Morphology-Dependent Evolutions of Sizes, Structures and Catalytic

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Morphology-Dependent Evolutions of Sizes, Structures and Catalytic Activity of Au Nanoparticles on Anatase TiO Nanocrystals 2

Dan Li, Rui You, Min Yang, Yuanxu Liu, Kun Qian, Shilong Chen, Tian Cao, Zhenhua Zhang, Jie Tian, and Weixin Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00262 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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The Journal of Physical Chemistry

Morphology-Dependent Evolutions of Sizes, Structures and Catalytic Activity of Au Nanoparticles on Anatase TiO2 Nanocrystals Dan Li1, Rui You1, Min Yang1, Yuanxu Liu2, Kun Qian1, Shilong Chen1, Tian Cao1, Zhenhua Zhang1, Jie Tian3, Weixin Huang1* 1

Hefei National Laboratory for Physical Sciences at Microscale, CAS Key

Laboratory of Materials for Energy Conversion, Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China. 2

School of Pharmacy, Anhui University of Chinese Medicine, Anhui Academy of

Chinese Medicine, Hefei 230012, China. 3

Engineering and Materials Science Experiment Center, University of Science and

Technology of China, Hefei 230026, China.

AUTHOR INFORMATION Corresponding Author *Phone number: +8655163600435. Email: [email protected]

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Abstract: Au nanoparticles with different loadings were prepared on anatase TiO2 nanocrystals with various morphologies predominantly exposing {001} facets (denoted as TiO2{001}), {100} facets (denoted as TiO2{100}) and {101} facets (denoted as TiO2{001}) by deposition-precipitation method. Sizes, structures and catalytic activity in low-temperature CO oxidation of the resulting Au/TiO2 catalysts were comprehensively characterized. Nucleation, growth and agglomeration of Au particles on TiO2 supports were observed to depend on TiO2 morphologies due to the morphology-dependent defect structures of TiO2 nanocrystals and subsequent AuTiO2 interactions. Au particles mainly homogeneously nucleate and grow on these three TiO2 nanocrystals with Au loadings of 0.2%-1%. With the increase of Au loadings to 2% and 5%, Au particles mainly agglomerate on TiO2{001}, mainly homogeneously nucleate and grow on TiO2{100}, and both nucleate and grow and slightly agglomerate on TiO2{101}. Such morphology-dependent behaviors are associated with the morphology of TiO2. The electronic effect of supported Au particles on CO adsorption was observed, in which fine Au nanoparticles exhibit electronic structures deviating from that of bulk Au and decreased the adsorption capacity of CO. Meanwhile, fine Au nanoparticles are less able to activate surface lattice oxygen at the Au-TiO2 perimeters than large Au nanoparticles and exhibit a lowered intrinsic catalytic activity in lowtemperature CO oxidation. These results nicely exemplify morphology-dependent metal-oxide interactions and catalysis.

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1. Introduction Au catalysis has been extensively studied and well-established to be very structuresensitive.1-5 Au-oxide interaction plays a key role in determining structures of oxide-supported Au catalysts. Structures and catalytic activity of supported Au catalysts have been reported to sensitively depend on the structures of employed oxide supports, such as defect structures6 and surface hydroxyls groups.7 Such a sensitivity to oxide structures makes preparations of oxidesupported Au catalysts sometimes not reproducible among different research groups due to the complexity of oxide structures. Therefore, it is of great importance to elucidate the effects of oxide structures of oxide-supported Au catalysts. Au/TiO2 catalysts are one of the earliest and most extensively studied system of Au catalysis. A volcano shape-dependence of their catalytic activity in low-temperature CO oxidation on the Au particle size has inspired many fundamental studies to understand the growth and structures of Au nanoparticles on TiO2 that mostly use a rutile TiO2(110) single crystal as a model surface for TiO2.8-11 The catalytic activity of Au nanoparticles supported on rutile TiO2(110) surface also exhibited a volcano shape-dependent catalytic activity in lowtemperature CO oxidation on the Au particle size.12,13 Au nanoparticles prepared by Au vapor deposition were observed to preferentially nucleate at surface oxygen vacancy sites of reduced rutile TiO2(110) surface with charge transfer from the substrate to the Au clusters, and such a charge transfer effect strongly modulates the electronic structure of supported Au nanoparticles and greatly enhances their stability.14,15 It was also reported that an oxidized rutile TiO2(110) surface with oxygen adatoms adheres Au clusters more strongly than a reduced rutile TiO2(110) surface with surface oxygen vacancy sites.16 However, studies of growth and structures of Au 3



nanoparticles on other TiO2 single crystal surfaces are quite limited although the TiO2 surface structures strongly depend on the crystal planes. Meanwhile, the Au vapor deposition method commonly used to prepare Au nanoparticles on flat TiO2 single crystal substrates differs from the co-precipitation and precipitation-deposition (DP) methods commonly used to prepare Au/TiO2 powder catalysts. Recently, uniform oxide nanocrystals with various morphologies and preferentiallyexposed facets have been used to investigate facet-dependent surface chemistry and catalysis of oxide-involved catalysts.17,18 Anatase TiO2 nanocrystals with preferentially-exposed {100}, {101} and {001} facets, respectively denoted as TiO2{100}, TiO2{101} and TiO2{001}, were successfully synthesized, and facet-dependent TiO2 catalysis have been demonstrated in thermal catalysis and photocatalysis by TiO2 and TiO2-supported metal catalysts.19-27 The surface energies of anatase TiO2 {001}, {100} and {101} surfaces were calculated as 0.90, 0.53 and 0.43J/m2, respectively,28 while oxygen vacancy formation energies of anatase TiO2 {001}, {100} and {101} surfaces were calculated as 4.57, 4.0 and 4.15 eV, respectively.29-31 The different orders of surface energies and oxygen vacancy formation energies among these surfaces are due to the surface reconstructions after the creation of oxygen vacancies. Liu et al. reported that Au/TiO2-nanocrystals catalysts with a similar average Au particle size of 3.6 nm prepared by DP method with urea as the precipitant exhibited an obvious TiO2 facet-dependent catalytic activity in low-temperature CO oxidation following an order of Au/TiO2{100} > Au/TiO2{101} > Au/TiO2{001}.24 Later, by employing in situ DRIFTS of CO adsorption at ambient temperature, they also observed facet-dependent Au-TiO2 interaction, Au species and reduction/oxidation pretreatment effects in Au/TiO2-nanocrystals catalysts and proposed TiO2 4


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facet-dependent band-structure bending and electron flows of different Au-TiO2 interfaces.26 By employing in situ DRIFTS of CO adsorption at 120 K capable of probing the structures of both Au and TiO2, we found that the TiO2 facet-dependent Au-TiO2 interaction and Au species depend mainly on the surface reducibility and surface O vacancy concentration of the TiO2 support.25,27 In this paper, we comparatively studied the nucleation and growth behaviors of Au nanoparticles on TiO2{100}, TiO2{101} and TiO2{001} nanocrystals via DP method as a function of Au loadings and the structures and catalytic activity in low-temperature CO oxidation. Strong TiO2 facet effects were observed to affect the structures and catalytic activity in CO oxidation of resulting Au nanoparticles. Our findings clearly demonstrate that the sizedependent structures and catalytic performance of Au/TiO2 catalysts are closely related with the involved TiO2 facets, which deepens the fundamental understandings of both preparation processes and structure-activity relation of Au/TiO2 catalysts.

2. Experimental Section 2.1 Catalyst preparation Synthesis of anatase TiO2{001} nanocrystals 20: Ti(OBu)4 (25 mL) and 40 wt % HF(aq) (3 mL) were mixed under stirring at room temperature (RT). The solution was then transferred into a 50 mL Teflon lined stainless steel autoclave and kept at 180 C for 24 h. The white precipitate produced was collected by centrifugation, washed repeatedly with EtOH and ultrapure H2O, and dried at 70 C for 12 h. The acquired powder was dispersed in 0.1 m NaOH (aq) (700 mL), stirred for 24 h at RT, centrifuged, and washed repeatedly with ultra-pure H2O until 5



the aqueous solution had pH values of 7 ~ 8. Synthesis of anatase TiO2{100} and TiO2{101} nanocrystals 21: TiCl4 (6.6 mL) was added dropwise into 0.43 mol/L HCl (aq) (20 mL) at 0℃. After stirring for an additional 0.5 h, the solution was added dropwise into 5.5 wt % NH3 (aq) (50 mL) under stirring at RT. Then, an appropriate amount of 4 wt % NH3 (aq) was used to adjust the pH value of the solution to 6 ~ 7, after which the system was stirred at RT for 2 h. The precipitate produced was filtered, washed repeatedly with ultra-pure H2O until no residual Cl- could be detected, and dried at 70 C for 12 h to acquire Ti(OH)4. Then, Ti(OH)4 (2.0 g) and (NH4)2SO4 (0.5 g for synthesis of TiO2{100}) or NH4Cl (0.2 g for synthesis of TiO2{101}) were dispersed in a mixture of ultrapure H2O (15 mL) and iPrOH (15 mL) under stirring at RT and the mixture transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180℃for 24 h. The white precipitate obtained was collected, washed repeatedly with ultra-pure H2O. Au/TiO2 catalysts: Au/TiO2 catalysts with various Au/TiO2 weight ratios were prepared by the conventional deposition–precipitation method, employing HAuCl4 as the Au precursor. Typically, the desired amount of HAuCl4 (aq) ,TiO2 (1.0 g), and ultra-pure H2O (50 mL) were co-added into a three-necked flask and mixed under stirring at 60℃for 15 min. An appropriate amount of NH3 (aq) was added to adjust the pH value to 7 ~ 7.5, after which the system was stirred at 60℃for 1 h. The solid was then filtered, washed with ultra-pure H2O several times, dried at 60℃under vacuum for 12 h, and calcined at 400℃ for 2 h under ambient air. 2.2 Sample characterizations The loadings of Au on the catalysts were analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES). Powder X-ray diffraction (XRD) patterns were recorded in 6


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the 2θ at range 20–80° on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 0.15406 nm) operating at 40 kV and 50 mA. Transmission electron microscopy (TEM), STEM and HRTEM were performed with a JEOL JEM-2100F instrument at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high-performance electron spectrometer using monochromatized Al Kα (h  1486.7 eV) as the excitation source, and the likely charging of samples was corrected by setting the C 1s binding energy of the adventitious carbon to 284.8 eV. Electron paramagnetic resonance (EPR) spectra were recorded on a JEOL JES-FA200 EPR spectrometer (9.063 GHz, X-band) at 130 K with employed microwave power, modulation frequency and modulation amplitude of 0.998 mW, 100 kHz and 0.35 mT, respectively. In situ DRIFTS measurements of CO adsorption were performed on a Nicolet 6700 FTIR spectrometer equipped with an in-situ low-temperature and vacuum DRIFTS reaction cell (Harrick Scientiﬁc Products, Inc.) in order to enhance the chemisorption with minimum interference of gas-phase molecules. The DRIFTS spectra were measured with 256 scans and a resolution of 4 cm−1 using a MCT/A detector. A 50 mg amount of catalyst was loaded on the sample stage of the reaction cell. Prior to adsorption experiments, the samples were degassed at a base pressure of 0.01 Pa and cooled to the desirable temperature, whose spectra were taken as the background spectra. CO was then admitted into the reaction cell to desirable pressures via a leak valve, and the DRIFTS spectra were recorded after the chemisorption reaches the steady state. 2.3 Catalytic activity evaluation 7



Catalytic performances of various Au/TiO2 catalysts in CO oxidation were evaluated in a home-made fixed-bed reactor with a quartz tubular microreactor. The catalyst underwent no pretreatment prior to the catalytic reaction. Typically, desired amounts of as-synthesized catalysts without any pretreatments were placed in a quartz tube reactor. The reaction gas mixture (1% CO, 1% O2, 98% N2) was fed at a rate of 30 mL/min. The catalyst was heated to the desired reaction temperatures at a rate of 2 °C/min and then kept for 30 min to reach the steady state. The composition of the eﬄuent gas was analyzed with an online GC-14C gas chromatograph equipped with a 5A column coupling to a TCD detector, and the CO conversion was calculated from the change of CO concentrations in the inlet and outlet gases.

3. Results and Discussion Figure 1 presents representative TEM and HRTEM images of as-synthesized TiO2{001}, TiO2{100} and TiO2{101} nanocrystals. The morphologies of these nanocrystals are quite uniform. The sizes of TiO2{001} nanocrystals (Figure 1A) are between 40-60 nm. TiO2{100} nanocrystals (Figure 1B) are with a length distribution of 20-50 nm and a width distribution of 10-15 nm. TiO2{101} nanocrystals (Figure 1C) are of sizes between 15-30 nm. The lattice fringes resolved in the HRTEM images all arise from those of anatase TiO2. All these microscopic results agree with previously reported results.20,23 According to the previously proposed procedure,24 the percentages of {001} facets in TiO2{001}, {100} facets in TiO2{100} and {101} facets in TiO2{101} nanocrystals were estimated to be all around 80% (Supporting Information). 8


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The BET specific surface areas of TiO2{001}, TiO2{100} and TiO2{101} nanocrystals are 102 m2/g, 99 m2/g and 108 m2/g respectively. XPS characterizations (Figure S1) do not detect any signals of F, S and Cl involved during the synthesis, demonstrating the negligible amounts of these elements on synthesized anatase TiO2 nanocrystals. Au/TiO2 catalysts with calculated 0.2%-5% Au/TiO2 weight ratio were prepared by employing various TiO2 nanocrystals as supports. The actual Au loadings in Au/TiO2 catalysts were analyzed by ICP-AES and the results (Table S1) show that the actual Au loadings are similar to the calculated values. In the XRD patterns (Figure S2), all Au/TiO2 catalysts only display diffraction patterns of anatase TiO2 (JCPDS card No. 89-4921) and no diffraction peaks attributing to Au could be identified. The XPS characterization results (Figure S3) show that all Au/TiO2 catalysts exhibit a single Au 4f component with the Au 4f7/2 binding energy at 83.2 eV, a typical value of metallic Au supported on TiO2.25 The Au 4f XPS peak grows with the Au loading. Meanwhile, all Au/TiO2 catalysts exhibit a Ti 2p3/2 binding energy at 458.7 eV and an O 1s binding energy at 530.0 eV arising from TiO2. Figures 2-4 present representative high-angle annular dark-field (HAADF)-STEM images and Au nanoparticle size distributions of various Au/TiO2 catalysts. The Au nanoparticle size distributions in each catalysts were acquired by counting more than 100 Au nanoparticles. Au nanoparticles in all catalysts are mainly in a spherical morphology, but the sizes and size evolutions as the function of Au loadings vary obviously with the TiO2 facets. As shown in Figure 2, Au nanoparticles exhibit obvious bimodal size distributions in Au/TiO2{001} catalysts with 0.2%-1% Au loadings and single size distributions in Au/TiO2{001} catalysts with larger Au loadings. In detail, Au nanoparticles exhibit a bimodal size distribution of 1.5 9



±0.5 and 4.0±1.1 nm in 0.2%-Au/TiO2{001} catalyst, a bimodal size distribution of 1.5±0.6

and 3.9 ± 1.3 nm in 0.5%-Au/TiO2{001} catalyst, a bimodal size distribution of 1.5 ± 0.7 and 4.6 ± 1.0 nm in 1%-Au/TiO2{001} catalyst, a single size distribution of 4.6±1.5 nm in 2%Au/TiO2{001} catalyst, and a single size distribution of 5.1±1.7 nm in 5%-Au/TiO2{001} catalyst. Au nanoparticles in Au/TiO2{100} catalysts (Figure 3) exhibit single size distributions of 2.3±0.9 nm in 0.2%-Au/TiO2{100}, 2.4±0.9 nm in 0.5%-Au/TiO2{100}, 2.3±0.8 nm in 1%Au/TiO2{100}, 2.1±0.8 nm in 2%-Au/TiO2{100}, and 2.4±0.9 nm in 5%-Au/TiO2{100} . It can be seen that Au nanoparticles have similar average sizes in all Au/TiO2{100} catalysts. Au nanoparticles in Au/TiO2{101} catalysts (Figure 4) also exhibit single size distributions of 1.9 ± 0.7 nm in 0.2%-Au/TiO2{101}, 1.9 ± 0.8 nm in 0.5%-Au/TiO2{101}, 1.8 ± 0.7 nm in 1%-

Au/TiO2{101}, 2.1±0.6 nm in 2%-Au/TiO2{101}, and 2.6±1.3 nm in 5%-Au/TiO2{101}. It can be seen that the average Au nanoparticle sizes in Au/TiO2{101} catalysts initially do not vary much for Au loadings up to 1% but then increase with the Au loading further increasing. The above microscopic characterization results clearly demonstrate that the size evolutions of Au nanoparticles supported on TiO2 sensitively depend on the TiO2 facets for Au/TiO2 catalysts prepared by DP methods employing HAuCl4 as the Au precursor and ammonia water as the precipitation agent. We made a more detailed Au nanoparticle size analysis by classifying particle sizes into three categories: below 2 nm, between 2 and 5 nm, and above 5 nm (Table S1). The percentages of Au nanoparticles of these three categories do not vary much in Au/TiO2{001} catalysts with Au loadings up to 1%, then the percentage of Au nanoparticles above 5 nm increases greatly at the expense of that of Au nanoparticles of 10


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2-5 nm in 2%-Au/TiO2{001}, and then both percentages of Au nanoparticles of 2-5 nm and above 5 nm increase greatly at the expense of that of Au nanoparticles below 2 nm in 5%Au/TiO2{001}. The percentages of Au nanoparticles of these three categories do not vary much in Au/TiO2{100} catalysts with Au loadings up to 5%. The percentages of Au nanoparticles of these three categories do not vary much in Au/TiO2{101} catalysts with Au loadings up to 1%, then the percentage of Au nanoparticles of 2-5 nm increases greatly at the expense of that of Au nanoparticles below 2 nm in 2%-Au/TiO2{101}, and then the percentage of Au nanoparticles of above 5 nm increase slightly at the expense of that of Au nanoparticles of 2-5 nm in 5%-Au/TiO2{101}. Therefore, Au nanoparticles mainly undergo a homogeneous nucleation and growth process on TiO2{001} with Au loadings of 0.2%-1% and then mainly agglomerate with Au loadings of 2%-5%; moreover, two types of nucleation sites are present on TiO2{001} surfaces lead to the bimodal size distributions of Au nanoparticles. Au nanoparticles mainly undergo a homogeneous nucleation and growth process on TiO2{100} with Au loadings of 0.2%-5%; and Au nanoparticles mainly a homogeneous nucleation and growth process on TiO2{101} with Au loadings of 0.2% to 1% and then undergo both homogeneous nucleation and growth process and slight agglomeration with Au loadings of 2%5%. Defects on TiO2 are strongly relevant to the nucleation and growth of Au nanoparticles. The defect structures of TiO2 and Au/TiO2 samples were characterized by EPR that is sensitive to defects (Figure 5). TiO2{001}, TiO2{100} and TiO2{101} exhibit features of g=2.02–2.026 arising from the O- 2 species,32-34 g=2.004–2.008 arising from the oxygen vacancy with one electron (F1+ color center).35,36 The O211



species likely result from the adsorption of O2 in the ambient atmosphere on the surface F1+ color centers of TiO2 and indicates the presence of surface F1+ color centers on TiO2 while the observed F1+ color center is likely located in the subsurface/bulk region of TiO2. Two additional features of g = 1.987 and g=1.968 respectively assigned to the subsurface/ bulk Ti3+ bulk species and surface Ti3+ surface species

37-39

appear for TiO2{100} and TiO2{101} but not for TiO2{001}. As

demonstrated by Figure 5 and a direct comparison of EPR spectra of various TiO2 nanocrystals (Figure S4), TiO2{100} exhibits the largest number of defects while TiO2{001} exhibits the least. The loading of Au affects both defect structures and concentrations of TiO2 nanocrystals. As shown in Figure 5A, upon a loading of 0.2% Au on TiO2{001} nanocrystals, both F1+ color center and O- 2 features attenuate while a weak Ti3+ surface signal emerges. The F1+ color center and O- 2 features of TiO2{001} keep decreasing with the Au loading up to 1% and then do not change, but the Ti3+ surface feature keeps increasing. As shown in Figure 5B, all features of O- 2, F1+ color center, Ti3+ surface and the subsurface/bulk Ti3+ bulk features of TiO2{100} nanocrystals are present on Au/TiO2{100} catalysts and keep decreasing with the Au loading increasing up to 5%. As shown in Figure 5C, all features of O- 2, F1+ color center, Ti3+ surface and the subsurface/bulk Ti3+ bulk features of TiO2{101} nanocrystals are present on Au/TiO2{101} catalysts and keep decreasing with the Au loading increasing up to 1% and then decreases slightly with the Au loading further increasing to 5%.

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The intensity variations of defects of TiO2 nanocrystals in Au/TiO2 catalysts are in accordance to the nucleation/growth and agglomeration processes of supported Au nanoparticles. As the defects of TiO2 keep decreasing for Au/TiO2{001} catalysts of Au loadings up to 1%, for Au/TiO2{100} catalysts of Au loadings up to 5% and for Au/TiO2{101} catalysts of Au loadings up to 1%, the Au nanoparticles mainly undergo the nucleation/growth processes. As the defects of TiO2{001} in Au/TiO2{001} catalysts with the Au loadings of 2%-5% do not decrease, the supported Au nanoparticles mainly undergo the agglomeration process. Similarly, as the defects of TiO2{101} in Au/TiO2{101} catalysts with the Au loadings of 2%-5% decrease slightly, slight agglomeration processes, in addition to the nucleation/growth process, occur for the supported Au nanoparticles. These results demonstrate that Au nanoparticles preferentially nucleate and grow at the defective sites of TiO2 supports. TiO2{100} nanocrystals afford the largest number of defects and thus can sustain the homogeneous nucleation and growth of Au nanoparticles up to a 5% loading while TiO2{001} nanocrystals have the least number of defects and thus can only sustain the homogeneous nucleation and growth of Au nanoparticles up to a 1% loading. When Au nanoparticles mainly agglomerate in Au/TiO2{001} catalysts of Au loadings above 1%, the EPR feature of O- 2 species almost vanishes although that of F1+ color centers

still exists and does not change. This suggests that the defects

associated with O- 2 species, i.e., the surface F1+ color centers, should play the dominant role in the nucleation and growth of Au nanoparticles. The decreased F1+ color centers upon Au loading should mainly come from those at subsurface regions 13



of TiO2 that are involved in the nucleation and growth processes of supported Au nanoparticles while the remaining F1+ color centers should come from those in the bulk that are not involved. In addition to the similar type of defect structure leading to the formation of Au nanoparticles of around 2 nm on all TiO2 nanocrystals, another type of defect structure exists on TiO2{001} to form Au nanoparticles larger than 4 nm and is likely large clusters of surface F1+ centers. Similar defect structures were previously observed on CeO2 nanocrystals.40 The Ti3+ surface defects emerging in Au/TiO2{001} catalysts is tentatively proposed to form at large Au nanoparticles-TiO2{100} perimeter sites. It was reported that large Au nanoparticles are more capable of activating surface lattice oxygen of oxide supports.41 Types of adsorption sites on both Au and TiO2 surfaces of Au/TiO2 catalysts were probed via CO adsorption. Figure 6 shows in situ DRIFTS spectra of CO adsorption on various TiO2 and Au/TiO2 catalysts with a CO pressure of 200 Pa at 123 K. Vibrational bands at 2181 and 2107 cm-1 arise respectively from CO adsorbed at Ti5c sites of TiO2 (denoted as CO-Ti(IV))22 and at Au0 sites (denoted as CO-Au0)27. The strong signals of CO-Ti(IV) species also give corresponding weak vibrational bands of

13CO-Ti(IV)

at around 2129 cm-1. However, after

comparing the intensity variations between the features at 2181 and 2129 cm-1, we can identify that the vibrational feature at 2129 cm-1 contains other contributions, i.e., CO adsorbed at partially positively-charged Au sites on Au nanoparticles (denoted as CO-Au+)27. These Au+ sites are generally associated with Au nanoparticles with very fine sizes whose lattice contracts and d charge at the Au atom site depletes relative to bulk Au.42 The CO adsorption results demonstrate that the fraction of Au+ sites in Au/TiO2 catalysts decreases rapidly as the Au 14


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loading increases. This corresponds well to occurrences of growth and agglomeration of supported Au nanoparticles at large Au loadings. The vibrational peak positions of adsorbed CO species do not vary much with Au loadings and TiO2 morphologies, but their intensities do. As shown in Figure 7A, the vibrational feature of CO-Ti(IV) species is much weaker on bare TiO2{001} nanocrystals than on TiO2{100} and TiO2{101} nanocrystals although they exhibit similar surface areas. This indicates that much less Ti5c sites are available for CO molecular adsorption on TiO2{001} nanocrystals than on TiO2{100} and TiO2{101} nanocrystals, which should be associated with their different surface compositions and structures. After the loading of Au nanoparticles, the vibrational feature of CO-Ti(IV) for Au/TiO2{100} and Au/TiO2{101} catalysts weakens initially rapidly and then slowly, while that for Au/TiO2{001} initially increases for 0.2%-Au/TiO2 and then decreases slowly but is always stronger than that for bare TiO2{001}. Thus, the Au nanoparticles reasonably cover some of the Ti(IV) sites on TiO2{100} and TiO2{101} nanocrystals but they unexpectedly create more Ti(IV) sites on TiO2{001} nanocrystals. We propose that the Au nanoparticles are capable of removing surface oxygen or other species to make the occupied and underneath Ti(IV) sites on TiO2{001} surface accessible for CO adsorption. This is supported by the emergence of Ti3+ surface defects in Au/TiO2{001} catalysts, and it could also be responsible for the existence of a second type of nucleation sites associated with formations of large Au nanoparticles. With the Au loading increasing, the initial rapid decrease of CO-Ti(IV) species mainly results from the nucleation processes at defects of TiO2{100} and TiO2{101} nanocrystals that tend to form 2D-like Au nanoparticles, and the subsequent slow decrease mainly results from the growth and agglomeration processes 15



in which 2D-like Au nanoparticles evolve into 3D-like Au nanoparticles. The vibrational feature of CO-Au species increases with the Au loading of Au/TiO2 catalysts (Figure 7B), but it can be seen that the vibrational feature of CO-Au species of Au/TiO2{001} catalysts with Au loadings above 1% increases much more rapidly than those of corresponding Au/TiO2{100} and Au/TiO2{101} catalysts. The Au-mass normalized vibrational feature intensity of CO-Au species were calculated and plotted as a function of the Au loadings of Au/TiO2 catalysts. As shown in Figure 7B, it does not vary much with the Au loadings for Au/TiO2{100} and Au/TiO2{101} catalysts, but obviously increases with the Au loading for Au/TiO2{001} catalysts with Au loadings above 1%. These observations suggest that despite their much larger sizes, Au nanoparticles in Au/TiO2{001} catalysts with Au loadings above 1% exhibit much more accessible Au sites for molecular CO adsorption than those in corresponding Au/TiO2{100} and Au/TiO2{101} catalysts. In addition to molecularly-adsorbed CO species, various types of carbon oxygenates are formed upon CO adsorption on TiO2 and Au/TiO2 catalysts (Figure 8) and the assignments are made based on previous reports

22,24,43,44.

CO adsorption mainly gives bicarbonate species

(1665 cm-1) on bare TiO2{001}, bicarbonate and bridged carbonate (1438 cm-1) species on bare TiO2{100} and TiO2{101} nanocrystals. The loadings of 0.2% and 0.5% Au on all TiO2 nanocrystals lead to significant appearances of additional species, including bicarbonate (1647 cm-1), formate (1595 cm-1), bidentate carbonate (1551 and 1345/1360 cm-1), monodentate carbonate (1533 and 1381 cm-1) and bridged carbonate (1476/1450/1438 cm-1). However, the vibrational features of all formed carbon oxygenates significantly attenuates for all 1%Au/TiO2 catalysts. With the further increase of Au loadings, only the features of bidentate 16


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carbonate species increase obviously while those of other species grow slowly or do not vary, and the vibrational features of bidentate carbonate species are stronger on Au/TiO2{100} and Au/TiO2{101} catalysts than on corresponding Au/TiO2{001} catalysts. The DFIFTS spectra of as-synthesized Au/TiO2 catalysts were recorded (Figure S5), and the results do not show much differences among as-synthesized Au/TiO2 catalysts with different Au loadings. Thus all observed surface species result from CO adsorption on Au-TiO2 interfaces and Au surfaces of Au/TiO2 catalysts. Inferred by their variations with the Au loading, the formed carbon oxygenate species in 0.2%- and 0.5%-Au/TiO2 catalysts are mainly located at Au-TiO2 interfaces while the bidentate carbonate species in 2%- and 5%- Au/TiO2 catalysts are mainly located on Au surfaces. The CO adsorption results demonstrate the structural evolutions of supported Au nanoparticles associated with their nucleation, growth and agglomeration processes on TiO2. The nucleation processes mainly occur in 0.2%- and 0.5%-Au/TiO2 catalysts, forming Au clusters on that exhibit more Au+ sites than Au0 sites and result in plenty of Au+-TiO2 interfaces, as evidenced by the relative intensities of CO-Au+ and CO-Au0 species and the intense carbon oxygenate species at Au-TiO2 interfaces. In 1%Au/TiO2 catalysts, Au clusters likely grow into Au nanoparticles that exhibit more Au0 sites than Au+ sites and result in Au0-TiO2 interfaces at the expense of Au+-TiO2 interfaces. The significant decrease of carbon oxygenate species at Au-TiO2 interfaces indicates that Au+-TiO2 interfaces are much more active in producing carbon oxygenate species upon CO adsorption than the Au0-TiO2 interfaces. With the further increase of Au loadings, Au nanoparticles further grow or agglomerate, and the Au0 17



sites dominate in the Au nanoparticles, resulting in the growth of CO-Au0 species and the bidentate carbonate species on Au surfaces. The formation of bidentate carbonate species on Au surfaces upon CO adsorption needs the participation of oxygen species. The much weaker bidentate carbonate species on large Au nanoparticles in 2%- and 5%-Au/TiO2{001} catalysts than on fine Au nanoparticles in corresponding 2%- and 5%- Au/TiO2{100} and Au/TiO2{101} catalysts can be associated with the less activity to adsorb O2 of large Au nanoparticles in as-synthesized Au/TiO2{001} catalysts. However, these large Au nanoparticles in 2%- and 5%-Au/TiO2{001} catalysts exhibit much stronger ability to adsorb CO than the fine Au nanoparticles in corresponding 2%- and 5%- Au/TiO2{100} and Au/TiO2{101} catalysts. This contrasts the general assumption that finer Au nanoparticles expose more low-coordinated Au atoms active in adsorbing CO45-47. Previously we observed that the electronic structure of Au nanoparticles supported on SiO2 affects the ability to adsorb CO,48 in which fine Au nanoparticles (< 2-3 nm) with abundant low-coordinated Au atoms but bulk Auunlike electronic structure do not chemisorb CO. We propose that although charge transfer from TiO2 to Au can help to modulate their electronic structure toward the bulk Au-like electronic structure, such an electronic effect exists more or less for lowcoordinated Au atoms on fine Au nanoparticles in 2%- and 5%- Au/TiO2{100} and Au/TiO2{101} catalysts, resulting in their weak ability to adsorb CO, as compared with those of large Au nanoparticles in 2%- and 5%- Au/TiO2{001} catalysts. Catalytic activity of various Au/TiO2 catalysts were evaluated in CO oxidation between 30 and 180 C. Figure 9 shows CO conversions as a function of reaction 18


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temperatures of various Au/TiO2 catalysts. It can be seen that the catalytic performance of Au/TiO2 catalysts sensitively depend on the TiO2 morphology. With the similar Au loadings, CO conversions of Au/TiO2{101} and Au/TiO2{100} are higher than that of Au/TiO2{001}. With the same type of TiO2 support, the dependence of CO conversions on the Au loadings varies with the TiO2 morphology. The Arrhenius plots of various Au/TiO2 catalysts were plotted using CO conversions below 30% within the reaction kinetics-controlled region (Figure S6), from which the apparent activation energies (Ea) were calculated (Table S1). Figure 10A presents Ea as a function of Au loadings for each type of Au/TiO2 catalysts. All studied Au/TiO2{001} catalysts exhibit a similar Ea of 29.21.0 kJ/mol, and all studied Au/TiO2{100} catalysts exhibit a similar Ea of 45.41.9 kJ/mol, and Au/TiO2{101} catalysts up to 2%-Au/TiO2{101} exhibit a similar Ea of 47.11.4 kJ/mol but 5%-Au/TiO2{101} catalyst exhibits a decreased Ea of 38.00.6 kJ/mol. Therefore, the structures and intrinsic catalytic activity of Au/TiO2 catalysts vary with the TiO2 morphology, which can be reasonably associated with TiO2 morphology-dependent nucleation/growth and structures of supported Au nanoparticles. Au/TiO2{001} catalysts of low Au loadings exhibit bimodal size distributions but a similar Ea to those of high Au loadings exhibiting single size distributions. Thus, the active site with an Ea value of 29.21.0 kJ/mol involves large Au nanoparticles on TiO2{001} instead of fine Au nanoparticles (1.50.5 nm). This is consistent with the results that Au/TiO2{100} catalysts and Au/TiO2{101} catalysts up to 2%-Au/TiO2{101} with fine supported Au nanoparticles exhibit much higher Ea. Meanwhile, the 19



decreased Ea of 5%-Au/TiO2{101} can also be related to the size increase of Au nanoparticles in 5%-Au/TiO2{101} (2.6±1.3 nm) than those in Au/TiO2{101} catalysts with lower Au loadings. Although the effects of TiO2 morphology on the active site can not be decided, our results suggest that the intrinsic catalytic activity of fine Au nanoparticles (below 2-3 nm) supported on TiO2 in low-temperature CO oxidation are not as high as that of large Au nanoparticles (2-5 nm) under our experimental condition. However, fine Au nanoparticles normally expose a high density of low-coordinated Au atoms at their corners, steps and edges, and thus can provide much higher density of active sites than larger Au nanoparticles. Generally the Au-TiO2 perimeter sites of Au/TiO2 catalysts are considered as the active site to catalyze low-temperature CO oxidation,49 but different reaction mechanisms and active species are proposed. Haruta proposed that the reaction takes place at the Au-TiO2 interface where CO adsorbs on Au nanoparticles and the support activates O2.2,50,51 Bond reported that Au cations are present at the Au-support interface and involved to catalyze low-temperature CO oxidation.52 Yates and coworkers observed a dual-perimeter site on Au/TiO2 catalysts,53 in which CO molecules adsorbed at Ti sites contribute to CO oxidation at very low temperatures. Au-TiO2 interfacial transfer of highly excited (hot) charge carriers is observed to accompany low-temperature CO oxidation.54-56 A coverage-dependent microkinetic analysis of CO oxidation catalyzed by Au/TiO2 nanocatalysts shows that the dominant kinetic pathway, activated oxygen species, and catalytic active sites all depend strongly on both temperature and oxygen partial pressure.57 Behm’s group proposed a highly stable 20


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atomic oxygen species, most likely surface lattice oxygen at the perimeters of Au nanoparticles, as the active oxygen species for CO oxidation above 253 K over Au/TiO2 catalysts.58 We propose that, under the reaction condition and within the size ranges of Au nanoparticles supported on TiO2 in this study, fine Au nanoparticles are less able to activate surface lattice oxygen at the Au-TiO2 perimeters than large Au nanoparticles and subsequently less catalytic active. Similar results were previously reported for Au nanoparticles supported on CeO2 with sizes varying from 1.7±0.6 to 3.7±0.9 nm during CO oxidation at room temperature.41 Fig. 10 B-D plot Au-mass normalized CO conversion rates as a function of reaction temperatures selected for the Arrhenius plots of various Au/TiO2. For the catalysts with similar Ea values and the same type of active site, their Au-mass normalized CO conversion rates under the same reaction conditions can be taken to roughly compare the Au-mass normalized densities of active site. Complex behaviors were observed, with 1%-Au/TiO2{001}, 2%-Au/TiO2{100} and 2%-Au/TiO2{101} exhibiting the largest Au-mass normalized CO conversion rates respectively in Au/TiO2{001}, Au/TiO2{100} and Au/TiO2{101} catalysts. These can be related to the facet-dependent size and structure evolutions of Au nanoparticles supported on TiO2{001}, TiO2{100} and TiO2{101} nanocrystals. Comparisons between catalytic activity (Fig. 9) and surface species formed upon CO adsorption (Figs. 6 and 8) of various Au/TiO2 catalysts suggest that molecularly-adsorbed CO, bicarbonate and bidentate carbonate species should be the surface species involved in lowtemperature CO oxidation. However, the reactivity of these species in low-temperature 21



CO oxidation conditions need to be further examined, as reported in previous literature.41,57 The above results comprehensively demonstrate facet-dependent evolutions of size, structure and catalytic activity of Au nanoparticles on TiO2 nanocrystals preferentially enclosed with different facets. Although Au nanoparticles are also present on other minor facets on each types of TiO2 nanocrystals, our comprehensive results demonstrate that the observed properties of various Au/TiO2-nanocrystal catalysts are mainly related to Au nanoparticles on the dominant TiO2 facets. Therefore, the complex catalysis of Au/TiO2 catalysts must partly arise from the various facets exposed on TiO2 supports. Consequently, morphology/facet engineering of TiO2 support provides as an effective strategy to fundamentally investigate the Au-TiO2 catalysis and meanwhile optimize the catalytic property of Au/TiO2 catalysts.

4. Conclusions Employing various types of anatase TiO2 nanocrystals as supports, we successfully identify morphology-dependent evolutions of size and structure of Au particles on TiO2. Au particles mainly homogeneously nucleate and grow on these three TiO2 nanocrystals with Au loadings of 0.2%-1%. With the increase of Au loadings to 2% and 5%, Au particles mainly agglomerate on TiO2{001}, mainly homogeneously nucleate and grow on TiO2{100}, and both nucleate and grow and slightly agglomerate on TiO2{101}. Such morphology-dependent behaviors are associated with the 22


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morphology-dependent defect structures of TiO2 nanocrystals and subsequent AuTiO2 interactions. The electronic effect of supported Au particles on CO adsorption was observed, in which fine Au nanoparticles exhibit electronic structures deviating from that of bulk Au and decreased the adsorption capacity of CO. Meanwhile, fine Au nanoparticles are less able to activate surface lattice oxygen at the Au-TiO2 perimeters than large Au nanoparticles and exhibit a lowered intrinsic catalytic activity in lowtemperature CO oxidation. These results add new experimental results for fundamental understandings of Au-TiO2 catalysis. ASSOCIATED CONTENT Supporting Information Available. Calculation method of the percentages of different crystal planes in these TiO2 nanocrystals; XPS spectra TiO2 nanocrystals; Au particle-size distributions, Au content and the apparent activation energies (Ea) of various Au/TiO2 catalysts; XRD patterns and XPS spectra of various Au/TiO2 catalysts; EPR spectra of various TiO2 nanocrystals; The DFIFTS spectra of as-synthesized Au/TiO2 catalysts; The Arrhenius plots of various Au/TiO2 catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Phone number: +8655163600435. Email: [email protected] Notes

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The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the National Key R & D Program of MOST (2017YFB0602205), the National Natural Science Foundation of China (21525313, 21761132005, 91745202, 21703227, 21872128, 21703001) and the Changjiang Scholars Program of Ministry of Education of China. REFERENCES (1) Fierro-Gonzalez, J. C.; Gates, B. C. Catalysis by Gold Dispersed on Supports: The Importance of Cationic Gold. Chem. Soc. Rev. 2008, 37, 2127-2134. (2) Takei, T.; Akita, T.; Nakamura, I.; Fujitani, T.; Okumura, M.; Okazaki, K.; Huang, J.; Ishida, T.; Haruta, M. Heterogeneous Catalysis by Gold. Adv. Catal. 2012, 55, 1-126. (3) Villa, A.; Dimitratos, N.; Chan-Thaw, C. E.; Hammond, C.; Veith, G. M.; Wang, D.; Manzoli, M.; Prati, L.; Hutchings, G. J. Characterisation of Gold Catalysts. Chem.

Soc. Rev. 2016, 45, 4953-4994. (4) Huang, W.X.; Qian, K.; Wu, Z.F.; Chen, S.L. Structure-Sensitivity of Au Catalysis.

Acta Phys.-Chim. Sin. 2016, 32, 48-60. (5) Huang, W.; Sun, G.; Cao, T. Surface Chemistry of Group IB Metals and Related Oxides. Chem. Soc. Rev. 2017, 46, 1977-2000.

24


Page 24 of 45

Page 25 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Risse, T.; Shaikhutdinov, S.; Nilius, N.; Sterrer, M.; Freund, H.-J. Gold Supported on Thin Oxide Films: from Single Atoms to Nanoparticles. Acc. Chem. Res. 2008,

41, 949-956. (7) Qian, K.; Fang, J.; Huang, W.; He, B.; Jiang, Z.; Ma, Y.; Wei, S. Understanding the Deposition–Precipitation Process for the Preparation of Supported Au Catalysts. J. Mol. Catal. A-Chem. 2010, 320, 97-105. (8) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. Gold Catalysts Prepared by Coprecipitation for Low-Temperature Oxidation of Hydrogen and of Carbon Monoxide. J. Catal. 1989, 115, 301-309. (9) Meyer, R.; Lemire, C.; Shaikhutdinov, S. K.; Freund, H.-J. Surface Chemistry of Catalysis by Gold. Gold Bull. 2004, 37, 72-124. (10)

Chen, M.; Goodman, D. W. Catalytically Active Gold: From Nanoparticles to

Ultrathin Films. Acc. Chem. Res. 2006, 39, 739-746. (11)

Pang, C. L.; Lindsay, R.; Thornton, G. Structure of Clean and Adsorbate-

Covered Single-Crystal Rutile TiO2 Surfaces. Chem. Rev. 2013, 113, 3887-3948 (12)

Valden, M.; Lai, X.; Goodman, D. W. Onset of Catalytic Activity of Gold

Clusters on Titania with the Appearance of Nonmetallic Properties. Science 1998,

281, 1647-1650. (13)

Laoufi, I.; Saint-Lager, M.-C.; Lazzari, R.; Jupille, J.; Robach, O.; Garaudée,

S.; Cabailh, G.; Dolle, P.; Cruguel, H.; Bailly, A. Size and Catalytic Activity of Supported Gold Nanoparticles: An In Operando Study during CO Oxidation. J.

Phys. Chem. C 2011, 115, 4673-4679. 25



(14)

Jiang, Z.; Zhang, W.; Jin, L.; Yang, X.; Xu, F.; Zhu, J.; Huang, W. Direct XPS

Evidence for Charge Transfer from A Reduced Rutile TiO2(110) Surface to Au Clusters. J. Phys. Chem. C 2007, 111, 12434-12439. (15)

Yu, X.; Xu, L.; Zhang, W.; Jiang, Z.; Zhu, J.; Huang, W. Synchrotron-Radiation

Photoemission Study of Growth and Stability of Au Clusters on Rutile TiO2(110)1×1. Chin. J. Chem. Phys. 2009, 22, 339-345. (16)

Matthey, D.; Wang, J.; Wendt, S.; Matthiesen, J.; Schaub, R.; Lægsgaard, E.;

Hammer, B.; Besenbacher, F. Enhanced Bonding of Gold Nanoparticles on Oxidized TiO2(110). Science 2007, 315, 1692-1696. (17)

Huang, W. Crystal Plane-Dependent Surface Reactivity and Catalytic Property

of Oxide Catalysts Studied with Oxide Nanocrystal Model Catalysts. Top. Catal. 2013, 56, 1363-1376. (18)

Huang, W. Oxide Nanocrystal Model Catalysts. Acc. Chem. Res. 2016, 49,

520-527. (19)

Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H.

M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638. (20)

Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. Synthesis of Titania Nanosheets

with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties. J. Am. Chem. Soc. 2009, 131, 3152-3153. (21)

Liu, L.; Gu, X.; Ji, Z.; Zou, W.; Tang, C.; Gao, F.; Dong, L. Anion-Assisted

Synthesis of TiO2 Nanocrystals with Tunable Crystal Forms and Crystal Facets 26


Page 26 of 45

Page 27 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Their Photocatalytic Redox Activities in Organic Reactions. J. Phys. Chem. C 2013, 117, 18578-18587. (22)

Chen, S.; Cao, T.; Gao, Y.; Li, D.; Xiong, F.; Huang, W. Probing Surface

Structures of CeO2, TiO2, and Cu2O Nanocrystals with CO and CO2 Chemisorption. J. Phys. Chem. C 2016, 120, 21472-21485. (23)

Chen, S.; Li, D.; Liu, Y.; Huang, W. Morphology-Dependent Defect Structures

and Photocatalytic Performance of Hydrogenated Anatase TiO2 Nanocrystals. J.

Catal. 2016, 341, 126-135. (24)

Liu, L.; Gu, X.; Cao, Y.; Yao, X.; Zhang, L.; Tang, C.; Gao, F.; Dong, L. Crystal-

Plane Effects on the Catalytic Properties of Au/TiO2. ACS Catal. 2013, 3, 27682775. (25)

Chen, S.; Zhang, B.; Su, D.; Huang, W. Titania Morphology ‐ Dependent

Gold–Titania Interaction, Structure, and Catalytic Performance of Gold/Titania Catalysts. ChemCatChem 2015, 7, 3290-3298. (26)

Liu, L.; Ge, C.; Zou, W.; Gu, X.; Gao, F.; Dong, L. Crystal-Plane-Dependent

Metal–Support Interaction in Au/TiO2. Phys. Chem. Chem. Phys. 2015, 17, 51335140. (27)

Li, D.; Chen, S.; You, R.; Liu, Y.; Yang, M.; Cao, T.; Qian, K.; Zhang, Z.; Tian,

J.; Huang, W. Titania-Morphology-Dependent Dual-Perimeter-Sites Catalysis by Au/TiO2 Catalysts in Low-Temperature CO Oxidation. J. Catal. 2018, 368, 163171.

27



(28)

Liu, G.; Yang, H. G.; Pan, J.; Yang, Y. Q.; Lu, G. Q.; Cheng, H.-M. Titanium

Dioxide Crystals with Tailored Facets. Chem. Rev. 2014, 114, 9559-9612. (29)

Cheng, H.; Selloni, A. Surface and Subsurface Oxygen Vacancies in Anatase

TiO2 and Differences with Rutile. Phys. Rev. B 2009, 79, 092101. (30)

Mattioli, G.; Filippone, F.; Alippi, P.; Bonapasta, A. A. Ab Initio Study of The

Electronic States Induced by Oxygen Vacancies in Rutile and Anatase TiO2. Phys.

Rev. B 2008, 78, 241201. (31)

Morgan, B. J.; Watson, G. W. A Density Functional Theory+ U Study of

Oxygen Vacancy Formation at the (110), (100), (101), and (001) Surfaces of Rutile TiO2. J. Phys. Chem. C 2009, 113, 7322-7328. (32)

Maurelli, S.; Vishnuvarthan, M.; Berlier, G.; Chiesa, M. NH3 and O2 Interaction

with Tetrahedral Ti3+ Ions Isomorphously Substituted in the Framework of TiAlPO5. A Combined Pulse EPR, Pulse ENDOR, UV-Vis and FT-IR Study. Phys. Chem.

Chem. Phys. 2012, 14, 987-995. (33)

Llansola-Portoles, M. J.; Bergkamp, J. J.; Finkelstein-Shapiro, D.; Sherman,

B. D.; Kodis, G.; Dimitrijevic, N. M.; Gust, D.; Moore, T. A.; Moore, A. L. Controlling Surface Defects and Photophysics in TiO2 Nanoparticles. J. Phys. Chem. A 2014,

118, 10631-10638. (34)

Micic, O. I.; Zhang, Y.; Cromack, K. R.; Trifunac, A. D.; Thurnauer, M. C.

Trapped Holes on Titania Colloids Studied by Electron Paramagnetic Pesonance.

J. Phys. Chem. 1993, 97, 7277-7283.

28


Page 28 of 45

Page 29 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35)

Zhang, Z.; Long, J.; Xie, X.; Zhuang, H.; Zhou, Y.; Lin, H.; Yuan, R.; Dai, W.;

Ding, Z.; Wang, X. Controlling the Synergistic Effect of Oxygen Vacancies and N Dopants to Enhance Photocatalytic Activity of N-Doped TiO2 by H2 Reduction.

Appl. Catal., A 2012, 425, 117-124. (36)

Zhang, M.; Jin, Z.; Zhang, J.; Guo, X.; Yang, J.; Li, W.; Wang, X.; Zhang, Z.

Effect of Annealing Temperature on Morphology, Structure and Photocatalytic Behavior of Nanotubed H2Ti2O4(OH)2. J. Mol. Catal. A-Chem. 2004, 217, 203-210. (37)

Kumar, C. P.; Gopal, N. O.; Wang, T. C.; Wong, M.-S.; Ke, S. C. EPR

Investigation of TiO2 Nanoparticles with Temperature-Dependent Properties. J.

Phys. Chem. B 2006, 110, 5223-5229. (38)

Howe, R. F.; Gratzel, M. EPR Observation of Trapped Electrons in Colloidal

Titanium Dioxide. J. Phys. Chem. 1985, 89, 4495-4499. (39)

Fittipaldi, M.; Gatteschi, D.; Fornasiero, P. The Power of EPR Techniques in

Revealing Active Sites in Heterogeneous Photocatalysis: The Case of Anion Doped TiO2. Catal. Today 2013, 206, 2-11. (40)

Chang, S.; Li, M.; Hua, Q.; Zhang, L.; Ma, Y.; Ye, B.; Huang, W. Shape-

Dependent Interplay between Oxygen Vacancies and Ag–CeO2 Interaction in Ag/CeO2 Catalysts and Their Influence on the Catalytic Activity. J. Catal. 2012,

293, 195-204. (41)

Chen, S.; Luo, L.; Jiang, Z.; Huang, W. Size-Dependent Reaction Pathways

of Low-Temperature CO Oxidation on Au/CeO2 Catalysts. ACS Catal. 2015, 5, 1653-1662. 29



(42)

Zhang, P.; Sham, T. X-ray Studies of the Structure and Electronic Behavior of

Alkanethiolate-Capped Gold Nanoparticles: The Interplay of Size and Surface Effects. Phys. Rev. Lett. 2003, 90, 245502. (43)

Liu, L.; Zhao, H.; Andino, J. M.; Li, Y. Photocatalytic CO2 Reduction with H2O

on TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and Exploration of Surface Chemistry. ACS Catal. 2012, 2, 1817-1828. (44)

Liao, L.-F.; Lien, C.-F.; Shieh, D.-L.; Chen, M.-T.; Lin, J.-L. FTIR Study of

Adsorption and Photoassisted Oxygen Isotopic Exchange of Carbon Monoxide, Carbon Dioxide, Carbonate, and Formate on TiO2. J. Phys. Chem. B 2002, 106, 11240-11245. (45)

Meier, D. C.; Goodman, D. W. The Influence of Metal Cluster Size on

Adsorption Energies: CO Adsorbed on Au Clusters Supported on TiO2. J. Am.

Chem. Soc. 2004, 126, 1892-1899. (46)

Weststrate, C.; Resta, A.; Westerström, R.; Lundgren, E.; Mikkelsen, A.;

Andersen, J. N. CO Adsorption on a Au/CeO2 (111) Model Catalyst. J. Phys.

Chem. C 2008, 112, 6900-6906. (47)

Weststrate, C.; Westerstrom, R.; Lundgren, E.; Mikkelsen, A.; Andersen, J. N.;

Resta, A. Influence of Oxygen Vacancies on the Properties of Ceria-Supported Gold. J. Phys. Chem. C 2008, 113, 724-728. (48)

Qian, K.; Luo, L.; Bao, H.; Hua, Q.; Jiang, Z.; Huang, W. Catalytically Active

Structures of SiO2-Supported Au Nanoparticles in Low-Temperature CO Oxidation. Catal. Sci. Technol. 2013, 3, 679-687. 30


Page 30 of 45

Page 31 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(49)

Haruta, M. Role of Perimeter Interfaces in Catalysis by Gold Nanoparticles.

Faraday Discuss. 2011, 152, 11-32. (50)

Haruta, M.; Daté, M. Advances in the Catalysis of Au Nanoparticles. Appl.

Catal. A 2001, 222, 427-437. (51)

Haruta, M. Catalysis of Gold Nanoparticles Deposited on Metal Oxides.

CatTech 2002, 6, 102-115. (52)

Bond, G. C.; Thompson, D. T. Gold-Catalysed Oxidation of Carbon Monoxide.

Gold Bull. 2000, 33, 41-50. (53)

Green, I. X.; Tang, W.; Neurock, M.; Yates, J. T. Spectroscopic Observation

of Dual Catalytic Sites during Oxidation of CO on a Au/TiO2 Catalyst. Science 2011, 333, 736-739. (54)

Park, J. Y.; Baker, L. R.; Somorjai, G. A. Role of Hot Electrons and Metal-

Oxide Interfaces in Surface Chemistry and Catalytic Reactions. Chem. Rev. 2015,

115, 2781-2817. (55)

Kim, S. M.; Lee, H.; Goddeti, K. C.; Kim, S. H.; Park, J. Y. Photon-Induced Hot

Electron Effect on the Catalytic Activity of Ceria-Supported Gold Nanoparticles. J.

Phys. Chem. C 2015, 119, 16020-16025. (56)

Nedrygailov, I. I.; Lee, H.; Lee, S. W.; Park, J. Y. Hot Electron Generation on

Metal Catalysts under Surface Reaction: Principles, Devices, and Application.

Chin. Chem. Lett. 2018, 29, 727-733.

31



(57)

Wang, Y.-G.; Cantu, D. C.; Lee, M.-S.; Li, J.; Glezakou, V.-A.; Rousseau, R.

CO Oxidation on Au/TiO2: Condition-Dependent Active Sites and Mechanistic Pathways. J. Am. Chem. Soc. 2016, 138, 10467-10476. (58)

Widmann, D.; Krautsieder, A.; Walter, P.; Brückner, A.; Behm, R. J. r. How

Temperature Affects the Mechanism of CO Oxidation on Au/TiO2: A Combined EPR and TAP Reactor Study of the Reactive Removal of TiO2 Surface Lattice Oxygen in Au/TiO2 by CO. ACS Catal. 2016, 6, 5005-5011.

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Figure captions Figure 1. Representative TEM with inserted schematic illustrations of corresponding morphologies and HRTEM images of TiO2{001} (A) , TiO2{100} (B) and TiO2{101} (C) nanocrystals. Figure 2. HAADF-STEM images and Au particle-size distributions of Au/TiO2{001} with different Au content: (A1) 0.2%, (A2) 0.5% , (A3) 1%, (A4) 2%, (A5) 5%; (B) Fitting curve of Au particle-size distributions of Au/TiO2{001} with Au content of 0.2%-5%. Figure 3. HAADF-STEM images and Au particle-size distributions of Au/TiO2{100} with different Au content: (A1) 0.2%, (A2) 0.5% , (A3) 1%, (A4) 2%, (A5) 5%; (B) Fitting curve of Au particle-size distributions of Au/TiO2{100} with Au content of 0.2%-5%. Figure 4. HAADF-STEM images and Au particle-size distributions of Au/TiO2{101} with different Au content: (A1) 0.2%, (A2) 0.5% , (A3) 1%, (A4) 2%, (A5) 5%; (B) Fitting curve of Au particle-size distributions of Au/TiO2{101} with Au content of 0.2%-5%. Figure 5. EPR spectra of (A) TiO2{001} and Au/TiO2{001} with Au content of 0.2%-5%; (B) TiO2{100} and Au/TiO2{100} with Au content of 0.2%-5%; (C) TiO2{101} and Au/TiO2{101} with Au content of 0.2%-5%. Figure 6. In situ DRIFTS spectra of CO chemisorption on various Au/TiO2 catalysts and TiO2 nanocrystals at 120 K and PCO =200 Pa: CO region. (A) Au/TiO2{001} and TiO2{001}, (B) Au/TiO2{100} and TiO2{100}, (C) Au/TiO2{101} and TiO2{101}.

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Figure 7. (A) Integrated peaks area of CO-Ti(IV) on Au/TiO2 catalysts as the function of Au content. (B) Integrated peaks area and Au-mass normalized peak area of COAu species on Au/TiO2 catalysis as the function of Au content. Figure 8. In situ DRIFTS spectra of CO chemisorption on various Au/TiO2 catalysts and TiO2 nanocrystals at 120 K and PCO =200 Pa: C-O vibrations region. (A) Au/TiO2{001} and TiO2{001}, (B) Au/TiO2{100} and TiO2{100}, (C) Au/TiO2{101} and TiO2{101}. Figure 9. CO conversion as a function of reaction temperature of (A) Au/TiO2{001}, (B) Au/TiO2{100} and (C) Au/TiO2{101} catalysts. Figure 10. (A) Apparent activation energies (Ea) of various Au/TiO2 catalysts as a function of Au loadings, and Au-mass normalized CO conversion rates as a function of reaction temperature selected for the Arrhenius plots of (B) Au/TiO2{001}, (C) Au/TiO2{100} and (D) Au/TiO2{101} catalysts.

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Figure 1. Representative TEM with inserted schematic illustrations of corresponding morphologies and HRTEM images of TiO2{001} (A) , TiO2{100} (B) and TiO2{101} (C) nanocrystals.

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Figure 2. HAADF-STEM images and Au particle-size distributions of Au/TiO2{001} with different Au content: (A1) 0.2%, (A2) 0.5% , (A3) 1%, (A4) 2%, (A5) 5%; (B) Fitting curve of Au particle-size distributions of Au/TiO2{001} with Au content of 0.2%-5%.

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Figure 5. EPR spectra of (A) TiO2{001} and Au/TiO2{001} with Au content of 0.2%-5%; (B) TiO2{100} and Au/TiO2{100} with Au content of 0.2%-5%; (C) TiO2{101} and Au/TiO2{101} with Au content of 0.2%-5%.

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Figure 6. In situ DRIFTS spectra of CO chemisorption on various Au/TiO2 catalysts and TiO2 nanocrystals at 120 K and PCO =200 Pa: CO region. (A) Au/TiO2{001} and TiO2{001}, (B) Au/TiO2{100} and TiO2{100}, (C) Au/TiO2{101} and TiO2{101}.

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Figure 7. (A) Integrated peaks area of CO-Ti(IV) on Au/TiO2 catalysts as the function of Au content. (B) Integrated peaks area and Au-mass normalized peak area of COAu species on Au/TiO2 catalysts as the function of Au content.

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Figure 8. In situ DRIFTS spectra of CO chemisorption on various Au/TiO2 catalysts and TiO2 nanocrystals at 120 K and PCO =200 Pa: C-O vibrations region. (A) Au/TiO2{001} and TiO2{001}, (B) Au/TiO2{100} and TiO2{100}, (C) Au/TiO2{101} and TiO2{101}.

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Figure 9. CO conversion as a function of reaction temperature of (A) Au/TiO2{001}, (B) Au/TiO2{100} and (C) Au/TiO2{101} catalysts.

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Figure 10. (A) Apparent activation energies (Ea) of various Au/TiO2 catalysts as a function of Au loadings, and Au-mass normalized CO conversion rates as a function of reaction temperature selected for the Arrhenius plots of (B) Au/TiO2{001}, (C) Au/TiO2{100} and (D) Au/TiO2{101} catalysts.

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TOC GRAPHICS

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Morphology-Dependent Evolutions of Sizes, Structures and Catalytic

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