Dynamic multinuclear sites formed by mobilized copper ions in NO _x selective catalytic reduction

Abstract

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NO ) with ammonia (NH), but the low-temperature rate dependence on copper (Cu) volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O)-mediated Cu→Cu redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…Besides key intermediate formation pathways described by Reactions (9) and (10), this mechanism also involves energetically feasible HONO (which can then lead to NH 4 NO 2 formation) and Cu-O-Cu hydrolysis steps to complete catalytic turnover [19]. A similar mechanism has been proposed by others without specifying how a charge balance can be maintained during the reduction of O 2 to 2O 2− and corresponding oxidation of two Cu(I) ions to Cu(II), nor how dinuclear intermediates split into two isolated Cu ions to complete a catalytic cycle [21]. These are some of the remaining questions for the low-temperature standard SCR mechanisms that require additional research.…”

“…First, as shown in Figure 12, for reaction temperatures up to 250 • C, instead of a linear SCR rate versus Cu loading correlation, an interesting linear SCR rate versus (Cu loading) 2 correlation was discovered by Gao et al [38], strongly suggesting that the reaction rate limiting step involves participation of two Cu ions. This surprising experimental result was later reproduced by Gounder, Ribeiro and coworkers [21]. Secondly, in a study on dry NO oxidation to NO 2 by Cu/SSZ-13, Ribeiro and coworkers discovered that isolated Cu ions do not catalyze this reaction, whereas CuxOy (x ≥ 2) clusters contribute linearly to the rate of NO oxidation per mole Cu at 300 • C [46].…”

“…In probing this low Cu-loading regime, critical new insights have been provided for understanding the standard SCR mechanism. Of special note is that reaction rates at low temperatures are limited by O 2 activation; notably, for the process by which Cu(I) is re-oxidized back to Cu(II) to finish redox cycling of the active sites [19,21]. In particular, a few important experimental findings led to the surprising proposal that rate-limiting, low-temperature O 2 activation requires participation of two Cu(I) sites.…”

“…This group's subsequent Science paper [21] provided many additional important details on the rates of complexed Cu-ion transport and the formation of the binuclear Cu complex. Significantly, Reaction (9) and especially the [Cu(NH 3 ) 2 ] + diffusion and Cu-O 2 -Cu binding steps, have been proven to be thermodynamically feasible from theoretical simulations [19,21]. At the lowest Cu loadings, formation of the Cu-O 2 -Cu intermediate is the rate-limiting, essentially controlled by the entropy costly diffusion of [Cu(NH 3 ) 2 ] + .…”

“…Now after 3 years, new and exciting results have appeared in literature, especially with respect to the developing molecular-level understanding of the nature of the active Cu species under realistic operating (i.e., situ/operando) conditions, as well as the SCR reaction mechanisms [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. As such, it seems timely to review these new fundamental understandings.…”