Effect of Ag, Pd and Co promoters on the Selective Catalytic Reduction (SCR) of NOx by ethanol over sulfated ceria-zirconia catalysts
Graphical abstract
Highlights
► Cex+ acidic species on the sulfated ceria-zirconia support are the most active sites for NOx-SCR by ethanol. ► Among the different organics formed under EtOH-SCR conditions, acetaldehyde, and to a lesser extent ethene, are directly involved in some pathways leading to N2, some of the final steps being close to those occurring in NH3-SCR. ► Ethylnitrite is a major product at low temperatures and is a secondary source for acetaldehyde. ► Supported Pd species have mostly an inhibiting effect on SCR activity and N2 selectivity due to the promotion of non-selective HC oxidation reactions. ► Supported Ag+ species, which are rather active for EtOH-SCR, deactivate to metallic silver under reaction conditions.
Introduction
Compared with other post-treatment technologies such as NOx adsorbers or urea-SCR, the intrinsic advantages of a lean-NOx catalyst technology, such as HC-SCR, are its simplicity in terms of engine control, after treatment system and infrastructure implementation requirements. The reductant efficiency in the catalytic reduction of NOx to N2 by hydrocarbons usually follows the order: alkanes < alkenes < oxygenated [1]. Among the class of oxygenated compounds, ethanol is one of the most interesting for mobile applications because on the one hand, it can be produced from renewable energy sources and blended with gasoline and/or diesel fuel, and on the other hand, it is less toxic than ammonia. With this reductant, high activities and selectivities to N2 were achieved at medium temperatures (300–400 °C) over Ag/Al2O3 [2] but also on Ag- or Ba-Y zeolites [3], [4]. Among the other investigated catalysts, ion-exchanged zeolites, such as Fe-, Co- or Cu- on BEA and Ag- on ZSM-5 have also shown some interest despite their lower activity [5], [6], [7].
The mechanism of NOx reduction with oxygenates on silver-promoted catalysts, namely Ag/Al2O3, has been investigated by many authors [2], [3], [8], [9], [10], but still remains partly unclear towards some points. This is widely due to the ill-defined state of silver during the HC-SCR reaction, which is strongly affected by interdependent parameters such as: the initial silver loading and salt used as precursor, the characteristics of the support, the reaction temperature, the concentration of reactants/contaminants (C-containing, NOx, O2, H2, SO2, H2O…) at the solid/gas interface. It has been proposed that Ag species in the +1 oxidation state promote the SCR reaction by increasing the yield of enolic/enolate species, which transform to the key isocyanate (NCO) intermediates by reaction with NO2 [2], [10]. On the other hand, metallic silver clusters were found rather inactive for NOx reduction while promoting the NO oxidation to NO2 and the HC depletion by total oxidation [8].
Apart from the considerable body of data available on Ag/Al2O3, literature studies devoted to the role of promoters and mechanisms of EtOH-SCR on other catalytic systems are much more scarce. In [11], we aimed to investigate the performances in ethanol-SCR of novel sulfated ceria-zirconia (SCZ) catalysts prepared by direct sulfation of the CexZr1−xO2 nanopowders (with x = 0, 0.2, 0.5, 0.8 and 1) in sulfuric acid. By comparison with the sulfated single oxides, the sulfated mixed oxides with intermediate Ce molar contents (typically between 0.2 and 0.8) displayed a superior behaviour, with DeNOx yields of the order of 40–50% between 300 and 400 °C [8]. The sulfation treatment was also found to be beneficial since it promoted the mild oxidation of the reductant instead of its total oxidation to CO2. The SCR-active sites on SCZ materials could possibly consist of [Ce(O,Zr)n⋯H]x+ adducts in the vicinity of a surface sulfate or bisulfate species [8], [12]. The intrinsic activity of Ce sites was found to be slightly higher when associated with tetragonal structures (or intermediate compositions) than with the cubic ones (Ce-rich compositions). By contrast, the sulfated zirconia catalyst with a monoclinic structure was found to be almost completely inactive under the same conditions [8].
Due to their intrinsic SCR activity, SCZ supports can be considered as a starting point to develop more complex, i.e. promoted, SCR catalysts. Hence, the aim of this article is to assess to what extent the reaction pathways and performances of SCZ catalysts could be affected by the presence of supported promoters such as Pd, Co and Ag. We focused on these metals because, on the one hand, Pd and Co supported SCZ catalysts showed an honourable activity in CH4-SCR [13] and on the other hand, Ag is obviously interesting for ethanol-SCR [2], [3]. In this study, the conditions of formation of important SCR intermediates produced during temperature-programmed surface reactions (TPSR) and steady-state experiments are investigated in order to assess the role of each promoter and to establish structure–properties relationships.
Section snippets
Catalysts
The Ce0.21Zr0.79O2 (CZ) nanopowder (Sbet = 180 m2/g) was kindly supplied by Rhodia (La Rochelle, France). The sulfated ceria-zirconia material (labeled SCZ28 hereafter) was obtained by: (i) reacting the CZ material (5 g) with a 0.5 M H2SO4 solution (100 mL) for 45 min; (ii) washing the slurry with diluted 0.05 M H2SO4; (iii) drying at 70 °C under vacuum for 24 h and calcination in air at 500 °C for 2 h. The resulting material has a specific surface area of 185 m2/g and a sulfate density of 2.0 SO42−/nm2.
Structural analysis
A detailed structural analysis of the sulfated ceria-zirconia support used in this study (SCZ28) and the corresponding Pd-promoted catalysts had been performed previously [12], [13]. From Fig. 1, it can be deduced that the XRD patterns of the Pd(0.24wt%)- and Co(1.03wt%)-promoted SCZ28 catalysts are similar to those of the support, only exhibiting the characteristic peaks of the tetragonal t structure (confirmed by Raman spectroscopy) with a crystallite size of 3–7 nm (as observed by TEM, [13]).
Conclusion
In this study, the effects of different metal promoters (Pd 0.24%, Co 1.03%, Ag 3.00%) on the activity of novel sulfated ceria-zirconia catalysts were investigated for EtOH-SCR.
From characterization studies, it was inferred that the Pd and Co promoters are mainly present on the acidic support as highly dispersed entities of structure [(MO)n⋯H]+ in co-existence with MOx/M0 clusters. For the Ag promoter, these species probably co-exist with a new Ag2SO4 cristalline phase, which decomposes above
Acknowledgments
This work has been done within the frame of the international research group (GDRI) on Environmental Catalysis, Renewable Energies and Sustainable Development (2006-2010, CNRS-PAN). We also greatly thank Rhodia-France for supplying the ceria-zirconia samples.
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