No oxidation reaction mechanisms over zeolite catalysts
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
“Lean-burn” engines such as diesel-powered vehicles and newer internal
combustion engines operate at high air/fuel ratios to increase fuel efficiency and
decrease CO and hydrocarbon emissions. However, the highly oxidizing environment
leads to increased nitrogen oxide (NOx) emissions, which cannot be effectively
reduced to N2 with conventional three-way catalytic converters. Over the past 30
years, the selective catalytic reduction of NOx with ammonia (NH3-SCR) using Fe- or
Cu-exchanged zeolite catalysts has been developed to address the “lean-NOx”
problem. This technology has since been commercialized and is currently one of the
most effective means of abating NOx emissions released from mobile or stationary
power sources. Small pore zeolites such as chabazite (CHA) are the state-of-the-art
materials because of their very high NOx conversion and exceptional hydrothermal
stability. Much of the research on NH3-SCR has focused on identifying the reaction
mechanism and kinetics, and although great progress has been made, many details
remain unknown or are widely debated. Numerous investigations have identified the
oxidation of NO to NO2 to be an important reaction in the mechanism, but the precise
kinetic role of the reaction remains unclear. The primary goal of the research
described in Part I of this dissertation (Chapters 1-5) has been to identify the
mechanism of the NO oxidation reaction over zeolites and other microporous
materials to better understand the catalytic sites responsible for activity, which will
hopefully aid in understanding the role of NO oxidation in NH3-SCR. At temperatures between 298 K and 423 K, it is shown that NO oxidation is
catalyzed by CHA zeolites in the proton (H+), sodium (Na+), and siliceous forms.
Additionally, microporous carbons and the metal-organic framework (MOF) material
Basolite A100 also showed substantial catalytic activity. Catalytic rates decreased with
increasing temperature to yield negative apparent activation energies between –24.9 kJ
mol-1 and –37.5 kJ mol-1. Reaction orders on all microporous materials at low
conversion were measured to be second order with respect to NO concentration and
first order with respect to O2 concentration; the same reaction orders are observed for
the gas-phase reaction. The catalytic properties of the samples are attributed to their
ability to stabilize a [N2O4]‡ transition state within the micropores through van der
Waals forces. Na-SSZ-13 samples exhibited faster catalytic rates than siliceous
chabazite due to the additional presence of electrostatic forces stabilizing the transition
state. An enhancement of catalytic rates on H-SSZ-13 was also observed and is the
result of more complex interactions due to the formation of NO+ and NO3
- in the
zeolite pores, which can also stabilize the [N2O4]‡ transition state.
Different reactivity for the zeolite samples was observed at temperatures
greater than 423 K. Reaction rates on H-SSZ-13 and Na-SSZ-13 now increased with
increasing temperature, a clear indication that a different reaction mechanism occurs
in this temperature regime. The reaction rates on siliceous CHA were low and
unaffected by temperature above 423 K, indicating that the material has minimal
catalytic activity in this temperature window and that framework aluminum atoms
with exchanged cations are necessary to observe significant activity. For all samples,
rates were proportional to NO and O2 concentrations, in contrast to the lowtemperature
catalytic rates (second order in NO and first order in O2). In-situ FTIR studies revealed that NO+ coordinated at framework sites in the zeolite pores (Si–O–
(NO+)–Al) plays a direct role in the catalysis. Furthermore, it was found that NO+ is in
equilibrium with gas-phase NO and that desorption of NO+ (as NO) yields an oxidized
acid site (Si–O•–Al). No evidence of NO+ formation was observed over the siliceous
zeolite samples. A working model of the NO oxidation reaction at high temperatures is
proposed for acidic zeolites that is consistent with the observed form of the rate
equation and the observed NO+ reaction intermediate.
Incorporation of copper into the zeolite framework resulted in substantially
higher activity (by ~1-2 orders of magnitude) compared to the acid zeolites. A number
of factors were observed to influence the activity of these samples, including zeolite
framework, Cu loading, and pretreatment conditions. Cu-ZSM-5 exhibited superior
NO oxidation activity compared to Cu-SSZ-13 and Cu-BEA. On Cu-SSZ-13, an
investigation of Cu loading on reaction rates showed that rates normalized per gram of
catalyst increased substantially with increasing Cu loading up to ~1 wt%. Cu loadings
beyond 1 wt% resulted in only slight increases in the rate. Meanwhile, rates
normalized per mol of Cu (turnover frequencies) decreased considerably with
increased Cu loading up to ~1 wt%, and were relatively independent of Cu loading at
higher levels. Samples reduced in a flow of 1% CO/He exhibited higher activity
compared to samples pretreated in 5% O2/He. Rates on Cu-zeolite samples pretreated
in 5% O2/He were observed to be first order with respect to NO and O2 concentrations.
For pre-reduced samples, the rate dependency on O2 concentration changed to half
order. DRIFTS spectra collected under in-situ conditions showed that different NxOy
surface species formed based on the Cu loading of the zeolite. A mechanism for the
reaction on Cu-zeolites pretreated in 1% CO/He is presented and discussed. Part II of this thesis (Chapters 6 and 7) investigates the chemistry of the
reverse water-gas shift (RWGS) reaction, which is an important reaction for the
conversion of CO2 to value-added products. Specifically, the reaction mechanism of
the RWGS reaction is examined on Fe/-Al2O3 and Fe-K/-Al2O3 catalysts at
temperatures between 723 K and 753 K and atmospheric pressure. It is shown that
both materials are excellent catalysts for the selective hydrogenation of CO2 to CO,
with selectivities in excess of 99%. Potassium had a remarkable effect on activity, as it
increased rates by a factor of ~3, changed the rate-determining step of the reaction as
verified by a change in the kinetic isotope effect using H2/D2, and changed the rate
orders with respect to CO2 and H2. Iron, on both catalysts, was found to reduce to the
+2 oxidation state under H2 flow, and oxidize to the +3 oxidation state under CO2 flow
based on in-situ XANES. Iron is mostly in the +2 oxidation state under continuous
equimolar flow of CO2 and H2. The catalysts were stable under excess H2 but
deactivated slowly under an equimolar mixture of CO2 and H2 (1-2%/h of the overall
reaction rate). Gas-switching experiments (CO2 or H2 only) and DRIFTS spectra
collected in-situ showed that stable intermediates formed on Fe-K/-Al2O3 but not on
Fe/-Al2O3. This suggests, but does not conclusively prove, that a redox mechanism is
the only reaction pathway on the Fe/-Al2O3 catalyst and is the predominant pathway
on the Fe-K/-Al2O3 catalyst. A detailed mechanistic analysis of the simpler Fe/-
Al2O3 showed that the redox mechanism is the main reaction channel for CO2
hydrogenation, but that there is a hydrogen pool on the surface of the catalyst that
provides or consumes hydrogen as needed to reform the sites that capture the oxygen
of CO2. We conclude that Fe/Al2O3 is a promising catalyst for practical applications of
the RWGS reaction at the conditions investigated.