Structure–activity relationships in NH₃-SCR over Cu-SSZ-13 as probed by reaction kinetics and EPR studies

Abstract

Cu-SSZ-13 catalysts with various Cu loadings were prepared via aqueous solution ion-exchange. The hydrated samples were characterized with Electron Paramagnetic Resonance (EPR). Cu²⁺ ion coordination numbers were obtained by analyzing the hyperfine structures, while Cu–Cu distances were estimated from line broadening of the EPR features. By examining EPR and temperature-programmed reduction (TPR) results, two Cu²⁺ ion locations are suggested. Standard NH₃-SCR, as well as non-selective NH₃ oxidation reaction with O₂, were carried out over these catalysts at high-space velocities. For the SCR reaction, intra-particle diffusion limitations are found. The kinetic data allow for reactant diffusivities to be estimated. However, clear structure–activity relationships for the SCR reaction cannot be derived due to this diffusion limitation. The slower NH₃ oxidation reaction, on the other hand, is kinetically limited at low temperatures, and, therefore allows for a correlation between Cu²⁺ ion location and reaction kinetics to be made.

Introduction

The abatement of environmentally harmful NO_x compounds (NO, NO₂, and N₂O) emitted from mobile or stationary power sources remains a challenging task for the catalysis community. In particular, conventional three-way catalysts used in the exhaust after-treatment technologies of internal combustion engines prove ineffective when the engine is operated under highly oxidizing conditions. However, in order to achieve better fuel efficiency and decrease CO₂ emission, operation under such fuel-lean conditions is a prerequisite for diesel engines and new-generation gasoline engines. The problem is daunting, since reduction chemistry (NO_x to N₂) has to be carried out under highly oxidizing conditions. Two elegant after-treatment techniques have been developed for reduction in NO_x from lean burn engines; namely lean NO_x traps (LNT) and selective catalytic reduction (SCR) with hydrocarbons (HC-SCR) or with ammonia (NH₃-SCR). For the NH₃-SCR technology, transition metal (in particular Fe and Cu) ion-exchanged zeolite catalysts have shown high activity and N₂ selectivity.

The most extensive studies have been carried out on Cu²⁺ ion-exchanged ZSM-5 (Cu-ZSM-5) zeolites, first shown to exhibit high NO decomposition rates and NO_x SCR activities in the 1980s [1], [2], [3], [4], [5], [6], [7]. Later, Cu²⁺ ion-exchanged beta zeolite (Cu-beta) was shown to have excellent activity in the SCR of NO_x with NH₃, and metal-exchanged beta zeolites are generally found to have greater hydrothermal stability than similar ZSM-5 catalysts [8]. Yet, none of these Cu-zeolite catalysts show durability sufficient for the automotive industry. In the year 2010, Cu²⁺-exchanged molecular sieves with Chabazite (CHA) structures, for example, Cu-SSZ-13 and Cu-SAPO-34, have been commercialized as NOx after-treatment catalysts in diesel-powered engines for transportation, due apparently to their much improved activity, selectivity, and durability [9], [10], [11], [12], [13], [14], [15], [16], [17], [18].

We and other researchers compared SCR activities and hydrothermal stabilities of Cu-SSZ-13 with other Cu²⁺ ion-exchanged zeolites (especially Cu-ZSM-5 and Cu-beta) [10], [13], [15]. Indeed, Cu-SSZ-13 shows comparable or even higher activity, higher N₂ selectivity and most impressively, much higher hydrothermal stability even under harsh treatment conditions; for example, extended hydrothermal treatments at 1073 K. There appears to be general consensus that these unique properties of Cu-SSZ-13 are due to its small-pore structure [10], [13], [15]. SSZ-13 adopts the $R\overline{3}m$ space group with a hexagonal unit cell described via the following face symbol: [4⁶·6²] + [4¹²·6²·8⁶]. The largest pore has an opening of 3.8 Å (8-membered ring). In contrast, ZSM-5 has medium size pore openings (∼5.5 Å, 10-membered ring), while beta has the largest pores (∼7 Å, 12-membered ring) [9]. This structural difference is significant in improving the stability of SSZ-13: (1) for zeolite materials, dealumination is the major cause of structural damage and activity loss. The dealumination product, Al(OH)₃, has a kinetic diameter of ∼5.03 Å [15]. Thus, it is possible that aluminum cannot escape from the SSZ-13 pores during hydrothermal treatments; even if a dealumination reaction occurs it can insert back to the framework during cooling (i.e., reversible dealumination) to maintain the integrity of the zeolite structure [15]. (2) Medium- and large-pore zeolites adsorb large quantities of hydrocarbons at low temperatures. This is inevitable in vehicle emission treatments, since significant quantities of hydrocarbons are generated during cold-start. As the temperature rises, however, reaction heat generated via burning of these molecules can thermally destroy zeolites. Again, the small-pore SSZ-13 greatly eliminates this deactivation mechanism since hydrocarbons with kinetic diameters larger than methane (3.8 Å) have at most very limited diffusion into the pores [15].

Apart from the commercial success, detailed catalyst structures, reaction mechanisms, and structure–activity relationships are still lacking. For example, there are debates even regarding the location of Cu²⁺, the catalytically active centers, within the SSZ-13 framework. Lobo and coworkers applied Rietveld refinement for TR-XRD data of their dehydrated Cu-SSZ-13 catalyst and proposed that Cu²⁺ ions are located only in six-membered ring windows with a coordination number of 3 and average Cu–O distances of 2.2–2.3 Å [11]. In a more recent study, they used EXAFS to analyze Cu-SSZ-13 and concluded that, for a hydrated sample, four lattice oxygen atoms coordinate to the Cu²⁺ ion with an average Cu–O distance of 2.02 Å, whereas for a dehydrated sample, the coordination number reduces to 3 at an average distance of 1.93 Å [16], [19]. We, on the other hand, have recently concluded that the copper locations within the SSZ-13 framework are affected by two factors: copper loading and moisture [14]. Both temperature-programmed reduction and FTIR coupled with CO and NO titrations reveal that there are two different Cu moieties in Cu-SSZ-13 as a function of Cu loading. For dehydrated samples and at low Cu loadings (at an ion-exchange (IE) level of ∼20%), only a single type of Cu²⁺ species exists which is rather hard to reduce to Cu⁺ (at ∼653 K). However, as the Cu loading increases, another Cu species develops, and the reduction temperatures are much lower (∼503 K). Furthermore, under wet-reducing conditions, the reduction temperatures for both species are significantly lower and are found at ∼523 K and ∼483 K, respectively. These findings are explained in the following way: at low Cu loading and under dehydrated conditions, Cu²⁺ ions are located close to the face of the 6-membered rings via formation of Cu–O bonds with lattice oxygen as suggested by Lobo et al. [11], [16], [19]. When exposed to gases that strongly interact with Cu²⁺ ions (e.g., NH₃), the Cu²⁺ ions are pulled slightly into the large cages [19]. However, as the Cu loadings increase, some Cu²⁺ ions also populate inside the large CHA cages next to 8-membered rings. Cu²⁺ ions next to 8-membered rings have easier access to hydrogen (as they are closer to the pore openings) perhaps explaining, in part, their lower reduction temperatures. Moreover, under wet-reducing conditions, the interaction between water and Cu²⁺ ions weakens interactions between Cu²⁺ ions and lattice oxygen, allowing both types of Cu²⁺ ions to move toward the large CHA cages and become more readily reducible. Note that this water-induced ion mobility is well known within the CHA cages [17], [20].

We note that understanding the Cu locations within SSZ-13 under wet conditions is more relevant to practical NH₃-SCR since engine exhausts typically contain ∼10% moisture. In the current study, we use Electron Paramagnetic Resonance (EPR), a technique ideally suited for studying Cu²⁺ exchanged zeolites, to study hydrated Cu-SSZ-13 catalysts at various Cu loadings in order to gain further insights into their locations. Also, NH₃-SCR and NH₃ oxidation kinetics are investigated over these catalysts at high-space velocity conditions for the development of structure–activity relationships.