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Article 2

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H Oxidation Mediated by Au-Doped Vanadium Oxide Cluster Cation AuVO : A Comparative Study with AuCeO 2

5+

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4+

Yan Zhang, Zi-Yu Li, Yan-Xia Zhao, Hai-Fang Li, Xun-Lei Ding, Hua-Yong Zhang, and Shenggui He J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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The Journal of Physical Chemistry

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H2 Oxidation Mediated by Au1−Doped Vanadium Oxide Cluster Cation AuV2O5+: A Comparative Study with AuCe2O4+

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Yan Zhang,† Zi-Yu Li,‡* Yan-Xia Zhao,‡ Hai-Fang Li, ‡ Xun-Lei Ding, †* Hua-Yong Zhang, † and

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Sheng-Gui He‡

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†

Department of Mathematics and Physics, North China Electric Power University, Beinong Road 2, Huilongguan, Beijing 102206, People’s Republic of China ‡

Beijing National Laboratory for Molecular Science (BNLMS),

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State Key Laboratory for Structural Chemistry of Unstable and Stable Species,

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Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

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*Author to whom correspondence should be addressed.

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E-mail: [email protected]. Phone: 86-10-62423763. Fax: 86-10-62559373.

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E-mail: [email protected]. Phone: 86-10-61771323. Fax: 86-10-61772872.

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ACS Paragon Plus Environment


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Abstract

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To clarify the relationship between the type of the oxide support and the activity of the gold

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doped oxide clusters toward H2 oxidation, a suitable closed shell system AuV2O5+ is chosen to have

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a comparative study with AuCe2O4+, the first closed shell cluster that is reactive toward H2

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oxidation. The reaction of AuV2O5+ with H2 was characterized by mass spectrometry and density

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functional theory calculations. The AuV2O5+ cluster is reactive toward H2 leading to the major

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product of V2O5H2+ (+ Au) whereas the product of AuV2O4+ (+ H2O) is completely absent in the

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experiment. This is in sharp contrast with the similar reaction system of AuCe2O4+ with H2, in

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which the formation of H2O was experimentally evidenced. Theoretical calculations revealed that

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the distinct reaction behaviours between AuV2O5+ and AuCe2O4+ can be attributed to the

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gold−metal bond strength which plays an important role in anchoring the gold atom. The weaker

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Au−V bond promotes the evaporation of Au, which has a negative effect on the total oxidation of

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H2 to H2O. This comparative study provides molecular level mechanisms to understand the

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important roles of the gold−metal bond in the oxidation of hydrogen molecule over metal oxide

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supports.

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1. Introduction

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The oxidation of hydrogen molecule (H2) has attracted much attention1,2 because of its role in

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H2O2 synthesis3,4, catalytic oxidation of hydrocarbons5 and the selective removal of CO from

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hydrogen streams6. Metal oxide supported gold catalysts have been shown to exhibit remarkable

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catalytic activity in H2 oxidation.7,8 Most studies mainly focus on the mechanism of H2

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dissociation9-15 and researchers have recognized that the gold atom usually acts as the active

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adsorption site and induces the H2 dissociation in collaboration with the separated lattice oxygen

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(O2–) of the metal oxide support. The subsequent oxidation after the H2 dissociation has also raised

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concern1,7,16 and it has been proposed that the perimeter interfaces of gold and oxide support such as

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TiO2 can provide active sites and enhance the H2O formation. However, the question about how the

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interaction between gold and metal oxide support controls the elementary steps and affects the

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activity and selectivity17,18 remains unclear so far. Owing to the complexity of the catalytic surface

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and the difficulty to uncover the mechanistic details in condensed phase, it is desirable to adopt an

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alternative approach to investigate how the support influences the reactivity and selectivity of the

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active sites.

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Gas−phase atomic clusters that can be studied under well−controlled and reproducible conditions

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serve as ideal models to reveal the nature of the active sites and reaction mechanism at a molecular

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level.19-34 The reactions of H2 molecule with gold doped metal oxide clusters have been previously

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studied.29,35,36 It has been revealed that the gold atom is the active adsorption site and facilities the

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heterolytic cleavage of H2 in collaboration with the separated O2– ions of the oxide support, after

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which the H atom is delivered to the OH unit to form H2O. These investigations have demonstrated

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that the branching ratio for the formation of H2O is remarkably dependent on the type of the oxide

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cluster support. For example, in the reaction of AuNbO4+ with H2, the production of H2O accounts 3



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for 14% in all products,36 while in the cerium oxide cluster systems such as AuCeO2+ and AuCe2O4+

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the corresponding BRs are 23% and 27% respectively32. However, the nature for the difference in

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the BRs of oxidation product H2O is not clear so far. Thus, it is quite interesting to investigate

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additional metal oxide cluster systems and compare with the reported study for a further

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understanding on the detailed mechanism in the oxidation of H2 into water. Considering that a

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strong synergistic effect between Au and vanadia was found in catalytic reactions37 and V2O5+

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cluster has a similar stoichiometric ratio to the Ce2O4+ in the reported polynuclear system of

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AuCe2O4+, herein, the reaction of AuV2O5+ with H2 has been studied by mass spectrometry and

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density functional theory calculations to clarify the effects of the interaction between Au and

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vanadium oxide support by comparing with the previous study of AuCe2O4+ with H2.

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2. Methods

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2.1 Experimental Methods

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The AuxVyOz+ oxide cluster cations were generated by laser ablation of a mixed−metal disk

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compressed with Au and V powders (molar ratio Au/V = 3/1) in the presence of 0.5% 18O2 seeded

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in a He carrier gas (6 atm). The AuV2O5+ clusters were mass−selected using a quadrupole mass

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filter (QMF)38 and entered into a linear ion trap reactor (LIT)39, where they were thermalized by

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collisions with a pulse of He gas and then reacted with a pulse of H2 or D2 for 1.9 ms.39,40 Before

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the prepared gases (H2, D2, and O2/He) were pulsed into the vacuum system, it was useful to pass

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them through copper tube coils at low temperature (~200 K, dry ice in ethanol) in order to remove a

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trace amount of water from the gas handling system. The LIT was formed by a set of hexapole rods

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and two cap electrodes. A reflection time−of−flight mass spectrometer (TOF−MS)41 was used to

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measure the species and abundances of the reactant and product ions.

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2.2 Computational methods

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The density functional theory (DFT) calculations with the Gaussian 09 program42 were

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employed to study the structures of AuV2O5+ cluster and the reaction mechanism with H2. The

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TPSS functional43 has been proved to perform well for the Au−V−O system;29,44-46 thus, the results

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by TPSS were given throughout this work. The TZVP basis set47 for V, O, H atoms and a D95V

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basis set combined with the Stuttgart/Dresden relativistic effective core potential (denoted as SDD

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in Gaussian software)48 for the Au atom were used in all the calculations. A Fortran code based on

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genetic algorithm49 was used to generate the initial guess structures of the clusters. The reaction

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mechanism calculations involved geometry optimizations of reaction intermediates (IMs) and

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transition states (TSs) through which the IMs transfer to each other. The initial guess structures of 5



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the TS species were obtained through relaxed potential energy surface scans using single or

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multiple internal coordinates.50 Intrinsic reaction coordinate (IRC) calculations were performed so

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that each transition state connects two appropriate local minima.51,52 Vibrational frequency

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calculations are performed to check that each of the IMs or TSs has zero and only one imaginary

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frequency, respectively. The zero−point vibration corrected energies (∆H0K) in units of eV are

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reported in this work. The natural bond orbital (NBO) analysis was performed with NBO 3.1.53

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3. Results

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3.1 Experimental Results

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The TOF mass spectra for the reactions of mass selected AuV2O5+ with H2 and D2 in the ion−trap

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reactor are shown in Figure 1. Before the reactions, only AuV2O5+ (labeled as 1,2,5 in Figure 1a)

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and a slight amount of its hydrate (labeled as 1,2,5H2O) could be found in the reactor. After the

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interactions of AuV2O5+ with 4 mPa H2 for about 1.9 ms (Figure 1b), three product peaks that can

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be assigned as AuV2O5H2+, V2O5H2+, and V2O5H+ were observed, which suggests the following

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three reaction channels: AuV2O5+ + H2 → AuV2O5H2+

(1)

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AuV2O5+ + H2 → V2O5H2+ + Au

(2)

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AuV2O5+ + H2 → V2O5H+ + AuH

(3)

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The above reaction channels were further confirmed by using a higher concentration of H2 with

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the pressure of 8 mPa (Figure 1c) and isotopic labelling experiment with D2 (Figure 1d). On the

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basis of a least−squares fitting process,54 the pseudo−first−order total rate constants k1 for the

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reactions of AuV2O5+ with H2 and D2 were estimated to be (5.1 ± 0.1) × 10−10 and (4.7 ± 0.1) ×

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10−10 cm3 molecule−1 s−1, respectively. The signal dependence of three product ions on the H2 or D2

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pressure could be derived and well fitted with the experimental data (Figure 2). The branching

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ratios of the above three channels are listed in Table 1. The theoretical collision rate constant

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[kcollision = 2π(e2α/µ)1/2, in which e is the charge of the cluster ion, α is the electric polarizability of

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the reactant molecule, and µ is the reduced mass]55 of the reaction (AuV2O5+ + H2) was calculated

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to be 1.5 × 10−9 cm3 molecule−1 s−1, corresponding to a reaction efficiency (Φ = k1/kcollision) of (34 ±

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1)%. The kinetic isotope effect (KIE), defined as k1(AuV2O5+ + H2)/k1(AuV2O5+ + D2), was

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determined to be (1.1 ± 0.1).

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3.2 Computational results

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The DFT calculated ground−state structure of AuV2O5+ has a closed−shell electronic structure

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with the Au atom terminally bonded with one O atom (1IS1 in Figure 3). Another closed−shell

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isomer 1IS2 has similar geometric structure and close energy (0.06 eV) with respect to 1IS1, so it is

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also a candidate for the ground−state structure of AuV2O5+. More low−lying isomers are given in

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Figure 3, from which one can find that the triplet state of each isomer of AuV2O5+ is much higher

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than the corresponding singlet state. Therefore, only the singlet state was considered for the

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potential energy profiles of the reaction (AuV2O5+ + H2), as shown in Figure 4.

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At the first step of AuV2O5+ (1IS1) reacting with H2, the H2 molecule is trapped tightly by the

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positively charged gold atom (natural charge: +0.78 e). With the formation of I1 (Figure 4), a high

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energy is released in this process (∆H0K = −1.20 eV). Natural bond analysis was carried out for all

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of the atoms labeled in I1 (Figure 4) along the reaction pathway. The detailed results are given in

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Table 2 and Table S1 (Supporting Information). In I1, the charge on each H atom is +0.08 e while

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the charge on the Au atom reduces 0.16 e (i.e., +0.62 e in I1, Table 2), indicating that the empty 6s

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orbital of the Au+ pulls about 0.16 electron from the σ orbital of H2 when forming I1, which

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weakens the H−H bond. H2 is slightly activated upon adsorption, which results in elongation of the

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H−H bond from 74 pm in the isolated H2 to 86 pm in intermediate I1. For another ground state of

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AuV2O5+ (1IS2), the terminally bonded gold atom can trap one H2 molecule releasing the similar

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binding energy with 1IS1. After a structure rearrangement (I8 → TS7 → I1, Figure S1 in the

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Supporting Information), the more stable intermediate I1 can be formed with a barrier of 0.68 eV,

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which is well below the adsorption energy of I8 (1.20 eV). Therefore, these two candidates would

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exhibit the uniform reactivity toward H2 due to the same intermediate (I1) formed in the reaction

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pathway. 8


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After the formation of I1, the gold atom delivers the attached H2 molecule to the terminal O2− ion

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and H−H bond cleavage occurs in terms of the heterolytic cleavage (I1 → TS1 → I2). During the

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step of H2 activation, the charge population on the H2 atom (see I1 in Figure 4) decreases, and that

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on the H1 atom increases significantly, which suggests that the H−H bond cleavage occurs in a

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heterolytic manner. The activation of H2 by other possible sites has also been explored and the

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calculated pathways are provided in Figure S2 (in the Supporting Information). The next step can be

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accompanied by the conversion of Au−O to Au−V with the Au atom approaching to the V1 (the left

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one) atom from 327 pm to 264 pm (I2 → TS2 → I3). This route is more favorable than the route of

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Au atom approaching to the V2 (the right one) atom, which has to surmount an overall reaction

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barrier of 0.05 eV (Figure S3). After a structural rearrangement (I3 → TS3 → I4), both of the two V

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atoms are four−coordinated, which is the favourite case for V atom,56 leading to the large energy

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decrease from I3 to I4. The structure of I5 is similar to I4 with a rotation of the AuH moiety around

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the Au−V bond. The reaction can terminate with the generation of AuH molecule from I5 to

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produce P2, as shown in reaction 3.

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Alternatively, the reaction can proceed from I5 by the transfer of the H atom bonded with Au to

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the cluster−support (V2O5) forming a second OH unit with a negligible barrier (I5 → TS5 → I6),

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leading to a more stable intermediate I6. The intermediate I6 has enough energy to evaporate an Au

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atom to form the experimentally observed V2O5H2+ cluster (P1, reaction 2). Alternatively, the deep

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potential well (−2.54 eV) of I6 means that the reaction intermediate has a relatively long life−time

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so that it has chances to be stabilized by collisions with bath gas (He) in the reactor, resulting in the

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formation of the association complex (AuV2O5H2+), as shown in reaction 1. All the reaction

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intermediates and transition states to P1, P2 and the association complex are all lower in energy

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than the separate reactants (AuV2O5+ + H2), corresponding to the observations of V2O5H2+, V2O5H+ 9



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and AuV2O5H2+ in the experiment (Figure 1). And all the transition states are confirmed by the

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vibration frequency calculations (Figure S4, Table S2). The lower energy of P1 than that of P2

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indicates that the elimination of Au is more favorable than that of AuH, which well rationalizes the

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experimentally observed branching ratios that the reaction 2 is the major channel as shown in Table

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1. It should be noticed that I6 can overcome a barrier of 1.12 eV to produce a H2O associated

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product AuV2O4H2O+ (I7). However, the formation of P4 (AuV2O4+ + H2O) from I7 is overall

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endothermic by 0.19 eV. As a result, no evidence of AuV2O4+ can be observed in the experiment.

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4. Discussion

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Investigating the relationship between the activity and selectivity of the active sites and the

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interaction between the gold and oxide supports is crucial for the rational design of effective

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catalysts. Recently, Meng et al.35 reported the reactions of closed−shell gold−doped heteronuclear

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oxide clusters AuCeO2+ and AuCe2O4+ with H2, and identified that the separated ion pair, Au+ and

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O2−, could promote the dissociation of H2, which paralleled the behavior of H2 dissociation on

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supported gold catalysts in condensed phase. This is the first example of thermal H2 activation by a

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closed−shell atomic cluster. Herein, we demonstrate that another similar closed−shell system

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AuV2O5+ could be reactive toward H2 oxidation and exhibit the same manner to dissociate H2

10

molecule. However, different reactivity and selectivity are observed for these two reaction systems.

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The vanadium system possesses a higher reaction rate with H2 and the evaporation of Au is much

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more facile whereas the H2O formation is not observed in the reaction system of AuV2O5+ with H2.

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The experimental results indicate that the rate constant of AuV2O5+ reacted with H2 is about 2

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orders of magnitude higher than that of the AuCe2O4+ system, as shown in Table 1, suggesting the

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higher reactivity of AuV2O5+ toward H2 than that of AuCe2O4+. The lower reactivity of AuCe2O4+

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can be attributed to the small adsorption energy of H2 on the reactant cluster AuCe2O4+ with a

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bridging gold atom. Only after the crucial structural rearrangement of the bridging−Au to the

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terminal−Au, the reaction system could store enough energy to overcome the subsequent energy

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barrier for the dissociation of H2. Therefore, it can be concluded that the cluster with a terminal gold

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atom possesses a higher reactivity toward H2 oxidation than that with a bridging gold atom.

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To understand the different selectivity of the two reaction systems, the role of the interaction of

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gold−metal (M = V/Ce) in the process of H2 oxidation has been studied. After the H−H bond

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cleavage induced by the (Au+−O2−) moiety, the Au atom is transferred from the O to the M atom. 11



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Due to the relativistic effect,57-59 the gold atom has a contracted and stabilized 6s orbital, which

2

tends to accept an electron. This tends to be a strong driving force to transform Au−O bonds into

3

Au−M bonds in reactions with various reductive molecules, such as CO54,60-63 and H2.35,36 The

4

intermediate I4 (Figure 4) has one Au−V bond as well as one OH unit, which is similar to the

5

dissociative intermediate involved in AuCe2O4+ system (I3 in Figure S8 of ref. 35). Such an

6

intermediate is crucial because different products will be generated from this intermediate. Sine V

7

has higher electronegativity (1.63, on the Pauling scale) and ionization potential (6.74 eV) than Ce

8

(1.12, 5.47eV), it is much more difficult for Au to extract electrons from V than that from Ce.

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Natural bond orbital analysis (Figure 5) indicates that, from the key intermediate I4, a part of

10

electrons accumulated on the Au atom gradually. However, the still positively charged Au and V

11

indicate a coulomb repulse between the two atoms which leading to a weak bonded Au−V. On the

12

contrary, in the Ce system, it is much easier for Au to extract electrons from Ce and a high electron

13

density is accumulated on the Au atom leading to a negatively charged Au and a much more

14

positively charged Ce. The electrostatic attraction between Au and Ce may lead to a much stronger

15

chemical bond of Au−Ce. Furthermore, the combined effect of two stronger Au−Ce bonds is also

16

the reason for the difficulty of Au evaporation in Ce system. Therefore, the evaporation of Au (or

17

AuH) from the “vanadium oxide support” is more flexible than that from the “cerium oxide

18

support”, as shown in Table 1. The missing of Au atom at this stage may hinder the formation of

19

H2O molecule on AuV2O5+. Additionally, the bond energy of Au−H (3.03 eV) is larger than that of

20

Au−V, resulting in that some AuH can directly split away from the AuV2O5H2+ (I5 in Figure 4)

21

before the hydrogen atom is transferred away from the Au atom to form H2O. In contrast, the bond

22

energy of Au−H is smaller than that of Au−Ce and the Au−Ce bond is quite strong to survive in the

23

AuCe2O4H2+ system. Subsequently, the H atom in AuH moiety may be transferred to the adjacent O 12


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atom (O−H bond energy is 4.46 eV, much larger than Au−H) and then the oxidation product H2O

2

molecule is formed. Thus, the comparative study provides convincing evidence that the interaction

3

between gold and metal has a very important effect on the selectivity of the hydrogen oxidation

4

over metal oxide supported gold catalysts.

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5. Conclusions

2

The reaction of Au1−doped vanadium oxide cluster cation AuV2O5+ with H2 has been

3

investigated by mass spectrometry in conjunction with density functional theory calculations. The

4

gold elimination was identified as the major channel in the experiment and the oxidation product of

5

H2O is not observed at all. This is in contrast with the previously reported closed−shell system of

6

AuCe2O4+ with H2 that the branching ratios for the formation of gold atom and H2O are almost

7

equal. Further theoretical calculations reveal that an intermediate structure with Au bonding to

8

metal (V/Ce) exists in both systems plays a crucial role in the oxidation process. The interaction

9

between Au and vanadium oxide support is weaker than that of Au−cerium oxide. As a result, Au or

10

AuH tends to split away from the AuV2O5+ + H2 reaction system without the formation of H2O,

11

which well rationalizes the experimental observation. Furthermore, the reactivity of the AuV2O5+

12

cluster that contains a terminal gold atom is superior to that of the bridging gold atom in AuCe2O4+

13

cluster. This study provides strictly molecular−level insights into the nature of the metal oxide

14

support dependence in H2 oxidation.

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Acknowledgment

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This work was supported by the National Natural Science Foundation of China (Nos. 91545122

3

and 21325314) and the Fundamental Research Funds for the Central Universities (JB2015RCY03).

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Supporting Information Available: The DFT calculated potential energy profile for the reaction

5

of H2 with the low−lying isomer 1IS2 (Figure S1); DFT calculated potential energy profiles for the

6

reaction of AuV2O5+ + H2 for other possible active sites (Figure S2); The DFT calculated potential

7

energy profile for the reaction of AuV2O5+ + H2 by the gold atom in intermediate I3 approaching to

8

the other V atom (Figure S3); Franck−Condon active vibrational mode (ν1) for the transition states

9

(Figure S4); DFT Calculated NBO Charges on the Atoms Labeled in I1 along the Reaction Pathway

10

of R(AuV2O5+ + H2) → P2 and P3 (Table S1); The First Ten Calculated Vibrational Frequencies for

11

all the Transition States at TPSS level (Table S2).

12 13 14 15 16 17 18 19 20 21 22 23 15



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6. References

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(1) Green, I. X.; Tang, W.; Neurock, M.; Yates, J. T. Low−Temperature Catalytic H2 Oxidation

3

over Au Nanoparticle/TiO2 Dual Perimeter Sites. Angew. Chem. Int. Ed. 2011, 50,

4

10186−10189.

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(2) Han, B.; Cheng, H.-S. Nickel Family Metal Clusters for Catalytic Hydrogenation Processes. Acta. Phys. −Chim. Sin. 2017. doi: 10.3866/PKU.WHXB201704172.

7

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7. Figure Captions

1 2

Figure 1. Time−of−flight mass spectra for interactions of mass−selected AuV2O5+ clusters with He

3

(a), H2 (b and c), and D2 (d) in an ion trap reactor for about 1.9 ms. Numbers x,y,z denote

4

AuxVy18Oz+. The relative signal magnitudes are amplified by a factor of 3 for 180 < m / z < 205.

5

Figure 2. Variations of the relative intensities with respect to the reactant gas pressures in the

6

reactions of AuV2O5+ with H2 (a) and D2 (b). The solid lines are fitted to the experimental data

7

points with the approximation of the pseudo−first−order reaction mechanism.

8

Figure 3. DFT calculated isomers for clusters AuV2O5+. The zero−point vibration corrected

9

energies (∆H0K in eV) of the singlet and triplet states with respect to the most stable isomer (i.e.,

10

1

11

Figure 4. DFT calculated potential−energy profiles for the reaction of AuV2O5+ + H2 to generate

12

the products V2O5H2+ + Au (P1), V2O5H+ + AuH (P2) and AuV2O4+ + H2O (P3). The relative

13

energies for intermediates (Is), transition states (TSs), and products (Ps) are given in eV.

14

Figure 5. (a) DFT calculated natural charge (e) on the gold, vanadium, and cerium atoms during the

15

reactions of AuV2O5+ + H2 and AuCe2O4+ + H2. (b) Three typical structures that with Au−M (M =

16

Ce and V) bonds are selected in each reaction system. The Group n (n = 1−3) contains two

17

corresponding typical structures from the two reactions respectively.

18

Table 1. Branching Ratios and Total Rate Constants (k1, in units of cm3molecule-1s-1) for the

19

Reactions of AuV2O5+ and AuCe2O4+ [ref. 35] with X2 (X = H and D).

20

Table 2. DFT Calculated NBO Charges (e) on the Atoms Labeled in I1 along the Reaction Pathway

21

R (Reactant) → I1 → I2.

IS1) are given below each structure. Bond lengths are given in pm.

23



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Page 24 of 37

1

Figure 1. Time−of−flight mass spectra for interactions of mass−selected AuV2O5+ clusters with He (a), H2 (b and c), and D2 (d) in an ion trap reactor for about 1.9 ms. Numbers x,y,z denote AuxVy18Oz+. The relative signal magnitudes are amplified by a factor of 3 for 180 < m / z < 205.

2

24


Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Variations of the relative intensities with respect to the reactant gas pressures in the reactions of AuV2O5+ with H2 (a) and D2 (b). The solid lines are fitted to the experimental data points with the approximation of the pseudo−first−order reaction mechanism.

1 2

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

1

Figure 3. DFT calculated isomers for clusters AuV2O5+. The zero−point vibration corrected energies (∆H0K in eV) of the singlet and triplet states with respect to the most stable isomer (i.e., 1IS1) are given below each structure. Bond lengths are given in pm.

2 3 4 5 6 7

26


Page 27 of 37

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Figure 4. DFT calculated potential−energy profiles for the reaction of AuV2O5+ + H2 to generate the products V2O5H2+ + Au (P1), V2O5H+ + AuH (P2) and AuV2O4+ + H2O (P3). The relative energies for intermediates (Is), transition states (TSs), and products (Ps) are given in eV.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. (a) DFT calculated natural charge (e) on the gold, vanadium, and cerium atoms during the reactions of AuV2O5+ + H2 and AuCe2O4+ + H2. (b) Three typical structures that with Au−M (M = Ce and V) bonds are selected in each reaction system. The Group n (n = 1−3) contains two corresponding typical structures from the two reactions respectively.

28


Page 28 of 37

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Branching Ratios and Total Rate Constants (k1, in units of cm3molecule-1s-1) for the Reactions of AuV2O5+ and AuCe2O4+ [ref. 35] with X2 (X = H and D). Branching ratios (%)

k1

AuV2O5X2+

V2O5X2++Au

V2O5X++AuX

AuV2O4++X2O

H2

22

74

4

-

5.1 ×10-10

D2

33

61

6

-

4.7 ×10-10

AuCe2O4X2+

Ce2O4X2++Au

Ce2O4X++AuX

AuCe2O3++X2O

H2

52

21

-

27

5.6 ×10-12

D2

34

21

-

45

4.7 ×10-12

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

Table 2. DFT Calculated NBO Charges (e) on the Atoms Labeled in I1 along the Reaction Pathway R (Reactant) → I1 → I2. Au

H1

H2

V1

V2

O1

O2

O3

O4

O5

R

0.78

0.00

0.00

0.87

1.18

-0.60

-0.21

-0.19

-0.41

-0.41

I1

0.62

0.08

0.08

0.89

1.17

-0.61

-0.22

-0.19

-0.41

-0.41

I2

0.40

0.41

-0.05

0.89

1.23

-0.46

-0.51

-0.17

-0.37

-0.37

30


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TOC Graphic

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Time-of-flight mass spectra for interactions of mass-selected AuV2O5+ clusters with He (a), H2 (b and c), and D2 (d) in an ion trap reactor for about 1.9 ms. Numbers x,y,z denote AuxVy18Oz+. The relative signal magnitudes are amplified by a factor of 3 for m / z < 205. 93x76mm (300 x 300 DPI)


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Variations of the relative intensities with respect to the reactant gas pressures in the reactions of AuV2O5+ with H2 (a) and D2 (b). The solid lines are fitted to the experimental data points with the approximation of the pseudo-first-order reaction mechanism. 169x209mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

DFT calculated isomers for clusters AuV2O5+. The zero-point vibration corrected energies (∆H0K in eV) of the singlet and triplet states with respect to the most stable isomer (i.e., 1IS1) are given below each structure. Bond lengths are given in pm. 42x13mm (300 x 300 DPI)


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DFT calculated potential-energy profiles for the reaction of AuV2O5+ + H2 to generate the products V2O5H2+ + Au (P1), V2O5H+ + AuH (P2) and AuV2O4+ + H2O (P3). The relative energies for intermediates (Is), transition states (TSs), and products (Ps) are given in eV. 155x144mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a) DFT calculated natural charge (e) on the gold, vanadium, and cerium atoms during the reactions of AuV2O5+ + H2 and AuCe2O4+ + H2. (b) Three typical structures that with Au-M (M = Ce and V) bonds are selected in each reaction system. The Group n (n = 1-3) contains two corresponding typical structures from the two reactions respectively. 94x34mm (300 x 300 DPI)


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TOC Graphic 132x106mm (300 x 300 DPI)


H2 Oxidation Mediated by Au1-Doped Vanadium ... - ACS Publications

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