Catalytic CO Oxidation by CO-Saturated Au Nanoparticles Supported

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Catalytic CO Oxidation by CO-Saturated Au Nanoparticles Supported on CeO: Effect of CO Coverage 2

Hyunwoo Ha, Hyesung An, Mi Yoo, Junhee Lee, and Hyun You Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09780 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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The Journal of Physical Chemistry

Catalytic CO Oxidation by CO-Saturated Au Nanoparticles Supported on CeO2: Effect of CO Coverage

Hyunwoo Ha†, Hyesung An†, Mi Yoo, Junhee Lee, and Hyun You Kim*

Department of Materials Science and Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 34134 Republic of Korea

†

These authors have equally contributed to this work

*To whom correspondence should be addressed: Prof. Hyun You Kim Email: [email protected]

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Abstract Understanding the reaction mechanism and the nature of the reactive species of heterogeneous catalysts under reaction conditions is the first step in the design of more consistent, reliable, and practical catalysts. We used density functional theory (DFT) calculations to study the mechanism of CO oxidation catalyzed by CeO2-supported Au nanoparticles (NPs) under reaction conditions by considering the sequential CO adsorption onto and CO saturation of Au NPs. We found that the Au9 NPs supported by CeO2(100) and CeO2(111) bind as many as 8 or 4 CO molecules, respectively. The last-bound CO molecule opens the fast CO oxidation pathway. The CO oxidation pathways constructed on both systems show that the reaction occurs at the AuCeO2 interface via the Mars-van Krevelen mechanism. We found that the most important O-CO-type intermediate was spontaneously formed at the Au-CeO2(100) interface upon the sequential binding of CO molecules onto the Au NPs. The reaction pathway therefore becomes relatively simpler than the CO oxidation pathways constructed with a first-bound single CO molecule. Although the O-C-O formation at the Au-CeO2(111) interface requires overcoming an activation energy barrier, the rate of CO oxidation shows that the Au/CeO2(111) was also highly reactive even at room temperature. Our findings show that the surface of Au NPs supported by CeO2 will be saturated with CO under CO oxidation conditions and that subsequent CO oxidation occurs at the Au-CeO2 interface. Our results suggest that the interaction between the catalysts and the reacting molecules should be more intensively studied to understand the catalytic performance of supported NP catalysts under reaction conditions.


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1. Introduction Computational catalysis studies have greatly improved our understandings of the reaction kinetics and mechanisms of heterogeneous catalysts (HCs), which have been obscured behind experimentally observed external catalytic properties.1-9 In general, the reliability of virtual model systems employed to DFT calculations determines the scientific value inherent in calculation results. Because a more reliable complicated calculation model would inevitably require more computational resources for analysis, appropriate DFT-based catalysis studies have always been chosen as a compromise between the accuracy of a model (morphological or physical similarity to real HCs) and the computation time required to study the model. Even a decade ago, unsupported tiny metal clusters or flat metal surfaces were considered a conventional model system for DFT-based catalysis studies to provide theoretical interpretations of catalytic properties of supported metal nanoparticles (NPs).10-12 Despite a morphological gap between theoretical model catalysts and real HCs, some insightful collaborative studies involving both theory and experiment have found important correlations between the energetics of a single individual catalytic step (usually adsorption of a reacting species) and the overall catalytic activity of HCs.1, 3, 9, 13-19 A combination of traditional BEP relations and Nørskov’s d-band center theory led to a simple reaction-descriptor-based catalyst screening; this design scheme has greatly accelerated the application of DFT-screeningbased catalyst design.1-3,

13

However, even such an acclaimed DFT-based catalysis design

strategy could not make predictions beyond the intrinsic activity of HCs. Recent findings on the dynamic evolution of HCs under reaction conditions (e.g. adsorption-induced surface segregation of bimetallic NPs,20-24 the dynamic formation of single-atom catalysts under reaction conditions,25-26 and dynamic changes in the morphology and surface concentration of NPs under 3 ACS Paragon Plus Environment


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reaction conditions27-28 suggest that the catalyst-environment interactions should be appropriately introduced into the framework of rational catalyst design at the atomic scale. Under these reaction conditions, the HC surface is covered with reactants and a steadystate reaction occurs at the reaction center. However, many DFT studies on the catalytic activity of oxide-supported metal classes of catalysts do not appropriately consider such a coverage effect. Finding the strongest binding sites for each reacting molecule and then constructing the reaction pathway using the most strongly bound reacting molecules is regarded as a general strategy in computation catalysis studies.29-33 For CO oxidation catalyzed by oxide-supported Au NPs, the first-binding CO molecule interacts most strongly with NPs and the last-binding molecule interacts most weakly with NPs. In general, the weakest-bound reactant can be easily used. Therefore, the catalytic pathway of CO oxidation built with the last-bound CO molecule would be energetically more accessible. In our previous reports on the catalytic pathway of CO oxidation by CeO2(111), supported Au NPs were studied using the first-bound single CO molecule, Au-CO*; we found that the high CO2 production energy from a reaction intermediate formed at the Au-CeO2 interface deactivated the Mars-van Krevelen (M-vK)-type CO oxidation pathway.31 We also found that the Au-CeO2 interfacial site can be activated for M-vK type CO oxidation by controlling the vacancy formation energy of CeO2.29-30 Given the recent findings on the vigorous environmental effect of the structure and the surface concentration of NP-based catalysts,21-24, 27-28, 34 our previous results based on simple single-molecule-based catalysis studies are presumably biased and not appropriate for describing the catalysis process under real CO oxidation conditions. Here, bringing the environmental effects into the framework of computational catalysis studies, we study CO oxidation-catalyzed CO-saturated Au NPs supported on ceria surfaces. We 4 ACS Paragon Plus Environment

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found that the surface of Au NPs supported on flat, defect-free CeO2(111) and CeO2(100) surfaces can be saturated with CO molecules; we also found that the last-bound CO molecule can be easily oxidized by the lattice oxygen of CeO2. Our results show that the structure of Au NPs and the energetics of CO oxidation are affected by the surface concentration of adsorbed CO molecules.

2. Computational details We constructed models of Au NPs supported on CeO2(111) and CeO2(100) surfaces. A two-layered FCC-type Au9 NP35 was supported on defect-free CeO2(111) 5×5×2 and CeO2(100) 3×3×2 slab models. The bottom slab composed of a trilayer of ceria was fixed during the geometric optimization. In a previous study, we found that an Au12 NP supported on the stepedge of CeO2(111) preferentially binds CO rather than O2 under CO oxidation conditions.30 We calculated the sequential binding energy, Ebind, of CO molecules on NPs. We also constructed the CO oxidation pathways catalyzed by the Au-ceria interfaces, which follow an M-vK-type oxidation mechanism.30, 33 Refer to Figure S1 for the geometry of optimized Au NPs on ceria surfaces. In a previous series of studies on catalytic CO oxidation by Au NPs supported on CeO2(111), we have consistently applied carefully optimized Au13 and Au12 NPs.

29-31

We

initially found the thermodynamically stable structural isomers of free-standing Au13 NPs and optimized them on CeO2(111) to ensure the structural robustness of Au NPs during catalytic CO oxidation and ultimately to study the intrinsic catalytic properties of Au/CeO2.31 A Au12 NP was specially used to study catalytic CO oxidation by Au NPs supported on the step-edge of CeO2(111).30 In this study, however, we used a highly crystalline Au9 NP for our comparative 5 ACS Paragon Plus Environment


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studies. We have tried to maintain the structural consistency of Au NPs on both CeO2(111) and CeO2(100) surfaces. Because our previous Au13 and Au12 NPs were specially optimized for CeO2(111), we used a crystalline Au9 NP stable both on CeO2(111) and CeO2(100). We performed spin-polarized DFT calculations with the Vienna ab-initio simulation package (VASP)36 and the PW9137 functional. To treat the highly localized Ce 4f-orbital, DFT+U38 with Ueff = 4.5 eV39-40 was applied. The interaction between the ionic cores and the valence electrons was described by the projector-augmented wave method.41 Valence electron functions were extended with the plane-wave basis to an energy cutoff of 400 eV. The Brillouin zone was sampled at the Γ-point. The convergence criteria for the electronic structure and the geometry were set to 10−3 eV and 0.01 eV/A, respectively. We used a Gaussian smearing function with a finite temperature width of 0.05 eV to improve convergence of states near the Fermi level. The location and energy of transition states (TSs) were calculated with the climbingimage nudged elastic band method.42-43 Details of the quality test of our ceria models and calculations settings and further details of the computational procedure are available elsewhere.29-31, 44

3. Results and discussion 3.1. Energetics of sequential CO binding on CeO2-supported Au NPs To describe the saturated surface state of supported Au NPs under real CO oxidation conditions, we calculated the change of the CO binding energy, Ebind, of Au NPs supported on CeO2(111) (Au/CeO2(111)) and CeO2(100) (Au/CeO2(100)) with an increasing number of binding CO molecules. Because the binding CO molecules would naturally prefer to be spatially


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separated from each other, we always calculated the strongest Ebind of each upcoming CO molecule by scanning the Ebind over all available binding sites of the Au NPs (Figure 1). In general, compared with Pt NPs, Au NPs relatively weakly bind CO; in addition, Au surfaces do not tightly bind CO.21, 30-31, 45-46 In our previous calculations on the sequential CO binding on unsupported Pt147 NPs, we found that regardless of the CO surface concentration, O2 molecules could not secure a binding site on Pt NPs. This result energetically confirms the wellknown CO poisoning of Pt NPs.46 This result also indirectly suggest that oxygen for CO oxidation should be supplied via a Pt-support interface. We found the same binding trend (CO poisoning of NPs) in our Au/CeO2(100) and Au/CeO2(111) catalysts (Figure 1). In Figure S2, the Ebind of the first-binding CO and O2 molecules clearly shows the energetic preference for CO binding on Au/CeO2(100) and Au/CeO2(111). Moreover, even the Ebind of the last-binding CO molecule was sufficiently strong to suppress the O2 binding at the same site (refer to the Ebind values of Figure 1 and Figure S2). The sequential Ebind values of CO molecules on Au/CeO2(100) and Au/CeO2(111), as shown in Figure 1, indicate that the surface of the Au NPs would be preferentially saturated with CO molecules under CO oxidation conditions. The Au NPs of Au/CeO2(100) catalysts can bind up to 8 CO molecules, where the Ebind of the last-binding CO molecule is -1.14 eV, which is still quite strong. By contrast, although the Au/CeO2(111) catalyst binds an initial 4 CO molecules with an average Ebind of -0.97 eV, it weakly binds the later CO molecules (5th to 9th). Because the entropic contribution to the Gibbs free energy of CO binding is +0.65 eV at 300 K, the Au NP of Au/CeO2(111) will not strongly bind more than 4 CO molecules. The interaction energies between Au NPs and ceria surfaces show that the Au-CeO2 interaction was at least 20 percent stronger on CeO2(100) than on CeO2(111) (Figure S1). This 7 ACS Paragon Plus Environment


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result suggests that the internal Au-Au interaction strength is relatively weak in Au/CeO2(100), which also explains why the Au NP of Au/CeO2(100) was disordered upon binding 8 CO molecules (see Figure 1a). Our results predict that the strong metal-support interaction weakens the interatomic cohesion inside the NPs. Therefore, the more flexible Au NPs of Au/CeO2(100) can be structurally adjusted to bind more CO molecules effectively, which was not allowed in Au/CeO2(111).

However,

more

detailed

high-precision

experimental

or

large-scale

computational studies are required to deduce a general statement on the relationship between the NP-support interaction and the interatomic cohesion strength of supported NPs. We note that our results were acquired for two-layered subnanometer Au9 NPs. The size of supported NPs could also affect the trend. The effect of NP-support interaction on the degree of the structural flux of the supported NPs would dissipate as a function of the size of NPs. For larger Au NPs, the AuAu interactions within mid- or upper-layer Au atoms of Au NPs would not be strongly affected by the NP-support interaction. For instance, a recent report by Rousseau et al. showed that the local atomic structure of an Au20 NP supported on CeO2(111) was disordered upon CO adsorption, forming a single-atom-like reaction species.25 On the basis of the various structural responses of our Au9 NP to CO saturation and the results of studies by Rousseau et al.,25 we postulate that the critical size for the support-NP interaction cannot affect the Au-Au interatomic cohesion strength that may exist between Au9 and Au20. Interestingly, our findings show that the saturated surface concentration of CO on the supported Au NPs varies depending on the surface index of the supporting oxide. Additionally, the presence of open interfacial sites in Au/CeO2(111) causes a critical difference in the CO oxidation pathway between Au NPs supported on CeO2(111) and CeO2(100). The results are discussed below. 8 ACS Paragon Plus Environment

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3.2. CO oxidation activated by CO-saturated Au NPs supported on CeO2(111) and CeO2(100) In our previous report on the mechanism of CO oxidation by Au13 NP supported on stoichiometric-CeO2(111) or reduced-CeO2(111), we studied and suggested three available CO oxidation pathways: oxidation of Au-CO* by O2 coadsorbed onto an Au NP with CO, oxidation of Au-CO* at the Au-CeO2 interface by the M-vK mechanism, and oxidation of Au-CO* by O2 bound at the Au-Ce3+ interfacial area.31 Among them, the M-vK mechanism of CO oxidation, oxidation of Au-CO* by lattice oxygen of CeO2(111) did not occur because of the high energy required for the CO2 desorption step, Edes.31 However, in subsequent studies, we found that some dopants that lower the vacancy formation energy, Evac, of CeO2(111), activate the M-vK mechanism of CO oxidation at the AuCeO2 interface.29 Moreover, the M-vK mechanism becomes the major CO oxidation mechanism and is catalyzed by a Au12 NP supported on the step-edge of CeO2(111).30 Considering that such structurally less-stable surface motifs like a step-edge represent the edge or vertex sites of large ceria NPs, we speculate that the M-vK mechanism of CO oxidation contributes a large portion of the experimentally observed CO oxidation activity of ceria-supported Au NPs.47-49 In this study, our structural model, an Au9 NP supported on CeO2(111) or on CeO2(100), reproduces an ideal model used in our previous study, meaning that our model could not possibly activate the oxidation of Au-CO* by the M-vK mechanism with the first-bound CO molecule.31 However, some changes occur as the surfaces of Au NPs become saturated with CO molecules. The formation of a bent O-C-O-type reaction intermediate from an Au-CO* and a lattice oxygen of CeO2 at the Au-CeO2 interface is the first step of the M-vK-type CO oxidation (refer 9 ACS Paragon Plus Environment


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to Figure 1a for detailed local geometry).30 On the basis of conventional computational studies, this step is found to be unusually exothermic.29-31 For Au NPs supported on the step-edge of CeO2(111), we found that the first formation of an O-C-O-type intermediate is exothermic (∆E = -0.82 eV) and requires 0.11 eV of Eb.30 These values were calculated with respect to the firstbound CO, the most strongly interacting molecule. Figure 1a shows the overall and detailed morphology of Au9/CeO2(100) saturated with 8 CO molecules. The most interesting feature of (Au9-8CO*)/CeO2(100) is that the O-C-O-type reaction intermediate was spontaneously formed at the Au-CeO2 interface upon the saturation of the Au9 NP. The right-side panel of Figure 1a shows that two O-C-O intermediates were formed at the Au-CeO2 interface (highlighted with dotted circles). Despite the formation of these intermediates, we attempted to find a stable state of an Au-CO* and a corresponding Eb of O-C-O formation. All the initial structures used in the trial spontaneously transformed into an O-C-Otype intermediate without an Eb. To form a O-C-O intermediate (Au-CO* + lattice oxygen → OC-O), the Au-bound CO molecule should be rotated toward the lattice oxygen of CeO2. The Eb usually arises from the early stage of rotation.29-30 As previously discussed, the Au9 NP on CeO2(100) was structurally locally disordered upon sequential CO adsorption. We believe that such adsorption-induced local structural fluxionality of the Au9 NP enabled the subsequent binding of CO molecules that spontaneously rotate to the lattice oxygen atoms and form O-C-O intermediates. The complete CO oxidation pathway constructed by the spontaneously formed O-C-O intermediate shows that the first CO oxidation occurs through a single step: CO2 desorption, which requires only 0.40 eV of Edes (S1 of Figure 2a). Given the negative entropic contribution to the Gibbs free energy of CO2 production (-0.64 eV at 300 K), the rate of the first CO2 10 ACS Paragon Plus Environment

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production will be high. Naturally, as a conventional case of M-vK-type CO oxidation, an oxygen vacancy was formed upon CO2 production (S1, Figure 2a). This vacancy was healed by an O2 molecule with Ebind of -1.17 eV (S2, Figure 2a). Under CO oxidation conditions, the empty Au atom that lost CO would again bind a CO molecule with Ebind of -1.39 eV, directly forming an O-C-O-type intermediate (S3, Figure 2a). The next CO2 desorption is spontaneous, with no barrier (confirmed by additional test calculations) and high exothermicity because of the energy acquired upon CO2 production and vacancy healing. The initial CO-saturated structure of Au9/CeO2(100) was later recovered by binding another 8th CO molecule (S5, Figure 2a). For catalytic CO oxidation by the M-vK mechanism which occurs at the Au-oxide interface, the vacancy formation energy, Evac, of oxide supports has been recognized as a reaction interpreter.30 However, in the case of DFT catalysis studies performed with the first-bound CO molecule, the Eb of O-C-O formation also accounts for the rate of CO oxidation, together with the Evac, which concerns the Edes of CO2.29-31 For our Au/CeO2(100) system, in which a O-C-O intermediate was spontaneously formed upon CO saturation, the Evac would have stronger description power for overall CO oxidation activity. We also constructed another CO oxidation pathway with the 7th-bound CO molecule, which also spontaneously formed an O-C-O intermediate. However, we observed a certain energy barrier (Eb = 0.52, TS1, Figure 2b) upon desorption of CO2 from the O-C-O intermediate. This barrier, which was not observed in Figure 2a, presumably originated from the local structural nature of the O-C-O intermediate used in Figure 2b (see the orange-dotted-circle highlighted local structure in the right-side panel in Figure 1a), indicating that the lattice oxygen needed for the formation of an O-C-O intermediate also participated in the Au-CeO2 interaction. We speculate that the Eb of 0.52 eV was required for migration of the underlying lattice O atom. 11 ACS Paragon Plus Environment


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By contrast, the O-C-O formation from an Au-CO* was not spontaneous on Au/CeO2(111), where the Au NP was not fully saturated with CO molecules (see Figure 2c). The last-bound CO molecule was sufficiently stable on an Au atom for the transformation of a AuCO* to an O-C-O intermediate to require a high Eb of 0.86 eV and for CO2 production to require an Edes of 0.45 eV (S0 to S2, Figure 2c). However, once an O-C-O intermediate was formed at the Au-CeO2 interface, the subsequent CO2 production, vacancy healing, and the second oxidation of CO are all exothermic and barrier-free, identical to the case of Au/CeO2(100). The relatively easier O-C-O formation on Au/CeO2(100) introduces a critical difference in CO oxidation activity between Au NPs supported on CeO2(100) and CeO2(111).

3.3. Microkinetic modeling of CO oxidation by Au NPs supported on CeO2(100) and CeO2(111) To quantitatively study the catalytic activity of Au NPs supported on CeO2(100) and CeO2(111), we performed microkinetic modeling to estimate the rate of catalytic CO oxidation as a function of CO partial pressure, p(CO), and temperature (refer to supporting information for details). In general, the entropic contribution of the Gibbs free energy of adsorption, –T∆S, was estimated from the standard entropy of a gas-phase molecule under the assumption that adsorbed molecules completely lost their entropy upon adsorption. In this case, the –T∆S negatively (qualitatively) affected the adsorption of a gas-phase molecule. However, recent experimental findings of Campbell and coworkers showed that a large portion of the gas-phase entropy of binding molecules is conserved even after adsorption.50 They have suggested the following linear relationship between the entropies of a gas-phase molecule and an adsorbed molecule: 12 ACS Paragon Plus Environment

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S0adsorbed-molecule = 0.7 S0gas phase-molecule - 3.3 R.50 In this case, the effect of the –T∆S on the Gibbs free energy of adsorption is limited to 30% of the conventional estimation. We note that Campbell and coworkers deduced the aforementioned relationship on the basis of oxide materials, not metal NPs.50 However, because their relation was found over various oxide materials and adsorbates,50 we applied the relation and constructed two types of rate maps for each system in Figure 3; these rate maps were estimated with different contributions of entropy change upon CO adsorption and CO2 desorption. Because the presence of the 8th- and 4th-bound molecules is mandatory for activation of subsequent CO oxidation by Au/CeO2(100) and Au/CeO2(111) catalysts, respectively, we took the average Ebind of a total of 8 or 4 CO molecules on each catalyst and calculated the surface concentration (physically equal to the possibility that all 8 or 4 CO molecules are stably adsorbed onto Au NPs simultaneously) and calculated the rate of CO oxidation. The rate maps presented as a function of temperature and p(CO) show that the geometrical difference of the CO oxidation pathways catalyzed by Au/CeO2(100) and Au/CeO2(111) (Figure 3) indicates a higher CO oxidation rate of Au/CeO2(100) (Figure 3a, 3b) than Au/CeO2(111) (Figure 3c, 3d) over general practical operating conditions of CO oxidation (300 ≤ T ≤ 600 K and 0.00 ≤ p(CO) ≤ 0.50 bar). We also found that Au/CeO2(100) is highly reactive for CO oxidation at temperatures below room temperature (Figure S3). In fact, the inclusion of the –T∆S term in the Gibbs free energy change alters the morphology of the rate map. Because the –T∆S lowers the Ebind of CO molecules on Au NPs, a less entropic change upon adsorption leads to the more stable adsorption of CO molecules onto the Au NPs. Therefore, in the rate maps constructed considering the findings of Campbell and coworkers50 (Figure 3b, 3d), the effect of p(CO) and temperature on the adsorption of CO 13 ACS Paragon Plus Environment


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molecules is dissipated and the parabolic rate map (Figure 3a, 3c) was transformed to a linear map (Figure 3b, 3d). Moreover, for Au/CeO2(100), in which the –T∆S term accelerated the CO2 desorption, the rate of CO oxidation decreases upon the application of Campbell’s findings50 (less entropic contribution to the Gibbs free energy change upon CO2 desorption, Figure 3b). However, for Au/CeO2(111), where the –T∆S term lowers the binding of CO molecules on the Au NPs, the lower penalty from Campbell’s findings50 increases the surface concentration of CO molecules and indicates a higher rate under the same temperature and p(CO) conditions (Figure 3d).

3.4. General Discussion The overall total rate of CO oxidation by experimentally synthesized CeO2-supported Au NPs would be an integrated function of Au NPs supported on various structural moieties of CeO2 supports, which are usually also nanosized. The estimated rate of CO oxidation represents the rate of a single reaction site so that the overall rate should be re-estimated by considering the surface fraction of each reaction site. The specific and mass rates of CO oxidation would decrease with increasing size of Au NPs. In the case of Au/CeO2(100), where the structural fluxionality of the Au NP is attributed to the formation of a specific active site, the Au size effect would be more prominent. The formation of O-C-O intermediates may be easier in smaller Au NPs, which are generally structurally more flexible than larger Au NPs. In sub-nanometer-sized Au clusters, the activity of some reactive sites at the Au-CeO2 interface may be indicative of the overall activity of the Au clusters. However, as the size of the Au NPs increases, the absolute site fraction of the Au-CeO2 interface, which can spontaneously produce the O-C-O intermediate under CO 14 ACS Paragon Plus Environment

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oxidation conditions, will decrease. We believe that the size of Au NPs supported on CeO2 is closely related to the contribution of the proposed reaction pathway to total CO oxidation rate. However, the reaction pathway of the CO oxidation reaction occurring at the interface between Au NPs and CeO2, which is covered with CO in an actual CO oxidation reaction environment, may not be significantly different from that shown in Figure 2. Surface defects of CeO2(111) promotes activation at the Au-CeO2 interface and provide a rapid CO oxidation pathway. In this study, our primary goal was to bring the effect of environmental factors into the framework of the computational catalysis study. Therefore, to simplify the model system, we have not considered the effects of defects at this stage. We are now turning our attention to a more realistic model system with defects and vacancies. The results will be reported in due course. Although Pt NPs supported on oxides are prone to CO-poisoning, they usually actively catalyze CO oxidation. In our preliminary study, we also found that the O-C-O intermediate can be formed spontaneously at the Pt-CeO2 interface upon multiple CO adsorption on the Pt9 NP of Pt/CeO2(100). We believe that CO oxidation proceeds through the M-vK mechanism in both Au and Pt NPs supported on CeO2.

4. Summary Considering the presence of pre-adsorbed surface CO molecules on the Au NPs of ceriasupported Au NPs, we constructed a more reliable structural starting point for DFT-based CO oxidation studies. The catalytic pathway of CO oxidation and microkinetic modeling concurrently confirmed that such consideration of the surface coverage of reactant gives rise to a totally different interpretation of the catalytic performance of ceria-supported Au NPs. Of 15 ACS Paragon Plus Environment


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particular interest is the spontaneous formation of a reaction intermediate on the Au/CeO2(100) system that dramatically simplifies the reaction pathway and accelerates the reaction. Our DFT studies clearly demonstrate how the NPs supported on CeO2(100) are superior to those on CeO2(111). The degree of structural fluxionality of the Au9 NPs on CeO2(100) and CeO2(111) directly affects the surface coverage of CO on the Au NPs under CO-rich conditions. The more structurally flexible Au NP on CeO2(100) can bind more CO molecules and easily activates the later-binding CO molecules, leading to the formation of an O-C-O intermediate. However, our results were based on subnanometer sized Au9 NPs. The size of NPs would also affect the surface CO coverage of NPs, structural fluxionality of NPs, and the nature of NPsupport interactions. Although the catalytic performance of supported metal NPs is an averaged function of cooperatively operating structural and chemical factors, our findings at least demonstrate that environmental factors should be appropriately considered in DFT-based catalysis studies. The catalyst-reactant interaction should be more intensively considered in the design of betterperforming catalysts under reaction conditions.

Supporting Information. Supplementary binding geometries and reaction energetics. This material is available free of charge via the Internet at http://pubs.acs.org.


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Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2017R1A2B4009829). This work was conducted under the framework of the research and development program of the Korea Institute of Energy Research (B7-2431). This research was supported by Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2017M3A7B4042235). This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Computing time was provided by the National Institute of Supercomputing and Network/Korea Institute of Science and Technology Information (KSC-2016-C3-0037).



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Figure 1. Energetics of sequential CO adsorption onto (a) Au/CeO2(100) and (b) Au/CeO2(111). CO Enbind represents the binding energy of the nth CO in the presence of pre-bound (n-1)th CO

molecules. Under CO oxidation conditions, in which a catalyst is exposed to a mixture of CO and O2, we found that all the surface binding sites of Au/CeO2(100) would be saturated with 8 CO molecules. By contrast, Au/CeO2(111) cannot strongly bind more than 4 CO molecules. The inset in (a) shows the detailed local geometry of the last-bound CO at the Au-CeO2 interface. An O-C-O type intermediate was spontaneously formed at the interface.



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Figure 2. Complete CO oxidation pathways catalyzed by Au/CeO2(100) (a, b) and Au/CeO2(111) (c). All CO oxidation reactions occur at the Au-CeO2 interface by the late-bound CO molecule (8th and 7th bound CO molecules for (a) and (b), respectively, and the 4th-bound CO for (c)) via the Mars-van Krevelen mechanism. The more energetically accessible reaction pathway catalyzed by Au/CeO2(100), (a), does not require a high activation energy, and the first 26 ACS Paragon Plus Environment

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desorption of CO2 becomes the rate-determining step; by contrast, an 0.88 eV activation energy barrier, which originated from the formation of an O-C-O-type intermediate, was required for CO oxidation by Au/CeO2(111). The energetically less accessible CO oxidation pathway by the 7th-bound CO molecule at the Au-CeO2(100) interface, (b), requires 0.52 eV of activation energy for the first CO2 desorption (TS1). ∆En represents the reaction energy calculated by comparing the absolute energies of stage x and stage n - 1.



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Figure 3. Rate maps of CO oxidation as a function of reaction temperature and CO partial pressure, p(CO). (a, b) Au/CeO2(100) and (c, d) Au/CeO2(111). (a) and (c) were constructed under the assumption that the gas-phase entropy of CO was completely lost upon adsorption. (b) and (d) were constructed by considering the experimental finding of Campbell and coworkers that a large portion of the gas-phase entropy was conserved upon adsorption (see ref. 48). These rate maps show the per-site activity of the reaction sites at the Au-CeO2 interface, which limitedly presents under CO oxidation conditions. Because the atomic or areal fraction of the reaction site was not considered, the real activity of the total catalyst system would be many orders of magnitude lower than the values in the rate maps.


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TOC Graphic


Catalytic CO Oxidation by CO-Saturated Au Nanoparticles Supported

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