The Synergy between Metal Facet and Oxide Support Facet for

Letter pubs.acs.org/NanoLett

The Synergy between Metal Facet and Oxide Support Facet for Enhanced Catalytic Performance: The Case of Pd−TiO2 Muhan Cao,† Zeyuan Tang,† Qipeng Liu,† Yong Xu,† Min Chen,† Haiping Lin,*,† Youyong Li,† Elad Gross,¶ and Qiao Zhang*,† †

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, 199 Ren’ai Road, Suzhou, 215123, Jiangsu, People’s Republic of China ¶ Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel S Supporting Information *

ABSTRACT: The demand for catalyst with higher activity and higher selectivity is still a central issue in current material science community. On the basis of first-principles calculations, we demonstrate that the catalytic performance of the Pd−TiO2 hybrid nanostructures can be selectively promoted or depressed by choosing the suitable shaped Pd and TiO2 nanocrystals. To be more specific, the catalytic activities of Pd nanoparticles enclosed by (100) or (111) facets can be promoted more significantly when dosed on the TiO2(001) than on TiO2(101) under irradiation. Such theoretical prediction has then been further verified by the experimental observations in which the Pd(100)−TiO2(001) composites exhibit the highest catalytic performance toward the activation of oxygen among all the other shaped hybrid nanostructures. As a result, the selection of facets of support materials can provide an extra tuning parameter to control the catalytic activities of metal nanoparticles. This research opened up a new direction for designing and preparing catalysts with enhanced catalytic performance. KEYWORDS: Palladium, titania, synergy, interaction, O2 activation

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cubes enclosed by (100) facets exhibit much higher catalytic activities compared with octahedrons enclosed by (111) facets in formic acid oxidation.27,28 Similar facet-dependent catalytic performances have also been reported for Au,29 Pt,30 Ag,31 and TiO2 nanocrystals.32,33 Despite the success in the design and preparation of various catalysts, the demand for catalyst with higher activity and higher selectivity is still a central issue in current material science community. From both fundamental and practical perspectives, it is of essential importance to find some new mechanisms or new tuning parameters that can significantly improve the catalytic performance of catalysts. Herein, by using the Pd− TiO2 hybrid system as the model system we demonstrate that there is a strong correlation between the exposed metal facet and oxide support facet. Density function theory (DFT) simulations show that the TiO2(001) facet is a better electron donating support for the Pd nanoparticles than the TiO2(101) facet under irradiations. Correspondingly, the catalytic activities of Pd nanoparticles are dramatically enhanced when they are supported on the TiO2 (001) surface. The theoretical predictions have been further confirmed by the experiment results. This research opened up a new direction to the design

n the early stage of heterogeneous catalysis, small particles of active phase (e.g., metal) were deposited on an inert support to enhance their thermal stability.1,2 It was later realized that a suitable support material could also improve the activity and selectivity of the active phase because of the “strong metalsupport interaction (SMSI)” effect.3 Since then, much effort has been devoted to studying the metal−oxide hybrid composites. Recently, some metal−semiconductor (M-S) hybrid nanostructures, such as Pd−P25,4 Au−TiO2,5,6 Pt−Ta2O5,7 Au−CeO2,8 and Pt−CeO2,9 have been studied extensively to investigate the effects of semiconductor supports in various catalytic systems. For the photocatalytic applications, different precious metals or support materials,5,7 together with the particle size5 and loading amount,10,11 have been explored as the impacting factors to realize optimum catalytic performances on the basis of enhanced hole−electron separation on the active sites. The rapid development of nanoscience and nanotechnology in recent years spurred a dramatically increased interest in the catalytic applications of nanomaterials, partially because remarkable progress has been made in the controllable synthesis of nanoparticles. For instance, various shaped metal nanoparticles, such as cubes,12−14 cuboctahedrons,15,16 octahedrons,17,18 rods,19,20 wires,21,22 plates,23,24 and oxide nanoparticles25,26 with desired shapes and sizes have been successfully synthesized. It has been demonstrated that the catalytic properties of nanoparticles can be significantly improved by tuning their exposed facets. For example, Pd © XXXX American Chemical Society

Received: June 28, 2016 Revised: July 25, 2016

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Figure 1. (a,c,e,g) The LDOS of top layer Pd atoms of the Pd−TiO2 systems. The black line is for the neutral system and the red line is for the negatively charged system. (b,d,f,h) The top views of neutral Pd−TiO2 system, corresponding to (a,c,e,g), respectively.

are generated, the TiO2(001) facet behaves as a better electron donor than the TiO2(101) facet. It has been reported by Xiong and co-workers that Pd(100) facets have better catalytic performance than Pd(111) facets in the catalytic activation of O2.40 Our simulation results also confirmed the results. As shown in Table S2, without support the bond lengths of oxygen molecules on Pd(100) and Pd(111) are 1.408 and 1.340 Å, respectively, which are all larger than that of the gas phase (1.244 Å). The effect of TiO2 facets to the catalytic activities of Pd catalysts are then investigated via the O2 activation reactions. As shown in Figure 1b,d,f,h, in the absence of irradiation, the O−O distances of adsorbed O2 molecules are all further increased. The bond length of O2 molecules are Pd(100)−TiO2(001) > Pd(100)−TiO2(101) > Pd(111)− TiO2(001) > Pd(111)−TiO2(101). When the irradiation is switched on (extra electrons are included in DFT calculations), the O−O bonds are more stretched because the Pd → O2 electron transfer is enhanced by the photoexcited hot electrons (see Table S2). The underlying mechanism of such catalytic O2 activation is studied by the charge difference analysis, which indicates that the photoexcited hot electrons tend to occupy the π* antibonding orbitals and therefore results in the elongation of O−O bonds (Figure S1). It is worth pointing out that the trend of catalytic performance of these hybrid catalysts toward O2 activation remains unchanged, revealing that the activities of such hybrid catalysts are mainly controlled by the exposed facets of metal nanoparticles: Pd(100) > Pd(111), which is consistent with previous investigation reported by Xiong and co-workers.40 The facets of supporting oxides, therefore, play an important and subtle role to tune the catalytic activities. This result is of great interest in catalysis because the change of supporting materials (and their facets) is feasible, which may provide a rich selection of catalytic activities for given metal nanoparticles. To verify the theoretical prediction and study the metal− support interaction, a series of Pd−TiO2 composite catalysts have been prepared (see Experimental Section in the Supporting Information for details). A facile colloidal synthetic approach has been used to prepare Pd nanoparticles with controllable shape and size.41,42 To obtain the Pd−TiO2 composite, TiO2 was dispersed in the precursor solutions of Pd nanocrytals. As shown in Figure 2, uniform Pd nanocubes

and synthesis of oxide supported metal catalysts with improved catalytic performance. Theoretical calculations were conducted in the framework of DFT using the Vienna ab initio Simulation Package (VASP).34,35 The electron−ion interactions were described by using projector augmented wave (PAW) method.36 The generalized gradient approximation (GGA) was employed with the exchange-correlation functional of Perdew−Burke− Ernzerhof (PBE).37 An energy cutoff of 400 eV was used for the plane-wave expansion of the electronic wave function. The van der Waals density functional (vdw-DF) was applied to include the dispersion corrections for O−Pd interactions.38 The TiO2 surfaces were modeled with two-dimensional periodic slabs consisted by four atomic layers in which the bottom two layers of atoms were fixed to their bulk positions. The Pd nanoparticles, however, were mimicked by the nanorods consisting of four atomic layers as suggested by Yates and coworkers.39 More detailed information about the slab models can be found in Table S1. The irradiation effect was mimicked by adding extra electrons to the Pd−TiO2 hybrid systems. A vacuum region of 20 Å was selected to avoid interactions between translational periodic images in the z-direction. The conjugated-gradient algorithm was used to optimize the structures until the force on each atom was less than 0.01 eV/Å. In all calculations, the convergence of electronic structures was set to 10−4 eV. Figure 1 shows the local density of states (LDOS) of the top layer Pd atoms of the Pd−TiO2 systems and the adsorption configurations of O2 molecules on the Pd−TiO2 hybrid systems. Because of the Schottky barrier, under irradiation the Pd nanoparticles can extract the photoexcited electrons from the TiO2 surface. As a result, the empty states of top layer Pd atoms are shifted toward the Fermi level, which means that these empty states are activated, and the catalytic activities of hybrid catalysts for certain reactions should be promoted correspondingly. From the insets of Figure 1a,c,e,g, it is clear that the amount of left-shift of LDOS is system-dependent. To be more specific, the Pd atoms over the TiO2(001) facet are more activated than those on the TiO2(101) facet. This result can be interpreted by the fact that the work function of TiO2(001) facet (5.78 eV) is 0.98 eV lower than that of the TiO2(101) facet (6.76 eV). When photoexcited hot electrons B

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to characterize the surface chemistry of Pd NPs after the washing treatment, as shown in Figure S4. The obtained Pd NPs without any treatments exhibit the obvious representative absorption peaks of PVP at 1664 cm−1 (CO stretching vibration and C−N functional groups), 1433 cm−1 (CH2 scissoring) and 1288 cm −1 (C−N bending and CH 2 wagging).45,46 The intensity of these characteristic bands became much weaker after washing for three times, indicating most of surfactant PVP has been removed. The catalytic conversion of singlet O2 was first studied by employing the electron spin resonance (ESR) analysis. To ensure the equivalent surface atoms were used, an ICP measurement was conducted to determine the loading amount of Pd. The surface atoms were calculated based on the loading amount of Pd and the percentage of surface atoms (Table S3). A singlet O2 trapping agent, 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) was employed for the detection of singlet O2 species, because the oxidation derivative (4-oxo-TEMPO) can serve as a sensitive probe molecule that can be a direct evidence of singlet O2.47 As shown in Figure 3, the ESR results for different Pd nanoparticles and Pd−TiO2 composites clearly display the 1:1:1 triplet signal, which is the characteristic feature of 4-oxo-TEMPO.47 It is worth pointing out that the signal intensity of 4-oxo-TEMPO in the case of Pd cubes is significantly higher than that of Pd octahedrons (Figure 3). The similar results have been observed when the Pd−TiO2 composites were used as the catalyst. As shown in Figure 3a,b, when the same TiO2 support was used, the intensity of singlet O2 produced by the Pd Cubes−TiO2 is always higher than that produced by the Pd Oct−TiO2 composites. The remarkably improved catalytic activity can be attributed to the facetdependent activity of Pd nanoparticles in which the (100) facets of Pd nanoparticles are more active for the generation of singlet O2 than the (111) facets.28,40 In both cases of Pd cubes and octahedrons, introducing a TiO2 support into the system can dramatically raise the intensity of singlet O2 (Figure 3). Notably, the intensity of singlet O2 increased when the percentage of TiO2(001) facet increased. For example, when TiO2-1 (with ∼2% of (001) facet) was used as the support, the intensity of singlet O2 is about 1.4 times higher than that of naked Pd cubes. The intensity can be further promoted to ∼2.6 times higher when TiO2-4 (with ∼80% of (001) facet) was used as the support. The similar trend has been observed in the case of Pd octahedrons. The corresponding ESR intensity

Figure 2. TEM and HRTEM images of (a,b) Pd nanocubes and (d,e) octahedrons, TEM images of (c) Pd Cubes−TiO2-4 and (f) Pd Oct− TiO2-4.

with edge length ∼11 nm (Figure 2a) and Pd octahedrons with edge length ∼26 nm (Figure 2d) have been successfully prepared. As evidenced by the HRTEM images (Figure 2b,e), the as-prepared Pd cubes and octahedrons can be indexed as face-centered cubic (fcc) structures and are enclosed by welldefined (100) and (111) facets, respectively. To elucidate the influence of exposed TiO2 facets, TiO2 nanosheets with different percentages of exposed (001) facets have also been prepared. By varying the amount of HF, the percentage of (001) facets can be readily controlled in the range of ∼2%− 80%, which are named as TiO2-1−4 with the exposed TiO2(001) facet being 2%, 39%, 61%, and 80%, respectively (Table S3 and Figure S2).32,33,43 Also, these catalysts are denoted accordingly as Pd Cubes/Oct−TiO2-1−4. Commercial P25 has also been employed in this study to act as a support for Pd nanoparticles. The growth of Pd cubes and octahedrons on different TiO2 supports have been confirmed by TEM characterizations (Figures 2c,f and S3). It is clear that Pd nanoparticles can be well dispersed on TiO2 support. Because the ligands capped on metal NPs may form a barrier for the electrons transfer between the support and active metals,44 the as-obtained samples have been washed with acetone and ethanol for several times to remove most of the capping ligands (PVP) on the Pd NPs by centrifugation. Fourier transform infrared spectroscopy (FTIR) was employed

Figure 3. ESR spectra of (a) Pd nanocubes- and (b) octahedron-based samples in the presence of 4-oxo-TMP. C

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Figure 4. Yield of gluconic acid with (a) Pd nanocubes- and (b) octahedron-based samples as catalysts. Glucose, 15 mM; catalyst surface, 0.8‰. No other byproducts were observed in the HPLC analysis.

facet was used as the support. Consistent with the ESR data, P25 showed moderate enhancement, which is higher than that of TiO2 support with 2% exposed (001) facet but lower than that of TiO2 support with over 39% exposed (001) facet. The same trend has been observed in the case of Pd octahedrons in which higher percentage of exposed TiO2 (001) facets means higher catalytic activity. The photocatalytic oxidation of glucose can be quenched by the addition of carotene, suggesting that the active species are singlet O2. As shown in Figure 4, the quenching fractions of gluconic acid show a strong correlation with the percentage of exposed (001) facets, suggesting that TiO2(001) facets play important roles in triggering the activation of triplet oxygen. The catalytic test again confirms the synergetic effect between the (001) facet of TiO2 support and the Pd nanoparticles. It is worth noting that the size of Pd octahedrons does not make a big difference. As shown in Figure S6, Pd octahedrons with size around 13 nm have been prepared and used as the reference. In summary, through theoretic simulation we predict that there is a synergetic effect between metal facet and oxide support facet, which has been underestimated in previous studies. By constructing a series of Pd−TiO 2 hybrid nanostructures with controlled shapes, the theoretic prediction has been experimentally confirmed in the catalytic generation of singlet O2. It is found that the facets of both metal nanoparticle and oxide support are critical in determining the catalytic activity of the hybrid nanostructure. This research is important because it shed some new light on the design and synthesis of nanocatalyst with improved performance by adding a new tuning parameter.

obtained in the presence of TiO2-4 support is about 2.4 times higher than that of naked Pd octahedrons. It is worth pointing out that the ESR intensity obtained in the presence of P25 is lower than that of TiO2 support with exposed (001) facet over 39% (TiO2-2−4). To further confirm the generation of singlet O2, a quantitative amount of carotene, a well-known scavenger for singlet O2, was employed in the control experiments. As depicted in Figure S5, a drastic deterioration of the ESR intensity can be observed among all the samples when carotene was added. It reveals that carotene could effectively inhibit the formation of the probe molecules, suggesting that the active species in this Pd-based system are singlet O2. The above findings unambiguously reveal that the exposed facet of both Pd nanoparticles and TiO2 support played a critical role in determining the catalytic activity toward the generation of singlet O2. As a result, the improved catalytic activity came from the synergetic effect between the TiO2 support and the Pd nanoparticle, which is in good agreement with theoretical simulation. The synergetic effect has been further confirmed by the photocatalytic oxidation of glucose. In a typical experiment, Pd−TiO2 composite was first mixed with the glucose solution, followed by the bubbling with oxygen for 15 min to get an oxygen-saturated solution. The comparison basis among different catalysts is the surface Pd atoms. Also, the surface atoms of both cubes and octahedrons were kept equivalent. The oxidation reaction was conducted under irradiation of simulated solar light. The product, gluconic acid, was detected by using the high-performance liquid chromatography (HPLC). Consistent with the ESR data, gluconic acid can be detected in all cases, as depicted in Figure 4 and Table S4. The TONs and TOFs values are also given in Table S4. When bare Pd nanoparticles were used as the catalyst, Pd nanocubes showed higher activity than Pd octahedrons. Thirtyeight percent of glucose was converted to gluconic acid when Pd nanocubes were used as the catalyst, while only 13% glucose was converted over Pd octahedrons in 120 min, further confirming the facet-dependent activity of Pd nanoparticles. The presence of TiO2 support can significantly improve the catalytic activity of Pd nanoparticles. In the case of Pd nanocubes, the yield of gluconic acid within 120 min can be increased to ∼51% (a 1.3 times enhancement) when TiO2 nanoparticles with ∼2% (001) facets were used as the support. The yield of gluconic acid can be further raised to almost 100% when TiO2 nanosheets with a 61% and 80% exposed (001)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b02662. Detailed synthesis procedures and calculation methods, additional TEM images, catalytic data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.L.). *E-mail: [email protected] (Q.Z.). D

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.C., Z.T., and Q.L. contributed equally to this paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21401135) and the Natural Science Foundation of Jiangsu Province (BK20140304). We acknowledge the financial support from the Collaborative Innovation Center of Suzhou Nano Science and Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the SWC for Synchrotron Radiation Research. Q.Z. thanks the Open Research Fund of Jiangsu Key Laboratory of Environmental Material and Environmental Engineering (Yangzhou University). H.L. is grateful to the Natural Science Foundation of Jiangsu Province (BK20150305) and Soochow University (SDY2014A14) for funding. M.C. thanks the Postdoctoral Science Foundation of China (2016M591909).

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