Catalytic Consequences of Active Site Speciation, Density, Mobility and Stability on Selective Catalytic Reduction of Nox with Ammonia Over Cu-Exchanged Zeolites
Abstract
Selective catalytic reduction (SCR) of NOx using NH3 as a reductant (4NH3 + 4NO + O2 → 6H2O + 4N2) over Cu-SSZ-13 zeolites is a commercial technology used to meet emissions targets in lean-burn and diesel engine exhaust. Optimization of catalyst design parameters to improve catalyst reactivity and stability against deactivation (hydrothermal and sulfur poisoning) necessitates detailed molecular level understanding of structurally different active Cu sites and the reaction mechanism. With the help of synthetic, titrimetric, spectroscopic, kinetic and computational techniques, we established new molecular level details regarding 1) active Cu site speciation in monomeric and dimeric complexes in Cu-SSZ-13, 2) elementary steps in the catalytic reaction mechanism, 3) and deactivation mechanisms upon hydrothermal treatment and sulfur poisoning. We have demonstrated that Cu in Cu-SSZ-13 speciates as two distinct isolated sites, nominally divalent CuII and monovalent [CuII(OH)]+ complexes exchanged at paired Al and isolated Al sites, respectively. This Cu site model accurately described a wide range of zeolite chemical composition, as evidenced by spectroscopic (Infrared and X-ray absorption) and titrimetric characterization of Cu sites under ex situ conditions and in situ and operando SCR reaction conditions. Monovalent [CuII(OH)]+ complexes have been further found to condense to form multinuclear Cu-oxo complexes upon high temperature oxidative treatment, which have been characterized using UV-visible spectroscopy, CO-temperature programmed reduction and dry NO oxidation as a probe reaction. Structurally different isolated Cu sites have different susceptibilities to H2 and He reductions, but are similarly susceptible to NO + NH3 reduction and have been found to catalyze NOx SCR reaction at similar turnover rates (per CuII; 473 K) via a CuII/CuI redox cycle, as their structurally different identities are masked by NH3 solvation during reaction.
Degree
Ph.D.
Advisors
Ribeiro, Purdue University.
Subject Area
Aging|Energy|Analytical chemistry|Chemistry|Optics
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