Catalytic Abatement of Environmental Pollutants and Greenhouse Gases in Automotive, Natural ...

136 Pages·2016·23.75 MB·English

Catalytic Abatement of Environmental Pollutants and Greenhouse Gases in Automotive,
Natural Gas Vehicles, and Stationary Power Plant Applications

Qinghe (Angela) Zheng

Submitted in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
in the Graduate School of Arts and Sciences

COLUMBIA UNIVERSITY
2016
© 2016
Qinghe (Angela) Zheng
All rights reserved
ABSTRACT
Catalytic Abatement of Environmental Pollutants and Greenhouse Gases in Automotive, Natural
Gas Vehicles, and Flue Gas Applications
Qinghe (Angela) Zheng

The present dissertation covers three research topics on catalytic environmental emissions
control, including (1) aging and regeneration mechanisms of Rh- and Pd- model three-way
catalysts (TWC) for gasoline automotive emission control, (2) catalytic methane emissions
abatement from natural gas vehicles, and (3) scale-up of CO capture and methanation using dual
2
functional catalytic materials. The study resulted in two peer-reviewed publications, two future
papers and one patent application which is currently under review.
Modern TWC use supported two separate catalyst layers on a monolith containing one Pd and
the other Rh for the emissions control of CO, HC and NO . The rhodium (Rh) metallic component
x
(active for NO reduction) experiences the most severe oxidative thermal deactivation (forming
x
inactive Rh3+) during fuel cutoff, an engine mode (e.g., at downhill coasting) used for enhancing
fuel economy. In a subsequent switch to a slightly fuel rich condition, in situ catalyst regeneration
is accomplished by the reduction of the Rh3+ with H generated through steam reforming catalyzed
2
by residual Rh0 sites. The present thesis reports the effects of the deactivation and regeneration
processes on the activity, stability and structural properties of 0.5% Rh/Al O and 0.5% Rh/Ce O -
2 3 x y
ZrO (CZO) as model catalysts. Both materials are used to varying extents in modern TWC. A
2
very brief introduction of three-way catalysis and system considerations will be presented.
During simulated fuel cutoff, catalyst deactivation is accelerated with increasing aging
temperature from 800 °C to 1050 °C. Rh on a CZO support experiences less deactivation and faster
regeneration than Rh on Al O . Catalyst characterization techniques including BET surface area,
2 3
CO chemisorption, temperature programmed reduction, and x-ray photoelectron spectroscopy,
transmission electron microscopy, scanning-electron microscopy, and x-ray diffraction
measurements were applied to examine the role of metal-support interactions in each catalyst
system. For Rh/Al O , strong metal-support interactions leading to the formation of a stable
2 3
rhodium aluminate (Rh(AlO ) ) complex dominates during fuel cutoff, resulting in more difficult
2 y
catalyst regeneration (reduction). For Rh/CZO, Rh sites were partially oxidized to Rh O and were
2 3
relatively easy to be reduced to active Rh0 during regeneration.
Moderate Pd and support sintering of Pd-Ce O is experienced upon aging, but with a minimal
x y
effect on the catalyst activity. Cooling in air, following aging, was not able to reverse the metallic
Pd sintering by re-dispersing to PdO. Unlike the aged Rh-TWCs, reduction via in situ steam
reforming (SR) of exhaust HCs was not effective in reversing the deactivation of aged Pd/Al O ,
2 3
but did show a slight recovery of the Pd activity when CZO was the carrier. The Pd+/Pd0 and
Ce3+/Ce4+ couples in Pd/CZO are reported to promote the catalytic SR by improving the redox
efficiency during the regeneration, while no such promoting effect was observed for Pd/Al O . A
2 3
suggestion is made for improving the catalyst performance.
The use of natural gas for vehicle applications is growing in popularity due to advanced
fracking technology. Exhaust methane has been excluded from regulations since it does not
participate in photochemical reactions. New vehicle environmental regulations are expected for
controlling methane emissions given their contribution to the greenhouse gas effects. Methane is
extremely resistant to oxidation when the natural gas-fueled engine operates in the stoichiometric
mode with a supported Rh-Pd three-way catalyst (TWC). Furthermore, vehicles will still operate
with fuel cutoff (for enhanced fuel economy), resulting in thermal oxidative deactivation (1050 oC)
of the Rh metal in TWC to inactive Rh3+, resulting in a loss of both NO and methane abatement
x
activity. When the engine returns to the slightly rich mode, H generated by methane steam
2
reforming does not readily occur to reduce and regenerate the Rh. We report a solution to methane
emissions abatement by catalytic reforming of an injected aqueous solution of ethanol into the
simulated exhaust stream in TWC mode, which generates sufficient H to regenerate especially Rh
2
by reducing Rh3+ to its metallic active state.
Conventional CO capture and sequestration (CCS) in aqueous alkaline solutions is a very
2
energy-intensive process with relative unstable performance and low efficiency especially for
power plant effluents, and therefore there is a need for new approaches to control green house gas
emissions of CO . Here we report on progress with an advanced technology involving CO
2 2
adsorption from flue gas and synthetic natural gas production, via methanation, both performed at
the same temperature with the addition of renewable H and by using a dual functional material
2
(DFM). The stored H used is produced by water electrolysis during those times when solar, wind,
2
and other alternative energies generate excess power out of phase with the direct use of the
electricity. The DFM is composed of nano-dispersed CaO (or Na CO ) and Ru metal supported on
2 3
𝛾Al O carrier, respectively functioning as the CO adsorbent and methanation catalyst. The
2 3 2
present paper focuses on a laboratory scale-up study by using a simulated flue gas and
5%Ru,10%CaO/Al O and 5% Ru,10%Na CO /Al O DFM samples. The effects of DFM
2 3 2 3 2 3
preparation methods, Al O carrier materials (with different shapes and properties), and adsorption
2 3
and methanation conditions (feed compositions, flow rates, reaction temperatures) on the DFM
performance were examined. Samples were prepared using chloride precursor salts and showed
stable performance under pseudo scale-up conditions, with SASOL TH100 Al O (with the highest
2 3
BET surface area and pore volume/radius among the support materials) exhibiting the best
performance. Compared to Ru-CaO, Ru-Na CO based DFM materials showed improved CO
2 3 2
utilization and methanation production. Reaction conditions were explored to find optimized CO
2
adsorption and methanation.
Table of Contents
List of Figures .............................................................................................................................................................. iv

List of Tables ............................................................................................................................................................... xi

Nomenclature ................................................................................................................................................................1

1.
  Introduction .........................................................................................................................................................2

1.1.
  Catalytic emissions abatement of carbon monoxide, hydrocarbons, and nitrogen oxides .................2

1.1.1.
  Automotive emission control using a Rhodium-Palladium three-way catalyst ......................................2

1.1.2.
  Catalyst deactivations during fuel cutoff process ...................................................................................4

1.1.3.
  Catalyst regeneration by fuel rich operation ...........................................................................................5

1.2.
  Catalytic methane emissions abatement on natural gas vehicles ..........................................................7

1.3.
  Carbon dioxide emissions control in post-combustion flue gas ...........................................................11

1.3.1.
  Carbon dioxideemissions control by sorption/desorption based technologies .....................................11

1.3.2.
  Carbon dioxideemissions control using dual functional catalytic materials ........................................12

1.4.
  Objective of the thesis: Catalytic emission control in automotive, natural gas vehicles, and flue gas
applications .............................................................................................................................................................15

2.
  Experimental methodologies ............................................................................................................................17

2.1.
  Aging and regeneration study of automotive three-way catalysts ......................................................17

2.1.1.
  Catalyst materials ..................................................................................................................................17

2.1.2.
  Simulated fuel cutoff aging and fuel rich regeneration processes ........................................................18

2.1.3.
  Catalyst regenerability as measured at simulated fuel rich condition ...................................................20

2.1.4.
  Catalyst stability during simulated fuel cutoff aging-fuel rich regeneration cycle tests .......................20

2.1.5.
  Data analysis .........................................................................................................................................20

2.1.6.
  Catalyst characterization .......................................................................................................................21

2.2.
  Catalytic methane emissions abatement study ......................................................................................24

2.2.1.
  Catalyst materials ..................................................................................................................................24

2.2.2.
  In situ catalyst pre-reduction, aging, and regeneration .........................................................................24

2.2.3.
  Catalytic methane conversion activity tests ..........................................................................................26

i
2.2.4.
  Reaction thermodynamic modelling .....................................................................................................27

2.3.
  Scale-up CO adsorption and methanation study .................................................................................27

2
2.3.1.
  Material preparation ..............................................................................................................................27

2.3.2.
  CO adsorption-methanation cycle tests ...............................................................................................29

2
2.3.3.
  Process parametric study for the adsorption and methanation ..............................................................31

3.
  Results and discussion .......................................................................................................................................32

3.1.
  Aging and regeneration mechanisms of Rh-based gasoline three-way catalysts (TWC) ..................32

3.1.1.
  Reaction thermodynamics at simulated engine fuel rich condition ......................................................32

3.1.2.
  Catalyst deactivation and regeneration of Rh-TWCs ...........................................................................34

3.1.3.
  Rh-TWCs stability during fuel cutoff aging-fuel rich regeneration cycle tests ....................................36

3.1.4.
  Catalyst deactivation and regeneration mechanisms for Rh-TWCs .....................................................37

3.2.
  Aging and regeneration mechanisms of Pd-based three-way catalysts (TWC) .................................49

3.2.1.
  Aging-induced Pd sintering: the primary catalyst deactivation mode ..................................................49

3.2.2.
  Support sintering and Pd-support interaction: other catalyst deactivation modes ................................58

3.3.
  Catalytic methane emissions abatement on natural gas vehicles by steam reforming ......................65

3.3.1.
  Reaction thermodynamics for reforming of methane, propane, or ethanol ..........................................65

3.3.2.
  Catalytic methane reforming on fresh catalysts ....................................................................................67

3.3.3.
  Catalytic methane reforming on aged catalysts ....................................................................................68

3.3.4.
  Methane emissions abatement by catalytic oxidation on fresh and aged catalysts ...............................69

3.3.5.
  Catalyst regeneration by reforming of ethanol .....................................................................................71

3.3.6.
  Catalyst regeneration by reforming of propane ....................................................................................74

3.3.7.
  Periodic air aging of Pd-TWCs .............................................................................................................76

3.3.8.
  Regeneration of Rh-TWCs by ethanol reforming in the presence of Pd-TWC ....................................77

3.4.
  Scale-up CO capture and methanation with dual functional materials (DFM) ...............................78

2
3.4.1.
  Cyclic tests of CO adsorption and conversion with 5%Ru,10%CaO (or Na CO ) DFM on various
2 2 3
Al O carriers ......................................................................................................................................................78

2 3
3.4.2.
  Effects of reaction parameters on the CO capture and methanation capabilities ................................83

2
4.
  Conclusions ........................................................................................................................................................90

ii
4.1.
  Aging and regeneration mechanisms of Rh-based three-way catalysts (TWC) ................................90

4.2.
  Aging and regeneration mechanisms of Pd-based three-way catalysts (TWC) .................................91

4.3.
  Catalytic methane emissions abatement on natural gas vehicles by steam reforming ......................91

4.4.
  Scale-up CO capture and methanation with dual functional materials ............................................93

2
5.
  Significance, novelty, and future work ............................................................................................................94

References ....................................................................................................................................................................96

iii
List of Figures
Figure 1. A washcoated monolith automotive TWC catalyst. .......................................................................................2

Figure 2. The TWC conversion profile as a function of air-to-fuel ratio. ......................................................................3

Figure 3. A schematic of the unit operations in the exhaust system for a TWC with feed back control of air-to-fuel
ratio (λ). .................................................................................................................................................................4

Figure 4. Scanning Electron Microscopic (SEM) images of fresh (a) 0.5% Rh/Al O and (b) 0.5% Rh/CZO, (c) 3%
2 3
Pd/Al O , (d) 3% Pd/Al O , (e) 1% Pd/CZO, and (f) 0.5% Pd/CZO model TWC catalysts at µm scale. SEM
2 3 2 3
measurement condition: beam voltage of 20 kV, beam current of 10 µm, working distance of 12 mm, and 30
µm in scale. .........................................................................................................................................................17

Figure 5. Schematic of the packed bed flow reactor and analysis system. (MFC: Mass Flow Controller, GC: Gas
Chromatography, TI: Temperature Indicator). ....................................................................................................18

Figure 6. Schematic process flow diagrams of (a) simulated fuel cutoff aging-fuel rich regeneration cycle and activity
test; and (b) on-board gasoline engine fuel cutoff-fuel rich operation cycles. ....................................................20

Figure 7. Schematic catalytic flow reactor setup for in situ catalyst pre-reduction, aging and regeneration, CH
4
reforming/oxidation activity measurements, and cycle tests at different reaction conditions. ...........................25

Figure 8. Schematic process flow for the catalyst preparation of 5%Ru,10%CaO/Al O DFM on different Al O
2 3 2 3
support materials and 5%Ru,10%Na CO /Al O DFM on TH100 Al O support. ............................................28

2 3 2 3 2 3
Figure 9. Reactor setup for the CO adsorption-methanation cycle tests and process parameter study. .....................30

2
Figure 10. Schematic reaction process flow for CO adsorption-methanation cycle tests. .........................................31

2
Figure 11. Reaction Gibbs free energy as a function of reaction temperature (25 °C to 700 °C) at 1 atm. Assume ideal
gas behavior for the reactant and product gas components. Compound thermodynamic data with temperature
and pressure inputs is collected from I. Barin, Thermochemical Data of Pure Substances (3rd Edition) [157].33

Figure 12. (a) Main mole fractions of H , CO, and CH ; and (b) theoretical reactant (propane and water) conversions
2 4
as a function of reaction temperature (200 °C to 550 °C) at thermodynamic equilibrium conditions. Reactant
feed: 500 vppm propane, 10 vol-% steam, 8 vol-% CO , N in balance. ............................................................34

2 2
Figure 13. Catalyst activity of fresh and aged (a) 0.5% Rh/Al O and (b) 0.5% Rh/CZO Catalyst activity is plotted in
2 3
terms of H mole fraction as a function of reaction temperature (200 °C–550 °C). ...........................................34

2

iv