Influence Of Palladium Crystallite Size On The Course Of Denox Reaction
INFLUENCE OF PALLADIUM CRYSTALLITE SIZE ON THE COURSE OF DENOX REACTION
W. Ćwikla-Bundyra, D. Nazimek
Faculty of Chemistry, University of Marie Curie-Skłodowska
3 Marie Curie Skłodowska Sq, 20-031 Lublin, Poland
Introduction
The palladium catalyst is an interesting catalytic system in the reaction between CO and NO. The aim of the present paper was to examine the influence of crystallite size of palladium on course of NO reduction by CO and selectivity of the process.
Experimental
The investigated catalysts Pd/Al2O3 was supported by g-Al2O3 from St. Gobain STotal = 82.4 m2/g were obtained only by double impregnation of the alumina supports EDTA aqueous solution, following by PdCl3 solution [1,2]. The Pd loading was is measured by X-ray fluorescence (XRF). Mean crystallite size of Pd was determined by X-ray diffraction (XRD) (occurrence ± 30%) and from CO chemisorption measurements. Total surface areas of the all investigated catalysts examined were determined by the BET method [3].
Results
Table 1 gives the results obtained from adsorption measurement and other physico-chemical properties of the Pd/Al2O3 catalysts examined.
Table 1. The physicochemical properties of Pd/Al2O3 catalysts.
Sample
Metal loading, wt. %
Active surface area, m2/g
Crystallite size of Pd, nm
Pd/Al2O3 (CIM)
0.146
1.38
1.22
Pd/Al2O3 (DIM)
0.112
1.10
1.18
Pd/Al2O3 (DIM)
0.087
0.81
1.24
Pd/Al2O3 (DIM)
0.093
1.03
1.05
Pd/Al2O3 (DIM)
0.109
0.87
1.45
Pd/Al2O3 (DIM)
0.106
1.03
1.20
Pd/Al2O3 (DIM)
0.107
0.88
1.42
CIM – Classical Impregnation Method, DIM – Double Impregnation Method
Figure 1 shows the relation ship between crystallite size of palladium and reaction rate of DENOX reaction, at 723 K and XNO conversion = 50%. The reaction rates of NO consumption show a clear maximum as a function of the crystallite size of palladium.
Conclusions
The result described and presented in this paper lead to the following conclusions:
– the observed maximum in the rate of conversion of NO (selectivity towards N2=100%) over the range of Pd crystallite size studied probably correlates with the concentration of low – coordination centres of the palladium surface (probably B5 or B4 sites),
– it is probable that such B5 or B4 sites may play an important role in the NO + CO reaction through the dissociative chemisorption of NO and CO on active sites (including B5 or B4 sites),
– the obtained results of the dependence of the reaction rate of NO consumption on the crystallite size of Pd were are analogous to results for rhodium catalysts, was described in paper [3].
References
1. J. Barcicki, D. Nazimek, W. Grzegorczyk, T. Borowiecki, R. Frąk, M.Pielach, React. Kinet.Catal. Lett., 17(1981)169.
2. J. Ryczkowski, React.Kinet.Catal.Lett., 40(1989)189.
3. W. Ćwikła, D. Nazimek, Ads. Sci. & Technol., 16(1998) 773.
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