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Catalytic NO Oxidation in the Presence of Moisture Using Porous Polymers and Activated Carbon Mohsen Ghafari, and John D. Atkinson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05443 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 20, 2016
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Environmental Science & Technology
Catalytic NO Oxidation in the Presence of Moisture Using Porous Polymers and Activated Carbon
Mohsen Ghafari, John D. Atkinson* Department of Civil, Structural, and Environmental Engineering, State University of New York at Buffalo, NY 14260 * Email:
[email protected]. Phone: 716-645-4001. Fax: 716-645-3667.
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Abstract
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NO oxidation catalyzed by porous materials is difficult to implement under industrial conditions because
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moisture in combustion exhaust streams blocks oxidation sites, decreasing NO conversion. In this work,
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hydrophobic crosslinked polymers are tested as NO oxidation catalysts to overcome these negative
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impacts associated with moisture. Although activated carbons (ACs) outperform hyper-crosslinked
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polymers by > 88% and low-crosslinked polymers by > 463% under dry conditions, their NO conversion
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drops to 0% when 50% relative humidity is added. Performance of hyper-crosslinked and low-crosslinked
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polymers, however, decreases by only 19-35% and < 6%, respectively, for NO conversion in the presence
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of moisture. NO conversion differences between materials are attributed to differences in the catalysts’
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initial hydrophilicity and their proclivity to react with generated NO2, which also increases hydrophilicity.
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While the initial hydrophobicity of the polymers contributes to their consistent performance, it is their
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intrinsic ability to resist NO2 reduction reactions, compared to AC, that makes them the more viable
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catalyst for industrial application. Results suggest that the polymer hyper-crosslinking process improves
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steady-state NO conversion but increases NO2 surface reactivity and hydrophilicity. 100 100% Reduction
NO Conversion (%)
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60 2NO+O 2
2NO2
40 19% 20
Reduction
0 0
10
20
30
40
50
60
70
Time (h)
15 16
Polymer (Dry)
Activated carbon (Dry)
Polymer (Wet)
Activated carbon (Wet)
TOC/Abstract art
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Introduction
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Nitrogen oxides (NOx) are emitted from mobile and stationary combustion sources, contributing to acid
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rain and photochemical smog and damaging the human respiratory tract.1, 2 In 2014, stationary sources
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contributed 39% of total anthropogenic NOx emissions in the US.3 Selective catalytic reduction (SCR)
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and selective non-catalytic reduction are established for limiting these emissions, but high reaction
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temperatures (400-1000 oC), catalyst costs, and capital costs necessitate finding an improved control
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technology.4-7 Catalyst deactivation, NH3 slip, and N2O formation during SCR add to environmental
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concerns.8, 9
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Absorption is used industrially for SO2 control and is considered, and often implemented, for CO2
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control.10 Similarly, absorption may provide an alternative control option for NOx.11 While this idea was
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recognized > 40 yr ago,12,
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continued need for improved stationary source NOx control.14-19 NO, which comprises > 90% of NOx in
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combustion flue gas, has low solubility in water (< 1 mg/L).20 NO2, however, is up to 254 times more
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soluble (based on Henry’s Law constants at 25 oC11) and reacts with water to produce HNO3,21,
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improving absorption efficiency. NOx absorption, therefore, is only practical if NO is first oxidized to
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NO2.12, 15, 16, 22, 23
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Proposed NO oxidation strategies include using oxidizing additives (e.g., ozone), metal oxide catalysts, or
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plasma,23-25 but high costs from chemical or energy consumption inhibit their application.26,
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oxidation catalyzed by porous materials is a low cost alternative, requiring no additional chemicals.
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Activated carbon (AC) and zeolites have been fundamentally investigated as NO oxidation catalysts.28-30
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Accordingly, the NO oxidation reaction mechanism for porous catalysts is well understood, including the
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role of the catalyst’s physical/chemical properties on oxidation kinetics.19, 28, 31, 32 Atkinson et al. showed
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that oxidized AC catalysts rapidly achieve steady-state conversion because their functionalized surfaces
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prevent NO2 reduction.21 Loiland and Lobo showed that the NO oxidation rate for zeolite catalysts is
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NOx absorption has gained research traction over the past 5 yr due to
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22
NO
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inversely proportional to the catalyst’s pore size.33 Several studies show improved conversion for
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microporous catalysts after surface chemistry modification.34-36
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Past research provides the framework for understanding the NO oxidation mechanism and advancing
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towards an industrially applicable NOx control technology. However, nearly all studies addressing NO
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oxidation catalyzed by porous materials consider gas streams lacking moisture.29, 31, 33 While informative
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and fundamentally important, these studies have limited industrial value because combustion NOx is
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always mixed with moisture. Studies that have considered moisture show near-complete loss of oxidation
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performance due to adsorbed water blocking catalytic sites.37,
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highlights improved performance for hydrophilic catalysts, including zeolites and oxidized AC.29,
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While oft-mentioned, to date no study has proposed a solution to the water adsorption problem, which
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limits industrial consideration of NO oxidation and, more broadly, NOx absorption. Full-scale drying
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systems are not realistic for large stationary sources.
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Porous polymers are gaining popularity in environmental applications, especially as separation
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membranes, because of their diverse and modifiable physical and chemical properties.39, 40 In particular, it
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is straightforward to produce hydrophobic microporous polymers.41, 42 A hydrophobic catalyst should be
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immune to water adsorption, so there is value in investigating the role of porous polymers for NO
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oxidation. In this work, therefore, commercially available low-crosslinked and hyper-crosslinked styrene-
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divinylbenzene (St-DVB) beads are tested as NO oxidation catalysts in the absence and presence of
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moisture, with comparisons to microporous AC catalysts with strong NO oxidation performances under
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dry conditions. This paper moves beyond fundamental NO oxidation studies and towards the
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development of an industrially relevant catalyst that operates under flue gas conditions. It is not only the
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first effort at catalytic NO oxidation using porous polymers, but also the first effort at overcoming
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moisture-induced NO oxidation inhibition when using porous materials as catalysts.
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Compounding this, existing literature
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Materials and Methods
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2.1
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Low-crosslinked (XAD16N and XAD4 from Sigma-Aldrich and Acros Organics, respectively) and
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hyper-crosslinked (Dowex Optipore V493 and V503, DOW Chemical Company) beaded St-DVB
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copolymers were used. XAD16N and XAD4 were washed with methanol and water before use to remove
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residual monomers and preservatives. Granular ACs (Vapure410 and F400 from CABOT Norit
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Americans Inc. and Calgon Carbon Corporation, respectively) were used as representative AC catalysts
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for comparisons with crosslinked polymers.
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2.2
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Air, 1000 ppmv NO in N2, and 1000 ppmv NO2 in N2 were used as purchased. All experiments were
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performed at room temperature in a stainless steel reactor with 1.27 cm outer diameter and 10.16 cm
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length. For dry NO oxidation, 1 g of catalyst was packed in the reactor between glass wool and plastic
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mesh (Figure 1). 0.04 standard liters per minute (SLPM) NO in N2 and 0.04 SLPM air were mixed to
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provide 500 ppmv NO and 10.5 vol.% O2. For wet NO oxidation, air passed through a water saturator
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before mixing with NO, providing 50% relative humidity (RH) to the reactor, which again contained 1 g
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of catalyst (Figure 1). The saturator’s outlet tube was wrapped in heating tape (50 oC) to prevent water
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vapor condensation. Steady-state conversion occurred when there was < 1% change in outlet NO
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concentration over 1 h. NO2 breakthrough occurred when outlet NO2 was 1 ppmv. Total NOx adsorption
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capacity was calculated by integrating the NOx concentration profile, with comparison to the inlet
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concentration. For dry NO2 reduction, 0.08 SLPM NO2 in N2 passed through the reactor with 0.1 g of
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catalyst for 24 h.
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For all experiments, effluent NO and NOx (NO2 determined by difference) were continuously measured
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using a chemiluminescence detector (ECO PHYSICS CLD 82 Mh, Switzerland) with heated sample inlet.
Catalysts
NO oxidation and NO2 reduction
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Concentrations were recorded using ECO PHYSICS’ software. Graphical results and corresponding
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discussions use “NO Conversion” as NO2 percent in outlet NOx.
Stainless steel reactor Plastic mesh Glass wool NO or NO2 in N2
MFC
Catalyst Glass wool Plastic mesh
Air
MFC Heating tape NO/NOx detector Water saturator
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Figure 1: Schematic diagram (not drawn to scale) of NO oxidation/NO2 reduction experimental set-up; MFC = mass flow
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controller.
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2.3
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After NO oxidation or NO2 reduction, select samples were degassed at 120 oC in 0.08 SLPM N2 to
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analyze desorbed species. The reactor was wrapped in heating tape and the temperature was controlled
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using a manually calibrated variable autotransformer. Outlet gases were analyzed using the same NOx
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detector. Total desorbed NO and/or NO2 were determined by integration.
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2.4
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A gravimetric sorption analyzer (VTI-SA+, TA Instruments) measured water adsorption in N2. 5±0.5 mg
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of catalyst was dried by purging with N2 for 120 min at 30 oC, prior to adding moisture. Water adsorption
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at 50% RH and 25 oC, matching NO oxidation conditions, continued until a static weight was achieved (
