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 AuV 2 O 6 + cluster is active for activation and transformation of methane, the most stable alkane molecule, into formaldehyde under mild conditions. In contrast, the AuV 2 O 6 + cluster isomers with the Au(I) cation can only absorb CH 4 .

Experimental Method CH 4 Laser ablation of a mixed metal disk compressed with Au and V powders Laser Ablation (molar ratio : Au/V = 1/1) 0.5 % O 2 seeded in a Quadrupole He carrier gas Linear Ion Trap TOF Mass Filter AuV 2 O 6 + Schematic of Ion-Trap Reactor  To well resolve AuV 2 O 6 + ( m/z = 395 for 16 O) from Au 2 + ( m/z = 394 ), 18 O 2 was used as the oxygen 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 CH 4 , CD 4 , or CH 2 D 2 for around 1.2 ms.  The cluster ions ejected from the LIT were detected by a reflectron time-of-flight mass spectrometer (TOF-MS).

Cluster Reactivity Figure 1 . TOF mass spectra for the reactions of AuV 2 18 O 6 + (a), with 2 mPa CH 4 (b), 4 mPa CH 4 (c), 5 mPa CD 4 (d), and 4 mPa CH 2 D 2 (e) for 1.2 ms. The relative signal magnitudes are amplified by a factor of 3 for m / z < 400. The Au x V y O z + and Au x V y O z X + are labeled as x , y , z and x , y , z ,X, respectively. The weak AuV 2 O 6 H 2 O + in (a) is due to the reaction of AuV 2 O 6 + with residual water from the gas handling system. AuV 2 O 6 + + CH 4 → V 2 O 6 CH 3 + + AuH 53% (1) AuV 2 O 6 + + CH 4 → AuV 2 O 5 H 2 + + CH 2 O 18% (2) AuV 2 O 6 + + CH 4 → V 2 O 5 H + + AuOCH 3 (CH 2 O + AuH) 13% (3) AuV 2 O 6 + + CH 4 → V 2 O 5 H 2 + + AuOCH 2 (CH 2 O + Au) 16% (4)

i) An experiment to verify that the reactant ii) Reactions of AuV 2 O 6 + with CH 4 under cluster ions are thermalized different cooling gas pressures Figure : TOF mass spectra for the reactions of Figure : Time-of-flight mass spectra for the reactions of mass- mass-selected AuV 2 18 O 6 + (a) with 5 mPa CH 4 for selected AuV 2 O 6 + (a) with 5 mPa CH 4 for about 1.2 ms (b-e). about 1.2 ms (b-e). Before reacting with methane, Before reacting with methane, the AuV 2 O 6 + cluster ions had been the AuV 2 18 O 6 + cluster ions are cooled for 0.6 ms (b), cooled for 0.9ms under the cooling gas pressures of 3 Pa (b), 6 0.9 ms (c), 1.4 ms (d), and 1.9 ms (e), respectively. Pa (c), 9 Pa (d), and 12 Pa (e). The relative intensity of The Au x V y O z + and Au x V y O z X + species are labeled as the adsorption products (I ads/ I T , in which I ads is the intensity of x,y,z and x,y,z,X, respectively. AuV 2 O 6 CH 4 + , I T is the total ion intensity) is given.

Reaction kinetics for AuV 2 O 6 + + CH 4 Figure 2 : Variations of relative ion intensities with respect to the CH 4 pressure in the reaction of AuV 2 O 6 + with CH 4 .

The transformation channels have small kinetic isotopic effects: k act (CH 4 )/k act (CD 4 ) = 1.4 ± 0.2 k act (CH 4 )/k act (CH 2 D 2 ) = 1.2 ± 0.1. The theoretical collision rate constant (k coll ) between AuV 2 O 6 + and CH 4 is 9.8 × 10 −10 cm 3 molecule −1 s −1 . So the reaction efficiency , (k act /k coll ) ~ 80% for the reactive isomer of AuV 2 O 6 + with CH 4 . The adsorption rate constants k ads are almost identical for three different reactant gases CH 4 , CD 4 , and CH 2 D 2 , indicating the non dissociative methane adsorption on the clusters.

Structural Characterisation Collision Induced Dissociation (CID) AuV 2 O 6 + → AuV 2 O 4 + + O 2 Photon Induced Dissociation (PID) AuV 2 O 6 CH 4 + + hv → AuV 2 O 6 + + CH 4 Figure 3 : (a) CID spectra of AuV 2 18 O 6 + with 30 mPa Ar. Center-of-mass collisional energy ( E c ) is given. Peaks marked as “*” and “ ” are due to water impurities. (b) The relative ion intensity of Au ◆ + with respect to E c . (c) PID spectra of AuV 2 18 O 6 CH 4 + and AuV 2 18 O 6 CD 4 + at 425 nm.

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.

Low-Lying Isomers of AuV 2 O 6 + Figure . DFT calculated potential energy profiles for the reactions of methane with low-lying isomers of AuV 2 O 6 + (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 (∆H 0 ) are given in eV. Bond lengths in pm are given.

Conclusion Combined with the structural characterizations and quantum chemistry calculations, it is revealed that the Au III species with strong Lewis acid property is the active adsorption site and facilitates the C −H bond cleavage in collaboration with the adjacent O 2 − 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 M II −O 2 − (M = Pt, Pd, Mg) and M III −O 2 − (M = Sc, Y, Ln, Al). However, it has been scarcely reported for methane activation by gas-phase clusters previously. This study enriches the Au III 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 (Ag n -(CH 4 ) m ) + . To further react such activated complexes formed in trap with another gas we can pass O 2 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.