TiO Nanocatalyst with Strong Metal–Support Interaction for Highly

Article pubs.acs.org/JPCC

Pd/TiO Nanocatalyst with Strong Metal−Support Interaction for Highly Efficient Durable Heterogeneous Hydrogenation M. Imran, Ammar B. Yousaf, Xiao Zhou, Yi-Fan Jiang, Cheng-Zong Yuan, Akif Zeb, Nan Jiang, and An-Wu Xu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: Developing heterogeneous catalysts that promote activation of hydrogen and adsorption of reactants on the surface is crucial to improve the activity of hydrogenation reactions. Herein, we present the use of highly reduced TiO support for loading noble metal nanoparticles, which provides a synergistic effect on boosting catalytic activity. The low-temperature polymorph of titanium monoxide is used as support and promoter, which contains vacancies in both the Ti and O sublattices with ordered distribution. The lattice vacancies increase the donor density and enhance the adsorption of substrate and hydrogen and subsequently lower the activation energy required for the hydrogen dissociation. Moreover, the existence of strong metal−support interactions in Pd/TiO ascertained from HRTEM and XPS analysis is responsible for both the strong binding of Pd nanoparticles and improved electronic conductivity, resulting in enhanced catalytic performance. The assynthesized Pd/TiO catalyst has been evaluated for the hydrogenation of styrene and 4-nitrophenol (4-NP) and shows excellent performance. The rate constant for 4NP reduction over Pd/TiO is 0.05 s−1, which is higher than the previously reported data; moreover, the catalyst shows good stability up to 10 successive cycles. Hydrogenation of styrene is conducted at ambient conditions using Pd/TiO as catalyst, which exhibits remarkably high turnover frequency (TOF) of 4838 h−1. The catalyst can be recovered and recycled several times without a marked loss of activity. The results show that our Pd/TiO catalyst is one of the highest activity among reported heterogeneous hydrogenation catalysts. The obtained Pd/TiO catalyst may hold great potential in catalysis for organic transformations and other reactions.

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INTRODUCTION Heterogeneous catalytic hydrogenation of olefins is one of the most important reactions that finds wide industrial applications including fine and bulk chemicals synthesis, petroleum refinery, and pharmaceutical products.1,2 These reactions are generally highly selective and easy to work up as the catalyst can be recovered and recycled.3 Noble metal based hydrogenation catalysts have been usually utilized, but their high cost and limited availability force us to find alternative options.4 Ultralow amount of noble metals loading on support is one important way to synthesize heterogeneous catalyst, but the activity could decreases if the support does not play a promoter role in catalytic reactions.5 Metal oxides have long been used as support and promoter for loading catalysts (metals, nitrides, carbides, sulfides, etc.) for hydrogenations, dehydrogenations, oxygenation, and acidcatalyzed reactions.6 The activity and selectivity of these systems are often found to be strongly dependent on the particle morphology, surface structure, defects in the support, and presence of poisons or promoters. In most cases, pure stoichiometric oxide support does not promote the catalytic activity of the system as compared to lattice defects. Nonstoichiometric support could endow noble metal nano© 2017 American Chemical Society

particles new physiochemical properties, thereby improving the catalytic activities because defective metal oxide support plays a functional role unlike its conventional role only as support.7,8 The lattice vacancies at the metal oxide surfaces or in the bulk alter the geometric and electronic structure as well as chemical properties of the system.9 Previous studies have revealed that transition-metal nanoparticles supported on oxide support play a critical role in promoting the catalytic activity, which is known as the strong metal−support interaction (SMSI) affecting both the activity and selectivity.10 In this case, oxide support participates in catalytic reactions by activating the reactants and presenting them to the metal nanoparticles at the boundary of metal/support and accelerates the reaction.11 Among many metal oxide supports investigated in catalysis, TiO2, a reducible support that is well-known to induce SMSI, has drawn the most attention due to its excellent activities in energy conversion and applied catalysis.12 TiO2 offers the opportunities for tuning material properties to create specific sites that influence the catalytic behavior. Modification of TiO2 Received: October 11, 2016 Revised: December 22, 2016 Published: January 5, 2017 1162

DOI: 10.1021/acs.jpcc.6b10274 J. Phys. Chem. C 2017, 121, 1162−1170

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mm) and quartz chromatographic column was used to determine the final products. Working conditions were as follows: sampling inlet temperature: 260 °C; programming temperatures: 60−260 °C at a rate of 6 °C/min; ion source temperature: 180 °C; electron energy: 70 eV. Synthesis of Titanium Monoxide (TiO). A mixture of commercial Ti and TiO2 powders with molar ratio of 1:1 was milled for 1 h under an air atmosphere using a Fritsch Pulverisette planetary ball milling. The mass ratio of the ball to the powder mixture was 25:1. The angular velocity of the supporting disc and vials was 317 and 396 rpm, respectively. The powders were further heated at 900 °C in evacuated and sealed quartz ampules for 24 h, and the furnace was slowly cooled down to room temperature to get the final product. Synthesis of Titanium Monoxide Supported Pd Nanoparticles. TiO supported Pd nanoparticles were prepared by an impregnation and reduction route. In a typical synthesis, 0.1 g of TiO was dispersed in distilled water followed by dropwise addition of K2PdCl4 aqueous solution (nominal weight content of 1 wt % Pd). The mixture was stirred vigorously at room temperature for 12 h. The resulting solid was filtered out, washed carefully with water and ethanol, and dried. Afterward, the product was calcined at 350 °C (temperature ramp: 5 °C/min) for 2 h in the presence of 5% H2/Ar gas. The synthesis of 1 wt % Pd/TiO2 nanoparticles is the same except the use of TiO2 in place of TiO. Catalytic Reduction of 4-Nitrophenol. The catalytic reduction of 4-NP was carried out using NaBH4 as a reducing reagent. Typically, 1 mL of catalyst dispersion (0.1 mg mL−1) and 7 mL of freshly prepared NaBH4 solution (0.1 M) were sequentially added to 60 μL of 4-NP aqueous solution (10 mM) under magnetic stirring at room temperature (25 °C). The progress of the reaction was monitored by UV−vis absorption spectra at regular intervals with a Shimadzu UV2550 UV−vis spectrometer. Hydrogenation of Styrene. Hydrogenation reactions were carried out at hydrogen atmosphere at room temperature. Typically, styrene (1.0, 2.0, 3.0, and 4.0 mmol), absolute ethanol (10 mL), 1,3,5-trimethylbenzene as internal standard, and 6 mg of catalyst were added to a round-bottom flask. The mixture solution was stirred at room temperature (25 °C) under 1 atm H2 flow rate of 40 mL min−1 for an appropriate time. The progress of the reaction was monitored by gas chromatography (GC-MS), and the extent of conversion was calculated on the basis of area ratio of substrate and product by the internal standard method.

by doping with various metals is one method to create oxygen vacancies and improve its catalytic activity.13 Titanium monoxide (TiO) is one such strongly nonstoichiometric form that contains a large concentration of vacancies in both metal and oxygen sublattices.14 TiO crystallizes in NaCl type cubic structure with compositions ranging from TiO0.7−1.25 at 1400 °C and TiO0.9−1.25 at temperatures below 1000 °C.15 The lowtemperature polymorph has the ordered distribution of vacancies with monoclinic symmetry. The vacancies are ordered within every third (100) plane, and one of two sites is vacant, resulting in the theoretical concentration of 16% vacancies.16 Until now, magnetic, electric, and structural properties of TiO17 have been well studied; nevertheless, TiO supported catalyst has not been reported to date. We report the first example of TiO supported Pd nanoparticles for heterogeneous catalysis. The hydrogenation of styrene and 4-nitrophenol is studied as a model reaction to evaluate the catalytic performance. This study focuses on how the hydrogenation activity is drastically enhanced by the Pd/TiO catalyst with abundant lattice vacancies and strong metal−support interactions (SMSI). Characterization studies using high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) were performed to check SMSI in as-synthesized Pd/TiO nanocatalyst. Additionally, electron paramagnetic resonance (EPR) and magnetic measurement were conducted to explain the role of oxygen vacancies in TiO support for the enhancement of catalytic performance. As novel support for Pd nanoparticles in heterogeneous catalysis, the catalytic properties were also compared with those of other reported available catalysts.

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MATERIALS AND METHODS Materials. Titanium powder with 100−200 mesh size and titanium dioxide (anatase TiO2) powder were purchased from Sigma-Aldrich. K2PdCl4 (99.9% metal basis), 4-nitrophenol 1,3,5-trimethylbenzene, sodium borohydride (NaBH4), and styrene were purchased from Sinopharm Chemical Reagent. All other chemical reagents were of analytical grade and used as received without further purification. Characterization. The morphology of the particles was observed by scanning electron microscope (SEM, JSM 6700F, JEOL). Transmission electron microscopic (TEM) images and high-resolution transmission electron microscopic (HRTEM) images were carried out on a JEM-2100F field emission electron microscope at an accelerating voltage of 200 kV. The X-ray powder diffraction (XRD) patterns of the products were performed on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.541 78 Å). The operation voltage was maintained at 40 kV and current at 200 mA. The X-ray photoelectron spectroscopy (XPS) was carried out on a PerkinElmer RBD upgraded PHI-5000C ESCA system. The UV−vis spectra at room temperature were recorded for the samples ranging from 200 to 800 nm wavelength range. A Plasma Quad 3 (VG Elemental, USA) was used for inductively coupled plasma−mass spectrometry (ICP-MS) analysis. The electron paramagnetic resonance (EPR) spectra were recorded on a JEOL JES-FA200 EPR spectrometer (140 K, 9064 MHz, 0.998 mW, X-band). The magnetic measurement was carried out with a superconducting quantum interference device magnetometer (SQUID, quantum design MPMS XL-7). Gas chromatography−mass spectroscopy (GC-MS, Trio-2000, Micromass, U.K.) with column BPX 70 (size 28 m × 0.25

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RESULTS AND DISCUSSION TiO belongs to the class of highly nonstoichiometric interstitial compound which is unique in the sense that it contains a large amount of both cation and anion vacancies. Depending on the oxygen content and heat treatment, the distribution of vacancies can be ordered or disordered in TiO, resulting in two different polymorphs of TiO, both closely related to the NaCl structure. In a disordered cubic phase, atoms and vacancies are randomly distributed, while in low-temperature polymorph, the vacancies are ordered within every third (100) plane and one of two sites is vacant, leading to the theoretical concentration of 16% vacancies.4 Different methods have been reported to synthesize low-temperature polymorph of titanium monoxide.17 Here, mechanochemical treatment and heating method was used to prepare TiO.15 The equimolar mixture of TiO2 and Ti powder was milled for 1 h in an air atmosphere 1163

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Figure 1. TEM image (a) and HRTEM images of (b−e) of as-obtained Pd/TiO catalyst. Yellow arrows point to the interface between Pd and TiO.

followed by heating the mixture at 900 °C in evacuated and sealed quartz ampules for 24 h. The obtained low-temperature polymorph of TiO was further used as support for Pd loadings. The main motivation of this choice relies on the fact that TiO exhibits a high concentration of lattice vacancies, which could boost the catalytic performance through strong metal−support interactions (SMSI) and increasing the donor density. An impregnation and reduction method was utilized for the synthesis of Pd/TiO (see Materials and Methods). The morphology of as-synthesized TiO and Pd/TiO measured by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) shows that the size of TiO is in micrometers (Figure S1) with palladium nanoparticles uniformly deposited on its surface (Figure 1). It can be seen that Pd nanoparticles are uniform shape with an average size of approximately 10 nm (Figure S1). A closer examination of the as-obtained Pd/TiO catalyst by high-resolution TEM (HRTEM) images reveals that the lattice fringes with a spacing of 0.22 nm correspond to the (111) atomic plane of metallic Pd (Figure 1b−e). A lattice spacing of 0.37 nm is assigned to the (011) plane of TiO. HRTEM images reveal that strong metal−support interaction (SMSI) was developed between TiO and Pd NPs.18 The SMSI decoration effect produces two kinds of Pd NPs; some of Pd NPs are partially embedded in TiO skin, while others are completely covered by TiO thin shell, notably a spillover and reconstruction of TiO around Pd NPs clearly observed. Figure 2 shows the XRD patterns of TiO and Pd/TiO samples. The diffraction peaks corresponding to the crystal planes of (100) (15.8°), (011) (23.7°), (111) (30.2°), and (102) (46.7°) are readily assigned to TiO (JCPDS No. 231078). The lattice parameters are a = 5.85 Å, b = 9.36 Å, and c = 4.15 Å with monoclinic symmetry having a space group A2/ m.19,20 In the case of Pd/TiO, no significant Pd signals were observed due to ultralow (actual weight content: 0.89 wt % determined by ICP-MS) loadings. As compared to pristine TiO, the intensity of Pd/TiO decreases, suggesting the surface disorder of TiO takes place (Figure 1) which is caused by Pd catalyzed instant hydrogenation of TiO. During H2 reduction by calcinations, Pd2+ was first reduced to Pd(0) by H2 and loaded on TiO, and then H2 spontaneously dissociates on the

Figure 2. XRD patterns of as-synthesized TiO and Pd/TiO samples.

Pd surface to generate active hydrogen species, which diffuse into and interact with TiO lattices to produce hydrogenated TiO with surface disorder, as confirmed by HRTEM observations (Figure 1). This Pd catalyzed instant hydrogenation of TiO leads to the reconstruction of TiO surface and a spillover of TiO thin shell onto Pd NPs, thus leading to strong metal−support interactions.18 Figure S1d shows TEM image of obtained Pd/TiO2 sample having uniform distribution of Pd NPs deposited on the surface of TiO2 with an average size of approximately 5−10 nm. HRTEM images of Pd/TiO2 are displayed in Figure S2b−d for comparison with Pd/TiO. As shown in Figure 1 and Figure S2a, thin layer of TiO encapsulates Pd NPs, leading to strong metal−support interaction. In the case of Pd/TiO2, Pd NPs are anchored on the surface of TiO2, and no spillover of TiO2 layer can be observed around Pd NPs, in contrast to the Pd/TiO sample. X-ray photoelectron spectroscopy (XPS) analysis was used to evaluate the surface composition and valence states of Pd/TiO nanocatalyst. The representative XPS survey scan spectrum 1164

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Figure 3. Survey XPS spectrum of Pd/TiO (a). High-resolution XPS spectra of Ti 2p (b), O 1s (c), and Pd 3d (d) orbitals.

Figure 4. Isothermal magnetization of (a) TiO and (b) Pd/TiO measured against applied field between −40 and 40 kOe at a temperature of 4 K.

by the final states, and the binding energy value of Pd0 much depends on the particle size.24,25 Pure Pd(0) has a binding energy of 335.5 eV (3d5/2) and 340.9 eV (3d3/2); Pd2+ has binding energy of 336.2 eV (3d5/2) and 341.4 eV (3d3/2). XPS analysis reveals a chemical shift occurs toward lower binding energy for metallic Pd(0);26 this binding energy shift is attributed to the interaction between Pd and highly reduced TiO support because the homogeneous particle size excludes the final-state effect. SMSI can promote migration of TiO over the surface of the Pd, resulting in partial or complete encapsulation, which significantly affects the chemisorption of substrate.27 The HRTEM images of Pd/TiO clearly present structural reconstruction of TiO around Pd NPs, and an increased ratio of Pd2+/Pd0 is evident proof for the SMSI. XPS analysis of Pd/TiO2 sample is also provided for comparison (Figure S3), and the survey XPS spectrum reveals the presence of Ti, O, and Pd elements. Figure S3b exhibits the Ti2p binding

(Figure 3a) indicates the existence of Ti, O, and Pd elements. Figure 3b exhibits the Ti2p binding energy (453−461 eV) region with Ti2+, Ti3+, and Ti4+ oxidation states.21 The appearance of Ti3+ and Ti4+ is ascribed to the surface oxidation of TiO upon exposure to air. The low-temperature polymorph of TiO crystallizes in NaCl type cubic structure with monoclinic symmetry and ordered vacancies on every third (100) plane, resulting in the theoretical concentration of 16% vacancies.20 The corresponding O 1s spectrum is displayed in Figure 3c; the O 1s spectrum is broad and asymmetric and can be deconvoluted into four peaks, indicating the presence of oxygen bonded to titanium (Ti−O) and Pd (Pd−O) chemisorbed and physisorbed H2O and CO2 molecules.22,23 The high-resolution XPS spectrum of the Pd 3d region (Figure 3d) reveals the existence of metallic Pd(0) with binding energies at 3d5/2 = 335.1 eV and 3d3/2 = 340.4 eV. It is noted that XPS spectrum of Pd nanoparticles is significantly affected 1165

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Figure 5. (a) UV−vis spectra of time-dependent reduction of 4-NP over Pd/TiO catalyst (actual weight content of Pd = 0.89 wt %); inset: reaction kinetics of the same reaction. (b) Recyclability of Pd/TiO catalyst up to 10 successive reaction cycles.

Table 1. Comparison of Reduction of 4-Nitrophenol over Pd/TiO with Reported Dataa

a

entry

catalyst

t (min)

1 2 3 4 5 6 7 8 9 10

Pd/TiO Pd/TiO2 Ag@Pd−Fe3O4 SPB/Pd PPy/TiO2/Pd Pt55Pd38Bi7 SBA-15/Pd Pd-ZnO Pd-CeO2 Pd/Al2O3

1 6 2 20 7 14 10 1.5 2

k (s−1) 50 10 33 4.41 12.2 4.3 12

× × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−3

39 × 10−3 63 × 10−4

molar ratio of NaBH4/4-NP

ref

10 10 909 100 111 1111 0 3000 42 50

our study our study 35 36 37 38 39 40 41 42

k: rate constant; t: conversion time.

energy region between 453 and 461 eV, and the Ti4+ oxidation state is observed, indicating the existence of TiO2. The O 1s spectrum shown in Figure S3c can be deconvoluted into two peaks: the major one indicates the presence of oxygen bounded to titanium (Ti−O) and the minor for oxygen bounded to Pd (Pd−O). The ratio of oxygen bounded to titanium is much higher than oxygen bounded to Pd. The high-resolution XPS spectrum of the Pd 3d region reveals that most of Pd species is in the form of metallic Pd(0), while a small amount of Pd(II) is observed (Figure S3d). It is noted that the ratio of Pd2+/Pd0 in Pd/TiO2 is lower than that in the Pd/TiO sample. The lattice vacancies (both titanium and oxygen sublattices) occurring in TiO are involved in the repolarization of the Ti 3d orbitals to stabilize its monoclinic symmetry. The repolarization contributes to the enhancement of Ti−Ti bonding interactions and electrostatic stabilization by increasing electron density at oxygen vacancies and depletion at titanium vacancies.28 The dbands are well separated from s-bands, and the Fermi level lies in the t2g bands which overlap with the eg bands.29 Isothermal magnetization measured vs applied field between −40 and 40 kOe at a temperature of 4 K shows a lack of remnant magnetization, suggesting there is no ferromagnetic impurity in TiO; there is no obvious change after Pd loadings (Figure 4). The temperature-dependent magnetic susceptibility of TiO exhibits the existence of Pauli paramagnetism due to itinerant delocalized d-electrons.30 The electron paramagnetic resonance (EPR) measurement of TiO fails to detect the presence of uncompensated magnetic moments, thus confirming the itinerant behavior of delocalized d-electrons (Figure S4). This itinerant feature of delocalized delectrons in TiO makes it a suitable support for noble metal

loading and subsequent use for heterogeneous hydrogenation reactions. In pure stoichiometric oxide, oxygen ions carry two electrons each (O2−); however, if an oxygen atom is missing from its place in the crystal (vacancy), there are two extra electrons which can act as a double electron donor. The increased electron density in TiO serves as active reduction sites, which significantly affect the electronic energy levels and promote partial dissociation and activation of adsorbed species and expose more active centers for catalytic hydrogenation reaction. Our recent work on oxygen-deficient TiO2−x and ZnO1−x has also shown significant increase in visible light photocatalytic activity due to oxygen vacancies.31,32 To evaluate the catalytic hydrogenation performance of Pd/ TiO catalyst, we chose the reduction of p-nitrophenol (4-NP) to p-aminophenol (4-AP) in NaBH4 aqueous solution at room temperature as a model reaction. 4-NP exhibits a strong characteristic peak at 317 nm under neutral conditions which shows a shift toward higher wavelength (401 nm) due to the formation of 4-nitrophenolate ion as the alkalinity of the solution increases (Figure S5).19 After addition of a certain amount of Pd/TiO catalyst, the peak intensity at 401 nm rapidly decreases, and the color of the solution disappears after 60 s. At the same time, a new peak at 300 nm appears which is ascribed to 4-AP molecules (Figure 5a). The UV−vis spectra were recorded to monitor the transformation of 4-NP to 4-AP. In this experiment, the amount of NaBH4 was in large excess as compared to 4-NP and considered constant during the reaction. The metal particles supported on titanium monoxide started the catalytic reduction by relaying electron from the donor BH4− to the acceptor 4-NP after the adsorption onto the catalyst surface. The lattice 1166

DOI: 10.1021/acs.jpcc.6b10274 J. Phys. Chem. C 2017, 121, 1162−1170

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Figure 6. (a) Hydrogenation of styrene over TiO, 0.89 wt % Pd/TiO, and 1 wt % Pd/TiO2 at 1 atm H2 and 25 °C. (b) Recycling of Pd/TiO catalyst used for hydrogenation of styrene.

ethylbenzene in approximately 48 min under experimental conditions. The high activity of Pd/TiO may be due to welldispersed Pd NPs intimately anchored on the support, thus preventing agglomeration or aggregation of Pd NPs. The superior catalytic activity could also be ascribed to the presence of lattice vacancies in TiO with delocalized electrons, which communicate with Pd NPs. The highly delocalized electrons increase the donor density and favor the adsorption of styrene and hydrogen, which in turn reduces the energy barrier associated with the cleavage of H−H bond and boosts the catalytic activity. SMSI plays a key role in the catalytic hydrogenation over Pd/TiO catalyst as the SMSI significantly affects the catalytic performance of supported catalyst.43 It is apparent from XPS results that the shift of binding energies for Pd species indicates the coexistence of two Pd species (Pd0/Pd2+) in Pd/TiO, thus suggesting communication between metal and support indeed occurs. HRTEM images of Pd/TiO already reveal the formation of an extended interface between Pd NPs and TiO support. Previous studies have assigned the enhanced catalytic activities of supported metal catalysts due to SMSI as the perimeter interfaces between the metal NPs and oxide support serve as active sites for catalysis.44 It is noted that SMSI occurs in our Pd/TiO catalyst at a much lower temperature than previous reports.45 Taken together, the obtained Pd/TiO catalyst exhibits improved catalytic performance in the liquid-phase selective hydrogenation of styrene to ethylbenzene. The effect of styrene concentration on the conversion was investigated (Figure S5). The catalyst exhibits superior activity at the concentration of 1 and 2 mmol of styrene, achieving complete conversion (100%) in about 48 min for 2 mmol of styrene. At a higher concentration of styrene, the conversion decreased to 73% for 3 mmol of styrene and 43% for 4 mmol of styrene. The hydrogenation of styrene over Pd/TiO2 was also investigated for comparison. As shown in Figure 6a, the hydrogenation of styrene was only 12% after 48 min reaction as compared to Pd/TiO (100%), implying that SMSI and oxygen vacancies in Pd/TiO play a promoter role in the catalytic hydrogenation of styrene. The turnover frequency (TOF), the moles of product formed per mole of noble metal per hour, and previously reported data are given in Table 2 for comparison. The actual weight content of Pd in Pd/TiO catalyst was detected to be 0.89 wt % by ICP-MS analysis. The TOF value calculated for our Pd/TiO catalyst was 4838 h−1. This TOF is one of the highest values as compared to previous reports for

vacancies in TiO serve as the shallow donors to improve the donor density and electrical conductivity and enhance the adsorption of these species and subsequent conversion into product.33,34 TiO alone does not convert the reactant into product (Figure S6), thus just providing a synergistic effect to ultralow amount of Pd NPs and greatly promoting the reduction rate. The perimeter interfaces between Pd NPs and the reduced TiO support may act as active centers to enhance the activity due to SMSI. For the evaluation of reduction rate constant, the pseudo-first-order kinetics with respect to 4-NP is reasonable assumption, as the amount of NaBH4 is constant during the whole reaction. The inset in Figure 5a shows the representative plot of ln(A/A0) versus reaction time (t) for the catalytic reduction of 4-NP to 4-AP over Pd/TiO catalyst. Here A and A0 represent the concentrations of 4-NP at time t and t = 0, respectively, which can be obtained from the decrease of peak intensity at 401 nm with time. The apparent rate constant was calculated from the ln(A/A0) vs time plot and is shown in Table 1 together with the reported data for comparison. From Table 1, it is clearly seen that the rate constant for our Pd/TiO catalyst (0.05 s−1) is much higher than that of previously reported catalysts for 4-NP reduction. Recyclability and stability of the catalyst is an important aspect to evaluate catalyst applications. To perform the catalytic recycling, after reaction completion, the catalyst was separated and washed thoroughly with water and ethanol, followed by drying and redispersed in water for subsequent catalytic experiments. Our Pd/TiO catalyst shows the good stability for the conversion of 4-NP to 4-AP up to 10 successive cycles without obvious loss in reactivity (Figure 5b). The catalytic reduction of 4-NP over 1 wt % Pd/TiO2 was also examined under the same conditions for comparison (Figure S7). The 4-NP reduction over Pd/TiO2 takes much long time (6 min) with a rate constant of 0.01 s−1, as compared to Pd/TiO (1 min) with a rate constant of 0.05 s−1, which further supports that itinerant delocalized delectrons of TiO and SMSI play important roles in promoting the catalytic activity. The catalytic activity of as-synthesized Pd NPs supported on TiO was further investigated using hydrogenation of styrene to ethylbenzene as a model reaction. We chose 1,3,5-trimethylbenzene as internal standard and ethanol as solvent at room temperature under 1 atm H2. Figure 6a displays the conversion (%) of styrene (2 mmol) with TiO and Pd/TiO as catalyst. Pd/ TiO catalyst exhibits complete conversion of styrene to 1167

DOI: 10.1021/acs.jpcc.6b10274 J. Phys. Chem. C 2017, 121, 1162−1170

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turnover frequency of 4838 h−1. Pd catalyzed instant hydrogenation of TiO induces the reconstruction of TiO surface and a spillover of TiO thin skin to Pd NPs, consequently resulting in strong metal−support interactions (SMSI). The origin of enhanced catalytic activities of Pd/TiO catalyst is attributed to the SMSI which accelerates hydrogen adsorption, dissociation, and subsequent hydrogenation reactions. Moreover, the lattice vacancies on TiO support enhance the catalytic hydrogenation by facilitating dissociative adsorption of H2 onto Pd nanoparticles. We anticipate that defective metal oxide as support and promoter could find great potential applications in heterogeneous catalysis for chemical transformations.

Table 2. Comparison of Turnover Frequency with Various Reported Catalystsa catalyst

TOF (h−1)

entry

substrate

product

1

styrene

ethylbenzene

Pd/TiOb

4838

2

styrene

ethylbenzene

Pd/TiO2c

580

3

styrene

ethylbenzene

972

4 5 6 7 8

styrene styrene styrene styrene styrene

ethylbenzene ethylbenzene ethylbenzene ethylbenzene ethylbenzene

9 10 11

styrene styrene styrene

ethylbenzene ethylbenzene ethylbenzene

commercial 26 wt % Pd/C Pd/PEG Pt/Al2O3 Pd/MOF-5 Fe3O4-NC-PZS-Pd single atoms Pd/ TiO2 Pd/ZIF-8 (PdCl2/bpy)10 Pd nanosheets

ref our study our study 46

660 138 682 1792 8973

47 48 49 50 46

307 6944 454

49 51 52

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10274. SEM and TEM images of TiO, TEM image of Pd/TiO, TEM image of Pd/TiO2, HRTEM image of Pd/TiO, and HRTEM images of Pd/TiO2, XPS spectrum of Pd/TiO2, electron paramagnetic resonance (EPR) spectra of TiO and TiO2, absorption spectra of 4-NP with and without NaBH4 addition, 4-NP reduction using TiO as catalyst, UV−vis spectra of time-dependent reduction of 4-NP over 1 wt % Pd/TiO2, effect of styrene concentration on conversion (%) over Pd/TiO (PDF)

a

TOF: turnover frequency calculated as the number of moles of product per mole of Pd per hour. Reaction conditions: H2, room temperature (25 °C). bThe actual weight content is 0.89 wt % Pd. c The weight content is 1 wt % Pd.

hydrogenation of styrene using various catalysts (see Table 2). Control experiments using TiO as support in the absence of Pd failed to give any activity, which further confirms that TiO alone does not proceed the reaction but plays a role as a support and promoter in boosting the hydrogenation reaction of styrene. Collecting the catalyst from the reaction system and reusing it for subsequent reaction cycles are indispensable in practical catalytic applications. Compared with homogeneous catalysts, the heterogeneous catalysts can be separated from the reaction system by centrifugation or filtration. In this work, the recycling tests were performed under the same reaction conditions as described above except using the recovered catalyst. Each time, the catalyst was isolated from the reaction solution at the end of the reaction, washed with water and ethanol, and dried at 60 °C under vacuum. The results of recycling use of Pd/TiO catalyst for styrene hydrogenation are presented in Figure 6b. The catalyst was successfully recycled for five runs and displayed a negligible decline in activity for styrene hydrogenation, giving 100% conversion to ethylbenzene. These results are very promising for further applications of highly reduced TiO support, which has never been used in catalysis. The observed behavior of reduced oxide supported Pd NPs may be extended to different catalytic reactions, which not only shows the capability to assist noble metals but also has a great potential to reduce the high cost of commercial noble metal catalysts while also conferring prolonged durability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph (86) 551-63602346 (A.W.X.). ORCID

An-Wu Xu: 0000-0002-4950-0490 Author Contributions

M.I. and A.B.Y. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Basic Research Program of China (2011CB933700) and the National Natural Science Foundation of China (51561135011, 51572253, 21271165). This work is also supported by CASTWAS President’s Fellowship programme.

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REFERENCES

(1) Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani, S. M.; Sykes, E. C. Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations. Science 2012, 335, 1209−1212. (2) Lawrie, L. Handbook of Industrial Catalysts; Springer Science & Business Media LLC: New York, 2011. (3) Li, X. J.; Zhang, W. P.; Liu, S. L.; Xu, L. Y.; Han, X. W.; Bao, X. H. Olefin metathesis over heterogeneous catalysts: Interfacial interaction between Mo species and a Hβ−Al2O3 composite support. J. Phys. Chem. C 2008, 112, 5955−5960. (4) Zang, W. T.; Li, G. Z.; Wang, L.; Zhang, X. W. Catalytic hydrogenation by noble-metal nanocrystals with well-defined facets: A review. Catal. Sci. Technol. 2015, 5, 2532−2553. (5) Twigg, M. V.; Spencer, M. S. Deactivation of supported copper metal catalysts for hydrogenation reactions. Appl. Catal., A 2001, 212, 161−174.

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CONCLUSIONS In this work, we have successfully synthesized titanium monoxide supported palladium nanoparticles for heterogeneous catalysis. Reduction of 4-nitrophenol and hydrogenation of styrene are studied as model reactions to evaluate catalytic performance. Reduction of 4-nitrophenol is accomplished within very short time (60 s) with the rate constant of 0.05 s−1. The catalyst exhibits good stability even up to 10 successive conversion cycles. Moreover, the obtained Pd/TiO catalyst shows highly efficient styrene hydrogenation at room temperature and 1 atm H2, achieving 100% conversion in 48 min with 1168

DOI: 10.1021/acs.jpcc.6b10274 J. Phys. Chem. C 2017, 121, 1162−1170

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DOI: 10.1021/acs.jpcc.6b10274 J. Phys. Chem. C 2017, 121, 1162−1170