Catalytic CO Oxidation on Single Pt-Atom Doped Aluminum Oxide

Jun 8, 2015 - Single platinum-atom catalysts exhibit extraordinary activity in a large number of reactions. However, a consensus regarding the molecul...
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Catalytic CO Oxidation on Single Pt-Atom Doped Aluminum Oxide Clusters: Electronegativity-Ladder Effect Xiao-Na Li,† Zhen Yuan,†,‡ Jing-Heng Meng,†,‡ Zi-Yu Li,†,‡ and Sheng-Gui He*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: Single platinum-atom catalysts exhibit extraordinary activity in a large number of reactions. However, a consensus regarding the molecular origin of Pt catalysis is far from being reached. Here, benefiting from the study of atomic clusters, we propose the Electronegativity-Ladder (E-Ladder) effect to account for the origin of Pt catalysis. The concept was obtained from the study of single Pt-atom doped aluminum oxide clusters PtAl3O5−7−, which are catalytically active in CO oxidation by molecular O2. The undoped aluminum oxide clusters, however, cannot drive such a catalytic cycle. The reactions have been identified by mass spectrometry and density functional theory calculations. The key to drive the cycle lies in the unique structure of PtAl3O6−, in which the Pt atom that is not fully oxidized can coexist with the highly oxidative oxygen-centered radical (O−•). After the oxidation of one CO by PtAl3O7−, the resulting PtAl3O6− can also oxidize a second CO. The E-Ladder effect originates from the well-fitting electronegativity of the Pt atom (2.28) in between that of the Al atom (1.61) and the O atom (3.44), and this effect promotes the generation of an unpaired electron localized O−• radical, which results in the oxidative nature of PtAl3O6− toward CO. Thus, the large enthalpy in the catalytic reaction (2CO + O2 → 2CO2) can be distributed much more evenly into several elementary reactions in the Pt−Al−O system than in the pure Al−O system.

1. INTRODUCTION Single platinum-atom doped oxide catalysts exhibit extraordinary catalytic activity in a large number of reactions, including CO oxidation,1,2 water−gas shifts (CO + H2O → CO2 + H2),3−5 oxidation of HCHO,6 hydrogenation,7 and so on. However, a consensus regarding the molecular origin of the unique activity triggered by the introduction of single Pt atoms has not been reached. Various proposals have been suggested to account for the origin of Pt catalysis. Some proposals highlighted the importance of the positively charged Pt centers, which were formed through the electron transfer from Pt atoms to oxide support.2,5,7 Atomically dispersed Pt-O species serving as catalytic centers or at least involved in the catalysis were also emphasized.1 The activation of atomic oxygen next to the Pt dopant was proposed to be critical to decrease the bond activation energy.8 Furthermore, the low-coordinated and the partially unoccupied 5d orbital of the Pt atom can also be responsible for the excellent performance.9 A general understanding of Pt catalysis is highly desirable to guide an improved design and optimization of single Pt-atom doped oxide catalysts. Isolated gas-phase clusters studied under controlled and reproducible conditions may serve as good models to uncover the mechanistic details in the related condensed phase.10−18 Heteronuclear oxide clusters are being actively studied to understand the mechanistic nature of doped catalysts.19−36 Study of single Pt-atom containing heteronuclear oxide clusters © XXXX American Chemical Society

is very important to explore the molecular origin of Pt catalysis. However, the reported studies scarcely touched the topic.23 In this study, the single Pt-atom doped heteronuclear aluminum oxide clusters PtAl3O5−7− were prepared and their catalytic reactivity toward CO oxidation, a simple, but a very important, model reaction in heterogeneous catalysis,10 is investigated to explore the molecular origin of Pt catalysis. The Pt−Al−O system is chosen because aluminum oxide supported platinum catalysts represent a tremendous success in industrial and environmental applications, especially in catalytic removal of CO from automobile exhaust.37 The inert substrate θ-Al2O3 supported isoelectronic single Pt atom in the condensed phase38 and the free Pt atom39 in the gas phase reported previously cannot catalyze CO oxidation using molecular O2 at room temperatures due to the presence of a barrier. The pure Al-O40 clusters and even the early transition-metal atoms (Y26 and V30) doped aluminum oxide clusters can catalyze CO oxidation using only N2O as oxidant, because the bond enthalpy of N2O (N2−O = 1.73 eV) is much smaller than that of O2 (O−O = 5.16 eV). In this study, the single Pt-atom doped aluminum oxide clusters PtAl3O5−7−, however, are catalytically active in CO oxidation under ambient conditions utilizing molecular O2, the activation of which is considered as the Received: May 2, 2015 Revised: May 26, 2015

A

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The Journal of Physical Chemistry C critical step in catalytic CO oxidation.41 The exciting results reported herein can serve as a good opportunity to explore how and why a single Pt atom works catalytically.

2. METHODS 2.1. Experimental Methods. Details of the experimental setup can be found in previous studies,42−44 and only a brief outline of the experiments is given below. The PtxAlyOz− cluster ions were generated by laser ablation of a Pt/Al mixed-metal disk (molar ratio: Pt/Al = 3/1) in the presence of O2 (0.4%) seeded in a He carrier gas (8 atm). The cluster ions of interest were mass-selected using a quadrupole mass filter and then entered into a linear ion trap (LIT) reactor, where they were thermalized by collisions with a pulse of He gas and then interacted with a pulse of CO, O2, or the gas mixture of CO and O2 for around 0.6−0.8 ms. The instantaneous pressure of He gas in the LIT reactor was around 15 Pa, and the partial pressures of the reactant molecules ranged from about 5 mPa to more than 2 Pa, depending on the nature of reaction systems. The temperature of cooling gas (He), reactant gases (CO, O2, or the gas mixture of CO and O2), and the LIT reactor was around 298 K. The cluster ions ejected from the LIT were detected by a reflectron time-of-flight (TOF) mass spectrometer. The pseudo-first-order rate constants in the elementary reactions and in the catalytic cycle can be determined by leastsquares fitting.20 To calculate the reaction efficiencies (the possibilities of reaction upon each collision), the collision rate constants were calculated as kcollision = 2π(e2α/μ)1/2, in which e is the charge of the cluster ion, α is the electric polarizability, and μ is the reduced mass.45 2.2. Theoretical Methods. Density functional theory (DFT) calculations using the Gaussian 0946 program were carried out to investigate the mechanistic details of catalytic CO oxidation mediated by PtAl3O5−7−. In order to find an appropriate functional for the Pt−Al−O system, the bond dissociation energies of Pt−O, PtO−O, Pt−C, Al−O, O−O, and Al−C were computed by various functionals and compared with available experimental data (Table S1, Supporting Information). It turns out that the TPSS functional47 was the best overall; thus, the results by TPSS were given throughout this work. The TZVP basis set48 for Al, C, and O atoms and a D95V basis set49 combined with the Stuttgart/Dresden relativistic effective core potential (denoted as SDD in Gaussian software) for the Pt atom were used in all the calculations. A Fortran code based on a genetic algorithm50 was used to generate initial guess structures of PtAl3O5−7−. The reaction mechanisms were studied for PtAl3O6,7− + CO and PtAl3O5− + O2 systems. The relaxed potential energy surface scan was used extensively to obtain good guess structures for the intermediates and the transition states along the pathways. The transition states were optimized using the Berny algorithm.51 Intrinsic reaction coordinate calculations52,53 were performed so that each transition state connects two appropriate local minima. Vibrational frequency calculations were carried out to check that intermediates and transition states have zero and only one imaginary frequency, respectively.

Figure 1. Elementary and catalytic reactions of atomic cluster ions. Mass spectra for elementary reactions of mass-selected PtAl3O7− (a), PtAl3O6− (d), and PtAl3O5− (f) with CO or O2 are shown in panels b, e, and g, respectively. Panel c shows the spectrum for the interaction of PtAl3O7− with N2. Panel h shows the mass spectrum for reactions of PtAl3O5− with a gas mixture of 36 mPa CO and 36 mPa O2. The PtxAlyOz− species are labeled as x,y,z. The time periods for all of the reactions were 0.8 ms.

represent the three major isotopes of Pt: 194Pt (32.967%), 195Pt (33.832%), and 196Pt (25.242%). The slight instability of the radio frequency used in the mass-selection procedure causes relative intensity variation of the Pt isotopomers. Upon the interaction of PtAl3O7− toward CO (Figure 1b), the co-appearance of PtAl3O6− and PtAl3O5− can be identified, which are not observed upon the interaction of PtAl3O7− with N2 (Figure 1c). This indicates that PtAl3O7− may oxidize two CO molecules consecutively to produce PtAl3O6− and PtAl3O5−. PtAl3O7− + CO → PtAl3O6− + CO2 −

PtAl3O6 + CO →

PtAl3O5−

+ CO2

(1) (2)

The interaction of PtAl3O6− with CO gives rise to PtAl3O5− (Figure 1e). This confirms that the signal of PtAl3O5− in Figure 1b is produced through the reaction of the resulting PtAl3O6− in reaction 1 with CO. The interaction of PtAl3O5− with O2 produces PtAl3O7− (Figure 1g). PtAl3O5− + O2 → PtAl3O7−

PtAl3O7−

(3)

PtAl3O6−

The reactions of + CO, + CO, and PtAl3O5− + O2 are also studied with different reactant pressures, and the results are provided in Figures S1−S3 in the Supporting Information. For reaction PtAl3O7− + CO, as the partial pressure of CO increases, the relative ion intensity of PtAl3O7− decreases and those of PtAl3O6− and PtAl3O5− increase gradually (Figure S1). The relative ion intensities of PtAl3O7−, PtAl3O6−, and PtAl3O5− are about 70%, 21%, and 2.2%, respectively, when the partial pressure of CO is 205 mPa,

3. RESULTS 3.1. Experimental Results. The TOF mass spectra for the interactions of the mass-selected PtAl3O5−7− cluster ions with reactant gases (CO, O2, or the gas mixture of CO and O2) are shown in Figure 1. The multiple peaks for each cluster B

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PtAl3O6− and PtAl3O7− increase gradually. This confirms that the catalytic cycle really takes place. Note that the rate constants for CO oxidation are higher in the cycle than that in the elementary reactions (Table 1). This is rationalized by the different temperatures during the catalysis and in the elementary reactions.54 In addition, CO adsorption cannot be negligible (Figures S6−S8, Supporting Information) and such behavior was usually observed in Pt-containing catalysts in the condensed-phase55 as well as in the gas-phase studies.56 3.2. Theoretical Results. Aiming to provide insights into the molecular origin of Pt catalysis, theoretical calculations have been performed to study the mechanisms of CO oxidation catalyzed by PtAl3O5−7−. The lowest-lying structures of PtAl3O5−7− have the triplet spin multiplicity (Figure 3). PtAl3O7− and PtAl3O6− contain an oxygen-centered radical (O−•), which is highly reactive toward small molecules,57−59 such as alkanes and CO.

as shown in Figure S1 (Supporting Information). This provides further evidence that the weak signal of PtAl3O5− in Figure 1b is indeed produced from the consecutive oxidation of two CO molecules by PtAl3O7−. The pseudo-first-order rate constants for reactions 1−3 can be well fitted (Figures S1−S3, Supporting Information), and the results are given in Table 1. The collision Table 1. Pseudo-First-Order Rate Constants (in unit of 10−10 cm3 molecule−1 s−1) Determined from Elementary and Catalytic Reactionsa reactions

elementary

catalytic

PtAl3O7− + CO → PtAl3O6− + CO2 PtAl3O6− + CO → PtAl3O5− + CO2 PtAl3O5− + O2 → PtAl3O7−

0.085 0.16 1.74

0.40 0.90 1.69

a

The uncertainties of the absolute and relative rate constants are within ±40% and ±20%, respectively.

rate constants 45 for reactions 1, 2, and 3 are calculated to be 6.4 × 10−10, 6.4 × 10−10, and 5.4 × 10−10 cm3 molecule−1 s−1, respectively, that correspond to the efficiencies of about 1.4, 2.5, and 32%. PtAl3O5 − 7−

2CO + O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2CO2 Catalysts

(4)

The elementary reactions 1−3 comprise a cycle of catalytic CO oxidation mediated by PtAl3O5−7− (reaction 4). To further support the cycle of catalytic CO oxidation, the reactions of PtAl3O5− with the gas mixture of CO and O2 are performed (Figure 1h and Figure S4 in the Supporting Information). Starting from PtAl3O5−, the co-appearance of PtAl3O7− and PtAl3O6− can be determined. This indicates that the newly formed PtAl3O7− from reaction 3 can further oxidize CO to produce PtAl3O6−. The formation of PtAl3O7− from the reaction of PtAl3O6− with O2 can be neglected because the signal of PtAl3O7− is not detected upon the interaction of PtAl3O6− with O2 (Figure S5, Supporting Information). The reaction kinetics for the identified catalysis herein is shown in Figure 2. As the partial pressures of reactant molecules increase, the relative ion intensity of PtAl3O5− decreases and those of

Figure 3. Calculated lowest energy structures of PtAl3O7−, PtAl3O6−, and PtAl3O5−. The spin density distributions are shown in the parentheses. Other lower energy isomers can be found in Figures S11−S13 in the Supporting Information.

The calculated pathway for the reaction PtAl3O7− + CO is shown in Figure 4a. The positively charged Pt atom (natural charge: +0.793 |e|) can trap CO tightly at the first step, and a large energy is released in this process (I1, ΔH0 = −2.24 eV). Natural bond orbital analysis indicates that −0.16 |e| charge is transferred from CO to PtAl3O7− upon adsorption. The formation of a bent CO2 unit is the bottleneck of the reaction, and an absolute barrier of 1.58 eV is required (I1 → TS1 → I2). The final release of the gas-phase CO2 has to suffer from a significant rearrangement of the cluster to guarantee that the Pt atom is still connected with the nearby O atoms in the resulting PtAl3O6−. The oxidation of CO by PtAl3O6− (Figure 4b) is very similar to that by PtAl3O7−, in which the Pt atom traps and delivers CO for oxidation by the “lattice” oxygen nearby. Furthermore, CO oxidation by the direct participation of the O−• radical, a highly reactive species toward CO oxidation,10,20,26,30,59−62 is also considered (Figures S16 and S17, Supporting Information), and all the calculations are favorable overall. Molecular O2 adsorption induced by the Pt atom in PtAl3O5− is moderate (I11 in Figure 5, ΔH0 = −0.52 eV), and the O−O bond is activated from 122 (free O2 molecule) to 129 pm. This process is accompanied by the transfer of −0.319 |e| charge into the 2π* orbital of O2. The subsequent O−O bond elongation follows a downhill pathway characterized by negligible barriers (111 → TS10 → I12 → TS11 → I13 → TS12 → I14). I14 contains a peroxide unit (O22−, O−O bond: 150 pm), and I14 is stable enough (ΔH0 = −2.03 eV) to dissociate an O22− unit with a barrier of only 0.38 eV (I14 → TS13 → I15). The facile dissociation of molecular O2 by PtAl3O5− may be contributed to the rather stable structure of the resulting PtAl3O7− (ΔH0 = −3.08 eV), which is the thermodynamic driving force to

Figure 2. Variation of ion intensities with respect to the partial pressures of CO in the reactions of PtAl3O5− with a gas mixture of CO and O2 is shown. The ratio of partial pressure of CO to O2 was 1:1, and the reaction time was 0.8 ms. The data points were experimentally measured, and the solid lines were calculated on the basis of rate constants determined from least-squares fitting. C

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has to surmount a barrier of 2.06 eV, which is rather high and less favorable than O2 dissociation induced by the Pt atom.

4. DISCUSION The capability to transfer at least two O atoms consecutively to CO from a single cluster can catalyze CO oxidation by molecular O2. However, clusters with the consecutive Otransfer reactivity have been scarcely reported20,63−73 and the available cases focus on late transition-metal or noble-metal (NM) containing systems. Early transition-metal oxide clusters (Sc 2 O 4 − , 6 0 Ti x O y ± , 5 9 , 7 4 VO 3 7 5 /V 4 O 1 0 + , 4 3 Mn x O y , 7 6 ZrxOy±59,62,77), some late transition-metal oxide clusters (FexOy ± 78−80 , CoxOy±,78,81 NixOy±,82 CuxOy0,−,63,78,83 and so on), lanthanide-metal oxide clusters (La2O4−,60 EuO+,70 YbO+,70 and CexOy±61,84), and the main-group metal oxide clusters Al2Oy±,40 in contrast, can transfer only one O atom efficiently to CO (see ref 10 for details). Even the early transition-metal atoms doped aluminum oxide clusters (YAlO 3 +26 and AlVO4+30) can transfer only one O atom to CO. Thus, N2O has usually been used as oxidant in catalytic CO oxidation.26,30,65,68,70,74,75,77,80,83 In this study, the unique reactivity of PtAl3O7− to oxidize two CO molecules consecutively is clearly attributed to the importance of single Pt-atom doping. The consecutive O-transfer reactivity of PtAl3O7− in CO oxidation lies in the unique structure of PtAl3O6−, in which the Pt atom that is not fully oxidized can coexist with the highly oxidative O−• radical (Figure 3). Thus, after the transfer of one O atom from PtAl3O7− to CO, the resulting PtAl3O6− can also oxidize a second CO (Figure 1). The presence of O−• in PtAl3O6− as well as in PtAl3O7− can be unambiguously identified upon the interactions of both clusters with n-C4H10 (Figure S9, Supporting Information). The appearance of products PtAl3O7H− and PtAl3O6H− indicates that one H atom in n-C4H10 is abstracted by O−•.57,58 The experimental results undoubtedly support the theoretical calculations. In contrast, with the substitution of the Pt atom in PtAl3O7− with an Al atom, the pure aluminum oxide cluster Al4O7− can also oxidize one CO molecule (Figure S10, Supporting Information) but gives rise to the inert Al4O6−. Then, the catalytic cycle of CO oxidation mediated by Al4O5−7− is terminated.

Figure 4. DFT-calculated potential energy profiles for reactions PtAl3O7− + CO (a) and PtAl3O6− + CO (b). The relative energies of intermediates (I1−I10), transition states (TS1−TS9), and products are in unit of eV. The structures of I1−I10 are shown. See Figures S14 and S15 in the Supporting Information for detailed information on cluster structures and mechanisms.

PtAl3O7− + CO → PtAl3O6− + CO2

ΔH0 = −1.90 eV (5a)

−

−

Al4O7 + CO → Al4O6 + CO2

ΔH0 = −1.18 eV (5b)

−

PtAl3O6 + CO →

PtAl3O5−

+ CO2

ΔH0 = −0.90 eV (6a)

−

Al4O6 + CO →

Figure 5. DFT-calculated potential energy profile for the reaction PtAl3O5− + O2. The relative energies of intermediates (I11−I17) and transition states (TS10−TS16) are in unit of eV. The structures of I11−I17 and the resulting PtAl3O7− are shown. The green atoms indicate O atoms from molecular O2. See Figure S18 in the Supporting Information for detailed information on cluster structures and mechanisms.

Al4O5−

+ CO2

ΔH0 = 0.67 eV (6b)

PtAl3O5−

+ O2 → PtAl3O7

Al4O5− + O2 → Al4O7−

−

ΔH0 = −3.08 eV

ΔH0 = −5.35 eV

(7a) (7b)

For the first time, benefiting from the study of atomic clusters, we propose the Electronegativity-Ladder (E-Ladder) effect to account for the unique structure of PtAl3O6−. The schematic drawing of the E-Ladder effect is described in Figure 6. The electronegativity of the Pt atom (2.28) sits in between that of the Al atom (1.61) and the O atom (3.44).85 After the oxidation of the first CO by PtAl3O7−, the released single electron (O−• + CO → CO2 + e−) is accepted by the Pt atom

collapse O2. In addition, O2 can be trapped tightly by the Al atom in PtAl3O5− (I25 in Figure S19, ΔH0 = −2.78 eV, Supporting Information). Significant charge (−0.619 |e|) is transferred from PtAl3O5− to O2, and the O−O bond is elongated to 139 pm. The subsequent O−O bond dissociation D

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Figure 6. Schematic drawing for the consecutive oxidation of two CO molecules by atomic clusters with (a) and without (b) E-Ladder.

cannot drive such a catalytic cycle. Here, we propose the Electronegativity-Ladder (E-Ladder) effect to account for the catalytic reactivity of PtAl3O5−7−. The E-Ladder effect originates from the well-fitting electronegativity of the Pt atom (2.28) in between that of the Al atom (1.61) and the O atom (3.44), and this effect promotes the generation of the unpaired electron localized O−• radical, which can account for the oxidation reactivity of PtAl3O6−. Thus, the large enthalpy in the catalytic reaction (2CO + O2 → 2CO2, ΔH0 = −5.88 eV) can be distributed much more evenly into several elementary reactions in the Pt−Al−O system than in the Al−O system. The generality of this E-Ladder effect obtained from the study of the Pt−Al−O system will be extended to other NM doped oxides as well as reactions other than CO oxidation.

due to the relatively strong electron-withdrawing ability (Figure 6a). Thus, such a Pt atom that is not fully oxidized can coexist with the unpaired electron localized O−• radical, which has been reported highly oxidative toward CO oxidation10,59−62 and can be used to account for the oxidative nature of PtAl3O6−. The reaction of PtAl3O6− with CO is exothermic (reaction 6a), and the released single electron (O−• + CO → CO2 + e−) is left on the Al atom in PtAl3O5− (Figure 6a). In contrast, without the E-Ladder of the Pt atom (Figure 6b), the released single electron after the oxidation of the first CO by Al4O7− is directly left on Al atoms86 (Figure S20, Supporting Information) because all the O atoms in the resulting Al4O6− have been fully reduced to O2−. Al4O6− has only the reductive nature, and the reaction of Al4O6− with CO is endothermic (reaction 6b). Thus, the E-Ladder effect originates from the well-fitting electronegativity of the Pt atom in between that of the Al atom and O atom, and this effect promotes the generation of the unpaired electron localized O−• radical and results in the oxidative reactivity of PtAl3O6−. Thus, the large enthalpy in catalytic reaction (2CO + O2 → 2CO2, ΔH0 = −5.88 eV) can be distributed much more evenly into several elementary reactions in the Pt−Al−O system (reactions 5a, 6a, and 7a) than in the Al−O system (reactions 5b, 6b, and 7b). This E-Ladder effect can also be used to account for the reactivity of single Au-atom doped aluminum oxide clusters AuAl3O3−5+ reported previously (Au electronegativity: 2.54).20 In contrast, Y and V are not the efficient “ladder” because of their much weaker electron-withdrawing ability (electronegativity: Y/1.22 and V/1.63).85 Thus, after the transfer of the O−• radical from YAlO3+26 and AlVO4+30 to CO, the resulting YAlO2+ and AlVO3+ have only the reductive nature. Furthermore, this E-Ladder effect will have general implications for other NM doped systems according to the fact that the electronegativity (2.20−2.54)85 of most NM atoms localizes in between that of non-NM atoms and an O atom. Future works by doping of other NM will be performed with the purpose of extending the generality of this E-Ladder effect and providing fundamental information on NM catalysis.

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving additional mass spectra, data analysis, DFT calculated cluster structures, and reaction mechanisms and a table giving related bond dissociation energies by experiments and DFT calculations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04218.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-10-62536990. Fax: +86-10-62559373. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major Research Plan of China (Nos. 2013CB834603 and 2011CB932302), the Natural Science Foundation of China (Nos. 21325314 and 21303215), and the Institute of Chemistry, Chinese Academy of Sciences (No. CMS-PY-201306).

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5. CONCLUSION In summary, catalytic CO oxidation by molecular O2 mediated by single Pt-atom doped aluminum oxide clusters PtAl3O5−7− was investigated to explore the molecular origin of Pt catalysis. The reactions have been characterized by mass spectrometry and density functional theory calculations. The key step to drive the cycle lies in the reactivity of PtAl3O6− toward CO oxidation. The Pt atom in PtAl3O6− cannot be fully oxidized, and such a Pt atom can coexist with the highly oxidative oxygen-centered radical (O−•). This unique structure results in the oxidative nature of PtAl3O6− toward CO. In contrast, the pure aluminum oxide clusters reported previously and in this study (Al4O5−7−)

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