Synthesis and Characterization of Copper-Exchanged Zeolite Catalysts and Kinetic Studies on NOx Selective Catalytic Reduction with Ammonia

Description

<p>Although Cu-SSZ-13 zeolites are used commercially
in diesel engine exhaust after-treatment for abatement of toxic NO_x
pollutants via selective catalytic reduction (SCR) with NH₃,
molecular details of its active centers and mechanistic details of the redox
reactions they catalyze, specifically of the Cu(I) to Cu(II) oxidation
half-reaction, are not well understood. A detailed understanding of the SCR
reaction mechanism and nature of the Cu active site would provide insight into
their catalytic performance and guidance on synthesizing materials with
improved low temperature (< 473 K) reactivity and stability against
deactivation (e.g. hydrothermal, sulfur oxides). We use computational,
titration, spectroscopic, and kinetic techniques to elucidate (1) the presence
of two types of Cu²⁺ ions in Cu-SSZ-13 materials, (2) molecular
details on how these Cu cations, facilitated by NH₃ solvation,
undergo a reduction-oxidation catalytic cycle, and (3) that sulfur oxides
poison the two different types of Cu²⁺ ions to different extents at
via different mechanisms. </p><p><br></p>

<p> </p>

<p>Copper was exchanged onto H-SSZ-13 samples with
different Si:Al ratios (4.5, 15, and 25) via liquid-phase ion exchange using
Cu(NO₃)₂ as the precursor. The speciation of copper
started from the most stable Cu²⁺ coordinated to two anionic sites
on the zeolite framework to [CuOH]⁺ coordinated to only one anionic
site on the zeolite framework with increasing Cu:Al ratios. The number of Cu²⁺
and [CuOH]⁺ sites was quantified by selective NH₃
titration of the number of residual Brønsted acid sites after Cu exchange, and by
quantification of Brønsted acidic Si(OH)Al and CuOH stretching vibrations from
IR spectra. Cu-SSZ-13 with similar Cu densities and anionic framework site
densities exhibit similar standard SCR rates, apparent activation energies, and
orders regardless of the fraction of Z₂Cu and ZCuOH sites,
indicating that both sites are equally active within measurable error for SCR. </p><p><br></p>

<p> </p>

<p>The standard SCR reaction uses O₂ as the
oxidant (4NH₃ + 4NO + O₂ -> 6H₂O + 4N₂)
and involves a Cu(I)/Cu(II) redox cycle, with Cu(II) reduction mediated by NO
and NH₃, and Cu(I) oxidation mediated by NO and O₂. In
contrast, the fast SCR reaction (4NH₃ + 2NO + 2NO₂ -> 6H₂O
+ 4N₂) uses NO₂ as the oxidant. Low temperature (437 K)
standard SCR reaction kinetics over Cu-SSZ-13 zeolites depend on the spatial
density and distribution of Cu ions, varied by changing the Cu:Al and Si:Al
ratio. Facilitated by NH₃ solvation, mobile Cu(I) complexes can
dimerize with other Cu(I) complexes within diffusion distances to activate O₂,
as demonstrated through X-ray absorption spectroscopy and density functional
theory calculations. Monte Carlo simulations are used to define average Cu-Cu
distances. In contrast with O₂-assisted oxidation reactions, NO₂
oxidizes single Cu(I) complexes with similar kinetics among samples of varying
Cu spatial density. These findings demonstrate that low temperature standard
SCR is dependent on Cu spatial density and requires NH₃ solvation to
mobilize Cu(I) sites to activate O₂, while in contrast fast SCR uses
NO₂ to oxidize single Cu(I) sites. </p><p><br></p>

<p> </p>

<p>We also studied the effect of sulfur oxides, a
common poison in diesel exhaust, on Cu-SSZ-13 zeolites. Model Cu-SSZ-13 samples
exposed to dry SO₂ and O₂ streams at 473 and 673 K. These
Cu-SSZ-13 zeolites were synthesized and characterized to contain distinct Cu
active site types, predominantly either divalent Cu²⁺ ions exchanged
at proximal framework Al sites (Z₂Cu), or monovalent CuOH+ complexes
exchanged at isolated framework Al sites (ZCuOH). On the model Z₂Cu
sample, SCR turnover rates (473 K, per Cu) catalyst decreased linearly with
increasing S content to undetectable values at equimolar S:Cu molar ratios,
while apparent activation energies remained constant at ~65 kJ mol^-1,
consistent with poisoning of each Z₂Cu site with one SO₂-derived
intermediate. On the model ZCuOH sample, SCR turnover rates also decreased
linearly with increasing S content, yet apparent activation energies decreased
monotonically from ~50 to ~10 kJ mol^-1, suggesting that multiple
phenomena are responsible for the observed poisoning behavior and consistent
with findings that SO₂ exposure led to additional storage of SO₂-derived
intermediates on non-Cu surface sites. Changes to Cu²⁺ charge
transfer features in UV-Visible spectra were more pronounced for SO₂-poisoned
ZCuOH than Z₂Cu sites, while X-ray diffraction and micropore volume
measurements show evidence of partial occlusion of microporous voids by SO₂-derived
deposits, suggesting that deactivation may not only reflect Cu site poisoning.
Density functional theory calculations are used to identify the structures and
binding energies of different SO₂-derived intermediates at Z₂Cu
and ZCuOH sites. It is found that bisulfates are particularly low in energy,
and residual Brønsted protons are liberated as these bisulfates are formed.
These findings indicate that Z₂Cu sites are more resistant to SO₂
poisoning than ZCuOH sites, and are easier to regenerate once poisoned. </p>

Links & Downloads

1. 10.25394/pgs.7464596.v1

Tags

Environmental Chemistry

Synthesis of Materials

Catalysis and Mechanisms of Reactions

Reaction Kinetics and Dynamics

catalysis

reaction kinetics

reaction mechanisms

selective catalytic reduction

zeolite

SSZ-13

titration

active site

spectroscopy

density functional theory

computation

chabazite

copper

sulfur

operando

ammonia

nitrogen oxides

Additional Fields

Identifer	oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7464596
Date	16 January 2019
Creators	Arthur J. Shih (5930264)
Source Sets	Purdue University
Detected Language	English
Type	Text, Journal contribution
Rights	CC BY 4.0
Relation	https://figshare.com/articles/Synthesis_and_Characterization_of_Copper-Exchanged_Zeolite_Catalysts_and_Kinetic_Studies_on_NOx_Selective_Catalytic_Reduction_with_Ammonia/7464596