Elsevier

Catalysis Today

Volumes 293–294, 15 September 2017, Pages 129-135
Catalysis Today

Ru-coated metal monolith catalyst prepared by novel coating method for hydrogen production via natural gas steam reforming

https://doi.org/10.1016/j.cattod.2016.11.016Get rights and content

Highlights

  • Deposition-precipitation was used for highly dispersed Ru-coated metal monolith.

  • Ru catalyst layer was coated uniformly and stably on the FeCrAlloy monolith surface.

  • Solution pH had a major effect on the particle size and dispersion of Ru catalyst.

  • Ru-coated monolith catalyst (pH 7) had large surface area and high Ru dispersion.

  • Monolith catalyst (pH 7) showed better catalytic activity than pellet catalyst in NGSR.

Abstract

Ru-coated FeCrAlloy monolith catalysts were prepared by a deposition-precipitation (DP) method with different solution pHs and evaluated the catalytic activity in natural gas steam reforming (NGSR) reaction for H2 production. The characteristics of prepared monolith catalysts as to the crystallite size, surface area, pore size distribution, metal dispersion, and morphology of Ru particles were analyzed by XRD, BET, BJH, CO-chemisorption, and SEM, respectively. The solution pH had a major effect on the precipitation formation and the particle size and dispersion of Ru catalyst coated on the monolith surface. At pH 7, nano-sized Ru particles with high BET surface area (112 m2/g) and metal dispersion (23.5%) were formed uniformly on the monolith surface. Among the prepared monolith catalysts, the monolith catalyst prepared at pH 7 showed the highest CH4 conversion in NGSR. Despite the less amount of Ru metal coated on the metal monolith, the monolith catalyst showed a good catalytic performance equivalent to the 2 wt% Ru/Al2O3 pellet catalyst due to the high availability of the Ru active metal in NGSR. It was also confirmed that the Ru catalyst layer coated on the FeCrAlloy monolith had good adhesion via ultrasonic vibration test.

Introduction

Metallic structured catalytic reactors are being considered an alternative to conventional packed bed reactors. Various types of metallic structure such as monolith, foam, metal plate, and microchannel are available. These structured catalysts provide a larger ratio of geometric reaction surface area to reaction volume (S/V) and minimize radial concentration and temperature gradients as well as pressure drop [1]. Due to these advantages, it is feasible to apply a structured catalyst for rapid and highly exothermic and endothermic reactions while retaining a compact reactor design [2], [3]. There is considerable research not only on environmental catalysts for automotive exhaust gas treatment, but also on fuel processor applications for H2 production [4], [5], [6], [7]. Compact fuel processor design is required in applications where there is limited space, i.e. residential fuel cell system [8], [9]. Here, compact design can efficiently utilize and integrate heat between each unit reactions in H2 production (e.g., steam reforming, water gas shift, and preferential CO oxidation).

Although the need and interest in development of metallic structured catalytic reactors has continuously increased, commercialization has been delayed due to two primary technical difficulties in stably coating metallic structure surfaces with catalyst [10], [11], [12], [13], [14]. First, the catalyst coating layer easily detaches due to differences in the thermal expansion coefficients between the metal substrate and ceramic catalyst layer; subsequently, the activity of the structured catalyst deteriorates. Second, there is a lack of reliable technology to form a stable catalyst layer on metallic structured surface. Especially, for precious metal catalysts, the coating technology should be such that the catalyst availability is maximized.

In our previous study on metallic structured catalyst development, we developed an electrochemical pretreatment for metal surface to form a uniform metal oxide layer on a metal substrate [15]. This technique increases the metal surface roughness providing anchoring sites that enhance adhesion between the metal substrate and the coating layer. Since this treatment increases the surface area of the metallic structured substrate, the catalyst can be highly dispersed on the surface [4], [16]. After the electrochemical surface pretreatment of the FeCrAlloy substrate containing Al, an Al2O3 layer is uniformly formed on the metal substrate via calcination.

Here, the metallic monolith catalyst is applied in natural gas steam reforming (NGSR), which is a typical H2 production reaction. Using precious metals like Ru and Pt for the commercial pellet catalysts applied in the existing fuel processor increases the cost of the residential fuel cell system. Thus, a coating technology for a uniform and stable attachment of a small amount of precious metal catalyst on metal substrate is required to develop a metallic structured catalyst having high activity and long-term stability for NGSR reaction.

Montebelli et al. [17] have reviewed coating methods for catalytic activation of metallic structured substrates. In previous studies, various coating methods, such as thermal spraying, electrophoretic deposition, and the general catalyst coating methods of washcoating (or dip-coating) and impregnation (IMP), have been introduced. The review highlights the necessity of selecting the most suitable catalyst deposition techniques based on substrate geometry. With the dip-coating method, there are problems of catalyst accumulation at the channel corner of monolith or pore blocking in the foam. It is difficult to control the coating amount using the IMP method. When a large amount of catalyst is needed, this method can increase the number of coating required due to the low catalyst amount per impregnation. An additional drawback with both the dip-coating and IMP method is that most of the active metal accumulates on the outer side of the monolith; here, water evaporates fastest due to capillary forces in the drying process [18], [19]. Finally, it is difficult to apply the thermal spraying method to small channels and there are non-uniformity problems with foam coating.

To overcome limitations of the conventional catalyst coating methods, the deposition-precipitation method (DP), which can produce a uniform coating of catalyst layers with simple coating amount adjustments, is applied. In our previous study, we evaluated the effects of catalyst coating methods on the activity of monolith catalysts in NGSR reaction [20]. The results show that the Ni-coated FeCrAlloy monolith catalyst prepared by the DP method has a higher activity and long-term stability than that prepared by the washcoating method. The stability of the Ni-coated FeCrAlloy monolith catalyst is attributed to the uniformity, large specific surface area, and high metal dispersion of the Ni catalyst layer.

Here, the Ru-based catalyst is highly dispersed on FeCrAlloy monolith surface by the DP method for use as the structured catalyst of the small-scale fuel processor. The performance of this catalyst was compared to that of the commercial pellet catalyst. There are various preparation parameters for the DP method such as pH of the preparation solution and the types of precursors and precipitants. Among these parameters, we investigated the pH of the preparation solution. The effect of the solution pH is closely associated with the size and dispersion of the Ru catalyst particles. We also determined the optimum pH conditions for preparation of the Ru-coated monolith catalyst with high Ru dispersion.

Section snippets

Catalyst preparation

The monolith was prepared by rolling the corrugated and flat strips using FeCrAlloy foil (Goodfellow, thickness = 0.05 mm) containing 5 wt% Al. The diameter and height of the monolith were 22 mm and 20 mm, respectively, while cell density was 690 cpi (cells/in2). Based on our previous research, we carried out electrochemical surface treatment and calcination (900 °C, 6 h) to produce a uniform Al2O3 layer on FeCrAlloy monolith surface [15]. Al2O3 sol as a support layer was washcoated repeatedly up to the

Effect of solution pH

Preparation parameters in DP method, such as aging temperature and time, types of precursors and precipitant, and solution pH, have significant effects on dispersion, size, and shape of Ru particles. The effect of pH variation in precursor solution on Ru catalyst coated on FeCrAlloy monolith surface was evaluated by varying the precursor solution pH from 6 to 10. The Ru crystallite size, BET specific surface area, total pore volume, and Ru dispersion of the prepared monolith catalyst were

Conclusions

The deposition-precipitation (DP) method is favorable to the uniform coating of highly dispersed Ru catalyst on the surface of FeCrAlloy monolith. The dispersion and size of the Ru particles formed on the monolith surface depended on the solution pH which had a principal effect on the precipitate formation. The Ru-coated FeCrAlloy monolith catalyst prepared at pH 7 showed an excellent CH4 conversion and long-term stability in NGSR due to the large surface area and high Ru dispersion on the

Acknowledgements

This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20123010040010). This research was supported by C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M3D3A1A01064927).

References (24)

  • C. Cristiani et al.

    Catal. Today

    (2009)
  • B.P. Barbero et al.

    Chem. Eng. J.

    (2008)
  • L. Kiwi-Minsker et al.

    Catal. Today

    (2005)
  • L. Zhou et al.

    Int. J. Hydrogen Energy

    (2009)
  • C. Fukuhara et al.

    Appl. Catal. A

    (2009)
  • C. Fukuhara et al.

    Appl. Catal. A

    (2013)
  • R.M. Heck et al.

    Chem. Eng. J.

    (2001)
  • B.R. Johnson et al.

    Catal. Today

    (2007)
  • Y. Hiramitsu et al.

    Appl. Catal. A

    (2015)
  • K.Y. Koo et al.

    Catal. Today

    (2011)
  • V. Meille

    Appl. Catal. A

    (2006)
  • P. Benito et al.

    Appl. Catal. B

    (2015)
  • Cited by (0)

    View full text