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Theoretical Investigation of Metal‐Support Interactions on Ripening

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Theoretical Investigation of Metal‐Support Interactions on Ripening DOI: 10.1002/cnma.201800052 Full Paper 1 2 3 4 Supported Metal Nanoparticles 5 6 7 Theoretical Investigation of Metal-Support Interactions on 8 9 Ripening Kinetics of Supported Particles 10 [a, d] [a, b, c] 11 Sulei Hu ( ) and Wei-Xue Li ( )* 12 13 Abstract: The stability of supported metal particles is one of ature and half-life time of ripening, a fact that should be 14 the key issues for successful industrialization of catalysts. A prevented. Moreover, supported particles with low surface 15 theoretical study of Ostwald ripening of supported particles energy and/or high sublimation energy of supported particles 16 and its dependence on metal-support interactions, sublima- would have a high onset temperature and a long half-life 17 tion energy, and surface energy is reported. Two distinct time. Compared to the metal particle-support interaction and 18 metal-support interactions are differentiated: metal particle- surface energy, the metal atom-support interaction and 19 support and metal atom-support. Although strong metal sublimation energy are most influential to the overall 20 particle-support interaction (small contact angle) stabilizes ripening resistance. The present work highlights the impor- 21 the supported particles and improve the ripening resistance, tance of interplay between two types of metal-support 22 strong metal atom-support interactions decrease the total interactions on the overall stability of supported particles. 23 activation energy and dramatically lower the onset temper- 24 25 26 27 Introduction down the sintering rate and prolonging the life time of 28 supported particles under reaction conditions are vital for 29 Transition metal particles on supports are widely used to industrialization of laboratory catalysts. 30 catalyse chemical reactions in heterogeneous catalysis.[1] It is Sintering and stability of supported particles have been 31 often prepared at small size for higher surface area, which is studied extensively in past. It was found that among others, 32 particularly important for precious platinum group metals.[2] varying surface structure and composition of supports can 33 Exposed numerous low coordination sites and accompanied effectively improve the sintering resistance.[6] Taking Pt particles 34 quantum size effect make the catalysts intrinsically more active as example, the sintering occurred easily on inert alumina 35 and selective.[3] However, the cost is rapid increase of the support,[7] but was inhibited in a large extent on more active 36 specific surface energy destabilizing significantly the corre- ceria support.[8] Strong metal-support interaction (MSI) was 37 sponding particles. To minimize the specific surface energy, essential here to stabilize the supported particles by forming 38 small particles tend to sinter to the larger ones via particle strong PtÀOÀCe bond at the interface.[9] Recent studies indicate 39 migration and coalescence[4] and/or Ostwald ripening.[5] Slowing that strong MSI can also promote catalytic activity through so- 40 called interface-confined effect[10] or electrical interaction.[11] On 41 [a] S. Hu ( ), Prof. W.-X. Li ( ) the other hand, MSI could not be too strong, since it might 42 State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics. result in the formation of the metal-support solution, which Chinese Academy of Sciences. [12] 43 Dalian 116023, China deactivates the catalysts otherwise. For particles in confined 44 E-mail: [email protected] space, though particle migration and coalescence is prevented, 45 [b] Prof. W.-X. Li ( ) growth of the particles still happened via Ostwald ripening Department of Chemical Physics, School of Chemistry and Materials Science, [5,13] 46 iCHeM, CAS Excellence Center for Nanoscience through gas phase, where corresponding sublimation en- [14] 47 University of Science and Technology of China ergies matter. It was also reported that morphology of 48 Hefei 230026, China supported particles affects the sintering process, due to the [c] Prof. W.-X. Li ( ) [15] 49 Hefei National Laboratory for Physical Sciences at the Microscale change of surface energy by adsorption of reactants. Never- 50 University of Science and Technology of China theless, theoretical investigation of the metal-support interac- 51 Hefei 230026, China tion, sublimation energy and surface energy on Ostwald [d] S. Hu ( ) 52 University of Chinese Academy of Sciences ripening are valuable, and quantitative study of their influence 53 Chinese Academy of Sciences on ripening kinetics is not reported yet. 54 Beijing 100049, China Ostwald ripening of supported metal particles describes the 55 Supporting information for this article is available on the WWW under growth of the large particles with expense of the smaller ones, https://doi.org/10.1002/cnma.201800052 56 This manuscript is part of a Special Issue on Selective Nanocatalysis. Click which has been extensively studied. In this process, the metal 57 here to see the Table of Contents of the special issue. atoms detach from the smaller particles with higher chemical ChemNanoMat 2018, 4, 510–517 510 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley VCH Montag, 07.05.2018 1805 / 108931 [S. 510/517] 1 Full Paper 1 potential, diffuse on the support and attach to the larger ones 2 with lower chemical potential. In earlier time, Wynblatt and 3 Gjostein[16] generalized the Ostwald ripening theory of the 4 particles in solution developed by Lifshitz, Slyozov,[17] and 5 Wagner[18] (so called LSW theory) to the gas-solid interface. 6 They defined the critical radius, a key parameter describing the 7 ripening behaviour, under the interface-control and diffusion- 8 control limit. Later on, a revised theory applicable to the small 9 particle sizes was proposed.[19] More accurate evaluation of the 10 critical radius subject to the mass conservation[20] and efficient 11 algorithm allowing a larger time step to solve the rate 12 equation[21] were developed. In presence of reactants, the 13 ripening could be promoted significantly via the metal-reactant 14 complexes, and corresponding theory was derived in our 15 previous work.[22] In this context, formation of the metal- 16 reactant complexes was subject to the metal-support inter- 17 action, and influence of reactant adsorption on surface energy 18 of supported particles was included in corresponding ripening 19 theory as well. These provide a unique framework to explore Figure 1. (a) Average and maximum diameter, standard deviation versus ripening time. (b) Normalized volume, dispersion and particle number versus 20 the influence of the metal-support interaction and surface ripening time, and half-life time (green star) is indicated. Thermal temper- 13 À1 21 energy on ripening kinetics. ature=500 K, Etot =2eV,a=908, vs =610 s , initial ˚ 2 ˚ 3 22 We report here a theoretical investigation of influence of <d0 > = <2R0 > =3 nm, rsd0 =16.7%, g=0.094 eV/A , W=18.27 A , the ratio between diffusion length and particle radius L =1.2. 23 the metal-support interaction, surface energy and sublimation r 24 energy on Ostwald ripening of supported metal particles. 25 Ostwald ripening under both isothermal and temperature 26 programmed conditions were considered. Corresponding half- simulation. We note that at the earlier stage of ripening, there 27 life time and onset temperature of ripening are extracted to is a large number of smaller particles with higher chemical 28 quantify the dependence of the ripening resistance on the potential, and the driving force for ripening is larger. As a result, 29 metal-support interaction, sublimation energy and surface the dispersion decreases more rapidly at beginning but 30 energy.For metal-support interaction, we differentiate in partic- becomes slowly later. Different from the dispersion, the particle 31 ularly two different metal-support interactions, metal particle- number decay faster. Specifically, at the time for 50% decrease 32 support interaction (MPSI) and metal atom-support interaction of dispersion, corresponding particle number drops down to 33 (MASI). Surprisingly, these two MSIs present a completely almost 10% of its initial value. The reason behind is that the 34 different and actually opposite influence on rate of ripening, disappearance of smaller particles with less number of atoms 35 which is important for rational design of supported nano- contributes little to the variation of dispersion. Since variation 36 catalysts. of the particle number is more sensitive to time, corresponding 37 half-life time t1/2 is defined to quantify the sintering rate and 38 facilitate the comparison below. In this particular case, corre- 39 Results and Discussion sponding half-life time is 50 days. 40 Ostwald ripening under temperature programmed condi- 41 Ripening under (Non)Isothermal Conditions tion tells the thermal resistance, and the corresponding results 42 are plotted in Figure 2. It can be found that the maximum and 43 We first studied the ripening of supported metal particles with average diameter remains same with its initial value before 44 initial average curvature diameter <d0 > = <2R0 > =3 nm and 600 K, and afterward it starts to increase rapidly with ramping 45 relative standard deviation rsd0 =16.7% under the isothermal temperature (Figure 2a). Corresponding standard deviation ˚ 2 46 condition of 500 K. Etot =2.0 eV, g=0.094 eV/A and a=908 shows a modest increase. In line with rapid increase of the 47 were used in simulation. As expected, both the maximum and average size, the dispersion and particle
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