Effects of dioxygen pressure on rates of NO_x selective catalytic reduction with NH₃ on Cu-CHA zeolites

Highlights

• NO_x SCR rates on Cu-CHA show a Langmuirian dependence on O₂ pressure.

• In operando X-ray absorption quantifies Cu(I) and Cu(II) distributions during SCR.

• First- and zero-order rate constants reflect the oxidation and reduction limit.

• Rates increase with Cu ion density as more sites become pairable and thus active.

• Transient XAS quantifies oxidation and reduction rates and Cu active sites.

Abstract

At low temperatures (<523 K), the selective catalytic reduction (SCR) of NO with NH₃ on Cu-exchanged zeolites occurs via elementary steps catalyzed by NH₃-solvated Cu ions, which are reactive intermediates in a Cu^II/Cu^I redox cycle. SCR rates are typically measured under “standard” reaction conditions, in which O₂ is the oxidant and present at partial pressures (~10 kPa O₂) that cause both single-site Cu^II reduction and dual-site Cu^I oxidation to behave as kinetically relevant steps. As a result, “standard” SCR rates (per g) transition from a second-order to a first-order dependence on isolated Cu content (per g) among Cu-CHA zeolites with increasing Cu content and thus spatial density (Si/Al = 15, Cu/Al = 0.08–0.37), as dual-site Cu^I oxidation steps limit SCR rates to lesser extents. On a given Cu-CHA catalyst, SCR rates (per Cu, 473 K) show a Langmuirian dependence on dioxygen pressure when varied widely (0–60 kPa O₂), enabling the isolation of first-order or zero-order kinetic regimes with respect to O₂. These kinetic regimes respectively correspond to limiting conditions in which either Cu^I oxidation or Cu^II reduction becomes the dominant kinetically relevant step, consistent with in operando X-ray absorption spectra. First-order rate constants (per Cu) increase approximately linearly with Cu density, reflecting the dual-site requirement of O₂-assisted Cu^I-oxidation steps. Zero-order rate constants (per Cu) increase more gradually with Cu density, in part reflecting the increasing fraction of isolated Cu ions that are able to form binuclear intermediates and thus participate in SCR turnovers. Combining steady-state and transient kinetic data with in operando spectra provides a methodology to quantitatively describe the rate dependences of SCR reduction and oxidation processes on Cu-zeolite properties such as Cu ion density as shown here, and others including the zeolite framework topology and its density and distribution of Al atoms.

Introduction

Selective catalytic reduction (SCR) with ammonia is a strategy used to abate nitrogen oxide (NO_x, x = 1,2) emissions from diesel and lean-burn engines, and was implemented commercially about a decade ago with the advent of Cu-CHA zeolite catalysts that have sufficient hydrothermal stability to withstand the harsh environments encountered in automotive exhaust applications [1], [2], [3]. Typical protocols to evaluate catalysts for practical application have focused on the measurement of NO_x conversion profiles as a function of temperature [4], including identifying the onset of the rapid increase in NO_x conversion with increasing temperature (i.e., “light-off”) observed at low temperatures (<473 K) and the decrease in NO_x conversion with increasing temperature (i.e., “seagull”-shaped dip) observed at intermediate temperatures (523–623 K) [5], [6], [7], [8]. Such macroscopic observations are manifestations of the underlying kinetic and mechanistic details of the SCR reaction on Cu-zeolites; they depend on the microscopic details of catalyst composition and active site arrangement. On Cu-CHA zeolites, these details include the number and spatial distribution of Cu cations exchanged at anionic framework sites introduced by Al substitution, and the fraction of Cu present nominally as monovalent [Cu^IIOH]⁺ (i.e., ZCuOH) and divalent Cu^II (i.e., Z₂Cu) sites that respectively charge-compensate one and two framework Al centers [4], [9], [10], [11].

The stoichiometry of the SCR reaction using molecular oxygen as the oxidant, which is also referred to as the “standard” SCR reaction, is:

$$4\mathrm{N}\mathrm{O}+4\mathrm{N}{\mathrm{H}}_3+{\mathrm{O}}_2\to 4{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}$$

Rates of “standard” SCR are typically measured at ambient pressures with NO and NH₃ present in equimolar quantities (e.g., 0.03 kPa each), and O₂ is present in large stoichiometric excess (e.g., 10 kPa), to resemble the concentrations present in lean-burn exhaust. The observed coexistence of both Cu^I and Cu^II species under “standard” SCR reaction conditions, and the sensitivity of their proportions to catalyst composition and structure and to the specific reaction conditions used, points to a mechanism involving a redox cycle between these two Cu states [12]. At temperatures below ~ 523 K and under SCR-relevant conditions, exchanged Cu ions in Cu-CHA zeolites are fully solvated by NH₃ ligands, as evidenced by phase diagrams constructed from first-principles thermodynamic analyses of density functional theory (DFT)-based free energies of Cu-ligand complexes [9], by in situ X-ray emission spectra (XES) that show Cu-N coordination in their oxidized (Cu^II) and reduced (Cu^I) states [7], [13], and by in situ X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of their oxidized and reduced states that are indistinguishable from those for four-fold and two-fold Cu-amine coordination complexes [9], [14], respectively. The coordination of Cu ions to ammonia ligands and not to zeolite framework oxygen atoms is further evidenced by the absence of second-shell Cu-T atom (T = Si, Al) scattering in EXAFS spectra collected in operando [9] and the absence of perturbed zeolite framework T-O-T vibrations in diffuse reflectance IR (DRIFTS) spectra collected in situ [15]. Thus, during low temperature NO_x SCR on Cu-CHA zeolites, ammonia-solvated Cu ions are present as cationic complexes that are ionically bonded to anionic zeolite framework centers.

The preponderance of evidence is consistent with a reduction half-cycle that consumes both NO and NH₃, which proceeds by NO attack on Cu^II-amine complexes to form nitrosamine (H₂NNO)-like intermediates that decompose to yield N₂, H₂O, Cu^I-amine complexes, and an excess proton [16]. The excess proton formed upon reduction of Z₂Cu sites resides on the zeolite lattice at framework oxygen adjacent to an Al site, while that formed upon reduction of ZCuOH is rejected as H₂O [9]. Evidence for the reduction mechanism of mononuclear Cu^II-amine complexes includes DFT calculations of plausible Cu^II reduction pathways [9], [14], [16], [17], transient XAS [14], [16] and TPR measurements [18] that detect complete and stoichiometric reduction of Cu^II to Cu^I only when both NO and NH₃ are present, and NH₃ titrations that quantify the zeolitic H⁺ sites formed upon reduction of Z₂Cu sites [9]. The details of the oxidation half-cycle are less well understood [6], [14], [19], [20], although O₂ must be consumed to balance stoichiometry. The oxidation of pairs of Cu^I complexes with O₂ is well-known in the homogeneous Cu literature [21], [22], [23]. Mononuclear Cu^I(NH₃)₂ complexes can react with O₂ to form NH₃-solvated binuclear Cu^II di-oxo complexes (e.g., (NH₃)₂Cu^II(O₂)Cu^II(NH₃)₂) [6], [19], consistent with DFT calculations of plausible dual-site Cu^I oxidation pathways with O₂ [6], [19], [24], [25] and with EXAFS spectra of Cu-CHA zeolites after their NH₃-solvated Cu^I ions are oxidized by O₂ that show second-shell Cu-Cu scattering at distances characteristic of binuclear Cu^II di-oxo complexes [19]. NH₃-solvated binuclear Cu^II di-oxo complexes can be reduced to two Cu^I sites while consuming two equivalents of NO and NH₃ to balance stoichiometry [19], via a sequence of elementary steps whose precise details have yet to be clarified.

Under low temperature “standard” SCR conditions, reaction rates on Cu-CHA (per mass or per pore volume catalyst) increase monotonically with the spatial density of isolated Cu ions (per mass or per pore volume catalyst), transitioning from a second-order dependence on Cu density in the dilute Cu limit to a first-order dependence on Cu density at higher Cu densities [9], [19], [26], [27]. In operando X-ray absorption spectra (XAS) provide direct evidence that Cu^I(NH₃)₂ is the most abundant reactive intermediate (MARI) at low Cu densities under “standard” SCR conditions [19], implying that Cu^I(NH₃)₂ oxidation with O₂ is the dominant kinetically-relevant process in the kinetic regime characterized by SCR rates that show a second-order dependence on Cu density. This interpretation is consistent with transient XAS experiments performed to measure O₂-assisted oxidation rates of Cu^I(NH₃)₂ complexes in Cu-CHA, which are best described by rate equations that are second-order in Cu^I(NH₃)₂ concentration [19]. Transient XAS experiments were also used to quantify the fraction of Cu^I(NH₃)₂ complexes that were oxidized by O₂ in the asymptotic limit of long reaction times, and this fraction was found to increase monotonically with Cu spatial density [19]. These findings indicate that O₂-assisted oxidation steps require two Cu^I(NH₃)₂ complexes, nominally isolated from one another, to co-locate within a single CHA cage to form a binuclear Cu^II di-oxo complex. DFT calculations indicate that transport of Cu^I(NH₃)₂ through an 8-MR window into an adjacent CHA cage that already contains a Cu^I(NH₃)₂ complex occurs with lower barriers (~35 kJ mol⁻¹) [19] than the measured range of apparent SCR activation barriers (~40–80 kJ mol⁻¹) [6], [14], [19], and that subsequent reaction steps to form binuclear Cu^II di-oxo complexes are even more facile [19], [24], [28].

Quantitative measurements that distinguish between the number of Cu sites that participate in SCR redox cycles, and the kinetics of reduction and oxidation processes catalyzed by such Cu sites, are difficult to extract from SCR rates measured under “standard” conditions that invariably convolute the kinetic contributions of both the oxidation and reduction half-cycles [12], [14], [16], [19], [29]. Here, we interrogate the catalytic function of Cu-CHA zeolites under reaction conditions perturbed far from those typical of “standard” SCR, in order to isolate either Cu^II reduction or Cu^I oxidation as the dominant kinetically-relevant process. Given that O₂ is only involved as a reactant in the oxidation half-cycle, we hypothesize that dual-site Cu^I oxidation becomes the dominant kinetically-relevant step at dilute O₂ pressures and becomes kinetically-irrelevant at higher O₂ pressures. We use in operando XAS measurements to quantify the prevalent Cu^II and Cu^I fractions as a function of reaction conditions and Cu density. Apparent Cu^II reduction and Cu^I oxidation rate constants are determined from regression of rate data to an empirical model that captures the observed Langmuirian dependence of SCR rates on O₂ pressure, which allows extracting kinetic parameters that describe the effects of Cu ion spatial density on SCR oxidation and reduction processes.