Center for Renewable Energy Science & Technology (CREST) at the

University of Texas at Arlington:Value-Added Hydrogen Generation

with CO2 Conversion

PI Names: Rick Billo and Krishnan RajeshwarPresenter Names: Krishnan Rajeshwar & Fred MacDonnell

May 9-13, 2011

Project ID # PD080

This presentation does not contain any proprietary, confidential, or otherwise restricted information

2

• Project start date: 8/15/08 • Project end date: 5/31/11• Percent complete: 90 %

• Barriers addressed– DOE: D, E, Y, Z, AA, AB – Direct photochemical reduction of H+

and CO2 to useful fuels, such as H2 and CH3OH, still faces a number of technical challenges. Some of these include:• Lack of efficient photocatalysts• Difficulty in driving multi-electron

processes• Coupling these reductions to useful

oxidative reactions• Total project funding

– DOE share: $2,387,463.00

– Contractor share: $596,865.00

• Project is fully funded

Timeline

Budget

Barriers

• Interactions/ collaborations• none

• Project lead : UT Arlington

Partners

Overview

Task 1 DOE Identified Barriers: Oxide Semiconductors for Solar Energy Conversion

3

Task 1 Project Objectives:New Oxide Semiconductors for Photocatalysis and Photoelectrochemical Catalysis

SubTask Number Project Milestones

Task Completion Date

Progress NotesOriginal Planned Revised Planned Actual Percent

Complete

1

Proof-of-concept of electrodeposition of BiVO4

Dec 2009 Dec 2009 Dec 2009 100 Complete

2

Combustion synthesis of BiVO4 nanoparticles March 2010 March 2010 March 2010 100 Complete

3Use of BiVO4 for CO2photoreduction Sept 2010 Sept 2010 Sept 2010 100 Complete

4

Combustion synthesis of Bi2WO6 and AgBiW2O6

Dec 2010 May 2011 May 2011 90 Nearly Complete

5

Photodeposition of Pt nanoislands on AgBiW2O6

Dec 2010 March 2011 March 2011 100 Complete

6

Optimizing Bi2WO6and AgBiW2O6 for CO2photoreduction

Dec 2010 May 2011 May 2011 90 Nearlycomplete

4

Materials for Solar Energy Conversion

• Heterogeneous Photocatalysts

• Oxide semiconductors for solar energy conversion and CO2 photoreduction

• Demonstrate combustion synthesis as an energy- and time-efficient synthesis method

• Demonstrate its applicability to the synthesis of bismuth vanadate and silver bismuth tungstate

• Demonstrate the use of silver bismuth tungstatefor mild syngas generation using photo-electrochemistry

Project Objectives

5

• Exothermic and fast reaction

• Products are homogenous and crystalline

• High surface area

• Simplicity of the process

No special equipment is required• Possibility to incorporate “dopants” in

situ in the oxide

Energy input for synthesis process comes from reaction exothermicity

Advantages of Combustion Synthesis of Semiconductors

6

• Self-propagating high-temperature synthesis (SHS)-name coined by A. G. Merzhanov.

• Synonyms: Combustion synthesis, solution combustion synthesis, gel (or sol-gel) combustion, fire or furnace-less synthesis etc.

• Pechini method (U.S. Patent 3,330,697, July 11, 1967) based in resin intermediates, i.e., on ignition of the resin, the organic portion is removed, leaving the mixed product behind.

• This method not widely used for solar applications yet.

Process Genesis and Variants

7

3.0

2.0

1.0

0.0

Comparison between the band edges of selected semiconductors (at pH 1) andthe redox potentials for water splitting.

TiO2WO3

Bi2WO6BiVO4

AgBiW2O8Bi2Ti2O7

Selected Semiconductor Photocatalysts

-1.0

Pote

ntia

l / V

vs.

NH

E

3.2

eV

2.8

eV

2.8

eV

2.4

eV

2.7

eV

3.1

eVH+/H2

O2/H2O

8

Combustion Synthesis of Bismuth Vanadate

To furnace

Anneal and grind

VOSO4VCl3

Sample Precursor Fuel

BV1

Bi(NO3)3.5H2O + VCl3

Urea

BV2 Glycine

BV3 Citric acid

BV4

Bi(NO3)3.5H2O + VOSO4

Urea

BV5 Glycine

BV6 Citric acid

BiVO4 (R)

9

light

A

A-

D

D+

VB

CB

h+

e-

Charge carrier recombination

Band gap energy

Reduction reaction

Oxidation reaction

Semiconductor

Schematic diagram of the heterogeneous photocatalytic process occurring on an illuminated semiconductor particle.

Heterogeneous Photocatalysis

10

Use of Bismuth Vanadate for Dye Photodegradation

Bismuth vanadate is not suitable for hydrogen generation but is a good photocatalyst for environmental remediation.

11

• Precursor mixture of sodium tungstate, bismuth nitrate, and silver nitrate in nitric acid

• Urea used as fuel

• Mixture of fuel and precursor dried and ignited

• Final product washed, dried, and ground to powder

Combustion Synthesis of Silver Bismuth Tungstate

12

Photogeneration of SyngasUsing Combustion Synthesized AgBiW2O8

1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.500.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

(Ahυ

)1/2 (c

m-1eV

)1/2

hυ (eV)

2.73

Tauc plot for combustion synthesized AgBiW2O8. The inset show the percent transmittance data of the sample before (black line) and after photodeposition of 1wt % Pt (blue line), along with the corresponding sample photographs

High-resolution core level X-ray photoelectron spectra in the Ag 3d (a), Bi 4f (b), W 4f (c), and O 1s (d)

binding energy regimes for AgBiW2O8 powder prepared by

combustion synthesis using urea as fuel.

380 378 376 374 372 370 368 366 364 362

1.0x105

1.5x105

2.0x105

2.5x105

3.0x105 (a)

CP

S

Binding Energy / eV

Ag 3d5/2

Ag 3d3/2

168 166 164 162 160 158 1560

1x105

2x105

3x105

4x105

5x105

6x105

7x105 (b) Bi 4f7/2

Bi 4f5/2

CP

S

Binding Energy / eV

40 39 38 37 36 35 34 33 32

2.0x104

4.0x104

6.0x104

8.0x104

1.0x105

(c)

CP

S

Binding Energy / eV

W 4f5/2

W 4f7/2

536 534 532 530 528 526

2x105

2x105

3x105

3x105

4x105

4x105

5x105

5x105

(d) O 1s

Binding Energy / eV

CP

S

High-resolution core level X-ray photoelectron spectra in the Ag 3d (a), Bi 4f (b), W 4f (c), and O 1s (d) binding energy regimes for AgBiW2O8 powder prepared by combustion synthesis using urea as fuel.

13

H2 and CO Photogeneration

Evolution of photogenerated syngas (H2+CO) sustained by in situ formation of CO2 from HCOOH.

0 50 100 150 200 2500

10

20

30

40

50

60

70

80

90

100

H2

CO

Am

ount

of g

as e

volv

ed (%

)Irradiation (min)

CO2

High-resolution core level X-ray photoelectron spectra in the Ag 3d (a), Bi 4f (b), W 4f (c), and O 1s (d) binding energy regimes for AgBiW2O8 powder prepared by combustion

synthesis using urea as fuel.

Selected GC runs for detection of gaseous products during the photocatalytic reaction of AgBiW2O8 - 1 wt.% Pt in 0.1 M HCOOH

14

15

2 H+

H2

HCOO-

HCOO●h+

e-

AgBiW2O8

Pt CO2 + 2H+

CO + H2O

HCOO

CO2 + H+

H2O

●OH

Syngas Photogeneration by AgBiW2O8-Pt

QY H2 = 3.0 %QY CO = 0.8 %

QY CO2 = 4.5 %

Quantum yields (QY %) were measured by chemical actinometry using 0.006 M K3Fe(C2O4)3.3H2O) in 1 N H2SO4

3.2 x 10-2 moles of photons/h

100% xphotonsofmol

formedproductofmolQY =

0.5 g AgBiW2O8/Pt in 200 mL HCOOH 0.1 M

tVmproductofmolPCP =PCP CO2 = 14.4 x 10-3 mol/g·L·h

PCP H2 = 10 x 10-3 mol/g·L·h

PCP CO = 2.2 x 10-3 mol/g·L·h

PCP = Photocatalytic performance

Cumulative: CO2 = 2.8 x 10-2 mol/g.L, CO = 3 x 10-3 mol/g.L, H2 = 1.0 x 10-2 mol/g.L

16

• Time and energy-efficient method developed for two new-generation oxide semiconductor photocatalysts: bismuth vanadate and silver bismuth tungstate.

• Silver bismuth tungstate has been used for mild syngas photogeneration.

• Further materials optimization will continue.

Task 1: Summary for Heterogeneous Photocatalysis

Barton CE, Lakkaraju PS, Rampulla DM, Morris AJ, Abelev E, Bocarsly AB. Using a One-Electron Shuttle for the Multielectron Reduction of CO2 to Methanol: Kinetic,Mechanistic, and Structural Insights. J. Am. Chem. Soc. 2010; 132: 11539-51.

HN N

OHO

H2O

+ e- + e- + H+ N+ CH3OHCO2 +

Task 2: Molecular photocatalystsfor CO2 Remediation and CH3OH Formation

Inspiration: Bocarsly and coworkers demonstrate electroreduction of CO2 using pyridine as a electocatalyst

Critical pH effects

Strategy: Generate reduced pyridinium ions using thephotochemistry of Ru-polypyridyl complexes

NN N

NN NRu2+

N

N N

2+

NN N

NN NRu2+

N

NN

2+

ptpbα ptpbβ

17

Task 2 DOE Identified Barriers: Homogeneous Photocatalytic CO2 reduction and Methanol Formation

18

SubTaskNumber Project Milestones

Task Completion Date

Progress NotesOriginal Planned Revised Planned Actual Percent

Complete

1Synthesis of three Ru-photocatalysts Dec 2009 Dec 2009 100 complete

2

Electrocatalyticscreening of three

photocatalysts March 2010 March 2010 100 complete

3

PhotophysicalEvaluation of the Photocatalysts June 2010 Oct 2010 Oct 2010 100 complete

4

Demonstration of Photocatalytic CO2reduction to MeOH Sept 2010 Dec 2010 March 2011 100 complete

5

Optimizing /Screening of

Photochemical CO2reduction Dec 2010 March 2011 May 2011 90

Nearly complete

Task 2 Project Objectives:Develop Homogeneous Molecular Photocatalysis to Convert CO2 to methanol

19

Proposed Homogeneous Photocatalytic Cycle for CO2 Reduction to Methanol

NN N

NN NRu2+

N

N NHNN N

NN NRu2+

N

N NNN N

NN NRu3+

N

N N

NN N

NN NRu2+

N

N N

2+

2+

2+

2+

MLCT1MLCT0

Ru+

Ru2+

D

D+

NN N

NN NRu2+

N

N N

2+

Ru+

O

OH+ H+

+ CO2

NN N

NN NRu2+

N

N N

2+ OH

hν

+ H+

+ D

2 hν+2 D

2 hν

+ H+

+2 D

- MeOH

OH

-H2O

NN N

NN NRu2+

N

N N

2+ O

H

NN N

NN NRu2+

N

N N

2+ OH

HNN N

NN NRu2+

N

N N

2+ OH

HH

hν (480 nm)

+ H+

CO2 + 6 D + 6H+

CH3OH + 6 D+ + H2O

6 hν

20

21

Performance of [(bpy)2Ru(ptpbβ)]2+ as Homogeneous Catalysts for CO2 reduction

0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

-0.10

-0.05

0.00

0.05

0.10

0.15

pH = 2

50 mV/s

0.5 mM beta - 0.2 M NaF

Curre

nt / m

A.cm

-2Potential / V vs. Ag/AgCl

Au electrode

-0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9-1.0

-0.5

0.0

0.5

1.0

1.5

2.010 mM Pyr- 0.2 M NaF + 35ul 0.1 M H2SO4

Curre

nt /

mA.

cm-2

Potential / V vs. Ag/AgCl

0.6 0.4 0.2 0.0 -0.2 -0.4

-0.05

0.00

0.05

0.10

0.15

pH = 2

50 mV/s

0.5 mM beta - 0.2 M NaF + H2SO4

Curre

nt /

mA.

cm-2

Potential / V vs. Ag/AgCl

Pt electrode

Au electrode

pH = 5.5

pH = 2

Pt electrode

pH = 2

EC’ mechanism: redoxprocess followed by a catalytic chemical reaction

[(bpy)2Ru(pbtpβ)]2+ + 1e- + 1H+ [(bpy)2Ru(pbtpβH]2+

[(bpy)2Ru(pbtpβH]2+ + CO2 [(bpy)2Ru(pbtpβ)]2+ + CHxO2-y

vs. Pyridine

[(bpy)2Ru(ptpbβ)]2+

CO2 Reduction via Electrocatalysis with Ruthenium Photocatalysts

[(bpy)2Ru(ptpbα)]2+ vs. [(bpy)2Ru(ptpbβ)]2+

22

Effect of concentration on the voltammetric behavior ofthe first electroreduction process of [(bpy)2Ru(ptpbα)]2+

in DMF + 1 M buffered water (pH ~ 5.5). Scan rate = 50mV/s. Working electrode = glassy carbon disk of 1 mmdiameter.

Comparison of the voltammetric behavior of [(bpy)2Ru(ptpbα)]2+ and[(bpy)2Ru(ptpbβ)]2+ in CO2 saturated solution. Scan rate = 5 mV/s. Thevoltammogram with CO2 without any Ru complex is shown in black dot line.

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2

0.00

0.01

0.02

0.03

0.04

0.05

CO2 sat. (no complex) Rupbtpα + CO2 sat. Rupbtpβ + CO2 sat.

Curre

nt / m

A cm

-2

Potential/V vs. Ag/AgCl

Photoreduction of CO2 by [(bpy)2Ru(ptpbβ)]2+

330 335 340 345 350 355 360

Time / s0 5 10 15 20 25 30

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

∆ Ab

sorb

ance

660665670675680685690695

564 nm 344 nm

Demonstration of efficient photocatalytic cycles: The three consecutive and reversible photochemical changes of [bpy)2Ru(pbtpβ)]2+ in CO2 saturated DMF/TEA (0.25 M)/H2O (1 M) are shown from left to right. The photoreduced catalyst is manifested in the growth of a band at 544 nm at expense of the 344 nm band characteristic of the unreduced photocatalyst. Each photoreduction requires 30 s to be completed and is separated from the next by a 5 min CO2 bubbling under dark. The period of bubbling in the dark is provoking the removal of the reduced CO2 species coordinated to the complex and thus releasing the complex to be able to work again as photocatalyst in the next irradiated cycle.

Photochemistry of [(bpy)2Ru(ptpbβ)]2+ CO2 Photocatalytic Conversion

23

Difference absorbance spectra of [(bpy)2Rupbtpβ]2+ (2.2 x 10-4 M) in N2 saturated DMF/TEA (0.25 M)/H2O (1 M) solutions during photolysis in the time domain for 0 to 200 s. Peaks pointing down indicate bands disappearing while those pointing up correspond to new bands appearing as a consequence of the photochemical reaction.

0.30

0.25

0.20

0.15

0.10

0.05

0.00

-0.05

Abso

rban

ce /

a.u.

900800700600500400300Wavelength / nm

0 5 s 10 s 15 s 25 s 35 s 50 s 200 s

458564

437

388

344

308

Custom Photoreactor: Water-jacketed glass reaction vessel with optical probe, stir bar and lamp consisting

of 120 ultra bright diode in circular pattern (wavelength 470 nm)

Photochemistryof [(bpy)2Ru(ptpbα)]2+ and [(bpy)2Ru(ptpbβ)]2+

in DMF/ water or MeCN /water

α and β complexes inactive at pH 5. Donor does not function below pH 5 24

Conditions: In a typical experiment, a water jacketed reaction vessel of 50 mL total volume was filled with ~25 mL MeCN or DMF that is 1.0 M H2O (pH 5), 0.20 M TEOA, and 100 µM Ru complex. The solutions are degassed, saturated with CO2, and then placed in the photochemical reactor for irradiation. Samples of both the headspace and solution are collected at various time periods and subjected to gas chromatographic (GC) analysis for product detection.

GC Analysis: Standards run using 1 C/minute temperature ramp from 110 C to 150 C using a Haysep DB column. Black line is methane, domestic natural gas, green line

NN N

NN NRu

NN N

NN NRu

HN

HN

N

OHO

CO2

CH3OH

[Ru(phen)3]2+

2+ +

TEOAred TEOAox

Ru*hν

Photochemistry of [Ru(phen)3]2+ and Pyridine in Water at pH 5

Conditions: 100 uM [Ru(phen)3]2+, 0.1M pyridine, 0.20 M TEOA, pH 5.0 then saturated with CO2 by extensive bubbling and placed under CO2 atm. We are currently working to quantify and optimize this reaction.

MeOH detected by GC analysis!

25

Cyclic voltammetry of [(bpy)2Ru(ptpbβ)]2+ on a Au electrode Effect of [H+] on the Electrocatalysis

working electrode: Au (1mm dia.)

0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

-0.1

0.0

0.1

0.2

0.3

0.4

[H+]

50 mV/s

W.E. = Au (1 mm dia.)Init Cell Vol. = 1.3 mL

1 mM beta - 0.2 M NaF (1.5 mL) + x µL 0.1 M H2SO4

750 µL 650 µL 550 µL 450 µL 350 µL 250 µL 200 µL 150 µL 100 µL 60 µL 20 µL 7 µL

Cur

rent

/ m

A.c

m-2

Potential / V vs. Ag/AgCl0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

-0.1

0.0

0.1

0.2

0.3

0.4

50 mV/s

pH 2

pH 6[H+]

1 mM beta - 0.2 M NaF (1.5 mL) + x µL 0.1 M H2SO4

W.E. = Au (1 mm dia.) 7 µL 1 mM beta 100 µL 250 µL 750 µL 0.5 mM beta

Cur

rent

/ m

A.c

m-2

Potential / V vs. Ag/AgCl

Selected CVs

26Ep vs. pH shifts positively by 60 mV per unit of pH for pHs between 6 and 2

[(bpy)2Ru(pbtpβ)]2+ + 1e- + 1H+ [(bpy)2Ru(pbtpβH]2+

27

Electrocatalytic reduction of CO2 by [(bpy)2Ru(ptpbβ)]2+ at pH 2

During ~ 30 h the potential holds in the 0V/-0.2V rangesustained by an EC’ mechanism (1e- reduction processforming [(bpy)2Ru(pbtpβH)]2+ and followed by a chemicalreaction that regenerate the initial [(bpy)2Ru(pbtpβ)]2+

complex). At times longer than 30 h the potential shiftsto -0.7V because all the [(bpy)2Ru(pbtpβ)]2+ complex hasbeen converted to [(bpy)2Ru(pbtpβH)]2+

Comparative CV runs

0.6 0.4 0.2 0.0 -0.2 -0.4

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

10 mV/s

0.5 mM beta - 0.2 M NaF (1.5 mL) - pH 2

W.E. = Pt (1 mm dia.)

CO2, no complex 0.5 mM beta 0.5 mM beta + CO2

Cur

rent

/ m

A.c

m-2

Potential / V vs. Ag/AgCl

Voltammetric profiles of 0.5 mM [(bpy)2Ru(ptpbβ)]2+ in 0.2 MNaF aq. media (adjusted to pH 2 with H2SO4) in N2 (dash blueline) and CO2 (b) saturated solutions respectively. Scan rate =10 mV/s. A blank run of CO2 saturated supporting electrolytewithout the complex is shown in black dot line.

Galvanostatic Electrolysis

Potential evolution during galvanostaticelectrolysis of 5 mM [(bpy)2Ru(pbtpβ)]2+ in 0.1 M KCl aq. solution (pH 2) under constant CO2

bubbling. Applied current = 0.45 mA (cathodic).

CH3OH Detected as Product in Electrocatalysis

Method: Galvanostatic Electrolysis under CO2 bubbling

Electrolyte: 0.1 M KCl (@ pH 2 w/1M HCl)[Ru(phen)2pbtpβ] = 5 mM

W.E.= Pt foil (8.4 cm2)C.E.= Pt in separated compartment Ref. E.= Ag/AgCl, KCl sat.

Applied current : -0.45 mA

6 8 100

10

20

Inte

nsity

/100

0

time / min

0 h 1 h 2 h 4 h 6 h

Selected GC runs of samples taken duringthe Ru(phen)2pbtpβ2+ + CO2 electrolysis.

Comparison of GC-MS data of a 2 h electrolysis sample (a) with authentic spectra of 10 mM CH3OH(b)

(a)

(b)

0 2 4 6 8 100

10

20

30

40

50

60

70

Inte

nsity

/100

0

time / min

beta-pH 2-CO2- 4 h pyr-pH 5.5-CO2- 23 h

GC comparison of CO2 conversion to CH3OH using 5 mMRu(phen)2pbtpβ2+ (blue line) and 10 mM pyridine (red line).

GC-Mass Spectrometryof MeOH from

catalysis

GC-Mass Spectrometryof system ‘spiked’ with

MeOH

28

29

• Ruthenium complexes with internal pyridyl functions prepared and characterized.

• Complexes competent for electrocatalytic CO2 reduction to methanol. Optimum performance at pH 2.

• Ru α and Ru β photocatalysts inactive for photochemical CO2reduction at pH 5 and experimental constraints precluded testing at pH 2.

• Simple homoleptic complex, [Ru(phen)3]2+, is able to function at pH 5 with pyridine added as co-catalyst.

Methanol is produced photochemically!!!• Controlling variables including ligand isomerism (ptpbα vs ptpbβ),

solvent composition (DMF/MeCN/H2O), solution pH, and choice of donor, need further investigation and optimization.

Task 2: Summary