NH3 storage isotherms:
a path toward better
models of NH3 storage
on zeolite SCR
catalysts
Josh Pihl, Stuart Daw
Oak Ridge National Laboratory
presentation to:
2014 DOE Crosscut Workshop on Lean Emissions Reduction Simulation
Dearborn, MI
April 29, 2014
2 Presentation name
High NOx conversion SCR systems require
effective control of catalyst NH3 inventory
• NH3 inventory on catalyst must be managed to:
– maximize NOx conversion
• requires high NH3 coverages
– minimize NH3 slip
• prevent excessive NH3 emissions
• efficiently utilize urea (or other NH3 source)
• NH3 management critical for approaches based on
cycles of NH3 production and consumption
– LNT-SCR
– passive SCR (TWC-SCR)
• Accurate NH3 storage models required to develop
and calibrate operating strategies
– urea dosing
– lean/rich cycle times
250 °C
350 °C
300 °C
Commercial small pore Cu zeolite (GM vehicle)
aged: 700 °C 4 h
350 ppm NH3, 350 ppm NO, 5% H2O, 10% O2, bal. N2
GHSV: 60k h-1
3 Presentation name
Accurate models of NH3 storage needed to
develop high NOx conversion SCR systems
• Equilibrium NH3 storage capacity varies significantly with:
– gas composition (NH3 concentration)
– temperature
– catalyst age
• high T capacity drops by half over vehicle life
• Inventories under SCR operating conditions further
complicated by:
– NH3 consumption:
• reactions with NOx
• NH3 oxidation
– catalyst utilization
• Models must accurately capture these dependencies
– focus of collaborative work between PNNL and ORNL Commercial small pore Cu zeolite (GM vehicle)
aged: 700 °C 4 h, 800 °C 6 h, or 800 °C 16 h*
350 ppm NH3, 5% H2O, bal. N2
GHSV: 60k h-1 *S. Schmieg, S. Oh, C. Kim, D. Brown, J. Lee, C. Peden, D. Kim, Catal. Today,
184, 252-261, 2011.
4 Presentation name
Industry partners have identified NH3 storage
models as a high priority research area
• 2013 CLEERS priority survey results
– recipients evaluated 32
emissions control topics
• scale: 1 (low) to 3 (high)
– 24 survey responses received
• Respondents ranked NH3 storage,
oxidation, and release as a high
priority topic:
– rankings:
• #1 of 7 urea SCR topics
• #1 of 12 NOx control topics
• #3 of all 32 survey topics
– score: 2.50
CLEERS 2013 Industry Priority Survey Results
5 Presentation name
Current strategies for measuring NH3 storage
create challenges for model calibration
• Commonly used experimental protocols* rely on
transient operating conditions that confound:
– thermodynamics (energies, capacities)
– kinetics (reaction rates)
– transport (diffusion limitations)
• No agreement on model strategies in literature
– number of NH3 storage sites
– energetics of adsorption
• Langmuir, Temkin, Freundlich ...
• Complicated data sets and uncertain model
structures make parameter estimation challenging
– easy to generate “reasonable” fits to data
– resulting parameters neither unique nor
globally valid
(a) adsorption (b) desorption (c) TPD
NH
3 co
nce
ntr
atio
n (
pp
m)
NH
3 m
ole
fra
ctio
n (
x10
5 )
T (K)
T (K)
700C 4h
800C 16h
TPD simulations
*E. Tronconi, L. Lietti, P. Forzatti, S. Malloggi, Chem. Eng. Sci. 51, 2965, 1996
6 Presentation name
Modified experiment to measure equilibrium NH3
storage isotherms and isolate energetics
• Measure NH3 released during isothermal stepwise desorption
– eliminates kinetic and transport effects during storage measurements
• Repeat at multiple temperatures to generate family of isotherms
NH3 in NH3 out
7 Presentation name
Modified experiment to measure equilibrium NH3
storage isotherms and isolate energetics
• Measure NH3 released during isothermal stepwise desorption
– eliminates kinetic and transport effects during storage measurements
• Repeat at multiple temperatures to generate family of isotherms
NH3 in
NH3 out
NH3 desorbed
8 Presentation name
Analysis of NH3 isotherms to isolate energetics of
adsorption
• Now that we have isotherms, what do we do with them?
– could try to fit storage model parameters directly,
but ...
– need to define model structure
• Stuart’s inspiration: calculate adsorption enthalpies from
isotherms using Clausius-Clapeyron equation
𝑑 ln𝑃𝑁𝐻3
𝑑1𝑇
=∆𝐻𝑎𝑑𝑠
𝑅
– commonly used in analysis of sorption materials
• DHads vs. NH3 coverage plots generate insights into
storage site characteristics and modeling strategies:
– number of sites
– relative abundance of sites
– energetics of adsorption at each site
Commercial small pore Cu zeolite (GM vehicle)
aged: 700 °C 4 h
variable NH3, 5% H2O, bal. N2
GHSV: 60k h-1
9 Presentation name
Calculation of DHads
from NH3 storage isotherms
1. measure isotherms
3. linearly interpolate to generate
isocoverage curves
4. fit slopes to estimate
DHads vs. coverage
2. transpose isotherms
to lnP vs. coverage
𝑑 ln𝑃𝑁𝐻3
𝑑1𝑇
=∆𝐻𝑎𝑑𝑠
𝑅
10 Presentation name
DHads
vs. coverage trends for simple (single site)
isotherms guide selection of modeling approaches
Temkin
𝐾0𝑒(−∆𝐻0
𝑎𝑑𝑠 1−𝛼𝜃 𝑅𝑇 )𝑃 = 𝜃(1−𝜃)
∆𝑯𝒂𝒅𝒔 = ∆𝑯𝟎𝒂𝒅𝒔
(𝟏 − 𝜶𝜽)
Langmuir
𝐾0𝑒(−∆𝐻0
𝑎𝑑𝑠 𝑅𝑇 )𝑃 = 𝜃(1−𝜃)
∆𝑯𝒂𝒅𝒔 = ∆𝑯𝟎𝒂𝒅𝒔
Freundlich
𝐾0𝑒(−𝛼𝑅𝑇 ∆𝐻0
𝑎𝑑𝑠 )𝑃(𝑅𝑇 ∆𝐻0
𝑎𝑑𝑠 ) = 𝜃
∆𝑯𝒂𝒅𝒔 = ∆𝑯𝟎𝒂𝒅𝒔
𝐥𝐧 𝜽𝑲𝟎
Measured
∆𝑯𝒂𝒅𝒔 = 𝒇 𝜽
11 Presentation name
DHads
vs. coverage trends for simple (two site)
isotherms guide selection of modeling approaches
2 site Langmuir isotherms
DHads,1 = -85 kJ/mol; DHads,2 = -30 kJ/mol
N1/Ntotal: 0.7
Measured
0.5 0.3
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Calculated DHads
vs. coverage provides starting
point for model structure & parameter estimation
• Measured DHads vs. coverage consistent with
two site Langmuir isotherm
– DHads = -85 and -30 kJ/mol
– roughly equal site distribution
• Approach facilitates storage parameter
estimation
– started with measured enthalpies and
site distributions
– manually adjusted adsorption entropies
– estimated isotherms not terribly far off
– fidelity of isotherm estimates could be
improved through parameter optimization
• Other model modifications may be needed
– H2O competitive adsorption
measured estimated
measured
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Investigating impact of H2O and hydrothermal
aging on measured NH3 adsorption enthalpies
• Impact of H2O during adsorption:
– significantly reduces DHads on the lower
energy site
– calculated DHads shows more apparent
scatter without H2O
• due to changes in Cu oxidation state?
• Impact of hydrothermal aging:
– NH3 without H2O: trends consistent with
conversion of high energy sites to low
energy sites
– NH3 with H2O: trends less clear
• interaction between changes in
energetics of NH3 adsorption and
H2O adsorption?
• Need better understanding of variability and
uncertainty in DHads measurements
H2O 800 °C 16 h
H2O 700 °C 4 h
no H2O 700 °C 4 h
no H2O 800 °C 16 h
H2O
aging
14 Presentation name
Acknowledgements
• Funding provided by DOE Vehicle Technologies Office
– Program managers: Ken Howden, Leo Breton, and Gurpreet Singh
Next steps
• Quantify uncertainty in adsorption enthalpy estimates
• Elucidate impacts of H2O on NH3 adsorption
– measure H2O adsorption energetics
– look into competitive adsorption in Langmuir isotherm
• Further investigate hydrothermal aging impacts
– identify model parameter changes required to account for aging
• Develop NH3 storage model consistent with isotherm data
– incorporate into full SCR model with PNNL
• Apply similar approach to HC traps
