ReviewPlasma methods for preparing green catalysts: Current status and perspective
Graphical Abstract
What will happen if you put a catalyst precursor in a neon tube? You will get a nice catalyst with the use of fewer chemicals and less energy.
Introduction
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 homoge neous 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.
Section snippets
Nucleation and crystal growth under the influence of a cold plasma
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
How does catalyst preparation using cold plasma meet the requirement of green chemistry?
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
New catalysts using cold plasma preparation
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
Future development
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
Conclusions
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
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Published 5 March 2016
This work was supported by the National Natural Science Foundation of China (20990223 and 21476157).