Catalytic CO Oxidation by Noble-Metal-Free Ni2VO4,5– Clusters: A

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 1133−1138

pubs.acs.org/JPCL

Catalytic CO Oxidation by Noble-Metal-Free Ni2VO4,5− Clusters: A CO Self-Promoted Mechanism Li-Na Wang,†,‡,§ Xiao-Na Li,*,†,§ and Sheng-Gui He†,‡,§ †

State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Beijing 100190, P. R. China J. Phys. Chem. Lett. Downloaded from pubs.acs.org by WEBSTER UNIV on 02/27/19. For personal use only.

S Supporting Information *

ABSTRACT: Catalytic CO oxidation is an important model reaction in gas-phase studies to provide a clear structure−reactivity understanding in related heterogeneous catalysis, whereas CO oxidation catalyzed by noble-metal (NM) free species has been scarcely reported, and the fundamental aspects are elusive. Herein a CO self-promoted mechanism of catalytic CO oxidation by O2 mediated with the Ni2VO4,5− clusters was experimentally identified and theoretically rationalized. The catalysis was characterized by mass spectrometry and quantum chemistry calculations. Ni2VO5− can oxidize CO to generate an oxygen-deficient product Ni2VO4−, which can only adsorb CO to give rise to Ni2VO4CO−, and the oxidative reactivity of Ni2VO4− can be boosted by the adsorbed CO. This finding reinforces the significance that the attached CO can modify the electronic structure of the Ni2 unit in Ni2VO4CO− and make the Ni2 unit behave like NM atoms to store the released electrons in an oxygen atom transfer process.

I

simultaneously (Mechanism II). Recently, for catalytic CO oxidation mediated with the CuAl4O7−9− clusters,23 we identified that the oxygen-rich24 CuAl4O9− cluster is kinetically less favorable to oxidize CO into CO2, whereas the adsorption of another CO on CuAl4O9CO− is crucial to dissociate the O− O bond of the superoxide unit in CuAl4O9− and promote CO oxidation (Mechanism III). In this Letter, for the catalysis over the Ni2VO4,5− clusters (Mechanism IV), in contrast, the relatively oxygen-rich Ni2VO5− cluster can oxidize CO to generate an oxygen-deficient product Ni2VO4−, which is thermodynamically hindered to oxidize CO into CO2. We discovered that the introduction of another CO to form Ni2VO4CO− is essential to make the Ni2 unit resemble the behavior of a noble-metal (NM) atom25,26 to store electrons and promote CO oxidation. Note that scarce examples of NMfree species have been reported to be capable of catalyzing CO oxidation by O2,20,23 and the fundamental aspects of the nature of the chemically covered CO are elusive. The CO-promoted event may be ubiquitous in heterogeneous processes,27−30 and this finding could be practically important to tailor the design of efficient and cost-effective catalysts using 3d metals. Laser-ablation-generated Ni 2 VO4,5 − and Ni2VO 3,4CO − clusters were mass-selected, cooled, and then interacted with CO or O2 in an ion trap reactor (Figure 1). On the interaction with CO (Figure 1b), Ni2VO5− can oxidize CO to generate

dentifying the fundamental mechanisms that can correlate the reactivity of a catalyst with the nature of the catalytically active site is a central subject of heterogeneous catalysis; however, it remains a big challenge to obtain a molecular-level mechanism of condensed-phase catalysts due to the complexity of the bulk materials. A catalytic event takes place microscopically on an active site that generally involves a limited number of atoms.1,2 In this case, atomic clusters that are considered as the intermediate matter to bridge atoms and their bulk counterparts can act as individual active sites of real-life catalysts.2−8 Cluster reactions can be performed under isolated conditions to provide mechanistic understandings of important catalytic processes that take place on the surfaces of bulk catalysts. Catalytic CO oxidation by molecular O2 is an important reaction in heterogeneous catalysis9 and represents one type of the extensively studied reactions in the gas phase to capture a clear structure−reactivity relationship of condensed-phase catalysis.10−12 Different mechanisms of catalytic CO oxidation by O2 mediated with atomic clusters have been identified (Scheme 1). The most common mechanism (Mechanism I) corresponds to the consecutive oxidation of two CO molecules to generate an oxygen-deficient product MxOy−2q, which can react with O2 to regenerate MxOyq. This mechanism has been demonstrated for cluster systems Au6O2−,13 Au3(CO)2−5−,14 PtnOm− (n = 3−6; m = 0−2),15 Pd6O4+,16 AuAl3O5+,17 PtAl3O7−,18 Cu5−,19 Cu2VO5−,20 and AuTi2O3−6.21 Coadsorption of two CO molecules22 on an Au2O2− cluster is essential to rupture the O−O bond of O2 and forms two CO2 molecules © XXXX American Chemical Society

Received: January 7, 2019 Accepted: February 25, 2019 Published: February 25, 2019 1133

DOI: 10.1021/acs.jpclett.9b00047 J. Phys. Chem. Lett. 2019, 10, 1133−1138

Letter

The Journal of Physical Chemistry Letters

implied that Ni2VO4− cannot oxidize CO into CO2 but can adsorb a CO molecule to give rise to Ni2VO4CO− (reaction 2) that may oxidize CO to generate Ni2VO3CO− (reaction 3). This event was evidenced by the experiments (Figure 1d,e) in which only Ni2VO4CO− can be clearly identified with the exposure of mass-selected Ni2VO4− to low-pressure CO (Figure 1d, 37 mPa CO). With the increase in CO pressure to 77 mPa (Figure 1e), the appearance of the Ni2VO3CO− signal indicated that Ni2VO3CO− could come from the reaction of Ni2VO4CO− with CO. The negligible signal intensity of Ni 2 VO 4 (CO) 2 − demonstrated that the Ni2VO4CO− species generated in the ion trap prefers to oxidize CO rather than absorb CO (Figures S1 and S2). Ni2VO4CO− can also be generated by seeding CO in the carrier gas and be mass-selected (Figure 1f). Figure 1g confirmed that Ni2VO4CO− can indeed oxidize CO to generate Ni2VO3CO−. Note that the cluster-source-generated Ni2VO4CO− species can have different reactivities in the reaction with CO (see details in Figure S3). The reaction of Ni2VO3CO− with O2 can regenerate Ni2VO5− (Figure 1i, reaction 4). Thus the elementary reactions 1−4 can comprise a CO self-promoted catalysis mediated with the Ni2VO4,5− clusters (Figure 2a). The signal of Ni2VO3− suffered from

Scheme 1. Identified Mechanisms for Catalytic CO Oxidation by O2 over Atomic Clusters

Figure 2. (a) Proposed CO self-promoted catalysis. (b) Variation of ion intensity with respect to the partial pressures of CO for reaction Ni2VO5− + CO.

low abundance during cluster generation (Figure S4). It is experimentally challenging to mass-select 58Ni2VO3− from species such as V2O7− to characterize the reactivity. Moreover, the Ni2VO3− cluster cannot be generated starting from Ni2VO5−, Ni2VO4−, and Ni2VO4CO− (Figure 1), and thus Ni2VO3− is absent in the catalysis.

Figure 1. Time-of-flight mass spectra for the interactions of massselected Ni 2 VO 5− (a), Ni 2 VO 4− (c), Ni 2 VO4 CO − (f), and Ni2VO3CO− (h) with CO (b,d,e,g) or O2 (i). The reaction time is 1.7 ms. The NixVyOz− and NixVyOzCO− species are denoted as x,y,z and x,y,z,CO, respectively. Peaks marked by the asterisk are from the water impurity.

Ni 2VO5− + CO → Ni 2VO4 − + CO2

(1)

Ni 2VO4 − + CO → Ni 2VO4 CO−

(2)

−

−

Ni 2VO4 CO + CO → Ni 2VO3 CO + CO2 −

Ni 2VO3 CO + O2 →

Ni2VO4− and CO2 (reaction 1), whereas the adsorption product Ni2VO5CO− cannot be clearly observed (Figure S1, Supporting Information). The absence of product Ni2VO3−

Ni 2VO5−

+ CO

(3) (4)

The pseudo-first-order rate constants (k1, in units of 10−11 cm3 molecule−1 s−1) for elementary reactions 1−4 can be well1134

DOI: 10.1021/acs.jpclett.9b00047 J. Phys. Chem. Lett. 2019, 10, 1133−1138

Letter

The Journal of Physical Chemistry Letters fitted (Figure 2b and Figure S3), and the determined values are 1.4, 1.9, 3.5, and 6.9, respectively, corresponding to the reaction efficiencies31 of about 2, 3, 5, and 10%. For reaction Ni2VO5− + CO (Figure 2b), the good fit of the experimental data confirms the validity of model Ni2VO5− → Ni2VO4− → Ni2VO4CO− → Ni2VO3CO−. Thus four species Ni2VO5−, Ni2VO4−, Ni2VO4CO−, and Ni2VO3CO− should be involved in the catalysis (Figure 2a). Density functional theory (DFT) calculations have been carried out to rationalize the experimental results and provide insights into the roles of the attached CO in promoting the catalysis. The lowest-lying structures for the Ni2VO5−, Ni2VO4− (Figure 3a), and Ni2VO3− clusters (Figure S5) are

binding energy of 1.12 eV (I4, Figure 3c). CO oxidation has to surpass a barrier of 0.68 eV to form a bent CO2− unit (I4 → TS3 → I5) attached on the Ni2 unit. However, the direct desorption of CO2 from I5 (I5 → TS4′ → I6′) is an energetically demanding process to surmount a positive barrier of 0.36 eV, which is challenging to overcome under thermal collision conditions. In contrast, the Ni site of I5 can preferably trap a second CO molecule (I6, ΔH0 = −2.41 eV); then, a gasphase CO2 molecule can be breezily evaporated to generate Ni2VO3CO−. The calculated results well rationalize the experimental observation that Ni2VO4CO− rather than Ni2VO4− can oxidize CO to generate CO2. The reaction of the resulting Ni2VO3CO− with O2 is also thermodynamically and kinetically favorable to regenerate Ni2VO5− (Figure S9); then, the cycle is closed. The rates of internal conversion from the initial adsorption intermediates to the final products in each elementary step (Figure 2a) were estimated by using the Rice−Ramsperger−Kassel−Marcus32 (RRKM) theory. The RRKM results showed that the rates of internal conversation (1.0 × 108 to 3.5 × 1010 s−1) are much larger than those of collision between the clusters and the He bath gas [∼3 Pa, (1.1 to 1.3) × 106 s−1] in the ion trap. Thus each of the individual steps in Figure 2a can take place. In contrast, the rate of collision between Ni2VO4CO− and the high-pressure He carrier gas (∼3000 Pa, 1.1 × 109 s−1) in the cluster formation channel is larger than that of internal conversion (1.0 × 108 s−1). Thus most of the initially formed Ni2VO4CO− species (IS10, Figure S5) can be stabilized. The Ni 2 VO 4 CO − intermediate in the ion trap can carry the binding energy between Ni2VO4CO− and CO. This result well rationalizes the fact that the cluster-source-generated Ni2VO4CO− species have different reactivities with respect to the ion-trap-generated Ni2VO4CO− intermediate (Figures S3, S7, and S8). The metal−CO interplay28−30 that may extensively exist in heterogeneous catalysis has been highlighted to have a substantial influence on the structure and the reactivity of related catalysts. The chemically coated CO was typically regarded as a strong poisoning agent and inhibited either the activity or selectivity of catalysts.33 Recently, the unexpected enhancements of catalytic performance mediated with CO adsorption have been identified in real-life processes34−41 and even in gas-phase studies.42−45 Herein we demonstrated that the attached CO can modulate the reaction from endothermic (for Ni2VO4− + CO, direct CO2 desorption) to exothermic (for Ni2VO4CO− + CO, CO-assisted CO2 desorption). Figure 3c shows that the ability of I5 to evaporate a gas-phase CO2 molecule lies in the ability of the product Ni2VO3− to accommodate the left electrons46 that are stored originally in the CO2− unit (−0.94 e). NBO analysis shows that without the attachment of another CO, the Ni2 unit in the product Ni2VO3− accommodates the released electrons (−0.82 e, Figure 4) reluctantly in a single step, and the barriers are challenging to surmount under ambient conditions. In contrast, with the CO addition, the Ni2 unit in Ni2VO3CO− also acts primarily as the oxidative center but can accept the transferred electrons (−0.78 e) readily and step by step. This can be rationalized by the fact that the attached CO creates a good charge separation of the Ni2 unit [OC-Ni (0.05 e)−Ni (0.74 e)] in I6 with respect to that in I5 [Ni (0.54 e)−Ni (0.80 e)], and thus I6 has a larger dipole moment (2.59 D) than that of I5 (1.54 D). A large dipole moment can generate a local electric field47 and facilitate charge transfer. A previous study demonstrated that the oxygen-deficient and NM-free metal

Figure 3. (a) DFT-calculated lowest-energy structures for the Ni2VO5−, Ni2VO4−, Ni2VO4CO−, and Ni2VO3CO− clusters and (b,c) the potential energy profiles for elementary reactions 1−3. See the details in Figures S5−S9. The bond lengths are given in picometers. The zero-point vibration-corrected energies (ΔH0, eV) with respect to the separate reactants are given. MNixVyOz− is labeled as Mx,y,z, and M denotes spin multiplicity.

in the quintet spin states. The Ni−Ni bond in four catalytically relevant species suffers from a remarkable stretching (232 pm in Ni2VO3CO− versus 272 pm in Ni2VO4CO−), the process of which is bound to induce a significant change of their electronic structures. CO can be trapped by the positively charged Ni site [+0.91 e, according to natural bond orbital (NBO) analysis] in Ni2VO5− with a binding energy of 0.37 eV (I1, Figure 3b); then, CO is oxidized by a terminal oxygen atom easily (I1 → TS1 → I2 → TS2 → I3). CO2 desorption favorably takes place to generate Ni2VO4−. The low-lying Ni2VO5− isomers (IS2 and IS3 in Figure S5) could also be populated in the cluster source, and the DFT method may not be accurate enough to determine the lowest-lying structure in the experiment. Thus their reactions with CO were also studied and have been demonstrated to be favorable (Figure S6). The half-naked Ni site of Ni2VO4− functions as the preferred trapping site for CO adsorption with a large 1135

DOI: 10.1021/acs.jpclett.9b00047 J. Phys. Chem. Lett. 2019, 10, 1133−1138

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The Journal of Physical Chemistry Letters

in electronic structure of 3d metals will bring about a remarkable difference in their reactivity in the catalysis. In summary, a CO self-promoted mechanism in the catalytic CO oxidation by O2 mediated with the Ni2VO4,5− clusters was identified. The key step to drive the catalysis lies in the reactivity of the Ni2VO4CO− species, which can be boosted by the attachment of another CO. The adsorbed CO can modify the electronic structure of the Ni2 unit, which behaves like a noble-metal atom to store the released electrons in an oxygen atom transfer process. In cluster catalysis, this finding is among the first to demonstrate the reactivity resemblance of 3d metals to noble metals upon CO adsorption.

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Figure 4. DFT-calculated natural charges on related moieties along the pathways shown in Figure 3c.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00047.

oxide clusters cannot oxidize small molecules.24 In contrast, with the introduction of NM atoms, the oxygen-deficient species, such as RhAl2O2+,26 can be highly reactive in CO oxidation, benefiting from the remarkable electron-storage ability of NM atoms. Metal carbides can display NM-like behavior in many catalytic processes.48,49 Herein we identified that the adsorbed CO is vital to modify the electronic structure of the oxygen-deficient complex Ni2VO4CO− and makes the OC−Ni2 moiety in Ni2VO4CO− resemble the behavior of an NM atom in terms of electron storage during an oxygen atom transfer process. Note that the variation of the NBO charges on the V atom is small (Figure S10), whereas the V atom can be four- and three-fold coordinated with the O atoms (VO4 ↔ VO3), (Figure 3a), indicating that the V atom also plays an important role to store and release the O atoms during the catalysis. CO prefers to interact with positively rather than negatively charged metal sites to act as an electron donor.25,50,51 Herein the attached CO is positively charged (+0.20 e) in I6, whereas it becomes negatively charged (−0.14 e in Ni2VO3CO−) during CO2 desorption (Figure 4). This indicates that the attached CO can share the burden with the Ni2 unit to function as another oxidative center to withdraw electrons. Orbital analysis demonstrates that back-donation (3d → 2π*) contributes to the interaction of Ni2VO3− and CO. This can be reflected by the binding energies of CO on the Ni2VO3− (2.10 eV), Ni2VO4− (1.12 eV), and Ni2VO5− (0.37 eV) clusters. This tight interaction is essential to lead to an exothermic process for reaction Ni2VO4CO− + CO. CO represents one of the most important ligands in coordination chemistry and organometallics,12 and the capability of the attached CO to tune the electronic structures and then improve the reactivity of atomic clusters has been reported.44,45 Recently, we identified that the attached CO on CuAl4O9CO− can stabilize the Cu atom of CuAl4O9CO− around the +I oxidation state under reaction conditions and promote the oxidation of another CO.23 Herein we highlighted that the attached CO is dynamic in charged states and the stored electrons in CO can be released [−0.14 e in Ni2VO3CO− → +0.12 e in Ni2VO5CO− (I1) ] during O2 activation. For the catalysis mediated with the Cu2VO3−5− clusters, the Cu2 unit is dynamic in terms of electron storage and release in the catalysis and functions effectively as an Au atom.17 The Ni2 unit is also dynamic under catalytic conditions (+1.8 e in Ni2VO5− → +0.6 e in Ni2VO3CO−), whereas this dynamic behavior has to be assisted by CO adsorption, indicating that a subtle change

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Detailed descriptions of experimental and theoretical methods, the analysis of experimental data, the calculated cluster structures, and the reaction pathways (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-10-62536990. Fax: +86-1062559373. ORCID

Li-Na Wang: 0000-0002-9194-3527 Xiao-Na Li: 0000-0002-0316-5762 Sheng-Gui He: 0000-0002-9919-6909 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (nos. 21773254, 21773253, and 21573246), the Youth Innovation Promotion Association CAS (No. 2016030), and the Beijing Natural Science Foundation (2172059).

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