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Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NO x ) with ammonia (NH 3 ), 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 2 )-mediated Cu I →Cu II 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., (Copyright © 2017, American Association for the Advancement of Science.)

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.

Purpose: To evaluate healing rates and complications in patients treated with temporary external fixation (EF) and subsequent open reduction and internal fixation (ORIF) for high-energy distal femur or proximal tibia fractures., Methods: Retrospective analysis of prospectively collected data 1999 to 2005. Demographic data and injury severity score were obtained from medical records. Factors reviewed included perioperative complications (nonunion, postoperative infection, loss of fixation) and time to radiographic and clinical union., Results: Forty-seven patients with 16 distal femur and 36 proximal tibia fractures were treated using temporary EF. Patients subsequently underwent ORIF (mean time from EF to ORIF = 5 days, range 1-23 days). Thirty-five fractures were open (Gustilo I = 8, II = 6, IIIA = 3, IIIB = 13, IIIC = 5) and 17 closed. Forty patients with 44 fractures reached 1-year follow-up. Of these, 36 patients with 40 (91%) fractures had healed both radiographically and clinically. The mean postoperative follow-up time was 14 months (range 3-68). Eight (16%) deep infections occurred, all in open fractures (Gustilo I = 2, IIIB = 3, IIIC = 3), with one patient requiring above knee amputation. Other complications included one hematoma, two malunions, one fixation failure, and one pin site infection. One patient died as a result of a stroke., Conclusions and Significance: Temporary bridging EF offers the advantage of early soft tissue and bone stabilization without the potential local risks of immediate ORIF in severely injured soft tissues, or the potential systemic risks in a severely traumatized patient. The 16% infection rate in this study, all occurring in open fractures, falls within the reported range for grade III open fractures (15%-20%). We conclude that the initial treatment of high-energy periarticular knee fractures with bridging EF, followed by planned conversion to internal fixation is a safe option in patients who are unsuitable for initial definitive surgery.