Sequential Oxidation of Group 6 Transition Metal Suboxide Clusters
Caroline Chick JarroldDepartment of Chemistry, Indiana University
April 18, 2023
Transition Metal Oxide ClustersTransition Metal Oxide Clusters
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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.
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
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.