Coating powdered copper catalyst with yttria sol

https://doi.org/10.1016/j.matchemphys.2011.02.025Get rights and content

Abstract

A commercial Y2O3 sol was tested as a binder for coating CuZnAl catalyst powder onto microchannels of a stainless steel plate (SSP). Coated plates were used to fabricate microchannel reactors that generate hydrogen via the steam reforming of methanol (SRM). Washcoating slurries were prepared by suspending catalyst powders into the sol. Slurry parameters, such as solid content, binder content, pH value, and stir time, were optimized to achieve a stable catalyst coating and good SRM performance. The expected stable coating could be obtained from neutral (pH 7) Y2O3 slurry that is required for a negligible dissolution of the copper component of the catalyst. The experimental coating stability generally improved with the slurry stir time. Observed improvements were attributed to a dispersion of catalyst powders in the slurry through a two-step mechanism: the mechanical disassembly of agglomerated CuZnAl powders into primary particles, and the repelling of dissembled particles through adsorption of positively charged Y2O3 binders. A reasonable reaction temperature of 280 °C was found for 95% conversion of methanol in SRM from the resulted microchannel reactors. A low CO fraction of 0.3% was also found in the hydrogen-rich gas reformed.

Research highlights

► The neutral Y2O3 sol is an effective binder for coating powders of CuZnAl catalyst. ► A particle size ratio of 15 for catalyst to binder is suggested for stable coating. ► Sufficient stirring is an important step in the catalyst slurry preparation.

Introduction

Hydrogen fuel cells are devices that efficiently convert chemical energy from hydrogen oxidation into electricity. However, the delivery (storage and transportation) of H2 fuel is major obstacle to their wide commercialization. The following steam reforming of methanol (SRM) has been recommended for the on-board supply of H2:CH3OH + H2O  3H2 + CO2, ΔH° = +49 kJ mol−1The SRM approach may be benefited with high energy density of liquid methanol and the production of H2 at low temperatures. Recently, methanol reforming using powdered CuZnAl catalysts in microchannel reactors (MCRs) has been suggested as a means for powering portable electronics devices [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The potential advantages of MCRs include compactness, uniform temperature, effective mass transfer, and a low pressure drop during operation.

Deposition of the catalyst plays an important role in the fabrication of MCRs for SRM. Generally, CuZnAl powders are deposited onto microchannels of catalytic plates by the coating method [14], [15]. Oxide sols and organic binders have been used to improve the suspension of catalyst powders in slurries. Table 1 compares the SRM performance of various binders used to fabricate MCRs. Obviously, the activity of the catalyst (in terms of T95, the temperature required for a 95% conversion of methanol) and the fraction of CO (FCO) in SRM reformates are influenced by the binder used.

We are interested in applying commercial sols as binders for MCR fabrication. A commercial ZrO2 sol (available at pH = 4) has been tried to bind powders of SRM catalyst. Experimental results indicated that the coating stability and SRM activity were significantly affected by the acidity of the resulted slurry [16]. Slurries with pH > 4 were too viscous, due to ZrO2 gelation, to produce a stable coating. Slurries with pH < 4 were sufficiently fluid (<4 cP) for a stable coating. However, the resulted catalytic plates showed poor SRM performance because the catalysts were partially dissolved in the acidic slurry.

Oxide sols of rare earth metals have been used as binders for catalyst coatings [17], [18]. A layer of CeO2–Y2O3 was precoated onto a monolithic substrate by Luo et al., and was found active for toluene oxidation after being impregnated with Pd [17]. Yttria has also been suggested in literature as a promising promoter for SRM performance [19], [20]. Here, we explore the use of a commercial yttria sol as a binder to prepare catalytic plates for the SRM process. Preliminary studies demonstrated that catalytic plates coated with Y2O3 sol indeed showed good SRM activities [21]. In this manuscript, we describe the design of the slurry preparation and propose a mechanism which leads to a stable coating.

Section snippets

Fabrication of the reforming catalyst

A powdered catalyst of Cu4Zn5Al1 (weight ratios of respective metal oxides) was prepared in house. Zinc nitrate and copper nitrate were sequentially precipitated onto γ-Al2O3 powders (from Kaiser, with a surface area of 230 m2 g−1) suspended in water at pH 9. The obtained CuZnAl catalyst was calcined at 400 °C for 4 h.

Catalyst washcoating

The calcined catalyst powders were converted into catalyst slurries by dispersion in a commercial Y2O3 sol (14 wt%) supplied by Nyacol Nano Tech. Inc. According to the supplier's

The coating of Y2O3 sol onto SSP foil

The physical properties of the commercial Y2O3 sol have been characterized for this study. Zeta potential measurements indicated that the sol had an IEP of 8.4. Therefore, Y2O3 in the as-received sol (pH  7) should be stabilized as positively charged particles due to proton adsorption. Fig. 1 shows the TEM micrograph of a diluted Y2O3 sol. The yttria sol was composed of spherical particles with an average diameter of 10 nm.

Acidity of the Y2O3 sol solution was found to affect the adhesion

Conclusions

In this study, a commercial Y2O3 sol was tested as a binder for coating CuZnAl catalyst powders onto the surface of SSP. The powders were suspended into the sol under controlled conditions to facilitate the coating process. Slurry parameters were optimized for the stability of the catalyst coating as well as the performance of the SRM reaction. The following conclusions can be drawn from this study:

  • 1.

    The Y2O3 sol (with an oxide diameter of d = 10 nm) has been demonstrated to be an effective binder

Acknowledgements

The authors appreciate the financial support of this project by the National Science Council (NSC 97-2627-M-155-001) and the Minister of Education (distinguished research centers). We are also grateful to Mr. Kang Tsao of Nyacol Nano Tech. Inc. for his immediate supply of Y2O3 sol. The corresponding author of this manuscript wants to expresses his appreciation to the Fuel Cell Center of Yuan Ze University for his postdoctoral fellowship from 2006 to 2008.

References (24)

  • P.J. de Wild et al.

    Catal. Today

    (2000)
  • G.G. Park et al.

    Catal. Today

    (2005)
  • Y. Kawamura et al.

    Chem. Eng. Sci.

    (2006)
  • T. Terazaki et al.

    J. Power Sources

    (2005)
  • J. Bravo et al.

    Chem. Eng. J.

    (2004)
  • M.S. Lim et al.

    J. Power Sources

    (2005)
  • A. Kundu et al.

    Chem. Eng. Sci.

    (2008)
  • S.M. Hwang et al.

    Appl. Catal. A

    (2007)
  • P. Reuse et al.

    Chem. Eng. J.

    (2004)
  • X. Yu et al.

    J. Power Sources

    (2005)
  • P. Alphonse et al.

    J. Colloid Interface Sci.

    (2009)
  • V. Meille

    Appl. Catal. A

    (2006)
  • Cited by (6)

    • Studies on the synthesis and sintering of nanocrystalline yttria

      2014, Ceramics International
      Citation Excerpt :

      Yttria is a ceramic that finds extensive application in sensors, optics and as a transparent material [1–6]. It is used as a catalyst in the selective catalytic reduction of NOx with methane [7] and as a “catalyst binder” in hydrogen fuel cell [8]. Yttria also finds application as a ceramic insert in the tools for cutting gray cast iron [9].

    • A well-dispersed catalyst on porous silicon micro-reformer for enhancing adhesion in the catalyst-coating process

      2014, International Journal of Hydrogen Energy
      Citation Excerpt :

      Nevertheless, using inorganic binder has some disadvantages. Chen et al. [12] reported that catalytic activity was significantly affected by the acidic sol because the catalysts were partially dissolved in the acidic slurry. Karim et al. [6] also mentioned that the low pH of the catalyst slurry caused catalyst dissolving.

    • Microstructured catalytic hollow fiber reactor for methane steam reforming

      2015, Industrial and Engineering Chemistry Research
    • Microstructured catalytic hollow fiber reactor for methane steam reforming

      2015, Industrial and Engineering Chemistry Research
    View full text