alternative materials to replace platinum in catalytic and...
TRANSCRIPT
Alternative Materials to Replace Platinum in
Catalytic and Electrocatalytic Applications
Jingguang Chen
Department of Chemical Engineering
University of Delaware
Newark, DE 19711
Outline of Presentation
- Brief overview of Pt utilization in catalysis and
electrocatalysis
- Example in electrocatalysis: reducing Pt loading for H2
production from water electrolysis
- Example in catalysis: replacing Pt in conversion of
biomass-derived oxygenates
Abundance of Elements of Catalytic Interests
Pt-group metals (Pt, Ir, Pd, Rh, Ru) are expensive and limited in supply
Needs of Pt in Catalysis and Electrocatalysis
- Demand in Heterogeneous Catalysis:
Pt catalysts are used in many chemical and refining processes
- Demand in Emerging Clean Energy Technologies:
Pt electrocatalysts are required in low-temperature fuel cells,
electrolyzers, and photoelectrochemical cells in significant amounts
- Research Efforts in Solving “Pt Challenge”:
I. Replace Pt with alternative materials with similar activity and stability
II. Reduce loading of Pt using monolayer catalysts and electrocatalysts
I. Replace Pt with Transition Metal Carbides
• Physical properties of carbides:
– High hardness, wear resistance
– High temperature stability
– Excellent electrical conductivity
• Chemical properties of carbides:
– “Similar” catalytic activity to
Pt-group metals
IV V VI
Levy & Boudart, Science, 181 (1973) 547
Oyama, “Transition Metal Carbides and Nitrides”, (1996)
Hwu & Chen, Chemical Reviews 105 (2005) 185
Chen Research Group: Angew. Chem. Int. Ed. 49 (2010) 9859
Thompson Research Group: J. Catalysis, 272 (2010) 235
Davis Research Group: J. Catalysis, 282 (2011) 83
II. Reduce Pt Loading with Monolayer (ML) Pt
Challenge: Identify substrates with Pt-like bulk properties Esposito & Chen, Energy & Env. Sci. (2011)
Bridging
“Materials Gap”
- Thin films
- Supported catalyst
Single Crystal
Model Surfaces
- UHV studies
- DFT modeling
Bridging
“Pressure Gap”
- Reactor studies
- Electrochem cells
Research Approaches
- Avoid trial-and-error, empirical approach, i.e., randomly picking elements
from the Periodic Table
- Use theory and model systems to obtain design principles for identifying
catalysts with little or no Pt, while maintaining Pt-like activity and stability
Bridging
“Materials Gap”
- Thin films
- Supported catalyst
Single Crystal
Model Surfaces
- UHV studies
- DFT modeling
Bridging
“Pressure Gap”
- Reactor studies
- Electrochem cells
Examples of of reducing and replacing Pt :
1. H2 production from water electrolysis with monolayer Pt
2. Conversion of biomass-derived oxygenates with Pt-free catalysts
Example 1: Reducing Pt Loading
H2 from Water Electrolysis on ML Pt/WC
Esposito, Hunt & Chen,
Angew. Chem. Int. Ed.
49 (2010) 9859
• H2 is a mobile energy carrier
• H2 has a high gravimetric energy density
• No CO2 emission when H2 is made from the electrolysis of
water using renewable energy such as solar
Motivation for Water Electrolysis
Hydrogen Production from Water Electrolysis
Oxygen Evolution Reaction (OER)
Overall Reaction
Hydrogen Evolution Reaction (HER)
EoH+/H2=0.0 V vs. NHE
EoH2O/O2=+1.23 V vs. NHE
Cat
ho
de
(HE
R C
atal
yst
)
Power Input
e- e-
(-) (+)
H2O
H2(g)
½ O2(g) + 2H+
An
od
e
(OE
R C
atal
yst
) Schematic electrolysis cell
Challenge: HER requires relatively large Pt particles (~ 5nm)
Questions of Using ML Pt/WC as Electrocatalysts
- What is the descriptor responsible for making Pt the optimal
catalyst for the hydrogen evolution reaction (HER)?
- Does ML Pt/WC meet such descriptor for high HER activity?
- Is ML Pt/WC stable under the relatively harsh HER
conditions?
HER Activity and Hydrogen Binding Energy (HBE)
[1] Data from: Norskov, Bligaard, Logadottir, Kitchin, Chen, Pandelov, Stimming, J.Electrochem. Soc., 152 (2005) J23-26.
•Classic volcano curve observed for the HER is explained by
Sabatier’sPrinciple[2] (Volmer Step)
(Tafel Step)
[2] P. Sabatier, Catalysis in Organic Chemistry, D. Van Nostrand Company, New York, 1922.
(Weak) (Strong)
Surface HBE (eV)
WC(001) -0.99
Pt(111) -0.46
1 ML Pt-WC(001) -0.43
DFT-calculated per-atom hydrogen
binding energy (HBE) for WC, Pt, and 1
ML Pt-WC surfaces with a hydrogen
coverage of 1/9 ML.
d-band density of states
DFT Prediction: Similar HBE Values between
Monolayer Pt-WC and Bulk Pt
Pt WC
1 atomic
layer of Pt
Experimental Verification: HER Activity of Pt/WC
As Pt coverage nears 1 ML, the
activity of WC electrodes reach
that of Pt foil
WC Foil
Pt Foil
Combined DFT and experimental results have identified monolayer Pt on WC
as electrolysis catalyst of similar activity with significant reduction in cost
Esposito, Hunt & Chen, Angew. Chem. Int. Ed. 49 (2010) 9859
Adhesion of ML Pt in the Pt/WC system
•Use DFT to compare adhesion of Pt atoms to WC and Pt surfaces:
Pt-(Substrate) > Pt-Pt
Pt-(Substrate) < Pt-Pt
ML configuration
favored
Particles
favored
Binding Energy Outcome
Pt
migration
ML surface atoms Substrate Binding energy
/ eV
(M-X^) - (M-M) BE
/ eV
Pt
Pt(111) -5.43 0.00
C(0001) -4.12 1.31
WC(0001) -6.59 -1.16
W2C(0001) -6.51 -1.08
ML Pt/WC Shows Excellent HER Stability
•Physical characterization of ML Pt-WC
surface further confirms that the Pt ML is
stable on WC under HER conditions.
SEM images taken before and after
extended stability tests
XPS Pt 4f spectra and atomic Pt4f/W4f signal ratio
before and after extended stability tests
From Model Thin Films to Catalytic Particles
Challenge: A synthesis technique to deposit ML Pt on WC particles
Transmission Electron Microscopy of
Atomic Layer Deposition (ALD) of Pt on WC
A thin film of Pt is deposited on WC particles at 50 ALD cycles
HER Activity of ALD Pt/WC Particles
- Similar to thin film results, low loading of Pt (10 ALD cycles) show similar
HER activity as 10 wt% Pt/C catalyst
- Elemental analysis reveals Pt loading of 10 ALD cycle Pt/WC is a factor of
~10 less than 10 wt% Pt/C
Extension to Other ML Metal/Carbide Catalysts
Volcano relationship reveals other potential catalysts: ML Pd/WC and Pd/Mo2C
Extension to Other Electrochemical Devices
- WC is electrochemically stable in the pH and potential range for HER
- Other applications depend on pH and E range
Weidman, Esposito & Chen, J. Electrochem. Soc. 157 (2010) F179
Bridging
“Materials Gap”
- Thin films
- Supported catalyst
Single Crystal
Model Surfaces
- UHV studies
- DFT modeling
Bridging
“Pressure Gap”
- Reactor studies
- Electrochem cells
Examples of of reducing and replacing Pt :
1. H2 production from water electrolysis with monolayer Pt
2. Conversion of biomass-derived oxygenates with Pt-free catalysts
Example 2: Replacing Pt
Skoplyak, Barteau & Chen,
ChemSusChem 1 (2008) 524
Pt Catalysts for Biomass-derived Oxygenates
Ni/WC(0001)
Ni/Pt(111)
Replacing Ni/Pt with Ni/WC for Pt-free Catalysts
Advantages of replacing Ni/Pt wth Ni/WC: lower cost; higher stability Humbert, Menning & Chen, Journal of Catalysis, 271 (2010) 132
Similar Reaction Pathways on Ni/WC and Ni/Pt
Glycolaldehyde
Acetaldehyde Ethylene glycol Acetic Acid
Sorbitol
HO
O
HO OH
OH
OH
GlucoseMannitol
Hydrolysis
isomerization
H2
Hydrogenation
OH
OH
Ethylene glycol
+other
polyols
OH
HO
OO
HOOH
O
OH
n
Cellulose
O
H2O
Fructose
CH2OH
OCH2OH
OH
OH
HO
H2
Hydrogenation
OHOH
OH
OH OH
OH
OHOH
OH
OH OH
OH
-H2O
Dehydration
H2
Hydrogenation
H2
HydrogenolysisLight alkanes
CO2, etc.
H+
C-C cleavage+oxdationOrganic acids (unidentified)
O
OH OO
OH OH
HMF DHM-THF
OH
Conversion of Cellulose on Pt-free Catalysts
Conversion of cellulose to ethylene glycol on Ni-WC & Ni-W2C: Ji, Zhang 7 Chen, Catalysis Today, 147 (2009) 77
Cellulose Conversion to Chemicals on Ni-W2C
Results: 100% conversion, 61% EG yield, (6 MPa H2; 518 K; 30 min)
Ji, Zhang, & Chen, Angew. Chem. Int. Ed. 47 (2008) 8510
Conclusions and Challenges
- Promising results are obtained in reducing Pt loading using
monolayer Pt on carbides for electrocatalysis, achieving
about a factor of ~10 in Pt reduction
- Pt-free catalysts are demonstrated for conversion of
biomass-derivatives, using less expensive metal (Ni, Co,
etc.) supported on carbides.
- Significant challenges exist for achieving large-scale
applications in catalysis and electrocatalysis:
- synthesis of high surface area carbides (critical for activity)
- deposition of monolayer metal on carbides (critical for saving Pt)
- resistance to carbon deposition (critical for catalysis)
- long-term stability in solution (critical for electrocatalysis)
Acknowledgement
Collaborators: Prof. Barteau (Univ. Delaware); Prof. Willis (Univ. Conn)
Funding: Department of Energy