Production of hydrogen by steam reforming of methanol over copper-based catalysts: The effect of cesium doping

doi:10.1016/j.apcata.2006.03.026

Applied Catalysis A: General

Volume 306, 7 June 2006, Pages 22-28

https://doi.org/10.1016/j.apcata.2006.03.026 Get rights and content

Abstract

The catalytic production of hydrogen by steam reforming of methanol was investigated using samples of copper oxide supported on alumina (Cu/Al₂O₃) and promoted with cesium (Cu-Cs/Al₂O₃). The effects of cesium content and reaction temperature on the catalytic activity were investigated. The Cu-Cs/Al₂O₃ catalysts exhibited higher activity and stability as compared to the undoped ones. The catalyst containing 2 wt% of cesium was the most active and at 300 °C the methanol conversion reached 94 mol% and the hydrogen selectivity 97 mol% with no detectable formation of CO. After an ageing treatment at 400 °C, methanol conversion was still close to 100% with the cesium-doped catalyst, while the undoped catalyst drastically deactivated. X-ray powder diffraction (XRD) and XPS measurements indicate that cesium prevents the reduction of copper oxide into metallic Cu, by the hydrogen produced, and inhibits the formation of CuAl₂O₄ spinel upon thermal treatment.

Introduction

The production of hydrogen by steam reforming of methanol is of great interest for the development of fuel cell (polymer electrolyte fuel cells, PEFC) powered engines. The direct use of hydrogen for fuel cell application is hindered by problems of storage, safety, refuelling, etc. These problems can be overcome by the production of hydrogen on the vehicle from a suitable high energy/density liquid fuel, such as methanol [1], [2]. The steam reforming of methanol is feasible thermodynamically at ambient temperature. It corresponds to the following global reaction:CH₃OH(g) + H₂O(g) → 3H₂(g) + CO₂(g) $Δ H_{R} ° = + 49 kJ mo l^{- 1} (298 K)$ Under these conditions, 3 mol of hydrogen are produced for 1 mol of fuel. However, this reaction being endothermic, it consequently requires an external energy contribution.

Hydrogen produced by steam reforming of methanol unfortunately contains a significant amount of CO (>100 ppm) as a by-product formed during the reaction [3], [4]. As for the application of PEFC, even traces of CO (20 ppm) in the reformed gases deteriorate the Pt electrode and the cell performance is worsened [5]. The steam reforming of methanol can also lead to the formation of toxic and undesirable products which limit the H₂ production, such as formic acid (HCOOH), formaldehyde (CH₂O) and dimethylether (CH₃OCH₃) [6], [7].

The majority of works devoted to the production of hydrogen using methanol were carried out on catalysts containing copper and zinc deposited on alumina [8], [9].

The introduction of zinc into Cu/Al₂O₃ catalysts is known to limit the sintering and improving the dispersion of copper [10].

A number of materials are being developed to replace Cu/Zn/Al catalysts. Oxide supported precious metals including Pd, Pt and Rh have received a great deal of attention [11]. These materials are very active, however, unlike Cu-based catalysts, these precious metals-based catalysts have poor CO₂ selectivity, yielding primarily CO and H₂ during the methanol steam reforming [12].

The objective of the present work is to study the promoting effect of alkaline compound like cesium and potassium on Cu/Al₂O₃ catalysts. In particular, the effect of the catalyst composition on its overall activity will be determined in relationship with its physico-chemical characteristics. The effect of reducing and ageing treatment of the catalysts on its activity will also be investigated.

Section snippets

Catalysts preparation and characterisation

Catalysts were prepared by impregnation of a support γ-Al₂O₃ (133 m²/g) with an aqueous solution of the corresponding metal precursor salt: Cu(NO₃)₂·3H₂O salt, KOH and Cs₂CO₃ were used as copper, potassium and cesium precursor, respectively. In each case, the slurry was heated slowly at 80 °C under continuous stirring and maintained at that temperature until nearly all the water was evaporated. The solid residue was calcined during 2 h at 400 °C for copper-based and copper-potassium based

γ-Alumina activity

At first, it was verified that the γ-alumina support alone does not present any noticeable conversion of methanol into H₂, CO₂ and CO. According to the literature [4], γ-alumina does not offer specific sites for the adsorption of methanol but only sites favourable for the dissociation of water. On these sites, turnover frequencies lie between 0.1 and a few seconds, which means that the duration of the total catalytic cycle is about 1 s. In these conditions the diffusion rate is always higher

Conclusion

The addition of Cs to a Cu/Al₂O₃ catalyst increases its activity and stability for hydrogen production from methanol steam reforming. The hydrogen formation is equal to the hydrogen thermodynamic value for 2 wt% of Cs. The increase in reaction temperature also improves methanol conversion. Hydrogen yield is quantitative in the temperature range 350–500 °C for a CO concentration equivalent to less than 0.2 vol%. The results of XRD and XPS show that the addition of Cs oxide prevents copper oxide

Acknowledgement

This work was carried out in the frame of the scientific project REALISE whose financial contribution is greatly acknowledged.

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Production of hydrogen by steam reforming of methanol over copper-based catalysts: The effect of cesium doping

Abstract

Introduction

Section snippets

Catalysts preparation and characterisation

γ-Alumina activity

Conclusion

Acknowledgement

Int. J. Hydrogen Energy

Catal. Today

J. Power Sources

J. Power Sources

JSAE Rev.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.