Catalysis is extensively applied in the chemical industries. A catalyst is involved in the production of most of the important chemicals. With depleting petroleum resources, the development of renewable energy and increasing concern about the environment, catalysts will play an even more important role in the future. The use of a catalyst is in the list of the twelve principles of green chemistry ^{[1, 2]}.

Catalysis now has a core comprising the sciences of chemistry, chemical engineering, and materials science. The world market for many catalysts is blooming, and there are significantly increasing publications on their catalysts. Two kinds of catalysts are applied at present: heterogeneous and homogeneous catalysts. A heterogeneous catalyst means that the catalyst is spreaded or dispersed in a different phase, which is normally on a porous solid substrate or a porous solid support material with a high surface area. Alumina, silicon dioxide, titanium dioxide, activated carbon, zirconia, micro- and mesoporous materials, polymer porous materials and others have been used as the substrate. High activity and excellent long term stability with high selectivity are needed for a good heterogeneous catalyst. Catalytic reactions can be affected by many factors including size ^{[3, 4, 5]}, phase structure ^{[6, 7]}, shape ^[7], catalyst-support interaction ^{[3, 6]}, interface ^{[8, 9, 10]}, surface properties ^{[11, 12, 13, 14, 15]}, reaction condition, feedstock and others. In many cases, it is not easy to reach the goal of high activity, high selectivity and excellent stability. Great efforts have been made towards the understanding of the fundamental issues of catalysis and also towards the development of new catalysts and novel catalyst preparation technology that give controllable size and structure in order to achieve the optimum catalytic properties.

One of the challenges in the field of catalysis is its increasing pollution to air, water and land and the high consumption of materials and energy during the preparation of a heterogeneous catalyst. Although the use of a catalyst is one of the green chemistry principles, the catalyst preparation is not really green. Fig. 1 shows the procedure of the preparation and application of a heterogeneous catalyst. Each step can cause pollution or excessive consumption of materials and energy. A hazardous chemical or hydrogen is required for the reduction of the catalyst. Also, present catalyst preparation is time consuming. A quick preparation is desired. Catalyst preparation in a green way is necessary. The objectives of green catalyst preparation should follow the 12 principles of green chemistry: (1) prevent waste; (2) the reactions used must meet the requirement of atom economy as much as possible; (3) less hazardous catalyst preparation; (4) design benign catalyst or catalyst precursor; (5) use of benign solvents and auxiliaries; (6) design energy efficient catalyst preparation; (7) use of renewable feedstocks as much as possible; (8) reduce derivatives, which would become necessary with the development of organic porous materials; (9) consider the applications of the catalyst prepared; (10) design catalyst that can be easily re-generated when deactivated; (11) develop inherently benign catalyst preparation to prevent accidents; (12) develop real time analytic technology or in situ characterization for catalyst preparation. Some of these objectives are long-term ones that cannot be achieved easily. For example, most of the catalyst preparation methods now rely on a trial and error approach. It is still a challenge to design the catalyst from the beginning, although theoretical studies have made some progresses ^{[5, 15, 16]}. There are many works conducted to meet the objectives above. Innovation in catalyst preparation has been a hot topic for a long time. Microwave heating ^{[17, 18]}, use of plasmas ^{[3, 19, 20, 21]}, ionic liquids ^[22], ultrasonic treatment ^[23], electron beams ^[24], electrostatic field ^[25], biochemicals ^[26] and others have been employed and developed. Among these innovations, the use of plasmas has received remarkable attention. Patents and publications with the keywords of plasma and catalyst have recently increased significantly. In this article, we summarize the progress in the plasma methods of green catalyst preparation. Differences between plasma preparation and thermal treatment are discussed. Future development is addressed.

Fig. 1. Preparation and application of a heterogeneous catalyst.

With the supply of sufficient electrical energy to a gas, the gas will be ionized and an electric gas discharge or discharge plasma will be created. Based on the energy of the discharge plasma, thermal plasma or cold plasma is generated. Thermal plasma is an equilibrium one where the bulk temperature reaches several thousands of degrees Celsius. Cold plasma is a non-equilibrium one where the bulk temperature remains as low as room temperature but the electron temperature reaches several thousands of degrees Celsius or even more ^{[20, 27]}. Both thermal plasma and cold plasma have been found useful for catalyst preparation. Thermal plasma is normally applied for the preparation of oxide support materials in a rapid way with a smaller particle size compared to the use of thermal treatment ^[27]. The cold plasma has been more investigated for catalyst preparation because of its low temperature operation with highly energetic electrons. Based on the electrode configuration and operation condition, cold plasmas can be very different. Glow discharge and dielectric barrier discharge (DBD or silent discharge) are two conventional cold plasma phenomena that have been employed for heterogeneous catalyst preparation. Glow discharge can be easily created in a low vacuum (e.g., 100 Pa) by putting two electrodes into the gas. One electrode is connected to a high voltage generator (several hundred volts) and the other electrode is grounded. This has been extensively applied for neon lights, surface cleaning treatment and others. Based on the application, the distance between the electrodes in the glow discharge is varied, ranging from several millimeters up to several meters. DBD has been applied for the industrial production of ozone, and used in a plasma TV and others. Different from glow discharge, one or two electrodes within the DBD are attached to quartz plate(s) or the dielectric material(s) with a thickness of millimeters. The distance between the two electrodes is normally less than 10 mm depending on the operational pressure. A large distance would cause the discharge to be unstable if the pressure is around atmospheric pressure or higher. There exist many micro-discharges within the DBD under normal conditions. Once DBD is initiated at any location within the gap between the electrodes, charge accumulates on the dielectric to form an opposite electric field and interrupts the current flow in a few nanoseconds to generate micro-discharges. The duration of the current pulse depends on the pressure, properties of the gases and the dielectric material. The energy applied for DBD is higher than that for glow discharge. Therefore its operational temperature will be higher than room temperature. In some cases, cooling is needed for DBD application. Compared to DBD, glow discharge is always operated near room temperature. At a specific condition (e.g., low pressure), DBD can change into glow discharge (with a uniform glow instead of pulses from micro discharges). Fig. 2 shows a schematic of the plate DBD setup. It can be tubular if the electrodes and dielectric materials are tubular. If one removes the quartz or dielectric material, glow discharge is formed.

Fig. 2. Schematic of the plate DBD setup. If the quartz plate is removed, this changes to glow discharge.

Cold plasma contains many active species like electrons, ions, radicals, excited species and neutral species. In our present understanding, the highly energetic electrons represent the most important species of cold plasma and are mostly responsible for the reactions involved in the catalyst preparation. Other plasma species may have some influences too. This causes a complex physics and chemistry. The interaction between plasma active species and the heterogeneous surface is one of the hot topics in plasma physics and chemistry. Because of the limited space here, we will not discuss this important issue. One can look up the progress in recent articles ^{[28, 29, 30, 31, 32, 33, 34]}.

How can cold plasma help catalyst preparation or how can cold plasma make a difference? First, the reactions between the active plasma species and catalyst precursors are much faster than those in thermal catalyst preparation. This induces rapid nucleation of the crystals under cold plasma. How fast is this? We imagine it would be as fast as lightning, as shown in Fig. 3. Also, cold plasma operates at low temperatures (as low as room temperature with glow discharge). This causes slow crystal growth. This helps the preparation produces small catalyst size or high dispersion. Fig. 4 shows a comparison between cold plasma decomposition and thermal calcination. Both processes are used for the preparation of ZrO₂ ^[35].

Fig. 3. Reactions with the electrons during cold plasma catalyst preparation can be as fast as lightning.

Fig. 4. Comparison between DBD plasma decomposition and thermal calcination. (a) DBD plasma decomposition for ZrO2 synthesis [35]. The images were taken at different times with sparks at different locations. Micro-combustion was initiated because a carbon template was used and the carbon was ignited by the active oxygen species in the DBD plasma [35]. (b) Representative image of thermal calcination. A temperature gradient was established for the heating.

Second, it is well known that cold plasma is excellent for surface treatment or modification. The catalyst powder can be highly electrically charged. This would induce unusual characteristics. Specific hydrates can be formed, leading to a unique structure and enhanced catalyst-support interaction after the thermal treatment of the plasma formed hydrates ^{[3, 36]}.

Third, cold plasma has shown excellent ability to decompose the precursors rapidly at low temperatures for catalyst preparation or synthesis of support material. Specific structured oxides have been obtained ^{[37, 38]}. Fig. 5 shows SEM images of urchin-like ZnO obtained from a 10-min oxygen DBD plasma decomposition of zinc nitrate with activated carbon with no use of extra chemicals or pH control. Washing and the following drying were not needed. The obtained ZnO possessed a hierarchical structure with two distinct morphologies, nanosheets and sub-micron rods. Such urchin-like ZnO can be made using other methods. For example, a hydrothermal method was applied for the preparation of urchin-like ZnO from ZnSO₄×7H₂O and NaOH by the addition of vitamin C at 190 °C for 10 h with washing using deionized water and drying at 80 °C ^[39]. Xiao et al. ^[40] reported a synthesis of urchin-like ZnO also by hydrothermal method. Zinc acetate dihydrate and hexamethylenetetramine were dissolved in an alcohol-water solution by stirring at 60 °C for 1 h. The resulting slurry was then transferred into a Teflon-lined stainless steel autoclave and kept at 130 °C for 6 to 14 h. The obtained precipitate was thoroughly washed with distilled water and then dried at 60 °C in air overnight ^[40]. Wu et al. ^[41] reported a hydrothermal preparation of urchin-like ZnO at pH = 12. They mixed solutions of Zn(NO₃)₂ and (NH₄)₂CO₃ and stirred with a magnetic stirrer. The obtained precipitate was separated from the liquid and rinsed thoroughly in water to remove residual NO₃^- and NH₄⁺ ions. The rinsed precipitate was dried in air and then dispersed in water. The precipitate-containing water was transferred into a Teflon-lined autoclave with the addition of NaOH solution (1 mol/L). The autoclave was placed in an oven heated to 200 °C for the hydrothermal synthesis of ZnO for 15 h. The autoclave was cooled down to room temperature. The product was then removed, rinsed with water, and dried at 60 °C in air ^[41].

Fig. 5. SEM images of the ZnO sample from DBD plasma decomposition with micro combustion [37].

With no additional chemicals, DBD decomposition of multiple precursors can make highly dispersed oxide nanoparticles on another oxide support, leading to unique catalytic activity ^[42].

All these differences suggest the unique characteristics of nucleation and crystal growth under the influence of cold plasma. There exist many opportunities (with challenges) for fundamental studies and applications.

Green catalyst preparation is a long-term goal for chemists and engineers. Any progress towards this goal will contribute to the achievement of the final goal, no matter whether the progress is small or significant. Catalyst preparation using cold plasma can make contributions by the following.

(1) With no auxiliary chemicals, we can make a catalyst with smaller catalyst size or higher dispersion using the non-hydrogen cold plasma preparation with the use of the single precursor (like nickel nitrate) in a simple and rapid way.

An example is the plasma-prepared Ni catalysts supported on Al₂O₃ ^{[43, 44, 45]}, SiO₂ ^{[46, 47, 48]}, MgO ^{[49, 50, 51]}, MgAl₂O₄ ^{[52, 53]} and others ^{[54, 55]}. These catalysts were prepared by decomposing nickel precursor into nickel oxide using DBD plasma, followed by thermal hydrogen reduction ^{[47, 48, 49, 52, 53]}, or by forming a specific nickel hydrate using glow discharge, followed by thermal hydrogen reduction ^{[44, 45, 46]}. The obtained catalyst showed improved activity and enhanced coke resistance in the dry reforming of methane, steam reforming of methane, CO and CO₂ methanation and other reactions. Neither promoter nor alloy element nor complex treatment was needed ^{[3, 21]}. The benefits of the non-hydrogen plasma catalyst preparation include: smaller catalyst size, more Ni(111) on the Ni based catalysts and enhanced nickel-support interaction ^{[3, 21]}. Obviously, the non-hydrogen plasma prepared Ni catalysts show performance as good as those made by the hydrogen plasma ^{[56, 57, 58, 59, 60, 61, 62, 63, 64]}.

(2) Cold plasma can remove the template at low temperatures for the synthesis of micro- or mesoporous materials ^{[65, 66, 67, 68]}. It can remove templates that react with active species like excited oxygen species, electrons and ozone ^[67]. This leads to a ‘dry’ template removal for the synthesis of porous materials in place of using a template that generates now a huge waste of water. Micro-combustion (Fig. 4) has also been developed with carbon template as discussed above ^{[35, 37]}. This micro-combustion has many uses because it can avoid the sintering of porous materials, which occur frequently when the template is removal thermally. This is very important for catalyst applications. Moreover, the micro-combustion helps to avoid sintering for the formation of a phase structure that is normally obtained at high temperatures. For example, the obtained mesoporous ZrO₂ from micro-combustion exhibited a monoclinic lattice structure, which is normally generated only at temperatures over 1000 °C ^[35].

(3) With no use of hazardous reducing chemicals or hydrogen, a non-hydrogen cold plasma gives a green electron reduction using glow discharge as the electron source. Reduction is one of the most frequently applied methods for catalyst preparation. Present catalyst reduction needs hydrogen and hazardous chemical reducing agents. An alternative clean reduction technology is desired. Electrons would be the greenest reducing agent if it can be applied for the catalyst preparation. In this regard, the room temperature electron reduction with non-hydrogen glow discharge or radio frequency discharge plasma as the source of electrons has been developed. We recently summarized the progress in this topic ^[24]. The room temperature electron reduction via non-hydrogen discharge is excellent for size control with fast nucleation and slow crystal growth. It is a simple, easy, cheap, and energy-efficient way to reduce metal ions. It is also useful to load noble metal particles in the channels of ordered porous materials, like SBA-15, with no complex chemical modification ^[69]. The room temperature operation makes this very useful for the preparation of noble metal catalysts supported on heat sensitive substrates like porous organic materials ^{[24, 70]}, conducting polymers ^[71], ultrahigh surface area carbon ^{[72, 73, 74]} and peptides ^[75]. Fig. 6 shows images of an electron reduced Pt on a high surface area activated carbon sample (1200 m²/g) ^[73]. Compared to the hydrogen thermally reduced sample, the electron reduced one has a much higher dispersion with enhanced stability ^[73].

Fig. 6. TEM images of hydrogen reduced Pt/activated carbon (a, c, e), and (argon plasma) electron reduced Pt/activated carbon (b, d, f) samples. (a, b): As synthesized; (c, d, e, f): After sintering at 500 °C (in Ar) for 3 h. Pt loading: 3%; Hydrogen reduction was conducted at 250 °C for 2 h [73].

(4) With no use of auxiliary chemicals, we can make a catalyst with enhanced stability using the non-hydrogen cold plasma preparation in a simple and rapid way. With the present glow discharge or radio frequency discharge technology, the non-hydrogen-based electron reduction can only reduce metal ions with a positive standard electrode potential ^{[24, 76]}. For those cases with a negative standard electrode potential, the glow discharge or radio frequency discharge treatment can modify the hydrate and the interaction between the precursor and the support material ^[36]. By a combination with thermal treatment, an excellent catalyst can be prepared in an efficient and effective way ^{[36, 44, 46]}. In particular, the thermal calcination or oxidation of the plasma treated catalysts can lead to catalysts with improved activity and enhanced stability ^{[77, 78]}.

On the other hand, for those metal ions that cannot be reduced by the electrons of non-hydrogen glow discharges ^[76], hydrogen cold plasma reduction can be applied to meet the requirement of green chemistry ^{[56, 57, 58, 59, 60, 61, 62, 63, 64, 79]}. The hydrogen plasma reduction is more powerful than either electron reduction or thermal hydrogen reduction. It uses energetic hydrogen species, electrons, and probably other strong reducing species for the reduction. It is rapid and convenient. It can be initiated at room temperature and operated near room temperature. It is a remarkable enhancement for the conventional thermal hydrogen reduction. The hydrogen plasma-made catalysts also showed improved low temperature activity and enhanced stability, which help to reduce waste and enhance energy efficiency ^{[56, 57, 58, 59, 60, 61, 62, 63, 64]}.

(5) It is well known that cold plasma can be used as a powerful and convenient surface treatment method for the easy modification of acidity or hydrophilicity or functionalization with no auxiliary chemicals. It has been applied for the surface treatment of the support materials for the catalyst preparation with no need of a hazardous chemical treatment or modification ^{[80, 81, 82, 83, 84]}. Acid or basic or chemical treatment was avoided ^[80].

(6) Catalyst preparation with a controllable structure is a dream of chemists. However, it is a challenge to design and prepare a catalyst with a controllable structure in a clean and energy efficient way. In this regard, the cold plasma catalyst preparation can make a difference. If one needs a Ni catalyst or a noble metal catalyst with the (111) face as the principal face, he or she can make it using cold plasma that always creates this face ^{[3, 21, 24, 36, 44, 46, 47, 48, 49, 52, 53, 69, 70, 72, 75]}. It is not easy to find an illustration. Fig. 7 presents an illustrative image of the formation of Pd(111) on graphene using non-hydrogen plasma reduction ^[24].

Fig. 7. Formation of Pd(111) on graphene [24].

(7) With no use of an auxiliary chemical, cold plasma preparation creates catalysts (normally metal oxides) with structured characteristics ^{[37, 38]} or special dimension ^[85] in a simple and easy way.

(8) Amorphization activation finds increasing catalyst applications ^[86]. DBD plasma is an easy way for the amorphization with no need of high pressure processing or energy intense treatment ^{[87, 88, 89]}.

New catalysts are always desired. The cold plasma preparation is promising for the creation of new catalysts in a rapid way with less use of auxiliary or hazardous or expensive chemicals. In particular, the cold plasma can be used to prepare catalysts that cannot be easily obtained with the conventional preparation methods. Three illustrative cases are presented to demonstrate the superior capability of the cold plasma preparation. The catalysts discussed below are not easy to prepare by the conventional methods. Because of the limited space, we do not discuss here the detailed properties of these catalysts. Further information will be reported in our future publications.

A peptide-based catalyst ^{[90, 91, 92, 93, 94, 95, 96]} has been recently a highlight for potential applications in photocatalytic water splitting. An increasing study of this kind of catalysts is expected. One of the challenges is how to load the catalyst nanoparticles on the peptide. Using room temperature electron reduction, we recently achieved the loading and highly dispersion of Au, Pt and Pd nanoparticles on peptide ^{[75, 96]} with the cooperation of Aarhus University ^[97] and Johns Hopkins University ^[75]. Argon grow discharge was employed as the resource of electrons. During the electron reduction, the peptide motif KLVFF (Aβ_16-20) was self-assembled into two dimensional membranes, while the metal ions in solution were simultaneously reduced by electrons to form nanoparticles. The obtained metal nanoparticles with size of less than 2.5 nm were well distributed in the resulting peptide thin films. Fig. 8 shows a photo of the Au/peptide film. Fig. 9 presents the TEM images of the Pt/peptide and Pd/peptide films ^[75]. Interesting, Pt(111) can be clearly identified.

Fig. 8. Au nanoparticle/peptide film.

Fig. 9. (a) TEM micrographs of Pt/Aβ16-20 films (using a 100 µmol/L Aβ16-20 and 100 µmol/L H2PtCl6 hybrid solution); (b) TEM micrographs of Pd/Aβ16-20 films (using a 300 µmol/L Aβ16-20 and 100 µmol/L Pd(NO3)2 hybrid solution) [75].

This simple approach is a promising strategy to create hybrid thin films that integrate functional inorganics into biomolecule scaffolds ^{[75, 96]}.

The preparation of a noble metal catalyst that can be easily recovered or separated from the reaction mixture is always desired. For a reaction in a liquid or solution, the best way is to make a floating catalyst. However, it is a big challenge to do so. Using room temperature electron reduction, we have been able to prepare such floating noble metal catalysts easily. To do so, poly(vinylpyrrolidone) (PVP) was added into the solution. A floating Au nanoparticle/PVP film was rapidly formed on the water surface within a few minutes using an argon glow discharge. Fig. 10 shows a photo of the floating Au/PVP film.

Fig. 10. Floating Au/PVP film.

The catalytic activity of the floating Au/PVP film was confirmed by the detection of glucose from glucose oxidation at 25 °C. The reaction was exposed to the air. The details will be reported in our future work.

MOF-based catalysts have received significant attention due to the designable framework and high surface area of MOFs ^{[70, 98, 99]}. One of the efforts in MOF-based catalysts is to use the MOF as the catalyst support material. However, the thermal stability of a MOF is not very good. A careful treatment is needed with the hydrogen reduction at elevated temperatures. In this regard, the rapid room temperature electron reduction with an argon glow discharge has been applied ^[70]. To highly disperse the catalyst into the MOF cavities, the effect of the solvent is significant. The use of an organic solvent is helpful for highly dispersion. From the viewpoint of green chemistry, water or a clean solvent is favored. In case an organic solvent has to be used, a clean one should be chosen. Fig. 11 presents a photo of the MOF membrane before and after loading of silver nanoparticles on the MOF membrane with the room temperature electron reduction. Fig. 12 is the TEM images of the nanoparticles. Because the MOF was deposited on a plastic chip by a 3D printing way ^[100], the surface was not smooth causing some aggregation of the particles. The obtained silver nanoparticles were mostly in the range of 10-15 nm. Ag(111) can be clearly identified.

Fig. 11. Cu-BTC on ABS before (left) and after (right) loading of silver nanoparticles.

Fig. 12. TEM images of the nanoparticles on a MOF.

Cold plasma catalyst preparation is a new direction and attracts increasing attention. Because of its multidisciplinary characteristics and the complexity of plasma physics, the mechanism for the interaction between plasma species and catalyst or catalyst precursor is still not clear. The difficulty in the measurement of the energy and density of the plasma active species (including electrons) make the investigation even more difficult. More fundamental studies are needed.

The developed cold plasma catalyst preparation mostly relies on glow discharge and DBD. Both discharges have a size limitation and a fine powder is needed. This would limit future applications. New plasma generation technologies have to be developed for this issue, although studies have been very helpful for the understanding of some fundamental issues like the effect of size and structure.

A broad interest in the cold plasma catalyst preparation with the use of a heat sensitive substrate as the support exists. One can expect increasing applications in peptide, DNA, protein, MOF, COF and high surface area carbon supported catalysts.

The floating metallic catalyst can be further applied as a photo-catalyst and plasmon promoted catalyst. This unique catalyst would create a new catalyst system using a gas-liquid interface for the catalytic reaction. This recyclable catalyst is also promising to economize on noble metals.

The DBD plasma has more applications in the preparation of multi-oxide catalysts. It can avoid the drawbacks of thermal calcination at high temperatures. The DBD plasma has more applications in the preparation of carbon-based catalysts ^[101]. Template removal by the active species of the DBD or by micro-combustion will find more application too.

The significant progress in green catalyst preparation using cold plasmas was summarized. The most reported catalysts were nickel, iron, cobalt, platinum and palladium catalysts on various support materials. The cold plasma preparation uses less chemicals and gives improved dispersion, enhanced catalyst-support interaction, changed morphology, enhanced coke resistance and promoted stability for many reactions, including dry reforming, steam reforming, CO methanation, FT synthesis, CO₂ methanation, glucose oxidation, electrochemical oxidation and reduction. In particular, cold plasma preparation can easily generate catalysts with the (111) face as the principal face. This is excellent for catalysts that require more (111) faces, like the nickel catalyst for CO₂ reforming. Cold plasma preparation is normally fast. This helps to save energy and to reduce labor cost. The low temperature operation is very useful for catalyst preparation with heat sensitive substrates. This creates many opportunities in catalyst preparation. This also promotes the use of biological macromolecules as catalyst or catalyst support. With development in plasma generating technology, we expect a rapid future progress in cold plasma catalyst preparation.