Sequential Oxidation of Group 6 Transition Metal Suboxide Clusters Caroline Chick Jarrold Department of Chemistry, Indiana University November 30, 2015

FundingDOE Dr. Jennifer Mann Sarah Waller David Rothgeb

Transition Metal Oxide Clusters November 30, 2015 Important in numerous catalytic applications including photocatalytic decomposition of water, CO 2 reduction, CH 4 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?

November 30, 2015 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 – E int 0 + E int - e - BE = hv – e - KE I (  )      2 Computational Approach: Density Functional Theory (DFT) Calculations Triple-  quality calculations using B3LYP with refined basis sets (K. Raghavachari, N. Mayhall)

November 30, 2015 Q sym Anion ground state Neutral ground state Neutral excited state hv eBE = hv – eBE = E 0 – E - Anion Photoelectron Spectroscopy

November 30, 2015 PE spectra of Mo 3 O 6 - and W 3 O 6 - consistent with high- symmetry structures Mo 3 O 6 - h = 3.49 eV W 3 O 6 - h = 3.49 eV D. W. Rothgeb, E. Hossain, A. T. Kuo, J. L. Troyer, and C. Chick Jarrold, Journal of Chemical Physics, 131, Article (2009).

November 30, (0.00) (-1.36) (-2.72) (-4.08) 0.05 (1.36) (-5.44) e a1a1 e e a1a1 a1a1 a1a1 e e e Mo 3 O 6 - W3O6-W3O6- Electronic structure of C 3h rings are predicted to be virtually identical. Anticipate similar chemical properties.

November 30, mJ/pulse 532 nm Clusters, reaction products UHP He (4 -7 atm) Metal target UHP He (4 -7 atm) Trace to 3000 Pa H 2 O or D 2 O Cluster Reactivity Studies High-pressure (0.5 to 0.8 atm) fast-flow reactor coupled to cluster source Mass spec

November 30, 2015 Mass/charge (amu/e - ) Mo 2 O y - Mo 3 O y - W2Oy-W2Oy- W3Oy-W3Oy- Initial mass distributions of anions generated in cluster source Ion Intensity

Reactivity studies M 3 O y ‾ + D 2 O  M 3 O y+1 ‾ + D 2  M 3 O y+1 D 2 ‾ + D Mass/charge (amu/e - ) 6 9 Mo 3 O D 2 O  Mo 3 O 6 D 2 - Mo 3 O D 2 O  nothing Mo 3 O 7,8 - + D 2 O  Mo 3 O 8,9 D 2 - W 3 O D 2 O  W 3 O D 2 W 3 O D 2 O  W 3 O 7 D 2 - W 3 O D 2 O  W 3 O 8 D 2 - Ion Intensity

Mass/charge (amu/e - ) 6 9 Ion Intensity W 3 O 6 ‾ and Mo 3 O 6 ‾ are essentially identical Why are apparent reactivities different? Reactivity studies M 3 O y ‾ + D 2 O  M 3 O y+1 ‾ + D 2  M 3 O y+1 D 2 ‾ + D 2

3.49 PE Spectra of Mo 3 O y ‾ and W 3 O y ‾ (y = 2 – 6) Electron Counts Electron Binding Energy (eV) y = 2 – 4, spectra appear different y = 5 – 6, spectra appear comparable

November 30, 2015 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 2 A 0.00 eV ADE =2.67 eV W3O4W3O4 N1 1 A 0.00 eV A1 2 A  0.14 eV ADE=2.11 eV A2 2 A 0.26 eV ADE=2.37 eV A0 4 A 0.26eV ADE=2.35eV A2 4 A 0.25 eV ADE=2.37 eV N0 3 A 0.36eV N2 3 A 0.38 eV N0 1 A 0.42eV N1 3 A  0.10 eV A1 4 A 0.54 eV ADE=1.81 eV N2 1 A 0.67 eV W3O4‾W3O4‾

Electron Counts Electron Binding Energy (eV) 3.49 PE Spectra of Mo 3 O y ‾ and W 3 O y ‾ (y = 2 – 6) X X

Lowest energy isomers Anions Neutrals Mo 3 O y ‾ W 3 O y ‾ Mo 3 O y W 3 O y Not the whole picture! W 3 O y ‾ and W 3 O y structures closer-lying energetically than Mo 3 O y ‾ and Mo 3 O y 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 Mo 3 O y ‾ bridge bonds have the HIGHEST NEGATIVE CHARGE of all the O-atoms – Trap for –H?

November 30, 2015 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 Mo 3 O y ‾ clusters. Previous computational/experimental reactivity studies: Bridge oxygens provide kinetic trap of H- atoms in cluster-water addition complexes. Infer: M 3 O y - + H 2 O  M 3 O y H 2 mechanism involves bridge bond flexibility/breakage, traps involve bridging oxygens.

Mo 3 O 7 H 2 - W3O7H2-W3O7H2-W3O7H2-W3O7H2- Not observed in experiment

November 30, 2015 Summary Reactivities of Mo 3 O 6 ‾ and W 3 O 6 ‾ clusters toward water are strikingly different, in spite of similar molecular and electronic structures, similar PE spectra. PE Spectra of Mo 3 O 3 ‾ versus W 3 O 3 ‾ are different- Mo 3 O 4 ‾ versus W 3 O 4 ‾ While DFT computational results are not satisfactory for M 3 O 3 ‾/ M 3 O 3 (M = Mo, W) species, general results suggest that spectroscopic differences reflect greater M-O-M bond stability in Mo 3 O y ‾ clusters relative to W 3 O y ‾. DFT results also suggest W 3 O y ‾ clusters more easily isomerize. Cluster oxidation mechanism by water likely involves bridge to terminal bond transformations, which is higher barrier in the case of Mo 3 O y ‾ clusters, resulting in trapped water addition complexes. THANK YOU

November 30, 2015

2A12A1 2 A ′ ADE =2.90 eV 2 B eBE =2.87 eV 2 A eBE =2.67 eV mulliken charges of to -.88 for Mo-O bridges and -.84 to for the Mo-O terminal bonds. For W-O bridges, I am seeing to -.80 and to for W-O terminal bonds. Basically the disparity is less between the different W-O bonds.

Mass spectrometer/PES apparatus November 30, 2015 Detachment region “Hole burning” region November 30, 2015