Shape Evolution of Metal Nanoparticles in Binary Gas Environment

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Shape Evolution of Metal Nanoparticles in Binary Gas Environment Jun Meng, Beien Zhu, and Yi Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00052 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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The Journal of Physical Chemistry

Shape Evolution of Metal Nanoparticles in Binary Gas Environment Jun Meng1,2, Beien Zhu*1 and Yi Gao*1 1Division

of Interfacial Water and Key Laboratory of Interfacial Physics and

Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800 China. 2University

of Chinese Academy of Sciences, Beijing 100049, China

*Email: [email protected]; [email protected].

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Abstract

To control the shape and structure of metal nanoparticle (NP) is a crucial strategy to improve its catalytic properties, but the understanding and quantitative description of the structure reconstruction of the catalysts under reaction conditions has not been achieved. Previous works are mostly focused on the single gas conditions, which is apparently not the case in the real catalytic reactions. In this work, a multi-scale structure reconstruction model is established to describe the equilibrium structures of metal NPs in mixed gas environment quantitatively. Taking NO and CO reaction as a model system, the structures of the Pd, Pt and Rh NPs in a large range of temperature and pressure are fully presented. Moreover, we show the variation of 𝑃 : 𝑃

plays

the critical role in determining the structures and therefore the number of active sites of the NPs at certain conditions. This work provides an efficient model to guide the future experiments in the real reactions.


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Introduction Metal nanoparticles (NPs) have drawn lots of research interests because of their remarkable catalytic performance in a large range of chemical reactions. One important issue is the shape control of metal NPs, since the physical properties and catalytic performance of metal NPs mainly depend on their sizes, shapes and structures. 1-2 To improve the activity of metal NPs, many experimental procedures have been developed to prepare metal NPs with specific shapes.3-6 However, well-defined NPs may change their shape in the reactive environment,7-17 resulting in dramatically altered catalytic performance.18-24

Using

time-resolved

high-resolution

transmission

electron

microscopy, a periodic refacetting of the Pt NPs was observed in the CO oxidation reaction.12 Eren et al. have reported that the Cu(111) surface decomposed into clusters when exposed to CO gas, and the cluster-covered Cu(111) surface was very active in dissociating water.21 Thus, it is a general idea now that the study of the morphology change of metal NPs in reactive environment is a key step to understand and control the real catalytic properties. However, up to date, most of the experiments and theoretical studies are limited in the condition of single gas, while in a real catalysis the NPs are mostly exposed to mixed gas conditions. NO and CO as the major pollutants of car exhausts, cause great damage to our living environment. One of the most promising way to eliminate the exhaust toxic gases is the reduction of NO by CO.25-26 The three-way catalysts including Pd, Pt, and Rh have been used in the automobile industry to oxidize CO to O2 and reduce NO to N2 simultaneously.27-29 Both NO and CO can influence the equilibrium shape of metal 3 ACS Paragon Plus Environment


NPs.10, 12, 19-23, 30-31 As a typical instance, Newton et al. have observed the size and shape changes of Pd NPs during NO/CO cycling.23 It is also known that the different facets of metal nanoparticles exhibit quite different catalytic activities. Goodman and coworkers have investigated the NO+CO reaction on Pd(111) and (100) facets. 32 At temperatures from 525K-650K, 𝑃 /𝑃

from 1:16 to 16:1, and total pressure of 2-

17 Torr, they found out that Pd(111) always shows a higher activity than Pd(100) for both CO2 and N2O production, while Pd(100) gives a higher selectivity for N2 than Pd(111). Nakao et al. discovered that Pd(110) is more active than Pd(111) surface in the pressure range of 10-2~10-1 Torr.33 Thus, the understanding of the morphology change of the metal NPs under CO and NO would be critical for the tuning and controlling its catalytic activities for real applications. In this work we present a multi-scale structure reconstruction model (MSRM) to quantitatively describe the equilibrium structures of metal NPs larger than 3nm interacting with mixed gas. In our previous studies, the corresponding single gas model gave precise description of metal NPs under single gas environment.13-15 We have discovered that the introducing of water vapor change the shape of Cu NPs,13 which have been observed by the in situ TEM experiments.9 Besides, CO, NO, O2, or H2 also has influences on the equilibrium shape of metal NPs depending on the temperature and the pressure.14-15 In order to figuring out how the mixed gas could affect the shape of metal NPs, taking NO + CO reaction as a model system, the shape evolutions of Pd, Pt and Rh NPs under NO and CO environment are investigated. The results show that the structures of Pd, Pt and Rh NPs could be not only adjusted by the temperature, pressure 4 ACS Paragon Plus Environment

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but by the partial pressure ratio of NO to CO (𝑃 ⁄𝑃 ). Under certain conditions, the structures of the NPs are highly dependent on the 𝑃 ⁄𝑃

and largely differ from the

ones in pure NO or CO environment. The partial pressure of the mixed gas, together with the temperature and the total pressure, should be considered as crucial parameters in the rational design of nanocatalysts. Our model provides an effective and useful method for advancing the structure reconstruction study from single gas effects to mixed gas effects. Computational Details According to the Wulff rule, the equilibrium shape of a nanoparticle(NP) could be constructed if the surface tensions of the NP’s facets are known. 34 When the NP is exposed to the reaction environment, the surface tensions 𝛾 the interface tensions 𝛾

should be revised as

, considering the interactions between the NP surfaces and

the adsorbates.14 To obtain revised interface tension, Fowler-Guggenheim (F-G) adsorption isotherm35 is employed to deal with the localized monolayer adsorption with the lateral interactions = 𝑃𝐾 𝑒𝑥𝑝 (1) where 𝜃 is the coverage of the adsorbates on the NP surface, 𝑃 is the gas pressure, 𝐾 is the equilibrium constant, 𝑧 is the number of the nearest neighbor sites, 𝑤 represents the interaction energy between two adjacent adsorbates, 𝑅 is the gas constant, and 𝑇 is the temperature. When the NP interacts with binary gases, an extended competitive adsorption isotherm can be derived based on this isotherm. 36 5 ACS Paragon Plus Environment


(

)

(

)

= 𝑃 𝐾 𝑒𝑥𝑝

(

)

(

)

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(2) = 𝑃 𝐾 𝑒𝑥𝑝

(3) where the subscripts A and B correspond to the two component A and B, respectively. 𝑤 (𝑤 ) represents the lateral interaction between A(B) molecules. 𝑤

represents the

interaction energy between A molecule and B molecule.𝐾 and 𝐾 is the equilibrium constant of the adsorption process for gas A and B, respectively, which are defined as 𝐾 = 𝑒𝑥𝑝 −

_

_

_

𝐾 = 𝑒𝑥𝑝 −

_

_

_

(4)

(5) where the 𝐸 𝑆

_

/𝑆

_

_

/𝐸

and 𝑆

is the adsorption energy of a gas A/B molecule.

_

_

/𝑆

is the entropy of gas A/B before and after adsorption,

_

respectively. By solving the above equations, we can get the value of 𝜃 and 𝜃 . Then the interface tension 𝛾 𝛾

= 𝛾

+

could be obtained by the following equation (

_

( A

+ AB

))

(

_

( AB

+

B

))

(6) 𝐴

is the surface area per atom. In this work, the surface tension 𝛾

, the adsorption energy 𝐸

and the lateral

interaction 𝑤 are computed by spin-polarized density functional theory (DFT) calculations using the Vienna Ab-initio Simulation Package (VASP). Three low-index 6 ACS Paragon Plus Environment

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facets (100), (110) and (111) are considered in this study to build the atomic structures of the NPs. The Perdew, Burke, and Erzernhof functional (PBE) is used to calculate the surface tension 𝛾

. By optimizing a series of (1×1) slab models with increasing

layers, the accurate surface energy could be extracted. The surface energies per atom are presented in Table S1 in the supporting information. Revised gradient-corrected functional of Perdew, Burke and Erzernhof (RPBE) is used to get the adsorption energies 𝐸

and the lateral interactions 𝑤 of NO/CO on

Pd/Pt/Rh surfaces. The adsorption energy 𝐸

is obtained by a (4×4) slab model.

Combined with the calculation of a (1×1) slab model, the lateral interaction energy 𝑤 between the same kind of adsorbates can be derived in such way ( × )

𝑤=

( × )

(7) where 𝐸

( × )

represents the monomolecular adsorption energy, 𝐸

( × )

is

the adsorption energy at 1 monolayer adsorption. The adsorption sites are carefully tested in the (4×4) and (1×1) slab models and the most stable structures are used to calculate the lateral interaction. In order to obtain the lateral interaction between NO and CO molecules, denoted by 𝑤

, a (2×2) slab model with the coadsorption of NO and CO molecules is

established. NO and CO coadsorbed on the surfaces side by side and the total coverage is 1 monolayer. The 𝑤

can be derived by solving several equations. Details of

the calculation methods and the results of the adsorption energies 𝐸 interaction 𝑤

, 𝑤

and 𝑤

are displayed in the ESI file. 7

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, lateral


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The slab models are separated with a 15 Å vacuum space along Z axis. Each model contains 6 atom layers and the bottom 2 layers are fixed. For all the DFT calculations, the cutoff energy of plane wave is 400 eV, the convergence of the electronic selfconsistent is 10-5 eV, and the force convergence criteria in a conjugate-gradient algorithm is 0.05 eV/Å respectively. The Kpoints are set as (2×2×1), (4×4×1) and (6×6×1) for the (4×4), (2×2) and (1×1) slab models, respectively. When the gas molecules adsorbed on the surfaces, the entropy 𝑆 as zero. The entropy of gas phase molecules 𝑆 𝑆 = 𝑆(𝑇, 𝑃 ) − 𝑅𝑙𝑛(

is assumed

is described by

)

(8) 𝑃 is 1 atm, and 𝑆(𝑇, 𝑃 ) is the entropy at 1 atm with different temperature. The 𝑆(𝑇, 𝑃 ) is fitted by data from the NIST-JANAF Thermochemical Tables37. Applying this multi-scale construction model, the equilibrium shape of Pd, Pt and Rh NPs under NO and CO environment are predicted. All the constructed NPs are around 10 nm, 28000 atoms. Results & Discussion The equilibrium structures of Pd, Pt and Rh NPs are first constructed with fixed partial pressure ratio ( 𝑃 /𝑃

= 1: 1) , which is a common 𝑃 /𝑃

in real

application due to the stoichiometry of the reaction 2NO + 2CO → N2 + 2CO2. The temperature is ranging from 200 K to 1200 K and the total pressure is ranging from 1 Pa to 10000 Pa. Fig. 1 shows the structure evolution of a Pd NP with respect to the temperature and the total pressure. Generally, at a constant total pressure, the shape of 8 ACS Paragon Plus Environment

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the Pd NP changes from a rhombic dodecahedron (blue region) to a truncated octahedron (red region) with increasing temperature. At a constant temperature, the shape of the Pd NP changes from a truncated octahedron (red region) to rhombic dodecahedron (blue region) by increasing the total pressure. The dynamic response of Pd NP to temperature and pressure is attributed to the change of coverage of NO and CO molecules on each of the three low-index facets under different conditions.

Fig. 1

(a) Contour plot of the structures of a 10nm Pd NP changing with the

temperature and the total pressure (𝑷𝑵𝑶 /𝑷𝑪𝑶 = 𝟏: 𝟏). Different structures are distinguished by the structure index number from 1 to 6. The black area indicates the negative interface energies area. (b) The typical structures of the Pd NP labeled by the structure index number. (c) The fractions of (111), (100) and (110) facets of the typical Pd NPs. Taking 1000 Pa as an example, which is a normal condition for the NO+CO reaction to take place,29 the coverage of NO and CO on the Pd (100), (110) and (111) surfaces, and the corresponding interface tensions are shown in Fig. 2. When the temperature is higher than 850 K, the Pd NP almost exposes (111) and (100) facets 9 ACS Paragon Plus Environment


entirely (structure 6 in Fig. 1). The (111) facet, as the dominant surface, occupies 85% of the surface region. Since the temperature is so high that the gases could not adsorb on the NPs (Fig. 2), the corresponding morphology is thus the same as the structure in vacuum. There is a large morphology change of the Pd NP at the temperature ranging from 700K to 850 K. As the temperature decreases, more NO and CO molecules could adsorb steadily on Pd surfaces. NO is the dominant adsorbate compared to CO due to its stronger adsorption energy (Table S2), as shown in Fig. 2a. The coverage of NO increases rapidly on (110) facet, resulting in the rapid decline of the interface energy of (110) facet (Fig. 2b). The proportion of (110) facets keeps to enlarge and reaches 100% when the temperature is smaller than 700 K (from structure 6 to 1). It is very interesting to see that the interface tension becomes negative when the temperature is below 600 K. As discussed in our previous work and in the literatures,14, 31 when the interaction between the adsorbates and the surface is strong enough, the interface tension could be negative. This will break up the NP into smaller nanoclusters. This prediction is in consistent with the experimental observation that Pd atoms with lower coordination numbers induced by NO.23


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Fig. 2

(a) The coverages of NO and CO on Pd (100), (110) and (111) surfaces,

changing with the temperature. (b) The interface energies between Pd surfaces with NO and CO gases, changing with the temperature. The total pressure is 1000 Pa (𝑷𝑵𝑶 /𝑷𝑪𝑶 = 𝟏: 𝟏). Above we present the morphologies that the Pd NPs could have under different conditions at given 𝑃 /𝑃 . One interesting and important thing in this work is to see how the 𝑃 /𝑃

may alter the structure at given temperature and total pressure. The

experiments have shown the metal NPs display different catalysis selectivity under different partial pressure ratio of NO to CO.32 To investigate the effect of 𝑃 /𝑃

on

the structure of Pd NPs, the equilibrium structures of a 10 nm Pd NP are constructed with different 𝑃 /𝑃

at different conditions.



Fig. 3

The structures evolution of a 10nm Pd NP changing with the temperature

and 𝑷𝑵𝑶 /𝑷𝑪𝑶 ratio under a total pressure of (a) 10 Pa, (b) 100 Pa, (c) 1000 Pa and (d) 10000 Pa. The structure evolution of Pd NP under pure CO and pure NO environment are displayed at the bottom and the top. The structures are represented by the structure index number. The typical structures of the Pd NP labeled by the structure index number have been shown in Fig. 1(b). The results of pure CO and pure NO are also provided to make comparison. From Fig. 3 we can see an overall conclusion that whether the variation of the 𝑃 /𝑃 would induce the structure reconstruction depends on the structures of the NP in pure CO and pure NO environment. If the structure of the NP in pure NO is the same as the one in pure CO environment, then it is the same for all the 𝑃 /𝑃 . There is no difference between the structure of the NP in single gas and in mixed gas. As we can 12 ACS Paragon Plus Environment

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see there are “solid colour” regions in Fig. 3. On the other hand, the 𝑃 /𝑃

has

significant effect on the structure of the NP if the structure in the pure NO environment is different to that in the pure CO environment. The “gradient colour” regions can be found in the middle of each graph in Fig. 3. As an example, at 10 Pa and 580K, the Pd NP is enclosed by 15% (100) and 85% (111) facets labelled by red colour (structure 6 in Fig. 1-b) in the pure CO environment, while under the same condition, it is enclosed by 88% (110) and 12% (111) facets labelled by cyan colour (structure 2 in Fig. 1-b) in the pure NO environment. Since NO has greater adsorption energies on Pd surfaces than CO, under this condition it has greater effects to decrease the interface energies, especially for the (110) facet. That is why the Pd NP has a large (110) facet in pure NO environment. When the 𝑃 /𝑃

increases from 1:10 to 10:1, there is a transition

procedure that the structure of the Pd NP gradual changes from the shape in pure CO to the shape in pure NO. In this transition procedure, the proportion of (110) facet of the Pd NP keeps growing, meanwhile the (111) and (100) facet keeps declining. So it is very interested to see that the NP goes through a complete red-yellow-light greendark green-cyan structure evolution. Apparently, the structure of the NP could be very different at different 𝑃 /𝑃 . Since different facet of the NP has different activity and selectivity, the change of the facet fraction caused by the 𝑃 /𝑃

also influences the NP’s catalytic property.

Notice that the boundaries between the “solid color” regions and the “gradient color” regions shift with temperature and pressure. The 𝑃 /𝑃

, together with the

temperature and pressure should be taken into consider as critical parameters in the 13 ACS Paragon Plus Environment


rational design of nanocatalysts. It actually provides more possible ways to tailor the desired structure and property of the catalyst. In the case of CO+NO reaction, the Pd(111) surface is the surface attracting lots of interests.28, 32, 38 If one wants to have more (111) facets when using Pd NPs as catalysts in this reaction, the structure 3 and 4 of the NP in Fig. 1-b are good candidates. The structure 5 and 6 are not good choice although they have the largest (111) facet fractions since the adsorption of CO and NO need to be weak enough to have those structures. According to our work, the structure 3 and 4 can be achieved by different ways. It could be done by controlling the temperature and total pressure at fixed 𝑃 /𝑃 tuning the 𝑃 /𝑃

(e.g. 1:1). It could also be done by

at the temperature and total pressure beneficial to the reaction.

Besides the structure, the 𝑃

/𝑃

could also influence the size of the NP. As we

discussed above, the black region in our contour plots representing the dispersion of the NP induced by the negative interface tension. In Fig. 3 it can be seen that under certain condition (e.g. 700 K, 10000 Pa) the NP transfers from a blue region to a black region with the increased 𝑃 /𝑃 . This indicates the NP disperses from bigger one to smaller ones. A similar phenomenon have been reported by Newton et al. in their experimental study of the size and shape changes of Pd NPs during NO/CO cycling. 23 At 673 K, the Pd NP was found to sinter in pure CO environment and to disperse in mixed CO+NO and pure NO environment. Their observations could be well understood by our results.


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Fig. 4

(a) Contour plot of the structures of a 10nm Pt NP changing with the

temperature and the total pressure (𝑷𝑵𝑶 /𝑷𝑪𝑶 = 𝟏: 𝟏). (b) The structures evolution of the Pt NP changing with the temperature and 𝑷𝑵𝑶 /𝑷𝑪𝑶 ratio under a total pressure of 10 Pa. Different structures are distinguished by the structure index number from 1 to 6 in (a) and (b). (c) Typical structures of Pt NP labeled by the structure index number. (d) Contour plot of the structures of a 10nm Rh NP changing with the temperature and the total pressure (𝑷𝑵𝑶 /𝑷𝑪𝑶 = 𝟏: 𝟏). (e) The structures evolution of the Rh NP changing with the temperature and 𝑷𝑵𝑶 /𝑷𝑪𝑶 ratio under a total pressure of 10 Pa. Different structures are distinguished by the structure index number from 1 to 6 in (d) and (e). (f) Typical structures of Rh NP labelled by the structure index number.



For Pt and Rh NPs, similar transformation trends have been observed. The structure of Pt and Rh NPs under NO and CO environment are shown in Fig. 4. Although the general evolution trends of Pt and Rh NP structure are consistent with Pd NP, there are still some differences. DFT calculations shows that the adsorption energies and the lateral interactions of NO and CO are different on Pd, Pt and Rh surfaces (Table S2). There is a great difference between the shape of Rh NPs in pure CO and mixed gas condition, because NO has much larger adsorption energies than CO on Rh (Table S2). The results show that the dispersion of Rh NPs from CO to mixed gas condition is easier than Pd and Pt. Meanwhile, there is a considerable region of negative surface energies for Rh NP under NO and CO environment, which means nanoclusters could be easily formed on Rh surfaces under NO and CO conditions. The changing of the facet ratio for Pd, Pt and Rh are consistent in general, except that the structure 2 of Pt NP is enclosed by (110) and (100) surfaces, Pd NP is enclosed by (110) and (111) surfaces and Rh NP is enclosed by (110), (111) and (100) facets. The effect of the 𝑃 /𝑃

on the structures are also investigated at 10 Pa shown in Fig (4-b,e)

(Results of other pressures are also presented in the ESI file). The results show that the partial pressure ratio of the mixed gas has a general effect on the structure of different metal NP. It should be mentioned the work of our MSRM model is not limited in NO and CO environment, but a general model suitable for other systems. Further application of this model to other binary or even more complex systems will be done in our future works. 16 ACS Paragon Plus Environment

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Conclusions In summary, a multi-scale structure reconstruction model (MSRM) is proposed to predict the equilibrium structures of metal NPs under binary gas environment. Taking NO and CO reaction as a model system, the shape evolution of Pd, Pt and Rh NPs under NO and CO binary gas environment is investigated. With a fixed partial pressure ratio of 𝑃

/𝑃

= 1 , the shape evolution of Pd, Pt and Rh NPs in a large range of

temperature and pressure is fully presented, which changes between a rhombic dodecahedron and a truncated octahedron. More important, we show the equilibrium structures could be largely alerted not only by the temperature and total pressure but also by tuning the partial pressure ratio of NO to CO. It is very necessary to take into consider the effect of the partial pressure ratio of the mixed gas when tuning the structure and catalytic property of the NPs. On the other hand, it offers more possible ways to design the catalysts. Our study shows the importance to study the structure reconstruction under mixed gas condition and also paves the way for the future studies in this area by offering a quantitative model.

Supporting Information. The following file is available free of charge. Computational details of the lateral interaction energies. Data of surface tensions, surface area, adsorption energies, and lateral interactions. Structure evolution of 10nm Pt (Rh) NPs change with the temperature and the 𝑃 /𝑃

ratio with a total pressure of 100Pa, 1000Pa and

10000Pa. 17 ACS Paragon Plus Environment


Notes The authors declare no competing financial interests. Acknowledgments This work is supported by National Natural Science Foundation of China (11604357, 21773287, 11574340). B.Z. also thanks for the development fund for Shanghai talents (Y439011011), and Natural Science Foundation of Shanghai (16ZR1443200). The authors also thank the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501. The computational resources utilized in this research were provided by Shanghai Supercomputer Center, National Supercomputer Centers in Tianjin, Shenzhen.


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Shape Evolution of Metal Nanoparticles in Binary Gas Environment

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