The role of H2O and oxidized copper species in methanol steam reforming on a Cu/CeO2/Al2O3 catalyst prepared by one-pot sol–gel method

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Abstract

A Cu/CeO2/Al2O3 catalyst prepared by a single-step sol–gel method was studied for methanol steam reforming (SRM) to gain information on the oxidation state of Cu and the role of oxidized Cu species. The catalyst was tested in a new laboratory-made flow apparatus that allowed on-line analysis of the products and measurement of the oxidation state and dispersion of Cu in situ. The catalyst pre-reduced in H2 was more active and selective than the not pre-reduced one. The oxidation state, evaluated after SRM tests, suggested the presence of a layer of oxygen atoms on the metallic Cu surface. Treating the pre-reduced catalyst with H2O vapour at 250 °C caused dissociative H2O chemisorption and led to a partial oxidation of Cu. The H2O oxidized sample reacted with CH3OH producing CO2 and H2 as long as oxygen was present on the Cu surface. The results support an oxidation–reduction mechanism.

Research highlights

▶ Treating a Cu/CeO2/Al2O3 catalyst with H2O at 250 °C oxidizes Cu superficially. ▶ The H2O oxidized catalyst reacts with CH3OH at 200–300 °C producing CO2 and H2. ▶ After SRM test, Cu average oxidation state of 0.45 is measured. ▶ The catalyst is more active and selective for SRM after pre-reduction with H2.

Introduction

Methanol steam reforming (SRM, reaction (1)), methanol partial oxidation (POM, reaction (2)) and a combination of these reactions named oxidative methanol steam reforming (OSRM) are the most suitable processes that allow to produce H2 from methanol for feeding fuel cells on board of electric vehicles [1].CH3OH(g) + H2O(g) = CO2 + 3H2 ΔH° = 49.5 kJ mol−1CH3OH(g) + (1/2)O2 = CO2 + 2H2 ΔH° = −192.3 kJ mol−1

These reactions are activated by catalysts generally based on metallic Cu or Pd dispersed on an oxide matrix. Cu based catalysts give the highest performances in terms of selectivity and low temperature activity [1], [2]. The metallic Cu phase is generally dispersed in pure or mixed oxides such as ZrO2 [3], [4], [5], [6], MnOx [7], ZnO/Al2O3 [8], [9], [10], [11], [12], [13], Cr2O3/Al2O3 [14], ZrO2/Al2O3 [15], and ZnO/ZrO2 [16], [17].

Catalysts based on Cu/CeO2 were recently proposed for SRM and OSRM [18], [19], [20], [21], [22], [23], [24]. These systems, being particularly active for the preferential oxidation of CO with O2 in the presence of hydrogen (CO-PROX) [25], [26], [27], [28], [29], [30], could help to reduce the CO concentration in the gaseous streams coming from the OSRM reactor. The effectiveness of this type of catalysts is partly due to the high Cu dispersion that can be attained in the presence of CeO2 [18], [27], [31], [32], [33]. Moreover it is known that the reducibility of CuOx species increases when CeO2 is used as support or added to other support oxides [27], [34], [35], [36]: this effect, that is probably related to the high oxygen mobility in CeO2, contributes to increase the catalytic activity in the processes of SRM and CO PROX [19], [20], [27], [28]. Another important feature of ceria as support is its ability to fix part of Cu in the 1+ oxidation state [26], [33], [36], [37], [38]: this state is probably involved in the oxidation of CO [26], [35], [39], [40] and can play an important role also in the mechanism of SRM [8]. It must be noted that both Cu dispersion and Cu reducibility are greatly affected by the preparation method [26], [34], [37], [41]. Analyzing literature data on Cu/CeO2 systems, it clearly appears that some points deserve further investigation, such as the oxidation state of the active phase, the role played by the oxide support and the effect of the preparation method.

A new preparation method of Cu/CeO2/Al2O3 catalysts, based on a single-step sol–gel procedure, was described recently by some of us [18], [25]. This method allowed to obtain catalysts with very high surface area and structurally organized mesoporosity, that showed high catalytic activity and good thermal stability for the PROX reaction [25]. One of these catalysts was studied for methanol reforming and showed high activity in terms of hydrogen production rate [18]. The results of this study suggested an involvement of CeO2 in the catalysis through an effect of either H spillover or a CeO2 assisted oxidation of Cu to Cu+ [18]. In this paper a Cu/CeO2/Al2O3 catalyst prepared by the same method is studied for SRM with the aim of gaining information on the oxidation state of Cu and the role of oxidized species in the reaction mechanism. A new experimental apparatus is used for this purpose, that allows, during the same experiment, to test the catalytic activity and to characterize, in situ, the redox properties and Cu metal dispersion.

Section snippets

Experimental

The catalyst EMCe14Cu7 containing 13.8 wt% Ce and 6.6 wt% Cu was prepared by a single-step sol–gel method starting from Al sec-butoxide, Cu and Ce stearate, and treated at 550 °C in air flow as described previously [18]. The material was found to have a surface area of 360 m2 g−1 and to contain CeO2 and CuO phases highly dispersed in an alumina phase of low crystallinity [18].

Temperature programmed reduction (TPR), Cu dispersion measurements and catalytic tests were carried out in a laboratory made

SRM test on EMCe14Cu7

SRM tests were carried out on both the as-prepared catalyst EMCe14Cu7 and on the same sample pre-reduced with H2 as described in Section 2, EMCe14Cu7-PR. Data obtained with the as-prepared catalyst are shown in Fig. 2, Fig. 3. In Fig. 2A we report the quantitative evolution as a function of time of the MS signals representative of methanol (masses 32, 31, 30 and 29) and in Fig. 2B those of H2O (m. 18 and 17) and H2 (m. 2). Temperature was kept linearly increasing with time. The signals of

Conclusions

The present work has confirmed some concepts on Cu based catalysts for SRM and has given new information on the mechanism of the reaction. It has been shown that in a Cu/CeO2/Al2O3 catalyst the metallic Cu phase active for SRM can be obtained from reduction of CuO directly in the reaction environment, but the Cu phase obtained by the reaction of CuO with CH3OH has different catalytic properties in comparison with that produced by the reaction with H2. These differences have been related to

References (60)

  • I. Ritzkopf et al.

    Appl. Catal. A: Gen.

    (2006)
  • A. Szizybalski et al.

    J. Catal.

    (2005)
  • S. Esposito et al.

    Appl. Catal. A: Gen.

    (2010)
  • J. Papavasiliou et al.

    Catal. Commun.

    (2005)
  • M. Turco et al.

    Appl. Catal. B: Environ.

    (2007)
  • M. Turco et al.

    J. Catal.

    (2004)
  • J.K. Lee et al.

    Appl. Catal. A: Gen.

    (2004)
  • C. Horny et al.

    Chem. Eng. J.

    (2004)
  • S. Velu et al.

    J. Catal.

    (2000)
  • B. Lindström et al.

    Chem. Eng. J.

    (2003)
  • B. Lindström et al.

    Appl. Catal. A: Gen.

    (2002)
  • P.H. Matter et al.

    J. Catal.

    (2005)
  • P.H. Matter et al.

    J. Catal.

    (2004)
  • M. Turco et al.

    Appl. Catal. B: Environ.

    (2009)
  • P. Yaseneva et al.

    Catal. Today

    (2008)
  • R. Pérez-Hernandez et al.

    Int. J. Hydrogen Energy

    (2007)
  • J. Papavasiliou et al.

    Appl. Catal. B: Environ.

    (2007)
  • S. Patel et al.

    Chem. Eng. Sci.

    (2007)
  • H. Oguchi et al.

    Appl. Catal. A: Gen.

    (2005)
  • A. Mastalir et al.

    J. Catal.

    (2005)
  • E. Moretti et al.

    Appl. Catal. A: Gen.

    (2008)
  • Z. Liu et al.

    J. Mol. Catal. A: Chem.

    (2007)
  • C.-Y. Shiau et al.

    Appl. Catal. A: Gen.

    (2006)
  • G. Avgouropoulos et al.

    Appl. Catal. B: Environ.

    (2005)
  • Y. Liu et al.

    Catal. Today

    (2004)
  • S. Patel et al.

    Fuel Process. Technol.

    (2007)
  • X. Zhang et al.

    J. Mol. Catal. A: Chem.

    (2003)
  • M. Fernández-García et al.

    J. Catal.

    (1997)
  • P. Djinović et al.

    Appl. Catal. A: Gen.

    (2009)
  • C.S. Polster et al.

    J. Catal.

    (2009)
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