Catalytic CO oxidation over Au nanoparticles supported on CeO2

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Catalytic CO oxidation over Au nanoparticles supported on CeO nanocrystals: Effect of the Au-CeO interface 2

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Hyunwoo Ha, Sinmyung Yoon, Kwangjin An, and Hyun You Kim ACS Catal., Just Accepted Manuscript • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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Catalytic CO oxidation over Au nanoparticles supported on CeO2 nanocrystals: Effect of the Au-CeO2 interface Hyunwoo Ha, ‡,∥ Sinmyung Yoon,†,∥ Kwangjin An,†,* and Hyun You Kim, ‡,*

†

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology

(UNIST), Ulsan 44919, Republic of Korea ‡

Department of Materials Science and Engineering, Chungnam National University, 99 Daehak-ro,

Yuseong-gu, Daejeon 34134, Republic of Korea *To whom correspondence should be addressed (E-mail: [email protected] and [email protected])

RECEIVED DATE

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ABSTRACT Gold nanoparticles (NPs) have attracted attention due to their superior catalytic performance in CO oxidation at low temperatures. Along with the size and shape of Au NPs, the catalytic function of Au-catalyzed CO oxidation can be further optimized by controlling the physicochemical properties of oxide-supporting materials. We applied a combinatorial approach of experimental analyses and theoretical interpretations to study the effect of a surface structure of supporting oxides and the corresponding CO oxidation activity of supported Au NPs. We synthesized Au NPs (average d ≈ 3 nm) supported on shape-controlled CeO2 nanocrystals, Au/CeO2 cubes and Au/CeO2 octahedra for experimental analyses. The catalysts were modeled as Au/CeO2(100) and Au/CeO2(111) via density functional theory (DFT) calculations. The DFT calculations showed that the O-C-O type reaction intermediate could be spontaneously formed at the Au-CeO2(100) interface upon sequential multi-CO adsorption, accelerating CO oxidation via the Mars-van Krevelen mechanism. The additional kinetic process required for O-C-O formation at the Au-CeO2(111) interface slowed down the reaction. The experimental turnover frequency (TOF) of the Au/CeO2 cubes was 4 times greater than that of the Au/CeO2 octahedra (under 0.05 bar CO and 0.13 bar O2). The increasing TOF as a function of CO partial pressure and the positive correlation between the reducibility of CeO2 and the catalytic activity of Au/CeO2 catalysts confirmed the theoretical prediction that CO molecules occupy the surface of Au NPs and that the oxidation of Au-bound CO occurs at the Au-CeO2 interface. Through a comparative study of DFT calculations and in-depth experimental analyses, we provide insights into the catalytic function of CeO2-supported Au NPs towards CO oxidation depending on the shape of CeO2 and ratio of CO/O2.

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KEYWORDS Au, CeO2, nanoparticles, CO oxidation, Mars-van Krevelen mechanism

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1. Introduction Catalytic carbon monoxide (CO) oxidation has been studied for several decades as a probe reaction in heterogeneous catalysis.1-6 The CO oxidation reaction is of practical importance for pollution control purposes in many industrial processes, including CO removal from exhaust gases and preferential CO oxidation for fuel cell devices. Tremendous research on CO oxidation has been performed using various metal nanoparticles (NPs), including Pt, Pd, and Au, to investigate the size-dependent activity and interfacial effect between the metal and the support.7-10 Historically, gold (Au) has been regarded as an inert metal for catalysis. However, since Haruta et al. reported the high activity of oxide-supported Au NPs for CO oxidation at low temperatures,11-14 supported Au catalysts have been applied to various catalytic reactions, including oxidation, hydrogenation, and water-gas shift reaction.15-17 Strong metalsupport interactions also cause dramatic changes in the catalytic performance of supported metal NPs.18-21 CeO2 has been widely used as an excellent supporting oxide due to its redox behavior and oxygen storage capacity.22-24 In particular, the easy formation of oxygen vacancies in CeO2 and reversible reduction of Ce4+ ions to Ce3+ affect the binding energy and the site of reacting molecules, as well as the activation energy barrier, leading to significant changes in the reaction kinetics and mechanism. Trovarelli et al. reported that the surface atomic arrangements of CeO2 influenced the ratio of Ce3+/Ce4+ ions, changing the oxygen storage capacity of CeO2.25 The reversible change in the oxidation state of CeO2 depending on the reaction environment creates interesting catalytic effects when CeO2 is combined with Au nanoparticles.26-27 Many studies have described the great enhancement in the CO oxidation rate due to the interfacial interactions between Au and CeO2, such as charge transfer under diverse redox conditions.23-31 Several previous studies have reported that the morphology of CeO2 affects the catalytic activity of Au/CeO2 catalysts.28-31 For example, Si et al. prepared CeO2 4


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NPs with controlled shapes that were terminated by either the {100} and {110} surface or both.29 The resulting Au NPs supported CeO2 nanorods ({100} and {110}), nanocubes ({100}), and nanopolyhedra ({100} and {111}), and they induced different activities of the water-gas shift reaction.29 Because the chemical nature of CeO2 can vary with surface termination, these studies predicted that the catalytic activity of supported metal NPs could be tuned by controlling the physicochemical properties of the CeO2 support. Heterogeneous catalysts usually operate under dynamic conditions. Catalysts for hightemperature reactions suffer from physical deactivation due to Ostwald ripening or the migration and coalescence of metal NPs.32-33 Strong chemical interactions between the reacting molecules and NPs sometimes reorganize the catalyst surface and even the surface concentration of bimetallic NPs.34 Over the last decade, computational methods have been recognized as a novel tool for designing better catalysts.35 Density functional theory (DFT) calculations combined with experimental analysis provide insights into the reaction mechanism and help to extract key design factors.36-37 Several recent studies on the identification of environmental factors in the framework of computational catalysts have provided interesting structural and chemical information about heterogeneous catalysts under real operation conditions.38-42 For example, the dynamic formation of single-atom active sites during CO oxidation over CeO2-supported Au NPs38, 42 or CO-induced surface segregation of Pd in AuPd bimetallic alloys,40 and NPs39 supply theoretical interpretations of the behavior of heterogeneous catalysts under reaction conditions. We have consistently studied the catalytic activity of Au/CeO2 catalysts toward CO oxidation using DFT calculations in several theoretical model systems.28,

41, 43-45

Using

subnano-to-nano-sized model systems: unsupported Aun clusters (n = 1–10, 13, 19, 20, 25, 38, and 55)45 and Au9~13 clusters supported on stoichiometric (STO)-CeO2(111),28,41 vacated5


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CeO2(111),28 doped-CeO2(111),44 stepped-CeO2(111),43 or on STO-CeO2(100),41 we have suggested the important role of the Au-CeO2 interface for activation of CO oxidation, which is consistent with recent experimental findings. Especially, in our most recent report, we performed DFT calculations to study CO oxidation rates and corresponding mechanisms on Au9 NPs supported by either CeO2 (100) or CeO2(111).41 Taking a step forward from conventional DFT-based catalysis studies, in which single CO molecules are considered to explore the reaction pathway and energetics, we calculated the sequential competing binding energy of CO and O2 on a Au9 NP and modeled the CO-saturated NP for CO oxidation studies.41 Based on the adsorption and corresponding CO oxidation modeled on CO-saturated Au/CeO2, we found that the more structurally flexible Au/CeO2(100) support binds more CO molecules and easily activates O‒C‒O intermediates, accelerating the overall CO oxidation. The reaction pathways were explained by calculating the O-C-O formation energy via the Mars-van Krevelen (M-vK) mechanism, in which an oxygen at the Au-CeO2 interface oxidized the Aubound CO, Au-CO*. Herein, we apply a combinatorial approach of theory and experiments to provide a complete mechanistic understanding of CO oxidation catalyzed by Au/CeO2. Based on our theoretical understanding on the catalytic function of the Au-CeO2 interface, we combine stateof-the-art experimental synthesis and analysis techniques, to confirm the essential role of the Au-CeO2 interface for CO oxidation. From DFT calculations performed with CO-saturated Au NP models, we derived rate maps of CO oxidation for Au/CeO2(100) and Au/CeO2(111) catalysts as a function of temperature and CO partial pressure, p(CO). The rate maps were reproduced by experimental studies carried out over 3-nm-sized Au NPs deposited on cubic or octahedral CeO2 nanocrystals in a batch mode reaction. We controlled the morphology of CeO2 by using either a cube with (100) facets or an octahedra with (111) facets via modified 6


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hydrothermal synthesis.46-48 While Au NPs showed negligibly low activity for CO oxidation, the turnover frequency (TOF) of the Au/CeO2 cubes was much higher than the Au/CeO2 octahedra, which was in good agreement with the DFT results. By conducting the CO oxidation reaction under different reaction environments by controlling the reaction temperature and the ratio of CO and O2, we further verified the theoretical interpretations using a real catalytic reaction over well-defined Au/CeO2 catalysts.

2. Methods 2.1 Materials Cerium nitrate hexahydrate (Ce(NO3)·6H2O, 99.99%), oleylamine (technical grade, 70%), and sodium phosphate tribasic dodecahydrate (Na3PO4∙12H2O, 98%) were purchased from Sigma-Aldrich. Hydrogen tetrachloroaurate hydrate (HAuCl4·xH2O, 99.8%) was obtained from Strem Chemical. Sodium hydroxide (NaOH, pellets, 98%), ethylene glycol, and ethanol were purchased from Samchun Chemical Co. All chemicals were used as received without any further purification.

2.2 Preparation of CeO2 nanocrystals CeO2 cubes were synthesized according to the reported hydrothermal method.46-47 The 9.6 g of NaOH dissolved in 40 mL of deionized (DI) water was placed in a Teflon cup, followed by missing with 0.868 g of Ce(NO3)3∙6H2O under vigorous stirring for 30 min to form a white slurry. The Teflon cup was placed inside a stainless steel autoclave reactor and kept in an oven at 453 K for 24 h. After the reaction, the solid product was separated by centrifugation with excess deionized water and washed with ethanol. The CeO2 octahedra were also synthesized using the hydrothermal method with some modification.48 The 0.0038 g of Na3PO4‧12H2O 7


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dissolved in 40 mL of DI water was load in the Teflon cup, followed by mixing with 0.434 g of Ce(NO3)3∙6H2O by stirring for 30 min. The hydrothermal reaction occurred in an oven at 473 K for 20 h. The precipitates were separated by centrifugation with DI water and washed with ethanol. The resulting CeO2 powders were dried at 353 K for 8 h and calcined at 673 K for 4 h at a heating rate of 5 K/min.

2.3 Preparation of Au/CeO2 catalysts The microwave-assisted reduction method was used to accelerate the homogeneous nucleation and deposition of Au NPs on CeO2.49 Typically, 40 mg of CeO2 powder with controlled shapes was dissolved in 40 ml of ethylene glycol, and the mixture solution was sonicated for 1 h for complete mixing. After adding 1 mg of HAuCl4·xH2O and 60 µL of oleylamine to the CeO2 solution, the mixture was transferred to the Teflon vessel and placed in the microwave reactor (MARS, CEM Corporation). The reaction temperature was monitored using an optic probe type thermometer during the microwave-assisted reaction. At 800 W of the power (2.45 GHz) for 30 s, reduction occurred by producing Au/CeO2 catalysts. The resulting catalysts were obtained by centrifugation and re-dispersed in ethanol.

2.4 Experimental characterization Transmission electron microscopy (TEM) was performed using a JEM-1400 instrument and a JEM-2100F (JEOL) instrument with acceleration voltages of 120 kV and 200 kV, respectively. Energy-dispersive X-ray spectroscopy (EDS) was used for elemental analysis (Oxford instrument, X-Max 80T). Inductively coupled plasma-optical emission spectrometry (ICP-OES) using a 700-ES model instrument (Varian) was used to determine the amount of Au. Before ICP-OES measurements, the concentration of Au was calibrated using a standard Au 8


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solution (Sigma-Aldrich), and the catalyst was dissolved in aqua regia and diluted with water. X-ray photoelectron spectroscopy (XPS) analysis was conducted using K-alpha radiation; an Escalab 250Xi instrument (ThermoFisher) with an Al Kα X-ray radiation source. The spectra were obtained under 3 × 10-8 mbar of pressure with a turbo molecular pump. A flood gun was used to compensate for the charging effect. X-ray diffraction (XRD) patterns were collected by a powder X-ray diffractometer PANalytical X’Pert Pro instrument (Philips) using a Cu Kα Xray radiation source operating at 40 kV and 30 mA. N2 adsorption experiments were conducted using a BELSORP-max model to measure the BET surface area. Before analysis, the catalysts were pretreated in a vacuum at 423 K for 12 h. Temperature-programmed reduction (TPR) was performed with a thermal conductivity detector (TCD) connected to a gas chromatograph (GC, Agilent 7820A) with a mixed flow of H2/N2.

2.5 Catalytic CO oxidation activity of Au/CeO2 Catalytic CO oxidation was carried out in a batch reactor. Thin films of the Au/CeO2 catalysts were prepared by drop-casting onto a silicon wafer and treatment with a UV lamp to remove surfactants. Two mercury (Hg) lamps emitting photons of 184 and 254 nm with intensities of 8.68 and 5.58 eV, respectively were used to irradiate the NPs for 1 h. UV treatment successfully removed only the organic molecules, such as oleylamine, without damaging the NPs, and the organic surfactants were photothermally decomposed by UV light at 184 and 254 nm while open to atmosphere by generating an organic-free surface on the NPs.9 The batch reactor was maintained under a vacuum up to 1 × 10-8 Torr (1 × 10-11 bar) using rotary and turbo pumps. The gases were introduced into the reactor containing 0.05 bar (40 Torr) of CO and 0.13 bar (100 Torr) of O2 balanced with He (0.83 bar, 620 Torr). All gases were circulated using a circulating pump at a rate of 5.5 L/min, and equilibrium was established after 30 min 9


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of recirculation. The reactants and products were detected by GC (YL-6500) equipped with TCD with a Carboxen 1000 column (Supelco). The reaction was conducted at 473–513 K, and the CO conversion was determined by the converted amount of CO to CO2 per unit catalyst weight determined by ICP-OES. The reaction rate was maintained below 20%, which corresponded to the initial reaction rate in a kinetically controlled regime.50 To obtain a rate map according to the CO partial pressure and temperature, the CO partial pressure was adjusted from 0.05 to 0.20 bar, and the reaction temperature was varied from 400 to 600 K using a boron-nitride heater. To calculate a turnover frequencies (TOF), the concentration of the Au NPs was measured by ICP-OES, and the surface area of Au was calculated according to the average diameter of Au NPs (3 nm) in the experiment. The TOF was determined by the number of moles of CO2 converted per gram of catalyst. The CO2 partial pressure was calculated as the integral of the areas indicated by GC and considering the reaction temperature and the volume (1 L) inside the reactor.

2.6 Density functional theory calculations We constructed a consistent model of Au NPs supported on CeO2(111) and CeO2(100) surfaces using a two-layered Au9 NP and defect-free CeO2(111) 5×5×2 and CeO2(100) 3×3×2 slab models.40 Based on Jenkins and coworkers’ DFT calculations on a stable structure of Au clusters on CeO2(111),51 we used a two-layered Au9 NP, and its overall morphology was the same on CeO2(100) and CeO2(111) during CO oxidation. The bottom trilayer of the ceria was fixed during the geometric optimization. Details of the Au/CeO2 model construction and reliability test results are available elsewhere.40, 43-44 The G of sequential CO adsorption on the Au9 NP was calculated as follows: ΔG = E n∙CO+Au9 /CeO2

E Au9 /CeO2 10


n E CO +ΔμCO

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where E(system) and n are the DFT-estimated total energy of a corresponding system and the number of adsorbed CO molecules, respectively. The chemical potential of CO and ΔμCO , the chemical potential difference, is given by μCO T, p = ΔH 0K, p0 →T, p0 ΔμCO

TΔS 0K, p0 →T, p0 + kT ln

μCO T, p

where p0 is set to 1.013 bar and μCO 0K, p0

p

p0

E CO E(CO) . Tabulated temperature-dependent

enthalpy and entropy values of CO were adopted from the NIST chemistry web-book52 and NIST-JANAF thermochemical tables.53 Microkinetic modeling was performed to construct a rate map of CO oxidation from the DFT calculation results. We estimated the entropic contribution to the G of CO adsorption by the experimentally verified Campbell’s model.54 The following linear relationship between the entropy of a gas-phase molecule and an adsorbed molecule was applied: S0adsorbed molecule = 0.7 S0gas‐phase molecule

3.3 R . Details regarding the microkinetic modeling

are available elsewhere41 and also summarized in supporting information. We performed spin-polarized DFT calculations with the Vienna Ab-initio Simulation Package (VASP)55 and the PW91 functional.56 To treat the highly localized Ce 4f orbital, DFT+U57 with Ueff = 4.5 eV was applied.58-59 The interaction between the ionic cores and the valence electrons was described using the projector-augmented wave method.60 Valence electron functions were extended with the plane-wave basis to an energy cutoff of 400 eV. The Brillouin zone was sampled at the -point. The convergence criteria for the electronic structure and 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 the convergence of states near the Fermi level. The location and energy of transition states (TSs) were calculated with the 11


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climbing-image nudged elastic band method.61-62

3. Results and discussion

3.1 DFT-studied CO oxidation pathway on Au/CeO2(100) and Au/CeO2(111) Figure 1a and 1b show the G of n·CO adsorption on Au/CeO2(100) and Au/CeO2(111) plotted as a function of ΔμCO , which itself is a function of temperature and p(CO). This is a conceptual phase diagram of Au/CeO2 catalysts under CO oxidation conditions. The available range of ΔμCO at 400, 500, and 600 K (0.05 ≤ p(CO) ≤ 0.20 bar, set to be consistent with the experimental conditions) is denoted under the ΔμCO axis. The binding energy, Ebind, of O2 on both Au/CeO2 catalysts was always weaker than Ebind of CO in all ranges of CO coverage, meaning that the surface of the Au NPs would be covered with CO under CO oxidation conditions. We found that the 8CO-Au/CeO2(100) and 4CO-Au/CeO2(111) models represented the majority phases under CO oxidation conditions. As discussed in our previous report, the relatively weaker Au-Au bond of the Au9/CeO2(100), resulting from the stronger Au-CeO2(100) than Au-CeO2(111) interaction, strengthened the Au-CO interaction of Au/CeO2(100), leading to higher CO coverage of Au/CeO2(100).41 Further CO oxidation pathways were estimated based on these CO-saturated Au/CeO2 models. Because the Au NPs were covered with CO molecules, CO oxidation occurred at the Au-CeO2 interface via the M-vK mechanism (Figure 2a and 2b). We attempted to determine the available binding sites for O2 molecules near the Au-CeO2 interface. However, Ebind of O2 was consistently positive. Several theoretical studies, including our previous study, have reported the energetically favorable adsorption of O2 at the interface of Au and TiO2 or CeO2.28, 63-64

Under-coordinated Ti ions or reduced Ce3+ ions formed upon oxygen vacancy formation 12


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might anchor O2 at Au-oxide interfacial sites and utilize for oxidation of Au-bound CO, AuCO*. However, we found no available case of O2 chemical adsorption at the Au-CeO2 interfaces of our catalyst models. Very recently, Schlexer et al. reported that TiO2-supported Au NPs are positively charged, thus stabilizing an oxygen vacancy by acting as an electron reservoir.65 We also found that our Au9 NPs on CeO2(100) and CeO2(111) were positively charged and that the released electrons were localized on adjacent Ce ions (Figure 3). However, although the Au9 NPs of Au9/CeO2(100) had a greater positive charge than that of Au9/CeO2(111) (Figure 3), we also found that the newly formed Au-Ce3+ interface in Au9/CeO2(100) cannot strongly bind O2 (as mentioned above, Ebind was positive). Presumably, the strong electronic interaction between the Au NPs and CeO2 is not sufficient for facile adsorption and activation of O2 at the Au-CeO2 interface. In their early study of O2 activation by the unsupported Au12 cluster, Nørskov and coworkers found that the low-coordinated Au corner site binds and activates an O2 molecule.66 In addition, we recognized that the O2 binding site (Au-Ce3+) found on the Au13/CeO2(111) system reported in our previous study also involves a low-coordinated Au site.28 Presumably, the O2 adsorption and activation at the Au-CeO2 interface is limited to the specific sites that provide both the reduced Ce3+ ion and the low-coordinated Au. Another interesting theoretical and experimental finding of Li and coworkers showed that the surface of CeO2-supported Au NPs is covered with CO molecules under CO oxidation conditions.42 Based on DFT calculations and environmental TEM analyses, they also confirmed that CO molecules rather than O2 molecules occupy the Au sites at the Au-CeO2 interface.42 Their interpretations and experimental findings are consistent with our previous theoretical findings. We also experimentally observed a positive correlation between the p(CO) and TOF of Au/CeO2 catalysts, presenting a dominant effect of CO on the activation of CO oxidation (discussed below). 13


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Although we excluded the initial O2 binding and activation at the Au-CeO2 interface in our current study, such O2 activation occurs during CO oxidation by the M-vK mechanism. The second half of the M-vK mechanism of CO oxidation (Figure 2a and 2b) involves O2 activation at a Au-CeO2 interfacial site adjacent to an oxygen vacancy and the Au-CeO2 interface strongly binding an O2 molecule. This step is generally regarded as vacancy healing, although the oxygen atom protruding from the original oxygen vacancy occasionally sits near the Au-Ce3+ interface. The protruding oxygen atom of the O2 that heals the vacancy can oxidize another Au-CO*. This step is usually faster than the first step of the M-vK mechanism of CO oxidation. A 2~3 nm Au NP supported on CeO2 may have closely packed pairs of reaction sites at the Au-CeO2 interface; therefore, adjacent pairs of oxygen vacancies could be formed. Under high p(O2) oxidation conditions, an O2 molecule may heal two adjacent oxygen vacancies at once, as proposed by Schlexer et al.65 The DFT-estimated energetics and pathways of CO oxidation at the Au-CeO2(100) and Au-CeO2(111) interfaces suggested that Au/CeO2(100) is a better catalyst for CO oxidation (Figure 2a and 2b). The original data published in our previous report41 were adopted and reorganized as schematic diagrams, as shown in Figure 2a and 2b. Because the surface of Au NPs of Au/CeO2(100) was covered with 8 CO molecules, the O-C-O type intermediate formed spontaneously at the Au-CeO2(100) interface upon adsorption of the last CO molecule, and only 0.4 eV was required for desorption of the first CO2 molecule. In contrast, an activation energy barrier, Eb, of 0.86 eV was required to convert the last and fourth bound CO molecule on the Au NP of Au/CeO2(111) into the O-C-O type intermediate. Based on the DFT-estimated energetics of CO oxidation by Au/CeO2(100) and Au/CeO2(111), we performed microkinetic modeling and presented the rate maps for both catalysts as a function of p(CO) and temperature (discussed below). 14


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The oxygen vacancy formation energy, Evac, of an oxide support is often used as an energetic descriptor of the activity of M-vK type CO oxidation. Indeed, the low Evac of the stepedge oxygen ions of CeO2(111) or the oxygen ions of doped CeO2(111) facilitates the M-vK type CO oxidation mechanism at the Au-CeO2 interface. Figure 3 presents the electronic accumulation or depletion accompanied by the Au-CeO2 interaction. As discussed above, the relatively stronger electronic interactions between Au and CeO2(100) leads to the reduction of three Ce4+ ions into Ce3+ ions, whereas Ce4+ ions in CeO2(111) were relatively weakly reduced upon Au deposition. As previously discussed by Behm and coworkers,65 positively charged Au NPs on oxide supports can act as electron reservoirs that take up the released electrons upon oxygen vacancy formation.65 The positively charged Au NPs therefore facilitate the formation of oxygen vacancies from the lattice of CeO2. We also found a positive correlation between the presence of reduced Ce ions (positively charged Au NPs) and the Evac values. The Evac values of CeO2(111) (2.63 eV) and CeO2(100) (1.92 eV) decreased to 2.52 eV and 1.77 eV upon deposition of Au NPs on CeO2. Based on our theoretical findings and the previous discussion by Behm and coworkers,65 the electronic interaction between the supported Au NPs and the supporting CeO2 lowers the Evac of CeO2, activating the oxygen ions at the Au-CeO2 interface for CO oxidation. The corresponding experimental findings of the modified Evac upon Au deposition on CeO2 will be discussed below.

3.2 Structural analysis of CeO2 nanocrystals and Au/CeO2 catalysts CeO2 nanocrystals with either a cubic or octahedral structure were synthesized using the hydrothermal method. NaOH and Na3PO4‧12H2O were used as a structure-directing agent, which determined the final structure of CeO2 nanocrystals. Figure 4a and 4c show TEM images of as-synthesized CeO2 cubes and octahedra, respectively, demonstrating the morphological 15


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homogeneity of both CeO2 nanocrystals. The calculated sizes of CeO2 cubes and CeO2 octahedra by the Scherrer equation in the XRD pattern were 52.37 and 75.29 nm, respectively (Figure S1). The specific surface area (SBET) measured by the N2 adsorption was 6.84 and 5.21 m2‧g-1 for the CeO2 cubes and octahedra, respectively (Figure S2). In Figure 3b and 3d, HRTEM images show well-defined CeO2 morphologies and the corresponding atomic lattices. The d-spacings derived from the HR-TEM images were 2.7 and 3.1 Å for the CeO2 cubes and octahedra, consistent with the (100) and (111) orientations, respectively. The fast Fourier transformation (FFT) patterns (insets in Figure 4b and 4d) demonstrated the highly crystalline nature of CeO2. The TEM and HR-TEM images of the Au/CeO2 catalysts, shown in Figure 5a and 5c, show the attachment of 3-nm-sized Au NPs on the surface of CeO2. Although Au NPs were not loaded on the entire surface of CeO2, intimate contacts were created between the Au NPs and CeO2 nanocrystals, as confirmed by the HR-TEM images (Figure 5b and 5d). The observed spacing of 2.4 Å in the HR-TEM images indicated that the Au NPs preferentially had exposed (111) facets. Well-dispersed Au nanoparticles on CeO2 were further confirmed by EDS mapping (Figure S3). The concentration of Au in the Au/CeO2 cubes and Au/CeO2 octahedra determined by ICP-OES was 1.24 and 1.56 wt%, respectively. To confirm whether Au/CeO2 catalysts were changed or not after the UV irradiation for the removal of organic surfactants, XPS and TEM studies were carried out.67-68 The dominant elements of oleylamine were carbon and nitrogen, thus we collected XPS for N1s and C1s after the UV irradiation for 2 h (Figure S4). The C-to-Au and N-to-Au were substantially decreased as a function of the time of UV treatment. It was also confirmed that the treatment time of 1 hour was sufficient to remove oleylamine around the NP. TEM images of Au/CeO2 cubes deposited on a SiN grid after UV treatment for 1 h show that original size and shape of both Au NPs and CeO2 nanocubes were preserved (Figure S5). From these results, it was demonstrated that a significant amount of 16


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oleylamine surfactant was removed and the original morphologies of Au and CeO2 remained after the UV treatment.

3.3 Catalytic performance of Au/CeO2 in CO oxidation Catalytic CO oxidation was carried out over Au/CeO2 catalysts deposited on a silica substrate in a batch-type gas reactor. The reactant gases, CO and O2, were introduced with partial pressures of 0.05 and 0.13 bar, respectively. We found that the CO oxidation activity of unsupported Au NPs was marginally small. The Au NPs were activated on CeO2 nanocrystals, qualitatively indicating the critical role of Au-CeO2 interface formation. Figure 6a shows the TOFs of the Au/CeO2 cubes and Au/CeO2 octahedra for CO oxidation. The overall TOF of the Au/CeO2 cubes was higher than that of the Au/CeO2 octahedra in the temperature range of 473‒513 K. The TOF of the Au/CeO2 cubes was 0.69 s-1 at 513 K, which was 4 times higher than that of the Au/CeO2 octahedra (0.17 s-1). The Arrhenius plots of the Au/CeO2 catalysts are shown in Figure 6b (detailed values are provided in Table S1). The activation energies, Ea, of the Au/CeO2 cubes and the Au/CeO2 octahedra were 26.87 kcal‧mol-1 (1.17 eV) and 31.88 kcal‧mol-1 (1.38 eV), respectively. Although these values were higher than the DFT-estimated Ea values, the trend was well conserved. The catalytic activity of CO oxidation was much higher over the Au/CeO2 cubes than the Au/CeO2 octahedra, indicating that CeO2 (100) facets provided greater facilitation of Au-catalyzed CO oxidation, compared with CeO2 (111) facets. Because the surface oxygen of CeO2 participates in CO oxidation, the change in the oxidation state of Ce4+ to Ce3+ and the corresponding reduction behavior of different CeO2 surfaces are important for understanding the reaction mechanism and promoting effect of CeO2. To measure the oxygen release capacity of CeO2 nanocrystals and Au/CeO2 catalysts, we conducted the H2-TPR experiment. In detail, 40 mg of CeO2 or Au/CeO2 powder was pretreated 17


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at 353 K under Ar flow (50 cm3/min) for 30 min. The 4% H2/N2 gas mixture was introduced as a reference flow at a rate of 50 cm3/min from room temperature to 1173 K (5 K/min). In Figure 7, the characteristic peaks of the CeO2 and Au/CeO2 catalysts are shown in the H2-TPR plot in the temperature range of 293‒1173 K. Two main peaks were observed in the CeO2 cubes and octahedra, with the highest peaks (985 K for cubes and 1000 K for octahedra) originating from the lattice oxygen ions of CeO2 and the peaks in the middle (749 K for cubes and 763 K for octahedra) from the surface oxygen ions.69-70 The observation of peaks for the CeO2 cubes at a temperature 15 K lower than that at which the CeO2 octahedra peaks appeared suggested that the oxygen ions of the CeO2 cubes were released more easily and utilized for CO oxidation. In both Au/CeO2 catalysts, additional peaks were observed at low temperatures (at 434 K for Au/CeO2 cubes and 467 K for Au/CeO2 octahedra) due to splitting of the surface oxygen peak. Because splitting was not observed in the single CeO2 nanocrystals, it was attributed to the AuCeO2 interaction. In particular, the surface oxygen ions at the interface between Au and CeO2, interfacial oxygen ions, the chemical nature of which was most strongly affected by Au NPs, were responsible for the low-temperature peaks. Similar observations have also been reported for the temperature shift of the surface oxygen peak of oxide supports upon NP deposition.7172

Because Au NPs donate electrons to the CeO2 support upon deposition,28, 43 the electron-rich

interfacial oxygen ions can be easily released and utilized for CO oxidation. The interfacial oxygen peak of the Au/CeO2 cubes appeared at a temperature 33 K lower than that at which the peak of the Au/CeO2 octahedra appeared, demonstrating that the reduction occurred more easily in Au/CeO2 cubes. The changes in oxidation states in the CeO2 and Au/CeO2 catalysts were also confirmed by XPS (Figure S6 and S7). While binding energy shifts were clearly observed in the Au 4f spectra for both Au/CeO2 catalysts in XPS after the CO reaction, the peak shift was much greater in the Au/CeO2 cubes than in the Au/CeO2 octahedra. In addition, the 18


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calculated Ce3+ concentrations (Ce3+ / Ce3+ + Ce4+) by peak deconvolution of the Ce 3d spectra were 26.1 and 23.9% for the Au/CeO2 cubes and the Au-CeO2 octahedra, respectively (Figure S7). The relative Ce3+ concentration was higher in the Au/CeO2 cubes than in the Au/CeO2 octahedra, indicating that the concentration of oxygen vacancies in the Au/CeO2 catalysts depended on the morphology of CeO2. Because the empty 4f orbitals of the Ce ions attract and localize extra electrons from Au NPs, the higher concentration of localized hot electrons at the interface of Au-CeO2(100) than at Au-CeO2(111) could increase its activity toward CO oxidation. Previous studies showed that the flow of hot electrons across the metal–oxide interface is correlated with the catalytic activity.73-74 Kim et al. reported that the flow of hot electrons generated on the interface of Au-CeO2 during light irradiation was responsible for the change in the CO oxidation activity and was dependent on the size of the Au NPs.74 Our experimental findings: the appearance of new H2-TPR peaks at low temperature, the relatively low H2-TPR peak temperature observed in the Au/CeO2 cubes compared with the Au/CeO2 octahedra, and the higher Ce3+/Ce4+ ratio in Au/CeO2 cubes, clearly confirm our theoretical interpretation of the correlation between the electronic interactions between Au and CeO2 and the Evac. We are convinced that the interface-mediated M-vK mechanism of CO oxidation occurs at the Au-CeO2 interface.

3.4 Construction of experimental TOF maps and comparison with DFT-estimated rate maps We constructed CO oxidation TOF maps of experimentally synthesized Au/CeO2 catalysts by scanning their TOF over various T-p(CO) combinations (Figure 8a and 8b). The CO partial pressure was varied from 0.05 to 0.20 bar with an interval of 0.05 bar, and the partial pressure of O2 was preserved under 0.21 bar. The reaction temperature was changed from 400 19


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to 600 K, with a temperature gap of 50 K. Moreover, the corresponding rate maps of the Au9/CeO2(100) and Au9/CeO2(111) model catalysts were also estimated based on microkinetic modeling (Figure 8c and 8d). Despite the discrepancies between the experimental TOF and the theoretical rate values originating from the structural gap between experimentally synthesized catalysts and theoretical model catalysts. The overall morphology of the TOF/rate maps consistently presented a higher catalytic activity for Au/CeO2(100) and the Au/CeO2 cubes than for Au/CeO2(111) and the Au/CeO2 octahedra. Several structural factors may be attributed to the overall higher rate values predicted in the theoretical rate maps. We note that the relatively lower Eact predicted in the DFT calculations increased the theoretical rates. Moreover, because an Au9 cluster supported on CeO2(100) or CeO2(111) bound to 8 or 4 CO molecules and arranged them close to the AuCeO2 interfacial area, formation of the O-C-O intermediate and CO2 production were energetically easily accessible. However, in the cases of larger experimentally synthesized Au NPs supported on CeO2 cubes or CeO2 octahedra, the NPs could not bind a sufficient number of CO molecules to saturate the Au-CeO2 interfacial area because of the weaker CO binding energy of the larger Au NPs. As predicted by Green et al.,75 subsequent surface diffusion could be involved in the CO oxidation pathway transporting Au NP-bound CO molecules to the AuCeO2 interfacial area. The concentration of reactive sites at the Au-CeO2 interfacial area was also higher in our model catalyst. We regard that two reaction sites could be produced upon the adsorption of 8 CO molecules on the Au/CeO2(100) catalyst (Figure 2a).41 However, the reaction site/adsorbed CO molecule ratio would be different in experimental Au/CeO2 cube catalysts. If the surface of the Au NPs activates CO oxidation by the Langmuir-Hinshelwood (LH) mechanism, competitive binding of CO and O2 on the Au NPs will occur, and the overall 20


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rate will be highly affected by the relative surface coverage of CO and O2. Because the Au-CO interaction is stronger than the Au-O2 interaction, the overall CO oxidation rate predicted by the L-H mechanism should increase as an inverse function of p(CO), as the binding sites of O2 on Au NPs are secured upon increases in the p(O2)/p(CO) ratio. However, the increasing experimental TOFs of our Au/CeO2 catalysts as a function of p(CO) showed that CO binding was critical for the activation of CO oxidation and that the oxygen for CO oxidation was supplied by the other part of the catalyst rather than the Au surface (Figure 8a and 8b), representing a general feature of the M-vK mechanism of CO oxidation: the supporting oxide supplies the oxygen for CO oxidation. At a higher p(CO), additional surface sites of the Au NPs can be occupied by CO molecules, and thus, further O-C-O formation easily occurs. Because of the strong CO binding and narrower surface area of the small Au9 cluster, the effect of p(CO) was relatively de-emphasized in the theoretical rate maps (Figure 8c and 8d). No rate changes appeared following the increases in p(CO) (Figure 8c and 8d). Our comparative study of experimental TOF and theoretical rate maps provides indepth information regarding the mechanism of CO oxidation catalyzed by Au/CeO2 catalysts. The morphology and trend of the TOF maps reveal the detailed chemical features of Au/CeO2 catalysts under CO oxidation conditions. In contrast to the general understanding that an easily CO-poisoned catalyst may lose its catalytic activity upon increasing p(CO), we find that a high p(CO) can be rather beneficial for activation of the interface-mediated M-vK mechanism of CO oxidation. Presumably, the later-bound CO molecules, which are forced to be adsorbed on the Au NPs due to high p(CO), will exhibit a low Ebind and thus easily undergo oxidation by interfacial oxygen ions at the Au-CeO2 interface.

4. Conclusion 21


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In summary, the catalytic function of Au NPs supported on CeO2 cubes (modeled by Au/CeO2(100)) and CeO2 octahedra (modeled by Au/CeO2(111)) for CO oxidation was studied to improve our understanding of the facet-dependent catalytic promoting power of CeO2. Based on DFT calculations and experimental results, the Au/CeO2 cubes consistently exhibited much higher CO oxidation activity than the Au/CeO2 octahedra. The DFT calculations confirmed the activation of the Au-CeO2 interface-mediated M-vK mechanism of CO oxidation. The H2-TPR measurement results showed that the superior catalytic activity of Au/CeO2 cubes was attributed to the facilitated oxygen release capacity of the Au-CeO2(100) interface, which aided CO oxidation via the M-vK mechanism. Based on experimental analysis (H2-TPS and XPS) and theoretical interpretation of the electronic interactions between Au and CeO2 and its effect on Evac, we found that the electronic interactions between Au and CeO2 could modify the Evac of the Au/CeO2 catalysts. Through both DFT-based microkinetic modeling and experiments, TOF/rate maps were constructed with varying reaction conditions (p(CO) and reaction temperature). We confirmed the superior assisting power of the CeO2(100) surface for activation of CO oxidation at the Au-CeO2 interface. Although the CO preference of Au NPs, caused CO poisoning, a positive correlation between p(CO) and the experimental TOF of CO oxidation was observed in both the Au/CeO2 cubes and the Au/CeO2 octahedra. This finding suggests that the oxygen for CO oxidation is supplied by the Au-CeO2 interface and that the increased CO concentration at the Au-CeO2 interface enhances the CO oxidation activity of Au/CeO2 catalysts. The results of our combinational study of theory and experimentation suggest that the detailed catalytic response of metal/oxide class heterogeneous catalysts examined by scanning the TOF as a function of p(CO), p(O2), and temperature will provide interesting points for understanding their catalytic functions under CO oxidation conditions, which is critical for the rational design of catalysts with superior performance. 22


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Author Information Corresponding Author *E-mail: [email protected] (K.A.) *E-mail: [email protected] (H.Y.K.)

Author Contributions ∥

These authors contributed equally

ORCID Hyunwoo Ha: 0000-0002-3874-8669 Sinmyung Yoon: 0000-0003-2103-8772 Kwangjin An: 0000-0002-5239-0296 Hyun You Kim: 0000-0001-8105-1640

Notes The authors declare no competing financial interest.

Supporting Information Details of microkinetic modeling and additional structural and catalytic analysis results.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2017R1A2B4009829 and NRF-2017R1A4A1015360) and the Basic Science Research Program (2017M1A2A2043138). This work was conducted under the framework of the research and development program of the Korea Institute of Energy 23


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Research (B7-2431). This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, and the Scientific Data and Computing Center, a component of the Computational Science Initiative, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Computing time was also provided by the National Institute of Supercomputing and Network/Korea Institute of Science and Technology Information (KSC-2017-C3-0009).

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Assisted Reduction: Rapid Synthesis of High-Activity Supported Catalysts. ACS Nano 2011, 5, 8049-8061. (50) Koo, J. H.; Lee, S. W.; Park, J. Y.; Lee, I. S., Nanospace-Confined High-Temperature Solid-State Reactions: Versatile Synthetic Route for High-Diversity Pool of Catalytic Nanocrystals. Chem. Mater. 2017, 29, 9463-9471. (51) Zhang, C.; Michaelides, A.; King, D. A.; Jenkins, S. J., Positive Charge States and Possible Polymorphism of Gold Nanoclusters on Reduced Ceria. J. Am. Chem. Soc. 2010, 132, 2175-2182. (52) NIST Chemistry WebBook. http://webbook.nist.gov (accessed October 15, 2018) (53) Chase, M. W. NIST-JANAF Thermochemical Tables, 4th ed.; Journal of Physical and Chemical Reference Data Monograph No.9; American Institute of Physics: Woodbury, NY, 1998; pp 1-1951. (54) Campbell, C. T.; Sellers, J. R. V., Enthalpies and Entropies of Adsorption on WellDefined Oxide Surfaces: Experimental Measurements. Chem. Rev. 2013, 113, 4106-4135. (55) Kresse, G.; Furthmuller, J., Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. (56) Perdew, J. P.; Wang, Y., Accurate and Simple Analytic Representation of the ElectronGas Correlation Energy. Phys. Rev. B 1992, 45, 13244-13249. (57) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P., ElectronEnergy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505-1509. (58) Kim, H. Y.; Hybertsen, M. S.; Liu, P., Controlled Growth of Ceria Nanoarrays on Anatase Titania Powder: A Bottom-Up Physical Picture. Nano Lett. 2017, 17, 348-354. (59) Park, J. B.; Graciani, J.; Evans, J.; Stacchiola, D.; Ma, S.; Liu, P.; Nambu, A.; Sanz, J. F.; Hrbek, J.; Rodriguez, J. A., High Catalytic Activity of Au/CeOx/TiO2(110) Controlled by the Nature of the Mixed-Metal Oxide at the Nanometer Level. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 4975-4980. (60) Blochl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. (61) Henkelman, G.; Uberuaga, B. P.; Jonsson, H., A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901-9904. (62) Henkelman, G.; Jónsson, H., Improved Tangent Estimate in the Nudged Elastic Band 29


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Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978-9985. (63) Duan, Z.; Henkelman, G., Calculations of CO Oxidation over a Au/TiO2 Catalyst: A Study of Active Sites, Catalyst Deactivation, and Moisture Effects. ACS Catal. 2018, 8, 1376-1383. (64) Duan, Z.; Henkelman, G., CO Oxidation at the Au/TiO2 Boundary: The Role of the Au/Ti5c Site. ACS Catal. 2015, 5, 1589-1595. (65) Schlexer, P.; Widmann, D.; Behm, R. J.; Pacchioni, G., CO Oxidation on a Au/TiO2 Nanoparticle Catalyst via the Au-Assisted Mars-van Krevelen Mechanism. ACS Catal. 2018, 8, 6513-6525. (66) Falsig, H.; Hvolbæk, B.; Kristensen, I. S.; Jiang, T.; Bligaard, T.; Christensen, C. H.; Nørskov, J. K., Trends in the Catalytic CO Oxidation Activity of Nanoparticles, Angew. Chem. Int. Ed. 2008, 47, 4835-4839. (67) Aliaga, C.; Park, J. Y.; Yamada, Y.; Lee, H. S.; Tsung, C. K.; Yang, P. D.; Somorjai, G. A., Sum Frequency Generation and Catalytic Reaction Studies of the Removal of Organic Capping Agents from Pt Nanoparticles by UV-Ozone Treatment. J. Phys. Chem. C 2009, 113, 61506155. (68) Baker, L. R.; Kennedy, G.; Van Spronsen, M.; Hervier, A.; Cai, X. J.; Chen, S. Y.; Wang, L. W.; Somorjai, G. A., Furfuraldehyde Hydrogenation on Titanium Oxide-Supported Platinum Nanoparticles Studied by Sum Frequency Generation Vibrational Spectroscopy: Acid-Base Catalysis Explains the Molecular Origin of Strong Metal-Support Interactions. J. Am. Chem. Soc. 2012, 134, 14208-14216. (69) Carabineiro, S. A. C.; Bastos, S. S. T.; Orfao, J. J. M.; Pereira, M. F. R.; Delgado, J. L.; Figueiredo, J. L., Exotemplated Ceria Catalysts with Gold for CO oxidation. Appl. Catal. AGen. 2010, 381, 150-160. (70) Liu, Y.; Liu, B. C.; Wang, Q.; Liu, Y. X.; Li, C. Y.; Hu, W. T.; Jing, P.; Zhao, W. Z.; Zhang, J., Three Dimensionally Ordered Macroporous Au/CeO2 Catalysts Synthesized via Different Methods for Enhanced CO Preferential Oxidation in H2-Rich Gases. RSC Adv. 2014, 4, 59755985. (71) Venezia, A. M.; Pantaleo, G.; Longo, A.; Di Carlo, G.; Casaletto, M. P.; Liotta, F. L.; Deganello, G., Relationship between Structure and CO Oxidation Activity of Ceria-Supported Gold Catalysts. J. Phys. Chem. B 2005, 109, 2821-2827. (72) Lai, S. Y.; Qiu, Y. F.; Wang, S. J., Effects of the Structure of Ceria on the Activity of 30


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Figure 1. DFT-calculated G of multiple CO adsorption on (a) Au/CeO2(100) and (b) Au/CeO2(111). nCO denotes the Au/CeO2 model with n adsorbed CO molecules. Under practical CO oxidation conditions (p(O2) = 0.21 bar and 0.02 bar ≤ p(CO) ≤ 0.20 bar), 8 CO molecules can be stabilized on the Au NP of Au/CeO2(100), whereas 4 CO molecules can be stabilized on the Au NPs of Au/CeO2(111).

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Figure 2. The schematic CO oxidation pathways catalyzed by (a) Au/CeO2(100) and (b) Au/CeO2(111) with 8 or 4 adsorbed CO molecules show that CO oxidation occurs at the AuCeO2 interface, although an additional O-C-O formation step (step 2, b) is required for Au/CeO2(111). The original data for CO oxidation are adopted from our previous report (ref 41).

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Figure 3. Electronic interaction between Au9 NPs and CeO2 surfaces. Bader charge analysis of (a) Au9/CeO2(100) and (c) Au9/CeO2(111).  represents the excess charge (e) originating from the Au-CeO2 interaction. The positive  values of the Au9 NPs represent the amount of electrons donated from the Au9 NPs to CeO2. More than half of donated electrons from Au to CeO2 was localized to three Ce ions with negative . The geometries of orbitals of CeO2(100), (b), and CeO2(111), (d), participating in the electronic interaction between Au and CeO2 (iso3

surfaces of ρ = 0.06e/Å ). Red and blue surfaces represent the orbitals with accumulated electrons and the orbitals that lost electrons, respectively.

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Figure 4. TEM and HR-TEM images of (a, b) CeO2 cubes and (c, d) CeO2 octahedra. Inset of (b, d): fast Fourier transformation (FFT) pattern of CeO2.

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Figure 5. TEM and HR-TEM images of (a, b) Au/CeO2 cubes and (c, d) Au/CeO2 octahedra.

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Figure 6. (a) TOFs of CO oxidation over Au/CeO2 catalysts with different-shaped CeO2: Au/CeO2 cubes (red) and Au/CeO2 octahedra (blue). The TOFs were calculated by dividing the turnover number (TON) by the reaction time. (b) Arrhenius plots obtained by TOFs of Au/CeO2 catalysts at different temperatures.

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Figure 7. H2-TPR of CeO2 nanocrystals with different shapes and corresponding Au/CeO2 catalysts.

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Figure 8. Reaction TOF/rate maps of Au/CeO2 catalysts presented as a function of CO partial pressure and reaction temperature. Experimental TOF maps of (a) Au/CeO2 cubes and (b) Au/CeO2 octahedra. DFT-estimated rate maps of (c) Au/CeO2(100) and (d) Au/CeO2(111). Small red crosses in (a) and (b) denote the (p(CO), T) combinations at which the experimental data were acquired.

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TOC Graphic

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Catalytic CO oxidation over Au nanoparticles supported on CeO2

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