Ind. Eng. Chem. Res. 2005, 44, 2955-2965
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Influence of the Operating Parameters on the Selective Catalytic Reduction of NO with Hydrocarbons Using Cu-Ion-Exchanged Titanium-Pillared Interlayer Clays (Ti-PILCs) Jose´ L. Valverde, Antonio de Lucas, Fernando Dorado,* Amaya Romero, and Prado B. Garcı´a Departamento de Ingenierı´a Quı´mica, Facultad de Quı´micas, Universidad de Castilla-La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain
Recently, the suitability of titanium-pillared clays as catalyst supports has been explored, because of their interesting physical properties. In this work, we have studied the influence of the operating parameters on the selective catalytic reduction of NO by propylene over a characterized Cu-ion-exchanged titanium-pillared interlayer clays (Ti-PILCs). NO reduction was favored by a C3H6 concentration increase when the combustion of this hydrocarbon was complete. NO reduction was observed to be promoted by a small amount of oxygen when the oxygen concentration was less than that required for complete C3H6 combustion (critical oxygen concentration). For oxygen concentrations higher than this critical value, the effect of oxygen concentration was different, depending on the reaction temperature, compared to that corresponding to the maximum NO conversion. Two different rate-limiting steps exist in the SCRNO, when Cu-Ti-PILCs are used as catalysts, depending on the reaction temperature: surface reaction control at low temperatures and external mass-transfer control at high temperatures. 1. Introduction Nitrogen oxides (NOx) are emitted primarily both from stationary and automotive sources and contribute largely to a variety of environmental problems, such as the formation of acid rain and the resultant acidification of aquatic systems, the photochemical reaction in the stratosphere that is destroying the ozone in the atmosphere, and the harmful impact of NOx on the respiratory system of human beings.1,2 To alleviate these problems, the emissions of nitrogen oxides must be seriously controlled. In 1986, Iwamoto et al.3 reported the activity of CuZSM-5 for the catalytic decomposition of NO. These authors observed that its catalytic activity decreased sharply as the NO concentration decreased, and they noted that the catalyst suffered from severe deactivation in the presence of oxygen. Later, they demonstrated that Cu-ZSM-5 showed significant activity when it was used for a real lean-burn engine; moreover, the rate of reaction increased in the presence of oxygen. It was observed that the reaction that was occurring in this case was not the decomposition of NO but rather the reduction of NO by hydrocarbons contained in the emission gases. In 1990, Held et al.4 and Iwamoto5 proposed that the reduction of NO over Cu-ZSM-5 could be greatly enhanced in an excess of oxygen by the presence of small amounts of hydrocarbons. Following this discovery, many catalysts (such as various types of solid acids and bases) were demonstrated to be active catalysts for this reaction.6 This reaction, which is generally called selective catalytic reduction of NOx by hydrocarbon (HC-SCR), is a challenging subject that has attracted much attention recently and has been * To whom correspondence should be addressed. Tel.: +34-926 29 53 00. Fax: +34-926 29 53 18. E-mail:
[email protected].
described as a potential method to remove NOx from natural-gas-fueled engines, such as lean-burn gas engines in co-generation systems.7 SCR of NOx by hydrocarbons can also find important applications for learnburn (i.e., O2-rich) gasoline and diesel engines, where the noble-metal three-way catalysts are not effective in the presence of excess oxygen.8 A large number of catalysts, such as V2O5-WO3 (or MoO3)/TiO2, other transition-metal (e.g., iron, chromium, cobalt, nickel, copper, niobium, etc.) oxides, and doped catalysts, as well as zeolite-type catalysts (e.g., H-ZSM-5, Fe-Y, Cu-ZSM-5), have been found active in this reaction. Early studies were concentrated primarily on developing catalysts with ZSM-5 as the porous support.9-11 However, the structure of ZSM-5 is not hydrothermally stable, and in the presence of water steam and SO2, which are the normal conditions in real applications, deactivation was severe.12 On the other hand, vanadium-based catalysts are very active in this reaction;13 however, major disadvantages remain, such as their toxicity and high activity for the oxidation of SO2 to SO3 (which causes corrosion) and plugging of the reactor and heat exchangers. Hence, there are continuing efforts in regard to developing new catalysts. Generally, two main different mechanisms have been reported in the literature, i.e., the NO decomposition mechanism14,15 and the NO reduction mechanism.16 The first mechanism proposes the direct decomposition of NO on copper, with hydrocarbon playing the role of oxygen elimination.16 This hypothesis currently seems limited to copper, even if it is mentioned primarily for catalysts that are based on noble metals, especially platinum-based catalysts. On the other hand, the NO reduction mechanism is widely proposed to occur on various catalysts, especially those based on zeolites. In this case, the hydrocarbon generates one or more intermediate species that act as reducing agents toward
10.1021/ie0488997 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/18/2005
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NO.16-18 The first step will be oxidation of NO to NO2 or to surface nitrates. These species should be the best species to oxidize the saturated hydrocarbon to give a CxHyOzN compound, which will react afterward with a NOx species to give N2, CO2, and H2O. However, the influencing parameters (e.g., the types of catalysts, the reaction conditions, and types of reductant) should be considered carefully, to develop a reaction mechanism. Pillared interlayer clays (PILCs) represent a class of microporous solids that have found a wide range of potential applications in catalytic, adsorption, and separation processes.19 These materials present better hydrothermal stability than ZSM-5 in the SCR of NO by hydrocarbons. The preparation of PILCs involves the introduction of bulky inorganic or organic clusters into the interlayer region of the clay. The intercalated species, when heated, are converted to the corresponding metal oxide clusters, which are rigid enough not only to prevent the interlayer spaces from collapsing, but also to generate micropores larger than those of conventional zeolites.20 Despite the interesting catalytic properties of titaniumbased catalysts, Ti-PILCs have received considerably less attention than other pillared clays. Among the pillared clays, Ti-PILCs have the following remarkable characteristics: (a) high interlayer spacing (∼16 Å, versus ∼10 Å for Al-PILCs or Zr-PILCs21,22); (b) high thermal and hydrothermal stability, comparable to that of Al-PILCs and Zr-PILCs;23 (c) large pore sizes, which allow further incorporation of active species without hindering pore diffusion;24 and (d) intercalating TiO2 between the SiO2 tetrahedral layers is practically the only way to increase the surface area and acidity of the clay support.25 Recently, the suitability of pillared clays as catalyst supports has been explored, especially because of the textural and acidic properties of these solids. Pillared clays have been applied in practice as catalysts for cracking, oligomerization, and disproportionation reactions.26 In recent studies, the catalytic behavior of these materials in the epoxidation of allylic alcohol,27-29 the transformation of m-xylene,30 the selective dehydration of 1-phenylethanol,31 and the hydroxylation of phenol32 has been explored. Ti-PILCs have been specifically used as catalysts for the SCR of NOx.32-37 Concretely, Ti-PILCs modified with copper present a higher activity in this reaction, because of their excellent properties (high thermal stability, high surface area, and appropriate acidity values). Because of the potential applications of titaniumpillared clays for HC-SCR, attention has been focused recently on the performance and reaction mechanism of these catalysts.38 A fundamental understanding of the reaction kinetics of NO HC-SCR is essential for the development of a catalyst that has a practical application. The present work shows the results obtained in the kinetic studies of the catalytic reduction of NO using propylene as a reducing agent over Cu-ion-exchanged Ti-PILCs. 2. Experimental Section 2.1. Synthesis of Ti-PILCs. The starting clay was a purified-grade bentonite (Fisher Company), with a particle size of
