Sequential Oxidation of Group 6 Transition Metal Suboxide Clusters

Caroline Chick JarroldDepartment of Chemistry, Indiana University

April 18, 2023

April 18, 2023

FundingFundingDOEDOE

Dr. Jennifer Mann

Sarah Waller

David Rothgeb

Transition Metal Oxide ClustersTransition Metal Oxide Clusters

April 18, 2023

April 18, 2023

• Important in numerous catalytic applications including photocatalytic decomposition of water, CO2 reduction, CH4 value addition

• Optimization of catalysts impeded by general lack of atomic-scale interactions involved in processes

• Localized bonding Cluster models for experimental and computational studies

• What can we glean from spectroscopic studies of these clusters?

April 18, 2023

April 18, 2023

Experimental Approach:

•Anion photoelectron spectroscopy (PES) of transition metal SUBOXIDE clusters

Mass-selected, internally cold ions are photodetached with fixed frequency laser:

e- KE = hv – EA – Eint0 + Eint

- e-BE = hv – e-KE

I ()2

Computational Approach:

•Density Functional Theory (DFT) CalculationsTriple- quality calculations using B3LYP with refined basis sets (K. Raghavachari, N. Mayhall)

April 18, 2023

Qsym

Anion ground state

Neutral ground state

Neutral excited state

020406080100120

00

.51

1.5

22

.53

3.5

Electron Counts

Ele

ctro

n B

ind

ing

En

erg

y (e

V)

Ele

ctro

n K

ine

tic E

ne

rgy

(eV

)

2.9

92

.49

1.9

91

.49

0.9

90

.49

hv

eBE = hv – eBE = E0 – E-

Anion Photoelectron Spectroscopy

April 18, 2023

0

45

90

135

180

1.5 2 2.5 3 3.5

1.5 2 2.5 3 3.5

1.5 2 2.5 3 3.5

PE spectra of Mo3O6- and W3O6

- consistent with high-symmetry structures

Mo3O6-

h = 3.49 eV

W3O6-

h = 3.49 eV

0

40

80

120

160

1.5 2 2.5 3 3.5

D. W. Rothgeb, E. Hossain, A. T. Kuo, J. L. Troyer, and C. Chick Jarrold, Journal of Chemical Physics, 131, Article 044310 (2009).

April 18, 2023

0.00(0.00)

-0.05(-1.36)

-0.10(-2.72)

-0.15(-4.08)

0.05(1.36)

-0.20(-5.44)

e

a1

e

e

a1

a1

a1

e

e

e

Mo3O6- W3O6

-

Electronic structure of C3h rings are predicted to be virtually identical.

Anticipate similar chemical properties.

April 18, 2023

5-10 mJ/pulse5-10 mJ/pulse532 nm532 nm

Clusters, reaction products

UHP He (4 -7 atm)

Metal target

UHP He (4 -7 atm)

Trace to 3000 Pa H2O or D2O

Cluster Reactivity StudiesHigh-pressure (0.5 to 0.8 atm) fast-flow reactor coupled to cluster source

Mass spec

April 18, 2023

150 200 250 300 350 400 450 500

350 400 450 500 550 600 650 700 750

Mass/ charge (amu/e-)

Mass/charge (amu/e-)

Mo2Oy-

Mo3Oy-

W2Oy-

W3Oy-

Initial mass distributions of anions generated in cluster source

Ion

Inte

nsity

6

6

5

5

4

4

3

3

2

2

2

6

5

4

65

4

8

3

7

9

87

9

Reactivity studies M3Oy‾ + D2O M3Oy+1‾ + D2

M3Oy+1D2‾ + D2

550 600 650 700 750

0

50

70

110

Mass/charge (amu/e-)

6 9

300 350 400 450

55

80

95

0

Mass/charge (amu/e-)

Mo3O5- + D2O Mo3O6D2

-

Mo3O6- + D2O nothing

Mo3O7,8- + D2O Mo3O8,9D2

-

W3O5- + D2O W3O6

- + D2

W3O6- + D2O W3O7D2

-

W3O7- + D2O W3O8D2

-

Ion

Inte

nsity

550 600 650 700 750

0

50

70

110

Mass/charge (amu/e-)

6 9

300 350 400 450

55

80

95

0

Mass/charge (amu/e-)

Ion

Inte

nsity

W3O6‾ and Mo3O6‾ are essentially identical

Why are apparent reactivities different?

Reactivity studies M3Oy‾ + D2O M3Oy+1‾ + D2

M3Oy+1D2‾ + D2

W3O3-

Mo3O4-

Mo3O5-

Mo3O6-

Mo3O3-

W3O4-

W3O5-

W3O6-

1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.5 1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.51 1.5 2 2.5 3 3.5

Mo3O2- W3O2

-

1 1.5 2 2.5 3 3.5 1 1.5 2 2.5 3 3.5

3.49 PE Spectra of Mo3Oy‾ and W3Oy‾ (y = 2 – 6)

Ele

ctro

n C

ount

s

Electron Binding Energy (eV)

y = 2 – 4, spectra appear different

y = 5 – 6, spectra appear comparable

April 18, 2023

Representative computational results-

Numerous close-lying structures found for both anions

and neutrals

Multiple close-lying spin states (including antiferromagnetically

coupled) found for each structure

A0 2A0.00 eV

ADE =2.67 eV

W3O4

N1 1A0.00 eV

A1 2A0.14 eV

ADE=2.11 eV

A2 2A0.26 eV

ADE=2.37 eV

A0 4A0.26eV

ADE=2.35eV

A2 4A0.25 eV

ADE=2.37 eV

N0 3A0.36eV

N2 3A0.38 eV

N0 1A0.42eV

N1 3A 0.10 eV

A1 4A 0.54 eV

ADE=1.81 eV

N2 1A0.67 eV

0.00

0.25

0.50

2.00

2.25

2.50

2.75

W3O4‾

W3O3-

Mo3O4-

Mo3O5-

Mo3O6-

Mo3O3-

W3O4-

W3O5-

W3O6-

1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.5 1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.5

1 1.5 2 2.5 3 3.51 1.5 2 2.5 3 3.5

Mo3O2- W3O2

-

1 1.5 2 2.5 3 3.5 1 1.5 2 2.5 3 3.5

Ele

ctro

n C

ount

s

Electron Binding Energy (eV)

3.49 PE Spectra of Mo3Oy‾ and W3Oy‾ (y = 2 – 6)

X X

1.2 1.4 1.6 1.8 2 2.2 2.4

Computation based simulationData

Electron Binding Energy (eV)

Lowest energy isomers

Anions Neutrals

Mo3Oy‾ W3Oy‾ Mo3Oy W3Oy

Not the whole picture!

W3Oy‾ and W3Oy structures closer-lying energetically than Mo3Oy‾ and Mo3Oy analogs

Structures with more M-O-M bridge bonds are relatively MORE STABLE for molybdenum oxide clusters than for tungsten oxide clusters.

O-atoms in Mo3Oy‾ bridge bonds have the HIGHEST NEGATIVE CHARGE of all the O-atoms – Trap for –H?

April 18, 2023

Structure-Reactivity Conclusions

Spectroscopic evidence: Tungsten suboxide cluster structures more interchangeable than molybdenum suboxide structures.

Computational/Spectroscopic comparisons: Bridge bonds are more stable and more charged in Mo3Oy‾ clusters.

Previous computational/experimental reactivity studies: Bridge oxygens provide kinetic trap of H-atoms in cluster-water addition complexes.

Infer: M3Oy- + H2O M3Oy+1

- + H2 mechanism involves bridge bond flexibility/breakage, traps involve bridging oxygens.

Mo3O7H2-

WW33OO77HH22--

Not observed in experiment

April 18, 2023

Summary

Reactivities of Mo3O6‾ and W3O6‾ clusters toward water are strikingly different, in spite of similar molecular and electronic structures, similar PE spectra.

PE Spectra of Mo3O3‾ versus W3O3‾ are different- Mo3O4‾ versus W3O4‾

While DFT computational results are not satisfactory for M3O3‾/M3O3 (M = Mo, W) species, general results suggest that spectroscopic differences reflect greater M-O-M bond stability in Mo3Oy‾ clusters relative to W3Oy‾.

DFT results also suggest W3Oy‾ clusters more easily isomerize.

Cluster oxidation mechanism by water likely involves bridge to terminal bond transformations, which is higher barrier in the case of Mo3Oy‾ clusters, resulting in trapped water addition complexes.

THANK YOUTHANK YOU

April 18, 2023

April 18, 2023

2A1

2A′

ADE =2.90 eV

2BeBE =2.87 eV

2AeBE =2.67 eV

mulliken charges of  -0.92 to -.88 for Mo-O bridges and -.84 to -0.75 for the Mo-O terminal bonds.    For W-O bridges, I am seeing -0.85 to -.80 and -0.81 to -0.78 for W-O terminal bonds.   Basically the  disparity is less between the different W-O bonds.

Mass spectrometer/PES apparatus

April 18, 2023

Detachment region

“Hole burning”

region

April 18, 2023