Plasma Chemistry and Catalysis in Gases and Liquids

Vasile I. Parvulescu, Monica Magureanu, Petr Lukes

• Publisher: Wiley-VCH
• Year: 2013
• Print ISBN: 9783527330065
• eBook ISBN: 9783527649549
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Filling the gap for a book that covers not only plasma in gases but also in liquids, this is all set to become the standard reference for this topic. It provides a broad-based overview of plasma-chemical and plasmacatalytic processes generated by electrical discharges in gases, liquids and gas/liquid environments in both fundamental and applied aspects by focusing on their environmental and green applications and also taking into account their practical and economic viability.

With the topics addressed by an international group of major experts, this is a must-have for scientists, engineers, students and postdoctoral researchers specializing in this field.

Chapter 1

An Introduction to Nonequilibrium Plasmas at Atmospheric Pressure

Sander Nijdam, Eddie van Veldhuizen, Peter Bruggeman, and Ute Ebert

1.1 Introduction

1.1.1 Nonthermal Plasmas and Electron Energy Distributions

Plasmas are increasingly used for chemical processing of gases such as air, combustion exhaust, or biofuel; for treatment of water and surfaces; as well as for sterilization, plasma deposition, plasma medicine, plasma synthesis and conversion, cleaning, and so on. These plasmas are never in thermal equilibrium – actually, we know of no exemption – and this fact has two main reasons.

1) It is easier to apply electromagnetic fields than to uniformly heat and confine a plasma. However, electromagnetic fields naturally transport charged species whose concentrations and energies therefore naturally vary in space, particularly, close to the walls of the container. Generically, the species in such a plasma are not in thermal equilibrium.

2) It is energy efficient to not feed energy equally into all degrees of freedom within a gas or plasma, such as into the thermal displacement, rotation, and vibration of neutral molecules, but only into those degrees of freedom that can efficiently create the desired final reaction products for the particular application. Therefore it is frequently preferable to accelerate only electrons to high velocities and let them excite and ionize molecules by impact while keeping the gas cold. If the electron energy distribution is appropriate, some reactions can be triggered very specifically.

In this manner, the nonthermal nature of the plasmas that are created electromagnetically is made into an asset. By varying gas composition, electrode and wall configuration, and circuit characteristics more energy can be channeled into specific excitations and reactions. Recent examples include the optimization of the pulsed power source for ozone generation in streamer corona reactors [1], or dual frequency RF-generated plasmas [2].

To elaborate the physical understanding further, Mark Kushner has proposed a workshop at the Gaseous Electronics Conference (GEC) 2011 on how the electron energy distribution within a discharge can be tailored for a specific application. A joint approach to this question by theory and experiment now seems within reach because of the large progress of theory in recent years.

1.1.2 Barrier and Corona Streamer Discharges – Discharges at Atmospheric Pressure

The past has mainly seen an experimental approach by trial and error, also guided by some physical understanding. Within the limited space available here, we will review some setups and their physical mode of operation. A common theme is the avoidance of plasma thermalization in the form of arcs and sparks. Variations over two basic approaches are used very commonly and will make the main theme of this review: the corona discharge and the barrier discharge. In a barrier discharge, large currents are suppressed by dielectric barriers on the electrodes. Basically, the discharge evolves only up to the moment when so much charge is deposited on the insulator surfaces that the field over the gas is screened. In a corona discharge, the discharge expands from a needle or wire electrode into outer space where the electric field decreases and finally does not support a discharge anymore. The discharge then has to feed its current into the high-ohmic region of the nonionized gas, which limits the current as well. These two basic principles have seen many variations in the past years and decades. For example, in corona discharges, short and highly ramped voltage pulses create much more efficient streamers that do not cease due to the spatial decrease of the electric field away from the curved electrode but due to the final duration of the voltage pulse.

Both discharge types can (but need not) operate at atmospheric pressure. This poses an advantage as well as a challenge. The advantage lies in the fact that no expensive and complex vacuum systems are required. This makes the design of any reactor a lot simpler, not only when the operating gas is air but also when other gases (such as argon or helium) are used. The challenge consists of the observation that characteristic length scales within the discharge can be much smaller than the discharge vessel and that the discharge can therefore form complex structures, rather than a more or less uniform plasma. These structures have to be understood and used appropriately. For instance, the initial evolution of streamer discharges follows similarity laws [3]: when the gas density is changed, the same voltage will create essentially the same type of streamer, but on different length and timescales. Therefore, streamer fingers and trees grow in a similar manner at 10 µbar as at 1000 mbar, but 10 µbar corresponds to an atmospheric altitude of 83 km where the so-called sprite streamers have a diameter of at least ∼10 m, while at 1000 mbar, the minimal streamer diameter is ∼ 100 µm and conveniently fits into typical experiments.

1.1.3 Other Nonthermal Discharge Types

There is a large variety of nonthermal plasmas. They can be classified into different discharge types, although definitions used by different authors vary significantly. The plasmas or discharges can be classified according to their time dependence (transient or stationary), importance of space charge effects or of heating of the neutral gas species, and presence of a surface close to the discharge. The most important nonthermal plasmas along with their energization method and typical applications are listed in Table 1.1.

Table 1.1 Overview of Nonthermal Discharge Types and Their Most Common Applications

This table is intended to give a general idea, but it is far from complete. A further complication is that definitions are used in different ways. For example, in Ref. 8, Braun et al. use what they call a microdischarge for ozone generation, whereas the microdischarges as intended in Table 1.1 are much smaller. The microwave discharge made by Hrycak et al. [28] qualifies much more for the term plasmajet than for microdischarge. More information on the different types of microdischarges is given in [29]; some examples of the use of microdischarges are given in Section 1.4.4.

In many transient discharges, the different discharge types can occur after each other. For example, a discharge can start as an avalanche and then become a streamer, which can develop into a glow and finally into an arc discharge. When applying a DC field between two metal electrodes, a discharge at high pressure will become a thermal arc if the power supply can deliver the current. Nonthermal discharges are, by definition, almost always transient.

An essential feature of a cold nonthermal discharge is its short duration. Therefore, the largely varying timescales of the processes inside the discharge must be considered. The excitation timescales, which often range from picoseconds to a few microseconds, are clearly not the timescale necessary for preventing thermalization as thermalization occurs in millisecond-order timescales. The critical timescale is basically the characteristic time of the glow-to-spark transition. This transition time can highly depend on conditions such as voltage amplitude and gas composition but is often in the order of a (few) hundred nanoseconds [30]. Dielectric barrier discharges (DBDs) are a well-known example of how (dielectric) barriers can reduce current density and n_e to keep the gas temperature of the discharge low.

Like streamer and avalanche discharges, Townsend and glow discharges are cold discharges. They usually occur as a stationary discharge but have to be preceded by another discharge such as a streamer or avalanche discharge to ignite. In Townsend and glow discharges, electrons are emitted from the electrode and are then multiplied in the gap. In the case of a Townsend discharge, the electron multiplication takes place in the whole gap, while in a glow discharge, space charge concentrates the multiplication in the cathode sheath region. Electrons are freed from the cathode by the temperature of the cathode itself or by secondary emission either due to the impact of energetic positive ions or due to photons or heavy neutrals.

Several cold atmospheric pressure discharges operate in helium. This is not a coincidence as He has a thermal heat conductivity that is about 10 times larger than that of most other gases, which renders heat removal from the discharge to be more efficient. Other methods for efficient heat removal include strongly forced convection cooling in flow stabilized discharges and creation of discharge with a large area-to-volume ratio (microplasmas, see also further) to make the heat losses to the walls more efficient.

1.1.3.1 Transition to Sparks, Arcs, or Leaders

Avalanches, Townsend, streamer, and glow discharges are examples of cold discharges. This means that the heavy particle temperature is not much above room temperature and definitely far below the electron temperature (T_e

T_i ≈ T_n where e,i, and n stand for electron, ion, and neutral, respectively). At even higher currents, at higher pressures, or with longer pulse durations, these discharges can transform into spark, arc, or leader discharges. These are hot discharges, the heavy particle temperature is close to the electron temperature and can reach thousands of Kelvin (T_e ∼ T_i ≈ T_n). In applicatio...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Preface
• List of Contributors
• Chapter 1: An Introduction to Nonequilibrium Plasmas at Atmospheric Pressure
• Chapter 2: Catalysts Used in Plasma-Assisted Catalytic Processes: Preparation, Activation, and Regeneration
• Chapter 3: NOx Abatement by Plasma Catalysis
• Chapter 4: VOC Removal from Air by Plasma-Assisted Catalysis-Experimental Work
• Chapter 5: VOC Removal from Air by Plasma-Assisted Catalysis: Mechanisms, Interactions between Plasma and Catalysts
• Chapter 6: Elementary Chemical and Physical Phenomena in Electrical Discharge Plasma in Gas–Liquid Environments and in Liquids
• Chapter 7: Aqueous-Phase Chemistry of Electrical Discharge Plasma in Water and in Gas–Liquid Environments
• Chapter 8: Biological Effects of Electrical Discharge Plasma in Water and in Gas–Liquid Environments
• Chapter 9: Hydrogen and Syngas Production from Hydrocarbons
• Index