Balance in Adsorption Energy of Reactants Steers CO Oxidation

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Balance in Adsorption Energy of Reactants Steers CO Oxidation Mechanism of Ag13 and Ag12Pd1 Nanoparticles: Association Mechanism versus Carbonate-Mediated Mechanism Hyun You Kim,† Sang Soo Han,‡ Ji Hoon Ryu,† and Hyuck Mo Lee*,† Department of Materials Science and Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Korea, and Center for Materials Measurement, Korea Research Institute of Standards and Science, 209 Gajeong-Ro, Yuseong-gu, Daejeon, 305-340, Korea ReceiVed: NoVember 24, 2009; ReVised Manuscript ReceiVed: January 19, 2010

CO oxidation is a very useful reference reaction in catalysis by nanoparticles (NPs). Two reaction modelssan association mechanism (AM) and a carbonate-mediated mechanism (CMM)shave been suggested for CO oxidation catalyzed by small NPs. It is still unclear, however, when and why these mechanisms preferentially operate. With the Ag13 crystalline NPs and the Ag12Pd1 core-shell NP, using a density functional theory calculation and a microkinetic reaction model, we found that the different reaction mechanisms can operate with different reaction intermediates accompanied by a balance in the adsorption energy of reactants. Variations in the adsorption energy of adsorbates can result from the interchange of two reaction mechanisms, even in a single NP. An AM operates when both reactants interact strongly with a NP, whereas the contribution of the CMM in CO oxidation increases when a CO molecule interacts weakly or not at all with a NP. 1. Introduction Many chemical reactions are catalyzed by small metal nanoparticles (NPs). For example, Pt NPs in the 2-5 nm range are a practical catalyst for fuel cell applications1–4 and Au NPs catalyze many selective oxidation reactions.5–11 Several bimetallic NPs have also been proposed as a designable system.2,12–22 Prior to developing and validating the catalytic properties of new NP catalysts, it is important to have an exact understanding of the reaction mechanisms. In this context, CO oxidation is regarded as the most general and simplest oxidation reaction for validating the reactivity of oxidation catalysts,8,10,12,15,20,23–26 and thus has been considered to be a reference reaction for testing and designing nanocatalysts with atomic precision. For example, Haruta’s pioneering finding that small gold NPs can catalyze CO oxidation even at or below room temperature27,28 was a turning point in the development of catalysis by metal NPs. Various reports on the improved catalytic properties of doped oxide materials23,25,26 and endohedral bimetallic NPs12,20 have also referred to their CO oxidation reactivity. Recent reports have attributed the origin of the excellent catalytic activity of NPs to the fact that they have smaller dimensions than previously anticipated.5,10,29–33 Kaden et al. reported that the TiO2 supported Pd20 (consisting of 20 atoms) NP shows maximum activity for CO oxidation.29 Vajda et al. showed that Pt8-10 NPs are highly active for oxidative dehydrogenation of propane.30 Hutchings and co-workers found that very small Au NPs consisting of around 10 atoms each are responsible for the CO oxidation reactivity of iron-oxidesupported Au NPs.10 On the basis of experimental and theoretical studies, Lee et al. showed that oxide-supported Au6-10 NPs catalyze selective epoxidation of propene.31 Given the recent * To whom correspondence should be addressed. Phone: +82-42-3503334. Fax: +82-42-350-3310. E-mail: [email protected]. † KAIST. ‡ Korea Research Institute of Standards and Science.

findings that very small subnanometer NPs are catalytically active, the importance of these small NPs is gaining increasing credibility. To date, several CO oxidation mechanisms have been reported using some sort of catalyst. CO oxidation by oxide materials clearly follows the Mars-van Krevelen mechanism (in which the surface oxygen atom of the oxide oxidizes the CO molecule).23,25,26 On the other hand, the CO oxidation by metal catalysts uses a gas-phase O2 molecule as an oxygen source. The dissociative adsorption of the O2 molecule and subsequent oxidation of CO molecules by the Langmuir-Hinshelwood mechanism34 reportedly occurs on metal surfaces (as a result of the dissociation mechanism of CO oxidation).35,36 This mechanism has been extensively studied in Pt and Pt-based bimetallic nanoalloys,3,37–40 since dissociative O2 adsorption is a required condition for fuel cell catalysts. As the interest in catalysts and catalysis has been extended to small NPs, the process of CO oxidation by metal NPs has been studied in depth. Liu et al.35 reported that a reaction called the association mechanism (AM), which is not accompanied by the dissociative adsorption of an O2 molecule, is dominant on a stepped Au surface. When the AM of CO oxidation is activated, coadsorbed O2 and CO molecules evolve into a fourcenter intermediate metastable (MS) state (O-O-CO), and the MS state is dissociated into a gas-phase CO2 molecule and a residual O atom.35,41 We recently showed theoretically that a residual O atom is directly removed by a gas-phase CO molecule as a result of the Eley-Rideal mechanism,34 and proved quantitatively that the activation energy barrier of O2 dissociation by small NPs is much higher than the energy barrier for an AM.12 Another study, by Falsig et al.,9 has confirmed that an AM results from the high CO oxidation activity of Au NPs. Another CO oxidation mechanism that does not incorporate O2 dissociation has been reported in very small NPs. Hakkinen and Landman42 showed that the negatively charged Au2- NP facilitates CO oxidation by forming a carbonate-like intermediate (CLI). Hagen et al.,43 Wallace and Whetten,44 and Presianni et al.45 also reported a CLI during CO oxidation by small Au NPs.

10.1021/jp9111553  2010 American Chemical Society Published on Web 02/02/2010

CO Oxidation Mechanism of Ag13 and Ag12Pd1 NPs

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TABLE 1: Reaction Energy of CO Oxidation by a CMM Catalyzed by the Studied Neutral NPsa Ag13-COh Ag12Pd1-Ih

O2 Ead (eV)

CLI E form (eV)

2 E CO de (eV)

E2nd (eV)

Etotal (eV)

-0.61 -0.71

-4.04 -3.65

+0.25 +0.14

-1.82 -2.00

-6.22 -6.22

a O2 2nd CLI 2 Ead is the energy of adsorption of O2 on the NPs, Eform is the formation energy of the CLI, ECO is de is the energy of desorption of CO2, E the energy released in the second CO oxidation by the residual O atom, and Etotal is the total reaction energy. Here, it is noticeable that the Ag13-Ih structure changes into the Ag13-COh during the CMM process.

In this procedure, a gas-phase CO molecule binds directly to a preadsorbed O2 molecule, leading to the formation of a CLI. The CLI subsequently dissociates into CO2 and a residual O atom. We call this procedure the carbonate-mediated mechanism (CMM) of CO oxidation. Although CO oxidation is a very useful reference reaction in catalysis by NPs, the reaction mechanism has been studied and implemented only as the occasion demands. To the best of our knowledge, the AM and the CMM of CO oxidation have never been studied together or integrated into a broader framework. It is still unclear when and why these mechanisms preferentially operate. Therefore, in this work, we use a density functional theory (DFT) calculation and a microkinetic reaction model to examine the operational conditions of two CO oxidation mechanisms. The effects of the balance in the adsorption energies of the reactants and the extra charge are discussed in depth. We use crystalline Ag13 NPs with icosahedron (Ih) and cubo-octahedron (COh) structures adopted from our previous study on the AM of CO oxidation by the Ag13 and Ag12X1 (X ) Pd, Pt, Au, Ni, or Cu) core-shell NPs.12 Moreover, we also consider the Ag12Pd1-Ih core-shell NP, which was proposed in our previous study as a robust and reactive CO oxidation catalyst.12 2. Computational Methods We performed GGA-level spin-polarized DFT calculations with the atomic orbital based DMol3 code.46,47 The exchangecorrelation energy was functionalized with the RPBE functional.48 The Kohn-Sham equation was expanded in a doublenumeric quality basis set with polarization functions (DNP). The orbital cutoff range was 5.0 Å. The DFT semicore pseudopotential49 was used to treat the core electrons of Ag and Pd atoms. We also used a Fermi smearing method with a window size of 0.007 hartree (1 hartree ) 27.2114 eV). The energy, force, and displacement convergence criterion were set to 10-5 hartree, 0.002 hartree/Å, and 0.005 Å, respectively. The transition state calculations were performed using synchronous transit methods, the linear synchronous transit, and quadratic synchronous transit,50,51 in combination with the conjugate gradient minimization algorithm52 for subsequent refinement. The transition state was confirmed by imaginary vibrational frequency. 3. Results and Discussion The CO oxidation procedure of an AM is better known than that of a CMM. An AM of CO oxidation by NPs is accompanied by strong coadsorption of CO and O2 molecules on the surface of the NPs.9,12,35 In this case, the association of the coadsorbed CO and O2 molecules into a four-center intermediate MS state is the rate-determining step.9,12,35 Because the sound operation of an AM requires the coadsorption of CO and O2 molecules, a NP that interacts weakly with one of the reactants is not a good catalyst for CO oxidation by an AM.12 Because the whole reaction mechanism of CMM has not been proposed thus far, we propose the following sequential procedure

for the CMM of CO oxidation, on the basis of the Eley-Rideal mechanism:

O2(g) + / S O2*

(R1)

CO(g) + O2* S CO3*

(R2)

CO3* S CO2(g) + O*

(R3)

O* + CO(g) S CO2(g) + /

(R4)

. The first reaction, R1, is unactivated; hence, it is fast and in equilibrium.9,12 The formation of the CLI, R2, is highly exothermic by ∼4 eV (see Table 1). Therefore, the reaction R2 would be very fast and thus it does not slow down CO oxidation. The reaction R4 is spontaneous and barrierless.12 Therefore, the life span of a residual O atom (O*) is negligible. Because the third reaction, R3, which involves CO2 desorption, is the only endothermic step (see Table 1), we expect that the rate of CO2 formation by the CMM would be equal to the maximum rate of the reaction R3. Sabatier activity (SA) of CO oxidation by the CMM was calculated with the maximum rate of the R3, rate(R3)max, as follows:

(

SA ) kT ln

kT rate(R3)max h

)

(1)

at T ) 273 K, p(CO) ) 0.01 bar, and p(O2) ) 0.21 bar (details on the microkinetic model of CO oxidation can be found in the Supporting Information). Additional transition state calculations on the Ag13-COh and Ag12Pd1-Ih NPs confirm our hypotheses. We found that the R2 is barrierless and the barrier for the R3 is equal to the thermodynamic energy difference (in both cases, the transition state is located below the final state). In our previous studies on CO oxidation by Ag13 NPs, we found that, in the presence of a preadsorbed O2 molecule on the surface, the Ag13-COh cannot strongly bind a CO molecule,12 indicating that an AM of CO oxidation would not operate at room temperature. However, for the activation of a CMM, the rejection of a CO molecule is no longer a limitation; rather, it is a necessity. Figure 1a shows the CMM of CO oxidation by the Ag13-COh. It suggests that if a NP binds an O2 molecule strongly enough to prevent its thermal desorption (that is, where the adsorption energy of the O2 molecule is higher than the free energy of an O2 molecule at a given temperature) the adsorbed O2 binds a CO molecule and then forms the CLI with high exothermicity. Note that this is the only possible form of CO-O2 coadsorption, because a CO molecule does not interact strongly with the Ag13-COh. The desorption of CO2 from the CLI is uphill by 0.25 eV, but the energy of desorption is within a tolerable range.

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Figure 1. CO oxidation by a CMM on (a) the Ag13-COh and (b) the Ag12Pd1-Ih. The energy of the NP plus the two CO molecules and the one O2 molecule is taken to be zero. Ex denotes the energy of the xth state relative to the initial state. In both cases, the total reaction energy released (Etotal ) -6.22 eV) is equal to the energy of oxidation of the two gas-phase CO molecules.

Figure 2. Mulliken charge distribution of the reaction intermediates: (a) the CLI on the Ag13-COh, (b) the CLI on the Ag12Pd1-Ih, and (c) the four-center intermediate MS state on the Ag12Pd1-Ih.

The formation of such a CLI as an intermediate of CO oxidation has also been reported in the CO oxidation by Au, V, Cr, Mn, Mo, or W doped rutile TiO2(110)23,25 and Au doped CeO2(111).26 Because the CO oxidation by metal oxides follows the Mars-van Krevelen mechanism,23,25,26 the formation of a surface oxygen vacancy, as well as the subsequent vacancy healing by a gas-phase O2 molecule, is an underlying factor of the CO oxidation reaction. The localized excess electrons at the surface oxygen vacancy of oxide materials facilitate the spontaneous vacancy healing by a gas-phase O2 molecule.23,25,26 This facilitation is natural because an O2 molecule behaves like an electron acceptor (theoretically, an O2 molecule can accept two more electrons).12,53 On the other hand, a CO molecule is known as an electron donor.15,20 Thus, the adsorption of a CO molecule on the top of a preadsorbed partially electron-rich O2 molecule donates more electrons to the O2 and fully activates the O2 (further stretching the O-O bond). As a result, the CO and O2 evolve to the CLI. Figure 2 shows the Mulliken charge distribution of the CO-O2 coadsorption intermediates of CO oxidation found in the Ag13-COh and Ag12Pd1-Ih. Clearly, the CLI is holding more electrons. In the case of the CLI, there is no contact between a CO molecule and a NP (see Figure 2). This lack of contact prevents electron exchange between a CO molecule and

a metal atom, and thereby localizes more electrons at the CLI. The surface of the NPs is electron-rich owing to the low coordination of the surface atoms. In addition, the fact that a CMM is generally reported in negatively charged small Au NPs, which interact more strongly with the electron acceptor O2 than with the electron donor CO, supports the hypothesis that O2 molecules should be predominantly adsorbed on the catalyst for the CLI formation. Table 2 shows the calculated SA for the CMM and the AM of CO oxidation of the NPs in question. The SA for the CMM of the Ag13-COh (-0.25) confirms that the Ag13-COh clearly facilitates the CO oxidation by a CMM rather than an AM (note that the Ag13-COh cannot oxidize CO by the AM of CO oxidation). On the other hand, in the case of the Ag13-Ih, with the CLI on the surface, the Ih NP changes to the COh NP. Figure 2 shows that the CLI positively charges three Ag atoms in contact with it in a relatively equal manner. Therefore, the depletion of unpaired electrons by the CLI formation and the electrostatic forces among the positively charged Ag atoms initiate structural change from the Ih to the more stable12,15 COh structure. In the case of the Ag13-Ih, the high CO oxidation reactivity that originates from the AM rapidly diminishes as the Ih structure evolves to the amorphous state.12 However, the high reactivity of the Ag13-COh associated with a CMM of CO oxidation does not diminish because the COh structure is conserved after the reaction (see Figure 1a). We have suggested that Ag12X1 core-shell type NPs could be a robust and reactive catalyst for CO oxidation.12 A severe activity drop induced by the reaction-driven structural change of the Ag13-Ih during the AM of CO oxidation can be prevented by adding a heterogeneous core atom, and the Ag12Pd1-Ih in particular is the best model for structural robustness and high catalytic reactivity. From Figure 1b and Table 2, we found that the Ag12Pd1-Ih is a good catalyst for CO oxidation by a CMM. Actually, the SA (-0.14) of CO oxidation by a CMM is better than that (-0.65) by an AM. However, since the energy of the CO adsorption of the Ag12Pd1-Ih is considerably high (-0.57 eV), the surface adsorption sites on the Ag12Pd1-Ih would be competitively occupied by CO and O2 molecules. Therefore,

CO Oxidation Mechanism of Ag13 and Ag12Pd1 NPs

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TABLE 2: Calculated SA of the Studied Neutral NPs Ag13-Ih -0.23 Ih transforms to COh when the CLI forms

AM CMM a

Ag13-COh

a

does not operate -0.25

Ag12Pd1-Ih a

-0.65a -0.14

From ref 12.

TABLE 3: Charge-Dependent Adsorption of O2 and CO Molecules on the Ag13-COh charge state

+2

+1

0

-1

-2

-0.28 -0.46 -0.61 -0.63 -0.86 energy of adsorption O2 COa -0.68 -0.54 -0.31 -0.31 -0.23 a Represents the energy of adsorption of a single CO molecule on NPs, not derived from the O2-CO coadsorption energy.

although the SA of a CMM is better than that of an AM, both mechanisms would systematically contribute to the CO oxidation reactivity of the Ag12Pd1-Ih. The results suggest that the adsorption energies of CO and O2 molecules of small NPs would determine which mechanism of CO oxidation operates more favorably. The moderate adsorption energy of both reactants is mandatory for the operation of an AM, whereas strong O2 adsorption and weak CO adsorption are necessary for a CMM. In addition, because each reactant donates electrons to the catalyst (CO) or attracts electrons (O2) from the catalyst, the charge state of the NPs affects the balance of the adsorption energy of both reactants and thereby leads to the interchange of the CO oxidation mechanism. Table 3 shows the variation of the adsorption energy of the adsorbates with the charge state of the Ag13-COh. Clearly, an O2 molecule interacts strongly and a CO molecule weakly with a negatively charged NP and vice versa. We postulate that a CMM preferentially operates on negatively charged NPs, which is supported by the fact that the CLI has been experimentally reported in negatively charged small NPs that strongly bind an O2 molecule and weakly interact with a CO molecule.42–44 Additionally, we deduce that the different CO oxidation mechanisms operate with different supporting materials (usually metal oxides) because each supporting material charges the supported NPs in a different manner. 4. Concluding Remarks The CO oxidation reaction is the simplest and most popular reaction for validating the catalytic activity of NPs. However, the CO oxidation reactivity of small NPs cannot be estimated by reference to a single reaction mechanism. Rather, different reaction mechanisms can independently contribute to the total reactivity. The adsorption of both reactants and the influence of the charge state of small NPs should be systematically considered. We found that the balance in the adsorption of the CO and O2 molecules is the factor that concerns the contribution of two reaction mechanisms on CO oxidation catalyzed by Ag13 and Ag12Pd1 NPs. An AM operates when both reactants interact strongly with the NPs, and the contribution of a CMM in CO oxidation increases when the CO molecule interacts weakly or not at all with the NPs. Of course, strong O2 adsorption is a minimum requirement for both reaction mechanisms. The balance in the adsorption energy can also be adjusted by charging the NPs. Among the negatively charged NPs, a CMM of CO oxidation would be the dominant mechanism.

Note that caution is needed to generalize our findings to large NPs or supported NPs because our model study is based on subnanometer sized unsupported crystalline Ag and Ag-Pd NPs which have not been considered as the preferential system for CO oxidation. Additionally, the larger NPs would be relatively more weakly charged than smaller ones with the same amount of electrons and thereby their CO oxidation mechanism would not be highly affected by their charge state. The presence of the support-NP interface and structural interaction between them also should be systematically considered. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (the Ministry of Education, Science and Technology, MEST) (No. 2009-059348) and the Nano R&D program through the KOSEF funded by the MEST (No. 20090082472). Supporting Information Available: The microkinetic model for CO oxidation by a CMM. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Nat. Chem. 2009, 1, 37. (2) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Science 2007, 315, 220. (3) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Top. Catal. 2007, 46, 249. (4) Shao-Horn, Y.; Sheng, W. C.; Chen, S.; Ferreira, P. J.; Holby, E. F.; Morgan, D. Top. Catal. 2007, 46, 285. (5) Turner, M.; Golovko, V. B.; Vaughan, O. P. H.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson, B. F. G.; Lambert, R. M. Nature 2008, 454, 981. (6) Roldan, A.; Gonzalez, S.; Ricart, J. M.; Illas, F. ChemPhysChem 2009, 10, 348. (7) Gruene, P.; Rayner, D. M.; Redlich, B.; van der Meer, A. F. G.; Lyon, J. T.; Meijer, G.; Fielicke, A. Science 2008, 321, 674. (8) Kung, M. C.; Davis, R. J.; Kung, H. H. J. Phys. Chem. C 2007, 111, 11767. (9) Falsig, H.; Hvolbaek, B.; Kristensen, I. S.; Jiang, T.; Bligaard, T.; Christensen, C. H.; Norskov, J. K. Angew. Chem., Int. Ed. 2008, 47, 4835. (10) Herzing, A. A.; Kiely, C. J.; Carley, A. F.; Landon, P.; Hutchings, G. J. Science 2008, 321, 1331. (11) Chretien, S.; Buratto, S. K.; Metiu, H. Curr. Opin. Solid State Mater. Sci. 2007, 11, 62. (12) Kim, H. Y.; Kim, D. H.; Ryu, J. H.; Lee, H. M. J. Phys. Chem. C 2009, 113, 15559. (13) Kim, H. Y.; Kim, H. G.; Kim, D. H.; Lee, H. M. J. Phys. Chem. C 2008, 112, 17138. (14) Kim, H. Y.; Kim, H. G.; Ryu, J. H.; Lee, H. M. Phys. ReV. B 2007, 75, 212105. (15) Graciani, J.; Oviedo, J.; Sanz, J. F. J. Phys. Chem. B 2006, 110, 11600. (16) Khan, N. A.; Uhl, A.; Shaikhutdinov, S.; Freund, H. J. Surf. Sci. 2006, 600, 1849. (17) Sheth, P. A.; Neurock, M.; Smith, C. M. J. Phys. Chem. B 2005, 109, 12449. (18) Studt, F.; Abild-Pedersen, F.; Bligaard, T.; Sorensen, R. Z.; Christensen, C. H.; Norskov, J. K. Science 2008, 320, 1320. (19) Studt, F.; Abild-Pedersen, F.; Bligaard, T.; Sorensen, R. Z.; Christensen, C. H.; Norskov, J. K. Angew. Chem., Int. Ed. 2008, 47, 9299. (20) Gao, Y.; Shao, N.; Bulusu, S.; Zeng, X. C. J. Phys. Chem. C 2008, 112, 8234. (21) Pyykko, P.; Runeberg, N. Angew. Chem., Int. Ed. 2002, 41, 2174.

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