The role of AgOAl species in silver–alumina catalysts for the selective catalytic reduction of NO_x with methane

Abstract

We examined the role of silver and alumina in Ag–alumina catalysts for the selective catalytic reduction (SCR) of NO_x by methane in gas streams containing excess oxygen. A cogelation technique was used to prepare Ag–alumina materials with high dispersion of silver even at high metal loadings (>10 wt%) and after air calcination at 650 °C. Typically, a part of silver is present as fine nanoparticles on the alumina, whereas another part is ionic, bound with the alumina as [AgOAl] species. Dilute nitric acid leaching was used to remove the silver particles and all weakly bound silver from the surface of these materials. Complementary structural characterization was performed by HRTEM, XPS, XRD, and UV–vis DRS. We found that the higher the initial silver content, the higher the amount of the residual [AgOAl] species after leaching. NO–O₂-TPD tests identified that silver does not modify the surface properties of the alumina. The SCR reaction-relevant NO_x adsorption takes place on alumina. Temperature-programmed surface reaction (TPSR) and kinetic measurements at steady state were used to check the reactivity of the adsorbed NO_x species with methane and oxygen to form dinitrogen. Only the alumina-adsorbed nitrates react with CH₄ to produce N₂ in the presence of oxygen, beginning at ∼300 °C as found by TPSR. Moreover, the SCR reaction rates and apparent activation energies are the same for the leached and parent Ag–alumina catalysts. Thus, metallic silver nanoparticles are spectator species in CH₄-SCR of NO_x. These catalyze the direct oxidation of methane at temperatures as low as 300 °C, which explains the lower methane selectivity for the SCR reaction measured over the parent samples.

Introduction

The selective catalytic reduction (SCR) of NO to N₂ with hydrocarbons is a promising technology for NO_x removal, having attracted much attention for nearly two decades. Initial studies were focused on zeolites loaded with metals, such as Cu, Co, Ga, In, and Pd [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], since zeolites are well known to stabilize metal ions, the presence of which is deemed necessary for the SCR reaction. However, zeolite-based catalysts suffer from deactivation in water vapor- and sulfur dioxide-containing exhaust gas streams, which renders them less attractive for practical applications. Metal oxide catalysts have been examined as alternatives due to their high hydrothermal stability. A recent review by Burch et al. [13] discusses these types of catalysts, which include rare earth oxides (REO) and alumina- (or other oxides) supported Pt, Ga, In and Pd catalysts. The platinum group catalysts are active at low temperatures, but are limited by a narrow operating temperature window in which they display good selectivity. They are also more selective to undesired N₂O, a potent greenhouse by-product. Ga and In catalysts are limited by loss of activity in the presence of water; and Pd catalysts are inhibited by excess of oxygen in the exhaust gas stream. Other reports in the literature point to the enhancement of both activity and stability using sulfated alumina or zirconia as supports for Pd [14], [15], [16], Mn [17] and Co [18], [19] for the SCR of NO with methane. The acidity of these supports was considered key to stabilizing the active state of the metal species.

Since Miyadera et al. [20] reported that silver/alumina catalysts exhibited relatively good activity and selectivity for NO reduction to N₂ and moderate resistance to water and sulfur dioxide, many studies have been performed on this catalyst system [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. The most commonly used reductants are propane/propene [20], [21], [22], [23], [24], [25], [26], [27], but there have also been reports on using higher hydrocarbons [28], [29], [30], [31], [32], [33]; oxygenated hydrocarbons [34], [35], [36], [37], [38], [39], [40]; and even methane [41] as reducing agents. Higher hydrocarbons were found to shift the active temperature window to lower values and increase the tolerance to water [28]. In particular, using octane as the reductant has attracted much interest in the recent literature [30], [31], [32], [33], with reported light-off temperature as low as 250 °C and optimal C:N ratios of 4–6. Interestingly, it has been reported that gas-phase reactions past the catalyst bed can contribute to the reduction of nitric oxide to dinitrogen [30], [31]. However, a significant drawback of using octane as a reductant is the significant formation of CO accompanying the NO reduction reaction. With oxygenated hydrocarbons, high NO conversions can be obtained at 250–400 °C; however, the main problem is the formation of a large amount of harmful nitrogen-containing byproducts [34], [35], [36], [37], [38], [39], [40]. An important finding for the Ag/alumina system is that the activity for NO reduction is strongly correlated to the silver loading, and the most active Ag–alumina catalysts have been reported to contain 1.2–3 wt% Ag [23], [24], [25], [26], [31], [33], [41]. Structural analysis identified oxidized silver in the SCR-active Ag/alumina catalysts of intermediate silver loading, whereas metallic silver particles or $\mathrm{Ag}_n{}^0$ clusters were dominant in the high-silver content alumina catalysts, which were less selective for NO reduction and were good for the direct combustion of hydrocarbons [23], [24], [25], [26], [29], [41]. This also holds true for methane. In their study of methane combustion over Ag/ZrO₂, Kundakovic et al. [42] found that metallic Ag nanoparticles (oxygen-covered) are excellent catalysts for the direct oxidation of methane.

In addition to the change in the silver oxidation states, Wang et al. [43] explained the loading effect of silver on alumina as relating to acidity changes of the alumina surface. Finally, another type of structure, silver–aluminate, has been suggested as important for NO SCR [22], [23]. Nakatsuji et al. [44] obtained a AgAlO₂/Al₂O₃ catalyst by hydrothermal treatment at high temperature that was even more active than the respective Ag/Al₂O₃ catalyst. However, it has also been observed that silver phases exhibit significant mobility under SCR reaction conditions [26], [27], thus contradicting the proposal of stable silver aluminate.

The proper synthesis method is crucial to the preparation of catalysts with desirable silver structures. Impregnation methods are limited by silver particle aggregation at relatively high silver loadings [23], [24], [25], [26], and the catalysts thus prepared have a very narrow active temperature window. In contrast, coprecipitation-gelation methods have been found to better disperse and stabilize oxidized silver, possibly due to better interaction of silver species with the alumina during the gelation process [41], [45], [46]. A broader active temperature window was reported for a 5 wt% Ag–Al₂O₃ prepared by a sol–gel technique [46]. In our previous work, Ag–alumina catalysts prepared by a single step co-gelation method [41] were active for the SCR of NO with methane in excess O₂. Silver ions and silver oxide clusters were found to dominate in the SCR-active (3.2 wt% Ag) catalysts by UV–vis DRS [41].

Thus, evidence points to oxidized silver on alumina as the NO-SCR sites and attributes the loss of selectivity to the presence of silver nanoparticles, which catalyze the direct oxidation of the hydrocarbon, including methane [23], [41], [42]. However, this general picture cannot explain how methane is activated on silver aluminate structures. Is the presence of silver particles needed for this? How does the presence of silver modify the alumina surface? In a recent paper [47], the importance of Ag⁺ ions for CH₄-SCR of NO was shown using Ag-ZSM-5. Activation of CH₄ was suggested to occur on isolated Ag ions in the zeolite. It is of interest to investigate the role of silver ions for CH₄-SCR of NO in Ag–alumina catalysts with no silver particles present. To our knowledge, no such particle-free Ag–alumina catalysts have been examined in the literature. Even with the most active, low-content silver–aluminas, some silver particles were always present, depending on the preparation method and calcination temperature.

In this work, dilute nitric acid leaching was used to successfully remove metallic silver particles from the as-prepared Ag–alumina catalysts. The resulting AgOAl surfaces were characterized and compared with unmodified aluminas, and in kinetic experiments the role of silver particles for the CH₄-SCR of NO was clarified by comparing the leached and parent catalysts.