Elsevier

Applied Catalysis B: Environmental

Volume 127, 30 October 2012, Pages 129-136
Applied Catalysis B: Environmental

Active size-controlled Ru catalysts for selective CO oxidation in H2

https://doi.org/10.1016/j.apcatb.2012.08.010Get rights and content

Abstract

We propose an activation method to control Ru particle size in supported Ru catalysts, viz. commercial Ru/Al2O3 and home-made Ru/SiO2, in order to increase the catalytic activity for the preferential CO oxidation (PROX). Ru particle size is controlled by adjusting pre-treatment conditions and it affects the catalytic activity for the PROX over supported Ru catalysts. Several measurements: inductively coupled plasma-atomic emission spectroscopy (ICP-AES), bright-field transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), CO chemisorption, and O2 chemisorption were conducted to characterize the catalysts. The co-presence of H2 and O2 is essential for controlling the Ru particle size accurately. The PROX activity especially at low temperatures increases with increasing particle size of Ru, which seems to be closely related to the adsorption behavior of O2 on Ru surface.

Highlights

► The Ru dispersion can be controlled through thermal treatment in the presence of H2 and O2. ► There exists an optimum Ru particle size in Ru/Al2O3 for the selective CO oxidation in H2. ► The reduced Ru catalyst is superior to the oxidized one for CO oxidation.

Introduction

Recently, metal and/or metal oxide nanoparticles with particles ranging from 1 to 20 nm have attracted considerable attention in various fields because of their unique properties [1], [2], [3], [4], [5], [6]. The control of the particle size, which in turn changes the surface-to-volume ratio of nanoparticles, is also an important issue in catalysis because all the catalytic reactions occur at the metal and/or metal oxide surface [7], [8]. Furthermore, the catalyst performance is sensitive to the particle size of active metal or metal oxides in the catalyst because the surface structure and electronic properties of the nanoparticles can be significantly altered in this size range [7], [8], [9], [10], [11], [12], [13], [14]. The formation of nanoparticles in the presence of surface-capping agents and the subsequent anchoring of these particles on a support is a well-known methodology employed in the case of nanocatalysts [9], [10], [11]. Moreover, it has been reported that the physical or chemical pre-treatment of the supported metal or metal oxide catalysts can be carried out to control their particle size [12], [13], [14].

With the increasing need for a highly efficient energy conversion system, fuel cells have attracted considerable attention, and consequently, there is a need to develop a high-performance fuel processor, in which various hydrocarbons are transformed into H2. Preferential CO oxidation (PROX) is an essential step in a fuel processing system for a low-temperature polymer electrolyte membrane fuel cell (PEMFC). The purpose of this step is to remove the residual CO in an H2-rich stream, because CO can degrade the electrochemical performance of the Pt-based anode of PEMFC [15], [16]. In the PROX system, CO oxidation (CO + (1/2)O2  CO2) occurs predominantly, rather than H2 oxidation (H2 + (1/2)O2  H2O). Moreover, two hydrogenation reactions (CO + 3H2  CH4 + H2O, CO2 + 4H2  CH4 + 2H2O) and the water-gas shift reaction (CO + H2O  CO2 + H2) occur because all the reactants (CO, CO2, H2, and H2O) coexist.

While searching for PROX catalysts, some researchers reported the high catalytic activity at low temperatures through the addition of promoters in the case of Pt-based catalysts [17], [18], [19], [20], [21]. Nevertheless, Ru-based catalysts have been proposed as active ones because of their excellent performance at low temperatures [22], [23]. Besides noble metal catalysts, active studies on non-noble metal PROX catalysts have been underway owing to their low cost [24], [25], [26], [27], [28]. Compared to the number of studies on Pt-based catalysts, few studies have been conducted on the improvement of the catalytic performance of Ru-based catalysts [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. Some factors, such as the types of Ru precursors [29], [30], pre-treatment conditions [29], [30], [31], [32] and types of supports [30], [31], [33], [34], [35] were examined in terms of their effect on the PROX activity. However, thus far, no detailed study has been carried out to examine the effect of the Ru particle size on the PROX activity.

Herein we propose an activation method for controlling the Ru particle size in order to increase the catalytic activity of Ru catalysts in PROX. In the case of supported Ru catalysts, the Ru particle size is controlled by changing the composition of the pre-treatment gas mixture, and the catalytic activity in PROX can be significantly improved by using activated Ru-based catalysts.

Section snippets

Catalyst preparation

Ru/Al2O3 catalysts were purchased from Aldrich. The Ru concentration in Ru/Al2O3 was 5 wt.%. Silica was prepared by the calcination of silica gel (Sigma–Aldrich) at 773 K in air and utilized as a support. Ru/SiO2 catalysts were prepared by the incipient wetness impregnation method. The Ru concentration in Ru/SiO2 was determined to be 1.08 wt.%. Ruthenium nitrosylnitrate (Ru(NO)(NO3)3·xH2O, Aldrich) was utilized as a Ru precursor. Reduced Ru/Al2O3 (denoted as Ru/Al2O3(R)) and Ru/SiO2 (denoted as

Results and discussion

To determine the structural and electronic state of Ru/Al2O3 catalysts, Ru k-edge X-ray absorption near-edge structure (XANES) spectra were obtained. Fig. 1(A) shows the Ru k-edge XANES spectra of reduced Ru/Al2O3 (Ru/Al2O3(R)) and activated Ru/Al2O3 (Ru/Al2O3(A)) with Ru references such as Ru foil and RuO2. The spectra of Ru/Al2O3(R) and Ru/Al2O3(A) show the presence of Ru metal characterized by two oscillations above the edge jump; moreover, no significant difference is observed between Ru/Al2

Conclusion

In this study, we proposed an activation method to increase the Ru particle size in supported Ru catalysts and used these catalysts for selective CO oxidation in an H2-rich stream. The activated Ru catalyst showed higher catalytic activity than the fresh catalysts; this can be attributed to the controlled increase in the Ru particle size. The co-presence of H2 and O2 is essential for controlling the Ru particle size accurately. In particular, an activated commercial 5 wt.% Ru/Al2O3 catalyst can

Acknowledgements

This research was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2010-0029617). Experiments at PLS were supported in part by a Korea Research Foundation Grant funded by the Korean Government (MEST) (KRF-2007-412-J04001) and POSTECH. This work was completed with Ajou University research fellowship of 2010 (S-2010-G0001-00059).

References (46)

  • P. Poizot et al.

    Nature

    (2000)
  • R. Makiura et al.

    Nature Materials

    (2009)
  • J.A. Rodriguez et al.

    Science

    (2007)
  • S. Crossley et al.

    Science

    (2010)
  • Y. Zhai et al.

    Science

    (2010)
  • J. Kim et al.

    Angewandte Chemie International Edition

    (2006)
  • A.T. Bell

    Science

    (2003)
  • R. Schlögl et al.

    Angewandte Chemie International Edition

    (2004)
  • C.-X. Xiao et al.

    Angewandte Chemie International Edition

    (2008)
  • J. Kim et al.

    Advanced Materials

    (2009)
  • S.H. Joo et al.

    Nano Letters

    (2010)
  • J.R.A. Sietsma et al.

    Angewandte Chemie International Edition

    (2007)
  • G.L. Bezemer et al.

    Journal of the American Chemical Society

    (2006)
  • I.H. Son et al.

    Journal of Catalysis

    (2002)
  • R. Farrauto et al.

    Annual Review of Material Research

    (2003)
  • E.D. Park et al.

    Catalysis Today

    (2009)
  • E.-Y. Ko et al.

    Angewandte Chemie International Edition

    (2007)
  • S. Alayoglu et al.

    Nature Materials

    (2008)
  • A.U. Nilekar et al.

    Journal of the American Chemical Society

    (2010)
  • Q. Fu et al.

    Science

    (2010)
  • B. Qiao et al.

    Nature Chemistry

    (2011)
  • S.H. Oh et al.

    Journal of Catalysis

    (1993)
  • Y.H. Kim et al.

    Catalysis Today

    (2009)
  • Cited by (18)

    • TiO<inf>2</inf> and ZrO<inf>2</inf> supported Ru catalysts for CO mitigation following the water-gas shift reaction

      2018, International Journal of Hydrogen Energy
      Citation Excerpt :

      Above 120 °C both catalysts show activity towards the methanation of CO, which is evident in Fig. 7. Kim et al. [16] reported that increasing CO conversions during the PROX reaction over 5 wt % Ru-Al2O3 catalysts above 170 °C may be due to either the water-gas shift reaction (CO + H2O ↔ CO2 + H2) or CO methanation (CO + 3H2 ↔ CH4 + H2O) taking place. Over these Ru-Al2O3 catalysts, CO methanation occurred simultaneously with PROX, which was reported to still be beneficial for CO removal, especially at high temperatures.

    • Metal phosphate-supported RuO<inf>x</inf> catalysts for N<inf>2</inf>O decomposition

      2016, Journal of the Taiwan Institute of Chemical Engineers
      Citation Excerpt :

      Although ruthenium is expensive, Ru- or RuOx-based heterogeneous catalysts have been demonstrated to be useful in ammonia synthesis [1–3], CO methanation [4–6], Fischer–Tropsch synthesis [7–9], oxidation of HCl to Cl2 [10], hydroconversion of hydrocarbons [11], selective oxidation of methane [12] and alcohols [13–16], catalytic conversion of bio-oil [17], ozonation of organic compounds in water [18,19], combustion of VOCs [20–22], CO oxidation [23–25], and N2O decomposition [26–32].

    View all citing articles on Scopus
    View full text