Papri Chakraborty 24.09.2016 Introduction In this paper In this - - PowerPoint PPT Presentation

• J. Am. Chem. Soc., 2016, 138 (30), pp 9437–

9443 DOI: 10.1021/jacs.6b03940

Papri Chakraborty 24.09.2016

Introduction

In this paper

In this paper, by doping Au atoms into gas-phase vanadium oxide clusters, it has been demonstrated that the Au(III) cation in the AuV2O6

+ cluster is

active for activation and transformation of methane, the most stable alkane molecule, into formaldehyde under mild conditions. In contrast, the AuV2O6

+ cluster isomers with the Au(I) cation can only

absorb CH4.

Experimental Method

Laser Ablation Quadrupole Mass Filter Linear Ion Trap CH4 TOF

Schematic of Ion-Trap Reactor

Laser ablation of a mixed metal disk compressed with Au and V powders (molar ratio : Au/V = 1/1) 0.5 % O2 seeded in a He carrier gas

AuV2O6

+

• To well resolve AuV2O6

+ ( m/z = 395 for 16O) from Au2 + ( m/z = 394 ),18O2 was used as the

• xygen source to generate the clusters.
• The mass-selected cluster ions entered into a linear ion trap (LIT) reactor where they

were confined and cooled by collisions with a pulse of He gas for 0.9 ms and then interacted with a pulse of CH4, CD4, or CH2D2 for around 1.2 ms.

• The cluster ions ejected from the LIT were detected by a reflectron time-of-flight mass

spectrometer (TOF-MS).

Figure 1. TOF mass spectra for the reactions of AuV2

18O6 + (a), with 2 mPa CH4 (b), 4 mPa CH4 (c), 5 mPa CD4 (d),

and 4 mPa CH2D2 (e) for 1.2 ms. The relative signal magnitudes are amplified by a factor of 3 for m/z < 400. The AuxVyOz

+ and AuxVyOzX+ are labeled as x,y,z and x,y,z,X, respectively. The weak AuV2O6H2O+ in (a) is due to the

reaction of AuV2O6

+ with residual water from the gas handling system.

Cluster Reactivity

AuV2O6

+ + CH4 → V2O6CH3 + + AuH 53% (1)

AuV2O6

+ + CH4 → AuV2O5H2 + + CH2O 18% (2)

AuV2O6

+ + CH4 → V2O5H+ + AuOCH3 (CH2O + AuH) 13% (3)

AuV2O6

+ + CH4 → V2O5H2 + + AuOCH2 (CH2O + Au) 16% (4)

Figure : Time-of-flight mass spectra for the reactions of mass- selected AuV2O6

+ (a) with 5 mPa CH4 for about 1.2 ms (b-e).

Before reacting with methane, the AuV2O6

+ cluster ions had been

cooled for 0.9ms under the cooling gas pressures of 3 Pa (b), 6 Pa (c), 9 Pa (d), and 12 Pa (e). The relative intensity of the adsorption products (Iads/IT, in which Iads is the intensity of AuV2O6CH4

+, IT is the total ion intensity) is given.

Figure : TOF mass spectra for the reactions of mass-selected AuV2

18O6 + (a) with 5 mPa CH4 for

about 1.2 ms (b-e). Before reacting with methane, the AuV2

18O6 + cluster ions are cooled for 0.6 ms (b),

0.9 ms (c), 1.4 ms (d), and 1.9 ms (e), respectively. The AuxVyOz

+ and AuxVyOzX+ species are labeled as

x,y,z and x,y,z,X, respectively.

i) An experiment to verify that the reactant cluster ions are thermalized ii) Reactions of AuV2O6

+ with CH4 under

different cooling gas pressures

Reaction kinetics for AuV2O6

+ + CH4

Figure 2 : Variations of relative ion intensities with respect to the CH4 pressure in the reaction of AuV2O6

+ with CH4.

The transformation channels have small kinetic isotopic effects: kact(CH4)/kact(CD4) = 1.4 ± 0.2 kact(CH4)/kact(CH2D2) = 1.2 ± 0.1. The theoretical collision rate constant (kcoll) between AuV2O6

+ and CH4 is

9.8 × 10 −10 cm3 molecule−1 s−1 . So the reaction efficiency , (kact/kcoll) ~ 80% for the reactive isomer of AuV2O6

+ with CH4.

The adsorption rate constants kads are almost identical for three different reactant gases CH4, CD4, and CH2D2, indicating the non dissociative methane adsorption on the clusters.

Structural Characterisation

Figure 3 : (a) CID spectra of AuV2

18O6 + with 30 mPa Ar. Center-of-mass collisional energy (Ec) is given. Peaks marked as “*”

and “ ” are due to water impurities. (b) The relative ion intensity of Au ◆

+ with respect to Ec. (c) PID spectra of AuV2 18O6CH4 +

and AuV2

18O6CD4 + at 425 nm.

AuV2O6CH4

+ + hv → AuV2O6 + + CH4

AuV2O6

+ → AuV2O4 + + O2

Photon Induced Dissociation (PID) Collision Induced Dissociation (CID)

Structures and Reaction Mechanisms

Figure 4 : DFT calculated potential energy profile for reactions 1–4. The relative energies (in eV) of the reaction intermediates (I1–I6), transition states (TS1–TS5), and products (P1–P4) are with respect to the separate reactants (R). Some bond lengths (in pm) are given.

Figure . DFT calculated potential energy profiles for the reactions of methane with low-lying isomers of AuV2O6

+ (within

0.3 eV, IS2-IS7). The relative energies of the reaction intermediates (I7−I18)and transition states (TS6−TS11) with respect to the separated reactants (∆H0) are given in eV. Bond lengths in pm are given.

Low-Lying Isomers of AuV2O6

+

Conclusion

Combined with the structural characterizations and quantum chemistry calculations, it is revealed that the AuIII species with strong Lewis acid property is the active adsorption site and facilitates the C −H bond cleavage in collaboration with the adjacent O2 − anion through the mechanism of cooperative Lewis acid −base pairs. Such mechanism of cooperative effect of Lewis acid −base pairs has been proposed for methane activation in many condensed-phase systems including MII −O2 − (M = Pt, Pd, Mg) and MIII −O2 − (M = Sc, Y, Ln, Al). However, it has been scarcely reported for methane activation by gas-phase clusters previously. This study enriches the AuIII chemistry at a strictly molecular level and provides a fundamental basis to transform methane under mild conditions. Transformation of the activated methane into formaldehyde has been identified.

Future Plan

• In our instrument we can change

the gases in the trap. It can be reacted with clusters .

• If we react methane with bare

silver clusters, methane adsorption complex might be detected (Agn-(CH4)m)+ . To further react such activated complexes formed in trap with another gas we can pass O2 doped He/ CO doped He in the He cell situated just after the trap. Reaction in trap → Activated complex → Reaction in He cell → Product Cluster can act as the catalytic centre.

• Fe and Ir have high affinity for CO, so reaction of Fe and Ir clusters with

carbon mono-oxide can be done.

Future Plan

SID gives quantitative idea about appearance energy thresolds.

By carbon mono-oxide reaction in the trap it is possible to form cluster- carbonyl

• complexes. There might be a possibility to do SID on the cluster – carbonyl
• complexes. If we get sequencial CO loss, it might be possible get idea about the

dissociation energy thresolds of the complexes.