Facet Effect of Single-Crystalline Pd Nanocrystals for Aerobic

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Facet Effect of Single-Crystalline Pd Nanocrystals for Aerobic Oxidation of 5-Hydroxymethyl-2-furfural Da Lei, Kai Yu, Meng-Ru Li, Yuling Wang, Qi Wang, Tong Liu, Pengkun Liu, Lan-Lan Lou, Guichang Wang, and Shuangxi Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02839 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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ACS Catalysis

Facet Effect of Single-Crystalline Pd Nanocrystals for Aerobic Oxidation of 5-Hydroxymethyl-2-Furfural

Authored by Da Leia,b,1, Kai Yua,1*, Meng-Ru Lic, Yuling Wanga, Qi Wanga, Tong Liua, Pengkun Liua, Lan-Lan Loub, Guichang Wangc**, Shuangxi Liub,d***

a

MOE Key Laboratory of Pollution Processes and Environmental Criteria, College of

Environmental Science and Engineering, Nankai University, Tianjin 300350, People's Republic of China b

Institute of New Catalytic Materials Science and MOE Key Laboratory of Advanced Energy Materials Chemistry, School of Materials Science and Engineering, National Institute of Advanced Materials, Nankai University, Tianjin 300350, People's Republic of China c

d

College of Chemistry, Nankai University, Tianjin 300071, People's Republic of China

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People's Republic of China

1

These authors contributed equally to this work

* Corresponding author: Tel: +86-22-85358635; E-mail: [email protected] ** Corresponding author: Tel: +86-22-23503824; E-mail: [email protected] *** Corresponding author: Tel: +86-22-23509005; E-mail: [email protected]

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Abstract Single-crystalline Pd nanocrystals enclosed by {111} or {100} facets with controllable sizes were synthesized and originally employed as catalysts in the aerobic oxidation of 5-hydroxymethyl-2-furfural (HMF). The experimental results indicated that the particle size and exposed facet of Pd nanocrystals could obviously influence their catalytic performance. The size-dependent effect of Pd nanocrystals in this reaction could only be derived from the different Pd dispersion. Therefore, the facet effect of Pd nanocrystals was firstly investigated in this work through experimental and theoretical approaches. It could be found that Pd-NOs enclosed by {111} facets more efficient than Pd-NCs enclosed by {100} facets for the aerobic oxidation of HMF, especially for the oxidation step from 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) toward 5-formyl-2-furancarboxylic acid (FFCA). The TOF value of Pd-NOs(6 nm) was 2.6 times as high as that of Pd-NCs(7 nm) and 5.2 times higher than that of commercial Pd/C catalyst for HMF oxidation. Through the density functional theory (DFT) calculation, the notably enhanced catalytic performance of Pd-NOs could be mainly attributed to the lower energy barrier in the alcohol oxidation step (from HMFCA to FFCA) and higher selectivity for O2 hydrogenation to produce peroxide.

Keywords Facet effect; Size-dependent effect; Single-crystalline Pd; Aerobic oxidation; 5-Hydroxymethyl-2-furfural.

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1. Introduction Biomass is currently the only carbon-based renewable energy which has considerable potential to replace petroleum as a feedstock for chemical industry because

of

its

abundance,

renewability

and

worldwide

distribution1-4.

5-Hydroxymethyl-2-furfural(HMF), one of the most important biomass platform chemicals, can be obtained from inexpensive and plentiful cellulosic derivates such as fructose and glucose (via isomerisation to fructose), even directly from cellulose5-8. Its final oxidation product, 2,5-furandicarboxylic acid (FDCA) is one of the selected top value added chemicals from biomass.9 It could be used as a monomer for the production of new polyesters and nylons10 and was considered as a promising alternative for terephthalic acid, which was widely used in polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) plastic production.11 From green chemistry point of view, converting HMF to FDCA using O2 as oxidant and H2O as reaction medium under ambient pressure was a sustainable and promising route. Compared with transition metal12-15 and enzyme16,17 catalysts, noble metal nanocatalysts18-40 usually exhibited desirable activity and FDCA selectivity under mild conditions. Thus, in the last decade, the studies focused on noble metal nanocatalysts have received much attention. Au18-21, Pt22-27, Pd28-31,41, Ru32-35,42 and their bimetal nanoparticles36-40 were used as catalysts for the aerobic oxidation of HMF. Some researchers had reported the influences of noble metal type and particle size on the catalytic performance for the aerobic oxidation of HMF. Davis et al.43 compared the reactivity of Pt, Pd and Au catalysts for the oxidation of HMF and

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found that Au nanoparticle catalyst exhibited the best efficiency in the oxidation of HMF to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), however, Pt and Pd nanoparticle catalysts were more efficient in the oxidation of HMFCA to 5-formyl-2-furancarboxylic acid (FFCA) and FDCA. It indicated that Au was more active for the oxidation of aldehyde side chain, while Pt and Pd were more active for the oxidation of alcohol side chain. Siankevich et al.23 prepared a series of polyvinylpyrrolidone (PVP) stabilized Pt nanoparticles with variable particle sizes from 2.0 nm to 5.0 nm and investigated the size-dependent effect on the catalytic performance of Pt nanocatalysts for the oxidation of HMF. It was found that smaller Pt particle size could lead to better HMF oxidation activity. A similar conclusion was also drawn in PVP stabilized Pd nanoparticle catalyzed HMF oxidation reaction by Siyo et al.28. It has been well-established that the catalytic activity of nanocrystal catalysts is connected with size44, while the selectivity is most influenced by the exposed facets45,46. However, to the best of our knowledge, there is no research focused on the facet effect of single-crystalline noble metal nanocrystals in this reaction system. In the last two decades, the shape-controlled synthesis of metal nanocrystals has been well researched47-61. Pd nanoparticles with specific shapes that exposing low index facets, such as nanocube enclosed by {100} facets and nanooctahedron enclosed by {111} facets, have been well synthesized and used to estimate the facet effect in different reactions, including CO oxidative coupling62, formic acid oxidation63, CO oxidation64, oxygen reduction reaction65, and so on.

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In this work, {111}- and {100}-faceted single-crystalline Pd nanooctahedrons (Pd-NOs) and nanocubes (Pd-NCs), respectively, with controllable particle sizes in the range of 6-18 nm, were synthesized through a hydrothermal method. The as-synthesized single-crystalline Pd nanocrystals were originally used as catalysts in the aerobic oxidation of HMF. The facet effect and size-dependent effect of these single-crystalline

Pd

nanocrystals

were

systematically

investigated

through

experimental and theoretical approaches. Moreover, through reaction kinetic simulation and periodic density functional theory (DFT) calculation, we attempt to provide insights into the mechanism of facet effect on HMF aerobic oxidation over Pd nanocatalysts.

2. Experimental 2.1 Materials All chemicals were obtained from commercial sources and used as received. HMF (97%) and FDCA (98%) were obtained from Heowns Biochemical Technology Co.,Ltd. HMFCA (98%) and FFCA (98%) were purchased from Matrix Scientific and Toronto Research Chemicals Inc., respectively. 2,5-Diformylfuran (DFF, 98%) was supplied by Sun Chemical Technology Co.,Ltd. Sodium tetrachloropalladate (Na2PdCl4), phosphoric acid (H3PO4) and commercial 5 wt% Pd/C were purchased from Aladdin. PVP was obtained from Tianjing Guangfu Fine Chemistry Research Institute. L-ascorbic acid, citric acid, potassium chloride (KCl) and potassium bromide (KBr) were supplied by Kewei Chemical Industry Co.,Ltd in Tianjin. 2.2 Characterization

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Transmission electronic microscopy (TEM) images of single-crystalline Pd nanocrystals were taken on an FEI Tecnai G2 F20 instrument. The samples were dispersed by ultrasonic and dropped onto a formvar stabilized with carbon support film for observation. Inductively coupled atomic emission spectrometry (ICP-AES) analysis was performed on a Thermo Fisher Scientific Inc., IRIS Intrepid II XSP instrument. X-ray photo-electron spectroscopy (XPS) spectra were detected on a Kratos Axis Ultra DLD spectrometer employing a monochromatized Al Kα X-ray source (hν=1486.6 eV). The pulse CO chemisorption experiments were taken on a Micromeritics ChemiSorb 2750 analyzer with a TCD detector at 303 K. The reaction solutions were analyzed using an Agilent 1200 series high-performance liquid chromatography (HPLC) equipped with a Sepax Carbomix H-NP10:8% column (column oven at 338 K) and a UV-Vis detector operating at 271 nm. 1 mmol/L H3PO4 aqueous solution was used as mobile phase at a flow rate of 0.6 mL/min. 2.3 Computational methods Methods: The Vienna ab initio simulation package (VASP)66-68 was applied to investigate the HMFCA dehydrogenation reaction on Pd nanocrystal surfaces by the self-consistent periodical DFT calculations with the projected augmented wave (PAW)69 pseudopotentials. All the electronic structures were calculated using the Perdew-Burke-Ernzerhof (PBE)70 form of the generalized gradient approximation (GGA) expended in a plane wave basis with kinetic cut-off energy of 500 eV. The climbing image general nudged elastic band (CI-NEB)71 method was employed to locate the transition states (TSs). Spin polarization was included in the calculations.

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Models: The Pd nanocrystal surfaces with exposed {111} and {100} facets were modeled by the p (4×4) unit cell of four layers of which the uppermost two layers were relaxed, and the model catalysts were named as Pd(111) and Pd(100), respectively. The vacuum space of 2.0 nm was applied in case of the spurious interactions normal to the surface. The 3 × 3 × 1 Monkhorst-Pack k-point mesh72 was used in the surface Brillouin zone. The adsorption energy (Eads), activation energy (Ea), and reaction energies (∆E) were calculated by the following three formulas: Eads=EA/M-EA-EM, Ea=ETS-EIS, and ∆E= EFS-EIS, respectively. Here, EA, EM, EA/M, ETS, EIS and EFS mean the calculated energies of the adsorbate, substrate, adsorption system, transition state (TS), initial state (IS), and final state (FS), respectively. 2.4 Synthesis of single-crystalline Pd nanocrystals The syntheses of Pd-NOs and Pd-NCs with different particle sizes were based on the work of Shao et al65. 2.4.1 Synthesis of Pd-NOs with different particle sizes Pd-NOs(6 nm): PVP (105 mg, nPVP/nPd=4.9) and citric acid (180 mg) were dissolved in the mixture of deionized water (DI water, 5 mL) and ethanol (3 mL). This solution was preheated at 80 ºC for 5 min under magnetic stirring. Then 3 mL of Na2PdCl4 aqueous solution containing 57 mg of Na2PdCl4 was added. The obtained solution was further stirred at 80 ºC for 3 h. After cooling to room temperature, Pd-NOs(6 nm) were separated by centrifugation, washed with DI water and acetone, and then re-dispersed in DI water for further usage. Pd-NOs(18 nm): Pd-NCs(7 nm) were used as cubic seeds in the synthesis of

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Pd-NOs(18 nm). PVP (105 mg, nPVP/nPd=4.9), HCHO (100 µL, 40%) and cubic Pd seeds (1 mg) were suspended in 8 mL of DI water. This solution was preheated at 60 ºC for 5 min under magnetic stirring. Then 3 mL of Na2PdCl4 aqueous solution containing 57 mg of Na2PdCl4 was added. The obtained solution was further stirred at 60 ºC for 3 h. After cooling to room temperature, Pd-NOs(18 nm) were separated by centrifugation, washed with DI water and acetone, and then re-dispersed in DI water for further usage. 2.4.2 Synthesis of Pd-NCs with different particle sizes Pd-NCs(7 nm): PVP (105 mg, nPVP/nPd=4.9), L-ascorbic acid (60 mg), KCl (185 mg) and KBr (5 mg) were dissolved in 8 mL of DI water. This solution was preheated at 80 ºC for 5 min under magnetic stirring. Then 3 mL of Na2PdCl4 aqueous solution containing 57 mg of Na2PdCl4 was added. The obtained solution was further stirred at 80 ºC for 3 h. After cooling to room temperature, Pd-NCs(7 nm) were separated by centrifugation, washed with DI water and acetone, and then re-dispersed in DI water for further usage. Pd-NCs(16 nm): The synthesis procedure of Pd-NCs(16 nm) was very similar with that of Pd-NCs(7 nm), but KCl (185 mg) was replaced with KBr (595 mg). 2.5 HMF oxidation reactions 0.4 mmol of HMF, 1.6 mmol NaHCO3 and desired amount of Pd-NOs or Pd-NCs catalysts were suspended in 20 mL DI water and this reaction mixture was heated to 90 ºC and bubbled with O2 flow (25 mL/min) under magnetic stirring for a certain time. Then 50 µL reaction mixture was taken out when needed and diluted with 5 mL

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of DI water for HPLC test. Liquid samples were syringe-filtered (0.2µm PTFE membrane) and detected using an Agilent 1200 HPLC.

3. Results and Discussion 3.1 Characterization of single-crystalline Pd nanocrystals Figure 1 describes the HRTEM images of Pd-NOs and Pd-NCs with different particle sizes. It could be clearly observed that all the samples were of high monodispersity and well-defined shape. In addition, the histograms of particle size distributions indicated a very narrow size distribution of Pd particles in these samples. The average particle sizes were about 6.1 nm and 18.4 nm for Pd-NOs(6 nm) and Pd-NOs(18 nm), respectively. And Pd-NCs(7 nm) and Pd-NCs(16 nm) consisted of particles of about 7.4 nm and 16.5 nm in size, respectively. Moreover, the images in Figures 1(b) and 1(f) confirmed that Pd-NOs were single crystals and had a {111} interplanar distance of 2.2 Å. For Pd-NCs, the well-resolved lattice spacing of 2.0 Å, corresponding to the interplanar distance of the {100} plane of Pd, could be clearly observed in Figures 1(d) and 1(h). The as-synthesized Pd-NOs(6 nm) and Pd-NCs(7 nm) were characterized by XPS and the high-resolution XPS spectra of Pd 3d, Cl 2p and Br 3d regions are shown in Figure 2. It could be found that no Cl 2p and Br 3d peaks were detected in these two samples, which suggested the thorough removal of KCl and KBr from the Pd nanocrystals. From the XPS spectra shown in Figures 2(a) and 2(b), the predominant Pd 3d5/2 and Pd 3d3/2 peaks with binding energies at 335.2 eV (334.9 eV) and 340.4eV (340.2 eV), respectively, could be assigned to Pd0 species in Pd-NOs(6 nm)

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(Pd-NCs(7 nm)), which indicated that Pd2+ from Na2PdCl4 precursor had been reduced to Pd0 during the synthesis process. It could also be observed that, besides Pd0 species, there were ca. 16% and 28% of Pd2+ species existed on the surface of Pd-NOs(6 nm) and Pd-NCs(7 nm), respectively, which were mainly derived from the long-time storage of sample before XPS characterization31. It should be noted that the amount of Pd2+ species on Pd-NCs(7 nm) surface was larger than that of Pd-NOs(6 nm), which could be explained by the higher performance of Pd {100} facets for O2 dissociation. 3.2 HMF oxidation over Pd nanocrystals These Pd-NOs and Pd-NCs catalysts were employed as catalysts in the aerobic oxidation of HMF. The dosage of Pd nanocatalysts was 4% based on the molar ratio of total Pd to HMF, which was a slightly higher catalyst loading compared with the ever reported researches31,37,41,43 because of the larger particle size of these Pd nanocrystals. The reaction profiles are described in Figure 3. It could be found that Pd-NOs enclosed by {111} facets exhibited obviously higher catalytic performance than Pd-NCs enclosed by {100} facets with similar particle size. For example, full HMF conversion and 91.3% FDCA yield could be achieved over Pd-NOs(6 nm) within 4 h, while only 33.1% FDCA yield and 98.3% HMF conversion could be obtained over Pd-NCs(7 nm) through 12 h reaction. In addition, for the catalysts with the same crystal shape, those with smaller particle size obviously performed better. Moreover, a serious amount of side-reaction products (about 20%) were generated through ketonization and condensation reaction19,37,43,73,74

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over Pd-NOs(18 nm) and Pd-NCs(16 nm) after 12 h reaction, which could be attributed to the lack of active sites derived from the lower percentage of surface Pd atoms in these catalysts. No DFF was detected in these reaction solutions due to the alkaline reaction conditions. Furthermore, no obvious morphology and size change were observed by TEM for these Pd catalysts after the oxidation reactions (as shown in Figure S1 of Supporting Information). In order to quantitatively evaluate the catalytic performance of these Pd-NOs and Pd-NCs catalysts, the commercial 5 wt% Pd/C was used as a reference catalyst, and the turnover frequency (TOF) values were calculated from the results at 5 min reaction using equation (1) as follows. In this equation, Pd dispersion is surface atom percentage of Pd catalyst. Pd dispersion of Pd-NOs(6 nm) and Pd-NCs(7 nm) were determined by geometrical configurations and average particle sizes, and the results are shown in Table 1. The calculated Pd dispersion was 23.87% for Pd-NOs(6 nm) and 15.36% for Pd-NCs(7 nm), respectively. Pd dispersion of commercial Pd/C was 54.73% determined through CO chemisorption. The higher value could be due to the smaller Pd particle size in commercial Pd/C. nHMF/nPd means the molar ratio of HMF to Pd. The calculated TOF values were 4.02 min-1 for Pd-NOs(6 nm), 2.02 min-1 for Pd-NCs(7 nm), and 0.78 min-1 for commercial Pd/C, respectively. Evidently, Pd-NOs(6 nm) exhibited the highest TOF value, which was 2.6 times higher than that of Pd-NCs(7 nm) and 5.2 times higher than that of commercial Pd/C catalyst. Compared with the ever reported catalysts22,30,31,37,41,43,75, the TOF value of Pd-NOs(6 nm) was also one of the highest results in the aerobic oxidation of HMF catalyzed by

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Pd nanocatalysts. ( ) =

(%)×( /) ()× !

Eq. (1)

3.3 Effect of surface Pd atom percentage on the catalytic performance The results in Figure 3 demonstrated the obvious facet-dependence and size-dependence of HMF oxidation over these obtained Pd nanocrystals. It is well known that the Pd nanocrystals with different particle sizes or exposing different facets would possess variable surface Pd atom percentage. Therefore, before the investigation of facet effect of Pd nanocrystals, the influence of surface Pd atom percentage on the catalytic performance should be studied firstly. Table 1 lists the surface Pd atom percentage of all the Pd-NOs and Pd-NCs nanocrystals calculated based on the geometrical configurations and average particle sizes. In order to investigate the effect of surface Pd atom percentage on the catalytic performance, the aerobic oxidation experiments of HMF were carried out over desired amount of Pd nanocatalysts with an equal quantity of surface Pd atom. In these experiments, the molar ratio of surface Pd atoms to HMF was consistent at 0.6% and the reaction profiles are shown in Figure 4. It could be found from Figure 4, these Pd-NOs or Pd-NCs nanocrystals with different particle sizes exhibited very similar catalytic performance for the aerobic oxidation of HMF. >98% HMF conversion and ~63% FDCA yield could be achieved over Pd-NOs(6 nm) and Pd-NOs(18 nm) through 4 h reaction (Figure 4(a) and 4(c)). While Pd-NCs(7 nm) and Pd-NCs(16 nm) only gave ~80% HMF conversion and ~10% FDCA yield at the same reaction time (Figures 4(b) and 4(d)). In addition, as shown

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in Figure 4(c) and 4(d), the yields of by-products over Pd-NOs(18 nm) and Pd-NCs(16 nm) were significantly decreased compared with the results described in Figure 3(c) and 3(d), which could be attributed to the increased catalyst loading in the present control experiments. By comparing the results in Figure 3 and Figure 4, the following conclusions can be drawn. Firstly, the size-dependent effect of these Pd nanocrystals could be derived from the difference of surface Pd atom percentage. With an identical amount of surface Pd atoms, the Pd-NCs or Pd-NOs nanocrystals with different particle sizes exhibited

similar

catalytic

performance.

More

importantly,

an

obvious

facet-dependent effect of these single-crystalline Pd nanocrystals was discovered for the aerobic oxidation of HMF. Pd-NOs enclosed by {111} facets exhibited notably improved catalytic activity compared with Pd-NCs enclosed by {100} facets. 3.4 Reaction kinetic model for Pd facet-dependent oxidation of HMF The reaction pathway from HMF to FDCA is shown in Scheme 1. Since no DFF was detected in the reaction process (shown in Figure 3 and Figure 4), the oxidation from

HMF

to

FDCA

was

considered

through

the

following

pathway

(HMF→HMFCA→FFCA→FDCA) over these Pd-NCs or Pd-NOs nanocrystals. According to the reported methods23,25, a mechanism model was built to describe the catalytic oxidation of HMF and determine the rate constants of each step. The oxidation from HMF to HMFCA was confirmed as first order with respect to HMF by using the differential method. Equations (2)~(5) contained the rate equations for each species consumed and formed, where CHMF, CHMFCA, CFFCA, CFDCA were the

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concentrations of the participating reactants and products, and k1 to k3 were the catalytic constants of each reaction step. " "#

= −% &'()

" *+ "# " *+ "# " /*+ "#

Eq. (2)

= % &'() − %, &'() -

Eq. (3)

= %, &'() - − %. &)) -

Eq. (4)

= %. &)) -

Eq. (5)

To estimate the kinetic parameters of different oxidation steps, the least squares fitting algorithm of MATLAB lsqcurvefit was used, and the following assumptions have been taken. i) the temperature is regulated and assumed to be stable at 90 ºC throughout the reaction; ii) the properties and the amount of the active species of the catalyst remain constant during the catalytic reaction; iii) the reaction system is considered as a homogeneous solution; iv) all dehydrogenation reactions are irreversible; v) the oxygen concentration is sufficiently and invariable in the liquid phase. The accuracy of this method can be appreciated from the fitting shown in Figure S2 (Supporting Information). Table 2 lists the estimated experimental kinetic parameters for Pd-NOs(6 nm) and Pd-NCs(7 nm) based on the reaction results described in Figure 3. In this model, k1 and k3 represent the oxidation reactions of the aldehyde groups in the reactant, and k2 corresponds to the oxidation of alcohol group. It could be found that all the rate constants of Pd-NOs(6 nm) were much higher than

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those of Pd-NCs(7 nm), which indicated that Pd-NOs(6 nm) exhibited notably improved catalytic activity compared with Pd-NCs(7 nm). In particular, for the step of alcohol group oxidation, these two catalysts exhibited the largest difference in catalytic performance. For the Pd-NOs(6 nm) catalyst, k2 was much higher than k1 and k3, which indicated that Pd {111} facets exhibited the higher catalytic activity for the oxidation of alcohol group. On the contrary, for the Pd-NCs(7 nm) catalyst, k2 was much lower than k1 and k3, which could be confirmed from the higher HMFCA yield over Pd-NCs catalysts described in Figure 3 and Figure 4. From these reaction kinetic simulation results, the conclusions could be drawn as follows. The facets of single-crystalline Pd nanocatalysts had an obvious influence on the catalytic performance for aerobic oxidation of HMF. Pd-NOs enclosed by {111} facets exhibited notably improved catalytic performance for the oxidation of alcohol group. While, for Pd-NCs enclosed by {100} facets, inferior activity for the oxidation of alcohol group was obtained, and the oxidation from HMFCA to FFCA became the rate determining step. In order to further confirm the improved catalytic activity of Pd-NOs for the oxidation of alcohol group, a control experiment was designed and details of the reaction procedure were as follows. In a typical experiment of HMF oxidation over Pd-NCs(7 nm), the reaction was terminated by cooling down the reaction solution to room temperature after 6 h reaction. Then the Pd-NCs(7 nm) catalyst was separated from the reaction solution by centrifugation. After adding a certain number of Pd-NOs(6 nm) catalyst, the reaction was continued under the same conditions. The

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reaction results are shown in Figure 5. As shown in Figure 5, in the first 6 h, the reaction profile was almost exactly coincided with the result over Pd-NCs(7 nm) described in Figure 3(b). After 6 h reaction, 92.9% HMF was converted, but most of HMF was oxidized to HMFCA (HMFCA yield was 61.3%). The yields of other products including FFCA and FDCA were relatively low and less than 16.4% after 6 h oxidation. While, after replacing Pd-NCs(7 nm) with Pd-NOs(6 nm), an obvious acceleration occurred in this oxidation reaction. The concentrations of HMFCA and FFCA in the reaction solution were significantly decreased, and the yield of FDCA increased from 16.4% to 60.0% after another 6 h reaction. These results further confirmed the difference of facet-dependent activity between Pd-NOs and Pd-NCs. 3.5 DFT calculation for Pd facet-dependent oxidation of HMFCA Now that the dominant facet of Pd nanocrystals could significantly influence their catalytic performance for HMF oxidation, which was successfully proved by the experimental results, especially for the oxidation of alcohol group in HMF, moreover, there is no report regarding the different performance of Pd facets in this reaction, the reaction mechanism on different facets of Pd nanocrystals was studied through DFT calculation in this section. According to the previous research works76-78, the adsorbed structures of furfural and furfuryl alcohol were flat on Pd surface by adsorption of furan ring. Figure 6 and Figure 7 show the structures and energy barriers of HMFCA dehydrogenation, respectively, and the adsorption was mainly due to the coordination effect of

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furan-rings and O atoms of -CH2OH and -CH2O groups with metal surface. A two-step dehydrogenation mechanism occurred in the oxidation of alcohol group of HMFCA79-81, that is, the dehydrogenation of O-H bond in -CH2OH group firstly, and then the dehydrogenation of C-H bond in -CH2O group. In the first dehydrogenation step, as shown in Figure 6(a) and Figure 7, an adsorbed alcohol HMFCA* was formed by adsorption of furan-ring and O atom of -CH2OH group. The O atom was adsorbed on the top site of Pd atom in IS. Then, the OH- adsorbed on bridge site captured the dissociated hydrogen from O-H in -CH2OH group and H2O was generated. Finally, the generated HMFCA-H* was adsorbed on a bridge site of Pd surface through the O atom of -CH2O group. The adsorption energy of HMFCA adsorbed on Pd(100) was notably higher than that on Pd(111) (-0.98 eV vs. -0.79 eV, as shown in Figure 7) due to the lower surface free energy of Pd(100). But the lengths of O-H, C-H and C-O bonds in -CH2OH group of HMFCA were similar on both Pd(100) and Pd(111), which indicated the adsorption was mostly contributed by furan-ring instead of -CH2OH group. The energy barrier of HMFCA* dehydrogenation to HMFCA-H* step was 0.23 eV on Pd(111), which was much lower than that on Pd(100) (0.45 eV). In the second dehydrogenation step, the -CH2O group of generated HMFCA-H* was attacked by OH-, which could capture the dissociated hydrogen from -CH2O group and generated a H2O molecule similar with the first step. The FS of this step was that the -CH2O group flatted absorbed on Pd surface. The adsorption energy of the adsorbed HMFCA-H* on Pd(100) was also obviously higher than that on Pd(111)

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(-0.99 eV vs -0.79 eV), which led to the fact that the energy barrier of HMFCA-H* dehydrogenation on Pd(111) was found to be 0.92 eV, which was slightly lower than that on Pd(100) (0.99 eV). Moreover, the energy barriers of the second dehydrogenation step on Pd(100) and Pd(111) were much higher than those of the first step. It indicated that the dehydrogenation of α-C hydride was much harder compared with the dehydrogenation of hydroxyl group, because HMFCA-H* was adsorbed on Pd surface through the O atom of -CH2O group rather than the carbon atom. Pd(100) exhibited prohibitively higher barriers in the both dehydrogenation steps compared with Pd(111), which could be mainly attributed to the strong adsorption between Pd(100) and reaction intermediates (HMFCA and HMFCA-H as shown in Fig. 7) and OH-82,83. The strong adsorption between OH- and Pd(100) competed with the ability of OH- drawing away hydrogen atom from -CH2O group84,85. In addition, as shown in Figure 7, the reaction energy of first dehydrogenation step (∆E1) on Pd(111) and Pd(100) were both marginal (0.00 eV and -0.02 eV). However, for the second dehydrogenation step, the reaction energy (∆E2) on Pd(111) was -0.72 eV, which was more exothermic than that on Pd(100) (-0.46 eV). It indicated that Pd(100) was the thermodynamically unfavorable facet in this reaction. The results of DFT calculation for Pd facet-dependent oxidation of HMFCA demonstrated that Pd(111) was more active than Pd(100) in the oxidation of HMFCA toward FFCA. 3.6 O2 conversion mechanism in different facets of Pd nanocrystals

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It has been widely demonstrated that Pd-NCs enclosed by {100} facets exhibits more efficient catalytic performance in the gas-phase oxidation of CO64,86 and alcohols87-90 compared with Pd-NOs enclosed by {111} facets. However, in the present work, Pd-NOs catalysts exhibited higher catalytic activity for the liquid-phase aerobic oxidation of HMF than Pd-NCs. Because of the significant difference of reaction mechanism between liquid-phase oxidation and gas-phase oxidation, the investigation on O2 conversion mechanism over Pd nanocrystals enclosed by different facets was another important part to reveal the facet-dependent effect of Pd nanocrystals for aerobic oxidation of HMF. According to the reported researches, O2 should be adsorbed on catalyst surface and dissociated to O atoms (as described in Eq. (6)) in the gas-phase reaction of alcohols87-90. While, for the liquid-phase aerobic oxidation of HMF, O2 participated in the reaction through hydrogenation (Eqs. (7) and (8)) to compose peroxide and drew away the extra electrons produced in this reaction91-93. , ∗ = 2∗

Eq. (6)

, ∗ + 3∗ = 3∗

Eq. (7)

3 ∗ + 3∗ = 33∗

Eq. (8)

In order to demonstrate the reaction mechanism of O2 on Pd nanocrystals enclosed by different facets, Pd(111) and Pd(100) were modeled as p (2×3) unit cell because of the smaller molecular size of O2. Both the dissociation and hydrogenation paths of O2 on Pd(111) and Pd(100) surfaces were calculated through a DFT method and the geometrical structures of IS, TS, and FS for O2 dissociation and

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hydrogenation on Pd(111) and Pd(100) are shown in Figure 8. According to the previous researches82,83,94, the most stable adsorption site of O2 is top-bridge on Pd(111) and bridge-bridge on Pd(100), respectively, while OOH was preferentially adsorbed on a top-top-bridge site on both facets (as described in Figure 8). It could be found from Figure 8 that the O-O bond lengths of O2 on Pd(100) and Pd(111) were 1.417 Å and 1.365 Å, respectively. This indicated that the O–O bond of O2 on Pd(100) was weaker than that on Pd(111) and O2 molecules should be more likely to dissociate on Pd(100) instead of Pd(111). The potential energy diagrams for O2 hydrogenation and dissociation reactions on Pd(111) and Pd(100) are described in Figure 9. For Pd(111) catalyst, it could be observed from Figure 9(a) that the energy barriers of hydrogenation path (O2* + H* = OOH*) and dissociation path (O2* = 2O*) were similar. The hydrogenation path has an energy barrier of 0.73 eV, slightly lower than that of dissociation path (0.76 eV). Moreover, the hydrogenation path was more exothermal with a reaction energy of -0.97 eV compared with the dissociation path (-0.26 eV). These results indicated that the adsorbed O2 on Pd(111) surface preferred to be hydrogenated and produced OOH, which could be further hydrogenated to compose H2O2. These generated H2O2 could participate in the oxidation reaction of HMF, draw away the extra electron and complete the catalytic cycle. For the catalyst of Pd(100), however, there was a noticeable difference in energy barrier between the two reaction paths. The energy barrier of hydrogenation path was very high (0.95 eV), moreover, this process was endothermic with a reaction energy

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of 0.57 eV. However, the energy barrier of dissociation path was much lower (0.12 eV) and this process was exothermal with a reaction energy of -0.86 eV. These indicated that the adsorbed O2 on Pd(100) surface tended to be dissociated to atom state, which should be unfavorable to the oxidation of HMF because of the following reasons. Firstly, these adsorbed O atoms cannot participate in the catalytic cycle of HMF oxidation reaction. Secondly, these adsorbed O atoms may cause valence state change of Pd0, further interfere with the adsorption and desorption of the reactants, intermediates and products94.

4. Conclusions In summary, the PVP stabilized single-crystalline Pd-NCs and Pd-NOs with different particle sizes were synthesized through a hydrothermal method and characterized by HRTEM and XPS. These obtained Pd nanocrystals were used as catalysts in the aerobic oxidation of HMF. The facet effect and size-dependent effect of these Pd nanocrystals on the catalytic performance were originally investigated. It was found that the size-dependent effect of these Pd nanocrystals was derived from the different surface Pd atom percentage. By controlling the amount of surface Pd atoms to be identical, the Pd nanocrystals with same shape but different particle sizes exhibited very similar catalytic performance for HMF oxidation. Compared with Pd-NCs enclosed by {100} facets, Pd-NOs enclosed by {111} facets exhibited notably enhanced catalytic activity for the aerobic oxidation of HMF. The reaction kinetic simulation results revealed that Pd-NOs exhibited better catalytic performance in the oxidation of alcohol group, while Pd-NCs performed better in the

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oxidation of aldehyde groups instead of alcohol group. Among these Pd nanocatalysts, Pd-NOs(6 nm) exhibited the highest catalytic activity with a calculated TOF up to 4.02 min-1, which was 2.6 times as high as that of Pd-NCs(7 nm) and 5.2 times higher than that of commercial Pd/C catalyst. The DFT calculation results indicated that Pd-NOs had a lower energy barrier in the alcohol oxidation step (from HMFCA to FFCA) compared with Pd-NCs. In addition, Pd-NOs showed a higher selectivity for O2 hydrogenation to produce peroxide, which could draw away the extra electrons and complete the catalytic cycle. However, Pd-NCs tended to dissociate the adsorbed O2 to O atoms, which had a negative effect on the oxidation of HMF.

Associated Content Supporting Information TEM images of Pd-NOs and Pd-NCs catalysts after oxidation reaction, the reaction kinetic simulation through the least squares fitting algorithm of MATLAB lsqcurvefit, recycling tests of Pd-NCs(7 nm) and Pd-NOs(6 nm), EPR spectra of spin-trapped radicals over Pd-NOs(6 nm) and Pd-NCs(7 nm).

Acknowledgements This work was supported by the National High Technology Research and Development Program of China (Grant 2012AA063008), National University Student Innovation Program (Grant 201510055105) and Tianjin Municipal Natural Science Foundation (Grants 14JCQNJC06000, 14JCZDJC32000 and 15JCTPJC63500). Computational support was provided by Beijing Computing Center (BCC).

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Table 1 Calculation for surface atom percentages of Pd nanocrystals a Surface Pd atom Pd nanocrystal

mb

Nt c

Ns d percentage (%)

Pd-NOs(6 nm)

23

8119

1938

23.87

Pd-NCs(7 nm)

19

25327

3890

15.36

Pd-NOs(18 nm)

67

200531

17414

8.68

Pd-NCs(16 nm)

45

352485

23234

6.59

a

Assuming that all the nanocrystals are perfect cubic or octahedral morphology.

b

m defines as the number of equivalent edge atoms including corner atoms.

c

Nt represents the total number of atoms in a single Pd nanocrystal.

Nt(Pd-NOs)=m(2m2+1)/3, Nt(Pd-NCs)=4m3-6m2+3m. d

Ns represents the number of surface Pd

atoms. Ns(Pd-NOs)=4m2-8m-6,

Ns(Pd-NCs)=12m2-24m+14.

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Table 2 Kinetic parameters estimated for Pd-NOs(6 nm) and Pd-NCs(7 nm) Rate constant (h-1) Catalyst k1

k2

k3

Pd-NOs(6 nm)

1.92

4.69

0.96

Pd-NCs(7 nm)

0.40

0.09

0.29

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Figure 1. HRTEM images with particle size distribution of Pd-NOs and Pd-NCs catalysts. Pd-NOs(6 nm): (a), (b); Pd-NCs(7 nm): (c), (d); Pd-NOs(18 nm): (e), (f); Pd-NCs(16 nm): (g), (h). Figure 2. High-resolution XPS spectra of Pd 3d, Cl 2p and Br 3d regions for Pd-NOs(6 nm) and Pd-NCs(7 nm). Figure 3. Reaction profiles for HMF oxidation over Pd-NOs and Pd-NCs catalysts at ambient pressure. Reaction conditions: ntotal

Pd/nHMF

(mol/mol)=4%, nNaHCO3/nHMF

(mol/mol)=4, 90 ºC, O2 bubbling (25 mL/min). Figure 4. Reaction profiles for HMF oxidation over Pd-NOs and Pd-NCs catalysts at ambient pressure. Reaction conditions: nsurface Pd/nHMF (mol/mol)=0.6%, nNaHCO3/nHMF (mol/mol) = 4, 90 ºC, O2 bubbling (25 mL/min). Figure 5. Reaction profiles for HMF oxidation catalyzed by Pd-NCs(7 nm) (1~6 h) and Pd-NOs(6 nm) (6~12 h). Reaction conditions: nsurface Pd/nHMF (mol/mol)=0.6%, nNaHCO3/nHMF (mol/mol) = 4, 90 ºC, O2 bubbling (25 mL/min). Figure 6. Geometrical structures of IS, TS, and FS for the two-step dehydrogenation of HMFCA on Pd(111) and Pd(100). The energies in parentheses are activation energies of the reaction step. Figure 7. Potential-energy profiles for the oxidation from HMFCA to FFCA. Figure 8. Geometrical structures of IS, TS, and FS for O2 dissociation (a) and hydrogenation (b) on Pd(111) and Pd(100). The energies in parentheses are activation energies of the reaction step. Figure 9. Potential-energy profiles for O2 hydrogenation and dissociation reactions on

32


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ACS Catalysis

(a) Pd(111) and (b) Pd(100). Scheme 1. The reaction pathway from HMF to FDCA.

33


ACS Catalysis

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

Figure 1.

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

ACS Catalysis

35


ACS Catalysis

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

Page 36 of 45

Figure 2. (a) Pd 3d

Pd-NOs(6 nm)

Pd-NCs(7 nm)

(b) Pd 3d

334.9

335.2

340.2

340.4

341.6 336.3

341.8

346

344

342

340

336.5

338

336

334

332

330

346

344

342

Binding Energy (eV)

(c) Cl 2p

340

338

336

334

332

330

Binding Energy (eV)

(d) Cl 2p

Pd-NOs(6 nm)

Pd-NCs(7 nm)

212 210 208 206 204 202 200 198 196 194

212 210 208 206 204 202 200 198 196 194

Binding Energy (eV)

Binding Energy (eV)

(e) Br 3d

82

80

78

76

74

72

Pd-NOs(6 nm)

(f) Br 3d

70

82

68

66

64

Binding Energy (eV)

80

78

Pd-NCs(7 nm)

76

74

72

70

Binding Energy (eV)

36


68

66

64

Page 37 of 45

Figure 3. 100

100

(a) Pd-NOs(6 nm)

60

HMF HMFCA FFCA FDCA Other

40

20


80

Conversion/Yield (%)


80

0

60

(b) Pd-NCs(7 nm)

40

20

0

0

2

4

6

8

10

12

0

2

4

Time (h)

6

8

10

12

Time (h)

100

100


60

40


80


80


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

ACS Catalysis

(c) Pd-NOs(18 nm)

20

0

(d) Pd-NCs(16 nm)

60

40

20

0 0

2

4

6

8

10

12

0

2

Time (h)

4

6

Time (h)

37


8

10

12

ACS Catalysis

Figure 4. 100

100

(a) Pd-NOs(6 nm)

60




80

80


40

20

0

60

(b) Pd-NCs(7 nm)

40

20

0 0

2

4

6

8

10

12

0

2

4

Time (h)

6

8

10

12

Time (h) 100

100


(c) Pd-NOs(18 nm) 80

60


80


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

Page 38 of 45


40

20

0

(d) Pd-NCs(16 nm)

60

40

20

0 0

2

4

6

8

10

12

0

2

Time (h)

4

6

Time (h)

38


8

10

12

Page 39 of 45

Figure 5. catalyst replacement

100

Pd-NCs(7 nm)


80


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

ACS Catalysis

60

40

Pd-NOs(6 nm)

20

0 0

2

4

6

8

Time (h)

39


10

12

ACS Catalysis

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

Figure 6.

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Figure 7. 0.5

0.0

TS

Pd(111) Pd(100)

0.3

Energy barrier (eV)

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

ACS Catalysis

HMFCA(g)

-0.3

Ea2 = 0.92 eV

TS Ea1 = 0.23 eV

-0.5 -0.8 -1.0 -1.3 -1.5

Ea1 = 0.45 eV

HMFCA*

Ea2 = 0.99 eV

HMFCA-H* ∆E1 = 0.00 eV ∆E1 = -0.02 eV

-1.8

Reaction coordinate

41


∆E2= -0.46 eV

FFCA*

∆E2 = -0.72 eV

ACS Catalysis

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

Figure 8.

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Figure 9. 0.0

O2(g) + H*

O2* = 2 O* O2* + H* = OOH*

Energy barrier (eV)

-0.5 -1.0

TS

-1.5 -2.0

O2* + H*

-2.5

Ea1 = 0.73 eV Ea2 = 0.76 eV

2O* + H*

OOH*

-3.0

(a) Pd(111)

∆E = -0.26 eV

∆E = -0.97 eV

-3.5

Reaction coordinate 0.0

O2(g) + H*

O2* = 2 O*

-0.5

Energy barrier (eV)

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

ACS Catalysis

O2* + H* = OOH*

-1.0 TS

-1.5

OOH*

-2.0

∆E = 0.57 eV

Ea2 = 0.12 eV

-2.5 -3.0 -3.5

O2* + H*

Ea1 = 0.95 eV 2O* + H*

(b) Pd(100) Reaction coordinate

43


∆E = -0.86 eV

ACS Catalysis

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

Scheme 1.

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ACS Catalysis

Graphical Abstract

45


Facet Effect of Single-Crystalline Pd Nanocrystals for Aerobic

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