Catalytic reduction of NO by propene over LaCo_1−xCu_xO₃ perovskites synthesized by reactive grinding

Abstract

One series of LaCo_1−xCu_xO₃ perovskites with high specific surface area was prepared by the new method designated as reactive grinding. These solids were characterized by N₂ adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), H₂-temperature programmed reduction (TPR), O₂-temperature programmed desorption (TPD), NO + O₂-TPD, C₃H₆-TPD, NO + O₂-temperature programmed surface reaction (TPSR) under C₃H₆/He flow as well as catalytic reduction of NO activity tests. The catalytic performance of unsubstituted sample is poor with a maximum conversion to N₂ of 19% at 500 °C at a space velocity of 55,000 h⁻¹ (3000 ppm NO, 3000 ppm C₃H₆, 1% O₂ in helium) but it is improved by incorporation of Cu into the lattice. A maximal N₂ yield of 46% was observed over LaCo_0.8Cu_0.2O₃ under the same conditions. Not only the abundance of α-oxygen but also the mobility of β-oxygen of lanthanum cobaltite was remarkably enhanced by Cu substitution according to O₂-TPD and H₂-TPR studies. The better performance of Cu-substituted samples is likely to correspond to the essential nature of Cu and facility to form nitrate species in NO transformation conditions. In the absence of O₂, the reduction of NO by C₃H₆ was performed over LaCo_0.8Cu_0.2O₃, leading to a maximal conversion to N₂ of 73% accompanied with the appearance of some organo nitrogen compounds (identified as mainly C₃H₇NO₂). Subsequently, a mechanism involving the formation of an organic nitro intermediate, which further converts into N₂, CO₂ and H₂O via isocyanate, was proposed. Gaseous oxygen acts rather as an inhibitor in the reaction of NO and C₃H₆ over highly oxidative LaCo_0.8Cu_0.2O₃ due to the heavily unselective combustion of C₃H₆ by O₂.

Introduction

The purification of automobile exhaust gases, especially from NO which can cause acid rain and photochemical smog, is regarded as one of the main objectives for catalytic control of air pollution. Several series of catalytic materials including supported noble metals [1], metal oxides [2], [3], mesostructured alumino-silicates [4], [5], pillared clays [6] and active carbon [7] were investigated as catalysts for NO reduction. Recently, special attention has been paid to the use of perovskite-type mixed oxides as candidates for NO elimination due to their excellent redox properties and non-stoichiometric structure. Libby early pointed out their potential application for NO reduction [8].

Up to now, some success has been met in the study of the selective catalytic reduction of NO with CO, one of the reducing gases present in the effluent of gasoline engines, using perovskite-type oxides as catalysts [9], [10], [11]. In contrast, there are much less reports on NO reduction by hydrocarbons, which are also reducing gases present in the gasoline engine exhaust.

Menezo et al. [12] pointed out that NO reduction efficiency over La_0.59Sr_0.39MnO₃ is poor, with a maximum NO conversion to N₂ of 5% at 350 °C despite an appreciable propene conversion above 95% with a flow of NO–C₃H₆–O₂–H₂O at a space velocity of 26,500 h⁻¹. It was found that the addition of alumina as a diluent produced a cooperative effect which improved NO conversion up to 20% by simultaneously shifting the temperature of the maximum conversion from 350 °C to 400 °C. The reduction of NO by a mixture of propane and propene over YFeO₃ was investigated by Lentmaier and Kemmler-Sack [13] who achieved a maximum NO reduction of 5% at 400 °C with a space velocity of 20,000 h⁻¹ in the presence of 2% oxygen. Moreover, NO conversion can be increased up to 10% by sulfation of YFeO₃ with (NH₄)₂SO₄ solution. Furthermore, Buciuman et al. [14] described catalytic behavior of a series of Mn-based perovskites in a NO–C₃H₆–O₂–H₂O mixture at a space velocity of 14.4 cm³/(g s) with a maximum NO conversion to N₂ of 20%. Thus, according to the literature, the catalytic activity of perovskite-type oxides for NO reduction by C₃H₆ in the presence of O₂ still needs to be improved.

LaCoO₃ was proposed as a good potential catalyst for the simultaneous reduction of NO and oxidation of unburnt hydrocarbon in as early as 1974 [15]. Subsequently, Co-based perovskites were applied in the hydrocarbon oxidation in our previous work [16], [17], [18]. In the present work, attempts to use Co-based perovksites as catalysts for the SCR of NO by C₃H₆ have been performed. Furthermore, copper containing catalysts are of special interest as they are active in a wide range of reactions for the transformation of nitrogen oxides, as reviewed by Centi and Perathoner [19]. Low coordination isolated Cu ions are regarded as the active sites for SCR of NO over Cu-zeolites [4] and our recent work showing that Cu/MCM-41 was also an active catalyst [5] indicates that the surface environment of the Cu ion is of crucial importance for the catalyst activity. Introducing Cu cations into the B sites of a perovskite structure is thus likely to yield good catalytic performances.

The major traditional drawback of perovskites is the low specific surface area (usually several m²/g) due to their preparation which involves a rather high temperature (often as high as 800 °C) to ensure the formation of the crystalline phase. This suppresses their activity and to some degree limits their application [20]. A new preparation method called reactive grinding was developed in our group for the synthesis of perovskites at room temperature via high-energy ball milling, resulting in a relatively high surface area, on the order of 100 m²/g when grinding additives are used [16], [17], [18], [21], [22].

Herein, one series of LaCo_1−xCu_xO₃ perovskites with various atomic ratios x and possessing high specific surface areas was prepared by the reactive grinding method. The aim of this work is therefore to study the influence of Cu substitution in the B site of ABO₃ solids on their physicochemical properties, emphasizing the oxygen mobility on the surface and in the lattice as well as the catalytic properties in NO reduction by propene in the presence of oxygen. It is then hoped to clarify the role of oxygen in NO reduction and propene oxidation, to determine the correlation between physicochemical properties and catalytic behavior and, finally, to propose a reaction mechanism for the catalytic reduction of NO by propene over these perovskites.