Elsevier

Catalysis Today

Volume 138, Issues 3–4, November 2008, Pages 141-146
Catalysis Today

Partial oxidation of CH4 with air to produce pure hydrogen and syngas

https://doi.org/10.1016/j.cattod.2008.05.013Get rights and content

Abstract

The gas–solid reaction between methane and the lattice oxygen of Ni, Co, and Fe-oxides loaded on various support materials produced a synthesis gas (hydrogen and carbon monoxide) at 600–800 °C. Metal oxides were reduced to metals or lower valence oxides, and they were re-oxidized to oxides by introducing air after the reaction. Thus, production of hydrogen or synthesis gas free from nitrogen can be achieved alternatively without using pure oxygen. As a metal oxide, Fe2O3 and Rh2O3-loaded on Y2O3 exhibited the highest H2 selectivity of 60.1% with a moderate CH4 conversion of 54% and a high lattice oxygen utilization of 84% at 800 °C.

Introduction

Hydrogen production from natural gas has generally been conducted by steam reforming (SR) (reaction (1)) [1].CH4+H2O3H2+COΔH298°=+206kJ/molSince reaction (1) is highly endothermic and runs at high temperatures above 1073 K, the reaction necessitates a large amount of energy, and as a result a large amount of CO2 is emitted into the atmosphere.

In contract, the catalytic partial oxidation of CH4 (POM) (reaction (2)) is an exothermic reaction and has the advantage that it is performed at relatively lower temperatures. Since Ashcroft et al. recalled attention to this reaction in 1990 [2], this field of chemistry has attracted much interest of many researchers [2], [3], [4], [5].CH4+12O22H2+COΔH298°=36kJ/mol

Application of Ni-loaded catalysts used to steam reforming has been investigated. However, in the catalytic partial oxidation, the catalysts have suffered from sintering of Ni and carbon deposition [6], [7]. Nearly all the noble metal catalysts can produce syngas, showing superior advantages to Ni-loaded catalysts with regard to both activity and carbon resistance [8]. However, noble metals are very costly and their reserves are limited in the world. Thus, it may be somewhat difficult to apply them to industrial processes. To overcome this problem, Takehira et al. [9] have applied a solid-phase crystallization method to Ni-loaded MgO-Al2O3 catalyst. This catalyst has highly dispersed Ni metal and high durability against coke formation. However, pure oxygen is typically required for POM, and in the commercial operation of POM, a costly oxygen plant must be installed [10], [11].

As an alternative process, in order to obtain pure hydrogen from CH4, catalytic decomposition of CH4 over Ni/SiO2 is proposed (reaction (3)), and carbon formed could potentially be removed by introducing steam to give H2 and CO (reaction (4)) [12].Step1CH4+Cat.C/Cat.+2H2ΔH298°=+75kJ/molStep2C/Cat.+H2OCat.+CO+2H2ΔH298°=+131kJ/mol

However, such a process yields a large amount of carbon on the catalyst bed, which causes a large pressure drop between the front end and the bottom of the bed, making operation difficult in a fixed bed reactor. In addition, if steam is used to re-generate the carbon-formed catalyst, the advantage of such a process against steam reforming would be diminished.

The redox properties of NiO (reactions (5) and (6)) dispersed with different inorganic binders including MgO, Y-SZ and Ni–Mg–Al mixed oxides have been investigated [13], [14], [15]. The reactions have been studied by means of thermogravimetric measurement during redox cycles at a constant temperature. Ni–Mg–Al mixed oxides have been reported to exhibit excellent regenerability in cyclic use [15].Step 1 NiO + CH4  Ni + CO + 2H2 ΔH = +205 kJ/molStep2Ni+12xAir(O2+N2)NiOx+12xN2ΔH=241kJ/mol

Recently, the gas–solid reaction between methane and lattice oxygen of oxides to give synthesis gas was reported by Otsuka et al. (reactions (7), (8), (9)) [16].Step 1 CeO2 + xCH4  CeO2−x + xCO + 2xH2Step 2 CeO2−x + xCO2  CeO2 + xCOCeO2−x + xH2O  CeO2 + xH2

Similarly, the redox cycle, the reduction of Fe2O3 with CH4 (reaction (10)) and the subsequent oxidation of iron metal with H2O (reactions (11) and (11′)) or CO2 has been proposed [17], [18], [19], [20], [21], [22], [23].Step 1 Fe2O3 + 3CH4  2Fe + 3CO + 6H2 ΔH = +239.1 kJ/molStep 2 3Fe + 4H2O  Fe3O4 + 4H2 ΔH = +151.2 kJ/mol2Fe + 3H2O  Fe2O3 + 3H2 ΔH = +99.4 kJ/mol

The studies have shown that the lattice oxygen of iron oxide or cerium oxide exhibits high activity for methane oxidation to give synthesis gas, and that there is no danger of an explosion occurring with pure oxygen. However, these reactions with H2O or CO2 are highly endothermic and consume a large amount of energy, necessitating a higher reaction temperature.

Shikong et al. have reported that La0.9Sr0.1FeO3 catalyst shows high catalytic activity for the redox cycle between reaction (12) and reaction (13) by using air at 900 °C [24]. Wei et al. have reported that CeO2 catalyst exhibits high catalytic activity in the same reaction by using air above 865 °C [25]. In addition, reduction of metal oxides such as WO3 [26] and LaFeO3 perovskite [27], [28] with CH4 and oxidation of the reduced metal oxide has been reported.Step1CaHb+McOdaCO+b2H2+cMStep2M+12mAir(O2+4N2)MOm+2mN2However, the gas–solid reaction between methane and the lattice oxygen of oxides to the synthesis gas runs at a high temperature of nearly above 865 °C and consumes a large amount of energy. Metal oxides are rapidly deactivated by repeated runs of the reduction of oxide and re-oxidation of metal due to metal sintering.

This paper deals with the oxidation of CH4 using lattice oxygen of transition metal oxides and re-oxidation of reduced metals by air in order to develop a process that can be carried out at a lower temperature. We found in particular that Fe2O3-loaded catalysts with a small amount of co-loaded Rh2O3 exhibit high and constant catalytic activities for repeated reduction and oxidation cycles.

Section snippets

Catalyst preparation

The catalyst supports used in this study were CeO2, Y2O3, SiO2 (Wako Pure Chemical Industries Ltd.), Al2O3 (Sumitomo Chemical Co.), MgO (1000A; Ube Industries Ltd.), TiO2 (P25; Japan Aerosil Co.), and La2O3 (Nacalai Tesque, Inc.). CeO2 and Y2O3 were prepared by thermal decomposition of Ce(NO3)3·6H2O and (CH3COO)3Y·4H2O (Wako Pure Chemical Industries Ltd.) at 600 °C under air for 5 h, respectively.

The supported Fe2O3-Rh2O3/Y2O3 catalyst was prepared by impregnating the suspended Y2O3 with an

Effects of metal oxide and support material on the reduction behavior with methane

Previous studies of the reduction of Fe2O3 with CH4 were carried out with a bulk iron oxide with small amounts of additives [19]. The amount of lattice oxygen utilized to oxidize CH4 could be maximized without any additives. In this simple system, the reaction with methane would proceed at a higher temperature, and bulk oxides tend to aggregate when they are reduced metallic species. In addition, bulk oxide generally has a small surface area.

Supports having high surface areas are commonly used

Conclusion

Fe2O3-loaded Y2O3 catalyst promoted with a small amount of Rh2O3 produced H2 and CO by the reaction with CH4, and reduced iron was regenerated by air (Ar:O2 = 4:1). Thus, without using pure oxygen, H2 or synthesis gas free from nitrogen could be produced.

Acknowledgment

This work is supported in part by a Grant-in-Aid for Scientific Research (C) from JSPS and MEXT.HAITEK(2007).

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