Plasma methods for preparing green catalysts: Current status and perspective

Abstract

Most current catalyst preparation methods cause pollution to air, water and land with the use of hazardous chemicals, lengthy operation time, high energy input and excessive water usage. The development of green catalyst preparation is necessary to prevent and eliminate waste from each step of the catalyst preparation. We summarize recent progress in the application of cold plasmas for green catalyst preparation. Cold plasma preparation can reduce the catalyst size, improve the dispersion and enhance catalyst-support interaction with the use of less or no hazardous chemicals. These improvements also lead to the enhancement of catalyst activity and stability. An alternative room temperature electron reduction with a non-hydrogen plasma as an electron source was developed for the reduction of noble metal ions in which no hazardous chemical reducing agent or hydrogen was needed. This creates many opportunities for the development of supported catalysts with heat sensitive substrates, including metal organic frameworks (MOFs), covalent organic framework (COFs), high surface area carbon, peptide, DNA, proteins and others. A novel floating metal catalyst on a water (or solution) surface has been established. Template removal using low temperature cold plasmas also leads to the formation of high surface area porous materials with characteristics that are normally only obtainable with high temperature calcination, but sintering can be avoided. Micro combustion has been developed for the removal of carbon template using cold plasma. This is promising for preparing many structured oxides in a simple way with no use of auxiliary chemicals. Many opportunities exist for the use of cold plasmas to make multi-metallic oxides. Some future development ideas are addressed.

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.