Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites

Abstract

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein's rule. Further, exchanged Cu(II) ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu(II)OH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.

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

“…It is now well-documented that in dehydrated Cu/SSZ-13, Cu 2+ -2Z with Cu ions located in windows of 6-membered rings (6 MR) are the energetically most favorable configuration. As such, Schneider and coworkers predicted that exchanged Cu 2+ ions first saturate these 2Al sites before populating unpaired, or 1Al, sites as Cu(OH)] + [17]. Note that the relative populations of Cu 2+ -2Z and [Cu(OH)] + -Z are dependent both on Si/Al and Cu/Al ratios of the Cu/SSZ-13 material, as shown in Figure 2.…”

“…In comparison to NO x and H 2 O, interactions between Cu ions and NH 3 are much stronger; this renders a generally agreed scenario that, under low-temperature NH 3 -SCR reaction conditions, Cu-NH 3 complexes are abundant and some of these are key intermediates for reaction [15,17,20]. Therefore, it is of considerable relevance to study species formed by adsorption of NH 3 on Cu/SSZ-13 and identify their mechanistic involvement in NH 3 3 adsorption and purging at 150 • C, the apparent discrepancy can be understood from the gas-phase NH 3 pressure differences, where the presence of gas phase NH 3 allows for higher coordination with NH 3 ligands.…”

“…Unless Cu is present at levels exceeding the ion-exchange capacities of the particular SSZ-13 composition used, the current general consensus is that, in the freshly prepared specimens, Cu is present as isolated ions, including Cu 2+ balanced by two nearby framework negative charges (abbreviated as Cu 2+ -2Z, where Z represents a charged zeolite framework site) and a [Cu(OH)] + species balanced by one framework negative charge (as [Cu(OH)] + -Z) [17,[31][32][33]. It is also generally agreed, based on almost identical X-ray absorption near edge structure (XANES) spectra with model Cu salt solutions, that these ions are fully solvated as [Cu(H 2 O) 6 ] 2+ and [Cu(OH)(H 2 O) 5 ] + complexes in hydrated ambient samples [20,34,35].…”

“…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.…”

Response to Comment on “Selective anaerobic oxidation of methane enables direct synthesis of methanol”

“…Similarly, after saturation with water, zeolites that work as adsorbents can be dried in a flow of dry gas, despite the positive value of the standard DG (12). Numerous reactions, including water splitting on the surfaces, are inhibited by standard values of DG but are nonetheless feasible (13).…”