8/10/2019 Alternative Materials to Replace Platinum

1/30

Alternative Materials to Replace Platinum inCatalytic and Electrocatalytic Applications

Jingguang Chen

Department of Chemical EngineeringUniversity of Delaware

Newark, DE 19711 jgchen@udel.edu

8/10/2019 Alternative Materials to Replace Platinum

2/30

8/10/2019 Alternative Materials to Replace Platinum

3/30

Abundance of Elements of Catalytic Interests

Pt-group metals (Pt, Ir, Pd, Rh, Ru) are expensive and limited in supply

8/10/2019 Alternative Materials to Replace Platinum

4/30

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

8/10/2019 Alternative Materials to Replace Platinum

5/30

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) 547Oyama, Transition Metal Carbides and Nitrides, (1996)Hwu & Chen, Chemical Rev iews 105 (2005) 185Chen Research Group: Angew. Chem. Int. Ed. 49 (2010) 9859Thompson Research Group: J. Catalysis , 272 (2010) 235

Davis Research Group: J. Catalysis , 282 (2011) 83

8/10/2019 Alternative Materials to Replace Platinum

6/30

II. Reduce Pt Loading with Monolayer (ML) Pt

Challenge: Identify substrates with Pt-like bulk properties

Esposito & Chen, Energy & Env. Sci . (2011)

8/10/2019 Alternative Materials to Replace Platinum

7/30

BridgingMaterials Gap

- Thin films

- Supported catalyst

Single CrystalModel Surfaces

- UHV studies

- DFT modeling

BridgingPressure Gap

- Reactor studies

- Electrochem cells

Research Approaches

- Avoid trial-and-error, empirical approach, i.e., randomly picking elementsfrom the Periodic Table

- Use theory and model systems to obtain design principles for identifyingcatalysts with little or no Pt, while maintaining Pt-like activity and stability

8/10/2019 Alternative Materials to Replace Platinum

8/30

BridgingMaterials Gap

- Thin films- Supported catalyst

Single CrystalModel Surfaces

- UHV studies- DFT modeling

BridgingPressure Gap

- Reactor studies- Electrochem cells

Examples of of reducing and replacing Pt :

1. H 2 production from water electrolysis with monolayer Pt2. Conversion of biomass-derived oxygenates with Pt-free catalysts

Example 1: Reducing Pt Loading

8/10/2019 Alternative Materials to Replace Platinum

9/30

H2 from Water Electrolysis on ML Pt/WC

Esposito, Hunt & Chen, Angew. Chem. Int. Ed .49 (2010) 9859

8/10/2019 Alternative Materials to Replace Platinum

10/30

H2 is a mobile energy carrier

H2 has a high gravimetric energy density No CO 2 emission when H 2 is made from the electrolysis of

water using renewable energy such as solar

Motivation for Water Electrolysis

8/10/2019 Alternative Materials to Replace Platinum

11/30

Hydrogen Production from Water Electrolysis

Oxygen Evolution Reaction (OER)

Overall Reaction

Hydrogen Evolution Reaction (HER)

Eo H+/H2 =0.0 V vs. NHE

Eo H2O/O2 =+1.23 V vs. NHE

C a t

h o

d e

( H E R C a t a l y s

t )

PowerInpute- e-

(-)(+)

H2O

H2(g)

O 2(g) + 2H +

A n o

d e

( O E R C a t a l y s

t )

Schematic electrolysis cell

Challenge: HER requires relatively large Pt particles (~ 5nm)

8/10/2019 Alternative Materials to Replace Platinum

12/30

Questions of Using ML Pt/WC as Electrocatalysts

- What is the descriptor responsible for making Pt the optimalcatalyst 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 HERconditions?

8/10/2019 Alternative Materials to Replace Platinum

13/30

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 bySabatiersPrinciple [2] (Volmer Step)

(Tafel Step)

[2] P. Sabatier, Catalysis in Organic Chemistry, D. Van Nostrand Company, New York, 1922.

(Weak)(Strong)

8/10/2019 Alternative Materials to Replace Platinum

14/30

Surface HBE (eV)WC(001) -0.99Pt(111) -0.46

1 ML Pt-WC(001) -0.43DFT-calculated per-atom hydrogenbinding energy (HBE) for WC, Pt, and 1ML Pt-WC surfaces with a hydrogencoverage of 1/9 ML.

d-band density of states

DFT Prediction: Similar HBE Values betweenMonolayer Pt-WC and Bulk Pt

Pt WC

1 atomiclayer of Pt

8/10/2019 Alternative Materials to Replace Platinum

15/30

Experimental Verification: HER Activity of Pt/WC

As Pt coverage nears 1 ML, the

activity of WC electrodes reachthat of Pt foil

WC Foil

Pt Foil

Combined DFT and experimental results have identified monolayer Pt on WCas electrolysis catalyst of similar activity with significant reduction in cost

Esposito, Hunt & Chen, Angew. Chem. Int. Ed . 49 (2010) 9859

8/10/2019 Alternative Materials to Replace Platinum

16/30

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 configurationfavored

Particlesfavored

Binding Energy OutcomePt

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

8/10/2019 Alternative Materials to Replace Platinum

17/30

ML Pt/WC Shows Excellent HER Stability

Physical characterization of ML Pt-WCsurface further confirms that the Pt ML isstable on WC under HER conditions.

SEM images taken before and afterextended stability tests

XPS Pt 4f spectra and atomic Pt4f/W4f signal ratiobefore and after extended stability tests

8/10/2019 Alternative Materials to Replace Platinum

18/30

From Model Thin Films to Catalytic Particles

Challenge: A synthesis technique to deposit ML Pt on WC particles

8/10/2019 Alternative Materials to Replace Platinum

19/30

Transmission Electron Microscopy ofAtomic Layer Deposition (ALD) of Pt on WC

A thin film of Pt is deposited on WC particles at 50 ALD cycles

8/10/2019 Alternative Materials to Replace Platinum

20/30

HER Activity of ALD Pt/WC Particles

- Similar to thin film results, low loading of Pt (10 ALD cycles) show similarHER 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

8/10/2019 Alternative Materials to Replace Platinum

21/30

Extension to Other ML Metal/Carbide Catalysts

Volcano relationship reveals other potential catalysts: ML Pd/WC and Pd/Mo 2C

8/10/2019 Alternative Materials to Replace Platinum

22/30

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

8/10/2019 Alternative Materials to Replace Platinum

23/30

BridgingMaterials Gap

- Thin films- Supported catalyst

Single CrystalModel Surfaces

- UHV studies- DFT modeling

BridgingPressure Gap

- Reactor studies- Electrochem cells

Examples of of reducing and replacing Pt :

1. H 2 production from water electrolysis with monolayer Pt2. Conversion of biomass-derived oxygenates with Pt-free catalysts

Example 2: Replacing Pt

8/10/2019 Alternative Materials to Replace Platinum

24/30

Skoplyak, Barteau & Chen,ChemSusChem 1 (2008) 524

Pt Catalysts for Biomass-derived Oxygenates

8/10/2019 Alternative Materials to Replace Platinum

25/30

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 stabilityHumbert, Menning & Chen, Journal of Catalysis , 271 (2010) 132

8/10/2019 Alternative Materials to Replace Platinum

26/30

Similar Reaction Pathways on Ni/WC and Ni/Pt

Glycolaldehyde

AcetaldehydeEthylene glycol Acetic Acid

8/10/2019 Alternative Materials to Replace Platinum

27/30

Sorbitol

HOO

HO OHOH

OH

Glucose Mannitol

Hydrolysis

isomerization

H2

Hydrogenation

OH

OH

Ethylene glycol

+other

polyols

OH

HOO O

HO OH

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)

OOH O

OOH OH

HMF DHM-THF

OH

Conversion of Cellulose on Pt-free Catalysts

Conversion of cellulose to ethylene glycol on Ni-WC & Ni-W 2C:Ji, Zhang 7 Chen, Catalysis Today , 147 (2009) 77

8/10/2019 Alternative Materials to Replace Platinum

28/30

Cellulose Conversion to Chemicals on Ni-W 2C

Results: 100% conversion, 61% EG yield, (6 MPa H 2; 518 K; 30 min)

Ji, Zhang, & Chen, Angew. Chem. Int. Ed. 47 (2008) 8510

C l i d Ch ll

8/10/2019 Alternative Materials to Replace Platinum

29/30

Conclusions and Challenges

- Promising results are obtained in reducing Pt loading using

monolayer Pt on carbides for electrocatalysis, achievingabout 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-scaleapplications 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 )

8/10/2019 Alternative Materials to Replace Platinum

30/30

Acknowledgement

Collaborators: Prof. Barteau (Univ. Delaware); Prof. Willis (Univ. Conn)

Funding: Department of Energy