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J. Phys. Chem. C 2010, 114, 2282–2292
Promotion Effects in the Reduction of NO by CO over Zeolite-Supported Rh Catalysts Muriel Lepage,†,‡ Tom Visser,† Fouad Soulimani,† Ana Iglesias-Juez,† and Bert M. Weckhuysen*,† Inorganic Chemistry and Catalysis group, Department of Chemistry, Utrecht UniVersity, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands, and AdVanced Technology DiVision, Toyota Motor Europe, Hoge Wei 33B, 1930 ZaVentem, Belgium ReceiVed: October 30, 2009; ReVised Manuscript ReceiVed: December 19, 2009
Rh nanoparticles supported on a series of zeolite Y samples containing different monovalent (H+, Na+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations have been used as model systems to investigate the effect of promoter elements in the reduction of NO by CO. Infrared (IR) spectroscopy with NO as a probe molecule allowed monitoring of the electronic changes in the local environment of Rh. The IR bands corresponding to linearly adsorbed NO, i.e., linear and dinitrosyl species, were found to shift to lower wavenumbers with increasing ionic radius-to-charge ratio of the cation. Simultaneously, a lower ignition temperature for NO reduction was observed. In addition, the relative intensity of the bridge-bonded NO band as compared to the total absorbance of Rh-bonded NO species decreased with increasing Lewis acidity of the cation, as expressed by the Kamlet-Taft parameter R. The latter observation matches with similar trends, observed in a previous study (Lepage, M., et al., J. Phys. Chem. C 2008, 112, 9394) for the same catalysts using CO as the IR probe molecule, which could be related to the Rh activity for the CO oxidation reaction. The samples of the present study also showed different catalytic activities, although a straightforward correlation between the results obtained with NO IR spectroscopy and the catalytic reduction of NO by CO could not be established. Introduction Emissions from cars have been regulated in Europe since the 1970 Directive 70/220/EEC showing, among others, increasingly stricter requirements concerning the abatement of nitrogen oxides (NOx).1,2 For this purpose, Rh has been until today the precious metal of choice in three way catalyst (TWC) applications where it is used to reduce NO with CO or hydrocarbons.3-13 It has been selected for its high activity and selectivity toward N2 formation, its good thermal stability, and its non-negligible resistance to poisons in spite of its elevated and highly fluctuating price. The need for an enhanced catalytic performance has pushed the catalyst material designers to develop better support materials, as they have been shown to have a strong influence on the precious metal catalytic activity, their dispersion, their valence state, and their aging behavior, i.e., their tendency to sinter, get poisoned, and be involved in solid state reactions.9,14-19 Promoters, such as alkaline and alkaline earth metals, have been introduced to improve the TWC activity.7,8,10,12,20-22 In previous papers, we have shown that these promoting elements have a direct impact on the electron density of supported Pt and Rh nanoparticles, which was directly reflected in their catalytic activity for the CO oxidation reaction.23-26 As Rh is mainly introduced in TWCs for the reduction of NOx and as it has been observed that the effect of Ba can be different on Pt- and Rh-based catalysts,21 it is therefore of utmost interest to verify in a systematic manner how promoters, favoring the CO oxidation in excess of oxygen, influence the reduction of * To whom correspondence
[email protected]. † Utrecht University. ‡ Toyota Motor Europe.
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NO. The NO reduction by CO was also shown to be sensitive to the Rh particle size and the CO oxidation to be structure dependent.5,27-30 Additionally, it was demonstrated that alkaline metals promoting Rh/Al2O3 catalysts had an influence on the precious metal dispersion when applied by wet impregnation.31 Furthermore, we have demonstrated that the trends observed in the CO IR signal for two series of zeolite Y- and silica-supported Pt catalysts were similar.24 Therefore, it was decided to study the NO reduction by CO by means of Rh dispersed on the same series of zeolite Y supports with a varying composition, the charge compensating cation being modified via ion exchange. The goal of this work is to elucidate support effects on the activity of Rh for the NO reduction reaction by CO in a systematic manner. For this purpose, we have studied Rh-based model catalysts making use of zeolites as supports in which different monovalent (H+, Na+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations are present. By using NO as a probe molecule in combination with IR spectroscopy, it will be shown that an increasing electron density on the supported Rh nanoparticles, indirectly induced by alkali and earth alkaline metal ions via the framework oxygen atoms, promotes the bonding of NO in a bridge position. However, as opposed to the clear effect of the cation promoters on the activity of Pt and Rh for the CO oxidation reaction in excess of oxygen, no systematic effect on the activity of Rh for the NO reduction by CO reaction could be observed. Experimental Section 1. Catalyst Preparation. The starting material for ion exchange was a Na-Y material from AKZO Nobel with a Si: Al ratio of 2.3. The zeolite support materials with H+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, and Ba2+ were obtained by five consecutive ion exchanges for 24 h with an aqueous 0.1 M
10.1021/jp910371j 2010 American Chemical Society Published on Web 01/12/2010
Promotion Effects in the Reduction of NO by CO
J. Phys. Chem. C, Vol. 114, No. 5, 2010 2283
TABLE 1: Summary of the Rh-Based Model Catalysts under Study and Their Main Characteristicsa ICP-AES +
TEM
EXAFS
sample
Na exchange (%)
Rh (wt %)
particle size (nm)
average particle size (nm)
Rh-Rh coordination number
average particle size (nm)
Rh/H-Y Rh/Na-Y Rh/K-Y Rh/Rb-Y Rh/Cs-Y Rh/Mg-Y Rh/Ca-Y Rh/Sr-Y Rh/Ba-Y
NA 100 78 65 72 66 77 83 83
0.21 0.33 0.32 0.36 0.49 0.32 0.38 0.39 0.45
