Insight into the Role of Unsaturated Coordination O2c-Ti5c-O2c Sites

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Insight into the Role of Unsaturated Coordination O2c-Ti5c-O2c Sites on Selective Glycerol Oxidation over AuPt/TiO2 catalysts Pengfei Yang, Jiahao Pan, Yanan Liu, Xinyi Zhang, Junting Feng, Song Hong, and Dianqing Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03438 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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Figure 1 (A) Schematic illustration of TiO2 nanocrystal with the increase of HF addition. XRD patterns (B) and Raman spectra (C) for TiO2 nanocrystals enclosed by different facets. Structure models of (101) plane (D) and (001) plane (E) of anatase TiO2. 99x93mm (300 x 300 DPI)

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Figure 2 HRTEM images of Au1Pt3/TiO2 catalysts with different TiO2 crystal planes: (A, B) Au1Pt3/TiO2– 101; (C, D) Au1Pt3/TiO2–101–001; (E, F) Au1Pt3/TiO2–001. Insets of (A), (C), (E) are images of statistics of particle distribution and Gaussian fitting curves. (G), (H), (I) and (J) are elemental mappings of Au1Pt3/TiO2–101. (K), (L), (M) and (N) are elemental mappings of Au1Pt3/TiO2–101-001. (O), (P), (Q) and (R) are elemental mappings of Au1Pt3/TiO2-001. 129x92mm (300 x 300 DPI)


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Figure 3 Curves of glycerol oxidation performance over Au1Pt3/TiO2–101, Au1Pt3/TiO2–101–001 and Au1Pt3/TiO2–001. Abbreviations: DHA, dihydroxyacetone; GLYAD, glyceraldehyde; GLYA, glyceric acid. Conditions: 0.3 M glycerol, glycerol/metal = 500:1, 0.1 MPa O2, T = 60oC. 129x90mm (300 x 300 DPI)


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Figure 4 (A) In situ CO2 FT-IR spectra of TiO2-101, TiO2-101-001, TiO2-001; Illustration of monodantate carbonate on (101) plane (B) and bidentate carbonate on (001) plane (C). 129x68mm (300 x 300 DPI)


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Figure 5 In situ FT-IR spectra of formaldehyde at programmed raised temperatures over TiO2-101 (A), TiO2-101-001 (B) and TiO2-001 (C). 60x127mm (300 x 300 DPI)


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Scheme 1 Illustration of glyceraldehyde isomerization and oxidation over pristine TiO2 and corresponding catalysts terminated with (101) plane (A) and (001) plane (B) 99x99mm (300 x 300 DPI)


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Figure 6 XPS spectra for Au1Pt3 supported on TiO2-101, TiO2-101-001, TiO2-001, and the ratio of compound form to correspond metallic form. 119x86mm (300 x 300 DPI)


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Figure 7 Kinetic studies of glyceraldehyde oxidation reaction on (A) Au1Pt3/TiO2-101, (B) Au1Pt3/TiO2-101001, and (C) Au1Pt3/TiO2-001. Condition: 0.015 M glyceraldehyde solution, glyceraldehyde: metal=500:1, 0.1 MPa O2, 60oC, 1000 rpm. 129x52mm (300 x 300 DPI)


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Scheme 2 A possible reaction pathway for the oxidation of glyceraldehyde over the Au1Pt3/TiO2-001 catalyst. 129x93mm (300 x 300 DPI)


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Table of Content 85x44mm (300 x 300 DPI)


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Insight into the Role of Unsaturated Coordination O2c–Ti5c– O2c Sites on Selective Glycerol Oxidation over AuPt/TiO2 catalysts Pengfei Yang1, Jiahao Pan1, Yanan Liu1, Xinyi Zhang1, Junting Feng1, 2*, Song Hong3, and Dianqing Li1*

1 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China 2 Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, China 3 Center for Instrumental Analysis, Beijing University of Chemical Technology, Beijing, China

* Corresponding author Address: Box 98, 15 Bei San Huan East Road, Beijing 100029, China Tel: +86 10 64436992

Fax: +86 10 64436992

E–mail address: [email protected] (Junting Feng) [email protected] (Dianqing Li)

ABSTRACT: In this work, we synthesized a series of TiO2 enclosed with different terminated facet and investigated the catalytic performance on glycerol oxidation catalysts over Au1Pt3/TiO2 under base–free condition. To our surprise, Au1Pt3/TiO2 catalyst presented the support facet–dependent catalytic performance. Different TiO2 crystal planes apparently affect the oxidation products distribution especially the selectivity of glyceraldehyde and glyceric acid but have inconspicuous influence on activity of glycerol oxidation. Primarily, the coordination unsaturated sites O2c–Ti5c– O2c of (001) facet highly dissociated C=O bond of aldehyde group in a bidentate form characterized by in–situ CO2 and formaldehyde Fourier Transform Infrared


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Spectroscopy. In another aspect, Auδ+ species were generated on the interface between AuPt nanoparticles and the (001) facet of TiO2 due to the strong interaction, which facilities the insertion of oxygen species during the process of oxidation to glyceric acid. With the aid of bidentate intermediates and Ti5c–O2c–Auδ+ active sites, the ability of glyceraldehyde further oxidation is enhanced due to the decreased active energy of glyceraldehyde over Au1Pt3/TiO2–001, revealed by the kinetic studies of glyceraldehyde oxidation. These findings provide a unique perspective and feasible approach to manipulate the products distribution in multistep continuous oxidation reactions. KEYWORDS: Glycerol Selective Oxidation, Shaped TiO2, Crystal–Plane–Effects, Unsaturated Coordination Sites, Bidentate Intermediates

1. INTRODUCTION Selective oxidation of glycerol, which is the main byproduct of biodiesel via transesterification of triglycerides with alcohols, not only is an efficient way to produce high value–add chemicals but also has great academic appeals1–3. Nevertheless, the pathway of glycerol selective oxidation is extremely complex involving the issues of oxidation position—primary versus secondary hydroxyl group—and the depth of oxidation to aldehyde, carboxyl acid or even cleavage products4, 5. Consequently, it is an enormous challenge to achieve a satisfying selectivity towards a single desired product under high conversion. Supported precious metal catalysts are well known as the most active materials for glycerol oxidation6–12. Heterogeneous catalytic reaction usually happens on the surface of catalyst, herein the surface composition and structure of supported catalysts would remarkably influence catalytic performance13–16. Initially, metallic active sites have been intensively investigated and an evident impact on the course of reaction and the resulting distribution of products involving the primary and secondary hydroxyl group oxidation products as well as the products with different extent of oxidation depth were well reported17–22. Recently, the properties of supports, which were normally neglected,


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are drawing more attentions to devise the constitution and structure of catalysts23, 24. In the case of supported AuPt catalysts, basic oxide supports have superiority of activation for glycerol oxidation but a lower selectivity towards C3 products because of the C–C cleavage, whereas the acidic oxide supports distinctly reduce the C–C cleavage but have a relative lower oxidation activity with high selectivity of glyceraldehyde and low selectivity of glyceric acid. It is thus concluded that the nature of support also has great effects on both activity and selectivity of products, but it is still phenomenological on intrinsic relationship between the support structure and the oxidation behavior of the glycerol molecule. From the view of microscopic structure, the different acid/base properties are radically caused by disparate coordination structures25, 26 and an escalating endeavor has been devoted to reveal the role of surface coordination unsaturated sites in some certain reactions. Kemnitz and co–authors27 demonstrated the unsaturated 4–fold and 5–fold coordinated Al–sites greatly improve the activity of ethene hydrophenylation under mild conditions. Furthermore, Cong’s group28 found the coordinative unsaturation 5–fold Al sites of amorphous alumina can anchor the subnanometric Ru particles with strong interaction, which lead to a high turnover frequency of benzene hydrogenation. Besides, coordination unsaturated Ce3+ and the resulting oxygen vacancy on the surface of CeO2 support has been found to reform the reaction pathway through a formate intermediate in CO2 methanation over Ru catalysts29, resulting in a high selectivity of methanol instead of CH4. Herein, the accurate control of unsaturated sites on the surface of supports is very likely to manipulate the pathway to obtain the different products distribution of glycerol oxidation. The adjustment of support morphology, especially the metal oxide, with different terminated facets is an effective way to obtain the certain coordination unsaturated sites30. Because of the long–range ordered structure of crystal and the asymmetry of terminated facet, the surface coordination of oxygen and metal atoms usually stay uniform and unsaturated. Consequently, the catalytic performance of metal oxides with different morphology as both support and catalyst shows conspicuous crystal–plane– dependent effects31,

32

. Dong et al.33 studied the crystal–plane effects on Au/TiO2


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catalyst and found that the strong electronic interaction occurs on the interfaces of Au nanoparticles and (100) facet of TiO2, resulting in a decrease of active energy of CO oxidation. Huang et al.34 demonstrated the crystal–plane–controlled selectivity in propylene oxidation over different morphologies of Cu2O nanoparticles. Under identical conditions, partial oxidation occurs on Cu2O (111) facet with a major product of acrolein, whereas combustion reaction is dominant with a product of CO 2 on Cu2O (100). Besides, Cu2O rhombic dodecahedra exposing (110) crystal planes is the most selective for propylene oxide. However, up to date, barely research focus on the adsorption and activation behavior of reactants as well as intermediates on the coordination unsaturated sites over different planes in the multistep glycerol oxidation reaction. It thus inspires us to proceed the fundamental research on the relationship between the coordination structure of support and the catalytic behavior in selective oxidation of glycerol to push our understanding at the molecular/atomic level. In this work, on the basis of good selectivity towards C3 products achieved by acidic oxides, we chose titanium dioxide as the catalyst support and tuned its morphology to regulate the surface coordination structure. Specifically, by increasing the ratio of (001) facet of anatase TiO2, an escalated amount of surface unsaturated O2c–Ti5c–O2c sites was obtained. The Au1Pt3 nanoparticles, which are demonstrated a marvelous glycerol oxidation performance by previous literatures19, 23, 24, were further immobilized on the surface of TiO2 nanocrystal. As expected, the Au1Pt3/TiO2 catalysts shows a conspicuous crystal–plane effect on glycerol oxidation performance. Surprisingly, the selectivity of glyceric acid is gradually increased by coordination unsaturated Ti5c–O2c sites of (001) facet while a totally inverse phenomenon is observed in case of glyceraldehyde, which implies the main contribution of coordination unsaturated sites reflected in the deep oxidation of C=O bond. From the results of in situ Fourier Transform infrared spectroscopy (in situ FT–IR) with CO2 and formaldehyde as the probe molecule, the different adsorption form of aldehyde group was observed on both pristine TiO2 and corresponding catalysts enclosed by different facets. Besides, Ti5c– O2c site also has a strong interaction with Au1Pt3 nanoparticles particularly the Au species, which facilitates the H–abstraction to achieve final acid products. The kinetic


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studies and possible mechanism of support–facet–dependent glycerol oxidation are also proposed. Most meaningfully, the role of unsaturated Ti5c–O2c site on the intermediate adsorption behavior revealed in this work deepens the understanding the glycerol oxidation process, which provides a unique perspective and feasible approach to manipulate the product distribution in multistep continuous oxidation reactions.

2. EXPERIMENTAL SECTION 2.1 Synthesis of titanium dioxide. Anatase TiO2 nanocrystals enclosed with variant ratio of (001) facet were prepared by a hydrothermal synthesis method with different HF amount as described in ref. 35. To prepare TiO2 enclosed with (001) facet, 20 mL titanium tetrisopropanol was used as the Ti source and mixed with 5 mL 48% HF in a Teflon–lined autoclave stirring for 30 minutes. The autoclave was then transferred to an electric oven and kept at 200oC for 24 h. Subsequently, the autoclave was cooled to room temperature in water flow. The white precipitates were collected by high–speed centrifugation and washed by ethanol and deionized water until the pH reached 7. After dried at 80oC overnight, the final powder was calcined at 450oC for 2 h with the rate of 10oC/min, named as TiO2–001. For TiO2 exposed with (101) and (101)–(001) planes, the same procedures were conducted but using 2 mL of H2O and 2 mL of 48% HF, respectively. 2.2 Preparation of Au1Pt3 supported catalysts. The Au1Pt3/TiO2 catalysts were prepared by sol–immobilization method. Firstly, the solution of polyvinyl alcohol (PVA, the weight ratio of PVA to Au–Pt is 1.2:1) was added into 100 mL lab–made deionized water. A certain volume of HAuCl4 and H2PtCl4 solution with the mole ratio Au to Pt=1:3 was added into the previous PVA solution. Subsequently, a freshly prepared 0.1 M aqueous NaBH4 (NaBH4/metal fraction = 5) was added to form a dark–brown sol for 30 minutes of generation. The colloidal Au1Pt3 nanoparticles were then immobilized onto the obtained TiO2 materials with different facet by tuning the isoelectric point at pH 3~4 with 0.1 M H2SO4 solution. After immobilization for 1 h, the slurry was filtered washed by DI water 3 times and dried at 80oC overnight. The three catalysts were


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named as Au1Pt3/TiO2–101, Au1Pt3/TiO2–101–001 and Au1Pt3/TiO2–001. 2.3 Characterization methods. The crystal structure of a series of TiO2 and corresponding supported catalysts were identified by X–ray diffraction (XRD) (Shimadzu XRD–600) using a Cu Kα radiation source (λ = 0.154 nm) in the range from 20o to 80o with a scan step of 10o/min. Raman measurement was carried out using a Raman spectroscopy (Renishaw, inVia–Reflex, 633 nm). The morphology of the synthesized TiO2 and supported catalysts were characterized by JEOL JEM–2100F high–resolution transmission electron microscope (HRTEM). Cs–corrected STEM images were obtained by JEOL ARM200F microscope equipped with a probe–forming spherical–aberration corrector. Metal content was measured by a Shimadzu ICPS–7500 inductively coupled plasma emission spectrometer (ICP–AES). The X–ray photoelectron spectroscopy (XPS) spectra were collected using a Thermo VG ESCALAB 250 spectrometer equipped with an Al Kα anode, with the calibration peak being the C 1s peak at 284.6 eV. Fourier transform infrared spectra (in–situ FTIR) were recorded on a Bruker Tensor 27 spectrometer installed with a highly sensitive MCT detector and a heating chamber equipped with KBr windows. 40 mg samples were firstly tableted with the IR mold to be used in the later measurements. For CO2 in situ FT–IR measurements, the sample was pretreated at 100oC under a flow of He for 1 h to remove the gaseous impurities absorbed on the surface. After collecting the background spectra, the flow was switched to CO2 at a rate of 40 mL/min for 1 h. The spectra were continuously collected during the absorption of CO2 until the intensity of absorbed CO2 stable. After removing the gaseous and physical absorbed CO2 with the He for 0.5 h, spectra were collected in the range 2000−1000 cm−1 with 64 accumulation scans at room temperature. For in–situ formaldehyde FT–IR, formaldehyde solution was used as the formaldehyde source. The procedures of pretreatment are same as that of in situ CO2 FTIR. After cooling to the room temperature, the background spectra data were collected with a temperature programmed raising to 100oC at a rate of 10oC/min every 10oC. Again cooled to room temperature, a bubble equipment was induced before the sample cell and the formaldehyde gas was bubbled by He flow to get into the sample cell. Then, the spectra were collected in the range of 4000−400 cm-1 with accumulation


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scans every 10oC under a programmed raise of temperature of 10oC/min. 2.4 Glycerol oxidation test The liquid–phase aerobic oxidation of glycerol oxidation was performed in a 50 mL quartz glass reactor equipped with a heating block and magnetic stirrers. In a typical reaction, 10 mL 0.3 M glycerol solution and requisite amount of catalyst (glycerol/active metal molar ratio was 500:1) were added. The reactor was purged with continuous oxygen flow for 30 s and kept O2 pressure at 1 bar. Subsequently, the glass reactor was kept in a heating block which is preheated to 60oC. After a specific time, the samples were rapidly cooled in a cold–water bath and filtered to remove the solid catalyst. The reaction mixture was analyzed by high–pressure liquid chromatography (HPLC, Shimadzu LC–20AT) equipped with an ultraviolet (210 nm) and a refractive index detector in series. An Agilent Zarbox SAX column (4.6 mm × 250 mm, 5 μm) was employed for products separation with a mixed solution of H3PO4 (0.1% w/w) in H2O/acetonitrile (1/2, v/v) (1.0 mL/min) as the eluent at 30oC. The reaction mixture was diluted 20 times with ultrapure water to be tested. An external calibration method was used for the quantification of the reactants consumed and products generated. 2.5 Kinetic study The oxidation kinetic study was carried out in sequential measurements as a function of the reaction temperature from 60oC to 75oC with conversion below 15%. Typically, 5 mL 0.3 M glycerol solution and a certain amount of catalyst (glycerol/active metal molar ratio was 500:1) were added into glass reactor and other procedures are same as glycerol oxidation test described in 2.4. Time−conversion plots at various temperatures and Arrhenius plots were employed for the calculation of the activation energy of glycerol oxidation over series catalysts. For the activation energy calculation of glyceraldehyde, same method was applied but 5 mL 0.015 M glyceraldehyde solution and the amount of catalyst (glyceraldehyde/active metal molar ratio was 100:1) were used.

3. RESULTS AND DISCUSSIONS


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3.1 Characterization of pristine TiO2 and supported Au1Pt3 catalysts.

Figure 1 (A) Schematic illustration of TiO2 nanocrystal with the increase of HF addition. XRD patterns (B) and Raman spectra (C) for TiO2 nanocrystals enclosed by different facets. Structure models of (101) plane (D) and (001) plane (E) of anatase TiO2.

With the aid of capping agent HF, the growth rate in [101] and [001] direction of TiO2 are controlled (as described in 2.1 and Figure 1A) during the hydrothermal synthesis process36. The prepared TiO2 nanocrystals have a well–crystallized anatase structure and the main diffraction peaks in the pattern are identical with the standard card [JCPDS No. 21–1272] (as shown in Figure 1B). With the increasing amount of HF addition, the XRD peak intensities were aggrandized indicating that the fluoride improves the crystallization of TiO2 nanocrystals. It is worth noting that the full width of (004) diffraction peak was broadened, while that of (200) was narrowed, which reveals the decrease of thickness in the direction of [001] and the increase of length along [100] direction. As listed in Table 1, the thickness of TiO2 nanoparticles decreases


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from 29.8 nm to 19.9 nm, while the length increases from 37.7 nm to 64.3 nm, according to Bragg's Law. Therefore, the percentage of the exposed (001) facets can be estimated by the equilibrium truncated octahedron model of an anatase TiO2 single crystal and the detailed calculations for the ratio of (001) facet are illustrated in Figure S1.The result reveals the percentage of (001) increased from 24% to 55% with the addition of HF. Table 1 Physical properties of obtained TiO2 nanomaterials HF

average thickness

average length

percentage

(mL)

(nm)

(nm)

of (001)

TiO2–101

0

29.8

37.7

24%

TiO2–101–001

2

20.6

49.7

46%

TiO2–001

5

19.9

64.3

55%

Sample

Raman spectroscopy was then employed for characterizing the surface structure of synthesized TiO2 nanoparticles. According to the results of Raman spectroscopy (Figure 1C), all these TiO2 nanocrystals have similar peaks appearing at 144, 394, 514, and 636 cm−1, which indicates an anatase phase of TiO2 in according with XRD. Compared to (101) facet, the number of the symmetric stretching vibration modes of O−Ti−O decrease when the exposed (001) facets exist and correspondingly, the intensity of the Eg peaks in the Raman spectra obviously decreased while the A1g increased. It has been reported that the peaks of Eg, B1g and A1g are attributed to symmetric stretching vibration, symmetric bending vibration and antisymmetric bending vibration of O–Ti–O, respectively. As illustrated in structure models (Figure 1D and 1E), the (101) facet exposes both saturated O3c– Ti6c– O3c and unsaturated O2c– Ti5c– O2c sites, whereas only unsaturated O2c– Ti5c– O2c sites are distributed on the top layer of the (001) surface, thus leading to an increase of antisymmetric structure37. In consequence, three TiO2 nanocrystal samples with an increasing ratio of (001) facet identified by various characterizations were successfully prepared. Supported precious metal nanoparticles have shown tremendous superiority in catalytic glycerol


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oxidation reported by previous studies and the bimetallic Au–Pt with a mole ratio 1:3 was demonstrated to be an effective catalytic materials in terms of activity and selectivity in the absence of base. To eliminate the discrepancies of nanoparticle size as much as possible, the supported bimetallic Au1Pt3 catalysts were prepared by a colloidal method. The metal loading are 0.96%, 0.92% and 0.95% from the results of ICP–AES for Au1Pt3/TiO2–101, Au1Pt3/TiO2–101–001 and Au1Pt3/TiO2–001, respectively, which is approximate to the desired value (1%). There are no diffraction peaks of metal components except the peaks of TiO2 in XRD patterns of synthesized Au1Pt3/TiO2 catalysts (Figure S2). This indicates that the bimetallic Au1Pt3 nanoparticles have a small size, which cannot be detected by Bragg diffraction.

Figure 2 HRTEM images of Au1Pt3/TiO2 catalysts with different TiO2 crystal planes: (A, B) Au1Pt3/TiO2–101; (C, D) Au1Pt3/TiO2–101–001; (E, F) Au1Pt3/TiO2–001. Insets of (A), (C), (E) are images of statistics of particle distribution and Gaussian fitting curves. (G), (H), (I) and (J) are elemental mappings of Au1Pt3/TiO2–101. (K), (L), (M) and (N) are elemental mappings of Au1Pt3/TiO2–101-001. (O), (P), (Q) and (R) are elemental mappings of Au1Pt3/TiO2-001.

The morphology, exposed plane and particle size of pristine TiO2 nanocrystals


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(Figure S3) as well as synthesized Au1Pt3/TiO2 catalysts (Figure 2) were characterized by HRTEM. Consistent with the result of XRD patterns, the nanocrystal size of TiO2 increased with the amount of HF. Furthermore, the capping agent HF apparently shaped the terminated facet of obtained TiO2 with a distinct difference in lattice fringes. Figure S3B shows that sample TiO2–101 has a clear lattice fringe with an interplannar spacing of 0.35 nm, corresponding to a typical (101) facet of anatase TiO2. In Figure S3F, the sample TiO2–001 exhibits the fringe with a lattice spacing of 0.24 nm for the (001) planes. In case of the catalysts, bimetallic Au1Pt3 nanoparticles highly dispersed on the surface of TiO2 support with a small particle size between 2 nm and 3 nm. It is not surprising that TiO2 supports with different terminated facet did not significantly affect the mean size of supported Au1Pt3 nanoparticles by using sol–immobilization method. Besides, the Au and Pt components in each catalyst show a high alloying extent from mapping of spherical aberration corrected Transmission Electron Microscope (Figure 2G ~ R), suggesting the conformity on bimetallic structure of Au1Pt3 nanoparticle over different TiO2 supports. From the result of CO–FTIR (shown in Figure S4), carbon monoxide molecule linearly absorbed on the site of Au and Pt species with a small amount of bridged CO on Pt terrace for the three samples38–40, further implying the resemblance of metallic sites. As the consequence, the similarity of metal nanoparticles has built up a good basis for studying the effects of different TiO2 exposed facet.

3.2 Selective Glycerol Oxidation Performance


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Figure 3 Curves of glycerol oxidation performance over Au1Pt3/TiO2–101, Au1Pt3/TiO2–101–001 and Au1Pt3/TiO2–001. Abbreviations: DHA, dihydroxyacetone; GLYAD, glyceraldehyde; GLYA, glyceric acid. Conditions: 0.3 M glycerol, glycerol/metal = 500:1, 0.1 MPa O2, T = 60oC.

Glycerol oxidation is a sequential reaction that can follow multiple paths, which brings such a difficulty controlling selectivity of different products. Tuning the structure of catalysts is extensively regarded as the most effective method to manipulate the catalytic performance; therefore, the performance of series Au1Pt3/TiO2 catalysts in glycerol oxidation under base–free condition was investigated. All the catalysts have a good carbon balance (>95%) and no obvious glycolic acid is formed. Figure 3A shows the corresponding glycerol conversion curves as a function of time, which exhibits an almost indiscrimination over Au1Pt3/TiO2 catalysts with different support facets. As expected, the selectivity of dihydroxyacetone (DHA), the oxidation product of secondary hydroxyl, mainly keeps at a low and comparatively stable terrace which is much lower than that of glyceraldehyde and glyceric acid, hinting that the oxidation of primary hydroxyl group is dominant over AuPt based catalysts (Figure 3B). Besides, glyceraldehyde has a large amount initially but decreases during the process of reaction, in contrast with glyceric acid (shown as Figure 3C and 3D). It is generally accepted that


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glycerol oxidation over supported AuPt catalysts undergoes dehydrogenation to an aldehyde or ketone intermediate, followed by oxidation to the acid product. Surprisingly, a notable distinction of products selectivity (Figure 3B, C and D) can be observed over Au1Pt3/TiO2–101 and Au1Pt3/TiO2–001 during the whole reaction process. It is interestingly found that the Au1Pt3/TiO2–001 sample obviously lowers the selectivity towards glyceraldehyde and promotes glyceric acid selectivity. A table reporting the selectivity to all the products at iso-coversion is listed as Table S1. At the conversion around 50%, the selectivity of glyceraldehyde is only 25% over Au1Pt3/TiO2–001, much lower than the value of 44% over (101) facet. According to previous literature23, 24, the acidic supports such as TiO2 have difficulty in the further oxidation of aldehyde group. Since the sol–immobilization condition (the amount of PVA and NaBH4, volume of solution and stirring speed) were kept the same for Au1Pt3/TiO2 catalysts, the above differences are more likely to originate from the nature of the support itself. Herein, the reasonable explanation on a decrease of glyceraldehyde selectivity just by tuning the terminated facet particularly the coordination structure is worthy of attention and being investigated to further understand the glycerol oxidation reaction.

3.3 Investigation of TiO2 surface coordination structure on intermediates adsorption form.

Figure 4 (A) In situ CO2 FT–IR spectra of TiO2–101, TiO2–101–001, TiO2–001; Illustration of


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monodentate carbonate on (101) plane (B) and bidentate carbonate on (001) plane (C).

Carbon dioxide is a common probe molecule and widely used to investigate the surface properties of catalytic materials41. Due to the linear symmetrical of CO2 molecule, different absorption types of CO2 are sensitive to the surface structure and can be detected by infrared spectroscopy. Herein, in situ CO2 FT–IR spectra of anatase TiO2 have been conducted to identify the properties of different terminated facet of TiO2 particularly the surface coordination structure (shown in Figure 4). Notably, the kinds of carbonates developed on divergent crystal plane show a significant difference, turning out the distinction of the interactions of CO2 with TiO2. Various adsorbed CO2 species including monodentate carbonate33, 35 (m–CO32-, at 1540−1548 cm-1, 1512 cm1

and 1348 cm-1), bicarbonate (HCO3-, at 1409 cm-1) and carboxylate (CO2-, at 1665 cm-

1

and ) are formed on the TiO2–101. The small shoulder peaks at 1602 cm–1 and 1560

cm–1 are attributed to bidentate carbonate (b–CO32-), which indicates that the bidentate carbonate is not the major type of absorbed CO2. Both m–CO32- and b–CO32- are generated on TiO2–101–001, while for TiO2–001 sample, b–CO32- is dominant with a small amount of m–CO32- in contrast with TiO2–101. Only weak CO2- at 1650 cm-1 and HCO3- at 1425 cm-1−1435 cm-1 were observed on TiO2–101 and TiO2–001. It was reported in the previous works33,

42

that the formation of absorbed carbon dioxide

species required different surface structures. Specifically, CO2- and HCO3- are developed by CO2 absorbed on Ti3+ and Ti–OH- sites, respectively. The m–CO32species are preferable to be generated on O2- sites, however b–CO32- need both O2- and adjacent Ti sites (Ti–O2- pair). As previous discussion, the (001) facet of anatase TiO2 have more coordination unsaturated sites than that of (001). The b–CO32- species become more dominant with the increase of (001) facet in the order of TiO2–001 > TiO2–101–001 > TiO2–101, while the m–CO32- species decreases. It indicates that CO2 predominately coordinates with Ti5c−O2c pairs on the surface of (001) facet to create b– CO32- species, whereas m–CO32– are formed by the coordination of O2c or O3c site without the participation of adjacent Ti sites. Moreover, the C=O bond of CO2 is broken during the formation of b–CO32-, which


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implies that the degree of C=O bond dissociation is higher on Ti5c−O2c sites of (001) by bidentate than that on (101) facet by monodentate. Considering the glycerol oxidation performance that (001) plane lowers the selectivity of GLYAD and heightens the selectivity of GLYA, the degree of absorbed C=O bond dissociation may plays a decisive role in the process of GLYAD oxidation to GLYA.


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Figure 5 In situ FT–IR spectra of formaldehyde at programmed raised temperatures over TiO2–101


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(A), TiO2–101–001 (B) and TiO2–001 (C).

Considering that CO2 is a liner nonpolar molecule and the GLYAD has difficulties to be directly introduced to FT–IR system due to the low saturated vapor pressure, formaldehyde as the probe molecule is employed to investigate the absorption behavior of aldehyde group. Besides, many studies43, 44 have reported that the certain function group such as aldehyde group can be adsorbed by supports. Herein, the species of adsorbed formaldehyde are investigated on pristine TiO2 at first. The FT–IR spectra of formaldehyde at different temperatures from 30oC to 100oC over different facet of anatase TiO2 are shown in Figure 5. The differences in two regions around 3000 ~ 2700 cm-1 and 1600 ~ 1200 cm-1 respectively associated with C–H bonds and C–O bond indicate the formation of various absorbed formaldehyde species on different TiO2 surface structure. In case of TiO2–101 sample at a low temperature of 30oC (shown in Figure 5A), the major peaks at 2912cm-1, 2863cm-1 and 2762 cm-1 are assigned to the C–H stretching mode (vC–H). Besides, the peaks at 1415 cm-1 and 1253 cm-1 are assigned to the deformation vibration mode of δC–H and δH–O–C of molecularly adsorbed formaldehyde species. A small amount of dissociated adsorbed formate species whose peaks at 2923 cm-1 (vC–H), 1358 cm-1 (vO–C–O) and 1305 cm-1 for HCOO- can also be detected45–48. With the raise of temperature, the peak intensities of molecularly adsorbed formaldehyde species vanish while that of dissociated adsorbed formate species increase. The transformation from molecular adsorption to dissociated adsorption indicates a higher energy is required to dissociate formaldehyde on the TiO2 surface. Notably, the ratio of peaks at 1413 cm-1 to 1354 cm-1 of (001) facet (shown in Figure 5C), which belong to molecularly adsorbed and dissociated formaldehyde species respectively, is much lower than that of TiO2–101 and TiO2–101–001 sample. Furthermore, the disappearance temperature (approx. 70oC) of the peak at 1413 cm-1 owing to molecularly adsorbed formaldehyde species over (001) facet is the lowest among three samples, suggesting the disappearance of molecularly adsorbed species. These results demonstrate the (001) facet of TiO2 has the highest ability of dissociating


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the formaldehyde. In addition, a couple of new peaks occur with the raising of temperature, indicating the formation of a variety of absorbed species. The peak appearing at 2954 cm-1 is owing to the combination of vas O–C–O (asymmetrical stretching vibration) and δip

C–H

(in–plane deformation vibration mode). The ones at 2923 cm-1,

2737 ~ 2740 cm-1 and 1378 cm-1 as well as the previously mentioned 1305 cm-1 are originated from the absorbed formate species (HCOO-). Furthermore, the peaks at 2846 ~ 2850 cm-1, 2830 ~ 2831 cm-1 and 1587 cm-1 are attributed to bidentate formate species, while the 1554 cm-1 one is originated from monodentate formate species. For TiO2–101 sample, both bidentate and monodentate formate species are increased with the temperature rising. TiO2–101–001 sample has familiar characters with TiO2–101 but a higher ratio of bidentate formate species. As further increase of (001) facet, bidentate formate species become the main dissociated formate species on TiO2–001 sample, same as the adsorbed CO2 species. Besides of the adsorption form on pristine support, the effects of metallic nanoparticles on the adsorption behavior should also be considered. From the result of formaldehyde FTIR of Au1Pt3 based catalysts (Figure S5), molecularly adsorbed formaldehyde species are reduced, which illustrates the Au1Pt3 nanoalloy particles could further facilitate the formation of dissociated formate species. The bidentate species of aldehyde are also generated on coordinative unsaturation Ti5c–O2c sites of (001) facet after loading metallic nanoparticles, but a slightly blue shift of C–O bonds peaks was observed for about 35cm-1, which shows a strong interaction between absorbed aldehyde species and metallic nanoparticles. It is worth noting that the coordination unsaturated sites of (001) facet plays a decisive role in glyceraldehyde absorption and activation. Quite similar with the absorption of CO2 over the structure O2c–Ti5c–O2c on (001) surface, bidentate formate species are formed by coupling Ti5c site with oxygen atom and O2c sites with carbon atom of C=O in aldehyde group. Because of the specific absorption form of C=O bond, the aldehyde group has a higher degree of dissociation on (001) facet than that on (101), which facilitate the further oxidation of GLYAD to GLYA.

3.4 Insight into the effects of metallic nanoparticles on glyceraldehyde oxidation


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With no doubt, the catalytic activity and/or the selectivity of catalysts are strongly influenced by the metal nanoparticles in many reactions as well as glycerol oxidation. The O2c–Ti5c–O2c coordinative unsaturation sites are demonstrated to be favorable for glyceraldehyde oxidation to the acidic products but the roles of metallic nanoparticles are still undefined. Therefore, the catalytic performance over pristine TiO2 and supported catalysts are investigated utilizing the aqueous solution of glyceraldehyde as the reaction substrate.

Table 2 The oxidation performance of GLYAD over Au1Pt3 supported TiO2 with different ratio of (001) facet

Samples

DHA

GLYA

GLYCA

Carbon

Selectivity

Selectivity

Selectivity

balance

(%)

(%)

(%)

(%)

75 53 48 20

56 49 55 10

0 0 0 50

0 0 0 0

56 49 55 60

46

2

83

1

86

45

1

88

1

90

Conversion (%)

TiO2–101a TiO2–101–001a TiO2–001a Au1Pt3/TiO2–101c Au1Pt3/TiO2–101– 001c Au1Pt3//TiO2–001c

a

The weight of TiO2 = 0.0587 g, the concentration of GLYAD = 0.015 M, 0.1 MPa O2, T = 80oC, t

= 4 h. b

AuPt catalysts, the concentration of GLYAD = 0.015 M, GLYAD/metal (Au and Pt) = 500:1, 0.1

MPa O2, T = 80oC, t =4 h.

As listed in Table 2, it is interesting that glyceraldehyde isomerization to dihydroxyacetone occurs on the surface of TiO2 but oxidation occurs over AuPt/TiO2 catalysts, indicating that oxidation reaction still needs the participation of Au1Pt3 nanoparticles. From the data of glyceraldehyde conversion, the conversion of isomerization are decreased in the order of TiO2–101 > TiO2–101–001 > TiO2–001. The C=O bond partially dissociates in monodentate species on (101) facet followed by the


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H–transfer of secondary hydroxyl group process to produce dihydroxyacetone. However, in bidentate species on (001) facet, the oxygen atom of aldehyde group is chelated with unsaturated Ti5c site, which creates a barrier of the H–transfer process. After loading the AuPt nanoparticles, the isomerization is distinctly suppressed and the oxygen can be activated to motivate the oxidation of glyceraldehyde (as illustrated in Scheme 1). Besides, the selectivity of glyceric acid over (001) facet is still higher than that of the others consistent with the catalytic glycerol oxidation performance. The carbon balance are less than 90%, indicating other side reactions also exist in glyceraldehyde oxidation but the products were not determined except glycolic acid under this HPLC condition. The gaseous products were also qualitative analyzed by gas chromatography and CO2 is determined as the side gas product during the reaction.

Scheme 1 Illustration of glyceraldehyde isomerization and oxidation over pristine TiO2 and corresponding catalysts terminated with (101) plane (A) and (001) plane (B)


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Figure 6 XPS spectra for Au1Pt3 supported on TiO2–101, TiO2–101–001, TiO2–001, and the ratio of compound form to correspond metallic form.

From the result of glyceraldehyde oxidation, even though there are different adsorbed aldehyde species on (101) and (001) facet, the oxidation process still need the participation of bimetallic nanoparticles, indicating the nonnegligible effects of metal nanoparticles. Therefore, XPS analysis is applied to investigate variation of the chemical state of Au and Pt supported on TiO2 with different ratio of (001) facet. In the region of Ti 2p (as shown in Figure 6C), two major peaks at 485.6 and 464.3 eV are owing to typical characteristic of Ti4+ 2p3/2 and 2p1/2 with two small shoulders at 485.1 and 463.5 eV for Ti3+ 2p3/2 and 2p1/2. The ratio of Ti3+/ (Ti3++Ti4+) is declined with the increase of (001) facet, constant with the result of CO2–FTIR (Figure 5) that the TiO2– 101 sample has the most amount of CO2- species coordinated with Ti3+. In the spectra of Au 4f and Pt 4f, the variation of cationic Auδ+ (at 83.5 and 87.2 eV) and Pt2+ (at 72.0 and 75.2 eV) contents show quite a distinct tendency in different samples. The Pt2+ content slightly fluctuates from 34% to 32%, while the Auδ+ content ascends from 20% to 28% with the augment of (001) ratio. The Au0 species are oxidized to Auδ+ on the surface of TiO2 (001) facet without any oxidizing gas or high temperature treatment, which could be ascribed to the strong interaction between Au1Pt3 nanoparticles


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especially Au species with coordination unsaturated sites O2c–Ti5c–O2c. As mentioned above, the wavenumber of liner CO peaks linking with Au increase from 2083 cm-1 to 2087 cm-1 (from the result of CO–FTIR, shown in Figure S4), which indicates that an increase of positive metal content. Besides, in the structure of O2c–Ti5c–O2c, the coordination unsaturated O2c site generally has a tendency of combining the Au species. Herein, Ti5c–O2c–Auδ+ sites are formed in the interface between metal nanoparticles and TiO2 supports, which could facilitate the activation of oxygen activation and the oxidation of aldehyde group. Apparent activation energy values of glyceraldehyde oxidation over Au1Pt3/TiO2– 101, Au1Pt3/TiO2–101–001, and Au1Pt3/TiO2–001 catalysts are obtained with a conversion lower than 15%, as shown in Figure 7. With no doubt, Au1Pt3/TiO2–001 catalysts has the lowest active energy of glyceraldehyde oxidation (16.46 kJ/mol for (001) facet and 28.84 kJ/mol for (101) facet). The result further proved the Au1Pt3/TiO2–001 has a dramatic ability to further oxidation of glyceraldehyde with the aid of Ti5c–O2c–Auδ+ sites and bidentate absorbed intermediates. Besides, the activation energy for glycerol oxidation were also calculated and it only exhibits slight difference with the value of 41.32 kJ/mol for Au1Pt3/TiO2-101, 43.15 kJ/mol for Au1Pt3/TiO2-101001 and 37.58 kJ/mol for Au1Pt3/TiO2-001, respectively (shown in Figure S6). Apparently, the range of variation of activation energy for glycerol oxidation is much narrower than that of glyceraldehyde oxidation, indicating that the coordination unsaturated O2c-Ti5c-O2c site have great effects on the activation of C=O rather than COH. This result is closely corresponding with the catalytic performance in Figure 3, which exhibits an almost indiscrimination in conversion but obvious difference in the further oxidation of aldehyde group.


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Figure 7 Time−conversion plots at various temperatures for Au1Pt3/TiO2–101 (A), Au1Pt3/TiO2– 101–001 (B), Au1Pt3/TiO2–001 (C), and Arrhenius plots (D) for glyceraldehyde oxidation

Scheme 2 A possible reaction pathway for the oxidation of glyceraldehyde over the Au1Pt3/TiO2– 001 catalyst.

Based on the above evidence and previous reports in the literatures6,

49

, a possible

mechanism considering bidentate intermediates for glyceraldehyde oxidation over Au1Pt3/TiO2–001 is proposed (shown as Scheme 2). Initially, the glyceraldehyde molecules are adsorbed on the surface coordination unsaturated sites of (001) facet in a bidentate form. Meanwhile, the oxygen dissociates on the metal nanoparticles and reacts with solvent H2O to produce active hydroxyl species. Because of the formation


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of bidentate intermediates, the electric charge of carbon atom in aldehyde group becomes more positive than monodentate. Under effects of the interface sites Ti5c–O2c– Auδ+, positive carbon center is easily attacked by the active hydroxyl species, followed by a C–H cleavage and desorption to produce the final acidic products. Thus, the surface structure especially the unsaturated coordination sites of TiO2 are the origins for a higher glyceraldehyde oxidation rate over Au1Pt3/TiO2–001 catalysts which brings a different products distribution compared with Au1Pt3/TiO2–101.

4. CONCLUSION A series of supported bimetallic Au1Pt3 catalysts supported on anatase TiO2 with different terminated facet (101) and (001) facet are prepared to investigate the crystal– effect on catalytic glycerol oxidation under base–free condition. Different TiO2 crystal planes have little influence on the activity of glycerol oxidation but great effects on the oxidation product distribution. Glyceraldehyde is firstly produced at the beginning of the reaction over the series catalysts. Subsequently, the oxidation of aldehyde group is accelerated over Au1Pt3/TiO2–001 and hence the selectivity of glyceric acid significantly increases. Two main reasons are conclude by in–situ CO2 and formaldehyde FT–IR, XPS, and corresponding kinetics studies. The C=O bond of aldehyde group highly dissociates with a formation of bidentate species on the surface coordination unsaturated structure O2c–Ti5c–O2c of TiO2 (001) facet, while monodentate species are dominant on (101) facet. Moreover, a strong interaction between Ti5c–O2c site and Au1Pt3 nanoparticles particularly the Au species is observed with a formation of Ti5c–O2c–Auδ+ active sites. With the aid of bidentate intermediates and Ti5c–O2c–Auδ+ active sites, the active energy of glyceraldehyde oxidation is decreased from 28.84 kJ/mol for (101) facet to 16.46 kJ/mol for (001) facet, indicating the enhanced ability of glyceraldehyde further oxidation over TiO2 based catalysts which is usually considered to have a high glyceraldehyde selectivity. Besides, AuPt nanoparticle plays an essential role in oxygen activation and insertion of hydroxyl species and C–H cleavage on glyceraldehyde oxidation, otherwise isomerization to hydroxyl acetone occurs on the surface of pristine TiO2. The findings reported by this article reveal the


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role of unsaturated O2c–Ti5c–O2c site on the intermediate adsorption and deepen the comprehension of glycerol oxidation process. Moreover, it provides a unique perspective and feasible approach to manipulate the product distribution in such multistep continuous oxidation reactions. SUPPORTING INFORMATION This material is available free of charge via the Internet at http://pubs.acs.org. Additional experimental data (Figures S1−S6) ACKNOWLEDGMENT This work was supported by National Key Research and Development Program of China (2017YFA0206804), the National Natural Science Foundation and the Fundamental Research Funds for the Central Universities (BHYC1701B, JD1816). REFERENCES (1) Villa, A.; Dimitratos, N.; Chan–Thaw, C. E.; Hammond, C.; Prati, L.; Hutchings, G. J. Glycerol Oxidation Using Gold–Containing Catalysts. Acc. Chem. Res. 2015, 48, 1403−1412. (2) Zhou, C. H.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q. Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals, Chem. Soc. Rev. 2008, 37, 527–549. (3) Dodekatos, G.; Schünemann, S.; Tüysüz, H. Recent Advances in Thermo−, Photo−, and Electrocatalytic Glycerol Oxidation, ACS Catal. 2018, 8, 6301−6333. (4) Behr, A.; Eilting, J.; Irawadi, K.; Leschinski, J.; Lindner, F. Improved Utilisation of Renewable Resources: New Important Derivatives of Glycerol. Green Chem. 2008, 10, 13−30. (5) Katryniok, B.; Kimura, H.; Skrzyńska, E.; Girardon, J.; Fongarland, P.; Capron, M.; Ducoulombier, R.; Mimura, N.; Paul, S.; Dumeignil, F. Selective catalytic oxidation of glycerol: perspectives for high value chemicals. Green Chem. 2011, 13, 1960–1979. (6) Davis, S. E.; Ide, M. S.; Davis, R. J. Selective Oxidation of Alcohols and Aldehydes over Supported Metal Nanoparticles. Green Chem. 2013, 15, 17−45. (7) Porta, F.; Prati, L. Selective Oxidation of Glycerol to Sodium Glycerate with Gold–on–Carbon Catalyst: An Insight into Reaction Selectivity. J. Catal. 2004, 224, 397−403. (8) Carrettin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Kiely, C. J.; Attard, G. A.; Hutchings, G. J. Oxidation of Glycerol Using Supported Gold Catalysts. Top. Catal. 2004, 27, 131−136. (9) Villa, A.; Veith, G. M.; Prati, L. Selective Oxidation of Glycerol under Acidic Conditions Using Gold Catalysts. Angew. Chem., Int. Ed. 2010, 49, 4499−4502. (10) Liu, S. S.; Sun, K. Q.; Xu, B. Q. Specific Selectivity of Au–Catalyzed Oxidation of Glycerol and Other C3−Polyols in Water without the Presence of a Base. ACS Catal. 2014, 4, 2226−2230. (11) Liang, D.; Gao, J.; Sun, H.; Chen, P.; Hou, Z. Y.; Zheng, X. M. Selective oxidation of glycerol with oxygen in a base–free aqueous solution over MWNTs supported Pt catalysts, Appl. Catal. B: Environ. 2011, 106, 423– 432. (12) Kapkowski, M.; Bartczak, P.; Korzec, M.; Sitko, R.; Szade, J.; Balin, K.; Lelątko, J.; Polanski,


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J. SiO2–, Cu–, and Ni–supported Au nanoparticles for selective glycerol oxidation in the liquid phase. J. Catal. 2014, 319, 110–118. (13) Xie, X.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. Low–temperature oxidation of CO catalysed by Co3O4 nanorods. Nature 2009, 458, 746−749. (14) Aneggi, E.; Llorca, J.; Boaro, M.; Trovarelli, A. Surface–structure sensitivity of CO oxidation over polycrystalline ceria powders. J. Catal. 2005, 234, 88−95. (15) Gu, X. K.; Bin Liu, B.; Greeley, J. First–Principles Study of Structure Sensitivity of Ethylene Glycol Conversion on Platinum. ACS Catal., 2015, 5, 2623–2631. (16) Zhao, K.; Zhang, L. Z.; Wang, J. J.; Li, Q. X.; He, W. W.; Yin, J. J. Surface Structure–Dependent Molecular Oxygen Activation of BiOCl Single–Crystalline Nanosheets. J. Am. Chem. Soc. 2013, 135, 15750–15753. (17) Bianchi, C.; Canton, P.; Dimitratos, N.; Porta, F.; Prati, L. Selective Oxidation of Glycerol with Oxygen using Mono and Bimetallic Catalysts Based on Au, Pd and Pt Metals. Catal. Today 2005, 102, 203–212. (18) Villa, A.; Wang, D.; Veith, G. M.; Prati, L. Bismuth as a Modifier of Au−Pd Catalyst: Enhancing Selectivity in Alcohol Oxidation by Suppressing Parallel Reaction. J. Catal. 2012, 292, 73−80. (19) Brett, G. L.; He, Q.; Hammond, C.; Miedziak, P. J.; Dimitratos, N.; Sankar, M.; Herzing, A. A.; Conte, M.; Lopez–Sanchez, J. A.; Kiely, C. J.; Knight, D. W.; Taylor, S. H.; Hutchings, G. J. Selective Oxidation of Glycerol by Highly Active Bimetallic Catalysts at Ambient Temperature under Base–Free Conditions. Angew. Chem., Int. Ed. 2011, 50, 10136−10139. (20) Lei, J. Q.; Duan, X. Z. Qian, G.; Zhou, X. G.; Chen, D. Size Effects of Pt Nanoparticles Supported on Carbon Nanotubes for Selective Oxidation of Glycerol in a Base–Free Condition. Ind. Eng. Chem. Res. 2014, 53, 16309−16315. (21) Tongsakul, D.; Shun Nishimura, S.; Ebitani, K. Platinum/Gold Alloy Nanoparticles–Supported Hydrotalcite Catalyst for Selective Aerobic Oxidation of Polyols in Base–Free Aqueous Solution at Room Temperature. ACS Catal. 2013, 3, 2199–2207. (22) Ning, X. M.; Li, Y. H.; Yu, H.; Peng, F.; Wang, H. J.; Yang, Y. H. Promoting role of bismuth and antimony on Pt catalysts for the selective oxidation of glycerol to dihydroxyacetone. J. Catal. 2016, 335, 95–104. (23) Xu, C. L.; Du, Y. Q.; Li, C.; Yang, J.; Yang, G. Insight into effect of acid/base nature of supports on selectivity of glycerol oxidation over supported Au–Pt bimetallic catalysts. Appl. Catal. B: Environ. 2015, 164, 334–343. (24) Villa, A.; Campisi, S.; Mohammed, K. M. H.; Dimitratos, N.; Vindigni, F.; Manzoli, M.; Jones, W.; Bowker, M.; Hutchings, G. J.; Prati, L. Tailoring the selectivity of glycerol oxidation by tuning the acid–base properties of Au catalysts. Catal. Sci. Technol. 2015, 5, 1126–1132. (25) Wu, Z. L.; Mann, A. K. P.; Li, M. J.; Overbury, S. H. Spectroscopic Investigation of Surface– Dependent Acid–Base Property of Ceria Nanoshapes. J. Phys. Chem. C, 2015, 119, 7340–7350. (26) Diallo–Garcia, S.; Osman, M. B.; Krafft, J. M.; Casale, S.; Thomas, C.; Kubo, J.; Costentin, G. Identification of Surface Basic Sites and Acid–Base Pairs of Hydroxyapatite. J. Phys. Chem. C 2014, 118, 12744–12757. (27) Ahrem, L.; Scholz, G.; Gutmann, T.; Calvo, B.; Buntkowsky, G.; Kemnitz, E. Direct Observation of Coordinatively Unsaturated Sites on the Surface of a Fluoride–Doped Alumina Catalyst. J. Phys. Chem. C 2017, 121, 12206−12213. (28) Tang, N. F.; Cong, Y.; Shang, Q. H.; Wu, C. T.; Xu, G. L.; Wang, X. D. Coordinatively


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Unsaturated Al3+ Sites Anchored Subnanometric Ruthenium Catalyst for Hydrogenation of Aromatics. ACS Catal. 2017, 7, 5987−5991. (29) Wang, F.; He, S.; Chen, H.; Wang, B.; Zheng, L. R.; Wei, M.; Evans, D. G.; Duan, X. Active Site Dependent Reaction Mechanism over Ru/CeO2 Catalyst toward CO2 Methanation. J. Am. Chem. Soc. 2016, 138, 6298−6305. (30) Pan, J.; Liu, G.; Lu, G. Q.; Cheng, H. M. On the True Photoreactivity Order of {001}, {010}, and {101} Facets of Anatase TiO2 Crystals. Angew. Chem. Int. Ed. 2011, 50, 2133–2137. (31) Hu, Z.; Liu, X. F.; Meng, D. M.; Guo, Y.; Guo, Y. L.; Lu, G. Z. Effect of Ceria Crystal Plane on the Physicochemical and Catalytic Properties of Pd/Ceria for CO and Propane Oxidation. ACS Catal. 2016, 6, 2265−2279. (32) Yu, J. G.; Low, J. X.; Xiao, W.; Zhou, P.; Jaroniec. M. Enhanced Photocatalytic CO2−Reduction Activity of Anatase TiO2 by Coexposed {001} and {101} Facets. J. Am. Chem. Soc. 2014, 136, 8839−8842. (33) Liu, L. C.; Gu, X. R.; Cao, Y.; Yao, X. J.; Zhang, L.; Tang, C. J.; Gao, F.; Dong, L. Crystal– Plane Effects on the Catalytic Properties of Au/TiO2. ACS Catal. 2013, 3, 2768−2775. (34) Hua, Q.; Cao, T.; Gu, X. K.; Lu, J. Q.; Jiang, Z. Q.; Pan, X. R.; Luo, L. F.; Li, W. X.; Huang, W. X. Crystal–Plane–Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen. Angew. Chem. Int. Ed. 2014, 53, 4856–4861. (35) Liu, L. J.; Jiang, Y. Q.; Zhao, H. L.; Chen, J. T.; Cheng, J. L.; Yang, K. S.; Li, Y. Engineering Coexposed {001} and {101} Facets in Oxygen–Deﬁcient TiO2 Nanocrystals for Enhanced CO2 Photoreduction under Visible Light. ACS Catal. 2016, 6, 1097−1108. (36) Ong, W. J.; Tan, L. L.; Chai, S. P.; Yong, S. T.; Mohamed, A. R. Highly reactive {001} facets of TiO2–based composites: synthesis, formation mechanism and characterization. Nanoscale, 2014, 6, 1946–2008. (37) Tian, F.; Zhang, Y. P.; Zhang, J.; Pan, C. X. Raman Spectroscopy: A New Approach to Measure the Percentage of Anatase TiO2 Exposed (001) Facets. J. Phys. Chem. C 2012, 116, 7515−7519. (38) Henao, J. D.; Caputo, T.; Yang, J. H.; C. Kung, M. C.; H. Kung, H. H. In Situ Transient FTIR and XANES Studies of the Evolution of Surface Species in CO Oxidation on Au/TiO2. J. Phys. Chem. B 2006, 110, 8689–8700. (39) Lang, H. F.; Maldonado, S.; Stevenson, K. J.; Chandler, B. D. Synthesis and Characterization of Dendrimer Templated Supported Bimetallic Pt–Au Nanoparticles, J. Am. Chem. Soc. 2004, 126 , 12949–12956. (40) Benvenutti, E. V.; Franken, L.; Moro, C. C. FTIR Study of Hydrogen and Carbon Monoxide Adsorption on Pt/TiO2, Pt/ZrO2, and Pt/Al2O3. Langmuir 1999, 15, 8140–8146. (41) Liu, Y. N.; Zhao, J. Y.; He, Y. F.; Feng, J. T.; Wu, T.; Li, D. Q. Highly efficient PdAg catalyst using a reducible Mg–Ti mixed oxide for selective hydrogenation of acetylene: Role of acidic and basic sites. J. Catal. 2017, 348, 135–145. (42) Su, W.; Zhang, J.; Feng, Z.; Chen, T.; Ying, P.; Li, C. Surface Phases of TiO 2 Nanoparticles Studied by UV Raman Spectroscopy and FT–IR Spectroscopy. J. Phys. Chem. C 2008, 112, 7710– 7716. (43) Miller, K. L.; Falconer, J. L.; Medlin, J. W. Effect of water on the adsorbed structure of formic acid on TiO2 anatase (101). J. Catal. 2011, 278, 321–328. (44) Zhang, C. B.; Liu, F. D.; Zhai, Y. P.; Ariga, H.; Yi, N.; Liu, Y. C.; Asakura, K.; Flytzani– Stephanopoulos, M.; He, H. Alkali–Metal–Promoted Pt/TiO2 Opens a More Efficient Pathway to


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Formaldehyde Oxidation at Ambient Temperatures. Angew. Chem. Int. Ed. 2012, 51, 9628–9632. (45) Raskó, J.; Kecskés, T.; Kiss, J. Formaldehyde formation in the interaction of HCOOH with Pt supported on TiO2. J. Catal. 2004, 224, 261–268. (46) Collins, S. E.; Baltanás, M. A.; Bonivardi, A. L. An infrared study of the intermediates of methanol synthesis from carbondioxide over Pd/β–Ga2O3. J. Catal. 2004, 226, 410–421. (47) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gra1tzel, M. Formic Acid Adsorption on Dry and Hydrated TiO2 Anatase (101) Surfaces by DFT Calculations. J. Phys. Chem. B 2000, 104, 1300– 1306. (48) Araiza, D. G.; Gómez–Cortés, A.; Díaz, G. Reactivity of methanol over copper supported on well–shaped CeO2: a TPD–DRIFTS study. Catal. Sci. Technol. 2017, 7, 5224–5235. (49) Zope, B. N.; Hibbitts, D. D.; Neurock, M.; Davis, R. J. Reactivity of the gold/water interface during selective oxidation catalysis. Science 2010, 330, 74–78.

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Insight into the Role of Unsaturated Coordination O2c-Ti5c-O2c Sites

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