not desired, one can control the operation period or duty cycle to keep the bulk temperature low (with possible intermittent operations) without the need for external cooling. A key characteristic of cold plasmas is their far-fromequilibrium state at relatively low temperatures, typically in the range of 300−1000 K. Nonequilibrium means that the electrons in the cold plasma can be very “hot” but the heavier species, such as ions, atoms, and molecules, remain “cool” (or at very modest temperatures). The combination of reactivity of plasma species, nonequilibrium state, and low-temperature operation enables nonthermal plasmas for catalyst preparation in unique ways with enhanced reactivity of the surfaces in contact with the plasma species. As a result, low-temperature plasmas show superior characteristics that allow the production of structures and induce processes at surfaces more efficiently in a more controlled way, in comparison with the traditional thermal methods. In this review article, we aim to summarize the recent advances in catalyst preparation with various plasmas. We mostly focus on the fundamental issues and attempt to explain why plasmas can improve the catalyst preparation process and also the resulting catalytic properties. If readers prefer more details on specific topics, other recent review articles are available on the green chemistry aspects,3 carbon surface modification,5 Ni catalysts,6 Fischer−Tropsch cobalt catalysts,7 supported catalysts,8 fuel cell catalysts,9 and photocatalysts.10 Finally, the challenges and future development will be addressed.

Figure 1. Schematic representation of the state of matter in a pressure−temperature diagram. Figure reproduced from ref 12. Copyright 2015 AIP Publishing LLC.

2. PLASMAS AND PLASMA REACTIONS FOR THE PREPARATION OF HETEROGENEOUS CATALYSTS A plasma or gas discharge is normally created by applying a high voltage to a gas or a gas mixture for ionization. The first man-made gas discharge was generated by Francis Hauksbee in 1705 by charging and discharging an evacuated sphere containing a small amount of mercury.11 The first industrial gas discharge was developed by Siemens in 1857 for ozone production. The term “plasma” was coined by Irving Langmuir in 1928. Figure 1 shows a schematic representation of the status of the matters with the corresponding ionized states.12 By modification of the nature of the reactants, the plasma can shift the thermodynamically unfavorable reactions to become favorable even near ambient conditions. An example is that one can use cold plasma to create zirconia with a monoclinic structure at relatively low gas temperature (less than 150 °C).13 This kind of monoclinic structure is normally only obtained at temperatures over 1000 °C. Other examples include the formation of graphene14 and other carbon nanostructured materials via plasmas at low temperatures.15,16 Depending on how the power is coupled into the plasma, the electrode configuration, or the dielectric material applied, a variety of different plasmas can be obtained, including direct current (dc) and alternative current (ac) glow discharges, radio frequency (rf) discharges, microwave discharges, dielectric barrier discharges (DBDs), gliding arcs, arcs, plasma jets, and plasma torches. All of these discharges have been applied for catalyst preparation. Arcs, plasma jet, and plasma torch are thermal plasmas. Gliding arcs have characteristics of both thermal and nonthermal plasmas. The others are normally cold plasmas. Figure 2 shows the classification of plasmas and their common applications in catalyst preparation. The details of these plasmas can be found in the literature.1−12,17−21

Figure 2. Overview of plasma types and their most common applications in catalyst preparation.

Because electrons possess much higher mobility in comparison to ions and other plasma species, the exposed surfaces within the plasma will attain a negative charge. This further sets up an electric field, which slows down the electrons and accelerates the positive ions. This in turn establishes a dynamic equilibrium, with equal fluxes of electrons and ions. As a result, a region of positive charge, the so-called “sheath”, develops in front of the surface.11,22,23 The sheath is important in plasma catalyst preparation, as it determines the fluxes and energies of the (charged) species reaching the surface. The neutral gas is also affected in the sheath, as it determines the energy distribution of the electrons and ions influencing the chemical reactions. Thus, the plasma−surface interactions are determined, to a large extent, by the electric field distribution in the plasma and especially in the sheath as well as by the closely related plasma characteristics, such as the power coupling with the plasma, the gas pressure, the chemical composition of the plasma, etc.11 Figure 3 shows comparative illustrative images of 2094


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these reactions depends on the electron energy, electron density, gas temperature, gas pressure, and properties of the gases, liquids, and solids present in the reactor. In comparison to the conventional thermal preparation, the reactions during the plasma catalyst preparation are much more dynamic. An enhanced energy transfer can be induced from plasma species to the catalyst surfaces by reactions with ions, electrons, and excited species. However, it is unclear how each individual plasma species affects the catalysts prepared. An understanding of the plasma catalyst preparation process is still in the early R&D stage. There are many opportunities in plasma chemistry for catalyst preparation. Recently, there has been some nice progress in experimental and theoretical studies on plasmaenhanced catalytic reactions, with which the plasma reactions occur simultaneously during the catalytic reactions.28−36 Note that, in the plasma catalyst preparation, the plasma is used only for the catalyst preparation and is not intended to be directly involved in catalytic reactions. The achievements or methods of the plasma catalytic reactions, however, can be applied to studies on plasma catalyst preparation. For example, optical emission spectra were employed by Tu and his co-workers to analyze the chemical intermediates.29 They can be used for the analysis of important intermediates during the plasma catalyst preparation. In addition, in situ DRIFTS-MS33 is a very useful tool for plasma catalyst preparation. Bogaerts, Neyts, and coworkers found that a large number of H atoms can be dissolved (absorbed) in the metal catalyst during the plasma hydrocarbon conversion.37 This conclusion can explain why hydrogen plasma causes an enhanced catalyst reduction. One typical issue for the plasma catalyst preparation is that the solid immersed in the plasma is normally a porous material. Only a few papers in the literature have dealt with porous materials in plasmas. A study by Neyts, Bogaerts, and coworkers38 suggests that plasma species can be formed inside the pores of the structured catalysts, with pore sizes above 10 μm. The ionization mainly takes place inside the pore, due to the strong electric field and high electron temperature. The ionization is more pronounced for larger pores and at higher applied voltages, leading to a high plasma density near and in the pore, as observed by Hensel et al.39,40 If the pore size is 10 μm or lower, no significant ionization takes place inside the pore. The plasma species might still be able to penetrate into the pore due to diffusion and migration and then interact with the catalyst surface.38,41 A recent study indicated that microdischarges can be formed inside both micrometer- and nanometer-sized pores, yielding ionization inside the pore. For the micrometer-sized pores, the ionization mainly occurs inside the pore, while for the nanometer-sized pores the ionization is strongest near and inside the pore.42 Some porous catalysts such as zeolites possess an intense “natural” electric field (much stronger than any electric field used for the generation of gas discharges). This kind of “natural” electric field can induce a strong back-corona when the zeolite is placed in the corona discharges, which can help the plasma preparation of zeolitebased catalysts. The plasma can be used for any step of the catalyst preparation, including the preparation or treatment of the support, drying, decomposition, oxidation, reduction, and activation and also the regeneration of deactivated catalysts.3 It is excellent for surface treatments. The plasma treatment of the catalyst surface includes both a temporary treatment and permanent treatment. A temporary treatment just induces some changes in the surface by plasmas. A typical example is the

Figure 3. Comparison of thermal calcination (a) and plasma catalyst preparation (b).

thermal calcination and plasma catalyst preparation. The newly formed nanoparticles in the plasma are primarily negatively charged. This prevents particle agglomeration and enables the production of plasma-generated nanoparticles with particle sizes less than those from the thermal methods. Narrow size distributions have also been observed from the catalysts prepared via plasmas. The electrons in the plasma play an important role in catalyst preparation by direct surface reactions. The electrons can recombine with metallic ions to form metallic nanoparticles,3,24 similar to the reduction of metal ions by an electron beam.25 A convenient electron reduction with non-hydrogen gas discharge (such as helium and argon) as a cheap electron source has been thus developed for those metal ions with positive standard electrode potentials (which can attract electrons easily).3,24 In addition, the electrons induce many other reactions or effects in the catalyst preparation with plasmas. Electron collisions with molecules26 create highly reactive growth precursors, making the nanoparticle growth largely irreversible and driving the reactions at low temperatures, thus allowing materials to grow far from their chemical equilibrium. Electrons can react with water (even if only in trace amounts on the catalyst support) to generate hydrated electrons and radicals, which reduce the metal ions (the catalyst precursors) to form nanoparticles. Although the setup and operation of the plasma for catalyst preparation is typically not complicated, the reactions that occur are complex. Of particular importance are electroninduced reactions.26 All of the chemicals involved, as well as moisture and decomposition products of the catalyst precursors, can react with electrons, ions, and molecules and induce further reactions. For example, the products of the reactions between electrons and ethanol in an argon plasma may be the major reducing agents for the reduction of palladium ions.27 If hydrogen plasma is employed, a synergistic reduction effect will be achieved, since both hydrogen radicals and electrons are strong reducing agents for the reduction of metal ions. The hydrogen radicals can readily be generated by the dissociation of hydrogen via electron collisions. If a nonhydrogen plasma is used, the electron-related reactions will be more complex. In addition to the electron-induced reactions as mentioned above, the reactions among atoms, ions, and molecules26 may also contribute to the plasma catalyst preparation. Moreover, photochemical reactions may also take place, which may affect the catalyst preparation. The rate of all 2095


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load noble-metal particles into channels of ordered porous materials, such as SBA-15, with no need for complex chemical modification.53 Nanowires can be directly made for gold (Figure 4) and silver.53 The length of the produced Au and Ag

hydrophilic modification. One can use air plasma to treat a hydrophobic surface (such as a Nafion membrane) to become hydrophilic temporarily. This temporary modification is very helpful for the impregnation or ion exchange with hydrophobic catalysts or supporting materials. No hazardous chemicals are needed for this modification. On the other hand, permanent treatment is normally needed when the catalyst is to be chemically functionalized or to be doped with plasmas, when the catalyst precursor is to be decomposed, or when the catalyst itself needs to be reduced by the plasma or to be treated for other reasons. Since the plasma is full of reactive species, the nucleation and crystal growth under plasma are very different from growth in a thermal reactor. A fast nucleation but a slow crystal growth under cold plasmas has been observed for the preparation of noble-metal catalysts with plasma reduction.3,24 The lowtemperature operation of cold plasmas determines the unique kinetics of crystal growth. If a thermal plasma is applied, a synergy effect can be induced with the combination of a thermal effect and plasma effect. A typical advantage of cold plasmas is their low operating temperature. However, there are some potential negative effects as well. Because of the low-temperature operation, the residues from the plasma decomposition or the plasma reduction will remain on the catalyst or the catalyst support. Part of the undecomposed precursors may cause distortions of the structure generated by the plasma. If moisture is present, a specific catalyst precursor hydrate has been observed.43 Therefore, in many cases, thermal treatment6 or washing44 has to follow the plasma operation. For Ni and Co catalysts, metal oxides are normally obtained. They have to be thermally reduced by hydrogen at elevated temperatures.44−46 These issues can be avoided by using external heating or by improving the plasma-generating methodologies or changing the plasmagenerating gas.47 Interestingly, thermal treatment or calcination of the plasma-treated samples always leads to catalysts with different particle sizes and different structures.48,49 This means that the plasma treatment has already caused significant permanent changes in the catalyst. Additionally, one can directly use metal rods (on a length scale of 10−3−100 m), metal particles, or graphite rods as the catalyst precursors via thermal plasma evaporation or plasma deposition. This will avoid the potential problems created by the impurities of the conventional catalyst precursors such as nitrates. This kind of plasma preparation can be also used to fabricate core/shell metal oxide structures in a rapid way.50 In comparison to the conventional ways, the plasma method is simpler and more convenient. This may create new ways to prepare core/shell catalysts. In the following discussion, we explain how plasmas operate and illustrate their use in the context of catalyst preparation.

Figure 4. Au nanowires from room-temperature plasma reduction.

nanowires is determined by the loading amount of the metal. The room-temperature operation makes it very useful for the preparation of noble-metal catalysts supported on thermal sensitive substrates such as porous organic materials,54 fibers,55 conducting polymers,56 ultrahigh-surface-area carbon,57,58 gels,59 and peptides.60−63 A floating metal composite can be even made on water by room-temperature electron reduction,64,65 which leads to a moving catalyst system on the liquids. Room-temperature electron reduction has also shown its ability to prepare bimetallic noble-metal catalysts with very different structures, in comparison to the bimetallic catalysts obtained by hydrogen reduction. For example, a Pd-rich shell/ Au-rich core structure with abundant surface coordination unsaturated Pd atoms has been created by room-temperature electron reduction with well-dispersed Au−Pd nanoparticles.66 The obtained AuPd bimetallic catalyst shows a high activity in the selective oxidation of benzyl alcohol with a rate constant of 0.50 h−1, which is 12.5 and 2 times higher than those of Au and Pd monometallic catalysts, respectively. Moreover, highly active and stable carbon-supported PtPd alloy catalysts were recently prepared by room-temperature electron reduction.67 The alloy nanoparticles thus prepared show a particle size of around 2.6 nm and a core−shell structure with Pt as the shell. With this structure, the breaking of O−O bonds and desorption of OH are both promoted for the oxygen reduction reaction (ORR). In comparison with the commercial Pt/C catalyst prepared by conventional methods, the mass activity of the Pt-Pd/C catalyst for ORR is shown to increase by a factor of ∼4. After a 10000cycle durability test, the Pt-Pd/C catalyst is shown to retain 96.5% of its mass activity,67 which is much more stable than that of the commercial Pt/C catalyst. As described above, electrons tend to react with molecules and other species before they reach the metal ions for the reduction. Therefore, room-temperature electron reduction using glow discharge as the electron source is normally operated under vacuum. It can effectively reduce the metal ions with positive standard electron potentials. For metal ions with negative standard electron potentials, hydrogen plasma or other reducing plasmas have to be used. In this regard,

3. PLASMA REDUCTION An important application of plasmas is the reduction of metal ions for catalyst preparation. Electrons, hydrated electrons, hydrogen radicals, and many other reducing active species within plasmas can lead to a rapid reduction of metal ions.3,24,27 Using a non-hydrogen glow discharge as the cheap electron source, a room-temperature electron reduction has been developed for the reduction of metal ions, as summarized in our previous review articles.3,24 This room-temperature electron reduction is excellent for size control with fast nucleation and slow crystal growth.51,52 It can be used to 2096


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Gehl et al.76 successfully applied an rf plasma under vacuum to completely remove the organic ligand sphere from spincoated arrays of cobalt−platinum particles. An et al.77 applied a liquid plasma to remove hexadecyltrimethylammonium bromide (CTAB) for the production of hydrogenated TiO2 photocatalysts with high crystallinity and porous structure suitable for solar irradiation. To do so, they first made an amorphous TiO2/CTAB aggregate. An electrode system with a gas injection channel for the introduction of oxygen gas was applied to initiate the plasma underwater. During the plasma removal of CTAB by reactive plasma species, hydrogenated TiO2 crystals were formed at the same time. The obtained catalyst shows high photocatalytic efficiencies for degradation of reactive black 5, rhodamine B, and phenol under solar light irradiation, up to 10 times higher than those of commercial TiO2. Moreover, cold plasma has been applied to clean the catalyst surface via the removal of contaminants. Catalysts are normally porous nanomaterials and nanoparticles with large surface areas. They tend to adsorb/absorb contaminant molecules on their surfaces, because of their inherent high surface energy. The contaminants sometimes induce a significant negative influence on the catalytic properties and have to be cleaned. Chen et al.78 used atomic oxygen, generated in oxygen plasma, to remove the graphitic carbon on the tungsten carbide (WC) in order to synthesize high-quality and carbon-free WC with improved performance for electrocatalytic methanol oxidation and hydrogen evolution. Duriyasart et al.79 employed a nonthermal plasma jet to clean and activate porous TiO2 nanoparticle assemblies. To do so, a helium atmospheric pressure plasma jet was generated in ambient air. Highly active oxygen species are thus formed by the reactions between the plasma species (such as He+ ions, metastable He, and electrons) and ambient air. •O and •OH radicals were considered to be the active species because of the existence of O2 and trace moisture in air. Both are capable of oxidizing contaminants. A significant advantage of the plasma jet is that it can penetrate deep inside porous materials. In addition, it is operated at room temperature and atmospheric pressure, which would cause less damage to fragile materials. For the 100% removal of the contaminants, 1 h of treatment was found to be sufficient.79 Poudyal et al.80 reported a surface cleaning of SmCo5 nanochips and CoFe2O4 nanoparticles using a capacitively coupled pulsed rf discharge. Nanoparticle aggregation during the plasma treatment was found to be reduced when glass beads were included with the nanoparticles during treatment. A very important application of plasma template removal is in the synthesis of various microporous and mesoporous materials. Liu et al.81−83 recently developed a template removal technique using a DBD plasma in air or oxygen for the synthesis of zeolites. Characterization of the materials confirms that the dissociation of template molecules by active species (such as electrons and excited oxygen species) and oxidation of radicals (from the dissociation) by active oxygen species (such as ozone and excited oxygen species) are the major reactions. The infrared image confirms that the DBD template removal is conducted at a temperature of around 125 °C. The thermal effect can thus be ignored. This DBD method has also been applied to the removal of the carbon template for the synthesis of ZrO2 at relatively low gas temperature (less than 150 °C) with a monoclinic structure.13 During the DBD removal of the carbon template, a spark was observed, leading to micro-

Karuppiah and Mok applied Ar/H2 DBD plasma for the reduction of precursors of Ni/γ-Al2O3 and CeO2-Ni/γ-Al2O3 catalysts.68 With a 4 h reduction, highly active ultrafine Ni nanoparticles with uniform dispersion were achieved by this plasma reduction. The use of CeO2 further enhances the Ni− support interaction. Benrabbah et al.69 reported a hydrogen DBD plasma reduced Ce0.58Zr0.42O2-supported Ni catalyst for CO2 hydrogenation at low temperature. The plasma-reduced catalyst shows an enhanced metal−support interaction, which improves the activity slightly. However, if the H2/Ar DBD plasma is applied for the reduction of supported NiO catalyst, instead of the precursor, it does not change the size of Ni nanoparticles.70 Additionally, Teng et al.71 reported a hydrogen hot filament plasma reduction of TiO2 powders to produce black TiO2. Black TiO2 possesses a greater solar absorption, especially in the visible and near-infrared region, with increasing numbers of active sites, which ensure excellent photocatalytic activity for the black TiO2 in the photo-oxidation of organic molecules in water. It was confirmed that the formed oxygen vacancy and the formation of Ti−H bonds are the major causes forming a disordered layer and the coloration of black TiO2. Qi et al.72 reported a DBD plasma reduction of Pd/C catalyst, followed by hydrogen treatment at 300 °C. The obtained catalyst shows excellent catalytic activity for CO oxidation. The plasma reduction with thermal treatment under hydrogen can completely remove the residual Cl ions and give predominantly metallic Pd nanoparticles without aggregation. Hydrogen plasma has also been used to handle liquid systems. For example, Luo et al.73 used a solution plasma assisted hydrogen reduction in the ionic liquid 1-butyl-3methylimidazolium chloride ([BMIM]Cl) as a medium and KClO4 as a {100}-facet director for the preparation of facecentered-cubic indium nanocubes. Nanocubes of average size 6.8 nm were obtained with excellent plasmonic catalytic activity toward hydrogen generation from aqueous N2H4. In addition to the enhanced catalyst dispersion, cold plasma reduction has been also applied to the surface enrichment of the catalyst. Di et al.74 reported a plasma reduction of Pd/FeOx catalysts with DBD plasma, initiated under ambient conditions. A mixture of argon and hydrogen was applied as the plasmaforming gas with 50% hydrogen. The plasma-reduced catalysts show the surface enrichment of Pd species, a larger pore diameter of the FeOx support, and abundant oxygen vacancies, which cause an enhanced low-temperature activity for CO oxidation. Desorption of the hydroxyl radicals and gas-phase reactions can also significantly accelerate reduction under discharge plasma conditions.23

4. PLASMA OXIDATION AND DECOMPOSITION 4.1. Plasma Removal of Organic Ligands, Template, and Contaminants. Organic molecules, such as polyvinylpyrrolidone (PVP), are frequently used for the stabilization of nanoparticles or as a solvent, surfactant, or template during the synthesis of nanoparticles and micro-/mesoporous materials. The template has to be removed for the synthesis of micro-/ mesoporous materials. In some cases, the organic ligand has some negative effects and must be removed.75 Removing the ligand molecule or template is a challenging task, since the close-packed layers form a complex, difficult to clean surface morphology with narrow gaps, cavities, and curvatures with small radii.76 In some cases, the template removal thermally destroys the porous structure of the material. Cold plasma removal can be a promising alternative to thermal treatment. 2097


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present in the inner part of the crystal because the ordered arrangement of atoms cleaves suddenly at the crystalline surface.88 In addition, the calcination process may cause aggregation of zeolite nanoparticles from the high-temperature-induced Si−O−Si bridging. Therefore, the template removal by thermal treatment at elevated temperatures may not always be suitable. In this case, the template removal by cold plasmas is an excellent alternative. El-Roz et al.88 reported such a cold plasma removal under low pressure to remove the organic template (TEA cations) from Beta zeolite nanocrystals. A complete template removal within several minutes is achieved. The method was also employed for the preparation of the H form of the zeolite from the ammonium form. They found that water molecules adsorbed in the microporous structure play an important role in the degradation of the organic template because of the generation of very reactive oxygen-containing species. 4.2. Plasma Oxidation. Plasma oxidation can be operated at lower temperatures and avoids the potential problems (sintering) of conventional thermal oxidation. Plasma oxidation can be applied for the regeneration of coked catalysts with no significant change in the catalyst structure.89 Atmosphericpressure cold plasma oxidation was also employed to activate Au/TiO2 coated on a glass substrate.90 The activated catalyst was applied to the photocatalytic oxidation of CO in air under visible light (λ >420 nm). It was confirmed that the oxygenactivated catalyst shows enhanced photocatalytic activity, although argon plasma activation yields the same Au particle size (4 nm). The oxygen plasma activation results in a high content of surface oxygen, which favors the formation of superoxide ions (O2−) by accepting hot electrons, generated from the absorption of visible light through the local surface plasmon resonance of Au NPs. A unique application of cold plasmas is the surface oxidation of metal-based catalysts. It can change the top surface of the metal into porous nanostructured materials, which is a basic requirement for catalyst applications. In this regard, Gao et al.91 applied a low-pressure oxygen plasma for oxidation of polycrystalline Cu foils. A highly roughened surface with obvious pore structures and high oxygen content (∼41%, atomic percentage) was formed. The enhanced adsorption of halides with higher coverage has been achieved. This further enhances the adsorption of carbon dioxide and stabilizes the carboxyl intermediate via the transfer of charge from the Cu surface to CO2. An enhanced electrocatalytic performance was achieved.91 A similar work has been reported by Koh et al.,92 who applied the oxygen plasma to create nanoislands on gold film for an enhanced electrocatalytic reduction of carbon dioxide to carbon monoxide. Additionally, Mistry et al.93 carried out a plasma oxidation pretreatment of silver foil, which enhances the number of low-coordinated catalytically active sites and dramatically lowers the overpotential. An enhanced activity of CO2 electroreduction to CO has been achieved. 4.3. Plasma Decomposition. A basic operation for catalyst preparation is to decompose catalyst precursors such as nitrates. The conventional decomposition is normally conducted at elevated temperatures in the absence/presence of air or oxygen. This process is also called “thermal treatment” or “calcination”. Decomposition, volatilization, and phase transitions can take place simultaneously during the thermal calcination. Plasma decomposition can be a good alternative to thermal decomposition. Both thermal and nonthermal plasmas can be employed. Thermal plasma decomposition combines the

combustion. It has been used for the preparation of ZnO with an urchin-like structure.84 This removal of the carbon template can be extended to make various porous metal oxides in a simple and rapid way. Figure 5 shows the color changes during the DBD plasma template removal for the synthesis of MCM41 and ZSM-5.81

Figure 5. Color changes of the samples during DBD template removal. Figure reproduced with permission from ref 81. Copyright 2015 Elsevier BV.

Yang et al.85 reported oxygen glow discharge plasma removal of a polymeric template, Pluronic P123 (EO20PO71EO20), near room temperature (below 50 °C) and in a short operation time (2 h) for the synthesis of SBA-15. The plasma-made SBA-15 exhibits a larger surface area of 1025 m2 g−1 with larger pores and micropore volume in comparison with those by conventional calcination (550 °C and 5 h, 827 m2 g−1). In addition to less structural shrinkage, the plasma-made SBA-15 shows significantly increased silanol density from 5.4 to 6.6−7.6 mmol g−1, which leads directly to higher amine loading from 1.8 to 3.0 mmol g−1 and causes a 77% greater CO2 capacity and 60% higher CO2/N2 selectivity, in comparison to the conventionally treated sample.85 El-Roz et al.86 reported a one-step postsynthesis method for the structural stabilization of germanium (Ge) silicate molecular sieves (Ge-rich BEC-type zeolite) with glow discharge plasma treatment below 120 °C for a short period (∼30 s). During their plasma template removal, partial extraction of Ge from the framework and incorporation of Si and Ti in micropore space occur. The obtained material is further calcined at 550 °C, which completes the incorporation of replacing cations in the zeolite framework. Chien et al.87 used a nitrogen atmospheric pressure plasma jet (APPJ) to remove the structure-directing agent (SDA; tetrapropylammonium hydroxide (TPAOH) was applied as the SDA in this case) from as-synthesized zeolites. The APPJ plasma removal can be completed in less than 60 s, which is much less than that with the existing thermal calcination, whose durations range from hours to days. It was suggested that the active species generated in the nitrogen plasmas readily reacts with the SDA, resulting in defragmentation, which allows the removal of the SDA by convective flow from the APPJ. The highly reactive plasma also results in a pronounced Q3 to Q4 transformation in the pure-silica zeolite MFI.87 An important application for plasma template removal is in the synthesis of nanosized zeolites. Nanosized zeolites are normally thermally sensitive materials with just a few unit cells and a relatively high surface proportion of atoms. Thus, when the zeolite nanocrystals are subjected to high-temperature treatments, these atoms can behave differently from those 2098


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ACS Catalysis plasma effect and thermal decomposition. The operation for thermal plasma decomposition has to be limited to a short period. Long-term operation would cause sintering. For example, Su et al.94 reported a controlled synthesis of MgO and SnO2 with different nanostructures by a direct current arc discharge in a short reaction time. Nanowires, nanobelts, nanocubes, and nanodisks can be controllably synthesized by changing the content of air in the buffer gas. An increasing amount of work has been reported with the cold plasma decomposition, especially with the DBD plasmas. The cold plasma decomposition normally is initiated at room temperature.44,46,95−103 It can be operated near ambient conditions for some cases.104 The DBD-decomposed Ni and Co catalysts usually show higher activity with smaller size (higher dispersion) for Ni catalysts,44,46,97−103 in comparison to those made by thermal decomposition. An enhanced coke resistance has been achieved with DBD-made Ni catalysts for methanation, CO2 re-forming, and steam re-forming.46,99,105 The enhanced coke resistance has resulted in a significantly lower steam/methane feed ratio for steam re-forming of methane,46,106 which is extremely important for energy savings in the production of syngas and ammonia. Table 1 shows an

Figure 6. Images of the Co3O4/HZSM-5 samples by plasma decomposition (10-P, 20-P, and 30-P) and by conventional thermal decomposition (10-C, 20-C, and 30-C) with a comparison of the impregnated sample before decomposition (30-F). 10, 20, and 30 denote the Co3O4 loading in wt %. P represents plasma. C denotes conventional thermal decomposition. Figure reproduced with permission from ref 97. Copyright 2017 Elsevier BV.

Table 1. Conversions of CO and Hydrogen for CO Methanation over Ni/SiO2 Catalysts with 10 wt % Loading of Nickela 10Ni/SiO2-C

10Ni/SiO2-P

reaction temperature (°C)

CO conversion (%)

H2 conversion (%)

CO conversion (%)

H2 conversion (%)

350 400 450

14.2 47.5 64.0

32.2 57.3 70.8

40.5 76.3 70.3

59.4 80.9 76.0

The DBD plasma decomposition can be also applied to the preparation of multimetal oxides. For example, Liu et al.112 prepared MnCeOx catalysts using O2/N2 DBD discharge, initiated at room temperature, with nitrates of cerium and manganese as the precursors. The O2 concentration in discharge plays a key role in determining the catalyst properties. A relatively low O2 concentration (10% was found to be optimal) allows the synthesis of MnCeOx catalyst with optimum physicochemical properties, leading to superior catalytic behavior for NO oxidation with the highest NO conversion of ∼80.5% at 275 °C. The stronger interaction between manganese oxides and ceria with the formation of a poorly crystallized Mn−O−Ce phase was observed. This was considered to be the major reason for the enhanced catalytic properties, together with abundant Ce3+ species and active oxygen species formed by DBD decomposition of nitrates and organics.112 El-Roz et al.113 reported a VCl3 DBD plasma decomposition to make V clusters on nanosized zeolite, such as EMT, FAU, and Beta, followed by the oxygen DBD plasma conversion of the V clusters into highly dispersed vanadium oxide clusters. The thus-prepared photocatalyst shows a high activity for methanol photo-oxidation. It is the high dispersion and low oxidation potential of vanadium sites supported on zeolite Beta that lead to the enhanced activity. In addition, plasma decomposition has a special capability in the creation of nanostructured materials. Jiang et al.114 carried out a plasma-enhanced solid-state low-temperature synthesis of spinel LiMn2O4 with superior performance for lithium ion batteries. rf plasma was applied with a reaction temperature of 500 °C at a pressure of 66 Pa. Lithium hydroxide was used as a lithium source and manganese dioxide as a manganese source with a stoichiometric molar ratio of 1:2. The reaction only takes

a

Abbreviations: C, prepared by thermal calcination; P, prepared with DBD plasma decomposition). Table reproduced with permission from ref 99. Copyright 2013 Elsevier BV.

illustrative comparison of catalytic activities of Ni/SiO2 for CO methanation.99 The DBD-prepared catalyst presents a much higher activity with a significantly improved coke resistance. Figure 6 shows images of the Co3O4/HZSM-5 samples by plasma decomposition and by conventional thermal decomposition.97 From the images, it can be seen that the DBD plasma can effectively decompose the catalyst precursor even at a high Co loading. A slight color difference can be also seen in the samples with 10 wt % loading. The difference is due to the size difference of the catalysts. With a high loading of up to 30 wt %, the color is almost the same. The DBD-plasma-decomposed catalysts have different structures, in comparison to those prepared by thermal decomposition.44,46,95−103 Additionally, it is observed that a very different carbon structure is formed on the DBD-plasmadecomposed Ni catalyst in comparison to that on the thermally decomposed catalyst.6,107−109 In addition to smaller size and different structure, the plasma can induce a large change in the catalyst shape as well. It was confirmed that the DBD decomposition of WO3·nH2O at a total time of 75 min at low temperature leads to the formation of WO3 nanolamella.110 No hazardous chemicals were needed. CuO nanofibers have also been made this way via the decomposition of Cu(OH)2.111 2099


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preparation by plasma deposition is that it can use a metal rod or graphite rod to directly deposit the catalyst on the supporting materials. It can avoid the potential effect of the catalyst precursor or capping agent. The simplicity and high reproducibility of the direct vaporization of metallic materials onto the oxide supports are other advantages of plasma deposition. Therefore, a plasma-deposited catalyst is more suitable for studies on the intrinsic activity of the catalyst.10 Park’s group10 recently reviewed the applications of arc plasma deposition (APD) for catalyst preparation. The greatest advantage of APD is that it is relatively easy to generate a large number of particles with sizes of a few nanometers. It is also easy to control the size of the nanoparticles on the nanometer scale by controlling the APD parameters, such as the number of arc plasma pulse shots, the arc discharge voltage, and the arc discharge condenser capacitance. The APD method provides a simple and easy method for the direct and dry deposition of metallic nanoparticles on a variety of substrates, including twodimensional thin films and three-dimensional powders.10,123,124 Machida and co-workers developed a catalyst preparation method based on pulsed arc plasma deposition.20,125−127 In 2014, they reported a deposition of highly dispersed cerium oxide subnanoparticles onto γ-Al2O3 using a pulsed arc-plasma process.128 The as-prepared and thermally aged Ce/Al2O3 catalysts exhibit much higher activities for CO oxidation in comparison to those prepared using a conventional wet impregnation process. The plasma-deposited catalyst contains coordinatively unsaturated (cus) Ce in a lower oxidation state (3+), which leads to an enhancement in the activity. They also carried out a synchronous dual-mode arc plasma deposition of a very small amount of Cr and Cu (0.07 wt % each) onto CeO2, followed by subsequent thermal aging at 900 °C for 25 h.129 The obtained catalyst shows high catalytic activities for CO−O2 and CO−NO reactions. The activities were superior to or comparable with those of platinum-group-metal catalysts. They found that Cu+ substitutes Ce on the (111) surface of CeO2 to form asymmetric 3-fold oxygen coordination sites, whereas Cr3+ is incorporated into four- or five-coordinated surface sites. Cr3+ increases the surface concentration of Cu+, which plays a key role in CO chemisorption and catalytic oxidation. These synergistic functions of Cu and Cr are possible reasons the catalytic activity is enhanced 3-fold in comparison with that of Cu/CeO2. A nanometer-thickness Rh overlayer on a metal foil surface has also been developed as a highly efficient three-way catalyst.130 Todoroki et al.131 reported the synthesis of Pt-Ni thin-film ORR catalysts on highly oriented pyrolytic graphite (HOPG) supports with low Pt loading. They first prepared Pt-Ni thin films using synchronous APD of Pt and Ni, with which nanometer-sized metal particles can be made via an arc discharge incident on the target materials. Much thinner films were synthesized in comparison to those by magnetron sputtering. Then electrochemical dealloying was used to dissolve excess Ni from the APD-prepared Pt-Ni thin films in 0.1 M HClO4 solution. The nanostructures of the Pt-Ni catalysts obtained via this technique are strongly dependent on the composition of the pristine alloy. In particular, a 2−3 nm sized Pt−Ni nanoparticle stacking thin film (NPSTF) structure was fabricated for the APD-prepared Pt2Ni8 thin film after the dealloying processing. The Pt mass activity of Pt-Ni NPSTF is 10-fold greater than that of typical commercial Pt/C catalysts.131

about 30 min. This work suggests that cold plasma could speed up the syntheses of spinel- and perovskite-like catalysts.

5. PLASMA SPRAY Plasma spray is a kind of thermal plasma preparation technique. It uses a dc arc or rf plasma to generate a stream of hightemperature ionized gas as the spraying source. The catalyst precursor(s), normally in powder form, is (are) carried in an inert gas stream into the plasma jet where it (they) will be heated and react. They are then propelled toward the substrate or condensed immediately into the particles. Liquid or gaseous hydrocarbons can be mixed together in the gas stream as the carbon source if a carbon particle or carbon shell would be needed. The spraying of liquid feedstock in the form of submicrometer particles or chemical precursors in a solvent is now available.115 The plasma spray allows the design of composite materials consisting of different combinations of plasmasprayed catalytic and protective layers with minor limitations on the complexity of the monolithic substrate geometry.116 Abatzoglou’s group developed a plasma spray method for the one-step preparation of carbon-supported Co and Fe catalysts for Fischer−Tropsch synthesis.117−119 They mixed Co or Fe powder (as the catalyst precursor) with mineral oil (as the carbon precursor) to form a homogeneous suspension. The mixture was then introduced directly into the rf plasma spray by a peristaltic pump. Co/C and Fe/C catalysts were directly obtained.118 The performance of these plasma-synthesized catalysts was improved with significantly higher CO conversions, in comparison to the identical C-supported Co and Fe catalysts prepared by impregnation or precipitation. The plasma spray has also been applied to the preparation of TiO2 photocatalysts.120 The high energy of the plasma spray is also very useful for alloy formation. Gulyaev et al.121 adapted the plasma arc spray method for the direct synthesis of palladium−cerium−carbon composite (PdCeC). The composite was then calcined with carbon burnout in air at 600, 700, and 800 °C. Such a synthesized PdCeOx composite shows a high activity for oxidation of carbon monoxide with the ability to oxidize CO at temperatures as low as room temperature. The TOF value over the plasma-made composite is 2−3 times higher than that of the catalyst prepared by chemical methods. 6. PLASMA DEPOSITION Plasma deposition for catalyst preparation includes direct plasma deposition, plasma-enhanced atomic layer deposition (PEALD), plasma-enhanced chemical vapor deposition (PECVD), and others. Direct plasma deposition uses both thermal plasma and cold plasma based on the catalyst precursors or the deposition processes. Liquid cold plasma can be also applied to the plasma deposition. In the discussions below, “plasma deposition” refers to direct plasma deposition. An important application of thermal plasmas is for the deposition of metal(s) or metal oxide(s) or related compounds on the porous materials for catalyst applications. With the plasma deposition, the metal to be deposited serves as the cathode, where the metal evaporates, is accelerated, and is deposited onto the substrate. Plasma deposition is usually applied to the coating but a new setup has been able to handle powder samples.122 With an improvement in the plasmagenerating system, it can now be used to deposit nanoparticles onto porous substrates. A big advantage of the catalyst 2100


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density of 30 mA/cm2, and a photovoltage greater than 600 mV were achieved under simulated solar illumination. Sustained photoelectrochemical water oxidation was observed with no detectable degradation after 24 h. The reduced silicon oxide thickness provides more intimate interfacial contact between the light absorber and catalyst, which was suggested to be the reason for the enhanced performance of the nanotextured structure, in comparison to planar Si. In addition, plasma deposition has been applied to deposit the catalyst on the metallic support. For example, PECVD has been applied to deposit cobalt spinel (Co3O4), ruthenium oxide (RuO2), mixed oxides (RuO2/Co3O4), and iron oxide (Fe2O3) on knitted wire gauze, made from Kanthal steel (wire diameter d = 0.11 mm) with Fe (73.7%), Cr (21%), Al (5.3%), and trace amounts of Mn, Ni, Cu, and Co.141 The obtained catalysts were tested for CO2 methanation. The RuO2 catalyst possesses the highest yield of methane, while the Fe2O3 catalyst favors CO production through a reverse water-gas shift reaction. PECVD was recently applied to the fabrication of NiOx and NiOx(OH)y thin films on titanium as substrate using either oxygen or air as the plasma-forming gas.142 The substrate was etched in HCl for 3 min and washed in deionized water before deposition. Ni(II) acetylacetonate was used as the catalyst precursor. The organic precursor was introduced into the reaction chamber using N2 as the carrier gas. Oxygen or air was introduced by a separate supply line simultaneously. The decomposition of the organic precursors takes place in either an inductive N2/O2 or N2/air plasma. The enhanced activity of NiOx(OH)y films for the oxygen evolution reaction is related to the amount of hydroxide moieties on the electrode surface: the higher the content of hydroxide moieties, the more accelerated the conversion to the active Ni(OH)2/NiOOH precursor phase and the lower the resulting overpotential. PECVD was also used to grow vertically oriented graphene (VOG) from the carbon-containing precursor on a plain carbon rod.143 The obtained VOGs were then applied to physically separate and improve the stability of electrodeposited Ag nanoparticles. In this way, the coalescence and agglomeration of silver nanoparticles is avoided, thereby enhancing the electrocatalytic stability. Microwave plasma-enhanced chemical vapor deposition (MPECVD) is a frequently used technique for the preparation of various carbon materials, such as diamond and CNT. MPECVD was recently applied to the synthesis of selfsupporting porous CoNPs@C nanosheets as efficient and stable bifunctional electrocatalysts for water splitting.144 To do so, electrochemical deposition was carried out to deposit the Co species nanosheet precursor on carbon cloth. Then the asprepared Co species nanosheet precursor was converted into CoNPs@C nanosheets by MPECVD with methane plasma. The CoNPs@C electrode shows high HER and OER activity with long-term stability and is thus proposed as an excellent bifunctional catalyst for the overall water splitting.

Moreover, pulsed APD is excellent for catalyst preparation in a size-controllable way. It has been also employed to fabricate uniformly dispersed FeOx atomic clusters on a porous carbon substrate.132 The obtained FeOx/C composite was tested as a cathode material in a rechargeable Li-O2 battery under different current rates. A significant improvement in battery performance in terms of both cycle life and reaction rate has been achieved. In addition, such a prepared cathode material stabilizes the cathode and reduces side reactions. Metal oxide supported metal catalysts have very important applications in industry. How to optimize oxide-supported metal catalysts remains a great challenge because of the difficulty in the preparation of catalysts with controllable structure. In this regard, Ma and co-workers applied the arc plasma deposition method to make an excellent model Pt/ NbOx/C catalyst with tunable structural and electronic properties for the ORR.133 From this model catalyst, some fundamental issues have been clearly investigated. They confirmed that Pt interacts with the Nb in unsaturated NbOx owing to the oxygen deficiency in the metal/metal oxide interface, whereas Pt interacts with the O in nearly saturated NbOx and further interacts with Nb when the oxygen atoms penetrate into the Pt cluster. They also confirmed that the Pt− Nb interactions do not benefit the inherent activity of Pt toward the ORR and the Pt−O interactions improve the ORR activity by shortening the Pt−Pt bond distance. Pt donates electrons to NbOx in both Pt−Nb and Pt−O cases. The resultant electron efficiency stabilizes low-coordinated Pt sites, thereby stabilizing small Pt particles. Recently, a one-step solution plasma process (SPP) was developed to make carbon-encapsulated highly pure Ni, Co, and Fe nanoparticles (MNPs/C) without any additional reductant, agent, or treatment, using benzene as the carbon source.134 A pair of Co, Ni, or Fe rods with diameters of 1 mm (purity 99.9%) was used as electrodes of the discharge plasma and also as precursors of MNPs. The obtained MNPs have an average diameter of 5 nm with good crystalline structure. Carbon capsules with spherical shapes (containing onion-like layers) were characterized by uniform sizes (ranging from 20 to 30 nm). This method has also been applied to the preparation of carbon-supported NPs.135−138 For PEALD, Su et al.139 used it to deposit Cu3N nanocrystals on the commercial CNTs with a length and outer diameter of 3−12 mm and 20−40 nm, respectively. Copper(II) hexafluoroacetylacetonate (Cu(hfac)2) and NH3 gas were used as precursors of copper and nitrogen, respectively. Ultrahighpurity nitrogen was used for purging and as a carrier gas for the copper source. A 13.56 MHz radio frequency power source was used to generate the plasma for the PEALD. The particle size and loading of Cu3N were precisely controlled by the number of ALD cycles. The deposited Cu3N@CNT catalysts are more ORR active under alkaline conditions, showing comparable mass activity with respect to the commercial Pt/C catalyst. Such highly efficient ORR catalysts follow a mixed two- and four-electron ORR process because of the synergistic effect of the highly selective two-electron ORR process by the Cu3N nanoparticles and pseudo four-electron ORR process by the nitrogen-doped CNTs. PEALD has been applied to deposit cobalt oxide onto nanotextured p+n-Si devices, which enables efficient photoelectrochemical water oxidation and effective protection of Si from corrosion at high pH (pH 13.6).140 A photocurrent density of 17 mA/cm2 at 1.23 V vs RHE, a saturation current

7. PLASMA SYNTHESIS OF SULFIDES, NITRIDES, PHOSPHIDES, AND OTHER SPECIFIC COMPOUNDS The catalyst applications of sulfide, nitride, phosphide, and other specific compounds have drawn increasing interest worldwide.145−149 Plasmas have found extensive applications in the synthesis of sulfides, nitrides, and phosphides in a rapid way with less use of chemicals. In particular, plasmas can be applied to the syntheses of some compounds that cannot be easily obtained using conventional methods.150 2101


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ACS Catalysis Ouyang et al.151 directly converted metal Ni foam into hierarchical 3D nickel nitride (hNi3N) by a facile and simple radio-frequency N2 plasma activation technique. In their work, commercial Ni foam is used as the nickel source and nitrogen gas as the nitrogen source. A short duration (≤15 min) and relatively low temperature (≤450 °C) are required to achieve highly purified hNi3N. The as-obtained hNi3N presents a remarkably porous structure with high surface area and homogeneous coral size distribution. Such an Ni3N electrode has exhibited remarkable oxygen evolution reaction (OER) catalytic activity and superior long-term stability. This group also used the N2 rf plasma treatment of NiMo alloy films to create 3D hierarchical porous nickel molybdenum nitride (NiMoN) on carbon cloth for the electrocatalytic HER.152 They found that the plasma treatment allows the synthesis of bimetallic nitrides in considerably shorter time and lower reaction temperature. Because of the high roughness factor, superior mass, and electron transport, and possibly a synergistic effect of the Ni, Mo, and N, the obtained NiMoN catalyst exhibits outstanding HER performance with a small overpotential of around 109 mV to acquire a current density of 10 mA cm−2 with an outstanding durability at different current densities. Figure 7 shows schemes of the fabrication process of

subsurface reduction reactions needs to be minimized to obtain WS2 with well-controlled composition (S/W ratio of 2). Transition-metal phosphides are promising high-performance catalysts. However, the syntheses of these phosphides normally need reactions at high temperatures (>1000 °C). Wang and co-workers156 applied hydrogen DBD plasma to synthesize supported and unsupported phosphides under mild conditions. The active species within the hydrogen plasma can reduce not only the metal oxide precursor but also the phosphorus oxide. The oxide precursors can thus be converted into metal phosphides in a short period. Liang and Alshareef157 recently summarized their PH3 plasma-assisted rapid preparation of transition-metal phosphides (TMPs). In comparison to conventional synthetic methods, a wide spectrum of precursors including metals, hydroxides, and oxides can be directly converted into TMPs under plasma treatment at low temperature (100−300 °C) and in a rapid manner (30 s to 20 min). Various monometallic, bimetallic, or even more complex TMPs with tailored nanostructures can be easily synthesized. Figure 8 shows the structural characterization of NiFe-DH, NiFeP, and α-NiFeOH/NiFeP on Ni foam fabricated with PH3 plasma.158 The obtained amorphous NiFeOH/NiFeP/Ni foam shows enhanced performance for the OER because the intense electronic interactions between NiFe hydroxide and NiFeP significantly lower the adsorption energy of H2O on the hybrid.158 As a result, the hybrid catalyst can deliver a current density of 300 mA cm−2 at an extremely low overpotential (258 mV, after Ohmic-drop correction), along with a small Tafel slope of 39 mV decade−1 and outstanding long-term durability in alkaline media. Because of their high-energy characteristics, plasmas have been also used for the facile synthesis of other specific compounds such as carbon-based structured materials, fluorides, borides, and others,157,159 which cannot be easily synthesized by conventional methods. For example, Seo et al.160 applied an rf inductively coupled plasma CVD system to make vertical graphene nanosheets from honeycomb with graphite paper and Ni foam as the growth substrates. To do so, the honeycomb was first melted by heating at 100 °C. The substrates were then coated evenly with the liquefied honeycomb. A gas mixture of Ar and hydrogen was used as the plasma-generating gas. After 9 min of processing at 400 °C (due to the plasma-heating effect), graphene nanosheets were obtained. In addition, the cold plasma can be a useful tool for carbonization,161,162 which has important applications in the preparation of carbon materials as catalysts or catalyst supports.

Figure 7. (a) Schematics of the fabrication process of 3D hierarchical porous nickel molybdenum nitride (NiMoN). SEM images of (b,e) carbon cloth, (c, f) NiMo alloy deposited for 7200 cycles, and (d, g) NiMoN treated under N2 plasma for 15 min. Figure reproduced with permission from ref 152. Copyright 2016 John Wiley and Sons, Inc.

the 3D hierarchical porous nickel molybdenum nitride (NiMoN) with the characterization results.152 A rapid synthesis of cobalt nitride nanowires has been also conducted using the N2 plasma treatment.153 Highly efficient and low-cost catalysts for oxygen evolution have been obtained. Zhao et al.154 carried out a 10 min sulfidation of Cd(NO3)2 and Zn(NO3)2 using DBD plasma with 10% H2S/Ar as the plasma-forming gas at ∼150 °C. Highly dispersed sulfide catalysts on Al2O3 have been obtained with high performance for hydrogen production from H2S with reduced energy costs. Groven et al.155 reported a low-temperature PEALD process for 2D WS2 synthesis based on a ternary reaction cycle consisting of consecutive WF6, H2 plasma, and H2S reactions. Strongly textured and nanocrystalline WS2 is grown at 300 °C. They found that the composition and crystallinity of these layers depends on the PEALD process conditions. The hydrogen plasma is essential for the deposition of WS2, as it enables the reduction of −W6+Fx surface species. In addition, the effect of

8. SURFACE TREATMENT Cold plasmas have been extensively applied to catalyst surface treatment or modification. The purposes of such treatment include the generation of surface vacancies, defects, doping, and roughed surface, increase in the active sites, and modification of the surface functional groups. Surface treatment with cold plasmas is a rapid and powerful way to make changes in the catalyst surface. In some cases, a short plasma treatment (4 s) can lead to an enhanced activity that needs 6 h if thermal treatment at 450 °C is applied instead.163 Plasma treatment has many applications in doping as well. Qiu and co-workers reported a nonthermal dielectric barrier discharge (DBD) plasma modification of anatase TiO2 nanosheets near room temperature.164 Bharti et al.165 reported that a simple air plasma treatment of a Fe- and Co-doped TiO2 thin film causes the formation of Ti3+ and oxygen vacancies in TiO2. 2102


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Figure 8. Structural characterization of NiFe-DH, NiFeP, and α-NiFe-OH/NiFeP on Ni foam. (a) XRD patterns of three NiFe-based electrocatalysts. The asterisks mark the diffraction peaks from Ni foam substrate. (b, c) SEM and TEM images of NiFe-DH nanoplates. (d, e) SEM and TEM images of NiFeP nanoplates. (f, g) SEM and TEM images of α-NiFe-OH/NiFeP hierarchical nanostructures. (h) TEM image and SAED pattern (inset) of α-NiFe-OH/NiFeP in the rectangle marked in (g). (i) HRTEM image of NiFeP/NiFe-OH. (j) HADDF image of α-NiFe-OH/ NiFeP and the corresponding EELS elemental maps showing the distribution of Ni, Fe, and P. Figure reproduced with permission from ref 158. Copyright 2017 American Chemical Society.

Tripathi and Islam166 found that a 10 min oxygen plasma treatment of iron catalyst on a P-type silicon (100) substrate can significantly increase the nucleation sites of carbon nanotubes (CNTs), which can change the orientation of CNTs. When the CNT growth was carried out over the substrate, not treated with plasma, a horizontal network of CNTs was formed. Plasma treatment prior to CNT growth leads to the formation of vertically aligned CNTs. Zhu, Jang, and co-workers reported an oxygen DBD plasma treatment of Au/TiO2 catalysts, synthesized via the deposition−precipitation method at atmospheric pressure.167 The plasma-treated Au/TiO2 catalyst has small particle size and narrow particle size distribution with plentiful surface hydroxyl groups. It shows a significant improved activity for CO oxidation. The improved activity is attributed to the large amount of low-coordinated Au species and interface active sites as well as abundant surface hydroxyl groups. Pastor-Pérez et al.168 found that an rf plasma treatment of Pt/CeO2 increases the electronic density on platinum nanoparticles and reduces the ceria surface, leading to a significant enhancement in water dissociation and CO activation for the water-gas shift reaction. An important application of plasma treatment is catalyst preparation with metallic substrates. Yang et al.169 used oxygen rf plasma to treat a thin layer of alumina at room temperature for 15 min, prior to iron nanoparticle formation. The plasmatreated alumina is more resistant to iron bulk diffusion in comparison to untreated alumina, which causes an increase in the height of carbon nanotube forests from ∼0.2 to >2 mm. DBD plasma, initiated in air at room temperature, has been employed to create a roughened surface of antimony tin oxide (ATO) nanoparticles.170 Such treated ATO nanoparticles were then applied to deposit Pt@Pd with high dispersion and narrow

particle size distribution. The Pt@Pd core−shell catalyst was prepared using the polyol method and shows a significant improvement toward ORR activity and durability. Mistry et al.171 used hydrogen and oxygen plasma to treat electropolished polycrystalline Cu foils in short times (less than 10 min). The so-obtained oxidized copper catalysts show lower overpotentials for carbon dioxide electroreduction. A 60% selectivity of ethylene has been achieved. They confirmed that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of Cu+ is the key to lowering the onset potential and enhancing ethylene selectivity. The morphology of the catalyst surface can be easily controlled by the plasmas. Gao et al.91 also reported a low-pressure plasma oxidation of polished Cu foils to produce a roughened surface with porous structures. Plasma is excellent for the treatment of various carbon materials for defect modification, surface functionalization, and others.5,172 Zhong et al.173 etched an iron- and nitrogencodoped porous carbon (Fe-N/C), made from catalytic carbonization of chitin with the assistance of FeCl3 and ZnCl2, using air rf plasma for only 120 s. The etched Fe-N/C shows activity comparable to those of Pt catalysts for the ORR. The plasma etching removes the less stable sp3 and amorphous sp2 carbons, which exposes the more active catalytic FeN4 centers and leads to a transformation of a small fraction of ironbased nanoparticles into FeN4 species. This in turn leads to the enhancement in the catalytic activity. Tian et al.174 applied an rf discharge plasma to create defects on three-dimensional N-doped graphene based materials. A highly active metal-free catalyst for the hydrogen evolution reaction has been obtained. Ding et al.175 employed a nitrogen glow discharge plasma to treat the graphene-encapsulated Pt nanocrystals. Nitrogen doping was successfully achieved. The 2103


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difficult to apply to these materials. However, cold plasmas have been demonstrated as a promising way to do so. El-Roz et al.88 reported the preparation and activation of nanosized zeolites, as discussed above. Yang et al.57,58 applied room-temperature glow discharge for the reduction of Pt ions on ultrahigh-surface-area carbons. Argon rf plasma was applied to the reduction of platinum ions in a chloroplatinic complex anchored onto a polypyrrole chain at room temperature after impregnation.56 It was confirmed that the plasma reduction is more efficient than the chemical reduction with sodium borohydride. The plasma-reduced Pt/polypyrrole catalyst shows a high dispersion and effectively catalyzes the aqueous reduction of nitrates with H2 to yield N2 with a very low selectivity to undesired nitrites and ammonium byproducts. Luo et al.179 used an air DBD plasma, initiated at room temperature, to prepare a 0.320 wt % Mn-doped SiO2 catalyst on paper substrates for cataluminescence sensor applications. A high activity with long durability has been achieved. Plasma has also shown the capability to form porous TiO277 and TiO2 coatings on polymer fibers.180 Baba et al.180 deposited photocatalytic anatase TiO2 thin films at low temperature on polymer optical fibers (POFs) using an atmospheric-pressure microwave plasma process. The TiO2-coated POFs exhibit good photocatalytic activity with a degradation rate constant evaluated at 0.046 h−1. This method overcomes the challenge of forming crystalline transition-metal oxide coatings on polymer substrates by using conventional methods. On the other hand, cold plasma is capable of enabling the complete and fast polymerization of various organic molecules in the solid state within only a few minutes near room temperature.181 Such plasma polymerization has applications to catalyst preparation. Senthilnathan et al.182 synthesized electron-rich acetonitrile polymers (ANPs) using a submerged liquid plasma (SLP) process. The SLP was also applied to dope nitrogen on graphene. The nitrogen-functionalized graphene (NFG) and ANPs were then used to form a Au-ANPs-NFG nanohybrid with ANPs as the reducing agent to directly reduce Au3+ under ultraviolet (UV) light. The pyridinic and pyrrolic nitrogen present in the NFG effectively chemisorbs or binds with ANPs through π−π or σ−π interactions, while the ANPs well control the Au nanoparticle formation in a size of ∼5 nm. The obtained Au-ANPs-NFG nanohybrid shows a good activity for the selective oxidation of benzyl alcohol to benzaldehyde in both suspended and immobilized forms. A room-temperature glow discharge plasma polymerization of biomolecules such as peptides was recently developed.183 Biofilms have been thus obtained in a simple and rapid way. This plasma polymerization of biomolecules can combine with the plasma reduction of metal ions for the preparation of highly dispersed metallic nanoparticle/biomolecule composites.60,184 In this way, the synthesis of biomolecule-stabilized nanoparticles has been successfully developed. In comparison to other organic or polymer stabilizers, biomolecules will not cover the active surface of the nanoparticles. Instead, the Ncontaining and other functional groups of biomolecules may significantly improve the catalyst properties. In addition, the biomolecule can be an excellent doping agent for the preparation of supported catalysts. These composites have been confirmed to be excellent catalysts with small particle sizes. For example, the peptide in the composite can promote electron transfer and also improve CO2 adsorption for photocatalytic water splitting and CO2 reduction with water to CO under irradiation of visible light.62

nitrogen-doped graphene encapsulated Pt nanocrystals show enhanced electrocatalytic performance for methanol oxidation, in comparison to the commercial catalysts and undoped Ptgraphene composite catalysts. Dou et al.176 treated graphene-supported Co9S8 nanoparticles with NH3 rf plasma. They found that NH3 plasma treatment not only leads to nitrogen doping onto both Co9S8 and graphene but also partially etches the surface of both Co9S8 and graphene. The nitrogen doping can efficiently tune the electronic properties of Co9S8 and graphene, while the surface etching can expose more active sites for electrocatalysis. Significantly enhanced electrocatalytic performance for the ORR and OER has been achieved. Especially, the plasmatreated Co9S8/G shows excellent ORR activity, close to that of commercial Pt/C catalysts. Figure 9 shows a scheme of the plasma treatment of Co9S8/graphene.

Figure 9. Illustration of the preparation of nitrogen-doped Co9S8/ graphene. Figure reproduced with permission from ref 176. Copyright 2016 Royal Society of Chemistry.

Li et al.177 also used a hydrogen DBD to treat a mixture of graphene oxide (GO) and boric acid to prepare boron-doped reduced GO hybrid under atmospheric pressure. H2 was chosen as the working gas mainly because the hydrogen radical has a much higher reduction capability in comparison to molecular hydrogen and other working gases (such as Ar and CO2). This method is useful for the preparation of doped rGO catalysts. Gao et al.178 successfully applied the oxygen DBD plasma to modify the internal surface in one-dimensional channels of mesoporous SBA-15 particles. The silanol groups on the surface of the channels are thus intensely activated. After that, a large amount of amine groups can be effectively grafted in a short time (2 h). The highly dispersed amine groups inside the channels can be easily revealed by the location of in situ coordinated Ag nanoparticles. The amine-modified SBA-15 as obtained via the DBD plasma treatment shows a better CO2 adsorption capacity (1.26 mmol/g), while it is only 0.95 mmol/ g with the conventional amine-modified SBA-15 for 18 h modification.

9. PLASMA PREPARATION WITH SOFT AND TEMPERATURE-SENSITIVE SUPPORTING MATERIALS There is an increasing interest in the use of soft and temperature-sensitive materials as catalyst supports. Highsurface-area carbon and nanosized zeolites are also sensitive to heat. The conventional thermal treatment is therefore very 2104


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ACS Catalysis Plasma polymerization is a promising tool for innovations in organic−inorganic hybrid catalysts.62 It will be useful for future catalyst design to mimic the natural way of converting solar energy into chemicals and fuels. Carbonization of the polymer/ metal or polymer/metal oxide is also a great way to prepare carbon-coated or carbon-based catalysts.

10. PLASMA ETCHING AND PLASMA DOPING Nonthermal plasmas are excellent for etching and doping, which further enhance the generation of vacancies and improve the catalytic activity.185 The plasma etching also helps to remove the unwanted species in order to expose more active sites.173 The doping of TiO2 can facilitate the formation of charge trap centers or avoid the formation of charge recombination centers. Bharti et al.165 reported that a simple air plasma treatment of Fe- and Co-doped TiO2 thin film can enhance the optical absorbance and absorption region of TiO2 films, while keeping them transparent. The air plasma treatment causes enhanced optical absorbance and optical absorption region, as revealed by the formation of Ti3+ and oxygen vacancies in the band gap of TiO2 films. With an increase in treatment time from 0 to 60 s, Ti3+ and oxygen vacancies increase in the Fe- and Co-doped TiO2 films, leading to increased absorbance. However, a red shift (from 3.22 to 3.00 eV) was observed in Fe-doped TiO2 films. In contrast, Codoped TiO2 films exhibited a blue shift (from 3.36 to 3.62 eV) due to a Burstein−Moss shift. Qiu et al.164 reported a DBD plasma modification of anatase TiO2 nanosheets with exposed {001} facets near room temperature. Ar, H2, and NH3 were used as the plasma forming gases. They found that the modified TiO2 nanosheets possess a unique crystalline core−amorphous shell structure (TiO2@TiO2−x) with improved visible and nearinfrared light absorption. Both surface Ti3+ and substituted N were doped into the lattice of TiO2 through NH3 plasma discharge, whereas the oxygen vacancy or Ti3+ (along with the oxygen vacancy) was generated after Ar or H2 plasma treatment. TiO2@TiO2−x from NH3 plasma with a green color shows the highest photocatalytic activity under visible light irradiation in comparison with the products from Ar plasma or H2 plasma. This suggests a synergistic effect of reduction and simultaneous nitridation in the NH3 plasma.164 Moafi et al.186 used a glow discharge to treat Ce-doped ZnO synthesized by a precipitation method. They found that the plasma treatment does not change the phase structure of the catalyst but the catalyst morphology is completely changed to a nanoflower by an Ar plasma and to nanorod-like structures by He and N2 plasmas. However, O2 plasma does not change the nanoparticle shape of Ce-doped ZnO but does reduce the catalyst size slightly. The plasma-treated catalysts show enhanced photocatalytic performance for dye elimination. Moreover, metal-free electrocatalysts have been extensively investigated as alternatives to noble-metal Pt catalysts for the oxygen reduction reaction (ORR) and OER in fuel cells or metal−air batteries. These electrocatalysts are usually deposited on a 3D conductive support such as carbon paper or carbon cloth to facilitate mass and electron transport. For practical applications, it is desirable to fabricate catalysts in situ on the carbon fiber support to simplify the fabrication process for catalytic electrodes. Plasmas can be a useful tool for this purpose. Wang et al.187 reported an in situ exfoliated, edge-rich, oxygen-functionalized graphene on the surface of carbon fibers using Ar plasma treatment (Figure 10). In comparison to pristine carbon cloth, the plasma-etched carbon cloth possesses

Figure 10. In situ exfoliation of edge-rich and oxygen-functionalized graphene from carbon fiber by the plasma treatment. Figure reproduced with permission from ref 187. Copyright 2017 John Wiley and Sons, Inc.

a higher specific surface area and an increased number of active sites for the OER and ORR. It also shows good intrinsic electron conductivity with excellent mass transfer performance. They found that the plasma-etched carbon cloth has a low overpotential, comparable that of to Pt-based catalysts, as a result of both defects and oxygen doping. Panomsuwan et al.188,189 directly synthesized metal-free nitrogen-doped carbon nanoparticles (NCNPs) using a solution plasma process. Cyano-aromatic molecules (benzonitrile, 2-cyanopyridine, and cyanopyrazine) were used as a single-source precursor of the catalyst.188 The obtained NCNPs with particle sizes of 20−40 nm show an interconnected hierarchical pore structure with a high specific surface area (210−250 m2 g−1). Different carbon/nitrogen mole ratios of the organic precursors yield nitrogen-doping levels in NCNPs from 0.63 to 1.94 atom %. The obtained NCNPs exhibit a significant improvement in terms of both onset potential and current density under alkaline and acidic conditions for the oxygen reduction reaction (ORR). The predominant distribution of graphitic-N and pyridinic-N sites on NCNPs plays an essential role in enhancing the ORR activity and the selectivity toward a four-electron-reduction pathway. The nitrogen precursors were found to have a significant effect.190 An excellent robust long-term durability and strong methanol tolerance have been achieved, in comparison with those of a commercial Pt/carbon catalyst. Figure 11 shows the formation mechanism of NCNPs via a solution plasma process.188 Undoped carbon nanoparticles can be also synthesized from liquid benzene using the same procedure.188,191 The obtained carbon nanoparticles have a lower ORR activity in comparison to NCNPs. Halogen doping can improve the ORR activity with the use of a mixture of benzene (C6H6) and organics containing halogen atoms as the precursors (i.e., hexafluorobenzene (C6F6), hexachlorobenzene (C6Cl6), and hexabromobenzene (C6Br6)).192 If acrylonitrile is applied, subsequent thermal annealing is needed for a high electrocatalytic activity for the ORR.189 The solution plasma process has been also exploited to improve the large-area growth and quality of 2D nanostructures, such as heteroatom-CNs and graphene.193 The solution plasma process has also been used to synthesize a boron−carbon−nitrogen nanocarbon catalyst with a control2105


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ACS Catalysis

conductivity and create more active defects for the OER. In comparison to pristine Co3O4, the engraved Co3O4 exhibits a much higher current density and a lower onset potential. The specific activity of the plasma-engraved Co3O4 nanosheets (0.055 mA cm−2BET at 1.6 V) is 10 times higher than that of pristine Co3O4. Molybdenum disulfide (MoS2) is a promising alternative to noble-metal catalysts for the hydrogen evolution reaction (HER). It contains active edge sites and an inert basal plane. Activating the MoS2 basal plane can further enhance the HER activity. However, not many ways can be used to do so. In this regard, Nørskov and co-workers197 reported activation and optimization of the basal plane of monolayer 2H-MoS2 for the HER by introducing sulfur (S) vacancies and strain with argon rf plasma treatment. They demonstrated that that strained Svacancies in the basal plane of 2H-phase monolayer MoS2 act as a new highly active and tunable catalytic site for HER. This is the first report of using S-vacancies in the basal plane to introduce gap states, which can be varied using the S-vacancy concentration and elastic strain. With optimized combinations of strain and S-vacancies, the strained S-vacancy sites exhibit a TOFS‑vacancy (at 0 V versus RHE) of 0.08−0.31 s−1, higher than those of the MoS2 edge sites (0.013−0.02 s−1) and [Mo3S4]4+ clusters (0.07 s−1). Layered double hydroxide (LDH) based catalysts have been considered to be a promising for the OER. However, the stacking structure of LDHs causes less exposure of active sites. Liu et al.198 used water DBD plasma to destroy the electrostatic interaction between the host metal layers and interlayer cations, which leads to a fast exfoliation. The etching effect of the plasma can also generate multiple vacanices in the as-exfoliated ultrathin LDH nanosheets. An enhanced activity for the OER has been achieved.

Figure 11. Schematic representations of the formation mechanism of NCNPs via the solution plasma process. Figure reproduced with permission from ref 188. Copyright 2016 Elsevier BV.

lable bond structure for enhanced ORR activity.194 The synergistic effect of N and B in an uncoupling bond state improves the formation of new active sites for the ORR performance by changing the electronic structure of the base carbon. Meanwhile, when B and N are bonded together, the BCN catalyst contributes to a reduced ORR activity by forming a balanced electronic structure in carbon. The BCN nanocarbon with an uncoupling bond state exhibits an enhanced ORR activity under alkaline conditions.194 Tian et al.174 made a highly active metal-free N-doped graphene catalyst for the hydrogen evolution reaction using a self-assembly hydrothermal/annealing method with subsequent rf plasma-etching process. The prepared catalyst possesses a 3D porous network with large surface area, high level of N doping, and abundant structural defects. This catalyst exhibits a low overpotential of 128 mV at 10 mA cm−2, a small Tafel slope of 66 mV dec−1, and a large exchange current density of 1.1 × 10−1mA cm−2 in acidic electrolyte, which are superior or at least comparable to those of most metal-free carbon catalysts and some state of the art transition-metal-based catalysts reported to date. This method has been also employed for the preparation of S-doped graphene catalyst for the HER.195 Additionally, Co3O4, which has the mixed valences Co2+ and Co3+, has been investigated as an efficient electrocatalyst for the oxygen evolution reaction (OER). Proper control of the Co2+/ Co3+ ratio in Co3O4 leads to modifications in its electronic and thus catalytic properties. Wang et al.196 used argon rf plasma to engrave Co3O4 nanosheets at low temperatures. The plasma treatment not only produces higher surface area but also generates oxygen vacancies on the Co3O4 surface with more Co2+ formed. The increased surface area ensures that the Co3O4 has more sites for the OER, while the generated oxygen vacancies on the Co3O4 surface improve the electronic

11. CONCLUSIONS AND OUTLOOK From the discussions above, the following conclusions can be made. (1) Plasma is powerful in catalyst preparation. The plasma is full of active species, including electrons, ions, radicals, excited species, and photons, which can play an active role in catalyst preparation or catalyst treatment. (2) Under the influence of plasma, nucleation and crystal growth for the catalyst preparation are very different from those for the conventional thermal way. Fewer chemicals and less energy are needed with the use of plasma. In most cases, the plasma catalyst preparation takes less time as well. (3) Some thermodynamically unfavorable reactions can easily take place with plasmas, which make the synthesis of unique compounds such as sulfides, nitrides, and phosphides under mild conditions possible. (4) Plasma is a useful tool for oxidation, reduction, decomposition, surface cleaning, surface treatment, template removal, etching, doping, coating, alloying, and others. It can be an excellent alternative when one meets difficulties with the conventional methods. Plasma is typically useful for catalyst formation on metallic substrates and also on temperature-sensitive supporting materials. (5) Using thermal plasmas, one can directly use the pure metal or pure oxide or graphite as the catalyst precursor. This can avoid the possible impurity issues associated 2106


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ACS Catalysis with using conventional catalyst precursors and ensure the reproducibility of the catalyst. It is very helpful for an intrinsic study of the catalyst. (6) A significant benefit of the plasma catalyst preparation technique is the smaller size or greater dispersion of the resulting nickel catalysts with enhanced catalyst−support interaction and improved catalytic properties. The plasma-prepared catalyst can be a model for improvement of industrial catalysts. (7) Plasma polymerization is a promising tool for innovation in organic−inorganic hybrid catalysts. The carbonization of the polymer/metal or polymer/metal oxide is also a great way to prepare carbon-coated or carbon-based catalysts. This will become increasingly important with the development of catalyst preparation via 3D printing.199,200 We have to acknowledge that plasma catalyst preparation is still in the beginning stages of development, although there have been an increasing number of publications and patents on this topic recently. Many challenges remain. The biggest challenge is the poor understanding of the plasma physics and plasma chemistry associated with the nonequilibrium thermodynamics. No tool can be used to measure the density and energy of electrons and other active or intermediate species during the catalyst preparation. In situ catalyst characterization is even more difficult in the presence of plasmas. The present theoretical studies of plasmas have many limitations with porous materials and liquid-containing multiphase systems. Another great challenge is the plasma generation technology. The plasma catalyst preparation deals with materials from fine powders to large particles and even to membranes. All of these materials are full of functional groups and defects with complex porous networks, which require uniform and stable plasma treatments. The plasma generator design must consider the specific requirements of various catalysts. In conclusion, significant progress has been made in catalyst preparation with plasmas since 2000. Much more progress can be expected in the near future on the basis of the accomplishments today. We would anticipate that the following areas or studies will lead to further progress in the development of plasma catalyst preparation. (1) The metallic nanoparticles or the metal oxide nanoparticles generated from the plasma reactions would possess different specific structures with different defect or vacancy status, in comparison to those nanoparticles from the conventional preparations. The plasmagenerated nanoparticles will not only show different reactivities or catalytic properties but also generate many fundamental future studies of defect chemistry. This provides a convenient model system for studies on defect chemistry in a systematic way. (2) Steam re-forming of methane still represents the highest energy input in chemical production. The nickel catalysts produced by plasmas will further show industry what kind of catalyst structure and size will be the best to reduce the steam/methane feed ratio and thereby cut down the energy input for steam re-forming. The plasmaprepared nickel catalysts will be extended to more applications in CO2 re-forming and methanation with increasing studies on Fe and Co catalysts. (3) Plasma catalyst preparation with size, structure, and site control will be an important future direction. Site control

(4) (5)

(6)

(7) (8) (9)

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has not been considered to be an important influencing factor at present. It is challenging to do so with the conventional catalyst preparation approach. With the development of plasma-localized preparation,201 catalysts with site control are becoming a reality. Plasma is excellent for the preparation of catalysts with mixed phases and heterojunctions, especially for photocatalyst preparation. Plasma shows unique capabilities in the preparation of sulfide, nitride, phosphide, alloy, and multimetallic catalysts and others. This will lead to more applications as catalysts in the future. Cold plasma will have more future applications in catalyst preparation with thermally sensitive substrates such as porous polymers, biomolecules, nanosized zeolites, and ultrahigh-surface-area catalysts. Increasing applications of plasmas will be found in the preparation of various nanostructured catalysts, including those with metallic substrates and core−shell catalysts. Plasma will be more extensively applied for doping, coating, and surface treatment of various catalysts. Because of the importance of catalysts, the R&D of plasma catalyst preparation will inspire further investigations of plasma physics and chemistry, which will lead to further fundamental innovation in the sciences.

AUTHOR INFORMATION

Corresponding Author

*E-mail for C.J.: [email protected]; [email protected]. ORCID

Chang-jun Liu: 0000-0001-9918-1638 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A donation from ABB Switzerland is appreciated. Support from the National Key Research and Development Program of China (2016YFF0102503, 2016YFB0600900), the National Natural Science Foundation of China (21536008, 91334206, 21476157, 20990223, 20225618) and the Tianjin Municipal Natural Science Foundation (16JCYBJC19500) is greatly appreciated.

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ACS Catalysis

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