Review
Review of plasma catalysis on hydrocarbon reforming for hydrogen production—Interaction, integration, and prospects

https://doi.org/10.1016/j.apcatb.2008.06.021Get rights and content

Abstract

This paper, for the first time, reviews the development of the application of plasma catalysis, the combination of plasma and thermal catalysis, on hydrocarbon reforming for H2 generation. It has been experimentally demonstrated that this novel technique results in a synergistic effect, i.e., the performance achieved with plasma catalysis is better than the summation of plasma-alone and catalysis-alone, indicating its promising application on H2 generation. To gain better insights and provide useful information for further optimization, the interactions between plasma and thermal catalysis possibly leading to the synergism observed in different plasma catalysis systems are comprehensively reviewed. The thermal and non-thermal plasmas are suggested as the suitable plasma source driving the two-stage and single-stage plasma catalysis systems, in which catalyst are located downstream and inside the plasma reactor, respectively. In the latter case, the non-thermal plasma should play the role as exciting the reactants to vibrational state instead of converting them. Because of the either too high reduced field or too low electron density to effectively generate vibrationally excited species in the existing non-thermal plasmas, a novel power supply, which could separately control the electron density and electron energy is proposed to serve as the ideal plasma source.

Introduction

Under the pressure of global warming and diminishing fossil fuel reserves, the newly developed energy must meet the requirements of being environment-friendly and renewable simultaneously. With these restrictions, H2 is one of the most promising alternative energy sources not only because H2 can be produced from biomass but also because H2 is compatible with fuel cell, an environmental clean and highly efficient power generation device. The importance of H2 for the future energy can be easily understood from the fact that it is listed as a primary energy source in the energy development strategy in many developed countries.

Currently, catalytic hydrocarbon reforming process is the most well-developed and economical technique for H2 production. In addition to the hydrocarbon, the other reactant for the reforming process could be either steam or O2, which is known as the steam reforming or partial oxidation reaction. The former is endothermic but the latter is exothermic. When these two reactions are combined with a net reaction enthalpy change of zero, it is termed as the auto-thermal reaction. To date, methane is the most commonly adopted hydrocarbon for H2 generation. For example, in 2005, the steam methane reforming accounts for 80% of the hydrogen produced in the United State [1]. In addition to methane, a variety of hydrocarbons, such as propane, methanol, ethanol, and dimethyl ether (DME), etc., are currently under investigation. The catalytic reforming process is typically operated at high temperature (400–1000 °C) and pressure (>10 atm.) [2], [3]. The commonly used catalysts include Ni, Co, Ir, Pd, or Ru metal(s) supported on metal oxides, such as Al2O3, CeO2, MgO and TiO2 [4]. However, due to the requirement of high temperature, the catalytic reforming process is limited in some applications, e.g., vehicles, in which rapid ignition/response is essential. Moreover, the deactivation of expensive catalyst resulted from the coke formation is also a problem to be resolved.

In early 1990s, application of plasma technologies on hydrocarbon reforming to generate H2 has been gradually attracting attention because of the following characteristics: fast ignition, the compatibility for a broad range of hydrocarbons, removal of the catalyst sensitivity to trace impurity in the gas stream, and compactness [5]. The plasma could be operated at low pressure or atmospheric pressure. Although the low pressure plasma, such as radio frequency (RF) plasma or microwave (MW) plasma, could achieve high hydrocarbon conversion and good H2 selectivity, the low H2 production rate and extra energy requirement for vacuum device restrict its practical use. The atmospheric plasma can be divided into two types, i.e., thermal plasma and non-thermal plasma, depending on the gas temperature. In thermal plasma, like plasma torch, all the charged species (electrons and ions) and neutral species (atoms, molecules, radicals and excited species) are approximately in thermal equilibrium. The typical gas temperature ranges from 1,000 to 10,000 K. On the other hand, non-thermal plasma, such as corona discharge, dielectric barrier discharge (DBD) and surface discharge, are in thermally non-equilibrium state, in which the electron temperature (10,000–100,000 K or 1–10 eV) is much higher than the gas temperature (∼100 K).

Plasmas contain energetic electrons and a variety of chemically active species which can greatly promote the reforming chemistry. However, the H2 selectivity obtained with plasma technologies is generally lower than that achieved with the traditional catalytic reforming process [6]. It is one of the most important bottlenecks need to be resolved for further industrial application.

Recently, the plasma catalysis technique, which is constructed by integrating plasma and thermal catalysis, has been developed. This novel technique combines the advantages of high products’ selectivities from thermal catalysis and the fast startup from plasma technique. The main applications of plasma catalysis include hydrocarbon reforming for hydrogen production and gaseous pollutant removal. For the latter application, two excellent review papers have been published [7], [8]. Nevertheless, no review paper is available on the topic of hydrocarbon reforming so far, which is the main motivation of this review paper.

In terms of hydrocarbon reforming, it has been experimentally demonstrated in various studies that the plasma catalysis could result in synergism [9], [10], [11]. For hydrocarbon conversion rate, a synergistic factor ranging from 1.1 to 2.4 can be found in the above-mentioned literatures. The synergistic factor is defined as the ratio of the performance achieved with plasma catalysis to the summation of the performance obtained with plasma-alone and catalysis-alone. In other words, the synergistic effect occurs when the synergistic factor is greater than 1. Moreover, at a fixed applied voltage and frequency, the synergistic factor firstly increases with temperature but decreases at high temperature region, i.e., there is an optimum temperature. To clarify the major causes resulting in the synergism, it is essential to understand the interactions between plasma and catalysis. Moreover, finding out the main reasons leading to the synergism could provide useful information for further performance enhancement as well.

The plasma catalysis technique can be classified into two systems, i.e., single-stage and two-stage, depending on the position of catalyst. The single-stage type is constructed by packing catalyst pellets within the plasma zone or coating catalyst on the surface of electrode(s). The catalysts could completely or just partially overlap with the plasma zone. In this manner, the plasma and catalysis could directly interact with each other.

As for the two-stage type, the plasma zone is located either upstream or downstream the catalyst bed, which is termed as plasma preprocessing and plasma postprocessing, respectively. In the former case, the most adopted configuration for two-stage plasma catalysis, the plasma provides chemically reactive species for further catalysis or pre-converts reactants into easier converted products to accelerate the catalysis. While in the plasma postprocessing, the function of plasma is to convert the residual reactants and destroy the undesired byproducts generated from thermal catalysis. Comparison of these two systems for auto-thermal reforming of C8H18 has been made by Sobacchi et al. [9]. Their experimental results indicate that the plasma preprocessing performs as a better configuration from the viewpoint of hydrogen yield. In the following text, the two-stage plasma catalysis system represents the plasma preprocessing type unless specified otherwise.

This paper is the first review paper focusing on hydrocarbon reforming for H2 generation via plasma catalysis. The interactions between plasma and catalysis as well as the possible causes leading to the better performance are comprehensively reviewed and discussed. The prospects for the further development and optimization are also highlighted.

Section snippets

Interactions between plasma and catalysis in two-stage system

The interactions between plasma and catalysis in two-stage system are relatively simple since all the short-lived active species (excited species, radicals and ions) generated in plasma vanish before they reach the catalyst. In such a configuration, plasma mainly plays the role to change the gas composition fed into the catalyst reactor. For example, C2 compounds such as C2H2, C2H4 and C2H6 are usually generated as byproducts in the CH4 reforming process via non-thermal plasma. Pre-converting CH

Interactions between plasma and catalysis in single-stage system

The interactions between plasma and catalysis become rather complicated once the catalyst pellets are packed within plasma zone. The interactions can be elucidated from two aspects: (1) the influence of packing catalysts on plasma characteristics and (2) the influence of plasma discharge on catalysis.

Thermal plasma or non-thermal plasma?

How the plasma and catalysis are combined with each other would affect the interactions between them and also the overall performance. To date, a comprehensive comparison between single-stage and two-stage plasma catalysis systems is still unavailable. Although it is difficult to determine which one is better based on available data, some intrinsic characteristics of these two different systems could provide useful information for comparison.

In terms of thermal plasma, such as microwave torch,

Conclusions

Plasma catalysis has shown a promising potential for the application of hydrocarbon reforming to produce hydrogen. This novel technique not only combines the advantages of high products’ selectivity from thermal catalysis and the fast startup from plasma technique but also results in a synergism enhancing the energy efficiency for hydrocarbon reforming. However, it needs to be emphasized that the suitable plasma source depends on the configuration of plasma catalysis system. The ideal plasma

Acknowledgements

The authors gratefully acknowledge the financial supports provided by Industrial Technology Research Institute (ITRI) and National Science Council of Taiwan (NSC 95-EPA-Z-008-002 and NSC 97-NU-7-008-001).

References (90)

  • J.R. Rostrup-Nielsen et al.

    Adv. Catal.

    (2002)
  • L. Bromberg et al.

    Int. J. Hydrogen Energy

    (2000)
  • M.G. Sobacchi et al.

    Int. J. Hydrogen Energy

    (2002)
  • S. Broer et al.

    Appl. Catal. B: Environ.

    (2000)
  • J. Niu et al.

    Catal. Commun.

    (2006)
  • V. Tufano et al.

    Appl. Catal. B: Environ.

    (1993)
  • C.U.I. Odenbrand et al.

    Appl. Catal.

    (1986)
  • S. Futamura et al.

    Catal. Today

    (2002)
  • H.H. Kim et al.

    Appl. Catal. B: Environ.

    (2005)
  • H.H. Kim et al.

    Appl. Catal. B: Environ.

    (2008)
  • U. Roland et al.

    Appl. Catal. B: Environ.

    (2005)
  • M. Magureanu et al.

    Appl. Catal. B: Environ.

    (2005)
  • M. Magureanu et al.

    Appl. Catal. B: Environ.

    (2007)
  • W.L. Morgan et al.

    Comp. Phys. Commun.

    (1990)
  • E. Por et al.

    Chem. Phys. Lett.

    (1991)
  • T. Nozaki et al.

    Catal. Today

    (2004)
  • T. Nozaki et al.

    Catal. Today

    (2004)
  • Y. Zhang et al.

    Plasma Chem. Plasma Process.

    (2000)
  • J.G. Wang et al.

    Catal. Today

    (2004)
  • Z.H. Li et al.

    J. Mol. Catal. A: Chem.

    (2004)
  • Y.P. Zhang et al.

    Catal. Commun.

    (2004)
  • M.H. Chen et al.

    Catal. Today

    (2004)
  • C. Wagner

    Adv. Catal.

    (1970)
  • G. Foti et al.

    J. Electroanal. Chem.

    (2002)
  • C.J. Liu et al.

    J. Catal.

    (1998)
  • S. Futamura et al.

    Catal. Today

    (2006)
  • C. Subrahmanyam et al.

    Appl. Catal. B: Environ.

    (2006)
  • C. Subrahmanyam et al.

    Appl. Catal. B: Environ.

    (2006)
  • M. Magureanu et al.

    Appl. Catal. B: Environ.

    (2007)
  • U.S. Department of Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program, Multi-Year Research, Development...
  • J. Ogden, Review of Small Stationary Reformers for Hydrogen Production, Report to the International Energy Agency,...
  • A. Haryanto et al.

    Energy Fuel

    (2005)
  • B. Pietruzka et al.

    Catal. Today

    (2004)
  • H.H. Kim

    Plasma Process. Polym.

    (2004)
  • J. Van Durme, J. Dewulf, C. Leys, H. Van Langenhove, Appl. Catal. B: Environ. 78 (2008)...
  • Y. Iwasaki et al.

    J. Chem. Eng. Jpn.

    (2006)
  • T. Nozaki et al.

    Energy Fuel

    (2006)
  • D.L. Baulch et al.

    J. Phys. Chem. Ref. Data

    (1994)
  • S. Toby et al.

    J. Phys. Chem.

    (1985)
  • W.O. Heath, S.E. Barlow, T.M. Berqsman, D.L. Lessor, T.M. Orlando, A.J. Peurrung, R.R. Shah, Development and Analysis...
  • J.H. Whealton, R.L. Graves, Exhaust remediation using non-thermal (plasma) aftertreatments: a review, Proceedings of...
  • J.S. Chang et al.

    IEEE Trans. Ind. Appl.

    (2000)
  • S.K. Kang et al.

    IEEE Trans. Plasma Sci.

    (2003)
  • T. Takuma

    IEEE Trans. Electr. Insul.

    (1991)
  • M.A. Lieberman et al.

    Principles of Plasma Discharges and Materials Processing

    (2005)
  • Cited by (0)

    View full text