Understanding ammonia selective catalytic reduction kinetics over Cu/SSZ-13 from motion of the Cu ions

Abstract

Cu/SSZ-13 catalysts with Si/Al = 6 and various Cu/Al ratios are synthesized with solution ion exchange. Catalysts are characterized with surface area/pore volume measurements, temperature programmed reduction (TPR), and electron paramagnetic resonance (EPR) spectroscopy. Catalytic properties are examined using NO oxidation, ammonia oxidation, and standard ammonia selective catalytic reduction (NH 3-SCR) reactions. Prior to full dehydration of the zeolite catalysts, hydrated Cu 2+ ions are found to be very mobile as judged from EPR. NO oxidation is catalyzed by O-bridged Cu-dimer species that form at relatively high Cu loadings and in the presence of O 2. For NH 3 oxidation on samples with low to intermediate Cu loadings, transient Cu-dimers are the low-temperature ( 300 C) active centers, while these dissociate to monomers at 350 C and above and become active centers. For the much more complex standard SCR reaction, transient Cu-dimers are the active sites for reaction temperatures < 250 °C at very low Cu loadings (Cu/Al  0.016). Between ~250 and 350 °C, these Cu-dimers become less stable causing SCR reaction rates to decrease. At temperatures  350 °C, Cu 2+ monomers that had migrated to faces of 6-membered rings are the active sites. At intermediate Cu loadings, monomeric Cu 2+ ions are also active in SCR in the low-temperature regime; these are proposed to be located within CHA cages and next to 8-membered rings, likely in the form of [Cu(OH)] +. At high Cu loadings (i.e., more than one Cu 2+ ion in each unit cell), stable Cu-dimers form and these do not dissociate at temperatures above 350 °C. These moieties effectively occupy CHA cage space and block pore openings causing decreased efficiency of the catalysts. Also these moieties are highly active in catalyzing the NH 3 oxidation reaction thus causing SCR selectivities to decrease above ~450 °C. Finally, our kinetics results strongly support a redox mechanism for standard SCR.

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

“…Furthermore, complexity is also evident in the nature of the catalytically active Cu species, as first recognized from SCR kinetics. For example, Figure 10 displays steady state NO x light-off curve for a Cu/SSZ-13 catalyst with relatively low Cu loading, allowing low-and high-temperature reaction regimes to be clearly resolved [19,38]. Importantly, the NO x conversion decrease with increasing temperature between~250 and 350 • C indicates a temperature-dependent change in the nature of active Cu sites, which has now been repeatedly demonstrated via in situ/operando XAS.…”

“…5 ] + ions are likely spectroscopically indistinguishable by XANES, indirect evidence does exist for the reversible reaction described in Reaction (1) from electron paramagnetic resonance (EPR) measurements. Figure 3 presents EPR spectra collected during dehydration of a Cu/SSZ-13 sample with Si/Al = 6 and Cu/Al = 0.032 [38]. At such Al abundance and low Cu loadings, it is expected that [Cu(H 2 O) 6 ] 2+ is the only Cu species in the hydrated form and it will be converted exclusively to Cu 2+ -2Z upon dehydration.…”

A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

“…Figure 1 shows the NO conversion and N 2 O selectivity measured in a microreactor in the temperature range 160-550 • C. The NO conversion shows a bimodal pattern, with a first maximum around 300 • C and a second maximum around 400 • C, similar to earlier observations. 39,40 The decreasing NO-conversion above 400 • C can be ascribed to an enhanced direct oxidation of ammonia to NO that occurs in this temperature range. It is noted that this particular 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 NH 3 , 50,56,72 the rate constants for the SCR reaction in this temperatures are evaluated.…”

“…These two locations for the Cu ions are well known from structural analysis of Cu-CHA catalysts, although some controversy exists on which form actually constitutes the active site. 6,12,28,34,[39][40][41]44,52,72,73 However, when adsorbates are present, the Cu ion is lifted out of the 6-ring plane and the preference for the 6-ring location diminishes. 42,43,74 This is also observed for the formation of Cu 2+ −NO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 (step 1), where the difference in energy for the Cu located in the 8-ring is only 0.08 eV lower compared to the Cu in the 6-ring, showing that there is no real preference for location of the Cu in the 6-ring or 8-ring in this case.…”

“…39,40 As the propensity for Cu-dimer formation increases with Cu loading, this may indicate that Cu-dimers could enhance the rate of the NH 3 -SCR reaction. A possible scenario is that the oxygen leaves a single O-atom on each Cu-ion.…”