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Single Molecule Nanocatalysis Reveals Facet-Dependent Catalytic Kinetics and Dynamics of Pd Nanoparticles Tao Chen, Sheng Chen, Ping Song, Yuwei Zhang, Hongyang Su, Weilin Xu, and Jie Zeng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00087 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 18, 2017
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Single Molecule Nanocatalysis Reveals Facet-Dependent Catalytic Kinetics and Dynamics of Pd Nanoparticles Tao Chen‡a,b, Sheng Chen‡c, Ping Songa, Yuwei Zhanga, Hongyang Suc, Weilin Xu*a, Jie Zeng*c a
State Key Laboratory of Electroanalytical Chemistry, & Jilin Province Key
Laboratory of Low Carbon Chemical Power, Changchun Institute of Applied Chemistry, Chinese Academy of Science, 5625 Renmin Street, Changchun 130022, P.R. China. b
c
University of Chinese Academy of Science, Beijing, 100049, P.R. China.
Hefei National Laboratory for Physical Sciences at the Microscale Collaborative
Innovation Center of Suzhou Nano Science and Technology, Center of Advanced Nanocatalysis (CAN-USTC) and School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026 (P.R. China)
ABSTRACT: By studying the nanocatalysis of individual Pd nanoparticles (Pd NPs) in two shapes (cube with (100) facet and octahedron with (111) facet) with single-turnover resolution, the facet-dependent activities and dynamics were observed apparently. The results indicate that Pd octahedrons possess higher intrinsic catalytic activity per site than Pd nanocubes. Within a competitive Langmuir-Hinshelwood mechanism, the facet-dependent activities are derived at single particle level, and the facet-dependent adsorption behaviors of substrate molecules on Pd(111) and Pd(100)
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are revealed and clarified by theoretical calculation. Furthermore, the facet-dependent restructuring behaviors of the particle surfaces were also observed. This study gives deeper insight into Pd-based nanocatalysts.
KEYWORDS: Pd nanoparticles, single molecule, nanocatalysis, fluorescence microscopy, shape effect
1. INTRODUCTION The effect of shape on catalytic activity of nanocatalysts has attracted remarkable attention.1-7It has been well established that the catalytic properties are greatly influenced by the arrangement manner of surface atoms or crystal facets exposed.8,9 But for the mechanism study, it still remains a challenging goal to understand how surface crystal facets influence their properties in nanocatalysis research.1,4,10,11 Recently, the shape effect on catalytic properties of nanoparticles has been investigated extensively at ensemble level.4,6,8,12-18 By synthesizing monodisperse nanoparticles with uniform shapes and examining their reactivity and selectivity, researchers have derived tremendous insights into the facet-dependent heterogeneous catalysis.19 Pd nanoparticles, as an important type of catalysts in industry and organic synthesis,20,21 have been studied as models for the clarification of shape and size effect on catalytic properties.5,18,22-24 In recent reports, Pd nanocubes with (100) facet exposed and Pd octahedrons with (111) facets have been demonstrated to possess different reactivity and selectivity in many reactions at ensemble level.4,6,12,14,18,23 However, the studies about the facet effect are mostly limited at the ensemble level, 2 ACS Paragon Plus Environment
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which could only give out the apparent average information of the catalysis. For the deeper investigation of the facet/shape-dependent catalytic kinetics and dynamics, it is very desirable to examine the catalytic process at the single nanoparticle level to reveal more facet/shape-dependent catalytic properties which usually cannot be revealed from ensemble measurement. Recently, the single molecule fluorescence microscopy (SMFM) has been used extensively to elucidate the catalytic kinetics and dynamics of individual nanoparticles with single-turnover resolution.25-35 With this technique, we have studied the size dependent and surface-atom-type dependent catalytic properties of Pd nanocubes.36,37 Here, we adopt the same technique to further reveal the facet/shape-dependent catalytic properties of individual Pd nanoparticles with different shapes/facets (nanocubes with (100) facet and octahedrons (111) facet) towards fluorogenic reduction reaction of resazurin by H2 at single nanoparticle level. The results indicate that Pd octahedrons possess higher intrinsic catalytic activity per site than Pd nanocubes. On the basis of the two-site Langmuir-Hinshelwood mechanism and theoretical calculation, we explored the underlying reaction kinetics and the further study reveals that the surface restructuring rate, which was pivotal in catalysis, were facet-dependent apparently. We believe that our work provides a instruction to the synthesis of nanocatalysts and the investigation of the reaction mechanism.
2. EXPERIMENTAL SECTION
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2.1 Materials and synthesis: All materials were used as received, Pd cubes and octahedrons with 22 nm edge length were synthesized by a seeded growth with 10-nm Pd cubes as seed (Supporting Information),38,39 and characterized by transmission electron microscopy (TEM) with an accelerating voltage of 200 kV.
2.2 Single-Molecule Experiments: The setup of the single molecule experiment and the preparation of flow cell have been introduced in detail by previous literatures (Supporting Information).25,36 Specially, for each movie, it was collected on a fresh microreactor. The substrate resazurin was diluted from the high concentration of resazurin with the H2-saturated PBS solutions (pH~7.4). A series of movies were collected at different substrate concentrations. These movies were analyzed with a home-written IDL program to obtain the fluorescence intensity time trajectories of individual particles by integrating every signal counts of each frame over an area of ~1×1 um2.
3. RESULTS AND DISCUSSION The 22-nm Pd nanoparticles (nanocubes and octahedrons) were synthesized according to literatures (Supporting Information).38,39 Figure 1a and b show the typical transmission electron microscopy (TEM) images of Pd nanocubes and octahedrons, both of which exhibited an average edge length of ~22 nm and a narrow size distribution (Figure S1). We employed resazurin reduction by hydrogen to investigate the catalytic performance of Pd nanoparticles from ensemble level. As shown in Figure S2, both of these two types of Pd nanoparticles are active for 4 ACS Paragon Plus Environment
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resazurin reduction by hydrogen in aqueous solution. Apparently, the average activity per Pd nanocube is higher than that per Pd octahedron for this reaction.
Figure 1. (a) Typical TEM image of Pd nanocubes. (b) Typical TEM image of Pd octahedrons. The scale bars represent 10 nm in both (a) and (b). (c) Experimental diagram of single molecule and single nanoparticle optical measurements under TIRF microscopy. (d) Part of a catalytic fluorescence time trajectory of a single Pd nanoparticle. τ is the time interval between two nearby fluorescence intensity bursts, which means the reaction time between sequential reaction events.
To reveal the catalytic properties of individual Pd nanoparticles during the reaction (Figure 1c), shape-monodispersed Pd particles (cubes or octahedrons) were first immobilized sparsely on the surface of a clean quartz slide inside a microfluidic channel and then flowed in the substrate solution containing resazurin (B) from 0.1 to 2.0 nM and saturated H2 (A). The product resorufin molecules were excited to be fluorescent by a circularly polarized 532 nm laser, and acquired on electric multiplying charge-coupled camera (EMCCD) operating at the rate of 10 frames/second, which was equipped on a total internal reflection fluorescence (TIRF) microscope. Thanks to the wide-field imaging, we can record the signals of many 5 ACS Paragon Plus Environment
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individual nanoparticles simultaneously, each nanoparticle gave out fluorescence bursts repetitively, and each burst means one fluorescent product resorufin formed on the surface of this Pd nanoparticle. Figure 1d shows part of an exemplary catalytic fluorescence time trajectory from one Pd nanocube. The trajectory contained many fluorescence intensity bursts, each of them indicates the formation of one fluorescent product molecule and represents one catalytic turnover. The time interval (τ) of two adjacent bursts is the time needed for the formation of single product molecule; the values of τ are stochastic, but their distributions and average values can reflect the underlying catalytic reaction kinetics. It should be noted here, the narrow bursts on trajectories indicate that the new product molecules dissociated very fast after their formation on nanoparticle surface. Then the time interval τ between two neighbor bursts could be approximately regarded as the time needed to complete a catalytic turnover.40 From a trajectory as shown in Figure 1d, the average turnover of frequency (TOF) of a single Pd particle can be derived directly by counting the number of bursts within a given time with TOF= -1. To probe the kinetic mechanism of these two shape-different Pd nanoparticles, we examined their activities by conducting catalytic turnovers with different concentrations of reactant. Figure 2a gives the concentration-dependent TOF for both Pd nanocubes and octahedrons. Apparently, the TOFs initially increased rapidly with the increase of substrate concentration. When the concentration of substrate reach a certain value (0.5 nM for nanocubes and 0.7 nM for octahedrons), TOFs of both catalysts reduced inversely. It can be clearly seen that the catalytic reactivity of 6 ACS Paragon Plus Environment
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individual Pd nanocubes was much higher than that of individual Pd octahedrons, indicating higher catalytic activity of (100) facet than (111) facet of Pd nanoparticles for this reduction reaction.41 This result is consistent with traditional ensemble experiments (Figure S2). The decay of TOFs at higher substrate concentrations could be attributed to a competitive adsorption between resazurin and hydrogen on Pd nanoparticles.20,29 As expected, the reactant of resazurin at high concentration can occupy predominately the surface active site and prevent the access of hydrogen to the active site for reaction. 42
Table 1. Kinetic parameters of Pd NPs with different shapes γeff
γeff/S×104
aA
aB
Ead[a]
Ead[a]
(s-1)
(s-1nm-2)
(mM-1)
(nM-1)
(H2)
(Resazurin)
Nanocube (100)
0.34±0.10
1.17±0.34
1.53 ±0.4
4.8 ±1.2
-0.928
-2.547
Octahedron (111)
0.26±0.08
1.55 ± 0.32
1.55 ±0.7
3.1 ±1.0
-0.856
-1.903
Pd shape
[a] Note: Ead represents the adsorption energy for H2 and resazurin molecule, respectively. Unit: eV
According to the previous work, such plots of TOFs as a function of resazurin concentration coincide with a competitive Langmuir-Hinshelwood mechanism, which has been widely used for heterogeneous catalysis.42 In this model, two substrate molecules maintain fast adsorption equilibrium on the surface of nanoparticles. Then they react with each other and convert into products. Based on this mechanism, TOF is connected with the conventional kinetic parameters as (SI):25,29 ∞
a Aa B
B TOF=-1 =1/ 0 τf(τ) dτ=γeff (1+aA A+a A
B B)
2
(2)
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Where f(τ) is the probability density function of τ, and γeff is the effective catalytic rate constant, representing the reactivity of an entire Pd nanoparticle; aA, aB are the adsorption equilibrium constants of H2 and resazurin, respectively; [A], [B] are the concentrations of H2 and resazurin, respectively. Equation (2) predicts a variation of -1 with the resazurin concentration at constant H2 concentration, consistent with the experimental data shown in Figure 2a. The kinetic parameters (Table 1) derived from these two types of Pd nanoparticles give insight into the reaction kinetics: (1) The catalytic rate constant (γeff) of 22-nm Pd nanocubes is larger than that of 22-nm Pd octahedrons, consistent with the apparent observation from the ensemble experiment (Figure S2). However, the higher activity per Pd nanocube may result from its larger surface area or more surface atoms or active sites, as the activity of per total surface area (γeff/S, S is the total surface area) of Pd nanocubes is lower than that of Pd octahedrons (Table 1); in addition, both the per total surface atoms activity (γeff/NS, NS is the number of total surface atoms) and the per total atoms activity (γeff/NT, NT is the number of total atoms in a whole particle) of Pd octahedrons are higher than that of Pd cubes (Table S1), all indicating that Pd octahedrons possess higher intrinsic catalytic activity per site than Pd nanocubes. (2) The hydrogen adsorption constant (aA) is almost independent of the shape of Pd nanoparticles, as both
Pd
nanocubes
and
octahedrons
exhibited
similar
aA value.
The
shape-independence of aA probably could be due to the small H2 molecule fast dissociated spontaneously into protons or tiny atomic hydrogen (H) when adsorbed onto much larger Pd surface.7,43 (3)The adsorption constant (aB) of resazurin on Pd 8 ACS Paragon Plus Environment
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nanocubes with (100) facets is larger than that on Pd octahedrons with (111) facets, probably due to the higher surface energy of (100) facet than that of (111) facet.13 As the structures of resazurin and resorufin are similar, the strong binding between the (100) facets and the resazurin molecules means the strong binding of resorufin molecules to (100) facets, which may prevent the access of substrate to the active site. This could explain the lower catalytic activity (γeff/S) of (100) facets than that on (111) facets. To further clarify the adsorption properties of reactant on these two kinds of Pd nanoparticles, theoretical calculation of adsorption energy (Ead) of H2 and resazurin on Pd (100) and (111) facets was performed (SI and Table 2). The results show that H2 adsorbed on different facets dissociate into two atomic hydrogen (H) directly, consistent with previous results.7,43 However, resazurin molecule adsorbed on Pd surface invokes great changes for molecule itself and Pd surface (Table 2), indicating large interaction between Pd surface and the substrate resazurin molecule, with a spontaneous surface-recostruction. Moreover, as Table 1 shows, the adsorption energies for resazurin molecule on the two facets were larger than those for H2, indicating the stronger adsorption of resazurin than H2. The calculation also shows that the adsorption energy of the two reactants on (100) facet is larger than that on (111), confirming the observed stronger adsorption of resazurin (aB in Table 1) on (100) facet than that on (111) facet. In addition, the theoretical result reveals a tiny difference in the H2 binding over the two Pd facets, with -0.928 eV on (100) vs. -0.856 eV on (111), consistent with the observation (aA in Table 1). 9 ACS Paragon Plus Environment
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Table 2. The adsorption of H2 and resazurin on Pd(100) and Pd(111). Initial Structure
Final optimized Structure/the pink part represents the change in structure
Pd nanocube with (100) facets Resazurin
H2
Pd octahedrons with (111) facets Resazurin
H2
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Figure 2. (a)Plots of turnover of frequency (TOF) of single nanoparticle versus different resazurin concentration at saturated H2 solution,, red: 22-nm Pd nanocubes; black: 22-nm Pd octahedrons. Each data point averages from the many nanoparticles (> 80), error bar is sem. Solid lines are fitting with equation (1) with red line: γeff = 0.34 ± 0.1 s-1, aA= 1.53 ± 0.4 mM-1, aB = 4.8 ± 1.2 nM-1 and black line: γeff = 0.26 ± 0.08 s-1, aA= 1.55 ± 0.7 mM-1, aB=3.1 ± 1.0 nM-1. (b) Distribution of γapp from individual Pd nanoparticles at [B] = 0.5 nM and H2-saturated solution, red: 22-nm Pd nanocubes; black: 22-nm Pd octahedrons. Solid lines are a Gaussian fit with center at 0.044 ± 0.001 s-1 (red line), 0.038 ± 0.001 (black line) and fwhm of 0.0225 (red line), 0.019 (black line). (c,d) Distribution of the turnover time τ for a single particle of 22-nm Pd nanocube (c) and 22-nm Pd octahedrons (d); solid line is single exponential equation fit and gives the decay constant of γapp= 0.045 ± 0.003 s-1 (c) and 0.037 ± 0.004 s-1 (d).
Furthermore, the catalytic activity of individual Pd nanoparticle can be quantified and the activity distribution of many individual nanoparticles also can be determined following two-site Langmuir-Hinshelwood mechanism. The interval time τ, the time needed to complete a reaction turnover, can be evaluated by Equation 3: ( ) = (%& Here,
--
! " # $
' ! "& # $)
exp *+ (%&
! " # $
! "& # $)
'
, =
-- exp (+ -- )
(3)
= /" 0/$ 1⁄(1 + /" 0 + /$ 1)2 , is the apparent product
formation rate constant of resorufin on the surface of single nanoparticle. Figure 2c 11 ACS Paragon Plus Environment
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and 2d show the typical histogram distributions of τ from one Pd nanocube and one Pd nanooctahedron, respectively. The solid curves are the fitting results based on Equation (3) to obtain γapp for individual nanoparticles. Finally, the distribution of γapp for many nanoparticles can be determined as shown in Figure 2b. The γapp of both Pd nano-cubes and nano-octahedrons are both distributed broadly, suggesting large static activity heterogeneity among individual Pd nanoparticles. To compare the heterogeneity of γapp quantitatively, a parameter, heterogeneity index (h, in percentage, is the value of the full width at half maximum (fwhm) of the Gaussian distribution divided by the average), was used to evaluate the spread of values from the average, which the larger heterogeneity index h is, the greater heterogeneity the catalytic activity has.29,44 The values of h for Pd nano-cubes and Pd nano-octahedrons are almost the same (hcube = 44 ± 8 %, hoctahedron = 42 ± 4 %), indicating the static activity heterogeneity of these two shapes of Pd nanoparticles with the same size is not sensitive to the shapes/facets. The center of the Gaussian distribution, xc, of Pd nano-cubes is larger than the xc of Pd nano-octahedron, that is, the apparent rate constant, γapp, of Pd nano-cubes is larger than that of latter, indicating the product formation on individual Pd nano-cube is faster than that on individual Pd nano-octahedron, which is in accordance with the above results related to γeff shown in Table 1 or the ensemble experiment (Figure S2). Moreover, the underlying dynamic activity fluctuations of single nanoparticles can be determined by the temporal variations frequency of their catalytic turnovers from the single-turnover trajectory. Recently, the temporal activity fluctuations of 12 ACS Paragon Plus Environment
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single Au and Pt nanoparticles have been demonstrated at single molecule level,45,46 and these activity fluctuations of nanoparticles are the results of the surface atoms restructuring which occurred spontaneously or induced by catalysis. It should be noted here that the surface atom restructuring may have some contribution from the oscillatory kinetics of surface coverage of reactants on nanoparticles.47 In addition, the timescale of the surface atoms restructuring equals to the timescale of the activity fluctuations, which can be obtained by analyzing their autocorrelation functions Cτ(m) = /< ∆τ2>.25,48 Here, m is the number of catalytic turnover index in the sequence and ∆τ(m)= τ(m) – . In the presence of catalytic activity fluctuations of Pd nanoparticles, Cτ(m) presents a decay behavior and gives the decay time constant, suggesting the underlying fluctuation correlation time.
Figure 3. (a)(b) Autocorrelation function Cτ(m) of m from the catalytic turnover trajectory of a single Pd nanocube (a) and Pd octahedron (b) at 1nM resazurin concentration and H2-saturated solution. Solid line is the single exponential equation fitting and gives the decay constant (a) 8.7 ± 1.2 turnovers and (b) 2.4 ± 0.6 turnovers. inset: autocorrelation function Cτ(τ) of τ for Pd nanocubes (a) and Pd octahedrons (b); solid line is the single exponential fitting and gives the decay constant (a) 267 ± 72s and (b) 97 ± 32 s. Each date is averaged from > 50 trajectories. (c) Plots of the activity fluctuation rate of Pd octahedrons (red dots) and Pd nanocubes (black dots) versus TOF. Solid lines are fittings from the horizontal lines at 0.004 ± 0.003 s-1 (red line, Pd nanocubes) and 0.013 ± 0.004 (black line, Pd octahedrons).
For a single Pd nanocube and Pd octahedron, the Figure 3a and b show both their
Cτ(m) exhibit a single exponential decay behavior, suggesting the temporal catalytic 13 ACS Paragon Plus Environment
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activity fluctuations of the Pd nanoparticles. The decay constants of Cτ(m) are 2.4 ± 0.6 turnovers for the individual Pd nanocube and 8.7 ± 1.2 turnovers for the individual Pd octahedron. To further determining the facet effect on fluctuation timescale of the surface atom restructuring, the x-axis from the turnover index m can be converted into real catalytic turnover time by
multipling the average catalytic turnover time of each
particle in these patterns.29 As shown in the insets of Figure 3a and b, the corresponding autocorrelation function Cτ(m) of the microscopic catalytic reaction times of Pd octahedrons and nanocubes could be derived from their turnover trajectories. When averaged over large quantities of trajectories, their behaviors of single exponential decay were preserved. The two corresponding average decay time constants or fluctuation correlation times were 267 ± 72 s for Pd nanocubes and 97 ± 32 s for Pd octahedrons, respectively, representing the time scales of activity fluctuation. The two values revealed the underlying dynamic restructuring time scales of the surface atoms on Pd nanoparticles. We further explored the TOF-dependent fluctuation rate of the catalytic activity (obtained by inversing the fluctuation correlation time) with the two shapes of Pd nanoparticles. From Figure 3c, we found that the fluctuation rate is mostly independent of the TOF, sharp contrast to Au nanoparticles.25 In this case, the average fluctuation rate of Pd octahedrons was 0.013 ± 0.004 s-1 (Figure 3c), which was almost three times faster than that of Pd nanocubes (0.004 ± 0.003 s-1). The difference demonstrates that Pd nanocubes are more stable than Pd octahedrons since the sharp corners and edges of Pd octahedrons would be easily restructured under chemical 14 ACS Paragon Plus Environment
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perturbations.49 We could also draw a conclusion that the restructuring rate of the surface atoms of Pd particles is facet -dependent apparently in this case.
4. CONCLUSION In summary, two types of 22-nm Pd nanoparticles (nanocubes with (100) facets and octahedrons with (111) facets) have been studied here to reveal the facet effect on catalytic reactivity, activity distribution and kinetic dynamics at single particle level. The results indicate that Pd octahedrons possess higher intrinsic catalytic activity per site than Pd nanocubes, and both the catalytic reactions by two facet-different Pd nanoparticles follow the same two-site Langmuir-Hinshelwood mechanism. Interestingly, it was found that these two types of nanoparticles exhibited the same adsorption ability to small substrate molecules such as hydrogen but different ability to the size-larger resazurin molecules probably due to different binding patterns. Theoretical calculation further clarified the observed adsorption difference. For these two types of Pd nanoparticles, it was found that the activity fluctuation or heterogeneities could be mainly attributed to the spontaneous surface reconstruction, while the catalysis-induced surface reconstruction was negligible. The spontaneous fluctuation of Pd octahedrons was faster than that of Pd nanocubes, indicating the reconstruction of active sites was not directly correlated with the surface energy under our experiment conditions. The new knowledge obtained here provides fundamental instruction towards the controllable synthesis of nanocatalysts with high activity and stability. Also, this single molecule catalysis could deepen our understanding of the 15 ACS Paragon Plus Environment
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underlying catalytic kinetics and dynamics on different facets, which was complementary to the previous ensemble measurement. We believe that this work is of great help to the further design of nanocatalysts and the mechanism study.
ASSOCIATED CONTENT
Supporting Information. The following files are available free of charge. TEM characterization of Pd cubes and Pd octahedrons, Ensemble measurement of Pd nanoparticles catalyzed reaction, Table S1, Derivation of the various kinetic equations of single nanoparticle catalysis.
AUTHOR INFORMATION
Corresponding Authors *
[email protected]; *
[email protected] Author Contributions ‡These authors contributed equally.
Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Work was funded by the National Basic Research Program of China (973 Program 2014CB932700), National Natural Science Foundation of China (U1601211, 21633008, 21422307, 21433003, 21573215, 21503212, and 21573206), “the 16 ACS Paragon Plus Environment
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