Active size-controlled Ru catalysts for selective CO oxidation in H2
Graphical abstract
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).
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